Sulfur present in fuels gets converted to SO2, and in the presence of a catalyst the
502 is converted to SO3, which reacts with water vapor to form sulfuric acid vapor. Sulfuric acid causes environmental damage through corrosion. SO2 and
503 are together referred to as SOx. The level of SOx depends on the amount of sulfur present in the fuel. Typically, 95% of the sulfur converts to SO2 and 1-3% converts to SO3. Historically SOx pollution has been controlled through dispersion through the use of tall stacks. However, in cases where this is not adequate, reduction methods such as flue gas desulfurization (FGD) are used. FGD involves the use of scrubbers to remove SOx emissions from the flue gases. These are classified as either regenerable or nonregenerable, depending on how the byproducts are disposed of. In regenerable systems, the sulfur or sulfuric acid is recovered. However, these are expensive processes and are justified only in large high-sulfur coal-fired plants. Wet scrubbers using chemicals such as lime soda, magnesium oxide, and limestone are widely used in large utility plants. Because many of these chemical processes occur beyond the boiler boundary, they are not discussed here.