Effect of central secondary air on flow and combustion characteristics of 600-MWe down-fired boiler: From laboratory to industrial site

•Flow and combustion characteristics of down-fired boiler with central secondary air supply are studied.•Central secondary air ratio that is beneficial to the flow field symmetry and penetration depth is given.•Reasons for the burnout of burner nozzle and the blending proportion limitation of bituminous coal are obtained.•Technical improvement ideas for increasing the blending proportion of bituminous coal are proposed.