Dynamic control system and method for multi-combustor catalytic gas turbine engine Number:7,152,409 from the United States Patent and Trademark Office (PTO) owispatent

Title: Dynamic control system and method for multi-combustor catalytic gas turbine engine

Abstract: According to one aspect, a method of controlling a multi-combustor catalytic combustion system is provided for determining a characteristic of a fuel-air mixture downstream of a preburner associated with a catalytic combustor and adjusting the fuel flow to the preburner based on the characteristic. The characteristic may include, for example, a measurement of the preburner or catalyst outlet temperature or a determination of the position of the homogeneous combustion wave in the burnout zone of the combustor.

Patent Number: 7,152,409 Issued on 12/26/2006 to Yee,   et al.

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Inventors:	Yee; David K. (Hayward, CA), Reppen; Dag (Chandler, AZ)
Assignee:	Kawasaki Jukogyo Kabushiki Kaisha (Kobe, JP)
Appl. No.:	10/758,879
Filed:	January 16, 2004

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Current U.S. Class:	60/777 ; 431/7; 60/39.281; 60/723
Current International Class:	F02C 9/00 (20060101); F23R 3/40 (20060101)
Field of Search:	60/39.27,39.281,723,777 431/7,170

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Primary Examiner: Casaregola; L. J.
Attorney, Agent or Firm: Morrison & Foerster LLP

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Parent Case Text

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CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefit of earlier filed provisional patent application, U.S. application Ser. No. 60/440,940, filed on Jan. 17, 2003, and entitled "DYNAMIC CONTROL SYSTEM AND METHOD FOR MULTI-COMBUSTOR CATALYTIC GAS TURBINE ENGINE," which is hereby incorporated by reference as if fully set forth herein.

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Claims

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The invention claimed is:

1. A method of controlling a multi-combustor catalytic combustion system comprising the acts of: determining a temperature downstream of a preburner associated with a catalytic combustor in a multi-combustor system, wherein the preburner includes two or more fuel stages and wherein fuel flow to the two or more fuel stages is determined based upon a fixed fuel split schedule during an ignition sequence; and adjusting the fuel flow to the preburner based on the temperature.

2. The method of claim 1, wherein the preburner includes a flame burner.

3. The method of claim 1, wherein the preburner includes one or more fuel orifices that are sized proportional to the airflow of the combustor.

4. The method of claim 1, wherein one or more fuel orifices supplying fuel to a catalyst of the catalytic combustor are sized proportional to the airflow of the combustor.

5. The method of claim 1, wherein the system includes at least a second preburner associated with at least a second catalytic combustor, and the fuel flow to each preburner is proportional to the airflow through each combustor.

6. The method of claim 5, wherein closed loop control on a single preburner is used to determine fuel flow to all preburners in the multi-combustor system.

7. The method of claim 1, wherein the act of adjusting the fuel flow to the preburner includes closed loop control on the preburner outlet temperature.

8. The method claim 1, wherein the act of adjusting the fuel flow to the preburner includes closed loop control on a catalyst inlet temperature.

9. The method of claim 1, wherein the act of adjusting the fuel flow to the preburner includes closed loop control on a catalyst outlet temperature.

10. The method claim 1, wherein the system includes at least a second preburner associated with at least a second combustor, and the act of adjusting the fuel flow to the preburner compensates for combustor-to-combustor variations.

11. The method of claim 10, wherein the combustor-to-combustor variations include a variation in at least one of preburner ignition delay, catalyst light-off temperature, and a position of homogeneous combustion in a burnout zone.

12. The method of claim 11, wherein the fuel flow is adjusted to vary the position of a homogeneous combustion wave in the burnout zone.

13. The method of claim 12, wherein the position of the homogeneous combustion wave in the burnout zone is determined by dual UV' sensors disposed in the burnout zone.

14. The method claim 1, further including the act of adjusting an airflow through at least one of the preburner and the combustor.

