Co-gasification of plastic wastes and soda lignin in supercritical water

•A synergistic effect was found in co-gasification of soda lignin and PE.•The order of gasification efficiency of various plastics: PE > PC ≈ PP > ABS.•Much alkane gases were generated from degradation of plastics, especially PE and PP.•The presence of soda lignin promoted the reforming of alkanes and H2 production. In this study, the co-gasification of soda lignin produced from black liquor and various plastics in supercritical water was investigated experimentally. The effect of the mixing ratio of soda lignin/PE, temperature, reaction time, total concentration and plastic types was studied. A synergetic effect was found in co-gasification of PE and soda lignin, and the maximum influence was obtained at a mixing ratio of 50:50. Increasing the temperature and reaction time, and reducing the concentration improved the gasification efficiency and hydrogen production from co-gasification of PE/soda lignin (50:50). The H2 yield increased over 5 times to 57.0 mol/kg with temperature increasing from 500 °C to 750 °C, and the highest H2 yield of 63.3 mol/kg was obtained at 700 °C when the concentration was reduced to 5 wt%. The gasification efficiencies of different plastics were distinct and the order is: PE > PC ≈ PP > ABS, and the addition of soda lignin improved all the gasification efficiencies. The alkali salts in soda lignin also catalyzed the reforming of CH4 and C2H6 that generated from the depolymerization of plastics, especially PE and PP and improved the hydrogen production.