FR2992058A3 - Tempering furnace for treatment of metal product e.g. steel tube, has enclosure with burners that use pre-heated hot air of preset temperature from central rotating regenerator and subjected to on/off control - Google Patents

Abstract

The tempering furnace has an enclosure with burners that use pre-heated hot air of 550[deg] C from a central rotating regenerator. The burners are subjected to on/off control. A system is provided for deriving combustion air into an exhaust gas flue circuit with low speed. The envelope is provided with a unit to guard against excessive temperature. A supported flue of exhaust gas is provided in the envelope.

Description

The present invention relates to a furnace for use in the treatment of metal products such as steel tubes. The furnace must treat tubes of 5 different steel grades by monitoring various parameters of its operation, and in particular: an adequate average temperature, a uniformity of the temperature, a holding time in a temperature range determined by the metallurgists. The furnace and its control / control process are designed to ensure the thermal and therefore metallurgical success of each product. The key factor in the process is to meet the heating objectives for all products, while maximizing the level of production, minimizing operational costs (energy, maintenance, utilities, etc.) and minimizing pollutant emissions. Furnace productivity, heating quality, operational flexibility and control of polluting emissions are therefore the most important features of the income process. The productivity is related to: - The capacity of the heating equipment to maximize heat transfer to the products, while respecting uniformity tolerances. The length of the furnace and the energy recovery over flue gas conditions the useful efficiency of the furnace, and must be optimized according to the representative workbook. The selection of the heating equipment and its location on the oven also has an impact on the oven's productivity. - The availability of the oven, which mainly depends on the maintenance operations and the various scheduled or non-scheduled shutdowns. An optimized design furnace can bring very significant annual gains by improving productivity. For quality heating, the heat treatment requires a certain level of temperature uniformity in the products, in their thickness as well as in their length. Depending on the treatment objectives and the products to be processed, special heating curves may be required, such as long-time holding curves up to 45 min. The heating quality depends mainly on the selected burner technology (extent of the combustion zone, etc.), the location of these burners on the furnace (zoning), the shape of the enclosure and the heating system. burner control. Another sought-after quality of the furnaces of income is their flexibility of operation. The flexibility or flexibility of the furnace is its ability to handle the treatment of a succession of heterogeneous product batches with regard to: 25 o Their dimensions (diameter, length), o The type of steel, o Their position in the furnace (centered or quinconces) while respecting the heat treatment objectives (average temperature and uniformity at the time of removal). The improvement in flexibility has a positive impact on the quality of the treatment, and on the annual productivity of the oven. In addition, a more flexible oven consumes less energy because the energy just needed for product processing is provided by the burners. Flexibility is mainly related to achievable transition times between two batches of products.

This depends on: o the choice of technology: a furnace with direct flames in the enclosure (associated with a circulation turbines ramp in maintenance) ensures the best flexibility, since a change of temperature setpoint or change of format and rate of load will have a direct consequence on the heating demand, without delay of inertia. o the configuration of the burners in the oven, and the control system (Level 1 and Level 2 optimization). The constraints are the inertia of the furnace and the ability of the heating equipment to treat each batch of products individually. In addition, environmental constraints apply to the use of kilns, particularly with regard to pollutant emissions. Standardization furnaces are important sources of emission from the integrated plant. Two species are particularly concerned: o CO2, the main greenhouse gas o NOx, having a negative impact on the ecosystem (acid rain, photochemical pollution, eutrophication), On the stratospheric ozone layer and on human health . The amount of CO2 generated by the furnaces is directly related to the energy consumption, while the NOx emissions are created in the heart of the flames, at high temperature and for specific mixing conditions. In addition, a high oxygen content in the fumes promotes NOx emissions. In a furnace at a temperature below 900 ° C, NOx emissions are not a critical element of the design, because the flame temperatures are lower than in a rotary hearth furnace at 1300 ° C for example. The high-speed burners on the market make it possible to comply with European and French legislation without difficulty. The furnace of income is an energy, mechanical and electrical system whose maintenance is often complex and expensive. The furnace of income is characterized by: o many control and measurement organs o many parts subjected to high thermal and thermomechanical movements and stresses o Availability to be as close as possible to 100% Maintenance is an element essential for optimal performance while maximizing equipment availability.

