-- understanding electric arc furnace operation

8/14/2019 -- Understanding Electric Arc Furnace Operation

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Introduction

Over the past two decadesthe use of electric arc furnaces(EAFs) for the production ofsteel has grown dramaticallyin the United States. In 1975EAFs accounted f or 20% of thesteel produced; by 1996 thefigure had risen o 39% andby the year 2000 (or shortlythereafter) could approach50%. There are tw o majorreasons for thi s rend-lowercapital cost for an EAF steel-making shop and significantlyless energy equired toproduce steel by the EAFversus the blast urnace/basicoxygen furnace method ofthe integrated steelmaker.EAFs range in capacity fro ma few tons to s many as 400tons, and a steelmaking shopcan have ro m one to fivefurnaces. In brief, EAFs can beeither ac or dc powered andthey mel t steel by applyingcurrent to a steel scrap chargeby means of graphite elec-trodes. It requires about 360to 400 k w h of electricity o melta ton of steel; consequently,

(Start of Power Supply)Arc Ignition Period

Long Arc !

Main Melting Period

these furnaces use a remendousquantity of power. Transformerloads may each 150 MVA.

The features of EAFs aredescribed i n a prior CMPTechCommentarytitledIntroduction to Electric ArcFurnace Steelmaking (TC-107713).The purpose f this TechCommenraw(TC-107714) is to give util ities amore comprehensive understand-ing of the electrical operationsand energy usage, and to reviewsome of he innovations that remaking the EAF a very energy-efficient steel melter.

n !

Tapping Spout

Hot Swt

Meltdown Period

L Liquid BathMolten Metal Formation

Period

I-l!

Short Arc ! athMeltdown-Heating Peiiod

Figure 1. Steel Melting Cycle.

Typical Steelmaking Cycle

Figure 1 shows a typical heatcycle, commonly referred t o as thetap-to-tap cycle, for the EAF. Thecycle starts wi th the chargingof the furnace it h steel scrap. Afterthe furnace is charged and the oofis in place, the operator lowers theelectrode or electrodes, each ofwhich has its own regulator andmechanical drive. Current is initiat-ed and the lectrodes bore throughthe scrap to fo rm a pool of liquidmetal. The scrap helps to protectthe furnace lining from he high-

intensity arc during meltdown.Subsequently, the arc is lengthenedby increasing the voltage o maxi-mum power. Most modern urnacesare equipped with water-cooledpanels in the upper half of hesidewall, rather than refractories,which allow for longer rcs andhigher energy nput to the urnace.In the final tage, when there is anearly complete metal pool, the rcis shortened o reduce radiationheat losses and to avoid refractorydamage and hot spots. After rnelt-down, oxygen is injected o oxidizethe carbon n the steel or thecharged carbon. In some opera-


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1OOO-

900-

800 -

Utm-High

700 I600 -

Highooo-

400

300 - Medium200

100Low

Examples

TtimNucor HidaanchaparralNorthstar/BHP

CO-Steel RaritanInlandLukens

Ameri-SteelCaparo Steel

~~ ~~

Figure 2. EAF Power Classifications.

whemical

6045%

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

tions, oxygen inject ion 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 nitrogen and 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 Needs

Institute 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 modern EAFsfound in steelmaking shops are atleast 500 kVA per to n and the trendis toward ultra-high-power urnacesin the ange of 900 to 1000 kVA perton of urnace capacity.

A typical energy balance

(Sankey diagram) for modern EAFis shown in igure 3. Dependingupon the meltshop peration, about60 to 65% of the total nergy is elec-tric, the remainder being chemicalenergy arising rom th e oxidation ofelements such as carbon, i ron, andsilicon and he burning of naturalgas with oxy-fuel burners. About53% of the total energy eaves thefurnace in the liquid steel, while theremainder is lost to the slag, wastegas, and cooling. I

Just a decade ago tap-to-taptimes had decreased from over 2

TechCommentarv

The International Iron and 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 consum ption inthe range of 360 to 400 kWhhon.EAF operations ut ilizing scrappreheating such as the CONSTEELQProcess and h e Fuchs Shaft fur-nace can achieve even ower cycletimes. Most new EAF shops nowaim for tap-to-tap times of between

50-60 minutes. These times arerapidly approaching those or basicoxygen furnace operations used inintegrated steel mills.

Charge Materials

tially charged 100% scrap to thefurnace. Although most EAF steel-makers producing long products,such as rebar and merchant bar,continue to use a ll scrap, someEAF shops today are supplement-ing the harge with other materials

for 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, ir on carbide, and pig iron.For more information on scrapquality and direct reduced iron seeCMP Report 95-1. The trend towardthe use of direct-reduced materialswill continue to grow s morehigh-quality scrap conta ining lowlevels of residuals or undesirableelements becomes scarce.

