Patent Application
20100089066
Embodiments of the present invention generally relate to low pollution combustion processes, and to combustors with burners that support such combustion processes, and in particular to combustion processes and combustors for gas turbine engines. However, the principles of the invention are also applicable to other combustion apparatus, such as steam raising boilers.
All gas turbine engines employ some sort of fuel injector or burner to deliver a stream of fuel into the combustion chamber. In many cases, the burner will also deliver a stream of air for combustion of the fuel. Many prior art burners for gas turbine engines have supplied liquid fuels such as kerosene or diesel to their combustors, but burners have also been adapted to deliver one or more of a variety of other fuels including:
A problem that arises with burners in general is a build-up of carbon on the end of the fuel nozzle, or even ingress of combustion gases to the fuel system via the fuel nozzle. In some cases, a burner can deliver two different types of fuel into the combustion chamber at different times. For example, the burner may include a fuel nozzle that supplies a gaseous fuel (often the main fuel) and a pilot fuel nozzle that supplies a liquid fuel. The hot combustion products within the combustion chamber may particularly affect the fuel nozzle that is not being used; e.g., gaseous combustion products may enter the fuel supply path of the fuel nozzle, and/or solid combustion products such as carbon may be deposited on a fuel nozzle. Carbon deposition can even build up while fuel is being supplied through a nozzle. To help prevent hot combustion products from damaging or blocking the fuel nozzle when it is not in use, it is normally necessary to purge the nozzle, e.g., by supplying a proportion of the compressor delivery air through it, for example. However, the need for positive forward purge of the fuel nozzle increases the complexity of the design of the gas turbine engine's fuel system and the burner.
The problem of climate change has received much attention in recent years, and when fossil fuels are burned to power gas turbine engines and boilers, carbon dioxide is of course produced by the combustion process. Therefore, combustion systems capable of safely and reliably burning hydrogen (or fuels with a high free hydrogen content) are required, in order to reduce CO2 emissions and the impact of climate change.
Allied with the above, an important concern in modern gas turbine engine combustor and burner design is to increase combustion efficiency and to reduce atmospheric pollution attributable to engine emissions. Mono nitrogen oxides (collectively known as “NOx”, which contribute to the production of photochemical smog and acid rain, and in themselves are potent “greenhouse” gases) are produced during combustion of fuels in air at temperatures of the order of 1850 degrees Kelvin (1850K) or more, therefore most combustors generating low emissions operate with full load flame temperatures below 1900K. A common approach to achieving relatively low flame temperatures is to use the so-called “lean premix” combustion process, in which excess air and fuel are mixed together before reaching the flame zone to give a combustion process with a lean equivalence ratio and a full load flame temperature of the order of 1850K.
The equivalence ratio is defined as the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio. Stoichiometric combustion occurs when all the oxygen is consumed in the reaction, and there is no molecular oxygen in the combustion products. Hence, if the equivalence ratio has a value of one, the combustion is stoichiometric. If it is less than one, the combustion is lean with excess air, and if it is greater than one, the combustion is rich, with incomplete combustion.
A problem with lean premix combustion processes is that as the load on the engine is reduced down to engine idle speeds, the fuel flow must also be reduced, and the fuel/air mixture tends to become leaner. The result of leaner mixtures is reduced combustion stability, leading to flame blow-out or combustion oscillations if the fuel flow is turned down too much relative to the air flow. A known way of at least partially overcoming this problem is to have pilot burners in the combustors that operate with diffusion flames at low loads, but of course, this expedient produces more emissions due to increased combustion temperatures in the diffusion flames. Alternatively, or additionally, the compressor can be provided with variable inlet guide vanes which act at low loads to reduce compressor delivery air flow to the burners, but this adds cost and complexity to the engine.
In aero gas turbine engines, one lean-burn concept currently being researched for liquid fuel can be classified as Lean Pre-mixed Pre-vaporised (LPP). The LPP concept has been shown to have pleasingly low NOx emissions, but the main disadvantage when used for hydrogen and other high flame speed fuels is the risk of flame flashback from the combustion chamber to the premixing chamber. The risk of auto-ignition may also be an issue with elevated combustion air temperatures associated with high pressure ratio engines and with reheat combustors (otherwise known as sequential combustors).
Another lean-burn concept currently being researched is Lean Direct Injection (LDI, sometimes referred to as “micro-diffusion”). The main difference from LPP is that instead of the fuel being premixed with the air before injection to the combustion chamber, it is injected directly into the flame zone. This means that there is no potential for flame flashback or auto-ignition. However, the lack of pre-mixing means that it is necessary for the fuel to be mixed quickly and uniformly with all the combustion air in the flame zone so that the flame temperature is low and the NOx formation levels are comparable to LPP. The LDI concept employs a large number of small flames. Each of the multiple fuel injectors has an air swirler associated with it to provide quick mixing and a small re-circulation zone for burning. The use of multipoint fuel injection provides quick, uniform mixing of the fuel and combustion air and the small multi-burning zones provide for reduced burning residence time, resulting in low NOx formation levels. See, e.g., NASA Report No. TM-2002-211347 “A Low NOx Lean-Direct Injection, Multipoint Integrated Module Combustor Concept for Advanced Aircraft Gas Turbines”, by Robert Tacina, Glenn Research Center, Cleveland, Ohio, Changlie Wey, QSS Group, Inc., Cleveland, Ohio, and Peter Laing and Adel Mansour, Parker Hannifin, Andover, Ohio.
U.S. Pat. No. 6,928,823 B2 A is another example of a gas turbine combustor in which fuel and air are supplied into a combustion chamber as a plurality of coaxial jets. In one embodiment, the exit of each fuel supply nozzle is concentrically located just within the entrance of a cylindrical air supply nozzle, which corresponds to a premixing tube of a conventional premixing combustor. According to the description of U.S. Pat. No. 6,928,823 B2, low NOx combustion equivalent to lean premixed combustion is achieved, together with reduced flashback potential, by producing coaxial jets in which the air flow envelopes the fuel, the fuel flows into the combustion chamber, mixes with an ambient coaxial air flow to become a premixed air fuel mixture having a proper stoichiometric mixture ratio, and then comes in contact with high-temperature gas and starts to burn. However, there is no disclosure of relative dimensions, velocity ratios, or momentum ratios, and some of the air delivered to the combustor is not only used to cool combustion chamber walls, but also passes into the chamber as dilution air.
