Generalized derivation of multi-step kinetic models for polymer condensed-phase pyrolysis: Application to poly(ethylene terephthalate)

From a circular economy perspective, plastic waste (PW) is a valuable source of chemicals, energy vectors and energy. This work presents a general methodology to derive multi-step kinetic models for the condensed-phase thermal decomposition of polymeric networks starting from fundamental physics-based models. This methodology, initially developed for vinyl polymers, is here extended and generalized to account for transient phenomena, interplay of molecular and radical pathways, and reaction systems where depolymerization does not play a major role. The approach is applied to PET as it is among the main components of PW, affects product quality, alters the release of chlorinated and nitrogenated compounds, and a validated physics-based semi-detailed mechanism (PET-85-700) is available. The lumping methodology allows to derive a multi-step model (PET-18-22) that employs 18 species and 22 condensed-phase reactions, achieving an 80 % reduction in species and 97 % decrease in reactions. This model is validated by comparison with PET-85-700 and literature experimental data on mass-loss profiles, volatile yields, and char characterization, demonstrating satisfactory agreement with the semi-detailed model predictions at a lower computational cost. The resulting CHEMKIN-like condensed-phase model is attached as Supplementary Materials and available online in the CRECK Modelling Lab GitHub repository. Extending the proposed approach to other polymers and coupling it with existing subsets in the CRECK kinetic framework (e.g., biomass, polyvinylchloride, polyethylene, polypropylene, polystyrene) offers a powerful tool to model thermochemical recycling of PW and biomass/PW mixtures.