Abstract:
A dual fuel premix nozzle and method of operation for use in a gas turbine combustor is disclosed. The dual fuel premix nozzle utilizes a fin assembly comprising a plurality of radially extending fins for injection of gas fuel and compressed air in order to provide a more uniform injection pattern and homogeneous mixture. The premix fuel nozzle includes a plurality of coaxial passages, which provide gaseous fuel and compressed air to the fin assembly. When in liquid fuel operation, the gas circuits are purged with compressed air and liquid fuel and water pass through coaxial passages to the tip of the dual fuel premix fuel nozzle, where they inject liquid fuel and water into the secondary combustion chamber. An alternate embodiment includes an additional gas fuel injection region located along a conically tapered portion of the premixed fuel nozzle, downstream of the fin assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a fuel and air injection apparatus and method of operation for use in a gas turbine combustor for power generation and more specifically to a device that reduces the emissions of nitrogen oxide (NOx) and other pollutants by injecting gaseous fuel into a combustor in a premix condition while including liquid fuel capability. 
     2. Description of Related Art 
     In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of emissions, especially nitrogen oxide (NOx) and carbon monoxide (CO). Lower combustion emissions can be attributed to a more efficient combustion process, with specific regard to fuel injectors and nozzles. Early combustion systems utilized diffusion type nozzles that produce a diffusion flame, which is a nozzle that injects fuel and air separately and mixing occurs by diffusion in the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature. An improvement over diffusion nozzles is the utilization of some form of premixing such that the fuel and air mix prior to combustion to form a homogeneous mixture that bums at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. Some examples of prior art found in combustion systems that utilize some form of premixing are shown in FIGS. 1 and 2. 
     Referring to FIG. 1, a fuel nozzle  10  of the prior art for injecting fuel and air is shown. This fuel nozzle includes a diffusion pilot tube  11  and a plurality of discrete pegs  12 , which are fed fuel from conduit  13 . Diffusion pilot tube  11  injects fuel at the nozzle tip directly into the combustion chamber through swirler  14  to form a stable pilot flame. Though this pilot flame is stable, it is extremely fuel rich and upon combustion with compressed air, this pilot flame is high in nitrogen oxide (NOx) emissions. 
     Another example of prior art fuel nozzle technology is the fuel nozzle  20  shown in FIG. 2, which includes a separate, annular manifold ring  21  and a diffusion pilot tube  22 . Fuel flows to the annular manifold ring  21  and diffusion pilot tube  22  from conduit  23 . Diffusion pilot tube  22  injects fuel at the nozzle tip directly into the combustion chamber through swirler  24 . Annular manifold ring  21  provides an improvement over the fuel nozzle of FIG. 1 by providing an improved fuel injection pattern and mixing via the annular manifold instead of through radial pegs. The fuel nozzle shown in FIG. 2 is described further in U.S. Pat. No. 6,282,904, assigned to the same assignee as the present invention. Though this fuel nozzle attempts to reduce pollutant emissions over the prior art, by providing an annular manifold to improve fuel and air mixing, further improvements are necessary regarding a significant source of emissions, the diffusion pilot tube  22 . The present invention seeks to overcome the shortfalls of the fuel nozzles described above by providing a fuel nozzle that is completely premixed in the gas circuit, thus eliminating all sources of high NOx emissions, while providing the option for dual fuel operation through the addition of liquid fuel and water passages. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a fuel nozzle for a gas turbine engine that reduces NOx and other air pollutants during gas operation. 
     It is another object of the present invention to provide a premixed fuel nozzle with an injector assembly comprising a plurality of radially extending fins to inject fuel and air into the combustor such that the fuel and air premixes, resulting in a more uniform injection profile for improved combustor performance. 
     It is yet another object of the present invention to provide, through fuel hole placement, an enriched fuel air shear layer to enhance combustor lean blowout margin in the downstream flame zone. 
     It is yet another object of the present invention to provide a fuel nozzle for a gas turbine engine that is premixed when operating on gaseous fuel and has the additional capability of operating on liquid fuel. 
     It is yet another object of the present invention to provide a premixed fuel nozzle with improved combustion stability through the use of a plurality of fuel injection orifices located along a conical surface of the premixed fuel nozzle. 
     In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross section view of a fuel injection nozzle of the prior art. 
     FIG. 2 is a cross section view of a fuel injection nozzle of the prior art. 
