Three-dimensional computational fluid dynamics simulations are performed for a 650 ℃ grade 1 000 MW ultra-supercritical swirl-opposed firing boiler with low NO_x combustion in the furnace. The flow and combustion characteristics in the furnace and the NO_x concentration in the flue gas are investigated under various conditions of overfire air. It is shown that setting a staggered overfire air and injecting the lower OFA with the angle 15° downward into the furnace is helpful to reduce NO_x concentration and improve the burnout rate. A lower overfire air ratio results in a higher temperature in the region of the combustor and a shorter distance between the high-temperature region and the heating surface. As the overfire air ratio increases

the NO_x concentration of the flue gas at the exit of the furnace first decreases and then increases

and the optimal overfire air ratio is around 33.9%.