Sustainable chemical recycling of waste plastics into olefins through low-pressure hydrothermal liquefaction and microwave pyrolysis: Techno-economic analysis and life cycle assessment

Plastics pose environmental challenges in landfills, persisting for extended periods spanning thousands to millions of years. Consequently, research into plastic depolymerization has gained significance, aiming to address the problem of plastic waste management and the increasing demand for plastics. This study proposes a novel process for recovering olefins, specifically ethylene and propylene, from waste polyethylene (PE) and polypropylene (PP) through low-pressure hydrothermal liquefaction (LP-HTL) and microwave steam pyrolysis (MSP) chemical recycling. Mixed waste PE and PP undergo LP-HTL to produce gas and oil. Subsequently, the oil from the LP-HTL undergoes cracking via MSP to enhance olefin recovery. Olefin compounds produced through distillation serve as refrigerants. The results demonstrated the production of 39.75 wt% C2H4 and 13.32 wt% C3H6, achieving a total recovery of 53.07 wt% of olefin materials. The levelized cost of ethylene (LCOE) in the proposed process was calculated at 0.89 USD/kg C2H4, equating to a 72.86 % reduction compared with that in flash pyrolysis. Furthermore, the life cycle assessment (LCA) results indicated reduced 100-year global temperature potential and global warming potential (GTP100 and GWP100) emissions of 2.46 and 2.55 kg CO2 eq/ kg C2H4, respectively, approximately 90 % lower than that in the flash process. Thus, the proposed process, with its energy efficiency and high recovery rates, can serve as a benchmark for future plastic depolymerization endeavors aimed at achieving a circular carbon economy.