Abstract:
A method to facilitate controlling flame-holding margins in a turbine engine is provided. The method includes coupling at least one turbine nozzle segment within the turbine engine, wherein the at least one turbine nozzle segment includes at least one vane extending between an inner band and an outer band. The method also includes positioning at least one fuel injection orifice in a surface of the at least one vane, channeling a fuel through the at least one fuel injection orifice into a compressed fluid flow to establish a jet penetration height, and defining an operating window by adjusting an operating parameter of the fuel to reduce the jet penetration height and to facilitate reducing the flame-holding margins.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to turbine assemblies, and more particularly, to methods and systems to facilitate controlling flame-holding margins during turbine operation. 
         [0002]    Generally, turbine assemblies used in power generation systems are designed for use with a particular fuel. More specifically, known turbines are designed to achieve mandated nitrous oxide (NOx) emission levels when operating. However, as the cost of gaseous fuels has increased, while gas fuel supplies have become more difficult to secure, at least some known turbines have had to operate with alternative fuels. 
         [0003]    At least some known methods of operating turbine assemblies economize fuel consumption by increasing the temperature of the fuel supplied to the turbines using waste heat. By increasing the temperature of the fuel supplied to the turbine, less energy is required to bring the fuel to a turbine operating temperature. For example, in a turbine having an operating temperature of 2500° F., fuel having an initial temperature of 100 0  F is heated to between about 300 to 400° F. using waste heat, and then additional energy is required to heat the fuel to the 2,500° F. operating temperature. Thus, pre-heating the fuel using waste heat facilitates decreasing the quantity of energy necessary to reach an exhaust temperature that produces a corresponding desired amount of power. 
         [0004]    Using these methods with non-design gas fuels may decrease the fuel nozzle&#39;s flame-holding margins below the desired allowable limits. Flame holding may damage the fuel nozzle, creating hot streaks that exceed the local maximum operating temperature of turbine engines, thus causing turbines to fail. Moreover, exceeding flame-holding margins may also limit the useful life of the fuel nozzles and/or may cause damage to the combustor lining. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    In one embodiment, a method to facilitate operating within flame-holding margins in a turbine engine is provided. The method includes coupling at least one turbine nozzle segment within the turbine engine, wherein the at least one turbine nozzle segment includes at least one vane extending between an inner band and an outer band. The method also includes positioning at least one fuel injection orifice in a surface of the at least one vane, channeling a fuel through the at least one fuel injection orifice into a compressed fluid flow to establish a jet penetration height, and defining an operating window by adjusting an operating parameter of the fuel to reduce the jet penetration height and to facilitate increasing the flame-holding margins. 
         [0006]    In another exemplary embodiment, a system to facilitate operating within flame-holding margins in a turbine engine is provided. The system includes at least one turbine nozzle segment coupled within the turbine engine, where the at least one turbine nozzle segment includes at least one vane extending between an inner band and an outer band. The system also includes a design fuel from a fuel source, at least one fuel injection orifice defined in a surface of the at least one vane, where the at least one fuel injection orifice is designed to optimize turbine performance using the design fuel. A non-design fuel is channeled through the at least one fuel injection orifice into a compressed fluid flow to establish a jet penetration height, and an operating window facilitates reducing the jet penetration height and facilitates increasing the flame-holding margins. 
