Although hydrothermal treatment dewatering (HTD) has become an important direction of efficient and clean utilization of lignite, there are still few studies on the downstream combustion characteristics based on HTD process. In particular, the influence mechanism of HTD on the combustion properties of upgraded lignite is not clear. At the same time, conventional flue gas treatment (FTD) is a relatively mature lignite pretreatment technology, and is selected for comparative analysis. Here, lignite was treated by HTD and FTD, and the physicochemical structure and combustion reactivity of upgraded lignite were studied. Results showed that HTD and FTD processes can both promote dehydration and deoxidation of lignite, thereby improving the coal quality. HTD has a better effect in terms of dehydration, and is related to the removal rate of the hydrophilic O-containing functional group. In terms of aromaticity and coal-rank, upgraded coals treated by HTD are less than that of FTD. This is likely due to that hot water, to a certain extent, has a hydrogen supply function, and can stabilize free radicals produced by pyrolysis groups and avoid condensation and crosslinking of pyrolysis free radicals. Another interesting result is that the comprehensive combustion characteristics indexes of upgraded lignite are increased by HTD and decreased by FTD, indicating that HTD process promotes the combustion reactivity of upgraded lignite. These findings are consistent with the results of kinetic analysis. Therefore, In general, compared with FTD, HTD seems to be a more effective way for lignite pretreatment. While considering relatively effective dehydration and deoxidation, the combustion reactivity of upgraded coals is improved. Besides, the factors that affect combustion characteristics are complex and multi-faceted. The change mode of microstructure and, to a lesser extent, in effective coal rank and the concentration of oxygen functional groups have a more important impact on combustion reactivity for upgraded lignite
