Kinetics of rapid pyrolysis and hydropyrolysis of a sub-bituminous coal

Nineteenth Symposium (IntemalJonal) on Combustion/The Combustion Institute, 1982/pp. 1131-1138

KINETICS OF RAPID PYROLYSIS A N D HYDROPYROLYSIS OF A SUB-BITUMINOUS COAL K. R. DOOLAN AND J. C. MACKIE

Department of Physical Chemistry, University of Sydney, NSW 2006, Australia AND

M. F. R. MULCAHY AND R. J. TYLER

CSIRO Division of Fossil Fuels, P.O. BOX 136, North Ryde, NSW 2113, Australia Coal pyrolysis in both argon and hydrogen atmospheres has been investigated in the temperature range 1100-2300K using a shock tube. Particles of size <10 p~m diameter were studied. Particle heating rates of 10r Ks -~ were achieved giving particle heating times of < - i 5 0 p,s. This high rate of heating enabled the devolatilisation kinetics to be decoupled from particle heat-up effects. Particle cooling rates due to the rarefaction wave in the shock tube were 5 • 105 Ks -1. Residence times ranged from 0.25-1.3 ms and total pressures from 17-35 atmospheres. CI----Cr hydrocarbons and benzene, toluene and xylene yields were determined for pyrolysis in both argon and hydrogen/inert gas mixtures. Major gaseous products included CH4, C2H4, Call6, 1, 3-butadiene and benzene. C2H~ only became important at temperatures above 1500K and was attributed to secondary gas phase decomposition of the volatilised species. In the presence of hydrogen the major effect observed was an increase in the yields of CH4 and C6H6. CzHz yields were markedly lower in H2. The results were fitted by a first order evolution and secondary decomposition model which included equations to allow for particle heatup and quench. The model enabled kinetic parameters for volatile species evolution and decomposition to be evaluated. The implications of these results to coal pyrolysis are discussed.

Introduction Insight into the mechanism of coal pyrolysis under fast heating conditions is fundamental to both the design of efficient combustion systems and development of the flash pyrolysis process for the production of liquid fuels. There have been many recent studies of rapid 1 13 pyrolysis of coalsbut nearly all of these were limited to heating rates <10 ~ Ks - I and used coal particles of sizes tens of microns and larger. In these studies significant reaction may occur during initial heating up of the particles. Extraction of rates of evolution of individual volatile species and their secondary gas phase reactions is more straightforward, however, if the particles are isothermal. We report very rapid pyrolysis data obtained over a temperature range of 1100-2300K for a subbituminous Millmerran coal at heating rates as large as 10r Ks -1 and residence times of about 1 ms, using a laboratory shock tube. Yields and evolution rates of the light hydrocarbons were determined when the coal was heated in both inert gas and

hydrogen atmospheres. Coal particle sizes were kept to <10 ~m diameter so that particle heating could be essentiaU~, uncoupled from the rates of devolatilization of the light hydrocarbons.

Apparatus and Methods The shock tube and its application to coal heating studies have been described previously. 14A5 For pyrolysis kinetics studies the shock tube was used in the single pulse mode. The dried coal was separated on a Bahco air centrifuge then suspended in the test section of the shock tube with the driven gas which was either argon (CIG ultrahigh purity) or 50% H 2 in inert gas, namely either Ar or N 2 (CIG high purity mixture). A fast-acting relief valve (opening time ~10 ms) set in the end wall opened under the excess pressure of the reflected shock to allow collection of a small sample of product gases for gas chromatographic and mass spectrometric analyses. The uniform hot gas residence time behind the reflected shock was varied by using either hydrogen driver into H J i n e r t gas

