When a combustible material is burned, most of it is released into the air. Coal and other combustible materials release carbon dioxide which contributes to global warming, sulfur dioxide which can cause acid rain, fine particles of dust which can cause respiratory disease, and other pollutants. Scrubbers are devices used to trap pollutants and stop them from going into the air.
Wet scrubbers use fluids to dissolve toxic gasses. The gas flowing through the smokestack is mixed with water vapor. The gas dissolves in the vapor, which condenses and flows into a filter material such as ground limestone. The dissolved gas being targeted sticks to the filter, cleaning the water, which starts the process again.
Dry scrubbers introduce a slurry of fine particles into the combusted gas stream. The particles react with targeted toxins in the gas. The gas then runs through a filter--basically a cloth bag. The filter allows gas to flow through, but stops the fine particles, which are attached to the large particles from the slurry. The filter will also stop dust from flowing out into the air.