To reduce NOx emissions without introducing an obvious increase in the carbon content of fly ash, a novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired boiler. This approach mainly consisted of moving fuel-lean nozzles from the arches to the front/rear walls, rearranging staged air, and introducing separated overfire air (SOFA). The aim of this work was to evaluate the overall performance of the novel combustion system relative to different SOFA ratios (i.e., 15, 20, 25, and 30%) and different SOFA locations in the upper furnace (1.0, 2.0, and 3.0 m above the arches) using numerical simulations and experimental measurements. Both numerical and experimental results showed that, with increasing the SOFA ratio from 15 to 20%, NOx emissions were greatly reduced but the carbon content in the fly ash increased slightly. With a further increase from 20 to 30%, NOx emissions slightly decreased but the carbon content in the fly ash increased substantially. Considering both the environmental and economic effects, 20% was chosen as the optimal SOFA ratio. With increasing the SOFA location height in the upper furnace from 1.0 to 3.0 m above the arches, the average gas temperature after the superheater and the carbon content in the fly ash at the furnace outlet somewhat increased but NOx emissions decreased. Considering various factors, the location (2.0 m above the arches) was chosen as the optimal SOFA location. The performance of this boiler in actual operation was good after performing modifications with the optimal SOFA ratio and location.