METHOD FOR STAGED COMBUSTION OF A FUEL AND FIREHEAD

Description

[0001] The present invention relates to a method for the staged combustion of a fuel and a combustion head for the staged combustion of a fuel.

[0002] The combustion of fossil fuels in combustion plants produces, among other combustion products, nitrogen oxides such as NO and NO2. In the following, we will refer only to NO. These and other pollutant emissions can be influenced and reduced through design measures in the burners. The reaction mechanisms that lead to such nitrogen oxides are largely known and are generally divided into thermal and prompt NO formation, as well as NO formation resulting from the oxidation of the chemically bound nitrogen in the fuel.

[0003] It is known that, according to the so-called Zeldovich mechanism, thermal NO depends on the residence time of the reactants in the combustion zone and, to a large extent, on the combustion temperature itself. The combustion temperature is linked to the fuel / air ratio λ. The maximum combustion temperature occurs at a fuel / air ratio λ=1. This is also referred to as the stoichiometric ratio. The combustion air contains exactly enough oxygen for the fuel to burn completely. A fuel / air ratio λ < 1 is referred to as a rich mixture; there is too much fuel. A fuel / air ratio λ > 1 is referred to as a lean mixture; there is an excess of air. In both cases, the combustion temperature drops again and consequently less thermal NO is formed.

[0004] In addition to thermal NO, the formation of prompt NOx also plays a significant role. Prompt NO is formed by hydrocarbon radicals (CH) formed as intermediates in flames, which are present as intermediates in the combustion of carbon-containing fossil fuels. The CH radicals react with atmospheric nitrogen to form hydrogen cyanide (HCN), which is further converted to NO in very rapid formation reactions. A proven method for suppressing the formation of free CH radicals and thus the formation of prompt NO is lean combustion or superstoichiometric combustion. Lean combustion refers to combustion with an excess of air, i.e., with λ > 1.

[0005] Compared to thermal NO, prompt NO is produced in small amounts, but is crucial for reducing NO formation, especially in ultra-low NO applications.

[0006] It is also known that the recirculation or recirculation of exhaust gases produced during combustion has a positive effect on reducing nitrogen oxide formation. The recirculated, cooled exhaust gas reduces both the flame temperature itself and the O2 partial pressure in the combustion zone. Both effects contribute to reducing NO formation. However, the mixing of increasing exhaust gas quantities tends to destabilize the continuous combustion process.

[0007] EP 1 754 937 B1 and EP 2 037 173 B1 show combustion heads that achieve NO reduction. These are primarily single-stage combustion processes that only allow for further NO optimization and flame stabilization to a limited extent. DE 195 09 219 A1 shows a combustion head for two-stage combustion with a superstoichiometric air-gas mixture in the first stage and a substoichiometric air-gas mixture in the second stage. DE 44 27 104 A1 shows a process for the staged combustion of a fuel. A highly superstoichiometric primary flame is formed within a burner tube.A second quantity of fuel is supplied downstream for a main flame in such a way that a stoichiometrically complete combustion takes place. EP 3 078 910 B1 shows a gas burner with multi-stage combustion with a strongly overstoichiometric primary flame within a burner tube and a slightly overstoichiometric main flame at the outlet of the burner tube.

[0008] The primary combustion head features a swirler and main and stabilizing gas nozzles. All combustion air flows through both combustion stages. The gas burner achieves a reduction in thermal NOx.

[0009] In a combustion head, a distinction can usually be made between so-called mixing zones and so-called combustion zones.

[0010] In a mixing zone, different fluids are mixed that are not (yet) combusted. In a mixing zone, the conditions required for combustion are usually not met. This can be the case, for example, if the flow velocity of the ignitable mixture is significantly higher than the flame velocity.

[0011] A combustion zone is an area in which the conditions required for combustion are present. A combustion zone exists when an ignitable mixture (e.g. fuel-combustion air mixture, fuel-combustion air-exhaust gas mixture, fuel-oxidizer mixture, fuel-oxidizer-exhaust gas mixture) is present, the flow velocity of the ignitable mixture and the flame speed are essentially the same, and a temperature is present that is equal to or greater than the ignition temperature of the ignitable mixture. The more general term oxidizer encompasses the term combustion air, but also includes, for example, ambient air enriched with additional oxygen. In areas where these conditions are not met, ignition or combustion cannot occur. Mixing zones often merge into combustion zones without a clear spatial separation.

[0012] For these and other reasons, there is a need for the present invention. It is an object of the invention to provide energetically advantageous combustion with minimized NO emissions. This object is achieved by the features of the independent claims.

[0013] The objects and features of the present invention will become clear in the following description of embodiments, which is given with reference to the accompanying figures, in which: Fig. 1 shows a highly schematic side view of a combustion head; Fig. 2 schematically shows in perspective parts of a combustion head from a fuel supply side; Fig. 3 schematic perspective parts of the combustion head of Fig. 2 shows from one side of the flame; Fig. 4 schematically shows a side view of a combustion head; Fig. 5 schematically shows a sectional view of a front portion of a combustion head; and Fig. 6 schematically shows a front view of a combustion head.

