conversion of existing furnaces requires TYPICAL RESULTSFOR SINGLEUtility Advantages havlng enough space In the transformer ELECTRODE FURNACES Reduced voltage fluctuatlon lessens room for the thyristor, reactor, and auxillary the problem of fllcker thereby greatly Florida Steel-Tampa Plant reduclng or elimlnatlng the need for costly equipment. Also, sufficient space must This is a 35-ton, 14 MVA, 11 MW meltbe available under the furnace bottom for VAR compensation. down dc furnace thatwas converted from Offsetting the advantages, capltal coststhe secondaryconductor and cooling equlpment for the bottom electrode. acto dc. Electrodeconsumptlon provided for a dc installation are higher than for an ac system; however, reduced operating costs Bottom electrodedesign has varied with one of the greatest advantages,dropping from 10 Ibs./billet-tonto 3'/2 Ibs. Inaddimany typesbeing used. (See Figure 3.) can recoup this difference Theseincludeconductive refractorieswith tion, the furnace became simpler and safer a copper external shell, and the multi-plnto operate ... operators don't change AC vsDC FURNACES electrodes as often,and spend less time type, amanifold having conductive rods passing through the hearth. Both of these on topof the furnace. Also, very little sidewall AC Advantages DC Advantages have been usedon medium-sized furnaces refractory spraying is required. EnvironReduced = Lower less than 40 kA. A third type with currents mentally, while there is little dtfference in installation electrode consists of alargediameter steel plate fitted the volume of dust generated, there are costs consumption (single electrode vs in the furnace bottom, the steel being waterfewer holes In the roof No bottom m Lower lining cooled where it emergesfrom the furnace. three) whch permits easiercapture of electrode wear Bath stirring Higher power rn Better temperarating for DATA FOR OPERATING SINGLE-ELECTRODE DC FURNACES bigger heats ture distribution rn Less noise us. Japan Less network disturbance Nucor Florida Steel TOW Tokyo Steel w Lessenergy Darlington Tampa Toyohashi Kyushu consumption Sept 88 Jan 86 March 85 start-up Jan 89 Despite the many of dc diameter Shell 7.0m 4.57madvantages 3.8m 3.8m operation, it is only recently that thesecapacity Rated 32t 32t 3ot 13Ot furnaces have gained a foothold. The Tapping weights 30t20-1 130-35t 30-35t 30-35t development of low-cost semiconductor Transformer primary was the needed breakthrough. 22kV rectifiers 22kV 13.8kV 13.8kV voltage Max melting power 11.5 MW MW11 15MW 2 x30MW electrode current Max 37 kA 40kA 100-12okA 42kA DC ARC FURNACEDESIGN 16"diameter Electrode 18" 18" 28 I' FEATURES Electrode regulating hydraulic hydraulic hydraulic hydraulicsystem Essentially, the equipment neededfor dc Roof wtr-cooled wtr-cooled wtrcooled wtr-cooled melting has the same configuration as Sidewall wtr-cooled wtr-cooled wtr-cooled wtrcooled that of a conventional ac furnace shown in Bottom electrode air-cooled air-cooled air-cooled air-cooled Figure 1. Theexceptions are theaddition ConversionlNew of the bottom electrode (anode), a dc Conversion installation Conversion Conversion New reactor, and a thyristor rectifier (Figure 2.) all ofwhich add to the cost of a dc furnace. High Voltage n Figure 7. Conventional ac electric arc furnace. Figure 2. Layout of dc arc furnace. ’ speclfcally forsafety. Advantagesclantled for the furnace are: 50% reductlon In electrode consumptlon rn 5 to 10% reduction in power consumption rn Greatly reduced refractory ccnsumption Uniform melting Reduction of flicker leveland flicker frequency byhalf Production of 23 heatdday and 75,000 MT/mo-rated capacity-were achieved TOPY-Toyohashi Works (Japan) early in 1990. Visitorsto the operation conThis furnace is veryclose in size and firm that veryimpressive results are being power to the FloridaSteel furnace. Following attained wlththis furnace. two yearsof operation, TOPY published results showing cost reductions of 53% in electrode consumption, 5% in power, and 30% in refractorywear. Together these NEW DC FURNACESIN THE UNITED savings add up to $6.00/ton. Electrode STATES consumption has been about 3.2 IbsAon, As of early 1991,there are two newdc and bottom life about 330 heats per arc furnaces being installed in the