Energy & Fuels 2000, 14, 1119-1120
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Communications Effect of Calcium Catalyst on Coal Nitrogen Removal during Pyrolysis Zhiheng Wu,* Yoshikazu Sugimoto, and Hiroyuki Kawashima† Department of Surface Chemistry, National Institute of Materials and Chemical Research, Tsukuba 305-8565, Japan Received March 15, 2000. Revised Manuscript Received July 15, 2000 Introduction NOx and N2O are harmful pollutants emitted from coal combustion. Coal combustion comprises two main stages, i.e., coal devolatilization and subsequent combustion of the volatiles and char. Since coal pyrolysis or devolatilization occurs at the primary stage of coal combustion, efficient conversion of coal nitrogen (coal-N) to inert substance such as N2 in the pyrolysis stage can reduce NOx and N2O emissions in the subsequent combustion process. From such a point of view, we have studied extensively N2 formation from many coals with different ranks during a fixed-bed pyrolysis, and found that inherent iron in some low rank coals can remarkably promote N2 formation from coal-N.1,2 On the other hand, calcium is well-known as an active catalyst for coal desulfurization and gasification,3 and the effect of calcium catalyst on NOx and N2O emissions during coal combustion has also been widely investigated. 4-7 However, almost no attention to the effect of the calcium on coal-N release under pyrolysis conditions has been paid so far, and the influence of calcium catalyst on the fate of coal-N, especially on N2 formation during pyrolysis, remains unclear. In a previous paper,2 it has been reported that calcium catalyst added to a demineralized Chinese lignite shows almost no effect on N2 formation during pyrolysis at 400 °C/min up to 1000 °C. In the present study, we focus on making clear the effect of calcium catalyst on N2 formation during a slow heating rate (10 °C/min) pyrolysis of an Australian brown coal, and report for the first time that calcium catalyst can selectively convert coal-N to N2 in a temperature-programmed fixedbed pyrolysis. Experimental Section Yallourn (YL) brown coal with size fraction of 99.9999%, 100 cm3/min) at 10 °C/min up to 1000 °C and then soaked for 30 min. Pyrolysis products were separated into gas, tar, and char. N2 in the gas was on-line analyzed in 5-min intervals with a micro gas chromatograph (Microsensor Technology, Inc., M200) equipped with a thermal conductivity detector. Nitrogen in the char (char-N) was determined with a conventional combustion-type elemental analyzer. Yields of N2 and char-N are expressed in percent of total nitrogen in feed sample. Two experiments were carried out for each condition, and excellent reproducibility was obtained.
Results and Discussion Figure 1 shows the effect of calcium addition on the rate of N2 formation from YL coal. With YL raw coal, N2 started to evolve at about 500 °C and showed two maximums at 550 and 800 °C. When 3.0 wt % calcium catalyst was added to YL coal, N2 formation started at almost the same temperature as that for YL coal. The addition of calcium suppressed N2 formation at temperatures below 650 °C, but another two peaks of N2 formation appeared at around 700 and 950 °C, and N2 formation showed a maximum at around 700 °C. (8) Asami, K.; Ohtsuka, Y. Ind. Eng. Chem. Res. 1993, 32, 1631-1636.
10.1021/ef000051h CCC: $19.00 © 2000 American Chemical Society Published on Web 08/15/2000
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Communications Table 1: Effects of Demineralization and Calcium Addition on Yields of N2 and Char-N in the Pyrolysis of YL Coal at 1000 °C yields, % sample
N2
char-N
yields, % sample
N2
YL 60 21 YLD 15 YL/Ca 70 13 YLD/Ca 55 (+40)a a Difference in yield by calcium addition.
Figure 2. Effect of calcium, iron, sodium, and magnesium addition on the rate of N2 formation from YL demineralized coal.
