EP2029951B1 - Method for monitoring lateral burners of a heating furnace - Google Patents

Description

The invention relates to continuous steel reheating furnaces, and more particularly the control of burners of such reheating furnaces.

BACKGROUND OF THE INVENTION

Steel reheating furnaces are used to heat steel semi-finished products such as slabs, billets, blooms, beam-blanks and ingots to a suitable temperature in order to prepare these products for rolling, forging or extrusion operations, or more generally to any other hot forming operation.

Many types of arrangements have already been proposed for the burners fitted to the reheating furnaces. So-called front burners have been proposed, which are arranged in front of recesses for lower level burners, or in front of vault noses for higher level burners. The front burners then form a row of a plurality of burners arranged at the same abscissa of the length of the furnace. The advantage of such arrangements is to implement a substantially homogeneous heat input, but with the disadvantage of high rigidity. Indeed, with the rows of burners arranged in the vaulted nose, the flames heat several steel products at a time, over the whole width of the oven during the progression of these, so that it is practically impossible to have different instructions from one product to another, thus prohibiting any flexibility in the use of the reheating furnace. In addition, the axial length of the vaulted noses or lower recesses corresponds to a length that is lost for heating, which de facto decreases the heating power likely to be implemented for a given length. Finally, the structure of such furnaces is relatively expensive, especially for ovens of great length, with the further risk of a certain fragility of certain parts of the furnace, particularly in the area adjacent to the nose of the vault, which fragility can induce a risk of breakage.

It has also been proposed to use so-called vault burners, which are also arranged in transverse rows by being integrated into the vault of the oven. The advantage over the front burner arrangement is that the zone directly underlying the vault is heated, so that the iron and steel products are heated by radiation, without direct impact of the flames on the products. This makes it possible to improve the homogeneity of the heating, and to obtain an operation all the more satisfactory that the reheating furnace is heavily filled with iron products to be heated. However, the vault burners have several drawbacks, the first of which is the emission of NOx pollutants resulting from the very hot flames emitted by the vault burners. Since the vault burners emit a substantially rotating flame around the burner axis, flame velocities generally remain low, so that some NOx confinement can not be avoided in the upper zone of the reheat furnace. In addition, the use of vault burners is generally done with a common supply of all the burners of the same row or the same group of adjacent rows, so that it is excluded to decouple the arch burners . This lack of flexibility is particularly unfavorable when products of cross-section significantly less than the width of the furnace are introduced into the reheating furnace. Indeed, the small steel products are generally arranged at the vertical vertical plane of the reheating furnace, so that the side areas adjacent to the side walls of the furnace are spaces without products to heat, and therefore create at these spaces areas of overheating that can be dangerous, especially if the temperature in these lateral areas reaches temperatures close to 1600 ° C, because there is then a risk of irreversible vitrification of the refractory material constituting the enclosure of the oven. Finally, the arch burners of the same row induce a thermal field which is essentially distributed in a vertical plane, so that it is excluded to be able to simultaneously heat several steel products.

The aforementioned disadvantages of the front burners and arch burners have led the designers of reheating furnaces to turn to the use of side burners integrated into the side walls of the oven. Indeed, the direct advantage of the side burners is to be able to align the flames on the products, so that it is theoretically possible to heat treat each product with the flame located at the same abscissa at a given moment.

For example, we can refer to the document JP-10 168 514 A , which discloses an upper and lower side burner furnace arranged symmetrically with respect to the median vertical plane of the furnace. The teaching of this document is simply that the lower and upper burners (of the heat storage type) are individually controlled to maintain the temperature prevailing in the vicinity of each burner set at a predetermined value.

Among the side burners, the lower burners, by reference to the circulation level of the iron and steel products in the reheat furnace, however, pose problems in the zones of the furnace where the calamine which falls from the underside of the steel products forms an excessive heap on the oven floor, this heap disrupting the action of the surrounding burners and hindering the flow of fumes.

Such areas occur in particular whenever the steel products undergo a significant thermal and / or mechanical shock in the furnace.

