Kinetics of rapid pyrolysis of a calcium-exchanged brown coal and of a calcium model compound

Twentieth Symposium (International) on Combustion/The (]ombustion Institute, 1984/pp. 1463-1469

KINETICS OF R A P I D PYROLYSIS O F A C A L C I U M - E X C H A N G E D B R O W N C O A L A N D OF A C A L C I U M M O D E L C O M P O U N D K. R. DOOLAN AND J. C. MACKIE

Department of Physical Chemistry University of Sydney, Sydney, N.S.W., 2006 Australia The pyrolysis of calcium exchanged Yallourn hrown coal has been carried out under rapid heating conditions in a shock tube which generated heating rates of 107 K s~. The results are compared with the pyrolysis of untreated Yallourn brown coal and with calcium acetate chosen as a model for earboxylate group decomposition. Light hydrocarbon yields from the two coals were the same within experimental error. It is suggested that these products arise from secondary decomposition of tars of similar composition. Compared to the untreated coal, increased COz and decreased CO were found for the calcium-exchanged coal. The primary products of thermal decomposition of calcium acetate were found to be acetone, carbon dioxide, methane and ketene for temperatures in the range 900-1500 K. The overall rate of decomposition and primary product yields were fitted by means of a detailed model consisting of 16 chemical reactions. For the initial decomposition reactions of the ealciuln acetate low Arrhenius parameters were found. These parameters were similar to those found for formation of volatile products from the coals when based on a first order kinetic coal decomposition model.

Introduction

Experimental

Brown coals contain some 25% wt/wt oxygen so that oxygen containing species are the principal pyrolysis products. An understanding of the kinetics and mechanism of pyrolysis of coals under rapid heating is of fimdamental importance in coal combustion. The pyrolysis bchaviour of brown coals is thought to be greatly influenced by the cation constituents of the coal. Certain Victorian brown coals of which Yallourn is an example contain only small amounts of metal cations so that most of the carboxyl groups are in the acid form in the as-mined coal. All of the carboxyl group protons I can be replaced by alkali or alkaline earth cations to produce coals with high metal cation content analogous to similar rank cx)als mined elsewhere in Australia and in the United States. The present study was undertaken to compare the pyrolysis of un-exchanged Yallourn coal with the pyrolysis of the fully exchanged calcium form. The devolatilization behaviour of brown coals is thought to be markedly dependent on the environment of the - - C O O groups. We have therefore chosen to investigate, under similar conditions to those used for the coals, the pyrolysis of calcium acetate as a model system for the carboxylate group decomposition from calcium exchanged brown coals.

The shock tube and its application to rapid coal heating studies have been described elsewhere. 2'3'4 The untreated Yallourn coal and its calcium exchanged form were sieved to obtain the <45 Ixm fractions. Analyses of the coals are given in Table I. The untreated coal was air dried overnight at 100~ C; the calcium-exchanged coal was vacuum dried at 90~ C. The calcium acetate supplied by BDH Chemicals Ltd (not less than 97% dried calcium acetate) was first coarsely sieved to obtain the <500 txm fraction and air dried at l l 0 ~ to remove the residual 3% water. Samples of coal or calcium acetate were then loaded into a circulation cell4 which employed Stokes sedimentation to reduce the sizes of the largest suspended particles to <6 Ixm for the coals and <10 I~m for the calcium acetate. These small particles in suspension in argon were blown into the test section of the shock tube and subsequently heated by the reflected shock. Gas samples were collected through a fast opening relief valve attached to the end plate of the shock tube s which opened ~10 ms after arrival of the incident shock front. Yields of light hydrocarbons, acetone and CO were determined by gas chromatography (CC); COz and ketene were determined by infrared (IR) spectroscopy and water by mass spectrom-

1463

1464

COAL COMBUSTION

TABLE I Analysis of Yallourn brown coal (<45 Ixm fraction) Air dried basis (% wt/wt). Moisture Ash

