Innovation in electric arc furnaces scientific basis for selection

Innovation in Electric Arc Furnaces Yuri N Toulouevski · Ilyaz Y Zinurov Innovation in Electric Arc Furnaces Scientific Basis for Selection 123 Dr Yuri N Toulouevski 303-84 Oakridge Court Holland Landing ON L9N 1R4 Canada y.toulouevski@sympatico.ca Dr Ilyaz Y Zinurov B Khmelnitsky Str 25, Apt 54 Chelyabinsk 454047 Russia akont-project@yandex.ru ISBN 978-3-642-03800-6 e-ISBN 978-3-642-03802-0 DOI 10.1007/978-3-642-03802-0 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2009939072 © Springer-Verlag Berlin Heidelberg 2010 This work is subject to copyright All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer Violations are liable to prosecution under the German Copyright Law The use of general descriptive names, registered names, trademarks, etc in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use Cover design: WMXDesign GmbH, Heidelberg Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Preface Selection of innovations for each plant as well as selection of directions of further development is one of the crucial problems both for the developers and for the producers of steel in EAF Ineffective selection leads to heavy financial losses and waste of time In practice, this happens quite frequently The main objective of this book is to help the readers avoid mistakes in selecting innovations and facilitate successful implementation of the selected innovations The entire content of the book is aimed at achieving this objective This book contains the critical analysis of the main issues related to the most widespread innovations in EAF The simplified methods of calculations are used for quantitative assessment of innovations These methods are explained by numerous examples Considerable attention is given to the new directions of development which the authors consider to be the most promising In the process of writing of the book, its content was discussed with many specialists working at metallurgical plants and for scientific research and development organizations The authors express deep gratitude for their valuable observations and considerations A number of the important issues covered in the book are debatable The authors would like to thank in advance those readers who will consider it possible to take the time to share their observations Their input will be really appreciated and taken into account in further work Our heartfelt thanks go to G Toulouevskaya for her extensive work on preparation of the manuscript for publication Ontario, Canada Yuri N Toulouevski v Contents Modern Steelmaking in Electric Arc Furnaces: History and Prospects for Development 1.1 General Requirements for Steelmaking Units 1.1.1 Process Requirements 1.1.2 Economic Requirements 1.1.3 Environmental and Health and Safety Requirements 1.2 High-Power Furnaces: Issues of Power Engineering 1.2.1 Maximum Productivity as the Key Economic Requirement to EAF 1.2.2 Increasing Power of EAF Transformers 1.2.3 Specifics of Furnace Electrical Circuit 1.2.4 Optimum Electrical Mode of the Heat 1.2.5 DC Furnaces 1.2.6 Problems of Energy Supply 1.3 The Most Important Energy and Technology Innovations 1.3.1 Intensive Use of Oxygen, Carbon, and Chemical Heat 1.3.2 Foamed Slag Method 1.3.3 Furnace Operation with Hot Heel 1.3.4 Use of Hot Metal and Reduced Iron 1.3.5 Single Scrap Charging 1.3.6 Post-combustion of CO Above the Bath 1.4 Outlook 1.4.1 World Steelmaking and Mini-mills 1.4.2 The Furnaces of a New Generation 1.4.3 Consteel Process References 1 11 13 13 14 14 15 16 17 17 18 20 20 20 22 23 Electric Arc Furnace as Thermoenergetical Unit 2.1 Thermal Performance of Furnace: Terminology and Designations 2.2 External and Internal Sources of Thermal Energy: Useful Heat 2.3 Factors Limiting the Power of External Sources 25 25 27 28 vii viii Contents 2.4 Key Role of Heat Transfer Processes Reference The Fundamental Laws and Calculating Formulae of Heat Transfer Processes 3.1 Three Ways of Heat Transfer: General Concepts 3.2 Conduction Heat Transfer 3.2.1 Fourier’s Law Flat Uniform Wall Electrical–Thermal Analogy 3.2.2 Coefficient of Thermal Conductivity 3.2.3 Multi-layer Flat Wall 3.2.4 Contact Thermal Resistance 3.2.5 Uniform Cylindrical Wall 3.2.6 Multi-layer Cylindrical Wall 3.2.7 Simplifying of Formulae for Calculation of Cylindrical Walls 3.2.8 Bodies of Complex Shape: Concept of Numerical Methods of Calculating Stationary and Non-stationary Conduction Heat Transfer 3.3 Convective Heat Exchange 3.3.1 Newton’s Law: Coefficient of Heat Transfer α 3.3.2 Two Modes of Fluid Motion 3.3.3 Boundary Layer 3.3.4 Free (Natural) Convection 3.3.5 Convective Heat Transfer at Forced Motion 3.3.6 Heat Transfer Between Two Fluid Flows Through Dividing Wall; Heat Transfer Coefficient k 3.4 Heat Radiation and Radiant Heat Exchange 3.4.1 General Concepts 3.4.2 Stefan–Boltzmann Law; Radiation Density; Body Emissivity 3.4.3 Heat Radiation of Gases 3.4.4 Heat Exchange Between Parallel Surfaces in Transparent Medium: Effect of Screens 3.4.5 Heat Exchange Between the Body and Its Envelope: Transparent Medium 3.4.6 Heat Exchange Between the Emitting Gas and the Envelope Energy (Heat) Balances of Furnace 4.1 General Concepts 4.2 Heat Balances of Different Zones of the Furnace 4.3 Example of Heat Balance in Modern Furnace 4.4 Analysis of Separate Items of Balance Equations 4.4.1 Output Items of Balance 29 30 31 31 32 32 35 38 39 40 41 42 43 47 47 47 48 49 50 52 56 56 57 60 61 62 63 65 65 66 69 70 70 Contents ix 4.4.2 Input Items of Balance Chemical Energy Determination Methods 4.5.1 Utilization of Material Balance Data 4.5.2 About the So-Called “Energy Equivalent” of Oxygen 4.5.3 Calculation of Thermal Effects of Chemical Reactions by Method of Total Enthalpies References 72 73 73 74 75 80 81 81 83 84 88 88 89 90 91 91 92 4.5 Energy Efficiency Criteria of EAFs 5.1 Preliminary Considerations 5.2 Common Energy Efficiency Coefficient of EAF and Its Deficiencies 5.3 Specific Coefficients η for Estimation of Energy Efficiency of Separate Energy Sources and EAF as a Whole 5.4 Determining Specific Coefficients η 5.4.1 Electrical Energy Efficiency Coefficient ηEL 5.4.2 Fuel Energy Efficiency Coefficient of Oxy-gas Burners ηNG 5.4.3 Energy Efficiency Coefficient of