These significant disadvantages in a reheat furnace will be better understood with the aid of the following description of an oven of traditional type with tubular stringers, with reference to Figures 1 to 4 .

The Figures 1 and 2 illustrate a furnace F with tubular longitudinal members, of traditional design, which makes it possible to implement a heating of the steel products circulating in said furnace concerning both the upper and lower faces of each of the products conveyed.

The furnace F comprises a heat insulated enclosure 10 whose upper and lower walls (or hearth) are denoted 11 and 12, and the left and right walls, with reference to the direction of progression of the iron products marked P in the tunnel (direction indicated by the arrow 100), are noted 13 and 14.

The figure 1 which is a section along II of the figure 2 , illustrates various equipment associated with the conveyance of iron and steel products in the tunnel, as well as other external equipment associated with the evacuation of fumes and the control of the process, while the figure 2 , which is a section along II-II of the figure 1 , allows to better distinguish the arrangement of side burners, here provided both at the upper and the lower level by reference to the level of conveyance of P steel products.

The enclosure 10 includes an upstream charging zone 15 and a downstream transfer zone 16, which are equipped with doors not shown here. A charging system 17 causes the products P to be heated in the charging zone 15, and a dewatering system 18 discharges the heated products P at the dewatering zone 16.

The reheating furnace is equipped with conveying means marked 20 which make it possible to advance the steel products P inside the tunnel from upstream to downstream of said tunnel, said steel products being subjected to the action of the lateral burners. to undergo the desired progressive heating within their mass. As this is best seen on the figure 2 the steel products P are supported by movable spars 21 and fixed spars 22 arranged alternately in the longitudinal direction of the furnace. The movable spars 21 have pins 23 which pass through the bottom wall 12 of the furnace and are fixed on a sole 25 resting on a frame 26. The fixed spars 22 have bowling pins 24 which are anchored in the lower wall 12 of the oven enclosure. As this is best seen on the figure 1 , the frame 26 is equipped with a plurality of rollers 27, 28, respectively interposed between the frame 26 and the sole 25, and between the frame 26 and supports 29 with an inclined plane 29.1 The upper rollers 27 are driven by means not represented here. When the frame 26 is pushed by means of cylinders not shown here at its upstream end (as shown by the arrow), the lower rollers 28 roll on the inclined planes 29.1 of the supports 29, which induces a lifting of the sole 25 and movable spars 21, and consequently an uprising of steel products P which then rest only on said movable spars 21 being disengaged from the fixed spars 22. The rollers 27 are then driven and can then make a shift of one not in the longitudinal direction of the furnace of the movable assembly and raised and therefore the steel products supported by it, after which the rollers 27 are stopped and the frame is unlocked, so that the rollers 28 back down on the inclined planes 29.1 supports 29, for a new axial position of all steel products. Such a step-by-step conveying process, according to a square or rectangular cycle, is well known in the field of steel furnaces.

With regard to the external equipment of the reheating furnace, an upstream chimney 31 for evacuation of flue gases has been illustrated, the outlet of which is connected to a network 32 for evacuating the smoke from combustion, these fumes being here discharged to energy recovery means, with for example a heat exchanger 33 transmitting the energy of the fumes, then discharged through a chimney 34, to the combustion air supplied by a fan 35 through the exchanger The process control is provided from a control center 38 which is connected in particular by a line 37 to different thermo-couples 36 integrated in the vault of the oven chamber, which are used to monitor the temperature. at different abscissae of the furnace in order to comply with a heating curve which is predetermined in function of the steel products concerned.

The side burners equipping furnace F allow to heat both the upper face and the lower face P steel products passing at the flame of said burners. For a better identification of the different side burners equipping the oven, use a two-letter notation followed by a number, the first letter being G for a burner associated with the left side wall or D for a burner associated with the right side wall , and the second letter being S for a side burner associated with the upper level or I for a side burner associated with the lower level, the corresponding number for its index 1, 2, ... k, ... n of the row of burners substantially disposed in a common vertical plane, from upstream to downstream of the furnace. In the present case, there are seven rows of lower and upper burners, but it is naturally only an example, to the extent that we can provide a different number of burners, or provide two or three burners by bowling in certain areas of the oven. With this notation, the cup of the figure 2 This makes it possible to distinguish four lateral burners, with the upper and lower burners GS2 and GI2 on the left, and the upper and lower burners DS2 and DI2 on the right.

