CATALYTIC FUEL-AIR INJECTOR WITH BLUFF-BODY FLAME STABILIZATION

CROSS-REFERENCE

[0001] This application claims the benefit of U.S. Patent No. 11/361 ,626 filed on February 24, 2006, which application is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates to a fuel-air injection device for use in a combustion system, as in a gas turbine engine. More specifically this invention is directed toward a shaped duct for injecting catalyst effluent into a combustion chamber, the shaped duct containing a small bluff-body for stabilizing (anchoring) combustion of the effluent. When the bluff body is properly positioned within the duct, and when the duct is properly shaped, as explained herein, the catalyst effluent is directed outward toward the walls of the combustor, even in the absence of swirl, forming a flame-anchoring recirculation zone with dimensions much greater than the bluff body diameter.

BACKGROUND OF THE INVENTION

[0003] One attractive option for low-emissions combustion of fuel with air is to use the method and apparatus described in U.S. Patents 6,358,040 and 6,394,791. These

patents describe an air-cooled catalytic reactor comprising metal tubes having catalyst-coated exterior surfaces. In operation fuel is mixed with air in fuel-rich proportions and contacted with the catalyst, while a separate air stream passes through the tubes' interiors to cool the catalyst. At the reactor exit (the downstream end of the tubes) the cooling air stream mixes with the catalytically-reacted fuel-rich stream to create a fuel-lean mixture for combustion completion.

[0004] In conventional (non-catalytic) lean-premixed combustion, a swirler would typically be used to inject the fuel-lean mixture into the combustor. When a swirler is used, the flame is anchored because the swirler induces recirculation (backmixing) of hot combustion products within the combustor. The recirculating, hot combustion products continuously contact and ignite the incoming fuel-air mixture, thus anchoring the flame in the vicinity of the recirculation zone, as is well known in the art. An additional effect of the swirler is to direct the incoming fuel-air mixture outwards toward the walls of the combustor, thus rapidly "diffusing" the mixture into the combustor, making effective use of the combustor' s volume. [0005] In the catalytic combustion system described by the '040 and '791 patents, however, the use of a swirler is undesirable since all fuel and air would typically be premixed (and partially pre-reacted) upstream of me swirler, at the catalyst exit. This premixed mixture is highly reactive, and may combust upstream of the swirler vanes if it auto-ignites or if a flame propagates upstream from the main combustor (flashback). This puts the swirler vanes at risk of overheating, and it is therefore preferable to avoid placing swirler vanes in the premixed fuel-air mixture downstream of the catalyst.

[0006] Without swirl, an alternate method must be used to provide for flame anchoring. The simplest means of flame anchoring is the use of a dump. Although

effective in stabilizing combustion, injecting a combustible fuel-air mixture axially into a combustion chamber without swirl, in the same direction as the bulk flow through the combustor, leads to a high-velocity jet transiting the combustor from inlet to exit, without significant expansion into the combustion chamber volume. In this case, velocities in the jet remain high and the residence time for a fluid particle transiting the combustor, from inlet to exit, remains relatively short even with an overly long combustor. One might consider correcting this problem by using a diffuser duct just upstream of the combustor, but without swirl a diffuser duct would be impractϊcally long since it would be constrained to a spread angle of less than about 10-degrees (half-angle) to prevent flow separation.

[0007] One solution that does not require swirl is the large flame-holding cone to direct the flow outward. Here, the term "large" means that the bluff body (cone) diameter is a significant fraction of the injector duct diameter, i.e. sufficiently large to redirect flow direction. In general, we shall consider a "large" bluff body to be one that has a diameter (at its largest cross-sectional) greater than about 70% of the injector duct's diameter (at its smallest cross-sectional). The disadvantage of a large cone is that it requires significant cooling air to ensure that it will not overheat in the event of flashback or autoignition. In many low-emissions combustion systems very little cooling air is available, and the use of significant cooling air can increase NOx emissions since cooling air must be taken from the primary fuel-air mixture, making it less lean.

[0008] At low flame temperatures, such as are possible with a catalytic combustor, long combustor residence times are needed for combustion completion in the gas- phase, downstream of the catalyst. Thus it is known in the art that a large flame holding cone can stabilize ultra-low emissions in a catalytic combustion system of the

type described in the '040 and '791 patents... In prior studies, a large flame-holding cone 2.6-inchs in diameter in a 3-inch diameter injector duct, followed by an 8-inch diameter combustor was used to anchor combustion downstream of the catalyst, and gas-phase combustion was completed in a cylindrical combustor having about 30 ms residence time. For comparison, combustors in aeroderivative machines typically have residence times below 10-20 ms, and industrial or large-frame machines typically have combustor residence times below 20-30 ms. Thus, it is yet to be demonstrated that even large cones can achieve low- enough residence times for many engines.

