SERI/CP - 622 -1096

UC CATEGORY; 61 a

SPECIALISTS' WORKSHOP ON

FAST PYROLYSIS OF BIOMASS

PROCEEDINGS

OCTOBER 19 - 22, 1 9 80

FOXPINE INN

COPPER"MOUNTAIN, COLORADO

SPONSORED By:

SoLAR ENERGY RESEARCH INSTITUTE

FOR THE

u.S. DEPARTMENT OF ENERGY
Preface

Table of Contents

Page

1

Workshop Summary

James Diebo1d~ Solar Energy Research Institute • . . . • . • 3

Heat Transfer Perspectives

T. Reed~ J. Diebold~ and R. Desrosiers~ Solar Energy

Research Institute . . • • • . . . • . . • • 7

Fast Pyrolysis of Fossil Fuels

Theory of Hydrocarbon Pyrolysis to Form Olefins

S. Narayanan~ Stone &Webster Engineering Corporation ••• 21

Commercial Hydrocarbon Cracking Practices

R. Corrtr'act.or E. I. du Pont de Nemours & Company • • . . . 45

Flash Pyrolysis and Hydropyrolysis of Coal

M. Steinberg and B. Bhatt, Brookhaven National Laboratory. 57

Low Temperature Pyrolysis of Biomass

Introduction to Pyrolysis of Biomass

F. Shafizadeh~ University of Montana • 79

Direct Formation of Pyrolysis Oils from Biomass

H. Kosstrin, Stone &Webster Engineering Corporation • . . 105

The Conversion of Biomass Derived Pyro1ytic Vapors

to Hydrocarbons

T. Frankiewicz~ Occidental Research Corporation.

High Temperature Pyrolysis of Biomass

High Temperature Electric Arc, RF Energy, and Combustion

Gas Heated Pyrolysis of Powder Biomass

D. Goheen, Crown Zellerbach Corporation ...

123

137

High Radiative Heat Flux Pyrolysis of Thin Biomass

K. Lincoln, Ames Research Center, NASA.. • •••.. 153

Bench Scale Radiant Flash Pyrolysis of Biomass

M. Antal, L. Hofmann, J,. Moreira, Princeton University .. 175

A Comparison of Heating Technique on the Pyrolysis of

Douglas Fir

K. Voorhees and L. Hendricks, Colorado School of Mines .. 183

Microwave Pyrolysis of Biomass

B. Krieger, University of Washington ......•.... 191

iii
Product Compositions and Kinetics for Rapid Pyrolysis

of Cellulose

W. Peters, M. Haja1igo1, J. Howard, and J. Longwell,

Massachusetts Institute of Technology • •• • • . • • • 215

Ablative Pyrolysis of Macropartic1es of Biomass

J. Diebold, Solar Energy Research Institute

Fluidized Bed Pyrolysis to Gases Containing 01efins

J. Kuester, Arizona State University ••••

• • • . • • 237

253

Pyrolysis in a Tubular Entrained Flow Reactor at

China Lake

C.Benham and J. Diebold, Solar Energy Research

Institute • • • . • . • 271

An Experimental Investigation into Fast

Biomass Using an Entrained Flow Reactor

M. Bohn and C. Benham, Solar Energy

Institute • • • • • • . . • • • • •

Pyrolysis of

Research

• • . • • • 287

Flash Pyrolysis of Biomass in Sweden

C. Ekstrom and E. Rensfe1t, The Royal Institute

of -Te chnol ogy . . • • • • • • • •• •••• 303

Cyclone Reactor for Flash Pyrolysis of Solid Particles

J. Lede, F. Verzaro, B. Antoine, and J. Vi11ermaus,

Chemical Engineering Science Laboratory • • • • • • • • • • 327

Fast Pyrolysis on a Molten Lead Bath

J. Skaates, Michigan Technological University •.•.••• 347

Other Considerations

Preparation of Powdered Feedstock from Biomass with Steam

Q. Nguyen and G. Noble, Iotech Corporation Ltd. • . • 365

The Embritt1ement Treatment of Wheat Straw for Conversion

to a Powder

A. Ghazee and N. Hecht, University of Dayton Research

Institute • • • • 375

Purification and Uses of Fast Pyrolysis Gases made from

Biomass

M. Graboski , Colorado School of Mines

Attendees • • • • • • • •

iv

• 393

•• 415
PREFACE

The Solar Energy Research Institute (SERI) is deeply committed to the

thermal conversion of biomass to fuels and petrochemicals. One of

the most promising techniques involves the very rapid pyrolysis of

biomass and this specialists' workshop was organized to explore in

depth the phenomena of fast pyrolysis. This workshop brought

together most of those who are currently working in or have published

significant findings in the area of fast pyrolysis of biomass or

biomass-derived materials, with the goal of · attaining a .bet t er

understanding of the dominant ·mechani sms which produce olefins,

oxygenated liquids, char, and tars. In addition, background papers

were given in hydrocarbon pyrolysis, "slow' pyrolysis of biomass, and

techniques for powdered-feedstock preparation in order that the other

papers did not ·need to introduce in depth these concepts in their

presentations for continuity. In general, the authors-were requested

to present summaries of experimental data with as much interpretation

of that data as ,pos s i bl e with regard to mechanisms and process

variables such as heat flux, temperatures, partial pressure,

feedstock, particle size, heating rates, residence time, etc. The

atten.dees were limited primarily to the other authors in order' to

have a small informal gathering. After presentation of all the

papers, time was set aside for a round table discussion of the

pyrolysis mechanisms and how they can be manipulated to produce

valuable 'f uel and petrochemical products. The discussion also

addressed those research areas which would , be most fruitful to the

future application of fast pyrolysis of biomass and biomass-derived

materials to more useful fuels and petrochemi.cals. The round table

discussion was, off the ' record to encourage as much discussion as

possible~ It is hoped that these proceedings will constitute a

useful reference volume to those pursuing fast pyroLysd s , In order

to make the proceedings available as soon as possible, the individual

papers were not subjected to editing by SERI and are reproduced

herein as they .were submitted by the authors.

The financial support and technical direction of Simon Friedrich of

the Biomass Energy Systems Division of the U.S. Department of Energy

through Don Stevens and Gary Schiefelbein of Pacific Northwest

Laboratories of Richland, WA are gratefully acknowledged.

James Diebold

Biomass Thermal Conversion

and Exploratory Branch

SERI

1
WORKSHOP SUMMARY

James Diebold, Workshop Olairman

SERI, Golden, CO 80401

This workshop brought together what is believed to be most of the

individuals who have been or who are now involved with the study of

the fast pyrolysis of biomass. These individuals have approached

fast pyrolysis with a number of different methods and reaction

conditions. As evidenced by the relative yields of char, tar,

oxygenated liquids, and gases (including olefins), it appears that

different reaction conditions favor different product distributions.

There was a general agreement that very rapid heating of the biomass

to temperatures above 7000C greatly increased the gas yields at the

expense of the char and tars favored at low heating rates and low

final temperatures. The actual measurement of the heating rate of

the biomass during fast pyrolysis is quite difficult due to the

relatively short time involved with the very high heating rates,

e s g , , at 10,OOOoC/s the biomass is pyrolyzed in 30 to 50

milliseconds. This difficulty in temperature measurement of the

pyrolyzing biomass is reflected by the differences in kinetic

parameters derived by different researchers utilizing different

experimental techniques.

Another complicating factor in comparing pyrolysis data from . several

sources is that there has been very little, if any, attempt to

standardize the feedstocks used in fast pyrolysis. Thus, it has been

difficult to separate the effects of feedstock from experimental

apparatus and techniques. To obtain a uniform sample of a

heterogeneous material such as hygroscopic biomass presents a real,

non-trivial problem due to differences in moisture content,

lignin/cellulose/homocellulose ratios, volatile oils or terpene

content, ash content, and ash composition within even the same

species or the same plant. For example, a pine tree will have a high

concentration of pitch near an old injury, which would be expected to

greatly increase the olefin content of the pyrolysis gases. Refined

biomass products such as microcrystalline cellulose and powdered

Kraft lignin are thought to be very uniform materials, but since they

are not chemically the same as raw biomass their use does not

necessarily lead to valid conclusions relati ve to the pyrolysis of

unp rocassed biomass materials. The use of a central repository

having several "standardized" biomass feedstocks would help in the

comparison of pyrolysis data. The "st anda rdLaatLon'", storage, and

distribution of the feedstocks would require a significant effort for

which none of the attendees currently had resources available. Until

a central biomass repository is established, the most practical, low

cost solution to the non-uniform feedstock problem may well be to use

the highly processed, microcrystalline cellulose as a standard to

compare different pyrolysis techniques and process conditions. The

evaluation of more practical biomass feedstocks would then be

relative to this standard cellulosic feedstock in the same reactor

3
and process conditions. The use of a standard in this way will

increase the experimentation a bit, but several researchers have

already reported fast pyrolysis results with this type of "standard"

material.

The chemical mechanisms involved in the fast pyrolysis of biomass are

not yet well understood. However, in an attempt to visualize the

general effect of process variables, the overall mechanisms shown

below are thought to have sufficient validity for consideration. Of

considerable importance in the visualization of biomass pyrolysis is

that there are many competing reactions which result in different

products. These many reactions may be divided into two major

categories: molecule splitting versus molecule building. The

molecule building reactions are favored by the close proximity of the

molecules found in condensed phases (liquid or solid) or by higher

partial pressures which encourage bimolecular reactions. The

molecule splitting or cracking reactions are favored by a separation

of the molecules, e.g., a low partial pressure in the vapor phase,

which makes the bimolecular reactions less probable. Figure 1

illustrates this pyrolysis concept where the desired route to olefins

is shown as a horizontal sequence of reactions within' -the reccangul.ar

box. In this desired sequence, the biomass is first depolymerized to

a sufficiently low molecular weight to allow it to vaporize from the

liquid primary tar surface. High surface temperatures and a low

partial pressure for the tar vapors encourage this volatilization to

occur before the primary tar liquids rearrange and/or polymerize to

more thermally stable secondary tar liquids. These primary and

secondary tar liquids are apparently quite viscous at low pyrolysis

temperatures and can retain many of the macroscopic features of the

biomass cell walls. If the secondary tar liquids are not quickly

heated to higher temperatures, they further polymerize and rearrange

to form solid charcoal (having macroscopic cellular features, water,

and carbon oxides. The primary tar vapors will tend to form vaporphase derived tars by polymerization if the temperatures are only

moderately high and/or if the partial pressure is high. Transient

oxygenated fragments in the vapor phase are formed by cracking any of

the previously mentioned tars or the refractory water soluble

' oxygena t ed compounds. These transient fragments would be very active

species and rapidly crack to form hydrogen, carbon oxides, and

hydrocarbons if the temperatures are high and the partial pressures

are low. Higher partial pressures of these active species encourage

the molecular building processes which result in the vapor-phasederived tar and the refractory water solubles. The vapor-phaseerived tars can undergo further' molecular growth with extended

esidence times, high temperatures and partial pressures to form a

apor-phase-derived, solid carbon black (which can condense into

. 1~m spheres and then form agglomerates of up to about 200 ~m in

diameter). The olefins are relatively inert with respect to the

other gases, but can undergo vapor phase polymerization to form

aromatic liquids and polyaromatic solids (carbon black) with high

temperatures, high partial pressures, and long residence times. The

temperatures, times, and partial pressures in Figure 1 should only be

considered qualitative at this time; future work in fast pyrolysis

4
HighT

Slow

HlghP

co

CH.. Oleflns H,

co.

HighT

Slow

HlghP

HighT

F•••

LowP

High THigh T

&Iow Slow

Low P High P

Water Soluble

Refractory

Oxygenated

Compounds (V)

b · ~~~"'~ ~.ti

low T Secondary VJ4 High T ,M. 4I.T

Slow Slow Slow

High p Tar (l) Low P M.~.P

11.iw

:

Charcoal (B)

co•• H.C

Low P < 75 kPa <High P

Low Temp < 400°C < High Temp

Fast < 0.1 sec <Slow

High T High T Transient

Biomass (5) low T .... ,Primary F••t .... Primary F... lit.. Oxygenated

r Tar (L) Low P r Tar (V) Low P r Fragments V

FIG. 1. REACTIONS COMPETING WITH

FAST PYROLYSIS REACTIONS
will help to better quantitatively establish these values as well as

to modify some of these proposed mechanisms. It should be noted that

not all of the attendees were in agreement with these overall

mechanisms, but at the moment the concepts seem to explain most of

the experimental results reported at the workshop.

Other areas for future research efforts which were discussed by the

attendees included tar and char characterization, heat transfer, and

feedstock preparation including the possibility of prehydrogenation

followed by pyrolysis.

6
PERSPECTIVES IN HEAT TRANSFER REQUIREMENTS AND MECHANISMS

FOR FAST PYROLYSIS

T.B. Reed, J.P. Diebold, and R. Desrosiers

SERI, Golden, CO

ABSTRACT

When biomass is heated slowly it produces charcoal, tars and gases,

but as the heating rate increases less char is produced and at

sufficiently high rates very little, if any, char is produced. Since

the slowest step in gasification of biomass is the conversion of char

to gas, rapid heating of biomass will be very important in

gasification processes.

Methods of attaining the high heat fluxes of interest in fast

pyrolysis are discussed and put into relative order of magnitude

perspective. The concept of transfer by solid convection is

introduced for consideration as a practical method for fast

pyrolysis. Equations are presented which describe a non-char forming

pyrolysis model as a function of heat flux, time, and thermal

properties of the biomass.

INTRODUCTION

In the last few years it has become increasfngly evident that the

yields of volatiles from the pyrolysis of biomass" (and coal) increase

with the rate of heating [1,2,3]. In fact, there now seems to be

good evidence that at sufficiently high heating rates, cellulose and

probably all biomass can be nearly all converted to volatiles

[4,5]. Since once formed, the conversion of char to gas is the most

difficult step in gasification of biomass or coal, the absence of

char has far reaching implications for the future of gasification.

If we accept the conclusion that sufficiently rapid heating can

completely convert biomass to volatile materials only, we should then

examine all possible methods for achieving high heat transfer rates

and determine which ones are likely to hold the most promise for

practical fast pyrolysis systems. We summarize many possible methods

and examine their suitability below.

HEAT TRANSFER CHARACTERISTICS OF FAST PYROLYSIS OF BIOMASS

Energy Requirements

Thermogravimetric (TGA) experiments show that pyrolysis of dry

biomass begins at about 2000C and is essentially complete at 500oC.

The energy required in this temperature range is more difficult to

7
estimate because it varies, depending upon the products formed. In

slow pyrolysis the techniques of differential scanning calorimetry

(DSC) show that pyrolysis can be exothermic (self-heating) over a

part of this temperature range if char is formed [6].

Under the rapid heating conditions of fast pyrolysis however, it has

not been possible to measure the heat consumed in vaporizing

biomass. For the sake of argument, consider fast pyrolysis to be a

sequence of steps involving sensible and latent heating of the

biomass. Consider that the biomass is first sensibly heated without

undergoing phase changes or chemical reactions. At the reaction

temperature, the biomass is depolymerized to form a solid which

subsequently melts. If the temperature is high enough, the melted

biomass can then vaporize. The melting and vaporization may actually

occur as a single step (sublimation). Since enthalpy is a point

function and independent of path, the energy required for a singlestep or a two-step path to the vapor state would be identical. The

overall standard heat of reaction of fast pyrolysis (reactants

starting at 250C and the final products ending up at 250C) was

determined by the work of Diebold and Benham at the Naval Weapons

Center, China Lake, CA to be essentially zero with municipal-trash

derived organics. The heat required to produce vapors from biomass

is given by:

llh

P = cllT + LX.llh

1. m.

1.

+ LX.llh

1. v.

1.

+ [Lx.llhf - llhf ]

1. i feedstock

(1)

where c is the heat capacity of the solid biomass,

in temperature of the biomass needed for pyrolysis,

of each pro due t

species in the

llT is the increase

llh is the heat

m.

J.

llh is the heat of vaporization v.

1.

the mass fraction

formation of each

of melting of each liquid species,

of each vapor species and xi is

species, and llh f is the heat of

solid phase.

Unfortunately, the number of species in the vapor phase is enormous;

the heats of formation and vaporization are not known and the

vaporization occurs over a range of temperature. We can obtain an

estimate for the heat of fast pyrolysis from the following

approximation. The heat capacity of biomass is about 2.7 J/gOc.

Thus, it requires about 1350 Jig to sensibly heat biomass to 5000C

without chemical reaction, melting, vaporization, or pyrolysis.

Assume that the average molecular weight of the vapors is 100 and

that vaporization occurs at 500 oC. From Trouton' s rule to estimate

the heat of vaporization and Reed's rule to estimate the heat of

melting, the entropy of sublimation is 104 Jlmole oK. Ftom this we

estimate the heat of sublimation of biomass is 803 Jig. Then the

heat required to vaporize biomass would be 2157 Jig or about 2000

Jig, if the energy associated with the chemical reactions is

negligible. This corresponds to a little more than 10% of the heat

of combustion of the biomass.

8
If however, the pyrolysis step does not require the vaporization of

the decomposition product and a mechanical means is used to wipe the

tars away, then the heat input to the biomass is less. If the

primary tars have a higher molecular weight than the vapors, sayan

average molecular weight of 162, using Reed's rule their heat of

fusion would be estimated to be about 40 J/g at 5000C. Using the

same sensible heat as before of 1350 J/g, the pyrolysis to tar

liquids would require 1390 J/g or about 1400 J/g which is about a

third less than required for pyrolysis followed by vaporization.

Subsequent vaporization of the tar liquids would require the same

total input of energy to the system as in the above example, but

considerably less energy would need to be transferred to the

decomposing biomass itself.

Heat Flux Requirement

There are a large number of competing reactions involved with fast

pyrolysis. For example, if the initially formed tar molecules do not

vaporize quickly enough, they will have a tendency to crosslink and

form thermally stable larger tar · molecules which in turn can

eventually form char. The relative rates of the volatile-forming and

the char-forming reactions have been studied extensively. The

volatile forming reaction is favored over the char forming reaction

by a ratio of only two to one at 3000C, but at 6000C the volatile

forming reaction is favored by fifty to one [7]. Thus, if the

heating rate is very fast so that the biomass spends an insignificant

amount of time at the lower temperatures, nearly all volatiles can be

produced. Since volatiles are more apt to be the precursors of

olefins than is char, high heat fluxes causing very rapid heating

rates are therefore desirable to produce olefins from biomass. With

very thin, two-dimensional cellulose samples blackened with 2% carbo~

black, it has been found that radiant heat flu~es of 6.3 W/cm

produced 33 per cent by weight char whereas 46 W/cm reduced the char

produced to 3% and 12,500 W/cm2 further reduced the char level to

about 1% [1]. Three-dimensional biomass samples will require

significantly higher heat fluxes to achieve the same degree of

volatization due to heat sink effects. Thus, the central problem of

fast pyrolysis is to supply about 2000 J/g to a biomass surface at

about 5000C with a flux somewhat above 50 W/cm. 2

Ablative Cooling

The physical mechanism of fast pyrolysis appears to be one of

ablative cooling. As the surface temperature of the biomass

increases to that necessary for pyrolysis, depoLyme r Laat.d.on of the

lignocellulosic material produces a liquid tar on that surface. If

the liquid tar is not mechanically removed and is allowed to remain

on the biomass surface, it will absorb a considerable amount of

energy as it further pyrolyzes and/or vaporizes. The initial

presence of the pyrolysis products and their subsequent movement away

from the biomass surface form a protective film which adversely

affects the ability to transfer heat into the biomass. This

mechanism is called ablative cooling and is the mechanism that

9
enables rockets to reenter the atmosphere while sacrificing their

ablatively cooled nose cones. In fact, wet wood was demonstrated to

have a resistance to high heat fluxes superior to that of most solid

materials including quartz or sapphire. While it is difficult to

estimate the magnitude of this effect without specifying an exact

experiment, calculations for transpirational cooling (a similar

phenomenon) show a decrease of the heat transfer coefficient to 10%

or less under conditions of strong transpirational (and ablational)

cooling [8]. Gaseous heat transfer from flames and arcs to any

surface tends to have a low eff iciency [ 9] • Due to the ou twar d

movement of gases it is expected that heat transfer will be even

lower to pyrolyzing biomass surfaces. On the other hand, radiation

heating can reach the biomass surface through the transpirational

boundary layer provided that the gases f ormed are not opaque at the

active wavelengths. Solid conduction and convection are more

complex. While the gases and liquids formed interfere with heat

transfer, the pressures and velocities involved can rapidly remove

these products from the interface. It may be possible to pyrolyze

only to the liquid phase and to remove that liquid from the

pyrolyzing solid prior to vaporization and/or further cracking.

Transient Heating

Consider a rod (such as a wooden dowel) heated uniformly at one end

only and vaporizing with a surface regression rate of v. The

temperature T at a distance x from this end is given by

(2) x-vt

erfc(- 1/)

2(at) 2

a

-vx

+ e 1/ x + vt - 2 [erfc( 1;)

2(at) 2

T -T

o

T -T P 0

</> =----

k where, a is the thermal diffusivity of the material (a = -- , p is

pc

the density, k is the thermal conductivity and c is the heat

capacity), To is the initial temperature of the rod, and Tp is the

temperature of pyrolysis. The temperature profiles at specified

times are shown in Fig. 1.

(3)

-vx

a </> = e

Fortunately, after a relatively short time the temperature reaches

the steady state described by

It is possible to calculate various characteristic thermal states

from this distribution and the thermal properties of the biomass.

The steady state pyrolysis velocity is related to the rate of heat

input I in J/cm2s by

v = I/pAh (4) . P

The limiting temperature in eq. (2) can be integrated to give the

10
FIGURE I. HEAT PENETRATION IN PYROLYSIS

3.0

20

a = 0.82 • In-3 em2/see

v =1.0 em/sec

I = 1280 w/em2

15

2.0

zx

a

o 5 10

o

1.0

1.0

vx

- .- e a at t =co

r:1'o° 0.8 I a.. I- I-

...... II ...... -e0.6 UJ

0:

:J

t-

-c

0:

UJ

a..

Z

UJ 0.4

IUJ

U

:J

UJ

0:::

0.2

DISTANCE PEN~TRATEDJJ1M
quantity of heat, q in J/cm2 which must enter the biomass before T

p is reached at the surface

q = It

o

CXl

pc f (T - T )dx

o 0

(T - T ) P 0

kp~h

P

I (5)

The time to reach steady state, to' is calculated from the heat

transfer and this heat input according to

t

o

q

I

(T - T ) popkp~h

12 (6)

Note that this time is inversely proportional to the square of the

heat transfer rate, thus putting a high premium on heat transfer

rate.

Although the thermal wave in Fig. 1 extends to CXl , a straight line

approximation intersecting the x axis at

the same

\IX

- = 2 would have nearly a

heat content, ho' under this curve. We define the heat penetration

as this distance where

2a

x =-

o v

(7)

We illustrate these thermal parameters in Table I using the thermal

prope~1ies of yellow pine, and for comparison purposes use velocities

of 10 cm/s as typical of slow pyrolysis (10) and 1 cm/s as typical

of fast pyrolysis.

Note particularly that the heat penetration is exceedingly small for

fast pyrolysis. This small amount of heat penetration may appear to

be contradictory, but is the result of the biomass-surface regression

rate being nearly ' equal to the rate of heat penetration. Char

formation is thought to be a much slower reaction than tar formation

in fast pyrolysis. For slow pyrolysis, the hot layer available for

pyrolysis is 1000 times larger with a significant amount of time

spent at low pyrolysis temperatures where the char forming reaction

dominates to form large amounts of char and lesser amounts of

volatile tars.

HEAT TRANSFER MECHANISMS

Heating with Atoms, Photons and Electrons

Heat can be transferred

gaseous flame), photons

electron welding) or by

to a surfa~e by a flux of atoms (as in a

(as in a furnace), or electrons (as in

combinations of these mechanisms. Various

12
TABLE" I

THERMAL PARAMETERS IN SLOW &FAST PYROLYSIS (YELLOW PINE)

SYMBOL QUANTITY RELATION SLOW FAST UNITS

v Pyro 1ysi s Rate 10-3 1 em/s

I Heat Transfer v~hp 1.3 1280 W/em2

x

o Heat Penetration 2a.lv 16,000 16 11m

Heat in Wood . k Tlv 370 0.73 J/em2

q I--'

0.6 x 10- w 3

to Time to Tp q/l 570 s

Heating Rate dT 1 8 ·x 105 °Cls dt

Assumptions: e = 2.8 JIg 003 C; ~hp = 2000 JIg; Tp

= 500 C, p = 0.64 g/em

-3 J em · -3 2 k =1.47 x 10 0c s em2 ; a. =0.82 x 10 em Is
devices using these mechanisms are illustrated in Fig. 2 [9]. We

will first comment on the three mechanisms briefly before examining

their application to biomass.

In the discussion of heat transfer from directed energy flows to a

surface, it is important to distinguish between the energy flux, E,

and that which is actually transferred, 1. The ratio of the two is

the heat transfer efficiency, n = I/E. It is also important to

distinguish between the role of temperature and heat transfer.

Although the temperature of the beam must be higher than that of the

surface for heat to flow, and in general, the higher the temperature

difference the. higher the flow; there are situations in which a

higher temperature difference decreases heat flow (e.g., the

Leidenfrost effect as exemplified by water "dancing" on a very hot

surface). It is important to recognize that pyrolysis depends on I,

the actual heat transferred to the surface, not on T, the

temperature of the source or E, the available energy flux. Although

biomass heating rates are often listed in degrees/sec, it is

important to recognize that this is usually the rise time of the heat

source or of a temperature sensor, not of the biomass. We seldom

actually know what temperatures exist at the biomass surface itself

during such experiments, especially at very high heating rates which

involve values of to of less than 100 ms.

Gaseous Convection Heating

Heat Transfer by gaseous convection is a relatively difficult

approach to attain high heat fluxes. As indicated in Fig. 3, with a

temperature difference of SOOoC, the free convective heat flux can be

as low as 0.3 W/cm2 and gradually increases to a little over 1 W/cm2

as the velocity of the gases approaches that of forced convection (l

W/cm2 = 317S BTU/ft2hr). With the same temperature difference,

forced convective heat fluxes can be as high as 14 W/cm2 which is

about the same as the radiative heat flux from a iooo?c black body.

However if the biomass particle is pyrolyzing to produce a cloud of

gas and vapors which surround the particle, the heat transfer by

convection will be very severely reduced because of transpirational

cooling.

Higher heat transfer rates can be achieved with impinging flames. In

a typical Bunsen flame the ener9' flux of the flowing gases just

above the cone is about 400 W/cm for a flame velocity of 103 cm/s

[9]. Measurements of actual heat transfer to cold surfaces from such

flames show that the heat transfer efficiency in the projected area

of the flame ~s about 10% thus giving a directed heat transfer of

about 40 W/cm. While useful for making coffee or soldering, in

general, this heat transfer rate is too low for most industrial

applications. In particular, such a flame directed at a large piece

of wood would produce charcoal and volatiles and so is too low in

intensity for "fast pyrolysis" directed at producing only volatiles

from other than very small or thin particles of biomass.

14
• • 4---- II ELECTRONS ---....-- PHOTONS --.

ATOMS -------------+• II

IN-

"COHERENT+-4- COHERENT - ..--- II SOLIDS ---;-_. LIQUID-.I-.I-------- GASES -------...

.-.- POTENTIAL _ ........ ' •• KINETIC -.

ENERGY ENERGY

~P'I ,

"

o 0

o 0

o 0

o 0

o 0

o 0

SOLDERING

IRON

FRICTION

SAW

OPEN CONSTRICTED

SHAPED FLAMES CONSTRICTED ARCS OPEN

CHARGE fLAMES ARCS

ELECTRON

(ION)

BEAMS ARC

IMAGE

LASERS

SOLID SOLID LIQUID MAGNETIC ELECTRO-

_ CON- -.....- CON- --...- CON- -4__.~-- PRESSURE INDUCED :----..._...... CON- -..- STATIC -..

DUCTION VECTION VECTION GASEOUS CONVECTION VECTION ACCELERATION

DIRECTED HEAT TRANSFER FROM ATOMS. ELECTRONS 8 IONS
w

cm 2

10,000 -focussed co2 laser

-electric arc

sol id convection

with nichrome wire 1000

_ oxy-acetylene flame

2000°C (net to cold surface)

"50 0 Suns"

solar collector

bunsen burner

-(net to cold

surface)

gaseous free convection

with 6.T= 500 °c

gaseous forced convection

with 6 T: 500 -c

100

oL~! 1

of fast

p olysis

p nomen

10

solar insolation 0.1

w

cm 2

black body

radiation

FIG. 3. HEAT TRANSFER PERSPECTIVES

16
The flame heat transfer intensity can be increased a factor of ten by

using oxygen instead of air (increasing flame velocity) or by using

jet burners such as rocket engines. These higher intensities come

close to converting wood or cellulose completely to volatiles and

have been used in ornamental wood erosion, where the different

relative rates of the volatilization of spring and summer wood

produces ridges of the more resistant _wood. In general, the low

efficiencies of heat transfer from directed flames make them

unsuitable for fast pyrolysis.

Most of the devices of Fig. 2 are unsuitable for fast pyrolysis for

various theoretical or practical reasons. Electric arcs and electron

beams are diff:lcult for biomass heating because biomass is not an

electrical conductor. Liquid jets transfer too much momentum and

would be too costly for fast pyrolysis, though they might be used in

cutting wood in special instances.

Solid Conduction and Convection

-The conduction 6f heat from a solid has the potential for delivering

high heating rates and a familiar device relying on solid heat

conduction is the soldering iron. The amount of heat transferred by

solid heat conduction is proportional to the thermal conductivity and

to the temperature difference, but inversely proportional to the

thickness involved. A high thermal conductivity metal (such a~

tung~ten) at high temperatures is capable of delivering heat at 10

W/cm , with a temperature difference of 20000C and a 2 mm thickness,

and so could be considered for fast pyrolysis. The materials and

control problems in such a heat source would be formidable, however.

Convection is general Ly thought of as the motion of liquid or gases,

but in a sense, a moving hot wire is a solid convection heat transfer

agent. With a 0.025 cm diameter wire, moving at a velocity of 20

cm/s , we have achf.eved rapid pyrolysis of wood in the laboratory with

penetration rates of about 3 cm/s (11). If we assume a heat of

pyrolysis requirement of 2000 J/g, this implies a heating rate of

3500 W/cm2 from this moving wire.

Radiation Heat Transfer

The heat delivered by radiation has a very wide range. Black body

(incoherent) radiation delivers heat proportional to the absolute

temperature raised to the fourth power. A furnace at 10000C delivers

a maximum heat :El ux of 14.9 W/cm2, which we think may be marginal for

fast pyrolysis and so it is common experience that a large piece of

wood will char during pyrolysis in such a furnace. The temperature

of the sun is about 55000K and the heat flux at the surface of the

sun is 5,200 W/cm2, far above the requirement for fast pyrolysis.

However, since we are a considerable distance from the sun, the heat

flux. actually received at the earth's surface is closer to about 0.11

W/cm2• Point focusing solar collectors can achieve about 500 suns,

or a heat flux. of - 55 W/cm2 and so could marginally achieve fast

pyrolysis.

17
The laser (producing coherent radiation) is not limited by the laws

of black body radiation gnd has in practice reached steady heat

transfer intensities of 10 W/cm2 and pulsed values a thousand time~

high~r. Carbon dioxide lasers can easily be focused to give 10

W/cm fluxes and are now being used to produce art objects of carved

wood using fast pyrolysis. UnfortunatelYJ the cost and efficiency of

lasers at present prevents them from being a practical heat source

except in such special applications.

Fluidized Bed

Heat transfer in a fluidized bed has the characteristics of many of

the previously mentioned heat transfer mechanisms. With a mildly J

fluidized bed, the pyrolyzing particles are surrounded by hot

radiating bed-particles. However, the heat capacity of the bedparticles is small so that a relatively large number of them must be

used to transfer heat from the source to the biomass if a high

uniform temperature is to be maintained. With an increase in bed

turbulence, the importance of heat transfer by gaseous convection and

intermittent solid-solid conduction is increased. With very high

turbulence as in entrained flow, the bed-particles may be superfluous

particularly if the heat source is the hot wall of the reactor. If

the bed-particles have a high velocity relative to the biomass, the

high heat transfer rates achievable with solid eonvection may be

attained unless the bed-particles become attached to the biomass to

form a protective coating.

Momentum Transfer During Fast Pyrolysis

In all heat transfer processes there is also a nec:essary associated

momentum transfer. For photons and electrons, this momentum transfer

can be very small but it is higher with gases and can be very high

with liquids and solids. The momentum transfer can be very important

in removing the products of pyrolysis such as tar and/or any char

that may form, in order to permit further pyrolysis.

Bulk Heating of Biomass

The above mechanisms describe the transfer of heat: to a surface by

moLe cuLe s , atoms, electrons, and photons. In a.ddition, one can

envisage the bulk heating of materials, and a widely used example of

such heating is induction heating in which currents are induced in

metals and semiconductors for very rapid heating.

Microwave heating (as in microwave ovens) is another example of bulk

heating in which the high frequency field couples to the dielectric

moment of the material being heated by displacement currents.

Unfortunately biomass has a relatively low dielectric constant which

presents difficulties in using this heat transfer mechanism.

A second bulk heating method is the adiabatic compression of biomass

in .which a sudden application of pressure generates internal heat

through friction and gas heating. This mechanism iB important in the

18
densification of biomass for fuel, but would not be useful for fast

pyrolysis because of the necessary confinement of the biomass.

CONCLUSION

We have listed here the various mechanisms that are available for the

surface and bulk. heating of biomass and shown those that are most

suitable for fast pyrolysis. These are high velocity gas flames,

direct conduction, solid convection, fluidized beds, and radiant

heating. The use of conduction and solid convection, in particular,

would appear to be a very fruitful area for additional consideration

because very high heat fluxes are readily available with relatively

inexpensive technology (e.g., compared to electrically operated

lasers).

19
REFERENCES

1. Lincoln, K.A. "Flash-Pyrolysis of Solid-Fuel Materials by

Thermal Radiation", Pyrodynamics, Vol. 2, pp, 133-143 (1965).

2. Llewellen, P.C., W.A. Peters, and J.B. Howard "Cellulose

Pyrolysis Kinetics and Char Formation Mechanisms", Fire and

Explosion Research: Sixteenth Symposium on Combustion. The

Combustion Institute, pp. 1471-80 (1976).

3. Antal, M.J. Jr., W.E. Edwards, H.L. Friedman, F.E.

Study of the Steam Gasification of Organic Wastes"

Grant No. R804836010 to Princeton University by U.S.

Cincinnati, Ohio 45268.

Rogers "A

University

EPA, IERL,

4. Diebold, J.P. "Preliminary Results in the Fast Pyrolysis of

Biomass to Lower Olefins", Petroleum Chemistry Division of ACS

Symposium on "Alternate Feedstocks for Petrochemicals", Las

Vegas, NV, August 24-29, 1980. (Also appears in more detail in

Solar Energy Research Institute report SERI/TR-332-586, June

1980).

5. Diebold, J.P. "Thermochemical Conversion of Biomass to

Gasoline. September 1980 Update" DOE's 11th Biomass

Thermochemical Conversion Contractor's Meeting at Pacific

Northwest Laboratories (September 23, 1980).

6. Milne, T.

Principles

Chapter 5.

"A Survey of Biomass Gasification. Volume II -

of Gasification", Solar Energy Research Institute.

SERI/TR 33-239 (1979).

7. Bradbury,

Pyrolysis

(1979).

A. ,

of

Y. Sakai,

Cellulose",

8. Eckert, E.R.G., R.M. Drake, Jr. "Analysis of Heat and Mass

Transfer", McGraw Hill, 1972, p. 453.

9. Reed, T.B. "Directed Heating with Atoms, Electrons and

Photons", ASM Technological report NY 9.1. 62; or Science and

Industry (N.V. Philips' Gloeilampenfabrieken) ~1 (1965).

10. Schaffer, E.L. "An Approach to the Mathematical Prediction of

Temperature Rise within a Semi-infinite Wood Slab subjected to

High Temperature Conditions", Pyrodynamics,1., 117-132 [1965).

11. Diebold, J.P. "Ablative Pyrolysis of Macroparticles of

Biomass", Proceedings of the Specialists Workshop on the Fast

Pyrolysis of Biomass, Copper Mountain, CO October 19-21, 1980.

20
THEORY OF HYDROCARBON PYROLYSIS TO FORM OLEFINS

S. Narayanan

Stone &Webster Engineering Corporation

Boston, Massachusetts 02107

INTRODUCTION

The basic concept of a "mechanism" of a reaction is to postulate a procedure by

which a particular reaction proceeds. One can have many mechanisms postulated.

There are two basic approaches in the literature with regard to the pyrolysis of

hydrocarbons, they are radical modelling and the molecular schemes.

Radical Modelling. Rice (1] postulated a radical mechanism which is successfully applied to hydrocarbon pyrolysis and is used to predict yields at low

conversion levels. It postulates that hydrocarbon dec ompositi.on is initiated

by the formation of a radical, which being active abstracts a hydrogen atom from

the parent alkane and it subsequently decomposes to an olefin and a radical.

This propagates the reaction. . Radical interaction tends to terminate the

reac t i on ,

Molecular Schemes. For very complex reactions, where the radical formation is

too difficult to determine experimentally ' or theoretically, a molecular

mechanism is postulated and tested. This mechanism simply involves evaluating

all plausible reactions of a decomposing hydrocarbon.

A generalized reaction rate for many reaction steps put together, known as

global reaction rate, of a decomposing component is arrived at only experimentally~ Most hydrocarbons decompose by a first order type relation.

Accordingly one can write:

dc =kc dt (1)

where k Ae E/RT

Once we know the rate coefficient for a particular ' decomposition reaction, we

can find out a given time how much of the component will be converted for a given

tempera t.ure ,

Examining the level of conversion of a component becomes difficult when the

compound decomposing is a complex one. Therefore, some generalized techniques

have to be developed to predict conversion.

Most organic compounds, when heated above ' 450 0C, decompose into simpler

produc ts . Such thermal dec;omposi tions have been studied extensively. The

evidence so far accumulated has shown that several classes of organic compounds,

such as hydrocarbons, ketones, aldehydes, ethers, and amines, decompose homogeneously and by an approximately first-order relationship. Self-inhibition

(decrease in calculated first-order rate constant with increasing conversion)

is sometimes observed. The energies of activation of these processes lie in the

range between 50,000 and 67,000 calories/g-mole.

21
The occurrence of free radicals as intermediates in cracking reactions has been

well established by now. Rice and Herzfeld C2] developed a free radical

mechanism, which was later modified by Kossiakoff and Rice C3J to predict the

product distribution from hydrocarbon decomposition reactions. This theory

also gives plausible explanation for the approximately first-order kinetic

relationship and the experimental activation energy, which is considerably

lower than the carbon-carbon bond energy.

Rice-Herzfeld-Kossiakoff Mechanism

The strength of the carbon-carbon bond in paraffins is 80 kcal/g-mole, whereas

the carbon-hydrogen bond is 100 kcal{g-mole. Rice indicates that because of

this 20 kcal/g-mole difference in the bond energies, the decomposition of a

paraffin will exclusively involve the rupture of the carbon-carbon bond to

produce two free radicals. The yields of the products from the cracking would

then depend upon the subsequent reactions of these free radicals formed in the

prllnary step. An llnportant limitation of this mechanism is that it is valid for

small conversions only. Any organic compound, unless it is one of the few very

simple ones, can yield a great variety of compounds which will in turn decomr

pose; if, therefore, more than a small fraction of the original compound is

decomposed, the whole problem will become hopelessly involved.

According to Rice, the primary decomposition products can be estimated from

experllnental measurements in which the conversion of the feed is kept below

20 percent. The main features of the theory are as follows. As a first step,

it is assumed that a hydrogen atom is removed from a paraffin molecule by attack

by an alkyl radical. The radical thus formed is assumed to dissociate

unimolecularly into an olefin and a free radical through the rupture of the much

weaker carbon- carbon bonds. Since the methyl and ethyl radicals are much more

stable than the higher radicals, the free radicals so formed, with the exception

of the methyl and ethyl radicals, decompose very rapidly into olefins leaving

methyl groups, ethyl groups or atomic hydrogen. These small radicals are then

assumed to form methane, ethane, or hydrogen, by abstracting a hydrogen from the

original paraffins and thus continuing the chain. The following additional

assumptions are made:

(a) The large radical formed in the primary step decomposes without reacting

with any other hydrocarbon. This implies that the rate constant of the

bimolecular reac tion is much smaller than the unimolecular dissociation rate

constant.

(b) The kind of initial radicals formed depends upon the number and nature of

the carbon-hydrogen bonds present. The activation energy of the removal of a

secondary and tertiary hydrogen are assumed to be, respectively, 2 and 4 kcals

less than required for the removal of a primary hydrogen. The frequency factors

for all reactions are assumed to be the same. The initial rate of formation of

each type of radical is taken to be proportional to the rate constant for that

reaction multiplied by the number of bonds of that type present in the original

hydrocarbon.

(c) In order to llnprove the agreement of the theoretical predictions with the

experimentally measured values, Kossiakoff and Rice later modified the theory.

22
small saturated molecule and a new radical. Radicals may also stabilize themselves by decomposing to ethylene and another reaction species, (E.2), and

(E.4).

Termination: Reaction chain is ended by the collision of free radicals to

either combine or disproportionate to stable molecules (E.S).

In hydrocarbon pyrolysis, main reaction products usually originate from the

chain propagation steps. Thus, for ethane pyrolysis methane, ethylene and

hydrogen would be the major reaction products.

The first reaction, (E.I), is known as the initiation reaction. (E.2), (E.3),

and (E.4) are known as the propagation reactions and (E.S) is known as the

termination reaction. Besides these a host of other side reactions have been

observed by chemists which are established as occuring at higher temperatures of

SOOoC and the like.

C

*

2H4 + H* C2H3 + H2

C2H~ < C2H2 + H*

(E.6)

Further radical reaction schemes involving the formation of carbon and its

reaction have also been reported as:

(E.7)

The overall reaction ethane is considered as

The forward reaction is favored at lower partial pressures.

Propane Cracking

The thermal cracking propane has been established to proceed as follows:

C3Ha r CH3* + C2HS* Initiation (P.I)

C2HS* + C3HS C2H6 + C3H7

* Propagation O(P. 2 )

H* + C3HS H2 + C3H7

* Propagation (P.3)

CH * + C

3HS I CH4 + C 3

H7* Propagation (P.4) 3

C3H7

* .. CH3

*+ C2H4 Propagation (P.S)

C3H7* .. H* + C3H6 Propagation (P.6)

C3H7* + CH3* CH4

* C3H6 or C4HI O Termination (P.7)

CH * + CH * ) C2H6 Termination (P.S) 3 3

25
Comparing this with that of ethane decomposition scheme, we observe with

increasing paraffinic series the reaction steps are becoming more complex.

Simple overall stoichimetric relations for propane are:

C3Ha ---..,... C3H6 + H2

C3Ha'~ C2H4 + CH4

N-Butane Cracking

Normal butane decomposition follows the following mechanism.

(P.9)

(P.lO)

C

4Hl O ;. 2C2HS* Initiation (NB.l)

C

4HIO + C2HS* ) C2H6 + C4H9* Propagation (NB.2)

C4HIO + C2HS* ) C2H4 + C

4Hl O + H* (NB.3)

C4Hl O + H* ) H2 + C4H9* (NB.4)

C4H9* C2H4+ C2HS* (NB.S)

C4H9* C3H6 + CH3* Propagation (~B.6)

C4Hl O + CH * CH4 + C4H9* (NB.7) 3

2CH * )- C2H6 Termination (NB.a) 3

2C2HS* C4Hl O (C2H4 + C2H6) (NB.9)

2C4H9* ) CaHl a (C4Ha

+ C

4HlO) (NB.IO)

CH * + C4H9* ) C

SHI 2; C2HS + C4H9*-+C6HI4 (NB.l1) 3

Comparing the sets of reaction, the normal butane cracking has a larger number

of reaction steps. This brings out the point; namely, the reactions' sequence

multiplies with increasing carbon number of the paraffin series. The relative

speed of the reactions or radical concentrations are too difficult to determine.

Simple overall reactions of N-Butane can be written as:

nC4HIO ---T 2C2H4 + H2

nC4HIO----+ C2H4 + C2H6

nC4Hl O ---r C3H6 + CH4

Iso-Butane Cracking

(NB.l2)

(NB.13)

(NB.l4)

While normal butane has a (primary) carbon-carbon rupture, iso is more refractory and accordingly its decomposition does not involve major (secondary)

carbon-carbon break up.

(iB.I)

26
In their modification they assume that any free radical with six or more carbon

atoms, prior to rupture, can coil around and react with itself to produce an

isomer.

(d ) The free radicals formed in the isomerization step are then assumed to

break at the carbon-carbon bond relative to the free radical site. If there is

more than one such bond, the mechanism which forms a radical of greater

stability is preferred.

The theory has been developed with observations made on paraffins containing up

to six carbon atoms. It cannot be used for higher paraffins without proper

verification. A number of simplifications have been made to reduce the number

of reactions possible and as such it cannot be expected to give precise results.

However, the theory provides good agreement with the broad nature of

experimental facts.

As shown below, this mechanism gives a plausible explanation for the approximate

nature of the decomposition process and also predicts an activation energy much

less than the energy required to break a carbon-carbon bond. Considering the

case in which only one carbon-carbon bond in the free radical is broken before

the resulting radical is converted into a paraffin, we have the following steps:

E kcal/g-mole

K1 j

M ,. R1+R radical formation 80 (2) 1

Rl+M

K2 RlH+R2 ) Hydrogen transfer 8 (3)

R2

K3 R01+dl >. fission 38 (4)

Rl+R2

K4 RlR2 ) products chain termination 0 (5)

In the first step, the decomposing hydrocarbon, M, splits into two free radicals

which involves the rupture of a carbon-carbon bond.

The radical Ri is assumed to play no part in the reaction. In the second step

the radical Rl abstracts a hydrogen from the parent molecule to form a molecule

and a large free radical R2. Fission at a carbon-carbon bond in the third step

converts the R2 radical into an olefin (011) and a small radical which continues

the chain. Termination occurs through the collision of Rl and R2•

Woinsky (4] simulated the kinetics of thermal cracking of high molecular weight

normal paraffins on a digital computer. The products and free radicals heavier

than the ethyl radical were lumped into various groups depending on whether they

were olefins or paraffins and whether they were heavier or lighter than the

feed. The primary reactions involved could thus be represented by 32 equations.

Secondary reactions were neglected and as such the model is applicable at low

conversions only. The rate constants for the intermediate reactions were

approximated from the literature.

23
This model, although more detailed than the mechanism proposed by Rice, has a

number of limitations. With the exception of hydrogen, methane and ethane, the

model does not give individual values of the product yields and a number of

constants in the model have been assigned arbitrary values. Moreover, the

predictions from the model will be only as good as the frequency factors and

activation energies used to determine the rate constants for the intermediate

reactions, which to date have not been determined very accurately. However,

considering the approximations involved, the results presented by Woinsky were

in good agreement with the experimental data.

Gavalas [S] presented a mechanism that uses the chain propagation reactions

alone to determine the concentrations of the various species in the reacting

system. The initiation and termination reactions were taken to have an effect

only on the overall kinetics. All possible reactions were written in matrix

notation and by using both the long chain approximation and the customarily made

steady-state approximation, the resulting set of simultaneous differential

equations could be solved on a digital computer . In theory, this method is

capable of handling large sets of reactions. It can even take into account the

polymerization, isomerization, and the reaction of olefins with free radicals.

However, due to the limited knowledge of intermediate rate constants, this

method is of little value.

LIGHT · HYDROCARBON CRACKING

Ethane Cracking

Ethane is the most suitable feedstock for ethylene production for it has a high

ultimate yield of ethylene. Ethane is recovered from refinery or natural gas or

obtained as a by-product during naphtha or gas oil cracking. The simplified

mechanism by which ethane cracking proceeds is expressed as

*

C2H6 .. ) 2CH3

*

CH3 + C2H6 CH4 + C2HS*

C2HS* C2H4 .", + H*

C2H6 + H* ) H2 + C2HS*

i: C

4H10 or C

2H4 + C2H6 2 C2HS >

(E.l)

(E.2)

(E.3)

(E.4)

(E.S)

I~ is inte~e~tin~ t~ ?ote that the radicals CH!, H* and C2HS* ~ecause of their

hi.gh r e ac t i.vtty i.mt i at;e , propagate, and t ernnnate the r eac t i.ons . The formation of H* radical is something characteristic to ethane cracking.

Initiation: Free radicals are produced by the rupture of parent molecule at its

weakest bond, (E.l).

Propagation: Newly formed radicals abstract H from parent compound to form

24
--7- iC3H7* + CH *

3

iC4Hg* -_.~ C3H6 + CH * (iB.Z) 3

iC4Hg* ---7 iC4HS + H* (iB.3)

iC H * + H*-~ iC4HS + HZ (iB.4) 4 9

iC4Hg

*+ CH *-+ iC4HS + CH4 (iB.5 ) 3

iC

rlH

7* + H* ----7 C~H6 + HZ (iB.6)

H * + H*--+ C 4 (iB.7) 3

iC4Hg* + H*----r iC4HIO (iB.S)

iC4H9* + iC4H9* CSHI S (iB.9)

LIGHT HYDROCARBON MIXTURES CRACKING

The mixtures' cracking technology is more complicated due to the interaction of

the various radical species. When light hydrocarbon mixtures crack, the conversion levels and produc t yields are influenced by the ability of large

radicals effectively to utilize the smaller free radicals, thus during ethane

and propane cocracking, propane starts decomposing first. But as the reaction

proceeds when ethane starts decomposing, the radicals generated by ethane are

preferentially consumed by the propane and propylene formed for its fuller

decomposition. The hydrogen radicals produced in greater amount during ethane

cracking have greater ability to decompose heavier hydrocarbons and,

accordingly, heavier hydrocarbons decompose faster when cracked with ethane or

hydrocarbons which produce H* radicals during their decomposition.

HEAVY HYDROCARBON CRACKING

From the studies of vapor phase pyrolysis of individual hydrocarbons, the rate

of disappearance of reactant is generally independent of surface to volume ratio

of the reactor and pressure. Thus, the disappearance can be considered to be

unimolecular and to follow a first order mechanism. The integrated form of the

first order equation is

where

1 Z.3 log (I~)

-1

Kt = rate coefficient, Sec ,evaluation at temperature t

e = time in Sec

~= fractional disappearance of reactant

(6)

Tne rate coefficient varies with temperature in accordance with the Arrheni~s

equation:

k = Ae-E/ RT . (7)

The rate coefficient-temperature-conversion relationships similar to those

27
which hold for light hydrocarbons apply to the constituents found in the heavier

feedstocks such as naphtha and gas oils.

While considering the heavier hydrocarbons, it 1S convenient to relate their

rate coefficients to that for normal pentane at the same temperature. If we

designate any normal paraffin of carbon number greater than 5 by subscript i

k = A -E./RT . . e 1

1 1

Dividing one by the other

(8)

(0)

Study of available data indicates that the activation energies for all normal

paraffins of five or more carbon atoms can be assumed to be equal without

ser10US error. Such assumptions causes the exponential factor to become unity

so that

(1)

If we further assume that the frequency factor fOli this class of compound is a

power function of carbon number n ,as A = g(n)

where g and h are constants we may write

(12)

Estimating the values of h for three specific components a curve can be

established for a particular family. Such a set of curves for normal paraffins,

brancher paraffins, alkyl cyclohexanes, alkyl cyclopentanes, and normal alpha

olefins is shown in Fig. 1.

REAVY HYDROCARBON MIXTURES CRACKING

The distillate feedstocks like naphtha, kerosine, and gas oil are extensively

used in conmercial practice. Each Eeedstock contains well over 75 separately

identified compounds. So in order to establish their conversion, we may have to

know the kinetic rate constants of 75 different species, plus the exact yield

from each one. We already observed that even in the pyrolysis of binary

mixtures, unpredictable synergistic interactions can result. To develop an

exact reaction mechanism amidst all these complications would be a formidable

task for distillate feedstocks.

While the measure of the percentage conversion is easy to obtain for pure

components~ it is a difficult parameter to estimate for a distillate feedstock.

A kinetic severity function or KSF is normally used. KSF is a measure of the

conversion of distillate related to n-pentane conversion, and indicates how

28
I

tn

I

C\J

::1:-'

&0

U

I

C

10

(\J

J:

lO

o

I

C

.¥

:::: 1.0

.¥

RATE COEFFICIENT OF PENTANE

VERSUS TElv\PERATURE AND CARBON

750 800

TEMPERATUREo C

850

0.1

1 NORMAL PARAFFINS

2 BRANCHED PARAFFINS (lCH3 GROUP)

:3 BRANCHED PARAFFINS (2CH3 GROUP)

4 ALKYL CYCLOHEXANES

5 ALKYL CYCLOPENTANES

6 NORMAL ALPHA OLEFINS

2 4 6 8 10

CARBON NUMBER

29

12 14
much n-pentane would have converted under identical conditions of temperature

and time.

The product distribution is normally presented for distillate feedstocks as a

function of severity. A tyical curve for naphtha cracking is shown in Fig. 2.

This severity diagram can be divided into three zones. In zone 1, the zone of

low severity up to KSF of about 1, the principal reaction in progress are those

involved in the primary disappearance of saturates in the feedstocks.

In zone 2, primary reactions continue, but the secondary reactions become

dominant. This zone extends up to a KSF of 2.4. Yields of hydrogen, methane,

total C2 's, and butadiene continue to increase but at a gradually diminishing

rate.

The rates of decomposition of propylene and butylenes reach such levels that

their rates of disappearance catch up to and then exceed their rates of

formation. As a resul t , their concentrations in the reaction mixture pass

through maxima.

The propylene peak typically occurs at a KSF of about 1.7 for naphtha cracking.

The butylene peak usually appears at lower severities, although feedstocks rich

in isoparaffins which yield isobutylene on primary decomposition tend to displace the peak to a severity level comparable to the propylene peak.

In zone 3, primary reactions have virtually ceased and all further changes in

constitution of the mixture are attributed to secondary reactions. Yields at

C6+ pass through a minimum as the original saturates become exhausted and the

formation of stable aromatics from the degradation of propylene and C4's adds to

C6+ yields as the severity is increased.

The production of hydrogen plus methane continues to rise but the C2' sand

butadiene curves eventually pass through maxima. Butadiene yield typically

peaks at a KSF of about 2.5, while C2

1s pass through a broad maximum at about

KSF = 5.

PILOT PLANT SIMULATION

In 1965, Stone &Webster decided to proceed with a laboratory scale hydrocarbon

pyrolysis program. The principal objective of this work was to study the

pyrolysis reactions, simulate commercial cracking conditions, and to determine

the effect of important process variables such as hydrocarbon partial pressure,

residence time, and temperature profile on the product yield patterns obtained

from different feedstocks. Instrumentation on-line analytical capability was

developed for effluent monitoring. Laboratory analytical techniques were

adopted from commercial practice for feed and product inspection.

The bench scale pyrolysis unit is as shown in Fig. 3. Hydrocarbon feedstock and

water are moved from the feed tanks by positive displacement pumps via separate

heaters to the superheater and reactor. The reaction coil is housed in a

multi-heating zone electric furnace.

Following the completion of cracking reactions, the products are rapidly

quenched, then condensed at ambient temperature. The vapors are sent to an

electrostatic precipitator, dried and chilled to -40°F. The cracked gas is

30
o I 2 3

KINETIC SEVERITY

.API~THA CRACKING TYPICAL VARIATION

()F PI'~ODUCT DISTRIBUTION ltVITH CRACKING SEVERITY

· 36 I

IZone 1: Zone 2

• 32

0 28

J--

:r:

(!)

.- 24

hJ

:~....,. w .. H2 I--' + CH4 Cl 20

-I

t1J

-

>- 16

Jz

l.l.J

Z 12

0

a..

0 8

()

4

80 - 18,265 FIGURE 2
bench scale pyrolysis unit

STEAM FEED

GENERATOR VA-PORIZER

RANAREX

GAS HOLDER

MEOH I

•

~Dv.A#

I

: GAS

I CHROMATO·

------- METERS GRAPH

PRECIPITATORS

CONDENSER

CONDENSER

QUENCH POT PUMP PUMP

3 fj ,E

W

N

FIGURE 3 STONE a WEBSTER • r
metered and c~llected. Liquid products from the condensers are collected and

weighed separately allowing complete material balancing.

The features of the pilot plant are tabulated.

Details of Pyrolysis Unit

Reactor type

Reactor length

Furnace

Maximum operating temperature

Pressure range

Hydrocarbon throughout

Dilution steam

Product recovery

Features that can be simulated

Features that can not be

simulated

Helical

Variable 4-120 feet

Electrical resistance with six

heating zones

1,000 C 2

0-5 70 kg/em g

1-6 kg/h

1-6 kg/h

Indirect quench

Gradual condensation to -40°C

Gas collection

Temperature profile

Pressure profile

Residence time

Steam/hydrocarbon ratio

Coil outlet pressure

Mass velocity

Reynolds number

Heat flux

A variety of feedstocks were used for the bench scale tests. They are listed in

Table 1. These include pure components and major feedstocks of commercial

interest ranging from light condensates to high vacuum distillates.

The importance of bench scale cracking lies in its ability to reproduce test

results and generate dependable simulation of the conmer ci a L processes. The

long operational history of the bench scale unit proved that both requirements

were successfully met. Typical experimental and predicted values are shown in

Table 2.

Influence of Residence Time

To obtain the same amount of conversion at a sh0rter resident time, the temperature level of the reaction has to be increased. This increase in temperature

shifts the equilibrium so as to favor higher yields of light olefins. Ethylene

yields from ethane, propane, butane, and naphtha cracking at the same conversion

level increases when the residence time is decreased. See Fig. 4.

Influence of Hydrocarbon Partial Pressure

The influence of decreasing the hydrocarbon partial pressure provides an

environment favoring progress of forward reactions like

33
SUMMARY OF

BENCH SCALE FURNACE TESTS

CONVENTIONAL FEEDSTOCKS

PURE COMPONENTS

NGL, NAPHTHAS, RAFFINATES

KEROSENE, AGO, VGO

WAXY DISTILLATES, RESIDUES, CRUDES

RANGE

C2H6 - C14

SG 0.62 - 0.76

SG 0.77 - 0.91

NO. TESTS

180

200

150

20

HYDROGENATED FEEDSTOCKS COKER & FCC NAPHTHAS

AGO

VGO

PYROLYSIS FUEL OILS

50

.,.J-17, ., .....;

SYNTHETIC FEEDSTOCKS

TABLE 1

COAL LIQUIDS

SYN CRUDE FRACTIONS

SHALE OIL

25

STONE 81 WEBSTER
COMPARISON OF PREDICTED

AND EXPERIMENTAL YIELD PATTERNS

FROM BENCH SCALE PYROLYSIS TESTS

COMPONENTS lNT. % FEED BASIS

FEED STOCK HEAVY G'AS OIL AND NAPHTHA MIX

PREDICTED EXPERIMENTAL

RUN No. SOCA-2

COMPONENTS

HYDROGEN 0.51 0.58

METHANE 9.57 9.70

ACETYLENE 0.20 0.25

ETHYLENE 21.49 21.25

ETHANE 3.65 3.50

METHYL ACEl: + PROPADIENE 0.55 0.48

PROPYLENE 14.09 13.80

PROPANE 0.42 0.46

1,3- BUTADIENE 4.83 4.73

OTHER C4 - s 5.93 6.30

TOTAL C4 +LIGHTER 61.24 61.00

C5-400F GASOLINE 25.66 24.60

FUEL OIL 13.10 14.40

T(')T ~_ 100.GO 100.00 -.

TABLE 2 STONE 8c WEBSTER •
50 I I TYPICAL

48 r I

I

I ETHANE - 60% CON.

I

I ONCE THROUGH

46 I I

..-...

I I ETHYLENE

44 I

I I

42 I I I YIELDS • I

... I

40 I PROPANE - 93% CON.I

I

38 I I C I

... 36 I I LLI BUTANE - 96% CON. I -

>- 34 I I

I I w LLI 32 I 0'\ I Z I I

LLI

.... 30 I I I >- I I

J: 28 NAPHTHA - KSFA 3.5 I I

l- I I I LLI 26 I I I

I I I

24 I I

I USC-S I USC-M VG I

22 I DESIGN I DESIGN DESIGN:

I I

20

0.4 0.6 0.8 1.0 1.2

RESIDENCE TIME (SECONDS)

7!5 - !5469 A

FJ - 4 STONfi & WEB5!ER •
and discouraging the formation of by-products by condensation reaction between

radicals. Thus, high ratios of olefins to saturates are favored and ratios of

acetylenes and diolefins to mono-olefins tend to increase.

One method of decreasing hydrocarbon partial pressure in industry is to increase

the steam to hydrocarbon ratio during pyrolysis. By selecting an optimum steam

to hydrocarbon ratio, the formation of heavy liquid, tars, and coke are reduced.

All commercial pyrolysis of hydrocarbons for olefin production is carried out in

the presence of steam, which serves three important functions.

1. It lowers the hydrocarbon partial pressure and thereby encourages higher

selectivity to the desired olefinic products.

2. It reduces the partial pressure of high boiling aromatic hydrocarbons in the

zone of high conversion, lessening the tendency to form coke within the cracking

coils and deposit tars on the walls of downstream heat exchange surfaces and

piping.

3. It has sufficient oxidizing effect on the tube metal to diminish significantly the catalytic effect of iron and nickel which otherwise would promote

carbon forming reactions.

The steam to hydrocarbon ratio varies with the hydrocarbons selected for

pyrolysis. A typical set of values are shown below:

Feed

Ethane

Propane

Naphtha

Gas Oil

Kg Steam/Kg Hydrocarbon

0.3 0.4

0.4 I'J 0.5

0.5 ,-.J 0.6

0.7 rJ 1.0

In reactors characterized by low pressure drop, the hydrocarbon partial

pressure gradually increases through the coil and reaches its maximum at the

outlet.

THE DEMONSTRATION UNIT

The bench scale unit was limited, since it could not provide information on the

prolonged effect of the furnace unit, the fouling characteristics, the tubular

reactor behaviour from start of run to end of run conditions, the heat transfer

aspects. Since the efficient utilization of the feedstocks and fuel have to be

ascertained before commercial units are designed and built, a logical step was

to test the pilot plant results in a semicommercial plant.

Stone &Webster's demonstration unit consisted of a preheat furnace, a furnace

filled with commercial sized coils, effluent exchangers, quenching facilities,

and a single stage quenching facilities and a single stage of gas compression.

The cracking furnace was basically one module of a commercial design with

respect to coil dimensions and metallurgy and number, arrangement and type of

burners,etc. The effluent heat exchange and quenching systems closely simulated commercial operation. Compressed cracked gas was delivered into an

operating ethylene plant for recovery of the contained ethylene as well as other

37
valuable products. Essential features of this unit are shown in Fig. 5.

From the tests in the demonstration unit, Stone &Webster could not only examine

the yield predicted by the pilot plant unit but also could develop a radiant

coil which had the best combination of residence time, hydrocarbon partial

pressure, and temperature profile leading to an ultra selective coil.

A number of extended duration runs were made to prove the operability of the

furnace and to examine the performance transferline exhanger and quench systems

when processing a number of feedstocks including heavy gas oils and waxy

distillates.

STONE & WEBSTER COMMERCIAL USC FURNAC~

With the extensive knowledge of basic theoretical analysis on the pyrolysis

reaction examined in the pilot plant and tested out in the demonstration unit,

Stone & Webster developed several Ultra Selective Coil designs, capable of

pyrolysis many feedstocks to maximize the possible olefinic production. A

typical furnace with radiant coil layout is shown in Fig. 6.

Radiant coils are laid out vertically in the furnace and fired from both the

sides.. The burners are so located to provide uniform heat distribution. The

radiant coils absorb as much as 40 percent of the fired duty and the waste heat

is recovered in the convection section where the hydrocarbons are preheated.

Additional heat recovery in the convection section lend itself to high pressure

steam production which provides energy to run the compressors.

Fig. 7 indicates a typical set of tube metal temperature profiles prevailing

under those pyrolytic conditions. Precise predictions of these temperature

profiles are possible by the understanding of the conversion of the feedstock as

it passes along the length of the coil.

The coke formation steps near the walls of the reactor is developed by the

temperature profile that prevails on the inside wall of the pyrolysis tube. The

coke precursor synthesis procedure is then modeled by examining the possible

stable small radicals present at the gas boundary layer and the aromatic

molecules. Coking mechanisms are developed and are incorporated along with the

mechanistic models of pyrolytic conversion. Such a model then predicts the

decomposition of the feedstock from start of run condition to the end of run

condition.

Stone & Webster have by now built 90 units allover the world with variety of

feedstocks (Table 3). This experience and expertise is available for extension

into realms of biomass pyrolysis.

38
PRIMARY

0.0. SE~

TO SEC. SEf?

TO HVY Q.O.

SURGE

DRUM

HVY.

Q.O.

EXCH .

.11

150#

STM

HEAVY

Q.O.

PUMP

demonstration cracking unit

carrington, england

cracking seclion

CRACKING

FURNACE STEAM

_t?_ DRUM BFW

--·+-~r-~·:~~NCH ! · FITTING

t. .T

I I

• •

I

USX •

EXCH I ..

I

FEED

PUMP

250 PSIG STM . ------4--+ ---~

PREHEAT r-....... FURNACE i

•

i

usc

FEED

TANK

FIGURE 5 STONE &: WEBSTER ..

54ASA
WALL BURNERS (TYP.)

SIGHT DOORS (TYP.)

r-

.--

J

1=-r ....

RADIANT I'""

COIL INLET (

\ r • \

~~

- - 1+ + + + + +~ n 0 0 0 0

+ + + + + +

- 1+0+0+0+0+0+

+ + + + + +

- I

I-J+o+o+o+o+ ... :t-

+ + + + + +

-

'r' 'I' r ... 'r ~t} -.- . . . . EFFLUENT TO TLX

RADIANT

COIL OUTLET

SPRING

SUPPORTS

I+--USX

EXCHANGER

o 0

STONE & WEBSTER USC

TYPE CRACKING FURNACE

Q

RADIANT COILS

NDUCED DRAFT --~,.-.. :AN AND STACK

CONVECTION COILS ~~

FLOOR BURNERS

WALL BURNERS (TYP.)

PLATFORMS (TYP.)

_____________________ GRADE

STONE 8c WEBSTER ~. FIGURE 6
1040

980

.pI-'

U

0

.. en 920 (;IJ

860

800

TUBE METAL TEMPERATURE

VS COIL LENGTH

-- CLEAN COIL

AVERAGE FOULED COIL

o 25 50

PERCENT LENGTH

FIGURE 7

75 100
USC EXPERIENCE BASED ON :

80-17,061

COMMERCIAL TEST FURNACE

BENCH SCALE FURNACE

COMMERCIAL INSTALLAliONS

TOTAL STONE & WEBSTER

OLEFIN UNITS BUILT

TABLE 3

6 YEARS

800 RUNS

40 FEEDSTOCKS

12 YEARS

500 TEST SERIES

ETHANE THRU 1100°F EP

17 OPERATING

6 DESIGN/CONSTRUCTION

18 BILLION LB/YR C2 H4

90

STONE & WEBSTER •
1. Rice, F.O., J. Arn. Chern. Soc., Vol. 55 (1933) p. 3035

2. Rice, F.D., Herozf~ld, K.F., J.Arn. Chern. Soc., Vol. 56 (1934) p. 284

3. Kossiakoff, A., Rice, F.D., J. Am. Chern. Soc., Vol. 65 (1943) p. 590

4. Woinsky, S.G., Ind. Eng. Chern. Pro c , Des. and Develop., Vol. 7 (1961)

p. 529

5. Gavalas, G.R., Chern. Eng. Sci., Vol. 21 (1966) p. 133

6. "Manufacturing Ethylene," Zdonik, S.B., Green,

compendium of articles from Oil and Gas Journal.

Company, Tulsa, Oklahoma.

43

E.J. & Hallee, L.P. A

The Petroleum Publishing
COMMERCIAL HYDROCARBON CRACKING PRACTICES

R. M. Contractor

E. I. du Pont de Nemours & Company

Central Research & Development Department

Wilmington, Delaware 19898

ABSTRACT

Steam cracking of different hydrocarbons, as practiced commercially,

to produce ethylene and other primary chemical feedstocks is

summarized. The process technology involved, and the effects of

feed quality and the process parameters on product yields are

briefly described. The feedstocks trends and feed flexibility for

ethylene plants are also ,di s cus s ed .

INTRODUCTION

Pyrolysis, or steam cracking of hydrocarbons, is the primary source

of ethylene worldwide. Hydrocarbon feeds as different as ethane to

gas oils are used in commercial crackers. Besides ethylene, the

cracked products include, in various ratios, other important primary

petrochemicals such as propylene" butadiene and BTX as well as fuels

ranging from hydrogen and methane to heavy fuel oil. Although the

design of an ethylene plant and operating parameters change substantially with different feeds, the same general processing scheme

is used. Hydrocarbon feed is cracked in the presence of steam at.

high temperatures and low pressures in tubular furnace coils. The

furnace effluent is rapidly quenched, heat recovered and liquid

products separated out. The gaseous products are compressed, acid

gases and water are removed and the desired individual products are

separated in a series of fractionation steps.

-

The current installed ethylene capacity in the United States is about

16.8 million tons per year [1]. These ethylene plants also supply

large percentages of the nation's propylene and butadiene needs as

well as about 10% of benzene demand as petrochemical industry feedstocks. The plants are among the largest, most complex and expensive

facilities of the hydrocarbon processing industry. Today's world

scale steam crackers often have ethylene capacities in excess of

500,000 tons per year.

45
PROCESS DESCRIPTION

A block flow diagram of naphtha orgas oil fed ethylene plant is

shown in Figure 1. The feed is mixed with steam in a steam-to-hydrocarbon weight ratio of 0.2 to 1.0 range, and the mixture is heated to

a cracking temperature of about 750°C to 900°C in a tubular furnace.

The steam is basically an inert diluent and is added for two reasons.

It lowers the hydrocarbon partial pressure, which improves selectivity

to olefins; and it increases the furnace run length by minimizing

coke formation on the reactor tube walls. The total residence time

in the furnace is generally in the range of 0.2 to 0 .7 seconds.

After the cracking reaction has progressed as far as desired, the

furnace effluent is rapidly cooled in the transfer line heat exchangers to stop further reaction. The heat is removed as high pressure

steam and is used to drive compressor turbines. The cracked products

are further cooled in a quench tower by a circulating quench liquor

stream. Here, the dilution steam and any liquid products are condensed and separated into different boiling range fractions;

additional heat is recovered as well.

The cracked gas is then compressed from a little over atmospheric

pressure to around 3500 kPa in a series of multistage compressors.

Water and hydrocarbons condensed between stages are separated from

the cracked gas in interstage separators and returned to appropriate

recovery units. Before the final compression stage, the gas is

treated to remove carbon dioxide and hydrogen sulfide, which can

contaminate the product and affect further processing. After

compression, the cracked gas is dried to remove the last traces of

water.

The product recovery section involves chilling the gas and processing

it through a sequence of condensate separation, fractionation and

heat exchange to recover ethylene, propylene, a butadiene rich C4

stream, hydrogen and methane. There exists numerous process variations in the product recovery scheme. Small amounts of acetylene in

the cracked gases is usually converted to ethane and ethylene by

hydrogenation. Other unwanted products such as methyl acetylene and

propadiene are also hydrogenated. The location of the acetylene

conversion step varies depending on the importance of butadiene

recovery. Ethane is usually recycled to extinction. Propane may be

recycled or sold as fuel. The C4 stream may be sent for butadiene

extraction and pyrolysis gasoline fraction is hydrotreated and fed

to an aromatics extraction unit or used as a blending stock for

gasoline.

RAW MATERIAL

Ethane, propane and heavier paraffins recovered from natural gas as

well as LPG, naphtha, kerosene and gas oil fraction of virgin crude

are the primary feedstocks for steam crackers. A small amount of

the nonaromatic raffinate from aromatics extraction units is also

46
COMPRESSION

GAS DRYING

ACID GAS

REMOVAL

C4 S PRODUCT

RESIDUE GAS

lit

CRACKED FUEL OIL

ETHYLENE PRODUCT

CRACKED GASOLINE

PROPYLENE PRODUCT

DEMETH.

