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Energy & Fuels 1996, 10, 1022-1027
Role of Iron Catalyst in Fate of Fuel Nitrogen during Coal Pyrolysis Hiroshi Mori, Kenji Asami, and Yasuo Ohtsuka* Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan Received March 6, 1996X
The fate of fuel nitrogen during the pyrolysis of brown and bituminous coals at 900 °C, in the presence of iron precipitated from FeCl3 solution, has been studied with a fluidized bed reactor. The catalysts at 0.2-0.7 wt % Fe promote N2 formation, whereas they lower nitrogen conversions to HCN, NH3, N-containing oil, tar, and char. The nitrogen distribution is not changed significantly by increasing iron loading. The effect of the iron on nitrogen distribution is larger with brown coal, the conversion to N2 being ≈50 and ≈20% for brown and bituminous coals, respectively. The size of iron particles is smaller in brown coal chars, which appears to lead to a larger catalytic effect. Comparison of nitrogen mass balances in the presence and in the absence of iron shows that N2 comes from not only volatile nitrogen but also char nitrogen. The X-ray diffraction measurements reveal the presence of graphitized carbon as well as cementite (Fe3C) in brown coal chars. These observations suggest that carbon atoms in the char substrate interact with fine particles of metallic iron and that interstitial iron nitrides formed in this process are decomposed to N2.
Introduction
Experimental Section
The nitrogen present in coal (coal-N) is emitted as NOx and N2O in pulverized coal-fired plants and fluidized bed combustion.1-4 As is well-known, NOx has been implicated in acid rain and photochemical smog formation and N2O is involved in the greenhouse effect and the ozone layer depletion.3,4 If coal-N can be converted efficiently to N2 upon pyrolysis, fuel NOx and N2O emissions during subsequent combustion can be reduced.
Coal Samples. Loy Yang and Blair Athol coal from Australia, denoted as LY and BA coal, respectively, were used in the present work. These samples were air-dried at room temperature, ground, and sieved to coal particles with size fraction 150-250 µm. The ultimate and proximate analyses are given in Table 1. Catalyst Addition. Iron catalysts were mainly loaded onto both coals by the precipitation method using an aqueous solution of FeCl3, since this method has made it possible to incorporate finely dispersed iron particles into coal.7,8 The procedure has been described in detail elsewhere6,7 and is thus simply explained below. After a predetermined amount of Ca(OH)2 powder was added into an aqueous mixture of coal particles and FeCl3 solution, the resulting iron-bearing coal was separated by filtration, then washed with deionized water, and finally dried in a stream of N2 at 110 °C. The iron was free from chloride ion contamination because the chloride in FeCl3 could be completely removed as water-soluble CaCl2.8 Excess Ca(OH)2 on the iron-bearing coal was also removed by repeated water-washing after filtration, although a slight amount of Ca, e0.5 wt %, was retained. In a different preparation, a nanophase iron oxide catalyst, with a mean particle size of 3 nm,9 produced by Mach I, Inc., was added to BA coal by mixing in ethanol at room temperature, followed by evaporation of the ethanol in vacuo at 50 °C. Pyrolysis and Product Analysis. Pyrolysis runs were carried out with a quartz-made fluidized bed reactor, the bed zone being 2.5 cm i.d. and 35 cm long. A quartz-made filter for gas dispersion was fritted onto the bottom of the reactor. The reactor was heated electrically with a glass-made transparent furnace, which made it possible to watch the fluidization state of coal particles during pyrolysis. About 5 g of the sample was first fluidized in atmospheric He, then heated at
We have shown that iron precipitated on brown coal can promote coal-N conversion to N2 during fluidized bed pyrolysis under atmospheric pressure and achieve high conversion of 50%.5,6 This novel method would be more effective for the reduction of the NOx and N2O emissions in fluidized bed combustion, since such pollutants come from coal-N alone in the process. The present work focuses on making clear the effect of iron catalysts on the distribution of N2, HCN, NH3, and N-containing liquids evolved during the pyrolysis of coals, on examining the relationship between the catalytic effect and the coal type, and on elucidating the mechanism of the iron-catalyzed nitrogen removal as N2. * Corresponding author. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, June 1, 1996. (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) Takashita, M.; Sloss, L. L.; Smith, I. M. In N2O Emissions from Coal Use; IEAPER/06; IEA Coal Research: London, 1993. (5) Ohtsuka, Y.; Mori, H.; Nonaka, K.; Watanabe, T.; Asami, K. Energy Fuels 1993, 7, 1095-1096. (6) Ohtsuka, Y.; Mori, H.; Watanabe, T.; Asami, K. Fuel 1994, 73, 1093-1097.
