Towards circular plastics chemical recycling of high-density polyethylene-rich municipal waste using hydrothermal liquefaction

Plastic waste presents significant environmental challenges, driving research into chemical recycling. High-density polyethylene (HDPE) is particularly difficult to recycle among common plastics due to its stability. Hydrothermal liquefaction (HTL) technology has emerged as a promising method to address these issues. This study applies HTL to convert post-consumer plastic waste, predominantly HDPE (93 wt%), into oil, gas, and char. The process achieved high oil yields (68–87 wt%), with limited gas (10–30 wt%) and minimal char (2–3 wt%). The oil properties were found to be adjustable based on reaction parameters: lower temperatures (430 °C) and shorter reaction times (1 min) produced heavier oils with a high saturated alkanes content, whereas higher temperatures (460 °C) and extended reaction times (60 min) yielded lighter oils with an increased aromatics content. Additionally, a higher water-to-plastic ratio favored alkenes production, resulting in a heavier oil with a limited aromatics fraction. This research illustrates the strong potential of HTL to transform real post-consumer plastic waste streams into new plastic monomers, thereby closing the loop in the chemical recycling of polyolefins and fostering an innovative approach for urban-industrial symbiosis and a circular economy.