Patent Application
20080078183
Because the invention proposes to inject liquid fuel through or in the vicinity of the swozzle vanes, the prior art liquid fuel/water/atomizing air cartridge (
The burner assembly is divided into four regions by function including an inlet flow conditioner 1, an air swirler assembly with fuel injection (swozzle assembly) 2, an annular fuel air mixing passage 3, and a central diffusion flame natural gas fuel nozzle assembly 4.
Air enters the burner from a high pressure plenum 6, which surrounds the entire assembly except the discharge end, which enters the combustor reaction zone 5. Most of the air for combustion enters the premixer via the inlet flow conditioner (IFC) 1. The IFC includes an annular flow passage 15 that is bounded by a solid cylindrical inner wall 13 at the inside diameter, a perforated cylindrical outer wall 12 at the outside diameter, and a perforated end cap 11 at the upstream end. In the center of the flow passage 15 is one or more annular turning vanes 14. Premixer air enters the IFC 1 via the perforations in the end cap and cylindrical outer wall.
The function of the IFC 1 is to prepare the air flow velocity distribution for entry into the premixer. The principle of the IFC 1 is based on the concept of backpressuring the premix air before it enters the premixer. This allows for better angular distribution of premix air flow. The perforated walls 11, 12 perform the function of backpressuring the system and evenly distributing the flow circumferentially around the IFC annulus 15, whereas the turning vane(s) 14, work in conjunction with the perforated walls to produce proper radial distribution of incoming air in the IFC annulus 15. Depending on the desired flow distribution within the premixer as well as flow splits among individual premixers for a multiple burner combustor, appropriate hole patterns for the perforated walls are selected in conjunction with axial position of the turning vane(s) 14. A computer fluid dynamic code is used to calculate flow distribution to determine an appropriate hole pattern for the perforated walls. A suitable computer program for this purpose is entitled STAR CD by Adapco of Long Island, N.Y.
To eliminate low velocity regions near the shroud wall 202 at the inlet to the swozzle 2, a bell-mouth shaped transition 26 is used between the IFC and the swozzle.
Experience with multi-burner dry low emissions combustion systems in heavy-duty industrial gas turbine applications has shown that non-uniform air flow distribution exists in the plenum 6 surrounding the burners. This can lead to non-uniform air flow distribution among burners or substantial air flow maldistribution within the premixer annulus. The result of this air flow maldistribution is fuel/mixture strength maldistribution entering the reaction zone of the combustor, which in turn results in degradation of emissions performance. To the extent that the IFC 1 improves the uniformity of air flow distribution among burners and within the premixer annulus of individual burners, it also improves the emissions performance of the entire combustion system and the gas turbine.
After combustion air exits the IFC 1, it enters the swozzle assembly 2. The swozzle assembly includes a hub 18 and a shroud 20 connected by a series of air foil shaped turning vanes 23 which impart swirl to the combustion air passing through the premixer. In an example embodiment of the invention, each turning vane 23 contains a primary, natural gas fuel supply passage 21 and a secondary, liquid fuel supply passage 22 through the core of the air foil. These fuel passages distribute natural gas fuel to primary fuel injection holes 24 and liquid fuel to secondary fuel injection holes 25, which penetrate the wall of the air foil. These fuel injection holes may be located on the pressure side, the suction side, or both sides of the turning vanes 23. Natural gas fuel enters the swozzle assembly 2 through inlet port 29 and annular passage 27 which feeds the primary turning vane passages 21. Liquid fuel enters through a suitable inlet port (not shown) and annular passage 28 which feeds the secondary turning vane passages 22. Alternatively liquid fuel is fed through a liquid fuel cartridge as described above and illustrated in
Since the swozzle assembly 2 injects the natural gas fuel or the liquid fuel through the surface of aerodynamic turning vanes (airfoils) 23, the disturbance to the air flow field is minimized. The use of this geometry does not create any regions of flow stagnation or separation/recirculation in the premixer after fuel injection into the air stream. Secondary flows are also minimized with this geometry with the result that control of fuel/air mixing and mixture distribution profile is facilitated. The flow field remains aerodynamically clean from the region of fuel injection to the premixer discharge into the combustor reaction zone 5. In the reaction zone, the swirl induced by the swozzle 2 causes a central vortex to form with flow recirculation. This stabilizes the flame front in the reaction zone 5. However, as long as the velocity in the premixer remains above the turbulent flame propagation speed, flame will not propagate into the premixer (flashback); and, with no flow separation or recirculation in the premixer, flame will not anchor in the premixer in the event of a transient causing flow reversal. The capability of the swozzle 2 to resist flashback and flame holding is extremely important for application since occurrence of these phenomena would cause the premixer to overheat with subsequent damage.
The invention takes advantage of the above-described existing design of the swozzle to make it capable of running on liquid fuel so that water injection is not required for NOx abatement and dedicated air supply is not required for atomizing liquid fuel. In this regard, the swozzle swirler structure is a dual core hollow casting where both internal passages 21, 22 have conventionally been used for gas fuel only. In accordance with an example embodiment of the invention, one of the existing internal passages or cores 22 is modified for use for injecting liquid fuel through a modified swirler vane in the manner of an air blast/pressure atomizer.
An embodiment of the modified swozzle vane is depicted by way of example in
Another example embodiment of a modified swozzle vane according to the invention is depicted in
Thus, the intention is to inject fuel through the swirler vanes instead of the central fuel nozzle. This provides a highly distributed source of liquid fuel compared to the conventional swozzle design. Since there are several vanes and the fuel is more highly distributed than when injected through the center fuel nozzle, it can potentially atomize due to the interaction with the incoming combustion air and no specially pressurized atomizing air stream is required. In one embodiment, as illustrated in
The location, number and characteristics of the atomizers and/or slots can be optimized for achieving the desired fuel-air mixing level at the diffusion tip location which is downstream of the vanes trailing edge. It may be noted that some or all of the atomizers may be placed on the center body between the vanes as opposed on the vanes themselves or an addition thereto, thus allowing even more flexibility in achieving optimum fuel-air mixing. This alternative is illustrated in FIG. 10. As illustrated therein, a pressurized fuel passage 202 is defined in the center body 218 and communicates with pressurized chambers 250 in the swirler vanes 223 via passages 207. Additionally, a nozzle tip 254 is threaded to a threaded hole 255 in the center body 218 so as to be in direct communication with the pressurized fuel passage 202. Thus, liquid fuel injection in this embodiment is from both the nozzle tip 254 on the center body outer wall and from the vanes at 252.
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.
