Patent Publication Number: US-2023136865-A1

Title: Methods of operating a turbomachine combustor on hydrogen

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. Non-Provisional patent application Ser. No. 17/513,066 having a filing date of Oct. 28, 2021, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     FIELD 
     The present disclosure relates generally to methods of operating a turbomachine combustor on hydrogen. In particular, the present disclosure relates to methods of burning high levels of hydrogen and/or exclusively hydrogen within a gas turbine combustor. 
     BACKGROUND 
     Turbomachines are utilized in a variety of industries and applications for energy transfer purposes. For example, a gas turbine engine generally includes a compressor section, a combustion section, a turbine section, and an exhaust section. The compressor section progressively increases the pressure of a working fluid entering the gas turbine engine and supplies this compressed working fluid to the combustion section. The compressed working fluid and a fuel (e.g., typically natural gas) mix within the combustion section and burn in a combustion chamber to generate high pressure and high temperature combustion gases. The combustion gases flow from the combustion section into the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a rotor shaft connected, e.g., to a generator to produce electricity. The combustion gases then exit the gas turbine via the exhaust section. 
     Traditional gas turbine engines include one or more combustors that burn a mixture of natural gas and air within the combustion chamber to generate the high pressure and temperature combustion gases. As a byproduct, oxides of nitrogen (NOx) and other pollutants are created and expelled by the exhaust section. Regulatory requirements for low emissions from gas turbines are continually growing more stringent, and environmental agencies throughout the world are now requiring even lower rates of emissions of NOx and other pollutants from both new and existing gas turbines. 
     Burning a mixture of natural gas and high amounts of hydrogen and/or burning pure hydrogen instead of natural gas within the combustor would significantly reduce or eliminate the emission of NOx and other pollutants. However, because hydrogen burning characteristics are different than those of natural gas, traditional combustion systems and methods are not capable of burning high levels of hydrogen and/or pure hydrogen without issue. For example, burning high levels of hydrogen and/or pure hydrogen within a traditional combustion system could promote flashback or flame holding conditions in which the combustion flame migrates towards the fuel being supplied by the nozzles, possibly causing severe damage to the nozzles in a relatively short amount of time. 
     As such, an improved method for burning high levels of hydrogen and/or pure hydrogen within a gas turbine combustor is desired and would be appreciated in the art. 
     BRIEF DESCRIPTION 
     Aspects and advantages of the methods in accordance with the present disclosure will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the technology. 
     In accordance with one embodiment, a method of operating a combustor of a turbomachine on a total fuel input that contains a concentration of hydrogen that is greater than about 80% to generate outlet combustion gases having an outlet temperature is provided. The combustor includes a combustion chamber that has a primary combustion zone and a secondary combustion zone. The method includes injecting, with at least one fuel nozzle, a first mixture of air and a first fuel containing hydrogen into the primary combustion zone of the combustor to generate a first flow of combustion gases having a first temperature. The method further includes injecting, with one or more premix injectors disposed downstream of the fuel nozzles, a second mixture of air and a second fuel containing hydrogen into the secondary combustion zone of the combustor as a cross-flow to generate a second flow of combustion gases having a second temperature. The method further includes separately injecting a third fuel into secondary combustion zone. The third fuel ignites and mixes with the first flow and the second flow of combustion gases to generate outlet combustion gases having a third temperature. 
     In accordance with another embodiments, a method of operating a combustor is provided. The method includes injecting, with at least one fuel nozzle, a first mixture of air and a first fuel into a primary combustion zone of the combustor to generate a first flow of combustion gases having a first temperature. The method further includes injecting, with one or more premix injectors disposed downstream of the fuel nozzles, a second mixture of air and a second fuel into a secondary combustion zone of the combustor as a cross-flow to generate a second flow of combustion gases having a second temperature. The method further includes separately injecting a third fuel into the secondary combustion zone. The third fuel ignites and mixes with the first flow and the second flow of combustion gases to generate outlet combustion gases having a third temperature. 
     In accordance with yet another embodiment, a method of operating a combustor of a turbomachine on a total fuel input that contains a 100% concentration of hydrogen to generate outlet combustion gases having an outlet temperature is provided. The combustor includes a combustion chamber that has a primary combustion zone and a secondary combustion zone. The method includes injecting, with at least one fuel nozzle, a first mixture of air and hydrogen into the primary combustion zone of the combustor to generate a first flow of combustion gases having a first temperature. The method further includes injecting, with one or more premix injectors disposed downstream of the fuel nozzles, a second mixture of air and hydrogen into the secondary combustion zone of the combustor to generate a second flow of combustion gases having a second temperature. The method further includes separately injecting a flow of pure hydrogen into the combustion chamber of the combustor to generate a third flow of combustion gases having a third temperature. 
     These and other features, aspects, and advantages of the present methods will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the technology and, together with the description, serve to explain the principles of the technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present methods, including the best mode of making and using the present systems and methods, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG.  1    is a schematic illustration of a turbomachine, in accordance with embodiments of the present disclosure; 
         FIG.  2    illustrates a cross-sectional view of a combustor suitable for use in the turbomachine of  FIG.  1   , in accordance with embodiments of the present disclosure; 
         FIG.  3    illustrates a bundled tube fuel nozzle suitable for use in the combustor of  FIG.  2   , in accordance with embodiments of the present disclosure; 
         FIG.  4    illustrates a fuel injection assembly suitable for use in the combustor of  FIG.  2   , in accordance with embodiments of the present disclosure; 
         FIG.  5    illustrates a graph of flame temperature over outlet temperature vs. the percentage of hydrogen present in a total fuel input to a combustor, in accordance with embodiments of the present disclosure; 
         FIG.  6    is a graph of a percentage of total fuel input supplied to immersed injectors vs. the percentage of hydrogen present in the total fuel input to the combustor, in accordance with embodiments of the present disclosure; 
         FIG.  7    is a flow diagram of a method of operating a combustor of a turbomachine on a total fuel input that contains a concentration of hydrogen that is greater than about 80% to generate outlet combustion gases having an outlet temperature, in accordance with embodiments of the present disclosure; 
         FIG.  8    is a flow diagram of a method of operating a combustor, in accordance with embodiments of the present disclosure; and 
         FIG.  9    is a flow diagram of a method of operating a combustor of a turbomachine on a total fuel input that contains a 100% concentration of hydrogen to generate outlet combustion gases having an outlet temperature, in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the present methods, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation, rather than limitation of, the technology. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present technology without departing from the scope or spirit of the claimed technology. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other implementations. Additionally, unless specifically identified otherwise, all embodiments described herein should be considered exemplary. 
     The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the subject technology. As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. 
     The term “fluid” may refer to a gas or a liquid. The term “fluid communication” means that a fluid is capable of flowing or being conveyed between the areas specified. 