In the past, EAF shops essen-

Oxygen Usage

Much of he EAF productivitygain achieved in the past decadeis related to increased oxygen use.With the increased availabi lity oflower cost oxygen due o new irseparation technologies, oxygenuse in the EAF has grown. Oxygenusage has increased rom about300 scfhon (8.8Nm3honne) n 1985to as much as 1300 scfhon (37.4

Nm3honne), aving 50 to 100 k w hof electric energy per ton of teelproduced and reducing tap-to -taptimes by 3 to 6 minutes. The rela-tionship between lectric energyand oxygen consumption for theEAF is shown in Figure 4. It is nowcommon for between 0 to 40%of the total energy input o the EAFto come from oxy-fuel urners andoxygen lancing.

Oxy-fuel burners are currentlystandard equipment on EAFs. Thefirst use of burners was for meltingthe scrap a t the slag door where arc

heating 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. Burners are ofteninstalled to help melt crap in thesecold spots. This results in moreuniform melting and decreases hetime required o reach a 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

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natural gas and carbon dioxide. existing furnace electrics. A good transformer secondary voltage sAdvantages for bott om tirring foamy slag practice can llow volt- increased (sometimes o as high asinclude yield increases of 0.5 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 higher arc voltages and owerof about 5 minutes, energy savings nace sidewalls. Energy losses can electrode currents. Some of theof about 10 to 20 kWhhon, and be minimized when eactance is benefits attributed o ac operationreduced electrode consumption. associated wit h the primary circuit . with supplemental reactance are:

Supplementary reactors are

ing voltages on the EAF secondarycrapreheating being used to increase the operat-

Of the otal energy consumed circuit by connecting a reactor indirectly in EAF steelmaking,about 20% normally leavesthe furnace i n the wastegases, see Figure 3. The losscan exceed 130 kWhhon 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 nstal ledon several new installationsin recent yeap. The CON-STEEL@ rocess utilizes aconveyor to continuouslyfeed scrap into the EAF. Hotfurnace gases leaving theEAF travel counte rcurrent othe scrap on the way o 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 u pthrough the shaft. Energy

usage has been reduced y15 to 20% over conventionalEAF operat ions using hesetechnologies.

High Voltage ACOperations

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

Utility System:- Generation- Distribution

Supply Metering

Substation:- Transformer- P.F. Correctidn- Surge Protection- Supplemental Reactors

EH.V. Distribution(Cables)

Furnace Metering

Furnace Transformer

_I Metering for ArcRegulation

Secondary Conductors

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

operating voltages. An acfurnace electrical circuit s Figure 6. Simplified Schematic of he ac EAF Arc Circuit.

shown in Figure 6. Energylosses in the econdary circuit aredependent on the econdary circuitreactance and t o a greater extenton the econdary circuit current. Ifpower can be supplied at a highervoltage, the current will be lowerfor the same power input ate.Operating with a lower secondarycircuit current wil l also give lowerelectrode consumption. Thus, it isadvantageous to operate at as higha secondary voltage as is practical.Of course, his is limited by arc 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 is made possiblebecause a large storage device splaced in the ircuit, which in effectacts as an electrical flywheel duringoperation. The insert ion of theseries reactor drops the secondaryvoltage to lim it he 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 years a numberof dc furnaces have beeninstal led. Essentially, th eequipment needed for dcmelting has the same configu-ration as that of a convention-al ac furnace shown in Figure

7. The exceptions are theaddition of a bot tom electrode(anode), a dc reactor, and athyristor rectifier all of whichadd to the cost of the dc fur-nace. These furnaces use nlyone graphite electrode wit hthe return electrode installedin the bottom of he furnaceand operate wit h a hot heelpractice in order to ensure anelectrical path to the returnelectrode. Figure 8 shows sev-eral types of return lectrodes.These include conductiverefractories wi th a copperexternal base plate and themultipin type made up of con-ductive rods passing throughthe hearth and connected toa botto m 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 emerge rom thefurnace. During startup ro mcold conditions, a mixtureof scrap and slag is used toprovide an initi al electrical

path. Once his is melted n, thefurnace can be charged with scrap.Advantages claimed for dc furnacesover ac furnaces include:

50% reduction i n elec-trode consumption.5 to 10% reduction i npower consumption.Reduced refractoryconsumption.Uniform melting.50% reduction in flicker.

Techcommentary 4


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Hiaholtage nrC I-

AC Furnace

High Voltage AC Line

DC Furnace~~

Figure 7. Layouts for ac and dc Furnaces.

