Ash content is not inherent in coal. Ash content refers to the complex chemical changes that occur in minerals during the combustion of coal at high temperatures, in addition to the formation of gaseous compounds that escape from the combustible parts, ultimately forming substances mainly composed of silicon and aluminum oxides. The temperature and combustion conditions are different, and the amount and composition of generated ash also vary. It can be seen that ash is the residue left after a series of complex reactions of decomposition and synthesis during combustion. Therefore, the results of ash content determination are referred to as ash yield or ash content, rather than ash content.