Patent Publication Number: US-9851107-B2

Title: Axially staged gas turbine combustor with interstage premixer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     TECHNICAL FIELD 
     The present invention generally relates to an apparatus and method for enhancing combustion efficiency, increasing turndown and reducing nitrous oxide (NOx) and carbon monoxide (CO) emissions through axially staged combustion. More specifically, the present invention is directed towards a gas turbine combustion liner and way of injecting fuel and air into a combustion liner after a first stage of combustion has occurred. 
     BACKGROUND OF THE INVENTION 
     In a typical gas turbine engine, a compressor having alternating stages of rotating and stationary airfoils is coupled to a turbine, which also has alternating stages of rotating and stationary airfoils. The compressor stages decrease in size, and as the volume decreases, the air passing therethrough is compressed, raising its temperature and pressure. The compressed air is then supplied to one or more combustors which mixes the air with fuel and ignites the mixture to form hot combustion gases. The hot combustion gases are directed into a turbine, where the expansion of the hot combustion gases drives the stages of a turbine, which is in turn, coupled to the compressor to drive the compressor. The exhaust gases can then be used as a source of propulsion, as typical in an aircraft engine, or in powerplant operations to turn a shaft coupled to a generator for producing electricity. 
     The exact type and size of combustion systems used in a gas turbine engine can vary depending on a variety of factors such as engine geometry, performance requirements, and fuel type. Each combustor typically includes at least one fuel injection means and ignition source. The gas turbine engine may have a single combustor or a series of individual or inter-connected combustors. 
     Combustion systems however do not always burn all of the fuel particles or do not completely burn the fuel particles, which results in higher emissions. Therefore, what is needed is a way of more completely mixing and burning the fuel particles to obtain the maximum energy output from the burned fuel while minimizing the resulting emissions. 
     SUMMARY 
     In accordance with the present invention, there is provided a novel and improved method and apparatus for an axially staged combustion system. The combustion system comprises a combustion liner having a first combustion chamber, a transition duct in communication with the combustion liner and a premixer positioned generally axially between the combustion liner and the transition duct. The premixer comprises a plurality of channels and a plurality of fuel injectors positioned proximate the channels for injecting fuel into the channels to mix with a passing air flow. 
     In an alternate embodiment, a premixer for injecting a fuel/air mixture into a combustor downstream of a first combustion chamber is disclosed. The premixer comprises a plurality of vanes oriented in both a tangential and axial direction, forming channels therebetween, and a plurality of fuel injectors positioned proximate the channels such that fuel and air pass through the channels positioned radially outward of the combustion liner, is imparted with a swirl, mix and is directed radially inward proximate an outlet end of the combustion liner. 
     In yet another embodiment of the present invention, a method of providing low emission operation for a gas turbine combustor is disclosed. The method comprises providing a flow of fuel and air to form a first fuel/air mixture and burning the first fuel/air mixture within the first combustion chamber. The method also includes providing a flow of fuel and air through a premixer to generate a second fuel/air mixture proximate an inlet region of a transition duct, where the second fuel/air mixture is mixed and auto-ignited with the hot combustion gases from the first combustion chamber. 
     Additional advantages and features of the present invention will be set forth in part in a description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from practice of the invention. The instant invention will now be described with particular reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention is described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a cross section view of a combustion system of a gas turbine engine of the prior art; 
         FIG. 2  is a cross section view of a combustion system of a gas turbine engine in accordance with an embodiment of the present invention; 
         FIG. 3  is a cross section view of a combustion system in accordance with an alternate embodiment of the present invention; 
         FIG. 4  is a detailed cross section view of a portion of the combustion system of  FIG. 2  in accordance with an embodiment of the present invention; 
         FIG. 5  is a partial cross section view of the premixer portion of the combustion system of  FIG. 2  in accordance with an embodiment of the present invention; 
         FIG. 6  is a perspective view of an aft portion of the combustion system of  FIG. 2  in accordance with an embodiment of the present invention; 
         FIG. 7  is an alternate perspective view of the aft portion of the combustion system of  FIG. 6  in accordance with an embodiment of the present invention; 
         FIG. 8  is a side elevation view of the aft portion of the combustion system of  FIG. 7  in accordance with an embodiment of the present invention; 
         FIG. 9  is a detailed elevation view of a channel in the premixer in accordance with an embodiment of the present invention; and, 
         FIG. 10  is a flow diagram outlining a process for providing low emissions for an axially staged combustion system in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The subject matter of the present invention is described with specificity herein to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different components, combinations of components, steps, or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. 
