2014 Volume 54 Issue 6 Pages 1244-1250
In ironmaking high temperature processes, solid, liquid, and gas phases coexist and interact one another. The gas flow in a packed bed containing liquid and powder depends on their distributions, and the packing structure. Some parts of the liquid and powder phases passing through of the packed bed accumulate. Excessive accumulation may clog the gas flow in the packed bed. The flow and accumulations behaviors of powder and liquid phases coexisting in a packed bed have yet to be clarified.
Thus the phenomena of liquid and powder accumulation and clogging were experimentally investigated in the present study. The effect of the wettability and powder diameter on their accumulation was mainly studied. The results revealed that the wettability of packed material with liquid had significant effect on the accumulation of powder through the form of the liquid holdup.
Permeability in blast furnace is directly connected to the productivity, efficiency, and stability of the blast furnace process, and it should be controlled strictly. Molten slag and molten iron descend through the coke bed of the dripping zone in the blast furnace, and the powders formed by the degradation of coke or combustion of pulverized coal simultaneously this zone. The gas flow in the packed bed depends on the distributions of liquid and powder, and the packing structure of the bed.
The relation between the permeability of gas and the packed bed structure had been studied since the first half of the 20th century through several experiments. The results of these studies were summarized as Eq. (1) by Ergun.1) This equation is widely used for estimating the gas flow and the gas flow resistance in blast furnace simulators.
| (1) |
Even though the physical properties of the powder or liquid affect their accumulation and clogging in the vacancy of the packed bed, their effective values are determined by the packed bed structure and gas flow rate of the blast furnace process. The blast furnace process is a large-scale process, in which gas is injected into a moving bed of ore and coke particles over a height of 20 m. The behavior of liquid and powder phases in blast furnaces has been studied by many researchers. If the amount of powder (diameter: 5 mm or less) increases, the permeability decreases, and discontinuous phenomena such as hanging and abnormal descending of liquid and solid phases occur owing to clogging of the gas flow.3) In the lower part of the blast furnace where hot gas is injected at high velocity, an uneven flow or accumulation of liquid causes flooding in the packed bed. Drag force and stagnation of liquid phase also cause unsteady liquid flow and flooding.4) Flooding is a phenomenon that prevents normal descending of liquid and solid phases, and becomes a factor of instability during the process. The stagnant liquid in a packed bed is called holdup, and it is divided into static holdup which is stagnant one, and dynamic holdup which is moving. Static holdup is affected by the wettability between the liquid and packed bed material such as adhesional wetting4) and immersional wetting.5) A method for predicting flooding generation using the contact angle and the physical properties of the liquid is proposed. However, there is little data about the physical properties of slag in a blast furnace and the wettability between coke and melts. The wettability between ash and molten slag/iron is different from that between carbon and molten slag/iron. Moreover, ash is condensed on the surface of coke particle during the gasification reaction of carbon, namely the wettability changes. Therefore, it is difficult to evaluate the flooding phenomena using the existing measured values of wettability. The prediction of the flooding phenomena from the physical properties of the liquid in a blast furnace has not yet been done successfully.4,6)
Since the 1980s, pulverized coal has been used as a reducing agent in blast furnaces but the effect of powder in the lower part of the furnace on productivity has posed a problem. A solid shell layer formed by ash or char from the pulverized coal in the inner part of the raceway causes gas flow resistance.7) Analysis of the powder flow by fluid dynamics simulation has been proposed.8) In recent years, there has been an increase in research works to predict the generation of unburned char from the properties of ash in pulverized coal.9)
When the powder contacts the liquid phase, it may be entrapped on the surface of the liquid. The accumulation behavior of the powder is affected by the wettability between the powder or packed bed material and the liquid phase. However, little research has been conducted on the behavior of powder in a packed bed containing the liquid phase. This study investigates the effects of the wettability of the packed material and powder diameter on the accumulation of the liquid and powder in a packed bed.
In blast furnaces, gas, solid, liquid, and powder coexist in a region below the cohesive zone. In order to investigate the relation between the accumulation behaviors of liquid and powders, a cold model experiments were carried out to observe the powder flow. This study mainly focused on the effect of wettability on liquid and powder behaviors, thus the powder passed downward through a wet packed bed due to gravity in the experiments.
2.1. Experimental Apparatus and SpecimenThe experimental apparatus is shown in Fig. 1. A water-repellent cylindrical acrylic cylinder with an inner diameter of 90 mm was used as a container. A net of 3.6 meshes of φ0.5 mm wire was installed at the bottom of the cylinder to support packed particles (high-purity alumina balls of φ10 mm). Water-repellent treatment was carried out on the alumina balls using fluorinated water repellents. The balls were placed randomly, and a packed bed of 100 mm height was formed in the cylinder. The porosity of the packed bed was 0.45. In order to distribute the powder in the packed bed uniformly, a powder diffuser comprising glass balls of φ20 mm and the net was placed over the acrylic cylinder. A funnel with 30 g of the powder was placed over the diffuser. The lower end of the funnel was placed 10 mm above the center of the diffuser. Alumina balls of φ0.3, 0.5, and 1.0 mm with high and low wettability were used as the powder.
