Patent Publication Number: US-2007107437-A1

Title: Low emission combustion and method of operation

Description:
BACKGROUND  
      The invention relates generally to combustors, and more particularly, to a low emission combustor and method of operation.  
      Various types of gas turbine systems are known and are in use. For example, aeroderivative gas turbines are employed for applications such as power generation, marine propulsion, gas compression, cogeneration, offshore platform power and so forth. Typically, a gas turbine includes a compressor for compressing an air flow and a combustor that combines the compressed air with fuel and ignites the mixture to generate a working gas. Further, the working gas is expanded through a turbine for power generation. Typically, the combustor section is arranged coaxially with the compressor and turbine sections. Further, the design of the combustor section may be selected based upon the operational layout of the gas turbine. For example, the combustor employed in a particular gas turbine may be a can combustor, an annular combustor or a can-annular combustor.  
      Moreover, the combustors for the gas turbines are designed to minimize emissions such as NO x  and carbon monoxide emissions. In certain systems, lean premixed combustion technology is employed to reduce the emissions from such systems. Typically, NO x  emissions are controlled by reducing the flame temperature in the reaction zone of the combustor. In operation, low flame temperature is achieved by premixing fuel and air prior to combustion. Unfortunately, the window of operability becomes very small for such combustors and the combustors are required to be operated away from the lean blow out limit. As a result, it is difficult to operate the premixers employed in the combustors outside of their design space. Moreover, when sufficiently lean flames are subjected to power setting changes, flow disturbances, or variations in fuel composition, the resulting equivalence ratio perturbations may cause loss of combustion. Such a blowout may cause loss of power and expensive down times in stationary turbines.  
      Furthermore, lean premixed combustion may cause fluctuations in the position of the heat release zone leading to high fluctuations in pressure. Such fluctuations may reach high amplitudes and result in substantially higher NO x  emissions that may damage the combustor hardware.  
      Accordingly, there is a need for a combustor that has reduced NO x  emissions while operating at full power. It would also be advantageous to provide a combustor for a gas turbine that will work on a variety of fuels, while maintaining acceptable levels of pressure fluctuations across the turbine load.  
     BRIEF DESCRIPTION  
      Briefly, according to one embodiment a combustor is provided. The combustor includes a combustor liner and a swirl premixer disposed on a head end of the combustor liner and configured to provide a fuel-air mixture to the combustor. The combustor also includes a plurality of tangentially staged injectors disposed downstream of the swirl premixer on the combustor liner; wherein each of the plurality of injectors is configured to introduce the fuel-air mixture in a transverse direction to a longitudinal axis of the combustor and to sequentially ignite the fuel-air mixtures from adjacent tangential injectors.  
      In another embodiment, a gas turbine system is provided. The gas turbine system includes a compressor configured to compress ambient air and a combustor in flow communication with the compressor, the combustor being configured to receive compressed air from the compressor and to combust a fuel stream to generate a combustor exit gas stream. The gas turbine system also includes a turbine located downstream of the combustor and configured to expand the combustor exit gas stream. The combustor includes a swirl premixer disposed on a head end of the combustor to induce a core swirl of a fuel-air mixture within the combustor and a plurality of tangential injectors disposed downstream of the swirl premixer; wherein each of the tangential injectors is configured to introduce fuel-air mixtures in a transverse direction to a longitudinal axis of the combustor to facilitate sequential ignition of the fuel-air mixtures through the injector.  
      In another embodiment, a method of operating a combustor is provided. The method includes generating a core swirl flow of a fuel-air mixture within the combustor through a swirl premixer disposed at a head end of the combustor and transversely introducing fuel-air mixtures downstream of the swirl premixer through a plurality of injectors. The method also includes sequentially igniting the fuel-air mixtures introduced through each of the injectors by utilizing heat from previous burnt gases from an adjacent injector. 
