EP0661499B1 - Real time control of a burner for gases with differing characteristices, especially for a metallurgical furnace for reheating - Google Patents

Abstract

From the fuel stream feeding the burner 1 of the furnace M, the system samples a given flow rate F2, the combustion of which develops a power of at least 100 th/h, which is burnt with defined but excess oxidant air flow rate F1 in a calorimeter 6, a probe 9 is used to measure, in the complete-combustion smoke, a quantity representative of the content of at least one of the components of the smoke and, on the basis of this quantity, a computation unit 10 is used to determine a feed set-point for the burner 1, in particular the oxidant air intake flow rate (a) or the mixing proportions of a plurality of combustible feed gases. This system operates in parallel and simultaneously with the heating furnace M. It allows instantaneous correction of the feed set-points of the burners of the furnace in order to adjust them in real time in accordance with fluctuations in the characteristics of fuels such as industrial gases currently used for this purpose. <IMAGE>

Description

The subject of the present invention is a method and a device for real-time adjustment of a burner supplied with a fuel variable characteristics.

As is known, it is necessary to heat the semi-finished products steelworks to be rolled, which is carried out in furnaces reheating.

In the case of so-called "integrated" steel factories (i.e. those which carry out by themselves all the necessary operations steel fabrication from ore preparation to rolling of semi-finished products), the energy used most often comes from combustible gases co-produced in different reactors in the plant steel industry itself. This is how in the oven burners of heating of steel products, a mixture is conventionally used gases from emissions from a coke oven, blast furnaces, and the steelworks, each of these sources providing a gas, of different content in combustible elements, such as hydrogen for example, and of contents different in impurities, especially sulfur, and somewhat charged with dust, despite the filtering undergone.

This gas mixture is produced in principle to obtain specific level thermal properties, such as PCI (Power Lower Calorific), the PCO (Combustive Power) which is deduced therefrom, or the Wobbe index for example. But the composition of the gas mixture may be subject to fluctuations resulting from variations in the reactors that supply the components. So, for example, the content of hydrogen fluctuates quite commonly because different sources supply of this gas mixture do not have a composition constant over time. When these compositions fluctuate, a low change, particularly in the composition of the coke oven gas (which is rich in hydrogen) has significant repercussions on its PCO among others. However, the latter must be constantly known with good precision if we want to ensure a particular temperature in the enclosure of the reheating oven, which can also be modified using the regulation of the combustion air flow with which the burners are provided.

Thus, also, the content of impurities, in particular sulfur, fluctuates for the same reasons over time, which results in composition fluctuations, especially in sulfur oxides, gases combustion and therefore the atmosphere of the reheating furnaces.

It has been found that too high a content of impurities, in particular of sulfur oxides in the atmosphere of a reheating furnace containing steel products caused the appearance of surface defects said steel products, in particular when they are made of steel austenitic stainless.

The problem therefore consists in controlling the combustion of a mixture gaseous, moreover charged with dust, and whose characteristics can quickly vary randomly around mean values (lower calorific value, combustive power, density, content of sulfur, etc ...).

The problem also consists in controlling the mixture of various co-produced combustible gases, the impurity contents of which are generally very different, so that the content of impurities, especially in sulfur oxides, in the atmosphere of reheating either constant or kept below a limit predetermined.

To this end, devices have already been proposed called "comburimeters", "wobbmeters" and batteries of analyzers. These systems theoretically achieve the goal, but in the case using fuels with very variable characteristics and / or loaded with impurities, they appear either ineffective or have disadvantages in operation, or both. So the comburimeters are sensitive to high humidity and impurities, and by therefore unusable with steel gases without maintenance constant. Wobbmeters have long response times and a poor accuracy, which add to the humidity problems and impurities. Finally, analyzer batteries are devices technically very sophisticated, with sampling and gas treatment before analysis, requiring maintenance followed by Qualified staff. Their cost is very high and they are sensitive to corrosion if the gases contain sulfur.

The object of the invention is to provide a system capable of solving the problems posed while eliminating the above disadvantages of known prior devices.

