Influence of Cohesive Zone Thickness on Gas Flow in Blast Furnace Analyzed by DEM-CFD Model Considering Low Coke Operation

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1 See discussions, stats, and author profiles for this publication at: Influence of Cohesive Zone Thickness on Gas Flow in Blast Furnace Analyzed by DEM-CFD Model Considering Low Coke Operation Article November 2013 DOI: /srin CITATIONS 4 READS 26 6 authors, including: Tatsuro Ariyama Tohoku University 106 PUBLICATIONS 892 CITATIONS SEE PROFILE Available from: Tatsuro Ariyama Retrieved on: 10 May 2016

2 Influence of Cohesive Zone Thickness on Gas Flow in Blast Furnace Analyzed by DEM CFD Model Considering Low Coke Operation Tatsuya Kon, Shungo Natsui, Shohei Matsuhashi, Shigeru Ueda, Ryo Inoue, and Tatsuro Ariyama Reduction of the reducing agent aiming at the mitigation of carbon dioxide emissions decreases the gas permeability in blast furnace. Favorable control of burden distribution and optimization of packed bed might mitigate decreasing permeability. Especially, decreasing thickness of cohesive zone would be effective. In present study, the influence of the cohesive zone thickness on gas flow and pressure distribution was investigated using the DEM CFD model to evaluate the effects of adoption of a thin layered cohesive layer structure on gas flow and permeability changes in the cohesive zone during low coke ratio operation. Reducing the thickness of the cohesive zone can effectively increase permeability in the cohesive zone even in the thin coke slit of the low coke rate operation, and improvement in the permeability of the cohesive zone can be realized more effectively in combination with appropriate coke mixed charging. 1. Introduction Reduction of the blast furnace reducing agent rate is a key challenge for ironmaking and is an object of ceaseless research. In recent years, reduction of reducing agent ratio has also been an object of increased research activity from the viewpoint of reducing CO 2 emissions. New processes such as blast furnace top gas recirculation are now being developed in order to reduce the coke rate. [1,2] Since the coke maintain void fraction of the moving bed, deterioration of blast furnace permeability becomes a concern when the coke rate is reduced. In particular, securing a stable gas flow in the cohesive zone by use of a thinner coke layer is an issue. Figure 1 shows a schematic diagram of the gas flow in the vicinity of the cohesive zone. Solid circles denote softening ore layer. In the cohesive zone, permeability decreases due to softening and contraction of the ore layer. The gas flow is generally secured by forming coke slits between the ore layers by layered charging, as illustrated in Figure 1a. However, when the coke rate is reduced, the coke slits become relatively thinner, as shown in Figure 1b, and as a result, permeability decreases. In recent years, mixed charging has attracted attention as a means of improving the permeability of the cohesive zone [ ] T. Kon, S. Natsui, S. Matsuhashi, S. Ueda, R. Inoue, T. Ariyama Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai , Japan tie@tagen.tohoku.ac.jp DOI: /srin by charging nut coke in the ore layer, [3] or by actively charging lump coke in the ore layer, [4] as shown in Figure 1c. Addition of coke in the iron ore layer increases reduction rate of the ore and that also increases the melting temperature of iron ore and decreases thickness of the cohesive zone. Moreover, when coke particles exist in the cohesive layer as shown here, it is thought that the coke has the effect of moderating the decrease in the void ratio due to softening and contraction of the ore, and thereby reduces the increase of pressure drop in the cohesive layer. [5] The effect of coke mixing has also been confirmed in load softening tests under load. [5] In the low reducing agent rate operation, coke mixing in the ore layer decreases thickness coke layer, and layer structure might be disappeared as shown in Figure 1d. In order to stabilize gas flow at cohesive zone with no coke slits, gas flow in soften layer should be enhanced by decreasing thickness of cohesive zone. The authors had performed an analysis of the gas flow in a packed bed, which reproduces the layered distribution that is a characteristic feature of the blast furnace by applying a DEM-CFD model. [6] Although permeability decreases when the coke rate is reduced, that research confirmed that the gas flow changes and the degree of influence differs depending on differences in the packed bed structure in layered charging