EP0088717A1 - Method to optimize the operation of a furnace - Google Patents

Abstract

1. Method to optimize the operation of an industrial furnace used for heating metallic material such as a soaking pit, a pusher type furnace or a walking beam type furnace, wherein the ratio of the fuel flow rate to the oxydant is adjusted in order to maintain a set residual amount of oxygen or carbon monoxyde in the exhaust gases, characterized in that at the start the working characteristics of the furnace are established for different temperatures, different furnace loads and different residual amounts of oxygen or carbon monoxyde in the exhaust gases, in that these characteristics are memorized, in that during normal furnace operation the actual furnace temperature and the oxydant flow rate are measured and in that the fuel flow rate is adjusted in conformity with the memorized characteristics in order to have the desired residual amount of oxygen or carbon monoxyde in the exhaust gases.

Description

The invention relates to a method and a device for optimizing the operation of an oven, in particular an industrial oven used for heating metallic materials (Pit oven, beam oven, pushing oven etc.). It more particularly relates to the optimization of the ratio between the fuel and the oxidizer, with the aim of having in the exhaust gases only compounds which are neither oxidizing nor oxidizable.

Optimizing this report is essential from an energy and metallurgical point of view. To ensure complete combustion, it is common practice to operate ovens used to heat steel ingots with an excess of oxygen (0.2 to 0.5%). However, the Applicant has found that this excess oxygen is only partially found in the exhaust gases, the remaining quantity of oxygen having oxidized the metal, which can cause weight losses of the order of 2 %. If, on the other hand, the supply of oxygen is reduced, the oxidation of the metal is reduced, but the combustion is no longer complete.

It is obvious that a manual adjustment of the ratio between the oxidant and the fuel does not give convincing results. Therefore, automatic regulation systems have been developed. Unfortunately the fuel = oxidizer x constant i.e. that one admits the existence of a fixed relationship between the fuel and the oxidizer, whatever the operating conditions of the furnace. However due to the complex characteristics of a flue oven, provide heat of gases, temperature of the product which is heating) this equation is only verified for a well-defined oven operating regime.

To overcome this drawback, the exhaust gases are constantly analyzed and the ratio between the fuel and the oxidant is readjusted, as a function of the determined components (C0, C02, 02). To carry out this analysis, there are mainly two systems which are distinguished above all by the mounting of the detector.

In a first system, the detector is in the form of a long refractory tube housing a detection cell and a thermocouple. The detector must be in direct contact with the gases to be analyzed and preferably be placed in the middle of the flue. Given the thickness of the flue walls covered with refractories, this last condition is difficult to achieve given the length of the refractory tubes offered on the market. The temperature variations to which this probe is subjected strongly influence the measurement results, so a correction as a function of temperature is essential.

To this is added a complicated technique for mounting these detectors which do not withstand any thermal shock and which usually have to be replaced without the oven being able to be stopped. After each detector replacement, a delicate calibration must be repeated.

In a second system, the detector is in the form of a measuring cell which is mounted in a chamber of small dimensions, in which the temperature is kept constant at around 800 ° C. A pump sucks the fumes to be analyzed through a sampling probe fitted with filters and discharges them through the analysis chamber. In this system, the depth of the sample can be obtained using refractory tubes, which are cut to the desired length.

Correction of the analysis results as a function of temperature is not necessary. Replacing the detector does not pose the problem of thermal shock. The calibration is done quite easily by injecting a gas of known composition into the analysis chamber.

The disadvantages consist in a complex sampling and treatment of smoke with probes, filters, purges and valves which require a lot of maintenance and which represent as many possibilities of faults as for example the air intake in the vacuum lines.

The object of the invention is to propose a method for regulating an oven which does not have the defects described above, namely a limited lifetime of the measuring cells, high installation and purchase costs and reliability. of questionable operation.

This object is achieved by the process according to the invention in which either the flow rate of the fuel or that of the oxidant is measured and where the flow rate of the oxidant resp. fuel, following a preset response curve.

Preferential embodiments of the invention are described in the subclaims.

The advantages obtained by the invention essentially consist in that the optimal ratio between the fuel and the oxidizer is not determined by means of fragile gas analysis devices, but by electronic components which calculate the optimal conditions for operation based on the characteristics of the oven which have been carefully noted once and for all. These characteristics can be checked once a year or in the event of operating anomalies. The production costs of the _b _y-stème are reduced and the apparatus requires practically no maintenance.

• La figure 1 montre un schéma de principe illustrant la régulation d'un four Pit.
• La figure 2 montre un diagramme de combustion optimal relevé sur un four Pit.

