"Redistribution" Effect of Lumpy Zone for Gas Flow in BF

Availableonlineatwww.sciencedirect.comScienceDirectJOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2007, 14(6): 01-07Redistribution"Effect of Lumpy Zone for Gas Flow in BFZHU Qing-tian, CHENG Shu-sen(School of Metallurgical and Ecological Engineering, University of Science and Technology beijing, Beijing 100083, China)Abstract, The gas flow from tuyere to raceway zone by blasting involves three distributional zones, such as drippingcohesive, and lumpy zone. The gas flow distribution in lumpy zone directly affects the gas utilization ration andsmooth operation in the blast furnace. However, the furnace closeness brings about great difficulty in the study ofhigh-temperature gas flow. The charging and blasting system affecting the gas flow and whether the top gas flowdistribution could reflect its inner condition as well as the furnace state, such as hanging or scaffolding, which havebecome the main problems for the research on gas flow. Recently, several researches overseas studied gas flow dis-ribution using the numerical simulation method, however, such a research was rare amongst the natives. In thisstudy, the flow model of gas in cohesive and lumpy zone was established using the numerical simulation software anthe gas flow distributions with uniform distribution of burden permeability, scaffolding of wall, and nonuniformcharge level were analyzed. As a result, the effects of cohesive zone and lower parts on the gas flow are very limitedand the charge level largely affects the distribution of top gas flow. Therefore, it was found that the distribution oftop gas flow could hardly reflect the inner gas flow. The process is called"redistribution"effect, which means thatthe gas flow after passing through the raceway, dripping, and cohesive zone is distributed when it flows into theKey words, blast furnace; gas flow: lumpy zone: numerical simulationboILCConstant, C=0 in freeboard, and C=l in packe-Turbulence dissipation raterbed;ped factor of particled.-Effective diameter of particlea, o. Effective Prandtl number for k, t, respective-ic energy, m/s-Pressure of gas, Pa-Static pressure drop of gas in packed bed, PaViscosity coefficient of gas, Pa.8:u,velocity in z, r direction, m/s-Void fraction of charge layer.The rational distribution of radial gas flow in a tem as top operation pf the blast furnace is impor-blast fumace is of great importance to stable smooth op- tant in the distribution of gas flow.eration and improving gas utilization. It is one of theThe furnace closeness and reaction complexity bringbasic methods for blast furnace operation that adopts about great difficulty in the study of distribution of gasappropriate blasting and charging systems to control flow. In this study, the inner furnace state, distributionand adjust the distribution of gas flown. The sound of gas flow, and cohesive zone shape are estimated bydistribution of gas flow is to achieve the highest gas monitoring the distribution of top gas. However, theutilization efficiency and the lowest fuel consumption distribution of gas flow from bottom to top containson the basis of the smooth operation of the furnace. thend raceway zone,According to long-term practice, the charging sys- mid di中国煤某化二ck, and upper dis-CNMHGFoundation Item: Item Sponsored by National Natural Science Foundation of ChinBiography,ZHUQing-tian(1981-),Male,Master:Bmall,bluersky1@yahoo.com.cr,RevisedDate:September15,2006Journal of Iron and Steel Research, Internationaol,14tribution in the upper shaft. The gas flowing intothe lumpy zone after going up the raceway zone,Outletdripping zone, and cohesive zone is redistributed un-der the effect of uniform burden distribution, Theburden of stronger permeability promotes the expan-ding of gas flow, whereas it retrains the flowing andeven leads to hanging, channeling, and so on. Thedistribution of gas flow near the charge level is af-fected by the charge level. Therefore, it is doubtfulthat the distribution of top gas flow could well andtruly reflect the inside states of the blast furnace.During recent years, numerical simulation forblast furnace has been developed, with which the“ black box” began to be opened under the monitoring of models. Many models have been developed toestimate the mutual influences between burden andCentergas distribution 2J, In this study, the flow gas modelin cohesive and lumpy zone was established using thenumerical simulation software and "redistributioneffect on gas flow during ascending procespacked bed was analyzed. In addition, the actual in-Flg. 1 Schematic diagram of gas flow modelformation of top gas flow is studied to play