EP0926434A1 - Burner with low nitrogen oxide emission using recycled gas feed - Google Patents

Abstract

An annular pipe (4) containing oxygen-poor gas surrounds the concentric feed pipes for fuel and combustion air in the burner nozzle (1). Outside this pipe is secondary air (3). The inside wall of the outlet orifice (5) is expanded radially relative to the outside wall of the outer concentric pipe by an annular thickness d which is at least 25 mm and is 3 to 8% of the diameter (D) of the nozzle. Combustion starts at the outlet to the nozzle. The secondary air is at least 85% of the total air supplied and is at a temperature of at least 500 degrees C. The outlet to the annular pipe diverges to form an angle alpha between 2 and 10 degrees to the axis XX' of the burner. The exit speed of the oxygen-poor gas is less than that of the primary air, and of the order of the secondary air, between 10 and 50 m/s. The oxygen-poor gas is recycled exhaust gas, or mixture of exhaust gas and water vapor. The annular orifice is continuous, or discontinuous with a number of openings. Obstacles to deviate the secondary air flow are placed around the outside of the annular pipe. These obstacles form a continuous collar in the form of a diverging cone. Burner containing such nozzles.

Description

The present invention relates to a low emission burner of nitrogen oxide with recycled gas circuit.

The technical sector of the invention is the production of burners can use one or more fuels simultaneously or not, with special arrangements for air supply ducts and / or other gases.

The main application of the invention is its use in burners for rotary tube ovens, in which at least one fuel and primary combustion air are brought in at at least two concentric conduits arranged around a conduit central, the rest of the so-called secondary air is brought outside of these ducts.

In a preferred embodiment, these burners comprise at least two annular ducts supplying the primary air, one under form of a swirling air stream with components peripheral flow, and the other as an axial air stream having substantially axial flow components; which two primary air flow ducts are arranged radially at the outside of any fuel line including in particular at least an annular pulverized coal supply duct; and leads him central, inside which a feeding rod of at least starting fuel, is partially blocked by a flame stabilizer in the form of a flange surrounding its opening central where said rod opens, and having openings for the passage of a small part of the primary combustion air is at most 20% of this air, so that in the central area downstream said stabilizer, no significant combustion of the combustible, qualifying the said central dead zone: such a burner is described and claimed in European patent 421 903 published on April 10 1991 and filed by the same applicant as for the present invention.

The present invention can in fact be considered as a improvement made to such burners and more generally to all burner that can be described as modern comprising said conduits supply of fuel and primary combustion air representing a maximum of 15% of the total air, which ducts forming together a nozzle intended to be mounted on an orifice provided for this in the wall of the enclosure forming the hearth of the oven or chamber combustion, and around which secondary air is brought representing at least 85% of the total air and having a temperature of over 500 ° C.

The objective of the present invention is to reduce the formation of nitrogen oxides which are formed, either by the oxidation of nitrogenous compounds existing in the fuel and called NO _X fuel, or in the combustion air itself by oxidation of nitrogen which occurs in the presence of atomic oxide or other oxidizing radicals, when the temperature is very high in the combustion furnace and qualified as thermal NO _X ; which objective must be achieved by maintaining a good range for adjusting the burner operating speed with a stable flame.

Many methods and devices have been developed, and some have been patented, such as in particular those reducing the content oxygen in the flame by the use of recirculation of fumes present in the hearth in order to recombine them with the air of combustion: such recirculation using suction systems and injection of this flue gas has been known for a long time and is sometimes combined depending on the case with the staging of the supply in combustible.

One can quote for example in this field the requests for previous patents from EGCI PILLARD, applicant for present invention, such as patent application FR2706985 published on December 30, 1994 and entitled "liquid fuel burner or gas with very low nitrogen oxide emissions ", or the demand for patent FR2518222 published on October 6, 1995 and entitled "development of very low gaseous fuel burner emission of nitrogen oxide "; reference may usefully be made to the introduction and description of these two patent applications for fully understand the interest and the various known possibilities of implementation to reduce the emission of nitrogen oxide: this reduction is in fact more and more sought after by burner users, in particular due to existing environmental standards and releases to the atmosphere, which are becoming increasingly restrictive.

