Blast Furnace Design of the Future

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I SPECIAL FEATUREBlastFurnacedesignofthefutureJ

ABSTRACTHotmetalcostsaredeterminedbyrawmaterialandenergyelectricity)prices,laborcostsanddepreciation.Low.costhotmetalcanonlybeproducedat lowcokeandhighfuel injectionrates.A highproductivityalsocontributesto low.cost hotmetal.Thiscouldbe achievedwithhighoxygenenrichment.Theseoperatingconditionsresult in higherloadingsto the blast furnacelining. Theseloadings include thermal,mechanicalandchemicalcomponents.Modernblastfurnacesshouldbeabletocopewiththeseincreasingdemands.Thetargetcampaignlife of newdesignsto-dateis >20yearsandtheavailability>95%.Furthermore,t's of paramountimportancethattheinternalprofileoftheblastfurnaceismaintainedduringtheentirecampaignasanydegradationwill immediatelyhaveanegativeinfluenceon low-costhot metalproductionrequirements.This paperaddressesthe performanceanddesignoftheblastfurnacebottomandhearth,tuyerearea,bosh,belly,stackandthroatarmor.Examples of various designs andperformanceswill bediscussedandincludean analysis of bottomand hearthcarbongrades,ceramiccupandcoolingsystemaswellasbosh,bellyandstacklining&coolingsystems.

KeyWordsBlastFurnace,Hearth,Bosh,Stack,CoolingSystem

38 STEEL&METALLURGYANUARY2012

Introduction

Low-cost hot metal can be achieved by:. Lowcokeate 300 kg/THM);. High fuel injection rate (PCI > 200

kg/THM) ; /". Highoxygenrate (>30%);. High productivity (> 3.0 THM/m3WV/d);. Stable operations

availability (>95%).and high

The blast furnace must also be able tocope with various raw materialcompositions of sinter, pellets and lumpare.Determining the sizing and profilingparameters of a new blast furnace andmaintaining these parameters during thecampaign are critical to enable low-costhot metal operations.These parameters include for examplethroat, belly, and hearth diameter, boshand stack angle, working height, sumpdepth, number of tuyeres, etc. Referenceis made to Figure 1.The blast furnace design has beenfacing many improvements in the lastdecades and campaigns> 20 years arerecorded to-date. The bosh, stack andthroat armor were critical areas in the

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High-efficiency and capacity coolingsystem(s) and advanced instrumentationand control are additional requirementsfor low-cost hot metal production.Process and OperationsAn advanced blast furnace designincludes instrumentation and controlsystems for monitoring the process,operations and lining performance.Figure 2 is a schematic illustration of atypical instrumentation layout includinggas, pressure and temperaturemeasurements, etc. Theseinstrumentation and control systemshave been useful in understanding theblast furnace process, operations andlining performance. A post-mortemanalysis should be performed during(interim) relines and repairs. The liningcan also be monitored duringmaintenance shut-downs and include -for example - (hearth) core sampling orlining ultra-sonic nondestructivethickness measurements.The blast furnace is a high-temperature,pressurized counter-current reactor.Abrasive raw materials are descendingand gradually softening due to(s)meltingwhilst high temperature gasesare ascending through the burden butalso along the lining. The tuyere levelflame temperature is > 2000C and theblast furnace operational pressure is 2- 4 Bar(g). This can result in highthermal loadings to the lining.The blast furnace is continuous processbut with batch elements, such ascharging and tapping.Furthermore, the blast furnace processis sensitive to de-stabilization due tovariations in raw materials quality,equipment failures, (unscheduled)shutdowns, burden slips, castingdeficiencies and 'gasjets'.This results in dynamic processconditions. Average process conditionsare well-understood and reported inindustry. Designers, however, must also

SPECIAL FEATURE

fl Top gas: CO, C~, Ha.temperature (total)

