Abstract: A process for reducing the level of nitrogen oxides (NO.sub.x) in a fossil fuel (e.g. coal) combustion flue gas has a selective non-catalytic reduction (SNCR) treatment stage followed by a selective catalytic reduction (SCR) treatment stage. The level of NO.sub.x produced by a power plant, particularly a coal fired power plant, varies depending upon the plant load. A set predetermined non-zero amount of first nitrogenous treatment agent is constantly introduced for the SNCR stage during all periods of plant operation. Additional first nitrogenous treatment agent is introduced for the SNCR stage only during periods of relatively higher NO.sub.x production, such as during peak load periods. A second nitrogenous treatment agent is added for the SCR stage up to the maximum SCR NO.sub.x reducing capability of the SCR stage based upon the total amount of NO.sub.x present in the flue gas, subject to limitations on the amount of allowable ammonia slip exiting the plant. Once the total amount of NO.sub.

Abstract: A process and apparatus for limiting the production of nitrogen oxides (NO.sub.x) during the combustion of a fossil fuel (e.g. natural gas, fuel oil and coal) in a cyclone type boiler includes the injection of water into the secondary air supply. The water is quickly vaporized into steam as the temperature rises, simultaneously cooling the surrounding air predominately as a result of the latent heat of vaporization, thus reducing the quantity of heat contained within the combustion air delivered to the flame. To avoid quenching combustion, substantially all of the water is vaporized into steam prior to exiting the cyclone section. For natural gas and fuel oil, preferably about 2.5 to 10.0 gallons of water are injected per 100 lbs of fuel. Water is injected through existing ports originally provided in cyclone boilers either for use as secondary air calibration ports or as oil deslagging system ports.