We could also cite European patent 0279913 published on August 31, 1988 and filed under German priority by the company SONVICO OFFICE ENGINEER entitled "burner for combustion of liquid or gaseous fuels ": this document teaches a device providing air in three-stage form (air primary, secondary air, tertiary air) with a supply of flue gas between secondary air and tertiary air in order to delay the progress of combustion and prevent reaching flame temperatures which acts against the formation of nitrogen oxides which in the case of liquid or gaseous fuels, are essentially thermal.

In fact, in the burner described in this document, most of the air is brought inside the cylinder defined by the duct providing said fumes, unlike the present invention for which the primary air represents at most only 15% of the total air, and only the tertiary air is located outside of it and supplied by nozzles converging towards the axis of the hearth (unlike also with the present invention for which the mixing of outside air with flame); such burners may include fuel lances external to the primary and secondary air ducts, and to the smoke duct for injecting fuel into the interior of the combustion chamber and in a convergent manner. It is therefore a particular burner in which the staged supply of tertiary air and the addition of recycled fumes make it possible to lower the temperature of the flame and therefore reduce the formation of essentially thermal NO _X.

However, the configuration and the means described allowing such addition of smoke do not make it possible to sufficiently reduce the rate of NO _X for burners in which the primary air already represents less than 15% of the total air, as for the burners object of the main application of the present invention as indicated in the introduction and in particular using solid fuel; and moreover in an application of cement kilns, the flame temperature should not be reduced as sought in this document of the prior art: on the contrary in the applications of the invention, if one wishes to reduce the oxides of nitrogen you must keep a radiant flame at the same time.

The objective of the present invention is in fact to reduce, even more than by the devices known to date, the formation of NO _X, both thermal and fuel for such burners comprising at least two substantially concentric fuel supply conduits and in primary combustion air representing at most 15% of the total air, which ducts together form a nozzle at the outlet of which the start of combustion can take place and around which secondary air is brought representing at least 85% total air and having a temperature of more than 500 ° C.: according to the present invention, this objective is achieved with a burner comprising at least four independent supply circuits, one of which feeds an annular duct surrounding all of said fuel and primary air supply ducts, inside which oxygen-depleted gas is injected, and outside which secondary air is brought, the inner edge of the outlet orifice of said annular duct being offset radially with respect to the outer edge of the outermost concentric duct of the nozzle by an annular hub with a thickness of at least 25 mm and from 3 to 8% of the diameter of said nozzle.

According to a preferred embodiment, the outlet speed of the oxygen-depleted gas is lower than that V _c of the primary air, and of the order of magnitude of that V _a of the secondary air; in addition, the outlet opening of the annular duct may diverge and form an angle α of between 2 and 10 ° relative to the axis of the burner, and said oxygen-depleted gas is preferably recycled flue gas, or steam or a mixture of steam and fumes.

According to a particular embodiment, said burner can also have outside the duct protection outermost concentric and toward the outlet thereof, obstacles capable of deflecting and diverting the flow of current from the axis of the nozzle secondary air.

The result is new low-emission burners of nitrogen specifically adapted to rotary tube ovens and allowing in particular to improve the following burners the main application stated in the introduction and more particularly those of the type described in patent EP421903.

On the one hand, the presence of obstacles in the crown outside on the burner, and above all and on the other hand the addition of a annular duct, which surrounds all of the supply conduits in fuel and primary air, is supplied with gas depleted in oxygen, and is placed at a minimum distance from the duct the outermost primary air supply, away from it the secondary air flow and delays the energy exchange, and therefore their mixture, between these various flows: these indeed arrive at different speeds, the speed of the secondary air being of the order of 10 to 50 meters per second, while that of primary air is around 150 to 200 meters per second.

In the absence of the screen according to the invention, created by the gas depleted in oxygen or the fumes and / or water vapor, the two air flows then of course mix very quickly and, enriching in oxygen the mixture of combustible air, causes oxidation of nitrogen in combustion air and / or fuel to NO _X just as quickly: on the other hand, with such a screen produced according to the invention, an unexpected sort of insulating guide during a longer or shorter journey depending on the particular embodiments as described below, between the secondary air and the primary air, thus delaying their mixing. Such a smoke and or water vapor screen, the outlet of which is offset from that of the outermost primary air, and having a lower speed than this, facilitates the creation of eddies between said depleted gas and this primary air, creating a stabilizing annular space maintaining the primary air and the fuel inside the guide thus formed and delaying its mixing with the secondary air all the more.

