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Chemical Processes During the

Starved Air Incineration of

Municipal Solid Waste

Wing Tsang and James A. Walker

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Gaithersburg, MD 20899

October 1 984

Prepared for: U,S. DEPARTMENT OF COMMERCE

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inal Bureau of Standards

iersburg, MD 20899

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NBSIR 84-2938

CHEMICAL PROCESSES DURING THE

STARVED AIR INCINERATION OF

MUNICIPAL SOLID WASTE

Wing Tsang and James A. Walker

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Gaithersburg, MD 20899

October 1 984

Prepared for: U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Final Report

Chemical Processes During the Starved Air Incineration of

Municipal Solid Waste

Wing Tsang and James A. Walker

Chemical Kinetics Division

National Bureau of Standards

Gaithersburg, Maryland 20899

Supported by the Department of Defense, Naval Civil Engineering Laboratory,

Port Hueneme, California 93043

Chemical Processes During the Starved Air Incineration of

Municipal Solid Waste

Wing Tsang and James A. Walker

Chemical Kinetics Division

National 3ureau of Standards

Washington, DC 20234

Abstract

Thermal analytical measurements on the pyrolytic and oxidative decomposition of municipal solid waste and its constituent parts (cellulose, sawdust,

newsprint and polyethylene) have been carried out. Under pyrolytic conditions

thermogravimetric analysis of municipal solid waste indicated minimum yields

of char in the 1 0% range. Evolved gas analysis yielded information regarding

the nature of the volatile gases in terms of total FID sensitive hydrocarbons,

CO, C0 2 and CH^. From material balance considerations it is concluded that

the volatile gases includes large quantities of acidic carbonyl and alcohol

compounds. The total quantity of volatile gases not including the directly

analyzed light gases, is of the order of 60$. The overall process is thermally neutral. This is in good agreement with measurements using larger

samples. Under oxidative conditions MSW ignited very easily and the reaction

goes rapidly to completion. Combination of the experimental data and literature information leads to a description of the chemical processes in starved

air incineration.

1
Introduction

This paper contains the results of an investigation into the nature and

heat content of the substances formed during the pyrolysis and partial

oxidation of municipal solid waste. The information is to be used as inputs

to a computer model for starved air incineration currently being developed by

C. A. Kodres at the Naval Civil Engineering Laboratory (1). A schematic of a

starved air incineration system can be found in Figure I. During operation,

partial oxidation with under and/or overfired air occurs in the primary

combustion chamber. Complete combustion is effected in the secondary combustion chamber and followed by heat recovery. Such devices offer advantages

with respect to completeness of combustion. A key variable in modeling such

systems is the nature and heat content of the gas that is carried over into

the secondary combustion chamber. This is not a simple and straightforward

problem as evidenced by the fact that even for as thoroughly studied a subject

as coal the nature and rates of the aevolatization process is still a subject

of resear ch , controversy and uncertainty (2). In the following, we begin with

background material on the nature of municipal waste. This will be followed

by a discussion of reaction mechanisms for the decomposition of the generic

chemical components and how such considerations will be reflected in the

heating values of the product gases. We then describe our thermal analytical

studies. Finally, we discuss the implication of our results in terms of

earlier work and conclude with a number of recommendations.

Background

Municipal solid waste is a heterogeneous mixture of organic and inorganic

substances. Tables la-d contains representative data (1, 3~5). It is

indicative of the difficulties inherent in attempting to describe its proper2
heat

exchange

and stack

Fuel and air

>, secondary

combustion

over fire ajr Primary Combustion

Chamber

Municipal j solid i___

waste

underf i re air

ash

i Y

!

Figure I: Schematic of starved air incinerator

Chamber

3
Table I: Representative Data Regarding

the Composition of Municipal Solid Waste

Quad-City Solid Wastes Project First Annual Report Fact Sheet (%) (3)

paper 39.8 - 53.66

wood 1 .69 - 3.45

plastic 2.33 ~ 2.97

glass 3.73 " 9.96

metal 6.95 - 12.91

stone-sand 6.16 - 8.53

* organics 20.22 - 26.76

rags 2.2 - 7.6

Municipal Waste Generation in 1971 (4)

paper 31 .8

wood 3*7

plastics -3.4

glass 9.7

metals 9.5

food wastes 17.6

yard wastes 19.3

rubber and leather 2.4

textiles 1 .

4

miscellaneous inorganic 1 .4
c) Composition of Organic Waste at NAS, Jacksonville (1)

paper 34.7

corrugated bases 27.9

plastics 14.8

food waste 17.1

textile 1 .2

grass 2.1

wood 2.2

d) Composition of MSW obtained January 10-21, 1984 from the Baltimore County

Resource and Recovery Facility; 10 samples (5)

initial moisture 22 - 52.7

metal (magnetic) .3 - 6.4

metal (non-magnetic) .6 - 1 .9

\ wood, vegetable .2 - 2.5

textiles .5 - 13.8

plastics 3.3 - 12.8

paper 57.9 - 67.7

glass, ceramic • 3 - 3.5

fines 9.7 - 23.6

metals 1 .2 - 10.7

Assume to be food, yard wastes and miscellaneous organics.

5
ties. In terms of chemical effects we note the presence of large quantities

of cellulosic products; specifically, newsprint, wood, food and yard wastes.

In the case of wood and yard wastes there are also considerable quantities of

lignin. Together with plastics, rubber, textiles and leather they represent

other types of organic polymers. They differ from cellulose in terms of a

much lower oxygen content. This leads to higher heating values. From the

generic chemical point of view the processes occurring in the primary combustion chamber are dominated by depolymerization and condensation reactions of

cellulose and with varying contributions from other man-made and natural

polymers. The inorganics are not of concern with respect to heating values.

