Flue-Gas Desulfurization (FGD)

Definition - What does Flue-Gas Desulfurization (FGD) mean?

Flue-gas desulfurization (FGD) is the removal process of sulfur dioxide (SO2) from flue gas emissions, often chemically. Sulfur dioxide in gases is produced by the combustion of fossil fuels and many industrial processes such as gasoline refining as well as cement, paper, glass, steel, iron and copper production. Sulfur dioxide emissions are a primary contributor to acid rain and have been regulated by every industrialized nation in the world.

FGD processes vary depending on the amount of SO2 involved, the solution being used to absorb the SO2, and the particular equipment used in the absorption tower.

Corrosionpedia explains Flue-Gas Desulfurization (FGD)

Flue-gas desulfurization is an absorption process by which sulfur dioxide in flue gas from fossil-fuel power plants and from the emissions of other sulfur-oxide-emitting processes can be controlled. SO2 is removed from flue gases by a variety of methods, common ones being:

  • Wet scrubbing using a slurry of alkaline sorbent, usually limestone, lime or seawater to scrub gases
  • Spray-dry scrubbing using similar sorbent slurries
  • Wet sulfuric acid process recovering sulfur in the form of commercial-quality sulfuric acid
  • SNOX flue gas desulfurization removing sulfur dioxide, nitrogen oxides and particulates from flue gases
  • Dry sorbent injection systems

Most FGD systems employ two stages: one for fly ash removal and the other for SO2 removal. In wet scrubbing systems, the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a wet scrubber, and then into the SO2 absorber. However, in dry injection or spray drying operations, the SO2 is first reacted with the sorbent, and then the flue gas passes through a particulate control device.

The flue gas exiting the absorber is saturated with water and still contains some SO2. These gases are highly corrosive to any downstream equipment such as fans, ducts and stacks. Two methods that may minimize corrosion are:

  • Reheating the gases to above their dew point
  • Using materials of construction and designs that allow equipment to withstand the corrosive conditions

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