Reduced-order condensed-phase kinetic models for polyethylene thermal degradation

In a circular economy perspective, plastic wastes (PW) can be a source of chemicals, energy vectors and fuels. Pyrolysis, gasification, and partial oxidation technologies can be employed to produce chemicals, fuels, or energy. Modelling the thermochemical valorization requires first the definition of suitable condensed phase pyrolysis mechanisms for each constituent. This work proposes a reduced and a multi-step condensed-phase kinetic model for polyethylene (PE) thermal degradation for CFD applications. The former model employs 50 species and 480 reactions, while the latter involves 10 species and 10 reactions. The degradation rate and the selectivity to the different products is obtained from a validated semi-detailed model. The kinetic mechanisms are complemented by the thermochemistry of gas, liquid, and solid-phase species, accounting for phase-transition through pseudochemical reactions. Model validations are performed by comparison with experimental data in terms of mass loss, heat fluxes and product distribution profiles. Extending the proposed approach to other polymers and considering secondary gasphase reactions offers a powerful tool to model PW chemical recycling processes.