EP1828679B1 - Combustion method with cyclic supply of oxidant - Google Patents

Abstract

The invention concerns a combustion method for industrial furnace comprising an arrangement of two substantially parallel and symmetrical burner assemblies (G, D). Each burner assembly comprises a fuel injector (10<SUB>G</SUB>, 10<SUB>D</SUB>) and three oxidant injectors (1<SUB>G</SUB>, 2<SUB>G</SUB>, 3<SUB>G</SUB>, 1<SUB>D</SUB>, 2<SUB>D</SUB>, 3<SUB>D</SUB>) arranged at increasing distances from the fuel injector. An oxidant supply system cyclically distributes a specific flow of oxidant among some at least of the second and third injectors of the burner assemblies (2<SUB>G</SUB>, 3<SUB>G</SUB>, 2<SUB>D</SUB>, 3<SUB>D</SUB>). The amount of nitrogen monoxide produced upon combustion is thus reduced, while ensuring a good distribution of the heating power in the furnace.

Description

The present invention relates to a combustion process for an industrial furnace.

The distribution of the heating power over a given furnace surface, the reduction of the quantity of nitrogen oxides produced and the stability of the combustion flame (s) generated in the furnace are among the principal stakes in the technology of combustion furnaces.

Indeed, the energy efficiency and the profitability of an industrial combustion furnace are higher for furnaces of great capacity. This is why the surface to be heated can be large. This is usually the top surface of a feed of raw materials or melt contained in a tank. It is then difficult to distribute the heating power delivered by the flame (s) of combustion in a substantially uniform manner over the entire surface, so as to avoid the formation of colder zones which would be harmful vis-à- screw of the melt or the subsequent process of treating it. For this, it is known to have several burners in an oven at specific locations above the tank. In particular, two burners can be arranged parallel to one another, with respective horizontal flames and directed in the same direction. Another possibility is to have the burners facing each other in pairs, with respective flames directed toward each other within each pair.

dans laquelle k est une constante numérique, exp désigne la fonction exponentielle, E_a est une énergie d'activation positive, R désigne la constante des gaz parfaits et T est la température locale.Furthermore, the amount of nitrogen oxides (NO _x) produced in a combustion flame depends on the local concentrations of oxygen and nitrogen, denoted [O _2] and [N _2]. In particular, an evaluation of the quantity of thermally produced nitric oxide (denoted [NO] _th ) is given by the following formula:

where k is a numerical constant, exp is the exponential function, E _a is a positive activation energy, R is the ideal gas constant and T is the local temperature.

In order to reduce the amount of thermal nitric oxide, it is known to use an oxidizer substantially free of nitrogen. Thus, an oxygen-enriched oxidant is used instead of air. Nevertheless, the resulting reduction of nitrogen oxides is insufficient compared to the regulations in force.

To further reduce the amount of nitrogen oxides produced, it is also known, especially after US5,522,721 and EP 0 524 880 cyclically vary the oxidant flow rate and / or the fuel flow rate that feed the flame. The ratio between the instantaneous local concentrations of oxygen and fuel in the flame is then different from the stoichiometry of the combustion reaction. The local temperature is therefore lower, and, according to relation (1), a further reduction in the amount of thermally produced nitric oxide results therefrom. But the flow rate variation parameters, such as the amplitude, the frequency and the phase of the variations of each flow, are difficult to adjust to obtain a satisfactory heating efficiency and a low release of carbon monoxide (CO). Indeed, carbon monoxide is toxic and polluting, and comes from incomplete combustion when the instantaneous local concentration of oxygen in the mixture is too low compared to the instantaneous local concentration of fuel.

