Conversion of polyethylene waste into clean fuels and waxes via hydrothermal processing (HTP)

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The exponential growth of plastic production and a low recycling rate (<9%) over the last 60 years have resulted in vast plastic waste accumulation in landfills and the environment. About 40% of the 350 million tons of global plastic waste generated in 2015 was polyethylene (PE). Converting PE waste into useful products can help reduce PE waste accumulation. Hydrothermal processing (HTP) of PE into oils was studied previously; however, comprehensive reaction pathways were not reported, and the products were not evaluated for potential fuel and wax applications. In this study, various PE wastes were converted via HTP at 425–450 °C for reaction times of 0.5–4 h into clean waxes or ultra-low sulfur fuels. Two-dimensional gas chromatography (GC×GC) and FTIR were used for determining the chemical compositions of the products. A mixture of paraffin and α-olefin waxes was obtained with a high yield, 97 wt. %, at 425 °C and 30 min. The highest oil yield, 87 wt. %, for producing fuels was obtained at 425 °C and 2.5 h. The diesel fraction (HTP diesel) met all requirements for No. 1-D ultra-low sulfur diesel and had a high cetane number of 61 and a low aromatic content of 18 vol. %. Mixtures up to 50 vol. % of HTP diesel in commercial diesel also met all requirements for No. 2-D ultra-low sulfur diesel. Comprehensive reaction pathways for PE conversion were inferred from detailed GC×GC analyses of reaction intermediates. The HTP method is more energy-efficient and has lower greenhouse gas emissions than incineration and pyrolysis, and is a promising method for reducing PE waste accumulation.