     As used herein, the terms “upstream” (or “forward”) and “downstream” (or “aft”) refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows, and “downstream” refers to the direction to which the fluid flows. The term “radially” refers to the relative direction that is substantially perpendicular to an axial centerline of a particular component, the term “axially” refers to the relative direction that is substantially parallel and/or coaxially aligned to an axial centerline of a particular component, and the term “circumferentially” refers to the relative direction that extends around the axial centerline of a particular component. 
     Terms of approximation, such as “about,” “approximately,” “generally,” and “substantially,” are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value, or the precision of the methods or machines for constructing or manufacturing the components and/or systems. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value, or the precision of the methods or machines for constructing or manufacturing the components and/or systems. For example, the approximating language may refer to being within a 1, 2, 4, 5, 10, 15, or 20 percent margin in either individual values, range(s) of values and/or endpoints defining range(s) of values. When used in the context of an angle or direction, such terms include within ten degrees greater or less than the stated angle or direction. For example, “generally vertical” includes directions within ten degrees of vertical in any direction, e.g., clockwise or counter-clockwise. 
     The terms “coupled,” “fixed,” “attached to,” and the like refer to both direct coupling, fixing, or attaching, as well as indirect coupling, fixing, or attaching through one or more intermediate components or features, unless otherwise specified herein. The terms “directly coupled,” “directly fixed,” “directly attached to,” and the like indicated that a first component is joined to a second component with no intervening structures. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive-or and not to an exclusive- or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     Here and throughout the specification and claims, range limitations are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. For example, all ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. 
     As used herein, the term “pure hydrogen” may refer to a gas containing almost entirely (or entirely) hydrogen not mixed with air or other oxidants, such as greater than 90% hydrogen with some natural contaminants or 100% hydrogen containing little to no contaminants. Also, as used herein, “pure fuel” may refer to a mixture of fuels in the absence of air or other oxidants, such as hydrogen and natural gas (e.g., 80% hydrogen and 20% natural gas, or such as 70% hydrogen and 30% natural gas, or such as 60% hydrogen and 40% natural gas, or such as 50% hydrogen and 50% natural gas). The natural gas may be ethane, propane, methane, or others. Additionally, as used herein, the term “hydrogen” may refer to diatomic hydrogen (H 2 ), such as hydrogen gas not containing any carbon. 
     Referring now to the drawings,  FIG.  1    illustrates a schematic diagram of one embodiment of a turbomachine, which in the illustrated embodiment is a gas turbine  10 . Although an industrial or land-based gas turbine is shown and described herein, the present disclosure is not limited to a land-based and/or industrial gas turbine unless otherwise specified in the claims. For example, the technology as described herein may be used in any type of turbomachine including but not limited to a steam turbine, an aircraft gas turbine, or a marine gas turbine. 
     As shown, gas turbine  10  generally includes an inlet section  12 , a compressor section  14  disposed downstream of the inlet section  12 , a plurality of combustors  17  (shown in  FIG.  2   ) within a combustion section  16  disposed downstream of the compressor section  14 , a turbine section  18  disposed downstream of the combustion section  16 , and an exhaust section  20  disposed downstream of the turbine section  18 . Additionally, the gas turbine  10  may include one or more shafts  22  coupled between the compressor section  14  and the turbine section  18 . 
     The compressor section  14  may generally include a plurality of rotor disks  24  (one of which is shown) and a plurality of rotor blades  26  extending radially outwardly from and connected to each rotor disk  24 . Each rotor disk  24  in turn may be coupled to or form a portion of the shaft  22  that extends through the compressor section  14 . The compressor section  14  further includes a plurality of stationary vanes (not shown), which are arranged in stages with the rotor blades  26  and which direct the flow against the rotor blades  26 . 
     The turbine section  18  may generally include a plurality of rotor disks  28  (one of which is shown) and a plurality of rotor blades  30  extending radially outwardly from and being interconnected to each rotor disk  28 . Each rotor disk  28  in turn may be coupled to or form a portion of the shaft  22  that extends through the turbine section  18 . The turbine section  18  further includes an outer casing  31  that circumferentially surrounds the portion of the shaft  22  and the rotor blades  30 , thereby at least partially defining a hot gas path  32  through the turbine section  18 . The turbine section  18  further includes a plurality of stationary vanes (not shown), which are arranged in stages with the rotor blades  30  and which direct the flow against the rotor blades  30 . 
     During operation, a working fluid such as air flows through the inlet section  12  and into the compressor section  14  where the air is progressively compressed by multiple compressor stages of rotating blades and stationary vanes, thus providing pressurized air to the combustors  17  of the combustion section  16 . The pressurized air is mixed with fuel and burned within each combustor  17  to produce combustion gases  34 . The combustion gases  34  flow through the hot gas path  32  from the combustion section  16  into the turbine section  18 , in which energy (kinetic and/or thermal) is transferred from the combustion gases  34  to the rotor blades  30 , causing the shaft  22  to rotate. The mechanical rotational energy may then be used to power the compressor section  14  and/or to generate electricity. The combustion gases  34  exiting the turbine section  18  may then be exhausted from the gas turbine  10  via the exhaust section  20 . 
       FIG.  2    is a schematic representation of a combustor  17 , as may be included in a can annular combustion system for the heavy-duty gas turbine  10 . In a can annular combustion system, a plurality of combustors  17  (e.g., 8, 10, 12, 14, 16, or more) are positioned in an annular array about the shaft  22  that connects the compressor section  14  to the turbine section  18 . 
     As shown in  FIG.  2   , the combustor  17  may define an axial direction A that extends along an axial centerline  170 . The combustor may also define a circumferential direction C which extends around the axial direction A and the axial centerline  170 . The combustor  17  may further define a radial direction R perpendicular to the axial direction A and the axial centerline  170 . 
       FIG.  2    illustrates a combustor  17  having one or more exemplary fuel injection assemblies  80  (also referred to as an axial fuel staging (AFS) system), as discussed further herein. The combustor  17  includes a combustion liner  46  that defines a combustion chamber  70 . The combustion liner  46  may be positioned within (i.e., circumferentially surrounded by) an outer sleeve  48 , such that an annulus  47  is formed therebetween. At least one fuel nozzle  40  may be positioned at the forward end of the combustor  17 . Fuel may be directed through first fuel supply conduits  38 , which extend through an end cover  42 , and into the fuel nozzles  40 . The fuel nozzles  40  convey the fuel and compressed air  15  into a primary combustion zone  72 , where combustion occurs. In some embodiments, the fuel and compressed air  15  are combined as a mixture prior to reaching the primary combustion zone  72 . 
