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Energy & Fuels 2003, 17, 694-698
Formation of N2 from Pyrrolic and Pyridinic Nitrogen during Pyrolysis of Nitrogen-Containing Model Coals Zhiheng Wu* New Energy and Industrial Technology Development Organization, Tokyo 170-6028, Japan
Yoshikazu Sugimoto and Hiroyuki Kawashima Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan Received September 4, 2002. Revised Manuscript Received March 18, 2003
Two model coals, one containing pyrrolic nitrogen and another containing pyridinic nitrogen, have been used to study N2 formation from different nitrogen structures during coal pyrolysis. The formation of N2 during pyrolysis of the model coals with and without a catalyst has been investigated in a fixed-bed reactor at 2.5-10 °C/min up to 1000 °C. The pyrolysis results show that calcium and iron catalyze both pyrrolic and pyridinic nitrogen in model coals to N2. Calciumcatalyzed N2 formation occurs at temperatures higher than 800 °C; however, the iron catalyzes N2 formation at temperatures below 800 °C. The remarkable formation of N2 results in low residual nitrogen in char. It is suggested that the calcium and iron catalysts catalyze N2 formation mainly through solid-phase reactions of pyrrolic and pyridinic nitrogen forms in char.
Introduction In recent years, considerable attention has been paid to the research on the fate of coal nitrogen (coal-N) during pyrolysis, since it is related with NOx and N2O emissions during coal combustion. According to previous publications, coal-N is initially released as tar, which is then decomposed into HCN and NH3,1-3 and the nitrogen distribution among these products depends strongly on the pyrolysis conditions and coal type.4,5 However, little attention to N2 formation has been paid so far. If coal-N can be removed efficiently as harmless N2 during pyrolysis, this method would contribute to reduction of NOx and N2O emissions, because such pollutants originate mostly from coal-N. From such a point of view, we have studied extensively N2 formation from many coals with different ranks during a fixed-bed pyrolysis6 and found that ironcontaining minerals present in some low-rank coals,7,8 and calcium, iron, molybdenum catalysts can catalyze N2 formation from char nitrogen.9-11 However, we do not know which kind of nitrogen is catalyzed to N2 by the metals, because pyrrolic and pyridinic nitrogen * Corresponding author. E-mail:
[email protected]. (1) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710-720. (2) Niksa, S. Energy Fuels 1995, 9, 467-487. (3) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. In Proceedings of the Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 1259-1267. (4) Takagi, H.; Isoda, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1999, 13, 934-940. (5) Kidena, K.; Hirose, Y.; Aibara, S.; Murata, S.; Nomura, M. Energy Fuels 2000, 14, 184-189. (6) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 477-482. (7) Wu, Z.; Ohtsuka, Y. Energy Fuels 1997, 11, 902-908. (8) Wu, Z.; Ohtsuka, Y. Energy Fuels 1996, 10, 1280-1281.
forms are reported to coexist in coal.12-14 To solve this problem, we have successfully prepared model coals containing well-defined nitrogen forms, that is, pyrrolic and pyridinic nitrogen,15 and furthermore, these model coals have a carbon structure similar to that of natural brown coal but contain no minerals. It would be effective to study the reactivity of different nitrogen functionality using the model coals. Therefore, the present work focuses on making clear the effect of calcium and iron catalysts on N2 formation from different nitrogen forms, pyrrolic and pyridinic nitrogen, during a temperatureprogrammed pyrolysis of nitrogen-containing model coals. Experimental Section Samples. Two model coals containing well-defined nitrogen forms with size fraction of 45-74 um were used in this study. The preparation and characterization of the nitrogen-containing model coals have been described in detail elsewhere15 and is thus simply explained below. A mixture of cellulose with 2-hydroxycarbazle and cellulose with 6-hydroxyquinoline was heated in an autoclave at 0.5 °C/min up to 200 °C under a pressure of 50 kg/cm2 He and held for 25 h at that temperature. During the compaction heat treatment, the mixture was (9) Wu, Z.; Sugimoto, Y.; Kawashima, H. Energy Fuels 2000, 14, 1119-1120. (10) Wu, Z.; Sugimoto, Y.; Kawashima, H. Energy Fuels 2002, 16, 451-456. (11) Tsubouchi, N.; Oshima, Y.; Xu, C.; Ohtsuka, Y. Energy Fuels 2001, 15, 158-162. (12) Kambara, S.; Takarada, T.; Yamamoto, Y.; Kato, K. Energy Fuels 1993, 7, 1013-1020. (13) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896-906. (14) Wo´jtwicz, M. A.; Pels, J. A.; Moulijin, J. A. Fuel 1995, 74, 507516. (15) Wu, Z.; Sugimoto, Y.; Kawashima, H. Fuel 2001, 80, 251-254.
