WO2016061067A1

WO2016061067A1 - Swirl jet burner

Info

Publication number: WO2016061067A1
Application number: PCT/US2015/055278
Authority: WO
Inventors: Masoud Hakak KHADEM
Original assignee: Eclipse, Inc.
Priority date: 2014-10-13
Filing date: 2015-10-13
Publication date: 2016-04-21
Also published as: CN107002988A; US20160102857A1

Abstract

A burner includes a combustor having a generally cylindrical, hollow shape extending along a centerline, and a closed end. A fuel tube that is connectable to a fuel supply has at least one opening fluidly connecting an interior of the combustor with the fuel source such that fuel can be delivered from the fuel source into the interior of the combustor. At least one air inlet disposed to provide an air stream into the interior of the combustor. The air stream is arranged and configured to create a vortex flow structure within the combustor.

Description

SWIRL JET BURNER

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/063,037, which was filed on October 13, 2014, the contents of which are hereby incorporated herein in their entirety by this reference.

FIELD OF THE DISCLOSURE

[0002] The present disclosure relates to burners and, more specifically, to burners having a combustor.

BACKGROUND OF THE DISCLOSURE

[0003] Industrial furnaces operate at high temperatures, typically in the range of about 2400-3000°F, to promote furnace and process thermal efficiency. As a result, furnace and flame temperatures tend to be high, resulting in the generation of significant amounts of NOx emissions.

[0004] Efforts to control NOx emissions have resulted in the development of various NOx control technologies. Such technologies inhibit NOx formation by modifying flame stoichiometry and the overall combustion process. Exemplary NOx control technologies include oxygen-enriched air staging, in which oxygen-enriched air is introduced in stages into the combustion process, exhaust gas recirculation, in which exhaust gas is introduced into the primary combustion zone to reduce the flame temperature, fuel staging, in which the fuel is introduced in stages into the combustion process, and other methods such as oscillating and pulsed combustion. Although some of these controls have been partially effective at controlling NOx emissions, they may not sufficiently address NOx formation and reduction at the burner.

BRIEF SUMMARY OF THE DISCLOSURE

[0005] The present invention relates to flame stabilization in a burner using a decaying swirling air flow created by tangential jets in an annulus combustor, which in the illustrated embodiments is formed between an outer shell and a gas pipe. A swirling motion imparted into air or an air/fuel mixture passing through the burner and provided to a burner outlet is used to form flue gas recirculation (FGR) zones in the vicinity of the flame, which leads to a lower flame temperature and a lower rate of nitrogen oxide (NOx) formation. Gas that is radially injected into the axial-tangential air stream may further maximize air/fuel mixing and increase combustion efficiency.

[0006] In the disclosed embodiments, a wide range of swirl numbers can be achieved at the burner outlet. The swirl number is controlled by changing the number of tangential jets, the diameter of the jets, the incoming flow angle, or the length of the annulus, independently. In these respects, numerical simulations and experimental investigations have shown that reducing the air flow rate results in slightly lower swirl number while maintaining the burner stability at lower inputs, which results in very high turn-down ratios.

[0007] In one aspect, therefore, the disclosure describes a burner. The burner includes a combustor having a generally cylindrical, hollow shape extending along a centerline, and a closed end. A fuel tube that is connectable to a fuel supply has at least one opening fluidly connecting an interior of the combustor with the fuel source such that fuel can be delivered from the fuel source into the interior of the combustor. At least one air inlet disposed to provide an air stream into the interior of the combustor. The air stream is arranged and configured to create a vortex flow structure within the combustor.

[0008] In another aspect, the disclosure describes a method for operating a burner. The method includes providing a combustor having a generally hollow cylindrical shape defining a closed end and an open end, the combustor defining an interior volume therein. The method further includes injecting at least one stream of air into the interior volume in a direction that is tangential to a circular cross section of the combustor adjacent the closed end, and inducing a swirling flow of the at least one stream of air into the combustor. A flow of fuel is provided into the combustor such that the flow of fuel mixes with the swirling flow of the at least one stream of air to form a swirling flow of a combustible mixture, and the swirling flow of the combustible mixture is expelled from the open end of the combustor. When the swirling flow of the combustible mixture is burning, at least a portion of combustion products created by the combustible mixture is admitted back into the swirling flow of the combustible mixture as the combustible mixture is expelled from the combustor. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIGs. 1 A and IB are schematic representations of a burner in accordance with the disclosure.