S0887-0624(96)00035-7 CCC: $12.00
(7) Ohtsuka, Y.; Asami, K.; Yamada, T.; Homma, T. Energy Fuels 1992, 6, 678-679. (8) Asami, K.; Ohtsuka, Y. Ind. Eng. Chem. Res. 1993, 32, 16311636. (9) Zhao, J.; Huggins, F. E.; Feng, Z.; Lu, F.; Shah, N.; Huffman, G. P. J. Catal. 1993, 143, 499-509.
© 1996 American Chemical Society
Fate of Fuel Nitrogen during Coal Pyrolysis
Energy & Fuels, Vol. 10, No. 4, 1996 1023
Table 1. Ultimate and Proximate Analyses of Coal Samples Used ultimate analysis, wt % (daf)
proximate analysis, wt % (db)
coal
code
C
H
N
S
O
ash
VM
FC
Loy Yang Blair Athol
LY BA
65.9 78.6
4.7 4.4
0.60 2.0
0.3 0.3
28.5 14.7
0.5 7.5
51.0 28.7
48.5 63.8
Figure 1. Formation of N2, NH3, and HCN during the pyrolysis of Loy Yang coal at 900 and 1000 °C.
Figure 2. Formation of N2, NH3, and HCN during the pyrolysis of Blair Athol coal at 900 °C.
600-700 °C/min up to a predetermined temperature, and finally soaked for 10 min. Pyrolysis temperatures of g900 °C were selected in the present study, since the previous study has revealed that the highest nitrogen conversion to N2 is achieved at 900 °C when LY coal with precipitated iron is pyrolyzed between 600 and 900 °C.5 Any fluidizing agents such as sand were not used to avoid their possible influence on the fate of coal-N during pyrolysis. The procedure has been described in more detail elsewhere.5,6 Pyrolysis products were first condensed as tar and oil with two traps, and the gas was collected in a plastic bag. Tar was almost solid at room temperature and mostly soluble in tetrahydrofuran. Oil comprised liquid hydrocarbons and water. The residue remaining in the fluidized bed after pyrolysis was recovered as char. The detailed procedure for product separation has been described in the previous paper.5,6 The total nitrogen in tar or char, denoted as tar-N or charN, respectively, was determined with a conventional elemental analyzer. The nitrogen in oil, denoted as oil-N, was also determined using a total nitrogen analyzer with a chemical luminescence cell. The amount of N2 in the gas was determined by gas chromatography with a thermal conductivity detector. In order to determine the amounts of HCN and NH3 formed, another experiment was performed separately in the same manner as above; the reactor effluent was bubbled directly into deionized water, and dissolved CN- and NH4+ ions were analyzed with a specific ion electrode. The yields of oil, tar, and char are expressed in weight percent of dry, ash-free, and catalyst-free coal. Nitrogen conversion to N2, HCN, NH3, oil-N, tar-N, or char-N is expressed in percent of the total nitrogen content in the feed sample. Catalyst Characterization. The actual iron loading was determined by atomic absorption spectroscopy after extraction of the metal from the iron-loaded coal with hot HCl, and it is expressed as weight percent of iron metal on a dry coal basis. The X-ray diffraction (XRD) measurements of iron-bearing coals and chars were made with Mn-filtered Fe KR radiation. Transmission electron microscopy (TEM) was carried out to determine the size of iron particles on the chars after pyrolysis.
