Nitrogen Transformations during Secondary Coal Pyrolysis - Energy

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Energy & Fuels 2001, 15, 1512-1522

Nitrogen Transformations during Secondary Coal Pyrolysis Haifeng Zhang and Thomas H. Fletcher* Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602 Received June 5, 2001. Revised Manuscript Received August 10, 2001

A CO/H2/O2/N2 flame was operated under fuel-rich conditions in a flat flame reactor to provide a high-temperature, oxygen-free environment to study secondary reactions of coal volatiles. The distributions of fuel nitrogen in the devolatilization products of four coals, ranging from high volatile bituminous to lignite, were obtained at gas temperatures ranging from 1159 to 1858 K. It was found that the initial nitrogen released was contained almost exclusively in the tar for all coals. Release of nitrogen from the char as light gases started at a later stage than tar nitrogen release. During secondary reactions, the nitrogen contents in the coal tars were higher than the nitrogen contents in the parent coals at temperatures below 1300 K. A rapid decay in the tar nitrogen content was observed between 1300 and 1600 K, followed by a much slower decrease in nitrogen content at temperatures above 1600 K. Nitrogen release from the coal tar can be described with first-order kinetics using the same rate constant for all the coals studied. Nitrogen release followed different routes in the tar and in the char, despite similar nitrogen functionalities in both products. Thermal decomposition of char was found to be an important source for nitrogen release at high temperatures. For low rank coals, NH3 was released earlier than HCN. For high rank coals, NH3 was released at the same time as HCN.

Introduction The major source of NOx from coal combustion is the nitrogen present in the coal itself, since NOx production by the thermal mechanism (thermal NOx) is effectively inhibited by regulating flame temperatures.1 In commercial pulverized coal furnaces, coal nitrogen is released in three stages. During primary devolatilization, the volatile-N is released with the tar.2 In the second stage, the volatiles undergo secondary reactions in hot, fuel-rich conditions that convert part of the nitrogen in the tar into HCN.3-5 Since tar has a strong propensity to form soot at high temperatures and long residence times, the rest of the nitrogen in the tar will be incorporated into the soot. At the same time, nitrogen trapped in the char is expelled by thermal dissociation induced by higher particle temperatures. In the third stage, oxygen reacts with char, liberating all additional nitrogen by chemical conversion to NOx.6 All of the nitrogen released will end up in the combustion products. However, volatile-N, unlike char nitrogen, is amenable to reduction to N2 through inexpensive tech* Author to whom correspondence should be addressed. (1) Niksa, S. Twenty-Fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1994; pp 537-544. (2) Chen, J. C.; Niksa, S. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 12691276. (3) Nelson, P. F.; Kelly, M. D.; Wornat, M. J. Fuel 1990, 70, 403407. (4) Chen, J. C. Effects of Secondary Reactions on Product Distribution and Nitrogen Evolution from Rapid Coal Pyrolysis. Ph.D. Dissertation, Stanford University, Stanford, CA, 1991, (5) Ledesma, E. B.; Li, C. Z.; Nelson, P. F.; Mackie, J. C. Energy Fuels 1998, 12, 536-541. (6) Pershing, D. W.; Wendt, J. O. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1977; pp 389-399.

niques such as burner and air flow configuration modifications which can reduce NOx emission by 5080%.7 The most economical combustion modification to reduce NOx is air staging.8 In air staging, the combustion air is distributed at different elevations along the furnace wall to establish alternating fuel-rich and fuellean zones.8 Air staging promotes the conversion of volatile-N to N2, hence minimizing NOx formation by delaying the mixing of the air (oxygen) supply with volatile-N.9 This delay gives the primary coal volatiles (tar and light gas) ample time to undergo secondary reactions. Since aerodynamic control methods such as air staging totally rely on the availability of volatile nitrogen in the gas phase, the incorporation of the released tar nitrogen into the soot has an adverse effect on NOx reduction. It is clear that secondary pyrolysis significantly influences the ultimate NOx production in industrial furnaces. Consequently, a detailed investigation into the nitrogen transformations during secondary pyrolysis and the effects of the tar-soot transition on nitrogen release is critical for design and implementation of new pollution control strategies. The current study investigates nitrogen evolution at high-temperature, high heating rate conditions relevant to industrial furnaces. Previous studies of secondary coal pyrolysis have been conducted in entrained flow drop tube reactors under (7) Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; Elsevier: New York, 1993. (8) Man, C. K.; Russell, N. V.; Gibbins, J. R.; Williamson, J. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43 (3), 1139-1142. (9) Van der Lans, R. P.; Glarborg, P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23, 349-377.

