Impact of Carbon-in-Ash on Mercury Removal across Particulate Control Devices in Coal-Fired Power Plants

Emissions of mercury to the environment, in particular air emissions, are of increasing concern in the United States. The U.S. EPA has surveyed sources of mercury emissions, and coal-fired power plants were found to discharge the largest amount of mercury into the atmosphere as compared to other man-made sources. In December 2000, the U.S. EPA made a determination to regulate mercury emissions from coal-fired electric utility boilers. Compliance with the as-yet-to-be-determined regulations may necessitate additional air-pollution control devices being installed at utility power plants.There is a considerable amount of data in the literature showing that high levels of mercury removal can occur in existing control devices. Particulate control devices such as electrostatic precipitators and fabric filters are capable of removing mercury, but the level of removal is highly variable, depending on coal type, boiler configuration, mercury speciation in flue gas, and operating conditions in the control device. The composition of the fly ash has been shown to be an important factor in mercury removal (and oxidation). However, the evidence has been, until recently, largely anecdotal. In this paper, we review recent literature on the effects of fly ash carbon content on mercury and present the results of current sampling campaigns on full-scale coal-fired power plants where mercury control processes are being demonstrated. Correlations of Hg removal with carbon content are developed for pulverized-coal-fired units burning bituminous coals.