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Energy & Fuels 2002, 16, 920-924
Effectiveness of Hydrothermal Treatment of Coal with Lime for Removal of Trace Elements Koichi Matsuoka,* Aki Abe, Yohsuke Suzuki, and Akira Tomita Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai, 980-8577, Japan Received November 26, 2001
The possibility of applying chemical cleaning method to remove trace elements from coal was examined. The hydrothermal reaction with CaO followed by HCl washing was carried out for three different coals. Most of the mineral matter was removed by this treatment, except a small amount of clay and pyrite. The removal percentage of trace elements was generally lower in comparison with that of mineral matter. This is because a substantial portion of trace elements is associated with organic matter or mineral matter that could not be removed by this treatment.
Introduction Coal contains trace amount of toxic metals such as As, Hg, Se,and Cd. These trace elements from coal-fired power plants become environmental hazards. Although most of the trace elements are removed in particulate removal and desulfurization systems, small amounts are still emitted into the atmosphere.1,2 To reduce the trace elements emission to much lower level, the following two attempts are made. One is capture of trace elements in flue gas by using sorbents such as activated carbon and several kinds of mineral matter,3-7 and the other is removal of trace elements prior to the combustion.8-12 In the case of the capture by sorbents, a specific sorbent is needed for each element and absorption characteristics depend on several conditions such as temperature and flue gas composition. Therefore, complex facilities are needed for complete removal of trace elements from flue gas. On the other hand, coal cleaning, which attempts to reduce mineral matter, may also be able to reduce trace elements, as trace elements are * Corresponding author. Fax: +81-22-217-5626. E-mail: matsuoka@ tagen.tohoku.ac.jp. (1) Sloss, L. L.; Gardner, C. A. Sampling and analysis of trace emissions from coal-fired power stations; IEA Coal Research, 1995; pp 1-74. (2) Davidson, R. M.; Clarke, L. B. Trace elements from coal; IEA Coal Research, 1996; pp 1-60. (3) Carey, T. R.; Hargrove, O. W., Jr.; Richardson, C. F.; Chang, R.; Meserole, F. B. J. Air Waste Manage. Assoc. 1998, 48, 1166-1174. (4) Moritomi, H.; Yosiie, R.; Sonoda, K. Proc. 7th China-Japan Coal C1 Chemistry 2001, 423-426. (5) Scotto, M. V.; Uberoi, M.; Peterson, T. W.; Shadman, F.; Wendt, J. Fuel Process. Technol. 1994, 39, 357-372. (6) Felsvang, K.; Gleiser, R.; Juip, G.; Nielsen, K. K. Fuel Process. Technol. 1994, 39, 417-430. (7) Ho, T. C.; Lee, H. T.; Chu, H. W.; Hopper, J. R.; Bostick, W. D. Fuel Process. Technol. 1994, 39, 373-388. (8) DeVito, M. S.; Rosendale, L. W.; Conrad, V. B. Fuel Process. Technol. 1994, 39, 87-106. (9) Akers, D.; Dospoy, R. Fuel Process. Technol. 1994, 39, 73-86. (10) Blanchard, L. J.; Robertson, J. D.; Srikantapura, S.; Parekh, B. K.; Huggins, F. E. Prepr. Pap.sAm Chem Soc., Div. Fuel Chem. 1995, 40, 828-832. (11) Davidson, R. M. Coal Prep. 1998, 19, 159-176. (12) Smit, F. J.; Shields, G. L.; Moro, N.; Jha, M. C.; Feeley, T. J. Coal Prep. 1998, 19, 231-241.
considered to be mainly associated with mineral matter such as illite, kaolinite, and pyrite in coal.13,14 In the case of physical coal cleaning,8,9,11,12 trace elements were removed to some extent, though removal yields were not so high. This is probably due to low removal yields of mineral matter achieved by physical cleaning.8,9,11,12 Blanchard and others10 applied chemical cleaning using NaOH or HNO3 to remove trace elements. This method was more effective than physical cleaning in reducing mineral matter and trace elements, but the removal yield was still at an unsatisfied level. Thus, it is desired to develop an alternative method to remove more effectively mineral matter and trace elements. Recently, we have proposed a new chemical coal cleaning method that utilizes hydrothermal reaction of mineral matter with inexpensive lime (CaO) followed by HCl washing.15 The removal percentage of mineral matter by this method was around 90%. Thus, the chemical cleaning by this method is expected to remove trace elements to a higher extent. In this study, we examined the effectiveness of this chemical cleaning method for trace elements. Experimental Section Sample Preparation. Newlands (NL), Wallarah (WA), and Ermelo (ER) coals were used in this study. Table 1 presents the analyses of the three coals. The pulverized coal particles (
