Control system for a melting process

Abstract

Disclosed is a control system for a melting process in an electric arc furnace for melting a metallic material that minimizes desired process properties such as the melting time or the total power consumption of the melting process. The system includes a processing unit adapted for receiving or collecting measured data of at least one process variable, determining the current state of the process, performing an optimization of the melting process, determining a process input based on the result of the optimization, and controlling the melting process with the process input. A method is also presented herein.

Claims

1. A control system for controlling a melting process in an electrical arc furnace for melting a metallic material, comprising an electromagnetic stirrer for stirring the melt and a power supply unit arranged to supply power to the stirrer, characterized in that the control system comprises: a stirrer control unit, operatively connected to the power supply unit of the stirrer, for controlling the power supply in response to control values for the power supply to the stirrer, at least one sensor arranged to measure the power supply to the stirrer, a processing unit arranged to receive measured data regarding process variables from said at least one sensor arranged to measure the power supply to the stirrer and to: perform an optimization of the melting process with regard to time and/or energy consumption based on a predefined optimization problem including a state model of the melting process which relates process inputs, including power supply to the stirrer, to at least one state of the process, a loss function subject to said state model and an initial start condition, determine the state of the process based on said optimization problem using said initial start condition, and arranged to perform the following steps: determining reference values of a control signal for the power supply to the stirrer based on a result of the optimization, providing said reference values to the stirrer control unit for controlling the stirring of the metallic material, collecting measured data of at least one process variable reflecting the melting process, determining the current state of the process based on the state model, the determined previous state, and the determined previous control signal, determining a corrected current state of the process based on the measured data using a state observer, performing an optimization of the melting process with regard to time and/or energy consumption based on said optimization problem using the corrected current state as start condition for the optimization, and repeating the above steps during the process until a desired state of the process has been achieved.

2. The control system according to claim 1, comprising one or more electrodes for melting a metallic material and a power supply unit arranged to supply power to the electrodes, characterized in that the control system comprises: an electrode control unit, operatively connected to the power supply unit of the electrodes, for controlling the power supply in response to control values for the power supply to the electrodes, at least one further sensor arranged to measure the power supply to the electrodes, a processing unit arranged to receive measured data regarding said process variables from said at least one further sensor arranged to measure the power supply to the electrodes and to: perform an optimization of the melting process with regard to time and/or energy consumption based on a predefined optimization problem including a state model of the melting process which relates process inputs, including power supply to the electrodes, to at least one state of the process, a loss function subject to said state model and an initial start condition, determine the state of the process based on said optimization problem using said initial start condition, and arranged to perform the following steps: determining reference values of a control signal for the power supply to the electrodes based on a result of the optimization, providing said reference values to the electrode control unit for controlling the melting of the metallic material.

3. The control system according to claim 1, comprising wall or bottom mounted injection units arranged for supplying oxygen to the melt and a supply unit arranged to supply a flow of oxygen to the injection units, characterized in that the control system comprises: a flow control unit, operatively connected to the supply unit arranged to supply a flow of oxygen to the injection units, for controlling the flow of oxygen in response to control values for the oxygen supply to the injection units, at least one further sensor arranged to measure the oxygen supply to the injection units, a processing unit arranged to receive measured data regarding said process variables from said at least one further sensor arranged to measure the oxygen supply to the injection units and to: perform an optimization of the melting process with regard to time and/or energy consumption based on a predefined optimization problem including a state model of the melting process which relates process inputs, including oxygen supply to the injection units, to at least one state of the process, a loss function subject to said state model and an initial start condition, determine the state of the process based on said optimization problem using said initial start condition, and arranged to perform the following steps: determining reference values of a control signal for the control signal for the oxygen supply to the injection units based on a result of the optimization, providing said reference values to the flow control unit for controlling the oxygen supply to the injection units.