• on prélève sur le courant de combustible alimentant le brûleur un débit de combustible donné dont la combustion complète produit une puissance d'au moins 100 thermies/h,
• on brûle ce débit de combustible avec un débit d'air donné en excès de manière à assurer la combustion complète du combustible,
• on mesure dans les fumées de combustion la valeur d'une grandeur représentative de la teneur de l'un au moins des composants choisis parmi l'oxygène résiduel et le soufre,
• on compare cette valeur à une valeur de référence prédéterminée représentative de l'état de marche voulu du brûleur,
• et si l'écart est supérieur à un seuil prédéterminé, on agit sur le réglage de l'alimentation du brûleur, de manière à réduire cet écart.

According to the invention, the method of real-time adjustment of an oven burner supplied with a gaseous fuel of variable characteristics around a standard state and with an adjustable combustion air flow rate, is characterized in that at during burner operation:

• a given flow of fuel is taken from the fuel stream supplying the burner, the complete combustion of which produces a power of at least 100 therms / h,
• this flow of fuel is burned with a flow of air given in excess so as to ensure complete combustion of the fuel,
• the value of a quantity representative of the content of at least one of the components chosen from residual oxygen and sulfur is measured in the combustion fumes,
• this value is compared with a predetermined reference value representative of the desired operating state of the burner,
• and if the difference is greater than a predetermined threshold, action is taken to adjust the burner supply, so as to reduce this difference.

• ladite grandeur reflète l'écart à la stoechiométrie (par exemple, la teneur résiduelle en O₂,ou la température des fumées),
• à partir de cette grandeur mesurée, on détermine la valeur du PCO réel instantané du combustible, ou de toute autre propriété thermique exprimant les caractéristiques instantanées réelles du combustible,
• et on agit sur le réglage de l'alimentation du brûleur en appliquant à la régulation du débit d'entrée d'air comburant dans le brûleur une consigne représentative de ladite valeur.

According to a first main variant of the method according to the invention, in addition:

• said quantity reflects the deviation from stoichiometry (for example, the residual O _{2 content} , or the temperature of the fumes),
• from this measured quantity, the value of the instantaneous real PCO of the fuel is determined, or of any other thermal property expressing the actual instantaneous characteristics of the fuel,
• and one acts on the adjustment of the burner supply by applying to the regulation of the combustion air inlet flow rate to the burner a setpoint representative of said value.

This instruction is advantageously a "correction coefficient" of the combustion air flow calculated from the instantaneous real PCO ratio measured / standard PCO, this standard PCO being that corresponding to the standard fuel composition around which the composition is likely to fluctuate.

According to a second main variant of the method according to the invention, when said fuel is a mixture adjustable in proportions of several fuels containing levels of impurities different sulfur, said quantity measured is the oxide content of sulfur fumes and we act on the adjustment of the burner supply in modifying the mixing proportions of said fuels to maintain said sulfur oxide content in the vicinity of said reference value predetermined.

• d'une part, un mini-four constitué par au moins :
  • un tunnel de combustion de longueur suffisante pour que la combustion complète d'un combustible séparée à l'une de ses extrémités soit achevée à son autre extrémité,
  • un brûleur, monté à l'une des extrémités du tunnel et dimensionné pour développer une puissance calorifique d'au moins 100 th/h, et
  • des moyens de réglage de l'alimentation dudit brûleur ;
• d'autre part, à l'autre extrémité du tunnel laissée ouverte pour permettre l'évacuation des fumées de combustion, des moyens pour mesurer une grandeur représentative de la teneur de l'un au moins des composants desdites fumées, et une unité de calcul recevant en entrée ladite mesure et déterminant, sous la forme d'un signet disponible à sa sortie, une consigne de réglage de l'alimentation dudit brûleur.

The device for implementing the method according to the invention, hereinafter called "correctometer", comprises, in accordance with the invention:

• on the one hand, a mini-oven consisting of at least:
  • a combustion tunnel of sufficient length so that the complete combustion of a fuel separated at one of its ends is completed at its other end,
  • a burner, mounted at one end of the tunnel and dimensioned to develop a calorific power of at least 100 th / h, and
  • means for adjusting the supply of said burner;
• on the other hand, at the other end of the tunnel left open to allow the evacuation of combustion fumes, means for measuring a quantity representative of the content of at least one of the components of said fumes, and a calculation unit receiving as input said measurement and determining, in the form of a bookmark available at its output, an instruction for adjusting the supply of said burner.

