Patent Publication Number: US-8113001-B2

Title: Tubular fuel injector for secondary fuel nozzle

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to gas turbine combustors and, more particularly, to improvements in gas turbine combustors for reducing air pollutants such as nitrogen oxides (NOx). 
     Gas turbine engines typically include a compressor section, a combustor section, and at least one turbine section. The compressor compresses air that is mixed with fuel and channeled to the combustor. The mixture is then ignited generating hot combustion gases. The combustion gases are channeled to the turbine, which extracts energy from the combustion gases for powering the compressor, as well as for producing useful work to power a load, such as an electrical generator. 
     Existing dry low NOx (DLN) combustion systems have a secondary fuel nozzle that provides a flame that supports the primary flame. The fuel/air mixture coming out of the secondary fuel nozzle is not fully premixed and contributes to the NOx production from the gas turbine. 
     It would be desirable to increase the air/fuel mixedness in the secondary fuel nozzle to enable NOx reduction from the gas turbine. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an exemplary embodiment, a secondary fuel nozzle for a gas turbine includes a fuel manifold coupled with a plurality of annular fuel passages, and a tubular fuel injector in fluid communication with the fuel manifold and disposed surrounding the plurality of annular fuel passages. The tubular fuel injector includes a plurality of axially oriented air slots and a plurality of fuel injection holes disposed between the plurality of air slots. The plurality of fuel injection holes are oriented such that fuel from the fuel manifold is injected in at least a circumferential radial direction to mix with air flowing through the plurality of air slots. 
     In another exemplary embodiment, a secondary fuel nozzle for a gas turbine includes a fuel manifold coupled with a plurality of annular fuel passages, and a tubular fuel injector in fluid communication with the fuel manifold and disposed surrounding the plurality of annular fuel passages. The tubular fuel injector includes a plurality of axially oriented air slots disposed about a circumference of the tubular fuel injector and a plurality of fuel injection holes disposed between the plurality of air slots. The plurality of fuel injection holes include axially oriented injection holes and radially oriented injection holes such that fuel from the fuel manifold is injected in both a radial direction and an axial direction to mix with air flowing through the plurality of air slots. 
     In still another exemplary embodiment, a fuel injector is provided for a secondary fuel nozzle in a gas turbine. The fuel injector includes axially oriented air slots and a plurality of fuel injection holes disposed between the air slots. The plurality of fuel injection holes include axially oriented injection holes and radially oriented injection holes such that fuel input through the plurality of fuel injection holes is injected in both a radial direction and an axial direction to mix with air flowing through the air slots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a prior art known dry low NOx combustor; 
         FIG. 2  is a partial cross sectional view of a prior art secondary premixed/diffusion fuel nozzle; 
         FIG. 3  illustrates a peg arrangement for the prior art secondary fuel nozzle; 
         FIG. 4  illustrates the arrangement of fuel discharge holes in the peg of the prior art secondary nozzle; 
         FIG. 5  illustrates a prior art manifold for fuel premix; 
         FIG. 6  is a perspective view showing a fuel nozzle tubular fuel injector; and 
         FIG. 7  is a close-up view of the tubular fuel injector. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a prior art combustor for a gas turbine  12 , which includes a compressor  14  (partially shown), a plurality of combustors  16  (one shown for convenience and clarity), and a turbine represented by a single blade  18 . Although not specifically shown, the turbine  18  is drivingly connected to the compressor  14  along a common axis. The compressor  14  pressurizes inlet air, which is then reverse flowed to the combustor  16  where it is used to cool the combustor  16  and to provide air to the combustion process. Although only one combustor  16  is shown, the gas turbine  12  includes a plurality of combustors  16  located about the periphery thereof. A transition duct  20  connects the outlet end of each combustor  16  with the inlet end of the turbine  18  to deliver the hot products of combustion to the turbine  18 . 
     Each combustor  16  comprises a primary or upstream combustion chamber  24  and a secondary or downstream combustion chamber  26  separated by a venturi throat region  28 . The combustor  16  is surrounded by a combustor flow sleeve  30 , which channels compressor discharge air flow to the combustor. The combustor is further surrounded by an outer casing  31 , which is bolted to the turbine casing  32 . 
