Catalytic combustor having fuel flow control responsive to measured combustion parameters

A gas turbine combustor (23) includes a catalytic combustion stage (22) receiving a first portion (18) of a total oxidizer flow (16) and a first portion (30) of a total fuel flow (29) and discharging a partially oxidized fuel/oxidizer mixture (40) into a post catalytic combustion stage (24) defined by a combustion liner (58). The combustor further includes an injector scoop (54) having an injector scoop inlet (56) in fluid communication with an opening (56) in the combustion liner for receiving a second portion (20) of the oxidizer flow. A fuel outlet (e.g. 64) selectively supplies a second portion (42) of the total fuel flow into the second portion of the oxidizer flow. The injector scoop includes an injector scoop outlet (66) in fluid communication with the post catalytic combustion stage and discharges a fuel/oxidizer mixture (44) into the partially combusted fuel/oxidizer mixture at an angle relative to the flow axis to impart a swirl to the fuel/oxidizer mixture as it enters the post catalytic combustion stage.

FIELD OF THE INVENTION

This invention relates generally to gas turbines, and more particularly, to a catalytic combustor for a gas turbine.

BACKGROUND OF THE INVENTION

Catalytic combustion systems are well known in gas turbine applications to reduce the creation of pollutants, such as NOx, in the combustion process. One catalytic combustion technique known as the rich catalytic, lean burn (RCL) combustion process includes mixing fuel with a first portion of compressed air to form a rich fuel mixture. The rich fuel mixture is passed over a catalytic surface and partially oxidized, or combusted, by catalytic action. Activation of the catalytic surface is achieved when the temperature of the rich fuel mixture is elevated to a temperature at which the catalytic surface becomes active. Typically, compression raises the temperature of the air mixed with the fuel to form a rich fuel mixture having a temperature sufficiently high to activate the catalytic surface. After passing over the catalytic surface, the resulting partially oxidized rich fuel mixture is then mixed with a second portion of compressed air in a downstream combustion zone to produce a heated lean combustion mixture for completing the combustion process. Catalytic combustion reactions may produce less NOx and other pollutants, such as carbon monoxide and hydrocarbons, than pollutants produced by homogenous combustion.

U.S. Pat. No. 6,174,159 describes a catalytic oxidation method and apparatus for a gas turbine utilizing a backside cooled design. Multiple cooling conduits, such as tubes, are coated on the outside diameter with a catalytic material and are supported in a catalytic reactor. A portion of a fuel/oxidant mixture is passed over the catalyst coated cooling conduits and is oxidized, while simultaneously, a portion of the fuel/oxidant enters the multiple cooling conduits and cools the catalyst. The exothermally catalyzed fluid then exits the catalytic oxidation zone and is mixed with the cooling fluid in a downstream post catalytic oxidation zone defined by a combustor liner, creating a heated, combustible mixture.

Typically, gas turbines using catalytic combustion techniques are designed to operate using a fuel having a certain heating value within a predetermined range. The heating value is the amount of energy released when the fuel is burned. However, it may be desired to operate the gas turbine using fuels having heating values outside the predetermined range. If the heating value of the fuel is lower than the predetermined range, the flow rate of the fuel must be increased to obtain the same temperature in the combustion zone.

DETAILED DESCRIPTION OF THE INVENTION

In some applications, it may be desired to operate a gas turbine using a fuel having a heat capacity rating lower than the rating of a fuel normally used to fire the gas turbine. For example, a gas turbine may be designed to operate efficiently with a fuel having a relatively higher heating value (high BTU rating) such as natural gas, instead of a fuel having a lower heat capacity rating (low BTU rating), such as syngas. However, to operate such a gas turbine using a lower BTU fuel, a higher flow volume of fuel may be required to maintain a desired heat output in the combustor. Fuel supply and fuel mixing channels configured for operation with a relatively high BTU rated fuel may be too small to support an additional fuel volume required to operate the gas turbine with the lower BTU fuel. Because of the comparatively large surface area required for catalytic combustion, pressure drop through the combustion system is an important design consideration. By using a lower BTU fuel, a total flow rate of fuel through a catalytic portion of a catalytic combustor will need to be increased significantly compared to using a higher BTU fuel, resulting in an unacceptable pressure drop through the catalytic portion of the catalytic combustor catalyst. Another area of concern when using a low BTU fuel is the fuel injection system of the combustor. Significant changes in the fuel flow rates will require a change in the fuel injection system to obtain an optimized fuel air mixture at the catalyst section of the combustor. Inadequate fuel mixing may result in a decrease in catalytic reaction performance and may result in overheating. The inventors of the present invention have innovatively realized that a catalytic gas turbine designed for operation with a higher BTU fuel may be operated with a lower BTU fuel by injecting a portion of the lower BTU fuel supplied to a catalytic combustor into a post catalytic combustion stage downstream of a catalytic combustion stage. Advantageously, the gas turbine may be operated using fuels having a wider range of heating values than is possible using a conventional catalytically fired gas turbine.