Coke Charged Along with Scrap 5.4.4 Determining the Specific Coefficients η by the Method of Inverse Heat Balances 5.5 Tasks of Practical Uses of Specific Coefficients η References Preheating of Scrap by Burners and Off-Gases 6.1 Expediency of Heating 6.2 Consumptions of Useful Heat for Scrap Heating, Scrap Melting, and Heating of the Melt 6.3 High-Temperature Heating of Scrap 6.3.1 Calculation of Potential of Electrical Energy Savings 6.3.2 Sample of Realization: Process BBC–Brusa 6.4 Specifics of Furnace Scrap Hampering Its Heating 6.5 Processes of Heating, Limiting Factors, Heat Transfer 6.5.1 Two Basic Methods of Heating 6.5.2 Heating a Scrap Pile in a Large-Capacity Container 6.5.3 Heating on Conveyor 6.6 Devices for Heating of Scrap: Examples 6.6.1 Heating in Charging Baskets 6.6.2 DC Arc Furnace Danarc Plus 6.6.3 Shaft Furnaces 6.6.4 Twin-Shell Steelmelting Units References 93 93 94 95 95 96 97 98 98 99 102 105 105 108 110 111 113 Replacement of Electric Arcs with Burners 7.1 Attempts for Complete Replacement 115 115 x Contents 7.2 Potentialities of Existing Burners: Heat Transfer, Limiting Factors 7.3 High-Power Rotary Burners (HPR-Burners) 7.3.1 Fundamental Features 7.3.2 Two-Stage Heat with HPR-Burners 7.4 Industrial Trials of HPR-Burners 7.4.1 Slag Door Burners: Effectiveness of Flame Direction Changes 7.4.2 Two-Stage Process with a Door Burner in 6-ton Furnaces 7.4.3 Two-Stage Process with Roof Burners in 100-ton and 200-ton EAFs 7.5 Oriel and Sidewall HPR-Burners 7.6 Fuel Arc Furnace (FAF) 7.7 Economy of Replacement of Electrical Energy with Fuel References Basic Physical–Chemical Processes in Liquid Bath: Process Mechanisms 8.1 Interaction of Oxygen Jets with the Bath: General Concepts 8.2 Oxidation of Carbon 8.3 Melting of Scrap 8.4 Heating of the Bath Jet Streams: Fundamental Laws and Calculation Formulae 10.1 Jet Momentum 10.2 Flooded Free Turbulent Jet: Formation Mechanism and Basic Principles 10.3 Subsonic Jets: Cylindrical and Tapered Nozzles 10.4 Supersonic Jets and Nozzles: Operation Modes 10.5 Simplified Formulae for Calculations of High-Velocity Oxygen Jets and Supersonic Nozzles 10.5.1 A Limiting Value of Jets’ Velocity 10.6 Long Range of Jets Reference 122 124 127 131 135 137 139 141 141 142 144 146 149 149 150 150 151 152 152 154 157 157 159 159 160 162 165 167 169 170 170 Bath Stirring and Splashing During Oxygen Blowing 9.1 Stirring Intensity: Methods and Results of Measurement 9.2 Mechanisms of Bath Stirring 9.2.1 Stirring Through Circulation and Pulsation 9.2.2 Stirring by Oxygen Jets and CO Bubbles 9.3 Factors Limiting Intensity of Bath Oxygen Blowing in Electric Arc Furnaces 9.3.1 Iron Oxidation: Effect of Stirring 9.3.2 Bath Splashing 9.4 Oxygen Jets as a Key to Controlling Processes in the Bath References 10 117 120 120 120 122 Contents 11 12 13 xi Devices for Blowing of Oxygen and Carbon into the Bath 11.1 Blowing by Consumable Pipes Submerged into Melt and by Mobile Water-Cooled Tuyeres 11.1.1 Manually Operated Blowing Through Consumable Pipes 11.1.2 BSE Manipulator 11.1.3 Mobile Water-Cooled Tuyeres 11.2 Jet Modules: Design, Operating Modes, Reliability 11.2.1 Increase in Oxygen Jets Long Range: Coherent Jets 11.2.2 Effectiveness of Use of Oxygen, Carbon, and Natural Gas in the Modules 11.3 Blowing by Tuyeres Installed in the Bottom Lining 11.3.1 Converter-Type Non-water-Cooled Tuyeres 11.3.2 Tuyeres Cooled by Evaporation of Atomized Water 11.3.3 Explosion-Proof Highly Durable Water-Cooled Tuyeres for Deep Blowing References 171 171 172 172 174 177 178 181 183 183 184 187 191 Water-Cooled Furnace Elements 12.1 Preliminary Considerations 12.2 Thermal Performance of Elements: Basic Laws 12.3 Principles of Calculation and Design of Water-Cooled Elements 12.3.1 Determining of Heat Flux Rates 12.3.2 Minimum Necessary Water Flow Rate 12.3.3 Critical Zone of the Element 12.3.4 Temperature of Water-Cooled Surfaces 12.3.5 Temperature of External Surfaces 12.3.6 General Diagram of Element Calculation 12.3.7 Hydraulic Resistance of Elements 12.4 Examples of Calculation Analysis of Thermal Performance of Elements 12.4.1 Mobile Oxygen Tuyere 12.4.2 Elements with Pipes Cast into Copper Body and with Channels 12.4.3 Jet Cooling of the Elements 12.4.4 Oxygen Tuyere for Deep Blowing of the Bath References 193 193 193 197 197 199 200 200 202 204 204 Principles of Automation of Heat Control 13.1 Preliminary Considerations 13.2 Automated Management Systems 13.2.1 Use of Accumulated Information: Static Control 13.2.2 Mathematical Simulation as Method of Control 13.2.3 Dynamic Control: Use of On-line Data 13.3 Rational Degree of Automation References 217 217 217 217 218 221 227 228 207 207 209 212 213 215 244 14 Off-gas Evacuation and Environmental Protection 2−2 1−1 3 Q2−2 GAS.PH = 14630; VCO = VCO = 104 m /min (1.73 m /s) QCO = 12,700 kJ/m CO – thermal effect of reaction (14.5) 3−3 3−3 ×t3−3 ×V 3−3 Q3−3 GAS = 14,630 + 104 × 12700/60 = 36,643 kW; QGAS = c GAS GAS 3−3 = 36,643/18.1 × c3−3 Hence tGAS Taking into consideration the composition of gases at the cross-section (3–3) and using the method of consecutive approximations we find c3−3 = 1.57 kJ/m3 × ◦ C 3−3 ∼ and tGAS = 36,643/18.1 × 1.57 ∼ = 1290◦ C Negative Pressure at the Inlet of the Stationary Gas Duct Necessary for Prevention of Uncontrolled Emissions Through the Electrode Ports To directly evacuate the entire amount of primary gases at the moments of their most intensive formation, the gas exhausters must provide minimum required rarefaction (negative pressure) −p2−2 MAX , Pa, at the cross-section (2–2) In the past, the typical error in the design of gas evacuation systems was using of gas exhausters with a quite sufficient productivity, but with a pressure not big enough to create such rarefaction As a result, the uncontrolled emissions from the freeboard could not be eliminated Hydraulic calculation of the portion of the gas duct between cross-sections (1–1) and (2–2) aimed to determine the value −p2−2 MAX has significant specifics, which were not encountered in the calculation of water-cooled elements, Chap 12 The main difference is that the amount of primary gases increases downstream due to infiltration of air through the gap between the roof elbow and the gas duct In the water-cooled