         [0007]    In yet another exemplary embodiment, a turbine engine is disclosed. The turbine engine includes a nozzle assembly including an inner band, an outer band, and at least one vane extending between the inner band and the outer band. The vane includes a plurality of fuel injection orifices designed to optimize performance using a design fuel and are configured to channel a non-design fuel therefrom to facilitate controlling flame-holding margins. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a perspective cross-sectional view of an exemplary turbine nozzle assembly; 
           [0009]      FIG. 2  is an enlarged perspective view of a swirler assembly used in the turbine nozzle assembly shown in  FIG. 1 ; 
           [0010]      FIG. 3  is an enlarged perspective view of a portion of the swirler assembly shown in  FIG. 2 ; 
           [0011]      FIG. 4  is an enlarged cross-sectional view of an exemplary fuel jet and flame-holding margin; 
           [0012]      FIG. 5  is a schematic diagram of an exemplary operating window for operating a nozzle assembly with a non-design gas fuel; and 
           [0013]      FIG. 6  is a schematic diagram of an alternate operating window for operating a nozzle assembly with the same non-design gas fuel used in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 1  is a cross-sectional view of an exemplary nozzle assembly  10 . In the exemplary embodiment, nozzle assembly  10  is divided into four regions by function including an inlet flow conditioner (IFC)  12 , a swirler assembly  14  with fuel injection, an annular fuel fluid mixing passage  16 , and a central diffusion flame fuel nozzle assembly  18 . Nozzle assembly  10  also includes a high pressure plenum  20  having an inlet end  22  and a discharge end  24 . High pressure plenum  20  circumscribes nozzle assembly  10 . Discharge end  24  does not circumscribe nozzle assembly  10 , but rather discharge end  24  extends into a combustor reaction zone  26 . IFC  12  includes an annular flow passage  28  that is defined by a solid cylindrical wall  30 . Wall  30  defines an inside diameter  32  for passage  28 , and a perforated cylindrical outer wall  34  defines an outside diameter  36 . A perforated end cap  38  is coupled to an upstream end  40  of nozzle assembly  10 . In the exemplary embodiment, flow passage  28  includes one or more annular turning vanes  42 . During operation, compressed fluid enters IFC  12  via perforations in end cap  38  and cylindrical outer wall  30 . Moreover, it should be understood that in the exemplary embodiment nozzle assembly  10  defines a premix gas fuel circuit that enables design gas fuel and compressed fluid to be mixed prior to combustion. 
         [0015]    As used herein the term “design gas fuel” is the particular gas fuel originally selected to enable a corresponding turbine engine to achieve a demand power output by design. Non-design gas fuels may also be used to power turbine engines and are different from design gas fuels. In the exemplary embodiment, the design gas fuel is a methane-based gas. However, it should be appreciated that in other embodiments the design gas fuel may be any gas fuel that facilitates powering a turbine engine as described herein. In the exemplary embodiment, non-design gas fuels include no-methane based gas fuels, such as, but not limited to, liquefied natural gas (LGN), non-design gas fuels and process gas fuels. 
         [0016]      FIG. 2  is an enlarged perspective view of an exemplary swirler assembly  14  that may be used with nozzle assembly  10  (shown in  FIG. 1 ).  FIG. 3  is an enlarged perspective view of a portion of swirler assembly  14 . Swirler assembly  14  includes a plurality of vanes  44  that each extend between a radially outer band  46 , having an outer surface  48 , and a radially inner band  50 , having an outer surface  52 . Each vane  44  includes a suction sidewall  54  and a pressure sidewall  56 . Suction sidewall  54  is convex and defines a suction side of vane  44 , and pressure sidewall  56  is concave and defines a pressure side of vane  44 . Sidewalls  54  and  56  are joined at a leading edge  58  and at an axially-spaced trailing edge  60  of vanes  44 . 
         [0017]    Suction and pressure sidewalls  54  and  56 , respectively, extend longitudinally, in span between radially inner band  50  and radially outer band  46 . A vane root  62  is defined as being adjacent inner band  50 , and a vane tip  64  is defined as being adjacent outer band  46 . 
         [0018]    It should be understood that turning vanes  44  impart swirl to compressed fluid passing through swirler assembly  14 . Moreover, turning vanes  44  each include a primary fuel supply passage  66  and a secondary fuel supply passage  68  defined in a core (not shown) of each vane  44 . In the exemplary embodiment, each vane pressure side  56  includes a plurality of gas fuel injection orifices  70  and a plurality of secondary gas fuel injection orifices  72 . In the exemplary embodiment, orifices  70  and  72  each have a substantially circular shape, and orifices  70  have a larger cross-sectional area than orifices  72 . It should be understood that primary injection orifices  70  and secondary injection orifices  72  penetrate pressure sidewall  56  of each vane  44 . Moreover, it should be appreciated that in other embodiments, fuel injection orifices  70  and  72  may be located on vane suction side  54 , or on both pressure and suction sides  56  and  54 , respectively. Furthermore, although orifices  70  and  72  are described as having a substantially circular shape, in other embodiments, orifices  70  and  72  may have any shape, orientation, or configuration that enables nozzle assembly  10  to function as described herein. 
