Patent Publication Number: US-2015059348-A1

Title: System and method for controlling fuel distributions in a combustor in a gas turbine engine

Description:
BACKGROUND 
     The subject matter disclosed herein relates generally to gas turbines, and, more particularly to systems and methods for operating gas turbines. 
     Gas turbine engines include one or more combustors, which receive and combust air and fuel to produce hot combustion gases. Some gas turbine engines produce undesirable emissions, such as oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). In some circumstances, it may be desirable to operate the gas turbine engine at a reduced rate or power level. However, when operating at reduced rates, it is difficult to maintain low levels of emissions. For example, the temperature within the combustor may be too low to completely combust fuel when the gas turbine engine is operating at a reduced rate, and as a result, the gas turbine engine may produce undesirable emissions. 
     BRIEF DESCRIPTION 
     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. 
     In a first embodiment, a gas turbine engine system includes a plurality of combustors arranged circumferentially about a rotational axis of the gas turbine engine. A first combustor includes one or more fuel nozzles and one or more fuel injectors positioned generally downstream from the one or more fuel nozzles. The first combustor also includes a first valve disposed along a fuel delivery line between a fuel circuit and the first combustor, the first valve being configured to adjust a first flow of the fuel to the first combustor. The first combustor also includes a second valve disposed along the fuel delivery line between the first valve and at least one of the one or more fuel injectors, the second valve being configured to adjust a second flow of the fuel to at least one of the one or more fuel injectors. 
     In a second embodiment, a method of operating a gas turbine engine is provided. The method includes the steps of directing fuel to a plurality of combustors using a controller, wherein each of the plurality of combustors is configured to receive fuel via one or more fuel nozzles and one or more fuel injectors, wherein the one or more fuel nozzles are positioned proximate to a first end of each of the plurality of combustors and the one or more fuel injectors are positioned proximate to a second end of each of the plurality of combustors. The method may also include stopping a first flow of fuel to a subset of the plurality of combustors using the controller, and adjusting a second flow of fuel to the one or more fuel injectors of at least one of the plurality of combustors that is not in the subset using a controller. 
     In a third embodiment, a system includes instructions disposed on a non-transitory, machine readable medium, and the instructions are configured to direct fuel to a plurality of combustors, wherein each combustor is coupled to a plurality of fuel nozzles positioned proximate to a first end of the combustor and at least one fuel injector positioned proximate to a second end of the combustor. The system also includes instructions to control a first valve to stop a first flow of fuel to a subset of the plurality of combustors, and to control a second valve to adjust the second flow of fuel to the at least one fuel injector of at least one of the plurality of combustors that is not part of the subset. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a block diagram of an embodiment of a gas turbine system; 
         FIG. 2  is a partial side cross-sectional view of an embodiment of a gas turbine system; 
         FIG. 3  is a schematic illustration of an embodiment of a gas turbine system having a plurality of control devices to adjust the flow of fuel within the gas turbine system; 
         FIG. 4  is a schematic illustration of an embodiment of a gas turbine system having a plurality of control devices to adjust the flow of fuel within a plurality of combustors; 
         FIG. 5  is a schematic illustration of an embodiment of a gas turbine system having a plurality of control devices to adjust the flow fuel within a plurality of combustors; 
         FIG. 6  is a schematic illustration of an embodiment of a gas turbine system having a plurality of combustors arranged into a plurality of sectors, and a plurality of control devices to adjust the flow of fuel within the plurality of combustors; and 
         FIG. 7  is a front perspective view of an arrangement of combustors within a gas turbine system, in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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. 
