|Identification of (and Responses to) Potential Effects of SCR and Wet Scrubbers on Submicron Particulate Emissions and Plume Characteristics (85 pp, 1.37 MB) (EPA/600/R-04/107) August 2004
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 and sulfur dioxide. 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 sulfur dioxide oxidation to sulfur trioxide 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 sulfur trioxide 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 sulfur trioxide measurement methods and the resulting uncertainties in many existing sulfur trioxide emissions data. Size distributions and visible plume formation are discussed, including empirical predictions of conditions—such as coal type, sulfur content, and presence of pollution control equipment—that may lead to greater potential for visible plume formation.
Four general approaches provide a high probability for successful sulfur trioxide removal: (1) alkali injection into the furnace, (2) humidification at the electrostatic precipitator (ESP) inlet to reduce the temperature to below the acid dew point, (3) alkali injection combined with humidification at the ESP inlet, and (4) a separate wet particulate control device such as a wet ESP. All of these approaches have been tested in demonstration scale, but all must be considered new technologies that will require adjustments or identifications of procedures to deal with specific sites. The first three have balance of plant issues associated with them; that is, increased risks of unforeseen negative effects on plant operation. Other concepts of interest, but less fully tested, include alkali injection in the duct leading to a wet FGD scrubber and an electrostatically augmented mist eliminator. Various devices are being offered that have the purpose of contacting sorbents with flue gas for removal of sulfur trioxide as well as other gaseous pollutants.
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