         [0019]    During operation, primary fuel passage  66  and secondary fuel passage  68  distribute gas fuel to primary injection orifices  70  and secondary injection orifices  72 , respectively. Gas fuel enters swirler assembly  14  through inlet port  74  (shown in  FIG. 1 ) and annular premix gas fuel passages  76  and  78  (shown in  FIG. 1 ). Annular premix gas fuel passages  76  and  78  supply primary  66  and secondary  68  fuel supply passages, respectively. The gas fuel mixes with compressed fluid in swirler assembly  14 , and fuel/air mixing is completed in annular premix passage  16  (shown in  FIG. 1 ). Passage  16  is defined by a nozzle hub extension  80  (shown in  FIG. 1 ) and a nozzle shroud extension  82  (shown in  FIG. 1 ). It should be appreciated that most of the compressed fluid for combustion enters nozzle assembly  10  via IFC  12 , and is channeled through swirler assembly  14  after exiting IFC  12 . After exiting annular premix passage  16 , the fuel/air mixture enters combustor reaction zone  26  wherein the mixture is ignited. It should be understood that there are a plurality of nozzle assemblies  10  in an annular array about a turbine housing (not shown). It should also be appreciated that the term “fluid” as used herein includes any medium or material that flows, including, but not limited to, gas and air. 
         [0020]    Premix nozzle assembly  10  operates as a “soft fuel nozzle.” As used herein the term “soft fuel nozzle” refers to combustion pressure waves that may feed back into fuel passages  76  and  78  (shown in  FIG. 1 ). Thus, in the exemplary embodiment, a fuel nozzle pressure ratio, defined as the fuel supply pressure divided by the combustor pressure, is less than about 1.07 to 1.10. Above such a pressure ratio range, nozzle assembly  10  is called a “hard fuel nozzle” and damaging dynamics may be produced. Another type of damaging dynamics may be produced when the fuel nozzle pressure ratio is below an acceptable minimum fuel nozzle pressure ratio. Such damaging dynamics are capable of quickly ruining a turbine engine. Thus, in the exemplary embodiment, a safe fuel nozzle pressure ratio range is between the minimum fuel pressure ratio and 1.07 to 1.10. It should be understood that altering the non-design gas fuel temperature may influence dynamics development. 
         [0021]      FIG. 4  is an enlarged cross-sectional view of an exemplary gas fuel jet  84  and a corresponding flame-holding margin  86 . As non-design gas fuel is injected through primary fuel injection orifice  70  in an injection angle that is substantially perpendicular to pressure side  56 , the non-design fuel forms fuel jet  84 . As fuel jet  84  exits orifice  70 , it encounters rapidly-moving, compressed fluid cross-flow  88  which forces fuel jet  84  to flow substantially parallel to pressure side  56 . Once configured, fuel jet  84  engenders flame-holding margin  86  development. It should be appreciated that fuel jet  84  flows substantially parallel to pressure side  56  and is distanced from pressure side  56  by a penetration height PH. Penetration height PH represents the relative, maximum non-design gas fuel jet penetration height into compressed fluid cross-flow  88 . By reducing non-design gas fuel jet penetration height PH and by reducing non-design gas fuel reactivity relative to design gas fuel reactivity, flame-holding margin  86  may be stabilized, substantially reduced, or eliminated. It should be appreciated that non-design gas fuel may also be projected onto non-vane  44  surfaces such as, but not limited to, an outer band inner surface  47  (shown in  FIG. 2 ) and outer surface  52  (shown in  FIG. 2 ), which may also cause flame holding, flashback, or both. If the momentum of fuel jet  84  is not within an acceptable range, a flow disturbance (not shown) may develop that causes a separation in the downstream swirler assembly  14 . 
