EP3074695B1 - Burner for metal heating furnace or for thermal treatment furnace - Google Patents

Description

The present invention relates to a combustion system generating a heat flux for heating materials, in particular for furnaces for heating steel products.

A combustion system of this type is known from EP 0 994 302 , corresponding to FR 2 784 449 , on behalf of the applicant company.

It is known that heat treatment furnaces, in particular reheating, or holding, are intended to carry products, in particular slabs, blooms and the like, at the temperatures required for example for rolling or for obtaining a given metallurgical structure.

It is also known that the quality of the treatment of a product, for example by rolling or heat treatment, requires a precise and uniform temperature inside the product, this temperature depending on the type of treatment desired or on the chemical composition. of the product to be treated.

For example, in furnaces for re-heating metallurgical products, the average temperature level is obtained by passing the products through so-called heating zones which are characterized by a high thermal flux, which gives rise to a large temperature heterogeneity in the products. reheated, in particular in ovens equipped with axisymmetric flame burners according to the state of the art.

In order to obtain the temperature uniformity required for their subsequent treatment, the products leaving the heating zones pass through an equalization zone in which the heat input is very low, at zone temperatures close to the charging temperature, which makes it possible to obtain an equalization of the temperatures in the thickness of the products. For economic reasons, it is not possible to have too long a residence time in this equalization zone and this equalization time is a compromise between the maximum value of heterogeneity accepted and the costs arising from the construction. of this area of the oven.

A first solution to improve the homogeneity of the heat flux provided by the axisymmetric burners to the products in the oven results from the development of the flame spreader according to EP 0 994 302 . Since then, global or local regulations on limiting the release of pollutants, for example NOx, have considerably reduced the limits of acceptable emission values for discharges, which necessitates the improvement of burner technology.

The flame spreader according to EP 0 994 302 provides a significant improvement over axisymmetric flame burners by distributing the heat flow of the flame over a large area, parallel to the plane of the products.

The spread flame burner makes it possible to limit the temperature gradient at the surface of the products which are positioned in an oven fitted with such burners substantially parallel to the flame spreading plane.

• de réduire la durée de la phase d'égalisation des températures des produits, donc la longueur de la zone des fours de réchauffage dans laquelle cette égalisation de température est opérée ;
• de limiter les risques de surchauffe localisée du produit grâce à l'absence de zone très chaude ou de point chaud dans la flamme. Cette caractéristique permet l'amélioration de l'état métallurgique final du produit traité ;
• de répartir la combustion sur un volume plus important que celui réalisé par les brûleurs axisymétriques, ce qui permet de mieux contrôler le mélange des réactifs et des produits de combustion dans l'enceinte du four. Ceci réduit les émissions de polluants générés par la combustion et réduit la formation d'oxydes à la surface des produits réchauffés.
• de réduire la hauteur du laboratoire du four grâce à la réduction de la dimension de la flamme perpendiculairement au plan des produits.
• de remplacer un nombre important de brûleurs implantés sur la voûte du four par un nombre plus réduit de brûleurs implantés sur les parois du four. Le circuit de distribution de combustible et de comburant est plus réduit et il est réalisé pour un coût plus faible.

This burner allows:

• reduce the duration of the product temperature equalization phase, therefore the length of the zone of the reheating ovens in which this temperature equalization is carried out;
• to limit the risks of localized overheating of the product due to the absence of a very hot zone or hot spot in the flame. This characteristic allows the improvement of the final metallurgical state of the treated product;
• to distribute the combustion over a larger volume than that produced by the axisymmetric burners, which allows better control of the mixture of reactants and combustion products in the oven enclosure. This reduces the emission of pollutants generated by combustion and reduces the formation of oxides on the surface of the heated products.
• reduce the height of the oven laboratory by reducing the size of the flame perpendicular to the plane of the products.
• replace a large number of burners located on the top of the oven with a smaller number of burners located on the walls of the oven. The fuel and oxidizer distribution circuit is more reduced and it is produced at a lower cost.

These advantages have been recognized by users of spread flame burners according to the state of the art, however the shape of the following tunnel EP 0 994 302 limits the intake of ambient fumes at the root of the fuel jets, resulting in a local overheating zone of combustion products near the tunnel, this high temperature increases NOx emissions.

