Glycerol-assisted co-solvothermal liquefaction of food waste: Investigation of biocrude's fuel properties with proposed reaction mechanisms

The conversion of biowaste into biofuels has garnered attention in recent years due to the pressing need for sustainable energy solutions in the US. Owing to the rising volume of food waste (FW), producing biofuel from FW can address both waste management issues and provide a sustainable fuel alternative. Solvothermal liquefaction (STL) is a proven cutting-edge thermochemical pathway for producing biocrude oil from biowaste, potentially reducing dependence on fossil fuels. In addition, owing to low cost and ability to act as an in-situ hydrogen donor that stabilizes reactive intermediates, glycerol has potential as a liquefaction solvent. Prior work on co-solvent studies is typically based on one or two arbitrarily selected co-solvent loadings, which cannot distinguish a genuine synergistic optimum from a monotonic trend, and the influence of solvent composition remains unresolved. This study, therefore, examines the co-solvothermal liquefaction (Co-STL) of FW across the full glycerol–water composition range, from pure glycerol to pure water, to determine how solvent composition governs biocrude yield and quality. In this study, glycerol and water were mixed at ratios of 1:0, 3:1, 1:1, 1:3, and 0:1 (v/v), and the Co-STL was performed at 280-320 °C for 30 minutes. For all experiments, a 1:10 (w/v) FW-to-solvent ratio was maintained. The maximum amount of biocrude was obtained at 320 °C (50.06 ± 3.96 wt.%), with a heating value of 34.15 ± 0.55 MJ/kg, utilizing solely glycerol as the solvent. The thermogravimetric analysis revealed that biocrude obtained from water-based liquefaction contained a higher proportion of light fractions, e.g., gasoline/naphtha, kerosene, and diesel, compared to pure glycerol solvolysis. According to the GC-MS report, the produced biocrude is primarily composed of hydrocarbons, ketones, aldehydes, phenols, esters, alcohols, nitrogen-containing compounds, and organic acids. Based on ex-situ GC-MS and FT-IR analysis, reaction mechanisms for biocrude formation at varying glycerol concentrations were proposed. The findings indicate that STL is a promising and sustainable technology for converting FW into biocrude, aiding in advancing bioeconomy goals.