A 3-MW pilot-scale facility was used to study the effect of separated overfire air (SOFA) on NOx emissions from utility boilers. Experiment results showed that NOx emissions decreased as the excess air ratio in the main combustion zone first decreased, but remained unchanged when the excess air ratio was above a certain value; the carbon content in fly ash increased with the decrease of the excess air ratio in the main combustion zone; NOx emissions decreased and the heat loss due to unburned carbon increased as the residence time in the reduction zone (the zone in the furnace from the middle of combustion zone to the middle of the SOFA zone) increased. Based on the experiment results, a residence time of 2.12-2.68 s is recommended for the reduction zone, Besides, numerical simulations were conducted about the pilot-scale facility and a 600-MW boiler firing the blended coal of Shenhua coal (80 wt %) and Baode coal (20 wt %). The simulations, together with the experiments, showed that there was a critical excess air ratio with value of similar to 0.8 for the minimum NOx emissions with relatively low unburned carbon. This critical value, together with the residence time of 2.35 s, which is in the range recommended by our experiments, were used in the retrofit of the 600-MW boiler by SOFA technology, leading to a reduction of NOx emissions as high as 60%.