Impacts of CO2 atmosphere on property of the biochar from pyrolysis of lignin

CO2 can be used to activate biomass for production of activated carbon at high temperatures (usually above 700 °C). At typical pyrolysis temperatures, CO2 might also involve in biomass pyrolysis, which was investigated via pyrolysis of lignin at temperatures from 400 to 700 °C in N2 and CO2 atomsphere, respectively. The results suggested that CO2 interfered with the pyrolysis process, facilitating formation of bio-oil and biochar but not gases via suppressing further gasification of volatiles. This reserved more mono-ring phenolics in bio-oil and also formed more heavier phenolics through secondary condensation routes. As for biochar, CO2 enhanced the aromatization process, generating the carbon-rich biochar of higher heating value. The in-situ characterization of the functionalities of biochar during pyrolysis suggested that CO2 accelerated removal of the aliphatics like -OH, -C-H, CO, and C-O-C. This rendered the biochar of hydrophobic surface but less fragmentized structures than that obtained in N2.