The maintainability of a furnace must be taken into account from the moment of its design, in order to integrate both preventive maintenance tools and functionalities allowing to space up maintenance actions as much as possible.

It is obvious that the heating means are preponderant in the performance of the oven. The range of possible heating technologies for this type of application is wide: o Cold air burners (front or vault) o Air burners preheated by a centralized recuperator or by a centralized regenerator (front or vault) o Automatic burners recuperative (frontal only) o Regenerative burners (front only) o Oxygen burners (front or vault). The cold air burners are eliminated because of the overall low energy efficiency obtained. Oxyfuel burners are eliminated: the cost of oxygen generates operational costs that can not be made profitable by improving performance. On the other hand, the low smoke flow rate penalizes the heating uniformity, especially at medium / low temperature. Regenerative burners are eliminated: the energy gain is real, but the technology involves installing double the power (cyclic operation, half of the burners operates at a time). The associated additional cost is not very profitable and poses problems of implantation and uniformity of heating. Self-recuperative burners are a good candidate.

The principle of these burners is to re-suck about 80% of the fumes generated around the burner head. The recovered fumes pass through an internal exchanger preheating the combustion air. However, this solution proves to be inadequate in the case of furnaces, for the following reasons: o The self-recuperative burners are difficult to implant in the middle of the oven, because their larger size would require the installation of a long nose of vault, causing loss of heating length. This central zone can nevertheless be equipped with air burners preheated by a small centralized recuperator. o The previous mixed solution provides energy efficiency equivalent to a hot air solution with central recuperator (preheated to 350 ° C). Indeed, the fumes come out of the self-recuperative at more than 500 ° C and can no longer be recovered, while the fumes after recovery are exhausted to less than 400 ° C. An object of the invention is to provide a revenue furnace and a control method for increasing the performance of the income processes. For this purpose, according to the invention, there is provided a furnace with a chamber provided with hot air burners preheated to about 550 ° C by a centralized rotary regenerator, the burners being controlled in all or nothing. These characteristics make it possible to have a relatively low energy consumption while having a uniform heating in the enclosure. Depending on additional features, possibly combined in whole or in part: o The oven includes a combustion air discharge system in the flue system at low speed, to ensure proper operation of the fan and exhaust. This improves the process at low speed. o The enclosure is equipped with protection against over-temperature (hot air, hot smoke). o The oven includes an exhauster for the forced draft of fumes at low temperature. o The enclosure is equipped with a chimney flue system, which is potentially "cold" (low natural draft). This allows optimal oven operation: o In high temperature production (> 750 ° C), o During processing temperature transitions, o During low temperature pilot lights, o During oven starts and stops. The invention also relates to a method for controlling such a furnace.

Other features and advantages of the invention will emerge on reading the following description of particular non-limiting embodiments of the invention. Reference is made to the accompanying drawings, in which: FIG. 1 is a diagrammatic view of a centralized regenerative burner assembly; FIG. 2 is a Sankey diagram of this system in a given configuration, FIG. 3 is a diagrammatic sectional view of the enclosure of the furnace, FIG. 4 is a heating diagram with the position in meters in the furnace on the abscissa and the temperature on the ordinate; FIG. 5 is a diagram illustrating the FIG. FIG. 6 is a perspective view in longitudinal section showing the flow of fumes in the enclosure, FIG. 7 is a histogram comparing a pulse control in all or nothing of the burners and a proportional control. FIG. 8 is a timing diagram showing the firing times of the burners; FIG. 9 is a diagram illustrating the enclosure of the furnace and its division into a plurality of control zones; FIG. 0 is a heating diagram, like that of Figure 4, showing a heating curve in a particular example.

