FR3114324A1 - DIRECT FLAME PREHEATING SECTION FOR CONTINUOUS METAL STRIP TREATMENT LINE - Google Patents

Abstract

DIRECT FLAME PREHEATING SECTION FOR CONTINUOUS METAL STRIP TREATMENT LINE Direct flame preheating section for continuous metal strip treatment lines (B) comprising a connecting zone between an active zone (14) equipped with burners capable of operating in so-called flameless mode and a recuperative zone (11) for preheating the strip by exchange with the combustion fumes coming from the active zone, said connecting zone having chambers (18, 19) capable of directing the flow of the fumes so that they flow frontally with respect to the strip in exit from the active zone and at the entrance to the recuperative zone according to the direction of smoke flow. Figure for abstract: 7

Description

DIRECT FLAME PREHEATING SECTION FOR CONTINUOUS METAL STRIP TREATMENT LINE

Designation of the technical field concerned

The invention relates to continuous horizontal or vertical lines for annealing or galvanizing metal strips, and more particularly to the vertical direct flame preheating sections of these lines, sometimes called "NOF sections", NOF being the abbreviation of English “ Non Oxydizing Furnace ”, or Four non oxidizing, or “ section DFF ”, DFF being the abbreviation of English “ Direct Firing Furnace ”, or Four à feu direct.

The aim of the invention is that the preheating section makes it possible to carry out effective preheating of the strip with good uniformity of temperature and surface condition over the width of the strip. It also aims to avoid or control the interaction between the combustion reagents and the strip surface, while limiting atmospheric emissions.

Technical problems to which the invention responds

A direct flame preheating section is generally placed at the entrance to a furnace of a hot-dip galvanizing line or an annealing line.

Referring to the diagram of Figure 1 of the accompanying drawings, one can see schematically and partially represented, a galvanizing line according to the state of the art. From the entrance of the line, according to the direction of movement of the strip, we find a section 1 of preheating with direct flame, a section 2 of heating with radiant tubes, a section 3 of holding with radiant tubes, a section 4 of slow cooling, a rapid cooling section 5, an aging section 6, an oven outlet section 7 and a coating section 8.

The direct flame preheating section has the following main features:

. Heat the strip from room temperature to a desired temperature, for example varying from 500°C to 750°C for steel, depending on its grade.

. Remove the oil introduced by the cold rolling process and the oxide fines formed during or after the cold rolling present on the strip.

. Prepare the strip surface for the coating process by removing surface oxides.

The direct flame preheating section consists of two zones: an active zone where the burners are installed that allow the strip to be heated to the temperature defined by the thermal cycle, and a recuperative zone where the strip is preheated to a temperature below 250° C to avoid its oxidation, and this by exhausting the heat contained in the fumes coming from the active zone.

Referring to the diagram of Figure 2 of the accompanying drawings, an enlargement of the preheating section of Figure 1 can be seen. atmospheres between the ambient air and the atmosphere present inside the oven.

It is followed by a recuperative vertical zone 11 in which the strip is preheated by the combustion fumes. In this area, as in the entire preheating section, the fumes circulate in the opposite direction to the strip. In the vicinity of the entrance to the recovery zone, close to the airlock 10 for separating atmospheres, an outlet 12 makes it possible to conduct the fumes towards an additional energy recovery zone, not shown, outside the preheating section, at the means of an enhancer also not shown. The fumes leave the preheating section at a temperature generally between 700°C and 900°C.

The additional energy recovery zone further exhausts the fumes by lowering their temperature even further. It may include a heat exchanger making it possible to transfer heat energy from the flue gases to another fluid, for example the air used to supply the burners of the preheating section and thus limit fuel consumption.

The direct flame preheating section can be horizontal or vertical, depending on whether the strip runs through it horizontally or vertically. On a horizontal line, the preheating section is generally horizontal, but it can also be vertical, in particular to limit the length of the line.

In a horizontal preheating section, the active zone and the recovery zone follow each other without changing the direction of the strip. The fumes from the active zone thus flow towards the recuperative zone while maintaining a good distribution of the fumes over the strip width.

In a vertical preheating section, as shown in figure 2, the active zone and the recuperative zone are generally on two different belt strands, one rising for the recuperating zone and the other descending for the active zone. In the upper part of each zone is arranged a deflector roller 31, 32 for a change of direction of the strip at 90 degrees. Between the two deflecting rollers, the belt runs horizontally in the same clockwise direction. At the exit from the active zone, the temperature of the furnace is very high, for example 1350°C. To prevent the deflector rollers from being exposed to this temperature level, they are placed in a separate zone, in which the temperature is lower. The fumes travel from the active zone to the recovery zone in at least one connection tunnel 13, without passing through this separate zone 30 where the deflector rollers are placed thanks to restrictions 33, 34 installed at the entrance and exit of the latter on the up and down strands of the strip.