and mixed charging. Reduction of the cohesive layer thickness by improving the melting behavior of the ore layer is considered effective for increasing permeability. [7] However, the effect of a thin cohesive layer, and the quantitative relationship between changes in the packed bed structure and permeability, had 1146 steel research int. 84 (2013) No. 11 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Layered charging Cohesive zone Cohesive zone ore ore Mixed charging Nut coke mixing Nut coke Coke ore ore Gas Flow Coke layer slit Figure 1. Estimation of gas flow change by charging method and coke rate. not been clarified. The DEM-CFD model developed by the authors can reproduce the changes in the packed bed structure and permeability of the cohesive zone due to mixed charging. In present research, the influence of changes in the cohesive zone on gas flow and pressure distribution was studied using the DEM-CFD model to evaluate the effects of adoption of a thin-layered cohesive layer structure on gas flow and permeability changes in the cohesive zone during low coke ratio operation. 2. Fundamental Equations and Calculation Conditions DEM is used in kinematic analysis of non-steady solid flows and similar applications, and makes it possible to track the behavior of independent particles by solving a motion equation for all solid particles. [8] As the fundamental composition of the model used here was explained in detail in a previous report, [9] the description here will be limited to a general outline. Considering restrictions on computing capacity, in DEM with this model, the coke and ore particles were modeled as clusters, and an expanded particle diameter was adopted. [9] The actual particle size was used in the gas flow calculations in the CFD, as described in the following. In this model, the motions of solid particles and the gas phase fluid are calculated by DEM and CFD, respectively, and solutions for particle motion and gas flow are obtained by momentum transfer and exchange between the solid and gas phases (i.e. particles and fluid). particle and a wall. Contact force is calculated by the Voigt model, which is approximated by a spring, dashpot, and friction slider. In the tangential direction, slider, which expresses the maximum friction force m on particles, is installed. The equation of motion for particles considering the contact force acting between two particles is expressed as follows for translational displacement u i. m i d 2 u i dt 2 þ h du i dt þ Ku i þ f D þ m i g ¼ 0 ð1þ Rotational displacement c i is expressed by Equation 2. d 2 w I i i dt 2 þ dw i hr2 i dt þ KR2 i w i ¼ 0 where, t, f D, h, m,r,i, and v are time, an external force term, the viscosity coefficient of the dashpot, and the particle mass, radius, moment of inertia, and turning angle velocity, respectively. K and h are a spring constant and the viscosity coefficient of the dashpot. Although the particles handled by DEM have a spherical shape, blast furnace burden materials have irregular shapes. In order to represent coke and iron ore layers, 0.6 million particles were used in the calculation. And for mitigating calculation load, single particle with rolling friction a [10] was employed it present study. The shapes of the particles were expressed by changes in internal friction corresponding to coke, sintered ore, etc. by setting the coefficient of rolling friction on an inclined surface as a parameter showing the shapes of particles. Parameters of DEM calculation is summarized in Table 1. ð2þ 3. Fundamental Equations of DEM Model In DEM particles, translational motion and rotational motion change depending on the stress and moment from the contact points between two particles and between a 4. Fundamental Equations of CFD Model The blast furnace model is based on the DEM-CFD model developed by Natsui et al. [11] As CFD calculations, in ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 84 (2013) No

