Energy & Fuels 2002, 16, 451-456
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The Influence of Mineral Matter and Catalyst on Nitrogen Release during Slow Pyrolysis of Coal and Related Material: A Comparative Study 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 July 23, 2001. Revised Manuscript Received October 27, 2001
Formation of N2 during slow heating rate, temperature-programmed pyrolysis of a brown coal and a biodegraded leaf has been studied with a fixed-bed reactor in high-purity He. Formation profile of N2 depends strongly on type of sample. Demineralization by acid washing drastically changes the formation profiles and decreases nitrogen conversion to N2 for both the samples. Addition of iron to the demineralized brown coal promotes N2 formation and restores the profile to that of its original one. On the other hand, addition of calcium catalyst onto the demineralized biodegraded leaf promotes N2 formation and restores the profile to that of its raw sample. These results lead to the conclusion that inherent iron in the brown coal or inherent calcium in the biodegraded leaf is responsible for its remarkable formation of N2, respectively.
Introduction As is well-known, in pulverized coal combustion, 7595% of NOx emissions originates from coal nitrogen (coal-N),1,2 and in fluidized-bed combustion, all of the NOx and N2O emissions come from coal-N.3,4 NOx has been implicated in acid rain and photochemical smog formation, and N2O is involved in the greenhouse effect and ozone layer depletion. Since a majority of coal-N evolves from the initial pyrolysis stage of coal gasification and combustion, it is important to obtain a fundamental understanding of nitrogen release during pyrolysis in order to adequately apply a denitrogen strategy. When coal is pyrolyzed, coal-N is initially released as tar consisting of heterocyclic nitrogen compounds.5-8 The tar subsequently undergoes secondary decomposition reactions into HCN and NH3,9,10 and * Corresponding author. E-mail:
[email protected]. (1) Unsworth, J. F.; Barratt, D. J.; Roberts, P. T. In Coal Quality and Combustion Performance; Coal Science and Technology Vol. 19; Elsevier: Amsterdam, 1991; pp 579-590. (2) Boardman, R.; Smoot, L. D. In Fundamentals of Coal Combustion for Clean and Efficient Use; Coal Science and Technology Vol. 20; Elsevier: Amsterdam, 1993; pp 433-509. (3) Wo´jtowicz, M. A.; Pels, J. R.; Monlijn, J. A. Fuel Process. Technol. 1993, 34, 1-71. (4) Takeshita, M.; Sloss, L. L.; Smith, I. M. In N2O Emissions from Coal Use; IEAPER/06; IEA Coal Research: London, 1993. (5) Solomon, P, R.; Colket, M. B. Fuel 1978, 57, 749-755. (6) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P.; Lafleur, A. L. Energy Fuels 1988, 2, 775-782. (7) Chen, J. C.; Castagnoli, C.; Nikasa, S. Energy Fuels 1992, 6, 264271. (8) Johnsson, J. E. Fuel 1994, 73, 1398-1415. (9) Nelson, P. F.; Buckley, A. N.; Kelly, M. D. Twenty-fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1992; pp 1259-1267. (10) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710-720.
nitrogen distribution among those products depends on pyrolysis conditions such as heating rate and pyrolysis temperature.11,12 On the other hand, we have as yet very little information as to nitrogen conversion to N2 during pyrolysis, even though efficient conversion of coal-N to harmless N2 in pyrolysis can contribute to NOx and N2O reduction during subsequent combustion. From such a point of view, we have studied extensively N2 formation from many coals with different ranks during a fixedbed pyrolysis,13 and found that inherent iron-containing minerals present in some low rank coals can promote N2 formation during a fast pyrolysis at 400 °C/min and 1000 °C.14,15 Recently, we have also found that calcium catalyst can catalyze char nitrogen release and change nitrogen conversions to HCN and NH3 during pyrolysis of model coals containing pyrrolic or pyridinic nitrogen.16 However, the role of calcium-containing minerals on N2 formation and the details of catalyzed N2 formation by iron, calcium, and other catalysts remain unclear. Therefore, the purpose of the present study is to make clear the influence of minerals and catalyst on N2 formation during a temperature-programmed pyrolysis at a slow heating rate. Experimental Section Samples. Two samples with size fraction of 99.9999%, 100 cm3/min) at 2.5 °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 at 5-min intervals with a high-speed 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. Nitrogen conversions to N2 and char-N are expressed in percent of total nitrogen in feed sample. The reproducibility of the conversions determined by duplicating the experiment were within (1% for N2 and (2% for char-N. Minerals and Catalyst Characterization. The crystalline species of inherent minerals and added catalysts in char after pyrolysis at 1000 °C were measured by X-ray diffraction (XRD) analysis. The particle size and dispersion of Fe and Mo catalysts after pyrolysis were also observed by transmission electron microscopy (TEM).