A first embodiment is directed to a method for burning fuel with air in a combustion chamber with low production of NOx emissions. The method includes supplying fuel and air to a burner associated with the combustion chamber, the amount of air supplied being at least as much as required for stoichiometric combustion of the fuel and all subsequent dilution of the combustion process in the combustion chamber. The method also includes combusting the fuel with the air in a flame and a post-flame reaction by injecting the fuel and all the air through the burner directly into the combustion chamber in a substantially unmixed state as a fuel stream inside an air stream. The air is immediately available in the proximity of the flame during the combustion process to act as a heat sink and so that the combustion process in the flame is followed by fast dilution and the post-flame reaction continues as a lean-burn reaction. The thickness of the fuel stream being such that the fuel burns in the combustion chamber at or close to the stoichiometric fuel/air ratio as a diffusion flame with a high surface-to-volume ratio.
A second embodiment is directed to a combustor including a combustion chamber and at least one burner that in use supplies a fuel stream surrounded by an air stream in a substantially unmixed condition directly into the combustion chamber to burn therein as a diffusion flame with low production of NOx. The at least one burner includes an air supply nozzle and at least one fuel supply nozzle located within the air supply nozzle. The amount of air supplied by the air supply nozzle being at least as much as required for stoichiometric combustion of the fuel and all subsequent dilution of the combustion process in the combustion chamber. The air is immediately available in proximity of the flame during the combustion process to act as a heat sink and the combustion process in the flame is followed by fast dilution, with a post-flame reaction continuing as a lean-burn reaction. The at least one fuel supply nozzle is dimensioned to eject the fuel stream with a thickness such that the fuel burns in the combustion chamber at or close to the stoichiometric fuel/air ratio as a diffusion flame with a high surface-to-volume ratio.
A third embodiment is directed to a burner for use in a combustor.
A fourth embodiment is directed to a gas turbine engine including a combustor.
Exemplary embodiments of the invention will now be described, with reference to the accompanying diagrammatic drawings, in which:
In the drawings, like parts are given like reference numbers.
A combustion process for burning fuel in a combustion chamber is provided and includes the steps of:
supplying fuel and air to a burner associated with the combustion chamber, the amount of air supplied being at least as much as required for stoichiometric combustion of the fuel and subsequent dilution of the combustion process in the combustion chamber; and
injecting the fuel and all the air from the burner directly into the combustion chamber in a substantially unmixed state as a fuel stream within an air stream, the thickness of the fuel stream being such that the fuel burns in the combustion chamber as a diffusion flame with a high surface-to-volume ratio at or close to the stoichiometric fuel/air ratio.
Embodiments of the invention also provide a combustor comprising a combustion chamber and at least one burner that in use supplies a fuel stream surrounded by an air stream in a substantially unmixed condition directly into the combustion chamber to burn therein as a diffusion flame, the burner comprising an air supply nozzle and at least one fuel supply nozzle located within the air supply nozzle, the amount of air supplied by the air supply nozzle in use being at least as much as required for stoichiometric combustion of the fuel and subsequent dilution of the combustion process in the combustion chamber. At present, it is believed that to keep NOx production during combustion to low levels, the arrangement should be such that at the exit of the fuel supply nozzle, no part of a fuel stream emerging therefrom is more than about 2.5 mm from a boundary between the fuel stream and the air stream.
The amount of air supplied through the air supply nozzle is substantially in excess of the amount required for complete combustion, preferably between roughly 150% to roughly 250% of the amount required for complete combustion.
To facilitate low emissions without any substantial premixing of the fuel and air before combustion, the burners in the embodiments disclosed herein are intended to produce not only a small diffusion flame with a high flame surface-to-volume ratio in the combustion chamber, but also provide a sufficiently large mass of air adjacent the flame to act as an effective heat sink. The flame therefore loses heat rapidly to the adjacent air that issues from the air nozzle, leading to a low diffusion flame temperature, of the order of 1850K, or even less. Note also that sufficient air is supplied through the air supply nozzle to both support stoichiometric combustion and to be a fast-acting diluent that reduces the combustion temperature without quenching the combustion process. Furthermore, the embodiments are intended to facilitate a combustion process in which combustion products have only short residence times in the zone of maximum temperature, meaning that they are expected to produce less NOx in comparison with known lean burn combustion processes operating at the same maximum temperature. Moreover, it is expected that in such diffusion flames, the air and fuel will burn at or close to the stoichiometric fuel/air ratio throughout the range of engine loads, so the combustion process should produce low emissions even at engine idle speeds, thereby eliminating the need for separate pilot and main burners and variable inlet guide vanes.
The relative velocities of the fuel and air streams as they exit from their respective nozzles have an influence on obtaining the desired low emissions. As the ratio Vair/Vfuel is increased (where Vair is the velocity of the air stream and Vfuel is the velocity of the fuel stream), the flame length is shortened and the base of the flame moves closer to the nozzle tip. This reduces both the residence time of the combustion products in the flame and also the volume of the combustion zone. At present it is believed that the ratio Vair/Vfuel should be greater than unity (Vair/Vfuel>1).
Absolute velocity is also an important factor in reducing NOx emissions, with higher velocities, at least in the subsonic flow regime, resulting in lower NOx emissions for the same Vair/Vfuel ratio. It is believed that the invention is applicable for air velocities in the subsonic, supersonic and hypersonic air flow regimes.
Further factors having a bearing on the performance of the embodiments disclosed herein are the relative values of the delivery pressures of the fuel and the air and the pressure in the combustion chamber. If the air delivery pressure is Pa and the pressure in the combustion chamber is Pc, the pressure difference Pa-Pc necessary to deliver air into the combustion chamber may be termed δpair. Similarly, if the fuel delivery pressure is Pf, the pressure difference Pf-Pc necessary to deliver fuel into the combustion chamber may be termed δpfuel. At present it is believed that the ratio δpfuel to δpair should be less than one (δpfuel/δpair<1). The pressure differences δpair and δpfuel will be of the order of 4% to 5% of the air delivery pressure Pa and the fuel delivery pressure Pf. In a gas turbine engine, the air stream will be provided at the delivery pressure of the final stage of the associated compressor and the fuel must obviously be delivered at a similar pressure. Such pressures may be in the range of roughly 5 bar to roughly 30 bar.
