Abstract:
A fuel nozzle and gas turbine combustor capable of operating on multiple fuels with reduced carbon build-up to the fuel nozzle and adjacent combustor components is disclosed. The fuel nozzle incorporates a reconfigured gas fuel assembly and mixing tube to eliminate known areas of recirculation. Furthermore, the liquid fuel assembly includes reconfigured spray characteristics to further reduce droplet interaction with the mixing tube.

Description:
TECHNICAL FIELD  
       [0001]     This invention generally relates to gas turbine combustion systems and more specifically to a fuel nozzle having dual fuel capability.  
       BACKGROUND OF THE INVENTION  
       [0002]     Land-based gas turbine engines, which are primarily used for generating electricity, include a combustion system that mixes fuel with compressed air from the engine compressor and contains the reaction that generates hot combustion gases to drive a turbine. The combustion system injects a fuel, typically natural gas or a liquid fuel, to mix with the compressed air. Combustion systems which inject either fuel type are typically referred to as dual fuel combustors. This type of combustion system offers flexibility to the engine operator with regard to which fuel to use, depending on fuel availability, fuel costs, and level of emissions allowed. While natural gas fired gas turbine engines have become increasingly popular due to lower levels of NOx emissions produced, not all regions of the world in which gas turbine engines operate are regulated by emissions nor is natural gas a desired fuel choice for economic reasons.  
         [0003]     While dual fuel combustion systems provide the flexibility to operate on different fuel types, they have exhibited some shortcomings, especially during the liquid fuel operation. More specifically, the combustor hardware surrounding the liquid fuel nozzle has been known to exhibit carbon buildup over a period of time. Build up of carbon has resulted in poor performance and damage to the fuel nozzles and combustion liner components requiring premature repair and replacement. Often times, engine operators have been required to limit the amount of time operating on liquid fuel in order to limit the amount of carbon buildup.  
         [0004]     A specific example of a fuel nozzle known to exhibit carbon buildup is shown in  FIG. 1 . Fuel nozzle  10  includes gas tip  11  and liquid nozzle  12 , which includes a plurality of concentric tubes  13 ,  14 , and  15 . Inner tube  13  contains a liquid fuel such as oil, while middle tube  14  contains water, and outer tube  15  contains air. Surrounding liquid nozzle  12  is gas tip  11  that injects a gaseous fuel through injection holes  17  to mix with the surrounding air in mixing tube  16 . Whether fuel nozzle  10  is operating on liquid fuel or gaseous fuel, the fluids mix in mixing tube  16 . It is during the liquid fuel operation that this prior art design has exhibited carbon buildup along the tip region of fuel nozzle  10  and along mixing tube  16 . The carbon buildup is a result of recirculation zones within mixing tube  16 , particularly along the interface between fuel nozzle  10  and mixing tube  16 , such that liquid fuel droplets are redirected to impinge on the tip of fuel nozzle  10  and along mixing tube  16 , adhering to the surface and forming carbon deposits. Over time, the carbon deposits build-up to a level that impairs fuel nozzle and combustor performance, requiring repair and replacement.  
         [0005]     Referring to  FIGS. 2 and 3 , a second prior art fuel nozzle  30  is shown in detail and is the subject of U.S. Pat. No. 5,833,141. In order to prevent the carbon build up exhibited in fuel nozzle  10 , fuel nozzle  30  was positioned such that the liquid nozzle portion extended the full length of the mixing tube and is combined with an additional outer swirler  31  and therefore reduced the possibility of recirculation of liquid fuel droplets onto the fuel nozzle or mixing tube  36 . While this design has proven to reduce the amount of carbon buildup, it requires modifications to the gas/air swirler of the prior art fuel nozzle  10 , including extending the swirler channel and incorporating an additional outer swirler.  
