Abstract:
A system includes a fuel supply system. The fuel supply includes a primary fuel supply, a fuel additive supply, and a common pipeline coupled to the primary fuel and fuel additive supplies. The primary fuel supply includes a primary fuel having a first average molecular weight. The fuel additive includes a fuel additive having a second molecular weight that is greater than the first average molecular weight. The common pipeline is configured to direct a mixture of the primary fuel and the fuel additive into a fuel nozzle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to gas turbines, and more specifically, to systems and methods for controlling fuel flow in fuel nozzles. 
         [0002]    Gas turbine systems generally include a compressor, a combustor, and a turbine. The compressor compresses air from an air intake, and subsequently directs the compressed air to the combustor. In the combustor, the compressed air received from the compressor is mixed with a fuel and is combusted to create combustion gases. The combustion gases are directed into the turbine. In the turbine, the combustion gases pass across turbine blades of the turbine, thereby driving the turbine blades, and a shaft to which the turbine blades are attached, into rotation. The rotation of the shaft may further drive a load, such as an electrical generator, that is coupled to the shaft. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
         [0004]    In one embodiment, a system includes a fuel supply system. The fuel supply includes a primary fuel supply, a fuel additive supply, and a common pipeline coupled to the primary fuel and fuel additive supplies. The primary fuel supply includes a primary fuel having a first average molecular weight. The fuel additive includes a fuel additive having a second molecular weight that is greater than the first average molecular weight. The common pipeline is configured to direct a mixture of the primary fuel and the fuel additive into a fuel nozzle. 
         [0005]    In a second embodiment, a gas turbine engine includes a compressor, a combustor, a fuel supply system, and a turbine. The compressor is configured to compress air. The combustor comprises at least one fuel nozzle and is configured to receive the air from the compressor and to combust the air and a fuel mixture to generate combustion products. The fuel supply system is configured to supply the fuel mixture to the at least one fuel nozzle. The fuel supply system includes a primary fuel supply, a fuel additive supply, and a common pipeline coupled to the primary fuel supply and the fuel additive supply. The primary fuel supply includes a primary fuel having a first average volumetric heating value. The fuel additive includes a fuel additive having a second average volumetric heating value that is greater than the first average volumetric heating value. The common pipeline is configured to mix the primary fuel and the fuel additive to form the fuel mixture and to direct the fuel mixture to the combustor. The turbine is configured to receive the combustion products from the combustor. 
         [0006]    In a third embodiment, a method includes detecting an operating parameter related to combustion of air and a fuel mixture within a combustor, determining if the operating parameter is desirable using a sensor and a controller, and adjusting a flow rate of a fuel additive based on a measurement of the operating parameter. The fuel additive has a first volumetric heating value that is greater than an overall volumetric heating value of the fuel mixture. The fuel mixture includes a primary fuel and a fuel additive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0008]      FIG. 1  is a schematic diagram of an embodiment of a gas turbine system having a fuel supply system with features to improve flame stability; 
           [0009]      FIG. 2  is a perspective view of an embodiment of fuel nozzles of the fuel supply system, illustrating an arrangement of the fuel nozzles within a combustor of the gas turbine system; 
           [0010]      FIG. 3  is a block diagram of an embodiment of the fuel supply system of  FIG. 1 , illustrating a fuel additive supply containing higher hydrocarbons (HHCs) and/or diluents to improve flame stability within the combustor; 
           [0011]      FIG. 4  is a partial cross-sectional view of an embodiment of the fuel nozzle of  FIG. 1  with a plurality of swirl vanes to mix fuel and air for delivery into the combustor; 
           [0012]      FIG. 5  is a perspective view of an embodiment of the swirl vane of  FIG. 4 ; and 
           [0013]      FIG. 6  is a partial cross-sectional view of an embodiment of the fuel nozzle of  FIG. 1  with a plurality of pilot tubes configured to mix fuel and air for delivery into the combustor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0015]    When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0016]    By way of introduction, a distinction should be drawn between the terms “energy output” and “energy density”. The term “energy output” may refer to a rate of energy produced by combustion of a fuel. Accordingly, the energy output of a system may be increased by increasing a flow rate of the fuel. On the other hand, the terms “energy density” and “heating value” refer to intensive properties of the fuel. The energy density may be volumetric (i.e., energy produced per unit volume), molar (i.e., energy produced per mole of substance), or have another suitable basis. Notably, changing the flow rate of the fuel may have no impact on its energy density. 
