ES3018150T3 - Method for staged combustion of a fuel and combustion head - Google Patents

Abstract

Method: Combustion head for the staged combustion of a fuel with combustion air supply (28) to a burner tube (12). A first quantity of fuel is supplied to form a primary flame (24) within the burner tube. A second quantity of fuel is supplied downstream to form a main flame front (26). The main flame front (26) is stabilized downstream of the burner tube (12) and at a certain distance from it. The fuel supplies are designed so that the primary flame (24) burns with a stoichiometry greater than 1.5, in particular greater than 2.0. (Automatic translation with Google Translate, no legal value)

Description

DESCRIPTION

Procedure for the staged combustion of a fuel and combustion head

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

[0002]The combustion of fossil fuels in combustion plants also produces nitrogen oxides, e.g., NO, NO2, in addition to other combustion products. Hereinafter, NO will be referred to briefly only. These and other pollutant emissions can be influenced and reduced by design measures in the burners. The reaction mechanisms leading to such nitrogen oxides are widely known and are generally differentiated into thermal and instantaneous formation of NO, as well as formation of NO as a result of the oxidation of chemically bound nitrogen in the fuel.

[0003]In this regard, it is known that thermal NO according to the so-called Zeldovich mechanism depends, on the one hand, on the residence time of the reactants in the combustion zone and, on the other hand, to a large extent on the combustion temperature itself. The combustion temperature is linked to the fuel/air ratio A. The maximum combustion temperature is set at a fuel/air ratio of A = 1. This is also known as the stoichiometric ratio. The combustion air contains just enough oxygen for the fuel to burn completely. A fuel/air ratio of A < 1 is called a rich mixture, i.e. there is too much fuel. A fuel/air ratio of A > 1 is called a lean mixture, i.e. there is too much 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 flash NOx also plays a significant role. Flash NOx is produced by hydrocarbon radicals CH formed as flame intermediates, which are present as intermediate products during the combustion of carbon-containing fossil fuels. The CH radicals react with nitrogen in the air to form hydrocyanic acid (HCN), which in turn is converted into NO in very fast formation reactions. A proven method for suppressing the formation of free CH radicals and thus the formation of flash NOx is lean combustion or overstoichiometric combustion. Lean combustion refers to combustion with excess air, i.e. with A > 1.

[0005]Instantaneous NO is produced in small quantities compared to thermal NO, but it is a decisive factor in reducing NO formation, especially in ultra-low NO applications.

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

[0007] EP 1754 937 B1 and EP 2 037 173 B1 show combustion heads that achieve a reduction in NO. These are primarily single-stage combustion processes that only allow for limited NO optimization and flame stabilization. DE 195 09 219 A1 shows a combustion head for two-stage combustion with an overstoichiometric air-gas mixture in the first stage and a substoichiometric air-gas mixture in the second stage. DE 4427104 A1 shows a method for the staged combustion of a fuel. A highly overstoichiometric primary flame is formed in a burner tube. A second quantity of fuel is fed downstream for a main flame such that stoichiometrically complete combustion occurs. EP 3078910 B1 shows a gas burner with multi-stage combustion with a highly overstoichiometric primary flame inside a burner tube and a slightly overstoichiometric main flame at the burner tube outlet.

[0008]The primary combustion head features a turbulence generator, as well as main gas nozzles and a stabilizer. All combustion air flows through both combustion stages. The gas burner achieves a reduction in thermal NOx.

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

[0010]In a mixing zone, various fluids that have not yet combusted are mixed. In a mixing zone, the conditions necessary for combustion are usually not met. This may be the case, for example, if the flow velocity of the flammable mixture is significantly higher than the flame velocity.

[0011]A combustion zone is a region in which the conditions necessary for combustion are met. A combustion zone exists if a flammable 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 flammable mixture and the flame velocity are essentially equal, and there is a temperature equal to or higher than the ignition temperature of the flammable mixture. The generic term oxidizer encompasses the term combustion air, but also includes, e.g., ambient air enriched with additional oxygen. Ignition and combustion cannot occur in regions where these conditions are not met. 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. An objective of the invention is to provide energy-efficient combustion with minimized NO emissions. This objective is achieved by the features of the independent claims.

