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Publication numberUS3561743 A
Publication typeGrant
Publication dateFeb 9, 1971
Filing dateOct 17, 1967
Priority dateOct 17, 1967
Also published asDE1803047A1
Publication numberUS 3561743 A, US 3561743A, US-A-3561743, US3561743 A, US3561743A
InventorsLippitt David L, Schroeder David L
Original AssigneeGen Electric
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Use of stack gas as oxygen potential measurements to control the bof process
US 3561743 A
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Description  (OCR text may contain errors)

United States Patent [72] inventors David L. Schroeder Schenectady: David L. Lippitt, Scotia, NY.

[21] Appl. No. 675,877

[22] Filed Oct. 17, 1967 [45] Patented Feb. 9, 1971 [7 3] Assignee General Electric Company a corporation of New York [54] USE OF STACK GAS AS OXYGEN POTENTIAL MEASUREMENTS TO CONTROL THE BOF PROCESS 12 Claims, 10 Drawing Figs.

Primary Examiner-Charles W. Lanham Assistant ExaminerJohn E. Roethel AttorneysPaul A. Frank, R. R. Brainard, Louis A. Moucha,

Melvin M. Goldenberg, Frank L. Neuhauser and Oscar B. Waddell ABSTRACT: This invention relates to a control method and apparatus for a basic oxygen furnace steel making facility of the type having an oxygen lance for supplying oxygen to a molten metal bath within the furnace and means for moving the oxygen lance relative to the molten metal bath for controlling the partition of oxygen supplied by the lance to the molten metal bath. During the course of a heat, conditions are continuously sensed which are indicative of the oxygen content of the molten bath, the oxygen lance height and the oxygen lance flow rate to provide input data relative to the partitioning of the lance oxygen within the molten bath in the BOF vessel. This input data is processed by suitable computation means which derives measurements representative of the temperature, carbon content and oxygen content of the molten bath during the course of the heat, as well as corrective output signals for the lance height and/or lance oxygen flow-rate to optimize the process. The output control signals thus derived are then fed back in a dynamic, closed loop to control of the furnace to thereby optimize its operation. In a preferred embodiment of the invention the continuously sensed condition is the oxygen potential of the gases emanating from the furnace. The sensing is accomplished by a gas analyzer which is positioned ahead of any stack gas cleaning equipment located in the stack of the furnace, and which accordingly has a high response speed. The gas analyzer derives output measurement signals indicative of the oxygen partitioning of the lance oxygen, and these output measurement signals are employed to automatically and continuously control the position of the lance and the flow rate of the oxygen supplied therethrough to optimize the same during a heat.

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T/re/r A torney USE OF STACK GAS AS OXYGEN POTENTIAL MEASUREMENTS TO CONTROL THE BOF PROCESS BACKGROUND OF INVENTlON 1. Field oflnvention This invention relates to a new and improved control method and apparatus for a basic oxygen furnace steel making facility.

More specifically, the invention relates to a new and improved, dynamic control method and apparatus for dynamically and continuously controlling a basic oxygen furnace during the course of a heat to optimize its operation in a manner so as to assure production of molten steel which is within desired end-point temperature, carbon content and oxygen content specifications.

2. Description of Prior Art The art of steel making has advanced from the early medieval crucible processes requiring days and to the open hearth process requiring from 8 to hours to the modern basic oxygen furnace steel making process which requires a blowing time of less than minutes. The inherent speed of this dynamic process demands an ability to sense, judge and initiate control action within seconds, and places an extremely severe requirement on human operators in order to bring a heat in on desired end-point specifications. By the term heat" is meant the product of one run (normally requiring about 25 minutes) ofa basic oxygen furnace where the molten metal product is formed by an initial charge placed in the furnace and comprised of quantities of hot metal, scrap, lime, ore, and spar, and oxygen blown into the furnace at some known rate for a given period of time and from some set lance position. By the term lance" is meant the tool (which is in the shape of a lance) with which oxygen is blown into the mass of molten metal within the furnace. The furnace, which is maintained at a high temperature level in the neighborhood of 2,200 to 3,000 F., processes the charge to produce some quantity of steel, of some analysis and at some end-point temperature, along with some slag, flue gases and losses to thereby complete a heat. The speed at which a heat is conducted places an extremely severe demand on a human operator. Therefore, some type of automatic sensing and control over the furnace is required during the course of the heat in order to bring the heat in on some specified end-point temperature, carbon content and oxygen content.

The basic oxygen furnace (hereinafter referred to as BOF) steel making process is a selective oxidation distribution process. In the process undesirable impurities in a liquid metal charge received from a blast furnace, are removed. The oxidation reactions that occur generate enough heat to melt a substantial amount of scrap, and to elevate the temperature of the resultant molten bath to about 3,000 F. Oxidation or reduction is involved in all of the chemical reactions taking place in the BOF vessel during the course of a heat. These oxidations occur selectively and fall into three fairly definite characteristic periods (phases) that occur during the course of a heat. These periods are the slag formation period, the carbon boil period, and the metal refine period. They result from the way in which oxygen blown into the BOF vessel is partitioned (divided) between the reactions occurring in the slag, gas and metal refining phases. In order to optimize a BOF shop operation and realize improvements in production, quality and yield, it is essential to exercise dynamic control over the operation of the furnace during each of these phases.

One form of known control of a BOF process (which is not a dynamic control) is identified as control by charge calculation. The charge calculation technique is a ballistic means for using thermodynamic calculations along with past operating data to compute the weights of the various materials to be supplied to the BOF vessel and an oxygen blowing schedule. The purpose of this is to achieve the best thermodynamic charge and blowing rate for a prescribed heat. It is aimed at statistically bringing in a higher percent of heats on desired end-point specifications and can also reduce the standard deviation from a desired end-point bath carbon and temperature achieved at first turn-down. By first turn-down is meant the point at which the BOF vessel is tilted to do sampling of the molten bath. Errors in the input data to the charge calculation and the variants in the oxygen blowing trajectories over a campaign of heats, places limitations on the success of the charge calculation technique. Strictly open-loop charge calculation control measures have been used a number of times in the past with varying results. Because it is an open-loop method, the charge calculation technique does not react to in-process variations such as slopping or undue sparking which might occur during blowing of a particular heat. Therefore, it does not satisfy the need for a dynamic control of the BOF process.

