Abstract:
The present invention discloses an apparatus and method for reducing the carbon monoxide emissions emitted by a pilot injector of a gas turbine combustor. Multiple embodiments of a means for establishing a recirculation zone are disclosed whereby a portion of the fuel and air mixture from the pilot injector recirculates and a flame is held, thereby increasing the local reaction temperature at the desired location and lowering carbon monoxide emissions.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to gas turbine combustors and more specifically to an apparatus and method for reducing carbon monoxide emissions from gas turbine combustors.  
         [0002]     In recent years government officials have passed more restrictive regulations regarding powerplant emissions, especially those for oxides of nitrogen (NOx) and carbon monoxide (CO). Each of these emissions are well known to contribute to air pollution and regulators continue to set lower levels of acceptable emissions. There are various means to comply with these lower emissions requirements, which vary depending on the powerplant location. Such means include passing the exhaust gases through a catalyst, which serves to transform the carbon monoxide and remaining hydrocarbons into water and carbon dioxide, utilizing lower flame temperature combustors, or limiting the amount of operating time of the powerplant. The latter is the most unfavorable option as it limits the amount of revenue that can be generated. However, the other technologies such as a catalyst and lower flame temperature combustors can be expensive as well.  
         [0003]     Complying with environmental requirements is especially a concern when the powerplant is operating at a load point other than its preferred condition. Powerplants are designed to operate most efficiently at the “full-load” condition, that is when they are generating the most power possible, and it is at this condition that they are designed to produce the lowest emissions. However, there are many times when power demand is lower and it is more desirable to operate at a lower power setting, such that only the power demanded is actually supplied, thereby saving on fuel costs. It has been determined that when powerplants operate at conditions other than their most efficient, or design point, emission levels can go out of compliance with local regulations. This is especially true for NOx and CO and the present invention described herein addresses CO emissions reductions. Carbon monoxide from gas turbine combustion systems can typically be caused by a number of factors including inadequate burning rates, inadequate mixing of fuel and air prior to combustion, or quenching of the combustion products in surrounding cooling air. When combustion gases migrate towards a region containing cooler air, the temperature of this air, which is cooler than that of the hot combustion gases, prevents any further chemical reactions from occurring and CO will remain in the exhaust gases.  
         [0004]     In order for powerplants to run at lower load conditions, where emission levels can be higher, it is necessary to be able to control the amount of emissions that will result when the combustion system is not operating at its preferred design point. A condition at which higher CO emissions are especially prevalent is at lower power settings. At these lower power settings, the combustion systems are operating at a lower fuel flow and often times burning in a different region than that of the full power condition that may not be as efficient. Therefore, in order to operate a powerplant with reduced CO emissions throughout its operating envelope, it is necessary for the combustion system to be able to provide adequate mixing such that the CO is not quenched and the combustion reactions are completed.  
         [0005]     The present invention seeks to overcome the shortcomings of the prior art by providing an apparatus and method of reducing carbon monoxide emissions for a gas turbine combustion system.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention discloses an apparatus and method for reducing the carbon monoxide emissions emitted by a pilot injector of a gas turbine combustor. The pilot injector provides the main flame source for igniting a fuel/air mixture in the combustor and at lower power settings is the only source of hot combustion gases necessary to drive the turbine. The preferred embodiment of the pilot injector comprises a radial swirler, at least one fuel injector, a passageway formed between first and second spaced walls, a means for establishing a recirculation area adjacent to the pilot injector, and a generally annular extension protruding into the combustor thereby providing a region for the CO to burnout prior to interacting with surrounding air flows and becoming quenched. It is in this recirculation area, of lower pressure, that the pilot flame will anchor and burn. As a result, the pilot flame is anchored separate from the main fuel air mixture, which would quench the reaction processing CO emissions from the pilot flame. Furthermore, the pilot flame is anchored further upstream so as to establish a greater residence time in which the pilot flame is to burn and complete the reactions to minimize CO formation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a cross section view of a combustor utilizing the present invention.  
         [0008]      FIG. 2  is a detailed cross section of a portion of the combustor shown in  FIG. 1  in accordance with the preferred embodiment of the present invention.  
         [0009]      FIG. 3  is a further detailed cross section of a portion of the combustor shown in  FIG. 2  in accordance with the preferred embodiment of the present invention.  
         [0010]      FIG. 4  is a section view taken from  FIG. 1  looking axially upstream in accordance with the preferred embodiment of the present invention.  
         [0011]      FIG. 5  is a detailed cross section of a portion of the combustor shown in  FIG. 1  in accordance with a first alternate embodiment of the present invention.  
         [0012]      FIG. 6  is a section view taken from  FIG. 1  looking axially upstream in accordance with a first alternate embodiment of the present invention.  
         [0013]      FIG. 7  is a detailed cross section of a portion of the combustor shown in  FIG. 1  in accordance with a second alternate embodiment of the present invention.  