     The combustion liner  46  may contain and convey combustion gases to the turbine section  18 . The combustion liner  46  defines the combustion chamber  70  within which combustion occurs. As shown in  FIG.  2   , the combustion liner  46  may extend between the fuel nozzles  40  and an aft frame  118 . The combustion liner  46  may have a cylindrical liner portion and a tapered transition portion that is separate from the cylindrical liner portion, as in many conventional combustion systems. Alternately, the combustion liner  46  may have a unified body (or “unibody”) construction, in which the cylindrical portion and the tapered portion are integrated with one another. Thus, any discussion of the combustion liner  46  herein is intended to encompass both conventional combustion systems having a separate liner and transition piece and those combustion systems having a unibody liner. Moreover, the present disclosure is equally applicable to those combustion systems in which the transition piece and the stage one nozzle of the turbine section  18  are integrated into a single unit, sometimes referred to as a “transition nozzle” or an “integrated exit piece.” 
     The combustion liner  46  may be surrounded by an outer sleeve  48 , which is spaced radially outward of the combustion liner  46  to define an annulus  47  through which compressed air  15  flows to a head end of the combustor  17 . Heat is transferred convectively from the combustion liner  46  to the compressed air  15 , thus cooling the combustion liner  46  and warming the compressed air  15 . 
     In exemplary embodiments, the outer sleeve  48  may include a flow sleeve  110  at the forward end and an impingement sleeve  112  at the aft end. The flow sleeve  110  and the impingement sleeve  112  may be coupled to one another. Alternately, the outer sleeve  48  may have a unified body (or “unisleeve”) construction, in which the flow sleeve  110  and the impingement sleeve  112  are integrated with one another in the axial direction. As before, any discussion of the outer sleeve  48  herein is intended to encompass both conventional combustion systems having a separate flow sleeve  110  and impingement sleeve  112  and combustion systems having a unisleeve outer sleeve. 
     The forward casing  50  and the end cover  42  of the combustor  17  define the head end air plenum  122 , which includes one or more fuel nozzles  40 . The fuel nozzles  40  may be any type of fuel nozzle, such as bundled tube fuel nozzles  200  ( FIG.  3   , often referred to as “micromixers”) or swirler nozzles (often referred to as “swozzles”). For example, the fuel nozzles  40  are positioned within the head end air plenum  122  defined at least partially by the forward casing  50 . In many embodiments, the fuel nozzles  40  may extend from the end cover  42 . For example, each fuel nozzle  40  may be coupled to an aft surface of the end cover  42  via a flange (not shown). As shown in  FIG.  2   , the at least one fuel nozzle  40  may be partially surrounded by the combustion liner  46 . The aft, or downstream ends, of the fuel nozzles  40  extend through a cap plate  44  that defines the upstream end of the combustion chamber  70 . 
     The fuel nozzles  40  may be in fluid communication with a first fuel supply  150  configured to supply a first fuel  158  to the fuel nozzles  40 . In many embodiments, the first fuel  158  may be a fuel mixture containing natural gas (such as methane, ethane, propane, or other suitable natural gas) and hydrogen. In other embodiments, the first fuel  158  may be pure natural gas or pure hydrogen (e.g., 100% hydrogen, which may or may not contain some amount of contaminants), such that the first fuel is not a mixture of multiple fuels. 
     In exemplary embodiments, the first fuel  158  and compressed air  15  may mix together within the fuel nozzles  40  to form a first mixture of compressed air  15  and the first fuel  158  before being ejected (or injected) by the fuel nozzles  40  into the primary combustion zone  72 . The first mixture of the first fuel  158  and compressed air  15  may be injected into the primary combustion zone  72  and ignited to generate a first flow of combustion gases  164  having a first temperature. 
     As discussed below, during operation of the combustor  17  on a total fuel input that comprises a high amount of hydrogen (e.g., greater than about 80%), the temperature of combustion gases within the primary combustion zone  72  (e.g., the first flow of combustion gases  164 ) may be the lowest temperature of any of any combustion gases within the combustion chamber  70  (e.g., lower than the combustion gases within the secondary combustion zone  74 ). Operated in this way, the temperature of combustion gases within the primary combustion zone  72  may be a lower temperature than combustion gases in the secondary combustion zone  74 , which may advantageously enable the combustor  17  to operate on high amounts of hydrogen without creating potentially damaging flame holding and/or flashback conditions. 
     The forward casing  50  may be fluidly and mechanically connected to a compressor discharge casing  60 , which defines a high pressure plenum  66  around the combustion liner  46  and the outer sleeve  48 . Compressed air  15  from the compressor section  14  travels through the high pressure plenum  66  and enters the combustor  17  via apertures (not shown) in the downstream end of the outer sleeve  48  (as indicated by arrows near an aft frame  118 ). Compressed air travels upstream through the annulus  47  and is turned by the end cover  42  to enter the fuel nozzles  40  and to cool the head end. In particular, compressed air  15  flows from high pressure plenum  66  into the annulus  47  at an aft end of the combustor  17 , via openings defined in the outer sleeve  48 . The compressed air  15  travels upstream from the aft end of the combustor  17  to the head end air plenum  122 , where the compressed air  15  reverses direction and enters the fuel nozzles  40 . 
     In the exemplary embodiment, a fuel injection assembly  80  is provided to deliver a second fuel/air mixture and/or a flow of pure fuel (e.g., 100% fuel, such as hydrogen, not mixed with air) to a secondary combustion zone  74 . For example, a second flow of fuel and air may be introduced by one or more premix injectors  100  to the secondary combustion zone  74 , and a flow of supplemental fuel may be introduced by one or more supplemental or immersed injectors  104 . 
     The primary combustion zone  72  and the secondary combustion zone  74  may each be portions of the combustion chamber  70  and therefore may be defined by the combustion liner  46 . For example, the primary combustion zone  72  may be defined from an outlet of the fuel nozzles  40  to the premix injector  100 , and the secondary combustion zone may be defined from the premix injector  100  to the aft frame  118 . In this arrangement, the forward most boundary of the premix injector  100  may define the end of the primary combustion zone  72  and the beginning of the secondary combustion zone  74  (e.g., at an axial location where a second flow of fuel and air are introduced). 
     Such a combustion system having axially separated combustion zones is described as an “axial fuel staging” (AFS) system. The fuel injection assemblies  80  may be circumferentially spaced apart from one another on the outer sleeve  48  (e.g., equally spaced apart in some embodiments). In many embodiments, the combustor  17  may include four fuel injection assemblies  80  spaced apart from one another and configured to inject a second mixture of fuel and air into a secondary combustion zone  74  via the premix injector  100  and configured to inject a flow of pure fuel (e.g., a fuel mixture or pure hydrogen) via the immersed injector  104 , in order to increase the combustion gases  34  and temperature thereof. In other embodiments, the combustor  17  may include any number of fuel injection assemblies  80  (e.g., 1, 2, 3, or up to 10). 