4. The control system according to claim 1, comprising at least one gas burner arranged for preheating solid metallic material, melting solid metallic material and for heating the melt, and a supply unit arranged to supply a flow of burner gas to the gas burner, characterized in that the control system comprises: a flow control unit, operatively connected to the supply unit arranged to supply a flow of burner gas to the burner, for controlling the flow of burner gas in response to control values for the burner gas supply to the burner, at least one further sensor arranged to measure the burner gas supply to the burner, a processing unit arranged to receive measured data regarding said process variables from said at least one further sensor arranged to measure the burner gas supply to the burner and to: perform an optimization of the melting process with regard to time and/or energy consumption based on a predefined optimization problem including a state model of the melting process which relates process inputs, including burner gas supply to the burner, to at least one state of the process, a loss function subject to said state model and an initial start condition, determine the state of the process based on said optimization problem using said initial start condition, and arranged to perform the following steps: determining reference values of a control signal for the control signal for the burner gas supply to the burner based on a result of the optimization, providing said reference values to the flow control unit for controlling the flow of burner gas to the burner.

5. The control system according to claim 1, comprising at least one injection unit for adding carbon powder into the melt and a supply unit arranged to supply a flow of carbon powder to the melt, characterized in that the control system comprises: a flow control unit, operatively connected to the supply unit arranged to supply carbon powder to the at least one injection unit for adding carbon powder into the melt, for controlling the flow of carbon powder in response to control values for the carbon powder supply to the at least one injection unit for adding carbon powder into the melt, at least one further sensor arranged to measure the supply of carbon powder to the at least one injection unit for adding carbon powder into the melt, a processing unit arranged to receive measured data regarding said process variables from said at least one further sensor arranged to measure the supply of carbon powder to the at least one injection unit for adding carbon powder into the melt and to: perform an optimization of the melting process with regard to time and/or energy consumption based on a predefined optimization problem including a state model of the melting process which relates process inputs, including supply of carbon powder to the at least one injection unit for adding carbon powder into the melt, to at least one state of the process, a loss function subject to said state model and an initial start condition, determine the state of the process based on said optimization problem using said initial start condition, and arranged to perform the following steps: determining reference values of a control signal for the control signal for the supply of carbon powder to the at least one injection unit for adding carbon powder into the melt based on a result of the optimization, providing said reference values to the flow control unit for controlling the flow of carbon powder to the at least one injection unit for adding carbon powder into the melt.

6. A control system for a melting process in an electric arc furnace for melting a metallic material, comprising: at least one sensor; an electromagnetic stirrer; and a processing unit arranged to: i) receive, from the at least one sensor, measurements of at least one process variable reflecting the melting process, ii) determine a current state of the melting process based on a model of the melting process, a previous state of the melting process, a previous control input, and the measurements of the at least one process variable, iii) determine a current process input which minimizes a desired process property, wherein the determining comprises minimizing the desired process property with respect to all allowed values of process inputs and utilizing constraints involving the current state of the melting process and a desired end state of the melting process, and iv) control the melting process utilizing the current process input to control the electromagnetic stirrer.

7. The system according to claim 6, wherein the processing unit is further arranged to repeat i) to iv) until the desired end state of the melting process has been obtained.

8. The system according to claim 6, wherein the current process input is used to control an electrode power supply unit, an oxygen flow control unit, a burner gas supply unit, and a solid material supply unit.

9. The system according to claim 6, wherein the electric arc furnace comprises one or more electrodes for melting a metallic material, and the process inputs further includes power supply to the one or more electrodes.

10. The system according to claim 6, wherein the electric arc furnace comprises wall or bottom mounted injection units arranged for supplying oxygen to the melt, and the process inputs further includes oxygen supply to the injection units.

11. The system according to claim 6, wherein the electric arc furnace comprises at least one gas burner arranged for heating the melt, and the process inputs further includes burner gas supply to the gas burner.

12. The system according to claim 6, wherein the electric arc furnace comprises a carbon injection unit adding carbon powder into the melt, and the process inputs further includes carbon powder supply to the carbon injection unit.

13. The system according to claim 6, wherein the at least one process variable reflects a temperature of the melting process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawing in which:

(2) FIG. 1 shows a cross section of an electric arc furnace according to one embodiment of the invention.

(3) FIG. 2 shows a flowchart of the melting process according to one embodiment of the invention.

