EAF Operation

8/8/2019 EAF Operation

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IntroductionOver the pasttwo decadesthe use of electric arc furnaces(EAFs) for the production ofsteel has grown dramaticallyin the United States. In 1975EAFs accounted for 20% of thesteel produced; by 1996 thefigure had risen o 39% andby the year 2000 (or shortlythereafter) could approach50%. There aretwo majorreasons for thisrend-lowercapital cost for an EAF steel-making shop and significantlyless energy equired toproduce steel by theEAFversus the blast urnace/basicoxygen furnace method ofthe integratedsteelmaker.EAFs range in capacity froma few tons tos many as 400tons, and a steelmaking shopcan have rom one to fivefurnaces. In brief, EAFs can beeither ac or dc powered andthey melt steel by applyingcurrent to a steel scrap chargeby means of graphiteelec-trodes. It requires about360to 400 kwh of electricity o melta ton of steel; consequently,

(Start of Power Supply)Arc Ignition Period

LongArc !Main Melting Period

these furnaces use aremendousquantity ofpower. Transformerloads may each 150 MVA.The features of EAFs aredescribed in a prior CMPTechCommentarytitledIntroduction to Electric ArcFurnace Steelmaking (TC-107713).The purpose f thisTechCommenraw(TC-107714) is to give util itiesamore comprehensive understand-ing of the electrical operationsand energyusage, and to reviewsome of he innovations thatremaking theEAF a very energy-efficient steel melter.

n !

TappingSpoutHot Swt

Meltdown Period

L Liquid BathMolten Metal FormationPeriod

I-l!

Short Arc ! athMeltdown-Heating Peiiod

Figure 1. Steel Melting Cycle.Typical SteelmakingCycle

Figure 1 shows a typical heatcycle, commonly referred to as thetap-to-tap cycle, for theEAF. Thecycle starts wi th the chargingof the furnace it h steel scrap. Afterthe furnace is charged and theoofis in place, the operator lowers theelectrode or electrodes, eachofwhich has its own regulator andmechanical drive. Current is initiat-ed and thelectrodes bore throughthe scrap to forma pool of liquidmetal. The scraphelps to protectthe furnace lining from he high-

intensity arc during meltdown.Subsequently, the arc is lengthenedby increasing the voltageo maxi-mum power. Most modern urnacesare equipped with water-cooledpanels in the upper half ofhesidewall, rather than refractories,which allow for longerrcs andhigher energy nput to theurnace.In the finaltage, when there isanearly complete metal pool, thercis shortened o reduce radiationheat losses and to avoid refractorydamage andhot spots. After rnelt-down, oxygen is injectedo oxidizethe carbon n the steel or thecharged carbon. In some opera-


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1OOO-900-800 -

Utm-High

700 I600 - Highooo-400300 - Medium200100 Low

ExamplesTtimNucor HidaanchaparralNorthstar/BHP

CO-Steel RaritanInlandLukens

Ameri-SteelCaparo Steel

~~ ~~

Figure 2. EAF Power Classifications.

whemical6045%Figure:3. Energy Patterns n an Electric Arc Furnace.

tions, oxygen injection s startedas soon as a liquid pool of etalis formed. The decarburizationprocess is an important ourceof energy. In addition, the carbonmonoxide that volves helps oflush nitrogenand hydrogen outof the metal. It also foams the slag,which helps to minimize heat lossand shields the arc-thereby reduc-ing damage to refractories.

Energy NeedsInstitute classifies EAFs based onthe power supplied er ton of fur-nace capacity. The ower classifica-tion ranges and some representa-tive furnace installations are shownin Figure 2. Most modernEAFsfound insteelmaking shopsare atleast 500 kVA per ton and the trendis toward ultra-high-powerurnacesin the ange of 900 to 1000 kVA perton of urnace capacity.A typical energy balance(Sankey diagram) for modern EAFis shown in igure3. Dependingupon the meltshop peration, about60 to 65% of the totalnergy iselec-tric, the remainder being chemicalenergy arising rom th e oxidation ofelements such as carbon, iron, andsilicon and he burning of naturalgas with oxy-fuelburners. About53% of the total energyeaves thefurnace in the liquidsteel, while theremainder is lost to the slag, wastegas,and cooling. IJust a decade ago tap-to-taptimes haddecreased from over 2TechCommentarv

