gas exiting the absorber is saturated with water and still contains some . These gases are highly corrosive to any downstream equipment such as fans, ducts, and stacks. Two methods that may minimize corrosion are: (1) reheating the gases to above their dew point, or (2) using materials of construction and designs that allow equipment to withstand the corrosive conditions. Both alternatives are expensive. Engineers determine which method to use on a site-by-site basis. Scrubbing with an alkali solid or solution is an acid gas, and, therefore, the typical sorbent slurries or other materials used to remove the from the flue gases are alkaline. The reaction taking place in wet scrubbing using a (limestone) slurry produces calcium sulfite () and may be expressed in the simplified dry form as: + → + When wet scrubbing with a (hydrated lime) slurry, the reaction also produces (calcium sulfite) and may be expressed in the simplified dry form as: + → + When wet scrubbing with a (magnesium hydroxide) slurry, the reaction produces (magnesium sulfite) and may be expressed in the simplified dry form as: + → + To partially offset the cost of the FGD installation, some designs, particularly dry sorbent injection systems, further oxidize the (calcium sulfite) to produce marketable (gypsum) that can be of high enough quality to use in wallboard and other products. The process by which this synthetic gypsum is created is also known as forced oxidation: + 2 + → A natural alkaline usable to absorb is seawater. The is absorbed in the water, and when oxygen is added reacts to form sulfate ions and free . The surplus of is offset by the carbonates in seawater pushing the carbonate equilibrium to release gas: + + → + 2 + → + In industry caustic () is often used to scrub , producing sodium sulfite: 2 + → + Types of wet scrubbers used in FGD To promote maximum gas–liquid surface area and residence time, a number of wet scrubber designs have been used, including spray towers, venturis, plate towers, and mobile packed beds. Because of scale buildup, plugging, or erosion, which affect FGD dependability and absorber efficiency, the trend is to use simple scrubbers such as spray towers instead of more complicated ones. The configuration of the tower may be vertical or horizontal, and flue gas can flow concurrently, countercurrently, or crosscurrently with respect to the