[0013]The purposes and characteristics of the present invention will become clear from the following description of embodiments, which is given with reference to the attached figures, in which:

Fig. 1 shows, in a very schematic manner, a side view of a combustion head;

Fig. 2 shows, schematically in perspective, parts of a combustion head from a fuel supply side;

Fig. 3 shows, schematically in perspective, parts of the combustion head of Fig. 2 from one side of the flame;

Fig. 4 shows, schematically, a side view of a combustion head;

Fig. 5 shows, schematically, a sectional view of a front section of a combustion head; and

Fig. 6 shows, schematically, a front view of a combustion head.

[0014]Aspects and embodiments are described below with reference to the drawings, in which identical or similar references are generally used to refer to identical or similar elements. In the following description, numerous specific details are set forth to provide a complete 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 implemented with a lesser extent of the specific details. In other cases, elements are shown schematically to facilitate the description of one or more aspects of the embodiments. The following description should not be construed as limiting. It should be noted that the representation 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 "up," "down," "top side," "bottom side," "left," "right," "front side," "rear side," "vertical," "horizontal," etc., is not to be construed as limiting. Components of the embodiments may be positioned in various orientations; the directional terminology is used for illustrative purposes only. It is understood that other embodiments may be utilized and that structural or logical changes may be made without departing from the concept of the present invention.

[0016] Multi-stage combustion processes have been known in practice for a long time. However, the approaches known to date are currently not sufficient to continue to meet the increasingly stringent NO requirements imposed on 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, 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 is provided for the staged combustion of a fuel by feeding combustion air into a burner tube according to claim 1. The fuel may be a gas or a liquid fuel. A first quantity of fuel is fed into the burner tube to form a primary flame. A second quantity of fuel may be fed downstream to form a main flame front. The main flame is stabilized downstream of the burner tube and at a distance from it. The fuel feeds are designed such that the primary flame burns with a stoichiometry greater than 1.5, in particular greater than 2.0. This makes it possible to achieve a very low flame temperature. Virtually no instantaneous NO is formed. The main flame is slightly overstoichiometric. The stoichiometry may be between 1.03 ... 1.18. The temperature of the main flame can be significantly reduced by recirculating the exhaust gases within the combustion chamber.

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

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

[0020]In another embodiment, a portion of the combustion air is swirled. This generates turbulent combustion air. A first partial amount of the first fuel quantity is discharged into the air turbulence generation region. This creates a turbulent lean air/fuel mixture. Very good mixing can be achieved. In this region, the flow rate is high and the mixture is lean, so that ignition conditions do not exist. In the next step, the flow rate of the turbulent lean air/fuel mixture is reduced. A second partial amount of the first fuel quantity is fed to the slowed turbulent lean air/fuel mixture.

[0021]There is further provided a combustion head for the staged combustion of a fuel according to claim 7. The provided combustion head enables the method to be carried out. The combustion head is designed to combust a first feed quantity of the fuel in a superstoichiometric primary flame. A second feed quantity of the 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 regulated independently of each other and thus ensure a very low nitrogen oxide combustion in a wide load range.

[0023]The following figures show exemplary configurations 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 very schematic manner, 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 feed line 20 and a second fuel feed line 22. The arrows symbolize the inlet of the fuel flow. In operation, an overstoichiometric primary flame 24 is formed within the swirl device 14 and a main flame or main flame front 26 is formed at a distance from the combustion head 10, both of which are symbolically represented in Fig. 1 by a flame. Thus, the combustion head 10 is used for staged combustion of fuel. The fuel may be gaseous. The fuel may be natural gas. The fuel may include hydrogen. In addition to use as a pure gas burner, a dual-fuel burner is also possible, in which liquid fuel can be burned in addition to gaseous fuel. A burner for liquid fuel only is also possible. The following description generally refers, but is not limited to, an embodiment as a gas burner.

[0025]In the representation of Fig. 1, the combustion air 28 is supplied to the burner tube 12 from the right. The right end of the burner tube 12 in the representation is therefore the upstream end. The burner tube 12 can be essentially cylindrical. The combustion air 28 flows through the burner tube 12 and leaves it at the open end, left in the representation, of the burner tube 12, i.e. the downstream end. The main flame front 26 is formed downstream of the combustion head 10. Here is the combustion or fire chamber, which is not shown in more detail.

