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Particulate Control Technologies

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 percent of the fly ash contained in the exhaust gas.

A bag house 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 percent 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 its Gallagher plant in Indiana where it recently replaced the ESPs with bag houses.

What happens to the captured fly ash?