10.1021/ef010118g CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001

Nitrogen Transformations during Secondary Coal Pyrolysis

inert conditions,2,10 with heated grid reactors,8 and with fluidized bed reactors followed by laminar flow reactors.5 Experiments on a hv bituminous coal showed that the N/C ratio was quite different for the tar and soot, indicating that soot is not a simple accumulation of PAHs in the tar.10 Incorporation of the N-containing PAH into the soot was faster than the non N-containing PAH during the initial soot formation process. The nitrogen content of the soot was found to decrease with increasing temperature. Two mechanisms were suggested for such a decrease, including the liberation of N-containing gas species from the soot and the incorporation of PAH with successively lower nitrogen content during soot growth. The second aspect was suggested to have a larger effect on reducing the soot N/C ratio. As much as 25% of the volatile-N was reported to be incorporated into soot for a hv bituminous coal and 10% for a subbituminous coal.2 It was also observed that nitrogen incorporation into soot occurred early during secondary pyrolysis, and that the fraction of coal nitrogen integrated into the soot remained constant, even though the soot yield increased steadily with increasing temperature. Consequently, the nitrogen content of the soot decreased throughout secondary reactions, which is consistent with Wornat’s observation. The major N species in the gas phase was found to be HCN. Haussmann et al.11 also reported about 20-30% of volatile-N trapped in the soot for a bituminous coal. However, pyrolysis experiments in a flat flame burner showed much less nitrogen fraction trapped in the soot, and no significant changes of nitrogen composition in the soot with residence time were noticed.12 The nitrogen functionality of the tar was examined by pyrolyzing a German bituminous coal in a fluidized bed and performing size exclusion chromatography (SEC) on different molecular mass fractions.13 The nitrile group, not present in the raw coal and the tars produced at 600 and 700 °C, appeared in the SEC fractions of the tar produced at 800 °C, which coincides with the temperature at which N-containing model compounds begin to decompose. XPS analysis of the tars also indicated the conversion of pyridinic nitrogen to nitrile nitrogen in the range of 600-800 °C. The presence of reactive species and H was suggested as a reason for the earlier release of nitrogen gas species (HCN, NH3, HNCO, etc.) from the coal than from the model compounds. Heated grid experiments on some bituminous coals demonstrated the different nitrogen release patterns during high-temperature pyrolysis.8 At relatively low temperatures (1000-1200 °C), volatile nitrogen (mostly contained in tar) fractional yields were approximately equal or slightly less than the total volatile yields. However, at higher temperatures (1400 °C above), there was additional release of nitrogen with very little total (10) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1988; pp 135-143. (11) Haussmann, G. J.; Kruger, C. H. Evolution and Reaction of Fuel Nitrogen During Rapid Coal Pyrolysis and Combustion. Presented at the Spring meeting of the Western States Section of The Combustion Institute, Livermore, CA, 1989. (12) Rigby, J.; Ma, J.; Webb, B. W.; Fletcher, T. H. Energy Fuels 2001, 15, 52-59. (13) Li, C. Z.; Buckley, A. N.; Nelson, P. F. Fuel 1997, 77, 157-164.

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mass loss. The “secondary” nitrogen release (defined by these authors to be any reactions occurring after the primary tar release) occurred at a much slower rate than the “primary” nitrogen release. This “secondary release” was associated with a reduction of hydrogen content in the char. Ledesma’s5 experiment on the thermal cracking of coal tars is the only study of nitrogen release from tars free from the effects of residual char and from transport effects from the coal surface. Primary tars were generated at 600 °C in a fluidized reactor and subsequently thermally decomposed in a tubular reactor connected with the fluidized reactor. HCN was found to be the dominant nitrogen species from tar cracking. A considerable amount of NH3 and HNCO was also identified. This is the only experiment where a significant amount of HNCO has been reported. The fraction of soot-N was not reported, but was likely less than 10% based on the nitrogen balance. Recently, the N-containing PAH (NPAH) in the tars of a bituminous coal and a subbituminous coal were characterized according to their fused aromatic ring numbers using gas chromatography coupled with a chemiluminescence detector.14 It was found that the initial depletion of N-containing species was mainly attributed to direct conversion to soot during the early stage of secondary pyrolysis. Neutralization and mass transformation of polar compounds (carboxyl-substituted NPAH) appear to be responsible for an observed increase of NPAC in the middle stage of secondary reactions. The decrease of NPAC, after reaching a maximum at the late stage of secondary pyrolysis, indicates the successive predominance of polymerization and ring rupture reactions, which lead to the release of HCN. Yu’s results also confirm the findings of Axworthy and co-workers,33 in that the stability of NPAC does not necessarily correspond to the activation energy associ(14) Yu, L. E.; Hildemann, L. M.; Niksa, S. Fuel 1999, 78, 377385. (15) Axworthy, A. E.; Dayan, V. H.; Marin, G. B. Fuel 1978, 57, 2935. (16) Freihaut, J. D.; Proscia, W. M.; Mackie, J. C. Compos. Sci. Technol. 1993, 93, 323-347. (17) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710-720. (18) Xu, W. C.; Tomita, A. Fuel Process. Technol. 1989, 21, 25-37. (19) Leppalahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43, 1-45. (20) Takagi, H.; Isoda, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1999, 13, 934-940. (21) Nelson, P. F.; Buchley, A. N.; Kelly, M. D. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 1259-1267. (22) Brill, T. B.; Brush, P. J.; James, K. J.; Shepherd, J. E.; Pfeiffer, K. J. Appl. Spectrosc. 1992, 46, 900-911. (23) Schafer, S.; Bonn, B. Fuel 2000, 79, 1239-1246. (24) Rudiger, H.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Fuel 1997, 76, 201-205. (25) Friebel, J.; Kopsel, R. F. W. Fuel 1999, 78, 923-932. (26) Mackie, J. C.; Colket, M. B.; Nelson, P. J. Phys. Chem. 1990, 94, 4009-4106. (27) Mackie, J. C.; Colket, M. B.; Nelson, P.; Esler, M. Int. J. Chem. Kinet. 1991, 23, 733-760. (28) Schafer, S.; Bonn, B. Fuel 2000, 79, 1239-1246. (29) Van der Lans, R. P.; Glarborg, P.; Dam-Johansen, K. Prog. Energy Combust. Sci. 1997, 23, 349-377. (30) Freihaut, J. D.; Proscia, W. M.; Seery, D. J. Energy Fuels 1989, 3, 692-703. (31) Blair, D. W.; Wendt, J. O. L.; Bartok, W. Sixteenth Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, 1976; pp 475-489. (32) Rees, D. P.; Smoot, L. D.; Hedman, P. O. Eighteenth Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, 1980; pp 1305-1311.

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Zhang and Fletcher

Table 1. HCN and NH3 from Pulverized Coal Pyrolysis and Combustion Experiments (modified from van der Lans et al.9) nitrogen products HCN is the dominant product

both HCN and NH3 detected; HCN is the primary components with small amount of NH3 for lower rank coals

more NH3 is formed than HCN

more HCN is found in high rank coals, while the distribution of NH3 increases toward lower rank coals, and can become larger than HCN a

apparatus

conditions

arc-jet fired entrained flow reactor [11] heated grid [16] radiant flow reactor [4] entrained flow reactor [30] entrained flow reactor [17] pyroprobe [31] laboratory-scale combustora [32] drop-tube reactor [33] fluidized bed [21] pyroprobe in an air-staged entrained flow furnace [34] radiant flow reactor [35] fluidized bed [5] electrically heated furnace [36] flat-flame burnera [37] fixed bed [19] TG-FTIR [17] laboratory-scale combustora [38] fixed bed [25]