     Referring initially to  FIG. 1 , a cross section view of a gas turbine combustion system  100  of the prior art is depicted. The typical gas turbine combustion system  100  includes a casing  102  coupled to a compressor discharge plenum  104 . Contained within the casing  102  is a combustion liner  106  and one or more fuel injectors  108 . The fuel injectors are typically secured to and are in fluid communication with a cover  110 , which also provides an end to the casing  102 . Fuel and compressed air from a compressor (not shown) mix and burn within the combustion liner  106  with the resulting hot combustion gases discharged through a duct  112 . Air from compressor plenum  104  passes along an outer wall of the combustion liner  106  as the air is directed towards the forward end of the combustor. 
     The present invention is shown in detail in  FIGS. 2-10  and can be applied to a variety of gas turbine combustion systems, as shown in  FIGS. 2 and 3 . The present invention provides an apparatus and method for providing high combustor efficiency and low nitrous oxide operation of a gas turbine combustor through an axially staged combustion system. Referring initially to  FIG. 2 , a gas turbine combustion system  200  in accordance with an embodiment of the present invention is shown in cross section. The combustion system  200  comprises an outer case  202  secured to a compressor discharge casing  204 . Contained within the outer case  202  and discharge casing  204  is a flow sleeve  206  and a combustion liner  208 . The flow sleeve  206  regulates the quantity of air provided for the combustion process as well as to straighten the flow of air passing along the combustion liner  208  to better direct the air for cooling of the combustion liner and for use in the combustion process. More specifically, the flow sleeve  206  regulates the quantity of air utilized through a series of metering holes  210  positioned about an aft end of the flow sleeve  206 . 
     The combustion liner  208  has an inlet end  212 , an opposing outlet end  214 , and a first combustion chamber  216  positioned therebetween. The combustion liner  208  is in fluid communication with a transition duct  218 , which receives the hot combustion gases from the combustion liner  208  and directs the gases into an inlet of a turbine (not shown). 
     As shown in  FIG. 4 , the outlet end  214  of the combustion liner  208  passes the exhaust of hot combustion gases to a premixer  220 , which is positioned generally between the combustion liner  208  and the transition duct  218 . The premixer  220  provides a homogeneously mixed flow of fuel and air to a second combustion stage  222  that is spaced axially downstream from the first combustion chamber  216 , but upstream of the transition duct  218 . 
     Referring now to  FIGS. 4-6 , the premixer  220  will be discussed in greater detail. The premixer  220  has an annular opening  221  through which compressed air enters and is directed into a plurality of channels  224 , which are spaced a distance apart, as shown in  FIGS. 5, 6 and 9 , and formed between vanes  225 . Referring to  FIGS. 4 and 5 , and as will be discussed below, the premixer  220  also has a plurality of fuel injectors  226  for directing fuel into one or more of the channels  224 , where channels  224  are formed between vanes  225 . For the embodiment shown in  FIGS. 4, 5, 7, and 8 , there are 24 equally spaced channels  224  in the premixer  220  with the channels  224  being oriented in both an axial and tangential direction to induce a swirl and enhance mixing of the air passing therethrough. However, it is to be understood that the exact size, shape, orientation, and spacing of the channels can vary depending on specific combustor requirements. For example, it is envisioned that the quantity of channels  224  could vary from approximately twelve channels to approximately 48 channels. 
     The channels  224  are important to the overall effectiveness of the premixer  220  by providing axial, circumferential, and radial mixing. However, the channels  224  can vary in size and shape from a channel opening  226  to a channel outlet  228 . That is, for the embodiment shown, the channel  224  has an axial, tangential and radial component, but the exact size, shape, and quantity of channels can vary. As shown in  FIGS. 7-9 , in which a portion of the premixer outer wall is removed for clarity, the channel  224  generally maintains a constant slot height, which for the embodiment shown, is approximately two inches. However, this slot height can vary in both height and taper for alternate embodiments of the present invention. 