[0014] Aspects and embodiments are described below with reference to the drawings, wherein like or similar reference numerals are generally used to refer to like or similar elements. In the following description, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects of the embodiments. However, it may be apparent to one skilled in the art that one or more aspects of the embodiments may be practiced with a lesser level of specific detail. In other instances, elements are shown in schematic form to facilitate describing one or more aspects of the embodiments. The following description is therefore not to be considered limiting. It is noted that the illustration of the various elements in the figures is not necessarily to scale.

[0015] Directional terminology used in the description with reference to the drawings, such as, for example, "top," "bottom," "top," "bottom," "left," "right," "front," "back," "vertical," "horizontal," etc., is not intended to be limiting. Components of embodiments may be positioned in a number of different orientations; the directional terminology is used for illustration purposes only. It is understood that other embodiments may be utilized and structural or logical changes may be made without departing from the concept of the present invention.

[0016] Multi-stage combustion processes have long been known in practice. However, the approaches currently available are not sufficient to continue meeting the ever-increasing NO requirements for the operation of combustion plants in the long term. A more intensive NO reduction is possible through staged combustion according to the disclosure. With appropriate controllability, the NO reduction can also be ensured over a wide load range and / or for different fuels and / or for different combustion chambers.

[0017] A method for the staged combustion of a fuel with the supply of combustion air into a burner tube according to claim 1 is provided. The fuel can be a gas or a liquid fuel. A first quantity of fuel is supplied to form a primary flame within the burner tube. A second quantity of fuel can be supplied downstream to form a main flame front. The main flame stabilizes downstream of the burner tube and at a distance from the burner tube. The fuel supplies are designed such that the primary flame burns with a stoichiometry greater than 1.5, in particular greater than 2.0. This allows a very low flame temperature to be achieved. Virtually no prompt NO is formed. The main flame is slightly overstoichiometric. The stoichiometry can be between 1.03 and 1.18.The temperature of the main flame can be significantly reduced by exhaust gases recirculated within the combustion chamber.

[0018] In one embodiment, the first fuel quantity can be controlled independently of the second fuel quantity. This ensures a superstoichiometric primary flame over a wide load range.

[0019] In one embodiment, the first fuel quantity supplied can be significantly less than the second fuel quantity supplied. The first fuel quantity can be approximately between 3% and 15% of the total fuel quantity, i.e., the sum of the first fuel quantity and the second fuel quantity. Preferably, the first fuel quantity is between 5% and 10% of the sum of the first fuel quantity and the second fuel quantity.

[0020] In a further embodiment, a portion of the combustion air is swirled. This creates swirled combustion air. A first portion of the first fuel quantity is delivered into the air swirl region. This creates a swirled, lean air / fuel mixture. Very good mixing can be achieved. In this region, the flow velocity is high and the mixture lean, so that no ignition conditions exist. The flow velocity of the swirled, lean air / fuel mixture is reduced in the next step. A second portion of the first fuel quantity is fed to the slowed-down, swirled, lean air / fuel mixture.

[0021] Furthermore, a combustion head for the staged combustion of a fuel according to claim 7 is provided. The provided combustion head enables the implementation of the method. The combustion head is configured to combust a first supplied amount of fuel in a superstoichiometric primary flame. A second supplied amount of fuel is combusted in a slightly superstoichiometric main flame.

[0022] A supply of the first quantity of fuel and a supply of the second quantity of fuel can preferably be controlled independently of each other and thus ensure very low nitrogen oxide combustion over a wide load range.

[0023] The following figures show exemplary embodiments of combustion heads according to the invention with which the method according to the invention for the staged combustion of a fuel can be carried out.

[0024] Fig. 1 shows in a highly schematic form a side view of a combustion head 10. The combustion head 10 comprises a burner tube 12, a swirl device 14, first fuel nozzles 16a, 16b, second fuel nozzles 18, a first fuel supply 20 and a second fuel supply 22. Arrows symbolize the incoming fuel. During operation, a superstoichiometric primary flame 24 forms within the swirl device 14 and a main flame or main flame front 26 spaced from the combustion head 10, both of which are each formed by a flame in the Fig. 1 are represented symbolically. Combustion head 10 thus serves for the staged combustion of fuel. The fuel can be gaseous. The fuel can be natural gas. The fuel can comprise hydrogen. In addition to use as a pure gas burner, a dual-fuel burner is also possible, in which, in addition to gaseous fuel, liquid fuel can also be burned. A burner for liquid fuel only is also possible. The following description generally refers, in a non-limiting manner, to an embodiment as a gas burner.

[0025] The burner tube 12 is shown in the Fig. 1 Combustion air 28 is supplied from the right. The right end of the burner tube 12 in the illustration is thus the upstream end. The burner tube 12 can be essentially cylindrical. The combustion air 28 flows through the burner tube 12 and exits it at the open end of the burner tube 12, which is the left end in the illustration, the downstream end. The main flame front 26 forms downstream of the combustion head 10. This is where the combustion chamber or furnace (not shown) is located.

