Regular Article
Effects of Operation Parameters on Cohesive Zone in COREX Melter Gasifier
2016 Volume 56 Issue 9 Pages 1559-1562
Details
2016 Volume 56 Issue 9 Pages 1559-1562
The cohesive zone plays very important role in the operation of COREX melter gasifier. A two-dimensional 1/30 scale thermal dynamic model, in which paraffin and corn are used to simulate DRI, coke and lump coal respectively, has been established to study the cohesive zone of COREX melter gasifier. A set of different operation parameters, such as discharging rate (melting rate), DRI to lump coal and coke volume ratio, blast temperature as well as blast volume, were taken into account. The effects of these mentioned parameters on cohesive zone position and thickness in COREX melter gasifier have been analyzed in detail.
COREX melter gasifier is a complex reactor in which the gas-solid-liquid multiphase pyro reaction occurs,1,2,3) being divided into free board zone, packed bed, cohesive zone, semi-coke bed, raceway and deadman etc. The cohesive zone plays critical roles in ore melting, primary slag formation and secondary gas distribution etc. Unfortunately, there are fewer researches of cohesive zone in melter gasifier, so the study focuses on the experience of blast furnace (BF).
The cohesive zone has been studied extensively experimentally in BF. Generally, these studies can be classified into two kinds, including cold simulation and thermal simulation.
In cold simulation, the cohesive zone is almost all artificial predefined to study the gas distribution. Chen et al.4) established a two-dimensional cold model based on the BF of Ansteel to study the permeability index of burden, pressure distribution and gas flow pattern under different shape of cohesive zone. Wu et al.5) also established a 1/30 scale cold model to study the static pressure and pressure drop of BF under different cohesive zone shape. Hiroshi Takahashi et al.6) studied the unsteady behavior with bridging/slipping of solid bed in low reducing agents rate operation of BF using a two dimensional cold model. And they also simulated the effect of cohesive zone on unsteady behavior by setting a sand layer with lower gas permeability. In general, these models can be applied for specific cohesive zone, whereas it is difficult to provide different cohesive zone position and shape when the furnace operating condition changes.
In thermal simulation, the material with low melting point is used to simulate ore. When the hot blast is blown in, the qusi-ore will be melted to form the cohesive zone. IRITA Toshiyuki7) built a thermal dynamic model based on Muroran 4 BF, in which the paraffin was used to simulate ore. To observe the internal state, the front side of the apparatus was covered by two-layer heat-resistant glass, and the back side was equipped with 160 thermocouples and pressure sensors. The materials were charged by the movable armors and further discharged according to the set speed, and then the corresponding relations among burden distribution, heat flow ratio, blast volume and cohesive zone shape were obtained. Tamura8) built a two-dimensional 1/9 scale thermal model based on Sakai BF. The front side of the apparatus was also covered by heat-resistant glass to permit observation of the furnace state, and then the corresponding relations among blast temperature, blast volume, ore to coke ratio and cohesive zone were obtained. Fukushima T9) indicated that the cohesive zone shape would be changed through changing the distribution of ore to coke ratio using thermometric, manometry and resistance measuring methods.
At present, the DRI and coke mixed charging pattern is being employed in COREX melter gasifier, which is significantly different from BF and may cause its cohesive zone to have certain difference. Based on the above analysis, a thermal dynamic model, in which paraffin and corn are used to simulate DRI, coke and lump coal respectively, has been established to study the effects of discharging rate (melting rate), DRI to lump coal and coke volume ratio, blast temperature and volume on the cohesive zone position and thickness. As the experiment starts, hot blast is blown into the model through the tuyeres, and then the raceway is formed. With the hot blast ascending, the cohesive zone would be formed. This system can be used to performance the interaction among cohesive zone, semi coke bed and raceway. The author also finds that there is collapse phenomenon in raceway under some conditions, and it has certain effect on cohesive zone position and thickness.
A two-dimensional 1/30 scale thermal dynamic model of Baosteel’s COREX 3000 melter gasifier has been developed. Figure 1 shows a schematic diagram of the experimental apparatus, including the main apparatus, discharging system, hot blast supply system and digital image acquisition system.
Experimental apparatus. 1. Two-dimensional melter gasifier model, 2. Computer, 3. High-speed camera, 4. Tuyere, 5. Mechanical transmission mechanism, 6. Furnace bottom baffle, 7. Gas distributor, 8. Air heater, 9. Flowmeter, 10. Air blower.
The main apparatus are mainly made of stainless steel. Especially, the front side of the apparatus is made of heat-resistant glass to observe the descending and melting behaviors of quasi-coke and quasi-DRI. To enhance the heat preservation effect of the furnace, the display panel is covered by two-layer heat-resistant glass and where around the furnace is covered by thermal insulation material except for the display panel.
