Abstract:
A fuel injector is cooled by steam during operation of a combustor. The injector includes a conduit for channeling fuel into the combustor, and a jacket covering the conduit through which steam is channeled for cooling the injector. The steam may be mixed with the fuel, or kept apart from the fuel in the injector by flowing through passageways in the jacket. Cooling can be further improved by expanding the steam through an orifice prior to entry into the injector.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to industrial power turbines, and, more specifically, to low NOx combustors therein. 
     Industrial gas turbine engines may be used alone for producing power by rotating an electrical generator, for example, or may be used in a combined cycle with a steam turbine. Industrial gas turbines are continually being developed for increasing thermal efficiency, increasing specific work, reducing exhaust emissions, and reducing overhead and running costs associated therewith. 
     Higher thermal efficiency may be effected by increasing the firing temperature of the combustor. However, the higher combustion gas temperature increases the difficulty of reducing NOx (nitrogen oxide) emissions therefrom. NOx is one of several undesirable exhaust emissions, also including unburned hydrocarbons and carbon monoxide, which are reduced by various means. 
     Axially staged combustion is one method for reducing undesirable exhaust emissions while increasing the firing temperature. NOx emissions can be reduced by this method when compared to a single stage combustor. Axial staging is effected by providing fuel injection at several axial locations in a combustor correspondingly configured for this purpose. Primary fuel and air is injected at the upstream or dome end of the combustor in a first stage. As required, for meeting high power operation, additional or secondary fuel and air are injected at an axially downstream location to provide axially staged combustion. 
     An axially staged combustor provides low NOx operation in a dry configuration without the complexity of steam injection used in past generation industrial power turbines. However, the second stage, or secondary, fuel injectors required in axial staged combustion are necessarily located downstream in the combustor and are subject to heating by the combustion gases first generated by burning of the fuel and air mixture from the primary fuel injectors. 
     The secondary fuel injectors may be cooled using the fuel flow through these injectors or a portion of compressor bleed air for the injectors, but these techniques are of limited efficacy. During low power operation, the secondary fuel injectors may not be called upon to provide substantial fuel flow, so that insufficient fuel is available for cooling the secondary fuel injectors. Moreover, bleeding of compressor air for cooling the secondary injectors correspondingly decreases the overall efficiency of the engine. 
     Accordingly, it would be desirable to provide improved cooling of a secondary fuel injector in a dry, low NOx, axially staged combustor. 
     BRIEF SUMMARY OF THE INVENTION 
     A fuel injector is cooled by steam during operation of a combustor. The injector includes a conduit for channeling fuel into the combustor, and a jacket covering the conduit through which the steam is channeled for cooling the injector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial schematic view of a portion of an industrial gas turbine engine including a compressor, combustor, and turbine with fuel injector cooling in accordance with one embodiment of the invention. 
     FIG. 2 is an enlarged, partly sectional, elevational view of a steam cooled fuel injector for the combustor illustrated in FIG.  1 . 
     FIG. 3 is a partly sectional, elevational view of an alternative embodiment of the steam cooled fuel injector illustrated in FIG.  2 . 
     FIG. 4 is a partly sectional, elevational view of another alternative embodiment of the steam cooled fuel injector illustrated in FIG.  2 . 
     FIG. 5 a perspective view of another alternative embodiment of the fuel injector illustrated in FIG. 2, shown in a portion of the combustor in which it is employed. 
     FIG. 6 a partly sectional alternative embodiment of the fuel injector illustrated in FIG. 2 for use in a stepped combustor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a portion of an industrial gas turbine engine  10  configured for powering an electrical generator (not shown), for example. Engine  10  is axisymmetrical about a longitudinal or axial centerline axis  12  and includes in serial flow communication a multistage axial compressor  14 , combustor  16 , and high pressure turbine  18 . 