With reference to the figures, the CMI Greenline revenue furnace is precisely designed to meet these industrial constraints, thanks to a combination of dedicated technologies. The oven has an enclosure with a burner-free inlet airlock, arch nose, screen and airlock designed to optimize heating uniformity. The enclosure is equipped with high-speed, low-NOx low-temperature front burners (unlike the so-called "Flameless" technologies), impulse-controlled in all or nothing thanks to the algorithm developed by CMI, ONOFFSoft. This algorithm optimizes the ignition sequences of the burners to avoid pressure surges in the air and gas pipes, and on the furnace pressure.

The enclosure is divided into 3 zones in its length and 5 zones in its width, ie 15 heating zones in total, to adapt the heater to a wide range of lengths and diameters of tubes. Silicon carbide curved crown burners are provided at the end of the furnace to provide excellent thermal uniformity in maintenance. The oven is also equipped with stirring fans in the holding zone, to ensure even heating, especially on difficult heating curves with more than 40 minutes of holding time. This more specifically contributes to ensuring uniformity of heating in the enclosure. The use of rotary central regenerative burners makes it possible to offer a specific consumption that is more than 10% lower than standard optimized techniques (centralized recuperator, or self-recuperative burners), ie a substantial annual saving of natural gas. With this solution, the combustion air can be preheated to about 550 ° C in income at 700 ° C, an overall efficiency of the system of the order of 80%. This system is based on the well known technique of alumina porous matrix regenerators, with multiple industrial applications in the field of block heating, aluminum smelting, glass furnaces, etc. The big advantage of this system in the case of kilns is the performance at "low" flue gas temperature. On the other hand, the hot air generated at about 550 ° C is fully compatible to feed the high-speed burners, without resorting to very high temperature special valves. In recent years, CMI has implemented several ovens with impulse burners and air at 550 ° C. CO2 emissions are minimized, as they are linked to energy consumption. The choice of high-speed enclosure burners incorporates NOx constraints. Note that the impulse control minimizes emissions because the flames are still operating at their rated speed.

The oven design incorporates the following elements: o lightweight concrete refractory materials, to lower the inertia of the oven. o design with direct flame burners, ie directly heating the oven enclosure. A setpoint change will be directly perceived by the heating equipment, without delay. o judicious positioning of thermocouples (TC) in the enclosure. It is recommended here for example 2 thermocouple ramps, at the beginning of the zone and at the end of the zone: the end-of-zone TCs are used because they are linked to the objective of thermal success of the tubes, and the start-of-zone CTs. serve to anticipate the transitions between two lots. This positioning is applied to the heating zone, and the end of heating / holding zone. Several so-called "Intelligent Maintenance" modules are provided and arranged to enable, for example, monitoring of the energy performance of the furnace, the number of cycles of the ON-OFF valves, the nominal performance of each burner, the concordance of the positioning of the valves with respect to the expected, etc. The oven is equipped with a low-speed combustion air blower discharge system to provide optimal air and oven pressure control at any rate. Oven control is provided via a self-correcting loop of air / gas ratio regulation, via a dedicated algorithm and 3 gas flow measurements.

The design of the furnace and the process, as well as the advantages offered, are described in more detail below. The oven profile has been designed to ensure heating uniformity while determining the best technical and economic solution in terms of energy efficiency, operating flexibility, ease of maintenance and NOx emissions. The proposed configuration consists in associating a centralized regenerative system (RC) and a control technology of the heating equipment. By way of example, the RC system, such as that known under the name ECOREG from Jasper GmbH, is illustrated in FIG. 1 The principle is as follows: A porous matrix of cylindrical form, separated into quarters, is installed in a continuous rotation tank to form the rotary regenerator. o The tank is fed from the bottom with cold air, which passes through the matrix vertically upwards. The air comes out between 500 ° C and 550 ° C depending on the oven speed. o The tank is fed from the top by the fumes coming from the oven, which cross the matrix vertically downwards. The fumes come out between 120 ° C and 250 ° C depending on the speed of the oven. o The air and fumes are of course separated inside the tank.