         [0017]    The present disclosure is directed toward systems and methods to improve flame stability within combustors of gas turbine systems. In particular, a fuel additive may be added to a primary fuel to form a fuel mixture, and the fuel mixture may then be directed to the combustor for combustion. The composition of the fuel mixture may be varied in order to adjust certain properties (e.g., energy density or heating value) of the fuel, which, in turn, may adjust the pressure, temperature, length, volume, flame front shape, or another parameter of the combustion flame. In certain embodiments, the fuel additive includes a higher hydrocarbon (HHC) with a higher molecular weight, density, and/or volumetric energy density than the primary fuel. For example, the primary fuel may be mostly methane, and the fuel additive may include ethane, propane, or butane. 
         [0018]    As will be appreciated, the primary fuel is often a mixture of several components, such as hydrocarbons (e.g., alkanes), sulfur (e.g., thiols), and/or nitrogen (e.g., amines). These components define average properties (e.g., molecular weight, energy density, etc.) of the primary fuel. The primary fuel is typically not homogenous, and the average properties may change over time, often unpredictably. Unfortunately, these unpredictable changes result in combustion instabilities within the gas turbine system. For example, the energy density (e.g., heating value) of the primary fuel may decrease, resulting in fluctuations in flame temperature, flame pressure, and flame volume. Thus, it is now recognized that the addition of a fuel additive may help reduce these fluctuations within the primary fuel. In other words, the fuel additive can help change the average energy density, molecular weight, volumetric flow rate, and other properties of the primary fuel. In this manner, the fuel properties can help stabilize the flame, thereby reducing combustion dynamics and increasing the efficiency of the gas turbine system. 
         [0019]    Turning now to the figures,  FIG. 1  illustrates a block diagram of an embodiment of a gas turbine system  10  having a fuel supply system  11  with features to improve the operability of the gas turbine system  10 . For example, the fuel supply system  11  may supply at least one fuel additive (e.g., one or more HHCs) for stabilizing a flame within the gas turbine system  10 . In the illustrated embodiment, the fuel supply system  11  includes a fuel manifold  12 , which may route or flow a primary fuel and one or more fuel additives. Throughout the discussion, a set of axes will be referenced. These axes are based on a cylindrical coordinate system and point in an axial direction  14 , a radial direction  16 , and a circumferential direction  18 . For example, the axial direction  14  extends along a longitudinal axis  20  of the gas turbine system  10 , the radial direction  16  extends away from the longitudinal axis  20 , and the circumferential direction  18  extends around the longitudinal axis  20 . 
         [0020]    As illustrated, the gas turbine system  10  includes a compressor  22 , a combustor  24 , and a turbine  26 . The compressor  22  receives air  28  from an intake  30  and compresses the air  28  for delivery to the combustor  24 . A portion of the air  28  is routed to a fuel nozzle  32 , where the air  28  may premix with a fuel (e.g., a fuel mixture)  34  before entering the combustion zone. As shown, the fuel  34  is supplied by the fuel manifold  12 . The fuel manifold  12  may also supply a fuel additive (e.g., one or more HHCs) to adjust the composition of the fuel  34 . As noted earlier, the composition of the fuel  34  may be adjusted in order to improve the stability of the flame within the combustor  24 . 
         [0021]    The air  28  and the fuel  34  are fed to the combustor  24  at a ratio suitable for combustion, emissions, power output, and the like. The mixture of the air  28  and the fuel mixture  34  is subsequently combusted in the combustor  24 , thereby producing hot combustion products. The hot combustion products enter the turbine  26  and force blades  36  of the turbine  26  to rotate, thereby driving a shaft  38  of the gas turbine system  10  into rotation. The rotating shaft  38  provides the energy for the compressor  22  to compress the air  28 . More specifically, the rotating shaft  38  further rotates compressor blades  39  attached to the shaft  38  within the compressor  22 , thereby compressing the air  28  that is fed into the compressor  22 . In addition, the rotating shaft  38  may rotate a load  40 , such as an electrical generator or any device capable of utilizing the mechanical energy of the shaft  38 . After the turbine  26  extracts useful work from the combustion products, the combustion products are discharged to an exhaust  42 . 