[0026]The amount of fuel exiting the first fuel nozzles 16a, 16b can be small relative to the amount of fuel exiting the second fuel nozzles 18. If only a small amount of fuel is combusted in the primary flame 24 in a strongly overstoichiometric manner, a second substoichiometric combustion phase is not necessary. The spaced-apart main flame 26 can therefore also be generally overstoichiometric. With the combustion head 10 according to the invention, a general substoichiometric combustion zone is not generated, as in staged combustion with a substoichiometric and an overstoichiometric combustion zone and the necessary residence time of the gases in these zones for NO reduction. The method according to the invention provides a strongly overstoichiometric primary flame and a slightly overstoichiometric main flame.

[0027]The swirling device 14 is arranged inside the burner tube 12. The swirling device 14 may be open at both ends. A longitudinal axis of the burner tube 12 and a longitudinal axis of the swirling device 14 may be parallel to each other or located one above the other, so that the swirling device 14 is located in the center of the burner tube 12 and is uniformly radially spaced from the inner wall of the burner tube. A part of the combustion air 28 flows outside the swirling device 14 through the burner tube 12, another part of the combustion air 28 flows through the swirling device 14.

[0028]The swirling device 14 comprises a swirling body 30, swirling blades 32 and a perforated partition 34. The swirling body 30 may have an essentially cylindrical shape. The perforated partition 34 may extend substantially perpendicular to the longitudinal axis of the swirling body 30 and divide an interior space of the swirling body 30 into a first region 36 and a second region 38. The first region 36 may be located upstream of the second region 38. The perforated partition 34 may cause a pressure loss. In this way, the flow velocity downstream of the perforated partition 34 may decrease locally.

[0029]The swirling blades 32 may be arranged only in the first region 36. The second region downstream of the partition 34 may be free of swirling blades 32. Several swirling blades 32 may be provided.

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

[0031]The first fuel nozzles 16a, 16b are arranged within the swirling body 30. They are connected to the first fuel feed conduit 20. The first fuel feed conduit 20 allows the quantity of fuel/combustion gas flowing to the first fuel nozzles 16a, 16b to be regulated, as shown with a symbol 40 in Fig. 1. This regulation is separate and independent of a regulation 42 in the second fuel feed conduit 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 swirling device 14. The first fuel nozzles 16a, 16b may comprise further fuel nozzles - hereinafter referred to as supporting fuel nozzles or supporting gas nozzles 16b - which are located in the first, upstream region 36 of the swirling device 14.

[0033]The support fuel nozzles 16b can be arranged evenly distributed between the swirling vanes 32. The support fuel nozzles 16b can be arranged essentially parallel to a longitudinal axis of the burner tube 12. The swirling vanes 32 cause strong turbulence of the combustion air 28. The fuel exiting the support fuel nozzles 16b, which is also called support gas, is thus very efficiently premixed with a portion of the combustion air 28 for the primary flame 24. The result is a swirling fuel/combustion air mixture. The fuel feed through the support fuel nozzles 16b can be designed such that a turbulent lean fuel/air mixture is formed. The support fuel nozzles 16b can discharge a first partial quantity of the first quantity of fuel. The supporting fuel nozzles 16b may have openings for discharging the fuel. The openings may be arranged such that the fuel is discharged at least partially inward essentially in a radial direction, i.e., in a direction essentially perpendicular to the wall of the swirling body 30. Due to the high flow velocities of the swirling combustion air and the high proportion of air relative to the amount of fuel or gas, the ignition conditions for the swirling fuel-air mixture in the region of the swirling blades 32, i.e., in the upstream region 36, are not yet met.

[0034]The partition 34 can be designed to slow down the swirling fuel-combustion air mixture. The partition 34 can have openings for this purpose. For this purpose, the partition can be designed essentially as a grid. The geometry of the partition 34 can be designed to reduce the flow velocity of the swirling fuel-combustion air mixture, without significantly impairing the formation of turbulence. The partition 34 reduces the absolute flow velocity of the swirling, premixed primary air and thus ensures ignition of the primary flame 24, which is additionally enriched with a second proportion of the first quantity of fuel in this region.

[0035]The primary fuel nozzles 16a may be evenly distributed in the downstream region 38. In this way, the primary fuel nozzles 16a are located downstream of the partition 34, in a region of lower flow velocity. The primary fuel nozzles 16a may be arranged substantially perpendicular to a longitudinal axis of the burner tube 12. The primary fuel nozzles 16a may be evenly distributed in a jet ring. A plurality of primary fuel nozzles 16a may be provided. In the downstream region 38, the primary fuel nozzles 16a discharge the second portion of the first quantity of fuel, referred to as primary gas, into the fuel-air mixture formed in the blade portion of the swirling body 30, or in other words, in the first region 36, thereby generating the flammable mixture for forming the primary flame 24. The primary fuel nozzles 16a may have openings for discharging the fuel. The openings may be arranged on the side of the primary fuel nozzles 16a. The side openings may be arranged such that the fuel is discharged substantially in a tangential direction.

