Upgrading biochar by co-pyrolysis of heavy bio-oil and apricot shell using response surface methodology

Abstract In this study, heavy bio-oil (HB) and apricot shell (AS) were used as raw materials to produce biochar by co-pyrolysis in a tube furnace. Three operational factors were assessed, namely pyrolysis temperature (400–700 °C), residence time (5–20 min) and percentage of HB (0–100%), with the effects on the yield and properties of biochar comprehensively investigated using a response surface methodology. Results showed that the mass yield, fixed carbon content, carbon content, and high heating value (HHV) of co-pyrolysis biochar (550 °C, 12.5 min, and an AS/HB mass ratio of 1:1) were 29.33%, 81.84%, 82.91%, and 32.97 MJ/kg, respectively. The experimental values for the co-pyrolysis biochar were 1.52%, 5.30%, 1.82%, and 1.12 MJ/kg higher than their corresponding theoretical values, respectively. However, the oxygen content of co-pyrolysis biochar was 1.71% lower than the theoretical value. Meanwhile, the C/H ratio of biochar improved with increasing pyrolysis temperature, residence time and percentage of HB, indicating higher biochar aromaticity. The evolution of surface functional groups indicated that an increase in pyrolysis temperature and residence time reduced the intensity of C O and C–O absorption peaks, while an increase in the percentage of HB enhanced the intensity of the C–O absorption peak of biochar. In general, compared to the pyrolysis of HB and AS alone, co-pyrolysis of HB and AS exerted a synergistic impact, improving the mass yield and fuel quality of biochar. Finally, the relationships between properties (mass yield, element content, proximate analysis parameters, and HHV) and the three varied factors were established, providing guidance for effective upgrading of biochar by co-pyrolysis of HB and biomass.