An Overview of the Behavior of Selected Trace Metal Species During Coal Combustion and Gasification

Title III of the 1990 Clean Air Act Amendments classifies 189 species as "Air Toxics." Of these, eleven are heavy metals found in coal and are therefore of potential concern for electric utilities. The emission of these species is directly related to their volatility and the associated enrichment in either the vapor or submicron-sized particle phases. To minimize emissions, a utility must therefore minimize the vaporization of these elements, minimize their association with submicron particles, and maximize the capture of the smallest ash particles, either through combustion process modifications or the use of sorbents. In this paper, we present an overview of the results of our prior studies describing the partitioning of the inorganic air toxics species as a function of ash particle size during coal combustion. The results of this work suggest that species such as arsenic, selenium, and occasionally chromium may partially volatilize during combustion. We also present new data indicating that during entrained gasification of an Illinois #6 coal, arsenic, selenium, antimony, and mercury were the only air toxics metals exhibiting measurable volatility. To address the minimization of air toxics emissions, a controlled bench-scale study emphasizing arsenic and chromium was conducted. Vaporization experiments with pure compounds suggested that the oxide forms of these elements would dominate, and would be amenable to capture by selected sorbents