Another way to achieve further reduction in the amount of nitrogen oxides produced is to inject a major portion of the oxidant and the fuel at two locations in the furnace separated from each other by a relatively large distance. A combustion performed under these conditions is called "staged" (see for example EP 0 748 981 or WO 02/081967 ). A small portion of the oxidant is further injected near the fuel outlet to stabilize the combustion regime. The main part of the oxidant and the fuel are then gradually mixed in a spread volume where the jets overlap. In this way, a gap effect is still obtained between the ratio of the local concentrations of fuel and oxidizer on the one hand, and the stoichiometry of the combustion reaction on the other hand. In addition, this stoichiometric difference effect is superimposed on a dilution effect. The local temperature, and therefore the amount of nitric oxide, are thus also reduced. But in this staged combustion configuration, the position of the flame in the vertical direction is particularly unstable. The heating efficiency of the fired material is then reduced and vault refractories can be damaged.

An object of the present invention is therefore to provide a combustion process which does not have the disadvantages mentioned above, or for which these disadvantages are reduced.

• un injecteur de combustible ;
• des premier, deuxième et troisième injecteurs de comburant respectivement disposés à des distances croissantes de l'injecteur de combustible.

Thus, the invention proposes a combustion process for an industrial furnace according to which two burner assemblies are arranged substantially horizontally, parallel to one another and symmetrically with respect to a median plane passing between the two assemblies. Each burner assembly includes:

• a fuel injector;
• first, second and third oxidizer injectors respectively disposed at increasing distances from the fuel injector.

An oxidizer feed system cyclically distributes a determined flow rate of oxidizing between at least the second and third injectors of the two burner units.

Since the burner assemblies are substantially horizontal, the flame produced in the furnace is itself contained in a horizontal plane. In this way, the heat generated by the flame is efficiently transferred to the furnace charge without excessively heating a vault structure disposed above the furnace at a particular location thereof. Premature wear of the arch structure is thus avoided.

The oxidant is thus introduced into the oven at three points for each of the two burner units. The first point of introduction of the oxidant is constituted by the first injector, which is the closest to the corresponding fuel injector. It allows to generate a first incomplete combustion of the fuel, which is then completed by the oxidant introduced by the second and third injectors. The first injector also generally stabilizes the combustion regime at its output. For each burner assembly, the third point of introduction of the oxidant is the farthest from the fuel injector, and the second oxidizer injector is located at a distance from the intermediate fuel injector between the distances of the first and third injectors .

The oxidant preferably has an oxygen content greater than 30% by volume, and even greater than 70% by volume.

The total flow rate of oxidant introduced into the furnace is distributed between the first, second and third injectors of the two burner units. A determined part of this total flow is injected by the second and third oxidizer injectors, with a distribution between at least some of these which is variable cyclically. The determined part of the total flow rate of oxidant which is injected by the two second and the two third oxidizer injectors is substantially constant. It may possibly vary, but much more slowly than those of the individual flows of the second and / or third oxidizer injectors which are variable. Thus, a specific fraction of the oxidant is injected into the furnace by some of the second and / or third injectors at a given instant, then is injected by the other second and / or third injectors at a later time. The oxidant injection obtained by a device according to the invention is therefore alternated between some of said second and / or third injectors.

The cyclic distribution of the oxidant flow rate between some of the second and third injectors of the two burner units is preferably carried out at a frequency of less than 1 hertz. The oscillation period of the flame in the oven is then greater than 1 second. The inventors have observed that such conditions provide a particularly stable combustion.

In the staged combustion that is obtained, the fuel and the oxidant that are introduced into the furnace are diluted by recirculation of the exhaust gas in the combustion zone. For this, a main part of the oxidant is introduced into the furnace at a great distance from the fuel introduction locations. By thus delaying the mixing between the fuel and the oxidant, the oxidant is strongly diluted with ambient gases present in the furnace before entering the main combustion zone. However, to stabilize the flame, it is necessary to also introduce oxidant into the furnace near each fuel introduction location. The part of oxidant that is introduced near the fuel is called primary flow, and that which is introduced at a distance from the fuel is called secondary flow.

Advantageously, the oxidizer supply system feeds the first injectors respectively of each burner assembly with respective primary oxidant flow rates at each instant.

Each burner assembly generates a flame in the furnace, but when the two burner units are not too far apart, their respective flames are united and form a single combustion volume. Such a single flame is obtained, in particular, when the distance between the respective fuel injectors of the two burner units is less than 30 times the diameter of each fuel injector. In the following, generally denote by flame the total volume in which combustion occurs, it being understood that this volume can be divided into two parts for a significant separation distance between the two burner units.

