Co-hydrothermal carbonization of polyvinyl chloride and moist biomass to remove chlorine and inorganics for clean fuel production

This work proposes an innovative integrated process to produce clean solid biofuel from chlorinated wastes and polyvinyl chloride (PVC). The PVC and pinewood sawdust were used as parent materials. We studied the effect of parameters such as the hydrothermal temperature, the residence time and the particle size (PS) of the sawdust on the dechlorination efficiency (DE), the inorganics removal efficiency (RE), and the HHV of hydrochar. The co-hydrothermal carbonization (Co-HTC) process was performed by mixing the PVC and pinewood sawdust at a mass ratio of 1:9. For the DE, the most important factor was the hydrothermal temperature, followed by residence time and particle size of pine sawdust. The DE could reach about 84% by the co-HTC at a temperature of 260 °C for 120 min. The particle size of pine sawdust has noticeable effect on DE because of heat and mass transfer. The DE was decreased from 79.17% to 71.12% when the PS was increased from 0.22–0.49 to 0.6–0.9 mm. The RE of inorganics from pine sawdust was significantly promoted because the addition of PVC enhanced the acidity of the reaction system, regardless of the co-HTC operating parameters investigated in this work. The temperature increase is conducive for the removal of K and Na. The maximal RE of Al, Ca, and Mg increased significantly from 49.39%, 49.19% and 41.86 to 97.61% (Da-260-30), 98.59% (Da-260-90) and 97.66% (Dc-260-30), respectively. The maximal RE of Fe, K and Na increased from 49.79%, 50.80% and 47.44% to 92.82% (Da-220-30), 92.32% (Dc-260-30) and 87.43% (Dc-260-30), respectively. The oxygen-containing functional groups decreased with the increase of HTC temperature (220–260 °C), residence time (30–90 min) and particle size (0.22–0.49 to 0.6–0.9 mm), resulting in the weakening absorption ability of hydrochar for inorganics. The addition of PVC and the temperature increase are not conducive to the formation of porous hydrochar. Nevertheless, the residence time extension and particle size growth could increase the porosity of hydrochar. The hydrochar with low chlorine, low inorganics content and improved higher heating value (HHV) of 24–30 MJ/kg was similar to bituminous coal, which could be utilized as clean solid biofuels. A high-energy yield of 74–81% was achieved by this co-HTC process. These results show that the co-HTC of PVC with biomass was feasible for clean biofuel production, because the chlorine and inorganics could be removed effectively by the positive synergistic effect. This work provides a new viewpoint for the development of WtE and biomass upgrading technologies.