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Show INTRODUCTION The combustion of coal can produce many compounds, both organic and inorganic, that are inlcuded on the Clean Air Act Amendments (CAAA) list of 189 hazardous air pollutants (HAPs). These include chlorinated dioxins and furans, other semivolatile organic compounds, and numerous volatile organic species-in addition to trace metals. Data from many sources have been collected and show that emissions of these HAPs are varied and likely affected by numerous variables including coal type and source, type of boiler, the specific combination and types of air pollution control devices (APCDs) and firing rate. In order to mitigate the potential public and environmental risk associated with emissions of these toxic compounds it is important first to understand the mechanisms by which they are formed and the physical and chemical transformations they undergo during and after combustion, and before escaping through the stack. This paper describes the results of a series of efforts which have been synthesized towared this goal. In the first, emissions of HAPs were measured from two coal-fired utility-scale combustors. Separately, data from other measurement studies were compared and analyzed to identify the differences and similarities between different combustion systems. Finally, a model was developed and employed to predict the physical and chemical behavior of metals in coal ash during coal combustion. The synthesis of these efforts provides some reasonably simple explanations for emissions of specific HAPs, for others the explanations are less clear. However, in all cases it appears that a direction for further study is indicated and this paper attempts to focus that direction. A metal contained in coal ash can follow several pathways after combustion. Depending on the chemical state of the metal, its relative volatility, and the local chemical and thermal environment it may end up in one of four streams: 1) metals that remain in the boiler. These are primarily metals that are retained in the ash during combustion and become part of either bottom ash or slag. 2) metals that leave the combustion process as fly ash components (entrained particles). These metals are primarily retained in the ash, but may include metals that volatilize and later condense on ash particles. 3) metals that are released from the ash, are chemically transformed and subsequently condense. 2 |