Since inherent minerals, especially iron-containing minerals in low rank coals, can promote N2 formation during pyrolysis,2 the effect of calcium catalyst on N2 formation cannot be clarified when calcium was added onto YL raw coal, and the synergistic effect of the inherent minerals with the added calcium catalyst makes N2 formation profiles complicated (Figure 1). To eliminate the influence of inherent minerals and make clear the effect of calcium catalyst on N2 formation, YL coal was demineralized, and then the calcium was added onto the demineralized coal by the same way used for YL raw coal. The effects of demineralization and calcium addition on the rate of N2 formation for YL coal are illustrated in Figure 2. Comparison of N2 formation profiles from YL raw coal and YLD coal in Figures 1 and 2 reveals that the demineralization drastically changed the formation profiles. Two large peaks of N2 formation existed at around 550 and 800 °C for YL coal (Figure 1) were removed by demineralization, and only two small peaks existed at around 650 and 1000 °C for YLD coal (Figure 2). The comparison points out that minerals present in YL coal play crucial roles in N2 formation. When 3 wt % of calcium was added onto YLD coal, N2 formation was suppressed at temperatures below 750 °C. On the other hand, at the temperatures higher than 800 °C, N2 formation increased drastically as the increase of temperature, and reached a maximum at around 950 °C. This observation demonstrates that calcium can selectively catalyze N2 formation from coal-N at 800-1000 °C. Ash content in YL coal was decreased from 1.6 to 0.15 wt % by demineraliztion. Fe was the major metal in YL coal followed by Mg, Ca, and Na, and their contents are 0.38, 0.19, 0.17, and 0.07 wt % on a dry coal basis. ICP analysis of the HCl leaching solution reveals that 90% of Fe or Ca, 70% of Mg or Na were removed by the demineralization. To confirm the effects of Fe, Mg, and Na on N2 formation, we also pyrolyzed YLD coal with addition of Fe, Mg, and Na, and the results are also plotted in Figure 2. The Fe drastically promoted N2 formation at temperatures between 450 and 800 °C, and showed a maximum at around 700 °C. But Mg and Na showed almost no positive effect on N2 formation. These observations suggest that the two peaks of N2 formation at 550 and 800 °C observed for YL coal and the large peak at 700 °C observed for YL/Ca sample in Figure 1 may arise from the catalysis of coal-N by inherent iron in YL coal. Since Ca catalyzed N2 formation at a temperature between 800 and 1000 °C (Figure 2), the peak existing at around 950 °C for YL/Ca sample in Figure 1 probably comes from the catalysis of coal-N by calcium. Table 1 summarizes the effects of demineralization and calcium addition on the yields of N2 and char-N after
char-N 54 18 (-36)a
pyrolysis of YL coal at 1000 °C. When YL coal was pyrolyzed at 1000 °C, 60% of nitrogen in YL coal was released as N2, and 21% of nitrogen was retained in char. Addition of 3 wt % calcium increased N2 from 60% to 70%, whereas decreased char-N from 21% to 13%. On the other hand, the demineralization drastically decreased N2 from 60% to 15%, but increased char-N from 21% to 54%. When calcium was added onto YL demineralized coal, N2 increased from 15% to 55% but char-N decreased from 54% to 18%. It should be noted that the increased amount of N2 (40%) is roughly the same as the decreased amount of char-N (36%) by calcium addition for YLD coal. Since the release of volatile nitrogen, that is, nitrogen in tar, HCN, and NH3, during coal pyrolysis is almost complete at around 600 °C,1 and calcium-catalyzed N2 formation occurred at temperatures higher than 800 °C (Figure 2). These observations suggest that N2 originates mainly from char-N. The comparison of N2 yield obtained from YL coal (60%) and YLD/Ca coal (55%) suggests that the catalytic effect of externally added calcium is not as large as inherent iron in YL coal. In our previous study,2 we have reported that Ca had almost no effect on N2 formation during the pyrolysis of a demineralized lignite. The inconsistency was probably caused by different soaking time at high-temperature range (800-1000 °C), where calcium-catalyzed N2 formation occurred. In our previous work, Ca-loaded samples were pyrolyzed at 400 °C/min, and soaked for 2 min at 1000 °C. However, in the present study the heating rate is only 10 °C/min, and the hold time at 1000 °C is 30 min. It means that the hold time at a temperature higher than 800 °C under above two different heating conditions is 2.5 and 50 min, respectively. When YLD/Ca was pyrolyzed at 400 °C/ min and held for 2 min at 1000 °C, Ca showed almost no effect on N2 formation as described in our previous paper,2 and N2 yield increased as the increase of soaking time at 1000 °C. Since calcium-catalyzed N2 formation occurred at high temperature (800-1000 °C), a relatively long soaking time in this temperature range is necessary for Ca to react with char-N through a solid-phase reaction. The mechanism of iron-catalyzed nitrogen removal during pyrolysis of iron-loaded brown coal and iron-containing low rank coals has been proposed. Fine particles of metallic iron with the mean size of 20-30 nm react with char-N to form iron nitrides, and the later subsequently decompose to form N2.2,9 The mechanism of calcium-catalyzed N2 formation during coal pyrolysis is not clear at present. At temperatures above 800 °C, the added calcium catalyst would exist in the form of CaO, and the CaO may react with char-N to form some precursors and then decompose to N2. Further study is required to elucidate the mechanism of the calcium-catalyzed N2 formation.
Conclusions In summary, we have found for the first time that calcium catalyst can selectively catalyze coal-N to N2 during a fixed bed pyrolysis of an Australian brown coal. It is likely that the calcium catalyzes N2 formation mainly from char-N through a solid-phase reaction. EF000051H (9) Mori, H.; Asami, K.; Ohtsuka, Y. Energy Fuels 1996, 10, 1022-1027.
