The Electric Power Research Institute (EPRI) is conducting research to investigate mercury removal in utility flue gas using sorbents. Bench-scale and pilot-scale tests have been conducted to determine the abilities of different sor-bents to remove mercury in simulated and actual flue gas streams. Bench-scale tests have investigated the effects of various sorbent and flue gas parameters on sorbent performance. These data are being used to develop a theoretical model for predicting mercury removal by sorbents at different conditions. This paper describes the results of parametric bench-scale tests investigating the removal of mercuric chloride and elemental mercury by activated carbon. Results obtained to date indicate that the adsorption capacity of a given sorbent is dependent on many factors, including the type of mercury being adsorbed, flue gas composition, and adsorption temperature. These data provide insight into potential mercury adsorption mechanisms and suggest that the removal of mercury involves both physical and chemical mechanisms. Understanding these effects is important since the performance of a given sorbent could vary significantly from site to site depending on the coal- or gas-matrix composition.

Sorbents for removing mercury from flue gases of coal-fired power plants are presently being evaluated due to potential regulation of mercury emissions under Title III of the 1990 Clean Air Act Amendments. Laboratory tests have been conducted to evaluate the adsorption characteristics of potential sorbents and the effects of flue gas constituents on these characteristics. This paper presents a theoretical model that combines the adsorption characteristics measured in the lab with mass transfer considerations to predict mercury removal by the duct injection process in actual flue gas streams. The model was used to determine the effect of various sorbent properties on mercury removal when injecting a powdered sorbent upstream of either an electrostatic precipitator (ESP) or fabric filter. Mercury removal is expected to differ between these configurations since the mass transfer conditions are different in an ESP and fabric filter. The model was used to determine when mercury removal is limited by mass transfer and when it is limited by sorbent capacity. This information defines conditions when removal can be improved by reducing particle size or increasing sorbent capacity. In both cases, removal can be increased by injecting more sorbent.

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The emission rates for 27 major, minor and trace elements from four coal-fired electrical generating stations are presented. The flow rates of each element in each process stream entering and leaving the power plant are given for three of the stations. The exit streams include bottom ash, collected ash and fly ash and vapors in the exhausted flue gases. Elemental material balances were calculated to assess the reliability of the results. The elemental compositions of the particulates in the flue gas were determined as a function of particle size for two stations, one of which was included in the previous material balance study. The two stations studied during this phase used hot-side and cold-side electrostatic precipitators, respectively, to collect fly ash. The concentration effects were evaluated to determine whether the gas temperature during particulate collection affects the distribution characteristics of the elements enriched at the fly ash surfaces.

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