Thermochemical conversion of sunn hemp biomass and mixed plastic waste via co-liquefaction

Global interest has surged in the co-liquefaction of oxygen-rich lignocellulosic biomass with hydrogen-rich plastic into oil, a valuable precursor for biofuel and biomaterial. This study investigates the co-liquefaction of modelled mixed plastic waste with sunn hemp ( Crotalaria juncea ) biomass (SHB) for oil production. Initially, SHB was co-liquefied with polyethylene terephthalate (PET) under varying conditions, including temperature (300–340 °C), residence time (30–90 min), and biomass-to-plastic ratio (1:2–2:1), to optimise the process using Response Surface Methodology. The predicted optimal conditions at 324 °C, a 1:2 biomass-to-plastic ratio, and a 30 min residence time yielded a maximum theoretical oil output of 57.79 wt%. Under these conditions, experimental validation resulted in an oil yield of 55.73 ± 1.11 wt% (close to the predicted 57.79 wt%), demonstrating a positive synergistic effect. The co-liquefaction of SHB with binary and ternary plastic mixtures produced oil yields of 22–62 wt%. Notably, the SHB + PET + flexible polyurethane (PUF) (58.33 wt%) and SHB + PET + PUF + rigid polyurethane (62.36 wt%) combinations achieved the highest yields, exhibiting corresponding higher heating values of 27.18 and 27.63 MJ/kg, respectively. Co-liquefaction also improved oil yields from lower-performing plastics such as polypropylene, polystyrene, and low-density polyethylene. Polyurethane (PU) co-liquefaction improved overall process efficiency via enhanced oil, carbon, and energy recovery compared to pure SHB processing. Detailed characterisation indicated that PU-derived nitrogen-containing compounds enhanced oil formation through increased heteroatomic reactivity. Finally, co-liquefaction of lignocellulosic biomass with mixed plastic waste offers a more viable approach for waste valorisation than relying on sorting, paving the way for energy and carbon recovery.