Dynamic Analysis of Gas and Solid Flows in Blast Furnace with Shaft Gas Injection by Hybrid Model of DEM-CFD

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1 ISIJ Internatonal, Vol. 51 (2011), No. 1, pp Dynamc Analyss of Gas and Sold Flows n Blast Furnace wth Shaft Gas Inecton by Hybrd Model of DEM-CFD Shuno NATSUI, 1) Sheru UEDA, 1) Hrosh NOGAMI, 2) Junya KANO, 1) Ryo INOUE 1) and Tatsuro ARIYAMA 1) 1) Insttute of Multdscplnary Research for Advanced Materals, Tohoku Unversty, Senda Japan. 2) Department of Chemcal Enneern, Ichnosek Natonal Collee of Technoloy, Takanash, Hasho, Ichnosek Japan. (Receved on July 28, 2010; accepted on September 17, 2010) In order to mtate CO 2 emssons from steel ndustry, decreasn coke rate by shaft as necton such as top as recycln s a favorable way. The concepton based on oxyen blast furnace s able to brn several profts for ntensfyn as reducton and decreasn coke rate by massve coal necton. In these processes, as necton from auxlary tuyere plays an mportant role to nect reducn as or make up heat balance n the upper part. Therefore, the effect of shaft as necton s consdered to be so mportant factor to realze the above processes. In the present study, dynamc behavors of as and sold flow, and stress dstrbuton between partcles n the blast furnace wth as necton at dfferent shaft levels were three-dmensonally examned by DEM-CFD. Snce the permeablty resstance of burden n blast furnace s domnant for as flow, as necton from auxlary tuyere s restrcted to specfed areas due to the nsuffcent horzontal nertal as force compared wth upwards as. Althouh these results are slhtly nfluenced by the number of auxlary tuyeres and as velocty, the overall behavors do not chane. It was estmated that shaft as dd not dffuse unformly. The penetraton of shaft as nto center of the blast furnace s lmted to perpheral zone. Thus, penetraton area of shaft as n horzontal secton s almost proportonal to rato of shaft as and as from the conventonal tuyere. Then, the chane of stress dstrbuton between partcles was calculated and as penetraton effect was quanttatvely clarfed. KEY WORDS: ronmakn; blast furnace; shaft as necton; low reducn aent operaton; dscrete element method; computatonal flud dynamcs. 1. Introducton Reducton of CO 2 emssons from the steelmakn ndustry s a serous subect. Prmary method of reducn CO 2 emsson s low reducn aent operaton n blast furnace. The concepton of shaft as necton and oxyen blast furnace s a favorable way to decrease coke rate remarkably. Reardn shaft as necton of reducn as, some excellent researches had been carred out by method of necton of reformed coke oven as or partal oxdaton as. 1 3) The valdty of ths process (NKG) was confrmed by the expermental blast furnace. 1) It was reported that shaft as necton nto the blast furnace enabled the decrease of coke rate from 571 to 374 k/thm n the expermental blast furnace operaton. 1) Moreover, oxyen blast furnace process had been nvestated around ,5) In ths process, cold oxyen was nected from burner to ntensfy the coal necton. In ths process, as necton from auxlary tuyere was utlzed to make up heat balance n the upper part of blast furnace. There were some dfferences on the shaft as necton level between the above processes correspondn to ther purpose. Recently, New Blast Furnace n ULCOS (Ultra Low CO 2 Steelmakn) has been developed and some campans have been carred out by the expermental blast furnace. 6,7) The basc conceptons of above representatve processes are shown n F. 1. Totally, shaft as necton can be consdered to be an mportant technoloy to expand the blast furnace capablty. However, the common subect n shaft as necton s the effectveness of as penetraton n the actual blast furnace, and the nfluence of the necton condton should be examned. In the present study, three dmensonally dynamc behavor of as flow and sold moton caused by as necton was analyzed by a hybrd model of dscrete element method (DEM) and contnuum model (CFD). 8) The analyss by DEM-CFD model for the blast furnace s consdered to provde useful nformaton on the effectveness of shaft as n- F. 1. Conceptons of varous shaft as necton processes. 51