Still another known control technique for the BOF process is identified as dynamic end-point control. U.S. Pat. No. 3,236,630 and U.S. Pat. No. 3,181,343 are illustrative ofcontrol systems employing this technique. In this system of control, the dynamic control of end-point carbon and temperature requires exact input temperature and carbon values upon which to base control calculations and logic. To be of greatest usefulness, input carbon and temperature data values should be obtained some time near the end of the heat. Having determined the carbon and temperature of a bath at a particular point in the course of a heat, and knowing the carbon-versustemperature trajectory that a particular heat is following, control actions can be taken in the following manner. Where an initially program trajectory (if followed to first turn-down) would produce a heat in which the temperature and/or carbon content would be too low, then the control logic should raise the lance and increase the oxygen blowing rate. This would result in correcting the temperature and/or carbon content so that the heat at the point of turn-down would end up within end-point specifications. Alternatively, the trajectory, as initially programmed, might lead to a heat in which the carbon and/or temperature is too high at first turn-down. Under these conditions the control logic should call for the addition of a coolant such as limestone, scrap, spar, etc., to be added to the heat. The resultant new trajectory would then lower the carbon and/or temperature so that at the point of turn-down the heat is within end-point specifications. Thirdly, it is of course possible that the programmed trajectory would bring a specific heat in within desired end-point specifications at the point of first turn-down.

The absolute performance of the above briefly-described type of end-point control depends upon the allowable tolerances in the desired end-point carbon, the degree and direction outside of specifications the heat would have been if the initially programmed trajectory had been followed, and the rapidity with which the flux batching system can deliver prescribed coolant such as lime, ore, or scrapat a desired point in a heat. Using this dynamic end-point controlled technique, a dynamic determination ofthe bath carbon is continuously obtained during the course of a heat by subtraction of the carbon leaving the stack of the furnace vessel from the initial carbon. The integration of carbon removal from the start of the heat in this manner appears to statistically improve certain types of high carbon heats. However, the technique has definite weaknesses in that the error in the measurement of the initial carbon content of the charge generally is of the same magnitude as the specified end-point carbon. Additionally, the integration of carbon loss based on stack gas analysis is itself subject to error. Further, it provides no means for preventing undesired slopping or sparking during the earlier phases of a heat with their consequent material wastages.

From the above description it will be appreciated that the existing control techniques for the BOF steel making process are lacking for a number of reasons. What is required is a dynamic control which will maximize the production rate and quality of each heat produced while simultaneously decreasing labor costs and the raw material requirements by avoiding slopping and sparking. By improving the logistics of the steel making operation and reducing the number of off-specification heats at first turn-down, the production rate of a shop can be increased. The positive prevention of slopping during a blow (heat) and the minimization of the amount of iron oxide in the slag present in a heat at first turn-down (tapping) leads to greatly increased metallic yields. The controlled blowing of heats to specified end-point carbon content, temperature and oxygen content satisfied the primary quality requirements of BOF shops and leads to increased refractory life and minimum nitrogen and oxygen in the steel bath. It also greatly facilitates further processing (molding, rolling, etc.) by controlling oxygen content of the molten metal at the point of first turn down. The present invention provides a dynamic control for the BOF process having the above-listed desirable capabilities.

SUMMARY OF INVENTION It is therefore a primary object of the present invention to provide a new and improved control method and apparatus for a basic oxygen furnace steel making installation which can dynamically control the operation of the installation throughout the running ofa heat.

A further object of the invention is to provide a new and improved dynamic control method and apparatus for automatically and continuously controlling a basic oxygen furnace to optimize its operation during the course of a heat in a manner so as to controllably produce molten steel having a desired end-point temperature, carbon content and oxygen content.

In practicing the invention a new and improved dynamic control method and apparatus is provided for a basic oxygen furnace steel making facility having an oxygen lance for supplying oxygen to the molten metal bath within the furnace and means for moving the oxygen lance relative to the molten metal mass for controlling the partition of oxygen supplied to the molten bath. During the course of the heat conditions are continuously sensed which are indicative of the oxygen content of the molten bath. From these input measurements, output control signals are continuously derived for controlling the partitioning of the lance oxygen within the molten bath during the course of the heat. These output control signals are then supplied back to dynamically control the furnace to thereby optimize its operation.

In a preferred embodiment of the invention, the condition that is continuously sensed is the oxygen potential of the gases emanating from the furnace. This is done by analyzing the stack gases emanating from the furnace at a position ahead of any stack gas cleaning equipment located in the stack and deriving output measurement signals indicative of the oxygen partitioning of the lance oxygen. These output measurement signals are then fed back to control the position of the oxygen lance and the flow rate of oxygen supplied therethrough in order to optimize partitioning of the oxygen and hence the BOF process.

Other objects, features and many of the attendant advantages of this invention will be appreciated more readily as the same becomes better understood by reference to the following detailed description, when considered in connection with the accompanying drawings, wherein like parts in each of the several FIGS. are identified by the same reference character, and wherein:

BRIEF DESCRIPTION OF DRAWINGS FIG. I is a functional schematic diagram of a basic oxygen furnace steel making facility constructed in accordance with the invention;

FIG. 2 is a schematic diagram indicative of the chemical reactions taking place in a BOF vessel during the course of a heat;

FIG. 3 is an idealized trajectory plotting a factor identified as the oxygen utilization factor (O.U.F.) versus time, and illustrates the manner in which oxygen is utilized during the three readily identified, primary phases of a BOF process;

FIG. 4 is a functional block diagram of a new and improved dynamic control system constructed in accordance with the present invention, and it illustrates the manner in which the system is applied to the BOF facility shown schematically in FIG. 1;

FIGS. 5. 6, and 7 are graphs illustrating plots of the lance oxygen flow rate and lance height versus time, as well as oxygen utilization factor (O.U.F.) and mole fraction of carbon monoxide occurring within the BOF vessel in the course of a heat, and illustrate such plots for a nonslopping heat, a mildly slopping heat and a badly slopping heat;

FIG. 8 is a plot of the blowing time versus bath carbon level for constant carbon increments;

FIG. 9 is a plot of three representative carbon-temperature trajectories illustrating end-point control of low carbon heats; and

