Patent Publication Number: US-8991187-B2

Title: Combustor with a lean pre-nozzle fuel injection system

Description:
TECHNICAL FIELD 
     The present application relates generally to gas turbine engines and more particularly relates to a combustor with a lean pre-nozzle fuel injection system for mixing fuel and air upstream of the fuel nozzles. 
     BACKGROUND OF THE INVENTION 
     In a gas turbine engine, operational efficiency generally increases as the temperature of the combustion stream increases. Higher combustion stream temperatures, however, may produce higher levels of nitrogen oxide (“NO x ”) and other types of emissions that may be subject to both federal and state regulation in the United States and also subject to similar regulations abroad. A balancing act thus exists between operating the gas turbine engine in an efficient temperature range while also ensuring that the output of NO x  and other types of regulated emissions remain below the mandated levels. 
     Several types of known gas turbine engine designs, such as those using Dry Low NO x  (“DLN”) combustors, generally premix the fuel flows and the air flows upstream of a reaction or a combustion zone so as to reduce NO x  emissions via a number of premixing fuel nozzles. Such premixing tends to reduce overall combustion temperatures and, hence, NO x  emissions and the like. 
     Premixing, however, may present several operational issues such as flame holding, flashback, auto-ignition, and the like. These issues may be a particular concern with the use of highly reactive fuels. For example, it is possible for a flame to sustain in the head-end upstream of the fuel nozzles with any significant fraction of hydrogen or other types of fuels. Any type of fuel rich pocket thus may sustain a flame and cause damage to the combustor. Other premixing issues may be due to irregularities in the fuel flows and the air flows. 
     There is thus a desire for an improved combustor design. Such a combustor design should promote improved fuel-air premixing, particularly with the use of highly reactive fuels. Such combustors designs should promote such good mixing while maintaining emissions below mandated levels and avoiding or limiting issues such as flame holding, flashback, auto-ignition, and the like 
     SUMMARY OF THE INVENTION 
     The present application thus provides a combustor for combusting a flow of fuel and a flow of air. The combustor may include a number of fuel nozzles, a lean pre-nozzle fuel injection system positioned upstream of the fuel nozzles, and a premixing annulus positioned between the fuel nozzles and the lean pre-nozzle fuel injection system to premix the flow of fuel and the flow of air. 
     The present application further concerns a method of providing a number of flows of fuel and a flow of air in a combustor. The method may include the steps of injecting a flow of a premix fuel into a premixing annulus, providing the flow of air into the premixing annulus, premixing the flow of the premix fuel and the flow of air into a premixed flow along the premixing annulus, providing the premixed flow to a number of fuel nozzle, and injecting a further flow of fuel into the premixed flow along the number of fuel nozzles. 
     The present application further provides a combustor for combusting a flow of fuel and a flow of air. The combustor may include a number of fuel nozzles with each of the fuel nozzles including a bellmouth, a lean pre-nozzle fuel injection system positioned upstream of the fuel nozzles, and a premixing annulus positioned between the fuel nozzles and the lean pre-nozzle fuel injection system to premix the flow of fuel and the flow of air. The premixing annulus may expand in the direction of the fuel nozzles. 
     These and other features and improvements of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a known gas turbine engine. 
         FIG. 2  is a side cross-sectional view of a known combustor. 
         FIG. 3  is a side cross-sectional view of a combustor with a lean pre-nozzle fuel injection system as may be described herein. 
         FIG. 4  is a side cross-sectional view of a fuel nozzle for use with the combustor with the lean pre-nozzle fuel injection system of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like elements throughout the several views,  FIG. 1  shows a schematic view of gas turbine engine  10  as may be described herein. The gas turbine engine  10  may include a compressor  15 . The compressor  15  compresses an incoming flow of air  20 . The compressor delivers the compressed flow of air  20  to a combustor  25 . The combustor  25  mixes the compressed flow of air  20  with a compressed flow of fuel  30  and ignites the mixture to create a flow of combustion gases  35 . Although only a single combustor  25  is shown, the gas turbine engine  10  may include any number of combustors  25 . The flow of combustion gases  35  is in turn delivered to a turbine  40 . The flow of combustion gases  35  drives the turbine  40  so as to produce mechanical work. The mechanical work produced in the turbine  40  drives the compressor  15  and an external load  45  such as an electrical generator and the like. 
     The gas turbine engine  10  may use natural gas, various types of syngas, and/or other types of fuels. The gas turbine engine  10  may be anyone of a number of different gas turbine engines offered by General Electric Company of Schenectady, N.Y. The gas turbine engine  10  may have different configurations and may use other types of components. Other types of gas turbine engines also may be used herein. Multiple gas turbine engines, other types of turbines, and other types of power generation equipment also may be used herein together. 
       FIG. 2  shows a simplified example of a known combustor  25 . Generally described, the combustor  25  may include a combustion chamber  50  with a number of fuel nozzles  55  positioned therein. The fuel nozzles  55  may be premixing nozzles with one or more swirlers  60  thereon. The swirlers  60  aid in the premixing of the flow of air  20  and the flow of fuel  30 . An incoming air path  65  may be defined between a liner  70  of the combustion chamber  50  and a casing  75 . A transition piece  80  may be positioned downstream of the combustion chamber  50 . Other types of combustor configurations are known. 
