Kinetic and thermodynamic evaluation of plastic waste degradation for energy recovery

Abstract

Plastic consumption with effective recycling is essential for sustainable life-cycle management. In recent years, energy recovery and value-added product generation from plastic waste have gained increasing attention, with pyrolysis emerging as a promising thermochemical pathway for plastic degradation. This study investigates the thermokinetics of two waste plastic streams: mechanically recycled plastics, high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polyethylene terephthalate (PET), and landfill-dumped mixed waste plastic (MWP) and multilayer plastic (MLP), to evaluate operational parameters for pyrolysis and hydrothermal liquefaction (HTL) to optimize their conversion into valuable products. Thermogravimetric analysis (TGA), conducted at a heating rate of 10 °C/min, revealed that HDPE exhibited the highest weight loss (98%) at a peak temperature of 494 °C with a degradation time of 13 minutes. PET has a low conversion rate (80.53%) than HDPE, LDPE, and PP, resulting in a larger residue. The Coats-Redfern model-fitting method, applied with various models, was used to calculate the kinetic triplet and thermodynamic properties. Among the tested models, F1 (First-order) and F2 (Second-order) reaction models were identified as the most suitable for recycled plastics, while D2 (two-dimensional diffusion) was found to be appropriate for dumped plastics. The activation energies of mechanically recycled plastics were estimated to range from 154 to 295 kJ/mol, whereas landfill-dumped plastics ranged from 117 to 171 kJ/mol. Analysis of degradation temperature, duration, and weight loss (%) obtained from TGA provided a reference point for determining the optimal operating parameters (reaction time and temperature) for pyrolysis, thereby minimizing solid residue and enhancing the efficiency of plastic conversion into oil and gases.