Particulate Control Technologies
Coal contains varying amounts of naturally occurring noncombustible mineral material that remains after combustion. Most of this material exits the boiler with the exhaust gas in a form that is commonly referred to as fly ash. The material that remains is collected in the bottom of the boiler. Hence the term bottom ash. There are two types of technologies used to capture fly ash – electrostatic precipitators and baghouses.
An electrostatic precipitator (ESP) works like a big fly ash magnet. The exhaust gas enters the precipitator where the fly ash particles in the gas receive a negative electric charge. The negatively charged particles are then attracted to a series of positively charged metal plates. Electrostatic precipitators typically remove over 99 percent of the fly ash from the exhaust gas.
A baghouse works much like a vacuum cleaner. As its name implies, a baghouse consists of a series of cloth bags that filter the fly ash from the exhaust gas. Like an ESP, a baghouse typically removes more than 99 percent of the fly ash in the exhaust gas.
All of Duke Energy’s coal-fired generating units currently use ESPs to capture fly ash. In the near future, Duke Energy will be replacing the existing ESPs at the Gallagher generating facility with baghouses.
What happens to the captured fly ash?
Fly ash from coal-fired power plants is a valuable byproduct. Fly ash is used in coal mine reclamation projects, in the manufacturing of cement, as filler in concrete, as structural fill and as a road base. The use of fly ash conserves natural resources and the energy required to extract and process those resources. Duke Energy recycles much of the fly ash produced at its coal-fired power plants. The fly ash that is not recycled is placed in permitted landfills or ash basins.