     Gas turbine systems in accordance with the present disclosure may be configured to operate at reduced rates or power levels (e.g., turn down), while maintaining suitably low emissions. The primary emissions typically produced by gas turbine engines of gas turbine systems include oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO), which are subject to various federal and state regulatory limitations. Emissions may be reduced and/or maintained within regulatory compliance by certain operational conditions within the gas turbine system. For example, NOx and CO emissions may be kept within compliance if flame temperatures within a combustor of the gas turbine system are maintained at certain levels. The flame temperature within the combustor is highly dependent upon the fuel/air ratio, and thus, the temperature and emissions may be controlled by adjusting the fuel flow within the combustor. In some circumstances, however, it may be desirable to operate the gas turbine system at a reduced rate or power level. For example, during off-peak hours it is impractical and expensive to operate the gas turbine system at full power. Additionally, completely stopping and restarting the gas turbine system is a lengthy process and can impact the durability of system components. Thus, it is generally preferred to turn down the gas turbine system, rather than stopping the gas turbine engine, during periods of low demand. The reduced power level may be achieved by decreasing the fuel flow to the combustor. However, when operating at such reduced power levels, it can be particularly difficult to maintain emissions compliance. For example, the temperatures within the combustor may be too low to complete combustion of the fuel, which may result in an increase in emissions. 
     Certain turn down methods that enable the gas turbine system to remain emissions compliant may generally result in a decrease in power level to only about 40% of normal output. The present disclosure provides systems and methods to enable the gas turbine system to operate at very low power levels, while maintaining suitably low emissions. For example, systems and methods in accordance with the present disclosure may enable the gas turbine system to remain emission compliant and turn down to about 15%, 20%, 25%, or 30% of normal output. By providing one or more control devices (e.g., valves), the flow of fuel may be directed and adjusted in a manner that enables the gas turbine system to achieve very low power levels and low emissions. For example, valves may be controlled to adjust the flow of fuel to certain fuel injectors and/or to certain combustors within the gas turbine system in a manner that results in reduced rates and fuel/air ratios that maintain low emissions. More particularly, in some turn down operations discussed herein, the flow of fuel to certain downstream fuel injectors (e.g., late lean injectors) may be reduced and the flow of fuel to at least one of the combustors in the gas turbine system may be reduced or stopped. Such turn down methods may also enable the gas turbine system to quickly return to full power if demand increases. Additionally, in some embodiments of the present disclosure, the flow of fuel may be adjusted to shut down the combustor in a manner that reduces thermal stress on the components along the hot gas path, as described in more detail below. 
     Turning to the drawings,  FIG. 1  illustrates a block diagram of an embodiment of a gas turbine system  10 , which may be configured to operate at low power levels while maintaining suitably low emissions. The systems and methods described herein may be used in any turbine system, such as gas turbine systems, and is not intended to be limited to any particular machine or system. As shown, the system  10  includes a compressor  12 , a turbine combustor  14 , and a turbine  16 . The system  10  may include one or more combustors  14  that include one or more fuel nozzles  18  configured to receive a liquid fuel and/or gas fuel  20 , such as natural gas or syngas. The system  10  may also include one or more fuel injectors  22  (e.g., late lean fuel injectors or LLI&#39;s) positioned generally downstream from the one or more fuel nozzles  18  and configured to inject the fuel  20 , or a mixture of fuel  20  and air, into the combustor  14 . The system  10  may include a controller  23  that is generally configured to control the flow of fuel to the one or more fuel nozzles  18  and/or to the one or more LLI&#39;s  22 . The controller  23  may be any suitable engine controller that is configured to send and/or to receive signals from the gas turbine system  10  and to control the flow of fuel within the gas turbine system  10 . 
     The turbine combustors  14  ignite and combust a fuel-air mixture, and then pass hot pressurized combustion gases  24  (e.g., exhaust) into the turbine  16 . Turbine blades are coupled to a shaft  26 , which is also coupled to several other components throughout the turbine system  10 . As the combustion gases  24  pass through the turbine blades in the turbine  16 , the turbine  16  is driven into rotation, which causes the shaft  26  to rotate. Eventually, the combustion gases  24  exit the turbine system  10  via an exhaust outlet  28 . Further, the shaft  26  may be coupled to a load  30 , which is powered via rotation of the shaft  26 . For example, the load  30  may be any suitable device that may generate power via the rotational output of the turbine system  10 , such as an electrical generator, a propeller of an airplane, and so forth. 