The flow of smoke in the existing connection tunnel configurations leads to heterogeneous smoke distribution over the width of the strip. This leads to a temperature heterogeneity over the strip width and a disparity in the concentration of chemical species at the surface of the strip. This results in a different surface condition over the width of the strip leaving the preheating section.

The direct flame burners of the active zone must ensure the preheating of the strip with good temperature uniformity over the width of the strip. They must have low energy consumption and emit few polluting emissions, particularly nitrogen oxides (NOx).

The burners must also be able to operate in reducing mode, i.e. undersupplied with oxidizer, in order to minimize the presence of oxygen near the strip and thus prevent its oxidation. If it is accepted that a low oxygen level close to the band of a few hundred ppm is acceptable, it is still advisable to try to approach zero oxygen near the band.

With the emergence of high strength steels, the content of alloying elements such as Mn, Si and Al has increased. These oxygen-hungry elements oxidize easily. Despite an overall reducing atmosphere in the preheat section and in downstream sections, such as radiant tube heating and holding sections, oxides of these alloying elements inevitably form in these sections under normal operating conditions. In a galvanizing line, if these oxides are present on the surface of the strip before it is immersed in the zinc bath, they lead to coating defects. To remedy this problem, it is known to carry out a selective oxidation, or pre-oxidation, of these alloying elements in the preheating section so as to avoid their diffusion on the surface of the strip. The oxides formed are then reduced in the radiant tube sections. This requires slightly oxidizing conditions at the outlet of the preheating section, with fine control of the air/gas ratio of the burners. It is also necessary to have a uniform temperature (+/-10°C) over the width of the strip so that the nature and thickness of the oxide layer are constant over the width of the strip.

In addition, to limit investment and maintenance costs, the number of burners and their control and regulation devices must be reduced.

Existing solutions do not make it possible to combine all these requirements. The invention overcomes these problems.

Technical background

In a vertical direct flame preheating section according to the state of the art, the fumes travel from the active zone to the recuperative zone in at least one connection tunnel according to three configurations.

In the first configuration illustrated in FIGS. 2 and 3, the connection tunnel 13 is longitudinal, that is to say it connects the active zone 14 and the recuperative zone 11 by a horizontal section extending in the direction of scrolling of the strip B. FIG. 3 corresponds to a view from above according to the section plane CC of FIG. 2. In this configuration, the two vertical strands of the strip at the level of the tunnel constitute obstacles to the flow of smoke that some of these must circumvent. Swirls of smoke (vortices) form in places, particularly at the entrance to the recuperative section in the direction of smoke flow. This results in a heterogeneous smoke distribution over the width of the strip leading to a difference in temperature and surface condition over the width of the strip.

In the second configuration illustrated in FIG. 4, according to a sectional view similar to that of FIG. 3, a lateral connection tunnel 13a, 13b is arranged on each side of the preheating section. The smoke inlets on the side of the active zone 14 and their exits on the side of the recuperative zone 11 are carried out laterally, on the sides of the strip B. This results in an asymmetry over the width of the strip, the distribution of the smoke being greater on the edges of the strip than in its centre.

In the third configuration illustrated in FIG. 5, according to a sectional view similar to that of FIGS. 3 and 4, the suction of the fumes at the outlet of the active section 14 is carried out symmetrically on each side of the strip, but the reinjection of those -Ci, at the entrance of the recuperative section 11, is carried out laterally on a single side of the strip. This results in an asymmetry in the distribution of the fumes over the width of the strip.

The burners that equip the vertical direct flame preheating sections are grouped into two main categories, the so-called front burners and the so-called side burners according to their position relative to the strip.

The so-called front burners are placed opposite the band. Two types of front burners are distinguished: front burners with nozzle mixing and front burners with premixing. The front burners develop a flat, short spiral flame so as to avoid impacting and oxidizing the strip. This technology is the most widespread, in particular because it makes it possible to modulate the temperature profiles over the width of the strip by adjusting the distribution of heat between the burners. However, this technology is costly in terms of investment and maintenance, since it requires a large number of burners to cover the entire width of the strip (between three burners and nine burners depending on the strip width and the unit power of the burners) and a system complex regulation for adjusting power and the air/gas ratio per burner. These burners operate with hot air when they are front burners with nozzle mixing (typically air preheated to 550°C) or with cold or slightly preheated air (temperature below 300°C) when it comes to front premix burners. Generally, with front burners, at least one zone of the preheating section is equipped with premixed burners, which leads to overconsumption of fuel compared to hot air burners.