1131

1132

COAL COMBUSTION MECHANISMS AND PYROLYSIS

1600 -

M2

1200 800

r

40O

I"i

~1600 I

i

I

I

I

M1

800 [ 400

0

1

2 3 4 PARTICLE

5 6 7 8 9 DIAMETER/jura

10

FIG. t. Histograms of the two coal particle size distributions M1 and M2. (residence times 0.25-0.7 ms) or helium driver into Ar (residence times 0.6-1.3 ms). Residence times were determined by measuring the pressure signals from Kistler piezotron gauges. The cooling transient was monitored by following the decay in the pressure signal with time after arrival of the reflected rarefaction wave from the driver. Bulk gas properties were calculated from the NASA Chemical Equilibrium Composition program. ~6 Incident and reflected shocked gas properties were computed from the measured incident shock velocity. The coal used in these studies was a sub-bituminous Millmerran coal with the following ultimate analysis: carbon 79.1% wt daf, hydrogen 6,5%, nitrogen 1.2%, sulfur 0.6%, oxygen 12.6% (by difference). Ash content was 17.7% d.b. To determine the possible effects of particle size variation on the pyrolysis kinetics we have studied two size distributions, M1 and M2. Size distributions which were determined by electron microscopy are shown in Figure 1. To determine the susp e n d e d mass of coal, n e e d e d for yield determinations, coal particles were circulated throughout the test section in the normal way up to the point at which the shock would have been fired. The suspension was then allowed to settle out on to a aluminium foil and electron microscope grids placed on the floor of the test section. The

mass distribution in the test section was then determined gravimetrically. The suspended coal concentration was found to decrease slowly and linearly from the point of injection for about 40 cm upstream. Sampling was confined to a length of <8 cm from the point of injection and concentration was essentially uniform over this length. In all experiments the coal mass fraction in carrier gas was always maintained <0.03. Some of the suspended coal sedimented out prior to firing the shock. Part of this sediment was rapidly ( - i 0 ~s) re-entrained by the strong shearing flow immediately behind the shock front. Determination of the net mass of suspended coal heated by the shocked gas was made both by calibrating the sedimented and re-entrained masses by light attenuation measurements and by theoretical analysis of the particle dynamics using the equations of shock wave re-entrainment presented by Merzkirch and Bracht.17 From sedimentation studies the initially suspended mass of coal was found to be subject to a random error of ---10%. Systematic differences between calculated and measured values of the mass of coal re-entrained by the shock wave added another 20% uncertainty to the net mass of coal suspended. This led to an absolute

0 /0

.o o I

/

/

//~3 C2H2 0

/

"i:~" "" I

I

I

I

I

X •

o~

-W ~ 0

1000

Y.-'~

C2H6 I

1600

1800

2 200

T,/K FIG. 2. Yields of the designated products (symbols) obtained on pyrolyses in Ar as function of T5 the reflected shocked gas temperature. Full curves are theoretical fits to the data (see text). Residence times in range 0.7 to 1.1 ms.

KINETICS OF RAPID PYROLYSIS AND HYDROPYROLYSIS

5

model for heat transfer to fine coal particles. We found that the mass average particle size of distribution M1 of radius 1.6 ~xm will heat up to 0.9T 5 in 150 Ixs. Thus the particles are essentially at the stated T5 for most of the hot gas residence time. However, in modelling the evolution of the volatiles, finite heating rates of the particles have been taken into account (see below). Figures 2 and 3 show the yields of CH 4, C2H 4, C2H 6 and C2H2 produced by inert gas and hydrogen pyrolyses (50% H 2 + 50% Ar or N2) respectively. Figures 4 and 5 show the corresponding yields of C3H6, C3H8, l;3-butadiene, C4H s and C6H6. Total pressures in both sets of runs varied from 17 atm to 35 atm between temperatures of ll00K and 2300K, respectively. For Ar and Hz/ Ar carrier gases residence times are approximately constant over the temperature range at 1 ms and 0.7 ms respectively. For the H J N 2 carrier gas, residence times decrease progressively from 0.6 ms to 0.25 ms as T5 increases. Comparison of Figs. 2 and 3 will show that substitution of H 2 for half the inert gas results in a marked shift towards formation of CH 4. This trend has been reported previously in longer residence time pyrolyses. 12,1s Above 1600K acetylene becomes an important

C2~ 4

x ~/^~ /

3 2

x.~x

*~

~x

X

X

x C2H6

I-1

9

\5 9

CH4 6~

r'l ,_,3

9

? B

O

~176

2

e2H2

1

0

9

Z

9

n n ~ "=

_ 9

1200---

.~

1600

0

.0

_o-'7."

Ts/K2000

1133

I

Fzc. 3. Yields of the designated products obtained on pyrolyses in 50% H2/inert gas. Open symbols and X - 50% Ar, closed symbols and * - 50% N2. Residence times for 50% Ar 0.7 ms. Residence times for 50% N z range from 0.25-0.7

ms.

V

0.0"5 2I5 ~0

error of 30% in the yields quoted below although repeatibility of the data was + 10%.

