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Bottom ash is part of the non-combustible residue of combustion in a furnace or incinerator. In an industrial context, it usually refers to coal combustion and comprises traces of combustibles embedded in forming clinkers and sticking to hot side walls of a coal-burning furnace during its operation. The portion of the ash that escapes up the chimney or stack is, however, referred to as fly ash. The clinkers fall by themselves into the water or sometimes by poking manually, and are cooled.
The clinker lumps get crushed to small sizes by clinker grinders mounted under water and fall down into a trough from where a water ejector takes them out to a sump. From there it is pumped out by suitable rotary pumps to dumping yard far away. In another arrangement a continuous link chain scrapes out the clinkers from under water and feeds them to clinker grinders outside the bottom ash hopper.
More modern systems adopt a continuous removal philosophy. Essentially, a heavy duty chain conveyor (SSC) submerged in a water trough below the furnace which quenches hot ash as it falls from the combustion chamber and removes the wet ash continuously up a de-watering slope before onward discharge into mechanical conveyors or directly to storage silos.
Alternatively, bottom ash can be conveyed using the dry technology, the MAC (Magaldi Ash Cooler) System, originally introduced since mid '80s by the Italian company Magaldi Power S.p.A. and in recent years proposed also by Clyde Bergemann's DRYCON system and United Conveyor Corporation's VAX Vibratory Ash Extractor. These systems eliminate water usage in the cooling and conveying of bottom ash. The system cools ash using only a small controlled amount of ambient air.
Bottom ash may be used as an aggregate in road construction and concrete, where it is known as furnace bottom ash (FBA), to distinguish it from incinerator bottom ash (IBA), the non-combustible elements remaining after incineration. It was also used in the making of the concrete blocks used to construct many high-rise flats in London in the 1960s.