Reference

NBS

PUBLICATIONS

NBSIR 85-3218

NAT'L INST. OF STAND & TECH

A 111 Ob 3 Ml DM

Pyrolysis of Cellulose, An

Introduction to the Literature

Toshimi Hirata

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

August 1985

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U.S. DEPARTMENT OF COMMERCE

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NBSIR 85-3218

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INTRODUCTION TO THE LITERATURE

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Toshimi Hirata

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

August 1985

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

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director
'
TABLE OF CONTENTS

Page

LIST OF FIGURES iii

Abstract 1

1. INTRODUCTION 1

2. CHANGES IN FINE STRUCTURE WITH HEATING 3

3. CHEMICAL CHANGES DURING ACTIVE PYROLYSIS 6

4. PYROLYSIS REACTION KINETICS AND MODELS 10

3. SUMMARY AND CONCLUSIONS 15

6. REFERENCES 17

ii

LIST OF FIGURES

Page

Figure 1 - Chemical Formula of Cellulose 28

Figure 2 - 1-6 Anhydro- 0-D Glucopyranose (Levoglucosan) 29

Figure 3 - Chain Mechanism of Cellulose Pyrolysis... 30

iii

PYROLYSIS OF CELLULOSE, AN INTRODUCTION TO THE LITERATURE

Toshimi Hirata

Abstract

Topics related to cellulose pyrolysis are briefly surveyed under several

headings. The principal aim is to give the reader some grasp of the issues

involved and provide a guide to the relevant literature. The headings

include: changes in cellulose fine structure with heating, chemical changes

during pyrolytic weight loss, and kinetic modeling of pyrolysis. Principal

emphasis is on the last area; it is concluded that no current kinetic model

adequately predicts both the observed changes in degree of polymerization and

the weight loss during heating.

1. INTRODUCTION

Pyrolysis of cellulose has been studied by many researchers in relation

to fire-proofing of cellulosic fabrics and wood, of which cellulose is a

principal component, in relation to charcoal production and in relation to

gasification of biomass. Furthermore, many results on changes in physical

properties, especially mechanical and visco-elastic properties, on production

of useful materials, on gasification, and on thermal liquefaction have

recently been reported in studies relating to the pyrolysis. There are

inconsistencies among these results which have led to different

interpretations and proposed models of the pyrolysis.

1
Approximately one half of wood consists of cellulose; the rest mainly

consists of lignin (25 to 35 percent) and hemicelluloses (15 to 25 percent)

with the exact content of each depending on the tree species. Cellulose is a

simple linear polymer of repeating glucose units all identically linked

together as shown in Figure 1. It rapidly decomposes to produce highly

inflammable volatiles in a narrow temperature range around 300°C, as inferred

from the ready combustibility of paper. On the other hand, lignin, an

aromatic macromolecule whose elemental unit is phenyl propane, and

hemicelluloses, whose main components are pentosans, are constructed with a

variety of linkages. Lignin is generally the most stable wood component

whereas hemicelluloses are comparable in stability to cellulose as shown by

numerous thermogravimetric (TG) studies [1-10]. The pyrolysis of cellulose is

thus a principal source of the flammable gases formed from gasifying wood.

The pyrolysis mechanism and kinetics of cellulose decomposition under

pyrolytic conditions are complex and are not yet sufficiently known. The

mechanistic and kinetic aspects have to be more satisfactorily clarified to

allow exact predictions of combustibility and to aid in development of more

effective fire-proofing of cellulosic and wood materials, as well as for other

purposes such as the production of energy from biomass. An overview of the

literature on pyrolysis should be useful in helping clear up some of the

confusion and in obtaining clues as to the major difficulties in the

clarification of the mechanism of cellulose pyrolysis. This survey is limited

to anaerobic degradation conditions in order to exclude the additional

complexities which oxygen introduces.

