Hydrothermal dewatering of lignite to improve the slurry-ability, rheology, and stability of coal-water slurry

Xiaolongtan lignite was upgraded by hydrothermal dewatering (HTD). The main factors affecting slurry-ability of the lignite including coal property, oxygen functional groups, surface hydrophilicity, and particle size distribution were analyzed. The effect of HTD on solid concentration, rheology, and stability of the upgraded coal was also investigated. The results show that a substantial amount of moisture was removed, oxygen content decreased, and the coal rank was enhanced by HTD upgrading. The contact angle between coal and water increased after removal of oxygen functional groups by HTD, thereby improving surface property of the upgraded lignite. A typical bimodal distribution of the lignite particle size was observed. The mean particle diameter of lignite decreased and the lignite particles became more regular after HTD. HTD upgrading significantly improved slurry-ability of the lignite. The solid concentration of raw coal was 44.09%, while that of the upgraded coal after HTD increased to 61.94%. The consistency coefficient K decreased, while the rheological index n increased for the CWS prepared from the upgraded coals. HTD upgrading decreased the apparent viscosity, and maintained the shear-thinning behavior of pseudo-plastic fluid. Moreover, as the water separation ratio decreased and formation of hard sedimentation was delayed, stability of the CWS was enhanced after HTD upgrading. Overall, the physico-chemical properties of Xiaolongtan lignite were significantly modified after HTD, thus a high-quality slurry fuel with high solid concentration, superior pseudo-plastic behavior, and good stability could be achieved.