       SUMMARY AND OBJECTS OF THE INVENTION  
       [0006]     The present invention improves upon each of the prior art dual fuel nozzles by providing a fuel nozzle designed to reduce carbon buildup while having a relatively simple fuel nozzle configuration. The present invention positions the injection point of the liquid fuel portion approximately halfway in a mixing tube and utilizes a reconfigured spray angle and air swirler and alternate mixing tube to eliminate recirculation areas found in the prior art fuel nozzle.  
         [0007]     A fuel nozzle for use in a dual fuel gas turbine combustion system is disclosed having a fuel nozzle axis, nozzle tip, and comprising a liquid fuel assembly having coaxial tubes for flowing a liquid fuel, water, and compressed air and a gas fuel assembly comprising a nozzle body that injects a gaseous fuel to mix with surrounding compressed air. The first, second, and third tubes of the liquid fuel assembly and the nozzle body of the gas fuel assembly each extend to proximate the nozzle tip.  
         [0008]     The present invention dual fuel nozzle is designed to operate in a gas turbine combustor comprising a combustion liner with a cap assembly fixed to a first end of the combustion liner. The cap assembly has a plurality of openings located about the combustion liner center axis, with each of the openings having a convergent—divergent mixing tube with a forward tube end and aft tube end and a collar positioned adjacent the forward tube end of the mixing tube. The dual fuel nozzles of the present invention are arranged in an annular array about the center liner axis corresponding to the openings in the cap assembly and extend into the mixing tubes to a position approximately halfway between the forward tube end and the aft tube end. Positioning the dual fuel nozzle of the present invention in this location in combination with optimizing the spray orientation of the liquid fuel assembly and reconfigured mixing tube ensures that liquid fuel droplets will not contact the fuel nozzle surface or mixing tube wall, thereby minimizing carbon buildup along said surfaces.  
         [0009]     It is an object of the present invention to provide a gas turbine combustor that can operate on multiple fuel types and exhibit reduced carbon deposits.  
         [0010]     It is another object of the present invention to provide a dual fuel nozzle that injects a liquid fuel that does not recirculate and impinge on the fuel nozzle tip or cap assembly mixing tube wall.  
         [0011]     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  
       [0012]      FIG. 1  is a cross section of a dual fuel nozzle of the prior art.  
         [0013]      FIG. 2  is a cross section of an alternate dual fuel nozzle of the prior art.  
         [0014]      FIG. 3  is an elevation view of an alternate dual fuel nozzle of the prior art.  
         [0015]      FIG. 4  is a cross section view of a gas turbine combustor in which the present invention can operate.  
         [0016]      FIG. 5  is a perspective view of a dual fuel nozzle in accordance with the present invention.  
         [0017]      FIG. 6  is a perspective view taken in cross section of a dual fuel nozzle installed in a combustor in accordance with the present invention.  
         [0018]      FIG. 7  is a detailed cross section of a dual fuel nozzle in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     The present invention is shown in detail in  FIGS. 4-7  and is preferably operated in conjunction with a dual stage combustion system such as that shown in  FIG. 4 . A gas turbine combustor  50  capable of operating on multiple fuels comprises an outer case  51 , a sleeve  52 , an end cover  53  fixed to a forward end of case  51 , and a generally cylindrical combustion liner  54 . The combustion liner comprises a first end  55 , a second end  56  and a cap assembly  57  fixed to combustion liner  54  proximate first end  55  and located generally within combustion liner  54 . Furthermore, combustion liner  54  also comprises a first combustion chamber  58 , a second combustion chamber  59 , and a venturi  60  separating chambers  58  and  59 . Further details of cap assembly  57  can be seen in detail in accordance with  FIG. 7 . Cap assembly  57  has a plurality of openings  61  located about center line axis A-A, with each of openings  61  having a mixing tube  62  and collar  63 . Mixing tube  62  has a forward tube end  64  and an aft tube end  65 , with aft tube end  65  proximate opening  61  and collar  63  positioned adjacent forward tube end  64  of mixing tube  62 .  
