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

Abstract

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 polymersincluding 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), polyethylene terephthalate (PET), polycarbonate (PC), polypropylene (PP), polystyrene (PS) and polyurethane foam (PUR)using batch HTL reactors at 350 °C. Based on mass balances and oil 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 depicts that these materials interact with miscanthus oil changing its composition. Bisphenol-A based polymers as PC and epoxy resins both contribute for the formation of monomer-like structures in the oil. PET increases the presence of carboxyl groups, while polyamides and PUR increase significantly the oil yield, modifying oil composition towards nitrogen-containing molecules. PUR co-HTL was found to increase both oil carbon and energy yields, leading to process improvement when compared to pure miscanthus processing.

Component additivity model for plastics—biomass mixtures during hydrothermal liquefaction in sub-, near-, and supercritical water

“…These are true predictions (not an assessment of goodness of fit) in that none of these data were used to determine the model parameters. We found 48 experimental oil yields published for HTL (t ≥ 15 min) of mixtures containing PP, PET, PS, and/or PC with and/or without biomass ( Wu et al., 2017 ; Raikova et al., 2019 ; Souza Dos Passos et al., 2020 ; Seshasayee and Savage, 2021b ). One of the studies ( Raikova et al., 2019 ) was not done isothermally but rather used a fixed heating rate to the desired final temperature and then immediate cooling.…”

“…We have not yet tested for potential interactions between lipids and the plastics used herein, but that would be useful for future work. Another oil yield that was greatly overpredicted is from a recent study from Biller's lab ( Souza Dos Passos et al., 2020 ). This mixture was 50 wt% PS, which gave an oil yield of 86 wt% at 350°C in our earlier experimental work ( Seshasayee and Savage, 2020 ).…”

“…The experimental oil yield reported for the mixture was just 16 wt%. We used DCM to recover oil after HTL, whereas the Biller study ( Souza Dos Passos et al., 2020 ) used methanol. To determine whether the different solvents were causing the widely different oil yields, we ran an experiment for HTL of PS at 425°C, 30 min with methanol as the recovery solvent (following the recovery method of the Biller study ( Souza Dos Passos et al., 2020 )) and measured an oil yield of 13.32 ± 0.72 wt%.…”

“…We used DCM to recover oil after HTL, whereas the Biller study ( Souza Dos Passos et al., 2020 ) used methanol. To determine whether the different solvents were causing the widely different oil yields, we ran an experiment for HTL of PS at 425°C, 30 min with methanol as the recovery solvent (following the recovery method of the Biller study ( Souza Dos Passos et al., 2020 )) and measured an oil yield of 13.32 ± 0.72 wt%. An analogous experiment with DCM as recovery solvent (following the recovery method of the Biller study ( Souza Dos Passos et al., 2020 )) for HTL of PS at 425°C, 30 min gave an oil yield of 47.03 ± 0.13 wt%.…”

“…Hence, determining oil yields from plastic-biomass mixtures over a range of temperatures is vital for application of HTL for MSW. Previous studies ( Yuan et al., 2009 ; Wu et al., 2017 ; Raikova et al., 2019 ; Hongthong et al., 2020 ; Souza Dos Passos et al., 2020 ; Seshasayee and Savage, 2021a , 2021b ) indicate the existence of synergistic interactions between plastics and biomass that lower the depolymerization temperature of plastics and hence achieve higher oil yields for the mixtures under subcritical temperatures than would be anticipated from HTL of the components individually. Accordingly, we investigate herein the HTL of binary mixtures and then use these results to develop a model that can predict oil yields from HTL over a range of temperatures for mixtures containing synthetic and natural organic polymers.…”