         [0022]    As noted previously, the fuel supply system  11  supplies a fuel additive to one or more fuel nozzles  32  in order to improve combustion stability.  FIG. 2  illustrates an arrangement of the fuel nozzles  32  within the combustor  24  of the gas turbine system  10 . As shown, six fuel nozzles  32  are mounted to a head end  44  of the combustor  24 . However, in other embodiments, the number of fuel nozzles  32  may vary. For example, the gas turbine system  10  may include 1, 2, 3, 4, 5, 10, 50, 100, or more fuel nozzles  32 . 
         [0023]    As illustrated, the six fuel nozzles  32  are disposed in a concentric arrangement. That is, five fuel nozzles  32  (e.g., outer fuel nozzles  46 ) are disposed about a central fuel nozzle  48 . As will be appreciated, the arrangement of the fuel nozzles  32  on the head end  44  may vary. For example, the fuel nozzles  32  may be disposed in a circular arrangement, in a linear arrangement, or in any other suitable arrangement. 
         [0024]    In certain embodiments, the fuel supply system  11  may supply the fuel additive to a certain subset of the fuel nozzles  32 . For example, the central fuel nozzle  48  (e.g., pilot fuel nozzle) generally may have a greater influence on combustion dynamics, and it may be desirable to supply the fuel additive to the central fuel nozzle  48 . However, certain embodiments of the fuel supply system  11  may supply the fuel additive to all of the fuel nozzles  32 . In addition, the fuel supply system  11  may supply a similar or different HHC to each of the fuel nozzles  32 . The components of the fuel supply system  11  are discussed below with respect to  FIG. 3 . 
         [0025]      FIG. 3  illustrates a block diagram of an embodiment of the fuel supply system  11 . The fuel supply system  11  includes a primary fuel supply  50  and a fuel additive (e.g., HHC) supply  52  coupled together by a common pipe  54 . That is, the primary fuel  50  and the HHC  52  combine to form the fuel mixture  34  that is directed to the fuel nozzles  32 . The HHC  52  may be supplied from storage tanks coupled to the fuel supply system  11 . As noted earlier, the fuel additive  52  may be any fuel having a greater molecular weight, density, and/or energy density than the primary fuel  50 . For example, the primary fuel  50  may include methane, and the HHC  52  may include ethane, propane, butane, another alkane, an alkene, alkyne, or any other suitable hydrocarbon. It should be noted that the HHC  52  is often a mixture of various components, and may include hydrocarbons, thiols, amines, and the like. In certain embodiments, the HHC  52  may include any species with more than one carbon molecule (e.g., C 1 +). The HHC  52  may have at least, on average, 1, 2, 3, 4, 5, or more carbon atoms per molecule than the primary fuel  50 . Certain HHCs  52  have a higher heating value (HHV) in the range of 1500 to 11000 BTU/cubic foot. 
         [0026]    The combustor  24  may be designed to combust the fuel mixture  34  to produce a specific total energy output. As noted earlier, the energy density of the primary fuel  50  may vary, which results in a variable total energy output. In order to stabilize and maintain the total energy output, the flow rate and/or the composition of the fuel mixture  34  may be varied. As will be appreciated, stabilization of the energy output may improve the operability and efficiency of the gas turbine system  10 . 
         [0027]    The flow rate of the fuel mixture  34  can be increased or decreased by adding a diluent  56  and/or the HHC  52  to achieve a desired total heat output. In order to maintain an approximately constant total heat output, the HHC  52  may be used to increase the energy density of the fuel mixture  34 , thereby reducing the total flow rate of the fuel mixture  34 . Depending on the desired total energy output, the flow rate of the HHC  52  may be less than approximately 40, 30, or 20 percent of the total flow rate of the fuel mixture  34  by volume. However, in certain embodiments, it may be desirable to adjust the total energy output without changing the flow rate of the fuel mixture  34 . 
         [0028]    In certain embodiments, the fuel nozzle  32  may operate using the primary fuel  50  without the HHC  52  until it is desired to adjust the energy density of the fuel mixture  34 . For example, the gas turbine system  10  may include a plurality of operating modes, such as a startup mode, a steady-state mode, a low load mode, a medium load mode, a high load mode, a transient mode, a shut-down mode, or any other mode, each having a controlled ratio of primary fuel  50  to the HHC  52 . For example, each of these modes may include a mixture of primary fuel  50  and 1, 2, 3, 4, 5, or more fuel additives  52  (i.e., HHCs). The primary fuel  50  and one or more fuel additives  52  may change for each mode, or they may be partially or entirely the same. Furthermore, the ratio among the primary fuel  50  and one or more fuel additives  52  may change from one mode to another. 