Experiment apparatus.
The wettability of alumina balls with ion-exchanged water is shown in Fig. 2. The contact angles of untreated and water-repellent alumina substrates are 78° (high wettability) and 108° (low wettability), respectively. Figure 3 shows alumina powder contacting water. For high-wettability powder, water penetrates between the powder particles, consequently, aggregating the powder. For low-wettability powder, wet powder particles cover the surface of the droplets.
Contact angle of water droplet on untreated or water repellent treated alumina substrates.
Wetting phenomena of untreated or water repellent treated alumina powder on water drop.
In order to prepare a wet packed bed, the packed bed was wetted with 10.0 g or 6.0 g of ion-exchanged water uniformly, and the average amounts of the water in the packed beds were 15.7 and 9.4 mg/cm3, respectively. Figure 4 shows the water distribution in the packed bed. In high-wettability packed bed, the liquid was distributed uniformly. A water layer covered the surface of packed particles, and water bridging between the particles was observed. On the other hand, in low-wettability packed bed, water droplets accumulated at the bottleneck of the packed bed. This indicated that the liquid was not uniformly distributed, and the surface of the packed material was generally dry except for the part holding the water droplets.
Appearance of wet packed bed of the alumina ball.
The effect of the powder diffuser was evaluated. After the diffuser was set below the funnel, the powder particles passed through it. The distribution of powder flow after passing through the diffuser is shown in Fig. 5. The vertical axis indicates the collection rate defined as;
| (2) |
Effect of the diffuser on distribution of passed powder.
No powder particles remained in the diffuser. Although the powder was fed only to the central part, the powder distributes fairly uniform over the top of the packed bed.
First, the certain amount of the water was fed to the packed bed to form the wet packed bed. The diffuser and the funnel were set above the packed bed. Then the pinchcock of the funnel was removed and the powder fell through the diffuser onto the packed bed. The powder flow inside the packed bed was observed using a high-speed camera. The frame speed was 400 fps. After certain time elapsed, the weights of the water and the powder flowed out from the packed bed were measured. Powder and water holdups were defined as the weight of the powder and water existing in the packed bed after the experiment. For comparison, the same procedure but using a dry packed bed that contains no water was carried out.
Powder and liquid holdups were observed from the outside of the acrylic cylinder. Figure 6 shows the states of powder and liquid holdups in the packed bed containing water of 15.7 mg/cm3, and diameter of powder particle is 0.5 mm. The wettability of the packed material and powder leads to the difference in powder accumulation. Schematics of the effect of wettability on powder accumulation and clogging are shown in Fig. 7. In the dry packed bed, less powder accumulation was observed compared to the wet packed beds.
Powder accumulation in wet packed bed.
Schematic sketch of powder clogging and accumulation in a packed bed which have different wettability.
In the case of high-wettability powder in high-wettability packed bed, the powder was wetted immediately on contact with the packed material because the packed material was covered with water. Powder particles with high wettability and low velocity are easily entrapped on the surface of the packed material at the contact points. Furthermore, the powder particles were entrapped to form multiple layers, and this would accelerate the powder accumulation.
Contrarily to the high-wettability packed bed, water exists as droplets in the low-wettability packed bed. In the case of high-wettability powder in the low-wettability packed bed, the powder particles were entrapped in the water droplets and aggregated with one another. The weight of water droplets increased with the increase in the entrapped powder, and the droplets drops downward from the initial position. Further, the powder particles assimilated by the liquid droplets formed powder clusters, and these clusters easily clogged the bottleneck part of flow paths in the packed bed.
In the case of low-wettability powder in the high-wettability packed bed, the powder adheres to the packed material surface owing to adhesion forces of the liquid. However, once the surface of the packed material was covered by a layer of the powder particles, no more powder adhered to the packed particles. Meanwhile, as the apparent diameter of the packed material grew larger owing to powder adhesion, the powder easily accumulated around the contact point between the packed particles. Additionally no aggregation behavior to form the cluster was observed.
In the case of low-wettability powder in the low-wettability packed bed, the powder did not assimilated in the droplets. The water droplet, however, moved due to the impact force and vibration of powder. Some droplets with powder particles on their surface were able to exist at the bottleneck stably, and these droplets closed the flow paths in the packed bed.
3.2. Effect of Powder Diameter on AccumulationFigure 8 shows the weights of powder holdup in the packed bed containing a water of 15.7 mg/cm3. The powder holdup ratio is defined as;
| (3) |
Influence of diameter of powder particle on holdup ratio.
For high-wettability powder, the holdup ratio increased with an increase in the powder diameter. In the high-wettability packed bed, most of the powder was entrapped in the packed bed. As shown in Fig. 7, multi-layer accumulation easily occurred in this condition. Meanwhile for the low-wettability packed bed, the holdup ratio was less than that of high-wettability packed bed since the frequency of the powder in contact with the liquid was lower than that in the high-wettability packed bed condition. Water dripping from the packed bed caused by powder flow was observed. In general, since the liquid adhesion force becomes large compared to the gravity as the particle diameter becomes smaller, the cohesive force between the powder particles in the cluster increases. However, large particles occlude easily the bottleneck of packed bed.10) It appears that this effect is evident in the cluster including the large particles.
