In modern coal-fired power plants, hard coal is burnt to produce energy. This process yields several by-products that can be used in the building and building materials industries due to their quality.
In the case of dry-bottom boilers, coal is ground to particles of < 90 µm and is then blown into the boiler at a temperature of about 1200 °C.
Depending on its origin, the coal contains between 65 % and 95 % of carbon and 5 % to 35 % of non-combustible rock. Whereas the carbon is burnt, the mineral rock material is left as ash after the combustion process.
Due to the relatively low temperature in the boiler, the ash particles are only partially melted. 85 % to 90 % of these ash particles are entrained in the flue gas stream. The ash cools in the gas stream and forms spherical, mainly amorphous particles – coal fly ash. This fly ash is separated from the flue gas in a series of electrofilters from where it is transported into silos via conveyor lines.
The remaining 10 % to 15 % of the sintered particles conglomerate until they are so heavy that they fall to the bottom of the boiler and from there into a water bath. This material is called bottom ash.