900 ppm O2, bit. and subbit. coal in N2, 14 coals inert atmosphere, 6 coals inert atmosphere, 14 coals inert atmosphere, Argonne Premium coals inert atmosphere, 20 coals substoichiometric; bit. coals in argon, subbit. and lignite inert atmosphere; 3 coals inert atmosphere, bit. and subbit. coals slightly oxidizing atmosphere, bit. and subbit. coals inert atmosphere, bit. coal oxidizing atmosphere; mv bit. coal Ar/O2 flame, subbit. coal inert atmosphere; slow heating rate; Russian coal inert atmosphere; Argonne Premium coals Ar/O2/CO2; 48 coals; various stoichiometry inert atmosphere, low rank coals

Combustion experiments rather than pyrolysis experiments.

ated with a given pyrolysis condition. Two-ring NPAC (such as quinoline) in the coal tars, assumed to form from the reaction of pyridine and acetylene, appear to be the most stable species during severe secondary pyrolysis. Secondary reactions of tar and thermal decomposition of char at high temperatures result in the release of nitrogen species into the gas phase. The major gas species are identified as HCN, NH3, HNCO, and N2.2,5,13,16-18 HCN and NH3 are by far the most important nitrogen species in pulverized coal burners and fluidized reactors, although some slow heating pyrolysis experiments on fixed bed did show N2 as the dominant species.15,19,20 HNCO yields corresponding to 15% of the total volatile-N were reported in a fluidized bed pyrolysis experiment.13 There is still controversy over the origins and interactions of HCN and NH3 during coal pyrolysis. Some researchers believe HCN and NH3 are generated from a similar source since the temperature of initial HCN and NH3 formation is very close;21 others assume that NH3 is converted to HCN under severe conditions.4,22 Recently, more and more researchers have begun to believe that HCN may be the primary nitrogen species during pyrolysis and that NH3 is partly formed from HCN through hydrogenation.5,17,19,23,24,25 The absence of NH3 from the decomposition products of N-containing model compounds was explained by the lack of donatable hydrogen atoms in these aromatic compounds.26,27 Enhanced HCN conversion to NH3 by adding small amounts of water (hydrogen donor) was also reported.28 The hydrogenation of HCN to NH3 is further complicated by the fact that more NH3 has been found in experiments with relatively high concentrations of oxygen-containing species (O2, O, OH, etc.).29 The relatively higher NH3 yield associated with low rank coals under inert conditions may be somewhat correlated with the higher oxygen content in the parent coal. (33) Phong-Anant, D.; Wibberley, L. J.; Wall, T. F. Combust. Flame 1985, 62, 21-30.

Table 2. Proximate and Ultimate Analyses of the Coals Used proximate analysis (wt %)

coal Illinois No. 6 Utah Black Thunder Knife River a

rank

volatile matter ash (daf)

hvCb 12.3 hvBb 9.8 subC 6.8 lignite 11.2

48.8 49.3 52.3 74.7

ultimate analysis (wt %, daf) C H N Oa S 75.7 81.4 76.6 70.8

5.2 5.9 5.0 8

1.5 1.6 1.1 1.0

12.8 10.5 16.9 21.9

4.6 0.5 0.5 1.5

O ) 100 - (C + H + N + S).

The relative amounts of HCN and NH3 can be affected by many factors such as coal rank, heating rate, temperature, local stoichiometry, and even experimental apparatus. Table 1 shows a summary of reported HCN and NH3 yields from different coal pyrolysis and combustion experiments. These results can be summarized as follows: HCN is predominant in high-temperature, high heating rate, inert entrained-flow systems; however, in slow heating rate fixed bed experiments, more NH3 is identified. Strong rank dependence of HCN and NH3 release is demonstrated in entrained flow systems and fluidized bed experiments, with more NH3 release for low rank coals than for high rank coals. The large variation in the reported HCN and NH3 yields at various conditions shows that more understanding is needed. Therefore, additional research is necessary on the release of HCN and NH3 in industrially relevant systems. Experimental Section Coal Samples and Preparation. Four coals, Illinois No. 6, Utah, Black Thunder, and Knife River, were used in the pyrolysis study. The analyses of these coals are shown in Table 2. All the coals were pulverized and sieved to the 45-75 µm diameter size range. Coal samples were dried for 2 h at 105 °C prior to the experiment. Apparatus and Operation. The flat flame burner (FFB) used in this study was described thoroughly by Ma,39 so only



Figure 1. Schematic of the flat flame burner (FFB) with the gas analysis system. a brief description of the apparatus was included in this paper. A Bomem MB-155 FTIR gas analysis system was added and used in connection with the suction probe in the FFB system to allow on-line gas measurement.40 Figure 1 shows the revised FFB system. A syringe-type particle feeder was used to provide a steady coal feed rate (∼1 g/h) and to allow an accurate measurement of the total amount of sample fed in each experiment. This flow rate ensured single-particle behavior in the reactor. The particles from the feeder were entrained in N2 and injected about 1 mm above the burner surface through a metal centerline tube. Particles passed through the flame zone and reached pyrolysis temperatures later in the reactor;39 the flame is therefore only used as a heat source. The openended reactor can be raised and lowered relative to the level of the sampling probe to achieve desired residence times. All the reaction products were collected by a water-cooled probe connected with two vacuum pumps. Nitrogen quench jets at the probe tip quenched the reaction products instantly to freeze the reaction. Nitrogen was also transpired through a porous (34) Kambara, S.; Takarada, T.; Toyoshima, M.; Kato, K. Fuel 1995, 74, 1247-1253. (35) Niksa, S.; Cho, S. Energy Fuels 1996, 10, 463-473. (36) Kremer, H.; Schulz, W. Twenty-First Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, 1986; pp 12171222. (37) Peck, R. E.; Midkiff, K. C.; Altenkirch, R. A. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1984; pp 1373-1380. (38) Chen, S. L.; Cole, J. A.; Kramlich, J. C.; McCarthy, J. M.; Pershing, D. W. Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1988; pp 1135-1145. (39) Ma, J. Soot Formation During Coal Pyrolysis. Ph.D. Dissertation, Brigham Young University, Provo, UT, 1996. (40) Zhang, H. Nitrogen Evolution and Soot Formation During Secondary Coal Pyrolysis. Ph.D. Dissertation, Brigham Young University, Provo, UT, 2001.

