EP0775868B1 - Gas burner for a furnace for reheating steel industry products - Google Patents

Abstract

The burner consists of a central gas feed channel, surrounded by a concentric air feed channel. A nozzle (7) has its front surface facing towards the interior of a furnace. The nozzle has two sets of orifices (10) in concentric rings of different diameters (D1,D2), one for gas and one for air. Both orifices are linked to the respective feed channels. The gas feed orifices lie concentrically with the axis (X) of the nozzle. The air feed orifices are in a spiral line to create a swirling flow and create a depression which draws the gas through the nozzle. The diameters of the two rings of orifices are in a ratio of 0.45-0.5 : 1. The face of the nozzle has a recessed section (14) which contains the two rings of orifices and is connected to its main front face (13) by a conical surface (15) at an angle of 20-40 deg., preferably 30 deg.

Description

The present invention relates to a gas burner, of the type used. in furnaces for heating steel products, such as slabs for example.

These burners are high power burners, supplied with particularly in steel gas (mixed gas, coke oven gas for example), and are arranged in the roof of the heating furnaces.

We already know different types of burners of this kind, which generally have several concentric channels fuel gas and air supply, opening at the nose of the burner, downstream of which combustion occurs.

For example, we know burners of the type "jet burner" very high gas ejection speed, greater than 100 meters per second. In this type of burner, the air and the combustion gas burn in the combustion chamber combustion of the burner, and at the burner outlet, we are in the presence of hot smoke, combustion is complete.

This type of burner is commonly used in ovens requiring direct heating of products, for example in hoods caking agglomeration, because the very high gas ejection speed allows to heat a very localized area.

Such a type of burner is not suitable for furnaces reheating of steel products because the heating they provide is very localized direct heating. Their use would cause temperature heterogeneity on the product to be heated with risk important appearance of black streaks and risk of causing problems during the subsequent rolling operation (in the case of reheating the slabs).

We also know the burners called "flat flame" or so-called "flat flame". In this type of burner, the combustion air is brought in rotation around the gas supply. Document EP-A-0 674 135 shows such a burner.

The flame due to the combustion of the air / gas mixture is generated downstream of the burner nose, in the burner, i.e. in the housing fitted into the internal lining of the oven and into which the burner.

This tulip-shaped workpiece allows you to tackle the flame against the walls of said opener and against the walls of the roof of the oven.

This type of burner, allowing indirect heating of the product by radiation from the roof of the oven, presents a number disadvantages.

On the one hand, the weak gas impulse in this type of burner leads to poor mixing of the fumes inside the oven and thus heterogeneity of temperatures between the oven roof and the product steel to be heated.

In addition, this type of burner, by design, has a flared flat flame and therefore a certain black hole in the center of said flame, where it does not generate a flame. This phenomenon reinforces the problem of temperature heterogeneity.

This problem of temperature heterogeneity is very important because the heating of the steel product is effected by radiation of the heat stored in the roof of the oven, all temperature heterogeneity on the vault affects the product steel.

On the other hand, this type of burner is not multi-fuel. he is only intended to operate on steel gas and cannot be suitable for running on natural gas.

This type of burner requires a tulip-shaped burner, it is relatively expensive to operate due to the high cost of this type of workman. In addition, this type of workman is relatively fragile.

Finally, this type of burner has a low efficiency thermal, in the sense of its capacity to generate a rise in temperature in the oven, because the useful heating surface on the roof of the oven of each burner is relatively small.

We also know the so-called "ball flame" burners within from which the flame develops in the opening in the form of a ball.

The flame due to the combustion of the air / gas mixture is generated downstream of the burner nose, into the burner, in the form of a ball.

A big disadvantage of this type of burner is its very low adjustment latitude in terms of heating power.

We still know burners direct heating and length of variable flame.

This type of burner has three series of orifices, a first for the passage of gas, a second for the passage of air, arranged in circle around the gas passage openings, generating a short flame, with a length of the order of 2 meters, and a third series of orifices for the air passage, located at the periphery, generating a long flame, of a length of the order of 4.5 meters.

This type of burner is a burner for direct heating of steel products. There is therefore a drawback related to this function, to know the heterogeneity of temperatures in the product to be heated, certainly less important than in the case of "jet burners".