The International Ironand Steel

hours to 70-80 minutes for the effi-cient melt shops. Continuingadvancements in EAF technologynow make it possible to melt heatof steel n less than one hour withelectric energy consumption inthe range of 360 to 400 kWhhon.EAF operations utilizing scrappreheating such as the CONSTEELQProcess and he FuchsShaft fur-nace can achieve evenower cycletimes. Most new EAF shops nowaim for tap-to-tap times of between50-60 minutes. These times arerapidly approaching thoseor basicoxygen furnace operations used inintegrated steel mills.Charge Materialstially charged100% scrap to thefurnace. Although mostEAF steel-makers producing longproducts,such as rebar and merchant bar,continueto use all scrap, someEAF shops today are supplement-ing the harge with other materialsfor producing higher quality prod-ucts. This is the case for someproducers of high-quality ars andhighly formable heet productsfor automobiles. These chargematerials include direct reducediron, iron carbide, and pig iron.For more informationon scrapquality and direct reduced iron seeCMP Report 95-1. The trend towardthe use of direct-reduced materialswill continue to grows morehigh-quality scrap containing lowlevels of residuals or undesirableelements becomes scarce.

In the past, EAF shops essen-

Oxygen UsageMuch of he EAF productivitygain achieved in the past decadeis related to increased oxygen use.With the increased availability oflower cost oxygen due o new irseparation technologies, oxygenuse in the EAF has grown. Oxygenusage has increased rom about

300 scfhon (8.8Nm3honne) n 1985to as much as 1300scfhon (37.4Nm3honne), aving 50 to 100kwhof electric energy per ton of teelproduced and reducing tap-to-taptimes by3 to 6 minutes. The rela-tionship between lectric energyand oxygen consumption for theEAF is shown in Figure 4. It is nowcommon for between0 to 40%of the total energy inputo the EAFto come from oxy-fuel urners andoxygen lancing.Oxy-fuel burners are currentlystandard equipment onEAFs. Thefirst use of burnerswas for meltingthe scrap at the slag door wherearcheating was fairly inefficient. Inultra-high-power UHP) furnaces, tis common for cold spots to existin the areas lying between theelectrodes on th e periphery of thefurnace bottom. Burnersare ofteninstalled to help melt crap in thesecold spots. This results in moreuniform meltingand decreases hetime required o reacha flat bath.Also, burners are beneficial for heat-ing the cold spot around the taphole of eccentric bottom tappingfurnaces. Typically burners areinstalled in the side wall and roof of

2


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5 3506,5

0 600 1200Oxygen Consumption(Standardcubic feetperton)

-igure 4. Eleptrical Energy vs. Oxygen Consumption.the furnace as well as in the slagdoor. Productivity increases of5 to20% have been reported from theuse of burners.Oxygen lancing has alsobecome an ntegral partof EAFmelting operationsover the pastdecade. Modern furnaces useoxygen lances to cut scrap, decar-burize (refine) the ath, and foamthe slag. Energy savings ue tooxygen lancingarise from bothexothermic reactions (oxidation ofcarbon and iron) and dueo thestirring of the bath whicheads totemperature and compositionhomogeneity of he bath. Oxygenlances can be f two forms, watercooled andconsumable. Water-cooled lances are generally used ordecarburizing; however, in somecases they are used or scrap cut-ting. The first consumable lanceswere operated manually throughthe slag door. Today, remote-controlled lance manipulators areavailable to optimize the injectionprocess. These obotic units, Figure5, can be usedwith multiple, indi-vidually controlled,consumablelances for scrap burning and decar-burizing, as well as for injectingoxygen, carbon, and lime.Foamy Slag Practice

At the tar! of meltdown theradiation from the rc to the side-walls is negligible ecause the elec-trodes are surrounded by the scrap.As melt ing proceeds, the efficiencyof heat ransfer to the scrap andTwhCo rn rnen ta r v

bath declines as moreheat is radiated fromthe arc to the side-walls. By covering thearc in a layer of slagthe arc is shielded andmore energy is trans-ferred to the bath.Oxygen injected withgranular coalor car-bon produces carbonmonoxide (CO) whichfoams theslag. Insome cases, only car-bon is njected and treacts with the ironoxide in the slag toproduce CO. This iscalled a oamy slagpractice and is nowcommonly used byEAF operators. Whenfoamed, the slag covernormally increasesfrom 4 inches (0.1meter) to 12 inches(0.3 meter) thick.Claims for thermal efficiencyangefrom 60 to 90%with slag foamingcompared to 40% without foamyslag. If a deep oamy slag isachieved, it is possible o increasethe arc voltage considerably. Thisallows a greater rate f power input.Slag foaming is usually carried outonce a lat bath is chieved.However, with hotheel operations(residual liquid steel in the furnacebottom) it is possible o start foam-