[0026] The amount of fuel from the first fuel nozzles 16a, 16b can be small compared to the amount of fuel exiting the second fuel nozzles 18. If only a small amount of fuel is burned at a high overstoichiometric level in the primary flame 24, a second substoichiometric combustion stage is not necessary. Therefore, the spaced-apart main flame 26 can also be superstoichiometric overall. A generally substoichiometric combustion zone, as in staged combustion with substoichiometric and superstoichiometric combustion zones and the residence time of the gases in these zones necessary for NO reduction, is not generated with the combustion head 10 according to the invention. The method according to the invention provides a high overstoichiometric primary flame and a slightly superstoichiometric main flame.

[0027] The swirl device 14 is arranged within the burner tube 12. The swirl device 14 can be open at both ends. A longitudinal axis of the burner tube 12 and a longitudinal axis of the swirl device 14 can be parallel to each other or superimposed, so that the swirl device 14 is located centrally in the burner tube 12 and is radially evenly spaced from the inner wall of the burner tube. Part of the combustion air 28 flows outside the swirl device 14 through the burner tube 12, while another part of the combustion air 28 flows through the swirl device 14.

[0028] The swirl device 14 comprises a swirl body 30, swirl vanes 32, and a perforated partition 34. The swirl body 30 can be substantially cylindrical. The perforated partition 34 can run substantially perpendicular to the longitudinal axis of the swirl body 30 and divide an interior of the swirl body 30 into a first region 36 and a second region 38. The first region 36 can be located upstream of the second region 38. The perforated partition 34 can cause a pressure loss. It can thus locally reduce the flow velocity downstream of the perforated partition 34.

[0029] The swirl vanes 32 may be arranged only in the first region 36. The second region downstream of the partition wall 34 may be free of swirl vanes 32. A plurality of swirl vanes 32 may be provided.

[0030] The swirl body 30 can have a larger diameter in the first region 36 than in the second region 38. A conical section can be provided in the transition between the first region 36 and the second region 38.

[0031] The first fuel nozzles 16a, 16b are arranged within the swirl body 30. They are connected to the first fuel supply 20. The first fuel supply 20 allows for the regulation of the amount of fuel / combustion gas flowing to the first fuel nozzles 16a, 16b, as indicated by a symbol 40 in Fig. 1 This control is separate and independent of a control 42 in the second fuel supply 22.

[0032] The first fuel nozzles 16a, 16b may comprise primary fuel nozzles, hereinafter also referred to as primary gas nozzles, 16a, which are located in the second, downstream region 38 of the swirl device 14. The first fuel nozzles 16a, 16b may comprise further fuel nozzles—hereinafter referred to as support fuel nozzles or support gas nozzles 16b—which are located in the first, upstream region 36 of the swirl device 14.

[0033] The support fuel nozzles 16b can be evenly distributed between the swirl vanes 32. The support fuel nozzles 16b can be arranged substantially parallel to a longitudinal axis of the burner tube 12. The swirl vanes 32 cause a strong swirl in the combustion air 28. The fuel flowing out of the support fuel nozzles 16b, which is also referred to as support gas, is thus highly efficiently premixed with a portion of the combustion air 28 for the primary flame 24. This creates a swirled fuel / combustion air mixture. The fuel supply through the support fuel nozzles 16b can be configured to form a swirled, lean air / fuel mixture. The support fuel nozzles 16b can deliver a first portion of the first fuel quantity. The support fuel nozzles 16b can have bores for delivering the fuel.The bores can be arranged such that the fuel is at least partially discharged substantially radially inwards, that is to say in a direction substantially perpendicular to the wall of the swirl body 30. Due to the high flow velocities of the swirled combustion air and due to the high air content in relation to the amount of fuel or gas, the ignition conditions of the swirled fuel / combustion air mixture are not yet present in the region of the swirl vanes 32, that is to say in the upstream region 36.

[0034] The partition wall 34 can be designed to slow down the swirled fuel / combustion air mixture. The partition wall 34 can have openings for this purpose. The partition wall can be designed essentially like a grid. The geometry of the partition wall 34 can be designed to reduce the flow velocity of the swirled fuel / combustion air mixture while leaving the swirling largely undisturbed. The partition wall 34 reduces the absolute flow velocity of the swirled and premixed primary air and thus ensures the ignition of the primary flame 24, which is additionally enriched in this area with a second portion of the first fuel quantity.