2 ISIJ Internatonal, Vol. 51 (2011), No. 1 ecton n the actual blast furnace. Snce, the moton of each partcle can be ndvdually calculated n DEM, the nfluence of structure of burden layers can be accurately represented. 9) In the present model, t s easy to chane the necton ponts and as necton condtons, and the dynamc behavor of as sold flows throuh the layers of coke and ore can be vsually clarfed. As case studes, addtonal as was nected from upper and lower part of shaft chann the number of auxlary tuyere, and as and sold moton and stress dstrbuton were analyzed. Totally, dynamc behavor of as and sold flows and penetraton effect of shaft as were studed. 2. Structure of Present Model 2.1. Formulaton of Partcle Moton by DEM The basc equatons of DEM n the present study are as ven below. Contact forces are descrbed by the Vot model that conssts of sprn, dash pot and slder n F. 1(a). At tme t, the translatonal dsplacement u and the rotatonal dsplacement of the partcle wth mass m by the contact force that works between two partcles are shown as follows. m d 2u du η Ku f m 0...(1) 2 dt dt 2 d ϕ I R d ϕ 2 2 η KR ϕ 0...(2) 2 dt dt where K, h, I, R, f and are the stffness, a dampn coeffcents, the moment of nerta, radus of partcle, the partcle flud nteracton force and ravtatonal acceleraton. The contact force F actn on a partcle to s expressed as:...(3) F ( e d )...(4) where e and d are the elastc force expressed by the sprn and the dampn force expressed by the dash pot, and n and s denote normal and shear drectons, respectvely. The movement of the partcles s calculated by the fnte dfference method by the tme ncrement for the contact forces. The contact force s obtaned from the relatve dsplacement ncrement for a dscrete tme Dt. Δun e e K Δu, d η nt, nt, Δt n n nt, n...(5) Δt K n F ( e d ) nt, nt, nt, st, st, st, ν 1 ν 2Db E E 1, η 2 n mk n...(6) where E and n denote the Youn s modulus and Posson s rato of a partcle, and D b s the radus of the contact crcle. When the shear force s larer than the statc frcton force, the follown shear stress employs....(9) where, m denotes the maxmum statc frcton coeffcent. In order to represent the shapes of coke and ore, rolln frcton whch mtates the shear stress actn on the contact crcle was ncorporated n the model. 2 d ϕ...(10) 2 ( st, t ) dt T M...(11) where T and M are the receved torque of a partcle and the functon of the rolln frcton coeffcent. The rolln frcton coeffcent a s decded dependn on the physcal propertes of the partcle surface. In fact, t s dffcult to treat all actual partcles n a blast furnace as DEM partcles because the number of partcles that can be handled practcally n DEM s lmted by the capacty of the computer. Therefore, the partcle number was reduced by cluster approxmaton, n whch two or more partcles are represented by one partcle, as shown n F. 2(b) ) 2.2. Formulaton of Flud Flow by Contnuum Model The movement of the flud s descrbed by the contnuty equaton and the Naver Stokes equaton. t I 3 T R F, M D F st, st, 8 α t F ( ερ ) ( ερ ν ) 0 μ F st, t nt, t b n ( ερ ν) ( ν ) ερ ν ε p εμ ν...(12) 2 f...(13) K s e e K Δu, d η st, st, Δt s s st, s Δu 22 ( ν )( 1 ν ) 22 ( ν )( 1 ν ) 8Db E E s Δt...(7) 1, η s 2 mk s...(8) F. 2. Dscrete element model for the numercal smulaton ncludn as flow. 52

3 ISIJ Internatonal, Vol. 51 (2011), No. 1 where, e, n, p, r and m denote the vod fracton, velocty vector, pressure, densty, and vscosty of a flud, respectvely. The local vod fracton s obtaned from the partcle locatons calculated by DEM. The source term f takes nto account the nteracton force between flud and partcles. The as flow equatons are common to the as from ordnal tuyere and auxlary tuyere at shaft as necton Partcle Flud Interacton When the calculaton reon s dvded nto mcroscopc cells n order to calculate the as flow n the blast furnace, the flud velocty s treated as superfcal velocty n Eqs. (12) and (13). In ths study, the vod fracton n the packed bed s calculated from the locaton of the partcles obtaned as a result of the partcle movement analyss by DEM. The pressure drop s expressed by the mult-dmensonal Erun s equaton for a packed bed. 13,14) In ths study, only the as velocty s consdered, as t was assumed that the partcle velocty n the packed bed s suffcently small n comparson wth the as velocty. μ ( 1 ε) p 150 d ε p 2 ρ ( 1 ε) 175. ν ν d ε p (14) where, d p denotes the representatve partcle dameter n a cell. The pressure drop n the cell s consdered to be the total of the momentum exchane of the partcles. Therefore, a cluster partcle used n DEM receves the total dra forces of the actual partcles whch are ncluded n the cluster, as shown n F. 2(b). For the partcle flud nteracton force, Erun s equaton s appled n the low vod fracton reon and the Wen and Yu equaton s appled n the hh vod fracton reon. 15) f NC ε ν p p ν...