According to a first variant of the device according to the invention, the adjustment means make it possible to control the fuel flow and the combustion air flow and the calculation unit evaluates the thermal properties actual fuel snapshots.

According to a second variant of the device according to the invention, the adjustment means make it possible to control the proportions of the mixture of combustibes and the means of measuring said representative quantity are a probe for measuring the content of sulfur oxides in fumes.

Thus the invention provides for supplying the burner of the correctometer arranged at the end of the combustion tunnel in air and fuel regulated with controlled flow rates, and to correlate in a simple way the quantity measured at the real time setting of the furnace burner supply reheating. In other words, we measure instantaneous variations a fuel property, such as its PCO or its content impurities, caused by fluctuations in its composition, for determine, and apply instantly, the corrections to be made to the burner supply.

According to the first variant of the invention, the correction is applied at the combustion air inlet flow rate in the oven burner (s), to allow the power delivered by this burner to be kept constant, despite the vagaries of the combustible PCO that feeds it.

According to the second main variant of the invention, when the fuel is an adjustable mixture, the correction is applied to the mixture of combustibes, to allow to maintain in the vicinity of a predetermined reference value the content of sulfur impurities in the atmosphere of the metallurgical furnace.

The invention solves the problem posed by means of a simple system, economical and robust, not very sensitive to the environment, requiring no that reduced maintenance and very low operating cost. He imports however, for reasons that are not yet fully understood, that the burner used is capable of developing a power of at least 100 th / h, otherwise the measured quantity cannot be correctly evaluated in the smoke and the invention produces unusable results industrially.

• la Figure 1 est un bloc diagramme schématique illustrant le procédé conforme à l'invention de réglage en temps réel des brûleurs d'un four de réchauffage industriel ;
• la Figure 2 est une vue en coupe longitudinale axiale et élévation partielle d'une forme de réalisation de correctomètre de mise en oeuvre du procédé selon l'invention.

The invention will be clearly understood in the light of the description which follows given with reference to the appended drawings which illustrate a first embodiment thereof by way of nonlimiting example, and in which:

• Figure 1 is a schematic block diagram illustrating the method according to the invention for real-time adjustment of the burners of an industrial heating furnace;
• FIG. 2 is a view in axial longitudinal section and partial elevation of an embodiment of a correctometer for implementing the method according to the invention.

The installation shown schematically in Figure 1 includes a metallurgical furnace M for reheating steel semi-finished products, by example of steel slabs, equipped, in known manner, with one or more several burners 1, supplied with combustible gas (g) from a source of gaseous fuel 2 via a flow control 3 of gas controlled by a Co setpoint given by the oven driver of heating M. The combustion air inlet (a) in the burner 1 is controlled by a flow control 4 connected to the atmosphere by an external outlet F5. The air flow (a) is continuously adjusted to the gas flow (g) using of a reference signal C1 which the gas control 3 sends to the air control 4. This adjusts the heating power of the burners while ensuring that complete combustion conditions are respected.

As can be seen, the "correctometer" 5 according to the invention takes its place in this assembly between the gas source 2 and the control 4 of air flow.

This correctometer 5 consists essentially of a comburimeter oven 6 and a calculation unit 10. The comburimeter 6 comprises, as will be seen in more detail below, a mini-burner 7 of nominal power of at least 100 th / h and opening into a room combustion chamber 8 in the form of a tunnel, provided at its other end with the sensor of a probe analyzer 9. The burner (preferably at high impulse to promote obtaining homogeneous fumes) receives on a input a mass flow of combustible gas F2 constant and perfectly controlled, coming from source 2 and, on its other input, a flow external combustion air correspondent F1 also at a flow rate constant and perfectly known mass, but in excess compared to needs for stoichiometric combustion of the gas flow F2.

The combustion tunnel 8 is open at its end opposite that of the burner 7 to allow evacuation to the atmosphere F3 (or else in the reheating furnace M itself) of the combustion products. The latter are analyzed before evacuation by the probe 9 which determines the content of free oxygen. The measurement by the probe 9 of the residual O ₂ in the combustion products reflects, as will be seen, the difference in stoichiometry. The value of this measurement is used by the computer 10 to determine a thermal characteristic of the gas, taken in its real state, like its instantaneous PCO, and to develop a signal C2, here in the form of a correction coefficient to be applied to the flow rate setpoint. of air (a) of control 4, which is set to the standard PCO value of the gas to be burned.