a basicrole in optimizing the burden distribution to achieve is-coordinate form[3-5:he rational distribution of gas flow in blast furnaceContinuity equationin favor of high productivity and stability.a()+:(prv)=0(1)1 Modelam equationThe gas distribution in cohesive zone and lumpy t direction, azone of the blast furnace is mainly studied. The shaft02(a)+13(m)=az|"az+is axisymmetric and the furnace diameter is uniformI aPin this model (Fig. 1). In this physical model, oreand coke layers are laid alternately and the ores in r direction: a(puo)+13the lower stack are softened and molten to form acohesive zone. The coke layers between the moltingencop i/*ore layers turn into" coke window"of the gas flow.Ergun equationIn fact, cohesive zone shape is various and isnot only simply invvs, and the move- z dir(+√+u)ment of matters in dripping zone is complicated. But(3)this model is mainly used to study the effect of r direction=(1+f2√a2+)lumpy zone, the cohesive zone, and the factors ofk-e turbulence model equationslower parts on the distribution of gas flow, and the Turbulent kinetic energygas going up through the cohesive zone is redistribu- ated under the effect of lumpy zone. So these short- az(puk)+10,(prok)=a fa akiz032comings are irrelevant because of gas flow redistri-13{bution"in the lumpy zone.,+G-cThe model is based on the differential for中国煤化工(4)the continuity, momentum, k-e turbulence model, aCN Gaeland Ergun equations, The pressure drop of gas flehrough the burden layers is represented by the erg-un equation. These equations are described in an axarl+E(CG-Cape)No 6Redistribution"Effect of Lumpy Zone for Gas Flow in BFdue to better radial permeability than axial one.f1=150(1-e)Fig 3 shows the simple path of gas flow in different)2Elayers.1=1-51-°,In the uppcr part of Fig. 2, the gas flow goescircuitously through the alternant ore and coke lay-aG=leleers, until charge level goes to freeboard as top gas.arWhen approaching the charge level, the gas flow inAeff=A+CPr k/ethe center is stronger than that nearer the wall dueC.=0. 09, C=1.44, C,=1. 92, 0=1.0, 0=1.3. to the lower level and litter gas resistance in the cen-An example of R=3. 4 m, z=10 m is taken, and ter part under the effect of inclination angle.the boundary conditions are given by the following:To analyze the differences of distribution be-(1)The volumetric flow rate of gas in the inlettween inner and top gas flow, the radial distribution1. 6X10 m" /h and pressure of outlet is 1X10 Pai of gas flothe parallel surface with charge let(2)The centerline of the blast furnace is considered of z=7.5 m(A), horizontal surface of z=7.5m(as symmetry axis;(3) Furnace stave is considered (B), and charge level are compared, and the resultas the wall (no slippage of wall). The parameters of is shown in Fig 4. As can be seen from Curve A incoke, ore, and gas are shown in Table 1. And Fig 4, gas velocity in a single layer is nearly uni-with the fluid software, the gas distribution of stock form, which adequately reveals the"redistributioncan be simulated and calculatedeffect of the uniform burden layer. However, thedifferences of gas flow in coke and ore layer can beTable 1 Properties of burden and gasobtained from Curve b in Fig 4: There is the ore(kg·m-3)p/(Pa:s)layer in the center of z=7. 5 m, and therefore, oreBurden0.50390.72and coke layers appear alternately in the horizontale0.42100.77surface of z=7. 5 m. From the fluctuant curve in this2 Results and DiscussionCokeOre2.1 Uniform distribution of burdenWith the same thickness of ore and coke layersand uniform radial distribution of permeability, theresult of simulation is shown in Fig. 2. From thevector field in the lower part, the gas flow is strorger in the "coke windows "than in the ore areashen going up through the cohesive zone. For theeffect of cohesive zone, gas flow is stronger near thetop of the cone than at the lower part, where the gasflow is almost negligible. However, after going intethe burden layer of uniform permeability, the gastittie/!cOkeflow is redistributed. This phenomenon is calledredistribution"effect of the lumpy zone. From themid part of Fig. 2, the uniform distribution andnearly fixed direction of gas flois a result of "redistribution". In blast furnace, theore and coke layers are distributed alternately. gasflow through the ore layers of lower permeability is中国煤化7intended to be upright with burden level for theCNMHGshortest path and least pressure drop Whereas gasflow is promoted in the coke layers and gas pene-Fig2 Effect of aniform burden dlstrlbutlon ontrates a coke layer with deflexion toward the wallistribution of gas flowJournal of Iron and Steel Research, International(bad