elements, the water flow rate remained constant along the duct length Due to this specific of the portion of the duct under consideration, to determine the maximum value of negative pressure −p2−2 (here and further the index “max” is dropped), we should use the Bernoulli equation It expresses the law of conservation of energy of the incompressible gas flow When it is applied to the problem under consideration, this equation can be rearranged as follows: (p + ρv2 /2)2−2 = (p + ρv2 /2)1−1 − ζ8 (ρv2 /2)1−1 (14.9) The first two terms of Eq (14.9) represent the energy of flow at the cross-sections (2–2) and (1–1) equal to the sum of potential energy of pressure p and kinetic energy ρv2 /2 The indices at the parenthesis are applicable to all values inside the parenthesis The physical meaning of Eq (14.9) is that the energy of the flow at the crosssection (2–2) is equal to the energy at the cross-section (1–1) less irreversible losses of energy, which is converted to heat when the flow of the primary gases collides with the flow of infiltrated air The amount of losses is determined by the last term of Eq (14.9), where ζ is the coefficient of local resistance of the gap between the elbow and gas duct This coefficient depends on geometrical parameters of the gap Calculations 245 and can be calculated using a simplified formula: ζ8 = 0.7 + 15 L/dh , (14.10) L – width of gap, m dh – hydraulic diameter of roof elbow Formula (14.10) was obtained as a result of the same studies which led to formula 2−2 (14.6) for determining VGAS By solving Eq (14.9) for p2−2 , we obtain the following: p2−2 = p1−1 + (ρv2 /2)1−1 × (1 − ζ8 ) − (ρv2 /2)2−2 (14.11) Let us assume that the pressure of the gases under the roof and their velocity in the furnace freeboard is equal to zero; the diameter of roof opening is equal to the diameter of the roof elbow, and quite insignificant hydraulic resistance due to friction in the elbow can be ignored Under these conditions, using the corresponding Bernoulli’s equation for the cross-sections (0–0) and (1–1) we can obtain the following expression: p1−1 = −(ρv2 /2)1−1 × (1 + 0.5), (14.12) where 0.5 is the coefficient of local resistance for gases entering the roof elbow Let us find the actual velocity of gases at the temperature of 1650◦ C, v1−1 GAS.t The density of the gases at this temperature is ρ 1−1 = 0.18 kg/m3 The actual amount of gases per second is v1−1 GAS.t = 313 × (1650 + 273)/273 × 60 = 36.7 m /s For the diameter of the roof elbow of dh = 1.25 m and its cross-section area equal to F 1−1 = 1.23 m2 : v1−1 GAS.t = 36.7/1.23 = 29.9 m/s Using expressions (14.12 and 14.10), we find P1−1 = – 0.18 × 29.92 × 1.5/2 ∼ = − 121 Pa; ζ = 0.7 + 15 × 0.080/1.25 = 1.66 Let us find the actual velocity of gases at the cross-section (2–2) v2−2 GAS.t The density of the gases at this cross-section at t2−2 = 570◦ C is ρ 2−2 = 0.43 kg/m3 The amount of gases per second recalculated for this temperature is as follows: 1−1 = 19.0 × (570 + 273) 273 = 59 m3 /s VGAS.t For the cross-section area of the gas duct F 2−2 equal to twice the area of the elbow F 1−1 , the actual velocity of gases is v1−1 GAS.t = 59.0/1.23 × = 24 m/s Then, using Eq (14.11), let us find the unknown maximum rarefaction (negative pressure): p2−2 = − 121 + (0.18 × 29.92 /2)(1–1.66)–0.43 × 242 /2 = −300 Pa We must point out that as the diameters of the roof opening, the roof elbow, and the gas duct decrease, the required negative pressure p2−2 sharply increases 246 14 Off-gas Evacuation and Environmental Protection 14.3.4 Energy Problems The most important feature of the modern systems of gas evacuation of the EAFs is a quite unusual for other steelmaking units ratio between the amount of process gases formed inside of the furnace and the amount of gases emitted into the atmosphere after the gas purification As it was shown in Sect 14.3.3, the maximum intensity of evolution of gases in the freeboard of 120-ton furnace, including the infiltrated air, is about 19,000 m3 /h When these gases enter the bag filters for dust removal, their amount increases due to the dilution with air up to 1.4–1.5 million m3 /h, i.e., by 70– 80 times As a result, in the modern EAFs, about 10% of entire electrical energy used for steelmaking is consumed for evacuation and purifying of quite insignificant, in essence, amount of primary process emissions Such excessive energy consumptions for gas purification are caused mostly by a necessity to capture the secondary uncontrolled emissions by low-efficiency exhaust hoods located above the furnace The modern environmental standards for control of air pollution with dust require complete absence of any visible dust emissions above the still mill, which is controlled by the sensitive optical instruments To satisfy these requirements, huge amounts of dust-laden air must be exhausted through the hood Satisfactory operation of the exhaust hoods is hampered by the large distances vertically from the furnace to the hood, the presence of the crane, and the basket during the scrap charging between them, as well as the air flows along the shop building which entrain the dust-laden gases To diminish the effect of these flows, the areas of the aperture in the shop walls for the rail carriers are minimized; the apertures are enclosed with air curtains; the air delivery uniformly distributed along the shop perimeter is provided by the systems of forced ventilation, and the hood configuration is improved Nevertheless, the studies and the accumulated experience show that even under the most favorable conditions, to ensure the complete capturing of the secondary uncontrolled emissions, the velocity of suction of air– gas mixture through the open from below hood aperture must not be less than m/s In 120-ton furnaces, with the area of hood aperture equal about 200 m2 , such velocity of suction results in the volumes of gases passing through the hood, which are shown above (200 × × 3600 = 1,440,000 m3 /h) One of the trends in reduction of energy costs for gas evacuation is the installation of furnaces in the small-volume air-tight doghouses, which excludes the usage of hoods In the 1980s, such doghouses were widespread They enclosed not only the furnace itself, but also the zone of tapping of metal into the ladle