Patent Application
20100071373
The invention generally relates to a gas turbine engine, and more particularly to a pilot burner capable of achieving low-NOx emissions during operation.
Combustion engines such as gas turbine engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to this energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustor section. The products of combustion are directed through a turbine section, where they expand and turn a central rotor.
A variety of combustor designs exist, with different designs being selected for suitability with a given engine and to achieve desired performance characteristics. One popular combustor design includes a centralized pilot burner (hereinafter referred to as a pilot burner or simply pilot) and several main fuel/air mixing apparatuses, generally referred to in the art as injector nozzles, swirlers, main swirlers or main swirler assemblies, arranged circumferentially around the pilot burner. With this design, a central pilot flame zone and a mixing region are formed. The stream of mixed fuel and air flows out of the mixing region, past the pilot flame zone, and into a main combustion zone of a combustion chamber, where combustion occurs. Energy released during combustion is captured by the downstream components to produce electricity or otherwise do work.
During operation, the pilot burner selectively produces a stable flame that typically is anchored in the pilot flame zone, while the fuel/air mixing apparatuses produce a mixed stream of fuel and air in the above-referenced mixing region. In many designs of conventional pilot burners, the stabilization of the pilot flame is recognized to occur due to a strong, central recirculation zone. An undesired effect of the recirculation zone is the occurrence of regions of higher than average temperatures which result in relatively high levels of undesired combustion products, such as oxides of nitrogen (NOx). Due to concerns and regulations about reduction of undesired emissions from gas turbines, a number of approaches have been taken toward reduction of such emissions while maintaining a suitable efficiency and stable combustion.
Among the approaches taken to address a balanced, low-NOx burner (also referred to by some as a nozzle) is an approach taught in U.S. Pat. No. 5,735,681. That patent teaches a fuel-air nozzle in which a premixed fuel-air mixture is swirled gently by low swirl jets of air introduced tangentially, upstream of the exit port of the fuel-air nozzle. As the fuel/air mixture moves downstream from the fuel nozzle, the flow stream diameter increases, the axial flow velocity decreases and the flame is stated to be positioned where the flame speed matches the flow rate of the fuel-air mixture. This is stated to occur without recirculation which would normally result in an anchoring of flame at a point near the fuel nozzle. The patent further states that “[b]ecause the fuel-air mixture is weakly swirled only at the outside edges of the burn zone, complete burning is possible and NOx emissions are minimized.” U.S. Pat. No. 5,879,148 describes an another approach where a flow balancing insert in introduced into a central passage which surrounds an annular passage of a swirler to produce a stable flame without recirculation.
Despite such approaches in the art, there remains a need to develop an efficient and flexible pilot burner suitable for commercial gas turbine engines, and more particularly for lean premixed gas turbine engines.
The invention is explained in following description in view of the drawings that show:
The present invention provides a flexible, superiorly controllable multi-passage pilot burner for gas turbine and other applications that may benefit from such pilot burner's low-NOx operation. To achieve this, various embodiments of the present invention include an inner and an outer mixing passage, each of these having separate inlets for a fluid oxidant, such as air, and separately controlled fuel outlets supplying fuel to the respective passages. The mixing of the fluid oxidant and fuel occurs in each respective mixing passage rather than at a common upstream locus. By providing for separate controls of fuel to each mixing passage, the pilot burners are considered to be fuel-profilable. Further, separate fluid oxidant flows are provided to the inner and outer mixing passages, and these flows may be modified in various embodiments through modifications at or near the entrances to the respective mixing passages to control flow velocity through the respective passage. Also, by appropriate design within the constraints of the features of the present invention, the flow of pre-mixed fuel/oxidant leaving the pilot burner diverges in a generally expanding conical flow pattern and there is no need for, nor use of, a flame stabilizer nor flow recirculation zone to stabilize the flame. Rather, stable flame and combustion are achieved by initial selling of fluid oxidant and, as needed during operation, modification of fuel supplied to the separately controlled fuel outlets. Another benefit of the present invention is that the pilot burner is able to operate at a lower fuel/oxidant ratio than conventional pilot burners. Specifically the present pilot burner is able to provide stable combustion at a fuel/oxidant ratio that may be as low as the fuel/oxidant ratio of the main burners. Also in contrast with other approaches, in various embodiments flow-balancing inserts are not required. Thus, elegant solutions are achieved toward design of a pilot burner that has a broad flow range between blow-off and flashback, and yet that also consistently achieves low-NOx combustion.
The appended figures and descriptions of them below are meant to be illustrative of aspects of the invention without being limiting.
The outer annular mixing passage 180 is disposed radially outwardly from the inner mixing passage 160. A cylindrical casing 185 separates the two mixing passages 160 and 180, and also supports at least one outer swirler element 186 disposed in the outer annular mixing passage 180. The other more outward ends of the outer swirler elements 186 are fixedly attached to an outer swirler casing 187. The swirler elements 186 are disposed at a desired angle transverse to the axial flow axis 161 so as to impart a desired swirl velocity (i.e. rotating about axis 161) to fluids passing the swirler elements 186.