The cyclic variations in the distribution of the oxidant flow between at least some of the second and third injectors cause a horizontal displacement of the flame in the furnace. Depending on the separation distance between the two burner units and on the shape of the variation curves of the oxidant flow rates of the second and third injectors, the displacement of the flame consists of a flapping thereof between two positions or in an oscillation of the flame between two configurations. In general, the cyclic variations of the distribution of the gases in the furnace improve the stability of the flame, especially in the vertical direction, by moving the flame alternately in a substantially horizontal direction.

Finally, the displacement of the flame contributes to further improving the distribution of the heating power throughout the volume of the furnace: a heat transfer to the load of the furnace is obtained, which is more uniform thanks to an effect of average in the time of the thermal contributions taking place at each point of the furnace.

The invention also provides an oven adapted to implement a method as described above.

• la figure 1 illustre la configuration d'un four adapté pour mettre en oeuvre l'invention ;
• la figure 2a est un diagramme de variation des débits de comburant des premiers, deuxièmes et troisièmes injecteurs d'un four selon la figure 1, conformément à un premier mode de mise en oeuvre de l'invention ;
• la figure 2b illustre deux configurations de la flamme obtenues à des instants différents pour les variations de débits représentées à la figure 2a ;
• les figures 3a et 3b correspondent respectivement aux figures 2a et 2b pour un second mode de mise en oeuvre de l'invention ; et
• la figure 4 illustre différentes configurations de flamme correspondant à des perfectionnements du deuxième mode de mise en oeuvre de l'invention.

Other features and advantages of the present invention will emerge in the following description of two nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

• the figure 1 illustrates the configuration of an oven adapted to implement the invention;
• the figure 2a is a diagram of variation of the combustive flow rates of the first, second and third injectors of an oven according to the figure 1 according to a first embodiment of the invention;
• the figure 2b illustrates two configurations of the flame obtained at different times for the flow rate variations represented at figure 2a ;
• the Figures 3a and 3b correspond to the Figures 2a and 2b for a second embodiment of the invention; and
• the figure 4 illustrates different flame configurations corresponding to improvements of the second embodiment of the invention.

For the sake of clarity of the figures, the dimensions of the devices shown are not in proportion with real dimensions. In particular, dimensions measured in these figures which are associated with distinct real directions are not transposed according to the same scale ratio.

The figure 1 represents a vertical wall 101 of a furnace 100, for example a melting furnace of raw materials. The furnace 100 may be batch-operated, with separate stages of loading, heating and discharging of the furnace, or continuous operation, with permanent flows of raw material loading and melt output. F denotes the trace of the free surface of material charged to the wall 101 of the oven.

The fuel and oxidizer injectors are disposed on the wall 101, with respective directions of fluid outlet substantially horizontal. They are aligned on a horizontal line located at a height h above the trace F. h is preferably between 250 mm (millimeters) and 550 mm.

• deux injecteurs de combustible, référencés 10_G et 10_D, sont disposés respectivement sur les parties de paroi G et D, à une même distance d₁₀ du plan médian P, mesurée horizontalement ;
• trois injecteurs de comburant, référencés 1_G, 2_G et 3_G, sont alignés dans la partie de paroi G, respectivement à des distances d₁, d₂ et d₃ du plan médian P. Les distances des injecteurs de la partie de paroi G au plan médian P satisfont par exemple la relation suivante : d₁ < d₁₀ < d₂ < d₃. Les injecteurs 10_G, 1_G, 2_G et 3_G sont généralement situés sur une même ligne horizontale ; et
• trois injecteurs de comburant 1_D, 2_D et 3_D, respectivement identiques aux injecteurs 1_G, 2_G et 3_G et disposés symétriquement à ces derniers sur la partie de paroi D.