Results

L~ Z~ ~ C

AI~~I

A I

u

6H6

I

•

;o,F ~0

I

Z~

~X~.~..~ .,~x o X" x

x

CH

;46

/ +

Pyrolysis Yields Principal gaseous hydrocarbon products of both inert gas and hydrogen pyrolyses were CH4, C2H 4, C~H6, C3H6, 1, 3-butadiene (C4H6), butenes and benzene. Traces of C5---C7 hydrocarbons and toluene and xylenes were also observed. Olefines predominated over the corresponding alkanes. C2H 2 and soot were important products at high temperature in the inert gas pyrolysis together with traces of propadiene and propyne. The C5--C 7 chain compounds were largely absent in hydropyrolyses. Yield data are reported as a function of the reflected shocked gas temperature, T5. In a previous study 15 we established the validity of the Nusselt

0-75 o ~ 0"5 >0'25 0

1000

j

_

~ 1/,,00

~ 1800

3 8j 2200

T,/K FIG. 4. Yields of the designated products obtained in pyrolyses in argon. Yield of C~H6 is for 1,3-butadiene. Residence times in range 0.7 to 1.1

ms.

1134

COAL COMBUSTION MECHANISMS AND PYROLYSIS

2.5

2.0

C6 H6

1.5

& 1-0

z~ i

"~ 0 T~X

0/ 1-2

I

I

I ._

i

I

~--C4H6 ux X ~

I

I C3H~ o

+_,.

tion using a highly simplified kinetic model. The yield data of Figs. 2-5 exhibit a plateau region above about 1900K that is, at temperatures at which rapid secondary decomposition of the product hydrocarbons would be expected to take place. This phenomenon is a consequence of the finite cooling time associated with the reflected rarefaction wave and modelling of the yield data must explicitly allow for the influence of a finite rate of cooling on devolatilization and decomposition of the volatiles. The rarefaction wave is treated as an adiabatic expansion and temperature time profiles are obtained from pressure-time measurements under the assumption of isentropic flow. In most single pulse shock tube studies the pressure drop is assumed to be linear through the expansion, n~ and the temperature profile is obtained from the usual isentropic relations. From a series of measurements of pressure profiles in cooling transients we find the expression TJTg = 1 + Kt

IN

__C3H8"3"8 + 0 1000

_+ 1400

T,/K

t 1800

I

I 2200

Fro. 5. Yields of the designated products obtained on pyrolyses in 50% H2/inert gas. Open symbols and crosses - 50% Ar closed symbols 50% N2. Residence times as for Fig. 3.

product (Fig. 2) along with gas phase produced soot. Production of acetylene and soot, however, is greatly inhibited in the presence of hydrogen. Benzene yields (Figs 4 and 5), on the other hand, are enhanced in hydropyrolysis. Under the very short residence times of the 50% H2/N 2 pyrolyses the C6H6 yield is markedly favoured towards the highest attainable temperature of 1800K. However, under these conditions particle heat-up becomes a significant proportion of the total residence time whereas at the longer residence times generated during H2/Ar pyrolysis, the increase in benzene is not nearly so marked. 1,3-butadiene (Figs 4 and 5) has not been commonly reported as a product of rapid devolatilisation of coal. However, Meuzelaar 19 reports significant amounts of 1,3-butadiene in Curie-point pyrolysis mass spectra of coals of various rank from South West Utah.

Kinetic Modelling It is of interest to attempt to separate the combined effects of volatile evolution and decomposi-

(1)

to be a more precise relation for the variation of gas temperature T., with time t, through the expansion wave with our experimental arrangement. We find that for Ar, K/S-1 = 80 + 0.13(TJK), and for 50% H2/Ar, K/s -1 = 280, where 1100 < T5 < 2300K. The rate of evolution of each volatile species (i in number) will depend on the temperature, Tpj, of each particle size class (j in number). This rate is assumed to be first order in the amount of product yet to be produced. Since yield data have been obtained over temperatures at which appreciable decomposition of the ith volatile takes place, the evolution rate of this volatile also includes a first order decomposition term. If Yi is the yield of the ith volatile, then

dY,/dt = k~i (Y*- Y,) - ka, Yi

(2)

where Y* is the maximum yield of i (t ~ :r kvi and kdi are Arrhenius rate constants for evolution and decomposition of the ith product, respectively. also,

k,~, = Av, E

fJ exp (-E~JRTpj)

