Bottom ash

Bottom ash is part of the non-combustible residue of combustion in a power plant, boiler, furnace or incinerator, made up of clinkers and other combustible traces that collect at the bottom of a coal-burning furnace, distinguishing it from fly ash, which escapes up the chimney. It forms mainly from burning coal in industrial facilities, with clinkers falling into a hopper at the furnace base and then being cooled; US power plants generated about 9.7 million short tons of bottom ash in 2017 alone. Once processed through dry ash handling, it can be reused in concrete and brick manufacturing, road construction, cement kiln production and horticultural growing media, and it was used to make concrete blocks for high-rise housing in London during the 1960s. Most US facilities still store bottom ash in ponds, though some use dry landfill disposal, and coal ash carries concentrated trace elements such as arsenic, cadmium and mercury that pose a groundwater contamination risk.