This solution offers the following advantages: o Very high efficiency: the combustion air can be preheated to 550 ° C with fumes coming out of the oven at 650 ° C. o More compact system than a tube bundle recuperator, thanks to the porous matrix offering a very favorable surface / volume ratio. A smoke exhauster is necessary, but since the fumes are cold, the size of the exhauster is reasonable. o With air at 550 ° C, it is possible to use piping, valves and standard instrumentation, without the use of "noble" materials such as stainless steel. The burners are provided internally, and the pipes by an external insulator. This combination is original and has never been applied as such in the past.

The energy balance under the same conditions as above is given in Figure 2. The furnace chamber can be broken down into four sections: o The airlock in which the fumes transmit their energy by radiation and convection, according to a ratio that depends on the treatment temperature. At more than 700 ° C, the radiation becomes predominant. o The heating section, in which the temperature difference between products and enclosure remains high. Heating demand in this area is typically very high in most production cases. o The End of Heating / Maintenance section, in which the temperature of the products tends to the treatment temperature, asymptotically. For long hold times, this area provides the desired uniformity. o Finally the maintenance section, in which the heating uniformity tolerance is reached, at the desired average treatment temperature. More precisely: o the airlock is without a burner, o the Heater section is equipped with a row of 25 front burners. o the end of heating / maintenance section equipped with 25 front burners, and separated upstream by a vaulted nose and downstream by a suspended refractory wall. This partition is essential for a good separation of flue gases in the sections. The height of the partitions is determined according to the largest diameter of the treated tubes, to which is added a margin of safety. The end of the section is equipped with 5 stirring turbines, to ensure uniformity of temperature for long holdings (40 to 45 minutes). o the airlock or Uniformisation is equipped with front burners, also equipped with 5 stirring turbines, and 7 burners of sole to compensate for the lower losses, in particular by the rollers. This cutting into sections allows precise control of the heating curve over the length of the oven. The homogeneity of longitudinal temperature on the tubes during maintenance is ensured by: o The configuration of the burners in the enclosure and the associated control / control (cascade control, as explained below). o The 2 ramps of 5 turbines on both sides of the separation wall, allowing circulation / mixing of fumes at the end of the oven. This combination secures the desired level of holding uniformity, +/- 3 ° C during the entire hold, and +/- 2.5 ° C at the end of the oven. The heating power per section, and the speed of the turbines are dimensioned for the thermal success of the products in the different cases of sizing production. A heating curve is illustrated in FIG. 4. The following paragraph explains how the control / control method achieves excellent heating uniformity of the tubes for the wide range of holding times while finely controlling the combustion ( air / gas ratio). The control cascade summarizes how the heating equipment is operated, from the combustion air blower and the natural gas regulator 20, to each burner. Four control loops are implemented in cascade, as illustrated in FIG. 5: The burners are controlled individually, under the control of the ONOFFSoft method of controlling the burners 25 (see below). o The process zones make it possible to control the heating profile in the width of the oven (see below). o The air / gas ratio of the Sections (Heater, End of Heater / Hold and Hold) is finely controlled (see below). o Finally, the air and gas service pressures are regulated independently regardless of the furnace demand (see below). This combination is of course in accordance with the EN746-2 safety standard on combustion plants. The advantages of the control cascade are summarized in Figure 5. These are discussed below. All burners are operated in All-or-Nothing 10 (ON / OFF), which brings the following advantages: o The flames always have the same shape: same length for the frontal ones, same cover for the burners of vault. The heating quality is therefore constant, regardless of the load and the temperature level of the oven. o NOx emissions are minimized because the burners still operate at rated speed. The circulation of fumes under the products is favored, which also improves the uniformity of heating (see FIG. o Zone thermocouple measurements are more reliable because they do not depend on the burner regime. Indeed a short or long flame locally influence the thermocouple, and does not account for the average real temperature level of the area. The ONOFF control consists in igniting the burners a fraction of the time at rated speed. On average, the energy released into the enclosure is the same as proportionally controlled burners (see Figure 7).

