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A method to regulate overfire air passing through an overfire air duct and entering a flue gas stream in a combustion system including: directing overfire air into an inlet of the overfire air duct, passing the overfire air through the duct and discharging the overfire air into the flue gas stream i

A method to regulate overfire air passing through an overfire air duct and entering a flue gas stream in a combustion system including: directing overfire air into an inlet of the overfire air duct, passing the overfire air through the duct and discharging the overfire air into the flue gas stream in the combustion system; adjusting a flow rate of overfire air entering the inlet using a damper adjacent the inlet, and moving the damper parallel along an axis of the overfire air duct to increase and decrease the overfire air entering the inlet, wherein the damper has an open position at which the damper is extended out of the inlet and a closed position in which the damper is substantially in the inlet and blocking air entering the inlet.

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1. 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 oute

1. 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, andat least a portion of the overfire air flows through the outer overfire air duct regardless of the position of the damper. 2. A method as in claim 1 wherein cooling air flows between the duct and the damper when the damper is in the closed position. 3. A method as in claim 1 wherein the damper is mounted on a rod coaxial to the inner overfire air duct and the damper moves along an axis of the rod. 4. A method as in claim 1 wherein the damper abuts a U-shaped bracket when in the open position. 5. A method as in claim 1 wherein the damper is moved manually. 6. A method as in claim 1 wherein the damper is moved automatically. 7. A method to control overfire air passing through inner and outer overfire air ducts wherein the outer overfire air duct envelops the inner overfire air duct and entering a flue gas stream in a combustion system, the method comprising: directing overfire air into the inlets of the inner and outer overfire air ducts, passing the overfire air through the 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 of the inner overfire air duct using a damper adjacent the inlet, wherein the damper is aligned with an axis of the inner overfire air duct and is movable axially between an open position and a closed position and wherein at the open position at which the damper is extended out of the inlet of the inner overfire air duct and at the closed position the damper is substantially in the inlet and blocking air entering the inlet to the inner overfire air duct;moving the damper between the open and closed position and along the axis of the overfire air duct to control an amount of the overfire air entering the inlet of the inner overfire air duct, andat least a portion of the overfire air flows through the outer overfire air duct regardless of the position of the damper. 8. A method as in claim 7 wherein the damper is mounted on a rod coaxial to the axis of the overfire air duct and the damper moves with the rod. 9. A the method of claim 8 wherein the rod is supported by a bracket which is at least one of a U-shaped bracket and a spoke bracket, and the rod moves with respect to the bracket. 10. A method as in claim 7 wherein an actuator moves the damper axially to various positions including and between the open and closed positions. 11. A method as in claim 10 wherein the actuator is manually operated to move the axial position of the damper. 12. A method as in claim 10 wherein the actuator is remotely controlled to move the axial position of the damper. 13. A method as in claim 10 wherein the damper is a polygon in cross section. 14. A method as in claim 13 wherein the damper has a shape selected from a group consisting of simple, convex polygon shape in cross-section, a sphere, a football shape in cross-section, and an oval in cross-section.