(3)

J where A~i and Evi are the Arrhenius parameters, )~ is the fraction of particles in the jth class, and the summation is over all size classes of the coal distribution. To allow for a finite heating rate, the particle temperature is given by T~ = Tg - (Tg - T.jo) exp (-313t/~)

(4)

KINETICS OF RAPID PYROLYSIS AND HYDROPYROLYSIS

1135

TABLE I Fitted Arrhenius parameters for formation (A,, E.~) and decomposition (Aa Ea) of the light hydrocarbons together with fitted values of maximum yields (Y,)

Product CH4 CH4 C2H, CzH4 CaH6 C3H6 C6H6 C6H6 C4H8' C,H~~

Pyrolysis gas Ar 50% H2/Ar Ar 50% H~/Ar Ar 50% HJAr Ar 50% H~/Ar Ar 50% H~/Ar

A~" 7.0 1.1 9.1 4.4 9.5 8.6 9.3 9.9 6.9 1.9

x 104 x 10s x 104 x 10s x 103 x 104 x 104 x 105 X 104 x 10~

E~,~

Ad,=

99.3 83.4 80.3 87.2 64.1 69.1 92.8 111 76.3 81.6

4.9 5.6 2.2 2.9 9.6 1.1 5.3 4.1 2.0 1.2

x 108 x 108 X

109

x 10~ x 108 X 10l~ x 10r x 10s x 10~ x 10~

Edi~

Y*

204 200 222 222 209 204 167 196 218 228

0.164 0.124 0.064 0.052 0.043 0.030 0.037 0.029 0.013 0.015

"Units are s-L bUnits are kJ mol-L 1,3-hutadiene. where T., o is the particle temperature at arrival of the refle~cted shock front; r~ is the jth particle radius. For Millmerran coal we ~ have found 15 that 13 = (2.5 --- 1.0) x 10- s m z s-1. The Arrhenius parameters A~, E~i, Ad~, Eal, together with Y* were chosen as adjustable parameters, Y~' being assumed to be independent of temperature. To reduce computer time, size distribution M1 was approximated by 3 representative size classes chosen as mass average sizes. Equations (2)-(4) were integrated numerically using the Gear method zl for times t ~< tr~s, the hot gas flow time, during which Tg = T5. From t = t ~ this set of equations was integrated together with equation (1) which accounts for the rarefaction wave cooling. Integration was continued for 3 ms after which the gas temperature T~ <~ 0.5T 5. The adjustable parameters were least-~quares fitted to the experimental yield data by this procedure. In Figs. 2-5 the full curves are the theoretical yield curves obtained by nonlinear least-squares fitting where the yields have been scaled to the daf weight of coal in each case. The best-fit Arrhenius parameters for each product together with the maximum yield are given in Table 1. To check for possible influence of particle size on the rate data, the best-fit Arrhenius parameters obtained for CH 4 and C6H 6 from the 1 ms residence time argon pyrolyses of size distribution M1 were used to compute theoretical profiles for size distribution M2. Experimental results for particle sizes M2 are compared in Fig. 6 with the calculated values, both at residence times of 0,3 ms. Agreement between this theoretical profile and the best-fit profile is seen to be reasonable suggesting that particle size does not have a strong influence

on the kinetic parameters. The lower predicted yields from small particles when large particle data are used probably arise from systematic errors in the determination of the mass of sedimented coal re-entrained by the shock. In both cases, the curves in Fig. 6 agree, however, within the stated precision of the data. Discussion

Under our conditions of small particle size and very rapid heat-up, effects of differences between 1"00

&

A

0"75

0-50

z x f s" z~ ,,,/

/

C6H6

-1= 0-25 41

~

I

I

I

I

I

I

2

f 0

t~ ~

1

-

1200

I

1600

I

T,/K

..... 1

2000

I

__,__ I

I

24.00

FIG. 6. Yields of CH~ and C6H8 obtained in argon pyrolyses at residence times of 0.3 ms together with the best-fit theoretical profiles (full curves) and the theoretical yield curves computed from data obtained at longer residence times and larger size distribution (see text).

