Evaluation of the Effect of SCR NOx Control Technology on Mercury Speciation

The U.S. Environmental Protection Agency (EPA) performed an Information Collection Request (ICR) in 1999 to gather additional information on the control and emission of mercury from coal-fired power plants. The ICR data indicates that a significant, but highly variable, amount of mercury removal can occur across a power plant’s conventional air pollution control (APC) equipment used for the capture of particulate matter, nitrogen oxide (NOx), and sulfur dioxide (SO2) emissions. Such equipment includes electrostatic precipitators (ESP), fabric filters (FF), and flue gas desulfurization (FGD) systems. One of the more important operating variables that influence the degree of co-benefit mercury capture in these devices is the chemical speciation of the mercury in the flue gas. The mercury in coal-fired power plant combustion flue gas exists as elemental, oxidized, or particulate-bound species. Although elemental mercury is not readily captured, oxidized and particulate mercury can be effectively removed by conventional APC equipment. Recent testing has indicated that the selective catalytic reduction (SCR) devices used for the control of NOx emissions may further enhance mercury capture in existing pollution control equipment by oxidizing elemental mercury across the SCR catalyst. However, the degree of SCR mercury oxidation is quite variable and appears to be both coaland catalyst-specific. Also uncertain at this time is whether the oxidation capacity degrades after extended operation of the SCR catalyst. Since the number of coal-fired power plants equipped with SCR controls is expected to increase significantly in response to current and future NOx regulations, the potential for SCR systems to enhance the removal of mercury could play an important role in a plant’s strategy to comply with future restrictions on mercury emissions. This paper presents the results of recent testing sponsored by the Department of Energy’s National Energy Technology Laboratory (DOE/NETL), and other organizations, to study the impact of SCR systems on mercury capture and to identify the important design and operational parameters that affect capture performance.