15. The method of claim 14, wherein the act of adjusting the airflow through at least one of the preburner and the combustor includes adjusting dilution holes in the preburner.

16. The method of claim 14, wherein the act of adjusting the airflow through at least one of the preburner and the combustor includes varying at least one of a bypass valve and a bleed valve associated with the combustor.

17. The method of claim 14, wherein in a closed loop fuel control, the preburner is used to determine fuel flow to at least a second preburner associated with at least a second combustor.

18. A method of controlling a multi-combustor catalytic combustion system comprising the acts of: varying at least one of a fuel flow and an airflow to a plurality of combustors; and controlling the location of a homogeneous combustion wave in each of the plurality of catalytic combustors.

19. The method of claim 18, wherein the fuel flow or the airflow is varied based upon feedback from an ignition delay calculation.

20. The method of claim 18, wherein the fuel flow is varied based upon feedback from at least one of a measure of a catalyst inlet gas temperature, catalyst exit gas temperature, and combustor airflow.

21. The method of claim 18, wherein the airflow is varied based upon feedback from at least one of a measure of a catalyst inlet gas temperature, catalyst exit gas temperature, and combustor fuel flow.

22. The method of claim 21, wherein the airflow to each combustor is varied by a bypass valve.

23. The method of claim 21, wherein the airflow to each combustor is varied by a bleed valve.

24. The method of claim 18, wherein at least one of the fuel flow and the airflow is varied based upon feedback from two W sensors placed in the burnout zone of at least one combustor.

25. The method of claim 24, wherein at least one of the fuel flow and the airflow is varied based upon feedback from two sets of two UV sensors placed in the burnout zone of two combustors.

26. The method of claim 25, wherein the two combustors include a minimum mass flow combustor and a maximum mass flow combustor of the plurality of combustors.

27. The method of claim 18, wherein at least one of the fuel flow and the airflow is varied based upon feedback from a measure of the relative uniformity of the exhaust gas temperature.

28. The method of claim 18, wherein at least one of a fuel flow and an airflow to a preburner is varied, the preburner being associated with at least one of the catalytic combustors.

29. The method of claim 18, wherein at least one of a fuel flow and an airflow to the catalyst is varied.

30. A method of controlling a multi-combustor catalytic combustion system comprising the acts of: determining a first characteristic of operation for at least one combustor in a multi-combustor system; determining a second characteristic of operation for the multi-combustor system; and controlling the system based upon feedback from the first characteristic and the second characteristic, wherein the first characteristic includes the position of a homogenous combustion wave.

31. A method of controlling a multi-combustor catalytic combustion system comprising the acts of: determining a first characteristic of operation for at least one combustor in a multi- combustor system; determining a second characteristic of operation for the multi-combustor system; and controlling the system based upon feedback from the first characteristic and the second characteristic, wherein the second characteristic includes a measure of CO emissions.

32. The method of claim 31, wherein the second characteristic includes a measure of CO emissions from all combustors in the multi-combustor system.

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Description

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BACKGROUND

1. Field of the Invention

The invention relates generally to combustion control systems, and more particularly to dynamic control systems and methods for use with multi-combustor processes as they relate to and are utilized by gas turbine engines with catalytic combustors.

2. Description of the Related Art

In a conventional gas turbine engine, the engine is controlled by monitoring the speed of the engine and adding a proper amount of fuel to control the engine speed. Specifically, should the engine speed decrease, fuel flow is increased causing the engine speed to increase. Similarly, should the engine speed increase, fuel flow is decreased causing the engine speed to decrease. In this case, the engine speed is the control variable or process variable monitored for control.

A similar engine control strategy is used when the gas turbine is connected to an AC electrical grid in which the engine speed is held constant as a result of the coupling of the generator to the grid frequency. In such a case, the total fuel flow to the engine may be controlled to provide a given power output level or to run to maximum power with such control based on controlling exhaust gas temperature, turbine inlet temperature, or some other engine fundamental. Again, as the control variable rises above a set point, the fuel is decreased. Alternatively, as the control variable drops below the set point, the fuel flow is increased. This control strategy is essentially a feedback control strategy with the fuel control valve varied based on the value of a control or process variable compared to a set point.

