Insights into PVC-promoted hydrothermal carbonization of manure: Dechlorination, inorganic metals removal, and combustion behaviors

Hydrothermal carbonization (HTC) can serve as a promising route to effectively co-valorize biomass and plastic wastes for solid fuel production. However, manure-derived hydrochar for fuel application remains a significant challenge due to its high ash content and low energy density. Polyvinyl chloride (PVC), as a chlorine-containing plastic, subjected to HTC commonly results in the generation of massive acidic wastewater. Herein, we proposed a PVC-promoted HTC route for high energy–density hydrochar production. By utilizing PVC as an in situ acidic additive, the HTC condition was optimized at 275 °C, 30 min, with a PVC addition of 10 %. The hydrochar obtained under the optimized condition exhibited a higher heating value (HHV) of 25.89 MJ/kg, comparable to bituminous coals, and achieve a high energy recovery rate of nearly 70 %. Moreover, the excellent removal efficiencies of inorganic metals and chlorine both exceeding 95 % (except for Fe at 86 %) were observed, owing to the strong interactions between dechlorination and inorganic metals removal. Importantly, it was evidenced that acidic reaction environment was demonstrated to enhance the carbonization degree and modify the hydrochar formation mechanism. Furthermore, the high comprehensive combustion index S of 3.22 × 10−7 %2/( °C3·min2) and low average Ea of 134.20 kJ/mol were achieved, indicating the favorable combustion behaviors. Overall, this work systematically and comprehensively investigated the PVC-promoted HTC of manure waste for advanced solid fuel production and examined its combustion characteristics, unveiling the hydrochar formation mechanism.