Patent Publication Number: US-7594400-B2

Title: Catalytic oxidation module for a gas turbine engine

Description:
FIELD OF THE INVENTION 
     This invention relates to a catalytic oxidation module for a gas turbine engine, and, in particular, to a catalytic oxidation module comprising a plurality of tubular elements. 
     BACKGROUND OF THE INVENTION 
     Catalytic combustion systems are well known in gas turbine applications to reduce the creation of pollutants in the combustion process. As known, gas turbines include a compressor for compressing air, a combustion stage for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. For example, U.S. Pat. No. 6,174,159 describes a catalytic oxidation method and apparatus for a gas turbine utilizing a backside cooled design. Multiple cooling conduits, such as tubes, are coated on the outside diameter with a catalytic material and are supported in a catalytic reactor portion of the combustor. A portion of a fuel/oxidant mixture is passed over the catalyst coated cooling conduits and is oxidized, while simultaneously, a portion of the oxidant enters the multiple cooling conduits and cools the catalyst. The exothermally catalyzed fluid then exits the catalytic oxidation system and is mixed with the cooling fluid outside the system, creating a heated, combustible mixture. The tubes used in such catalytic reactors are typically exposed to extreme temperature and vibration conditions which may adversely affect the integrity and service life of the tubes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other advantages of the invention will be more apparent from the following description in view of the drawings that show: 
         FIG. 1  is a functional diagram of a gas turbine engine having an improved catalytic oxidation module. 
         FIG. 2  is a perspective view of an exemplary bundle of tubular elements that may be used in the catalytic oxidation module of the gas turbine engine of  FIG. 1 . 
         FIG. 3  is a partial cross sectional view of the tubular elements of  FIG. 2 . 
         FIG. 4  is a partial end view of the tubular elements of  FIG. 2 . 
         FIGS. 5-7  show exemplary slot configurations of the tubular elements of  FIG. 2 . 
         FIG. 8  depicts differential thermal expansion between the elements of  FIG. 2   
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a gas turbine engine  10  having a compressor  12  for receiving a flow of filtered ambient air  14  and for producing a flow of compressed air  16 . The compressed air  16  is separated into a combustion mixture fluid flow  24  and a cooling fluid flow  26 , respectively, for introduction into a catalytic combustion module  28 . The combustion mixture fluid flow  24  is mixed with a flow of a combustible fuel  20 , such as natural gas or fuel oil for example, provided by a fuel source  18 , prior to introduction into the catalytic combustion module  28 . The cooling fluid flow  26  may be introduced directly into the catalytic combustion module  28  without mixing with a combustible fuel. Optionally, the cooling fluid flow  26  may be mixed with a flow of combustible fuel  20  before being directed into the catalytic combustion module  28 . 
     Inside the catalytic combustion module  28 , the combustion mixture fluid flow  24  and the cooling fluid flow  26  are separated, for at least a portion of the travel length, L, by one or more conduits, such as tubular elements  30 , having respective inlet ends  42  and an outlet ends  44 . The tubular elements  30  may be retained in a spaced apart relationship by a tubesheet  33 . The tubular elements  30  are coated with a catalyst  32  on the side exposed to the combustion mixture fluid flow  24 . The catalyst  32  may include, as an active ingredient, a precious metal, Group VIII noble metals, base metals, metal oxides, or any combination thereof. Elements such as zirconium, vanadium, chromium, manganese, copper, platinum, palladium, osmium, iridium, rhodium, cerium, lanthanum, other elements of the lanthanide series, cobalt, nickel, iron, and the like may be used. 
     The tubular elements  30  may be coated on respective outside diameter surfaces with a catalyst  32  to be exposed to a combustion mixture fluid flow  24  traveling around the exterior of the elements  30 . In a backside cooling arrangement, the cooling fluid flow  26  is directed to travel through the interior of the tubular elements  30 . While exposed to the catalyst  32 , the combustion mixture fluid flow  24  is oxidized in an exothermic reaction, and the catalyst  32  and the pressure boundary element  30  are cooled by the unreacted cooling fluid flow  26 , thereby absorbing a portion of the heat produced by the exothermic reaction. 
     Alternatively, the tubular elements  30  may be coated on the respective interiors with a catalyst  32  to expose a combustion mixture fluid flow  24  traveling through the interior of the tubular elements  30 , while the cooling fluid flow  26  travels around the exterior of the tubular elements  30 . Other methods may be used to expose the combustion mixture fluid flow  24  to a catalyst  32 , such as constructing a structure to suspend the catalyst in the combustion mixture fluid flow  24 , constructing a structure from a catalytic material to suspend in the combustion mixture fluid flow  24 , or providing pellets coated with a catalyst material exposed to the combustion mixture fluid flow  24 . 
