Homogeneous and heterogeneous reaction mechanisms and kinetics of mercury oxidation in coal-fired flue gas with bromine addition

Abstract

A detailed kinetic model consisting of homogeneous mechanism with 352 elementary reactions and a new heterogeneous mechanism on fly ash surface was developed to predict mercury oxidation in coal-fired flue gas with bromine addition. The heterogeneous mechanism involves the catalytic oxidation activity of unburned carbon (UBC) and Fe2O3 within fly ash, and includes 38 irreversible elementary reactions governing the adsorption, oxidation and desorption of mercury on fly ash. The competitive adsorption among flue gas compositions is also involved in the heterogeneous mechanism. Kinetic parameters of some homogeneous reactions in the Hg/Br and Hg/Cl sub-mechanisms have been updated based on the recent experimental data obtained by the improved aqueous chemistry methods. This kinetic model was validated by comparison to lab-, pilot- and full-scale experimental data. The comparison shows that the model predictions are in good agreement with the experimental data. The rate of production analysis illustrates that the heterogeneous pathway is dominant for mercury oxidation in coal-fired flue gas with bromine addition. The sensitivity analysis and reaction path analysis indicate that the dominant pathway of heterogeneous mercury oxidation by bromine on fly ash surface is Hg0→StHgBr(s)→HgBr2, whereby a brominated carbon site partially oxidizes Hg0 into StHgBr(s) which is subsequently oxidized into HgBr2. NO and SO2 weakly inhibit mercury oxidation. H2O exhibits an inhibitory effect on heterogeneous Hg oxidation because of the elimination of brominated carbon sites.