However, they have important effects with regard to catalyzing or transforming

the nature of the decomposition process. This can be an important uncertainty. In addition, on a weight basis the most important inorganic compound

is water. Since it has no heating value its principal role will be as a

diluent. Unfortunately, the quantities present can vary greatly. In any

model, water content must be an adjustable parameter.

The general question of the variability of municipal solid waste has been

treated in detail at the National Bureau of Standards (5). Over a two week

period, samples of municipal solid waste at the Baltimore County Resource

Facility in Cockeysville , Maryland which have passed through the primary

shredder were collected daily. The total moisture content was determined and

after reduction to particle sizes of 2 mm or less 40 samples were prepared for

measurement. Properties determined included residual moisture, furnace ash,

bomb ash and higher heating value. The important conclusion reached was that

while moisture and ash showed wide variations, close to 40$, the higher

heating value (moisture and ash free) of the combustible fraction showed

variations of only 4$. This is indicative of the kind of variation in

6
combustion related properties that can be expected at a particular locality

and over a limited period of time. It is a necessary condition for serious

work in this area. Much of our experimental results will be based on these

samples

.

There is considerable qualitative understanding of the mechanism of

decomposition of the various components in municipal solid waste. The

quantitative situation is, and probably will remain so for many years,

unsatisfactory. This is due to the problems inherent in studying chemical

processes involving organic solids. Specifically, during the decomposition

process compounds are produced which are chemically and thermally less stable

than the starting material. There is competition between escaping from the

solid (volatilization) or undergoing further reactions (condensation, ultimately leading to char formation). As a result it is not possible to isolate

individual processes for study and then creating via computer simulation

directly observable events. Instead, all that is available are global

observations which are influenced strongly by the physical parameters of the

experiments. These include heating rates, sample size, sample state, etc.

Nevertheless the existing data are suggestive and set a basis for further

work.

Cellulose begins to decompose at temperature above 300 °C. From differential scanning calorimetric (DSC) measurements, the reactions appear to be

mildly endothermic. Interestingly , the degree of endothermicitly is controlled

by the crystallinity of the sample (6). From our DSC tests with newsprint the

heat of reaction is close to neutral. However note that we are describing a

devolatilization process. The heat measured in DSC experiment represent that

which is transferred to the sensing elements. The exact significance with

respect to the total system is unclear. The initial depolymerization product

7
is levoglucosan (1,6 anhydro-g-D-glucopyranose) and a number of its isomers.

This is accompanied and followed by dehydration processes leading to unsaturated compounds such as levoglucosenone , furfural etc. The relative importance of these processes is also influenced by crystallinity, with dehydration

being favored under completely amorphous conditions (6). At somewhat higher

temperatures, fission of the basic structure occurs leading to the formation

of the lighter gases. Condensation of the unsaturated compounds leads to the

build up of a carbonaceous char. The overall result is the production of

scores (7) of compounds whose relative concentrations are strongly controlled

by the manner by which the experiments are carried out. Thus it is customary

to speak of products in general terms as consisting of a char, tar and

volatile gases. These are very indistinct classes. There are small quantities of volatiles in the char. Tar contains substances that are readily

volatilizable at higher temperatures. Thus reports of product yields in these

terms must be related to the experimental methodology.

Under combustion conditions the initial process involves the ignition of

the volatile gases. If oxidation is slow this is followed by the slow

combustion of the char. However, in some systems ignition is followed by such

rapid temperature increases that practically all the material is volatized and

burned and very little residue is left. Under such rapid burning conditions

mass transfer effects become controlling and combustion is by layers and the

unburnt layers have the same composition as the initial sample.

Lignin is the principal non-polysaccharide found in vegetative matter. It

is introduced into municipal solid waste through its presence in wood and yard

wastes. It is a polymer with a phenolic structure. There are considerable

similarities in structure between lignite and lignin and there may well be

similarities in degradation processes. From analytical pyrolysis studies (8)

3
methoxyphenols , furans as well as low molecular weight volatile gases such as

acetone and acetaldehyde have been found. Due to its cross linked aromatic

structure char yields are much higher than that for cellulose (9). Plastics

are a significant portion of municipal solid waste. Due to its higher heating

value (in comparison to MSW) it is of more importance in terms of heating

value than its concentration present may suggest . Our thermal analytical

studies suggest that most of the plastic present in MSW is polyethylene. The

mechanism for the degradation of such compounds is "unzipping” to produce

monomer units (such as ethylene from polyethylene). However as in the case

with the degradations of all organic solids char formation is a possible side

reaction. As for other man-made polymers, they follow the same general

pattern, unzipping to smaller fragments and condensation to form char. In

common with lignin, cross linked polymers have a greater tendency towards char

formation.

We now consider heating values. These are the key parameters for

modeling the thermal behavior of energy generating systems. Furthermore,

because heats of combustion (on a weight basis) are strongly dependent on

elemental composition the complexity of the chemistry which is the charactei

istic feature of municipal solid waste degradation is considerably simplified.

Table II contains a summary of data (10) regarding the heats of combustion of

substances commonly found in municipal solid waste. In accordance with their

basic cellulosic nature, wood, paper and plants have heats of combustion close

to the 4165 cal/g value of ac-cellulose (10). The higher value for wood and

plants is due to the presence of lignin whose heat of combustion is approximately 6000 cal/gm (10). The lower value for magazine paper can be attributed

to the presence of inorganics such as kaolins with very low heating values.