In addition, by this addition of fumes and / or water vapor, the mixture of primary air and fuels is thus depleted in oxygen, which certainly has a NO _X reduction effect, already obtained and produced in many previous patent applications as mentioned above, but here, it is obtained without wanting to cool the flame too much (water vapor creates a thermal screen reflecting in particular infrared): however such a depletion normally weakens the stability of the flame while, according to the present invention, the creation of an annular stabilizing space forming a guide makes it possible, despite this depletion, to maintain good stability of the said flame, which is an additional advantage of such a screen arranged according to the present invention.

Water vapor, by creating a heat shield, enhances the effect of insulation between primary air and secondary air by delaying their mix, all the better since it does not consume energy unlike water spray.

We could cite other advantages of this one, but those mentioned above already show enough to prove its novelty and interest.

The description and the figures below represent examples of realization of the invention, but have no limiting character: others achievements are possible, within the scope and scope of the invention, in particular by using various configurations of known burners with air and air supply ducts fuel constituting the central nozzle.

Figure 1 is a simplified longitudinal sectional view of a burner according to the invention.

Figure 2 is a sectional view of another embodiment of a burner according to the invention.

Figure 3 is a front view of a type of burner of the Figure 1 and 2 with a continuous annular duct.

Figures 4a and 4b are front views of the same burners of Figures 1 and 2 as for Figure 3 but with an annular duct discontinuous.

Figure 5 is a longitudinal sectional view of a burner according to the invention and used as an improvement of a burner such as described in patent EP421903.

Figure 6 is a simplified perspective view in cutaway of a powder fuel supply line to create separate flames, completing a particular embodiment of a burner according to the invention.

The present invention applies to any burner comprising at at least two substantially concentric supply lines fuel and primary combustion air representing at most 15% of the total air, which ducts together form a nozzle 1 with an axis XX 'as shown in Figures 1 to 4 by a simple cylinder of diameter D and specified in a particular embodiment on the figure 5.

Combustion 2 therefore takes place downstream from the outlet of this nozzle 1 around which is brought the secondary air 3 representing at least 85% of the total air and having a temperature of more than 500 ° C. A general external representation of an example of a complete burner can be viewed according to Figure 1 of European patent EP421903.

According to the present invention, such a burner comprises at least four independent supply circuits, one of which supplies an annular duct 4 surrounding the assembly 1 of said fuel and primary air supply ducts, inside which is injected oxygen-depleted gas, and outside of which the secondary air is brought 3. The internal edge 5 ₁ of the outlet orifice 5 of said annular duct 4 is offset radially with respect to the external edge 6 ₁ of the concentric duct the more external of the nozzle 1 by an annular hub 6 of thickness "d" of at least 25 mm and from 3 to 8% of the diameter D of the said nozzle 1 in order to generate a ripple by the creation of Karman vortex, all by not being too thick to avoid problems of fouling of the external surface of the hub 6 and too great a break in aerodynamic flow which could go against the desired goal; a person skilled in the art taking cognizance of the effect sought by the present invention is able to determine the characteristics of said annular hub 6 without it being necessary to give all the explanations in this description.

As shown in Figure 1, the outlet port 5 of the conduit annular 4 can diverge and form an angle α between 2 and 10 ° relative to the axis XX 'of the burner in order to spread the air even more secondary 3 and thus delay its mixing with that constituted by the primary air and the fuel supplied by the conduits of the nozzle 1.

According to the representation of FIG. 1, the outlet orifice of said annular duct 5 can also be concentric without divergence with respect to the XX 'axis.

The exit velocity V _g of the gas depleted in oxygen or smoke is of the order of magnitude that that V _a of secondary air 3, that is to say from 10 to 50 meters per second, preferably of the order of 15 meters per second, which is significantly lower than that V _c of the primary air which is supplied at a sufficiently high pressure to obtain an outlet speed of the order of 150 to 200 meters per second, or in general of the order of 170 meters per second. The definition of oxygen-depleted gas is that of a gas composed of less than 21% oxygen: it can therefore be the recycled smoke gas leaving the combustion chamber where the burner according to the invention is located, or smoke gas from other sources, or even water vapor, or a mixture of steam and smoke.

According to different possible embodiments, the outlet orifice 5 of said annular duct 4 can be a continuous open crown as shown in FIG. 3 or discontinuous and have several openings, either rectangular in shape over the entire thickness e of duct 4 such as shown in Figure 4a, either in the form of orifices 5 ₂ of rounded shape, such as circular, shown in Figure 4b.