[0035] The primary fuel nozzles 16a can be evenly distributed in the downstream region 38. The primary fuel nozzles 16a are thus located downstream of the partition wall 34 in a region of lower flow velocity. The primary fuel nozzles 16a can be arranged substantially perpendicular to a longitudinal axis of the burner tube 12. The primary fuel nozzles 16a can be evenly distributed in a spray ring. A plurality of primary fuel nozzles 16a can be provided. In the downstream region 38, the primary fuel nozzles 16a deliver the second portion of the first fuel quantity, referred to as primary gas, to the fuel-air mixture formed in the bladed part of the swirl body 30, or in other words, in the first region 36, thus generating the ignitable mixture for forming the primary flame 24. The primary fuel nozzles 16a can have bores for delivering the fuel.The holes may be arranged laterally on the primary fuel nozzles 16a. The lateral holes may be arranged such that the fuel is discharged substantially tangentially.

[0036] The ratio of the opening area of ​​the entire bores in the primary fuel nozzles 16a to the opening area of ​​the entire bores in the auxiliary fuel nozzles 16b can determine a ratio of primary gas to auxiliary gas, taking into account the supply lines to the primary fuel nozzles 16a and the auxiliary fuel nozzles 16b. The ratio can be selected depending on the overall geometry and the fuel quality or fuel composition. The ratio can be approximately 1:1. Approximately half of the fuel flowing through the first fuel supply 20 can be delivered via the primary fuel nozzles 16a in the region 38, and approximately half of the fuel flowing through the first fuel supply 20 can be delivered via the auxiliary fuel nozzles 16b in the region 36.

[0037] The separate controllability of primary and support gas by control device 40 relative to the controllability of the second and main fuel quantity flowing through second fuel supply 22, as well as the design of swirl body 30, primary and support fuel nozzles 16a, 16b, and partition 34, can generate a primary flame 24 with a stoichiometry λ>>1 over a wide load range. In one embodiment, the stoichiometry of primary flame 24 is λ>1.5. In another embodiment, the stoichiometry of primary flame 24 is λ>2.

[0038] Due to the resulting very low combustion temperatures, it has been proven that almost no thermal and no prompt NO is produced within the primary flame 24.

[0039] However, such low combustion temperatures also always generate flame instabilities that must be controlled. The reaction rate depends exponentially on the temperature in the flame zone and on the turbulence within it. The reaction rate is reduced by imperfect mixing of fuel and oxidizer. Flame instability occurs when the flow velocity in the axial direction is greater than the turbulent flame velocity.

[0040] For a stable primary flame 24, the prior supply of the support gas via the support fuel nozzles 16b into the swirled combustion air and thus the enrichment and premixing of the primary air with fuel, the type and position of the primary gas introduction, the ratio of support and primary gas, and the geometry and position of the partition wall 34 in the unbladed part 38 of the swirl body 30 in the illustrated embodiment are important. Other means can be provided to achieve a stable primary flame with a stoichiometry greater than 1, in particular greater than 1.5 or even greater than 2.

[0041] Furthermore, the cylindrical, unbladed part of the swirl body 30, the area 38 in Fig. 1 , designed so that the primary flame 24 forms in a defined area which is protected from the remaining combustion air 28 which flows outside the swirl body 30 through the burner tube 12.

[0042] The second fuel nozzles 18, also referred to as main gas nozzles, are located outside and downstream of the swirl device 14. The second fuel nozzles 18 are connected to the second fuel supply 22. The second fuel supply 22 allows for the control of the amount of fuel / combustion gas flowing to the second fuel nozzles 18. The second fuel quantity comprises the majority of the total fuel quantity and is therefore also referred to as the main fuel quantity or main gas. The controllability of the main gas is indicated by the symbol 42 in Fig. 1 shown.

[0043] The second fuel nozzles 18 are located within the burner tube 12. The second fuel nozzles 18 can be located at the downstream end of the burner tube 12 and can be flush with it. The second fuel nozzles 18 can be evenly distributed over the inner circumference of the burner tube 12. In Fig. 1 Not shown is an annular delta disc that can fill a gap between the burner tube 12 and the second fuel nozzles 18 at the downstream end of the burner tube. The delta disc is arranged with respect to Fig. 4-6 explained in more detail.

[0044] The second fuel nozzles 18 are designed to ensure a high fuel exit velocity. The resulting momentum transports the fuel as far as possible into the combustion chamber and forms a combustion zone spaced from the combustion head 10. The main gas can be discharged essentially in the flow direction, i.e., parallel to the longitudinal axis of the burner tube 12. For this purpose, the second fuel nozzles 18 can have an opening on one end face. A baffle can determine the opening on the end face. The design of the second fuel nozzles 18 leads to the formation of the main flame or main flame front 26, which is spaced from the downstream end of the combustion head 10 and is stably formed in the combustion chamber (not shown in detail). Due to the arrangement of the second fuel nozzles with a coaxial outflow direction relative to the burner tube axis, the main flame 26 can have a slender and elongated flame shape.Internal exhaust gas recirculation, which is explained in more detail below, can inject exhaust gases into the hot zones of the main flame 26 and thus into the areas of greatest NO production. This reduces NO production in the main flame.

[0045] The fuel supplies may be designed and arranged to provide ignition energy for the spaced main flame 26 from the primary flame and recirculated exhaust gases to ignite the mixture of main fuel, combustion air or generally oxidant and recirculated exhaust gas and to ensure a continuous, stable progress of the oxidation reactions.