To maximize combustion efficiency and minimize the possibility of flashback or premature combustion, the air supply nozzle and the fuel supply nozzle are shaped and located relative to each other such that they form a venturi arrangement for air flowing between an inside surface of the air nozzle and an outer surface of the fuel nozzle. The resulting venturi effect increases the speed of the air stream and exerts a suction effect on the fuel nozzle. To form the venturi arrangement, the air nozzle may comprise a contracting nozzle and a region of restricted cross-sectional area in the air nozzle may be defined between the inside surface of the air nozzle and the outer surface of the fuel nozzle. Nevertheless, the venturi arrangement will comply with the requirement of subsonic, supersonic, or hypersonic configuration, depending on the air flow injection regime chosen for specific combustor types. For example, if the flow is supersonic, the venturi nozzle shape will converge to a minimum area, then diverge to the exit.
In the above venturi arrangement, the fuel nozzle is preferably set back (i.e., recessed) with respect to the exit of the air nozzle, the fuel nozzle being nested within an inlet portion of the air nozzle. The distance by which the fuel nozzle is set back relative to the air nozzle exit is preferably sufficient to shield the fuel nozzle from the combustion products and is likely to be a few millimeters (a range of roughly 5 mm to 10 mm may be typical). Recessing the fuel nozzle relative to the air nozzle tends to prevent hot combustion products from contacting the fuel nozzle because the air stream provides positive shielding. That is, the air stream flows past the fuel nozzle before it flows through the air nozzle, thereby shielding the fuel nozzle from combustion products in the combustion chamber. However, the set back distance should not be so large as to enable the air stream and the fuel stream to mix to any significant extent before emerging into the combustion chamber, because this may have undesirable consequences for flame characteristics, leading to higher NOx emissions, and/or flashbacks.
The fuel is selected from the group consisting of liquid and gaseous fuels, such as natural gas (CH4), coke oven gas (COG), medium calorific value (MCV) fuel, integrated gasification combined cycle (IGCC) high hydrogen gasification gas, pure hydrogen, fuels containing fine particles of solid fuel such as coke dust, powdered or pulverized solid fuel in a gaseous stream (such as steam, nitrogen or air) to simulate a gaseous fuel, powdered or pulverized solid fuel in a liquid stream (such as water) to simulate a liquid fuel, or a pre-vaporized conventional liquid fuel.
The invention is capable of expression in many different forms.
In a first, currently preferred embodiment, the burner comprises a circular air supply nozzle and a circular fuel supply nozzle nested concentrically within the air supply nozzle. In this embodiment, the following burner size relationships are believed to maximize combustion efficiency and reduce pollution relative to the prior art:
0.5 mm to 3.5 mm, preferably 1.0 mm to 3.0 mm, for the fuel nozzle,
4.0 mm to 11.0 mm, preferably 4.8 mm to 10.5 mm, for the air nozzle.
In view of the above small burner size, it will be necessary to assemble groups or matrices of such burners together in the combustion chamber in order to produce sufficient power for normal commercial processes. For example, an upstream wall of an annular combustion chamber in an industrial or aero gas turbine engine could have groups or matrices of such burners arranged around it. In such groups or matrices, the burners should be positioned sufficiently far apart to ensure that the small diffusion flame each burner produces does not merge to any significant extent with any of the flames produced by neighboring burners. In such an arrangement, a typical distance between the centres of adjacent fuel nozzles should be of the order of two to three times the diameter of the air nozzles.
In a second embodiment, the burner comprises an annular air supply nozzle and a circular row of circular fuel supply nozzles centrally located within the radial height of the air supply nozzle and angularly spaced apart from each other around the air supply nozzle. Although the term “annular” usually means circular or substantially circular, when used throughout this description and claims the term should be interpreted as broad enough to embrace other circuitous shapes such as elliptical, polygonal, rectangular, and the like.
In this second embodiment, the radial height of the annular air supply nozzle must be sufficient to produce an annular air stream that enables the required cooling of the individual flames produced by the fuel supply nozzles. A radial height similar to the diameter of the circular air supply nozzle of the previous embodiment may be sufficient for this task. Such a burner would deliver the fuel streams into the combustion chamber as small diameter cores of fuel embedded in an annular air stream. The dimensions of the individual fuel supply nozzles within the annular air supply nozzle will be as for the previous embodiment, and the distance between neighboring fuel supply nozzles will also be the same. It will be evident that the amount of power such a burner can produce will be greater than a single burner of the preceding embodiment and will be determined by the diameter of the annular air supply nozzle and the number of fuel supply nozzles nested inside the annular air supply nozzle. In the context of gas turbine engine technology, such a burner could be particularly suitable for so-called “can” or “cannular” types of combustion chamber, in which each upstream wall of a combustion chamber is provided with one of the burners. Alternatively, a number of such burners could be equally spaced around the upstream wall of an annular combustion chamber.
A third embodiment features a burner having an annular air supply nozzle and an annular fuel supply nozzle centrally located within the radial height of the air supply nozzle, the fuel and air supply nozzles being concentric. Such a burner would look like an “annular sandwich”, and would deliver the fuel stream into the combustion chamber sandwiched between a radially outer air stream and a radially inner air stream. In this third embodiment, the radial height of the fuel supply nozzle should be such as to produce an annular flame having a radial thickness which facilitates rapid heat loss to the inner and outer air streams and therefore the desired low flame temperatures. Likewise, the radial height of the annular air supply nozzle must be sufficient to produce inner and outer annular air streams that enable the required cooling of the annular flame produced by the fuel supply nozzle. A radial height of the annular air supply nozzle similar to the diameter of the circular air supply nozzle of the previous embodiment may be sufficient for this task, provided the annular flame is sufficiently thin in the radial direction. Possibly, the radial height of the annular fuel supply nozzle could be in the range of roughly 0.5 mm to 3.5 mm. However, in view of the above considerations, it is preferred that the radial height of such an annular fuel supply nozzle is in the range of roughly 0.5 mm to 1.0 mm.