         [0020]     Fixed to end cover  53  and arranged about center liner axis A-A, is a plurality of fuel nozzles  66 , with each nozzle corresponding to an opening  61  in cap assembly  57 . Fuel nozzles  66 , which are shown in greater detail in  FIGS. 5-7 , have a fuel nozzle axis B-B, a nozzle tip  67 , and comprise a liquid fuel assembly  68  and a gas fuel assembly  69 .  
         [0021]     Liquid fuel assembly  68  comprises a plurality of generally concentric tubes that extend to proximate nozzle tip  67  and are coaxial with fuel nozzle axis B-B. A first tube  70  extends substantially along fuel nozzle axis B-B and contains a liquid fuel such as No. 2 diesel fuel. Surrounding first tube  70  is a second tube  71  that preferably contains water and surrounding second tube  71  is a third tube  72  that contains compressed air.  
         [0022]     Gas fuel assembly  69  comprises a nozzle body  73  that is generally conical and tapers generally inward at nozzle tip  67  towards fuel nozzle axis B-B and surrounds third tube  72  of liquid fuel assembly  68 . Nozzle body  73  has a first wall  74 , a second wall  75 , and a plurality of swirler vanes  76  extending therebetween, and contains natural gas that passes between third tube  72  and nozzle body  73  and is injected into a passing flow of swirling compressed air by a plurality of injection holes  77 . Nozzle body  73  is positioned within collar  63  and mixing tube  62  such that a portion of second wall  75  is in contact with collar  63 . As with liquid fuel assembly  68 , nozzle body  73  of gas fuel assembly  69  also extends to proximate nozzle tip  67 .  
         [0023]     The present invention incorporates multiple improvements to the mixing tube region of cap assembly  57  and nozzle body  73  to discourage recirculation of liquid fuel droplets and thereby reduce the amount of carbon deposits on nozzle tip  67  and mixing tube  62 . The first improvement to mixing tube  62  is with respect to the tube shape. Mixing tube  62  has generally conical first and second portions with first portion  62 A converging towards a mixing tube throat  78  and second portion  62 B diverging from the mixing tube throat. The use of a converging—diverging mixing tube geometry directs the initial air flow away from from the mixing tube walls.  
         [0024]     The second improvement to mixing tube  62  constitutes a plurality of air injection holes for cooling and for providing a film of air to mixing tube  62  to prevent liquid fuel droplets from adhering to the tube. First portion  62 A has a plurality of first cooling holes  79  and second portion  62 B has a plurality of second cooling holes  80 . In the preferred embodiment, plurality of first cooling holes  79  are oriented generally perpendicular to first portion  62 A as shown in  FIG. 7 . Alternatively, plurality of second cooling holes  80  are oriented at an angle α relative to mixing tube  62  and towards aft tube end  65  of mixing tube  62 . Second cooling holes  80  are oriented at angle α in order to provide a film of cooling air along second portion  62 B. It is preferred that angle α is between 15 and 45 degrees. Cooling air enters through plurality of first holes  79  and impinges along the outer portion of nozzle body second wall  75 . The cooling air then flows through passage  81  that is created between nozzle body second wall  75  and mixing tube first portion  62 A. This cooling air provides a stream of fluid to prevent recirculation of fuel onto second portion  62 B of mixing tube  62 . Furthermore, should any fuel droplets penetrate this stream of air from passage  81 , a film of air is covering second portion  62 B, thereby preventing these fuel droplets from bonding to second portion  62 B and causing a carbon build-up.  
         [0025]     The final appreciable improvement of the present invention relates to the position of fuel nozzle tip  67  relative to the new and improved mixing tube design. In order to prevent interaction between fuel droplets and second portion  62 B of mixing tube  62 , fuel nozzle tip  67  is positioned approximately halfway between forward tube end  64  and aft tube end  65  of mixing tube  62  at mixing tube throat  78 . The fuel nozzle is positioned such that the spray angle from liquid fuel assembly  68  in combination with the surrounding streams of air significantly avoids mixing tube second portion  62 B.  
         [0026]     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.