In FIG. 5, the nozzle 1 of axis XX ′ is that described in patent EP421903 already mentioned previously, and constituting a preferred embodiment of the burner according to the invention: the outlet orifices of the primary air supply conduits are arranged radially outside any outlet of the fuel conduit, both by the rod 10 located in the axis of the central conduit 8 as by the peripheral conduit 9 into which is preferably sent pulverized coal. Said outlet orifices supply the nozzle 1 with axial air and swirling air through at least two concentric ducts 12, 13 and the central duct 8, inside which is positioned a nozzle holder 10, is partially blocked by a stabilizer of flame 8 in the form of a flange surrounding its central opening 10 ₁ where said nozzle holder 10 opens; said flame stabilizer 8 has openings for the passage of part of the primary combustion air, ie at most 20% of this air, so that, in the central zone 2 ₁ located downstream of said stabilizer 8, does not produces no significant combustion of the fuel qualifying said central zone 2 ₁ as dead.

Preferably, the quantity of primary air sent to said central dead zone 2 ₁ is between 1% and 20% of the total primary air and preferably less than 10%, and the proportion of primary combustion air is between 2 and 10% of the total combustion air and preferably less than 6%.

The flame stabilizer 8 is preferably axially offset behind the set of concentric openings of outlet of primary air and solid or gaseous fuels.

The flame stabilizer 8 is disposed at the front end of a nozzle holder 10 inside which is in fact a liquid fuel supply rod, essentially used for starting the burner. The axial air outlet opening 13 is the one located radially as far outside as possible from the set of primary air supply conduits and has an outer edge 14 which projects axially forward and constitutes the internal edge 6 ₁ of the hub 6 separating it from the external channel 4 of oxygen-depleted gas.

To complete the screen effect, created by the supply of oxygen-depleted gas, such as smoke and / or water vapor, between the secondary air 3 and the primary air, in order to help reduce the formation of nitrogen oxide, the secondary air itself can also be staggered by adding outside the protection 7 of the outermost concentric duct, in fact that 4 of the supply of smoke or depleted gas in oxygen, and towards the outlet orifice 5 thereof, obstacles 7 ₁ capable of deflecting and deviating from the axis XX 'of the nozzle 1 the flow of the secondary air current 3; such obstacles 7 ₁ can form a continuous crown in the form of a cone diverging from the external continuous surface of the protection 7 or consist of separate blocks forming a notched crown from the front.

Furthermore, it was found that the training of several combustion flames resulting from the formation of multiple veins of solid fuel at the outlet of the burner nozzle, ends, in addition to the screen defined above, to a reduction in the formation of oxide nitrogen, this reduction being particularly increased when applies the invention to a burner equipped with several conduits supply of axial air and swirling air which are arranged at the outside of the solid fuel supply duct, such as described above.

Said burner has at least two obstacles 67 for this, such as shown in Figures 5 and 6, provided in the conduit fuel supply 9, and which divide it into at least two channels in the vicinity of the outlet of this conduit, i.e. of the burner nozzle 1, so as to cause the separation of the incident vein of solid fuel in at least two veins for allow the formation of at least two flames.

With reference to FIGS. 5 and 6, in a particular mode of embodiment, said conduit 9 for supplying solid fuel has an upstream part located on the right in these figures, which is provided an annular cross section; due to the presence, such as represented in FIG. 6, of five obstacles 67 in the form of a profiled wedge triangular, the conduit 9 is divided into five channels 91, 92, 93, 94, 95 which are each delimited by, on the one hand, the external face 66A of the wall 66 separating the conduit 9 from the conduit 11, on the other hand the internal face 68A of the wall 68 separating the conduit 9 from the conduit 12, and also faces lateral 73 obstacles 67 in the form of a wedge; these extend to from their tapered or profiled rear end 70, and their section transverse widens or increases from their leading edge 70 to their terminal section 71 corresponding to the end or front face of the nozzle.

In order to limit the speed increase resulting from the decrease in the free passage section of solid fuel in said channels, and even to maintain the speed of solid fuel substantially identical in the channels and compared to that in the upstream part of the supply duct 9, a section of substantially constant passage of solid fuel between the part upstream of the completely annular section duct and the downstream part fitted with obstacles 67 and divided into five channels: the internal radius of the vein remains constant for example due to a cylindrical surface 66A, the external radius 61 corresponding to the radius of the face 68A external to the vicinity of the outlet openings of channels 91 to 95, is greater than radius 60 of the upstream part of the duct 9 of annular section, in order to compensate for the reduction in free cross-section resulting from the presence of these obstacles 67. The height (measured radially) of the separate channels 91 to 95 by said obstacles 67 of the fuel supply duct 9 solid, is then more important than the height of the part of the duct located upstream of obstacles.