     After the flows  24 , 26  exit the catalytic combustion module  28 , the flows  24 , 26  are mixed and combusted in a plenum, or combustion completion stage  36 , to produce a hot combustion gas  38 . In an embodiment of the invention, the flow of a combustible fuel  20  is provided to the combustion completion stage  36  by the fuel source  18 . The hot combustion gas  38  is received by a turbine  40 , where it is expanded to extract mechanical shaft power. A common shaft  41  may interconnect the turbine  40  with the compressor  12  as well as an electrical generator (not shown) to provide mechanical power for compressing the ambient air  14  and for producing electrical power, respectively. The expanded combustion gas  43  may be exhausted directly to the atmosphere or it may be routed through additional heat recovery systems (not shown). 
     The catalytic oxidation module  28  of  FIG. 1  provides improved performance as a result of the retaining features of the tubular elements  30  that are shown more clearly in  FIGS. 2-4 .  FIG. 2  shows a perspective view of an exemplary bundle  50  of tubular elements  30  that may be used in the catalytic oxidation module  28  of the gas turbine engine  10  of  FIG. 1 . In the past, bundled tubular elements  30  have been used in catalytic combustors  28 , wherein respective inlet ends  42  of the tubular elements  30  have been retained spaced apart from one another by attaching, such as by welding or brazing, an upstream end of each of the elements  30  to a tubesheet  33 . At the outlet ends  44 , the tubular elements  30  have included an expanded cross section regions  46  having an outer surface  48  in contact with an outer surface  48  of expanded cross regions  46  of adjacent tubular elements  30  to maintain a spaced relationship among the tubular elements  30  and provide support for the elements  30  within the bundle  50  to provide a defined space in the combustion mixture catalytic reaction channels as well as vibration control. 
     However, such configurations have proven unreliable in the past due to conditions such as engine or flow induced dynamics, heat extremes, and differential heat induced expansion among the respective elements  30 . For example, the expanded cross section regions  46  of the elements  30  are subject to wear (e.g. fretting or fret corrosion) where the surfaces  48  of the regions  46  contact one another. Although the expanded cross section regions  46  maintain the tubular elements  30  in a spaced relationship at respective outlet ends  44 , such a configuration provides little self-containment of the tube elements  30  within in the module  50 . For example, if an element  30  becomes dislodged from an upstream tubesheet  33 , the expanded cross section region configuration cannot prevent the element  30  from traveling downstream and potentially causing catastrophic damage to the turbine  40  or other parts of the engine  10 . A downstream tubesheet may be used to retain the elements at a downstream end of the bundle, but such a tubesheet may be subject to heat extremes and may introduce flashback and flame holding problems at the outlet ends  44 . 
     The elements  30  may be joined, such as by welding or riveting, areas of contact, such as expanded cross section contact points  52 , at the outlet ends  44  of the tubular elements  30 . However, it has been discovered that elements  30  in the bundle  50  may expand and contract in a longitudinal direction at different rates due to differential heating. Such heat induced relative movement may cause stresses in joined contact points  54  sufficiently high enough to cause the joints, such as welds  56 , to fail. If the elements  30  are retained at respective inlet ends  42  by the tubesheet  33  and at respective downstream ends by attachment to a downstream tubesheet, heat induced longitudinal expansion may cause bowing of the tubular elements  30  being restrained at both ends  42 ,  44  from moving in a longitudinal direction. The inventors have innovatively realized that by forming flexible fingers  58  in the ends  42 ,  44  of the elements  30 , containment of the elements  30  at the ends  42 ,  44  may be achieved while still being capable of accommodating differential expansion and vibration. 
     As shown in the perspective view of  FIG. 2 , the partial cross sectional view of the tubular elements of  FIG. 3 , and the partial end view of the tubular elements of  FIG. 4 , each of the tubular elements  30  includes a respective end portion  60  comprising a plurality of spaced apart longitudinal fingers  58 . The fingers  58  of each tubular element  30  may be joined to abutting fingers  58  of respective adjacent elements to retain the tubular elements  30  at the end portions  60  with sufficient flexibility to allow relative movement between the adjacent tubular elements  30 . For example, as shown in  FIG. 8 , differential thermal expansion  100  of adjacent elements  30  joined at contacting fingers  58  may be accommodated as indicated by dotted lines  98  showing positions of the joined fingers  58  when one of the elements  30  has expanded longitudinally with respect to the adjacent attached element  30 . 
     The fingers  58  may be joined by forming a weld  56  (for example, using capacitance discharge welding, gas tungsten welding, or brazing techniques) between contact points  52  or contact areas of the abutting fingers  58  near the respective outlet ends  44  of the tubular elements  30 . In an embodiment of the invention, the weld  56  may be formed as wide as an arc width  94  of the finger  58 , and may extend upstream from the outlet end about 20 to 30 mils. In another embodiment, the fingers  58  may be joined by riveting. The fingers  58  may be formed integrally with a remainder of the tubular element  30  or may be joined, such as be welding, to an end of the tubular element  30 , so that the fingers  58  are spaced apart around a perimeter of the end of the element  30  and extend longitudinally away from the end of element  30 . 
