Effect of waste liquid produced from the hydrothermal treatment of both low-rank coal and sludge on the slurryability of coal sludge slurry

•More than 85 wt% carbon was retained in the HT solid products.•TOC and pH value increased rapidly with an increase in the amount of sludge.•Alkanes, phenols and heterocyclic were the main organics in the HT waste liquid.•Heterocyclic increased and amines slightly decreased with the sludge amount increased.•The better slurryability of CSS were prepared using the HT waste liquid.

Effects of temperature and the amount of sludge on the physicochemical properties of the waste liquid products produced from the hydrothermal treatment (HT) of both low-rank coal and sludge were investigated. The slurryability of coal sludge slurry (CSS) prepared with HT solid and waste liquid product was also analyzed. An increase in the amount of sludge led to a decrease in the solid yield, calorific value and fixed carbon of solid products decreased, particularly the solid yield which reached 63.8 wt% in the HT of sludge at 300 °C. However, more than 85 wt% carbon was retained in the solid products, which is beneficial for the energy densification. The Total Organic Carbon (TOC) content increased significantly with an increase in the amount of sludge and attained a concentration of 60 g/L. The same trend was observed in the pH value of waste liquid, owing to the presence of ammonium nitrogen organics. Alkanes, phenols, and heterocyclic were the main organic compounds produced from the HT of Inner Mongolia Coal (IMC), Yunnan Coal (YNC) and sludge at 300 °C, respectively. CSS prepared using the HT waste liquid instead of water has a higher fixed-viscosity concentration and better stability due to the presence of dissolved organic compounds and ions.