FR2847969A1 - Combustion of fuel and a combustive agent injected through furnace wall comprises regulation of injection speeds to create flame extending above and/or below furnace charge - Google Patents

Abstract

The combustion of a fuel and a combustive agent injected through a lateral wall of a furnace containing a charge to be heated which is displaced in an essentially perpendicular plane to the wall generates a flame extending above and/or below the charge. The speeds of injection of the fuel and of the combustive agent are regulated in a manner to obtain a heating of the charge by the flame thus created over a heating length, measured from the wall and perpendicular to it, as a function of these speeds.

Description

Method of heating a continuous oven

  The present invention relates to a combustion process in a furnace, preferably continuous and in particular a furnace for reheating steel products or a furnace for secondary glass melting or a furnace for melting non-ferrous metals. It relates more particularly to a method of combustion of a fuel and of an oxidizer injected through a wall, in particular a side wall, of an oven comprising a charge to be heated which moves in a plane substantially perpendicular to said side wall, this combustion generating a flame extending above and / or below said charge (or laterally, optionally on either side of the product if it moves in a direction having a vertical component. profession will adapt the invention by itself in this case by giving the words used the desired meaning).

  Reheating furnaces in the steel industry, for example, are used to reheat steel products from, for example, continuous casting and bring them to the rolling temperature (1100 to 1300 ° C). The reheated products are, for example, billets, blanks or slabs. It can also be flat products which are reheated before passing through a heat treatment oven and in particular sheets before annealing-pickling or galvanizing. Whatever oven is considered, it is generally sought to heat the charge contained in this oven so as to ensure the most uniform temperature possible in the product.

  In furnaces for heating steel products, it is thus sought to ensure homogeneous heating of the whole product, and in particular the part of the product located at the heart thereof. On some billet, bloom or slab ovens, the width of the oven is at least 7m, up to 15m, making it difficult to heat the center of the oven.

  This problem is all the more difficult to solve if the products such as (6) and (7) are placed in staggered rows as illustrated in FIG. 1. It is then necessary to bring more energy to the center of the oven where the mass flow rate of the product is more important than on the sides near the walls of the oven.

  One solution chosen by the oven manufacturers consists for example of installing niches (5) which cover the entire width of the oven and which are equipped with burners (2) whose flame develops along an axis parallel to the axis (8) flow of smoke, in the same direction or in the opposite direction thereof (against the current; it is for this purpose that arrow 8 in Figure 1 indicates the two possible directions of flow).

  This solution is illustrated in Figure 2 (sectional view in a vertical plane) where a recess (15) and front burners (2) are shown. This solution significantly increases the investment cost of the installation, whether for a new oven or for an existing oven to which we want to add burners (case of a repair of an oven also called "revamping" ").

  Another solution consists in heating the center of the product with burners placed on the transverse walls (3) as illustrated in Figure 1. The initial axis 10 of injection of fluids (10) of the burner (3) makes an angle a with the axis (9) perpendicular to the vertical plane which corresponds to the main smoke flow. a is between -80 and + 80, generally between -20 and +20. Preferably, this angle a is substantially equal to 0 degrees. The initial axis of injection of the burner fluids is then perpendicular to the vertical plane containing the axis of the main flow.

  In practice, according to the invention it has been found that whatever the value of oc, it is necessary to give a sufficient boost to the fluids ejected from the burner. Indeed, if the impulse is too weak, we see that the flame (4) is strongly deflected and develops near the wall. Such a flame (4) risks damaging the wall of the oven considerably and must therefore be avoided at all costs.

  To verify on an existing installation that the pulse of the burner is sufficient, the skilled person can for example visualize the flame by an optical access on the furnace (door or other). This verification can only be done once the burner is installed, in operation, on a given oven and in a given total power configuration. A satisfactory setting on a given oven and / or for a given power of the burner and / or for a given power of the burners upstream of the burner concerned (upstream relative to the direction of the flue gas flow), may prove to be completely inappropriate if one of the conditions where the verification was made is no longer fulfilled (either a change in the power of the burner or the power of the burners located upstream of the burner concerned). Finally, this check must be made individually for each of the burners concerned, these can, for example, be adjusted to individual powers different from one burner to another. This shows in particular that no adjustment can be transposed from one oven to another and that the whole operation must be repeated for each oven and / or each type of load, with the need to provide optical access for each burner, which is often a constraint

unacceptable.

