Page 306 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 285

Solution

1. The discussion on wet scrubbing flue gas desulfurization process is presented here.
Flue gas desulfurization (FGD) is a process by which sulfur is removed from the com-
bustion exhaust gas. Wet scrubbing FGD using lime/limestone is the most commonly
used method of removing sulfur oxides resulting from the combustion of fossil fuels.
It is also the method that is best suited to control SO emissions from copper smelters.
x
SO is a symbol meaning oxides of sulfur (e.g., SO and SO ).
x 2 3
The FGD processes result in SO removal by inducing exhaust gases to react with a
x
chemical absorbent as they move through a long vertical or horizontal chamber. The
absorbent is dissolved or suspended in water, forming a solution or slurry that can be
sprayed or otherwise forced into contact with the escaping gases. The chamber is
known as a wet scrubber, and the process is often referred to as wet scrubbing FGD.
More than 60 different FGD processes have been developed, but only a few have
received widespread use. Of the systems currently in operation, over 90% use lime or
limestone as the chemical absorbent. In a lime slurry system, the sulfur dioxide reacts
with lime to form calcium sulfite and water. For cases where limestone is used instead
of lime, the sulfur dioxide reacts with limestone to form calcium sulfite, water, and
carbon dioxide gas.
Wet scrubbing FGD typically removes 90+% of the sulfur dioxide in a flue gas stream.
A few problems have arisen in the operation of the lime or limestone wet scrubbing
FGD systems, and US EPA’s Industrial Environmental Research Laboratory in
Research Triangle Park, North Carolina, has been successful in developing solutions.
Current efforts are directed toward using the limestone more efficiently, removing
more SO from the exhaust gases, improving equipment reliability, and altering the
2
composition of the waste sludge so that it can be more easily disposed of in landfills.
Calcium sulfite that is formed during the scrubbing process presents another impor-
tant problem. This substance settles and filters poorly, and it can be removed from the
scrubber slurry only in a semiliquid or pastelike form that must be stored in lined
ponds. The US government has developed a method to solve this engineering problem
through a process called forced oxidation.
Forced oxidation is a defined as a process in which sulfite-containing compounds are
further oxidized to sulfate compounds by aeration with air or pure oxygen to promote
dewatering, ease of handling, and/or stability in the waste product. Forced oxidation
requires air to be blown into the tank that holds the used scrubber slurry, composed
primarily of calcium sulfite and water. The air oxidizes the calcium sulfite to calcium
sulfate.
The calcium sulfate formed by this reaction grows to a larger crystal size than does cal-
cium sulfite. As a result, the calcium sulfate can easily be filtered to a much drier and
more stable material, which can be disposed of as landfill. In some areas, the material
may be useful for cement or wallboard manufacture or as a fertilizer additive.
Another problem associated with limestone wet scrubbers is the clogging of process
equipment as a result of calcium sulfate scale. Forced oxidation can help control scale
by removing calcium sulfite from the slurry and by providing an abundance of pure
gypsum (calcium sulfate) to rapidly dissipate the supersaturation normally present.
The process also requires less fresh water for scrubber operation, which is scarce in
many western US locations. Current experiments at the US Research Triangle Park