Overfire air tube damper for boiler and method for regulating overfire air

A damper and overfire air duct in a combustion system having a combustion structure defining a flue gas passage, the damper and overfire air duct including: an inlet to the overfire air duct and an outlet to the duct discharging overfire air into the flue gas passage, and the damper aligned with an axis of the overfire air duct, and having an open position axially distal to the inlet and a closed position at least partially in the inlet and duct, wherein the damper is movable axially between the open and closed positions.

BACKGROUND OF THE INVENTION

This invention relates generally to secondary air injection to combustion systems and, particularly, to dampers for secondary air tubes in fossil fuel fired boilers.

Combustion systems are used in numerous industrial environments to generate heat and hot gases. For example, boilers and furnaces burn hydrocarbon fuels, e.g., oil and coal, in stationary combustors to produce heat to raise the temperature of a fluid, e.g., water. Industrial combustors typically employ various burner elements to combust the fuel and air injectors to provide combustion air to ensure complete combustion of the fuel. A typical industrial furnace, whether gas or fossil fired and hereafter referred to as a boiler, typically includes a lower combustion zone and a generally vertically extending flue gas passage.

The air introduced into a combustion system may be staged. Primary air is mixed with the fuel as both are injected into a combustion zone. Secondary air (also known as overfire air) may be injected into a combustion chamber downstream (in the direction of flue gas flow) of the primary combustion zone. The secondary air may be used to burnout any unburned hydrocarbons remaining from the primary combustion zone.

Overfire air is typically injected into the flue gas at a location in the flue gas passage downstream of the combustion zone. The combustion air provided to the combustion zone may be reduced to suppress flame temperature in the combustion zone and NOx formation. Suppressing combustion temperature creates excessive unburned hydrocarbons in the flue gas. The overfire air, introduced above the primary combustion zone, completes combustion of the unburned hydrocarbons which are then converted to carbon dioxide and water.

In conventional boilers, the overfire air is introduced to the flue passage through injection ports in the front or side walls or both of the boiler. The amount of secondary air (overfire air) needed for effective burnout may vary depending on the operating condition of the combustion system. To adjust the amount of secondary air, dampers are closed or opened to vary the amount of secondary air flowing from the secondary air tubes into the flue passage. However, conventional dampers tend to either shut off secondary air flow or allow substantial amounts of air flow. Conventional dampers tend not to effectively allow for adjustable amounts of secondary air. There is a long felt need for an improved damper for a secondary (overfire) air system.

BRIEF DESCRIPTION OF THE INVENTION

A damper and overfire air duct has been developed for a combustion system having a combustion structure defining a flue gas passage, the damper and overfire air duct including: an inlet to the overfire air duct and an outlet to the duct discharging overfire air into the flue gas passage, and the damper aligned with an axis of the overfire air duct, and having an open position axially distal to the inlet and a closed position at least partially in the inlet and duct, wherein the damper is movable axially between the open and closed positions.

An overfire air duct has been developed for a combustion system having a combustion structure defining a flue gas passage, the damper and overfire air duct comprising: an inlet to the overfire air duct and an outlet to the duct discharging overfire air into the flue gas passage, and the damper aligned with an axis of the overfire air duct, and having an open position axially distal to the inlet and a closed position at least partially in the inlet and duct, wherein the damper is movable axially between the open and closed positions.

A method has been developed to regulate 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; moving the damper parallel to an axis of the overfire air duct to increase and decrease the overfire air entering the inlet, wherein the damper having 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.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is schematic diagram of a combustion system10, e.g., a boiler, with a sidewall removed to show the interior combustion zone12and flue gas duct14. The combustion system10may be a large hollow structure11, that is more than one, two or even three hundred feet tall. The combustion system10may include a plurality of combustion devices16, e.g., an assembly of combustion fuel nozzles and air injectors, which mix fuel and air to generate flame in the combustion zone12. The combustion device16may include burners, e.g., gas-fired burners, coal-fired burners and oil-fired burners. The burners may be arranged on one or more walls, e.g., front and back walls, of the structure11of the combustion system10. The burners may be situated in a wall-fired, opposite-fired, tangential-fired, or cyclone arrangement, and may be arranged to generate a plurality of distinct flames, a common fireball, or any combination thereof. Air for the burners may flow through an air duct(s)17on an outside wall(s) of the structure11.

