Patent Application
20110131998
This invention relates to system and method for injecting fuel in secondary fuel nozzles in gas turbine systems.
Numerous regulations requiring reductions in the amount of emissions, especially nitrogen oxide (NOx) and carbon monoxide (CO), have been enacted by governmental agencies in an effort to reduce pollution from gas powered turbines. More efficient combustion processes, such as those provided by Dry Low NOx (DLN1) systems, can lead to lower combustion emissions.
Some early combustion systems utilized diffusion type nozzles that produce a diffusion flame. This is a type of nozzle that injects fuel and air separately and mixing occurs by diffusion in a flame zone. Diffusion type nozzles produce high emissions since fuel and air burn stoichiometrically at high temperatures.
To improve upon diffusion nozzles, fuel and air can be premixed such that the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than the diffusion type flame producing lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto.
The secondary fuel nozzle 10 also includes a plurality of discrete pegs 12, which are fed with fuel from a fuel delivery conduit 13. Pegs 12 are provided to disperse the fuel—i.e., to spray the fuel into the secondary air 20—to achieve a premix of fuel and air upstream of the combustion chamber. In this regard, pegs 12 act as injectors for injecting the fuel into the secondary air 20. Conventionally, round fuel pegs 12 are used with round fuel dispersion holes 22 to disperse the fuel as illustrated in
Referring back to
It will be appreciated that the pegs in the conventional secondary fuel nozzle can lead to a substantial pressure drop in the air flow since they are in the flow area of the secondary air. For example, when there are six pegs in the secondary fuel nozzle, the pegs block approximately 29% of the flow area. The resulting pressure drop can adversely affect overall efficiency of the turbine, which in turn can lead to increased emissions. Also, pegs limit ability to vary the delivery of fuel. Moreover, pegs limit flexibility to adapt to wide range of fuel volumetric flows.
A non-limiting aspect of the present invention relates to a secondary fuel nozzle which includes a nozzle body arranged deliver fuel for combustion in a gas turbine and a nozzle housing surrounding the nozzle body. The nozzle housing defines one or more air passages at predetermined lengthwise positions of the nozzle body such that the secondary air entering the nozzle housing flows from the one or more air passages towards one end of the nozzle body. A premix fuel unit, which is provided upstream of the one or more air passages, is arranged to inject fuel delivered by the nozzle body into the secondary air to premix the fuel and air prior to combustion.
Another non-limiting aspect of the present invention relates to a gas turbine, which includes a combustion chamber and a secondary fuel nozzle arranged to provide premixed fuel and air into the combustion chamber. The above described secondary fuel nozzle can be used in the gas turbine.
The invention will now be described in greater detail in connection with the drawings identified below.
For ease of reading, terms “secondary fuel nozzle”, “fuel nozzle” and “nozzle” will be used interchangeably in this disclosure. As explained above, conventional nozzles include pegs in the flow area of the secondary air which can cause a substantial pressure drop in the air flow leading to many disadvantageous affects. In one or more embodiments of the inventive nozzle, fuel is injected upstream of the secondary air entry location. This removes the need for the pegs to deliver fuel for premixing.
In
Injecting fuel upstream of the secondary air entry location removes the need for the fuel delivery pegs of the conventional secondary fuel nozzle. That is, the inventive secondary fuel nozzle can be free of fuel delivery pegs in the air flow area downstream of the air passages, which minimizes air flow blockage. The removal of pegs in the flow area correspondingly reduces the pressure drop which in turn enhances combustion efficiency and leads to lower emissions.
Other advantages also follow the inventive fuel nozzle. For example, injecting fuel upstream provides more length for fuel mixing. The reduced pressure drop can allow for a higher fraction of the combustor air flow to be used in the secondary fuel nozzle. There are also potential turndown, dynamics, and CO emissions benefits for increasing secondary air and fuel splits. Increasing the secondary air-fuel splits will help to have more stable secondary flame and will thus improve turndown and dynamics performance. Reducing pressure drop through the secondary fuel nozzle in turn reduces over all pressure drop that increases overall system efficiency. Upstream fuel injection improves the flexibility of using fuel nozzle designs for a wider range of fuel volumetric flows. Since fuel can be injected in a co-flow manner, it reduces blockage behind fuel injection that causes recirculation zones and flame holding in peg-style fuel injection. This arrangement also provides more fuel area with minimum blockage effect enhancing fuel flexibility of the system as low energy content fuels that require more area can be utilized in such nozzle.
While it is preferred that the fuel nozzle be free of pegs downstream of secondary air entry location, the invention does contemplate that at least some fuel injection can occur downstream. For example, a combination of upstream and downstream fuel injection is possible if the air flow blockage due to downstream fuel injection mechanism is minimal, i.e., it is at or below some predetermined tolerance level. It may be that in some instances, providing some fuel injection downstream may be beneficial in the overall premixing of fuel and air process. The downstream fuel injection, if provided, can be via pegs or other mechanisms.
For simplicity of illustration, the nozzle housing 210 which surrounds the nozzle body 16 is omitted in these figures. The locations of the air passages 220 are illustrated by the secondary air 20 entering the nozzle housing 210 at predetermined lengthwise positions of the nozzle body 16. Secondary air 20 flows rightward towards one end of the nozzle body 16.
The first embodiment includes a premix fuel unit 500 positioned upstream of the air passages 220. The premix fuel unit 500 injects the fuel delivered by the nozzle body 16 into the secondary air 20 so as to premix the fuel and secondary air prior to combustion in the combustion chamber.
