US20090249789A1

US20090249789A1 - Burner tube premixer and method for mixing air and gas in a gas turbine engine

Info

Publication number: US20090249789A1
Application number: US12/099,241
Authority: US
Inventors: Baifang Zuo; Abdul Rafey Khan; Christian X. Stevenson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2008-04-08
Filing date: 2008-04-08
Publication date: 2009-10-08
Also published as: CN101556043A; JP2009250604A; DE102009003453A1; FR2929688A1

Abstract

A burner for use in a gas turbine engine includes a burner tube having an inlet end and an outlet end; a plurality of slots formed in the burner tube and configured to introduce air flows tangentially into the burner tube and impart swirl to the air flows; a plurality of fuel passages extending axially along the burner tube; and a plurality of fuel injection holes provided to each fuel passage. At least one of the fuel injection holes of each fuel passage is configured to inject a fuel flow tangentially into the burner tube between air flows of adjacent slots to form a fuel and air co-flow. A method of mixing air and fuel in a burner of a gas turbine is provided. The burner includes a burner tube including a plurality of slots formed in the burner tube. The method includes introducing air flows tangentially into the burner tube through the slots and imparting swirl to the air flows; and injecting fuel between air flows of adjacent slots to form fuel and air co-flows. This eliminates jet cross flow, which tends to cause flame holding. The tangentially entered fuel and air co-flow layers then flow down axially with quick mixing and dump to a combustor for a stable premixed combustion.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No. DE-FC26-05NT42643 awarded by the Department of Energy. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to an air fuel mixer for the combustor of a gas turbine engine, and to a method for mixing air and fuel.

BACKGROUND OF THE INVENTION

Gas turbine manufacturers are regularly involved in research and engineering programs to produce new gas turbines that will operate at high efficiency without producing undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. The oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone. The rate of chemical reactions forming oxides of nitrogen (NOx) is an exponential function of temperature. If the temperature of the combustion chamber hot gas is controlled to a sufficiently low level, thermal NOx will not be produced.
One method of controlling the temperature of the reaction zone of a combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is not formed.
There are several problems associated with dry low emissions combustors operating with lean premixing of fuel and air in which flammable mixtures of fuel and air exist within the premixing section of the combustor, which is external to the reaction zone of the combustor. There is a tendency for combustion to occur within the premixing section due to flashback, which occurs when flame propagates from the combustor reaction zone into the premixing section and causes the flame to hold inside the wake flows behind the fuel injection columns (jet cross flow) or vane trailing edges, or autoignition, which occurs when the dwell time and temperature for the fuel/air mixture in the premixing section are sufficient for combustion to be initiated without an igniter. The consequences of combustion in the premixing section are degradation of emissions performance and/or overheating and damage to the premixing section, which is typically not designed to withstand the heat of combustion. Therefore, a problem to be solved is to prevent flashback or autoignition resulting in combustion within the premixer.
In addition, the mixture of fuel and air exiting the premixer and entering the reaction zone of the combustor must be very uniform to achieve the desired emissions performance. If regions in the flow field exist where fuel/air mixture strength is significantly richer than average, the products of combustion in these regions will reach a higher temperature than average, and thermal NOx will be formed. This can result in failure to meet NOx emissions objectives depending upon the combination of temperature and residence time. If regions in the flow field exist where the fuel/air mixture strength is significantly leaner than average, then quenching may occur with failure to oxidize hydrocarbons and/or carbon monoxide to equilibrium levels. This can result in failure to meet carbon monoxide (CO) and/or unburned hydrocarbon (UHC) emissions objectives. Thus, another problem to be solved is to produce a fuel/air mixture strength distribution, exiting the premixer, which is sufficiently uniform to meet emissions performance objectives.
Still further, in order to meet the emissions performance objectives imposed upon the gas turbine in many applications, it is necessary to reduce the fuel/air mixture strength to a level that is close to the lean flammability limit for most hydrocarbon fuels. This results in a reduction in flame propagation speed as well as emissions. As a consequence, lean premixing combustors tend to be less stable than more conventional diffusion flame combustors, and high level combustion driven dynamic pressure fluctuation (dynamics) often results. Dynamics can have adverse consequences such as combustor and turbine hardware damage due to wear or fatigue, flashback or blow out. Accordingly, another problem to be solved is to control the combustion dynamics to an acceptably low level.
Lean, premixing fuel injectors for emissions abatement are in use throughout the industry, having been reduced to practice in heavy duty industrial gas turbines for more than two decades. A representative example of such a device is described in U.S. Pat. No. 5,259,184. Such devices have achieved progress in the area of gas turbine exhaust emissions abatement. Reduction of oxides of nitrogen, NOx, emissions by an order of magnitude or more relative to the diffusion flame burners of the prior art have been achieved without the use of diluent injection such as steam or water.
As noted above, however, these gains in emissions performance have been made at the risk of incurring several problems. In particular, flashback and flame holding within the premixing section of the device result in degradation of emissions performance and/or hardware damage due to overheating. In addition, increased levels of combustion driven dynamic pressure activity results in a reduction in the useful life of combustion system parts and/or other parts of the gas turbine due to wear or high cycle fatigue failures. Still further, gas turbine operational complexity is increased and/or operating restrictions on the gas turbine are necessary in order to avoid conditions leading to high-level dynamic pressure activity, flashback, or blow out.
In addition to these problems, conventional lean premixed combustors have not achieved maximum emission reductions possible with perfectly uniform premixing of fuel and air.

