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See also Utility Air Pollution Regulations

Control technologies capable of capturing SO2 include dry scrubbers, wet scrubbers and semi-dry scrubbers.

In the case of a typical wet scrubber, flue gas coming from the boiler is saturated with a slurry containing limestone reagent. This type of SO2 control is characterized by high capital cost, low operating cost and high performance.

Dry scrubber processes inject particles of alkaline sorbent into the flue gas, producing a dry solid by-product. The flue gas leaving the absorber is not saturated in this process. Dry scrubber systems can be grouped into three categories: spray dryers, circulating spray dryers and dry injection systems. The circulating dry scrubber (CDS) technology operates at similar temperatures, but is based on separately feeding dry hydrated lime and water into a fluidized bed reactor. Here the SO2 removal takes place in a bed of moistened powder.

Eco Power Solutions’ Comply 2000 is a multi-pollutant removal system.
 Photo courtesy Eco Power Solutions.


Another scrubber option known as dry sorbent injection (DSI) involves injecting a reagent in dry powdered form (hydrated lime, sodium bicarbonate or Trona) into the flue gas upstream of existing particulate control equipment. DSI is praised for its simplicity and low capital costs, but is limited in performance and requires reagent injection rates above stoichiometry that are often necessary for removal efficiencies.

One semi-dry FGD process uses a circulating fluid bed (CFB) dry scrubber or a lime spray dryer. Lime spray dryer technology operates by spraying a slurry of slaked lime reagent into the flue gas. The flue gas is cooled to 30 to 40 degrees above its saturation temperature as the slurry droplets are dried. As a result, when flue gases come out of the spray dryer, they are present as a dry powder product that is collected in a bag house. Semi-dry FGD technologies are characterized by capital costs that are about half that of wet FGD. They have higher operating costs than wet FGD, but lower operating costs than DSI. (Power Engineering, 6/1/2011)