The so-called side burners are placed on the side of the band. They develop a flame across the width of the oven, parallel to the strip. This technology is simpler and more economical, as it requires only one burner per row to cover the entire width of the strip on one side. In addition, the mode of regulation of the air/gas ratios is practiced by section, for a set of burners. These burners operate in hot air (up to 450°C) with savings in fuel. On the other hand, these burners according to the state of the art have fairly high NOx emission levels, typically 250 mg/Nm3 at 3% oxygen compared to 120 mg/Nm3 for front burners. In addition, the temperature heterogeneity of their flame over the width of the preheating section is affected by the process and must be corrected by means other than the burner itself. Thus, the temperature difference over the width of the strip can vary between +/-20°C under moderate conditions of production and temperature at the outlet of the preheating section (600°C), to +/-50°C for outlet temperatures around 720°C.

In an attempt to overcome this problem, there are hybrid preheating sections combining the two categories of burners. In the last zone, the side burners are replaced by front cold air premix burners. This solution makes it possible to correct the problem of temperature heterogeneity at the outlet of the preheating section, but it has the same other drawbacks mentioned above.

In addition, these front or side burners according to the state of the art take up a classic design. Combustion between gas and air is initiated in a combustion tunnel and develops in the furnace according to a thermal and chemical distribution that is more or less difficult to control over the width of the strip. The applicant is not aware of any burner operating in flameless mode in the preheating sections of the continuous lines. The characteristics of the flameless combustion mode, resulting from diffuse combustion, have been widely studied and the limits are fairly well identified. In a confined environment, on the other hand, this mode of combustion is difficult to apply, because it requires combustion chamber volumes in line with the large quantity of recirculated flue gases necessary to obtain diffuse combustion.

According to a first aspect of the invention, there is proposed a direct flame preheating section for a continuous metal strip processing line comprising a connecting zone provided for circulation of the combustion fumes coming from an active zone equipped with burners to a recuperative zone for preheating the strip by exchange with said fumes, the burners being capable of operating in so-called flameless mode. Said connecting zone comprises an outlet chamber capable of directing the flow of fumes so that they flow frontally with respect to the strip at the outlet of the active zone and an inlet chamber capable of directing the flow of fumes so that they flow frontally with respect to the strip at the entrance to the recovery zone, in the direction of flue gas flow.

The outlet chamber is arranged at the outlet of the active zone, in the direction of smoke flow and is provided arranged for a withdrawal of smoke, the inlet chamber is arranged at the entrance to the recuperative zone and is provided arranged for a smoke injection, the connecting zone further comprising two turning chambers each arranged to cause the flow of smoke to make a 90 degree turn between an inlet opening and an outlet opening, a first turning chamber communicating directly with the exit chamber and a second turning chamber communicating directly with the entry chamber, and two connecting tunnels provided arranged for smoke circulation, a first connecting tunnel directly connecting the exit opening of the first chamber with an inlet opening of the inlet chamber and a second connecting tunnel directly connecting an outlet opening of the outlet chamber and the inlet opening of the second c room.

The two circuits are more or less symmetrical to obtain a balanced distribution of the fumes on the two sides of the belt, contributing to good temperature uniformity.

The two outlet openings of the outlet chamber are arranged facing each other and frontally with respect to a circulation of the strip in the active zone and the two inlet openings of the inlet chamber are arranged opposite each other. Opposite and frontally in relation to movement of the band in the recovery zone.

This arrangement favors the distribution of the smoke flow over the width of the strip in the connecting zone and over the length of the active and reactive zones. This results in better homogeneity in temperature and in surface condition over the width of the strip compared to a solution where the injection and/or the extraction of the fumes is carried out laterally, in a direction parallel to the direction defined by the width of the strip. the band.

In addition, the absence of the band in the chambers in which the smoke flow makes a 90 degree turn contributes to the homogeneity of the smoke distribution over the width of the band.

The width and length dimensions on a horizontal plane of the connecting zone chambers where the band is located are the same as those of the active and recuperative zones which extend. Thus, the section of the chamber which extends the recuperative zone is smaller than that of the chamber which extends the active zone. The chambers intended to direct the flow of fumes, their openings and the connecting ducts between the chambers are dimensioned so that the fumes flow into the chambers where the strip is located in a direction perpendicular to one side of the strip and from so that the distribution of fumes is homogeneous over the width of the strip.