Plastics have higher heats of combustion and the lower value for phenoL-f orr.a9
Table II: Heats of Combustion of Constituents of Municipal

Solid Waste (10)

paper

:

newsprint 4433 cal/g

corrugated box 3915

magazine 2921

brown 4034

wood: birch 4775

cherry- 4370

fir 5038

maple 4533

pine 4592

plants

:

grass clippings 4369

leaves 4439

plastics

:

polyethylene 11093

polypropylene 11082

phenol-formaldehyde resin 7554

rubber 1081 4

10
ldehyde resins is due to the presence of oxygen. There are a wide variety of

methods for the prediction of the heats of combustion (11). A very simple

correlation has been found by Shafizadeh and DeGroot (9) and is particularly

applicable for the present situation. It has the form

AH 2 5°( combustion) (cal/gm) = 94.19(56 C) + 55.01

As noted by these authors the success of this correlation is due to a

fortuitious cancellation of different effects. Measurements at NBS (12) on

the variability of MSW samples led to a heating value of 5200 ± 200 cal/g on a

dry and ash free basis. This is suggestive of a mixture of cellulose and

plastics (polyethylene) with the latter at the 10-15? level.

In the above we have dealt with fairly well defined starting materials. A

considerably more difficult problem is the definition of the heat of combustion of the products of pyrolysis and partial oxidation. There is very little

literature on this question. In principle if we can enumerate all the

compounds that are formed during the pyrolytic process, then since many

individual heats of combustion have been measured and others can be estimated

with high accuracy, the thermochemistry can be defined. Unfortunately in view

of the enormous number of chemicals and the undefined nature of the tar and

char it is not possible to proceed in this manner. It is interesting to note

that even in an area such as coal pyrolysis this question is still open. In

Table III we summarize literature results (9, 13, 14) expressed in terms of

percentage char, tar and gases and their heating values. Considering the

nature of the experiments and of the starting materials results are not in

consistent. The heating value of char is in the 6500-8000 cal/g range or very

close to lignite. Char yields are lowest in pure cellulose, higher in MSW inc

11
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12
highest with lignin. The heating value of the gases is much lower than that

for tar and char and is due to the presence of C0 2 and CO. There is a large

difference between the amount of tar detected in the saw dust and MSW experiments. We are not aware of any chemical basis for this difference. It is

suspected that this may arise from differences in carrying out the experiments. The samples of saw dust are probably more closely packed and since the

sample size is a factor of 15 larger, total reaction time is much longer.

An important parameter for the present purposes is the heat of pyrolysis.

In the case of the sawdust results (14) all of the heating value is recovered.

In the MSW experiments (13) considerable organics were lost during the work up

of the results, and thus an accurate value is not obtainable. In addition,

Kung and Kalekar (15) have fitted a mathematical model to experimental results

on wood pyrolysis and derived a heat of reaction of -48.4 cal/g. This is about

]% of the heating value of wood or other cellulosic substances and thus can

probably be ignored for the present application. From a more theoretical

point of view these results are not unexpected when one compare the heat of

combustion of hydrocarbons and their heats of formation. The latter represents the heat of reaction from the elements and for non-oxygenated carbons

can be considered the reverse of the pyrolysis reaction.

Experimental

In order to increase the information base on the pyrolysis and partial

oxidation of municipal solid waste we have carried out a series of experiments

using thermo-analytical methodology. At the outset it should be pointed out

that for systems such as MSW pyrolysis truly meaningful data can only be

obtained from full scale units. The present data and those mentioned earlier

can be useful only in the sense of setting limits and suggesting the type of

measurements in full scale units that can yield the most significant results.

13
However, as will become clear subsequently, in combination with earlier work

our results are very suggestive. In the following we begin by describing the

experimental apparatus, this will be followed by a tabulation of the results,

and conclude with an interpretation of the observations.

Thermal analytical methods are frequently used today in order to characterize complex condensed phase mixtures (15). The basic technique involves

heating a sample at a controlled rate and then observing the response of the

system through measurements of an appropriate property (weight change,

enthalpy change, dimension, evolved gases, etc). The thermogram so produced

is as characteristic as a boiling point or infrared spectrum. Furthermore,

through careful analysis one can obtain significant information on the

physico-chemical processes that are occurring. A special advantage of the

method is that by concentrating on a particular property one drastically

simplifies the problem of interpretation. One could for example carry out

complete spectroscopic analysis of the sample. In view of the complexity of

some systems such data will be extremely difficult to interpret. For the

present purposes, our interest will be concentrated on weight loss during

pyrolysis and the nature of the gases evolved. The connection between these

properties and combustion phenomena is obvious. Indeed, the time scales and

conditions are very akin to the situation during the initial steps of the

decomposition process in the primary combustion chamber of a starved air

incinerator

.

The thermogravimetric experiments were carried out with a DuPont 1090

Thermal Analysis System. Some Differential Scanning Calorimetric (DSC)

experiments were also carried out with an appropriate attachement. Schematic

drawings of the two instruments can be found in Figure II(A-B). In evolved

gas analysis, gaseous products are monitored. An apparatus have been con14
PRESSURE DSC CELL

-—Top

TC Junction

Heating

Block

Cover

Thermoelectric Oise

Sample Pan

Chrome! Oise Alumel Wires

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Figure II

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Figure

III:

Evolved

Gas

Analyzer
structed and tested. It involved coupling the effluent from the thermogravicnatic analyzer with appropriate gas analysis equipment. A schematic of the

apparatus can be seen in Figure III. During operation the effluent from the

thermogravimetr ic analyzer, (TGA) is split into three streams. The first

stream is fed directly into a flame ionization detector and thus gives a

direct measure of the total hydrocarbons that are present. Samples from the

second stream are analyzed for the light gases; CH^, CO and C0 2

. The CO and

C0 2 are converted to CH4 by a hydrogenation apparatus with a ruthenium

catalyst. This is placed immediately before the flame ionization detector. A

third stream was vented so as to control the magnitude of the signal from the

FID. We have also attempted to determine H

2 concentration using a thermoconductivity detector. Unfortunately, due to the lower sensitivity of such a

detector compared to flame ionization detection we were not able to determine

its presence. Nevetheless, there is no question that it is not a major light

gas decomposition product. The lines exiting from the TGA were kept at 230

°C. The intention was to make sure that no deposits are formed and all

detectable products are analysed. This appears to have been achieved and we

believe that there is very little condensation in the lines.