inner wall of the probe in order to minimize tar or soot deposition on the probe walls. A virtual impactor at the end of the collection probe was used to aerodynamically separate the large, dense char particles from the small and low-density soot particles. A cyclone installed behind the virtual impactor was used for char collection. The condensed tar aerosols or soot particles were collected on polycarbonate filters with pore sizes of 1 µm, installed immediately after the virtual impactor. Glass fiber filters were also placed immediately behind the polycarbonate filters as additional support and to monitor any condensables that passed through the polycarbonate filters. Tar or soot samples collected on the polycarbonate filters were carefully scraped off the filters to avoid the use of solvents. The tar or soot samples were sealed in argon-filled vials before the analysis to prevent oxidation or degradation. The reaction gas stream, after passing the filters, was directed into the gas cell on the FTIR and was analyzed instantly. Reactor Temperature Control. Temperature is a critical parameter for secondary pyrolysis. Significant secondary reactions usually start at temperatures as low as 1100 K for a coal system. Such low temperatures were achieved by using carbon monoxide (CO) for the fuel in the FFB. CO has much broader flammability limits (12.5% to 75%, by volume in air) than the commonly used hydrocarbons. Therefore, the temperature of a stable CO flame can be easily adjusted to as low as 1100 K even at very fuel-rich conditions. Such stable low-temperature flames cannot be maintained by hydrocarbon fuels such as natural gas. CO flames also minimize the steam production, which has an adverse effect on FTIR analysis of the combustion gas. In practice, only a small amount of hydrogen was added to the fuel stream to enhance combustion stability, since OH is needed for realistic CO combustion rates. The reactor temperatures were regulated by changing the amount of

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dilution with N2, and by changing the equivalence ratio. Using CO as the major fuel, the temperature can be easily adjusted from 1100 to 2000 K in the FFB to facilitate the pyrolysis experiments, with less than 1 mol % steam in the post flame gases. Equivalence ratios varied in a linear fashion from 1.45 at the 1159 K condition to 1.09 at the 1858 K condition. Product Characterization. The fraction of carbon, hydrogen, nitrogen, and sulfur in the condensed products (tar, soot, and char) were determined on a dry basis using a Leco CHNS-932 elemental analyzer. The oxygen content was calculated by difference. Proximate analysis was performed on the coal and the char following the standard practice set by the American Society for Testing Materials (ASTM). A detailed description of these analysis techniques can be found elsewhere.40 Noncondensable gases were quantified by an FTIR coupled with a 10 m multi-reflection gas cell (Infrared Analysis, Inc.). All spectra were acquired with a resolution of 1 cm-1 and spectral range of 500-4000 cm-1. Standard calibration gases purchased directly from vendors were first diluted to the appropriate range with ultrahigh-purity N2. Spectra of these diluted calibration gases were then collected under the same condition as the pyrolysis gases and certified with a quantitative FTIR database.41 Special care was made to ensure that the concentration range of each gas was within the dynamic range, where the detection response to the concentration is linear. Therefore, the spectra were easy to work with mathematically. These “secondary” reference gas spectra were then used in the calibration of the noncondensable gases in the FFB. There are two advantages to using the “secondary” reference gas spectra in the calibration of the combustion gases. First, the reference gas spectra from a quantitative FTIR database cannot be used directly to quantify the same species in the FFB. The reference gas spectra and the combustion gas spectra were collected on different spectrometers and at different geographical locations (at different total pressures). Hence, there are always subtle differences regarding the peak shape and the peak position of a certain species in the reference gas and in the combustion gases. These subtle differences in peak shape and position cause large zigzag-shaped “subtraction garbage” when the reference spectra are subtracted from the combustion gas spectra. The “subtraction garbage” significantly overlapped the absorption peaks of other species left in the combustion gas spectra, making the quantitative analysis almost impossible. The use of the “secondary” reference spectra eliminates this problem. Due to the extremely low concentrations of nitrogen-containing species in the FFB (PPB-level) and the significant overlap of the nitrogen-containing species with other species, such exactly matched “secondary” reference gas spectra are necessary to completely subtract each species in the combustion gas spectra to get accurate measurements of the weakly absorbed or significantly overlapped nitrogen species in this experiment. Second, the “secondary” reference spectra were updated frequently (once a month) to offset any shifts of the peaks caused by the machine drifts over time. All the “secondary” reference spectra were certified with the same reference spectra in the quantitative FTIR database. This procedure ensures the consistency of these “secondary” reference spectra and, at the same time, avoids the problems of any possible decay of the reference gases in the tank either due to physical absorption/desorption or chemical reactions. The spectral region of 3250-3400 cm-1 was selected for the quantification of HCN concentrations. The strongest absorption band of HCN at 712 cm-1 was not used due to the significant overlap of this peak with the CO2 peak at 680 cm-1. A strong C2H2 absorption band was also found at 3250-3400 cm-1. These C2H2 peaks were removed by subtraction before (41) Hanst, P. L. QASOFT Version-32; Infrared Analysis, Inc.: Anaheim, CA, 1999.

Zhang and Fletcher

Figure 2. Cumulative distribution of the coal nitrogen incorporated into various products for Black Thunder. the HCN peaks could be quantified with satisfactory accuracy. For NH3, the strongest peaks at the spectral range of 9001000 cm-1 were used for the quantification. Ethylene, with a much higher concentration, had to be subtracted out before the quantitative analysis of NH3. By using a liquid N2-cooled MCT detector and a 10 m gas cell at 1 cm-1 resolution, the detection limit of the FTIR was as low as 50 PPB for certain types of gases (including NH3, C2H4, and C2H2). The detection limits of other species including HCN were about 100 PPB. At least 10 spectra were recorded for the noncondensable gases in the FFB at each experimental condition. Several spectra were recorded at 10 scans (40 s) for the determination of NH3 in order to minimize the “memory effects” due to the selective adsorption or desorption of NH3 from the cell walls (made of borosilicate glass).41,42 The other spectra were recorded at 144 or 256 scans to obtain an excellent signal-to-noise ratio (S/N) to quantify other species without the “memory effect”.

