There are three main factors that cause errors in ash content determination:

① The degree of oxidation of pyrite; ② The degree of decomposition of carbonates (mainly calcite); ③ The amount of sulfur fixed in ash.

In order to measure reliable ash yield, it is necessary to completely oxidize pyrite, decompose calcite, and minimize the reaction between sulfur trioxide and evolved calcium. To achieve this, the following measures can be taken

（1） Using slow ashing method to completely oxidize and discharge sulfides in coal before carbonate decomposition, avoiding the production of calcium sulfate;

（2） During the ashing process, good ventilation is always maintained, and sulfur oxides are discharged as soon as they are generated. Therefore, it is required that the muffle furnace is equipped with a chimney, ventilation holes on the furnace door, or a small gap is opened on the furnace door to allow natural air circulation in the furnace;

（3） The coal sample should be laid flat in the ashtray to avoid local thickness, incomplete combustion, and the fixation of sulfur dioxide generated by sulfides in the bottom coal sample by calcium oxide produced by upper carbonate fractionation;

（4） Burn at a sufficiently high temperature (>800 ℃) for a sufficient amount of time to ensure the decomposition of carbonic acid and complete expulsion of carbon dioxide.