Process Chemistry of Br Addition to Utility Flue Gas for Hg Emissions Control

This paper describes a detailed reaction mechanism tor Br/Hg/Cl chemistry in coal-derived flue gas and interprets the Hg oxidation performance across a broad range of Br addition rates in recent field tests at plants Miller, Milton R. Young, and Monticello that burn low-rank coals. The dominant channels of the homogeneous Hg chemistry with Br are analogous to those for Cl, whereby a Br atom partially oxidizes Hg-0 into HgBr, which is then oxidized into HgBr2 by Br-2. Mercury also oxidizes heterogeneously on unburned carbon (UBC) with Br species. This mechanism is analogous to the surface mechanism for Cl species, except that (i) elemental mercury (Hg-0) adsorption is faster on brominated sites and (ii) the higher Br atom concentrations in flue gas promote recombination reactions that maintain very low surface coverages of Hg/Br species. Therefore, the accelerated Hg-0 adsorption rate on brominated UBC promotes Hg-0 oxidation at the hottest gas cleaning temperatures but does not enhance the production of particulate Hg (HgP). The amount of HgP was predicted to increase for progressively greater loss-on-ignition (LOI) levels, although the removals of this form of Hg by eletrostatic precipitators (ESPs) are always low for low-rank coals.