FEED COOLING

C, ACETYLENE

REMOVAL

C. ACETYLENE

REMOVAL

RECYCLE

OIL QUENCH

ETHANE RECYCLE

H.P.

STEAM

EFFLUENT

EXCHANGE

EFflUENT

EXCHANGE

DILUTION STEAM

DILUTION

STEAM

PYROLYSIS

"--- NAPHTHA

Figure 1 - TYPICAL PROCESSING STEPS OF AN ETHYLENE UNIT

CRACKING LIQUID FEEDSTOCKS (2)
used. Although there have been several articles published in

technical journals within the last few years regarding the advantages

of using vacuum gas oil, as of now it is not a significant factor as

a feedstock for the commercial crackers.

Different feeds produce different slates of products. In general,

the higher the molecular weight of the feed, the easier it cracks.

Thus, ethane crackers give only 50 to 60% conversion per pass under

high severity cracking conditions whereas naphtha and gas oils give

very high conversions at lower severities [Table I]. Usually, this

means that a given furnace, designed for one type of feed, cannot

readily be used for other feeds without some penalty .

TABLE I

FEEDSTOCKS FOR OLEFINS PRODUCTION

% Rela.

Conv. % H2 % C2H4 Lb. Feed Steam Plant

Feed Pass in Feed Yield Lb. C2!!4- Feed Invest.

Ethane 50-60 20 83 1.2 0.2-0.4 IX

Propane 75-90 18 45 2.2 0.3-0.5 1.15X

n-Butane 95 17.2 41 2.4 0.3-0.5 1.2X

Naphtha High 15.5 34 2.9 0.5-0.8 1.45X

Gas Oil High 13.5 27 3.7 0.8-1. 0 1.64X

Vac. Gas Oil High 12.4 21 4.8 1. 0-1. 2 1.84X

Another important parameter ,of the feed is its hydrogen/carbon ratio.

It is a measure of feedstocks ethylene potential. The lighter hydrocarbons with their higher hydrogen/carbon ratio, or wt.% H2, give

greater ethylene yields. The basic hydrocarbon type in the feed also

has an important effect on its crackability and yield pattern.

Paraffins 'a r e the easiest to crack. Straight chain paraffins give

highest ethylene yields whereas branched paraffins give high propylene

yields. Cyclo paraffins decompose by ring rupture and give ethylene

yields similar to those of iso paraffins. The ring structure of

aromatic compounds has a high thermal stability and resists thermal

decomposition. However, side chains break off readily resulting in

high concentration of benzene in pyrolysis gasoline. Also there are

tar and coke forming tendencies due to ring condensation.

The amount of feed required to produce a pound of ethylene increases

rapidly with heavier feedstocks. Also, heavy feeds require more

diluent steam because of their higher tendency for coking. The

product recovery section is considerably more complex for ethylene

plants operating on heavy feeds because of large quantities of byproducts produced. Thus, for a given ethylene capacity, the size of

the plant and investment are significantly higher for heavy feedstocks.

48
However, besides investment, the selection of the feed for an

ethylene plant depends on the relative costs and availability of the

feed as well as on the market value of the by-products.

Typical pyrolysis yields of ethylene and other 'by- pr oduct s from

commercial crackers operating with different feeds are shown in

Figure 2. As the feed gets heavier and ethylene yield decreases,

yields of other products increase. Some of the by-products such as

propylene, butadiene and BTX are high value petrochemicals and the

remaining have fuel value. The dotted line in the figure shows the

typical split between chemicals and fuels, and indicates that even

gas oil crackers can produce over SO% chemical yields. Thus, for

a constant ethylene capacity plant, very large quantities of other

chemicals and fuel are produced with liquid feeds. Even though

ethylene plants are high energy consumers, gas oil crackers make so

much fuel product that they are invariably net fuel makers.

MAJOR 'PROCESS VARIABLES

The typical yield pattern shown in Figure Z can be significantly

altered by changing process conditions. Major process variables

affecting yields are cracking severity, residence time and hydrogen

partial pressure.

Severity

Severity is a measure of how hard the feed is cracked or how much of

the feed is cracked, and is a function of cracking temperature and

hydrocarbon residence time in the furnace coil. Figure 3 shows the

typical effect of increasing cracking severity on product distrIbution for a liquid feedstock. As the cracking severity is increased,

the yields of HZ, CH4, ethylene and higher mono olefins, the ,pr i mar y

products of the cracking reactions, start increasing. Simultaneously,

yields of CS+ liquid products start decreasing because of cracking

of liquid feed. As thetolefins concentration build up in the reactor,

they start cracking, hydrogenating, polymerizing and condensing at

increasing rate and produce acetylenes, diolefins, light saturates and

aromatics. Tge diolefins and acetylenes would also react further in

similar manner. Soon the yields of olefins reach a peak value and

then start ' decl i ni ng because of these secondary and tertiary reactions.

C4 olefins yields peak first, then C3 and eventually ethylene. As

the olefins yields start declining, the yields of CS+ liquid products

reach a low value and then start increasing because of polymerization

and condensation r eact Lons., Yields of HZ, CH4 and benzene keep

increasing with increased severity because of their high thermal

stability.' Thus, different product slates can be obtained by selecting different cracking severities. This gives olefins producers

some flexibility in responding to market demands. At very high

severity, the tar build up in the CS+ fraction begins to foul up

the reactor and the t ransfer line heat exchanger at increasing rate.

The maximum severity is usually limited by this factor. Very low

49
FEED

ETHANE 20 r---------------------::::::;IlI".,l!I_--,

40 60 80 100 SEVERITYYiELD ilT %

20

PROPANE 18

n-BUTANE CZH4

N

::c f

16 Q

NAPHTHA ,.. V1

0

14

GAS Oil

Figure 2 - TYPICAL PYROLYSIS YIELDS

FROM DIFFERENT FEEDS

Figure 3- TYPICAL EFFECT

OF SEVER ITY ON

YIELDS
severity decreases the furnace capacity because of poor feed conversion per pass. Also, once the furnace is designed, there is

generally not a great deal of flexibility in changing severity

without paying some economic penalty.

Residence Time

In general, short residence time favors the desired primary cracking

reactions and minimizes the less desired secondary reactions. The

selectivity to olefins increases at the expense of fuel products as

the residence time decreases. Thus, with advances in tube metallurgy

and furnace design, the design residence times in commercial naphtha

crackers have been decreasing for the last two decades from about

2 seconds to 0.2 seconds.

Hydrocarbon Partial Pressure

Pyrolysis selectivity towards ethylene and butadiene has been found

to be favored by low hydrocarbon partial pressure in the furnace coil.

The partial pressure is controlled by the reactor outlet pressure and

by the diluent steam to feed ratio. Selected partial pressure is a

compromise between steam costs, product gas compression costs and

increased ethylene yields. Once again, the hydrocarbon partial

pressure cannot be changed to a large extent without paying significant economic penalty, once the plant is designed to operate at a

certain partial pressure.

FEEDSTOCK TRENDS

Based on economics and availability, historically, light hydrocarbons

recovered from natural gas have been the primary choice as feedstocks

for ethylene plants in the United States. In 1975, about 75% of

ethylene produced in the nation came from crackers operating on these

light hydrocarbon feeds. However, as it became apparent that the

United States natural gas production was peaking out, ethylene producers started considering use of naphtha and gas oil for additional

capacity. Most new ethylene plants built in the last five years were

designed for these liquid feeds. This trend towards use of heavier

feedstocks is generally expected to continue in the future [Figure 4].

The situation is reverse in Western Europe and Japan. Traditionally,

naphtha has been the feedstock of choice for ethylene plans in these

areas because of limited production of natural gas and availability

of naphtha at low costs due to lower gasoline demand IFigure5].

Recently, however, increased amounts of light hydrocarbons are being

fed to ethylene plants. The percentage of ethylene produced from

these lighter feeds is expected to grow in the future I4,5] with

increased availability of NGL and LPG from the North Sea and Middle

East and increased demand for naphtha for the growing gasoline market.

Various assumptions and factors normally considered in making these

p~ojections are discussed below.

51
800

700

600

0::

ct

au

>- 500

0::

au

0..

CI)

..J

au

0:: 400

0::

ct

CD

Z

o

3300

200

100

NAPHTHA a

GAS OIL

•••••••••••••••• :•••••

..... " <-:.:-:-:-:-:.:-:-:-:.

,.

Nj~PHTHA

BUTANE

PI~OPANE

- ETHANE

1977 1980 1985 1990 2000

o L.---'---.-....-L-__L---L.. --I.----J~_---I'---......&-___L_ _

Figure 4 - U. S.OLEFIN PLANT FEEDSTOCKS (3)

52
u.s.

EUROPE

JAPAN

PRESENT

LPG

LPG

1985-90

Figure 5 - FEEDSTOCKS FOR ETHYLENE PLANTS

53
The supply of natural gas liquids is limited. U.S. production of

natural gas is expected to decline and the gas is getting drier.

World supply of LPG, primarily from Middle East, is expected to

greatly increase. U.S. gasoline consumption has peaked, but the

phaseout of leaded gasoline and expected rapid increase in demand

for premium unleaded gasoline will keep the heavy straight run naphtha

and aromatic feedstocks in tight supply. Low octane light virgin

naphtha and raffinate of reformate have only fuel value and should be

available for ethylene production. Raffinates are, however, rich in

isoparaffins which are poor feeds for ethylene production. Distillate demand is expected to keep rising because of increased demand

for jet fuel and diesel for automobiles.

It is generally believed that petrochemical producers will be able to

compete for these feedstocks with the fuel market. However, temporary

glut and shortages will almost certainly develop from time to time in

each category. Therefore, ability to use a variety of different feeds

for ethylene plants will become increasingly important. Meanwhile, the

incentive to use even heavier fractions of crude oil as feedstocks for

ethylene production will gr~w. Several efforts are underway for extending the commercially viable range of feedstocks by developing

processes to crack vacuum gas oils, whole crude and even resids [6-9].

FEEDSTOCK FLEXIBILITY

Most existing ethylene plants are designed for a single feed and

normally they cannot be operated on a different feed without some

penalty. Design of each major area of an ethylene plant changes substantially with a change in feedstock. For example, for a constant

capacity ethylene plant, a change to a heavier feed usually:

• Increases the amount of feed and requires higher steam

to feed ratio.

• Increases the furnace duty.

• Requires lower severity furnace with lower temperature

and shorter residence time.

• Results in lower percentage of heat removed as high

pressure steam in transfer line heat exchangers.

• Increases complexity of the quench tower. With liquid

feeds, a distillation column is required to separate large

quantities of pyrolysis gasoline and fuel oil products.

• Increases average molecular weight of the gaseous

products and increases compressor duty.

54
• Increases complexity of the gaseous product -recovery

section because of higher flow rates and because of

the importance of recovering increased quantity of other

olefins,

Also, as ethylene plants are often a part of a major complex with a

certain degree of interdependency regarding utilities and materials

between the ethylene plant and surrounding facilities, even a minor

change in feedstock composition requires prior extensive assessment

of its impact. However, ethylene producers usually find ways to

crack some amount of ot her-e.thanedesd.gn feed from time to time, sometimes after minor additional investment.

New facilities can be designed to provide significant feedstock

flexibility. Although this flexibility increases the plant investment

and ope-rating costs, ethylene producers have begun to find it prudent

to provide more flexibility, particularly on their newer plants.

'REFERENCES

1. Wett, T., "Ethylene Report", The Oil and Gas Journal, Sept. 1980,

pp. 59.,-64.

2. Zdonik, S. B., Bassler, E. J., and Hallee, L. P., "How Feedstocks

Affect Ethylene", Hydrocarbon Processing, Feb., 1974, pp. 73-81.

3. Cobb, C. B., "Petrochemicals in an Energy Constrained Economy",

Paper presented at 88th National Meeting of AIChE, June 1980,

Philadelphia, PAt

4. Scopes, H. M., "Feedstocks in the Eighties: A European

Perspective", Paper presented at 88th National Meeting of AIChE,

June 1980, Philadelphia, PAt

5. Ichikawa, M. and Kaike, H., "Feedstocks for Japanese Petrochemical

Industries", Paper presented at 88th National Meeting of AIChE,

June 1980, Philadelphia, PAt

6. Ishikawa, T. and Keister, R. G., "A Petrochemical AlternativeACR", Hydrocarbon Processing, December 1978, pp. 109-113.

7. Wing, M., "Why not Crack Crude?", Chemtech, January 1980,

pp. 20-22.

8. Greene, R. B., "Vacuum Gas Oil Cracking", 9th World Petroleum

Congress Proceedings of PD 19 (1), Tokyo, 1975.

9. Kunii, D., et aI, "Production of Olefins by Thermal Cracking of

Heavy Oils ....•.. ,", 9th World Petroleum Congress Proceedings

of PD 19 (3), Tokyo, 1975.

55
FLASH PYROLYSIS AND HYDROPYROLYSIS OF COAL

by.

Meyer Steinberg a~dBharat Bhatt

Process Sciences Division

Department of Energy and Environment

Brookhaven National Laboratory

Upton, New York

October 1980

Fbr presentation at "S~ecialists~

Workshop on Fast Pyrolysis of Biomass

Copper Mountain, Colorado

This work was performed under the auspices of the Department of Energy,

Washington, D.C. under contract DE-AC-02-76CH00016.

57
FLASH PYROLYSIS AND HYDROPYROLYSIS OF COAL

by

Meyer Steinberg and Bharat Bhatt

Process Sciences Division

Department of Energy and Environment

Brookhaven National Laboratory

Upton, New York

Flash Pyrolysis

Pyrolysis of coal is usually referred to as a process of thermal

decomposition in the absence of air or oxygen over a wide range of

temperatures. There is a considerable body of literature dating back

to early 1920's on the pyrolysis of coal with a rapid growth of

interest in the last two decades. It is the purpose of this discussion to briefly summarize the major findings and to bring in the more

recent work especially that relating to developing larger scale

pyrolysis and rapid hydropyrolysis processes.

Coal is the product of very slow decomposition of organic matter

deposited in prehistoric times. The younger coals will have gone

through less decomposition and hence will contain more volatile matter

than the older coals. The decomposition can be accelerated artificially by increasing the temperature. When heated at conventional

rate, (1~10oC/sec) in an inert atmosphere, coal begins to decompose at

350 to 400 0C into a carbon rich· residue and a hydrogen rich volatile

fraction. The volatile fraction consists of various gases and

liquids. The amount of volatile matter released and the relative

proportions of its various components depend upon the process

parameters such as temperature, heat up rate~ pressure, particle size,

type of coal, etc. Higher heating rates (10~ to 106 °C/sec) tend to

increase the volatile yield substantially. Pyrolysis with higher heat

up rate is known as flash pyrolysis. The developments in pyrolysis

and flash pyrolysis have been reviewed in several pUblications.~1,2)

Various heating methods, such as laser, microwave, flash tube, plasma,

electric arc, shock tube, electric current and entraining gas, have

been applied in the experimental work.

The amounts of volatile matter obtained by various investigators

were compared to the volatile matter content determined by proximate

analysis. Their ratio varied from 0.75 to 1.36 for several coals.(1)

58
Comparison of volatile yields obtained by Mentser et a1(3) with the

ASTM determined volatile matter is given in Table 1. Volatile yields

in excess of the ASTM volatile matter content, were obtained.

Figure 1 shows weight loss as a function of temperature for various

coa1s.(3) As the temperature increases, weight loss increases upto

a certain temperature, after that a drop in weight loss is observed in

a certain range of temperature, before rising again. The fraction of

tar and gas obtained for Pittsburgh hvAb coal are shown in Figure 2 as

a function of temperature.(3) As the temperature increases, tar

yield decreases and gas yield increases. The -gas composition for the

Pittsburgh hvAb coal as a function of temperature, are shown in Figure

3.(3)

II. Kinetics of Mechanisms

The exact description of the complex decomposition and transport

phenomena involved in coal pyrolysis is not yet available. A simple

model proposed by many authors is a first order decomposition occuring

throughout the particle. The rate constant is typically correlated

with temperature by the Arrhenius expression. There is little

agreement on the observed rate constants, with several orders of

magnitude discrepancy at a given temperature. The value of apparent

activation energies vary from 2 to over 50 kca·l/mole. Differences in

equipment, experimental procedures and various coals used seem to be

the reasons for the disagreement.

III. Flash Hydropyrolysis

Following early work, in the 1960's on the rapid gas phase

hydrogeneration of coal for the synthesis of liquid hydrocarbons,

preliminary bench scale experiments in a 3/4-in. diameter x 8-ft long

downflow reactor was undertaken at Brookhaven National Laboratory in

1974.(4) When hydrogenating lignite at temperatures and pressure up

to 700 0 c and 1500 psi, the results_indicated significant yields of

liquids, especially benzene, and gaseous hydrocarbons, particularly

methane and ethane. The liquid yields increased for reaction

residence times less than 30 seconds.

Preliminary process design and economic evaluation of a flash

hydropyro1ysis process(5) indicated a reasonable return on .ca pi t a 1

investment especially for chemical feedstock production. This gave

encouragement to the further investigation and development of a flash

hydropyrolysis process (FHP) which featured a one step, non-catalytic,

rapid deep hydrogeneration system for conversion of coal to synthetic

liquids and gaseous fuels. In late 1976 a larger, versatile, and

highly instrumented entrained downf1ow tubular reactor was constructed

and placed into operation.(6,7)

During this period several university research laboratories have

contributed to an understanding of the rapid coal hydrogenation

reaction, notably, R. Graff et a1.(8) at City University of New York

and J.B. Howard et a1.(9) at MIT. In addition, three

federally-supported industrial programs have been undertaken at Cities

59
Table 1

COMPARISON OF VOLATILE YIELDS FROM RAPID PYROLYSIS

OF VARIOUS COALS WITH ASTM VOLATILE MATTER CONTENTS

Volatile matter content-wt%

by ASTM from peak Increase

Coal Source analysis weight loss factor

Pochontas No. 3 16.8 18.5 1.10

Lower Kittanning 25.3 30.8 1.22

Pittsburgh 35.1 47.9 1.36

Colchester Ill. No.2 48.0 55.8 1.16

Rock Springs No. 7-1/2 37.7 42.4 (plateau) 1.12

60
,1

1200 1400

I

600 800 1000

TEMPERATURE. 'c

400

Ol.-~_"'---'----lL..-~-.t_-L... ~-L_""""~

200

so

10

.

en 30 en

o

-J

s20

-

W

:3:

....

z

w 40 (.J - a::

w

CL.

Figure 1. Devolatilization of Bituminous Coals by Rapid Heating (3)

61
60 r----r----.----,.-_~-....__-_

I- 50 z

LLJ

o

0:::

LLJ

e, 40

lX

(.!)

l!J3= 30 . .

o

-I

LLJ

-

>- 20

Io

::>

o

10 0

GAS

OL...--...l-_....J.._-.L__l..-_..L-_-J

600 70C 800 900 lOCO 1100 1200

TEMPERATURE.oC

Figure 2. Yields of Tar and Gas from Devolatilization

of Pittsburgh hvAb coal (3)

62
70 ,--__,..--..,..----r---r-----,r----,

.~---' 8

800 900 1000 1100 1200

TEMPERATURE.·C

700

CO2

o "--_.-..L__.........__....r.-__.a...-_-.-.:._..;;;..--J

600

10

.. z

o

i= SO

-

U)

o

0-

x:

820

en

a:

t!)

so

...

Z

I.LI -

50

I.LI

0-

W

...J

10

Figure 3. Composition of Gas from Devolatilizatiop

of Pittsburgh hvAb coal (3)

63
Service Research and Development Company on a short residence time

(CS-SRT) hydrogenation process,(10) at the Institute of Gas

Technology on a high speed riser cracking (IGT-RC) process using a

coil reactor(11) and at Rocketdyne Corp. on a rocket type reactor

system.(12)

The I-in. 1.0. tubular reactor, 8-ft long, equipped with four

sectional clam shell electric heaters followed by 4-ft of cooling

section is constructed of Inconel 617, a high Cr-Ni alloy. Hydrogen

can be supplied up to 5 lbs/hr and preheated to a maximum of 9000c in

a 1/4-in. diameter electrical resistance heated hairpin tube. The

reactor tube maxim~ operating conditions are 4000 psi and 800 0C or

2500 psi and 9000C. An on-line process gas chromatograph can analyze

10 chemical components every eight minutes with sample taps every two

feet along the length of the reactor and in the traps and vent lines.

A char trap maintained at 3000c separates out the char and avoids

condensation of liquid hydrocarbons. This is followed by a

water-cooled trap which separates the oils (> Cg) and a low

temperature trap (-400C) which separates the condensabla BTX « Cg).

The remaining gases are vented up a stack. A schematic of the

apparatus is shown in Figure 4.

The general trend of yields for the FHP conversion of coal

(lignite) to products collected at the exit of the tubular reactor are

shown for the light liquid product BTX « Cg, mainly benzene) in

Figure 5,' and for the main gaseous hydrocarbon products, methane and

ethane in Figure 6. It appears that the yield (in terms of fraction

of carbon converted) of liquid remains at a relatively low level at

any given pressure until temperatures in the order of 6500C is

reached. The BTX yield tends to rise and reach a maximum in the order

of 10% at temperatures ranging from 7250C to 8000C after which the

yield declines. Thus, a dynamic equilibrium appears to be established

between the formation and the decomposition of the liquid

hydrocarbons. In the case of the gaseous hydrocarbons, the yield

appears to continually rise both as a function of pressure and

temperature reaching values as high as 50 to 60%. However, these are

not maximum yields since they have been measured at the reactor exit

which is across the entire length of the reactor for coal residence

times ranging from about 9 to 12 seconds. The yields have been found

to vary along the length of the reactor going through a maximum in a

. number of cases.

Concerning the flash hydropyrolysis operation, it is calculated

that the average coal particle (100 dia) heat-up rate from ambient

feed temperature to reactor temperature is > 20,000 °C/sec when mixed

at the entrance to the reactor with 7500c preheated hydrogen. The

average cool-down or quench rate of the reaction mixture from the

heated reaction zone, through the air cooled quench zone to the 3000C

char trap is approximately 200 °C/sec. However, the initial cool-down

rate at the exit of the reaction zone may be higher. The maximum

yields were determined from the highest measured value along the

length of the reactor using the sample taps. One example of the

product yield as a function of coal particle residence times is given

64
PRODUCT CONDENSERS

H2 SUPPLY_, _H....;::2-+-_

TRAILER 30~

COMPRESSOR

PRESS. REG.

CHAR

TRAP

SAMPLE

TAPS

-- -

~O

GAS

VENT

GAS

METER

FLOW

CONTROL I

REFRIG.

SAMPLE

TAP

SCHEMATIC FLOWSHEET OF ENTRAINED TUBULAR REACTOR EXPERIMENT

Figure !~

65
500 550 600 650 700 750 800 850

0.20

ill

c

a.J 015

II:

a.J

>

Z

0

U

Z

0

III

II: 0 .10 ct

U

Z

0

i=

u

ct

II:

u,

0.05

3000psig

TEMPERATURE ·C

FLASH HYDROPYROLYSIS OF LIGNITE

BTX vs TEMPERATURE

PRESS. 500 - 3000 psill

COAL RESIDENCE TIME 9-12 sec

Figure 5

66
0.7

TOTAL HC GAS o 0.6 a:

80.

5

CDs 0.4 a:

z 0.3 o

t=

a: 0.2,..I_-=,.""..:;:::::::;;~::::::=------------------:n

I>.. C---------

0.1

500 1000 1500 2000 2500

PRESSURE (psiQ)

FLASH HYDROPYROLYSIS OF LIGNITE

TOTAL He GAS VI. PRESSURE

TEMP 700°- 800°C

COAL RESIDENCE TIME 9-12 see

Figure 6

67

3500 4000
in Figure 7 at 3000 psi and 8250c. As the residence time increased

from 2 to 10 seconds, BTX and ethane yield decreased while methane

yield went through a maximum around 7 seconds and this also decreased.

In the temperature range of 7500 to 800 0C and pressure range of

2000 to 2500 psi, the maximum yield for BTX peaks out at coal particle

residence times varying from 2 to 7 seconds with the shorter residence

times at the higher temperatures.

Because of the higher boiling range of the heavier oils (> Cg),

these are determined only by collection at the end of the reactor. As

shown in Figure 8 for North Dakota lignite at temperatures in the

range of 7250 to 7500C and 2000 psi, the maximum yield of the heavier

liquid hydrocarbons (> Cg), is approximately equal . to that of the BTX

yield. However, above 7500C, the> Cg's decrease much more rapidly

with increasing temperature than theBTX. Since one of the probable

products of decomposition or hydrocracking of the heavier liquids is

BTX, there appears to be a relationship between the measured yields of

these two liquid products. At even higher temperatures, (> 7750C) the

heavier li~uids (> Cg's) decrease rapidly primarily due to

hydrogenation and rapid formation of gaseous hydrocarbons.

As the reactor temperature increases above 8750C, the principal

product remaining is methane with smaller amounts of ethane. The

maximum gaseous hydrocarbon yield occurs between 2.4 and 7 seconds

coal particle residence time over a range 6f pressures as given in

Figure g. The almost linear nature of the curves shows a yield

increase of from 2.8 to 3.8% per 100 psi of total system pressure.

This pressure effect is thermodynamically in agreement with that

predicted for the hydrogen-carbon reaction producing CH4 and C2H6.

Since the feed ·ratio of hydrogen to coal is usually in the order

of 1 lb of hydrogen to 1 1b of coal, the product concentration in the

process gas stream are far below thermodynamic equilibrium values.

Due to this dilute phase, the methane gas usually is below 5 mole %

concentration in the equipment while the equilibrium concentration is

84% at 7000C and 4000 psig and 32% at 9000C and 500 psig. By

decreasing the hydrogen to coal feed ratio in the reactor to 0.2 at

2500 psig and 8750C, a methane concentration as high as 33% was

observed. The thermodynamic equilibrium concentration under these

conditions is 61%, thus, the methane reached 54% of the equilibrium

value. At very low feed ratios we have exceeded the equilibrium

concentration. The rapid hydropyrolysis of coal is not equilibrium

limited.

Two maximum yield runs are shown in Table 2. One run listed is

the highest liquid yield run (19.9% liquid and 64.5% total conversion)

and the other is the highest gaseous yield run (88.5% gaseous HC and

90.5% total conversion).

Additional data has been obtained with a sub-bituminous coal.

The yield structure appears similar. However, the sub-bituminous gave

a higher gaseous yield at lower pressures than the lignite.(13)

68
FLASH HYDROPVROLYSIS OF LIGNITE

FRACTION CARBON CONVERTED

VI

RESIDENCE TIME

RUN NO. ISS

TEMPERATURE -825 OC

PRESSURE-3000 psi

0.9

0.8

0.7

0

a:: 0.6

LLI

:>

z

0

(,J 0.5

z

2 TOTAL H.C.

a:: GAS

(,J 0.4

z

0

i=

(,J 0.3

u,

0.1

2 4 5 6 7

RESIDENCE TlME (sec)

8 9 10 II

Figure 7

69
0.22

0.20

018

0

w

l- 016 a::

w

>z 0.14 0

o

s 0.12

« 0.10 0 o

z

0

i= 0.08

e: 0.06

0.04

002

FLASH HYDROPYROLYSIS OF LIGNITE

LIQUID YIELD YS. TEMPERATURE

PRESSURE - 2000 psig

o BTX (~Cs)

• OILS (~Cg)

775 800 825 850

TEMPERATURE "C

750

oL----l.....-__.L-__---L_---===:c=====~___J

725

Figure 8

70
1.0

0

LLI

LLI 0.8

; 0.6

8

z

04

L!..

0.2

FLASH HYDROPYROLYSIS OF LIGNITE

GASEOUS HYDROCARBON YIELDS

vs.

PRESSURE

TEMPERATURE-B5Q-9OQOC

RESIDENCE TIME-1.2-2.4 sec. 0

7.0 - 9.4 2.4-7.0 sec.sec. 0

•.>:

:~~/

1500 2000 2500

PRESSURE (psig)

1000 OL-------....I..-..------~--------L...-- ___I,;..__J

500

71
Table 2

FLASH HYDROPYROLYSIS OF NORTH DAKOTA LIGNITE

Maximum Liquid and Maximum Gas Yield Runs

Reactor temp. (OC)

Reactor pressure (psig)

H2 feed rate (lb/hr)

Coal feed rate (lb/hr)

Residence time (sec)

Product Yields (% Carbon

CO

CH4

C2H6

Total HC gas

BTX

C9

Total liquid

Total

Heat of reaction (kcal/gm coal)

Hydrogen consumption (lb/lb coal)

Effective carbon conv. (%)

Effective energy eff. (%)

Max. Liquid Run Max. Gas Run

775 875

2000 2500

0.82 0.88

0.98 0.95

7.1 4.7

3.4 1.3

30.9 88.0

10.3 0.5

41.2 88.5

10.2 0.7

9.7 0.0

19.9 0.7

64.5 90.5

- 0.70 - 1.3

0.077 0.20

58.8 62.4

75.9 100.0

72
The nitrogen and sulfur distribution and balances were measured.

The lignite is a relatively low S content coal (0.5% S). With FHP the

major portion, 48 to 77% of initial S remains in the char. The oil is

found to contain < 0.1% S. For the nitrogen (initial N content is

0.9%), less than half remain~ in the char and the oil contains < 0.2%

N.

Mixing recycled char with the lignite feed and addition of iron

oxide to the lignite improved the yield of BTX and ethane.

A reaction model has been developed in an attempt to obta~n

generalized expressions for the hydrocarbon component yields as a

function of pressure, temperature and residence time.(14,15) The

results of the non-linear regression analysis show that a first-order

chemical reaction model with respect to C conversion, with a

production and a decomposition step for each of the major products,

satisfactorily describes the dilute phase hydrogenation. The activation energy for the initial products formation was estimated to be

about 43,000 cal/mole and the power of hydrogen partial pressure was

found to be +0.14. Activation energies indicate mainly a chemical

reaction controlled mechanism. The use of these rate equations in

conjunction with heat balance expressions are applied to the design of

a hydrolysis reaction ves~el. Additional experimental, design, and

analysis work is needed to obtain a much more detailed understanding

of the reactor engineering.

Process design and preliminary economic estimates were made for

three versions of the FHP process system:(14) (1) producing only

liquid products for motor gasoline fuel, (2) producing both liquid and

gaseous hydrocarbon products for motor gasoline and pipeline gas, and

(3) producing only gaseous products for pipeline gas. For comparable

overall values of coal conversions (62%)', the minimum production cost

is obtained for the combined production of motor gasoline and pipeline

gas. The largest part of the capital cost of the FHP plant, amounting

up to 35% of the plant, is required by the equipment needed for

recycling and conditioning the hydrogen process gas. The FHP process

has the advantage of allowing effective gas-solid and gas-liquid

separatio~ operations. The hydrocarbon products can be readily

separated from the unconverted char, oils, and process gas by

condensation from the gas phase. The FHP system also has the distinct

advantage of versatility and process flexibility. The product slate

and the production rate of gaseous and liquid' hydrocarbon fuel

products can be altered in the same reactor by adjusting the reactor

operating conditions.

IV. Comparison of Flash Pyrolysis and Hydropyrolysis Systems

An attempt has been made to compare the yields and product distribution from several pyrolysis and hydropyrolysis systems. The data

are listed in Table 3.

73
Table 3

FLASH PYROLYSIS AND HYDROPYROLYSIS OF COAL

COMPARISON OF PROCESSES

Heating Residence Yield-% Carbon Conversion

Pressure Temp. , rate, Time Total Gas BTX Oil Tar

Reactor System Coal Type atm °c °C/sec °C/sec Gas+Liquid

Electrically heated Pittsburgh vacuum 700 8300 50 1Z 38

tube, Muentser et al HVB(1973)

Garret-occidental Pittsburgh 1 atm 580 Z800 4Z 7 35 -....J

+:-- system--heat with HVB (COZ,HZ)

char

Flash hydropyrolysis Lignite and 130 (HZ) 775 ZO,OOO 7 60 1 40 10 10

entrained flow with HZ Sub-bituminous 170 (HZ) 900 ZO,OOO 5 89 Z 88 1

BNL, IGT, CS

Rockwell--short res. Pittsburgh 100 (HZ) 1040 50,000 0.1 60 Z7 33 total liquid

time, hydrogen HVB 70 (HZ) 980 50,000 Z 56 45 11

entrainment

Avco gasifier Bituminous 37 1315 >50,000 0.1 49 49

char combustion and HVB (HZO,NZ'COZ) (HZ'CO,COZ)

steam gasification

l)Liquefaction mode.

Z)Gasification mode.
Mentser et al(3) heated small quantities of coal in an

electrically heated resistive screen cylinder in vacuum. The heating

rate was of the order of 8300 0e/sec. Maximum yields of volatile

matter including tars and gases were obtained in the range of 700 0 to

8000e. The volatile matter yields were higher than the ASTM

analyses. The tar yields at 700 0 c are up to 40% of the carbon content

in the coal while the gas yields are in the order of 12%. As the

temperature increased to 12000 c tar yields decreased and gaseous

yields increased.

The Garrett-Occidental System heats coal at a rate of 28000e/sec

with recycled hot char and inert gas.(16) This process which

reached pilot plant scale operates at 5800C and near atmospheric

pressure. The yield amounted to 35% of the carbon to heavy liquid

(tar) products and 7% to gas. The heavy liquids would have to be

upgraded by hydrocracking for producing a saleable lighter liquid

fuel.

Data for the flash hydrogenation indicate higher heat up rates

(20,OOOoe/sec) in an entrained tubular reactor. For maximizing liquid

products, temperatures of 775 0e and hydrogen pressure up to 130 atm

are required. Liquid yields are up to 20% of carbon converted to

benzene and lights oils and 45% to gas of which up to 31% consists of

methane. At higher temperatures of 8750 C and 170 atmup to 90% of the

carbon in the coal can be converted to methane.

A form of a high capacity entrained reactor is the Rockwell(17)

design which employs a rocket engine type mixing device for contacting

the preheated hydrogen and the coal. In the mode of maximizing

gaseous product, 56% of the coal carbon can be converted to gaseous

and liquid products (45% to gas and 11.3% to lighter liquids such as

benzene) at residence times of 2 seconds. It is also claimed that for

very short residence times, on the order of 0.1 sec or less, gas

yields are reduced and up to 33% liquids can be obtained. However,

these liquids appear to be heavier tarry substances.

The Avco-Gasifier concept(18) is a rapid gasification process

using high temperature combustion products in a high temperature

pyrolyzer. Unconverted char is combusted with oxygen and the products

are mixed in a secondary pyrolyzer with steam. The products are

mainly H2 and CO, which is a synthesis gas useful for subsequent

catalytic conversion to hydrocarbons. The overall carbon conversion

for the process is 49%.

Summary of Observations

A brief summary of the major findings of pyrolysis and

hydropyrolysis are as follows:

1. Rapid pyrolysis of coal usually produces larger yields of

volatile matter including liquid and gaseous hydrocarbons than slow

pyrolysis.

75
2. Rapid pyrolysis in vacuum or with an inert atmosphere yields

lower conversions of coal to liquid and gaseous hydrocarbons than

hydropyrolysis in a hydrogen atmosphere.

3. Pyrolysis in a higher pressure inert atmosphere tends to

decrease overall yields.

4. Rapid pyrolysis in an inert atmosphere produces larger

amounts of heavier liquids and tars than hydropyrolysis.

5. Hydropyrolysis at higher hydrogen pressures produ~es higher

overall yields.

6. Rapid pyrolysis at higher temperatures tend to decrease

yields and produce lower molecular weight species.

7. Rapid hydropyrolysis at higher temperatures produces more

gaseous hydrocarbons and lower liquid yields.

8. Longer residence times in rapid hydropyrolysis tends to

produce more methane. Shorter residence times tend to produce more

liquids. There are optimum residence times for maximizing either

liquid or gaseous yields.

9. Pyrolysis in an inert atmosphere at longer residence times

tend to increase conversions but then becomes constant after

devolatilization is complete.

10. Heating rate is a function of the method of heating.

Entrained gas heating with hydrogen produces the highest heating rates

and produces the highest yields.

11. Lower rank coals tend to be more reactive and produce higher

yields of gaseous and liquid hydrocarbons than higher rank coals.

12. From a process economics point of view, pyrolysis in an

inert atmosphere requires the need for' a consumer of residual char,

and an upgrading of heavier liquids.

13. In a flash hydropyrolysis process, the char can be used to

produce hydrogen consumed the process. A considerable fraction of the

cost of the process is involved in production, recovery and recycle of

hydrogen.

References

1. Anthony, D.B., and Howard, J.A., "Coal Devolatilization and

Hydrogasification" AIChE Journal 22, No.4, 625-56 (1976).

2. Belt, R.J., and Bissett, L.A., "Assessment of Flash Pyrolysis

and Hydropyrolysis" METC/RI-79/2 Morgantown Energy Technology Center,

West Virginia (July 1978).

76
3. Mentser, M., O'Donnel, H.J., Ergun, S., and Friedel, R.A.,

"Devolatilization of Coal by Rapid Heating", ACS Preprint, Division of

Fuel Chemistry, Vol. 18, No.1, pp. 26-35 (1973).

4. Steinberg, M. and Fallon, P., "Coal Liquefaction by Rapid Gas

Phase Hydrogenation", BNL 19507, Brookhaven National Laboratory,

Upton, New York, (November 1974) and American Chem. Soc., 169th

National Meeting (Philadelphia, Pa.), Petroleum Chemistry Division 20,

No.2, pp. 542-52 (April 1975).

5. Steinberg, M., Sheehan, T.V., and Lee, Q.E., "Flash

Hydropyrolysis Process for Conversion of Lignite and Gaseous

Products", BNL 20915, Brookhaven National Laboratory, Upton, New York

(October 1975) and American Chem. Soc., 170th National Meeting (New

York, N.Y.), Ind. and Eng. Chem. Division (April 4-9, 1976). Also

published in Syn. Fuels Processing, A.H. Pelofsky, editor, Marcel

Dekker, Inc., (New York, N.Y.) pp. 163-92 (1977).

6. Fallon, P. and Steinberg, M., "Flash Hydropyrolysis of

Coal--The Design, Construction, Operation and Initial Results of a

Flash Hydropyrolysis Experimental Unit", BNL 50698 (January 1977)

presented at 173rd National American Chern. Soc. Meeting (New Orleans,

La.) March 20-25, 1977).

7. Steinberg, M. and Fallon, P., "Flash Hydropyrolysis of Coal",

Quarterly Reports Nos. 1 to 5, BNL 50677, 50707, 50779, 50821, 50824

(January 1977 to March 1978).

8. Dobner, S., Graff, R.A., and Squires, A.M., "Flash

Hydrogenation of Coal, Yield Structure of .Illinois No.6 Coal at 100

atm.", Fuel 55, 114 (1976).

9. Anthony, D.B. and Howard, J.B., "Coal Devolatilization and

Hydrogasification", AIChE J. 22, 625 (1976).

10. Greene, M.I., Ladelfa, C.J., and Bivacca, S.J.,

"Benzene-Ethylene-SNG from Coal via Short Residence Hydropyrolysis",

Cities Service Research and development Company, Cranbury, New Jersey

(March 1978) presented at 175th National AIChE Meeting (Anaheim,

Calif.) (March 12-17, 1978).

11. Dunea , D.A., Beeson, J .L., and Oberle, R.D., "Research and

Development of Rapid Hydrogenation for Coal Conversion to Synthetic

Motor Fuels (Rise Cracking of Coal)", FE-2307-34 (August 1978)

Institute of Gas Technology, Chicago, Illinois, Annual Report for the

period of April 1, 1977 to March 31, 1978.

12. Oberg, C.L., Combs, L.P., and Silverman, J., "Coal Conversion

by Flash Hydropyrolysis and Hydrogasification", Proceedings of the

13th Intersociety Energy Conversion Engineering Conference (San Diego,

Calif.) 1, pp. 402-8 (August 20-25, 1978).

77
13. Fallon,P.T., Bhatt, B., and Steinberg, M., "The Flash

Hydropyrolysis of Lignite and Sub-Bituminous Coals to both Liquid and

Gaseous Products", BNL 26210, Brookhaven National Laboratory, Upton,

N.Y. (May 1979) accepted in Fuel Technology (in press).

14. Steinberg, M., Fallon, P., Dang, V.D., Bhatt, B., Ziegler,

E., and Lee, Q., BNL 25232, Reaction, "Process and Coal Engineering

for the Flash HydropyroLysds of Coal", BNL 25232, Brookhaven National

Laboratory, Upton, New York, (November 1978).

15. Bhatt, B., Fallon, P., and Steinberg, M., "Reaction

Modelling and Correlation for Flash Hydropyrolysis of Lignite", BNL

27574, Brookhaven National Laboratory, Upton, N.Y., (March 1980).

16. Sass, A., "Garrett's Coal Pyrolysis Process", Chem. Eng.

Progress, 70, No.1, 72-3 (1974).

17. Silverman, J., Freidman, J., and Kahn, D.R., ·"The CS/R

Advanced SNG Hydrogasification Process", 15th Intersociety of Energy

Conversion Engineering Conference (IECEC) Vol. 2, 952-958 (Aug. 18-22,

1980).

18. White, G.A., Stechler, D.B., Rosenberg, C.W., and Gannon,

R.E., "Indirect Liquefaction via the Avco Coal ,Gasi f i ca t i on System",

15th IECEC, Vol. 2, 978-84 (Aug. 18-22, 1980).

78
INTRODUCTION TO PYROLYSIS OF BIOMASS

F. Shafizadeh

Wood Chemistry Laboratory

Department of Chemistry University of Montana

Missoula, Montana 59812

ABSTRACT

COMPOSITION OF BIOMASS

Pyrolysis of biomass involves different materials and methods and pro- vides a variety of products. An analytical relationship between these

factors provides the basic knowledge which is needed for new development.

Biomass consists of different types of dead and living plant cells,

the structure and composition of which varies for different parts and

species of the plant. The living cells as in the green leaves contain

some proteins in their protoplasm, considerable water in the vacuoles

which maintains their turgidity and less cellulosic cell wall materials.

The woody tissues contain some living (parenchyma) cells, but are largely

79
composed of dead (prosenchyma) cells which consist of several layers and an intercellular material (middle lamella). Figure 1 shows a

diagramic presentation of a wood fiber (tracheid).

FIG. 1. Diagrammatic pre~entation of wood fiber

(tracheid).

Cellulose microfibrils embedded in a matrix of hemicelluloses and

lignin form the main components of the cell walls [1] and biomass

in general. In addition to these materials, there are some lipids and

hydrocarbons (terpenes) which are soluble in ether and various types of phenolic compounds, carbohydrates and proteins, which may be soluble

in benzene, alcohol or water. The leaves and bark generally contain

more extractives and less cell wall materials than wood and woody tissues. Table 1 shows the approximate analysis of several types of biomass.

The cellulose component, which is a macromolecule composed of linearly linked B - (l~4) D-glucopyr~nose units, is the same in all types of biomass except for the degree of polymerization (DP). However, the

nature of the hemicelluloses and lignin could vary. Aggregation of

the linear chains of the cellulose macromolecules within the microfibrils provide a crystalline structure which is highly inert and

inaccessible to the chemical reagents. Consequently, it is very difficult to hydrolyze the cellulose molecules by acids or enzymes.

80
-:fABLE ,

App roximate Ana 1ys is 0 f Some Bioma ss Speci es

Species Total Solvent Water Ltqnin Hemi- Cellulose

ash sol. sol. cellulose

Softwood 0.4% 2.0% 27.8% 24.0 % 41.m;

Hardwood 0.3 3.1 19.5 35.0 39.0

~Jhea t straw 6.6 3..7 7.4 16.7 28.2 39.9

Rice straw 16.1 4.6 13.3 11.9 24.5 30.2

Bagasse 1.6 0.3 20.2 38.5 3B.1

In contrast to cellulose, the hemicelluloses are amorphous, have a

lower DP and could be preferentially hydrolyzed under relatively mild

conditions, known as prehydrolysis. Acetyl-4-0-methylglucuronoxylan

(xylan) forms the main hemicellulose of the hardwoods and glucomannans

(mannan) form the principal hemicellulose of the softwoods.

Lignin is a randomly linked, amorphous, high molecular weight phenolic

compound which is more abundant and polymeric in softwoods than in

hardwoods. Furthermore, the softwood lignin contains guaiacyl propane

units (phenolic groups having one methoxyl group). Hardwood lignin, in addition to this, contains syringyl propane units (with two methoxyl groups). The higher content of "acetyl and methoxyl groups in hardwoods

explains why this material has been used in destructive distillation

processes to obtain acetic acid and methanol.

The soluble components could be removed by stepwise solvent and water

extraction, but isolation of the other cell wall components in the

native form is very difficult, if not impossible. After the solvent

extraction, lignin could be removed by selective oxidation to leave

a combination of cellulose a d hemicelluloses called holocellulose.

Alternatively, the hemicellulose component could be removed by controlled hydrolysis to leave a combination of cellulose and lignin called

lignocellulose. Complete hydrolysis of the carbohydrate leaves the

lignin as a dark partially decomposed residue called Klason lignin. It

is also possible to break the lignin bonds (presumably with the carbohydrates) so that it could be removed with solvents at high temperatures as organosolv lignin.

COMPETING PYROLYTIC PATHWAYS

Different components of biomass display different thermal properties

and the thermal properties of the aggregate represents the sum of its

major organic components [2].

81
As discussed later on rapid heating, the carbohydrates break down at

the glycosidic link to provide lower molecular weight volatile pyroly- sis products, whereas lignin is mainly charred to a carbonaceous residue. This phenomenon is observed on thermogravimetry of wood and its

components shown in Figure 2 [2J.

100 t----niiiiii"i";;~.......~rnrn~--..;;::------------

--

c

CI)

o

...

CI)

0.

50

200 300

Temperature (OC.)

400 500

FIG. 2. Thermogravimetry of cottonwood and its components.

Furthermore, thermal properties of the components and aggregate are highly influenced by the presence of inorganic materials and various

treatments. As these materials are heated, the availability of sufficient energy for alternative pathways at higher temperatures results

in a series of complex, concurrent and consecutive reactions which

give a variety of products.

The Arrhenius plot for the rates of weight loss of cellulose in air

and nitrogen is shown in Figure 3 [3]. These data indicate a transition at - 300°C which reflects··the existence of two different pathways. As shown in Figure 4, the rate of pyrolysis followed by weight loss under isothermal conditions, shows an initial period of acceleration

and proceeds much faster in air than in an inert atmosphere. As the

pyrolysis temperature is increased, the initiation period and the

difference between pyrolysis under nitrogen and air gradually diminish

and disappear at 310°C when pyrolysis by the second pathway takes over.

82
,. " -. . .

lnk

. -2

-3

-4

.,

0 ...... , ........

.........

'b~.Q..-::----.- " . .

< , --.---.

......... !2.....

.... .Q . ------- '~...... . -5

...0..............

....(1

1.70 1.75

x 103 (OK-l )

1.80

FIG. 3. Arrhenius plot for the first order reaction in the isothermal

degradation of cellulose in air (~) and nitrogen (~~-).

---------

300

"- Ai r

--------------- ----2.?~t"

---.... - --

200

Time (min)

...

""""

"

'" ,,

....

"....,

....

:-",

,

-,

" ,

"'~<ft "f',

" ,

,

....

100

0.2

1.0

0.9

0.8

0.7

0.6

0.5

0,4

w-w""

wo-w"" 0.3

FIG. 4. First order plot for the residual cellulose weight (normalized)

versus time~ Plots at 310°C and 325°C for air ~nd nitrogen

are similar.

83
The general pathways for pyrolysis of cellulose [4,5] are shown in

Figure 5.

Anhydrosugars, Tar

1

Cellulose Gases and Low MWt Products

T Water, Char, CO 2, CO

FIG. 5. Competing pathways for cellulose pyrolysis.

LOWER TEMPERATURE REACTIONS

The reactions in the first pathway, which dominates at lower temperatures, involve reduction in molecular weight or DP by bond scission,

appearance of free radicals, elimination of water, formation of carbonyl, carboxyl and hydroperoxide groups (especially in air), evolution of

carbon monoxide and carbon dioxide, and finally production of a charred

residue. The mechanism and kinetics of these reactions which contribute

to the overall rates of pyrolysis of cellulosic materials have been

individually investigated [3]. Reduction in the degree of polymerization of cellulose on isothermal heating in air or nitrogen at a temperature within the range of l50-l90°C has been measured by the visicosity method

as shown in Figure 6. This data has been correlated with rates of bond

scission given in Table 2 and used for calculating the Arrhenius plot shown in Figure 7.

Figure 8 shows the rate of production of carbon monoxide and carbon

dioxide at 170°C in air and in nitrogen. The rate of evolution of these

gases is much faster in air than in nitrogen and, furthermore, accelerates on continued heating. It is constructive to compare the initial

linear rates for the emission of these gases with the rates of bond

scission obtained for depolymerization at 170°C, discussed before.

As can be seen in Table 3, for air the rate of bond scission approxi- mately equals the rate of production of carbon dioxide plus carbon

monoxide in moles per glucose unit. In nitrogen, however, the rate

of bond scission is greater than the combined rates of carbon monoxide and carbon dioxide evolution.

It is assumed that carbon dioxide and carbon monoxide may be formed by decarboxylation and decarbonylation, respectively, as well as by other

competing reactions. The significance of the former reactions was

determined by measuring the net rate of accumulation of carboxyl and

carbonyl groups in cellulose on heating in air at 190°C. The results

84
... A

A 150·C .

... 16O·C

• 170·C

• 180·C

• 19O·C

400 --~-----':=====..L• •

800

1200

2000

10 20 30 40

Tine (hr)

FIG. 6. Viscosity average degrees of polymerization (Pv) of cellulose

heated in air at 150°C-190°C.

TABLE 2

Rate Constants for the Depolyrnerization of Cellulose in Air and Nitrogen

Temperature.Pfl

ko X 107 in Nz

mole/162 g min"

ko X 107 in Air

mole/162 g min

150

160

170

180

190

1.1

2.8

4.4

9.8

17.0

6.0

a.i

15.0

29.8

48.9

a 162g represents 1 mole of monomer unit.

85
- 12

•

-13

l nk -14

-15

- 16

-,, ,

0,, , ,

'0, , , , <,

6',,

.... ,0

, , ....... , ,

0 ...... ,

2.15 2.2 2.25 2.3

1. x 103

T.

2.35

FIG, 7. Arrhenius plot for the rate of bond scission in air (--) and

nttr,ogen (---).

TABLE 3

Initial Rates of Glycosidic Bond Scission and Carbon Monoxide and Carbon Dioxide Formation

at 170°C

Reaction

Bond scission

CO evolution

CO2 evolution

Rate X 105 in N2•

mole/162 g hr

2.7

0.6

0.4

Rate X 105 in Air.

mole/162 g hr

9.0

2.1

6.4

30

l1lIOOlel

162 gm 20

10

200 400 600 800

Time (hr)

FIG. 8. Yields of CO and C02 from heating cellulose at l70QC:(o)C0

2 in N2; (0 )CO in N2; (.)CO2 inai r ; (.)CO inai r . .

86
shown in Figure 9 indicate an almost linear rate of formation on heating for 50 hr. On heating for longer periods, the rate of accumulation

of carboxyl groups falls off, and "the rate of accumulation of aldehyde

groups is increased. The latter effect, however, could be due to the

sodium chlorite method employed for the oxidation and measurement of

the aldehyde groups. These data show the production of carboxylic and

aldehyde groups on oxidation of cellulose in air. There was also a

very small increase in the number of oxidlzed functions on heating in nitrogen. In order to check the extent of decarboxylation at the relatively lower pyrolysis temperatures, samples of low-OS (degree of sub- stitution) carboxylcellulose with carboxyl groups at Cl, C2 and C3 and

C6 were prepared and pyrolyzed. The results shown in Figure 10 are,

by and large, inconclusive, except for the sample oxidized at C2 and

C3, which showed a definite reduction in carboxyl-group content. Thus,

although both carboxyl and carbonyl groups, as well as carbon dioxide

and carbon monoxide, are formed during low-temperature pyrolysis, the

production of the "l at t er compounds could not be directly related to

the formation of the former functions.

The thermal degradation of cellulose in air, similar to that of synthetic polymers, may involve a free-radical mechanism. We were not

able to observe these radicals, but were able to monitor the formation

of hydroperoxide groups on heating cellulose in air. The hydroperoxide functions are simultaneously formed and decomposed, and their concentration rapidly c1 imbs unti 1 a steady state is reached. Figure 1\' shows

10

8

6

nillO lei

162 gm CARBOXYL

4

LACTONE

ALDEHYDE

10 20 30

Time (hr)

40 50

FIG. 9. Incorporation of oxidized groups in l l at 190°C---v---

represents number of lactone and carboxyl groups formed in

nitrogen at 190°C.

87
the development of the steady-state concentration in air at 170°C during a period of about 100 min. It also shows the rate of decay in nitrogen

over a similar period. This decay of the hydroperoxide function appeared

to follow first-order kinetics with a rate constant of 2.~X10-2 min-l

at 170°C. From the steady-state concentration of 3.0xlO- mole/162g, the rate of hydroperoxide decomposition is th~s 7.5x10-7 mo1e/162 g

min. When compared with the initial rate of bond scission in air of

1.5xlO-6 mo1e/162 g min at 170°C (Table 2), it is apparent that the

hydroperoxide formation could make a significant contribution to bond

scisson.

o []

20 \

~~"

15 •

"---.-------.

0I0101e/

162 901

10 ~--

•

-~

5

10 20 30 40 50 60

FIG. 10. Effect of heating on carboxyl content of carboxyl ce1lu1oses -i n nitrogen at 190°C. (0) l-carboxyl cellulose; (.) 2,3-

carboxyl cellulose; (D) 6-carboxy1 cellulose; (.) prepared by heating cellulose in air at 190°C.

6

•

4 nvro1e/

162 901

( x 102)

2

• •

I I I I f I

30 60 90 0 30 60 90 180

Time (mi n. )

FIG. 11. Rate of formation and decay of hydroperoxide groups in cellulose at 170°C. Formation is in air and decay is in

nitrogen.

88
Based on the above considerations and in analogy with studies onradiation of carbohydrates, it is proposed that three stages are involved

in the low temperature pathway; initiation of .pyrolysis, propogation

and product formation. As shown in Figure 12, the initiation period apparently involves the formation of free radicals facilitated by the

presence of oxygen or inorganic impurities. Subsequent reactions of

the free radicals could lead to bond scission, oxidation and decompo- sition of the molecule, producing char, water, carbon monoxide and carbon dioxide. Figure 13 shows the nature of the i-nitiation, propagation,

and decomposition reactions involved in the thermal decomposition of

cellulose by this pathway.

Initiation

Hea t

Initiator (l) ----. ICell-H + O2 - Cell-+ H0 2-

I- + O2 - 10 2-

Cell-H + I- - Cell- + IH

Cell-H + 10 2- - Cell- + 10 2H

P ropagation

Cell- + O2 --+ Cell O2-

Cell O2-+ Cell-H --+ Cell 0 2H + CellFormation of Products

Cell 02H --+ Cell 0- , Cell O2-

Cell OzH --+ Products

Cell Oz-, Cell 0 - - Pr oducts

FIG. 12. The thermal autoxidation of cellulose in air.

FIG. 13. Possible mechanism of formation and decomposition of

cellulose hydroperoxide in air.

89
TRANSGLYCOSYLATION

At temperatures above 300°C, cellulose is decomposed by an alternative

pathway which provides a tarry pyrolyzate containing levoglucosan

(1,6-anhydro-B-D-glucopyranose), other anhydroglucose compounds, randomly linked oligosaccharides, and glucose decomposition products [6J.

Figure 14 shows the g.l.c. analysis of the tarry pyrolyzate after

trimethylsilylation of the free hybroxyl groups [6,7J. The mechanism

for the formation of these compounds has been established by extensive

investigation of the pyrolytic reactions of phenylglucosides and other

related model compounds. [5,8-10J.

k

9

o

r:n n

5 10 15 20 25 30 35 40

Retenti on time ( min)

FIG. 14. G.l.c. of tar from pyrolysis of (trimethylsilyl)ated cellulose

(column 2,110 to 275 0 at 4°/min). [a,5-(Hydroxymethyl)-2- furaldehyde; b, 1,4:3,6-dianhydro-~-D-g1ucopyranose; c, 2,3-

dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one; d, unknown;

e, 3,5-dihydroxy-2-methyl-4H-pyran-4-one; f, unknown

9,1,5-anhydro-4-deoxy-D-glycero-hex-l-en-3-ulose; hand

i, unknown; j, levogluc'osan; k, 1,6-anhydro-B-:-D-g1ucofuranose; 1, a-D-glucose; m, B-D-glucose; n, 3-deoxy-D-erythro-hexosulose; and 0, .Q-D-g1 ucosy1-1 evogl ucosans.] .

90
It has been shown that pyrolysis ofa glycosidic compound as shown in

Figure 15 starts with cleavage of the glycosidic group and condensation

of the sugar moiety, which is decomposed on further heating. The sequence of these reactions as a function of temperature could ·be determined by thermogravimetry (TG) [5] and differential thermal analysis (OTA) as shown for phenyl S-O-glucopyranoside in Figure 16.

r .OH\O,-0-0-L

H~OH

1EVapC'lratiof,. i Evaporation

HoD + Q-Glucosans Decomposition

- Products

FIG. 15. Pyrolytic reactions of phenyl S-D-glucopyranoside.

The individual reactions which take place within each thermal event,

characterized by changes in mass (TG) and/or energy (OTA), are then

followed by various scanning or isothermal analytical methods. In

Figure 15, cleavage of the phenolic group is established by quantitative recovery of the free phenol and condensation of the sugar unit is established by recovery and chemical ·analysi s of intermediate pyrolysis products [8].

It has been shown that the reaction proceeds through a heterolytic mechanism because it is influenced by variation of the electron density producted through the introduction of various substituents on the aglycone [5,8]. This conclusion eliminated the possibility of a homolytic mechanism (free radical cleavage of the glycosidic bond), although

further breakdown and charring .of the sugar units involve free radical

100r-"""""""-----....,....-.:.....----------------,

ie !"": f! at Ll "C

FIG. 16. Thermal analysis of phenyl f3-:-0-glucopyranoside.

91
formation. It also indicated that the thermolysis, as a heterolytic

process, must be sensitive to the effect of acidic and alkaline reagents.

Investigation of a variety of glycosides containing deoxy, amino and

acetamidogroups and the corresponding fully acety1ated compounds [8,9]

showed that the substituent on the sugar moiety, in addition to the

inductive effect on the glycosidic bond, changes the availability and

reactivity of the transg1ycosy1ation sites, and the acety1ated compounds

are considerably more stable than the parent compounds with free hydroxyl

groups. These data and production of randomly linked condensation pro- ducts [8,11] confirmed that the heterolytic cleavage of existing glycosidic bonds proceeds through a transg1ycosy1ation process involving nucleophilic displacement of the glycosidic groups by one of the free

hydroxyl or amino groups of the sugar molecule. Thermolysis of 1evoglucosan (1,6-anhydro-S-D-g1ucopyranose) [6] reducing and non-reducing disaccharides [10] (cellobiose and trehalose), cellulose [6] and xy1an [12] and the isolation and structural investigation of the products showed that the transg1ycosy1ation reactions give mixtures of anhydrosugars and randomly linked oligosaccharides.

There is a close similarity between the thermolysis of phenyl glucoside (Figures 15 and 16) and the thermolysis of cellulose (Figure 17) which

starts at 300°C and proceeds rapidly until most of the substrate is

evaporated. Also chromatographic analysis of the tarry pyro1yzate (Figure 14) shows the presence of similar products. These products,

as shown in Figure 18, are formed by the intramolecular substitution

of the glycosidic linkage in cellulose by one of the free hydroxyl groups

(transg1ycos1y1ation). Subsequent inter- and intra- molecular transglycosy1ations provide several anhydrosugars. and randomly linked oligosaccharides, which could dehydrate and decompose on further heating. The initial substitution requires changes in the conformation of the

sugar units and increased flexibility of the molecule. This could be

achieved at the elevated temperatures by reduction of molecular weight, breaking of hydrogen bonds and glass trans·ition which are expected to

activate the molecule.

TG

HJO ISO 200 250 300

OTG

450

1 10

90

80

t esce -aeere, "C

FIG. 17. Thermal analysis of untreated cellulose.

92
1.4:3,6-DIANHYDRIDE

H

1,6-ANHYDRID£ ' (FURANOSE)

~""TO-H~°:elO~CH20HO'

OH n

CE LLUlOSE HO OH

}\/~~ f1H

H

2 H~CHPHo f

OH HO z

o o OH OH E

14-ANHYDRIDE 1,6-ANHYDRIDE 1,2-ANHYDRIDE S

' 0, 0 -~ i

z , H 2

!

1M

o

HO

FIG. 18. Pyrolysis of tellulose to anhydrosugars and other

compounds by transglycosylation reactions.

KINETICS OF CELLULOSE PYROLYSIS

Kinetic studies have shown that at higher temperatures the tar-forming reactions accelerate rapidly and overshadow the production of char and

gases. 'The data in Table 4 shows the production of diminishing amounts

of char and increasing amounts of tar, the anhydrosugars and other

compounds (that could be hydrolyzed to reducing sugar) as the oven

temperature is raised' from 3000 to 500°C [13]. At this time, it should

be pointed 'out that evaporation of levoglucosan and the volatile pyroly- sis products is highly endothermic (see Figure 17 and Ref. 13).. Thus,

the increased oven temperature could raise the rate of heat transfer

but not necessarily the temperature of the ablating substrate. As

shown in Fi.gure 19 '[14], the oven temperature is reached when the rapi d evaporation and the accompanied change in entropy is over. In other

words, at the higher temperatures the pyrolysis process is controlled by the rate of heat transfer rather than the kinetics of the chemical reaction. Material transport presents another major obstacle to the investigation of chemical kinetics, because if the products of primary reactions are not removed, they could undergo further decomposition

reactions.

93
400 /--4070C

I

I

I

,

300

200

100

40rc

2 3 4

Time in oven (min)

5 6

FIG. 19. Temperature of pyrolysis as measured by a thermocouple in

cellulose sample. Final temperatures Tf are indicated.

(---) temperature measured in empty boat. The cooling

curve follows removal of a sample from the oven at 335°C.

Table 5 shows the difference between the yield of different pyrolysis products in vacuum, which removes the primary volatile products, and

in nitrogen at atmospheric pressure which allows more decomposition

of the anhydrosugars [6]. It also shows the effect of inorganic catalysts in changing the nature of the reactions and products. In view

of these considerations, the chemical kinetics of cellulo-se pyrolysis

has been investigated within the limited temperature range of 260-340°

and under vacuum in order to obtain chemically meaningful data. Under

these conditions, the chemical kinetics of cellulose pyrolysis could

be represented by the three reaction model shown in Figure 20 [14].

In this model, it is assumed that the initiation reactions discussed

before lead to the formation of an active cellulose, which subsequently

TABLE 4. Effect of temperature on the products from pyrolysis of

cellulose powder under vacuum.

Oven Pyrolysis Percent Yield from Cellulose

Temp Time levo- l,6-Anhydro- Reducing

(Ge) (min) Char Tar glucosan fl-D-gluco- Sugar

furanose

300 180 21 60 34 4 47

325 60 10 70 38 54

350 30 8 70 38 4 52

375 10 6 70 38 59

400 5 5 77 39 4 60

425 4 4 78 40 4 S9

4S0 3 4 78 39 4 57

475 3 3 80 38 4 58

500 3 3 81 38 4 57

94
decomposes by two competitive first order reactions, one yielding

anhydrosugars (transglycosylation products) and the other char and

a gaseous fraction.

TABLE 5. Analysis of the pyrolysis products of cellulose at 300°C

under nitrogen.

Condition

1.5 Hm Hg,

Atm. pressure 1. 5 Hm Hg 5\ SbC1 3

Q)ar 34.2\a 17.8\a 25.8ta

Tar 19.1 55.8 32.5

levoglucosan 3.57 28.1 6.68

1,6-anhydro-B-D- 0.38 5.6 0.91

glucofuranose =

~-glucose trace trace 2.68

hydrolyzable materials 6.08 20.9 11.8

a Th e percentages are based on the origin al amount of cellulose.

where

k .

1.

Cellulose -

Weell

k Volatiles

"Active cellulose";: Wv

wA Char + Gases

w w e q

-d(Wee11) z ki[wcell]

dt

d(WA) ki[weell]- (kv + kel[WA]

d(Wel • O.35ke[WA]

For pyrolysis of pure cellulose under vacuum the rate constants k. k • . 1.' Y

and k

c were found to correspond with k

i =1.7 lo2l e---(58,OOO/RT) min-l

k

y = 1.9 X l016e---(47,300/RT) min-1 and k

e

= 7.9 X 1011e---(36.000/RT) min-1

FIG. 20. Pyrolysis model for cellulose.

95
DEHYDRATION REACTIONS

As discussed before, detailed analysis of the pyrolysis tar (see

Figures 14 and 18) shows the presence of 1evog1ucosan, its furanose

isomer (1,6-anhydro-S-D-g1ucofuranose) and their transg1ycosy1ation products as the main components. In addition to these compounds, the

pyro1yzate contains minor amounts of a variety of products formed

from dehydration of the glucose units. The dehydration products de- tected include 3-deoxy-D-erythro-hexos-u1ose, 5-{hydroxymethy1)-2- fura1dehyde, 2-fura1dehyde (furfural), other furan derivatives, 1evoglucosenone {1,6-anhydro-3,4-dideoxY-B-D-g1ycerO-hex-3-enopyranose, 1,5-anhydro-4-deoxy-D-g1ycero-hex-1-en-3-u1ose and other pyran derivatives. The dehydration products are important as intermediate compounds in char formation.

The 3-deoxy-D-hexosu1ose, which is the initial dehydration product of hexoses plays a significant role as an intermediate in the production of furan derivatives. Isomers of this compound have been identified

among the-pyrolysis products of cellulose [6,7], D-g1ucose, D-fructose

and D-xy1ose [5]. Moreover, the oligosaccharides formed from the

pyro1ytic transg1ycosy1ation of carbohydrate derivatives or the condensation of the reducing sugars often show u.v. and i.r. absorption bands that could be due to the presence of 3-deoxy-hexosu1ose units

or its tautomeric forms I10J. -

In view of the above considerations, pyrolytic reactions of 3-deoxy- D-eryth,ro-hexosulose (3-deoxy-glucosulose) were investigated with

dynamic thermal analysis and parallel chemical methods in order to gain

better understanding of the dehydration, rearrangement, decarboxylation and charring, which could ensue after the initial formation of this

compound. The resulting data showed that 3-deoxy-D-erythro-hexosulose decomposes rapidly within the range of [15] 100-200°C as compared to

D-g1ucose [16], levog1ucosan and glycosides [8] which decompos~ at the

nuch higher temperatures of 250-350°C. Within this range, a small

fraction of the compound gives 5-(hydroxymethy1)-2-furaldehyde, pyru- va1dehyde, glycolaldehyde, glyceraldehyde and glucometasaccharinic acid as monomeric products of dehydration, fission and rearrangement; while most of the compound undergoes condensation and chars.

As in aqueous reactions [14], the dehydration reactions are strongly catalyzed by presence of acidic reagents. G.l.c. analysis (see Figure 21) has shown that acid catalyzed pyrolysis of cellulose at -350°C

produces a pyro1yzate containing 1evoglucosenone (instead of levoglucosan) as the major component and 1,4:3,6-dianhydro-a-D-g1ucopyranose, 2-fura1dehyde and 5-(hydroxymethyl)-2-fura1dehyde as minor components [17]. Levog1ucosenone, formed by dehydration reactions shown in Figure 22, could be separated by fractional distillation, and is a highly reactive compound that could be obtained by pyrolysis of waste paper, treated with mineral acids. It could be readily converted to a variety of vinyl, substituted and addition compounds. Some of these reactions

are already investigated, but the industrial applications of this compound still remain to be explored.

96
A FLRFLIW.

8 LEVOGLUCOSHOt£

C t,J4:3J 6-DIANHYDRo-a-Di...LX:OPYRAf'OSE

0 5-HvDROXYt-'ElHYL-2-

FLRA1..IEHYIE

B

A

C

D

I ~~ ~LA o 10 20

TWE (MIN)

30

FIG. 21. G.1.c. analysis of the pyrolyzate from cellulose + 2%

H4P04at 350°C.

Cellulose

N

/t

H~1OHtt l~::nhYdro-a- 1 Levoglucosan Levoglucosenone .D-g ucopyranose

FIG. 22. Dehydration of cellulose and glucose derivatives to

levoglucosenone.

97
fISSION AND DISPROPORTIONATION REACTIONS

On further heating, fission of the sugar units at higher temperatures accompanied by dehydration, disproportionation, decarboxylation and

decarbonylation provides a variety of carbonyl, carboxyl and olefinic

compounds as well as water, carbon dioxide, carbon monoxide and char

[5,18,19]. The analyses of these products are closely similar to those

obtained from pyrolysis of levoglucosan shown in Table 6 [20]. These

compounds may be divided into three categories. The first category

includes furan compounds, water and char, which are the expected products of the better understood acid catalyzed dehydration reactions of

carbohydrates under aqueous acid conditions [4]. The second category

includes glyoxal, acetaldehyde and other low molecular weight carboxyl

compounds, which are similar to the alkaline catalyzed fission products

of the sugar molecule formed through the reverse aldol condensation

mechanism. The products formed through these pathways are further

randomized by disproportionation, decarboxylation, and decarbonylation

reactions to provide a th ird category which is characteristic for the

pyrolytic reactions, especially at the elevated temperatures.

TABLE 6. Pyrolysis products of 1,6-Anhydro~S-D-Glucopyranose at 600°C.

Pyrolysis product ----- - -- - - -- -yi el d- - - - --- ---- --- Neat +ZnC1Z +NaOH

Aceta1dehyde 1.1 0.3 7.3

Furan 1.0 1.3 1.6

Acrolein 1.7 <0.1 2.6

"'.ethanol 0.3 0.4 0.7

2.3-Butanedione 0.5 0.8 1.6

Z-Butenal 0.7 O.Z 2.Z

l-Hydroxy-Z-propanone . 0.8 <0.1 1.1

Glyoxal 1.4 <0.1 4.9

Aceti c ad d 1.7 0.7 1.5

2-Fura 1dehyde 0.9 3.0 0.4

5-Methyl-Z-furaldehyde 0.1 0.3

Carbon dioxide 2.9 6.8 5.7

\.later 8.7 ZO.l 14.1

Char 3.9 29.0 16.0

Balance (tar) 74.3 36.8 40.3

98
In the absence of acidic or alkaline catalysts, both fission and

dehydration products are formed. However, as expected, addition

of these catalysts promote the formation of one type of product

at the expense of the other. .

The chemical reactions involved were further investigated by the synthesis of levoglucosan labeled at Cl, C2, and C6 and tracing of the

major pyrolysis products to theoriginal positions of the sugar molecule

[19J. The resulting data shown in Table 7 indicate the major dehy- dration and fission pathways shown in Figures 23 and 24. These schemes

are .more complex and less specific than the single pathways generally

postulated for normal reactions, because at high temperatures, the

energy barrier becomes less significant and the thermal decomposition

could proceed throuqh different routes[4].

r~ OLIi

I

HCOH

I

HOCH

I

nCCH I

I H~O~ I I

-0:1

2

I

CHOH

I

HCOH

I

H-DCH

--.--~ I

HCOH i

I '

HCO---.J

I

LIi

20H

)

CHOH

II

CHOH

+

CHO

I

HCOH

i

HCOH

I

CH20H

1

)

----4

THO

HCOH

I

COH

II

CH2

I

CH

'"

!i 2

COH

I

I

COH

H

GI 2

,.

I

~3

c=o

I

c==o

I

CH3

CHO

I

COH

II

01

I

CH20H

1

CHO

I

c=o

I

012 I

CH20H

CHO

I

HCOH

I

c=:o

I

aH. 3

1

co

4-

CH20H I

c:=O

I

013

1

FIG. 23. Degr3dation of levoglucosan to volatile products.

99
TABLE 7. Percentage of the pyrolysis products traced to the labeled

carbons of 1,6-anhydro-S-D-glucopyranose.