A) on prélève sur le courant eux issu de la source 2 un débit massique de gaz donné et constant (flèche F2) ;

B) on brûle ce débit de gaz F2 avec un débit massique d'air F1 donné et constant en excès, afin d'assurer la combustion complète du gaz. Cette combustion est réalisée dans le four 6 par le brûleur 7. On rappelle un point important les alimentations en gaz combustible et en air comburant sont régulées à des débits constants et parfaitement connus ;

C) on mesure, dans les fumées de combustion complète à l'extrémité ouverte de la chambre de combustion 8, par un capteur approprié 9, une grandeur représentative de l'écart à la stoechiométrie, à savoir dans l'exemple décrit, la teneur en O₂ résiduel.

D) à partir de cette grandeur mesurée, on détermine dans le calculateur 10 la valeur du PCO réel instantané du gaz, on déduit le rapport PCO réel/PCO standard, ce dernier correspondant à la composition standard du gaz combustible autour de laquelle sa composition réelle est susceptible de fluctuer et ce rapport définissant un coefficient de correction C2 ;

E) et on utilise ce coefficient de correction C2 pour déterminer la consigne de régulation du dispositif de commande 4 du débit d'entrée d'air de combustion dans le brûleur du four industriel M.

The successive stages of the process targeted by the invention, by means of the real-time correctometer device 5 of the burners fitted to the oven 1, are linked as follows:

A) a constant and given mass flow of gas is taken from the current from the source 2 (arrow F2);

B) this flow of gas F2 is burned with a mass flow of air F1 given and constant in excess, in order to ensure complete combustion of the gas. This combustion is carried out in the oven 6 by the burner 7. An important point is recalled, the supplies of combustible gas and of combustion air are regulated at constant flow rates and perfectly known;

C) in the complete combustion fumes at the open end of the combustion chamber 8, a quantity representative of the difference in stoichiometry, namely in the example described, is measured by a suitable sensor 9 in residual O ₂ .

D) from this measured quantity, the value of the instantaneous real PCO of the gas is determined in the computer 10, the real PCO / standard PCO ratio is deduced, the latter corresponding to the standard composition of the combustible gas around which its real composition is likely to fluctuate and this ratio defining a correction coefficient C2;

E) and this correction coefficient C2 is used to determine the regulation setpoint of the control device 4 for the combustion air inlet flow rate in the burner of the industrial oven M.

The magnitude representative of the deviation from stoichiometry can be other than the residual oxygen content in the fumes after complete combustion, these measurements being carried out by probes known per se and which do not require a more detailed description for be understood and implemented by the skilled person.

Variations in the composition of the gas, in particular its content in hydrogen, cause as we said correlative fluctuations and of the magnitude representative of the deviation from stoichiometry corresponding to these variations during combustion with excess air. In detecting in real time the fluctuations of this representative quantity, we can deduce, knowing the flow rates of combustion air and gas fuel in the comburimeter, fluctuations in the composition of the combustible gas. By means of a simple calculation giving its PCO value instantaneous, we can therefore adjust in real time the value of setpoint for control 4 of the combustion air inlet flow in the burners of the reheating oven, in order to maintain in the latter the conditions of a desired stoichiometric or other combustion.

Instantaneous real PCO (or correlative correction coefficient which is used to act on the command setpoint 4) can be determined either by calculation or by reading pre-established charts with the same relative air and gas flow rates for different values of the quantity representative.

In summary, the burners of the reheating oven are set for a Standard PCO given gas to burn. The combustion air flow is adjusted to this standard PCO to ensure desired combustion, even when varies the flow of combustible gas, for example to modify the oven temperature by intervening on the calorific power delivered by the burners. To maintain the desired quality of combustion that the combustion air flow (a) instantly follows possible modifications that are imposed by the setpoint Co on the control 3 of the flow rate of gas to be burned (g). Correctometer 5, according to the invention intervenes at the 2nd, finer level, to correct the effects of fluctuations in the composition of the gas which modify its properties thermal, for example his real PCO without realizing it at once.

Figure 2 shows an embodiment of the comburimeter oven 6 according to the invention.

