2015 Volume 55 Issue 9 Pages 1866-1875
The importance of energy saving in the ironmaking process is widely recognized. Many energy saving efforts related to ironmaking have already been carried out, and further energy savings by conventional methods are hardly to be expected. The oxygen blast furnace is considered to be a promising process in terms of flexibility of energy use and advantages related to CO2 mitigation. Focusing on energy saving, in this study, the optimum configuration of the ironmaking process based on the oxygen blast furnace was investigated by numerical approaches and case studies.
First, because productivity can be greatly improved in the oxygen blast furnace, blast furnace inner volume can be reduced while maintaining the same production rate. Because the downsized oxygen blast furnace makes it possible to relax burden material strength requirements, energy consumption for agglomeration in the coke oven and sintering machine can also be reduced. Therefore, a DEM simulation was carried out to confirm the effect of the burden load reduction in the downsized condition. It was found that the compressive stress in the downsized oxygen blast furnace was 20–30% less than that in the conventional blast furnace. The energy flow in the ironmaking process was also investigated by using a material and energy balance model, considering the functions of an integrated steel works. It was found that the energy consumption of the ironmaking process based on the energy saving oxygen blast furnace could be reduced by 5.3% while maintaining the same energy supply to downstream processes.
Energy saving in steel works has become a serious subject from the viewpoints of societal responsibility and environmental issues in the steel industry. In particular, demands for energy saving in ironmaking processes are increasing, as large amounts of coal and other fossil fuels are consumed to reduce iron oxide and produce pig iron. Technical developments over the past decades, such as high pressure operation, hot blast operation, injection of various kinds of reducing agents through the blast furnace tuyeres and precise burden distribution control, have contributed to decreased energy consumption and improved energy efficiency. However, conventional energy saving in blast furnaces is reaching its limit, and further large reductions in energy consumption are hardly to be expected.
Looking at the overall ironmaking process, iron ore and coal are agglomerated by the sintering machine and coke oven to produce sinter and coke, respectively, so as to secure the burden properties required in the blast furnace. On the other hand, the inner volume of blast furnaces has gradually been enlarged in recent years, requiring higher burden properties. For example, in Japan, half of all operating blast furnaces are large blast furnaces with inner volumes exceeding 5000 m3, and a blast furnace with an inner volume of 6000 m3 has been constructed in Korea. Historically, blast furnaces have been enlarged in the radial direction while keeping the height constant in order to avoid increased pressure drop. However, with the upscaling of blast furnaces, nonuniformity in the radial direction tends to become more pronounced, as only the diameter is enlarged. The large blast furnace offers excellent production efficiency, however high strength, high quality burden materials are required to suppress this nonuniformity in the radial direction and secure stable operation. This raises energy-related issues, as increasing the strength of coke and sinter requires more agglomeration energy in the carbonization and sintering processes. Moreover, with the depletion of high quality natural resources, the quality of iron ore and coal has deteriorated in recent years. This change in resource quality also increases the agglomeration energy required to produce high quality burden materials and suggests that it will be difficult to supply the high strength burden materials necessary for current large blast furnaces in the future. Thus, the ironmaking process based on the large blast furnace must be reviewed from the viewpoints of both energy and natural resource issues.
