Patent Publication Number: US-6339925-B1

Title: Hybrid catalytic combustor

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to combustors, and more specifically, to hybrid combustors for providing a substantially uniform fuel and air mixture. 
     Combustors for gas turbines typically comprise a combustion chamber together with burners, igniters, and fuel injection devices. Combustors for gas turbines have traditionally operated in a non-premixed mode in which a fuel (e.g., natural gas) and an oxidant (e.g., air) are completely separated as the reactants enter the flame. In general, non-premixed combustors are stable over a wide range of operating conditions and at low fuel-air ratios. A drawback of non-premixed combustors, however, is that high temperatures in the reaction zone lead to increased production of nitrogen oxides (NOx). 
     In premixed combustors, the fuel and the oxidant are completely premixed before combustion. The production of NOx in premixed flames is minimized because localized high temperatures in the reaction zone are avoided. A drawback of premixed combustors is that at low loads, premixed combustors produce higher levels of carbon monoxide (CO) and unburned hydrocarbons (UHCs) and are also not as stable compared to non-premixed combustors. Although the flame stability in premixed combustors can be improved through mechanical and aerodynamic means (e.g., fuel nozzles having a bluff body with a broad flattened surface for causing recirculation of the flow of the fuel and air mixture having swirlers), premixed combustors generally lack the stability of non-premixed combustors. 
     An approach for stabilizing premixed combustors is the application of a catalyst in the combustor to initiate and promote gas phase combustion, which combustion has been referred to sometimes as “catalytic combustion”, catalytically stabilized combustion, or “catalytically stabilized thermal combustion.” A drawback of catalytic combustors is that their maximum operating temperature may be limited by the thermal stability of the catalytic materials or the mechanical supports. Another drawback is that non-uniformities in the fuel-air mixture, for example, from a fuel nozzle, result in areas of localized overheating if the fuel-air mixture is too rich, or areas of low catalyst activity if the fuel-air mixture is too lean. 
     Therefore, there is a need for hybrid combustors which provide stable high and low levels of operation while minimizing emissions of NOx at high levels of operation and minimizing emissions of CO or UHCs at low levels of operation. In addition, there is a need for fuel nozzles for providing a substantially uniform fuel and air mixture. 
     SUMMARY OF THE INVENTION 
     A hybrid combustor, for providing stable high and low levels of operation while minimizing emissions of NOx, CO, and UHCs, includes a casing having a chamber, a catalytic combustor disposed in the chamber, and a non-premixed combustor disposed in the chamber. The hybrid combustor may comprise a fuel nozzle comprising a casing having a chamber, and a body supportable in the chamber to define a passageway between the body and the casing. The passageway has an inlet for receiving a stream of air and an outlet for discharging a stream of fuel and air, and the body includes a tapering downstream portion. Desirably, flow separation of the fuel and air mixture from the body (i.e., recirculation of the fuel and air mixture in the passageway or chamber) is inhibited whereby a generally uniform fuel and air mixture is provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic sectional view of a hybrid combustor of the present invention; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG. 1; 
     FIGS. 3A and 3B are tables of the results of adiabatic flame temperature for catalytic versus premixed burner paths for various fuelair ratios at 0 percent, 3 percent, and 10 percent air leak around the flame; 
     FIG. 4 is a diagrammatic sectional view of an alternative embodiment of a hybrid combustor of the present invention; 
     FIG. 5 is a cross-sectional view taken along line  5 — 5  of FIG. 4; 
     FIG. 6 is a diagrammatic sectional view of an alternative embodiment of a hybrid combustor of the present invention; 
     FIG. 7 is a cross-sectional view taken along line  7 — 7  of FIG. 6; 
     FIG. 8 is a side elevation view, in part section, of a fuel nozzle of the present invention; 
     FIG. 9 is a graph of a concentration profile of three fuel-air ratios measured diametrically across the downstream end of the fuel nozzle shown in FIG. 8; 
     FIG. 10 is an end view of an assembly having seven fuel nozzles shown in FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 diagrammatically illustrates one embodiment of a hybrid catalytically stabilized dry low NOx combustor  10  that may be used in, for example, a gas turbine (not shown). Hybrid combustor  10  provides stable high and low levels of operation while minimizing emissions of NOx, CO, and UHCs. In this exemplary embodiment, a catalytic combustor  20  is arranged substantially to run in parallel and substantially simultaneously with a non-premixed (e.g., diffusion flame) combustor  30 . 