[0036]The ratio of the opening area of all of the orifices of the primary fuel nozzles 16a to the opening area of all of the orifices of the support fuel nozzles 16b may determine a ratio of primary gas to support gas, taking into account the feed conduits to the primary fuel nozzles 16a and the support fuel nozzles 16b. The ratio may be selected based on the overall geometry and the quality or composition of the fuel. The ratio may be approximately 1:1. Approximately half of the fuel flowing through the first fuel feed conduit 20 may be discharged through the primary fuel nozzles 16a in the region 38, and approximately half of the fuel flowing through the first fuel feed conduit 20 may be discharged through the support fuel nozzles 16b in the region 36.

[0037] The ability to separately regulate the primary and support gas by the regulating device 40 in relation to the ability to regulate the second main fuel quantity flowing through the second fuel feed line 22, as well as the design of the swirl body 30, the primary and support fuel nozzles 16a, 16b and the partition 34 can generate a primary flame 24 with a stoichiometry A>>1 over a wide load range. In one embodiment, the stoichiometry of the primary flame 24 is A>1.5. In another embodiment, the stoichiometry of the primary flame 24 is A>2.

[0038] Due to the resulting very low combustion temperatures, practically no thermal or instantaneous NO is produced in the primary flame 24.

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

[0040] In order to obtain a stable primary flame 24, the pre-feeding of the support gas through the support fuel nozzles 16b into the swirling combustion air and thus the enrichment and premixing of the primary air with fuel, the type and position of the introduction of the primary gas, the ratio of support gas to primary gas, and the geometry and position of the partition 34 in the bladeless portion 38 of the swirling body 30 in the embodiment shown are important. Other means may 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, bladeless part of the swirl body 30, the region 38 in Fig. 1, is designed so that the primary flame 24 is formed in a defined region which is protected from the remaining combustion air 28, which flows out of 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 swirling device 14. The second fuel nozzles 18 are connected to the second fuel feed line 22. The second fuel feed line 22 makes it possible to regulate the amount of fuel/flue gas flowing into the second fuel nozzles 18. The second fuel quantity comprises, in this respect, the majority of the total fuel quantity, which is why it is also referred to as the main fuel quantity or main gas. The regulating capacity of the main gas is represented by the symbol 42 in Fig. 1.

[0043] The second fuel nozzles 18 are located within the burner tube 12. The second fuel nozzles 18 may be located at and terminate with the downstream end of the burner tube 12. The second fuel nozzles 18 may be evenly distributed around the inner periphery of the burner tube 12. Not shown in Fig. 1 is an annular delta disk that may fill a distance between the burner tube 12 and the second fuel nozzles 18 at the downstream end of the burner tube. The delta disk is explained in more detail in Figs. 4-6.

[0044] The second fuel nozzles 18 are designed to ensure a high fuel exit velocity. The resulting pulse transports the fuel as far as possible into the combustion chamber and forms a combustion zone at a distance 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. The second fuel nozzles 18 can, for this purpose, have an opening on a front side. A deflector can define the opening on the front side. The configuration of the second fuel nozzles 18 leads to the formation of the main flame or main flame front 26, which is at a certain distance from the downstream end of the combustion head 10 and is formed in a stable manner in the combustion chamber, which is not shown in more detail. Due to the arrangement of the second fuel nozzles with an outlet direction coaxial with the burner tube axis, the main flame 26 can have a slender, elongated flame shape. Internal exhaust gas recirculation, explained in more detail below, can inject exhaust gases into the hot zones of the main flame 26 and thus into the regions of highest NO production. This reduces NO production in the main flame.

[0045] The fuel feed ducts may be designed and arranged so as to provide ignition energy to the main flame 26 spaced from the primary flame and the recirculated exhaust gases in order to ignite the mixture of main fuel, combustion air or general oxidizing agent and recirculated exhaust gases and ensure a continuous and stable progress of the oxidation reactions.

