Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven off during combustion. Analysis is fairly straightforward, with the coal thoroughly burnt and the ash material expressed as a percentage of the original weight. It can also give an indication about the quality of coal.
Ash content may be determined as air dried basis and on oven dried basis. The main difference  between the two is that the latter is determined after expelling the moisture content in the sample of coal.