In the case of low-wettability powder in the high-wettability packed bed, the holdup ratio of powder particles with 1.0 mm diameter was less than that of the powder particles with 0.5 mm or 0.3 mm diameter. Low-wettability powder particles adhered to the surface of the packed material; however, the adhesive force due to liquid was small. Therefore, the kinetic energy became larger with increase in the particle size. Therefore the holdup ratio decreased in the 1.0 mm case.
In the case of low-wettability powder in the low-wettability packed bed, the powder holdup ratio was highest for 0.5 mm particle size. In this case, since the obstruction factor of packed bed was the presence of water droplets, it is necessary to discuss its behavior.
The weights of water that have dropped from the packed bed till the steady state has been attained are shown in Fig. 9. In the case of low-wettability powder in the low-wettability packed bed, the amount of dropped water shows the least value for a powder diameter of 0.5 mm. As shown in Fig. 7, the water droplets in the low-wettability packed bed dropped owing to the impact force from the powder flow. The number of powder particles increased with the decrease in the particle diameters, for the same weight of powder. Thus the contact frequency between the droplets and powder increased with decreasing the particle size. On the other hand, the impact force of powder particle on the droplet increased with increasing the particle size. The amount of water dripping for the same weight of powder was determined by the balance between the contact frequency and the impact force. Clogging of the low-wettability packed bed was caused by the liquid droplets held at the bottlenecks of the packed bed; therefore, powder accumulation increased with the decreasing liquid dripping.
Influence of powder diameter on weight of liquid dropped from packed bed.
In the case of high-wettability powder in the low-wettability packed bed, the liquid dripping decreased with increasing the powder size. As shown in Fig. 7, high-wettability powder particles aggregated with the liquid. Larger particles easily formed larger powder cluster. Larger cluster is likely to clog the packed bed. In this case, the amount of liquid holdup is dependent on the amount of holdup of the cluster; therefore, the liquid dripping decreased with increasing powder size.
For the high-wettability packed bed, little liquid dripping was observed since the affinity was high.
3.3. Relationship between Liquid and Powder HoldupsThe effect of the amount of liquid in the packed bed on powder holdup is shown in Fig. 10. The powder particle diameter was 0.5 mm. In the case of the high-wettability packed bed, the surface of the packed material was wet; therefore, almost all the powder particles were entrapped. However, in the case of the low-wettability packed bed, water droplets existed at bottlenecks; therefore, the contact frequency between the liquid and powder was dependent on the amount of liquid. The powder holdup increased with increasing amount of water in the packed bed.
Holdup ratio of powder in wet packed bed.
As shown in Fig. 11, water dripping increased with increasing initial water holdup for low-wettability packed bed. Contrarily, the dripping water ratio with respect to the amount of initial water was almost the same for high-wettability packed bed. The volume of single water droplet was independent of the amount of water holdup; however, the number of droplets decreased with decreasing amounts of water holdup.
Influence of wettability on weight of liquid dropped from packed bed by powder flow.
The moving bed in the lower part of the blast furnace consists of coke, and liquid slag and molten iron descend into this bed. The powder formed by degradation of coke or unburned char of pulverized coal flow in the coke bed. Since the specific gravity of molten iron is high, and the wettability of molten iron with carbon is low, there would be little static holdup of molten iron. However, since the density of slag is as low as about 3000 kg/m3 and its viscosity is relatively high, the amount of molten slag holdup varies with respect to its wettability. Since the concentration of ash on coke surface increases with the gasification reaction or degradation of coke, it is necessary to take the wettability of coke ash with molten slag/iron. As coke ash consists mainly of oxides, the wettability of slag and ash would be high. If their wettability are high, the liquid phase uniformly distributes in the packed bed, even when the amount of liquid is less. As shown in Fig. 7, the behavior of powder significantly varies with the changes in the liquid phase distribution. In the lower part of the blast furnace, powder would easily accumulate and clog the moving bed owing to liquid holdup.
A solid layer called “the bird’s nest” is formed around the raceway, lowering the permeability.7) Unburned char and ash from the injected pulverized coal, and dripped liquid droplets coexist in the coke bed around the raceway. The results of present experiment show powder–liquid phase interaction in the packed layer. From these results, it can be said that the powder–liquid phase interaction affects the formation of bird’s nest.
When considering the gas permeability in a furnace, it is important to analyze the behaviors of powder and liquid as well as the interaction between them.
In order to understand the behaviors of liquid or powder in a packed bed, the phenomena of accumulation and clogging were experimentally investigated in the present study. The liquid and powder behaviors were discussed from the view point of wettability among the materials. The following conclusions were obtained.
(1) The wettability of a packed bed affects the distribution of the liquid phase.
(2) The behavior of clogging and the accumulation of powder vary with respect to the amount of liquid phase and the wettability of packed material. Especially, the wettability of packed bed particles affects the accumulation of powder significantly.
A portion of this research was carried out with an ISIJ research promotion grant. Authors wish to express deep appreciation for this generous support.