2
2. CHANGES IN FINE STRUCTURE WITH HEATING

Heating cellulose above 200°C first causes changes in the fine structure,

in other words, in properties such as the crystalline fraction and crystallite

form, length of the molecular chains and the accessibility. The accessibility

is here defined as the proportion of hydroxyl groups available to a reagent;

it depends on the fine structure, and is measured by the absorption rate, the

reactivity, and so forth. Ordering seems to occur in the amorphous region

before cleavage of the covalent bonds. This has been suggested as the cause

of an increase in the crystallinity [1-4], a rise in the temperature of glasstransition [15], softening [16], a decrease in the accessibility [17], and

changes in the mechanical properties [18-20].

Another type of change which causes an increase in the crystallinity,

namely, chemical decomposition in the amorphous region, probably occurs later

than the above rearrangement of chains. Since the products of pyrolysis of

the amorphous region escape by volatilizing from the system, the percent

crystallinity increases. The view that the thermal decomposition occurs first

in the amorphous region has been supported by several experiments: the

increase in crystallinity with heating as seen by x-ray scattering [12,13,21-

23], by the interpretation that the leveling off of the degree of

polymerization (DP) during heating is caused by decomposition of the amorphous

region or weak linkages and that this result then indicates the length of the

crystallite [12,13,24,27], by a decrease in the molecular accessibility

measured by the reactivity with phenolic resin [28] and by other ways [22-29],

such as moisture regain; finally, it is also supported by a proposed model for

zeroth-order weight loss [30].

3
The main initial reaction of the pyrolysis in the amorphous region seems

to be cleavage of glucosidic bonds and dehydration [22,24,31,32]; dehydration

has been interpreted as leading to intermolecular cross linking [24,29,32).

Following these reactions, levoglucosan (see Figure 2) as a sort of "monomer"

appears to be produced in some amount from the less ordered region [33]. This

conjecture is consistent with the results of Lipska and Parker [34,35], Lipska

[36], and Lipska and Woodley [37], who have reported an initial rapid decrease

in the number of glucose units in cellulose as determined by hydrolysis.

Rapid shortening of the cellulose chains by cleavage of the glucosidic bonds

at the initital stage of heating has been pointed to as the cause of the

decrease in DP by many workers [12,13,17,24-28,33,38-55].

The idea that interchain cross-linking occurs (via dehydration) has been

supported by several reports. These include the finding that water production

by pyrolysis is associated with cross linking [56], that crosslinking is

responsible for an increase in the mechanical strength with heating

[28,39,57], that it causes an extraordinarily high apparent activation energy

of 145 kcal/mole as determined from changes in the mechanical strength in the

temperature range 115° to 175°C [58], and that cross-linking causes an

increase in the dimensional stability [29,57]. Reported increases in the

mechanical strength [18,20] and in the dimensional stability [12] which have

otherwise been interpreted as being due to hydrogen bonding formed during

heating, may be also interpreted as being due to interchain cross linking.

Most reported values of the leveling off DP obtained by heating roughly

approximate those obtained by cellulose hydrolysis [59-70]. The length of the

cellulose chains in a crystallite is considered to depend upon their

arrangement in it. (There are several theories about this arrangement, i.e.

,

those of folded chains [68-73] and spiral chains [74-77], in addition to the

4
well-known fringed micellar model). The leveling-off values of DP found by

hydrolysis, which differ depending on the source of the cellulose, agree with

the size of the crystallite determined from x-ray measurements [68,78],

electron microscopy [64], and from absorption [79]. Thus, it has been

verified that the leveling-off of the DP found in hydrolysis is caused by the

fine structure. The leveling-off of the DP found in pyrolysis, however,

cannot be explained satisfactorily by the interpretation that it is based

purely on the fine structure. For example, it is difficult to explain the

large temperature dependency of the leveling-off DP [45,49-51,55] by this

interpretation; also it is difficult to explain the actual values which can be

about 10 times larger than the value of DP corresponding to a single

crystallite length [17].