     As shown in  FIG.  2   , each fuel injection assembly  80  may include a premix injector  100 , an immersed injector  104 , a second fuel supply conduit  102  that supplies a second fuel (such as pure hydrogen or a natural gas and hydrogen mixture comprising greater than 80% hydrogen) to the premix injector  100 , and a third fuel supply conduit  103  that supplies a pure fuel (e.g., a fuel mixture or pure hydrogen) to the immersed injector  104 . For example, each premix injector  100  may be in fluid communication, at least partially via the second fuel supply conduit  102 , with a second fuel supply  152  configured to supply a second fuel  160  to each premix injector  100 . In many embodiments, the second fuel  160  may be a fuel mixture containing natural gas (such as methane, ethane, propane, or other suitable natural gas) and hydrogen. In other embodiments, the second fuel  160  may be pure natural gas or pure hydrogen (e.g., 100% hydrogen), such that the second fuel includes no other fuels mixed therein. Similarly, each immersed injector  104  may be in fluid communication, at least partially via the third fuel supply conduit  103 , with a third fuel supply  154  configured to supply a third fuel  162  to each immersed injector  104 . In exemplary embodiments, the third fuel  162  may be pure fuel (e.g., a fuel mixture or pure hydrogen), such that the third fuel includes no other fuels or air mixed therein. 
     Because the fuel nozzles  40 , the premix injectors  100 , and the immersed injectors  104  are separately fueled (e.g., via the fuel supplies  150 ,  152 , and  154 ), they may allow the combustor a wide range of operational flexibility. For example, each of the fuel nozzles  40 , the premix injectors  100 , and the immersed injectors  104  may be supplied with a different fuel or fuel mixture. Particularly, in exemplary embodiments, each of the fuel nozzles  40 , the premix injectors  100 , and the immersed injectors  104  may be supplied pure hydrogen or a fuel mixture that contains mostly hydrogen (e.g., greater than 80% hydrogen) and natural gas (such as methane, ethane, propane, or other natural gas). However, it should be appreciated that, in some embodiments, each of the fuel nozzles  40 , the premix injectors  100 , and the immersed injectors  104  may be fueled by the same fuel supply, such that the same fuel mixture or pure fuel is supplied to all of the fuel nozzles  40 , the premix injectors  100 , and the immersed injectors  104 . 
     As used herein, the term “premix” may be used to describe a component, passage, or cavity in which fuel and air are mixed together prior to being injected into the combustion chamber  70 . In many embodiments, each premix injector  100  may fluidly couple the high pressure plenum  66  to the secondary combustion zone  74 . For example, compressed air  15  from the high pressure plenum  66  may enter the premix injector  100  where it is mixed with the second fuel  160  prior to being injected into the secondary combustion zone  74 . For example, in exemplary embodiments, each premix injector  100  may extend through the outer sleeve  48 , the annulus  47 , and the combustion liner  46  and into the secondary combustion zone  74 . Specifically, the premix injectors  100  may each extend radially from the high pressure plenum  66 , through the outer sleeve  48 , the annulus  47 , and the combustion liner  46 , such that the premix injector  100  is capable of delivering a second flow of fuel and air to the secondary combustion zone  74 . The premix injectors  100  may be coupled to the combustion liner  46  and/or the outer sleeve  48 , such that each premix injector  100  introduces the second fuel/air mixture as a jet entering a cross-flow (such as at an angle, oblique, orthogonal, slantwise, diagonally, transverse, or nonparallel) of the combustion products  164  produced in the primary combustion zone  72 . The second fuel/air mixture(s) are ignited by the combustion products  164  from the primary combustion zone  72  and burn in the secondary combustion zone  74 . 
     The premix injector  100  may be coupled to the outer sleeve  48  and may extend through the outer sleeve  48  and the combustion liner  46 . In one embodiment, a boss (not shown) supporting the premix injector  100  functions as a fastener for securing the outer sleeve  48  to the combustion liner  46 . In other embodiments, the premix injector  100  may be coupled to the outer sleeve  48  in any suitable manner, and the outer sleeve  48  may have any suitable number of components coupled between the flange of the forward casing  50  and the turbine nozzle in any suitable manner that permits the fuel injection assembly  80  to function as described herein. 
     In exemplary embodiments, the second fuel  160  and compressed air  15  may mix together within the premix injectors  100  to form a second mixture of compressed air  15  and the second fuel  160  before being ejected (or injected) by the premix injectors  100  into the secondary combustion zone  74 . The second mixture of the second fuel  160  and compressed air  15  may be injected (e.g., as a cross-flow, such as generally radially) into the secondary combustion zone  74  and ignited to generate a second flow of combustion gases  166  having a second temperature. 
     The immersed injector  104  may extend radially through the premix injector (e.g., through the center of the premix injector  100 ) and into the secondary combustion zone  74 . For example, the immersed injector  104  may extend radially into the secondary combustion zone  74  of the combustion chamber  70 , such that the immersed injector  104  is directly exposed to combustion gases during operation of the combustor  17 . As described above, although the immersed injector extends through the premix injector  100 , the immersed injector  104  may be fluidly isolated from the premix injector  100 . In this arrangement, the immersed injector  104  may separately inject a flow of third fuel  162  (e.g., a fuel mixture, such as hydrogen and natural gas, or entirely fuel, such as pure hydrogen not mixed with air or other oxidants) directly into the secondary combustion zone  74  to generate a third flow of combustion gases  168 . In this way, the third fuel  162  may be introduced by the immersed injector  104  (or supplemental injector) as a supplemental fuel that generates additional combustion gases  34  proximate the exit of the combustion chamber  70  (e.g., closer to the aft frame  118  than the end cover  42 ), which may advantageously allow the combustor  17  to generate outlet combustion gases  172  having an outlet temperature without any potentially dangerous flashback or flame holding events. As should be appreciated, the third fuel  162  may be airless (or oxidant-less), such that no air or other oxidants are mixed therein. In this way, the third fuel  162  may be a pure fuel or pure hydrogen. In exemplary implementations, the first flow of combustion gases  164 , the second flow of combustion gases  166 , and the third flow of combustion gases  168  may mix together within the secondary combustion zone  74  to form outlet combustion gases  172  ( 34  in  FIG.  1   ) having an outlet temperature. The outlet combustion gases  172  may exit the combustor  17  via the aft frame  118  and enter the turbine section  18  of the gas turbine  10 . 
     Although the immersed injector  104  in  FIG.  2    extends radially through the premix injector  100  and into the secondary combustion zone  74 , it should be understood that the immersed injector  104  may be axially spaced apart and disposed downstream from the premix injector  100  with respect to the flow of combustion gases. For example, the immersed injector  104  may be disposed axially between the premix injector  100  and the aft frame  118  with respect to the axial centerline  170 . In such embodiments, the immersed injector  104  may extend independently through the outer sleeve  48 , the annulus  47 , the combustion liner  46 , and into the secondary combustion zone  74 . 