FIG. 10 is a plot showing the correlation of the oxygen content of the stack gases to the molten metal bath carbon content.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Construction of BOF Facility FIG. 1 of the drawings is a schematic functional diagram of a basic oxygen furnace steel making facility, and illustrates the several major components of such facility which are employed in conjunction with a suitable general purpose digital computer for use in practicing the present invention. The facility shown in FIG. 1 is comprised by a conventional BOF closed hood vessel shown at 11 in which a charge of molten metal. scrap, etc., shown at 12 had been deposited by a suitable charging or loading apparatus illustrated schematically at 13. While a closed hood facility has been illustrated, the invention may be practiced in conjunction with an open hood system with equal ease, as will be described more fully hereinafter. The hood of the BOF vessel 11 is connected with a stack 14 having a suitable gas cooling and cleaning equipment I5 disposed therein which cools and cleans the gases emanating from the furnace vessel 11 prior to discharging them into the atmosphere. As is conventional with a BOF facility, an oxygen lance shown at 16 is moveably disposed within the vessel II when it is tilted to its upright or blowing position. The oxygen lance 16 may be raised or lowered within the vessel II by a suitable height controlling mechanism shown at 17, and the flow rate of the oxygen supplied through the lance I6 is con trolled by a suitable flow rate control shown at 18,

As will be described more fully hereinafter, in conducting a heat in the BOF facility in accordance with the invention, certain measurements must be made in order to determine the oxygen content of the molten metal mass 12 within the vessel 11. For this purpose, instrumentation means shown broadly at 19 and 21 are provided. The instrumentation means are comprised by a gas analyzer for obtaining output signals indicative of the oxygen partial pressure, the mole fraction of carbon dioxide and/or the mole fraction of carbon monoxide present in the gaseous atmosphere. The instrumentation means also includes a stack gas flow rate meter 21 for measuring the flow rate of the gases. At a particular point in the course of a heat, it is desirable to obtain an actual temperature measurement of the temperature of the molten metal mass 12 within the vessel 11. For this purpose, means are provided shown at 22 for inserting a disposable bomb thermocouple into the furnace. Also, it may prove essential at a particular point in the course of the heat to provide to the molten metal mass 12 in the vessel l1 additives such as spar, ore, or lime shown generally at 23 by means of a suitable automatically operated conveyor and weighing hopper system shown generally at 24. These additives can act as Coolants to the molten metal mass so as to in effect lower its temperature, and bring it in at the end ofa heat on a desired end-point temperature.

The exact details of the construction of the furnace vessel 11, the charging and loading equipment I3, the nature of the gas cooling equipment IS, the construction of the lance height raising and lowering mechanism, the lance oxygen flow rate control, the bomb thermocouple injecting mechanism, and the conveyor and weighing hopper assembly for injecting coolants into the vessel are all well known in the art, either having been described in the published literature or placed in use or on sale, so that a further detailed description of their construction and operation is believed unnecessary.

Operation of BOF Facility The BOF steel making process, as stated previously, is a selective oxidation and distribution process wherein during the process undesirable impuritiesin the liquid metal received from the blast furnace are removed. During the process, the oxidation reactions that occur generate enough heat to melt a substantial amount of scrap and to elevate the temperature of the resultant molten metal bath to a high value in the neighborhood of 3,000 F. The primary reactions that occur in the BOF vessel during the course ofa heat are shown in FIG. 2 of the drawings. As will be determined from an examination of FIG. 2, oxidation or reduction is involved in all of the chemical reactions. These chemical reactions fall into three fairly definite and readily identifiable characteristic periods (phases) which occur during the course of a heat. These periods are the slag formation phase, the carbon boil phase and the metal refining phase. They result from the way in which oxygen blown into the BOF vessel 11 by the oxygen lance is partitioned (divided) between the reactions occurring in the slag, gas and metal phases.

FIG. 3 of the drawings is a plot of an idealized oxygen utilization trajectory in which an oxygen utilization factor is plotted versus time for the course of a heat conducted in a BOF process. The oxygen utilization factor (O.U.F.) will be defined more fully hereinafter, as well as the manner of its derivation, but for the purpose of the instant disclosure, it can be considered to be indicative of the partitioning of the lance oxygen between the several reactions that occur in the BOF furnace in the course of a heat. This oxygen utilization factor (O.U.F.), in the particular embodiment of the invention disclosed, is in effect a measurement of the partitioning of the lance oxygen between the several reactions occurring in the vessel, and can be correlated with the carbon content of the molten metal bath. As will be apparent hereinafter, by varying the lance height and/or the lance oxygen flow rate in accordance with the measured oxygen potential of the stack gases, the partitioning of the lance oxygen between the several reactions occurring in the vessel can be selectively controlled to obtain optimum process performance.

From an examination of FIG. 3 it can be seen that during the first period of the blow (heat) extending from 6 to 12 minutes, the lance oxygen is primarily used to selectively form the oxides FEO SIO MNO, etc. These combine with the flux material to form a slag. A properly oxidized slag (one containing proper contents of FEO, SIO MNO, etc.) is essential to the stability of the BOF process and its refining capability. If an excessively oxidized slag is formed, slopping (ejection of slag from the vessel) might occur. If a slag is formed that is insufficiently oxidized, the maximum elimination of phosphorus from the molten metal bath will not occur.

As the slag formation phase ends, the partitioning of lance oxygen begins to change to almost entirely oxidation of carbon into the gas phase as carbon monoxide (CO) and carbon dioxide (C0 The transition from the slag formation phase to the carbon boil period lasts from 2 to 4 minutes. During the ensuing 8 to 10 minutes the carbon within the metal reacts with the lance oxygen as fast as oxygen can be supplied to the molten metal bath (resulting in the violent boiling action from which this period derives its name). The substantial consumption of the lance oxygen during this phase of the heat is best il- Iustrated in FIG. 3 wherein it will be seen that the oxygen utilization factor (O.U.F.) drops substantially to zero or below during the carbon boil period. The limiting factor controlling decarbonization rate during this carbon boil period is the supply of oxygen to the bath. Hence, during this phase, an idealized control would maximize the supply of oxygen to the bath. Eventually, as the carbon concentration decreases below a certain level, the rate of carbon diffusion to the reaction sites with oxygen can no longer keep up with the rate at which oxygen is being supplied to the bath. At this point, carbon diffusion becomes a rate limiting phenomenon which determines the rate of decarbonization.