  In all these operations, an incorrect setting of the oven can have very important consequences for the user customer, since there is a risk of significant damage to the side wall 11 or 12 of the oven by the flame 4, (FIG. 1).

  The present invention proposes to provide a method for adjusting the pulse of the fluids of such a burner located on a wall in particular a lateral wall of an oven, pulse 10 which can be different for each burner, without the need for optical access for check the flame setting. This process is applicable whatever the geometry of the furnace, the geometry of the burner and the power upstream of the burner (relative to the flue gas flow). According to the invention, the impulse of these burners is calculated relative to the speed of the main flow. (The fumes that pass through the oven, often against the flow of 15 advancing the load).

  In particular, this process is applicable for burners whose oxidant is air, or air enriched with oxygen, preferably containing more than 90% vol. oxygen, in particular oxygen supplied by VSA type adsorption apparatuses and in particular oxygen containing a complement of 1 to 5% of Argon and from 1 to 5% of nitrogen.

  The method according to the invention is characterized in that the injection speed Vfiluide of at least one of the oxidizing fluids and / or fuel injected into at least one burner is greater than or equal to approximately: Vfluid> trans (CoS (a ) Lchauf) 1I Porans 0.5 V - 2 DP fluid formula in which: * VraNs designates the flow speed of the fumes in the oven (in general, against the direction of advancement of the parts to be treated in the oven .

  * -c the angle made by the axis of injection of fluids at the outlet of the burner relative to the axis perpendicular to the side walls of the furnace.

  * The, (f denotes the width of the oven (measured from the wall of the oven where the burner concerned is installed) on which we want to obtain optimal heating as defined above.

  * Pra.s represents the density of the fumes in the oven.

  e * Pjtide represents the density of the fluids considered (oxidizer or fuel). D represents the diameter of the burner (or of the circle of equal surface when the burner does not have a circular section) in the case of a burner with coaxial injection of oxidizer and fuel in its equivalent diameter, that is ie the diameter of the circle 10, the surface of which is equal to the sum of the surfaces of the fuel injection and oxidant injection openings in the case of separate injections.

  Preferably, we will choose Vfuel Vtrans ((CS) Lchauf), (Ptrans) 0.5 V Vtrans ((CS) Lchauf) l, (Ptrans) 0.5 VfIIII (os a = COUf'1 (Ptans) O5 and Vcomb = 2 con 2 D Pfe 2 Dfe According to a first variant, the method according to the invention, in which at least one burner is installed in each side wall of the oven substantially opposite, said walls being distant from the width of the Lfour oven, is characterized in that the heating length Lchauf is between 10% / and 100% / 1/2 (Lfour), preferably between 50% and 100% / 1/2 (Lfour).

  According to another variant, the method according to the invention, in which at least one burner is installed in each side wall of the oven substantially offset, in particular in staggered rows, the distance LbUoc between the injection axes of said burners in a direction perpendicular to said walls being between 25% / and 100% / o of 1 '/ 2 (Lfour) and preferably between 33% and 50% / o of 1/2 (Lfou r), is characterized in that the heating length Lchauf will be between 10% and 75% / of Lfour, preferably between 50% / o and 75% of Lfour.

  To assess this distance L block, proceed as follows either P the power of the burner (RW), or Q the power per unit of volume (expressed in HW / m3) accepted by the oven, according to the manufacturer's standards, or V the volume of the parallelepiped (or approximate) of width Lbloc, of height equal to the height of the corresponding zone of the furnace and of the width equal to Lchauf.

  According to the invention, this distance will be chosen such that: P braleur Lfour> Lb locor Lchauf xH zone xQ while preferably we will have: Lfour> Lbloc According to the invention, Q will be chosen between 0.1 and 0.4 MW / m3 and preferably between 0.2 and 0.3 MW / m3.

  The power of the burners will be directly linked to the width of the oven: the wider the oven 10, the more it will be necessary to choose a large power for each burner.

  According to another variant, the method according to the invention, in which at least two burners are used, each placed in one of the side walls of the oven (at least one burner per side wall, with generally an even number of burners with the same number of burners per wall), the distance Lbloc between the injection axes of said burners in a direction perpendicular to said walls being at least equal to 1/2 (Lfo0u) is characterized in that the heating length Lchauf will be included between 10% and 100% of Lfour, preferably between 50% and 100% of Lfou.