In a fourth embodiment, the burner comprises an elongate air supply nozzle having a height and a length and a row of circular fuel supply nozzles centrally located within the height of the air supply nozzle, the length of the elongate air supply nozzle being many times its height, so that this embodiment is a linear analogue of the above-described second embodiment.
In a fifth embodiment, the burner comprises an elongate air supply nozzle having a height and a length and an elongate fuel supply nozzle centrally located within the height of the air supply nozzle, the lengths of both the supply nozzles being many times their height, so that this embodiment is a linear analogue of the above-described third embodiment. If both nozzles are straight, such a burner would look like a sandwich or “a slot within a slot”.
Regarding the relative dimensions of the supply nozzles required to obtain low flame temperatures using burners according to the fourth and fifth embodiments, the considerations that apply are similar to those already mentioned in respect of the previous embodiments.
The elongate nozzles of the fourth and fifth embodiments can be straight-sided, curved or even wavy, say in the form of a sine wave or similar. Hence, for a given space, a wavy elongate fuel supply nozzle would have an increased length relative to a straight elongate fuel supply nozzle and hence increase the flame's surface to volume ratio.
In all the above embodiments, burners, or (in the case of burners comprising a number of individual fuel supply nozzles) individual fuel supply nozzles can be supplied with different types of fuel and interspersed with each other to enable the combustor to operate on two or more different fuel types (e.g., liquid and gaseous), either simultaneously or at different times during an operating cycle.
It would be possible, though not presently preferred, to locate an air swirler in or before the air supply nozzle to swirl the air stream before it emerges into the combustion chamber. Alternatively, a longitudinal axis of the air supply nozzle may angled with respect to a longitudinal axis of the fuel supply nozzle. This can provide a global swirl component to the air stream without the need for a separate air swirler component. However, it is generally preferred at present that the air stream does not have a substantial swirl component because this can create regions of lower pressure in the combustion chamber that can encourage the hot combustion products to move towards the burner.
Although size ranges have been quoted for the dimensions of the air and fuel supply nozzles of the embodiments, optimum combustion characteristics will be obtained by matching particular nozzle dimensions to particular fuels. However, if it is desired to use two or more types of fuel in the same burner, good combustion characteristics are nevertheless obtainable by making appropriate compromises between the ideal nozzle dimensions for the different fuel types. Fuel supply nozzles using the same fuel can be supplied from a common source through a manifold.
The present invention also embraces a burner provided for use in any of the combustors described above and a gas turbine including any of the combustors described above.
Although the combustors and burners of the embodiments described herein have particular application to gas turbine engines, the same principles are applicable at lower absolute air and fuel delivery pressures to various other types of domestic and industrial equipment, such as gas central heating boilers and steam-generating boilers
An annular passage 14 is defined between the inner surface 10 of the convergent air passage 12 and the tapered front end 5 of the fuel supplier 2. The annular passage is in fact a venturi with a throat at dotted line T. The velocity of the air stream increases as it flows through the passage 14 because of the venturi effect, the air flow through the burner being represented in
The shape of the convergent air nozzle portion 12, and in particular its radiused leading (i.e. upstream) edge 12a and its frusto-conical surface, can be selected to avoid any, or any excessive, turbulence as the air flows through the passage 14 and out of the nozzle exit 8. The tapered part 5 of the fuel nozzle 4 can also be designed to avoid turbulence and in the present case is tapered like the frustum of a cone. As shown, the upstream edge 12a of the air nozzle 12 is roughly axially aligned with a mid-portion of the tapered part 5 of the fuel nozzle 4. However, the exact amount by which the externally tapered fuel nozzle 4 projects into the internally tapered air nozzle 12 may vary upwards from zero and will depend upon the amount of flow constriction it is desired to create in the venturi, and where it is desired that the venturi throat T should be located. These will in turn depend on the type of fuel it is desired to burn, pressure differentials across the burner, and other factors. Computer-aided analysis or trial-and-error can determine such matters.
It is emphasised that the burner 1 of
It should also be noted that the present invention can be used in gas turbines equipped with sequential or reheat combustion systems, in either or both of the combustors.
The gaseous fuel for each burner 1 is supplied from a fuel source (not shown) along the central passage 3 of the fuel supplier 2 at a predetermined flow rate. At the same time, combustion air is supplied into air chamber 6 from an air source (not shown, but which in a gas turbine engine would be its high pressure compressor). The combustion air will flow through the air nozzle 12 at a flow rate determined by the pressure difference between air chamber 6 and combustion chamber 7, the air velocity past the fuel nozzle 4 being increased by the venturi effect, as mentioned above.
The relative values of the delivery pressures of the fuel and the air and the pressure in the combustion chamber have an influence on correct functioning of the invention. If the air delivery pressure is Pa and the pressure in the combustion chamber is Pc, the pressure difference Pa-Pc necessary to deliver air into the combustion chamber may be termed δpair. Similarly, if the fuel delivery pressure is Pf, the pressure difference Pf-Pc necessary to deliver fuel into the combustion chamber may be termed δpfuel. At present it is believed that the ratio δpfuel to δpair should be less than one (δpfuel/δpair<1). The pressure differences δpair and δpfuel will be of the order of 4% to 5% of the air delivery pressure Pa and the fuel delivery pressure Pf, respectively. In a gas turbine engine, the air stream will be provided at the delivery pressure of the final stage of the associated compressor and the fuel must obviously be delivered at a similar pressure. Such pressures may be in the range of roughly 5 bar to roughly 30 bar.