The transition from the carbon boil phase to the metal relining phase is again marked by a characteristic change in partitioning ofthe lance oxygen. At this point in a heat, the portion of the oxygen used for decarbonization is decreasing. Accordingly, the oxygen utilization factor (O.U.F.) trajectory will continually increase through the metal refining period with an increasing portion of the lance oxygen going into the reaction changing CO into CO and also into the reaction forming FeO. This final metal refining period generally lasts less than three minutes during which all subsequent process steps to determine end-point characteristics of the heat, must be carried out.

FIGS. 5, 6, and 7 are plots obtained from reduced data which illustrate the trajectories that certain measured process variables including O.U.F. tend to follow during the course of a heat in a semiclosed hood BOF vessel of the type shown schematically in FIG. 1. Of these, FIG. 5 illustrates a trajectory for a nonslopping heat, FIG. 6 a trajectory for a mildly slopping heat, and FIG. 7 illustrates a trajectory for a badly slopping heat. From a consideration of FIGS. 5, 6, and 7, it will be appreciated that there is a pronounced difference between the trajectories of slopping and nonslopping heats. Large changes occur in the ratio of carbon monoxide to carbon dioxide in the gases emanating from the BOF vessel. Similarly, large changes occur in the amount of lance oxygen which goes to make carbon monoxide and carbon dioxide during the course of a heat. Accordingly, it will be appreciated that it becomes possible to make use ofthese process variables individually or in combination to dynamically control the BOF process.

Control Theory In order to implement a dynamic, on-line process control. some arrangement must be provided for gathering control information data for use in following process response and reac tion kinetics. Setting of the oxygen lance height and the oxygen blowing rate at preset fixed values will lead merely to the generation of random, after the fact data since the lance oxygen partition between the gas-slag-metal reactions cannot be a priori predicted from these particular parameters if they are preset. A preferred means of obtaining control information is to regulate the flow at predetermined (preprogrammed) lance oxygen partition values (by appropriate adjustment of the lance oxygen height above the molten metal and the oxygen flow rate), and determining the response of the process under these preset conditions. Thereafter, corrective adjustment (signals) are derived to optimize the process, and are fed back to control the oxygen partitioning. Analysis of the stack gas emanating from the BOF vessel during the course of a blow provides a feedback O.U.F. signal indicative of the partitioning of the lance oxygen. Control can then he achieved by varying the lance height and/or the lance oxygen blowing rate to maintain or correct a precalculated desired lance oxygen partition trajectory and thereby optimize the process. 1

Control System Construction FIG. 4 of the drawings is a functional block diagram of a dynamic, on-line process control apparatus for carrying out the novel control method of the invention. In FIG. 4, the BOF vessel is shown at 11 together with the molten metal bath I2 being processed during the course of a heat, the oxygen lance 16, the stack 14 for carrying away gases emanating from the vessel 11 during the process, and gas cleaning equipment 15. The lance height and lance oxygen blowing rate sensing and control mechanisms are shown at l7, 18 together with an online general purpose, digital computer 25. The computer 25 includes suitable analog-to-digital and digital-to-analog converters comprising a part thereof for converting analogue measurements to digital values useable by the computer, and for reconverting digital values processed by the computer into analogue signals suitable for use with a particular control in question. in addition to the lance height and lance oxygen blowing rate input data, the computer 25 has supplied thereto output measurements from the stack gas analyzer 19 (to be described more fully hereinafter), a conventional stack gas flow rate sensor 21 where a facility is not designed to operate at a constant stack gas flow rate, and the output temperature measurement of a disposable bomb thermocouple 22.

The computer processes the input data in a manner to be described more fully hereinafter from the array sensing instruments and derives output controlling signals for the lance height and lance oxygen blowing rate control l7, 18. Where it is desired to completely automate an installation, a coolant addition control 26 can be provided for automatically supplying additives to the molten metal bath in the vessel 11 at a desired point in a heat. Also a turn-down control 27 may be provided for automatically turning down the vessel and pouring out the molten metal bath into suitable receptacles such as molds, ladles, etc., for receiving the processed metal. Lastly, an automatic charge control 28 can be included for automatically supplying a new charge to the vessel 11 in response to suitable coitrol signals supplied thereto from the computer 25. These last mentioned controls all are within the state of the art of the steel making industry, but require for their actuation necessary control signals from a control computer. The provision of these control signals at an appropriate point in the BOF process of course require the kind of dynamic, on-line control made available by the present invention.

Process Control Philosophy As stated before, the basic oxygen furnace practice uses oxygen to refine impure metals. Ideally, in the refining process, the following objectives are to be attained.

l. Desired end-point conditions of both the molten metal and slag with respect to:

a. Bath carbon b. Bath temperature c. Bath oxygen d. Minimum FeO in the slag ll Prevent slopping.

lll. Minimize sparking.

lV. Detect unmelted scrap.

V. Best possible (optimize) oxygen utilization.

The above-listed objectives can best be obtained by dynamic control of the partition of the lance oxygen between the different chemical reactions which occur in the BOF process. Such dynamic control can be obtained only by a direct closedloop control of the process and to provide such closed-loop control of the process, it is necessary:

I. To obtain a process signal that gives an indication of the lance oxygen partitioning.

ll. Manipulate the lance oxygen partitioning to obtain optimum process performance.

in the embodiment of the invention hereinafter described, the above-listed requirements are realized through:

1. Measurement and correlation of the oxygen potential in the stack gases with the lance oxygen partition and with the carbon content of the molten metal bath.

ll. Control of lance oxygen partition by variation of lance height and/or lance oxygen flow rate.

lll. Determining the difference between an integrated heat balance based on oxygen utilization and initial molten metal temperature and comparison to the instantaneous molten metal temperature as measured by a disposable bomb thermocouple to determine if unmelted scrap is still present as a cross check on the operation of the dynamic control.

N. If required, the addition of material (lime. ore, spar, scrap, etc.) at a point during the course of the heat in order to obtain the desired end-point conditions.

V. Automatic turnoff of the lance oxygen to provide a specified carbon content and turn-down of the vessel at a point in the heat when desired specified end-point conditions can be obtained.

