The most commonly used standard methods for determining moisture content in air dried coal samples are heating drying and azeotropic distillation.

（1） The heating drying method can be divided into drying weight loss method and direct weight method. The dry weight loss method involves heating and drying coal samples at 105-110 ℃ to determine moisture content based on the mass loss of the samples. The method is simple and suitable for older coal, but due to the prolonged heating time in the air (1-2h), the coal samples are prone to oxidation and weight gain, resulting in lower measurement results, especially for younger coal. To overcome this disadvantage, two measures are generally adopted. One method is to heat in a vacuum or inert atmosphere (such as nitrogen) to avoid contact between coal and oxygen; Another approach is to appropriately increase the heating temperature while minimizing the heating time to weaken the degree of oxidation, as specified in the national standard GB212-77 "Industrial Analysis Methods for Coal", which involves baking at 145 ℃± 5 ℃ for 1 hour to determine the moisture content of brown coal.

The direct weight method is used to measure the moisture in coal samples heated and driven out in dry nitrogen gas at temperatures ranging from 105 to 110 ℃. The moisture is absorbed by an absorption tube containing an absorbent, and the increase in weight of the absorption tube is used to determine the moisture. The specific reason is that there is no oxidation problem during heating in the nitrogen flow, so the results are also relatively accurate. However, the instrument equipment and measurement steps are more complicated than the dry weight loss method.

（2） The azeotropic distillation method suspends coal samples in an organic solvent (such as toluene or xylene) that is insoluble in water. It is heated and distilled, and the water in the coal is carried out by the organic solvent vapor and condensed and collected in a graduated receiving tube. The water content is determined based on the volume of collected water. It is a very accurate method, but the sample amount is large, and the solvents used are toluene or xylene, which are flammable and toxic substances. Moreover, the measurement results of the two commonly used solvents for young lignite - toluene and xylene - are inconsistent, and xylene has a high boiling point (140 ℃). At this temperature, some organic compounds in young lignite will decompose, and the product moisture will cause the measurement results to be higher.