SO2 is usually removed by spraying
the hot exhaust gases with a slurry of limestone, dolomite, or (less frequently) lime. The process is called
'wet scrubbing', and it involves many separate dissolution, oxidation, and neutralization processes.
For wet scrubbing with CaCO3 slurry, the most important reactions can be summarized by
SO2(g) + H2O(l) 
SO2·H2O(aq)
SO2·H2O(aq) + CaCO3(s) 
CaSO3(s) + H2O(l) + CO2(g)
CaSO3(s) + 2 H2O(l) + 1/2 O2(g) CaSO4·2 H2O
The final reaction produces gypsum, which has economic value.
In fact, some power plants earn additional income by selling the gypsum to industry for use in plaster, wallboard, and Portland cement.
It's interesting to note that although wet scrubbing
has been used since the 1920's and 1930's in Great Britain's coal-fired power plants, scrubbing didn't
catch on in the United States until the Clean Air Act Amendments of 1970 and 1977, for economic reasons.
Author: Fred Senese senese@antoine.frostburg.edu