The fuel injected by the premix fuel unit 500 is fed from the nozzle body 16, and specifically, from one or more fuel feed conduits 13 provided within the nozzle body 16. As seen in
The fuel feed conduit 13 can run lengthwise from a base 28 to near the nozzle tip 14—nearly the whole length of the nozzle body 16 as seen in
While a single fuel feed conduit 13 is illustrated for simplicity in
A central passage 26 can also be provided within the nozzle body 16. The central passage 26 delivers fuel to feed the diffusion pilot tube 11 to inject fuel directly into the combustion chamber 18.
The premix fuel unit 500 of the first embodiment includes a first plate 510 positioned upstream of the air passages 220 and a second plate 520 positioned further upstream so as to form a fuel plenum 550 with a volumetric space defined between the plates 510, 520. A plurality of fuel injection holes 530 are provided on the first plate 510. The premix connection conduits 540 connect the fuel plenum 550 to the fuel delivery conduits 13 so that the fuel plenum 550 can be filled with the fuel which is injected into the secondary air 20 via the plurality of fuel injection holes 530.
The separation between the first and second plates 510, 520 should be enough to create the volumetric space needed for uniform fuel distribution inside the fuel plenum 550 from which the fuel contained within the space can be injected into the air stream 20. Not having sufficient volume inside the fuel plenum 550 can cause fuel non-uniform distribution inside the fuel plenum 550 leading to non-uniform fuel injection through the fuel injection holes 530 that will results in reduced air-fuel mixing necessary for lower emissions. In addition non-uniform distribution could also affect flame holding capability of the nozzle. In one embodiment, the volumetric space of the fuel plenum 550 is laterally defined by the first and second plates 510, 520 and radially and circumferentially by the nozzle body 16 and the nozzle housing 210.
The fuel injection holes 530, in a simple arrangement, can be arranged circumferentially as illustrated in
The distribution of the fuel injection holes 530 need not be regular. For example, to minimize wetting of the surface of the nozzle body 16, the fuel injection holes 530 can be distributed such that more are provided at radially further distances from the center. The holes 530 are not limited to any particular shape as well—they can be circles, squares, rectangles, arcs, and so on. In addition, the sizes of the holes 530 are not limited. One or more factors regarding the fuel injection holes 530—distribution, shape, size—can be varied to provide a desired fuel injection pattern and amount so as to optimize the premixing of fuel with air to achieve efficient combustion.
One difference between the two embodiments is in the premix fuel unit. The premix fuel unit 600 of the second embodiment includes a plate 610 with a plurality of fuel injection holes 630 located upstream of the air passages 220 much like the first plate 510 of the first embodiment. However, instead of forming a fuel plenum with another plate, the second embodiment premix fuel unit 600 includes a fuel manifold 620 positioned further upstream of the air passages 220. The fuel manifold 620 delivers fuel from the nozzle body 16 to the fuel injection holes 630 so as to inject fuel into the secondary air 20.
As seen in
The fuel manifold 620 also includes a plurality of distribution connections 660. As seen in
In
It should be noted however that the invention does contemplate leaving one or more fuel injection holes 630 unconnected to the extent that the likelihood of the flame holding region from occurring is minimal, i.e., the likelihood is maintained at a predetermined threshold level or below. For example, a separate mechanism to seal some or all unconnected fuel injection holes 630 can be provided.
Similar to the first embodiment, the plurality of fuel injection holes 630 can arranged to be varied in a variety of ways. As seen in
One advantage of the second embodiment's fuel manifold 620 over the first embodiment's volumetric space 550 is that possibility of fuel leak is reduced. For example, the first and second plates 510, 520 may be welded only on the inner circumference. On the outer circumference, the clearance gap between the plates and nozzle body 210 may be closed with a seal. In the event of a seal failure, fuel may leak into the air stream. However, in case of second embodiment, even when the seal fails, leakage is less likely since fuel is supplied through distribution connections 660. Thus possibility of fuel leak is reduced. In addition, also in the event of the seal failure, air may rush into to the volumetric space between the plates 510, 520 in the first embodiment. However, in the case of the second embodiment, air will not come in contact with fuel since fuel is passed through the closed distribution connections 660.
Indeed, in some instances, it may actually be more preferable to vary the lengths of the fuel extension tubes 710 as illustrated in
Yet further, the cross-sectional shape and/or size along a length of any one fuel extension tube 710 need not remain constant. The view of the fuel extension tubes 710 in
This variation also allows the flexibility of determining the characteristics of the fuel extension tubes 710 connected to the fuel injection holes 630 including lengths, shapes, sizes, and shapes along lengths of individual fuel extension tubes 710. There also is the option of not providing the fuel extension tube 710 for some of the fuel injection holes 630.
While not illustrated, it is allowable to have one or more fuel extension tubes 710 that extend such that fuel is injected downstream of the air passages 220. Preferably, the air flow blockage caused by such extension tubes 710 is kept at or below a predetermined level. Also while not illustrated, the extension direction of the fuel extension tubes 710 can vary as well. While it may be preferred that the fuel extension tubes 710 extend in a direction that is substantially parallel to the nozzle body 16, in some instances, it may be that having some fuel extension tubes 710 extending in a non-parallel direction may promote better total fuel and air premix.
The inventive secondary fuel nozzle embodiments may be used to premixed fuel and air into a combustion chamber 18 of a gas turbine for combustion.
It will thus be appreciated that the inventive secondary fuel nozzle provides many benefits. A non-exhaustive list of such advantages include: removal of pegs in the flow area correspondingly reduces the pressure drop which can enhance combustion efficiency and lead to lower emissions; injecting fuel upstream provides more length for fuel mixing; the reduced pressure drop allows for a higher fraction of the combustor air flow to be used in the secondary fuel nozzle; turndown, dynamics, and CO emissions benefits for increasing secondary air and fuel splits; and upstream fuel injection improves the flexibility of using the inventive fuel nozzle design for a wider range of fuel volumetric flows including using fuel with lower energy content.
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 embodiments. The invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