[0009] It is the purpose of this invention to provide such an alternate means of flame anchoring and to further provide an alternate means of "diffusing" the non-swirling mixture into the combustor, making effective use of the combustor's volume. This is especially important for ultra-low emissions combustion, where burning occurs at low flame temperatures, as would typically be the case in applications of catalytic combustion. In fact, the advantage of the catalytic combustion system described in the '040 and '791 patents is that catalytic pre-reaction of a portion of the fuel enables complete combustion (with low emissions of CO and unburned hydrocarbons) even at flame temperatures below 2600 F where "thermal" NOx emissions are negligible. [0010] In addition to a flame stabilization mechanism, having an additional fuel-air stage will provide system flexibility for the gas turbine operation. The additional fuel- air stage (pilot) provides a means of starting the engine and provide a means of assuring flame stability during the load shedding. In load shedding, the engine load is suddenly removed and the main fuel through catalytic modules will be suddenly be reduced to respond back to this sudden change in load. In this transient operation the

system should not loose the flame. The pilot flame (extra fuel-air stage) will provide this stability.

SUMMARY OF THE INVENTION

[0011] It has now been found a pilot fuel injector placed within the post catalyst duct of a catalytic reactor can serve as a small bluff body to stabilize not only a pilot flame but direct the surrounding flow outward towards the combustor wall if placed within a flared injector duct. Such placement induces outward motion of the primary flow towards the combustor walls for effective "fill" of the combustor volume without the need for swirler vanes or large cones. Combustion is stabilized by the resulting gas recirculation and by contact with the pilot flame.

[0012] The present patent thus provides a system to fully utilize combustor volume and minimize the combustor size required for required burnout of fuel values.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Fig. 1 provides a schematic of a combustion system according to the present invention.

[0014] Figures 2 through 5 provide a depiction of CFD results for respective curvatures as described herεinbelow.

DETAILED DESCRIPTION OF THE INVENTION

[0015] As shown in Figure 1, exit flow from RCL reactor 20 flows through postmix duct 21 and enters combustion zone 22. Flow duct 42 in pilot 41 supplies the pilot flow of fuel and air to combustion zone 22. Downstream combustion zone 22. Central pilot 41 is located within duct 21. Flow Surface 43 of pilot 41 serves as a

small bluff body for dump stabilized combustion and provides an area of pilot flow recirculation. The combustion heating and resulting expansion of the pilot flow together with flared exit 32 of duct 21 wall 31 forces the postmix flow from reactor 20 to follow the flared exit and fill combustion zone 22 volume.

[0016] In this configuration, gas exiting from the reactor 20 flows through a postmix channel 21 and enters the downstream combustion zone 22. There is a central pilot 41, located at the center of duct 22. Pilot 41, has flow duct 42, where fuel and air may enter the combustion zone as premixed for low NOx emission operation. The end surface 43 of pilot 41 provides an area of flow recirculation to anchor the flame at the end of pilot 41. The resulting heating and expansion of the pilot flow pushes the surrounding flow outward the nature of flared exit 32 with radius R on duct 31 expands the flow to fill the combustor and at the time creates a large scale recirculation in the combustion zone 22.

[0017] In general, for a given RCL and central pilot, there are several key parameters for an effective downstream system design, (a) The axial location of the pilot surface 43 with respect to the curvature 32 (b) The curvature R of flared end 32 (c) the mass flow rate through the central pilot 42. [0018] PILOT DESIGN:

[0019] Axial Location — The best optimum operations are when the axial location of the pilot surface 43 is in line (flushed) or recessed with respect to the beginning of the curvature 32.

[0020] Mass flow through Pilot Hole 42 - Ideally, for a stable flame, the surface area 43 requires no mass flow in order to maintain a stable central recirculation. However, during the engine start up and load shedding there is a requirement for having a second flame. In addition, the pilot surface area, such as surface 43 is required to be

cooled. Thus a value such as 1-5% of the RCL air flow is recommended to flow through hole 42. The phi (=equivalence ratio) for this flow may be below 0.45 to minimize NOx level at typical combustor inlet temperatures.

[0021] The central pilot flow through pilot duct 42 may used for engine start up and for load shedding.

[0022] FLARED POST MDC DESIGN:

[0023] Ideally one needs to select a large curvature radius, R for location 32. For a given RCL exit diameter, the minimum acceptable value for R is readily selected by trial and error to achieve the flow such that the downstream combustor zone is filled.

This can be achieved experimentally by fabricating different size R curvature for 32 or by running analytical tools, such as CFD to determine acceptable flow expansion.

Figure 2 shows the flow field for four different R values.

[0024] CFD Test Results for Curvature R= 0.1", 0.3", 0.75", 1.4" are provided in

Figures 2, 3, 4, and 5, respectively. The relevant parameters in generating the CFD models are as follows:

V =141.16 ft/s = 44 m/s Pressure = 17 atm T = 950 K = 677 C Reynolds stress model Incompressible ideal gas Pilot diameter- 1.63 inches Postmix diameter - 3.4 inches Combustor diameter — 8 inches

[0025] Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the invention should not be limited to the description of the preferred versions contained herein.