The air stream that flows through the air nozzle 12 represents the amount of air that is required for stoichiometric combustion of the fuel stream and subsequent dilution of the combustion process, the excess air being sufficient to act as a heat sink effective to lower the temperature of the above-mentioned diffusion flame sufficiently to minimise production of nitrogen oxides (NOx). In fact, all the air and the associated fuel acts as a heat sink to reduce the flame temperature. A suitable flame temperature is of the order of 1850 degrees Kelvin or less. The necessary condition to obtain such a relatively low temperature in a diffusion flame is that the flame must lose heat rapidly to the surrounding air, and the invention achieves this by burning the fuel in a small flame that has a relatively high surface to volume ratio when compared with the flames of existing premixed low NOx burners. For example, assuming use of a methane fuelled burner matrix like
Another of the controlling factors in the production of pollutants during the combustion process is the length of time the combustion products stay in the zone of high flame temperatures—the so-called “residence time”. Because the small diffusion flame of the invention minimises the residence times of combustion products in the zone of high flame temperatures, combustors using the invention are projected to have lower NOx emissions than combustors using known ultra-low-NOx lean premix technology. Furthermore, use of a small diffusion flame ensures that the air and fuel burn at the air/fuel ratio associated with the maximum flame speed (which is normally on the fuel rich side of stoichiometric when both air and fuel are supplied at the same temperature) across the entire range of loads experienced by a gas turbine engine. Therefore, the invention should enable stable and oscillation-free low-NOx combustion even at very low load conditions, such as so-called “idle” conditions. Hence, it is envisaged that pilot fuel burners and their associated fuel system may not be needed when the main fuel burners are designed in accordance with the present invention.
Burners in accordance with the invention can be optimised for use with a particular fuel. For example, hydrogen is notorious for having high flame speeds, which makes hydrogen burners prone to flashback, and a high burning temperature, which tends to produce high NOx levels in conventional combustion regimes, but it is expected that burners with nozzle sizes optimised for use with hydrogen fuel will not require either dilution of the fuel with a diluent such as nitrogen to reduce flame speed, or injection of diluents such as steam into the combustion chamber to control NOx.
Alternatively, the burners can be designed to allow use of a number of fuels without changing the hardware. Although the nozzle sizes, etc., will not be optimum for all the fuels, acceptable emission levels will nevertheless be achieved. For example, using a fuel nozzle exit diameter of 1.4 mm, the predicted maximum flame temperature is 1730K for natural gas fuel and 1740K for pre-vapourised kerosene, both examples being at 100% load for a given engine and the same hardware configuration.
It is a common expedient in gas turbine engine combustion chamber design to introduce dilution air after the primary combustion zone. However, in the present invention, all the combustion air required for each flame, including the dilution air, is supplied through the air nozzles 12 and is therefore immediately available in the proximity of the flame, i.e., the combustion process in the flame is followed by fast dilution and the post-flame reaction continues as a lean-burn reaction. For example, after dilution by the immediately adjacent air, the flame temperature is reduced from the above-mentioned 1850K or less to about 1100-1200K and the reaction continues through the combustion chamber to achieve a turbine inlet temperature of about 1600K.
Another advantage of the invention is that the fuel is injected directly into the combustion zone, with no pre-mixing in a separate premixing chamber; therefore no flash-back is expected and auto-ignition problems associated with high compression ratio engines or sequential (pre-heat/re-heat) combustors are likely to be avoided.
A further advantage associated with the present invention is that the use of small diffusion flames avoids the type of combustion instability that result in dynamic pressure oscillations in combustion chambers containing larger diffusion flames or lean-burn combustion processes.
The present invention could also be used to facilitate simplification of gas turbine engine systems by reducing or eliminating the need for fuel staging and the associated use of variable inlet guide vanes at the entrance to the compressor to improve the engine's “turndown” characteristic—i.e., to extend the load range over which low emissions can be achieved.
Yet another advantage of the invention arises because the air stream is effective as a protective air purge and shield to prevent combustion products such as carbon from building up on the end of the fuel nozzle 4. This positive shielding effect inhibits the flow of hot combustion products through the final nozzle 8 back to the fuel nozzle 4, and reduces or obviates the need for a separate supply of air for the sole purpose of purging fuel nozzles.
For a gas turbine engine burning a gaseous fuel, the diameter d of the fuel nozzle exit is in the range of roughly 0.5 mm to roughly 5 mm, preferably roughly 0.5 mm to roughly 3 mm. For example, for methane fuel (CH4) the diameter would be about 1.4 mm. More generally, the optimum diameter will depend on the type of fuel that is to be used. For example, when burning pure liquid fuels, such as kerosene or diesel, it will probably be advantageous to use smaller fuel nozzle exit diameters, e.g., in the range of roughly 0.2 mm to roughly 0.5 mm.
To obtain the correct excess of air when CH4 fuel is used, the exit diameter D of the air nozzle 12 will be about 7.2 mm, assuming subsonic air flow in the air nozzle. More generally, for each fuel type, the ratio of the exit diameter of the fuel nozzle to the exit diameter of the air nozzle will be a function of the desired air/fuel velocity ratio, i.e.,
d/D=f(air velocity/fuel velocity)
and from this relationship the exit diameter of the air nozzle may be in the range of roughly 4 mm to roughly 15 mm, again assuming subsonic air flow in the air nozzle The desired air/fuel velocity ratio in turn depends on the allowable NOx content of the combustion products. The following is the expected correlation that gives the NOx emissions for a given fuel at given conditions of inlet temperature, inlet pressure and air/fuel ratio:
NOx=f(Vairn1,Vfueln2,dfueln3)
where
NOx is the NOx emissions in parts per million by volume (ppmv)
Vair is the air exit velocity in metres per second (m/s)
Vfuel is the fuel exit velocity (m/s)
dfuel is the fuel nozzle exit diameter in millimetres (mm)
n1, n2 and n3 are power exponent values (positive or negative)
The axial distance L between the exit 8 of the air nozzle 12 and the exit of fuel nozzle 4 is selected to enhance the positive shielding effect mentioned above. In practice, the axial distance L may not be very critical and in some circumstances the exits of the fuel nozzle and the air nozzle 12 might be substantially coplanar. More typically, however, distance L in the case of a burner like that of
The individual burners 1 in the arrays of
Further design considerations allied to the present invention will now be explained.
As known to people skilled in the art, in a gas turbine operating at full load, the air mass flow to the combustor is fixed and this flow will be the same for any given fuel, assuming that the turbine flow capacity is not a limiting factor.
