Abstract:
A fuel/air mixing tube for use in a fuel/air mixing tube bundle is provided. The fuel/air mixing tube includes an outer tube wall extending axially along a tube axis between an inlet end and an exit end, the outer tube wall having a thickness extending between an inner tube surface having a inner diameter and an outer tube surface having an outer tube diameter. The tube further includes at least one fuel injection hole having a fuel injection hole diameter extending through the outer tube wall, the fuel injection hole having an injection angle relative to the tube axis. The invention provides good fuel air mixing with low combustion generated NOx and low flow pressure loss translating to a high gas turbine efficiency, that is durable, and resistant to flame holding and flash back.

Description:
FEDERAL RESEARCH STATEMENT 
       [0001]    This invention was made with Government support under Contract No. DE-FC26-05NT42643, awarded by the Department of Energy. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to premixed direct injection nozzles and more particularly to a direct injection nozzle having good mixing, flame holding and flash back resistance. 
         [0003]    The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. It is well known in the art that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. One method of controlling the temperature of the reaction zone of a heat engine combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. 
         [0004]    There are several problems associated with dry low emissions combustors operating with lean premixing of fuel and air. That is, flammable mixtures of fuel and air exist within the premixing section of the combustor, which is external to the reaction zone of the combustor. Typically, there is some bulk burner tube velocity, above which a flame in the premixer will be pushed out to a primary burning zone. However, certain fuels such as hydrogen or syngas have a high flame speed, particularly when burned in a pre-mixed mode. Due to the high turbulent flame velocity and wide flammability range, premixed hydrogen fuel combustion nozzle design is challenged by flame holding and flashback at reasonable nozzle pressure loss. Diffusion hydrogen fuel combustion using direct fuel injection methods inherently generates high NOx. 
         [0005]    With natural gas as the fuel, premixers with adequate flame holding margin may usually be designed with reasonably low air-side pressure drop. However, with more reactive fuels, such as high hydrogen fuel, designing for flame holding margin and target pressure drop becomes a challenge. Since the design point of state-of-the-art nozzles may approach 3000 degrees Fahrenheit bulk flame temperature, flashback into the nozzle could cause extensive damage to the nozzle in a very short period of time. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    The present invention is a premixed direct injection nozzle design that provides good fuel air mixing with low combustion generated NOx and low flow pressure loss translating to a high gas turbine efficiency. The invention is durable and resistant to flame holding and flash back. 
         [0007]    According to one aspect of the invention, a fuel/air mixing tube for use in a fuel/air mixing tube bundle is provided. The fuel/air mixing tube includes an outer tube wall extending axially along a tube axis between an inlet end and an exit end, the outer tube wall having a thickness extending between an inner tube surface having an inner diameter and an outer tube surface having an outer tube diameter. 
         [0008]    The tube further includes at least one fuel injection hole having a fuel injection hole diameter extending through the outer tube wall, the fuel injection hole having an injection angle relative to the tube axis, the injection angle being generally in the range of 20 to 90 degrees. The fuel injection hole is located at a recession distance from the exit end along the tube axis, the recession distance being generally in the range of about 5 to about 100 times greater than the fuel injection hole diameter, depending on geometric constraints, the reactivity of fuel, and the NOx emissions desired. 
         [0009]    According to another aspect of the invention, a fuel/air mixing tube for use in a fuel/air mixing tube bundle is provided. It includes an outer tube wall extending axially along a tube axis between an inlet end and an exit end, the outer tube wall having a thickness extending between an inner tube surface having a inner diameter and an outer tube surface having an outer tube diameter. It further includes at least one fuel injection hole having a fuel injection hole diameter extending through the outer tube wall, the fuel injection hole having an injection angle relative to the tube axis, the inner diameter of said inner tube surface being generally from about 4 to about 12 times greater than the fuel injection hole diameter. 
         [0010]    According to yet another aspect of the invention, a method of mixing high hydrogen fuel in a premixed direct injection nozzle for a turbine combustor is provided. The method comprises providing a plurality of mixing tubes attached together to form the nozzle, each of the plurality of tubes extending axially along a flow path between an inlet end and an exit end, each of the plurality of tubes including an outer tube wall extending axially along a tube axis between said inlet end and said exit end, the outer tube wall having a thickness extending between an inner tube surface having a inner diameter and an outer tube surface having an outer tube diameter. 