[0010] FIGS. 2A and 2B are schematic representations of an alternative embodiment for a burner in accordance with the disclosure.

[0011] FIGS. 3A-3D are outline and fragmentary views of a burner in accordance with the disclosure.

[0012] FIG. 4 is a graphical representation of the effect of burner length on NOx emissions in a burner in accordance with the disclosure.

[0013] FIG. 5 is an outline view of a burner arranged in a shroud in accordance with the disclosure.

[0014] FIGS. 6 and 7 are graphical representations of swirl number distribution in an annulus in two burner embodiments in accordance with the disclosure.

[0015] FIGS. 8A and 8B are graphical representations of velocity fields within two burner embodiments in accordance with the disclosure.

[0016] FIG. 9 is a graphical representation of swirl number along the respective length of two burner embodiments in accordance with the disclosure.

[0017] FIG. 10 is a graphical representation of swirl number with respect to a respective axial distance along two burner embodiments in accordance with the disclosure.

[0018] FIG. 1 1 A is an outline view of an alternative embodiment of a burner arranged in accordance with the disclosure.

[0019] FIG. 1 IB is a fragmented view of the burner shown in FIG. 11 A.

[0020] FIG. 12 is an outline view of a burner arranged with a manifold in accordance with the disclosure.

DETAILED DESCRIPTION

[0021] The present disclosure relates to a burner design that creates a stable swirling of air or an air/fuel mixture. The burners disclosed herein rely primarily on the geometry of a combustor of the burner to achieve the desired flame characteristics. Advantageously, the disclosed combustor geometries are simpler than existing designs and require no moving parts. In other words, the burners described herein have been developed to use air and/or fuel swirling to obtain better mixing than existing burners such that burner efficiency can be improved, and the effect of FGR to minimize NOx emissions can be maximized.

[0022] A schematic representation for a design of a burner 100 is shown in FIGS. 1 A and IB. The burner 100 includes an air lance or air inlet 102 and a gas or fuel inlet 104, which are provided to a generally elongate, cylindrical combustor 106 having an outlet opening 108 at one longitudinal end. As shown, the combustor 106 has a tubular shape having a closed end 110 opposite the outlet opening 108. The combustor 106 has a frusto-conical shape or wall 1 12 that interconnects a combustor wall having a diameter D₀ to the outlet opening having a smaller diameter, Dj.

[0023] For creating a geometry in which air and fuel meet and mix with tangential momentum, and for creating low pressure regions (vortexes or vortex flow structures) to take advantage of internal flue gas recirculation near the outlet of the combustor, where combustion occurs, the air and fuel inlets 102 and 104 are provided at an angle relative to centerline 1 14 of the combustor 106. More specifically, the air inlet 102 is disposed at an acute angle, θι, relative to the centerline 1 14, and at an axial location, l_ls along the centerline 1 14 of the combustor such that air entering the combustor 106 carries momentum

components in both the radial and axial directions with respect to the centerline 114.

Similarly, the fuel inlet 104 is disposed at an acute angle, 0₂, relative to the centerline 1 14, and at an axial location, 1₂, along the centerline 1 14 of the combustor such that fuel entering the combustor 106 also carries momentum components in both the radial and axial directions with respect to the centerline 114. The angles and axial locations of the air and fuel inlets 102 and 104 may be the same or different. In the burner embodiment shown in FIG. IB, the air and fuel inlets are disposed tangentially with respect to the circular outer wall of the combustor 106.

