How do coal-fired power plant scrubbers work? --- Gil, Onancock, Va.
Marshall Brain Answers:
If coal were pure carbon, then the only thing we would have to worry about at coal-fired power plants is the carbon dioxide formed when coal burns. Unfortunately, coal often contains other chemicals. Sulfur is one common element found in coal. When burned, the sulfur in coal turns into sulfur dioxide. When sulfur dioxide mixes with moisture in clouds, it creates acid rain (see How acid rain works for details).
Scrubbers help to remove most of the sulfur oxides from the smoke stack exhaust of a power plant. The smoke from the burning coal first goes through a particulate filter to remove soot and ash. See this page (scroll down) to view different types of filters. Then it enters a scrubber. In the scrubber, there is a water spray that produces a cloud of fine water droplets. The water is mixed with crushed limestone, which reacts with the sulfur and pulls it out of the exhaust.
The unexpected side effect is that the calcium in the limestone plus the sulfur form gypsum (CaSO4-2H2O). Gypsum is best known as the material inside gypsum wallboard.
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