Coal contains varying amounts of naturally occurring noncombustible mineral material that remains after the coal is burned. Most of this material exits the boiler with the exhaust gas in a form that is commonly referred to as fly ash. The remaining unburned material is collected in the bottom of the boiler. Hence the term bottom ash. There are two types of technologies used to capture fly ash and prevent its release to the atmosphere – electrostatic precipitators and bag houses.
An electrostatic precipitator (ESP) works like a big fly ash magnet. The exhaust gas enters the precipitator where a negative electric charge is imposed on the fly-ash particles. The negatively charged particles are then attracted to a series of positively charged metal plates. Electrostatic precipitators typically remove over 99% of the fly ash contained in the exhaust gas.
A bag house (also referred to as a fabric filter) works much like a household vacuum cleaner. As its name implies, a bag house consists of a series of cloth bags that filter the fly ash from the exhaust gas as the gas passes through the bags. Like an ESP, a bag house typically removes more than 99% of the fly ash contained in the exhaust gas.
Duke Energy uses ESPs to capture fly ash at all of its coal-fired power plants except at the Gallagher plant in Indiana, where the ESPs were recently replaced with bag houses, and in North Carolina at Rogers Energy Complex Unit 6, which was originally designed with a bag house.
Dry Ash Handling
Dry ash handling describes the process of transporting and placing coal ash in an engineered landfill. Duke Energy manages coal ash in this manner at a number of its generating facilities in both the Carolinas and the Midwest.
All of Duke Energy’s dry ash landfills are permitted by the states in which they are located. When a portion of a landfill reaches its capacity, it is covered with soil and seeded to manage rainwater infiltration, or otherwise capped in accordance with the permit conditions.