Abstract:
The invention is a method and apparatus for use therewith for a main burner of a gas turbine. The method employs catalytic combustion to support main combustion. More specifically, a rich fuel/air mixture is catalytically oxidized with the resulting reacted mixture being made lean by having additional air added thereto. The resulting lean mixture is then combusted in the presence of the main mixture that is also lean thereby supporting combustion of the main mixture. The method allows for enhanced turndown of a lean main mixture.

Description:
FIELD OF THE INVENTION 
     The present invention is generally directed to combustion, and more specifically to a method of operating a main burner wherein the main combustion occurring therein is supported by a catalytic pilot that oxidizes a fuel rich mixture and a main burner for use therewith. 
     BACKGROUND 
     Power is generated in a gas turbine engine by the expansion of heated gases against a rotating turbine. To accomplish this heating and expansion a gas turbine has at least one combustor having at least one main burner positioned therein. The main burner combines a fuel and air into a fuel/air mixture and combusts the mixture thereby creating the expanding hot gases. Combustion of the mixture generally occurs by a flame mechanism. 
     A problem commonly associated with the operation of gas turbines employing a flame mechanism is that at high flame temperatures, particularly above 2800 degrees F., oxygen and nitrogen present in the air combine by a thermal formation mechanism to form pollutants such as NO and NO 2 , collectively referred to as NO x . In a gas turbine, temperatures of most common fuels combusting in air can easily exceed this value. Accordingly, it has been an objective of gas turbine combustion system designers to develop methods and associated apparatuses for combustion that produce reduced temperatures at or below 2800 degrees F., so that such thermal formation of NO x  is limited. 
     Modern combustion methods employed in gas turbine combustors reduce flame temperatures, and thereby NO x , by using excess air to create lean fuel/air mixtures, e.g. mixtures that contain more air than needed to fully combust all the fuel present. Quantitatively, the mixture has a fuel/air equivalence ratio less than one. The equivalence ratio is the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio, where the stoichiometric coefficients are calculated for the reaction giving full oxidation products CO 2  and H 2 O. An equivalence ratio greater than one defines a fuel-rich fuel/air mixture, and an equivalence ratio less than one defines a fuel-lean fuel/air mixture. For any given substantially premixed fuel/air mixture, the combustion temperature will be at its highest temperature when the fuel/air mixture being combusted has a fuel/air equivalence ratio of about one. 
     The more excess air added to and well mixed in a fuel/air mixture, the leaner the resulting fuel/air mixture becomes and the lower the flame temperature of that mixture. However, if too much excess air is added the resulting fuel/air mixture will become so lean that it will not homogeneously combust. In this situation, the mixture is said to have reached its lower flammability limit. Therefore, excess air to limit flame temperature can only be added to a well mixed fuel/air mixture until this limit is reached. 
     In order to obtain the benefits of lower flame temperatures in fuel/air mixtures, the fuel/air mixture being combusted must be substantially mixed. Typically, the lower the unmixedness the lower the NO x  that will be produced. While unmixedness defines a continuum such that mixtures can only be categorized as being mixed to some degree, a “substantially premixed mixture” can be defined based on the fuel/air mixture&#39;s potential to produce a certain level of NO x  when combusted within the context of acceptable NO x  production based on existing environmental regulation. In other words, the mixture is mixed sufficiently to produce a level of NO x  that will meet current environmental regulations. 
     Thus based on current environmental regulation, substantially premixed fuel/air mixtures are mixtures wherein the average variation of fuel/air ratio from the mean is less than about 20 percent of the mean value and more preferably in the range from about 10 percent to about 2 percent, with less than 2 percent being a practical minimum. Mean fuel/air ratio refers to the average fuel/air ratio as measured at various points in the region of interest. Variation from the mean refers to the magnitude of the difference between the mean and the measured fuel/air ratio at some single measured point, and the average variation from the mean is the average of all measured variations from the mean. For a combustible fuel/air mixture the region of interest is generally immediately prior to combustion. 