         [0011]    The method further provides for injecting a first fluid into the plurality of mixing tubes at the inlet end; injecting a high-hydrogen or syngas fuel into the mixing tubes through a plurality of injection holes at angle generally in the range of about 20 to about 90 degrees relative to said tube axis; and mixing the first fluid and the high-hydrogen or syngas fuel to a mixedness of about 50% to about 95% fuel and first fluid mixture at the exit end of the tubes. 
         [0012]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0013]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0014]      FIG. 1  is a cross-section of a gas turbine engine, including the location of injection nozzles in accordance with the present invention; 
           [0015]      FIG. 2  is an embodiment of an injection nozzle in accordance with the present invention; 
           [0016]      FIG. 3  is an end view of the nozzle of  FIG. 2 ; 
           [0017]      FIG. 4  is an alternative embodiment of an injection nozzle in accordance with the present invention; 
           [0018]      FIG. 5  is an end view of the nozzle of  FIG. 4 ; 
           [0019]      FIG. 6  is a partial cross-section of a fuel/air mixing tube in accordance with the present invention. 
           [0020]      FIG. 7  is an example of a fuel/air mixedness method in accordance with the present invention. 
       
    
    
       [0021]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring now to  FIG. 1  where the invention will be described with reference to specific embodiments, without limiting same, a schematic illustration of an exemplary gas turbine engine  10  is shown. Engine  10  includes a compressor  11  and a combustor assembly  14 . Combustor assembly  14  includes a combustor assembly wall  16  that at least partially defines a combustion chamber  12 . A pre-mixing apparatus or nozzle  110  extends through combustor assembly wall  16  and leads into combustion chamber  12 . As will be discussed more fully below, nozzle  110  receives a first fluid or fuel through a fuel inlet  21  and a second fluid or compressed air from compressor  11 . The fuel and compressed air are then mixed, passed into combustion chamber  12  and ignited to form a high temperature, high pressure combustion product or gas stream. Although only a single combustor assembly  14  is shown in the exemplary embodiment, engine  10  may include a plurality of combustor assemblies  14 . In any event, engine  10  also includes a turbine  30  and a compressor/turbine shaft  31 . In a manner known in the art, turbine  30  is coupled to, and drives shaft  31  that, in turn, drives compressor  11 . 
         [0023]    In operation, air flows into compressor  11  and is compressed into a high pressure gas. The high pressure gas is supplied to combustor assembly  14  and mixed with fuel, for example process gas and/or synthetic gas (syngas), in nozzle  110 . The fuel/air or combustible mixture is passed into combustion chamber  12  and ignited to form a high pressure, high temperature combustion gas stream. Alternatively, combustor assembly  14  can combust fuels that include, but are not limited to natural gas and/or fuel oil. Thereafter, combustor assembly  14  channels the combustion gas stream to turbine  30  which coverts thermal energy to mechanical, rotational energy. 
         [0024]    Referring now to  FIGS. 2 and 3 , a cross-section through a fuel injection nozzle  110  is shown. Nozzle  110  is connected to a fuel flow passage  114  and an interior plenum space  115  to receive a supply of air from compressor  11 . A plurality of fuel/air mixing tubes is shown as a bundle of tubes  121 . Bundle of tubes  121  is comprised of individual fuel/air mixing tubes  130  attached to each other and held in a bundle by end cap  136  or other conventional attachments. Each individual fuel/air mixing tube  130  includes a first end section  131  that extends to a second end section  132  through an intermediate portion  133 . First end section  131  defines a first fluid inlet  134 , while second end section  132  defines a fluid outlet  135  at end cap  136 . 
         [0025]    Fuel flow passage  114  is fluidly connected to fuel plenum  141  that, in turn, is fluidly connected to a fluid inlet  142  provided in the each of the plurality of individual fuel/air mixing tubes  130 . With this arrangement, air flows into first fluid inlet  134 , of tubes  130 , while fuel is passed through fuel flow passage  114 , and enters plenum  141  surrounding individual tubes  130 . Fuel flows around the plurality of fuel/air mixing tubes  130  and passes through individual fuel injection inlets (or fuel injection holes)  142  to mix with the air within tubes  130  to form a fuel/air mixture. The fuel/air mixture passes from outlet  135  into an ignition zone  150  and is ignited therein, to form a high temperature, high pressure gas flame that is delivered to turbine  30 . 