It can be seen that the level of pollution, in particular the level of NOx emitted, had to be improved compared to EP 0 994 302 so as to keep this spread flame burner technology at the best level by anticipating the regulatory changes expected in terms of pollutant emissions in the various countries of the world.

An object of the invention is to improve the design of a spread flame burner which makes it possible to obtain better transmission homogeneity of the heat flux generated by said flame, in order to reduce the temperature heterogeneity induced in the products. to be reheated, and which makes it possible to obtain better heat transfer and to reduce the quantity of pollutants emitted, in particular NOx.

The invention provides an answer to this problem by providing users with a new spread flame burner technology for heating steel products which, while retaining or improving the shape of the spread flame, improves the distribution of the heat flow to the product and significantly reduce emissions of pollutants, especially NOx.

The invention is defined in the claims. The invention solves said problem by means of a burner according to claim 1 and by a method of use according to claim 6.

Preferably, the ratio of the pulses between the oxidizer and fuel is between 5 and 50, depending on the characteristics of the reactants, in particular between 30 and 50 for natural gas, or between 3 and 15 for lean gas.

1. a/ produire une flamme étalée par la combinaison de l'injection de combustible par l'orifice de combustible et de l'injection de comburant par les orifices de comburant du premier ensemble,
2. b/ étendre le volume de la réaction issue des jets des orifices du premier ensemble et de l'orifice de combustible avec le comburant issu directement des orifices du deuxième ensemble, ou avec le comburant qui a préalablement été recirculé dans le four et s'est dilué lors de cette recirculation avec les produits de combustion du four dans un plan vertical,
3. c/ réaliser cette dilution par les recirculations de produits de combustion de façon à mélanger les réactifs dans un volume important de fumées avant de réaliser l'oxydation du combustible par le comburant résiduel pour réaliser l'expansion de cette zone de réaction dans un volume important et limiter la production de points chauds,
4. d/ réaliser une combustion du combustible et du comburant dilués, en particulier avec les produits de combustion produisant un niveau de NOx limité.

Advantageously, the angles of inclination of the geometric axes of the oxidant supply channels, and the diameters of these supply orifices are chosen so as to:

1. a / produce a flame spread by the combination of fuel injection through the fuel orifice and injection of oxidant through the oxidant orifices of the first assembly,
2. b / extend the volume of the reaction originating from the jets of the orifices of the first assembly and of the fuel orifice with the oxidant coming directly from the orifices of the second assembly, or with the oxidant which has previously been recirculated in the furnace and has diluted during this recirculation with the combustion products of the furnace in a vertical plane,
3. c / achieve this dilution by the recirculation of combustion products so as to mix the reactants in a large volume of smoke before oxidizing the fuel with the residual oxidizer to achieve the expansion of this reaction zone in a large volume and limit the production of hot spots,
4. d / achieve combustion of the diluted fuel and oxidizer, in particular with the combustion products producing a limited level of NOx.

Advantageously, a burner according to the invention is characterized by the combination of the relative positions of the fuel and oxidant injection orifices, the diameter of the injection orifices, the speed of the fluids coming from these orifices during operation, and the angle of the supply channels so that the jets of fuel, oxidizer and recirculated combustion gases can combine to control their point of convergence and mixing.

According to the invention, the axes of the oxidant supply orifices are included in horizontal planes, substantially parallel to the plane of the products, and are inclined, with respect to the axial direction, by an angle (a) for the orifices of the second set, and an angle (b) for the orifices of the first set.

The angle (a) of the geometric axes of the pairs of orifices in the second set is between 5 ° and 18 °, and the axes are divergent. The angle (b) of the axes geometrical pairs of orifices of the first set is between 10 ° and 20 °, and the axes are divergent.

The expression “geometric axis of an orifice” is to be understood as designating the geometric axis of the injection channel leading to the orifice.

Preferably, the pairs of oxidant supply orifices open into an outlet plane substantially coincident with a plane corresponding to the internal face of the furnace.