[0046]Both gas inlets, i.e. the fuel feed line 20 for the primary and support gas for the primary flame 24 and the fuel feed line 22 for the main gas for the main flame 26, are regulated separately by gas regulating devices 40 and 42 in the embodiment shown. In this way, the gas quantity in the primary flame 24 and in the main flame 26 can be adjusted separately from each other, thus individually regulating the stoichiometry in the respective combustion zone. This allows the establishment of a stable and overstoichiometric primary combustion zone and thus the formation of an almost NO-free primary flame 24 in 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. The combustion head 10a may have the same features as those described for the combustion head 10 shown in Fig. 1. The combustion head 10a may represent an implementation of the combustion head 10. Therefore, the same references are used as in Fig. 1. The description of Fig. 2 is essentially limited to details that do not follow from the representation of Fig. 1. The burner tube 12 is not shown in Fig. 2.

[0048]The second fuel supply line 22 of the combustion head 10a is designed as a tube, which has a connecting flange 44 for connection to a fuel supply. Smaller tubes 46 or main gas lances 46 extend from the second fuel supply line 22. The main gas lances 46 guide the fuel from the second fuel supply line 22 to the second fuel nozzles 18 and terminate therewith. In the embodiment shown, 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 feed line 22 merges into a fuel pipe 48, which can pass through the center of the swirling body 30 parallel to the longitudinal axis of the swirling body 30. The fuel pipe 48 is preferably designed as a central fuel pipe. The fuel pipe 48 conducts the main gas to the first upstream region. Downstream of the branch of the main gas lances 46, a gas separator plate 50 isolates the second fuel feed line 22 from the downstream fuel pipe 48. The gas separator plate 50 is arranged in the second fuel feed line 22/fuel pipe 48 and is arranged substantially perpendicular to a longitudinal axis of the second fuel feed line 22/fuel pipe 48.

[0050]Downstream of the gas separator plate 50, the first fuel feed line 20 opens into the fuel pipe 48. Downstream of the gas separator plate 50, the fuel pipe 48 thus serves to conduct the first quantity of fuel. Smaller pipes 52, so-called supporting gas lances, extend downstream of the gas separator plate 50. The supporting gas lances 52 conduct the fuel from the first fuel feed line 20 to the supporting gas nozzles 16b. In the embodiment shown, the combustion head 10a has three supporting gas nozzles 16b. The supporting gas nozzles 16b are arranged within the swirl body 30. The swirl vanes 32 can be seen next to the supporting gas nozzle 16b.

[0051]Fig. 2 also shows by way of example a shape of the swirling body 30. In the first region 38 with the swirling vanes 32 and the support gas nozzles 16b, the swirling body 30 is designed cylindrical with a first diameter. In the second region 38 without swirling vanes, the swirling body 30 is designed 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]The swirl body 30 is slidably mounted on the fuel tube 48 via a swirl body inner tube 54, which allows, for example, adaptation to different combustion chamber geometries and process parameters. By axially displacing the swirl body 30 on the fuel tube 48, the ratio of the air quantities flowing through the swirl body 30 and exiting the gap formed by the outer diameter of the swirl body in the region 38 and the inner diameter of the delta disk 66 can be influenced within limits.

[0053]Although the burner tube 12 is not shown in Fig. 2, it should be understood that the combustion air 28 flows from the front right to the rear left in the representation of Fig. 2, 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 one side of the flame. The parts already illustrated with reference to Figs. 1 and 2 are not described again in detail. All features described so far also apply to the combustion head 10a shown in Fig. 3. In Fig. 3, the conical part of the swirling body 30 is not shown and parts of the swirling body 30 enveloping the downstream region 38 have been omitted to allow the parts lying within the swirling body 30 to be seen. As in Fig. 2, the burner tube 12 is not shown.

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

[0056]In the embodiment shown, the fuel line 48 ends downstream in a cylindrical fuel distributor 58. The fuel distributor 58 can also be referred to as the primary gas distributor 58, since at this point the fuel line 48 still only transports the primary gas. In the embodiment, four primary gas nozzles 16a are arranged in the form of spokes on a lateral surface of the primary gas distributor 58 in the primary gas distributor 58. The primary gas nozzles 16a are arranged at regular intervals and point away from the fuel line 48 or the primary gas distributor 58 towards the burner tube 12, not shown.