In a typical non-catalytic combustion system using a diffusion flame burner or a simple lean premixed burner, the combustor has only one fuel injector. In such systems, a single valve is typically used to control the fuel flow to the engine. In more recent lean premix systems however, there may be two or more fuel flows to different parts of the combustor, with such a system thus having two or more control valves. In such systems, closed loop control may be based on controlling the total fuel flow based on the required power output of the gas turbine while fixed (pre-calculated) percentages of flow are diverted to the various parts of the combustor. In addition, the desired fuel split percentages between the various fuel pathways (leading to various parts of the combustor) may either be a function of certain input variables or they may be based on a calculation algorithm using process inputs such as temperatures, airflow, pressures, and the like. Such control systems offer ease of control due primarily to the very wide operating ranges of these conventional combustors and the ability of the turbine to withstand short spikes of high temperature without damage to various turbine components. Moreover, the fuel/air ratio fed to these combustors may advantageously vary over a wide range with the combustor remaining operational.

The configuration of industrial gas turbines with conventional, non-catalytic combustors, varies from simple single-silo configurations, i.e., one combustor as discussed above, to multiple-combustor configurations. The application of industrial, or otherwise, gas turbine engines with catalytic combustion, however, has been limited to the single-silo configuration. For example, the Kawasaki M1A-13X and the GE 10 (PGT 10B) gas turbine engines. A properly operated single-silo catalytic combustion system may provide significantly reduced emissions levels, particularly of NO.sub.x over conventional diffusion flame or lean premixed burners. Unfortunately, however, such systems may have a much more limited window of operation compared to conventional diffusion flame combustors. For example, fuel/air ratios above a certain limit may cause the catalyst to overheat and lose catalytic activity in a very short time. In addition, the catalyst inlet temperature may have to be adjusted as the engine load is changed or as ambient temperature or other operating conditions change to keep NO.sub.x production low.

The application of catalytic combustion in a multi-combustor configuration poses several additional problems. For example, in a multi-combustor configuration there typically are variations from combustor-to-combustor due to manufacturing or design differences that may lead to variations in pre-burner ignition, catalyst light-off, and/or homogeneous combustion in the burnout zone across the multiple combustors. Additionally, the combustor sizes are typically reduced to prevent combustor-to-combustor physical interference adding complexity to the design of the combustors. Combustor size reduction can be achieved through flame-holders in the burn-out zone and single-stage catalyst designs. To supplement the single stage catalyst designs, pre-burners with increased turn-down ratios are generally used. These design changes will require more complex control of the pre-burner and/or post catalyst homogenous combustion burnout zone. What is needed therefore is a method and system for controlling catalytic combustion in a multi-combustor system.

BRIEF SUMMARY OF THE INVENTION

According to one aspect, a method of controlling a multi-combustor catalytic combustion system includes determining a characteristic of a fuel-air mixture downstream of a preburner associated with a catalytic combustor and adjusting the fuel flow and/or airflow to the preburner based on the characteristic. The characteristic may include, for example, a measurement of the preburner or catalyst outlet temperature or a determination of the position of the homogeneous combustion wave in the burnout zone of the combustor.

According to another aspect, a method of controlling a multi-combustor catalytic combustion system includes the acts of determining a first characteristic of operation for at least one combustor of the system, determining a second characteristic of operation for the whole system, and controlling the system based upon feedback from the first characteristic and the second characteristic. The first characteristic may include a catalyst exit temperature or the like and the second characteristic may include a measure of CO emissions or the like.