     Compressor blades may be included as components of the compressor  12 . The blades within the compressor  12  are coupled to the shaft  26 , and will rotate as the shaft  26  is driven to rotate by the turbine  16 , as described above. An intake  32  feeds air  34  into the compressor  12 , and the rotation of the blades within the compressor  12  compress the air  34  to generate pressurized air  36 . The pressurized air  36  is then fed into the one or more fuel nozzles  18  and/or the LLI&#39;s  22  of the turbine combustors  14 . The one or more fuel nozzles  18  mix the pressurized air  36  and fuel  20  to produce a suitable mixture ratio for combustion (e.g., a combustion that causes the fuel to more completely burn) so as not to waste fuel or cause excess emissions. As described in more detail below, the system  10  may be configured to operate at very low power levels while maintaining suitably low emissions. 
       FIG. 2  is a partial cross-sectional side view of an embodiment of the combustor  14  of the gas turbine system  10 . As shown, the gas turbine system  10  may be described with reference to a longitudinal axis or direction  38 , a radial axis or direction  40 , and a circumferential axis or direction  42 . The gas turbine system  10  includes one or more fuel nozzles  18  disposed within a head end  43  of the combustor  14 . The one or more fuel nozzles  18  may also be generally positioned proximate to (e.g., near, adjacent, etc.) a first end  44  of the combustor  14 . Further, the combustor  14  may include one or more late lean injectors  22  (LLI&#39;s) positioned proximate to a second end  46  of the combustor, the second end  46  being located generally downstream from the first end  44  in a direction of flow of hot combustion gases toward the turbine  16 . 
     As shown in  FIG. 2 , one or more control devices, or valves  64 , may be provided to control the flow of fuel  20 . The valves  64  may be arranged in any suitable manner. For example, in the depicted embodiment, at least one valve  64   a  is disposed along a fuel delivery line  62  (e.g., manifold) between a fuel circuit  60  and the combustor  14 , and the valve  64   a  is positioned so that the valve  64   a  may adjust delivery of fuel  20  to the combustor  14  (e.g., to the one or more fuel nozzles  18  and to the LLI&#39;s  22 ). 
     Additionally, one or more valves  64   b  may be provided to enable an additional level of control or independent control of the flow of fuel  20  to the LLI&#39;s  22 . In the illustrated embodiment, the valves  64   b  are disposed along the fuel delivery line  62  between the valve  64   a  and the LLI&#39;s  22 . As shown, the LLI&#39;s  22  may be structurally supported by a liner and/or flow sleeve  47  surrounding a transition zone  48  of the combustor  14 . The LLI&#39;s  22  are configured to provide fuel  20  to the combustor  14  at one or more axial stages, or regions, along the longitudinal axis  38  of the combustor  14 . The LLI&#39;s  22  may be configured to inject fuel  20  into the combustor  14  as shown by arrows  50 , the fuel  20  being injected in a direction that is generally transverse to a flow direction  52  within the combustor  14 . Such a configuration creates local zones of stable combustion within the combustor  14  during operation of the gas turbine system  10 . Additionally, the flow of fuel  20  to the LLI&#39;s  22  may also be adjusted by valves  64   a ,  64   b  in a manner that facilitates turn down while maintaining suitably low emissions, as described in more detail below. 
     As discussed above, the one or more LLI&#39;s  22  may be disposed at one or more axial stages or regions of the combustor  14 . In some embodiments, multiple LLI&#39;s  22  are disposed circumferentially  42  about the combustor  14  at a single axial stage along the longitudinal axis  38  of the combustor  14 . In certain embodiments, multiple LLI&#39;s  22  are disposed circumferentially  42  about the combustor  14  at multiple axial stages along the longitudinal axis  38  of the combustor  14 . Thus, a first axial stage may include one or more LLI&#39;s  22 , and a second axial stage may include one or more LLI&#39;s  22 . The LLI&#39;s  22  may be arranged in any suitable manner. For example, the LLI&#39;s  22  of the first axial stage and the LLI&#39;s  22  of the second axial stage may be circumferentially  42  staggered with respect to one another. The axial stages may also include the same number or a different number of LLI&#39;s  22 . 