without the precipitated iron. At 900 °C and without the catalyst, NH3 was the predominant product followed by HCN. On the other hand, the presence of the iron at a low loading of 0.73 wt % not only increased the total amount of N2, NH3, and HCN but also drastically changed the product distribution. The amount of N2 increased 12-fold by catalyst addition, viz., N2 was the predominant product, whereas NH3 and HCN decreased. When iron loading was increased further to 2.8 wt %, the total amount of N2, NH3, and HCN increased slightly, but the distribution of these products was almost unchanged. Figure 1 also shows that an increase in pyrolysis temperature from 900 to 1000 °C has no significant effect on the distribution of N2, NH3, and HCN. The formation of N2, NH3, and HCN from BA coal at 900 °C is illustrated in Figure 2, where the precipitated and nanophase catalysts are used. In the absence of catalyst, NH3 was the dominant product as observed for the LY coal, but the total amount of N2, NH3, and HCN was much larger with the BA coal. When a slight amount of iron, 0.24 wt %, was precipitated onto BA coal, N2 increased 3.3-fold, but NH3 and HCN decreased, and the total amount of N2, NH3, and HCN was almost unchanged. The selectivity for N2 formation was decreased with BA coal. No significant change in the distribution of these products was observed by increasing iron loading further to 1.2 wt %. The use of the nanophase catalyst with 1.9 wt % Fe increased N2 slightly. Nitrogen in Oil and Tar. Figure 3 shows the effect of iron loading on the yields of oil and tar evolved at 900 °C from LY and BA coal. Both yields were higher with LY coal as expected from the more volatile matter (Table 1). Oil yield was unchanged by catalyst addition irrespective of the coal type. Tar yield with LY coal decreased to a small extent with increasing iron loading, whereas the yield with BA coal was almost constant. The same loading dependence of both yields for LY coal was also observed at 1000 °C. The nitrogen content in oil or tar as a function of iron
Results N2, NH3, and HCN. Figure 1 shows the amounts of N2, NH3, and HCN formed from LY coal with and
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Mori et al.
Figure 3. Effect of iron loading on yields of oil and tar formed at 900 °C.
Figure 6. Nitrogen distribution from pyrolysis of Loy Yang coal at 900 and 1000 °C.
Figure 4. Effect of iron loading on nitrogen contents in oil and tar evolved at 900 °C.
Figure 7. Nitrogen distribution from pyrolysis of Blair Athol coal at 900 °C.
Figure 5. Yield of char at 900 °C and the nitrogen content as a function of iron loading.
loading is given in Figure 4. The percent of N (%N) in both oil and tar without the catalyst was higher with BA coal. The result corresponded to a higher nitrogen content in BA coal (Table 1). The presence of the precipitated iron at a low loading of 0.7 or 0.2 wt % lowered the %N in the oil from LY or BA coal, but a further increase in iron loading had no effect on the nitrogen content irrespective of the coal type. The %N in the tar from LY or BA coal was also lower in the presence of the iron, but the catalytic effect was smaller for tar than for oil. The elemental analysis of tar revealed that the atomic ratio of H/C was almost unchanged by catalyst addition regardless of the coal type. Nitrogen in Char. Figure 5 illustrates the effect of the iron on char yield and nitrogen content at 900 °C. The yield for LY coal tended to increase slightly with
increasing iron loading, whereas the yield for BA coal appeared to be independent of the loading. On the other hand, the presence of the iron at low loadings of 0.20.7 wt % lowered the %N in both chars, and the degree of the lowering was larger with LY char. When LY coal was pyrolyzed at 1000 °C, the catalytic effect of 0.73 wt % Fe on char yield and nitrogen content was almost the same as that at 900 °C; the iron did not change char yield significantly, but it lowered the nitrogen content from 0.88 wt % in the original char to 0.27 wt %. Nitrogen Distribution. Figures 6 and 7 show the nitrogen distribution from pyrolysis of LY and BA coal, respectively. Nitrogen mass balance fell within the reasonable range of 100-108% in every case. In the pyrolysis of the original LY coal at 900 °C, about 50% of coal-N was retained in the char, and nitrogen conversions to oil-N and NH3 were higher among volatile nitrogen, the conversion sequence of NH3 > HCN > N2 being observed. The nitrogen distribution in the absence of catalyst was unchanged by increasing the temperature to 1000 °C. When BA coal without catalyst was pyrolyzed at 900 °C, as is seen in Figure 7, a distribution similar to that of the original LY coal was observed; about 65% of the nitrogen was retained in the char, oil and NH3 being the main products.