     During operation of the combustor  17  on a total fuel input that comprises a high amount of hydrogen (e.g., greater than about 80%), the temperature of combustion gases within the secondary combustion zone (e.g., outlet combustion gases  172 ) may be the highest temperature of any of any combustion gases within the combustion chamber. Specifically, the temperature of combustion gases  166 ,  168  within the secondary combustion zone  74  may be a higher temperature than combustion gases  164  in the primary combustion zone  72 , which may advantageously enable the combustor  17  to operate on high amounts of hydrogen (or entirely on hydrogen) without experiencing potentially damaging flame holding and/or flashback conditions. 
       FIG.  3    provides a cross-sectional side view of a portion of a bundled tube fuel nozzle  200 . In exemplary embodiments, the one or more fuel nozzles  40  shown in  FIG.  2    may each be a bundled tube fuel nozzle  200 . As shown in  FIG.  3   , the bundled tube fuel nozzle  200  includes a fuel plenum body  202  having a forward or upstream plate  204 , an aft plate  206  axially spaced from the forward plate  204  and an outer band or shroud  208  that extends axially between the forward plate  204  and the aft plate  206 . A fuel plenum  210  is defined within the fuel plenum body  202 . In particular embodiments, the forward plate  204 , the aft plate  206  and the outer band  208  may at least partially define the fuel plenum  210 . In particular embodiments, the fuel supply conduit  38  may extend through the forward plate  204  to provide fuel (such as pure hydrogen or a fuel mixture comprising greater than 80% hydrogen) to the fuel plenum  210 . In various embodiments, the bundled tube fuel nozzle  200  includes a cap plate  212  axially spaced from the aft plate  206 . A hot side  214  of the cap plate  212  is generally disposed adjacent or proximate to the primary combustion zone  72 . The cap plate  212  may be unique to each bundled tube fuel nozzle  200  or may be common among all the bundled tube fuel nozzles  200  (e.g., such as the cap plate  44  shown in  FIG.  2   ). 
     As shown in  FIG.  3   , the bundled tube fuel nozzle  200  may include a tube bundle  216  comprising a plurality of premix tubes  106 . Each premix tube  106  may extend through the forward plate  204 , the fuel plenum  210 , the aft plate  206 , and the cap plate  212 . The premix tubes  106  are fixedly connected to and/or form a seal against the aft plate  206 . For example, the premix tubes  106  may be welded, brazed or otherwise connected to the aft plate  206 . Each premix tube  106  includes an inlet  220  defined at an upstream end  222  of each respective tube  106  and an outlet  224  defined at a downstream end  226  of each respective tube  106 . Compressed air from the head end  122  may enter each of the premix tubes  106  at the inlet and may be mixed with fuel before being expelled into the primary combustion zone. For example, each premix tube  106  defines a respective premix flow passage  228  through the bundled tube fuel nozzle  200 , in which fuel (such as pure hydrogen or a fuel mixture comprising greater than 80% hydrogen) may be mixed with compressed air. In particular embodiments, one or more premix tubes  106  of the plurality of tubes  106  is in fluid communication with the fuel plenum  210  via one or more fuel ports  230  defined within the respective premix tube(s)  106 . 
       FIG.  4    illustrates an enlarged perspective view of a fuel injection assembly  80 , in accordance with embodiments of the present disclosure. As shown the fuel injection assembly  80  may include a premix injector  100 , an immersed injector  104 , a second fuel supply conduit  102  that supplies a second fuel to the premix injector  100 , and a third fuel supply conduit  103  that supplies a third fuel to the immersed injector  104 . The second fuel and the third fuel may be pure (i.e., unmixed) hydrogen or a mixture of natural gas and hydrogen with greater than 80% hydrogen (e.g., diatomic hydrogen gas). The second fuel and the third fuel may be the same fuel or may be different fuels. In some embodiments (not shown), the fuel injection assembly  80  may only include a singular fuel supply conduit that supplies fuel to both the immersed injector  104  and the premix injector  100 . As shown, the premix injector  100  extend radially between a radially outer end  175  and a radially inner end  177 . 
     In exemplary embodiments, the premix injector  100  may include end walls  182  axially spaced apart from each other and side walls  184  extending between the end walls  182 . For example, the side walls  184  extend axially between the end walls  182  along the axial direction A. The end walls  182  of the premix injector  100  may include a forward end wall and an aft end wall disposed oppositely from one another. The side walls  184  may be spaced apart from one another and may each extend axially between the forward end wall and the aft end wall. In many embodiments, one set of walls (e.g., the side walls  184 ) may be generally arcuate or curved, and the other set of walls (e.g., the end walls  182 ) may be generally straight. In some embodiments, as shown, the end walls  182  and the side walls  184  may collectively define a geometric stadium shaped area, i.e., a rectangle with rounded ends, that outlines and defines a perimeter of the opening  186 . In other embodiments (not shown), the end walls  182  may be straight such that end walls  182  and the side walls  184  collectively define a rectangular shaped area. 
     An opening may be defined between the end walls  182  and the side walls  184  of the premix injector  100 . In many embodiments, the premix opening  186  may be longer in the axial direction A than in the circumferential direction C, thereby advantageously allowing the opening  186  to introduce a large amount of fuel and air into the combustion chamber  70  without having the premix injector  100  impede a large portion of the annulus  47  through which it extends. For example, in various embodiments, the opening  186  may have a variety of cross-sectional shapes, such as but not limited to a rectangle, oval, stadium shape (e.g., a rectangle having arced or curved ends), or other suitable shapes. Although the premix injector  100  and the opening  186  are illustrated as having a geometric stadium shape, it should be understood that the premix injector  100  and its opening  186  may have a different shape (e.g., a round shape) or that the opening  186  may have a shape that is different from the outermost perimeter of the premix injector  100 . 
     A plurality of ribs  178  may extend within the opening  186  of the premix injector  100  and may at least partially define a plurality of premix passages  180  each extending between an air inlet  174  disposed at the radially outer end  175  and an outlet  176  disposed at the radially inner end  177 . For example, the plurality of ribs  178  may include at least one axial rib  179  extending along the axial direction A between the end walls  182  (e.g., from the forward end wall to the aft end wall). Additionally, or alternatively, the plurality of ribs  178  may include circumferential ribs  181  axially spaced apart from one another and each extending between the side walls  184 . As shown in  FIG.  4   , the at least one axial rib  179  and one or more of the circumferential ribs  181  may couple to the immersed injector  104  (e.g., at a base  190  of the immersed injector  104 ). In many embodiments, the immersed injector  104  may be spaced apart from both the end walls  182  and the side walls  184  and may extend from within the opening  186  (e.g., from a center point of the opening  186 ). The plurality of ribs  178  may couple to, and at least partially support or suspend, the immersed injector  104  within the opening  186  (e.g., at the base  190  of the immersed injector  104 ). 
