Characterization and Thermochemical Conversion of Mixed Plastic Waste (Plasmix) Through Pyrolysis and Hydrogasification

The thermochemical valorization of non-recyclable mixed plastic waste represents a promising strategy for waste management and energy recovery. This work experimentally investigates the hydrogasification of Plasmix, the residual fraction of mixed plastic waste not mechanically recyclable, and compares it with conventional pyrolysis at 550 °C and 2.1 bar. Hydrogasification of low-density polyethylene (LDPE) was also investigated to assess the influence of feedstock composition. Feedstocks were characterized by elemental, proximate, and thermal analyses, while product distribution, methane evolution, and condensable products were evaluated by online gas analysis and gas chromatography–mass spectrometry. For Plasmix, hydrogasification increased the gas yield from 53.1% during pyrolysis to 74.0% and methane yield from 4.2 to 13.3%, while reducing the liquid fraction from 29.6 to 14.4%. Methane evolution indicated a two-stage mechanism involving initial devolatilization followed by hydrogen-assisted conversion of residual carbonaceous species. LDPE hydrogasification achieved a gas yield of 85.3% and nearly complete feedstock conversion. These results demonstrate that hydrogasification promotes methane-rich gas production from mixed plastic waste under relatively mild operating conditions, providing a basis for integration with innovative steam methane reforming units, potentially allowing the hydrogen required for hydrogasification to be internally generated while achieving net hydrogen production.