[0046] In the illustrated embodiment, both gas connections—that is, fuel supply 20 for primary and support gas for the primary flame 24 and fuel supply 22 for the main gas for the main flame 26—are controlled separately by gas control devices 40 and 42. This allows the gas quantity in the primary flame 24 and the main flame 26 to be adjusted separately, thus allowing the stoichiometry in the respective combustion zone to be individually controlled. This allows the creation of a stable and superstoichiometric primary combustion zone and thus the formation of a virtually NO-free primary flame 24 over a wide load range, as well as adaptation to different combustion chambers.

[0047] Fig. 2 shows schematically in a perspective view parts of a combustion head 10a from a fuel supply side. Combustion head 10a can have the same features as for the Fig. 1 Burner head 10a may represent an implementation of the burner head 10. Therefore, the same reference numerals as in Fig. 1 used. The description of the Fig. 2 is essentially limited to details that emerge from the presentation of the Fig. 1 not emerge. In the Fig. 2 the burner tube 12 is not shown.

[0048] The second fuel supply 22 of the combustion head 10a is designed as a tube having a connecting flange 44 for connection to a fuel supply. Smaller tubes 46 or main gas lances 46 extend from the second fuel supply 22. The main gas lances 46 conduct the fuel from the second fuel supply 22 to the second fuel nozzles 18 and terminate therewith. In the illustrated embodiment, the combustion head 10a has six second fuel nozzles 18. The main gas lances 46 extend outside the swirl body 30.

[0049] The second fuel supply 22 merges into a fuel pipe 48, which can pass through the center of the swirl body 30 parallel to the longitudinal axis of the swirl body 30. The fuel pipe 48 is preferably designed as a central fuel pipe. The fuel pipe 48 carries the main gas in the first upstream region. Downstream of the branch of the main gas lances 46, a gas separator plate 50 seals the second fuel supply 22 from the downstream fuel pipe 48. The gas separator plate 50 is arranged in the second fuel supply 22 / the fuel pipe 48 and is located substantially perpendicular to a longitudinal axis of the second fuel supply 22 / the fuel pipe 48.

[0050] Downstream of the gas separation plate 50, the first fuel supply 20 opens into the fuel pipe 48. Downstream of the gas separation plate 50, the fuel pipe 48 thus serves to guide the first quantity of fuel. Downstream of the gas separation plate 50, smaller pipes 52, the so-called support gas lances, branch off. The support gas lances 52 guide the fuel from the first fuel supply 20 to the support gas nozzles 16b. In the illustrated embodiment, the combustion head 10a has three support gas nozzles 16b. The support gas nozzles 16b are located within the swirl body 30. The swirl vanes 32 can be seen next to the support gas nozzle 16b.

[0051] Fig. 2 An exemplary shape of the swirl body 30 can also be seen. In the first region 38 with the swirl vanes 32 and the support gas nozzles 16b, the swirl body 30 is cylindrical with a first diameter. In the second region 38 without swirl vanes, the swirl body 30 is cylindrical with a second diameter. In one embodiment, the first diameter is larger than the second diameter. Both regions 36, 38 can then be connected to one another by a conical region.

[0052] Swirl body 30 is movably mounted on fuel tube 48 via a swirl body inner tube 54, allowing, for example, adaptation to different combustion chamber geometries and process parameters. By axially displacing swirl body 30 on fuel tube 48, the ratio of the air volumes flowing through swirl body 30 and exiting from the gap formed by the outer diameter of the swirl body in region 38 and the inner diameter of delta disk 66 can be influenced within limits.

[0053] Even if in Fig. 2 the burner tube 12 is not shown, it is to be understood that the combustion air 28 in the illustration of the Fig. 2 flows from the right front to the left rear, both through the swirl body 30 and outside the swirl body 30. The main gas lances 46 are located in the air flow.

[0054] Fig. 3 shows schematically in a perspective view the combustion head 10a from a flame side. Already with reference to Figuren 1 and 2 The parts described above will not be described in detail again. All features described so far also apply to the combustion head 10a as shown in Fig. 3 shown. In Fig. 3 the conical part of the swirl body 30 is not shown and parts of the swirl body 30 that enclose the downstream region 38 are cut away to allow the representation of parts located within the swirl body 30. As in Fig. 2 the burner tube 12 is not shown.

[0055] To start the combustion process, a direct, electrical ignition 56 can be provided, which is used only initially for (e.g., initial) ignition. Once a flame has formed and stabilized, further ignition of the fuel-air mixture occurs through reaction from the flame. In one embodiment, the ignition device 56 is attached to one of the support gas nozzles 16b.

[0056] In the illustrated embodiment, the fuel pipe 48 ends downstream in a cylindrical fuel distributor 58. The fuel distributor 58 can also be referred to as a primary gas distributor 58, since at this point the fuel pipe 48 only carries the primary gas. In this embodiment, four primary gas nozzles 16a are arranged radially on a lateral surface of the primary gas distributor 58. The primary gas nozzles 16a are arranged at equal intervals and point away from the fuel pipe 48 or the primary gas distributor 58 toward the burner tube 12 (not shown).

