Co-hydrothermal carbonization of polyvinyl chloride and rice straw for solid fuel production: Investigation into effect of solid to water ratio using Box-Behnken design and response surface methodology

Co-hydrothermal carbonization (co-HTC) of polyvinyl chloride (PVC) and biomass is a promising method for the responsible disposal of both waste streams. Temperature and holding time were considered as the dominant factors in the production of chlorine-free and energy-dense hydrochar. However, the effect of solid-to-water ratio (S:W), which determines wastewater generation remains unclear. This study investigated the effect of S:W ratio and its synergistic effect with temperature and holding time on dechlorination efficiency (DE), higher heating values (HHV), and energy yield (EY) of hydrochar from blended PVC and rice straw (RS) using design experiment methodology. Two optimal conditions for the highest DE and HHV were identified as 250 °C-60 min − 1:5 and 250 °C-90 min-1:10, respectively. The likely optimal condition for EY was 220 °C-60 min-1:5. Statistically significant synergistic interaction was found between temperature and holding time in terms of the DE, although the effect of holding time became less pronounced with increasing temperature. RS was found to wrap the PVC intermediates during co-HTC, resulting in co-hydrochars with identical surface functional properties to RS-derived hydrochar. Notably, this study found that the S:W ratio is proportional to the enhancement of DE and HHV, as well as the removal of HCl-soluble alkali and alkali earth metals (AAEMs). Lowering the water content in co-HTC engenders similar behavior to pyrolysis, where the released HCl played an autocatalytic effect in the dechlorination of PVC, thus enhancing the DE in the elimination pathway.