METHOD FOR OPERATING AN ELECTRIC ARC FURNACE

Abstract

A method for operating an electric arc furnace having at least one electrode, the method including the following steps: introducing material that is to be melted in the form of an actual mass flow into the electric arc furnace and feeding electrical energy via at least one electrode into the electric arc furnace in order to melt the introduced material depending on a previously determined, necessary electrical energy input. The necessary electrical energy input into the arc furnace is determined depending on the mass flow input into the furnace.

Claims

1-8. (canceled)

9. A method for operating an electric arc furnace having at least one electrode, the method comprising the steps of: introducing material as an actual mass flow into the electric arc furnace, wherein the electric arc furnace is an electric reduction furnace for melting and reducing the material introduced; feeding electrical energy via the at least one electrode into the electric reduction furnace to melt the introduced material according to a required electrical energy input determined previously; and determining the required electrical energy input into the electric reduction furnace depending on the mass flow q.sub.mactual input into the furnace, wherein the required energy input is determined as a power setpoint value P.sub.setpoint and is introduced into the electric reduction furnace by either a power open-loop control or a power closed-loop control, wherein the step of determining the required energy input comprises the following sub steps: predefining or determining a specific energy demand as an energy demand parameter k.sub.1, wherein the determining is effected depending on a predefined expected energy value for operation of the electric reduction furnace, the mass flow q.sub.mactual input into the electric reduction furnace, and/or depending on properties of the input material; predefining or determining thermal energy stored in a vessel of the electric reduction furnace in relation to a mass of slag and to a mass of the material tapped from the electric reduction furnace as a loss parameter k.sub.0; and determining the required electrical energy input as a power setpoint value P.sub.setpoint for the electric reduction furnace depending on the mass flow q.sub.mactual input into the electric reduction furnace, the energy demand parameter k.sub.1 and the loss parameter k.sub.0.

10. The method as claimed in claim 9, wherein the properties of the material introduced into the electric arc furnace which are taken into account when determining the specific energy demand are at least one of the group consisting of: chemical composition, temperature and/or moisture content of the input material, and whether the input material had previously already been subjected to a prereduction.

11. The method as claimed in claim 9, wherein the determination of the required electrical energy input P.sub.setpoint is effected using a predefined functional relationship between the power setpoint value and the parameters q.sub.mactual, k.sub.0 and k.sub.1, and optionally also a correction factor k.sub.2.

12. The method as claimed in claim 11, wherein the functional relationship is represented as follows:
P.sub.setpoint=(k.sub.0+k.sub.1*q.sub.mactual)*k.sub.2

13. The method as claimed claim 9, wherein the determination of the required electrical energy input P.sub.setpoint and of at least individual parameters from among those required for calculating said energy input is effected continuously during the operation of the electric reduction furnace.

14. The method as claimed in claim 13, wherein the individual parameters are the mass flow q.sub.mactual, the specific energy demand k.sub.1 and/or the loss parameter k.sub.0.

15. The method as claimed in claim 9, wherein actual power input into the electric reduction furnace is set to a power setpoint value in the power open-loop control or is controlled to the power setpoint value in the power closed-loop control, in each case by varying an ignition angle of a power converter as actuator or by varying a setting of a tap switch of a transformer of the electric reduction furnace as actuator.

Description

[0033] Six figures are appended to the description, wherein

[0034] FIG. 1 illustrates the method according to the invention for operating an electric arc furnace;

[0035] FIG. 2 illustrates the energy balance in an electric arc furnace;

[0036] FIG. 3 illustrates a functional relationship between the energy or power to be fed to the furnace and the mass flow input into the furnace;

[0037] FIG. 4 illustrates the closed-loop control of the electrical power according to the present invention;

[0038] FIG. 5 illustrates a method for operating an electric arc furnace from the prior art; and

[0039] FIG. 6 illustrates the effects of a material input into the electric arc furnace on the profile of the electrical energy in the prior art.

[0040] The invention is explained in detail below in the form of exemplary embodiments with reference to FIGS. 1 to 4 mentioned.

[0041] FIG. 1 shows the function blocks for the open-loop control and closed-loop control of the material input and energy input according to the invention into an electric arc furnace.

[0042] By means of an input possibility of the open-loop control of the material input 401, the operator can predefine a setpoint value 501; q.sub.m setpoint for the material input or the mass flow. According to the desired material input, at least one material conveying device is actuated and the amount of material actually introduced is measured by means of at least one weight detector 303. The function block 401 calculates the actual material input 502; q.sub.m actual as a function of time.

[0043] In an advantageous closed-loop control, the desired material input (setpoint value) is compared with the actual material input (actual value) and the difference is fed to a controller, which adapts a manipulated variable for the material conveying device 302 such that the control difference becomes as small as possible.

