Question 5.26: Wet scrubbing using lime/limestone is one of the feasible pr...

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 combustion 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_{x} emissions from copper smelters. SO_{x} is a symbol meaning oxides of sulfur (e.g., SO_{2} and SO_{3}).

The FGD processes result in SO_{x} removal by inducing exhaust gases to react with a 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_{2} from the exhaust gases, improving equipment reliability, and altering the 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 important 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 calcium 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 are directed toward testing various forced oxidation designs to find the best oxidation system using the least energy.

2. The chemical reactions of the wet scrubbing FGD process are discussed and presented next. As stated previously, in a lime slurry system, the sulfur dioxide and sulfur trioxide react with lime (CaO) to form calcium sulfite and water, based on the following reaction:

SO_{x} + CaO + H_{2}O → CaSO_{3} + H_{2}O

When limestone (CaCO_{3}) is used instead of lime, it results in a similar chemical reaction, but also yields carbon dioxide:

SO_{x} + CaCO_{3} + H_{2}O → CaSO_{3} + H_{2}O + CO_{2}

In the forced oxidation reaction, the oxygen in air oxidizes the calcium sulfite CaSO_{3} to calcium sulfate CaSO_{4} as present in the following reaction:

CaSO_{3} + H_{2}O + 0.5  O_{2} → CaSO_{4} + H_{2}O

where SO_{x} = SO_{2} + SO_{3}, CaO = lime, calcium oxide, CaSO_{3} = calcium sulfite, CaCO_{3} = limestone and, CaSO_{4} = calcium sulfate.