  Another aspect of the invention is to be able to adapt, during operation, the pulse of the burners as a function of the power delivered to the furnace to take account of variations in said power.

  According to a first variant, when the total heating power of the furnace is varied in order to adapt to the parts to be treated in said furnace, the momentum of the fuel and of the oxidant is varied in the same direction as that according to which the power delivered to the oven varies.

  According to another variant, the amount of movement of the fuel and / or of the oxidant is increased when the heating length Lchauf of the corresponding burner is to be increased. In all the variants of the invention, the burner used is either a burner with separate injection of fuel and oxidizer, or a burner with coaxial injection of fuel and oxidizer, or a burner in which the fuel is premixed at least partially and the oxidizer before injecting this mixture into the burner.

  We can also predict that from 2 to 80% in flight. argon is injected with the fuel and / or the oxidizer.

  In addition, the angle - between the axis of injection of the burner fluids and the axis perpendicular to the side wall of the furnace is between -80 and +800, preferably between -20 and +20, again more preferential equal to about 00.

  Preferably also, the fuel will be preheated, around 300 ° C, to improve the energy efficiency, preferably by heat exchange with the fumes from the oven.

  The invention will be better understood with the aid of the following exemplary embodiments given in conjunction with the figures which represent: FIG. 1: a top view (schematic) of a reheating oven according to the prior art with products arranged in staggered rows, FIG. 2: a sectional view of a burner installed in a niche of the voting cross member, FIG. 3: a schematic representation of a flame according to the invention and of its end driven by the fumes, FIG. 4: an exemplary embodiment of the invention with side burners on either side of the load, FIG. 5: a schematic sectional view of a coaxial injection of the fluids into the burner, FIG. 6, an exemplary embodiment of the invention with burners distant from each other, FIG. 7: an embodiment of the invention with staggered flames of different lengths, FIG. 8: an exemplary embodiment of the invention with staggered flames 30, staggered.

  Fig. 9: an example of a metallurgical heating furnace equipped with the process according to the invention.

  In Figure 3 is shown a flame 22 in a transverse flow of speed Vtrans. This flow consists of fumes composed of the products of combustion of the burners located upstream of the burner concerned (upstream by referring to the direction of circulation of the fumes which is that of the transverse flow Vtrans). The burner is located on the side wall 11. The oxidizers and fuels are injected along the axis 21 making an angle U. with respect to the axis 20 perpendicular to the axis of the main flow, defined by Vtrans. This angle may be different for the different fluids injected into the furnace via the burner: for example, the fuel and the oxidant may each have a different injection angle when injected separately, or identical if the burner is of the coaxial injection type. In the following description, given by way of example, all the fluids have the same angle cc, but the person skilled in the art will easily know how to transpose to each of the fluids if this angle aX is different.

  The angle Ct is between -80 and +80 degrees, preferably between and +200, even more preferably, the angle (x is substantially close to zero. The term "heating length", below designated by Lchauf designates the length of the flame, measured from the wall 11 of the furnace, in a direction along the axis 20. According to the invention, the characteristics of injection of fluids are defined below burner located in particular side wall to obtain optimal heating over the Lehauf length, while preserving the integrity of the oven and in particular the integrity of the side wall 11 o the burner is installed.

  Typically, the flow (in the direction Vtran,) is composed of combustion products, resulting from the combustion of burners located upstream of the burner considered.

  These burners located upstream can be burners whose oxidant is air, air enriched in oxygen, pure oxygen. The nature of the oxidizer can moreover vary from one burner to another.

  The speed Vtrans is therefore perfectly defined by the power delivered upstream of the burner concerned and by the dimensions of the furnace in the section where the burner is installed.

  Figure 4 shows a top view of the oven, where the burners concerned are located.

  The burners which generate the flames 26 and 27 respectively are located on either side of the oven. In this case, the length to be heated for each of these burners is, for example, half the width of the oven, ie Lfour / 2.

  According to the invention, the burners' pulse must be such that the associated heating length is between 10% and 150% of Lfour / 2. Below 10%, the flame will be too close to the wall and may endanger the integrity of the oven.

  As it is preferable to preferably supply energy to the center of the oven, the associated heating length will be between 50% and 100% of Lfour / 2.

  when the products which are reheated in the oven are arranged symmetrically with respect to the longitudinal axis of the oven (for example as in fig. 7). In the case where this load is not symmetrical (as is the case in fig. 4) we will prefer to have our heating lengths between approximately 100% and 150% of 2 Lfour.