Results and Discussion The coal nitrogen was balanced within 10% at temperatures above 1500 K for all coals except for the Illinois No. 6 coal. However, there was a significant deficit (about 20%) in the nitrogen balance at temperatures below 1400 K. Figure 2 shows the cumulative nitrogen distributions with temperature for the Black Thunder coal. The nitrogen distributions for the Utah and Knife River coals were similar.40 However, for the Illinois No. 6 coal, only 80% of the coal nitrogen was accounted for over the temperature range in the current study. At low temperatures (less than 1400 K), the deficit in the nitrogen balance is likely due to inaccuracies in the determinations of the nitrogen fraction in the tar. This assumption is supported by the facts that no significant amount of noncondensable nitrogen species were identified in the FTIR spectra at low temperatures and the freshly generated tar, sticky in nature, is more likely to deposit on the walls of the collection tubing during sampling. Another reason for not balancing the nitrogen in the system may be N2 formed from coal nitrogen; N2 has been reported as a major nitrogen species from coal devolatilization in fixed-bed or fluid(42) Compton, S. V.; Compton, D. A. C. Practical Sampling Techniques for Infrared Analysis, Chapter 8: Quantitative AnalysisAvoiding Common Pitfalls; CRC Press: Ann Arbor, MI, 1993.



Figure 3. Nitrogen release versus mass release for Illinois No. 6, Utah, and Knife River.

ized-bed reactors.43,44 The presence of HNCO could not be detected in this experiment due to overlapping bands of CO and CO2. However, scoping experiments using the same FTIR system in drop tube experiments (in N2 at c.a. 1050 K) for a South Banko subbituminous coal indicated the absence of HNCO in entrained flow pyrolysis.40 Total Nitrogen Release. As discussed previously, only the nitrogen released to the gas phase during pyrolysis can be reduced by economical aerodynamic methods such as air staging. Therefore, the fraction of volatile nitrogen largely determines the overall NOx reduction efficiency for coal combustion. Figure 3 shows the total nitrogen release versus mass release for the coals at selected conditions in this study. The total nitrogen release is defined as one minus the fraction of coal nitrogen in char. For bituminous coals such as Illinois No. 6 and Utah, the nitrogen release was comparable with the mass release during the early stage of devolatilization, followed by a delayed nitrogen release in the middle stage. In the final stage, nitrogen continued to evolve from the char even though the mass release was completed. This observation is consistent with the coal nitrogen structure and the sequence of pyrolysis. For bituminous coal, tar is the major transporting medium for both the nitrogen and the volatiles during the early stage of pyrolysis. Since the fraction of fuel nitrogen evolved as tar is almost directly proportional to the tar fraction,45 a comparable release rate of nitrogen and the total mass is not surprising. The apparent delay of nitrogen release in the middle stage of pyrolysis can be explained by the chemical structure in char. A small amount of aliphatic side chains remain in the char after the primary tar release;46 this material can be released as light gas at moderate temperatures (above 1000 K). The nitrogen release was delayed since coal nitrogen resides principally in tightly bound ring structures, which break at higher temperatures. The continued release of nitrogen from char after the mass release ceased was also observed in previous studies.8,47 (43) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 477-482. (44) Phong-Anant, D.; Wibberley, L. J.; Wall, T. F. Combust. Flame 1985, 62, 21-30. (45) Perry, S. T.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Energy Fuels 2000, 14, 1094-1102. (46) Perry, S. T.; Hambly, E. M.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Proc. Combust. Institute 2000, 28, 2313-2319.

Figure 4. Nitrogen incorporated into tar and soot vs temperature.

At this stage, nitrogen was released through ring rupture due to the high particle temperatures. The nitrogen release pattern is different for the lignite. In the early phase of devolatilization, the fractional nitrogen release rates were much slower than the fractional release of overall mass, as indicated in Figure 3 at the earliest stages of mass release for the Knife River lignite. The early volatiles released from low rank coals were dominated by light gases (e.g., CO, CO2, and H2O) that were relatively free of nitrogen species. As pyrolysis proceeded, the particle temperatures rose and the nitrogen was released when the aromatic ring structures were volatilized or ruptured. Nitrogen Evolution in Tar and Soot. The results in this study showed that almost all the volatile nitrogen was contained in the tar during primary pyrolysis (at temperatures less than 1000 K). Noncondensable nitrogen species in the gas phase were negligible even at 1159 K. Light gas nitrogen release started at a later stage than tar nitrogen release for coals of all types. Two major sources were identified for the release of light gas nitrogen: secondary reactions of tar and thermal decomposition of char at elevated temperatures. As expected, the fraction of nitrogen incorporated into the tar was much lower for the low rank coals, which have a much lower tar yield (see Figure 4). At 1159 K, 8% of the coal nitrogen was found in the initial tar for the Black Thunder coal, and only 3% for the Knife River lignite. In contrast, over 20% of the nitrogen was released with the tar for the Illinois No. 6 and Utah coals. In addition, about 75% of the nitrogen in the tar was released during the subsequent secondary reactions for the Illinois No. 6 coal. At the same temperature range, only 50% of the tar nitrogen was released for the Black Thunder coal and the Knife River lignite. Since tar yields are so low for the low rank coals, it is reasonable to believe that the effects of the secondary reactions of the tar on the nitrogen evolution for low rank coals should not be significant. The nitrogen ratio (RN), defined as the nitrogen content in the tar plus soot divided by the nitrogen (47) Pohl, J. H.; Sarofim. A. F. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1976; pp 491501.

