Supercritical water gasification of polyolefins and polystyrene-based polymers

The efficient valorization of plastics is one of the key challenges for contributing to the pollution mitigation and promoting the circular economy of resources. This work focuses on the supercritical water gasification (SCWG) of polyolefins (polyethylene, polypropylene) and polystyrene-based polymers at two different temperatures (450 °C and 600 °C). The impact of polymer type and temperature on product formation was investigated. Polystyrene exhibited poorer gasification compared to polyolefins, with a gas yield of 5.2 mol/kg at 600 °C, whereas polyolefins achieved a gas yield of around 20 mol/kg at the same temperature. A few similar patterns of behavior were nevertheless observed across all polymers: raising the temperature from 450 °C to 600 °C resulted in improved gasification and a preferential conversion to methane (>70 mol%). In each case, the proportion of carbon recovered in the aqueous phase was very low, less than 1% of the carbon initially introduced into the process. Furthermore, for each polymer, an oily organic phase was recovered at 450 °C while a char residue was recovered at 600 °C. All the chars exhibited similar carbon content and amorphous structure. In contrast, the organic phases recovered at 450 °C were very different depending on the polymer introduced. Polyethylene oil was rich in olefins and paraffins, polypropylene oil contained saturated aliphatic, unsaturated aliphatic but also cyclic and aromatic compounds and PS oil was mainly composed of benzene and naphthalene derivatives. Thus, SCWG of plastics at 450 °C seems more promising than at 600 °C for the production of plastic monomers and intermediates for the chemical industry.