Results and Discussion Effect of Mineral Matter on Nitrogen Release. The changes of ash and metal content in YL and MS by demineralization are illustrated in Table 2, where the corresponding demineralized samples were denoted as YLD and MSD. The demineralization decreased ash (17) Asami, K.; Ohtsuka, Y. Ind. Eng. Chem. Res. 1993, 32, 16311636.
Table 2. Changes of Ash and Metal Content in Sample by Demineralization code
ash, wt % (db)
Fe
YL YLD MS MSD
1.6 0.2 18.6 1.1
0.38 0.05 0.13 0.06
metal content, wt % (db) Ca Na Mg Al Si 0.17 0.02 8.10 0.19
0.07 0.03 0.24 0.01
0.19 0.04 0.70 0.02
0.02 0.01 0.08 0.05
0.10 0.10 0.53 0.22
K 0.01 0.00 1.68 0.08
Figure 1. Effect of demineralization on formation of N2 during pyrolysis of YL sample (dry ash free basis).
content from 1.6 to 0.2 wt % with YL, and 18.6 to 1.1 wt % with MS. Iron was the major metal in YL minerals followed by Mg and Ca. On the other hand, Calcium was the richest metal in MS, followed by K and Mg. It is to be noted that HCl washing removed almost 90% of Fe in YL, and 98% of Ca in MS. Figure 1 shows the effect of demineralization on N2 formation during temperature-programmed pyrolysis of YL sample at 2.5 °C /min and 1000 °C. Formation of N2 from YL started at about 500 °C, and the formation increased with increasing temperature, and reached a maximum at around 540 °C. After that, as the increase of temperature, the formation decreased but showed a second maximum at around 790 °C. Formation of N2 from YLD started at 100 °C higher than that of YL, that is 600 °C, and showed only one small peak at around 650 °C. Comparison of the two profiles from YL and YLD reveals that the demineralization drastically changed N2 formation profile, that is, decreased the lowtemperature peak and removed the high-temperature one. This observation points out that minerals present in YL play an important role in N2 formation within the temperature range of 500 to 850 °C. Figure 2 gives N2 formation profiles from MS and MSD during pyrolysis at the same conditions with YL and YLD. When MS was heated, N2 evolution started at around 700 °C, and N2 increased with the increase of temperature, and showed a maximum at about 810 °C. With MSD sample, almost no considerable formation of N2 was measured at temperatures below 800 °C, and N2 increased gradually with temperature until 1000 °C, and decreased during soaking at that temperature. It is clear that the demineralization of MS also removed N2 formation peak at 810 °C, and suppressed N2
Nitrogen Release during Slow Pyrolysis of Coal
Energy & Fuels, Vol. 16, No. 2, 2002 453
Figure 2. Effect of demineralization on formation of N2 during pyrolysis of MS sample (dry ash free basis).
Figure 4. Effect of iron addition on formation of N2 during pyrolysis of MSD sample (dry ash free basis).
Figure 3. Effect of iron addition on formation of N2 during pyrolysis of YLD sample (dry ash free basis).
Figure 5. Effect of calcium addition on formation of N2 during pyrolysis of MSD sample (dry ash free basis).
