Abstract:
A gas turbine combustion system having reduced emissions and improved flame stability at multiple load conditions is disclosed. The improved combustion system accomplishes this through complete premixing, a plurality of fuel injector locations, combustor geometry, and precise three dimensional staging between fuel injectors. Axial, radial, and circumferential fuel staging is utilized including fuel injection proximate air swirlers. Furthermore, strong recirculation zones are established proximate the introduction of fuel and air premixture from different stages to the combustion zone. The combination of the strong recirculation zones, efficient premixing, and staged fuel flow thereby provide the opportunity to produce low emissions combustion at various load conditions.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates in general to gas turbine combustion systems and specifically to a gas turbine combustion system that can operate at significantly lower load conditions while having stable combustion and lower emissions. 
   2. Description of Related Art 
   In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location and mixing effectiveness. 
   Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles produce high emissions due to the fact that the fuel and air burn stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics. 
   An enhancement in combustion technology is the utilization of premixing, such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. An example of a premixing combustor of the prior art is shown in  FIG. 1. A  combustor  8  has a plurality of fuel nozzles  18 , each injecting fuel into a premix cavity  19  where fuel mixes with compressed air from plenum  10  before entering combustion chamber  20 . Premixing fuel and air together before combustion allows for the fuel and air to form a more homogeneous mixture, which will burn more completely, resulting in lower emissions. However, in this configuration the fuel is injected in relatively the same plane of the combustor, and prevents any possibility of improvement through altering the mixing length. 
   An alternate means of premixing and lower emissions is through multiple combustion stages, which allows for enhanced premixing as load increases. Referring now to  FIG. 2 , an example of a prior art multi-stage combustor is shown. A combustor  30  has a first combustion chamber  31  and a second combustion chamber  32  separated by a venturi  33 , which has a narrow throat region  34 . While combustion can occur in either first or second combustion chambers or both chambers, depending on load conditions, the lowest emissions levels occur when fuel, which is injected through nozzle regions  35 , is completely mixed with compressed air in first combustion chamber  31  prior to combusting in second combustion chamber  32 . The amount of load turndown is limited by the decreasing flame temperature as the load is decreased, making the flame unstable to the point where flashback occurs into the first combustion chamber. Therefore, this multi-stage combustor with a venturi is more effective at higher load conditions. While a full load condition is the most common operating point for land-based gas turbines used for generating electricity, often times electricity demands do not require the full load of the generator, and the operator desires to operate the engine at a lower load setting, such that only the load demanded is produced, thereby saving fuel costs. Combustion systems of the prior art have been known to become unstable at lower load settings while also producing unacceptable levels of NOx and CO emissions at lower load settings, especially below 50% load. This is primarily due to the fact that most combustion systems are staged for most efficient operation at high load settings. The combination of potentially unstable combustion and higher emissions often times prevents engine operators from running engines at lower load settings, forcing the engines to either run at higher settings, thereby burning additional fuel, or shutting down, and thereby losing valuable revenue that could be generated from the part-load demand. A further problem with shutting down the engine, is the additional cycles that are incurred by the engine hardware. A cycle is commonly defined as the engine passing through the normal operating envelope. Engine manufacturers typically rate hardware life in terms of operating hours or equivalent operating cycles. Therefore, incurring additional cycles can reduce hardware life requiring premature repair or replacement at the expense of the engine operator. What is needed is a system that can provide flame stability and low emissions benefits at a part load condition, as well as at a full load condition, such that engines can be efficiently operated at lower load conditions, thereby eliminating the wasted fuel when high load operation is not demanded or incurring the additional cycles on the engine hardware when shutting down. 
