Abstract:
The present invention discloses a novel apparatus and method for a mixing fuel and air in a gas turbine combustion system. The mixer helps to mix fuel and air while being able to selectively increase the fuel flow to a shear to a shear layer of a pilot flame in order to reduce polluting emissions. The mixer directs a flow of air radially inward into the combustion system and includes two sets of fuel injectors within each radially-oriented vane. A first plurality of fuel injectors operate independent of a second plurality of fuel injectors and the second plurality of fuel injectors are positioned to selectively modulate the fuel flow to the shear layer of the resulting pilot flame.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/708,323 filed on Oct. 1, 2012. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention generally relates to a system and method for improving combustion stability and reducing emissions in a gas turbine combustor. More specifically, improvements in a combustor premixer and fuel injection location are provided. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location and mixing effectiveness. 
         [0004]    Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics. 
         [0005]    An enhancement in combustion technology is the utilization of premixing, such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns 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. An example of a premixing combustor of the prior art is shown in  FIG. 1 . A combustor  8  has a plurality of fuel nozzles  18 , each injecting fuel into a premix cavity  19  where fuel mixes with compressed air  6  from plenum  10  before entering combustion chamber  20 . Premixing fuel and air together before combustion allows for the fuel and air to form a more homogeneous mixture, which will burn more completely, resulting in lower emissions. However, in this configuration the fuel is injected in relatively the same plane of the combustor, and prevents any possibility of improvement through altering the mixing length. 
         [0006]    An alternate means of premixing and lower emissions can be achieved through multiple combustion stages, which allows for enhanced premixing as load increases. Referring now to  FIG. 2 , an example of a prior art multi-stage combustor is shown. A combustor  30  has a first combustion chamber  31  and a second combustion chamber  32  separated by a venturi  33 , which has a narrow throat region  34 . While combustion can occur in either first or second combustion chambers or both chambers, depending on load conditions, the lowest emissions levels occur when fuel, which is injected through nozzle regions  35 , is completely mixed with compressed air in first combustion chamber  31  prior to combusting in the second combustion chamber  32 . Therefore, this multi-stage combustor with a venturi is more effective at higher load conditions. 
         [0007]    Gas turbine engines are required to operate at a variety of power settings. Where a gas turbine engine is coupled to drive a generator, required output of the engine is often measured according to the amount of load on the generator, or power that must be produced by the generator. A full load condition is the point where maximum generating capacity is being drawn from the generator. This is the most common operating point for land-based gas turbines used for generating electricity. However, often times electricity demands do not require the full capacity of the generator, and the operator desires to operate the engine at a lower load setting, such that only the load demanded is being produced, thereby saving fuel and lowering operating costs. Combustion systems of the prior art have been known to become unstable at lower load settings, especially below 50% load, while also producing unacceptable levels of NOx and CO emissions. This is primarily due to the fact that most combustion systems are staged for most efficient operation at high load settings. The combination of potentially unstable combustion and higher emissions often times prevents engine operators from running engines at lower load settings, forcing the engines to either run at higher settings, thereby burning additional fuel, or shutting down, and thereby losing valuable revenue that could be generated from the part-load demand. 
         [0008]    A further problem with shutting down the engine is the additional cycles that are incurred by the engine hardware. A cycle is commonly defined as the engine passing through the normal operating envelope. Engine manufacturers typically rate hardware life in terms of operating hours or equivalent operating cycles. Therefore, incurring additional cycles can reduce hardware life requiring premature repair or replacement at the expense of the engine operator. What is needed is a system that can provide flame stability and low emissions benefits at a part load condition, as well as at a full load condition, such that engines can be efficiently operated at lower load conditions, thereby eliminating the wasted fuel when high load operation is not demanded or incurring the additional cycles on the engine hardware when shutting down. 