ing much ooner.Post Combustion

CO gas is produced n largequantities in the EAF both fromoxygen lancing and lag foaming.If the CO is not ombusted in thefurnace freeboard then t must be

burned in the fourth hole vacua-tion system conveying the off-gasesfrom the furnace to the baghouse.The heat of ombustion of CO toC 0 2 s three timesreater than theheat of combustion of to CO.Thus, this represents avery largepotential energy source or theEAF.If theCO is burned n the EAF it ispossible to recover the heat whilereducing the heatload on the off-gassystem. This is called post combus-tion. Results of studieshave shownthat bypracticing post combustion,i.e., injecting oxygen nto theEAF toburn theCO to COz,35 o 60 percentof the heat in the off-gas can berecovered. EAF operators are nowmoving toward adopting thispractice and typical lectric energysavings are about 0.1 kWh/scf(4 kWh/Nm3)of oxygen injected).EAF Bottom Stirring

For conventional ac furnacesthere is little natural electricallyinduced turbulencewithin the bathcompared to dc furnaceswhichhave more convection stirring. fthere is ittle bath movement, largepieces of scrap can take aong timeto melt and mayequire oxygenlancing. The concept f stirring thebath isnot a new one and recordsindicate that electromagnetic coilswere used for stirring trials s earlyas 1933.Today mostEAF stirringoperations use inert gas as the stir-ring medium. The gas s introducedthrough the bottomf the furnaceusing porousplugs. In a conven-tional ac furnace, three plugsareused with a plug located midwaybetween each of the hFee elec-trodes. Primarily argon or nitrogengases are used; however,sometrials havebeen conducted with

Figure 5. Lance Manipulator.3


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natural gas and carbon dioxide. existing furnace electrics. A good transformer secondary voltagesAdvantages for bottom tirring foamy slag practice can llow volt- increased (sometimes o as high asinclude yield increases of0.5 to 1%, age increases of up o 100%without 800 to 1100 volts) allowing operationaverage tap-to-tap ime savings adversely affecting lare to the fur- at higherarc voltages and owerof about 5 minutes, energy savings nace sidewalls. Energy losses can electrode currents. Some of theof about10 to 20 kWhhon, and be minimized when eactance is benefits attributed o ac operationreduced electrodeconsumption. associated with the primary circuit. with supplemental reactanceare:Supplementary reactors areing voltages on the EAF secondarycrapreheating being used to increase the operat-

Of the otal energy consumed circuit by connecting a reactor indirectly inEAF steelmaking,about 20% normally leavesthe furnace in the wastegases, see Figure 3. The losscan exceed130kWhhon ofsteel produced. A significantportion of this nergy can berecaptured by using the ff-gas to preheat the scrap. Twomethods for preheatingscrap, the CONSTEEL@process and he Fuchs shaftfurnace have been nstalledon several new installationsin recent yeap. TheCON-STEEL@ rocess utilizes aconveyor to continuouslyfeed scrap into theEAF. Hotfurnace gases leaving theEAF travel countercurrent othe scrap on the wayo thebaghouse thereby preheatingthe scrap. The Fuchs furnacehas a shaft situated on top ofthe EAF which holds a crapcharge that is preheated bythe off-gases rising upthrough theshaft. Energyusage has been reduced y15 to 20% over conventionalEAF operations using hesetechnologies.High Voltage ACOperations

Some EAF steelmakershave retrofi tted their hopsand installed new powersupplies to obtain higher

Utility System:- Generation- DistributionSupply MeteringSubstation:- Transformer- P.F. Correctidn- Surge Protection- Supplemental ReactorsEH.V.Distribution(Cables)Furnace Metering