Process Control implementation In order to realize part of the above-listed step Number I of the control process, the lance height and lance oxygen blowing rate sensing and control l7, 18 of FIG. 4 is provided. This device may in fact comprise a lance hoist drum and shaft manufactured and sold by Kaiser Engineers of Linst, Austria together with a position-regulated adjustable voltage drive having a positioning accuracy of plus or minus one-half inch. The position references of this adjustable voltage drive should be settable by the computer 25. The oxygen flow rate sensor can comprise a conventional, commercially available flow meter of the type manufactured and sold by Republic Engineering Corporation. The control on the flow rate of the lance oxygen maybe comprised 'by any suitable valving arrangement, but it is necessary that the oxygen flow rate reference be settable by the computer 25. If required. the stack gas flow rate sensor 21 may comprise a conventional, commercially available flow rate sensor of the type using an orifice plate and suitable means for sensing the pressure drop across this orifice plate.

In order to determine the oxygen potential in the stack gas, the stack gas analyzer 19 is employed. The stack gas analyzer 19 comprises more than'one instrument since it in effect must provide output signals indicative not only ofthe oxygen partial pressure (P0 in the stack gas but also the percent of CO and CO in the stack gas. in order to determine the oxygen partial pressure (Po of the stack gases, a commercially available instrument known as the General Electric "Oxysensor" Mark I is employed. The GE. Oxysensor" is an industrial instrument that continuously measures the oxygen potential of gaseous media. The instruments characteristics are such that it provides: (a) a logarithmically increasing signal with linearly decreasing oxygen partial pressure; (b) extremely low drift: (0) continuous fast response; (d) ability to process corrosive and particle laden gas streams without adversely affecting the instruments sensitivity.

The G.E. Oxysensor" operates on the principle that an electrical potential (volts) will be developed between two electrodes immersed in two gases having different oxygen partial pressures; the gases being separated by an oxygen-ion conducting calcium-stabilized zirconium oxide electrolyte. The measured open cell voltage of the Oxysensor" is related to the oxygen partial pressure in the two gases by the Nernst equation. The temperature of the Oxysensor is preset and the oxygen partial pressure in the reference gas is known. Therefore, by measuring the open cell voltage between the two gases, the oxygen partial pressure in the gas to be measured can be calculated'by means of the'N'ernst equation. This measured oxygen partial pressure is directly related to the oxygen concentration and chemical composition of the gas being measured. The GE. Oxysensor" has been designed to operate on industrial gas samples with a minimum of filtering required and includes its own filters, dryers, and pump for gas movement. A needle valve and gas flow meter is also included for regulating and measuring the gas flow rate. The cell temperature is maintained at 850 C plus or minus 5 C by a solid state proportional controller. The cells voltage output is treated in a self-contained amplifier circuit so that it can either be read directly or outputted to subsequent processing instrumentation such as a computer or recorder. The Oxysensor is manufactured and sold commercially by the Instrument Department of the General Electric Company located in Lynn, Mass. Because the Oxysensor is a commercially available instrument, a further description of its characteristics is believed unnecessary.

The percent CO and percent CO in the stack gas may be measured by a conventional, commercially available infrared analyzer such as that manufactured and sold by the Leeds and Northrup Company. ln particular, a Leeds and Northrup Model No. 7804-A6-A5 lnfra-Red Analyzer is quite satisfactory for measuring these parameters.

Determination of Oxygen Potential in Furnace Vessel Gases The oxygen potential of the gases emanating from a BOF vessel during the course of a heat is expressed in terms of the oxygen utilization factor (O.U.F.) Since the O.U.F. is a'measure of the oxygen potential of the vessel gases it in effect is indicative of the capability of these gases to react with the molten metal bath, and hence is indicative of the oxygen content formance. 2 5

In a closed-hood system, such as that depicted by FIGS. 1 and 4, not enough oxygen is present in'the stack gases to oxidize all of the carbon monoxide (CO) to carbon dioxide (C0,). The resulting oxygen potential is in equilibrium with the CO to CO ratio and is measured continuously with a GE.

Oxysensor" unit and the CO and/or CO measured thereafter with an infrared sensor or other convenient sensors along with the measurement of the gas flow rate in the stack gas and the lance oxygen flow rate. Using these measurements, the O.U.F.

for a closed-hood system can be determined from the following equations.

The flow rate balance is given by: t RA uu RA (A02) The oxygen balance is given by: f R. m SW2) RA (A02) I (.%02 Eco The carbon balance is given b: u: ea SIUZ) t The Oxysensor measurement is processed according to:

E. the voltage of the electrode in contact with the gas sample with respect to the voltage of the electrode in contact with the reference gas T= electrolyte temperature in "K In the above calculation of the O.U.F. for a closed-hood system, the equilibrium free oxygen concentration in the vessel gases is assumed to be low enough to be neglected.

FIG. 3 of the drawing shows an idealized trajectory plotting the O.U.F. versus time for a particular heat. FIG. 5 of the drawing illustrates how the O.U.F. trajectory can be employed in controlling the lance height (LH) in FIG. 5. and the lance oxygen flow rate (1 in FIG. 5 to thereby control the lance oxygen partitioning and optimize the process performance. By comparing the O.U.F. plot shown in FIG. 5 for a nonslopping heat to either FIGS. 6 or 7 for mildly slopping and badly slopping heats, it will be appreciated that O.U.F. signal can be employed to correct the lance height and/or lance oxygen flow rate at particular points in the course of a heat to optimize process performance. For example, it will be seen, from a comparison of these FIGS, that at around 8 minutes from the start of a heat during the transition from the slag formation phase to the carbon boil phase, it is desirable for the lance to be lowered further into the molten metal, and the flow rate to be drastically increased. This is to accommodate the increased consumption of oxygen during the carbon boil phase as mentioned earlier. Accordingly, it will be seen how the instant system provides a dynamic, closed-loop control of the oxygen partitioning through the feedback of suitable control signals derived from the O.U.F. by the computer 25 to control the operation of the lance height and lance oxygen blowing rate control l7, 18.

In an open-hood system, more than enough oxygen from the aspirated air is present in the stack gases to oxidize all of the carbon monoxide to carbon dioxide. The free oxygen still present in the stack gases after combustion of carbon monoxide to carbon dioxide, as well as the resulting carbon dioxide. is measured continuously along with the gas flow rate in the stack and the lance oxygen flow rate. From these measurements the oxygen utilization factor (O.U.F.) is calculated from the following equations.