The invention will be explained in more detail with the aid of figures which represent only one embodiment.

• Figure 1 shows a block diagram illustrating the regulation of a Pit furnace.
• Figure 2 shows an optimal combustion diagram taken from a Pit oven.

In FIG. 1 is shown a Pit 1 furnace comprising a device 2 for evacuating combustion fumes which pass through two heat exchangers 3 and 4 serving to preheat the fuel and the oxidizer. The fuel in this case is blast furnace gas (supplied via line 5a) enriched with natural gas (supplied via line 5b) so as to maintain a constant PCI (intrinsic heat capacity). The constant energy capacity of the fuel is ensured by a dosing system 5c. It should be noted in passing that when using pure natural gas or liquid fuel, there is no preheating.

The fuel flow is controlled by a fan 6a and by a valve 6c controlled by a servo motor 6b. This device is preferably located upstream of the exchanger 4.

As oxidizer, it is possible to use air optionally enriched with oxygen, the flow rate of which is likewise controlled by a fan 7a and a valve 7c controlled by a servo-motor 7b. The valve 7c is located between the heat exchanger 3 and a flow meter 15. The fuel and the oxidizer are then supplied via the lines 8 resp. 9 to burner 10 of the oven. A pressure sensor 11 monitors the pressure inside the furnace and transmits the measured values to a pressure regulating system 12a controlling a servomotor 12b, which acts on a valve 12c, preferably mounted downstream of the exchanger heat 4, in the flue evacuation chimney.

The temperature of the combustion fumes is continuously monitored upstream and downstream of the heat exchangers 3,4 by the sensors 13 b and 13 c and safety devices 16a and 16b bring ambient air into the device. smoke evacuation 2 when the measured temperatures exceed predetermined limits. It should also be noted that the temperature of the fuel as well as that of the oxidizer are measured downstream of the heat exchangers 3 and 4, by the sensors 13d, 13e.

• On fait fonctionner le four à sa température d'utilisation normale, qui est dans le cas présent d'environ 1300°C. On surveille la composition des fumées d'échappement à l'aide d'un analyseur de gaz 17 du genre décrit précédemment. On ajuste de façon connue le rapport entre le combustible et le comburant de sorte à avoir une concentration résiduelle d'oxygène choisie d'avance. Lorsqu'on introduit p.ex. une charge de lingots froids dans le four, la température mesurée par l'intermédiaire du capteur 18 chute. On augmente de façon connue le débit de combustible et de comburant et on ajuste leur rapport en fonction des mesures faites par l'analyseur de gaz 17 de sorte à retrouver la concentration résiduelle d'oxygène voulue. Au fur et à mesure que le lingot chauffe, la demande d'énergie diminue et il y a de nouveaux réglages à faire. On trace ensuite pour une température constante du four le diagramme qui montre quel débit de combustible correspond à quel débit de comburant pour obtenir une concentration résiduelle d'oxygène ou de CO donnée dans les fumées de combustion.

For the implementation of the invention, the procedure is then as follows:

• The oven is operated at its normal operating temperature, which in this case is about 1300 ° C. The composition of the exhaust fumes is monitored using a gas analyzer 17 of the type described above. The ratio between the fuel and the oxidizer is adjusted in a known manner so as to have a residual oxygen concentration chosen in advance. When, for example, a load of cold ingots is introduced into the oven, the temperature measured by means of the sensor 18 drops. The flow rate of fuel and oxidizer is increased in known manner and their ratio is adjusted according to the measurements made by the gas analyzer 17 so as to find the desired residual oxygen concentration. As the ingot heats up, the demand for energy decreases and there are new adjustments to be made. A diagram is then drawn for a constant temperature of the furnace which shows which flow of fuel corresponds to which flow of oxidant in order to obtain a given residual concentration of oxygen or of CO in the combustion fumes.

In FIG. 2, this curve is shown for an excess of oxygen of 0.4% with the air flow rate on the abscissa and the combustible gas flow rate on the ordinate. The oven temperature was kept constant at 1320 ° C. Although a priori this curve can be of any shape we see that it is linear in the present case. The Applicant has found that this response curve is always close to a straight line in the normal operating ranges of a Pit furnace. This circumstance simplifies the automation of the operation of this oven, but in no way limits the scope of the invention which is in fact independent of the shape of this curve.

Once the response curve (s) for the temperature (s) and residual oxygen or CO concentration (s) have been noted and possibly stored in the calculation system 24, the gas analyzer is short-circuited 17 and the oven is operated automatically. The data defining the residual quantity of oxygen or of CO desired in the combustion fumes is entered into the calculation system 24. The device 22 is adjusted to the desired temperature. The actual oven temperature, measured using the sensor 18, is compared to the desired temperature in a comparator 21 which transmits, depending on the result, an opening or closing signal to the servomotor 7b coupled to the valve 7c.

