Moisture re-adsorption characteristics of hydrochar generated from the Co-hydrothermal carbonization of PVC and alkali coal

Abstract

Our previous work reported to simultaneously remove the alkali and alkaline earth metals, and chlorine by co-hydrothermal carbonization (co-HTC) of high-alkali coal and polyvinyl chloride (PVC) (Fuel Process Technol, 199(2020)106277). This work is to investigate on the moisture re-adsorption characteristics of those hydrochar obtained at various co-HTC operating conditions. The results show that the water re-adsorption capacity of hydrochar was decreased with the circumstance temperature rising from 20 °C to 35 °C, while it was increased with the air humidity increasing from 50% to 80%. During the co-HTC, the addition of PVC could effectively inhibit the moisture re-adsorption behavior of hydrochar resulting from the adhere of the hydrophobic PVC to the surface of hydrochar as evidenced by SEM. The circumstance temperature had a significant influence on the effective diffusivity D over the humidity. The equilibrium moisture content (EMC) and effective diffusivity (D) could be linearly correlated with surface area and most probable pore size, respectively. The EMC of hydrochar was reduced about 0.6 mg/g as the BET surface area decreased 1 m2. The reduction of oxygen-containing groups could reduce the adsorption equilibrium time and adsorption heat. Besides, the higher circumstance humidity would cause hydrochar to release higher adsorption heat. As the humidity was increased from 70% to 80%, the heat released by 1 g hydrochar was increased about 13.82–19.63 kJ/mol. Due to the lower EMC and adsorption heat, 300 °C was supposed to be a suitable temperature for the co-HTC to obtain hydrophobic hydrochar.