4 Maximum particle number 600,000 Diameter of particle (coke, ore) 0.300, m Young s modulus of coke 1.00 GPa Young s modulus of ore In lumpy zone 1.00 GPa In cohesive zone 0.02 Poisson s of particle (coke, ore) 0.21, 0.24 Apparent density (coke, ore) 1100, 4000 kg m 3 Rolling fiction coefficient (coke a coke, ore a ore ) 2.50, 1.25 Contact friction coefficient (particle particle, particle wall) 1.0, 0.7 Time step dt s Table 1. Parameters used in DEM calculation. addition of the term f g for the force of interaction between the particles and fluid in the Navier Stokes equation, the Ergun equation (Equation 6 (e 0.8)) or the Wen Yu equation (Equation 6 (e > 0.8)) was used in the external force term of that equation, corresponding to the void fraction. The fundamental equations of the CFD analysis are ðer gþþrðer ðer gvþþðv rþer g v ¼ erp þ em g r 2 v þ f g f g ¼ N i C g p e ðv p i v g Þ 8 m g ð1 eþ r g e 2 d 2 ½150 ð1 eþþ1:75 ReŠ >< p ðe 0:8Þ C g p ¼ 3 4 C m g ð1 eþ >: D r g e 3:7 d 2 Re p ðe > 0:8Þ ( C D ¼ 24½1 þ 0:15 Re0:687 Š=Re½Re 1000Š 0:43½Re > 1000Š Re ¼ v pi v g rg ed p m g In the above Equation 3 8, e, n, p, r g, m g, f g, C D, and Re are the void fraction, a velocity vector, pressure, density, viscosity, a fluid drag term, a fluid drag coefficient, and the Reynolds number, respectively. In Equation 5, N i, n pi, and n g are the number of particles in a cluster, the particle velocity, and the gas velocity. F D is given by the following ð3þ ð4þ ð5þ ð6þ ð7þ ð8þ equation as the reaction of Equation 5. F D ¼ pd2 p C g p 6ð1 eþ ðv g v pi Þ 5. Calculation Conditions Figure 2a shows the profile of a 5000 m 3 class blast furnace as the object of the calculation, together with the CFD calculation grid and the positions of the set cohesive zone and raceway. The calculation region is a threedimensional half-section divided longitudinally by a frictionless wall. In the case of layered charging, a packed bed was formed by alternately charging ore and coke from the mouth of furnace at a coke ratio of 350 kg thm 1 as the base case or 240 kg thm 1 assuming future low coke rate operation. In layered charging, the particle number was set so as to obtain a coke layer thickness of 1 m at the furnace top. Inversed V shaped cohesive zone was observed in dissection of blast furnace. [12] Thickness of cohesive zone is depending on softening and melting temperature of iron ore, therefore that might be decreased by controlling physical property of iron ore. As illustrated in Figure 2a, an inverted-v shaped cohesive zone with a vertical thickness of either 4 or 2 m was set at the bottom of the blast furnace shaft, and softening of the ore was simulated by reducing the Young s modulus of the ore on reaching the top of the cohesive zone. In order to express melting, the solid ore particles were eliminated at the bottom plane of the cohesive zone. In this study, void fraction in the packed bed is calculated from the location of the particle obtained as a result of the particle movement analysis by DEM. Since liquid phased is distributed in the melting zone, void fraction in the melting zone was multiplied by 0.8. [11] ð9þ 1148 steel research int. 84 (2013) No. 11 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 Figure 2. Object of simulation and calculation grid for modeling of blast furnace, showing change in Young s modulus of burden. As a total of 40 tuyeres was assumed in the furnace bottom, 20 tuyeres were set in the half-section model used in the simulation, and a spherical raceway position with a diameter of 1.6 m was given in front of the tuyere positions. The descent velocity of the solid was controlled by successively eliminating coke at a constant speed as it reached the raceway. The migration velocity of the packed bed was increased by a factor of 120 by accelerating the furnace feed and discharge in comparison with an actual blast furnace. The fact that acceleration of the descent velocity has a little effect on the descent behavior of a packed bed structure as a whole has already been reported. [13] In gas flow calculations, the force of interaction based on the actual particle diameters of coke and ore was used. The condition of the CFD calculations is shown in Table 2. In the CFD used to calculate gas flow, horizontal blowing of a set quantity of gas toward the center from tuyere was used. The gas was assumed to be isothermal and no reaction, and the influx rate of the gas from each tuyere was the identical. Blast volume was set at a specified pressure loss in the furnace, which is derived from blast velocity, area of tuyere and permeability. 6. Change in Particle Properties The Young s modulus and contact friction coefficient of the particles comprising the DEM model are the characteristic physical property values of coke and ore particles. These values were obtained by measurement of the Diameter of particle (coke, ore) 0.050, m Apparent density (coke, ore) 1100, 4000 kg m 3 Number of grids (h,r,q) ( ) Inlet area of tuyere m 2 Inlet gas velocity 87.2 m s 1 Viscosity of gas (m) Pa s Density of gas (r g) 1.20 kg N m 3 Time step (CFD) 7.2 s Table 2. Parameters used in CFD calculation. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 84 (2013) No