     Additionally, the immersed injector  104  may extend radially through the opening  186  (such as through a center point of the opening  186 ) of the premix injector  100  and directly into the combustion chamber  70 . The immersed injector  104  may have a generally contoured aerodynamic shape (such as a teardrop shaped cross-section) in order to minimize disruption to flow of the combustion gases around the immersed injector  104  during operation of the combustor  17 . For example, the immersed injector  104  may define an airfoil  188  extending radially from a base  190  at the radially inner end  177  to a tip  192 . The entire airfoil  188  (e.g., from the base  190  to the tip  192 ) may be disposed within the secondary combustion zone  74 . Additionally, the immersed injector  104  may include a leading edge  194 , a trailing edge  196 , and side surfaces  198  extending between the leading edge  194  and the trailing edge  196 . In many embodiments, the leading edge  194  and the trailing edge  196  may face the end walls  192  (but be spaced apart therefrom), and the side surfaces  198  may generally face the side walls  184  (but be spaced apart therefrom). During operation of the immersed injector  104 , the combustion gases may engage the airfoil  188  at the leading edge  194  and may travel along the side surfaces  198  to the trailing edge  196 . 
     One or more fuel ports  199  may be defined on the side surface(s)  198  to inject pure fuel (such as hydrogen) directly into the combustion chamber  70 . For example, the one or more fuel ports  199  may be in fluid communication with the third fuel supply  154 . In this way, the immersed injector  104  may receive fuel from the third fuel supply  154  and may expel (or inject) the fuel into the secondary combustion zone via the fuel ports  199 . In some embodiments, as shown in  FIG.  3   , the fuel ports  199  may be arranged in a row and may be generally aligned (e.g., along a common axis) on the side surface  198  of the airfoil  188 . Contrary to both the fuel nozzles  40  and the premix injector  100 , in exemplary embodiments, the immersed injector  104  does not introduce (or inject) a premix flow of air and fuel into the combustion chamber  70 . Rather, the immersed injector  104  introduces pure fuel (such as pure hydrogen not mixed with air) into the combustion chamber  70 , which advantageously allows the combustor  17  to operate on high amounts of total hydrogen. 
     In exemplary embodiments, the immersed injector  104  is surrounded by one or more premix passages  180  of the plurality of premix passages  180 . For example, the immersed injector  104  may at least partially define a boundary of one or more of the premix passages  180 , such that the premix passages  180  surrounding the immersed injector are collectively defined by the plurality of ribs  178 , one of the side walls  184 , and the immersed injector  104 . Positioning the immersed injector  104  within the opening  186 , and surrounded by one or more of the premix passages  180 , advantageously allows the airfoil  188  to be cooled by the mixture  166  of air and fuel exiting the premix passages  180  of the premix injector  100 . 
       FIG.  5    is a graph  500  of flame temperature over outlet temperature (expressed as a percentage) vs. the percentage of hydrogen present in the total fuel input to the combustor  17 , in which line  502  is the temperature of outlet combustion gases  172 , line  504  is the temperature of the first flow of combustion gases  164  generated by the fuel nozzles  40 , and line  506  is the temperature of the second flow of combustion gases  166  generated by the fuel injection assemblies  80  (i.e., the premix injectors  100  and the immersed injectors  104 ). The total fuel input may include all of the fuel that is supplied to the combustor  17  (including the fuel supplied to the fuel nozzles  40 , the premix injectors  100 , and the immersed injectors  104 ). The horizontal line  502  may be the outlet temperature of the outlet combustion gases  172  (e.g., the combustor exit temperature of the combustion gases). As shown by the horizontal line  502 , the outlet temperature may be unchanged regardless of what percentage of the total fuel input consists of hydrogen. The line  504  may be the flame temperature at the outlet of the fuel nozzles  40  within the primary combustion zone  72 , e.g., the flame temperature of the first flow of combustion gases  164 . Additionally, the line  506  may be the flame temperature at the outlet of the premix injector  100  within the secondary combustion zone  74 , e.g., the flame temperature of the second flow of combustion gases  166 . 
     As shown, as the percentage of hydrogen present in the total fuel input supplied to the combustor increases above 80%, the flame temperature of the second flow of combustion gases  166  may increase above the flame temperature of the first flow of combustion gases  164 , which advantageously allows the combustor to operate on high amounts of hydrogen without entering a flame holding or flashback condition. 
     In many embodiments, the outlet temperature of the outlet combustion gases  172  may be between about 2500° F. and about 3100° F. In other embodiments, the outlet temperature of the combustion gases  172  may be between about 2600° F. and about 2900° F. In some embodiments, the outlet temperature of the combustion gases  172  may be between about 2650° F. and about 2800° F. 
       FIG.  6    is a graph  600  of the percentage of total fuel input supplied to the immersed injectors  104  vs. the percentage of hydrogen present in the total fuel input to the combustor  17 , in which the line  602  represents the amount of fuel supplied to the immersed injectors  104 . As depicted, when the percentage of hydrogen in the total fuel input supplied to the combustor  17  is present at levels above 80%, the immersed injectors  104  may be supplied with fuel (e.g., hydrogen) unmixed with air or other oxidants. When operating on 100% hydrogen, about 10% of the total fuel input may be supplied to the immersed injectors  104 . This advantageously allows the combustor  17  to maintain the required outlet temperature without causing the fuel nozzles  40  or the premix injectors  100  to enter flashback and/or flame holding conditions that could otherwise be caused by operating on high amounts of hydrogen. 
     Referring now to  FIG.  7   , a flow diagram of one embodiment of a method  700  of operating a combustor  17  of a turbomachine  10  on a total fuel input that contains a concentration of hydrogen that is greater than about 80% to generate outlet combustion gases  172  having an outlet temperature is illustrated in accordance with aspects of the present subject matter. In general, the method  700  will be described herein with reference to the combustor  17 , the bundled tube fuel nozzle  200 , the fuel injection assembly  80 , and the graphs  500 ,  600  described above and with reference to  FIGS.  1  through  6   . However, it should be understood that the method  700  may be utilized with any suitable combustor for a turbomachine without deviating from the scope of the present disclosure. Additionally, although  FIG.  7    depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure. 
     As shown in  FIG.  7   , the method  700  may include a step  702  of injecting, with at least one fuel nozzle  40 , a first mixture of air and a first fuel  158  containing hydrogen (e.g., diatomic hydrogen gas) into the primary combustion zone  72  of the combustor  17  to generate a first flow of combustion gases  164  having a first temperature. For example, the first mixture of compressed air  15  and the first fuel  158  may be delivered to the primary combustion zone  72  by the fuel nozzles  40  (which, in some embodiments, may be bundled tube fuel nozzles  200  in accordance with  FIG.  3   ). In various embodiments the first fuel  158  may be a mixture of natural gas (such as methane, ethane, propane, or other natural gas) and hydrogen. In exemplary embodiments, the first fuel  158  may include a high concentration of hydrogen (such as greater than about 80% hydrogen), with the remainder of the first fuel  158  being one or more natural gases. 
