Patent Application
20100281876
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, therefore the NOx generated by these reactions is also called thermal NOx. 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 potential 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 may be 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 should 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 may 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 flame 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 may result in a reduction in flame propagation speed as well as emissions. As a consequence, lean premixing combustors may tend to be less stable than more conventional diffusion flame combustors, and high levels of combustion driven dynamic pressure fluctuation (also called combustion dynamics) often results. Combustion 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 may 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 may have not achieved maximum emission reductions possible with perfectly uniform premixing of fuel and air exiting the premixer and entering the reaction zone.
According to an embodiment, a premixer for a gas turbine combustor comprises a first passage configured to inject a highly reactive fuel; and a second passage configured to inject an inert gas or a less chemically reactive fuel or a mixture of both, wherein the second passage is configured to form a layer of the inert gas or the less reactive fuel or the mixture of both that blankets a layer of the highly reactive fuel.
According to another embodiment, a premixer for a gas turbine combustor comprises a plurality of nozzles, each nozzle comprising a pair of concentric tubes, the pair of concentric tubes comprising a first tube configured to inject a highly reactive fuel and a second tube surrounding the first tube and configured to inject an inert gas or a less reactive fuel or a mixture of both that blankets the highly reactive fuel.
According to still another embodiment, a method of forming a combustible mixture for a gas turbine combustor comprises injecting a highly reactive fuel; and injecting an inert gas or a less reactive fuel or a mixture of both, wherein the inert gas or the less reactive fuel or the mixture of both blankets the highly reactive fuel.
Referring to
An inert gas or a less reactive fuel or a mixture of both 8 is provided from the second passage 6. The inert gas may be, for example, nitrogen (N2), or steam (H2O), or carbon dioxide (CO2) or a combination thereof. The first, or fuel, passage 4 is configured to keep the fuel 10 away from the hub and shroud section 2 where the fuel 10 could be trapped and cause flame holding. As shown in detail in
The flame holding margin may be increased by blanketing the highly reactive fuel 10 by the layer of inert gas or less reactive fuel or mixture of both 8 as shown in
The passages 4, 6 are used to slowly diffuse and form the layer of highly reactive fuel 10 covered by the inert gas or less reactive fuel 8 and retard the reaction between the fuel air interface. As shown in detail in
Flame holding may also be prevented for the highly reactive fuel 10 by controlling the air fuel interaction near the fuel introduction space. The passages 4, 6 are also configured to control the air/fuel mixing interface to control ignition.
The blanketing of a highly reactive fuel by an inert gas or a less reactive fuel can also be used in a concentric tubes arrangement as shown in
A combustor 14 includes a premixer comprising nozzles 16. Each nozzle 16 includes concentric tubes, including an inner tube 18 and an outer tube 20. The inner tube 18 provides fuel 22, for example highly reactive fuel, and the outer tube 20 provides an inert gas or less reactive fuel 24 that blankets the fuel 22. A flow of air 26 is provided to the combustor 14 to form a combustible mixture with the fuel 22. The air flow 26 may include a swirl 28 and a counter-swirl 28. A flashback arrestor 30 may also be provided.
The premixer can be used for machines that require high hydrogen fuels or high hydrogen syngas fuels.
Referring to
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.