     FIG. 3 is a perspective view of the present invention. 
     FIG. 4 is a cross section view of the present invention. 
     FIG. 5 is a detail view in cross section of the injector assembly of the present invention. 
     FIG. 6 is an end elevation view of the nozzle tip of the present invention. 
     FIG. 7 is a cross section view of the present invention installed in a combustion chamber. 
     FIG. 8 is a perspective view of an alternate embodiment of the present invention. 
     FIG. 9 is a detail view in cross section of an alternate embodiment of the injector assembly of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A dual fuel premix nozzle  40  is shown in detail in FIGS. 3 through 6. Dual fuel premix nozzle  40  has a base  41  with three through holes  42  for bolting premix fuel nozzle  40  to a housing  75  (see FIG.  7 ). Extending from base  41  is a first tube  43  having a first outer diameter, a first inner diameter, a first thickness, and opposing first tube ends. Within premix fuel nozzle  40  is a second tube  44  having a second outer diameter, a second inner diameter, a second thickness, and opposing second tube ends. The second outer diameter of second tube  44  is smaller than the first inner diameter of first tube  43  thereby forming a first annular passage  45  between the first and second tubes,  43  and  44 , respectively. Dual fuel premix nozzle  40  further contains a third tube  46  having a third outer diameter, a third inner diameter, a third thickness, and opposing third tube ends. The third outer diameter of third tube  46  is smaller than said second inner diameter of second tube  44 , thereby forming a second annular passage  47  between the second and third tubes  44  and  46 , respectively. Third tube  46  contains a third passage  57 . 
     Dual fuel premix nozzle  40  further comprises an injector assembly  49 , which is fixed to first and second tubes,  43  and  44 , respectively, at the tube ends thereof opposite base  41 . Injector assembly  49  includes a plurality of radially extending fins  50 , each of the fins having an outer surface, an axial length, a radial height, and a circumferential width. Each of fins  50  are angularly spaced apart by an angle α of at least 30 degrees and fins  50  further include a first radially extending slot  51  within fin  50  and a second radially extending slot  52  within fin  50 , a set of first injector holes  53  located in the outer surface of each of fins  50  and in fluid communication with first slot  51  therein. A set of second injector holes,  54  and  54 A are located in the outer surface of each of fins  50  and in fluid communication with second slot  52  therein. Fixed to the radially outermost portion of the outer surface of fins  50  to enclose slots  51  and  52  are fin caps  55 . Injector assembly  49  is fixed to nozzle  40  such that first slot  51  is in fluid communication with first passage  45  and second slot  52  is in fluid communication with second passage  47 . Premix nozzle  40  further includes a fourth tube  80  having a generally conical shape with a tapered outer surface  81 , a fourth inner diameter, and opposing fourth tube ends. Fourth tube  80  is fixed at fourth tube ends to injector assembly  49 , opposite first tube  43  and second tube  44 , and to third tube  46 . The fourth inner diameter of fourth tube  80  is greater in diameter than the third outer diameter of third tube  46 , thereby forming a fourth annular passage  82 , which is in fluid communication with second passage  47 . 
     Nozzle  40  further includes the capability of operating under dual fuel conditions, gas or liquid fuel, through the use of additional concentric tubes. Within third tube  46  is a fifth tube  56  having a fifth outer diameter, a fifth inner diameter, a fifth thickness, and opposing fifth tube ends. The outer diameter of fifth tube  56  is smaller than the inner diameter of third tube  46  such that third passage  57 , which is formed between third tube  46  and fifth tube  56 , is annular in shape. The fifth tube  56  further includes a means for engagement  60 , such as threading, located at the fifth tube end proximate base  41 . Located coaxial to and within fifth tube  56  is sixth tube  61 . Sixth tube  61  has a sixth outer diameter, a sixth inner diameter, a sixth thickness, and opposing sixth tube ends. The outer diameter of sixth tube  61  is smaller than the inner diameter of fifth diameter  56  thereby forming a fifth annular passage  62 . Sixth tube  61  further includes a swirler  63  located on its outer diameter at a sixth tube end, proximate the nozzle tip cap assembly  59 , such that a swirl is imparted to the fluid flowing through fifth annular passage  62 . A means for engagement  64  is located at an end of sixth tube  61 , opposite of swirler  63 . Sixth tube  61  also contains a passage  65  contained within its inner diameter. When assembled, fifth tube  56  and sixth tube  61  are each fixed to housing  75 , shown in FIG. 7, through the means for engagement  60  and  64 , respectively. In order to allow fifth tube  56  and sixth tube  61  to fit within nozzle tip cap assembly  59 , the cap assembly, which is fixed to fourth tube  80 , has a seventh outer diameter and seventh inner diameter such that the seventh inner diameter has substantially the same inner diameter as that of third tube  46 . The use of a conical shaped tube as fourth tube  80  allows a smooth transition in flow path between injector assembly  49  and cap assembly  59  such that large zones of undesirable recirculation, downstream of fins  50 , are minimized. If the recirculation zones are not minimized, they can provide an opportunity for fuel and air to mix to the extent that combustion occurs and is sustainable upstream of the desired combustion zone. 
