Abstract
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 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 Hg0 into HgBr, which is then oxidized into HgBr2 by Br2. 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 (Hg0) 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 Hg0 adsorption rate on brominated UBC promotes Hg0 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 electrostatic precipitators (ESPs) are always low for low-rank coals.
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