(4) FIG. 3 shows possible melting profiles, where the amount of solid (x.sub.1) and liquid (x.sub.2) steel is shown together with one of the process inputs (u.sub.1, u.sub.2, u.sub.3, u.sub.4, u.sub.5) that affect the EAF, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 illustrates an electrical arc furnace 1 (the EAF may be a DC EAF or an AC EAF and is hereinafter called EAF) arranged for melting metallic material such as scrap being loaded into the EAF prior to the beginning of the melting process. The EAF further comprises one or more electrodes 4, a vessel 28 covered with a retractable roof through which the one or more graphite electrodes 4 enter the furnace 1 and a power supply system 12 operatively connected to the electrodes. At least one apparatus for electromagnetic stirring 2 (hereinafter called stirrer) of molten metal 3 in the EAF 1 is arranged on an outer surface, preferably the bottom surface, of the EAF vessel 28.

(6) A power supply system 8 is operatively connected to the stirrer 2. The power supply system 12 operatively connected to the electrodes 4 and the power supply system 8 operatively connected to the stirrer 2 may be two separate power supply systems, but it can also be the same system for both purposes. At least one control unit 31, including hardware, one or more memory units, one or more processing units 11 (i.e. processors), and software, is operatively connected to the power supply system 8 to control the operation of the stirrer 2. The at least one control unit 31 (e.q., control system), arranged to control the stirrer 2 and the electrodes 4, is operatively connected to the power supply system 8 and may also be connected to the power supply system 12 to control the operation of the electrodes 4, however a separate control system can also be arranged for this purpose.

(7) The EAF 1 operation starts with the furnace being charged with scrap metal, wherein the meltdown commences. The electrodes 4 are lowered onto the scrap and an arc is struck thereby starting to melt the scrap. Lower voltages are selected for this first part of the operation to protect the roof and walls of the furnace from excessive heat and damage from the arcs. Once the electrodes 4 have reached the heavy melt at the base of the furnace and the arcs are shielded by slag the voltage can be increased and the electrodes 4 are raised slightly, thereby lengthening the arcs and increasing power to the melt. This enables a molten pool of metal to form more rapidly, reducing tap-to-tap times. Injection units 5 are arranged in the EAF walls or bottom for injecting oxygen provided by oxygen supply unit 15 into the molten metal. One or more gas burners 6 (and gas burner supply unit 17) are arranged in EAF to provide extra chemical heat to the scrap and the molten metal inside the EAF. Both processes accelerate the scrap meltdown. Also means for injecting carbon 7 (e.g., injection units) provided by carbon supply unit 20 into the molten metal 3 are arranged in the EAF 1. The stirrer 2 is arranged to accelerate the molten metal 3 which will further accelerate the scrap meltdown and the tap-to-tap times.

(8) The concept of an integrated EAF control is to control the EAF 1 in such a way that stirring control 9, oxygen injection unit (e.g. a lance) control 16 (e.g., oxygen flow control unit), gas burner control 21 (e.g., gas burner flow control unit), off-gas control 18 (e.g., carbon flow control unit), and electrode power supply control 13 are all integrated as one control strategy.

(9) Given is an initial state where the EAF is loaded with metallic material (i.e. scrap) (x.sub.1, x.sub.2, etc.) for t=0, and by solving the optimization problem as shown below, reference values for the process inputs u for 0tt.sub.f can be determined.

(10) $\begin{array}{cc}\mathrm{mi}\underset{u}{n}{t}_f& \left(\mathrm{loss}\mathrm{function}\right)\\ \mathrm{Subject}\mathrm{to}& \\ x\left(k+1\right)=f_s\left(x\left(k\right),u\left(k\right),d\left(k\right)\right)& \left(\mathrm{state}\mathrm{model}\mathrm{of}\mathrm{th}e\mathrm{melting}\mathrm{process}\right)\\ x\left(t_f\right)=x_f& \left(\mathrm{end}\text{condition/end}\mathrm{state}\right)\\ x\left(0\right)=x_0& \left(\text{startcondition/initialstate}\right)\\ x_{1}x{x}_h& \\ u_{1}u{h}_h& \end{array}$

(11) By solving the above problem, the process will beneficially change from the initial state x(0), which like any state typically is a vector, to the desired end state in shortest possible time. Hence, optimal process inputs are determined at the time when melting of the metallic material begins. For each instant t.sub.k during the melting process the appropriate process input u(t.sub.k) is applied to the process. This approach relies on the state model of the melting process which relates process inputs (such as power supply to the stirrer or to the electrodes, oxygen flow, burner-gas flow or injected carbon) to at least one state (x) of the process, a loss function subject to the state model and an initial start condition, since no feedback from the real EAF is used. Once the initial start condition x.sub.0 has been defined it is used to perform an optimization of the melting process with regard to time and/or energy consumption including determining the state of the process based on the optimization problem presented above.

