Kinetic modeling of homogeneous mercury oxidation:: The importance of NO and H2O in predicting oxidation in coal-derived systems

This paper develops and evaluates an elementary reaction mechanism for homogeneous Hg-0 oxidation that accounts for major interactions among Cl-species and other pollutants in coal-derived exhausts. Most importantly, interactions among NO and Cl-species were found to exert a strong and previously unrecognized impact on homogeneous Hg-0 oxidation under some but not all conditions. The proposed oxidation mechanism is subjected to quantitative evaluations against all the available laboratory datasets that characterize homogeneous Hg-0 oxidation when HCl is the primary chlorinated species. The simulations depict the reported extents of oxidation for broad ranges of HCl and temperature within useful quantitative tolerances without any heuristic parameter adjustments. The predicted pool of Cl-atoms was found to be governed by the chemistries of moist CO oxidation, Cl-species transformations, and NO production. However, an artificial initiation scheme was needed to depict the temperature dependence observed in one set of literature data. This indicates that either the kinetic mechanisms are incomplete or that heterogeneous initiation came into play under these test conditions. The evaluations further show that Hg oxidation is primarily through a Cl atom recycle process, with Cl and Cl-2 concentrations both playing an important role. Oxygen weakly promotes homogeneous Hg oxidation, whereas moisture is a stronger inhibitor. NO can promote or inhibit homogeneous Hg oxidation, depending on its concentration. In the presence of NO, extents of Hg oxidation increased for progressively, faster quenching. Conversely, without NO, extents of oxidation diminished for faster quenching.