Furnace Transformer

_IMetering for ArcRegulationSecondary Conductors

R= Resistance X,= Inductive ReactanceX,= Capacitive Reactance Zo= Arc Impedance

operating voltages. An acfurnace electrical circuit s Figure6.Simplified Schematic of he acEAFArcCircuit.shown inFigure 6. Energylosses in the econdary circuit aredependent on the econdary circuitreactance and to a greater extenton the econdary circuit current. Ifpower can besupplied at a highervoltage, the currentwill be lowerfor thesame power input ate.Operating with a lowersecondarycircuit currentwil l also give lowerelectrode consumption. Thus, it isadvantageous to operate at as higha secondary voltage as is practical.Of course, his is limited byarc flareto the furnace sidewall and the

series with the primary windings ofthe EAF transformer. This allowsoperation at a power actor ofapproximately 0.707 which is thetheoretical optimum for maximumcircuit power. This ismade possiblebecause a large storage device splaced in the ircuit, which in effectacts as an electrical flywheel duringoperation. The insertion of theseries reactor drops thesecondaryvoltage to lim ithe amount ofpower transferred to the arc. In orderto compensate for this, the furnace

More stable arc han forstandard operations.Electrode consumptionreduced by 10%.

Secondary voltageincreased by 60 to 80%.DC Furnaces

In recent yearsa numberof dc furnaces have beeninstal led. Essentially, theequipment needed fordcmelting has the same configu-ration as that of a convention-al ac furnace shown inFigure7.The exceptions are theaddition of a bot tom electrode(anode), a dc reactor, and athyristor rectifier all of whichadd to the cost of thedc fur-nace. These furnaces use nlyone graphite electrode withthe returnelectrode installedin the bottom ofhe furnaceand operate with a hot heelpractice in order to ensure anelectrical path to the returnelectrode. Figure8shows sev-eral types of return lectrodes.These include conductiverefractories with a copperexternal base plateand themultipin typemade upof con-ductive rodspassing throughthe hearth and connected toa bottom steel plate. A th irdtype consists of one o fourlarge diameter teel rods fit-ted in the furnace bottom, thesteel rods being water cooledwhere they emergerom thefurnace. During startup romcold conditions, amixtureof scrap and slag is used toprovide an initial electricalpath. Once his is meltedn, thefurnace can be chargedwith scrap.Advantages claimed for dc furnacesover ac furnaces include:50% reduction in elec-trode consumption.5 to 10% reduction inpower consumption.Reduced refractoryconsumption.Uniform melting.50% reduction in flicker.

Techcommentary 4


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Hiaholtage nrC I-

AC Furnace

High VoltageAC Line

DC Furnace~~

Figure7. Layouts for ac and dc Furnaces.

PW icraphiteElectrode T I

0 0 0Conductiveulti-Pinin le PieceRefractories Metallic detallicConductor conductor

Figure 8. Bottom Electrode Designsor dc Furnaces.

Reducing ElectricalDisturbancesThe melting process involvesthe use of large quantities f energyin a short ime (1-2 hr) and n someinstances the process has causeddisturbances (flicker & harmonics)in power grids but this problem isbeing minimized with the installa-tion of modernurnaces andimproved operating practices.

TechCommentan.

Many ways existo reduce theeffects of arc disturbances. Theseare determined by the utili tyystemto which the furnace orurnaces areto be connected, and theyare influ-enced mainly by the ize and stabili-ty of the powerrid. Some sizableshops requireno particular flickercontrol equipment.It is quitepossi-ble that, if a furnace shop is fedfrom a 220 kV or higher system witha short-circuitcapacity of 6500 MVAor more, the utility will experience

very little loaddisturbance, and thesteelmaker can have considerableflexibility in configuring his internalplant powersystem. Most utilitiesrequire power factor orrection.Shops with large EAFs would morethan likely use static capacitors;synchronous condensers of suffi-cient capacity would be prohibitive-ly expensive for amultifurnaceshop. Before such systems areinstalled, transient analysis isrequired to determine:

1) Capacitor bankconfiguration.2) Need for harmonic tuning ofsections.3) Switching procedure (This simportant to avoid a powerfactor penalt and does noteliminate flic r).As mentioned earlier, use of dcEAFs and improved operatingpractices such as scrap preheating,foamy slags, and use of hotheels

all work o reduce flicker. If addi-tional regulation s needed, installa-tion ofa static var control SVC)may be required. Many EAF shopshave installed SVC systems notonly to minimizelicker problemsbut also to increase productivity.Ladle Refining Furnace

During the pastecade, theEAF has evolved into a fast andlow-cost melter of crap with themajor objective being higher ro-ductivity in order to reduce fixedcosts. In addition, refining opera-tions to improve product qualityare (for the most part) carried outin a ladle refining furnaceLRF).This allows the AF to concentrateon melting thecrap and removingimpurities via oxidation reactions.Temperature and chemistry adjust-ments are carried out moreoptimally in the LRF. For moreinformation on the operationand role of theRF see CMPTechCommenraryCMP-071.EAF Dust

