Gas Phase Mercury Oxidation by Halogens (Cl, Br, I) in Combustion Effluents: Influence of Operating Conditions

Control of mercury emissions is one of the major challenges faced by power generation in coal burning and incineration plants, due to the increasing emission control regulations in the electricity generating sector. This study focuses on the elimination of mercury from the combustion flue gases via the oxidation of elemental mercury (non-soluble) into its oxidized form (soluble) by the addition of halogens (chlorine, bromine and iodine). A detailed reaction mechanism is developed and comparisons of mercury loss versus halogen, NO, SO2 and H2O presence in a typical combustion effluent stream are presented. The influence of different air-fuel equivalence ratios is also illustrated. The removal of mercury is evaluated with an elementary reaction mechanism (957 reactions, 203 species) developed from fundamental principles of thermodynamics and statistical mechanics. Thermochemistry and rate constants are from literature, or calculated at the M06-2X/aug-cc-pVTZ-PP (mercury species), and CBS-QB3 (nonmercury species) levels of theory. Rate constants are calculated by application of the Canonical Transition State Theory (CTST). Pressure dependence of chemically activated reactions is included by the QRRK analysis for k(E) and Master Equation for fall-off. Thermochemistry on Hg halides, oxides, and Hg-NOx-X and Hg-SOx-X (X=Cl,Br) has been determined and kinetics incorporated in the mechanism. Results show that bromine and iodine are more effective than chlorine at oxidizing mercury due to competition for chlorine by hydrogen. Other results show that NO and SO2 are observed to inhibit mercury conversion, that moderate changes in H2O have a slight impact on mercury oxidation, and that the air-fuel ratio significantly influences the conversion of mercury by the halogens.