IMPROVEMENTS FOR LD-CONVERTER & ELECTRIC ARC FURNACES

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1 IMPROVEMENTS FOR LD-CONVERTER & ELECTRIC ARC FURNACES Franz Rubenzucker a Richard Krump a Markus Borz a Kurt Hölzl a Michael Traugott a a voestalpine mechatronics gmbh (vatron), voestalpine Str.3, 4031 Linz, Austria Abstract A bundle of three measurement and control devices developed by vatron helps the operator not only to get information from his steelmaking process but also to control and optimize it. ArCOS/DynArCOS Electrode Control Systems with more then 125 references worldwide helps to control and optimize the wide range of AC arc furnaces. IRIS fast, reliable slag detection has a number of advantages for the steel production process. It permits optimum differentiation between slag and steel. LOMAS which carries out continuous gas analysis during combustion processes at high temperatures (up to 1800 deg. C ), extreme dust loads (up to 2000 g /Nm 3 ) and in corrosive and reducing environments forms an ideal basis for process control. 1 DynArCOS - DYNAMIC ARC OPTIMIZATION 1.1 Introduction In this paper a novel concept for an energetic optimized control of a three phase electric arc furnace is presented. For an existing three phase electric arc furnace at the Böhler - Uddeholm company, a control system based on this concept was designed and installed. Measurements in this plant in closed loop show the basic functionality of the new controller design and confirm the theoretical approaches about that. The objective of this work was to achieve an energy optimized meltdown process by using an intelligent control concept. The control which is presented in this work had the task to calculate the impedance set-points in such a way, that the electrical energy is ideally transferred into the scrap with respect to thermal influences to the shell of the furnace itself. Besides preferring optimised energy input into the scrap, it is also important to keep the wear of the lining material and panels as small as possible. To achieve this objective, the temperatures of the water cooled panels are measured. With a fuzzy algorithm the temperatures of each cooling panels and their time gradients are transformed into a so called abrasion signal. This abrasion signal is the control variable of a superior control loop. In the following, talking about regulation we always consider a closed loop control circuit. 1

2 1.2 State of the art principles of regulation Fig. 1.1 shows the possible regulator modes impedance, arc-resistance and arc-voltage control. Each mode is combined with an inherent melt down automatic program and a cascaded controller loop for Adaptive Setpoint Optimization (ASO) which considers changes in arc stability. At the beginning of a new basket set-points for short arcs, high in current are selected to get good initial arc stability. With the increasing melting progress, set-points for longer arcs, lower in current will be chosen. Active Power Time / Meltdown Progress Fig.1.2: Active Power and Total Harmonic Distortion vs. time. Total Harmonic Distortion Fig. 1.1: Principles of Regulation Influences of the process Several unfavourable factors reduce the stability of the arc during first ignition on the scrap. Irregular scrap surfaces as well as a bad ionization at this moment lead to gaps in arc burning. Arctears and short-circuits define the furnace operation. Due to generated harmonics of more than 40%, the operation reactance appears much higher. Fig. 1.2 shows how Active Power input increases during Melt Down [MD] due to improvements in arc stability although the set-point of the Electrode Regulation System remains unchanged. Arc stability improves during melt down. At the same time the THD (Total Harmonic Distortion) value decreases with increasing arc stability. 1.3 Optimizing strategies for an electrode regulator DynArCOS is an overlay optimizing control system. It processes additional information like cooling water temperature of the panels, basket weight and required melting energy according to the scrap type. Some of the functions for dynamical meltdown optimisation are: Melt-down Automatic This module changes automatically transformer voltage taps and/or reactor taps in accordance with melt-down procedures according to predefined setpoint values, stored in the ArCOS System. It features: Energy input for each selected melting step Maximum or minimum time for each melting step THD as an indicator for the melt progress Interrupts such as SVC overloads, natural gas burner failures, too high flicker, 2

