Using bromine gas to enhance mercury removal from flue gas of coal-fired power plants

Bromine gas was evaluated for converting elemental mercury (Hg-0) to oxidized mercury, a form that can readily be captured by the existing air pollution control device. The gas-phase oxidation rates of Hg-0 by Br-2 decreased with increasing temperatures. SO2, CO, HCl, and H2O had insignificant effect, while NO exhibited a reverse course of effect on the Hg-0 oxidation: promotion at low NO concentrations and inhibition at high NO concentrations. A reaction mechanism involving the formation of van der Waals clusters is proposed to account for NO's reverse effect. The apparent gas-phase oxidation rate constant, obtained under conditions simulating a flue gas without flyash, was 3.61 x 10(-17) cm(3)center dot molecule(-1)center dot s(-1) at 410 K corresponding to a 50% Hg-0 oxidation using 52 ppm Br-2 in a reaction time of 15 s. Flyash in flue gas significantly promoted the oxidation of Hg-0 by Br-2, and the unburned carbon component played a major role in the promotion primarily through the rapid adsorption of Br-2 which effectively removed Hg-0 from the gas phase. At a typical flue gas temperature, SO2 slightly inhibited the flyash-induced Hg-0 removal. Conversely, NO slightly promoted the flyash induced Hg-0 removal by Br-2. Norit Darco-Hg-LH and Darco-Hg powder activated carbons, which have been demonstrated in field tests, were inferred for estimating the flyash induced Hg-0 oxidation by Br-2. Approximately 60% of Hg-0 is estimated to be oxidized with the addition of 0.4 ppm of gaseous Br-2 into full scale power plant flue gas.