     Primary nozzles  36  provide fuel delivery to the upstream combustion chamber  24  and are arranged in an annular array around a central secondary nozzle  38 . Each of the primary nozzles  36  protrudes into the primary combustion chamber  24  through a rear wall  40 . Secondary nozzle  38  extends from a rear wall  40  to the throat region  28  in order to introduce fuel into the secondary combustion chamber  26 . Fuel is delivered to the primary nozzles  36  through fuel lines (not shown) in a manner well known in the art. 
     Combustion air is introduced into the fuel stage through air swirlers  42  positioned adjacent the outlet ends of nozzles  36 . The swirlers  42  introduce swirling combustion air, which mixes with the fuel from nozzles  36  and provides an ignitable mixture for combustion on startup, in chamber  24 . Combustion air for the swirlers  42  is derived from the compressor  14  and the routing of air between the combustion flow sleeve  30  and the wall  44  of the combustion chamber. The cylindrical wall  44  of the combustor is provided with slots or louvers  46  in the primary combustion chamber  24 , and similar slots or louvers  48  downstream of the secondary combustion chamber  26  for cooling purposes, and for introducing dilution air into the combustion zones to prevent substantial rises in flame temperature. The secondary nozzle  38  is located within a centerbody  50  and extends through a liner  52  provided with a swirler  54  through which combustion air is introduced for mixing with fuel from the secondary nozzle. 
     Referring now to  FIG. 2 , a gas-only secondary fuel nozzle assembly  56  is illustrated. Fuel is supplied to sustain a flame by diffusion pipe P 1  and to sustain a premixed flame by pipe P 2  which, at the inlet to the secondary fuel nozzle assembly  56 , are arranged concentrically relative to each other. 
     The following will primarily describe the premix fuel secondary nozzle assembly  56 . A rearward component, or gas body,  58  includes an outer sleeve portion  60  and an inner hollow core portion  62  provided with a central bore forming a premix fuel passage  64 . A plurality of axial air passages  68  are formed in a forward half of the rearward component  58  in surrounding relationship to the premix fuel passage  64 . A like number of radial wall portions (e.g., four) are arranged about the end of sleeve portion  60  and each includes an inclined, radial aperture  70  for permitting air within the liner  52  to enter a corresponding air passage  68 . The rearward end of component  58  is adapted to receive the fuel pipes P 1 , P 2 , respectively, as shown in  FIG. 2 , within a mounting flange  77 . 
     A plurality of radial holes  78  are provided about the circumference of the forward portion of component  58 , permitting a like number of radial gas injector tubes (pegs)  80  to be received therein to thereby establish communication with the premix fuel passage  64 . Each peg  80  is provided with a plurality of apertures or orifices  82  so that fuel from the premix passage  64  may be discharged into a premixing area  90  between the secondary nozzle assembly  56  and liner  52  for mixing with combustion air within the liner. The pegs  80  are designed to distribute fuel into the airflow. Good mixing of fuel and air in the premixing area  90  is necessary to reduce nitrogen-oxide (NOx) emissions. A flame holding swirler  116  which may or may not be integral with the nozzle is located at the forward end of the secondary nozzle, extending radially between the reduced diameter forward end  108  and the liner  52  for swirling the premixed fuel/air flowing within the liner. Combustion air will enter the secondary nozzle assembly  56  as shown by arrows in  FIG. 2  (above  38 ) and via holes  70 , and fuel will flow through the premix passage  64 , pilot bore and pilot orifice  98 . This fuel, along with air from swirler slots  96 , provides a diffusion flame sub-pilot. At the same time, a majority of the fuel supplied to the premix passage will flow into the gas injectors  80  for discharge from orifices  82  toward the liner  52  where it is mixed with air. 
     As illustrated in  FIGS. 3-4 , premixing of fuel with air as performed in prior art secondary fuel nozzles may include the plurality of pegs  80 , equally spaced around the periphery of the secondary nozzle body  75  in the premixing volume  90 . Each peg  80  may include a central cavity  85  running the length of the peg. The inner end of each peg may be attached to the nozzle body at the location of the radial fuel holes, thereby establishing communication between the fuel cavity in the nozzle body and the central cavity of the peg, as previously described with respect to  FIG. 2 . Along a downstream surface of the peg  80 , a plurality of the fuel discharge holes  82  are provided from the central internal cavity  85 , thereby providing for discharge of premix fuel into the airflow between the secondary nozzle body  75  and the liner  52 . Three radially-located fuel discharge holes  82  are provided along the downstream side of the peg  80 . Positioning of the hole location along the row of holes was varied. In this prior art secondary nozzle, six pegs are evenly distributed around the circumference of the secondary nozzle body  75 , with three orifices for fuel dispersal along the downstream side of the peg. However, the effective mixing of fuel and air is not complete. More complete mixing of the fuel and air can lead to lower NOx emissions and more stable combustion. 
