Bromine Chloride as an Oxidant to Improve Elemental Mercury Removal from Coal-Fired Flue Gas

The equilibria and kinetics of the reaction between bromine (Br-2) and chlorine (Cl-2) to form bromine chloride (BrCl) at various temperatures were determined. BrCl was employed to oxidize elemental mercury (Hg-0) under simulated flue gas conditions. The removal of Hg-0 from the gas phase by a homogeneous gas-phase oxidation reaction and the heterogeneous reactions involving flyash were investigated. The second-order gas phase rate constant was determined to be 2.3(+/- 0.2) x 10(-17) cm(3).molecules(-1).s(-1) at 373K. The reaction of Hg-0/BrCl was significantly accelerated in the presence of flyash, and the estimated Hg-0 removal efficiency in the presence of 0.6 ppmv 130 and 20 g/m(3) flyash was up to 90%. Unexpectedly, the major product was found to be HgCl2, rather than HgBr2, indicating that bromine in part acted as the accelerant in Hg-0 oxidation in BrCl/Br-2/Cl-2 system by facilitating the formation of intermediates. As a result bromine consumption is much less than if only bromine gas is utilized alone. These results were helpful not only for understanding the mechanism of Hg-0 removal in coal-fired flue gas but also in any atmosphere in which bromine and chlorine species coexist.