permeability) zones at a height of 5.5-6.0 m(assuming the total pressure drop of stock in everycase is 1×105Pa)From the isobar of different cases it is foundthat thOredrop of these burden layers incre(15%to80%)idth of“BP”ses. In addition, the pressure drop grads of betterpermeability zone relatively at the same height alsoincrease. It is a result of reducing the scale of stronger permeability zone and strengthening of the gasflow. When the"BP"zone is wider, the pressure dropgrads are larger. For example, if the width of " BPzone is 80% of stock radius, the pressure drop ofthe layer of0.5 m in thickness is up to0.8×10°PTherefore, the force of gas in this zone of stock is solarge that it is easy to overcome the gravity of thestock and ultimately leads to hanging.The right side in Fig. 5 shows the radial distrFig 3 Flow direction of gas in burden layersbution of gas flow at different height, where x5. 75 m is the center of"BP"zone and this profile re-flects the distribution of gas flow inside the "BPCharge levelcanseen trom(B)Horzontal surfacecases at z=5. 75 m, the gas flow is weaker insidethe“BP”outside the zone at the() Paralleled with charge levelsame height. The strongest gas flow is near the raBP"zone is thin and the radial distribution of gasflow surrounding the"BP"zone caused by its"redis-tribution"effect is nonuniform. comparing the distri-bution of gas flow at z=5 m with that at z=6. 5 mRadius/mthe effect of the width of“BP” zone being20%ofFlg4 RadIal velocity distribution of gas nowstock radius on gas flow distribution in the zone of0.5 m below and above the“BP” zone is weak,Socase,the physical velocity of gas flow in the ore lay- the Fig. 5(b), the gas flow in the ascending processers is stronger than that in the coke layers, because rapidly becomes uniform distribution. When thef lower void fraction of the ore layers. The distBPzone is wider such as Fig. 5(c) and (d), thebution of gas flow of charge level shows the influ- gas flow of z=5 m and z=6. 5 m in the center isence of charge level shape on the distribution of gas stronger and near the wall is weaker, because of theflow. Gas flow in the center is stronger and de- effect of bad permeability in partial stock, but thescends toward the wall as a result of the inclination gas flow of these two cases becomes uniform distri-angle. The gas flow will be strengthened in the low- bution at the height of z=7 m and z=7.5 m afterefore,distributionredistribution" Therefore the effective distanceeffect of charge level is largerof"BP"zone for the distribution of gas flow of thewhole stock is limited, which is lengthened with2.2 Bad permeability of partial stockthe thickness of theThe bad permeability of partial stock will alter stock中国煤化工 n of gas flow willthe distribution of the inner gas flow and increase beCNMH Gution with“ redisthe pressure drop, even leading to hanging and tributionthe different figures of disere are same pressure dropsNo 6"Redistribution"Effect of Lumpy Zone for Gas Flow in BF14 x10"Aa12814息1810201x012I=6.5 mL9 10Pa005101.520253035(a)No"BP"zone: (b)Width of"BP zone is 20% of stock radius(c) Width of "BP zone is 50% of stock radius (d) Width of"BPzone is 80% of stock radiusFlg. 3 Effect of bad permeability of partial stock on isobar(Left)and radial distribution(Right) of gas flowof stock, the average flow rate lessens with wideningaccording to Fig. 6, the ascending gas flowBP"zone width. Namely, if the layer of bad permea- near the wall is blocked by the salamander, thenbility is wider, the total mass rate of gas flow be- surrounds the lump to redistribute gas flow. So nearcomes less. There is a good agreement between this the lump, the gas flow is stronger, whereas it canresult and practice that blast furnace does not easily be neglected inside the lump. After going throughaccept the blast volume when the permeability of the accretion, the gas will flow toward the wall andstock is poor. These conclusions can be applied ap- gas flow"redistribution"partly takes place till theproximately to the states of scaffolding, hanging, gas flow and permeability distribution of the wholeburden layer are consistent.From the above-mentioned discussion, it can be2.3 Scaffolding in upper shaftconcluded that the effect of the distribution of theThe scaffolding of blast furnace will lead to the primary gas flow is very limited and the distributionchange in interior shape, and consequently affect the of gas flow in the lumpy zone tends to be determineddistribution of gas flow. The large-scale scaffolding by burden distribution. As a result, if the distancein the wall will form the new interior lining and the from the lump to charge level is large, the distribu-actual inner diameter of furnace diminishes to have tion of top gas cannot reflect the scaffolding state inthe stronger gas flow. A lump will change the par- lower part because of"redistribution".tial distribution of gas flow, which may lead to中国煤化工large-scale scaffolding and hanging. Fig. 6 shows the 2.4distribution of gas flow in partly lump stage, theCNMHheight of which is x=5. 