The addition of selective catalytic reduction (SCR) systems and wet flue gas desulfurization (FGD) scrubbers to coal-fired boilers has led to substantial reductions in emissions of nitrogen oxides (NO(sub x)) and sulfur dioxide (SO(sub 2)). However, observations of pilot- and full-scale tests of these technologies reveal potential adverse side effects that may produce operational and particulate emissions problems. The indirect effects of SCR technology of immediate interest center around its catalytic enhancement of SO2 oxidation to sulfur trioxide (SO(sub 3)) and subsequent increases of sulfuric acid aerosols, which can produce visible near-stack plumes and acid aerosol mists. This report summarizes the current state-of-the-science concerning SO(sub 3) formation processes and methods to minimize such formation. Corrosion of plant components such as air preheaters and other balance of plant problems are discussed, as are problems associated with operation of air pollution control equipment. Also discussed are the limitations of SO(sub 3) measurement methods and the resulting uncertainties in many existing SO(sub 3) emissions data. Size distributions and visible plume formation are discussed, including empirical predictions of conditionssuch as coal type, sulfur content, and presence of pollution control equipmentthat may lead to greater potential for visible plume formation.

Title from title screen (viewed on Dec. 13, 2010). "August 2004." Includes bibliographical references (p. 65-71). "EPA-600/R-04/107."