Hydrothermal Co-Liquefaction of Synthetic Polymers and Miscanthus giganteus: Synergistic and Antagonistic Effects

Synthetic polymers constitute one of the main carbon-containing wastes generated nowadays. In this study, combined hydrothermal liquefaction (co-HTL) is evaluated for 1:1 mixtures of Miscanthus giganteus and different synthetic polymers—including poly-acrylonitrile-butadiene-styrene (ABS), bisphenol-A-based epoxy resin, high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyamide 6 (PA6), polyamide 6/6 (PA66), poly(ethylene terephthalate) (PET), polycarbonate (PC), polypropylene (PP), polystyrene (PS), and polyurethane foam (PUR)—using batch HTL at 350 °C. Based on oil yields and composition, a comprehensive discussion of observed interactions is presented. The results show that even though polyolefins do not depolymerize under these conditions, the oil products depict that these materials interact with miscanthus biocrude changing its composition. Bisphenol-A-based polymers as PC and epoxy resins both contribute to the formation of monomer-like structures in the biocrude. PET increases the presence of carboxyl groups, while polyamides and PUR increase significantly the oil yield, modifying the biocrude composition toward nitrogen-containing molecules. PUR co-HTL was found to increase oil, carbon, and energy yields, leading to process improvement when compared to pure miscanthus processing.