     In a combustor, the air stream and the fuel stream must form a fuel/air mixture prior to combustion. To mix two flowing fluid streams to form a single flowing stream, the individual streams must be brought into contact and travel some distance together. If mixing is done within a duct, the length of the duct will determine the degree of unmixedness. Generally speaking, the longer the duct the lesser the degree of unmixedness. 
     As a lean fuel/air mixture is made ever leaner but above the mixture&#39;s lower flammability limit, the rate of combustion associated with the mixture decreases, i.e. the flame is becoming less robust. In order to maintain the flame, the environment within the flame must be made ever more conducive to combustion, e.g. the flow velocity must be reduced, otherwise the flame could be blown out, much like one blows out a candle. In a gas turbine when the fuel/air mixture has been leaned to the point that the rate of combustion of the mixture is too low to sustain combustion under the existing conditions, the extinguishing of the flame by its environment is termed blowout. Flame anchoring, i.e. the ability to provide proper environmental conditions to support a flame, and flame stability thus become problematic for fuel-lean combustion. 
     The management of combustion within a gas turbine operating on lean fuel/air mixtures to avoid blowout and assure flame anchoring and stability is complex. Gas turbines are generally designed to operate at a given or peak condition, i.e. an optimum condition which is highly efficient. However, during startup or at other times, it may be desirable to operate at other, or off-peak, conditions. Therefore, a gas turbine must have the ability to transition from the peak condition to off-peak conditions. This ability to go from a peak to off-peak condition is generally referred to by those skilled in the art as the ability to turndown the gas turbine. 
     Turndown is accomplished by reducing the fuel supply to the combustor, thereby making the fuel/air mixture being combusted therein leaner. As the gas turbine at its peak condition is already operating with a fuel/air mixture that is quite lean to meet current environmental standards, when the fuel/air mixture is made ever leaner to achieve the desired off-peak operating condition, sustaining combustion within the combustor becomes ever more problematic. In some cases, turndown is simply insufficient to permit acceptable off-peak operation conditions. 
     To increase the ability of a gas turbine to turndown, pilots can be used to support combustion within the combustor. Specifically, the pilots are supporting what is termed main combustion. Pilots that use flames operate at very favorable fuel/air mixtures, which may even be at fuel/air ratios at or near 1.0, providing highly stable and high temperature flames. Initially, pilot emissions were a small percentage of the overall emissions from the gas turbine. Currently, however, gas turbines have main combustion occurring at such lean fuel/air mixtures that NO x  discharge is acceptable, and it is the emissions from these flame based pilots that must be further reduced to reduce overall gas turbine NO x  emissions. 
     Conventional catalytic pilots on the other hand are highly stable but operate at lower temperatures, because of catalyst material considerations, thereby producing less NO x  than flame pilots. However, these lower temperatures hamper the ability of the catalytic pilot to support combustion of lean fuel/air mixtures. 
     Based on the foregoing, it is the general object of the present invention to provide a method and apparatus for use therewith to support main combustion that overcomes the problems and drawbacks of the prior art. 
     SUMMARY OF THE INVENTION 
     The method of combustion utilizes catalytic oxidation to support flame burning of a lean fuel/air mixture. In the method of the invention a first, second, and third air, and a first and second fuel are provided such that: the first fuel and first air have a fuel/air equivalence ratio greater than 1; the first fuel in combination with the first air and second air have a fuel/air equivalence ratio less than 1; and the second fuel and the third air have a fuel/air equivalence ratio less than 1. 