     The fuel circuit  60  may supply the fuel  20  to the fuel nozzles  18  and/or to the LLI&#39;s  22 . The fuel  20  may be delivered to the fuel nozzles  18  and/or to the LLI&#39;s  22  via the fuel delivery line  62 . It should be understood that multiple fuel circuits and/or multiple fuel delivery lines  62  may be incorporated into the systems of the present disclosure. As indicated above, one or more valves  64   b  may be provided to independently adjust the flow of fuel  20  to the LLI&#39;s  22 . In the embodiment of  FIG. 2 , one valve  64   b  is provided for each LLI  22 , although any suitable configuration is envisioned. In some embodiments, one valve  64   b  may adjust the flow of fuel  20  to more than one LLI  22 . In some embodiments, one valve  64   b  may adjust the flow of fuel  20  to all of the LLI&#39;s  22  circumferentially  40  arranged in a single axial stage. Thus, the LLI&#39;s  22  of one axial stage may be operated together. In some embodiments, one valve  64   b  may adjust the flow of fuel  20  to each of the LLI&#39;s  22  of two or more axial stages. Thus, the LLI&#39;s  22  of multiple axial stages may be operated together. In some embodiments, one valve  64   b  may adjust the flow of fuel  20  to all of the LLI&#39;s  22  of the combustor  14  or to the LLI&#39;s  22  of multiple combustors  14  of the gas turbine system  10 . 
     The controller  23  may be in communication with the one or more valves  64 . The controller  23  is configured to provide a signal  70  to the valves  64  to open, close, or modulate the valves  64 . Thus, in the illustrated embodiment, the controller  23  controls the valves  64  to adjust the flow and delivery of fuel  20  to the entire combustor  14  and/or to separately control the flow of fuel  20  to the LLI&#39;s  22 . The various valves  64  of the combustor  14  may be positioned in any suitable arrangement and may be adjusted in any suitable manner to enable low turn down, as described in more detail below. 
       FIG. 3  is a schematic illustration of an embodiment of the gas turbine system  10 . As shown, the controller  23  is configured to control one or more control devices, or valves  64 . The one or more valves  64 , in turn, affect or adjust the flow of fuel  20  to various components (e.g., fuel nozzles  18  and LLI&#39;s  22 ) of the combustors  14  of the gas turbine system  10 . For example, in certain turn down operations, the one or more valves  64  may first reduce the flow of fuel  20  to the LLI&#39;s  22  of one or more combustors  14 . When a certain threshold fuel flow rate or temperature is achieved in one or more of the combustors  14  (e.g., as monitored by a sensor or other monitoring device integrated into the system  10 ), the one or more valves  64  may subsequently stop the flow of fuel  20  to at least one of the combustors  14  of the gas turbine system  10 . As described in more detail below, these components of the gas turbine system  10  may be arranged in various configurations and may be operated via various methods to enable very low turn down while maintaining suitably low emissions. 
     The controller  23  may independently control operation of the gas turbine system  10  by electrically communicating with the one or more valves  64  and/or other flow adjusting features of the gas turbine system  10 . The controller  23  may also electrically communicate with one or more sensors, as described in more detail below. The controller  23  may include a distributed control system (DCS) or any computer-based workstation that is fully or partially automated. For example, the controller  23  may be any device employing a general purpose or an application-specific processor, both of which may generally include memory circuitry for storing instructions related to combustion parameters, such as flame temperatures and fuel flow rates. The processor may include one or more processing devices, and the memory circuitry may include one or more tangible, non-transitory, machine-readable media collectively storing instructions executable by the processor to perform the methods and control actions described herein. Such machine-readable media can be any available media that can be accessed by the processor or by any general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by the processor or by any general purpose or special purpose computer or other machine with a processor. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a machine, the machine properly views the connection as a machine-readable medium. Thus, any such connection is properly termed a machine-readable medium. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions comprise, for example, instructions and data which cause the processor or any general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions. As discussed below, the controller  23  may use information provided via input signals received from one or more sensors to execute instructions or code contained on the machine-readable or computer-readable storage medium and generate one or more output signals  70  to the various valves  64 . For example, based on the execution of the instructions or code contained on the machine-readable or computer-readable storage medium of the controller  23 , the output signals  70  may be used to control the flow of fuel  20  within the gas turbine system  10 . 
