AISTech 2005 Proceedings Volume I - PDF document

497 AISTech 2005 Proceedings - Volume I Effect of Temperature on Coke Properties and COTobias Hilding, Veena Sahajwalla, Richard Sakurovs, NSC/CRI test has been recently adopted by ASTM, and is also being considered for ISO standard. Accordingly, a high CSR coke is believed to prevent the coke from breaking down, improve the permeability of gas and liquid and increase the productivias well as decrease the specific coke consumption. Many empirical correlations based on ash chemistry and the CSR tests have been developed in past. Each steelworks relies on their own empirical experience for the interpretation of the CSR test resultsDespite widespread popularity, there are well known concerns about the effectiveness of CSR measurements in predicting coke behaviour. Alternatively, coke quality is also tested in an experimental blast furnace facility in order to obtain a comprehensive understanding of coke behaviour. Even though these tests are time consuming, tedious and very expensive, the information generated is of great value in terms of their suitability due to simulation of realistic conditions of blast furnace process. Coke reactivity can be influenced by many physical and chemical properties of coke e.g. porosity, carbon structure and constituent minerals. For example, coke pore structure is modified by growth and/or coalescence behaviour of pores . Another modification of carbon structure involves coke graphitisation which is believed toIron is believed to graphitize as well as catalyze carbon gasification reactions, while the effects of other minerals such as those containing alkalis on the coke degradation behaviour is less certain. Alkalis are known to influence the solution loss reaction such that carbon reaction rates are significantly enhanced by potassium8 -10. Potassium content of coke varies at different locations of an operating blast furnace, e.g. alkalis are completely vaporised at very high temperatures of the raceway, while at the centre of the blast furnace hearth coke often contains high concentration of condensed alkalis such that alkali concentration in deadman cokecould reach up to 30 % of total coke ash 8, 11, 12. Potassium adsorption in coke could cause irregular swelling, increased stress, modification of surface area, microstrength, and possibly size degradation. However, alkalis are not believed to affect the crystallite height of the carbons signficantly. Generally, potassium is believed to weaken the coke strength at high temperaturesConcentration gradients of alkali in coke lump has been reported to cause stress abrasion of alkali-rich layer of coke. On the other hand, a recent study, coke containing up to 5% of alkalis did not display any adverse impact on the coke strength (CSR)Despite several indicators relating alkali content and coke strength, there are uncertainties regarding their extent of impact particularly on the mechanisms of coke weakening by alkalis. However, coke gasification in a blast furnace is believed to occur preferentially on the coke’s surface, and thus raises concerns about the interpretation of the conventional CSR test res

ults fofurnace operationIn the EBF, coke strength will be influenced by carbon structure which in turn could be modified by reactions inside the EBF including those with recirculating alkalis and other gases. The main aim of the current study is to investigate the effect of alkali reactions on coke behaviour in EBF, and to distinguish the thermal effects on carbon structure of coke from those of alkalis reactions. The Coke strength as well as the abrasion behaviour of cokes in the EBF were also related to carbon structure of coke as measured by using X-ray diffraction. EXPERIMENTAL Experimental Blast Furnace In the current study, coke samples were obtained from a dissection study based on 10 campaign of the Experimental Blast Furnace (EBF) at Luleå using high CSR coke. The CSR and CRI values of coke used were 68.8 and 23.15 respectively while fuel and coke rates were of the order of 500 kg/thm and 350 kg/thm respectively. The blast temperature of EBF was around 1200during this campaign. The EBF has a working volume of 8.2 m and a diameter of 1.2 meter at the tuyere and is 6 meter high from tuyere as detailed elsewhere. Approximately 20-35 pieces of coke samples after quenching the EBF with nitrogen were collected from several vertical and radial locations in the EBF. However, in this study, coke samples only from centre line locations were used. Coke samples from various locations were selected to represent various zones of the EBF as illustrated in Figure 1a. For example, sample code KL10NC indicates a coke sample of center line at 10 layer. Figure 1b illustrates the temperature of coke layers based on separate probe measurements. Based on thermal profile, cokes KL05 & KL10C, KL15C & KL20C were considered to represent the stock line and thermal reserve zone cokes respectively, while cokes KL25C & KL30C represented the cohesive zone (Figure 1b). Coke sample KL35C represents the bosh coke. Horizontal Tube Furnace The raw coke was heated in a custom made horizontal tube furnace as shown in Figure 2. Internal diameter of the furnace tube is50 mm. The furnace heat is supplied by Super-Kanthal heating elements connected with a low voltage-high current power supply. An external thermocouple indicates the temperature of interest and hence that of the sample inside the working tube. The coke sample KL01 was used to represent the feed coke. The lump coke was crushed to smaller particles in a size range from of 0.6 to mm and placed in a graphite holder. Sample holding rod was kept near the outer edge of the furnace while the furnace was beingheated at the rate of 3/minute. A slow heating rate is used to avoid thermal shock in the alumina tube. The furnace was purged with argon to remove any residual oxygen till the furnace approached the predetermined annealing temperature, followed by slowly moving the sample assembly to a pre-determined location in the furnace. A CCSD camera was used to monitor the physical state of the sample during annealing. Each sample was held for 30 minutes at each annealing temperature being 12001500C. After completing the annealing, the assembly was slowly pushed out and held near thto cool down for more than 30