The chambers of the connection zone in which the flow of fumes makes a 90 degree turn are located between the rising strand and the falling strand of the strip. They are located at the same level on the height of the preheating section as the chambers where the strip is located and they are aligned with them longitudinally, according to the direction of movement of the strip in the line. The horizontal space usually available between the active zone and the recuperative zone of a direct flame preheating section according to the state of the art is sufficient for the installation of the two chambers in which the flow of the fumes makes a 90 bend. degrees. This space can nevertheless be slightly increased, if necessary, to obtain a good distribution of the fumes and a flow of the latter in a direction perpendicular to the direction defined by the width of the strip over the width.

According to a second aspect of the invention, the burners are of the direct flame side type, said burners being capable of operating in flameless mode.

This type of combustion is very low emissive in the ultraviolet range. The flame is almost invisible to the naked eye, hence the term flameless mode. The limits of the flame are less well defined, because the products of combustion are very homogeneous and mix with the smoke from the oven.

In flameless mode, the combustion is strongly diluted in several volumes of smoke. This mode of operation is accessible either by recirculating the fumes locally within the combustion chamber itself or by taking part of the fumes elsewhere, for example directly from the chimney, and by reinjecting them into the burner. This last possibility is however more complex to implement. To obtain sufficient recirculation locally within the combustion chamber itself to operate in flameless mode without calling on external recirculation, it is necessary to have an injection of air and gas at high speeds into the combustion chamber. combustion. The geometry of the burner and that of the combustion chamber create recirculations of the products of combustion towards the burner, thus diluting the oxidizer and the fuel with the products of combustion before the reaction.

In normal operation, i.e. outside the furnace temperature rise and fall phases, during line stoppages and restarts, the internal temperature of the active zone is greater than 850°C. The burners therefore mainly operate in flameless mode.

The combination of burners operating without flame and of a connecting zone between the active zone and the recuperative zone of the preheating section according to the invention makes it possible to obtain good uniformity of temperature and surface condition over the width of the strip from its entry into the preheating section to its exit. This combination is necessary to obtain this good homogeneity over the width of the strip at the exit of the preheating section, because a significant heterogeneity present on the strip at the entrance to the active zone which would result from a connection zone depending on the state of the technique could not be corrected in the active zone. Indeed, the volume combustion of the side burner flameless mode does not allow the power delivered to the strip to be adjusted across its width.

The temperature difference over the strip width is thus limited to approximately +/- 10°C at the outlet of the preheating section, which makes it possible to obtain mechanical properties and a homogeneous oxide layer over the width of the strip. , in the case of selective oxidation.

Operation in flameless mode makes it possible to limit the temperature reached by the combustion products compared to a combustion mode with flame. Thus, in operation with an air factor of 0.95, the burner according to the invention in operation in flameless mode makes it possible to lower the hot point in the flame to approximately 1450° C., i.e. barely 100° C. above refractory temperature. For comparison, for the same operating conditions, the front burners according to the state of the art have flame temperatures exceeding 1700 °C.

Since the formation of NOx is directly linked to the flame temperature, the burner according to the invention has a significantly lower NOx emission rate than that of burners according to the state of the art during operation in flameless mode. . In addition, analyzes of chemical species within the flame show better homogeneity compared to conventional combustion. The low local oxygen level also contributes to the reduction of the level of NOx.

Switching to flameless mode from a temperature of 850°C ensures good combustion in the volume of the chamber, this temperature level allowing self-ignition of the fuel. Below this temperature, the burner operates in flame mode with a slightly oxidizing combustion setting.

The burner according to the invention is capable of operating with combustion air preheated up to 600° C., without significant impact on NOx emissions. Energy harvesters now have an efficiency that makes it possible to reach preheated air temperatures close to 600°C. However, the production of NOx on conventional burners is very dependent on air temperature levels with an exponential evolution curve. The air temperature on these burners is therefore limited. This evolution of NOx as a function of air temperature is much flatter and linear in diffuse combustion, which makes it possible to raise the air temperature to 600°C. This higher air temperature limits fuel consumption and promotes the recirculation of fumes and the homogeneity of species in the combustion chamber.

The preheating of the combustion air can be carried out in a heat exchanger in which the fumes leaving the preheating section have circulated. Although cooled by an exchange with the strip in the recuperative zone, their temperature level is still sufficient to ensure the preheating of the combustion air.

The burners have an axial direction at the intersection of a vertical plane and a horizontal plane, and comprise a diffuser through which fuel injection ducts pass for operation in flameless mode and oxidizer injection ducts. Said oxidizer injection ducts emerge from the diffuser closer to the axis of the burner than said fuel injection ducts for operation in flameless mode. The burners have oxidizer injection ducts which emerge from the diffuser on the vertical plane which are divergent and others which emerge from the diffuser on the horizontal plane which converge towards the axis of the burner.