  FIG. 5 schematically represents in section a burner with coaxial injection of fuel and oxidizer. The fuel is injected at the center into the injector 31 and the oxidizer is injected around the fuel, this oxidizer can be air, oxygen-enriched air, pure oxygen, with a possible addition of argon to this oxidizer so as to increase the impulse of the oxidizer and therefore of the flame, according to the aim pursued by the invention: it is possible to add from 2% vol to 80% vol of argon. FIG. 6 represents the case of burners distant from each other, on either side of the load. The burners 41, 43 have their respective flames 42, 44 which extend beyond the center line of the furnace, with a large heating width Lchauf 25 and therefore a significant pulse.

  FIG. 7 represents the case of offset flames 51, 52, 53 having respective heating lengths Lchaufl, Lchauf2 and Lchauf 3, corresponding here to decreasing burners of power (the burner generating the flame 51 being the most powerful), according to the areas to be heated or according to the specifications imposed.

  FIG. 8 represents an exemplary embodiment with staggered flames 61, 62, 63, 64 and burners placed in staggered rows with Lchauf heating lengths greater than Lfour / 2. Finally, FIG. 9 represents an exemplary embodiment of a heating furnace 5 for billets 75 which enter the furnace laterally through the inlet zone 76 to enter the smoke evacuation zone 79, fumes which are used to preheat billets 75, against the current. The fumes are evacuated through the orifice 77 for evacuating the fumes.

  In the heating and / or preheating zone 80, the burners 71, 72, 73, 74 according to the invention are arranged. The so-called equalization zone 81 includes burners 82 with aero10 combustibles to equalize the temperature of the billets 75 which are then ejected from the furnace by the exit zone 78.

  The invention will be better understood using the following nonlimiting examples:

Example 1

  The oxidizer used can be air, oxygen-enriched air or pure oxygen. In a reheating furnace (7.2 m wide and 1.3 high) with a power of 30 MW supplied by burners whose oxidizer is air preheated to 20,300 C., a smoke flow is measured about 33000 Nm3 / h ..

  The burnt gases, resulting from the combustion of natural gas and air, have a density measured ptrans = 0.28 kg / m3 at 1000 C (equivalent to 1.3 kg / m3 at 0 C) The incident speed of the gases is 4.5 m / s.

  The air / fuel burners of the furnace are replaced with oxygen / fuel burners such as burners sold under the trade name ALGLASS with a nominal power of 1 MW, shown diagrammatically in FIG. 5, with an opening diameter 32, oven side, equal to 11 cm.

  The burners are oriented along the axis perpendicular to the flue gas flow, ie c (x = 0. The density of oxygen is 1.4 kg / m3 at 25 ° C. and normal pressure.

  The width of the oven is approximately 7 m. Burners are used on either side of the oven, a burner located on the side to provide heating over a distance between 10% and 100% of the half-width of the oven, preferably between 50% and 100 % of the width of the oven. The following results are obtained by applying the formula described above:% / of Lfour heats 10% 0.36 m 4 m / s 50% 0 1.8 m 22 m / s 100% 0 3.6 m 47 m / s By injecting oxygen at the speeds mentioned above, many Lchauf heating lengths are obtained as defined in the table above.

  Example 2: In the example described below, natural gas was chosen as fuel (if the fuel used is fuel oil, we must consider the density and the speed of the atomized jet in the calculation below).

  In the case of natural gas, preheated to 300 ° C, the density of the fuel is: Pfuel = 0.8 kg / m3 at 0 C therefore Pcombustible = 0.38 kg / m3 at 300 C.

  If the burner used is a burner sold under the trade name "ALGLASS", with a diameter D equal to 11 cm (fig. 5), we obtain: (o. 1 'L Speed Length Natural Gas / Lfio) heats 10% 0 , 36 m 7 m / s 50% 1.8 m 42 m / s 100% 3.6 m 90 m / s The table is interpreted as in example 1.

  Example 3: The addition or substitution of argon in the oxidizer can make it possible to increase the mass of the oxidant injected and therefore to promote mixing, and thus to obtain better heating. Finally, if argon is used to replace nitrogen in the air, the removal of nitrogen will cause a significant reduction in the pollution generated by the flame (substantial reduction in NOS).