Therefore, all the secondary uncontrolled emissions were concentrated in the doghouse To provide passage for the charging baskets and the ladle for steel tapping, these doghouses were equipped with tight-closing doors of different design Due to relatively small volume of doghouses, the amount of gases evacuated from them, assuring the complete capturing of the secondary emissions, reduced by a number of times compared to the hoods At the same time, the doghouse protected the personnel from the furnace noise Calculations 247 Despite the advantages noted above, with the further increase in furnace productivity, the use of the doghouses had to be ceased This was caused by the following reasons At every heat, massive doghouse overhead half-doors for the passing of the charging baskets and half-doors for passing of the ladle car must be moved repeatedly As the number of heats per day was about 40, even insignificant delays of the technological process related to these additional operations caused unacceptable drop of the furnace productivity Besides, the necessity to work inside the doghouse greatly complicates the maintenance of such furnace elements as oxy-gas burners and devices for oxygen, carbon, and lime injection into the bath The doghouse prevents the use of manipulators at the working platform, particularly to control temperature and composition of the metal Recently, a number of effective innovations to reduce volumes of uncontrolled emissions and energy costs for gas evacuation were developed and implemented Furnace operation modes with closed door became widespread In the course of the heat, the door opens only when it is necessary and for a short time, for example to remove slag When the reliable sensors for gas pressure under the roof became available, the automatic control of draft of gas exhausters to maintain the optimum pressure during the entire heat has been successfully implemented The operation of the furnace with the closed door and with maintaining of the low negative pressure under the roof allows sharply reducing air infiltration into the furnace and completely eliminating the secondary uncontrolled emissions through the electrode ports and the annular gap between the roof and the shell of the furnace Amount of the secondary emissions at the tapping can be sharply reduced, if the exhaust hood capturing them is located close to the ladle Despite the attempts to develop such design, the satisfactory solution has not been found so far However, other possibilities exist In the most modern furnaces designed for charging the entire amount of scrap with one basket, the duration of a single dust–gas emission does not exceed 3–4 The emission during the tapping has approximately the same duration Short duration of these single emissions combined with total lack of the frequent escape of gases through the electrode ports enables a new approach to the solution of the problem of capturing of the secondary emissions under the shop’s roof The new approach is to use the so-called deep accumulating hoods which are the air-tight pockets built-in the top part of the shop building The volume of these pockets is approximately 10 times greater than the volume of the regular hoods This allows accumulating the entire single emission of the dust-laden gases at the scrap charging, as well as at the tapping In comparison to air, these gases have higher temperature and lower density, hence they can remain in the pockets long enough Therefore, they can be exhausted gradually as opposed to the very short time period of emission as in the regular hoods The highest intensity of the secondary emissions is reached during the charging of scrap containing large amounts of oil and plastics In these moments, the power of the exhausters has to be approximately doubled compared to the other periods of the heat This significantly increases the costs of the gas purification, since it requires 248 14 Off-gas Evacuation and Environmental Protection the respective increase in its carrying capacity Substitution of the hoods with the accumulating pockets allows to stretch exhausting of a single emission in time and, therefore, to sharply reduce the required maximum power of the exhausters and energy costs Significant contribution into increase in the energy expenditures in the gas evacuation system comes from a simplest but quite cost-ineffective method of the cooling of the primary emissions by diluting them with cold air As calculations show, the amount of air at the temperature of 25◦ C, which has to be mixed with m3 of hot gases to cool them down from 1600 to 150◦ C, is approximately 15 m3 Unfortunately, incomparably more effective method of cooling of the primary emissions by atomized water is not used in all cases, since it requires strict control of the amount of injected water and atomization quality in the course of the heat Water must evaporate completely Otherwise, unacceptable moisturizing and plugging of the bag filters occur If the potential of the reviewed innovations is fully utilized, they allow to significantly reduce the ultimate volumes of emissions and costs for their capturing and purification Nonetheless, the energy problem of gas evacuation in the EAFs is quite far away from being satisfactorily resolved As the capacity and productivity of the furnaces increase and the environmental protection standards become stricter, even further aggravation of this problem can be expected This will require new, more radical approaches to its resolution 14.4 Use of Air Curtains The EAF operation mode with closed door, which became widespread, eliminates the air infiltration into the freeboard However, this mode has significant disadvantages It is necessary to eliminate quite efficient mobile door burners located at the manipulators In the absence of such burners, the complete melting of the bath near the sill is somewhat delayed Accordingly, the moment when it becomes possible to control the temperature and the composition of the bath, is pushed back as well This, in turn, delays the tapping The caked slag–metal build-up forms on the sill near the closed water-cooled door The removal of this build-up is associated with certain difficulties Pushing the slag–metal build-up into the bath