- Coal is not a clean-burning substance. It produces sulfur dioxide, which causes acid rain; nitrogen oxide, which causes smog; carbon dioxide, which contributes to the greenhouse effect; particles of dirt, which can cause respiratory distress; and many other pollutants. Scrubbers are currently used in most coal plants. They do not prevent air pollution, but they do reduce it by filtering out some of the most harmful pollutants.
Wet Coal Scrubbers
- There are two types of scrubbers used: wet and dry. Both use limestone, which reacts with sulfur to remove it from the air. In wet scrubbers, limestone powder is mixed with water and sprayed into the smokestack. As hot gasses from the coal plant blow through it, they come in contact with the water vapor. Dust clings to the water, and sulfur dioxide reacts with the limestone trapped in the vapor. The water vapor then precipitates out of the stack, where it can be filtered out, removing some of the worst pollutants. Some of the water vapor actually remains in the smokestack, which is why many stacks release white plumes. Those plumes are composed of steam and hot gasses.
Dry Coal Scrubbers
- Dry coal scrubbers are a different way to do the same thing. Instead of being mixed with water a lot of water, the limestone is blown into the stack dry or with a bit of moisture. Sulfur dioxide still reacts with the particles of limestone, which trap the chemical. The hot gas then flows through a filter. The limestone particles with their trapped sulfur are too big to get through the filter, as are particles of dust. This removes them from the air stream before it leaves the smokestack. (eHow. com)