(15) C p μ ( 1 ε) [ 150 ( 1 ε ) Re] ( ε ) ρεd p...(16) 3 μ ( 1 ε) C Re ( ε 08. ) D ρε d p ( Re ) / Re (Re 1 000) C D...(17) 043. (Re 1000) Re ν ν p p...(18) where, N, n p and n denote the number of actual partcles ncluded n a cluster partcle and the velocty vectors of the partcle and the as, respectvely. 3. Procedure of Smulaton 3.1. DEM-CFD Model for the Blast Furnace A blast furnace of m 3 nner volume wth 40 tuyeres was taken as the obect for present smulaton. The profle of the blast furnace s shown n F. 3. In order to decrease number of partcle n DEM, semcrcle model whch has a flat wall n front wth no frcton was employed. However, ths model s based on the three dmensonal structure. μ ρεd F. 3. Schematc daram of three dmensonal calculaton reon of blast furnace nner volume over m 3. Raceways are represented by 20 spheres wth 1.6 m n dameter whch placed equally-spaced on wall at tuyere level. Partcles of coke entraned n the raceway dsappeared at arbtrarly-specfed nterval. The p ron and sla layer n the hearth are not consdered. The curved lne whch denotes meltn zone was placed from 1.0 to 7.4 m over tuyere level as shown n F. 3, and partcles of ore dsappeared on the lne. The top surface of burden descends wth consumpton of ore at meltn zone and coke at raceways. When surface of burden places at 1 m below from stock lne, coke and ore partcles are chared alternately. Gas phase was assumed to be sothermal and ncompressble. Moton of burden partcles was calculated by the DEM and, then dstrbuton of vodae was derved from poston of partcles. Dependn on the vodae n blast furnace, as flow was calculated by CFD. Gas dra force on partcle n DEM was enerated from as flow. The detals of the DEM-CFD model were descrbed n the prevous papers. 8) In ths study, the method based on numerc strct technque (Controlled Volume method) proposed by Okaya et al. was adopted for dervaton of vodae. 16) By the applcaton of the prevous study, three tmes of radus was employed for the sphercal control volume radus around partcles for calculatn vod fracton n the blast furnace. 8) Accordnly, the characterstcs such as the vodae dstrbuton n blast furnace can be well represented wthout losn the local nformaton on the packed bed Applcaton of DEM-CFD Model for Shaft Gas Inecton Several shaft as necton processes whch have been proposed untl now are shown n F. 1. 1,4,6) The necton pont levels are dfferent each other for each purpose. Wth 53

4 ISIJ Internatonal, Vol. 51 (2011), No. 1 reference to these prevous processes n the past, the calculaton condtons were selected. The auxlary tuyere was placed at 16 m as the upper level and 8 m as the lower level above the tuyere. The latter condton was determned after consderatons of reducn as necton such as NKG and New Blast Furnace n ULCOS. 1,6) The former s correspondn to necton of preheatn as n the oxyen blast furnace. 4) Table 1 shows condton of calculaton n CFD module. Table 2 shows the condton of as volume dstrbuton from ordnary tuyere and auxlary tuyere. The as volume nected from auxlary tuyere at upper and lower part of shaft was 30% of total as volume. For shaft as necton at upper level, 12 or 6 tuyeres were placed at 16 m above the tuyere level, namely, 6 or 3 n the semcrcle. For lower level necton, 20 or 10 tuyeres were placed at 8 m above the tuyere level, namely, 10 or 5 n the semcrcle. Table 3 shows condton for DEM. In order to mtate Table 2. Table 1. Table 3. Calculaton condtons of CFD. Tuyere and shaft as necton condtons. Calculaton condtons of DEM. the computaton load, the dameter of partcle was enlared. Frstly, coke was dsappeared n front of tuyere and coke and ore chared alternately for 60 s n DEM tme, stable