The oxygen blast furnace (OBF) process is a unique process which uses pure oxygen instead of hot blast. It was proposed by JFE Steel Corporation (former NKK) in the 1980 s.1,2) Some campaigns were carried out with an experimental blast furnace, and its features were verified. Pure oxygen injection into the tuyeres and injection of preheat gas were applied in the experimental blast furnace, and finally operation with high productivity under a high pulverized coal rate was successfully achieved.1) It was found that the oxygen blast furnace makes it possible to double productivity owing to operation in a nitrogen-free condition compared with the conventional blast furnace. Yamaoka studied the effectiveness of the oxygen blast furnace using a 1-dimensional numerical simulation and experiments with the experimental blast furnace.3,4) Murai proposed an expanded concept of the oxygen blast furnace aiming at CO2 mitigation.5) In addition to the previous oxygen blast furnace, the application of waste plastics injection and top gas recycling were studied, and a numerical simulation confirmed that CO2 emissions could be remarkably reduced compared with the conventional blast furnace. A similar type of oxygen blast furnace was proposed in the ULCOS (Ultra Low CO2 Steelmaking) project in Europe. In the proposed concept of the ULCOS-TGR (Top Gas Recycling) BF,6) the CO2 in the blast furnace top gas is captured and stored in a geological trap (CCS: Carbon Capture and Storage), and the remainder of the top gas is reinjected into the blast furnace as an additional reducing gas. The effect of CO2 mitigation in the ULCOS-TGR BF was verified using the experimental blast furnace in LKAB.6) Arasto studied the effect of CO2 mitigation by the oxygen blast furnace with TGR and CCS using a material and energy balance model of an integrated steel works and clarified the effect of CO2 mitigration.7) The concepts of the oxygen blast furnace proposed by Murai and the ULCOS-TGR BF are clearly effective for mitigation of CO2 emissions from the ironmaking process, however may cause a shortage of energy supply to the downstream processes. Saru studied the CO2 emission and available downstream energy of the oxygen blast furnace with TGR and CCS using a material and energy balance model, and reported that the available downstream energy decreased as recycling gas volume increased.8) Because this means that additional sources of energy, for example, natural gas or electric power, are required in the downstream processes, it is not certain that these processes are suitable for energy saving in the integrated steel works in the future. Therefore, the favorable configuration of the oxygen blast furnace considering the actual energy balance in the integrated steel works must be reconsidered from the viewpoint of energy saving in the overall steel works.
In this study, the favorable configuration of the ironmaking process based on a newly proposed oxygen blast furnace was investigated aiming at minimizing the energy consumption of the integrated steel works. First, the features of the oxygen blast furnace were reviewed. Then, the concept of an ironmaking process based on the improved oxygen blast furnace aiming at energy saving was suggested, while paying close attention to energy balance in the integrated steel works. As described above, the productivity of the oxygen blast furnace is much improved in comparison with the conventional blast furnace. This indicates the possibility of downsizing the blast furnace while maintaining the same production rate. Therefore, the effect of burden load reduction in an oxygen blast furnace with a reduced inner volume, which is one of the key elements for energy saving, was examined by a DEM (Discrete Element Method) simulation. The energy consumption of the ironmaking process was then evaluated by using a material and energy balance model of the ironmaking process. Finally, the changes in the operation of the energy saving type of oxygen blast furnace were investigated, and favorable operating conditions were newly proposed.
The oxygen blast furnace is characterized by injection of pure oxygen gas into the tuyeres instead of hot blast. The oxygen blast furnace has several features. Figure 1 shows a typical example on the comparison of an oxygen blast furnace with a conventional blast furnace. First, the oxygen blast furnace can be operated with higher productivity. In the oxygen blast furnace, the concentration of reducing gases such as CO and H2 increases and the bosh gas volume decreases owing to operation under a nitrogen-free condition. In general, the productivity of blast furnaces is limited by the flooding of slag or the reduction rate of ore. The productivity of the oxygen blast furnace is almost double that of the conventional blast furnace because the limitations of the ore reduction rate and slag flooding are loosened. Ohno reported that the maximum productivity of 5.1 t/dm3 was achieved in a campaign with the experimental blast furnace.1) The direct reduction ratio in the oxygen blast furnace is suppressed and hidrogen reduction is enhanced due to the increase of the hydrogen content derived from intensified injectants including hydrogen. Therefore, the solution loss reaction in the blast furnace can be reduced. This is effective for suppressing coke degradation.
Typical comparison of oxygen blast furnace and conventional blast furnace.