[0057]The primary gas nozzles 16a may have holes 60. Each primary gas nozzle 16a may have several holes 60. Two holes 60 are shown. However, there may also be more or fewer holes. The holes 60 are arranged in the primary gas nozzles 16a such that the primary gas exits essentially in a tangential direction. The orientation of the holes 60 can be coordinated with the arrangement and design of the swirling vanes 32 such that the primary gas is discharged with the flow of the swirling fuel/combustion air mixture in the first region 36. The primary gas exits through the lateral holes in a tangential direction, determined by the swirling direction. Additionally or alternatively, the primary gas nozzles 16a may have an axial hole from which the primary gas also exits.

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

[0059]Fig. 4 shows a schematic side view of a combustion head 10b. The combustion head 10b may correspond to the combustion head 10 and/or the combustion head 10a. The parts already illustrated with reference to Figs. 1 to 3 are not described again in detail. In the side view, individual parts have been omitted in order to better visualize the details. The part of the swirl body 30 facing the observer is omitted so that the internal structure can be observed. The region where the first fuel feed conduit 20 opens into the fuel pipe 48 is cut away.

[0060]In the side view of Fig. 4, openings 64 are visible in the primary gas nozzles 16a. The openings 64 are designed to discharge the fuel substantially inward in a radial direction. The primary gas nozzles may additionally or alternatively have axial openings.

[0061]Fig. 4 also shows the burner tube 12. The burner tube 12 may be closed at its downstream end by an annular delta disk 66, which extends radially inwardly from the burner tube 12. In the embodiment shown, the delta disk 66 has a plurality of radially inwardly oriented directing means 68. The openings of the second fuel nozzles 18 may terminate flush with the delta disk 66. The design of the delta disk 66, which is described in more detail with reference to Fig. 6, serves for internal recirculation of the exhaust gases in the main flame 26. The recirculation may be effected, in this case, by that part of the combustion air 28 which flows past the swirl device 14 and directly impinges on the ring of the annular delta disk 66. As a result, negative pressure zones and vortex areas are formed on the downstream side of the steering means 68, i.e. on the sides of the steering means 68 facing the combustion chamber. The recirculated exhaust gases are then injected into the hot zones of the main flame 26. In these zones, the recirculated exhaust gases reduce the temperature and partial pressure of O2. Both effects contribute to reducing the formation of NO, or to the formation of NO.

[0062]The amount of fuel exiting the primary gas nozzles 16a and the support gas nozzles 16b is small relative 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 amount of fuel.

[0063]In some embodiments, 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 may be between 1.075 and 1.2. The combustion zones of the primary flame and the spaced-apart main flame are each over-stoichiometric. Locally, the fuel flow entering the combustion chamber axially from the second fuel nozzles 18 may lead to the formation of sub-stoichiometric zones before the fuel gas, air and recirculated exhaust gases have sufficiently mixed.

[0064]The decrease in NO values is the result of the extremely low NO combustion in the partially premixed and very lean primary flame in combination with the spaced-apart main flame, which cannot develop the high temperatures harmful to NO formation due to the intensive mixing of the internally recirculated exhaust gases and the reduction of the O2 partial pressure in the mixture. The advantage is the formation of a slender, but not too long flame, which efficiently decouples the heat released during fuel combustion by chemical enthalpy conversion to the surrounding cooled walls of the combustion chamber by radiation and convection.

[0065]Fig. 5 shows, schematically, a sectional view of a front section of the combustion head 10b. The parts already illustrated with reference to Figures 1 to 4 are not described again in detail.

[0066]The inner tube of the swirl body 54 is visible, which is guided above the fuel tube 48. In this way, the swirl body can be moved longitudinally and fixed in position by means of a screw 70. The movement capacity allows a better adaptation to different combustion chambers in which the main flame front 26 is formed.

[0067]The perforated partition 34 is arranged in the region 38. The perforated partition 34 is positioned and designed such that, in the described embodiment, the primary flame 24 is stabilized or held securely in the region 38 of the swirling 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 centrally located, from which the primary gas nozzles 16a with their holes 60 extend. Behind it is the perforated partition 34. In the illustrated embodiment, the openings are implemented by two concentric rows of holes, the holes being circular. It should be understood that the openings can also have a different shape. The ratio of the opening areas to the total area can also be different from that shown. The perforated partition 34 serves to reduce the flow velocity of the swirling air/fuel mixture from the region 36 of the swirling body 30. The partition 34 is delimited by the wall of the swirling body 30. The support gas nozzles 16b located behind it can be seen through the openings.