       SUMMARY 
       [0009]    The present invention discloses a mixer for premixing fuel and air prior to combustion in combination with precise staging of fuel flow to the combustor to achieve reduced emissions at multiple operating load conditions. The mixer operates so as to selectively increase the fuel flow to a boundary layer of a pilot flame, thereby increasing the stability of the pilot flame for use in ignition of other fuel injected into the combustor. More specifically, in an embodiment of the present invention, a premixer for a gas turbine combustor is disclosed. The premixer comprises an end cover having multiple fuel plenums contained therein and a radial inflow swirler. The radial inflow swirler comprises a plurality of vanes oriented at least partially perpendicular, relative to the longitudinal axis of the combustor. The plurality of vanes each have a plurality of fuel injectors in fluid communication with the multiple fuel plenums of the end cover. The premixer further comprises an inner wall and outer wall, both of which extend from a direction generally perpendicular to the longitudinal axis and transition to a direction generally parallel with the longitudinal axis. 
         [0010]    In an alternate embodiment of the present invention, a method of tuning a pilot flame in a gas turbine combustor is disclosed. The method comprises providing a cover for the combustor having multiple fuel plenums and passageways for flowing fuel from the plenums. The method also provides a radially inflowing swirler coupled to the cover and having a plurality of vanes oriented in a generally radial direction relative to a combustor axis where each vane has a plurality of fuel injectors with the fuel injectors in fluid communication with a first fuel plenum and a second fuel plenum where the fuel from the second fuel plenum is controlled independent of the fuel from the first fuel plenum so as to provide a radial staging of fuel to the fuel injectors within each of the vanes. 
         [0011]    In yet another embodiment of the present invention, a method of operating a combustion system to improve ignition of the combustor main fuel injectors is provided. The method provides for a way of increasing the fuel/air ratio to a shear layer of the pilot flame through fuel injection through a second set of fuel injectors such that a main combustion flame can be more easily lit upon injection of fuel from the main set of fuel injectors. 
         [0012]    The premixer of the present invention is positioned within a combustor casing, where the combustor has a longitudinal axis, and the casing is in fluid communication with the engine compressor. In an embodiment of the invention, the premixer includes a radial inflow swirler having a plurality of fuel injectors with staged fuel injection so as to modulate the fuel/air mixture in a shear layer for igniting fuel injected by a main set of fuel injectors. 
         [0013]    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 
         [0014]    The present invention is described in detail below with reference to the attached drawing figures, wherein: 
           [0015]      FIG. 1  is a cross section view of a gas turbine combustion system of the prior art. 
           [0016]      FIG. 2  is a cross section view of an alternate gas turbine combustion system of the prior art. 
           [0017]      FIG. 3  is a cross section view of a combustion system in accordance with an embodiment of the present invention. 
           [0018]      FIG. 4  is a perspective view of a portion of the combustion system in accordance with an embodiment of the present invention. 
           [0019]      FIG. 5  is a cross section view of the portion of the combustion system of  FIG. 4  in accordance with an embodiment of the present invention. 
           [0020]      FIG. 6  is an end view of the portion of the combustion system of  FIG. 4  in accordance with an embodiment of the present invention. 
           [0021]      FIG. 7  is a cross section view of an end cover and swirler portion of the combustion system of  FIG. 3  in accordance with an embodiment of the present invention. 
           [0022]      FIG. 8  is a detailed cross section view of a portion of the end cover and swirler depicted in in  FIG. 7  in accordance with an embodiment of the present invention. 
           [0023]      FIG. 9  depicts the process of operating a combustion system in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    By way of reference, this application incorporates the subject matter of U.S. Pat. Nos. 6,935,116, 6,986,254, 7,137,256, 7,237,384, 7,308,793, 7,513,115, and 7,677,025. 
         [0025]    The preferred embodiment of the present invention will now be described in detail with specific reference to  FIGS. 3-9 . Referring now to  FIG. 3 , a gas turbine combustion system  300  in accordance with an embodiment of the present invention is shown. Combustion system  300  is mounted to a casing (not shown), which is coupled to a compressor plenum of an engine for receiving compressed air from a compressor. 
         [0026]    The combustion system  300  extends about a longitudinal axis A-A and includes a flow sleeve  302  for directing a predetermined amount of compressor air along an outer surface of combustion liner  304 . Main fuel injectors  306  are positioned radially outward of the combustion liner  304  and are designed to provide a fuel supply to mix with compressed air along a portion of the outer surface of the combustion liner  304 , prior to entering the combustion liner  304 . 
