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Remarkable Formation of N2 from a Chinese Lignite during Coal Pyrolysis Zhiheng Wu and Yasuo Ohtsuka* Research Center for Carbonaceous Resources, Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan Received March 25, 1996. Revised Manuscript Received August 20, 1996 Nitrogen release during coal pyrolysis influences the NOx and N2O emissions in the subsequent combustion process.1-3 Previous studies on the fate of coal nitrogen during pyrolysis in inert gas atmospheres have shown that NH3 and HCN are the major gas products.4-10 Little attention to N2 formation has been paid so far,10-12 however, although it is expected that efficient conversion of coal nitrogen to N2 can reduce the NOx and N2O emissions, in particular from fluidized bed combustion where such pollutants originate from coal nitrogen alone. In this paper, we report high conversion of coal nitrogen to N2 upon pyrolysis and clarify the factors controlling N2 formation. Ten coals with different ranks and nitrogen contents ranging 0.7 to 2.2 wt % (daf) were pyrolyzed with a quartz-made fixed bed reactor with infrared image lamps attached. A Chinese lignite was used mainly among them, and the ultimate analysis was as follows: C, 72.0; H, 5.0; N, 1.7; S, 0.4; O, 20.9 wt % (daf). About 0.5 g of coal particles with fraction size 150-250 µm, held into a graphite-made cell in the reactor, were heated in high-purity He to a predetermined temperature and soaked for 2 min. Heating rate and pyrolysis temperature were 400 K/min and 1000 °C, respectively, unless otherwise stated. Special care was taken to ensure that the entire system was free from any leakage before every run. Pyrolysis products in the reactor effluent were first condensed as tar by two cold traps, and the remainder was collected as gas in a plastic bag laminated with aluminum foil. N2, HCN, and NH3 in the gas were analyzed, and the nitrogen contents in the tar and the char remaining in the reactor were deter* Author to whom correspondence should be addressed (e-mail
[email protected]). (1) Hjalmarsson, A.-K. In NOx Control Technologies for Coal Combustion; IEACR/24; IEA Coal Research: London, 1990. (2) Boardman, R.; Smoot, L. D. In Fundamentals of Coal Combustion for Clean and Efficient Use; Smoot, L. D., Ed.; Coal Science and Technology 20; Elsevier: Amsterdam, 1993; pp 433-509. (3) Wo˘ jtowicz, M. A.; Pels, J. R.; Moulijn, J. A. Fuel Process. Technol. 1993, 34, 1-71. (4) Blair, D. W.; Wendt, J. O. L.; Bartok, W. Sixteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1976; pp 475-489. (5) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749-755. (6) Freihault, J. D.; Zabielski, M. F.; Seery, J. D. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 1159-1167. (7) Baumann, H.; Mo¨ller, P. Erdoel, Erdgas, Kohle 1991, 44, 2933. (8) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; pp 1259-1267. (9) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710-720. (10) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels 1993, 7, 1013-1020. (11) Phong-Anant, D.; Wibberley, L. J.; Wall, T. F. Combust. Flame 1985, 62, 21-30. (12) Ohtsuka, Y.; Furimsky, E. Energy Fuels 1995, 9, 141-147.
mined. N-conversion to each component was calculated on the basis of total nitrogen in feed coal. The major nitrogen forms of gas products released at 600 °C were HCN and NH3, whereas only a slight amount of N2 was formed. When the temperature was increased up to 1000 °C, however, N2 was the dominant product regardless of coal type. Figure 1 shows the effect of coal rank on N-conversion to N2 at 1000 °C. The conversion was independent of carbon content in the coal and the highest (≈50%) for a Chinese lignite among the 10 coals examined. There was no relationship between N2 formation and nitrogen content in coal or the amount of volatile matter released at 1000 °C. When the height of coal particles in the fixed bed was reduced to one-fifth by decreasing the feed weight of the lignite from 0.5 to 0.1 g, the conversion to N2 at 1000 °C was almost unchanged, which suggests insignificant secondary decomposition reactions of volatile N compounds into N2 during their passage through the bed. These reactions may take place on reactor materials such as stainless steel, since Fe and Ni are catalytically active for NH3 decomposition at 900 °C.13 To remove this possibility, no metal materials were used in the present reactor, and the whole system was made of less active quartz and graphite. Nitrogen distribution at 1000 °C of a Chinese lignite without and with demineralization is illustrated in Figure 2, where “dem” means the coal demineralized by HCl washing at 50 °C. Nitrogen mass balance of 9899% was excellent. Demineralization changed the nitrogen distribution drastically; N-conversion to N2 decreased considerably from 48 to 13%, whereas char-N increased from 37 to 68%, though tar-N, HCN, and NH3 were not changed largely. Figure 2 also shows that the increase in heating rate from 400 to 1400 K/min does not change the nitrogen distribution of the raw coal significantly. In other words, heating rate in this range is an insignificant factor in high conversion to N2. The amount of volatile matter evolved at 1000 °C from a Chinese lignite was almost unchanged by demineralization. This suggests no significant changes in the pore and surface structures which may result in different speciation of N-containing compounds upon pyrolysis. It is therefore likely that some minerals in the lignite promote N2 formation. The ashes from the raw and demineralized samples were thus analyzed by X-ray diffraction (XRD) measurements. The results are summarized in Table 1, where low-temperature ash (LTA) and high-temperature ash (HTA) are prepared with a plasma asher and by burning up at 815 °C, respectively. The comparison of LTA and HTA of the raw coal reveals the appearance (13) Leppa¨lahti, J.; Simell, P.; Kurkela, E. Fuel Process. Technol. 1991, 29, 43-56.