The experimental procedure involves placing a 3“5 milligram sample in the

pan of the thermogravimetric analyzer, purging the system for 15 minutes with

N

2 to exhaust the air with the atmosphere to be used and then proceeding with

the experiments. This involves ramping the temperature at rates of 20 to

100°C/min. The two direct measurements are the thermogravimetric and flame

ionization results. The former data represent the weight lose as a function

of temperature (or a function of time). Characteristic traces can be seen in

Figures IV and V. Note the presence of two distinct processes. The first i.3

due to the decomposition of the cellulosic component while the second is lue

17
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IONIZATION

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19
to the degradation of the plastic. This has been established through the use

of neat samples. In the case of the evolved gas analysis, the direct FID

results show the analogous two peaks which resemble the derivative curves.

These are due to the gases first from the decomposition of the cellulosic

components and then that from the plastic(s). Note the much larger relative

amount of the latter in comparison to the thermal analytical trace. This is

due to the higher sensitivity of the flame ionization detector to pure

hydrocarbons. The stream (see Figure III) that was used to determine quantitative amounts of CH^, CO, and CO2 contain information regarding the time

history of the evolution these gases. Since this information was not desired

we mix the gas in a small 200 cc cylindrical chamber which was initially

evacuated. The volume of the chamber was close to that of the lines. Both

are commensurate with the flow rate. For example, if flow rate in this stream

was 5 cm^/min, then with a volume of 200 cm3 experiments were carried out for

40 minutes. When all the gases in the lines are dumped into the chamber

complete mixing was achieved. A small sample - 5 cm3 was then taken for

analyses and replicate runs were used for checking results. All residue left

in the thermogravimetric analyzer was either oxidized in place or in a

differential scanning calorimeter. The latter leads to a heating value of the

char and was of the order of 7000-8000 cal/g, a characteristic value. The

remnant was then weighed. In either case the amount of char is thus determined. A summary of all our results can be found in Table IV-VI. These

include experiments with heating rates of 20-100 C/min and involve studies

with newsprint, filter paper, plastic, and sawdust as well as the RDF samples

mentioned earlier. Most of the experiments were carried out with the latter.

The reason for this will become obvious subsequently.

20
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O S- CO S, s s. S- S- rH

03 S- 03 03 03 03 03 03 03 nJ c 03 O Q O O CM O ~ O 4H O O o 2 l-H o O 33 o H c O Vi Cl-I Vi a V*. O Vi O Vi

21
Table

V:

Summary

of

Data

on

Municipal

Solid

Waste

Pyrolysis

on s i- n OV Vi CO Vi CM IS* O Vi Ov V* \ O CO n c— =r cm <— 0- cm on on O0

O • • o •=r CO n on MO T— on

CM r~ =r

C on

•rH Vi O vi. n vi MO ** CM Vi V*

E -=r on CO OV co n t— .=r T— t— O MO CO

CM -=X C- -=r • • • • e • • • O • • • CM T— -=r CM o -=r T CM o

C

•rH£ C-- -=r vi CM VI =r vi on vi on

CM CO on on VO r- n <- CM CM O-v O • • o on t— r— O -=r T Ov

CM CM on

CM

C

•rHENoO

CM

O

o

o

CM

=r vo

CM VO

•=r vi

on lTi mo viVO CT> on

i— VO o vi

<— CO

MO Vi

o c-

^r

CM ViVO

o-n CO on CM

C\J CM in cm

MO Vi

=r in

CM Vi

VO CMo

CM

O Vi

CM MO O ViCM MO

CM Vi O MO

on

is* CO

E on c~- CM V* n v. MO Vi CO Vi r— Vi

s. co on o- o C— MO <— VO o on 0- -=r O • • 20 on

CM =r CM •“ cr> -=r

CO MO

mo in on

on vi

on t— C— Vi

mo -=r in vi

<— CO

-=T Vi

o on

in

CM

Vi

COMOon c

o

JOg

CO CO o 0 0 0 0 0

CD 0 0 0 0 rH rH •K rH 1—1 t—

1

rH MO JD bO MO MO MO MO •iH

X N •rH £ •rH •rH •*—t •rH *— G>

CO c 4J 4J G> ^ G> bO 0

CD 0 o 0 0 ^—

0 C 0 C 0 E 3

i—I i—

1

MO 3 • 3 c 3 O 3 O 3 •V MO Q. G N MO G MO bO MO o MO MO MO MO MO £

£ <D so ••H 0 £ G £ MO £ G £ G E C O

CO MO s 4_> O O O O G O 0 O 0 O o 0

co £ CO o o 0 o 0 0 0 0 MO G 0 3 *.

1 • 1 o 1 1 1 G •» Cti C 4-> 4-5 MO bO c 0 c C - G - c 0 C

Ci 4-5 0 2 £ £ o c o o bO O bO O 0 o

cc G cc O

' MO 0 MO bO MO E MO £ MO MO

<D rH 0 G CO G £ G G w G i—

1

G

Q. G G 0 0 — 0 0 0 0 0

O 0 4-5 C 0 O Q O O CM 0 3T 0 4J 0

G o o O MO l-H O O no O

a. CO 4-5 G O Cij Vi o Vi CJ V- O Vi H Vi

22
utw/oOS

ujiu/o

02

u

tui/

o

02

utlu/

o

02

utui/

o

02

utuj/

o

001

utlu/

o

02

uiui/

o

001

co cm **KO CT\ O r- c~- »* ^=r %*

CO

cO C— T vo on r— VO O cO a-

• • • • • • • • • on <— LO on

a- co a* is* on ft* cm is* CO r- oO «— <— to- <— a-

-=r t— a- on on

CM

on co LH v*. CO IS* on is* CO 15* •=r ft* co o CM 10- in co T- o o <- o a- on t— OO CM