         [0029]    It may also be desirable to increase the flow rate of the fuel mixture  34  without changing the total heat output. As shown, a diluent supply  56  is coupled to the common pipe  54 . The diluent  56  may be any material having a lower average molecular weight, density, and/or energy density than the primary fuel  50 . For example, the diluent may be steam, nitrogen, another inert gas, an alcohol, ketone, or any other suitable material. Thus, the addition of the diluents  56  into the primary fuel  50  decreases the energy density of the fuel mixture  34 . In a manner similar to the HHC  52  above, a flow rate of the primary fuel  50  may be decreased and a flow rate of the diluent  56  may be increased by a greater amount that impacts the pressure drop across the fuel nozzle  32 . 
         [0030]    In order to adjust the flow rates of the primary fuel  50 , the HHC  52 , and the diluent  56 , control valves  58 ,  60 , and  62  are disposed along their respective flow paths. The control valves  58 ,  60 , and  62  are communicatively coupled to a controller  64 . As shown, the controller  64  includes a processor  66  and memory  68  to execute instructions to control the combustion dynamics by adjusting the control valves  58 ,  60 , and  62 . These instructions may be encoded in software programs that may be executed by the processor  66 . Further, the instructions may be stored in a tangible, non-transitory, computer-readable medium, such as the memory  68 . The memory  68  may include, for example, random-access memory, read-only memory, hard drives, and the like. In certain embodiments, the controller  64  may execute instructions to control the ratio of the primary fuel  50  to the HHC  52  in each operating mode of the gas turbine system (e.g., startup mode, steady-state mode, etc.) 
         [0031]    The combustion dynamics and flame stability are largely affected by the energy output and energy density of the fuel mixture  34 . However, the flame stability is affected by a myriad of other operating parameters, such as flame temperature, pressure fluctuations, flow rates, pressure drops, and the like. Accordingly, it is desirable to monitor certain operating parameters and adjust the flow rates of the primary fuel  50 , the fuel additive  52 , and/or the diluent  56  in response to the monitored operating parameters. 
         [0032]    As shown, a sensor  70  is disposed upstream of the fuel nozzle  32  and another sensor  72  is disposed within or downstream of the fuel nozzle  32 . The sensors  70  and  72  monitor various operating conditions of the fuel supply system  11  and the fuel nozzle  32 , such as pressure, flow rates, pressure differentials, flame temperature, flame length, flame volume, and the like. The sensors  70  and  72  are communicatively coupled to the controller  64 , which may adjust the control valves  58 ,  60 , and  62  based on the operating parameters detected by the sensors  70  and  72 . For example, the controller  64  may determine that an operating parameter is not desirable and may execute instructions to adjust the control valves  58 ,  60 ,  62  in order to adjust the operating parameter toward a desired range. 
         [0033]    In certain embodiments, the sensors  70  and  72  may detect a pressure drop or differential across a portion of the fuel nozzle  32 . As noted earlier, the pressure differential may affect combustion dynamics, and thus, it is desirable to monitor and adjust the pressure differential. In a similar manner to adjusting the energy density of the fuel mixture  34 , the pressure differential across the fuel nozzle  32  may be varied by changing the composition of the fuel mixture  34 . Certain HHCs  52  have a greater heating value (i.e., energy per unit volume) than the primary fuel  50 . Thus, the HHC  52  and the primary fuel  50  may be mixed in certain ratios to reduce the flow rate of the fuel mixture  34  while maintaining an approximately constant total heat output. Lower flow rates typically have lower pressure drops through orifices, and thus, adding HHCs  52  to the fuel mixture  34  may decrease the pressure drop across the fuel nozzle  32 . Decreasing the pressure drop may be desirable, for example, to shift the heat release location and reduce combustion dynamics. 
         [0034]    As noted earlier, changes in the operating conditions of the combustor  24  may result in combustion instabilities. Accordingly, it may be desirable to adjust the pressure differential across the fuel nozzle  32 , while maintaining an approximately constant energy output. For example, a flow rate of the HHC  52  may be increased while a flow rate of the primary fuel  50  is decreased, such that the additional energy output contributed by the HHC  52  is substantially offset by the decreased energy output contributed by the primary fuel  50 . In this manner, the resulting flow rate of the fuel mixture  34  is decreased and the density of the fuel mixture  34  is increased. As will be appreciated, the decreased flow rate may decrease the pressure drop across the fuel nozzle  32 . The increased density may have a positive effect on flame location or shape. In summary, the composition and/or flow rate of the fuel mixture  34  may be changed to adjust the pressure differential across the fuel nozzle  32 , while maintaining an approximately constant energy output. 