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Figure 6. Effects of temperature on the chemical structure characteristics of tars from the Illinois No. 6 coal. The numbers above each bar represent the measured value; these measurements are divided by the value in the parent coal to obtain the relative value plotted on the y-axis. Figure 5. [N]tar/[N]coal vs temperature for the coals in this study.

content in the parent coal (daf), is a convenient gauge to track the rank dependence of nitrogen release. Since the nitrogen content in the parent coal is a constant, the ratio actually reflects the change of nitrogen evolution in tar or soot:

RN )

[N]tar + soot [N]daf coal

(1)

Pyrolysis experiments on 12 bituminous coals in a TGA by Solomon and Colket48 showed values of RN of unity. Freihaut et al.16 reported that bituminous coal tars from a heated grid reactor displayed an almost constant RN, irrespective of the extent of tar evolution, during the early phase of devolatilization. Values of RN for the bituminous coal tars started at 1.0 and rose to 1.1 with increasing temperature (to 1300 K), suggesting that the initial tar is a collection of random samples from the coal. The value of RN increases because the early tar reactions consist of release of aliphatic side chains and oxygenated species prior to significant nitrogen release. However, low rank coal tars exhibited smaller nitrogen content than that in the parent coal, i.e., RN was initially as low as 0.4 and increased with increasing temperature to values of 0.8 at 1300 K. The smaller RN in low rank coal tars suggests that the nonpolar polycyclic aromatic compounds (PAC) are preferentially released as tar during pyrolysis for low rank coals. The exact nature of the delayed nitrogen release in low rank coal tars is not obvious, but it maybe related to the early cross-linking reactions that occur only in low rank coals.49 Some nitrogen-containing PAC (PACN) may be trapped in the large clusters during the early cross-linking, making them too large to vaporize as tar during pyrolysis. Nitrogen Release from Tar during Secondary Reactions. The nitrogen release pattern from tars observed here during secondary pyrolysis was different than that reported in the literature during primary pyrolysis. Figure 5 shows the nitrogen ratio (RN) versus temperature for the four coals used in this study. A (48) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749-755. (49) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio M. A.; Deshpande, G. V. Energy Fuels 1988, 2, 405-422.

similar trend is observed for all the coals: the nitrogen ratios were higher than unity below 1300 K, followed by a rapid decay between 1300 and 1600 K, finally decreasing at a much slower rate at temperatures above 1600 K. The striking similarity of the nitrogen release from tars of different coal types indicates that the reactivity of the tar nitrogen functionalities during secondary pyrolysis is largely rank independent. Previous studies already have shown that the nitrogen functionalities in coal tar are very similar for coals ranging from brown to bituminous3. One-ring to tworing nitrogen-containing aromatics such as pyrrole, pyridine, quinoline, indole, and some nitriles were identified as major components. The observed nitrogen evolution from tar reflects the combined effect of these compounds. Since the nitrogen functionalities in tar seem to be somewhat coal independent, and since secondary tar pyrolysis reactions are homogeneous gasphase reactions, the nitrogen decay in tar is not expected to be strongly dependent on the original coal properties. It is interesting that the nitrogen content in tar is higher than the nitrogen content in the parent coal (daf) during the early stage of secondary pyrolysis. Such enrichment of nitrogen in tar has important implications and must be properly treated in order to predict the nitrogen release during coal pyrolysis. The delayed nitrogen release from tar can be justified by examining the chemical structure data obtained from these tar and soot samples (reported previously50). Figure 6 shows some of the chemical structure data derived from 13C NMR analysis for the parent Illinois coal and the tars at 1159 K, 1281, 1411, and 1534 K respectively. The relative values plotted on the y-axis represent the measured value divided by the value in the parent coal; this permits examination of trends on the same scale. For convenience, the measured values of the lattice parameters for each condition are shown above each bar in the figure. A description of the 13C NMR analysis technique and the definitions of lattice parameters are given by Solum and co-workers.50 It was found that the first set of reactions of tar was loss of side chains and oxygen functional groups (almost free of nitrogen). The side chains per aromatic cluster (50) Solum, M. S.; Sarofim, A. F.; Pugmire, R. J.; Fletcher, T. H.; Zhang, H. Energy Fuels 2001, 15, 961-971.


in the tar was 0.9 at 1159 K, and dropped rapidly to zero at 1411 K. At the same time, the number of bridges and loops per cluster dropped only slightly due to the loss of side chains. Nitrogen in tar usually exists in tightly bound ring structures, which react at higher temperatures. Therefore, since the tar released carbon, hydrogen, and oxygen in the form of side chains, but not nitrogen from the aromatic clusters, high nitrogen ratios were observed during the early stage of secondary pyrolysis. As the secondary reactions proceeded at temperatures above 1400 K, ring opening reactions became significant, where the tightly bound nitrogen in aromatic rings was released (usually as HCN) quickly from the tar. The fast decay of tar nitrogen is demonstrated clearly by the sharp slope of the tar nitrogen profiles in Figure 5 between 1400 and 1600 K. Nitrogen in the Soot. During the final stage of secondary pyrolysis, the nitrogen release rate from tar became much slower. This is most likely caused by the changes of chemical structure in tar. The 50% increase in the number of bridges and loops from 1280 to 1411 K, shown in Figure 6, is a measure of potential crosslinking sites, suggesting that ring polymerization reactions have taken place. Ring polymerization reactions, which lead to soot formation, accelerated at temperatures higher than 1400 K. Figure 6 shows the molecular weight per cluster and aromatic carbon per cluster increased about four times from 1411 to 1534 K. In addition, 13C NMR analysis showed that the aromaticity in tar was 89% at 1534 K. This means that at temperatures higher than 1534 K, the clusters in the tar were not only larger, but also more interconnected. The rate of soot formation in a coal system is sensitive to temperature and is very fast at temperatures higher than 1500 K.39 Therefore, a portion of the nitrogen in the tar could be integrated into the soot before being released through ring rupture. The large, interconnected networks developed in tar at high temperatures are thought to retard the further release of nitrogen, resulting in a significant amount of coal nitrogen incorporated in the soot under severe conditions. At 1752 K, about 6% of the volatile nitrogen was incorporated into soot for the Knife River lignite, 12% for the Illinois No. 6 coal, and 20% for the Utah coal. At 1858 K, 6% of the volatile nitrogen was found in the Black Thunder soot. These results are comparable with Chen’s finding,4 where 25% of volatile-N was reported to be integrated into soot for a hv bituminous coal and 10% for a subbituminous coal. Haussmann11 and Chen and Niksa2 reported that the fraction of coal nitrogen integrated into soot is constant even though soot yields increase dramatically during secondary pyrolysis. The nitrogen analysis in this study seems to support this idea. At temperatures above 1600 K, the nitrogen fraction in soot (there was almost no tar at these conditions) reached an asymptotic value which is coal dependent. Recently, Yu and co-workers14 reported that in the early stages of secondary pyrolysis, the tar nitrogen was converted into soot nitrogen, and that polymerization and ring rupture reactions dominated at higher temperatures. This means that the final fraction of coal nitrogen integrated into soot is largely determined by the early soot formation process.