Neat. 5' Na.OH 5\ %ne1z poun4

I_He Z_14 e 6_ He I_He 2_14e 6.He 1_14e 2_ 14e 14 6- e

ral::!e~yde 60 .8 103.4 35 .8 30.2 100 47 73.0 86 .0 95.8 16 .6

2 • .!- t.!,;a..~e ~i o n 24.8 54.6 31.3 16 .5 31 .0 56.5 64 .8 57 .7 26.9

l:, ;.: valc!ehy~. 27.3 26 .3 19 .1 23.3 42.0 30 .7 49.7 46.7 29 .6

;: e t a. c! de 10 .1 30.5 36 .0 6 .3 29.2 55.1 4.4 7.3 29.8

C:'y.::x al 15 .4 19 .2 6.9 25.5 48.3 36 .0 29 .5 28.2 28.2

:: a.: ':::"I. c!ioxicSe 34.3 24 .5 6 .3 31 .2 17.7 8.9 43 .7 33.3 9.5

Ca =. :-.o:,.o x i de 21 .1 18 .9 16 .8 38 .4 18.0 13 .7 36.7 27 .6 11 .4

r

CH eH::>H CHO CHOH

I I I U

HCOif HCOH HCOM COH

I I I I

HOCH

[~ ll" [" I .~ I --? -~ HCOH HCOH HCOH HfOH

I 111')- Ht

HCO U H HCOH

I I

00:2 -0:

2 CH2 CH2

/ 1

co CH20 CH20H CH20H

+ I I

CHO C=O c=o

I I I

L l~H10 CH H

I

CH HCOH

1\ II I

CH CH CH2

FIG. 24. Production of 2-furaldehyde under alkaline conditions.

100
CONCLUSIONS

The above data indicate that pyrolysis at lower temperatures (below

300°C) favors the productiori of char, water, CO2, and CO; whereas

heating at higher temoeratures (300-500°C) favors the production of

tar, containing anhydrosugars, oligosaccharides and some pyran and

furan dehydration products. Flash pyrolysis 'at still high tempera ... tures (above 500°C) results in the fission, dehydration, disproportionation, decarboxylation and decarbonylation reactions, which provide a mixture of low molecular weight, gaseous or volatile products.

Indirect evidence shows that ,t hese materials are formed from the

secondary reaction of the tar and high temperature interaction of

char with water and carbon dioxide (see Figure 5). However, the

possibility of direct conversion of cellulose molecules to the low

molecular weight fission and disproportionation products could not be entirely eliminated.

,ACKNO\~LEDGMENT

The author is pleased to acknowledge the contributions of the postdoctoral fellows, research associates and graduent students who are

named in the following list of 'ref erences . He is also grateful to

the National Science Foundation and the National Bureau of Standards

for their support of this program.

101
REFERENCES

1. Shafizadeh, F. and McGinnis, G.D. "Morphology and Biogenesis

of Cellulose and Plant Cell Walls" Advan. Carbohydr. Chem.,

26, 297 (1971). ----

2. Shafizadeh, F. and McGinnis, G.D. "Chemical Composition and

Thennal Analysis of Cottonwood," Carbohyd • Res., l§.., 273 (1971).

3. .Shafi zadeh, F. and Bradbury, A.G.W. "Thenna1 Degrada t i on of

Cellulose in Air and Nitrogen at Low Temperatures," ~. ~.

~. Sci., 23, 1431 (1979).

4. Shafizadeh, F. "Pyrolysis and Combustion of Cellulosic Materials, "

Advan. Carbohydr. Chem., £l, 419 (1968).

5. Shafizadeh, F. "Industrial Pyrolysis of Cellulosic Materials,"

~. Polym. ~., 28, 153 (1975).

6. Shafizadeh, F. and Fu, Y.L. "Pyr ol ys i s of Cellulose," Carbohyd.

Res., 29, 113 (1973).

7. Shafizadeh, F., et al. "1,5-Anhydro-4-Deoxy-D-glvcero-hex-lEn-1-Ulose and Other Pyro lysis Products of Cellulose," Carbohyd.

Res., 67, 433 (1978).

8. Shafizadeh, F. Susott, R.A., and McGinnis, G.D., "Pyrolysis of

Substituted Phenyl S-D-glucopyranosides and 2-Deoxy-a-D-arabinohexopyranosides, " Carbohyd. Res., n., 63 (1972).

9. Shafizadeh, F., ~1eshriki, M.ll., and Susott, R.A., "Thennolysis

of Phenyl Glycosides," ~. Org. Chem., 38, 1190 (1973).

10. Shafizadeh, F. and Lai, Y.I., "Thermal Rearrangements of Cellobiose

and Trehalose, " Carbohyd. Res., li, 57 (1973).

11. Lai , V.I. and Shafizadeh, F., "Thermolysis of Phenyl S- D- gl ucODYranoside Catalyzed by linc Chloride," Carbohyd. Res., 38, 177 (1974).

12. Shafizadeh, F., McGinnis, G.D. and Philpot, C.W., "Thermal Degradation of Xylan and Related Model Compounds, " Carbohyd. Res.,

~, 23 (1972).

13. Shafizadeh, F., et al., "Production of Levoglucosan and Glucose

from Pyrolysis of Cellulosic Materials,11 ~. ~. Polym., Sci.,

£l, 3525 ( 1979) .

14. Bradbury, A.G.~J., Sakai, V. and Shafizadeh, F., "A Kinetic Model

for Pyrolysis of Cellulose," ~. ~ppl. Polym . , Sci., 23, 3271 (1979).

15. Shafizadeh, F. and Lai, Y.I., "Thermal Degradation of 3-DeoxyD-erythro-Hexosulose," Carbo~yd. Res., 40,263 (1975).

102
16. Shafizadeh cF, and Lai, Y.Z., "Base-cata1yzed, Pyrolytic

Rearrangement of Some Monosaccharides, II Carbohyd. Res., 26, . 83 (1973) ..

17. Shafi zadeh, F., Furneaux, R.H., and Stevenson, T.T., II Some

React ions of Levog1 ucosenone, II Ca rbohyd. Res., Zl, 169 (1979).

18. Shafizadeh, F., Lai, Y.Z., and McIntyre, C.R., "Thermal Degradation of 6-Ch1oroce11u1ose and Cellulose-Zinc Chloride Mixture," .'l.. ~. Po1ym. Sci., 22,1183 (1978). It

19. Shafizadeh, F. and Lai , Y.Z., "Therma1 Degradation of 1,6-

Anhydro-B-D-g1ucopyranose, ~. Org. Chern., 37, 278 (1972).'

20. Shafizadeh, F., Philpot, and Ostojic, N., "Thermal Analysis

of 1, 6-Anhydro-B-D-g1 ucopyranose, II Carbohyd. Res., l§., 279 (1971) .

. 103
DIRECT FORMATION OF PYROLYSIS OIL FROM BIOMASS

by

Herbert M. Kosstrin, Ph.D.

Stone &Webster Engineering Corporation

ABSTRACT

An extensive experimental program of the Fluidized Bed Pyrolysis of

Biomass to Clean Energy was carried out by Energy Resources Co.

This paper describes the analytical model developed to predict the

yield of liquid fuels from biomass. It is found that secondary

reactions play a large part in the determination of the final liquid

product yeild.

BACKGROUND

The production of liquid fuels from biomass has been accomplished

via various means on several pilot plant size reaction systems.

Various amounts of pyrolysis oil have been produced by Occidental's

Flash Pyrolysis Process (1 )Georgia Tech t s " Fixed Bed Partial

Oxidation, (2) Lawrence ~erk1ey's Labs' Acid Hydrolysis(3), and

ERCO's Fluidized Bed Pyrolysis Process(4). The quality and quantity

. of pyrolysis oils produced by these several processes vary. In

addition, reaction conditions within each process will change the

output quality and quantity of the pyrolysis oils produced. This

change ·i n the quanity and quality of produced pyrolysis oils is due

to the effect of secondary reactions. This paper will discuss the

work on Fluidized Bed Pyrolysis of Biomass preformed by Energy

Resources 'Co. Inc. (ERCO) of Cambridge, Massachusetts, ' under EPA

contract 68-03-0240.

INTRODUCTION

The production of biomass liquids, as practiced in the ERCO bubbling

fluidized bed reaction system, is a rapid pyrolysis. The solids

entering the system are well mixed and can have a residence time on

the order of several minutes; while the gases and vapors evolved are

flowed through the bed in plug flow and have a stay time on the

105
0.80 Oil Yield

Ul

PRODUCT YIELD FLEXIBILITY _0.60 '"d

r-l

~*- (1)

AT VARIOUS TEMPERATURES u:C 'r; B e 0.40 :>t

U'l ... +J -0 Q)

Q)>

0

tf80

.20

;::l

'00H

Pot

0.00 Ul

800°F 1,400oF 1,700oF .r;

Ul

:>l

...-l \0 0 0

.-I

Gas Yield Char Yield

H

0.80 0.80 :>l

Pot

0.60 0.60

-

- ~~ ~*- u-o u:C B Q) 0.40 B e 0.40 U'l t U'l ... r-f

-0 Q) -0 Q)

C1l> Q)

>

Q)C Q)

C

LL8 0.20 LL

8 0 .20 ::>oH

0.00I

II

, 0.00

800°F 1,100oF 1,400oF 1,700oF 800°F 1,400oF 1,700oF
order of seconds. A series of parametric studies were conducted in

the ERCO 20 in. diameter fluidized bed pilot facility. The

parameters studied were gaseous residence timet particle size t and

reaction temperature. Figure 1 shows the data from the pyrolysis of

sawdust. It is evident that the major parameter of interest is

reaction temperature. Although variations of a factor of three (3)

were obtained in gaseous residence timet and of an order of

magnitude in particle size t these parameters did not influence the

data to the extent of the reaction temperature.

With this experimental data , a reaction model has been developed

that predicts the formation of pyrolysis oils from biomass.

MODELING

It is clear from the data obtained that the modeling of pyrolysis

behavior in bubbling fluidized beds must account for secondary

reactionst referring primarily to cracking and carbon deposition on

particles and the associated conversion of part of the oils or tars

to char and gases. This requirement arises because the oil yields

observed for all of the materials studied decrease with increasing

temperature t whereas the amount of oil actually produced as a

primary product and observable in simplified experiments that are

designed to minimize t or even eliminate secondary reactionst is

known to increase monotonically with increasing temperature. In

other words t the concept of oil destruction by secondary reactions

is required to explain the decrease of oil yield with increasing

temperature. Oil lost in this way is assumed to be converted to

char and gas. These characteristics of the behavior are illustrated

qualitatively in Figure 2 where the solid curves represent product

yields from the fluidized bed pyrolyzer and the dashed curves

represent yields obtained under simplified conditions of no

secondary reactions. Thus t the differences between the two types of

yields .r epr esent oil destruction by cracking and an associated and

equal amount of char plus gas formation. The extent of the

secondary reactions is seen to be significant only above some

critical t emperatarre , and to increase with increasing temperature

until the attainment of the limits impos~d by the depletion of the

oil and the completion of the primary decomposition.

The general overall reaction mechanism from refuse to final products

will follow the general scheme in Figure 3. The refuse is

decomposed by a series of n primary reactions into primary productst

each of which is acted upon by a series of secondary reactions as

displayed in Figure 3. Char is assumed to be formed as the solid

residue of the primary reactions and as solid material deposited in

the course of ·secondary reactions.

107
e

-

"">-

la/OIL

... - .. --- --------

ICI CHAR

ibl G':'S

TeMPERATURE

"' ....... ...~""­ ... ....

TEMPEi=lATURE

... ... -- TEMPERATURE

Yields with secondary rnctions.

as in fluioizea bed pvroiysis.

_____ _ Vietds WithOut !ec:]ndaty a c r. . I n d ea

on lv ~,imarv oecomCOSUion or ;:yrolvsrs.

Figure 2 Schematic representation of effect of secondary reactions on pyrolysis

yields at different temperatures.

108
pnlMAnv PUIMAnV SECONUARV SECONIlARY

n[ACTIONS rUOIlUCIS nEACllONS rnooucrs

i 1

( 'a

fit

)

,

) Ih

• II .

······

S

· Ul · oM · s::: (2) 1m · liE ruse ) 2 ) 1m «J

..c:

CJ

Q)

:g

s::: 0

OM n) ) 3

+l

CJ

.--~ «J

Q)

0

M ....-I M

«J

,..

. Q)

~NiI

t·

N

a

>0

(Y"l

(Nt ) N

N

h

tN

h Q)

-- _..

· J.f ·

t

··· OM

..UIMAflV

· r:. .

ellAn

·

N

m

·

Nm

t·

ADDITIONAL

C.IAn
Although the figure shows one volatile product for each primary or

secondary reaction, the concept is sufficiently general to account

for the possibilities that a given reaction may form multiple

products or that a given product may be formed from more than one

reaction.

This general reaction mechanism is needlessly complicated for a

first approximation of what occurs in a waste pyrolysis system. To

enable a reasonably simple determination. of meaningful results we

can condense the above model as follows:

k

gas

The primary volatile products are assumed to be only gas and oil,

the formation of each of which is described by a global rate

constant. Some of the oil is assumed to be converted to gas and

char by a global secondary reaction having a rate constant k'. The

terms oil and gas obviously include numerous compounds.

Given the foregoing picture, a model is developed below which

describes oil yield from the fluidized bed pyrolyzer by considering

both the primary pyrolysis and secondary reactions.

PRIMARY PYROLYSIS

As a first approximation, the particles are assumed to heat up

instantaneously and to remain isothermal during the run. This

assumption has been used in the analyses given here, but for cases

where it is inadequate it can be replaced by calculation of the

temperature distribution within the refuse elements as a function of

time. Based on recent data on cellulose pyrolysis, pyrolysis of the

various materials being studied in the ERCO program is assumed to be

adequately represented as a single reaction first order in the mass

of pyrolyzable material remaining, and having an Arrhenius type of

temperature dependence. Thus:

(1)

and

110
where t is time, Voi l is the oil yield expressed as weight fraction

of refuse fed, V~il ~s the ultimate or asymptotic value approached

by Voi l as time approaches a large value the exact size of which

depends on temperature (i.e., Voi l ->V~il as kt becomes much larger

than 1), koil is the first order rate constant for primary oil

production, in the units of s-1; (k) oil and E il are kinetic

parameters commonly referred to as theOpre-exponentPaT factor (8-1)

and activation energy (kcal/mole), respectively, R is the gas

constant (kcalj.°K-mole) and T is absolute temperature (OK).

Solution of Equation (1) for a refuse element fed (and hence heated)

at time t and then held at the experimental temperature until t

= T gives the following

V

*

oi l = Voi l [1 - exp{-kOi l (T - t)}] (3)

In the case of a pyrolyzer into which refuse is fed at a constant

rate m (kg/hr) during the interval t=O to t = T, the value

of T - t for the different refuse elements range from 1" for those

fed at t=O to 0 for those fed at t=T. If the pyrolyzer is an

isothermal and constant temperature fluidized bed, the cumulative

oil yield at the end of the run of duration 1" is

T*

Cumulative Oil Yield = J mV il [l-exp{-k l(T-t)l]dt (4) o 0 oi

Recognizing that the ultimate oil yield for the run would have been

mV~il T and defining $ as the fractional completion of the oil

formation process, i.e.,

$= Cumulative Oil Yield/m~oilT *

solution of Equation 4 gives

(5)

Asymptotic forms of this equation are useful in the analysis of

data.

When koi l T « 1, $ koi lT/2. When koilT » 1, $ 1 - 1/koi 11"t.

III
SECONDARY REACTIONS

In accordance with the above discussion, some of the oil is assumed

to be converted to char and gas according to the reaction

Oil -) Char + Gas

The rate of this reaction is assumed to be proportional to the

concentration of oil vapor, and the fluidized bed is assumed to be

well mixed with respect to solids and plug flow with respect to

gas. The bed is isothermal and of constant temperature, and all

entrained and carried-over char particles have properties

characteristic of the bed temperature and residence time that would

have been experienced if the particles had not been entrained. The

average gas velocity prevails everywhere throughout the bed.

Accordingly, the rate of oil evolution by pyrolysis is constant

throughout the bed while the concentration of oil vapor increases

from zero at the bottom of the bed to its largest value at the

top. Figure 4 illustrates this approximate picture. .

If a material (oil) balance is made around the fluidized-bed

section, then the concentration of oil at any point x above the bed

bottom (x 0) is given by: (In the remainder of the derivation of

the model, the subscript oil is omitted for convenience.)

m • k' • a

when

*

V oil . • m

-kt (1 - e ) k' a [1 - exp {- • ,l. x] v vol (6)

.

m

k

M

k'

v

x

ultimate value of oil yield expressed as weight

fraction of

refuse fed (kg oil/kg refuse)

= refuse feed rate (kg/hr)

= first order rate constant for primary decomposition of

refuse

(s-1)

= molecular weight of oils (kg/kg mole)

= rate constant for oil consuming secondary reaction

(m/s)

= gas velocity through bed (m/s)

= distance from bottom of bed (m)

112
' .

Rate of

Formation

By Pyrolysis

,

,, , Concentration: ,

Without Secondary , , Reactions , ,

,

,

,

C · , oncantranon: .,

VVith Secondary ,

R . , . eactions ,

.:

,

,

,

o

-o

a:

-

=

-

o

-e

c

.2

-

=...

..

Q)

(,)

e

o

u

...

o....

:>

Distance From Bottom of Bed

Figure 4 Assumed distribution of oil vapor i~ fluidized bed.

113
vol = volume of bed (m3)

a = surface area of solids in bed (m2)

Further, if we let e be the cumulative oil yield up to time t (i.e.,

total oil yield carried from the bed during an experiment of

duration t) expressed as a fraction of ultimate oil yield, then

{'c h .M • v • A • dt o x e=~-.....::..-..:.:.._------

•

m • Voi l • t

where A = cross-sectional area of bed, i.e., vol/h (m2)

and h = bed height (m) during run.

Substituting for cx=h from Equation 6, integrating, and rearranging

terms, we have

e = vA [

k'. a 1 - e

-k'a

aV] (1 1 --kt

or e = ~' x where

1 -kt $ = 1 - kt(1 - e ) is the same as (Equation 5),

representing the oil yield in the absence of secondary reactions.

1 -

-k'a

e---vA

k'a

vA

is the fraction of oils consumed by secondary

reactions.

k'a When vA and kt are both « 1, as at low temperatures, 1/2kt

and e 1/2kt. Therefore, parameters for k (ko' E), the primary

oil production rate constant, can be derived from the lower

temperature data. At highter temperatures kt becomes large enough

so that 1 and e R,A/vA and thus parameters for k', the

secondary oil-consuming reaction rate constant, can be estimated

from the higher temperature data.•

For convenience of computations, the above equation may be rewritten

as:

(7)

114
where q> = 1 -

(1 - e -kt)

kt

(1 -k't'

q>' 1 - - e )

k't'

k = k e -E/RT

o

-E' /RT k' = ko e

t' a/vA

The time t in the correcation is a general parameter which indicates

the duration of the experiment. In short experiments, the time of

feeding and the time of sampling, in general, will not be the

same. A complete derivation of the changes in Equation 7 may be

found in Reference 4·. With the inclusions of these minor changes,

Equation 7 becomes

. -k' a/va -kT = V* [l-e ][1_{1-e S}e-k(ts - t f)] Voi l oil k'a/va kT

s

where the new parameters are

(8)

T

S sampling time

time between the beginning of feeding and

the beginning of sampling.

(ts - t s) represents the contribution during sampling of all

material fed prior to the begining of sampling.

EVALUATION OF PARAMETERS

The above model was used to analyze the pyrolysis data. Let us look

at the parameters that must be determined. Restating Equation 8

where

-k'a/va -kT * [1-e ] [l-{ t~e ~}e-k(ts-tf)] Voi l k'a/va

s

(8)

V

*

oil

t'

Mass of oil leaving the bed during the sampling

period, per unit mass of the refuse fed (lb/lb).

Value of Voil for an infinitely long residence time

of the solids in the bed with no secondary

reactions.

a/vA, sec

115
(1 -k 't')

lP' 1 - e - - k't'

-k'ts) (l - e -k(t - t ) - 1 - e s f k't

S

k k -E/RT - e

0

, -E' /RT k' - k e

0

The data from the individual pyrolysis experiments give us the

parameters Voil, rs' t', t s - tf' and T. R is the universal gas

constant defined as R = 1.987 cal/ok-mole, with the temperature (T)

in degrees Kelvin. V* must be between 0 and 1. oil

The remaining parameters must be fitted to the data. These are kO'.

kO' E', E V~il. A complete computer fit of these five parameters

was accomplished.

Figure 5 shows the "Computer Curve" that was generated by fitting

the model of Equation 8 with the data that are available from our

Sawdust experimental results. The values for the five fitted

parameters for all materials are given in Table 1. For materials

which did not have sufficient low temperature data, a point was

added of zero production of pyro1ytic oil at 100°C. This assumption

of no pyro1ytic oil production at 100°C is perfectly valid for the

~ime period that we are considering.

A composite plot of curves for all Phase I materials that were curve

fitted is shown in Figure 6. The curves for the different materials

all have the same common shape, a steep increase in the pyro1ytic

oil production followed by a more gentle decrease in the yield of

pyro1ytic oil. The temperature at which the curves change slope

varies between 200°C and 400°C. The shape of the curves confirms the

approach that was taken in describing the reactions. There is a

series of rapid primary reactions that form the pyrolytic oil from

the feed. Then, after a certain temperature is attained, the

secondary reactions become dominant and decompose the pyro1ytic oil

to a gas and a char product.

116
TABLE 1

PARAMETERS OF FITTED VARIABLES

E E' ko k ' V~IL KCAL/MOLE-oK KCAL/MOLE-oK SEC-1 M?SEC

Sawdust (SAW) 43.8 17.4 1013 3.8 .3"0

Paper (PAP) 43.8 15.7 8.9 x 1012 5.0 .604

Paper + Sawdust (PAS) 43.3 17.1 9.9 x 1012 4.8 .413

..... Corncobs (COB) 26.0 13.8 4.92 x 1010 1.25 .256 .....

........

Municipal Solid Waste (MWI) 29.7 14.5 3.4 x 1010 2.88 .377

Waste Oil (WOL) 27.2 17.5 1.01 x 109 4.93 .692

Rice Straw 38.5 17.1 3.67 x 1011 2.75 .336

Cotton Gin Waste 38.4 22.7 1.76 x 1012 3.11 .299

Wheat Straw 43.0 16.1 4.58 x 1012 3.64 .316

Industrial Sewage Sludge 43.5 20.0 1.98 x 1012 3.46 .274

Pine Bark 39.8 16.6 4.98 x 1012 3.42 .333
CONCLUSIONS

The model agrees very well with our data. This not only confirms

the presence of secondary reactionst but also describes the rates at

which these reactions take place.

The primary reactions at the lower temperatures dominate the overall

reaction to such an extent that the peak in the curve occurs at

almost the ultimate oil yield for that temperature. This again

points to rapid primary reactions. After this rapid build-up in

pyrolytic oil product t the primary oil yield would slowly increase

with temperature asymptotically to a final value as predicted by a

single reaction step. Near this maximum of the oil yield curve, the

secondary reactions start to play an important role and dominate the

overall reaction. This is evident in the decrease of the pyrolytic

oil yield curve.

The differences in the peak pyrolytic oil production among the

various materials is a result of the different ultimate oil yields

that would be expected from the structure of the molecules at the

temperature that the reaction occurs.

The pyrolytic oil curvet if plotted against a normalized Voil/V*oil'

would all peak at approximately the same value because of the rapid

primary reactions. The shape of the curves is in very good

agreement with the model and the work on primary cellulose pyrolysis

conducted with the MIT strip heater.(5) The data from MIT shows an

activation energy of 33.4 Kcal/mole-~ with a pre-exponential factor

of 6.79 x 109 sec-l for cellulose.

The data from a wide variety of cellulosic-based materials gives

activation energies of between 27.2 and 43.8 Kcal/mole-~ with preexponential factors in the order of 109 to T 1013 sec-I.

Considering that our materials are not the pure orcellulose that MIT

used, the agreement is excellent.

The curve labeled PAS is a mixture of paper and sawdust. The

pyrolytic oil yield falls between the two curves for individual

paper and sawdust. This is as should be expected since it is a

combination of the two materials. But more important, this

indicates that the exact shape of the curves and the ultimate oil

yield will be closely related to the amount of orcellulose that

occurs in the mixture for all of the cellulosic-based materials.

The parameters for pre-exponential factor (k' ) and activation

o

energy (E') for the secondary reactions are very similar among the

various materials indicating that the secondary reaction mechanism

can, indeed, be approximated by the single reaction step. The model

shows agreement with the data.

118
MATERIAL: SAWDUST (SAW)

... •

'.5

•••

0

w .... w

u, 0 u. 0 OJ

0

Z

X ••3 II:

u.

...=

c

..J

w '.2

..J

0

'.1

TEMPERATURE (DEGREES C)

Figure 5 Model curve for pyrolysis oil yield data of sawdust.

119
•• t

t.'

o

. lO.I

c

Z

x

:x: ••• lL.

..:

!

o

.J

IU

>-

.J '.Z

o

•••

J

WOL

2.. 4"

7EMPERATURE lOEG;:::E:5 Cl

It• . 1.0.

Figure 6 Model curve for pyrolysis oil yield data for various feed materials.

120
REFERENCES

1.

2.

3.

4.

5.

Cheremisinoff & Morregi (Ed.), Energy From Solid Wastes, Ch. 9,

Clean Liquid & Gaseous Fuels From 'Or gani c Wastes, Marcel

Deicker, 1976.

Tatom et a1 - "Utilization of Agricultural, Forestry, and Animal

Wastes for the Production of Clean Fuels," EPA Draft Final

Report contract 68-02-1485, April 1975.

Ergun & Figueroa, "Direct Liquefaction of Biomass, Correlative

Assessment of Process Development," presented at the 3r d Annual

Biomass Energy Systems Conference, June 5-7, 1979.

Kosstrin et a1 - "Pilot Scale Pyrolytic Conversion of Mixed

Wastes to Fuel," EPA Draft Final Report, March 1980. Contract

68-03-2340.

Lewellen, Peters & Howard - "Cellulose Pyrolysis Kinetics and

Char Formation Mechanism," Sixteenth Symposium (Int.) on

Combustion, Pittsburgh Pennsylvania, 1976.

121
THE CONVERSION OF BrOMASS DERIVED PYROLYTIC VAPORS

TO HYDROCARBONS

Theodore C. Frankiewicz

Occidental Research Corporation

2100 S.E. Main

Irvine, CA 92714

ABSTRACT

A new process concept, termed pyrolysis/catalysis is ,int roduced for

the conversion of biomass to organic liquids of commercial value. The

effective catalyst is a high Si02/A1203 ratio zeolite in its acid form

which is supported on a low surface area, non-acidic solid. Using

tetrahydrofurfural alcohol (Thfa) as a compound modeling biomass

pyrolysis vapors, oxygen free - organic liquid yields of up to 70%

were demonstr~ted for this catalytic conversion. An integrated

pyrolysis/ catalysis process using ~- methylglucoside in water as

feed resulted in 30% of the feed carbon being converted to organic

liquids of interest.

INTRODUCTION

The research reported in this paper originated in Occidental Research

Corporation's Resource Recovery program whose objective was the

extraction of energy and materials from municipal solid waste (MSW).

MSW organics are more representative of biomass in general than one

might initially expect. Various studies on the composition of trash

indicate that 90-95% of the organics are derived from paper, biomass

waste, and processed cellulosics.[1],[2] Rubber plus plastics com- ' prise less than 10% of the organics by weight and are frequently

present at a less than 5% level. Because the carbohydrate content is

so high, MSW organics can be assumed chemically to be cellulose in the

presence of known levels of impurities. Work conducted with pure

cellulose and with MSW in ORC laboratory and bench-scale pyrolysis

reactors suggests this assumption is viable at least for the study of

thermal reactions.

The chemical structure of cellulose, shown in Figure 1, is one of

repeating glucose monomeric units joined by a relatively rigid p - glucoside linkage. Upon heating, the glucoside linkages may sever

and the fragments formed would be capable of rearranging into organic

123
o

acids (R-C-OH), acetals

H

I

(R-O-~-O-R),

R

OH 0

I 1\

keto-alcohols (R-C - C-R), and

polyhydric alcohols of varying molecular weights. Faster

N

(R- ~HJ -R)

heating rates and higher temperatures tend to increase the ,yield of

gaseous products and decrease tar and char formation.[3] It is

interesting to note that relatively few oxygen-free products have been

detected in cellulose pyrolysis experiments. Thermochemical calculations using the techniques of Benson [4] indicate why this is case.

The results, see Figure l(b), show that the carbon-carbon bonds in

cellulose are weak compared to -C-OH and ~O-H bonds. Thus, a set of

strictly thermal reactions are not likely to lead to the formation of

oxygen free hydrocarbons in high yields. Furthermore, the oxygenated

fragment molecules formed upon pyrolysis also contain -C-O- bonds of

equal or greater stability than -C-C-. This suggests that thermal

conversion of these fragments to oxygen free hydrocarbons is similarly

unlikely.

A major objective of the pyrolysis/catalysis process concept is to

convert the highly oxygenated molecular fragments from the initial

pyrolysis into oxygen-free hydrocarbons by catalytically rather than

thermally breaking and rearranging -C-C- and -C-O- bonds. This is

shown schematically in Figure 2.

Table 1 lists by functional characteristics, the kind of molecules

that have been identified as cellulose pyrolysis products under a

variety of conditions.[5] Generally any given species is produced

in low yields which vary with the 'pyr olysis conditions used.

A characteristic common to most, if not all, organic oxygen functionalities of the molecular classes listed in Table I is their

susceptibility to nucleophilic attack. As such, some form of acid

catalyzed decomposition or transformation is possible for this

broad spectrum of functional groups. Crysta1line aluminosilicate

minerals, i.e., zeolites, are well-known for their surface acidity and their ability to induce chemistry characteristic of acid

catalysis. As will be shown, zeolites are good candidates for a

catalyst which is capable of inducing a series of inter/intramolecular rearrangements, additions, scissions, etc. These can

ultimately result in the desired deoxygenation of the oxygenated

molecular fragments present in pyrovapor originating from MSW or

biomass.

The feed material contemplated for use in this process was MSW

organics which had been cleaned of most metals and glass through

conventional shredding and air classification technology. Typically this material contains 20-30% moisture and possesses an MAF

124
TABLE 1. Cellulose Pyrolysis products which have been

identified by various investigators are listed

by functional characteristics [5].

Types of Molecule Number Identified

Poly-functional oxygenates

Furan &Furan derivatives

Simple acids &esters

Simple carbony1s - Ketones

Aldehydes

Olefinic Carbony1s

Hydrocarbons

Carbohydrate PolYmers

FIGURE 1

CELLULOSE STRUCTURE AND BOND STRENGTHS

(8)

[

CH 2 0 H OH CH 2 OH J o 0 HO~O~O\.

o-L--7--o 1 .n~O-- CH ,OH . HO OH ,

15

12

10

6

7

4

3

?

(b)

H

H

o

I H - C- H

H O-H

102

125

H

H- C-H

I

o

H

OH
stoichiometry of C6HI004.[6] After drying and comminution to millimeter sized particlest the MSW organics are fed to a pyrolysis

reactor at 500-550°C for periods of 0.5 to 5 seconds. At these

temperaturest the products obtained are water (15%)t char (30%)t

noncondensible gases (lO%)t and oxygenated hydrocarbons (45%). The

latter were found to have the stoichiometry C5HS02[6J. Char yields

are high due to the use of a solid heat carrier which provides considerable surface for char forming reactions. In an absolute sense t

these"yields may not be comparable to those where indirect heating is

used.

As shown in Figure 2t the non-condensible gas and vaporous hydrocarbons are then to be passed over the appropriate zeolite catalyst for

the production of substantial yields of high grade fuels and chemicals.

The purpose of this research was to identify the characteristics

required for the catalyst to be used in pyrovapor conversion and to

demonstrate the process concept.

A priori t the search for an appropriate catalyst was limited to high

Si02/A1203 ratio zeolites for several reasons. Firstt the surfaces

are strongly acidic and capable of inducing the kind of chemistry

required for pyrovapor conversion.[7] Second t these highly acidic

surfaces are not very susceptible to poisoning by organic bases since

the latter tend to decompose on strong acid surfaces above 350 uC.[SJ

Third t this class of zeolites is stable to steam and inorganic acids

at temperatures up to 900°C. FinallYt at Si02/A1203 ratios of 15 or

above t zeolite surfaces change from hydrophilic to hydrophobic.[9J

That is the ~H of adsorption becomes higher for hydrocarbons than for

water. Such a change is significant for the conversion of oxygenated molecules to hydrocarbons since "at this point:

• water adsorption will not be limiting hydrocarbon reactions

• surface mobility of unreacted heteromolecules will be

enhanced t and

• hydrocarbon intermediates will be retained on the catalyst

surface where desired hydrocarbon formation reactions may

occur.

The activity of de-aluminized mordenite t for example t has been studied

by several laboratories and rather striking changes in reaction

character are observed as the Si02/A1203 ratio increased.[9 t 10J Of

particular interest is that mordenite with R = Si02/A1203 < 15 is

capable of dehydrating alcohols but not effective at aggregatively

condensing them to higher carbon number products. If R > l5 t hydrocarbon formation reactions are observed and the overall reaction

becomes decidedly exothermic.

126
FIGURE 2

PYROLYSIS - CATALYSIS IS AN APPROACH

FOR GENERATING HYDROCARBONS

CELL - 0

500°C, 5 SEC.

OXYGENATED INTERMEDIATES

POLY-ALCOHOLS

CARBOXYLIC ACIDS

ETHERS

ALDEHYDES

KETONES

l

o -CELL

CHEMICALS

CATALYST

(ZEOLITE, 350°C 1)

... ...... HIGH GRADE FUELS

BTX

PHENOLICS

LOW MW PARAFFINS

127

LOW MW ALCOHOLS

NAHPTHA MIXTURES

LIGHT OILS
EXPERIMENTAL

Two sets of apparatus were used in this work. The first, shown in

Figure 3 is a pulsed micro reactor connected through a heated transfer

line to a microprocessor controlled gas chromatograph. Carrier gas for the g.c. was routed through the micro reactor. The latter was a

0.25" 0.0. stainless tube which contained 0.1 to 1.0 cc of catalyst.

Just upstream of the reactor was a heated chamber where either liquids

or solids could be injected and vaporized or pyro1yzed at the desired

temperature. Products flowed directly to the g.c. where they were

analyzed with a two column program summarized in Table 2. The second

apparatus consisted of a larger tubular reactor containing up to 109.

of catalyst which did not require a carrier gas and could be fed

continuously. Products from the continuous reactor were then collected (gaseous) or trapped (liquid) and analyzed off-line according to the program of Table 2.

Although the purpose of the work under discussion is to develop

pyrovapor conversion chemistry, the actual use of pyrovapor in this

early work was neither practical nor desirable. Not practical because

in-situ pyrovapor generation on this small scale complicates the

experimental procedure and requires careful between run apparatus

cleaning to prevent plugged lines and skewed analyses. The complicated nature of pyrovapor would make the extraction of chemical

reaction information from the experiments considerably more difficult

than it is already. Therefore, several compounds were examined for

their suitability as model pyrovapor components. These included

glycerol, aldol, 3-hydroxy-2-butanone, acetol, furfural, and tetrahydrofurfury1 alcohol. The last molecule, thfa, was ultimately chosen

for study because it is similar in structure to about one fourth of

those molecules which have been identified as cellulose pyrolysis

products. It also has a stoichiometric composition of C5HI002,

similar to the C5H802 reported for pyro1ytic oil, and the reaction of

this molecule on the catalyst surface involves intermediates resembling those expected from most of the C4 and C5 po1yfunctiona1 oxygenates

known to to be cellulose pyrolysis products.

The structure of the molecule is

/0\

H2C CH-CH2-0H

I I

H2C- CH2

As will be seen in results to be discussed, the behavior of this

molecule over a zeolite catalyst closely resembles the behavior

of pyrovapor genarated from the considerably more complicated molecule ~-methy1g1ucoside.

128
TABLE 2. Time program functions for analyzing pulsed micro-reactor products with

the in-line g.c.

TIME

IN

MINUTES

o

. 1

2.2

11 .7

18.0

37.0

60.0

92.0

PROGRAM

EVENT

STATUS

System set for start of

experiment

Column II brought in line

to provide audible inject

command

Column II isolated to

trap gases

Column II brought in

line to elute trapped

gases

Column II isolated for

remainder of program

Oven temperature is

increased to 180°C at

a rate of 10GC/minute

Oven temperature is

increased to 230°C at

a rate of 10°C/minute

Oven temperature is

increased to 270°C at

a rate of 10°C/minute

COLUMN

I

FUNCTION

In Line.

In Line.

H2

Separated

In Line.

C02,C2H4,C2H6,C2H2 Separated.

In Line.

In Line.

H20,C31S

Separated

In Line.

C4 IS, CSls, C6 1S

Separated

In Line.

C7IS, C8IS, Cgls

Separated

In Line.

C101s, etc.

Separated

COLUMN

II

FUNCTION

Isolated.

In Line.

Isolated.

°2,N2,CH4,&CO

Trapped.

In Line.

°2,N2,CH4,&CO

Separated.

Isolated.

Col. I: 1/8 11 x 72 11 Poropar Q, 60-80 Mesh

Col. II: 1/8 11 x 84 11 13X Molec. Seive, 80-100 Mesh

129
Figure 3

Flow Diagram Of Pulsed Micro-Reactor Apparatus For Use

In The Catalytic Conversions Of Oxygenated Hydrocarbons

To Fuels And Chemicals

DETECTION

SIGNALS

MICROPROCESSOR

INJECTION PORT

FIGURE 4

HYDROCARBON YIELDS VS. AMOUNT OF FEED FOR Thfa

OVER A SERIES OF ZEOLITES AT 375°C

60

50

Io

:::>

Q

o

a: 40 Go

:I:

g

z 30 o

III

a:

-c

o

11' 20

10

Thfa FEED

• Intermediate Port Zeolite With MgO.

AI203 Spinel Support

o H-Mordenite With Si02Support

H-Mordenite With MgO.AI20a Spinel Support

• de-AI Mordenite With MgO-AI20s

Spinel Support

100 200 300 400 500 600 700 600

Il. MOLES CARBON INJECTED

130
RESULTS &DISCUSSION

As a result of several screening experiments, two zeolites were chosen

for further study. The first, mordenite, is a large port zeolite

whose 6.4Apore dimensions result from crystal channels bounded by 12

membered rings of oxygen atoms covalently bonded to the tetrahedral

silica-alumina framework. Mordenite was used in its hydrogen form

both as received from Norton Co., Si02/A1203=10, and in a de-aluminized form, Si02/A1203=62. The second ~eolite was an intermediate

port zeolite with Si02/A1203=38 and 5.7A pore openings defined by

10-membered rings of oxygen atoms in the crystal framework. Examples

of intermediate port zeolites are clinoptilolite, TMA offretite,

silicalite, and ISM-5.

Figure 4 shows the relative ability of the two zeolites to convert

the pyrovapor model compound, Thfa, to oxygen free hydrocarbons.

These experiments, performed in the pulsed micro-reactor clearly show

the benefit of using the smaller, more shape selective zeolite pores

to effect the desired chemistry. The pore dimensions of the intermediate port zeolite are too small to permit the formation of condensed

ring hydrocarbons (coke) within the pores and this appears to be

critical to maintaining catalytic activity. Not surprisingly, rates

of coke formation are a major problem for this technology since

hetero-atom containing hydrocarbons are known coke precursers.

The amount of catalyst coking observed was found to be a strong

function of both the acid strength and surface area of the support

solid used in conjuction with the active zeolite. Table 3 summarizes data in support of this conclusion. All runs listed in this

table were conducted at 375°C with a weight hourly space velocity

(WHSV) of 1 hr- 1. Vapor/catalyst contact time was ~l sec. Surface

acidities were bracketed through the use of a series of Hammett

indicators whose colors are affected by the acid strength of surface

adsorption sites.[ll]

Experiments 38 and 42 from the table were identical and show the

lowest catalyst coke loadings at the conclusion of the experiment.

In each case, a neutral, low surface area solid support was used with

the zeolite. Run 39 used a high surface area silica only slightly

more acidic than the beads of Run 38, yet coke formation almost

tripled. The use of neutral alumina with surface acidity very similar

to that of fumed silica but with very low surface area again reduced coke formation. The worst result is Run 6 with 45% coke yield at a

very early point in the experiment. In this case, the A1203 support

used was both acidic and of high surface area.

Material balances for three stmilar experiments conducted at temperatures from 325° to 425°C are given in Table 4. In each case the experiment was continued until catalyst activity was .negligible

as evidenced by the cessation of gas production. In general catalyst

activity was very high during the first 10% of the run, fairly uniform

for the middle 80% and then dropped dramatically during the last 10%.

131
TABLE 3. Hammett acidity functions (Ho), surface areas,

and level of coking observed in Thfa conversion

for support solids used with an intermediate port

synthetic zeolite. All catalyst formulations

contained 10% active zeolite, 90% support solid

Feed to Reacted

Support Surface Catalyst Carbon

Expt Solid Ho Area (BET) Ratio on Catalyst

38 Amorphous

silica beads >6.8 <10m2/g 1.0 2.2%

42 Amorphous

silica beads >6.8 <10m2/g 1.0 1.6%

39 Fumed

sil ica 3.3-4.0 150m2/g 1.0 4.7%

B A1203 3.3-4.0 <10m2/g 1.0 3. 1%

35 Kaolin 1.5-3.3 150m2/g o. 1 16%

6 A1203 <1.5 150m2/g O. 1 45%

51 Sil ical ite >3.3 20am2/g 1.5 14%

Experiment 38 at 375°C was the most successful run with the longest

catalyst life, lowest coke yield, and highest oxygen free hydrocarbon

yield. At lower temperatures, Thfa decomposed first to a tar-like

substance which subsequently charred and deactivated the catalyst.

At 425°C, zeolite surface cracking activity was too high and the

resulting carbon formation quickly blocked the catalyst pores.

Catalyst coke levels were determined by an oxidation procedure where

both steam and oxygen were used to oxidize carbon residues at temperatures of 550-600 uC. After oxidation, the catalysts were found to be

indistinguishable in activity from the fresh" preparations.

Since the vapors produced in pyrolysis of biomass organics will be

quite damp, several runs were made using 50% Thfa/50% H20 as a feed.

Experiment 41, also listed in Table 4 shows that neither catalyst

performance nor catalyst life are affected by the high relative steam

pressures.

132
TABLE 4. Material balances for the conversion of Thfa over 10%

intermediate port Zeolite/90% low surface Si02 at a

WHSV = 1 hr- l .

Feed to Distribution of Carbon in P~oducts

Catalyst %Oxygen Catalyst Oxygenated

Expt Temp. Ratio Removal Gaseous Coke Hydrocarbons Hydrocarbons

48 325°C .67 46% 1% 7% 53% 39%

42 375°C · 1.0 78% 1% 2% 27% 70%

38 375°C 1.3 68% 3% 2% 32% 63%

49 425°C .67 52% 5% 3% 48% 44% I-'

w

w

The following expt. used 50% Thfa/50% H2O as a feed

41 375 QC 1.0 75% 2% 2% 26% 70%
In order to demonstrate the conversion of a more complicated

biomass related molecule, an aqueous solution of 33% by weight of ~- methylglucoside (C7H1205) was prepared. This was fed into the

dual chamber reactor shown in Figure 5. The first chamber was

maintained at 550°C and vapor residence time, based on the volume

of steam generated at 1 atmosphere pressure, was 'U5 seconds. The

steam and organic vapor generated were then fed into the second

chamber maintained at 375 uC and contacted with the 10% intermediate

port zeolite, 90% amorphous silica catalyst. Residence times in

the catalyst reactor were 1 sec. The overall product distribution

obtained after 1 hr. of running was

Pyrolysis char

Catalyst coke

Gaseous product

Aqueous soluble organics

Organic liquid

15%

11%

20%

24%

30%

of carbon in feed stream

These results indicate that a good yield of oxygen free organic liquids can be produced via the coupling of pyrolytic and catalytic

conversion steps for the treatment of biomass related molecules.

Figure 5

Schematic Diagram Of 2-Stage Pyrolysis/Catalysis

Reactor For Continuous Conversion Of Pyrolysis

Vapors To Hydrocarbon Products

Feed

Sta inles s

Pyroly sis

React or

60-80

Mesh Cataly st,

Quart z Rea ctor

Furnac e I

5 50·C

134

Furn ace II

3 75·C

Gaseous Produ ct Coll ec tion

2 Ph as e Liq uid Pr odu ct

Septum

Produc t Collection Syr inge
CONCLUSION

The use of high silica/alumina ratio zeolites for converting complex,

highly oxygenated hydrocarbons to oxygen free hydrocarbons was shown

to be technically feasible. Highly oxygenated organic vapors are

generated in the pyrolysis of biomass at 500-550°C and these vapors

may be catalytically converted over acidic zeolites to useful hydrocarQon liquids. Intermediate port zeolites with pore openings around

5.7A appear to be most useful for effecting the conversion of interest

since they are relatively resistant to coking. A low surface area,

low acid strength support for the active zeolite is crucial to

maintaining a reasonable catalyst lifetime by further reducing coke

formation on the catalyst.

References

1. Kaiser, E.R., Combustion, Feb. 1977, p.26.

2. Jackson, F.R., Recycling and Reclaiming of MSW, Noyes Data

Corp., N.J., 1975.

3. Lewellen, P.C., Peters, W.A., and Howard, J.B., 16th SYmposium

on Combustion, The Combustion Institute, Pittsburgh, PA., 1977.

4. Benson, S. W., Thermochemical Kinetics, J. Wiley &Sons, N.Y.,

1968.

5. Molton, P.M., and Demmitt, T.F., Reaction Mechanisms in Cellulose

Pyrolysis, a Literature Review, Battelle Pacific Northwest

Laboratories (BNWL-2297)~ Richland, Washington, 8/77.

6. Pober, K. and Bauer, F.H., Chemtech, March 1977, p. 164.

7. Chang, C.D., and Si1vestic, A.J., Journal of Catalysis 47,

249-259 (1977).

8. Butter, S.A., Jurewicz, A.T., and Kaeding, W.W., U.S. Patent

3,894,107, 7/8/75.

9. Kiovski, J.R. et.al., 5th North American Catalysis Society Meeting, Pittsburg, PA, 1977.

10. Givens, E.N., Plank, C.J., and Rosinski, E.J., U.S. Patent

4,052,472, 10/4/77.

11. Athinson, D., and Curthoys, G., Chern. Soc. Re. ~(4),

475-497(1979).

Acknowl edqement : The. contr-ibut-ions of Ryan Nel son to this experlmenta1

program are gratefully' acknowl edged. ... .

135
HIGH TEMPERATURE ELECTRIC ARC, RF ENERGY AND COMBUSTION

GAS HEATED PYROLYSIS OF POWDERED BIOMASS

David W. Goheen

Central Research Division

Pioneering Research Group

Crown Zellerbach Corp.

Camas, Washington 98607

ABSTRACT .

Lignocellulosic materials are found almost everywhere on the earth's

surface and, being part of the biological system, have the capacity to

regenerate on a cyclic basis. Many proposals aimed at production of

energy and chemicals from these renewable biomass ~aterials are being

presently studied. Nearly 20 years ago, high temperature pyrolysis of

finely divided lignocellulose under controlled conditions was examined

as a possible source of hydrocarbon chemicals and promising results

were obtained. The basic products were acetylene and ethylene along

with methane. Calculations showed theoretical yields of more than 40%

for acetylene and yields up to 15% were obtained experimentally.

137
BACKGROUND

At the present time, most chemicals of commerce are prepared from p~ .

troleum, but it is now clear that alternative sources of chemical feedstocks will need to be developed if our modern industrialized societies

are to be maintained. This report is a summary of work, described in

more detail elsewhere(l) and undertaken some twenty years ago, aimed

at production of hydrocarbon chemicals from renewable lignocellulosic

materials. While the work at the time did not result in a competitive

commercial process, it should be emphasized that, even during the

period of major petrochemical expansion and incredibly low crude petroleum prices, it was found that acetylene could be produced fro~ renewable lignocellulose at almost competitive costs to processes based

on natural gas and petroleum.

In analyzing the potential of the world's biomass as a source of energy

and chemicals, the tendency until very recently has been to dismiss

the biomass as being incapable of any sizeable contribution to future

needs. This erroneo~s assumption needs to be corrected at every

opportunity. On a global basis, carbon is fixed by photosynthetic

processes annually on a tremendous scale. By assuming that the carbon dioxide in the atmosphere is roughly in an equilibrium state, it

can be calculated that carbon is fixed at a rate of between lOll and

10 12 metric tons per year and is converted back to carbon dioxide at

approximately the same rate. This continuous removal and buildup is

often referred to as the natural carbon cycle.

Thus, by tapping only a small portion of the carbon in the cycle mankind could theoretically solve all energy and material demands that

can be foreseen both now and in the future. The problem is, of course,

of a practical nature. A major portion of the biomass consists of

materials that are widely scattered or are obtained as dilute aqueous

solutions so that it is costly to collect them in one place suitable

for conversion.

Lignocellulose from woody plants to a large extent overcomes the

collection problem and, in fact, wood represents just about the only

practical way that incident solar energy can be collected and stored

for long periods at the present time. Thus, it represents a very

usable and valuable raw material for potential conversion to energy

and chemical products. One important consideration is the amount of

wood available for conversion. Presently, the cut of wood in the

world is about 8 x 10 8 metric tons and this figure is expected to increase to about 2 x 109 metric tons by the year 2000(2). That this

cut can be reached and probably exceeded on a sustained basis was

pointed out as long as thirty years ago by E. Glesinger in a book,

"The Comin~ Age of Wood"(3). Glesinger believed that, world-wide, up

to 14 x 10 metric tons of wood could be produced annually if sound

timber management practices were adopted. It is reasonable to expect

that the 2 x 10 9 metric ton figure will be reached by the year 2000.

It is difficult to estimate the fraction of the total cut that does

not end up as a finished product. But taking into account the amount

138
of bark that is known to be discarded, the amount of lignin separated

during chemical pulping, saw kerfs and tops and limbs that are not

even brought in from the forest, a figure of about 33% is probably

conservative. Thus, there are now available ~t least 2.6 x 108 metric

tons of unused woody material and by the year 2000 there should be

6.7 x 108 metric tons of unused wood available for conversion. This

theoretically could be obtained without increasing the annual cut that

already occurs for all purposes for which wood is used. These figures

are of roughly similar magnitude to crude petroleum production and

greatly exceed the tonnage of all chemical products now obtained from

petrochemical processing. In this connection, it is instructive to

point out that in the United States more than four times as much cellulose is produced from chemical pulping of wood than all the synthetic

plastics and polYmers produced by the petrochemical industries.

The very large amount of unused organic material from trees has

attracted the attention of many investigators and numerous schemes to

convert it to more useful, lower molecular weight materials have been

made. Procedures are known for conversion of cellulose and lignin

into many chemicals of commercial value, but very few processes have

been able to compete with petrochemical procedures for making the same

chemicals. This was especially true in the two decades following

World War II when petroleum was incredibly plentiful and inexpensive.

Almost all of these conversion procedures have attempted the stepwise

breakdown of the large naturally occurring polymers. Economic exploitation of these procedures has been hampered, not only by very low

crude petroleum costs (prior to 1973) but also by the fact that the

conversion schemes resulted in a multiplicity of products with low

yields for each product.

There is, however, a completely different approach.to the production

of chemicals from lignocellulosic materials. By using inexpensive

residues already collected such as sawdust, and subjecting them to

very severe conditions so that the materials are degraded to a single

product or, at most, a very few products that are easy to collect and

purify~ one can eliminate some of the economic problems.

Such a study on the rapid, high temperature pyrolysis of lignocellulosic

materials' was undertaken by the Crown Zellerbach Corporation nearly

20 years ago. Since World War II, a number of plants have been built

that produce acetylene and ethylene by the rapid, high temperature

pyrolysis of saturated hydrocarbons, particularly methane that occur

in natural gas. A description of one of the procedures can be found

in u.S. Patent No. 2,792,437(4). When natural gas is heated in a regenerative type furnace at temperatures of 1,204-1,482°C with retention times of .0001-0.2 second, the main product is acetylene. When

the temperature is reduced to 927-1,204°C and the retention time is

0.01-0.2 second, a mixture of acetylene and ethylene is obtained; and

when the temperature is lowered still further to 677-925°C and the

retention time is 0.01-2.0 seconds, the main product is ethylene.

It has been known for many years that formation of acetylene is endothermic. It has also been known that, under · equilibrium conditions,

139
it is the only thermodynamically stable hydrocarbon at high temperatures (in excess of 100o-1200°C); however, the thermal decomposition

of hydrocarbons into acetylene is not a reaction that goes to complete

equilibrium(5). Acetylene, although formed at high temperatures, decomposes rapidly into carbon and hydrogen. This explains the need for

high temperatures with very fast quenching when hydrocarbons are

pyrolyzed to produce acetylene.

Many processes for accomplishing the pyrolysis of diverse hydrocarbons

and even mixtures of carbon and hydrogen have been developed. Heating

can be by resistance furnaces, regenerative furnaces, induction

furnaces (6, 7), electric arcs(8,9), plasmas(lO), and partial

combustion(11,12). In all these reports, hydrocarbons (or carbon in a

hydrogen atmosphere) were the feedstock for acetylene production.

Only one pertinent reference was found to the high temperature pyrolysis of wood(13). In this case, wood was subjected to conventional

charring at relatively low temperatures to give gaseous products, including methane. The gaseous mixture was then cracked yielding unsaturated hydrocarbons. No references, other than our work, to the

rapid, ,high temperature pyrolysis of wood were uncovered.

As mentioned above, the possibility of producing acetylene led us to

try rapid, high temperature pyrolysis of lignocellulosic materials

since it was felt that only one or two products would be produced.

Consequently, a laboratory research program, followed by a pilot plant

evaluation of the economics of acetylene production was undertaken.

EXPERIMENTAL WORK AND DISCUSSION

Various ways of subjecting small samples of powdered lignocellulosic

materials to very high temperatures were first investigated. The first

experiments were conducted by blowing powdered kraft lignin through an

arc in a "four-way" glass tube with dry nitrogen. A "four-way" glass

tube of 2Qmm inside diameter was fitted with two copper wires about

1 cmapart in the middle of the tube. The wires were connected to a

transformer capable of producing a potential of 15,000 V. The system

was, purged with nitrogen and an arc was struck between the wires. A

sample of kraft lignin was introduced into one of the arms of the glass

apparatus and this was carried through the arc by means of the nitrogen stream. The gases from the cell were passed through an infra-red

cell and analyzed by IR spectroscopy. Carbon monoxide, carbon dioxide,

hydrogen cyanide, and acetylene were found to be present in the gases.

Quantitative data were not obtained in the first experiment, but

acetylene was shown to be formed and was about 2.5% of the total gas

stream. The same experiment was used in subsequent quantitative

experiments. In these, weighed-amounts of lignin were blown through

the arc and helium was used as the sweep gas to prevent formation of

hydrogen cyanide found when nitrogen was used as the carrier. The

gases were collected in a calibrated volumetric cylinder by displacement of a saturated brine solution. Various types of lignin and also

wood and cellulose were tried. The results are shown in Table 1.

140
TABLE 1

Amount of Volume % Acetylene Yield of

Material Pyrolyzed Material Collected in the Gases Acetylene in %

Precipitated Kraft Lignin 0.4g 1,90Oml 2.5 14

Desugared Calcium

Base Spent Liquor Solids 0.6g 1,60Oml 0.3 0.9*

Desulfonated Calcium Base

Spent Liquor Solids 0.4g 1. 750ml 2.5 13

Lignocarbon from Heating

Calcium Base Spent Liquor

with Acid 0.4g l,60Oml 0.8 4* ....

.p-.

.... Powdered Douglas-Fir Wood 0.4g l,70Oml 2.5 13

Powdered Cellulose O•.5g l,80Oml 3.5 15

* Gases contained a considerable amount of H2S.
These experiments showed that several lignocellulose materials gave unexpectedly large yields of acetylene when pyrolyzed in short times at

high temperatures. Contrary to expectations, the presence of oxygen

in the lignocellulose did not prevent formation of substantial amounts

of hydrocarbons.

With this apparatus, there was no way of estimating the temperature of

the arc, so a different heating method was devised. The arc in the

"four-way T-tube" was replaced with a coil wound from No. 28 tungsten

wire. The coil of about 1/8" diameter and one inch long was connected

to copper wires and sealed into the system with rubber stoppers.

After purging with helium, a current from a variable transformer was

applied and the coil was heated to incandescence drawing 500 watts at

20 volts. The temperature was estimated by an optical pyrometer to be

between 2000 and 2500°C. As before, kraft lignin was blown through

the coil and a yield of 23% acetylene based on the lignin was obtained.

The tungsten coil became brittle and broke after one run, probably

owing to formation of tungsten carbide. A tantalum coil was also tried

but this also broke after one run.

In the experiments using the arc and heated filaments, acetylene was

the ~in organic product, and only small amounts of carbon monoxide,

carbon dioxide, and free carbon were found. The yields were promising

but the exact values were uncertain owing to the difficulty in determining the exact amount of material actually heated to the high

temperatures. Thus, it was decided to investigate other furnace designs that could pyrolyze larger samples at known temperatures.

Induction furnaces, heated by means of high frequency currents in

water-cooled copper coils, appeared to offer an attractive means of

generating high temperatures in small, easily controlled regions.

Consequently, an Rf generator was secured and experiments to design a

small suitable laboratory furnace were carried out.

A 4-inch quartz tube was first tried. This was wrapped with a coil of

l/4-inch copper tubing with eleven turns. Water was passed through

the tubing to keep it from melting; The ends of the coil were

attached to the output terminals of the Rf generator. Taps were first

used to drive the grids off the oscillator tubes of the generator, but

it was soon found that a coil of about two turns of the copper tubing

about l/2-inch away from the main coil and connected to the griddriving input terminals of the oscillating circuit worked better and

gave balanced loads.

A carbon crucible 3" x 3" was used as the conductor to be heated and

it was placed in the quartz tube near the middle of the copper coil.

This design was found to be too large for production of high temperatures, so a smaller tube and crucible were used. With a 2 l/2-inch

quartz tube and a carbon crucible of I-inch diameter and 3-inches long,

a temperature of l650°C could be reached using the full output of the

Rf generator. However, this type of furnace was not suitable for

pyrolyzing lignin in a helium atmosphere. The high temperature and

voltage caused ionization of the helium and the arc so formed actually

142
perforated the quartz tube. Argon was even worse as a carrier. Carbon

dioxide did not ionize but reacted rapidly with the carbon crucible to

form carbon monoxide. Thus, the latter gas was used as a carrier as it

neither ionized nor reacted with the crucible.

A few experiments on lignin pYrolysis at temperatures of about 1500°C

gave much carbon black but very little acetylene. It became obvious

that this design resulted in far too long a residence time for the

gases, so this approach was abandoned.

It was found that flake graphite had just the right properties to be

intensely heated. In one test, a porcelain crucible containing flake

graphite placed in a matrix of alumina was melted into the alumina

(m.p., about 2093°C). Thus, a new furnace designed to utilize graphite as the heating element was designed in the following way: Two

alumina blocks of about 2 1/2-inches in outside diameter were prepared.

The bottom block had a hole which just fitted the combustion tube.

Above this block was placed the other alumina block which had inside

hole about 1/2 ,larger than the combustion tube. The space between the

tube and block was filled with graphite. When the system was placed

inside the copper coil and the Rf generator turned on, it was found

that rapid and easy production of any temperature up to the melting

point of the combustion tube could be obtained (above 1760°C). The

hot graphite attacked the combustion' tubes but they were found to hold

up for two or three runs. ~ainting them with magnesium zirconate

helped and allowed the tubes to be used for six or seven runs. A tube

of silicon carbide was found to be very resistant to the graphite and

initial pyrolyses were used with this tube in the newly designed

furnaces.

The pyrolysis was very simple. The tube was loosely packed with

broken pieces of combustion, the Rf generator turned on and the graphite was heated to the desired temperature. Then lignin was slowly

dropped into the heated zone by means of a water-cooled copper tube

extending to about 2-inches above the heated zone. ' Several experiments gave only about 4% acetylene yields. The reason for low yields

was finally found to be porosity of the silicon carbide tubes, and

use of these was discontinued as more than half the gases produced

were being lost. Thus, porcelain combustion tubes painted with magnesium zirconate were used for subsequent runs. A series of runs was

made using measured helium flow rates (ionization and arcing were not

a problem in this design) and variable temperatures from 1760°C down

to 1040°C. Results are shown in Table 2.

An inspection of the table shows that yields of acetylene and ethylene

almost reverse themselves from the highest to the lowest temperatures

and that a large fraction of methane formed in the pyrolytic reaction

was not cracked even though the temperatures were high enough to do so.

In an attempt to determine why the methane was not sufficiently

cracked, a series of runs on natural gas (>90% methane) in the same

furnace was made using about 10-15% methane in helium. Gases were

collected in a 3Q-inch weather balloon and the extent of pyrolysis

determined by IR analysis. Results were shown as partial pressures in

143
TABLE 2

Amount of Time of Temp. Vol. of Helium Flow Y1d. of Y1d. of Y1d. of

Run Lignin, g Addn. , sec. °c Gas, ml. Rate m1/min. C2H2 , % C2H4 , % CH4 , %

1 2.9 30 sec. 1760 5,800 2,700 7.9 2.1 13.4

2 3.0 30 sec. 1704 6,900 2,700 8.0 2.8 16.1

3 3.0 30 sec. 1538 7,000 2,700 8.6 2.5 16.5

t-' 4 3.0 30 sec. 1482 6,100 2,700 5.8 2.2 15.0 ..,...

..,...

5 3.0 30 sec. 1371 6,000 2,700 6.2 2.5 14.9

6 3.0 30 sec. 1260 7,300 2,700 5.2 4.7 22.7

7 3.0 30 sec. 1149 7,200 2,700 3.5 7~2 23.1

8 3.0 30 sec. 1040 7,900 2,700 3.2 6.5 22.1
the gases collected in the balloon. The pyrolyses were run unt~l back

pressure of the balloon equalled the pressure in the natural gas line.

Data are shown in Table 3.

These data show that pYrolysis of methane is appreciable at about 1200-

l260°C for this furnace design and proceeds very well at l3l6°C. It

is of interest to note that the partial pressure of acetylene reached

a maximum and then held steady with increasing temperature,. while the

methane partial pressure steadily decreased with temperature. This

was taken to mean that for any particular furnace design, there is a

maximum allowable acetylene concentration. As the methane decomposes,

it goes through an acetylene intermediate and finally is converted to

carbon and hydrogen. It follows that a shorter hold time and higher

temperature could allow greater concentrations of acetylene to be

developed. This was shown to be the case in earlier experiments where

partial pressures of acetylene in excess of 30mm were obtained by both

the arc and heated coil pyrolyses.

From the data on methane (Table 3) it can be seen that at 1038°C, only

a small 'amount of acetylene is produced, whereas the data on lignin

pyrolysis (Table 2) shows that more acetylene is produced than from

the methane at the same temperature. From this, the conclusion was

drawn that acetylene from lignin and wood is formed by two different

paths, the first by direct pyrolysis of the large molecules into

smaller fragments and the second by pyrolysis of methane formed by more

extensive degradation of the large molecules. This conclusion is

supported by work on hydrocarbons. It is easier to make ethylene and

acetylene from large hydrocarbon molecules than it is from methane(5).

In addition to kraft lignin, other raw material feeds were studied.

Cellulose worked well, but wood served even bet t er, Results on wood

are shown in Table 4.

Based on these laboratory results, a decision to build a bigger furnace and determine the commercial feasibility of preparation of

acetylene from wood was made. It was realized that the yields obtained in the small furnace were probably not high enough to be commercially interesting, but it was felt that in a larger unit, better heat

control and transfer would result in increased yields by converting

more of the methane which was always formed in the laboratory sized

unit. Theoretical consideration also led to the assumption that

larger yields were possible. These were:

1. Wood has an elemental composition of close to 50% C, 6.0% H2 , and

44% O2 •

2. This gives an empirical formula of Ctt . 1 7 H.5.g O2 • .,.

3. Assuming that oxygen of the wood ends up equally or CO and H20 and

that carbon formation is not a major side reaction, the decomposition can be expressed as Ctt . 1 7 H .5 . 9 0 2 . 7 -+1.4lC:zH2 + 1.35 CO + 1.35

H20 + 0.19 H2 and for 100 lb. of wood, 37 lb. C2H2 + 38 lb. CO +

24.5 lb. H2 0 + 0.5 lb. H2 •

145
TABLE 3

Nat. Gas Helium Pressure of Pressure of Pressure of

Run Setting* Setting* Temp. , °c Methane, mm Ethylene, mm Acetylene, mm

1 4.3 12.0 R.T. 142 0 0

2 4.3 12.0 1,037 134 7.0 1.5

3 4.3 12.0 1,204 131 6.0 13.0

4 4.3 12.0 1,316 90 2.5 24.0

5 4.3 12.0 1,427 39 3.0 24.0

6 4.3 12.0 1,538 14 2.0 24.0

TABLE 4

Feed Material

Powdered Douglas-Fir Wood

Powdered Douglas-Fir Wood

Amt.•

2g

2g

Ternp.,oC

1427

1482

Yield of

Methane, %

18.5

15.8

Yield of

Ethylene, %

4.6

5 .7

Yield of

Acetylene, %

13.0

11.4
4. The yield of acetylene could be as much as 37-40% of the wood. A

yield of 25-26% was assumed to be reasonable. This was based on

two points.

a. Our laboratory apparatus gave about 1/2 the yield of acetylene

from methane that is expected from a commercially perfected

acetylene reactor using methane. Assuming a proportionate

improvement for a commercially perfected reactor using wood,

the laboratory yields of 10-15% could be doubled.

b. The calculated theoretical yield from wood was about 40%.

Commercial acetylene yields from methane is about 65% of theoretical. A similar yield from wood, thus would be about 26%.

A furnace was designed with a tangential reaction burner. Heat was

supplied by burning natural gas and sawdust (30 mesh and less) was

transported to the hot zone by a carrier gas (steam, CO 2 , etc.).

Product gases were removed through a 3/4-inch diameter graphite reaction tube fitted with a moveable quench probe. Acetylene was determined by gas chromatography. A schematic diagram is 'shown in Figure 1.

This reactor proved to have several undesirable features and was modified several times and finally a straight jet-type, burner-reaction

design was adopted as the best. This is shown in Figure 2.

Numerous runs showed to optimum O2 to methane (for heating) ratio to

be 1:1.4. Highest acetylene yields were found to be at low sawdust

rates but acetylene concentrations in the product gas were found to

increase as the sawdust feed rate increased. Thus, optimum sawdust

feed rate became a compromise between obtaining high yields and high

acetylene concentrations in the product gas stream, A series of runs

was made to determine the decomposition .r a t e s of acetylene under the

optimum conditions of temperature, flow rate, and firing gas mixture

ratios. These were plotted as a curve of acetylene concentration

versus time. From this, the amount of wood converted to acetylene

but not necessarily appearing as product acetylene was calculated.

This was found to be 46%, which is in good agreement with the assumed

theoretical yield of about 40%. Owing to excessive acetylene decompositions ·in .allof the furnaces, we were not able to approach this

yield. The best yields actually obtained in the jet-type reactor were

12% by weight of acetylene accompanied by 4% ethylene.

An experiment substituting hydrogen and carbon monoxide for natural

gas for firing still produced acetylene showing that the hydrocarbon

was not being produced by incomplete conbustion of methane used for

firing. Some calculations were made concerning the energy absorbed in

converting sawdust to acetylene. In our jet furnace, this was found

to be 3,180 BTU/lb. of acetylene obtained. This figure was obtained

from temperature, flow rates~ and chromatographic analyses. This

figure is considerably lower than the 5,150 BTU/lb. of acetylene

necessary in the partial methane combustion processes. The figures

refer only to energy required to convert the feedstock and do not take

into account furnace losses and energy required to reach reaction

147
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temperature.

In our furnace, it was found necessary to use relatively low temperatures (1204-1260°C) and relatively long reaction times. These combined to give low acetylene yields. The low temperatures meant that

heat transfer was relatively slow to the wood particles so that initially formed acetylene decomposed before all the wood reached pyrolysis temperature. Thus, the presence of oxygen in the wood was shown

to be a disadvantage. The ratio of added oxygen to methane had to be

lower than the stoichiometric value since much oxygen was available

from the wood and the temperatures attainable by chemical energy were

lower than would be optimum for rapid heat transfer and conversion of

the wood particles to gaseous products.

The possibility of using electric arc or corona discharge in combination with the combustion process was considered. After a complete

economic feasibility showed that yields in excess of 30% acetylene

would be necessary to compete on an equal basis with acetylene from

natural gas, it was decided to terminate our study without building a

large plant.

SUMMARY

Although we did not produce a commercially competitive process, we

came close. The economic evaluation showed that production costs of

acetylene from wood were about 25% higher than for acetylene from

methane. Taking into consideration present day natural gas and wood.

waste costs, the acetylene from wood looks more attractive. Future

plants to make hydrocarbons from renewable lignocellulose plants

appear to be a certainty. These may be based on conventional hydrolysis or gasification routes, but may very well be based on rapid, high

temperature pyrolyses similar to that described in this report.

150
REFERENCES

1. Goheen, D. W. and Henderson, J. T., The Preparation of Unsaturated

Hydrocarbons from Lignocellulose Materials, Cellulose Chern. Technol.

12, 363-372 (1978).

2. Karlivan, V. P., New Aspects on Production of Chemicals from

Biomass, Paper presented at World Conference on Future Sources of

Organic Raw Materials" Toronto, Canada (July 10-13, 1978).

3. Glesinger, E., The Coming Age of Wood, Simon and Shuster, NY (1979).

4. Goins, R. R. and Begley, J. W., U.S. Pat. No. 2,793,437, May 14,

1957.

5. Lockwood, D. C., Pet. Ref. 39, No. 11, 223-226, 1960.

6. Weaver, T., Chern. Eng. Prog. ~, No.1, 35-39 (1953).

7. Baddour, R. F. and Iwasyk, J. M., Ind. and Eng. Chern. Pro. Design

and Dev. !, No.3, 169-176 (1962).

8. Pettyjohn, E. 5., Nat. Pet. News, 38, No. 32, 597-604 (1946).

9. Schoch, E. P., Univ. of Texas, Pub. No. 5011 (1950).

10. Leutner, H. W. and Stokes, C. 5., Ind. and Eng. Chern. ~, No.5,

341-342 (1961).

11. Hale, A. B. and Parrish, S. E., U.S. Pat. No. 2,868,856, Jan. 13,

1959.

12. Elliot, J. and McClure, H. H., U.S. Pat. No. 2,945,074, July 12,

1960.

13. Cedarquist, K. N., Swedish Pat. No~ 115,120, Oct. 9, 1945.

151
HIGH RADIATIVE HEAT FLUX PYROLYSIS OF THIN BIOMASS

K. A. Lincoln

Ames Research Center, NASA

Moffett Field, California

ABSTRACT

Some recent experimentation on the pyrolysis of cellulose resulting

from intense thermal radiation is compared with similar work conducted

15-20 years earlier by the author and colleagues. Various aspects of

flash-heating by high radiative flux are discussed, and several thermal

radiative sources are compared. A refocused carbon arc, a xenon flash

lamp, and lasers were used to pyrolyze cellulose over a wide range of

radiant heat fluxes. Results show that the heating rate significantly

influences the composition of the pyrolysis products. Critical attention is given to the methods of analyzing the vapors produced from

flash-heating and to the interpretation of the resulting data.

INTRODUCTION

Historically the work reported on here received its impetus soon after

World War II when the various effects of nuclear weapons were being

vigorously investigated. · One of those effects is fire damage; it is of

greater importance at high weapon yield because the fire-ignition range

exceeds the blast-damage range. With solid fuels, pyrolysis is usually

the first step in the sequence of pyrolysis, .ignition, combustion, and

fire spread. In most burning of solid fuels it is the pyrolysis step

that actually provides the reactants for the subsequent combustion reactions. Thus, it was concluded that if the composition of the pyrolysis

products could be determined and if the factors that influence thermal

decomposition ascertained, we would be in a better position to postulate

combustion-reaction mechanisms and establish a more scientific basis for

fire retardants and flame inhibitors.

