Microwave pyrolysis of biomass for low-oxygen bio-oil: Mechanisms of CO2-assisted in-situ deoxygenation

Abstract

This work used CO2 as a co-reaction gas to realize in-situ deoxygenation of the bio-oil produced from microwave pyrolysis of biomass, and the mechanisms were thoroughly analyzed. Results showed that the bio-oil obtained at 550 °C (N2 atmosphere) had the highest yield of 29.24 wt% with the higher heating value (HHV) of 20.27 MJ/kg. The introduction of CO2 atmosphere reduced the oxygen content of the bio-oil by 12.68 wt% and increased the HHV by 26.43%. The oxygen mass balances demonstrated that the oil-O decreased by 31.45%, and the gas-O increased by 29.28% at CO2 atmosphere. The specific components in the bio-oil and pyrolytic gas were characterized. Furans in the bio-oil could react homogeneously with CO2 to generate CO at CO2 atmosphere, which was the main route to realize in-situ deoxygenation of the bio-oil. Meanwhile, stearic acid was selected as a model compound of the bio-oil to verify the action mechanisms of CO2 atmosphere. CO2 inhibited the decomposition of stearic acid to reduce the polycyclic aromatic hydrocarbons (PAHs) content in the bio-oil and promoted the generation of alkenes and CO. This study provides a novel perspective for the production of low-oxygen and high-quality bio-oil from biomass using CO2 reaction atmosphere.