Energy & Fuels, Vol. 15, No. 6, 2001 1519 Table 3. Best-Fit Kinetic Parameters for the Nitrogen Decay in the Tar AN (s-1)

EN (kJ/mol)

R∞ (unitless)

5.90 × 108

220

0.36

Nitrogen Decay Kinetics from Tar. The similar nitrogen decay profiles from tar for coals of different types make it easy to model using coal-independent parameters. An empirical first-order reaction mechanism was devised to fit the data in this study.

( )

EN dRN ) -AN exp (R - R∞) dt RT N

(2)

where AN and EN are the empirical preexponential factor and activation energy, respectively. It is also assumed that RN will reach an asymptotic value of R∞ at severe conditions to account for the observed slow rate of nitrogen release rate from soot. The coalindependent kinetic parameters derived from fitting the data in Figure 5 are shown in Table 3. The model prediction using these kinetic parameters is also shown in Figure 5. The nitrogen incorporated into tar can be easily calculated if the tar yields are known. The tar nitrogen yields (tarN) can be calculated using a secondary reaction model that is able to account for the tar decay and soot formation:40

tarN )

mN,tar [N]tarmtar ) ) RN ‚ ytar mN,coal [N]coalmcoal

(3)

where m represents mass and y represents fractional yield. It should be emphasized that the kinetic parameters derived from this study are empirical, and caution must be taken if these parameters are used in conditions significantly different from those used in this study. However, these results do provide insight into the nitrogen release mechanism in tars at high temperature and rapid heating conditions. Nitrogen Species in the Gas Phase. The relative amounts of HCN and NH3 formed during coal pyrolysis can have a substantial impact on the final fuel-N conversion. For example, Schafer and Bonn23 recently reported that at typical fluidized bed temperatures, NH3 is mainly converted to NO, but that HCN can either be converted to NO or N2. At higher temperatures typical of pulverized coal furnaces, the HCN/NH3 ratio may not affect the final amount of NOx formed.9 It is still not clear from previous studies whether HCN and NH3 are released independently from different functionalities in the coal, or whether, and to what degree, NH3 is a secondary product of HCN hydrogenation.5 FTIR measurements in this study showed that HCN and NH3 are the dominant nitrogen species evolved during coal pyrolysis. Figures 7 and 8 show the measured release of HCN and NH3 as a function of temperature in this study, measured at the 1 in. sampling height. For the higher rank coals, 32% of the nitrogen in the initial Illinois coal was transformed into HCN and NH3 by 1750 K, as was 42% for the Utah coal. Experiments conducted at 1858 K at the 3 in. location indicated that 50% of the Illinois coal nitrogen and 56% of the Utah coal nitrogen was transformed into HCN and NH3.40 The maximum amount of nitrogen incorpo-

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Figure 7. Yields of HCN and NH3 versus temperature for Illinois No. 6 and Utah (collected at 1 in. above the burner).

Figure 8. Yields of HCN and NH3 versus temperature for Black Thunder and Knife River (collected at 1 in. above the burner).

rated into the initial tar (shown in Figure 4) was only 25% for the Illinois coal and 20% for the Utah coal. Obviously, secondary reactions of tar cannot account for all the nitrogen released into the gas phase. Actually, the fraction of nitrogen released as tar is almost insignificant for low rank coals. The coal nitrogen found in the tars from the low temperature experiments is only 8% for the Black Thunder coal and 3% for the Knife River lignite (see Figure 4), while as much as 45% of the coal nitrogen was found in the form of HCN or NH3 at the highest temperatures in Figure 8. Thermal decomposition of char plays a critical role in nitrogen release at high temperatures. However, the nitrogen release rate from char was much slower than the tar nitrogen release. A substantial amount of the coal nitrogen was still trapped in the char even at the most severe conditions examined in this study. Observations regarding the release of gas nitrogen species during coal pyrolysis are listed below. 1. The release of HCN is very sensitive to temperature. A very fast release rate of HCN was observed during 1400-1600 K. However, HCN release slowed at temperatures above 1700 K. The release rate of NH3 was much slower than that of HCN and demonstrated

Zhang and Fletcher

a strong rank dependence. NH3 yields and release rates from low rank coals are greater than observed in high rank coals in this system. 2. A number of researchers suggested that NH3 is a secondary product formed from HCN hydrogenation.23,51-53 This reaction can occur in the gas phase,54 on the char surface,17 or on the soot surface.5 However, the mechanism of NH3 formation from HCN at a particular surface seems less convincing by examining the data in the current study. From Figures 7 and 8, it can be seen that the profiles of HCN release are similar for the Illinois No. 6, Utah, and Black Thunder coals. However, the NH3 yield from the Black Thunder coal is three times higher than that of the Illinois No. 6 coal and 30% higher than that of the Utah coal at 1858 K. Bassilakis et al.17 suggested that the major reaction pathway of NH3 is the reaction of HCN with the coal hydrogen in the pore structures of coal. A longer contact time in the pore structure results in a higher conversion rate from HCN. Since the same particle size was used for all the coals in this study, the formation of NH3 from HCN in the pore structure of the char seems unlikely. Ledesma5 proposed that soot may provide the required surface for the conversion from HCN to NH3. Apparently, this is not the case in this study, since the soot yield from the Black Thunder coal is much less than that from the Illinois No. 6 and Utah coals. Van der Lans and co-workers9 suggested that NH3 is formed from other nitrogen compounds (like HCN) by reaction with oxygen-derived radicals, since more NH3 has been found in experiments with larger amount of oxygen. The higher NH3 yield in pyrolysis of low rank coals is also attributed to their higher oxygen content. The data from the current study are consistent with the idea that NH3 formation may be enhanced by oxygen radicals, since more NH3 was formed from coals having a higher oxygen content. 3. The substantial amounts of NH3 found in this study contradict data from previous studies, where negligible amounts of NH3 were reported in entrained flow systems.2,11,48 In the data presented in Figures 7 and 8, NH3 release from high rank coals commenced at the same temperature as HCN. For low rank coals, NH3 release started at an even lower temperature than HCN. Therefore, it is unlikely that these initial amounts of NH3 were formed from HCN. However, the possibility of the formation of NH3 from HCN hydrogenation at more severe conditions cannot be excluded. The early occurrence of NH3 from low rank coals strongly suggests that there is a unique source of NH3 that is significant only in low rank coals. It is possible that certain types of quaternary nitrogen in coal may be the source for the early release of NH3 in this study, since greater amounts of quaternary nitrogen are found in low rank coals than in higher rank coals. From this study, it is clear that nitrogen release during coal pyrolysis is a very complicated process. The relative amount of HCN and NH3 is more dependent on the reactor configuration (entrained flow vs fluidized (51) Leppalahti, J. Fuel 1995, 74, 1363-1368. (52) Rudiger, H.; Greul, U.; Spliethoff, H.; Hein, K. R. G. Fuel 1997, 76, 201-205. (53) Friebel, J.; Kopsel, R. F. W. Fuel 1999, 78, 923-932. (54) Baumann, H.; Moller, P. Erdol, Kohle, Erdgas, Petrochem. 1991 44, 29-33.