Because typical fire kindling fuels - such as wood, paper, and cotton

cloth - are of biomass origin, cellulose is their chief component. It

has been found that small amounts of inorganic impurities have a large

effect on the thermal decomposition reactions of cellulosic materials,

and so care must be exercised in choosing only the same sample mater~al

when comparing results. Thus, for the purpose of providing a standard

153
supply of samples having uniform physical and chemical properties, specially prepared a-cellulose was formed into sheets of several discrete

thicknesses with known amounts of carbon black included. For the work

reported here, . the samples contained 2% carbon black by weight, which

provided an absorbance greater than 90% for the various radiant heat

sources. This paper reports on work performed 15-20 years ago by the

author [1) and colleagues [2), and compares it with recent results

employing newer instrumentation but using the same supply of a-cellulose.

EXPERIMENTAL

Radiant Heating

Inasmuch as the emphasis in this paper is on the response of materials

to thermal radiation, it would be well to briefly review some of the

salient features of high-intensity radiant heating. When the irradiance level is extreme on an opaque body of high surface absorptance,

energy deposition is limited to material very near the exposed surface,

and ablation effects predominate over the relatively inconsequential

diffusion of heat into the solid. Thus, to make flash-heating feasible,

a sample must possess certain physical characteristics. For maximum

temperature rise, a material must have a high optical absorptance in

the spectral range of the radiant source and, if the entire sample is

to be vaporized, it must have a high surface-to-volume ratio.

Among the advantages of radiant heating for flash-pyrolysis is that it

is "clean," that is, only the light-absorbing material is heated; the

container and other surrounding objects remain cool and do not contribute to the vapor products. The radiant pulses are very reproducible

and can be very short and very intense, thereby enhancing the detection

of certain thermally produced, reactive intermediates. The disadvantage

of this type of heating is that it is difficult to measure the temperature of the heated material. We have met with some success using fast,

electronic optical pyrometers, but care must be exercised to exclude

light from the incident radiation and from induced light-emitting

processes.

Radian t Thermal Sources

Among .the earlier achievements of the weapon-effects work was the simulation of the thermal irradiance exposures from nuclear weapons, using

a refraction-optics carbon-arc thermal source [3); this source produced

irradiance levels up to about 92 W/cm2 on a 2.3-cm diameter uniform spot.

The outstanding feature of this refocused arc was that the pulse shape

and intensity could be controlled with shutters. The spectral distribution of the radiation approximated that of a 5500 K blackbody.

Martin [2) has done an enormous amount of work using this thermal source

to (1) measure the temperature profiles and to determine the ignition

parameters of cellulose in air and (2) determine the pyrolysis products

resulting from exposures of cellulose in inert atmospheres.

At a somewhat later time the quartz xenon flashlamp (flashtube) was

introduced as a source of thermal radiation for the pyrolysis work [4).

154
Although this lamp has a spectral distr~bution similar to that of the

carbon arc, the distribution varies with the applied voltage and

approximates a 5000-7000 K blackbody; thus, the energy maximum is in

the visible. The pulse shape and intensity can be varied to some

extent by the electrical parameters in the power supply [5], but, most

important, the output is very reproducible. To receive maximum heat

flux the sample material was placed in a glass (or quartz) tube and

positioned at the center of the helical flashlamp. The maximum peak

irradiance employed was about 25 kW/cm2 per surface. This had the

advantage that a thin sheet of cellulose (up to about 3 cm2 ) could be

irradiated on both sides simultaneously.

The advent of the laser ushered in a new chapter in radiant pyrolysis.

Although elegant optics can be incorporated into experiments, only very

simple optics are required to direct the energy exactly where it is

needed on a sample material. In this case the thermal source can be

located remote from the target, and a sample can be heated conveniently

inside a vacuum chamber or inert gas atmosphere. The greatest advantage is that the laser beam can be focused to heat a small part of a

bulk material or to increase the flux density, and attenuators can be

inserted easily into the beam to reduce the flux. With pulsed lasers

it is very easy to monitor the energy of each pulse as well as the

.shape of the pulse. The latter can be varied somewhat by the electrical

parameters in a manner similar to that of the flashlamp.

In the cellulose work reported here we have used neodymium-glass lasers

(1.06 ~m) only in the normal (or burst) mode to achieve longer heating

pulses and to avoid, for example, plasma effects, inverse bremsstrahlung, and multiphoton absorbtion, all of which tend to prevail with the

Q-switched mode. The beam cross-section, which is nongaussian and

fairly uniform, was focused to 1-6-mm-diameter spots. Flux densities

in the 2500-kW/cm2 range on a l-mm-diameter spot are easily reached,

but in this work much lower values were used to simulate the phenomena

of interest.

The disadvantage of normal-mode lasers is the very irregular intensity

variations with time in their output, which is composed of a train of

approximately l-~sec spikes of random intensities. Despite this, the

overall pulse shape and energy are very reproducible from puise to

pulse. Depending on the application, the monochromaticity mayor may

not be a disadvantage.

Instrumentation for Vapor Analysis

As might be expected, early on in this work the vapor pyrolysis products

of cellulose were analyzed entirely by gas chromatography [6]. Figure 1 is a schematic of the exposure cell employed by Martin in his

experiments in which he used the carbon-arc thermal source to pyrolyze

cellulose directly in the stream of the helium carrier gas prior to its

entry into the column of the gas chromatograph. The chamber was kept

small to prevent the accumulation of volatiles in front of the sample;

the helium flowed through the chamber continuously before, during, and

after the exposure, thus serving as .a nonoxidizing atmosphere to sweep

away the products as they were formed. The stripping column was in the

155
LEAD GASKET

QIJARTZ WINDOW \

........ ..........~.

RADIANT

BEAM

.........................

TO CHROMATOGRAPH

STRIPPING COLUMN

MICA

SAMPLE

HEATER

ALUMINUM BLOCK

t HELIUM CARRIER

FIGURE 1. GAS CHROMATOGRAPHY EXPOSURE CELL FOR RADIANTLY HEATED SAMPLES.

stream to preclude the introduction of tarry substances and heavy viscous liquids from entry into the chromatographic column. In a smaller

number of experiments the xenon flashlamp was employed as the thermal

source. We found that small pieces of 2-mil cellulose sheets could be

completely vaporized in a single flash that had a total energy as low

as 8 J/cm2 incident on both surfaces [4]. This provided a very convenient means of injecting the vapor products directly into the gas

chromatograph, for the sample could be held in a segment of quartz

tubing in the carrier-gas stream and positioned at the center of the

helical flashlamp. This is clearly very similar to the procedure with

the carbon-arc source, but it permitted extension to much higher flux

densities.

Although gas chromatography is extremely effective in separating the

various components in a gaseous mixture, it cannot detect short-lived .

species or transient intermediates or condensable components that fail

to pass through the chromatography column. For this reason we turned

to direct-inlet mass spectrometry and combined a flashlamp with a

time-of-flight mass spectrometer [4], as shown in Figure 2. Here the

samples are located within the ion-source housing and are vaporized by

the brief thermal pulse directly into the ionizing electron beam with

no slits or baffles between the point of vaporization and the ion gun.

This instrumentation provided us not only with in situ analysis of the

vapors, but also with time-resolved mass spectra, because the spectrometer generated up to 40 mass spectra per millisecond. Although this

arrangement permits line-of-sight travel of molecules from the sample

to the ionizing region, there are, nonetheless, ample opportunities

for wall collisions and condensations on the walls of the test tube.

156
BACKING PLATE

ELECTRICAL LEAD·THRUS

"orr-RING

-,

FLASHTUBE --

QUARTZ ROD TEST TUBE

FIGURE 2. ION SOURCE OF TIME-OF-FLIGHT MASS SPECTROMETER

WITH FL4SHTUBE-HEATED, SOLID-SAMPLE INLET.

Consequently, when the pulsed laser was devel~ped, we coupled one to the

mass spectrometer [7] as shown in· Figure 3. Now with the vaporizing

material much closer to the ion gun, we could get a much better analysis of the condensables and short-lived species. .

, FOCUSED LASER BEAM

WINDOW

GRIDS (3)

ELECTRON BEAM

BACKING PLATE

VAPORS " ::

ELECTRICAL FEED-THRUS

.PORT TO

AUXILIARY VACUUM PUMPS

SAMPLE'

SAMPLE HOLDER

QUICK-CONNECT COUPLING

FIGURE 3. ION SOURCE OF TIME-OF-FLIGHT MASS SPECTROMETER

MODIFIED FOR LASER VAPORIZATION OF SOLID SAMPLES.

157
At a later time the instrument shown in Figure 4 was developed at Ames

Research Center; it is described in [8]. As before, this instrument

was built around a time-of-flight mass spectrometer with provision for

both pulsed- and CW-laser heating; it also has provision for varying

the distance between the sample and the ion gun. This latter feature

enabled us to measure the velocities of the vapor species. We found

that the laser-produced vapors emerge from a planar surface (no crater

formed) in an adiabatic free-jet expansion, just as though they had

originated in a reservoir and exited via an orifice the size of the

spot on the material from which they were vaporized by the laser. This

is the subject of another paper [9], and is mentioned here only to give

a more complete picture of the vaporization phenomenon.

FILAMENT,~-,

. ,

ELECTRON BEAM

ION DEFLECTOR rn

BACKING PLATE

ELECTRON

GRIDS

C::::> 'j

ELECTRON

TRAP

IONS

TOF MASS

SPECTROMETER

TO 60 Q/sec

PUMP

TO VACUUM

PUMP

TO 400 Q/sec PUMP

.JI1III-CW LASER

BEAM

TO 270 Q/sec

PUMP

======= :;:.7\,(' "

-========~"" ' " LOADING

PORT~ " , ~.,.~c... LASER " .

'O~ ~EAM -.

I LASER F~ - - - - - - - -<> - - -- - - --'~~ BEAM

SPLITTER

FIGURE 4. MASS SPECTROMETER AND VAPORIZATION CHAMBER FOR

LASER-HEATED SAMPLES.

RESULTS AND DISCUSSION

Table 1 compares the pyrolysis products of cellulose that resulted from

very different rates of heating. The first three columns summarize

some of the work performed by Martin [2] in the early 1960's, using gas

chromatography to analyze the volatiles and wet chemistry techniques for

the tar fractions. Martin found the latter to be mostly levoglucosan

(l,6-anhydro-8-D-glucopyranose), which is the monomer of a-cellulose.

Complete listings of the approximately one dozen compounds that constitute the bulk of the volatile organics, along with their concentrations

and the pyrolysis details, are included in his paper. It was concluded

158
that the rate of heating makes a profound difference in the products.

This is also borne out by the data in column 4 of Table 1 by the results

of flash-heating with the xenon flashtube, with its much higher irradiance, although the difference is more in relative concentrations

rather than in kinds of compounds. Evidently the reactions leading to

char formation are repressed, and the tars are vaporized at the higher

flux. This observation is substantiated by some very recent work utilizing the Ames equipment on the same cellulose material. Figure 5

shows that the kinds of species are the same, but that their relative

concentration is changed by the incident power level. This is the

typical response of polymeric materials - higher flux breaks the polymer into smaller fragments (i.e., a larger proportion of low-molecularweight compounds).

TABLE 1. PYROLYSIS PRODUCTS OF CELLULOSE

(Analyzed by Gas Chromatography)

Thermal sources

Resistance Carbon Carbon Xenon

furnace arc arc flashtube

Irradiance, W/cm2 6.3 46 12,000

(peak)

Temperature, °c 25 incr. -300 >600 >600

to 350

Exposure time 2.7 hr 10 sec 4 sec 1/2 msec

Atmosphere In vacuo He He He

Products based on weight

percent of decomposed

sample:

H2O 19 32 16 28

CO 3 3 13 37

CO2 ? 9 11 4

Volatile organics -51 3 6 30

Tar 2 19 51 None

Char 25 -33 -3 -1

CO/CO2 (molar ratio) 0.52 1.9 14.4

Table II summarizes the recent laser-mass spectrometric experimentation on a-cellulose performed at Ames Research Center. These data

should correspond closely with those of column 4 of Table 1 except for

the small difference introduced by expressing the products in weight

percent in one table (Table 1) and in mole fractions in the other.

However, there is a significant difference in the amount of volatile

organics detected by the gas chromatograph and the mass spectrometer.

Since th~ latter instrument makes an in situ measurement and detects

nearly all of the vapors, we can infer that something was lost on the

way to the column in the earlier work. Figure 5 also shows that highmolecular-weight compounds are present even at the high irradiance

exposure. Parenthetically, it should be pointed out that most of the

159
100

rIPEAK POWER 250 kW/cm 2

fffI I I

PEAK POWER 6.7 kW/cm 2

lfI I I I I I

o

80

60

60

20

40

40

>-

20 w

Z

0 we,

c 100 w

N

...J

:2: 80

a::

o

z

20 40 60 80

m/2

100 120 140

FIGURE 5. ABRIDGED MASS SPECTRA OF LASER-VAPORIZED a-CELLULOSE.

TABLE 2. PYROLYSIS PRODUCTS OF LASER-VAPORIZED CELLULOSE

(Analyzed by in situ Mass Spectrometry)

Peak irradiance, kW/cm2

Energy/pulse, J

~xposure time

Atmosphere

6.7

1.1

0.4 msec

In vacuo

250

1.1

0.4 msec

In vacuo

Mol fraction of products:

H20

CO

CO2

Volatile organics

Tar

Char

CO/C02

0.07

0.03

0.02

0.88

None

None

1.5

0.15

0.07

0.02

0.76

None

None

3.2

160
low m/z values in the mass spectra are fragments of higher-molecularweight compounds, but the high m/z values verify that molecular

weights of at least that value are present. The mass' peaks at m/z 126

and 144 are believed to be from levoglucosan (MW = 162), an example of

a volatile compound that would not be detected by the GC system.

To avoid certain errors in acquiring and intrepreting data from the

in situ mass spectrometry, some subtle but important instrumental

factors must not be overlooked, otherwise the relative concentrations

of the condensable vapors with respect to the noncondensables can

be severely distorted. This can be best explained by referring to

Figure 6, which shows the envelope of one of the laser pulses from

which the above data were derived plus the resulting total vapor pulse

2

VAPOR DENSITY

1

TIME, msec

o

I I

j.-TIME WINDOW-+1

1

I

I

FIGURE 6. LASER PULSE AND RESULTING VAPOR PULSE FROM a-CELLULOSE.

as it emanated from the cellulose target and was detected by the mass

spectrometer shown in Figure 4. Despite the irregular shape of the

laser pulse in this case, the vapor cloud comes out in a smooth' pulse '

which tails out for 100 msec or more. The condensable vapors are

quickly lost to. the container walls, but the noncondensable gases persist within the vacuum chamber and cause the long tail whose decay rate

is dependent on the speed of the pumping units and the geometry of the

system. When spectra are accumulated continuously (as was the case in

all of the earlier work), the results tend to exaggerate the amount of

noncondensables present. Recently this problem was avoided by gating

the mass spectrometer 11on" only during a time window of less than

1 msec when the vapor cloud emanating directly from the sample passed

through the ion source; as a result, the spectrometer is "off" by the '

time most reflected vapors can reach the ion source, thus obviating the

above discrimination against the condensables.

A large number of mass spectra were obtained from the flash-pyrolysis

of a-cellulose, but they have not as yet been translated into their

complete chemical components; however, some comparisons of spectra

obtained in the earlier work with those obtained recently with the

equipment depicted in Figure 4 (where the distance from the sample to

the ion gun was about 50 cm) are very revealing. The recent continuous

161
spectra are quite similar to those obtained from the flashtube-heated

samples via the arrangement shown in Figure 2. On the other hand the

recent "window" spectra are nearly identical with those from the earlier

laser-heated samples via the equipment depicted in Figure 3. In the

latter case there is a much greater abundance of high-molecular-weight

materials (i.e., more readily condensable organic compounds) evident in

the spectra. This difference in spectra is not due to the difference

in heating, but to the proximity of sample to ion gun - a factor that

tends to give the condensables equal detectability. Hence, it is now

evident that the condensable vapors and transient species were adequately included in the analyses only when the sample was close to the

ion source or only when the vapor cloud was analyzed during the appropriate time window in cases where the sample was remote from the ion

source.

This brief discussion of instrument parameters is included to point out

the importance of the sampling techniques in any analysis designed to

measure reliably all the products produced by the flash-pyrolysis of

biomass. The above provides strong evidence that a large percentage of

the condensable materials were lost to detection in the earlier work in

which the cellulose was heated inside a quartz tube by a coaxial flashtube. This was the case in which the tubulation was an appendage to the

mass spectrometer or in the carrier stream of the gas chromatograph;

hence, we are drawn to the conclusion that many of the high-molecularweight compounds (volatile organics) are missing or surpressed in the

gas chromatography results of Table 1; this is particularly true of the

data in column 4.

CONCLUDING REMARKS

This work has substantiated the premise that the rate of heating of

cellulose makes a profound difference in the composition of the pyrolysis products. It can be inferred from this that pyrolysis proceeds

along different reaction paths as the heating rate changes, but caution

must be exercised in attributing all the results to this explanation.

For example, the amount of char decreases with increasing irradiance to

the point where none is produced by laser flash-heating. The surface

temperature is believed to exceed 600°C but not by very much. This

is because the preponderance of unpyrolyzed organic molecules present

in the vapor indicates that the temperature is significantly below that

required to vaporize carbon; hence, it is unlikely that the char is

vaporized. Moreover, as the char decreases the tar fraction tends to

increase, and so these factors suggest that a change in reaction path

occurs as the irradiance (and consequent heating rate) is increased.

Martin [2] has suggested that the char layer becomes hot enough to

react endothermically with the mixture of pyrolysis products passing

through it. This explanation is plausible at the heating rates produced

by the carbon are, but would appear less likely with the very high

irradiances at short times produced by the laser. Under these conditions the tar also disappears, but this does not necessarily reflect a

further change in the pyrolysis reaction path. In the first place,

thermal cracking appears to occur because there isa shift toward more

162
smaller molecules with an increase in irradiance. Second, much of the

tar really does not disappear, because'the in situ spectra clearly show

the presence of large molecules, even at the highest irradiance; this

indicates that many of those components that make up the tar at lower

temperatures are vaporized and escape the heated zone fast enough to

avoid cracking. Note that under these conditions the high-temperature

zone is extremely thin. The ,above observations and the data indicating

that above some irradiance level a l~rge increase produces only a small

change in vapor composition support the conclusion that a maximum ablation temperature is reached and that no further change in reaction

mechanism occurs.

It is important to emphasize that in the flash pyrolysis of materials

by pulsed, high-flux thermal sources, only a thin layer of the surface

of the exposed material is vaporized; thus, it is necessary to clean

off the surface or remove it before a meaningful relationship between

the vapor products and the bulk material can be established. We have

found it most convenient to hit the surface of the sample with several

"shots" from the laser prior to analyzing the vapor products produced

by a subsequent laser "shot" on the same exposed surface. When samples

are so thin that they are vaporized by a single flash, this is not

possible; consequently, the vapors may contain a high percentage of

surface contaminates.

Another caveat pertains to unsuspected errors that can be introduced by

the sampling and analytical techniques employed to measure the relative

concentrations of the vapor products from flash-pyrolysis. Where possible, it is best to make in situ measurements close to the vapor source

under conditions that do not cause discrimination against high or low

molecular weights or against condensable components. In this work we

found that although the mass spectrometer itself caused little discrimination, the manner of injecting the vapors into the ion source was

extremely critical.

163
REFERENCES

1. Lincoln, K. A., "Flash-Pyrolysis of Solid Fuel Materials by Thermal

Radiation," Pyrodynamics (1965), pp. 133-143 ;

2. Martin, S. B., "Diffusion-Controlled Ignition of Cellulosic Materials by Intense Radiant Energy," Tenth International Symposium on

Combustion, The Combustion Institute, 1965, pp. 877-896.

3. Broida, T. R., "The Production of Intense Beams of Thermal Radiation by Means of a High-Current Carbon Arc and Relay-Condenser

Optical System," USNRDL-4l7, Nov. 1953.

4. Lincoln, K. A., "Flash-Vaporization of Solid Materials for Mass

Spectrometry by Intense Thermal Radiation," Anal. Chem, I!... (1965),

p , 541.

5. Lincoln, K. A., "Thermal Radiation Characteristics of Xenon Flashtubes," Appl. Opt. 1 (1964), pp. 405-412.

6. Martin, S. and Ramstad, R. W., "Compact Two-Stage Gas Chromatograph

for Flash Pyrolysis Studies," Anal. Chem. 11 (1961), p . 982.

7. Lincoln, K. A., "Mass Spectrometric Studies Applied to Laser-Induced

Vaporization of Polymeric Materials," High Temperature Technology.

Proceedings of the Third International Symposium, 1967,

Butterworths, London, pp. 323-332.

8. Lincoln, K. A., "A New Mass Spectrometer System for Investigating

Laser-Induced Vaporization Phenomena," Int. J. Mass Spectrom. Ion

Phys. 13 (1974), p. 45.

9. Covington, M. A., Liu, G. N., and Lincoln, K. A., "Free-Jet Expansions from Laser-Vaporized Planar Surfaces," AlAA J. 15 (1977),

pp. 1174-1179.

164
Experimental Procedure

The system was adjusted for maximum light intensity before any experiments were conducted. At the focus of the shallow ellipse, light profiles were scanned with a calorimeter when possible. Athermocouple

set inside the reactor was used to do the final adjustment and to give

a crude evaluation of the flux level after the addition of the spherical mirror. With the spherical mirror in place, physical space

limitations prevented the use of the calorimeter.

One and a half to two grams of white "Avicel" PHl02 cellulose were

weighed out and loaded in the feeder. The heater tapes and light were

turned on and the system allowed to warm up. The downward gas flow

through the feeder was set to between 50 and 70 cc/min. Following

this, the carrier gas was fed downward through the top of the reactor

at about 500 cc/min, purging the system of air for several minutes.

All of the gas flows were monitored with rotometers.

The feeder was then started and run until all the 100-200 ~m diameter -

cellulose particles had passed through the reactors. A typical run

lasted about five minutes, depending on the amount of material in the

feeder and the rate of feed.

After a run, the material collected in the bucket and the condensors

was weighed. Material deposited on the reactor walls and tubing was

collected by washing these surfaces with methanol followed by acetone.

The volume of gas collected in the sample bag was also measured. The

collected gases were then analyzed with a chromatograph.

Results

Several runs were performed and the amount of pyrolysis was minimal.

To increase the residence time of the particles in the "hot spot" the

light source was changed to a 2000W bulb, and a front surface spherical mirror was placed behind the reactor "to increase the integrated

flux incident on the surface of the particles. With careful adjustment

of this spherical mirror the peak flux was increased to 150 W/cm2. The

light profile remained almost unchanged. These conclusions were

reached by running the light bulb at a low voltage and using a

thermocouple probe which had been "calibrated" against the

calorimeter.

Under these circumstances there was an increase in the amount of cellulose which underwent pyrolysis. Nevertheless, more than 50% of the

material was collected in its original form.

To impro~e the light absorptivity, black carbon was added to the white

cellulose in a 1 to 9 blend by weight. At least 40% of the material

was still not pyrolyzed.

165
Another configuration of the reactor was attempted. The steam was fed

into the reactor from the bottom and now traveled counter~current to

the direction of flow of the cellulose particles. This was intended

to slow down the particles; thereby increasing their residence time in

the light flux. Very high steam flows (up to 3 g/min) were used and

no significant improvement in the pyrolysis was observed. One problem

was the difficulty in handling particulates with different diameters.

The terminal velocity of the particles is a function of the square of

the diameter [19], thus their deceleration is a strong function of

their size.

The experimental system described in the preceding sections was capable

of marginal performance at best. The temperature of the quartz wall

around the focus was not homogeneous due to the aSYmmetry of the configuration. This caused many particulates to miss the most intense

radiation zone due to convection currents which developed in the gas

flow.

To overcome the problems associated with this configuration an improved

pyrolysis system was assembled, as described in the following section.

THE VERTICAL AXIS SYSTEM

Experimental Set UP.

The vertical axis system employed two ellipses with major and minor

semi-axes of 49.8 and 33.4 cm respectively to focus the light from a

5 kW tungsten-halogen bulb. A flat, front-surface mirror was used to

redirect the light beam to a vertical orientation. This was to permit

the use of an optical bench to support one ellipse and the bulb.

Figure 4 illustrates the configuration.

The main advantages of this new configuration were the sYmmetry of the

ellipses with respect to the reactor's axis, and its ability to position a cylindrical mirror concentric to the reactor. This mirror

increased the length of the intense light zone by trapping some of the

radiation inside the upper part of the reactor.

A minor complication with the system was the necessity of cooling the

flat mirror to enable it to withstand large light intensities without

shattering. The mirror lay against a water-cooled copper plate which

functioned as a heat exchanger. Thermocouple readings showed that

without cooling, the center of the mirror was heated to above 200°C

by the focused light.

The cylindrical mirror was prepared by platinum ion sputtering both

sides of a quartz tube. Radiant energy entered the reactor and was

trapped within this cylinder. The mirror trapped a significant amount

of radiation, since a temperature high enough to initiate wood pyrolysis [20] existed even at four inches above the focus. Unfortunately,

the half-life of the platinum coating was not very long due to the

degradation which occurred at these high temperatures. To extend the

cylindrical mirror life, the feasibility of increasing the coating's

166
a- water inlet

b- water outlet

c- gas inlet

d- gas outlet

e- vibrating feeder

f- light source

9- cylindrical mirror

h- quartz reactor

i- plane mirror

j- cooling mirror jacket

FIGURE 4. EXPERIMENTAL LAYOUT FOR THE

VERTICAL AXIS SYSTEM

167
thickness, or of using

is being investigated .

first experiments which

a different material for a preliminary coating,

The cylindrical mirror was not used with the

employed a two inch diameter reactor.

The ellipsordal mirrors used in the hori zontal system had to be replaced by larger units because of the additional heat generated by the

5 kW light bulb. Even these bigger ellipses required air cooling to

prevent the temperature of their surface (aluminum coated with rodium)

from rising above 250°C. This is the upper operating temperature

recommended by the manufacturer. As noted earlier, sYmmetrical ellipses are not the most efficient system for collecting light radiated

by extended sources, but the availability of relatively inexpensive

large units outweighed the loss in efficiency. The decrease in

efficiency is calculated to be on the order of 10% of what could be

achieved with the best system investigated.

The first reactor used in this system was fabricated from two inch

diameter quartz tubing. It consisted of a main body with spherical

ball joints at the top and the bottom, as shown in Figure 4. This

reactor was designed to accommodate an increased throughput of biomass. It was also hoped that the larger diameter would prevent the

particulates from hitting the wall during their downward journey.

The particulate biomass material was introduced into the top of the

reactor either by the vibrating feeder described earlier or by an

Acrison model l50Z-A screw feeder .

The experimental procedure was the same as before; however the superheated steam, CO2, or N2 was introduced into the bottom of the reactor

flowing upward countercurrent to the direction of the falling particulate matter. The teflon bags used previously for gas collection were

replaced by a liquid displacement system to allow for greater accuracy

in gas volume measurement.

Results

It soon became clear that very few of the falling particles were passing through the "hot zone" along the central axis of the reactor. Most

fell along the wall. Temperature profiles obtained with the thermocouple showed that the center of the reactor was hotter than the wall

area. This caused convection currents, moving outward from the central axis, to carry particulates out of the intense light zone. Even

using a 5 kW bulb, the spot diameter was small compared with the two

inch reactor diameter.

In view of the difficulty, a one inch diameter reactor was built, and

the five inch long cylindrical mirror added. All of the other parts

of the system remained the same.

With this arrangement, higher temperatures at the wall than at the

center of the reactor were recorded. If pronounced convection currents

formed, they would push particles to the center, thus improving pyrolysis. It is also evident that the particulates will travel at least

168
three inches .in a zone where the temperature is high enough to induce

pyrolysis.

Several runs, using Avical PHl02 cellulose, Kraft lignin an~ ground

corn cob material were condycted, but the amount of pyrolysls was

never complete. Mass balances and gas analyses were performed and

data from the best runs are presented in Table II.

Discussion of Results

From these results it is obvious that a good mass balance was achieved,

but the amount of pyrolysis was never better than 50% by weight of

the feed material. Complete pyrolysis is not an imperative result,

since biomass can be submitted to more than one pass through the

reactor in commercial applications.

The fact that the concentrated radiant energy available at the inner

focus of the downward facing ellipse has the characteristics of both

an upward and a downward facing beam has been a source of some difficulties. The effectiveness of a cavity as a radiant energy trap is

reduced because the focused radiant energy has no dominant orientation.

Also, a cavity designed to trap upward directed radiation tends to

obscure the downward radiation. Although deep ellipsoids are much less

expensive than large paraboloids, the complexity and operat.tng charac....

teristics of ellipsoidal systems make them less desirable than systems

based on paraboloids.

The solids collected after an experiment consisted of unpyrolyzed feed

material and char. A method o-f separating these two components has not

yet been devised, but from visual inspection it appears that the mix~

ture is mainly unpyrolyzed biomass. If the assumption is made that

all of this mixture is unconverted feed material, then subtracting the

total solids collected from the original sample weight gives the

amount of solid material that was actually pyrolyzed. Using this

method, it can be shown that the percentage yields of liquids from the

feed material is 54%, 30%, 69%, and 24% for runs 1, 2 3 and 4 respec.... tively.

The high percentage yield of liquids from Run I is probably due to the

fact that the lignin, which was the feed material, is soluble in the

cleaning solvents. This solubility caused some material which should

have been accounted for as unpyrolyzed solids to be added into the

liquids, thus distorting the results.for this run.

A possible explanation for the difference in liquid production between

the vacuum run and the remaining two runs is based on the heat transfer

phenomena occurring in the reactor. It is well known [21] that solid

biomass undergoes pyrolysis at temperatures below 500°C and that gas

phase reactions should occur at temperatures of about 700°C. According

to the currently accepted mechanism for cellulose pyrolysis [22,23], . most of the pyrolysis products shoul d be complex organic molecules

(sugars). The high liquid yield obtained from the vacuum experiment

s.eems to Lndfcat;e that in this run the reactor was performing as ex....·

pected, so that no gas phase reactions were occurring. .

169
TABLE II

EXPERIMENTAL RESULTS FROM VERTICAL AXIS SYSTEM

Run 1 Run 2 - Run 3 Run 4

Day 4/29/80 5/9/80 5/13/80 5/29/80

Sample material Lignin PH-l02 PH-l02 Wood

Cellulose Cellulose

Sample weight (gm) 1. 01555 2.2303 2.4518 2.0504

Total solids collected 0.6454 1.0409 2.0951 0.7630

(gm)

Total liquids collected 0.2008 0.3552 0.2456 0.3058

(gm)

Carrier gas (at N2 N2 N2 N2 SO cc/min)

Steam flow (gm/min) 0.898 1.25 vacuum 2.09

Total gas collected 500 1500.1 n.a. 969

(cc)

Total gas produced -- 709.5 n.a. --

(cc)

Gas elementary -- 35.5% C n.a. -- composition (volume base)

-- 28.5% H n.a. -- (volume base)

-- 31. 7% a n.a. -- (vo lume base)

Gas detailed CO2 -- 10.6% -- 8.04

analysis H2 -- 1.99 -- 17.10

(% by weight) N2 -- 0 ..... -. 27.29

CO -- 69.1 -- 35.14

CH4 -- 6.21 -- 7.20

C2H4 -..... 7.18 -- 3.95

C2H6 -- 0.85 -~ 0.38

C3H6 -- 3.01 -- 0.41

C3H8 -- 0.26 -- 0.13

C

4 -- 0.78 -- 0.36

Total: 100.00 100.00

Gas calorific

value (Btu/Scf) 491.5

170
The inert gas which flowed upwards through the reactor in the other two

runs may have had an adverse effect on the reactor's temperature profile. Ideally, the inert should help to decouple the solid and gas

phase reactions by diluting and cooling the evolved volatile matter so

that no gas phase reactions will occur. However, the inert will absorb heat .from the falling particles and the wall by convection and

conduction. If enough heat is transferred to the inert, it could in

turn act to heat the pyrolysis products instead of cooling them, and

thus cause gas phase reactions to occur. The occurrence of gas phase

reactions would explain the lower liquid production from these two

inert runs.

CONCLUSIONS

There are no inexpensive optical sources which adequately simulate the

intense radiant energy flux available at the focus of a good solar furnace. Research efforts which require high flux levels on the bench

should plan on constructing or accessing an arc image furnace with a

xenon lamp. Many of the problems discussed in this pap~r could have

been avoided through the use of an arc image furnace.

The products of radiant flash pyrolysfs may be either liquid sYrups or

permanent gases, dependlng upon reactor conditions. Syrups are favored

by low gas phase temperatures and short residence times, whereas permanent gases are favored by high gas phase temperatures. Thus the ability of the radiant flash pyrolysis reactor to decouple the gas from the

solid phase temperature makes it unusually flexible, providing for the

selective production of either liquids or gases. Clearly much more

research is required to better define the limits of selectivity, and

to identify markets for the syrups.

REFERENCES

1. Antal, M. J., Feber, R. C. and Tinkle, M. C., "SYnthetic Fuels

from Solid Wastes and Solar Energy," Proceedings of the World

Hydrogen Energy Conference, Miami (1976).

2. Antal, M. J., "High Temperature Pyrolysis Using Solar Process

Heat," Report on the SYmposium and Workshop on the SMwth Solar

Thermal Test Facility, Houston (1976), ALO/370l-76/1.

3. Antal, M. J., "Tower Power: Producing Fuels from Solar Energy,"

in Towards a Solar Civilization, R. Williams ed., MIT Press,

Cambridge (1978), pp. 80-84.

4. Antal, M. J., "Tower Power: Producing Fuels from Solar Energy,':'

Bull. Atom. Sci., Vol. 32, (1976) pp. 58-62.

S. Antal, M. J .., "The Conversion of Urban Wastes," International

Symposium on Alternative Ene~gy Sources, Barcelona, Spain (1977).

171
6. Antal, M. J., "Results of Recent Research on the Use of Pyrolysis/

Gasification Reactions of Biomass to Consume Solar Heat and Produce a Useable Gaseious Fuel," Proceedings of the STTF Users

Association 1978 Annual Meeting, Denver (1978).

7. Antal, M. J., "Solar Flash Pyrolysis: Syngas from Biomass," Proceedings of the Solar High Temperature Industrial Processes

Workshop, (1978) Atlanta.

8. Antal, M. J., "The Effects of Residence Time, Temperature and

Pressure on the Steam Gasification of Biomass," American Chemical

Society Division of Fuel Chemistry (1979), Honolulu.

9. Antal, M. J., "Synthesis Gas Production from Organic Wastes by

Pyrolysis/Steam Reforming," IGT's Conference on Energy from Biomass and Wastes (1978), Washington.

10. Antal, M. J., "Biomass Energy Enhancement. A Report to the President's Council on Envoronmental Quality" (1978).

11. Antal, M. J., Rodot, M., Royere, C., and Vialaron, A. "Solar

Flash Pyrolysis of Biomass," ISED Silver Jubilee 1979 International

Congress, (1979) Atlanta .

12. Moreira, J. R., and Antal, M. J., "Gasification of Biomass as a

Source ·of Synfuels for Developing Countries," 2nd Miami International Conference on Alternative Energy Sources, (1979) Miami.

13. Antal, M. J., Rodot, M., Royere, C. and Via1aron, A., "Bi omas s

Gasification at the Focus of the Odeillo (Fran~e) 1 MWth Solar

Furnace," American Chemical Society. Symposium on Thermal Conversion

of Solid Wastes and Biomass (1979), Washington, D.C.

14. McDanal, A. J., "Optical Analysis of Paraboloidal Concentrator with

Secondary Reflectors," 1979 International Congress - Joint Meeting

with the American Section of the International Solar Energy Society, May 28 - June 1 (1979).

15. Handbook of Chemistry and Physics, 52nd Edition, The Chemical

Rubber Co., Cleveland, Ohio, 1971/72.

16. General Electric, Commercial and industrial Lamp Catalog.

17. Rabl, A., Solar Energy (1976), pp. 93-111.

18. Sherber, M. S., "High Irradiance Pyrolysis of Cellulose with Applications to Biomass Gasification with Solar Process Heat," BSE

Thesis, Princeton University, Princeton, N.J., unpublished.

19. Bird, R. B., Steward, W. E., Lightfoot, E. N. "Transport Phenome.,.

na ," John Wiley and Sons, Inc. (1960), p. 60.

172
20. Antal, M. J., "Biomass Energy Enhancement - A Report to the President's Council on Environmental Quality," (1978) Princeton

University, Princeton, N.J.

21. Antal, M. J., "Thermochemical Conversion of Wood (Biomass) to

Liquid and Gaseous Fuels," Internal Report, Department of Mechanical and Aerospace Engineering, Princeton University, Princeton,

N.J., 1979.

22. Shafizadeh, F., "Industrial Pyrolysis of Cellulosic Materials,"

Proceedings of the 8th Cellulose Conference, T. E. Timell

(editor), Interscience, (1975), N.Y.

23. Broido, A., "Kinetics of Solid Phase Cellulose Pyrolysis," in

"Thermal Uses and Properties of Carbohydrates and Lignins,"

Shafizadeh, F. et ale (ed.), Academic Press, New York, 1976.

173
BENCH SCALE RADIANT FLASH PYROLYSIS OF BIOMASS

M. J. Antal

L. Hofmann

J. R. Moriera

Princeton University

Department of Mechanical and Aerospace Engineering

Princeton, NJ 08544

ABSTRACT

The results of continuing research at Princeton University on the radiant flash pyrolysis of biomass as a source of fluid fuels, industrial

feedstocks and chemicals is described in this paper. Bench scale

sources of intense, visible radiant energy have been used to simulate

the concentrated solar flux available at the focus of solar towers.

Windowed transport reactors have been developed, which act as cavity

receivers for the focused radiant energy and provide a means for direct use of the radiation to rapidly pyrolyze the entering biomass.

Detailed results of bench scale experiments are presented, which suggest the use of concentrated radiant energy as a selective means for

the production of sugar related syrups from biomass by flash pyrolysis.

INTRODUCTION

The flash pyrolysis of biomass requires a source of high quality heat.

Early work at Princeton [1-7] identified concentrated solar radiation

as an attractive means for rapidly heating the biomass and thereby

achieving flash pyrolysis. When used in th~s fashion the .solar heat

input is effectively stored in the form of a fluid fuel. This storage

aspect overcomes some of the problems associated with the intermittent

nature of solar heat. The energy of the biomass feedstock is preserved by the use of an external heat source for pyrolysis, so that the

product energy leaving the reactor is equal to or greater than the

feedstock energy entering the reactor. Because the solar heat input

is only about 10% of the biomass energy throughput, the cost of the

solar concentrator plays only a small role in the overall economics of

the system. Thus the useful energy output of a solar concentrator

system is magnified by about a factor of ten when the solar heat is

used to pyrolyze biomass; in contrast with electric power production

which reduces the useful energy output of the concentrator by a factor

of three. Experimental work at Princeton suggests that the yields of

valuable chemicals such as ethylene may be enhanced through the use of

175
radiant energy for flash pyrolysis [8,9] . Economic projections [10,

11] indicate that the enhanced production of chemicals from biomass

using conversion methods based on radiant flash pyrolysis should be

very attractive to the hydrocarbon processing industry. The use of

concentrated solar radiation for biomass pyrolysis also holds promise

for developing countries [12].

This paper describes bench scale experimental work using artificial

sources of intense radiant energy which is being performed to support

the development of radiant flash pyrolysis reactors. Because of our

experience using the Odeillo lMwth horizontal beam solar furnace [13],

we initially chose to build an artificial source of intense radiant

energy with a horizontal beam. One of the reactors taken to Odeil10

was used with this horizontal beam furnace to recreate the Odei110

experiments under more controlled conditions. This reactor operated

vertically, with the flux of radiation directed normal to the flow of

the biomass particles. As discussed in the following sections, this

experimental configuration was only marginally successful. More recent work has emphasized the development of vertical beam furnaces

operating with axially concentric, vertical windowed transport reactors. This work has enjoyed better success, and has revealed the

possibility of achieving very high yields of liquid products due to

the unique ability of concentrated radiant energy to sustain two

characteristic temperatures (gas and solid) within the reactor.

SOURCES OF INTENSE RADIANT ENERGY

Since solar radiation as intense as 1000 W/m2 is sometimes available,

and concentration ratios of 10,000 have been achieved [14], some experimental solar furnaces are able to supply radiation in excess of

1000 W/cm2. To simulate these intense energy fluxes in the laboratory

it was necessary to develop an optical system which could focus light

from an artificial source. Currently available high intensity light

sources are tungsten-halogen lamps, xenon lights, and carbon arcs.

These lights produce radiation with a spectrum similar to that of a

black body radiating at a temperature of about 3000°C, 6500°C or

5500° C, respectively. According to 't he Stefan Bo1tzman Law for a

black body radiator, these temperatures produce theoretical maximum

radiation intensities of 450, 10,000 and 5000 W/cm2.

Xenon lights are the most intense of the three, but they have several

disadvantages: a small arc size which leads to a small, intense spot

of radiation when the light is properly focused, very expensive, complex RF equipment for lamp ignition, and a high level of ultra-violet

radiation requiring various protective measures. Carbon arcs have

light intensity instabi1ities associated with fluctuations in the distance between the carbon rods; they also produce undesirable gases

when burned in air. Halogen-tungsten bulbs are the least intense of

these lights, but can easily be ordered in a variety of physical sizes,

configurations, and power levels. They are also the least expensive

option, and present the advantage of simple control of the light intensity through a conventional variac.

176
Previous experiments had shown that biomass particles several hundred

microns in diameter could be pyrolyzed by solar radiation of about

100-200 W/cm2, if the residence time of the particles in the flux was

on the order of l/lOth of a second [13]. This light intensity should

be attainable from tungsten-halogen bulbs, since their brightness

temperature, T, is approximately 32000K, and the emissivity of tungsten

is .45 [IS]. The heat radiated by a grey body is given by q = saT4

where a = 5.7x 10-8 W/m2°K4. It is easily shown that q = 270 W/cm2

for a tungsten halogen bulb. For our application, tungsten halogen

bulbs ranging in power from 750 to 5000 Wwere chosen.

THE OPTICAL SYSTEM

Since large plastic Fresnel lenses are very inexpensive, and narrow

beam flood lights up to 1000 Ware commercially available, an optical

system which could utilize these two components was considered. Several of the flood lights could be distributed in a circle around the

reactor. A separate Fresnel lens would focus each light onto the

reactor's wall. This design was abandoned due to the necessity of

keeping each Fresnel lens, which absorbs as much as 10% of the incident light, in a cooling bath, as well as the separate positional

adjustments needed for each of the several sources.

Another configuration was analyzed in which several flood lights would

illuminate a large parabolic mirror. This mirror would then concentrate all of the light into its focus. Since the typical divergence

of the narrow beam flood light is at best S° [16], the theoretical

concentration ratio would be 400 times less than what could be

achieved with solar radiation [17]. Some radiation intensity measurements were performed with the flood light and a 36 inch parabolic

mirror. Light intensities no higher than 15 W/cm2 were obtained.

Previous work with elliptical mirrors having an acceptance angle as

large as 70% of the total solid angle had provided peak light intensities of 40 W/cm2 [18], with the light distributed over several square

centimeters. This system, together with a second ellipse, was the one

finally chosen for use as a solar simulator. The second ellipse used

the image produced at the secondary focus of the first ellipse as a

light source, and re-imaged it to the primary focus of the second

ellipse. This system was able to provide flux intensities up to

150 W/cm2, distributed over an area very similar to that of the tungsten filament. For a point source, this arrangement should provide its

maximum intensity when two sYffiffietrical ellipses are used and set at a

distance such that their secondary focii overlap.

Syffimetrical ellipses can be very difficult to use in practical applications, because the large solid angle required for efficient light

collection in one of the ellipses will interfere with the reactor

mounted at the main focus of the other. Also, although there are a

variety of relatively inexpensive tungsten filament light bulbs available up to 5000 W, they all present an almost constant irradiance per

square centimeter. This means the higher the power of a bulb, the

larger its filament area. Since large filaments depart appreciably

177
from point sources, it

in an optical system.

rithm was developed to

is difficult to anticipate their magnification

For these reasons a computer simulation algoinvestigate the best optical configuration.

The algorithm was based upon ray tracing techniques, selecting a few

thousand random rays through a Monte-Carlo calculation. Due to the bidimensional geometry used by the code, only an approximate simulation

was performed for those configurations which have a lack of symmetry.

A three dimensional code could easily be constructed to overcome this

approximation, but the time required for computation would increase

significantly. Several important points were learned from the code,

some of which can be extracted from the data shown in Table I .

1. The largest dimension of the filament must always be parallel

to the y-axis, instead of the x-axis (see Figure 2). Orienting the filament in this manner gives a better image (small

magnification) and a higher light collection efficiency.

2. A configuration of two symmetrical ellipses does not necessarily give the maximum light collection efficiency for an

extended light source. This is because some of the light

emitted from the filament extremes will not be collected by

the second mirror, even if their angle of departure, measured

with respect to the line that contains both focii, is small.

3. The overall efficiency of the system increases when the

ellipses are closer than the ideal position for a point

source, since less light is missed by the second mirror.

4 . The best distance of separation for the ellipses is not well

defined since the size of the spot (magnification) is not

strongly affected by the first 10% variation from the "ideal"

position for a point source.

S. The addition of a spherical mirror to re-image light to the

focus of the first ellipse, Fl, theoretically can increase

the system efficiency. From a practical point of view its

inclusion is almost useless when deep ellipses are used,

since the ellipses usually have a hole to allow for bulb

positioning and the light re-imaged to Fl will either escape

through this hole or will intercept the bulb base before

reaching the primary reflector surface. A significant increase in light collection is observed only when a shallow

ellipse is being used. A shallow ellipse plus a spherical

mirror can be as efficient as a deep ellipse.

6. A two paraboloid system can also be used and the light collection efficiency as well as the magnification compete with

the two ellipse system, provided a spherical mirror is added

to re-image the forward light back to the focus. In particular, a commercial very narrow beam flood light with a

divergence of S°, plus a large paraboloid provided light

intensities of the order of 50% of what was achieved with a

two ellipse system.

178
TABLE I

COMPUTER CODE RESULTS

Large ellipse + shallow ellipse

filament along yl); 02) = 173.6 cm

Large ellipse + shallow ellipse

filament along y; D = 163.6 cm

Large ellipse + shallow ellipse

filament along y; D = 153.6 cm

Large ellipse + shallow ellipse

filament along x; D = 153.6 cm

Symmetrical shallow ellipses

filament along y; D = 149.1 cm

Symmetrical shallow ellipses

filament along y; D = 149.2 cm

plus spherical mirror3)

Symmetrical shallow ellipses

filament along y; D = 149.2 cm

plus spherical mirror4)

Solid angle

efficiency of 1st

ellipseS)

(%)

75.5

78.2

76.9

77.6

46.5

47.5

53.8

Total efficiency

of the system

(%)

34.8

37.5

41.4

30.0

21.5

31.6

31.5

Image size along

y direction

(cm)

.90

1.12

1.35

1.58

1.08

.90

.90

1) Filament length, 3 cm.

2) 0, distance between primary focal points of the two ellipses.

3) Spherical mirror, radius = 5 cm, thickness = 4 cm.

4) Spherical mirror, radius = 6 cm, thickness = 4 cm.

5) Provides useful test of the corle. Geometrical efficiency for point source

is 75%.
In summary, it is apparent that for f~lament sizes up to 5 cm the

double ellipse system is relatively insensitive to slight variations

in mirror displacement from the "best'! configuration. (Figure 1)

Several measurements of light intensity distribution were made using

a water cooled calorimeter or a thermocouple, and compared against

the results from the computer code calculations. Figure 3 displays

a comparison between typical experimental and calculated light inten

sity profiles.

THE HORIZONTAL BEAM SYSTEM

Experimental Set-Up

The main components of the optical system used for the hori zontal beam

experiment are a deep and shallow ellipse pair. The arrangement of

the system is presented in Figure 2. The deep ellipse with semi-axes

of 46.7 and 22.1 cm was used to collect light from the tungsten bulb.

The shallow ellipse with semi-axes of 49.8 and 33.4 cm produced a

focused bulb image inside the reactor. An optical bench was used to

support the deep ellipse and the light bulb. The shallow ellipse was

attached to atable. All three components could be displaced in the

x, y and z directions. In the later experiments a 90 mm diameter

spherical mirror was added to the system. This mirror reconcentrated

the light after it crossed through the reactor in an attempt to use

the light flux twice.

The light source for the first system was a 650 Wquartz halogen bulb.

In all the pyrolysis systems the light bulb received electrical power

from a 120 V/60A variac to permit control of the light intensity.

The reactor used in the first pyrolysis experiment was fabricated from

a one inch diameter quartz tube, 15 inches long, as depicted in

Figure 1. Quartz was used as the material for all the reactors due to

its ability to transmit and withstand intense radiant energy fluxes.

A quart z bucket was placed in the bottom of the reactor to catch char

and any unpyrolyzed material . The inlet and outlet lines of the reactor were wrapped with electrical heater tape and asbestos. Thermocouples and variacs allowed the temperature to be kept between 160 and

180°C. This prevented steam condensation but did not initiate pyrolysis of the biomass feed.

After leaving the reactor, the evolved volatile matter passed through

a series of two condensers located in ice-water baths. The gases that

left the second condenser were collected in teflon gas tight sample

bags for later analysis.

Biomass, along with a continuous flow of inert gas, was introduced into

the top of the reactor via a vibrating feeder. The gas flow was needed

to aid in the solids feeding process, and also to prevent a flow of

pyrolysis products or steam back into the feeder.

180
r: r2' r\' \ 0 e ()

F2 )

FIGURE 1. BEST CONFIGURATION FOR A TWO ELLIPSE SYSTEM

supporting

rod

y

ht bulb

x

opt icat

v ibrati ng

feeder

FIGURE 2. SKETCH OF THE HORIZONTAL AXIS SYSTEM.

181
2 (W(cm )

(mV)

Calorimeter

Reading

36

27

18

9 j

x'

/

20

• • • experimental

results

Monte Carlo

simulation

2 (W/cm )

-20 -15 -10 -50S 10 15 20

Z-axis (mm)

(mV)

3li experimental Calorimeter • • results

Reading Monte Carlo

27 simulation

18

9

-20 -15 -10 -50S 10 15 20

y-axis(mm)

FIGURE 3. MEASURED LIGHT INTENSITY PROFILES (2000 WBULB AT 50 VAC).

182
A Comparison of Heating Techniques

on the Pyrolysis of Douglas Fir

Kent J. Voorhees and Lloyd J. Hendricks

Department of Chemistry and Geochemistry

Colorado School of Mines

Golden, Colorado 80401

ABSTRACT

The effect of heating rate and heating times on the

product distribution of the light hydrocarbons (methane,

ethene, ethane, ethyne, propene, propyne~allene and butene)

for the pyrolysis of Douglas Fir has been investigated.

The methane/ethene ratio was affected to the greatest extent by varying the heating rate. A large methane/ethene

ratio was observed for the slowest heating ratio (0.25°C/

msec). The concentration ofethane-ethyne was observed to

increase dramatically as longer constant temperature heatting times were employed. It was speculated that char degradation produced the larger quantities of ethane-ethyne.

A review of the literature indicates there are many

different techniques available for conducting pyrolysis experiments. It appears that many of these approaches were

designed simply to provide heat without serious consideration of the resulting pyrolysis parameters (heating rate,

heating time, etc.). Because the product distribution in

the pyrolysate is dependent on these parameters, it is almost impossible to make any direct comparison between the

various pyrolysis processes. In general, there has been no

serious attempt to define the overall effect of the pyrolysis parameters on the product distribution. This paper

presents an initial study on the determination of the effect of heating rate and heating times on the product distribution for the light hydrocarbons (methane, ethene,

ethyne-ethane, propene, propyne-allene and I-butene) resulting from pyrolysis of Douglas Fir.

183
EXPERIMENTAL

The product analyses were conducted using a Hewlett

Packard 5710 gas chromatograph with a 1/8" x 8' stainless

steel Chromo sorb 101 column~ The column was routinely temperature programmed from 30°C to 220°C at 12°C/min. Helium

was used as the carrier gas. The signal from a flame ionization detector was electronically integrated using a Columbia Scientific integrator. Response factors as defined

by Dietz [lJ were used in all calculations.

The pyrolysis experiments for the heating rate studies

were done on a Chemical Data System Pyroprobe unit using

the filament probe. Heating profiles for the pryoprobe

have been reported [2J to be non-linear during the last 20

percent of the heating process. Therefore, 10000C was chosen for the final temperature with expectation that ~g

quantity samples would be completely pyrolyzed before

reaching the non-linear portion of the temperature profile.

The preparation of the pyrolysis samples was accomplished by first grinding the wood to pass a 120 mesh sieve

followed by suspending 5 mg of the particles in 1 ml ethyl

alcohol. The suspended mixture (2 ~l) was carefully applied across the center portion of the ribbon. Removal of

the alcohol was accomplished by preheating the sample for

20 sec at 125°C. The interface for the pyroprobe was maintained at 200°C and final pyrolysis interval was 5 sec.

The heating time parameter was evaluated using a 10.6

micron continuous-wave carbon dioxide laser, with a maximum

power output of about 20 watts. Fifteen watts were used for

this study and was attained with a pressure of 20 torr in

the 6~foot tube at a discharge current of 37.5 rna at 19.5

kV. The beam from the laser was focused down to about 1 mm

diameter using a short focus germanium lens. Heat dissipation in the irradiated substrate prevents the size of the

heated spot being made smaller than this size. At this

power level and spot size, the power density was calculated

as 7.4kW/cm2 • This spot size is much larger than theory

would predict. Estimating the actual power density for the

focused spot in air using the I-inch focus lens suggests

that the power level was about 0.80 MW/cm 2 •

The sampling chamber for the laser pyrolysis products

consists of an outer chamber, containing a nitrogen atmoshere at slightly more than atmospheric pressure, and an

inner microchamber of about 0.6 cm 3 volume. The chamber lS

illustrated in Figure 1. The inner chamber is sealed to

the sample material 'wi t h a Teflon gasket. The focused

laser beam was admitted at the front of this chamber

through a sodium chloride window. At the bottom of the inner chamber is a jet for introduction of the sweep gas and

at the top is the gas-sample exit. Because of the design

of the microchamber, any leakage is always in such direction that the inert atmosphere of the outer chamber is

184
forced to pick up sample from the laser plume and carry it

to the collection point.

The outer chamber, besides surrounding the sample in

an inert atmosphere, also contained a drive screw attached

to the sample-holder. This mechanism positioned the sample

precisely for organic microstructure mapping, as well as

rapidly moving the sample such that a fresh surface was

supplied for pyrolysis. The equipment has been described

in more detail [3J.

The temperature measurement of the focused laser beam

was made using an optical pyrometer [4J. The sensitive

spot of the pyrometer has a size comparable to the laser

spot size. Therefore, two readings of the weight-average

temperature obtained at two different sample-to-pyrometer

distances yields sufficient data, when combined with the

projected spot size data, for the temperature distribution

parameters to be calculated. A piece of fired Colorado

clay crucible wall was used for the temperature measurement sample since it has a much slower ablation rate than

most materials. This method gave a maximum temperature of

2255°C on that material, with a standard deviation of 1.5~

mm in the spacial distribution of the temperature. With

Douglas Fir, the temperatures were about 245°C higher than

for the clay, but the rapid ablation rate prevented a complete temperature distribution measurement. The higher indicated temperatures in the wood were probably due to the

very deep crater produced in the wood, which served as a

more efficient black-body cavity than the shallower cavity

in the clay crucible.

RESULTS AND DISCUSSIONS

The data illustrating the effect of heating rate on

product distribution is shown in Table 1. The values represented in this Table are weight percentages for these

compounds only. A sum of these weights when compared to

all compounds, Cg and smaller, comprised about 40 percent

of the total weight. The numbers in Table 1 are averages

of at least three pyrolysis runs.

The methane/ethene ratio appeared to be most severely

affected by heating rate. At the fastest rate (~ 40°C/min)

the methane value was lower than the ethene value. As the

heating rate was decreased, a reversal was observed with

the methane/ethene ratio reaching a final · value of approximately 4:1 ratio at a heating rate of 0.25°C/msec. The

other products monitored remain fairly constant.

An important experimental observation was recorded

during these experiments. It is completely possible to

shift the product ratios report in Table 1 at a given heating rate by simply doubling sample size. It cannot be

overemphasized that near monolayers of finely divided

185
Table I

Summary of Heating Rate Studies with Pyroprobe

Time (msec)

(heating time

to 1000°) 25 100 500 2000 5000

Compound

methane 23.8 39.4 40.8 44.4 41. 7

ethene 38.5 36.8 28.1 23. 9 10.5

ethane/

acetylene 8.4 8.6 8.8 6.1 6.6

propene 9.0 8 . 5 8. 7 9.1 7.6

propyne/

allene 3.0 3.6 4.2 4.8 10.2

butene 14.3 11. 6 14.1 11. 3 17.8

Log Time (msec) 1. 40 2.00 2.70 3.30 3.70

material must be applied to the ribbon in order to mlnlmlze

the heat. transfer problem.

The effect of heating time was measured using the C02

laser as the pyrolysis source. Table II summarizes the

measured temperature for various heating times. The temperatures of greater than 2000 0C for the 10 sec exposure

probably did not represent the pyrolysis temperature of the

sample. A more realistic assessment would be for the pyrolysis to occur at temperatures less than 10000C. This is

the approximate magnitude of temperature observed for the

short pyrolysis times. The temperature increase observed

for longer pyrolysis times probably result as a direct consequence of char buildup. The char can act as an insulation barrier and therefore reduce the amount of energy deposited to the sample.

Table II

Maximum Crater Temp in Wood as

Function of Laser Pulse Duration

Time in

Sec

1

2

5

10

186

Observed

Temp °C

1100

1500

1920

2000
Table III summarizes the product distribution as a

function of laser exposure time. Again, the numbers represented in Table III are averages of at least 3 separate

measurements. Heating time seems to affect the ethaneethyne concentration to the greatest extent. The increase

in these products has been speculated as arising from char

degradation to produce ethyne. This seems quite logical

based solely on observed temperatures as a function of

heating time. An experiment where primarily char was degraded at various heating times showed fairly uniform concentration of products with ethane-ethyne concentration

higher than those from the short time experiments recorded

in Table III. Table IV summarizes these data . . We believe

the comparison between Table III and IV clearly indicate

the importance of the cha~ layer in producing high quantities of ethane-ethyne. .

Table III

Summary of Laser Exposure-Time Studies

Time (sec)

Compound

1 2 5 10

methane

ethene

ethane/

acetylene

propene

propyne/

allene

butene

26.1 31.3 31.8 34.7

28.8 40. 28.9 23.4

8 . 2 8.6 23.7 30.6

9.3 7 . 8 5. 5 3.4

3.4 4.1 3.8 3.4

24.2 8.4 8.0 4. 8

Table IV

Laser Char Pyrolysis

methane

ethene

ethane/acetylene

propene

propyne/allene

butene

187

5 sec

32. 5

14.4

45.9

2 • 5

2. 3

2.3

10 sec

37.1

15.0

39.1

3.1

2. 3

3. 3
The craters produced from a .prolonged laser burn

seemed to have the highest concentration of char around the

edges of the hole. Therefore, if a wood sample could be

prepared in a cylinder such that the maximum diameter of

the wood sample was slightly less than the focused laser

spot, much of the char formation and subsequent char degradation could be minimized. Laser pyrolysis (2 sec exposure) of a rod shaped portion of Douglas Fir 1 mm in diameter was conducted followed by product analysis. The products observed for this experiment are summarized in Table

V. The product ratios for this analysis resembled the

shorter exposure times rather than the two second run on

the slab of Douglas Fir. Visually, the char layer formed

was much thinner than those observed in the craters.

Table V

Summary of Various Laser Exposures

Rod- Fresh

2 sec Surface 2 sec 1: 1 sec~':

methane 35.1 32.8 31. 3 34.8

ethene 28.3 26.7 40. 23.2

ethane/acetylene 6.1 24.8 8.6 12.1

propene 10.2 5.3 7.8 11.1

propyne/allene 2 . 5 3.4 4.1 2. 7

butene 17.7 6.9 8.4 15.9

~':slab

The actual quantitative char formation was determined

by physically removing the char followed by weighing. Likewise the aerosols were determined by trapping in a glass

wool-Tenex trap. Table VI lists these two quantities plus

the volatiles as determined by difference.

The data reported in this study represents a preliminary investigation; however, they demonstrate the important

fact that significant product ratios can result from changing the pyrolysis parameters. A continuing study is presently underway to investigate other species as a function

of the same pyrolysis parameters.

Table VI

Percentages of Laser Pyrolysis Products

gas 80.0% tar 4.1%

Total 99.9%

188

Char 15.8%
t

INLET

PATH

AGE

E~A~K~-III~ E=5====!:.

TO GC

LASER BEAM -- --........ AXIS

CARRIER GAS ABOVE l

ATMOSPH ERIC' PRESSU~E

Figure 1. Laser Collection Apparatus
REFERENCES

1. W.A. Dietz, J. Gas Chromatogr., 5, 68 (1967).

2. G. Wells, K.J. Voorhees and J.H.-Futrell, Anal. Chern.,

52, 1782 (1980). ----

3. ~J. Hendricks, submitted to Rev. Sci. Inst.

4. L.J. Hendricks and S.P. Zobrist, submitted to Appl.

Optics.

190
MICROWAVE PYROLYSIS OF 'BI OMASS

BY

Barbara Krieger

University of Washington

Microwave pyrolysis of biomass can occur by two mechanisms. The

electromagnetic radiation can be absorbed by the biomass itself in

proportion to the dielectric-loss factor. Or, in the presence of a

low pressure gas, the radiation can cause arcing (ionization) in the

gas surroundi~g the solid and a plasma can be formed.

The principal findings for dielectric-loss pyrolysis are as

follows: The applicator geometry and design cause high temperatures

to occur in dielectric-loss pyrolysis. Principally gases are formed

in the perpendicular reactor geometry. For a recent design, using an

angled reactor, mild pyrolysis to high molecular weight tars occurs.

The gases typically contain -40% of the original biomass wei~ht

as carbon oxides and 5-10% of the original weight as H2 and hydrocarbons

such as methane and ethylene. As carbon content increases from cellulose < biomass < lignin, the residue (char) increases from 10 to 33%.

Tar yields are quite variable, with 15-31% being typical. Less than

half the tar reflects the chemical structure of the biomass (levoglucosan for cellulose; phenols, guaiacols for lignin). The remainder

is condensed products such as naphthalene and anthracene. Using an

angled reactor in the applicator shows increased phenolic compounds in

the tar.

The principal findings for microwave plasma pyrolysis are as

follows: The process is primarily one of gasification. The gases

contain up to 14% (volume) of acetylene, in contrast to those described above. The size of the pellet affects all product yields,

with smaller particles unexpectedly showing increased char and

decreased gas and tar yields (for an equivalent batch reaction

time to that of a large particle).

191
MICROWAVE DIELECTRIC-LOSS AND PLASMA PYROLYSIS BIOMASS

by Barbara Brockett Krieger

Univ. of Washington BF-IO

Dept. of Chern. Engrg. Seattle, Washington 98195

INTRODUCTION - MICROWAVE DIELECTRIC-LOSS PYROLYSIS

Microwave pyrolysis of biomass can occur by two mechanisms.

In the appropriate microwave reactor, the electromagnetic

radiation can be absorbed by the biomass itself in proportion to the dielectric-loss factor which quantifies the

ability of the material to dissipate electrical energy as

heat. Or in the presence of a low pressure gas above a certain power level, the radiation can cause arcing (ionization) in the gas surrounding the solid and a plasma can be

formed (I-G). Dielectric-loss heating occurs in the domestic microwave oven and in many drying applications including

bonding and drying of wood(7). Details of the physical

principles causing microwave heating and pyrolysis are described in Refs. (8-18).

The amount of microwave power absorbed depends on the local

electric field strength and the temperature- and

moisture-dependent dielectric constant of the material

placed in the field (8-l2). When the material does not absorb the radiation strongly (biomass-like substances) and

the penetration depth is large, the field is not attenuated

and uniform volume heating occurs. This results in a temperature distribution quite different from that found in heating by conduction (19). The heating is especially rapid

for materials with high dielectric loss factors such as

water or other materials with large 'd i pol e moments. Owing

to volumetric heating, a pot~ntial advantage of microwave

heating would be to process the rather large particles in

which biomass is generally found. Refuse treatment (15) and

waste rubber reclamation (20,2l) have been found to be economic using microwave heating.

Wen and Tone (22) suggest that high temperature and high heating rate favor the production of gases from coal over tar

and char production. This concept may indeed apply to biomass, but the exact regimes have not been demonstrated.

However, when large 'particles of biomass experience intense

heati ng, the very low thermal conducti vi ty causes only the

external particle surface to be rapidly heated. The interior is heated slowly at a rate governed by the thermal diffusivityof wood (23) . If the conclusions of Wen and Tone

apply to wood, the slow heating rates and lower temperatures

at the particle interior will favor tar and char production,

192
thus decreasing gas yield from large particles of biomass.

The forest products industry and others (24,25) have found

particle size reduction to be a costly process. Thus, a

principal motivation for this work has been 't o determine if

microwave heating of biomass: gasifies large particles rapidly avoiding size reduction steps; gives unusual results;

or offers an advantage over conventional pyrolysis using

conduction heating.

EXPERIMENTAL APPARATUS

The details of the experimental apparatus, procedure, and

reaction product analysis appear elsewhere(14,16). A waveguide applicator consists of a quartz tube through the broad

face of an S-band waveguide. This configuration offers the

possibility of a continuous process, easier scale-up, and

easier product recovery than the resonant cavity exemplified

by the home microwave oven. Although these applicators are

quite efficient (26), the extremely low dielectric-loss factor of dry biomass (10-2 for wood versus 10+2 for waterRef. 27) causes low absorption. For reasons described in

detail elsewhere (7,14,16), a batch-type microwave reactor

containing a single large cylindrical particle (1 cm diam, 1

cm high) is used. Helium sweeps the reaction products to a

cold trap, and to continuous CO and total hydrocarbon analyzers. Tar fractions and grab samples of the gas are analyzed by gas chromatography or GC- mass spectrometry.

Initial reactor pressures are 1-1.6 atm. Total batch reaction times ar-e 1-5 minutes but most devolatilization is completed after one minute.

The characteristics of the reactants are given in Table 1

(increasing carbon content, left to right). These substrates are under-utilized wood or agricultural materials

found in the Pacific Northwest. The anisotropy of biomass

was removed by using small particles pelletized to the indicated densities. As discussed in Schwartz, et a1 (20), inorganic additives can increase microwave power absorption.

Composition and amounts of these additives are also described in Table 1.

RESULTS

Photographs taken of split pellet cross-sections after very

short reaction times show the black char coloration to be

radially uniform (28). Identical pellets, heated rapidly by

conduction from the surface (28) react in a she11- progressive manner with a sharp front between the black char and

light- colored lignin. This demonstrates that the tempera~

ture distribution in dielectric-loss pyrolysis is the reverse of that found in heating by conduction, unless the re193
TABLE 1 - Reactant Characteristics

Substrate4

1 Red 2

Cellulose Alder

Wheat2 Straw

Cotto2

Wood 3 Coal Ligninl

Elemental: (wt. %)

Carbon 44.45 45.74

Hydrogen 6.17 6.65

Oxygen 49.38 37.46

Ash Content (wt. %) 0.01 0.29

Pellet Density (g/ cm3) 1.13 0.92

Pelletizing Pressure (psi.) 6,000 6,000

46.43 47.48 51.85 69.83

6.70 6.22 4.66 5.67

33.71 35.86 17.45 24.50

5.52 0.48 8.23 1.00

0.93 0.88 0.97 0.83

6,000 6,000 2,000 2,000

10btained from Rydholm (1965).

2

Analyzed by Schwarzkopf Microanalytical Laboratory.

3Analyzed by Northwest Laboratories Inc.

4 . All substrates are pre-dr1ed.

Additive

NaOH HCOONa NaHC03 Si02 Ca(OH)2

WT. % 0.6 2.8 6.6 8.3 6.7 1% 1%

Substrate Red Alder Red Alder Red Alder Lignin Kraft Lignin

WT. % 1.0 1.0 1.0 1%

Substrate Lignin Lignin Lignin Organosolv Lignin

TABLE 2 - Length of Induction Period As A Function of Composition

Substrate Red Alder Wheat Cotton Lignin Cellulose Coal Straw Wood

Type of

Additive

Wt. % of

additive

NaOH HCOONa NaHC03

0.6 2.83 6.65 8.27 6.7

Induction

time (min.) 2.4 1.85 1.40 1.30 1.9

194

o 00 o o 00
action is highly exothermic.

Although microwave irradiation at any desired power level

can be accomplished instantaneously, because the absorption

is temperature- dependent {via the temperature-dependent dielectric-loss factor}, the heating process exhibi ts an .. induction period" of 'l ow absorption and slow temperature rise

{14,20}. During the period of maximum absorption {as indicated by power meters}, the pyrolysis of cellulose, lignin,

or biomass is rapid even for large pellets (28). An approximate, apparent first order rate coefficient for the lignin

weight loss process is 0(10 ) min-l to be compared with approximately (50) min-l from the weight loss data of Iatridis

and Gavalas (29).* These are to be compared with a simple

apparent rate for conduction heating, calculated as described in Russel, et al (30). For the thermal properties of

lignin pellets used in this study, an approximat e conduction

constant is O{0.5} min-I. Bradbury, et al (31) report appar ent rate coeff icients for heating of 2.92 min-l for f inely powdered cellulose in an oven. These results are consistent with the conclusion that microwave heating of large

particles is indeed rapid.

Table 2 summarizes the length of the induction period for

microwave pyrolysis of several substances. Experiments were

attempted to decrease the induction period (28). High density pellets were more quickly and successfully pyrolyzed

(14,16). Additives to red alder (Table 1) shortened the

induction period. Wheat straw with a higher inorganic (ash)

content absorbed microwaves readily, however, coal (high

ash) did not pyrolyze rapidly. This is attributed to the

higher devolatilization temperatures of coal rather than

poor absorption (28).

The presence of addi ti ves caused increased char production

at the expense of the tar for sodium hydroxide additives to

lignin. The resul ts were similar for carbonates, silicates,

and organic sodium salts. Since water is an effective absorber of microwave energy (responsible for the rapid cooking of foods in microwave ovens), water-retaining sal ts such

as Ca{OH~ were used as additives. Wet substrates were also

pyrolyzeB. Since the loss factor of water was approximately

10+4 times that of wood, the water immediately evaporated

(observed by a pressure rise) but reaction took place after

the induction period observed for dry lignin. In some experiments, pellets fractured owing to extremely rapid water

loss. As a result of poor absorption and unsuccessful attempts to improve it, high input power levels (1-2 kW) were

necessary to accomplish biomass pyrolysis.

The applicator geometry influences, the power absorbed by the

*The latter study used small lignin particles heated on a

wire screen in two sec to a temperature of 750 C •

195
sample. Two different configura~ions have been tried, an

appl icator through which the reactor is" angled" (schematically shown in Ref. 14) and an applicator through which the

reactor is perpendicular (shown in Ref.16). Although the

former applicator absorbs less power itself, the power absorbed by the sample is small unless high input power levels

are used. Mild pyrolysis of the biomass results, with high

char yields and large quantities of tar being produced. The

latter configuration causes more power to be absorbed by the

sample and lower (0.8-1.5 kW) input power levels are required. Since the reactor walls (quartz) and gases surrounding the pellet are relatively transparent to microwave

radiation,the walls and gases were cooler than the pellet in

each case. Verification of this inverse temperature profile

appears in Allan, et al (14).

Table 3 reports product yields exclusive of tar composition

under identical conditions using the "perpendicular" reactor. weight loss ~time and temperature-time profiles appear

elsewhere(14,16,28). Reported in Ref. 16 are comparisons

between conventional and microwave pyrolysis products including tar composition.

DISCUSSION - DIELECTRIC-LOSS PYROLYSIS OF BIOMASS

The discussion will be limited to differences between biomass substrates under microwave pyrolysis since these conclusions are generalizable to other systems. Differences

between microwave and conventional pyrolysis results are indicated as they apply to general pyrolysis mechanisms. It

should be noted that results of any kind are related to the

large particle size and density of the reactant.

