Fuel injector

Abstract

A method of creating a swirling flow of fuel-lean premixed fuel and air without passage of the flow through swirler vanes is taught. A supply of fuel is mixed with a sufficient quantity of air to form a fuel-lean mixture. The mixture is passed tangentially into an annular flow passage creating a spiraling flow without bulk flow recirculation. The spiraling flow of fuel and air is passed into a combustion chamber. Reacting a fuel-rich mixture of fuel with air in the presence of a catalyst to produce heat and a reaction product may form the supply of fuel. The reaction product is mixed with air to form the fuel-lean fuel-air mixture and then the mixture is passed into an annular chamber in a direction nominally perpendicular to the chamber axis of rotation, thereby creating a flow rotating about the axis of rotation.

Description

CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No. 60/626,542 filed Nov. 10, 2004.

FIELD OF THE INVENTION

The present invention is directed to a method and system for stable combustion of fuels with low NOx emissions. More particularly, the present invention is directed toward systems for lean premixed combustion. In another embodiment, the present invention is directed toward systems for low emissions catalytic combustion for gas turbine engines.

BACKGROUND OF THE INVENTION

To achieve stable combustion of very lean premixed fuel-air mixtures, as is necessary to achieve lowered NOx emissions, hot combustion gases must be re-circulated into contact with the incoming fuel-air mixture. The required recirculation can be accomplished by various means, most typically by a dump and/or swirl. In a dump, combustor fuel and air are injected in a direction generally perpendicular to the combustor exit plane and expand on entry with recirculation. Without use of a flame holder in the flow stream, flame length is longer than desirable in smaller gas turbines. A flame holder is not acceptable in a gas turbine system since, without cooling, a flashback event can damage the flame holder sending it downstream into the turbine blades. Therefore, swirl stabilization is used with fuel injection downstream of the swirler hardware.

Unfortunately, at typical gas turbine compressor discharge temperatures, swirl stabilization techniques alone cannot achieve flame stability at low enough flame temperatures to achieve ultra-low NOx levels. The problem is particularly acute in axial flow annular aero-engine type combustors because flame length must be very short. Catalytic combustion can achieve superior combustion stability without re-circulation, even at very low flame temperatures, but flame length can be long. In addition, available catalytic materials cannot operate at the combustion temperatures required for many present gas turbine systems let alone those required for the advanced gas turbines under development.

Accordingly, catalytic combustion systems are typically designed to enhance downstream flame stability by partial pre-combustion of fuel to increase the reactivity of the fuel-air mixture entering the combustion zone. In such a system, if the resulting reactive mixture is passed through a swirler, flashback through the swirler can occur damaging the swirler. Although a dump stabilized combustor can be designed to tolerate flashbacks, dump stabilization without a flame holder results in a much longer flame length than can be accommodated in many modem gas turbines.

It has now been found that combustion of a hot catalytically reacted lean fuel-air mixture can be swirl stabilized to achieve a short, compact flame without use of flow through swirler vanes.

Based on the foregoing, it is the general object of the present invention to overcome or improve upon the problems and drawbacks of the prior art.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for stable low NOx combustion of an admixture of fuel and air, which fuel preferably has been partially oxidized in the presence of a catalyst. The admixture is directed into a spiral flow path within an annular flow chamber whereby a swirling flow is created for passage into a combustion chamber, wherein combustion is stabilized by flow re-circulation. In the present invention, air is passed tangentially into an annular flow chamber and spirals around a central hub towards the exit leaving as a swirling flow. A preferred fuel injector of the present invention comprises an annular flow chamber having a tangential entry for air. The center hub forming the annulus in conjunction with an outer tubular chamber wall is sized to avoid flow separation on the hub wall and to block re-circulation of combusting flow into the annular passage. It may be either open or closed ended. If closed ended the closed face is advantageously cooled with a flow of cooling fluid, preferably air.

A spark or torch igniter or other known means may be used to establish combustion in the downstream combustion chamber. Thereafter, combustion of incoming fuel and air is self sustaining being ignited by contact with hot re-circulating flow. In one preferred method of the present invention, a fuel-rich fuel-air mixture is reacted in the presence of a catalyst, as for example is described in U.S. Pat. No. 6,358,040 (incorporated herein by reference), and the reacted fuel rich mixture is then mixed with air to form a fuel-lean fuel-air admixture. Thus in the method of the present invention air and fuel in a fuel lean ratio pass tangentially into an annular flow passage creating a swirling flow without bulk flow recirculation of air mixed with fuel. The swirling flow is passed into a downs stream combustion chamber. The fuel may be mixed with the air before or after entering the annular passage. Fuel and the air may be provided from any convenient source.