The fuel and oxidant injection ducts are arranged so as to obtain the desired distribution of fuel and oxidant in the volume of the combustion chamber delimited by one face of the strip and the side and front walls of the furnace to obtain flameless combustion. The resulting volumetric combustion makes it possible to obtain a good distribution of the combustion products over the width of the strip and thus a good temperature uniformity over it.

At the exhaust from the injection ducts, the oxidizer spreads vertically and contracts horizontally. Fuel jets have a lower momentum than oxidizer jets. The fuel is sucked up by the oxidizer with which it reacts, constituting an airflow envelope which protects the strip from oxidation. The pulse of the oxidizer jets draws in the fumes in the same way to recirculate them.

Thus, although the strip is located in the immediate vicinity of the burners, the axis of the burners being typically around 400 mm from the strip, the presence of oxygen in the vicinity of the strip and its oxidation are avoided.

This near-band oxygen criterion is critical for the use of flameless side burners in a preheating section, because flameless burners generally require larger combustion chamber volumes to achieve maximum flue gas recirculation. If the confinement of the chamber does not allow this, the combustion spreads and the residual oxygen present within the flame pollutes the strip.

For an application in the preheating section, it is therefore not enough to homogenize the oxygen level in the flame as in a conventional flameless burner. It is also necessary not to increase the size of the reaction zone. In other words, the width of the flame should not be increased. However, flameless combustion is generally more extensive than conventional combustion.

The flameless combustion regime relies on the necessary presence of a high-intensity recirculation zone around the reagent jets in the furnace enclosure. The fuel and air jets must therefore have sufficient momentum to be able to entrain and mix with the aspirated fumes. The pulses of the oxidizer and fuel jets implemented according to the invention guarantee an overall smoke recirculation rate of six around the jets, which is sufficient for flameless combustion. This implies that, on average, the jet of oxidizer or fuel is diluted in six volumes of smoke.

In addition, flameless burners do not have a combustion tunnel. However, this contributes to initiating reactions very early on a conventional burner. The harmful consequence for an application of flameless burners in the preheating section would be to impact the wall located in front of the burner, which would accelerate its degradation. For this reason it is necessary to also contain the length of the flame.

The arrangement of the oxidizer and fuel injection ducts of the burners according to the invention makes it possible to meet these constraints.

The oxidizer injection ducts of the burners which emerge from the diffuser on the vertical plane are divergent at an angle of between 2 and 12 degrees, and preferably of seven degrees.

The oxidizer injection ducts of the burners which emerge from the diffuser on the horizontal plane diverge at an angle of between 1 and 5 degrees, and preferably of three degrees.

The fuel injection ducts of the burners for operation in flameless mode converge towards the axis of the burner.

They converge at an angle of between five and fifteen degrees and preferably eleven degrees.

These angles on the fuel and oxidizer ducts, combined with the injection speeds and the impulse of the jets, are particularly suitable for the usual dimensions of a direct flame preheating section. The impulse and the injection angle of the air jets are predominant, the impulse of the fuel jets being weaker.

Flameless burners are unstable when cold. Indeed, on these burners, the flames are unhooked and develop diffusely in the oven. When cold, when the auto-ignition temperature is not reached, this poses a problem, because in the event of a flame stall, the burner secures the entire area of the preheating section. This must then be purged in order to be able to restart. It is therefore necessary to have a heating mode that is very stable when cold to raise the temperature of the oven.

To meet this constraint, the burners have a fuel injection duct for operation in flame mode which extends along the axial direction of the burner and which emerges from the diffuser in the axis of the burner.

The burners also have an annular combustion air supply duct around the fuel injection duct for flame mode operation. This air contributes to the attachment of the flame.

The burners according to the invention are particularly suitable for operation with natural gas and steelmaking gas, in particular coke oven gas also called COG for "Coke oven gas".

The burner according to the invention makes it possible to obtain NOx levels of less than 100 mg/Nm ³ at 3% oxygen for a furnace at 1350° C., a combustion setting with a lack of air and air from the combustion preheated to 600°C. The residual oxygen close to the strip is of the order of 20 ppm over the entire width of the strip.

Burners placed at the entrance to the active zone, according to the direction of travel of the strip, operate in a stoichiometric atmosphere while the others, the majority of the burners, operate in a lack of air. The operation in a stoichiometric atmosphere of these burners makes it possible to produce fumes which will burn/crack the hydrocarbons present on the surface of the strip. The operation in lack of air of the other burners makes it possible to obtain reducing fumes which will reduce the iron oxides present on the surface of the strip.

