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Kinetics of Volatile Product Evolution in Coal Pyrolysis: Experiment and Theory Michael A. Serio,* David G. Hamblen, James R. Markham, and Peter R. Solomon Advanced Fuel Research, Inc., East Hartford, Connecticut 06108 Received September 15, 1986.
Revised Manuscript Received October 29, 1986
Relatively few studies have addressed the kinetics of individual volatile species evolution, particularly at high-temperature, high-heating-rate conditions. In addition to the sparsity of species evolution data, substantial controversy surrounds the wide variation (factors of 1000) in reported kinetic rates for both overall weight loss and species evolution. The aim of this study was to usé data from three types of reactors, each with different heating characteristics, to develop a more accurate reactorindependent, heating-rate-independent, and coal-independent set of kinetic parameters. Toward this end, several steps were taken to obtain better measurements of the pyrolysis rates and heat-transfer rates for coal. In addition to improvements to the experiments, improvements were also made to a previously described functional group (FG) model for coal pyrolysis. Two submodels were added to describe (a) the cracking of hydrocarbon species released in primary pyrolysis and (b) the equilibration of oxygén-, hydrogen-, and carbon-containing species at high temperatures. Comparisons of data obtained in the three reactors with the predictions of the improved FG model are presented for six coals. In general, the agreement of the FG model and the data is quite good for all the pyrolysis products at temperatures below 1100 °C. As the temperature increases above 1100 °C, secondary reactions, including soot formation and gasification, begin to play an important role. This léads to overprediction of olefins, CH4, H20, C02, and tar and underprediction of CO, H2, C2H2, and benzene. The results for weight loss during primary pyrolysis are in reasonable agreement with predictions of a single first-order model for primary pyrolysis weight loss that uses a rate constant k = 4.28 X 1014 exp(-54570/fiT) s'1 This indicates that the rate of primary pyrolysis is much higher at elevated temperatures (>700 °C) than predicted by commonly used rate expressions. 11.
Introduction
for the discrepancies. A major contributor has been inaccurate knowledge of particle temperatures, which has
Coal devolatilization is important because it is the initial step in a coal conversion process, accounting for up to 70% weight loss of the coal. It is also the process that is most dependent on the organic properties of the coal. Knowledge of the individual species evolution is important for a number of reasons. The composition of the species must be known to model the energy released by oxidation of the volatiles in combustion or gasification. For example, up to 30% of the volatiles weight can consist of C02 and pyrolytic H20. Knowledge of the amounts and rates of the individual species is important in gasification or mild gasification where product composition is of concern. It is also important in pollution control by staged combustion or sorbent addition. Finally, knowledge of the species
(1) Anthony, D. B.; Howard, J. B. AIChE J. 1976, 22, 625-656. (2) Howard, J. B.; Peters, W. A.; Serio, . A. “Coal Devolatilization
Information for Reactor Modeling”; Final Report, EPRI Project No.
986-5, 1981. (3) Howard, J. B. Chemistry of Coal Utilization·, Elliott, M. A., Ed.; Wiley: New York, 1981; Chapter 12, pp 665-784. (4) Gavalas, G. R. Coal Science and Technology 4: Coal Pyrolysis·, Elsevier Scientific: Amsterdam, The Netherlands, 1982. (5) Solomon, P. R.; Colket, . B. Symp. (Int.) Combust., [Proc.] 1978, 17th, 131-143. (6) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Krause; J. L. Symp. (Int.) Combust., [Proc.] 1982, 19th 1139-1149. (7) Solomon, P. R.; Hamblen, D. G. EPRI Final Report No. 1654-8, 1983. (8) Solomon, P. R.; Hamblen, D. G. Prog. Energy Combust. Sci. 1983, 9, 323-361. (9) Solomon, P. R.; Hamblen, D. G. Chemistry of Coal Conversion·, Schlosberg, R. H., Ed.; Plenum: New York, 1985; Chapter 5, pp 121-251. (10) Solomon, P. R.; Serio, . A.; Carangelo, R. M.; Markham, J. R. Fuel 1986, 65, 182-194. (11) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. Process Des. Deo. 1978, 17, 37-46. (12) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Symp. (Int.) Combust., [Proc.] 1978, 17th, 117-130. (13) Campbell, J. H. Fuel, 1978, 57, 217-223. (14) Juntgen, H.; van Heek, K. H. Fuel, 1968, 47, 103-117. (15) Juntgen, H.; van Heek, K. H. Fuel Process. Technol. 1979, 2,
evolution provides added understanding of pyrolysis mechanisms and their relation to coal structure. Recent reviews1"4 of the coal pyrolysis literature have identified numerous studies on the kinetics and amount of total volatile yield. Some of these studies have addressed the individual volatile species5"30 and measured
the kinetics of species evolution.6'20·29,30 Only a few studies have been performed at high temperatures (>600 °C) and high heating rates (>103 K/s),6'12·14·16·20·28'30 even though these are the conditions of interest in most coal conversion processes. In addition to the sparsity of species evolution data,
