Kinetic study of heterogeneous mercury oxidation by HCl on fly ash surface in coal-fired flue gas

A detailed chemical kinetic model consisting of a homogeneous mechanism with 279 elementary reactions and a new nineteen-step heterogeneous mechanism for reactions on fly ash surfaces was developed to predict mercury oxidation under various quench rates and gas compositions characteristic of coal-fired flue gas. The Arrhenius parameters of some of the homogeneous reactions in the Hg/Cl sub-mechanism have been modified based on recent experimental data that is unbiased by additional Hg oxidation in the impinger solutions used in aqueous chemistry methods. The heterogeneous model includes the catalytic effects of unburned carbon (UBC) and Fe2O3 within fly ash and the competitive adsorption of various flue gas species. This model was validated by comparison with experimental data. The comparison shows that model predictions are in good agreement with the experimental data conducted by four different groups. Sensitivity analysis indicates that the dominant channel of heterogeneous mercury oxidation by HCl on Fe2O3 surface is Hg0 → FeHgCl(s) → HgCl2. The main reaction pathway for heterogeneous mercury oxidation on fly ash is Hg0 → StHgCl(s) → HgCl2, which suggests that UBC exhibits much higher catalytic oxidation activity for mercury oxidation than Fe2O3. The effect of HCl on mercury oxidation shows that mercury oxidation for higher UBC level are much more sensitive to HCl concentration variations than that for lower UBC level. SO2 mainly weakly promotes heterogeneous mercury oxidation on fly ash, and the presence of H2O exhibits inhibitory effect on heterogeneous Hg oxidation through the elimination of chlorinated sites.