[0024] Geometrical parameters contributing to fuel/air mixing and to creation of recirculation zones have been determined to include the distance from the end of the combustor to the air and gas jets, respectively, li and 1₂, the air and fuel injection angles, respectively, θι and 0₂, and the combustor and outlet opening diameters, respectively, D₀ and Dj. Other factors that may affect burner performance include the achievable turn-down ratio of the burner. As used herein, "turn-down ratio" refers to the ratio of maximum heat output that can be achieved by the burner to the minimum level of heat output at which the burner will operate efficiently or controllably. [0025] An alternative embodiment for a burner 200 is shown in FIGS. 2 A and 2B. The burner 200 includes an air inlet 202 and a fuel inlet 204. In this embodiment, the air inlet 202 is provided tangentially relative to a generally elongate, cylindrical combustor 206. Fuel is provided to the combustor 206 through a fuel tube 204, which is concentrically disposed within the combustor 206 and extends along a combustor centerline 214 from a closed end 210 to an outlet opening 208. The combustor 106 has a cylindrical shape having a diameter Di, which is larger than an outlet diameter D₂ of the fuel tube 204. The relative position and dimensions of the combustor 106 and fuel tube 204 create a generally cylindrical, annular space 216 extending axially along the combustor 206 and radially between the outer shell of the combustor 106 and the fuel tube 204.

[0026] During operation, air provided to the combustor 206 swirls as it travels along the combustor 206 within the annular space 216. The swirling air in the annular space 216 mixes with fuel exiting the fuel tube 204 at the outlet 208 to form a combustible mixture that forms a flame. The swirling flow of air in the annular space 216 is created when a tangential velocity component is added to the velocity vector of air entering the combustor 206 through the tangential air inlet 202.

[0027] For swirling jets, different flow regimes may be identified depending on the degree of swirl present in the jet. The term "swirl number" is known in the art and is used herein to refer to a ratio of the maximum tangential velocity to the axial velocity of a flow. Thus, the swirl number is a non-dimensional parameter that is indicative of the amount or extent of swirling within a flow. In the burner 200, for low swirl numbers (i.e. when the maximum tangential velocity is of the order of 50% or less of the axial centerline velocity), the air flow or jet behaves in a similar way as for the non-swirling case, even though some modification in the mean and fluctuating velocity distributions, jet width or spreading are present. However, when the swirl becomes strong (i.e. when the tangential velocity becomes larger than the axial velocity), formation of a recirculation zone may result in a reverse flow. This phenomenon is usually referred to as a vortex breakdown flow regime. For creating a vortex breakdown regime downstream of a pipe, i.e., after the outlet of the burner, tangential air jets are injected into the annular space 216 of the combustor 206.

[0028] In general, higher swirl numbers will result in a more expanded flame at the outlet

208, which in turn increases a zone of recirculation for combustion products to be entrained within the flame near the outlet of a combustor. Flue gas recirculation (FGR) lowers flame temperature and, thus, NOx production. However, excessive FGR entrainment may destabilize combustion and even cause flame blow-out. On these bases, a range of swirl numbers for a given burner configuration at which NOx emissions can be sufficiently reduced while maintaining flame stability can be determined and implemented for a particular burner configuration.

[0029] An alternative embodiment for a burner 300 is shown in FIGS. 3A-3D. The burner 300 includes four air inlets 302 arranged tangentially around a closed-end cross section 310 of a combustor 306. The combustor 306 includes a converging, frusto-conical outlet 309. A fuel inlet device for providing fuel to the interior of the combustor 306 is embodied in the illustrated embodiment as a fuel tube 304. The fuel tube 304 is

concentrically disposed within the combustor 306 and extends along a combustor centerline

314 from a closed end 310 to an outlet opening 308. The combustor 306 has a cylindrical shape having a diameter D_1} which is larger than an outlet diameter D₂ of the fuel tube 304.

The relative position and dimensions of the combustor 306 and fuel tube 304 create a generally cylindrical, annular space 305 extending axially along the combustor 306 and radially between the outer shell of the combustor 306 and the fuel tube 304.

[0030] During operation, air provided to the combustor 306 swirls as it travels along the combustor 306 within an annular space 305 formed between the walls of the combustor 306 and the fuel tube 304. The swirling air in the annular space 305 mixes with fuel exiting the fuel tube 304 at an outlet opening 308 of the combustor 306 to form a combustible mixture that, when ignited, produces a flame. The swirling flow of air in the annular space 305 is created when a tangential velocity component is added to the velocity vector of air entering the combustor 306 through the tangential air inlets 302. In the illustrated embodiment, four pipes form the air inlets 302. The air inlets 302 are tangentially connected to the end of the combustor 306. The fuel tube 304 is formed as a closed-end tube having its closed end disposed at least partially within the combustor 306 along the centerline 314. A number of holes 316 are drilled in the wall of the fuel tube 304 to permit gas or another fuel to enter the internal space of the combustor 306 and mix with the air travelling therein.