.~ ~ ~ - CO,o"i"""""(""'"~ ~', Burden levef,~ ~.' Heat Flux Monitoring:6x8 panels in total,.~ ~ , In-burden probes: CO,CO:I,~, temperature!~ ~ . Shaftpressure:2 rows,6 positions~~ ~ ~ ~!e~ . Blast:volum&,J)l'essure.Oz. temperature- - ~ '~. Hearth:refractory,hell.~ing watertemperat""" Slag:compositionndt/mlny-:= lroo; tnperature: composition and t/mini'~ ~

Figure2: Blast furnace instrumentation

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have a good understanding of thedynamic process conditions'fluctuations since these imposesignificantly higher thermal andmechanical loadings to the lining.

development has often been reportereflect loss of accretion and rasolidification of a new accretion duhigh-efficiency cooling system.believe, however, that it is more likthat this reflects the consequences ohightemperature .gas-jet' impinginthe lining.Local, incidental upset processconditions can result in loadings thatare lOx higher compared to averageconditions. An example is illustrated inFigure 3: this dynamic temperatureProcess conditions have been monitoat many plants and bosh and sta

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I SPECIAL FEATURE

temperature fluctuations have oftenbeen observed exceeding> 100C /minute.Table 1 lists actual 'fatigue limits'of various lining material grades andactual temperature measurements. It isclear that highconductive, ductile liningmaterial such as copper and graphiteare required to cope with thesetemperature fluctuations.The hearth is also facing demandingprocess and operating conditions andthe hearth lining performance is criticaltoday. We have been monitoring andassessing the performance of varioushearth designs and operationsworldwide including large block and hotpressed brick designs and includingbottom air, water and oil cooling as wellas hearth shell spray, jacket and castiron stave cooling.The operating range included all-cokeas well as lowcoke ultra-high PCIoperations, dry-hearth and singletaphole operations and low-grade tohigh-grade raw materials (pellets,sinter, lump ore and combinationsthereof) .Despite all scientific experiments andmodels, actual performance of the hearthis difficult to forecast due to the largevariety of influencing factors. Manyplants run the hearth in campaignextension mode requiring modificationsto the process and operations andsometimes interim repairs.Bottomand HearthThe bottom and hearth performance is

40 STEELMETALLURGYJANUARY 2012

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Figure 5:hearth shell jacket cooling designs

the limiting factor at many existingplants for low-cost hot metal in the 21stcentury. The condition of the bottomand hearth usually determines the timingof relines.

increase, whilst the hearth volume ialso reduced at low coke rate due toreduction of the permeability of thehearth.

This is amongst others due to the factthat the blast furnace working volumeand production were increased duringprevious relines, whilst increases of thehearth were limited due to casthouseand foundation constraints. The hearth'productivity' will then effectively

Advanced scientific CFD modeling andtwo-phase flow experiments aredeveloped to simulate the hearthoperations and process and demonstratesignificant hot metal and slag flow andvelocity differences depending on thegeometry, tapping cycle and hearthvoidage.

Table 1: Temperature fluctuations [OCImin]

MaterialExperimentsGraphite,Copper 500SiliconCarbide,CastIronAIP3' ChromeCorundum 5ProcessMeasurementsSinterBurden> 90%Mixed Burden 50%/50% 150PelletBurden> 70% 180


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I SPECIAL FEATUREIt is important to design a large hearthvolume to support stable operations.Also stable casthouse practice isrequired to minimize liquid levelfluctuations. The hearth volume can beincreased by adopting conical shellsegments instead of a cylindrical shellalthough the hearth diameter at thetuyere level will have to be in proportionwith the intended production level.The shell and bottom seal-plate must beprotected and water cooling hasdemonstrated to be the most effectivemethod if considering:. Coolingefficiencyand capacity. Designredundancyandmaintenance. CapitalinvestmentandoperationalcostsAdvanced bottom and hearth designsinclude bottom water cooling and hearthshell jacket cooling as this providesmore space within the shell.We have evaluated many hearth shelljacket cooling designs comprising anglebars, circular pipes, Uprofiles orcassettes (Figure 5).The most optimum design includesshaped profiles, which results inminimum dry areas and enablesinstallation of grouting andthermocouple nozzles.High-efficiency taphole jacket coolingsystems have also been developed tocope with high heat load dynamicprocess conditions and to minimize shelldeformations.Alternatively, copper stavecoolers could be in this area.The bottom water cooling systemincludes two circuits with a series ofparallel pipes. These pipes areembedded in high conductive SiCcastable within grooves of graphiteshapes. Conventional designs includecarbon ramming, but this introduces therisk of evaporation of volatiles, whichjeopardizes the thermal conductivity of