The burners of the preheating section are thus distributed according to at least two regulation zones. The atmosphere in the section is controlled along the active zone by varying the air/gas ratio in the different regulation zones.

Some flat products coming onto the market, especially third generation steels, require selective pre-oxidation of the strip surface. To obtain this pre-oxidation, the preheating is carried out in several stages with one stage in a very slightly oxidizing zone. In this, the combustion must be finely tuned around the target air/gas factor, generally between 1.01 and 1.05. The new burner design according to the invention is compatible with this use. The distribution of near-band oxygen is very homogeneous, at +/- 0.1%. It is thus possible to produce an identical selective oxidation over the entire width of the strip, especially since the temperature uniformity of the strip is also improved. The thickness of the oxide layer on the steel is thus controlled by simple management of excess air in this area. The advantage of this characteristic is interesting, because it avoids a complex chamber dedicated to the selective oxidation of the strip.

According to a second aspect of the invention, there is proposed a continuous treatment line for a metal strip comprising a direct flame preheating section as described previously.

Brief description of figures

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

is a schematic overview of a galvanizing line with a direct flame preheating section according to the state of the art,

is an enlargement of the preheating section of figure 1,

is a schematic view from above and in section of the preheating section according to Figure 2,

is a schematic view from above and in section of a preheating section according to a second example of the state of the art,

is a schematic cross-sectional top view of a preheating section according to a third prior art example.

is a schematic view similar to Figure 2, but for a direct flame preheating section according to one embodiment of the invention,

is a schematic cross-sectional top view of the preheating section similar to those of Figures 3 to 5, but for the preheating section according to Figure 6,

is a schematic front view of the diffuser of a burner according to one embodiment of the invention,

is a schematic cross-sectional view in three dimensions of one half of the diffuser according to figure 8.

Detailed description of the invention

The embodiments described below being in no way limiting, variants of the invention may in particular be considered comprising only a selection of 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 from the state of the prior art. This selection includes at least one feature, preferably functional without structural details, or with only part of the structural details if only this part is sufficient to confer a technical advantage or to differentiate the invention from the state of the prior art .

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

Referring to the diagram of Figure 6 of the accompanying drawings, there can be seen a schematic view of a direct flame preheating section according to the invention. A connection zone 13 ensures the fluidic connection between the recuperative zone 11 and the active zone 14 equipped with side burners 15.

The nature of the connection zone 13 is similar to that of the active and recuperative zones in that it comprises a metallic outer casing and an inner lining made of refractory materials.

The connection zone 13 comprises two chambers 18, 19 in which the strip circulates, the chamber 18 at the entrance to the recovery zone 11, in the direction of flow of the fumes, for the rising strand and the chamber 19 at the exit from the zone active for the descending strand.

The connection zone 13 also includes two other chambers 20, 21 intended to direct the flow of the fumes facing the strip by causing them to make a 90 degree turn, the chamber 20 on the side of the riser and the chamber 21 on the side of the descending strand. They are arranged in the central part of the connection zone, between the rising strand and the falling strand of the strip.

Due to the suction produced by the smoke extractor, the flow of smoke leaves the chambers 19, 21 arranged on the side of the active zone 14 and enters the chambers 18, 20 arranged on the side of the Zone 11 recovery.

As illustrated in Figure 7, each of the chambers 18, 19 in which the strip circulates comprises two openings 22, 23, respectively 24, 25, arranged opposite each other, facing the strip, through which the fumes enter or leave. . In each of the chambers 20, 21 intended to direct the flow of fumes, one of the openings 23, respectively 25 (the one in connection with the chambers 18, 19 where the band circulates), is arranged facing the band and a second opening 26, respectively 27 is arranged at 90 degrees on a side face of said chamber.

The connecting zone 13 comprises two connecting ducts 28, 29 which channel the fumes from the active zone 14 to the recuperative zone 11. The first duct 28 connects the chambers 18 and 21 and the second duct 29 connects the chambers 19 and 20. These sheaths comprise a metal outer casing and an inner lining of refractory materials.

On its upper part, the connecting zone 13 is connected to a chamber 30 in which are placed two deflecting rollers 31, 32 for the path of the strip. Two restrictions 33, 34 limit the circulation of fumes in the chamber 30 so that the latter remains at a moderate temperature suitable for the deflector rollers.