In designing a burner matrix as discussed above, a desired air velocity at the air nozzle exits is selected, for example, 160 m/s. From the air mass flow to the combustion system and the desired air velocity, the total area of the air exit nozzles will be calculated. For a given air nozzle exit diameter the total number of holes is calculated; for example to replace a prior art type of low NOx burner in an existing gas turbine engine might require 151 burners of the
Some optimization will be required to achieve the required NOx emissions. The procedure could be performed by selecting a larger (or smaller) air nozzle exit diameter, for example 10.5 mm (or 4.8 mm). In this case, the total number of holes to replace the prior art burner will change to 71 air holes (or 340 air holes). The corresponding NG fuel hole diameter will be 2.04 mm (or 0.93 mm). That is to say, a matrix of 151, 7.2 mm diameter air nozzles with 1.40 mm diameter fuel nozzles could be replaced by either:
The actual number of such burners used in a gas turbine engine combustion chamber will of course depend not only on the power that the gas turbine is required to generate, but also upon the fuel that is being burnt and upon a design trade-off between the complexity of the combustion system and the degree to which it is considered desirable to avoid production of environmental pollutants.
It will be appreciated by those skilled in the gas turbine combustion art that it is necessary to vary the amount of power produced by a gas turbine engine. This can readily be achieved in the present invention by reducing the amount of fuel supplied through the fuel nozzles, without unduly affecting flame stability or turndown characteristics. Alternatively, or additionally, if deemed to be a requirement for operation of the engine, a fuel staging strategy could be applied to an array or matrix of burners, such as shown in
During periods of time when no fuel is flowing through any particular fuel supplier 2 in the arrays of burners in
As was the case for burner 1 of
In an alternative configuration (not shown), the air supply nozzle 112 could be fed by an annular combustion air supply passage, with the fuel supplier 102 being coaxially located within it. Since the fuel supplier 102 is annular, it would divide the air supply passage into an inner annular air passage and an outer annular air passage.
As was the case for the embodiments of
As shown in
Burner 101 is preferably configured for a gaseous fuel such as hydrogen (H2), natural gas (CH4), coke oven gas (COG) or medium calorific value (MCV) fuel, for example. Depending upon its diameter and nozzle dimensions, one such gas burner 101 would be equivalent to several to many of the burners 1 of
Although the burners 401 are linear and arranged in parallel, they can alternatively be formed to various other linear shapes as desired. For example, fuel and air nozzles could be formed into straight-sided geometrical shapes or wavy lines.
Referring now to
The embodiment shown in
Whereas the above description has focused on versions of the invention suitable for gas turbine combustion systems, the invention also has the potential to be used for steam boilers with a gaseous stream containing particles of solid fuel such as coke dust or pulverised solid fuel in a liquid stream (e.g., water), in addition to normal fuels.
A further use of the invention could be for domestic central heating boilers.
Diameter of air nozzle exit 7.2 mm,
Air velocity at air nozzle exit 150 m/s
Air supply temperature, five test sets, 723K (450° C.)
Air supply temperature, one test set, 823K (550° C.)
Diameter of fuel nozzle exit 1.4 mm
NOx sensor probe station: 180 mm downstream of the burner air nozzle exit.
The graph shows the levels of NOx (in parts per million by volume on a logarithmic scale) produced by the burner over a range of air delivery pressures from 5 bar to 30 bar and a range of flame temperatures from about 1570K to about 1870K.
The design point for the burner with regard to the desired flame temperature is 1600K, as indicated by a circle on the line representing a set of tests using an air delivery pressure of 30 bar and an air delivery temperature of 823K.
NOx production during the tests varied from about 0.38 ppmv at the design point to about 2.0 ppmv at a flame temperature of 1867K, an air delivery pressure of 10 bar and an air delivery temperature of 723K.
It is also notable that at a second gas sensor probe station, 680 mm downstream of the burner air nozzle exit, sensed levels of carbon monoxide (CO) and unburned hydrocarbons were zero, indicating that combustion of the fuel was complete before that station.
The graph shows the levels of NOx (in parts per million by volume on a logarithmic scale) produced by the burner over a range of air delivery pressures from 5 bar to 30 bar and a range of flame temperatures from about 1400K to about 1865K.
The design point for the burner with regard to the desired flame temperature is again 1600K, as indicated by a circle on the line representing a set of tests using an air delivery pressure of 30 bar and an air delivery temperature of 823K (550 degrees C.).
NOx production during the tests varied from about 9.3 ppmv at the design point to about 51 ppmv at a flame temperature of 1840K, an air delivery pressure of 15 bar and an air delivery temperature of 723K.
Regarding the set of tests carried out at an air delivery pressure of 30 bar and an air delivery temperature of 823K, it is notable that at a flame temperature of 1400K, the hydrogen fuel burnt with NOx levels at or below 1 ppmv. This implies that with suitable burner design (e.g., smaller fuel nozzle hole diameter) or other combustion parameter adjustments to obtain more rapid flame heat dissipation, it may be possible to burn pure hydrogen at 1600K with NOx levels at or below 1 ppmv.
The above described embodiments are purely exemplary, and modifications can be made within the scope of the invention as claimed. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments. Each feature disclosed in the specification, including the claims and drawings, may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise.
Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
Various aspects and embodiments of the present invention will now be defined by the following numbered clauses:
1. A method for burning fuel with air in a combustion chamber with low production of NOx emissions, comprising the steps of:
supplying fuel and air to a burner associated with the combustion chamber, the amount of air supplied being at least as much as required for stoichiometric combustion of the fuel and all subsequent dilution of the combustion process in the combustion chamber; and
combusting the fuel with the air in a flame and a post-flame reaction by injecting the fuel and all the air through the burner directly into the combustion chamber in a substantially unmixed state as a fuel stream inside an air stream, wherein the air is immediately available in the proximity of the flame during the combustion process to act as a heat sink and so that the combustion process in the flame is followed by fast dilution and the post-flame reaction continues as a lean-burn reaction, the thickness of the fuel stream being such that the fuel burns in the combustion chamber at or close to the stoichiometric fuel/air ratio as a diffusion flame with a high surface-to-volume ratio.
2. The method of clause 1, further comprising the step of subjecting the air to a venturi effect before it leaves the burner, thereby to increase the speed of the air stream and exert a suction effect on the fuel.
3. The method of clause 1, wherein the amount of air supplied is between approximately 150% to 250% of the air required for complete combustion.