     The dual fuel premix nozzle  40 , in the present embodiment, injects fluids, such as natural gas and compressed air, or liquid fuel, water, and compressed air, depending on the mode of operation, into a combustor of a gas turbine engine for the purposes of establishing a premix pilot flame and supporting combustion downstream of the fuel nozzle. One operating embodiment for this type of fuel nozzle is in a dual stage, dual mode combustor similar to that shown in FIG. 7. A dual stage, dual mode combustor  70  includes a primary combustion chamber  71  and a secondary combustion chamber  72 , which is downstream of primary chamber  71  and separated by a venturi  73  of reduced diameter. Combustor  70  further includes an annular array of diffusion type nozzles  74  each containing a first annular swirler  76 . In the gas only combustor operation, the dual fuel premix nozzle  40  of the present invention is located along center axis A-A of combustor  70 , upstream of second annular swirler  77 , and is utilized as a secondary fuel nozzle to provide a pilot flame to secondary combustion chamber  72  and to further support combustion in the secondary chamber. In gas operation, flame is first established in primary combustion chamber  71 , which is upstream of secondary combustion chamber  72 , by an array of diffusion-type fuel nozzles  74 , then a pilot flame is established in secondary combustion chamber  72  when fuel and air are injected from nozzle  40 . Gaseous fuel flow is then increased to secondary fuel nozzle  40  to establish a more stable flame in secondary combustion chamber  72 , while flame is extinguished in primary combustion chamber  71 , by cutting off fuel flow to diffusion-type nozzles  74 . Once a stable flame is established in secondary combustion chamber  72  and flame is extinguished in primary combustion chamber  71 , fuel flow is restored to diffusion-type nozzles  74  and fuel flow is reduced to secondary fuel nozzle  40  such that primary combustion chamber  71  now serves as a premix chamber for fuel and air prior to entering secondary combustion chamber  72 . The present invention, as operated on gas fuel, will now be described in detail with reference to the particular operating environment described above. 
     In the preferred embodiment, nozzle  40  operates in a dual stage dual mode combustor  70 , where nozzle  40  serves as a secondary fuel nozzle. The purpose of the nozzle is to provide a source of flame for secondary combustion chamber  72  and to assist in transferring the flame from primary combustion chamber  71  to secondary combustion chamber  72 . In this role, the second passage  47 , second slot  52 , and second set of injector holes  54  and  54 A flow a fuel, such as natural gas into plenum  78  where it is mixed with compressed air prior to combusting in secondary combustion chamber  72 . During engine start-up, first passage  45 , first slot  51 , and first set of injector holes  53  flow compressed air into the combustor to mix with the gaseous fuel. In an effort to maintain machine load condition when the flame from primary combustion chamber  71  is transferred to secondary combustion chamber  72 , first passage  45 , first slot  51 , and first set of injector holes  53  flow fuel, such as natural gas, instead of air, to provide increased fuel flow to the established flame of secondary combustion chamber  72 . Once the flame is extinguished in primary combustion chamber  71  and securely established in secondary combustion chamber  72 , fuel flow through the first passage  45 , first slot  51 , and first set of injector holes  53  of premix nozzle  40  is slowly cut-off and replaced by compressed air, as during engine start-up. 
