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.