1136

COAL COMBUSTION MECHANISMS AND PYROLYSIS

gas and particle temperatures on the devolatilization kinetics are minimal. Although a first order kinetics model of devolatilization of coal has been shown to have limitations 5'9 it nevertheless has been found to give an adequate representation of the light hydrocarbon yields produced in our very short residence time inert gas and hydrogen pyrolyses. The Arrhenius parameters for the devolatilization rate constants are in general remarkably similar to the first order rate constants reported in weight loss studies of rapid pyrolysis of bituminous coals. 2z The values also lie within the range found by Szydlowski et a113 for overall formation of light hydrocarbons from shock heated U.S. bituminous coals. The Arrhenius parameters obtained for decomposition of the products are typical of those reported in the literature for overall chain reactions of the individual species. For example, shock tube decomposition of ethylene in neon has been foundz3 to be first order with respect to [C2H4] with activation energy 210 -+ 7 k J mo1-1 and half-order with respect to [Ne]. The pseudo-first order preexponential factor is 4 x 108 s -1 close to the values reported in our study. On the other hand the Arrhenius parameters for volatiles evolution, Avi and Eve, are not indicative of molecular processes, nor are the low values of both Ave and Eve likely to result from a complex chemical mechanism. A chain mechanism for volatiles formation in principle could lead to a low observed value of Evi but should give Avi around 1012 s -t. Nor is it likely that a multiple set of competing chemical reactions would give rise to the observed values. The overall activation energy of such a mechanism can be shown to approach the activation energy of the lowest member of the set at pyrolysis temperatures. Volatiles evolution would involve competing bond breaking reactions and our observed Eve'S are too low to be associated with bond rupture. The values of A~i and Evi rather suggest the possibility that even for these small particle sizes, the devolatilization kinetics are being strongly influenced by physical processes during the coal decomposition. 9,24 Indeed, the observed Arrhenius parameters for devolatilization are typical of values obtained for diffusion controlled thermal degradation of synthetic polymers. 2z The major influence of hydrogen at partial pressures of 8.5-17.5 atm appears to be in increasing the values of kvi over those obtained from inert gas pyrolyses. The substantial increases in CH4 yield and devolatilization rate in hydropyrolysis might be attributed in part to the enhanced reaction of methyl groups in the coal via CH 3 - R + H2 ~ CH4 + RH coal In contrast, with the exception of propene and 1,3butadiene, kai values are virtually constant and

independent of the nature of the pyrolysis gas. From the rate data shown in Table I it may be deduced that the kdi become comparable with the kvi at temperatures as low as 1400K. Thus modelling studies at or above this temperature should include provision for product decomposition. The plateau in yields observed at T > 1800K can also be explained in terms of the derived values of E~i and Edi. Although the cooling rate in the rarefaction wave is high (dTJdt = - 5 • l0 s Ks -1 initially) only the product decomposition reactions with their relatively large activation energies will be rapidly frozen.2~ Devolatilization with its low activation energy, will proceed with an appreciable rate for at least 1.5 ms after arrival of the rarefaction wave head. The least squares fits were sensitive to variations in the Arrhenius parameters Avi, Evi, Adi, Edl. Variation of the Y* values by 20%, however, made very little difference to the quality of.the fits. The Av~ value varied in direct proportion to the variation in the value of Y*, whereas the other parameters remained essentially unchanged. Probably little significance can be attributed to the fitted values of Y* owing to the insensitivity in fitting. Likewise, the differences in values of u with and without H2, (Table I), which are mostly in the opposite sense from those observed experimentally, are not significant statistically and are to be attributed to the insensitivity in fitting Y* to the data. In principle the Y* could be derived a priori from the coal structure if sufficiently detailed knowledge were available. However, despite the apparently high computed values of the Y*, (Table I), for the same coal pyrolysed at 1200K in nitrogen at 1 s residence time, Tyler12 has obtained yields of products exceeding 0.5 Y* and in the case of CzH4, exceeding the computed Y*. Implicit in the application of uncoupled equations (2) for evolution of the ith product is the assumption that its decomposition does not produce one of the set of i products. Ideally, a more elaborate decomposition model would incorporate a coupled multiple reaction sequence. However, the simple model represented by equation (2), when account is taken of finite heating and cooling rates has been shown to be adequate in explaining the observed short residence time yield data.

