Mechanistic insights into Cl transformation and calcium-based dechlorination during pyrolysis of waste plastics: Kinetics, product analysis and process optimization

Pyrolysis presents a promising pathway for the chemical recycling of waste plastics. However, the pyrolysis of chlorine-containing plastics can generate hazardous chlorinated compounds. Therefore, it is critical to investigate the migration, transformation, and removal of chlorine during pyrolysis to enable environmentally sustainable recycling. In this study, we combined kinetic analysis with in-situ FTIR and TG-FTIR-MS to elucidate the mechanisms of chlorine migration and transformation during the pyrolysis of chlorinated mixed plastics (CMPs). An efficient dechlorination process using calcium-based additives was also developed. The results indicate that the pyrolysis of CMPs occurs in two distinct stages (Stage Ⅰ and Stage Ⅱ), with average apparent activation energies (Ea) of 166.69 kJ/mol and 243.86 kJ/mol, respectively. The process followed mechanism functions involving diffusion, nucleation, and phase boundary reactions. Initially, weak C–Cl and C–H bonds ruptured preferentially, releasing HCl. This was followed by synergistic cleavage of the main C–C chains, generating radicals such as methyl (·CH₃), butyl (·C₄H₉), propylene (·C₃H₅), and phenyl (·C₆H₅). The combination of these radicals with free chlorine radicals (·Cl) resulted in the formation of diverse and complex chlorinated organic compounds. For dechlorination, Ca(OH)₂ was identified as the optimal calcium-based additive, achieving a chlorine removal efficiency of 97.75 %, attributed to its self-decomposition behavior. This study clarifies the transformation pathways of chlorinated pollutants during pyrolysis and proposes a practical calcium-based dechlorination strategy to support sustainable chlorine management in the valorization of plastic waste.