[0031] The number, size and location of the holes 316 is adjusted and selected to provide a desired gas pressure. As shown, the holes 316 are disposed symmetrically around a cross section of the fuel tube 304 at an axial location that is selected. Alternatively, the axial location, orientation, and arrangement of the holes can be different than the arrangement shown. The plane on which the injection holes are located may fall anywhere on the gas pipe, and there may be multiple planes, e.g., there may be holes at both the start and end of the gas pipe. The optimum gas pressure will maximize the effect of mixing time and gas penetration in the air stream. For example, for the burner shown in FIGS. 3 A-3D, sixteen 2mm-holes were found to minimize thermal NOx emissions while also producing a gas pressure drop of 14 inches of water.

[0032] A graph 400 showing the effect of the outlet swirl number on thermal NOx formation is shown in FIG. 4. For conducting the experiment, a burner similar to the burner 300 (FIG. 3D) without a converging outlet 309 was constructed and tested with a relatively long combustor (1.5m). NOx emissions were measured for various combustor lengths. The obtained data is shown in FIG. 4, where burner length (in meters) is expressed along the horizontal axis and NOx concentration (in g/L) is expressed in the vertical axis 403. Two curves were produced, each representing different combustion mixtures. A first curve 402 represents an experiment where 37% excess air is provided relative to stoichiometric combustion mixtures. A second curve 404 represents 55% excess air. As can be seen from the graph 400, a correlation exists in which longer burner length contributes to an increase in NOx emissions. For lengths shorter than 0.4 meters, for the particular combustor diameter tested, the flame was extinguished. It is believed that as the combustor length decreases from 1.5m to 0.4m, NOx production is lowered significantly as a result of higher swirl number and more FGR associated with shorter combustor. For combustor lengths shorter than 0.4m, excessive FGR extinguishes the flame and the burner becomes unstable.

[0033] The role of the converging cone 309 (FIG. 3) at the end of the combustor is also notable. It is contemplated that reducing the outlet diameter (as a result of the cone) of the combustor decreases the outlet swirl number and increases the outlet velocity of the fuel/air mixture exiting the combustor 306 through the outlet opening 308. The increased outlet velocity shifts the flame anchorage location to a position ahead of the gas injection holes 316. For illustration, it is noted that without the cone, the flame is anchored at the fuel injection holes. This means that mixing time is increased and the flame is more exposed to the FGR. Also, lower swirl number helps the stability of the burner and higher outlet velocity prevents flash-back.

[0034] With the converging cone 309, the burner can operate in a semi-premixed condition. Semi-premixed condition, as used here, means that only a portion of the fuel required for stoichiometric combustion can be provided through the fuel tube disposed in the combustor. The remaining fuel can be provided to the active flame at the end of the combustor. In one test that was conducted, gas was injected into the combustor from a gap or opening in the gas line inside the combustor (far from the outlet) and it was observed that, while the gas pressure was significantly lowered (the gap was much bigger than the nozzle holes), NOx emissions and the flame anchor location were unaffected. The cone angle, as measured by an acute angle extending from the outer periphery of the combustor towards the centerline, can range between 0 and 90 degrees. As shown, the angle is about 13 degrees. On the basis of the experiment, we determined that the flame dynamics, stability and emissions are all strong functions of the size and the location of the recirculation zone created as a result of the swirling air flow, and equally the swirl number near the combustor outlet.

[0035] To lower carbon monoxide emissions, the burner 300 may be operated in conjunction with a shroud 500, as shown in FIG. 5. The shroud 500 may be added to the outside of the burner 300 to help lower carbon monoxide from the burner. In the illustrated embodiment, the shroud 500 has a cylindrical shape that is open on both ends and is concentrically disposed relative to the combustor 306. A diameter of the shroud 500 is larger than the diameter of the combustor 306 to form an annular volume 502 therebetween. During operation, exhaust gas produced by combustion is at least temporarily enclosed within the annular volume to provide an opportunity for carbon monoxide (CO) present therein to further oxidize into carbon dioxide (C0₂). In the illustrated embodiment, while use of the shroud 500 may increase NOx generation due to increased dwell time of combustion gases in the annular volume 502, and also an increased flame temperature, but CO emissions are reduced by the further oxidation of CO into C0₂ such that a balance may be selectively controlled between NOx and CO production by adjusting the dimensions and other structural features of the shroud 500, for example, the diameter and/or length of the shroud. As the length of the shroud increases the temperature and residence time of the hot flue gases increases, which will cause a reduction in CO emissions and an increase in NOx emissions. Similarly, as the diameter of the shroud increases the temperature of the flue gases decreases, increasing CO emissions and reducing NOx emissions.

