Effects of flue gas constituents on mercury speciation

Beginning with the 1990 Clean Air Act Amendments, there has been considerable interest in mercury emissions from coal-fired power plants. This past year, the U.S. Environmental Protection Agency (EPA) issued both the Mercury Study Report to Congress and the Study of Hazardous Air Pollutant Emissions from Electric Utility Steam-Generating Units, which make clear that EPA views mercury in the environment as a serious issue and that coal-fired utilities are a major source of mercury. For the past 4 years, EPRI and the U.S. Department of Energy (DOE) have funded research on mercury measurement, control, and chemistry at the Energy and Environmental Research Center (EERC). The primary goal of bench-scale work was to determine what flue gas constituents affect mercury speciation, specifically how mercury speciation affects measurement methods and the ability of mercury sorbents to absorb mercury. A bench-scale test rig was designed and built to simulate flue gas conditions. The baseline simulated flue gas consisted of O2, CO2, H2O, and N2. Other flue gas constituents tested include SO2, HCl, NO, NO2, HF, Cl2, and fly ash. The mercury was delivered to system as either elemental mercury (Hg0) or mercury(II) chloride (HgCl2) via temperature-controlled permeation tubes. EERC bench-scale data clearly show that the type of fly ash is important in determining mercury speciation in flue gas streams. Not surprisingly, there appear to be a number of interactions between various flue gas constituents that affect mercury speciation. Depending on concentration, there is clearly an interaction between NO–NO2 and fly ash, and it is possible that the interaction may be related to the ratio of NO:NO2. However, it has been shown that when NO–NO2 is tested without fly ash, there is no conversion of Hg0 to Hg2+. Bench-scale tests clearly show that the chemistry of mercury is very complex and that more research is needed to understand what is occurring. However, it is equally clear that the development of effective mercury sorbents and the ability to accurately model mercury speciation are dependent on understanding mercury chemistry, thermodynamics, and kinetics.