Only changes occurring before the main pyrolysis process, i. e. , before

the actual weight loss, have been discussed so far. Studies of changes in the

fine structure during the main pyrolysis process are largely lacking in the

literature. A gradual and almost linear decrease in DP with the progress of

the main weight loss has been reported by Lipska [41], and by Lipska and

Martin [42] who succeeded in measuring viscosity-average DP despite the

presence of insoluble ingredients in their samples. This change in the DP was

thought to be caused by the unzipping of cellulose chains whose number

remained constant during the pyrolysis.

It has been contended that cellulose molecules pre-melt and lose their

ordered structure at temperatures near 300°C [19]; the active weight loss

process of cellulose in TG, with usual heating rates, begins beyond 300°C. On

the other hand, Weinstein and Broido [80] have reported results showing that

approximately one half of the initial crystallinity is retained after a 64

percent, heat-induced weight loss and that additives further increase the

5
crystallinity in the residue. However, in a study by Inagaki and Katsuura

[81], it was found that the crystalline structure disappeared in the presence

of a reactive additive before the beginning of the rapid weight loss

process. In any event, the changes in the fine structure at high temperatures

or during the rapid weight loss process remain uncertain, because the fine

structure present at high temperature, is not shown with certainty by

measuring properties of the residue at room temperature, as has typically been

done. Therefore, interpretation of the results of such studies will always be

open to question.

3. CHEMICAL CHANGES DURING ACTIVE PYROLYSIS

The first significant chemical reactions in the crystalline region of

cellulose definitely appear to be cleavage of glucosidic bonds followed by

production of anhydro-glucoses (for the most part, levoglucosan) ; however, the

presence of a primary product (unidentified) other than levoglucosan has been

proposed by Broido et al. [82].

Levoglucosan is produced from the pyrolysis of cellulose; it constitutes

the majority of the tar [14,22-24,31,33,44,83-98]. Its yield increases with

the crystallinity [13,14,31-33,86], and decreases with impurities [92,96] and

additives [31,45,82,83,87,88,91,93,94,98]. Furthermore, the idea that

levoglucosan is a primary product of cellulose pyrolysis is supported not only

by the large yield but also by the close similarity among the groups of

identified degradation products from the pyrolysis of cellulose and from

levoglucosan [42,99,100]. Levoglucosan has been assumed to function as the

"monomer" in cellulose pyrolysis and various chemical mechanisms for its

production have been proposed [31,83,84,93,98,101-110]. A mechanism based on

a glucose intermediate in the production of levoglucosan was refuted by

experimental evidence [31]. Depending on the position of the hydroxyl groups

6
which attack the glucosidic bonds, levoglucosan may be formed via an

intermediate of either 1, 2-anhydro- [83, 105, 109 ] or 1, 4-anhydro-glucose [109],

or directly without an intermediate [31,46]. No decisive proof of the real

course seems to have been presented yet [109].

In any case, the principal route of cellulose pyrolysis is assumed to be

a process proceeding from cleavage of glucosidic bonds to production of

levoglucosan; dehydration is considered to be a side reaction of lesser

significance. However, this side reaction is important for the flameretardation of cellulosic materials. One of the most effective means of

flame-retardation is a greater production of water and a minimization of

volatilized, inflammable levoglucosan in cellulose pyrolysis.

As stated above, the production of levoglucosan is very sensitive to

additives and impurities. However, few results [101,111] have been reported

regarding the chemical mechanism of action of any additive. Thus, the effects

of additives on the production of levoglucosan are not well understood and

require further study. Also, several chemical mechanisms have been proposed

for the dehydration reaction [46,52,112-114].

