Limestone And Lime As Reagents In The Aqueous Scrubbing Of Flue Gas For Sulfur Removal

The control of sulfur dioxide and fly ash in the emissions from coal-fired boilers is a significant segment of the national environ- mental protection program. Developing technologies to accomplish this objective presently emphasize aqueous scrubbing techniques. Should circumstances require aqueous scrubbing for fly ash control only, provision must nevertheless be made to counteract the presence of sulfur dioxide which can create an acidic and potentially corrosive environment. Major technologies for sulfur dioxide control by aqueous scrubbing include, l), recovery processes yielding sulfuric acid or elemental sulfur and 2), throw-away processes which produce a sludge consisting essentially of calcium sulfite or varying amounts of calcium sulfate.1/ Recovery processes are more sophisticated and best suited for locations where there are markets for sulfuric acid or sulfur, or where sludge disposal is an overriding consideration. Throw-away processes are generally less sophisticated but have a problem in that gas scrubbing systems may not operate for practical time periods without failures due to corrosion and to plugging by particulates and precipitates. To overcome this difficulty, some throw-away systems employ an aqueous solution of caustic in the gas scrubbing device to assure maximum water solubility of sulfur compounds, followed by the addition of lime to the scrubbing medium in a system external to the scrubber to produce the throw-away sludge and regenerate the caustic for recycle. Such systems are referred to as double alkali systems.2/