A method of decreasing the concentration of nitrogen oxides in a combustion gas flowing through a vessel including the steps of: generating a flue gas in a combustion zone of the vessel, the flue gas containing nitrogen oxides and carbon monoxide; providing overfire air into a burnout zone of the vessel from a first injector of the overfire air to oxidize at least some of the carbon monoxide in the flue gas; injecting a selective reducing agent concurrent with the overfire air at a level in the burnout zone downstream of the first injector of overfire air and downstream of the oxidization of the carbon monoxide; and reacting the selective reducing agent with the flue gas to reduce the nitrogen oxides.