Emissions of sulfur trioxide (SO 3 ) are a key component of plume opacity and acid deposition. Consequently, these emissions need to be low enough to not cause opacity violations and acid deposition. Generally, a small fraction of sulfur (S) in coal is converted to SO 3 in coal-fired combustion devices such as electric utility boilers. The emissions of SO 3 from such a boiler depend on coal S content, combustion conditions, flue gas characteristics, and air pollution devices being used. It is well known that the catalyst used in the selective catalytic reduction (SCR) technology for nitrogen oxides control oxidizes a small fraction of sulfur dioxide in the flue gas to SO 3 . The extent of this oxidation depends on the catalyst formulation and SCR operating conditions. Gas-phase SO 3 and sulfuric acid, on being quenched in plant equipment (e.g., air preheater and wet scrubber), result in fine acidic mist, which can cause increased plume opacity and undesirable emissions. Recently, such effects have been observed at plants firing high-S coal and equipped with SCR systems and wet scrubbers. This paper investigates the factors that affect acidic mist production in coal-fired electric utility boilers and discusses approaches for mitigating emission of this mist. INTRODUCTIONAs understanding of the adverse effects of air pollution has grown, so also has the complexity of coal-fired power plant design and operation, especially with regard to air pollution control systems. Control of air pollutant emissions is not only a legal requirement but also is becoming a financial necessity, as salability of effluents and trading of emissions increase the direct monetary value of emissions control. The days when one must only consider the nuisance value of fly ash are long past. 1 As plant complexity has increased, so have unexpected consequences of changing segments of the total chemical process that occurs between fuel preparation and ultimate emissions. One of the more discernible adverse consequences is the formation and emission of sulfur trioxide (SO 3 )/sulfuric acid (H 2 SO 4 ), as highlighted by the recent and well-publicized experiences of a power plant in Ohio. 2 Although not directly subject to emission limits, SO 3 is important to consider during the design and operation of coal-fired utility boilers for a number of environmental and plant performance reasons.The formation of SO 3 will occur during the combustion of sulfur (S)-bearing fuels such as coal and heavy fuel oils. Virtually all of the SO 3 converts to H 2 SO 4 as flue gas is cooled in the air preheater (APH). Relatively high concentrations of SO 3 /H 2 SO 4 in the boiler, stack, or plume can cause adverse impacts to plant equipment and to the environment. Impacts on plant equipment can include corrosion, fouling, and plugging and may require additional hardware or changes in operation to minimize SO 3 / H 2 SO 4 concentrations and the resulting adverse impacts.

Tests were completed at the Duke Power Cliffside Steam Station on the Unit 5 SCR system to increase NOx removal from an initial design value of 85% to an in-use operating level of 93%. These tests took place from May 24 to May 26, 2004 at the start of the third OTAG season for the SCR that was furnished by Riley Power, Inc., a Babcock Power Inc. company. Unit 5 is a balanced draft, subcritical boiler that operates at 590 MW firing eastern bituminous coal. Two SCR reactors are installed at the economizer outlet of the boiler including economizer bypasses for low load operation. Anhydrous ammonia is the reagent for NOx reduction. Each reactor had two initial catalyst layers when the unit was tested.