This device comprises the combustion tunnel chamber 8, of a sufficient length for the combustion of gas, initiated at one end, is completed at the opposite end, and the mini-burner 7 provided at a end of the tunnel 8. A nozzle 11 for excess combustion air inlet in the burner 7 and a fuel gas inlet pipe 12 open into the mixing chamber 13. Devices not shown allow adjustment of the gas mass flow to a value predetermined constant as well as the combustion air flow.

The tunnel 8 is thermally insulated by means of two layers superimposed 14 and 15 of insulation inserted between the wall of the combustion 8 and the outer casing 16 of the comburimeter 6. Au near the exit opening 17 of the tunnel 8, a probe analyzer 9 for measuring the residual oxygen content of combustion fumes crosses the envelope 16 and the two layers of insulation 14, 15.

The combustion chamber 8 has in practice a length of one meter about, not much more, but in any case sufficient to ensure that combustion is completed at the level of the probe 9, which is by example, a known type zirconium probe.

As already indicated, the characteristics of the gas (power calorific, combustive power, density) may fluctuate around those of the gas taken in its standard state.

However, the flow rate (Q) regulations for combustible gas operate on pressure measurements (ΔP) given by the usual pressure-reducing organs (venturi-diaphragm) from the conventional relations linking the flow rate to the DP for a given density ρ of the gas: like Q = k ΔP P .

But, we only know the density P of the gas in the state standard. This means that for a constant standard gas flow (i.e. ΔP constant), it is in fact a variable gas flow (g), in a therefore variable composition around the standard state, which is actually burned in burner 1, with a constant air flow. Setting work of the invention corrects the situation by allowing by calculation to continuously adjust the air flow (a) to the needs of combustion desired actual fluctuating flow rate (g) of the gas to be burned.

This adjustment is advantageously made by calculating a correction coefficient C2 used to calculate the flow control 4 air, with C2 = real PCO / standard PCO for example.

The invention can also be understood in view of the description which follows concerning a second embodiment given as non-limiting example.

The installation is identical to that previously described in difference except that the gaseous fuel source 2 is a mixer adjustable powered by multiple gaseous fuels co-produced in different reactors of a steel factory.

Gaseous fuels contain different rates sulfur impurities which are functions of the type of reactor from which they come.

The sulfur content of each gaseous fuel is known on average, by means of analysis known in themselves.

The "correctometer" 5 according to the invention is as before powered by source 2 which is an adjustable mixer and sends a setpoint for said mixer.

The "correctometer" 5 is identical to the correctometer previously described, with the difference that probe 9 measures the oxide content of sulfur from burner combustion gases 7. The value of this measurement is used to the computer 10 to determine the setpoint to be sent to the gaseous fuels, to maintain in the vicinity of a value of predetermined reference said content of sulfur oxides. The calculator 10 evaluates said instruction as a function, in particular, of the average rates known sulfur of each gaseous fuel.

C') on mesure, dans les fumées de combustion complète, par le capteur 9, la teneur en oxydes de soufre,

D') à partir de cette mesure, à l'aide du calculateur 10, on calcule et on envoie au mélangeur de gaz combustible une consigne de mélange apte à maintenir au voisinage d'une valeur de référence prédéterminée la teneur en oxydes de soufre dans les fumées de combustion.

The successive stages of the process targeted by the second embodiment of the invention, are linked as previously for stages A and B then in the following manner:

C ′) the sulfur oxide content is measured in the complete combustion fumes, by the sensor 9,

D ') from this measurement, using the computer 10, a mixture instruction is calculated and sent to the combustible gas mixer capable of maintaining the sulfur oxide content in the vicinity of a predetermined reference value. combustion fumes.

The advantage of the process according to the invention is that the result of the measurement does not depend on possible interactions in different areas of combustion of the installation, does not depend on unexpected air inlets in installation, emissions or possible absorption by products steelworks placed in the furnace.

Thus overall, the invention allows not only to optimize the operation of the burners 1 of the oven, but also to control the atmosphere of the oven, which has a decisive advantage. Indeed, like the sulfur character of this atmosphere, the more or less oxidizing character has a decisive influence on the quality of the rolled products. If the burner's operation is not sufficiently controlled, a drop for example in the hydrogen content in the combustible gas mixture will lead to an increase in the excess air in the combustion fumes, therefore in the furnace atmosphere . This excess of combustion air, not only will modify the heating temperature, but above all, by the corresponding increase in the O ₂ content of the furnace atmosphere, has a detrimental influence on the metallurgical quality of the products obtained after rolling. , far downstream.