       FIG. 4  is a schematic illustration of an embodiment of the gas turbine system  10  having a plurality of valves  64  configured to adjust the flow of fuel  20  within the plurality of combustors  14 . In the depicted embodiment, a first combustor  14   a  (e.g., a first combustor can) includes one or more fuel nozzles  18   a , which may be disposed within the head end  43  and positioned proximate to the first end  44  of the combustor  14   a . Additionally, the first combustor  14   a  includes one or more LLI&#39;s  22   a  positioned proximate to the second end  46  of the combustor  14   a . The gas turbine system  10  may include the controller  23 , which is configured to control the plurality of valves  64 . As shown, a first valve  64   a  may be disposed along the fuel delivery line  62  between the fuel circuit  60  and the first combustor  14   a . The first valve  64   a  may be configured to adjust the flow of fuel  20  to the first combustor  14   a . Additionally, a second valve  64   b  may be disposed along the fuel delivery line  62  between the first valve  64   a  and the LLI&#39;s  22 . The second valve  64   b  may be configured to provide an additional level of control and to independently adjust the flow of fuel  20  to the LLI&#39;s  22   a  of the first combustor  14   a . As discussed above with respect to  FIG. 2 , one valve  64   b  may be provided for each LLI  22  or for the LLI&#39;s  22  at each axial stage or for all of the LLI&#39;s of the first combustor  14   a , for example. As shown, a second combustor  14   b  (e.g., a second combustor can) may have a similar arrangement of fuel nozzles  18   b , LLI&#39;s  22   b , and valves  64  as the first combustor  14   a , although it should be understood that the various combustors  14  of the system  10  may have different arrangements and configurations. 
     As discussed above, the controller  23  may control the valves  64  to adjust the amount of fuel  20  that is delivered from the fuel circuit  60  to various components (e.g., the fuel nozzles  18  and/or the LLI&#39;s  22 ) of the combustors  14 . In some embodiments, the controller  23  may selectively operate the valves  64  based upon sensed combustion parameters in the combustors  14 . For example, in certain embodiments, one or more sensors  82  may be configured to sense flow rates of fuel  20  within the fuel delivery lines  62 . The information obtained by the one or more sensors  82  may be provided to the controller  23 , and the controller  23  may initiate various actions, such as opening or closing certain valves  64 . As described in more detail below, the controller  23  may partially close or shut (e.g., completely close) one or more of the valves  64  in one or more of the combustors  14  of the gas turbine system  10  in a manner that reduces fuel consumption and maintains emissions compliance. 
     With reference to  FIG. 4 , during a turn down operation, the controller  23  may control the valves  64  to reduce the flow of fuel  20  to certain portions of the gas turbine system  10 . In some embodiments, the controller  23  may control one or more valves  64   b  to reduce the flow of fuel  20  to one or more LLI&#39;s  22  of one or more of the combustors  14 . The flow of fuel  20  to the LLI&#39;s  22  may be reduced to a certain flow rate (e.g., a threshold rate) or the flow of fuel to the LLI&#39;s  22  may be reduced until a certain flame temperature (e.g., a threshold flame temperature) is achieved within the combustor  14 , for example. As discussed above, in some embodiments, one or more sensors  82  may be provided to detect fuel flow rates and/or temperatures within the combustor  14 . The information collected by the one or more sensors  82  may be used to determine or trigger subsequent steps in the turn down process. For example, when the flow of fuel  20  to the LLI&#39;s  22  of one or more of the combustors  14  reaches a certain threshold flow rate (e.g., a lower threshold flow rate) via valves  64   b , then the controller  23  may subsequently control the valves  64   a  to reduce or to stop the flow of fuel  20  to at least one of the combustors  14  of the gas turbine system  10 . Additionally, in some embodiments, emissions of the system  10  may be monitored by the sensor  82  or other suitable monitoring device. Thus, the controller  23  may be configured to dynamically adjust the flow of fuel to the LLI&#39;s  22  and/or to the fuel nozzles  18 , and/or to stop the flow of fuel to at least one of the combustors  14  to maintain emissions compliance (e.g., below an emissions threshold) during the turn down process. 