6 contact friction between coke, between coke and a mirror surface, and between coke and refractory brick and calculation from measurement of the angle of repose by DEM calculation. [13] The values of the actual particles were used for density, Young s modulus, and the Poisson ratio. In an actual blast furnace, the particle size, and physical properties change due to powdering and chemical reactions in the process of particle descent through the furnace. However, in this research, it was assumed that there are no changes in the particle properties of the charged particles. In the lumpy zone, the composition, properties, etc. of the burden gradually change due to particle temperature increase, reactions, etc. accompanying descent, but there are no large changes in the basic shapes of the coke and ore layer particles. On the other hand, in the cohesive zone, softening and contraction of the ore particles occur due to loading from above, temperature increase, and progress of the reduction reaction, and as a result, the shape of the ore particles changes greatly. Although it is difficult to express quantitatively the changes in the properties of ore particles in the cohesive zone, such as contraction and the like, the authors introduced a method for expressing ore particle softening and contraction in DEM by reducing the Young s modulus of these particles in the cohesive zone. [9] Because reduction of Young s modulus allows a decrease in the interparticle distance, the particle number per unit of volume increases, and it is possible to express the decrease in the apparent void fraction and permeability. As shown in Figure 2b, cohesive zone was expressed by changing the Young s modulus to 0.02 GPa at the cohesive zone position. The appropriateness of the change of this Young s modulus was confirmed in a previous report by comparison with the results of a load softening test. [9] In these calculations, fundamental structures of each packed bed were prepared by performing calculations from charging of coke and iron ore at a set ratio from the furnace top by bell-type charging equipment until both the solid flow and the gas flow stabilized, simulating consumption of the coke in the raceway. The conditions for layer composition are shown in Table 3. The base condition is layered charging. In order to investigate the effects of coke mixed charging, two mixed charging conditions were used in this research, namely, 50% mixed charging, in which 50% of the coke particles were mixed with the ore, and 100% mixed charging, in which all of the coke particles were mixed with the ore. However, in mixed charging, flat charging was used to prevent segregation of the mixed materials during charging. As mentioned above, two cases were examined for the cohesive zone thickness, namely, 4 and 2 m. The layer shapes (burden distributions) obtained by DEM calculation for cohesive zone thicknesses of 4 and 2 m are shown in Figure 3. Although deformation of the Coke rate [kg t 1 ] 350 Charging condition 240 Layered charging 50% coke mixing 100% coke mixing Thickness of cohesive zone [m] Table 3. Calculation conditions of moving bed structure. 7. Gas Flow and Pressure Distribution in Vicinity of Cohesive Zone The range of existence of the cohesive zone is determined by softening and dripping originating from the liquidus temperature, which is linked to the eutectic temperature and melting temperature of the mineral in solid phase, which in turn are related to the liquid phase composition of the iron ore such as sintered ore, etc. This eutectic temperature is strongly influenced by the composition of the oxide, the mineral phase and iron oxide concentration, and the ratio of iron (II) and iron (III) oxides, while the start of dripping at the bottom of the cohesive zone is influenced not only by the oxide composition, but also by the reduction and carburization condition of the iron. Ore meltdown behavior varies depending on the reducibility of the ore and the composition of the gas, and reduction of the thickness of the cohesive zone in order to maintain permeability becomes an important factor in low coke rate operation. [12] Figure 3. Calculated burden distribution by DEM steel research int. 84 (2013) No. 11 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