sold flow of burden was acheved wthout as flow. Then under the condton of Tables 1, 2, and 3, as and sold motons were calculated for 480 s. In the DEM calculaton tme was accelerated for 120 tmes, therefore the calculaton has been done for 4 s n DEM tme. 4. Calculated Results 4.1. Chane n Gas Flow and Gas Dra Force by Shaft Gas Inecton The structure of burden layers durn ore charn calculated by DEM s shown n F. 4. Dark and ray partcles represent ore and coke, respectvely. Ore partcles were chared around 3 s n DEM tme. It was confrmed that structure of burden layer was three-dmensonally represented by the above calculaton condtons. The calculated as flow n blast furnace for each case based on DEM-CFD s shown n F. 5. Fure 5(a) represents as flow wthout shaft as necton, and Fs. 5(b-1) and 5(b-2) are that wth shaft as necton from 12 or 6 auxlary tuyeres at upper part of shaft, namely 6 or 3 n the semcrcle. In Fs. 5(c-1) and 5(c-2) are that from 20 and 10 of tuyeres at lower part, namely 10 and 5 n the semcrcle respectvely. Each arrow denotes as vector composed of as velocty and drecton n each cell. The lare whte arrow shows the necton level. It s thouht that the penetraton reon of as s almost domnated by the momentum dstrbuton because the dffuson dstance of nected as s much smaller than that of convecton effect. 1 4) From F. 5, the nfluence of shaft as necton was observed around the necton level n each case. Especally, n case of Fs. 5(b-1) and 5(b-2), the as flow nearby the wall deformed by the shaft as necton. The upward as was slhtly pushed to center. The nfluence of auxlary tuyere on as flow s slhtly stron n F. 5(b-2), n whch the as velocty from the auxlary tuyere s much hher than F. 5(b-1). For every case, n the upper part above the necton level, the as flow almost drected to upward, and the nfluence of nected as dsappear above the necton level. The as velocty n the central part at the throat was ntensfed by the nfluence of nclned burden surface n every case as shown n red colored vector n F. 5. The three-dmensonal as vector at the necton level s shown n F. 6, and the horzontal as velocty s shown n F. 7. In F. 6, the drecton and velocty of as can be F. 4. Calculated burden layers n blast furnace by DEM. 54

5 ISIJ Internatonal, Vol. 51 (2011), No. 1 F. 8. Gas dra force dstrbuton for each partcle at level of shaft as necton, 2 m and 4 m (0 m: shaft as necton level). stron could be clearly reconzed. In case of (c-1) n F. 7, the horzontal as flow was weak, because the as velocty from the auxlary tuyere was low. Althouh these results were calculated from the momentum balance of as, the penetraton area of nected as could be qualtatvely estmated from as velocty dstrbuton. The dra force by as flow on partcle at the level of 0 m, 2 m and 4 m above auxlary tuyere s shown n F. 8. Snce the as dra force on partcles reflects the nerta force of as and the packn state n the packed bed, t can be consdered to be an ndex of as penetraton obtaned as a soluton of DEM-CFD. Hh forced reons as shown n red color can be seen around the auxlary tuyere at necton level, and t s smlar to the horzontal as flow shown n F. 7. The area of red reon s dependent on the as velocty throuh the auxlary tuyere, and t s estmated that the penetraton reon of nected as s lmted n the vcnty of the auxlary tuyere. Especally, as shown n Fs. 8(c-1) and 8(c-2) at lower part necton of reducn as, the reon whch s affected by nected as s located only n the perpheral zone nearby the wall, because the dameter of the lower part s relatvely lare compared wth the upper part. Snce the permeablty resstance of the packed bed s so lare, velocty of nected as from auxlary tuyere s rapdly decreased. At 2 m and 4 m level above the necton level, the dra force dstrbuton becomes almost flat as shown n F. 8. These above results are consdered to be mportant to evaluate the effect of shaft as necton and determne the favorable necton pont n blast furnace. F. 5. Calculated as velocty vectors n blast furnace of base condton and shaft as necton. F. 6. Three-dmensonal representaton of as velocty vectors at level of shaft as necton. F. 7. Horzontal velocty of as flow at level of shaft as necton Pressure Dstrbuton at Shaft Gas