Preferably, each of the two sets of orifices comprises two groups of two orifices whose axes are located in a plane parallel to the horizontal plane passing through the axial direction of the burner, the planes of the axes of the orifices 8 or 8 'of the second set being located at a distance Y _{8 from} said horizontal plane, and the planes of the axes of the orifices 9 or 9 'of the first set being located at a distance Y ₉ , and the ratio between the distances Y ₉ to Y ₈ is advantageously between 0.4 and 0.7.

The holes 8 and 8 'of the second set are preferably at a distance from the axial vertical plane less than the distance from this plane of the holes 9 and 9' of the first set, the ratio of the distances possibly being between 0.5 and 0.7.

The burner can be characterized by the presence of two oxidizer boxes capable of being supplied by independent circuits, and capable of supplying respectively the two sets of orifices, and a third set of orifices situated radially inside the orifices of the first two sets, which are adapted to obtain a long spread flame, while the third set of orifices is suitable for obtaining a short spread flame.

The burner can be characterized by the presence of two oxidizer boxes supplied by independent circuits, and respectively supplying the two sets of orifices, and a third set of orifices located radially inside the orifices of the first two sets, which provide a long spread flame, while the third set of orifices provides a short spread flame.

The burner may include a fuel injection rod, produced by a plurality of tubes for the use of several fuels of different natures.

• Fig. 1 est une coupe schématique selon le plan vertical I-I de Fig. 2, passant par la direction axiale d'un brûleur selon l'invention ; pour simplifier les canaux d'injection ont été représentés en trait plein, bien que situés en dehors du plan de coupe.
• Fig. 2 présente le brûleur vu de face, de l'intérieur du four.
• Fig. 3 présente une coupe schématique du brûleur par un plan horizontal, en vue de dessus ; pour simplifier, les canaux d'injection ont été représentés en trait plein, bien que situés en dehors du plan de coupe.
• Fig. 4 est une vue en coupe de dessus, semblable à Fig. 3, illustrant les panaches de fluide sortant par les divers orifices ; pour simplifier, les canaux d'injection ont été représentés en trait plein, bien que situés en dehors du plan de coupe.
• Fig. 5 est une vue en coupe de dessus, semblable à Fig. 4, illustrant le volume de la flamme amorcée par les jets de comburant du premier ensemble avec le jet de combustible, et les courants de recirculation.
• Fig. 6 est une vue de dessus, semblable à Fig. 4, illustrant le volume de la flamme avec les jets de comburant du deuxième ensemble et les courants de recirculation.
• Fig. 7 est une coupe schématique selon le plan vertical VII-VII de Fig. 8, semblable à Fig. 1, d'une variante de brûleur selon l'invention, et
• Fig. 8 est une vue de face, de l'intérieur du four, du brûleur de Fig. 7.

The invention consists, apart from the arrangements set out above, of a certain number of other arrangements which will be more explicitly discussed below in connection with embodiments described with reference to the appended drawings, but which are not in no way limiting. In these drawings:

• Fig. 1 is a schematic section along the vertical plane II of Fig. 2 , passing through the axial direction of a burner according to the invention; to simplify the injection channels have been shown in solid lines, although located outside the cutting plane.
• Fig. 2 shows the burner seen from the front, from inside the oven.
• Fig. 3 presents a schematic section of the burner in a horizontal plane, seen from above; for simplicity, the injection channels have been shown in solid lines, although located outside the cutting plane.
• Fig. 4 is a sectional view from above, similar to Fig. 3 , illustrating the plumes of fluid exiting through the various orifices; for simplicity, the injection channels have been shown in solid lines, although located outside the cutting plane.
• Fig. 5 is a sectional view from above, similar to Fig. 4 , illustrating the volume of the flame initiated by the oxidizer jets of the first assembly with the fuel jet, and the recirculation currents.
• Fig. 6 is a top view, similar to Fig. 4 , illustrating the volume of the flame with the oxidizer jets of the second set and the recirculation currents.
• Fig. 7 is a schematic section along the vertical plane VII-VII of Fig. 8 , similar to Fig. 1 , of a burner variant according to the invention, and
• Fig. 8 is a front view, inside the oven, of the burner Fig. 7 .

