Abstract:
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.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to secondary air injection to combustion systems and, particularly, to dampers for secondary air tubes in fossil fuel fired boilers. 
         [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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 
       [0006]    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. 
         [0007]    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. 
         [0008]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram showing a side, cross-sectional view of a combustion system. 
           [0010]      FIG. 2  is a perspective view of an overfire air injector assembly. 
           [0011]      FIG. 3  is a side view, show in partial cross-section, of the overfire air injector assembly shown in  FIG. 2 . 
           [0012]      FIG. 4  is a perspective view of the side and inlet end of the inner cylindrical air duct. 
           [0013]      FIG. 5  is a cross-sectional side view of the inner cylindrical air duct shown in  FIG. 4 . 
           [0014]      FIG. 6  is cross-sectional view of inner cylindrical air duct taken along line  6 - 6  in  FIG. 5 . 
           [0015]      FIG. 7  is a front, side perspective view of an overfire air tube with a conventional damper. 
           [0016]      FIG. 8  is a side view showing in cross section the overfire air tube, shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  is schematic diagram of a combustion system  10 , e.g., a boiler, with a sidewall removed to show the interior combustion zone  12  and flue gas duct  14 . The combustion system  10  may be a large hollow structure  11 , that is more than one, two or even three hundred feet tall. The combustion system  10  may include a plurality of combustion devices  16 , e.g., an assembly of combustion fuel nozzles and air injectors, which mix fuel and air to generate flame in the combustion zone  12 . The combustion device  16  may 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 structure  11  of the combustion system  10 . 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)  17  on an outside wall(s) of the structure  11 . 
         [0018]    The fuel/air mixture  18  injected by the combustion devices  16  burns primarily in the combustion zone  12  and generates hot combustion gases that flow upward through the flue gas passage  14 . From the combustion zone  12 , the hot combustion gases flow into an optional reburn zone  20  into which additional (reburn) fuel  22  is supplied to the hot combustion gases to promote additional combustion. 
         [0019]    Downstream of combustion and reburn zones, overfire air (OFA)  24  is injected through an overfire air nozzle(s)  26  into the OFA burnout zone  28  in 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 gas  24  passes through a series of heat exchangers  30  and 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. 
         [0020]      FIG. 2  is a perspective view and  FIG. 3  is a side view, show in partial cross-section, of an overfire air injector assembly  32 . The air injector assembly forms the structure for the overfire air nozzles  26  shown in  FIG. 1 . The overfire air injector assembly  32  generally includes an OFA inlet port  34  that receives overfire air from the air duct  17  on an outer sidewall, e.g., the front and rear walls, of the structure  11  of the combustion system  10 . The air inlet port  34  may be arranged to face into the flow of the air in the duct  17 . For example, the inlet port may face downward into the upwardly flowing air in duct  17 . 
         [0021]    Turning vanes  36 , in the inlet port  34  of a hollow elbow conduit  42 , 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 structure  11  of the combustion system  10 . An annular flange  44  on the elbow conduit provides a coupling for a hollow frustoconical air duct  46  that extends towards a hollow cylindrical end section  48  of the overfire air injector assembly  32 . The cylindrical end section includes a flange  50  that provides a coupling mount for the assembly  32  to the wall of the structure  11  of the combustion system  10 . For example, the cylindrical end section  48  fits into a circular aperture in the structure wall and the flange  50  is bolted to a mounting ring on the wall and at the circumference of the wall aperture. 
         [0022]    The distal end  52  of overfire air injector assembly  32  is 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 end  52  and into the flue gas stream at the burnout zone  28 , as is shown in  FIG. 1 . An N-agent injector, e.g., a pipe (not shown) extending through and coaxial with the cylindrical end section  48 , is shown in  FIG. 1  and may be included in the overfire injector assembly  32 . 
         [0023]    An inner cylindrical air duct  54  extends through the frustoconical duct  46  and cylindrical end section  48 . The cylindrical air duct has an air outlet aligned with the distal end  52  of the cylindrical end section. The cylindrical air duct  54  has an inner overfire air passage  56  that extends through the duct from an inlet  58  to the duct. The duct inlet  58  may extend into the interior of a hollow elbow conduit  42 . An axially movable damper  60  for the air duct  54  is positioned at the inlet  58 . 
         [0024]    An annular outer overfire air duct  62  extends between the air duct  54  and an inner wall of the cylindrical end section  48  and an inner wall of the frustoconical duct  46 . A swirler  64 , e.g., radial array of vanes, may be positioned in the outer overfire air duct  62  to impart a rotation to the overfire air flowing through the outer duct  62 . While not shown, a swirler may be positioned in the inner overfire air passage  56 . An annular damper  66  may be near the inlet (aligned with flange  44 ) to the outer overfire air duct  62  to regulate the volumetric rate of overfire air through the duct  62 . The damper  66  may be adjusted, e.g., between closing offer overfire air flow to duct  62  and fully open to such air flow, by an actuator  40 . The actuator  40  may include a separate actuation arm and hydraulic servo for each damper/louver system controlled by the actuator  40 . 