261—293
(16) juntgen, H. Fuel 1984, 63, 731-737. (17) Weimer, R. F.; Ngan, D. Y. Prepr. Pap.—Am. Chem. Soc., Dio. Fuel Chem. 1979, 24(3), 129-140. (18) Fitzgerald, D.; van Krevelen, D. W. Fuel 1959, 38, 17-37. (19) Serio, . A.; Peters, W. A.; Sawada, K.; Howard, J. B. Prepr. Pap.—Am. Chem. Soc., Dio. Fuel Chem. 1984, 29(2), 65-76. (20) Doolan, K. R.; Mackie, J. C.; Mulcahy, M. F. R.; Tyler, R. J. Symp. (Int.) Combust. [Proc.] 1982, 19th, 1131-1138. (21) Morris, J. P.; Keairns, D. L. Fuel, 1979, 58, 465. (22) Tyler, R. J. Fuel, 1980, 59, 218-226. (23) Suuberg, E. M.; Scelza, S. T. Fuel, 1982, 61, 198-199. (24) Loison, R.; Chauvin, F. Chem. Ind. (London) 1964, 91, 269-275.
substantial controversy surrounds the wide variation (factors of 1000) in reported kinetic rates for both overall weight loss and species evolution. The reasons for the wide variations have been examined in several recent publications,8"10·30·31 and it appears that there are several causes *
To whom correspondence is to be addressed.
0887-0624/87/2501-0138$01.50/0
©
1987 American Chemical Society
Kinetics of Coal Pyrolysis some case to overestimation (by hundreds of degrees), of the particle temperatures. The reported rates are, therefore, reactor dependent because of the reactor dependence of the heat-transfer rates. A second cause has been that modeling of data using single first-order rate expressions has sometimes produced very low activation energies to account for what is really a spread in activation energies. In this case, the reported rates are heating-rate dependent. Extrapolation away from the conditions under which the experiments were done can lead to orders-ofmagnitude errors in the predicted rates. Finally, temperature gradients of hundreds of degrees within the
led in
particles for several high-heating-rate (106 K/s) experiments can produce low rates at high apparent measured particle temperatures. To address the need for describing species evolution in pyrolysis, previous work in this laboratory has been devoted to obtaining pyrolysis data for individual species and developing a general pyrolysis model.5"10,30 A number of reactors and coals have been used to obtain data over a wide range of heating rates, heat-transfer regimes, temperatures, and coal types. The studies have led to the interesting observation that when all conditions are held constant except for coal type, the kinetics for the evolution of individual pyrolysis species are relatively insensitive (varying by perhaps a factor of 5) to coal rank. This observation led to the development of a functional group (FG) model for coal thermal decomposition, which uses rank-independent kinetics and a distributed activation energy kinetic model to describe individual species evolution. The general framework of the model has been very successful in providing accurate descriptions of species evolution under a wide variety of conditions. However, the rates have changed from the original model, due to improved knowledge of coal particle temperatures during pyrolysis and the use of distributed rates for the kinetics. The aim of this study was to use data from three types of reactors, each with different heating characteristics, to develop a more accurate reactor-independent, heatingrate-independent, and coal-independent set of kinetic parameters. Toward this end, several steps were taken to obtain better measurements of the pyrolysis rates and heat-transfer rates for coal. A new technique was developed for measuring the temperature and emissivity of small coal particles by using Fourier-transform infrared (FT-IR) emission and transmission (E/T) spectroscotechnique was applied to determine particle py _io,32-36 (25) Shapatina, E. A.; Kalyuzhnyi, V. V.; Chukhanov, Z. F. “Technological Utilization of Fuel for Energy, 1. Thermal treatment of Fuels”. Reviewed by: Badzioch, S. Br. Coal Util. Res. Assoc., Mon. Bull. 1961, 25, 285. (26) Friedman, L. D.; Rau, E.; Eddinger, R. T. Fuel, 1968, 47, 149. (27) Peters, W.; Bertling, H. Fuel 1965, 44, 317-331. (28) Freihaut, J. D.; Seery, D. J. Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem. 1983, 28(4), 265-277. (29) Freihaut, J. D.; Solomon, P. R.; Seery, D. J. Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem. 1980, 25(4), 161-170. (30) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M. “Coal
Pyrolysis”, presented at the AIChE Symposium on Coal Pyrolysis, New Orleans, LA, Nov 1981. (31) Solomon, P. R.; Serio, . A. “Evaluation of Coal Pyrolysis Kinetics”, presented at NATO Workshop on Fundamentals of Physical Chemistry of Pulverized Combustion, Les Arcs, France, July 28-Aug 1, 1986. (32) Solomon, P. R.; Best, P. E.; Carangelo, R. M.; Chien, P.; Santoro, R. M.; Semerjian, H. G. “FT-IR Emission/Transmission Spectroscopy for
In-Situ Combustion Diagnostics”; presented at the 21st Symposium (International) on Combustion, Munich, Federal Republic of Germany, 1986. (33) Best, P. E.; Carangelo, R. M.; Solomon, P. R. Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem. 1984, 29(6), 249-258. (34) Solomon, P. R.; Carangelo, R. M.; Best, P. E.; Markham, J. R.; Hamblen, D. G. Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem. 1986, 31(1), 141-151.