3 Manual interrupt requests Adaptation to boring position and -speed To protect the roof one might use a lower voltage tap in the very beginning. Then, with advancing boring depth the system can increase the voltage for faster melt down. Additionally the resulting boring speed is used to determine the appropriate transformer tap matching the scrap type to avoid furnace wall stress by the arc-flair. Foaming slag management From the THD ArCOS calculates a so called slag index value, which is calculated by the following formula: I50 FSI = I750. K1 + K2 I50 RMS value of the current fundamental signal (50Hz band-pass) I750 RMS value of the current harmonic harmonics ( Hz band-pass) K1, K2 calculation parameters for adapting the furnace properties This value is used by DynArCOS as an indicator for selecting higher voltage taps. If foamy slag is correctly detected, higher productivity can be achieved automatically. Additionally the carbon injection for slag foaming can be directly controlled by using the slag-index in an intelligent fuzzy-logic controller. Hot spot protection By monitoring the temperature and the flow rate of cooling water circuits in each panel the system can detect hot spots. The temperatures and their time gradients are calculated and fed into a fuzzy control algorithm. A so called abrasion-value is generated for each panel. From these signals a sophisticated algorithm determines a refractory change value for each electrode controller set-point and the transformer tap-changer as well. Temperature T t weight hot grow fast down warm grow down cold grow nothing hot const down Panel to Electrode Furnace Operation Control Transformer Tapchanger Control Set-Point Control Tapchanger Set-Point Fuzzy Control weight Fig.1.3: Schematic function diagram for DynArCOS Hot-Spot control. Protection algorithms Flicker to handle critical periods THD to select the appropriate reactor tap Burner energy Limitation of maximum apparent power input if electric power supply system is weak. Anti-Resonance Circuit for LF In case of strong bath oscillations during Ladle Furnace operation (certain slag qualities) an additional Anti Resonance Regulation (ARR) detects such bath oscillations with the aid of a 3

4 frequency analyzer and sets respective countermeasures by actively bringing interferences into the system. Remote Maintenance Support Although ArCOS includes detailed information and optimizes electrode regulation, the system offers the option that outside specialists are able to assist via data link using a dial-up modem. ArCOS data base can be connected under real time conditions via life-mirror techniques. Files from ArCOS/ DynArCOS can be transferred easily to download heat reports. If customer s management agrees, even new set-point tables including system adjustments can be uploaded via this data link. 1.4 Conclusion The ArCOS/ DynArCOS Electrode Regulation Systems are used in modern arc furnace or ladle furnace operations with great success. This powerful tool is able to optimise electric furnace steel making, increase its productivity and to reduce conversion costs. More than 125 references worldwide emphasise the high quality standards realized by the experienced development and commissioning crew. 2 IRIS - INFRARED SLAG IDENTIFICATION 2.1 Introduction Optimised production, top quality, and cost and time savings are important tasks in today s steel industry. In their efforts to attain - and maintain - these ambitious goals, converter steel mills are looking for easy-to-integrate, intelligent solutions that focus on the critical points in the process - like on the tapping operation. Fast, reliable slag detection has a number of advantages for the steel production process: Minimises carryover slag volumes Saves deoxidising and alloying agents Reduces rephosphorisation Lessens wear-and-tear on the refractory lining of the steel casting ladle Improves steel desulphurisation Guarantees more accurate analyses Developed and field-tested by voestalpine mechatronics and voestalpine Stahl, this noncontact InfraRred Identification System for slag (IRIS) permits optimum differentiation between slag and steel. Moreover, the measuring principle of IRIS can also used during slag-dumping, to detect any steel before it is carried over and to ensure that it is kept in the converter (increases output). 2.2 Why was IRIS developed? Originally, an electromagnetic measuring system was used for detecting slag during tapping. Basically consisting of a coil system, the existing measuring system is placed directly 4

5 adjacent to the tapping hole. Because of this, the measuring system is exposed to high thermal and mechanical loads. The wiring arrangements are rather difficult here, as all cables have to be towards the outside through the rotating end journal. This exposed installation situation makes the system very maintenance-intensive. However, repairs can only be carried out when the converter is re-lined or cause a significant long stoppage, meaning that the system only has very limited availability. A further difficulty with the electromagnetic system is that owing to the design of the tapping hole, the system does not function when used with the most of new tapping holes until the hole has been used several times. The first few taps thus have to be performed without the aid of the early-warning system. For these reasons, an entirely new, non-contact measuring system was developed that has none of the above drawbacks. In addition, the IRIS system delivers much more information: Accurate slag detection during tapping State of the tapping hole Closing time of the gate unit Quantitative acquisition of the volume of carry-over slag Detection of residual steel during slag-dumping 2.3 System overview IRIS exploits the different emission behaviour of steel and slag in the infrared. Its system components (infra-red camera, visualisation PC and evaluation unit) are used to continuously observe/visualise the tapping operation in real time. If any slag is carried over, an alarm is generated, triggering the respective gate unit. Where no gate unit has been installed, the system can also be used on a stand-alone basis, e.g. for triggering the procedure whereby the converter is tilted back up. 2.4 IRIS - a telling comparison During the commissioning of an IRIS system, it was tested in over 1000 heats and directly compared with an electromagnetic system. In 80 % of all cases, IRIS responded sooner than the electromagnetic system - at exactly the right moment (N.B. the beginning of the slag-flow was determined from retrospective analysis of the tapping images). As may be seen from Fig. 2.1, IRIS responds an average of 0.5 s faster than the electromagnetic system. Fig. 2.1: Comparison between IRIS and electromagnetic system 5