[0057] The primary gas nozzles 16a can have bores 60. Each primary gas nozzle 16a can have multiple bores 60. Two bores 60 are shown, but there can also be more or fewer bores. The bores 60 are arranged on the primary gas nozzles 16a such that the primary gas flows out essentially in a tangential direction. The orientation of the bores 60 can be coordinated with the arrangement and design of the swirl vanes 32 such that the primary gas is discharged with the flow of the fuel / combustion air mixture swirled in the first region 36. The primary gas flows out of the lateral bores in a tangential direction, which is predetermined by the swirl direction. Additionally or alternatively, the primary gas nozzles 16a can have an axial bore from which primary gas also flows.

[0058] The second fuel nozzles 18, or main gas nozzles, are arranged in a circle around the downstream end of the swirl body 30. They have bores 62 on their end faces. The bores 62 are designed to ensure a high fuel exit velocity of the main gas, so that the main flame front 26 forms at a distance from the combustion head.

[0059] Fig. 4 shows a schematic side view of a combustion head 10b. Combustion head 10b can correspond to combustion head 10 and / or combustion head 10a. Already with reference to Figuren 1 bis 3 Described parts will not be described in detail again. In the side view, individual parts have been cut away to better illustrate details. The part of the swirl body 30 facing the viewer has been cut away to reveal the internal structure. The area where the first fuel supply 20 joins the fuel pipe 48 is cut open.

[0060] In the side view of the Fig. 4 Bores 64 are visible on the primary gas nozzles 16a. The bores 64 are designed to discharge the fuel essentially radially inward. The primary gas nozzles may additionally or alternatively also have axial bores.

[0061] In Fig. 4 The burner tube 12 is also shown. The burner tube 12 can be closed at its downstream end by an annular delta disc 66, which extends radially inward from the burner tube 12. In the illustrated embodiment, the delta disc 66 has a plurality of radially inwardly facing guide devices 68. The openings of the second fuel nozzles 18 can be flush with the delta disc 66. The configuration of the delta disc 66, which is described with reference to Fig. 6 described in more detail, serves for internal exhaust gas recirculation in the main flame 26. The recirculation can be brought about by the part of the combustion air 28 that flows past the swirl device 14 and directly strikes the ring of the annular delta disk 66. As a result, negative pressure zones and vortex regions form on the downstream side of the guide devices 68, i.e. on the sides of the guide devices 68 facing into the combustion chamber. The exhaust gases thus recirculated are injected into the hot zones of the main flame 26. In these zones, the temperature is reduced by the recirculated exhaust gas and the O2 partial pressure is reduced. Both effects contribute to reducing NO formation.

[0062] The amount of fuel from the primary gas nozzles 16a and the supporting gas nozzles 16b is small compared to the amount of fuel exiting the second fuel nozzles 18. It is preferably 3% to 15%, particularly preferably 5% to 10% of the total fuel quantity.

[0063] The excess air required for complete combustion of the fuel portions from the primary gas nozzles 16a, the support gas nozzles 16b, and the fuel nozzles 18 can be between 1.075 and 1.2 in certain embodiments. The combustion zones of the primary flame and the spaced-apart main flame are each superstoichiometric. Locally, substoichiometric zones may form due to the fuel flow entering the combustion chamber axially from the second fuel nozzles 18 before the fuel gas, air, and recirculated exhaust gas have sufficiently mixed.

[0064] The reduction in NO levels is achieved through extremely low NO combustion in the partially premixed, very lean primary flame combined with the spaced-apart main flame, which, due to the intensive mixing of internally recirculated exhaust gases and the reduction of the O2 partial pressure in the mixture, prevents high temperatures from developing that are detrimental to NO formation. The advantage is the formation of a slender, yet not too long flame, which efficiently couples the heat released during fuel combustion from the conversion of chemical enthalpy to the cooled surrounding walls of the combustion chamber through radiation and convection.

[0065] Fig. 5 shows schematically a sectional view of a front section of the combustion head 10b. Already with reference to Figuren 1 bis 4 described parts will not be described in detail again.

[0066] Visible is the swirl body inner tube 54, which is guided above the fuel tube 48. This allows the swirl body to be longitudinally displaced and secured in position with a screw 70. This displaceability allows for better adaptation to different combustion chambers in which the main flame front 26 forms.

[0067] The perforated partition 34 is arranged in the region 38. The perforated partition 34 is positioned and configured such that, in the described embodiment, the primary flame 24 is securely stabilized or held in the region 38 of the swirl body 30.

