Abstract:
A catalytic combustor ( 34 ) for a gas turbine engine ( 30 ). A fuel-air mixture ( 50 ) is reacted on a catalytic surface ( 54 ) of a catalytic heat exchanger module ( 36 ) to partially combust the fuel ( 48 ) to form heat energy. The fuel-air mixture is formed using compressed air ( 44 ) that has been pre-heated to above a reaction-initiation temperature in a non-catalytic cooling passage ( 46 ) of the catalytic heat exchanger module ( 36 ). Because the non-catalytic cooling passages ( 46 ) provide the necessary pre-heating of the combustion air, no separate pre-heat burner is required. Fuel ( 48 ) is added to the pre-heated air ( 44 ) downstream of the non-catalytic cooling passage ( 46 ) and upstream of the catalytic surface ( 54 ), thereby eliminating the possibility of flashback of flame into the cooling passages ( 46 ). Both can-type ( 60 ) and annular ( 80 ) combustors utilizing such a combustion system are described.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of combustion turbines, and more specifically to a gas turbine including a catalytic combustor, and in particular to a passively cooled catalytic combustor having improved protection against overheating and a wider operating range. 
     BACKGROUND OF THE INVENTION 
     In the operation of a conventional combustion turbine, intake air from the atmosphere is compressed and heated by a compressor and is caused to flow to a combustor, where fuel is mixed with the compressed air and the mixture is ignited and burned. This creates a high temperature, high pressure gas flow which is then expanded through a turbine to create mechanical energy for driving equipment, such as for generating electrical power or for running an industrial process. The combustion gasses are then exhausted from the turbine back into the atmosphere. Various schemes have been used to minimize the generation of pollutants during the combustion process. The use of catalytic combustion is known to reduce the generation of oxides of nitrogen since catalyst-aided combustion can occur at temperatures well below the temperatures necessary for the production of NOx species. 
     FIG. 1 illustrates a prior art gas turbine combustor  10  wherein at least a portion of the combustion takes place in a catalytic reactor  12 . Compressed air  14  from a compressor (not shown) is mixed with a combustible fuel  16  supplied through fuel injectors  18  upstream of the catalytic reactor  12 . Catalytic materials present on surfaces of the catalytic reactor  12  initiate the heterogeneous combustion reactions at temperatures lower than normal ignition temperatures. However, for certain fuels and engine designs such as natural gas lean combustion, known catalyst materials are not active at the compressor discharge supply temperature. A preheat burner  20  is provided to preheat the combustion air  14  by combusting a supply of preheat fuel  22  upstream of the main fuel injectors  18 . One such system is described in U.S. Pat. No. 5,826,429 issued on Oct. 27, 1998, incorporated by reference herein. Such pre-burn systems are costly and they add complexity to the design and operation of the combustor. 
     The surface reactions within the catalytic reactor release enough heat energy to cause auto-ignition and combustion of the remainder of the fuel in the gas stream beyond the catalytic reactor  12 , in a region of the combustion chamber called the burnout zone  24 . For modern high firing temperature combustion turbines, the amount of fuel reacted in the catalyst bed must be limited in order to prevent overheating of the materials within the reactor. In order to cool the catalytic reactor  12  and to limit the amount of conversion within the reactor, it is known to provide both catalyzed and non-catalyzed substrate passages through the catalytic reactor  12 . Such designs are described in U.S. Pat. No. 4,870,824 dated Oct. 3, 1989, and U.S. Pat. No. 5,512,250 dated Apr. 30, 1996, also incorporated by reference herein. The fuel-air mixture passing through the non-catalyzed passages serves to cool the catalytic reactor  12  while retaining the removed heat in the combustion gas stream. While such passive cooling is an improvement over previous designs, there remains a risk of the fuel-air mixture in the non-catalyst cooling passages igniting or of the flame traveling upstream into the non-catalyzed cooling passages. In such an event, the cooling action will be lost and the catalyst may overheat and fail. 
     SUMMARY OF THE INVENTION 
     Accordingly, an improved catalytic combustor is needed to reduce the risk of overheating of the catalytic reactor. Furthermore, a simple and cost effective catalytic combustor is needed for applications where the gas supply temperature is below the temperature necessary to activate the catalyst. 
     A combustor is described herein as having: a heat exchanger module having catalytic passages in a heat exchange relationship with non-catalytic passages; a fuel injection apparatus; and a means for directing combustion air in sequence through the non-catalytic passages, the fuel injection apparatus and the catalytic passages. Because the air traveling through the non-catalytic passages does not contain fuel, the risk of flash-back of the flame into these cooling passages is eliminated. 
