Abstract: Over Fire Air( OFA) ratio not only has strong influence on NO_x emission,but also on heat flux and combustion inside the large capacity boilers.To investigate such effects,three-dimensional CFD numerical simulations were conducted for a 1 000 MW ultra-supercritical double reheated tower boiler.3D body fitted hexahedra meshes were adopted to discretize the simulation domain.Temperature of membrane wall and the tubes were identified through user defined functions( UDF).Stochastic particle tracking model was used to calculate dispersion of the pulverized coal particles.The turbulence information was calculated through Realizable k-ε model.Discrete-Ordinates model was used for the radiation heat transfer.The simplified PDF model was used to solve the coupling of turbulence and chemical reactions.The results show that OFA ratio has significant influence on the temperature,pollutant emission,and heat flux.When OFA ratio increases from 0 to 40%,the temperature of burner area will firstly increase and then drop down; the temperature of OFA area will increase,and the original NO_x emission will decrease and reach the minimum value of 25% of OFA ratio.The heat absorption by the furnace membrane wall decreases by 12% with the increases of OFA ratio from 0 to 25%,then the heat absorption decreases slowly when OFA ratio increases.The heat absorption by the convection heat surfaces increases so that the total heat absorption doesn't change.Such trend shows the effect of OFA ratio on heat absorption of furnace has to be considered in the design of the large capacity boilers.