Abstract: A combustion boiler for burning fuel and producing heat to generate steam. A method of minimizing discharging of nitrogen oxides from a combustion boiler comprising the steps of spraying a reducing agent into an overfire air stream and supplying the overfire air stream to the combustion boiler. Vaporizing the sprayed reducing agent at least within about 0.1 seconds of the sprayed reducing agent entering the combustion boiler and reacting the vaporized reducing agent with the nitrogen oxides within the combustion chamber to reduce the nitrogen oxides and minimize discharge of nitrogen oxides from the combustion boiler. The reducing agent is nearly substantially instantaneous evaporated/gasified by the high energy of the reducing agent injection system.