Process evaluation of supercritical water liquefaction of mixed polyolefin wastes for integrated waste and energy systems

Highlights • Liquefaction process, potentially effective strategy, to combat plastic waste accumulation. • Process optimization led to aromatic hydrocarbons production as the primary constituents in product oil by supercritical water liquefaction. • Potential reaction pathways determined for HTL of mixed waste plastics. Landfill mixed waste plastics are difficult to recycle mechanically and often end up in open burning or uncontrolled disposal, especially in low- and middle-income countries. This study evaluates supercritical water liquefaction as a contamination-tolerant, water-based route to recover transport fuel range oils from real landfill-sourced mixed waste plastics and mechanically recycled HDPE, LDPE, and PP, including their blends. Batch experiments were carried out in a 400 mL stainless steel reactor at 425°C for 3 h and 450°C for 1 h, with a feed plastic-to-water mass ratio of about 1:2. Phase-separated products were quantified and characterized by thermogravimetric fractionation, Fourier transform infrared spectroscopy, and gas chromatography mass spectrometry. At 425°C and 3 h, oil yields reached 75 ± 4 wt% for HDPE, 70 ± 3.5 wt% for LDPE, 60 ± 3 wt% for PP and 22 ± 1.5 wt% for landfill mixed waste plastics, with mass closures of 92 to 95 percent in experiments with gas analysis. At 450°C and 1 h, resulted in enhanced gas formation and a pronounced shift toward aromatic hydrocarbons, with PP-rich systems exhibiting aromatic peak areas reaching about 90% and gasoline-range (C6 -C12) fractions of 56 to 78 wt% by thermogravimetric pseudo distillation. The results show structure-dependent reaction pathways in supercritical water: PP favors aromatic products, PE-rich feeds favor paraffinic and olefinic products, and blends with landfill-mixed waste plastics still deliver significant gasoline-range fractions. These findings indicate that supercritical water liquefaction can couple water-based chemical recycling of contaminated plastic streams with recovery of high-quality hydrocarbon fuels, with direct relevance for integrated energy and waste management systems at the energy environment nexus.