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

摘要

This paper describes a detailed reaction mechanism for 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 Monlieello that burn low-rank coals, The dominant channels of the homogeneous Hg chemistry with Br are analogous io those for Cl. whereby a Br atom partially oxidizes Hg~0 into HgBr. which is then oxidized into HgBr_2 by Br_2. Mercury also oxidizes hetcrogeneously 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 particulale 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 electrostatic prccipitalors (ESPs) are always low for low-rank coals.