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.