The classic difficulties of ONOFF are mainly the possible influence of the ignitions / extinctions of the burners on the pressures upstream (air and gas) and downstream (enclosure) of the burners (effects of jerk). To eliminate these disadvantages, each burner (front) or group of burners (vault burners grouped by 4) is considered independent with regard to the ignition sequences. The ONOFFSoft process will: o Retrieve the heat demands of each burner generated by the heating automaton. o The constrained ONOFFSoft optimization algorithm will search for optimal ignition sequences to ensure a smoothed average flow rate in the air / gas manifolds and in the flue gas circuit. ONOFFSoft determines the optimal cycle time, and "sub-cut" possibly the ignition sequences, to arrive at a better optimization (see Figure 8 where the numbers 1, 2_ to 8 each designate a burner or group of burners) . The ONOFFSoft process is implemented by a program installed on a dedicated server (standard configuration, no need for large CPUs), which communicates with the heating controller. Note that a Backup algorithm is programmed in the PLC, so the oven can run without the ONOFFSoft process, but with poorer performance. The control of the heating profile over the width of the oven is essential to ensure good uniformity at all speeds and regardless of the length of the load. To do this, the oven will be "divided" into 5 zones in the width of the oven, for each of the 3 heating sections, ie 20 process zones in total.

Each process zone is equipped with a thermocouple. We do not duplicate the thermocouples, because if one is faulty, the mesh is fine enough to use the neighboring thermocouple while waiting for the defective replacement. The burners are associated with each process zone (5 burners per process zone), but there is no additional winnowing / instrumentation per process zone, since the burners are individually controlled by ONOFF. Figure 9 illustrates the proposed process zone division. This combination brings a real flexibility of heating, because it is possible to change the combination of burners to a thermocouple to refine the heating profile. To better control the air / gas ratio of the installation, the 3 heating sections are each equipped with a dedicated gas supply manifold, with a so-called "self-corrective" control valve and a flow measurement. . On the combustion air side, there is also a flow rate measurement per diaphragm on each of the three supply manifolds. A control loop continuously corrects the air / gas ratio of each heating section, by cutting the hot air flow calculated on the zones (which depends on the number of burners on, the temperature and the operating pressure). and the flow rate calculated and measured on the gas.

This simple and original system ensures a permanent self-correction of the air / gas ratio, which brings the following advantages: o Optimum adjustment of the air / gas ratio, which avoids unnecessary overconsumption o Control of the oxygen level in the oven at desired level o Good burner combustion, to minimize NOx emissions, without CO emissions.

Finally, at the head of the control cascade, the operating pressure of natural gas and combustion air are regulated at fixed setpoints, which depend on: o The maximum air / gas ratio required on the heating sections o Temperature Combustion air o Reference air and gas pressures These pressures are regulated by the combustion air fan blades, and by a head valve downstream of the natural gas expansion block. Note that at low oven speed, the fan may not hold its set pressure, even fully closed sashes. To avoid this problem, a cold air and flue gas discharge valve ensures minimal fan speed. The other advantage of the discharge system is to be able to properly regulate the pressure of the oven even at very low speed, the air discharged into the flues used to "force" the exhaust smoke.