     In exemplary embodiments, the method  700  may include a step  704  of injecting, with one or more premix injectors  100  disposed downstream of the fuel nozzles  40 , a second mixture of air and a second fuel  160  containing hydrogen (e.g., diatomic hydrogen gas) into the secondary combustion zone  74  of the combustor  17  as a cross-flow to generate a second flow of combustion gases  164  having a second temperature. For example, the second mixture of compressed air  15  and the second fuel  158  may be delivered to the secondary combustion zone  74  by the premix injectors  100 . In various embodiments, the second fuel  160  may be a mixture of natural gas (such as methane, ethane, propane, or other natural gas) and hydrogen (such as diatomic hydrogen gas). In exemplary embodiments, the second fuel  160  may include a high concentration of hydrogen (such as greater than about 80% hydrogen), with the remainder of the second fuel  160  being one or more natural gases. 
     In exemplary implementations of the method  700 , the second temperature of the second flow of combustion gases  166  may be greater than the first temperature of the first flow of combustion gases  164 . This may advantageously allow the combustor  17  to operate on a total fuel input that contains a concentration of hydrogen that is greater than 80% without experiencing flame holding and/or flashback conditions. 
     In many embodiments, the method  700  may further include a step  706  of separately injecting a third fuel  162  as a pure fuel (e.g., diatomic hydrogen gas) into secondary combustion zone  74 . The third fuel  162  ignites and mixes with the first flow and the second flow of combustion gases  164 ,  166  to generate outlet combustion gases  172  ( 34  in  FIG.  1   ) having a third temperature. In exemplary embodiments, the third temperature of the outlet combustion gases  172  may be greater than the first temperature of the first flow of combustion gases  164  (such as 10%, 20%, 30%, or 40%) greater, which advantageously allows the combustor  17  to operate on high amounts of hydrogen without entering potential flashback conditions. In some embodiments, the third temperature of the outlet combustion gases  172  may be generally equal to the outlet temperature of the combustor  17  (e.g., within ±5%). In various embodiments, the third fuel may be a fuel mixture, such as natural gas (e.g., methane, ethane, or propane) and hydrogen (e.g., diatomic hydrogen gas). For example, the third fuel may contain a high amount of hydrogen, such as 50%, 60%, 70%, 80%, 90%, or 100% hydrogen. 
     In many implementations, the step  706  may be performed by the immersed injector  104  described above with reference to  FIGS.  2  and  4   . For example, in contrast with the fuel nozzles  40  and the premix injector  100 , which both deliver a mixture of fuel/air to the combustion chamber  70 , the immersed injector  104  may advantageously deliver only fuel (e.g., not mixed with air) to the combustion chamber  70 . The fuel may be a fuel mixture containing hydrogen and natural gas or may be only hydrogen not mixed with other fuels. In exemplary embodiments, as shown in  FIGS.  2  and  4   , the third fuel  162  may be injected at the axial location of the one or more premix injectors  100  into the secondary combustion zone  74 . For example, the immersed injector  104  may extend through a center point of the premix injector  100 . 
     Alternately, in other embodiments, the third fuel  162  may be injected downstream of the one or more premix injectors  100  into the secondary combustion zone  74 . For example, the immersed injectors  104  may be entirely separated from the premix injectors  100  and may extend into the combustion chamber  70  downstream of the premix injectors  100 . 
     In some implementations, the method  700  may include an optional step of increasing the concentration of hydrogen from about 80% to about 100% while maintaining the outlet temperature. In such a step, the combustor  17  may shift from operation on mostly hydrogen (with some natural gas mixed therein) to operation on entirely hydrogen (having no other fuels mixed therein). As a result of increasing the concentration of hydrogen from about 80% to about 100%, the first temperature of the first flow of combustion gases  164  may decrease and the second temperature of the second flow of combustion gases  166  may decrease while remaining equal to or above the first temperature. 
     In order to facilitate the transition from 80% to 100% hydrogen while maintaining the required outlet temperature and without creating a flame holding or flashback incident, as shown by  FIG.  5   , an amount of hydrogen supplied to the immersed injector  104  may be simultaneously increased to substitute for the loss of temperature in the first and second flow of combustion gases  164 ,  166 . For example, when operating the combustor  17  when the concentration of hydrogen available in the fuel has been increased from about 80% to about 100% (while maintaining the outlet temperature), the method  700  may include simultaneously increasing an amount of the third fuel  162  (e.g., hydrogen) injected into the secondary combustion zone without air such that the third fuel  162  comprises up to about 10% of the total fuel input to the combustor  17 . 
     In some embodiments, the method  700  may further include a step of mixing the first flow of combustion gases  164 , the second flow of combustion gases  166 , and the third flow of combustion gases  168  within the secondary combustion zone to generate the outlet combustion gases  172  having the outlet temperature. For example, the outlet temperature of the outlet combustion gases  172  may be between about 2500° F. and about 3100° F. In other embodiments, the outlet temperature of the combustion gases  172  may be between about 2600° F. and about 2900° F. In some embodiments, the outlet temperature of the combustion gases  172  may be between about 2650° F. and about 2800° F. 
     Referring now to  FIG.  8   , a flow diagram of one embodiment of a method  800  of operating a combustor  17  is illustrated in accordance with aspects of the present subject matter. In general, the method  800  will be described herein with reference to the combustor  17 , the bundled tube fuel nozzle  200 , the fuel injection assembly  80 , and the graphs  500 ,  600  described above and with reference to  FIGS.  1  through  6   . However, it should be understood that the method  800  may be utilized with any suitable combustor for a turbomachine without deviating from the scope of the present disclosure. Additionally, although  FIG.  8    depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure. 
     In many embodiments, the method  800  may include a step  802  of injecting, with at least one fuel nozzle  40 , a first mixture of air and a first fuel  158  into a primary combustion zone  72  of the combustor  17  to generate a first flow of combustion gases  164  having a first temperature. For example, the first mixture of compressed air  15  and first fuel  158  (such as a fuel mixture containing natural gas and/or hydrogen, such as diatomic hydrogen gas) may be delivered to the primary combustion zone  72  by the fuel nozzles  40  (which, in some embodiments, may be bundled tube fuel nozzles  200  in accordance with  FIG.  3   ). 
     The method  800  may further include a step  804  of injecting, with one or more premix injectors  100  disposed downstream of the fuel nozzles  40 , a second mixture of air and a second fuel  160  into a secondary combustion zone  74  of the combustor as a cross-flow to generate a second flow of combustion gases  166  having a second temperature. Additionally, the method  800  may further include a step  806  of separately injecting a third fuel  162  as a pure fuel into secondary combustion zone, the third fuel igniting and mixing with the first flow and the second flow of combustion gases  164 ,  166  to generate outlet of combustion gases  172  having a third temperature. 