The active zone 14 comprises a plurality of burners 15 according to the invention arranged on its side faces. Its average temperature is around 1350°C. The burners are placed in staggered rows on each side of the oven and in staggered rows on each side of the strip. Thus, the burners are arranged two by two on successive horizontal planes, but the position of the burners is different between two horizontal planes. On a first horizontal plane, a burner is arranged on a side face of the oven and on one side of the strip and the second is arranged on the opposite side face, and on the other side of the strip. We have the inverse in a second horizontal plane close to the first.

The horizontal distance between the axis of the burners and the belt is for example 400 mm. The vertical distance between two burners arranged on the same face of the active zone and on the same side of the strip is for example 750 mm.

The nominal power of a burner is for example 500 kW and is generally between 400 kW and 800 kW. It may be different along the length of the preheating section. However, all the burners often have the same nominal power and they operate in proportional mode to modulate the heat input over the length of the active zone.

The sizing of the burner takes into account various aspects which affect both the capacity of the line (number of tons per hour of steel strip to be heated), the implementation of the flameless combustion mode, the development of the flame desired in the furnace according to the bandwidth and the dimensions of the cross section of the active zone, as well as taking into account the conditions of use of the burner.

As illustrated in FIGS. 8 and 9, for this exemplary embodiment of the invention, the oxidant passes through four ducts 51, 52. For a burner power of 500 kW and air preheated to 600° C., these ducts can have a diameter of 21 mm. They emerge in a small mini-tunnel 53 through holes whose axes are 100 mm apart from the central axis of the burner. The length of the ducts 51, 52 must be at least three times their diameter to correctly establish the air jet at the exhaust of the duct. Hot air velocities are generally between 50 and 300 m/sec, and typically 200 m/sec. The divergent orientation of the vertical jets at 7° spreads the flame. The convergent orientation of the horizontal jets at 3° makes it possible to contract it. The more the divergence is increased, the more there is a risk of deteriorating the level of NOx. By increasing the convergence, there is a risk of disturbing the air flow and therefore of having an unstable flame. The range for which operation is optimum is thus quite narrow, with +/- five degrees for the divergent vertical jets and +/- two degrees for the convergent horizontal jets.

The air holes are grouped in pairs. They must be diametrically opposed along two axes, vertical and horizontal. The pairs of holes do not have to be identical. Greater flame spread will be obtained if the vertical and diverging air holes have a larger diameter. To maintain the same speed at the exhaust of the convergent and divergent oxidizer ducts, the diameter of the horizontal and convergent air holes is reduced in proportion to the increase in that of the vertical and divergent holes.

The outlet of the air jets is set back from the plane of the diffuser by approximately 60 mm. This mini tunnel 53 makes it possible to initiate the mixing of air with the fumes and locally lowers the partial oxygen level. Its diameter is 150 mm, i.e. 1.5 times the diameter on which the exhausts of the air ducts 51, 52 are arranged. Another utility of this tunnel is to improve the stability of the flame when the oven is cold.

Fuel injection is carried out by two conduits 54. The gas jets are diametrically opposed and placed in the upper and lower part on the outside of the diffuser 60 over a diameter of 250 mm. The two ducts 54 converge towards the axis of the burner at an angle of 11°. This feature allows the gas to be mixed with the fumes before being sucked in by the air jets. A similar principle would have been obtained by arranging the ducts 54 horizontally since the gas is sucked in by the air flow. The air/gas meeting point is approximately 30 cm from the diffuser.

The gas injection ducts 54 have a constriction at their end for speeding up the jet, the diameter of which is 15 mm. The exhaust gas velocity here is 50 m/sec for natural gas. It is generally between 20 and 100 m/sec. The gas exhaust orifices are spaced two to four times the distance between the two air exhaust orifices of the same pair, horizontal or vertical. Given the angle of inclination of the injectors, which can go up to 15°, the gas jets should not be spaced too far apart for reasons of space outside the oven.

The gas injection ducts 54 open into a small cavity to protect them from the radiation of the flame and the furnace, the gas speed being ensured by the constriction at the end of the duct.

For cold flame stability, a conventional axial gas rod 55, pierced with three rows of radial holes, is supplied with fuel instead of the two peripheral conduits 54 during the furnace temperature rise phases. As a variant, the axial gas pipe 55 is supplied with an air/gas premix. The fuel flow injected by the axial gas rod represents less than 10% of the overall fuel flow. The goal is to have as intimate a mixture as possible with the air. The tunnel 53 of the diffuser at the level of the air injection stabilizes the combustion. On the other hand, we lose the advantage of flameless operation. For this reason, this mode of operation is only used when the oven has a temperature below 850°C and with a slightly oxidizing combustion setting.