Many degradation products have been identified by various analytical

means, such as gas-liquid chromatography (GLC), mass spectrometry (MS), the

combined methods of GLC and MS, IR and UV spectrometry, thin layer

chromatography and nuclear magnetic resonance. The identified products of the

secondary pyrolysis of cellulose, i.e. the main pyrolysis products of

levoglucosan, may be classified into fixed gases such as hydrogen, carbon

monoxide, carbon dioxide, methane, acetylene, ethylene, and propane, as well

as semi-volatiles such as lower alcohols, water, ketones, aldehyde and

carbonyl compounds, aliphatic acids, hydrocarbons, and furans. The yield of

7
these products depends upon the atmosphere [25,88,90,115,116], and upon

additives [4,45,48,82,83,88,93,95,100,104,117-124]. Additives generally tend

to decrease the number of products [100].

Shafizadeh [93], and Shafizadeh and Lai [120] have presented chemical

decomposition mechanisms for the production of many of the products via

intermediates of 3-deoxy glucosan and its derivatives. The former compound

(3-deoxy glucosan) was first identified by Kato and Komorita [125] as a

product of levoglucosan pyrolysis. Furthermore, Shafizadeh et al. [126-129]

have shown that many products of cellulose pyrolysis are produced from the

pyrolysis of substances related to the intermediates. According to these

authors, the course of levoglucosn pyrolysis is directed either to a process

of condensation or of disproportionation depending on whether the reaction

conditions are acidic, basic, or neutral.

The pyrolysis reactons beginning with the cleavage of a glucosidic bond

of a cellulose chain are widely considered to be controlled by radical

mechanisms. Many of the proposed radical mechanisms have been discussed

extensively by Onishi and Sugisawa [108,109], and by Shafizadeh [107]. The

existence of radicals has been confirmed in various degradation experiments by

using electron spin resonance (ESR) analysis (93,120,130-132]. On the other

hand, Byrne et al. [104] have proposed an ionic mechanism for the pyrolysis in

which decomposition is caused by and develops with the formation of carbonium

ion produced by heterolysis of the glucosidic bond. Shafizadeh et al. [132]

have proposed heterolytic cleavage of a glucosidic bond in the pyrolysis of

xylose on the basis of ESR measurements thus giving support to the

interpretation by Byrne et al. Moreover, from measurements of radical

concentrations by ESR analysis, Shafizadeh [93] has presented the

interpretation that pyrolysis of cellulose is heterolytic in the temperature

8
range below 280 °C and homolytic in the higher temperature range in which the

active weight loss occurs. Shafizadeh and Bradbury [54] have also presented a

simple radical chain reaction mechanism which is composed of radical

initiation, propagation, and formation of products.

In an inert atmosphere, the pyrolysis of cellulose yields a stable

residual solid char plus liquid tars and gases. The char yield is found to

increase with the addition of various chemicals and it has been reported to

differ among types of cellulose [133]. It has been observed that cellulose

can be converted to volatiles without char formation during pyrolysis in

helium or in vacuo and this observation has lead to the interpretation that

char is not a primary product of the pyrolysis [134].

The chemical mechanisms proposed concerning char formation may be divided

into two types. The first type, due to Broido and Nelson [135,136], assumes

the char derives directly from cellulose molecules by intermolecular

dehydration; this is based on the "dehydrocellulose" concept of Kilzer and

Broido [137]. Their proposal has been supported by the interpretation of

thermal analysis curves [138,139] and agrees with the overall course of the

pyrolysis as proposed by Lewin et al. [32]. The second type of mechanism is

that the char is assumed to be formed not directly from the cellulose

molecules, but rather by polymerization of levoglucosan or its derivatives;

this has been proposed based on an increase in char formation from

levoglucosan with the addition of chemicals [41,42,100,140,141], as is

similarly observed for cellulose. This interpretation was based on results

for pure levoglucosan pyrolysis and seems to be somewhat idealized; the actual

reacton process of levoglucosan pyrolysis should be affected by the presence

of cellulose which has been activated by heating, and by water and other

9
products. The proponents of this mechanism seem to assume that levoglucosan

produced from cellulose begins to pyrolyze only after the completion of

conversion of cellulose molecules into levoglucosan.

