Effect of residence time on two-step liquefaction of rice straw in a CO2 atmosphere: Differences between subcritical water and supercritical ethanol

•Subcritical CO2-solvent pretreatment had the characteristics of hydrothermal and dilute acid.•Residence time had marginal impact on subcritical CO2-supercritical ethanol liquefaction.•CO2 promoted the carbonyl reaction in subcritical CO2-subcritical water.

This study investigated the influence of temperature and residence time on liquefaction of rice straw in subcritical CO2-subcritical water (subCO2-subH2O) and in subcritical CO2-supercritical ethanol (subCO2-scEtOH), considering the final reaction temperatures (270-345 °C) and residence times (15 and 30 min). Residence time was identified as a crucial parameter in the subCO2-subH2O liquefaction, whereas residence time had a marginal influence on subCO2-scEtOH liquefaction. When reaction conditions were 320 °C and 15 min, solvents have weak impact on the quality of bio-oil, HHV 28.57 MJ/kg and 28.62 MJ/kg, respectively. There was an obvious difference between the subCO2-subH2O and subCO2-scEtOH liquefaction mechanisms. In subCO2-subH2O, CO2 promoted the carbonyl reaction. In subCO2-scEtOH, supercritical ethanol have the function of donating hydrogen and promoting the reaction of hydroxyl-alkylation.