Hydrogen production from supercritical water gasification of soda black liquor with various metal oxides

Supercritical water gasification is an innovative handling method of black liquor, which can also produce hydrogen-rich gases. In this study, the catalytic activity of 14 common metal oxides on SCWG of black liquor were investigated and the catalytic mechanism was studied through X-ray photoelectron spectroscopy (XPS) analysis. All the tested oxides improved the gasification efficiency and V₂O₅, WO₃ and Co₂O₃ showed the highest performance. The H₂ yields of 21.67 and 21.03 mol/kg were achieved with the presence of Co₂O₃ and ZnO respectively. The gasification efficiency increased with the increasing V₂O₅ amount, but the H₂ fraction increased firstly and then decreased. The highest H₂ fraction of 44.59% was obtained with V₂O₅ loading amount of 45 wt%. The increase of reaction time improved the CE, but the further prolongation had little influence. The XPS analysis of CuO and Fe₂O₃ showed some of them were reduced to metal (Cu) or lower-valence oxides (Cu₂O, FeO). The metal oxides probably promoted black liquor decomposition by introducing reactive oxygen, and the generated metal also catalyzed the gasification. Accordingly, an oxidation reactor could be introduced to recycle the metal oxides, which may help to realize the complete gasification of black liquor or other refractory wastes at milder temperatures.