Behaviors of rice straw two-step liquefaction with sub/supercritical ethanol in carbon dioxide atmosphere

Abstract This study extended previous work investigating two-step liquefaction by supercritical ethanol of rice straw under CO 2 atmosphere at temperatures of 270–345 °C. Subcritical CO 2 –subcritical ethanol (SubCO 2 –SubEtOH) pretreatment decreased the content of lignin in the rice stalk from 22.94 to 21.43 wt%. The results showed that although oxygen-transfer reaction, transesterification, carbonylation, and other reactions may occur with the supercritical CO 2 –supercritical ethanol (ScCO 2 –ScEtOH) liquefaction reactions, transesterification was the main reaction. The “de-oxygen-transfer” reaction mainly comprised de-oxygenation and decarboxylation. For temperatures exceeding 320 °C, the bio-oil yield decreased because the effects of esters decreased. The residence time affected the H/C and O/C ratios to a minor extent. It was shown that the nucleophilic and hydrolytic functions of ethanol might be strengthened, generating higher amounts of ester, phenolic, acidic, and hydrocarbon derivatives in the bio-oil fraction.