Supercritical water gasification of an aqueous by-product from biomass hydrothermal liquefaction with novel Ru modified Ni catalysts

► Ru modified Ni/γ-Al2O3 catalyst was effective in catalyzing biomass SCWG for H2 production. ► H2 yield increased with increasing reaction temperature, but decreased with increasing WHSV. ► Activated carbon as a catalyst support was more resistant than γ-Al2O3 to alkali attack in SCW. ► Biomass ashes not only catalyzed SCWG reactions but also resulted in reactor plugging. ► Hydrothermal liquefaction combined with SCWG was an effective approach for sludge treatment.