Fluid oxidant, such as compressed air, enters the outer annular mixing passage 180 through an upstream annular port 181. Optionally, the port 181 may be flow-restricted such as by placement of an optional fluid-permeable cover 182. This may be a wire cloth, a wire screen, or plate with holes, or other cover that restricts flow but nonetheless allows a desired amount of fluid to pass through apertures 183 and then into the port 181 and into the outer annular mixing passage 180 to mix with fuel. Fuel may be supplied to outer annular mixing passage fuel outlet(s) 188 from a second fuel supply 182 via a second fuel line 184 that communicates with an outer fuel controller 190. These fuel outlets 188 are shown disposed along the outer swirler elements 186. However, this is not meant to be limiting, and more generally, outer annular mixing passage fuel outlets may be disposed anywhere in the outer annular mixing passage 180.
An optional diffusion fuel outlet 189 is also shown. It is noted that the specific features of this exemplary embodiment are not meant to be limiting. For example, instead of the swirler elements 186, other structures may be employed to provide a desired flow pattern.
During an exemplary operation, compressed air, as an exemplary fluid oxidant, enters the inner mixing passage 160 through at least one fluid-restricting inlet port 166, and fluid oxidant also enters the outer annular mixing passage 180 through port 181. The optional fluid-permeable cover 182 may be set into place over the port 181 by means known in the art, such as spot welding, during operational down times if it is determined after a period of operation that operations would be enhanced with relatively less flow entering the outer annular mixing passage 180. The percent open area may be varied in this optional fluid-permeable cover 182, and may be set, for example, between 10 percent and 90 percent, or between 30 percent and 60 percent, or any ranges therein.
Analogously, the inlet port(s) 166 may be further restricted, as may be determined to be desired after a period of operation, by inserting optional inserts 167 having a desired open area 169. These optional inserts 167 act to further restrict the flow velocity of the oxidant, thereby further reducing the axial flow velocity of the fuel/oxidant mixture exiting the inner mixing passage 160. Optional inserts 167 may be affixed by spot welding or other methods of attachment known to those skilled in the art. Thus, the oxidant flow rates through each of the passages 160, 180 may be tailored for a particular application, either before initial operation or after a period of operation where application-specific data is gathered and is then used to determine optimal oxidant flow rates.
Because in various embodiments there are independently controllable fuel supplies to the inner and the outer annular mixing passages, such pilot burners are considered fuel-profilable. In other words, the fuel profile of the fuel/oxidant mixture exiting the inner mixing passage may have a first fuel concentration (equivalence ratio) while the fuel profile of the fuel/oxidant mixture exiting the outer annular mixing passage may have a second, different fuel concentration (equivalence ratio), and these concentrations may be different from each other and may be varied. For example, the mixture may be leaner in the inner mixing passage than in the outer passage. As these different mixtures move further downstream and the effect of the relatively narrow but strong swirl of the outer annular mixing passage results in an outward substantially conical spreading pattern, as will be described more fully below, the relative fuel concentrations in the respective fuel/oxidizer mixtures provide for flexibility that may lead to more stable and clean operation.
For comparative purposes, it is noted that the overall swirl number of a conventional swirler assembly may be in the range of 0.65 to 0.7. In embodiments of the present invention the overall swirl number may be lower, about 0.4 to 0.5, however there is a narrow annular region of relatively stronger swirl, coming from the outer annular mixing passage due to the presence of at least one swirler element, that is effective to establish, further downstream in the pilot flame zone, a narrow strong swirl zone between unburned and burned fuel/oxidant mixtures to create a shear layer that continuously mixes the unburned and burned fuel/oxidant mixtures. Lower swirl of the mixture flowing from the inner mixing passage reduces the overall swirl number of the fuel-profilable pilot burner of the present invention.
Further, in various embodiments, the inner mixing passage 160 lacks a flow balancing insert. This is the case for the embodiment of
In one particular embodiment, meant to be illustrative and not limiting, a relatively narrow strong swirl zone is established between the unburned and burned mixtures to create a shear layer that continuously mixes the unburned and burned fuel/oxidant mixtures. This keeps the flame ignited while minimizing undesired regions of recirculation and/or high fuel/oxidant ratio and consequent undesired elevated NOx emissions
Embodiments of the present invention are used in gas turbine engines such as are represented by
All patents, patent applications, patent publications, and other publications referenced herein are hereby incorporated by reference in this application in order to more fully describe the state of the art to which the present invention pertains, to provide such teachings as are generally known to those skilled in the art.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Moreover, when any range is described herein, unless clearly stated otherwise, that range includes all values therein and all subranges therein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