    
    
     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 diagrammatical illustration of a gas turbine having a low emission combustor in accordance with aspects of the present technique;  
       FIG. 2  is a diagrammatical illustration of the process of operation of the gas turbine of  FIG. 1  in accordance with aspects of the present technique;  
       FIG. 3  is a diagrammatical illustration of the low emission combustor of  FIG. 1  in accordance with aspects of the present technique;  
       FIG. 4  is a diagrammatical illustration of a configuration of tangential injectors and the axial swirl premixer at head end employed in the combustor of  FIG. 3  in accordance with aspects of the present technique;  
       FIG. 5  is a cross-sectional view of another exemplary combustor in accordance with aspects of the present technique; and  
       FIG. 6  is a diagrammatical illustration of zones of fuel staging and sequential ignition achieved through the tangential injectors and the head end swirl premixer of  FIG. 3  in accordance with aspects of the present technique.  
    
    
     DETAILED DESCRIPTION  
      As discussed in detail below, embodiments of the present technique function to reduce emissions in combustors such as in can combustors and can-annular combustors employed in gas turbines. In particular, the present technique includes employing lean premixed fuel staging and flue gas recirculation within the combustor to enable a lean operation of the combustor with homogenous combustion to minimize emissions such as NO x  emissions. In a present embodiment, the lean premixed fuel staging enables a stable combustion with a substantially low flame temperature in the combustor to minimize emissions. Turning now to the drawings and referring first to  FIG. 1 a  gas turbine  10  having a low emission combustor  12  is illustrated. The gas turbine  10  includes a compressor  14  configured to compress ambient air. The combustor  12  is in flow communication with the compressor  14  and is configured to receive compressed air from the compressor  14  and to combust a fuel stream to generate a combustor exit gas stream. In addition, the gas turbine  10  includes a turbine  16  located downstream of the combustor  12 . The turbine  16  is configured to expand the combustor exit gas stream to drive an external load. In the illustrated embodiment, the compressor  16  is driven by the power generated by the turbine  16  via a shaft  18 .  
       FIG. 2  illustrates the process of operation of the gas turbine  10  of  FIG. 1 . In operation, the compressor  14  receives a flow of ambient air  20  and compresses the flow of ambient air  20  to produce a flow of compressed air  22 . In certain embodiments, a boost compressor may be employed to receive and compress the flow of ambient air  20 . Further, this flow of compressed air from the boost compressor is channeled towards the compressor  14  for further compression. As will be appreciated by one skilled in the art, depending on the operational layout, the compressor  14  may include a plurality of compressors for increasing the power output of the gas turbine  10 . For example, the gas turbine  10  may include a low-pressure compressor and a high-pressure compressor. Alternatively, the gas turbine  10  may include a low-pressure compressor, a medium-pressure compressor and a high-pressure compressor.  
      The compressed air flow  22  from the compressor  14  is then directed towards the combustor  12  for mixing and combustion with a fuel stream  24  and to generate a combustor exit gas stream  26 . In one embodiment, the combustor  12  includes a can combustor. In another embodiment, the combustor  12  includes a can-annular combustor. Further, the combustor exit gas stream  26  is expanded through the turbine  16  for driving an external load. In the illustrated embodiment, the combustor  12  employs fuel staging of the fuel stream  24  via a plurality of transverse injectors that will be described in detail below with reference to  FIGS. 3-6 . As used herein, the term “fuel staging” refers to ignition of the fuel-air mixture at different points as it travels through the combustor  12 .  
       FIG. 3  is a diagrammatical illustration of a low emission combustor  30  of  FIG. 1 . In the illustrated embodiment, the combustor  30  includes a combustor liner  32  and a swirl premixer  34  disposed on a head end of the combustor liner  32 . The swirl premixer  34  is configured to provide a fuel-air mixture to the combustor  30  and to induce a core swirl of the fuel-air mixture within the combustor  30 . In one embodiment, the combustor  30  includes a Dry Low NO, (DLN) combustor. In certain embodiments, the swirl premixer  34  is operated to induce the core swirl of the fuel-air mixture within the combustor  30  during a start-up, or acceleration, or a turndown condition of the combustor  30 .  