Likewise, an overly reducing atmosphere risks causing uptake of hydrogen in the products to be heated.

Thus, the method and the device according to the invention have the essential advantage of allowing real-time modification of the setpoint supply control of the oven burners, which was not possible with the previous techniques recalled at the beginning. So we check in instant composition of the furnace atmosphere.

The invention also allows very precise counting of thermies used by burners 1 whatever the variations in characteristics of the fuel used.

In addition to the substantial advantages already indicated, the method and the device according to the invention make it possible to carry out measurements (in particular oxygen and sulfur content, flow rate, pressure, etc.) in a very accessible, in places distant from the heating enclosure and by easily removable devices for possible maintenance.

Claims (10)

1. Method for adjusting, in real time, a burner, especially for a metallurgical furnace, which is fed with an adjustable stream of fuel, whose characteristics can vary about a standard state, and with an adjustable flow rate of combustion air (a), characterized in that, during operation of the burner (1):

   a given bleed (F2), the combustion of which develops a flame having a power of at least 100 thermies/h, is taken off the fuel stream (2),

   this bleed of fuel is burnt with a given stream of excess air (F1), ensuring complete combustion of the fuel,

   the value of a quantity representative of the content of at least one of the components, chosen from sulphur and residual oxygen, is measured in the combustion smoke,

   this value is compared with a predetermined reference value representative of the desired state of operation of the burner, and

   if the deviation is greater than the predetermined threshold, the burner feed is adjusted so as to reduce this deviation.
2. Method according to Claim 1, characterized in that the said measured quantity reflects the deviation from stoichiometry, in that the value of a thermal property expressing the instantaneous actual characteristics of the fuel is determined from this measured quantity and a set value, representative of the said value, is applied to the means (4) of regulating the incoming flow of combustion air (a) feeding the burner.
3. Method according to Claim 2, characterized in that the quantity representative of the deviation from stoichiometry is the residual oxygen content in the smoke after combustion.
4. Method according to Claim 2, characterized in that the thermal property of the fuel, expressing its actual instantaneous characteristics, is its combustion air consumption (CAC).
5. Method according to Claim 4, characterized in that the quantity applied to the means (4) for regulating the flow of combustion air (a) is a correction factor C2 defined as the instantaneous actual CAC/standard CAC ratio, this standard CAC value being that of the CAC of the fuel taken in its standard state.
6. Method according to Claim 1, characterized in that the said fuel being an adjustable mixture consisting of variable proportions of several fuels containing different amounts of sulphur-containing impurities, the said measured quantity is the content of sulphur oxides in the smoke and in that, depending on the said measured quantity, the burner feed is adjusted by modifying the proportions of a mixture of the said fuels in order to maintain the content of sulphur oxides at a predetermined level.
7. Apparatus (5) for the implementation of the method according to any one of Claims 1 to 6, characterized in that it comprises:

   on the one hand, a mini-furnace consisting of at least:

   a combustion tunnel whose length is sufficient for the complete combustion of a fuel, separated at one of its ends, to be completed at its other end,

   a burner mounted at one of the ends of the tunnel and sized in order to develop a heating power of at least 100 th/h,

   means for adjusting the feed for the said burner;

   on the other hand, at the other end of the tunnel left open to allow the combustion smoke to escape, means for measuring a quantity representative of the content of at least one of the components of the said smoke and a computing unit which receives as input the said measurement and determines, in the form of a signal available at its output, a set value for adjusting the feed for the said burner.
8. Apparatus (5) according to Claim 7, characterized in that the adjustment means allow the flow of fuel and the flow of combustion air (a) to be controlled and in that the computing unit evaluates the actual instantaneous thermal properties of the fuel.
9. Apparatus according to Claim 7, characterized in that the means for measuring the said representative quantity are a probe (9) for measuring the residual oxygen content in the smoke.
10. Apparatus according to Claim 7, characterized in that the adjustment means adjust the proportions of the fuel mixture and in that the means for measuring the said representative quantity are a probe for measuring sulphur oxide content in the smoke.

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