   SUMMARY AND OBJECTS OF THE INVENTION 
   The present invention discloses a gas turbine combustion system for reducing polluting emissions such as NOx and CO, while being able to provide stable combustion at lower load conditions. The combustion system contains a casing having a center axis, which is in fluid communication with the engine compressor, and an end cover fixed to the casing. In the preferred embodiment, the end cover contains a plurality of first injectors arranged in a first array about the end cover and a plurality of second injectors arranged in a second array about the end cover, with the second array radially outward of the first array. Located proximate the end cover is a first swirler having a plurality of passageways oriented generally perpendicular to the casing center axis for inducing a swirl generally radially inward to a first portion of the compressed air. Fuel, which is injected through the first and second injectors, mixes with the first portion of compressed air from the first swirler before entering a liner through a dome section. Additional fuel is also introduced to a second portion of compressed air through a plurality of third injectors located in a manifold of an aft injector assembly. The third injectors are divided into multiple circumferential sectors to allow for various fuel staging circumferentially around the aft injector assembly. To enhance mixing between fuel from the third injectors and second portion of compressed air, a second swirler is positioned adjacent the aft injector assembly for imparting a swirl to the second portion of compressed air. This fuel and air mixes in a second passage located between a first part of the liner and the dome prior to entering the liner and mixing with the fuel and first portion of compressed air from the first swirler region. Upon entering the liner, the premixture from the second passage must undergo a complete reversal of flow direction that causes strong recirculation zones at the forward end of the liner. These recirculation zones help to increase combustor stability by providing a region where a portion of the hot combustion gases can be entrained and recirculate to provide continuous ignition to the incoming premixed fuel and compressed air. Fuel flow to each of the first, second, and third sets of injectors is controlled independently to allow for fuel staging throughout various load conditions to control NOx and CO emissions at each load setting. 
   It is an object of the present invention to provide a combustion system having low NOx and CO at multiple operating conditions. 
   It is a further object of the present invention to provide a combustion system having a stable combustion process throughout all operating conditions. 
   In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross section view of a portion of a gas turbine engine containing a combustion system of the prior art. 
       FIG. 2  is a cross section view of an alternate combustion system of the prior art. 
       FIG. 3  is a perspective view of the present invention. 
       FIG. 4  is a cross section view of the present invention. 
       FIG. 5  is a detailed cross section view of the end cover of the present invention. 
       FIG. 6  is a detailed cross section view of a portion of the dome of the present invention. 
       FIG. 7  is a detailed cross section view of a portion of the aft injector assembly of the present invention. 
       FIG. 8  is a detailed cross section view of the aft injector assembly of the present invention. 
       FIG. 9  is a cross section view of an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The preferred embodiment of the present invention will now be described in detail with specific reference to  FIGS. 3-8 . Referring now to  FIGS. 3 and 4 , a gas turbine combustion system  40  of the present invention is shown. Combustion system  40  includes a casing  41  having a first end  42 , a second end  43 , and a center axis A—A. Casing  41 , which is mounted to an engine through flange  44 , is in fluid communication with compressed air from a compressor. 
   Referring now to  FIGS. 4 and 5 , an end cover  45  is fixed to casing first end  42 , with end cover  45  having at least one fuel source in fluid communication with at least one set of injectors. In the preferred embodiment a first fuel source  46  is in fluid communication with a plurality of first injectors  47 , where first injectors  47 , comprising at least two injectors, are arranged in a first array radially outward of center axis A—A. Furthermore, the preferred embodiment of end cover  45  also contains a second fuel source  48  in fluid communication with a plurality of second injectors  49 , where second injectors  49  are arranged in a second array radially outward of first injectors  47 . As with first injectors  47  it is preferred that second injectors  49  comprises at least two injectors. 
   Referring now to  FIGS. 4 and 6 , a dome  50  is located radially inward from casing  41 , thereby forming, a first passage  51 . Also located radially inward from casing  41  is a liner  53 , having a first part  54  located radially inward from dome  50 , thereby forming a second passage  55  between dome  50  and first part  54  of liner  53 . Dome  50  also contains a first opening  56 , an inner dome wall  57 , and an outer dome wall  58 , where inner dome wall  57  and outer dome wall  58  have a third passage  59  therebetween. An additional feature of dome  50  is the plurality of first feed holes  60  in outer dome wall  58  that extend from third passage  59  to first passage  51 . 
   Referring back to  FIGS. 4 and 5 , a first swirler  61  is positioned adjacent end cover  45  and has a plurality of passageways  62 . First swirler  61  is oriented such that a first portion of compressed air from the engine compressor passes through the plurality of passageways  62  prior to entering the liner. Passageways  62  are oriented generally perpendicular to the center axis A—A such that the first portion of compressed air is introduced radially into swirler  61 . 