According to the following flame spreader EP 0 994 302 , the fuel injection was carried out by means of orifices oriented in a horizontal plane towards the outside of the burner, the oxidant injection orifices were also inclined towards the outside of the burner to generate the spread flame. It has been observed that this arrangement favors the rapid mixing of the oxidant and the fuel near the front face of the burner, therefore the formation of local hot zones in the flame, which in these zones promotes the formation of thermal NOx.

According to the invention, the means for injecting the fuel and the oxidant have been improved in order to obtain a reduction in the NOx produced, while retaining a spread flame and in order to produce a dynamic of oxidation of the fuel slowed down to reduce the emission of pollutants.

Referring to Fig. 1 to 3 , it can be seen that the burner is composed of an oxidizer diffuser 1 installed in the side wall of the oven 2, the front face of which is substantially in alignment with the internal face of this oven wall according to the plane P, and d an oxidizer body 3 equipped with a connection flange 4 to a combustion oxidant supply circuit diagrammatically represented by an arrow 5. The fuel rod 6 is connected to a supply circuit 7 symbolically represented by an arrow .

The fuel rod 6, in particular rectilinear, opens out substantially in the plane of the wall of the furnace P along an orifice 10 with an axis perpendicular to this plane. The burner has an axial direction corresponding to the geometric axis of the rod 6 and the orifice 10. The rod 6 passes through the thickness of the diffuser 1.

It is possible to make a single-fuel rod as shown on Fig. 1-3 , or poly-fuel by making several feeds, for example with an orifice for natural gas and another orifice for another fuel. This arrangement of several injections for several fuels can be carried out in any form according to the state of the art. The fuel injection takes place in the axial direction of the burner with a central orifice, or in a direction parallel to the axial direction of the burner with an orifice located substantially in the axis of the burner.

The oxidizer body 3 supplies the oxidant injections according to two sets of four orifices, namely: two symmetrical orifices 8,8 and 9,9 with respect to a vertical plane and their symmetrical 8 ', 8' and 9 ', 9' by compared to a horizontal plane. The four holes 9, 9 'form a first set, and the four holes 8, 8' form a second set.

All injection ports according to Fig. 3 are located substantially in the plane of the wall of the furnace P. The geometric axes of the oxidant injection channels with orifices 8, 8 ′ of the second set are inclined at an angle (a) relative to the perpendicular to the plane P, the geometric axes of the injection channels of the first assembly with orifices 9, 9 ′ are inclined at an angle (b) relative to the perpendicular to the plane P.

The axes of the pairs of orifices 8, 8 ′ of the second set are contained in the same plane parallel to the horizontal plane Y ₁₀ , passing through the axis of the orifice 10, at a distance Y ₈ as illustrated by Fig. 2 . The axes of the pairs of orifices 9, 9 ′ of the first set are contained in the same plane parallel to the horizontal plane, at a distance Y ₉ .

The burner operation is shown diagrammatically on Fig. 4 which is an image on which are presented volumes associated with injections of reagents, these volumes being of different dimensions depending on the injection points 8, 8 ′, 9, 9 ′ and 10. It appears that the desired result is obtained by a particular combination of the positioning of the fuel and oxidant orifices, of the respective angles of the orifice supply channels with respect to the plane P, and with the axial direction of the burner, and of the pulses of each jet with respect to the neighboring jets. This makes it possible to control the reaction zones of the reagents diagrammed by the plumes identified by numbers in square brackets [8], [9] and [10] on Fig. 4 , the zone [10] corresponding to the fuel.

The oxidizer orifices 9 and 9 ′ shown on the Fig. 2 and 3 are located in the immediate vicinity of the fuel outlet 10 and the axes of their injection channels are inclined at an angle (b) between 10 and 23 ° relative to the perpendicular to the plane P. Said axes are included in a horizontal plane and distant from the center of the burner so as to give the flame its spread shape, that is to say not two independent and symmetrical flames but a single flame spread in the main directions given by the orifices 9 and 9 'as represented by [11] on Fig. 5 and specific to this type of spread flame burner.