10.1021/ef020184y CCC: $25.00 © 2003 American Chemical Society Published on Web 04/17/2003
Formation of N2 from Coal Pyrolysis always pressed by a piston under a pressure of 100 kg/cm2. Nitrogen structure in the model coals has been investigated using GC-NPD and GC-MS analyses of their high-temperature tars15 and also analyzed by pyrolysis GC-NPD and pyrolysis GC-MS of the model coals.16 All the analysis results have shown that one model coal, denoted as HC coal, contains only pyrrolic nitrogen, and another model coal, denoted as HQ coal, contains only pyridinic nitrogen. The results of the elemental analysis (wt % daf) of the two model coals were C, 69.6; H, 4.7; N, 1.3; O, 24.4 for HC coal and C, 71.0; H, 4.9; N, 0.9; O, 23.2 for HQ coal. Catalyst Addition. Iron and calcium catalysts were loaded onto the model coals. The calcium catalyst was added onto the model coals by mixing in a saturated aqueous solution of Ca(OH)2 at room temperature, followed by evaporation of the water in vacuo at 50 °C. The iron catalyst was precipitated from an aqueous solution of FeCl3 by using Ca(OH)2. A mixture of model coal particles and FeCl3 solution was stirred at room temperature, and a sufficient amount of Ca(OH)2 powder was then added to load the iron as FeOOH on the model coals.17 The iron-bearing sample was separated by filtration and washed repeatedly with boiling deionized water to remove excess Ca2+ and Cl- ions. The metal loading amount was 0.53.0 wt % for both Ca and Fe catalysts. Pyrolysis and Nitrogen Analysis. Pyrolysis experiments of the model coals with and without a catalyst were carried out using a fixed-bed reactor. About 0.2 g of the dried sample was heated under a flow of high-purity He (100 cm3/min) at 2.5 or 10 °C/min up to 1000 °C and then kept for 30 min at that temperature. Pyrolysis products were separated into gas, tar, and char. Tar was recovered by toluene from the tar trap containing cotton wool, and char remaining in the quartz cell was recovered and weighed. The concentration of N2 in the gas was on-line analyzed with a high-speed gas chromatograph at 5-min intervals. Nitrogen in the char was measured with a conventional, combustion-type elemental analyzer. Nitrogen conversions to N2 and char-N are expressed in percent of the total nitrogen in the feed sample.
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Figure 1. Effect of Ca addition on the rate of N2 formation during pyrolysis of HC coal at 10 °C/min and 1000 °C.
Results and Discussion Formation of N2 during Pyrolysis of Model Coal Containing Pyrrolic Nitrogen. Figure 1 shows the N2 formation profiles formed during the pyrolysis of HC coal, which contains pyrrolic nitrogen only. The effect of Ca addition on the rate of N2 formation is also plotted in Figure 1, where the rate is expressed as millimoles of N2 evolved per minute and grams of total nitrogen in the feed sample. With the HC coal, in the absence of the Ca, N2 started to evolve at about 600 °C and showed two peaks at 650 and 1000 °C. The addition of 0.5 wt % Ca catalyst promoted N2 formation at temperatures higher than 900 °C but decreased the formation rate a little at temperatures below 800 °C. When the Ca addition was increased to 3.0 wt %, the low-temperature peak of the N2 formation observed with the HC coal at around 650 °C was completely removed by the Ca addition, and a large peak of the N2 formation appeared at around 900 °C. This large peak of the N2 formation corresponds to the high-temperature peak of the N2 formation observed with HC/(Ca 0.5 wt %) sample, and the increase of Ca addition shifted this high-temperature peak from 1000 to 900 °C. It is evident that the Ca catalyst suppressed the N2 formation at temperatures (16) Wu. Z.; Sugimoto, Y.; Kawashima, H. J. Jpn. Inst. Energy 2001, 80, 97-105. (17) Asami, K.; Ohtsuka, Y. Ind. Eng. Chem. Res. 1993, 32, 16311636.
Figure 2. Effect of Fe addition on the rate of N2 formation during pyrolysis of HC coal at 10 °C/min and 1000 °C.
below 750 °C but promoted the N2 formation remarkably in the temperatures between 800 and 1000 °C. Since HC coal contains only pyrrolic nitrogen, this observation suggests that the Ca can catalyze pyrrolic nitrogen in model coal to N2 at the temperatures higher than 800 °C. Figure 2 gives the effect of iron catalyst on N2 formation during pyrolysis of HC coal at 1000 °C. The addition of 0.5 wt % Fe lowered the onset temperature of N2 formation by almost 100 °C and promoted N2 formation at temperatures between 450 and 1000 °C. The formation profile from the HC coal with the addition of 0.5 wt % Fe showed a large formation peak at around 650 °C, a shoulder peak at around 750 °C, and a small peak at around 1000 °C. Because iron catalyst has been
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Figure 3. Detailed N2 formation from HC coal during pyrolysis at 2.5 °C/min and 1000 °C.
reported to catalyze N2 formation from coal nitrogen at temperatures below 800 °C,10 the large peak together with the shoulder peak of the N2 formation from the HC coal with 0.5 wt % Fe addition probably comes from the catalysis of the iron. As to the small peak observed at around 1000 °C, the peak might be caused by presence of Ca catalyst, since a small amount (