     Air  20  is pressurized in compressor  14  and mixed with fuel  22  in the combustor for forming a fuel and air mixture  24  that is ignited to generate hot combustion gases  26  which flow downstream to turbine  18 . The turbine extracts energy from the hot combustion gases for powering compressor  14 . The engine also includes a power turbine (not shown) disposed downstream from high pressure turbine  18  which may be joined to the generator for producing electrical power. 
     Combustor  16  is in the exemplary form of a canannular combustor having a plurality of circumferentially adjoining tubular combustor cans which are joined together at the downstream ends in a common annulus for channeling the combustion gases to turbine  18 . 
     In FIG. 1, combustor  16  is configured for axially staging fuel  22  in axially spaced-apart primary or upstream fuel injectors  28 , and secondary or downstream fuel injectors  30 . Except for secondary fuel injectors  30 , engine  10  may be conventional in configuration and operation for effecting dry, low NOx emission combustion. 
     For example, primary fuel injectors  28  may be disposed in groups at the corresponding dome ends of the combustor cans in the form of premixers wherein fuel  22  is mixed with swirled air  20  and discharged in corresponding fuel and air mixtures  24  which are ignited for generating hot combustion gases  26 . Primary injectors  28  are operated full time from engine idle through high power operation, and secondary injectors  30  are operated only when desired for providing additional power above idle operation. The primary and secondary injectors allow the combustor to axially stage the fuel for maximizing combustor efficiency while minimizing exhaust emissions, including NOx. 
     Secondary fuel injectors  30  may be of various configurations, but are subject to heating by hot combustion gases  26  formed upstream therefrom. Since the secondary injectors may be operated with little or no fuel, depending upon operating requirements, suitable cooling must be employed to achieve useful life for these injectors without damage due to excessive heating during operation. 
     Accordingly, and in accordance with a preferred embodiment of the invention, secondary fuel injectors  30  in various configurations are cooled by bathing the individual injectors  30  in steam  32  produced in a suitable steam generator  34  which, for example, may be a boiler producing steam for a combined cycle steam turbine (not shown). Since steam  32  has a substantially lower temperature than that of hot combustion gases  26 , the steam is effective for cooling secondary injectors  30  against the heating effects of gases  26  during operation of the combustor. 
     In a gas turbine engine with axially staged combustion, the second stage fuel (or fuel and air mixture) injectors  30  are protected from overheating by using steam cooling. Steam cooling also delays onset of autoignition and prevents formation of carbon deposits. Delay of autoignition ensures proper mixing so that low NOx levels may be obtained. 
     There are several embodiments for steam cooling secondary injectors  30  in either open or closed steam circuits. In open circuit steam cooling, the steam is injected into the combustor after cooling the corresponding secondary injectors, whereas in closed circuit steam cooling, the steam is channeled separately from the fuel and air mixture and returned to steam generator  34  for reuse. When the steam is injected into the combustion gases, the fuel can be burned in a diffusion mode with steam NOx control, or the steam may be used for power augmentation of the gas turbine output if excess steam flow is permitted. 
     In the open circuit embodiments, steam temperature may be reduced by pre-expansion through an orifice, and the steam may also be used for film cooling the secondary injectors. Steam cooling effectiveness may be increased by varying the specific configuration of the various secondary injectors, and by including heat convection enhancers, such as turbulators, in the injectors. 
     Secondary injectors  30  may be of various configurations for channeling steam  32  in order to achieve cooling with varying fuel flow through the secondary injectors ranging from 0 to 100% of injector capacity, the fuel flow being controlled by the thermodynamic process required of the combustor and not by the cooling requirements of the fuel injectors themselves. Cooling is independently effected using steam  32  irrespective of any cooling capability of fuel  22  channeled through the secondary injectors during operation. 