The flow rate balance is given by:

The oxygen balance is given by:

s1 (02 S02) RA A02) l'o2) The carbon from vessel is iven b The Carbon balance given by: su t-o2) t 1/7, YA/'02 l v The mole fraction of oxygen in the stack gas from "Oxysen- The nitrogen balance is given by: sor measurement is given by: ail x2) RA (Ave) 4 FE The oxygen utilization factor is given by: Log S Log (02):: K

l OllzI-LF. l fir 1/2 \lCO \jcoz) The carbon from vessel is given by:

Where:

R, dry stack gas flow rate (measured in standard cubic feet per minute (scfm) R gas flow rate from vessel (scfm) R lance oxygen flow rate (scfm) l A,,, mole fraction oxygen in dry air (0.21 S mole fraction oxygen in dry stack gases R flow rate of air (scfm) A mole fraction nitrogen in dry air (0.78) 6 S mole fraction carbon monoxide in dry stack gas S mole fraction carbon dioxide in dry stack gases S mole fraction nitrogen in dry stack gases K, calculation parameter from Oxysensor" measurements K calculation parameter from Oxysensor measurements V mole fraction CO in vessel gas V mole fraction CO in vessel gas F 96,501

0, is the partial pressure of oxygen of the reference gas V co V e02 l The nitrogen balance is given by:

s x2) 2 RA m) The oxygen utilization factor is given by:

(EV C0 V CO2) Rn- By monitoring and controlling oxygen utilization factor (O.U.F.) during the heat, through variation of the lance height and/or oxygen flow rate as described above, the partition of oxygen in the course of a heat is controlled.

Control of End-Point Carbon and Temperature From a consideration of FIG. 3 of the drawings, it will be appreciated that only a short time is available to make corrections for off-carbon and off-temperature during the refine period in order to bring a particular heat in on specified endpoint carbon content and end-point temperature. A still shorter time interval is available for carbon control purposes in heats which are specified to have a high end-point carbon content. To be of practicalfieia control system for a BOF process must have a response time several times better than the short time interval available for carbon control even in higher carbon heats, in order to be able to hold the heat to end-point specifications. The blowing time required to obtain various changes in carbon concentration during the metal refine phase, is shown plotted versus carbon level in FIG. 8 of the drawings. From a consideration of FIG. 8, it will be appreciated that the higher the end-point carbon content desired at the time of turn-down, the faster the time response of the control has to be for a given carbon content specification range.

The dynamic control of end-point carbon and end-point temperature requires datum temperature and carbon values upon which to base control calculations and logic. To be of greatest usefulness, the datum carbon and temperature values should be taken sometime near the end ofa heat FIG. 9 of the drawings is a plot of the bath temperature versus bath carbon for three different heats identified as A, B, and C. In FIG. 9, the envelope shown in dotted outline form defines the endpoint tolerance allowed for the heats in question. Having determined the carbon and temperature (in a manner to be described more fully hereinafter), and knowing the carbonversus-temperature trajectory that the particular heat is following, control actions should be taken with respect to each of the heats in the following manner. In the case of heat A, if the initially programmed trajectory is followed to first turn-down, the temperature and/or carbon would be too low. Hence, for this heat, the control logic should raise the lance and increase the oxygen blowing rate so that the trajectory would be lifted to follow along the dotted line A-D. In this manner, the heat could be turned down within end-point specifications at point D. In the case of heat B, the heat will be within specifications if the programmed trajectory is followed and turn-down occurs at point E. With respect to heat C, the trajectory as initially programmed would lead to the carbon and/or temperature being too high at first turn-down. Hence, the control logic should call for the addition of coolants such as limestone, scrap, etc, to be added to the heat. The new trajectory would then be along the dashed line CF, and the heat could be turned down within specifications at point F.

The absolute performance of the above-described type of end-point control is dependent upon three major factors. These factors are the allowable tolerances in the desired endpoint carbon and end-point temperature, the degree and direction outside the specifications the heat would have been if the initially programmed trajectory had been followed, and the rapidity with which the additive batching system can deliver the prescribed coolant lime, ore, scrap, etc.

From a consideration of FIGS. 8 and 9 it will be appreciated that only a short time is available to make corrections for offcarbon and off-temperature heats during the refine period. A still shorter time interval is available for carbon control in the high carbon heats. Hence, the control system to be practical must have a response time several times better than this to be able to hold a heat to within end-point specifications. The blowing time required to obtain various changes in carbon concentration is shown plotted versus carbon level in FIG. 8 of the drawings. The higher the carbon end-point desired at turndown, the faster the time response has to be for a given carbon specification range. The temperature increase during this interval which normally at a lance oxygen blowing rate of 100 scfm/tons of iron is approximately constant. For end-point carbons above 0.5 percent, the change in lance oxygen partition which comes near the end of the carbon boil period has not yet occurred, and accordingly, some other method of correlating carbon datum must be used. With the end-point carbons from 0.5 percent to 0.3 percent there is insufficient time to correct for a heat which is coming in 12 F too low in temperature, and still maintain carbon specification with plus or minus 0.05 percent C.

Below about 0.3 percent carbon, the control philosophy outlined in FIG. 8 is possible. However, in order to use this control philosophy, the gas analysis equipment must produce a useable signal within about 15 seconds after a change in the reaction occurring in the BOF vessel. For this reason, the sen sor for the stack gas analyzer I9 is located before the main gas cleaning equipment in the vessel stack as shown in FIG. 4.

As stated previously, the determination of the molten metal bath carbon content by subtraction of carbon leaving the stack from the initial carbon is demonstratably unsatisfactory. First of all, the error in the measurement of the initial carbon content of the charge can be of the same magnitude as the specified end-point carbon. Secondly, integration of carbon loss based on stack gas analysis is itself subject to error. Hence, the integration of carbon removal from the start of a heat appears to have value only in statistically improving endpoint performance on high carbon heats (carbon content greater than 0.4 percent).

