Solvent Liquefaction of Fast Pyrolysis Bio-Oil Distillate Residues for Solids Valorization

Viable thermochemical biorefineries require valuable outputs with optimized carbon distributions. While catalytic refining of biomass pyrolysis oils can produce fuel-grade hydrocarbons, more carbon-efficient pathways are needed to sustainably produce both renewable hydrocarbons─including those suitable for sustainable aviation fuel (SAF)─and carbonaceous solid materials. Bio-oils of sufficient quality and stability can undergo distillation, and catalytic hydrotreatment can upgrade the distillates without the interference of high-molecular-weight coke precursors. To further utilize the residues, we tested solvent liquefaction for upgrading of bio-oil distillate residues. Pyrolysis bio-oils from a lignocellulosic (switchgrass) and an oleaginous/proteinaceous (spirulina) biomass were distilled, and the distillate residues underwent liquefaction at 300 °C in microreactors with various solvents (water, ethanol, NaOH (aq), and formic acid (aq)). Optimal solvent conditions were downselected based on gas chromatography-mass spectrometry (GC-MS) of the products. Larger-scale reactions in optimal solvents (100 mL Parr reactor, 300 °C, 1500 psi) produced oils and hydrochar, the latter of which can be calcined into coke for manufacturing applications. For spirulina oil residues, ethanol-based liquefaction produced a 46% yield of oil; this represents more than double the yield for the NaOH-based liquefaction of switchgrass oil residues (20%). The oils contained straight-chain aliphatic compounds, which can potentially improve the processability for SAF applications.