The" induction period" or period before appreciable gas evolution was noted on the pressure transducers varies considerably and affects the overall pyrolysis (35). The length

of the induction period is a function of the dielectric-loss

factor and the heat of reaction since these affect the intrapellet temperature distribution. The dielectric-loss

factor is itself temperature-dependent. The length of the

apparent induction period is also determined by the temperature at which devolatilization occurs. TGA and DSC data in

Ref. 33 and 34 for biomass and components show weight loss

occurs at lower temperatures for wood and lignin (starting

at 200-250 C) than for cellulose (starting at 350 C). This

explains lignin's shorter induction period in Table 2.

Density also affects microwave absorption (32) through its

effect on intrapellet temperature distribution. The efficient pyrolysis of lignin may be due in part to the exothermic heat of reaction* which causes a more rapid internal

*Although disagreement exists (36-39), the heats of reaction generally accepted indicate that pyrolysis of cellulose is endothermic, lignin is exothermic, and wood is approximately thermoneutral.

196
temperature rise despite the reduced pellet density indicated in Table 1. This is offset by the swelling**

behavior of lignin (or porosity increase) during devolatilization The decrease in density causes slower overall pyrolysis for the lignin .

Lignin, wheat straw and black cottonwood absorb approximately 80% of the forward power and exhibit short induction periods. The data in Table 2, however, suggest ash (inorganic)

content affects the induction period. Schwartz, et al (20)

also report that certain additives caused a more rapid and

controlled microwave curing process for rubber. Since lignin, wheat straw, and cotton wood have higher ash content,

this could also explain their rapid microwave pyrolysis

rate. Red alder does not react unless inorganic salts are

present. The induction time is found to decrease with increasing additive and after this period (noted by a sharp

pressure increase) reaction is vigorous. It is believed

that the inorganic salts and ash can improve the absorption

via: an increased dielectric-loss factor; acting as catalysts to lower devolatilization temperatures; or providing

additional heat of reaction. They cannot be regarded as improving conduction heat transfer since volume heating occurs. The relative importance of these effects is discussed

in Ref. (35) . -Li t t l e data exist regarding the

dielectric-loss factor of mixtures or its temperature dependence.

The reactions products for each of the substrates in Table 3

are presented with (approximately) increasing C/H ratio left

to right. The char yield follows the same trend. The high

COx content of the gases from cellulose and biomass pyrolysis reflect their high oxygen content. Calculated as C/H,

C/O, and H/O, the ratios increase from left to right in the

table. The trend in H2 and hydrocarbon content of the product gas follows all three ratios • The conclusion is that

char and gas yield is determined in part by C/H/O content in

high temperature microwave pyrolysis rather than by chemical

structure" Yields are similar to thoe in conventional pyrolysis (using much smaller particles) at high temperatures

and rapid heating rates.

The effect of additive concentration on the gas and char

yield is shown in Fig. 1. It can be seen that the char

yield is increased dt the expense of the tar yield since gas

yield is also increased. An elemental carbon and oxygen

balance was performed on the substrates and is plotted in

Fig. 2. The dotted line represents the calculated increase

**Lignin, like some coals, softens and swells whereas biomass and cellulose do not. The coal pyrolyzed in this

study was a Washington sub-bituminous coal and did not

sof ten.

197
TABLE 3 - Reaction Products Dielectric-Loss Heating (wt %)

6.7% NaOH+

Red Wheat Black

Fraction Cellulose Aldert Straw Cottonwood Coal Lignin

Residue 27. 25.8 17.8 10.6 33.5

t:l

25.5* H

Volatiles & 43. 27.9 31.4 t:l 19.6

Polymeric film (14.2)** ( 8.2) (11.4) z (12.8)

0

1-3

Water 23. "d 8.2 10.1 8.6

0 >

0 b:l

Gases 4. ::0 48.7 46.1 47.9 tI) 38.3 0

> ::0

H2

b:l 2.2 1.3 0.6 b:l

tI) 0.8

0

CO ::0 26. 24.4 28.4 21.9

"d

CO 2 4. 1-3 14.4 14.5 10.3 5.1 H

0

CH4

z 1.6 2.6 4.6 4.4

C2H2 0.2 1.2 2.1 4.9

C2H4 4.1 1.5 1.3 1.8

CZH6

0.2 0.6 0.6 0.1

tRed Alder couples only with-additive present .

*By -difference **( ) amount in cold trap.

TABLE 4 - Plasma Heating (wt %)

Red Wheat Black

Fraction Cellulose Alder Straw Cottonwood Coal Lignin

Residue 2.9 9.2 23.1 9.7 49.8 39.8

Volatiles and 21.4 17.0 14.2 Z2.9 13.6 19.8

Polymeric film

Water 9.1 6.2 13.0 8.3 5.6 8.6

Gases 66.6 67.6 49.8 59.1 30.9 31. 9 --- - ---- ---- ----

HZ 1.8 2.4 1.1 1.2 1.4 0.5

CO 45.9 41.5 Z4.3 32 .8 21.6 19.1

CO 2 5.9 11.1 16.1 13.0 3.6 4.0

CH4 2.1 7.1 1.8 2.9 1.1 2.3

CZHZ 7.7 4.4 4.4 5.9 1.9 4.6

CZH4 3.0 0.8 1.5 2.5 1.1 0 .7

C

ZH6 0.3 0.2 0.7 0.8 1.2 0.7

198
5.0

~-~-_.-~-~

---

1.0 2.0 3.0 4.0

HT. PERCENT OF NAOH

• C ELEMENT .. H ELEMENT

• 0 ELEMENT

w 55

en

~50 (L

tf)45

a:

t=)40

z

-35

I-

~30 u

ffi25

Q..

·20

f-

~15

10

·s

OL.---'------lL..o-......L-~-..&-~-~--'--.&....--'

1.0 2.0 3.0 4.0 5.0 o

HT. PERCENT OF NAOH

5

60

55 • RESIDUE

A PERMANENT CASES

50 • V()..RT JlES

X ~TER

~45

u

540

0

0:::

0...35

LL

°30

Iz

~25 a::

w '-0 0...2 '-0

Fig. 1 The product distribution from the pyrolysis of

lignin in dielectric loss mode as function of

the weight percent of sodium hydroxide added.

(The products are evaluated as weight percent of

the original substrate including the additive.)

Fig. 2 The elements in gas product from the pyrolysis

of lignin in dielectric loss mode as function

of the weight percent of sodium hydroxide added.

(The elements are evaluated as weight percent of

the gas products.) The dotted line is the increased

oxygen content attributable to the presence of NaOH.
in gas-oxygen content as a result of the presence of NaOH.

The gas is seen to contain a larger amount of elemental oxygen than would be accounted for by the NaOH. This phenomena

has been observed by Robertus et a1 in their large scale

studies of biomass. using mixed catalysts (40).

The tar components (16) indicate that one can expect fragments of the original chemical structure to appear in the

tars. The tar from microwave pyrolysis of cellulose contains 1evog1ucosan, the same compound found in conventional

pyrolysis (14,31,41). The tar obtained from lignin and biomass contains phenols, guaiaco1s, and creso1s - fragments

of the lignin polymer. This also applies to 1ignins generated with different pUlping processes. Organoso1v lignin

obtained from ethano1- water de1ignification is a lower molecular weight polymer than Kraft lignin (35). When the

former is pyro1yzed, a low molecular weight tar that is completely soluble in tetrahydfuran is produced. Kraft lignin,

a more condensed lignin, produces considerable (30-35%) char

and a high molecular weight tar which is not soluble in THF.

Tars from both lignin and biomass contain condensed aromatics such as naptha1ene and anthracene which are not fragments of the original macromolecule. According to Sakai et

a1(42), these components are formed rapidly in secondary reactions during the high temperature (>850C) gas phase pyrolysis of simple hydrocarbons and olefins. However, it is

worth noting that the initial internal surface area (about

5E+04 cm2 , measured by BET) of the pellet is much greater

(ana at high temperature) than the relatively cool quartz

reactor surface with a smaller (about 200 cm2) surface area.

It is reasonable to suggest that the increased temperature

and greater area would enhance the rate of char-surface reactions over reactor surface reactions and be a factor in

producing the condensed aromatics in the tar and the secondary reaction products in the gas.* · Since the pellet is

dense, the volatiles are at high pressure which would favor

second-order reactions such as polymerization and condensation. However, the . data are inconclusive as to the origin

of these species.

If one broadly classifies acetylene and condensed aromatics

as "high temperature products", Fig. 3 and Fig. 4 present

results that indicate NaOH causes a gas and tar product distribution characteristic of a lower temperature as the amount of NaOH additive is increased. The limited data are

inconclusive with 'r e s pe ct to mechanism; however, Robertus,

et a1 have investigated this aspect (40).

In Ref. 16, microwave pyrolysis of lignin is compared to

results of Iatridis and Gava1as (29) who used an e1ectrica1-

*Berthe1ot long ago noted that acetylene forms benzene when

passed over hot glass at high temperature.

200
1.0 2.0 3.0 4.0 5.0

WT. PERCENT OF NAOH

1.0 2.0 3.0 4.0 5.0

HT. PERCENT OF NROH

O~-L--...L_~--'-----iL.---r..----._-,,----..-"J

o

30 1.5

• cmBOH tnJOXJDE 1.4 • Pt"£Nll..

.. CffiBON 0Jox I DE .. GUUOCOL

• tETHANE 1.3 • CRfSll.. CCttP()Jt,I)S

25 X FCETYLEt-E X cCtmENSf0 RROMATICS

• ETHYLENE 1.2

en • (f.)

t- t-1 .1

u u

520 • ::>01 .0

0 0

0::: 0:::

Q.. a,

LL u,

015 0

t- tZ Z

W W

U u

a::: 0:::: N

~10 w

0 a, f-'

t- .-

3:

•

Fig. 3 The product distribution of gases from the

pyrolysis of lignin in dielectric loss mode

as function of the weight percent of sodium

hydroxide added. (The products are evaluated

. as weight percent of the original substrate

including the additive.)

Fig. 4 The product distribution of tars from the

pyrolysis of lignin in the dielectric loss

mode as function of the weight percent of

sodium hydroxide added. (The products are

evaluated as weight percent of the original

substrate including the additive.)
ly heated metal screen to rapidly pyrolyze small particles

of lignin The primary differences appear to be in the

higher gas yield and the . greater · e xt e nt of secondary reactions to form condensed aromatics ln microwave pyrolysis •

This is attributed to a high intrapellet temperature.

INTRODUCTION--MICROWAVE-INDUCED PLASMA PYROLYSIS OF BIOMASS

Above certain power levels or at low pressure, arcing or

ionization can occur in a microwave reactor. The resulting

electrons are accelerated by the alternating field (2450

MHz) and their kinetic energy or "electron temperature" becomes very high (l). Although the electron concentration

and its velocity determine the intensity of plasma heating,

in a microwave reactor the heating may not be independent of

the dielectric-loss factor of the-5ubstance, depending on

how strongly the substance absorbs. At low pressure, only

low power is needed to form a plasma, and primarily plasma

heating occurs (owing to poor absorption by biomass). If

solid biomass particles are present in a plasma, they experience rapid surface heating by electron bombardment.

Volatiles are produced and the electrons quite effectively

penetrate the "thick" boundary layer caused by volatiles

outflow.* However, owing to surface cooling by the carrier

gas, the rate of surface temperature rise is only 10-20

C/sec determined from optical pyrometer measurements

(28,62,65).

When solids are heated by a plasma, it is generally true

that the plasma does not penetrate the solid surface. Under

certain conditions (62), plasma heating of a solid can be

made to approximate a high heat flux which is spatially uniform and nearly constant with time. Since biomass is a

rather poor heat conductor, the interior of a large particle

is heated by conduction (as in combustion) and

shell-progressive reaction occurs{62,63,65). If the

gas-phase plasma reactions are known, the conclusions drawn

from experiments designed to manipulate only intrapellet

properties in a plasma reactor can be more widely applied to

conventional, large pellet devolatilization In particular,

since the high temperature, large pellet pyrolysis is limited by the rate of heat conduction in the pellet, the role of

thermal diffusivity and particl~ size can be investigated.

A review of attempts in the literature (49-61) to model the

behavior of thermally- thick substances such as cellulose or

biomass under rapid pyrolysis conditions appears in Ref.

(48). From this review, it is concluded that particle size

effects are extremely complex (57) and are generally imbedded in a dimensionless time. For this reason, our studies

*A rough comparison: net po!er to the biomass pellet in this

study is about 5 watts/cm '·:1 combustion level heat flux is

about 4-12 watts/cm2 for cellulose according to Ref. (43).

202
have been carried out for the same dimensionless time when

particle size is systematically varied.

Chemical reactions occuring in a plasma are severe fragmentation, rearrangement and free-radical reactions (6). These

reactions have many applications today and general discussions appear in several books (1-6). Specific reactions of

organic gases in plasma have been reviewed by Ref. (6) and

organic solids such as coal (44) and refuse (45) have also

been studied and reviewed in (46). The large particle of

complex composi tion is generally not treated although plasma

etching of semi-conductor surfaces has caused interest in

this area (47). Principal motivations for this study are:

to determine if plasma pyrolysis of large biomass parc i cl.es

offers advantages to conventional pyrolysis; and to understand the role of intrapellet properties especially for the

high heat flux of plasmas.

EXPERIMENTAL DETAILS-MICROWAVE-INDUCED PLASMA PYROLYSIS OF

BIOMASS

The apparatus details can be found elsewhere (46,62,65);

the microwave appl icator is the perpendicular conf iguration

described above. Typical inert carrier gas pressures are

less than 200 mm Hg and the batch reaction time is generally

less than 45 sec to prevent study of the char gasification

regime. Input power variation has been investigated (49)

but it is generally set at the minimum which maintains a

steady plasma Gases such as CO" and H2 are added to determine those reactions occuring inXthe gas phase and to uncover the role of free-radical reactions. Pellet surface

temperatures are measured by an infra-red optical pyrometer.

Substrate composition and preparation is similar to that

described earlier (28,46,62,64,65). Particle size varied

from powdered lignin to lignin pellets of several confLqu r ations and characteristic dimension (65).

Experimental Procedur~

The unraveling of product distribution variation in plasma

pyrolysis is complex owing to the interdependence of temperature profile in the pellet and particle size, and to other

effects such as residence time in the plasma or total pressure. Thus, a full factorial screening experimental design

(65) was conducted. The four variables used in the 24 design (66) were: plasma reactor pressure; residence time in

the plasma phase; particle size (characteristic dimension

and aspect ratio); heat flux (particle surface area and aspect ratio). Large and small pellets were reacted for the

same dimensionless time, 2 2 ---- t1/r 1 t 2/r 2 (1)

(See Refs.48-61). In a separate study, density (thermal

diffusivity) was also systematically varied (62,67).

203
Detailed product analysis was made as described in Ref. 16.

RESULTS - MICROWAVE-INDUCED PLASMA PYROLYSIS OF BIOMASS

Photographs of split pellet cross-sections after increasing

batch reaction times indicate that the char and unreacted

biomass are sharply separated by a black front that

progresses inward*. Eg. 1 has been verified using

the char thickness so measured. This data was also used to

verify a simple conduction model to predict trends in intrapellet temperature. The model used constant , properties

(different for char and lignin), and constant heat flux with

the char-forming temperature and heat flux as parameters.

It is described in Refs. 62 and 65. A typical product distribution in plasma pyrolysis is shown in Table 4. Table 5

briefly summarizes the effect of doubling particle size for

constant pellet density and constant dimensionless reaction

time using a factorial design. It was determined (62) that

solely a change in particle density could produce the alterations in overall product yield such as those in Fig. 5.

For species not strongly influenced by the plasma reactions,

yields of specific compounds changed with a change in density as shown in Fig. 6. Effects of varying substrate and

additive gases are reported in Ref. 28.

DISCUSSION- PLASMA PYROLYSIS OF BIOMASS

The results in Table 4 are consistent with the elemental

composition of the biomass pyro1yzed. The high oxygen and

hydrogen content of biomass and cellulose cause low char

yields: the ash in wheat straw and the high carbon content

as well as ash in lignin (and coal) are responsible for

their higher char yields. The lower overall char yields in

Table 4 (in contrast to those of dielectric-loss heating,

Table 3) are attributed to the high effective temperature of

plasma heating. The high gas yields are indeed remarkable

in view of the 1 cm particle size .

Acetylene is found in high concentration when compared to

thermal pyrolysis. 'A free radical mechanism of Mertz, et a1

(68) involving H atoms and CO is consistent with our findings since both added CO and H2 cause an increase in acetylene yi e1d (28).

The rate of weight loss in plasma pyrolysis is not as rapid

as that occuring in dielectric-loss pro1ysis (28). However,

by extending the heating period , the extent of gasification

is qui te high.

When pellet size is varied in a factorial design, the increase in fractional yield (independent of amount reacted) is

*This is consistent with very rapid reaction relative to

heat conduction.

204
rather dramatic (see Table 5) and opposite from that predicted for large pellets (48-61). For equal actual reaction

times of a small and large pellet, predictions are: more

char, less gas and tar from the large particle. For equal

dimensionless pyrolysis times , the gas yield is higher and

char yield lower in the large pellet. Examination of 's i mpl e

heat conduction behavior reveals that the large particle is

cooler at the interior and hotter at the exterior (where it

has reacted). Consideration of the heating rate as a function of radius shows that it is lower for the large pellet.

Thus the observed yields including the increased tar can be

explained by the intrapellet temperature profile.

The gas and tar composition from the two different sized

pellets reflects changes that show an increase in products

char acter i stic of both lower and higher temperature pyrolysis. Methane yield is increased but so is the yield of

condensed aromatics (65). Although the CZHZ yield increases

with increas ing particle size, the factorial design results

Lndi.cafe the -qa s phase residence time (time in the plasma)

also affects the acetylene yield, as expected from the mechanism of Mertz, et ale

Owing to the structure of a factorial design, Table 5 demonstrates the magnitude of gas composition change for solely a

change in particle size. This concept is extremely important for the utilization of biomass which occu~s naturally

and as a by-product in an, extremely wide variation of sizes.

Table 5 and Figs. 5 and 6 demonstrate also that the

dimensionless time is the important design variable for

large particle~It should be noted that 'f o r

appropriately chosen heat flux arid temperature condi tions to

give the desired products from a small particle, the use of

large particles in the same reactor will increase the tar

yield and cause a significant unreacted fraction. The role

of thermal diffusivity in altering the intrapellet heating

rate is clear from Fig. 5 where char, gas and tar production rates are altered by solely a change in density which

has been shown (23) to determine the magnitude of the thermal diffusivity of wood. The limited data on gas composition changes (Fig. 6) when pellet density is changed ~how

that products of interest (methane, ethylene) from large

pellets can be influenced by the Lnt r'ape Ll.et; heating rate.

OVERALL CONCLUSIONS

'The study of plasma pyrolysis has provided insights that relate to the behavior of biomass under combustion conditions

and has provided some data to illustrate the magnitude of

the particle size and intrapellet conductivity effects

(62,67) on product distribution. These conclusions have

Dimensionless time has been cast in many ways (48-61). The

simplest of ten is (time) (thermal

diffusivity)/(length*length) .

205
TABLE 5

FACTORIAL DESIGN RESULTS:

EFFECT OF DOUBLING PARTICLE SIZE ON

PRODUCT YIELDl FROM PLASMA PYROLYSIS

Weight Percent Yield

Particle Size

i. Change

Residue

Tar

Gases

Total

6mm

68.4%

17.7

0.5

0.6

0.7

8.5

1.3

2.1

0.3

0.2

100.3

13 mm

45.9%

27.5

3.3

1.1

2.1

14.5

2.3

2.5

0.6

0.3

100.1

33%

+ 55

+560

+ 83

+200

+71

+77

+ 19

+100

+ 50

2 Also Dependent On:

Particle external area I

(or heat flux)

Residence time in p.lasma phase

% Unreacted 11.4 20.7 + 82 Pprtic1e e~te~nal area ~or heat tlux)

lFractiona1 Yield is based on initial particle weight less weight

of unreacted lignin.

2Interactions determined from full factorial experimental design.

206
80 -- I r- 40

- - - HIGH DENSITY -- LCM DENS ITY RATIO = CH4

C~4 + ct'6

60 30 ..--

-c:

Q.)

u

,.",

Q)

a.

-40 CHAR

en 20

0 GAS I -l

W

-

0

N >- s -...,J 20 10

TAR sa

---..-.,-0

I I 0 40 60 80 100 120 140 0 2 4 6 8 10 12

TIME .(seconds) Xl0

TIME (SEC)

Fig. 5 Gross product distribution during reaction for

low (solid line) and high (broken line) density

pellet. 0 = char ± 6%. [J = condensible gases

± 7%. Ll = permanent gases ± 0.1%. '

Fig. 6 Ratio of weight percent yield of methane to

ethylene and ethane during reaction. 4t a high

density pellet (1.2 g/cm3). c low density

pellet (0.9 g/cm 3).
wide application since an important usage of biomass is for

fuel.

Plasma microwave pyrolysis is more easily executed than dielectric-loss microwave pyrolysis owing to the latter form of

heating being very dependent on hardware design. Both microwave heating processes cause primarily gasification with

low char yields especially in view of the large particles

studied. The tar and gas composition reflects high temperature cracking and rearrangement reactions in the pellet, or

in the case of plasma heating, in the gas phase. The plasma

pyrolysis results presented here have implications for fast

pyrolysis since even under the extremely severe heating conditions existing in 'the plasma, the physical properties of

the large pellet affect the product distribution.

Both the dielectric-loss and plasma mechanisms for microwave

heating accomplish rapid pyrolysis of large particles of biomass (wet or dry). The char and tar yield is low in both

mechanisms, owing to rather rapid heating despite the large

pellet size and high temperatures. This is consistent with

similar results in the literature for coal.

Size reduction of biomass is costly.' This study indicates

that microwave pyrolysis can somewhat overcome heat transfer

rate limitations and indeed speed the pyrolysis of large

fragments of materials. Therefore it offers certain processings advantages. However, for materials with low dielectric-loss factors, the requisite power to initiate pyrolysis reactions is so high that currently, unless specific

product distributions can be selected by the use of microwave heating, the scheme is not economically attractive.

Little flexibility is available in choosing the frequency

(which would improve coupling) since only certain bands are

available for high-power commercial units. However, additives of appropriate composition and dielectric properties

can alter the economics. The commercial feasibililty of microwave-induced reactions remains to be demonstrated. The

use of dielectric-loss heating in the laboratory, however,

to explore the role of heat transfer in biomass pyrolysis is

valid.

ACKNOWLEDGEMENTS

The author is indebted to Ricky Chan, Martha Graef, Steve

Slottee, Debbie Wiggins, and Donald Work for competent and

enthusiastic pe rformance of the experiments and analytical

work during their studies. Others who have provided assistance and insight are K.V. Sarkanen and G.G. Allan, Dave

Hanson, and Jim Callis. The National Science Foundation,

Division of Problem Focused Research has supported this proj ect.

208
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214
PRODUCT COMPOSITIONS AND KINETICS

FOR RAPID PYROLYSIS OF CELLULOSE

M. R. Hajaligol, W. A. Peters: J. B. Howard

and J. P. Longwell

Energy Laboratory and Department of Chemical Engineering

Massachusetts Institute of Technology

Cambridge, Massachusetts 02139

ABSTRACT

Systematic studies of the independent effects of temperature

(300-ll00 0C), solids residence time (0-30s), and heating rate

«100-15,000°C/s) on the yields, compositions, and rates of formation

of products from the rapid pyrolysis of 0.0101 cm thick sheets of

cellulose under 5 psig pressure of helium have been performed. Tem~

perature and sample residence time are the most important reaction conditions in determining the pyrolysis behavior, while heating rate

effects are explicable in terms of their influence on these two parameters. A heavy liquid product of complex molecular composition

accounted for 40 to 83 wt% of the volatiles above 400°C. Secondary

cracking of this material increased with increasing residence time or

temperature and was a significant pathway for production of several

light gases. For short solids residence time the more volatile products

included modest quantities of H2 (rul wt.%), C~, C2~, C2H6, C3H6

(ruO.2-2.S wt.% each), and light oxygenated liquids such as acetone/furan

mixtures, methanol, and acetaldehyde (ruO.8-l.S wt.% each), most of which

were formed primarily over the temperature range SOO-800°C. At temperatures above 750°C for all holding times CO dominated the product gases,

and attained a yield above 23 wt.% at 1000°C. Char yields .decreased

monotonically with increasing temperature 't o a minimum of ru3 wt.% at

temperatures of 700 and 800°C at solids residence times of 2 and Os

respectively.

INTRODUCTION

Previous research at M.I.T. and elsewhere [1,2]. indicates that

biomass pyrolysis offers promise for producing commercially interesting

quantities of high heating value gases, and liquids suitable for

replacing petroleum-based products such as distillate fuels, high

octane motor fuel additives, and olefinic feedstocks. Realization of

this potential, however, is critically dependent on identification of

reaction conditions achievable in practical scale equipment. There

have been many previous investigations of the pyrolysis of wood, cellulose and ot~er forms of renewable materials. However, there still

remains a great need for reliable systematic studies of the independent

effects of commercially interesting reaction conditions such as

temperature, heating rate, solids residence time, vapor phase residence

time, pressure, presence of reactive gas, and sample dimension, and

of the type of material, on the yields, compositions, and rates of pro-

*To whom correspondance may be addressed.

215
duction of pyrolysis gases, liquids and chars. This level of detail

is needed to gain better fundamental understanding of, and to provide

predictive modeling capability for, the thermal degradation of lignocellulosic and other biomass materials. It would also be useful in the

selection, design, and optimization of thermal processes for converting

renewable resources to clean fuels and chemical feedstocks. Further,

since pyrolytic decomposition supplies the volatiles that allow ignition

and support flaming combustion of most materials these data would also

be of interest in studies on materials flammability, flame spread and

stability, and related issues in fire research.

This paper presents recent results from an ongoing M.I.T. research

program aimed at providing data and interpretive models responsive to

the above needs. Effects of temperature, solids residence time and

heating rate on the yields, compositions and rates of formation of

products from the rapid pyrolysis of cellulose under a pressure of 5

psig of helium are the main focus. Subsequent communcations will

furnish similar information on wood, lignin, and hemicellulose, and

on cellulose at other pressures.

OBJECTIVES

The overall objective 9f this work was to determine quantitatively,

by experimental measurements and associated modelling efforts, how reaction conditions affect the rapid thermal decomposition behavior of

cellulose. Specific objectives were to measure product yields, compositions and rates of formation for ranges of reaction conditions of commercial and scientific interest including: temperature (300-1100°C),

heating rate « 100-lS,000oC/s), total pressure (0.0001-69 atm), sample

size (~100-400-~), and sample residence time (0-30 s).

APPARATUS AND PROCEDURE

Reactor Description

Laboratory scale batch reactors were employed in this work. These

devices were designed to allow: a) total product collection for direct

measurement of material and elemental balances; b) inversion of product

yields from rapid thermal processes to kinetic parameters via non-isothermal models; and c) separation of collaborative effects of heating

rate, residence time, sample size, extent of sample dispersion and

temperature.

A schematic of one such reactor and the product collection equipment is shown in Fig. 1. This reactor vessel is designed for atmospheric

pressure and vacuum pyrolysis work. It is a Corning Pyrex, cylindrical

pipe, nine inches in diameter and nine inches long, closed at each end

with stainless steel flanges, with electrical feedthroughs and gas inlet

and outlet ports. Another version of this reactor capable of operation

216
./

High Current

Heating Circuit

Vaccuum Pump

(for purging)

Gas

Chromatograph

Lipophilic Gas Trap

(-I" -e.)

Thermocouple

Recorder

Screen

Helium

Lipophilic Oxygen Pre-Trap

~Glass Reactor Vessel

G-!<\ss Wool "t:" 7i~

(-77°C;

Figure 1. Captive Sample Apparatus
of up to 1500 psig is also available for studies of elevated pressures.

The sample is heated inside a folded strip of 325 mesh stainless steel

screen held between two massive brass electrodes mounted within the

above vessel. The heating circuit consists .of 100 and 50 amp variable

transformers connected to two 100 amp relays which are in turn respectively activated by O-ls and a-60s timer switches. This system allows

independent variation of the following reaction conditions over the

indicated ranges; heating rates (lOO-lOO,OOO°C/s), final temperatures

(200-1l00°C), sample residence time at final temperature (0 - 00 s).

During operation most of the gas within the reactor remains close to room

temperature so that upon exiting the immediate neighborhood of the hot

stage, volatiles are rapidly diluted and quenched.

The time-temperature history of the sample over the entire run is

measured in each experiment using a rapid response (time constant = 0.003 s) type K (chromel/alumel) thermocouple fabricated by joining

25.4 urn diameter bare wires to give an approximately 76 urn diameter

bead. The', thermocouple is placed within the folded screen and its

output (millivolt range) is monitored by a fast response strip chart

recorder.

Experimental Procedure

The cellulose samplrs used in this work were approximately 100 mg,

thin strips of predried, low ash «0.007 wt.%) S &S No. 507 filter

paper of dimensions 2 cm x 6 cm x 0.0101 cm thick and elemental composition C: 43.96 wt.%,H: 6.23 wt.%, and 0: 49.82 wt.%. To begin a run

a sample is placed between two folds of a preweighed screen which is

then reweighed and inserted between the brass electrodes of the reactor. The thermocouple bead is inserted within the folds of the screen.

The reactor is evacuated to a pressure of 0.1 mm Hg and flushed 3 to 5

times with helium, and then set at the desired pressure. The sample

temperature is raised at a desired rate to a desired holding value which

is maintained until .the circuit is broken. The screen and remaining

solid material then cool primarily by radiation and natural convection

at an initial rate of around 200°C/s.

All products are collected in each run enabling total material

balances and elemental balances to be directly determined (see below).

The yield of char, which remains on the screen, is determined gravimetrically.

Those products which remain in the vapor phase within the reactor

include gases, light liquids, and a small amount of material retained

on glass wool at temperatures up to 100°C. The last is included in the

product fraction operationally defined as tar. These three materials

are collected by purging the reactor vessel with 3 to 5 volumes of

helium and transporting them to two downstream traps. The first trap

consists of a 36 cm long x 0.95 cm 0.0., U-Shaped tube packed with

IThe samples were dried in a dessicator over silica gel for at least 1

week prior to use.

218
glass-wool and is immersed in a bath of dry ice/alcohol (-77°C). The

second (downstream) trap of the same geometry and dimensions, is

packed with 50/80 mesh Porapak QS and is immersed in a bath of liquid

nitrogen (-l96°C). Those products which can be recovered from the

traps by warming them to lOO°C are analyzed gas chromatographically

using a 3.66 m x 0.64 em O.D. 50/80 mesh Porapak QS column, temperature

programmed from -70°C to 240°C at a rate of 16°C/min with helium carrier

gas at a 60 ml/min flow rate. '

Hydrogen, which is recovered by direct sampling of the reactor

atmosphere with a precision gas syringe, is analyzed on a 3.05 m x

0.32 em O.D. 80/100 mesh, spherocarb column, operated isothermally at

O°C using nitrogen carrier gas at a flow rate of 30 ml/min. A thermal

conductivity detector is used in both analyses.

Tar is operationally defined as material condensed: (a) within the

reactor vessel at room temperature on the walls and flanges, and (b)

in the glass wool trap described above and not evolved by heating to

100°C. It is recovered by washing the above locations with a 2:1

(v/v) mixture of methanol and acetone. Its yield is determined gravimetrically after evaporating the solvent.

Selected samples of char and tar are sent to a c0mmercial laboratory

for elemental analyses for carbon, hydrogen, and oxygen . .

RESULTS OF APPARATUS VALIDATION EXPERIMENTS

All the data reported in this paper are for the thin 'strip cellulose samples described above. All yields are presented as a percent

by weight of initial cellulose, unless otherwise specified. Selected

experiments were performed to assess the validity of the experimental

data and to determine the ranges of operating conditions where the

above apparatus exhibits minimal contributions from secondary reactions.

The results now follow.

Material and Elemental Balances

The apparatus described above gave very good total mass balances

and reproducibility in the product yield and composition data. In most

experiments the total material balance closure was 100+5%, although in

some runs only 90 to 95% of the original mass of the cellulose was

accounted for. Elemental balances for carbon, hydrogen and oxygen

were calculated for selected experiments from the data on volatiles

yields and compositions, and elemental analyses' of the product tar

and char. Typical results for four runs are presented in Table I along

with the total mass balances. The four balances are seen to be excellent for each of the cases. In this calculation the total amount of

nitrogen, sulfur, and ash were assumed to be virtually zero.

219
N

N

o

Table 1. Carbon. Hydrogen. Oxygen, and Total Mass Balances for Cellulose Pyrolysis

Products Peal< Temperature Holding Temperature III real< Temperature Peak Temperature

500·C 400·C 750·C 1000·C

s !! Q s !! Q s !! Q s !! Q

CO .99 .42 - .57 .25 .11 - .14 15.82 6.78 - 9.04 22.57 9.67 - 12 .9

CO, .3 .08 - .22 1.45 .40 - LOS 2.38 .65 - 1.73 3.36 .92 - 2.44

H,O 3.55 - .39 3.16 6.49 O. .12 5.77 8.72 - .97 7.75 9.22 - 1.03 8.19

CH. O. O. O. - O. O. O. - loll .83 .28 - 2.62 1.96 .66 - C,H. O. O. O. - O. O. O. - LOS 0.9 .15 - 2.18 1.87 .31 - C,H, 0.0 O. O. - O. O. O. - .17 .14 . 03 - . 28 .22 .06 - C,H, O. O. O. - O. - O. - .70 .6 .1 - .80 .69 .11 - H, O. - O. - O. O. O. - .36 - .86 - 1.18 - 1.18 - CH,OH .25 .09 .02 .14 .21 .08 .03 .1 1.03 .39 .13 .51 .98 .37 .12 .49

CH,CHO .01 .01 .0 O. .05 .03 O. . 02 1.58 .86 .14 .58 1.7 . • 93 .15 .62

C.+ Ethanol •00 O• 0.0 O. O. O. o. O. .29 .15 .04 .10 .38 .2 .05 .13

AC + FU 0.01 .04 .01 .02 .16 .1 .02 .04 1.00 .62 .10 .28 .82 .51 .08 .23

OIO(CH,COOHI .12 .05 • 01 •06 .0 O• O• O. .85 .34 .06 .45 .58 .23 .04 .31

Tar 16.31 1.5 .97 1.9 83.35 38.28 4.95 40.32 59.92 27.77 3.63 28.63 49.12 22.89 2.98 23.25

Char 83.63 38.13 5.34 40 .16 6.17 4.94 .24 .99 3.32 2.65 .1 .57 3.91 3.46 .13 .32

Total 105.25 46 .32 6.74 52.23 98.36 43.96 5.96 48.43 98.8 42.68 6.59 49.53 99.86 43.92 6.9 48.88

Closure 105\ 105\ 108\ 105\ 98\ 100\ 96\ 97\ 99\ 97\ 106\ 99\ 100\ 100\ 111\ 98\

(llHolding Time· 30.
Extent of Secondary Reactions on the Screen Heater

The wire mesh screen used to support and· heat the cellulose sample

could cause catalytic or other secondary reactions during the cellulose

decomposition. As part of the routine run procedure the screens are

prefired in helium which contains a small amount of oxygen impurity.

During this operation the latter is believed to react with chromium

in the s~ainless steel to produce a layer of chromia which probably

reduces the catalytic activity of the screen [3]. Nevertheless, to

more quantitatively assess the role of surface effects experiments

were performed to either passivate the screen to cracking or to augment

its opportunity to cause cracking. To the former end a pyrolysis ' run

was performed with a screen on which a layer of gold had been vacuum

deposited while to the latter, runs were carried out using up to five

layers of hot screen above the cellulose sample. Operating conditions

in all cases were 5 psig of helium pressure, 1000°C/s heating rate,

about IOOO°C final temperature, and zero sample holding time. The

results in all cases showed almost no difference in the product yields

and compositions, exc~pt that in the multiscreen experiments there were

changes in the small amount of the tar collected downstream of the

reactor. While the total tar make was essentially unchanged, the yield

of the latter constituent increased, from 2.8 wt.% with 2 layers of

screen (normal operation) to 3 and rvS.S wt.% respectively with 3 and 5

layers. It was thus concluded that, except possibly for a very minor

amount of cracking of heavier tar components to lighter ones, the surface

of the screen heater exerted little influence on the data obtained in

the above reactor.

Extent of Secondary Reactions Wi thin the Sample '

It is obvious that in any pyrolysis apparatus there is a maximum

sample dimension above which heat and mass transfer limitations will

exert a significant influence on the observed thermal decomposition

behavior. Experiments were performed with the present equipment to

determine if the conveniently available 0.0101 cm thick samples were

small enough so that these effects would be unimportant. Runs were

carried out under the conditions specified for the multiscr~en tests,

using samples of up to four times the above thickness. The results

showed that doubling the thickness of the sample has no significant

effect on the total cellulose decomposition or total yields of tar

and volatiles. However, a four fold increase in sample thickness gave

decreased tar production and a corresponding increase in total gas yield.

The latter results may arise from enhanced secondary cracking due to

increased tar and volatiles residence time within the thicker sample

and from temperature variations due to thermal lags between the

centerline and the surface of the sample. Heat transfer calculations

showed that during heating at rates of up to 1000°C/s the temperature

of the centerline of the 0.0101 cm thick sample should in the worse

case lag its surface temperature by no more than II-30°C.

221
Based on the above measurements and calculations it was thus concluded that the 0.0101 cm thick cellulose sheets are sufficiently small

that contributions to their pyrolysis behavior from intra-sample heat

and mass transfer effects can be neglected .

RESULTS AND DISCUSSION

Effects of Reaction Conditions on Product Yields and Compositions

Temperature

Data on the effects of temperature on the yields and compositions

of char, tar and lighter volatiles from the pyrolysis of thin sheets

of cellulose of the size and composition specified above are shown in

Figures 2-6. These results were obtained in so-called peak temperature

runs in which the sample was heated under 5 psig of helium, to the

temperature indicated on the abscissa at a nominal rate of 1000°C/s, and

then immediately allowed to begin cooling at an initial rate of 200°C/s.

Figure 2 presents the results for char, tar and gas (including water)

yields, which show that for these heating rates measurable decomposition

of cellulose begins between 300 and 400°C. The extent of decomposition

increases with temperature until almost 95 wt.% of the sample is converted

to volatiles at 750°C. Most of the weight loss takes place between sao

and 700°C. Above 750°C the yield of char decreases from 6% to about

3%between 800 and 900°C. Because of cracking of volatiles which occurs

at very high temperatures, it then increases very slowly with further

temperature increases, reaching about 4% at 1000°C.

Tar yield increases with temperature to a maximum at around 700°C

and then decreases with further temperature increases undoubtedly

because cracking reactions become more favored at higher temperatures.

For significant yields of volatiles by cellulose pyrolysis under

these conditions, the sample must remain above its decomposition

temperature for a time which depends on the heating and cooling rate,

final temperature, and operating pr~ssure. For zero holding time conditions, complete reaction can be achieved at or above 750°C for a

1000°C/s heating rate. Most of the decomposition occurs during the

heatup period. When peak temperature is reached, tar which could not

escape the hot reaction zone during the heat up period could participate

in secondary cracking to yield lighter volatiles . Tar yield in fact .

reaches a maximum at lower temperatures as holding time increases.

The total gas yield, which includes water, also increases as peak

temperature increases, but at the temperature where tar yield goes

through a maximum the slope of the gas yield vs. temperature curve

increases. This is probably because the tar is cracked primarily to

gases with little if any coke being formed.

222



The influence of peak temperature on the yields of the indicated

volatile pro~ucts is shown as follows: Fig. 3: CO (23.3), CO 2 (3.2),

H20 (8.2); F1g. 4: C2H4 (2.2), C2H6 (0.25), C3H6 (0.70);

Fig. 5: H2 (1.2), CH4 (2.6); and Fig. 6: acetone + furan (0.83),

methanol (1.02), acetaldehyde (1.55). The yields of these

products increase monotonically with temperature until the asympototes

indicated here in brackets are reached at peak temperatures ranging

from ~700oC for H20, to ~800°C for CO2, C3H6, acetaldehyde, methanol,

and acetone + furan, to 900-950°C for H , CH4 and C H4. The

data on CO yields (Fig. 3) exhibit some scatter that aris~s trom

interferences from air impurity during the gas chromatographic analysis.

It is nevertheless believed that a true asymptote for these conditions

is attained at around 1000-1100°C. The yields of the light oxygenated

liquids methanol, acetaldehyde, and acetone + furan may go through a

maximum as temperature increases beyond ~800-900°C but the scatter

in the data preclude establishing this unequivocally. The existence

of such maxima would not be unreasonable since these products can

decompose at temperatures as low as 500°C.

During the early stages of the cellulose decomposition (i.e. over

the peak temperature range 300-600°C at the 1000°C/s heating rate employed), the light volatiles are dominated by H20 and some CO, C02, and

light oxygenated liquids, but above 750°C CO is by far the most abundant gaseous product. From 300 to 600°C none of the above hydrocarbons,

H2 or the oxygenated light liquids from Fig. 6 attain yields of more

than a few tenths of a percent, although the latter four compounds especially acetaldehyde are found in much larger amounts than the hydrocarbons. However above the 650-750°C temperature range where tar yield

has peaked the yields of H2, CO, CB4, C2H4, and C2H6 increase markedly

suggesting that a significant, and perhaps the dominant contribution

to these products comes from secondary decomposition of the tar rather

than from primary degradation of the parent cellulose .

Solids Residence Time

With the present apparatus, solids residence time or holding time

is the period the sample and wire screen heater are maintained at some

desired maximum temperature before initiating cooling. Volatiles

generally have a very short residence time at final t~mperature because

of the design of the reactor vessel and hot stage (Fig. 1). Figures

7 to 9 show the effects of variations in holding time over the range

o to 30s on the yields of char, tar· and gases respectively. Sample

dimensions and all other operating conditions were as specified in the

first part of this section. Below 800°C, increasing holding time

at a constant temperature, augments both tar production (Fig. 8) and

total weight loss (i.e. 100-char yield, Fig. 7). A similar effect

is exerted on gas yields below about 750°C (Fig. 9). In each case

the magnitude of the effect increases as the maximum temperature

decreases. This behavior is believed to reflect the fact that at zero

holding times and lower temperatures, decomposition of the cellulose is

incomplete at the time final temperature has been reached. Providing

226


some holding time at these te~peratures therefore allows further

decomposition and hence the augmented yields of tar, and gas and the

decline in char production. At temperatures greater than the thresholds

indicated above there is essentially no effect of holding time since

the reactions have become sufficiently rapid to give the maximum conversion realizable at that temperature even in zero holding times.

Figures 8 and 9 show that at a given holding time the temperature

at which secondary cracking begins to' dominate the global tar yield

(i.e. the temperature of maximum tar production) corresponds quite well

to that at which gas yields begin to increase substantially with temperature. These findings further support the picture that secondary

reactions of tar play an important role in gas production under these

pyrolysis conditions.

The effects of holding time on the low temperature yields of all

the individual volatile products described above except water, are

generally similar to those for the gas as a whole, i.e., increased

yield with increased holding time. Water yield reaches an asymptote

ofS.l wt.% at a temperature of 600°C for holding times of 2 to 3 sand

remains independent of temperature above 600°C. The asymptotic yields

of CO2 (3.0), C2H4 (1.9), C2H6 (0.22), C3H6 (0.68), and CH4 (2.53) at

1000°C and 2-30 s shown here in brackets as wt.% are virtually the

same as those for zero 'hol di ng times as might be expected from Fig. 9,

which shows no effect of holding time on the total gas make above 800°C.

As temperature increased for these holding times the yields of acetaldehyde and of total light oxygenated liquids increased to maximum

values of 1.8 and 4.7 wt.% respectively at around 800°C and then

decreased with further temperature increases, undoubtedly due to increased

time for secondary cracking of the ,oxyge~ates at higher temperatures.

Further details on the effects of holding time on the yields of

individual volatile products determined in this work are presented

elsewhere [4].

Heating Rate

Data on the effects of variations in heating rate over the range

100°C/s to 10,000-lS,000°C/s, on yields of char, tar and gas for th~

other reaction conditions specified in the first part of this section

are plotted in Figures 10-12 respectively. At a given peak temperature

total conversion of the sample to volatiles (100 - char yield) increases

as heating rate decreases. Similar behavior is exhibited by the yields

of gas and, below about 750°C, of tar. These effects undoubtedly

arise because at the lower heating rates more time is available for

conversion reactions during the heatup period. Tar yields approach

85 wt.% of the cellulose at heating rates of < 100°C/s and the maximum

in the tar yield vs. temperature curve disappears at heating rates of

3S0°C/s and lower. These maxima have been interpreted as reflecting

competition between: (a) escape of freshly formed tar from the elevated

229

temperature environment of the hot stage and decomposing sample, and

(b) cracking of the tar in that environment. The maxima are believed

to disappear because at the lower heating rates there is adequate time

during the heatup period for most of the tar to be formed and then

escape the immediate neighborhood of the screen before temperatures

sufficiently high for extensive cracking to occur are attained.

Data have also been obtained on the effects of heating rate

on the yields of most of the specific volatile products listed earlier

and are presented elsewhere [4]. In general at lower temperatures ·

their yields increase with decreasing heating rate.

Other Reaction Conditions

The effects of total reactor pressure and of sample thickness on

product yields and compositions have also been studied and are

presented elsewhere [4]. In brief, it was found that at a given temperature, as pressure is increased above 5 psig yields of char and most

gases increase, tar yields decrease, while acetaldehyde, methanol, and

acetone + furan behave as the gases at lower temperatures and as the

tar at higher temperatures. This behavior undoubtedly reflects

secondary cracking of tar and, at elevated temperatures, of these

oxygenated light volatiles, and also implies that at higher temperatures,

the latter compounds can contribute to the net make of hydrogen, CO,

CO2, and hydrocarbon gases. Increasing sample thickness from 100 ~rn

to 193 ~m ha¢ relatively little effect on the product spectrum, but

a further increase to 400 ~m resulted in decreased tar production and

a corresponding increase in total gas yield.

Preliminary Kinetic Modeling

The data presented above, and other results on pressure effects

reported in detail elsewhere [4], indicate that at higher temperatures

and residence times secondary reactions of tar and of light oxygenated

liquids such as acetaldehyde contribute extensively to the rates and

extents of formation of light volatiles during the rapid pyrolysis of

thin strips of cellulose. A rigorous kinetic model for the production

of individual volatile products should therefore reflect multi-step

chemical processes as well as any coupled effects of physical transport

of tar or other volatiles in the neighborhood of, or possibly within,

the decomposing sample. Results of modeling efforts to this end are

presented elsewhere [4].

A more simplified modeling approach that has proved useful in the

past for correlating similar data on the rapid pyrolysis of coal

[5,6,7] was also investigated. In the simplest version of this analysis

it is assumed that the cellulose decomposition directly yields a given

reaction product i by a single independent reaction pathway also

designated i:

Cellulose

k. ____1-4) Product i

231

(1)
and that the kinetics of this process can be modeled by a unimolecular

first order reaction having a rate constant that may be written in the

Arrhenius formalism:

dV.-

1

dt = k -E./RT

.e 1

01

* (V. -V .) 1 1 (2)

The experimental data for yields of specific products were fitted

to this model, with a non-linear least squares regression code by

integrating the non-isothermal form of Eq. (2) numerically using the

experimentally measured time-temperature histories. Best fit values

were obtained for the global kinetic parameters k . , E., and V. *. . 01 1 1 Table 2 presents the results for total weIght loss and for several

volatile products obtained under the experimental conditions given

at the bottom of the Table. Theoretical yield curves calculated

from these parameters generally gave good fits to the experimental

yield data [4]. For many of the products the values derived for the

activation energy and pre-exponential factor are reasonable for typical

organic decomposition reactions [5,8]. However it is emphasized that

these parameters should be considered only' as useful tools for correlating the experimental data for the ranges of operating conditions

under which they were measured. They should not be viewed as reflecting

the detailed chemistry of cellulose decomposition to specific volatile

products [1].

CONCLUSIONS

The captive sample reactor described above is a useful tool for

studying the separate effects of temperature,heating rate, pressure,

and solids residence time on the rapid thermal decomposition behavior

of thin samples of cellulose, wood and other forms of biomass for pressures of 1 atm and above. Excellent material balances and data

reproducibility are obtained. With the aid of suitable models kinetic

parameters for the primary decomposition of the sample and secondary

reactions of its volatiles may be derived from the integral product

yields measured with this system.

Temperature and sample residence time are the two most important

parameters in determining the rapid thermal decomposition behavior of

cellulose. Heating rate effects can be interpreted in terms of their

influence on these two reaction conditions. Thus for a fixed solids

residence time and heating rate, gas yield always increases with

increasing temperature, until some asymptote is attained, tar yield

increases to some maximum and then declines with further temperature

increases, and char yield decreases to around 3 wt.% but at high

temperatures rises slightly probably due to carbon deposition from

secondary reactions. At lower temperatures char yields decrease and

gas and tar yields increase when either solids residence time is

increased or heating rate is decreased . At higher temperatures

232
Table 2. Summary of Best-Fit Kinetic Parameters for Single

Reaction Model of Cellulose Rapid Pyrolysis l

E,kcal/g-mole -1 Product Loglok , s V*,wt.%

Total Weight Loss 31.79 8.30 94.08

CH4 60.04 13.00 2.41

C2H4 49.82 10.82 2.07

C2H6 41.55 9.06 0.26

C3H6 60.67 14.93 0.67

H2 27.29 6.17 1.16

CH30H 49.35 13.42 0.92

CH3CHO 55.1 13.56 1.54

Butene &Ethanol 42.54 9.9 0.32

Acetone &Furan 43.04 11.07 0.81

CHO (Mainly Acetic Acid) 58.18 12.8 1.19

H2O 24.62 6.71 8.04

CO 52.74 11.75 21.64

CO2 23.42 5.39 3.08

1

Data are for the temperature range 300-1000°C, a nominal heating

rate of 1000°C/s, solids residence times of 0 s and a total pressure

of 5 psig. Sample size was 6 cm x 2 em x 0.0101 cm thick.

233
however these two parameters have no significant influence on these

yields because of the dominant influence of temperature alone.

The product spectrUm from the thermal degradation of cellulose

at a 1000°C/s heating rate, with zero holding time at final temperature,

is characterized by evolution of chemical water from an initial decomposition at around 330°C, followed by small amounts of CO and CO

beginning at around 400 and 440°C respectively. Yields ot most volatile

products except tar increase monotonically as temperature increases

and generally attain asymptotic values at temperatures between 700

and 900°C.

Modest quantities of H2 ("'1 wt.%), CH4, C2H4, ~2H6' C3H6, ("'0.2 -

2.5 wt.% each) and light oxygenated liquids such as acetone/furan

mixtures, methanol, and acetaldehyde ("'0.8 - 1.5 wt. % each) are formed

primarily over the temperature range 500-800oC. Under zero residence

time conditions carbon monoxide dominates the gaseous products above

750°C and attains a yield of above 23 wt.% at 1000°C.

Most of the tar product is material condensed in the reactor at

room temperature. Based on preliminary characterization by elution

chromatography and gas chromatography [9] it is believed to contain

at least several hundred compounds. Its yield can be from 60 to 83 wt.%

of the cellulose depending on temperature and solids residence time.

At higher temperatures (400 to 700°C depending on solids residence

time) the yield of this product decreases with further temperature

increases undoubtedly due to secondary cracking.

This secondary cracking of tar is believed to be a significant

pathway for production of HZ' CO, and several light hydrocarbon gases

such as CH4 , CZH4 , and C2H6 .

Char yield decreases monotonically .with increasing temperature

under these conditions to a minimum of "'3 wt.% at temperatures of

700 and 800°C at solids residence times of 2 and 0 s respectively.

At solids residence times from 0 to 30s further temperature increases

above 800°C result in modest increases in char yield, probably as a

result of carbon deposition arising from secondary cracking of tar

and light oxygenated volatiles.

ACKNOWLEDGEMENTS

Financial support for this work is provided by the United States

Department of Energy under Grant No. DE-FG02-79ET00084 administered by

the Solar Energy Research Institute and is gratefully acknowledged.

Dr. Alex Kotch is the Technical Monitor for this project. Financial

support for the construction of the reactor facility used in these

studies, and for complementary research on biomass pyrolysis liquids

was provided by the Edith C. Blum Foundation of New York and is also

234
greatly appreciated. Important contributions to the experimental and

modeling work have been made by Dr. H. D. Franklin. and Messers. R. Caron,

J. Curme, and P. Houghton. Mr." P. Bhadha provided valuable data on

the global chemical composition of the pyrolysis tars.

NOMENCLATURE

E. the activation energy for reaction i, kcal/g-mole 1

k . the pre-exponential factor for reaction i, s-l

01

t sample thickness, rom

p total reactor pressure, psig

R the gas constant, kcal/g-mole-oK

T the absolute temperature, oK

Vi the yield of product i at time t, wt.% of the cellulose

V. * 1

the ultimate yield of product i, (i.e. at t = ~), wt.% of the

cellulose

REFERENCES

1. Lewellen, P.C., W.A. Peters, and J.B. Howard, "Cellulose Pyrolysis

Kinetics and Char Formation Mechanism", Sixteenth Symposium

(International) on Combustion, The Combustion Institute Pittsburgh,

(1977), 471.

2. Peters, W.A., "Literature Incentives for the Production of Clean

Fuels ·and Chemicals from Biomass by Thermal Processing", In-house

Literature Review, M.I.T. Energy Laboratory, June (1978).

3. Yurek, G.J., Personal Communication (1978).

4. 'Haj al i gol , M.R., "Rapid Pyrolysis of Cellulose", Sc.D. Thesis,

Department of Chemical Engineering, Massachusetts Institute of

Technology, Cambridge, Massachusetts, in preparation (1980).

5. Suuberg, E.M., W.A. Peters, and J.B. Howard, "Product Composition

and Kinetics of Lignite Pyrolysis", Ind. Eng. Chern. Process Design

Develop., 12, 37 (1978).

6. Suuberg, E..M., W.A. Peters, and J.B. Howard, "A Comparison of the

Rapid Pyrolysis of a Lignite and a Bituminous Coal", in Thermal

Hydrocarbon Chemistry, p. 239, Advances in Chemistry Series,

No. 183, A.G. Ob1ad, H.G. Davis, and R.T. Eddinger, Ed., Am. Chem.

235
Soc., Washington. D.C. (1979).

7. Suuberg, E.M., W.A. Peters, and J.B. Howard. "Product Compositions

and Formation Kinetics in Rapid Pyrolysis of Pulverized Coal -

Implications for Combustion", Seventeenth Symposium (International).

on Combustion. 117, The Combustion Institute, Pittsburgh (1979).

8. Benson. S.W .• The Foundations of Chemical Kinetics, McGraw-Hill,

New York (1960).

9. Bhadha. P.M.• "Fractionation of High Molecular Weight Biomass

Pyrolysis Tars". Unpublished 10.90 Report, Department of Chemical

Engineering, Massachusetts Institute of Technology, Cambridge,

Massachusetts (1980).

236
ABLATIVE PYROLYSIS OF MACROPARTICLES OF BIOMASS

J.P. Diebold

Solar Energy Research Institute

1617 Cole Boulevard, Golden, CO 80401

ABSTRACT

Recent research has revealed that macroparticles of biomass can be

pyrolyzed with a "solid convection" heat transfer technique to result

in surface regression rates of over 3 cm/sec, which is 3 orders of

magnitude faster than that attained with conventional gaseous and

radiative heat transfer techniques. The extremely high heat fluxes

attained by this technique pyrolyze the macroparticles of biomass

without . forming the usual layer of char (char usually insulates the

substrate to force a slow pyrolysis reaction to occur with

macroparticles). This observed laboratory phenomena is being used in

the design of a unique 100 lb/hr research reactor to fast pyrolyze 1

em wood chips in which the chips will be entrained into a reactor

which utilizes centrifugally-enhanced, solid-convection heat transfer.

INTRODUCTION

In considering a chemical process, selecting a pro.cess which makes a

high valued product rather than a mediocre valued product has an

obvious economic advantage, if all other things are equal. That is

why the conversion of biomass ' wastes to high octane gasoline rather

than a boiler fuel was selected for further study at the Naval Weapons

Center in the early 1970's. Conceptually, the process involved the

very selective fast pyrolysis of the biomass to gases containing

significant amounts of reactive hydrocarbons such as ethylene and

propylene. These gases were compressed, purified, and then under heat

and pressure the reactive hydrocarbons were reacted with themselves

without catalysts to form low molecular weight polymers. The

po Lyme r Lzat Lon reaction is self-limiting such that about 90% of .the

polymers boil in the gasoline range. With polymer gasoline made in

this manner, but from pure ethylene, the unleaded octane number was

found to be in the low 90' s , Polymer gasoline made from pyrolysis

gases had the same distillation curve as that made from pure ethylene;

indicating that the initial presence of acetylene, propylene, ~nd

butenes in the pyrolysis gases has little effect on the product. This

allows the use of a very simplified gas purification system since the

various Cz+ hydrocarbons (larger than methane) do not need to be

separated from each other. Gasoline yields of about 75 gallons per

dry, ash-free ton of biomass were projected based on values obtained

from the pyrolysis of municipal solid waste derived feedstock (ECO II

by Combustion Equipment Associates) (1).

The process is relatively energy intensive such that the byproduct

237
fuel gases, tars, and char (if any) would all be burned for process

energy. The yields of the intermediate olefins were shown to increase

wi th the amount of steam usage. This means that any improvement in

the energy efficiency of the process, which can be converted to steam

production, can be related to a small incremental increase in the

potential yield of olefins. For example, a study · of the energy

required to recirculate carrier gases revealed that carbon dioxide

required significantly more energy than the use of steam or the

byproduct fuel gases. Furthermore, unless the capital expense of a

larger hydrocarbon absorption tower, compressor, and a gas turbine

(for energy recovery) were less than for a steam boiler, steam

appeared to be a more energy efficient carrier than diluent gas. This

conclusion is reinforced by the petrochemical industry's use of steam

as the diluent gas when cracking naphtha to ethylene. Other means of

increasing the system's efficiency would, of course, include the

extensive use of heat exchangers to recover heat for steam

generation. The low temperature waste heat, such as that formed by

steam condensation at atmospheric pressure, would be used to heat air

for feedstock drying (2).

In addition to these more obvious methods of increasing the energy

efficiency of the process, the possible elimination of unnecessary,

superfluous process steps was examined and found to have real

potential. Figure 1 shows that the energy required for the conversion

of 1 em chips to 100)lm powder is a srnal L portion of the energy

contained in the feedstock. However, in a typical industrial

situation the energy put into the biomass for grinding is in the form

of mechanical energy so that the true energy penalty for grinding is 3

to 4 times as high depending on thermal to mechanical conversion

efficiencies. For 100)lm particles, this amounts to 71;2 % of the

energy contained in the feedstock or 10 to 15% of the energy available

for process energy, which is quite significant. In addition, the hot

ground feedstock would most likely be put into temporary storage where

it would be allowed to cool and thereby lose all the energy put into

it by the grinding step. Consequently, one of the valid criticisms of

state-of-the-art fast pyrolysis is that a finely powdered feedstock is

necessary to obtain the high heating rates for the particles to avoid

the undesirable side reactions of pyrolysis such as char and water

formation. A significant decrease in the process energy required

could be attained if one could use 1 cm chips rather than 100 um

powders for fast pyrolysis feedstocks.

The last effort with the OrLna Lake entrained-bed pyrolysis reactor

before the program was phased out at the Naval Weapons Center was the

pyrolysis of pure cellulose, lignin, and birch flour. Due to

mechanical difficulties with the system, only a preliminary screening

of these feedstocks was obtaf.ned , The cellulose and the birch flour

both produced about 12 to 14% by weight of the desired hydrocarbon

product (C2+). This was considerably lower than had been attained

with the ECO II material and is thought to represent suboptimal

reaction conditions. The very significant results of this work

concern the very small amount of char formed and its physical shape.

From the cellulose pyrolysis, only 0.1% of the feedstock was converted

238
FIG. 1 Biomass Grinding Power Requirements

Adapted from Malian, G.M., and Finney, C.S.,

uNew Techniques In the Pyrolysis of Solid Wastes"

AIChE 73rd National Meeting (1972)

N

W

\.0

10,000

CIJ

N

.-tn

Q)

-u

.-...

:; 1,000

a.

CD

C)

cu

..

CD

> . c(

LHV =7700 Btu.

dry Ib

(17.9 kj)

9

100 ~.-.._.--....._-.a.. ....a-_-a.-_~ '----I~----II.-..-..I_---&._-...L._--L

o 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Percent of Energy for Grinding 1/2-lnch Biomass
to char and part of that is thought to have been reactor scale. This

result was predicted by the work done at MIT (3) in which a few

milligrams of cellulose were sandwiched between electrically heated

screens. However, the China Lake work demonstrated the phenomena in a

continuous flow reactor with over 5 kilograms of cellulose involved in

the reaction. This represents a scale-up of the phenomena by 4 or 5

orders of magnitude. The birch flour pyrolysis resulted in a 1 1/2 %

conversion to char which reflects the higher ash content of the wood

flour. The calculated ash content for both the cellulose and the

birch char is very similar at about 30%. This similarity in ash

content may indicate that the very fine ash had a catalytic effect to

cause condensation reactions to occur on its surface until the

resultant tars decomposed to char to block the movement of reactants

to the catalytically active sites. Examination of the birch-flour

char confirmed that it was not typical of slow char forming

reactions. Most of this char had no structure resembling the fibrous

feedstock. Instead, the char had the appearance of being an

agglomeration of 1 ~m spheroidal particles as shown in Figures 2

. through 4 (4,5). This type of char formation is .contrary to the

classical concept of pyrolysis and char formation, in which the virgin

feedstock has a char layer through which heat must be conducted in

counter flow to the off-gassing pyrolysis products. This concept of

pyrolysis, although valid for slow pyrolysis, does not appear to be

applicable to fast pyrolysis. One of the apparent anomolies noted in

the EGO II pyrolysis work at China Lake was that the char had about

the same bulk density as the EOO II feedstock (6). This was

disturbing at the time because the slow pyrolysis concepts require

that the feedstock particle be hollowed out by the process. This

removal of 85% of the mass should have greatly reduced the bulk

density even after allowing for some physical shrinkage of the outside

dimensions. The char anomaly was not resolved until the birch flour

char was examined microscopically and found not to be composed of

skeletal remnants. The lack of char formation on the pyrolyzing

surface of the biomass has a major impact on the ability to transfer

heat into the particle.

The formation of char on the surface of slowly pyrolyzing biomass

serves to act as a thermally insulating layer. Through this layer,

the pyrolysis gases pass in counter-current flow relative to the heat

needed for pyrolysis. This transpirational cooling of the char by the

gaseous pyrolysis products (mostly steam formed by the slow

dehydration reaction) greatly reduces the heat flux and consequently

the pyrolysis rates. The rate of pyrolysis as measured by the

equilibrium rate of advance of the char-wood interface is very slow

and is about 3 1/2 em per hour with radiative and free convective

exposure to temperatures of 500 to 10000 e (ASTM-El19) (7). In

contrast to this slow rate, the pyrolysis of 250 um particles in an

estimated 50 ms residence time in the China Lake entrained flow

reactor suggests a fast pyrolysis rate in excess of 900 cm per hour or

0.25 cm per second. Although this estimated rate is very high, the

maximum rate of pyrolysis probably was not attained due to the

intermittant contact of the turbulently entrained particle with the

hot tube wall.

240
241

Fig. 2 SEM Photograph

of Birch Char (147X)

Fig. 3 SEM Photogra~

of Birch Char (853X)
Fig. 4 SEM Photograph of Birch Char (2500X)

242
The importance of physical contact of the pyrolyzing biomass particle

with the hot wall is readily apparent with the proper assumptions. If

it is assumed that the wall temperature is 18000p 0v 10000 C) , that the

surface of the pyrolyzing biomass is at 6000p (3150C), and the

absorptivity is a generous 0.85, then the radiative heat flux to the

particle is about 35,000 BTU/hrft 2 (~12 w/cm2). One concept of

pyrolysis is that the biomass decomposes first to pyrolytic oils.

With the same relative temperatures, conducting heat through a thin

10 um layer of this pyrolytic oil having an assumed conductivity of

0.08 BTU ft/ft 2hrop results in a phenomenal heat flux of nearly 3 .106

BTU/hrft 2 (1000 w/cm2). Whether or not this high heat flux to the

virgin substrate could be attained in reality depends upon how well

the assumed conditions can be met. The reactor wall temperature would

be very difficult to maintain in a steady state since petrochemical

cracking furnaces typically have heat fluxes of only 10,000 to 50,000

BTU/hrft 2 (3 to 15 w/cm2) even in the radiative sections. Without

resorting to focused light energy sources (e.g., laser or solar power

tower), high heat fluxes to the wall could be supplied by condensing

sodium vapors as in a heat pipe. At these fluxes the temperature drop

through a 0.1 inch (0.25 em) stainless steel wall is on the order of

several hundred degrees. However, if the pyrolyzing particle were to

be in a dilute entrained flow and in relative motion to the hot

reactor wall, the heat flux to the particle could be much higher than

the average heat flux through the wall. This "forced solid

convective" heat transfer would allow the wall to recover its initial

temperature between the passage of each particle. The particle must

be kept in firm contact with the wall to maintain the high conductive

heat flux. Maintaining the liquid pyrolytic oil film at a minimum

thickness requires that the particle be forced to the wall to extrude

the oil out. Relative motion between the wall and the particle will

also aid in wiping off the oil (and any char) as it is formed. The

required conditions for fast pyrolysis of macroparticles appeared to

fit an externally heated cyclone reactor as will be discussed, but the

demonstration of the concept was needed before committing the

necessary resources to build a unique reactor design.

ABLATIVE HOT-WIRE PYROLYSIS

The demonstration of the physical rate at which pyrolysis proceeds is

very difficult in an entrained flow reactor. However, a few

conceptual changes to the discussion above allows for a very easily

constructed qualitative experimental apparatus. If the biomass

macroparticle is held stationary while the hot stainless steel surface

slides past , the physical rate of pyrolysis could be measured. Since

the biomass is rapidly offgassing, air will have a very difficult time

to diffuse into the pyrolysis site. If the quantity of pyrolysis

produc ts formed is kept small, they will cool before they have mixed

with sufficient air to ignite.

To demonstrate the fast pyrolysis of macroparticles (e.g., tongue

depressors) a hacksaw frame was fitted with a 0.010 inch (254 um)

diameter nichrome wire. The wire was electrically insulated from the

frame and attached to a transformer to provide about 20 volts. With

243
this simple arrangement, one can "saw" through biomass as though it

were a thermoplastic like polyethylene or polystryene. The sawing

action produces smoke which normally does not ignite. The width of

the cut is slightly less than the width of the cutting wire. If the

"sawing" action is started without the wire being hot, only a

negligible cut can be made. Turning on the electricity to the wire

results in the saw suddenly having a lubricated feeling as the biomass

pyrolyzes to form the oils and vapors. It is not definitely known at

this time if the biomass forms only liquids which then vaporize or if

some gases are formed concurrently with the liquid. However, the

formation of gases between the biomass and the hot steel surface would

drastically reduce the heat transfer rates. Microscopic examination

of the best "cut" (pyrolyzed) surfaces reveal no char present and that

the cell structure can be clearly seen through a light-colored tar

layer which appears as a coat of varnish. The hypothesis that wood

pyrolyzes first to tar with very little concurrent gas formation is

supported by the lack of a foamed appearance on the "cut" surface.

Thin pieces of biomass cut fastest with the least evidence of char

formation due to the wire cooling too much with the thicker

sections. The red hot wire is visibly cooled and darkened as it

passes through even thin biomass such as paper or wood veneers.

A more sophisticated experimental apparatus was constructed to obtain

quantitative data for the hot-wire-fast-pyrolysis phenomena. A small

DC motor was obtained and fitted with a double spool so that while the

nichrome wire was winding off one side of the spool, the other side

could be winding it back up. The wire passes over an electrically

conductive graphite pulley to the pyrolysis zone and then to another

graphite pulley. A microswitch was placed so that the deflection of

the hot wire could signal the start and end of the cut. This elapsed

time divided by the width of the biomass gives the physical rate of

pyrolysis. The wire also passes around an odometer wheel which trips

a microswitch for every revolution. By counting the blips on a

recorder, wire speed is ascertained. The pressure at the wire-biomass

interface is calculated from the biomass weight, the thickness of the

biomass and the wire. Measurement of the temperature of the wire as

it enters and exits the biomass would provide indications of heat of

reaction, heat transfer rates, and pyrolysis temperatures. It appears

that optical pyrometry is the most promising approach to the wire

temperature measurements. Automatic commercial optical pyrometers

unfortunately are made to look at larger "targets" than the thin

wire. Manually operated "disappearing filament" optical pyrometers

are impossible to operate accurately in the second or so available to

take the measurement. The little known technique of using infrared

sensitive film to record temperatures appears to be very promising for

the hot wire system, but technical difficulties and priorities have

thus far precluded obtaining quantitative wire temperature data.

However, with wire temperatures estimated to be at 1500 to 18000F (800

to 10000C), hand-held hardwood tongue depressors have been cut at

rates of 1 em/sec and 3 em/sec at wire speeds of 10 and 20 em/sec

respectively. With thin birch veneer, cutting rates of 3 1/2 em/sec

were attained with wire speeds of about 10 em/sec. Preliminary

transient heat transfer calculations suggest the biomass is heated

244
only to a depth of about 20 11m, which implies a heating rate of about

5 .105 °C/sec.

Other means of attaining ablative pyrolysis which were demonstrated

included a "friction saw" made of a smooth disk of stainless steel

fitted to a hand-held rotary saw. A relief was machined into the disk

to minimize the rubbing contact of the sides of the disk on the walls

of the cut. The friction of the circumference of the disk against the

wood caused it to locally heat enough to cause pyrolysis. The

charless cutting front passed through the wood fast enough to avoid

charring the wall of the cut. The resulting cut surface was very

smooth and had a polished appearance. The use of a friction saw to

cut aluminum and stainless steel has been known for quite some time,

but its usefulness to cutting wood is a newly recognized phenomena.

A hand-held tesla coil used for checking out electronics was also used

to demonstrate fast pyrolysis. With very short duration discharges

through .a wooden tongue depressor, very small, crooked, charless holes

were formed initially. Longer duration discharges resulted in larger

diameter holes which had charred walls and through which light could

pass.

Although much more data need to be generated with the hot wire

apparatus, the tentative conclusions are that: biomass can indeed be

pyrolyzed without the . formation of an insulative char layer and that

pyrolysis rates, which are 3 V2 orders of magnitude greater than slow

pyrolysis rates, can be achieved with the proper combination of steel

surface temperatures, relative motion, and pressure. Due to all of

the implications that fast 'pyr ol y s i s of rnacroparticles was ·i nde e d

demonstrated in the laboratory, an engineering demonstration of the

concept was given top priority.

ENGINEERING DEMONSTRATION OF ABLATIVE FAST PYROLYSIS OF MACROPARTICLES

Initial concepts for the engineering demonstration of the

macroparticle pyrolysis involved the use of 1 cm wood chips entrained

by a supersonic steam flow into a toroidally shaped reactor. The

chips would be centrifuged to the outside wall of the toroid where

they would be pyrolyzed. However, it was soon realized that thLs

design would use only a small portion of the toroidal surface for heat

transfer, i.e., the major diameter of the toroid • . Subsequently, the

tangential entry for the feedstock was changed from a simple angle to

a compound angle which would cause the particles to travel a helical

path as they traveled around the inside of the toroid. The particles

would then extract heat from the entire inside surface of the

toroid. The relative diameters of the toroid under consideration were

such that the particle was being subjected to extremely high "g"

forces due to the helical motion in the toroidal tube and relatively

low "g" forces due to the toroid itself. After comparing ease of

construction, it was ·de c i de d to straighten out the toroid to form a

simple cyclone reactor.