[0068] Fig. 6 shows a schematic front view of the combustion head 10b from the flame side, or in other words, from the combustion chamber. The fuel distributor 58 is arranged centrally, from which the primary gas nozzles 16a extend with their bores 60. Behind it is the perforated partition 34. In the illustrated embodiment, the perforations are realized by two concentric rows of holes, wherein the holes are circular. It should be understood that the perforations can also have a different shape. The ratio of the opening areas to the total area can also be different than shown. The perforated partition 34 serves to reduce the flow velocity of the swirled air / fuel mixture from the region 36 of the swirl body 30. The partition 34 is delimited by the wall of the swirl body 30. The supporting gas nozzles 16b located behind it can be seen through the perforations.

[0069] The second fuel nozzles 18 with their bores 62 are arranged at equal intervals on a circle around the central axis of the swirl body 30. The annular delta disk 66, which closes the burner tube 12, is arranged around this. From the inner circumference of the delta disk 66, the guide devices 68 extend radially inward. In the illustrated embodiment, three guide devices 68 are provided. The three guide devices 68 are evenly distributed over the inner circumference. The combustion head 10b can also have more or fewer guide devices 68, which can then also be evenly distributed over the inner circumference. In the illustrated embodiment, the guide devices 68 are triangular in shape and point with one tip radially inward. The triangles point with one tip away from the annular delta disk 66. As can be seen from Fig. 5 As can be seen, the guidance devices 68 are not in the plane of the Fig. 6 , but point away from the swirl body 30. They are angled.

[0070] The guide vanes 68 with the delta disk 66 are designed to create a vacuum zone that draws exhaust gases from the combustion chamber. The delta disk 66 and guide vanes 68 thus result in internal exhaust gas recirculation. The shape, like an angled triangle pointing away from the swirl body, creates "standing vortices" on the guide vanes 68, which contribute to stabilizing the main flame front 26. Consequently, the recirculated exhaust gases are injected into the hot zones of the main flame and thus into the areas of greatest NO production.

[0071] The geometry of the guide devices 68 was optimized to ensure that the highest possible internal exhaust gas quantity is drawn into the main flame 26. Both the number and geometry of the guide devices 68 must be considered for both the NO reduction effect and the stability of the main flame.

[0072] The annular delta disc 66 may have a plurality of protrusions 72 on its inner circumference between the guide devices 68. The protrusions 72 form a toothed geometry. Fig. 6Semicircular bulges are shown, but the toothing can be configured with other geometries. The toothing 72 is designed to create a larger surface area. The larger surface area leads to a larger contact area between the exhaust gas, combustion air, and main fuel, thereby creating a more intensive and uniform mixing of the fuel-air-exhaust gas mixture. This allows a more uniform distribution of exhaust-enriched and thus stoichiometrically more favorable combustion zones to form in the main flame 26. The inventors have discovered that this further reduces the overall formation of thermal NO.

[0073] As already mentioned, the second fuel nozzles 18 are shaped to achieve the highest possible exit velocities. For this purpose, an orifice plate can be provided in front of the axial opening of the fuel nozzle. The high momentum of the escaping gas can further increase the intensity of the mixture of internally recirculated exhaust gas and fuel. Further optimization is achieved by coordinating the position of the second fuel nozzles 18 with the geometry of the guide devices 68. The second fuel nozzles 18 are evenly distributed between the guide devices 68.

[0074] During operation, the advantageous, low-nitrogen combustion is achieved by first supplying combustion air 28 to the burner tube 12 with a downstream open end. A portion of the combustion air 28 is swirled in the swirl device 14 arranged in the burner tube 12. A first quantity of fuel is supplied directly into the swirl body 30 and mixed there with the swirled combustion air 28. A primary flame is formed in the swirled fuel / combustion air mixture within the swirl body. A second quantity of fuel is supplied downstream of the swirl device 14. A main flame front is formed, which stabilizes downstream of the burner tube and at a distance from the burner tube. The first quantity of fuel is regulated independently of the second quantity of fuel.

[0075] The separate fuel control makes it possible to achieve very low NO emissions over a wide load range. At lower loads, a different ratio of first fuel quantity to second fuel quantity may be optimal than at high loads. If the ratio of the two fuel quantities to one another is fixed, low NO emissions cannot be guaranteed over the entire load range of the burner. With the combustion head according to the invention, for example, at low loads, less primary gas / support gas can be supplied as a percentage of the main gas than at high loads. If there is no separate control, the first fuel quantity decreases proportionally less than the air quantity flowing through the swirl body at lower loads due to flow engineering, so that NO emissions can increase at lower loads even with a superstoichiometric primary flame.

[0076] Although specific embodiments have been illustrated and described, one of ordinary skill in the art will understand that a variety of alternative and / or equivalent implementations may be substituted for the specific embodiment shown and described without departing from the scope of the present invention as defined in the claims. This application is intended to cover all adaptations or variations of the specific embodiments discussed herein within the limits of the scope defined in the claims.

