Abstract:
A fuel injector and a combustor including a fuel injector that enables a combustor to burn a lean fuel/oxider gas mixture while providing low emissions of oxides of nitrogen (NO x ), and a method of combustion. The fuel injector includes three fuel/oxider flow path channels. The first channel includes a flow balancing insert and provides a relatively straight flow. The second channel includes at least one angled vane that imparts a swirl to the flow. The third channel is a central tube, the first channel annularly surrounding the third channel, the second channel annularly surrounding the first channel.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0001]    This invention was made in part with Government support under Grant Number DE-AC03-76SF00098 awarded by the Department of Energy to the University of California, Lawrence Berkeley National Laboratory. The Government may have certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0002]    This patent disclosure relates generally to combustors and burners, and, more particularly to low swirl injectors for use in a combustor or burner. 
       BACKGROUND 
       [0003]    Combustion is a major source of a class of pollutants including oxides of nitrogen, or NO x , (NO or nitric oxide, and NO 2  or nitrogen dioxide), which may contribute to acid rain, smog, and ozone depletion. NO x  emissions from combustion sources primarily consist of nitric oxide produced during combustion. When utilizing gaseous fuels, combustion processes that decrease the combustion temperature can greatly reduce the production of NO, and, accordingly, can have a large effect on the overall production of NO x . 
         [0004]    Various attempts have been made to re-engineer conventional non-premixed combustion systems to reduce emissions of oxides of nitrogen (NO x ). Flames in non-premixed combustion, that is, the combustion process wherein fuel and oxidizer (typically air) mix and burn concurrently, generally emit unacceptable levels of NO x , over 200 parts-per-million (ppm), substantially higher than regulations allow for certain applications. The heating and power generation industries have recognized the need to develop cleaner, premixed combustion systems in which gaseous fuel and oxidizer (typically air) mix prior to burning. 
         [0005]    Although separation of the mixing and burning processes provides the opportunity to control the fuel-to-air ratio delivered to the reaction zone, differences in flame dynamics between non-premixed and premixed arrangements do not typically allow the direct application of non-premixed technology to premixed systems. For example, although maintenance of a stable flame in the zone where the fuel and air are mixed is the important design criterion in non-premixed burners, the design requirements are different relative to mixing in premixed systems, that is, prior to the burning processes. In premixed systems, the flame propagates through the feed gas, which is a premix of fuel and air. The flame speed, or the speed at which a premixed flame propagates through the mixture, is a function of the fuel air equivalence ratio and turbulence intensities. In order to prevent “flash-back,” or propagation of the flame upstream against the premixed feed gas stream and into the body of the premixed burner, the velocity of the premixed feed gas in the burner has to be greater than the flame speed. 
         [0006]    To maintain a stable flame, an obstacle may be placed in the premixed feed gas to “anchor” the flame. The size of the flame anchor and their aerodynamic shapes can be optimized for given operating ranges and burner considerations. The anchor generates a zone of zero axial flow on its upstream side and turbulent flow on its downstream side. As the fuel flows around the obstruction, the flow becomes turbulent, creating several regions of reverse flow where the fuel flow is actually circling back in a direction opposite to the original flow. This pattern or “recirculation” is relatively stable and generally prevents blowout when the operating conditions of the burner are appropriately set. Blowout, or an extinguishing of the flame, may occur when the velocity of the fuel mixture is greater than the speed of the flame. Although these anchors can help to prevent flash-back, a flame can become unstable and “blow-off” the anchor if the velocities are too high. This is particularly true for lean flames, as they tend to blow-off easily due to the excess air in the feed gas. In premixed burners that support lean flame conditions, it is a challenge to design burners robust enough to eliminate both flash-back and blow-off occurrences. 
         [0007]    Swirling flows have also been used to stabilize combustion in a variety of burners, both premixed and non-premixed. Swirl in such burners is generally created either by generating tangential flow motion in a cylindrical chamber, as in cyclone combustion chambers, or swirling a co-axial air flow. In both cases, the function of the swirl is to create a torroidal recirculation zone. For non-premixed combustion, the role of the torroidal recirculation zone is to mix the fuel and air to allow for complete combustion, to stabilize the combustion process, to recirculate some fraction of the products, and to dictate the physical shape and length of the flame in these burners. In premixed burners, the torroidal recirculation zone created by the strong swirl creates a zone where the combustion zone is “anchored” due to an area of low flow velocities found within the torroidal recirculation zone. 