         [0014]      FIG. 8  is a section view taken from  FIG. 1  looking axially upstream in accordance with a second alternate embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     The present invention will now be described in detail with reference to  FIGS. 1-8 . Referring now to  FIG. 1 , a gas turbine combustor having a pilot injector in accordance with the present invention is shown in cross section. Combustor  10  comprises a casing  11 , an end cover  12 , a liner  13 , and a pilot injector  14 . The pilot injector is placed proximate the forward end of combustor  10  in order to provide the fuel source to establish a pilot flame in liner  13 . Pilot injector  14 , which is shown in greater detail in  FIGS. 2 and 3 , comprises a radial swirler  15 , a first wall  16 , and a second wall  17  in spaced relation such that a passageway  18  is formed therebetween. Passageway  18  has an inlet  19  and an outlet  20  and is oriented generally radially proximate inlet  19  and generally axially proximate outlet  20 . Adjacent pilot injector  14 , proximate outlet  20 , but within passageway  18 , is a means for establishing a recirculation area  21 . Pilot injector  14  also comprises at least one fuel injector, but preferably a first injector  22  and a second injector  23 , wherein first injector  22  is located proximate radial swirler  15 . An additional feature of the present invention is generally annular extension  26 , located proximate outlet  20  and extending into liner  13  a predetermined distance.  
         [0016]     Referring to  FIGS. 3 and 4 , means for establishing a recirculation area  21  is shown in greater detail. In the preferred embodiment of the present invention, means for establishing a recirculation area  21  comprises an annular ring  24  that is positioned along second wall  17  proximate outlet  20  of passageway  18 .  
         [0017]     During typical gas turbine combustor operation, fuel and compressed air are mixed together and the premixture is then ignited to form hot combustion gases to drive a turbine. One measure of combustor, and engine, performance is emissions levels, and more specifically, carbon monoxide (CO) levels. One skilled in the art of gas turbine combustion will understand that CO formation is a multi-step process of breaking down the carbon molecules in the fuel. More specifically, high temperatures, concentrations of O 2 , and large residence times are required for CO oxidation. However, this multi-step process can be interrupted by a quenching effect due to the combustor design. That is, the remaining oxygen atoms designed to react with the CO molecules to complete the reaction and form CO 2  are quenched or cooled prematurely. This typically occurs in regions where additional cooling air is mixed into the process. A means to ensure that this combustion process is completed, despite the addition of potential quenching effects, is to provide a mechanism for increasing the time in which CO is consumed. The present invention provides this mechanism.  
         [0018]     In operation, air under pressure passes around the outside of liner  13  and is directed towards inlet  19 . The air then passes through radial swirler  15  and mixes with a fuel from first injector  22 . The fuel and air mixture is then directed through passageway  18  and towards outlet  20 . Additional fuel may be provided from a second injector  23 , as shown in the preferred embodiment in  FIG. 2 , where second injector  23  injects the fuel into passageway  18  in a direction that is generally perpendicular to first injector  22 . Once the premixture exits passageway  18  it is ignited by an ignition system  25 . For the preferred embodiment of the present invention, ignition system  25  is located generally along the centerline of combustor  10 , but it can be placed wherever is most optimal for ignition purposes. However, proximate outlet  20 , the fuel and air mixture encounters annular ring  24 , which creates a recirculation zone at the outer diameter of the region directly downstream of passageway  18 . This recirculation zone, which contains a low pressure region, holds the flame and raises the local reaction temperature. Without this recirculation zone, the flame at this region, and hence the local reaction temperature, was quenched. In addition to the recirculation zone established by annular ring  24 , quenching is significantly reduced by the placement of generally annular extension  26  such that compressed air entering the combustor radially outward of extension  26  does not immediately interact with the flame from pilot injector  14 . This separation provided by extension  26  allows sufficient time and distance for the CO to burnout of the reaction. Extensive rig testing and computational analysis has determined that the optimal axial length of generally annular extension  26  for the combustor of the present invention is approximately three inches. This axial distance provides the time necessary for the CO to burnout prior to interacting with the surrounding airflow radially outward of extension  26 .  
         [0019]     A first alternate embodiment of the present invention is shown in  FIGS. 5 and 6 . A majority of the features of the first alternate embodiment are identical to those of the preferred embodiment. Therefore, only features unique to the first alternate embodiment will be discussed in further detail. Depending on the desired recirculation zone configuration, and resulting flame region, means for establishing a recirculation area can comprise a plurality of spokes  34  instead of an annular ring. In this first alternate embodiment, it is preferred that spokes  34  are positioned together in an axial plane along second wall  17  proximate outlet  20  of passageway  18  and extend from second wall  17  towards first wall  16 . This can be seen in partial cross section in  FIG. 5  and in full view looking axially upstream in  FIG. 6 . As a result of this configuration, a similar benefit regarding recirculation zone, local reaction temperature, and quenching is achieved, but the flame will develop radially along the whole length of the spoke as opposed to annularly behind the ring of the preferred embodiment.  
         [0020]     A second alternate embodiment of the present invention is shown in  FIGS. 7 and 8 . A majority of the features of the second alternate embodiment are identical to those of the preferred embodiment as well. Therefore, only features unique to the second alternate embodiment will be discussed in further detail. As with the first alternate embodiment, the means for establishing a recirculation area can be positioned in yet another configuration. In the second alternate embodiment, the means for establishing a recirculation area is a combination of annular ring  24  of the preferred embodiment as well as plurality of spokes  34  from the first alternate embodiment. This combination is shown in partial cross section in  FIG. 7  and looking axially upstream in  FIG. 8 . In this configuration, a plurality of spokes  34  are positioned together in an axial plane along second wall  17  proximate outlet  20  of passageway  18  and extend from second wall  17  towards first wall  16 . In between spokes  34  are sections of annular ring  24 . This configuration will allow the flame to anchor on the outer diameter of passageway  18  proximate annular ring  24  as well as along spokes  34 , due to the multiple recirculation zones formed by ring  24  and spokes  34 , thus increasing the local reaction temperature and lowering CO emissions.  
         [0021]     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.