         [0022]      FIG. 5  is a schematic diagram showing an exemplary operating window  90  for non-design gas fuels in premix nozzle assembly  10 . More specifically, operating window  90  is shown as a function of penetration height PH, temperature  92 , and fuel nozzle pressure ratio range  94 . In the exemplary embodiment, operating window  90  defines a regime specific to the non-design gas fuel used to facilitate preventing negative effects of flame-holding. 
         [0023]    Penetration height PH is proportional to ((ρ fuel ×V 2   fuel )/(ρ air ×V 2   air )) 1/2 , where ρ fuel  is the density of the non-design gas fuel, V fuel  is the velocity of the non-design gas fuel, ρ air  is the density of compressed fluid cross-flow  88  (shown in  FIG. 4 ), and V air  is the mean velocity of compressed fluid cross-flow  88 . Additionally, the mass flow rate of a gas fuel is given by ρ×A×V, where ρ is the density of the gas fuel, A is the effective cross-sectional area of fuel injection orifice  70  or  72  (shown in  FIG. 3 ), and V is the velocity of the gas fuel. By manipulating the parameters (ρ fuel , V fuel , ρ air , V air ) that determine penetration height PH, and the parameters (ρ, A, V) that determine the mass flow rate, a plurality of different operating windows  90  may be generated for each non-design gas fuel. The fuel nozzle pressure ratio, defined as the fuel supply pressure divided by the combustor pressure, is another parameter that facilitates determining or generating operating window  90 . The fuel nozzle pressure ratio may be adjusted by manipulating the gas fuel supply pressure which also allows manipulating or redefining fuel nozzle pressure ratio range  94 ; however, it should be appreciated that heating of the non-design gas fuel may be required. 
         [0024]    Operating window  90  may also be altered by changing the density of non-design gas fuel by manipulating the non-design gas fuel temperature  92 . By cooling the non-design gas fuel, more non-design gas fuel is able to flow through nozzle assembly  10  while remaining within fuel nozzle pressure ratio (FNPR) range  94  which is defined by the area enclosed by lines  102 ,  104 ,  106  and  108 . Likewise, changing the density of non-design gas fuel by increasing its temperature  92 , causes less non-design gas fuel to flow through nozzle assembly  10  while remaining within fuel nozzle pressure ratio (FNPR) range  94 . It should be understood that increasing the non-design gas fuel temperature  92 , not only decreases the fuel density, but also decreases the non-design gas fuel velocity. Decreasing the non-design gas fuel velocity also causes a related gas fuel pressure drop. In the exemplary embodiment, it is desirable to provide a cooler, non-design fuel that remains within fuel nozzle pressure ratio range  94 , to satisfy combustion dynamic requirements while still providing enough fuel for desired turbine engine output; however heating the non-design fuel may be required. Changing the fuel temperature enables the non-design fuel to operate at the desired range of engine output. In addition, changing the fuel temperature can also facilitate increasing the range of power output allowable for the design fuel. For example, cooling the fuel temperature can reduce fuel reactivity which facilitates reducing flame holding and auto-ignition propensity, thus providing additional operating margin. 
         [0025]    A flame-holding margin boundary  96  distinguishes between a flame-holding region  98  above boundary  96 , and a non-flame-holding region  100  below boundary  96 . It should be appreciated that flame-holding margin boundary  96  is fuel specific and depends on the reactivity of the non-design gas fuel used, and as such is different for every type of non-design gas fuel. Flame-holding region  98  indicates that there is a flame-holding margin  86  and non-flame-holding region  100  indicates that there is no flame-holding margin  86 . In the exemplary embodiment, thermal temperature lines of a non-design gas fuel including fifty percent hydrogen (H2) and fifty percent carbon monoxide (CO), at two different temperatures, are also illustrated. More specifically, a first thermal temperature line  102  represents a non-design gas fuel having a temperature  92  of about 300 0  F. A second thermal temperature line  104  represents the same non-design gas fuel at a temperature  92  of about 80 0  F. Moreover, fuel nozzle pressure ratio range  94  is defined by a lower pressure ratio limit  106  and an upper pressure ratio limit  108 . 