CM in on o

*=T

CO on in »* ^ VsO O ft* in is* 0- ft* CO -=r -=T r- in co CM CO t— -=r on OO on on cO in -=r O0

*

<

on

o <- O0 ft* O ft* 0- ft* Oo ft* n is* in a- on co in on <— in o oo on

on CM CO n CM to- T— T— on

t— -=r T- ft* CO O ft* O ft* CM CT"> ft* CM t- a co a co T— CO <— CO O O -=r

-=r r— on CM CM , t

t

on

ft* ft* ft*

to- CM a <— o- n on ft* «— ft* n on co on r- o on t— on Od n n • • • • • • • • • on -=r on

co

Od a on ft* co CO ft* 0- ft* ^=r dO ft* on a n oo CO cm on CM — o a OO cO

-=r r- on o CO co ,

t T“ on cO

23
Average

vs

vO vs

Cv vsC\l Vi vs

LTl -=T CM

CM on CM

C

•-H VS E >— VO LO «— \ CT> -=T m r— o . • > o on

CM

v*. =r vs co vs

O'- O <— O O CM VO CM • • • • =r cm

•=r vs. CM ov

LOon

o

1 in o \ O =r

o • • o ^r

CM

vs. O *— o vs on vs o vs n vs on vs. =r t— co vo t— vo r— o oon co on on on <— <— o

cm =r

c

•«Hs o CTO vs. ov O CO VS on ^

vs. vO CM vO vs. \ VO CO CM r- vO CO x— on O in <— o • • • • • • • • • • 20 on CM

CM

-=r T— OVon

c

•rHE in <— vO VS. C— VS. OV VS c— vs CO vs CTv VS \ CO 10- — vO vO — O O O CM O VO CM CM O on -=T , on CM CM , CM T— CM -=T

c

•rHs ov in vs

on o c\j ^r on v*. on C— VS \ vo on on t— -=T «— t— on o OO on

o • • • • • • • • • 20 on •=r CMon

/min cm in CM vs on vs =r vs CM vs on vs vs

o in oo in n vO tv <— OV <— n o <— -=r •=r o

CM •=r *=T tv on on T i— CTV

t— r— on

24
Table

VI:

Summary

of

Data

on

the

Oxidation

of

RDF

and

Constituents

too

c

03 V*. vs. vs. V*. 60 LO vo ^r CM on G • • O

Q

i—

(

Cl4

vs. Vi Vi VS. VS. Vi vs. V*. V3.

IT> LO in r— CM CO M3 MO LO CM -=r O o o • • • • • • • • o =r on on

CM V*. vs. ^r vs VS

on

vs

CTn vs

CM VS

on vs00 Vi

cr\ O £

•=r -3

- O LO O LO •=r O' r— T on CM o -=r -=r on -=r •=r on JIT LO 'O =r

CM 2

C

•H CM CM CM O

to 0 O CM CM CM c G z* vs. O O O CM CM 0 G Vi •iH •H G VS 0 G O O

•H •H 0 03 03 •H O r- •r-i Vi V*. Vi

g> 03 r- 03 <

03 O O O Vi VS. •H • — • » 1

r— •

LO CM

•o • > « G C • « • « c • - c c c •H m—

G C C c c c c c

0 H •rH s e •rH g iH rH rH 0 s E \ \ C e \ s E g E E E

s s 0 0 s \ O N N \ N \ N 0 0 0 0 0 0 O O O O O 0 O 0 0 0 0 0 0 O O O O O O O

CM CM <— *— CM CM CM CM CM CM CM CM

G 0)

03Q. 0

nJ 03 rH

1—

1

G Cl >1

03 G> rH <-* rH to G G 4-) G G Q. O 0) CD O to lJ >> 3 3 r-H rH Ctl 03 03 CD *tH O Q

to c Q-t Q. 30

25
These experiments involve a multiplicity of measurements and turned out

to be difficult to carry out. The main problem was due to the thermgravimetric analyzer being constructed in such a manner that it could not maintain

a pressure very much above ambient. Without a large pressure drop, the flow

rates through our three lines, vent, FID and light gas analysis, had a

tendency to vary. This is probably the main source of experimental scatter in

these experiments. We have attempted to minimize this effect by constantly

monitoring the flow rate and calibrating immediately before and after the

experiments with a standard mixture.

The results in Table IV involve a number of the elements in municipal

solid waste. For the present purposes there can be considered to be the

"pure" components, whose properties do not vary greatly. Thus the results

serve as a measure of the accuracy and precision of our determination. On

this basis we expect scatter about mean values of about 15$. For all these

substances the amount of non-combustibles is close to minimal. Polyethylene

is different than all the other organics in leaving no char and with a high

carbon content of 80$. The calculated value is 85.7$. Here again, we have a

marker of the accuracy of our determinations. The other organics are all

essentially cellulosic in nature and as could be expected have considerable

similarities. In all cases we do not collect all of the carbon that is

available. Cellulose with an empirical formula of C^H^qO^ has 44$ carbon.