     Hybrid combustor  10  may be configured to include a generally cylindrically-shaped casing  12  having a chamber  14  therein in which generally cylindrically-shaped catalytic combustor  20  is centrally disposed in chamber  14  and non-premixed combustor  30  is disposed between casing  12  and catalytic combustor  20 . 
     Catalytic combustor  20  may include a generally elongated, cylindrically-shaped casing or liner  22  having a chamber  24  therein. A preburner  26  is disposed adjacent to an upstream end of liner  22 , a catalytic reactor  25  is disposed adjacent to a downstream end of liner  22 , and one or more fuel injectors  28  are disposed in chamber  24  between preburner  26  and catalytic reactor  25 . Preburner  26  provides heat to initiate the catalytic process in catalytic reactor  25 . In addition, preburner  26  provides an additional means for producing heat and combustion gases in hybrid combustor  10  to allow hybrid combustor  10  to achieve various load targets with or without operation of catalytic reactor  25 . Furthermore, preburner  26  may comprise a non-premixed preburner burner or a premixed preburner burner. 
     In this exemplary embodiment, as shown in FIGS. 1 and 2, non-premixed combustor  30  is desirably disposed in an annulus formed between casing  12  and catalytic combustor  20  and is spaced-apart and concentrically disposed between casing  12  and liner  22  of catalytic combustor  20 . Although FIG. 2 illustrates an arrangement of six non-premixed burners  36 , any number of non-premixed burners may be used. Non-premixed combustor  30  may further comprise a plurality of non-premixed burners or a combination of non-premixed and premixed burners. In addition, the axial positions of non-premixed combustors  30  relative to catalytic combustor  20  may also be varied. 
     With reference to FIG. 1 again, in the operation of hybrid combustor  10 , a stream or supply of air is provided to an upstream end of casing  12 . A first portion of the stream or supply of air is provided to catalytic combustor  20  by being introduced through an upstream portion of liner  22  or through the wall forming the upstream portion of liner  22 . Fuel injectors  28  are positioned downstream of preburner  22  for introducing a stream or supply of fuel into the stream of air in catalytic combustor  20 . Once fuel is injected into the stream of air, the premixed fuel-air mixture then passes through catalytic reactor  25  which oxidizes the fuel-air mixture. In some configurations, gas phase combustion of the hot gases from the catalytic reactor may continue downstream of catalyst reactor  25 . 
     A second portion of the stream or supply of air and a second supply of fuel are provided to non-premixed burners  36  for combustion between casing  12  and liner  22  of catalytic combustor  20 . 
     Hybrid combustor  10  may also be operated in an alternative mode to promote gas phase combustion from a generally parallel premixed fuel-air mixture from non-premixed burners  36 . For example, instead of using non-premixed burners  36  to burn a supply of fuel, non-premixed burners  36  may provide a stream of premixed fuel and air that is passed through the annulus between casing  12  and catalytic combustor  20  for combustion downstream of catalytic combustor  20 . For example, the flames produced by non-premixed burners  36  may be extinguished by shutting off the supply of fuel, followed by re-introduction of the fuel through a nozzle of burner  36  without ignition. Air required for the premixed fuel-air mixture can continue to pass through either the annulus between casing  12  and catalytic combustor  20 , or through a porous upstream portion  16  of casing  12 . 
