Burner, especially for a direct-flame preheating section of a continuous metal-strip treatment line

Abstract

The invention relates to a "flameless" gas fuel burner (1) having a longitudinal axis (A) at the intersection of two perpendicular planes (P1, P2), comprising a diffuser (2) through which fuel-injection ducts (3) and oxidiser-injection ducts (4, 5) pass, the oxidiser-injection ducts (4, 5) opening out from the diffuser closer to the burner axis than the fuel-injection ducts (3) for operation in flameless mode, and further comprising a central fuel-injection duct (6) for operation in flame mode, which extends in the axial direction (A) of the burner, further comprising an internal combustion chamber (7) into which the central fuel-injection duct (6) opens, as well as a passage (8, 27, 28) supplying oxidiser, the internal combustion chamber comprising an ignition electrode (9) for starting combustion in the internal combustion chamber (7) of the mixture of fuel and oxidiser supplied thereto by the central fuel-injection duct (6) and the at least one duct (8, 27, 28) supplying oxidiser.

Description

BURNER, IN PARTICULAR FOR DIRECT FLAME PREHEATING SECTION OF CONTINUOUS LINE FOR PROCESSING A METAL STRIP Designation of the technical field concerned

[0001] The invention relates to industrial burners, intended in particular for continuous lines for annealing or galvanizing metal strips, horizontal or vertical, and more particularly to the direct combustion preheating sections of these lines, sometimes called "NOF sections", NOF being the abbreviation for "Non-Oxidizing Furnace", or "DFF section", DFF being the abbreviation for "Direct Firing Furnace".

[0002] The invention is an improvement of the solution described in the applicant's patent FR3114375, for a burner having combustion modes with flame and without flame. Technical problems addressed by the invention

[0003] In the direct combustion preheating sections of the steel strip processing lines, the burners must be capable of operating in reducing mode, i.e. underfed with oxidant, in order to reduce as much as possible the presence of oxygen near the metal strip and thus avoid its oxidation.

[0004] The ignition of the burners when the furnace is at low temperature, for example during its start-up, is generally facilitated by the application of super-stoichiometric conditions, i.e. by overfeeding the burners with oxidant.

[0005] However, to simplify installation and avoid a different operating mode in excess air for low temperature operating phases, it is advantageous if the burners can start in reducer mode.

[0006] In the preheating section, the combustion gases flow in the opposite direction to the metal strip, from the outlet of the preheating section to its inlet. This exposes the burners in the zones located at the inlet of the section to the combustion gases from the burners in the zones located upstream in the direction of flue gas flow. In addition, inert gases such as nitrogen are present in the furnace. The burner must therefore be able to ignite and heat up in a highly diluted environment.

[0007] Flameless mode is achieved at high temperatures, for example above 850°C, by recirculating a significant portion of the combustion gases inside the furnace to achieve a homogeneous and distributed reaction zone compared to an intense zone for flame mode combustion.

[0008] During startup and at low temperatures, the burners operate in flame mode. The high recirculation rate and the presence of inert gases such as nitrogen inside the furnace dilute the reaction zone and disrupt the flame base.

[0009] Without a continuous stabilization mechanism, such as a premix pilot burner, the flame may go out.

[0010] Premixing the oxidizer with the fuel downstream of the flame stabilizes the flame but leads to an intense reaction zone near the burner. This results in burner overheating and higher nitrogen oxide emissions.

[0011] Under these conditions, it is difficult to maintain a strong and stable flame, which also makes it difficult to detect the flame via a UV cell.

[0012] Existing solutions do not allow all these requirements to be combined without a pilot burner.

[0013] The invention overcomes these problems without requiring a pilot burner. Technical background

[0014] Burners exposed to the above requirements are generally equipped with a pilot burner used to ensure ignition of the burner under all conditions.

[0015] The pilot burner power is limited, typically 5 to 15 kW, as it often operates in premix mode, and is only used to ignite the burner and help stabilize the flame during the furnace warm-up procedure.

[0016] Pilot burners are only used until the autoignition temperature of the fuel is reached, after which they are extinguished and sometimes retracted from the burner.

[0017] Pilot burners that remain in place often require cooling when the furnace is in production, at its nominal operating temperature.

[0018] Pilot burners represent additional capital expenditures because they require a fuel and oxidizer circuit separate from the main burner circuits, in addition to a dedicated control unit.