   The combustion system of the present invention further contains an aft injector assembly  63 , which is shown in  FIGS. 4 ,  7 , and  8 . Aft injector assembly  63  contains a manifold  64  having at least one sector. In the preferred embodiment of the present invention, manifold  64  contains a plurality of sectors  65 ,  66 ,  67 , and  68 , with each of the sectors in fluid communication with a another fuel source  69 . Each of the sectors  65 ,  66 ,  67 , and  68  is isolated from adjacent sectors by a manifold wall  65 ′,  66 ′,  67 ′, and  68 ′ so that fuel supplied to one of the sectors does not flow into another sector of the aft injector assembly  63 . Valve means (not shown) permit the fuel flow to each sector to be controlled independent of the other sectors. Located in manifold  64  is a plurality of third injectors  70  that inject a fuel into second passage  55 . Each of the third injectors  70  is connected to only one of the sectors  65 ,  66 ,  67 , or  68 , so that all of the fuel that flows through a particular injector  70  during engine operation is supplied by a single sector  65 ,  66 ,  67 , or  68 . 
   The combustion system of the present invention utilizes premixing fuel and air prior to combustion in combination with precise staging of fuel flow to the combustor to achieve the reduced emissions at multiple operating load conditions. In operation, casing  41  is in fluid communication with compressed air from a compressor. First passage  51  between casing  41  and dome  50  receives a first portion of the compressed air. The first portion of compressed air then passes into third passage  59 , which is located between inner dome wall  57  and outer dome wall  58 , by way of a plurality of first feed holes  60 , in order to cool inner dome wall  57 . The first portion of compressed air then flows through a second opening  100  in a dome baffle  102 , and then enters first swirler  61 , passes through passageways  62 , and is directed generally radially inward toward center axis A—A, at which point fuel is introduced to the swirling air through first injectors  47  and second injectors  49 , with second injectors  49  located proximate passageways  62  of first swirler  61 . The fuel and air premixture from first injectors  47 , second injectors  49 , and first swirler  61  then passes through a fourth passage  71  that directs the premixture through first opening  56  in dome  50 . Meanwhile, a second portion of compressed air from the compressor passes through a second swirler  72 , which is located adjacent aft injector assembly  63 , and imparts the second portion of air with a swirl prior to mixing with fuel from aft injector assembly  63 . The second portion of compressed air and fuel from aft injector assembly  63  mixes in second passage  55  and then, due to the geometry of dome  50 , reverses direction prior to entering combustion zone  73 . Therefore, fluid in first passage  51  and second passage  55  travel in a direction generally opposite to that of combustion products flowing through liner  53 . The premixture from fourth passage  71  mixes with the premixture from second passage  55  proximate combustion zone  73 . Depending on the load condition, some or all of the fuel injectors may be in use, with all fuel injectors being used at the highest load condition. The fuel is injected at flow rates and at different stages in order to generate the necessary amount of premixing to maintain low emissions throughout the operating spectrum. 
   An alternate embodiment of the present invention is shown in cross section in FIG.  9 . Included is the addition of sleeve  80 , which is coaxial with center axis A—A and is used for directing the second portion of compressed air to more effectively cool liner  53 , as well as to smooth air flow non-uniformity from the engine compressor. Sleeve  80  is positioned radially outward of liner  53  and aft of dome  50  such as to form a fifth passage  81  between sleeve  80  and liner  53  that is in fluid communication with second swirler  72  and second passage  55 . In order to supply compressed air to fifth passage  81  to more effectively cool liner  53 , a plurality of second feed holes  82  are placed about sleeve  80 . Due to pressure changes across second feed holes  82 , a jet of air is created that impinges on the outside of liner  53  to cool the surface prior to the compressed air being directed through second swirler  72  and mixing with fuel from aft injector assembly  63 . It should be noted that all other elements of the alternate embodiment of the present invention are the same as the preferred embodiment, and therefore do not require further discussion. 
   While the invention has been described in what is known as presently the preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements within the scope of the following claims.