Oxidation and stabilization of elemental mercury from coal-fired flue gas by sulfur monobromide.

Sulfur monobromide (S(2)Br(2)) was employed as a task-specific oxidant to capture and stabilize elemental mercury from coal-fired flue gas. Its performances on the removal of Hg(0) were investigated with respect to the gas-phase reaction and particle-involved reactions. It was found that the gas-phase reaction between Hg(0) and S(2)Br(2) was rapid, and the determined second-rate constant was about 1.2(+/-0.2) x 10(-17)cm(3) molecules(-1) s(-1) at 373 K, which was about 30 times higher than that with sulfur monochloride. The pilot tests showed that the presence of fly ash in flue gas can accelerate the removal of Hg(0) significantly. It was predicted that about 90% of Hg(0) removal efficiency can be obtained with 0.6 ppmv S(2)Br(2) and 30 g/m(3) fly ash in flue gas, and the unburned carbon in fly ash played an important role for Hg(0) removal. The fates of S(2)Br(2) and mercury in the process were evaluated, and the product analysis and leaching tests indicated that mercuric sulfide was the main product of the converted Hg(0) by the direct reaction and consequent series reactions, which is more stable and less toxic than other mercury species. Also, the surplus S(2)Br(2) in flue gas could be captured and neutralized effectively by the alkali components in fly ash or FGD liquor, and its hydrolysis products (elemental sulfur and sulfide) were also helpful to the stabilization of mercury. The result indicated that S(2)Br(2) is a promising oxidant for elemental mercury (Hg(0)) oxidation and stabilization for mercury emission control.