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An experimental study of the release of nitrogen from coals pyrolyzed in fluidized-bed reactors
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 3205–3211
AN EXPERIMENTAL STUDY OF THE RELEASE OF NITROGEN FROM COALS PYROLYZED IN FLUIDIZED-BED REACTORS C. -Z. LI* and P. F. NELSON CSIRO Division of Coal and Energy Technology North Ryde, Australia 2113 E. B. LEDESMA and J. C. MACKIE School of Chemistry, University of Sydney Australia 2006
Nitrogen release has been measured from a suite of Australian, German, and U.S. coals pyrolyzed under rapid heating conditions in two bench scale reactors. The first reactor was a single-stage fluidized bed reactor; the second was a two-stage reactor in which a fluidized bed was coupled with a tubular reactor. The two-stage reactor enabled cracking reactions of the volatiles to be studied in isolation from reactions of the char. The results show that the gas-phase N-containing species, HCN and NH3, are formed from both brown and bituminous coals. In addition, HNCO is also observed from bituminous coals; HNCO is a plausible precursor for NH3 formation. At high temperatures, low-volatile coals give lower yields of HCN and NH3 than high-volatile coals. This observation, however, does not imply that tar cracking is the only source of HCN and NH3. In fact, the results obtained in the two-stage reactor suggest that a significant proportion of the HCN is released from the char and that tar cracking is not a significant source of NH3. A major part of the NH3 is produced by interactions of N-containing species with the char. These results, together with previous studies of N release, demonstrate that there is no simple relationship between N functionality in coals and the composition and rate of release of the nitrogen.
Introduction The formation of NOx from the combustion of coal is due predominantly to oxidation of the nitrogen contained in the heterocyclic structures of the coal. During the initial pyrolytic decomposition, coal N is released as volatile N species, such as HCN and NH3, as well as nitrogen-containing compounds in the tars, and these species are oxidized to NO or reduced to N2. The N remaining in the char is similarly converted to NO or N2 by processes that are still poorly understood. Improvement in our understanding of the way in which coal N is released and the role that N functionality plays in determining the composition and relative amounts of volatile and char N is necessary in attempts to improve predictive techniques for NOx formation. Recently, there have been some significant advances in our understanding of the chemistry of coal nitrogen during pyrolysis and combustion. Firstly, the N functionality has been extensively studied using X-ray photoelectron spectroscopy (XPS) [1–5] and X-ray absorption near edge spectroscopy (XANES) [6]. Although a consensus has not emerged *Current address: Department of Chemical Engineering, Monash University, Clayton, 3168, Australia.
from these studies as to the details of the functionality of nitrogen in coals, it is generally accepted that the nitrogen is overwhelmingly present in heterocyclic structures and that those structures of pyrrolic (5-membered) type are more common than those of pyridinic (6-member) type. Some of the problems in interpretation of the spectra arise from low signalto-noise ratio spectra and from the dependence of the results on sample preparation [1]. Secondly, the release of nitrogen during the pyrolysis of coals has been studied over a very wide range of conditions [1,7–10]. Nitrogen is released as nitrogen containing species in tar and also as HCN and NH3. The relative amounts of HCN and NH3 vary and appear to depend on, inter alia, reactor temperature, type, and material. The formation of NH3 has generated considerable debate because studies of pyrolytic decomposition of model compounds such as pyrrole and pyridine [11,12] release N as HCN and not as NH3. Bassilakis et al.[10] measured N species from the pyrolysis of the same set of Argonne premium coal samples in a TG-FTIR reactor and in an entrained-flow reactor. The ratios of NH3/HCN were much higher in the TG-FTIR reactor, which has much lower heating rates than the entrained flow reactor. It was postulated that HCN is the primary product and that NH3 is formed by
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COAL AND CHAR COMBUSTION TABLE 1. Properties of coal samples used
Coal name or code Laubag Fortuna Coal A Coal B Coal C Prosper Gottelborn Ensdorf Coal D Coal E Coal F Coag G Coal H Pocahontas No. 3
Country of origin Germany Germany Australia Australia Australia Germany Germany Germany Australia Australia Australia Australia Australia USA
wt%, daf
Ash wt%, dry
WMa
C
H
N
S
Ob
6.5 12.9 2.1 15.6 8.0 8.3 7.1 7.6 7.8 19.6 13.5 9.0 15.6 7.7
54.4 43.2 53.0 50.8 33.4 34.8 39.2 37.9 40.8 22.4 27.8 21.4 17.8 18.5
67.4 67.3 67.4 77.9 79.8 79.9 82.3 82.4 83.2 86.5 86.8 87.8 88.4 90.0
5.0 5.4 4.6 6.3 4.6 6.0 5.6 5.7 5.7 4.5 4.9 4.5 4.6 4.5
0.7 0.8 0.6 1.2 2.0 1.3 1.7 1.8 1.8 1.6 1.6 1.8 1.6 1.1
0.9 0.6 0.3 0.7 0.3 1.3 1.0 0.8 1.2 1.6 0.4 0.4 0.6 0.9
26.0 26.0 27.1 13.9 13.3 11.5 9.4 9.3 8.2 5.8 6.3 5.5 4.8 3.5
avolatile bby
matter yield difference
intraparticle transformations of the HCN. We have also recently found [13] that NH3 can react with many types of material commonly used for the construction of bench scale reactors at elevated temperatures (.7008C), complicating the quantification of NH3 from the pyrolysis of coal. In spite of these advances, significant problems remain in predicting the influence of coal N on NOx formation over the very wide range of combustion conditions employed in coal combustors. There does not appear to be any simple relationship between N functionality and NOx production, in part because the variations in functionality do not appear to be great. A NOx predictor index however, has recently been developed on the basis of functionality [14]. Formation of NOx from coal combustion strongly depends on the rate of N release. In addition, the effectiveness of some control techniques such as staged air combustion depend critically on a rapid release rate of the N since this must occur in a fuelrich region where the formation of N2 is favored. Nitrogen incorporated in the char, in the tar, and in simple gas-phase species will likely have very different fates during subsequent combustion. Since nitrogen can be released from coal either during primary pyrolysis or during the further cracking of tars and chars, knowledge of the relative contributions of these two routes to the overall nitrogen release is very important. At present, there is no simple way to predict the composition and release rate of the N species, although significant advances have been made [15]. The purpose of the present study was to investi-
