1. Hydrothermal dewatering (HTD) could remove carboxyl and hydroxyl groups in lignite, which begin to decompose after 200 °C and 230 °C respectively. 2. During HTD process, pore volume and surface area increase significantly at first and then decrease. The increase mainly derives from the development of meso-pore and is attributed to the evaporation of water and release of volatiles initially, and the decrease is ascribed to the destruction of lignite structure at high temperature. 3. The removal of free water, capillary water and molecular water is associated with the collapse of macro-pore, the damage of meso-pore and the removal of oxygen functional groups respectively.
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