Second, a large quantity of pulverized coal can be injected into the tuyeres, and simultaneously with this, the coke rate can be reduced. Since the combustion efficiency of the oxygen blast furnace is higher due to pure oxygen combustion, the oxygen blast furnace can be operated at a higher pulverized coal rate and lower coke rate. The maximum pulverized coal rate of 320 kg/t was achieved with the experimental blast furnace.1)
Third, the oxygen blast furnace can produce blast furnace gas (BFG) with a higher calorific value than the conventional blast furnace. The comparison of the composition and calorific value of the BFG is shown in Fig. 1. The calorific values of BFG in the conventional blast furnace and oxygen blast furnace are 3.0 MJ/Nm3 and 6.4 MJ/Nm3, respectively. High calorie BFG can be utilized effectively in other processes such as the power plant or as a chemical resource.
Conversely, the oxygen blast furnace also has some inherent limitations. Due to the decreased bosh gas volume, the heating potential of the solid materials in the shaft region is lower than in the conventional blast furnace,2) and to compensate for the insufficient heat supply in the shaft region, injection of preheat gas in the upper shaft is required. The effectiveness of preheat gas injection in heating up the materials in the upper shaft region was confirmed with the experimental blast furnace. Appropriate control of the flame can also be pointed out. In the oxygen blast furnace, an injectant such as pulverized coal is helpful for controlling the flame temperature in the raceway. However, some additional injection of top gas is required in order to control the flame temperature, since the pulverized coal injection rate is ultimately limited by the combustion efficiency of the coal. The injection of cold top gas does not contribute to a reduced coke rate, but rather, it can increase the reducing agent and energy consumption in the ironmaking process.
2.2. Basic Concept of Energy Saving Oxygen Blast FurnaceThe basic concept of the energy saving oxygen blast furnace can be represented as shown in Fig. 2. The energy saving oxygen blast furnace has been positively improved as follows in order to intensify energy saving compared with the previous oxygen blast furnace.
Progressive development of oxygen blast furnace to energy saving.
Focusing on the high productivity feature of the oxygen blast furnace, the inner volume can be reduced while maintaining the same pig iron production rate. For example, assuming the productivity of the oxygen blast furnace is doubled, this means that the inner volume of the blast furnace can be reduced to one-half. Besides the decrease in the direct reduction ratio, the downsized profile is thought to be effective for reducing the burden load from the upper furnace and the weakening of uniformity in the radial direction, which means that the requirement of high physical strength of the burden material can be relaxed. This leads to a reduction in consumption of agglomeration energy in the sintering machine and coke oven. Moreover, the shaft efficiency in an oxygen blast furnace with a downsized profile is improved by suppression of gas channeling in the reduced diameter of the blast furnace. Of course, the heat loss of the blast furnace can also be reduced at the high production rate.
The focus of the previous oxygen blast furnace was massive injection of pulverized coal and production of excess energy from the economic viewpoint. However, while also keeping the advantage of high productivity, energy saving has become a priority from the viewpoint of environmental and resource issues. Currently, hydrogen-rich injectants such as natural gas are available even in the steel works in Japan.9) For CO2 mitigation and controlling the flame temperature by decomposition heat in the raceway, natural gas is an attractive injectant in place of pulverized coal. Moreover, controlled co-injection of pulverized coal and natural gas rather than massive injection of pulverized coal preferably leads to a reduction in the specific oxygen gas volume owing to natural gas properties derived from decomposition heat. In this case, the injection of cold top gas into the tuyere can be avoided. The decrease of the oxygen requirement is directly connected with energy consumption through the oxygen plant.
These characteristics can be represented in the Rist diagram shown in Fig. 3. Compared with the operating line of the conventional blast furnace, in the oxygen blast furnace, Point E in Fig. 3 moves to point E’ due to the oxygen requirement, which is related to increased oxygen consumption for cold oxygen injection. If the pulverized coal injection rate is increased, Point E’ shifts even further downward. Although this means the direct reduction ratio decreases, the reducing agent increases. This was the direction of the previous oxygen blast furnace, however in this case, the input energy to the ironmaking process increases. Although the remarkable decrease of the coke rate has some benefits in terms of the steel production cost, this situation is not suitable from the viewpoint of energy saving. Since the energy saving oxygen blast furnace uses controlled co-injection of pulverized coal and natural gas, the operating line in Fig. 3 moves slightly upward. Then, as mentioned above, shaft efficiency improves thanks to the downsized blast furnace, and as a result, the gas utilization ratio increases.
