Interpreting enhanced Hg oxidation with Br addition at Plant Miller

This paper introduces the first detailed reaction mechanisms for Br/Hg chemistry in flue gas, and interprets the Hg oxidation performance across a broad range of Br injection rates in recent field tests at Plant Miller. Homogeneous chemistry with Br species is much faster than with Cl species because the Br atom concentrations at the furnace exit are three to four orders of magnitude greater. The dominant channels 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 also analogous to the surface mechanism for Cl species, except that (i) Hg⁰ adsorption is faster on brominated sites and (ii) the high Br atom concentrations promote recombination reactions which maintain very low surface coverages of Hg/Br species. Hence, Br addition does not promote the production of particulate-Hg. Catalytic Hg⁰ oxidation on SCR monoliths by Br species is more than 40 times faster than by Cl species and follows a similar mechanism. For all Br loadings over 10 ppmw in the field tests, essentially all Hg⁰ entering the SCR was oxidized.