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 atmospheric mercury. Predicting emissions of mercury and the speciation of mercury in combustion emissions cannot be done without a fundamental understanding of the chemical reactions of mercury in flue gas. In this work, chars generated from three coals were used as sorbent material for both elemental mercury 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°C). When exposed to a synthetic flue gas consisting of O2, H2O, CO2, and N2, both Hg0 and HgCl2 were adsorbed by coal char. The rank of the coal seemed to have a large effect on the adsorption of Hg0, but not on adsorption of HgCl2. The bituminous chars adsorbed similar amounts of Hg0, while the sub-bituminous char adsorbed almost an order of magnitude less. The amount of Hg0 adsorbed did not appear to be correlated with the sulfur content of the char. The rank dependence suggests that some other characteristic of the char, for example, pore structure or surface functional groups may 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 as a function of coal rank and a better agreement among all chars with respect to char surface area. This suggests that adsorption of HgCl2 on char may be by a physical adsorption process and, therefore, char surface area would be a good indicator of capacity for HgCl2.