In addition, a large number of types of industrial burners produce gases with high nitrogen oxide NOx contents, in particular greater than 300 ppm for oxygen contents of the order of 2% in smoke, which is a drawback in most ovens, as well as for the environment, these nitrogen oxides being released into the atmosphere with the smoke from the oven.

The object of the present invention is to propose a burner combining direct heating and indirect heating by radiation of the roof of the furnace, having a great latitude of adjustment of the power of heats, allowing to generate a homogeneous temperature of the roof of the furnace, which can use both steel and natural gas, while not producing too much nitrogen oxide.

• une première série d'orifices pour le passage du gaz, disposés en cercle sur une couronne de diamètre moyen D1 et en communication avec le canal central,
• une seconde série d'orifices pour le passage d'air, disposés en cercle sur une couronne de diamètre moyen D2 autour des orifices de passage de gaz, et en communication avec le canal annulaire d'alimentation en air, les orifices de passage d'air étant orientés en hélice pour créer un flux d'air tourbillonnant provoquant une dépression qui aspire le gaz, caractérisé en ce que :
• les orifices pour le passage du gaz sont coaxiaux à la direction axiale du nez du brûleur,
• le rapport des diamètres D1/D2 des couronnes est compris entre 0,4 et 0,5, de préférence égal à 0,46.

The invention therefore relates to a gas burner, in particular for a furnace for reheating steel products, comprising a central fuel gas supply channel, a concentric annular air supply channel and a burner nose, said nose burner comprising a front surface facing the interior of the furnace and:

• a first series of orifices for the passage of gas, arranged in a circle on a crown of average diameter D1 and in communication with the central channel,
• a second series of orifices for the passage of air, arranged in a circle on a ring of average diameter D2 around the orifices for the passage of gas, and in communication with the annular air supply channel, the orifices for passage of air being oriented in a helix to create a swirling air flow causing a vacuum which sucks in the gas, characterized in that:
• the orifices for the passage of gas are coaxial with the axial direction of the burner nose,
• the ratio of diameters D1 / D2 of the crowns is between 0.4 and 0.5, preferably equal to 0.46.

• le nez du brûleur comporte une partie en retrait vers l'intérieur du brûleur par rapport à la surface frontale du nez, sensiblement plane, perpendiculaire à l'axe du brûleur, dans laquelle débouchent les deux séries d'orifices, la partie en retrait et la surface frontale étant reliées entre elles par une surface de liaison ;
• la surface de liaison entre la surface frontale et la partie en retrait de la face amont du nez du brûleur est sensiblement en forme de tronc de cône, dont la grande base est située au niveau de la surface frontale;
• l'angle du tronc de cône de la partie en retrait est compris entre 20 et 40 degrés, de préférence égal à 30 degrés.

According to other features of the invention,

• the burner nose has a part recessed towards the inside of the burner with respect to the front surface of the nose, substantially planar, perpendicular to the axis of the burner, into which the two series of orifices open, the recessed part and the front surface being interconnected by a connecting surface;
• the connection surface between the front surface and the recessed part of the upstream face of the burner nose is substantially in the form of a truncated cone, the large base of which is situated at the level of the front surface;
• the angle of the truncated cone of the recessed part is between 20 and 40 degrees, preferably equal to 30 degrees.

The present invention also relates to an oven roof of reheating of steel products, characterized in that it comprises at least one housing 9, cylindrical with diameter D3, arranged in the internal lining of the oven roof, in which a burner is kept according to the preceding claims, and in that the ratio between the diameter D2 of the crown of the orifices for the passage of air from the burner and the diameter D3 of the housing 9 is between 0.6 and 0.8, preferably equal at 0.68.

According to another characteristic, the ratio between the diameter D3 housing fitted in the internal lining of the oven roof and the distance between the front surface of the burner nose and the top of the oven is between 0.9 and 1, preferably equal to 0.95.

• la figure 1 est une vue en coupe axiale d'un brûleur à gaz selon l'invention, implanté dans la voûte d'un four de réchauffage,
• la figure 2 est une vue en coupe à échelle agrandie du nez du brûleur,
• la figure 3 est une vue frontale du nez du brûleur.