The wall 101 is divided by a median vertical plane P in two parts, respectively left, denoted G, and right, denoted D. Injectors are located symmetrically on the two wall portions, as follows:

• two fuel injectors, referenced 10 _G and 10 _D are arranged respectively on the wall portions G and D, at the same distance d ₁₀ from the median plane P horizontally;
• three oxidizer injectors, referenced 1 _G , 2 _G and 3 _G , are aligned in the wall portion G, respectively at distances d ₁ , d ₂ and d ₃ of the median plane P. The distances of the injectors of the wall portion G at the median plane P satisfy, for example, the following relation: d ₁ <d ₁₀ <d ₂ <d ₃ . Injectors 10 _G, 1 _G , 2 _G and 3 _G are generally located on the same horizontal line; and
• three oxidizer injectors 1 _D, 2 _D and 3 _D, respectively identical to the injectors 1 _G 2 _G 3 and _G and arranged symmetrically to the latter on the wall portion D.

The injectors 10 _G , 1 _G , 2 _G and 3 _G form a first burner assembly, associated with the left part of the wall 101. For simplicity, this burner assembly is designated G in the following. Similarly, the injectors _10D , _1D , _2D and _3D form a second burner assembly, designated D and associated with the right portion of the wall 101.

The fuel introduced into the furnace 100 by the injectors _10G and _10D can be gaseous or liquid. In the case of a liquid fuel, the injectors 10 and _G 10 _D each incorporate a spray nozzle so as to produce jets of fuel droplets.

Preferably, the distance d ₁₀ between the fuel injector of each burner assembly, 10 _G or 10 _D, and the median plane P is less than 15 times the diameter of each injector 10 or _G 10 _D, denoted Φ _10. Under these conditions, a single flame common to the two burner units G and D is generated in the oven 100.

The oxidant introduced by the injectors _1G , _2G , _3G , _1D , _2D and _3D is a gas usually having an oxygen content greater than 70% by volume.

Preferably, the third oxidizer injector of each burner assembly is located at a distance from the fuel injector of said assembly at least ten times greater than the output diameter of the third injector. In other words: d ₃ -d ₁₀ > 10.Φ ₃ , where Φ ₃ denotes the exit diameter of injectors 3 _G and 3 _D. Thus, the oxidant jet of the injector 3 _G , respectively 3 _D , is sufficiently spaced from the fuel jet of the injector 10 _G , respectively 10 _D , to obtain a staged combustion.

All the injectors of each burner assembly are directed substantially horizontally, so that the flame produced is parallel to the surface of the melt bath contained in the furnace 100.

Advantageously, the oxidizer supply system supplies each of the first injectors respectively of each burner assembly, that is to say the injectors _1G and _1D , with a respective primary flow of constant oxidant. The oxidizer feed system is then simplified, as regards the feeding of injectors _1G and _1D . Preferably, the respective flow rates of the two injectors _1G and _1D are substantially equal: x _G = x _D , denoting by x _G and x _D the respective flow rates of the injectors _1G and _1D . By way of example, x _G and x _D each correspond to 10% of the total flow rate of oxidant injected into each burner assembly.

According to a first embodiment of the invention, described with reference to Figures 2a and 2b the oxidant flow rates of two injectors arranged symmetrically with respect to the median plane P are equal at each instant. Denoting by y _G, y _D, z _D z _G and the respective instantaneous flow rates of the injectors 2 _G, 2 _D, 3 _G and 3 _D, the following relationships are satisfied: y _G = y _D and z _G = z _D. In other words, the oxidizer supply system feeds the second injectors respectively of each burner assembly with respective secondary flows of oxidant substantially equal at each instant, and feeds the third injectors respectively of each burner assembly with respective tertiary flows of oxidant substantially equal every moment. For example, the supply system of injectors _2G , _2D , _3G and _3D may comprise two identical distribution boxes respectively assigned to each burner assembly G and D. These distribution boxes are coupled to a common control member variable, and each box has a movable partition wall of oxidant flows directed respectively to the second or the third injector.

The flame obtained is then centered on the median plane P and is symmetrical with respect thereto at each instant.