     In one embodiment, a combustor is described herein as including: a plurality of catalyst modules disposed in a generally circular pattern at the inlet of an annular combustor chamber within an engine casing; a seal between the plurality of catalyst modules and the engine casing for directing a flow of air into contact with non-catalytic surfaces of the respective catalyst modules; a plurality of fuel injectors associated with the plurality of catalyst modules for injecting a combustible fuel into the flow of air downstream of the non-catalytic surfaces to form a fuel-air mixture; and a plurality of catalytic surfaces formed on the catalyst modules for contacting the fuel-air mixture downstream of the non-catalytic surfaces and for causing a first portion of the fuel to combust within the respective catalyst modules and a second portion of the fuel to combust within the combustion chamber. 
     A gas turbine is described herein as including: a compressor for providing a flow of air; a combustor for combusting a flow of fuel in the flow of air to produce a flow of combustion gas; and a turbine for extracting energy from the flow of combustion gas; wherein the combustor further comprises: a catalyst module having a catalytic surface and a non-catalytic surface in heat exchange relationship there between; a fuel delivery apparatus; and a flow directing apparatus for directing the flow of air in sequence from the non-catalytic surface to the fuel delivery apparatus to the catalytic surface. 
     A method of combusting a fuel is described herein as including the steps of: providing a catalyst device having a catalytic surface in heat exchange relationship with a non-catalytic surface; directing fuel-free air over the non-catalytic surface to remove heat energy from the catalyst device and to pre-heat the fuel-free air; adding a combustible fuel to the fuel-free air to form a fuel-air mixture; and directing the fuel-air mixture over the catalytic surface to combust at least a first portion of the fuel-air mixture and to generate heat energy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the course of the following detailed description, reference will be made to the following drawings in which: 
     FIG. 1 is a schematic side sectional view of a prior art catalytic combustor. 
     FIG. 2 is a schematic illustration of a gas turbine engine incorporating a catalytic heat exchanger. 
     FIG. 3 is a partial cross-sectional view of a can-type combustor for a gas turbine engine incorporating a catalytic heat exchanger. 
     FIG. 4 is an end view of an annular-type combustion system incorporating a plurality of catalytic modules interspaced with a plurality of pilot burners. 
     FIG. 5 is a partial side sectional view of the combustion system of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An improved gas turbine engine  30  is illustrated in FIG. 2 as including a compressor  32 , a combustor  34  having both a catalytic combustion heat exchanger module  36  and a homogeneous burnout zone combustion chamber  38  as well as a fuel injection apparatus  40 , and a turbine  42 . Compressed air  44  is delivered from the compressor  32  to a fuel injection location through a first plurality of non-catalytic passages  46  in the catalytic module  36 . At the fuel injection location, the air  44  flows through a fuel injection apparatus  40  where a flow of combustible fuel  48  suitable for a combustion turbine is added to form a fuel-air mixture  50 . The fuel-air mixture  50  then passes through a second plurality of passages  52  in the catalytic module  36  where one or more surface-exposed catalyst materials  54  initiates the heterogeneous combustion of the fuel-air mixture  50 . The catalyst material defining the catalytic passages  52  may be any catalyst known in the art to be effective for the fuel being burned, for example, platinum or palladium deposited on a thin ceramic wash coat having a high specific surface area on a metal substrate. The catalytic passages  52  are sealed from and are in a heat exchange relationship with the non-catalytic passages  46 . The structure of the catalytic heat exchanger  36 , including the material defining the non-catalytic passages  46 , may be any metal or ceramic material known in the art to be useful in such a combustion environment. Combustion is completed in the burnout zone portion  38  of combustor  34 , and the hot combustion gas  56  is delivered to the turbine  42 , where it is used to generate mechanical energy in a manner known in the art. 
     Heat energy is generated within the catalytic module  36  by the heterogeneous combustion of the fuel-air mixture  50  within the catalytic passages  52 , and heat energy is removed from the catalytic module  36  by the pre-heating of the compressed air  44  as it passes through the non-catalyst passages  46 . In one embodiment, the compressed air  44  provided by the compressor  32  may be at about 750° F. and it may be pre-heated within the catalytic heat exchanger  36  to a temperature of about 950° F. Following combustion of at least a first portion of the fuel-air mixture  50  within the catalytic module  36 , the air temperature may have been increased to about 1,600° F. Following combustion of a second portion of the fuel-air mixture  50  within the combustion chamber burnout zone  38 , the temperature of the combustion gas  56  may have been increased to about 2,700° F. The compressed air  44  is pre-heated in the non-catalytic cooling passages  46  to at least a temperature sufficient to initiate the catalytic reaction within the catalytic passages  52 , thereby eliminating the need for any pre-burner. Furthermore, since the catalytic module  36  is passively cooled with fuel-free compressed air  44 , there is no concern about flashback or auto-ignition in the cooling channels  46 . Accordingly, the gas turbine  30  of FIG. 2 may be less costly to design and manufacture than prior art devices having a pre-burner, and it may be less prone to overheating due to unanticipated back-propagation of the flame. Because at least a portion of the fuel is burned in the catalytic reactor  36 , a stable, complete combustion process having NOx emissions of less than 3 ppm in the exhaust gas may be achieved. 