Fate of Fuel Nitrogen during Coal Pyrolysis
Energy & Fuels, Vol. 10, No. 4, 1996 1025 Table 2. Species Identified by XRD and Particle Size of Iron Catalysts
coal
iron loadinga (wt %)
temp (°C)
species identified by XRDb
LY LY LY BA BA
0.73 2.8 2.8 1.2 1.9g
900 e 900 900 900
Fe3C (w), R-Fe (w), Gd (s) not detectable Fe3C (m), R-Fe (w), Gd (s) Fe-Cf (w), R-Fe (vw) Fe-Cf (w), R-Fe (vw)
particle sizec (nm) 20-30 30-50 70-100 50-60
a Prepared by the precipitation method unless otherwise stated. XRD intensities designated as vw (very weak), w (weak), m (medium), and s (strong). c Determined by TEM. d Graphitized carbon. e Before pyrolysis. f Austenite. g Nanophase catalyst.
b
Figure 8. X-ray diffraction profiles for different samples derived from Loy Yang coal: (A) 2.8 wt % Fe-loaded coal; (B) 900 °C-char from the original coal; (C) 900 °C-char from A.
Figure 6 also shows that the addition of 0.7 wt % Fe to LY coal increases nitrogen conversion to N2 at 900 °C drastically, but in contrast, it decreases the partitioning to all other nitrogen such as NH3, HCN, oil-N, tar-N, and char-N. The iron catalyst at 2.8 wt % had a similar catalytic effect, though the distribution of these nitrogen components changed somewhat. No temperature dependence of the catalytic effect on the fate of coal-N was observed between 900 and 1000 °C. As shown in Figure 7, both the precipitated and nanophase catalysts promoted to some extent N2 formation from BA coal and decreased nitrogen conversions to NH3, HCN, and char-N. However, there were the major differences in the conversions to N2 and char-N between LY and BA coals, and the effect of iron catalyst on the nitrogen distribution was smaller for BA coal. Improvement of the catalytic effect for bituminous coal would be the subject of future work, since the nitrogen removal from the coal is of practical significance. Catalyst State and Char Structure. The XRD profiles for some samples from LY coal are provided in Figure 8, where diffraction intensities in the angle region of 50-60° are enlarged 2 times. No XRD signals due to iron species were observed for LY coal with 2.8 wt % Fe, which indicates high dispersion of the iron catalyst. It has been shown that the iron precipitated onto brown coal from FeCl3 solution exists in the form of fine particles of FeOOH.8 When the iron-loaded coal was pyrolyzed at 900 °C, the precipitated iron was completely reduced to Fe3C (cementite) and R-Fe by reducing gases such as H2 and CO evolved during pyrolysis (Figure 8C). Interestingly, the diffraction lines attributable to C(002) were detectable on the ironbearing char. This observation indicates the formation of graphitized carbon. Contrarily, the char derived from the original coal was amorphous (Figure 8B). The species identified by XRD are summarized in Table 2. The char after pyrolysis of LY coal with 0.73 wt % Fe also gave graphitized carbon as well as Fe3C and R-Fe. When BA coal with the precipitated iron was pyrolyzed at 900 °C, however, no formation of graphitized carbon or cementite was observed and austenite (denoted as Fe-C) was formed. The nanophase iron with FeOOH structure9,10 was also reduced to Fe-C and R-Fe. Table 2 also summarizes the average size of iron
particles determined by TEM observations. The size on the char derived from LY coal with 0.73 wt % Fe was as small as 20-30 nm. The increase in iron loading to 2.8 wt % increased the particle size slightly, which indicates a larger degree of catalyst agglomeration at higher loading. The particle size depended on the coal type, and it was larger for BA char in spite of lower loadings. The size was slightly decreased by use of the nanophase catalyst. Discussion Formation Routes of N2. Previous studies on the release of NH3 and HCN during coal pyrolysis have shown that NH3 is the dominant product at low heating rate,11,12 whereas HCN alone is formed on rapid heating.13,14 It has been suggested that NH3 arises from secondary reactions of HCN11,15,16 and that a larger extent of secondary reactions between volatiles and char leads to higher conversion to NH3.17 As is seen in Figures 6 and 7, NH3 was the predominant gaseous product without added iron, irrespective of coal type, suggesting the significance of secondary reactions of volatile nitrogen under the present conditions. Figures 6 and 7 also indicate that the iron suppresses mainly the formation of NH3, HCN, and oil-N with a corresponding increase in N2, which shows that the iron catalyzes the conversion of volatile-N to N2. The changes in the amounts of volatile-N and N2 by catalyst addition are summarized in Table 3, where each value gives the difference found between runs in the presence and absence of iron. The catalyst decreased the amount of volatile-N in all runs irrespective of coal type, iron loading, and pyrolysis temperature, whereas