5-6.0 m and width is 20% is the largest. Especially, when the bellless top isof shaft radiusused, the distribution of coke/ ore rate in charge lev-Journal of Iron and Steel Research, InternstionalVol. 14of the distribution of charge level, though the lumpyzone inside the furnace is uniform, As a result, theburden distribution in charge level will directly affect the distribution of top gas flow and the disturb-ance of charging on estimating the lower gas flowwith the distribution of top gas flow is very large.Taking into account all the above-mentionedphenomena of the distribution of gas flow, the dis-tribution of gas flow under the effect of cohesivezone and lower parts only is the initial condition ofgas flowing into the lumpy zone. As the lumpy zoneis thicker, permeability is poor, and burden distribution is changeful, the "redistribution"effect ofburden on gas flow is very serious. It is difficult thatFig. 6 Velocity field of gas in scaffolding of upper shaftthe top gas flow reflects the distribution of interiorel becomes more complicated. Fig. 7shows the dis-gas flow, even the special gas flow appearing insidetribution of gas flow when there is a single ring of the lumpy zone is rapidly redistributed in asceneto adapt to burden distribution. So that the distribuore on the surface of the charge level after charging. tion of gas flow after flowing out of the cohesiveAs can be seen, with the effect of the lower permea- zone will vary and the "redistribution"takes placebility of ore ring in the surface the gas flow nearthe charge level will take place in the"redistribuceaselessly. especially, the gas flow ascending nearl will be affected bytion"that the gas under the ore ring tends the roundore to go up the charge level. Particularly, in the permeability of the charge level to change the distri-center of the furnace, the charge level is lower and bution of top gas flow, which is called the chargeleve“ redistribution”, Therefore,the“ distribution”permeability is better, and hence, the gas flowblows out easily from the center of the charge level effect of lumpy zone gives prominence to burden dis-and the top gas flow is the strongest. When char- tributionging, the gas flow on the surface of the ore ring isWhen there is only a whole rational burden dis-tribution of the lumpy zone, the distribution of gasweaker, whereas the gas flow between the ore rings flow is sound. Therefore, adjusting the distributionor in the little ore/coke rate is stronger. So the dis- of gas flow of the whole lumpy zone with chargingtribution of top gas flow is changed by the chargelevel. Therefore, the top gas flow can efficiently re-system is a slower process. The lower adjusting is aflect the shape and distribution of charge level.timely and efficient accessorial means. In practice,with the lower part distribution of gas flow changedIt is found that the radial distribution of the gas flocharge level becomes“w”- shaped with the effectby blasting system, the wrong states of furnace canbe treated, such as volume and temperature of blastasing or decreasing, halting, and so on, Bethis system cannot dominate the distribution of gaflow in the whole lumpy zone. So burden operationis the key to achieve the rational distribution of gasflow. It is impossible to produce stably and highlyefficiently in blast furnace if there is no stable andrational burden system3 Conclusions中国煤化工 del developed hereCNMHGand Ae turbulenceor studying the“ redistribution”Fig 7 Effect of nonuniform charge leveleffect for the gas flow of lumpy zone. It plays a bas-ic role in the practice operation of blast furnace.No 6Redistribution"Effect of Lumpy Zone for Gas Flow in BF(2)The path of gas flow in ascending fluctu- of charge levelates periodicallyy. The gas flow in an ore layer is in-tended to be upright with burden level and pene- Referencestrates a coke layer with deflexion toward the wall.[1] WEN Xue-ming, MI Ke-qin, SHEN Zhen-shi, et al. Ironmak(3)The poor permeability of partial stock willng Production Process in BaoSteel [M]. Heilongjiang, China:increase the pressure drop grads and lead to the dif-Science and Technolagy Press of Heilongjiang, 1994 (in Chi-ficulty of accepting the blast volume, even lead tohanging, and so on. 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