In certain areas of the furnace F, particularly where the steel products are likely to undergo a thermal shock and / or mechanical important, there is an accumulation of calamine falling from the underside of the steel products.

This phenomenon, well known to those skilled in the art, has been particularly noted in furnaces having at least one zone where the tubular stringers are interrupted and offset laterally at this interruption, with in general an interlocking of the ends concerned in the manner of a comb.

Such a staggered arrangement is illustrated on the figure 3 which is a section of the furnace F by a horizontal plane, on which only the movable spars 21 have been shown for the sake of clarity. We distinguish on the figure 3 a Z zone schematically in dotted line corresponding to the interruption zone of the mobile spars, with a nesting of the downstream ends of the upstream spars 21 between the upstream ends of the downstream spars 21 '. This is interesting for equalizing the temperature over the length of the steel products: in fact, the heated steel products P have alternating superheated zones (at the support on the side members) and underheated zones (between the side members) so that the staggered arrangement allows the transition to warm up the under-heated areas and warm up the overheated areas. We then obtain a temperature profile corresponding to the curve T represented on the figure 3 , whose amplitude is greatly reduced, which reflects a better homogeneity of the temperature in the products P.

A zone Z of accumulation of scale can also be found in other parts of the furnace, rather in the downstream half of the furnace, in particular in the zone of de-ironing of the heated steel products.

The figure 4 , which is a section along IV-IV of the figure 3 , therefore at the level of the lateral offset zone Z between the longitudinal members 21, 21 'supporting the steel products P, illustrates the formation of a heap noted C of the calamine which fell from the lower face of the products P on the hearth 12 F. oven

The upper side burners GSk, DSk are shown in dotted lines because they are not directly concerned. On the other hand, the lower side burners GIk, DIk are themselves directly concerned. Indeed, as and. as the level of the scale stack C rises, the action of the lower burners GIk, DIk, and also, to a lesser extent, that of the lower burners which are directly adjacent to them, is disturbed, and the zone central parts P is less well heated. The circulation of fumes is also hampered by the obstacle that is the accumulation of calamine C.

As a result, it is necessary to perform a periodic cleaning of the oven. In addition, the campaign time between two descaling operations is de facto shortened, because it is necessary to stop the oven for about eight days (with three days to cool the oven, and three days to warm the oven).

OBJECT OF THE INVENTION

The invention aims to design a method of controlling side burners of a steel reheating furnace to overcome the aforementioned drawbacks, in particular to combat the formation of an excessive stack of scale on the oven floor.

The invention also aims to design a method for controlling such side burners that takes into account the heat demand, it being for example very important when restarting the oven.

GENERAL DEFINITION OF THE INVENTION

The aforementioned technical problem is solved according to the invention by a method of side burners of a steel reheating furnace, the furnace comprising a plurality of side burners arranged to heat iron products circulating from one end to the other of said oven, characterized in that the piloting relates to the first and second burners being substantially opposite at least one pair of lower side burners, with reference to the level of circulation of steel products in the furnace, in a zone Z of the furnace corresponding to an accumulation of calamine falling from said products on the hearth of the furnace, the flame of said first and second burners being directed towards said accumulation of scale, and the piloting is such that the first and second burners are preferably alternately ignited, so that the alternation of the mechanical action exerted by their flame favors the spreading of the scale on the hearth of the furnace, said first and second burners being for this purpose sized to developing a heating power higher than that of other side burners which only serve to heat the steel products circulating in the oven, according to claim 1.

Preferably, the piloting of the first and second burners is independent of the control of the other side burners of the oven, and is only engaged when the accumulation of calamine on the hearth of the furnace induces a disturbance considered excessive, on the action burners adjacent to zone Z.

The Z zone mentioned above may be a lateral offset zone between longitudinal members supporting the steel products, or a zone for turning the oven off.