Inecton The amount of as volume from ordnary tuyere chanes wth the shaft as necton. Consequently, the as flow and dstrbuton of pressure n the blast furnace were vared. The sobar planes n blast furnace wth shaft as necton are shown n F. 9. Top plane ndcate the surface of burden, and t s the bass of the sobar. Other planes are plotted n the equal pressure nterval, and the color represents relatve pressure. In the operaton wthout shaft as necton, the sobar planes are nearly flat and locate n equal dstance. In the present calculaton, the as condton was consdered to be sothermal and ncompressble, therefore, absolute value of reconzed by the shape of plane conssted of vector and color at the necton level, and the curved shape of horzontal as flow s related to the nected as velocty. The red color part denotes the ntensfed as flow n the horzontal drecton n Fs. 6 and 7. The blue reon n F. 7 s equvalent to the part where the unform upward as domnates. Most part n Fs. 6 and 7 conssted of blue or reen reon. It was estmated that the reon nfluenced by shaft as necton as shown n red color was restrcted n the vcnty of the auxlary tuyere. In case of F. 7(b-2), the red reon where the horzontal as flow was relatvely 55

6 ISIJ Internatonal, Vol. 51 (2011), No. 1 F. 9. Isobar planes n blast furnace wth shaft as necton. F. 11. Normal stress dstrbutons between partcles alon vertcal axs. F. 12. Calculated normal stress dstrbutons between partcles alon vertcal axs. F. 10. Chane n as dra force dstrbutons for each partcle n whole blast furnace. pressure drop s dfferent from that of the actual blast furnace. However, the nfluence of shaft as necton on sobar can be relatvely examned. Around the auxlary tuyere level, the shape of sobar plane s much dfferent from the other sobar planes. Snce the auxlary tuyeres n shaft are placed ntermttently, the pressure n the perpheral zone becomes uneven crcumferentally. The varaton of number of auxlary tuyere caused unevenness of sobar plane near the tuyere, however the unevenness was not observed at 2 m above the auxlary tuyere of shaft. The crcumferental unevenness of sobar plane affect only on pressure dstrbuton at the necton level. From F. 9, t was estmated that the horzontal as flow was so weak and as velocty to upward was almost flat except the necton level and burden surface. F. 13. Effect of shaft as necton on sold descendn velocty. decreases due to the shaft as necton. The dstrbuton of dra force s enerally dependent on the dstrbuton of as volume to the auxlary tuyere and the ordnary tuyere. In an actual condton, the purpose of the shaft as necton s to decrease of coke rate, so these behavors such as dra force and pressure dstrbuton s nfluenced by the coke rate. However, the am of the present research s to clarfy the fundamental dynamc behavor of as and sold at shaft as necton, and these above basc results are avalable for analyss of shaft as necton. The precse analyss consdern the nfluence of coke rate and burden dstrbuton s the subect remaned. The dstrbuton of normal stress between partcles alon vertcal axs s shown n F. 11. The stress of partcle was derved by summaton of force of all contact ponts dvded 4.3. Sold Moton and Normal Stress Dstrbuton at Shaft Gas Inecton The dstrbuton of dra force on partcle n whole blast furnace s shown n F. 10. Fure 10(a) s that wthout the shaft as necton, Fs. 10(b) and 10(c) show the state of as dra force at the upper part necton and the lower part necton, respectvely. Color of partcle represents the mantude of dra force on each partcle. The eneral tendency of the dra force on partcle s smlar to each other. Strctly, the dra force below the auxlary tuyere of shaft 56