4. PYROLYSIS REACTION KINETICS AND MODELS

An enormous amount of kinetic data has been presented for cellulose

pyrolysis. The major part is derived from weight loss measurements. However,

kinetic parameters of pyrolysis have been determined by other measurements

such as changes in the mechanical properties [39,58], decrease in the DP

[26,39,40,48-52,54,55,110], rates of gas release [54,142-144], changes in acid

concentration [145], and changes in peak temperature of the differential

thermal analysis curves [146,147]. For the most part, the kinetic parameters,

including those for weight loss were determined with the assumption of a

first-order reaction. Some authors [3,30,24-37,41,139,148-153] have reported

results obeying a zeroth-order rate law over the major part or over a limited

portion of the weight loss, depending on the heating conditions. Furthermore,

the weight loss rate with heating has been fitted to expressions other than a

first- or zeroth-order law [13,23,32,103,133,139,141,152-159]. Even if one

limits his viewpoint to apparent values of Arrhenius parameters, the pyrolysis

is found to consist of two or more stages [3,6,7,13,30,34-37,48,103,112,138,

139,142,148,149,151,152,154,155,158-161]. This suggests that the actual

kinetic process of cellulose pyrolysis is complex.

In addition to the complexity of the pyrolytic chemical reactions,

different calculating procedures [27,112,150,151,161-163] and experimental

conditions, (namely, heating rates) [150,151,153,157,162,164], atmospheres

[12,54,139,157,165,166], types of cellulose [13,23,32] and size of samples

[156,163] have given a variety of different values for kinetic parameters.

10
Moreover, in general, additives appear to substantially decrease the apparent

activation energy, to promote the onset and increase the rate of pyrolysis

(except in the case of boric acid as reported by Rogers and Ohlemiller [153]),

and to increase the char yield. The activation energy appears to decrease

with an increase in the heating rate [150,151,157,164], with a decrease in the

crystallinity [13,23,32], and it appears to be larger in an inert atmosphere

than in an oxidizing atmosphere [139].

The atmosphere used in pyrolysis also affects heat transfer within a

sample. Large temperature gradients in the solid may be due to a poorly

conducting atmosphere [157] as well as to sample size [164]. Different local

pyrolysis rates in a large solid sample as a result of temperature gradients

have been observed with x-rays by Murty and Blackshear [164,167]. Thus, the

rate of the pyrolysis can be ruled not only by chemical processes but also by

physical factors, such as temperature gradients and migration of

condensates. Maa and Bailie [156] have developed a mathematical model of the

pyrolysis based on a shrinking, unreacted-core in which the pyrolysis is

assumed to be determined in some regions by heat transfer control and in

others by kinetic control.

Attempts have been made to express the overall weight loss during

pyrolysis of cellulose more satisfactorily than with a simple power law, while

still using a single set of values for activation energy and pre-exponential

factor, by using rate laws for heterogeneous reactions [168]; the weight loss

has been shown to obey random nucleation [133,152], phase boundary [139,141],

or diffusion-controlled rate laws [153]. These results are sometimes useful

for prediction of the amount and rate of evolution of fuel vapors from

cellulose in fire conditions. However, they do not address the fact that a

rate expression based on a single step Arrhenius rate law neglects the complex

kinetic processes of the pyrolysis reactions as suggested above.

11
Few kinetic models based on true chemical reaction mechanisms have been

presented to date. Most data on changes in DP during pyrolysis have been

explained by using the following rate equation for isothermal random scission

of a linear polymer

i-i =kt (1)

P P

where p is the number average DP at the time t, pQ the initial number average

DP value, and k the rate constant of the random scission process. The

observed changes in the DP appear to obey this equation only in the early

stages, because of the leveling off of the DP. The majority of the

experimentally determined activation energy values for this random scission

fall into a range from 25 to 35 kcal/mole. As stated above, the leveling-off

of the DP during pyrolysis cannot be satisfactorily explained by reference to

the fine structure of cellulose.