gate the effects of pyrolysis conditions and coal characteristics on nitrogen distributions and release during coal pyrolysis. Experiments were carried out in two bench scale fluidized bed reactors. The configuration of the two-stage reactor allowed the thermal cracking of volatiles to take place in isolation from that of chars. The effects of coal rank and nitrogen functionality have been examined. Based on the experimental data, factors influencing the nitrogen distribution during coal pyrolysis—for example, nitrogen-containing aromatic ring size, substitution, and availability of donatable hydrogen—are discussed. Experimental Coal Samples A suite of Australian, German, and U.S. coals was studied, ranging in rank from brown to low-volatile bituminous coals. Table 1 gives detailed analyses for these coals. Coal Pyrolysis The particle sizes of the coal samples used for pyrolysis experiments were between 75 and 106 lm. The coal samples were dried under vacuum at ambient temperature for at least 24 h before pyrolysis experiments. Pyrolysis experiments were carried out in two bench scale quartz fluidized bed reactors in N2. The first reactor is similar to that described by Tyler [16]. Coal particles entrained in N2 from a fluidized bed feeder were injected through a N2-cooled stainless steel probe into the heated fluidized bed of zircon sand (106–150 lm) and heated at rates
RELEASE OF N FROM COALS
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to temperatures of 600–10008C. The transfer line between the two stages was heated electrically to minimize the deposition of tars. Additional N2 gas was added between the two stages to adjust the residence time of volatiles in the second stage. These residence times depended on temperature and were in the range of 0.7–1.0 s. This reactor is denoted as the two-stage reactor in the present study. Product Characterization
Fig. 1. HCN, NH3 and HNCO yields from the pyrolysis of Fortuna brown coal (v) and Go¨ttelborn bituminous coal (M) in the single-stage reactor as a function of temperature.
exceeding 104K s11. This reactor featured a relatively long (at least 13 cm) freeboard zone heated to similar temperature levels as the fluidized bed of zircon sand, where the nascent tars (and chars) are further thermally cracked. This reactor is denoted as the single-stage reactor in the present study. Gas residence times were 0.3–0.5 s. Particle residence times varied from as short as the gas residence time to much longer times, depending on the tendency of the coal to soften and adhere to the bed material. The fluid bed reactor is a useful device for investigating the detailed composition of the products of the initial stages of coal pyrolysis and of the subsequent thermal cracking of the volatiles. These processes are of great importance in the ultimate combustion of the coal. However, while the heating rate is similar to that found in coal combustors, the temperatures achieved are somewhat less than those pertaining to a full-scale furnace. As a consequence, the timescale for the processes are relatively longer in the fluid bed reactor than they would be in a practical system, where conversion of the volatiles may be complete in 0.1 s. In the second reactor, a fluidized bed reactor essentially identical to the single-stage reactor was coupled to an empty second-stage tubular reactor. The volatiles (tar plus gases) from the pyrolysis of coal in the first stage were transported to the second stage. Char particles were removed by particle settling between the two stages. The first stage was heated to 6008C to generate volatiles. The thermal cracking of volatiles at this temperature is believed to be minimal, especially for the release of nitrogen-containing species [1]. The second stage was heated separately
During a pyrolysis experiment, tar was collected with a liquid-nitrogen–cooled trap fitted with a thimble. The tar sample was recovered by extracting the trap with dichloromethane, as described previously [1]. The tar samples thus recovered and the char samples collected in the thimbles were submitted for nitrogen analysis. Quantification of HCN, HNCO, and NH3 was carried out using Fourier transform infrared (FTIR) spectrometry. Gas-phase products were collected in a multipass FTIR gas cell (7.2 m total path length). Spectra were recorded and concentrations of HCN and NH3 determined as previously described [1,8]. HNCO was identified by its absorbance in the region from about 2300 to 2200 cm11 after the absorbance of 13CO2 in the region had been subtracted [17]. Calibration of HCN and NH3 was carried out by diluting respective standard calibration gases with N2. The calibration of HNCO was taken from the literature [18] by assuming a linear increase in absorbance with light pass length and concentration. Results Figure 1 shows HCN, NH3, and HNCO yields from the pyrolysis of a brown coal and a bituminous coal in the single-stage fluidized bed reactor. These are typical results chosen from the pyrolysis of a suite of coals. As previously reported for pyrolysis in a fluidized bed reactor [1], a maximum is observed in the NH3 yield at a temperature of about 8008C. This decrease has been accounted for [19] by assuming that HCN is the primary N-containing product released from coals and that NH3 is produced from the HCN by hydrogenation reactions within the coal particle. The decrease at high temperatures is a consequence of the reduced time spent by the HCN within the particle. However, we have observed recently [13] that NH3 can be decomposed by quartz, zircon sand, and stainless steel at temperatures above 7008C. Hydrogen cyanide yields from the coals studied (ranging from 67 to 90% C, daf basis) were observed to increase with increasing temperature, and the rate of this increase was very rapid at temperatures above 9008C. Isocyanic acid (HNCO) yields from the pyrolysis
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COAL AND CHAR COMBUSTION
Fig. 2. HCN yields from the pyrolysis of coals at 10008C in the one-stage reactor as a function of coal rank. (v), Australian coals; (¶), German coals; (m), U.S. coal.