The cyclone reactor design is conceptually' very simple, as shown in

245
Figure 5. The biomass, as large as 1 cm particles, are accurately

metered to the steam .e j e c t or with a gravimetric weigh feeder. This

screw feeder is on an electronic scale which constantly adjusts a

volumetric feeder to deliver a desired mass rate of a nominal 100

lbs/hr (45 kg/hr). A twin screw in the feeder will be used with

finely powdered feedstock, whereas a 45 mm single screw will be used

with wood chips. The steam ejector uses 100 lbs/hr of superheated

steam at 200 psig (1450 kPa) and 14000F (7600C). The steam serves to

entrain the particles into the 6 inch (15 em) diameter by 28 inch (71

em) cyclone reactor at speeds up to 375 ft/sec (114 m/s), as well as

to dilute the gases and thereby reduce the partial pressure of the

pyrolysis products. If 1 cm cubes of biomass were to be fed at 100

lbs/hr (45.4 kg/hr) there would be 30 particles per second spaced an

average of 10 ft (3 m) apart in the steam ejector stream. In the

cyclone reactor, the biomass particles initially experience 19,000

I g ' S" to force them to the red hot wall, as shown in Figure 6. This

produces a force of about 30 lbs (140 N) under a 1 em cube of biomass

or about 140 psia (1000 kPa) between the wall and the cube. If the

tar -is wiped off the biomass as fast as it is formed, the tar will be

under the fast moving 1 em chip for only 0.1 ms , Three electric

heater systems are to provide the energy needed for pyrolysis. These

heater systems will be independently controlled and the energy

consumption independently monitored. Although the solid biomass

particles are trapped in the cyclone reactor for an estimated 300 ms

until the 1 cm particle is vaporized, the gases and vapors are free to

leave through the exit located on the center line of the cyclone. The

average gaseous residence time in the cyclone is estimated to be

between 75 and 175 ms , A purged three inch (7 1/2 em) diameter quartz

sight glass will be located near the tangential entry so that the

pyrolyzing particles may be photographed with SERI's 10,000 frame per

second movie camera.

Downstream of the cyclone reactor is a 2 inch (5 em) diameter vapor

cracker reactor which is 17 feet (5 m) long. The gases and vapors

will have an additional 70 to 150 ms residence time in the vapor

cracker. The vapor cracker will be electrically heated by six

independently controlled and monitored heaters. Due to the inability

of large solids to escape the cyclone reactor, all tar vapors will

have a chance to be cracked, whereas in a purely entrained-flow

situation the larger particles are still producing tar vapors at or

near the exit of the system.

Gas samples may be withdrawn from three locations in the cyclone

pyrolysis reactor and six locations in the vapor cracker. This

reactor system will be able to generate data related to the energy

requirements at the various stages of pyrolysis as well as determining

the intermediate product aiitribution of the gases at various

residence times and temperature histories (cracking severities). The

independently controlled heater sections will allow a variety of

temperature profiles to be forced onto the cyclone reactor and the

vapor cracker. The capabilities of this reactor system are very

similar to that used by the Europeans to study hydrocarbon cracking to

produce olefins (9). The use of Incoloy 800R for the cyclone reactor

246
FIG. 5 Ablative Macroparticle Pyrolysis

Flow Schematic

Clean Pyrolysis ....... .--.=

Gases

Steam

Biomass ·

Chips

Steam

Ejector

...

Q)

.Q

.c

:::s

...

u

tn

"'C

Q)

u

as

I:L

t
FIG. 6 Macroparticle Pyrolysis Concept

19000 g's

( 30 Ibs. force; 140 psi)

Pyrolyzing

Tar Vapors

1.4 in/sec

(

) )

) \

1 em Cube ----t.~ 375 ft/sec

Liquid Tar Layer
and the vapor cracker reflects naphtha cracking state-of-the-art

(10). The use of the gravimetric weight feeder system will allow the

instantaneous comparison of biomass flow into the reactor to gas flow

out of the system and provide several data points per experimental

run. The capabilities of, this new reactor system are . relatively

sophisticated compared to previous reactors.

Downstream of the vapor cracker, the pyrolysis stream will be quenched

and cooled to slightly below the steam dew point in a cyclonic

scrubber using high pressure water sprays. The small amount of

anticipated char, tar, and ash will accumulate primarily in this

cyclonic spray system. A small amount of char-tar-ash slurry will be

constantly slipstreamed to the flare for incineration. The cooled

gaseous stream will then go to an irrigated packed scrubber where

steam will condense and help to coalesce the anticipated entrained tar

mists. The cooled, clean gases will then pass to an orifice meter

before being flared off. The entire pyrolysis reactor system will be

operated at only slightly above atmospheric pressure to maximize the

olefin formation in the gases. Conversion of these gases into

gasoline, alcohols, or · petrochemical feedstocks will be addressed in

future programs.

The major components for this 100 lb/hr macroparticle engineering

demonstration have been ordered with deliveries to begin in midOctober 1980. The equipment will be assembled next to SERI's oxygen

gasifier unit with a probable completion date of late December 1980.

Shakedown and experimentation will commence in January 1981.

CONCLUSIONS

New insights into biomass fast pyrolysis have revealed that char

formation can be a secondary rather than a primary reaction, so that

virtually no char is formed with the proper heat flux and primary tar

removal. This hypothesis was shown to be valid when using a red-hot

wire drawn across macroparticles of biomass such as wood veneer and

tongue depressors. Pyrolysis rates 3 1/2 orders of magnitude greater

than slow pyrolysis were demonstrated. Because the heating is

localized, only the pyrolyzing surface is heated so that the size of

the particles is of very little importance. Additional data will be

generated with the hot wire apparatus to look at wire temperature,

speed, and pressure relationships.

A practical 100 lb/hr (45 kg/hr) reactor to utilize these insights

into fast pyrolysis has been designed and is being procured. This

engineering demonstration reactor will be able to pyrolyze particles

up to 1 cm in diameter and is the smallest size reactor practical for

these large particles. The system will be operating during FY81 to

demonstrate the ablative fast pyrolysis of biomass macroparticles to

produce gases rich in olefins for gasoline, alcohol, or petrochemical

production.

249
ACKNOWLEDGEMENTS

The close interaction with Dr. T.B. Reed in the development of the hot

wire pyrolysis concepts is acknowledged and was much appreciated. The

use of the friction saw for wood cut t Lng was conceived and

demonstrated by Dr. Reed. Mr. Michael Soltys suggested the use of the

tesla coil for fast pyrolysis. The SEM photos were taken by Mr.

Rowland McNeil of the Naval Weapons Center, China Lake.

REFERENCES

1. Diebold, J.P. and G.D. Smith "Noncatalytic Conversion of Biomass

to Gasoline", ASME sponsored Solar Energy Conference, San Diego,

CA, March 12-15, 1979., 79-Sol-29.

2. Diebold, J. and G. Smith "Commercialization Potential of the

China Lake Trash-to-Gasoline Process", O1apter 11 in Design and

Management for Resource Recovery, Volume 1, Energy from Waste,

edited by T.C. Frankiewicz. Ann Arbor Science Publishers.

(1980) •

3. Llewellen, P.C., W.A. Peters, and J.B. Howard "Cellulose

Pyrolysis Kinetics and Char Formation Mechanisms", Fire and

Explosion Research: Sixteenth Symposium on Combustion. The

Combustion Institute, pp. 1471-80, 1976.

4. Diebold, J.P. "Preliminary Results in the Fast Pyrolysis of

Biomass to Lower Olefins", ACS Division of Petroleum Chemistry

Sponsored Symposium on Alternate Feedstocks for Petrochemicals,

Las Vegas, NV, August 24-29, 1980.

5. Diebold, J.P. "Research into the Pyrolysis of Pure cellulose,

Lignin and Birch Wood Flour in the China Lake Entrained-Flow

Reactor", SERI/TR-332-586., June 1980.

6. Diebold, J.P., G.D. Smith, and C.B. Benham "Conversion of

Organic Wastes to Unleaded, High Octane Gasoline", Final Report

to the Environmental Protection Agency, contract EPA-IAG-06-0781,

September 1979.

7 • Schaffer, E.L.

Temperature Rise

High-Temperature

(1965).

"An Approach to the Mathematical Prediction of

Within a Semi-Infinite Wood Slab Subjected to

Conditions", Pyrodynamics, Vol. 2, pp. 117-132 ,

8. Diebold, J.P., C.B. Benham, and G.D. Smith "R&D in the

Conversion of Solid Organic Wastes to High Octane Gasoline"

presented to the American Institue of Aeronautics and

Astronautics sponsored Symposium on Alternate Fuels Resources,

Santa Maria, CA. March 25-27, 1976. AIAA Monograph Volume 20.

9. Van Damme, P., S. Narayanan, and G.F. Froment, "Thermal Cracking

of Propane and Propane-Propylene Mixtures: Pilot Plant Versus

Industrial Data", AI01e J., 21, 1065 (1975).

250
10. Chambers, L.E. and W. s. Potter "Design Ethylene Furnaces: Part

2: Maximum Olefins", Hydrocarbon Processing, March 1974, 95-100.

251
FLUIDIZED BED PYROLYSIS

TO GASES CONTAINING OLEFINS

J. L. Kuester

College of Engineering and Applied Sciences

Arizona State University

Tempe, Arizona 85281

ABSTRACT

Recent gasification data is presented for a system designed to produce liquid hydrocarbon fuel from various biomass feedstocks. The factors

under investigation were feedstock type, fluidizing gas type, residence

time, temperature and catalyst usage. The response was gas phase

composition. A fluidized bed.system was utilized with a separate regenerator-combustor. An olefin content as high as 39 mole %was

achieved. Hydrogen/carbon monoxide ratios were easily manipulated via

steam addition over a broad range with an autocatalytic effect apparent for most feedstocks.

INTRODUCTION

A process to convert biomass materials to quality liquid hydrocarbon fuels has been under development at Arizona State University since 1975.

An indirect liquefaction approach is utilized, i.e., gasification followed by catalytic liquid fuels synthesis. The advanta§6 of indirect liquefaction (vs. direct) is minimization of oxygenated compounds in the liquid hydrocarbon fuel product. The use of catalysts in the

liquid fuels synthesis results in very mild processing conditions,

i.e., low pressures, temperatures and residence times.

The potential products from the system are indicated on Figure 1. The

medium Btu pyrolysis gas (500 + Btu/SCF) conceivably could be used as a

fuel gas. The project objective however has always been to tailor the

gas composition with respect to carbon monoxide, hydrogen and olefins

for use as a synthesis gas for the liquid fuels system. The first

reactor in the liquid fuels system contains a Fischer-Tropsch type catalyst. The condensible hydrocarbon phase is a narrow range light

paraffinic fuel (Cs - Cl ) which can be readily tailored to match

diesel, kerosene or jet ruels. If a high octane gasoline is desired,

a conventional catalytic reforming step is used to achieve the desired

effect. '

253

A secondary condensible phase from the Fischer-Tropsch step is essentially a binary of normal-propanol and water. The off gases from the

liquid fuels reactors are heavy in the low molecularwe;ght paraffins (C, - C5) and thus are of high heating value. It is anticipated that

these would be recycled back to the gasification system.

The research scale process has a capacity of about 25 lbs/hr of feedstock. A flow diagram is shown in Figure 2. A fluidized bed with

separate regenerator is employed for the pyrolysis step. The heat

transfer medium can either be. catalytic or inert. The system (both

functionally and operationally) is a direct analogy to a catalytic cracker in a petroleum refinery which has been successfully employed

since the 19401s. A fluidized bed is also used for the Fischer-Tropsch step (to control the temperature). The reformer is a fixed catalytic bed. Equipment and procedure development have accompanied factor

studies for the chemical reactor systems. Previous experimental results for the process h~ve been reported in several publications [1-5J. These incl~de a 2 factorial design (temperature, feed rate)

for pyrolysis, a 2 central composite design (CO, H2, C?H4 feed compo- sition) and temperature, pressure, catalyst loading stu~ies for the

Fischer-Tropsch system and a 23 central composite design (temperature,

pressure, feedrate) for the catalytic reformer. In each case, the

responses were the product yields and composition. Additional physical

properties were reported (octane number, cetane number, heating values,

specific gravity, etc.). In this paper the following additional studies will be .presented:

1) gasification data for alternative feedstocks,

2) temperature effects on gasification performance, 3) steam and residence time effects on gasification performance, and

4) water gas shift catalyst effects.

FEEDSTOCK STUDY

A large number of feedstocks were investigated through the gasification

step. (Table 1). The industrial wastes refer to by-products of indus- trial processes. The forest residues' tested are all cut by the U. S.

Forest Service in the southwest United States for water conservation

purposes and burned in the field. Environmental pressures will preclude burning in the future. Eco-Fuel II is a preprocessed municipal

refuse. Almond prunings are cut and burned in the almond orchards and

thus represent an environmental problem. Russian thistle (tumbleweeds),

raw guayu1e, water hyacinth and peat represent materials that might be

harvested deliberately for energy production purposes. Some of the

feedstocks were tested at the initiative of the Principal Investigator

while others were at the request of the industrial concerns, government agencies and other University lpboratories. Elemental and ash analysis for some of the feedstocks are listed in Tables 2 and 3. As indicated,

the carbon, hydrogen, oxygen and nitrogen compositions are similar for

the biomass materials. The sulfur content is very low except for pre- processed municipal refuse (Eco-Fuel II). The ash content does vary

significantly for the materials, ranging from negligable for the synthetic polymers to over 15% for a few materials (e.g., Eco-Fuel II,

Russian thistle, water hyacinth).

255

Industrial Wastes

Sawdust

firbark

guayule bagasse

guayule cork

jojoba meal

Forest Residues

creosote bush

sugar sumac

Arizona cypress pringle manzanita

Wright silktassel

pointleaf manzanita

Urban Wastes

Eco-Fuel II

Agricultural Wastes

almond prunings

Energy Crops

Russian thistle

raw guayule water hyacinth

peat

Table 1

FEEDSTOCKS

257

almond hull s

almond shells

paper chips

polyethylene

polypropylene

shrub live oak

hairy mountain mahogany Utah juniper pinion pine

mesquite
Table 2

FEEDSTOCK ANALYSIS

(WI %)

% Loss

Sample r,1arking %Nitrogen %Sulfur %Ash On Ignition

Guayule - Cork 0.91 0.34 3.53 96.47

Guayule - Bagasse 0.66 0.11 3.27 96.73

Guayul e - Raw 0.81 0.18 5.14 94.86

Jojoba Meal 3.94 0.36 3.04 96.96

Water Hyacinth 1.87 0.53 18.97 81.03

Almond - Hulls 0.88 0.08 5.91 94.09

Almond - Shells 0.68 0.03 8.75 91.25

Almond - Cured Prunings 1.01 0.21 25.44 75.46

Eco II - Brockton 0.50 1.44 24.41 75.59

Sawdust 0.28 0.12 7.03 92.97

Paper Ch ips 0.13 0.08 0.58 99. 42

Russian Thistle 1.33 0.19 15.45 84.55

Peat 0.97 0.15 7.63 92.37

Polyethylene 0.09 0.17 0.04 99.96

Polypropylene 0.13 0.03 0.03 99.97

258
Table 3

FEEDSTOCK ANALYSIS

(WT%)

Paper Guayule

Eca Fuel II Chips Bagasse

(Brockton)

C 38.0 41.7 40.2

H 4.9 5.7 4.7

0 31.4 52.1 48.4

Table 4

PYROLYSIS OPERATING CONDITIONS

Temperature, of ••••••••••••• 1200-1700

Pressure, psig ••••••••••••••0-1

Residence Time, Sec ••

Feed Rate, lbs/hr •.

Heat Transfer Media • •

259

• • 3-6

.••• 2-15

. • • sand
Gasification operating conditions for the feedstock survey runs are listed in Table 4. The runs were performed over a period of several months and some equipment modifications were implemented (e.g. t feeder

modification). Pyrolysis gas composition data is shown in Tables 5-7.

Gas phase yield measurements varied from 50-97% but were subject to

some experimental error due to lack of a continuous feedback measurement from the solids feeder and occasional coating of the venturi gas flow meter.

The operating conditions for the gasification data should not be considered optimal but are representative of the state-of-the-art of the

system at the time the runs were perfor.med. Thus for a given feed- stock t improvements in performance are anticipated.

All the cellulosic feedstocks yield a gas with a heating value of about-500 Btu/SCF. The gas from the synthetic polymers has a much

higher heating value due to the absence of oxygenated compounds. - The

gas composition results are masked somewhat by the variation in opera- ting conditions for the different runs. However t several conclusions

can be drawn:

1) The more cellulosic type feedstocks yield the lowest total olefin

content (generally in the 5-15 mole %range).

2) Materials containing hydrocarbon materials (e.g. t oilst latex,

synthetic polymers) result in total olefin yields in the 10-25

mole %range.

3) Pure synthetic carbon chain polymers result in total olefin yields of over 30 mole %. 4) Hydrogen/carbon monoxide mole ratios of 0.25 to 0.95 are encountered for dry feedstocks without steam addition (excluding the synthetic polymers). 5) H?S was not detected for any feedstock. This includes high sulfur

materials such as Eco-Fuel II. This is of significance with

regard to potential effects on catalyst activity downstream.

Previous studies [1-5] on the system have indicated that an optimal pyrolysis gas composition for maximizing liquid hydrocarbon fuel

yields is 20 mole %+ olefins and a H2/CO ratio of 1-1.5. Selected

feedstocks are capable of producing tne desired amounts of olefins

(e.g. t guayule cork). Without steam addition t all the materials

(except the synthetic polymers) result in a suboptimal H2/CO mole

ratio.

TEMPERATURE STUDY

Gas phase composition results for paper chip feedstock are shown in

Figure 3. Only the major components are included in the analysis

(H 2t COt C H t C H t CH t CO). Recycle pyrolysis gas was used for fluidizati6n~ T~e6feed~tock2was dry and steam was not fed to the system. Howevert the recycle gas is saturated with water after passing through the wet scrubbing system. The results shown in Figure 3 indicate an apparent water gas shift effect with increasing tempera- ture with a corresponding decline in paraffin and olefin production. When considering that gas phase yields increase with increasing

260
Table 5.

PYROLYSIS GAS COMPOSITION (mole %) 1

Almond Almond Almond Poly- Polypro- Paper

Peat2 Feedstock: Hulls Shell s Prunings ethylene pylene Chips

H2 28.08 26.03 25.70 14.19 13.57 14.77 45.05

O2 0.00 0.00 O.16 0.00 0.00 0.23 0.00

CO 35.44 38.06 42.68 0.96 0.69 58.86 18.48

CO 2 13.92 12.15 5.97 0.23 0.00 3.27 16.29

H2S 0.00 0.00 0.00 0.00 0.00 0.00 0.00

CH4 14.96 17.21 14.80 43.56 42.43 14.76 10.69

N C2H2 0.05 0.06 0.17 0.61 1. 18 0.10 0.12 0'

C2H4 4.01 3.09 5.68 19.29 13.34 3.70 4.15

C2H6 1.29 1.72 1.05 6.78 6.13 2.26 1.88

C3 olefins 1. 23 0.54 0.21 5.30 9.77 1. 21 1. 21

C3H8 0.03 0.06 0.00 0.00 0.00 0.08 0.09

C4 olefins 0.12 0.10 0.08 0.59 3.64 0.18 0.19

C4HlO 0.00 0.00 0.01 0.02 0.06 0.00 0.01

C5H12 0.01 0.01 0.00 0.00 0.00 0.00 0.02

C5 + olefins 0.86 0.97 3.41 7.49 9.20 0.57 1.83

total unsaturated 6.27 4.76 9.55 33.28 37.13 5.76 7.38

H2/CO ratio 0.79 0.68 0.60 14.78 19.67 0.25 2.44

1 - water. nitrogen free basis

2 - Steam fluidization with recycle pyrolysis gas to sparges (other runs with tota 1 recycle gas)
Table 6.

PYROLYSIS GAS COMPOSITION (niole %)1

SU9 a r Raw Guayule Guayule Russian Water Jojoba F~ cds t ock : Fi r Bark Sumac Cork !!!!ill! Hyacinth ...!1lli... Eco-Fuel II --- "

2 16.58 2B.89 17. 28 25.02 20. 57 26.37 23.00 15.1 3 11.96 15.74

O2 0.1 0 0. 00 0. 00 0.07 0.17 0.00 0.00 0.42 0.41 0.05

CO 53.42 31. 88· 34.98 39. 61 22. 14 36.08 42.4 3 55. 57 37.56 50.40

CO2 2.99 10.57 8.51 6.11 3.77 14.62 13.94 5.31 10.32 3. 20

"25 0.00 0. 00 0. 00 0.00 0. 00 0.00 0.00 0.00 0. 00 0.00

CII4 18.07 15. 16 26.17 15. 36 26.03 16. 23 14. 34 16. 37 23.2 1 15.03

N C2

1lZ 0. 05 0.01 0.04 0.21 0. 05 0.00 0.10 0.00 0.00

0\ C

2

1l4 5. 71 5.75 5. 57 7. 14 14. 80 3. 21 3. 52 2.63 9. 15 6. 04 N

C21lG 1. 60 2.8 5 2. 31 0.63 4.79 .J . 69 1.62 2. 36 3. 44 3.60

C3 nlefins 0.37 1.8 1 1. 50 o.ee 3.04 0.61 0. 57 1.13 2.01 2. 03

C3"11 0.01 0. 41 0.05 0.13 0.16 0.02 0.02 0.09 0.03 0.10

C4 Illefins 0.1 8 0. 65 0. 56 0.0 6 0. 83 0.14 0.13 0.20 0. 45 0.51

( 4" 10 0.02 0.08 0.01 0.00 0.08 0.01 0.00 0.01 0.01 0.11

C511 ' 2 0. 00 0.00 0. 04 0.00 0.00 0.01 0.08 0.00 0.80 0.00

C

5 I ol r-f t ns 0.9 1 2. 20 2. 97 5.64 . 3.58 1.02 0.35 0.67 1.78 3. 13

t ot.il uflsa lura le d 7. 22 10.42 10. 64 13.05 22. 30 4.98 4.57 4.73 13. 39 11. 71

112' CO r.llIo 0. 31 0.9 1 0.4 9 0.63 0.93 0. 73 0.54 0. 27 0. 32 0. 31

1 - wat 111 t rnqen free bas i s
Table 7.

PYROLYSIS GAS COMPOSITION (mole %) 1

Hairy

Arizona Pringle Creosote Pinion Wright Utah Pointleaf Shrub Mountain

Feedstock: Cypress -!i!!L Silktasse1 Juniper Manzanita Maho9any

u 26.64 24.99 25.99 25.82 25.64 28.83 24.46 27.99 27.61 33.01 tO

2 0.07 0.04 0.03 0.44 0.04 0.05 0.34 0.07 0.04 0.06

CO 38.40 40.68 39.43 41.78 38.69 39.54 35.50 41.28 37.84 44.35

CO2 7.04 6.76 7.70 4.39 5.43 6.41 10.58 4.53 4.76 5.10

H2S 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

CH4 15.82 15.10 15.61 15.68 16.56 16.20 14.0B 16.86 15.32 12.11

N

0\ C

2H2 0.01 0.11 0.02 0.14 0.17 0.11 0.15 0.11 0.30 0.03 LU

C

2H4 6.40 6.29 6,48 6.33 7.30 6.56 5.64 5.56 10.26 2.61

C

2H6 1.65 1.29 0.93 1.87 1.68 1.60 2.00 I. 12 1.53 0.18

C

3 olefins 0.65 0.44 0.45 0.28 0.53 0.43 1.21 0.15 0.67 0.00

C3HB 0.02 0.01 0.05 0.01 0.05 0.02 O.OB 0.01 0.04 0.17

C4 olefins 0.21 0.10 0.18 0.08 0.15 0.12 0.29 0.09 0.16 0.01

C4"1O 0.03 0.02 0.02 0.01 0.03 0.04 0.12 0.01 0.02 0.00

C5H I2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 O~OO 0.00 0.00

C5 + olefins 3.07 4.17 3.11 3.17 3.74 0.11 5.56 2.22 1.44 2.36

total unsaturated 10.34 11. 11 10.24 10.00 11.B9 7.33 12.85 8.13 12.83 5.01

HZ/CO ratio 0.69 0.61 0.66 0.62 0.66 0.73 0.69 0.68 0.73 0.74

I - water, nitrogen free basis

temperature and that an optimal balance of carbon monoxide, hydrogen and olefin production is desired, it is apparent that temperature alone is insufficient as a control variable in the gasification system.

FLUIDIZING GAS, RESIDENCE TIME, CATALYST STUDY

It is difficult to isolate the effects of fluidizing gas type and composition and residence time in a fluidized bed system. Thus for a

given reactor size, lower residence times are achieved by increasing the gas throughput. This is constrained by proper particle size to

prevent excessive solids carryover. Experimentation in this area consisted of steam addition to the system with fixed reactor dimensions

and particle size. Results for various feedstocks are shown in

Tables 8-10. The steam was superheated to approximately 600°F via a

coil in the gasification system combustor. With this system, the

reactor temperature was lowered with a significant amount of steam

addition to the pyro1yzer. For fir bark and guayu1e cork feedstock

(Tables 8, 9), an increase in unsaturated compounds and H2/CO ratio

were observed with steam addition. An autocatalytic water gas shift

reaction effect is apparent in each case. This could be due to the

presence of mixed metal oxides in the ash content of the biomass (similar to commercial water gas shift catalyst composition). Eco-Fue1 II

feedstock did not exhibit as significant a water gas shift effect

(Table 10). However, the H2/CO ratio was increased by a factor of 7

when a section of the reactor overhead line was packed with a commercial water gas shift catalyst. In general, it appears that the H2/CO ratio can be manipulated with steam addition for virtually any feedstock candidate. If an autocatalytic effect is not present, a simple fixed bed shift catalyst section will achieve the desired result.

Olefin production appears more complicated. The most dramatic effect

occurred with guayu1e cork feedstock with an increase from 22 to 39%

with steam addition. This however corresponded to a substantia~

reduction in residence time (8 to 1 seconds) and may also be feedstock

dependent. Current research is aimed at lowering the residence time .to less than one second via a combination of steam addition and

recycle off gas from the liquefaction system. The liquefaction off gas normally would be depleted of hydrogen and olefins and thus use of

this gas would avoid an effective increase in residence time with respect to these components as is the case when recycling pyrolysis gas.

SUMMARY, CONCLUSIONS, CONTINUING RESEARCH

The particular experiments emphasized in this paper lead to the follow- ing conclusions:

1) A wide variety of feedstocks can be processed through the gasifica- tion system to a gas with a heating value of 500 + Btu/SCF.

2) Some feedstocks are more attractive than others with regard to pro- ducing a high olefin content. This appears to be related to hydrocarbon content of the material.

3) The H2/CO ratio can be manipulated over a wide range in the gasi- ficatlon system with steam addition. Some feedstocks require the

aid of a water gas shift catalyst while others appear to exhibit

265
TabJ e 8.

STEfu~EFFECT - FIR "BARK ,FEEDSTOCK

Operating Condjtigns:

temperature, of

pressure, psig residence time, sec.

feed rate, lbs/hr

heat transfer media

fluidizing gas

Pyrolysis Gas Composition

H

2

O2

CO

CO2

H2S

CH4

C2H2

C

2H4

C2H6

C3 olefins

C3H8

C4 olefins

C

4H10

CSH12

Cs + olefins

total unsaturated

H2/CO ratio

No Steam

1400

0.9

4

5

sand

pyrolysis gas

(mole %) 1:

16.S8

0.10

53.42

2.99

0.00

18.07

0.05

5.71

1.60

0.37

0.01

0.18

0.00

0.00

0.91

7.13

0.31

Partial Steam

1250

0.9

3.2

5

sand

pyrolysis gas + steam

29.24

0.14

44.41

3.33

0.00

10.60

0.03

7.30

0.54

1.93

0.06

0.46

0.00

0.00

1.89

11.58

0.66

1 - water, nitrogen free basis

266
Table 9.

STEAM-RESIDENCE TIME-RECYCLE EFFECTS

(Guayule Cork Feedstock)

No Steam

Operating Conditions:

temperature, of

pressure, psig

residence time, sec

feed rate, 1bs/hr.

heat transfer media

fluidizing gas

Pyrolysis Gas Composition (mole %)1;

H

2

O2

CO

CO2

H

2S

CH4

C2H2

C2H4

C2H6

C3 olefins

C3H8

C4 olefins

C4H10

C5H12

C5 + olefins

total unsaturated

H2/CO ratio

- water, nitrogen free basis

1300

0.9

8

5

70 mesh sand

recycle pyrolysis

gas

20.57

0.17

22.14 .

3.77

0.00

26.03

- 0.05

14.80

4.79

3.04

0.16

0.83

0.08

0.00

3.58

22.30

0.93

1200

0.9

1

5

70 mesh sand

recycle pyrolysis

gas + steam

24.51

0.47

10.87

10.76

0.00

10.95

0.04

16.67

3.24

6.49

0.25

2.65

0.21

0.00

12.88

38.73

2.25

267
Table 10..

STEAM EFFECT - ECO-FUEL II FEEDSTOCK

Steam + Water-Gas

No Steam -lliam Shift Catalyst

Operating Conditions'

temperature, OF 1250 1180 1260

pressure, psig 0.9 0.9 1.5

residence time, sec. 4 5 5

feed rate, lbs/hr 4 4 8

heat transfer media sand sand sand

fluidizing gas pyrolysis pyrolysis gas pyrolysis gas + steam + steam

Pyrolysis Gas Composition (mole %)1:

H2 16.81 23.62 42.59

O2 0.09 0.08 0.00

CO 50.62 45.24 17.18

CO2 2.30 3.74 12.52

H2S 0.00 0.00 0.00

CH4 15.92 11.34 16.45

C2HZ O.OZ 0.05 0.11

CZH4 7.14 7.90 5.03

C2H6 3.43 Z.26 3.67

C3 olefins 1.72 Z0 51 1.03

C3Ha 0.11 0.17 0.03

C4 olefins 0.25 0~7a 0.30

C4HlO 0.04 0.06 0.02

CSH1"Z 0.00 0.00 0.02

Cs + olefins 1.56 2.23 1.05

total unsaturated 10.67 13.42 7.52

HZ/CO ratio 0.33 0.52 2.48

1 - water, nitrogen free basis

268
an auto-catalytic effect to achieve the conversion.

4) H2S content (beyond the gasification system wet scrubber) is negli- gible for the feedstocks surveyed. 5) The water gas shift reaction appears to be enhanced with an

increase in pyrolysis reactor temperature over the range of 1300 - 1700°F.

Continuing research includes integrated system performance assessment,

alternative feedstock characterization and factor studies for gasifica- tion (e.g. t catalyst usage, alternate heat transfer media, steam usage, recycle effectst residence time study) and liquefaction (e.g., improved catalystst catalyst activity characterization). An additional task in

progress includes the characterization of various feedstocks by compound types and corresponding corre 1ati on with reactor system performance.

ACKNOWLEDGEMENTS

The work described in this paper is supported by the U. S. Department of Energy (Contract No. DE-AC02-76CS40202, Alternate Materials Utilization Branch). The U. S. Department of Agriculture (Grant No.

59-2043-0-2-094-0, Science and Education Administration) and The Arizona Solar Energy Commission (Contract No. 205-80). Feedstocks were

supplied by Golden Byproducts, Inc., Phillips Petroleum, Colony Farms,

U. S. Forest Service, Centro de Investigacion ·en Quimica Aplicada, Office of Arid Land Studies (University of Arizona), Environmental

Research Laboratory (University of Arizona), Combustion Equipment Associates, San Carlos Apache Indian Reservation and Weyerhaeuser Corporation. .

REFERENCES

1. Kuester, J. L., "Conversion of Cellulosic and Waste Polymer Material to Gasoline," presented at the American Chemical Society Symposium on Thermal Conversion of Solid Wastes, Residues and

Energy Crops, Washington, D.C., September, 1979.

2. Kuester, J. L. t "Conversion of Cellulosic Wastes to Liquid Fuels" presented at Engineering Foundation Conference on Municipal Solid

Waste as a Resource: The Problems and the Promise, Henniker,

New Hampshire, July, 1979 (published by Ann Arbor Press).

3. Kuester, J. Lot illiquid Hydrocarbon Fuels from Biomass," presented

at The Biomass as a Non-Fossil Fuel Source Symposium, American

Chemical Society, Honolulu, April, 1979 (published in Preprints and Symposium Series).

4. Kuester, J. Lot "Ltqui d Fuels from Biomass," presented at the AIM/

ASERC Conference on Solar Energy, Phoenix, November, 1978 (pub- lished in Proceedings).

269
5. Kuester, J. L. t JlConversion of Waste Organi-c Materials to Gaso- line~ JI Proceedings of the Fourth National Conference on Energy and

the Environment~ Cincinnati, October, 1976~

270
PYROLYSIS EXPERIMENTS AT CHINA LAKE USING

A TUBULAR ENTRAINED FLOW REACTOR

J. P. Diebold* and C. B. Benham**

*Biomass Thermal Conversion and Exploratory Research Branch~ SERI.

**Solar Thermal Research Branch, SERI.

271
ABSTRACT

Data were obtained from 38 pyrolysis tests conducted on a municipal

solid waste feedstock using an entrained flow pyrolysis apparatus.

Additional data were obtained from 5 tests made using birch flour and

cellulose as feedstocks. Conditions were sought which maximized quantities of olefins produced per unit mass of feedstock. Exploratory tests

were made varying reactor temperature, residence time, carrier gas composition, and dilution (ratio of carrier gas to feedstock flow rates).

No effects of carrier gas composition upon pyrolysis gas composition

were found with respect to olefin yields. In general, olefin production

was greater for shorter residence times and greater dilution of the feed

by steam and carbon dioxide carrier flow. The effects of temperature

upon total olefin yield were not clear due to the narrow range of reactor temperatures explored. However, in general total olefin yields were

relatively constant over the temperature range of 700-850°C for short

residence times of less than 0.2 sec. In addition to saturated and aromatic . hydrocarbons, nearly 20 g of olefinsper 100 s of feed were

obtained using the municipal solid waste feedstock, although a 14 lffo

yield of olefins was the average attained in the short residence time

reactors. For the birch flour and cel LuLoae feedstocks the olefin

yields were about 10 g per 100 g of feed. The greater olefin yields

obtained with municipal solid waste were attributed to the possible

presence of plastics in the feed and/or to the proprietary processing

used to produce ECO II.

INTRODUCTION

The pyrolysis data on municipal solid waste (MSW) presented in this

paper was obtained as part ofa project sponsored by the Environmental

Protection Agency and carried out at the 'Naval Weapons Center at China

Lake, California. The goal of the project was to develop a process for

producing gasoline from MSW. The process flowsheet, described in detail

in Ref. 1, required fast pyrolysis of the organic portion of MSW to produce significant quantities of unsaturated hydrocarbons (primarily olefins). These hydrocarbons would be compressed, separated from the other

gases, and polymerized to produce polymer gasoline. The .potential for

fast pyrolysis of MSW to produce large yields of olefins had been

reported by Finney and Garrett in Ref. 2, and this work along with data

from Refs. 3-6 formed the basis for the gasoline flowsheet.

The goals of the pyrolysis effort were to build a continuous-flow pyrolysis reactor which would supply pyrolysis gases . to the gas separation

and gasoline synthesis subsystems and to verify the large olefin yields

reported in. Ref. 2:

Subsequent to the EPA sponsored project, Diebold conducted further tests

with the pyrolysis apparatus using pure cellulose and birchwood flour as

feedstocks. This work was sponsored by the Solar Energy Research Institute at Golden, Colorado. The goal of this effort was to determine the

amounts of olefins obtainable from lignocellulosic feedstocks using the

China Lake entrained flow reactor.

272
Table 1. FEEDSTOCK PROPERTIES

Average Feedstock Composition Dry

Particle (wt% Dry Basis) Major

Size Empirical HHV

Material lJm Ash C H ° S N CI Formula (kJ/dry gram)a

ECO II 188 1,2.46 45.50 6.20 34.22 0.98 0.27 0.37 C6H9.7403.39 19.77

(drum T47)

ECO II 270 9.73 49.64 6.40 32.38 1.04 0.22 0.59 C6H9.2802.93 22.19

(drum T5l)

Dry Cellulose --- 12.0 39.11 5.47 43.42 0 0 0 C6HlO05 15.38

+ 12% Ash

Cellulose 100 0.03 43.44 6.39 50.11 0.001 0.02 0 C6HlO. 6°5.20 17.23

Avicel PH-102

Birch Flour < 180 0.40 48.75 6.36 44.40 0.02 0.08 0 C6H9.4004.11 19.38

N

a1 kJ/g = 430 Btu/lb. --..J

Vol

FEEDSroCKS

TM

The solid waste feed chosen for the pyrolysis evaluation was "ECD-II

fuel" produced by Combustion Equipment Associates, New York. During the

manufacture of ECO II (Ref. 7), the inorganic fraction of the trash is

removed first. Then, the organic fraction is treated with mineral acids

in a proprietary process to embrittle the cellulosic materials. The

embrittled material is then ball-milled at elevated temperatures (93 to

204°C). ECO II is a non-fibrous, free-fLowing material having an average particle size of about 250 ~m. An. analysis was performed on two of

the drums to determine variability, since the material shipped was the

product of a process variable study. The results of those analyses are

shown in Table 1 along with data ona hypothetical feedstock comprised

of cellulose and 12% ash. The moat noticeable difference between the

ECO II fuel and cellulose containing 12% ash is that the ECO II has 5 to

10% more carbon and 5 to 10% less oxygen. This difference may be due to

a combination of plastics content and pretreatment of the ECO II fuel.

Cellulose was purchased from the FMC Corporation, Philadelphia, Pa ,

under the trade name Avicel PH-I02. It is a white, crystalline, freeflowing powder having a nominal particle size of 100 um, Birch wood

flour was purchased having a particle size of < 180 JJID (minus

80 mesh). Properties of Avicel PH-I02 and birch flour are also listed

in Table 1.

APPARATUS

The pyrolysis apparatus consisted of a solids feeding system, pyrolysis

reactor, and quench system. The feed system consisted of a Vibra ScrewTM

feeder having a screw auger 2.54 cm in diameter. This unit had a variable speed transmission to vary the feed flowrate. The feed was

entrained from the screw by a carrier gas and pumped toward the pyrolysis reactor by an ejector also driven by carrier gas. Immediately

upstream of the pyrolysis reactor, the mixed flow was entrained by a

second ejector driven by superheated steam. This design allowed the use

of different gases to be used as carrier and allowed the ratio of carrier gas to solid flow rate to be varied. Since the screw feeder hopper

was not sealed, there was a concern ab out entrainment of air with the

feed which would preferentially oxidize the unsaturated hydrocarbons in

the pyrolysis reactor. Therefore, a slight positive pressure « 5 cm of

H20) was maintained on the screw by adjusting the f l owr a t e of carrier

gas used to entrain the solids from the end of t he screw. l~ile this

slight positive pressure ensured that no air was entrained with the

feed, it also made impossible attempts to achieve a complete mass balance for the system since some of the carrier gases escaped through the

hopper.

Two differently configured entrained-flow pyrolysis reactors were

used. The first design, depicted in Fig. 1, consisted of a 2.8 m long

coil of type 304 stainless steel tubing having an inside diameter of

275
1.6 em on the inlet of a cyclone separator. Both the coil and cyclone

were contained within a propane-fired kiln. Most of the char was

removed by the cyclone separator. This design provided about 0.3 to 0.4

seconds residence time in the coil and 5 to 10 seconds within the

cyclone.

A second pyrolysis reactor design (Fig. 2) was used to achieve shorter

residence times. The cyclone separator was removed from the kiln leaving only the coil within the kiln. Two different lengths of coil, 2.1 m

and 6.1 m, were used to provide even greater control over residence

time. Quenching of the pyrolysis gas was accomplished in a water bath,

and two cold traps were used to remove tars. An air-driven ejector provided air for the premixed-flame flare and simultaneously provided suction on the system to assist in pumping the gases through the system.

INSTRUMENTATION AIm DATA A~UISITION

Data recorded for each run included run time, weight of solids fed, carrier gas flow rate, steam flow rate, weight of char produced, pyrolysis

kiln temperature, pyrolysis exit gas temperature, pyrolysis gas flow

rate and pyrolysis gas composition. The weight of solids fed was determined by weighing the feed in the hopper before and after each run. The

collected char which included ash, was dried and weighed. Carrier gas

and steam flowrates were calculated from the measured pressures upstream

of sonic orifices. Temperatures were measured using type K thermocou~

ples--attached to the outside wall 'of the pyrolysis coil until run 14

and thereafter immersed in the pyrolysis gas flow at the coil outlet.

Pyrolysis gas flowrate was measured using a 1. 23 em diameter orifice

with corner pressure taps. The pressure drop across the ~rifice and the

average molecular weight of the gases (as determined by the gas chromatograph) were used to calculate the product gas mass f Lowra t e, Gas

composition was determined using a Model AGC-111H Carle gas

chromatograph.

LONG RESIDENCE TIME REACTOR RESULTS

Data was obtained from 13 tests using the reactor configured as shown in

Fig. 1 and using ECO-II as the feedstock. A data ' summary for these

t ests is presented in Table 2. As can be seen from the table, the tests

were conducted at several different operating conditions. Carrier gases

employed in the first 6 runs were methane, carbon monoxide, and mixtures

of hydrogen, methane and carbon monoxide. The next 6 tests used both

carbon dioxide and steam, and test 13 used a mixture of hydrogen, methane and carbon monoxide as well as steam. Pyrolysis temperatures varied

from 716 to 860°C. Carrier gas flowrates varied from 0.23 to 0.73 kg

per kg of dry feed, and moisture content of the feed varied from 1 to

15%. Due to a loss of carrier gas back through the screw feeder the

values for the yields of the gases used as carrier are of poor accuracy

and are not presented. Trends of comP9sition as a function of temperature for tests 7 through 12 are shown in Fig. 3. The production of ethylene generally increased wi th t emperat ure , whereas that of propylene

276
Table 2. PYROLYSIS DATA SUHHARr

Pyrol - IIt % Pyro l - kg/kg Dry Feed Dry Resi- Grams Product/1 00 Gra ms Dry Feed

ysls 1120 ys ls I~ et Carrier Fee1 dence

Reactor Feed In Temp. Feed Gas Carrier g/m Time C5'+ C2-C4 Run Type Type Feed ·C Kg/h Type Gas St eama Gas es Sec CH4 C2112 C2114 C2116 C3116 C3118 C4118 C6+ H2 CO CO2 Char Unsat~

1 Cyclone ECO II 1 760 3.83 CH4 0.37 0. 01 302 >5 ? 0.1 8. 2 1.0 1.8 ? 0.5 ? 0.3 24.1 6.7 24.0 10.6

2 DRUM IS 790 2.39 Mix 0.67 0.18 175 I? 0.1 11.6 1.0 1.4 0. 4 j ? ? 10.8 23. 7 13.5

3 T51 10 815 3.43 CO 0.45 0.11 297 6.6 0.2 7.4 0.6 0.7 0.3 0.6 ! 8.6 15. 7 8.6

4 10 850 3.98 CO 0. 29 0.11 461 4.1 0.2 4.3 0.2 0.2 0.1 0.4 4.1 15.0 4. 8

5 10 815 3.05 Mix 0.23 0. 11 330 ? 0. 2 7.8 0.6 0.5 0.1 ? 8.8 18.6 8.6

6 10 732 3.60 014 0.52 0.11 300 ? 0 .1 5.7 1.1 2.3 1. 3 3.6 0.5 19.4 5.4 20.5 9.4

7 1 732 4.22 CO2 0.24 0.67 175 4.3 3.6 0.1 5.7 0.8 1.7 0.7 2. 1 0.4 18.2 ? 12. 2 8.2

8 1 732 3. 86 I 0.24 0.74 148 3.9 7.0 0.3 5.9 0.8 1.4 0.7 1.8 0.6 20.6 I

12.3 8.3

9 1 788 4.08 0.29 0.70 147 3.7 5.2 0.3 7.3 0.6 1.0 0.4 3.2 0.8 23.0 12.2 9.0

10 1 682 3.91 0.33 0.65 176 4.6 3.4 0.2 5.2 0.9 2.2 1.3 3.4 0.4 18.8 18.9 8.9

11 1 552 3.85 0.31 0.74 237 6.3 0.8 0 1.5 0.3 0.7 0.3 1.6 0.1 7.0 27.7 2.5

12 1 860 1.71 0.73 1.1 3 96 5.8 5.2 0.6 6.9 0.3 0.3 0.1 4.0 1.3 21.8 4.6 7.9

13 1 716 3.12 Mix 0.28 0.91 138 4.6 ? 0.2 5.0 0.9 1.5 0.7 1.7 ? ? 7.4 7.0 7.4

14 Call ECO II 700 2. 84 CO2 0.73 1. 0 126 0.195 5.6 0. 5 7.6 0.7 2.8 0.6 2. 2 3.5 0.7 19.6 ? 11.7 13.1

15 6.1 m DRUM

j

740 4.22

j

0.46 0.7 172 0.178 3.2 0. 5 6.1 0.5 2.1 0. 1 1.7 3. 2 0.5 22.0

1

? 10.4

16 ! T51 810 3.09 0.57 0.9 120 0.171 4.2 0. 6 8.1 0. 8 3.3 0.2 2.5 1.8 0.7 30.1 ? 14.5

17 750 3.13 0. 99 1.2 95 0.133 5. 1 0.8 9.4 1.0 3.3 0.2 2. 2 3. 2 O.B 36.8 23. 0 15.7 N 18 730 1.77 1. 74 2.4 56 0.139 4.5 1.0 11.4 0.9 3.9 0. 2 2.5 3.4 0.9 35.9 ? 18.8 ........

........ 19 Coi l 730 3.43 0. 90 1.1 III 0.050 2.5 0.5 6.7 0.6 2. 0 0. 1 1.8 3.7 0.6 22. 2 16.1 I} .0

20 2.1 m 780 1.86 1.65 1.9 62 0.052 3.5 1.3 9.4 0.6 2. 3 0. 1 1.5 3.0 0.9 27.4 12.5 14.5

21 770 2.31 N2 1.03 1.6 68 0.045 5.2 1.6 9.5 0.9 2.5 0. 1 2.5 1.9 1.2 42.5 9.6 22. 7 16 ~1

22 10 760 1. 91 CO2 1.79 2.3 56 0.049 4.1 1.2 11.3 0.7 2. 3 0.2 2. 1 5.0 0.9 34.8 ? 17.9 16.9

23 1 720 2. 16 1.59 1.9 67 0.053 3.1 0.9 9.1 0.8 2.0 0.1 2. 0 5.8 0.7 27.1 12.7 14.0

24 700 2.14 1.61 1.9 68 0.054 3.0 1.0 9.1 0.7 2.4 0. 1 2.4 3.9 0.7 27.0 14.3 14.9

25 760 2.34 1.46 1.8 68 0.049 3.8 1.2 10.5 0.8 2. 5 0.1 2.3 2.3 0.8 32. 3 20. 4 16.5

26 830 2.72 1.26 1. 6 70 0.044 4.3 1.4 10 . 7 0.7 2.0 0.1 1.8 2. 9 0.9 34.7 10.3 15.9

27 810 2.12 1.63 2.3 55 0.043 3.9 1.4 10.7 0.8 1.7 0.1 1.5 5. 2 1.0 32.3 17. 2 15. 3

28 820 2.57 1. 30 1. 7 66 0.044 4.6 1. 3 10.3 0.8 2.4 0. 1 2.0 2.9 1.0 38.3 ? 16.0

29 820 3.47 0.99 1.2 86 0.042 3.9 1.3 9.1 0.7 1.6 0. 1 1.6 3. 4 0.9 30.2 25.0 13.6

30 770 3. 79 0.90 1.1 102 0.046 3. 3 0.5 8.1 0. 4 2.8 0.2 2.4 4.4 0.7 29. 6 17. 2 13. 8

31 840 2.20 1.54 1.9 60 0.046 4.5 1.4 10.0 0.6 2. 1 0.1 1.4 2. 8 1. 1 34. 3 18. 2 14.9

32 750 2.29 1. 50 1.8 69 0.051 3.3 1.1 9.2 0.8 2.8 0. 1 2.4 3.9 0.8 29.6 15. 8 15.5

33 ECO [[ 743 2.36 CO2 I. IS 1. 7 78 0.056 1. 6 0.7 6.0 0.5 1.0 0. 1 1.0 2. 1 0.3 13.2 ? 8.7

34 DRUM 732 2.75 I

1.13 1.5 96 0. 059 2.7 0.9 7.5 0.7 2.0 0.1 2.0 3.4 0.5 22.8 ? 12.4

35 T47 760 1. 25 1.23 3.1 48 0.064 3.1 1.2 8.7 0.5 1.6 0.1 1.9 2.8 0.6 22.9 20. 2 13.4

36 ! 738 3. 42 0.77 1.2 112 0.055 3.5 1.1 8.8 0.6 2.2 0.1 2.2 4.4 0.6 26.4 ? 14.3

37 Co li 761 0.84 1.83 4.7 33 0.198 6.2 2.1 13.5 0.6 1.7 0.1 1.8 3.1 1.0 36.4 ? 19.1

38 6.1 m 759 3. 81 0.71 1. 0 121 0.161 4.1 0.9 8.8 0.7 2.3 0.1 2. 1 3.4 0.6 26.0 18.5 14.1

39 Coil Cell. 0 744 5.6 3 CO2 0.16 0. 7 142 0.114 4.3 O. B 5.9 0.6 1.4 ? 0. 6 4.6 1.3 58.2 ? 0.001 8.7

40 6.1 m Cell I

754 2.26 I 0.40 1. 2 84 0.167 6.7 0.8 9.3 0.8 0.6 j 0. 2 1.2 2.5 79.2 I ? 10.9

41 l Bir ch 765 1.68 0.53 1.9 65 0.169 9.8 0. 7 8.6 0.5 0.9 0. 6 0.8 3.0 63.4 0.028 10.8

42 732 2.11 0.44 2.5 58 0.121 8.1 0. 9 7.1 1.1 1.8 1.5 1.7 1.9 59.0 0.016 11.3

43 704 1.52 0.52 4. 1 37 0.108 6.2 0.7 6.2 1.0 1.7 1.1 2. 6 1.8 53.1 0.015 9.7

aIncludes 1120 In feed .

and butylenes decreased. Although it is possible that some of the propylene and butylenes were themselves pyrolyzed at the higher t empeza-rtures to account for the increase in ethylene production, data in Ref. 8

suggest that these olefins as well as ethylene were more likely polymerizing at these conditions to form a mixture containing about half benzene and half "coal" tars. The increase with temperature of the unknown

material, backflushed by the gas chromatograph and labeled "C5 + C6 +",

suggests that this polymerization may indeed have taken place. The

location of the cyclone separator in the pyrolysis furnace gave the

gases a calculated residence time of 4 to 5 seconds, which appears to

have been long enough for polymerization of the heavier olefins to take

place at the higher temperatures.

Samples of char from run 10 (682°C) and run 12 (860°C) were tested for

heat of combustion in a Parr bomb. A value of 17.9 kJ/g (7710 Btu/lb)

was reported for the char from the lower temperature pyrolysis, and a

value of 15.1 kJ/g (6504 Btu/lb) was reported for the char formed at the

high~r temperature. This trend is consistent with the greater gasification occuring at the higher temperature. Based on the weight of the ash

recovered from these tests, the char from run 10 contained 58.7% ash and

that from run 12 contained 50.1%. Based on the ash content of the feed,

run 10 should have produced 16.6% char compared to the measured 18.9%.

As indicated previously, the use of steam and carbon dioxide as carrier

gases, rather than hydrogen, methane, and/or carbon monoxide, did not

appear to result in major shifts of the weights of desired pyrolysis

products. To verify this observation, run 13 was made using a carrier

gas consisting of steam and a mixture of 19% by volume hydrogen, 39%

carbon monoxide, and 42% methane. The pyrolysis products from this run

were not greatly different from those made using steam and carbon dioxide except that the C2+ hydrocarbons were all slightly lower than

usual. It appeared that if there is a small effect of carrier gas on

the desired products, carbon dioxide is a preferred carrier gas. This

implied that an important function of the carrier gas is one of diluting

the products to decrease bimolecular reactions (polymerization) and subsequent tar formation during pyrolysis.

The use of steam as part of the carrier gas allowed the system to operate with a decreased noncondensable carrier flowrate and decreased the

eventual dilution of the C2+ hydrocarbons with carrier gas.

SHORT RESIDENCE TIME REACTOR RESULTS

It was realized that the product gases were being held at the pyrolysis

temperature for such a long time (- 5 seconds) that a portion of the

olefins was polymerizing to tars and char. A L 9-centimeter diameter

stainless steel tube, 6.1 meters long was formed into a 41-centimeter

diameter coil for the new pyrolysis reactor as shown in Fig. 2. This

resulted in a calculated residence time in the pyrolysis coil of about

150 milliseconds with the flowrates then in use. A thermocouple probe

was placed at the exit of the pyrolysis coil so that the pyrolysis products impinged upon it. The product gases then passed through a

279

1.2-meter length of noninsulated 1.9-centimeter tubing which cooled the

gases below about 550° C before they entered the 38-centimeter diameter

cyclone separator. The cyclone separator was moved out of the kiln.

The steam accumulator was moved to inside the pyrolysis kiln and the

steam lines were shortened and better insulated; these changes produced

much higher steam temperatures (approaching the pyrolysis temperature). The cyclone and the length of tubing connected to the quench jug

were insulated with fiberglass. Runs 1.4 through 18 were made with this

arrangement. The pyrolysis results for the entrained tubular reactor

are shown in Table 2.

As will be discussed in detail, the data suggested that the residence

time in the pyrolysis coil was still too long, so a 2.1-meter length of

tubing was shaped into a shorter reactor. This length of coil resulted

in a calculated residence time of about 50 milliseconds. The kiln was

rebuilt at that time to provide higher pyrolysis temperatures. Runs 19

through 36 were conducted with this shorter reactor tube. The large

cyclone was removed after run 25 so that, through run 38, the char and

tars were both collected in the quench jug, as shown in Fig. 2.

Nineteen pyrolysis runs were made using drum T-51 ECO I I (a product of

Combustion Equipment Associates) for the feedstock with the long tubular

pyrolysis reactor, as shown in Table 2. These experiments resulted in

the realization that between 700 and 840°C pyrolysis temperature has a

relatively minor role in affecting. the pyrolysis products with residence

times less than 150 milliseconds. The controlling variable appears to

be definitely related to the relative concentration of the feed or more

specifically the partial pressure of the pyrolysis gases in the reactor. Figure 4 shows that with the 6.1-meter pyrolysis coil there is a

pronounced relationship of yield of product as a function of the weight

of feed per cubic meter of pyrolysis gases (at the pyrolysis temperature). Figure 5 is similar but shows data generated with the 2. I-meter

pyrolysis coil. In both cases, the more dilute pyrolysis conditions

gave rise to a greater production of desirable hydrocarbons as well as

to carbon monoxide and hydrogen by-product gases. Initially it was

assumed that the increased hydrocarbon yields were due to a decreased

residence time in the reactor. However, when the 2.1-meter coil

replaced the 6. I-meter. coil and reduced the residence time by two-thirds

with no further increase in hydrocarbon production, it was realized that

dilution was the key variable. In fact, the longer coil produced

slightly greater amounts of products at the same dilution values, indicating more complete pyrolysis of the larger feed particles. The residence times indicated were calculated using a very simplistic approach;

the volumetric flow rate of the gross pyrolysis gases and the steam (at

the pyrolysis temperature) were divided into the volume of the pyrolysis

reactor. The pyrolysis temperature was simplistically chosen to be that

of the gas stream exiting the reactor.

The minor role played by temperature in this "high temperature" pyrolysis regime is seen in Fig. 6. With a variation in pyrolysis temperature

of nearly 150°C, there appears to be an increase in the ethylene production of perhaps 10%, but no significant increase in overall (C2 + C3

+ C4) nor C2+ fractions was observed. It should be noted that the C2+

281
22

20

11

1&

=a

8 14

... "" U >

:::J c 12

= Cl

c en c:

10 en c:

:e r.:t

c:

< c:a 8

c =

- &

....

o

o

65:tS 9 DRY Eca n

MJ

2.1M PYROLYSIS COIL

DL---J~-..L_-..L_--&._....I._.-L._-""----'--""""----

680 700 7Z0 740 760 710 800 820 840 860

PYROLYSJS TEMPERATURE. °c

Figure 6. Products vs. Temperature

fraction includes significant amounts of ethane which can be recovered

and recycled to the pyrolysis reactor to be cracked to ethylene with

expected yields of over 90% as well as the CS

2 + C6 (aromatic liquids)

which could be used as a high octane gasoline blending stock.

The use of nitrogen as the carrier gas in run 21 was made primarily to

determine if the increase in the amount of carbon dioxide carrier gas

was responsible for the apparently large increase in ethylene formation. The gas chroma tograph, used for the analysis, elutes ethylene

immediately after carbon dioxide, and there is some overlapping of the

two peaks. There WqS concern that part of what was thought to be the

ethylene peak was actually only carbon dioxide. The use of nitrogen as

the carrier gas resulted in a relatively small amount of carbon dioxide

in the gross gases (3% by volume) and good peak separation, but did not

adversely affect the apparent production of ethylene. It was concluded

that the ethylene data were valid. This same run did alter the interpretation of the propane and propylene data and resulted in their values

being reduced slightly. These results ·are reflected in the data presented after run 13. No major shifts in the C2+ hydrocarbon fraction

282
were observed in changing the carrier gas from carbon dioxide to nitrogen in the run. There did appear to be significant increases in the

carbon monoxide and hydrogen production with this run and a reduction in

the amount of C5+ material.

The variable partial loss of the carbon dioxide carrier gas backing up

through the feed hopper makes the carbon and oxygen balances relatively

unreliable. The use of steam in the pyrolysis process and water in the

quench system makes the hydrogen balance difficult also. An examination

of run 21, which used nitrogen as the carrier gas, shows good closure on

the overall mass balance (1.006 kilograms out per kilogram in) and the

carbon balance 0.018 kilograms out per kilogram in). The amount of

water formed by pyrolysis needed to close the hydrogen balance is

0.108 kilogram of water per kilogram of dry feed. The oxygen balance

very nearly closes without the hypothesis of water formation; i. e.,

0.975 kilogram oxygen out per kilogram oxygen in or 0.009 kilogram water

per kilogram of dry feed. Small differences in feed composition and/or

a small error in the relatively difficult hydrogen analysis of the feed

probably account for these discrepancies. It is concluded that little

water is formed during fast pyrolysis of biomass and that the amount

will be greatly affected by the extent of the water gas shift reaction

taking place.

Experimental pyrolysis runs were conducted to evaluate the use of a different lot of ECO II feed (lot T-47) , the reduction of the amount of

carbon dioxide carrier gas used, and the use of the longer 6.1-meter

pyrolysis coil. Lot T-47 of ECO II appeared to be similar to t he :

T-51 lot, but with some differences: 188 um (T-47) vs , 270 um (T-51)

average particle size; 12.5% ash vs. 9.7% ash; and a heat of combustion

(higher heating value) of 19.8 kJ/g (8507 Btu/lb) vs. 22.2 kJ/g

(9550 Btu/lb). There has been some concern that the lower heating value

for lot T-47 was indicative of a lower plastics content and that this

would result in a significantly lower yield in desirable hydrocarbons,

since plastics like polyethylene are pyrolyzed at these conditions to

form very high yields of desirable olefins such as ethylene and propylene. Taking the pyrolysis runs as a whole, it appears that the

hydrocarbon production from lot T-47 is about equivalent to that from

lot T-51, with the result that slightly better energy conversions to

hydrocarbons were observed with lot T-47 due t~ the lower energy content

of the feed.

The use of the longer pyrolysis coil for runs 37 and 38 confirmed previous runs (14 through 18) with respect to relatively good hydrocarbon

production with a lower kiln temperature (llT) due to the larger heat

transfer surface and residence time. In fact, it was observed that the

temperature of the exiting pyrolysis gases (measured with a sheathed

thermocouple probe) was 25 to 50°C hotter than the pyrolysis kiln (also

measured with a sheathed thermocouple). This probably reflects a

slightly exothermic pyrolysis reaction occurring at the elevated temperatures and/or large temperature variations within the kiln itself.

Us i ng lot T-47 feed with 10% moisture gave similar results on a dry feed

basis as were obtained with lot T-51 having only 1% moisture, as shown

in Fig. 4.

283
In runs 35 through 37 the steam flow was irregular. The problem with

the steam flow was in part due to the "weeping" of the steam pressure

relief valve and in part due to a cracked fitting. This leakage and

pressure fluctuation resulted in uncertainties in the actual dilution

present in the pyrol.y s Ls reactor for these four runs. Run 28 has no

known boiler water leakage, although the steam pressure still had considerable fluctuation (125 ± 25 psi). The steam fluctuation was apparently due to a small change in the boiler waterline which allowed the

water to accumulate in a horizontal section until the surface tension

was overcome and the water was dumped into the hot section of the

boiler. These factors resulted in variable s team flows, which were

averaged for the purpose of comparing the runs.

PYROLYSIS TESTS USING CELLULOSE AND BIRCHWOOD FLOOR

A few tests were conducted with the short-residence-time pyrolysis reactor using pure cellulose and birchwood flour to determine if the high

olefin yields obtained from ECO II could be obtained from lignocellulosic biomass materials (Ref. 9). The preliminary data from these tests

are shown in Table 2. A comparison of the C2 - C4 unsaturated hydrocarbons (last column) "l i s t ed in Table 2 with the average values for ECO II

reveals that the cellulose and birch flour produced only about 70% of

the amount of these hydrocarbons as the ECO II feedstock, but at the

same time produced much more methane. The greater amount of hydrocar~

bons obtained from the MSW feedstock may be due to plastics in the

ECO II feed or may be caused by the chemical pr~treatment used in preparing the ECO"II material. Due to feeding problems, both the cellulose

and the birch flour tended to pass through the reactor unevenly which

probably resulted in a low effective dilution and consequently lower

olefin yields. Additional experimentation needs to be conducted with

both natural biomass feedstocks as well as biomass material which has

undergone the proprietary processing used to prepare ECO II. SERI now

has such a material in hand made from sugar cane biogasses (AGRIFUEL~ by

Combustion Engineering Associates) and will test it in the near

future. .The amount of char produced by cellulose was 0.1% and by birch

flour 1.5%-which is one to two orders of magnitude less than 't ha t

contained in ECO II.

284
CONCLUSIONS

Significant yields of unsaturated hydrocarbon (approximately 15 weight

percent) as well as saturated and aromatic hydrocarbons were obtained

from a commercially available pretreated municipal solid waste feedstock

(ECO-II). Results of tests conducted with an apparatus providing residence times greater than 5 seconds indicated that ethylene yields

increased with temperature to the highest pyrolysis temperature of

860°C. However the quantity of higher molecular weight olefins showed a

decrease after about 650°C. Using a pyrolysis reactor providing residence times less than 0.2 seconds, no clear temperature for maximum olefin production was found. However, larger amounts of carrier gas

relative to the quantity of solids fed apparently produced more olefins. One plausible explanation for this .result is that higher partial

pressures of reactive intermediates produced in the pyrolysis reaction

form tars and char rather than low molecular weight olefins. Variation

of carrier gas composition had no significant effect upon pyrolysis gas

composition indicating that shifting of products based upon equilibrium

chemistry does not occur at a rapid rate at the conditions employed.

Olefin yields obtained from pure cellulose and birch wood flour using

the short-residence-timepyrolysis reactor were about 70% those obtained

from ECo-U. The greater yields of olefins from the MSW feedstock is

believed to be due to the presence of plastics, although the chemical

pretreatment of this material may contribute to olefin production.. The

char yields from the cellulose and birch flour were only 0.1% and 1.5%

respectively, compared to 10 to 25% for the municipal solid waste

feedstock.

285
REFERENCES

1. Diebold, J. P., C. B. Benham, and G. D. Smith. "Conversion of

Organic Wastes to Unleaded, High-Octane Gasoline." Final Report.

EPA IAG-D6-0781. Cincinnati, OH: Industrial Environmental Research

Laboratory; 1980.

2. Finney, C. S., and D. E. Garrett. "The Flash Pyrolysis of Solid

Waste." Presented to 66th Annual Meeting of AIChE, paper 13-E.

November 1973.

3. Lincoln, K. A. "Flash-Pyrolysis of Solid-Fuel Materials by Thermal

Radiation." Pyrodynamics, 1.., pp , 133-143; 1965.

4. Brink, D. L., M. S. Masoudi, and R. F. Sawyer. A Flow Reactor Technique for the Study of Wood Pyrolysis. Presented at: Fall meeting,

Western States Section, The Combusion Institute, El Segundo, CA.

October 1973.

5. McFarland, J. M., et ale "Comprehensive Studies of Solid Waste Management." NERC Report No. 72-3, National Environmental Research

Corp. Contract 2 ROI-EC-00250-01, Environmental Protection Agency.

May 1972.

6. Mallon, G. M. and L. E. Compton. "Gasification of Carbonaceous

Solid," U.S. Patent 3,846,096. 1974.

7. Beningson, R. M., et ale Produc t Lon of ECO....FUEL®II frokm Municipal

Solid Waste. CEA/ADL Process. Proceedings of 80th AIChE National

Meeting, Boston, MA. Paper 30e. September 1975.

8. Dunstan, A. E., E. N. Hague, and R. V. Wheeler. "Thermal Treatment

of Gaseous Hydrocarbons; 1. Laboratory Scale Operation." Ind. Eng.

Chern., 26(3):307-314.

9. Diebold, J. P. "Research into the Pyrolysis of Pure Cellulose, Lignin, and Birchwood Flour in the China Lake Entrained-Flow Reactor",

SERI Report No. TR-332-586, Solar Energy Research Institute, Golden,

CO., June, 1980.

286
AN EXPERIMENTAL INVESTIGATION INTO FAST PYROLYSIS OF

BIOMASS USING AN ENTRAINED FLOW REACTOR

M. BORN AND C. BENHAM*

Abstract

Pyrolysis experiments were performed using an entrained flow pyrolysis

reactor and two different feedstocks-a powdered material derived from

municipal solid waste (ECO-II)TM and wheat straw. All entrained-flow

tests were conducted with reactors having a length of either 30 em. or

90 em. Steam was used as the carrier gas for all tests. Reactor wall

temperature was varied from 7000C to 14000C. Gas composition data

from the ECO-II tests were comparable to data reported previously by

Diebold using an ECO-II feedstock. Trends were found in ethylene yield

as a function of reactor wall temperature and residence time. The trends

suggest that it may be possible to correlate gas yield with a single parameter which includes temperature, reactor length, and steam-to-biomass

flow rate ratio. The important conclusion from the wheat straw tests is

that olefin yields are about one half that obtained from ECO-II. Evidence was found supporting the idea that high olefin yields from ECO-II

are due to the presence of plastics in the feedstock.

Batch experiments were run on wheat straw using a PyroprobeTM. The

samples were heated at a high rate (~20,000oC/sec.) to 10000C and held

at 10000C for a variable period of time from 0.05 seconds to 4.95 seconds. These results revealed that two different mechanisms occur. For

times up to 0.15 seconds, volume fractions of ethylene, propylene, and

methane increase while that of carbon dioxide decreases. After that

period, those species are no longer generated and only carbon monoxide

and hydrogen are produced. The change in mechanism may be related to

poor thermal contact of portions of the sample with the probe. Data at

0.15 seconds compare quite well with data from the 30 em. reactor at

10000C except for the lack of acetylene in the Pyroprobe data. This

suggests that the sample was not heated as rapidly by the Pyroprobe as

in the entrained flow reactor .which provides heating rates of approximately 50000C/sec.

*Solar Thermal Research Branch

Solar Energy Research Institute

Golden, Colorado

287
Introduction

Destructive distillation (pyrolysis) of biomass materials is an

ancient art which has become the object of renewed interest during the past few years. This renewed interest stems from the

continuing energy dilemma and the realization by many scientists

and engineers that biomass is a viable feedstock for producing fuels

and chemicals which are now derived from petroleum feedstocks.

The thrust of the effort within the Solar Thermal Research Branch

at SERI is toward producing diesel fuel from gases obtained from an

entrained flow pyrolysis reactor. The feedstocks of primary interest are agricultural non-food materials such as wheat straw and

corn stalks. A catalytic process for converting olefins, carbon

monoxide and hydrogen to liquid hydrocarbons was described by Kuester.

Since the hydrocarbon yield from a catalytic process is influenced

by the relative amounts of hydrogen, carbon monoxide and unsaturated

hydrocarbons, it is important for us to understand the interrelationship of the large number of pyrolysis variables which influence gas

composition.

Much of the effort in pyrolysis during the past few years has been

directed towards detailed studies of the pyrolysis mechanism (see

reference 2-9). Welker presents a review of the literature on pyrolysis and ignition of cellulosic materials. Typically, these

studies involved small samples of a given material heated at a few

hundred degrees centigrade per minute and resulted in gas composition,

char yield, and kinetics data. An entrained flow reactor on the other

hand is one practical implementation of continuous pyrolysis which

can provide heating rates on the order of. 1000oC/sec.

Several researchers have reported data from entrained flow pyrolysis

reactors. Finney and Garrett reported high yields (>20 wt %) of ethylene using a finely divided municipal solid waste feedstock in a steam

carrier flow, a reactor temperature of about 800°C, and a short residence time «1 second). Brink studied pyrolysis of wood, kraft black

liquor, and municipal solid waste in a reac~or havin§ a heating rate

of about 10aaoC/second, a maximum temperature of 840 C and a residence

time of about 3 seconds. The carrier gas used was nitrogen. Rensfelt

studied pyrolysis of a variety of feedstocks including wood, straw,

municipal solid waste, peat, coal and graphite. The reactor was capable of heating rates to laaaoC/second and temperatures were varied

from soaoc to 10aOoC. Carrier gases used included steam, hydrogen,

and nitrogen. Ethylene yields obtained using municipal solid waste

were comparable to those obtained by Diebold with pyrolysis tests on

municipal solid waste feedstock (ECO-II) which is a commercially available material from Combustion Equi~ment Associates, Ne~ York.

In the Diebold experiments pyrolysis temperatures ranged from 700 to

800oC, residence times from 50 to 200 ms, and ratios of steam to biomass

288
flow rates from 1:1 to 5:1. A combination of steam and carbon

dioxide was generally used as the carrier gas. Three runs were

made using nitrogen in place of carbon dioxide. The two primary

results obtained from this study were that large amounts of olefins

(>20 wt %) were obtained in the pyrolysis gases and that the greater

the dilution of the biomass with carrier gas, the greater the yield

of olefins. Diebold, in a separate study used the same apparatus to

obtain data using pure cellulose and birch flour as feedstocks. These

materials produced less than one-half the amount of olefins produced

using the ECO-II feedstock. McFarland obtained about 4 wt% ethylene

using large particles (1.25 cm.) of municipal solid waste in an entrained flow reactor operated at about BOaoe.

In spite of the considerable amount of data obtained from the previously mentioned studies, a clear picture of which variables are important

and what influence these variables have upon gas composition has not

been determined.

The SERI Entrained Flow Pyrolysis Reactor

As mentioned in the previous section, one of the goals of the SERI

program is to allow more extensive entrained flow reactor measurements than reported in previous works. Measurements of interest

include the effect of reactor length, reactor temperature, and carrier

flow to biomass flow ratio on pyrolysis gas composition. In this section we describe the SERI entrained flow reactor, instrumentation and

experimental procedures.

As shown in figure 1, biomass is metered into a mixing chamber by a

screw feeder where it is entrained by a flow of superheated steam.

Water flow rate delivered to the boiler/superheater is measured by a

rotameter and the temperature of the steam entering the mixer is also

monitored. For smooth delivery, the water is supplied to the boiler/

superheater by a piston tank pressurized to about 14 atmospheres by

nitrogen.

Flowmeterl

Air

Flare

Screw Feeder

o

!

Boiler

Superheater

Figure 1. SERI Ent r a i ned Flow Pyrolysis Reactor.

289
The entrained biomass/steam flow enters the reactor which is electrically heated vycor or stainless steel tubes with 13 mm. inside diameter.

Electrical heating has been provided either with a tube furnace for

the vycor tube or by nichrome coils placed near the steel reactor tube.

For the vycor tube, reactor temperature was determined by the temperature setting chosen on the furnace. For the steel tube, power input

to the nichrome coils was provided by variacs' and the temperature was

monitored by chromel/alumel thermocouples. The thermocouples are

supported by small steel tubes welded perpendicular to the reactor and

held in close contact to the reactor tube by high temperature ceramic

cement. Reactor length is adjusted from 15 to 150 cm. by adding or removing reactor sections. Power supplied by the variacs was measured.