Claims

1. A method for the stepped combustion of a fuel by supplying combustion air (28) into a burner tube (12), comprising: - supplying of a first amount of fuel to form a significantly overstoichiometric primary flame (24) into the burner tube (12), with a stoichiometry greater than 1.5, in particular greater than 2.0; - supplying a second amount of fuel downstream to form a slightly overstoichiometric main flame (26) into a combustion chamber, wherein a temperature of the main flame (26) is reduced by exhaust gases recirculated within the combustion chamber, and wherein the main flame (26) stabilizes downstream of the burner tube, wherein the main flame (26) is slightly overstoichiometric, preferably with a stoichiometry between 1.03 and 1.18, and wherein the supply of the second amount of fuel into the combustion chamber takes place with a higher fuel exit speed, such that, due to the resulting impulses, the main flame (26) stabilizes at a distance from the burner tube (12).

2. The method of claim 1, further comprising: - regulating the first fuel amount independently of the second fuel amount, wherein the regulation is such that the first fuel amount is approximately between 3 % and 15 %, preferably between 5 % and 10 %, of the sum of the first fuel amount and the second fuel amount.

3. The method according to claim 1 or 2, further comprising: - swirling a part of the combustion air (28) to produce a swirled combustion air; - supplying a first partial amount of the first fuel amount into the region of the swirled combustion air to form a swirled lean air / fuel mixture; - reducing a flow velocity of the swirled lean air / fuel mixture; and - supplying a second partial amount of the first fuel amount into the slowed, swirled lean air / fuel mixture.

4. The method according to claim 3, wherein the supply of at least a part of the first partial amount of the first fuel amount takes place in a radially inward direction.

5. The method according to claim 3 or claim 4, wherein the supply of at least a part of the second partial amount of the first fuel amount takes place in a tangential direction with the flow of the swirled fuel / combustion air mixture.

6. The method according to any one of the preceding claims, further comprising: - the formation of vortices, in particular standing vortices, in the region of the supply of the second fuel amount, such that exhaust gases are returned into the hot zones of the main flame front (26).

7. A combustion head (10) for stepped combustion of a fuel, having: a burner tube (12), configured to be flowed through by combustion air (28), wherein the burner tube (12) has a downstream open end; a swirl apparatus (14), arranged within the burner tube (12) to be flowed through by a part of the combustion air (28), with a swirl body (30) enclosing a first and a second region, wherein the first region (36) is located upstream of the second region (38), and swirl vanes (32) are arranged only in the first region; first fuel nozzles (16a, 16b), arranged within the swirl body (30) for supplying fuel to form a primary flame (24) within the swirl body (30); second fuel nozzles (18), arranged downstream of the swirl apparatus (14) for supplying fuel to form a free main flame front (26), wherein the second fuel nozzles (18) are configured such that the main flame front (26) stabilizes downstream of the combustion head (10) and at a distance therefrom; a first fuel supply (20), connected to the first fuel nozzles (16a, 16b); and a second fuel supply (22), connected to the second fuel nozzles (18), wherein the swirl body (30) and the first fuel nozzles (16a, 16b) are configured to obtain the primary flame (24) with a stoichiometry greater than 1.5, in particular greater than 2.0.

8. The combustion head according to claim 7, wherein the fuel amounts of the fuel, supplied by the first fuel supply (20) and by the second fuel supply (22), respectively, can be regulated independently of one another.

9. The combustion head according to claim 7 or 8, wherein the swirl apparatus (14) has a perforated partition wall (34) between the first region (36) and the second region (38).

10. The combustion head according to any one of claims 7 to 9, wherein the first fuel nozzles (16a, 16b) comprise primary fuel nozzles (16a), arranged in the second region (38) of the swirl body (30), and auxiliary fuel nozzles (16b), arranged in the first region of the swirl body (30).

11. The combustion head according to claim 10, wherein the support fuel nozzles (16b) are evenly distributed between the swirl vanes (32) and are configured to discharge fuel in a radially inward direction, to form a swirled fuel / combustion air mixture.

12. The combustion head according to claim 10 or 11, wherein the primary fuel nozzles (16a) are evenly distributed in a spray ring and are configured to deliver fuel in a tangential direction with the flow of the swirled fuel / combustion air mixture.

13. The combustion head according to any one of claims 10-12, wherein at least a part of the fuel exits from the first fuel nozzles (16a) via lateral bores (60) in the first fuel nozzles.

14. The combustion head according to any one of claims 10-13, wherein the first fuel supply (20) is connected, via a fuel pipe (48) in the swirl body (30), to the primary fuel nozzles (16a) and the auxiliary fuel nozzles (16b), wherein the fuel pipe (48) terminates in a fuel distributor (58) to which the primary fuel nozzles (16a) are attached.

15. The combustion head according to claim 14, wherein the swirl body (30) is slidingly arranged in the longitudinal direction on the fuel tube (48).

16. A combustion head according to any one of claims 7-15, further comprising: an annular delta disk (66), extending radially inwardly from the downstream end of the burner tube (12) and having a plurality of radially inwardly facing guide apparatuses (68).

17. The combustion head according to claim 16, wherein the annular delta disk (66) has a plurality of bulges (72) on its inner circumference between the guide apparatuses (68).