Investigation on co-combustion of coal gasification fine slag residual carbon and sawdust char blends: Physiochemical properties, combustion characteristic and kinetic behavior

•The fuel property of gasification fine slag improved by the froth flotation process.•Both synergistic and antagonistic effects exist between sawdust char and residual carbon.•Chemical components and carbon crystalline structure of samples were compared.•Addition of sawdust char into residual carbon significantly promoted the combustion performance.•The most effective mechanisms for multi-step co-combustion process of sawdust char and residual carbon were obtained.