formation drastically at temperatures between 700 and 950 °C. Comparison of N2 formation profiles from YL (Figure 1) and MS (Figure 2) indicates that the two formation profiles are greatly different. Two formation peaks of N2 from YL appeared mainly at temperatures lower than 800 °C; however, only one N2 formation peak from MS existed within temperatures of 700-1000 °C. This comparison also suggests that different metal in YL and MS samples would be responsible for their different formation profiles of N2. Effect of Iron Catalyst on Nitrogen Release. Iron catalyst and iron-containing minerals in low rank coals have been found to promote nitrogen conversion to N2 during fast pyrolysis;15,18,19 however, the details of ironcatalyzed N2 formation remained unclear. To know how iron-containing minerals in YL sample influenced N2 formation, 0.4 and 3.0 wt % of Fe was added onto YLD and pyrolyzed at the above-mentioned conditions. The
effect of Fe addition on N2 formation from YLD is illustrated in Figure 3. The addition of 0.4 wt % (almost the same amount in YL) Fe promoted N2 formation at temperatures between 500 and 800 °C. Formation profile of N2 with 0.4 wt % Fe also showed two maxima at 550 and 750 °C, and the profile was almost restored to that of YL sample (Figure 1). This observation demonstrates that the two peaks of N2 formation from YL raw sample, as shown in Figure 1, completely arise from the catalysis of iron-containing minerals present in YL, and iron-catalyzed N2 formation was conducted via two reactions at two different temperature ranges. When Fe loading amount was increased from 0.4 to 3.0 wt %, N2 formation was further increased; both the two peaks were shifted to low temperatures by 50 °C, and the onset temperature of N2 evolution was also lowered by 50 °C. To know the effect of iron catalyst on N2 formation for other sample, 0.4 wt % Fe was added onto MSD, and the result is illustrated in Figure 4. The addition of 0.4 wt % Fe also promoted N2 formation mainly at the temperatures between 500 and 800 °C. The formation
(18) Ohtsuka, Y.; Mori, H.; Watanabe, T.; Asami, K. Fuel 1994, 73, 1093-1097. (19) Mori, H.; Asami, K.; Ohtsuka, Y. Energy Fuels 1996, 10, 10221027.
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Wu et al. Table 3. Effects of Demineralization and Catalyst Addition on Nitrogen Conversion during Pyrolysis at 1000 °C nitrogen conversion to, % sample
N2
char-N
YL YLD YLD/Fe0.4% YLD/Fe3% YLD/Ca0.5% YLD/Ca3% YLD/Ca8% YLD/Mo3%
59 16 57 78 29 52 55 57
22 54 23 3 45 24 20 23
MS MSD MSD/Fe 0.4% MSD/Ca 8%
31 8 31 30
3 18 4 4
Table 4. Crystalline Species in Chars after Pyrolysis at 1000 °C
Figure 6. Effect of calcium addition on formation of N2 during pyrolysis of YLD sample (dry ash free basis).
sample YL YLD YLD/Fe 3% YLD/Ca 3% YLD/Mo 3%
graphitized carbon (s), quartz (w), R-Fe (w), Fe3C (w) no species graphitized carbon (s), R-Fe (m), Fe3C (m) CaO (m) Mo2C (s)
MS MSD MSD/Fe 0.4% MSD/Ca 8%
CaO (s), ? (m) no species graphitized carbon (s),R-Fe (m), Fe3C (w) CaO (s)
a
Figure 7. Effect of molybdenum addition on formation of N2 during pyrolysis of YLD sample (dry ash free basis).
profile showed two peaks at 560 and 740 °C, and was similar to that of YLD with 0.4 wt % Fe addition. However, comparing the formation profile of MSD/ Fe0.4% with that of MS raw sample (Figure 2), we can find that N2 formation from the two samples occurred at different temperatures, and the formation profiles were greatly different. This observation reveals that remarkable formation of N2 from MS (Figure 2) was catalyzed by other metal rather than iron-containing minerals. Effect of Calcium Catalyst on Nitrogen Release. Calcium is the richest metal in MS sample. To know how calcium-containing minerals influence N2 formation during pyrolysis, calcium catalyst was added onto MSD and then pyrolyzed. Figure 5 shows the effect of Ca catalyst on N2 formation during pyrolysis of MSD sample. The addition of 8.0 wt % of Ca (the same amount in MS) promoted N2 formation at temperatures between 750 and 1000 °C. Formation of N2 started at about 700 °C, and reached a maximum at around 840 °C. It is clear that the formation profile of MSD with Ca addition is very similar to that of MS raw sample
species identified by XRDa
s, strong; m, middle; w, weak.
(Figure 2), and this result shows that remarkable formation of N2 from MS come from the catalysis of calcium-containing minerals in MS sample at temperatures higher than 700 °C. The effect of calcium catalyst on N2 formation was also investigated using YLD sample. The influences of Ca loading amount are illustrated in Figure 6. The addition of 0.5 wt % Ca promoted N2 formation at temperatures higher than 900 °C, and showed almost no influence on low-temperature peak (around 650 °C) of N2 formation from YLD. On the other hand, 3.0 wt % addition of Ca removed the low-temperature peak but increased N2 formation at temperatures higher than 800 °C. When the amount of Ca was further increased to 8.0 wt %, N2 formation only increased a little at high temperatures, and the profile is very similar to that of YLD with 3.0 wt % of Ca addition. These results mean that large amount (>3.0 wt %) Ca can not only promote N2 formation at high temperatures (>800 °C), but also suppress N2 formation at low temperatures (