even further delays its melting near the sill Besides, the operation with closed door complicates the heat control as it deprives the operator of important visual information regarding the condition of the freeboard, Chap 13, Sect 13.3 The alternative to the closed door mode is using of air curtains, which is free from the disadvantages mentioned above However, to successfully substitute the door with the air curtain, the elimination of infiltration of the external air must be assured on condition that the jets of the curtain itself not enter the freeboard Such mode can be called the mode of complete aerodynamic locking of the door Besides, the curtain must not worsen the environmental situation at the working platform of the furnace 14.4 Use of Air Curtains 249 2 a a α α b Fig.14.4 Air curtain for 60-ton EAF, – delivery of compressed air (the rest of designations are given in the text) The development of efficient air curtains for EAFs was hindered by lack of sound scientific concepts regarding the very mechanism of their operation in the mode of complete aerodynamic locking and, consequently, by lack of calculation methods of such curtains Due to this reason, the purely empirical attempts to develop efficient curtain designs for the EAFs for long time did not yield positive results The studies conducted by the authors on the cold and hot models of various air curtains, as well as on the EAFs of 100- and 200-ton capacity, has shown that the mechanism of air curtain operation in the complete aerodynamic locking of the door has nothing in common with the air curtains, which are installed in the building doors to prevent penetration of cold external air [5, 6, 7] The obtained data allowed offering a simple calculation method and efficient design of the air curtains for the EAFs A double-sided air curtain at the slag door of the 60-ton EAF at the plant in Akko (Israel) is an example of such design, Fig 14.4 The required air pressure in front of the curtain nozzles does not exceed 0.3 bar, which corresponds to the initial jet velocity of approximately 190 m/s However, to reduce the overall dimensions of air ducts in the door zone, compressed air under pressure of 4–6 bar rather than fan air is supplied to the curtain To use the excessive compressed air pressure effectively and to reduce its flow rate, the injectors (1) are built-in into the curtain design In the injectors, the compressed air is mixed with the atmospheric air Due to this fact, the flow rate of the compressed air does not exceed 300 m3 /h (5 m3 /ton), which is only about 60% of the entire air used in the curtain 250 14 Off-gas Evacuation and Environmental Protection The low pressure (about 0.3 bar) air obtained in the injectors enters the collectors (2) with the nozzles spread over their height Air jets flowing from the nozzles converge and form flat flows a–b, directed at the angle α to the plane of the door aperture a–a, Fig 14.4 Such direction of the jets eliminates the possibility of curtain’s air penetration into the furnace freeboard Laws governing the jets in this curtain are different in principle from the free jets Under the negative pressure in the open slag door, they behave as the jets in the limited space With an increase in the distance from the nozzle the jet momentum reduces, as well as their kinetic energy which is expended to form the circulation zones on both sides of the flat flows a–b As a result, the static pressure field of the gases in the door zone changes substantially At the certain value of the ratio of the initial momentum of curtain jets ICUR to the momentum of the air flow IAIR , which would be infiltrated into the furnace without the curtain, the mode of complete aerodynamic locking the door arises In this mode, the static pressure differential between the circulation zones divided by the flat air flows of the current a–b becomes equal to zero, and flowing of air through the boundary a–b–a, Fig 14.4, stops Under these conditions, the curtain behaves as a partition practically impenetrable to the air flows Under this mode, the air exchange between the circulation zones is insignificant and occurs only through the turbulent pulsations The studies conducted has shown that the value of the ratio ICUR IAIR = K; (K > 1) , (14.13) which assures the complete aerodynamic locking of the door does not depend on the initial velocity of jets, the area of the open door, and the negative pressure in the door The criterion K is determined only by the basic geometrical relations between the door and the curtain such as angle α, Fig 14.4 In the calculations of the door curtains for EAF it can be assumed K ∼ = The calculations of the air curtains are performed using the following method Using formula (4), for the given height of the EAF roof over the sill level, the dimensions of the slag door, and the degree of its opening, let us find the amount of air infiltrated into the furnace freeboard without the curtain, VAIR , m3 /s Since in this case we find the amount of air not per minute but rather per second, the coefficient equal must be introduced into formula (14.4) instead of the coefficient equal 180 By dividing VAIR by the area of the open aperture F = ε × h × b, m2 , let us find the flow velocity of the air infiltrated through the aperture vAIR , m/s and its momentum IAIR = ρ × VAIR × vAIR , where ρ = 1.2 kg/m3 is air density outside the furnace Then, for K = 3, let us find the required momentum of the curtain ICUR = IAIR Assuming the initial jet velocity to be equal vCUR , let us find the air flow rate into the curtain: VCUR = ICUR /ρ × vCUR For the given number of the nozzles in the collectors, we find the overall area and the nozzle diameter using the values VCUR and vCUR The curtain momentum ICUR increases directly proportionally to vCUR Therefore, in order to reduce the air flow rate into the curtain, the velocity of the 14.4 Use of Air Curtains 251 a b Fig 14.5 Picture of gas flows in front of the 60-ton EAF slag door (a) without curtain and (b) with curtain air jets should be increased by reducing the nozzle diameter However, the increase in vCUR makes sense only up to 160–190 m/s At the higher velocities, due to pressure increase in the collectors, the injection coefficient in the