The characteristics and advantages will become more clearly apparent from the following description, given solely by way of example, made with reference to the appended drawings, in which:

• FIG. 1 is a view in axial section of a gas burner according to the invention, installed in the roof of a reheating oven,
• FIG. 2 is a sectional view on an enlarged scale of the burner nose,
• Figure 3 is a front view of the burner nose.

The burner 1 includes a gas supply box 2 fuel and air, formed of a central gas supply channel 3, oriented in the axial direction X of the burner, surrounded by an annular channel 4 air intake.

These two coaxial channels 3, 4 are supplied respectively in gas and in air by supply conduits 5, 6 respectively.

The burner 1 also includes a nose 7 also called diffuser, generally made of a ceramic material.

The nose 7 consists of a thick plate 8 which covers the axial ends of the different channels while ensuring sealing between them, and which is inserted into a housing 9, cylindrical with diameter D3, fitted into the internal lining of the oven roof, being there held by fixing means, not shown, on the wall of the oven.

The upstream face of the burner nose 7 has a surface frontal 13 flat, facing the interior of the oven.

The plate 8 of the burner nose 7 is traversed by a plurality gas and air outlet ports.

A first series of orifices 10, distributed in a circle on a crown C1, of average diameter D1, opens on the side of the rear face 11 of the plate 8 in the central gas supply channel 3.

The orifices 10, connected to the gas supply channel 3 are coaxial to the axial direction X of the burner.

In the embodiment shown, the plate 8 of the nose 7 of the burner is provided with six orifices 10, also distributed in a circle on the crown C1.

A second series of orifices 12, distributed in a circle on a crown C2 of average diameter D2 around the orifices 10, opens on the side from the rear face 11 of the plate 8 in the directory feed channel 4 in the air.

As seen in Figure 2, the orifices 12, connected to the channel 4 air supply, are made so that the axis of each orifice either located in a plane parallel to the axial direction of the burner and tangent to the crown C2, and inclined in this plane, by an angle α with respect to said axial direction.

Preferably, the angle α is between 15 and 25 °.

The orifices 12 are therefore oriented in a helix, so that the escaping air flow has a rotational movement around the X axis of the burner, creating a swirling air flow causing depression which sucks in the gas flow.

In the embodiment shown, the plate 8 of the nose 7 of the burner is provided with eight orifices 12, also distributed in a circle on the crown C2.

The burner nose 7 has a portion 14 recessed towards the interior of the burner with respect to the substantially planar front surface 13, perpendicular to the X axis of the burner, into which the orifices 10 open gas passage and the air passage orifices 12, the recessed part 14 and the front surface 13 being interconnected by a connecting surface 15.

The connecting surface 15 between the front surface 13 and the part set back 14 from the upstream face of the burner nose is substantially in the form of truncated cone, the large base of which is located at the level of the frontal surface 13.

The angle β of the truncated cone of the connecting surface 15 is between 20 and 40 °, preferably equal to 30 °.

To ensure that the burner according to the invention combines the function direct heating and indirect heating by radiation of the oven roof, the D1 / D2 ratio of crowns C1 and C2, respectively of the series of orifices 10 of gas passage and the series of air passage holes is included between 0.4 and 0.5.

If this ratio D1 / D2 is less than 0.4, the flame is of the type "jet", impacting directly on the product, and when using gas natural, there is separation of the flame and risk of extinction. If this report is greater than 0.5, the flame is very short, disturbed, and there is overheating of the workman, especially if you use natural gas.

In addition, the ratio between the diameter D2 of the crown C2 of the series of air passage holes 12 and the diameter D3 of the housing 9 fitted in the internal lining of the oven roof is between 0.6 and 0.8.

If this ratio D2 / D3 is less than 0.6, a flame is produced soft, poorly directed, sensitive to variations in the internal pressure of the oven. If this ratio is greater than 0.8, the flame is an airplane nozzle type torch, it there is a strong separation of the flame if natural gas is used, and at high flow the flame is blown out for all types of gas.

Finally, the relationship between the diameter D3 of the fitted housing 9 in the internal lining of the oven roof and the distance H between the front surface 13 of the burner nose and the top of the oven is between 0.9 and 1.