         [0035]    The diluent  56  may be used in a similar manner to adjust the pressure differential across the fuel nozzle  32 . That is, the flow rate of the diluent  56  may be increased and the flow rate of the primary fuel  50  may be decreased, such that the total energy output of the fuel mixture  34  remains the same. Certain diluents  56  have a lower energy density than the primary fuel  50 . Thus, the diluents  56  and the primary fuel  50  may be mixed with various ratios to increase the flow rate of the fuel mixture  34  while maintaining an approximately constant total heat output. Higher flow rates typically have higher pressure drops through orifices, and thus, adding the diluents  56  to the fuel mixture  34  generally increases the pressure drop across the fuel nozzle  32 . In certain configurations, the flow rate of the diluent  56  may be increased while the flow rate of the primary fuel  50  is decreased or remains the same. As a result, the energy density of the fuel mixture  34  is decreased. As will be appreciated, in such a configuration, the net effect on the pressure drop helps to mitigate the combustion dynamics. Accordingly, the addition of the diluent  56  may be designed to increase the pressure drop across the fuel nozzle  32 , which may be desirable to improve the stability of the combustion flame. 
         [0036]    The disclosed technique of holding one operating parameter constant while modifying another using the HHC  52 , the diluent  56 , or both, can be applied to various other operating parameters of the fuel nozzle  32 . For example, the pressure, energy density, flow rate, pressure drop, pressure drop, flame length, flame volume, and the like may be held constant while another parameter is simultaneously varied. In addition, the aforementioned operating parameters are given by way of example and are not intended to be limiting. That is, it may be desirable to adjust the flow rates of the primary fuel  50 , the HHC  52 , the diluent  56 , or any combination thereof to affect any other operating parameter related to combustion stability. 
         [0037]    It should be noted that the fuel additive supply  52  and the diluent supply  56  may be used independently or in combination with each other. That is, certain embodiments of the fuel supply system  11  may include the fuel additive supply  52  but not the diluent supply  56 . In addition, the use of the fuel additive supply  52  or the diluent supply  56  may be based on the orientation of the fuel nozzles  32  about the head end  44  (see  FIG. 2 ). For example, the central fuel nozzle  48  may have a greater impact on combustion dynamics and may benefit more from addition of the fuel additive  52 . On the other hand, it may be desirable to provide the outer fuel nozzles  44  with the diluent  56  in order to control the volume of the combustion flame. 
         [0038]    It may be desirable to modify the fuel composition at different operating modes (e.g., startup, steady-state, partial load, full load, etc.) of the gas turbine system  10 . For example, each operating mode may have a different ratio of the HHC  52  to the primary fuel  50 . As will be appreciated, fuel gas compressors use a lower molecular weight fuel (e.g., the primary fuel  50 ) during start up. During partial load conditions, the HHC  52  may be slowly introduced in order to increase the molecular weight and heating value of the fuel mixture  34 . At steady-state, the HHC  52  may be maintained in order to control the heating value of the mixture  34 , as explained earlier. For example, the HHC  52  may be between approximately 0 to 30, 5 to 25, 10 to 20, or 12 to 18 percent of the total flow of the fuel mixture  34 , depending on the operating mode of the gas turbine system  10 . In addition, the percent of the HHC  52  may be the same or different between the fuel nozzles  32  (e.g., central fuel nozzle  48  and outer fuel nozzles  46 ) 
         [0039]    The configuration of the fuel nozzles  32  may also vary, as described below with respect to  FIGS. 4-6 . For example,  FIG. 4  illustrates an embodiment of the fuel nozzle  32  with a plurality of swirl vanes  74  to mix the air  28  with the fuel mixture  34 . As shown, the fuel nozzle  32  includes an inner wall  76  defining a central passage  78 . Additionally, a shroud  80  surrounds the inner wall  76 , thereby defining an annular passage  82 . 