4. The method of clause 1, wherein no part of a fuel stream emerging from the burner is more than approximately 2.5 mm from a boundary between the fuel stream and the air stream.
5. The method of clause 1, wherein a distance from a centerline of a gaseous fuel stream to a boundary between the fuel stream and the air stream is in the range of approximately 0.5 mm to approximately 5 mm.
6. The method of clause 1, wherein a distance from a centerline of a gaseous fuel stream to a boundary between the fuel stream and the air stream is in the range of approximately 0.5 mm to approximately 3 mm.
7. The method of clause 1, wherein a distance from a centerline of a liquid fuel stream to a boundary between the fuel stream and the air stream is in the range of approximately 0.2 mm to approximately 0.5 mm.
8. The method of clause 1, wherein as the fuel and air streams leave the burner, the ratio of the velocity of the air stream to the velocity of the fuel stream is greater than unity.
9. The method of clause 8, wherein the air issues into the combustion chamber through an air nozzle of exit diameter D and the fuel issues into the combustion chamber through an fuel nozzle of exit diameter d, the ratio d/D being a function of the air/fuel velocity ratio.
10. The method of clause 9, wherein the NOx emissions for a given fuel at given conditions of burner inlet temperature, burner inlet pressure and air/fuel ratio are given by:
NOx=f(Vairn1,Vfueln2,dfueln3)
where,
NOx is the NOx emissions in parts per million by volume;
Vair is the air velocity at entry to the combustion chamber in meters per second;
Vfuel is the fuel velocity at entry to the combustion chamber in meters per second;
dfuel is the fuel nozzle exit diameter in millimeters (mm); and
n1, n2 and n3 are power exponent values (positive or negative).
11. The method of clause 1, wherein the ratio of a pressure difference across the burner necessary to deliver the air into the combustion chamber, to a pressure difference across the burner necessary to deliver fuel into the combustion chamber, is less than unity.
12. The method of clause 11, wherein the pressure differences across the burner are of the order of 4% to 5% of air delivery pressure and fuel delivery pressure, respectively.
13. A combustor comprising a combustion chamber and at least one burner that in use supplies a fuel stream surrounded by an air stream in a substantially unmixed condition directly into the combustion chamber to burn therein as a diffusion flame with low production of NOx, the at least one burner comprising an air supply nozzle and at least one fuel supply nozzle located within the air supply nozzle, the amount of air supplied by the air supply nozzle being at least as much as required for stoichiometric combustion of the fuel and all subsequent dilution of the combustion process in the combustion chamber, wherein the air is immediately available in a proximity of the flame during the combustion process to act as a heat sink and the combustion process in the flame is followed by fast dilution, with a post-flame reaction continuing as a lean-burn reaction, the at least one fuel supply nozzle being dimensioned to eject the fuel stream with a thickness such that the fuel burns in the combustion chamber at or close to the stoichiometric fuel/air ratio as a diffusion flame with a high surface-to-volume ratio.
14. A combustor according to clause 13, wherein the air supply nozzle and the at least one fuel supply nozzle are shaped and located relative to each other such that they form a venturi arrangement for air flowing between an inside surface of the air nozzle and an outer surface of the at least one fuel nozzle, whereby a venturi effect increases the speed of the air stream through the air supply nozzle and exerts a suction on the fuel emerging from the at least one fuel supply nozzle.
15. A combustor according to clause 14, wherein the venturi arrangement is shaped for subsonic air flow, or supersonic air flow, or hypersonic air flow.
16. A combustor according to clause 13, wherein the at least one fuel supply nozzle being dimensioned such that at an exit of the at least one fuel supply nozzle, no part of a fuel stream emerging therefrom is more than about 2.5 mm from a boundary between the fuel stream and the air stream.
17. A combustor according to clause 13, wherein an amount of air supplied through the air supply nozzle is between approximately 150% to approximately 250% of the air required for complete combustion.
18. A combustor according to clause 13, wherein the at least one fuel supply nozzle exit is recessed with respect to the air nozzle exit.
19. A combustor according to clause 18, wherein the at least one fuel supply nozzle nests within an inlet portion of the air supply nozzle.
20. A combustor according to clause 18, wherein the at least one fuel supply nozzle is recessed by a distance of approximately 5 mm to roughly 10 mm with respect to the air nozzle exit.
21. A combustor according to clause 13, wherein the at least one burner comprises a circular air supply nozzle and a circular fuel supply nozzle nested concentrically within the air supply nozzle.
22. A combustor according to clause 21, wherein for a gaseous fuel, the ratio of an exit diameter of the fuel nozzle to an exit diameter of the air supply nozzle is a function of the ratio of a desired air velocity at the burner exit to a desired fuel velocity at the at least one burner exit.
23. A combustor according to clause 21, wherein for a gaseous fuel, the exit diameter of the air supply nozzle is between approximately three times and approximately eight times the exit diameter of the fuel nozzle.
24. A combustor according to clause 21, wherein an exit diameter of the air supply nozzle is between approximately 5 times and approximately eight times an exit diameter of the fuel nozzle.
25. A combustor according to clause 21, wherein an exit diameter of the air supply nozzle is in the range of approximately 4 mm to 15 mm and an exit diameter of the fuel supply nozzle is in the range of approximately 0.5 mm to 5 mm.
26. A combustor according to clause 25, wherein the exit diameter of the fuel supply nozzle is in the range of approximately 0.5 mm to 3 mm.
27. A combustor according to clause 21, wherein for a liquid fuel, an exit diameter of the air supply nozzle is between approximately 20 times and approximately 30 times an exit diameter of the fuel supply nozzle.
28. A combustor according to clause 27, wherein the exit diameter of the air supply nozzle is in the range of approximately 4 mm to 15 mm and the exit diameter of the fuel supply nozzle is in the range of approximately 0.2 mm to 0.5 mm.