The present invention is better understood upon consideration of the detailed description below in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary gas turbine system;

FIG. 2 illustrates an exemplary catalytic combustion system;

FIG. 2A illustrates an exemplary catalytic combustion system;

FIG. 3 illustrates an exemplary catalytic combustion system with associated temperature and fuel concentration profiles;

FIGS. 4A, 4B, and 4C illustrate an exemplary catalytic combustion system with varying location of the post catalyst homogeneous wave;

FIG. 5 illustrates an exemplary control method for a multiple combustor system;

FIG. 6 illustrates an exemplary catalytic combustion system with UV sensors and a thermocouple sensor;

FIG. 7 illustrates an exemplary catalytic combustion system with a bypass valve and a bleed valve;

FIG. 8 illustrates an illustrates an exemplary control method for a multiple combustor system;

FIGS. 9A 9D illustrate exemplary operation of a combustor system with UV sensors;

FIG. 10 illustrates an exemplary control method for a multiple combustor system;

FIG. 11 illustrates an exemplary control method for a multiple combustor system; and

FIGS. 12 and 13 illustrate exemplary control methods for a multiple combustor system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a catalytic multi-combustor system and associated methods of operation. The following description is presented to enable any person of ordinary skill in the art to make and use the invention. Descriptions of specific applications are provided only as examples. Various modifications to the exemplary embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the examples shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

Exemplary methods and systems are described herein for improved control strategies for an efficient application of multi-combustor catalytic combustion system configurations for gas turbine engines. Various methods described herein address issues relating to igniting and controlling multiple pre-burners associated with the combustors as well achieving uniform homogeneous combustion in the burnout zone across multiple combustors.

FIG. 1 schematically illustrates an exemplary catalytic multi-combustor gas turbine system. Compressor 1-1 ingests ambient air 1-2 through a compressor bellmouth, and compresses the air to a higher pressure and drives the compressed air, at least in part, through two or more combustors 1-3 and through the drive turbine 1-4. Although only two combustors 1-3 are shown, the gas turbine engine may include any number of a plurality of combustors 1-3 located about the periphery of the gas turbine as is known in the art for conventional multi-combustor gas turbine engines. Each combustor 1-3 mixes fuel and air 1-2 and combusts the mixture to form a hot, high velocity gas stream that flows through the turbine 1-4. The high velocity gas stream provides power to drive turbine 1-4 and the load 1-5. Load 1-5 may be, for example, a generator or the like.

FIG. 2 is a close-up view of one combustor 1-3 of the multiple combustor configuration of FIG. 1. Specifically, as shown in FIG. 2, a catalytic combustor 2-6 is provided. In this example, catalytic combustor 2-6 includes four major elements that are arrayed serially in the flow path of at least a portion of the air from the compressor discharge 2-14. Specifically, these four elements include a preburner 2-20, for example a flame preburner (which is positioned upstream of the catalyst and which produces a hot gas mixture 2-7), a fuel injection and mixing system 2-8, a catalyst 2-10, and a burnout zone 2-11. The exiting hot gases from the combustion system flow into the drive turbine 2-15 to produce power that may drive a load. In one example, there are two independently controlled fuel streams, with one stream 2-24 directed to a preburner 2-20 and the other stream 2-25 being directed to the catalyst fuel injection and mixing system 2-8, as shown. Further, in some examples multiple preburner zones or fuel stages may be employed with additional independently controlled fuel streams for each fuel stage of preburner 2-20. For example. FIG. 2A illustrates an exemplary catalytic combustor 2-6a, which is similar to catalytic combustor 2-6 of FIG. 2 (and labeled similarly), except that catalytic combustor 2-6a includes a multi-stage preburner 2-20a having multiple fuel stream 2-24a, one for each stage of preburner 2-20a.