  The substitution of nitrogen in the air with argon or the addition of argon in pure oxygen makes it possible to reduce the injection rate of the oxidizer while maintaining the same heating length. Reducing the injection speed reduces the noise associated with the flame, reduces the entrainment associated with the flame. Reduction of entrainment is important in ovens containing many particles such as glass ovens. For example, a mixture of 40% Argon and 60% oxygen reduces the injection speed of the oxidizer by 5%. Preferably, the complement of Argon is pure oxygen, optionally with a residual nitrogen of between 1% and 5% by volume.

Claims (10)

  1. Method of combustion of a fuel and of an oxidizer injected through a wall (11), in particular a side wall, of an oven comprising a charge (6, 7) to be heated which moves in a plane substantially perpendicular to said wall (11), this combustion generating a flame (22, 26, 27) extending above and / or below said charge (6, 7), characterized in that the fuel injection speeds Vfi , and of the oxidizer Vcomb are adjusted so as to obtain a heating of the charge (6, 7) by the flame (22, 26, 27) thus created over a heating length Lchauf measured from said wall (11) and perpendicular to this, the injection speed Vflwi of at least one of the oxidizing fluids and / or fuel injected into at least one burner being greater than or equal to approximately: Vf2uide Vtrans ((COSa) Lchauf) 1.1 ( trans) 0.5 2 D Fluid formula in which: * V ,, a ,,, denotes the speed of flow of the fumes in the oven, * o e the angle made by the axis of injection of the fluids at the outlet of the burner relative to the axis perpendicular to said wall (11) of the furnace, * Lcia denotes the width of the furnace on which one wishes to obtain heating 20 optimum ,.   * P, ... represents the density of the fumes in the furnace, * Pfflid, represents the density of the fluids considered (oxidizer or fuel), o. D represents the diameter of the burner flue or its equivalent diameter. 25 2. Method according to claim 1, characterized in that V trans ((COSC) LIchauf) l, (Ptrans) 5 and V Vtrans (COS) chauf) l (Ptrans) o5 Vfuel 2 D P fue eomb 2 Dm 3. Method according to claim 1 or 2, wherein at least one burner is installed in two walls (11, 12) of the oven substantially opposite, and in particular two side walls of the oven, said walls being spaced from the width of the oven Lfo0, characterized in that the heating length Lcha, is between 10% and 100% of 1/2 (Lfo ^), preferably between 50% and 100% of '/ 2 (Lfo4).   4. Method according to claim 1 to 3, wherein the oven has two side walls 5, characterized in that there is at least one burner in each side wall (11, 12) of the oven, said burners offset state the one relative to the other and preferably arranged in staggered rows, the distance Lbloc between the injection axes of said burners in a direction perpendicular to said walls being between 25% and 100% of 1/2 (Lfou,) and preferably between 33% and 50% of 1/2 (Lfo) and in that the heating length Lchuf is between 10% and 75% of LfO .., preferably between 50% and 75% of Lfour.   5. Method according to one of claims 1 to 4, characterized in that at least two burners are used, each placed in one of the side walls (11, 12) of the oven, the distance LbIoc between the axes d injection of said burners in a direction perpendicular to said walls being at least equal to / 2 (Lfou and in that the heating length Lchuf is between 10% and 100% of LfoUr, preferably between 50% and 100% of Lfo ,.   6. Method according to one of claims 1 to 5, characterized in that when the total heating power of the oven is varied to adapt it to the parts to be treated in said oven, and in that one does vary the momentum (or pulse) of the fuel and / or the oxidant in the same direction as that according to which the power of the furnace varies.   7. Method according to one of claims 1 to 6, characterized in that the momentum of the fuel and / or of the oxidant is increased in order to increase the heating length Lrauf of the corresponding burner.   8. Method according to one of the preceding claims, characterized in that the burner used is either a burner with separate injection of fuel and oxidizer, or a burner with coaxial injection of fuel and oxidizer, or a burner called pre- mixture in which the fuel and the oxidizer are at least partially premixed before injecting this mixture into the burner.   9. Method according to one of the preceding claims, characterized in that from 2% to 80% by volume. argon is injected with the fuel and / or the oxidizer.   10. Method according to one of the preceding claims, characterized in that the angle oc between the axis of injection of the burner fluids and the axis perpendicular to said wall of the furnace is between O 'and 20.