• lesdits obstacles 67 sont en forme de nervures ou cloisons épaisses, de section transversale variable, de préférence de profil et de section transversale continûment variable, par exemple en forme de coin et de forme allongée le long de l'axe longitudinal commun des extrémités de conduits, pour limiter ou éviter l'abrasion du conduit et/ou des obstacles par les particules de combustible solide, pour éviter une perte de vitesse et la nécessité d'une mise en surpression du combustible solide, tout en séparant le flux de combustible solide en plusieurs veines 91 à 95 nettement séparées à la sortie de la buse ;
• ledit brûleur comporte au moins trois et de préférence au plus douze obstacles 67, en particulier de trois à six obstacles sensiblement régulièrement répartis le long de la périphérie ou circonférence du conduit annulaire et disposés à proximité de l'orifice de sortie du conduit d'alimentation 9 pour permettre la formation d'au moins trois flammes de forme et de taille similaire ou identique et réparties autour de l'axe longitudinal commun selon une symétrie de rotation ;
• ledit brûleur comporte un conduit d'alimentation 13 en air dit axial, éjecté par un orifice de sortie selon une direction sensiblement parallèle à l'axe longitudinal commun des conduits du fait en particulier de la surface interne 14₁ cylindrique du bord externe 14 qui déborde axialement vers l'avant des conduits d'air : cet air primaire est guidé par des cloisons ou nervures ou ailettes prévues dans le conduit 13, qui est disposé autour des conduits d'alimentation en combustible, solide 9 et en air tourbillonnant 12 ; ce conduit d'alimentation en air axial 13 comporte des cloisons ou nervures épaisses, dont le nombre est au moins égal au nombre d'obstacles 67 prévus dans le conduit d'alimentation en combustible solide 9, pour former au moins autant de canaux de passage de l'air axial que le brûleur comporte de canaux de passage du combustible solide, afin de permettre la formation d'au moins un jet d'air distinct, associé à chaque veine du flux sortant de combustible solide ;
• le nombre de canaux et de cloisons prévus dans le conduit d'alimentation en air axial 13 peut être un multiple du nombre de canaux et d'obstacles 67 prévus dans le conduit d'alimentation 9 en combustible solide, pour permettre la formation de plusieurs (au moins deux) jets d'air axial, associés à chacun des jets ou veines de combustible solide sortant de la buse du brûleur ;
• les canaux délimités dans le conduit d'alimentation en air 13 par les cloisons et/ou les orifices de sortie d'air et/ou les jets d'air sortant de ces canaux, sont décalés angulairement par rapport aux canaux délimités par les obstacles 67 du conduit d'alimentation en combustible solide 9 et/ou par rapport aux orifices de sortie de combustible solide et/ou aux veines de combustible solide sortant de la buse, dans le sens de rotation imprimée au combustible et aux flammes par le flux d'air tourbillonnant délivré par le conduit d'alimentation 12 en air muni d'ailettes ou déflecteurs ;
• les canaux et/ou les jets d'air axial sortant de ces canaux peuvent être inclinés axialement dans le sens de la rotation des flammes et du flux d'air tourbillonnnant, et/ou également divergents par rapport à l'axe longitudinal commun de la buse 1 de brûleur et de ces conduits d'alimentation.

According to preferred embodiments of the invention:

