Abstract:
A method of operating 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. Fuel injection staging sequences are disclosed that create the conditions necessary to provide stable combustion and reduced emissions at multiple load conditions.

Description:
TECHNICAL FIELD 
   This invention relates in general to gas turbine combustion systems and specifically to a method of operating a gas turbine combustion system at significantly lower load conditions while having stable combustion and lower emissions. 
   BACKGROUND 
   Gas turbine engines typically include a compressor, one or more combustors each having a fuel injection system, and a turbine section. In an engine having a plurality of combustors, they are typically arranged in an annular array about the engine. The compressor pressurizes inlet air, which is then introduced to the combustors, where it is used to cool the combustion chamber as well to provide air for the combustion process. The hot gases resulting from the combustion process are then directed to drive a turbine. For land-based gas turbines whose primary purpose is to generate electricity, a generator is coupled to the turbine shaft such that the turbine drives the generator. 
   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 carbon monoxide and oxides of nitrogen 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. However, advancements have been made with regards to fuel staging in an effort to lower emissions. For example, U.S. Pat. No. 5,551,228 discloses a method of operating a combustor involving assymetrical fuel staging within a combustor and axially staging fuel injection within a single fuel nozzle for reducing emissions. Furthermore, U.S. Pat. No. 5,924,275 discloses a method of operating a combustor that utilizes the addition of a center pilot nozzle in combination with the previously mentioned assymetrical fuel staging to provide reduced emissions at lower load conditions. While this staging method and combustor configuration is an enhancement, it is still limited in turndown capability, such that in order to achieve turndown to low part-load settings, the combustor often reverts to the higher emissions diffusion mode. 
   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 throughout the full operating conditions of the gas turbine engine, including a low part-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 seeks to overcome the shortfalls of the prior art by providing a combustion system that provides stable combustion having low NOx and CO emissions throughout all load conditions. This is accomplished through three dimensional fuel staging, including axial, radial, and circumferential staging such that fuel flow, mixing characteristics, and injection location are precisely controlled depending on the combustor requirements. 
   In the preferred embodiment of the present invention, the combustion system includes a plurality of injectors staged radially, axially, and circumferentially. The combustor 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. A plurality of third injectors are located in a manifold of an aft injector assembly, which is located axially downstream of the end cover and radially outward of the liner. The manifold of the aft injector assembly comprises at least one injection sector providing circumferential fuel staging. 
   The present invention creates the means by which low emissions stable combustion can occur at various load conditions while remaining in a premix mode at all load conditions. This is accomplished at combustor ignition by supplying fuel to first injectors and a first sector of the aft injector assembly with fuel flow gradually increasing to, the first injectors until crossfire is achieved between adjacent combustors. After crossfire has occurred, fuel flow gradually decreases to the first injectors and first sector of the aft injector assembly until a full engine speed no load condition is reached. At this point, fuel flow to the first sector of the aft injector assembly terminates while fuel flow remains to the first injectors and fuel flow is initiated to the second injectors, gradually increasing to a first part-load condition. Then, fuel flow gradually decreases to both first and second injectors while gradually increasing fuel flow to the first sector of the aft injector assembly. Fuel flow increases to each of these regions to a second part-load condition, at which point fuel flow gradually decreases to first and second injectors and the first sector of aft injector assembly, while initiating and gradually increasing fuel flow to a second sector of the aft injector assembly. From this point, as load increases, fuel flow gradually increases to both first and second injectors and first and second sectors of the aft injector assembly. 
   It is through precise axial, radial, and circumferential fuel staging described herein that low emissions and stable combustion is maintained throughout all points of the engine operating cycle. By decreasing fuel flow to active injectors when additional injectors are started, emissions levels are controlled. At the first part-load condition, when only first and second injectors are operating, flame temperature will tend to be higher due to the locally high operating fuel/air ratio. As a result, the higher flame temperature precludes the formation of CO while assuring a stable flame at the first part-load condition. Stability is ensured throughout engine load operation by the first injectors supplying fuel preferentially to a shear layer of the combustor. 
   It is an object of the present invention to provide a method of operating a combustion system having low NOx and CO at multiple operating conditions. 
   It is a further object of the present invention to provide a method of operating 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 the combustor in accordance with the present invention. 
       FIG. 2  is a cross section view taken through the aft injector assembly in accordance with the present invention. 
       FIG. 3  is a diagram depicting fuel flow rates for each of the fuel injectors as a function of engine condition. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a gas turbine combustion system  40  in accordance with the present invention is shown. A combustion system  40  is provided, including 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. 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. 
   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 . 
