Substantial upgrading of a high-ash lignite by hydrothermal treatment followed by Ca(OH) 2 digestion/acid leaching

•A new method is proposed to substantially upgrade high-ash lignite.•The method is effective for deoxygenation and demineralization of lignite.•The higher heat value of lignite increases by a factor of 1.7 by an optimized treatment.•This method can appreciably remove many trace elements from lignite.