The comparison between the numerical results of the cold air flow and the experimental data of a 670 t/h coal-fired boiler shows a good coincidence. The NO value of calculation is 12% less than the ones of measurement but the CO value is almost same between calculation and measurement. Mathematical models and calculation method are proved to be valid. The aerodynamic field in the furnace with various Over-Fire-Air (OFA) parameters and arrangements is calculated to analyze the effect of OFA after the OFA ports are added to the boiler furnace. The optimized OFA parameters and arrangements are gained and simulated in reacting flow. The result shows that the NO emission of the furnace with OFA is reduced and the NO concentration of outlet decreases from 638 mg/N m(3) to 570 mg/N m(3) (O-2=6%). (c) 2005 Elsevier B.V. All rights reserved.