Utilizing CO2 to suppress the generation of harmful chemicals from thermal degradation of polyvinyl chloride

Abstract

This study put a great emphasis on the role of carbon dioxide (CO2) in thermal decomposition of polyvinyl chloride (PVC). To do this, systematic experimental works were carried out to explore the influence of CO2 on pyrolysis of PVC by comparing with that in N2. First of all, a series of thermo-gravimetric analysis (TGA) of PVC in N2 and CO2 was performed to characterize the thermal deconstruction of PVC in N2 and CO2, which confirmed that any physical aspect (i.e., onset and end temperature of depolymerization of PVC) attributed by CO2 was nearly negligible as compared to the case in N2. This phenomenon was also fully evidenced by differential thermogram (DTG) and differential scanning calorimeter (DSC) because both DTG and DSC curves in N2 and CO2 were identical. However, the concentration profiles of CO evolved from the thermal degradation of PVC in N2 and CO2 was significantly different: the enhanced generation of CO occurred in the presence of CO2. This observation suggested that the genuine role of CO2 is to consume carbon source (i.e., acting as a carbon scavenger) for pyrolysis of PVC. It also provided a favorable condition for suppressing the formation of harmful chemical compounds such as benzene derivatives and polycyclic aromatic hydrocarbons (PAHs). Therefore, the mass portion of the oil in pyrolytic products produced from pyrolysis of PVC in CO2 was lower than that in N2. These particular thermal degradation behaviors may be explained by the genuine role of CO2: the enhanced thermal cracking of volatile organic carbons (VOCs) evolved from the pyrolysis of PVC. The findings of this study strongly suggest that CO2 can be effectively applied to thermal disposal of recalcitrant wastes since employing CO2 in thermal treatment provides a means for in-situ mitigation and/or reduction of harmful chemical species.