injectors drops and a substantial reduction of compressed air flow rate cannot be achieved Besides, the noise from the curtain rises to undesired level If the curtain momentum differs from the minimum required momentum for the given area of the open door aperture and from the average value of the negative pressure in it, then the mode of the complete aerodynamic locking of the aperture is disrupted At the lower momentum, the complete elimination of air infiltration into the furnace cannot be achieved If the momentum is significantly higher, the curtain jets begin to entrain the dust-laden gases from the furnace onto the working platform However, if the momentum fluctuates within ±20% from the required level, these phenomena are practically not detected The best results are assured when the curtains are used simultaneously with the automatic control of the gas pressure under the furnace roof Under the conditions of 60-ton EAF at USM, the efficient operation of the curtain could be observed visually When the curtain was turned off, the gases flow rising from the slag ladle is sucked into the open door at high velocity When the curtain is turned on, this flow, which is clearly visible due to its high dust content, passes by the door without being deflected toward it Simultaneously, the vortices of red fume appear at the door aperture; they pulsate very close to its exit, but not escape outside, Fig 14.5 a, b Due to elimination of air infiltration into the furnace, the use of the curtain increased the yield by 0.8% on average This result was confirmed with quite high accuracy by conducting the special experiments In these experiments, the small series of heats with and without the curtain were conducted alternating randomly numerous times The calculations show that the decrease in heat input from iron oxidation due to the observed increase in the yield compensates to a considerable extent the reduction of the heat losses due to elimination of cold air infiltration 252 14 Off-gas Evacuation and Environmental Protection into the furnace Therefore, when the curtain was used, the reduction of electrical energy consumption was insignificant The design of the curtain at USM turned out to be quite reliable During a few years of continuous operation it practically did not require any maintenance or repair [8] References Amoldo L, Khan M, Evenson E, et.al Results of Goodfellow EFSOP at Hylsa, Planta Norte, ISS Mexico, 2003 Cordova E, Khan M, Zuliani D et.al Advanced dynamic control technology for EAF steelmaking and other combustion processes, MPT International, 2007, No 2, 28–32 Stark C B, Gas cleaning apparatus and units for metallurgical production, Moscow, Metallurgia, 1990 Shults L A, Kochnov Y M, Kochnov M Y, Current state and development of systems for evacuation, utilization and purification of gases at large capacity high power electric arc furnaces, Ferrous Metals, 2006, October, 18–28 Kiselyov A D, Toulouevski Y N, Zinurov I Y, Increase in efficiency of gas evacuation from electric arc furnaces, Moscow, Metallurgia, 1992 Idelchik I E, Handbook for hydraulic resistances, 3-d issue, Moscow, Engineering, 1972 Toulouevski Y N, Air curtain for 60-t EAF at United Steel Mills Ltd 6th European Electric Steelmaking Conference, Düsseldorf, June 1999 Toulouevski Y N, Air curtain for EAF slag doors, MPT International, 2001, No 4, 128–129 Index A Absolute temperature, 26, 57, 63, 168 Kelvin’s degrees, 26 Actual electrical power, 8, 125, 136 power factor, 9, 11 Amount of motion, 159, 160 Arcing stability, Arcs in AC EAF’s, Automated management systems, 217–227 as operator’s guide, 227 Autonomous units equipped with gas burners, 106 B Balance studies, 92 Bath splashing, 154–156, 186, 229 methods of physical modeling, 154, 186 Bath stirring control, 152 Blowing by consumable pipes, 171–176 manipulator BSE, 172–174 manually, 172, 173 C Calculation analysis of thermal performance of elements, 207–214 elements with pipes cast into body, 209–212 mobile oxygen tuyere, 207–209 oxygen tuyere for deep blowing, 213–214 Calculations of air curtains, 248–251 Calculations of high-velocity jets and supersonic jets, 167–170 simplified formulae, 167–170 Causes hindering increase in burners power, 117–119 fixed direction of flame, 119 fuel underburning, 119 oxidation of iron, 119 Chemical energy of oxidation reactions, 69, 73 Chemical energy of slag formation, 85 Chemical reactions, 27, 66, 68, 71, 75–80, 89, 126, 136, 143, 146, 151 endothermic, 27, 67 enthalpy, 27, 67 exothermic, 26, 27, 67 thermal effect, 27, 75–80 Coefficient of convection heat transfer, 47 Combined coefficient of heat transfer, 50 Combining EAF and oxygen converters, 111 Common energy efficiency coefficient of EAF, 83–84 Conditions of heat transfer, 20, 29, 30, 100, 200 from arcs, 15 from burner flames to scrap, 29 Conducting active experiments, 92 Conduction heat transfer, 31, 32–46 non-stationary processes, 32 stationary processes, 32, 38, 41 Consteel steelmelting unit, 102 conveyor, 102–105 high-temperature heating of scrap, 102, 103, 104 ineffective scheme of heat transfer, 102 Consumptions of useful heat for, 94–95 melting of liquid metal, 94–95 scrap heating, 94–95 scrap melting, 94–95 Control of hot heel, 17 Convective transfer of heat energy, 31 Correction of coefficients of mathematical models, 220 “self-learning” systems, 220 Critical zone of element, 200, 204, 206 D Deep blowing into slag–metal boundary, 188 effectiveness of deep oxygen blowing, 188 SIP process, 188, 189 Y.N Toulouevski and I.Y Zinurov Innovation in Electrical Arc Furnaces, 10.1007/978-3-642-03802-0, C Springer Science+Business Media B.V 2010 253 254 Dependences between parameters of electrical circuit, 10–11 mode of maximum productivity, 11 most economical mode of heat, 11 Development of electric arc furnaces, 6–7 increase in productivity, 6–7 Devices for heating of scrap, 105–113 DC arc furnace Danarc Plus, 108–110 sectional shaft preheater, 110 shaft furnaces, 110–111 twin-shell steelmelting units, 111–113 Direct current (DC) arc, 9, 13 Direct evacuation systems, 230, 233, 236–245 carrying capacity, 236 effectiveness, 236 effect on economics of entire system, 236 hydraulic calculation, 244 simplified method of gas parameters calculation, 236–245 durability