Abstract: Dewatering and upgrading are the premise of large scale utilization of lignite.Hydrothermal de watering (HTD) is a non evaporative drying technology,which has advantages of energy conservation and consumption reduction.In order to provide a theoretical guidance for the HTD of lignite,it is of importance to investigate the removal behavior and distribution of different types of water in lignite and the water re absorption behavior of upgraded lignite.Two lignite samples from Inner Mongolia and Yunnan provinces in China were upgraded by the HTD method.Water contents and types in the raw lignite and the HTD coal samples were determined by isothermal drying method.The oxygen functional groups and pore structures of coal samples were characterized using the chemical titration and N2 adsorption methods to investigate the effect of HTD on water distribution in lignite.Moreover,the water adsorption isotherms of the HTD coal samples were obtained at 30 ℃ and analyzed by the modified BET model.The results showed that the HTD process removed mainly molecular water,and partly capillary water and free water in lignite by decomposing oxygen functional groups.The removal of oxygen functional groups and change of pore structures in the HTD coal samples affected the primary adsorption capacity for water (combined with oxygen functional groups by hydrogen bonds) and secondary adsorption capacity for water (adsorbed in multilayer and capillary condensation) respectively.In the HTD process,the water in lower rank Yunnan lignite was removed more,and the reductions of primary and secondary adsorption capacities for water were more remarkable compared with Inner Mongolia lignite.It implied that the HTD process was more suitable for lower rank lignite with high water content.