Wet Scrubbing Flue Gas Desulfurization

In wet scrubber systems an alkali sorbent solution (limestone or lime) is injected into the flue gas. The sorbent slurry is injected by using a spray tower. The CaCO3 slurry is sprayed from the top to the bottom and the hot flue gas flows from the bottom to the top of the SO₂ absorber (countercurrent flow system). The alkaline slurry reacts with the SO₂ into calcium sulfite, further oxidation by air injection oxidize the sulfite into sulfate (gypsum = calcium sulfate).

Limestone forced oxidation (LSFO) (page2) is a newer limestone injection process that reduces the scaling in the SO₂ absorber. Constantly purging of some circulating slurry is needed, because of high concentrations of TDS, TSS, heavy meals and chlorides. This purge stream is called the FGD blowdown effluent.    

In regenerable systems the used slurry is desaturated by addition of lime and recycled back to the SO₂ absorber. In once through systems the effluent from the SO₂ absorber goes to an equalization tank (for average contaminant concentrations and flow rates). Desaturation of the waste water takes place by adding an alkali sorbent. Chemicals are added to the reaction tanks for further oxidation, precipitation and agglomeration of solid particles. Ferric chloride is used for flocculation and inline polymer injection as coagulant to optimize separation of the solids from the effluent. The clarifier separates the solids from the effluent, the sludge is collected in the sludge tank. Dewatering of the sludge takes place for disposal to a truck prior to storage in a landfill or to use/sell the (solid) cake as by-product. The overflow from the clarifier is ready for further treatment after pH adjustment. The FGD process diagram gives a clear overview of the overall process.