         [0008]    Many attempts have been made to reduce NO x  emissions from combustion sources in hopes of reducing the air pollution associated with the burning of hydrocarbon fuels. Pollution reduction methods generally fall into two categories. One category of reduction methods involves post-combustion remediation technologies, such as Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR), to reduce pollution after it has been generated in the combustion zone. A second category of pollution reduction technologies concentrates on combustion modifications through burner design changes, such as burning lean, fuel-air staging, or flue gas recirculation, to reduce pollutant formation in the reaction zone. Taking into account tradeoffs with engineering considerations and other pollutants, low NO x  burners generally burn with as much excess air (i.e. “lean”) as possible, as this will reduce combustion temperatures and minimize thermal NO x  production, although some NO x  may still be produced by virtue other mechanisms. 
         [0009]    U.S. Pat. No. 5,879,148 to Cheng, et al., discloses a lean premixed burner which generates a stable flame by providing parallel flows of a thoroughly mixed fuel-oxidant mixture or feed gas through a central passage and an annular passage about the central passage. The central passage includes a flow balancing insert that introduces an additional pressure drop beyond that which would occur in the central passage absent the flow balancing insert. The annular passage includes one or more vanes oriented to impart angular momentum to feed gas exiting the annular passage. The low swirl creates a divergent flow pattern that stabilizes lean combustion, allowing for lower production of pollutants, particularly oxides of nitrogen. 
         [0010]    The swirl requirement of a weak-swirl burner (WSB) is different from that of other burners since the feed gas is premixed and flame stabilization is achieved through use of a divergent flow field instead of a torroidal recirculation zone. Due to the propagating nature of pre-mixed flames and the deceleration of the flow as it moves away from its source, the flame is able to aero-dynamically stabilize itself at the position where the local mass velocity balances the flame propagation speed. The weak-swirl stabilization mechanism does not apply to diffusion flames (not pre-mixed) because they do not propagate, but rather burn at the boundary where the air and fuel flows have diffused to the appropriate ratios for sustaining the combustion reaction. 
         [0011]    Industrial gas turbine manufacturers rely primarily on lean-premixed combustion technology to meet current engine emissions regulations. Lean-premixed combustion systems premix fuel and air prior to injection to prevent stoichiometric burning locally within the flame. This results in a more spatially uniform flame temperature that reduces the conversion of atmospheric nitrogen to oxides of nitrogen (NO x ). Operating at lean conditions, however, may result in increased carbon monoxide (CO) and unburned hydrocarbon emissions. Consequently, the combustion system must operate within a narrow flame temperature range to maintain NO x , CO and unburned hydrocarbon emissions at acceptable levels. The window of low emissions operability is close to the lean extinction limit, which can lead to high amplitude combustion pressure oscillations that may damage the combustor hardware. 
         [0012]    Lean premixed combustion systems first entered the gas turbine market in the early 1990&#39;s. Typically, NO x  levels were reduced to 25 to 42 ppm range on natural gas fuel. As air quality regulations continue to tighten, however, manufacturers are actively pursuing technologies that can meet customer demands of the future. In some parts of the United States, gas turbines must already meet regulations as low as 2.5 ppm NO x . To date, lean-premixed combustion systems have not been able to achieve these ultra-low levels. Rather, operators must rely on costly exhaust clean up technologies such as selective catalytic reduction. 
         [0013]    Accordingly, there exists a need for alternative designs for low NO x  emission weak swirl burners, particularly designs that are easily adaptable to a broad range of applications, including, for example, designs that meet the need for ultra-low NO x  emissions in mid-range industrial gas turbines without selective catalytic reduction. Such designs would be particularly advantageous if they were relatively simple and economical to scale, manufacture and operate. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    The disclosure describes, in one aspect, a low swirl injector for use with a combustor and a combustor including a low swirl injector that includes three different channels for providing fuel gas and oxidant gas to a flame zone of the combustor. The first channel provides a straight flow of fuel/oxidant gasses; a flow balancing insert, which is of a porous material or includes a number of openings therethrough, is disposed within the first channel and introduces an additional pressure drop beyond that which would occur in the first channel absent the insert. The second channel is disposed annularly about the first channel and includes at least one vane that imparts an angular momentum to the gas flow exiting the second channel. The third channel provides a feed gas that may be a fuel gas, such as the fuel gas of the first and second channels, or a premix of fuel gas and oxidant gas; the third channel may be disposed along the central axis of the injector, the first channel being annularly disposed about the third channel. A premix of fuel gas and oxidant gas may be provided to the first and second channels. Alternately, oxidant gas and fuel gas may be separately provided to the first and/or second channels, for example, the fuel gas being provided by one or more generally radially extending spokes that include at least one injector orifice; mixing of the fuel and oxidant gasses then occurs within the separate first and second channels. 
