External and internal water in coal quality

The moisture in coal can be divided into three types, based on its state of existence within the coal: external moisture, internal moisture, and bound water. Free moisture (surface moisture) refers to the water that adheres to the surface of coal particles and in capillaries with a diameter greater than 10-5 cm during coal mining, transportation, storage, and washing processes (denoted by the symbol Mf). The external moisture is combined with the coal mechanically; its vapor pressure is equal to that of pure water, making it easier to evaporate. When coal is placed in air at room temperature, the external moisture continuously evaporates until equilibrium is reached with the relative humidity of the air. The water lost at this point is external water. Coal that contains external moisture is called received coal, while coal that has only lost its external moisture is called air-dried coal. Coal quality testing is usually carried out using air-dried coal samples. Inherent moisture (moisture in air-dried coal; inherent moisture) refers to the moisture retained in coal when it reaches an air-dried state under certain conditions (abbreviated as Minh). The internal moisture is bonded to coal through physicochemical means; its vapor pressure is lower than that of pure water, making it difficult to evaporate, and it can only evaporate when heated to 105–110°C. Therefore, the water lost when the air-dried coal sample is heated to 105–110°C is the inherent moisture. Coal that has lost its internal moisture is called dry coal. The sum of the external moisture and internal moisture of coal is referred to as the total moisture of coal (total moisture, abbreviated as Mt). Water of constitution: The water of constitution in coal refers to the water that is chemically bound to minerals and remains after the determination of total moisture content; it is what is commonly known as crystalline water and bound water. The content of bound water is low, and it can only be removed at higher temperatures; therefore, in the industrial analysis of coal, bound water is generally not taken into account.

External moisture (Wwz) refers to the water that adheres to the surface of coal as well as in its large capillaries (with a diameter > 10-5 cm) during mining, transportation, and washing processes. It is mechanically connected to the coal and evaporates relatively easily; its vapor pressure is equal to that of pure water. When placed in air, it continues to evaporate continuously until the vapor pressure of the moisture in the coal reaches equilibrium with the relative humidity of the air. The moisture lost at this point is known as surface moisture. Coal containing surface moisture is called applied coal, while coal from which the surface moisture has been removed is called air-dried coal. The amount of surface moisture depends on factors such as the particle size of the coal, but it has no direct relation to the quality of the coal. (Internal moisture, Wnz) refers to the water that is adsorbed or trapped in the capillaries within the coal particles (with diameters less than 10-5 centimeters). Internal moisture is the water lost when air-dried coal is heated to 105–110°C; it is bound to the coal through physicochemical processes such as adsorption, and therefore evaporates more slowly, resulting in a vapor pressure lower than that of pure water. Coal from which the internal moisture has been removed is called absolutely dry or dry coal.