Ash

Coal ash does not contain, but containing inorganic substances such as minerals. Ash is the inorganic residue of coal combustion products, composed of metal oxides such as Fe2O3, MgO, Na2O, K2O, etc., and non-metal oxides such as SiO2, P2O5, etc.. Determination of ash is part of the proximate analysis. Principle of this designation is a number of examples that have been refined coal burned at temperatures in the propagation of a particular heating up obtained residue (ash). Obtained residue was weighed and the amount calculated in percent. Burning example on a standard method BS and AS carried out at a temperature of 815 degrees Celsius with the propagation of a particular heating until constant weight gained, but in general, the combustion is carried out for 3 hours, and is considered constant, except when the residue is still visible example yet burning with perfect, so that the burning must be added for 15 minutes. In the standard method ISO, burning example is done in two stages. The first firing was started from room temperature to a temperature of 500 degrees Celsius for 60 minutes, then being held at that temperature for 30 minutes (to brown coal and lignite should be held for 60 min), then heating was continued until the temperature of 815 + / - 10 degrees Celsius. In the standard method ASTM burning example is generally carried out at a temperature of 750 degrees Celsius for 4 hours, but at certain old coal combustion can be reduced or increased depending on the type of its instances. For example certain coke and coal are non-reactive baking time may need to be added, but for some old coal combustion can be less than 4 hours before the time when it had reached a constant weight. Value of the coal ash is always smaller than the value of mineral content. This happens because during combustion has occurred in the coal chemical changes, such as the evaporation of crystal water, carbon dioxide and sulfur oxides. Mineral content can be calculated from its ash content of the formula, one of which is the Parr Formula: MM = 1.08*A + 0.55*TS. 

Reference:
ISO 1171 - Solid mineral fuels - Determination of ash
ASTM D 3174 - Test Method for Ash in the Analysis sample of Coal and Coke from Coal