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temperatures in an entrained-flow reactor (EFR) in which extensive pyrolysis data had been obtained at temperatures up to 1600 °C and heating rates of approximately 10 000 K/s.6"9 A new heated tube reactor (HTR) was designed to provide a geometry that simplified the prediction of particle temperatures (up to 950 °C) with heating rates of 20000 K/s and also permitted direct measurement of particle temperature and velocity by FT-IR.10 Finally, a new thermogravimetric and evolved-gas analyzer (TGA/ EGA) was employed to provide data for the evolution of pyrolysis species at low heating rates (0.5 K/s) up to 900 °C.37
In addition to improvements to the experiments, improvements were also made to the FG model.38 Two models have been added to describe (a) the cracking of hydrocarbon species released in primary pyrolysis and (b) the equilibration of oxygen-, hydrogen-, and carbon-containing species at high temperatures. The paper presents comparisons of new data obtained in the three reactors with the predictions of the improved FG model. In addition, some previously published data from the EFR9 are also compared (using new particle time-temperature histories validated with the FT-IR particle temperature measurements) with predictions of the improved model.
Experimental Section Coals Examined. Experiments were performed with North Dakota (Zap) lignite, Gillette and Montana Rosebud subbituminous coals, and Pittsburgh No. 8, Kentucky No. 9, and Illinois No. 6 bituminous coals. Each was sieved to produce a -200, +325 or -200, +270 mesh size cut. The elemental analysis of each coal is given in Table I. Apparatus. Heated Tube Reactor (HTR). The HTR has been described previously.10 It consists of a 5.08 mm i.d. Inconel 702 tube, which is heated electrically. Coal entrained in cold carrier gas is injected into the heated section of the tube. After a variable residence time, the reacting stream is either ejected from the tube end for FT-IR temperature and velocity measurements10,32"36 or is quenched in a water-cooled section of tube for species evolution measurements. After cooling, the sample stream passes through a cyclone to separate char particles ( or
Or PARTICLE POSITION
(f),
cm
10. Pyrolysis product distribution as weight percent of daf coal for Beulah North Dakota lignite in the EFR in nitrogen at a furnace temperature of 800 °C. The injector or particle position corresponds to the distance above the optical window. The solid lines in parts a-e are predictions of the functional group model. The solid lines in part f are predictions of the timetemperature-position model. The dashed line in part a is weight loss prediction of a single, first-order model with k = 4.28 X 1014 exp(-54570/R71) s"1 and % VM (daf) = 40.
Figure
perature levels (1100, 1300, and 1600 °C) in Figure 11 (supplementary material) and Figures 12-15. The calculated temperature-position histories for each injector height are given in the last plot (f) of Figures 10-15. The time-position histories for Figures 10-15 are similar to that indicated in Figure 4c. In general, the agreement of the model with char, tar, and individual gas yields is good over the range of temperatures (800-1600 °C) and coal ranks (lignite-bituminous) examined. The tar yields appear to be overpredicted in some of the earlier EFR experiments (Figures 10b, lib, 13b, and 15b) due to incomplete collection of tar. If the amount of missing material is added to the tar, the agreement is fairly good.9 For the 1600 °C experiments in the EFR (Figures 12a and 14a), the “tar” collected is primarily soot. The predictions of the model for individual species evolution are best at low temperatures (1300 °C. The volatiles are assumed to be at the same temperature. Another feature of the FG model is the ability to predict the elemental and functional group composition of the char. If measurements are available, these predictions provide an additional check on the validity of the assumed functional group composition for a given coal, as well as the kinetic rates for product evolution. An example of this type of comparison is shown in Figure 17 for 1100 °C EFR data from Pittsburgh Seam bituminous (parts a, d), Gillette subbituminous (parts b, e), and North Dakota Lignite (parts c, f) coals. In general, the predictions for the elemental compositions are quite good; the largest disagreements in the elemental compositions are for the nitrogen, which is likely due to the error in measuring the relatively small amounts (~1 wt % daf) of this element. The oxygen predictions are off on occasion, which is probably a result of the fact that these data are obtained by difference and
CHAR
0 GAS
HO
20 0
are
INJECTOR POSITION
,
cm
Pyrolysis product distribution as weight percent of daf coal for Beulah North Dakota lignite (a-c) Etnd Kentucky No. 9 bituminous coal (d-f) in the EFR in nitrogen at a furnace Temperature of 1600 °C. The injector position corresponds to the distance above the optical window. The solid lines are predictions of the functional group model with the assumption of chemical equilibrium (see text).