The non-swirling lean admixture flow is then passed tangentially into an annular or spiral flow chamber such as to cause the flow to rotate rapidly (swirling) without bulk flow re-circulation. Alternately, fuel may be mixed with air within the annular flow chamber. The swirling flow may be passed into a combustion chamber where expansion may generate re-circulation for flame stabilization as with conventional swirl stabilized combustors. Combustion is also stabilized on the hub which acts as a dump stabilizer. Thus, combustion may, if desired, be stabilized without flow expansion. A combustor may use multiple fuel injectors as needed. Fuel injectors of the present invention are particularly advantageous for annular combustors such as employed on the Solar Taurus 60 gas turbine. With such a combustor, it is preferred that all injectors have the same direction of rotation. Preferably, the injector includes a reactor for fuel-rich partial oxidation of the fuel and a chamber for mixing the reaction products with air to form the fuel-lean mixture for passage to the swirl chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric representation of a fuel injector of the present invention.

FIG. 2 is an isometric representation of a cut-away of the fuel injector of FIG. 1 along the line 2-2.

FIG. 3 is a diagrammatic representation of a cut-away of the fuel injector of FIG. 1 along the line 3-3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts an isometric representation of a spiral flow chamber 12 of a fuel injector in accordance with the present invention. Spiral flow chamber 12 defines an inlet 14 and a center swirler hub 16. As shown in FIG. 2, center swirler hub 16 is depicted as open-ended but may be closed by a cooled plate as with conventional vane bladed swirlers.

FIG. 3 depicts a cross-sectional view near the inlet region 18 of a fuel injector 10 designed for a gas turbine axial flow annular combustor such as that of the Solar Turbines Taurus 60 gas turbine engine. Fuel injector 10 comprises catalyst module 20 positioned within conduit 22 feeding air and partially reacted fuel into mixing zone 24. Non-swirling flow 26 exits mixing zone 24 into spiral flow chamber 12 having swirler hub 16 thus creating a swirling exit flow exiting spiral flow chamber 12 at the end opposite of inlet 14. Swirler hub 16 also prevents reverse flow down the centerline.

Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the invention should not be limited to the description of the preferred versions contained herein.

Claims

1. A method of creating a swirling flow of fuel-lean premixed fuel and air without passage of the flow through swirler vanes comprising:

a) providing a supply of fuel;

b) providing a supply of air in sufficient quantity to form a fuel-lean mixture if mixed with the fuel;

c) passing the air tangentially into an annular flow passage creating a spiraling flow without bulk flow re-circulation and wherein the spiraling flow comprises the air mixed with the fuel in a fuel-lean ratio; and

d) passing the spiraling flow of fuel and air into a combustion chamber.

2. The method of claim 1 wherein the fuel is a partially oxidized fuel-rich fuel-air admixture.

3. The method of claim 2 wherein the fuel-rich admixure has been oxidized in the presence of a catalyst.

4. The method of claim 1 wherein the swirling flow is passed into a combustion chamber.

5. The method of claim 4 wherein multiple swirling flows rotating in the same direction are formed and passed into an annular combustor.

6. The method of claim 4 wherein the flow expands on entry into the combustor.

7. The method of claim 4 comprising the additional step of stabilizing combustion on a swirler hub.

8. The method of claim 6 wherein the swirl number is sufficiently high for vortex breakdown.

9. The method of claim 1 wherein the fuel is premixed with the air prior to passing into an annular flow passage to create a swirling flow.

10. The method of claim 1 wherein the fuel is injected into the air on passing into an annular flow passage.

11. A fuel injector for a combustor comprising a fuel-air mixing chamber and an annular spiral flow chamber having a tangential flow entrance connected to the fuel-air mixing chamber.

12. The fuel injector of claim 11 comprising a catalytic reactor for partial oxidation of fuel to feed partially reacted fuel to the fuel-air mixing chamber.

13. The fuel injector of claim 12 wherein the reactor is a fuel rich reactor.

14. The fuel injector of claim 11 wherein the spiral flow chamber contains an axial hub.

15. The fuel injector of claim 14 wherein the hub is recessed within the spiral flow chamber.

16. The fuel injector of claim 14 wherein the hub has an open end.

17. The fuel injector of claim 14 wherein the hub has a closed end.

18. A combustor comprising one or more fuel injectors each comprising a fuel-air mixing chamber and an annular spiral flow chamber having a tangential flow entrance connected to the fuel-air mixing chamber directing a direction of rotation.

19. The combustor of claim 18 wherein multiple fuel injectors all have the same direction of rotation.

20. The method of claim 1 further comprising:

e) reacting a fuel-rich mixture of fuel with air in the presence of a catalyst producing heat and a reaction product;

f) mixing the reaction product with air to form the fuel-lean fuel-air mixture; and

g) passing the mixture into an annular chamber in a direction nominally perpendicular to the chamber axis of rotation, thereby creating a flow rotating about said axis of rotation.