[0019] This currently limits their use to a few areas of the furnace, usually the first areas in the direction of gas flow.

[0020] The zones equipped with pilot burners are used for ignition and warming up, while the other zones are ignited later, one after the other, when their temperature reaches the auto-ignition temperature of the fuel due to the energy provided by the neighboring zone in operation.

[0021] This operating system is less flexible because most zones depend on their temperature to be in operation.

[0022] In a non-oxidizing furnace, combustion occurs in an oxidant-deficient environment to prevent oxidation of the metal strip. The reaction produces non-oxidized or partially oxidized species such as carbon monoxide and hydrocarbons. These must be further oxidized in the recovery zone of the preheating section or in a dedicated section generally called a post-combustion furnace.

[0023] At low load and low furnace temperature, it is preferable that the last zones, in the direction of gas flow, are in operation, so that the combustion gases exit at a relatively high temperature, facilitating the oxidation of unburned species in the recovery zone.

[0024] Working with a few zones equipped with pilot burners, which are generally far from the flue gas outlet, requires a larger afterburner to reheat the flue gases to a higher temperature suitable for the oxidation of unburned species.

[0025] According to a first aspect of the invention, there is provided a gaseous fuel burner capable of operating in so-called "flameless" mode, having a longitudinal axis at the intersection of two perpendicular planes, and comprising a diffuser crossed by fuel injection ducts for operation in flameless mode and oxidant injection ducts, said oxidant injection ducts emerging from the diffuser closer to the axis of the burner than said fuel injection ducts for operation in flameless mode, the burner further comprising a central fuel injection duct for operation in flame mode which extends in the axial direction of the burner, characterized in that it comprises a combustion chamber internal to the burner into which the central fuel injection duct opens at least partially as well as at least one passage supplying oxidant,the internal combustion chamber further comprising an ignition electrode capable of starting combustion in the internal combustion chamber at least partially of the mixture of fuel and oxidant supplied therein by the central fuel injection conduit and at least one conduit supplying oxidant.,

[0026] The internal combustion chamber creates a volume in which combustion can take place under stable and controlled conditions, independent of the temperature of the furnace and its atmosphere.

[0027] The use of an electrode makes it possible to ensure the ignition of this combustion by a simple and inexpensive device. It can also be used to check the presence of a flame by ionization.

[0028] The layout of the internal combustion chamber allows the central fuel injection duct to be used to supply fuel to it.

[0029] According to the invention, the internal combustion chamber forms a tunnel whose inlet diameter is greater than the outlet diameter, depending on the direction of flow of the fluids.

[0030] The internal combustion chamber is a converging tunnel, with a progressive reduction in size to reach a diameter smaller than that of the oxidizer disc.

[0031] This reduction in diameter increases the momentum of the gases leaving the internal combustion chamber. This results in a more intense and longer flame, better able to maintain the main combustion of the burner.

[0032] The average diameter of the internal combustion chamber depends on the desired power output. A larger combustion chamber allows for higher power output. However, the internal combustion chamber must not interfere with the flow of oxidizer to the diffuser's oxidizer injection ducts.

[0033] The size of the combustion chamber is also limited by its pressure drop. It must be large enough to limit the amount of oxidant entering the combustion chamber to ensure that enough oxidant passes through the diffuser's oxidant injection ducts. This maintains the aerodynamics of the burner's flameless operating mode.

[0034] Advantageously according to the invention, the downstream end of the internal combustion chamber, in the direction of flow of the fluids, is located in an axial duct of the diffuser opening into a flame tunnel.

[0035] The axial duct to the burner mechanically holds the downstream end of the combustion chamber. It also allows the combustion chamber outlet diameter to be substantially maintained over an additional length, that of the axial duct which is located beyond the internal combustion chamber. This additional length contributes to the holding of the flame coming from the internal combustion chamber.

[0036] According to the invention, the central fuel injection duct comprises fuel injection passages arranged at two levels along the duct.

[0037] The first level of radially injected fuel is intended to mix with oxidant to form the base of the flame in the internal combustion chamber.

[0038] The second level of fuel injected later is mainly intended to mix with oxidant brought out of the internal combustion chamber by the oxidant injection ducts of the diffuser. It aims to give the flame a good boost to protect it from the diluting environment.