minutes before taking the whole assembly out of the furnace. Figure1 a) Schematic of the experimental blast furnace and locationsof the coke excavation and their codes; b) The temperatureprofilesof theEBF estimated from several temperatureprobes,. Quartz WindowSampleThermocoupleGraphite TrayGas Outlet Gas Inlet AluminaTube GraphiteRod Figure2Schematic of horizontal tube furnace usedfor thermal annealing of EBF feed coke.TGAReactivity of EBF Cokes withCONon-isothermal reactivity was measured byusing ~70 mg of cokepowderin an Alcruciblewiththe help of NetzschSTA 409Thermal Gravimetric and Differential Thermal Analyser(TGA/DTA) located at Luleå University of Technology.All the EBF cokes werereacted underdynamic heatingup to1300C with a heatingrate of10C/minute and a CO flow rateof100 ml/minasdetailed elsewhere (26).XRD ofCokesAs carbon structure of coke has been often related to the reactivity as well as graphitizationdegree of cokeswas measured, andquantified in terms of the height carboncrystallites ofcoke (Lc). Siemens 5000X-ray diffractometer was usedto measure the carbon structural parameters including thedegreeofgraphitisation. Two small coke lumps (approximately 6-8cmwere selected from each layer and crushed to powder (micron)for the XRD as well as the TGA reactivitymeasurements.TheXRDpatternswereobtainedby recording the scattering intensities of coke powder byusing CopperK radiation (30kV,30 mA)as the X-ray source. Cokepowder was packed into an aluminium holder and scannedover an angularrange from 5-105° usingastep size of0.05° and collectingthe scattering intensity for 5 seconds at each step. TheLc valueswere evaluatedby analysing the002 carbon peak of theXRD pattern. Average stack height (Lc)of the 002 carbon peak was calculatedby usingScherrer’s = 0.89/ B Coswhereisthe wavelength of theX-ray radiation,is the fullwidth at half maximum intensity (FWHM) of the 002carbonpeakis the 002 carbonpeakposition.A sharper 002 peak indicatesa larger carbon crystallite and a greater degreeoforderingofthe carbon structure of coke or graphitisation. The carbon structure of cokes annealed in the laboratory furnace weremeasuredby using Philips X-ray diffractometer under similar conditions to thoseused forother EBF samplesexcept theXRD data was acquired at a smaller step size of 0.02 andused cobalt source. SEM /EDS AnalysisTwo sets of coke samples were examinedby using Scanning Electron Microscope (PhilipsXL30) equippedwith EnergyDispersive X-ray Analysis(EDS). Coke embedded in epoxy were mountedon aluminium support andcoated with a thin layer ofgold-palladium alloy using a Bal-tec MCS 010 sputter coaterbefore microscopic examination. Chemical composition of inorganicelements werealso analysedwith EDS at a number locations in the carbon matrix of coke.RESULTS and DISCUSSIONIn the EBF, several coke properties changed such as carbonstructure and constituentmineral phases including alkali phases andporosity as discussed below. Each coke property and the recation environment couldhave its owncontributionon the cokebehavior in anoperatingbalst furnace particularly its strength.EffectofEBF Reactions on Coke Carbon StructureCoke carbonreacts with upc