In one example, catalytic combustor 2-6 may generally operate in the following manner. The majority of the air from the gas turbine compressor discharge 2-14 flows through the preburner 2-20 and catalyst 2-10. Preburner 2-20 functions to help start up the gas turbine and to adjust the temperature of the air and fuel mixture prior to the catalyst 2-10 at location 2-9. For instance, preburner 2-20 heats the air and fuel mixture to a level that will support catalytic combustion of the main fuel stream 2-25, which is injected and mixed with the flame burner discharge gases (by catalyst fuel injection and mixing system 2-8) prior to entering catalyst 2-10. Preburner 2-20 may further be used to adjust the catalyst 2-10 inlet temperature by varying, for example, the fuel or air supply to the preburner 2-20. Ignition of each combustor 2-6 may be achieved by means of a spark plug or the like in conjunction with cross fire tubes (not shown) linking the various combustors 2-6 as is known in the art.

Partial combustion of the fuel/air mixture occurs in catalyst 2-10, with the balance of the combustion occurring in the burnout zone 2-11, located downstream of the exit face of catalyst 2-10. Typically, 10% 90% of the fuel is combusted in catalyst 2-10. For example, to fit the general requirements of the gas turbine operating cycle including achieving low emissions, while obtaining good catalyst durability, 20% 70% of the fuel is combusted in catalyst 2-10, and in one example between about 30% to about 60% is combusted in catalyst 2-10. In various aspects, catalyst 2-10 may consist of either a single stage (as shown) or a multiple stage catalyst including multiple catalysts 2-10 serially located within the combustor 2-6.

Reaction of any remaining fuel not combusted in the catalyst and the reaction of any remaining carbon monoxide to carbon dioxide occurs in burnout zone 2-11, thereby advantageously obtaining higher temperatures without subjecting the catalyst to these temperatures and obtaining very low levels of unburned hydrocarbons and carbon monoxide. After complete combustion has occurred in burnout zone 2-11, any cooling air or remaining compressor discharge air may be introduced into the hot gas stream at 2-15, typically located just upstream of the turbine inlet. In addition, if desired, air can optionally be introduced through liner wall 2-27 at a location close to the turbine inlet 2-15 as a means to adjust the temperature profile to that required by the turbine section at location 2-15. Such air introduction to adjust the temperature profile may be one of the design parameters for power turbine 2-15. Another reason to introduce air through liner 2-27 in the region near the turbine 2-15 would be for turbines with very low inlet temperatures at 2-15. For example, some turbines have turbine inlet temperatures in the range of 900 to 1100.degree. C., temperatures too low to completely combust the remaining unburned hydrocarbons and carbon monoxide within the residence time of the burnout zone 2-11. In these cases, a significant fraction of the air may be diverted through the liner 2-27 in the region near turbine 2-15. This allows for a higher temperature in region 2-11 for rapid and complete combustion of the remaining fuel and carbon monoxide.

FIG. 3 illustrates an example of a typical existing partial combustion catalyst system corresponding to the system shown in FIGS. 1 and 2 and will be discussed in greater detail below. In such systems, only a portion of the fuel is combusted within the catalyst and a significant portion of the fuel is combusted downstream of the catalyst in a post catalyst homogeneous combustion zone. Further examples of partial combustion catalyst systems and approaches to their use are described in co-pending patent application and prior patents, for example: U.S. patent application Ser. No. 10/071,749 to D. Yee et al.; U.S. Pat. Nos. 5,183,401, 5,232,357, 5,250,489, and 5,281,128 to Dalla Betta et al.; and U.S. Pat. No. 5,425,632 to Tsurumi et al., all of which are incorporated herein by reference in their entirety.

I. Igniting and Controlling Multiple Pre-Burners:

Igniters located within each combustor may ignite the flame or preburner of each combustor. For example, preburner 2-20 of FIG. 2 may be ignited by an igniter (not shown) located in combustor 2-6. In other configurations, an igniter may be located in every other combustor 2-6 with cross-fire tubes disposed between combustors 2-6, or any other combination of igniters and cross-fire tubes, such that each preburner 2-20 is in physical contact with a fully ignited preburner 2-20. Confirmation of preburner 2-20 ignition may be determined by a measurement of the pre-burner 2-20 exit temperature with a thermocouple, a UV-sensor disposed in the preburner 2-20 "flame" region, or any other suitable method to confirm preburner ignition.

