Co-hydrothermal carbonization of sewage sludge and rice straw to improve hydrochar quality: Effects of mixing ratio and hydrothermal temperature

This study assessed the effects of mass mixing ratio and hydrothermal temperature on the co-hydrothermal carbonization (co-HTC) of sewage sludge and rice straw regarding the yield, chemical composition, fuel properties, surface functional groups, and combustion behavior of the hydrochar. The co-HTC increased the hydrochar yield at 180 °C but decreased it at 220 and 260 °C. The co-HTC increased the hydrochar organic matter contents, higher heating values, fuel ratios, and combustion behavior (combustibility index) by 16 % to 63 %, 15 % to 85 %, 51 % to 321 %, and 30 % to 419 %, respectively. The co-HTC also enhanced the formation of oxygen-containing functional groups on the hydrochar surface. The synergistic and anti-synergistic effects of the co-HTC were more positive at 180 and 220 °C but more negative at 260 °C. The co-HTC of municipal sludge and rice straw is promising in converting waste materials into high-quality hydrochar as a clean solid fuel.