The dust exiting the furnacewith theoff-gases has been classi-fied as a hazardous waste (KO61by the Environmental ProtectionAgency (EPA) because t can con-tain lead, cadmium, chromium,and nickel. As a result, the dustmust be treated prior to isposalin order to meet EPA requirements.There are various methods ortreating thedust-for more infor-mation onhese processes referto CMP Report93-1.35


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Design Criteria For the Mo de mUH P ElectricArc Fumece W t h Auxiliaries, Be man K.,GottardiR., 3rd European Electric2ee1Congress, 1989.Direct Curren t Electric Arc Furnaces,MPTechCommentary CMP-063, January 1991.Electric Arc Furnace Dust-1993Overview,CMP Report 93-1, July 1993.Electric Arc Furnace Efficiency,MP Report92-10, December 1992.ElectricArc Furnace Technology, RecentDevelopments and Future Trends,Teoh L.L.,lronmaking and Steelmaking, Vol. 16,1989.Giving EAFs the Shaft to Recoup Energy,33 Metal Producing, November 1990.Ladle Re fining Furnaces, CMPTechCommentary CMP-071, August 1991. N e w Develop ments in ElectricArc Furnace

The Electric Power Research Institute(EPRI) conducts a technical researchand development programor theU.S. electric util ity industry. EPRlpromotes the development ofnewand improved echnologies to helpthe utili ty industry meet present andfuture electric energy needs in envi-ronmentally and economicallyacceptable ways. EPRl conductsresearch on all spects of electricpower production anduse, includingfuels, generation, delivery energymanagement and consewation,environmental effects, and energyanalysis.

Ne w Tools for Improved Operation ofHighEfficiency Electric rc Furna ces,Aderup M.,LjunggrenR., Frisk L., Anderson F?,GustafssonA., SamuelssonF?,1992 Electric FurnaceProceedings.Optimal Distribution of Oxygen in HighEfficiency ArcFurnaces,GripenbergH.,Brunner M., Iron andSteel Engineer, July 1990.Oxy-Fu el Burner Technology or Electric ArcFurnaces,Wells M.B., Vonesh F.A., Iron &Steelmaker, November, 1986.Reflections on the P ossibilities andLimitations of Cost Saving in Steel Productionin Electric Arc Furnaces, Klein K., Paul G.,Metallurgical Plant and Technology,Vol.1,1989.Review of N ew lronmaking Technologies andPotential for PowerGeneration,CMP Report95-1, February 1995.

The EPRl Center for MaterialsProduction (CMP) s an R&D applicationcenter funded by The Electric PowerResearch Institute andoperatedby Carnegie Mellon Research Institute,Carnegie Mellon University. CMP is aservice of the Industrial and Agricul-tura l Technologies and ServicesBusiness Unit of the Customer SystemsGroup of EPRI. The mission of theCenter is to discover, develop, and deliver high value technological advancesthrough networking and partnershipwith the lectricity industry.

EPRlPreston Roberts, Manager,Materials Production and FabricationCMPJoseph E. Goodwill, Director

This TechCommentarywas prepared byDr. Jeremy Jones, Consultant. Itsupercedesa 1987 TechCommentarywith a similar title.Technical review was provided by Robert J.Schmitt, Associate Director at CMP, andJoseph E. Goodwill, Director of CMP. Editedby JohnKollar, CMP

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LEGALNOTICEThis TechCommentarywas preparedand sponsored by The EPRl Centerfor Materials Production (CMP).Neither members ofCMP nor anyperson acting on their behal f(a) makes any warranty, expressed orimplied, with respect to the use ofany information, apparatus, method,or process disclosed in thisTechCommentaryor that such usemay not nfringe privately ownedrights; or (b)assumes any liabilitieswith respect to the se.of, or fordam-ages resulting from theuse of, anyinformation, apparatus, method, orprocess disclosed in hisTechCommentary.

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The EPRlCenter forMaterialsProductionCarnegie Mellon Research InstituteP.O.Box 2950Pittsburgh, Pennsylvania 15230-29504 412-268-3243FAX:412-268-6852

For additional copies of this Techcommentarycall ECAC4 1-800-4320-AMPKeyWords: ElectricArc Furnace, ElectricalOperationsApplicable SIC Codes 3312,332501997 Electric Power Research Institute, Inc.All rights resewed. Printed 2/97

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EAF Operation