(12) Provided that measurements of one or more process variable is available, information about the current state of the melting process can be obtained by estimations based on the measurements of the one or more process variables, allowing for the previous procedure to be repeated on regular or non-regular intervals until a desired state of the process has been achieved. In one embodiment, the steps at each interval are: A) collecting or receiving measurements of at least one process variable y(t.sub.k) reflecting the melting process, B) determining the current state of the process by means of estimation, based on the state model, the determined previous state {circumflex over (x)}(t.sub.k-1), the determined previous control signal u(t.sub.k-1), and the measurements y(t.sub.k) of the at least one process variable. A state observer (e.g. a moving horizon estimator or an extended Kalman filter) can be used to determine an estimation of the current state {circumflex over (x)}(t.sub.k) of the melting process. C) solving the above optimization problem with regard to the time and/or energy consumption using the estimated current state {circumflex over (x)}(t.sub.k) as the start condition for the optimization. The result of this optimization will be reference values of the control signals (process inputs) u(t) for t.sub.ktt.sub.f. Apply the first control signal in the obtained reference value, u(t.sub.k), to the process, D) determining reference values of a control signal for the process input (u.sub.x) based on the result of the optimization of the melting process, E) using the control signal for controlling the process input (u.sub.x) and thus the melting process, and F) repeating this procedure (steps A-E) during the process until a desired state x(t.sub.f) of the process has been achieved. In each iteration the estimated state {circumflex over (x)}(t.sub.k) replaces the initial state x.sub.0. Thus, in each iteration {circumflex over (x)}(t.sub.k)=x.sub.t.sub.k is set as the initial condition.

(13) Once the desired state of the process has been achieved the molten metal is tapped from the EAF for further processing, e.g. a continuous casting process.

(14) Possible process inputs and process variables of an EAF subject to control are listed in the tables below.

(15) Process Input (u.sub.x) Description

(16) TABLE-US-00001 u1 Power supply to the stirrer/EMS power u2 Power supply to the electrodes/Electric arc power u3 Oxygen flow (through injection unit, e.g. a lance) u4 Burner gas flow (oxygen + fuel) u5 Carbon injected

(17) Examples of process variables are listed in the table below.

(18) Process Variables Description

(19) TABLE-US-00002 y1 Flue gas temperature y2 Flue gas concentration y3 Cooling water temperature (stirrer and/or furnace) y4 Wall temperature y5 Melt temperature (if possible)

(20) If the process variables d.sub.(t) are known functions in time, it would be possible to calculate values for the control variables u.sub.(t) that minimizes the energy or time consumption for the melting process.

(21) In the Model Predictive Control (MPC)/feedback case, i.e. when a moving horizon estimator is used, examples of process variables d(k) are measurable signals that affect the process but which are not determined in the optimization. Such signals can for instance be measurable disturbances associated with the melting process.

(22) Examples of the current state of the process are at least one of the states listed in the table below

(23) State Description

(24) TABLE-US-00003 x1 Quantity of solid metallic material (kg) x2 Quantity of liquid metallic material (kg) x3 Carbon in solution in metallic material (kg) x4 Silicon in solution in metallic material (kg) x5 Solid slag quantity (kg) x6 Liquid slag quantity (kg) x7 Quantity of FeO in slag (kg) x8 Quantity Si0.sub.2 in slag (kg) x9 Concentration of carbon-monoxide (kg) in flue gas x10 Concentration of carbon-dioxide (kg) in flue gas x11 Nitrogen in gas-phase (kg) x12 Solid temperature (K) x13 Furnace relative pressure (Pa)

(25) Generally, energy savings related to EAF-technology can be divided into two types, e.g. metallurgical process improvements, such as oxygen supply, scrap preheating, post-combustion and bottom gas stirring, or EAF process and operation automation, e.g. electrode regulation or melt down control.