29. A combustor according to clause 13, comprising a plurality of can-type combustion chambers, wherein a plurality of the burners are grouped together in a head wall of each combustion chamber, the burners being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
30. A combustor according to clause 13, comprising a cannular-type combustion chamber, wherein a plurality of the burners are grouped together in each one of a plurality of head walls of the combustion chamber, the burners of each head wall being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
31. A combustor according to clause 13, comprising an annular type of combustion chamber, wherein a plurality of groups of burners are spaced around a head wall of the combustion chamber, the burners being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
32. A combustor according to clause 29, in which each group of burners is arranged as a matrix.
33. A combustor according to clause 30, in which each group of burners is arranged as a matrix.
34. A combustor according to clause 31, in which each group of burners is arranged as a matrix.
35. A combustor according to clause 29, wherein the distance between the centers of adjacent fuel nozzles is of the order of two to three times an exit diameter of the air nozzle.
36. A combustor according to clause 30, wherein the distance between the centers of adjacent fuel nozzles is of the order of two to three times an exit diameter of the air nozzle.
37. A combustor according to clause 31, wherein the distance between the centers of adjacent fuel nozzles is of the order of two to three times an exit diameter of the air nozzle.
38. A combustor according to clause 13, wherein the burner comprises an annular air supply nozzle and a circular row of circular fuel supply nozzles centrally located within a radial height of the air supply nozzle and angularly spaced apart from each other around the air supply nozzle.
39. A combustor according to clause 38, wherein the fuel nozzles are spaced sufficiently far apart from each other to prevent flames from neighboring fuel nozzles merging with each other.
40. A combustor according to clause 39, wherein the distance between the centers of adjacent fuel nozzles is of the order of two to three times the radial height of the air nozzle exit.
41. A combustor according to clause 38, wherein for a gaseous fuel, the radial height of the air nozzle exit is between approximately three times and approximately eight times the exit diameter of each fuel nozzle.
42. A combustor according to clause 41, wherein the radial height of the air nozzle exit is between approximately five times and approximately eight times the exit diameter of each fuel nozzle.
43. A combustor according to clause 38, wherein the radial height of the air nozzle exit is in the range of approximately 4 mm to 15 mm and the exit diameter of each fuel nozzle is in the range of approximately 0.5 mm to 5 mm.
44. A combustor according to clause 43, wherein the exit diameter of each fuel nozzle is in the range of approximately 0.5 mm to 3 mm.
45 A combustor according to clause 38, wherein for a liquid fuel, the radial height of the air nozzle exit is between approximately 20 times and approximately 30 times the exit diameter of each fuel nozzle.
46. A combustor according to clause 45, wherein the radial height of the air nozzle exit is in the range of approximately 4 mm to 15 mm and the exit diameter of the fuel nozzle is in the range of approximately 0.2 mm to 0.5 mm.
47. A combustor according to clause 13, wherein the burner comprises an annular air supply nozzle and an annular fuel supply nozzle centrally located within a radial height of the air supply nozzle, the fuel and air supply nozzles being concentric.
48. A combustor according to clause 47, wherein the radial height of the air nozzle exit is between roughly three times and roughly twelve times the exit height of each fuel nozzle.
49. A combustor according to clause 48, wherein the exit height of the air nozzle is between approximately five times and approximately twelve times the exit height of each fuel nozzle.
50. A combustor according to clause 47, wherein the exit height of the air supply nozzle is in the range of approximately 4.0 mm to 11.0 mm and the exit height of the fuel supply nozzle is in the range of approximately 0.5 mm to 3.5 mm.
51. A combustor according to clause 50, wherein the exit height of the air supply nozzle is in the range of approximately 4.0 mm to 11.0 mm and the exit height of the fuel supply nozzle is in the range of approximately 0.5 mm to 1.0 mm.
52. A combustor according to clause 38, comprising a plurality of can-type combustion chambers, wherein a head wall of each combustion chamber is provided with at least one burner.
53. A combustor according to clause 38, comprising a cannular-type combustion chamber, wherein each one of a plurality of head walls of the combustion chamber is provided with at least one burner.
54. A combustor according to clause 38, comprising an annular type of combustion chamber, wherein a plurality burners are spaced around a head wall of the combustion chamber.
55. A combustor according to clause 13, wherein the at least one burner comprises an elongate air supply nozzle having a height and a length, and a row of circular fuel supply nozzles centrally located within the height of the air supply nozzle, the length of the elongate air supply nozzle being many times its height.
56. A combustor according to clause 13, wherein the at least one burner comprises an elongate air supply nozzle having a height and a length, and an elongate fuel supply nozzle centrally located within the height of the air supply nozzle, the lengths of both the supply nozzles being many times their height.
57. A combustor according to clause 55, wherein at least one elongate nozzle is straight-sided.
58. A combustor according to clause 56, wherein at least one elongate nozzle is straight-sided.
59. A combustor according to clause 55, comprising a plurality of can-type combustion chambers, wherein a plurality of the burners are grouped together in a head wall of each combustion chamber, the burners being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
60. A combustor according to clause 56, comprising a plurality of can-type combustion chambers, wherein a plurality of the burners are grouped together in a head wall of each combustion chamber, the burners being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
61. A combustor according to clause 55, comprising a cannular-type combustion chamber, wherein a plurality of the burners are grouped together in each one of a plurality of head walls of the combustion chamber, the burners of each head wall being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
62. A combustor according to clause 56, comprising a cannular-type combustion chamber, wherein a plurality of the burners are grouped together in each one of a plurality of head walls of the combustion chamber, the burners of each head wall being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
63. A combustor according to clause 55 comprising an annular-type combustion chamber, wherein a plurality of groups of burners are spaced around a head wall of the combustion chamber, the burners being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
64. A combustor according to clause 56, comprising an annular-type combustion chamber, wherein a plurality of groups of burners are spaced around a head wall of the combustion chamber, the burners being spaced sufficiently far apart from each other to prevent flames from neighboring burners merging with each other.
65. A combustor according to clause 55, wherein the burners in each group of burners extend in parallel with each other.
65. A combustor according to clause 56, wherein the burners in each group of burners extend in parallel with each other.
66. A burner for use in a combustor according to clause 13.
67. A gas turbine engine including a combustor according to clause 13.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0709350.3 | May 2007 | GB | national |
This application is a continuation of International Application No. PCT/EP2008/055992 filed May 15, 2008, which claims priority to Great Britain Patent Application No. 0709350.3, filed May 15, 2007, the entire contents of all of which are incorporated by reference as if fully set forth.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2008/055992 | May 2008 | US |
| Child | 12618136 | US |
