In the process of preheating combustion of circulating fluidized bed,the combustion process of the preheated fuel in the down-fired combustor（DFC)is very important. In order to study the flow characteristics and combustion characteristics of the preheated fuel in the DFC,the computational fluid dynamics software Fluent was used,combined with experimental methods,to test and simulate the combustion process of the preheated fuel in the DFC under different secondary air nozzles. the differences in flow characteristics,temperature characteristics,component concentration distribution characteristics and nitrogen oxide emission characteristics under different air distribution methods were compared. The results show that during the combustion process of the preheated fuel in the DFC,the secondary air can entrain the flue gas and reflow in the upper part of the down-fired combustor to dilute the reactants,and the recirculation area is larger when the air is distributed at the central nozzle. Under different air distribution modes,the temperature distribution is different. The peak temperature in the DFC is 1 459 K when the air is distributed at the annular nozzle,while the temperature peak is 1 555 K when the central nozzle is distributed. At the same time,when the air is distributed at the annular nozzle,the high temperature area is smaller and the temperature distribution is more evenly. When the air is distributed at the annular nozzle,the mixing of preheating fuel and secondary air is more sufficient,and the reaction between high temperature gas and air is more intense,which is helpful to the ignition and heating of fuel. When the air is distributed at the central nozzle,the concentration of CO and H2 at the top of the DFC is higher,which helps to reduce NOx. What′s more,a higher temperature promotes the gasification reaction,forming a reducing atmosphere,which is conducive to further reduce NOx. When the central nozzle is used for air distribution,more nitrogen oxides are reduced,the NOx emission in the tail flue gas is 107×10-6. When the annular nozzle is used for air distribution,the NOx emission is 121×10-6.