This result is obtained by the combination of the relative positions of the fuel and oxidant injection orifices, the diameter of the injection orifices, the speed of the fluids from these orifices during operation, and the angle of the channels d feed in such a way that the fuel jets and mixture of oxidizer and combustion gases can combine to control their point of convergence and mixing. The fuel jets and the jets of oxidizer and recirculated combustion gas mixture have a more open conical shape than the plumes shown for the purpose of simplification on Fig. 4 , and by point of convergence, is meant the point of intersection of the fuel jet and the jets of oxidizing mixture and of recirculated combustion gases. By this means, it is possible to control the progressive oxidation of the fuel and the dilution of the reactants with the combustion products of the furnace.

For the burner according to the invention, an impulse ratio (mass flow multiplied by the speed) of the oxidizer jets on the fuel jets is chosen. The pulse ratio between the oxidizer and fuel is between 5 and 50, depending on the characteristics of the reactants, in particular between 30 and 50 for natural gas, or between 3 and 15 for lean gas.

Oxidation of the fuel injected into the furnace through the orifice 10, according to the plume [10] shown diagrammatically, occurs progressively with the oxidant injected through the orifices 9,9 'to spread the combustion in a large volume of flame, which causes a lowering of the average temperature of this flame. This phenomenon is accelerated by the recirculation of smoke from the oven as represented by arrows 12 and 13 on Fig. 6 , which allow time for the reactants to be mixed before combining, which increases the volume of the flame and contributes to slowing down the fuel oxidation phenomenon and lowering the average flame temperature. The dilution of the reactants, namely fuel and oxidizer, in the oven takes place with the combustion products or fumes present in this oven at a temperature typically between 850 ° C and 1450 ° C. The temperature of the oxidizer injected according to [8] and [9] is typically between 400 ° C and 650 ° C.

Unlike the flames of burners according to the state of the art, the combustion propagation of which is essentially surface with reaction zones at very high temperatures, according to the invention the oxidation reactions are carried out in volume because the mixtures are at temperatures above the auto-ignition temperature, that is to say that the temperature of the reaction chamber and / or the temperature of the reactants when they are introduced into the oven are sufficiently high to carry out these reactions.

The oxidation reactions of the reagents according to the invention being carried out in a larger volume, this results in a better temperature uniformity of this volume with fewer high temperature zones in the flame, which results in a significant reduction in the production of NOx. This phenomenon is characterized by the formation of a flame whose brightness is reduced compared to those obtained according to the state of the art, this being obtained by the recirculation of the combustion gases inside the oven with the reactants injected through holes 8, 8 ', 9 and 9'.

Fig. 6 presents the combustion control device produced by means of the injection orifices 8 and 8 ′, of the second set, arranged in planes parallel to the horizontal plane. The axes of the orifices 8 and 8 'are located at distances Y ₈ , greater than the distances Y ₉ of the holes 9 and 9' at the horizontal plane of symmetry Y ₁₀ of the burner.

1. 1/ l'étalement de la flamme dans le plan horizontal pour respecter la hauteur disponible dans le four et privilégier l'étalement horizontal de la zone de combustion,
2. 2/ une oxydation du combustible résiduel qui n'a pas réagi avec les jets de comburant 9,9',
3. 3/ l'induction de courants de recirculation comparables à ceux illustrés par les flèches 12 et 13 sur Fig. 6 afin de diluer plus encore les réactifs avec les fumées du four, ce qui ralentit la réaction d'oxydation du combustible et réalise cette réaction dans un volume de combustible plus important, ce qui permet ainsi de réduire les zones chaudes dans la flamme, et ainsi limiter les quantités de polluants produits, principalement les NOx.

The injection angles (a) of the geometric axes of the orifices 8 with respect to the perpendicular to the plane P are advantageously chosen between 5 ° and 18 ° so as to produce the following effects on the flame produced by the injections of the orifices 9, 9 'and 10:

1. 1 / spreading the flame in the horizontal plane to respect the height available in the oven and favor horizontal spreading of the combustion zone,
2. 2 / oxidation of the residual fuel which has not reacted with the 9.9 'oxidizer jets,
3. 3 / induction of recirculation currents comparable to those illustrated by arrows 12 and 13 on Fig. 6 in order to further dilute the reagents with the fumes from the oven, which slows down the oxidation reaction of the fuel and carries out this reaction in a larger volume of fuel, which thus makes it possible to reduce the hot zones in the flame, and thus limit the quantities of pollutants produced, mainly NOx.