         [0015]    Also disclosed is a method of combustion comprising the steps of establishing an axial gas flow of a fuel gas and an oxidant gas through a first channel and a flow balancing insert to establish a straight flow, establishing an axial gas flow of said fuel gas and said oxidant gas through a second channel and past at least one vane disposed within the second channel to impart an angular momentum to said gas flow, and directing a feed gas into a third channel having an outlet aligned to direct said feed gas into a mix zone, allowing said gas flow from the channels to interact in the mix zone, and opening said mix zone into a combustion zone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional view of an embodiment of injector according to the disclosure. 
           [0017]      FIG. 2  is a fragmentary view of the injector of  FIG. 1  taken along line  2 - 2  in  FIG. 1 . 
           [0018]      FIG. 3  is an isometric view of the injector of  FIG. 1 . 
           [0019]      FIG. 4  is a partially cross-sectioned plan view of an exemplary combustor including the injector of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Turning now to the drawings, there is shown a cross-section of a fuel injector assembly  10  including a low-swirl injector  12 . In the illustrated embodiment, the injector  12  includes three gas flow channels  14 ,  16 ,  18  adapted to supply gas flow to a flame zone  20 . The gas flow channels  14 ,  16 ,  18  are disposed within a housing  22  that includes an upstream opening  24  and a downstream mix zone  26  prior to an outlet  28  at the flame zone  20 , the channels  14 ,  16 ,  18  opening to provide gas flow to the mix zone  26 . The axial gas flow through the injector  12  is indicated generally by arrow  21 . 
         [0021]    The first channel  14  is defined by an elongated tubular structure  30  and provides a relatively straight gas flow profile that includes no swirl component. While the illustrated elongated tubular structure  30  is of a generally circular cross-section, the structure  30  may be of any cross-section, including, by way of example only, oval or octagonal. Gas flow enters the channel  14  through opening  32 , and exits the channel  14  at end  34 , the fuel gas and the oxidant gas of the gas flow being thoroughly mixed as the gas flow proceeds through the first channel  14 . 
         [0022]    Gas flow through the channel  14  is at least partially controlled by a flow balancing insert  36 , here disposed at downstream end  34 . The flow balancing insert  36  includes at least one opening  38  through which the gas flow proceeds from the first channel  14 . As gas flow exits the first channel  14  through the restriction of the openings  38  at the downstream end  34 , the gas flow exhibits a higher velocity than the gas flow entering the first channel  14  through opening  32 . It will be appreciated that the flow balancing insert  36  introduces an additional pressure drop across the tubular structure  30  than would occur absent the inclusion of the flow balancing insert  36 . 
         [0023]    The flow balancing insert  36  may be formed of any appropriate design and of any appropriate material. For example, the flow balancing insert  36  may be constructed from metal, or other rigid material in which one or more openings may be placed, and the insert may be in the form of a perforated or porous plate, a screen, a mesh, or a wire cloth. 
         [0024]    While the openings  38  may be of any appropriate size, shape, and configuration, in the illustrated embodiment, the openings  38  are generally round, and distributed uniformly about the insert  36 , as may be seen most readily in  FIG. 2 . As also shown in  FIG. 2 , the flow balancing insert  36  further includes an opening  39 , the significance of which will be apparent upon reading the disclosure with regard to the third channel  18 , below. 
         [0025]    As shown in  FIGS. 1 and 2 , the second channel  16  is provided annularly about the first channel  14 . The second channel  16  is defined by the elongated tubular structure  30  and an outer annular wall  40 , and includes an upstream entry  42 , and a downstream exit  44 . The outer annular wall  40  is formed, in part, by an annular sleeve  46 , the inner surface  48  of the annular sleeve  46  being continuous with the remainder of the outer annular wall  40 . 