Discharging system at the hearth bottom mainly consists of two parts, mechanical transmission and furnace bottom baffle. The mechanical transmission can control the moving speed of the baffle, further adjust the discharging rate.
The hot blast supply system with the motor power 10 kW limits the blast temperature error to 0.1°C.
Digital image acquisition system consists of high-speed camera device, lighting equipment and image display equipment. Stereo light source made up by two 1 kW prefocus lamps is adopted in lighting process, which offers light from both sides of model symmetrically. The high-speed camera and the computer cover all the process of image acquisition.
2.2. Experimental MaterialsIn the experiment, the paraffin and corn are used to simulate DRI, coke and lump coal respectively. After the materials are mixed proportionally, the materials are charged into the model from the top to a certain height. With the hot blast blown, the paraffin becomes softening gradually, till the paraffin is melted into droplet falling into the hearth. Then they will be discharged from the furnace bottom continuously. The properties of materials are listed in Table 1.
| Items | Paraffin | Corn |
|---|---|---|
| Bulk density, kg·m−3 | 480 | 800 |
| Heat capacity, kJ·kg−1·K−1 | 3.2 | 1.735 |
| Melting points, °C | 58–60 | – |
| Latent heat of fusion, kJ·kg−1 | 101.5 | – |
| Diameter, mm | 3 | 3 |
The discharging rate is determined based on the practical melting rate; blast volume is determined by the corrected Froude number;10,11) blast temperature is determined based on the mass balance and heat balance. The experimental scheme is shown in Table 2.
| Items | Values |
|---|---|
| Packed bed height, cm | 27 |
| Burden temperature, °C | 20 |
| Discharging rate, L·h−1 | 2.2, 4.4, 6.6 |
| Paraffin to corn volume ratio, – | 0.5:1, 0.75:1, 1:1 |
| Blast temperature, °C | 80, 100, 120 |
| Blast volume, m3·min−1 | 10, 14, 18 |
The experimental procedures are as follows.
1) The bed is packed, and high-speed camera is properly located, according to the experimental scheme.
2) Gas flow is set according to the experimental scheme and diverted until the blast temperature reaches steady state.
3) The gas heater is set to the desired setpoint, and image acquisition of high-speed camera is commenced, when gas flow is introduced to the apparatus.
4) When the majority of paraffin has melted away, an apparent steady-state is achieved, and then the gas heater is stopped followed by the gas flow.
5) Image acquisition is stopped and the photos are saved for later analysis.
6) For the quench runs, the apparatus is allowed to cool to room temperature and then the bed is excavated to undertake next experiment.
7) The experiment photos are analyzed by using the method developed in previous literature.12)
During the experimental process, the author finds that most experiments reach stable state after 30 min. Fortunately, the collapse phenomenon in raceway is observed under some experimental conditions in the lab, and after a period of time the raceway will be formed again, presenting certain periodicity. In this paper, this phenomenon is called raceway collapse.
3.2. Effects of Operation Parameters on Cohesive Zone Position and Thickness 3.2.1. Effect of Discharging Rate on Cohesive Zone Position and ThicknessFigure 2 shows the effect of discharging rate on the cohesive zone position and thickness.12) When the discharging rate changes from 2.2 to 4.4 L/h, the cohesive zone position and thickness decrease with the increase of discharging rate. The reason is that the descending rate of solid materials increases with the increase of discharging rate, causing the amount of materials falling into cohesive zone increases, while the hot blast volume remains constant not to provide sufficient heat to melt more paraffin. When the discharging rate is 6.6 L/h, a raceway collapse was observed, causing the dramatic decrease of cohesive zone thickness. It is the reason that too large discharging rate results in the rapid renewal of each zone and the furnace would be in an unstable state. Just as the BF is, to stabilize BF operation,13) it is important to prevent the formation of a sluggishly descending zone and scaffolds on the inner wall of the furnace and make the dead man as small as possible. And at the low heat level, the scaffolds would be formed. Sometimes the scaffolds would suddenly fall off, causing the furnace in an unsteady state. Similarly, at too high heat level, the ore at the upper part burden layer would be earlier melted and even bonded together, causing the formation of low gas permeability layers to form the unsteady behavior with bridging/slipping at the upper part of furnace.
Effect of discharging rate on the cohesive zone position and thickness.
The discharging rate can be used to reflect the COREX melting rate. It states that the cohesive zone position and thickness decrease with the increase of COREX melting rate. The packed bed height increases with decreasing the cohesive zone position, causing the indirect reduction fully developed and improving the gas utilization and production efficiency. The decrease of the cohesive zone thickness can reduce the pressure drop in furnace. When the melting rate is too high, the collapse phenomenon that does harm to production process would happen.