     The first fuel and first air are introduced into a first common area so the first fuel and first air can travel together and intermix to form a first mixture. The first mixture is then flowed over and brought in contact with a catalyst where the fuel in the first mixture is oxidized resulting in creation of a first reacted mixture and a heat of reaction. The first reacted mixture is then introduced to the second air in a second common area so the first reacted mixture and the second air travel together and intermix to form a second mixture. The second mixture is then combusted. Combusted as used herein means that the mechanism of burning is a flame. The second mixture, which is derived from the first fuel, the first air, and the second air, is lean because the constituents from which it is derived in combination are lean. It should be understood as explained above, the second mixture can not be so lean as to be below the lower flammability for the second mixture. 
     Simultaneously, the third air and the second fuel are introduced into a common area where the third air and second fuel travel together and intermix forming a third mixture. The third mixture is then combusted. The third mixture is also lean because the second fuel and third air from which the third mixture was derived have a fuel/air equivalence ratio less than 1. As with the second mixture, it is understood that the third mixture must have a fuel/air equivalence ratio above the lower flammability limit of the third mixture. 
     The method requires that the combusting second mixture be in contact with the combusting third mixture. The second mixture is combusted in contact with the third mixture when the two flames interact. In other words, the flames touch. It is preferred that the flame of the second mixture be substantially within the flame of the third mixture. 
     In an enhancement to the method, a heat of reaction generated during the catalytic reaction of the first mixture can be transferred into the second air. Dissipating excess heat into the second air stream can protect the catalyst used in the oxidation of the first fuel mixture, i.e. backside cooling the catalyst. Backside cooling a catalyst protects the catalyst and substrate on which it might be positioned from damage from the extreme temperatures generated in exothermic catalytic oxidation. 
     While the first, second and third airs are identified as separate airs; the airs could be from a common source. In addition while the first fuel and second fuel could be different, the two fuels preferably are the same. 
     When this method is applied to a gas turbine, the method permits a pilot to produce less NO x  than other standard flame pilots, but also provides a flame that is generally equally robust to that of flame pilots to support the main combustion. It is preferred that the combustion of the second mixture support, i.e. pilot, the combustion of the third mixture. As such, the ratio of first fuel to second fuel should be less than about 1:1 but greater than about 1:19. Preferably, the ratio should be less than about 1:4 and greater than about 1:9. These ratios permit this method to be employed within current gas turbine designs. 
     A ratio greater than 1:9 is preferred because the pilot can provide significant stability to the main combustion with pilot emissions being a small percentage of the overall emissions. At ratios greater than 1:1, the flows through the pilot can disrupt main combustion and overall pilot size to accommodate the flows therethrough become problematic. 
     A main burner in one aspect that can employ the above method comprises a catalytic pilot comprised of a first duct and a main mixer disposed within an interior area of a housing with the main mixer in fluid communication with a second duct with both the first duct and the second duct having exits positioned relative one to the another such that the exits cooperate to position a flame emanating therefrom in contact. More specifically, the catalytic pilot is comprised of a catalytic reactor in fluid communication with the first duct that has a first entrance, a second entrance, and an exit. The first entrance and second entrance are positioned coincident one with the other, or the first entrance is spatially upstream. The third duct is in fluid communication with the second entrance. 
     The first and second ducts have geometry to permit mixing to occur. As indicated above, mixing requires some finite length of the duct regardless of other geometric considerations. This length is a critical parameter that must be sufficient to permit the degree of mixing required by the application. In the second duct a fuel and oxidizer is mixed, and the first duct a reacted mixture and another oxidant is mixed. In terms of the method above, the first air and first fuel are mixed in the first duct to form the first mixture, and the first reacted mixture and the second air are mixed in the second duct to form the second mixture. 
     Mixing of the fluids within the ducts can be accomplished by any means such as entrainment or swirling. As those skilled in the art will appreciate, some mixing methods will require additional structure, such as swirlers, in the duct and other mixing methods such as entrainment will not. 
     Preferably, the catalytic pilot is positioned within a passage defined by the main mixer. In some applications, it might be desirable to make the main mixer an annulus and position the catalytic pilot within the vacant center region concentric therewith. When the catalytic pilot and the main mixer are positioned in this manner, the exit from the catalytic pilot should be spatially downstream from the exit of the main mixer. 