7 ore in the cohesive layer is larger than that in other regions, because the Young s modulus of the coke is constant and deformation of the coke is slight, there is little overall change in volume. Therefore, no effect of the thickness of the cohesive layer on the layer shape in regions above the cohesive zone can be confirmed with either layered charging or mixed charging. Next, the distribution of the void fraction in the furnace in the case of a coke rate of 350 kg t 1 is shown in Figure 4. In layered charging, the existence of a cohesive layer with a low void fraction and coke slits can be observed. When the thickness of the cohesive zone was changed from L ¼ 4 to 2 m, there was no change in the value of the void fraction in the cohesive layer part, and the layer thickness became thinner in that condition. In mixed charging, the cohesive layer and the coke slits are unified. Because the thickness of the coke layer is constant, the tendency is similar in case of the coke rate of 240 kg t 1, even though the number of coke slits decreases and the thickness of the cohesive layer shows a large increase. Figure 5 shows the gas flow vectors in the packed bed as calculated by the DEM-CFD model in layered charging with cohesive layer thicknesses of L ¼ 4 and 2 m. The object in this figure is the region from directly above the tuyeres to the central part of the shaft. The direction and intensity of the gas flow is shown by lines and colors, respectively, with red indicating a strong gas flow velocity. In the cohesive zone, a high gas flow velocity due to Figure 4. Void fraction in blast furnace calculated by DEM. Figure 5. Influence of cohesive zone thickness on gas flow vectors in layered charging. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 84 (2013) No

8 Figure 6. Influence of cohesive zone thickness on gas flow vectors in 100% coke mixing condition. the decreased void fraction can be confirmed. With the coke rate of 350 kg t 1, coke slit gas flows from the furnace center, shown on the left, to the furnace wall direction can be observed at the position of the coke slits in the cohesive zone. At L ¼ 2 m, the region where a high flow velocity exists, shown in red, has decreased in comparison with the case of L ¼ 4 m. On the other hand, with the coke rate of 240 kg t 1, this condition has changed to a condition in which the coke slit flow and flow through the cohesive layer coexist, and a gas flow impacting on the furnace wall can be observed at the bottom of the cohesive zone. When the thickness is reduced to L ¼ 2 m, the effect of the coke slits on the gas flow decreases due to shortening of the slits. Figure 6 shows the gas flow vectors in the furnace in 100% mixed charging with a coke rate of 240 kg t 1. The horizontal gas flows in the coke slit have disappeared because coke slits do not exist structurally under this condition, and gas flow through the cohesive layer region is observed. At L ¼ 4 m, the region of high flow velocities of 12 m s 1 and higher is widely distributed. At L ¼ 2m, the thickness of the region with a low void fraction is thinner, and the region of strong gas flow velocities has become smaller. Moreover, it can also be understood that the maximum flow velocity is smaller at L ¼ 2 m than at L ¼ 4m. Figure 7 shows the pressure distribution obtained by DEM-CFD calculation of each case as the iso-bar plane distribution. The planes shown here are the iso-bar planes at steps of 33 kpa, using the stock line at the furnace top as the pressure reference plane. With the coke rate of 350 kg t 1, the pressure at the top of the cohesive zone is 70 kpa, and the pressure in the shaft part is virtually unaffected by the thickness of the cohesive zone. Because the iso-bar planes in the cohesive zone part are closely spaced, it can be understood that pressure drop becomes large in the cohesive zone. As shown previously in Figure 5, with a coke rate of 350 kg t 1, the coke slits function effectively in maintaining the gas flow. When the thickness of the cohesive layer is reduced from L ¼ 4 to 2 m, the length of the coke slit gas flow direction is also reduced, and pressure drop in the cohesive zone improves remarkably. On the other hand, due to the increase in the ore/coke ratio when the coke rate is reduced to 240 kg t 1, the pressure difference at the top of the Figure 7. Iso-bar planes of moving bed in blast furnace with layered charging steel research int. 84 (2013) No. 11 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