     In the embodiment of  FIG. 4 , the flow of fuel  20  to the LLI&#39;s  22  of one or more of the combustors  14  may be reduced to zero (or nearly zero), and subsequently the valve  64   a  of the first combustor  14   a  may be controlled. In certain embodiments, the controller  23  may control the valve  64   a  of the first combustor  14   a  to reduce or stop the flow of fuel  20  to at least the first combustor  14  of the gas turbine system  10 . As a result, the fuel  20  may be directed to adjacent combustors  14 , such as the second combustor  14   b , which may increase the fuel/air ratio in the second combustor  14   b . Such methods may reduce operating power by effectively shutting down the first combustor  14   a  and forcing fuel  20  to the second combustor  14   b , so that the second combustor  14   b  has a higher flame temperature and achieves low emissions. Although only two combustors  14  are shown in  FIG. 2 , it should be understood that in certain embodiments, the controller  23  may control one or more valves  64   a  to stop the flow of fuel  20  to one quarter, one half, or any suitable fraction of the combustors  14  of the gas turbine system  10 . 
     Additionally, the gas turbine system  10  may be returned to full power by controlling valves  64   a  to increase the flow of fuel  20  to at least the turned down combustors  14  of the system  10 . The valves  64   b  may additionally be controlled to adjust the flow of fuel  20  to the LLI&#39;s  22 , thus increasing the power levels of the gas turbine system  10 . Because the gas turbine system  10  can be operated at very low turn down rates via the current methods, the gas turbine engine may not need to be fully shut down during periods of low demand or during off-peak hours. Thus, the gas turbine system  10  does not go through a lengthy start-up process to increase the power level. 
       FIG. 5  is a schematic illustration of another embodiment of a gas turbine system  10  having a plurality of combustors  14  and a plurality of valves  64  configured to adjust the flow of fuel  20  within the plurality of combustors  14 . In the depicted embodiment, the first valve  64   a  is positioned such that the flow of fuel  20  to the fuel nozzles  18  maybe adjusted without affecting the flow of fuel  20  to the LLI&#39;s  22 . Thus, the first valve  64   a  may be provided to independently control the flow of fuel  20  to the fuel nozzles  18 , while the second valve  64   b  may be provided to independently control the flow of fuel  20  to the LLI&#39;s  22 . In certain embodiments, such control may be achieved by positioning the first valve  64   a  along the fuel delivery line  62  between the fuel circuit  60  and the fuel nozzles  18  and by positioning the second valve  64   b  along the fuel delivery line  62  between the fuel circuit  60  and the LLI&#39;s  22 . With reference to  FIG. 5 , in a turn down operation, the flow of fuel  20  to the LLI&#39;s  22  of the first combustor  14   a  may be adjusted via the second valve  64   b . When a certain threshold is reached (e.g., flow rate, flame temperature, etc.), the flow of fuel to the fuel nozzles  18  may be separately adjusted by the first valve  64   a . However, in the illustrated embodiment, the first valve  64   a  does not affect the flow of fuel  20  to the LLI&#39;s  22 . Thus, the depicted system  10  provides additional operational flexibility. For example, the first valve  64   a  and the second valve  64   b  may be operated simultaneously or the flow of fuel  20  to the fuel nozzles  18  and the LLI&#39;s  22  may be fine tuned based system conditions. Additionally, such a configuration may be utilized to efficiently turn down and to fully shut down the gas turbine system  10 . In certain embodiments, the first valve  64   a  may be controlled to adjust the flow of fuel  20  to the one or more fuel nozzles  18 , without affecting the flow of fuel  20  to the LLI&#39;s  22 . Once the flow of fuel  20  to the one or more fuel nozzles  18  reaches a certain threshold (e.g., flow rate, flame temperature, etc.), the second valve  64   b  may be subsequently controlled to reduce the flow of fuel  20  to the LLI&#39;s  22 . Such a technique may be utilized to stop the flow of fuel  20  to one or more combustors  14  within the gas turbine system  10 . Furthermore, such a technique may allow for improved shutdown procedures for the gas turbine system  10 , as the thermal stress to components along the hot gas path may be reduced. 
     Additionally, although not shown in  FIG. 5 , in certain embodiments, an additional valve  64  may be provided upstream of the first valve  64   a  to adjust the flow of fuel  20  to at least one combustor  14  (e.g., to both the fuel nozzles  18  and to the LLI&#39;s  22  of the combustor  14 ), as in  FIG. 4 . Such a configuration would provide additional operational flexibility and control to adjust the flow of fuel  20  within the gas turbine system  10 . The gas turbine system  10  illustrated in  FIG. 5  would also enable relatively quick increase in power when demand increases. The valves  64  may adjust the flow of fuel  20  to the fuel nozzles  18  and/or to the LLI&#39;s  22  to increase the power, without having to go through a lengthy start-up process. 