Around the axial gas rod 55 for cold operation, an annular passage 56 of combustion air contributes to proper ignition of the burner and flame stability when cold. This annular passage is supplied with air like the peripheral conduits 51, 52. The combustion air flow in this annular passage is approximately 20% of the total combustion air flow. It is maintained for both burner operating modes, flame mode and flameless mode.

The diffuser can be made of a usual refractory material for this type of application, of the same nature as that of front flame burners according to the state of the art.

Of course, the invention is not limited to the examples which have just been described and many adjustments can be made to these examples without departing from the scope of the invention. In addition, the different features, forms, variants and embodiments of the invention may be associated with each other in various combinations insofar as they are not incompatible or exclusive of each other.

Claims (11)

Direct flame preheating section (1) for a continuous metal strip treatment line (B) comprising a connecting zone (13) provided for circulation of the combustion fumes coming from an active zone (14) equipped with burners ( 15) to a recuperative zone (11) for preheating the strip by exchange with said fumes, characterized in that the burners are capable of operating in so-called flameless mode and in that said connection zone comprises an outlet chamber (19 ) capable of directing the flow of fumes so that they flow frontally with respect to the strip at the outlet of the active zone and an inlet chamber (18) capable of directing the flow of fumes so that they flow frontally with respect to the strip at the entrance to the recovery zone, in the direction of flow of the fumes. Preheating section (1) according to Claim 1, in which the outlet chamber (19) is arranged at the outlet of the active zone (14), in the direction of flow of the fumes and is arranged for a withdrawal of fumes, the inlet chamber (18) is arranged at the inlet of the recuperative zone (11) and is arranged for an injection of fumes, the connecting zone (13) further comprising two turning chambers (20, 21) each arranged to perform a 90 degree turn on the flow of smoke between an inlet opening (26, 25) and an outlet opening (23, 27), a first turning chamber (21) communicating directly with the outlet chamber ( 19) and a second turning chamber (20) communicating directly with the inlet chamber (18), and two connecting tunnels (28, 29) arranged for smoke circulation, a first connecting tunnel (28) directly connecting the outlet opening (27) of the first chamber (21) with an inlet opening (22) of the chamber entrance (18) and a second connecting tunnel (29) directly connecting an exit opening (24) of the exit chamber (19) and the entrance opening (26) of the second chamber (20). Preheating section (1) according to Claim 2, in which the two outlet openings (24, 25) of the outlet chamber (19) are arranged facing each other and frontally with respect to a circulation of the strip in the active zone (14) and the two inlet openings (22, 23) of the inlet chamber (18) are arranged facing each other and frontally with respect to a movement of the tape in the recovery zone ( 11). Preheating section (1) according to one of the preceding claims, the burners (15) are of the direct flame side type, characterized in that said burners are able to operate in flameless mode. Preheating section (1) according to one of the preceding claims, the burners (15) having an axial direction (A) at the intersection of a vertical plane (V) and a horizontal plane (H), and comprising a diffuser (60) traversed by fuel injection ducts (54) for operation in flameless mode and oxidizer injection ducts (51, 52), said oxidizer injection ducts (51, 52) emerging from the diffuser closer to the axis of the burner than said fuel injection ducts (54) for operation in flameless mode, characterized in that the burners have oxidizer injection ducts (52) which open from the diffuser onto the vertical plane which are divergent and ducts (52) for injecting oxidizer which emerge from the diffuser on the horizontal plane which are converging towards the axis of the burner. Preheating section (1) according to Claim 5, characterized in that the ducts (52) for injecting oxidizer from the burners (15) which emerge from the diffuser (60) on the vertical plane (V) are divergent at an angle between 2 and 12 degrees, and preferably seven degrees. Preheating section (1) according to Claim 5, characterized in that the oxidizer injection ducts (51) of the burners (15) which emerge from the diffuser (60) on the horizontal plane (H) diverge at an angle between 1 and 5 degrees, and preferably three degrees. Preheating section (1) according to Claim 5, characterized in that the fuel injection ducts (54) of the burners (15) for operation in flameless mode converge towards the axis of the burner. Preheating section (1) according to Claim 5, characterized in that the fuel injection ducts (54) for operation in flameless mode converge towards the axis of the burner at an angle of between five and fifteen degrees and preferably eleven degrees. Preheating section (1) according to Claim 5, characterized in that the burners (15) have a fuel injection duct (55) for operation in flame mode which extends in the axial direction of the burner and which opens of the diffuser (60) in the axis of the burner. Line for the continuous processing of a metal strip, characterized in that it comprises a direct flame preheating section according to one of the preceding claims.