Hydrogen/Methane Production from Supercritical Water Gasification of Lignite Coal with Plastic Waste Blends

Supercritical water gasification (SCWG) is an innovative conversion method of coal, and mixing with plastic waste can improve the energy production and handle the waste. In this study, co-gasification of lignite and various plastics [polyethylene (PE), polypropylene (PP), and polycarbonate (PC)] in supercritical water was investigated experimentally using quartz reactors. The presence of lignite and these plastics improved the gasification efficiency of each other in co-gasification, indicating a synergistic effect of them in SCWG. The magnitude of the synergistic effect between lignite with PP and PE was higher than that with PC probably because of the higher H/C ratio. The optimal mixing ratio of lignite to these plastics were 1:1, where the maximum promotion of the gasification was obtained. In addition, the co-gasification also reduced the phenol content in the liquid product. Increasing the reaction temperature improved the gasification efficiency and H2 yield, and the improvement was more significant when it is above 650 °C. At 800 °C, the highest H2 yield of 24.17 mol/kg from co-gasification of lignite/PP was obtained. The decrease of the total concentration and prolongation of the reaction time favored the co-gasification of lignite/PP, but the increase of the reaction time had little influence when it is over 30 min. The increase of the reaction time also reduced the content of aromatic hydrocarbons in the liquid product but increased the tar/char formation in the solid residues. The observation of the solid residues by scanning electron microscopy found that some spherical particles (1-2 μm) were formed during co-gasification of PP and lignite.