11.6 1.0

Dry ash free basis (% wt/wt) Volatile matter 53.5 Fixed Carbon 46.5 C 66.9 H 4.6 N 0.6 S (total) 0.3 O (by difference) 27.6 Atomic H/C 0.83

Results Pyrolysis Yields--Coals

Carboxyl Content (meq/g) a

--COO (acid form) , --COO (Ca salt form)

unexchanged coal 3.31 0.18

clusion of finite particle heat-up rates was found to have negligible effect on the computed species concentration profiles and evaluated Arrhenius parameters. The cooling rate behind the rarefaction wave was approximately - 5 x 105 K s -1. The finite rate of rarefaction cooling5 was included in the kinetic modelling of calcium acetate decomposition.

calciumexchanged coal nil 3.49

"meq/g = milli-equivalents/gram. etry. Sampling of the product gases was confined to <10 cm from the end plate of the shock tube. Uncertainties in the product yields were -+10%. The uniform hot gas residence times behind the reflected shock fronts were determined by recording the pressure signals from a piezotron pressure transducer located near the end plate of the shock tube. Reflected shock temperatures were determined from the measured incident shock velocities using the NASA Chemical Equilibrium Composition program 8 and corrected for slowing down of the reflected shock front. The suspended densities of the coal and calcium acetate were determined by circulating up to the point of firing the shock and then allowing the suspended particles to settle onto sheets of aluminium foil placed on the floor of the test section. The sheets of aluminium foil were weighed before and after collecting the required particles. Heating of the suspended particles by the shocked gas is very rapid. ~ We have previously determined the time of heating of the coal particles to 0.95 T5 to be
Principal gaseous pyrolysis products were CO, COz,.CH4, C2H4, C6H6 together with smaller amounts of C2H6 and alkanes and alkenes of size C3--C 7. Water yields of about 6% were recorded for the un-exchanged coal but considerably smaller amounts of this product were evolved from the exchanged coal. Acetylene is an important product from both coals at temperatures above about 1500 K. Figure 1 shows the yields of CO and CO2 produced from both un-exchanged and exchanged coals as a function of temperature, Ts. Figure 2 shows the corresponding yields of CH4, C2H4 and C6H6. Total pressures varied from 12 atm to 28 atm between temperatures of 1000 K and 2200 K, respectively. The residence time was approximately constant at 1.3 ms. A significant enhancement of CO2 yield is obtained from the calcium-exchanged coal when compared with the essentially acid-form

-3~f I ~

I

I

I

I

4~

L 30

I

I

o

I ~

I

I

1000

1400 Ts/K

I

I

1800

I

I

2200

FIG. 1. Yields of CO (circles) and CO2 (triangles) from the pyrolysis of Yallourn brown coal as a function of reflected shock temperature, Ts. Filled symbols: fully exchanged calcium-form; Open symbols: as-mined coal.

RAPID PYROLYSIS OF CALCIUM FORM BROWN COAL

1465

Pyrolysis Yields--Calcium Acetate

/

i

,,~,,~,,,h~-n

t

t

I

I

Principal products of thermal decomposition of solid calcium acetate are acetone, ketene, methane and carbon dioxide. Traces of acetone were obtained in pyrolyses at temperatures below 900 K. At low extent of decomposition of the calcium acetate yields of methane and ketene were equal within experimental error. Electronic emission from the green arc bands of CaO was detected at temperatures above about 1300 K. Figures 3 and 4 show the yields of acetone, ketene, methane and carbon dioxide produced by pyrolyses of calcium acetate between 1000-1500 K, and residence times similar to those for the coal pyrolyses. Minor products include C2H4, C2H6 and CO, presumably arising from secondary gas phase reactions of the primary volatile products at the higher temperatures.

oo~

2

Kinetic Modelling--Calcium Acetate I

100-0

I

1400

T6/K

I

I

1800

I

2200

FIG. 2. Yields of benzene (triangles), methane (squares) and ethylene (circles)from pyrolysis of Yallourn brown coal as a function of reflected shock temperature, Ts. Filled symbols: fully exchanged calcium-form; Open symbols: as-mined coal. un-exchanged coal. On the other hand CO yields are lower for the exchanged coal than for the unexchanged coal. Within the experimental error of the data there is no significant difference, however, in yields of the light hydrocarbon products. Yields in the rising portions of the volatiles evolution curves (Figs 1 and 2) have been found to obey simple firstorder kinetics. Arrhenius parameters for evolution, A~ and E~, can be obtained from first-order fitting of this yield data and are presented in Table II.