[0036] For NOx creation to remain at the desired low rate, portions of the shroud may be left open 1100. In an embodiment of the burner, the shroud's open area is created by making holes in the shroud upstream of the base of the flame. The open areas in the shroud allow for flue gases to be drawn into the flame by the swirling flow and reduce flame temperature at the base of the flame where a significant portion of the total NOx from the burner is created. During operation the flue gases at the base of the flame reduce peak flame temperature to abate NOx creation, while the shroud creates an enclosure that promotes the oxidation of carbon monoxide into carbon dioxide. In one exemplary embodiment, the low NOx emissions of the burner without a shroud, under 20ppm NOx, are combined with the low CO emissions of the burner with a shroud, down to Oppm CO, while firing with 15% excess air. Flame stability was improved with a shroud as the total volume of flue gas recirculation (FGR), which describes the portion of flue gas that is recirculated into the flame, can be reduced for equivalent NOx production as the FGR that is drawn into the flame is directed to the base of the flame where it has the most effect on NOx production. The shroud also creates a high temperature zone around the flame that is above the auto-ignition temperature, which continually reignites the flame making it very stable. The amount of FGR drawn into the flame can be precisely controlled by the location of the open area and size of the open area in the shroud for specific firing environments. In at least one embodiment of the burner, the flow area of holes formed in the shroud are externally adjustable, for example, by a gate or other type of valve, to dynamically control the amount of FGR drawn into and/or around the flame.

[0037] Various additional experiments were conducted to determine the parameters affecting burner operation and efficiency. For example, to investigate the swirl number distribution along the length of the combustor and understand the effect of the converging cone on the swirl number, numerical modeling was performed on annuluses with and without a converging cone. The results are shown in graphical form in FIGS. 6 and 7. As can be seen in FIG. 6, the swirl number, which is arranged on the vertical axis, exponentially decreases along the length of the combustor as the distance from the air inlets (e.g., air inlets 302) increases, as represented by the horizontal axis. When a converging cone (e.g., 309) is added, swirl behavior changes and the outlet swirl number decreases when all other structures of the burner are maintained the same as a burner having the same structure but without a converging cone. The effect of the converging cone was also tested to determine its effect on NOx emissions, which were unexpectedly found to be 50% lower when the converging cone was added to the combustor. An additional unexpected effect of adding a converging cone was an improvement in the turn-down ratio, which improved from 10: 1 for a straight combustor to higher than 40: 1 for a combustor with a converging cone.

[0038] To further investigate the effectiveness of the converging cone on the performance of the burner, the length of the burner was further shortened to examine burner stability using CFD simulations and physical testing. The results show that by gradually reducing the outlet diameter using the converging cone, the burner remains stable, i.e., without an inward gas stream, as the outlet swirl number remains substantially unchanged. A CFD simulation of velocity profiles within the combustor are shown in FIGS. 8 A and 8B, where the velocity field is depicted for 0.4m and 0.2m long combustors, not including the length of the converging cone, which is about 75cm. The swirl decay for the burners with different lengths as shown in FIGS. 8 A and 8B stabilizes after a given distance such that, at the outlet of each burner, at the converging cone, the swirl numbers equalize. FIG. 9 is a plot of the same swirl numbers over a normalized burner length.

[0039] To determine the effect of the number of lances (air inlets) on the performance of the burner, simulations and experiments were performed to compare burners with two and four lances. The results of this investigation are shown in the graph of FIG. 10, in which swirl number is plotted on the vertical axis against burner length, which is plotted against the horizontal axis. Data points acquired are plotted as curves. A first curve 902 represents information acquired for a burner having two lances (air inlets), and a second curve 904 represents information acquired for a burner having four lances (air inlets). In both burners, the diameter of the air inlets was scaled to maintain substantially uniform air velocity at the outlet opening of the burner. As can be seen in FIG. 10, the outlet swirl number is preserved and, therefore, the burner stability and NOx emissions are also expected to be maintained, as discussed above. On the basis of these experiments, it is contemplated that burner dimensions can be arranged to preserve a desired swirl number with a single lance (air inlet).

