Fly ash is the fine particles that are resultant of coal combustion and other waste materials that are driven out of the boiler with the flue gases. The fine particles that are deposited at the bottom is called bottom ash.
Fly ash, also known as "pulverized fuel ash" in the United Kingdom, is one of the residues generated by coal combustion and is composed of the fine particles that are driven out of the boiler with the flue gases.