     In operation in this alternative mode, the unburned fuel-air mixture exits a mixing region  17  so that the unburned fuel-air mixture can then mix with the hot effluent gases in a downstream region  19  from catalytic combustor  20 . Desirably, through a combination of thermal and chemical interactions between the hot effluent gases from catalytic combustor  20  and the premixed fuel-air mixture, the premixed fuel-air mixture can be ignited and burned in region  19  downstream of catalytic reactor  25  and between a downstream portion of a venturi  15  disposed in chamber  14 . 
     Venturi  15  not only helps stabilize gas phase combustion by acting as a bluff body and creating a recirculation region, but Venturi  15  also increases local gas velocities at the exit of the mixing region  17  to prevent flashback of the flame into the fuel-air premixing region  18 . For hybrid combustor  10  shown in FIG. 1, completion of gas phase combustion might also occur further downstream in the combustor, for example, in region  13 . 
     From the present description, it will be appreciated by those skilled in the art that separate means, for example, one or more ports or fuel injectors, for introducing a supply of fuel to the second portion of the supply of air may be provided in addition to non-premixed combustor  30  having a plurality of non-premixed burners  36 . In addition, it will be appreciated that the venturi may have other configurations, for example, curved surfaces, as well as other types of bluff bodies may be positioned in chamber  14  for stabilizing a flame in chamber  14 . Furthermore, depending on the particular application, it may also be advantageous to introduce additional air at various locations through the downstream portion of casing  12 . 
     The amount of NOx produced by hybrid combustor  10  is dependent upon a number of conditions, which conditions may include the type of fuel used, the temperature profile of the flame, the operation pressure, and the gas residence time in the combustor. Furthermore, the design and operation of hybrid combustor  10  are a compromise between the desire to run catalytic combustor  20  at a low temperature to extend the life of catalytic materials and mechanical supports versus the need to prevent non-premixed combustor  30  from operating at excessive temperatures wherein high rates of NOx emissions are produced. 
     By using and combining existing data from independent tests of a catalytic combustor and from a premixed combustor, it is possible to estimate the amount of NOx that may be produced from a hybrid combustor that combines, in parallel, the use of these two different combustors. This tradeoff can be characterized by examining, 1) the variations in the air split between the catalytic path and the premixed path, and also, 2) the variations in the fuel-air ratios to the two paths. 
     FIGS. 3A and 3B illustrate a table showing the fuel-air ratios and their associated adiabatic flame temperatures for various air splits and fuel-air ratios for the catalytic path versus the premixed paths. These calculations were made by assuming a combustor pressure of about 15 atmospheres, an inlet air temperature of about 735 degrees Fahrenheit (F.), and an inlet fuel temperature of about 70 degrees F. With methane as the fuel, the adiabatic flame temperatures were estimated at the various fuel-air ratios using NASA CET89 thermodynamic code. 
     The calculations were made to achieve a final combustor exit temperature of about 2700 degrees F. with the final combustor temperature being an average mixture temperature for the gases from the catalytic and premixed paths. Accordingly, as the adiabatic flame temperature of the fuel-air mixture to the catalytic path is reduced (i.e., below 2700 degrees F.), the adiabatic flame temperature through the premixed path must be increased (i.e., greater than 2700 degrees F.) in order to achieve the same final desired mixture temperature of 2700 degrees F. 
     Observable from FIGS. 3A and 3B is that as the fraction of air to the catalytic combustor is reduced, less of an increase in fuel-air ratio from the premixed path is required to offset a decrease in fuel-air ratio from the catalytic paths. Using the tabulation of adiabatic flame temperatures in FIGS. 3A and 3B, an estimate of the total amount of NOx produced from the combined catalytic and premixed streams may be made by adding together the amount of NOx expected (from readily available data) from each of the two combustion paths. 