     The flow of air  20  may enter the combustor  25  from the compressor  15  via the incoming air path  65 . The flow of air  20  may reverse direction and may be premixed about the fuel nozzles  55  and the swirlers  60  with the flow of fuel  30 . The mixed flow of air  20  and the flow of fuel  30  may be combusted within the combustion chamber  50 . The flow of combustion gases  35  then may be exhausted through the transition piece  80  towards the turbine  40 . Depending upon the nature of the combustor  25 , the combustor  25  may use a primary fuel which may be a fuel gas passing through the swirlers  60 ; a secondary fuel and a tertiary fuel which may be a premixed fuel gas; and a lean pre-nozzle fuel injection system that may inject a small amount of fuel just upstream of the swirlers  60 . Other types of fuel circuits and configurations also are known. 
       FIGS. 3 and 4  show a combustor  100  as may be described herein. Similar to the combustor  25  described above, the combustor  100  includes a combustion chamber  110  with a number of fuel nozzles  120  positioned therein. In this example, a center nozzle  130  may be surrounded by a number of outer nozzles  140 . Any number of fuel nozzles  120  may be used herein. 
     Generally described, each of the fuel nozzles  120  may include a central fuel passage  150 , generally for a liquid fuel. The fuel nozzles  120  also may include a number of fuel injectors  160 . The fuel injectors  160  may be positioned about one or more swirlers  170 . The fuel injectors  160  may be used with a premix fuel and the like. Other types of fuel circuits may be used herein. The fuel nozzles  120  also may include a bellmouth  180  at an upstream end thereof for the incoming flow of air  20 . Any number or shape of the bellmouths  180  may be used. 
     The combustor  100  also includes an incoming air path  200 . The incoming air path  200  may be defined between a liner or a cap baffle  210  and a casing  220 . The cap baffle  210  may be attached to an end cap  230  and may expand in the direction towards an end cover  240  in a flared shape  245 . Likewise, the casing  220  may be flared such that the casing  220  has a larger diameter in the direction of the flow towards the end cover  240 . The cap baffle  210  and the casing  220  may define a premixing annulus  250 . The overall premixing annulus  250  thus expands towards the end cover  240  as well. The premixing annulus  250  may have a smooth turning portion  260  about the end cover  240  towards the fuel nozzles  120 . The premixing annulus  250  may provide diffusion or not. Other configurations may be used herein. 
     A lean pre-nozzle fuel injection system  270  also may be positioned about the incoming air path  200  between the cap baffle  210  and the casing  220  about the end cap  230 . The lean pre-nozzle fuel injection system  270  may have a number of fuel injectors  280 . The fuel injectors  280  may have an aerodynamic wing-like or streamlined shape  285  for optimized flame holding resistance. The fuel injectors  280  each may have a number of injector holes  290  therein. The number of fuel injectors  280  and the number of injection holes  290  may be optimized for premixing. Other configurations may be used herein. A premix fuel  300  may flow therein. 
     In use, the premix fuel  300  is injected via the fuel injectors  280  of the lean pre-nozzle fuel injection system  270  into the incoming flow of air  20  passing through the incoming air path  200 . The aerodynamic wing-like shape  285  of the fuel injectors  280  minimizes the risk of holding a flame on or behind the injectors  280 . The premix fuel  300  and the flow of air  200  thus premix into a premixed stream  310  along the length of the premixing annulus  250 . Because both the cap baffle  210  and the casing  220  expand in the direction towards the end cover  240 , the premixing annulus  250  slows the air and recovers some of the static pressure. This flared shape thus allows more diffusion than a typical cylindrical casing. The premixing also removes any rich pockets of fuel that might sustain a flame. The length of the premixing annulus  250  along with the number and the spacing of the injectors  280  thus provide improved premixing within the premixing annulus  250 . The premixed stream  310  will be fully mixed before exiting the annulus  250 . 
     The premixed stream  310  then turns about the turning section  260  and enters the fuel nozzles  120 . Because the flow of air  200  slows within the premixing annulus  250 , the premixed stream  310  turns easily about the turning section  260  into the fuel nozzles  120  without recirculation or flow deficits. As a result, the fuel nozzles  120  may use the bellmouths  180  as opposed to a traditional flow conditioner that may result in a lower pressure drop. The premixed stream  310  further mixes with the conventional flow of fuel  30  from the fuel injectors  160  or otherwise before being combusted in the combustion chamber  110 . 
     The premixing annulus  250  may flow a large percentage of the total fuel flow without negatively impacting emissions. Likewise, by unloading the fuel nozzles  120 , i.e., by taking fuel away, overall flame holding performance of the fuel nozzles also may be enhanced. The ability to modulate the percentage of the total fuel delivered to the lean pre-nozzle fuel injection system  270  over a wide range may provide pressure ratio control so as to deal with fluctuations in the fuel composition. The overall pressure ratio of the fuel nozzles  120  may be optimized for dynamics without changing the nozzle equivalent ratio and the like. Moreover, the size of the fuel injectors  160  also may be reduced. 
     The use of the fuel injectors  280  of the lean pre-nozzle fuel injection system  270  and the premixing annulus  250  thus reduces NO x  emissions, reduces the pressure drop, and provides increased fuel flexibility in terms of both MWI. (Modified Wobbe Index) capability and fuel reactivity. The lean pre-nozzle fuel injection system  270  thus may be fuel flexible including the use of highly reactive fuels such as hydrogen, ethane, propane, etc. 
     It should be apparent that the foregoing relates only to certain embodiments of the present application and that numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.