     As shown in  FIG. 3 , the end portions  60  of each of the tubular elements  30  may comprise an expanded cross section region  46  having an expanded cross section  62  larger than a nominal cross section  64  of the tubular element  30 . The expanded cross section region  46  may include a flared portion  70  transitioning from a nominal cross section  64  of the tubular element  30  to an expanded portion  72  having a larger cross section  62  than the nominal cross section  64 . A wall thickness  66  of the expanded region  46  (and a corresponding thickness the fingers  58  formed in the expanded region  46 ) may be configured to be thinner than a wall thickness  68  of a nominal cross section  64  of the tubular element  30  so that the fingers  58  formed in the expanded cross section region  46  have a flexibility greater than a flexibility of fingers that may be formed in a thicker, nominal cross section portion of the element  64 . The wall thickness  66  may be made thinner as a result of enlarging the nominal cross section  64  at an end of the element into an expanded cross section  62  in the expanded region  46 . For example, it has been experimentally determined that when a 0.01 inch thick, 0.188 diameter cylindrical tube is expanded to have a diameter of 0.244 inches, the wall thickness of the expanded portion is thinned to 0.0075 inches. The fingers  58  may extend longitudinally through the expanded region  72  into the flared region  70  of the expanded portion  46 . 
     In an aspect of the invention, the fingers  58  are defined by slots  74  comprising a rounded bottom portion  76 . The rounded bottom portion  76  may be configured as a semicircular shape having a radius  78  corresponding to half a width  80  of the slot  74 . Other configurations of slots  74  that may be used are shown in  FIGS. 5-7 .  FIGS. 5 and 6  show slots  74  having a variable slot width along a length of the slot  74 . For example,  FIG. 5  shows slots  74  comprising a slot width  86  at the outlet end  44  wider than a slot width  88  remote from the outlet end  44 .  FIG. 6  shows slots  74  comprising a slot width  90  at the outlet end  44  narrower than a slot width  92  remote from the outlet end  44 . The slots  74  may have relatively straight sides  87  or may be contoured, for example, as shown in the exemplary slots  74  of  FIG. 6 , so that the slots have a tear-drop shape. In another aspect of the invention shown in  FIG. 7 , the slots  74  may include an enlarged circular bottom portion  82 , for example, having a diameter  84  larger than the width  80  of the slot  74 . 
       FIG. 4  is a partial end view of the tubular elements of  FIG. 2 . In the exemplary embodiment shown in  FIG. 4 , the tubular elements  30  have round cross sections. Other cross section profiles may include square, rectangular, oval, hexagonal or other shapes known in the art. As shown in  FIG. 4 , the arc width  94  of each of the fingers  58  at the outlet end  44  is sized sufficiently large to allow welding fingers  58  of adjacent elements  30  together. The arc width  94  of each of the fingers  58  may be modified to achieve a desired flexibility or stiffness of the finger  58  so that a larger arc width  94  provides increased stiffness, and a relatively smaller arc width  94  provides increased flexibility. In an aspect of the invention, a total combined arc width of the respective arc widths  94  of each of the fingers  58  of the tubular element at the outlet end  44  comprises from about 85 percent to 15 percent of the perimeter  96  of the tubular element  30  at the outlet end  44 . Preferably, the total combined arc width of the fingers  58  comprises about 60 percent to 20 percent of the perimeter  96  of the tubular element at the outlet end  44 . Even more preferably, the total combined arc width of the fingers  58  of each tubular element  30  comprises about 50 percent to 40 percent of the perimeter  96  of the tubular element  58  at the outlet end  44 . 
     With reference to  FIG. 2 , a method of assembling a catalytic module  50  including tubular elements  30  having a plurality of spaced apart longitudinal fingers  58  formed in respective end regions  60  includes assembling the elements  30  into a bundle and joining end regions  60 , such as the expanded cross section regions  46 , of each of the tubular elements  30  in the bundle  50  at points of contact  52  among the tubular elements  30 . For example, the end regions  60  may be welded or riveted at the contact points  52 . After being joined, longitudinal slots  74  may be formed the end regions away from the joined contact points  52  to define joined fingers  58  between the slots  74  so that the joined fingers  58  remaining after forming the slots  74  are capable of retaining the tubular elements  30  at the respective end regions  60  with sufficient flexibility to allow relative movement between adjacent tubular elements  30 . The slots  74  may be formed by sawing, laser cutting, or abrading away portions of the element  30  in the end portion  60 . For example, an abrasive wheel may be configured to have a cross section corresponding to a desired slot contour, such as slots  74  having the configurations as shown in  FIGS. 5-7 . To provide increased resistance to cracking, the slots  74  may be formed to have a rounded bottom portion  76  as shown in  FIG. 3 . In another aspect shown in  FIG. 7 , the slots  74  may be formed to have an enlarged circular bottom portion  82  in each slot, such as by drilling a hole before or after forming the slot  74 , so that the hole intersects a bottom portion of the slot  74 . 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. For example, the fingers may be formed in respective inlet ends of the tubular elements and welded to fingers of adjacent tubular elements. In another aspect, straight tubes, not having an enlarged cross section region may be used. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.