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Changes in physicochemical properties of brown coal under hydrothermal modification

Hydrothermal drying of low-grade coals was first suggested in Austria as early as in 1920, when it envisaged treatment of a lump coal with 180–240 °C steam in an autoclave. Hydrothermal treatment (HTT) of brown high-ash coals at 200 to 250 °C causes certain changes in their structure and physicochemical properties, similar to those under low-temperature pyrolysis. Moreover, these changes occur at values 50–100 °C below the pyrolysis temperatures, and the HTT product is close to semi-coke in terms of its calorific value. During hydrothermal treatment, the surface of the coal phase becomes more hydrophobic, while the specific surface of the mineral fraction decreases and the swellability is lost, which leads to lower adsorption activity. Taken in combination, these effects significantly modify the processing properties of coal. When only brown coal-water separation is required, the HTT may be performed without adding free water; when changes the processing properties of coals are necessary, it is advisable to carry out hydrothermal treatment in suspensions containing approximately 50 % of free water. This treatment not only causes the removal of moisture, but also leads to certain other changes in the coal structure. The study was supported by a grant from the Russian Science Foundation (project No. 18-17-00169).