     The catalytic reactor within the catalytic pilot can be of almost any design. As discussed above, an additional step in the method is the transfer of some of the heat of reaction into the second fluid. In terms of the catalytic reactor, this step can translate into a backside-cooled catalyst. Backside cooling of a catalyst occurs where a catalyst, i.e. substance that promotes the desired reaction, is positioned on just one side of a two sided substrate and the catalytic reactor is designed to permit a flow of a fluid over both sides. This structure permits the heat generated by the exothermic reaction of the fuel/air mixture on the surface of the substrate having the catalyst to be conducted through the substrate to the other side and transferred into the fluid flowing in contact therewith. 
     The method and main burner could be used within the combustor of a gas turbine as well as other devices such as heaters. While the invention is discussed in the more conventional terms of fuel/air, the invention should not be considered so limited as any fuel and associated oxidant could be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are as follows: 
     FIG. 1 is a schematic cross-section of a gas turbine main burner of the present invention; 
     FIG. 2 is an end view of the catalytic pilot portion of the gas turbine main burner depicted in FIG. 1; and 
     FIG. 3 is an end view of the gas turbine main burner depicted in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1, the main burner generally referred to by reference  10  comprises a catalytic pilot generally referred to by reference number  11  positioned within a main mixer generally referred to by reference number  12  that is positioned within a housing  14 . The catalytic pilot  11  is comprised of catalytic reactor generally referred to by reference number  16  in fluid communication with a first duct  18 . The main mixer  12  is comprised of a mixer  20  with an integral fuel injector  22  in fluid communication with a second duct  24 . The first duct  18  and the second duct  24  are in fluid communication. 
     The catalytic pilot  11  is comprised of a housing  26  that defines an interior area  28  and an inlet  30  in fluid communication therewith. The interior area  28  defines an exit  32 . Positioned within the interior area  28  is a plurality of tubes  34 , each having an exterior surface  36 . The housing  26  has an interior surface  38  that in cooperation with the exterior surfaces  36  defines a single flow channel  40 . Each tube  34  has an exit  42 . As shown in FIG. 2, the tube exits  42  cooperate to define an exit  44  from the single flow channel  40 . 
     Continuing with FIG. 1, the inlet  30  is in fluid communication with the single flow channel  40 . Positioned on the exterior surface  36  between the inlet  30  and the single flow channel exit  44  is a catalyst  50 , such that a first mixture  48  enters the single flow channel  40  through the inlet  30  and passes over the catalyst  50  before exiting the single flow channel  40  through the exit  44 . The catalyst  50  is application specific; however, in a gas turbine utilizing a hydrocarbon based fuel a precious metal based catalyst such as platinum or palladium, i.e. a catalyst having a platinum or palladium element whether individually or in compound, would be appropriate. 
     The single flow channel exit  44  and the tube exits  42  are in fluid communication with the first duct  18 . The single flow channel exit  44  and the tube exits  42  are coincident with each other, i.e. in the same plane. The tube exits  42 , however, can be spatially downstream from the single flow channel exit  44 . 
     As shown in FIG. 2, the single flow channel exit  44  is subdivided into multiple openings, which are preferably discrete, with the openings interspersed around the tube exits  42 . The multiple discrete openings subdivide the first reacted mixture as it exits the single flow channel exit  44  and permits the interspersal of these openings around the tube exits  42  thereby promoting more rapid mixing of the first reacted mixture  48  exiting the single flow channel  40  and the second air  46  exiting the tubes  34  within the first duct  18 . The openings are defined by the outer surface of the tubes  34 . In the preferred embodiment, flared ends of the tubes  34  position the tubes  34  within the housing  26 ; however, other structures such as a grid could be used. 