BRIEF DESCRIPTION OF THE INVENTION

According to one embodiment of the invention, a burner for use in a gas turbine engine comprises a burner tube having an inlet end and an outlet end; a plurality of slots formed in the burner tube and configured to introduce air flows tangentially into the burner tube and impart swirl to the air flows; a plurality of fuel passages extending axially along the burner tube; and a plurality of fuel injection holes provided to each fuel passage. At least one of the fuel injection holes of each fuel passage is configured to inject a fuel flow tangentially into the burner tube between air flows of adjacent slots to form a fuel and air co-flow.
According to another embodiment of the invention, a method of mixing air and fuel in a burner of a gas turbine is provided. The burner includes a burner tube comprising a plurality of slots formed in the burner tube. The method comprises introducing air flows tangentially into the burner tube through the slots and imparting swirl to the air flows; and injecting fuel between air flows of adjacent slots to form fuel and air co-flows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a perspective of a burner according to an embodiment of the invention;

FIG. 2 schematically depicts a cross section of the burner of FIG. 1 along 2-2; and

FIG. 3 schematically depicts a cross section of the burner of FIG. 1 along 3-3.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-3, a burner 2 comprises a burner tube 4 having an inlet end 6 and an outlet end 8. A flange 10 is provided to the burner tube 4 for mounting the burner 2 into a gas turbine engine. It should be appreciated that the flange 10 may be integrally formed with the burner tube 4, or may be provided separately. It should also be appreciated that other mounting arrangements may be provided for the burner 2.
A plurality of slots 12 are formed in the burner tube 4. Air is introduced tangentially into the burner tube 4 through the slots 12. Each slot 12 has a spiral, or swirling, configuration to impart swirl to combustion air entering the slot 12. As shown in FIG. 2, the burner tube 4 comprises a plurality of fuel passages 14 that extend axially along the burner tube 4. A plurality of fuel injection holes 16 are provided to inject fuel tangentially into the burner tube 4 from the fuel passages 14. The fuel passages 14 are formed in the burner tube 4 between an exterior wall 36 and an interior wall 30.
Referring to FIGS. 2 and 3, a converging central body 18 is provided in the burner tube 4 to accelerate the mixture of air and fuel along the axis of the burner tube 4 and to maintain no flow separation at the surface of the central body 18. The central body 18 has a central body tip 32 adjacent to the outlet end 8 of the burner tube 4. The central body 18 also comprises a central passage 20 to allow purge air to flow through the central body 18 to prevent flame holding at the central body 18, for example at the central body tip 32.
Referring to FIG. 3, the slots 12 each have a slot end 22 that is tangential to the axis 34 of the burner tube 4 to minimize disturbance in the mixing of the air and fuel. As shown in FIG. 2, the injected fuel 24 is sandwiched between an air flow 26 from a slot 12 and air flow 28 from another slot 12. The injected fuel 24 co-flows with the air flows 26, 28 and is prevented from contacting the interior wall 30 of the burner tube 4. This will eliminate jet cross flow, which tends to cause flame holding. The tangentially entered fuel and air co-flow layers then flow down axially with quick mixing and dump to the combustor for a stable premixed combustion.
The fuel and air co-flows 24, 26, 28 radially enter the burner tube 4. The radial entry of the co-flows 24, 26, 28 generates no wake domain inside the burner tube 4 and eliminates any potential flame holding spots. The radial fuel and air co-flows 24, 26, 28 also provide axial vortex breaking to ensure good fuel and air mixing. The slot ends 22 are configured to allow the air flows 26, 28 to enter the burner tube 4 tangentially to the burner tube axis 34 and radially. The central body 18 prevents flame flashback at the center of the burner tube 4 and stabilizes the flame near the central body tip 32.
The burner 2 has simple geometry and may be manufactured at low cost. The burner 2 also provides high efficiency and low emission. The interior wall 30 of the burner tube 4 may also be protected by pure air or an air purge. The burner 2 also provides low air and fuel pressure drop and the gradual axial fuel injection avoids dynamics. The burner 2 may also be used with high hydrogen fuel and syngas.
The length and width of the slots 12, the number of fuel injection holes 16, the diameter of the fuel injection holes 16, the locations of the fuel injection holes 16, and the surface profile of the central body 18 may be designed using Computational Fluid Dynamics (CFD) to provide desired, or enhanced, air/fuel mixing and to avoid autoignition and flame holding in the burner. It should also be appreciated that although the burner has been shown as including, for example, four slots, that any plural number of slots may be provided.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A burner for use in a gas turbine engine, comprising:

