Flue Gas Desulfurization (FGD) is a process that removes sulfur dioxide (SO2) from the exhaust of power plants when burning fossil fuels. SO2 forms in the combustion process and is a leading contributor in the formation of acid precipitation. Capture and removal of SO2 is accomplished by devices known as scrubbers. These devices combine the sulphur compounds with a calcium containing sorbent, generally lime (CaO) or limestone (CaCO3), to create a slurry. This slurry can be safely discarded after treatment or turned into useful byproducts. The three primary types of Flue Gas Desulfurization processes used today by coal-fired power plants are wet scrubbers, spray dry scrubbers and those using sorbent injection.

Wet Scrubbers

The wet scrubber process is by far the most common Flue Gas Desulfurization method use today and can achieve a sulfur dioxide removal efficiency rate of 99%. This process involves spraying the flue gas with an aqueous slurry of lime (CaO) or limestone (CaCO3) in a spray tower or absorber. The SO2 is removed through a series of chemical reactions between the slurry and the SO2 to produce calcium sulfate (gypsum) and calcium sulfite. The resultant slurry has traditionally been mixed with fly ash from the power plant and a fixative lime and discarded in a landfill. However if a forced oxidation step is included either in the scrubber process or afterwards, the slurry can be turned entirely into gypsum and can then be sold and utilized in the manufacturing of wallboard, cement, and agricultural soil amendments.

Spray Dry Scrubbers

Spray dry scrubbers is the second most common method of Flue Gas Desulfurization, achieving an efficiency rating between 93-97%. This method uses a water based sorbent containing lime or calcium oxide that is sometimes referred to as lime milk. This lime slurry is atomized into a reactor vessel in the form of an extremely fine spray. The heat from the flue gases entering the vessel evaporates the water from the slurry and the newly hydrated lime reacts with the SO2 to form a dry mixture of calcium sulfate/sulfite. The benefits of this process include the elimination of any water treatment process due to complete evaporation. However, the technology is limited to the volume of flue gases produced from power plants in the 200 MW range and requires the use of the more expensive sorbent lime rather than limestone.

Sorbent Injection

The third method of Flue Gas Desulfurization is the Sorbent Injection method. The Sorbent Injection involves spraying a dry sorbent, usually limestone or hydrated lime (Ca (OH) 2) into the flue gases in the upper part of the furnace. The sorbent reacts with the SO2 and produces gypsum as a byproduct, which is later captured in a fabric filter or via electrostatic precipitators (ESP) together with unused sorbent and fly ash. Efficiency for this process Flue Gas Desulfurization can be as low as 50%. If humidification of the flue gases is added to the process and the sorbent is sprayed further along in the flue gas duct where temperatures have cooled considerably, the sulfur dioxide removal efficiency can be boosted to 80%. The advantages of this method include low capital and operating costs, ease of retrofitting and operating, and the absence of slurry or wastewater to mange.