What are flue gas desulfurization scrubbers and how do they remove sulfur dioxide from exhaust streams?

• A. Water scrubs dissolve SO2 into moisture.
• B. Filters capture SO2 without chemical change.
• C. Catalytic converters oxidize SO2 to sulfur trioxide.
• D. A sorbent such as lime or limestone reacts with SO2 to form gypsum.

Answer: (D)

The main idea is that flue gas desulfurization scrubbers remove sulfur dioxide by a chemical reaction with a lime- or limestone-based sorbent to form gypsum. In a wet FGD system, the exhaust gas is brought into contact with a slurry of calcium compounds. Sulfur dioxide reacts with the sorbent (often calcium carbonate or calcium hydroxide) to form calcium sulfite, which is subsequently oxidized to calcium sulfate dihydrate, or gypsum. Gypsum is a solid that can be collected from the scrubber, removing SO2 from the exhaust stream. This chemical conversion to a solid byproduct is what makes this method effective.

The other ideas describe physical filtration, simple dissolution, or oxidation that does not convert SO2 into gypsum, so they don’t explain how the scrubber specifically removes SO2.