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  Evaluation and Mitigation of Visible Acidic Aerosol Plumes From Coal-Fired Power Boilers, Final Report (103 pp, 1.71 MB) (EPA/600/R-06/156) September 2006

The formation of sulfur trioxide during the combustion of sulfur-containing fuels, particularly coal, can increase significantly following the installation and operation of selective catalytic reduction (SCR) systems for reduction of nitrogen oxides.

The increased sulfur trioxide formation can lead to adverse environmental impacts, including visible near-stack plumes and increased fine particulate matter emissions, primarily in the form of sulfuric acid aerosols. The extent of the problem in the electric utility sector is estimated based on the population of coal-fired utility boilers, the sulfur content of coal burned by each unit, and the likelihood that units will install SCR and flue gas desulfurization (FGD) systems.

Of the 363 large (250 megawatt electrical [Mwe]) generating plants in the eastern U.S., as many as 65 could experience visible sulfuric acid aerosol plumes or more serious problems after installation of SCR and FGD systems, based on the sulfur content of the coal historically used at those plants. As use of FGD systems increases, it is also likely that utilities will turn to higher sulfur coal, which can exacerbate this problem.

This report describes the mechanisms of sulfur trioxide and acid aerosol formation and removal across boiler convection sections, air preheaters, and wet FGD systems. It presents information from an exploratory study of the absorption of sulfur trioxide onto coal fly ash. A model of sulfur trioxide formation and emissions based on these mechanisms is shown to accurately predict the stack concentration of sulfur trioxide for a 1300 MWe pulverized coal-fired boiler, indicating that the mechanisms described have captured the fundamental behavior of sulfur trioxide in utility combustion and flue gas treatment systems. This information can provide the basis for developing mitigation approaches to reduce the impacts of sulfur trioxide formation across SCRs and the subsequent formation and emission of acid aerosols.


Andrew Miller

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