(26) A major component of EAF slag is iron oxide from steel combusting with the injected oxygen. Later in the heat, carbon (in the form of coke or coal) is injected into this slag layer, reacting with the iron oxide to form metallic iron and carbon monoxide gas, which then causes the slag to foam, allowing greater thermal efficiency, and better arc stability and electrical efficiency. Once flat bath conditions are reached, i.e. the scrap has been completely melted, refining operations take place to check and correct the steel chemistry and superheat the melt above its freezing temperature in preparation for tapping. More slag formers are introduced and more oxygen is blown into the bath, burning out impurities such as silicon, sulfur, phosphorus, aluminum, manganese and calcium, and removing their oxides to the slag. Removal of carbon takes place after these elements have burnt out first, as they have a greater affinity for oxygen. Metals that have a poorer affinity for oxygen than iron, such as nickel and copper, cannot be removed through oxidation and must be controlled through scrap chemistry alone, such as introducing the direct reduced iron and pig iron. A foaming slag is maintained throughout, and often overflows the furnace to pour out of the slag door into the slag pit.

(27) Expressed in an alternative way the inventive concept provides a method of controlling a melting process in an electric arc furnace, wherein the method comprises the steps of: i) receiving measurement data of at least one process variable associated with the melting process, ii) determining a current state of the melting process based on a previous state of the melting process, on previous process input and on the measurement data, iii) determining a current process input which minimizes a desired process property, wherein the determining comprises minimizing the desired process property with respect to all allowed values of process inputs and utilizing constraints involving the current state of the melting process and a desired end state of the melting process, and iv) controlling the melting process based on the current process input.

(28) The above steps are preferably repeated, i.e. iterated, until the current state of the melting process is equal to or essentially equal to the desired end state of the melting process.

(29) The iteration of the steps i-iv is in one embodiment made once every minute. It is also envisaged that the iteration of the steps i-iv can be made with longer intervals, for instance once every fifth minute, every tenth minute, or just at a very few instances during a melting cycle.

(30) A process property is to be understood to mean e.g. a total power consumption of the melting process for one melting cycle or a total time of a melting cycle. A process property could also mean e.g. a total time until about 100% of the metal is in liquid form in the electric arc furnace.

(31) In embodiments where the process property is the total power consumption of the melting process, the total power consumption can be minimized based on the following minimization problem.

(32) $\begin{array}{cc}\mathrm{mi}\underset{u}{n}{P}_f^{\mathrm{tot}}& \left(\mathrm{loss}\mathrm{function}\right)\\ \mathrm{Subject}\mathrm{to}& \\ x\left(k+1\right)=f_s\left(x\left(k\right),u\left(k\right),d\left(k\right)\right)& \left(\mathrm{state}\mathrm{model}\mathrm{of}\mathrm{the}\mathrm{melting}\mathrm{process}\right)\\ x\left(t_f\right)=x_f& \left(\mathrm{end}\mathrm{condition}\text{/}\mathrm{end}\mathrm{state}\right)\\ x\left(0\right)=x_0& \left(\text{startcondition/initialstate}\right)\\ x_{1}x{x}_h& \\ u_{1}u{h}_h& \end{array}$

(33) Preferably, the above-described control process is arranged to generate a plurality of process inputs such as process inputs u1-u5 for controlling various process parameters, as has been described hereabove.

(34) Furthermore there is provided a control system for controlling a melting process in an electrical arc furnace (EAF) for melting a metallic material, wherein the control system comprises:

(35) a plurality of sensors 10, 14, 19, 22, 26, 33 arranged to sense respective process variables 61, 62, 63, 64, 65, 66 of the melting process, and

(36) a processing unit arranged to: receive the process variables sensed by the plurality of sensors; determine a current state of the melting process based on a previous state of the melting process, on a previous process input and on the measurement data; determine a current process input which minimizes a desired process property, wherein the determining comprises minimizing the desired process property with respect to all allowed values of process inputs and utilizing constraints involving the current state of the melting process and a desired end state of the melting process;

(37) wherein the control system is arranged to control the melting process based on the current process input.

(38) In particular the control system controls process parameters of the melting process. Such parameters can be controlled e.g. by means of the stirrer power supply unit 8 for controlling the electromagnetic stirrer, the electrode power supply unit 13, the oxygen flow control unit 16, the burner gas supply unit 17, and the solid material supply unit 20.

(39) Although favorable the scope of the invention must not be limited by the embodiments presented but contain also embodiments obvious to a person skilled in the art.