(181d) Kinetic Modeling of Gas-Phase Mercury Oxidation by Halides in Combustion Effluents: The Importance of NOx

Emissions of mercury from electricity generator and incineration plants have become one of the major environmental control problems that these plants are facing because of the toxicity of mercury. The conversion of mercury from its elemental form to the oxidized form, using halogens i.e. formation of HgX2, allows the reduction of mercury emissions by current collection methods. In this work, an elementary reaction mechanism is developed for modelling mercury conversion to mercury halides and is used to study the influence of the NOx in this system. Thermochemical properties and kinetic parameters have been calculated for Hg and Halogen ? NOx species using B3LYP and CBS-QB3 methods. An elementary reaction mechanism is constructed with use of chemical activation analysis for association, insertion and addition reactions with quantum RRK analysis for k(E) and Master Equation analysis for fall-off. Unimolecular dissociation reactions are also treated with quantum RRK / Master Equation analysis. Results indicate that the presence of NO significantly decreases the oxidation of mercury by chlorine, because: (i.) NO is a stronger competitor for Cl as the initial NOx-Halogen atom bond is stronger than the Hg-Halogen atom bonds, (ii.) there are several, exothermic, catalytic cycles with NOx for loss of Cl atom, (iii.) HCl is less reactive with OH than HBr for loss (abstraction) of the H (regeneration of halogen atom). Similar catalytic cycles for loss of Br atom occur, but the overall thermochemistry is less favorable to formation of HBr relative to HCl. The reduced loss of Br to HBr relative to Cl to HCl, explains the observations that Bromine is more effective than Cl in oxidation of Hg and mercury conversion with Br in the presence of NO is obtained, even at higher NO concentrations.