High-quality, low-nitrogen bio-oil from kitchen waste via K2CO3-catalyzed solvothermal liquefaction in recycled ethanol-water co-solvent.

Kitchen waste (KW), comprising 30 %-60 % of municipal solid waste, could be converted to bio-oil via alkaline-catalyzed solvothermal liquefaction (STL) without energy-intensive drying. This study systematically investigated six catalysts (K2CO3, Na2CO3, KHCO3, NaHCO3, KOH, NaOH) for product distribution and nitrogen migration in STL versus hydrothermal liquefaction (HTL). Results demonstrate K2CO3's superiority in ethanol-water co-solvent, synergistically enhancing bio-oil yield to 57.18 % (calorific value 35.49 MJ/kg) while achieving directional denitrification - reducing nitrogen content to 22.99 wt.% via pH-driven protein deamidation. Sulfur content decreased to 0.13 wt.% through sulfide decomposition. Critically, this method optimized bio-oil composition: light fractions (<343 °C) reached 82.10 % and hydrocarbons increased to 14.47 %, significantly outperforming HTL. Moreover, ethanol solvent recycling maintained 35.16 % bio-oil yield after three reuse cycles (distillation and antioxidants required), exceeding conventional HTL conversion (67.72 %). This work establishes three advances: (1) K2CO3-ethanol synergy enables high-yield, low-nitrogen bio-oil; (2) Alkaline catalysis directionally removes N/S impurities; (3) Multi-cycle solvent reuse sustains efficient oil production, providing a sustainable pathway for wet waste valorization.