Newsprint has been found to contain 49$ carbon (10). Sawdust is probably in

an intermediate position. The consequence is that we are missing about 14$ of

the available carbon in all three cases. Most if not all this missing carbon

must be volatile material tied to oxygen groups. Most such compounds are not

detectable with the flame ionization detector. Specifically the groups of

interest are acidic, aldehydic and alcoholic moieties. Actually the FID

26
detector is not completely insensitive to the latter. The sensitivity factor

for methanol being about 0.3 to that for methane. An important question is

the distribution of these compounds. An examination of the structure of

cellulose show three alcohol and one polyether linkage (0-C-0). Note that in

the absence of oxygen the only possibility of acid formation is from the

polyether linkage. The original C-0 linkage can then appear in our system as

either alcohols or aldehydes and ketones. The ratio of alcohol and carbonyl

groupings compared to the acid is 3 to 1 . It is interesting to note that our

CO and CO 2 ratio in our filter paper study is of the order of 2 to 1 . This is

in line with the greater lability of organic acids. On this basis a zeroth

order approximation for our undetected carbon is a 3 to 1 mixture of CI-^O

(formaldehyde) and CH2O

2 (formic acid). Actually in terms of heating values

we could have expressed these as H

2

+ CO and H 2

+ CO? respectively since the

heat of reaction for these processes is negligible. Thus we postulate the

carbon oxygenated products from our filter paper pyrolysis to consist of on a

carbon weight basis of 10.2? equivalent formaldehyde and 3.^5 formic acid.

Note that no claim is made that such compounds are actually present in these

proportions. Rather, it is the characteristic groups that are of interest. On

a similar fashion we can consider our FID results as indicative of the concentration of C

2

Ki1 or equivalently CH 2 group present. The resulting mixture from

cellulose decomposition is thus summarized on Table VII. In deriving these

numbers we have also invoked mass balance for oxygen. Obviously in pyrolytic

situations oxygen must be bound. Since we have accounted for all the carbon

oxygen structures all that is left is water. Interestingly, this leads to a

satisfactory mass balance of H

2 and supports cur suggestion that under the

present conditions hydrogen is not a major product. We can now use this

composition to calculate the heat of combustion of the reaction prcc-cts. 7'.^

27
numbers are given in the second column of Table VII. The approximate nature of

such computations must be emphasized. We have for example lumped the alcohols

in with our FID hydrocarbons and formaldehyde. However it is possible to

demonstrate that no large errors are introduced since the lowered sensitivity

in the FID is matched by the lower heats of combustion of C-OH groups.

Futhermore, we do arrive at a number that is fairly close to the heat of

combustion of cellulose. Thus one suspects that the heat of pyrolysis is very

close to zero.

We now consider municipal solid waste samples. Most of our work was

carried out with these mixtures. This was due to the fact that the mixtures

are quite heterogeneous and problems with respect to reproducibility are

expected to be important. This is indeed the case and can be established

through inspection of the data from Table V. It is clear that the scatter of

the results is far in excess of the results from our "pure" samples. We

suspect that this is due to the small samples used in these experiments having

an amplifying effect on the heterogeneity of the mixture. With the larger

samples used in the earlier NBS calorimetric experiments it is probably much

easier to obtain a representative sample. In the present case, homogeneity

for each individual run is difficult to achieve. This is readily demonstrated

from our thermogravimetr ic and evolved gas analysis curves where the two peaks

which we have earlier identified as that due to the decomposition of

cellulosics and polyethylene respectively showed considerable variance. There

is apparently some degree of correlation between large measured total carbon

concentrations and high levels of plastics. This is however not the complete

story since it is not easy to make an exact distinction.

28
Table

VII:

Approx

lrnate

Composition

and

Heats

of’

Combustion

of

Products

from

Cellulose

Pyrolysis

29
In any case, we believe that an average of all the results will lead to

representative values which are appropriate to the pyrolysis products from

heated municipal solid wastes. The results are included in the next to last

column of our data on Table V. It is interesting to note that our average

carbon content, 42.2?, is 12.8? less than the value of 55? given as the

approximate carbon content of the combustible portion of the municipal solid

waste used in these studies in the earlier NBS report. This is almost the

same number that we obtained from the difference between calculated and

detected carbon in cellulose decomposition. Furthermore, we note that the

percentage of carbon as char from cellulose and municipal solid waste

pyrolysis are very close. Finally, the carbon from the FID sensitive organics

is about 11? higher than the value from the filter paper studies. When all of

this information is coupled with our earlier observation that polyethylene

decomposition leave no char, we believe that these results demonstrate that in

the pyrolysis of municipal solid waste the chemical process is simply the

depolymerization of the constituent fractions with out large synergestic

effects. The presence of large quantities of inorganic impurities does not

appear to have a major influence on the nature of the decomposition process.

Of course all this is based on rather gross observations. It is entirely

possible that if one looks at the individual compounds or groupings significant differences may occur. At the present level however these considerations

are unimportant. Table VIII contains a summary of the constituents and the

heats of combustion of the various components in the pyrolysis products from

MSW combustion. Here we have simply distributed the three undetected oxygena-

• ted substances in the same manner as in cellulose decomposition. The rationale behind the assignments has been given earlier in the discussion on the

heats of combustion of the products from cellulose pyrolysis. On this basis,

30
Table

VIII:

Approximate

Composition

and

Heats

of

Combustion

of

Products

from

Municipal

Solid

Waste

Pryolysis

to \

r—i03O

c

o

•H

03 on M3 c— o O M3 3 CM M3 CM M3 CO LTl JD CO c— *— CM O' O' S CM OO

o

4-> 03

CD

on y—

s

y~N

CTn co on vO N—

c\j on on t— on

r— r— on CO

T— T— C\J > V ' N 'w'

in LO v£> co in O

Vi. • • • • • • co O' on o CM o

3 CM <— 1— *— <—

o y—' y—

n

~ o

O CJ *n CJ 33 CM CM O 33 03

03

03 03 03 03

03

' a

-3 3

O Cl a O

3 3 3 c. -a 0 o

o SL. Vt. so SO o

cu II CM o o n 1 o o o

CM O

O O CM a c_> o x

31
we get a total heating value of 4956 cal/g or very close to the heating value

of the original MSW of - 5200 cal/g. The pyrolysis reaction is very close to

thermally neutral.