Fuel flow to the preburner 2-20 of each combustor 2-6 may be controlled during ignition of each preburner 2-20 and thereafter to control the outlet temperature of the preburner 2-20 as well as the inlet temperature of the fuel-air mixture entering the catalyst 2-10. In some examples, the preburner 2-20 of each combustor 2-6 may include more than two fuel stages adding complexity to the ignition and control process in a multi-combustor system. In one exemplary method of operation, theoretical flame temperature control is used in the first stage to control NO.sub.x. Such a method is described in more detail in co-pending U.S. patent application Ser. No. 10/071,749, which is incorporated herein in its entirety by reference. The fuel flow to the third stage is limited to zero while allowing the second stage to perform closed loop temperature control up to a limit of the fuel flow, outlet temperature, pre-burner temperature rise, or theoretical flame temperature of the second stage. The secondary fuel flow (or theoretical flame temperature) may then be fixed and third stage fuel flow commenced. Closed loop temperature control may then be performed on the outlet temperature of the pre-burner 2-20 to determine fuel flow to the preburner.

In another exemplary method of operation, the total fuel flow to the preburner is based upon closed loop control on the pre-burner 2-20 outlet temperature. The total preburner fuel flow is distributed to each stage of the preburner based on an exemplary fixed fuel split schedule as shown in the table below:

TABLE-US-00001 Total pre-burner fuel flow First stage pre- Second stage pre- Third stage pre- (mass/time) burner burner burner 0 100% 0% 0% 100 100% 0% 0% 200 50% 50% 0% 300 33% 67% 0% 400 25% 50% 25% 500 20% 40% 60%

It should be recognized by those skilled in the art that the above method and table are illustrative only and that other similar schedules and methods may be used within the scope of the invention to ignite and control multiple combustors. For example, different ratios for each stage may be used as well as fewer or additional preburner stages. Further, in addition to controlling the ignition process, the above methods may be used to control the catalyst inlet temperature and thereby the catalytic combustion processes downstream of the preburner.

Each preburner 2-20 of each combustor 2-26 in the multi-combustor system may similarly be controlled to ensure similar preburner outlet temperatures, catalyst inlet temperature, or catalyst outlet temperatures across the multiple combustors. Closed loop temperature control on preburner outlet temperature T34, catalyst inlet temperature T36, catalyst interstage or catalyst outlet temperature T37 (see FIG. 2) of each combustor may be used to control the preburner of each combustor through fuel valve control (of single or multiple valves for each stage), and thereby compensate for combustor-to-combustor variations within the multi-combustor system. One exemplary method for closed loop control based on catalyst outlet gas temperature T37 feedback is illustrated in FIG. 5.

As seen in FIG. 5, the multiple combustors of the combustor process 52 are controlled by determining a main fuel flow, i.e., to the catalyst, and a secondary fuel flow, i.e., to the preburner, from various factors such as temperature measurements, fuel flow and/or airflow calculations, and the like. In this example, a fixed fuel split schedule based on the total fuel flow to the combustor is output from block 5-6. Fuel schedules may have various schemes including fixed fuel schedules to determine fuel demand to the preburner and catalyst based on a control variable such as the engine load or the like.

Block 5-4 determines the main fuel flow Wf, main, i.e., to the catalyst, as the difference between the total fuel flow to the combustor and the sum of the respective fuel flows to the primary and secondary preburners. For example, the schedule of total fuel flow Wf, tot and fuel flow to the first stage fuel valve Wf, pri (or primary preburner) is input to block 5-4 from block 5-6. The fuel flow to the second stage fuel valve Wf,sec (or secondary preburner) determined from the output of the secondary fuel flow switch in block 5-14 (described below) is added to the primary preburner fuel flow Wf,pri.