     The above described nozzle construction provides for the sustained premixed mode of operation via a diffusion flame pilot. However, elevated emissions from a gas turbine is the result of insufficient mixing of air and fuel prior to burning in the combustion chamber. The existing peg design, described above, is not able to mix fuel and air properly to obtain the requisite degree of mixing for low emissions. Attempts to change the location of holes in the pegs have not been able to achieve satisfactory fuel and air mixing. 
       FIG. 5  illustrates a fuel distribution device  150  for a secondary fuel nozzle as described in U.S. Pat. No. 6,446,439 and U.S. Pat. No. 6,282,904 by Kraft et al. An annular fuel manifold  155  is mounted to a support sleeve  160  through support cylinders  165 . The manifold  155  presents a rectangular cross-section. The support sleeve  160  is affixed to the body of a secondary fuel nozzle (not shown) by welding. Fuel in the body of the secondary nozzle, passes through holes  170  in the support sleeve and through the support cylinders  165  into the hollow annular fuel manifold  155 . The annular fuel manifold  155  is positioned in an airstream  175  around secondary nozzle body (not shown). Fuel is distributed from the downstream face  180  of the annular fuel manifold through an array of apertures  185 . The apertures  185  may be at a first radial distance  186  or a second radial distance  187  within the airstream from a central axis  188 . The direction of the apertures  185  with respect to the airflow may be collinear or at an angle. However, the rectangular-shaped annulus limits the angles that the apertures may make with respect to the direction of the airstream. 
     The cylindrical-shaped annular fuel manifold  155  for fuel premix distribution may provide for radial and circumferential fuel distribution over the peg arrangement. However the annular manifold has limitations on mixing, stemming from the limited flow angles that may be created with respect to the airflow, and particularly with respect to the radial and axial distribution of fuel into the airstream. 
     Accordingly, there is a need to provide an alternate structure to improve the fuel-air premixing in the secondary nozzle to promote lower emissions and improved combustion dynamics. 
     With the existing fuel pegs used to inject fuel into the mainstream air, the axial length provided to mix fuel and air is not sufficient, and unmixedness remains until this fuel/air mixture enters the combustion zone. With reference to  FIGS. 6 and 7 , a tubular fuel injector  200  adds axial length to better mix fuel and air and also adds cross-flow injection of fuel to promote better mixing of fuel and air. 
     The tubular fuel injector  200  extends from the end cover assembly  130  and is in fluid communication with the fuel manifolds forming part of the end cover assembly  130 . The tubular fuel injector  200  is disposed surrounding the annular fuel passages of the fuel nozzle  132 . The tubular injector  200  includes a plurality of axially oriented air slots  202  and a plurality of fuel injection holes  204  disposed between the air slots  202 . With continued reference to  FIGS. 6 and 7 , the axially oriented air slots  202  are preferably formed in an oblong shape as shown with a major axis oriented in the axial direction. The air slots  202  are preferably evenly disposed about a circumference of the tubular fuel injector  200 . 
     The fuel injection holes  204  are oriented such that fuel from the fuel manifold is injected in at least a radial direction to mix with air flowing through the air slots  202 . Preferably, at least one of the fuel injection holes  204  is oriented axially such that fuel from the fuel manifold is injected in an axial direction to mix with the air flowing through the air slots  202 . In this context, the tubular fuel injector  200  includes an end surface  206  at a distal axial end (i.e., the end farthest from the end cover assembly  30 ). The axially oriented fuel injection holes  204  are shown disposed in the end surface  206 . 
     The fuel injection hole orientation thus provides a combination of cross flow and axial flow of fuel, which helps to improve the premixedness of fuel and air at the exit of the secondary fuel nozzle. Additionally, the pressure drop in the system is reduced, which helps to improve the gas turbine efficiency, resulting in more power produced for the same amount of fuel burnt. 
     The tubular fuel injector of the preferred embodiments provides added axial length for the fuel to mix with air providing for better mixedness. Additionally, the orientation of fuel injection holes provide for cross-flow injection of fuel into the air to provide a better mixture of fuel and air. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.