     A first, relatively simple embodiment of the secondary fuel injector is illustrated in FIG.  2 . The injector includes a tubular conduit  36  disposed at one end  31  of injector  30  in flow communication with a suitable fuel supply (not shown), for receiving fuel  22 , such as natural gas. Conduit  36  includes a side inlet through which a portion of compressed air  20  is received from compressor  14  (FIG. 1) for premixing with fuel  22  inside conduit  36 . The resulting fuel and air mixture  24  is discharged from an outlet at the distal end of conduit  36 . 
     A tubular jacket  38  covers the distal end portion of fuel conduit  36  and includes a radially-directed inlet  33  at end  31  disposed in flow communication with steam generator  34  to receive steam  32 . Steam generator  34 , together with corresponding conduits joined to jackets  38  of the secondary injectors, define means for channeling the steam through the jackets to cool the secondary injectors. 
     In the embodiment illustrated in FIG. 2, secondary fuel injectors  30  (only one of which is shown) extend in part into the combustion zone of combustor  16  (FIG. 1) in which combustion gases  26  flow during operation. Jacket  38  effectively forms an extension of fuel conduit  36  and is immersed in the flowing combustion gases inside the combustor. The fuel injector is bathed in the steam channeled therein for cooling the injector against the heating effect of the combustion gases. In this way, secondary injectors  30  are steam cooled downstream from primary injectors  28  and are effective for providing dry, low NOx combustion in axial stages without combustion gases from the first stage damaging the fuel injectors of the second stage. 
     In the FIG. 2 embodiment, the fuel and air are premixed in fuel conduit  36  and discharged into the plenum defined by the surrounding tubular jacket  38 , which also allows the fuel and air mixture to be premixed with cooling steam  32 . Jacket  38  includes an axial row of outlet holes  40  disposed in flow communication with the outlet of fuel conduit  36  for injecting both the fuel/air mixture  24  and steam  32  into the combustor. In this way, the steam is injected into the combustor for mixing with the combustion gases after cooling the secondary injectors in an open cycle. 
     In the open cycle, steam  32  is preferably pre-expanded in a corresponding orifice  42  in the outlet path of steam generator  34  prior to delivery to jacket  38  and prior to cooling secondary injector  30 . Pre-expansion of the steam reduces its temperature and therefore increases the cooling effectiveness of the steam resulting in more effective cooling of the secondary injectors. 
     The amount of steam cooling provided by steam injection correspondingly varies in response to the amount of fuel injected through the secondary injectors. At maximum fuel flow, a minimum amount of steam injection is required for cooling since the fuel itself provides cooling of the injector. At minimum or no secondary fuel flow through injectors  30 , a maximum amount of steam cooling and injection is required for cooling the secondary injectors. However, the maximum amount of steam injection is generally equivalent to the cooling capability of the maximum amount of fuel flow in order to minimize the need for steam injection and maximize overall efficiency of the engine. 
     FIG. 3 illustrates an alternative embodiment of secondary fuel injectors  44 , which similarly include a tubular fuel conduit  36  through which air  20  and fuel  22  are channeled in premixture  24 , with a surrounding tubular jacket  38  through which steam  32  is channeled. In this embodiment, the fuel and steam are channeled independently along the secondary injector in the space provided between concentric jacket  38  and conduit  36  therein. 
     Conduit  36  preferably includes a plurality of tubular fuel outlets  46  disposed in an axial row through corresponding ones of outlet holes  40  in jacket  38  for injecting the fuel into the combustor independently of the steam. In this embodiment, conduit  36  and jacket  38  have common or concentric outlets  46 ,  40 , respectively, for discharging together both the fuel and air mixture  24  and steam  32  into the combustor. The jacket holes  40  are larger than fuel outlets  46  for discharging steam  32  into the combustor around respective ones of fuel outlets  46 . Thus the steam is used to cool the fuel and air mixture  24  in a co-flowing arrangement, with the steam being ultimately injected into the combustor along with the fuel and air mixture. The steam passes through the annulus surrounding conduit  36  and acts as a shield for the fuel and air mixture flowing in conduit  36 , which preferably extends the full length of jacket  38  so that it is fully immersed in the cooling steam  32  within jacket  38 . 
