Regulating overfire air in a boiler using an overfire air tube damper

A method to control overfire air passing through an overfire air duct and entering a flue gas stream in a combustion system, the method comprising:

• 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, wherein the damper is aligned with an axis of the 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 and at the closed position the damper is substantially in the inlet and blocking air entering the inlet, and
• 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.