[0040] It has been determined that an even or generally symmetrical distribution of lances around the burner may promote efficient operation. As shown in FIGS. 11 A and 1 IB, an alternative embodiment for a burner 1000 includes an air distribution manifold 1002, a cross section of which is shown in FIG. 1 IB. Similar to the burner 300 (FIGS. 3A-3D), the burner

1000 includes a cylindrical combustor 106, which in this embodiment is constructed from two segments, a base segment 1004 and an end segment 1006 that also forms the outlet opening 308 and the frusto-conical outlet 309. A pair of flanges 1008 connects the base and end segments 1004 and 1006, which segments can be made from different materials such as mild steel for the base segment 1004 and stainless steel for the end segment 1006.

[0041] The distribution manifold 1002 in the illustrated embodiment fluidly connects a single air inlet opening 1010 to two lance outlet openings 1012. Air to the air inlet opening 1010 may be provided during operation from a blower or other air source. Each of the two lance outlet openings 1012 is formed at an end of a, respective, lance air passage 1014 having a major axis 1016 that is disposed generally tangentially at an offset relative to an inner diameter, D, of the combustor 306. An annular passage 1018 extends peripherally around a portion of the combustor 306 to fluidly interconnect the single air opening 1010 with the two lance air passages 1014. The general shape of the annular passage 1018, and also the placement of the lance air passages 1014 along the annular passage 1018, can be selectively arranged to provide a balanced, even flow of air from the air opening 1010 through each of the two lance outlet openings 1012.

[0042] In the illustrated embodiment, the manifold 1002 is constructed by thin- wall sheet metal and is structured to include two concentric cylinders with face and back plates to enclose a hollow cylindrical plenum into which the single air inlet and two lance outlets are formed. The manifold is designed such that the burner body is set concentrically in the manifold and the lances of the burner are located at the mid-plane between the back plate and face plate of the manifold. The lance outlets 1012 of the manifold are located such that they are set 180° from each other with one being between 70°-80° from the inlet and the other

100°-1 10° from the inlet, but other angles can be used. In one embodiment these angles are

75° and 105°, respectively. From a combination of physical and analytical testing, for example, using computational fluid dynamic (CFD) models, these angle ranges were determined to lead to approximately equal flow and pressure drop through each lance.

[0043] An alternative embodiment of a shroud 1020 disposed around the burner 1000 is shown in FIG. 12. In this illustration, a shroud 1022 is disposed around an open end of the combustor 306, similar to the shroud 500 shown in FIG. 5, but in this embodiment, the shroud 1022 includes a plurality of windows 1024 formed along a periphery of the shroud

1020 in general axial alignment around the outlet opening 308. Each of the windows 1024 has a generally rectangular shape, with rounded corners, and permits fluids to pass therethrough during operation. An open area of each window 1024 that is available for fluid flow can be fixed, based on the size and dimension of each window, or can be adjustable, as is in the embodiment shown in FIG. 12. In this embodiment, a collar 1026 having a hollow cylindrical shape is slidably disposed along an outer surface of the shroud 1022. The collar 1026 has a leading edge 1028 that moves, as the collar moves, towards or away from the burner 1000, manually or by action of an actuator 1030 to selectively cover a portion of the flow area provided by the windows 1024, thus effectively controlling the flow area available for fluid flow through the shroud 1022 depending on the operating conditions of the burner 1000. Control of the opening of the windows 1024 by the actuator 1030 may be carried out automatically based on the operating conditions of the burner, for example, flame

temperature, fuel flow rate, air temperature, emissions, and the like.