The fuel flow to the second stage fuel valve Wf,sec is determined in block 5-14 by switching between the output of closed loop feedback control based on catalyst outlet temperature T37 from block 5-18 and a fixed offset secondary fuel demand from block 5-12. The output of block 5-14 switches between the output from block 5-12 and block 5-18 based on the output of block 5-10. Block 5-10 determines if the system is operating in a steady state and if an air bypass valve of the system is at its maximum position, i.e., near a maximum in flow capability. In an example where a bypass valve is not included, the maximum may be set at zero. The fuel flow offset used in block 5-12 is determined in block 5-20 by a difference between the current secondary fuel demand and the secondary fuel demand from the base engine loading control logic output from block 5-6. The offset may be stored in a memory, for example, a non-volatile memory 5-22 or the like so that it may be recalled after the controller is reset.

The demand schedule for fuel flow to the secondary stage may be determined, at least in part, from catalyst exit temperature T37 and used as feedback in block 5-16. The output of block 5-16 in this example is in the form of a preburner outlet temperature demand T34. Accordingly, block 5-18 performs closed loop control on the preburner outlet temperature T34 and outputs the secondary preburner fuel flow demand to the secondary fuel flow switch in block 5-14.

Closed loop control may similarly by used with a measure of the catalyst inlet temperature (not shown in FIG. 5). Further, the multiple combustor feedback process depicted in FIG. 5 may include bypass valve logic 5-8 to control bypass valves. An exemplary bypass valve process is depicted in FIG. 7.

The feedback control methods described may be implemented in hardware, firmware, and/or software suitable to carry out the various methods. For example, firmware commands or the like may be used to address various fuel valves and combustors.

According to another exemplary method, the fuel flow to each combustor may be matched to the airflow of each combustor. Specifically, the primary, second, and third stage fuel manifolds of the preburner may include fuel flow orifices that are configured to "match" the fuel flow to the combustor airflow. For example, a combustor with more airflow would have a larger fuel orifice and a combustor with less airflow would have a smaller fuel orifice. The fuel flow orifices may then be tuned during factory acceptance testing, commissioning, and the like to match the combustor airflow. Tuning the fuel flow orifices may reduce the total number of fuel valves per combustor. For instance, in one example, a single fuel valve may be used for each pre-burner stage of each combustor. Closed loop temperature control on the pre-burner outlet temperature (or catalyst inlet temperature, etc.) measured from one combustor may be the same or similar for all combustors in the system. Closed loop temperature control of one combustor may therefore be used to similarly control all of the combustors based on the measurements of one combustor. Further, control may be based on a global measurement or characteristic of the system, for example, the emission levels or exhaust temperature of the system. In this example, however, there may still be combustor-to-combustor variation in mass flow because of the varying air and fuel flows to each combustor. In some instances, however, the range of minimum to maximum mass flow across the multiple combustors after tuning the fuel orifices may be too large leading to the performance of the maximum mass flow combustors barely meeting CO emissions limits and the minimum mass flow combustor nearly overheating the catalyst. In this case, the minimum and maximum combustor would be monitored and controlled. For example, increase T34/bypass flow until the minimum catalyst combustor is at its maximum temperature and then decrease T34/bypass flow until the maximum catalyst module is at its minimum temperature or until the bulk CO measurement rises.

Alternatively, according to another exemplary method, the airflow may be matched to the fuel flow to the combustor. For example, the pre-burner dilution holes could be "tuned" in a manner similar to matching the fuel manifold orifices in the previous example. Varying the size, shape, etc. of the dilution holes allows the airflow through the combustor to be varied. In this instance, the pre-burner may include tunable or adjustable dilution holes that may be designed, for example, to ensure that by tuning the dilution holes, i.e., opening and/or closing dilution holes, the aerodynamic and structural performance of the pre-burner are not compromised. The dilution holes may include, for example, a plurality of holes, an orifice that may be constricted, vanes to divert airflow, and the like. Closed loop temperature control on the pre-burner outlet temperature, for example, for any one combustor may be the same for all combustors in the system such that all the combustors may be controlled based on the closed loop temperature control of the one combustor. Unlike the previous example, which included tuning the fuel orifices to match the fuel flow to the airflow, tuning the airflow to match the fuel flow should result in similar mass flows from combustor-to-combustor.