a burner tube having an inlet end and an outlet end;

a plurality of slots formed in the burner tube and configured to introduce air flows tangentially into the burner tube and impart swirl to the air flows;

a plurality of fuel passages extending axially along the burner tube; and

a plurality of fuel injection holes provided to each fuel passage, wherein at least one of the fuel injection holes of each fuel passage is configured to inject a fuel flow tangentially into the burner tube between air flows of adjacent slots to form a fuel and air co-flow.

2. A burner according to claim 1, further comprising a central body coaxially disposed in the burner tube between the inlet end and the outlet end.

3. A burner according to claim 2, wherein the central body converges from the inlet end to the outlet end.

4. A burner according to claim 2, wherein the central body comprises a central passage.

5. A burner according to 1, wherein each slot comprises a slot end that directs the air flow tangential to the axis of the burner tube.

6. A burner according to 1, wherein each slot comprises a slot end that directs the air flow radially.

7. A burner according to claim 1, wherein each fuel passage is formed between an exterior wall and an interior wall of the burner tube.

8. A burner according to claim 7, wherein each fuel flow of each co-flow is separated from the interior wall of the burner tube.

9. A burner according to claim 2, wherein the co-flows are not separated from the central body.

10. A burner according to claim 1, wherein the number of slots corresponds to the number of fuel passages.

11. A burner according to claim 1, wherein the plurality of slots comprises four slots.

12. A method of mixing air and fuel in a burner of a gas turbine, the burner including a burner tube comprising a plurality of slots formed in the burner tube, the method comprising

introducing air flows tangentially into the burner tube through the slots and imparting swirl to the air flows; and

injecting fuel between air flows of adjacent slots to form fuel and air co-flows.

13. A method according to claim 12, wherein each fuel flow is separated from an interior wall of the burner tube by an air flow.

14. A method according to claim 12, wherein injecting the fuel comprises injecting the fuel tangentially into the burner tube.

15. A method according to claim 12, further comprising injecting air flow tangentially to the burner tube axis at ends of the slots.

16. A method according to claim 12, further comprising injecting air flow radially to the burner tube at ends of the slots.

17. A method according to claim 12, further comprising accelerating the co-flows through the burner tube along a central body coaxially disposed in the burner tube.

18. A method according to claim 17, wherein accelerating the co-flows comprises maintaining no flow separation at a surface of the central body.

19. A method according to claim 17, further comprising providing purge air through a central passage of the central body.