Table VI contains data on the oxidation of MSW and its constituent parts.

The key observation is the ease of ignition of MSW samples. One notes that

even with concentrations of oxygen as low as 2 %, half of the municipal solid

waste is converted to carbon dioxide. This is also indicative of the "all or

nothing" nature of the oxidation process. That is, one does not proceed

smoothly from reactant to ever more oxygenated products. Instead, ignition

occurs and the very high temperature converts practically all the available

carbon compounds into carbon dioxide. Of some interest is the high yield of

hydrocarbons in polyethylene oxidation. The hydrocarbon effluents from

polyethylene pyrolysis are apparently more difficult to ignite. In a complex

mixture such as municipal solid waste, however the high temperature attendant

upon the ignition of cellulose will in most cases ignite the polyethylene.

Thus the observations on polyethylene oxidation must be interpreted with care.

Discussion

We now consider our experimental results within the context of earlier

work. This is particularly significant because of the widely diverging

experimental configurations. In our experiments sample sizes are extremely

small; of the order of 3~5 milligrams. As soon as the material is volatilized

it is diluted and the conditions are such that possibilities of subsequent

reactions are minimized. This is very similar to the experimental configuration of Shafizadeh and DeGroot (9) and their char yields of 14.9% is reasonably close to our value of 10.5%. On the other hand the results of Hoffman and

Fitz deal with pyrolysis in a retort and with a sample of 168 grams. This is

equivalent to a scale up of a factor of 40000. Nevertheless one notes on a

32
weight basis the char yields of 17? to 25? and more important, tar yields in

the 60? level. This is in very close accord with our observations. In both

cases the intermediate tar has not had time to decompose to light gases or

condense to form char. It is likely that there are compositional differences

among the three broad categories. Indeed the observed H

2 concentration in the

retort experiment is probably correlatable with the somewhat larger char

yields. For the present purposes the lack of gross differences despite

profound changes in experimental configuration is encouraging. It should be

noted that in the retort experiment, the reported temperature is at best the

temperature of the wall of the reaction. Unfortunately, no time information

is given. Thus detailed comparison becomes impossible. Note that the higher

temperature runs are undoubtedly also effected by the much shorter residence

time of the volatilate. This may partly account for the lower char yields at

the higher temperature.

A different situation is encountered in the sawdust experiments. Here

samples of 2500 gms are used and yields of char and light gases are considerably higher than from the experiments with municipal solid waste. It is

possible that the very small size of the sawdust increases the possibility of

initial volatilates reacting and thus producing less tar.

We now consider the situation in the primary combustion chamber of a

starved air incinerator. The physical process involves the contact of low

temperature under fired air with a mixture of char and refuse. There is no

question that the refuse will ignite. The high temperature will assist the

combustion of the char, although we suspect that some char will be unburnt.

Thus at a certain level above the grate all oxygen will be exhausted and we

will have a rising current of hot N

2 , C0 2 and H 20 which will pyrolyze the

layers of refuse. The initial composition will be that which we have deter33
mined in the course of our thermal analytical experiments. As the pyrolysis

gases rise through the refuse they will face temperature at or below the 350 °C

level where pyrolysis begins. The important question is whether further

cracking occurs. The results of Hoffman and Fitz may set an upper limit. Note

that although sample size in an actual incinerator is probably a factor of

1000 more, the linear length is changed by a factor of 10. Since in an

incinerator the heat source is within the system itself and with the N

2 in the

air as a diluent, the actual time for pyrolysis of the volatilates (order of

seconds), in a starved air incinerator is probably not much longer than in the

retort. Furthermore there is the effect of the diluent N

2 in reducing the

possibility of condensation processes. Thus we would take our values for the

quantity of char as a lower limit while the data of Hoffman and Fitz will be

close to the upper limit.

As the pyrolyses products escape from the bed the presence of over fired

air will cause further combustion. The overall effect will be to heat the gas

and lower the heating value. Physically, we envision a diffusion flame

mechanism. A given portion of the pyrolysis gas will be oxidized while the

remainder will be raised to a higher temperature. As before an important

question is whether there will be extensive cracking. At this time, no

definitive conclusions cannot be drawn. At the quoted temperatures we suspect

extensive cracking of oxygenated compounds will occur. Fortunately the degree

of cracking of the pyrolysis gases does not have much an effect on the heating

values. For example the cracking of acetaldehyde follows the reaction

stochiometry 2CH^CH0 -> 2C0 + C 2 H 4 + 2H 2 and is a thermally neutral process.

Indeed, the manner by which we have expressed the results of our pyrolysis

experiments suggest this approximation. That is, we can express our equivalent HCH0 and HC00H as cracked to H

2 , CO and CO 2

. On the other hand the

34
equivalent C

2

Hjj will be carried into the secondary combustion chamber. It is

of course possible to define the composition in terms of an equilibrium

hypothesis. However residence times of 1 sec at 800-900°C is insufficient to

crack olefins or aromatics. Indeed, it is at this temperature that one begin

to make the lighter olefins from larger hydrocarbons in naptha crackers.

Summary and Recommendation

Experiments have been carried out with the aim of defining the products

of incomplete combustion of municipal solid waste. In combination with

earlier work we find the pyrolysis process to be close to thermally neutral.

The minimum char content is of the order of 10$ (wt. basis) of the total

available combustible organics. The volatilates can be expressed in terms of

equivalent quantities of C

2H^ (28.6$), HCHO (21.1$), HC00H (10.9$), C0 2

(15.6$) and CO (12.3$). For higher char yields reduce equivalent quantities

of C

2H4. The equivalent HCHO and HCOOH will under sufficiently severe

condition be cracked to CO, C0 2 and H2

. Oxidation will lead to complete

conversion of stochiometr ic quantities of fuel to C0 2 and H 20. It should be

emphasized that the above represent a gross simplification to a very complex

chemical situation. In conjunction with an appropriate model it can best

serve as a guide for the kind of monitoring that is most appropriate for the

optimization of such devices.

