Flue gas desulfurization (FGD) is a process that removes sulfur dioxide from the exhaust gases produced by burning fossil fuels, particularly coal. Modern systems capture 95 to 99% of sulfur dioxide before it leaves the smokestack, preventing it from entering the atmosphere where it would contribute to acid rain and respiratory problems. The process works by exposing exhaust gas to an alkaline substance, most commonly crushed limestone, which reacts with the sulfur dioxide and traps it as a solid byproduct.
How the Process Works
At its core, FGD relies on a simple chemical principle: acidic sulfur dioxide gas reacts with an alkaline material to form a neutral salt. In the most widely used design, exhaust gas flows into a large tower called an absorber, where it meets a spray of water mixed with finely ground limestone. The sulfur dioxide dissolves into this wet slurry and reacts with the limestone to form calcium sulfite, a chalk-like compound. When air is blown into the mixture (a step called forced oxidation), that calcium sulfite converts into calcium sulfate, better known as gypsum.
The spent slurry collects at the bottom of the absorber, where it’s mixed with fresh limestone to keep the reaction going. Before the cleaned gas exits the tower, it passes through a mist eliminator that catches any leftover liquid droplets so they don’t escape into the air. The whole system runs continuously, scrubbing millions of cubic feet of exhaust per hour at a large power plant.
While limestone is the dominant reagent because it’s cheap and widely available, plants sometimes use quicklime, hydrated lime, sodium carbonate, magnesium carbonate, or ammonia instead. Coastal power plants can even use seawater as the scrubbing medium, taking advantage of its natural alkalinity and dissolved minerals like chloride and iron that help oxidize the captured sulfur.
Wet, Dry, and Semi-Dry Systems
Not all FGD systems work the same way. The three main categories differ in how much water they use, how much sulfur dioxide they remove, and how complex they are to operate.
Wet scrubbers are the industry workhorse. The flue gas contacts a liquid slurry inside a dedicated absorber tower, and the result is a wet byproduct that can be processed into commercial-grade gypsum. These systems remove 95 to 98% of sulfur dioxide as standard, with the latest designs reaching 99% or higher. The tradeoff is water consumption. A 500-megawatt coal plant with a wet scrubber uses roughly 2,000 liters of makeup water per minute.
Semi-dry systems come in two forms: spray dryers and circulating dry scrubbers. Spray dryers atomize a lime slurry into the gas stream, and the heat of the exhaust evaporates the water before the mixture reaches a dust collector. Circulating dry scrubbers inject dry hydrated lime into an upflow reactor. Both types use about 60% less water than wet scrubbers. Spray dryers typically remove 90 to 95% of sulfur dioxide, while state-of-the-art circulating scrubbers can exceed 98%.
Dry sorbent injection is the simplest and cheapest option. A dry powder, usually lime or sodium bicarbonate, is blown directly into the exhaust duct. The reacted powder is then caught by the plant’s existing dust collector. These systems use little to no water, but their removal rates are lower: up to 60% for calcium-based sorbents and 80 to 90% for sodium bicarbonate. They’re best suited for smaller facilities or plants burning low-sulfur fuel.
What Happens to the Gypsum
A single large coal plant can produce hundreds of thousands of tons of synthetic gypsum per year. Rather than treating it all as waste, the power industry has turned FGD gypsum into a commodity. According to the American Coal Ash Association, 54% of total FGD gypsum production goes into drywall manufacturing, where it directly replaces mined natural gypsum. Panels made entirely from FGD gypsum meet European standards for density, moisture resistance, compressive strength, and surface hardness.
Another 8% goes into cement, concrete, and asphalt production, while 17% is used in land-based applications like structural fill, mine reclamation, and agriculture. As a soil amendment, FGD gypsum is especially useful for treating salt-affected soils. Its high calcium content displaces sodium in the soil, improving drainage and structure. As a fertilizer, it supplies calcium and sulfur, two nutrients essential for plant growth, along with smaller amounts of magnesium, potassium, and selenium.
For gypsum to qualify for wallboard production, it has to meet strict purity standards: at least 92% calcium sulfate dihydrate content, moisture below 10%, and very low levels of chloride and alkali metals. Plants that want to sell their gypsum rather than landfill it need to carefully control the oxidation step and manage impurities in the scrubbing loop.
Energy Cost of Running FGD
FGD systems don’t run for free. They impose what engineers call a “parasitic load,” consuming some of the electricity the power plant generates. For standard nonregenerable systems using lime or limestone, this energy penalty is 3 to 4.5% of the plant’s total energy input. That means a 500-megawatt plant effectively loses 15 to 22 megawatts of output to run its scrubbers, pumps, and fans.
More advanced regenerable systems, which recover the sorbent for reuse rather than producing a disposable byproduct, carry a steeper penalty. Magnesia slurry processes consume 5 to 10% of energy input, and some solvent-based regenerable designs can reach 12 to 25%. This is one reason limestone scrubbing dominates the market: it offers the best balance of removal efficiency, low energy cost, and a sellable byproduct.
Operational Challenges
The inside of an FGD system is a punishing environment. The scrubbing slurry is mildly acidic, loaded with dissolved chlorides and sulfates from the coal, and full of abrasive calcium particles. Over time, this combination corrodes metal surfaces and erodes pump impellers, piping, and spray nozzles. Plants use specialized alloys, including austenitic stainless steels and duplex stainless steels, in the most vulnerable areas, but material degradation remains one of the top maintenance concerns.
Scaling is another persistent problem. When calcium sulfate concentrations in the slurry get too high, gypsum crystals deposit on absorber walls, spray nozzles, and mist eliminators, restricting flow and reducing performance. Operators manage this by controlling slurry chemistry, maintaining proper pH (typically between 5 and 6), and running regular cleaning cycles. Chloride buildup from the coal also has to be managed, because high chloride levels accelerate corrosion and interfere with the limestone reaction.
Why FGD Became Standard
Sulfur dioxide is one of the primary precursors of acid rain. When released into the atmosphere, it reacts with water vapor to form sulfuric acid, which damages forests, acidifies lakes, corrodes buildings, and aggravates respiratory illness. Tightening air quality regulations over the past several decades made FGD a requirement rather than an option for most coal-burning facilities.
Current EPA standards set sulfur dioxide limits as low as 3.1 parts per million by dry volume for larger waste incineration units, a threshold that’s essentially impossible to meet without active scrubbing technology. Coal-fired power plants face similarly strict limits under Clean Air Act regulations. The combination of high removal efficiency (routinely above 95%) and the ability to produce a useful gypsum byproduct has made wet limestone scrubbing the default choice for large plants worldwide. At one refinery installation, a wet FGD system achieved 99.9% removal, reducing inlet concentrations from 2,100 parts per million to less than 2 parts per million in the stack exhaust.