[0069]The second fuel nozzles 18 with their openings 62 are arranged at regular intervals in a circle around the central axis of the swirl body 30. The annular delta disk 66 is located around it, which isolates the burner tube 12. The directing means 68 extend inward in a radial direction from the inner circumference of the delta disk 66. In the embodiment shown, three directing means 68 are provided. The three directing means 68 are distributed evenly around the inner circumference. The combustion head 10b can also have more or fewer directing means 68, which can also be distributed evenly around the inner circumference. In the embodiment shown, the directing means 68 are triangular in shape and have a point directed inward in a radial direction. The triangles point with a point away from the annular delta disk 66. As can be seen in Fig. 5, the steering means 68 are not located in the drawing plane of Fig. 6, but point away from the swirling body 30. They are at an angle.

[0070]The steering means 68 with the delta disk 66 are configured such that they cause the formation of a negative pressure zone, which extracts the exhaust gases from the combustion chamber. The delta disk 66 and the steering means 68 thus result in an internal recirculation of the exhaust gases. The inclined triangle-like shape pointing away from the swirl body generates "stationary vortices" in the steering means 68, which contribute to the stabilization of the main flame front 26. Consequently, the recirculated exhaust gases are injected into the hot zones of the main flame and thus into the regions of greatest NO production.

[0071]The geometry of the steering means 68 has been optimized such that the largest possible amount of internal exhaust gases is drawn into the main flame 26. In this regard, both the number and the geometry of the steering means 68 must be taken into account for the effect of NO reduction as well as for the stability of the main flame.

[0072]The annular delta disk 66 may have a plurality of protrusions 72 on its inner circumference between the steering means 68. The protrusions 72 form a toothed geometry. Fig. 6 shows semicircular protrusions, but the toothing may be designed in other geometries. The toothing 72 is designed to create a larger surface area. The larger surface area results in a larger contact area between the exhaust gases, the combustion air and the main fuel, which results in a more intense and uniform mixing of the fuel-air-exhaust gas mixture. This allows a more uniform distribution of exhaust gas-enriched and therefore more stoichiometrically favorable combustion zones in the main flame 26. The inventors have discovered that this further reduces the formation of thermal NO in general.

[0073]As already mentioned, the second fuel nozzles 18 have a shape that allows the highest possible exit velocities to be achieved. For this purpose, a deflector may be provided in front of the axial opening of the fuel nozzle. The high momentum of the exit gas can further increase the mixing intensity of the internally recirculated exhaust gases and the fuel. A further optimization is achieved by positioning the second fuel nozzles 18 in coordination with the geometry of the steering means 68. The second fuel nozzles 18 are arranged evenly distributed between the steering means 68.

[0074]In operation, advantageous, low-nitrogen combustion is achieved by first feeding combustion air 28 into the burner tube 12 with an open end located downstream. A part of the combustion air 28 is swirled in the swirling device 14 arranged in the burner tube 12. A first quantity of fuel is fed directly into the swirling body 30, where it mixes with the swirled combustion air 28. A primary flame is formed in the swirling fuel/combustion air mixture within the swirling body. A second quantity of fuel is fed downstream of the swirling device 14. A main flame front is formed which stabilizes downstream of the burner tube and at a certain distance from it. In this respect, the first quantity of fuel is regulated independently of the second.

[0075] Separate fuel regulation makes it possible to achieve very low NO emissions over a wide load range. At lower loads, a different ratio between the first and second fuel quantities may be optimal than at high loads. If the ratio between the two fuel quantities were fixed, a low level of NO emissions could not be guaranteed over the entire burner load range. With the combustion head according to the invention, for example, less primary gas/support gas as a percentage of the main gas can be fed at low load than at high load. If the regulation capacity were not independent, the first fuel quantity would decrease proportionally less at a lower load than the amount of air flowing through the swirl body, due to flow dynamics reasons, so that NO emissions could increase at a lower load even with an overstoichiometric primary flame.