     Referring back to FIG. 1, the first duct  18  is defined by a portion of the interior surface  38  of the housing  26 . The first duct  18  has a length l 1  that is non-zero and sufficient to permit the second air  46  exiting the tubes  34  and the first reacted mixture  48  exiting the single flow channel  40  through exit  44  to mix to a desired degree of unmixedness forming a second mixture  49 , which exits the catalytic pilot  11  through the exit  32 . 
     The inlet  30  is in fluid communication with a conduit  56 . It is the conduit  56  through which the first mixture  48  flows into the single flow channel  40  such that the first mixture  48  exclusively enters the single flow channel  40  and not a tube  34 . The first mixture  48  is comprised of first air  52  and first fuel  54  that has been injected therein. The first mixture  48  should be well mixed. Mixing can be accomplished by any means such as swirlers (not shown) or entrainment. As an option, a plenum  57  can be imposed between the conduit  56  and the inlet  30 . When a plenum  57  is used, the plenum  57  should extend around the exterior surface  64  and there should be additional inlets  30  such that the first mixture  48  can enter the single flow channel  40  at multiple locations. The use of a plenum  57  allows for a better entering flow distribution of the first mixture  48  within the single flow channel  40 . 
     The tubes  34 , each of which have an entrance  58 , are positioned such that the second air  46  exclusively enters the tubes  34 , and does not enter the single flow channel  40 . In the preferred embodiment, the tubes  34  penetrate the housing  26  such that the tube entrances  58  are not within interior area  28 . Where the tubes  34  penetrate the housing  26 , the penetration is sealed such that leakage is prevented from the single flow channel  40  around the tubes  34 . This assures that the second air  46  will exclusively enter the tubes  34  and the first mixture  48  will exclusively enter the single flow channel  40 . 
     The housing  14  has an inner surface  62  and the housing  26  has an exterior surface  64  that cooperate to define a region  66  wherein the main mixer  12  is positioned. In the preferred embodiment, the main mixer  12  is comprised of a mixer  20  with an integral fuel injector  22  in fluid communication with the second duct  24 . As shown in FIG. 3, the mixer  20  is a swirler that fills the cross-section of the region  66 . Other main burners such as those where the mixer and fuel injector are not integrated are considered within the scope of the invention. It is also not a requirement of the present invention that the mixer  20  completely fill the cross-section of the region  66 . 
     Continuing with FIG. 1, the second duct  24  must be of sufficient length l 2  to permit mixing of a third air  68  with a second fuel  70 . The length l 2  is measured from where the third air  68  and the second fuel  70  are brought into contact to the point at which a third mixture  72  is created, which has the degree of unmixedness desired. Fluids mixing within the first duct  18  and the second duct  24  must be isolated one from the other, until of course the fluids are combusted. 
     While a first air  52 , a second air  46  and a third air  68  have been discussed, it is understood that these airs could be derived from a single primary air  74 . Similarly, it is understood that the first fuel  54  and the second fuel  70  could both be obtained from the same fuel source. 
     In the method of the current invention as applied to the above main burner  11 , a first air  52 , a second air  46 , and third air  68  as well as a first fuel  54  and second fuel  70  are provided. The first fuel  54  and the first air  52  are proportioned such that if traveling together and intermixed a first mixture  48  would be formed having a fuel/air equivalence ratio greater than 1.0. The first fuel  54 , the first air  52 , and the second air  46  are proportioned such that if traveling together and intermixed a second mixture  49  would be formed having a fuel/air equivalence ratio less than 1.0. Finally, if the second fuel  70  and the third air  68  are proportioned such that if traveling together and intermixed a third mixture  72  would be formed having a fuel/air equivalence ratio less than 1.0. 