However, a recent 13C NMR analysis of tar-char pairs collected at increasingly severe conditions (up to 1650 K) from a bituminous coal indicated that values of C/Cl in corresponding tar and char samples are roughly similar (i.e., within a value of 5).46 In fact, the reacted tar from Perry’s experiment had slightly higher values of C/Cl than the char. Therefore, it is possible that the much slower rate of nitrogen release from char cannot be solely attributed to growth in the cluster size during coal pyrolysis. The large differences in nitrogen release rates from char and tar makes it reasonable to believe that there may be differences in reactivity for the nitrogen functionalities in the char and tar. That is to say, the nitrogen functionalities trapped in char are less reactive than their counterparts in tar. The char nitrogen may either be transformed to more refractory forms induced by heat during pyrolysis, or may be stabilized by nearby functional groups, making the char nitrogen extremely resistant to thermal decomposition. More research is needed to verify the validity of this hypothesis. Summary and Conclusion

Figure 9. [N/C]tar/[N/C]coal versus temperature for the tar and the char in this study.

bed) and local environment (N2 vs combustion gas) than on the coal properties (such as chemical structure and nitrogen compositions). Therefore, a simple correlation of the release of nitrogen species with nitrogen content in coal34 does not seem to be justified. Nitrogen Evolution from Char. After the tar escapes the coal matrix, nitrogen release takes different routes in the tar and in the remaining char, although the nitrogen functionalities in the tar and char are similar. Thermal decomposition of char is a major nitrogen release mechanism at high temperatures. Figure 9 shows the normalized N/C ratio versus temperature for the coals in this study. The normalized N/C ratio is obtained from the N/C ratio in the tar or char divided by the N/C ratio in the parent coal. The normalization is made to eliminate the data scatter caused by the type of coal, thus the normalized values can better reflect the release rate of the nitrogen during coal pyrolysis. A striking similarity of the nitrogen decay profiles for the four coals is noticed in the tar samples. The nitrogen decay profiles in the char samples are also similar to each other, but only decreased slightly in this temperature range (1159 K to 1858 K). The reason for the delayed nitrogen release in the char is not yet well understood. Chen4 suggested that the extensive aromatic ring structures built up in the char retarded the release of heteroatoms at elevated temperatures, a similar mechanism to that previously discussed for tar at the late stage of secondary reaction. The number of aromatic carbons per cluster (C/Cl) in the tar/soot samples, reported by Solum et al.,50 is plotted in Figure 9. This shows that the rate of nitrogen decay in the tar/ soot samples is proportional to the rate of growth of the aromatic cluster size. Unfortunately, 13C NMR analyses of the corresponding char samples were not performed.

The pyrolysis of four coals in a flat flame burner has provided a unique opportunity to examine the nitrogen transformations under high-temperature, high heating rate conditions. The results confirm much of what has been reported in previous studies. The initial released nitrogen is contained almost exclusively in the tar for coals of all types. If the initial volatiles consist mainly of tar, such as in bituminous coals, initial nitrogen release is related to initial mass release. For low rank coals where light gases such as CO and CO2 are released before tar, initial nitrogen release lags behind initial mass release. Release of light gas nitrogen (e.g., HCN and NH3) starts at a later stage than tar nitrogen release. The two major sources of light gas nitrogen at elevated temperatures are secondary reactions of tar and thermal decomposition of char. The confirmation of results reported previously in the literature lends confidence to the experimental results. The major new findings from the current study are summarized below: 1. The nitrogen release rate from tars during secondary pyrolysis is much greater than from the corresponding chars. 2. The nitrogen functionalities in both tar and char are thought to be similar. The carbon aromaticities and number of carbons per cluster in the tars and chars from this experiment were also similar. The difference in nitrogen release rates in the tar and in the char do not therefore appear to be caused by differences in aromatic structure. 3. The release of nitrogen from tar was largely coal independent during secondary pyrolysis, based on the four coals studied here. 4. The release of nitrogen from the tar due to secondary pyrolysis was successfully modeled using a simple first-order rate expression. 5. In the entrained flow pyrolysis experiments reported here, NH3 was released earlier than HCN for low rank coals. NH3 was released at the same time as HCN for high rank coals. It is not clear what chemical feature of low rank coals is responsible for this behavior.

1522


6. Significant amounts of NH3 were observed from all four coals. HCN yields from the three highest rank coals increased significantly at ca. 1400 K, while such corresponding increases in NH3 yields were not observed. The HCN yield from the lignite did not surpass the NH3 yield until ca. 1700 K.

Zhang and Fletcher

Acknowledgment. This work was supported by the DOE/UCR contract DE-PS22-97PC97200 and the Advanced Combustion Engineering and Research Center (ACERC) at BYU. EF010118G

Nitrogen Transformations during Secondary Coal Pyrolysis - Energy

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