FIG. 1illustrates a gas turbine engine10having a compressor12for receiving an oxidizer flow14, such as filtered ambient air, and for producing a compressed oxidizer flow16. The compressed oxidizer flow16may be separated into a first portion18of the compressed oxidizer flow for introduction into a catalytic combustion stage22of a combustor23, and a second portion20of the compressed oxidizer flow for introduction into a post catalytic combustion stage24of the combustor23. The first portion18of the oxidizer flow may be further separated into a backside cooling air flow26and combustion mixture air flow28. The combustion mixture airflow28is mixed with a first portion30of a combustible fuel29, such as natural gas or fuel oil, for example, provided by a fuel source32, prior to introduction into the catalytic combustion stage22. The backside cooling air flow26may be introduced directly into the catalytic combustion stage22without mixing with a combustible fuel29. In an aspect of the invention, the combustion mixture air flow28may comprise about 15% by volume of the first portion18of the compressed oxidizer flow16, and the backside cooling air flow26may comprise about 85% by volume of the first portion18to achieve catalytic combustion having desired combustion parameters.

Inside the catalytic combustion stage22, the combustion mixture air flow28and the backside cooling air flow26may be separated by a pressure boundary element36. The pressure boundary element36may be coated with a catalytic material38on a side exposed to the combustion mixture air flow28. While exposed to the catalytic material38, the combustion mixture air flow28is partially oxidized in an exothermic reaction. The backside cooling air flow26passing on an opposite side of the pressure boundary element36absorbs a portion of the heat produced by the exothermic reaction, thereby cooling the catalytic material38and the pressure boundary element36. After the flows26,28exit the catalytic combustion stage22, the flows26,28are mixed and further combusted in the post catalytic combustion stage24to produce a partially combusted mixture40.

In an aspect of the invention, a second portion42of the combustible fuel may be mixed with the second portion20of the compressed oxidizer flow16to form a post catalytic combustion mixture44for introduction into the post catalytic combustion stage24. The second portion42of the combustible fuel and the second portion20of the compressed oxidizer flow16may be provided to a flow directing element, such as an injector scoop54, for injecting the portions20,42into the post catalytic combustion stage24. The portions20,42may be mixed in the scoop54to form the post catalytic combustion mixture44before being injected into the post catalytic combustion stage24.

A controller34, responsive to a sensor49monitoring a parameter responsive to combustion in the post catalytic combustion stage24may be configured to control the portions30,42of the combustible fuel provided to the catalytic stage22and post catalytic combustion stage24, respectively. For example, as a result of using a lower BTU fuel in the gas turbine, combustion conditions in the post catalytic combustion stage24may be different from combustion conditions using a higher BTU fuel. The controller34may be configured to monitor changes in parameters (for example, as a result of using a lower BTU fuel) such as temperature, oxides of nitrogen (NOx) emission, a carbon monoxide (CO) emission, and/or a pressure oscillation and to adjust the portions30,42supplied to the respective stages22,24. For example, an amount of the second portion42supplied to the post catalytic combustion stage24may need to be increased when using a lower BTU fuel to more than that required when using a higher BTU fuel. The controller34may be configured to independently control valves31and41via respective control signals33and43, to regulate flows30,42in response to sensed combustion parameters. In another aspect of the invention, the controller34may be responsive to a sensor47sensing a temperature of the catalytic material38to control the portions30,42of the combustible fuel provided to the catalytic stage22and post catalytic combustion stage24, respectively. Other parameters indicative of combustion operations in the combustor23may also be monitored to determine an appropriate apportioning of the portions30,42provided to the respective stages22,24to achieve desired combustion conditions, for example, based on a BTU rating of a fuel used to fire the combustor23. If the combustor23is fueled with a fuel having a BTU rating within a predetermined range, it may not be necessary to provide the portion42of fuel and/or the portion20of the oxidizer to the post catalytic combustion stage24. In another aspect, the portion20of the oxidizer provided to the injector scoop54may be controlled by an air control valve72, such a hinged flap, operable to selectively control the portion20of the oxidizer entering the scoop54. For example, when using a fuel with a high BTU value, the air control valve72may be closed. When firing the combustor with a low BTU value fuel, the air control valve72may be opened to allow a desired flow of the portion20of the oxidizer to enter the scoop54.