9 cohesive zone is 110 kpa in comparison with the coke ratio of 350 kg t 1. Overall, pressure drop is remarkably large in comparison with that when the coke rate of 350 kg t 1. However, the effect of using a thin cohesive layer can be confirmed, as the relative pressure at the bottom of the cohesive zone is 250 kpa when L ¼ 4 m and on the order of 200 kpa when L ¼ 2m. Figure 8 shows the change in the pressure distribution under a condition of 100% coke mixed charging. In the case of mixed charging with a coke rate of 350 kg t 1, the relative pressure in the shaft part increases in comparison with the pressure distribution in layered charging, irrespective of the thickness of the cohesive zone. In the cohesive zone, pressure drop increases slightly when the cohesive zone thickness is L ¼ 4 m, but when L ¼ 2 m, pressure drop decreased as a result. With mixed charging at 240 kg t 1, large pressure drop can be observed in the cohesive zone when L ¼ 2 m. Thus, it can be understood that the influence of the cohesive zone thickness changes depending on the charging method and the coke rate. In order to study the changes in the gas flow in the vicinity of the cohesive zone, the horizontal gas flow velocities with coke rates of 350 and 240 kg t 1 are shown in Figure 9 and 10, respectively. The object of analysis is the gas flow at a position 4.06 m from the furnace center. These Figure 8. Iso-bar planes of moving bed in blast furnace with mixed charging. Coke rate 350kg/t L=4m L=2m 14 Layered charging 50% coke mixing 100% coke mixing Height from tuyere [m] Horizontal gas velocity [ m/s ] Figure 9. Horizontal gas velocity in cohesive zone region at 4.06 m position from center with 350 kg t 1 coke rate. ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 84 (2013) No

10 Height from tuyere [m] Coke rate 240kg/t Layered charging L=4m 50% coke mixing L=2m 100% coke mixing Horizontal gas velocity [m/s] Figure 10. Horizontal gas velocity in cohesive zone region at 4.06 m position from center with 240 kg t 1 coke rate. figures can be interpreted as follows: In layered charging, the flow velocity changes depending on the height, becoming high at the position of the coke slit and decreasing at the position of the coke layer. From this, the effect of the coke layer can be understood. The thickness of the cohesive layer affects the length of the coke slits. Namely, when the cohesive zone thickness is reduced to L ¼ 2 m, it can be understood that the horizontal gas flow decreases by half in comparison with L ¼ 4 m, the gas flow penetrates through the cohesive zone, and the ratio of the gas flow in the upward direction increases. When coke mixed charging is increased to 50% and then 100%, coke slits are eliminated, the coke slit flow weakens, and the gas flow velocity becomes homogeneous. The average value of the horizontal gas flow velocity with the coke rate of 240 kg t 1 is shown in Figure 10. Under the layered charging condition, the flow velocity becomes large in comparison with that at 350 kg t 1. This is due to the decrease in the number of coke slits as the ore/coke ratio increases. Although the gas flow velocity shows its maximum value at the position 8 m above the tuyeres, that value is also large in comparison with the case of 350 kg t 1.Theinfluenceof coke mixing is similar to that with 350 kg t 1. In order to study the effect of thin layers in the cohesive zone in low coke rate operation, the pressure drop in the furnace corresponding to each charging method was illustrated in graph form in Figure 11. Lines show the gas pressure drop related to top of the moving bed at a position 4.06 m from the furnace center with the coke rate of 240 kg t 1. Thin and bold lines denote L ¼ 4 and 2 m, respectively. Solid, broken, and dot lines denote layer, 50% mixing and 100% mixing charging conditions, respectively. Pressure drops of the conditions are similar each other, in the lumpy zone over 11 m from tuyere level, and that in lumpy zone is relatively small to the cohesive zone. Pressure loss is significantly high in the cohesive zone between 11 and 9 or 7 m over the tuyere. Layer structure controlled by burden charging and thickness of cohesive zone effects on the permeability of whole blast furnace. Thickness of cohesive zone is directly related to the pressure drop. When L ¼ 4 m, 50% mixing charging causes lowest pressure loss, that means coke slit is effective for gas flow even in the condition. However, L ¼ 2 m, pressure losses of 50 and 100% mixing are almost same each other. Gas would flow smoothly in coke mixed ore layer in thin cohesive zone, and influence of coke slit is relatively decreased. Height from tuyere [m] m thickness Layer 50% mixing 100% mixing 2m thickness Layer 50% mixing 100% mixing Coke rate 240kg/t Pressure [KPa] Figure 11. Longitudinal pressure distribution in 240 kg t 1 coke rate steel research int. 84 (2013) No. 11 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

