Thermodegradation characterization of microplastics: Dispersion effect and pyrolysis kinetics by artificial intelligence

The research of pyrolytic thermodegradation of plastic wastes has received much attention. However, detailed combustion behavior and pyrolysis kinetics of microplastics are still absent in the current research. Understanding the characteristics and kinetics of microplastic pyrolysis is vital for designing and optimizing operational conditions. Adding silica to high-density polyethylene (HDPE) microplastics can effectively avoid the agglomeration of microplastics during pyrolysis, thereby better elucidating their thermodegradation. In the experiment, HDPE microplastics are mixed with SiO₂ at different weight ratios (10 %, 20 %, 50 %, 70 %, and 100 %) to observe the dispersion effect on the microplastic thermodegradation. The results suggest that less than 20 % of PE in the mixture can retain microplastic pyrolysis characteristics. In combustion analysis, 10 % PE undergoes a faster thermo-oxidation reaction than pure HDPE. This study applies the independent parallel reaction (IPR) model and particle swarm optimization (PSO) algorithm to investigate microplastic pyrolysis kinetics. The pyrolysis activation energy is between 273.89 and 292.90 kJ‧mol⁻¹, with fit quality high than 95 %, and rises with decreasing dispersion degree and increasing heating rate. This indicates that microplastic thermodegradation has low activation energy and is affected by heat transfer rate. The Py-GC/MS analysis indicates that up to 83.6 % of alkene and 52.3 % of products comprise high carbon numbers (17-Pentatriacontene and 1-Octatriacontene), respectively, suggesting the suppression of the secondary reaction of microplastics in the pyrolysis reaction. The calculated kinetic parameters are beneficial for understanding microplastic thermodegradation characteristics and can provide useful information for reactor design in industrial pyrolyzers or gasifiers.