         [0026]    The area below boundary  96  that is above thermal temperature line  102  and is also within fuel pressure ratio range  94 , defines an operating window  90  corresponding to a turbine engine using the given non-design gas at a temperature  92  of about 300 0  F. Likewise, the area below boundary  96 , that is between boundary  96  and thermal temperature line  104 , and that is also within fuel pressure ratio range  94 , defines an operating window  90  for a turbine engine using the given non-design gas at a temperature  92  of about 80 0  F. As can be seen by comparing thermal temperature lines  102  and  104 , using lower temperature non-design gas facilitates decreasing jet penetration height PH by about a third. Thus, it can be seen that cooling the temperature  92  of non-design gas fuels facilitates reducing penetration height PH and substantially reduces or eliminates flame-holding margin  86 . Because cooling non-design gas fuels reduces the fuel momentum ratio and produces a cooler jet with a lower jet penetration height PH, flame-holding margin  86  is facilitated to be decreased. 
         [0027]    It should be understood that for fuel orifices having fixed geometries, such as fuel injection orifices  70  and  72 , pressure is proportional to (V 2   fuel ×ρ fuel )/2, where V fuel  is the velocity of the non-design gas fuel and ρ fuel  is the non-design gas fuel density. Thus, a fuel having a high volumetric flow requirement, such as a low Modified Wobbe Index (MWI) non-design gas fuel, may be sufficiently cooled to generate the same power output as the design gas, such that turbine engines with design gas fuel nozzles may operate as “soft fuel nozzles” using low MWI non-design gas fuels. It should be appreciated that the Wobbe Index is usually the heat content (LHV) divided by the square root of the molecular weight ratio. The Modified Wobbe Index (MWI) assumes that the gases are at different temperatures and the units would have a square root of absolute temperature. As used herein, a low MWI non-design gas fuel ranges from about 15 to 70, which is different than an MWI design gas. Methane-based natural gas fuels generally have an MWI of about 39 to 55 depending on fuel temperature and composition. Furthermore, it should be appreciated that the increased load of a turbine engine is satisfied by increasing the flame temperature. To satisfy increased turbine engine loading, a temperature  92  range for a non-design gas fuel may be decreased for one or more fuels with different MWI, due to the fuel pressure ratio. 
         [0028]    It should be appreciated that in the exemplary embodiment non-design gas fuel is provided to turbine engines through underground pipelines, thus ensuring a relatively constant, cool, non-design gas fuel supply. However, in other embodiments, any means may be employed to cool non-design gas fuel such that nozzle assembly  10  functions as described herein. Moreover, it should be appreciated that although the exemplary embodiment is described as a cooled, non-design gas fuel, in other embodiments, various compositions of non-design gas fuels may be used, and depending on the composition of non-design gas used, such fuels may be heated or cooled such that turbine engines with design gas fuel nozzles operate as “soft fuel nozzles.” When cooling the non-design gas fuel, proper consideration of the dew point for condensation and Joule-Thompson effect in valves should be considered which may limit the lower temperature or require reduction in concentration of some fuel constituents for cooling or both. It should be appreciated that cooling non-design gas fuel reduces the fuel&#39;s reactivity and potentially decreases flame holding an auto-ignition. 
         [0029]    It should be understood that some non-design gas fuels have a higher volumetric flow rate requiring larger fuel injection orifices than is permitted for existing design gas fuel operation. Consequently, operating turbines with non-design gas fuels having a higher MWI may require heating during premixing operations in nozzle assembly  10  with one premixed fuel passage,  76  or  78  (shown in  FIG. 1 ), per fuel nozzle assembly  10 , to facilitate maintaining the minimum allowable fuel nozzle pressure ratio and not to “flame-hold” with fuels having lower MWI indexes. As used herein, a high MWI non-design gas fuel has a higher MWI than design gas. 