oming CO as coke descends into lower part of the EBF.Figure3a compares theXRD patterns ofcokes from three representative locationsof the EBF and shows that width of002 carbon peakof coke in cohesive zone (KL35C)is sharper compared towidthof carbon peak of stock line coke (KL05C)and upperzone coke (KL15C).The backgroundintensityof lower zonecoke samples (KL035C) areless than those collected from upper zones of the EBF (KL05C). Low backgroundintensity is indicative of less proportionof amorphous carbon. The amorphous carbonof coke was found todeclinesluggishlytowards the cohesive zone, and changed rapidly as the cokedescendedfurtherdownbelow the cohesive zone. Thisimplied thatcoke carbon became increasingly ordered as the coke moved towards bosh region.Figure3b shows that the crystallite height (Lc)of theEBF coke increases linearly withEBF temperaturesbasedon probemeasurements particularly after 1200C. As expected, at temperatures less than1200C,the Lc valuesofcokes did not changesignificantly compared to the Lc value of feed coke, and was not linearly related to the EBF temperature. It maybe noted thatnormally, Lc values are not expected to increase significantly or linearly up to 1200C due to similar range of temperaturesexperiencedby coke in thecokemakingoven. Many factors including presence of alkalis and ironspecies could influence thecarbon structure of cokes while alkalis are notknown to have significant effecton the Lc values of changewith increasingtemperature. However, a linear correlation in Figure 3b clearly suggests the strong influenceof the temperature on the Lc valueof the coke compared to alkalis. 10152025303540Diffraction angle (2 Theta) 1200140016001800EBF temperature ofcokelayer,Lcvalues of coke,Angstrom(a)(b)Figure 3 (a)Illustration ofvariationin backgroundintensity of XRDpatternsof cokesamples from three locations,(b)relationship betweenLcvalues of coke and the coke layer temperatures in the EBF.MineralChemistry of EBF CokeThe chemicalcomposition of inorganic constituents is detailed elsewhere, while variation of KO andNaO of coke ash insidethe EBF is plotted in Figure4.Alkalis content of increases as it moves into the cohesive zone of theEBF. For example, KKL35Ccokewas approximately 20 times of that ofKL05C coke while theNaO contentofwas increased approximately 10 times. Variation of the rangeof alkalisvariation is in agreement with previous studies. Alkali could influence the surface area,chemical structure of coke by interactingwith coke in two ways either assimilating in the pores or intercalate with coke carbonIn order tounderstand the modeof adsorbed alkali in coke, samples from two locationsopposite endsof the EBF were examinedby SEM/EDS. Figure 4Alkali variationof the EBF coke ash.Temperature profile of coke bed layers is also shown.Alkalis in coke are generally associatedwith aluminosilicatesphases. Figure5 illustrates theSEM imagesof the stock line coke(KL10C) inwhich the alkali distribution is assumed to be similarto that of feed coke due toverylow temperatures.Due tocomplexity of the inhomogenpus dsitribution andvariablecompoistionof aluminisilicates, it is very difficult to clearly distniguthe variation o

f the alkali distribution indifferent partsof coke.Potassium contentof aluminosilicate phases(shown by crossmarks)of the stock line cokeweresimilar to typical potassium concentration of the alumnosilicates phases. Compariosnof alkalicomposition of aluminosilicate in Figure5a,5b and 5c indicated that alkali concentration of aluminosilicate containing alkaliphaseswas ina typical composition rangeof aluminosilicate phases throughout the coke as detailed elsewhere. Theseresultsfurther suggest that alkalidistribution of coke was not significantly altered in the upper parts of the EBF. This couldbe attributedto less adsorbptionof recirculating alkali phsaes often experienced by duchcarbonaceousmaterils at low temperaturesFigure6 illustrates a similar alkali distribution for a lower zone coke sample (KL35C), and indicated a higherpercentageof alkain the aluminosilicates phases compared to KL10C coke sample. Physical structure ofboth coke samples appeared to be similar asno apparent crackor significant changes inmacro pore were noticed in the SEM images.Alkali content of cohesive zone cokewasgenerally higher than stock line coke, surprisingly therewas no significant variation in the alkali content of aluminosilicate of the outer, middle or inner coreof the same coke. The EDS analysis of few locations in the carbon matrix of cohesive zone cokindicated an increased alkali phases in lower zone coke.It appears that recirculating alkali vapour could condenseonthe externalayer of coke surface, and is trapped by aluminosilicates of coke duringpenetration to coreof the coke. Original alkali constituentsof the cokepresent in the aluminosilicate become active and diffuse uniformly into the bulk coke matrix. Thus the alkali reactioccur throughout the carbon matrix of coke, and thus increased alkalisare notpreferentially retained in the external layer of tcoke sample from any location.On theother hand, chemical analysis of abraded portionof the externals layers of cokes indicatedthat alkali concentration of the outermost layer was similar to the alkali content of the bulk coke, which further confirmed thalkali was notpreferentially deposited on external layers. Therefore, it is reasonable to infer that abrasion behaviour of the coke inthis study might not benecessarilyrelated to the preferential enrichment of alkalis on the outer surface. Figure 5a) SEM images of external layerof theupper zone EBF coke (KLC10) sample, b) midlle layer and c) inner core of thecoke matrix. Figure6 a) SEM images of external layerof the cohesive zone EBF coke(KL35) sample, b) similar imagesof middle layer of thesame coke; and c) the innercore of coke matrix.Reactivityof the EBFCokesFigure 7 compares the non-isothermal reactivity of the EBF coke and shows that weight loss of lower zone cokes are consistentlygreater compared toupper zones coke samples of the EBF. This implies that reactivityof coke increases as it descends into thelower parts ofthe EBF. The cokereactivity is often relatedto the carbon structure, surface area and coke minerals. In Figure 3was seen that carbon structure of lower zoneEBF cokes wasmore ordered. Therefore,the increased reactivity of lower zone cokes