         [0025]      FIG. 4  is a perspective view of the side and inlet end  58  of the inner cylindrical air duct  54 ,  FIG. 5  is a cross-sectional side view of the duct  54  near the inlet end  58 , and  FIG. 6  is cross-sectional view of duct  54  taken along line  6 - 6  in  FIG. 5 . 
         [0026]    The damper  60  is axially mounted on a damper control rod  68 . The control rod and damper may slide in and out of the inlet  58  of the inner cylindrical duct  54 . The damper  60  is shown fully open in  FIGS. 3 ,  4  and  5 . The damper shown in phantom lines and designated as in position  60   a  in  FIG. 5  is shown in a closed position that substantially closes off the overfire air flowing through duct  54 . 
         [0027]    Even with the damper  60  at the fully closed (see damper in position  60   a , a cooling gap  70  may be formed between the outer periphery of the damper  60  and the inner wall of inlet  58  to the duct  54 . Air passes through the cooling gap while the damper is in a closed position  60   a  to cool the end of the duct  54  which is exposed to the radiant heat energy of the combustion in the combustion system. 
         [0028]    The rod  68  is supported by a U-shaped mounting bracket  72  having legs  74  that attach to a quarl ring  76 . The quarl is a furstoconical metal collar that guides the overfire into the inlet  58  from the elbow conduit  42  ( FIG. 3 ). The quarl  76  may be fixed to the inlet  58  such as by welding. A radial spoke bracket  78  provides a mount for the damper rod  68  that is opposite to the mount provided by the U-shaped bracket. The spoke bracket  78  has narrow spokes, e.g., three spokes, each with an outer radial end attached to an inside surface of the duct  54 . The inner ends of the spokes support a cylindrical bearing that supports the rod  68 . 
         [0029]    An actuator  82  (See  FIGS. 2 and 3 ) moves the damper  60  and optionally the rod  68  to position the damper with respect to the inlet of the  58  of the inner cylindrical duct  54 . The damper may be moved axially with respect to the duct  54  by manually moving a hand lever (such as is shown in  FIG. 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 passage  56 . In the fully open damper position shown in  FIGS. 3 ,  4  and  5  (see reference numeral  60 ), the damper allows a maximum rate of overfire air to flow through the passage  56 . By advancing the damper axially along the axis of the rod  68 , the rate of overfire air entering the passage  56  can be progressively reduced. By advancing the damper to closed position  60   a , the rate of overfire air is minimized such that only a minimal volumetric rate of air flows through passage  56 . The actuator allows the duct to be positioned at any axial location between the fully open position (see reference numeral  60 ) and the fully closed position (see reference numeral  60   a ). 
         [0030]    The position of the damper  60  with respect to the inlet  58  may be adjusted to account for changes in the operation of the combustion system  10 . 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. 
         [0031]    The shape of the damper  60  may 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 duct  54 . The damper may be circular in front view and preferably has a front view shape substantially similar to the interior cross-sectional shape of duct  54 . The damper may have a simple, convex polygon shape as shown in  FIG. 5 , and may be shaped as a sphere, “football” in cross-section, oval in cross-section, or other shape that slides into the open inlet  58  of the of the duct  54 . 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 passage  56  is dependent on the position of the damper with respect to the inlet. Preferably, the distance that the damper  60  is advanced towards the inlet  58  is proportional, and most preferably linearly proportional, to the reduction or increase in the overfire air rate entering the passage  56 . 
         [0032]      FIG. 7  shows the prior art and is a perspective view of the side and inlet end of an inner overfire air passage  154  having an inlet end  158 . A conventional disc damper  160  (sometimes referred to as a “flapper”) is mounted on a rod  168  that is transverse to the axis of the duct  154 . By turning the rod  168 , the disc damper  160  can be rotated from a fully open position (as shown in  FIG. 7  and by the solid line damper  160  shown in  FIG. 8 ) to a fully closed position (shown by the broken line damper  160   a  shown in  FIG. 8 ). A radial post  178  stops the damper in a fully open position and a corner block  180  stops the damper in a fully closed position. In the fully closed position  160   a , a small annular cooling gap  170  remains between the outer perimeter of the disc and the inner wall of the inlet  158  to the duct  154 . The cooling gap allows a small amount of overfire air to flow through passage  156  to provide cooling to the inlet  158  which is exposed to the radiant heat of the combustion flames in the combustion system. 
         [0033]    The conventional disc damper  160  tends not to provide proportional flow control for the overfire air flowing through the passage  156 . 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 position  160   a.    
         [0034]    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 damper  60  shown in  FIGS. 2 to 6 . The damper  60  shown in  FIGS. 2 to 6  and the axial movement of the damper  60  provides proportional flow control because axial movement of the damper can proportionally adjust the volumetric flow rate of overfire air in the passage  56 . For example, a mid-point in the movement of the damper  56  along the axis of the rod  68  reduces the overfire air through passage  56  to about one-half the volumetric airflow of the passage when the damper is fully extended away from the inlet  58  (as is shown in  FIG. 5 ). One advantage of the axial movement and shape of damper  60  over the shape and rotational movement of damper  162  is proportional control of overfire air in passage  56 ,  156 . 
         [0035]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.