A method of decreasing the concentration of nitrogen oxides in a combustion flue gas in which a nitrogen reducing agent is introduced together with the overfire air to mixes with the products of primary combustion along with the overfire air. The nitrogen agent reduced NOx as it passes through the temperature regime that is optimum for the NOx reduction as overfire air and flue gas mix. The transition from low to high temperature effectively eliminates ammonia slip. Additionally, the nitrogen agent may be mixed with the overfire air stream in such a manner that it is optimally shielded from early mixing with the products of primary combustion, where a portion of the overfire air reacts initially with any residual carbon monoxide (CO) that would otherwise interfere with the NOx reduction chemistry.