         [0026]    In order to impart an angular momentum or swirl to the gas flow exiting the second channel  16 , at least one vane  50  is disposed within the second channel  16 . In the illustrated embodiment, a plurality of such vanes  50  is provided with the elongated tubular structure  30  acting as a hub and the vanes  50  extending outward to the outer annular wall  40 . In an embodiment, the vanes  50  extend between the elongated tubular structure  30  and the annular sleeve  46 . In this way, the elongated tubular structure  30  with the flow balancing insert  36 , the annular sleeve  46 , and the vanes  50  extending between the structure  30  and the annular sleeve  46  may fabricated as a subassembly that may be disposed within a combustor  10  during assembly. 
         [0027]    Any appropriate number of vanes  50  may be provided, and the vanes  50  may have any appropriate structure and be disposed at any appropriate angle, so long as the vanes impart the desired angular momentum to the gas as it flows from the downstream exit  44 . In the embodiment illustrated in  FIGS. 1 and 2 , sixteen axial curved vanes  50  are disposed between a tubular structure  30  having a diameter on the order of 1.5 inches and an annular sleeve  46  having a diameter on the order of 2.75 inches. The vanes  50  present a vane angle on the order of 40° to 60°, here, 46° to 48°. Typical swirl numbers for gas flow exiting the second channel  16  are between 0.6 and 1.2, although alternate swirl levels may be provided, depending upon the gas flow and the design of the vane  50  arrangement. The non-dimensional swirl number, S, is defined as the ratio of axial flux of angular momentum to axial flux of linear momentum. 
         [0028]    The gas flow may be provided to the first and second channels  14 ,  16  by any appropriate arrangement. For example, a premix of fuel gas and oxidant gas may be provided to the upstream inlet  24  to the housing  22 . Alternately, separate fuel gas and oxidant gas may be provided to the housing  22 . In an embodiment, oxidant gas, typically air, is supplied to the housing  22  through upstream opening  24 . Fuel gas may be introduced at any appropriate opening or location to mix with the oxidant gas, so long as adequate residence time is provided within the injector  12  for efficient and effective oxidant gas/fuel gas mixing. Fuel gas may be provided through one or more passages  52  into the housing  22 . 
         [0029]    In the illustrated embodiment, a plurality of generally radially extending spokes  54  form at least a portion of the passage  52 . More specifically, the spokes  54  include a hollow interior  56  and at least one injection orifice  58  through which fuel gas flows into the housing  22 . Although sixteen such spokes  54  and a plurality of injection orifices  58  are shown, any number of such spokes  54  and/or orifices  58  may be provided, so long as adequate fuel gas is provided and distributed to allow for adequate mixing with the oxidant gas. The spokes  54  may extend into either the first channel  14  or second channel  16 , or both the first and second channels  14 ,  16 , as illustrated. In this way, the particular design and distribution of the spokes  54  and injection orifices  58  provides for controlled distribution and flow of fuel gas to the first and second channels  14 ,  16 . Although the same oxidant gas/fuel gas ratio or nominal equivalence ratio may be provided in both the first and second channels  14 ,  16 , the arrangement may be designed such that varied ratios may be provided between two channels  14 ,  16 . 
         [0030]    In order to provide fuel gas to the spoke arrangement, an annular passage  60  fluidly connects a fuel gas supply passage  62  in a supply line  64  with the spokes  54 . Thus, the fuel gas supply passage  62 , the annular passage  60 , the hollow interior of the spokes, and the orifices  58  together form a plurality of passages  52  that supply fuel gas to the interior of the housing  22 . Flow of fuel gas into the supply passage  62  of the supply line  64  may be provided by a valve  66 . Alternate arrangements are within the purview of the disclosure, however. By way of example only, although a single such valve  66  is illustrated, should alternate fuel gas flow passages be provided to first and second channels  14 ,  16 , a plurality of valves may be provided to control the flow of fuel gas to the various fuel gas flow passages. 
         [0031]    Turning now to the third channel  18 , an embodiment may include a pilot fuel injector  70  which forms the third channel  18 . The pilot injector  70  includes an elongated tubular structure  72  that extends from a source of feed gas to the mix zone  26 . Although the tubular structure  72  may be of any appropriate cross-section and any appropriate dimension, in an embodiment, the tubular structure  72  has a substantially annular cross-section with an interior diameter on the order of 0.5 inches. In an embodiment, the downstream end  74  of the pilot fuel injector  70  is disposed along the centerline of the injector  12 , although it may be alternately disposed. Feed gas may be provided to the fuel injector  70  through line  76  or the like and gas flow controlled by any appropriate structure, such as the valve  78  illustrated. The feed gas may be in the form of either pure fuel gas or a premix of fuel gas and oxidant gas. 