[0044] The use of the terms "a" and "an" and "the" and "at least one" and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term "at least one" followed by a list of one or more items (for example, "at least one of A and B") is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly

contradicted by context. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to,") unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0045] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

CLAIMS: We claim:

1. A burner, comprising:

a combustor having a generally cylindrical, hollow shape extending along a centerline, and a closed end;

a fuel tube that is connectable to a fuel source and having at least one opening fluidly connecting an interior of the combustor with the fuel source such that fuel can be delivered from the fuel source into the interior of the combustor;

at least one air inlet disposed to provide an air stream into the interior of the combustor, the air stream arranged and configured to create a vortex flow structure within the combustor.

2. The burner of claim 1 , wherein the combustor is configured to maintain the vortex flow structure through a length of the combustor along the centerline such that fuel from the fuel tube is mixed with the air stream, and to deliver a swirling mixture of air and fuel from an open end of the combustor.

3. The burner of claim 1, further comprising a converging nozzle disposed at an open end of the combustor opposite the closed end.

4. The burner of claim 1, wherein the at least one air inlet is disposed to inject the air stream in a tangential direction relative to a circular cross section of the combustor adjacent the closed end, such that the air stream has momentum components in both a radial direction and an axial direction with respect to the centerline.

5. The burner of claim 1, further comprising an additional air inlet providing an additional air stream into the combustor.

6. The burner of claim 1, wherein the fuel tube is disposed in the combustor and extends generally concentrically along the combustor.

7. The burner of claim 1 , wherein the fuel tube is disposed to inject fuel into the combustor in a tangential direction with respect to a circular cross section of the combustor.

8. The burner of claim 7, wherein the air stream and the fuel are provided in opposing directions into the interior of the combustor.

9. The burner of claim 1, further comprising an air distribution manifold having an air inlet opening in fluid communication with an air distribution passage, the air distribution passage being fluidly open to the at least one air inlet.

10. The burner of claim 9, wherein the air distribution manifold has a generally hollow cylindrical shape disposed around the combustor adjacent the closed end, the air distribution manifold forming internally an annular passage extending around the combustor and surrounding a central ring, and wherein the at least one air inlet is formed as a straight passage extending through the central ring between the annular passage and the interior of the combustor.

1 1. The burner of claim 10, wherein a second straight passage defining a second air inlet is formed in the central ring, the second straight passage being oriented in parallel with the at least one air inlet and disposed at an offset therewith such that the at least one air inlet and the second straight passage are both opposingly tangential at an offset relative to a circular cross section of the combustor.

12. The burner of claim 1 1, wherein the at least one air inlet is disposed between 70°-80° with respect to the air inlet opening and the second straight passage is disposed between 100°-110° with respect to the air inlet opening around a periphery of the combustor.

13. The burner of claim 1 , further comprising a shroud disposed around an open end of the combustor opposite the closed end.

14. The burner of claim 13, wherein the shroud has a generally cylindrical shape and forms a plurality of openings that are axially aligned with the open end of the combustor.

15. The burner of claim 14, further comprising a collar being slidably selectively disposed along the shroud, wherein the collar is configured to cover at least partially each of the plurality of openings.

16. The burner of claim 1 , wherein the burner includes one or two pairs of opposed air inlets, each pair of air inlets being disposed in parallel and at an offset distance such that each air inlet provides the air stream in a tangential direction with respect to a circular cross section of the combustor.

17. The burner of claim 1, wherein the combustor forms an open end opposite the closed end, the open end including an outlet opening formed at an end of a converging nozzle.

18. A method for operating a burner, comprising:

providing a combustor having a generally hollow cylindrical shape defining a closed end and an open end, the combustor defining an interior volume therein;

injecting at least one stream of air into the interior volume in a direction that is tangential to a circular cross section of the combustor adjacent the closed end;

inducing a swirling flow of the at least one stream of air into the combustor; providing a flow of fuel into the combustor such that the flow of fuel mixes with the swirling flow of the at least one stream of air to form a swirling flow of a combustible mixture;

expelling the swirling flow of the combustible mixture from the open end of the combustor; and

when the swirling flow of the combustible mixture is burning, admitting at least a portion of combustion products created by the combustible mixture back into the swirling flow of the combustible mixture as the combustible mixture is expelled from the combustor.

19. The method of claim 18, wherein the flow of fuel is provided through a fuel tube extending along the interior volume of the combustor.

20. The method of claim 18, wherein the flow of fuel is provided as a stream tangentially into the interior volume with respect to the circular cross section of the combustor adjacent the closed end.