Fig. 3. Cumulative nitrogen distribution during the pyrolysis of Fortuna brown coal in the single-stage reactor as a function of temperature.
of bituminous coals were observed to maximize at about 8008C. At temperatures above 8008C, HNCO is thermally unstable and will decompose to NCO and H. No detectable HNCO yields were observed from the pyrolysis of Pocahontas No. 3, a low-volatile bituminous coal with carbon content of about 90 wt. % daf. Detection of HNCO from the pyrolysis of brown coals was more difficult because of the very
Fig. 4. Cumulative nitrogen distribution during the pyrolysis of Go¨ttelborn bituminous coal in the single-stage reactor as a function of temperature.
large amounts of CO2 produced during the pyrolysis of these low rank coals; however, it seems that the amounts produced were considerably less than those produced from the bituminous coals. Data in Fig. 1 also suggest that the NH3/HCN yield ratio at temperatures ,8008C is higher from the pyrolysis of the brown coal than from the bituminous coal. Pyrolysis of other coals in the present study largely confirmed this finding. No NH3 was detectable in the product gases from the pyrolysis of Pocahontas No. 3 coal under the same conditions. Figure 2 summarizes the HCN yields at 10008C as a function of coal rank (carbon content). It is clear that the nitrogen distribution during coal pyrolysis is coal rank dependent. Low-volatile coals gave much lower HCN (and NH3) yields than the high-volatile coals. Figure 3 shows the nitrogen distribution in char, tar, and gas phase from the pyrolysis of Fortuna brown coal. Because of the observed [13] NH3 decomposition reactions on the reactor materials, NH3 yields at temperatures higher than 8008C were assumed to be the same as that at 8008C. It is noted that even at 10008C, the majority of coal nitrogen (.55%) remained in the chars. The proportion of coal nitrogen in char decreased monotonically with increasing temperature. Nitrogen in tar comprised only a small part of the coal nitrogen. There was a significant deficit in the nitrogen balance that amounted to some 17%. Possible reasons for this deficit are discussed later in this paper. Data in Fig. 4 for the Go¨ttelborn bituminous coal show similar trends to those in Fig. 3. The caking properties of the bituminous coals have resulted in
RELEASE OF N FROM COALS
Fig. 5. Comparison of HCN, NH3, and HNCO yields from the pyrolysis of an Australian bituminous coal (coal D) in the single-stage (v) and two-stage (M) reactors as a function of temperature.
the agglomeration of coal/char particles with the zircon sand during pyrolysis. Only a small portion of char was collected in the thimble and the majority of char was retained and heated in the bed until the termination of the experiment. The time-temperature history of this part of char was unknown. Analysis of the char collected in the thimble was therefore deemed to be unrepresentative of the whole char. Comparison of the data in Figs. 3 and 4 suggests that the proportion of coal nitrogen in tars increased considerably from the brown coal to the bituminous coal. This finding was further confirmed with other coals of similar ranks. Data in Figs. 3 and 4 also show that the nitrogen in tars would not be enough to account for the increase in HCN and NH3 yields at higher temperatures. Even in the case of the bituminous coals (e.g., Go¨ttelborn in Fig. 4), the maximum amount of nitrogen in tars accounts for only about 17% of the coal nitrogen, whereas nitrogen in HCN and NH3 at 10008C is more than 25% of coal nitrogen. Considering that the HCN yields still tend to increase with temperature beyond 10508C (the upper limit used in the present study), it is fair to conclude that part of HCN and NH3 must have come from the thermal decomposition of chars. Further experiments were then carried out in the two-stage reactor to confirm and quantify experimentally the contributions of the thermal decomposition of tars and chars to the observed HCN and NH3 yields. Figure 5 compares the HCN, NH3, and HNCO yields from the pyrolysis of an Australian bituminous coal (coal D) in the single-stage and two-stage re-
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actors. The data clearly show that the thermal cracking of volatiles (mainly tars) alone in the second stage did not produce the same amount of HCN as the pyrolysis of coal in the single-stage reactor at the same temperatures. The production of HCN from the thermal decomposition of tar was negligible at temperatures lower than 7508C. The difference in HCN yield from the pyrolysis in the two reactors seems to increase with increasing temperature: about 8% more of coal nitrogen was released as HCN in the one-stage reactor than in the two-stage reactor at 10008C. It is fair to conclude that char thermal decomposition is an important route for HCN formation. The production of NH3 from the thermal decomposition of tars in the second stage was observed to be not more than 2.5% of coal nitrogen. In fact, there is very little change in the NH3 yield in the second stage over that which is produced in the first stage. Hence, it appears that the NH3 observed during pyrolysis in the single-stage reactor derives mainly from primary coal pyrolysis and secondary decomposition of the chars. No significant difference in the HNCO yields from the pyrolysis experiments in the two reactors was observed. This indicated that the majority of HNCO was formed from the thermal decomposition of tars.
Discussion As discussed in the introduction, the rate and overall amounts of coal N released to the gas phase are very important in NOx formation. The results in Fig. 2 clearly show that the yields of HCN are a function of coal rank and decrease significantly for coals of higher rank (and of low volatility). At first glance, this would seem to be related directly to the lower volatile yields from these coals, and, as a consequence, to imply that thermal cracking reactions of the tars are the source of the volatile N species. Close consideration of the data presented, however, suggests that this picture is too simplistic. While it seems clear that tar cracking is an important source of HCN based on the results obtained with the two-stage reactor, it is also clear that it is not the only source. There is simply insufficient N present in the tars to account for the HCN produced. In fact, the data in Fig. 5 suggest that thermal cracking of tars at temperatures lower than 7008C produced negligible amounts of HCN and NH3. The production of HCN from tar alone (Fig. 5) became significant only at temperatures higher than 8008C. This is in agreement with the data shown in Figs. 3 and 4, where nitrogen distributions in tars remained almost unchanged up to this temperature. Experiments to be reported elsewhere, however, suggested that, as the temperature was raised to 9008C,
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COAL AND CHAR COMBUSTION