If this D3 / H ratio is less than 0.9, there is internal overheating and too much NOx production when using steel gas. Yes this ratio is greater than 1, the flame is badly directed and combustion is incomplete.

This configuration allows the flame to develop in the housing 9 arranged in the roof of the oven. This flame is not directly pressed against the wall of the oven roof, but develops at the interior of the oven in the form of a very obtuse angle cone, of the order of 160 to 170 °.

According to the values of the ratios D1 / D2, D2 / D3 and D3 / H, the flame develops inside the oven more or less pressed against the wall of the vault of the oven.

Thus a ratio of diameters D1 / D2 equal to 0.40 or equal to 0.50 ensures that the burner will effectively combine the function direct heating and indirect heating by radiation of the oven roof.

For example, a D1 / D2 ratio of 0.43 is very close to ideal if a steel gas is used, but is not completely satisfactory if you use natural gas.

A D1 / D2 ratio of 0.48, for example, is very close to ideal for natural gas, but causes flames if using a gas mixed at high flow and produces too much NOx.

The best compromise for multi-fuel use is ensured for a D1 / D2 ratio between 0.44 and 0.48, preferably equal to 0.46.

Similarly, the best compromise regarding the ratio of diameters D2 / D3 is reached when this ratio D2 / D3 is between 0.66 and 0.70, preferably equal to 0.68.

Finally, the relationship between the diameter D3 and the distance H is optimum when it is between 0.93 and 0.97, preferably equal to 0.95.

Such a burner makes it possible to obtain heating of the furnace according to two combined modes, direct heating and indirect radiant heating, and this at from a gas ejection speed equal to 40 meters per second.

Such a burner can thus operate with an ejection speed gases from 40 to 100 meters per second, which gives it great latitude to adjust the heating power, while ensuring homogeneous arch temperature.

In addition, such a burner is multi-fuel, that is to say that it retains its properties regardless of the type of gas used, from natural gas to steel gas.

Claims (7)

1. Gas burner, especially for a furnace for reheating iron and steel products, comprising a central combustible-gas-feed channel (3), a concentric annular air-feed channel (4) and a burner head (7), the said burner head (7) having a front surface (13) facing the inside of the furnace and:

   a first series of holes (10) for passage of the gas, arranged in a circle on a ring (C1) of average diameter D1 and in communication with the central channel (3),

   a second series of holes (12) for passage of air, arranged in a circle on a ring (C2) of average diameter D2 around the gas passage holes (10) and in communication with the annular air-feed channel (4), the air passage holes (12) being oriented in a helix in order to create a turbulent flow of air causing a partial vacuum which draws in the gas, characterized in that:

   the holes (10) for passage of the gas are coaxial with the axial direction (X) of the burner head,

   the ratio of the diameters D1/D2 of the rings (C1) and (C2) is between 0.4 and 0.5.
2. Gas burner according to Claim 1, characterized in that the burner head (7) has a part (14) set back towards the inside of the burner with respect to the front surface (13) of the said head, which is approximately plane, perpendicular to the axis of the burner, in which part the two series of holes emerge, the set-back part (14) and the front surface (13) being connected together by a linking surface (15).
3. Gas burner according to Claim 2, characterized in that the linking surface (15) between the front surface (13) and the set-back part (14) of the upstream face of the burner head is substantially in the form of a truncated cone, the large base of which is level with the front surface (13).
4. Gas burner according to Claim 3, characterized in that the angle β of the truncated cone of the set-back part is between 20 and 40 degrees, preferably equal to 30 degrees.
5. Gas burner according to Claim 1, characterized in that the ratio of the diameters D1/D2 of the rings (C1) and (C2) is equal to 0.46.
6. Roof of a furnace for reheating iron and steel products, characterized in that it comprises at least one cylindrical housing (9) of diameter D3, made in the internal lining of the roof of the furnace, in which housing a burner according to the preceding claims is held in place and in that the ratio of the diameter D2 of the ring of air passage holes of the burner to the diameter D3 of the housing (9) is between 0.6 and 0.8, preferably equal to 0.68.
7. Roof of a furnace for reheating iron and steel products according to Claim 6, characterized in that the ratio of the diameter D3 of the housing (9) to the distance H between the front surface of the burner head and the roof of the furnace is between 0.9 and 1, preferably equal to 0.95.

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