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Ash
the unburned residue of a fuel's mineral content after complete combustion. The ash content of black and brown coal ranges from about 1% to 45%; of oil shale, from 50% to 80%; of peat, from 2% to 30%; of wood, usually less than 1%; of other types of biomass, from 3% to 5%; and of fuel oil, often less than 0.15%, but sometimes more. The upper limit of mineral content determines the technical possibility and economic viability of using any given mineral as a fuel.
The presence of ash reduces the proportion of combustible material in a fuel. During combustion, a certain percentage of heat is lost through ash. In boiler units, molten ash settles on flue pipes, screens and other elements in the form of vitrified slag. Ash deposits on heating surfaces impede the transfer of heat from flue gases to water or steam, and increase the boiler's aerodynamic drag. Airborne ash corrodes boiler pipes and flues; when removed from flue gases, ash pollutes the atmosphere.
