Extraction of Low Rank Coal with Sub- and Supercritical Water

Abstract

Alaska coals were extracted with sub- and supercritical water (SCW) using a micro reactor. Conversion of coal was studied in the temperature range of 320–400°C, pressure 15–30 MPa and water density 0.06–0.7 g/cm³ for 0–2 h. The experiments were conducted to investigate the effects of reaction temperature, pressure and water density on gas and liquid products respectively. The results show that the coal conversion in supercritical condition was higher than in sub-critical condition. The hexane soluble liquid product of original coal reaches a maximum 18.8% in the reaction time of 90 min at 400°C and 30 MPa. The contents of CO, CH₄, and H₂ form especially well at supercritical condition. It is seen that supercritical condition was favourable to the hydrogen formation. With the increase of temperature, the sulfur in coal releases into the liquid and gas effluents. And the sulfur removal in liquid effluents is much greater than that in gas effluents. Higher water density causes higher conversion due to promotion of hydrolysis in coal extraction.