Towards the development of a chemical kinetic model for the homogeneous oxidation of mercury by chlorine species

The potential for regulation of mercury emissions from coal-fired boilers is a concern for the electric utility industry. Field data show a wide variation in the fraction of mercury that is emitted as a vapor vs. that retained in the solid products. The reason for this variation is not well-understood. Near the end of the flue gas path, mercury exists as a combination of elemental vapor and HgCl2 vapor. The data show that HgCl2 is more likely to be removed from the flue gas. Thus, the degree of oxidation is considered to be a critical factor that tends to reduce emission. Mercury is certain to exist as elemental vapor in the flame, with the oxidation occurring at some point in the post-flame environment. At present, the mechanism promoting this oxidation is not quantitatively known, particularly under the low chlorine concentrations afforded by many coals. In the present work, we measure mercury oxidation from a furnace operating between 860°C and 1171°C. These data are compared with similar results from the literature. The possible elementary reactions that may lead to oxidation are reviewed and a chemical kinetic model is proposed. This model yields good qualitative agreement with the data and indicates that mercury oxidation occurs during the thermal quench of the combustion gases. The model also suggests that atomic chlorine is the key oxidizing species. The oxidation is limited to a temperature window between 700°C and 400°C that is defined by the overlap of (1) a region of significant superequilibrium Cl concentration, and (2) a region where oxidized mercury is favored by equilibrium. Above 700°C, reverse reactions effectively limit oxidized mercury concentrations. Below 400°C, atomic chlorine concentrations are too low to support further oxidation. The implication of these results are that homogeneous oxidation is governed primarily by (1) HCl concentration, (2) quench rate, and (3) background gas composition.