7 ISIJ Internatonal, Vol. 51 (2011), No. 1 by surface area. 17) These values on the normal stress at specfed heht were averaed on each heht. Sold, broken, and chan lnes denote those wthout shaft as necton, wth the necton from upper part of shaft, and wth that from lower part of shaft, respectvely. Snce the normal stress dstrbuton was nfluenced by the blast furnace profle, 14) t shows the maxmum value at 7.0 m above the tuyere level, and the normal stress between partcles around bosh was slhtly mtated by the bosh anle. From F. 11, t was observed that these tendences alon the vertcal axs were lttle nfluenced by shaft as necton. Fure 12 shows the three-dmensonal normal stress dstrbuton between partcles n whole blast furnace. In F. 12, the stress network can be clearly observed. Hh normal stress reon concentrates manly n the deadman n three cases. As mentoned above, the shaft as necton has an nfluence n the vcnty of the auxlary tuyere throuh the as flow and dra force on partcles. Althouh these reons chane wth the number of auxlary tuyere and as velocty, they are restrcted to the specfed area. Also n F. 11, t was observed that the normal stress dstrbuton n whole blast furnace was lttle nfluenced by the shaft as necton. Moreover, F. 13 shows the descendn velocty of burden at the steady state calculated by DEM. As the descendn velocty chanes wth burden charn, 17) F. 13 denotes the descendn velocty of burden at steady state before charn. The descendn velocty s bascally dependent on the blast furnace profle and the meltn zone shape. From F. 13, t was found that the descendn velocty of burden was not nfluenced by the shaft as necton. 5. Evaluaton of Shaft Gas Inecton Based on the Calculated Results The concepton of as flow and penetraton of shaft as s shown n F. 14. At the necton level of the shaft as, the sobarc plane s curved n the vcnty of necton pont. The upward as was suppressed to the center by the nected as. However, the sobar plane became flat n the reon above the necton level. It means that the as velocty dstrbuton across the radus s almost unform. Accordnly, as shown n F. 14, t s estmated that the penetraton area of nected as s proportonal to the relatve as volume of nected as. Althouh ths study does not nclude the dffuson effect, the above results aree wth the expermental result by Nsho et al. 2) They confrmed the dffuson effect of nected as by the three-dmensonal cold model and theoretcal calculaton. Totally, the effect of shaft as necton of reducn as s dependent on the amount of nected as from ths analyss. The area whch the nected as can penetrate s almost determned by the as volume of nected as. In the blast furnace, snce the effect of the radal eddy dffuson s lmted, 2) the desn of shaft as necton pont and as volume as to shaft as necton of reducn as s very mportant. Generally, f the necton as property s much dfferent from that of the upward as from the ordnary tuyere, the nner state of blast furnace s dvded to the dssmlar zones. The behavor of the necton as has a reat nfluence on the effect of shaft as necton. On the other hand, reardn the as necton from upper part to control the heat flow rato such as oxyen blast furnace, the dffuson effect s not so mportant. The as necton from upper part s avalable technoloy to control the heat balance of the upper part of blast furnace. 6. Conclusons The as flow and sold moton n blast furnace wth shaft as necton were three-dmensonally analyzed by the hybrd model of DEM and CFD. The follown conclusons were obtaned. (1) The nfluence of shaft as necton s restrcted to perpheral zone due to the permeablty resstance of burden. The as velocty and the number of auxlary tuyere at shaft as necton have lttle nfluence on the penetraton effect to central part of blast furnace. Gas flow chane n horzontal drecton s observed only n the vcnty of the auxlary tuyere level. (2) The shape of the sobar planes n blast furnace was dstorted by shaft as at the necton level, however n the upper part above the necton pont the sobar planes became almost flat. It means that the penetraton effect to horzontal drecton s not snfcant. The momentum effect of shaft as was so small compared wth upwards as. (3) The shaft as necton has lttle nfluences on normal stress between partcles of the burden around bosh. The stress net work of burden and sold flow above the necton ponts were lttle nfluenced by the shaft as necton n case of the present calculaton condton. (4) From the dynamc behavor of shaft as, t was estmated that the necton as could not dffuse effectvely nto center of the blast furnace. The area occuped by nected as n horzontal secton was almost proportonal to rato of necton as volume to blast as volume from ordnary tuyere. 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