According to a particularly advantageous characteristic, the piloting of the first and second burners is in impulse mode and depends on the heat demand of the pair of burners concerned, with, when said heating demand exceeds a predetermined threshold, a determined time of recovery during which the first and second burners of said pair are both turned on.

Preferably, when the heat demand is lower than said predetermined threshold, the extinction of the second ignited burner of the pair concerned coincides substantially with the end of the cycle time between two successive ignitions of the first burner, so as to minimize the recovery time during which the first and second burners are both on.

Advantageously, when the heat demand is below said predetermined threshold, the first and second burners of the pair concerned are ignited exclusively alternately during the entire cycle time.

In particular then, when the heat demand is lower than said predetermined threshold, the first and second burners of the pair concerned are extinguished for a predetermined time after the extinction of the first ignited burner and before the ignition of the second burner extinguished. It can then be provided that the first and second burners of the pair concerned are extinguished for another determined time, or that the extinction of the second ignited burner of the pair concerned coincides substantially with the end of the cycle time between two successive ignitions of the first burner.

Finally, finally, it can be provided that the predetermined threshold associated with the heat demand is of the order of 50%.

Other features and advantages of the invention will emerge more clearly in the light of the description which follows, with reference to Figures 5 to 11 attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

• the figure 1 is a longitudinal section of a traditional tubular string heater, and the figure 2 is a cross-section along line II-II of the figure 1 this tubular string oven;
• the figure 3 is a top view of the oven in longitudinal section illustrating a zone Z lateral shift of spars, which is a preferred area for the calamine drop on the oven floor;
• the figure 4 is a section IV-IV of the figure 3 showing the accumulation of calamine in the central part of the aforementioned zone Z;
• the Figures 5 and 6 are sectional views similar to that of the figure 4 , illustrating the control method according to the invention of the relevant pair of lower side burners, with the alternating mechanical action of the flames on the accumulation of scale;
• the Figures 7 and 8 are diagrams illustrating the piloting of lower burners associated with descaling, respectively in the case of a low heat demand and a high heat demand;
• the figure 9 illustrates a variant of piloting in case of high heat demand;
• the Figures 10 and 11 finally illustrate two other variants of operation in case of low heat demand.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

We will now describe in more detail the pulse mode control method of the lower side burners associated with the descaling action according to the invention.

In its most general definition, the control method of the lower side burners associated with the descaling action according to the invention.

In its most general definition, the control method concerns the first and second burners essentially facing at least one pair of lower side burners in a zone Z of the corresponding furnace. an accumulation of calamine C falling from iron and steel products on the furnace floor, the flame of said first and second burners being directed towards said accumulation of scale, and the piloting is such that the first and second burners are switched on preferentially alternately, so that the alternation of the mechanical action exerted by their flame favors the spreading of calamine on the hearth of the furnace.

On the Figures 5 and 6 Thus, such an alternating action of the descaling burners has been illustrated in accordance with an essential characteristic of the invention.

In figure 5 , the first burner GIk is lit, and the second burner DIk is off. The flame of the burner GIk is directed on the accumulation of calamine C, and the mechanical action of this flame acts directly on a CG zone of the scale stack.

In figure 6 , the second burner DIk is lit, and the first burner GIk is off. The flame of the burner DIk is directed on the accumulation of calamine C, and the mechanical action of this flame acts directly on a CD zone of the scale stack.

It is therefore understood that the alternation of the ignition of the burners GIk, DIk favors the spreading of the scale on the hearth 12 of the oven. The optional GSk, DSk upper burners can be controlled continuously or alternately depending on the needs.

The descaling burners may be part of the reheat burners, with then a dedicated control, or will be burners exclusively reserved for descaling, with then a control that is independent of the control of the other side burners of the oven. The advantage of using burners specially dedicated to descaling is that we can provide burners sized to develop a higher heating power than other side burners that only serve to heat the iron products circulating in the oven. It will of course be possible to use more than one pair of burners dedicated to descaling, especially in areas with a high calamine concentration.

Preferably, the piloting of the descaling burners is only triggered when the accumulation of scale on the hearth of the furnace induces a disturbance considered excessive on the action of the burners adjacent to the zone Z.