     Referring now to  FIG.  9   , a flow diagram of one embodiment of a method  900  of operating a combustor  17  of a turbomachine on a total fuel input that contains a 100% concentration of hydrogen to generate outlet combustion gases  172  having an outlet temperature is illustrated in accordance with aspects of the present subject matter. In general, the method  900  will be described herein with reference to the combustor  17 , the bundled tube fuel nozzle  200 , the fuel injection assembly  80 , and the graphs  500 ,  600  described above and with reference to  FIGS.  1  through  6   . However, it should be understood that the method  900  may be utilized with any suitable combustor for a turbomachine without deviating from the scope of the present disclosure. Additionally, although  FIG.  9    depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure. 
     As shown, in exemplary embodiments, the method  900  may include a step  902  of injecting, with at least one fuel nozzle  40 , a first mixture of air and hydrogen into the primary combustion zone  72  of the combustor  17  to generate a first flow of combustion gases  164  having a first temperature. For example, the first mixture of compressed air  15  and hydrogen may be delivered to the primary combustion zone  72  by the fuel nozzles  40  (which, in some embodiments, may be bundled tube fuel nozzles  200  in accordance with  FIG.  3   ). 
     In many embodiments, the method  900  may include a step  904  of injecting, with one or more premix injectors  100  disposed downstream of the fuel nozzles  40 , a second mixture of air and hydrogen into the secondary combustion zone  74  of the combustor  17  as a cross-flow to generate a second flow of combustion gases  166  having a second temperature. For example, the second mixture of compressed air  15  and hydrogen may be delivered to the secondary combustion zone  74  by the premix injectors  100 . 
     In various embodiments, the method  900  may further include a step  906  of separately injecting a flow of pure hydrogen (e.g., not mixed with compressed air) into the combustion chamber  70  of the combustor  17  to generate a third flow of combustion gases  168  having a third temperature. In many implementations, the step  906  may be performed by the immersed injector  104  described above with reference to  FIGS.  2  and  4   . For example, contrary to both the fuel nozzles  40  and the premix injector  100 , which both deliver a mixture of hydrogen and air to the combustion chamber  70 , the immersed injector  104  may advantageously deliver only hydrogen (e.g., pure hydrogen not mixed with air) to the combustion chamber  70 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject technology is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 
     Further aspects of the invention are provided by the subject matter of the following clauses: 
     A method of operating a combustor of a turbomachine on a total fuel input that contains a concentration of hydrogen that is greater than about 80%, the method comprising: injecting, with at least one fuel nozzle, a first mixture of air and a first fuel containing hydrogen into a primary combustion zone of the combustor to generate a first flow of combustion gases having a first temperature; injecting, with one or more premix injectors disposed downstream of the fuel nozzles, a second mixture of air and a second fuel containing hydrogen into a secondary combustion zone of the combustor as a cross-flow to generate a second flow of combustion gases having a second temperature; and separately injecting a third fuel as a pure fuel into secondary combustion zone, the third fuel igniting and mixing with the first flow and the second flow of combustion gases to generate outlet combustion gases having a third temperature. 
     The method as in one or more of these clauses, wherein the third temperature is greater than the first temperature. 
     The method as in one or more of these clauses, wherein the third fuel is injected at an axial location of the one or more premix injectors into the secondary combustion zone. 
     The method as in one or more of these clauses, wherein the third fuel is injected downstream of the one or more premix injectors into the secondary combustion zone. 
     The method as in one or more of these clauses, wherein separating injecting the third fuel as a pure fuel comprises injecting hydrogen gas in a concentration greater than 80% of the third fuel, the balance being natural gas. 
     The method as in one or more of these clauses, wherein the amount of the third fuel injected into the secondary combustion zone comprises up to about 10% of the total fuel input. 
     The method as in one or more of these clauses, further comprising mixing the first flow of combustion gases, the second flow of combustion gases, and the third flow of combustion gases within the secondary combustion zone to generate outlet combustion gases having an outlet temperature. 
     The method as in one or more of these clauses, wherein the combustor comprises an end cover, the at least one fuel nozzle extending from the end cover and at least partially surrounded by a combustion liner, the combustion liner extending from the at least one fuel nozzle toward an aft frame, wherein the combustion liner defines a combustion chamber that includes the primary combustion zone downstream extending from the at least one fuel nozzle to the one or more premix injectors and the secondary combustion zone extending downstream from the one or more premix injectors to the aft frame. 
     The method as in one or more of these clauses, wherein the total fuel input comprises greater than about 80% hydrogen with a remainder of the total fuel input being natural gas. 
     The method as in one or more of these clauses, wherein the first fuel and the second fuel further comprise natural gas. 
     The method as in one or more of these clauses, wherein the outlet temperature is between about 2500° F. and about 3100° F. 
     A method of operating a combustor: injecting, with at least one fuel nozzle, a first mixture of air and a first fuel into a primary combustion zone of the combustor to generate a first flow of combustion gases having a first temperature; injecting, with one or more premix injectors disposed downstream of the fuel nozzles, a second mixture of air and a second fuel into a secondary combustion zone of the combustor as a cross-flow to generate a second flow of combustion gases having a second temperature; and separately injecting a third fuel as a pure fuel into the secondary combustion zone, the third fuel igniting and mixing with the first flow and the second flow of combustion gases to generate outlet combustion gases having a third temperature. 
     The method as in one or more of these clauses, wherein the third temperature is greater than the first temperature. 
     The method as in one or more of these clauses, wherein the third fuel is injected at an axial location of the one or more premix injectors into the secondary combustion zone. 
     The method as in one or more of these clauses, wherein the third fuel is injected downstream of the one or more premix injectors into the secondary combustion zone. 
     The method as in one or more of these clauses, further comprising mixing the first flow of combustion gases, the second flow of combustion gases, and the third flow of combustion gases within the secondary combustion zone to generate the outlet combustion gases having an outlet temperature of between about 2500° F. and about 3100° F. 
     A combustor of a turbomachine configured to operate on a total fuel input that contains at least 80% concentration of hydrogen, the combustor comprising: at least one fuel nozzle mounted to an end cover and configured to inject a first mixture of air and hydrogen into a primary combustion zone of the combustor; a combustion liner extending downstream from the at least one fuel nozzle to an aft frame; one or more premix injectors disposed downstream of the fuel nozzles, the one or more premix injectors coupled to the combustion liner and configured to inject a second mixture of air and hydrogen into a secondary combustion zone of the combustor; and one or more injectors configured to inject a flow of pure fuel into the secondary combustion zone of the combustor; wherein the at least one fuel nozzle generates a first flow of combustion gases having a first temperature, the one or more premix injectors generate a second flow of combustion gases having a second temperature greater than the first temperature, and the one or more injectors generate a third flow of combustion gases having a third temperature. 
     The combustor as in one or more of these clauses, wherein the one or more injectors are disposed at an axial location of the one or more premix injectors. 
     The combustor as in one or more of these clauses, wherein the one or more injectors inject pure hydrogen into the secondary combustion zone.