Figure
16.
are not corrected for sulfur. The predictions of the hydrogen functional group compositions are only fair, except for the aliphatic hydrogen. This may indicate that the FG
Kinetics of Coal Pyrolysis
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17. Elemental (a-c) and hydrogen
functional group (d-f) compositions as weight percent of daf coal for pyrolysis in the EFR in nitrogen with a furnace temperature of 1100 °C: (a, d) Pittsburgh seam coal; (b, e) Gillette subbituminous coal; (c, f) Beulah North Dakota lignite.
Figure
TEMPERATURE
(°C)
for North Dakota lignite, 200 X 325 Figure mesh, heated at 30 K/min (0.5 K/s) in the TGA/EGA. Upper solid lines are cumulative evolution data and predictions of the functional group model for cumulative product evolution. The theory lines are indicated with arrows. The dashed line, in part a is weight loss prediction of a single, first-order model with k = 4.28 X 1014 exp(-54570/RT) s"1 and % VM (daf) = 40. The lower solid lines Me mass evolution rate data (arbitrary scale). After the coal reaches 900 °C, it is quenched to 700 °C and the char is oxidized for elemental analysis. 18. Pyrolysis results
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0
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Figure 20. Pyrolysis results for Pittsburgh seam bituminous coal, 200 X 325 mesh, heated at 30 K/min (0.5 K/s) in the TGA/EGA.
Upper solid lines Eire cumulative evolution data. Dotted lines are predictions of the functional group model for cumulative product evolution. The theory lines are indicated with arrows. The dashed line in part a is weight loss prediction of a single, first-order model with k = 4.28 X 1014 exp(-54570/RT) s*1 and % VM (daf) = 50. The lower solid lines Me mass evolution rate data (Mbitrary scale). After the coeQ reaches 900 °C, it is quenched to 700 °C and the char is oxidized for elemental analysis.
model does not properly treat the creation of methyl groups and aromatic hydrogens (see assumption e in the discussion of FG model assumptions). Comparisons for the TGA/EGA. Data for major species evolution from pyrolysis of North Dakota lignite at 30 K/min (0.5 K/s) in the TGA/EGA are presented in Figure 18 along with predictions of the FG model. Results for Montana Rosebud subbituminous and Pittsburgh seam bituminous coal are presented in Figure 19 (supplementary material) and Figure 20, respectively. The agreement is generally good for the Zap and Rosebud coals except for H20, where the data is partially obscured by desorption of water from the apparatus. The predicted yields of CO and C02 are somewhat different than observed for the Zap lignite at higher heating rates, although the kinetic rates are satisfactory. Some significant deviations in individual gas yields are observed for the Pittsburgh seam coal, and the rate for tar evolution is a little high. The different yields of oxygenated species for the lignite and subbituminous coal in the TGA/EGA when compared to the HTR and EFR may be due to cross-linking reactions that eliminate C02, CO, and/or H20 and are affected by heating rate due to their competitive nature. For the bituminous coal, differences in oxygenated and hydrocarbon gas yields may be related to tar repolymerization reactions. In this case, the tar yield in the TGA/EGA is less than half of the value in the EFR. Discussion of Model Accuracy. In general, the agreement of FG model and the data is quite good for all the pyrolysis products at temperatures below 1000 °C, except for the Pittsburgh seam bituminous coal under slow heating (30 K/min) conditions. As the temperature in-
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vation energies, the rates plotted in Figure 21 represent the mean of the distribution. We have limited the temperature range in which the rates have been plotted to that in which our data have been taken. This plot illustrates what is apparent from the previously presented data, i.e. that in a given reactor the coal weight loss is the result of rapid evolution of tar, hydrocarbons, and loosely bound oxygenated species and slower evolution of tightly bound oxygenated species, hydrogen, HCN, and other minor species. The primary pyrolysis rate is determined by the evolution of the first group of species. For comparison with other investigations, the primary pyrolysis weight loss was modeled by assuming a single first-order process using a rate constant k = 4.28 X 1014 exp(-54570/RT) s'1.10 This is half of the tar rate expression and represents a compromise between the rapid evolution of tar (along with some of the loosely bound oxygenated species) and slower evolution of most of the gases. This simple model (shown as a dashed line) is compared with the char yield in Figures 5a-20a and is in good agreement with the average primary pyrolysis rate in each case. As discussed above, the yields are off when significant secondary pyrolysis of the char occurs. This rate expression gives primary pyrolysis rates that are among the highest reported.10