United campaign. Further improvements are States. Each features single-electrode a anticipated, with particular attention to increasing the lifeof the bottom electrode. furnace. FloridaSteel,at their Tampa plant, is replacing two existing30-ton furnaces The expectation is that payback on this conversion to dc will be less than 3 years. with one 17 ft. diameter 60-ton furnace powered by a37 MVA transformer. Output of the new furnace will be one 35-ton heat Tokyo Steel-Kyushu Works (Japan) This furnace,with a capacity of 130 tons about every 45 minutes.Electrode consumptron of 3.5 Ibs./billetton is expected, and 60MVA, is the largest operating single-electrodedc steelmaking furnace as which compares to 10 Ibs. on similar ac furnaces. No change is expected in elecof 1990. It uses the maximum diameter of trical consumption. commercially available electrodes, 28”, Charter Steel,at thew Saukville, permitting a current of about 100 kA. This Wisconsln plant, is installrng a60-ton, 17 ft. furnace, which began operation in Sepdiameter shellfurnace powered with a tember 1989, has a rated capacity of 1 miillon tonslyear. Anticipated savingsof $15 perton 42.3 MVAtransformer.Two355 k A , 12-pulse rectifiers willprovide the dc current. over ac furnace operation translates to a payback on the added cost of dc in lessthan Electrode diameter willbe 24”. Initial output of the furnace will be 250,000NT/year, two years. Features of the furnaceinclude uniform melting with the central electrode, and start-up is planned for June 1991. simple arrangement with the single mount and single secondary conductor, and an air-cooled bottom electrode designed fumes. Basedon three year’s operatlon, the declslon was made to replace the current furnace and a sister ac furnace of the same slze withone 60-ton, 37 MVA dc arc furnace.One feature of the new furnace will be the use of a 12-pulserectifier compared to the 6-pulse unit on the existing dc furnace. This will eliminate the lower level harmonics (7,9, 11) which will be beneflcial in reducing line disturbance. I Bottom Electrode U 0 Conductwe Relractories Conductor THREE-ELECTRODE DCARC FURNACES While mostdc arc furnace lnstallatlons employ a slngle, central electrode, 82-ton an capaclty, 83 MVA dlrect current threeelectrode arc furnace has been operating at SME in Trith St. Leger, France since December 1985. Design of this furnace is based on development work carried out by the French SteelIndustry Research Institute(IRSID).Eachofthethreetopelectrodes is matched to a bottom electrode In the hearth. Addltionally, each electrode has its own transformer-rectifier-reactor assembly. The top electrodes 22” are diameter, and each can draw amaximum of 40 kA. Lower current intensity in the electrodes and the fact that the arc IS directed toward the center of the furnace (instead of at the walls in an ac furnace) has resulted in a reduction of electrode consumption and wall wear that approximates that experlenced with singleelectrode dc furnaces. Experienceduringthe first half of 1990 puts electrode consumption at 2.4 Ibs./ton. Bottom life hasreached about 1500 heats with an expectation of reaching 2000. ESTIMATEDCOMPARATIVE CAPITAL COSTS FOR A 100”N FURNACE TO PRODUCE 700,000 TONSNR: AC DC $million $million Furnace i d electncs Burners Reactiveenergy compensation Total Cost 6.3 0.5 9.5 not needed 2.s (65 MVAR) 1.0(30MVAR) 9.6 10.5 Operabng Cost (less scrap) W5iton Payback on the additional $29Ron less than six months investment THREE-ELECTRODE SINGLE-ELECTRODE AC FURNACE FURNACE DC U 0 Multi-Pin Metallic Conductor Single Piece Metallic 1 Figure 3. Bottom electrode designs for dc furnaces. I Uneven Heat Dislrlbutinm Pattern in &Phase AC Furnace. Unlform Heat Distribution From Central. VerlicsI Arc in DC Furrucs. ~~ Figure 4. Temperature Distribution-dc furnace operation, particularly witha single electrode produces improved heat distribution. The ac furnace has hot spots on the furnace walls adjacent to the three electrodes typically causing high sidewall refractory wear. The dc furnace produces a uniform heat from a central arc. The resulting temperature profile also promotes convection stirring as anadded benefit. Kawasakl Steel has installedfurnace a with one topand three bottom electrodes whlch