     Channel  224  also has a slot length, which for the embodiment of  FIG. 5 , is the total length extending from annular opening  221  to outlet  228 . As for the width of channel  224 , the channel width can vary. In one embodiment, the channel  224  has a first slot width of approximately one inch, but then tapers to approximately 0.9 inches wide at a second slot width, which is located a short distance axially downstream of the fuel injectors  226 . The channel  224  then tapers to a larger channel opening to provide a velocity of approximately 50 meters per second or greater at the channel outlet  228 , or discharge plane, with the taper of the channel occurring at approximately a five degree angle. The five degree angle permits expansion of the fuel/air mixture while ensuring the flow within the channel  224  does not separate as separation of the flow can cause a flame to anchor in the premixer  220 . That is, the effective throat of the channel  224  can taper, either in a width dimension, a height dimension or both, in order to accelerate flow starting at inlet  221  through a channel area reduction to prevent flashback. However, depending on operating requirements, it is possible that the channel  224  does not need to taper. 
     In the embodiment of the present invention shown in  FIGS. 4-6 , the channel  224  also has a bottom surface, which is generally flat or generally conical. However, as discussed above, the specific geometry of the channel  224  can vary depending on the desired performance for the premixer component. More specifically, because the premixer  220  is passing a fuel/air mixture into a second combustion stage  222 , where, upon interaction of the fuel/air mixture with the hot combustion gases, auto-ignition occurs due to the high temperatures of the hot combustion gases. It is important that the channel has geometry such that the fuel/air mixture maintains a velocity of at least 50 meters per second in order to maintain sufficient margin to prevent a flashback from occurring. Depending on fuel composition, this value can be significantly higher. 
     As discussed above, the premixer  220  also includes a plurality of fuel injectors  226  for supplying fuel to an air stream to form the second fuel/air mixture. The fuel injectors  226  can be seen most clearly in  FIGS. 4 and 5 . An annular fuel manifold  230  is positioned radially outward of the channels  224  and contains a supply of fuel. Fuel injectors  226  are positioned to pass the fuel from the manifold  230  into one or more of the channels  224 . The exact quantity, size, spacing, and injection angle of fuel injectors  226  relative to the channels  224  will vary depending on the crossflow through the channels  224  and penetration requirements for when the second fuel/air mixture enters the second combustion stage  222 . For example, in the embodiment depicted in  FIGS. 4-7 , there are three fuel injectors  226  in the manifold  230  supplying fuel to each channel  224 , with the fuel being injected at approximately a 30 degree surface angle. The fuel is injected at an angle in this embodiment to avoid separation and recirculation after the point of fuel injection, so as to avoid any possibility of flame holding. The fuel injectors  226  are also positioned so as to not be directly exposed to hot combustion gases from the combustion liner in order to protect the fuel injectors and fuel manifold from damage that could occur due to the hot temperatures of the combustion gases as well as damage from an auto-ignition and burning of fuel within the premixer  220 . 
     The premixer  220  is positioned generally between the combustion liner  208  and transition duct  218 . However, as shown in  FIGS. 2 and 4 , a portion of the premixer  220 , is positioned radially outward of the outlet end  214  of the combustion liner  208 . More specifically, the flow of the fuel and air through the channels  224  of the premixer  220 , in addition to being imparted with at least a partial radial component due to the angles of the channels  224 , is also directed from the premixer  220  radially inward into the second combustion stage  222 . The forward and aft ends of the premixer  220  are positioned generally between the combustion liner  208  and the transition duct  218 , such that the combustion liner  208  is secured to the forward end of the premixer  220  while the transition duct  218  is secured to the aft end of the premixer  220 . 
     Referring now to  FIG. 5 , the premixer  220  may include additional flame stabilization features, such as a converging orifice plate  244  with a sudden expansion, aft of the channel opening to create a recirculation zone at the entrance of the second combustor. 