Figure 6: Taphole copper stave coolers

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Figure 7: General arrangement bottom and hearth

carbon ramming and the performance ofthe bottom cooling system.This design includes bottom watercooling pipes above the seal-plate anda graphite, semi-graphite and microporecarbon bottom course protected by ahigh alumina and chamotte 'arrestor'course. The hearth lower sidewallincludes graphite against the shell andhigh conductive micropore semi-graphit(ultra-micropore carbon) and the uppersidewall comprises (micropore)carbon asthis is exposed to low heat load levelsThis design also allows for a ceramiccup, which provides better resistanceagainst hot metal erosion, but alsoreduces the effective hearth volume and

A closed-loop cooling system will berequired to minimize contamination..Chillers' could be considered to reduceand stabilize the average cooling watertemperature. Alternatively, a more robustand economic water temperature controlsystem can be implemented to eliminatethe effects of seasonal and dailytemperature fluctuations.An example of an advanced bottom andhearth design is illustrated in Figure 7.


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could cause higher stresses and strainsin the hearth sidewall.Large blocks are used for the bottomand the' thin' hearth sidewall liningcomprises small blocks as this enablesto minimize the ramming thickness to5Omm.The authors recommend 'thin' hearthsidewall lining blocks < 1000 mm toreduce thermal stresses and strains.These cause (micro-)cracks and jointopenings, which catalyze lining erosionand reduce the campaign lifetime.The hot face temperature will also bereduced if the sidewall lining is 'thin'and this will reduce chemicaldegradation and chance of dissolutionof carbon refractory in hot metal.The performance of the hearth alsodepends on the (carbonaceou~refractory grades. We have executed anextensive benchmark test programmeand evaluated 14 carbonaceous gradesfrom USA, European, and Asianmanufacturers. The test programmeincludes' conventional' ISOmeasurements and also customizedexperiments to compare the performanceof the refractory upon exposure to morerealistic process conditions. The testprogramme is summarized in Table 2.Typical results are illustrated in Figure8 and Table 3. It is clear that theperformance depends on the materialselection.The advanced bottom and hearthdesigns are based on the results of thistest program and our 'thermal' designphilosophy reducing hot facetemperatures and reducing temperaturegradients.The design also accounts for minimumstresses and strains: high stresses cause(micro-) cracks, brittle layer, shelldeformation and sometimes shell cracks.Dynamic thermo-mechanical stress-strainanalysis computer models have beendeveloped to simulate the bottom and

SPECIAL FEATURE

Table2:Carbondepositionandsteamoxidation2.5

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Figure8:CorrosionPerformance

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hearth and the results are compared tactual field measurements at severadifferent designs using strain gaugeacoustic emission sensorscircumferential length measurementsanradial bars. The models anmeasurements are in good agreemenand confirm that shell stresses anstrains are highest at the interface othe bottom and hearth as indicated bthe FEA model illustration of Figure 9

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Figure9:Highshellstressandstrain

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Table2: Carbon refractoryestprogrammeGrades PropertyTestsCarbon PoresizedistributionPSD)Microporearbon CO-DisintegrationCO)Supermicroporearbon Ash,IronUltra-microporearbon ChemicalAnalysis(CA)Semi-graphite CarbonReactivityCR)Microporeemi-graphite BulkDensity/ApparentorosityGraphite X-RayDiffractionXRD)DoubleDensifiedGraphite ColdCrushingStrengthCCS)HotPressedBricks ThermalConductivityX,Y,Z](TC)PerformanceTests YounqsModulusE)Oxidationir RefractorinessnderLoad(RUL)OxidationSteam ThermalCoefficientfExpansionTCE)Alkali(ASTM) ThermalExpansionUnderLoad(TEUL)HotMetalCorrosion HotModulusofRuptureHMOR)SlagCorrosionFeO) PermanentinearChangePLC)