DCM-soluble tar yields decreased to one-fourth or one-half of their maxima for Fortuna and Go¨ttelborn, respectively. Taken together, these findings would appear to indicate that the thermal cracking reactions of tars at lower temperatures (e.g., ,8008C) to gases or soot did not result in the release of tar nitrogen and that the nitrogen in tar in fact was very stable up to about 8508C. This is probably a consequence of the nature of the cracking reactions at these low temperatures, which tend to decompose the more labile aliphatic groups and do not result in decomposition of the aromatic or heterocyclic structures. Hence, primary coal pyrolysis and/ or further decomposition of chars are important sources of HCN and NH3 formation, even at relatively low temperatures (e.g., ,8008C). If the lower-volatile yield is not solely responsible for the decrease in N release as a function of increasing coal rank (Fig. 2), other factors need to be considered. It seems likely that the size (extent of condensation of rings) of the nitrogen-containing heteroaromatic ring systems is an important factor governing the nitrogen release during pyrolysis. It is generally agreed that the sizes of aromatic ring systems in coal tend to increase with increasing rank. It has been shown [20] that aromatic (also including heteroaromatic) ring system distributions ultimately govern the thermal decomposition of the coal macromolecular network. The coordination number was shown to increase as the size of the aromatic ring system increases, making the macromolecular network more difficult to decompose [20]. Moreover, the increases in the size of aromatic or heteroaromatic ring systems would tend to stabilize the ring systems. The nitrogen in higher rank coals is more likely to be in larger heteroaromatic ring systems, making the ring systems difficult to be released from the coal and also difficult to be ruptured even at high temperatures (Fig. 2). Even though the nitrogen might be in pyrolic form, if the ring system containing the nitrogen is large, the ring system would be likely to have a high coordination number and therefore difficult to be released from the network. The large size of the ring system also implies that the ring system is stable, possibly making it more difficult to be ruptured than a smaller ring system containing pyridinic nitrogen. The HCN and NH3 formed during pyrolysis at lower temperatures (,7008C) are thought to have their origin in the small nitrogen-containing ring systems in coal. These smaller ring systems are relatively easily ruptured compared to the corresponding larger ones. The data in Fig. 1 showed that the HCN production covered a wide range of temperature, and it would be expected that more HCN production is likely at temperatures higher than the upper limit in the present study. Previous experimental data [2,21] have shown that HCN production continued at tem-
peratures higher than 12008C until the total release of coal nitrogen was observed. The observation that the production of gas-phase nitrogen-containing species (e.g., HCN) covers an extremely wide range of temperature is thought to reflect the fact that the nitrogen in coal is distributed in ring systems of a wide range of sizes. Ammonia formation has created considerable discussion in studies of the nitrogen distribution during coal pyrolysis. Since nitrogen exists in coal as heteroaromatics, the formation of NH3 requires the hydrogenation of nitrogen. Model compound studies [11,12] of the pyrolysis of N-containing aromatics do not result in NH3 formation. Various mechanisms have been proposed for the formation of ammonia. For example, Baumann and Mo¨ller [7] have suggested gas-phase HCN hydrogenation to form NH3. Similarly, it has been suggested [10,19] that HCN may be hydrogenated on the coal/char surface to form NH3. The observation of HNCO in the pyrolysis products is also interesting in the context of NH3 formation from the pyrolysis of coals. Isocyanic acid readily hydrolyzes [18] on alumina surfaces to yield NH3 and CO2 and such surface catalyzed reactions within the coal particle are possible during coal pyrolysis. The failure to detect HNCO from brown coals is consistent with this mechanism since these coals have much higher moisture contents. The dependence of ammonia yields on reactor types [10,19] and particle sizes [19] supports the contention that NH3 is largely a secondary product of the decomposition of coals and arises from interactions of other N-containing precursors with the coal surface. Given the weight of the experimental evidence supporting this conclusion, it is surprising that some attempts [14] have been made to link NH3 formation with specific functional groups in the coal and to derive a predictive NOx formation index based on the functional forms of N in coals. Based on the results of the present study and of previous work, we would contend that nitrogen-containing species produced during coal pyrolysis are very reactive and interconversion between them is more than possible. The extent of this interconversion depends to a great extent on reactor configuration and other experimental parameters, and hence any attempts to account for the formation of particular products from the decomposition of specific functional groups is highly problematic. The deficit in the N balance observed at high temperatures with the brown coal (Fig. 3) is possibly due to the formation of N2 gas. Previous work [22] has shown that significant amounts of N2 can be produced from Australian brown coals pyrolyzed in a thermobalance. Conclusions 1. Nitrogen distributions during coal pyrolysis, especially HCN and NH3 yields, were found to
RELEASE OF N FROM COALS
change with coal rank, with the low-volatile coals giving lower HCN and NH3 yields than the highvolatile coals. 2. Tar cracking and coal nitrogen functionality alone were found to be insufficient to predict the nitrogen distribution during pyrolysis. 3. Primary coal pyrolysis and/or further thermal decomposition of chars are major sources of HCN formation, even at temperatures lower than about 8008C. The importance of tar cracking reactions for the nitrogen distribution in the gas phase increases from brown coals to high-volatile bituminous coals. 4. Tar cracking does not appear to be an important source for ammonia formation. Hydrolysis of HNCO within the coal particle is a plausible source for NH3 formation, but reactions of other N-containing precursors are probably also involved. 5. While the formation mechanisms for HNCO remain uncertain, tar cracking reactions seem mainly responsible for the observed HNCO yields. 6. Sizes of the nitrogen-containing heteroaromatic ring systems are another important factor governing the nitrogen distribution during pyrolysis. Nitrogen in coal is believed to exist in heteroaromatic ring systems of a wide range of sizes resulting in the nitrogen release from coal covering an extremely wide range of temperatures. Acknowledgments The award of a Rothmans Foundation Fellowship to one of us (C.-Z. Li) is gratefully acknowledged.
REFERENCES 1. Nelson, P. F., Buckley, A. N., and Kelly, M. D., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1259–1267.