Downstream of the last reactor section, a cold water spray quenches

the reaction and condenses the steam carrier. At this point, pyrolysis gases and solid particles are separated. The gases then pass

through two tar traps consisting of dry ice/water baths and small gas

samples are drawn into 150 cc evacuated stainless steel sample

bottles downstream of the tar traps. A flow meter section then measures

pyrolysis gas molecular weight and mass flow before the gases are flared

with a bunsen burner. The entire system is operated slightly below atmospheric pressure due to suction provided by the air ejector. Gas analysis including hydrocarbons through Cs is provided by a Hewlett-Packard

model 5840 gas chromatograph.

Flow rate of the biomass particles is measured by a mass balance upon

which the screw feeder rests~ In addition to the reactor wall temperature probes, gas temperature is measured at several stations along the

reactor by chromel/alumel thermocouples ~ersed in the flow through the

reactor wall. The probes are made from 0.005" wire and are gold plated

to minimize emissivity and thereby reduce sensitivity to radiation from

the reactor wall. All data are recorded by a microcomputer based data

acquisition system. This system also controls the steel reactor temperature by closing a feedback loop between the wall temperature probes and

stepper motors' which turn the variacs. Gas sampling ports are also

provided at several stations along the reactor length to allow one to

measure gas composition as a function of extent of reaction.

Typically, the reactor and the steam are brought up to temperature together. When steady conditions are reached, the screw feeder is turned

on and the reactor temperature is allowed to re-equilibrate to the original temperature. At this time gas samples are taken. A run of

about five minutes is required to allow solids feed rate to be accurately determined, but for purposes of debugging feed or entrainment problems, runs of thirty minutes have been used.

Results

For purposes of comparing results from the SERI reactor with previous

experiments, references 13 and 14, we ran experiments with ECO II fuel

as the biomass feedstock. Temperatures from 7000 C to l2000C were used

with a 90 em. reactor and temperatures from 10000C to l4000C with a

30 em. reactor. Steam to biomass ratio was approximately 1:1. The

ECO II particles have an average size of 240 micron.

290
Diebold, reference 14, has shown that steam/biomass ratio can strongly

influence ethylene yield. We have taken data for 1:1 ratio from the

SERI reactor and from reference 14. Reference 13 does not report the

steam/biomass ratio used in those experiments. The feedstock used in

reference 13 was not ECO II but a municipal waste with a composition of

C6H9 304 2while ECO II is reported, reference 14, to have a composition

of C6H9.g03~. In addition, particle sizes may have been somewhat larger as t he s1ze reported in reference 13 was "<0. Smm". Residence times

were calculated for the SERI reactor according to the method ~sed in

reference 14 (volumetric flow rate of gases in the reactor at gas exit

temperature divided by reactor volume) while residence times used in

reference 13 were reported to be generally >0.4s.

Volume fraction of ethylene is shown in figure 2. The continuous

curves are the data for the SERI 90 em. and 30 em. reactors and from

reference 13 while the individual points are from reference 14. Absolute scaling of data from reference 14 is difficult because carbon

dioxide was used as a carrier for thcseexperiments and gas composition was reported on a carbon dioxide-free basis.

1400

CI

1000

Steam

Bio

800 1200

Reactor Temperature rc )

Residence

Reference Time

10

8

:I:

N

6 0

Q

4

Q)

-0 2 ::E

0

400 600

0 Present 1 1 0.012 s

0 Study 1 1 0.037 s

6- 14 1 1 0.05 s

"V 14 1 . 1 0.15 s

• 13 ? 0.4 s

Figure 2. Ethylene Yield vs. Reactor Temperature and Residence Time .

291
Figure 2 shows a well defined trend (except for the reference 14 data)

of ethylene peaks occurring, at low temperatures for longer residence

times. A decrease in ethylene yield at high temperatures is generally

regarded a result of secondary gas phase reactions which convert the unsaturated hydrocarbons to higher molecular weight hydrocarbons.

The dependence on residence time in an entrained flow reactor has not

been shown before, but can be explained in terms of heat transfer limitations.

Species evolution from the particles depends on the rate of increase

of particle temperature and the heating period (residence time). A

short reactor operating at a given temperature could produce a gas composition similar to that produced by a longer reactor operating at

a lower temperature. The exception would be the olefins which may

be produced in greater volume fraction in the short reactor due to

the higher heating rate. This appears to be the case in figure 2.

The ethylene peak for the 30 cm. reactor is higher than that for the

90 cm. reactor. Note that the longest residence time peak (from reference 13) is even higher but a direct comparison of yields from reference 13 should not be made because steam/biomass ratio is unknown

for those data. More data for other residence times and steam/biomass

ratio should confirm this dependence of gas composition on residence

time and temperature and could lead to a correlation between these

three parameters.

This same trend is exhibited in figures 3, 4, and 5 in which gas composition is plotted as a function of reactor temperature for the 90 cm.

reactor, for ·t he 30 cm. reactor, and for the data from reference 13, respectively. The trends, especially for carbon monoxide, hydro~en, methane,

and carbon dioxide are shifted towards higher temperatures in the short

reactor. A noticeable difference between reference 13 data and the

90 cm. data, figu.re 4 and 5, is the decrease in carbon monoxide and

the increase in hydrogen and carbon dioxide for the entire temperature

range. This suggests the water-gas shift reaction which could be related to catalytic materials in the solid waste or the long residence

times used in reference 13.

Experiments were also run with wheat straw as the biomass feedstock.

As discussed in an earlier section, wheat straw is a potential feedstock for a process upon which our research is focused. That process

is the pyrolysis of agricultural non-food products and the catalytic

conversion of the resulting pyrolysis gases (primarily hydrogen, carbon monoxide, and olefins) to diesel fuel.

Results for a 30 cm. reactor with particle size ~l50 micron, and steam

to biomass ratio of approximately 1:1 are shown in figure 6. The most

important result is that the wheat straw produces approximately half

the ethylene as does the ECO II feedstock. Possible reasons include

the presence of plastics in the ECO II as well as the fact that the

ECOII is chemically pre-treated by acid hydrolysis to assist in grinding. The latter explanation seems less plausible because tests with

an electrically heated platinum wire on small batches of acid treated

292
newsprint gave an increase in olefin yield from 10.1 to 11.2 wt 7. compared to non-treated newsprint. In addition, Finney has reported entrained flow reactor results on untreated municipal waste in which

half the paper fibers were removed, presumably increasing the fraction

of plastics in the feedstock. Those results indicated ethylene yields

of 21% by volume which is considerably higher than that reported for

ECO II yields. We therefore conclude that the presence of plastics is

responsible for the high olefin yields reported for fast pyrolysis of

municipal wastes.

10

C2H4

5 C3Hs

C2H2

0 C2He

60

40

20

o

700 800 900 1000 1100 1200

Reactor Temperature (0 C)

Figure 3. Gas Composition vs. Reactor Temperature, 90 cm. Reactor,

ECO II.

293
10

0-m 0-" zszau0-

; C2H2

0 @ . C3H6

C2H6

60-

c

CI)

- ~CO 0

:E

40

1100 1200 1300 1400

0......---.......-----.....---.....---......---..

1000

Reactor Temperature (0 C)

Figure 4. Gas Composition vs. Reactor Temperature, 30 em. Reactor,

ECO II.

294
10.-------===&===::::---------,

o

60

QI

-o

::IE 40

20

600 700 800 900

O......-...,--""'-----L----""'--.......-_-L- ..L-__---J

500

Reactor Temperature (0C)

Figure 5. Gas Composition vs. Reactor Temperature, Reference 13,

Solid Waste.

295
10

5

0'*"'d 2P ~C2H4

0 2 'i -,!tH~ic2H.

A CO2 f:= _4 : .>c: • H2

0 0 -g-- CH4

900 1000 1100 1200

20

o

700 800

60 ~A CO

:::.e 0 e:t A &

Q)

"0

:E 40

Reactor Temperature (0 C)

Figure 6. Gas Composition vs. Reactor Temperature, 30 cm. Reactor,

Wheat Straw.

To complement our reactor experiments we have been using an electrically heated p~a~i~um wire to , study fast pyrolysis of small batches of

prospective feedstocks. This method is useful for screening feedstocks as well as for quickly determining operating regimes in wh~cb

the reactor experiments may be interesting. The registered tradename of the device is Pyroprobe (Chemical Data Systems, Inc.) and has

been used by many researchers for similar purposes. Our work, however indicates that some caution in interpreting results from the

Pyroprobe is in order.

The Pyroprobe allows one to heat the platinum wire along a prescribed temperature-time curve. Specifically, the temperature can be programmed to .increase linearly to a given temperature, and then hold

that temperature for a given period of time. We used the device to

heat small (200~g) samples of fine wheat straw on a temperature ramp

of 20,OOOoC/sec up to 10000C and then hold the sample at that temperature for times ranging from 0.05 seconds to 4.95 seconds.

296
In figure 7, we plot the gas composition versus the time which the

probe was held at 10000C. There are clearly two regimes for holding

times less than or greater than 0.15 seconds. This suggests two

diff erent mechanisms. At 0".15 seconds the maximum ethylene, propylene, and methane mole fractions occur. For holding times longer than

this, the fraction of those components and also of carbon dioxide decreases at the same rate on the log-log scale while the hydrogen

increases and carbon monoxide fraction is essentially constant. The

linear decrease in the four species after 0.15 seconds holding time

can be shown to imply that either those four species cease to be generated or are generated at rates which are related in an unusual way.

The most likely explanation is that the four species are produced up

until 0.15 seconds and after that are diluted by the increasing amounts

of hydrogen and carbon monoxide.

The change in mechanism may be related to poor contact of the particles

with the probe. Microscopic examination of a probe after applying the

particles by what seems to be the most effective method (applying a

distilled water/wheat straw slurry to the element and then evaporating

the water) revealed that the particles contacted the probe in only a

very few isolated points. For holding times up to 0.15 seconds these

contact points are very rapidly heated and produce "i ncr ea s i ng quantities

of olefins (and decreasing quantities of carbon dioxide which is consistent with our low temperature reactor experiments). During this time,

it is possible that the portions of the sample not in direct contact

with the probe are heated much more slowly producing char. After 0.15

seconds, por~ions of the sample in direct contact with the probe have

completely pyrolyzed leaving the char to continue heating slowly and

producing hydrogen and carbon monoxide. In addition, gases evolved

from the particles in direct contact with the probe, could force the

particles from the probe thereby greatly reducing the heating rate of

the sample. A close examination of the residue left on the probe after

short holding times could help verify these explanations.

Comparing the gas composition after 0.15 seconds at 10000C (which seems

to be the gas composition most representative of the 20,000oC/sec. heating rate) with that for the 30 cm. reactor at 1000oC, figure 6, we find

they are quite similar with one exception. Pyroprobe data at these

conditions is consistently lacking any meaningful quantities of acetylene, while the reactor consistently yields acetylene. A lack of

acetylene implies a lower temperature reaction in the probe experiments

than in the reactor experiments. Although the probe was heated at

20,000oC/sec to 10000C it appears that the particles lagged the probe

temperature due to poor thermal contact with the probe. It is difficult

to determine the particle temperature except indirectly by comparison

with probe runs for higher final temperatures. However, an analysis of

particle heating rates for the entrained reactor suggests that heating

rates of 5000oC/sec may be reasonable and therefore we expect that the

particles heated with the probe were heated at less than 5000 oC/sec,

and achieved temperatures less than that achieved by the particles in

the 30 cm. reactor.

297
1.0 ~--------------- ....,

0.1

c

o.-..

u

l!

u.

G»

o

:E

0.01

0.001 L-_-'- J.... ........... .....

0.05 0.1 0.5 1.0 5.0

Time at 1000°C, sec

Figure 7. Gas Composition vs. Holding Time ClOOOOC), vfueat Straw,

20,OOOoC/sec.

298
Conclusions

Entrained flow reactor pyrolysis experiments on a municipal solid waste

feedstock CECO II) resulted in olefin yields comparable to that reported previously. However, trends in the data, namely ethylene yield as

a function of reactor temperature and residence time which had not been

seen before, were found. The trends suggest that it may be possible to

correlate gas yield with a parameter which includes temperature, reactor

length and steam/biomass ratio.

Experiments were also performed with an agricultural non-food product,

wheat straw. The most important result of those experiments was that

olefin yields were about half that of the ECO II feedstock. The most

likely explanation is the presence of plastics in the ECO II feedstock.

Pyroprobe experiments with wheat straw reveal that two mechanisms affect

the gas composition resulting from heating at 20,0000C/sec to lOOOoC

and then holding at 10000C for a variable period. For periods up to

0.15 seconds, volume fractions of ethylene, propylene, and methane increase while that of carbon dioxide decreases. After that period the

species are no longer generated and only hydrogen and carbon monoxide

are generated. The change in mechanism may be related to poor thermal

contact of portions of the sample with the probe. Data at 0.15 seconds

compare quite well with the 30 cm. reactor results at 10000C except for

the lack of acetylene in the pyroprobe data. This suggests that the

sample was not heated as rapidly by the pyroprobe as in the entrained

flow reactor.

Future Work

We plan to run more experiments on wheat straw and other agricultural

non-food products in the entrained flow pyrolysis reactor. Data from

these experiments will include gas yield as a function of reactor

length, reactor temperature, and steam/biomass ratio. Trends from

these data will be used to develop correlations which will be useful

for engineering scale-up of the reactor and to help explain pyrolysis

mechanisms. .

Acknowledgement

The assistance of Mr. G. Bessler in design and construction of the

apparatus, the assistance of Mr. R. Kemna in construction and oper- -

ation of the apparatus, and the assistance of Mr. P. Bergeron in

gas chromatographic analysis are qppreciated.

299
REFERENCES

1. Kuester, J.L., "Conversion of Cellulosic Wastes to Liquid Fuels,"

presented at the Engineering Foundation Conference on Municipal Solid Waste as a Resource, Henniker, New Hampshire, July

1979.

2. Antal, M.J., "The Effects of Residence Time, Temperature and

Pressure on the Steam Gasification of Biomass," pr,esented at

the American Chemical Society Symposium, Honolulu, Hawaii,

April 1979.

3. Baker, R.R., "Thermal Decomposition of Cellulose," Journal of

Thermal Analysis, Vol. 8, pp. 163-173, 1975

4. Broido, A., and Nelson, M.A., "Char Yield on Pyrolysis of Cellulose," Combustion and Flame, Vol. 24, pp. 263-268, 1975.

5. Stamm, A.J., "Thermal Degradation of ·Wood and Cellulose," Ind.

Eng. Chem., Vol. 48, pp. 413-417, 1956.

6. Lipska, A.E., and Parker, W. J ., "Kinetics of the Pyrolysis of

Cellulose in the Temperature Range 250-300 °C," Journal

of Applied Polymer Science, Vol. 10, pp. 1439-1453, 1966~

7. Antal, M.J., Friedman, H.L.,. and Rogers, F. E., "Kinetic Rates

of Cellulose Pyrolysis in Nitrogen and Steam," presented at

the Fall Meeting, Eastern Section, The Combustion Institute,

Hartford, Connecticut, November 1977.

8. Antal, M.J., Friedman, H.L., and Rogers, F.E., "Kinetics of

Cellulose Pyrolysis in Nitrogen and Steam," Combustion

Science and Technology, Vol. 11, 1979.

9. Lewellen, P.C., Peters, W.A., and Howard, J. B., "Cellulose

Pyrolysis Kinetics and Char Formation Mechanism," Sixteenth

International Symposium on Combustion, August 1976

10. Welker, J.R., "The Pyrolysis and Ignition of Cellulosic Materials:

A Literature Review," J. Fire and Flammability, Vol. 1, p , 12,

1970.

11. Finney, C.S., and Garrett, D.E., "The Flash Pyrolysis of Solid

Waste," presented at the 66th Annual Meeting of the AICHE,

paper l3-E, November 1973.

12. Brink, D.L., Masoudi, M.S., and Sawyer, R.F., "A Flow Reaction

Technique for the Study of Wood Pyrolysis," presented at the

Fall Meeting, Western States Section, The Combustion Institute,

El Segundo, California, October 1973.

300
13. Rensfe1t, E. et al., "Baade Gaai.;f; ;tcat;lon Stud;tes ~or Development

of ·Biomass Medium-Btu Gasification Processes," presented at

the Energy from Biomass and Wastes Symposium, Washington,

D.C., August 1978.

14. Diebold, J.P., Benham, C.B., and Smith, G.D., "Conversion of

Organic Wastes to Unleaded High Octane Gasoline," Final Report EPA IAG-D6-0781, Cincinnati, Ohio, Industrial Environmental Research Laboratory, 1980.

15. Diebold, J.P., "Research into the Pyrolysis of Pure Cellulose,

Lignin, and Birch Flour in the China Lake Entrained Flow

Reactor," SERI/TR-332-586, Golden, Colorado, 1980.

16. McFarland, J. M., et a L, , "Comprehensive Studies of Solid Waste

Management," SERL Report No. 72-3, National Environmental

Research Corp., Contract 2 R01-EC-00250-01, EPA, May 1972.

301
FLASH PYROLYSIS OF BIOMASS IN SWEDEN

+ C. Ekstrom, E. Rensfelt

The Royal Institute of Technology

Dept. of Chemical Technology

S-IOO 44 Stockholm, Sweden

ABSTRACT

Biomass has been shown to be a very special raw material for gasification g~v~ng a high yield of volatiles and a low yield of char which is

highly reactive. These properties are accentuated by flash pyrolysis,

a high heating rate process. Char yield is still lower (10 % of m.a.f)

and even more reactive. The char fraction after long residence time

(final carbonization of flash char) is almost independent of the

temperature between 650 and 10000C. Depending on temperature, residence

time and presence of a catalytic surface, the secondary gas reactions

can yield ' anyt hi ng between a syngas and a methane-rich and/or ethylenerich medium-BTU-gas. Ethylene and methane are shown to be at least

partly secondary products from thermal cracking of the tar fraction.

The main part of the hydrocarbon liquids initially produced are rather

easily converted even thermally to e.g. hydrocarbon gases and carbon

black. A residual fraction of the tar (5 % on m.a.f. basis) needs some

kind of a catalyst to be converted.

The paper describes the experimental apparatus and procedure used to

investigate flash pyrolysis. Experimental results are presented together

with a short commentry. Finally, the importance of these results for

the process development of fuel gas, syngas and chemical feedstock are

mentioned.

1. INTRODUCTION

Experimental studies on pyrolysis and gasification of peat, solid waste

and biomass were initiated in late 1974 in Sweden at the Royal Institute

of Technology. Experiments were conducted in small batch reactors with

heating rat,es from 10 to 1000C/min. The pyrolysis reactions were found

to be much faster than could be resolved by these heating rates (1).

Information ,r egar di ng the Garret (Occidental) Flash Pyrolysis Process

(solid waste) and a Russian milled peat flash pyrolysis process lead to

a dicision to investigate further the rapid-rate heating pyrolysis

(1976) (2), (3). In (1977) studies on secondary tar conversion were

initiated, to investigate both the production of ethylene thermally and

syngas and fuel gas catalytically (4), (5), (6).

This paper presents the basic experimental methods and apparatus used

+Present address: Studsvik Energiteknik AB,

Dept. for Chemistry and Environment

Environmental Chemistry

S-6ll 82 Nykoping, Sweden

303
together with the results mainly perta~n~ng wood. Results for peat and

solid waste have to some extent been published wlsewhere (1), (6), (7).

2. EXPERIMENTAL APPARATUS

The flash pyrolysis apparatus is shown in Figure 1. The powdered fuel

(particle size normally less than 0.6 mm) is fed continuously by the

screw feeder and falls through the electrically heated reactor tube

(500-10000C). The char particles then fall into the char hopper under

the reactor, while the finer char particles are separated by the cyclone

and the dust filter. The liquid products condense in the tar condenser

and the tar wate~ condenser after which the tarry aerosol is retained

in the cotton filter. The amounts of outlet gas is measured by the gas

meter. Gas samples, which are take~ after the cotton filter, are first

completely dried using a drying agent before being analyzed using a

gas chromatograph.

The fuel throughput range is approximately 0.1 to 0.5 kg dry fuel/hour.

Experiments have shown the limit where increased throughput results in

a decrease in the thermal decomposition of the fuel (4-7 g/m2,s related

to the heated reactor surface). Results from Russian experiments with

flash pyrolysis of peat also indicate a limit in the same order of

magnitude (3).

Normally nitrogen is used as carrier gas but other gases and/or steam

can also be used. Although not normally done, the carrier gas stream

not passing the screw feeder can be preheated. The total carrier gas

flow varies between approximately 5 and 65 ml/s at STP (5 ml/s through

the feeder).

In order to achieve a flow without too much pulsation, a vibrating

screw feeder is used. A tube parallel to the screw, ensures equalization

of pressure in the fule hopper and the rest of the system in case of

pressure fluctuation.

The vibrations from the screw feeder are damped by a bellow immediately

after the screw. The tube section before the reactor is cooled by air

in order to keep a reasonably low temperature above the reactor entrance.

The reactor consists of a replaceable inconel 600 tube (in some experiments a quartz tube) with an inner diameter of 2 cm and a heated length

of 1 m. It is heated electrically by eleven individually controlled

kanthal spirals wound round a quartz tube surrounding the reactor tube.

The temperature profile inside the reactor is measured by a chromelalumel thermocouple, adjustable to any height.

The residence time for a given particle size (the maximum speed relative

the gas stream is very much dependent upon particle size) can be varied

over a wide range by combining two,methods:

1. Using different carrier gas flows.

2. Varying the heated length of the reactor.

304
Experimental Apparatus

Screw feeder

Inconel tube

Kanthal spirals

Quartz tube.

Thermocouple

Fuel ho er

Insulation

Packed bed

[Z] Glass wool (pyrex)

GCanalysis

fl'

Cotton filter

ft n

Gas Nitrogen

Tar water condenser

Thermometer

Tar condenser

Therrnocoupte

Dust filter

-'" Char hopper

Figure 1

305
The thermal expansion of the reactor tube is taken up by a bellow before

the char separating system. The above described reactor oven has been

used since summer 1977. Previous to this, the oven consisted of six

vertical kanthal spirals which only were controlled as a whole unit,

i.e. the shape of the temperature profile and the heated length were

unvariable.

The whole char separating system, except the char hoppers, is kept at

300-S00oC by heating tapes in order to prevent condensation of the

liquid products, and at the same time keeping the temperature well

below the reactor temperature.

If blockage occures in the cyclone inlet, it can easily be removed

during an experiment by using the moveable cleaning wire.

The above described char separating system has been used since 1978.

In earlier experiments, e.g. those discussed below in paragraph 6, all

products passed the cyclone, the unseparated dust being caught at the

beginning of the tar condenser. This dust was removed and weighed

separately after the experiments. With this system, however, bigger

particles often gathered in the horizontal cyclone inlet if the carrier

gas flow was too low to blow them into the cyclone.

All liquids are condensed in two steps, the tar condenser and the tar

water condenser, after which tarry aerosol, that is always formed, is

caught in the cotton filter. The first step, the tar condenser, is

heated by heating tapes. The temperature at the inlet is kept between

300 and SOOoC, and is then lowered along the condenser to SO-lSOoC at

the outlet in order to achieve better separation between tar, carbon

black and tar water. Also, experiments have shown that, if the tar

condenser is heated as mentioned above, the amounts of aerosol is

markedly reduced compared to if the tar condenser is not heated. A

probable explanation is, that if the gas stream is cooled too rapidly

the water starts to condense at the same time as the tar. This yields

a very stable aerosol.

The tar condenser and the tar water condenser are filled with glass

wool and ceramic fillers respectively.

In the experiments presented in paragraph 5.1.2 the condensing system

was moved and a second vertical reactor for further conversion of the

gas phase (i.e. both gases and liquids at room temperature) was connected between the dust filter and the tar condenser. This reactor had a

quartz tube with a heated length of 1.1 m and an inner diameter of 3 cm.

It was heated by an oven similar to the pyrolysis oven but with 12

individually controlled k~nthal spirals.

Gas samples could in this case also be taken between the dust filter

and this second reactor.

3. EXPERIMENTAL PROCEDURE AND CALCULATION OF RESULTS

Before an experiment, the fuel hopper is loaded with dried fuel

(moisture content about 1.6 % for biomass, 1.4 % for peat and 0.5 % for

306
solid waste) and after that air in the fuel hopper is removed by

nitrogen. The screw feeder is then calibrated. All ovens and heating

tapes are heated to desired temperatures and the temperature profile

1n the reactor (and in the cracking reactor when that is being used)

1S measured. The carrier gas flow is measured by the "ga s meter.

An experiment is started by turning on th~ screw feeder. During experiment, the outlet gas amounts are continuously measured by the gas meter.

Normally three gas samples are taken during one run (0.5-1 h) and

analyzed for NZ' 02' HZ' C?, CO2, CH4, CZHZ' CZH4 and CZH6 contents.

Also, the temperature prof1les 1n the reactors are measured. The

temperatures from all thermocouples are continuously recorded. During

the experiments presented in paragraph 5.l.Z the temperature in the

cracking reactor was adjusted to three levels while the pyrolysis

reactor temperature was held constant. Then, one gas sample was taken

after the cotton filter for each temperature level, and two gas samples

were taken before the cracking reactor.

After the experiments, all separated char is taken and weighed; all

tubes in the char separation system, the cyclone, the char hoppers and

the reactor tube are brushed. The glass wool in the dust filter is

weighed before and after a run. The total amount of liquid products is

calculated from the weight differences between, before and after a run

of all parts in the condensing system, including the cotton filter. It

is not possible to achieve complete separation of the different

components carbon black, tar and tar water. Therefore, the percentage

contents of these components in the liquid phase are estimated visually

for each condenser and the yields of each component is calculated from

these estimates. They are therefore marked with dotted lines in Figure

4 and 5. The errors in these estimations are probably systematic, so it

should be possible to compare the results from different runs.

The yields of gaseous products and the dry product gas composition are

calculated from:

- Measured carrier gas flow before experiment

- Measured total gas flow during experiment

It is assumed that this gas flow is saturated with water vapour,

about Z vol-% at room temperature

- The analyzed gas samples

The average nitrogen content in the dry gas samples is compared to the

nitrogen content in the total dry gas flow during experiment calculated

by comparing this with the measured carrier gas flow. The average

between those two contents, or the one with the believed lowest experimental error, is then used for further calculations.

The total yield of all products, i.e. char, liquids and gases, is

compared to the total fuel feed calculated from the calibration of the

screw feeder. The total product yield is normally 90-100 % of the total

fuel feed.

The temperature referred to as the reactor temperature is the estimated

average temperature 80 cm from the reactor inlet (with maximum heated

307
reactor length) during a run. Depending on the shape of the temperature

profile, it is sometimes adjusted up to 100C. Examples of temperature

profiles, see Figure 2. (In the experiments discussed in paragraph 6

with the old reactor oven, see paragraph 1, the shape of the temperature

profile was somewhat different and therefore the average temperature

70 cm from the reactor inlet was used).

The solids residence time is calculated from the gas velocity in the

reactor and the maximum speed of the particles relative the gas, which

in turn is calculated from particle and gas data. For these calculations, the reactor is roughly divided into three zones based on the

temperature profile:

Zone 1: The temperature increases from 1000C to the reactor

temperature. The residence time in this zone was

0.1-0.3 seconds for biomass particles between 0.4

and 0.6 nnn.

Zone 2: Constant reactor temperature. The length of this

zone was varied between different runs, which

resulted in residence times from 0.04 to 1.4

seconds.

Zone 3: Temperature decreases from reactor temperature

to 40o-S000e at reactor exit. Residence time

was 0.1 to 0.3 seconds.

The used residence time is defined as the total residence time in these

three zones. In Figure 2 the temperature profiles for five of the

experiments presented in Figure 4 are shown, together with number of

heated kanthal spirals, carrier gas flows and the resulting residence

times of the solids. The residence time decreases rapidly with increased solid particle size, and this limits the maximum particle size that

can be used.

4. REACTIONS "INSIDE THE CHAR. PARTICLES

4.1 Results

Results from pyrolysis of wood at different heating rates are shown in

Figure 3. It can be seen that flash pyrolysis (heating rate in the

order of 10000e/s) at temperatures above 6000C results in less char

than pyrolysis with low heating rates (20 0C/min). It can also be seen

that the amount of char after final carbonization is almost temperature

independent above approximately 6S00e.

The weight loss by flash pyrolysis of wood at different solids residence

times are shown in Figure 4. At a final temperature of 760-77SoC the

necessary residence time for complete devolatilization is little more

than I s.

Char from flash pyrolysis has been heated slowly (20-S00C/min) to the

same final temperature as with flash pyrolysis and held there for 15

min (Figure 4) and 20-60 min (Figure 3) respectively. Other experiment~

308



have shown that this final devolatilization is completed in less than

10 min.

The results in Figure 3 and 4 show that high heating rate, i.e. flashpyrolysis, gives less char than low heating rate, over the whole

investigated temperature range providing that the residence time at the

final temperature is long enough for complete devolatilization. From

Figure 4 it can be seen that, for flash pyrolysis, the char yield after

complete devolatilization is nearly constant for residence times longer

th&n 0.7 s in the flash pyrolysis reactor. This may be the time

necessary for the particles to reach the final temperature and for the

primary reactions to be completed.

Experiments with stearn-gasification of char, has shown that char from

flash pyrolysis is much more reactive than char from pyrolysis with

low heating rate (2-3 times higher reaction rate at a given temperature).

This char property is related to the chemical char structure and not to

physical properties like surface area (9).

4.2 Discussion

The markedly reduced char yields from flash pyrolysis with final

devolatilization of the char compared to the char yields from pyrolysis

with low heating rate can be explained by the theory presented by

Shafizadeh and Brunner (10), (11). At low temperature « 3000C) slow

dehydration of cellulose takes place producing a product, which is more

stable than cellulose against the fast depolyrnerization to primary

volatiles, occuring at higher temperatures. High heating rates provide

shorter time for the dehydration to take place than low heating rates,

which results in a more unstable material left for depolyrnerization to

primary volatiles and therefore lower final char yields.

The char yield is also dependent on the mass transfer of primary

volatiles out of the char (12). Very slow escape of volatiles (for

example big particles) especially at low temperatures results in

secondary charring reactions.

The primary pyrolysis reaction leads in all cases to char as one product

in the range of experimental parameters that we have studied. Particle

size distribution and macro structure of the wood feed is reflected in

the properties of the produced char.

The very strong temperature dependence of the char yield from flashpyrolysis (Figure 3) at temperatures below 7S0oC is probably a kinetic

effect. Above 7S0oC, the solids residence time in the f lash pyrolysis

reactor is enough for almost complete devolatilization, i.e. the

temperature dependence on the reaction rate at higher temperatures

could not be seen in these experiments.

Such a strong dependence on temperature for the necessary time for

complete pyrolysis has also been shown by Shafizadeh at temperatures

between 300 and 42SoC (10).

312


It should be observed that our results do not show whether the chemical

reaction or the heat and mass transfer inside the particles is the ratedetermining step.

5. SECONDARY GAS PHASE RE1\CTIONS

5.1 Results

5.1.1 Flash Pyrolysis Experiments

The yields of volatiles at different solids residence times when

pyrolyzing wood at 760-7750C are shown in Figure 4. A slight maximum

yield of tar + carbon black (carbon black is formed in the gas phase

at higher temperatures, see part 5.Z.3) is found after 0.7-0.8 s.

The product distributions at different reactor temperatures are shown

in Figure 5. The total yield of the liquid fraction by flash pyrolysis

is about half that of the "primarily" produced liquids (1). This

corresponds to a much higher and with temperature rapidly increasing

gas yield.

The production of different gases as a function of reactor temperature

for biomass is shown in Figure 6. The CO -yield is nearly constant in

the measured temperature range and the Cn4-yield does not increase

markedly above a temperature of 850oC. The CzH4-yield is maximized

between 800 and 8500C for all investigated fuels.

The product gas composition at different reactor temperatures is showp

in Figure 7. The gas composition is far from that corresponding to

thermodynamic equilibrium for the two reactions:

CO + HZO COZ + HZ

CH4 + HZO CO + 3HZ

In order to reach thermodynamic equilibrium, both CO and CH4 should

react with some of the pyrogenetic water. The high content of hydrocarbons give the product gas a relatively high heating value (3300 to

4Z00 kcal/SCM).

5.l.Z Flash Pyrolysis followed by Further Conversion of the Gas Phase

In these experiments the gas phase from the flash pyrolysis reactor

passed a second cracking reactor after the char had been separated

(see end of paragraph Z).

The results for different conditions are shown in Figure 8, 9 and 10.

The temperature in the pyrolysis reactor was held constant while the

temperature in the cracking reactor was varied between 500 0C and 9500C

in each experiment. Results with pyrolysis temperatures between 5ZloC

and 9l30C are presented. As the conditions varied during one experiment,

only products that could be measured continuously, i.e. the gas (gas at

room temperature) could be used for calculating the results. Gas samples

were taken before the cracking reactor and after the liquid condensing

313

.\

system. . The only way to see if an increase in gas yield came from tar

and carbon black, and not from reactions between gases and pyrogenetic

water, was to calculate and compare the C-content in the product gas

from different conditions.

As can be seen in Figure 8, a small part of the tar and/or carbon black

from flash pyrolysis has been cracked, but that part increases, when

the temperature in the cracking reactor increases. Cracking at lower

or at the same temperature as in the flash pyrolysis reactor results

only in small reduction of the tar and/or carbon black yield.

The production of methane and ethylene is shown in Figure 9 and 10

respectively. A higher gas phase temperature than in the flash pyrolysis

reactor, after separation of the char, results in increased production

of both methane and ethylene. For ethylene, there is a maximum yield at

about 700oe, except for cracking after flash pyrolysis at low temperature (52l-5420C).

The product gas composition at a given temperature in the cracking

reactor was o·similar to the gas composition only after flash pyrolysis

at the same temperature, i.e. the gas, was still far from thermodYnamic

equilibrium.

5.2 Discussion

The volatiles that have escaped from the char can undergo further

secondary reactions in the gas phase to form different hydrocarbons,

hydrogen, carbonoxides and carbon black. The results presented above

clearly show that the rate at which primary volatiles are converted

increases markedly with increased temperature, especially above 500oC.

This has also been shown by Antal (14), (16) and Shafizadeh (10) for

cellulose. Antal has also shown that steam reforming reactions probably

do not dominate the secondary reactions.

When discussing results from pyrolysis experiments, it is important to

separate the influence of:

a) Heating rate

b) Solids residence time

c) Gas phase residence time

5.2.1 'Ther mal Cracking of Tar

As already mentioned the total yield of liquid products from flashpyrolysis is about half as much as that from pyrolysis with low heating

rate. In the latter case, volatiles formed at temperatures below 5000C

have time to leave the reactor without being further heated, i.e. undergoing a minimum of secondary gas phase reactions, while in the co-current

flash pyrolysis reactor, all the volatiles have to pass the hotest zone

where they can undergo secondary reactions. An experiment with flashpyrolysis of solid waste at about 500 oC, i.e. too low temperature for

rapid thermal cracking, also gave rather high yields of tar (about 30%).

These results are similar to those from the Occidental Flash Pyrolysis

Process (2), (17).

317



However, for wood, the tar yield is rather low after flash pyrolysis at

5000C (about 12 %). As can be seen in Figure 3 and 5, the total yield

of volatiles after flash pyrolysis also is rather low at 500oC, and

increases markedly with increased temperature up to 750oC. As mentioned

before, the rate of thermal cracking of tar increases markedly with

temperature, and these effects together makes the tar yield for biomass

rather constant over a wide temperature range. At high temperatures

(above 7500C) both the rate of primary pyrolysis and the rate of thermal

cracking are rather high. As can be seen in Figure 4, almost complete

devolatilization is achieved after 1 s, and the parallel rapid thermal

cracking of the tar and devolatilization of the fuel results in only a

slight maximum in total yield of tar and carbon black after 0.7-0.8 s.

The results presented in paragraph 5.1.2 show, that the remaining tar

after flash pyrolysis does not undergo considerable thermal cracking if

the gas phase pass a second oven at the some or somewhat lower temperature than in the flash pyrolysis reactor. Also, the amounts of tar left

after Antal~s pyrolysis experiments with low heating rate, followed by

thermal cracking, were similar to those found after flash pyrolysis

(14), (16).

The above mentioned indicates that only some of the hydrocarbons in the

tar can undergo rapid thermal cracking (below 9500C), while the others

decompose very slowly. Experiments has shown that these last mentioned

hydrocarbons, and also carbon black, can be converted by catalytic

steam reforming (4), (5), (6).

5.2.2 Formation and Breakdown of Lower Hydrocarbons

The results presented in paragraph 5.1.2 show that a great deal of the

methane and ethylene from flash pyrolysis is formed in the gas phase by

thermal cracking of tar. This has also been shown by Antal (14). The

rate of breakdown of C2H4 and other unsaturated hydrocarbons increases

markedly with temperature and partial pressure (13), (14), (15) which

explains the maximum yields of ethylene at 7000C for flash pyrolysis

followed by thermal cracking and 800-850oC for flash pyrolysis only.

It also increases in presence of pyrolyzing biomass (15).

Diebold ettal . have also shown, that the ethylene yield can be raised

by increased dilution of the gas phase (resulting in lower partial

pressure) (18).

The results in paragraph 5.1.2 also show, that increased temperature in

the pyrolysis reactor results in increased yield of methane and ethylene

after the thermal cracking step. This indicates, that the yield of

primary tar, from which those hydrocarbons are formed, increases with

increased pyrolysis temperature, or that high heating rates result in

the formation of methane and ethylene directly from the solid phase.

5.2.3 Formation of Carbon Black

Experiments with flash pyrolysis of peat and solid waste show that

. carbon black is formed .in the gas phase from hydrocarbons in the tar

(1), (6). When pyrolyzing wood, it is formed at all temperatures above

321
500oC, but for peat and solid waste it is formed first above 7500C and

6000C respectively. According to Lavrov (19) and Berezina and Magaril

(20) soot is formed from different hydrocarbons with benzpyrene as an

important intermediate.

It should be observed that carbon black is formed in the gas phase at

high temperatures and is condensated together with the tar. This is

not the case for char produced by secondary charring reactions, which

occure inside the char and at low temperatures. See paragraph 4.2.

6. COMPARISON OF WOOD, SOLID WASTE AND PEAT

Flash pyrolysis of all these fuels give principally the same products,

and they also seem to undergo the same reactions, although the yields

of the products differ.

The char yields after final carbonization of wood and the combustible

part of solid waste are nearly the same, a little more than 10 % of

m.f. and m.a.f. fuel respectively above 650oC. For low moor peat it is

much higher, 30-35 % of m.f. peat (1), (6). It is very interesting to

notice that the reduction in char yield due to high heating rate tends

to be 8-10 % on a m.a.f. basis for all fuels investigated. That means

a 50 % reduction in char yield for wood and solid waste but only 20 %

for peat.

The total yield of liquid products do not differ much between wood and

peat, about 35 % and 30 % respectively at 6000C and about 13 % for both

fuels at 1000oC. Solid waste give higher yields, about 45 % at 6000C

and about 30 % at 1000oC. A great deal of this difference is due to

the much higher yield of pyrogenetic water for solid waste.

Consequently, the product gas yield is much higher for wood than for

peat, and especially for biomass it increases markedly with increased

temperature. For solid waste the gas yield is about the same as for

peat, but it reaches its maximum at a lower temperature, about 800oC.

Although the gas yields differ for the investigated fuels the composition of the product gas are very similar. For wood the CO-content in

the dry product gas is higher and the H2-content is lower than for peat

and solid waste. Also, the maximum content of C H4 is about twice as

much for solid waste and peat as for wood. The tigh hydrocarbon content

for all fuels result in rather high heating values of the product gas.

7. POTENTIAL USES FOR FLASH PYROLYSIS

7.1 Chemical Feedstock - Ethylene

In a special study attempts were made to opt1m1ze the ethylene yield

from flash pyrolysis. A maximum of 4 weight % ethylene on a m.a.f. basis

was achieved (15). As mentioned in paragraph 5.2.2 the reaction rate

for the breakdown of ethylene increases with temperature and is also

higher in presence of pyrolyzing biomass. As has been seen above, the

322
yield of volatiles is independent of temperature above 6500C (with long

.pa r t i c1e residence time, Figure 3) and the ethylene is to a large extent

a secondary product from volatiles. This means that further studies

should be directed towards processes with a primary step with high

heating rate and long residence time for particles but short residence

time for gases and a moderate temperature (6000C). A fast fluidized bed

is' one example of this reactor type. The secondary step after high

temperature removal of fly ash can be fully qptimized regarding temperature and residence time for maximum ethylene yield. The important

questions regarding suitable methods for introducing heat to both these

steps have not yet been closely investigated. Some factors indicate

that indirect heating is most interesting:

a) Steam crackers for ethylene production operates at atmospheric

pressure, and the secondary step in a plant for making ethylene

from solid fuels would be rather similar to a conventional unit.

o b) The extremely low temperature (600 C) at which heat has to be

supplied to the primary gasifier.

7.2 Medium-BTU Gas

Production of syngas and fuel gas can also utilize flash pyrolysis as

an important step. Low exit temperature and a very rich medium-BTU

gas is possible by combining concurrent fluid bed gasification, high

temperature dust removal and suitable catalyst for product gas adjustment. Such processes have been presented lately and a PDU will be built

in Sweden (6), (21). System studies on the wood to methanol process

have indicated that oxygen blown processes are superior to airblown/

indirect heat transfer processes (21). Fuel gas production with indirect

heating might be interesting due to the smaller scale and the lower

reactor temperature.

ACKNOWLEDGEMENT

The authors are indepted to Professor Olle Lindstrom, by whom this

research work was initiated, for his many helpful suggestions and

encourageing support. Grant support from Swedish National Board for

Energy Source Development and Swedish Board for Technical Development

are gratefully acknowledged.

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323
3. Reprintseva, S.M., "An Experimental Investigation of the Thermal

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Pyrolysis/Steam Reforming", IGT-Conference on Energy from Biomass

and Wastes, Washington, D.C. (1978).

17. Preston, G.T., "Resource Recovery and Flash Pyrolysis of Municipal

Refuse", Waste Age, May (1976), p.p. 83-98.

18. Diebold, J., Smith, G., "Conversion of Trash to Gasolinert , NWC

Technical Publication 6022, Naval Weapons Center, China Lake,

California (1978).

19. Lavrov, I.V., "Some Questions of the Theory of the Pyrolysis of

Methane and possible Mechanisms of the Formation of Soot", Khimiya

Tverdogo Topliva, Vol. 10, No.2 (1976), p.p. 100-107.

20. Bereiina, Z.N., Magaril, R.Z., "The Formation of Pyrocarbon in the

Pyrolysis of Individual Hydrocarbons", Khimiya Tverdogo Topliva,

Vol. 11, No. 5 (1977), p.p. 156-159.

21. Waldheim, L., Rensfelt, E., "Development of a Swedish Biomass

Gasifier for Synthesis Gas Production", 11th Biomass Thermochemical

Conversion Contractors Meeting, U.S. DOE, Richland, Washington

(1980).

325
CYCLONE REACTOR FOR FLASH

PYROLYSIS OF SOLID PARTICLES

Application to the fast pyrolysis of

wood by use of solar energy.

J. LEDE, F. VERZARO, B. ANTOINE and J. VILLERMAUX

Laboratoire des Sciences du Genie Chimique

(Chemical Engineering Science Laboratory)

CNRS-ENSIC, 1, rue Grandville 54042 NANCY FRANCE

ABSTRACT

The purpose of the present paper is to present the results obtained on

the study of a continuous reactor suitable for fast pyrolysis of biomass (wood particles). The cyclone, the advantages of which are underlined, seems to be one the most adapted device for reactions occuring

in less than 1 second. The results presented are relative to theoretical and experimental investigations concerning chemical reaction,

residence time distribution of carrier gas and solid particles, heat

transfer processes and study of the competition between chemical process and conduction of heat within the reacting solid. The correlations

deduced from these studies are used for· the calculation of the characteristics of a fast pyrolysis cyclone reactor capable to operate in a

solar concentrator of a given power.

327
INTRODUCTION

A possible solution to the present problems of energy is to produce

synthetic fuels from biomass and particularly from wood. Different routes can be followed for energetic valorization of wood : combustion,

gasification (with air or O2), pyrolysis (slow and fast), hydrolysis,

fermentation and liquefaction. Among the thermal processes, fast pyrolysis is deserving attention because of the large yield of gases evolved, tar and char production being minimized by fast heating. The evolved gas has a higher heating value than in the case of usual gasification. It contains large percentages of CO and H2 and appreciable

amounts of hydrocarbons such as C2H4 and C2H2.

Gasification and fast pyrolysis reactions are endothermic and a source

of heat is required. Heat can be produced by the combustion of a part

of the wood feedstock (usual gasifiers), of an external fuel or of a

part of the reaction products. Use of concentrated solar energy would

bring the following advantages :

· Saving of biomass (up to 30 %) , the entire feedstock being used for

the reaction itself

· Saving the cost of an oxygen unit

• Gas evolved not diluted in N2 and CO2

Very c lean source of energy

· The focal volume is a region of high concentration of energy : these

are ideal conditions to ensure high heat flux at the wood surface (fast

pyrolysis conditions). Moreover, each gaseous mole~ule evolved, immediately penetrates i n t o a cold non absorbing gaseous medium : these are

favorable conditions for quenching of products.

· The energy evolved by combustion of the products represents the sum

of the solar energy stored during photosynthesis (formation of biomass)

and during the endothermic reaction (in the solar c o nc e n t r a t o r ) .

Our purpose is to study a new type of reactor for solar fast pyrolysis

of wood.

PROGRAMM OF THE STUDY

The problem will be e xamined with a chemical engineering approach as

follows :

· Research of the principal characteristics of the chemical reaction

itself

Selection of the type of reactor suitable for t he reaction

Hydrodynamic studies (residence time measurements for carrier gas and

solid)

328
• Studi~s of heat transfer processes in the reactor

· Theoretical and experimental investigation of competition between

heat transfer and chemical processes at the particle level.

· Prediction of the possible performances of a cyclone reactor operating in a given solar furnace.

PRINCIPAL DATA CONCERNING THE CHEMICAL REACTION

1. Analysis of gaseous products

The results of preliminary experiments have been published elsewhere

[1,2J. These experiments have been carried out with Douglas fir and

beech sawdust samples in discontinuous operation (free falling in an

electric furnace of known temperature or across the focal zone of an

image furnace) and in continuous operations (constant flowrate of sawdust in the image furnace device or in a small vortex reactor with

walls heated at 1323 K [3J.

A fairly good agreement has been observed in the results in account

of the different experimental conditions.

The gas contains a large proportion of CO and H2 (more than 70 % of the '

total volume) and appreciable amounts of C2H4 (around 7 % of total volume). H2 and C2H2 concentrations increase with temperature while CO,

CO2 and C2H4 concentrations decrease ' [2, 4J .

The mass yield of gas increases with temperature. Volume of gas evolved

above 1273 K corresponds to about 1 m3 NTP per kg of wood pyrolysed.

2. Water effects

Experiments made with wet samples show that water plays a significant

role above 973 K : mass yield of gas, H2 and CO proportions increase

12,4 \.

Water has also a significant importance when brought as steam, as proved by other types of experiments.

-3 The end cross section of a beech wood rod (5, 8, 10 and 16 x 10 m

diameter) is set up at the focus of an image furnace (made with 2 parabolic mirrors 1.5 m in diameter associated with a carbon arc discharge

providing a heat flux of the order of 3.5 x 106 w m- 2 at the focus).

The reactive part of the wood rod is placed inside a pyrex reactor fed

by a constant steam flowrate. The rod is automatically moved forwards

as the volatilization reaction proceeds, with a linear velocity of the

order of 3 x 10-4 m s-1. Gasi£itation yields are close to 100 % with

H2/CO ratio greater than 1 [5J.

Experiments with wet wood and under steam atmosphere show that continuous operation would profit by using steam as a carrier gas.

3. Heat of reaction

329
Reaction is endothermic above 973 K and the heat of reaction reaches

about 10 % of the heat of combustion of wOud at 1273 K [4J. This value

increases in the presence of water and reaches 30 % for a theoretical

complete reaction giving only CO and H2 (5600 KJ x Kg-i) [2J.

SELECTION OF A REACTOR ADAPTED TO FAST PYROLYSIS

The problem is to choose a continuous reactor suitable · for a reaction

occuring between solid particles (wood) and a gaseous phase (H20) for

reaction times smaller than one second. Figure 1 shows that among the

different gas solid reactors available, free falling reactors, transported beds and vortex devices (cyclone) are recommended for reactions

occuring between 10-3 and 10 s, with small particles s~zes (between

10-6 and 10-3 m). One can expect they are the most adapted to fast pyrolysis of biomass.

The best known vortex device is the cyclone which is generally used to

separate fine particles from a gas. Its advantages for past pyrolysis

are the following :

• Low residence time for solid (fast pyrolysis condition) and for carrier gas (quenching condition)

· The shape of the cyclone is such that it can be easily set up inside

a solar cavity

· Chemical Engineering studies of the cyclone operating as a chemical

reactor have not been extensively published, offering a large and interesting field of research

· One can expect that ashes and unreacted particles will be automatically separated from gaseous products (separator aspect of the cyclone).

In ,t h e conventional configuration (Figure 2) the gas solid mixture tangentially enters the cyclone and is forced into a constrained vortex.

Under the influence of centrifugal force, solid particles are forced

towards the walls. The gaseous vortex flow is reversed in the lower

portion of the cyclone leaving most of the entrained particles which

are collected at the bottom while clean gas escape at the top.

RESIDENCE TIME STUDIES

Studies of the hydrodynamics of carrier gas and wood particles have

been made at room temperature.

1 . Residence Time Distribution (RTD) of the gas

These experiments have been previously described [1,6J. Air flows in a

conventional cyclone (1.25 x 10-1 m diameter) under steady state conditions. A pulse of H2 is injected at the inlet. Acatharometer detec~or

records the outlet concentrations signal, the shape of which is analysed on a computer as a function of the gas flow rate.

330


The flow pattern in the cyclone can be represented by a plug flow zone

(fraction S of the total volume) followed by a well stirred zone (fraction a of the total flow rate) more or less by passed by the gas (figure 3). The relative importance of these two zones have been correlated as a function of the entrance Reynolds number Reo

10-7 Re 2

o

1 - a = 3.72 x 10- 1 1 Re 3

o

(Re is calculated with inlet duct o

( 1 )

(2)

hydraulic diameter).

Figure 3 shows that for Reo < 500 the cyclone can be assimilated to a

stirred reactor while for Reo> 3000 it works like a plug flow reactor.

Intermediate conditions must be avoided because of by pass.

2. Residence Times of particles

Two optical cells are set up at the inlet and at the bottom of the cyclone. Ea~h cell is made with a phototransistor facing an infrared

light emitting diode. A pulse of a small quantity of wood sawdust is .

introduced at the inlet of the cyclone (1.25 x 10-1 m diameter) and

passes across the first cell . . A pulse is delivered and starts a timer

which is stopped when the "particles leave the cyclone through the . 's e -

cond cell. A ~icroprocessor board computes the elapsed time and gives

the residence time of the particles on a display unit. Figure 4 gives

the results obtained with different particle sizes as a function of Reo .

The solid residence time t s is surprisingly higher than t

G for high valU~s of Reo. For low values of Reo, t s tends to the free falling time

wh1.le t

G -+ 00.

t

It seems advisable to work with ratios close to 1, in conditions

where results obtained on gas t G RTD could apply to solid particles.

These conditions correspond to Reo around 6000 where the cyclone is

working like a plug flow reactor (as shown by gas RTD measurements) .

HEAT TRANSFER STUDIES

1. Heat transfer between the heated walls and the carrier gas

These experiments have also been previously described [1, 6J. A small

stainless steel conventional cyclone (1.68 x 10-2 m diameter;

9.64 x 10-6 m3 volume and 3,2 x 10-3 m2 surface) fed by a known flowrate

of argon is placed at the focus of an image furnace operating with a

4000 W xenon lamp. Gas temperatures at the inlet (T ) and at the outlet

o

(TS) are measured by thermocouples while wall temperature (TW) is estimated with an optical pyrometer. The measurements are made· as a function of Re .'

o

A theoretical analysis taking into account the results obtained on gas

RTD leads to the following relationship [6J

333


TW - TS [ ------- = exp(- N) 1 - a +

TW - To (3 )

where N h S ~~-- is a number of transfer units.

Qm CPG

a and 8 are given by (1) and (2) when Re is known. h is the overall

o heat transfer coefficient

h

Nu beeing the Nusselt number calculated with d (hydraulic diameter of

o V H . the cyclone = 4 ).

S

For Re > o

T

W

- T

S 3000 one can verify that = exp(-N). TW - To

Experimental measurements of To' TS and Tw have been made for

600 < Reo < 1600 and the following relationship has been proposed(fig.5)

Nu

o

35 0.38 1. Reo (4 )

Values of Nuo are greater than those corresponding to a turbulent flow

in a cylindrical duct. For Reo · = 1500 the mean residence time (calculated at 988 K) is 6.2 x 10-2 s. This corresponds to a heating rate greater than 104 K S-l .

-2 -1 For Reo > 3000 h is of the order of 50 watts m K .

2 . Heat transfer between the gas and the particles

Wood particles can be heated by forced convection (heat transfer coefficient hc) and by free convection (heat transfer coefficient hL) .

For spheres, h

c

and hL can be estimated thanks to the correlations [7J

(5 )

(6)

For wood sawdust (dp

-4 2 x 10 m, '1'

s

336

1200 K, T

o

300 K) one calcu-

lates the following orders of magnitude

h

c

-2 -1 = 700 watts m h and m- 2 K-1 hL = 800 watts

3. Heat transfer between the particles and the heated walls

. Radiative heat transfer

The heat transfer coefficient hR can be estimated as [7J

(7)

with E 1200 K and TS = 300 K

one finds h R = 65 watts

-2 -1

m K

This transfer process is probably of minor importance.

. Direct transfer between the particles and the wall

This transfer process is probably the most efficient one because of the

strong frictions occuring when the grains forced to the walls undergo

intensive whirling motions on the heated surface. Experimental measurements of these effects would be of major importance but have not been

performed yet.

As a conclusion, many heat transfer processes contribute to a fast heating of the particles and we can assume that surface temperature

of particles is rapidly equal to the temperature of the carrier gas. The

validity of such an assumption can be checked by the results of chemical experiments : fast pyrolysis of sawdust has been carried out in a

small cyclone (1.68 x 10-2 m dia~eter). The gas temperature was measured to be 1143 K. The chemical analysis of gases gave results close to

those observed in an electric furnace at 1173 K, a value close to

1143 K 121.

ANALYSIS AT THE PARTICLE LEVEL (Study of the competition between chemical process and conduction of heat within the solid)

1. Theoretical point of view OiL

Consider a particle undergoing a high intensive heating at its e xternal

surface and suppose that the solid decomposes to gaseous products. A

model has b een proposed [8J to represent the thermal volatilization of

particles in conditions of heat transfer limitations between the external surface and the internal parts of the particles.

with four different boundary conditions :

constant heat flux at the surface ; 'c o n -

gas at constant temperature ; radiative

The problem has been treated

constant surface temperature

vective heat e xchange with the

e xchange with the hot wall.

338
Two extreme situations can be imagined :

. The chemical processes are much slower than heat conduction in the

solid. The volatilization is then controlled by chemi.cal reaction. The

temperature of the whole solid is nearly uniform and the reaction occurs in the whole volume : the particle is said to valatilize in "chemical regime ll

•

. The chemical processes are much faster than heat conduction. A steep

temperature gradient appears close to the surface, in the first internal layers, where the reaction occurs, while the deeper parts of the

particle remain cold. In such a case, volatilization is controlled by

the competititon between chemical reaction and diffusion of heat. Even

in this extreme condition, diffusion of heat must be taken into account

and the concept of a surface reaction is irrelevant : the particle is

said to volatilize in "ablation regime".

The processes involved in these two extreme situations can be respectively represented by two characteristic times :

the reaction time

Ps E -3 -1

t R = where r = k exp - ~T is the chemical reaction rate (Kg m s )

the thermal conduction time :

L

t

T

0 where L is

aT 0

fusivity

2

t

T L r

Let M 0 -

t PSa R T

In chemical regime

In ablation regime

the radius of the particle and aT.its thermal dif-

(8) be a thermal IIThiele modulusll

M « 1

M » 1

In the case where the surface is maintained at constant temperature

(equal to the temperature of the gas) one can show that the time

required for a particle to disappear in ablation regime is given by

( 9)

where b is a constant factor depending on the heat of reaction.

The experimental measurement of t F allows the calculation of M thanks

to the relation

4

b 2 L

M 0

2 2

a t

T F

339

(10)
According to the value of M, the volatilization regime (chemical or

ablation) can be determined.

t.r can be estimated (L~:J and with help of relations (8) and (10) it is

possible to calculate t R and to deduce the value of r, the chemical

rate that would be observed if no diffusional limitation occured.

Notice that it is also possible to determine the l inear ablation velocity uL thanks to the relation

= _1 [~r] 1/2

b p C s ps

(l1 )

2. Experimental determination of the volatilization regime in a cyclone

fed with calibrated sawdust

A stainless steel cyclone (1.68 x 10-2 m diameter) is heated at the focus of an image furnace. The cyclone is fed in continuous operation by

a constant flowrate of beech wood particles (1.5 - 2 x 10-4 m diameter)

fluidized in argon (known entrance Reynolds number Reo)' The outlet

temperature of the gas T and the unreacted fraction f s of wood are measured as a function of R~o'

Taking into account the results previously obtained on RTD of gas and

assuming the same behaviour for solid particles one can write [6J :

Re

o < 500 fs = (l2 )

Re > 3000

o

(l3)

-

t is the mean residence time.

Experimental results are shown on table 1.

TABLE 1

- Gas flowrate Solid t (s )

3· -1 flowrate Re T (K) (750 K) f tF(s) m s (NTP) Kg s-l 0 s s

1.33 10-4 2.37 -4 3200 1141 27 .5 x 10"':'3 0.113 53 x 10-3 x x 10

1.38 x 10-4 2.46 x 10-4 3320 1126 26.5 x 10-3 0.111 51 x 10-3

2.37 x 10-4 5.62 -4 7600 1088 15 10-3 0.423 60 x 10-3 x 10 x

340
The values of complete volatilization times tF are calculated according

to (13). The fact that t F is relatively independent of E is a proof of

the validity of the model.

From relation (10) it is possible to calculate M = 15 (with

= 5 x 10-2 sand b = 2.79 [6J) : this shows that fast py.roLys i s of

2 x 10-4 m particles of wood in a cyclone occurs in ablation regime

(M > 1).

The velocity of the ablation front uL is calculated to be of the order

of 2 x 10-3 m s-l.

The above estimations are only orders of magnitude that it would be inresting to confirm in further experiments with different particles

sizes.

PRESENT INVESTIGATIONS

This research is now being pursued along the following lines

1. Residence time of particles are studied in other cyclones of different sizes

2. The nature of products obtained by flash pyrolysis in electrically

heated cyclone fed by argon or by steam is compared

3. The behaviour of the cyclone is observed when the loading ratio

solid flow rate (Kg s-l) R = is increased. Ratios up to R = 10 can be

gas flow rate (Kg s-l)

achieved without appreciable alteration in the cyclone working conditions.

4. Different ways of heating the cyclone by solar radiation are studied.

Two possibilities can be considered

The cyclone, quite opaque, is set up in an absorbing cavity•

. The upper part of the cyclone is transparent (window) and ·concentrated solar radiation penetrates into the reactor which plays the role of

a cavity. The wood particles are heated up by the processes previously

described and also by direct absorption of light.

Only the first arrangement w~ll be considered in a first step.

A theoretical study has been made to calculate the distribution of radiative flux in a cavity inside which a reactor is set up [9J. Different cavities have been considered : cold cavities with specular or diffusive reflecting walls and hot cavities with diffusing walls. Calculations have been conducted in order to enhance absorption by lateral cylindrical walls with respect to the upper flat surface of the cyclone.

The hot cavity is found t~ give the best theoretical performance.

341
NUMERICAL APPLICATION

In so far as the correlations proposed in the present study are supposed to hold, one can predict the performances of a cyclone in which

fast pyrolysis of wood is carried out in a solar concentrator with a

given power P.

The calculations are made under the pessimistic assumption that heat

required to heat up water and wood at high temperature is not recovered.

The initial temperature of wood and · steam is chosen equal to 433 K.

The reactor is supposed to operate in conditions of plug flow assumption (Reo > 3000).

For a conventional cyclone with a D diameter

V 2.135 D3

S 11.66 D2

dH

4V

- 0.733 D

S

d D

- 0 3

Relations (3) allows calculation of TW required to ensure a given value

of TS (for example 1073 K)

Tw- Ts For Re > 3000 : exp(-N) o T - T

w 0

N

h S with h

No heat transfer measurement has been made with steam, but we can anticipate that the 1.35 factor in (4) depends on the nature of the gas.

Suppose as a first approximation that Nuo = 2 Pr ReO. 38 (for argon

Pr =0.67) .

Thanks to the general expression of Re

o

Re

o

4 Qm = 1f n

G d

o 0

(14)

we can wri.t.e

T - T

w 0

Log T - T

w s

x Pr

121. 5 _0 _

Cp Go nGO

1

R 0.62 eo

342

(15 )
with the following values for steam

= 1977 J

-1 -1 C Kg K

PGO

Pr 1.09

1..

0 29.5 10-3 -1 -1 x W m K

146 x 10-7 -1 -2

nG N s m

0

T - T

w 0

Logn T - T

w s

135.4

0.62 Re

o

(16 )

For example if Re

to calculate 0

T = 1473 K

P

3000 433 K 1073 K, it is possible

Suppose, now, the cyclone is set up in a cavity placed at the focus of

a solar concentrator with a given power P and that it is desired to

pyrolyse a massic flow rate R x Qm of wood. It is possible to calculate

Qm and D (cyclone characteristic).

P = R Q Cp (T - T ) + Q Cp (T - T ) + R Q ~H m s s 0 m G s 0 m

with P 6000 W

R 10

2800 J

-1 -1

Cp. Kg K

s

C

PG

2117 J Kg- 1 K- 1

3 -1 ~H = 14 x 10 J mole

(~ 9 Kg/hour)

The corresponding cyclone can be calculated to have a diameter

D = 2.2 x 10-2 m.

For P = 6000 Wand R = 1, the calculations give

Q = Q = 1.6 x 10-3 Kg m ms

-1

s (~ 6 Kg/hour)

and D 1.43 x 10- 1 m

343
For R = 10 the mean residence time is t = 28 x 10-3 s.

The size of particles entirely volatilized after a time t

F = t

28 x 10-3 s can be calculated thanks to relat~on (10). Let us choose

M = 20 in order to be in ablation regime, with aT = 1.4 x 10-7 m2 s-l

and b = 2.8 [6J we calculate :

d 1.6 x 10-4m

p

- -3 For R = 1, t = 1.1 sand d value increases to 10 m. p

All these calculations are, of course, orders of magnitude.

NOTATIONS

constant factor [at T + T

heat capacity of ] s 0

gas 2

heat capacity of gas (at To)

heat capacity of wood

c(YC1~ne+

. 0

gas mean resiaence time at 2

hydraulic diameter of the cyclone

particle diameter

inlet duct of the cyclone : hydraulic diameter

cyclone diameter

activation energy

unreacted fraction

overall heat transfer coefficient

forced convection heat transfer .c o e f f i c i e nt

free convection heat transfer coefficient

radiative heat transfer coefficient

preexponential factor

half initial thickness of a particle

thermal Thiele modulus

number of transfer units

Nusselt number (at T ) o Prandtl number

massic flow rate of gas

massie flow rate of solid

loading ratio

entrance Reynolds number

chemical reaction rate

internal surface of the

r

S

t

b

Cp G

C

PGo

Cp s

dH

dp

do

D

E

f s

h

hc

hL

h R

k

L o

M

N

Nu

o Pr

Qm

Qms

R

Re .

o

t

F time required for a particle to disappear

t

G gas mean residence time (at To)

t s solid mean residence time

t R chemical reaction time

t

T thermal conduction time

344
T inlet temperature

TO outlet temperature s

Tw wall temperature

u relative rate of gas and particle r linear ablation velocity

V volume of the cyclone

a stirred zone fraction (flow rate)

aT thermal diffusivity of wood

S plug flow zone fraction (volume)

llH enthalpy of reaction

E: emissivity of the solid

(at

T + T

of the gas ] s 0

nG viscosity 2

nGo viscosity of the gas (at To)

[at

Ts

+

To] A

G thermal conductivity 2 of gas

A thermal conductivitl of solid ] s Ts + To

PG density of the gas at 2

Ps density of solid

a Stephan constant

REFERENCES

[lJ J. LEDE, Proceedings of Solar Fuels Workshop, STTFUA Albuquerque (USA) (1979)

[2J J. LEDE, P. BERTHELOT, J. VILLERMAUX, A. ROLIN, H. FRANCOIS,

X. DEGLISE, Revue Phys. Appl., ~' 545 (1980)

[3J J. LEDE, F. VERZARO, J. VILLERMAUX, Revue Phys. Appl., ..!..~-, 535

(1980)

[4J X. DEGLISE, C. RICHARD, A. ROLIN, H. FRANCOIS, Rev. Gen. Therm.,

227 (1980)

[5J J. LEDE, B. ANTOINE, J. VILLERMAUX, C. RICHARD, H. FRANCOIS,

X. DEGLISE, ' to be published

[6J J. VILLERMAUX, F. VERZARO, J. LEDE, Part II, Rev. Gen. Therm.,

227 (1980)

[7J J. SZEKELLY, J.W. EVANS, H.Y. JOHN, Gas Solid Reactions, Academic Press (1976)

[8J J. VILLERMAUX, B. ANTOINE, Part I, Rev. Gen. Therm., 227 (1980)

[9J C. ROYERE, to be published

[1q] N. B. VARGAFTIK, Tables on the thermophysical properties of liquids and gases, John Wiley (1975)

345
FAST PYROLYSIS ON A MOLTEN LEAD BATH

J. M. Skaates

Department of Chemistry and Chemical Engineering

Michigan Technological University

Houghton, Michigan

HISTORY OF LEAD BATH PYROLYSIS

Molten lead as a heat source for biomass or fossil fuel pyrolysis has

a long history. Here we note only that in 1923 Henry Ford contracted

with Piron Coal Distillation Systems to build a plant in Detroit to

carbonize 4000 tons per day of coal by means of a steel conveyor belt

floating on a pool of molten lead [1].

The inventor of the lead bath pyrolysis process described in this

paper is Dr. Martin Fassell, a metallurgical engineer and former

director of research of Howmet. Dr. Fassell conceived the process

primarily as a means of disposing of mixed municipal solid waste and

generating a medium Btu gas.

In 1974 the Resource Recovery Systems Division of the Barber-Colman

Company, with Dr. Fassell as Division Manager, initiated a pilot plant

study of the process with the goal of maximizing the production of

fuel gas and a high value aromatic fraction. Test runs on wood chips,

agricultural wastes, and other types of biomass were carried in a

908 kg/day (1 ton/day) pilot plant unit. Development work on the

process continued until 1978, when Barber-Colman closed out the

Resource Recovery Systems Division and donated the lead bath pyrolysis

process, including all technical files, experimental units, and patent

rights, to Michigan Technological University. The Center for Waste

Management Programs (CWMP), a research division of Michigan Technological University, has continued development of the process on a limited

scale. Small-scale studies of lead bath pyrolysis have been carried

out to explore some of the major parameters affecting product

distribution.

DESCRIPTION OF THE PROCESS

Pyrolysis of biomass for the production of liquid and gaseous fuels

involves thermal degradation of the cellulose and lignin according to

a particular temperature-time path. Pyrolysis schemes that have been

proposed differ according to the following points:

1 . . method of heating the biomass

2. temperature-time-history of the biomass

3. temperature and environment encountered by the primary

gases liberated in biomass pyrolysis

Various methods of heating biomass are:

347
1. heating by contact with a heated solid surface

(rotating drum)

2. heating by contact with hot gases (fluidized bed)

3. heating by contact with a liquid metal surface

(molten lead bath)

4. heating in a molten salt bath

In the continuous lead bath pyrolysis process solid organic wastes are

subjected to a high-temperature, low-contact-time pyrolysis followed

by a rapid quench of the off-gas to preserve valuable chemical species.

The solid feed to the pyrolysis system is comminuted until 't he particles are 0.6 cm (1/4 inch) or less in the longest dimension. The feed

is then placed in an airlocked hopper and metered into the pyrolysis

furnace by a screw conveyor. The 980 kg/day pilot plant unit designed

by Dr. Fassell is a brick-lined refractory furnace 6.1 m (20 ft) long

(Figure 1). The feed from the screw conveyor drops onto the molten

lead hearth and is pyrolyzed at 7600C in the absence (or near absence)

of oxygen. The lead is kept in circulation by a turbine pump. A rake

mechanism sweeps the char to the end of the hearth where it is

conveyed out of the furnace to cool (in the absence of oxygen). The

residence time of the solid biomass on the hearth is 45 to 90 seconds,

depending on the speed of the raking mechanism.

The gas processing system used in the 908 kg/day Barber-Colman pilot

plant unit consisted of a water quench, followed by an oil scrubbing

system to recover benzene, toluene, and other high molecular weight

organics. The remaining gas was a medium Btu fuel gas, part of which

was used to fire the radiant heaters above the hearth. The technical

development files contain results of a number of runs of biomass feed

to the 908 kg/day pilot plant unic

Table 1

PRODUCTS FROM BIOMASS PYROLYSIS

Product

Percent of dry biomass for feed of

Rice· hulls Sugar Maple Sawdust

Water

Char

Ash

Gases (excluding

aromatic and olefins)

BTX

Olefins

Tars and pitch

Water-soluble organics

Unaccounted for

21.0

14.4

17.1

30.7

0.5

1.2

9.6

5.5

100.0

27 .7

18.3

0.9

38.1

0.7

1.5

0.6

2.3

9.9

100.0

In all cases the gas recovered from the scrubbers was a medium Btu

fuel gas having a heating value of about 18.6 MJ/m3 (500 Btu/scf).

The composition of the fuel gas is given in Table 2:

348

Table 2

COMPOSITION OF MEDIUM BTU FUEL GAS (DRY BASIS)

Component

Hydrogen

Carbon Monoxide

Carbon Dioxide

Nitrogen

Ethane

Mole Percent

36.8

35.2

·25 . 6

1.3

1.1

100.0

The Barber-Colman reports refer to energy balance calculations

indicating that the medium Btu fuel gas plus aromatics contain about

80% of the heating value of the biomass feed. Twenty percent of the

feed heating value would be required to fuel the pyrolysis furnace,

leaving sixty percent for other purposes.

STATUS OF THE PROCESS

The continuous lead bath pyrolysis process achieves in an economically

feasible way the objectives of rapid heating of biomass, a high

pyrolysis temperature and quick removal and quenching of primary

pyrolysis products. These are basic requirements if the yields of

valuable products are to be increased, and tar and char minimized.

The design of the furnace, the solid feed system, and the raking

mechanism are based on Dr. Fassell's life-long experience with hightemperature metallurgical operations.

Development of the continuous lead bath pyrolysis process was incomplete when Barber-Colman abandoned the project. The gas scrubbing

system on the pilot plant unit was primitive. As expected, considerable trouble was encountered with phenolic tar deposits on the gas

take-off line and in the scrubbers. The further use or disposal of the

char raked off the hearth was a question mark, as this char contained

from 1 to 2% lead by weight. Crucial variables, such as lead

temperature, water content of the biomass feed, and furnace atmosphere

had not been explored to determine their effect on product yield.

Finally, an economic evaluation of the process, including biomass

collection and transportation costs, capital cost of the pyrolysis

system, and cost of cleanup of waste . streams, had not been made.

DEVELOPMENT WORK.AT MICHIGAN TECH

Background

The Center for Waste Management Programs (CWMP), a mission-oriented

research unit of Michigan Technological University, was assigned the

task of evaluating and further developing the continuous lead bath .

pyrolysis process. Due to the cost of operating the 908 kg/day pilot

350
Wlit, CWMP chose to conduct experiments on a smaller scale to explore

the chemistry of the process. The questions addressed were:

1. Would gas production and benzene-toluene-xylene (BTX) yield

be significantly improved by operating the process at a

higher temperature?

2. What is the mechanism of contamination of the char by lead?

What methods would be effective in -separating the lead from

the char?

Although the scrubbing system on the pilot plant unit was crude and had

severe tar fouling problems, it was felt that these problems could be

solved by applying technology developed for coal gasification and for

the partial oxidation of heavy residua.