6 2.5 Additional information provided by IRIS As well as ensuring reliable slag detection, the system provides considerably more information on the tapping process. Slag detection The transition from steel to slag during tapping is visualised by a colour-change in the images from the infra-red camera. Steel is shown as green in the images, while slag is (red)-white. The following images show the abrupt transition taking place at the end of tapping: Fig. 2.2: Pure steel (green), Start of slag carryover (white), Pure slag-flow (white) Quantification of the slag carryover volume For every tapping operation, a slag index is computed that is directly proportional to the volume of slag transferred from the converter into the steel casting ladle. Steel detection during slag-dumping The same measuring principle is also used during slag-dumping. In this case, the system detects any steel carryover, enabling the slag-dumping operation to be finished in good time. In this way, the residual steel is kept in the converter, thereby helping to maximise output. Closing time of the gate unit The closing time of the gate unit is computed from the thermal images. This is important in order to be able to detect and remedy any functions in the gate unit in good time. State of the tapping hole Observation of the rheological behaviour of the steel during tapping provides indications regarding the state of the tapping hole. It also makes it possible to detect skull formation around the tapping hole in good time, so that unnecessary downtime for correcting the problem can be avoided. Excessive skull formation in the tapping hole can impair the functioning of the gate unit. Economic appraisal The using IRIS results in a number of economic benefits for the company. The following economic appraisal assumes that IRIS will be replacing an existing electromagnetic system. 6

7 It must be pointed out that where IRIS is implemented as the first-ever measuring system (i.e. where no early-detection system has been used hitherto), the potential savings are even greater still. Minimised carryover slag volumes Direct comparisons made with the electromagnetic system during the test phase showed that IRIS responded an average of 0.5 seconds sooner, resulting in around 100 kg less slag being carried over. Reduction in carryover slag volume in comparison with the electromagnetic system: approx. 100 kg per heat, i.e. approx. 0.07% of the tap weight Reduction in carryover slag volume when installed as the first-ever early-detection system: approx. 900 kg per heat, i.e. 0.6% of the tap weight Reduced rephosphorisation The reduction in the amount of slag being carried over leads in turn to reduced rephosphorisation, which makes it possible to achieve very low phosphorus contents without resorting to any additional measures. Achieves reductions of around 5-10 ppm as against the electromagnetic system Saves deoxidising agents Aluminium: approx. 4.5 kg per heat Positive influence on secondary metallurgy The small slag volumes, coupled with the fact that the volume of slag that is transferred from the converter into the steel casting ladle is known, are highly advantageous for the models used in secondary metallurgy. Less wear-and-tear on the refractory lining of the steel casting ladle approx. 4% greater durability approx. 5% reduction in refractory gunning material Increased output In the residual-steel plant operation mode (i.e. where the residual steel is detected by IRIS during slag-dumping), the residual steel remains in the converter and thus increases the output of the next heat, meaning that overall output can be greatly improved. Also, the service life of the slag ladle is significantly prolonged, as this comes into contact with crude steel much less. System availability Thanks to the high (nearly 100%) availability of the IRIS system, all the above-mentioned advantages are even more effective. The reason for this higher availability is basically to do with the fact that unlike the electromagnetic system, IRIS needs and has no direct installations in the converter. When needed, access for maintenance is possible at any time. Improved steel desulphurisation The small amounts of slag in the steel ladle, coupled with the fact that the lime volumes remain constant, together have a positive effect on steel desulphurisation during tapping (sulphur distribution (S%)/[S%]). A content of (FeO+MnO)<1% in the slag, which can easily be achieved using deoxidising agents where there is only a small level of slag carryover, brings about very good steel desulphurisation results. Positive environmental impact The production economics of the residual-steel cycle are optimised by enhanced output (less residual steel has to be separated from the slag and then fed back into the converter). The reduction in ladle-slag volumes means a corresponding reduction in the quantities of ladle-metallurgy slag that have to be aged on an intermediate dump (for at least 2 years) before the slag can be used for making rock-wool or in road construction. For voestalpine 7