A kinetic model based on a chain mechanism, which has been presented by

Hirata et al., appears to reasonably explain the overall changes in the DP

[49-51,55,110]. As shown in Figure 3, the mechanism consists of initiation of

random scission of the chain, unzipping to produce levoglucosan, and

termination by grafting between an activated chain fragment and a hydroxyl

group of cellulose; this mechanism predicts the leveling off of the DP as a

result of a stationary state between the initiation and termination

reactions. The Arrhenius parameters for the three reactions have been given

with the model [49, 110].

The weight loss process during pyrolysis has been explained by models

with and without definite chemical reaction mechanisms. Chatterjee and Conrad

[103] appear to be the first workers who interpreted the weight loss in terms

12
of a definite chain mechanism and determined the Arrhenius parameters of the

weight loss. According to their model, the weight loss reaction consists of

initiation of random scission of the chains and unzipping which proceeds along

an activated fragment, generated by the random scission, producing

levoglucosan. In the early stages, the weight loss rate is controlled by both

reactions and is predicted to be directly proportional to time. At later

stages, it is assumed to be determined by the unzipping only and to obey a

first-order reaction law. Furthermore, they have shown that a zeroth-order

weight loss can be caused by establishment of a stationary state between both

of the reactions in their model. Their model has been supported by early

stage weight loss measurements by several authors [13,22,23,32,

149,154,160]. Despite this support, however, the model is in strong

disagreement with observed changes in the DP. According to their model, the

DP should decrease at a rate larger than that implied by Eq. [1], since the

chains should be shortened by both random scission and unzipping. This

prediction from the model goes against all of the reported results on the DP

behavior.

Following the presentation of the Chatterjee and Conrad model [103],

Parker and Lipska [30] proposed a model based on a zeroth-order reaction as a

means to explain the zeroth-order weight loss rate observed over a relatively

long portion of the heating time. In the model they have assumed that, during

heating, scission of the chains first occurs at weak linkages, about every 500

units, until a constant number of active chains is produced; the active chains

then undergo an unzipping reaction while the number of active chains stays

constant. They propose that the weight loss due to pyrolysis essentially

obeys a zeroth-order law and they have reported several results supporting

their model [34-37], However, their model cannot explain the many reported

13
observations of first-order weight loss obtained without any accompanying

stage of zeroth-order reaction. Furthermore, the assumed constant number of

active molecules which was related to the fine structure, is inconsistent with

the temperature dependent leveling off of DP during pyrolysis

Okamoto [169] has presented a review of the kinetics of cellulose

pyrolysis. In his review, he has calculated Arrhenius parameters for random

scission initiation, propagation producing levoglucosan, and termination,

based on the assumption of a stationary state with a constant radical

concentration produced by a chain mechanism. A definite chemical mechanism

for the termination was not given in his radical chain scheme which was

derived from a model for depolymerizaion of polymers by Boyd [170 ]. Despite

this vagueness about the termination mechanism and a poor proof of the

existence of a constant radical concentration, the general mechanism by

Okamoto seems to have a good possibility of development into a viable

explanation of the kinetics of cellulose pyrolysis.

Another model not based on a definite chemical reaction mechanism has

been presented by Bradbury, Sakai, and Shafizadeh [171]. Their abstract and

somewhat ambiguous model consists of initiation which produces "active"

cellulose and two competing first-order reactions which yield from one path,

volatiles, and from the other, char (plus gases) from the same active

cellulose. They have obtained Arrhenius parameters for each reaction by

applying weight loss data to the model. This model will be of more

significance when a definite chemical mechanism is given to it.