     FIG. 3 is a partial cross-sectional view of a combustor that may be used in a gas turbine engine  30  as described with respect to FIG.  2 . The combustor  60  would be used in a can-type combustion system, as is currently known to be used in Siemens Westinghouse Power Corporation Model 501F gas turbine engines. In a Model 501F engine, sixteen such combustors  60  would be spaced circumferentially about an outlet end of a compressor, radially displaced from a longitudinal axis of the turbine. The combustors  60  would be housed in a generally cylindrical casing (not shown) which provides a flow communication for compressed air  61  between the compressor outlet (not shown) and an annular inlet opening  62  of combustor  60 . The compressed air  61  is then directed by the shell  63  of the combustor  60  over a non-catalytic surface  64  of a catalyst module  66  to a fuel delivery location  68 . While passing over the non-catalytic surface  64 , the compressed air  61  removes heat from the catalyst module  66 , thus pre-heating the compressed air  61 . At the fuel delivery location  68 , a fuel injection apparatus  70  introduces a flow of fuel into the pre-heated air to form a fuel-air mixture  72 . The fuel injection apparatus  70  may be a combination swirl vane/nozzle combination as is known in the art for injecting the fuel and pre-mixing the fuel and the air together to form the fuel-air mixture  72 . The fuel-air mixture  72  is pre-heated by contact of the compressed air  61  with the non-catalytic surface  64  to a temperature sufficiently high to initiate combustion of the fuel-air mixture  72  when it is next directed over a catalytic surface  74  of catalyst module  66 . Catalyst module  66  may be formed as a cross-flow device, as illustrated, wherein the non-catalytic passages and the catalytic passages are formed to be at approximately right angles to each other. Other designs may be envisioned wherein the non-catalytic passages and the catalytic passages are parallel to each other or are otherwise aligned to be in a heat-exchange relationship with each other. At least a first portion of the fuel-air mixture  72  is combusted within the catalyst module  66 , and a second and preferably completed portion of the fuel-air mixture  72  is combusted in a burnout zone defined by a generally tubular-shaped combustion chamber  76 . The hot combustion gas  77  is then directed to a transition piece (not shown) and into a downstream turbine, as shown in FIG.  2 . 
     The catalyst module  66  is illustrated in cross-section as having an annular ring shape. Alternatively, a plurality of such modules may be disposed in a side-by-side configuration around an annular inlet to the combustion chamber  76 . The main fuel injection upstream of the modules may be divided into stages that are turned on at different times as the engine load is increased and turned off as the engine load is decreased. A portion of the combustion air  61  is directed away from the main fuel injection apparatus  70  into a pilot burner  78 . The pilot burner is provided with one or two additional fuel lines  80  that may be used for engine startup and for low load operation. Fuel supply to the pilot burner  78  may be reduced or eliminated at higher loads or whenever the flame in the combustion chamber  76  is stable in order to reduce the overall emissions of the engine. For natural gas fuel applications, an alternative fuel such as hydrogen or propane may be added to the main fuel supply to facilitate the heat-up of the catalyst module  66 , since these are much easier to react catalytically than is methane. Once the catalyst module  66  has reached a desired temperature, the compressed air  61  will be heated to a temperature where the catalytic reaction of the natural gas-air mixture will occur, and the alternative fuel supply may be terminated. 
     A plurality of catalytic heat exchanger modules as described above may also be used in an annular-type combustion system such as the Siemens Model V84.3A gas turbine engine. FIG. 4 illustrates an end view of one such combustion system  80  where a plurality of catalytic heat exchanger modules  82  are spaced around an inlet to an annular combustion chamber  84 . Pluralities of pilot burners  86  are placed among the catalytic modules  82 , for example, with a pilot burner  86  between each two adjacent catalytic modules  82 . A seal  88  is made from the engine casing  90  to the catalyst modules  82  as may best be seen in FIG. 5, which is a partial side sectional view of the combustion system  80 . The seal  88  directs the flow of combustion air  92  into contact with non-catalytic surfaces  94  of the catalyst module  82  for removing heat there from. The pre-heated air is then directed by the engine casing  90  to the fuel injectors  96  for the injection of a combustible fuel downstream of the non-catalytic surfaces  94  to form a fuel-air mixture  98 . The inlet of the annular combustor structure  84  then directs the fuel-air mixture  98  over the catalytic surfaces  100  of catalyst member  82  where the combustion process is initiated to create heat energy. Combustion is completed downstream of the catalytic heat exchanger  82  in the burnout zone  102  and the hot combustion gasses  106  are directed out of the combustor to a turbine. The pilot burners  86  each have an outlet to the combustion chamber burnout zone  102  for stabilizing the combustion therein. 
     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. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.