         [0026]    Referring now to  FIGS. 4 and 5 , a cross-section through an alternative fuel injection nozzle  210  is shown. Nozzle  210  is connected to a fuel flow passage  214  and an interior plenum space  215  to receive a supply of air from compressor  11 . A plurality of fuel/air mixing tubes is shown as a bundle of tubes  221 . Bundle of tubes  221  is comprised of the same individual fuel/air mixing tubes  130  identified in  FIGS. 2 and 3 , and are attached to each other and held in a bundle by end cap  236  or other conventional attachments. Each individual fuel/air mixing tube  130  includes a first end section  131  that extends to a second end section  132  through an intermediate portion  133 . First end section  131  defines a first fluid inlet  134 , while second end section  132  defines a fluid outlet  135  at end cap  236 . 
         [0027]    Fuel flow passage  214  is fluidly connected to fuel plenum  241  that, in turn, is fluidly connected to the fluid inlets  142  provided in the each of the plurality of individual fuel/air mixing tubes  130 . With this arrangement, air flows into first fluid inlet  134 , of tubes  130 , while fuel is passed through fuel flow passage  214 , and enters plenum  241 , which is fluidly connected to individual tubes  130  via fluid inlets  142 . Fuel flows around the plurality of fuel/air mixing tubes  130  and passes through individual fuel injection inlets (or fuel injection holes)  142  to mix with the air within tubes  130  to form a fuel/air mixture. The fuel/air mixture passes from outlet  135  into an ignition zone  250  and is ignited therein, to form a high temperature, high pressure gas flame that is delivered to turbine  30 . 
         [0028]    Referring now to  FIGS. 2 through 5 , in full load operations for low NOx, the flame should reside in ignition zone  150 ,  250 . However, the use of high hydrogen/syngas fuels has made flashback a difficulty and often a problem. In order to avoid any flame holding inside the mixing tubes  130 , the heat release inside the mixing tube from the flame holding should be less than the heat loss to the tube wall. This criterion puts constraints on the tube size, fuel jet penetration, and fuel jet recession distance. In principal, long recession distance gives better fuel/air mixing. If the ratio of fuel to air in mixing tubes  130 , referred to herein as the mixedness of the fuel is high, and fuel and air achieve close to 100% mixing, it produces a relatively low NOx output, but is susceptible to flame holding and/or flame flashback within the nozzle  110 ,  210  and the individual mixing tubes  130 . The individual fuel/air mixing tubes  130  of tube bundle  121 ,  221  may require replacement due to the damage sustained. Accordingly, as further described, the fuel/air mixing tubes  130  of the present invention creates a mixedness that sufficiently allows combustion in an ignition zone  150 ,  250  while preventing flashback into fuel/air mixing tubes  130 . The unique configuration of mixing tubes  130  makes it possible to burn high-hydrogen or syngas fuel with relatively low NOx, without significant risk of flame holding and flame flashback from ignition zone  150 ,  250  into tubes  130 . 
         [0029]    Referring now to  FIGS. 6 and 7 , a fuel/air mixing tube  130  from tube bundle  121  or  221  is shown. Tube  130  includes an outer tube wall  201  having an outer circumferential surface  202  and an inner circumferential surface  203  extending axially along a tube axis A between a first fluid inlet  134  and a fluid outlet  135 . Outer circumferential surface  202  has an outer tube diameter D o  while inner circumferential surface  203  has an inner tube diameter D i . As shown, tube  130  has a plurality of fuel injection inlets  142 , each having a fuel injection hole diameter D f  extending between the outer circumferential surface  202  and inner circumferential surface  203 . In a non-limiting embodiment, fuel injection hole diameter D f  is generally equal to or less than about 0.03 inches. In another non-limiting embodiment, the inner tube diameter D i  is generally from about 4 to about 12 times greater than the fuel injection hole diameter D f . 