[0039] According to the invention, the central fuel injection duct comprises fuel injection passages which open into the combustion chamber internal to the burner.

[0040] This arrangement makes it possible to easily supply the fuel necessary to obtain combustion in the internal combustion chamber.

[0041] Advantageously according to the invention, the fuel injection passages may be axial and / or radial and / or inclined at an angle of between 0 and 90°% relative to the axis of the burner.

[0042] The arrangement of the fuel injection passages is chosen according to the nature of the gaseous fuel, in particular its calorific value, so that the flow of fuel into the internal combustion chamber leads to a stable flame of sufficient intensity.

[0043] Advantageously according to the invention, the central fuel injection conduit comprises at least one fuel injection passage which opens downstream of the internal combustion chamber to the burner, in the direction of flow of the fluids.

[0044] The supply of fuel downstream of the chamber limits the power released in it. This has the effect of limiting the temperature of the walls of the internal combustion chamber and thus increasing its durability over time.

[0045] Advantageously according to the invention, the burner comprises an oxidizer disc linked to the central fuel injection duct, the disc forming an annular passage with the internal combustion chamber in which oxidizer can circulate.

[0046] The injection of oxidant into the combustion chamber is carried out in such a way as to confine the flame away from the wall of the internal combustion chamber. The peripheral flow of oxidant allows the internal combustion chamber to be cooled and the reaction zone to be confined.

[0047] Advantageously according to the invention, the oxidant disc comprises passages through which oxidant can circulate.

[0048] These passages allow the oxidant to be optimally distributed in the internal combustion chamber. They are arranged axisymmetrically around the central fuel injection duct.

[0049] Advantageously according to the invention, passages of the oxidizer disc form grooves having a radial component and a tangential component to generate a swirling flow of the oxidizer downstream of the oxidizer disc.

[0050] The swirling flow resulting from the grooves promotes mixing of the oxidizer and fuel.

[0051] Advantageously according to the invention, the oxidizer disc comprises circular openings adjoining the ends of the proximal grooves of the burner axis.

[0052] Advantageously according to the invention, the oxidizer disc comprises cylindrical passages converging towards the axis of the burner, in the direction of flow of the oxidizer.

[0053] The injection angle is determined to form a stable flame base.

[0054] Advantageously according to the invention, the combustion chamber internal to the burner has a silicon carbide wall.

[0055] The use of silicon carbide allows for very good durability of the internal combustion chamber. It also allows for more intense combustion in the combustion chamber due to the higher temperature resistance of silicon carbide compared to that of refractory steels.

[0056] Other materials with good temperature resistance can be used, such as refractory steels. Brief description of the figures

[0057] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which:

[0058] is a schematic front view of the diffuser of a burner according to the state of the art,

[0059] is a schematic three-dimensional sectional view of one half of the diffuser according to the,

[0060] is a schematic enlargement and longitudinal section of an internal part of a burner according to a first exemplary embodiment of the invention,

[0061] is a view similar to that of the for a second exemplary embodiment of the invention,

[0062] is a view similar to that of the for a third exemplary embodiment of the invention,

[0063] is a view similar to that of the for a fourth exemplary embodiment of the invention, and

[0064] is a schematic front view of the injector of the central fuel injection duct and the oxidizer disc for a fifth exemplary embodiment of the invention. Detailed description of the invention

[0065] The embodiments described below being in no way limiting, it will be possible in particular to consider variants of the invention comprising only a selection of the characteristics described, subsequently isolated from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one characteristic, preferably functional without structural details, or with only a part of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0066] In the remainder of the description, elements having an identical structure or similar functions will be designated by the same references.

[0067] Referring to the diagram of the attached drawings, there is partially and schematically shown a burner 1 according to the state of the art, as described in the applicant's patent FR3114375, seen from the front, and oriented along a vertical plane P1 and a horizontal plane P2. This figure illustrates the orientation of the burner when it is positioned in a vertical preheating section of a continuous line, with a strip circulating vertically.

[0068] The fuel passes through two converging conduits 3, arranged on the plane P1. The oxidant passes through four conduits 4, 5, the conduits 4 arranged on the plane P1 being divergent and the conduits 5 arranged on the plane P2 being convergent.

[0069] The injection of the oxidant and the fuel through this set of conduits 3, 4, 5 makes it possible to obtain combustion in flameless mode when the furnace is at a sufficient temperature.

