Nitrogen-doped porous biocarbon materials originated from heavy bio-oil and their CO2 adsorption characteristics

To realize the high-value utilization of biocoke derived from heavy bio-oil, N-doped porous biocarbon materials were prepared through co-pyrolysis of biocoke with urea and KOH activation, and their physiochemical structures and CO2 adsorption capacity were analyzed. Compared with direct heat treatment of biocoke at 550 °C, N-doping at the same temperature would reduce the biocarbon yield and block its pores, but the pore structure could be recovered through further heating to 800 °C. KOH activation at 800 °C could produce highly porous biocarbon materials with BET surface areas of 1710.20–3361.45 m2/g in the expense of yield reduction. The mass ratio of KOH to biocarbon material significantly affect its pore formation and distribution. Moderate activation (ratio of 2, 550NBC-800-2) could generate more micropores, over activation (ratio of 3 and 4, 550NBC-800-3/4) could further increased its total surface area, but resulted in the damage/merging of micropores to mesopores. KOH activation was found to increase the O-containing groups and decrease the large ring systems in the biocarbon as well, but the etching of KOH would lead to the releasing of N previously existed/doped in the biocarbon. The CO2 adsorption performance of the biocarbon material was closely related to its pore structure and surface N-doping. At a high CO2 pressure (1 bar), the CO2 adsorption capacity of biocarbon materials was more dependent on its micropore areas. But at a low CO2 pressure (0.15 bar), N-doping, especially to form more N-5 in biocarbon, was observed to be more important than its micropores.