Kinetics, products and mechanism for polyethylene and plastic bag pyrolysis under nitrogen and carbon dioxide atmospheres

The thermal conversion technology has revealed new potential for the resource utilization of waste plastics. This study systematically investigates the pyrolysis mechanisms of polyethylene (PE) and real-world waste plastic bags (PBs) by integrating multiple analytical approaches, including thermogravimetric analysis, thermodynamic evaluation, and comprehensive characterization of gaseous, liquid, and solid products. The effects of key parameters including temperature and reaction atmosphere are thoroughly examined. With respect to the reaction atmosphere, kinetic analyses based on conversion-dependent methods (e.g., Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS)) consistently demonstrate that CO 2 significantly influences the pyrolysis process by elevating the apparent activation energy. When the pyrolysis atmosphere is changed from N 2 to CO 2 , the activation energy of PE increases from 133.54 to 141.54 kJ/mol to 162.3–171.5 kJ/mol. However, the activation energy of PBs remains between 140 and 150 kJ/mol. CO 2 can also reduce the carbon deposition rate in PBs residue from 9.64% to 1.77%. Furthermore, CO 2 selectively modulates the decomposition of PE by enhancing the formation of short-chain hydrocarbons and increasing the yield of gaseous products. Notably, a similar regulatory effect is also observed in the pyrolysis of waste PBs, a complex multicomponent system. These findings provide valuable insights for optimizing and controlling the pyrolysis of real-world mixed plastic wastes.