Energy conversion performance in co-hydrothermal carbonization of sewage sludge and pinewood sawdust coupling with anaerobic digestion of the produced wastewater.

Energy conversion and utilization of sewage sludge (SS) and lignocellulosic biomass are an important measure to deal with environmental pollution and resource utilization. Addressing the waste by-product in a clean way is essential. In this study, solid char fuel (hydrochar) was obtained through co-hydrothermal carbonization of SS with pinewood sawdust (PS), and methane gas was obtained through anaerobic digestion (AD) of hydrothermal carbonization wastewater (HTCWW). The energy conversion performance of the feedstock organics under different HTC conditions (temperature of 160 °C, 220 °C, and 280 °C; reaction time of 0, 2, and 4 h; feedstock liquid-solid mass ratio of 4:1, 10:1, and 16:1), and the mass and energy yields of hydrochar and methane and their influencing factors were emphasized. More than 60% of the energy in SS and PS can be recovered by coupling the HTC-AD process. With the increase in hydrothermal reaction temperature and reaction time, the mass yield of hydrochar decreased, but the higher heating value increased. The maximum energy yield of hydrochar was 86.47% under the HTC temperature of 160 °C, liquid-solid ratio of 10:1, and reaction time of 2 h. The HTCWW obtained at a lower temperature (160 °C) showed the highest cumulative methane yield of 304.16 mL-CH4/g-COD.