         [0032]    The fuel gas may be any appropriate gas, such as, for example, natural gas. Likewise, the oxidant gas may be any appropriate gas, such as, for example, air. 
       INDUSTRIAL APPLICABILITY 
       [0033]    The industrial applicability of the injector  12  described herein will be readily appreciated from the foregoing discussion. The injector  12  may be utilized to achieve ultra-low NO x  emissions in, for example, an industrial gas turbine without establishing a strong recirculation region. The injector  12  may present a low swirl concept that utilizes an aerodynamic flame stabilization mechanism in a diverging flow field where an unanchored flame is allowed to freely propagate at ultra-lean conditions. The lack of a strong recirculation region with a large recirculated mass of combustion products may also reduce the residence time in the primary flame zone  20  of the combustor  86 . This stability at very lean conditions and reduced residence time of the combustion products in the flame zone  20 , may contribute to ultra low NO x  emissions. 
         [0034]    Turning to  FIG. 4 , the disclosed injector  12  is illustrated with a combustor  86 . The injector  12  is disposed within a combustor housing  80  to which oxidant or air flow may be provided through an air inlet  82 . In turn, exhaust gas may be expelled to outlet  84 . In use, the gas flow exiting the first and second chambers  14 ,  16  and the feed gas  18  exiting the third chamber  18  interact and partially mix in the downstream, mix zone  26  of the injector  12 . As the gas flow then exits the injector  12 , it may expand radially outward into the combustor  86  in the illustrated embodiment. Upon ignition, a flame may be stabilized just downstream of the injector exit plane at the downstream outlet  28  and centered on a central axis of the injector  12 . Inasmuch as the central flow of the injector  12  may have no recirculation zone, the flame is held at the flame zone  20  and does not stabilize within the injector barrel. 
         [0035]    Beyond merely the provision of fuel gas and oxidant gas delivered to the injector  12 , several design features may provide design flexibility in establishing optimal performance of the injector  12 . For example, the flow balancing insert  36  and its openings  38  may be designed to achieve optimal performance in the form of flame stability and low emissions. The level of open area provided by the openings  38  through the flow balancing insert  36  within the first channel  14 , as well as the level of open area provided by the upstream opening  24  into the housing  22  may be adjusted in order to provide a desired gas flow through the first channel  14 , and a desired relationship to the gas flow through the second channel  16 . By way of example only, the extent of open area provided by the openings  38  through the flow balancing insert  36  may be on the order to 20%-50%, and may be determined based upon various injector characteristics and dimensions, including, but not limited to the size and shape of the openings  38  themselves. In an embodiment, the flow balancing insert  36  includes an open area of 40%-50% of the area covered by the insert  36 . The location of the flow balancing insert  36  with respect to the vanes  50  of the second chamber  16  may likewise be adjusted to provide desired flow characteristics. In an embodiment, the flow balancing insert  36  was disposed on the order of 0.9 to 1.2 inches from the trailing edge of the swirl vanes, and 2 to 3 inches from the upstream opening  24  into the housing  22 . 
         [0036]    Additionally, the arrangement of and/or flow level of fuel gas through the spokes  54  may be readily adjusted. By way of example only, spokes  54  may be provided with larger or smaller hollow interiors  56 , and/or larger or smaller injection orifices  58 . Alternate arrangements of injection orifices  58  may be provided, and/or injection orifices  58  may be provided that provide fuel gas flow to either or both of the first and second channels  14 ,  16 . 
         [0037]    The pilot injector  70  may provide added flame stability during light off, transients, and off-design operating conditions. For example, the pilot injector  70  may act as a pilot for ignition from light-off conditions, it may be utilized to accelerate the engine to idle speed or full speed in no-load conditions, or it may be utilized in sudden on-load, or off-load conditions. The pilot injector  70  may likewise be adjusted for desired injector characteristics. The size of the tubular structure  72  of the pilot injector  70  may be adjusted, as well as the flow through the pilot injector  70 . While flame stability may improve with increasing pilot injector  70  fuel flow rates, higher NO x  emissions may likewise result, however, low pilot fueling levels may provide a reasonable operating range with ultra-low NO x  emissions. It is envisioned that acceptable NO x  and CO emissions may result with pilot fueling on the order of 5% or less. 
         [0038]    It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the invention or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the invention more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the invention entirely unless otherwise indicated. 
         [0039]    Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
         [0040]    Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.