As indicated above, the so-called zone Z zone may be another zone of the furnace, in particular the zone of disruption where the iron and steel products always undergo a thermal or mechanical shock more or less important.

It is also interesting to provide that the control of the descaling burners GIk, DIk is in pulse mode, that is to say in all or nothing, and function of the heat demand of the pair of burners concerned.

It is recalled that the "heat demand" corresponds to the ratio between the ignition time of a burner and the cycle time, which cycle time corresponds to the duration between two successive ignitions of the same burner, for example the first burner of the concerned burner pair.

The control according to the heat demand makes it possible to take into account particular situations, for example a restart of the furnace for which the calorific demand is very high, or in the event of accumulations of calamine which are variable because of the warming of products iron and steel different.

Preferably, when the heat demand exceeds a predetermined threshold, the piloting of the descaling burners imposes a lap time Δt during which the first and second burners of the pair concerned are both switched on.

We will now refer to Figures 7 to 11 which illustrate various embodiments of such an impulse mode control method, with alternating, descaling burners.

On the diagrams of Figures 7 to 11 , there is illustrated an operation in all or nothing according to the pulse mode, the burner (s) being either on or off, the ignition corresponding to a constant ordinate for a determined duration and the extinction at a zero ordinate. For the burner or burners on the left the heating power is noted Qg, while for the burner or burners on the right the heating power is noted Qd. The maximum time interval represented, limited by a dotted vertical line, corresponds to the duration of the cycle time, that is to say to the time interval between two successive ignitions of the first burner or burners, in the kind of burners on the left.

In the present description, the term "low demand" means a heat demand (DC) which is below a predetermined threshold, for example of the order of 50%. Beyond this predetermined threshold, it will be considered that the heat demand (DC) is called "strong".

On the figure 7 the situation is that of a low heat demand (DC), so that the hatched rectangles corresponding to the ignition of the left or right burners are of short length.

Preferably, the control will then be chosen in such a way that the first and second burners of the or of each upstream pair concerned are ignited exclusively. alternately during the entire cycle time. There is then a time interval noted Δt ₁ during which the first and second burners of each pair are extinguished. The interval Δt ₁ , which is more or less long, thus separates the extinction of the first burners ignited from the ignition of the second or burners extinguished. We also see on the figure 7 that the extinction of the second or burners ignited from the or each upstream pair concerned substantially coincides with the end of the cycle time between two successive ignitions of the first burner (Δt ₂ = 0). This is only a special case, and we have illustrated on the figure 10 a variant in which, while having an ignition exclusively alternately during the entire cycle time, there is a time interval noted Δt ₂ during which the first and second burners of the or each upstream pair are extinguished after extinction the second burner is on.

The exclusive alternation which has just been described for the case of a low heat demand operation appears to be the most advantageous from the simulations carried out by the Applicant. The invention is however not limited to such a control mode, and it can be provided alternatively a recovery time Δt during which the first or the first and the second burners are all on. Such a variant is illustrated on the figure 11 . It is easy to understand that this variant is a priori slightly less efficient than driving according to the figure 7 or the figure 10 , in that the recovery time Δt corresponds to a simultaneous symmetrical action of the flames of the two burners, which "breaks" minus the accumulation of scale, so that recovery time Δt should in practice be reduced to a minimum .

The figure 8 corresponds to a mode of operation in case of high heat demand (DC), for example greater than 50%.

The length of each gray rectangle corresponding to a lighting period of the burner or burners left or right is then longer than previously. In this case, and according to a characteristic of the invention, there is a lap time Δt during which the first and second burners of the or each upstream pair concerned are both switched on.

The simulations carried out by the applicant tend to show that the best results are actually obtained by minimizing the above-mentioned recovery interval Δt. This optimization corresponds to the diagram of the figure 8 , for which the extinction of the second ignited burner of the or each upstream pair concerned substantially coincides with the end of the cycle time. This corresponds to a hypothesis where the time interval Δt ₂ is zero.