      Further, the combustor  30  includes a plurality of tangentially staged injectors such as represented by reference numerals  36 ,  38 ,  40  and  42 . In the illustrated embodiment, the combustor  30  includes four tangentially staged injectors  36 ,  38 ,  40  and  42 . However, a lesser or greater number of injectors may be employed in the combustor  30 . Further, the plurality of injectors  36 ,  38 ,  40  and  42  are arranged in a circumferentially staggered configuration on the combustor liner  32  to achieve the fuel staging within the combustor  30 . In one embodiment, the plurality of injectors  36 ,  38 ,  40  and  42  are staggered axially to achieve axial fuel staging within the combustor  30 . In the illustrated embodiment, each of the plurality of injectors  36 ,  38 ,  40  and  42  is configured to introduce fresh fuel-air mixture in a transverse direction to a longitudinal axis  44  of the combustor  30  and to sequentially ignite the fuel-air mixture. As used herein, the term “transverse” refers to a direction at right angles to the longitudinal axis  44  of the combustor  30  but off centerline of the combustor  30 . In certain embodiments, the injectors  36 ,  38 ,  40  and  42  may introduce the fuel-air mixtures in a direction at an angle to the longitudinal axis. The fuel injected through the plurality of injectors  36 ,  38 ,  40  and  42  includes natural gas, or hydrogen, or syngas, or a hydrocarbon, carbon monoxide, or combinations thereof. However, a variety of other fuels may be envisaged. In some embodiments, each of the injectors  36 ,  38 ,  40  and  42  have a dual or multiple fuel capability and employs the premixed-prevaporize feature for the fuel. Advantageously, the multiple fuel capability facilitates a backup fuel capability, particularly for liquid fuels such as distillates.  
      In the illustrated embodiment, each of the tangential injectors  36 ,  38 ,  40  and  42  include fuel inlets  46 ,  48 ,  50  and  52  for supplying the fuel-air mixtures to respective tangential injectors  36 ,  38 ,  40  and  42 . In addition, the injectors  36 ,  38 ,  40  and  42  may include associated valving to control the fuel supply to the injectors  36 ,  38 ,  40  and  42 . In certain embodiments, the injectors  36 ,  38 ,  40  and  42  may generate a swirling flow to accelerate the premixing process. In operation, the fuel-air mixtures introduced through the injectors  36 ,  38 ,  40  and  42  are ignited by utilizing heat from previous burnt gases from the injectors  36 ,  38 ,  40  and  42  and the heat released by the reaction of the swirl stabilized flame of the head end swirler.  
      Further, the plurality of injectors  36 ,  38 ,  40  and  42  are configured to induce a tangential momentum inside the combustor  30  to facilitate flame stabilization within the combustor  30  and supplementing the swirling flow that is generated by the head end swirler  34 . Thus, the core of the combustor  30  maintains a swirling movement and fresh lean mixtures are supplied perpendicular to the axis  44  of the combustor  30 . Additionally, the low swirl and tangential momentum of this fresh mixture of fuel and air induces a velocity substantially high enough to prevent flame holding on the combustor liner  32  or the tangential injectors  36 ,  38 ,  40  and  42  and to facilitate ignition of the fresh lean mixtures supplied through the injectors  36 ,  38 ,  40  and  42 . In the illustrated embodiment, the combustor  30  includes a plurality of dilution holes  54  disposed downstream of the injectors  36 ,  38 ,  40  and  42  for introducing dilution air to facilitate cooling of walls of the combustor liner  32 . The sequential ignition of the fuel-air mixtures supplied through the injectors  36 ,  38 ,  40  and  42  will be described below with reference to  FIGS. 4-6 .  