The figure 2a illustrates an example of variation of the rates y _G and y _D on the one hand, and rates z _G and z _D on the other hand. The abscissa axis represents the time, indicated in seconds, and the ordinate axis represents the fraction of the oxidizer flow rate of each burner assembly that is introduced by each injector thereof. It is assumed that the total oxidant flow rate of each burner assembly G or D is constant, and that x _G and x _D are also constant and are each equal to 10% of the flow rate of the corresponding burner assembly.

• état 1, dans lequel y_G = y_D = 10% et z_G =z_D = 80%,
• état 2, dans lequel y_G = y_D = 50% et z_G =z_D = 40%.

For example, y _G and y _D vary substantially sinusoidally between 10% and 50%, and z _G and z _D vary between 40% and 80%. The period of these variations is 2 seconds. The extreme configurations of the flame then correspond to the following states:

• state 1, where y _G = y _D = 10% and z _G = z _D = 80%,
• state 2, wherein y _G = y _D = 50% and z _G = z _D = 40%.

The mixing volume is greater in state 1 than in state 2. In accordance with figure 2b which represents the periphery 200 of the flame in a horizontal plane passing through the injectors, the state 1 corresponds to an extended flame, both in width and in length, and the state 2 corresponds to a narrower and shorter flame . For the sake of clarity, the flow introduced into the furnace by each oxidizer injector is indicated on the figure 2b . In state 1, the fuel and the oxidant are more diluted within the flame. The temperature is then lower, but a better coverage of the entire surface of the material is obtained. The heat transfer of the flame to the furnace charge is then particularly homogeneous. Conversely, the flame is more concentrated and intense in state 2.

A second mode of implementation is now described in relation to the Figures 3a and 3b . This second mode corresponds to an alternating oxidant feed between the two burner units. More particularly, the oxidizer feed system cyclically distributes a determined tertiary total flow of oxidant between said third injectors of the two burner units.

The oxidizer supply system can further supply each of the second injectors respectively of each burner assembly with a respective secondary flow of constant oxidant. A particularly simple implementation of the alternate supply of oxidant is thus obtained. In addition, the secondary flows of oxidant may be substantially equal.

The furnace and the burner units used above for the first embodiment of the invention can be taken over without modification for operation with alternating supply of oxidant. With the same notations and references, we now have: x _G = x _D = x / 2 and y _G = y _D = y / 2, where x denotes the total flow rate of oxidant introduced into the furnace 100 by the injectors _1G and 1 _D , and y denotes the total oxidant flow introduced by injectors _2G and _2D . x and y are respectively called primary and secondary total flows of the oxidant. Similarly, the tertiary total oxidant flow rate, that is to say the oxidant flow rate introduced by the injectors 3 _G and 3 _D , is noted. By way of example, x = 10%, y = 15% and z = 75%, expressed in percentages of the total flow rate of oxidant introduced into the oven. In general, x and y are substantially constant or vary much more slowly than the individual cyclically varying flow rates of injectors.

The oxidizer feed system may be a distribution box connected to the injectors 3 _G and 3 _D , which has a movable partition wall disposed between the oxidant flows directed respectively to the injectors 3 _G and 3 _D. The figure 3a illustrates such an operation, according to which the relation z _G + z _D = z is satisfied at every moment. The y-axis of the figure 3b is expressed as a percentage of the total flow rate of oxidant introduced into the furnace, that is to say x + y + z. z _G and z _D each vary between 10% and 65%. The period of flow variations is still 2 seconds.

• état 1, dans lequel z_G = 65% et z_D = 10%,
• état 2, dans lequel z_D = 10% et z_G = 65%.

The extreme configurations of the flame now correspond to the following states:

• state 1, in which z _G = 65% and z _D = 10%,
• state 2, wherein z _D = 10% and z _G = 65%.

The mixing volume and the flame have symmetrical configurations between the preceding states 1 and 2 ( figure 3b ). In each of these states, the flame is moved to the side of the 3 _G or 3 _D injector having the largest oxidant flow rate. Thus, the flame is moved to the left side in state 1, and to the right side in state 2. This lateral reciprocation of the flame stabilizes the height thereof, so that the flame remains at a substantially constant distance from the free surface of material charged on the one hand, and at a substantially constant distance from the vault of the oven on the other hand. These two distances can then be well controlled, which makes it possible to obtain a regular melting process and a slowing down of the refractory degradation of the vault.