[0044] The function block 402 determines the specific energy demand for the furnace 100 on the basis of measured or input material parameters 504. Said material parameters can be e.g. the composition and analysis of the feedstock materials, information concerning the prereduction, temperature, moisture content, or other values that are expedient for determining the specific energy demand. Furthermore, an expected value 503 can be predefined and taken into account in the determination of the specific energy demand. The specific energy demand can be compared with historical data from a technological database (403) and can optionally be concurrently adapted by a process model. As a result, the function block 402 transfers a coefficient k.sub.1 representing the specific energy demand to the downstream open-loop control steps. In this case, the coefficient k.sub.1 is preferably cyclically recalculated.

[0045] If e.g. material that has already been prereduced is introduced into the reduction furnace, the coefficient k.sub.1 decreases according to the material properties. In this case, the specific energy demand falls and the energy efficiency rises.

[0046] In the function block energy and mass balance 404, essentially the thermal energy stored in the furnace vessel 101 is determined from temperature and mass of the slag 102 contained and the metal 103 contained, and the electrical W.sub.v-el and thermal W.sub.therm losses are calculated. For this purpose, the function block 404 receives all required and expedient furnace process values 505 expedient assumptions and historical data from the technological data base 403.

[0047] FIG. 2 illustrates the manner of operation of the function block energy and mass balance 404 in greater detail.

[0048] The energy fed to the furnace vessel 101 results from the sum of the electrical energy W.sub.electr. and the chemical energy W.sub.chem fed in with the feedstock materials.

W.sub.in=W.sub.electr.+W.sub.chem  (2)

[0049] The energy dissipated from the furnace vessel results firstly from the thermal energy W.sub.exhaust gas dissipated by the hot exhaust gas, the electrical W.sub.v-el and thermal W.sub.therm losses, and also from the energy dissipated by the hot slag W.sub.slag and the hot metal W.sub.metal:

W.sub.out=W.sub.exhaust gas+W.sub.metal+W.sub.slag+W.sub.v-el+W.sub.therm  (3)

[0050] In this case, the thermal energy losses are substantially represented by thermal losses to cooling water and cooling air, and other thermal losses to the surroundings.

[0051] The difference between the energy fed in and the energy dissipated is a measure of the thermal energy W.sub.g stored in the furnace vessel 101.

W.sub.g=W.sub.in−W.sub.out  (4)

[0052] In this case, the stored thermal energy W.sub.g is substantially subdivided into the energy of the liquid metal and of the slag.

[0053] The mass of slag 102 and metal 103 contained in the furnace vessel 101 increases as a result of the material input q.sub.m actual fed to the furnace vessel and decreases as a result of the tapped metal q.sub.m metal and the tapped slag q.sub.m slag. Furthermore, the mass decreases to a small extent owing to dust losses q.sub.m dust.

q.sub.m in=p.sub.m actual  (5)

q.sub.m out=q.sub.m metal+q.sub.m slag+q.sub.m dust  (6)

q.sub.mg=q.sub.m in−q.sub.m out  (7)

[0054] Referring to FIG. 1 again:

[0055] The results of a discontinuous or continuous measurement of the temperature of the metal and/or slag, the bath level and/or the furnace temperature, and other expedient process measurements can additionally be used to correct the concurrent calculation of the energy and mass balance 404. The task of the function block 404 is firstly to provide the operator with these data for observing the process and secondly to keep in balance the energy W.sub.g stored in the furnace in relation to the mass of the slag q.sub.m slag and of the metal q.sub.m metal. For this purpose, the function block (404) calculates a coefficient k.sub.0 for the function block 405. In this case, the coefficient k.sub.0 is cyclically recalculated.

[0056] A good online visualization of the material/energy balance on a supervisory monitor improves the process supervision, increases the understanding of the metallurgical process and simplifies the operator control. This can be effected e.g. in the form of Sankey diagrams or similar diagrams or tables.

[0057] If the heat losses decrease for various reasons, the coefficient k.sub.0 decreases accordingly. The energy consumption falls and the energy efficiency rises.

[0058] The function block 405 determines a power setpoint value P.sub.setpoint; 507 from the material input q.sub.m actual fed in, the coefficient k.sub.0 and the coefficient k.sub.1 in accordance with a curve/table or mathematical function stored in the function block. FIG. 3 shows by way of example such a possible curve or a first-order mathematical function.

P=k.sub.0+k.sub.1*q.sub.m  (8)

[0059] Deviations of this function are conceivable depending on the embodiment and metallurgical process.