Representation of energy saving oxygen blast furnace in Rist diagram.
Totally, the energy consumption in the ironmaking process, including the sintering machine and coke oven, can be improved thanks to the features of the newly-designed oxygen blast furnace.
Some previous research examined the relationship between changes in the profile of the blast furnace and the burden load. The influence of inner volume on the burden load in blast furnaces was studied by numerical simulation based on the elastic-plastic theory proposed by Inada.10) It was shown that vertical stress at the tuyere level increases as the inner volume of a blast furnace increases. On the other hand, the simulated results by DEM described in later studies showed that the stress distribution in blast furnaces does not change gradually. The packed bed in the blast furnace is supported by a particle network consisting of particles receiving large stress.11) Fan studied the effect of the diameter of a blast furnace on stress distribution, considering a layered bed structure calculated by DEM.12) In previous studies, the coke rate of the conventional blast furnace was set to 350–400 kg/thm, while the coke rate of the oxygen blast furnace reached approximately 200 kg. To confirm the effect of burden load reduction in the downsized oxygen blast furnace, DEM simulation was carried out, considering the low coke rate condition. The stress distributions of isotropic shaped blast furnaces with inner volumes of 5000 m3 (conventional blast furnace) and 2500 m3 (downsized oxygen blast furnace) were calculated. The low coke rate conditions are expressed by the average bulk density reflecting the high ore-to-coke ratio.
The calculation conditions are shown in Fig. 4 and Table 1. The hypothetical profile based on the actual blast furnace was taken for a downsized oxygen blast furnace with an inner volume one-half that of the actual furnace. To reduce the calculation time, a 1/12 symmetrical sector model was used, as shown in Fig. 4. Here, slip wall conditions at the symmetry planes were applied. Coke and ore were expressed by one spherical particle, where the particle diameter was enlarged. The particle density was determined by the average bulk density of the burden materials in each case. The particles were charged from the top of the blast furnaces and discharged at the raceway. In this study, the raceway depth of the oxygen blast furnace was determined to be the same size as the conventional blast furnace. The bosh gas volume of the oxygen blast furnace is about 60% of the conventional blast furnace, however it is possible to control the raceway depth by adjusting the tuyere diameter.
Blast furnace profiles of two cases studied by DEM simulation model. (a) Conventional blast furnace (b) Downsized oxygen blast furnace.
| Conventional BF | Downsized OBF | |
|---|---|---|
| Inner volume | 5000 m3 | 2500 m3 |
| Particle diameter Dp | 0.2 m | 0.2 m |
| Particle density rS | 2400 kg/m3 | 2800 kg/m3 |
| Liquid density rL | 6700 kg/m3 | 6700 kg/m3 |
| Particle number N | 65000 | 40000 |
| Poisson’s ratio n | 0.2 | 0.2 |
| Restitution coefficient e | 0.46 | 0.46 |
| Sliding friction coefficient ms | 0.7 | 0.7 |
| Rolling friction coefficient mr | 0.12 | 0.12 |
| Normal stiffness kn | 4.0×107 | 4.0×107 |
| Shear stiffness kt | kn/[2(1+n)] | kn/[2(1+n)] |
| Time step dt | 10−4 s | 10−4 s |
| Discharging rate at raceway | 200 particles/s | 150 particles/s |
The calculation procedure of the DEM simulation model was the same as in the previous research by the authors.11) After the calculation, the normal components of the contact force vectors were summed up and divided by the surface area of each particle. The results are defined as compressive stress.
3.2. Burden Load Calculated by DEM in Downsized Blast FurnaceThe compressive stress distributions of the conventional blast furnace and the downsized oxygen blast furnace are shown in Fig. 5, and the longitudinal distributions of the compressive stress at the center region are shown in Fig. 6. In each case, a stress network is observed in the blast furnace, as shown in Fig. 5. The compressive stress on the particles varies widely, and most of the particles (about 90%) receive small compressive stress of less than 1 MPa, as shown Fig. 6. On the other hand, a small number of particles in the network structure received huge compressive stress. In particular, particles at the center of the bosh and hearth received large compressive stress.