In addition, the lateral back and forth of the flame provides a sufficient heat transfer uniformly between the flame and the furnace charge in a horizontal direction parallel to the wall 101.

Because of the velocity of the oxidant at the outlet of the injectors 3 _G and 3 _D , the flame is longer on the side of the injector 3 _G or 3 _D having the instantaneous flow rate of the highest oxidant. A good average coverage of the furnace surface by the flame results. For example, the oxidant is expelled by injectors 3 _G and 3 _D with a speed of between 20 ms ^-1 (meter per second) and 160 ms ^-1 , for example 90 ms ^-1 . In general, the average fuel and oxidant mixing distance, as well as the average distance at which the combustion occurs, located from the wall 101 of the furnace, are all the greater the greater the speed of expulsion of the furnace. oxidizer by injectors 3 _G and 3 _D is high.

In addition, at each alternation, the high flow rate of the oxidant introduced by one of the two injectors 3 _G or 3 _D causes a significant dilution of the fuel on the side of the median plane P which corresponds to this injector. Conversely, the fuel is more concentrated in a zone of the flame offset from the median plane P on the side of the injector 3 _G or 3 _D which has the instantaneous flow rate of the weakest oxidant. This area is marked A on the figure 3b , for the flame edges 200 corresponding to each of the two states 1 and 2. The zone A moves at each alternation between two symmetrical positions located on either side of the median plane P.

Since the flame is richer in Zone A, more soot is produced there. At the same time, zone A corresponds to the part of the flame that contributes the most to the heat transfer to the load at every moment.

The existence of such a zone A inside the flame may be favorable or harmful to the material that is being melted, in particular as a function of the chemical behavior of this material when the temperature is not uniform. According to an improvement of the second mode of implementation of the invention, it is possible to reduce or exacerbate the presence of such a zone A by varying the fuel flow of the injectors _10G and _10D at each alternation. For this, a fuel supply system can cyclically distribute a determined total flow of fuel between the fuel injectors of the two burner units.

Advantageously, the fuel supply system is coupled to the oxidizer supply system so that the total fuel flow is cyclically distributed between the fuel injectors of the two burner assemblies in phase or in phase opposition with respect to the distribution. cyclic tertiary total flow of oxidizer between the third injectors of the two burner units.

For example, another distribution box may be disposed at the inlet of injectors _10G and _10D . This other distribution box has a movable separation wall arranged between the fuel flows directed respectively to the injectors _10G and _10D .

The two distribution boxes, connected to the injectors 3 _G and 3 _D for the first, and the injectors 10 _G and 10 _D for the second, can then be controlled synchronously in opposition of phase: the fuel flow rate sent into the one of the two injectors 10 _G or 10 _D is maximum or minimum at the same time that the flow of oxidizer sent into the injector 3 _D or 3 _G on the opposite side is also maximum or minimum. A reinforcement of the zone A is thus obtained, which causes an increase in the brightness of the flame near the exit of the fuel injector _10G or _10D when the fuel flow therein is maximum. The fuel concentration is depleted on the side of the 3 _G or 3 _D injector for which the oxidizer flow rate is maximum. This increased depletion causes a shortening of the flame to its furthest point of the injectors.

Conversely, the two distribution boxes can be controlled synchronously in phase. The flow rate of fuel sent into one of the two injectors 10 _G or 10 _D is then maximum or minimum at the same time as the oxidizer flow rate sent into the injector 3 _G or 3 _D of the same side is also maximum or minimum. Zone A is then blurred and can be confused with the entire extent of the flame. It then oscillates between the two left and right sides with a greater amplitude of transverse displacement. At the same time, the flame is lengthened so that the two effects are combined to obtain an optimal scanning of the entire furnace surface by the flame. This results in an average heat transfer area with a particularly large load.