A detailed kinetic model has been developed to simulate the product yield data from pyrolysis of solid calcium acetate. Reactions considered are included in Table III. The gas phase methyl radical chain pyrolysis mechanism of decomposition of acetone is well established 9 and these reactions (5) to (8) and (16) have been incorporated into the model. Gas phase evolution of ketene from acetone and

0:I /

S

TABLE II Arrhenius parameters for formation of light volatile products from shock heated Yallourn brown coal and its fully exchanged calcium-form

Product CO CO2 CH4 C2H4 C6H6

Calciumexchanged coal log~o A~ E~ 5.1 4.6 5.8 6.0 6.3

65 45 90 90 130

Un-exchanged coal loglo A~ Ev 5.2 5.1 5.4 5.7 5.3

70 50 100 90 110

Units of A~ are s-l; units of Ev are kJ mo1-1.

I

11 O 0

1300

1500

Ts/K FIG. 3. Concentration of indicated species M from pyrolysis of calcium acetate as a function of reflected shock temperature, Ts. A, M = acetone; 9 M = ketene. The curves are theoretical fits to the experimental points using the reaction scheme of Table III.

1466

COAL COMBUSTION 6

CO:, j

/k ,

6 0

%

9-

X

4

1100

o

9

CH4

1300

1500

ts/K FIG. 4. Concentration of indicated species M from pyrolysis of calcium acetate as a function of reflected shock temperature, Ts. A, M = carbon dioxide; O, M = methane. The curves are theoretical fits to the experimental points using the reaction scheme of Table III.

the reactions leading to decomposition of ketene 1~ are also well understood and they, too, have been included in Table III as reactions (8)-(10). The unimolecular decomposition of acetone and the recombination of methyl radicals are both pressuredependent reactions. The pressures used in this study are less than the high pressure limit and therefore appropriate fall-off values of the rate constants of these reactions have been chosen. Calcium acetate has earlier been considered to decompose unimolecularly to form acetone and calcium carbonate l~A3 although Arrhenius parameters for this (probably) solid state decomposition do not appear to have been given previously. This reaction has been included in Table III [reaction (1)] and the Arrhenius parameters have been obtained by fitting the experimental yield data. It was found impossible to simulate the experimental ketene and methane yields using purely gas phase evolution and decomposition rate constants, species concentrations two orders of magnitude too low being predicted theoretically. We are forced to conclude that there is a direct solid state evolution mechanism for simultaneous production of ketene and methane. We have included this reaction (2) in the model and the Arrhenius parameters for this reaction were obtained by a fitting procedure. The possibility of CH3 radical attack on calcium acetate was also taken into

TABLE III List of reactions and rate data used in kinetic modelling the pyrolysis of calcium acetate Reaction Number

Reaction

log,o A a

E~b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Ca(OOCCHa)2(s) ~ CHaCOCHa + CaCO3(s) Ca(OOCCH3)~(s) ~ CH4 + CH2CO + CaCO3(s) CHa + Ca(OOCCHa)a--9 CH4 + CH3COOCaOOCCH2 CH3COOCaOOCCH~ ~ CH2CO + CHa + CaCO3(s) CHzCOCH3 ---* CHACO + CH3 CH3 + CHaCOCHa ~ CH4 + CH2COCHa CHACO ~ CHa + CO CH2COCHa--* CH2CO + CHa CHACO + M --* CHa + CO + M CH2 + CH2CO ~ Call4 + CO CaCOa(s) ~ CaO(s) + CO~ CHa + CHa ---> C2H6 CH2 + CH3 "* C2H4 + H H + C H 4 --"> CH3 + Ha CHa + H2 ~ CH4 + H CHACO + CHa ~ CHaCOCHa