Acknowledgments We thank ICI (Australia) for provision of hydrocarbon standards. J.C.M. and K.R.D. acknowledge the financial support of CSIRO and of the Australian Research Grants Committee. REFERENCES 1. HowaRD, J. B. AND ESSEr~HIGIJ, R. H.: Eleventh Symposium (International) on Combus-

KINETICS OF RAPID PYROLYSIS AND HYDROPYROLYSIS tion, p. 399, The Combustion Institute, 1967. 2. KIMBER, G. M. AND G~AY, M. D.: Combust. Flame 11 360 (1967). 3. JUNTCEN, H. AND VAN HEEK, K. H.: Fuel 47 103 (1968). 4. BADZlOCH, S. AND HAWKSLEY, P. G. W.: Ind. Eng. Chem. Process Design and Development 9 521 (1970). 5. ANTHONY, D. B., HOWARD, J. B., HOTrEL, H. C., AND MEISSNER, H. P.: Fifteenth Symposium (International) on Combustion, p. 411, The Combustion Institute, 1977. 6. KOBA'I'ASHI,H., HOWARD, J. B., SAROFIM,A. F. : Sixteenth Symposium (International) on Combustion, p. 411, The Combustion Institute, 1977. 7. UBHAYAKER, S. K,, STICKLER, D. B., VON RoSENBERC, C. W. AND GANNON, R. E.: Sixteenth Symposium (International) on Combustion, p. 427, The Combustion Institute, 1977. 8. SUURERG, E. M., PETERS, W. A. AND HOWARD, J. B.: Ind. Eng. Chem. Process Design and Development 17 37 (1978). 9. SUUBERG, E. M., PETERS, W. A. AND HOWARD, J. B.: Seventeenth Symposium (International) on Combustion, p. 117, The Combustion Institute, 1979. 10. SOLOMON, P. R. AND COLKET, M. B.: Seventeenth Symposium (International) on Combustion, p. 131, The Combustion Institute, 1979. 11. TYLER, R. J.: Fuel 58 680 (1979). 12. TYLER, R. J.: Fuel 59 218 (1980). 13. SZYDLOWSKI,S. L., WEGENER, D. C., MERKLIN,

1137

J. F. AND LESTER, T. W. : Short Residence Time

Pyrolysis and Oxidative Pyrolysis of Bituminous Coals. Paper presented at Thirteenth International Shock Tube Symposium, 1981. 14. CATHRO, W. S. AND MACKIE, J. C.: J.C.S. Faraday I 68 150 (1972).' 15. DOOLAN, K. R., MACKIE, J. C. AND WEISS, R. G.: Combust. Flame 1982 (in press). 16. GORDON, S. AND McBRIDE, B. J.: NASA SP273, 1971. 17. MERZKIRCH, W. AND BRACnT, K.: Int. J. Multiphase Flow 4 89 (1978). 18. SUUBERG, E. M., PETERS, W. A. aND HOWARD, J. B.: Fuel 59 405 (1980). 19. MEUZELAA~,H, L. C.: Characterization of Rocky

Mountain Coals and Coal Liquids by Computerized Analytical Techniques. Report to U.S.

20. 21.

22. 23. 24. 25.

Department of Energy, DOE No. DE-FG2280PC30242, 1980. TSCHUIKow-Roux, E. : Phys. Fluids 8 821 (1965). HALL, G. AND WATT, J. M.: Modern Numerical Methods for Ordinary Differential Equations, Clarendon Press, 1976. ANTHONY, D. B. AND HOWARD, J. B.: AIChE J. 22 625 (1976). GaY, I. D., KERN, R. D., KISTIAKOWSKY,G. B. AND N1KI, H.: J. Chem~ Phys. 45 2371 (1966). CHERMIN, H. A. G. AND VAN KREVELEN, D. W.: Fuel 36 85 (1957). DAVID, C.: Comprehensive Chemical Kinetics (Bamford and Tipper, Eds.) Vol. 14 Degradation of Polymers, Ch. 1, Elsevier, 1975.

COMMENTS J. Lahaye, CNRS, France. Referring to the small value of the apparent activation energy EA of devolitalization, you indicated that this step is not a chemical process. Actually, this small value of EA only shows that the kinetics is diffusion controlled, but gives no information on the process itself. Author's Reply. The kinetic parameters were derived using a highly simplified kinetic model with the assumption that the rate of devolatilization is first order in the amount of product yet to be produced. Although this model has obvious limitations, it nevertheless gave an adequate representation of the hydrocarbon product yields. As noted in our discussion section, the low values obtained for the Arrhenius parameters suggest that physical rather than molecular processes are strongly influencing the kinetics. We agree entirely that these parameters describe

only the overall evolution process and give no information on individual reaction steps.

