It should be noted that it is very hard to differentiate between SO 3 and H 2SO 4 in stack measurements in fact, the two quantities often are used interchangeably. Because scrubbers are not good at removing H 2SO 4 aerosols, most of the SO 3 converted to H 2SO 4 in the unit will exit the stack. Reduction is generally in the 30% to 40% range. The extent of reduction depends on characteristics of the wet scrubber’s design, in particular its temperature profile. The compound typically is converted to H 2SO 4 mist (vapor-phase sulfuric acid) because the temperatures in an FGD unit are below the H 2SO 4 dewpoint. SO 3 levels are reduced by wet FGD systems. Here, too, reductions typically range from 10% to 50%. Large SO 3 reductions that transfer a significant amount of SO 3 to the flyash may reduce an ESP unit’s capture efficiency. The extent of reduction primarily depends on the flue gas temperature and the composition of the flyash. SO 3 reduction by electrostatic precipitators. At lower temperatures SO 3 also can combine with ammonia slip from SCR or selective noncatalytic reduction (SNCR) systems to form ammonia bisulfate, which can deposit on heater surfaces and cause plugging and increased pressure drop. This reduction, however, is tempered by the increased risk of H 2SO 4 formation at lower (<500F) outlet temperatures. Reduction rates of 10% to 50% are typical. The faster the temperature quench and the lower the air preheater outlet temperature, the greater the SO 3 reduction. Although the extent of reduction varies greatly from unit to unit, it generally depends on the flue gas temperature and the type of heater (whether it is regenerative or tubular). In contrast to SCR systems, air preheaters reduce SO 3 levels. The chemical reactions occurring in a selective catalytic reduction system. Conversion levels are generally 0.5% to 1.5%, so our typical furnace with 2,000 ppm of SO 2 in its flue gas would have another 20 ppm of SO 3 added to the flue gas at the SCR outlet (Figure 3).ģ. The extent of conversion depends on the type of catalyst and on flue gas temperature, with higher temperatures producing more SO 3. The vanadia-based catalysts that SCR systems use to reduce NO x also oxidize SO 2 to SO 3. In other words, a typical furnace whose flue gas contains 2,000 ppm of SO 2 also would have about 20 ppm of SO 3 in the flue gas leaving the economizer. The conversion of SO 2 to SO 3 takes place in the furnace’s radiant section and backpass at a rate of about 1%. The extent of the conversion depends on the fuel’s composition, the flue gas temperature, the oxygen concentration (excess air) in the furnace, and the ash content of tube deposits. Inside the furnace, some SO 2 (a normal by-product of the combustion of any sulfur-containing fuel) is converted to SO 3 through reactions with atomic oxygen (O), molecular oxygen (O 2), and catalytic reactions with tube deposits (and local oxygen). The cost of SCR NOx control was $195 million. Gavin today-after complying with NOx SIP Call in 2001, and adding chemical injection for SO3 control. Because the variables reflect differences in fuel composition, component design, and plant operation, there is no one-size-fits-all solution to the problem.Ģc. SO 3 formation and concentrations depend on several variables that differ significantly from plant to plant, making predictions difficult. Of the factors affecting plume opacity, only SO 3 concentration can be controlled by a power plant. Under severe conditions, the sulfuric acid–containing plume may even reach ground levels. The higher the plume concentration, the greater the discoloration and the longer the plume remains in the atmosphere. In most cases, a plume is visible when H 2SO 4 aerosol concentrations in the flue gas exceed 10 to 20 ppm. The plume’s color and opacity depend on the aerosol concentration, aerosol size, sun angle, gas temperature, and atmospheric conditions. In sufficient concentration, the particulates scatter light and create a visible plume that is typically blue or brown-orange. When the H 2SO 4 cools, it forms fine particles of sulfuric acid aerosol. The plume from a power plant stack becomes opaque when SO 3 is converted into vapor-phase sulfuric acid (H 2SO 4). Courtesy: American Electric PowerĪny attempt to mitigate blue plumes requires an understanding of the relationship between SO 3 and stack opacity. Injecting trona (the raw material for soda ash) has brought SO 3 emissions under control. AEP’s General Gavin Plant battled the “blue plume” shortly after it installed SCR units.
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