Effects of gaseous agents on the evolution of char physical and chemical structures during biomass gasification

•Evolution of char structural features during biomass gasification was investigated.•The formation of micropores was facilitated in the process of CO2 gasification.•The aromatic layers of bio-chars preferred to grow laterally at high temperature.•CO2 favored the consumption of amorphous carbon during char-CO2 gasification.•Cross-linked carbon structures were easy to be consumed in char-H2O gasification.