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2. Kambara, S., Takarada, T., Yamamoto, Y., and Kato, K., Energy Fuels 7:1013–1020 (1993). 3. Kelemen, S. R., Gorbaty, M. L., and Kwiatek, P. J., Energy Fuels 8:896–906 (1994). 4. Wo´jtowicz, M. A., Pels, J. R., and Moulijn, J. A. Fuel 74:507–516 (1995). 5. Buckley, A. N., Kelly, M. D., Nelson, P. F., and Riley, K. W., Fuel Processing Technol. 43:47–60 (1995). 6. Mitra-Kirthley, S. M., Mullins, O. C., van Elp, J., and Cramer, S. P., Fuel 72:133–135 (1993). 7. Baumann, H. and Mo¨ller, P., Erdo¨l Erdgas Kohle 44:29–33 (1991). 8. Nelson, P. F., Kelly, M. D., and Wornat, M. J. Fuel 70:403–407 (1991). 9. Chen, J. C. and Niksa, S., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1269–1276. 10. Bassilakis, R., Zhao, Y., Solomon, P. R., and Serio, M. A., Energy Fuels 7:710 (1993). 11. Mackie, J. C., Colket, M. B. III, and Nelson, P. F., J. Phys. Chem. 94:4099–4106 (1990). 12. Mackie, J. C., Colket, M. B. III, Nelson, P. F., and Esler, M., Intl. J. Chem. Kinet. 23:733–760 (1991). 13. Li, C.-Z. and Nelson, P. F., Fuel, in press. 14. Kambara, S., Takarada, T., Toyoshima, M., and Kato, K., Fuel 74:1247–1253 (1995). 15. Niksa, S., Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 537–544. 16. Tyler, R. J., Fuel 58:680–686 (1979). 17. Nelson, P. F., Li, C.-Z., and Ledesma, E., Energy Fuels 10:264 – 265 (1995). 18. Du¨mpelmann, R., Cant, N. W., and Trimm, D. L., Appl. Catal. B: Environ. 6:291 – 296 (1995). 19. Wo´jtowicz, M. A., Zhao, Y., Serio, M. A., Bassilakis, R., Solomon, P. R., and Nelson, P. F., Proc. 8th Intl. Conf. Coal Sci., Vol. I, Elsevier, 1995, pp. 771–774. 20. Li, C.-Z., Wu, F., Xu, B., and Kandiyoti, R., Fuel 74:37–45 (1995). 21. Pohl, J. H. and Sarofim, A. F., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 491–501. 22. Phong-Anant, D., Wibberley, L. J., and Wall, T. F., Combust. Flame 62:21–30 (1985).
AN EXPERIMENTAL STUDY OF THE RELEASE OF NITROGEN FROM COALS PYROLYZED IN FLUIDIZED-BED REACTORS C. -Z. LI* and P. F. NELSON CSIRO Division of Coal and Energy Technology North Ryde, Australia 2113 E. B. LEDESMA and J. C. MACKIE School of Chemistry, University of Sydney Australia 2006
Nitrogen release has been measured from a suite of Australian, German, and U.S. coals pyrolyzed under rapid heating conditions in two bench scale reactors. The first reactor was a single-stage fluidized bed reactor; the second was a two-stage reactor in which a fluidized bed was coupled with a tubular reactor. The two-stage reactor enabled cracking reactions of the volatiles to be studied in isolation from reactions of the char. The results show that the gas-phase N-containing species, HCN and NH3, are formed from both brown and bituminous coals. In addition, HNCO is also observed from bituminous coals; HNCO is a plausible precursor for NH3 formation. At high temperatures, low-volatile coals give lower yields of HCN and NH3 than high-volatile coals. This observation, however, does not imply that tar cracking is the only source of HCN and NH3. In fact, the results obtained in the two-stage reactor suggest that a significant proportion of the HCN is released from the char and that tar cracking is not a significant source of NH3. A major part of the NH3 is produced by interactions of N-containing species with the char. These results, together with previous studies of N release, demonstrate that there is no simple relationship between N functionality in coals and the composition and rate of release of the nitrogen.
Introduction The formation of NOx from the combustion of coal is due predominantly to oxidation of the nitrogen contained in the heterocyclic structures of the coal. During the initial pyrolytic decomposition, coal N is released as volatile N species, such as HCN and NH3, as well as nitrogen-containing compounds in the tars, and these species are oxidized to NO or reduced to N2. The N remaining in the char is similarly converted to NO or N2 by processes that are still poorly understood. Improvement in our understanding of the way in which coal N is released and the role that N functionality plays in determining the composition and relative amounts of volatile and char N is necessary in attempts to improve predictive techniques for NOx formation. Recently, there have been some significant advances in our understanding of the chemistry of coal nitrogen during pyrolysis and combustion. Firstly, the N functionality has been extensively studied using X-ray photoelectron spectroscopy (XPS) [1–5] and X-ray absorption near edge spectroscopy (XANES) [6]. Although a consensus has not emerged *Current address: Department of Chemical Engineering, Monash University, Clayton, 3168, Australia.
from these studies as to the details of the functionality of nitrogen in coals, it is generally accepted that the nitrogen is overwhelmingly present in heterocyclic structures and that those structures of pyrrolic (5-membered) type are more common than those of pyridinic (6-member) type. Some of the problems in interpretation of the spectra arise from low signalto-noise ratio spectra and from the dependence of the results on sample preparation [1]. Secondly, the release of nitrogen during the pyrolysis of coals has been studied over a very wide range of conditions [1,7–10]. Nitrogen is released as nitrogen containing species in tar and also as HCN and NH3. The relative amounts of HCN and NH3 vary and appear to depend on, inter alia, reactor temperature, type, and material. The formation of NH3 has generated considerable debate because studies of pyrolytic decomposition of model compounds such as pyrrole and pyridine [11,12] release N as HCN and not as NH3. Bassilakis et al.[10] measured N species from the pyrolysis of the same set of Argonne premium coal samples in a TG-FTIR reactor and in an entrained-flow reactor. The ratios of NH3/HCN were much higher in the TG-FTIR reactor, which has much lower heating rates than the entrained flow reactor. It was postulated that HCN is the primary product and that NH3 is formed by
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COAL AND CHAR COMBUSTION TABLE 1. Properties of coal samples used
Coal name or code Laubag Fortuna Coal A Coal B Coal C Prosper Gottelborn Ensdorf Coal D Coal E Coal F Coag G Coal H Pocahontas No. 3
Country of origin Germany Germany Australia Australia Australia Germany Germany Germany Australia Australia Australia Australia Australia USA
wt%, daf
Ash wt%, dry
WMa
C
H
N
S
Ob
6.5 12.9 2.1 15.6 8.0 8.3 7.1 7.6 7.8 19.6 13.5 9.0 15.6 7.7
54.4 43.2 53.0 50.8 33.4 34.8 39.2 37.9 40.8 22.4 27.8 21.4 17.8 18.5
67.4 67.3 67.4 77.9 79.8 79.9 82.3 82.4 83.2 86.5 86.8 87.8 88.4 90.0
5.0 5.4 4.6 6.3 4.6 6.0 5.6 5.7 5.7 4.5 4.9 4.5 4.6 4.5
0.7 0.8 0.6 1.2 2.0 1.3 1.7 1.8 1.8 1.6 1.6 1.8 1.6 1.1
0.9 0.6 0.3 0.7 0.3 1.3 1.0 0.8 1.2 1.6 0.4 0.4 0.6 0.9
26.0 26.0 27.1 13.9 13.3 11.5 9.4 9.3 8.2 5.8 6.3 5.5 4.8 3.5
avolatile bby
matter yield difference
