Patent ID: 8176911

Claim:
A method to regulate overfire air passing through inner and outer overfire air ducts and entering a flue gas stream in a combustion system, wherein the inner and outer overfire air ducts are concentric, the method comprising: directing overfire air into the inlets of the concentric inner and outer overfire air ducts, passing the overfire air through the inner and outer concentric air ducts and discharging the overfire air into the flue gas stream in the combustion system; adjusting a flow rate of overfire air entering the inlet to the inner overfire air duct using a damper adjacent the inlet; moving the damper parallel along an axis of the inner overfire air duct to alternatively increase and decrease the overfire air entering the inlet of the inner overfire air duct, wherein the damper has an open position at which the damper is extended out of the inlet to increase the overfire air entering inner overfire air duct, and a closed position in which the damper is substantially in the inlet and decreasing the overfire air entering the inner overfire air duct, and at least a portion of the overfire air flows through the outer overfire air duct regardless of the position of the damper.