     NOx emissions are reduced through the use of this premix nozzle by ensuring that all fuel that is injected is thoroughly mixed with compressed air prior to reaching the flame front of the combustion zone. This is accomplished by the use of the fin assembly  49  and through proper sizing and positioning of injector holes  53 ,  54 , and  54 A. Thorough analysis has been completed regarding the sizing and positioning of the first and second set of injector holes, such that the injector holes provide a uniform fuel distribution. To accomplish this task, first set of injector holes  53 , having a diameter of at least 0.050 inches, are located in a radially extending pattern along the outer surfaces of fins  50  as shown in FIG.  3 . To facilitate manufacturing, first set of injector holes  53  have an injection angle relative to the fin outer surface such that fluids are injected upstream towards base  41 . Second set of injector holes, including holes  54  on the forward face of fins  50  and  54 A on outer surfaces of fin  50 , proximate fin cap  55 , are each at least 0.050 inches in diameter. Injector holes  54 A are generally perpendicular to injector holes  54 , and have a slightly larger flow area than injector holes  54 . Second set of injector holes  54  and  54 A are placed at strategic radial locations on fins  50  so as to obtain an ideal degree of mixing which both reduces emissions and provides a stable shear layer flame in secondary combustion chamber  72 . To further provide a uniform fuel injection pattern and to enhance the fuel and air mixing characteristics of the premix nozzle, all fuel injectors are located upstream of second annular swirler  77 . 
     Dual fuel premix nozzle  40  can operate on either gaseous fuel or liquid fuel, and can alternate between the fuels as required. Depending on gas fuel cost, gas availability, scheduled operating time, and emissions regulations, it may advantageous to operate on liquid fuel. When dual fuel premix nozzle  40  is operating in a liquid mode in a dual stage dual mode combustor, the annular array of diffusion type nozzles  74  of FIG. 7 are also operating on liquid fuel. Both the diffusion type nozzle  74  and dual fuel premix nozzle  40  alternate between liquid and gas fuels together. In the preferred embodiment of a dual stage dual mode combustor, when operating on liquid fuel, the start-up sequence to the combustor is similar to that of the gas fuel operation, but when increasing in load to full power, fuel nozzle operating conditions are slightly different. Liquid fuel is first flowed to the diffusion type nozzles  74  and a flame is established in primary combustion chamber  71 . Liquid flow is then decreased to diffusion nozzles  74  while it is directed to the dual fuel premix nozzle  40  to establish a flame in secondary combustion chamber  72 . The fuel flow is maintained in both the diffusion nozzles  74  and dual fuel premix nozzle  40  as the engine power increases to full base load condition, with flame in both the primary and secondary combustion chambers,  71  and  72 , respectively. At approximately 50% load condition, water can be injected into the combustion chambers, by way of the fuel nozzles, to lower the flame temperature, which in turn reduces NOx emissions. 
     With specific reference to the nozzle embodiment disclosed in FIGS. 3-6 in the liquid fuel operating condition, liquid fuel passes through passage  65  of sixth tube  61  and injects fuel into secondary combustion chamber  72 . Mixing with the liquid fuel in secondary combustion chamber  72 , at load conditions above 50%, is a spray of water that is also injected by nozzle  40 . Water flows coaxial to sixth tube  61  through fifth tube  56  via fifth annular passage  62 , and exits nozzle  40  in a swirling pattern imparted by swirler  63 , which is positioned in fifth annular passage  62 . Passages  45  and  47 , slots  51  and  52 , and first and second sets of injector holes  53 ,  54 , and  54 A, which flowed either natural gas or compressed air in the gas mode operation each flow compressed air in liquid operation to purge the nozzle passages such that liquid fuel does not recirculate into the gas or air passages. 
     An alternate embodiment of the present invention is shown in FIGS. 8 and 9. The alternate embodiment includes all of the elements of the preferred embodiment as well as a fourth set of injector holes  83 , which are in communication with fourth annular passage  82  of fourth tube  80 . These injector holes provide an additional source of gas fuel for combustion. The additional gas fuel from fourth set of injector holes  83  premixes with fuel and air, from injector assembly  49 , in passage  78  (see FIG. 7) to provide a more stable flame, through a more fuel rich premixture, in the shear layer of the downstream flame zone region  90 . Fourth set of injector holes  83  are placed about the conical surface  81  of fourth tube  80 , between injector assembly  49  and cap assembly  59 , and have a diameter of at least 0.025 inches. 
     While the invention has been described in what is known as presently the preferred embodiment, it is to be understood that one skilled in the art of combustion and gas turbine technology would recognize 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 within the scope of the following claims.