1. 1/ une augmentation du volume de réaction,
2. 2/ une température moyenne de la zone de réaction plus faible parce qu'elle se développe dans un volume de réactivité plus importante,
3. 3/ une réduction de l'émission de NOx thermiques à cause de la réduction du nombre et du volume des points chauds dans la flamme.

In fact, part of the oxidizer only reacts with the fuel after recirculation and dilution by the fumes, which causes:

1. 1 / an increase in the reaction volume,
2. 2 / a lower average temperature of the reaction zone because it develops in a greater volume of reactivity,
3. 3 / a reduction in the emission of thermal NOx due to the reduction in the number and volume of hot spots in the flame.

1. 1/ la position, le diamètre et l'angle des orifices et injecteurs de comburant du premier ensemble 9, 9' situés à proximité du plan de l'injecteur de combustible 10.
2. 2/ l'optimisation du nombre et des positions relatives des injecteurs de comburant 9,9' du premier ensemble, de leur angle d'inclinaison (b) et de leurs diamètres, et de l'injecteur de combustible 10, en combinaison avec les vitesses d'éjection des réactifs issus de ces injecteurs.
3. 3/ la position des injecteurs de comburant 8,8' du deuxième ensemble, leur angle d'inclinaison (a) et leurs diamètres pour réaliser l'étalement de la zone de réaction dans le plan horizontal et générer une recirculation secondaire du comburant introduit par les jets de ces orifices 8,8' et des fumées autour de la zone de réaction, 4/ le volume de la zone de réaction obtenue grâce aux injecteurs 9,9', aux injecteurs 8,8' et 10 permet d'obtenir un volume de réaction important dont l'homogénéité est bien adaptée au chauffage des produits sidérurgiques.

It appears that the optimization of the flame produced by this set of fuel injector 10 and of the two sets of oxidizer injectors 8,8 'and 9,9' is preferably carried out by a combination of the following arrangements:

1. 1 / the position, the diameter and the angle of the oxidizer orifices and injectors of the first set 9, 9 ′ located near the plane of the fuel injector 10.
2. 2 / optimization of the number and relative positions of the oxidizer injectors 9.9 ′ of the first set, of their angle of inclination (b) and of their diameters, and of the fuel injector 10, in combination with the reagent ejection speeds from these injectors.
3. 3 / the position of the 8.8 'oxidizer injectors of the second set, their angle of inclination (a) and their diameters to achieve spreading of the reaction zone in the horizontal plane and generate a secondary recirculation of the oxidant introduced by the jets of these 8.8 'orifices and fumes around the reaction zone, 4 / the volume of the reaction zone obtained thanks to the injectors 9.9', to the injectors 8.8 'and 10 makes it possible to obtain a large reaction volume whose homogeneity is well suited to the heating of steel products.

In a preferred embodiment of the invention, the distances Y ₉ and Y ₈ are in a ratio of between 0.4 and 0.7.

The orifices 8, 8 ′ of the second set are preferably at a distance from the axial vertical plane, passing through the axis of the rod 6, less than the distance from this orifices 9.9 ′ of the first set, the ratio of the distances which can be between 0.5 and 0.7.

The Fig. 7 and 8 present an alternative embodiment of the burner according to the invention in a flame modulation application, that is to say for the production of a burner capable of producing a long spread flame or a short spread flame according to its operation.

We see on Fig. 7 that the burner of the preceding figures is preserved, with its oxidizer body 3 supplying the pairs of orifices 8,8 'and 9,9' from the connection flange 4 to the circuit 5. A partition 14, in particular cylindrical, separates the oxidizer body 3 from another chamber 15, forming an oxidizer body supplied by the flange 16 from a circuit 17 summarily represented by an arrow. The oxidizer body 3 feeds the two sets of pairs of orifices 8.8 'and 9.9' whose positioning, angle of inclination and the diameter and speed of the fluid are determined to produce a long spread flame similar to that previously described, and a third set of orifices 18, distributed concentrically with the orifice 10 to produce a short spread flame. The orifices 18 are advantageously distributed, for example as on Fig. 8 six in number, on a circumference centered on the geometric axis of the fuel orifice 10.

