Supercritical Alcohol Liquefaction of Oleaginous Halophyte Seeds: Investigating the Roles of Lipids and the Lignocellulosic Matrix in Bio-Oil Formation and Properties

This study investigates the influence of feedstock biochemical composition and alcohol type on product distribution and bio-oil properties during supercritical alcohol liquefaction. Seeds from six oleaginous halophyte species and rapeseed were processed at 280 °C for 40 min in supercritical methanol or ethanol, at an alcohol-to-biomass mass ratio of 5.5:1. The bio-oils were characterized using physicochemical, elemental, GC–MS and GC-FID analyses, while Pearson and Spearman correlations examined composition–yield relationships. Bio-oil yields ranged from 30.94 to 79.62 wt.%, with lipid content showing a significant positive correlation with bio-oil yield for both alcohols. Methanol-derived bio-oils exhibited lower viscosities than those obtained with ethanol; the lowest values (5.02, 5.17, and 5.90 mPa·s) were obtained for Consolida regalis Gray, Papaver rhoeas L., and rapeseed, respectively. Methanol-derived rapeseed bio-oil exhibited an HHV of 38.36 MJ/kg and an ester content of 38.41 wt.%. Lower-lipid feedstocks yielded more viscous bio-oils with lower HHVs, reaching 27.02 MJ/kg. Liquefaction of raw seeds, exhausted seeds, and extracted oils suggested that other biochemical fractions may contribute to bio-oil formation and properties. Biochar ash contained appreciable amounts of alkali and alkaline-earth elements, suggesting potential valorization opportunities. These findings support further investigation of non-food oleaginous biomass for renewable fuel production.