         [0030]    It should be appreciated that controlling flame-holding margin  86  facilitates using non-design fuels that facilitate reducing nitrous oxide (NOx) emissions. More specifically, because controlling flame-holding margin  86  allows using a wider variety of non-design gas fuels, non-design gas fuels that inherently or more effectively reduce NOx and CO emissions when burned, may be used for combustor designs. For example, if a non-design gas fuel produces one part per million NOx when burned and the design gas fuel produces five parts per million NOx when burned, it may be better to operate a turbine engine using non-design gas fuel to reduce NOx emissions. Moreover, it should be appreciated that the design gas fuel may be heated to improve penetration, improve fuel mixing to decrease NOx emissions and reduce damaging dynamics. 
         [0031]    It is desirable to increase the fuel flexibility of turbine engines by operating turbine engines with less expensive, and more readily-available alternative non-design gas fuels, rather than design gas fuels. Such alternative non-design gas fuels include, but are not limited to, liquefied naural gas (LGN), syngas and process gas. Using alternative fuels requires providing sufficient flame-holding margin in premixed fuel nozzles. It should be appreciated that controlling jet penetration height produces acceptable flame-holding margin for operation on a single fuel passage and extends flame-holding margins. 
         [0032]      FIG. 6  is a schematic diagram showing an alternative exemplary operating window  110  for non-design gas fuel in diffusion nozzle assemblies (not shown). Diffusion nozzle assemblies mix non-design gas fuel and compressed fluid, and ignite the combination where it is mixed. Operating window  110  is shown as a function of fuel nozzle pressure ratio  112  and fuel nozzle flame temperature  114 . It should be understood that operating window  110  defines a regime specific to the non-design gas fuel used to facilitate preventing the negative effects of flame-holding. 
         [0033]    Thermal temperature lines of a non-design gas fuel including fifty percent hydrogen (H2) and fifty percent carbon monoxide (CO), at two different temperatures, are also illustrated. More specifically, a first thermal temperature line  116  represents a non-design gas fuel having a temperature of about 300 0  F. A second thermal temperature line  118  represents the same non-design gas fuel at a temperature of about 80 0  F. In the exemplary embodiment, operating window  110  indicates the minimum and maximum pressure ratio over which combustion dynamics are acceptable for diffusion nozzle assemblies. Increasing the fuel temperature decreases the fuel density such that fuel jet  84  velocity increases for the same non-design gas fuel flow to deliver the same mass of fuel to a combustion chamber. It should be understood that operating within operating window  110  provides adequate turbine engine power and does not engender development of damaging dynamics. 
         [0034]    In each embodiment, the above-described methods of controlling flame-holding margins facilitate increasing the range of fuel compositions that can be safely used in existing turbine engines with a reduced-cost, single-premixed circuit. Moreover, existing turbine engines may operate with different fuels without requiring installation of new fuel nozzles. Furthermore, non-design gas fuel heating and cooling circuits could be integrated into systems for minimizing cycle losses. Such circuits may include heating non-design gas fuel in an exhaust stack and cooling non-design gas fuel with make up water. More specifically, in each embodiment, changing the fuel temperature and operating within a fuel pressure ratio range, stabilizes, substantially reduces or eliminates flame-holding margins. As a result, fuel flexibility of existing turbine engines and operation of existing turbine engines with a single fuel circuit are provided. Accordingly, turbine performance and component useful life are each facilitated to be enhanced in a cost-effective and reliable manner. 
         [0035]    Exemplary embodiments of methods for controlling flame-holding margins are described above in detail. The methods are not limited to use with the specific turbine embodiments described herein, but rather, the methods can be utilized independently and separately from other components described herein. For example, the methods may be used with any utility, industrial or mechanical drive turbine. Moreover, the invention is not limited to the embodiments of the method described above in detail. Rather, other variations of the method may be utilized within the spirit and scope of the claims. 
         [0036]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.