Conclusions
Reciprocal Absolute Temperature (103) (K'·*·)
Figure
21. Comparison
of
mean
kinetic rates for individual
species evolution from Table II. The primary pyrolysis rates are indicated with heavier lines. The dashed line is a single, first-order rate with k = 4.28 X 10u exp(-54570/fiT) s'1. creases above 1000 °C, secondary reactions, including soot formation and gasification, begin to play an important role. This leads to overprediction of olefins, CH4, H20, C02, and tar and underprediction of CO, H2, and benzene (not shown). While the hydrocarbon cracking model accurately predicts the high-temperature behavior of paraffins, additional improvements will be required for the pyrolysis of olefins, acetylene, and ethylene to form benzene and further pyrolysis to form soot. Also, while the equilibrium model is satisfactory at high temperatures (>1300 °C) and long residence times, kinetic rates for gasification and secondary gas phase reactions will be required to improve the model predictions in the intermediate temperature range (1000-1300 °C). There are also variations in the quality of the model predictions with heating rate and coal type. In general, the results for the high-heating-rate experiments (HTR, EFR) are better than for the low-heating-rate experiment (TGA/EGA). These differences are probably due to competitive reactions involving oxygenated species or tar repolymerization reactions and will require a more sophisticated description of the coal pyrolysis chemistry than is currently in the FG model. The kinetic rates are more optimal for low-rank coals. Better accuracy could be achieved by adding rank-specific rates, but the model would lose some of its generality. Evaluation of Kinetic Rates. A plot has been made of the kinetic rates used in the functional group model in Figure 21. Since the kinetic expressions (given in Table II) are formulated with a Gaussian distribution of acti-
(1) The evolution of individual primary pyrolysis species in three different reactors was successfully predicted by a functional group model using validated particle temperature models and rate parameters that varied from species to species but were insensitive to the reactor, the temperature (from 350 to 1600 °C), the heating rate (from 0.5 to 20000 °C/s) and the coal rank (six coals of different rank). While the FG model does contain a large number of parameters, it is constrained by making predictions for a large number of species over a wide range of experimental conditions and coal rank. The model was also able to make good predictions of the char elemental and hydrogen functional group compositions. (2) Many of the secondary reactions were predicted by using a hydrocarbon cracking model to describe the cracking of paraffins and olefins to form light gas species and an equilibrium model to describe the reactions of oxygen-, hydrogen-, and carbon-containing species at high temperature. (3) The results for weight loss during primary pyrolysis are in reasonable agreement with predictions of a single first-order model for primary pyrolysis weight loss which uses a rate constant k = 4.28 X 1014 exp (-54570/RT) s'1. This indicates that the rate of primary pyrolysis is much higher at elevated temperatures (>700 °C) than predicted by commonly used rate expressions.
Acknowledgment. The authors gratefully acknowledge financial support of this work by the United States Department of Energy, Morgantown Energy Technology Center, under Contract Nos. DE-AC21-81FE05122, DEAC21-84MC21004, and DE-AC21-85MC22050.
Supplementary Material Available: Figure 8, pyrolysis results for North Dakota (Zap) lignite, 200 X 325 mesh, in the HTR at an equilibrium tube temperature of 700 °C; Figure 9, pyrolysis results for North Dakota (Zap) lignite, 200 x 325 mesh, in the HTR at an equilibrium tube temperature of 935 °C; Figure 11, pyrolysis product distribution as weight percent of daf coal for Beulah North Dakota lignite in the EFR in nitrogen at a furnace temperature of 1300 °C; Figure 19, pyrolysis results for Montana Rosebud coal, 200 X 325 mesh, heated at 30 K/min (0.5 K/s) in the TGA/EGA (4 pages). Ordering information is given on any current masthead page.