     Although a single fuel circuit  60  is shown in  FIG. 5 , it should be understood that multiple fuel circuits  60  may be provided. In some embodiments, the fuel  20  may be delivered from the fuel circuit  60  to the one or more fuel nozzles  18  located at the first end  44  of the combustor, and additional fuel  20  or a second different fuel (e.g., LLI fuel) may be delivered from a second different fuel circuit to one or more of the LLI&#39;s  22 . The LLI fuel may include any suitable fuel composition or alternate gas, such as refinery gases or gases having a reactivity higher than methane, for example. Such an arrangement may provide for increased flexibility in the types of fuel that can be utilized and may provide for additional flexibility in the ways in which the flow of fuel  20  can be controlled to enable the system  10  to run at reduced rates and maintain low emissions. The arrangement of the valves  64 , fuel nozzles  18 , and the LLI&#39;s  22  shown in  FIG. 5  would enable independent control of the different types of fuels to the fuel nozzles  18  and/or to the LLI&#39;s  22  within the combustor  14 . 
       FIG. 6  is a schematic illustration of an embodiment of a gas turbine system  10  having a plurality of valves  64  to adjust the flow of fuel  20  within the plurality of combustors  14 . In the depicted embodiment, the plurality of combustors  14  are arranged into sectors  90  (e.g., subsets of combustors  14 ). For example, a first combustor  14   a  and a second combustor  14   b  are arranged into a first sector  90   a , and a third combustor  14   c  and a fourth combustor  14   d  are arranged into a second sector  90   b . It should be understood that any suitable number of sectors  90  may be provided (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, or more), and that each sector  90  may include any suitable number of combustors  14  (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, or more). The sectors  90  may include adjacent combustors  14  or non-adjacent (e.g., alternating) combustors  14 . In some embodiments, the flow of fuel  20  to each sector  90  may be controlled by the first valve  64   a . With reference to  FIG. 6 , in a turn down operation, the flow of fuel  20  to the LLI&#39;s  22  of one or more of the combustors  14  may be reduced to a certain threshold via one or more second valves  64   b . As shown, one second valve  64   b  may be provided for each combustor  14 , although as discussed above, one second valve  64   b  may be provided for each LLI  22 , for the LLI&#39;s  22  of each sector  90 , or for the LLI&#39;s  22  for the entire gas turbine system  10 , for example. Once the threshold is reached via the valves  64   b , one or more of the first valves  64   a  may be controlled to adjust the flow of fuel  20  to one or more of the sectors  90 . In some embodiments, one or more of the first valves  64   a  may be controlled so that fuel  20  is only supplied to some of the sectors  90 . For example, one or more of the first valves  64   a  may be controlled so that fuel  20  is supplied to only one half of the sectors  90  and/or one half of the combustors  14 . As will be understood by one of skill in the art, the various combustors  14  and the various sectors  90  within the system  10  may have different arrangements and configurations. For example, any of the previous configurations illustrated in  FIGS. 4 and 5  may be used for each of the sectors  90 , and the sectors  90  of the gas turbine system  10  may have configurations different from one another. 
       FIG. 7  is a front perspective view of the gas turbine system  10  having a plurality of combustors  14  (e.g., 14 combustors) arranged circumferentially  42  about the longitudinal axis  38  of the gas turbine system  10 . The combustors  14  may be arranged into any suitable number of sectors  90 , and each sector  90  may include any number of combustors  14 . For example, as shown, the combustors  14  are arranged into four sectors  90   a ,  90   b ,  90   c ,  90   d , each sector  90  having four combustors  14 . As discussed above, each sector  90  may include a series of adjacent combustors  14 , or the sectors  90  may include non-adjacent combustors  14  (e.g., alternating combustors  14 , or every third, fourth, or fifth combustor  14 , etc.). The flow of fuel  20  may be controlled to certain combustors  14  and/or certain sectors  90 . For example, the flow of fuel  20  to one sector  90  or to any subset of combustors  14  may be reduced or stopped by adjusting one or more valves  64 . In some embodiments, the flow of fuel  20  to the combustors  14  of each sector  90  may be controlled by one valve  64 . For example, one valve  64  may adjust the flow of fuel  20  to all of the combustors  14  within one sector  90 . Such a configuration may enable efficient turn down with less hardware (e.g., fewer valves), and reduce the processing steps, for example. 