of tuyeres, 191 Dynamic control, 221–227 of decarburization of bath, 222, 223, 224 of heating of bath, 224 of heating scrap by burners, 224 of injection into bath, 225 measuring sondes, 222 predicting devices, 223, 224 use of on-line data, 223 E Effect of heat transfer conditions on, 30 power level external heat energy sources, 30 productivity and energy efficiency, 30 Effectiveness of blowing, 175, 176, 185, 188 above slag, 175, 176 submerged, 175, 176, 188 by tuyeres submerged below metal surface, 176 Effectiveness of use in modules, 181–183 of carbon, 181–183 of natural gas, 181–183 of oxygen, 181–183 Effect of screens, 61–62 Electrical energy efficiency of the secondary circuit, 88 Electric arc furnaces, 1–23, 25–30, 50, 65, 69, 81, 105, 117, 136, 152–156, 171, 175, 176, 180, 182, 187, 189, 193, 217, 220, 225, 229 of direct current, 13 large-capacity, 108 of a new generation, 20–21 Index ultrahigh-power, Electromagnetic compatibility with other energy consumers, 13 electrical energy quality deterioration, 14 Element heat balance condition, 199 Emitting ability of body surface, 58, 59 black bodies, 58, 59 gray bodies, 58, 59 white bodies, 58, 59 Energy consumptions for gas purification, 246 operation of exhaust hoods, 246 operation of furnace with closed door, 247 placement of furnaces in doghouses, 246 use of deep accumulating hoods, 247 Energy efficiency of oxy-gas burner, 91 Energy equivalent of oxygen, 74–75 Energy of natural gas, 67, 73, 85, 120, 138 Enthalpies of chemical reactions, 27, 66, 75, 76, 80 Enthalpies of heated substances (physical heat), 66 Enthalpy of final slag and slag removed, 67, 70 Enthalpy of metal charge, 67, 68 Enthalpy of metal before tapping, 70 useful heat of heat, 70 F Factors limiting intensity of oxygen blowing, 152–156 degree of overoxidation of bath, 154 effectiveness of use of oxygen, 153 iron oxidation, 152–154 yield, 153, 154, 156 Flooded free turbulent jet, 160–162 boundary layer, 160, 161, 162 expansion angle, 161 initial region, 161 jet boundary, 161 jet core, 160, 161, 162 kinetic energy, 160 main region, 161, 162 Forced turbulent motion of fluids, 50 Forms of heat transfer in scrap, 100 convective heat transfer between high-temperature gases, 100 heat efficiency coefficient of gases, 100 heat radiation between heated lumps, 100 volumetric overall heat transfer coefficient, 100 Fuel arc furnace (FAF), 135–137 calculated indices, 135 two-stage process mode, 135 Furnace electrical circuit, 8–11 Index inductive impedance, phase shift, 8–9 secondary circuit, 8, 9, 11 G General diagram of element calculation, 204 Gravitational stirring, 147 H Heat capacity, 25, 26, 44, 45, 51, 66, 70, 76, 94, 126, 199, 232, 238, 241, 242 Heat (energy) balances for, 65–80 bath, 65, 66, 67, 68, 69, 70, 71, 73, 76, 80 electro-technical zone, 65, 67–68 entire furnace, 66–67 entire heat, 65 freeboard, 65, 66, 67, 68, 71, 73, 74, 76, 78, 80 instantaneous balance, 65 offgas evacuation system, 65, 66, 68, 71 separate stages of heat, 65 Heat energy efficiency coefficient, 81 Heat energy of light rays, 57 Heat exchange between emitting gas and envelope, 63–64 Heat flux concentration on water-cooled surface, 194 degree of concentration, 194 Heat flux density, 32, 55, 71, 72, 195, 201, 213, 214, 225 Heat flux dissipation on water-cooled surface, 194 Heat flux rates, 197–198 maximum values of densities, 198 total maximum flux, heat generation, 198 Heating of bath, 146–147 heat flux densities from arcs, 146 intensive stirring of bath, 146, 147 Heating on conveyor, 102–105 advantages, 102, 103, 104 Heating scrap by offgases in charging baskets, 105–107 with/without recirculation of gases, 106, 107 Heating scrap pile in large-capacity container, 99–102 medium mass temperature of scrap, 100 possibilities of speeding up, 102 scrap heating durations, 101 Heat losses, 70, 71, 72 through furnace bottom, 72 with offgases, 70, 71 with water, 71, 72 Heat radiation, 56–64 255 continuous spectrum, 56 Heat transfer from burners flame to scrap, 89 Heat transfer intensification through wall, 55 High-power rotary burners, HPR-burners, 120–122 oriel, 132–135 sidewall, 132–135 High-temperature heating of scrap, 95–97 calculation of potential, 95–96 Hourly productivity of furnace, power-off time, power-on time, Hydraulic resistance of elements, 204–207 coefficient of friction resistance, 206 coefficient of local resistance, 206 friction resistance, 206 local resistance, 206 I Implementing new potentially dangerous innovations, 188 three basic conditions, 188 Increase of oxygen consumption, 14, 73, 75, 152, 153 Increase in oxygen jets long range, 178–181 results of stand tests, 179, 180 use of coherent supersonic nozzles, 178 use of pilot flame, 178, 179 Increasing durability of water-cooled elements, 193 Increasing intensity of heat transfer, 30 Increasing maximum secondary voltage of transformer, 12 Increasing output of mini-mills, Increasing power of EAF transformer, 7–8 specific power of furnaces, Indicator of durability of water-cooled element, 193 external surface temperature, 194 Injection carbon into bath, 154 reduction of iron oxides, 154 slag foaming, 154 Intensity of splashing, 155 dependence on position of tuyere, 156 Interaction of oxygen jets with bath, 141–142 jet immersion into melt, 142 reaction zone, 142 J Jet cooling of elements, 212–213 cylindrical heat-removing cell, 213 Jet modules, 69, 91, 92, 134, 170, 176, 177–183, 185, 186, 193, 198, 210 Jet momentum, 159, 165, 170, 249 256 L Laminar or viscous sub-layer, 49 laminar sub-layer turbulization, 49 Limiting value of jets velocity, 169–170 Long range of jets, 170 Losses of electrical energy, 70 Low-power oxy-fuel burners, 117–119 increase in number of burners, 117 oriel, 117 pilot flame, 118 sidewall, 117 M Material balance, 73–74 of carbon, 74 of heat, 73, 74 Mathematical simulation as method of control, 218–221 determinate models, 219, 220 statistic models, 219 Maximum critical temperature, 195 for copper wall, 195 for low-carbon steel wall, 195 Mechanisms of bath stirring, 150–152 circulation stirring, 151 by CO bubbles, 151–152 by oxygen jets, 151–152 pulsation stirring, 151 Melting scrap in iron-carbon melt, 144–146 diffusion melting, 144, 145 freezing of melt, 144, 145 intensive melting, 145 Melt stirring