     The combustion system  200  also comprises one or more fuel injectors positioned to inject a flow of fuel to mix with air within the combustion liner  208 . This first fuel/air mixture is ignited and burns in the first combustion chamber  216 , with the hot combustion gases formed as a result of the burning being directed axially downstream towards the outlet end  214  of the combustion liner  208 . A variety of fuel types can be burned in the combustion system  200 , including, but not limited to gaseous fuel or liquid fuel. 
     In other embodiments of the present invention, it is envisioned that fuel injectors  226  may not be placed within every channel  224 , but could be spaced in alternating channels or in another pre-determined pattern. Furthermore, alternate embodiments of the present invention may have a single or multiple fuel injectors  226  in their respective channel and the angle of fuel injection may also vary from the 30 degree angle of the embodiment shown in  FIGS. 4 and 5 . 
     In order to provide a combustion system capable of improved mixing and ensuring sufficient durability, it is necessary to configure the premixer  220  such that only the mixing of fuel and air occurs proximate the channel outlet  228  and there is no ignition. That is, ignition of the mixture from the premixer  220  should be restricted to the second combustion stage  222 . 
     The present invention is also directed towards a method of providing low nitrous oxide and carbon monoxide operation for a gas turbine combustor that also provides increased turndown. The gas turbine combustor has a combustion liner with a first combustion chamber and a premixer is positioned proximate the outlet end of the combustion liner for providing a subsequent fuel/air mixture to the hot combustion gases from the first combustion chamber. The method  1000 , which is outlined in  FIG. 10 , comprises providing a flow of fuel and air to form a first fuel/air mixture in a step  1002 . Then, in a step  1004 , the first fuel/air mixture is burned to form hot combustion gases in the combustion liner. In a step  1006 , a flow of fuel and air is provided through the premixer for generating a second fuel/air mixture. This second fuel/air mixture is injected into a second combustion stage which is positioned proximate an inlet region of the transition duct. Then, in a step  1008 , the second fuel/air mixture is mixed with the hot combustion gases from the combustion liner and this mixture is auto-ignited and burned in a step  1010 . 
     The present invention is not limited to use with a type of gas turbine combustor depicted in  FIG. 2 , but instead can be applied to a variety of combustion systems. For example, the present invention can be applied to a variety of commercially-available combustion systems, including, but not limited to, a single axially stage combustor  300 , such as a Dry-Low NOx 2.0/2.6 combustion system on the Frame  7 FA gas turbine engine produced by the General Electric Company and as depicted in  FIG. 3 . As discussed above, the exact size and shape of the premixer portion of the present invention will vary depending on the type of upstream combustion system. 
     The result of the process described herein uses the premixer to create an axially staged combustor with more complete burning of the fuel particles, leading to low Nox and CO emissions. Furthermore, the arrangement provides for increased turndown, allowing the engine to operate at lower load settings. 
     Due to the proximity of the premixer  220  to the combustion liner  208  and the associated need for the components to thermally expand and contract together, it is preferable that the premixer  220  be fabricated from materials capable of withstanding the operating temperatures of the combustion liner  208 . Therefore, such acceptable materials for the premixer  220  can include a nickel-based alloy. As shown in  FIGS. 2 and 4 , a portion of the premixer  220  is positioned axially between the combustion liner  208  and the transition duct  218 . Therefore, in addition to the premixer  220  being fabricated from high temperature capable materials, depending on the operating conditions of the combustion system, the inner surface of the discharge end of the premixer  220  may also be coated with a thermal barrier coating for providing additional capability against the high operating temperatures. The coating applied to a portion of the premixer, would be comparable to that also applied to the adjacent combustion liner and transition duct. 
     The present invention has been described in relation to particular embodiments, which are intended in all respects to be illustrative rather than restrictive. Alternative embodiments and required operations, such as machining of shroud faces other than the hardface surfaces and operation-induced wear of the hardfaces, will become apparent to those of ordinary skill in the art to which the present invention pertains without departing from its scope. 
     From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects set forth above, together with other advantages which are obvious and inherent to the system and method. It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and within the scope of the claims.