intraparticle transformations of the HCN. We have also recently found [13] that NH3 can react with many types of material commonly used for the construction of bench scale reactors at elevated temperatures (.7008C), complicating the quantification of NH3 from the pyrolysis of coal. In spite of these advances, significant problems remain in predicting the influence of coal N on NOx formation over the very wide range of combustion conditions employed in coal combustors. There does not appear to be any simple relationship between N functionality and NOx production, in part because the variations in functionality do not appear to be great. A NOx predictor index however, has recently been developed on the basis of functionality [14]. Formation of NOx from coal combustion strongly depends on the rate of N release. In addition, the effectiveness of some control techniques such as staged air combustion depend critically on a rapid release rate of the N since this must occur in a fuelrich region where the formation of N2 is favored. Nitrogen incorporated in the char, in the tar, and in simple gas-phase species will likely have very different fates during subsequent combustion. Since nitrogen can be released from coal either during primary pyrolysis or during the further cracking of tars and chars, knowledge of the relative contributions of these two routes to the overall nitrogen release is very important. At present, there is no simple way to predict the composition and release rate of the N species, although significant advances have been made [15]. The purpose of the present study was to investi-
gate the effects of pyrolysis conditions and coal characteristics on nitrogen distributions and release during coal pyrolysis. Experiments were carried out in two bench scale fluidized bed reactors. The configuration of the two-stage reactor allowed the thermal cracking of volatiles to take place in isolation from that of chars. The effects of coal rank and nitrogen functionality have been examined. Based on the experimental data, factors influencing the nitrogen distribution during coal pyrolysis—for example, nitrogen-containing aromatic ring size, substitution, and availability of donatable hydrogen—are discussed. Experimental Coal Samples A suite of Australian, German, and U.S. coals was studied, ranging in rank from brown to low-volatile bituminous coals. Table 1 gives detailed analyses for these coals. Coal Pyrolysis The particle sizes of the coal samples used for pyrolysis experiments were between 75 and 106 lm. The coal samples were dried under vacuum at ambient temperature for at least 24 h before pyrolysis experiments. Pyrolysis experiments were carried out in two bench scale quartz fluidized bed reactors in N2. The first reactor is similar to that described by Tyler [16]. Coal particles entrained in N2 from a fluidized bed feeder were injected through a N2-cooled stainless steel probe into the heated fluidized bed of zircon sand (106–150 lm) and heated at rates
RELEASE OF N FROM COALS
3207
to temperatures of 600–10008C. The transfer line between the two stages was heated electrically to minimize the deposition of tars. Additional N2 gas was added between the two stages to adjust the residence time of volatiles in the second stage. These residence times depended on temperature and were in the range of 0.7–1.0 s. This reactor is denoted as the two-stage reactor in the present study. Product Characterization
Fig. 1. HCN, NH3 and HNCO yields from the pyrolysis of Fortuna brown coal (v) and Go¨ttelborn bituminous coal (M) in the single-stage reactor as a function of temperature.
exceeding 104K s11. This reactor featured a relatively long (at least 13 cm) freeboard zone heated to similar temperature levels as the fluidized bed of zircon sand, where the nascent tars (and chars) are further thermally cracked. This reactor is denoted as the single-stage reactor in the present study. Gas residence times were 0.3–0.5 s. Particle residence times varied from as short as the gas residence time to much longer times, depending on the tendency of the coal to soften and adhere to the bed material. The fluid bed reactor is a useful device for investigating the detailed composition of the products of the initial stages of coal pyrolysis and of the subsequent thermal cracking of the volatiles. These processes are of great importance in the ultimate combustion of the coal. However, while the heating rate is similar to that found in coal combustors, the temperatures achieved are somewhat less than those pertaining to a full-scale furnace. As a consequence, the timescale for the processes are relatively longer in the fluid bed reactor than they would be in a practical system, where conversion of the volatiles may be complete in 0.1 s. In the second reactor, a fluidized bed reactor essentially identical to the single-stage reactor was coupled to an empty second-stage tubular reactor. The volatiles (tar plus gases) from the pyrolysis of coal in the first stage were transported to the second stage. Char particles were removed by particle settling between the two stages. The first stage was heated to 6008C to generate volatiles. The thermal cracking of volatiles at this temperature is believed to be minimal, especially for the release of nitrogen-containing species [1]. The second stage was heated separately
During a pyrolysis experiment, tar was collected with a liquid-nitrogen–cooled trap fitted with a thimble. The tar sample was recovered by extracting the trap with dichloromethane, as described previously [1]. The tar samples thus recovered and the char samples collected in the thimbles were submitted for nitrogen analysis. Quantification of HCN, HNCO, and NH3 was carried out using Fourier transform infrared (FTIR) spectrometry. Gas-phase products were collected in a multipass FTIR gas cell (7.2 m total path length). Spectra were recorded and concentrations of HCN and NH3 determined as previously described [1,8]. HNCO was identified by its absorbance in the region from about 2300 to 2200 cm11 after the absorbance of 13CO2 in the region had been subtracted [17]. Calibration of HCN and NH3 was carried out by diluting respective standard calibration gases with N2. The calibration of HNCO was taken from the literature [18] by assuming a linear increase in absorbance with light pass length and concentration. Results Figure 1 shows HCN, NH3, and HNCO yields from the pyrolysis of a brown coal and a bituminous coal in the single-stage fluidized bed reactor. These are typical results chosen from the pyrolysis of a suite of coals. As previously reported for pyrolysis in a fluidized bed reactor [1], a maximum is observed in the NH3 yield at a temperature of about 8008C. This decrease has been accounted for [19] by assuming that HCN is the primary N-containing product released from coals and that NH3 is produced from the HCN by hydrogenation reactions within the coal particle. The decrease at high temperatures is a consequence of the reduced time spent by the HCN within the particle. However, we have observed recently [13] that NH3 can be decomposed by quartz, zircon sand, and stainless steel at temperatures above 7008C. Hydrogen cyanide yields from the coals studied (ranging from 67 to 90% C, daf basis) were observed to increase with increasing temperature, and the rate of this increase was very rapid at temperatures above 9008C. Isocyanic acid (HNCO) yields from the pyrolysis
3208
COAL AND CHAR COMBUSTION
Fig. 2. HCN yields from the pyrolysis of coals at 10008C in the one-stage reactor as a function of coal rank. (v), Australian coals; (¶), German coals; (m), U.S. coal.
