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IDENTIFICATION AND RESPONSES TO POTENTIAL EFFECTS OF SCR AND WET SCRUBBERS ON SUBMICRON PARTICULATE EMISSIONS AND PLUME CHARACTERISTICS
Citation:
Farthing, W. E., P. M. Walsh, J. P. Gooch, W. Hinton, AND R. F. Heaphy. IDENTIFICATION AND RESPONSES TO POTENTIAL EFFECTS OF SCR AND WET SCRUBBERS ON SUBMICRON PARTICULATE EMISSIONS AND PLUME CHARACTERISTICS. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-04/107 (NTIS 2008-111682), 2004.
Impact/Purpose:
to publish information
Description:
Applications of selective catalytic reduction (SCR) systems and wet flue gas desulfurization (FGD) scrubbers on coal-fired boilers have led to substantial reductions in emissions of nitrogen oxides (NOX) and sulfur dioxide (SO2). 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 (SO3) and subsequent increases of sulfuric acid aerosols, which can result in visible near-stack plumes and acid aerosol mists. The report summarizes the current state-of-the-science concerning both SO3 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. In addition, limitations of SO3 measurement methods and the resulting uncertainties in many existing SO3 emissions data are discussed. Size distributions and visible plume formation are discussed, including empirical predictions of conditions, including coal type and 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 SO3 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) 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 technology 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, increase risks of unforeseen negative effects on plant operation. Other concepts of interest, but less fully tested, include (1) alkali injection in the duct leading to a wet FGD scrubber and (2) an electrostatically augmented mist eliminator. Various devices are being offered that place sorbents in contact with flue gas to remove SO3 and other gaseous pollutants.