Sulfur Dioxide Scrubbers

What is sulfur dioxide (SO₂)?

Sulfur dioxide is a colorless gas that can be released into the air through various manufacturing processes, particularly through the combustion of coal for power generation. SO₂ is also released to the air from natural sources, but on average, human activities are the predominant global source. Volcanic eruptions, for example, can release millions of tons of SO₂ during a single major event, but such massive eruptions are thankfully infrequent.

What are SO₂ scrubbers?

An SO₂ scrubber system is the informal name for flue gas desulfurization (FGD) technology, which removes, or "scrubs," SO₂ emissions from the exhaust of coal-fired power plants. A scrubber works by spraying a wet slurry of limestone into a large chamber where the calcium in the limestone reacts with the SO₂ in the flue gas. There are some variations in design of scrubbers. For example, some scrubbers may use other chemicals such as lime or magnesium oxide to react with the SO₂ in the flue gas.

How is SO₂ emitted through a coal-fired power plant?

Coal is not a pure carbon; most coal contains other chemicals, including sulfur. When coal is burned, the sulfur combines with the oxygen in the combustion air to produce SO₂, which is emitted into the air through a plant’s stacks, if not removed by a scrubber.

Why does SO₂ need to be scrubbed?

Sulfur dioxide is a reactive gas – so when it meets other gases within the earth’s atmosphere, a fine, secondary particle forms. Emissions from automobiles and trucks, industrial processes, wood-burning stoves and forest fires, and surface mining and agriculture activities also contribute to the formation of fine particles. An abundance of these fine particles, or SO_2, within the atmosphere could impact our health and the environment.

In 1971, the United States Environmental Protection Agency first established air quality standards relating to SO₂ and fine particles, and updates the standards periodically. All Duke Energy plants continuously monitor emissions to ensure air quality regulations are met.

How does a scrubber work?

Once sulfur is burned and produces SO₂, the exhaust gas passes through the scrubber where a spray mixture of limestone (or other chemical reagent) and water reacts with the SO₂. The reaction enables the SO₂ to be removed before it’s released into the atmosphere. Duke Energy’s newer scrubbers are typically designed to remove 95% or more of the SO₂ from the exhaust gas. The white plume that comes out of the stack is water vapor.

What happens to the SO₂ that is captured in a scrubber?

When SO₂ combines with limestone, a primary byproduct is calcium sulfate, commonly known as synthetic gypsum. A recyclable product, synthetic gypsum is used in the manufacturing of wallboard and cement, and as a soil amendment in agricultural and construction applications.

Much of the synthetic gypsum produced from Duke Energy’s scrubbers is reused in these and other applications. Unused byproducts are properly disposed of in approved landfills.

Managing synthetic gypsum

How does Duke Energy manage synthetic gypsum and FGD solids?

Gypsum is the main ingredient used in the manufacture of drywall. Duke Energy sells much of the synthetic gypsum produced at our generating facilities to drywall manufacturers. We also supply synthetic gypsum for use in agricultural applications and for use in manufacturing portland cement. The gypsum that Duke Energy produces that is not used in drywall manufacture, cement manufacture or agriculture is placed in onsite landfills that are permitted by the state where the landfill is located. All of the flue gas desulfurization (FGD) solids produced by Duke Energy are placed in onsite or offsite landfills that are permitted by the state where the landfill is located.