it increased N2 in every case. These observations point out that volatile-N is a source of N2. Furthermore, it is evident in Table 3 that the amounts of N2 increase, 3.0-3.2 and 2.9-4.0 mg/g of coal for LY (10) Huffman, G. P.; Ganguly, B: Zhao, J.; Rao, K. R. P. M.; Shah, N.; Feng, Z.; Huggins, F. E.; Taghiei, M. M.; Lu, F.; Wender, I.; Pradhan, V. R.; Tierney, J. W.; Seehra, M. S.; Ibrahim, M. M.; Shabtai, J.; Eyring, E. M. Energy Fuels 1993, 7, 285-296. (11) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710-720. (12) Ohtsuka, Y.; Furimsky, E. Energy Fuels 1995, 9, 141-147. (13) 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. (14) Chen, J. C.; Castagnoli, C; Niksa, S. Energy Fuels 1992, 6, 264271. (15) Baumann, H.; Mo¨ller, P. Erdo¨ l Erdgas Kohle 1991, 44, 29-33. (16) Wo´jtowicz, M. A.; Zhao, Y.; Serio, M. A.; Bassilakis, R.; Solomon, P. R.; Nelson, P. F. Proceedings of the 8th International Conference on Coal Science; Elsevier: Amsterdam, 1995; Vol. I, pp 771-774. (17) Leppa¨lahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43, 1-45.
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Mori et al.
Table 3. Changes in Amounts of Volatile Nitrogen and N2 Formed at 900 and 1000 °C by Iron Catalyst Addition
coal
iron loadinga (wt %)
LY LYd LY BA BA BA
0.73 0.73 2.8 0.24 1.2 1.9e
change in amount by catalyst additionb (mg/g(daf) of coal) volatile-Nc N2 -1.6 -1.4 -1.3 -2.7 -2.5 -2.2
+3.0 +3.2 +3.2 +3.1 +2.9 +4.0
a Prepared by the precipitation method unless otherwise stated. At 900 °C unless otherwise described. c Comprising NH3, HCN, oil-N, and tar-N. d At 1000 °C. e Nanophase catalyst.
b
Iron-Catalyzed Nitrogen Removal in Solid Phases. As shown in Table 2 and Figure 8, cementite and austenite were present as the bulk species in the iron-bearing LY and BA char, respectively. The presence of iron carbides points out that some of the carbon atoms in the char substrate can readily dissolve into iron particles. The degree of carbon dissolution is higher for LY char because the carbon content is larger in cementite than austenite. It should be noted that graphitized carbon is formed on the iron-bearing LY char (Figure 8). Since the metal-catalyzed graphitization of amorphous carbon proceeds through carbon dissolution into metal particles followed by the decomposition of metal carbides,21 the following mechanism can be suggested for the formation of graphitized carbon in the presence of the precipitated iron:
R-Fe + amorphous carbon f Fe3C Fe3C f R-Fe + graphitized carbon Figure 9. Proposed formation routes of N2 during coal pyrolysis in the presence of iron.
and BA coal, respectively, are always smaller than those of volatile-N decrease, 1.3-1.6 and 2.2-2.7 mg/g of coal for the corresponding coal. The difference points out that some of the N2 derived by catalyst addition comes from a source other than volatile-N, that is, char-N (and/ or precursors). This is also evidenced by the decrease in the conversion to char-N in the presence of catalyst (Figures 6 and 7). On the basis of the above discussion, some probable formation routes of N2 during the iron-catalyzed coal pyrolysis are proposed in Figure 9. Part of coal-N is released as volatile-N during the primary pyrolysis, and the remainder is retained as char-N.18 Volatile-N is finally released as HCN, NH3, oil-N, and tar-N. Volatile-N and/or the once-released products can subsequently be decomposed into N2 on the catalyst surface (route A). Iron-containing materials are catalytically active for NH3 decomposition.19 It is likely that an N2 formation via route A occurs mainly in the coal pores. When a mixture of the iron-free and iron-loaded LY coal was pyrolyzed with a fluidized bed reactor at 900 °C, the iron had no significant effect on the nitrogen distribution of the iron-free coal, which suggests that iron particles exist mostly in the coal pores. Another formation route of N2 is illustrated as route B via char-N in Figure 9. When nitrogen-enriched carbons derived from polyacrylonitrile were first mixed physically with fine particles of metallic iron with the average size of 20 nm, and then the resulting samples were heated in high purity He, a large amount of N2 was formed.20 This observation supports the occurrence of route B. The difference between N2 increase and volatile-N decrease by catalyst addition was larger with LY coal than BA coal (Table 3). This result shows larger contribution of route B to N2 formation from LY coal, which corresponds to the observed lower conversion of coal-N to char-N (Figure 6) in this case. (18) Solomon, P. R.; Colket, M. B. Fuel 1978, 57, 749-755. (19) Leppa¨lahti, J.; Simell, P.; Kurkela, E. Fuel Process. Technol. 1991, 29, 43-56. (20) Ohtsuka, Y.; Watanabe, T.; Asami, K. Proceedings of the 7th International Conference on Coal Science; 1993; Vol. 2, pp 11-14.