     The same calculations were also repeated by assuming 3 percent and 10 percent leakage of the total air into the hot gas flow path between the flame and the combustor exit, and are also illustrated in FIGS. 3A and 3B. Air leaks between the flame and combustor exit can be caused by leak paths in the seals between various combustor components which are not uncommon in commercial gas turbine combustors. Note that if an air leak exits between the flame and the combustor exit, the flame must fire at even higher temperatures to achieve a final temperature of 2700 degrees F. since the air leak will reduce the mixture temperature. For an example, it was estimated that with a 3 percent air leak, the mixture gas temperature before the leak must be 2750 degrees F. to give a final average temperature of 2700 degrees F. If the air leak were 10 percent, the mixture gas temperature before the leak must be 2878 degrees F. to give the same 2700 degrees F. average temperature. The calculations which include air leaks give a more realistic representation of temperatures which might be found in commercial gas turbine combustors. 
     FIGS. 4-7 show two alternative embodiments for hybrid combustors. FIGS. 4 and 5 illustrate a hybrid combustor  40  in which a non-premixed combustor  60  is centered within and surrounded by a catalytic combustor  50 . A plurality of preburners  56  are disposed adjacent to an upstream end of catalytic combustor  50 , a catalytic reactor  55  is disposed adjacent to a downstream end of catalytic combustor  50 , and a plurality of fuel injectors  58  are disposed between preburners  56  and catalytic reactor  55 . Non-premixed combustor  60  comprises a non-premixed burner  66  that may also be transitioned to provide a stream of premixed fuel and air. Desirably, a venturi  45  is provided at the downstream portion of non-premixed combustor  60  to prevent flash back of the flame into the fuel-air premixing region  48 . FIGS. 6 and 7 illustrate a hybrid combustor  70  in which catalytic combustor  80  and a non-premixed combustor  90  each occupy half of a cylindrically-shaped casing  72 . A preburner  86  is disposed adjacent to an upstream end of catalytic combustor  80 , a catalytic reactor  85  is disposed adjacent to a downstream end of catalytic combustor  80 , and a plurality of fuel injectors  88  are disposed between preburner  86  and catalytic reactor  85 . Non-premixed combustor  90  comprises a non-premixed burner  96  that may also be transitioned to provide a stream of premixed fuel and air. Desirably a venturi  75  is provided at the downstream portion of non-premixed combustor  90  to prevent flash back of the flame into fuel-air premixing region  78 . From the present description, it will be appreciated by those skilled in the art that other generally parallel configurations of a catalytic combustor and a non-premixed combustor may be employed. 
     FIG. 8 illustrates one embodiment of a fuel nozzle  100  for providing a generally spatially uniform fuel and air mixture (e.g., having a uniform distribution concentration of fuel and air) to a catalytic combustor in, for example, a gas turbine, and in particular for fuel injectors  28  shown in FIG. 1, fuel injectors  58  shown in FIG. 4, and fuel injectors  88  shown in FIG.  6 . 
     In this illustrated embodiment, fuel nozzle  100  includes a cylindrical outer casing  112  having a chamber  114  and a longitudinal axis L. A hub or body  120  is supported in casing  112  so that body  120  and casing  112  define an air flow path or passageway  130  therebetween. Passageway  130  includes an inlet  132  for receiving a stream or supply of air and an outlet  134  for discharging a stream or supply of fuel and air. Body  120  includes a tapered downstream portion  122  so that the cross-sectional area of passageway  130  increases when moving towards outlet  134 . 
     Body  120  may be supported and positioned in the center of the air flow path in a casing  112  by a plurality of struts  140  (only two of which are shown in FIG.  8 ). Fuel is supplied to the forward portion of body  120  and distributed into the air flow path by a plurality of apertures  152  in a plurality of fuel spokes or injectors  150 , which injectors  150  extend between casing  112  and body  120 . 