11 Pressure drop [KPa] Coke rate 350kg/t L=4m L=2m Coke rate 240kg/t L=4m L=2m Layered charging 50% coke mixing 100% coke mixing Layered charging 50% coke mixing 100% coke mixing Figure 12. Influence of cohesive zone thickness on pressure drop. 8. Control of Permeability in Furnace in Low Coke Rate Operation Pressure drop between top and raceway levels of coke rate 350 and 240 kg t 1 operation are compared in Figure 12. Under the base condition of a coke rate of 350 kg t 1, mixed charging weakens the effect of the coke slits and invites an increase in pressure drop in the furnace as a whole. This is due to the increase in the flow through the cohesive layer from the coke slit flow. However, when the thickness of the cohesive layer is reduced, pressure drop in the furnace decreases due to decreased pressure drop in the cohesive zone part. With the coke rate of 240 kg t 1, pressure drop increases greatly in the furnace as a whole. However, coke mixed charging also acts effectively to reduce pressure drop in layered charging because the strong local gas flows in the cohesive zone part are moderated and change to a more homogeneous gas flow. Furthermore, reduction of the cohesive zone thickness also has a large effect under this condition. In the condition with the coke rate of 350 kg t 1, the gas smoothly flows in the coke layer or coke structure in cohesive zone, and thus influence of variation of cohesive zone thickness from 4 to 2 m is relatively small compared to the condition of 240 kg t 1. In order to decrease pressure drop in low coke rate operation, decreasing thickness cohesive zone is an effective method. From this discussion, the importance of securing permeability in the cohesive zone in low coke rate operation can be confirmed. A charging method, which considers permeability in the cohesive zone is desired. It is considered that permeability in the cohesive zone can be increased even more effectively by further reduction of the thickness of the cohesive zone. Thus, adoption of a charging method such as mixed charging, etc. in combination with improvement of the reducibility and meltdown properties of the ore appears to be important in low coke rate operation. 9. Conclusion In the iron and steel industry, various improvements in blast furnace operation are demanded with the aim of realizing low reducing agent operation. For this, highly accurate simulation models which make it possible to understand the phenomena in the blast furnace in detail are considered necessary. The authors proposed a DEM- CFD model, which considers behavior in the cohesive zone, reflecting the packed bed structure of the blast furnace. An analysis of permeability in the vicinity of the cohesive zone in low coke rate operation was performed using this model. It was shown that reducing the thickness of the cohesive zone by improvement of ore properties can effectively increase permeability in the cohesive zone, and improvement in the permeability of the cohesive zone can be realized even more effectively by use of this technique in combination with appropriate coke mixed charging. Received: January 16, 2013; Published online: September 4, 2013 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 84 (2013) No

12 Keywords: blast furnace; cohesive zone; computational fluid dynamics; discrete element method; ironmaking References [1] T. Miwa, H. Okuda, M. Osame, S. Watakabe, K. Saito, EECR Steel, Metec InSteelCon, 2011, Düsseldorf 2011, ECCR-58, DVD. [2] J. P. Birat, EECR Steel, Metec InSteelCon, 2011, Düsseldorf 2011, ECCR-91, DVD. [3] E. A. Mousa, A. Babich, D. Senk, ISIJ Int. 2012, 51, 350. [4] A. Murao, Y. Kashihara, S. Watakabe, M. Sato, ISIJ Int. 2011, 51, [5] S. Watakabe, K. Takada, H. Nishimura, S. Goto, N. Nishimura, T. Uchida, M. Kiguchi, Tetsu-to- Hagané 2006, 92, 901. [6] S. Matsuhashi, H. Kurosawa, S. Natsui, T. Kon, S. Ueda, R. Inoue, T. Ariyama, ISIJ Int. 2012, 52, [7] K. Suzuki, H. Watanabe, M. Hayashi, K. Nagata, CAMP-ISIJ 2012, 25, 587. [8] P. A. Cundall, O. D. L. Strack, Geotechnique 1979, 29, 47. [9] H. Kurosawa, S. Matsuhashi, S. Natsui, T. Kon, S. Ueda, R. Inoue, T. Ariyama, ISIJ Int. 2012, 52, [10] S. Natsui, S. Ueda, M. Oikawa, J. Kano, R. Inoue, T. Ariyama, ISIJ Int. 2009, 49, [11] S. Natsui, H. Nogami, S. Ueda, J. Kano, R. Inoue, T. Ariyama, ISIJ Int. 2011, 51, 41. [12] S. Ueda, T. Miki, T. Murakami, H. Nogami, T. Sato, Tetsu-to-Hagané 2013, 99, 675. [13] S. Natsui, S. Ueda, Z. Fan, J. Kano, R. Inoue, T. Ariyama, Tetsu-to-Hagané 2010, 96, steel research int. 84 (2013) No. 11 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim