DIRECT FLAME PREHEATING SECTION FOR A CONTINUOUS METAL STRIP PROCESSING LINE

Abstract

Direct flame preheating section for continuous metal strip processing lines, comprising a connecting zone between an active zone provided with burners capable of operating in “no flame” mode and a recuperative zone for preheating the strip by exchange with combustion fumes originating from the active zone, the connecting zone having chambers capable of orienting the flow of fumes such that they flow head-on relative to the strip when exiting the active zone and entering the recuperative zone depending on the direction of flow of the fumes.

Claims

1. A direct flame preheating section for a continuous metal strip processing line comprising a connecting zone provided for circulating the combustion fumes coming from an active zone equipped with burners to a recuperative zone for preheating the strip by exchange with said fumes, characterized in that the burners are capable of operating in “no flame” mode and in that said connecting zone comprises an outlet chamber capable of orienting the flow of the fumes so that they flow head-on relative to the strip at the outlet of the active zone and an inlet chamber capable of orienting the flow of the fumes so that they flow head-on relative to the strip at the inlet of the recuperative zone, depending on the direction of flow of the fumes.

2. The pre-heating section according to claim 1, wherein the outlet chamber is arranged at the outlet of the active zone, in the direction of flow of the fumes, and is arranged for drawing off fumes, the inlet chamber is arranged at the inlet of the recuperative zone and is arranged for injecting fumes, the connecting zone further comprising two turn chambers each arranged to make the flow of the fumes turn 90 degrees between an inlet opening (26, 25) and an outlet opening, a first turn chamber communicating directly with the outlet chamber and a second turn chamber communicating directly with the inlet chamber, and two connecting tunnels arranged for circulating the fumes, a first connecting tunnel directly connecting the outlet 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 chamber.

3. The preheating section according to claim 2, wherein the two outlet openings of the outlet chamber are arranged opposite and head-on relative to a circulation of the strip in the active zone and the two inlet openings of the inlet chamber are arranged opposite and head-on relative to a circulation of the strip in the recuperative zone.

4. The preheating section according to claim 1, the burners being of the side, direct flame type, characterized in that said burners are capable of operating in no flame mode.

5. The preheating section according to claim 1, the burners having an axial direction (A) at the intersection of a vertical plane and a horizontal plane, and comprising a diffuser traversed by fuel injection ducts for operation in no flame mode and oxidizer injection ducts, said oxidizer injection ducts emerging from the diffuser closer to the burner axis than said fuel injection ducts for operation in no flame mode, characterized in that the burners have oxidizer injection ducts that emerge from the diffuser on the vertical plane and that are divergent and oxidizer injection ducts that emerge from the diffuser on the horizontal plane and that are convergent toward the burner axis.

6. The preheating section according to claim 5, wherein the vertical plane is parallel to the strip.

7. The preheating section according to claim 5, wherein the oxidizer injection ducts of the burners that emerge from the diffuser on the vertical plane are divergent at an angle of between 2 and 12 degrees, and preferably of seven degrees.

8. The preheating section according to claim 5, wherein the oxidizer injection ducts of the burners that emerge from the diffuser on the horizontal plane are convergent at an angle of between 1 and 5 degrees, and preferably of three degrees.

9. The preheating section according to claim 5, wherein the fuel injection ducts of the burners for operation in no flame mode are convergent toward the burner axis.

10. The preheating section according to claim 5, wherein the fuel injection ducts for operation in no flame mode are convergent toward the burner axis at an angle of between five and fifteen degrees and preferably of eleven degrees.

11. The preheating section according to claim 5, wherein the burners have a fuel injection duct for operation in flame mode that extends in the axial direction of the burner and that emerges from the diffuser in the burner axis.

12. A continuous metal strip processing line, comprising a direct flame preheating section according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Other features and advantages of the invention will become apparent on reading the following detailed description, for the understanding of which reference will be made to the appended drawings, in which:

[0079] FIG. 1 is a schematic overview of a galvanizing line with a direct flame preheating section according to the prior art,

[0080] FIG. 2 is an enlargement of the pre-heating section of FIG. 1,

[0081] FIG. 3 is a schematic top and sectional view of the preheating section according to FIG. 2,

[0082] FIG. 4 is a schematic top and sectional view of a pre-heating section according to a second example of the prior art,

[0083] FIG. 5 is a schematic top and sectional view of a pre-heating section according to a third example of the prior art.

[0084] FIG. 6 is a schematic view similar to FIG. 2, but for a direct flame preheating section according to one embodiment of the invention,

[0085] FIG. 7 is a schematic top and sectional view of the preheating section similar to those of FIGS. 3 to 5, but for the preheating section according to FIG. 6,

[0086] FIG. 8 is a schematic front view of the diffuser of a burner according to one embodiment of the invention,

[0087] FIG. 9 is a schematic sectional and three-dimensional view of one half of the diffuser according to FIG. 8,

[0088] FIG. 10 is a schematic side view showing the frontal shape of the flame with a burner operating in flame mode according to the prior art, for a vertical pre-heating section,

[0089] FIG. 11 is a schematic view showing the frontal shape of the flame with a burner operating in no flame mode according to the prior art, again for a vertical pre-heating section,

[0090] FIG. 12 is a schematic view showing the frontal shape of the flame with a burner according to the invention operating in no flame mode, again for a vertical pre-heating section.

DETAILED DESCRIPTION OF THE INVENTION

[0091] Since the embodiments described below are in no way limiting, it will in particular be possible to consider variants of the invention comprising only a selection of the features described, subsequently isolated from the other features described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection comprises at least one feature, preferably functional, without structural details, or with only a portion of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0092] In the rest of the description, elements having an identical structure or similar functions will be designated by the same reference signs.

[0093] Referring to the diagram of FIG. 6 of the appended drawings, a schematic view of a direct flame preheating section according to the invention can be seen. A connecting zone 13 ensures the fluidic connection between the recuperative zone 11 and the active zone 14 equipped with side burners 15.

[0094] The nature of the connecting zone 13 is similar to that of the active and recuperative zones in that it comprises a metal outer shell and an inner lining made of refractory materials.

[0095] The connecting zone 13 comprises two chambers 18, 19 in which the strip circulates, the chamber 18 at the inlet of the recuperative zone 11, in the direction of flow of the fumes, for the rising branch and the chamber 19 at the outlet of the active zone for the descending branch.

[0096] The connecting zone 13 also comprises two other chambers 20, 21 intended to orient the flow of the fumes facing the strip by making them perform a 90 degrees turn, the chamber 20 on the rising branch side and the chamber 21 on the descending branch side. They are arranged in the central part of the connecting zone, between the rising branch and the descending branch of the strip.

[0097] Due to the suction carried out by the fume exhauster, the flow of the fumes is exiting into the chambers 19, 21 arranged on the side of the active zone 14 and it is entering the chambers 18, 20 arranged on the side of the recuperative zone 11.

[0098] As shown in FIG. 7, each of the chambers 18, 19 in which the strip circulates comprises two openings 22, 23, respectively 24, 25, positioned opposite one another, facing the strip, through which the fumes enter or leave. In each of the chambers 20, 21 intended for orienting the flow of the fumes, one of the openings 23, respectively 25 (the one in connection with the chambers 18, 19 where the strip circulates) is arranged facing the strip and a second opening 26, respectively 27 is arranged at 90 degrees on a side face of said chamber.

[0099] The connecting zone 13 comprises two connecting ducts 28, 29 that 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 ducts comprise a metal outer shell and an inner lining made of refractory materials.

[0100] In its upper part, the connecting zone 13 is connected to a chamber 30 in which two deflector rollers 31, 32 are placed for the path of the strip. Two narrowed areas 33, 34 limit the circulation of the fumes in the chamber 30 so that the latter remains at a moderate temperature suited to the deflector rollers.

[0101] The active zone 14 comprises a plurality of burners 15 according to the invention arranged on the side faces thereof. Its average temperature is approximately 1350° C. The burners are staggered on each side of the furnace and staggered 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. In a first horizontal plane, a burner is arranged on one side face of the furnace 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. The reverse is true in a second horizontal plane adjacent to the first.

[0102] The horizontal distance between the axis of the burners and the strip 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.

[0103] The nominal power of a burner is for example 500 kW and is generally between 400 kW and 800 kW. It may be different over 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.

[0104] The dimensioning of the burner takes into account different aspects that affect both the capacity of the line (number of tons per hour of steel strip to be reheated), the use of the no flame combustion mode, the development of the flame desired in the furnace according to the strip width and the dimensions of the cross section of the active zone, as well as taking into account the conditions of use of the burner.