Compressive stress distribution in blast furnace. (a) Conventional blast furnace (b) Downsized oxygen blast furnace.
Longitudinal distributions of compressive stress in central region of blast furnaces.
Maximum stress has a strong effect on the degradation of burden materials. Therefore, attention was focused on the maximum stress in the simulation results. The maximum values of compressive stress were extracted from the results in Fig. 6. The longitudinal distributions of the maximum compressive stress are compared in Fig. 7. The illustration at the right side of Fig. 7 shows the outline of the conventional blast furnace and the downsized oxygen blast furnace on the same scale. Large values of maximum compressive stress were observed at a level slightly higher than the tuyeres in both cases. In that region, a contracted particle flow is formed in the deadman and near the wall because of the material discharge at the raceway, and this condition leads to the large compressive stress on the deadman region. The maximum compressive stress in the oxygen blast furnace with the downsized profile is about 20–30% smaller than that in the conventional blast furnace when the peak values of the distribution of the maximum compressive stress in both cases are compared. As a result, it was confirmed that the oxygen blast furnace with the reduced height has the potential to reduce the burden load.
Longitudinal distributions of maximum compressive stress in blast furnaces.
In this study, the material and energy balance model proposed in the previous study was applied in order to evaluate the energy consumption and flows in the ironmaking process.13) Figure 8 shows the structure of the material flow and energy flow model. The blast furnace model is based on a Rist diagram, which is commonly used for estimating actual blast furnace operation. The material and energy balance model includes all the processes in the total ironmaking process, such as the coke oven, sintering machine, electric power plant and cryogenic plant for oxygen production. The flow of materials, such as iron, carbon, oxygen and produced gases of BFG and COG, and the flow of energy, such as sensible heat and chemical enthalpy, between the processes in the ironmaking process are considered. This model makes it possible to evaluate the net inputs and outputs of energy based on the energy flow in the ironmaking process.
Schematic image of material flow and energy flow considered in material and energy balance model.
The energy consumption of ironmaking processes with various types of blast furnaces was evaluated in terms of “input energy,” “energy supply to downstream processes” and “energy consumption.” Input energy implies the sum of the enthalpy of the coking coal, pulverized coal, natural gas and other raw materials consumed in the coke oven, sintering machine and blast furnace. Energy supply to downstream processes is defined by subtracting the energy of the BFG and COG consumption in the ironmaking process from the total enthalpy of the gases produced by the coke oven and the blast furnace. Energy consumption is calculated by subtracting the energy supply to downstream processes from the input energy, which means the net energy consumption of the ironmaking process.
4.2. Simulation Conditions for Evaluating Energy Consumption in Ironmaking ProcessesConsidering the energy saving of an ironmaking process as described above, the favorable configuration of the oxygen blast furnace and surrounding processes was examined, assuming a process that takes full advantage of the features of the oxygen blast furnace. In this section, the concrete concept of energy saving in an ironmaking process based on the oxygen blast furnace is presented.
The directions for energy saving in the ironmaking process based on the oxygen blast furnace consist of various energy saving factors, as shown in Fig. 9. As mentioned in Chapter 3, the burden load in the oxygen blast furnace with the downsized profile is lower than that in the conventional blast furnace owing to the reduced height of the downsized furnace. Direct reduction decreases due to the large hydrogen content derived from injectants incuding hydorogen in the energy saving oxygen blast furnace, which leads to the suppression of the solution-loss reaction in the blast furnace. These features related to the burden make it possible to use lower strength burden materials. At the coke oven, allowing production of lower strength coke contributes to energy saving in coke-making. Conventionally, several methods such as flue temperature control or preheating of the coal have contributed to energy saving in the coke oven.14) Recently, medium temperature carbonization attracted attention as a technique for reducing carbonization energy.15) Although medium temperature carbonization is surely effective for reducing energy consumption in the coke oven, the coke product tends to have low strength and high reactivity characteristics, and thus is not suitable for the conventional blast furnace without additional treatment.16) On the other hand, the oxygen blast furnace with the downsized profile can possibly accept these coke products due to the decrease in coke degradation resulting from suppression of the solution-loss reaction and reduction of the burden load in the blast furnace. Therefore, by using the above method for reducing carbonization heat in combination with the oxygen blast furnace, a further energy saving in the coke oven can also be expected to some extent. In addition, it is also possible to apply high reactivity coke in the improved oxygen blast furnace to lower the thermal reserve zone (TRZ) temperature in the blast furnace, which is effective for improving gas utilization efficiency.
