FLUE GAS DESULFURIZATION

Flue gas desulfurization is a chemical process to remove sulfur oxides from the flue gas at coal-burning power plants. Many FGD methods have been developed to varying stages of applicability.
Their goal is to chemically combine the sulfur gases released in coal combustion by reacting them with a sorbent, such as limestone (calcium carbonate, CaCO3), lime (calcium oxide, CaO) or ammonia (NH3). Of the FGD systems in the United States, 90 percent use limestone or lime as the sorbent. As the flue gas comes in contact with the slurry of calcium salts, sulfur dioxide (SO2) reacts with the calcium to form hydrous calcium sulfate (CaSO42H20) or gypsum.
Certain material produced by some power plants in an oxidizing and calcium based process for air emission scrubbing is called FGD (or synthetic) gypsum. FGD gypsum is precipitated gypsum formed through the neutralization of sulfuric acid. While the material may vary in purity, which is defined as the percentage of CaSO4٠2H2O, it is generally over 94% when it is used in wallboard manufacturing. Because this material is very consistent when produced by power plants, wallboard manufacturers will often be located adjacent to the power plant to allow the FGD material to be delivered directly to the wallboard plants. This synergistic relationship not only is economically attractive, but it reduces the need to mine natural gypsum and therefore has a positive environmental impact.
FGD material can be wet or dry. Definitions related to FGD material can be found on this website by clicking on the tab “What are CCPs?” on the Home Page. The PDF file “Glossary of Terms” can be downloaded. As many different terms are used for FGD material, and operational differences between systems may create slightly different types of FGD, this Glossary of Terms is a reliable source of information.