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I SPECIALFEATUREOur analysis confirms this is mainly theresult of radial expansion of bottomMullite courses and insufficientexpansion. The strong Mullite coursesresult in high radial forces contributingto creation of (micro- )cracks inrefractory and deformation of shell andalso catalyze preferential flow as Mullitehas a strong erosion resistance. Theadvanced design incorporates chamotte,which can erode until processequilibrium is achieved. This results ina reduction of preferential ring flow.Radial forces are reduced since theYoungs modulus of chamotte is lowerthan Mullite and the advanced designincludes expansion provisions. Theseeffects are illustrated in Figure 10.The tap hole is one of the mostchallenging blast furnace components asit is exposed to an extremely violentenvironment: operations and processconditions are discontinuous at hightemperature and pressure (difference)and contains chemical, corrosive anderosive mechanisms.Low-cost hot metal operations willincrease the loading conditions as theproductivity will increase.This means that the taphole performancedesign must always be monitored andevaluated and 'lessons learned' must beidentified and implemented in thedesign.An advanced taphole designincorporates double densified graphiteagainst the shell, externaljacket cooling,large block micropore semi-graphite(ultramicropore carbon). A centercastable core provides maximumredundancy and minimizes operationalflaws such as gas-leakage and providesbetter resistance against oxygen lancingor oxidation due to leaking coolingelements. Alternatively, copper stavecoolers could be considered at thetaphole.TuyereAreaThe tuyere (cooler) area is an important


Mullitebottom courses

Figure 10: Erosion profiles

Figure 11: : Tuyere area performance

Figure 12: Lower bosh failures

Figure 13: Tuyere area design

Chamotte andhigh alumina

bottom courses

interface between the hearth and boshand stack. This area is exposed to hightemperatures, temperature fluctationsand erosion. Low-cost hot metalincludes high oxygen enrichment andPCI which could increase loadings tothe tuyere area.Many installations include tuyere areaceramics to prevent oxidation andprovide erosion resistance. Ceramicrefractorymaterials - such as SiC orcorundum - do, however, requireexpansion provision to prevent stresscrackingand ceramic refractorymaterialscan not cope with large temperaturegradients or temperature fluctations.Two examples of tuyere area andlower bosh failures are illustrated inFigure 12.An alternative design includes doubledensified, low iron and low ash contentgraphite in close contact to the tuyerecoolers. Additional copper plate coolerscan be considered if the spacingbetween the tuyere coolers is large.This design is illustrated in Figure 13.This design has proven campaign life>25 years at high productivity and ultra-high PCI and oxygen enrichment rates.Figure 11 illustrate the excellentperformance of this design.Long campaigns are achieved at highproductivity, high PCI and high oxygenenrichment rates since double densified,


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low iron and low ash content graphitehas a very high oxidation thresholdtemperature compared to othercarbonaceous and graphite grades.Furthermore, combining high conductivegraphite and high efficiency tuyerecoolers and copper plate coolers willresult in low hot face temperatures thuspreventing risk of oxidation.The low hot face temperature will alsopromote protective accretion eliminatingerosion.Bosh and StackTwo different lining designs have beendeveloped since the 1960's:. Cast iron and copper stave cooling. Copper plate cooling and refractoryThe performance of the designs dependon the application and local processconditions and operations. Cast ironstave cQolers cannot cope with hightemperatures and high temperaturefluctuations as this will cause materialdegradation and cracking despite manyimprovements of coating, cast iron gradeand refractory inserts. The applicationof cast iron stave coolers in the upperstack and throat armor, however haveproven to be very successful as thisarea is usually not exposed to hightemperatures or temperature fluctuations.These areas are mainly facing erosiondue to descending burden and impactdue to (incorrect) charging practice. Castiron stave coolers and SiC refractoryinserts have a good resistance againsterosion and impact and are thuspreferred in the upper stack and throatarmor. Copper stave coolers have beendeveloped since the 1980's and arecurrently installed in many plants inhigh-thermal load zones such as thebosh, belly and lower stack. Recentlyvarious plants have reported failure ofcopper stave coolers in these areas.We have performed literature surveysand experiments developed FEA andCFD computer models to evaluate and