8 Stahl Linz, this only applies in part at the moment, the slag is largely returned to the cycle by way of the scrap-chute (i.e. into the converter). 2.6 Summary IRIS performed highly satisfactorily in a test phase of over 1000 melts during which it was compared to an electromagnetic system. It delivers a number of tangible qualitative and economic benefits for the company. IRIS can be used with all known gate units or as a stand-alone device, both on converters and electric arc furnaces. 3 LOMAS - LOW MAINTENANCE GAS ANALYSING SYSTEM 3.1 Introduction LOMAS - LOw Maintenance Analyzing System is a high-tech system developed in the course of designing tailor-made industrial plants and paying off within a short space of time. In modern steel plants the gas concentration in the converter exhaust air is measured in order to monitor and control the production process thus improving product quality and saving energy. When using thermal recover flow systems for controlling the torch flare and gasometer operation gas analysis is indispensable. The main components measured for this purpose are oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), nitrogen (N2) and argon (Ar). Gas concentration measuring has to take place as close to the converter as possible in order to ensure maximum accuracy. Converter exhaust gas is very hot, heavily dust-laden and corrosive which made tapping and treatment very difficult in the past. But no longer so with our LOMAS, a gas tapping, treatment and analysis system with the following main characteristics: probe with cooling jacket and heated tapping tube alternating 2-probe operation completely heated gas tapping and gas treatment units connection with any currently available analysis device or mass spectrometer 3.2 LOMAS characteristics The design of the probes (Fig.3.1) heated gas tapping tube high-temperature and corrosion resistant special steel water cooled jacket heated inline filter with a large effective filter surface blowing-through and jet-cleaning system for filter and probe 8 Fig.3.1: Probes

9 LOMAS offers: high operational safety and long service life suitability for plants with very hot (up to 1800 C), heavily dust-laden (up to 2000 g/nm3) and corrosive process and waste gases The two probe operation Fig.3.2 Gas sampling probe 1, backwashing probe 2 Fig.3.3 Gas sampling probe 1, filter and jet cleaning probe 2 Fig. 3.4 Gas sampling probe 2, backwashing probe 1 Fig.3.5 Gas sampling probe 2, filter and jet cleaning probe 1 one probe is used for gas tapping while the second probe is jet cleaned and regenerated clean and dry process gas is back washed through the second probe for filter regeneration automatic, cyclic switching-over from one probe to the other LOMAS ensures: continuous measurement automatic cleaning of the filters and the probe a high degree of availability of the system low maintenance costs no oxygen peaks on the display during the jet cleaning operation constantly up-to-date measurements Gas tapping and treatment (Fig.3.6) heated throughout to maintain temperatures above the water and acid dew point up to the measuring gas cooler high-speed tapping of the measuring gas 9 Fig.3.6: LOMAS cubicle

10 LOMAS guarantees: high accuracy of the analysed values ease of maintenance and regenerability low maintenance operation ensured by keeping water and acid temperatures above dew point short reaction and response times of the system Gas analysis suitable for use in connection with various types of gas analysers and mass spectrometers best suited to your individual requirements means: free choice of analysing devices no special training for your service and maintenance personnel required optimisation of spare parts storage requirements LOMAS can be installed in different variations according the required application: BOF-converter LOMAS switch for converter gas switch-over station Blast furnace - LOMAS BLASTER for CO,CO 2,O 2,H 2 Vacuum degasser LOMAS VACU for CO,CO 2,O 2,H 2,Ar,N 2 in different analyzing variants: STANDARD analyzing with analysers for CO,CO 2,O 2,H 2 SUPREME analyzing with mass spectrometer for CO,CO 2,O 2,H 2,Ar,N 2 BENEFITS, as highest reliability and availability o alternating two probe operation o high accuracy in data measurement and data evaluation o multi gas tapping at different locations of the probes o availability of more than 99%, even under extremely hot and dust-laden gas conditions o short response time (T 90 less than 15 sec. depending on site conditions) o indication of adverse blowing conditions (CO, CO 2, H 2, O 2, N 2, Ar) easy of maintenance o routine checks only twice a month under normal operation conditions o calibration of analysers only all 4 weeks required o maintenance works only 1 times per year explosion protection o fast and easy detection of dangerous situations (increasing values of O 2 and H 2 during blowing process) o LOMAS leads to remarkable reduction of operation disturbances and make it a worthwhile investment. That has secured LOMAS a firm place in many a steel plant, chemical factory as well as in the rock-products industry. 10