         [0027]    Extending generally along the longitudinal axis A-A is a pilot fuel nozzle  308  for providing and maintaining a pilot flame for the combustion system. The pilot flame is used to ignite, support and maintain multiple stages of fuel injectors of combustion system  300 . 
         [0028]    Referring now to  FIGS. 3-5 , the combustion system  300  also includes a radially staged premixer  310 .  FIG. 4  shows a perspective view of the radial premixer  310  while  FIG. 5  shows a cross section of the radial premixer  310 . The premixer  310  comprises an end cover  312  having a first fuel plenum  314  extending about the longitudinal axis A-A of the combustion system  300  and a second fuel plenum  316  positioned radially outward of the first fuel plenum  314  and concentric with the first fuel plenum  314 . 
         [0029]    The radially staged premixer  310  also comprises a radial inflow swirler  318  comprising a plurality of vane  320  that are oriented in a direction that has at least a partial radial component thereto relative to the longitudinal axis A-A of the combustion system  300 . The radial orientation serves to direct airflow from the outer portions of the combustion system  300  inward into the combustor and towards the longitudinal axis A-A. The vanes  320  may also have a circumferential angle to them as shown by the swirler  318  of  FIG. 6 . The circumferential angle of the vanes  320  serves to help impart an angular momentum to the radially inward flow in order to enhance mixing of fuel and air. The vanes  320 , as depicted in FIGS.  4  and  6 - 8  have a generally rectangular cross section. However, the vanes  320  can have different cross sections such as an airfoil-shaped cross section, depending on the geometry of the radially staged premixer, fuel passageways, and manufacturing techniques. 
         [0030]    Referring now to  FIGS. 7 and 8 , the plurality of vanes  320  of swirler  318  each have a first plurality of fuel injectors  322  and a second plurality of fuel injectors  324 . That is, for the embodiment of the present invention depicted in  FIGS. 7 and 8 , each vane  320  has three fuel injectors  322  and a second fuel injector  324 . First plurality of fuel injectors  322  are in fluid communication with the first fuel plenum  314  in end cover  312  by way of a first passage  323  while the second plurality of fuel injectors  324  are in fluid communication with the second fuel plenum  316  by way of a second passage  325 . As such, the amount of fuel being injected by respective vanes  320  can be independently controlled through the first injectors  322  and second injectors  324 . 
         [0031]    In the embodiment of the invention disclosed in  FIGS. 7 and 8 , the first passage  323  is generally parallel to the longitudinal axis A-A, while the second passage  325  is oriented at an angle relative to the longitudinal axis A-A. The exact orientation of the first passage  323  and second passage  325  can vary depending on the size and shape of the end cover  312  and radial inflow swirler  318 . 
         [0032]    The exact size and spacing of the first plurality of fuel injectors  322  and second plurality of fuel injectors  324  can vary depending on the amount of fuel to be injected. For the embodiment shown in  FIG. 8 , the injector holes are generally perpendicular to the exit plane of the vanes  320 . The diameter of injector holes  322  and  324  can vary, but are generally in the range of approximately 0.030 inches-0.200 inches. 
         [0033]    The radial inflow swirler  318  further comprises a pair of walls extending from adjacent the plurality of vanes  320  in a direction which is initially generally perpendicular to the longitudinal axis A-A, thereby forming a premix passage  330 . The pair of walls comprise an inner wall  332  and an outer wall  334 , with the outer wall  334  spaced a distance from the inner wall  332  approximately equal to the axial length of the vane  320 . The inner wall  332  and outer wall  334  transition towards a direction that is generally parallel to the longitudinal axis A-A. For the embodiment depicted in  FIG. 5 , the premix passage  330  formed by the inner wall  332  and outer wall  334  maintains a generally constant cross section and provides a region in which fuel from the plurality of vanes  320  can mix with surrounding airflow. The inner wall  332  is essentially formed by a portion of the end cover  312  and the pilot nozzle while the outer wall  334  is fabricated from a formed sheet metal. However, it is envisioned that the inner wall  332  and outer wall  334  could each be separate from the end cover  312  and the geometry of the premix passage  330  can also vary, as may be required to provide the necessary fuel/air mixture to the combustion system  300 . 
