Abstract: A flue gas recirculation system for fire tube boilers includes a duct connected to the boiler gas discharge stack and to a fan. Another duct couples the fan to a specially designed burner in the boiler. Recirculated flue gas is injected downstream of the fuel/air mixture in the burner and cools the flame leading to substantially reduce the NO.sub.x content in the boiler stack emissions. The burner includes conventional damper and air diffuser systems and a plurality of openings near the inner end of the burner housing, through which the boiler fuel is admitted and mixed with the air. A plurality of slots are formed at the outlet end of the burner, the slots being coupled to an annular chamber surrounding the burner. The duct from the recirculation fan is coupled to such annular chamber. Recirculation of between about fifteen to twenty percent of the flue gas in the above-described device can result in NO.sub.