Ethanol addition during aqueous phase recirculation for further improving bio-oil yield and quality

•Aqueous phase recirculation in ethanol-water solvent was firstly investigated.•A comparison between recirculation in water and ethanol-water solvent was provided.•Ethanol addition improves the energy recovery efficiency and bio-oil quality. The increase of nitrogen content caused by the re-polymerization of nitrogenous compounds remains an important challenge during aqueous phase recirculation in the water solvent. In this work, aqueous phase recirculation was conducted in the ethanol-water solvent to evaluate the feasibility of ethanol addition in inhabiting the re-polymerization of nitrogenous compounds. The present study provides a comprehensive comparison of yield and quality between bio-oils derived from aqueous phase recirculation in the water solvent and ethanol-water solvent. The results showed that the addition of ethanol in aqueous phase recirculation displayed higher promotion in bio-oil yield (7%) compared with water solvent (3.55%). The energy recovery efficiency of bio-oil from ethanol-water solvent reached 87.43% after recirculation for three times, while that from the water solvent was 74.61%. The elemental analysis and gas chromatography-mass spectrometry analysis indicated that ethanol addition could inhibit the re-polymerization of nitrogenous compounds and thus decrease the nitrogen content in bio-oil. This resulted in lower NO and HCN emission peaks during the combustion process. Besides, the addition of ethanol increased the content of light molecular weight fractions and decreased the coke formation, which contributed to complete combustion and resulted in lower CO emission peaks. Overall, this work provides a feasible way for further improving the bio-oil yield and fuel quality through ethanol addition in solvent during aqueous phase recirculation.