Naturally, it will be possible alternatively to provide a non-zero time interval Δt ₂ , that is to say a time interval during which the first and second burners of the or each upstream pair concerned are extinguished after the extinction of the second burner. alight. This hypothesis is illustrated on the figure 9 . In this case, however, the higher the heat demand, the greater the recovery interval Δt, and then we will tend to lose more and more the advantages of alternation due to excessive recovery.

As a result, the diagrams of Figures 7 and 8 , respectively corresponding to a low and high calorific demand (DC) operation, illustrate a control which is considered optimal in view of the simulations carried out by the applicant.

It has thus been possible to design a control mode of the lower side burners associated with one or more areas of the oven with a high accumulation of scale on the sole of said oven which favors the spreading of the scale on said sole, and that according to the needs , while maintaining the maximum flexibility for the operation of the oven.

In addition to the direct benefits relating to the reduction of disturbances induced by an excessive build-up of scale on the furnace hearth and the improvement of the flue gas flow, the invention provides a significant increase in the campaign times between two descales of the furnace. reheating furnace, because calamine takes longer to reach the maximum permissible filling level.

Claims (11)

1. A method of controlling lateral burners of a steelmaking heating furnace, the furnace including a plurality of lateral burners arranged to heat steelmaking products traveling from one end of said furnace to the other, the method being characterized in that the control is applied to the essentially facing first and second burners (GIk, DIk) of at least one pair of lateral burners that are low relative to the travel level of the steelmaking products in the furnace, in a zone (Z) of the furnace that corresponds to an accumulation of scale dropping from said products onto the furnace bottom, the flames of said first and second burners being directed towards said accumulation of scale (C), and the control being such that the first and second burners (GIk, DIk) are preferably ignited in alternation, so that the alternation of the mechanical action exerted by their flames encourages spreading of the scale over the furnace bottom, said first and second burners being dimensioned for this purpose so as to develop heating power that is greater than the heating power of the other lateral burners that serve only to heat the steelmaking products traveling in the furnace.
2. A method according to claim 1, characterized in that the control of the first and second burners (GIk, DIk) is independent of the control of the other lateral burners of the furnace, and is engaged only when the accumulation of scale on the furnace bottom leads to disturbance that is considered to be excessive on the action of the burners adjacent to the zone (Z).
3. A method according to claim 1 or claim 2, characterized in that the zone (Z) is a lateral offset zone between length members (21, 21') supporting the steelmaking products.
4. A method according to claim 1 or claim 2, characterized in that the zone (Z) is a drop zone of the furnace.
5. A method according to claim 1 or claim 2, characterized in that the first and second burners (GIk, DIk) are controlled in pulsed mode and as a function of the demand for heat from the pair of burners concerned, and when said demand for heat exceeds a predetermined threshold there is provided an overlap time (Δt) during which the first and second burners of said pair are both ignited.
6. A method according to claim 5, characterized in that when the demand for heat is less than said predetermined threshold, extinction of the second ignited burner (DIk). of the pair in question coincides substantially with the end of the cycle time between two successive ignitions of the first burner (GIk), so as to minimize the overlap time (Δt) during which the first and second burners (GIk; DIk) are both ignited.
7. A method according to claim 5 or claim 6, characterized in that, when the demand for heat is less than said predetermined threshold, the first and second burners (GIk; DIk) of the pair in question are ignited exclusively in alternation throughout the entire cycle time.
8. A method according to claim 7, characterized in that, when the demand for heat is less than said predetermined threshold, the first and second burners (GIk; DIk) of the pair in question are both extinguished during a time (Δt₁) after extinguishing the ignited first burner (GIk) and before igniting the extinguished second burner (DIk).
9. A method according to claim 8, characterized in that, when the demand for heat is less than said predetermined threshold, the first and second burners (GIk; DIk) of the pair in question are extinguished for a time (Δt₂).
10. A method according to claim 8, characterized in that, when the demand for heat is less than said predetermined threshold, extinction of the ignited second burner (DIk) of the pair in question coincides substantially with the end of the cycle time between two successive ignitions of the first burner (GIk).
11. A method according to any one of claims 5 to 10, characterized in that the predetermined threshold associated with the heat demand is about 50%.

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