Flue gas desulfurization (FGD) systems — commonly referred to as “scrubbers” — remove sulfur dioxide (SO2) from the gases of the combustion process before they are exhausted to the atmosphere. There are two types of scrubber systems — “wet” and “dry” — referring to the byproduct produced, not the process itself. Both types of systems use water as the main component to reduce the temperature of the gas so that SO2 removal can be achieved.
Wet scrubbers use a mixture of limestone and water sprayed through nozzles (similar to shower heads) as the combustion gas enters the system. This mixture reacts with the SO2, eliminating it from the system and creating a safe, removable byproduct. Mirant has chosen wet scrubbers that will not only achieve 98 percent removal of SO2 but also produce gypsum, a recyclable material that can be used by other industries in the manufacturing of construction products, cement and wallboard. By selecting a process that yields a recyclable byproduct, Mirant will minimize landfill waste.
Dry scrubbers also involve spraying a mixture of limestone and water across the gas as it enters the system. The mixture reacts with SO2 to create a byproduct that is extracted. However, the SO2 removal efficiency of the dry scrubber process is only about 90 percent compared to wet scrubbers. Moreover, the dry scrubber system produces large quantities of solid waste that has no viable economic reuse and must be placed in a landfill.
For these reasons — much higher emissions reductions and a byproduct that avoids increased landfilling — wet scrubbers were selected as the better choice for Mirant and the communities in which we operate.