In the post catalytic combustion stage24, the post catalytic combustion mixture44and the partially combusted mixture40are mixed and further combusted to produce a hot combustion gas46. A turbine48receives the hot combustion gas46, where it is expanded to extract mechanical shaft power. In one embodiment, a common shaft50interconnects the turbine48with the compressor12as well as an electrical generator (not shown) to provide mechanical power for compressing the ambient air14and for producing electrical power, respectively. An expanded combustion gas52may be exhausted directly to the atmosphere, or it may be routed through additional heat recovery systems (not shown).

FIG. 2shows an injector scoop54in fluid communication with an opening56in a combustion liner58of the post catalytic combustion stage24ofFIG. 1. The injector scoop54may be disposed to receive the second portion20of the oxidizer flow16flowing around an exterior of the combustor liner58, while the first portion18may be directed to travel further upstream for introduction into a catalytic combustor stage (not shown). In an embodiment, the second portion20may comprise 15% to 20% by volume of the oxidizer flow16, while the first portion18may comprise 80% to 85% by volume of the oxidizer flow16. A fuel manifold64may be located in the scoop54for receiving the second portion42of the fuel29and injecting the second portion42into the second portion20of the oxidizer flow16to produce the post catalytic combustion mixture44. For example, the fuel manifold64may be located at the inlet56of the coop54to direct a plurality of fuel jets66into the second portion20of the oxidizer low16. In an aspect of the invention, the fuel jets66may be oriented to direct fuel perpendicularly into a flow direction of the second portion20of the oxidizer flow16.

The scoop54includes an outlet66in fluid communication with the opening56of the combustion liner for discharging the post catalytic combustion mixture44into the post catalytic combustion stage24to mix with the partially combusted mixture40flowing therethrough. In an aspect of the invention, the scoop54may be disposed at an angle68relative to a flow axis70through the post catalytic combustion stage24to impart a swirl, or helical motion, to the partially combusted mixture40as the post catalytic combustion mixture44enters the post catalytic combustion stage24. For example, the scoop may be disposed at an angle68between 15 degrees to 45 degrees relative to the flow axis70. By injecting the post catalytic combustion mixture44at an angle to the flow axis70(instead of injecting the post catalytic combustion mixture44coaxially with the flow axis70), improved mixing of the two mixtures40,44may be achieved, thereby improving flame stability. In an embodiment, a plurality of scoops54may be disposed circumferentially around the combustor liner58to inject the post catalytic combustion mixture44into the post catalytic combustion stage24through corresponding openings56in combustor liner58.

In an aspect of the invention, the injector scoop54may be configured as a ram injector scoop54configured to increase a velocity of a fluid flow therethrough. For example, an inlet56of the scoop45may comprise a larger cross sectional area than a cross sectional area of the outlet56so that a total velocity magnitude of the post catalytic combustion mixture44entering the post catalytic combustion stage24is accelerated to be greater than a velocity of an axial velocity of the partially combusted mixture40to avoid flame holding at the scoop outlet within the post catalytic combustion stage24. In an embodiment, the scoop54may be formed in the shape of a wedge having an inlet56at an upstream end60of the wedge and tapering to a thinner cross section at a downstream end62. The scoop54may be formed integrally with the combustor liner58or may be fabricated separately and attached to the combustor liner58, such as by brazing or welding.