A reference run was simulated to establish the heating curves and to quantify the energy performance of the furnace. The operating conditions are given in the table below. Grade Low carbon steel Dimensions 244.5x11.99x14: 10Ornm Targeted productivity 5-8 t / h - (6-0-tulaes / h) T ° treatment 696 ° C Holding time 10 min The heating curve is given in figure 10. The specific consumption is only 144 kWh / t, because the original solution retained allows to recover a very large part of the energy of the fumes. In this case of calculation, the fumes come out at about 150 ° C in the chimney, without dilution. The air is preheated in this case at 550 ° C. The furnace is intended to process multiple batches of tubes having different characteristics: o Format o Nuance o Maintenance time goal, related to metallurgy o Number of treatments (single or double) Annual oven productivity will be linked: o At the maximum possible oven rates for each batch: depends on the length of the oven and the 20 installed power per zone. This point has been described previously. o At the programmed or unscheduled shutdown rate of the line. o At the "distance" required between 2 batches of successive tubes, to ensure the success of the treatment for all the tubes. This last point contributes significantly to the annual productivity loss. The minimization of the distance between two successive batches must therefore be taken into account: In the scheduling of the production, so that the transitions between batches are the least important possible (ex: not to pass from a temperature of maintenance of 550 ° C to 800 ° C between two successive batches o The design of the oven These choices must make it possible to shorten the distance between two batches of tubes of less than half the length of the oven, ie about ten meters. Of course, the invention is not limited to the embodiments described but encompasses any variant within the scope of the invention.The furnace may comprise only a part of the features set out above in the description of the embodiment. prefer.

Claims (13)

REVENDICATIONS1. A furnace having an enclosure provided with hot air burners preheated to about 550 ° C by a centralized rotary regenerator, the burners being controlled in all or nothing. 2. A furnace according to claim 1 comprising a flue gas circuit and a combustion air discharge system in the low flue gas circuit. 3. Furnace of income according to claims 1 or 2, wherein the enclosure is equipped with means of protection against over-temperature (hot air, hot smoke). 15 4. A furnace according to any one of claims 1 to 3, comprising a centralized regenerative system and an exhauster for the forced draft of fumes at low temperature. 5. A furnace according to any one of claims 1 to 4, wherein the enclosure is provided with a flue gas assisted draft system. 6. A furnace according to any one of claims 1 to 5, wherein the furnace is divided longitudinally into: - an inlet lock without burner, - a section (Heater) equipped with a row of front burners, a section (end of heating / maintenance) equipped with front burners and end-of-section stirring turbines, and separated upstream by a vaulted nose and downstream by a suspended refractory partition, - an airlock of exit or of 'Standardization equipped with front burners, hearth burners and also equipped with mixing turbines. 35 7. A method of controlling an oven according to any one of claims 1 to 6, comprising the step of controlling the pulse burners in all or nothing so as to optimize the ignition sequences of the burners to avoid jolts pressure in the air and gas pipes, and on the oven pressure. 8. A method of controlling an oven according to claim 7, including the step of monitoring the energy performance of the furnace, the number of valve cycles, the nominal performance of each burner, the concordance of the positioning of the valves relative to the expected, etc. 9. A method of controlling an oven according to claim 7, wherein the control of the furnace is provided via a self-corrective loop of the regulation of the air / gas ratio, via a dedicated algorithm and gas flow measurements. 10. A method of controlling an oven according to claims 6 and 7, wherein a heating curve is controlled over the length of the oven to ensure temperature homogeneity along the length of heated products during maintenance. 11. A method of controlling a furnace according to claim 10, wherein a control loop continuously corrects the air / gas ratio of each section of the furnace, by intersecting the hot air flow calculated on the sections ( which depends on the number of burners ignited, the temperature and operating pressure) and the gas flow calculated and measured. 12. The method of controlling an oven according to claim 7, wherein the burners are controlled so as to ignite the burners by pulse of a fraction of the time at rated speed. 13. The method of controlling an oven according to claim 7, wherein the heating profile is controlled over the width of the oven to ensure good uniformity at any rate and regardless of the length of the load in the oven.