         [0040]    During operation of the fuel nozzle  32 , the air  28  flows through the annular passage  82  toward a combustion region  84 . As illustrated, the fuel mixture  34  (e.g., primary fuel  50  and HHC  52 ) flows through the central passage  78  and enters the annular passage  82  through orifices  86  of the swirl vanes  74 . For example,  FIG. 5  is a perspective view of an embodiment of the swirl vane  74  having orifices  86 . As noted earlier, the operability of the fuel nozzle  32  may be affected by the pressure drop of the fuel mixture  34  across the fuel nozzle  32 . For example, the stability of the combustion flame may be affected by the pressure drop of the fuel mixture  34  across the orifices  86 . Turning back to  FIG. 4 , sensors  88  and  90  are disposed within the fuel nozzle  32  to detect the pressure drop across the orifices  86 . In certain embodiments, the sensor  90  measure pressure detects fluctuations caused by the combustion of the fuel mixture  34  and the air  28 . The sensors  88  and  90  are communicatively coupled to the controller  64  of  FIG. 3 , which may adjust the control valves  58 ,  60 , and  62  in response to the pressure drop detected by the sensors  88  and  90 . For example, the pressure fluctuations may be used to determine respective flow rates of the HHC  52  and/or the diluent  56 . 
         [0041]    A mixture  92  of the air  28  and the fuel  34  flows to the combustion region  84 , where the mixture  92  combusts, forming a combustion flame  94 . As shown, the combustion flame  94  occupies a volume  96 , which may be adjusted by adding the HHC  52  or the diluent  56  to the primary fuel  50 , as explained in detail above. 
         [0042]      FIG. 6  illustrates another embodiment of the fuel nozzle  32  with a plurality of pilot tubes  98  to mix the air  28  with the fuel mixture  34  (e.g., primary fuel  50  and HHC  52 ). During operation of the fuel nozzle  32 , the air  28  flows through the central passage  78  and axially  14  into the plurality of pilot tubes  98 . The fuel mixture  34  flows through the annular passage  82  and radially  16  into the pilot tubes  98 . In certain embodiments, the fuel  34  may be supplied through one or more feed tubes. The air  28  and the fuel  34  mix within the pilot tubes  98 , and the mixture  92  of air  28  and fuel  34  is subsequently directed into the combustion region  84 . However, it should be noted that a variety of other configurations of fuel nozzles  32  with varying flow paths for the air  28  and the fuel  34  may be used. The sensors  88  and  90  are disposed within the annular passage  82  and the central passage  78 , respectively. The sensor  88  measures an upstream pressure of the fuel  34  (e.g., upstream of the pilot tubes  98 ), whereas the sensor  90  measures a downstream pressure of the fuel  34  (e.g., downstream of the pilot tubes). In certain embodiments, it may be desirable to adjust the composition of the fuel mixture  34  to improve the stability of the flame  94  based on the pressures detected by the sensors  88  and  90 . As explained earlier, the composition of the fuel  34  may be controlled by adjusting the control valves  58 ,  60 , and  62 , which in turn are controlled by the controller  64 . 
         [0043]    It should be noted that the embodiments of the fuel nozzles  32  and their respective geometries are not intended to be limiting. For example, the fuel mixture  34  may flow through the central passage  78  and air may flow through the annular passage  82  and vice versa. Indeed, the disclosed techniques may be applied to a variety of fuel nozzle designs, all of which fall within the scope and spirit of the present disclosure. 
         [0044]    Technical effects of the disclosed embodiments include the use of the HHC  52  to adjust a fuel composition of the fuel mixture  34 . The composition of the fuel mixture  34  is varied in order to adjust certain properties (e.g., energy density or heating value) of the fuel  34 , which, in turn, adjusts the pressure, temperature, length, volume, or another parameter of the combustion flame  94 . The HHC  52  has a higher molecular weight, density, and/or energy density than the primary fuel  50 , which enables various properties of the fuel mixture  34  to be adjusted. In particular, the energy density of the fuel mixture  34  may be adjusted while maintaining an approximately constant heat input to the combustor  24  by varying the flow rate of the HHC  52  and/or the diluents  56 . Additionally or alternatively, the pressure drop across the fuel nozzle  32  may be adjusted while maintaining an approximately constant energy output. These adjustments improve the stability of the combustion flame  94 , and subsequently, improve the efficiency and operability of the gas turbine system  10 . 
         [0045]    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 invention 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 have 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 languages of the claims.