The two sets of orifices 8.8 ′ and 9.9 ′ of oxidizer for producing the long spread flame are substantially identical to those described above. They are positioned radially outside the third set of orifices 18, as shown in Fig. 8 .

This third set of orifices 18, located radially inside the first two sets, makes it possible to obtain a short spread flame, close to the wall of the furnace 2, which will distribute its energy over the end of the product located near this wall and thus make it possible to control the distribution of thermal power on the product by the choice of the long spread flame produced by the orifices 8,8 'and 9,9' supplied by the elements 5 and 4 and 3, or with a short spread flame obtained by using the orifices 18, supplied by the elements 17 and 15 and 16.

The burners operating according to the invention therefore ensure the production of a spread and diluted flame which allows the dilution of the reactants before their oxidation with a low level of NOx production, this with a long spread flame, or with a single burner with a long or short flame spread.

This burner is particularly suitable for controlling the thermal profile of the product in the oven, for example according to the method described in EP 0 994 302 .

The tests carried out on the test platform have shown that the level of NOx produced by this type of burner, in particular in a long spread flame, is far below the limits set by current and future regulations. This very low level of NOx makes it possible to anticipate a regulatory limitation of pollutant emissions and thus the amounts of local taxes which will result therefrom.

Claims (6)

1. A burner for a reheating furnace for steel products, such as billets, blooms or slabs, or for a heat treatment furnace that is fitted with a fuel injection device and an oxidant supply body supplying oxidant supply ports (8) and (9), the burner having an axial direction:

   - the oxidant supply body has two sets of four oxidant supply ports (8, 8') and (9, 9'), each set having two ports (8, 9) located above a horizontal plane passing through the axial direction of the burner and two ports (8', 9') located beneath said plane, the ports (8, 8') in a second set being further away from said horizontal plane than the ports (9, 9') in the first set, the geometric axes of the supply ducts of the ports of the two sets having angles of inclination (a, b) in relation to said axial direction of the burner,

   - the axes of the oxidant supply ports fall within the horizontal planes, and are inclined in relation to the perpendicular to the output plane by an angle (a) for the ports (8, 8') of the second set and by an angle (b) for the ports (9, 9') of the first set, the axes of the oxidant supply ports being divergent,
   characterized in that

   - the injection device is designed to ensure central injection of the fuel through a port (10) parallel to the axial direction of the burner,

   - the angle (a) of the geometric axes of the pairs of ports (8, 8') of the second set is between 5° and 18°, and the axes are divergent,

   - the angle (b) of the geometric axes of the pairs of ports (9, 9') of the first set is between 10° and 20°.
2. A burner according to the preceding claim, wherein the pairs of oxidant supply ports (8, 8') and (9, 9') are adapted to open out into an output plane that is substantially equal to the plane (P) corresponding to the internal face of the furnace.
3. A burner according to any one of the preceding claims, wherein each set of ports comprises two groups of two ports (8, 8; 8', 8') (9, 9; 9', 9') located in a plane parallel to the horizontal plane Y₁₀ passing through the axial direction of the burner, the planes of the ports of the first set being located at a distance Y₉ from said horizontal plane Y₁₀ and the planes of the ports of the second set being located at a distance Y₈, and in that the ratio between the distances Y₉ to Y₈ is between 0.4 and 0.7.
4. A burner according to any one of the preceding claims, further comprising two oxidant boxes (3, 15) adapted to be supplied by independent circuits (5, 17) and adapted to be supplied respectively the two sets of ports (8, 8', 9, 9'), and a third set of ports (18) that are located radially inside the ports of the first two sets and that make it possible to obtain a long spread flame, while the third set of ports makes it possible to obtain a short spread flame.
5. A burner according to any one of the preceding claims, further comprising a fuel pipe (10) formed by a plurality of tubes for using several different types of fuel.
6. Use of a burner according to any one of the preceding claims, characterized in that the burner is adapted to have a momentum ratio between the oxidant and the fuel is between 5 and 50, depending on the characteristics of the reagents, and in particular between 30 and 50 for natural gas or between 3 and 15 for lean gas.

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