Effect of Operating Temperature

The 7600C operatlng temperature was chosen by Dr. Fassell to avoid two

problems. According to Dr. Fassell, furnaces with reducing atmospheres

operating below 7200C pose the risk of explosion in the event of

furnace failure and air in-rush. On the other hand,operating above

8000C on solid municipal waste would result in the softening and

melting of glass waste on the lead hearth. Since the latter is not a

factor with biomass feed, it was decided to explore biomass pyrolysis

at a series of temperatures up to 1000oe. A computer-aided search of

the literature on cellulose and lignin pyrolysis indicated that, to a

rough approximation, these components pyrolyze independently when

biomass is heated. Accordingly, samples o~cellulose (Whatman 50

filter paper) and lignin (Westvaco Indulin AT kraft pine lignin) were

studied. To trace the effect of high temperature pyrolysis on hydroxy

and methoxy groups attached to the aromatic ring in lignin, a model

compound (guaiacol, or 2-methoxy-phenol) was chosen for study [2].

The apparatus used in these studies consisted of a one-liter round

bottom quartz flask inserted in a 200-12000Cvertical Lindberg furnace

(~igure 2). The flask cover had ports for sample introduction, argon

purge, gas withdrawal, and a thermowell. The lead was contained in a

crucible machined from spectroscopic grade graphite. The purity of the

lea~ was 99.92%. Pyrolysis gases were removed through a quartz takeoff tube directly above the lead surface, and were analyzed by gas

chromatography. A typical chromatogram contained 20 to 24 peaks, of

which 14 were known. The unknown peaks were eluted after C02 and

before benzene. Total pyrolysis gas evolved could be determined by a

gas buret. Tars were collected on quartz wool inserted into the

electrically-heated quartz take-off-line.

Total gas production from the pyrolysis of cellulose, lignin, and

guaiacol is reported in Figure 3. Gas production increases more

steeply with temperature for guaiacol than for lignin or cellulose.

Pyrolysis gas composition as a function of pyrolysis temperature is

reported in Figures 4 to 11. Replicates of runs indicated that the

351










reproductibility of the reported percentages is about ± 3%. The gas

compositions determined are different in several aspects from those

reported for the pilot plant unit operating on sawdust and other

biomass (Table 1). The lower C02 content from the pilot plant unit

reflects removal of C02 in the water scrubber. The low hydrogen

content of the gas from the small experimental unit probably reflects

conversion to water by reaction with oxygen entrained with the entering

cellulose or lignin. Due to a more elaborate method of solid feed

introduction, the atmosphere in the pilot plant unit was probably more

reducing than that in the small experimental unit.

Of greatest interest is the increase in the yield of benzene and

toluene with increasing temperature. The yields from guaiacol were

significantly higher than from lignin or cellulose. The Barber-Colman

files contain a report of an anomalous run on wood chips where a high

yield (12%) of BTX was found. We have been unable to attain this yield

for cellulose or lignin in the temperature range 700 to 1000oC.

Few "literature data are available which can be directly compared with

our results. Iatridis and Gavalas [3] pyrolyzed a precipitated kraft

lignin in helium at 1 atmosphere over the temperature range 400 to

650oC. Their samples were held between two folds of a stainless steel

wire cloth functioning as a resistance heater. They found methane, CO,

C02, and methanol as major products, with single ring phenols reaching

a maximum yield of about 3% at 650oC. Their yields of CO and C02 at

6500 C are plotted in Figures 4 and 5.

Lead Contamination of Char

A study was made of factors affecting the contamination of char by lead

in the lead bath pyrolysis of wood waste [4]. One-quarter inch .

lengths of 0.8 cm dowel stock were pyrolyzed at temperatures from 7300 C

to 1050oC. The effect of residence time on the lead hearth was also

investigated, with times from 10 seconds to 120 seconds being studied.

The char produced had a void fraction of 0.4, and a surface area of

360 m2/g, as determined by BET adsorption of nitrogen. The char

samples were extracted with 7 M HN03, and the lead content of the

filtrate determined by atomic adsorption spectroscopy. It was found

that the lead content of the char was affected much more by pyrolysis

contact time than by pyrolysis temperature. After ten seconds of

pyrolysis at 8000 C the wood char contained 135 ppm lead. This

increased to 240 ppm after 120 seconds of pyrolysis.

Scanning electron microscope photographs of the internal and external

surfaces of the long time char revealed that bulk metallic lead did not

enter the interior of the char even after 100 minutes on the lead bath.

This conclusion was confirmed by an electron microprobe study of a

cross-section of a short time char sample. Although macroscopic drops

of lead near the external surface of the char were visible in the

electron microscope photographs, calculations showed that these

represented only a small fraction of the total lead content of the char.

This would indicate that physical adsorption of lead vapor on the pore

walls was the predominant mechanism of lead contamination.

362
Thus, a possible method of removing lead from the char would be to

blow the hot char with steam after it is raked off the lead hearth.

References

1. Piron, E. H., and Crawford, R. M., "ThePiron Coal Distiilation

Process" Proceedings of the International Conference on Bituminous

Coal, Carnegie Institute of Technology, November 15-18, 1926,

pp 729-746

2. Blankinship, Richard M., MSc Thesis, Michigan Technological

University, 1979

3. Iatridis, B. ,and Gavalas,G. R., Ind. Eng. Chem. Prod. Res. Dev.

l&, 127-130 (1979)

4. Alsgaard, David W., MSc Thesis, Michigan Technological University,

1979

363
Abstract

In the continuous lead bath pyrolysis process biomass is ground to less

than 0.6 cm in the longest dimension and fed by a screw conveyor onto

the surface of a molten lead bath maintained at 760oC. Gases liberated

in this high temperature, short contact time pyrolysis are water

quenched, and the aromatics removed in an oil scrubber. The gas

leaving the scrubber is a medium Btu fuel gas. The char formed on the

molten lead hearth is removed by a raking mechanism after a residence

time of 45 to 90 seconds.

To explore the effect of temperature on product distribution, samples

of cellulose, lignin, and a model compound (guaiacol) were pyrolyzed

on a pool of lead in a small laboratory unit. Pyrolysis temperatures

ranged from 7500C to 1000oC. Total gas production increased with

increasing temperature. Benzene yield from lignin increased from

0.4% at 7200C to 2% at 1000oC~

Separate studies of contamination of the char by lead revealed that

pyrolysis contact time was more important than pyrolysis temperature.

The predominant mechanism of char contamination by lead was adsorption

of lead vapor on the pore walls.

364
PREPARATION OF POWDERED FEEDSTOCK

FROM BIOMASS WITH STEAM

Q. Nguyen and G. Noble

Iotech Corporation Ltd., Ottawa, Ontario Canada

ABSTRACT

Pretreatment is prerequisite for effective enzymatic

saccharification of lignocelluloses. Steam explosion pretreatment fractionates lignocelluloses into three main

components: highly accessible cellulose which retains

the basic crystalline structure, hemicellulose rendered

hot water soluble and thermoplastic lignin soluble in

aqueous ethanol and dilute sodium hydroxide solution.

The biomass is subjected to saturated steam at 3450-

6900kP (500-l000psi) for 5 seconds to 5 minutes then

explosively released to atmospheric pressure. The sub~

strate is pulverized into a powder, 60% of which passes

through 200 mesh screen. Glucose and xylose yields of

90% and 75% of theoretical respectively are obtained after

24 hour enzymatic hydrolysis of steam exploded aspen.

Highly reactive lignin can be recovered with 80% yield.

The estimated operating cost of the steam explosion is

$7.50/0D ton of biomass. At this low cost, the Iotech

process achieves many pretreatment requirements in one

step. Total utilization of biomass is achieved since the

high quality lignin can be used for chemical feestock.

INTRODUCTION

Effective pretreatment at a low cost is a key factor in

the commercial utilization of lignocelluloses for chemicals

by biological conversion processes. The most desirable

pretreatment would give maxiMum yields of the three main

components of biomass, namely cellulose, hemicellulose and

lignin, in suitable forms for subsequent processing. One

of the major deterrents in the enzymatic saccharification

of biomass is the lignin-hemicellulose complex embeding

in the cellulose structure. To make the cellulose accessible, this lignin-carbohydrate complex must be broken up.

The pretreated biomass substrate should also be in a

finely divided form to achieve high specific surface area.

Delignification by pulping processes does not achieve

effective utilization of biomass since the use of high

value lignin is often neglected. Other methods for

enhancing cellulose accessibility such as: swelling with

sodium hydroxide solution, comminution, irradiation are

365
either ineffective or very expensive. Iotech Corporation

Ltd. has developed a patented pretreatment process which

satisfies the requirements mentioned above.

THE STEAM EXPLOSION PROCESS

The explosion processes for pulverizing permeable material

such as wood, coal, ore, minerals etc., have been known

for quite some time [1]. The basic principle is to

impregnate the material to be exploded with a suitable

fluid (usually steam) at high pressure and temperature

for a certain period of time in a pressure-tight cylindrical shape vessel, then suddenly release the pressure.

The decompression causes an instantaneous expansion of

the fluids occupying inside the pores or cells resulting

in size reduction of the exploded materials. High

temperature also weakens the structure of many materials,

adding to the degree of pulverization.

Extensive optimization tests have been carried out with

aspen wood (Populus Tremuloides) because it is the most

abundant hard wood species in Canada which has not yet

been fully utilized. Hybrid poplar clones have been

developed to give very high biomass yields with short

rotations.

In typical process conditions, green wood chips are

steamed at 3450-6900kP (500-l000psi) for 5 seconds to 5

minutes in the gun as shown in Figure 1. At the end of

the cook, the steam inlet is shut off and the discharge

valve is opened. The chips extrude through an orifice

at the gun nozzle and into a cyclone collector where

excess steam is flashed off and recovered. Exploded wood

has an average moisture content of 63% based on wet weight

as compared to 50% of input chips. The cyclone we use is

under-sized and unheated. There is a considerable amount

of steam blowing downwards and condensing on exploded

wood being collected in a bin placed beneath the cyclone.

With a properly designed collector, we believe that the

moisture content can be reduced to less than 60%.

The following variables were found to be significant to

the extent of size reduction: temperature, steaming time

and pressure drop when the explosion takes place.

Sufficient time must be allowed for the center of the

chips to reach the softening temperature, > 220oC, at

which the fiber structure fails~21. The degree of

pulverization was found to increase with larger pressure

drop. However, the effect of pressure was less significant beyond 4800kP (700psi).

366

Other less important variables were: chip size, moisture

content of chips, gun nozzle configuration. Heat pene~

trates to the center of the small chips faster than

to the center of large chips. With cooking time adjusted

accordingly, similar results were obtained from hammer

milled chips and commercial chips. Drier chips required

slightly shorter cooking time. The use of constricting

dies with similar cross-sectional area in the gun nozzle

such as: circular, star shape, perforated plate, had

little effect. Some of the dies created wore shear to the

extruded fibers but also slowed down the discharge rate

resulting in smaller pressure drop. Straws with their

delicate structure and low lignin content required less

severe treatment as compared to wood.

STE~ HYDROLYSIS OF LIGNOCELLULOSE

High pressure steam rapidly hydrolyses the acid groups

attached to the hemicellulose into acetic acid and uronic

acid which then attack the lignin-hemicellulose complex

and the amorphous cellulose regions. Fydrolysis occurs

at the glycosidic bonds, the bonds of lignin-hewicellulose

complex and the and ether linkages of lignin. Only

the amorphous cellulose regions are subjected to attack

by the available weak acids since the tight structure of

highly crystalline cellulose is impermeable. Steam

hydrolysis of lignocelluloses in the gun was found to be

first order reaction and the rate approximately doubles

at a temperature increase of 10oC. Prolonged cooking

also causes oxidation and degradation of hemicellulose

and condensation of lignin. Dehydration of xylose yields

furfural, some of which can condense with the lignin.

At high temperature the hydrolysis rate of hemicellulose .

is much faster than the degradation rate of the product.

Therefore, it is necessary to use high pressure steam and

short cooking time to obtain high yields. of xylose.

Because of the short cooking times required it is

advantageous to have the instant discharge feature of the

gun which enables one to terminate the reactions and to

cool down the product in a split second.

EXPLODED ASPEN WOOD AND ITS UTILIZATION

About 60% of exploded aspen wood passed through the 200

mesh screen (0.074 rom opening) of a Bauer-~cNett fiber

classifier. The degree of pulverization increases as

steam pressure goes up. The fiber classification results

of aspen wood processed at four different pressures are

shown in Table 1. The size distribution of input chips

is given in Table 2. Each sample was cooked to a similar

degree of hydrolysis. Finer fiber size may be obtained

368
if cooking time is increased for higher operating pressure

shots.

Tl\..BLE 1

FIBER CLASSIFICATION OF EXPLODED ASPEN WOOD

2000 3033 3792 4481 kP

Screen mesh (290) (440) (550) (650) psi

% exploded wood

48 < 1 < 28 46.3 34.4 23.2 4.4

100 < I" < 48 9.2 16.9 2.6 15.1

140 < 1 <100 15.5 4.5 22.8 18.8

200 < 1 < 140 2.6 4.9 5.3 4.3

"I (" 200 26.4 39.3 46.1 57.4

TABLE 2

SIZE DISTRIBUTION OF HMA~ER MILLED ASPEN WOOD

Tyler Equiv.

Screen Opening, rom Mesh Weight Percent

Retained by 9.51 0.371 in. 17.9

11 4.76 4 38.6

tl 3.36 6 22.6

II 1.00 16 16.5

Pass 1.00 16 4.4

Exploded aspen has the appearance of peat moss. The brown

colour is attributed to the hydrolysed lignin and the

sugars. Because of its finely divided form the product

can be dried quickly by blowing hot air over it. With the

presence of water soluble hemicellulose, the fibers terid

to stick t.oqe t.her in small lumps upon drying.

369
Exploded aspen can be fractionated into three main

components: cellulose, hemicellulose and lignin by

solvent extraction. Most of the hemicellulose can be

extracted with hot water, more than half of which is

free pentose. Under optimum conditions less than 20% of

the original hemicellulose is degraded. Over 90% of the

lignin can be extracted with O.lN NaOH solution at room

temperature and 80% is recovered by precipitation with

acid. About 20% of the original lignin is rendered water

soluble. A lignin fraction respresenting 51% of the

original amount in the wood was extracted with ethanol.

The lignin was fractionated with methanol and benzene

mixture and the number-average molecular weights of the

fractions were obtained with a vapour pressure osmometer

[3]. 19% of the above lignin had a molecular weight of

463 or lower. 39% had a molecular weight of 664 or lower,

and the highest molecular weight observed was only 3340.

The remaining lignin in the pulp had higher molecular

weight. The isolated lignin softens at 120-130oC, when

moisture is present the softening temperature drops to

90-100 oC.

The caustic washed pulp is over 90% alpha cellulose. The

cellulose retains the basic crystalline structure [4].

The degree of polymerization varies from about 800 to

the limit 200 depending on the extent of the hydrolysis.

The effects of temperature and cooking time on sugars

yields and on lignin extractable with caustic soda are

shown in Figure 2. At low processing temperature and

. pressure long cooking time is required to achieve maximum

cellulose accessibility, but considerable degradation of

hemicellulose also occurs and the l ignin is heavily

condensed and less reactive. The maximum yields of the

three components also do not arise at the same time. At

high temperature and short time conditions, 90% glucose

and 75% xylose yields and high quality lignin have been

obtained. The prospect of ethanol fermentation of xylose

is good, therefore, the potential ethanol yield from

exploded aspen is substantially increased. A lignin

formaldehyde resin has been successfully formulated and

tested. Figure 3 summarizes one of many possible ways of

utilization of exploded biomass.

COST ESTI.M..ATE

A detailed engineering design of the steam explosion

process gives an estimated operating cost of $7.50/0D ton

of biomass. The operating cost includes costs of capital,

maintenance, labour and steam. The cost of steam is a

relatively small factor in the cost economics when the

recovered steam can be utilized.

370


Energy requirements for comminuting dry pine shavings to

various screen sizes were reported by Millet et al [5].

At a size equivalent to Iotech exploded wood the energy

cost alone was estimated at $30/ton. This figure does not

include the cost of drying and capital related expenses

which would push the cost of milling even higher.

CONCLUSION

Steam explosion is a low-cost pretreatment process which

achieves several desirable effects in one step:

a) The carbohydrates are highly accessible to

enzymes.

b) The lignin is recovered in a suitable form

for conversion into useful chemicals.

c) The biomass is pulverized into a powder

thus increasing the active surface area.

d) The product is'sterilized by steam 'so that

risk of contamination in the downstream

biological processes is reduced.

REFERENCES

1. Meigs, D., "Explosion Unit Operation of the Process

Industries", Chern. & Met. Eng., (Feb. 1941), PP. 122-

125.

2. Baldwin, S.H. and Gorring, D.A.I., "The Thermoplastic

and Adhesive Behaviour of Thermornechanical Pulps from

Steamed Wood", Svenok Papperstidnig, II (1968) :18,

pp. 646-650. .

3. Brownell, H.H., Contract Report No. 65-80-528,

Forintek Canada Corp., Ottawa, Canada, 1980.

4. Marchessault, R.H ... and St.-Pierre, J., "A new

understanding of the carbohydrate system", CHEMRAWN I,

Pergammon Press, Toronto, Canada, 1978, pp. 613-625. ·

5. Millet, M.A., Baker, A.J., and Satter, L.D.,

"Physical and Chemical Pretreatments for Enhancing

Cellulose Saccharification", Biotechnol. & Bioeng.

Symp. No.6, John Wiley & Sons Inc., 1976, PP. 125-

153.

373
THE EMBRITTLEMENT TREATMENT OF WHEAT STRAW FOR CONVERSION TO A POWDER+

A. A. Ghazee

University of Dayton Research Institute

Dayton, Ohio 45469

N. L. Hecht

University of Dayton Research Institute

Dayton, Ohio 45469

Introduction

In this project conducted at the University of Dayton (UDRI), fifteen

pounds ·of wheat straw were treated with HCl and converted to a fin~

powder. The treatment used was based on the procedures established in

a previous program to develop cellulose ernbrittlement processes to produce RDF-powder from MSW. Twelve pounds of powder were sent to SERI

for evaluation in its pyrolysis facility. In addition powder samples

were analyzed to determine the effectiveness of the embrittlement process and characterize the powder obtained from the wheat straw.

Background - Cellulose Ernbrittlement

In recent years, we have begun to look with renewed interest upon a

variety of cellulose waste materials as a promising alternate source of

energy. Cellulose wastes are categorized as biomass materials which

in the 1800's served as a primary fuel source for the united States.

Biomass materials can be converted to liquid, solid, or gaseous fuels

by .a number of chemical, biological, and thermal processes.

Most of the organic waste in municipal and industrial refuse as well

as agricultural wastes are termed biomass since they are plant materials. Included in this category are waste paper and wood products

(newsprint, cardboard, furniture, packaging mat~rials, etc.), crop and

food processing residues (straw, corn cobs, husks, etc.), logging and

wood manufacturing residues (branches, stumps, bark, sawdust, etc.) as

well as manure and sewage sludge.

The major constituents of most biomass materials are cellulose, hemicellulose and lignin. The quantities of each of these constituents

vary depending on the type of plant material, the type of processing it

received and whether it is a paper or wood product. A tabulation of

the general composition for several important types of biomass materials is presented in Table I.

+This work was sponsGred by SERI

375
Biomass Material

TABLE I

BIOMASS MATERIAL COMPOSITIONS

Composition (\'

Mineral

Cellulose Hemicellulose Lignin Matter

Wases, Resins,

etc.

Hardwoods 40-55 24-40 18-25 1

SoftWood 45-50 25-35 25-35 1

Grasses, suaw, etc. 25-40 25-50 10-30 2-9

Leaves 15-20 85

Newsprint 40-50 20-40 18-30 1-4

Waste Paper 60-80 20-30 2-10 2-6

2-12

2-12

?

1:r

t:r

=

Most biomass materials are cellulosic compounds composed of 30-50 percent a-cellulose, 20-40 percent hemicellulose and 10-25 percent lignin.

Cellulose is a linear glucose polymer of D-anhydroglucopyranose units

linked by B- l - gl ucos i di c bonds as shown in Figure 1 . Hemicellulose is

a polysaccharide intimately associated with cellulose but with a lower

molecular weight. Lignin is a complex polyhydric polymer containing

condensed phenolic, open phenolic, condensed ether, open ether and alcoholic groups. These three major constituents are combined into a

fibrous structure which makes up the main body of most plant material.

Wood fiber serves as a good model to demonstrate the complex interrelationship of the cellulose, hemicellulose and lignin constituents. A

cross section of a wood fiber ash shown in Figure 2 consists of a central cavity (lumen) surrounded by two concentric layers. The inner

layer (middle layer) adjacent to the lumen consists of an inner and

outer skin which is mostly cellulose and hemicellulose encasing a

middle lamella which is mostly lignin. The outer layer (primary wall)

serves as an elastic membrane around the outside of the fiber and consists mostly of cellulose, hemicellulosic materials, and some lignin.

The zone between wood fiber cells is prinoipally lignin. The cellulose

fibers in the inner and outer skins are helical around the fiber axis.

The cellulose in the wood fiber has a fibrous morphology consisting of

crystalline and amorphous areas. The molecular weight and chain

length are variable and in most cases only average values can be determined.

Wood fibers, because of their pore structure and general cellular nature

have a large internal surface area and are highly adsorbent. They have

a particularly strong affinity for water. Dry cellulose fibrous materials under normal atmospheric conditions contain 15-30 percent moisture. Water can be in the fibrous structure as bound water, capillary

water or imbibed water. Bound water is held by adsorption on the surface of the cellulose crystallites. This water is strongly bonded to

the cellulose and no longer exhibits the properties of "free water".

Capillary water is found in the fiber pores but is not intimately bound

to the cellulose fibers. Imbibed water fills the lumen and coarse

pores of the fiber and remains as free water.

376
Figure 1. Chemical Structure of Cellulose

":"'M:\ - ....,

.'::..I'1'.'-. ...: •"".:.'..

,.:••••• i' , ' i~..

Figure 2. Cross Section of a Wood Fiber with

Adjoining Areas: (0) outer layer,

secondary wall; (m) middle layer,

secondary wall; (in) inner layer,

secondary wall; (lu) lumen; (p) primary wall; (iso) isotropic intercellular substance.

Thermal treatments alone can be used to degrade cellulosic materials.

Heating above 200°C (392°F) for long periods will result in .a loss of

fiber structure as well as loss of hygroscopicity. Significant degradation can even be observed at temperatures of 150~C (302°F). Initial

studies at the UDRI showed that thermal treatments resulted in moderate

embrittlement of cellulosic materials. It is believed that this

occurs by the formation of hydrogen or ether bonding between adjacent

cellulose molecules.

Treatment of cellulosic materials with oxidizing agents will also cause

a breakdown to products of lower molecular weight. These products can

vary from practically unchanged fibers to a friable powder. In screening studies performed at the UDRI it was found that exposure of

shredded newsprint to Cl2 (an oxidizing agent), resulted in substantial

embrittlement of paper. However, considerable Cl2 was adsorbed by the

paper when it was exposed to the embrittlement treatment. It would

appear that ozone or H202 might be a more desireableoxidizing agent

since it would not present potential corrosion or pollution problems if

it were adsorbed by or reacted with the celiulosic materiai during

treatment.

377
Mineral acid treatments have been found to be very effective for the

embrittlement of cellulosic materials. Nitric, sulfuric, and hydrochloric acid treatments have proved to be the best reagents in the

studies conducted at UDRI and inwork reported by Combustion Equipment

Associates (CEA). The work performed at CEA served as the basis for

the development of a proprietary process for 'converting MSW to a powder

fuel (Eco Fuel-II). The results of the studies at UDRI showed that

HCI was the most effective embrittlement agent tried. In addition, it

was found that longer treatment times and higher treatment temperatures

increased the embrittlement of the cellulose samples.

An understanding of the chemical processes which result in the embrittlement of cellulosic material by HCI treatment is of fundamental importance. Such an understanding would provide a basis for modification

and improvement of the present process, and could provide a rationale

for subsequent development of alternative processes which avoid the use

of expensive and highly corrosive agents such as HCI.

One of the anticipated results from the exposure of moist cellulosic

materials to HCI is the hydrolysis of glycoside bonds in the polysaccharide backbone. Vicosity measurements on cupriethylendiamine solutions prepared from HCI-treated filter paper have in fact confirmed

this expectation. Exposure to very low levels of HCI causes a dramatic

decrease in the solution viscosity, indicating a decrease in the degree

of polymerization (DP). In spite of this decrease in DP, however,

these lightly treated cellulosic samples were not significantly embrittIed. Based on these observations, it was concluded that embrittlement

was not solely the result of a.decrease in the DP of ·ce l l u l os e . Our

conclusion was reinforced by common experience with ·ov er - pul pi ng in the

paper industry. Paper produced from pulp having a low DP have certain

undesirable characteristics, but friability is not included as one of

these. Clearly some other chemical changes must occur subsequent to

the hydrolysis process.

A clue to the nature of the embrittlement process was provided by the

properties of filter paper which had been heavily treated with HCI.

Such samples failed to completely dissolve in the cupriethylenediamine

reagent. .Li ke the extent of embrittlement, the lack of solubility

appeared to qualitatively correlate with the level of HCI treatment of

the filter paper sample. However, meaningful viscosity measurements

could not be conducted with these samples owing to their lack of complete solubility.

Unfortunately time and funds were not available to conduct definitive

experiments which would indicate the cause for the reduced solubility

of the highly embrittled samples in the reagent. Moreover, a causal relationship between reduced solubility and embrittlement has not been

established for these extensively treated filter papers. Nonetheless,

we suspect that the observations that were made are related, and we

have developed a working hypothesis on the data available. Assuming

that the cross-linking occurs via the formation of ether linkages as

shown in Figure 3, the following reactions in the cellulose can be predicted:

378
ClH ClH if ~H OH ! i I i I 0 18 I HO...l.-.@-

ClH OH OH 0 0 OH OH Qi

I I I I bH~ I

, @- Hel I , ~- OH 0 0

OH CH 0

• 1 I 4 i CH He I i

OH OH OH OH Cl OH

Figure 3. Possible Embrittlement Mechanism; Hydrolysis and Crosslinking of Adjacent

Cellulose Chains by Oxygen Bridges.

1) Rigidization with attendant ease of fracture;

2) Decrease in solubility as the molecular shape changes

from a long extended form with readily accessible OH

groups to a more spherical form with reduced surface

area and inaccessible OH groups;

3) A rapid decrease in molecular weight or DP as the reactive glycoside linkages are cleaved, followed by a slowing of the decrease in DP as cross-linking takes place;

and,

4) A decrease in the quantity of retained HCl as the process temperature is increased. The ether cross-links

are thought to be formed by the reaction of OH groups

with chlorinated cellulose, and the subsequent loss of

HCl. Higher process temperatures should lead to more

complete cross-linking and loss of HCl.

Powder Preparation

The major emphasis of this project was the production of powder from

wheat straw for analysis. About 15 pounds of wheat straw were obtained

from SERI to be used as the feedstock for the embrittlement treatments.

The wheat straw was processed in the UDRI Pilot unit for the production

of minus' 45 mesh powder.

Five pound batches were embrittled in the reactor unit at a treatment

temperature of 3l0op using a 72 percent hydrogen chloride, 28 p~rcent

nitrogen reactant gas mixture for a treatment time of 5 minutes. The

embrittled straw was ball milled for 30 minutes and then screened for

10 minutes. A tabulation of the powder preparation conditions ,is presentedin Table II.

The reactor unit (shown in Figures 4 and 5), built by the UDRI, was designed to process 2.5 to 5 pounds of material with a continuous flow of

reactant gases. The reactor was constructed from a 30 gallon carbon .

steel drum coated with a 20 mil thickness of Emralon* 314 (50% Teflon -

50% epoxy). The ,r e a ct or 'i s supported in a Unistrut' frame such that it

could be loaded and unloaded by tilting it about a horizontal axis

*trade name

379
TABLE II

POWDER PROCESSING CONDITIONS

Quantity material processed

Processing

temperature

Processing

time

HCl flow

rate

N2 flow rate

HCl adsorbed

Ball mill time

Screening time

5 lb

5 minutes

0.36 ft 3/min

2.3% by wt

30 minutes

10 minutes

between two vertical Unistrut channels. The wheat straw was introduced

at the top and the lid was sealed with a specially designed silicone

rubber gasket and locking ring to prevent leakage. Three l800-watt

drum heaters were installed on the outside of the reactor for heating.

A stainless steel screen with 0.33 inch diameter holes was installed at

the bottom of the reactor to ensure an even distribution of the gas,

and a Teflon relief valve was installed on the lid to provide safety

from excessive pressures. All temperature measurements were made with

a digital thermometer.

Hydrogen chloride was supplied from a 60 pound HCl cylinder, and N2 was

drawn from an existing N2 cylinder bank. A 5 kW heater was used to

heat the N2. Two glass rotameters, 0-3.5 cubic feet/minute were used

to measure the rate of the reactant gases flowing through the reactor.

The scrubber unit consisted of a polyvinyl chloride-lined 55 gallon

drum containing 22.2 gallons of 0.15N sodium. hydroxide solution. The

unreacted gases were dispersed through a 6 inch diameter fritted glass

funnel into the NaOH solution. Efficiency of the scrubber was tested

by introducing a known quantity of HCl, and titrating a 0.34 ounce sample from a 16.9 ounce aliquot of the scrubber solution with O.lN HCl.

A series of tests showed that the titrimetric data could account for

approximately 95% of the quantity of HCl calculated from flow measurements.

After the reactor was charged with wheat straw, it was sealed and heated

to the desired. temperature with the drum heaters. Upon reaching the

specified temperature, a moderate flow of HCl and N was begun. The

HCl fow was maintained for a short time to provide the desired quantity

of this reactant. The reactor was then purged with N2 for an additional 0.5 hour. Gas flow rates and times were recorded from the control

380
PROCESS FLOW DIAGRAM

SAMPLE

PORT

-AIR

_ 3/4 - RU88ER

HOSE

.: ': ': II:, It " I

"

lV--ll---:I

;l:: ::

,I tI e l

l! :: :: " I, "

:: .:: :: II ,I "

:: I: ::

,. , I II

I' ,I ..

" _.J" "J '-- .... \..)1 ",' - ,

~ ~ :' \, "." 'I 'nv..:....;<

NoOH

SOLUTI

DRUM HEATER - IIBOO Watts)

55 GAL [)RUM-

(SCRUBBER)

(FRITTEO GLASS FUNNEll

"""';~=-Nz

l. 3/B· 1.0 TEFLON TUBING

Nz HEATERI5Kw)

318- 1.0 POLY- ETHYLENE

TUBING

- HCI CYLINDER

Figure 4. Process Flow Diagram for Pilot Reactor.

panel. Unreacted HCl and the N2 passed out the top of the reactor

through a 0.75 inch Teflon lined tube and into the scrubber unit described above. Two aliquots of the scrubber solution were titrated

with O.lN HCl to account for the amount of unreacted HCl. The input to

the reactor was calculated from the flow rate of HCl at the measured

pressure and temperature. Output of HCl from the reactor was calculated from the change in normality of the NaOH solution. The retention

of HCl could be determined by the difference in these two values.

After chemical treatment, the wheat straw was ball milled for 30 minutes. The ball mill unit consisted of a 13 quart ceramic jar filled

with 30 pounds of 0.5 x 0.5 inch ceramic cylindrical grinding pellets.

The jar was rotated on a pair of 2 inch diameter hard rubber rollers,

one of which was chain driven by a 186.4 watt electric motor. The '

ball milled powder was then screened through a Tyler RoTap sieve shaker

for 10 minutes to obtain ,minus 45 mesh powder.

Powder Characterization

Samples of the powder produced were obtained for chemical, physical,

and thermal analysis. In addition, samples were sent to the Galbraith

Laboratories, Inc. for selected chemical analysis.

381

Particle size distribution and morphology of the powder samples were

measured to better characterize the physical nature of the powder.

The particle size distribution was measured using a Tyler RoTap Sieve

Shaker and the results are shown in Table III. All of the data reported represent the average values calculated from five or more measurements.

TABLE III

PARTICLE SIZE DISTRIBUTION OF POWDERED WHEAT STRAW

Particle Size Quantity

mesh %

+45 34.7

-45+100 26.5

-100+200 32.7

-200 6.1

Using a scanning electron microscope and EDXA (Energy Dispersive X-ray

Analysis) the morphology and inorganic elements of the untreated ~traw,

the straw converted to powder, and ash samples from both were studied.

Representative electron micrographs are shown in Figures 6 through 13.

Figure 6. Wheat Straw at 100X

383



Figure 13. Ash from Powdered Straw at 300X.

The EDXA scans obtained for the untreated straw, powdered straw and the

ash from both materials are shown in Figures 14 to 17. The elements

identified for each sample are compiled in Table IV.

TABLE IV

ELEMENTAL ANALYSIS BY EDXA

Wheat Straw

Al

si

Cl

K

Powdered Straw Ash from Straw Ash from Powder

Al Si Si

Si Cl Cl

Cl K K

K Ca Ca

Fe*

*The Fe found in the ash from the powdered straw may be a contaminant

picked up during grinding.

387


Ash samples were prepared from the wheat straw and powder following

ASTM standard D 271. The wheat straw had an average ash content of

6.8% and the powdered straw had an average ash content of 9.5%. The

heat content of the wheat straw and the powdered straw was measured

using 1 gram samples in a Parr Adiabatic Bomb Calorimeter following

ASTM standard D 2015. The wheat straw had an average heat content of

7050 BTU per pound and the powdered straw had an average heat content

of 6931 BTU per pound.

The chemical analysis of both the wheat straw and the powdered straw

was conducted at the Galbraith Laboratories Inc. of Knoxville, Tennessee. The results of these analyses are presented in Table V.

TABLE V

CHEMICAL ANALYSIS

Element

C

°

H

N

Cl

SubTotal

Loss on Ignition

Wheat Straw

%

43.27

43.83

6.10

0 .12

0.38

93.70

94.18

Powdered Straw

%

41.51

40.90

5.69

0.15

2.70

90. 95

90.26

Discussion of Re su l t s and Conclusions

In this work, wheat straw was readily converted to a fine powder by the

use of cellulose embrittlement techniques developed for powdering MSW.

From the preliminary study conducted it appears that the wheat straw

is more easily embrittled than was observed for MSW subjected to the

same treatment conditions. The powder obtained consisted of short

choppy fibers primarily minus 45 mesh (354~) in size. The heat content

of the powder was 6931 BTU/per pound -and the ash content averaged

about 9.5 percent. Compared to the untreated wheat straw the powder

had a higher ash content and a slightly lower heat content.

In addition, the powder has a lower carbon, oxygen and hydrogen content

but a much higher chlorine content. It is believed that some hydrogen,

carbon and oxygen are volatilized during the embrittlement process

while some hydrochloric acid is adsorbed.

Procedures for converting wheat straw to a fine powder have been established. The powder obtained appears to be an effective feedstock for

biomass conversion processes. However, the mechanisms of the process

are not completely understood and need further elucidation. The identification of effective alternative embrittlements reagents, particularly reagents (RN03, H3P04 , etc.) that would not leave residues which

390
could be corrosive or detrimental to the environment are also needed.

It would appear that a variety of biomass materials, particularly cellulosewastes from industrial and agricultural sources (stalks, husks,

bark, wood and crop residue, etc.), could also be converted to powdered

feedstock for fuel production. However, more work is needed to develop

effective processing procedures for powdering the variety of biomass

materials which might be available for conversion.

Based on these observations the following recommendations for future

work are proposed: (1) further elucidation of the embrittment mechanism; (2) identification of alternate embrittlement reagents; and (3)

extension of the embrittlement process to other biomass materials.

391
PURIFICATION AND USES OF FAST PYROLYSIS

GASES MADE FROM BIOMASS

Michael S. Graboski

Chemical and Petro1e~-Refining Engineering Department

Colorado School of Mines

Golden, Colorado

ABSTRACf

The fast pyrolysis of biomass and MSW provides a product gas rich

in elefins. The raw product gas is useful for the manufacture of pure

ethylene and motor fuels. This paper explores gas cleanup and processing requirements for the production of ethylene and fuels and discusses

the economics of synthesis. It is concluded that alcohol motor fuels

represent an attractive conversion route for fast pyrolysis gas.

INTRODUCTION

Much has been written about the use of biomass as a supplement for

petroleum and natural gas energy sources. Depending on the conversion

route, that is, gasification, pyrolysis or liquefaction, a wide variety

of products are possible. Direct liquefaction of biomass is technologically far behind gasification and pyrolysis. Gasification and pyrolysis

represent distinctly different conversion paths. Figure 1 highlights

these differences. In gasification the aim is to produce synthesis gas

(a mixture of CO .an d HZ) for fuel or chemicals production by oxidizing

the carbon matter using air or oxygen, steam and COZ. Bulk chemicals

including methanol, ammonia and methane are the obvious chemical products from synthesis gas although the manufacture of higher alcohols

and hydrocarbons is certainly feasible. Pyrolysis on the other hand

yields a mixture of gases, tars and char by supplying heat indirectly

to the solid. The rate of heating dictates the product distribution.

Slow heating rates (10-100 C/min) yield a mixture of gas, tar and char.

The tar and · char yields are large. Flash pyrolysis of biomass with a

solid heating rate on the order of 1000 C/sec. provides a feedstock rich

in olefins and only a small amount of char and tars.

Olefins may be readily converted to many different petrochemicals

and fuels including alcohols, gasoline, plastics, glycols and ethers.

An important question is "which route is most attractive?" This depends

on a variety of factors including plant scale, location and technology.

PYROLYSIS TECHNOLOGY

A number of research and demonstration projects have had as their

purpose, the production of olefinic and paraffinic hydrocarbons from

the thermal decomposition of biomass. These include MalIan and Compton

(1974), Diebold and Smith at China Lake (1979), Benham at SERI, (1980),

Kuester at Arizona State (1978), and Antal at Princeton University

(1979) to name a few. The project which is farthest along developmentally and for which a large body of data are available in the open

literature for olefin production is that of Diebold. .

393
In the China Lake process, shown in Figure 2, pulverized biomass

is steam pyrolyzed in a short residence time, atmospheric pressure, high

temperature reactor similar to that employe~ in petroleum cracking.

Reaction temperatures of 1300-1500F and residence times of 10-1000

milliseconds have been employed in a reactor capable of processing several pounds of biomass per hour. The reactor used was 3/4-inch ID by

7 feet long. The principle biomass fuel employed is a preprocessed

municipal solid waste trade named ECO-II. Some tests have also been

conducted on cellulose and birch flour. Benham (1980) has recently

investigated various wood flours and straws in a similar apparatus. The

nature of the experimental apparatus employed is such that the reactor

is non-isothermal and the residence time is ill-defined. Additionally,

the heat is supplied indirectly through the vessel wall, confounding

the temperature history of the solid. It is therefore difficult to interpret the results of the experiments in a quantitative fashion to

determine the optimum operating conditions for ethylene yield.

Table 1 compares yields obtained from the China Lake Process with

those obtained for other hydrocarbon feedstocks. While the hydrocarbon

portions are similar in relative amounts, the biomass yields are lower

because of the large quantity of CO and C02 produced. These are a result

of the high oxygen content of the feedstock. Figure 3 gives a more detailed product analysis for ECO-II feedstock. Other feedstocks such as

cellulose and birch flour yield somewhat less olefins and almost no char.

More CO and C02 are produced because of the higher oxygen content.

While the composition of biomass and petroleum feedstocks greatly

differ, it is apparent that the same types of free radical reactions

take place since the product distributions are somewhat similar. Little

is known about the thermal degradation of oxygenated species. Experiments on the cracking of acetaldehyde show that the principal products

are light paraffins, olefins, carbon monoxide and hydrogen. Antal (1979)

and others have shown that olefins are not the primary pyrolysis products

issued from the solid but result from gas phase cracking. The effect of

ash components and feedstock pretreatment and preparation might have an

effect on the yield. The nature and amount of the heavy hydrocarbons

and char produced must be better quantified. More experimentation is

needed to model the chemistry.

GAS CLEANUP TECHNOLOGY

Figure 4 shows two possible schemes for pyrolysis gas cleanup.

Gas exiting the reactor must be rapidly cooled so that the pyrolysis

reactions are quenched. Experience shows that the rapidity of the

quench and ultimate ethylene yield are related. To minimize secondary

reactions, the product mixture must be cooled below 900-1000 F in tens

of milliseconds. This can be directly accomplished using oil or water

sprays at the expense of inefficient heat recovery in the production of

pyrolysis steam. Modern plants employ transfer line exchangers (Mol

(1973)), which are short-residence time, single pass heat transfer units.

394
Typically, the residence time is on the order of 10-20 milliseconds

total in these units with temperature reduced below 1000 -F in about 2

milliseconds. High pressure steam is readily recovered for use in the

pyrolysis plant. Transfer line -exchangers are used to reduce the temperature to between 600 F and 1100 F for gas and oil cracking respectively. In the later case, the temperature must be kept high enough

to eliminate fouling in the exchanger -due to condensation of heavy tars.

In the case of biomass feedstocks, the potential for fouling exists due

to a combination of tars, char, and ash. Care therefore must be taken

in the exchanger design to eliminate condensation problems. Following

the transfer line exchanger, the reaction mixture may have to be further

quenched depending upon the temperature of the gas mixture. The quench

may be accomplished using a light oil fraction or organic laden condensate derived from the plant.

In the case of MSW feedstock, considerable solid residue exits

the pyrolyzer. Hot cycloning of the gas appears to be advantageous for

recovering solids and permitting further heat recovery from the gas.

In the pyrolysis system cooling to 700 F to 900 F in the transferline exchanger should permit cycloning of residual char, ash and slag

without tar condensation. The dew point of the product mixture is not

well known and this temperature is therefore very tentative. The gassteam-tar mixture must then be cooled and scrubbed to remove the condensables.

When dealing with biomass feedstocks such as wood flours, the quantity of ash and char produced is negligible. In tqis instance, cleanup

can proceed as in theMSW case or can b~ simplified by direct quench to

an oil or water scrubber. The later scheme is less expensive but is

also less energy efficient. The feasibility of such a water quench/

scrubber has been demonstrated in the Union Carbide Purox process.

Table 2 compares features of oil and water scrubbing.

In either case, oil mist is carried through the separator in micron

size droplets, These can be recovered by a variety of techniques. The

two principle methods involve filter type devices termed mist eliminators and electrostatic precipitators (ESP). The latter are more cos~ly

but can give essentially complete recovery of oil from the gas stream.

Mist eliminators are much lower in efficiency. Table 3 gives some data

on oil recovery for scrubbers, mist eliminators and precipitators. A

good review of oil mist elimination techniques is given by Bennett

(1980).

Some tars produced: are water soluble. Little information has been

reported on the nature of the tars from biomass flash pyrolysis. Therefore, proper design of_a recovery system cannot be carried out at this

time. One alternative would be to recycle the tar-water mixture to the

cracker to provide additional ethylene. At this point, the gas consists

primarily of C6 and lighter components.

395
~EMICAL SYSTHESIS

The fast pyrolysis gas can be processed to yield a variety of products. Three simple alternatives are olefins recovery 'f or subsequent

chemical systhesis, gasoline manufacture and mixed alcohols manufacture.

Ethylene is an important intermediate in the petrochemical industry.

The major uses of ethylene (Stinson (1979)) are low density polyethylene

(27%), high density polyethylene (16%), ethylene glycol and .oxide (19%),

ethylene dichloride (15%), and ethylbenzene (8%). The bulk of the U.S.

ethylene comes from cracking of hydrocarbons. In 1979, the U.S. capacity was 33 billion pounds per year with an 88 percent onstream factor.

Ethylene demand is strongly dependent on the consumer economy. During

recessions, demand falls; in the current recession period, the onstream '

factor for the ethylene industry is approximately 66 percent•

. Ethylene demand is projected to grow to 41 billion pounds by 1985.

Capacity is expected to keep pace with demand. The trend in ethylene

production is toward heavier feedstocks which means more capital intensive facilities and high production costs. The price of ethylene has

moved strongly from l4¢/lb. in mid-1979 to 24¢/lb. in mid 1980 due to

increased feedstock prices. Large commercial plants typically are on

the order of one billion pounds per year, while small regional facilities are on the scale of ioo million to 500 million pounds per year.

The yield of ethylene from a 100 TPD biomass facility is estimated to

be 7 MM lb/year and 60 MM for a 1000 TPD facility. Because of higher

production costs, there must be a ready market within the region to

offset transportation costs. Therefore, olefin production is very site

specific.

Gasoline and alcohol production are suitable alternatives for fuel

purposes. Each has a ready, dependable market. Additionally, the politics of energy makes this type of venture attractive. Alcohol production has three important advantages over gasoline production. First,

the gallon output of an alcohol plant is greater than that for a gasoline plant. Second, the alcohol product may qualify for a 40¢/gallon

tax credit under the windfall profits tax bill. Third, the octane

rating of the alcohol product is much greater than the polygasoline

produced from olefins.

Table 4 (Christiansen (1980)) presents a comparison of several projections on liquid hydrocarbon demand through the year 2000. rhese

studies show that from the year 1985 to 2000, demand will continue to

grow. These studies are probably not realistic as the U.S. is currently

seeing a reduction in imports due to conservation resulting from pricing

and federal legislation mandating the production of higher performance

transportation vehicles. It is possible that there will be, more nearly,

a zero growth in consumption through 1985. This still means that an

import rate of 7 MM BB/D must be maintained.

396
Figure 5 (C&E NEWS, July 1980) shows the projected change in octane

requirements for the motor gasoline pool over the next decade. This

figure reflects a combination of the banning of lead and the higher

octane requirements of the new D~troit products. Currently, the EPA

requires a minimum octane (R+M)/2 of 87 for· the unleaded gasoline. However, better fuel economies mean higher compression ratios, and it is

expected that the future pool of unleaded gasolines will have a regular

grade with an octane of 89 and a premium grade with an octane of 93.

At this moment, refiners are facing pressures from all directions:

Detroit is pushing the compression ratios up because of tough EPA requirements on fuel economy, and at the same time, that government. agency

has limited the use of lead compounds to 0.8 gr/ga1 of gasoline

until October 1, ' 1980, when it will be phased down to 0.5 gr/gal. In

addition, the gasoline pool is competing for BTX (Benzene-Toluene-Xylene)

that might otherwise go to chemical feedstocks; therefore, it is facing

possible shortening supplies of aromatics. This gloomy overall picture

has left refiners with only a few alternatives to boost octane ratings.

The possibilities appear to be:

Lead compounds ~ike TEL and TML (tetraethyl lead and

tetramethyl lead)

MMT (methyl-cyclopentadienyl manganese tricarbonyl)

MTBE (methyl tert butyl ether)

TBA (tert butyl alcohol)

Reformate (Aromatic fraction consisting of C7-C9

compounds).

Alcohols; methyl fuel, methanol, and ethanol, Sun Tech

methano1- TBA blend

By definition, octane number is the percent of 2,2,4-trimethyl

pentane (isooctane) that must be blended with n-heptane to give the

same knock behavior in a laboratory engine as the fuel tested. RON

(Research Octane Number) reflects performance at full open throttle at

low speed (acceleration) and MON (Motor Octane Number) is a performance

index of even driving with full open throttle ·and high speed or partly

open throttle at high or low speed.

On October 27, 1978, EPA (Environmental Protection Agency) banned

MMT. The agency said that MMT producer Ethyl Corp. (Richmond, Va.)

failed to prove that the additive does not raise hydrocarbon levels in

auto emissions. According to EPA estimates, without MMT, refiners will

have to increase processing rates by 6% and ·de al with shortening supplies of aromatics. California was the first to react when the EPA

imposed a ban on MMT, on the grounds that unleaded gasoline containing

the additive did not meet the state's vehicle emissions standard for

397
hydrocarbons. Furthermore, the California Air Resources Board (CARB)

said that MMT emissions are toxic and blamed the additive for plugging

exhaust-gas recirculation valves and causing scale buildup.

With respect to TBA, the EPA approved its use in gasoline up to 7%

by volume, effective January 1979 (News of the Week 1979). Oxirane has

been using the alcohol at commercial scale for ten years and no consumerrelated problems have been reported. But TBA is not widely available

and the construction of a plant for the sole purpose of making TBA is

apparently not economically feasible. At the present time, the alcohol

is used mainly in Gulf Coast refineries and goes into gasoline distributed in the East via product pipelines, apparently because production

is not enough to treat gasoline in other areas.

MTBE, another octane booster, appears as a very attractive alternative because of its higher octane number and negligible water miscibility. Furthermore, the EPA approved the use of the additive as an

octane booster at concentrations of 7% or less by volume. But it is

produced from isobutylene and methanol, and because of limitations on

the amount of isobutylene produced in the u.s. by refineries and olefins

plants, only enough MTBE is available to make .up 3% of the gasoline

pool. Otfuer studies are more pesimistic.

For 1980, the unleaded gas demand will be 46%and will reach 64% of

the total U.S. gasoline consumption by 1984. Treating the entire 3.2

million bbl of unleaded gasoline expected to be used per day in 1980

would require 21.4 billion pounds per year of the ether. If the estimated production for 1980 is 1.3 billion pounds per year, that means

that only 6% of the unleaded gasoline would be treated at a 7% level

of MTBE.

Ethanol from fermentation is another alternative rece1v1ng considerable attention, but according to a study of the u.s. Department of

Energy (DOE), "if all practicably avaf.Labl.e farm land were used for farm

crop plantings in excess of those required for food production, the

ethanol produced from the crops and crop residues would satisfy no more

than 8% of today's (March 1978) total liquid fuels energy demand."

Economics of the process are improving since a 40¢/gal federal excise

tax is being considered.

One of the main reasons for the sudden popularity of MTBE is that

it will increase the refiners availability of aromatics. Reformate is

the most immediate answer to octane improvement. Furthermore, improved

technology and catalysts for catalytic reforming will allow better

yields and products to be achieved. But alcohol fuels produced from

coal or biomass have the potential to treat the u.s. gasoline production

without additional crude oil imports. Reformate, one of the highest

octane ingredients of gasoline, contains about 40% aromatics plus naphthenes, olefins, and straight-run paraffins. But higher severity oper~

ation, which reduces yield and boosts costs, is required to raise reformate's octane. In addition, refiners have to face competition from

398
the petrochemical industry. The chemical demand for BTX normally runs

about 3 billion gallons per year, while the amount consumed in gasoline

is roughly seven times that. There are other factors working against

the,use of BTX in the gasoline pool. Benzene has been listed as a hazardous pollutant under the Clean Air Act and nothing prevents the EPA from

banning benzene outright at some future point.' Refiners may choose not

to use it at all instead of installing controls and monitoring equipment.

Toluene and xylene have not been listed as hazardous and are not now

subject to regulation.

All these restrictions and bureaucratic obstacles leave refiners

with alcohols as the other alternative. Reformate, MTBE, and TBA will

be only minor components of an overall strategy to increase octane

ratings, whereas alcohols from coal and biomass will playa major role.

OELFIN RECOVERY -

The most commonly used scheme for olefin recovery utilizes cryo~

genic gas processing. A typical flow scheme is shown in Figure 6. The

raw gas off of the pyrolysis furnace must be compressed to 450 to 800

psia for processing. Because of the large quantity of COZ present in

the gas, the COZ along with other acid gases including HZS and HCl must

be removed at this point. For such applications, a variety of scrubbing

systems are available. A suitable choice is the Benfield hot carbonate

process. It has the important advantage that hydrocarbons are not soluble in the potassium carbonate scrubbing agent. The COZ must be removed

at this point because it will cause icing in the demethanizer. The gas

mixture is dried and cooled to -Z50 F where hydrogen is recovered.

The mixture is distilled in the demethanizer to recover CO, CH4 and HZ

as a fuel gas. ACZ splitter is then used to recover pure ethylene.

An ethane stream and a C! stream can be recycled for further cracking.

GASOLINE MANUFACTURE

In order to polymerize ethylene to gasoline, the raw gas must first

be compressed. After compression, the COZ must be scrubbed and the olefins concentrated. This can be accomplished by scrubbing with an aromatic absorption oil. , Acetylene must also be hydrogenated to ethylene.

This is a simple fixed bed step which can be accomplished with the available hydrogen in the steam. In the most complicated alcohol system,

these steps must also be employed.

The polymerization of olefins to form gasoline may be carried out

thermally or using a catalyst. Th~catalytic process involves a phosphoric acid catalyst. In the case of ethylene, depending on the temperature, naphthenes, aromatics and paraffins are formed along with olefins.

The light cut «llOC) is generally paraffinic while the heavy ends are

aromatic (>ZZOC)(Ipatieff, and Pines (1935)). Catalytic polymerization

takes place at 350 F to 750 F and 200 to 1500 psia.

399
Figure 7 shows a typical flow sheet for a cat-poly unit. Generally,

60-80 percent conversion per pass is attainable. Alkanes in the feed

along with CO, and H2 pass through the reactor intact along with unconverted olefins and therefore, a recycle stream is necessary. The liquid

polymer consists of a gasoline fraction with a boiling range between

190 'F and 360 F and a heavy gas/oil. By properly controlling the temperature the gasoline fraction can be maximized.

The motor octane number of the cat~poly gasoline produced from

ethylene is typically about 82. The research octane number is reportedly

higher. Compared with alkylates, the antiknock performance is inferior.

Upgrading might be necessary to produce a higher quality motor blending

fuel.

THERMAL POLYMERIZATION

Thermal polymerization requires temperatures on the 'order of 900-

1300 F and pressures between 800 and 1200 psi (Keith and Ward (1935».

The thermal process produces a produce containing about 2/3 olefins with

the remainder being naphthenes and paraffins. Additionally, a considerable amount of heavy oil is produced. Gasoline from the thermal process has a very low octane number, typically ranging from 68-72. Thus,

thermal polymer product would have to be reformed to increase the octane

level for blending. The flow sheet for the thermal process is similar

to that for the catalytic process.

PRODUCTION OF MIXED ALCOHOLS

Reasons

There are a number of reasons to consider the synthesis of mixed

alcohols from fast pyrolysis gas. These may be broadly divided into

economic and utlization classifications. Like the production case of

hydrocarbon fuels, the plant facility is not only eligible for a ,lO percent investment tax credit but also for an additional 10 percent energy

tax credit at least through 1985. In the case of the solid waste disposal facilities, the Windfall Profits Tax Bill allows the use of tax

exempt Industrial Development Bonds to build facilities to produce alcohol or other fuels. During this period of unprecedented high interest

rates, this is an important consideration. Finally, and most important,

the alcohol fuels tax exemption has been extended through 1992. This

tax credit amounts to 40¢/gallon of 95% alcohol and 30¢/gallon for 70%

alcohol added to gasoline in a 10% amount if the alcohol is derived

from sources other than coal, gas or petroleum. Alcohols produced chemically from fast pyrolysis gas ' appea r to satisfy the law.

From a utilization point of view, the alcohols also look attractive.

Based on the fast pyrolysis gas, a fuel blend containing the composition shown in Table 5 can be obtained.

Table 6 presents octane numbers for typical oxygenated fuels.

From Table 6, it can be seen that these compounds are capable of

400
providing substantial octane improvement on blending. Ethers are particularly attractive in that they boost the MON as well as the RON.

Figures 8, 9, 10, 11 (Keller, 1979) suggest that the alcohol blend

from fast pyrolysis will yie,ld a motor fuel with suitable vapor pressure,

volatility and phase separation resistance due to water.

Technology

Two principle synthesis routes for alcohol manufacture from olefins

are direct hydration using an acid catalyst and indirect hydration using

sulfuric acid. Additionally, higher alcohols, in this case principally

propanols, can be manufactured using the oxo-process and its modifications. For preparation of fuel grade alcohol, the indirect hydration

process appear~ to be the most useful.

Indirect Hydration

The indirect hydration process (Kirk-Othmer (1963)), as shown in

Figure 12, employs sulfuric acid as a reactant. At temperatures of 70-

80 C and acid strengths of 9S-99%, ethylene is rapidly adsorbed to yield

ethylhydrogen sulfates. Industrially, these ractions are carried out

at ISO-ZOO psi with feeds containing 3S% or more ethylene.

C2H4 + H2S04 ------C2HSS03

ZC2H4 + HZS04 ------(CZHS)Z(S04)

The quantity of the mono and di-sulfates present depend on the ethylene

partial pressure and liquid residence time.

The crude pyralY$~sgas contains ethylene, propylene and butylene

as well as saturates, CO, C02 and HZ. To carry out the above reaction,

propylene and butylene must be removed because these will tend to pplymerize. The most convenient way to effect this removal is to wash the

raw gas with cold, diluated H2S04. Typically, 8S% acid is satisfactory

to effect C) removal and 70% for C4 removal. Additionally, heavier tars

should be removed prior to ethylene adsorption.

A number of process improvements would greatly enhance process economics. These are listed in Table 7.

Past studies have shown that even at atmospheric pressure with low

olefin content gases, high ethylene recovery is possible. Bury and

Ollander studied the adsorption of ethylene from coke oven gas containing 3% ethylene and effected 70% removal in 1 l/Z minutes gas contact

time. This suggests that fast pyrolysis gas can be treated at or above

atmospheric pressure with a minimal amount of preprocessing. Eliminating the COZ stripping step and olefin concentration step are important

in process economics.

401
After adsorption, the mixed acid/ethylhydrogensulfate is diluted

with about 1.5 volumes of water and held for 3/4-1 hour at about 90 C.

The crude alcohol is stripped from the mixture and is sent to purification. The resulting acid stream, containing 60-70% aqueous acid is reconcentrated by vacuum evaporators to about 93%. The acid is brought

to full strength by the addition of oleum. The acid reconcentration

step is expensive and should be the target of research.

Distillation of the crude alcohol will yield the desired dry product. Little or no phase equilibria data are available for the design

of such a separator. A significant process improvement would be to replace the distillation step by a more suitable technology.

Economics

The economics of these 'pr oces s es have been recently evaluated by

SAl (1979). Table 8 gives efficiency data and Table 9 summarizes the

capital costs and economics for each process adjusted to a zero dump fee

for MSW. These economics may be unrealistic in terms of the fuel cost

because of the cost for preprocessing the solid waste. However, they

are satisfactory for comparison purposes.

For plant sizes of 100 ton/day and 1000 tons/day capacity, Table 9

shows the relative costs of the products in comparison to early 1980

prices. Considering the tax credit for alcohols, the 1000 ton/day capacity plant shows economic promise, particularly for alcohol manufacture.

Table 10 reflects a recent study by the Institute of Gas Technology

(C&E News, August 11, 1980). The point of Table 10 is that the cost of

imports and domestic fuels cannot be directly compared. There are a

number of benefits from domestic production which can be quantified including reduced inflation, competition, employment and national security.

The study suggests that a domestic synfuel barrel can be sold for twice

the price of an imported barrel with the same effect on the economy.

Implementation of an industry based on such reasoning can be accomplished

only by government assistance.

CONCLUSIONS

The production of an olefin rich synthesis gas is technically feasible. Such a gas is suitable for the recovery of ethylene, manufacture

of gasoline or mixed alcohols. The later route appears to be the most

economically feasible. A number of R&D areas have been identified which

must be investigated for process optimization.

402
REFERENCES

1. Antal, M.J., Edwards, W.E., Friedman, H.C., Rogers, F.E., "A Study

of the Steam Gasification of Organic Wastes," Final Report, EPG

University Grant No. R8048360l0 (1979).

2. Benham, C., Private Connnunication, 'Solar Energy Research Institute,

Golden, CO (1980).

3. Bennett, R., A Survey of Biomass Gasification Volume III, Ch.-!2,

Solar Energy Research Institute (1980).

4. Christiansen, A.P., "Evaluation of MEBP as a Gasoline Extender,"

M.S. Thesis, Colorado School of Mines, T-2303 (1980).

5. Dieboid, J., Smith, G., . "Commercialization Potential of the China

Lake Trash to Gasoline Process;" Municipal Solid Naste as a Resource:

The Prob~ems and Promise Conference, July 13-17, 1979.

6. Griswold, C.F., "Steam Pyrolysis of Tosco Shale Oil for Production

of Chemical Intermediates," M.S. Thesis, CPR, Colorado School of

Mines, Golden, CO (1976).

7. Ipatieff, U.N., Pines, Herman, "PolYmerization of Ethylene under High

Pressures in the Presence of Phosphoric Acid," I&EC'1:2, 1364 (1935).

8. Keith, P.C., Ward, J.T.,"3.5 Cents Per Gallon Ls'Hanuf acturd.ng Cost

of PolYmerized Gasoline," National Petroleum New~, 52-62 (Nov. 20,

1935).

9. Keller, J., "Alcohol Gas Motor Fuel?", Hydrocarbon Processing 58 (5),

127 (1979).

10. Keuster, J.L., "Urban Wastes as an Energy Source," Energy Systems:

An Analysis for Engineers and Policy Makers, Marcel Dekker (Jan.

1978).

11. Kirk Othmer Encyclopedia of Chemical Technology V8, 431-436, V16,

P568-571, John Wiley and Sons (1963).

12. MalIan, George, Compton, L.E., Occidental Petroleum, "Gaseous Hydrocarbons from Municipal Waste," U.S. 3,846,096 (Nov. 5, 1974).

13. Mol, A.; "Which Heat Recovery System?" Hydrocarbon Processing 52

(7), Pl09-ll2 (July 1973).

14. Science Applications Incorporated, "Evaluation of the NWL-Diebold

Pyrolysis Gas Process," SAl 1179-0l9-GLN (1979).

16. Stinson, S.C., "Ethylene Technology Moves. to Liquid Feeds," C&E

News, p. 32 (May 1979).

403
TABLE 1

COMPARISON OF YIELDS. Wi

SHALE OIL NAPHTHA

METHANE 19 19

CZ-UNSATURATES 30 Z6

C3 + C4-UNSATURATES 19 19

CARBON MONOXIDE

CARBON DIOXIOE

HYDROGEN

BIOMASS

6

16

6

40

15

TABLE Z FEATURD OF On. AND WATER SCRUBBING

Disposal of

purge liquid

Oil croptct

removal COlt-

~iderations

Makeup qtality

~et.llurgy

Source of scrUbbing

medium

Oil ~crubhin~

Thf'rmlll o'(idntion with

hen t reeeverv or reevcle after fractionRtion

Entealnment nnrJ saturntion of gns stream with

oil

Oil might require

frllctionation to achieve

proper boiling range

material

Carbon steel equipment is

probably adequate if no

weter condensetion oeeurs

Availeble if oil produced

by process can be used:

otherwise must be imported

404

WIlter Scrubbinli

Water treatment before

discharge

Oil entrainment from

H20

Condensate quality water

Water will be aeldie due

to contaminants in ges:

steinless steel scrubber

required

Readily available
TABLE 3 SUMMARY OP MIST ELIMINATION SURVEY RESULTS

Plate Packed s,ny Mist

DeYiee Serubber 5erubber lIrubber Venturi Eliminator Wet ESP

Pre_ure

O.24-0.Sd $'Op (In. H2O) 10 1-3 10 - 30 1 - 3

Droplet size (microns)

at percentile

removal 5 at 80 1.5 at 50 2 at 50 5 5 - 10 1

at 50 at 100

ClrC!Ulaticn

<,pm/I 000

elm) 2-50 2-50 30-JOO 5-20 3-SC! Variable

Capital oost Ca) (b)

OpeNti.. eost Power Power Power Power )flnimal :\finimal

Maintenanee

COlt Nominal Nominal Nominal . Nominal Nominal High

aplate IC!l'\.Cber 11the most expensive wet serubber: venturi is the least expensive wet serubb~.

!>wet ESP is the most expensive of aU devices considered..

Conne to five gpm/tt2 ot mist eliminator et'OS$-seetional area.

dPresure drop per foot of paeked heiJtht .

TABLE 4

E~rected Liquid Hydrocarbon De~a~d

(expressed as rr.; 1Jion barrels/day of crude oil equivalent)

1973 197'; 1985- Z::loa 20JO

(Pace) (Chern. Systerns) ( Lo\.; lmoorts;

( Che~ SystE!:7's)

G,Ho1ine t.7 (.6 6.6 9.~ ~.~

:.a~hth/l /JP-4il 0.2 0.5 0.4

A 1.4 1.3 l.~ 2.6 2.6 ~;~sel :.0. 2 2.7 2.5 .~ 3.6 3.6

Res : 1 Z.8 2.6 4.4 4.5 .5

:'s~ilalt. Lubes.

e tc. 0.7 0.6 1.6 1.5 1.5

Fuel :-.!s/Petl·oc:henicals 1.3 1.8 :.3 4.5 ·j . 5

T"- V ,1""_'I 16.3 15.7 20.7 25.7 25.7

OIL SO~~C£S

ir.:;l:rti 6 5.6 t1.6 • 10.0 6.0

Gc:::es tic ( ,-"de +

~l :;l 10.3 9.8 12.1 12.5 12.5 ~hale Oil 0 0 0 2.5 2.5

Co!1 !.i ~ui cs 0 0 0 1.0 5.0

T;)7;'L 15.3 15.7 20.7 25.0 26.0

a,e t,"octlcmi ca1 use of naphtha or gJS oil not incluJcd

405
TABLE 5

TYPICAL MIXED ALCOHOL FUEL FROM PYROLYSIS GAS

Wt. %

Ethanol

Isopropanol

Sec-Butanol

Diethylether

Di-isopropylether

Dibutylether

TABLE 6

73.0

10 .0

8.0

7.0

1.0

1.0

OCTANE NUMBER OF OXYGENATED HYDROCARBONS

COMPOUND RON MON

Methanol 106-115 86"':92

Ethanol 110 92

n-Propanol 90

Isopropanol 106 99

n-butanol 100

Sec-butanol no 92

Isobutanol 107

Tertbutanol(TBA) 113 110

MTBE 117 101

Di-isopropylether 103 99

TABLE 7

ALCOHOL SYNTHESIS RESEARCH

1. Eliminate or Minimize

A. Compression

B. CO Stripping

C. Etftylene Concentration

2. Develop New Alcohol Recovery System

3. Improve Acid Reconcentration Technology

406
TABLE 8

EFFICIENCIES FOR BIOMASS PYROLYSIS(MSW)

Product Mixed

Feedstock Ethylene Gasoline Alcohols

Biomass Feed, TPO 100 100 100

Biomass Fuel, TPO 44 44 44

Biomass for Power, TPD 13.5 5.1 7.4

TOTAL Biomass Feed, TPD 157.5 149.1 151.4

Products

Ethylene, TPO 10.0

Gasoline, BBL/O 106

Alcohol, GAL/O 7073

LPG,TPO 8.79

Fuel Gas, MMSCFD (HV) 1. 48(446) 1. 50(434) 1.39(434)

C/ Oil Fraction, TPD 3.30

Major Product Efficiency, % 16.4 24.3 25.4

Overall Efficiency, % 55.4 50.4 54.6

TABLE 9

SUMMARY OF ECONOMICS FOR

A MSW FAST PYROLYSIS FACILITY

It em

100 TPO capital cost, $mm*

yiel d

selli ng price at ** 2m DCF ROI

selling price ~~ 20%ROI for

25%reduction in capital

SID increase in fuel fee

25%increase in yield, ROI

selling price for 20 OCF ROI,

1000 TPD plant

current sell ing price

*Chemical Feedstock

Ethylene

8.2

10 TPD

SO. 48/1 b.

$0.36/lb.

+7c/1 b.

26 ;:

SO.22/lb.

$O.24Ilb.

Gasoline

11.0

4, 452 gal/O

$3.30/gal.

$2.60/gal.

+2U;/gal.

25%

52.14

$0.98/ga1.

Alcohols** *

9.1

7073 gal/D

S1.84/gal.

$1.29/gal.

+21¢!gal.

27%

S1.0S/gal.

$>l.OO/gal.

* *

* **

Assumes a zero price for MSWfeedstock

Alcohols may net an additional 40c/gailon credit

407
TABL.E 10

Benefits accrue from oil Import

reduction of 500,000 bbl per day

I p.f b.'f•• - v••, 1 v.,., ) Long., '."n

Oi, eel benelll .00 $3 6.49 $ 4000

Indlr eel !>eMma

0 11 price ellect 12.41 19.07 12.41

Inllalion etrect 9.92 23 .04 11.90

Employmenl etl oCl 1.32 n .n 678

Seeur Ity etlect 6 .71 6 .71 6.71

TOTAL $73.3' S 110 .23 $79.80

• L\., v"'-' Uri 1980oo1W\ $.outce: rift' Aft~ftN( u,,,llt ute

SL OW HEATING

MEG

CHARCOAL

fA ST HEATING

OLEf II/S

EJS: CHARCOAL

fUEL r.AS

GASIf ICAT ION

I a j r

SYNTHESIS GAS

GASIfiCATION

J 0"

ft9ure I. Bt~mass rrocesslng ou

408





SPECIALIST S' WORKSHOP ON FAST PYROLYSIS OF BIOMASS

OCTOBER 19-22, 1980

COPPER MOUNTAIN, COLORADO

Michael Antal

Department of Mechanical &

Aerospace Engineering

Princeton University

Princeton, NJ 08544

609/452-5136

Charles Benham

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231':'1754

Paul W. Bergeron

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-1768

Mark Bohn

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-1755

Rashmikant M. Contractor

E. I. du Pont de Nemours & Co.

CR & D Dept., Experimental Sta.

Wilmington, DE 19898

302/772-3111

Ray Desrosier

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-1436

James Diebold

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-7250

Clas G. V. Ekstrom

Dept. of Chemical Technology

Royal Institute of Technology

S-lOO 44 Stockholm

Stockholm, Sweden S-10044

08/787-8994

Ted Frankiewicz

Occidental Research Corporation

P. O. Box 19601

Irvine, CA 92713

714/957-7257

A. Ghazee

University of Dayton

Dayton, OR 45469

513/229-3016

David W. Goheen

Pioneering Research Group

Central Research Division

Crown Zellerbach Corporation

Camas, WA 98607

206/834-4444

Michael S. Graboski

Colorado School of Mines

Chemical & Petro~eum Refining

Engineering

"Gol den , CO 80401

303/279-0300 Ext. 2764

Herbert M. Kosstrin

P. O. Box 2325

245 Summer Street - 11th Floor

Boston, MA 02107

617/973-0309

Alex Kotch

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-1823

415
SPECIALISTS I WORKSHOP ON FAST PYROLYSIS OF BIOMASS

OCTOBER 19-22, 1980

COPPER MOUNTAIN, COLORADO

Barbara B. Krieger

University of Washington

Benson Hall BF-10

Seattle, WA 98195

206/543-2216 2250 (Message)

James Kuester

College of Engineering

Arizona State University

Tempe, AZ 85281

602/965-5071

J. Lede

Laboratorie des Sciences du

Genie Ghimique

1, Rue Grandville - 54042

Nancy Cedex, France

8-336-6623

Kenneth Lincoln

NASA-Ames Research Center

Mail Stop 234-1

Moffett Field, CA 94035

415/965-5373

Tom Milne

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-1440

S. Narayanan

Stone &Webster Engineering Corp.

One Penn Plaza

250 West 34th Street

New York, NY 10001

212/760-3619

Quang Nguyen

IOTECH Corporation

220 Lavrier Ave. - W

Ottawa, Ont. Canada ,KIP 529

613/238-4126

William A. Peters

M.I.T. Energy Laboratory

Room 66-460, M.I.T.

Cambridge, MA 02139

617/253-3433

Tom Reed

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-1437

John Scahill

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-7351

Fred Shafizadeh

School of Forestry & Department

of Chemistry

University of Montana

Missoula, MT 59801

406/243-6212

J. Michael Skaates

Department of Chemistry &

Chemical Engineering

Michigan Technological University

Houghton, MI 49931

906/487-2177

Mike Soltys

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231-1449

Clayton Smith

SERI

1617 Cole Boulevard

Golden, CO 80401

303/231.:..1200

416
SPECIALISTS' WORKSHOP ON FAST PYROLYSIS OF BIOMASS

OCTOBER 19-22, 1980

COPPER MOUNTAIN, COLORADO

Meyer Steinberg

Brookhaven National Laboratory

Upton, Long Island, NY 11746

516/345-3036

Don Stevens

Battelle Northwest

P. O. Box 999

Richland, WA 99352

509/375-2260

Kent Vorhees

Department of Chemistry

Colorado School of Mines

Golden, CO 80401

303/231-0300

,; u .s. GOVERNMENT PRINTING OFFICE, 1518 0--7 77 .0 8 4/ 4 3 7

417