PW

icraphiteElectrode T I

0 0 0Conductiveulti-Pinin le PieceRefractories Metallic detallic

Conductor conductor

Figure 8. Bottom Electrode Designs or dc Furnaces.

Reducing ElectricalDisturbances

The 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 modern urnaces andimproved operating practices.

TechCommentan.

Many ways exist o reduce theeffects of arc disturbances. Theseare determined by the utili ty ystemto which the furnace or urnaces areto be connected, and they are influ-enced mainly by the ize and stabili -ty of the power rid. Some sizableshops require no particular flickercontrol equipment. It is quite possi-ble that, if a furnace shop is fedfrom a 220 kV or higher system witha short-circuit capacity of 6500 MVAor more, the utility will experience

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

1) Capacitor bank configuration.2) Need for harmonic tuning of

sections.3) Switching procedure (This s

important 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 hot heels

all work o reduce flicker. If addi-tional regulation s needed, installa-tion of a static var control SVC)may be required. Many EAF shopshave installed SVC systems notonly to minimize licker problemsbut also t o increase productivity.

Ladle Refining Furnace

During the past ecade, theEAF has evolved int o a fast andlow-cost melter of crap with themajor objective being higher ro-ductivity in order to reduce fixedcosts. In addition, refin ing opera-tions to improve product qualityare (for the most part) carried outin a ladle refining furnace LRF).This allows the AF to concentrateon melting the crap and removingimpurities via oxidation reactions.Temperature and chemistry adjust-ments are carried out moreoptimally in the LRF. For moreinformation on the operationand role of the RF see CMPTechCommenrary CMP-07 1.

EAF DustThe dust exiting the furnace

with the off-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 prio r to isposalin order to meet EPA requirements.There are various methods ortreating the dust-for more infor-mation on hese processes referto CMP Report 93-1.3

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Design Criteria For the Mo de m UH PElectricArc Fumece W t h Auxiliaries, Be man K.,Gottardi R., 3rd European Electric 2 ee1Congress, 1989.

Direct Curren t Electric Arc Furnaces, MPTechCommentary CMP-063, January 1991.

Electric Arc Furnace Dust-1993 Overview,CMP Report 93-1, July 1993.

Electric Arc Furnace Efficiency, MP Report92-10, December 1992.

Electric Arc 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 Electric Arc Furnace

The Electric Power Research Institute(EPRI) conducts a technical researchand development program or theU.S. electric util ity industry. EPRlpromotes the development of newand improved echnologies to helpthe utili ty industry meet present and

future electric energy needs in envi-ronmentally and economicallyacceptable ways. EPRl conductsresearch on all spects of electricpower production and use, includingfuels, generation, delivery energymanagement and consewation,environmental effects, and energyanalysis.

N e w Tools for Improved Operation of HighEfficiency Electric rc Furna ces, Aderup M.,Ljunggren R., Frisk L., Anderson F?, GustafssonA., Samuelsson F?, 1992 Electric FurnaceProceedings.

Optimal Distribution of Oxygen in HighEfficiency ArcFurnaces, Gripenberg H.,Brunner M., Iron and Steel 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 Power Generation, CMP Report95-1, February 1995.

The EPRl Center for MaterialsProduction (CMP) s an R&D applicationcenter funded by The Electric PowerResearch Institute and operatedby Carnegie Mellon Research Institute,Carnegie Mell on 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 Fabrication

CMPJoseph E. Goodwill, Director

This TechCommentarywas prepared by

Dr. Jeremy Jones, Consultant. It supercedesa 1987 TechCommentarywith a sim ilar title.Technical review was provided by Robert J.Schmitt , Associate Director at CMP, andJoseph E. Goodwill, Director of CMP. Editedby John Kollar, CMP

I

LEGAL NOTICEThis TechCommentarywas preparedand sponsored by The EPRl Centerfor Materials Production (CMP).Neither members of CMP nor anyperson acting on their behal f(a) makes any warranty, expressed or

implied, w ith respect to t he 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 for dam-ages resulting from the use of, anyinformation, apparatus, method, orprocess disclosed in hisTechCommentary.

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The EPRlCenter forMaterialsProduction

Carnegie Mellon Research InstituteP.O.Box 2950

Pittsburgh, Pennsylvania 15230-29504 412-268-3243 FAX: 412-268-6852

For additional copies of this Techcommentarycall ECAC 4 1-800-4320-AMP

Ke y Words: Electric Arc Furnace, ElectricalOperations

Applicable SIC Codes 3312,3325

01997 Electric Power Research Institute, Inc.All rights resewed. Printed 2/97

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