Energy & Fuels 1993, 7, 1095-1096
1095
Selective Conversion of Coal Nitrogen to N2 with Iron Yasuo Ohtsuka,' Hiroshi Mori, Katsutoshi Nonaka, Takashi Watanabe, and Kenji Asami Research Center for Carbonaceous Resources, Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980, Japan Received June 3, 1993. Revised Manuscript Received August 3, 1 9 9 P
Brown coal with iron catalyst precipitated from FeCl3 solution has been pyrolyzed in He or H2 at 600-900 "C with a fluidized bed reactor. The presence of the iron at loadings of 3-7 wt % changes dramatically the fate of coal nitrogen during pyrolysis at 900 OC; 50-60% of coal nitrogen is readily converted to Nz, resulting in a low residual nitrogen in the char. Changing the fluidizing gas from He to Hz shows no significant effect on the conversion to N2. It is probable that ultrafine iron particles of around 50 nm in the reduced state are responsible for the present nitrogen removal. Nitrogen oxides (NO,) emitted from coal combustion to generate electricity have been implicated in acid rain and photchemical smog. Considerable effort has therefore been devoted to reduce the NO, emissions by combustion modification and flue gas However, since 75-95 % of the NO, emissions originates from coal nitrogen in pulverized coal-fired boiler~,~J the removal of nitrogen prior to coal combustion would be one of the most essential approaches to meet more stringent regulations on NO, emissions. As is well-known, coal nitrogen is present predominantly as the thermally stable aromatic ring structures like pyridines and pyrrole^.^^^ The physical removal seems almost impossible accordingly, but the conversion of coal nitrogen into inert substances may be feasible. The present article shows a novel nitrogen removal method of converting coal nitrogen mainly into N2 gas during the pyrolysis with iron in an inert atmosphere. Loy Yang brown coal of size fraction 150-250 pm was used; the ultimate analysis was C 65.9;H 4.7;N 0.60;S 0.27;and 0 28.5 wt % ' (daf). The iron catalyst was precipitated onto coal from an aqueous solution of FeCl3 by using Ca(OH)2; Ca(OH)2 powder was added into the mixture of coal particles and FeC13 solution during stirring at room temperature, and then the Fe-bearing coal was separated from the solution by filtration. The procedure has been described in more detail elsewhere.6 Pyrolysis experiments were carried out with a fluidized bed reactor; coal particles (5 g) were first fluidized in atmospheric He or HZof 0.2L(STP)/min,and then heated at 600-700 "C/min up to 600-900 "C by elevating the preheated, transparent electric furnace, the solid residence time being 10 min. The reactor effluent was collected as
* Author to whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, October 1, 1993. (1) Hjalmarsson, A-K. In NO, Control Technologies for Coal Combustion; IEA Coal Research London, 1990; IEACR/24, pp 1-62. (2) Unsworth, J. F.; Barratt, D. J.; Roberta, P. T. In Coal Quality and Combustion Performance; Coal Science and Technology Vol. 19; Elsevier: Amsterdam, 1991; pp 579-590. (3) Boardman, R.; Smoot, L. D. InFundamentals of Coal Combustion for Clean andEfficient Use;Smoot, L. D., Ed.; Coal Science Technology Vol. 20; Elsevier: Amsterdam, 1993; pp 433-509. (4) Solomon, P. R.; Colket, M. B. Fuel 1978,57, 749-755. (5) Whitehurst, D. D. In Organic Chemistry of Coal;Larsen, J. W., Ed.; ACS Symposium Series 71;American Chemical Society: Washington DC, 1978; pp 1-35. (6) Asami, K.; Ohtauka, Y. Ind. Eng. Chem. Res. 1993,32,1631-1636.
0887-0624/93/2507-1095$04.00/0
Temperature, "c Figure 1. Conversion of coal nitrogen to Nz during pyrolysis at different temperatures under atmospheric pressure. gas in a plastic bag after being passed two traps. For some materials recovered from the first trap heated at 120 "C, the fraction soluble in tetrahydrofuran is denoted as tar throughout the paper, the remainder being as coke which includes a small amount of char entrained from the reactor. The material condensed in the second trap cooled at -70 "C is denoted as oil, which includes water. The char remained in the reactor was also recovered. The material balance for every run revealed that 92-97 wt % of coal fed could be recovered as gas, oil, tar, coke, and char. The product distribution at 900 "C and 3-7 wt 5% Fe was gas, 27-31;oil, 9-14;tar,5-6; coke, 1-2;and char, 46-47 wt % . Coal nitrogen evolution was monitored in terms of N2 and the nitrogen contents of tar and char, but HCN and NH3 were not determined because the quantitative analysis was difficult due to being soluble in water evolved from coal and iron catalyst. The reproducibility of the results was within *5%. Figure 1 illustrates the effects of pyrolysis temperature, catalyst addition, and fluidizing gas on the conversion of coal nitrogen to Nz. The conversion without catalyst slightly increased with increasing temperature, and it was very small, less than 3% even at 900 "C. Major nitrogencontaining gas would be HCN in this temperature region.718 In contrast with the uncatalyzed pyrolysis, the conversion to Nz with catalyst, at 7 w t % Fe, increased considerably (7) Nelson, P. F.; Kelly, M. D.; Wornat, M. J. Fuel 1991,70,403-407. (8)Chen, J. C.; Castagnoli, C.; Niksa, S. Energy Fuels 1992,6 264-271.