Considerably improved carbon datum can be obtained in accordance with the present invention from the trajectory of the lance oxygen partition curve as represented by the O.U.F. signal. FIG. 10 of the drawings is a graph of the percentage carbon in the bath at the time of first turn-down plotted against the oxygen percentage in the stack gas measured some 10 seconds before the lance oxygen is turned off, and is an indication of the correlation between the oxygen in the stack gas and the bath carbon content. A correlation factor of 0.93 has been obtained by linearly fitting test data measurements of oxygen in the stack gas and bath carbon at the time of first turndown. Accordingly, it will be appreciated that the oxygen potential of the stack gas as measured by the stack gas analyzer provides, reliable end-point carbon datum values upon which to base control calculations and logic. The com puter processes this information and employs the same in determining the correct point of turn-off of the lance oxygen at the end of the heat.

With existing equipment, it is possible to drop a disposable bomb thermocouple into the upright BOF vessel without disrupting the heat. A bomb thermocouple such as the Jet BOB" manufactured and sold by the Lamp Glass Department of the General Electric Company located in Willoughby, Ohio, is suitable for this purpose. This device provides a means of obtaining a datum temperature value within three to five minutes before the projected end of a heat. At this point the scrap in the vessel should be completely melted. If, in fact, the scrap is melted, then the actual temperature value provided by the bomb thermocouple will compare favorably to a predicted end-point temperature based on a time integration of the input temperature of the charge and the amount of oxygen supplied to the vessel in the course of a heat. This provides a cross check to ascertain that indeed no unmelted scrap is present. Any difference between this integrated heat balance value and the actual molten metal temperature as measured by the disposable bomb thermocouple will indicate that corrective measures have to be taken in accordance with the philosophy depicted by FIG. 9 of the drawings.

Should it appear from the reading of the disposable bomb thermocouple that a projected heat is coming in at too high a temperature and/or too high a carbon content, the control may then take suitable measures through the coolants addition control 26 to add materials such as lime, ore, scrap, etc.. to bring the heat in on desired end-point conditions. Should it appear that the heat is coming in too low, then additional oxygen may be called for.

From the foregoing description, it can be appreciated that the invention makes available a dynamic, closed-loop control method and apparatus for a BOF facility wherein measurements taken of the stack gas can be used to detect the imminence of slopping and the presence of slopping before it can be seen outside the BOF vessel. The dynamic process control, by adjusting lance height, oxygen flow rate and possibly adding lime or other additives, can cause a proper slag to form, thus eliminating the slopping condition. Near the end of a heat, a bomb thermocouple dropped into the bath provides another process control input parameter. The actual bath temperature information plus bath carbon datum obtained from continuing offrgas oxygen measurements are used for endpoint bath temperature and end-point bath carbon control. The digital process control computer 25, in addition to its vital role in the dynamic, closed-loop control can be employed to provide indispensable services during the implementation stages in charge calculation, etc. It can also be employed in the gathering and logging of data required for control model refinement.

Accordingly, it can be appreciated from the above summary, that the invention attains the objective of a dynamic control which can maximize production rate and quality while simultaneously decreasing raw material and labor costs. By improving the logistics of the steel making operation, and by reducing the number of off-specification heats at first turndown, the production rate of a shop can be greatly increased. The prevention of slopping during the heat and the minimization of the amount of iron oxide formed in the slag at the time of turn-down, leads to increased metallic yield. The controlled blowing of heats to specified end-point carbon and end-point temperature values satisfies two of the primary quality requirements in BOF shops. The optimization of the oxygen flow rate leads to increased refractory life and minimizing excess nitrogen and oxygen in the bath. The control of the oxygen content of the bath at the point of turn-down is also made possible, and facilitates further processing steps and cleanliness of the resultant metal product.

It will be appreciated therefore that the invention provides a new and improved dynamic control method and apparatus for automatically and continuously controlling a basic oxygen furnace to optimize its operation during the course of a heat in a manner so as to controllably produce molten steel having a desired end-point temperature, carbon content and oxygen content.

Having described one embodiment of a new and improved method and apparatus for the dynamic control of a basic oxygen furnace steel making facility, it is believed obvious that other modifications and variations of the invention are possible in the light of the above teachings. It is therefore to be understood that changes maybe made in the particular embodiment of the invention described which are within the full intended scope of the invention as defined by the appended claims.

We claim:

1. A control apparatus for a basic oxygen furnace steel making installation of the type having an oxygen lance for supplying oxygen to a molten metal bath within the furnace and positioning means for moving the oxygen lance relative to the molten metal mass for controlling the partition of oxygen therein, the improvement comprising:

sensing instrument means for deriving output measurement signals indicative of the oxygen content of the molten bath comprising: stack gas analysis means positioned ahead of any stack gas cleaning equipment located in the stack for deriving measurement signals indicative of the oxygen potential of the gaseous atmosphere in the BOF vessel and the partitioning of the lance oxygen, and further comprising: means for providing measurement signals indicative of the oxygen lance height, the lance oxygen flow rate, and the stack gas flow rate said stack gas analysis means comprising, oxygen partial pressure measuring means for measuring and providing a measurement signal representative of the oxygen partial pressure of the gaseous atmosphere emanating from the furnace, carbon dioxide measuring means for measuring and providing a measurement signal representative of the mole fraction carbon dioxide present in the gaseous atmosphere emanating from the furnace, and means for providing a measurement signal representative of the mole fraction carbon monoxide present in the gaseous atmosphere emanating from the furnace.

the measurement signals thus provided being indicative of the oxygen potential of the gaseous atmosphere in the furnace and therefore indicative of the oxygen content of the molten bath,

feedback means including computation means for deriving from said measurement signals corrective output control signals for controlling partitioning of the lance oxygen,

means for coupling said corrective output control signals from said computation means back to the oxygen lance for dynamically, automatically, and continuously controlling the position of the lance and flow rate of the oxygen supplied therethrough during a heat, and

wherein the computation means comprises means for solving the following equation, and deriving its output control signals in accordance therewith:

VCO is the mole fraction of carbon monoxide produced in the furnace gases by the lanceoxygen, VCO is the mole fraction of carbon dioxide produced in the furnace gases by 2. A control apparatus according to claim 1 wherein the computation means further includes means for comparing an actual instantaneous measured O.U.F. signal to an initially programmed O.U.F. trajectory and for deriving therefrom corrective output control signals for adjusting the lance height and the lance oxygen flow rate to thereby dynamically and continuously control the partitioning of the lance oxygen during the course of a heat.