7.2 6.3 12.5 13.0 14.16 d 11.54 13.3 12.0 15.76 13.8 5.5 12.8 d 13.48 14.0 12.7 13.0

100 100 40 100 292 d 40.6 91 171 258 35 80 0d 0 49.4 47.8 0

"Units of A are s -t or cm 3 mol -t s -~ as appropriate. bUnits of E= are kJ mo1-1. ~ work. aFall-off values (see text).

Refer- Reaction ence Type c c ~ c 5 19 20 21 10 11 c 22 23 24 24 ~

I I I III I II III III II II I I! II II II III

RAPID PYROLYSIS OF CALCIUM FORM BROWN COAL account by including reactions (3) and (4). For the former, Arrhenius parameters were chosen as typical of methyl radical abstractions whilst the Arrhenius parameters for reaction (4) were chosen to be representative of the decomposition of a large radical. Other reactions included in the model involve the solid state decomposition of calcium carbonate and recombinations of gas phase radicals. The kinetic equations were integrated numerically using a Gear predictor-corrector method.14 At each temperature the rate constants for reactions (1), (2), (3) and (11) were progressively varied to best fit the concentrations of the major species acetone, ketene, COo and CH4. In Table III reactions of type I are those which critically affect the overall kinetics and product yields; type II reactions have minor effects on certain product yields; type III reactions do not have a significant effect on overall reaction rate or product yields. The computed profiles using the model of Table III are the full curves of Figs. 3 and 4.

Discussion

The main differences in the results for pyrolysis of the un-exchanged and calcium-exchanged coals in argon lie in the high temperature yields of CO and COo. A significant increase in the yield of COo and some decrease in CO yield are found for the calcium-exchanged form when compared with the un-exchanged coal. The ease with which pairs of carboxyl protons in the un-exchanged Yallourn coal can be replaced by Ca~ cations suggests that there is some ordered arrangement of pairs o f - - C O O H groups in the unexchanged coal. Consequently it should be possible for pairs o f - - C O O H groups to eliminate water and form an anhydride 15 which would then be expected to decompose to form CO and COo. Each pair of - - C O O H groups would ultimately be expected to eliminate one molecule each of HoO, CO and COo and in so doing half of the oxygen present in the - - C O O H groups would be expected to form COz. This is equivalent to a COo yield of 8% wt/wt daf coal as was observed. It is not possible for pairs of carboxyl groups in the calcium-exchanged coal to form anhydrides by elimination of water. In fact it appears that each carboxylate group in the calciumexchanged coal has eliminated one molecule of CO2 at temperatures above 1500 K where the oxygen content of the COo is equal16 to the oxygen content of the carboxylate groups. On this basis higher yields of CO and HoO and lower yields of COo would be expected from the un-exchanged coal when compared with the fully exchanged calcium form. The CO and HoO produced by pyrolysis of the calcium-exchanged coal would arise from the decomposition of other oxygen containing groups such as

