Patent Application
20070107437
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 NOx and carbon monoxide emissions. In certain systems, lean premixed combustion technology is employed to reduce the emissions from such systems. Typically, NOx 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 NOx emissions that may damage the combustor hardware.
Accordingly, there is a need for a combustor that has reduced NOx 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.
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.
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:
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 NOx 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
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
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
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
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
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
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.