       FIG. 4  is a diagrammatical illustration of an exemplary configuration  56  of tangential injectors employed in the combustor  30  of  FIG. 3 . As illustrated, the swirl premixer  34  is disposed at the head end of the combustor  30  (see  FIG. 3 ) and a plurality of injectors such as  36 ,  38 ,  40  and  42  are arranged in a staggered circumferential or axial configuration to achieve the fuel staging within the combustor  30 . The plurality of injectors  36 ,  38 ,  40  and  42  are configured to induce a torroidal movement of the fuel-air mixture via the fuel staging in addition to the core swirl generated by the swirl premixer  34 . Particularly, such staging is achieved by tangential injection of fresh fuel-air mixtures through the injectors  36 ,  38 ,  40  and  42 . In the illustrated embodiment, the injectors  36 ,  38 ,  40  and  42  introduce the fuel-air mixtures in a direction perpendicular to the longitudinal axis of the combustor. Alternatively, the injectors  36 ,  38 ,  40  and  42  may introduce the fuel-air mixtures in a direction at an angle to the longitudinal axis from about 0 degrees to about 45 degrees. In certain embodiments, the injectors  36 ,  38 ,  40  and  42  may be arranged in a staggered configuration to enable dynamics reduction within the combustor  30 . In some embodiments, a load staging capability may be achieved within the combustor  30  by operating a desired number of injectors  36 ,  38 ,  40  and  42 . In operation, a selected number of the injectors  36 ,  38 ,  40  and  42  may be turned on while the other injectors are run cold to facilitate a turndown condition of the combustor.  
      In operation, the core swirl generated by the swirl premixer  34  facilitates flame stabilization in the combustor  30  and enables start-up of the combustor  30  when the tangential injectors  36 ,  38 ,  40  and  42  are not in operation and only air is being supplied to the latter. Once the flame has been stabilized using the swirl premixer  34  at the head end of the combustor  30  and possibly a pilot flame, the swirl premixer  34  facilitates ignition propagation from the swirl premixer  34  to the injectors  36 ,  38 ,  40  and  42  as described below with reference to  FIGS. 5 and 6 . Further, once the ignition is propagated to the injectors  36 ,  38 ,  40  and  42  the combustor head end fuel may be reduced to a minimum thus enabling a highly premixed operation mode that is close to the lean blow out point of the premixer, while fuel is being supplied to full operation via the tangential injectors  36 ,  38 ,  40  and  42 .  
       FIG. 5  is a cross-sectional view  60  of another exemplary combustor having tangential injection of fuel. As described above, the combustor  60  receives a core swirl of air  62 . In this embodiment, the premixer  34  is disposed in the center of the combustor  60  and is aligned with the centerline  44 . The premixer  34  is configured to introduce the fuel-air mixture within the combustor  60 . In certain embodiments, the combustor may include an igniter (not shown) to ignite the fuel-air mixture during the startup condition of the combustor  60 . Additionally, fresh fuel-air mixtures are introduced in a transverse direction to the axis  44  of the combustor  60  via a plurality of injectors such as represented by reference numeral  64  disposed downstream of the swirler premixer  34 . In the illustrated embodiment, each of the plurality of injectors  64  receives fuel and air as represented by reference numerals  66  and  68  and this premixed mixture is introduced within the combustor  60  through each of the injectors  64 . The injection of fuel-air mixtures via the plurality of injectors  64  and the head end swirl premixer  34  introduces a tangential momentum of the mixture within the core of the combustor  60 . In the present embodiment, the upstream plenum of the combustor  60  functions as a large premixer and the reaction takes place downstream of the upstream plenum.  
      Additionally, the fuel-air mixtures are sequentially ignited by previous burnt gases from an adjacent injector and the heat released by the reaction of the swirl stabilized flame  70  of the head end swirl premixer  34 . Further, the combustion process is completed in a burn out zone where any balance combustion air may be introduced. In the illustrated embodiment, the toroidal movement of the fuel-air mixture within the combustor facilitates flame stabilization. In addition, the transverse injection of fuel-air mixtures facilitates self-sustaining ignition in the combustor  60  that will be described below with reference to  FIG. 6 .  