     The first fuel  54  and the first air  52  are introduced into a first common area, such as a conduit  56  where the first fuel  54  and the first air  52  travel together and intermix to form the first mixture  48 . The first mixture  48  is then oxidized in the presence of the catalyst  50  as the first mixture flows over and comes in contact therewith producing the first reacted mixture  48 . In the case of a gas turbine employing standard hydrocarbon fuels, the oxidation will be exothermic generating a heat of reaction. 
     The first reacted mixture  48  is then introduced into a second common area such as the first duct  18 , which also serves as a post mixing chamber for the cooling air flow  60  and the first reacted mixture  48 , along with the second air  46  where the first reacted mixture  48  and the second air  46  travel along and intermix to form the second mixture  49 . To form the second mixture  49 , the first reacted mixture  48  must not auto-ignite upon exiting the single flow channel exit  44  and contacting the second air  46 . Whether the first reacted mixture  48  will auto-ignite upon contact with the second air  46  is application specific and dependent upon such factors as the temperature and flow velocity of the first reacted mixture  48 . For a more complete discussion see U.S. patent application Ser. No. 09/527,708 titled “Method and Apparatus for a Fuel Rich Catalytic Reactor” that is assigned to the same assignee as the present application, namely Precision Combustion, Inc., and the disclosure of which is incorporated herein in its entirety. 
     The second mixture  49  is then combusted. Depending upon the conditions, the second mixture  49  may have to be ignited to begin combustion. The first duct  18  while depicted as being generally cylindrical, may in certain situations be non-cylindrical and even have a decreasing cross-section. A decreasing cross-section could assist in increasing the velocity of the first reacted mixture  48  and the second air  46  as the two mix to form the second mixture  49 , thereby decreasing potential of an autoignition event within the first duct  18 . If the cross-section is decreased as described above, a flame stabilizer such as a dump might be required at the end of the decreasing cross-section to anchor the combustion of the second mixture  49 . 
     Simultaneously with the above, the third air  68  and the second fuel  70  are introduced into a common area of mixer  20  so that the third air  68  and the second fuel  70  travel together and intermix to form the third mixture  72 . The third mixture  72  is then combusted. The combusting second mixture  49  is combusted in contact with the combusting third mixture  72 . 
     As those skilled in the art of combustion engineering will appreciate, adjustment of the fuel/air equivalence ratios within the parameters discussed above will determine the amount of NO x  produced by the main burner. As discussed above, NO x  formation occurs at elevated temperatures and fuel/air equivalence ratios can be adjusted to limit the resulting combustion temperatures resulting from the second and third mixtures. To achieve these NO x  reductions however, the mixtures must be highly mixed. The first and second ducts must be of sufficient lengths to permit the desired degree of mixing. Generally, the second fuel and third air, and the first reacted mixture and the second air must have an unmixedness no greater than about 20 percent with a range of between 2 and 10 percent being desired. Above these limits, the mixtures will not be burning as substantially premixed mixtures thus NO x  reductions will be minimized by significant high-temperature combustion within the mixture. 
     An additional consideration is the velocity of the second fuel and third air and the first reacted mixture and second air through the main mixer and the catalytic pilot, respectively. These mixtures must travel at sufficient velocity to prevent flashback, i.e. a flame traveling toward the fuel source of the flame, in this case entering the first and/or second duct. Velocity is also critical for the second and third mixtures. The velocity of these mixtures must allow for stable combustion. These velocities and calculations thereof are well within the knowledge and skill of those in combustion engineering. 
     As an option, a portion of the heat of reaction can be transferred into the second air  46 . The heat of reaction raises the temperature of the first reacted mixture  48 . If some of this heat of reaction is transferred to the second air  46 , the temperature of the second air  46  will be increased. As disclosed in U.S. patent application Ser. No. 09/527,708, the incorporation of the heat of reaction into the second air  46  will lower overall NO x  formation of the catalytic pilot. 
     While preferred embodiments have been shown and described, various modification and substitutions may be made without departing from the spirit and scope of the invention. Accordingly, it is understood that the present invention has been described by way of example, and not by limitation.