   The combustion system of the present invention further contains an aft injector assembly  63 , which is shown in FIG.  2 . Aft injector assembly  63  contains a manifold  64  having at least one region. In the preferred embodiment of the present invention, manifold  64  contains a plurality of regions  65 ,  66 ,  67 , and  68 , with each of the regions in fluid communication with a third fuel source  69 . Each of the regions  65 ,  66 ,  67 , and  68  is isolated from adjacent regions by a manifold wall  65 ′,  66 ′,  67 ′, and  68 ′ so that fuel supplied to one of the regions does not flow into another region of the aft injector assembly  63 . Valve means (not shown) permit the fuel flow to each region to be controlled independent of the other regions. 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 regions  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 region  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.  FIG. 3  discloses the method of operating the above described combustion system in order to obtain the reduced emissions. Referring now to  FIGS. 1 and 3 , compressed air from the engine compressor is flowed in a first direction adjacent liner  53 . The compressed air is then split into a first portion and a second portion. First passage  51  between casing  41  and dome  50  receives the first portion of compressed air and directs the air 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 to form a first mixture. At combustor ignition, fuel is supplied by first injectors  47  and a first sector of aft injector assembly  63 . The first mixture from first injectors  47  and first swirler  61  then passes through a fourth passage  71  that directs the first mixture through first opening  56  in dome  50  and into liner  53 . A second mixture is formed in second passage  55  from fuel injected by aft injector assembly  63  mixing with a second portion of compressed air. The second portion of compressed air is imparted with a swirl from second swirler  72 , located adjacent aft injector assembly  63 . In the preferred embodiment, the first sector of aft injector assembly  63  comprises two regions,  66  and  68 , which are not adjacent to one another. Regions  66  and  68  that comprise the first sector are positioned adjacent crossfire tubes (not-shown) for supporting the ignition of adjacent combustors. The second mixture passes through second passage  55  and then, due to the geometry of dome  50 , turns to flow in a second direction opposite of the first direction and into combustion zone  73 . Therefore, fluids in first passage  51  and second passage  55  travel in a direction generally opposite to that of combustion products flowing through liner  53 . Fuel flow gradually increases to first injectors  47  until crossfire is achieved between adjacent combustors. 
   After crossfire has occurred, fuel flow gradually decreases to first injectors  47  and decreases and terminates to the first sector of aft injector assembly  63  while the engine increases in speed. Then, while maintaining fuel flow to first injectors  47 , fuel flow is initiated to second injectors  49  until a full speed no load condition is achieved. Next, fuel flow gradually increases to first and second injectors,  47  and  49  respectively, until a first part-load condition is achieved. Then, fuel flow gradually decreases to first and second injectors,  47  and  49 , respectively, while fuel flow to the first sector of aft injector assembly  63  is re-established and gradually increases. 
   As load increases, fuel flow to both first and second injectors,  47  and  49  respectively, and first sector of aft injector assembly  63  increases to a second part-load condition. At this point, fuel flow gradually decreases to first and second injectors,  47  and  49  respectively, and the first sector of aft injector assembly  63 , while initiating and gradually increasing fuel flow to a second sector of aft injector assembly  63 . In the preferred embodiment, the second sector comprises two non-adjacent regions,  65  and  67 , which occupy a majority of aft injector assembly  63 , and therefore has a higher fuel flow rate than the first sector. As load increases beyond the second part-load condition to full load, fuel flow gradually increases to first and second injectors,  47  and  49  respectively, and first and second sectors of aft injector assembly  63 . 
   During normal operation the combustor will exhibit a certain level of noise due to the pressure fluctuations, harmonics of the combustor, and effects felt from being in fluid communication with the compressor discharge and turbine. Occasionally these noise levels can become high and if not avoided, they can cause serious damage to the combustion hardware. Engine monitoring apparatus measures, records, and determines if the combustor noise levels become potentially damaging. The present invention provides flexibility in avoiding these high noise levels by allowing for independent adjustment of the fuel stages as necessary to adjust the local heat release and corresponding flame temperature. Typically at least one fuel stage will be adjusted to compensate for the high noise levels. Measuring noise levels and adjusting fuel flow rates between fuel stages continues until the combustor noise levels are returned to an acceptable level. 
   As previously mentioned, higher flame temperatures minimize the formation of carbon monoxide. The preferred embodiment of the present invention controls NOx and CO emissions levels by precisely staging fuel flow between the axial, radial, and circumferential stages. Specifically, CO emissions at reduced power settings are minimized by adjusting fuel flow to at least first injectors  47 , and if necessary, second injectors  49  to minimize the interaction and surface area between fueled and unfueled zones in the combustor. As a result flame temperature increases high enough to assist in CO burn out. The surface area described is in fact the surface area of a shear layer in combustion zone  73  formed between the fuel/air mixture from fourth passage  71  and the fuel/air mixture from second passage  55 . Emissions levels are measured and fuel flow is readjusted as necessary to maintain CO emissions at their desired level. 
   The method of the present invention describes a combustion system operation that can provide flame stability and low emissions benefits throughout the full operating conditions of the gas turbine engine, including a low part-load condition. Therefore, the gas turbine can be operated efficiently at lower load conditions, thereby eliminating wasted fuel when high load operation is not demanded or incurring the additional cycles on the engine hardware when shutting down. 
   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.