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;. ",,'Figure 14: Advanced bosh, belly and stack design

SPECIAL FEATURE I

Cast !tonSt8YeCoolersIUpperStadt ICast IronSt8YeCooIetUpperMidcIeStade Graphite. SICI.Lowet~t~...~S-LowerStack Graphite. SICBettyBoshTuyere Band

IGtaphIte. SICGraphiteGraphIte

compare the performance of copperstave coolers and other designs andmaterials such as cast iron, SiC, highalumina and graphite. This included:. Friction abrasion experiments. Impact erosion experiments. Impact erosion CFD modeling. Burdendescendmodeling. Diffusion of Fe in Cu. Transient FEA thermo-mechanicalmodeling. Corrosion by Sulphur and halogensThe resistance of copper and cast ironagainst mechanical,thermal and chemical

attack mechanism reduces witincreasing temperature. It is expectethat the root cause of copper stavcooler failures is erosion by descendinburden and ascending gas. This coulbe enhanced by changes to thcohesive zone location.This implies that the copper stavcoolers are not adequately protected ban accretion. This contradicts thclassical theory of Konig, which indeedoes not account for dynamic proceand operations conditions. Burdedescend models also suggest thacopper stave coolers have a loweability to stabilize an accretion compareto copper plate cooler designs.Advanced bosh, belly and stack desigincludemachinedcopperplatecoolers,higconductivegraphiteand SiC (Figure14).

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Table 4: Refractory crushing temperaturesCompressibility Expansion Crushing

[%] at 1000C [%] Temperature [DC]Silicon Carbide 0.05 0.3 160HighAlumina 0.10 0.4 200Carbon 0.40 0.5 00)Graphite lOO 0.3 3300


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I SPECIALFEATUREThis design provides optimum synergiesbetween cooling and refractory and alsobetween high conductive graphite anderosion resistant SiC refractory.Alternative designs which only includeSiC and do not include graphiterefractory lining are vulnerable to stresscracking and fatigue due to thermalcycling (Figure 12). Crushingtemperatures of different refractorygrades are provided in Table 4.It is clear that graphite is required tocope with the high temperaturefluctuations that could occur in thebosh, belly and stack.SiC is required to provide protectionagainst erosion of descending burdenand ascending gas. Graphitesurrounding refractory in close contactwith machined copper plate coolerssecures low SiC temperatures.This results in low stresses and strain.In addition, special shaped SiC bricksand expansion provisions are used tofurther reduce stresses at the hot faceand eliminate the risk of spalling. Thisdesign is illustrated in Figure 15.The application of copper plate coolers,graphite and SiC has been demonstratedat > 50 plants worldwide and manyrecords have been achieved:. Campaignife: >25Years. Productivity: > 3.5THM/m3WV/24. Oxygennrichment:18%. Coalnjection: 260kg/THM. CokeRate: 250kglTHMTypical thickness measurements of TataSteel I]muiden blast furnace 6 areillustrated in Figure 16.Cooling SystemAdvanced blast furnace designs requireclosed-loop cooling systems at highpressure, which increases the coolingcapacity and efficiency. The coolingsystem design should be able to cope

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Figure15:Copperplatecoolers,refractoryandSiC

with normal process and operatconditions as well as incidental, lpeak heat load conditions. An examof a typical Mat load diagramillustrated In Figure 17. This reflnormal process and operatioconditions.This heat load diagram is indepenof the blast furnace design: the vaare determined by historical recordsmeasurements and computer modeand depend on parameters such

Figure16:: Infinitecampaignife records

HLO.HeatLoadDiagram(normaleatload)

UpperSfack1broatArmoor112

UpperMiddle StackLowerMiddle Stack

LowerSfack 41BellyBosh

1\JyeraandHearthBottom

10 20 30 40HeatLoadkWlm'J

Figure 17:Heat load diagram

productivity, pellet % and size ofblast furnace.Incidental, local peak heat load lecould be 10 x higher. Also, the averheat loss could be higher if rmaterials or burdening practiceschanged.The primary 'on-furnace' cooling sysdesign should enable adequate ldetection and leak searchifunctionality.