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Figure 1. Rank dependence of N-conversion to N2 at 1000 °C.
Figure 2. Effects of demineralization and heating rate on the nitrogen distribution of a Chinese lignite at 1000 °C. Table 1. Ash Contents and Species Identified by X-ray Diffraction Measurements sample raw coal
dem coald
type of ash content (wt %) asha LTA
7.7c
HTA
4.1
LTA HTA
1.6c 0.8
species identified by XRDb quartz (m), bassanite (m), kaolinite (w) anhydrite (s), Ca2MgAlFeO6 (m), Ca2MgFe2O6 (m), quartz (w), lime (vw), hematite (vw) quartz (vs), kaolinite (vw) quartz (vs)
aLTA, low-temperature ash; HTA, high-temperature ash. b XRD intensity designated vw (very weak), w (weak), m (medium), s (strong), and vs (very strong). c Including small amounts of coal residues. d Demineralized coal.
in HTA of both lime (CaO) and new species attributable to Ca2MgAlFeO6, Ca2MgFe2O6, and hematite (Fe2O3) as well as the transformation of bassanite (CaSO4‚1/2H2O) to anhydrite (CaSO4). The single and mixed species observed in HTA alone exist probably as the highly dispersed forms in the raw coal, since these species are not formed by the plasma ashing which allows mineral components to be liberated from coal in almost inherent forms. Table 1 also shows that the demineralization by HCl washing results in not only about 80% reduction in the ash content of LTA or HTA but also removal of anhydrite, lime, hematite, and mixed oxides from HTA. The latter can also be verified by the observation that the HCl solution after demineralization mainly contains the Ca and Fe cations. These observations point out that the Fe- and/or Cacontaining minerals inherently present in the lignite are responsible for the remarkable formation of N2. We have shown that fine particles of iron oxyhydroxide precipitated onto low-rank coals promote remarkably N2 formation during pyrolysis at 900-1000 °C.14-16 When a nanophase iron oxide catalyst with the particle size
of 3 nm was added to the demineralized lignite and then the resulting sample was pyrolyzed at 1000 °C, Nconversion to N2 increased 2-fold, though it did not reach the level for the raw coal. These results support the assumption that the Fe-containing minerals are catalytically active for N2 formation. Higher conversion to N2 for the raw coal than for the iron-loaded, demineralized coal suggests some possibilities. First, the Fecontaining minerals have a larger catalytic effect than the externally added iron. This is likely, since such minerals would be more finely dispersed due to the ionexchangeable forms.17 Second, the Ca species such as bassanite and ion-exchanged Ca catalyze the conversion to N2. Third, mineral components containing Fe and Ca (and Mg) show a synergistic effect on N2 formation. Investigation of the second and third possibilities will be the subject of future work. There are two formation routes of a large amount of N2 evolved from a Chinese lignite, as suggested in the selective conversion of coal-N to N2 with iron.15,16 One route is via tar-N and HCN. Since N-conversions to these nitrogen forms were increased by demineralization (Figure 2) and the externally added iron can catalyze the decomposition of tar-N and HCN into N2,15,16 some of the N2 arises from secondary decomposition reactions of these nitrogens on the surface of active mineral components. However, its contribution to N2 formation would be minor, since the sum of the amounts of tar-N and HCN increased by demineralization was only one-fifth of the amount of N2 decreased (Figure 2). Another source of N2 is char-N and/or precursors, since N2 formation starts after almost complete release of volatile-N,11,12 as observed in the pyrolysis of a Chinese lignite; N-conversion to volatile-N comprising tar-N, HCN, and NH3 at 600 °C was 12%, which reached ≈90% of that at 1000 °C, whereas the conversion to N2 at 600 °C was only 0.4%, which was