intensity, 149 methods and results of measurement, 149–150 Methods for recalculating standard enthalpies, 75–80 Minimum necessary water flow rate, 199–200 mass, 199 volumetric, 199 Mobile water-cooled tuyeres, 171–176 durability, 175, 176 Model of black body, 59 N Nozzle operation in design mode, 167 O On-line data, 221–227 continuous, 221, 222, 224 direct, 222, 223, 224 discrete or non-repeating, 221 indirect, 222, 223, 225 Overall coefficient of heat transfer, 54–55 Index range of variation in industrial devices, 55 Overall efficiency coefficient of primary energy, 138 Overall energy efficiency, 82 Oxidation of carbon, 142–144 bath decarburization process, 143 direct oxidation of carbon, 143 two-stage scheme, 143 P Possibilities of using visual information, 226 Power of sidewall burners, 90 Preheating of scrap in converters, 103 Preparing of primary emissions for cleaning, 233–248 cooling of offgases, 234 post-combustion of CO and H2 , 234 rate of CO burn-out, 235 suppression of dioxins, 235 water injection, 235 Price of natural gas, 3, 137 Primary emissions, 230, 231, 232, 233, 234, 235, 237, 247, 248 chemical energy, 232 composition, 231–233 dust content, 231, 232 intensity of formation, 230 physical heat, 232 temperatures, 231–233, 234, 235 total heat content, 232 Problem of graphitized electrodes, 12 electrode current carrying capacity, 12 increasing electrode diameter, 12 Problem of post-combustion of CO, 18–20 explosion-proof operation of offgas evacuation system, 18 increase in input of chemical energy, 18 oxygen consumption for post-combustion, 19 two-tier tuyeres, 19 Production safety, 187 Protecting from cylindrical and tapered nozzles, 162–165 coefficient taking into account jet contraction, 162 critical velocity, 163, 164 velocity coefficient, 161 PTI module, 177, 181 operating modes, 177–183 operating reliability, 178 reducing oxygen jet length, 177 R Radiant heat exchange, 56–64 dynamic thermal equilibrium, 57 Index Radiation constant of black body, 57 Radiation of severely dust-laden gases, 60–61 Rational degree of automation, 227–228 operator’s participation in process control, 227 Reduced emissivity of system of bodies, 62 Replacement of arc with burners, 115–139 economy, 137–139 FOS process, 115, 116 NSR process, 116 Requirements to steelmaking units, economic, environmental, health and safety, process, Ribbing, 54, 55, 194 S Scrap price, 3, Secondary or “uncontrolled” emissions, 230 composition, 231 dust content, 231 Sensors for detecting tuyeres position, 175 Slag door burners, 122–125 possibility to change flame direction, 124 Slag foaming mechanism, 15–16 consumption of carbon powder, 16 foaming ability, 16 stability of formed foam, 16 Solubility of oxygen in liquid iron, 142 Sources of dust-gas emissions, 229–230 carbon contained in bath, 229 charging of contaminated scrap, 229 metal evaporation in electric arcs zone, 229 oxy-gas burners, 229, 230 Sources of thermal energy, 27–28 external, 27–28 factors limiting power, 28–29 internal, 27–28 Specific energy efficiency coefficients, 86, 87 Specifics of steel scrap, 97–98 bales, 97 chips, 97–98 oil, 97–98 organic materials, 97, 98 Speed of sound, 47, 163, 164, 165, 166, 167, 168 Standard enthalpies of chemical reactions, 75, 76, 80 Standard volumes of gases, 26 Static control, 217–218, 220, 221, 227 of decarburization of bath, 220 effectiveness, 218 257 of heating of bath, 220 prescriptive type of control, 218 use of accumulated information, 219 Stationary tuyeres in bottom of furnace, 183 converter type non-water-cooled, 183–184 cooled by evaporation of atomized water, 184–186 Steelmelting unit BBC–Brusa, 96–97 continuous charge of scrap, 97 performance, 97 Subsonic jets, 161, 162–165, 167 Supersonic jets, 157, 165–167, 179, 180, 181 Supersonic nozzles, 166, 167–170, 175, 178, 179, 214 coherent nozzle, 166, 179 de Laval nozzle, 166, 179 overexpanded, 167 underexpanded, 167 T Temperature distribution in wall, 32–46 bodies of complex shape, 43–46 flat uniform, 32–35 multi-layer cylindrical, 41–42 multi-layer flat, 38–39 uniform cylindrical, 40–41 Temperature of water-cooled surfaces, 200–202 film boiling, 202 local or surface bubble boiling, 202 mixed process of heat exchange, 201 Thermal conductivity coefficient, 32, 37, 39, 46, 49, 194, 208, 209 for solids and fluids, 35 Thermal efficiency coefficient of electric arcs, 88 Thermal performance of elements, 193–197, 207–214 Thermal performance of furnace, 25–27, 50 Thermal resistances, 33, 35, 38, 39–40, 41, 43, 45, 49, 53, 54, 55, 97, 195, 197, 203, 204, 211, 212, 213 of contacts, 39 Total enthalpy of chemical compound, 77 values of total enthalpies, 77 Total enthalpy method, 77 determining resultant thermal effects, 77 Total thermal resistance, 39, 41, 54 Total useful energy consumption for heat, 85 Transition of laminar mode to turbulent one, 48 Reynolds criterion, 48 Two modes of fluid motion, 47–48 laminar mode, 47, 48 turbulent mode, 48 258 Two-stage heat with HPR burners, 122–131 with door burner, 125–127 with roof burners, 128–131 Two types of gas evacuation systems, 233 Types of wear of water-cooled element, 194 U Use of air curtains, 248–251 air curtain at slag door, 249 complete aerodynamic locking of door, 248, 250 efficient operation of curtain, 251 mechanism of operation, 248 Useful energies balance equation, 87 Useful energy consumption, 85 Index of chemical energy, 85 of electrical energy, 85 Useful heat, 27–28 of entire heat, 28 V Volt–ampere characteristic of arc, 9, 10 active resistance of arc, W Water-cooled tuyere in lining of bottom, 190 complete safety of installation, 190 cooling by water in closed circuit, 190 durability of lining adjacent to tuyere, 191 .. .Innovation in Electric Arc Furnaces Yuri N Toulouevski · Ilyaz Y Zinurov Innovation in Electric Arc Furnaces Scientific Basis for Selection 123 Dr Yuri N Toulouevski... required minimum of information in these fields of science are included in the book This information is presented in a rudimentary form yet not compromising strict scientific meaning Formulae for calculations... this book In certain cases, data, obtained not only in the modern steelmaking units but also in the obsolete open-hearth furnaces, are used When evaluating innovations for electric arc furnaces,