Energy & Fuels 1997, 11, 477-482
477
Nitrogen Distribution in a Fixed Bed Pyrolysis of Coals with Different Ranks: Formation and Source of N2 Zhiheng Wu and Yasuo Ohtsuka* Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan Received August 5, 1996X
Sixteen coals from brown to bituminous coal have been pyrolyzed in high-purity He at 400 K/min up to 1000 °C with a fixed bed reactor, and the nitrogen distribution has been examined in detail. Nitrogen mass balances fall within 96-103%. Not only volatile nitrogen (HCN, NH3, and tar) but also N2 is formed, and among these N2 is the dominant product for almost all of the coals. Conversion of coal nitrogen to N2 depends strongly on coal type. A German brown coal gives the highest conversion of ≈60%, followed by ≈50% from a Chinese lignite. No relationship between N2 formation and coal rank is observed. As N2 increases, volatile nitrogen decreases slightly, but char nitrogen decreases remarkably, which means a strong, reverse correlation between N2 and char nitrogen. When the lignite char devolatilized at 600 °C is heated to 1000 °C, conversion of the nitrogen to N2 proceeds almost exclusively. These observations show that N2 originates mostly from char nitrogen and/or precursors. The mechanism of N2 formation is discussed mainly in terms of the catalysis of solid phase reactions by Fe-containing minerals in low-rank coals.
Introduction Many studies on the fate of the nitrogen present in coal (coal-N) upon pyrolysis have been carried out, since understanding nitrogen evolution mechanisms is important for elucidating the formation of NOx and N2O during subsequent combustion. According to previous publications, coal-N is initially released as tar,1-5 which is then decomposed into HCN and NH3.6-12 The partitioning to HCN and NH3 depends on pyrolysis conditions.13,14 It appears that HCN alone is formed in rapid pyrolysis, in which secondary decomposition reactions are minimized,5,15 whereas NH3 is the dominant product in slow heating rate pyrolysis.3,10 * Author to whom correspondence should be addressed (e-mail
[email protected]). X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749-755. (2) Freihault, J. D.; Zabielski, M. F.; Seery, J. D. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 1159-1167. (3) Phong-Anant, D.; Wibberley, L. J.; Wall, T. F. Combust. Flame 1985, 62, 21-30. (4) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P.; Lafleur, A. L. Energy Fuels 1988, 2, 775-782. (5) Chen, J. C.; Castagnoli, C; Niksa, S. Energy Fuels 1992, 6, 264271. (6) Blair, D. W.; Wendt, J. O. L.; Bartok, W. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1976; pp 475-489. (7) Axworthy, A. E.; Dayan, V. H.; Martin, B. G. Fuel 1978, 57, 2935. (8) Baumann, H.; Mo¨ller, P. Erdoel Erdgas Kohle 1991, 44, 29-33. (9) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1259-1267. (10) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710-720. (11) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels 1993, 7, 1013-1020. (12) Ohtsuka, Y.; Furimsky, E. Energy Fuels 1995, 9, 141-147. (13) Johnsson, J. E. Fuel 1994, 73, 1398-1415. (14) Leppa¨lahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43, 1-45. (15) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Krause, J. L. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 1139-1149.
S0887-0624(96)00122-3 CCC: $14.00
However, N2 formation has been neglected in most studies, despite the fact that a high conversion to N2 during coal pyrolysis can be expected to reduce the NOx and N2O emissions from coal-N in the subsequent combustion process. There have been only a few papers which show that N2 is formed significantly at high temperatures of 1100-1400 °C.2,3,11 We have recently found that N2 is the predominant N-containing gas during the temperature-programmed pyrolysis of a subbituminous coal up to 1000 °C12 and that conversion of coal-N to N2 reaches ≈50% in the fixed bed pyrolysis of a Chinese lignite at 1000 °C.16 These observations indicate that N2 formation depends on coal type and pyrolysis conditions and that nitrogen distribution studies should take N2 into account. The objective of the present work is therefore to make clear the influence of coal type on the distribution of N2, HCN, NH3, and the nitrogen in condensed materials in the fixed bed pyrolysis of 16 coals with different carbon and nitrogen contents and to elucidate possible formation mechanisms and sources of N2. Experimental Section Coal Samples. Sixteen coals from various countries were used in this study. All of the samples from brown to bituminous coal were air-dried, ground, and sieved to coal particles with size fraction 150-250 µm. The ultimate and proximate analyses are given in Table 1, where all of the coals are represented as the corresponding code names throughout the paper. Pyrolysis Runs. The experiments were carried out with a fixed bed quartz reactor (4 cm i.d., 37 cm long) connected directly to two Pyrex traps. In every run, about 0.5 g of coal particles, after drying in a stream of N2 at 110 °C, were first charged into a rectangular graphite cell (6 mm wide, 46 mm long, 8 mm deep), which was then placed on a graphite holder held in the center of the reactor. After evacuation, high-purity (16) Wu, Z.; Ohtsuka, Y. Energy Fuels 1996, 10, 1280-1282.
© 1997 American Chemical Society
478 Energy & Fuels, Vol. 11, No. 2, 1997
Wu and Ohtsuka
Table 1. Ultimate and Proximate Analyses of Coals ultimate analysis, wt % (daf) coal
code
countrya
Rhein Braun Loy Yang Morewell Bienfait Wyoming Zalainuoer Taiheiyo Obed Mountain Illinois No. 6 Coal Valley Puertollano Upper Freeport Leopold Blair Athol Liddell Hunter Valley
RB LY MW BF WM ZN TK OM IL CV PL UF LP BA LD HV
GER AUS AUS CAN USA CHI JPN CAN USA CAN SPA USA GER AUS AUS AUS
a
proximate analysis, wt % (db)
C
H
N
S
O
ash
VM
FC
64.4 65.9 65.9 69.2 69.9 72.0 73.9 74.0 76.5 76.8 78.3 78.8 79.7 80.7 81.1 81.2
4.8 4.1 4.9 4.4 4.9 5.0 6.1 5.3 5.3 4.8 5.0 5.4 5.2 4.5 5.4 5.5
0.74 0.68 0.66 1.3 1.1 1.7 1.4 1.8 1.6 1.1 2.2 1.8 1.6 2.0 2.1 2.2
0.3 0.3 0.2 0.6 0.5 0.4 0.3 0.6 3.5 0.3 1.2 1.9 1.3 0.3 0.6 0.8
29.8 29.0 28.3 24.5 23.6 21.0 18.2 18.3 13.1 16.9 13.3 12.1 12.2 12.5 10.8 10.3
4.7 0.6 2.0 16.5 5.4 4.1 14.0 12.6 9.0 10.5 9.4 8.8 3.4 7.3 8.3 8.2
54.4 54.2 54.4 37.4 45.0 42.5 44.2 40.7 36.5 35.4 32.3 36.2 35.7 31.2 33.9 33.6
40.9 45.2 43.6 46.1 49.6 53.4 41.8 46.7 54.5 54.1 58.3 55.0 60.9 61.5 57.8 58.2
GER, Germany; AUS, Australia; CAN, Canada; USA, United States of America; CHI, China; JPN, Japan; SPA, Spain.
He (>99.9999%) flowed at 100 cm3(STP)/min into the whole system including the reactor and traps, and the exit gas was collected in a plastic bag laminated with aluminum foil to ensure that N2 concentration in the bag was