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

Claims (17)

1. Method for the staged combustion of a fuel by feeding combustion air (28) to a burner tube (12), comprising: - feeding a first quantity of fuel to form a strongly overstoichiometric primary flame (24) inside the burner tube (12) with a stoichiometry greater than 1.5, in particular greater than 2.0; - feeding a second quantity of fuel downstream to form a slightly overstoichiometric main flame (26) in 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) is stabilized downstream of the burner tube, wherein the main flame (26) is slightly overstoichiometric, preferably with a stoichiometry between 1.03 and 1.18, and the second quantity of fuel is fed into the combustion chamber at a high fuel output velocity, so that the main flame (26) stabilizes at a distance from the burner tube (12) due to the resulting momentum. 2. Method according to claim 1, further comprising: - regulating the first fuel quantity independently of the second fuel quantity, wherein the regulation takes place in such a way that the first fuel quantity is approximately between 3% and 15%, preferably between 5% and 10%, of the sum of the first fuel quantity and the second fuel quantity. 3. Method according to claim 1 or 2, further comprising: - swirling a portion of the combustion air (28) to generate turbulent combustion air, - feeding a first partial quantity of the first quantity of fuel to the turbulent combustion air region to form a turbulent lean air/fuel mixture; - reduce the flow velocity of the turbulent lean air/fuel mixture; and - feeding a second partial quantity of the first quantity of fuel into the turbulent lean air/fuel mixture. 4. Method according to claim 3, wherein the feeding of at least a part of the first partial quantity of the first quantity of fuel takes place in a radially inward direction. 5. Method according to claim 3 or claim 4, wherein the feeding of at least a part of the second partial quantity of the first quantity of fuel takes place in a direction tangential to the flow of the swirling fuel/combustion air mixture. 6. Method according to one of the preceding claims, further comprising: - forming eddies, in particular stationary eddies in the feed region of the second fuel quantity, so that the exhaust gases are fed back into the hot zones of the main flame front (26). 7. Combustion head (10) for the staged combustion of a fuel, having: a burner tube (12) configured to allow combustion air (28) to flow therethrough, wherein the burner tube (12) has an open end located downstream; a swirling device (14) arranged inside the burner tube (12) to be traversed by a part of the combustion air (28), with a swirling body (30) surrounding a first and a second region, wherein the first region (36) is located upstream of the second region (38), and only in the first region are swirling blades (32) arranged; first fuel nozzles (16a, 16b) arranged within the swirling body (30) for feeding fuel to form a primary flame (24) within the swirling body (30); second fuel nozzles (18) arranged downstream of the swirling device (14) for feeding fuel to form a free main flame front (26), the second fuel nozzles (18) being configured such that the main flame front (26) is stabilized downstream of and at a distance from the combustion head (10); a first fuel feed conduit (20), which is connected to the first fuel nozzles (16a, 16b); and a second fuel feed conduit (22) which is connected to the second fuel nozzles (18), wherein the swirling 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. Combustion head according to claim 7, wherein the quantities of fuel supplied by the first fuel supply line (20) or by the second fuel supply line (22) can be regulated independently of each other. 9. Combustion head according to claim 7 or 8, wherein the swirling device (14) comprises a perforated partition (34) between the first region (36) and the second region (38). 10. Combustion head according to 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 swirling body (30) and supporting fuel nozzles (16b) arranged in the first region of the swirling body (30). 11. Combustion head according to claim 10, wherein the supporting fuel nozzles (16b) are uniformly distributed between the swirling vanes (32) and configured to discharge fuel inwardly in a radial direction so as to form a swirling fuel/combustion air mixture. 12. Combustion head according to claim 10 or 11, wherein the primary fuel nozzles (16a) are uniformly distributed in a jet ring and configured to discharge fuel in a direction tangential to the flow of the swirling fuel/combustion air mixture. 13. Combustion head according to one of claims 10-12, wherein at least a part of the fuel outlet from the first fuel nozzles (16a) takes place through side holes (60) in the first fuel nozzles. 14. Combustion head according to one of claims 10-13, wherein the first fuel feed line (20) is connected to the primary fuel nozzles (16a) and the supporting fuel nozzles (16b) via a fuel pipe (48) in the swirl body (30), the fuel pipe (48) ending with a fuel distributor (58) to which the primary fuel nozzles (16a) are attached. 15. Combustion head according to claim 14, wherein the swirling body (30) is arranged on the fuel pipe (48) so as to be movable in the longitudinal direction. 16. Combustion head according to 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 several radially inwardly oriented steering means (68). 17. Combustion head according to claim 16, wherein the annular delta disk (66) has a plurality of protuberances (72) on its inner periphery between the steering means (68).