Coal ash, also known as coal combustion residuals (CCR), is a byproduct of burning coal for power generation. There are several alternatives to describe coal ash because it varies in composition, depending on the type of coal and combustion process used. Some synonyms for coal ash include fly ash, bottom ash, boiler slag, and flue gas desulfurization materials (FGD). Fly ash is the fine, powdery residue of burned coal that is carried out of the boiler with flue gases. Bottom ash is the heavier residue left after combustion has taken place. Boiler slag is molten ash that solidifies on the walls of the boiler. FGD materials are created during the process of removing sulfur dioxide from the flue gas.