Land Quality Coal Ash Management

Coal combustion products (CCPs) include coal ash, synthetic gypsum, also referred to as flue gas desulfurization (FGD) gypsum, and FGD solids. FGD systems (scrubbers) remove sulfur dioxide from the exhaust gas of coal-fired power plants and prevent its release into the air.

Coal ash is the unburned material that remains after coal is burned to generate electricity. Coal ash includes bottom ash, boiler slag, and fly ash. Bottom ash is generally gray or black with a consistency of coarse sand. Boiler slag is black and angular with a glassy appearance. Bottom ash and boiler slag settle to the bottom of the boiler during combustion where they are collected and transported usually by water in piping to an on-site storage facility. Fly ash is a fine lightweight powder usually tan to dark gray in color and similar in consistency to talcum powder. Fly ash exits the boiler as part of the exhaust gas and is captured in a mechanical collection device to prevent its release to the air and is transported by either water in piping or air in ductwork to an on-site storage facility.

Coal ash, made up mostly of silica, iron, calcium, and aluminum, is similar in composition to many of the rocks in the earth’s crust. Coal ash can also contain trace amounts of other substances that occur naturally in coal, such as arsenic, cadmium, lead, mercury, and selenium. These substances are also found naturally in soil and rock.

• View the Utilities Solid Waste Activities Group (USWAG) – Coal Ash Fact Sheet (pdf, 107 KB)

When coal is burned the sulfur that is present naturally in the coal is converted to sulfur dioxide (SO₂). Many of Duke Energy’s power plants employ what is commonly referred to as a scrubber, which is shorthand for flue gas desulfurization equipment or FGD, to remove the SO₂ from the exhaust gas and prevent its release into the air. Synthetic gypsum and flue gas desulfurization solids are both byproducts of the scrubbing process. There are several different types of scrubbing processes, and the type of scrubbing process determines of the type of FGD solids produced. Most of Duke Energy’s scrubbers are of the type that produces synthetic gypsum. The synthetic gypsum that is produced in a scrubber is nearly identical to the mineral gypsum found in rock formations around the world. It’s actually chemically more pure than mineral gypsum.

The CCPs that are produced at Duke Energy’s coal-fired power plants that are not sold or not otherwise beneficially reused are managed in the following ways.