Patent Application
20160102857
The present disclosure relates to burners and, more specifically, to burners having a combustor.
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.
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.
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.
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.
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.
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.
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.
A schematic representation for a design of a burner 100 is shown in
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 114 of the combustor 106. More specifically, the air inlet 102 is disposed at an acute angle, θ1, relative to the centerline 114, and at an axial location, ll, along the centerline 114 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, θ2, relative to the centerline 114, and at an axial location, l2, along the centerline 114 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
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, l1 and l2, the air and fuel injection angles, respectively, θ1 and θ2, and the combustor and outlet opening diameters, respectively, D0 and D1. 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.
An alternative embodiment for a burner 200 is shown in
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.
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.
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 de-stabilize 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.
An alternative embodiment for a burner 300 is shown in
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.
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
A graph 400 showing the effect of the outlet swirl number on thermal NOx formation is shown in
The role of the converging cone 309 (
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.
To lower carbon monoxide emissions, the burner 300 may be operated in conjunction with a shroud 500, as shown in
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 20 ppm NOx, are combined with the low CO emissions of the burner with a shroud, down to 0 ppm 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.
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
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
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
It has been determined that an even or generally symmetrical distribution of lances around the burner may promote efficient operation. As shown in
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.
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°-110° 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.
An alternative embodiment of a shroud 1020 disposed around the burner 1000 is shown in
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.
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.
The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/063,037, which was filed on Oct. 13, 2014, the contents of which are hereby incorporated herein in their entirety by this reference.
| Number | Date | Country | |
|---|---|---|---|
| 62063037 | Oct 2014 | US |
