Flue Gas Desulfurization (Wet Limestone Scrubbing)
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Removing SO2 from power plant exhaust to prevent acid rain
Overview
Flue gas desulfurization (FGD) removes sulfur dioxide from coal-fired power plant exhaust using limestone or lime slurry scrubbing. The wet limestone process is the most widely deployed FGD technology, removing >95% of SO2 and producing synthetic gypsum as a saleable byproduct. FGD is the primary technology responsible for reducing acid rain in North America and Europe since the 1970s. More than 300 GW of coal-fired power capacity worldwide is equipped with FGD systems.
Chemical Process
Flue gas from the boiler passes through an absorber tower where it contacts a recirculating slurry of crushed limestone (CaCO3) in water. SO2 dissolves in the slurry and reacts with limestone to form calcium sulfite. Forced oxidation with air converts calcium sulfite to gypsum (CaSO4.2H2O). The gypsum is dewatered by hydrocyclones and vacuum filters for sale to wallboard manufacturers.
CaCO3 + H2SO3 -> CaSO3 + CO2 + H2O
CaSO3 + 1/2O2 + 2H2O -> CaSO4.2H2O (forced oxidation to gypsum)
Raw Materials
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Limestone (CaCO3) — Quarrying (Alkaline sorbent (low cost, widely available))
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Air (O2) — Atmosphere (for forced oxidation) (Oxidizing agent to form gypsum)
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Water (H2O) — Plant water supply (Slurry medium)
End Products
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Synthetic gypsum (CaSO4.2H2O) — Wallboard/drywall manufacturing, cement additive (>95% purity, replaces mined gypsum)
Environmental Impact
FGD has reduced SO2 emissions from US power plants by over 90% since the 1970 Clean Air Act. Synthetic gypsum production displaces gypsum mining. However, FGD increases water consumption and generates wastewater containing mercury, selenium, and other trace elements. FGD wastewater treatment is now required under EPA regulations.
Safety Considerations
- ⚠ Confined space hazards in absorber towers and ductwork
- ⚠ Slippery surfaces from wet limestone slurry
- ⚠ SO2 gas is toxic -- respiratory protection required during maintenance
- ⚠ Acidic conditions in absorber can corrode equipment and structural steel
Recent Innovations
Seawater FGD uses natural alkalinity of seawater, eliminating limestone costs in coastal plants.
Dry and semi-dry scrubbing (lime spray dryer absorbers) reduce water consumption.
Multi-pollutant control combining SO2, NOx, and mercury removal in a single system is being developed.
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