1467

phenols. Despite the different limiting yields similar Arrhenius parameters were found for the formation of CO from un-exchanged and calcium-exchanged coals (Table II). So under these conditions, evolution of CO occurs at similar rates from both coals within experimental error. Light hydrocarbons are thought to arise from secondary gas phase cracking of evolved tar molecules, a The similarity between yields of light hydrocarbons from both the un-exchanged and calcium-exchanged coals suggests that tars of similar yield and structure are evolved from both coals. Cliff et ala found that tar yields and oxygen content of tars from pyrolysis of Yallourn coal in a fluid bed decreased to low values at temperatures around 1300 K. For the results reported here at 2000 K water yields were 6% wt/wt daf coal from the un-exchanged coal and about 2% wt/wt daf coal from the calcium-exchanged coal so that all of the oxygen originally present in the coal can be accounted for in the light volatile products CO, CO2 and H20. Consequently, at temperatures above 1300 K it is possible that similar tar intermediates are formed from both coals and that these tars are not affected by calcium content of the coal. At lower temperatures and heating rates Schafer and Tyler 17 found that tar yields from pyrolysis of brown coals decreased with increasing calcium content of the coals. However, at lower temperatures the tars contain significant amounts of oxygen and are likely to have different structural content to tars formed under our conditions. The low activation energies for evolution of the light hydrocarbons from the Yallourn coals studied here are largely similar to those values reported for rapidly heated US is and Australian 7 bituminous coals. These values are not typical of molecular processes and are thought to represent the overall values for a coupled slow tar evolution rate with a subsequent gas phase tar cracking, a Yallourn tars produced under rapid heating conditions contain insufficient oxygen4 to account for the observed yields of CO and CO2. Therefore the carbon oxides must be evolved directly from the coal. The low activation energies for CO and COo evolution probably reflect a co-operative pathway in the solid involving adjacent - - C O O groupings. Analysis of the calcium acetate decomposition kinetics may provide further support for the postulate of a rate-determining solid state process. Here the gas phase reaction kinetics of the volatile products are well understood. Nevertheless, it has been found to be impossible to fit our experimental concentration profiles by invoking purely isolated molecular processes. The experimental Arrhenius parameters for breakdown of the calcium acetate to its primary products are also very low and are atypical of individual molecular processes. Values of Ai, Eai (i = 1, 2 or 11) in Table III at least qualitatively

1468

COAL COMBUSTION

resemble the values found for breakdown of coals to their volatile products. Reactions (1) and (2) were independently varied in the fitting process but were found to have the same activation energies. Yakerson13 considered that the evolution of acetone from heated solid calcium acetate took place by a cooperative lattice reaction of low activation energy. Since we have found that both of the reactions which form acetone and ketene have the same activation energy, we suggest the possibility that in the solid compound a proton transfer could lead to the intermediate (I):

o/Ca\o 0

0

,.,';C ----~.+C'X~ H3

.\FH2C-

"OH

o

nor with oxygen-containing products evolved from these groups. Possibly other model compound studies based on phenols or aromatic acids derived from lignins might provide further information on chemical reaction pathways in the coal for elimination of CO and HzO. However, it must be borne in mind that, in addition to chemistry, physical processes such as mass transfer in the plastic coal matrix during devolatilization can have important influences on product yields. These physical effects might not be well simulated by synthetic compound pyrolyses.

/Co \

which would produce CaCOz and the intermediate enol. The latter would then either form acetone or methane and ketene via four-centre transition states. Another possibility is that intermediates (II) and (III) are involved and that both eliminate their respective products with similar activation energies. Since low rank coals contain more remnants of the chemical structures of their precursor materials than do bituminous and higher rank coals, model compound pyrolyses may be of assistance in the interpretation of the reactions of brown and other low rank coals. Thus, both for the two coals and the calcium acetate we find that the evolution of carbon dioxide takes place by reactions of low activation energy. From detailed kinetic modelling we may postulate that for calcium acetate, CO2 production arises via the facile elimination and subsequent decomposition of CaCO3. This suggests the possibility of an analogous cooperative route in the coal involving juxtaposed - - C O O groups also forming CaCO3 and leading to enhanced CO2 yields in the exchanged coal. For the model compound pyrolyses we see that CO evolution does not arise directly from decomposition of the carboxyl groups; instead CO is produced from reactions of evolved carbonyl compounds at higher temperatures. This, in turn, suggests that the CO evolved at low temperatures from the coals is not associated with carboxyl groups

0/ 0

H3t.;

(I)

0

Ca

,-"'_/C"o i-13C/,I_I(,,CH2

(II)