Fig. 3. Cumulative nitrogen distribution during the pyrolysis of Fortuna brown coal in the single-stage reactor as a function of temperature.
of bituminous coals were observed to maximize at about 8008C. At temperatures above 8008C, HNCO is thermally unstable and will decompose to NCO and H. No detectable HNCO yields were observed from the pyrolysis of Pocahontas No. 3, a low-volatile bituminous coal with carbon content of about 90 wt. % daf. Detection of HNCO from the pyrolysis of brown coals was more difficult because of the very
Fig. 4. Cumulative nitrogen distribution during the pyrolysis of Go¨ttelborn bituminous coal in the single-stage reactor as a function of temperature.
large amounts of CO2 produced during the pyrolysis of these low rank coals; however, it seems that the amounts produced were considerably less than those produced from the bituminous coals. Data in Fig. 1 also suggest that the NH3/HCN yield ratio at temperatures ,8008C is higher from the pyrolysis of the brown coal than from the bituminous coal. Pyrolysis of other coals in the present study largely confirmed this finding. No NH3 was detectable in the product gases from the pyrolysis of Pocahontas No. 3 coal under the same conditions. Figure 2 summarizes the HCN yields at 10008C as a function of coal rank (carbon content). It is clear that the nitrogen distribution during coal pyrolysis is coal rank dependent. Low-volatile coals gave much lower HCN (and NH3) yields than the high-volatile coals. Figure 3 shows the nitrogen distribution in char, tar, and gas phase from the pyrolysis of Fortuna brown coal. Because of the observed [13] NH3 decomposition reactions on the reactor materials, NH3 yields at temperatures higher than 8008C were assumed to be the same as that at 8008C. It is noted that even at 10008C, the majority of coal nitrogen (.55%) remained in the chars. The proportion of coal nitrogen in char decreased monotonically with increasing temperature. Nitrogen in tar comprised only a small part of the coal nitrogen. There was a significant deficit in the nitrogen balance that amounted to some 17%. Possible reasons for this deficit are discussed later in this paper. Data in Fig. 4 for the Go¨ttelborn bituminous coal show similar trends to those in Fig. 3. The caking properties of the bituminous coals have resulted in
RELEASE OF N FROM COALS
Fig. 5. Comparison of HCN, NH3, and HNCO yields from the pyrolysis of an Australian bituminous coal (coal D) in the single-stage (v) and two-stage (M) reactors as a function of temperature.
the agglomeration of coal/char particles with the zircon sand during pyrolysis. Only a small portion of char was collected in the thimble and the majority of char was retained and heated in the bed until the termination of the experiment. The time-temperature history of this part of char was unknown. Analysis of the char collected in the thimble was therefore deemed to be unrepresentative of the whole char. Comparison of the data in Figs. 3 and 4 suggests that the proportion of coal nitrogen in tars increased considerably from the brown coal to the bituminous coal. This finding was further confirmed with other coals of similar ranks. Data in Figs. 3 and 4 also show that the nitrogen in tars would not be enough to account for the increase in HCN and NH3 yields at higher temperatures. Even in the case of the bituminous coals (e.g., Go¨ttelborn in Fig. 4), the maximum amount of nitrogen in tars accounts for only about 17% of the coal nitrogen, whereas nitrogen in HCN and NH3 at 10008C is more than 25% of coal nitrogen. Considering that the HCN yields still tend to increase with temperature beyond 10508C (the upper limit used in the present study), it is fair to conclude that part of HCN and NH3 must have come from the thermal decomposition of chars. Further experiments were then carried out in the two-stage reactor to confirm and quantify experimentally the contributions of the thermal decomposition of tars and chars to the observed HCN and NH3 yields. Figure 5 compares the HCN, NH3, and HNCO yields from the pyrolysis of an Australian bituminous coal (coal D) in the single-stage and two-stage re-
3209
actors. The data clearly show that the thermal cracking of volatiles (mainly tars) alone in the second stage did not produce the same amount of HCN as the pyrolysis of coal in the single-stage reactor at the same temperatures. The production of HCN from the thermal decomposition of tar was negligible at temperatures lower than 7508C. The difference in HCN yield from the pyrolysis in the two reactors seems to increase with increasing temperature: about 8% more of coal nitrogen was released as HCN in the one-stage reactor than in the two-stage reactor at 10008C. It is fair to conclude that char thermal decomposition is an important route for HCN formation. The production of NH3 from the thermal decomposition of tars in the second stage was observed to be not more than 2.5% of coal nitrogen. In fact, there is very little change in the NH3 yield in the second stage over that which is produced in the first stage. Hence, it appears that the NH3 observed during pyrolysis in the single-stage reactor derives mainly from primary coal pyrolysis and secondary decomposition of the chars. No significant difference in the HNCO yields from the pyrolysis experiments in the two reactors was observed. This indicated that the majority of HNCO was formed from the thermal decomposition of tars.