• said obstacles 67 are in the form of ribs or thick partitions, of variable cross section, preferably of profile and of continuously variable cross section, for example wedge-shaped and elongated along the common longitudinal axis of the ends of conduits , to limit or avoid abrasion of the duct and / or obstacles by solid fuel particles, to avoid a loss of speed and the need for overpressure of the solid fuel, while separating the flow of solid fuel into several veins 91 to 95 clearly separated at the outlet of the nozzle;
• said burner comprises at least three and preferably at most twelve obstacles 67, in particular from three to six obstacles substantially regularly distributed along the periphery or circumference of the annular duct and arranged near the outlet orifice of the supply duct 9 to allow the formation of at least three flames of similar or identical shape and size and distributed around the common longitudinal axis according to a rotational symmetry;
• said burner comprises a supply duct 13 in said axial air, ejected by an outlet orifice in a direction substantially parallel to the common longitudinal axis of the ducts due in particular to the internal surface 14 ₁ cylindrical of the external edge 14 which projects axially towards the front of the air ducts: this primary air is guided by partitions or ribs or fins provided in the duct 13, which is arranged around the fuel, solid 9 and swirling air supply ducts 12; this axial air supply duct 13 comprises thick partitions or ribs, the number of which is at least equal to the number of obstacles 67 provided in the solid fuel supply duct 9, to form at least as many passage channels axial air that the burner has solid fuel passage channels, in order to allow the formation of at least one separate air jet, associated with each stream of the outgoing stream of solid fuel;
• the number of channels and partitions provided in the axial air supply duct 13 may be a multiple of the number of channels and obstacles 67 provided in the solid fuel supply duct 9, to allow the formation of several ( at least two) axial air jets, associated with each of the jets or streams of solid fuel leaving the burner nozzle;
• the channels delimited in the air supply duct 13 by the partitions and / or the air outlet orifices and / or the air jets leaving these channels, are angularly offset with respect to the channels delimited by the obstacles 67 of the solid fuel supply duct 9 and / or with respect to the solid fuel outlet orifices and / or the solid fuel streams leaving the nozzle, in the direction of rotation imparted to the fuel and to the flames by the flow of swirling air delivered by the air supply duct 12 provided with fins or deflectors;
• the axial air channels and / or jets exiting from these channels can be axially inclined in the direction of rotation of the flames and of the swirling air flow, and / or also divergent with respect to the common longitudinal axis of the burner nozzle 1 and these supply conduits.

Claims (10)

Burner comprising at least two substantially concentric conduits for supplying fuel and primary combustion air representing at most 15% of the total air, which conduits together form a nozzle (1) of axis XX 'at the outlet of which can start the combustion (2) and around which is brought the secondary air (3) representing at least 85% of the total air and having a temperature of more than 500 ° C, characterized in that comprises at least four independent supply circuits, one of which supplies an annular duct (4) surrounding the assembly (1) of said fuel and primary air supply ducts, inside which is injected gas depleted in oxygen, and outside of which the secondary air (3) is brought, the internal edge (5 ₁ ) of the outlet orifice (5) of said annular duct (4) being offset radially with respect to the external edge (6 ₁ ) of the outermost concentric conduit of the nozzle (1) by an annular hub (6) with a thickness d of at least 25 mm and from 3 to 8% of the diameter D of said nozzle. Burner according to claim 1, characterized in that the outlet orifice (5) of said annular duct (4) diverges and forms a angle α between 2 and 10 ° relative to the axis XX 'of the burner. Burner according to any one of Claims 1 to 2, characterized in that the exit speed V _g of the oxygen-depleted gas is lower than that V _c of the primary air, and of the order of magnitude of that V _a secondary air. Burner according to claim 3 characterized in that the exit speed V _g of the oxygen-depleted gas is of the order of 10 to 50 m per second. Burner according to any one of Claims 1 to 4, characterized in that said oxygen-depleted gas is smoke gas recycled, or steam, or a mixture of fumes and steam of water. Burner according to any one of Claims 1 to 5, characterized in that the orifice (5) of said annular duct (4) is continued. Burner according to any one of Claims 1 to 5, characterized in that the orifice (5) for exit from said annular duct (4) is discontinuous and has several openings (5 ₁ , 5 ₂ ). Burner according to any one of Claims 1 to 7, characterized in that it includes, outside the protection (7) of the outermost concentric duct and towards the outlet orifice (5) thereof, obstacles (7 ₁ ) capable of deflecting and deviating from the axis XX 'of the nozzle (1) the flow of the secondary air current (3). Burner according to claim 8, characterized in that said obstacles (7 ₁ ) form a continuous crown in the form of a cone which diverges from the continuous external surface of the protection (7). Burner according to any one of Claims 1 to 9, characterized in that the outlet openings of the primary air supply ducts are arranged radially outside any fuel outlet opening (9, 10), and supply the nozzle (1) in axial air and in air swirling through at least two concentric conduits (12, 13), a central conduit (8) being partially closed by a flame stabilizer (8) forming a flange surrounding its central opening (10 ₁ ) where a fuel supply rod (10) opens and having openings (15) for the passage of a small part of the primary combustion air.

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