amples can not be attributed to the carbon structure alone as increased crystalline orderof carbon is often believed to influethe reactivity adversely. Surface area of coke carbon did not increase consistently which implied that increased reactivity of thecoke samples can notbe totally attributed surface areaIt may be noted that at increased temperature a greaterproportion of alkalis are expected to bereleased from aluminosilicate slphases inside the coke.It is also interestingto note that reactivity of cokecould increasedue to catalytic influence ofalkalis evenwhen the carbon structure is more ordered.This implies that recirculating alkalis are playing a strong effecton the coke reactivityin theEBF such that they could enhance thereactivity withoutsignificantlymodifying the crystalline order of coke carbon. Figure7 Loss inweight of the EBF cokeswith increasingreaction temperatureof TGA/DTA.Effect of ThermalAnnealing on Carbon Structure In order to isolate the thermal effects on carbonstructure from the EBF reacting conditions including thepresence ofrecirculatialkalis, the feed coke sample was heated ina horizontalfurnace in the absenceofrecirculating alkalisor any other reactinggasessFigure8a compares the XRD spectra of theEBF feed cokes annealed at different temperatures. Narrowwidthof 002 carbonpeakof coke annealed at 1650C indicates a greater degree of crystalline orderof cokes compared to cokes annealed at lowertemperatures. Figure 8b clearly indicates a linear relationship between Lc values of annealed cokes and the annealingtemperatures. A similarlinear relation between Lc values of cokes and the EBF temperatures was also observed (Figure3b).Therefore, theLc valuesof EBF cokes and thefeed cokedisplayed a similar annealingbehaviourfor a similar temperature range.Figure9 shows that for a similar temperaturerange, the Lc values of cokes in bothfurnaces canbe linearly related.It maybenoted that this linear correlation might remain valid outside this temperaturerange as many other factors particularlycatalyticeffectof liquid iron at highertemperatures couldbe more significant in the EBF.A small difference inthe Lc valuesof cokes in two situationscanbe attributed to inhomogeneity of theEBF coke sample,which can be improvedby repeating the EBF cokemeasurements by analysing agreaterrangeof coke samples from EBF. Despite differences of the absolute values of Lc values of cokes treated under different conditions,growth of Lc values in bothcases can be considered tobe of similar orderof magnitude. These observations confirm ourpreviousopinion in relation tothermal effects of cokes in largeindustrial blast furnace, which the Lc valuesof tuyere cokes were also found tobe of the sameorderwhen annealed at similar temperatures. Furthermore, slightly higher Lc valuesof cokes in a horizontal furnace furtherendorses that recirculating alkalis could nothave contributed in graphitisationof coke in the EBF. 1020304050Diffraction Angle(2 Theta) 1200C 1500C 1660C 120015001650 1200140016001800Annealing temperatureof coke,Lc values ofcoke, Angstrom(a)(b)Figure 8: a) Effect of temperature on crystalline order of EBF feed coke heated in a horizontal furnace

