The limestone forced oxidization (LSFO) system is the most common wet FGD system in use today. A stable gypsum byproduct is produced which can be sold for use in the manufacture of wallboard or to the cement industry. In some cases, a disposable gypsum product is produced for landfill if there are no wallboard or cement plants within a reasonable distance.

The differences between gypsum products depend on the amount of impurities or the degree of dewatering. In addition, the LSFO system is less prone to scaling compared to a natural oxidization system and as a result, requires less outage time. The LSFO wet FGD process incorporates air injection to ensure a fully oxidized gypsum product. The introduction of air into the absorber reaction tank creates calcium sulfate (gypsum) as a marketable byproduct.

The absorber reaction tank, sometimes referred to as the recirculation tank, serves several functions. It is a reservoir of slurry to supply the absorber recirculation pumps and the absorber bleed pumps. The reaction tank is where the oxidation of the calcium sulfite to gypsum takes place. It serves as a storage tank to allow the limestone to dissolve and to promote gypsum crystal growth.

A wet FGD can be converted to forced oxidation using the existing reaction tank, or in some cases, an external oxidation tank can be used. This conversion will minimize scaling from utilization of stable system chemistry and creating a saleable byproduct which will decrease disposal costs.

Air sparge grids and air lances with mechanical agitators are the two generally applied methods of injecting oxidation air into the process. The method to inject oxidation air has an impact on plant costs and system operating requirements. The sparge grid is a multiple air header arrangement with evenly spaced bubble stations across the vessel plan area. The sparge grid is prone to nozzle plugging if the oxidation is not constantly passing through the absorber tank whenever it contains slurry. The lance system consists of air pipes directed to a region in the liquid jet created by side entry mixers. Figure 1 shows a typical air lance injection system.

Figure 1 - The lance system consists of air pipes directed to a region in the liquid jet created by side-entry mixers

The performance of the lance system is influenced by the energy of the fluid jet (mixer power) and the submergence depth (air blower power). The performance of the sparge grid is less dependent on the mixer power and is, to a much greater degree, influenced by submergence depth. When upgrading an existing wet FGD, a general review of the capacity available in the existing absorber recirculation tank is required. In some cases, a sparge grid or lance system can be retrofit into the existing tank. If there is not enough residence time available, an external oxidation air tank may be required.