Patent Grant
8256226
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.
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 produced will be at a much lower rate.
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 at an acceptable rate to remain in emission compliance.
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 objective's.
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.
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 air passages extending axially in the burner tube configured to convey air flows from the inlet end to the outlet end; a plurality of fuel passages extending axially along the burner tube and spaced around the plurality of air passage configured to convey fuel from the inlet end to the outlet end; and a radial air swirler provided at the outlet end configured to direct the air flows radially toward the outlet end and impart swirl to the air flows. The radial air swirler comprises a plurality of vanes to direct and swirl the air flows and an end plate. The end plate comprises a plurality of fuel injection holes to inject the fuel radially into the swirling air flows.
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 comprises a burner tube comprising an inlet end, an outlet end, a plurality of axial air passages, and a plurality of axial fuel passages. The method comprises introducing an air flow into the air passages at the inlet end; introducing a fuel into fuel passages; swirling the air flow at the outlet end; and radially injecting the fuel into the swirling air flow.
Referring to
The burner tube 4 comprises a plurality of air passages 12. The air passages 12 surround a central body 18 that comprises a central passage 20. The central body 18 is coaxial with an axis 34 of the burner tube 4. A plurality of fuel passages 14 are provided around the air passages 12. A radial air swirler arrangement 22 is provided at the outlet end 8 of the burner 2 to impart a swirl to the air flow 26 (
A plurality of fuel injection holes 16 are provided in the front plate 36 to inject fuel radially into the burner tube 4 from the fuel passages 14. The injected fuel 24 from the fuel passages 14 is mixed with the air flow 26 that is swirled by the vanes 28 of the radial air swirler arrangement 22. The fuel 24 is injected into the air flow where most of the air mass flow is concentrated in the thin annulus section 40 (
The central body 18 includes an end portion 42 that is configured to cut back a recirculation zone and accelerate the air flow 26 that might otherwise carry hot combustion products or reactants back into the burner tube 4 that could create local hot spots and result in damage. The central body 18 may be utilized for starting up on a second fuel or backup fuel, for example natural gas. It should be appreciated that the central body 18 may also be replaced by a liquid fuel cartridge or atomizer assembly for liquid fuels.
The injected fuel 24, 30 may be highly reactive fuel, for example pure hydrogen or various hydrogen/CO and hydrocarbon mixtures. Injecting the fuel 24, 30 in the radial swirling air flow provides rapid air fuel mixing that reduces emissions and prevents unpredictable flame holding and flash backs that may occur in premixed combustion systems.
It is possible to vary the fuel locations and penetration depths that will provide more control over the fuel distribution and mixing to reduce and control emissions. The fuel location can be changed depending on the reactivity of the fuels to provide distribution and mixing necessary for attaining low emissions.
Referring to
Referring to
As shown in
Referring to
Radial lean direct injection may comprise more than one swirler and fuel injection annulus to enhance mixing and tailor the combustor aerodynamic flow field, as shown in
The radial lean direct injection burner may inject highly reactive fuels, such as pure hydrogen or various hydrogen/CO and hydrocarbon mixtures, in the radial swirling air flow field that provides rapid air fuel mixing necessary for reducing emissions and prevent unpredictable flame holding and flash back issues that poses challenge in premixed combustion systems.
Air is introduced radially and swirled, fuel is injected radially into the air stream where most of the air mass flow is concentrated in the thin annulus section at the exit section of the burner. The use of fuel injection tubes makes it possible to vary fuel locations and penetration depths that can give more control over fuel distribution and mixing to reduce and control emissions. The number and/or location of the fuel injection passages, either fuel injection holes and/or fuel injection tubes, may be designed to improve fuel distribution and mixing to attain lower emissions.
The radial injection of fuel into a swirling air flow may also be used as a premixer for premix combustor design systems.
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.
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.
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| Number | Date | Country | |
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| 20100269507 A1 | Oct 2010 | US |