       FIG. 6  is a diagrammatical illustration of zones  80  of fuel staging and sequential ignition achieved through the tangential injectors of  FIG. 4 . In the illustrated embodiment, the sequential ignition is achieved through a premix-react-ignite mechanism inside the combustor. The sequential ignition with the swirl and toroidal momentum inside the combustor substantially reduces emissions from the combustor and facilitates operability over a relatively larger window of temperatures.  
      In the illustrated embodiment, for each of the injectors  36 ,  38 ,  40  and  42  the ignition can be characterized by four zones  80  that facilitate the flame stabilization and flue gas recirculation within the combustor. For example, the fuel and air introduced through the injector  40  is premixed in a premixing zone  82  and then subsequently in a mixing zone  84 . Further, the fuel-air mixtures are ignited in an ignition zone  86 . Once the temperature in the ignition zone  86  is high enough to sustain combustion, chemical reactions take place in a reaction zone  88 . Subsequently, the gases emerging from the reaction zone  88  enter a burnout zone  90 . Similarly, for each of injectors  36 ,  38  and  42  the ignition is facilitated via the premix-react ignite mechanism as described above.  
      In the illustrated embodiment, the emerging premixed gas and air velocity out of each of the tangential premixers  36 ,  38 ,  40  and  42  is substantially larger than the local flame speed, thus preventing the flame to flash back into the tangential premixers. Further, the premixing continues in the premixing zone  82  between the fuel and air supplied to each of the premixers  36 ,  38 ,  40  and  42 . Additionally, mixing with hot gases resulted from the combustion at the core of the combustor develops in the mixing zone  84 . As a result, the fresh mixtures are ignited spontaneously upon reaching the ignition conditions. Further, the momentum carries the burnt gases and mixes them completely with the core resulting in a homogeneous and complete reaction in the reaction zone  88 , where the core has a substantially higher axial momentum along the axis  44  (see  FIG. 3 ). This is achieved by inducing a low swirl and large axial momentum (i.e. low swirl number) in tangential premixing tubes. It should be noted that the momentum facilitates the swirling movement in the core and flame is stabilized using this arrangement. The core flame  70  is thus not scrubbing against the wall of the liner  42  and thus the walls of the said liner  42  are kept cooler.  
      In the illustrated embodiment, the fuel-air mixtures introduced at each location are continuously ignited from the previous burnt gases thus facilitating self-sustaining ignition within the combustor. Further, the premix-react-ignition mechanism employed by the injectors  36 ,  38 ,  40  and  42  facilitates a stabilized flame in the center of the combustor or a hot core while preventing the hot gas scrubbing of the liner and domeplate of the combustor. The tangential injectors  36 ,  38 ,  40  and  42  may be employed for sequential ignition for various fuel-to-air ratios for controlling stability, flue gas recirculation of partially or fully burnt gases. This will achieve lowering of the emissions and elimination of the aerodynamic flame stabilization requirement by introducing self-sustaining ignition.  
      The various aspects of the method described hereinabove have utility in different applications such as combustors employed in gas turbines. As noted above, the fuel staging achieved in a combustor via transverse introduction of fuel-air mixtures in the combustor facilitates flame stabilization away from the combustor walls. Further, the present technique enables reduction of emissions particularly NOx emissions from such combustors thereby facilitating the operation of the gas turbine in an environmentally friendly manner. In addition, the fuel staging described above may be employed with a variety of fuels thus providing fuel flexibility of the system while maintaining acceptable levels of pressure fluctuations across a required turbine load. Moreover, the technique described above may be employed in the existing can or can-annular combustors to reduce emissions and achieve a relatively high stability of the flame.  
      While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.