The fuel/air mixture18injected by the combustion devices16burns primarily in the combustion zone12and generates hot combustion gases that flow upward through the flue gas passage14. From the combustion zone12, the hot combustion gases flow into an optional reburn zone20into which additional (reburn) fuel22is supplied to the hot combustion gases to promote additional combustion.

Downstream of combustion and reburn zones, overfire air (OFA)24is injected through an overfire air nozzle(s)26into the OFA burnout zone28in the flue gas stream. A reducing agent, e.g., nitrogen (N-agent), may be injected into the flue gases with one or more of the streams of overfire air. Downstream of the OFA burnout zone, the combustion flue gas24passes through a series of heat exchangers30and a particulate control device (not shown), such as an electrostatic precipitator (ESP) or baghouse, that removes solid particles from the flue gas, such as fly ash.

FIG. 2is a perspective view andFIG. 3is a side view, show in partial cross-section, of an overfire air injector assembly32. The air injector assembly forms the structure for the overfire air nozzles26shown inFIG. 1. The overfire air injector assembly32generally includes an OFA inlet port34that receives overfire air from the air duct17on an outer sidewall, e.g., the front and rear walls, of the structure11of the combustion system10. The air inlet port34may be arranged to face into the flow of the air in the duct17. For example, the inlet port may face downward into the upwardly flowing air in duct17.

Turning vanes36, in the inlet port34of a hollow elbow conduit42, turn the overfire air to a direction, e.g., horizontal, that is preferably substantially perpendicular to the flow of flue gases moving up through the structure11of the combustion system10. An annular flange44on the elbow conduit provides a coupling for a hollow frustoconical air duct46that extends towards a hollow cylindrical end section48of the overfire air injector assembly32. The cylindrical end section includes a flange50that provides a coupling mount for the assembly32to the wall of the structure11of the combustion system10. For example, the cylindrical end section48fits into a circular aperture in the structure wall and the flange50is bolted to a mounting ring on the wall and at the circumference of the wall aperture.

The distal end52of overfire air injector assembly32is hollow and extends a short distance, e.g., one-half to three meters, beyond the wall of the structure and into the flue gas stream. Overfire air is discharged from the distal end52and into the flue gas stream at the burnout zone28, as is shown inFIG. 1. An N-agent injector, e.g., a pipe (not shown) extending through and coaxial with the cylindrical end section48, is shown inFIG. 1and may be included in the overfire injector assembly32.

An inner cylindrical air duct54extends through the frustoconical duct46and cylindrical end section48. The cylindrical air duct has an air outlet aligned with the distal end52of the cylindrical end section. The cylindrical air duct54has an inner overfire air passage56that extends through the duct from an inlet58to the duct. The duct inlet58may extend into the interior of a hollow elbow conduit42. An axially movable damper60for the air duct54is positioned at the inlet58.

An annular outer overfire air duct62extends between the air duct54and an inner wall of the cylindrical end section48and an inner wall of the frustoconical duct46. A swirler64, e.g., radial array of vanes, may be positioned in the outer overfire air duct62to impart a rotation to the overfire air flowing through the outer duct62. While not shown, a swirler may be positioned in the inner overfire air passage56. An annular damper66may be near the inlet (aligned with flange44) to the outer overfire air duct62to regulate the volumetric rate of overfire air through the duct62. The damper66may be adjusted, e.g., between closing offer overfire air flow to duct62and fully open to such air flow, by an actuator40. The actuator40may include a separate actuation arm and hydraulic servo for each damper/louver system controlled by the actuator40.

FIG. 4is a perspective view of the side and inlet end58of the inner cylindrical air duct54,FIG. 5is a cross-sectional side view of the duct54near the inlet end58, andFIG. 6is cross-sectional view of duct54taken along line6-6inFIG. 5.

The damper60is axially mounted on a damper control rod68. The control rod and damper may slide in and out of the inlet58of the inner cylindrical duct54. The damper60is shown fully open inFIGS. 3,4and5. The damper shown in phantom lines and designated as in position60ainFIG. 5is shown in a closed position that substantially closes off the overfire air flowing through duct54.

Even with the damper60at the fully closed (see damper in position60a, a cooling gap70may be formed between the outer periphery of the damper60and the inner wall of inlet58to the duct54. Air passes through the cooling gap while the damper is in a closed position60ato cool the end of the duct54which is exposed to the radiant heat energy of the combustion in the combustion system.