[0105] As shown in FIGS. 8 and 9, for this embodiment of the invention, the oxidizer passes through four ducts 51, 52. For a power of 500 kW of the burner and air preheated to 600° C., these ducts may have a diameter of 21 mm. They emerge in a small mini-tunnel 53 through holes whose axes are separated from the central axis of the burner by 100 mm. The length of the ducts 51, 52 must be at least three times their diameter in order to correctly establish the air jet at the outlet of the duct. Hot air speeds are generally between 50 and 300 m/sec, and typically 200 m/sec. The diverging orientation of the vertical jets at 7° makes it possible to spread the flame. The convergent orientation of the horizontal jets at 3° makes it possible to contract it. The greater the divergence, the greater the risk of worsening the NOx level. By increasing the convergence, there is a risk of disrupting the airflow and therefore of having an unstable flame. The range for which the operation is optimum is thus quite narrow, with +/— five degrees for the divergent vertical jets and +/−two degrees for the convergent horizontal jets.

[0106] The air holes are grouped in pairs. They must be diametrically opposite along two axes, vertical and horizontal. It is not necessary that the pairs of holes be identical. A greater spread of the flame will be obtained if the vertical and divergent air holes have a larger diameter. To maintain the same speed at the outlet of convergent and divergent oxidizer ducts, the diameter of the horizontal and convergent air holes is reduced in proportion to the increase in the diameter of the vertical and divergent holes.

[0107] The outlet of the air jets is set back relative to the diffuser plane by approximately 60 mm. This mini-tunnel 53 makes it possible to initiate the mixture of the air with the fumes and locally lowers the partial oxygen level. Its diameter is 150 mm or 1.5 times the diameter on which the outlets of the air ducts 51, 52 are arranged. Another usefulness of this tunnel is to improve the stability of the flame when the furnace is cold.

[0108] The fuel is injected through two ducts 54. The gas jets are diametrically opposite 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 are convergent toward the axis of the burner at an angle of 11°. This feature allows the gas to be mixed with the fumes before being aspirated by the air jets. A similar principle would have been obtained by arranging the ducts 54 horizontally since the gas is aspirated by the air flow. The air/gas meeting point is approximately 30 cm from the diffuser.

[0109] The gas injection ducts 54 have a recess at their end for the speed setting of the jet, the diameter of which is 15 mm. The gas speed at the outlet is here 50 m/sec for natural gas. It is generally between 20 and 100 m/sec. The gas outlet orifices are separated by two to four times the distance between the two air outlet orifices of the same pair, horizontal or vertical. Given the inclination angle of the injectors, which can range up to 15°, the gas jets should not be too far apart due to the space requirement outside the furnace.

[0110] The gas injection ducts 54 emerge into a small cavity making it possible to protect them from the radiation of the flame and the furnace, the gas speed being produced by the recess at the end of the duct.

[0111] For cold flame stability, a conventional axial gas pipe 55, pierced with three rows of radial holes, is supplied with fuel instead of the two peripheral ducts 54 during the temperature rise phases of the furnace. As a variant, the axial gas pipe 55 is supplied with air/gas premix. The flow rate of fuel injected by the axial gas pipe represents less than 10% of the overall fuel flow rate. The aim is to have the closest possible mixture with air. The tunnel 53 of the diffuser at the air injection allows the combustion to be stabilized. However, the advantage of no flame operation will be lost. Therefore, this mode of operation is only used when the furnace has a temperature below 850° C. and with a slightly oxidizing combustion setting.

[0112] Around the axial pipe 55 of gas for cold operation, an annular combustion air passage 56 contributes to the correct ignition of the burner and to the cold flame stability. This annular passage is supplied with air like the peripheral ducts 51, 52. The combustion air flow rate in this annular passage is approximately 20% of the total combustion air flow rate. It is maintained for the two modes of operation of the burner, in flame mode and in no flame mode.

[0113] The diffuser may be made of a common refractory material for this type of application, of the same nature as that of flame-resistant burners according to the prior art.

[0114] Of course, the invention is not limited to the examples that have just been described and numerous modifications 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 one another in various combinations insofar as they are not incompatible or exclusive of one another.