35
References

1. C. A. Kodres, "A Parametric Examination of the Heat Recovery Incinerator

at NAS Jacksonville”, TN No. N-1659, Naval Civil Engineering Laboratory,

Port Huenemes, California 93043.

2. See for example Symposium on ”Devoltalization of Carbonaceous Materials”

Division of Fuel Chemistry, 1 89th ACS National Meeting, St. Louis,

Missouri, April 8-13, 1984.

3. C. L. Mantell ”Solid Wastes: Origin, Collection, processing and disposal,

John Wiley and Sons, New York, 1975, pg. 17.

4. E. M. Wilson and J. M. Leavens, "Results of A National Survey of

Cellulosic Waste Characteristics and Processes for their Conversion to

Energy Forms”, in "Thermal Uses and Properties of Carbohydrates and

Lignins, F. Shafizadeh, K. V. Sarkenen and D. A. Tillman, ed., Academic

Press, New York, 1976, pg. 203.

5. K. L. Churney, E. S. Domalski, A. E. Ledford, J. C. Colbert, S. S. Bruce,

T. J. Buckley, R. C. Paule and M. L. Reilly, "Assessing the Credibility

of the Calorific Value of Municipal Solid Waste” NBSIR 84-2825 DoE,

National Bureau of Standards, Washington, DC 20234.

6. K. E. Cabradilla and S. H. Zeronan, "Influence of Crystallinity on the

Thermal Properties of Cellulose in Thermal Uses and Properties of

Carbohydrates and Lignins” F. Shafizadeh, K. V. Sarkanen and D. A.

Tillman, ed. Academic Press, New York, 1972.

7. F. D. Hileman, L. H. Wojcik, J. H. Futrell and I. N. Einhovin, "Comparison of the Thermal Degradation Products of a-cellulose and Douglas Fir

Under Inert and Oxidative Environments” in "Thermal Uses and Properties

of Carbohydrates and Lignins” F. Shafizadeh, K. V. Sarkanen, D. A.

Tillman, ed. Academic Press, New York, 1971, pg. 49.

8. W. J. Irwin, "Analytical Pyrolysis”, A Comprehensive Guide Marcel Dekker,

New York, pg. 352, 1975.

9. F. Shafizadeh and W. F. DeGroat "Combustion Characteristics of Cellulosic

Fuels” in "Thermal Uses and Properties of Carbohydrates and Lignins” (F.

Shafitadeh, K. V. Sarkenen, D. A. Tillman, ed.) Academic Press, New York,

pg. 29, 1972.

10. ASME Research Committee on Industrial and Municipal Wastes, "Thermodynamic Data for Waste Incineration”, The American Society of Mechanical

Engineers, New York, NY 10017, 1979.

11. R. C. Reid, J. M. Praunitz and T. K. Sherwood, "The Properties of Gases

and Liquids, McGraw-Holl, New York, 1977.

36
12. D. R. Kirklin, J. C. Colbert, K. L. Chruney, M. L. Reilly, D. D.

Thornton, R. V. Ryan, A. E. Ledford and E. S. Domdlski, "Test Procedure

for the Determination of the Gross Calorific Value of Refuse and Refuse

Dervied Fuel of Kilogram-Size Samples Using Constant Pressure Flow

Calorimeter”, NBSIR 82-2491, National Bureau of Standards, Washington, DC

20234.

13. D. A. Hoffman and R. A. Fitz, "Batch Retort Pyrolysis of Solid Municipal

Wastes, Environmental Science and Technology, 2, 1025, 1968.

14. J. A. Knight, "Pyrolysis of Sawdust" in "Thermal Uses and Properties of

Carbohydrates and Lignins" F. Shafizadeh, K. V. Sarkanen, D. A. Tillman

ed. Academic Press, New York, 1971, pg. 159.

15. H. C. Kung and A. S. Kalelkar, "On the Heat of Reaction in Wood

Pyrolysis", Combustion and Flame 20, 91, 1973*

37
NBS-1 T4A (REV. 2-ao) U.S. DEPT. OP COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR 84-2938 FINAL REPORT DECEMBER 1984

Chemical Processes During the Starved Air Incineration of Municipal Solid Waste

5. AUTHOR(S)

Wing Tsang and James A. Walker

5. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) T. Con tract/ Grant No. national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME ANO COMPLETE ADDRESS (Street. City. State. ZIP)

10. SUPPLEMENTARY NOTES

1 | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. A8STRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bi bliography or literature survey, mention it here)

Thermal analytical measurements on the pyrolytic and oxidative decomposition of municipal solid waste and its constituent parts (cellulose, sawdust,

newsprint and polyethylene) have been carried out. Ur^der pyrolytic conditions

thermogravimetric analysis of municipal solid waste indicate minimum yields of

char in the 10% range. Evolved gas analysis yielded information regarding the

nature of the volatilate in terms of total FID sensitive hydrocarbons, CO, C0 2

and CHi|. From material balance considerations it is concluded that the

volatilate include large quantities of acidic carbonyl and alcohol compounds.

The total quantity of volatilates not including the directly analyzed light

gases is of the order of 60%. The overall process is thermally neutral. This

is in good agreement with measurements using larger samples. Under oxidative

conditions MSW ignited very easily and reaction goes rapidly to completion.

Combination of the experimental data and literature information leads to a description of the chemical processes in starved air incineration.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Evolved gas analysis; refuse derived fuel; pyrolysis; combustion; oxidation;

incineration; cellulose; depolymerization

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38

IS. Price 20402.

34] Order From National Tecnnical Information Service (NTIS), Springfield, VA. 22161 $8 . 50