Thermochemical conversion and gasification of high-density polyethylene (HDPE) in supercritical CO₂: A sustainable approach to plastic waste management

This study explores the thermochemical gasification of HDPE in a Sc-CO₂ environment, presenting a sustainable pathway for plastic waste valorization. The unique physicochemical properties of Sc-CO₂—acting as both a solvent and reactive agent—enhanced polymer decomposition, facilitated selective syngas production, and promoted gas-phase reforming reactions. The effects of temperature, reaction time, and CO₂ pressure on carbon conversion efficiency (CE) and hydrogen conversion efficiency (HE) were systematically investigated. Optimal performance was achieved at 700 °C and 20 min, yielding a maximum CE of 65 % and an HE of 98 %, accompanied by significant increases in H₂ and CO production. Morphological and structural analyses revealed that Sc-CO₂ improved mass transfer, suppressed excessive tar and char formation, and promoted the formation of spherical carbonaceous structures. Comparative evaluation with conventional air and steam gasification demonstrated that Sc-CO₂ enabled effective operation at lower temperatures while enhancing product selectivity and carbon utilization. These results position Sc-CO₂ gasification as a promising alternative for integrated plastic waste valorization and carbon resource recovery. Further research into catalytic enhancement and process intensification is required to improve scalability and techno-economic viability.