Flue Gas Desulfurization (FGD) is an industrial process designed to capture and remove sulfur dioxide (SO2) from the exhaust gases, known as flue gas, produced by burning fossil fuels like coal or oil. This technology acts as a large-scale air pollution control measure, treating the gas stream before it is released into the atmosphere.
Why Sulfur Dioxide Removal is Essential
Sulfur dioxide is a major air pollutant that reacts in the atmosphere to create significant environmental and health hazards. When SO2 mixes with water and oxidants in the air, it forms sulfuric acid, the primary component of acid rain. Acid rain damages sensitive ecosystems by acidifying lakes and streams, harming aquatic life, and contributing to the decay of forests and man-made structures.
Exposure to SO2 gas presents a direct risk to human respiratory health, especially for sensitive populations such as children and asthmatics. The gas can irritate the mucous membranes of the eyes, nose, throat, and lungs, leading to symptoms like coughing and difficulty breathing. Furthermore, SO2 contributes to the formation of fine particulate matter, which can penetrate deep into the lungs and worsen existing cardiovascular and lung conditions.
Categorizing Flue Gas Desulfurization Systems
FGD technologies are broadly classified into three categories based on the moisture content of the scrubbing agent and the final byproduct: Wet, Semi-dry, and Dry systems. The choice among these systems often balances removal efficiency with cost, space requirements, and water availability. Wet FGD systems, which are the most common type globally, utilize a liquid slurry as the absorbent and offer the highest SO2 removal efficiency, often exceeding 95%.
Semi-dry systems, also known as Spray Dryer Absorbers (SDA), inject an atomized alkaline slurry into the flue gas. The heat evaporates the water, resulting in a solid, dry byproduct and requiring less water than wet systems. Dry FGD systems, which are simpler and more compact, inject a dry, powdered sorbent like hydrated lime directly into the gas stream. These systems are suited for facilities burning lower-sulfur fuels or those with less stringent emission limits because they have a lower removal efficiency than wet scrubbers.
Detailed Steps of Wet Scrubbing Technology
Wet scrubbing, particularly the Limestone Forced Oxidation (LSFO) method, is the most prevalent and effective FGD technology used in large coal-fired power plants. The process begins after the flue gas has been pre-treated to remove fly ash and enters a large vessel called an absorber tower. Inside the tower, the SO2-laden gas flows upward while being sprayed with a finely dispersed liquid slurry that contains an alkaline reagent, most commonly pulverized limestone (CaCO3) mixed with water.
This counter-current contact maximizes the surface area for the chemical reaction, absorbing the SO2 into the liquid phase. The sulfur dioxide dissolves in the water and reacts with the calcium carbonate from the limestone slurry, forming primarily calcium sulfite (CaSO3), which precipitates out of the solution.
To create a stable and marketable byproduct, the process includes forced oxidation. Compressed air is bubbled through the slurry, oxidizing the initial calcium sulfite product. This converts the calcium sulfite into calcium sulfate dihydrate (CaSO4 ยท 2H2O), which is chemically identical to mineral gypsum.
This resulting gypsum has a crystalline structure that is easier to dewater and process than the initial sulfite sludge. After scrubbing, the cleaned flue gas passes through a mist eliminator to remove any remaining liquid droplets before release through the stack. The slurry containing the newly formed gypsum is then continuously drawn off for further processing.
Handling and Repurposing FGD Byproducts
The solid material captured by the FGD system must be managed, and in the case of wet scrubbing, the valuable byproduct is synthetic gypsum. This synthetic gypsum possesses a high purity, often ranging from 96% to 99%, making it a viable substitute for natural gypsum. The most significant application for this material is in the construction industry, where it is used to manufacture wallboard, also known as drywall.
The captured material is refined through dewatering equipment, such as hydroclones and vacuum belt filters, which concentrate the crystals and remove excess water and impurities. Beyond construction, synthetic gypsum is also repurposed for agricultural use as a soil amendment to improve soil structure and water retention. While the marketable gypsum from wet scrubbers is recycled, the solid waste from dry or semi-dry systems (a mix of unspent sorbent, reaction products, and fly ash) is conditioned and disposed of in landfills.