Calcium based materials including quicklime, hydrated lime and limestone play an important role in air pollution control applications for the removal of acidic gases. The largest use of calcium based reagents is the removal of SO2 in industrial boiler applications. Calcium based materials can be introduced to the flue gas system in wet scrubbers, dry scrubbers or in direct injection systems.
Dry scrubbing with lime spray dryers and circulating fluid bed dryers
are used to remove SO2 by spraying atomized lime slurry into a vessel. The lime slurry reacts with the SO2, water is evaporated, reaction products are removed from the bottom of the vessel and the scrubbed flue gas flows to the particulate control device. At coal burning electric utilities, dry scrubbing is used primarily for low-sulfur fuels. SO2 removal rates of 95% or greater can be achieved depending on the situation. The high calcium content and excellent slaking characteristics of Mississippi Lime Standard Quicklime make it a preferred option for Dry FGD systems.
Wet scrubbing with either lime or limestone
is performed by spraying a reagent slurry through the flue gas stream. The sulfur dioxide is absorbed into the spray and then precipitated as wet calcium sulfite. Calcium sulfate or gypsum can be produced through forced oxidation of the calcium sulfite. Wet scrubbing is used primarily for high-sulfur fuels.
Direct injection of calcium based materials
is possible with calcium carbonate, quicklime and liquid calcium hydroxide products. Calcium reagents can be injected at various points in a flue gas system to control pollutants. Calcium carbonate (limestone) can be injected into the furnace with the coal as a means of SO2 control. This is commonly employed in CFB and stoker boiler applications. Liquid calcium hydroxide can be injected at various points within the system to mitigate SO2.