3. A control apparatus according to claim 2 further including charge control means for supplying an initial charge of known ingredients under known conditions to the furnace, and means supplying a charge control signal from said computation means to said charge control means for automatically controlling the operation thereof.

4. A control apparatus according to claim 2 further including coolant addition control means for supplying additives to the bath at a desired point in a heat, and coolant addition feedback means for supplying a coolant addition feedback control signal from the computation means to the coolant addition control means for automatically controlling the supply of additives to the bath at a desired point in a heat.

5. A control apparatus according to claim 2 further including turn-down control means for turning down the furnace at a desired point in a heat and pouring out the molten metal into appropriate receptacles, and additional feedback means for supplying a turn-down control output signal from said computation means to said turn-down control means for automatically controlling the operation thereof.

6. A control apparatus according to claim 2 further including charge control means for supplying an initial charge of known ingredients under known conditions to the furnace. means supplying a charge control signal from said computation means to said charge control means for automatically controlling the operation thereof, coolant addition control means for supplying additives to the bath at a desired point in a heat, coolant addition feedback means for supplying a coolant addition feedback control signal from the computation means to the coolant addition control means for automatically controlling the supply of additives to the bath at a desired point in a heat, turn-down control means for turning down the furnace at a desired point in a heat and pouring out the molten metal into appropriate receptacles, and additional feedback means for supplying a turn-down control output signal from said computationmeans to said turn-down control means for estsm llx na ps e Op at c thereof:

7. A control apparatus according to claim 2 wherein the computation means further includes means for producing an integrated heat output signal representative of the predicted bath temperature and based on a time integration of the initial bath temperature, the lance oxygen flow rate and the time that has elapsed from the beginning ofa heat, means for inserting a disposable bomb thermocouple in the molten metal bath at a point in time just prior to the end ofa heat. and means for supplying the actual instantaneous bath temperature signal from said bomb thermocouple to the computation means, said computation means further including means for comparing the predicted bath temperature output signal to the actual bath temperature signal supplied by the bomb thermocouple and for deriving integrated heat balance corrective output control signals for bringing the heat in on a desired end-point temperature.

8. A control apparatus according to claim 7 wherein the computation means includes means for comparing the actual O.U.F. signal to an initially programmed O.U.F. versus percent carbon content trajectory for the bath and deriving a cutoff control signal therefrom for terminating the supply of oxygen to the bath at the end ofa heat.

9. A control apparatus according to claim 8 further including charge control means for supplying an initial charge of known ingredients under known conditions to the furnace, and means for supplying a charge control means for automati cally controlling the operation thereof.

10. A control apparatus according to claim 8 further including coolant addition control means for supplying additives to the bath at a desired point in a heat and coolant addition feedback means responsive to an integrated heat balance corrective output feedback control signal from the computation means for automatically controlling the supply of additives to the bath at a desired point in a heat.

1 l. A control apparatus according to claim 8 further including turn-down control means for turning down the furnace at a desired point in a heat and pouring out the molten metal into appropriate receptacles, and additional feedback means for supplying a turn-down control output signal from said computation means to said turn-down control means for automatically controlling the operation thereof.

12. A control apparatus according to claim 8 further including charge control means for supplying an initial charge of known ingredients under known conditions to the furnace, means supplying a charge control signal from said computation means to said charge control means for automatically controlling the operation thereof. coolant addition control means for supplying additives to the bath at a desired point in a heat, coolant addition feedback means responsive to a first integrated heat balance corrective output feedback control signal from the computation means for automatically controlling the supply of additives to the bath at a desired point in a heat where required, means for supplying a second integrated heat balance corrective output feedback control signal from the computation means to the oxygen lance for controlling the position of the lance and the oxygen flow rate therethrough under situations where such integrated heat balance control is required in a heat prior to turn-down, turndown control means for turning down the furnace at a desired point in a heat and pouring out the molten metal into appropriate receptacles, and additional feedback means for supplying a turn-down control output signal from said computation means to said turn-down control means for automatically controlling the operation thereof.

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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3707069 *Oct 13, 1970Dec 26, 1972Air Pollution IndGas collector for steel furnace
US3816720 *Nov 1, 1971Jun 11, 1974Union Carbide CorpProcess for the decarburization of molten metal
US3871871 *Apr 28, 1971Mar 18, 1975Centre Nat Rech MetallMonitoring and control of pig iron refining
US4133036 *Feb 26, 1976Jan 2, 1979Republic Steel CorporationMethod and system for monitoring a physical condition of a medium
US4291379 *Nov 1, 1979Sep 22, 1981Renzo CappellettoMethod and an installation for regenerating molding sands
US4416691 *Mar 25, 1982Nov 22, 1983Kobe Steel, Ltd.Method for converter blow control
US4731732 *Aug 7, 1985Mar 15, 1988Aluminum Company Of AmericaMethod and apparatus for determining soluble gas content
US5610346 *Jan 5, 1996Mar 11, 1997Bethlehem Steel CorporationApparatus for storing and dropping expendable BOF sensors
US5984998 *Nov 14, 1997Nov 16, 1999American Iron And Steel InstituteApparatus providing non-intrusive method for obtaining real-time data about off-gas characteristics to permit analysis and/or dynamic control of a steelmaking process
US6171364Mar 21, 1997Jan 9, 2001Steel Technology CorporationMethod for stable operation of a smelter reactor
US6693947Sep 25, 2002Feb 17, 2004D. L. Schroeder & AssociatesPouring out all molten carbon steel produced to provide empty furnace, adding molten metal to empty furnace before next batch operation
CN102169326BMar 2, 2011Nov 13, 2013中冶南方(武汉)威仕工业炉有限公司System for optimizing optimal furnace temperature set value based on data mining
DE2707502A1 *Feb 22, 1977Aug 25, 1977Nippon Steel CorpVerfahren zum steuern der temperatur von geschmolzenem stahl und des kohlenstoffgehaltes in einem sauerstoffkonverter
EP2423336A1 *May 19, 2011Feb 29, 2012SMS Siemag AGMethod for controlling the temperature of the metal bath during the blowing process in a converter
Classifications
U.S. Classification266/80, 266/225, 96/422, 266/87, 266/243, 700/146, 75/385, 266/90, 266/86
International ClassificationC21C5/30, G05B13/02
Cooperative ClassificationC21C5/30, G05B13/02
European ClassificationG05B13/02, C21C5/30