Enhanced slurryability and rheological behaviors of two low-rank coals by thermal and hydrothermal pretreatments

The aim of this work is to study the slurryability of two low-rank coals (a Chinese YX lignite and a Chinese ZD sub-bituminous coal) and various upgraded coals or chars obtained by thermal and hydrothermal treatments. Thermal treatment was conducted in an air-isolated atmosphere at temperatures from 145 °C to 700 °C. Hydrothermal treatment of coal was carried out in an autoclave at temperatures from 150 °C to 300 °C with autogenous pressure from 0.8 MPa to 9.2 MPa. The two raw coals exhibited the poor slurryability, and a significant enhancement in the slurryability was achieved by both the thermal and hydrothermal treatments depending on the conditions. A coal or char slurry (CWS) with the solid loading of 59 wt.% was prepared from the YX lignite by the thermal treatment at 400 °C, which had the apparent viscosity of 1208 mPa s and the good stability. The apparent viscosity of CWS was found to be related to but not limited to the elimination of hydrophilic groups by pretreatment. The hydrothermal treatment was more effective to decompose the hydrophilic groups than the thermal treatment for YX lignite, while this was not the case for ZD coal. The enrichment of carboxyl-bound calcium in ZD coal enabled the hydrophilic groups to be more thermally refractory, and this calcium-combining structure appeared to have a negative effect on the slurryability and the dispersion stability. The CWSs prepared from the thermally treated YX coals or chars were less pseudoplastic and less easily subjected to a pseudoplastic change with increasing solid loading than those from the parent coal.