The actual air flow is measured using a flow meter 15, connected to a flow-voltage converter (analog or digital) 23 which communicates the measurement result to the calculation system 24. The system 24 transmits to the regulator 25, for each oxidant flow rate measured, the fuel flow rate required to have combustion in accordance with the residual oxygen or CO concentration desired in the combustion fumes. To further optimize the operation of the oven, the calculation system 24 is also connected to the temperature sensor 18 so that it bases its calculations on the data which are in agreement not with the chosen temperature, but with the instantaneous temperature of the oven. .

In the example of a Pit furnace, as described above, in which there is a simple linear relationship between the flow rates of fuel and oxidant in order to have optimal combustion, the calculation system 24 can be reduced to a simple analog circuit in which the measured air flow is multiplied by a factor equal to the slope of the line shown in fig.2 and a fixed value is subtracted depending on where the line crosses the ordinate axis. In this case, the operation of the oven is based on a single response curve corresponding to the operating temperature. chosen oven and no account is taken of instantaneous variations in the temperature thereof.

The optimum fuel value which corresponds to the measured air flow is transmitted to the regulator 25 which acts on a servo motor 6b connected to the valve 6c. The actual fuel flow is monitored using the flow meter 14 and compared, after conversion in the flow-voltage converter 26, with the set value calculated by the device 24. The regulator 25, which compares these two values, then transmits a correction signal to the servo motor.

We note that the air flow is not directly controlled in the example described, although a control is achievable with reduced means, since we measure this flow anyway. This is due to the fact that the air flow is indirectly controlled by the temperature measured in the oven.

In the example described, the fuel flow rate was adjusted in accordance with the oxidant flow rate. The reverse is also true.

It is obviously possible for multipurpose ovens, to store in the calculation system 24 a multitude of fuel-oxidizer diagrams for different oxygen and CO excess and then to easily switch from one operating regime to another by simple electronic settings.

Claims (9)

I..Process for optimizing the operation of an oven, in particular an industrial oven, in which the flow ratio between fuel and oxidizer is adjusted so as to maintain a fixed residual quantity of oxygen or CO in the flue gases combustion, characterized in that the oxidant flow is measured and that the fuel flow is adjusted accordingly in accordance with predetermined oven response curves. 2. Method according to claim 1, characterized in that one establishes the response curves with variable load while keeping the temperature of the furnace and the residual quantity of oxygen or CO in the combustion fumes constant. 3. Method according to claims 1 or 2, characterized in that establishes the response curves for each operating temperature of the furnace coming into consideration as well as for each residual amount of oxygen or CO in the combustion fumes. 4. Method according to one of claims 1 to 3, characterized in that the response curves are stored in memory in a computer which calculates as a function of the actual temperature of the furnace and of the oxidant flow rate, the fuel flow rate which corresponds to it to have the residual quantity of oxygen or CO desired in the combustion fumes. 5. Method according to one of claims 1 to 3, characterized in that the operating temperature of the oven is fixed and that the analog response curve of the oven is recreated, corresponding to this temperature and to the desired residual quantity d or CO in combustion fumes. 6. Method according to one of claims 1 to 5 wherein the flow rate of the fuel is measured instead of that of the oxidizer and where the flow rate of the oxidant is adjusted instead of that of the fuel. 7. Device for implementing the method described in one of claims 1 to 6, characterized in that it comprises means (22) for choosing an operating temperature of the oven, means (14,15) for measuring the fuel flows resp. oxidizer sent to the burner (10), means (6b, 6c, 7b, 7c) for influencing said flow rates, means (21) for comparing the actual temperature of the oven with the temperature chosen in advance and for controlling the increase resp. the reduction in the oxidizer flow rate when the oven temperature drops below resp. exceeds the chosen temperature, means (24) for calculating, as a function of the flow rate of the oxidant measured and of the operating temperature chosen, the flow rate of fuel resulting in the desired residual quantity of oxygen or CO in the combustion fumes and means (25,6b, 6c) for adjusting the fuel flow to the calculated value. 8. Device according to claim 7, characterized in that it comprises means (25) for comparing the actual fuel flow with that which has been calculated and for modifying the flow so that the difference between the two values tends towards zero. 9. Device according to claim 7, characterized in that the calculation means (24) are also connected to the temperature sensor (18) of the oven, so as to calculate the fuel flow rate not as a function of the operating temperature chosen , but based on the actual oven temperature.

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