Discussion As discussed in the introduction, the rate and overall amounts of coal N released to the gas phase are very important in NOx formation. The results in Fig. 2 clearly show that the yields of HCN are a function of coal rank and decrease significantly for coals of higher rank (and of low volatility). At first glance, this would seem to be related directly to the lower volatile yields from these coals, and, as a consequence, to imply that thermal cracking reactions of the tars are the source of the volatile N species. Close consideration of the data presented, however, suggests that this picture is too simplistic. While it seems clear that tar cracking is an important source of HCN based on the results obtained with the two-stage reactor, it is also clear that it is not the only source. There is simply insufficient N present in the tars to account for the HCN produced. In fact, the data in Fig. 5 suggest that thermal cracking of tars at temperatures lower than 7008C produced negligible amounts of HCN and NH3. The production of HCN from tar alone (Fig. 5) became significant only at temperatures higher than 8008C. This is in agreement with the data shown in Figs. 3 and 4, where nitrogen distributions in tars remained almost unchanged up to this temperature. Experiments to be reported elsewhere, however, suggested that, as the temperature was raised to 9008C,
3210
COAL AND CHAR COMBUSTION
DCM-soluble tar yields decreased to one-fourth or one-half of their maxima for Fortuna and Go¨ttelborn, respectively. Taken together, these findings would appear to indicate that the thermal cracking reactions of tars at lower temperatures (e.g., ,8008C) to gases or soot did not result in the release of tar nitrogen and that the nitrogen in tar in fact was very stable up to about 8508C. This is probably a consequence of the nature of the cracking reactions at these low temperatures, which tend to decompose the more labile aliphatic groups and do not result in decomposition of the aromatic or heterocyclic structures. Hence, primary coal pyrolysis and/ or further decomposition of chars are important sources of HCN and NH3 formation, even at relatively low temperatures (e.g., ,8008C). If the lower-volatile yield is not solely responsible for the decrease in N release as a function of increasing coal rank (Fig. 2), other factors need to be considered. It seems likely that the size (extent of condensation of rings) of the nitrogen-containing heteroaromatic ring systems is an important factor governing the nitrogen release during pyrolysis. It is generally agreed that the sizes of aromatic ring systems in coal tend to increase with increasing rank. It has been shown [20] that aromatic (also including heteroaromatic) ring system distributions ultimately govern the thermal decomposition of the coal macromolecular network. The coordination number was shown to increase as the size of the aromatic ring system increases, making the macromolecular network more difficult to decompose [20]. Moreover, the increases in the size of aromatic or heteroaromatic ring systems would tend to stabilize the ring systems. The nitrogen in higher rank coals is more likely to be in larger heteroaromatic ring systems, making the ring systems difficult to be released from the coal and also difficult to be ruptured even at high temperatures (Fig. 2). Even though the nitrogen might be in pyrolic form, if the ring system containing the nitrogen is large, the ring system would be likely to have a high coordination number and therefore difficult to be released from the network. The large size of the ring system also implies that the ring system is stable, possibly making it more difficult to be ruptured than a smaller ring system containing pyridinic nitrogen. The HCN and NH3 formed during pyrolysis at lower temperatures (,7008C) are thought to have their origin in the small nitrogen-containing ring systems in coal. These smaller ring systems are relatively easily ruptured compared to the corresponding larger ones. The data in Fig. 1 showed that the HCN production covered a wide range of temperature, and it would be expected that more HCN production is likely at temperatures higher than the upper limit in the present study. Previous experimental data [2,21] have shown that HCN production continued at tem-
peratures higher than 12008C until the total release of coal nitrogen was observed. The observation that the production of gas-phase nitrogen-containing species (e.g., HCN) covers an extremely wide range of temperature is thought to reflect the fact that the nitrogen in coal is distributed in ring systems of a wide range of sizes. Ammonia formation has created considerable discussion in studies of the nitrogen distribution during coal pyrolysis. Since nitrogen exists in coal as heteroaromatics, the formation of NH3 requires the hydrogenation of nitrogen. Model compound studies [11,12] of the pyrolysis of N-containing aromatics do not result in NH3 formation. Various mechanisms have been proposed for the formation of ammonia. For example, Baumann and Mo¨ller [7] have suggested gas-phase HCN hydrogenation to form NH3. Similarly, it has been suggested [10,19] that HCN may be hydrogenated on the coal/char surface to form NH3. The observation of HNCO in the pyrolysis products is also interesting in the context of NH3 formation from the pyrolysis of coals. Isocyanic acid readily hydrolyzes [18] on alumina surfaces to yield NH3 and CO2 and such surface catalyzed reactions within the coal particle are possible during coal pyrolysis. The failure to detect HNCO from brown coals is consistent with this mechanism since these coals have much higher moisture contents. The dependence of ammonia yields on reactor types [10,19] and particle sizes [19] supports the contention that NH3 is largely a secondary product of the decomposition of coals and arises from interactions of other N-containing precursors with the coal surface. Given the weight of the experimental evidence supporting this conclusion, it is surprising that some attempts [14] have been made to link NH3 formation with specific functional groups in the coal and to derive a predictive NOx formation index based on the functional forms of N in coals. Based on the results of the present study and of previous work, we would contend that nitrogen-containing species produced during coal pyrolysis are very reactive and interconversion between them is more than possible. The extent of this interconversion depends to a great extent on reactor configuration and other experimental parameters, and hence any attempts to account for the formation of particular products from the decomposition of specific functional groups is highly problematic. The deficit in the N balance observed at high temperatures with the brown coal (Fig. 3) is possibly due to the formation of N2 gas. Previous work [22] has shown that significant amounts of N2 can be produced from Australian brown coals pyrolyzed in a thermobalance. Conclusions 1. Nitrogen distributions during coal pyrolysis, especially HCN and NH3 yields, were found to
RELEASE OF N FROM COALS
change with coal rank, with the low-volatile coals giving lower HCN and NH3 yields than the highvolatile coals. 2. Tar cracking and coal nitrogen functionality alone were found to be insufficient to predict the nitrogen distribution during pyrolysis. 3. Primary coal pyrolysis and/or further thermal decomposition of chars are major sources of HCN formation, even at temperatures lower than about 8008C. The importance of tar cracking reactions for the nitrogen distribution in the gas phase increases from brown coals to high-volatile bituminous coals. 4. Tar cracking does not appear to be an important source for ammonia formation. Hydrolysis of HNCO within the coal particle is a plausible source for NH3 formation, but reactions of other N-containing precursors are probably also involved. 5. While the formation mechanisms for HNCO remain uncertain, tar cracking reactions seem mainly responsible for the observed HNCO yields. 6. Sizes of the nitrogen-containing heteroaromatic ring systems are another important factor governing the nitrogen distribution during pyrolysis. Nitrogen in coal is believed to exist in heteroaromatic ring systems of a wide range of sizes resulting in the nitrogen release from coal covering an extremely wide range of temperatures. Acknowledgments The award of a Rothmans Foundation Fellowship to one of us (C.-Z. Li) is gratefully acknowledged.
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