     In this illustrated embodiment, tapered downstream portion  122  of body  120  transitions from a cylindrical-shaped cross-sectional portion  124  to an ellipsoid-shaped cross-sectional portion  126 , and then to a conically-shaped cross-sectional portion  128  that terminates at a point  129 . This configuration minimizes flow separation of the fuel and air mixture from the surface of body  120  (i.e. recirculation of the fuel and air mixture). Desirably, a downstream inner surface  113  of casing  112  also diverges, slopes, or expands outwardly at an angle of about 3.5 degrees or less so that the cross-sectional area of passageway  114  further increases when moving towards outlet  134  while minimizing flow separation of the fuel and air mixture from inner surface  113 . 
     During operation, fuel nozzle  120  first reduces the cross-sectional flow area of the supply of air to a narrow annular region where fuel, for example, gas, is injected into the air flow. Then, the flow path is expanded through a diffuser section defined by sloped sides  113  of casing  112  and tapered downstream portion  122  of body  120 . 
     The geometry of fuel nozzle  100  minimizes flow separation in order to minimize the likelihood of recirculation of the fuel and air mixture, which recirculation would lead to a nonuniform fuel and air mixture, as well as the possibility that a gas phase flame could be anchored in the wake of fuel nozzle  100 . In addition, the overall geometry of fuel nozzle  100  desirably reduces the pressure losses to the air flow between the upstream end and the downstream end. 
     An experimental eight-inch fuel nozzle has been built and tested under fired and unfired conditions. The concentration of fuel and air from the fuel nozzle was first measured prior to firing of a preburner which was positioned upstream of the nozzle. The test operated at combustion air flowrate of 7 pounds/second, air preheat temperature of about 575 to 600 degrees F. (about 302 to 316 degrees C.), and combustor pressure of 7 atm. A diametrically traversing gas sampling probe was used to measure the fuel concentration profiles at the catalytic reactor inlet (i.e., downstream from the fuel nozzle). 
     Initially, the diametrically traversing probe was positioned to scan the direction from a 10:30 position (top left) to a 4:30 position (lower right, looking downstream). Without firing the preburner, three fuel flowrates of 0.028, 0.078, and 0.110 lb./sec. were used, corresponding to fuel-air ratios of 0.004, 0.011, and 0.016 lb./lb., respectively. The results of these measurement are shown in FIG.  9  and illustrate a generally uniform and constant fuel concentration across the diameter of chamber  114  for each of these three fuel flowrates. 
     The fuel nozzle was exposed to the operational thermal cycles of the preburner to determine if the nozzle was operable to withstand thermal stresses under actual test conditions. The preburner was ignited and cycled from about 650 degrees F. to 1100 degrees F. (about 343 degrees C. to 593 degrees C.) at a rate of about 25 degrees F./min (about 14 degrees C./min). After two thermal cycles of the preburner, a fuel concentration traverse was made at a fuel flowrate of 0.110 lb./sec. and compared to the concentration profile measured prior to the preburner cycles. No measurable changes in fuel uniformity were observed following the preburner cycles indicating that the fuel nozzle remained undamaged through the preburner thermal cycles and that the fuel nozzle continued to give excellent fuel concentration uniformities, i.e., a generally uniform fuel and air mixture. 
     The fuel nozzle has also been tested under fired catalytic combustor conditions. Thermocouple temperature measurements taken within the catalytic reactor and thermal imaging temperature measurements of the aft end of the catalytic reactor show the radial temperature profile in the reactor to be highly uniform. 
     A plurality of fuel nozzles  100  may be configured in an array or assembly  200  as shown in FIG.  10 . Such an arrangement of fuel nozzles  100  may be more advantageous under some conditions, e.g., when a single fuel nozzle may be prohibitively large or long. Other configurations of an array or assembly of fuel nozzle may also be employed, for example, an array or assembly having a different number of fuel nozzles  100 . 
     From the present description, it will be appreciated by those skilled in the art that while fuel nozzle  100  is desirable for use with catalytic combustors, fuel nozzle  100  may also be used in a premixed combustor, for example, by placing a bluff body or a V-gutter downstream from the fuel nozzle in order to anchor a flame. 
     While only certain features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.