II. Homogeneous Combustion in the Burnout Zone:

According to another aspect of the invention, multi-combustor catalytic combustion control methods and systems are provided to ensure uniform combustor-to-combustor homogeneous combustion in the burnout zone.

With reference again to FIG. 3, a linear schematic representation of a simplified partial combustion catalytic system is illustrated with the gas temperature and fuel concentrations at various locations along the flow path shown there below. Air 3-7 enters combustor 3-26 and passes through a fuel injection and mixing system 3-8 that injects fuel into the flowing air stream. A portion of the fuel is combusted in the catalyst 3-10 resulting in an increase in temperature of the gas mixture as it passes through catalyst 3-10. As can be seen, the mixture exiting catalyst 3-10 is at an elevated temperature. This fuel/air mixture contains remaining unburned fuel that undergoes auto-ignition in the post catalyst burnout zone 3-11. The burnout zone 3-11 includes the portion of the flow path downstream of the catalyst but prior to introduction of additional air and before the turbine where the gas mixture exiting the catalyst may undergo further reaction. The fuel is combusted in the burnout zone 3-11 to form final reaction products including CO.sub.2 and H.sub.2O with the temperature rising to the final combustion temperature 3-31 at homogeneous combustion process wave 3-30 (the region where the remaining uncombusted fuel exiting the catalyst is combusted). The resulting hot, high-energy gases in burnout zone 3-11 may drive the power turbine and load (e.g., 1-4 and 1-5 in FIG. 1).

The lower portion of FIG. 3 illustrates a graph with the gas temperature indicated on the ordinate and the position along the combustor, or flow path through the combustor, indicated on the abscissa. The position of the graph corresponding generally to the linear combustor diagram directly above it. As can be seen, the gas temperature increases as the mixture passes through catalyst 3-10 and a portion of the mixture combusts. Downstream of catalyst 3-10, however, the mixture temperature is constant for a period, typically referred to as the ignition delay time 3-32, t.sub.ignition, before the remaining fuel combusts to form the homogeneous combustion process wave 3-30. The combustion of the mixture in the burnout zone 3-11 thereby further raises the gas temperature.

Homogeneous combustion in the burnout zone is primarily determined by the ignition delay time of the gas exiting the catalyst. The ignition delay time and catalyst exit conditions may be controlled such that the position of the homogeneous combustion process wave can be moved and maintained at a desired location or range of locations within the post catalyst reaction zone. The location of the homogeneous combustion process wave 3-30 may therefore be moved by changing, for example, the gas composition, pressure, catalyst outlet/exit temperature, and the adiabatic combustion temperature. For example, by increasing the catalyst outlet temperature to move the location of the homogeneous combustion process closer to the catalyst or decreasing the catalyst outlet temperature to move it farther downstream from the catalyst. In this way, the present control system advantageously keeps the catalyst operation across multiple combustors within a desired operating regime for good catalyst durability while maintaining low emissions. Specifically, when operating in such a regime, emissions of NOx, CO, and unburned hydrocarbons may be reduced while the durability of the catalysts maintained.

In one example, the homogeneous combustion wave is located just downstream of the catalyst but is not so far downstream that a long reaction zone or volume is required of the combustor. Ignition delay time depends, at least in part, on the gas composition (i.e., fuel-to-air mixtures), gas pressure within the combustor, catalyst exit gas temperature, and adiabatic combustion temperature (the temperature of a fuel and air mixture after all of the fuel in the mixture has been combusted with no thermal energy lost to the surroundings). Of these four parameters, the latter two in particular, catalyst exit gas temperature and adiabatic combustion temperature, may be adjusted in real time by an exemplary control system to change the ignition delay within each combustor and compensate for variations from combustor