0 1993 American Chemical Society
1096 Energy & Fuels, Vol. 7, No. 6,1993
I
t
8 6o
.NP
Tar Other
Char
Iron loading, wt?6
Figure 2. Fate of coal nitrogen during pyrolysis in He at 900"C.
with increasing temperature, and it reached 50% at 900 "C. Thus, the amount of Nz formed at 900 "C was drastically enhanced by iron addition, and it was about 20 times that without iron. When the fluidizing gas in the iron-catalyzed pyrolysis at 900 " C was changed from He to Hz, as is seen in Figure 1,no further improvement on the conversion to Nz was observed. This finding is noteworthy from a practical point of view, because expensive Hz is unnecessary for this conversion. The fate of coal nitrogen during the pyrolysis in He at 900 " C is illustrated in Figure 2, where the difference in the nitrogen balance is labeled as "other", which includes HCN, NH3, and oil nitrogen. Nearly 60 % of coal nitrogen was converted into Nz even at a low loading of 3 wt % Fe. Further increase in the loading from 3 to 7 wt % showed almost no improvement on the conversion, but rather a slight decrease, which may suggest the presence of the optimum loading. Not only the tar yield but also the nitrogen content in the tar was lower in the presence of the iron catalyst, and consequently the conversion to tar nitrogen at 3 wt % Fe was less than half of that without iron (Figure 2). The char yield increased slightly from 44 wt % without iron to 46 and 47 wt % at 3 and 7 wt % Fe, respectively. On the contrary, the nitrogen contents in Fe-bearing chars were lower, and they were almost onethird of that without iron. Figure 2 shows as a result that the Fe-bearing chars retain only 18-21 % of coal nitrogen, whereas the original char does 50%. The change of the fluidizing gas from He to Hz had no significant effect on the conversion to char nitrogen, as observed in the formation of Nz (Figure 1). As is seen in Figure 2, the conversion of coal nitrogen to other nitrogen-containing compounds was also reduced by catalyst addition.
Ohtsuka et al.
It has been shown that the iron precipitated onto brown coal by the present method exists in the form of fine particles of FeOOH.6 The X-ray diffraction measurements revealed that FeOOH remained partly oxidized after the pyrolysis in He or H2 at 750 "C, but it was completely reduced to a-Fe and Fe& at 900 "C irrespective of the fluidizing gas. Thus, the kind of the gas did not affect any crystalline form of iron catalyst, indicating the evolution of sufficient reducing gases such as Hz, CO, and hydrocarbons for catalyst reduction. This is the probable reason why the use of Hz instead of He had no effect on the fate of coal nitrogen. The transmission electron microscope showed that the particle size of the iron catalyst was as small as 30-50 nm at 900 "C. The finely dispersed catalyst in the reduced state would be responsible for the present nitrogen removal. Some mechanisms can be speculated for the dramatically increased conversion to Na and consequent decreased residualnitrogen in the char with the iron catalyst. When coal is pyrolyzed, coal nitrogen can initially be either retained in the char or released as volatile nitrogen, which can then be decomposed into HCN, NH3, and tar nitrogen! Part of the volatile nitrogen may also be reincorporated into carbon and soot may be formed during the secondary decomposition,7~8which results in the increased nitrogen retention in the char. Since metal particles finely dispersed on coal can catalyze secondary decomposition reactions of the volatiles released during pyrolysis,s11 the present iron may change the fate of volatile nitrogen. In other words, the nitrogen may be converted selectively to Ne and nitrogen-free (or nitrogen-poor) carbon on the catalyst surface. Another speculated mechanism is that the iron catalyst may promote the reactions to extract Nz from char. The first mechanism may be more predominant, since the occurrence of the latter solid-catalyzed solidphase reactions is not so easy because of small chance of contact between char nitrogen and iron particles. In conclusion, when brown coal with fine iron particles is pyrolyzed in an inert gas at 900 "C, 50-60% of coal nitrogen can easily be converted to Nz with the corresponding low nitrogen retention in the char. Acknowledgment. We thank the Iketani Science and Technology Foundation for partial financial support and Miss N. Katahira for help with elemental analyses. (9) Tyler, R. J.; Schafer, H. N. S. Fuel 1980,59, 487-494.
(IO) Franklin,H. D.; Cosway, R. G.; Peters, W. A.; Howard, J. B. Znd. Eng. Chem. hocess Design Dev. 1983,22, 39-42. (11) Tomita, A,; Watanabe, Y.; Takarada, T.; Ohtsuka, Y.; Tamai, Y. Fuel 1985,64, 795-800.
