Thermal conversion of plastic waste into fuels and lubricant additives for hydrogen internal combustion engines: A systematic review

The escalating accumulation of plastic and microplastic wastes underscores the urgent need for innovative approaches to convert these pollutants into valuable products. Thermal conversion processes, including pyrolysis, plasma-catalytic pyrolysis and graphitization, have emerged as effective pathways to transform plastic waste into fuels and lubricant additives. This review provides a comprehensive discussion on the production of fuels and hydrogen via thermal conversion processes, emphasizing their applications in hydrogen-powered internal combustion engines. Among the thermal conversion techniques, steam gasification of blended plastic waste with a CaO catalyst demonstrated a high hydrogen fuel yield of 104 mmol/g_plastic. Meanwhile, pressurized batch pyrolysis and batch pyrolysis exhibited superior liquid fuel and polymer-originated oil production efficiencies, achieving yields of 97 wt% and 96.7 wt%, respectively. This review also underscores the significant potential of thermal conversion processes for producing lubricant additives from plastic waste. The conversion pathways are categorized into direct and indirect thermal conversion methods, each offering distinct advantages for lubricant performance enhancement. In direct thermal conversion, processes such as pyrolysis, flash joule heating and graphitization transform plastic waste into carbon-based materials, including carbon nanomaterials, graphite and graphene. These carbon derivatives are highly valued for their superior tribological properties, which significantly enhance lubricant performance by reducing friction, minimizing wear and improving overall mechanical efficiency. Conversely, indirect thermal conversion methods produce various functionalized hydrocarbons, heterocyclic nitrogen-enriched carbon additives and other high-value compounds. These derivatives improve lubricant characteristics by enhancing viscosity stability, oxidation resistance and thermal performance.