Wet FGD Technology A simplified process flow diagram of a conventional wet scrubber is shown in the figure below. The calcium in the slurry reacts with the SO 2 to form calcium sulfite or calcium sulfate. A portion of the slurry from the reaction tank is pumped into the thickener, where the solids settle before going to a filter for final dewatering to about 50 percent solids. Alternatively, gypsum can be produced from FGD waste, which is a useful by-product. In some installations, the flue gas is reheated to avoid corrosion downstream in the power plant.
Many scrubbers have gas-bypassing capability, which can be used for gas reheating.
In the early years of introduction of FGD technology, a spare absorber was included to allow full-load operation with one absorber out of service. Recent improvements in reliability have contributed to the elimination of the spare module in most installations. Presently, the largest capacity scrubber module can handle flue gas approximately equivalent to that of a 1,000 MW coal power plant. Because of their larger size and structure, gypsum crystals settle and dewater better than calcium sulfite crystals, reducing the required size of by-product handling equipment. The high gypsum content also permits disposal of the dewatered waste without fixation.