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 a slag tap boiler, the coal is ground to particles of < 90 µm and blown into the boiler at a temperature of about 1500 °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 high temperature, most of the ash melts in the boiler and falls into a water bath, where it solidifies into granules with a grain size of about 0 to 11 mm – known as boiler slag. Only a small portion of the ash is not melted completely and is entrained by the flue gas stream as coal fly ash.