E. Suuberg, Brown University, USA. Regarding the low values of activation energy and pre-exponentials for evolution of the gaseous species measured, it would not seem possible to rule out an explanation based on a distribution of activation energies as an alternative to the diffusion limitation explanation. Has this been explored? Author's Reply. First consider a discrete set of parallel first-order Arrher~ius rate constants, k~ = A, exp (-E,/RT) and define the apparent rate constant, k~pp, which would be obtained by fitting experimental yield data. If now we define the apparent or observed activation energy

1138

COAL COMBUSTION MECHANISMS AND PYROLYSIS d d(1/T)

then, n

Eo~=~k,E,/2k, t

t

If the Et are associated with bond breaking reactions and El < E2 < 9 . 9 then at typical pyrolysis temperatures (~600 ~ C), E~pp ---) El which must surely be larger than 200 kJ mol -~ even for rupture of very weak bonds. Furthermore, E ~ --> El, irrespective of temperature. In the case of a normally distributed set of activation energies where Eo is the peak in the distribution and (r is the standard deviation,

lution rate constants obtained from the larger particle size distribution underestimate the devolatilization rates of the smaller size distribution but any differences lie within the error limits of the data. When our kinetic parameters are used to compute light hydrocarbon yields obtained in second timescale flash pyrolyses with maximum sizes of 44, 90 and 125 Ixm, respectively, our rate constants increasingly overestimate devolatilization yields. These phenomena appear consistent with the postulate that evolution rates are increasingly limited by internal mass transport as particle size increases, and do indeed suggest that one would therefore need to work with even smaller particles than those used in the present studies to overcome transport limitations.

E:~ = Eo + (2/~r)'/2cr - cr~/nT. Now Ea~ is found experimentally to be about 100 kJ mo1-1, Eo as an average activation energy for bond rupture must be at least 300 kJ mo1-1. These values lead to (r ~ 40 kJ mo1-1 around 600 ~ C. This implies that weak bonds whose rupture activatiola energies are at least five standard deviations less than the average, are responsible---a most improbable occurrence. Thus distributions of activation energies directly associated with bond breaking reactions do not seem to provide an explanation for the low observed activation energies.

P. Kalson, Technion, Israel. Despite great efforts to separate heat-up and volatilization (by using very high heating rates) and to eliminate external transport resistances (by using very small particle sizes), you still observe activation energies and pre-exponential factors which are inconsistent with simple chemical reactions. If this is due to some internal transport limitation, as you claim, these results may be very significant in indicating the ultimate futility of such herculean efforts in attempting to observe actual chemical kinetic rates. Are there any circumstances in which one will be able to observe the actual kinetics of the chemical reaction itself? Author's Reply. To determine the extent to which our kinetic parameters may be transferred to other devolatilization data we have calculated, using our rate constants, evolution and weight loss yields obtained for other particle sizes and at varying residence times. For our own two sets of data the evo-

N. Gat, TRW-Space & Technology Group, USA. The energy balance equation (particle temperature use) and the mass balance equation (volatilization rate) are coupled in the sense that the temperature obtained from the former determines the mass loss calculated by the latter. In addition, however, since the volatilization process is endothermic, there should be a "latent heat" (L~) term in the energy equation to account for the heat absorbed by this endothermic reaction. Further, the particle mass is constantly changing so that the energy equations should account for these effects.

Author's Reply. With these very small particles heat transfer from hot gas to particles is very rapid. In addition the particles are highly dilute in a heat bath gas. Thus, this correction to the energy equation is very small and need not be included.

P. Solomon, Advanced Fuel Research, Inc., USA. If the low rate measured for pyrolysis is due to diffusion control then what eliminates the diffusion control to produce the higher rate for hydropyrolysis?

Author's Reply. This can be explained if the observed light hydrocarbon products arise largely from secondary gas phase cracking of tar molecules. Hydrogen would be expected to increase the rate of this gas phase tar cracking. Indeed, we have shown (Doolan and Mackie, Combust. Flame 1982 in press) that the rate of decomposition of n-octane in argon is increased by approximately a factor of ten when half of the inert gas is replaced by hydrogen.