     For an exemplary gas turbine combustor having a combustion gas temperature of about 2700° F., the amount of steam flow for cooling the secondary fuel injectors is relatively small, and no greater than about a few pounds per second of flow, as compared with over a thousand pounds per second of flow of air through the combustor for maintaining the temperature of the secondary injectors well below 1000° F. 
     FIG. 4 illustrates yet another embodiment of one of the secondary fuel injectors  48 , which is similar to the FIG. 3 embodiment but specifically configured for removing the steam from the combustor after the steam has cooled the secondary injectors without mixing with the combustion gases in a closed cycle. In this embodiment, jacket  38  is sealingly joined to fuel outlets  46  around outlet holes  40  and therefore sealingly surrounds internal conduit  36  for preventing discharge of the steam into the combustor. Only the fuel and air mixture  24  is discharged into the conductor. In this way, steam  32  may be circulated through the secondary injectors for cooling the injectors and then returned to steam generator  34  in a closed loop for reuse. 
     If desired, any one of the several secondary fuel injectors, such as injector  48 , may include a plurality of ribs or turbulators  50  disposed inside jacket  38  on the inner side, outer surface, or both, of conduit  36 , for increasing heat transfer and cooling of both conduit  36  and jacket  38 . Turbulators  50  trip the steam flow to enhance convection and thereby enhance cooling effectiveness. 
     FIG. 5 illustrates yet another embodiment of one of the secondary fuel injectors  52 , which again include fuel conduits  36  extending through corresponding jackets  38 . The jackets include outlets  40  sealingly joined with fuel outlets  46  of inner conduit  36  in a manner identical to that illustrated in FIG. 4, but the fuel injectors extend completely across the individual combustor can  16  in an independent and crossing configuration for again channeling steam  32  in a closed loop. 
     The fuel injectors may have individual circuits for feeding the steam and returning the steam to the steam generator  34 . Alternatively, the injectors may be linked across the combustor to create a network of feed and return lines. In a closed steam circuit, the energy recovered by using the steam for first cooling the fuel injectors may be applied to other parts of the gas turbine cycle, such as in an associated steam turbine. 
     FIG. 6 illustrates yet another embodiment of one of the secondary fuel injectors  54 , wherein combustor  16  includes a wall having a step or ramp  56  which faces in the downstream or aft direction, as shown by the horizontal arrow. The secondary injectors are spaced around the circumference of the combustor and each includes a fuel conduit  36  for injecting the premixed fuel and air into the combustor. Conduit  36  is disposed outside the combustor wall in flow communication with the combustor through an outlet  58  in the step. 
     Jacket  38  is in the form of a sheet metal baffle which adjoins a portion of outer surface of the combustor and through which fuel conduit  36  extends. Steam  32  is channeled through jacket  38  for cooling the jacket and is also channeled through the local portion of the combustor at fuel conduit  36  for cooling the entire secondary injector  54  with steam. Outlet  58  is suitably larger than conduit  36  for allowing a portion of cooling steam  32  to be injected into the combustor in an open cycle. If desired, outlet  58  may be sealingly joined to the outlet end of conduit  36  to prevent discharge of the steam into the combustor in effecting a closed cycle. 
     The various embodiments disclosed above introduce steam cooling of the secondary fuel injectors in various forms in open or closed cycles as desired. Relatively little steam is required for effectively cooling the injectors, and in the open cycle does not adversely affect the thermodynamic cycle. Axially staged dry, low NOx combustion is effected with or without steam discharge into the combustor, while cooling the corresponding secondary injectors. 
     Steam cooling may also be applied to other forms of the secondary fuel injectors with increasing levels of complexity compared with the relatively simple fuel injectors disclosed above. 
     While only certain preferred features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.