Transport fuel from waste plastics pyrolysis – A review on technologies, challenges and opportunities

Abstract

Despite extensive research in recent years and great potential to help with energy and waste management issues, thermochemical conversion technologies of mixed waste plastics (MWP) into transport engine fuel have not yet been developed in industrial and commercial settings. This study reviewed, discussed, and summarised research findings of over 280 up to date publications, emphasising pyrolysis technologies and their product yields, qualities, challenges and opportunities. The study investigated the suitability of pyrolysis oil for automobile engine application and summarised the experimental findings of diesel engine performance and emission characteristics. Furthermore, techno-economic assessment and key challenges for commercial fuel production from plastic pyrolysis are discussed. The review found that the pyrolysis of MWP produces oil yields of up to 90% by weight, and excellent fuel properties in the pyrolysis temperature range of 500 °C−550 °C. Polystyrene individually yields the highest oil yield of 97%, followed by low-density polyethylene, high-density polyethylene and polypropylene. However, polyethylene terephthalate and polyvinyl chloride plastics are not suitable for pyrolysis because they produce harmful chlorine gas and HCl that corrode the metallic parts of the pyrolysis reactor and condenser. Engine testing with MWP oil showed delayed combustion, high heat release rate and high cylinder pressure with similar power output and brake thermal efficiency. In contrast, NOx, UHC, CO and CO2 emissions were much higher. The study found that the pyrolysis plants must process at least 50 kT/year to earn an ROI of 25%. Additional research is required to resolve the remaining issues and challenges for industrial and commercial applications.