Thermodynamic and kinetic analysis of waste plastic pyrolysis: Synergistic effects and sustainability perspectives

In this study, low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), waste mixed plastics (WMPs) and WMPs with spent fluid catalytic cracking (sFCC) catalyst (WMPs/ sFCC) were investigated to simulate real-life pyrolysis and catalytic pyrolysis of waste plastics using Thermogravimetric analysis (TGA). TGA was performed under different heating rates i.e., 5, 10, 15 and 20 ˚C/min) in an inert nitrogen atmosphere. The pyrolysis kinetics are assessed using three model-free methods, Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS), and Starink, as well as two model-fitting methods, Coats–Redfern (CR) and Criado methods (master plots). The results showed that the WMPs exhibited a positive synergetic effect among the different types of plastics, leading to a notable reduction in degradation temperature and required activation energy. Moreover, adding sFCC catalysts significantly lowered the initial pyrolysis temperature (approximately 47 ˚C) of WMPs compared to direct pyrolysis. Moreover, the average activation energy of WMPs decreased by approximately 13.41 kJ/mole with the inclusion of the sFCC catalyst. The thermodynamic properties such ΔH^‡, ΔG^‡ and ΔS^‡ suggested that the process was endothermic, non-spontaneous and decreased in randomness during pyrolysis. This study promotes sustainability through a circular economy to convert waste into wealth. These findings offer valuable theoretical insights for reducing energy consumption in plastic pyrolysis and expanding the applications of sFCC catalyst.