Mercury emissions from coal-fired power plants represent 32% of the total anthropogenic mercury emissions in the United States (44.2 tons in 2004). In recent years, public concern has increased due to their long-term irreversible effects on the environment and human health and as a result, the U.S. Environmental Protection Agency (EPA) proposed in December 2011 the Mercury and Air Toxics Standards (MATS); which require U.S. natural gas and coal-fired power plants to install air pollution control devices to prevent 91% of the Hg present in flue gas from being released. In the United States., 40% of electricity from coal sources is produced in power plants that are already equipped with Selective Catalytic Reduction (SCR) units, which regulate NOx emissions with the co-benefit of Hg control (by oxidizing elemental mercury, Hg0, to Hg+2). Oxidized Hg can then be further removed by conventional wet flue gas desulfurization unit due to its high solubility in aqueous solutions.
One of the main achievements of my PhD work has been the integration of an atomic-scale model with bench-scale experiments to identify key factors in Hg oxidation as a co-benefit of the SCR unit. Widely employed materials in commercial SCR catalysts include titania-supported vanadium and tungsten oxides, i.e., V2O5-WO3-TiO2. Theoretical models were used to assess the role of each component in the SCR catalyst (namely, the support (TiO2), the active phase (V2O5) and the promoters (WO3)) toward Hg oxidation, and include both density functional theory and ab-initio thermodynamic calculations. The latter are applied to investigate the effects of temperature and flue gas composition (which is coal dependent) on the reactivity of the catalyst under realistic operating conditions. Additionally, results from Hg oxidation experiments carried out in a lab-scale packed-bed reactor are presented. The effects of flue gas composition, catalyst formulation, temperature and space velocity on the Hg oxidation efficiency of different SCR catalysts are quantified. Based on these results, a kinetic model was developed to determine parameters such as reaction orders and activation energies.