It is likely that the reaction between metallic iron and char-N occurs in a manner similar to that above. The scheme is provided in Figure 9. Iron particles could first react with pyrrolic and pyridinic nitrogen in the char substrate to form interstitial iron nitrides, which could subsequently decompose to R-Fe and N2 since the nitrides are thermally unstable under the conditions applied. It has been shown that fine particles of metallic iron promote solid phase reactions which extract N2 from condensed heterocyclic ring structures formed during the carbonization of polyacrylonitrile at 700-900 °C.22 The contribution of route B (Figure 9) to N2 formation is larger with LY coal than BA coal as discussed above, and the TEM observations reveal that the size of iron particles in LY char is as small as 20-50 nm, whereas the particle size in BA char is larger (Table 2). These findings show that the dispersion of the iron catalyst plays a crucial role for the solid-solid reaction between iron particles and char-N. It is well-known that a more finely dispersed metal has higher reactivity and larger mobility. The higher reactivity of the iron in LY char leads to a larger degree of carbon dissolution into metallic iron, which in turn causes a larger rate of formation of iron nitrides. Furthermore, since fine particles with large mobility could readily migrate in the char matrix, the solid-solid reaction can proceed to a large extent. The formation of graphitized carbon in the iron-bearing LY char alone (Figure 8) suggests that the migration of iron particles in the char matrix in this case is easier due to smaller particle size, which results in a larger extent of solid phase reactions. Conclusions Brown and bituminous coals, containing iron catalysts precipitated from FeCl3 solution, were pyrolyzed in a fluidized bed reactor at 900 °C in an inert atmosphere. The following conclusions were reached: (21) Oya, A.; Marsh, H. J. Mater. Sci. 1982, 17, 309-322. (22) Watanabe, T.; Ohtsuka, Y.; Nishiyama, Y. Carbon 1994, 32, 329-334.
Fate of Fuel Nitrogen during Coal Pyrolysis
(1) The iron at low loadings of 0.2-0.7 wt % increases nitrogen conversion to N2, whereas it decreases the conversion to other nitrogen-containing product components, such as HCN, NH3, liquid materials, and char. (2) The catalytic effect of iron depends on the coal type. It is found that the iron drastically promotes N2 formation from brown coal, but the effect is much smaller for bituminous coal. (3) Finely dispersed catalysts show the highest activities. (4) The nitrogen balance indicates two formation routes of N2, i.e., (a) secondary decomposition of volatile nitrogen and (b) the conversion of char nitrogen in solid phase. The contribution of the latter to N2 formation is larger with brown coal. (5) Not only Fe3C but also graphitized carbon exists in the iron-bearing brown coal chars. Solid phase reactions to extract N2 from char nitrogen proceed
Energy & Fuels, Vol. 10, No. 4, 1996 1027
possibly through a mechanism involving the formation and subsequent decomposition of iron nitrides. Acknowledgment. The present work was supported by a Grant-in-Aid for Developmental Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (No. 07555246), and by an International Research Grant sponsored by the New Energy and Industrial Technology Development Organization (NEDO), Japan. The authors acknowledge the assistance of Ms. Naomi Katahira and Ms. Ayumi Shoji in carrying out experiments. We are also grateful to Mr. Eiji Aoyagi of the High Voltage Electron Microscope Laboratory, Tohoku University, for the TEM measurements and to Dr. Bernard M. Kosowski of Mach I, Inc., for providing a nanophase catalyst. EF960035D