The rod68is supported by a U-shaped mounting bracket72having legs74that attach to a quarl ring76. The quarl is a furstoconical metal collar that guides the overfire into the inlet58from the elbow conduit42(FIG. 3). The quarl76may be fixed to the inlet58such as by welding. A radial spoke bracket78provides a mount for the damper rod68that is opposite to the mount provided by the U-shaped bracket. The spoke bracket78has narrow spokes, e.g., three spokes, each with an outer radial end attached to an inside surface of the duct54. The inner ends of the spokes support a cylindrical bearing that supports the rod68.

An actuator82(SeeFIGS. 2 and 3) moves the damper60and optionally the rod68to position the damper with respect to the inlet of the58of the inner cylindrical duct54. The damper may be moved axially with respect to the duct54by manually moving a hand lever (such as is shown inFIG. 2) or by a servomotor that is remotely controlled by a computer control system that may also controls other dampers and louvers for the air supply to the combustion system. The actuator positions the damper to regulate the volumetric rate of overfire air flowing through the inner overfire air passage56. In the fully open damper position shown inFIGS. 3,4and5(see reference numeral60), the damper allows a maximum rate of overfire air to flow through the passage56. By advancing the damper axially along the axis of the rod68, the rate of overfire air entering the passage56can be progressively reduced. By advancing the damper to closed position60a, the rate of overfire air is minimized such that only a minimal volumetric rate of air flows through passage56. The actuator allows the duct to be positioned at any axial location between the fully open position (see reference numeral60) and the fully closed position (see reference numeral60a).

The position of the damper60with respect to the inlet58may be adjusted to account for changes in the operation of the combustion system10. For example, as the load on the boiler changes, the damper may be adjusted axially in or out to reduce or increase the amount of overfire air entering the flue gases in the combustion system. Further, the damper may be adjusted to provide enhanced emission controls, e.g., nitrous oxide (NOx) control, which may be achieved by increasing or reducing the amount of overfire air entering the flue gases.

The shape of the damper60may be such that the outer perimeter of the damper has a diameter that is slightly, e.g., within one quarter inch, smaller than an inside diameter of the duct54. The damper may be circular in front view and preferably has a front view shape substantially similar to the interior cross-sectional shape of duct54. The damper may have a simple, convex polygon shape as shown inFIG. 5, and may be shaped as a sphere, “football” in cross-section, oval in cross-section, or other shape that slides into the open inlet58of the of the duct54. The shape of the damper and the movement of the damper by the actuator may be designed such that the rate of overfire air flow through the passage56is dependent on the position of the damper with respect to the inlet. Preferably, the distance that the damper60is advanced towards the inlet58is proportional, and most preferably linearly proportional, to the reduction or increase in the overfire air rate entering the passage56.

FIG. 7shows the prior art and is a perspective view of the side and inlet end of an inner overfire air passage154having an inlet end158. A conventional disc damper160(sometimes referred to as a “flapper”) is mounted on a rod168that is transverse to the axis of the duct154. By turning the rod168, the disc damper160can be rotated from a fully open position (as shown inFIG. 7and by the solid line damper160shown inFIG. 8) to a fully closed position (shown by the broken line damper160ashown inFIG. 8). A radial post178stops the damper in a fully open position and a corner block180stops the damper in a fully closed position. In the fully closed position160a, a small annular cooling gap170remains between the outer perimeter of the disc and the inner wall of the inlet158to the duct154. The cooling gap allows a small amount of overfire air to flow through passage156to provide cooling to the inlet158which is exposed to the radiant heat of the combustion flames in the combustion system.

The conventional disc damper160tends not to provide proportional flow control for the overfire air flowing through the passage156. In particular, the disc damper tends to rapidly allow substantially a full air flow through the passage as the disc is rotated away from the fully closed position160a.

There is a long felt need for an inlet damper that provides proportional flow control of overfire air entering an inner overfire air passage. This need is believed to be satisfied by the damper60shown inFIGS. 2 to 6. The damper60shown inFIGS. 2 to 6and the axial movement of the damper60provides proportional flow control because axial movement of the damper can proportionally adjust the volumetric flow rate of overfire air in the passage56. For example, a mid-point in the movement of the damper56along the axis of the rod68reduces the overfire air through passage56to about one-half the volumetric airflow of the passage when the damper is fully extended away from the inlet58(as is shown inFIG. 5). One advantage of the axial movement and shape of damper60over the shape and rotational movement of damper162is proportional control of overfire air in passage56,156.