Seventy percent of acid rain comes from sulphur dioxide that dissolves into water to form sulphuric acid. The rest comes from various oxides of nitrogen. The cleaning of coal was originally used to reduce costs from transporting inert material and improving the quality and uniformity of coal. This process has also found to be useful in reducing sulfur content. The process is performed gravitationally and is dependent of the density of the sulfur. It is only successful in removing pyrite sulfur due to its properties. Since pyretic sulfur content varies from region to region, so those with the highest percentage will be in the highest demand. Another way to reduce sulfur dioxide is to by burning coal that has low sulfur content. Fluidized Bed Combustion allows sulfur dioxide emissions to be reduced during the combustion process. A limestone or sand bed are crushed and fluidized. It is essential that a balance be established between the heat liberated within the bed from fuel combustion, and the heat removed by the flue gas as it leaves. The limestone is able to react with the SO2 and reduce emissions by 90%. Wet Flue Gas Desulfurization is a highly effective and cost effective system of flue gas desulfurization. The wet scrubber is located downstream of the boiler, and consists of limestone, lime, or sodium hydrogen. Limestone is the most popular choice. The flue gas enters the absorber and is re-emitted after being scrubbed, at which time the waste solids are removed and disposed of. Dry scrubbing involves the contact between drying gas and the atomized liquid (alkaline based). Upon contacting the flue gas, the drying gas will convert the atomized droplets into a dry product that can be separated and disposed of. The dry scrubbing process requires less power to complete than wet scrubbing.
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