         [0034]    The present invention provides a combustion system operable in a manner so as to improve ignition of the main injectors for the combustion system. Referring to  FIG. 9 , a method  900  of operating the combustion system to improve ignition of a main set of injectors is provided. 
         [0035]    In a step  902 , a flow of fuel is provided from the first fuel plenum and through a first set of fuel injectors of a radial inflow swirler in order to mix with a passing airflow. The fuel/air mixture travels through the premix passage and discharges into the combustion chamber, where in a step  904 , a pilot flame is established along the longitudinal axis of the combustor. The pilot flame is supported with fuel from the radial inflow swirler. 
         [0036]    As one skilled in the art understands, a flame inherently contains a shear layer. Generally speaking, a shear layer, or boundary layer is a region of flow in which there can be significant velocity gradient. The shear layer of a flame is the shared region between the outermost edge of the flame and the non-flammable surroundings or an adjacent flame. 
         [0037]    In a step  906 , fuel from the second plenum is directed through a second set of fuel injectors of the radial inflow swirler. By directing a supply of fuel to the second injectors in each of the vanes of the swirler, additional fuel is directed to the radially outward most region of the premix passage, adjacent the passage outer wall, and therefore increases the amount of fuel along the shear layer so that fuel/air ratio is locally increased. In operation, when fuel is supplied to the second injectors, this represents a fuel flow increase of approximately 5%-50% over the amount of fuel flowing through only the first set of fuel injectors of the radial inflow swirler. 
         [0038]    In a step  908 , fuel is provided to a main set of fuel injectors. For the embodiment of the present invention depicted in  FIG. 3 , the main set of fuel injectors comprises a set of annular fuel injectors positioned about the combustion liner  304  so as to inject a flow of fuel upstream and into a passing air stream. The fuel from the main injectors ignites as a result of the pilot flame, enhances the shear layer, and establishes a main combustion flame in a step  910 . 
         [0039]    As a result of the present invention, ignition of fuel from a main set of fuel injectors can occur more easily and reliably due to the ability to control the fuel/air ratio of the shear layer of the pilot flame. More specifically, by locally increasing the supply of fuel at an outermost radial location in the premix passage, the concentration of fuel in the shear layer of the resulting pilot flame is increased. As a result, the richened shear layer allows the main injectors to more easily and reliably ignite without the need for a lot of energy, which then results in lower pulsation levels during ignition of the main fuel injectors. 
         [0040]    An additional benefit of being able to locally richen the fuel flow to the shear layer is the ability to maintain a stable process of igniting the fuel being injected by the main injectors. That is, in a premixed combustion system, fuel flow levels are traditionally kept as lean as possible in order to reduce emissions. By locally adding fuel to the shear layer during a selective time period, a more fuel-rich mixture is established, thereby increasing the fuel/air ratio in the shear layer region. A more fuel-rich mixture provides more favorable conditions for ignition to occur and increases the stability of the flame. Once the flame is ignited, then the level of fuel richness can be reduced to a leaner mixture without jeopardizing the stability of the flame. 
         [0041]    Yet another benefit recognized through the radially fuel staging of the present invention is with respect to combustion noise. Combustion noise is a by-product of the combustion process. More specifically, fluctuations in the combustion process create unsteadiness in the heat release rate which generate sound. Combustion noise is also generated by non-uniformities in temperature due to unsteady combustion. Typically, leaner flames, or flames resulting from leaner fuel-air mixtures have generally more tendency for fluctuations and instabilities due to their lower levels of fuel. The shear layer region of a flame is typically sensitive to fuel/air mixture modulation. By modulating the fuel flow to the shear layer, the fuel/air mixture in the shear layer is more fuel-rich or fuel-lean, which can be an effective measure for reducing combustion instabilities. 
         [0042]    For example, for an embodiment of the present invention, noise levels associated with the combustion process disclosed herein without additional fuel provided to the shear layer of the pilot flame can result in generally high sound pressure levels at certain transient operating conditions. However, with the additional fuel provided to the shear layer, tests have shown combustion noise levels reduced to approximately 33% during the same transient operating conditions. 
         [0043]    While the invention has been described in what is known as presently the 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 within the scope of the following claims. 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. 
         [0044]    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.