[0060] In one advantageous embodiment, the coefficients k.sub.0 and k.sub.1 are continuously or cyclically captured and optimized by the open-loop control/process model, depending on interactions that occur.

[0061] Via an operator interface, the operator has the possibility of manually intervening in the automatic calculation of the values and correcting the latter if necessary. Moreover, it is conceivable, for different operating points of the process, to store different coefficients in an electronic table or within the database.

[0062] The function block of the power measurement (408) determines the actual power per electrode or per pair of electrodes from the current measurement (205) and the voltage measurement (206), by means of the mathematical relationship

P=U*I*cos Φ  (9)

[0063] In this case, the power factor cos Φ corresponds to the phase shift between current and voltage. Depending on the process and arrangement of the measuring instruments (205; 206), the assumption cos Φ=1 can be made for simplification. As a result, the relationship is simplified to

P=U*I  (10)

[0064] The function block 406 serves for the closed-loop control of the power input. Within the function block, firstly the control deviation is calculated from the difference between setpoint power (P.sub.setpoint) and actual power of the respective electrodes. Depending on the control deviation, a controller determines the manipulated variable (y) for the downstream ignition pulse generator (407), in accordance with a controller characteristic curve stored in the controller or an electronic calculation method. Said generator then adjusts the ignition angle (α) for the driving of at least one power converter. As a result, the output voltage of the power converter (204) is adjusted until the actual power corresponds to the setpoint power. In the event of limit values being undershot or overshot, the tap switch (203) of the furnace transformer (202) can additionally be increased or reduced, respectively. Moreover, it is conceivable, in a less advantageous but less expensive alternative embodiment, to dispense with the power converter (204) and to use only the tap switch (203) for power closed-loop control. For automatic correction of a remaining control difference, the actual value of the power P.sub.actual (as part of the furnace process values 505) can be fed back to the function block of the energy and mass balance (404) and—if necessary—the coefficient k.sub.0 can be adapted in the next cycle.

[0065] FIG. 4 shows a possible advantageous embodiment of a power control loop by means of a proportional-integral controller (PI controller).

[0066] Besides the driving of the power converter (204) by means of an ignition angle calculation and the phase gating of power semiconductors, a pulse width modulation, a chopper control, or pulse control or a comparable technology for controlling the energy input into the electric arc furnace is also possible.

LIST OF REFERENCE SIGNS

[0067] 100 Arc furnace [0068] 101 Furnace vessel [0069] 102 Slag [0070] 103 Metal [0071] 104 Electrode [0072] 105 Electrode position adjustment [0073] 106 Metal tapping [0074] 107 Slag tapping [0075] 201 Energy supply rail [0076] 202 Furnace transformer [0077] 203 Tap switch [0078] 204 Power converter [0079] 205 Current measurement [0080] 206 Voltage measurement [0081] 207 Circuit breaker [0082] 208 Disconnecting switch [0083] 301 Furnace bunker [0084] 302 Material conveying device [0085] 303 Weight detector [0086] 401 Open-loop control of the material input [0087] 402 Determination of the specific energy demand [0088] 403 Technological database [0089] 404 Energy and mass balance [0090] 405 Determination of the required energy input [0091] 406 Power closed-loop control [0092] 407 Ignition pulse generator [0093] 408 Power measurement [0094] 501 Material input (setpoint value) [0095] 502 Material input (actual value) [0096] 503 Expected value of the specific energy consumption [0097] 504 Material parameters [0098] 505 Furnace process values [0099] 506 Correction values [0100] 507 Furnace power (setpoint value) [0101] 508 Furnace power (actual value) [0102] W.sub.electr. Electrical energy [0103] W.sub.chem Chemical energy [0104] W.sub.in Energy fed in [0105] W.sub.g Stored energy [0106] W.sub.out Dissipated energy [0107] W.sub.dust Energy of the dissipated exhaust gas [0108] W.sub.v-el Electrical losses [0109] W.sub.therm Thermal losses [0110] W.sub.slag Energy of the discharged slag [0111] W.sub.metal Energy of the discharged metal [0112] q.sub.m setpoint Desired material input (setpoint value) [0113] q.sub.m actual Actual material input (actual) [0114] q.sub.m metal Tapped metal [0115] q.sub.m slag Tapped slag [0116] q.sub.m dust Dust losses [0117] q.sub.mg Stored material [0118] q.sub.m in Material fed in [0119] q.sub.m out Material fed out [0120] k.sub.0 Coefficient 0 [0121] k.sub.1 Coefficient 1 [0122] P.sub.setpoint Power (setpoint value) [0123] P.sub.actual Power (actual value) [0124] cos Φ Power factor [0125] α Ignition angle [0126] Y Manipulated variable