Effect of hydrothermal dewatering on the physico-chemical structure and surface properties of Shengli lignite

• Variations and relationships of changes among coal properties during HTD process were analyzed.
• Surface energy of coal sample was test and related its hydrophilicity.
• Coal properties were mainly changed by the decomposition of oxygen functional groups.

Shengli lignite from China was treated by hydrothermal dewatering (HTD) under 200–300 °C. The changes of physico-chemical structure and surface properties of Shengli lignite were tested and the relationship between these changes was analyzed. The equilibrium moisture content of Shengli lignite decreased from 29.75% in raw coal to 7.40% in HTD-300 (the sample treated by HTD under 300 °C) due to the decomposition of oxygen functional groups and the alternation of hydrophilic property of the coal sample. The coal upgrading by the HTD process was shown through the decrease in atomic ratios of oxygen to carbon (O/C) and hydrogen to carbon (H/C) due to the decomposition of functional groups and side-chains, which should be the important reason for the change of pore size distribution. The analysis of surface energy showed that the decrease of hydrophilicity was mainly caused by the reduction of acidic components, which provides another evidence for that the decomposition of carboxyl and hydroxyl resulted in the decrease of equilibrium moisture content.

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