Study on the effects of using a carbon dioxide atmosphere on the properties of vine shoots-derived biochar

This study analyzes the effects of using a different atmosphere (pure N2 or pure CO2) at two levels of absolute pressure (0.1 and 1.1 MPa) on the pyrolysis of vine shoots at a constant peak temperature of 600 °C. Recycling CO2 from residual flue gases into the pyrolysis process may be economically beneficial, since CO2 can replace the use of an expensive N2 environment. In addition, the use of a moderate pressure (e.g., 1.1 MPa) can result in higher carbonization efficiencies and an improvement in the pyrolysis gas (in terms of yield and composition). Results from our study suggest that the use of CO2 instead of N2 as pyrolysis environment led to similar carbonization efficiencies (i.e., fixed-carbon yields) and mass yields of biochar. The chemical properties related to the potential stability of biochar (i.e., fixed-carbon content and molar H:C and O:C ratios) were very similar for both pyrolysis atmospheres. Under an atmosphere of CO2, the yield of produced CO2 was drastically decreased at the expense of an increase in the yield of CO, probably as a consequence of the promotion of the reverse Boudouard reaction, especially at high pressure. The enhanced reverse Boudouard reaction can also explain the relatively high BET specific surface area and the macro-porosity development observed for the biochar produced under a CO2 environment at 1.1 MPa. In summary, the pressurized pyrolysis of biomass under an atmosphere of CO2 appears as a very interesting route to produce highly stable and porous biochars and simultaneously improving the yield of CO