Forms of water binding

When water combines with coal, some of it remains attached to the surface of the coal; this water evaporates through low-temperature heating or natural air circulation, and this water is known as external water. Another portion of the water exists in the form of crystalline water along with the minerals in the coal; this water must be removed by heating to a certain temperature in order to be eliminated as internal water. That’s all I know; I’ll wait for more experienced experts to answer and learn from them*

This crystalline water should be distinct from the internal water

Regarding the forms of water present in coal and the definition of total moisture, the water in coal can be divided into two categories based on its binding state: free water and bound water (i.e., crystalline water). Free water refers to the water that is bound to coal through physical adsorption or sorption. Bound water refers to the water that is combined with minerals in coal through chemical bonds; it is part of the mineral lattice, such as the bound water in calcium sulfate (CaSO4•2H2O) and kaolin (Al2O3•2SiO2•2H2O). The free water in coal can be evaporated at normal pressure by short-term drying at temperatures of 105 °C to 110 °C ； The crystalline water usually has to be removed at 200 °C, and in some cases even above 500 °C. The total moisture and air-dry moisture that we measure are only free water, that is, external moisture and internal moisture. Internal moisture refers to the water adsorbed in the capillaries within coal particles. External moisture refers to the moisture adsorbed on the surface of coal particles or in non-capillary pores.