         [0030]    The fuel injection inlets  142  have an injection angle Z relative to tube axis A which, as shown in  FIG. 6  is parallel to axis A. As shown in  FIG. 6 , each of injection inlets  142  has an injection angle Z generally in the range of about 20 to about 90 degrees. Further refinement of the invention has found an injection angle being generally between about 50 to about 60 degrees is desirable with certain high-hydrogen fuels. Fuel injection inlets  142  are also located a certain distance, known as the recession distance R, upstream of the tube fluid outlet  135 . Recession distance R is generally in the range of about 5 (R min ) to about 100 (R max ) times greater than the fuel injection hole diameter D f , while, as described above, fuel injection hole diameter D f  is generally equal to or less than about 0.03 inches. In practice, the recession distance R for hydrogen/syngas fuel is generally equal to or less than about 1.5 inches and the inner tube diameter D i  is generally in the range of about 0.05 to about 0.3 inches. Further refinement has found recession distance R in the range of about 0.3 to about 1 inch, while the inner tube diameter D i  is generally in the range of about 0.08 to about 0.2 inches to achieve the desired mixing and target NOx emission. Some high hydrogen/syngas fuels work better below an inner tube diameter D i  of about 0.15 inches. Further refinement of the invention has found an optimal recession distance being generally proportional to the burner tube velocity, the tube wall heat transfer coefficient, the fuel blow-off time, and inversely proportional to the cross flow jet height, the turbulent burning velocity, and the pressure. 
         [0031]    The diameter D f  of fuel injection inlet  142  should be generally equal to or less than about 0.03 inches, while each of tubes  130  are about 1 to about 3 inches in length for high reactive fuel, such as hydrogen fuel,and have generally about 1 to about 8 fuel injection inlets  142 . For low reactive fuel, such as natural gas, each of the tubes  130  can be as long as about one foot in length. Multiple fuel injection inlets  142 , i.e. about 2 to about 8 fuel injection inlets with low pressure drop is also contemplated. With the stated parameters, it has been found that a fuel injection inlet  142  having an angle Z of about 50 to about 60 degrees works well to achieve the desired mixing and target NOx emissions. It will be appreciated by one skilled in the art that a number of different combinations of the above can be used to achieve the desired mixing and target NOx emissions. For instance, when there are a plurality of fuel injection inlets  142  in a single tube  130 , some injection inlets may have differing injection angles Z, as shown in  FIG. 6 , that e.g. vary as a function of the recession distance R. As another example, the injection angles Z may vary as a function of the diameter D f  of fuel injection inlets  142 , or in combination with diameter D f  and recession distance R of fuel injection inlets  142 . The objective is to obtain adequate mixing while keeping the length of tubes  130  as short as possible and having a low pressure drop (i.e., less than about 5%) between fluid inlet end  134  and fluid outlet end  135 . 
         [0032]    The parameters above can also be varied based upon fuel compositions, fuel temperature, air temperature, pressure and any treatment to inner and outer circumferential walls  202  and  203  of tubes  130 . Performance is enhanced when the inner circumferential surface  203 , through which the fuel/air mixture flows, is honed smooth regardless of the material used. It is also possible to protect nozzle  110 , end cap  136 ,  236  which is exposed to ignition zone  150 ,  250  and the individual tubes  130  by cooling with fuel, air or other coolants. Finally, end cap  136 ,  236  may be coated with ceramic coatings or other layers of high thermal resistance. 
         [0033]    Referring now to  FIG. 7 , an example of mixing a high hydrogen/syngas fuel in a recessed injection nozzle is shown. Specifically, a desired mixing of low NOx emission (below 5 ppm) and low nozzle pressure loss (below 3%) is achieved, when the recession distance R of the fuel injection inlets  142  in the non-limiting example shown is about 0.6 to about 0.8 inches from the fluid outlet  135 . As described above, recession distance R may vary from generally about 1 to about 50 times greater than the fuel injection hole diameter. As can be seen, in the non-limiting embodiments shown, three fuel injection angles are shown, 30 degrees, 60 degrees and 90 degrees but, as described above, may vary generally in the range of about 20 to about 90 degrees. By the time the fuel/air mixture reaches fluid outlet  135 , fuel/air mixedness is at almost 80% with an injection angle Z at about 60 degrees, between 60% and 70% with an injection angle Z at about 30 degrees, while fuel/air mixedness is at about 50% with an injection angle Z of 90 degrees. 
         [0034]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.