Flame boundaries obtained when the distribution of fuel flows varies with the distribution of oxidizer flows are shown in Table 1. figure 4 . The traces 200a and 200b respectively correspond to variations in phase opposition and in phase. The trace 200 corresponds to a constant distribution of the fuel flow, and balanced between the two injectors 10 _G and 10 _D. It is represented in dotted lines for comparison. The traces 200, 200a and 200b all correspond to the total flows of fuel and oxidant identical. For the sake of clarity of figure 4 only the contour of the flame in state 1 defined above is represented for each case.

It is understood that many modifications and adaptations of the invention can be introduced with respect to the modes of implementation which have been described in detail. Such modifications or adaptations may in particular take account of particular characteristics, in particular geometrical characteristics, of the furnace in which the invention is implemented. In addition, the rate of variation of the oxidizer flow rates can be adjusted in a manner known to those skilled in the art, in particular to obtain a maximum burn rate and to reduce the amount of carbon monoxide produced.

Claims (12)

1. Combustion method in which two burner assemblies (G, D) are placed substantially horizontally, parallel to one another and symmetrically about a median plane (P) passing between the two assemblies, each burner assembly comprising:

   - a fuel injector (10_G, 10_D) ;

   - first (1_G, 1_D), second (2_G, 2_D) and third (3_G, 3_D) oxidant injectors placed respectively at increasing distances from the fuel injector (10_G, 10_D),

   and in which an oxidant feed system cyclically distributes a predefined flow of oxidant among at least some of the second and third injectors (2_G, 2_D, 3_G, 3_D) of the two burner assemblies.
2. Method according to Claim 1, in which the cyclic distribution of the oxidant flow among some of the second and third injectors of the two burner assemblies (2_G, 2_D, 3_G, 3_D) is carried out at a frequency below 1 hertz.
3. Method according to either of Claims 1 and 2, in which a distance between the respective fuel injectors (10_G, 10_D) of the two burner assemblies is shorter than 30 times the diameter of each fuel injector (Φ₁₀).
4. Method according to any one of Claims 1 to 3, in which the oxidant has an oxygen content above 30% by volume.
5. Method according to any one of Claims 1 to 4, in which the third oxidant injector (3_G, 3_D) of each burner assembly is located at a distance from the fuel injector (10_G, 10_D) of said burner assembly at least 10 times longer than the outlet diameter of said third injector (Φ₃).
6. Method according to any one of Claims 1 to 5, in which the oxidant feed system supplies each of the first injectors (1_G, 1_D)of each burner assembly with oxidant at a constant respective primary flow rate (x_G, x_D).
7. Method according to any one of Claims 1 to 6, in which the oxidant feed system supplies the first injectors (1_G, 1_D) respectively of each burner assembly with oxidant at respective primary flow rates (x_G, x_D) that are substantially equal at any time.
8. Method according to any one of Claims 1 to 7, in which the oxidant feed system supplies the second injectors (2_G, 2_D) respectively of each burner assembly with oxidant at respective secondary flow rates (y_G, y_D) that are substantially equal at any time, and supplies the third injectors (3_G, 3_D) respectively of each burner assembly with oxidant at respective tertiary flow rates (z_G, z_D) that are substantially equal at any time.
9. Method according to any one of Claims 1 to 7, in which the oxidant feed system cyclically distributes a predefined total tertiary oxidant flow among the third injectors (3_G, 3_D) of the two burner assemblies.
10. Method according to Claim 9, in which the oxidant feed system supplies each of the second injectors (2_G, 2_D) respectively of each burner assembly with oxidant at a constant respective secondary flow rate (y_G, y_D).
11. Method according to either of Claims 9 and 10, in which a fuel feed system cyclically distributes a predefined total fuel flow among the fuel injectors (10_G, 10_D) of the two burner assemblies.
12. Method according to Claim 11, in which the fuel feed system is coupled with the oxidant feed system so that the total fuel flow is cyclically distributed among the fuel injectors (10_G, 10_D) of the two burner assemblies in phase with or in phase opposition to the cyclic distribution of the total tertiary oxidant flow among the third injectors (3_G, 3_D) of the two burner assemblies.

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