[0070] The burner also includes a central fuel injection duct 6 for operation of the burner at lower temperature, in flame mode. An annular passage around this duct 6 allows an oxidant supply.

[0071] There, the burner of the is partially and schematically represented, in a perspective and sectional view according to the plane P1. It allows to better visualize the inclination of the fuel injection conduits 3, and of the diverging conduits 4 of oxidant, in the diffuser 2.

[0072] The oxidant conduits 4, 5 open into a combustion tunnel 13 for better flame retention.

[0073] Referring to the diagram of the attached drawings, an enlargement of an internal part of a burner 1 can be seen, making it possible to schematically visualize an internal combustion chamber 7 of the burner according to a first exemplary embodiment of the invention.

[0074] The combustion chamber is substantially cylindrical, with a longitudinal axis coincident with axis A of the burner.

[0075] The internal combustion chamber comprises a first segment 15 of larger diameter connected by a second conical segment 16 to a third segment 17 of smaller diameter than that of the first segment.

[0076] The third segment 17 is located in an axial conduit 12 connecting to a flame tunnel 13.

[0077] The chamber is held in position by the connecting conduit 12 as well as support and guide means not shown.

[0078] Fluids flow roughly from left to right, as shown by arrow 14.

[0079] The upstream face 18 of the internal combustion combustion chamber 7 comprises passages 8 allowing the entry of oxidant into the chamber.

[0080] These passages can be distributed on the upstream face 18, being more or less distant from the axis A of the burner. A clearance between the upstream face 18 and the central fuel injection conduit 6 can form a passage.

[0081] The central fuel injection duct 6 comprises radial passages 19 for injecting fuel into the internal combustion chamber, for example here four cylindrical holes.

[0082] The oxidant entering the internal chamber 7 through the passages 8 and the fuel entering it through the passages 19 mix in the internal combustion chamber 7.

[0083] An ignition electrode 9 generates an electric arc when energized to initiate combustion in chamber 7.

[0084] The flow rates of oxidant and fuel entering the combustion chamber, as well as the flow velocities therein, are such that combustion is not complete in the internal chamber. It continues outside the internal chamber by the escape of fluids outside the internal chamber through the downstream opening 21.

[0085] The flame which starts in the internal chamber thus extends outside of it, into conduit 12, into tunnel 13 and then into the furnace.

[0086] In this example, the downstream end of the central fuel injection conduit 6 also comprises an axial passage 20 arranged substantially at the inlet of the flame tunnel 13. The supply of fuel via the axial passage 20 makes it possible to obtain a very stable flame of sufficient power to prevent any extinction of the main flame.

[0087] For example, 60% of the fuel flow supplied by the central fuel injection conduit 6 passes through the radial passages 19 and 40% through the axial passage 20 and the oxidant flow passing through the passages 8 makes it possible to obtain a mixture with this fuel flow having 5% excess air compared to stoichiometric combustion.

[0088] In a second embodiment shown schematically in, the central fuel injection duct 6 extends into the flame tunnel 13. In addition, the axial passage is replaced by new radial passages 22 arranged at the downstream end of the duct.

[0089] This arrangement of the passages 22 allows a more intense mixing of the fuel arriving from the passages 22 with the oxidant arriving via the conduits 4 and 5.

[0090] The main flame present in the tunnel 13 is thus brighter, leading to a shorter and more intense flame, compared to that obtained with an arrangement according to the first example of embodiment of the.

[0091] A third example of embodiment is shown schematically in. In this one, the internal combustion chamber 7 is open on its upstream end 23 in the direction of flow of the oxidant.

[0092] The central fuel injection duct 6 comprises an injector 33 comprising a set of eight passages 24 having a radial component and an axial passage 25, all of these passages opening into the internal combustion chamber 7.

[0093] The passages 24 are inclined at an angle of 75° relative to the axis A of the burner and the section of the axial passage 25 is here 10% of the total section of the passages 24.

[0094] The injector 33 carries a disc 26 of oxidant arranged in the internal combustion chamber 7. It forms an annular passage 27 with the internal combustion chamber in which oxidant can circulate.

