Interactions between vapor-phase mercury compounds and coal char in synthetic flue gas

Data suggest that coal-fired power plants are a significant source of atmos pheric mercury. Predicting emissions of mercury and the speciation of mercu ry in combustion emissions cannot be done without a fundamental understandi ng of the chemical reactions of mercury in flue gas. In this work, chars ge nerated from three coals were used as sorbent material for both elemental m ercury and mercuric chloride. The temperature of the source as well as the char sorbent was carefully controlled at either 343 or 433 K (70 or 160 deg rees C). When exposed to a synthetic flue gas consisting of O-2, H2O, CO2, and N-2, both Hg-0 and HgCl2 were adsorbed by coal char. The rank of the co al seemed to have a large effect on the adsorption of Hg-0, but not on adso rption of HgCl2. The bituminous chars adsorbed similar amounts of Hg-0, whi le the sub-bituminous char adsorbed almost an order of magnitude less. The amount of Hg-0 adsorbed did not appear to be correlated with the sulfur con tent of the char. The rank dependence suggests that some other characterist ic of the char, for example, pore structure or surface functional groups ma y be important for adsorption of elemental mercury. Adsorption of HgCl2 was higher by a factor of two for the bituminous chars and 50 times higher for the sub-bituminous char. The adsorption of HgCl2 showed less differences a s a function of coal rank and a better agreement among all chars with respe ct to char surface area. This suggests that adsorption of HgCl2 on char may be by a physical adsorption process and, therefore, char surface area woul d be a good indicator of capacity for HgCl2. (C) 2000 Elsevier Science B.V. All rights reserved.