(III) Acknowledgments

We are greatly indebted to Mr. H. N. S. Schafer for preparing the calcium-exchanged coal. We thank Mr. R. J. Tyler and Dr. M. F. R. Mulcahy for helpful discussions. The financial support of the Australian Research Grants Scheme is acknowledged. REFERENCES "1. SCHAFER,H. N. S.: Fuel 58, 667 (1979). 2. CATHRO, W. S. ANn MACKIE, J. C.: J. C. S. Faraday I 68, 150 (1972). 3. DOOLAX,K. R., MACKIE, J. C. AND WEISS, R. G.: Combust. Flame 49, 221 (1983). 4. CLIFF, D. I., DOOLAN, K. R., MACKIE, J. C. AND TYLER, R. J.: Fuel (in press). 5. DOOLAN,K. R. AND MACKm, J. C.: Combust. Flame 50, 29 (1983). 6. GORDON, S. AND McBI~IDE, B. J.: Computer Program for Calculation of Complex Chemical Equilibrium Compositions, Rocket Performance, Incident and Reflected Shocks, and Chapman-Jouguet Detonations, NASA Report No. SP-273, 1971. 7. DOOLAN,K. R., MACKIE, J. C., MULCAHY,M. F. R. ANDTYLER, R. J.: Nineteenth Symposium

RAPID PYROLYSIS OF CALCIUM FORM BROWN COAL

8.

9. 10. 11.

12. 13. 14.

15.

(International) on Combustion, p. 1131, The Combustion Institute, 1982. DOOLAN, K. R. AND MACKIE, J. C.: Kinetics of Pyrolysis of Coals and Coal Model Compounds, Paper presented at the 14th International Symposium on Shock Tubes and Waves, University of Sydney, August, 1983. ERNST, J., SPINDLER, K. AND WAGNER, H. GG.: Ber. Bunsenges Phys. Chem., 80, 645 (1976). WAGNER, H. GG. AND ZABEL, F.: Ber. Bunsenges Phys. Chem. 75, 114 (1971). MACKIE, J. C. AND DOOLAN, K. R.: High Temperature Kinetics of Thermal Decomposition of Acetic Acid and its Products, Int. J. Chem. Kinet., (in press). REED, R. I., J. Chem. Soc., 4423 (1955). YAKERSON,V. I.: Akad. Nauk. SSSR, Bull. Divn. Chem. Sci.: 13, 914 (1963). HALL, G. AND WAa'r, J. M.: Modern Numerical Methods for Ordinary Differential Equations, Clarendon Press, Oxford, 1976. HURD, C. D.: The Pyrolysis of Carbon Compounds, Reinhold, N.Y., 1929.

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16. SCRAPER, H. N. S.: Fuel, 58, 673 (1979). 17. TYLER, R. S. AND SCHAFER, H. N. S.: Fuel 59, 487 (1980). 18. SZYDLOWSKI,S. L., WEGENER, D. C., MERKLIN, J. F. AND LESTER, T. W., Proceedings of the 13th International Symposium on Shock Tubes and Waves, Ed. C. E. Turner and J. H. Hall (State University of New York Press, Albany, N.Y., 1982) p. 860. 19. KERR, J. A. AND PARSONAGE, M. J.: Evaluated Kinetic Data on Gas Phase Hydrogen Transfer of Methyl Radicals, Butterworths, London, 1976. 20. SZlROVlCZA,L. AND WALSH, R.: J. C. S. Faraday I 70, 33 (1974). 21. SOLLY, R. K., GOLDEN, D. M. AND BENSON, S. W.: Int. J. Chem. Kinet. 2, 11 (1970). 22. GIANZER, K., QUACK, M. AND TROE, J., Chem. Phys. Letters 39, 304 (1976). 23. PILLING, M. J. AND ROBERTSON, J. A.: Chem. Phys. Letters 33, 336 (1975). 24. WESTBROOK, C. K., DRYER, F. L. AND SCHUG, K. P., Combust. Flame 52, 299 (1983).