     The embodiments described above provide examples of techniques for operating the gas turbine system  10  at a reduced rate or power level while maintaining emissions compliance. It should be understood that the controller  23  may be configured to gradually turn down or change the flow of fuel to the various parts of the gas turbine system  10  in any suitable order or sequence. Thus, the controller  23  may control the turn down process by sequentially or gradually reducing the flow of fuel to one or more of the LLI&#39;s  22 , reducing the flow of fuel to one or more of the fuel nozzles  18 , and/or turning off (e.g., stop) the flow of fuel to one or more combustors  14  or sectors  90  of combustors  14  in any sequence or order. For example, the controller  23  may first reduce the flow of fuel to one or more LLI&#39;s  22  and then stop the flow of fuel to a subset of the combustors  14  (e.g., one or more, but not all). In some embodiments, the controller  23  may first stop the flow of fuel to the subset of the combustors  14  (e.g., one or more, but not all) and then reduce the flow of fuel to one or more LLI&#39;s  22  that are coupled to other combustors  14  (e.g., active combustors, combustors that are not part of the subset) of the gas turbine system  19 . Additionally, in some embodiments, certain steps of the turn down process may be carried out simultaneously. For example, the flow of fuel to some or all of the LLI&#39;s  22  may be reduced as the flow of fuel to the subset of the combustors  14  is reduced. Furthermore, as noted above, the gas turbine system may include sensors  82  or other monitoring and processing devices that are configured to monitor various features of the gas turbine system  10 , including flow rates, temperature within one or more of the combustors  14 , and/or emissions produced by the gas turbine system  10 . Thus, the gas turbine system may be configured to progressively and dynamically change the fuel flow to the LLI&#39;s  22  and the fuel nozzles  18  and/or to stop the flow of fuel to the subset of combustors  14  in response to monitored temperature and/or emissions levels, thus facilitating turn down while maintaining emissions compliance (e.g., a temperature or emissions threshold). For example, the controller  23  may increase the flow of fuel to one or more LLI&#39;s  22  if the monitored temperature and/or emissions level exceeds a pre-programmed threshold. 
     As indicated above, in some circumstances, it may be desirable to operate the gas turbine system  10  at a reduced rate or power level. For example, during off-peak hours, it may be impractical and expensive to operate the gas turbine system  10  at full power. Additionally, completely stopping and restarting the gas turbine system  10  is a lengthy process and can impact the durability of system components. However, when operating at such reduced power levels, it can be particularly difficult to maintain emissions compliance. Thus, the present disclosure provides systems and methods to enable the gas turbine system  10  to operate at very low power levels, while maintaining suitably low emissions. For example, systems and methods in accordance with the present disclosure may enable the gas turbine system to remain emission compliant and turn down to about 15% of normal output (e.g., 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90%, or any ranges therebetween). By providing one or more control devices (e.g., valves), the flow of fuel may be directed and adjusted in a manner that enables the gas turbine system  10  to achieve very low power levels and low emissions. For example, valves may be controlled to adjust the flow of fuel to certain fuel injectors and/or to certain combustors within the gas turbine system  10  in a manner that results in reduced rates and fuel/air ratios that maintain low emissions. The above embodiments are provided as examples and are not intended to be limiting. Thus, any suitable number and arrangement of valves to adjust the flow of fuel to the various components (e.g., the fuel nozzles and/or the LLI&#39;s) may be utilized in accordance with the present disclosure. Such turn down methods may also enable the gas turbine system  10  to quickly return to full power if demand increases. Additionally, in some embodiments, the flow of fuel may be adjusted to shut down the combustor in a manner that reduces thermal stress on the components along the hot gas path. Technical effects of the presently disclosed embodiments include the ability for the gas turbine system  10  to operate at a low power level, while maintaining emissions compliance. 
     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.