A semi-detailed pyrolytic gas-phase kinetic model for the volatiles of polyethylene thermal degradation

In a circular economy approach, plastic waste is a source of valuable chemicals and energy vectors. Thermochemical technologies such as pyrolysis, gasification, and combustion enable the valorization of even complex and contaminated waste streams. While condensed-phase degradation governs overall reactivity, accurately modeling the gas-phase reactivity of pyrolysis products is essential for scaling up valorization processes in industrial reactors. Isolating the pyrolysis behavior of volatiles first allows the decoupling of complexities associated with the low-temperature oxygen reactivity. This work presents a semi-detailed kinetic model to address the pyrolytic gas-phase reactivity of volatiles formed during the thermal degradation of polyethylene (PE). The model builds on a validated multi-step condensed-phase kinetic model and employs established lumping approaches. Short-chain compounds are modeled with high detail, while long-chain ones are described by surrogate species representative of diesel-cuts (NC16H32) and waxes (NC30H60). The reactivity of short chains is described through the comprehensive CRECK kinetic model by incorporating recent experimental data to refine reaction pathways of C5-C7 olefins. Due to the lack of experimental data for longer olefins, their reactivity is modeled by analogy to the shorter ones, ensuring an asymptotic behavior with increasing carbon numbers. The semi-detailed model is validated against experimental data on PE pyrolysis, assuming instantaneous mixing of the inert inlet flow with released volatiles, followed by a segregated plug-flow behavior. Validation across different reactor setups confirms the model’s capability to predict detailed product distributions. Despite minor discrepancies, the proposed model effectively captures experimental trends. Future work will address modeling the reactivity in oxygen-containing environments.