[0095] The mixture obtained in the internal combustion chamber of the oxidant and the fuel supplied by the passages 24, 25 and 27 allows ignition thereof by the electrode 9. The inclination of the fuel passages 24 promotes rapid mixing with the oxidant in the internal combustion chamber, advantageous for the reliability of the ignition of the internal combustion and the maintenance thereof. The axial passage 25 allows an elongation of the flame favorable to the main combustion, in the tunnel 13 and in the furnace.

[0096] A fourth embodiment is shown schematically in, close to that of the. It is distinguished from the latter by a larger diameter oxidizer disc 26, hence the absence of significant flow on the outside of the oxidizer disc. The profile of the internal combustion chamber 7 is also slightly different.

[0097] The oxidizer disc has passages 28 for the entry of the oxidizer into the internal combustion chamber, conducive to good ignition and good flame retention.

[0098] A fifth embodiment is shown in. Only the central fuel injection conduit 6 and the oxidizer disc 26 are shown in front view and in section at the level of radial fuel passages 19. The oxidizer disc 26 comprises four circular passages 32 for supplying oxidizer into the internal combustion chamber 7 of the burner. These passages are inclined and they converge towards the axis A of the burner. They are arranged opposite the fuel injection passages 19 in the internal combustion chamber 7. The passage 34 is intended to receive the ignition electrode 9.

[0099] The innovation lies in the procedure for igniting and heating the burner using a central fuel injector ignited by an electrode instead of a pilot burner.

[0100] The gas injector design is a coaxial burner with a power that can be more than half of the total power of the main burner. The fuel injector design is not limited to a coaxial configuration.

[0101] Of course, the invention is not limited to the examples just described and many adjustments can be made to these examples without departing from the scope of the invention. In addition, the various features, forms, variants and embodiments of the invention can be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive.

Claims

Gaseous fuel burner (1) capable of operating in so-called "flameless" mode, having a longitudinal axis (A) at the intersection of two perpendicular planes (P1, P2), and comprising a diffuser (2) crossed by fuel injection ducts (3) for operation in flameless mode and oxidant injection ducts (4, 5), said oxidant injection ducts (4, 5) emerging from the diffuser closer to the axis of the burner than said fuel injection ducts (3) for operation in flameless mode, the burner further comprising a central fuel injection duct (6) for operation in flame mode which extends in the axial direction (A) of the burner, characterized in that it comprises a combustion chamber (7) internal to the burner into which the central fuel injection duct (6) opens at least partially as well as at least one passage (8, 27, 28) supplying oxidant,the internal combustion chamber further comprising an ignition electrode (9) capable of starting combustion in the internal combustion chamber (7) at least partially of the mixture of fuel and oxidant supplied therein by the central fuel injection conduit (6) and the at least one conduit (8, 27, 28) supplying oxidant., Burner according to claim 1, characterized in that the internal combustion chamber (7) forms a tunnel (10) whose inlet diameter is greater than the outlet diameter, depending on the direction of flow of the fluids. Burner according to one of the preceding claims, characterized in that the downstream end (17) of the internal combustion chamber (7), in the direction of flow of the fluids, is located in an axial conduit (12) of the diffuser opening into a flame tunnel (13). Burner according to claim 1, characterized in that the central fuel injection duct (6) comprises fuel injection passages (19) arranged at two levels along the duct. Burner according to claim 4, characterized in that the central fuel injection conduit (6) comprises fuel injection passages (19) which open into the combustion chamber (7) internal to the burner. Burner according to the preceding claim, characterized in that the fuel injection passages (19) can be axial and / or radial and / or inclined at an angle of between 0 and 90° relative to the axis (A) of the burner. Burner according to one of claims 5 or 6, characterized in that the central fuel injection conduit (6) comprises at least one fuel injection passage (20, 21) which opens downstream of the combustion chamber (7) internal to the burner, in the direction of flow of the fluids. Burner according to one of the preceding claims, characterized in that it comprises an oxidant disc (26) linked to the central fuel injection conduit (6), the disc forming an annular passage (27) with the internal combustion chamber in which oxidant can circulate. Burner according to the preceding claim, characterized in that the oxidant disc (26) comprises passages (28) through which oxidant can circulate. Burner according to claim 9, characterized in that the oxidant disc (26) comprises passages (32) of cylindrical shape converging towards the axis (A) of the burner, in the direction of flow of the oxidant. Burner according to one of the preceding claims, characterized in that the combustion chamber (7) internal to the burner has a wall (31) made of silicon carbide.

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