| Mercury emissions from coal-fired power plants account for 40% of the anthropogenic mercury emissions in the U.S. Every year, 48 tons of mercury are emitted into the atmosphere and only 27 tons are captured in the existing Air Pollution Control Devices (APCDs). The speciation of mercury largely determines the amount of mercury capture in control equipments. Conversion of insoluble elemental mercury (Hg°) into more soluble oxidized mercury (Hg2+) facilitates its removal in wet scrubbers. Past studies suggest that an added supply of OH radicals may enhance the mercury oxidation process. This study demonstrates that the application of H2O 2, as a source of OH radicals, accelerates the oxidation of Hg° into Hg2+. A comprehensive reaction mechanism was compiled from various literature sources and kinetic modeling was carried out to establish the oxidation pathways with the effect of H2O2 addition. The sensitivity analysis of the reaction mechanism indicates that the supply of OH radicals increases the formation of atomic Cl, which accelerates the formation of HgCl2, and then enhancing the oxidation process. The optimum temperature range and the amount of H2O2 required for the maximum oxidation were established. The effects of other flue gas pollutants on the Hg° oxidation were also analyzed. The flue gas NO was found to inhibit the Hg° oxidation, since it competed for the supplied H2O2. However, the effect of SO2 in flue gas was not significant because of the regeneration of OH radicals in the SO2 mechanism. The model predictions in this study were critically evaluated by uncertainty analysis with Monte-Carlo simulations. The Monte-Carlo analysis provided a clear evaluation of the model predictions and established uncertainty limits on the predicted Hg° oxidation. The model was also tested with three sets of experimental data from past research. The surface chemistry of the particles in the flue gas could be important and a preliminary analysis was conducted to study the possible effects of sulfur species and calcium component in the fly ash. The interaction of oxidized mercury with calcium oxide in fly ash could be important in Hgp formation. The oxidized mercury formed by the enhanced oxidation would probably lead to greater adsorption on fly ash, leading to removal in particle control devices. The added hydrogen peroxide effectively oxidized flue gas Hg° as well as NO, thus the oxidation of two pollutants can lead to a potential multi-pollutant control for bituminous-coal power plants. |