At present, no single kinetic model for change in the DP or loss in the

weight appears able to satisfactorily explain both types of behavior. These

kinetic models have been constructed in the manner of those which succeed in

explaining the pyrolysis of synthetic linear polymers. However it should be

14
noted that at some point in the overall reaction, cellulose pyrolysis is

different from that of synthetic polymers. Many synthetic polymers fuse

before they decompose, while cellulose pyrolysis proceeds in the solid

state. In the molten state the concentration of some activated centers such

as free radicals, pertinent to calculating a bimolecular reaction rate such as

for termination, can readily be obtained by assuming the sample volume is

proportional to a monomer unit number. But, this assumption cannot be applied

to cellulose pyrolysis, because the volume may not be directly proportional to

the monomer unit number in the system. Furthermore, the concentration in the

above sense may be meaningless due to the negligible mobility of activated

centers in the solid state. Therefore, a realistic bimolecular termination

mechanism must be independent of the mobility in the case of cellulose. Thus,

a satisfactory explanation of both changes in the DP and in the weight, based

on a realistic chemical mechanism for the solid state pyrolysis remains for

the future. This model would provide a basis for exact predictions of various

properties of cellulose pyrolysis.

5. SUMMARY AND CONCLUSIONS

The literature suggests that changes in cellulose during heating occur

first in the amorphous region; ordering or rearrangement of the molecules

increases the crystallinity, and cleavage of glucosidic bonds and dehydration

occur as the first chemical changes. The former reaction leads to production

of volatile levoglucosan and the latter causes cross-linking to increase the

mechanical strength. These pyrolytic reactions increase the relative

crystallinity. Following the amorphous region, the crystalline region

decomposes also initially at a glucosidic bond and then produces a large

amount of levoglucosan yielding active weight loss. In addition to volatiles

15
(tar), fixed gases and char are simultaneously evolved from the pyrolysis.

With additives or impurities or a decrease in the crystallinity, the yield of

volatiles decreases and that of char increases.

The majority of the reported kinetic data have been obtained from weight

loss and, to a lesser extent, from DP measurements. It is difficult to find

satisfactory agreement among the data. Some indicate that pyrolysis proceeds

in more than one step. These facts suggest that pyrolysis is a complex

reaction sequence. Several kinetic models have been proposed in order to

explain the overall behavior. However, no single kinetic model appears to

succeed in a fully consistent explanation of all aspects of the pyrolysis

behavior. Thus, there remains a need for a kinetic model, based on a

realistic solid state chemical mechanism, capable of exact predictions of the

pyrolysis behavior of cellulose.

16
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17
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27
X 5

O

X

oCVJ X

(J

28

FIGURE

1

-

CHEMICAL

FORMULA

OF

CELLULOSE
29

FIGURE

2

-

1-6

ANHYDROB-D

GLUCOPYRANOSE

(LEVOGLUCOSAN)
or

CNtOH oh

OH CHiOH tO

FIGURE 3 - CHAIN MECHANISM OF CELLULOSE PYROLYSIS

(REF. 49, 110)

30
i»l 1 4A I R EV. 2-8C) DEPT. OF COMM. 1. 8IBLI0GRAPHIC DATA

St.EE T (See instructions)

T I T LE AND SUBTITLt

PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date NBSIR 85-3218 August 1985

Pyrolysis of Cellulose, an Introduction to the Literature

5. AUTHOR(S)

T. Hirata

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No.

national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

Gaithersburg, MD 20899

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

10. SUPPLEMENTARY NOTES

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

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

bibliography or literature survey, mention it here)

Topics related to cellulose pyrolysis are briefly surveyed under several headings.

The principal aim is to give the reader some grasp of the issues involved and

provide a guide to the relevant literature; 171 references are cited. The headings include: Changes in cellulose fine structure with heating; chemical changes

during pyrolytic weight loss and kinetic modeling of pyrolysis. Principal emphasis

is on the last area; it is concluded that no current model adequately predicts

both the observed changes in degree of polymerization and the weight loss during

heating.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key

bibliographies; cellulose; kinetics; pyrolysis; reaction models

words by semicolons)

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

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| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

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