Energy & Fuels 2004, 18, 1605-1606
1605
Several Distinct Types of HCl Evolution during Temperature-Programmed Pyrolysis of High-Rank Coals with Almost the Same Carbon Contents Naoto Tsubouchi,* Shinya Ohtsuka, Hiroyuki Hashimoto, and Yasuo Ohtsuka Research Center for Sustainable Materials Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan Received January 6, 2004. Revised Manuscript Received May 19, 2004 The chlorine present in coal, denoted as coal-Cl, is usually in the range of 100-3000 mg/kg-coal(dry)1 and emitted as HCl during coal pyrolysis and gasification. It is well-known that HCl affects the fate of trace elements, such as alkali metals and mercury, in these processes,2,3 and that it causes corrosion problems on gas turbine materials and deterioration of fuel cell performance in integrated gasification combined cycle and fuel cell technologies under development.4,5 According to X-ray absorption near-edge structure spectra of coals,6,7 it has been reported that most of the coal-Cl is present as CaCl2‚ 6H2O, a saturated NaCl solution in the pores and microcracks, and organic hydrochlorides, and that there are no significant differences in the spectra of bituminous coals. It may be important to examine the relationship between such Cl functionalities and HCl formation, in terms of designing an efficient removal method of coalCl and HCl. However, only quite limited information on this topic has been provided so far.8-10 In this communication, we determine in situ how HCl evolved during temperature-programmed coal pyrolysis, show several distinct types of HCl evolution from three high-rank coals with almost the same carbon contents, and discuss them mainly in terms of Cl functionality. Three coals with carbon contents of 90-92 wt %(daf)s YRB and JET coals from Australia, and HGI coal from Vietnamswere sieved to particles with a size fraction of 150-250 µm. The chlorine and ash contents were 0.14 wt %(daf) and 9.8 wt %(dry) for YRB coal, 0.081 wt %(daf) and 7.4 wt %(dry) for JET coal, and 0.021 wt %(daf) and 10.2 wt %(dry) for HGI coal, respectively. A mixture of activated carbon and a chlorine-containing compound was prepared by physical mixing and impregnation. The * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Davidson, R. M. Chlorine and Other Halogens in Coal. Report IEAPER/28, IEA Coal Research, London, 1996. (ISBN 92-9029-276-8.) (2) Galbreath, K. C.; Zygarlicke, C. J. Environ. Sci. Technol. 1996, 30, 2421-2426. (3) Thompson, D.; Argent, B. B. Fuel 2002, 81, 555-570. (4) Scott, D. H. Advanced Power Generation from Fuel Cellss Implications for Coal. IEA Coal Research, London, 1993, Report No. IEACR/59. (5) Mitchell, S. C. Hot Gas Cleanup of Sulphur, Nitrogen, Minor and Trace Elements. IEA Coal Research, London, 1998, Report No. IEACCC/ 12. (ISBN 92-9029-317-9.) (6) Huggins, F. E.; Huffman, G. P. Chlorine in Coal. Coal Science and Technology, Vol. 17; Elsevier: Amsterdam, 1991; pp 43-61. (7) Huggins, F. E.; Huffman, G. P. Fuel 1995, 74, 556-569. (8) Herod, A. A.; Hodges, N. J.; Pritchard, E.; Smith, C. A. Fuel 1983, 62, 1331-1336. (9) Fynes, G.; Herod, A. A.; Hodges, N. J.; Stokes, B. J.; Ladner, W. R. Fuel 1988, 67, 822-830. (10) Shao, D.; Hutchinson, E. J.; Cao, H.; Pan, W.-P.; Chou, C.-L. Energy Fuels 1994, 8, 399-401.
reason for using activated carbon as a support is that the amount of HCl evolved from the carbon is negligibly small and, thus, no significant effect on HCl formation is observed from any chlorine-containing compounds used in this work. Two inorganic chlorides and two organic hydrochlorides were selected, according to earlier work on Cl-functional forms in coal.6,7 Each compound was mixed mechanically with activated carbon (ash, 0.87 wt %(dry)). In the impregnation method, the carbon was first immersed into an aqueous solution of CaCl2 or NaCl at room temperature, and then the water was slowly removed at low temperatures of 450 °C from the three coals may result from other Cl functionalities than the inorganic chlorides and organic hydrochlorides mentioned previously. There may be some possibility about the Cl sources, which are probably insoluble in water, because the HCl profile at >350 °C for the YRB coal was almost unchanged before and after water washing. Hydrogen chloride complexes attached to basic pyridinic groups may be considered as one possibility. The nitrogen form is the most dominant functionality in high-rank coals.11 Compared to the hydrogen chloride complexes attached to quaternary ammines, there may be stronger interactions between the pyridinic form and the HCl, which may lead to the highertemperature HCl formation. The HCl peak observed at the highest temperature of 580 °C, irrespective of the type of coal, may originate from the Cl functionality that has the strongest interactions with the char matrix. The functionality might be organic chlorine inherently present in high-rank coals, for example, covalent C-Cl bonds in condensed aromatic structures. The organic chlorine could also be formed by secondary reactions of HCl evolved at lower temperatures with carbon active sites in the nascent char in the pyrolysis process. Determination of the presence of covalent C-Cl bonds in coal and char should be the subject of future work. In conclusion, HCl evolved during the temperatureprogrammed pyrolysis of three high-rank coals with almost the same carbon contents shows very different rate profiles and provides at least four peak temperatures, which make it possible to speculate about the presence of different Cl functional groups in the coals. EF040003N (11) Kelemen, S. R.; Gorbaty, M. L.; Kwiatek, P. J. Energy Fuels 1994, 8, 896-906.