3.2.2. Effect of DRI to Lump Coal and Coke Volume Ratio on Cohesive Zone Position and ThicknessFigure 3 shows the effect of DRI to lump coal and coke volume ratio on cohesive zone position and thickness. When the volume ratio changes from 0.5 to 0.75, the cohesive zone position and thickness increase with the increase of volume ratio. The reason is as follows. The amount of DRI increases with increasing DRI to lump coal and coke volume ratio, leading to the cohesive zone permeability to deteriorate and further to prolong the heat transfer process. As a result, the cohesive zone thickness increases. With the increase of volume ratio, the dripping zone becomes relatively inattentive, blown and dropped easily. However, its supporting role has not yet completely loosed and only the raceway expands easily, thus improves the heating position, leading to the increase of cohesive zone position. When the volume ratio changes from 0.75 to 1, the cohesive zone position decreases with the increase of volume ratio while the thickness increases. The reason is that the coke located in the dripping zone loses the supporting role with increasing volume ratio, causing the operation of the furnace to be in an unstable state especially when the volume ratio is 1. To be accompanied by a collapse phenomenon, the cohesive zone position decreases. Like the above analysis, the cohesive thickness increases with the increase of DRI content.
Effect of paraffin to corn volume ratio on the cohesive zone position and thickness.
It is shown that the cohesive zone position and thickness increase with increasing volume ratio under low volume ratio. The increase of the cohesive zone position and thickness may cause the gas utilization insufficiency and against the pressure drop. The collapse that does harm to production will happen when the volume ratio is too high. Consequently, the burden distribution should be adjusted to improve the stability of packed bed in actual operation.
3.2.3. Effect of Blast Temperature on Cohesive Zone Position and ThicknessFigure 4 shows the effect of blast temperature on cohesive zone position and thickness. When the blast temperature changes from 80 to 100°C, the cohesive zone position and thickness increase with increasing blast temperature. This is because the gas moving upward contains a lot of heat under high blast temperature, and more materials falling into cohesive zone can be melted. When the blast temperature changes from 100 to 120°C, the cohesive zone position and thickness decrease with the increase of blast temperature. The reason is that the furnace running state is in an unstable state and a collapse phenomenon was observed when blast temperature is 120°C.
Effect of blast temperature on the cohesive zone position and thickness.
It states that the cohesive zone position and thickness increase with increasing raceway gas temperature, but there is a critical temperature value based on the above analysis. When raceway gas temperature is too high, a collapse will appear. The raceway gas temperature can be reduced by injecting pulverized coal into the furnace or mixing certain cold nitrogen into pure oxygen. Thus, the collapse can be controlled and the furnace state will be in a stable state.
3.2.4. Effect of Blast Volume on Cohesive Zone Position and ThicknessFigure 5 shows the effect of blast volume on cohesive zone position and thickness. It is clearly seen that when the blast volume changes from 10 to 18 m3/h, the cohesive zone position and thickness increase. The reason is that the heat capacity increases with increasing blast volume. The more heat the hot blast carries, the more paraffin would be softened or melted. The frequent collapse phenomenon can be observed when the blast volume is 18 m3/h, causing the position slight increase from 14 to 18 m3/h.
Effect of blast volume on the cohesive zone position and thickness.
It states that the cohesive zone position and thickness increase with increasing raceway gas volume. If the gas volume is too large, a collapse will appear. It is important to ensure the tuyere to provide blast well-distributed and to avoid too large gas volume being produced by some tuyere.
In this paper, a 1/30 scale thermal dynamic model of COREX has been established based on similarity theory. Using this model, the effects of discharging rate (melting rate), paraffin to corn volume ratio (DRI to lump coal and coke volume ratio), blast temperature (raceway gas temperature) and blast volume (raceway gas volume) on cohesive zone position and thickness are analyzed, and the following knowledge is obtained.
(1) The cohesive zone position and thickness decrease with increasing discharging rate. The decrease of cohesive zone thickness can reduce the pressure drop in furnace. When the melting rate is too high, the collapse that does harm to production will happen.
(2) The cohesive zone position and thickness increase with increasing DRI to lump coal and coke volume ratio. When the volume ratio reaches a certain value, a collapse will happen.
(3) The cohesive zone position and thickness increase with the increase of blast temperature. When raceway gas temperature is too high, a collapse will appear. The raceway gas temperature can be reduced by injecting pulverized coal into the furnace or mixing certain nitrogen into pure oxygen. Thus, the collapse decreases and the furnace state will be in a stable state.
(4) The cohesive zone position and thickness increase with increasing blast volume. If the gas volume is too large, a collapse will appear. It is important to ensure the tuyere to provide blast well-distributed and to avoid too large gas volume being produced by some tuyere.
This project is supported by Scientific Research Fund of Hebei College of Industry and Technology, and yet supported by the Fundamental Research Funds for the Central Universities (N090402021).
