CO2-looping in biomass pyrolysis or gasification
Thermochemical technologies for converting biomass into energy or chemicals mainly consist of combustion, pyrolysis, and gasification. This review summarizes the recent advances in biomass pyrolysis and gasification by using CO2 as a reaction medium. The CO2-looping pyrolysis or gasification of biomass is compared with conventional processes. In the integrated valorization of biomass by pyrolysis or gasification, CO2 can play a vital role in each stage, mainly including biomass pyrolysis, biomass/biochar gasification, biochar activation, and tar cracking/reforming. CO2 as a reaction medium can significantly improve the thermal efficiency of biomass pyrolysis. Pyrolysis in CO2 results in deep decomposition of biomass compared to pyrolysis in N2. Also, CO2 has an affinity to react with hydrogenated and oxygenated groups, leading to biochar with a higher specific surface area (SBET). Thus, exploiting CO2 as a reaction medium in biomass pyrolysis provides an attractive option for enhanced generation of syngas and tuned adsorption capability of biochar. In addition, the CO2 pyrolysis of biomass can enhance the thermal cracking of harmful organic compounds, thus suppressing of the formation of benzene derivatives (e.g., volatile organic compounds) and polycyclic aromatic hydrocarbons. In general, CO2 gasification of biomass serves a dual purpose of reducing pollution and generating syngas. In addition, introducing CO2 with steam as a gasifying agent can enhance CO production. The internal mineral species (e.g., K, Ca) in biomass can also improve the reactivity of char gasification and tar reforming in the entire gasification processes.