in the presence of argon, b)correlation between Lc values of annealed cokes and temperatures. 1020304050Lc values of coke in the EBF, AngstromLc values of coke in laborator y furnace, AngstromFigure9 Correlation betweengraphitisation in laboratoryand experimental blast furnace at similar temperatures.Implications ofGraphitization and Alkalization onCokeBehaviorin the EBFFigures 10 illustrates that the coke strength(CSR) decreases as coke moves towards lower parts of the EBF. It maybe noted thatthe CSR values of the EBF reacted cokes arehigher than CSR valueof feed coke. This is because during the CSR measurement ofthe EBF samples, cokes were not reacted with CO prior to tumbling step of the standard CSR test. The CRI component of theCSR test was omittedmainly to evaluate the impact of EBF reactions on the coke strength. Figure9further shows that abrasionindex of lowerzonecokes are higher compared toupper zone cokes. Thissuggests that erosion tendency of coke alsoincreases inthe EBF athigher temperatures. Coke strength coulddependon many factors including porosity and carbon structurePorosity measurements basedon light optical microscopy suggested that percentageof both macro and micro pores marginallydecreased even in the lower zones coke rather than indicating any increase under theEBF test conditions. Consequently, it was inferred thatreduction in coke strength in the EBF couldnot be attributed toporosity changes.Figure11a shows that CSR values of the EBF canbe linearly related tothe coke crystallite height while abrasion index is onlyinfluenced until Lc values exceeds certain limit or the relationship becomesmore apparent at higher temperature. Thismeans, thecoke will not abradeuntil it exceeds certaindegreeofgraphitisation.Figure12a show that decreased CSR valuesof coke cannotbe consistently related to increasing alkali content particularlywhen thealkali content of coke exceeds more than19%.On theotherhand, the abrasion index is not influenced by potassium until it exceeds morethan 20%. The carbonstructure of cokeprovides a consistent correlationwith coke deterioration in the EBF such that the increased graphitisation accelerated coke weakening as well as its abrasion tendency.Even though, cokereactivity is consistently increased fromtop tobottom,the lack ofany observation of any cracks or fissuresorchange in porosity suggest that coke reactivity mightnotbe havingsignificant impact on the degradationof coke strength. The EBF observationswere also consistent with previous studies inwhich cokegraphitisationwas shown to affect coke degradationOur results indicate that coke alkalis could catalyse the reactivity but might not necessarily have a strong effect on cokegraphitisationandhenceon the coke strength.In summary, thestudy highlights that EBF alkalis are contributing in increasing thereactivity of coke without having any adverse impact on strength. The studyprovides an insight inorder to manipulate the cokebehaviour by controlling the properties of coke minerals. For example iron species can catalyse as well as graphitise the cokewhile this study indicates that alkalis could catalyse the reactivity but not graphitise thecarbon struc

ture. However,further studiesare required to demonstrate the predominant factors affecting the coke reactivity and graphitisation, andhence optimise theirimpact on coke finegeneration. This strong effect of cokegraphitisation onfines generationneeds tobe furthervalidatedbyexperiences ofa wide variety of metallurgical cokesunder differentoperating blastfurnaces.On thebasis of thisstudy,we can also conclude that temperatureeffects on cokegraphitisation and their consequences on cokeweakeningbecomes important only at temperatures above 1200C. The conventionalCSR test is conducted at1100C, hencecannot account for the impact of graphitisation behaviouron coke finesgeneration and its behaviour in blast furnace. Figure10Variationof cokestrength (CSR , based on I-drum test)andabrasion indices of the EBF cokes in EBF. (a)(b) Figure11 a) Effect of carbonstructureon coke strength and b)on the abrasion index of the EBF cokes. Figure12 a) Effect ofpotassium of EBF coke ash on theCSR, and b) on the abrasion indices of the same cokes. CONCLUSIONS Physical and chemical properties of cokes samples excavated from a dissection study of experimental blast furnace using a high CSR coke were measured. Evolution of coke properties particularly carbon structure was related to carbon structure of coke annealed in a laboratory furnace and their implications on coke behaviour (e.g. CSR/abrasion) were studied. Following conclusions were made. 1.The coke reactivity in the EBF was accelerated by the presence of recirculating alkalis in the coke, which increased as the coke descended in the furnace. 2.The growth of carbon crystallite height of coke in the horizontal furnace was found to be of similar order as observed under EBF reactions conditions under a similar range of temperatures. Comparison of carbon structure of laboratory treated cokes and the EBF excavated cokes indicated that coke graphitisation in the EBF is strongly influenced by temperature and not influenced by the presence of recirculating alkalis or reacting gases. 3.The deterioration of coke quality such as coke strength (CSR) and abrasion propensity were related to coke graphitisation, alkalization and reactivity such that coke graphitisation was shown to have a strong impact on coke weakening. 4.The study further implied that alkalis have a potential to modify the coke reactivity without affecting their graphitisation behaviour. The study highlights the limitations of the CSR test for assessing the coke behaviour in an operating blast furnaces as it can not simulate impact of graphitisation of cokes which becomes significant only at much higher temperatures. ACKNOWLEDGEMENTS The authors would like to thank MEFOS Metallurgical Research Institute AB and LKAB for providing the samples and the opportunity to conduct this research, Jernkontoret and Swedish Energy Agency for financial support. A part of this work was undertaken as part of the Cooperative Research Centre for Coal in Sustainable Development (CCSD) Research Program 5.1 (Ironmaking). Authors also appreciate technical support provided by Mr N M Saha-Chaudhury from the University of New South REFERENCES 1)Coke - Determination of Coke Rea

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