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The cooling system could also be ufor 'heat flux monitoring' proccontrol. This has demonstrated to beeffective tool to control and stabioperations, which is a pre-requisite

MIddleStackRow 2270

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1I low-cost hot metal.

The bosh, belly and stack copper platecoolers are usually coupled in series of4 - 5 platecoolersandan arrayof theseseries are combined in a 'panel'.The bosh, belly and stack are dividedin 20 - 48 panels depending on the sizeof the blast furnace.The upper stack cast iron stave coolersand hearth shell jacket cooling includea re-circulating cooling system withwater temperature control (Figure 19).This eliminates temperature fluctuationsof the hearth shell jacket cooling andminimizes the chance of condensationof the burden moisture in the upperstack. This will prolong the campaignlife and contribute to stable process andoperations.Instrumentation and Control

21st Century low-cost stable operationscan only. be accomplished by improvedinstrumentation and measurementcontrol. This allows the process to bemonitored and studied online. Theincreased predictability and repeatabilityof the process allows the process to befurther optimized and stabilized, withproductivity rates reaching new highsand campaign lifetimes growing longerand longer. This improvement is anecessity to remain competitive in themodern iron and steel industry. Modernblast furnace instrumentation includes:. Hearth Monitoring (Thermocouples). HeatFluxMonitoring. Above BurdenProbes (ABP). Short In burdenprobes (IBP). TopGas analysisfor ABPand IBP. Refractory (Ceramic)WearRods. Bosh and Stack

(Thermocouples)Monitoring

. ProfileMeter

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Figure 19: Cooling system

Upper Stack

Middle Stack

Lower Stack

Belly

Bosh

Health

STEEL&METALLURGY JANUARY 2012 49


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I SPECIALFEATURE. Topscan camera. Radar Stockline Recorder. MechanicalStocklineRecorder. Stack Pressure 'Hoogovens' design. Tuyere Cooling water Flow lowpressure. TuyereCoolingwater Flow highpressure. Top Pressure. Taphole Thermocouples. Tuyere CoolingWater Temperature. SkinFlow. Under Tuyere temperatureThe data are processed in a Level 2control system. It is advised toimplement a modular approach withdedicated modules individually designedto be of optimum benefit to theoperations of that specific area. Eachmodule should be designed specificallyfor the operator in mind, using theexperience of current and previousoperators with a combination ofcenturies of experience to capture thevery best of the relevant informationand present it in the most useful format.Although the modules are area-specific,there is of course an overlap betweenareas, and where this occurs theinformation is shared as much aspossible to give the relevant and timelyinformation to the operator. A modernconfiguration is illustrated in Figure 21.The Operator Advisory and ControlSystem (OACS) for the Blast Furnace iscontinuously improved. The OACS is anintegrated part of a modern Level IIsystem and is capable of identifyingprocess deviations at a very early stage.It is also designed to compare historicaland calculated data in order to avoidmajor and costly incidents. The system


Figure 20: State ofthe art blast furnace control room

Figure 21: Typical Level 2 system configuration

advises and gives guidance to operatorsand process operators to help in takingthe correct actions to keep the furnaceoperating in the most optimum andeconomicway.Conclusions. Low-cost hot metal requires high

productivity stable process andoperations, low coke and high oxygenenrichment and fuel injection rates;

. Low-cost hot metal requires a stableblast furnace profile and 20 yearstable lining performance;. Modern blast furnace designsinclude state-of-the-art bottom andhearth refractory grades including

high conductive micropsemigraphite (ultra micropcarbon), external hearth shell jcooling, bottom water cooling;

. Bosh, belly and stack desincluding machined coppercoolers, graphite and SiC refrahave demonstrated to enablecost hot metal;

. Upper stack and throat armoriron stave coolers enablecampaigns;. Advanced instrumentationcontrol secure appropriate prand operations controlmonitoring of the liperformance..

Blast Furnace Design of the Future

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