started-upIn December 1990. Another furnace hasbeen ordered by Nakayami Steel. ’ , I REFERENCES 1. CMP Report 86-8, DC Arc Furnace Melting Performance Analysis 2. Design, Installationand Operation of the World’s First Direct Current Arc Furnace in Steel Mill Service-l&SM, May 1986 .. . . . FUTURE OF DC ARC FURNACES 3. Single-ElectrodeDC Furnace at Florida Steel-l&SE. January 1987 Whlle it is not clear thatdc arc furnaces are the “waveof the future”for steel melting, results obtained in the past five years with furnaces ranging in size from 30 to 130-tonscapacity have been very impressive. Interestin these furnaces is high on the part of many domestic steelto assume that makers, and it is reasonable many more dc furnaces will be installed. Today’s electrode technology limits diameter to a maximum of 28 inches, allowing a dc current of approximately 100 kA, and restricting power to 75 MVA for a single electrode furnace. This Inturn limits the size ofsuch afurnace to well under the 200-tons capacity of current large ac furnaces: Perhaps the three-electrodedc furnaceswill fill thisgap. Another possibility is to develop larger diameter electrodes. At present, the dc furnace offers a potential for substantial savingsto the steelmaking industry In ranges up to 75 MVA.Utilitiescan look forwardto fewer electrical disturbances in the powergrid. 4. The DC ArcFurnace of Societe Metallurgiguede I‘Escant: FirstMonth of Operation-ME, June 1987 5. DC Arc Furnaces-U.I.E. 11, March 7, 1988 6. Technical and Economic Aspects Regardingthe Practical Applications of a DC Electric Arc Furnace-H. Essman and Dieter Grumberg of Brown Boveri 7. Mini-millsTestthe DC Route-MBM Supplement, October, 1989 8. Single Electrode DC Arc Furnace Operation at Florida Steel CorporationD. Meredity, Florida Steel Corp.and s. E. Stenkbvest, ASEA Metallurgy 9. Start-up of the World‘s Largest DC Arc Furnace at SME-AIM€ Advanced Technology Symposium, October 5,1989 10. Technological AdvantagesOffered by Direct Current Arc Furnaces-International Steel Technology,December 1988 11. Economics and Technology of Electric Arc Furnace, CMP TechCommentary, CM P-059 i 12. Electric Arc Furnace Steelmaking, CMP TechCommentary, CMP Vol. 1, No. 3 13. Understanding Electric Arc Furnace Operationsfor Steel Production, CMP TechCommentary, CMP Vol. 3, No. 2 The Center forMaterials Production The ElectricPower Research Institute (CMP) is an R&Dapplicationscenter (EPRI) conducts technical a research funded by the Electric Power Research US. and development program for the Institute and operated by Carnegie electric utility industry. EPRl promotes Mellon ResearchInstitute of Carnegie the developmentof new and improved is to technologies to help the utility industry Mellon University. CMP’s goal meet present and future electric energy develop and transfertechnical information that addresses the concerns of needs in environmentallyand ecoU.S. materials producing companies nomically acceptableways. EPRl conducts research on all aspects of use, regarding productivity, quality, energy efficiency, and the environment. including fuels, generation, delivery, energy management and conservation, EPRI environmental effects, and energy Robert Jeff ress analysis. Manager, Industrial Program CMP Joseph E. Goodwill Director This Techcommentarywas prepared by Dick Hurd and John Kollar, CMP. Technical review wasprovided by Robert J. Schmitt, ManagerofTechnicalProjects, and . Joseph E. Goodwill, Director,CMP. Applicable SIC Codes: 3312,3313,3315,3316,3317 @Copyright 1991 Electric Power Research Institute, Inc. All rights reserved Materials Production Printed 1/91 Carnegie Mellon Research Institute 4400 Fifth Avenue Pittsburgh, PA 15213-2683 412-268-3243 LEGAL NOTICE This Techcommentary was prepared and sponsored bythe Center for Materials Production (CMP). Neither members of CMP or any person acting on their behalf: (a) makes anywarranty expressed orimplied with respect to the use of anyinformation, apparatus, method, or processdisclosed in this Techcommentary or thatsuch use may not infringeprivatelyownedrights; or (b) assumes anyliabilities with respect to the use of, or for damages resultingfrom theuseof, any information, apparatus, method, or process disclosed in this TechCommentary.