Energy saving factors in ironmaking process based on energy saving oxygen blast furnace.
Energy saving in the sintering process can also be expected by using lower strength materials. The fine coke consumption in a sintering machine affects the strength of the sinter product. Figure 10 shows the relationship between fine coke consumption for sintering and sinter strength in an actual sintering machine. It has been found that the tumbler index of sinter decreases by 0.4% when fine coke for sintering is reduced by 1 kg/t-sinter. It is estimated that a tumbler index decrease of 2% corresponds a fine coke reduction of 10%. This means that the relaxation of the strength limitation for burden materials is helpful for reducing agglomeration energy consumption in the sintering machine. For the same reason, the ratio of lump ore, which is lower in strength than sinter, can be increased.17) Accordingly, a further decrease in the energy consumption in the sintering machine can be expected.
Relationship between fine coke consumption for sintering and sinter strength.
Moreover, the heat loss of the downsized oxygen blast furnace is lower than that of the conventional blast furnace owing to the high productivity and small size of the furnace. The shaft efficiency of the downsized oxygen blast furnace is improved because gas channeling is largely suppressed by the small diameter.
The oxygen blast furnace produces high calorie BFG, which increases the efficiency of electric power generation. Therefore, a comparable energy saving effect can be expected in the power plant by using high calorie BFG from an oxygen blast furnace. Nakagawa reported similar results regarding the increase in power plant efficiency by oxygen enrichment.18,19)
Summarizing the concepts described above, the operating conditions of ironmaking processes based on the conventional blast furnace and the oxygen blast furnaces in this study are shown in Table 2. Also, some calculated results are shown in Table 2. In Table 2, the first case is the ironmaking process based on a typical conventional blast furnace as a reference, where the coke rate is 345 kg/thm and the pulverized coal rate is 150 kg/thm. The second case is the previous type of oxygen blast furnace, which was studied in JFE Steel1,5) in the past and was characterized by massive pulverized coal injection into the tuyere at the rate of 300 kg/thm. In this case, BFG is also injected into the tuyere to some extent to control the flame temperature in the raceway, and preheat gas injection into the shaft region is applied. The last 3 cases are the energy saving oxygen blast furnaces suggested in this paper. Compared with the previous oxygen blast furnace, the pulverized injection rate is reduced moderately and natural gas is preferably injected into the tuyere. Use of natural gas is extremely effective for controlling the flame temperature in place of BFG. All the energy saving factors adopted in these energy saving oxygen blast furnaces are shown in Fig. 9. To progressively clarify the effect of relaxing material strength limitations, which is possible owing to the burden load reduction, three cases with different energy saving ratio conditions were considered. Here, “energy saving ratio derived from material strength limitation” means the total energy saving ratio consisting of the ratio of decrease of energy consumption in the coke oven and the sintering machine and the increase in the lump ore ratio in the ore burden made possible by relaxation of burden strength requirements. In this study, three energy saving ratios were investigated, i.e., 0%, 5% and 10%. The flame temperature in the raceway in the oxygen blast furnace was adjusted to a relatively high level compared with the conventional blast furnace due to the use of pure oxygen. The top gas temperature was adjusted between 150°C and 200°C to avoid condensation of water related to the dew point.
| Conventional BF | Previous OBF | Energy saving OBF | ||||||
|---|---|---|---|---|---|---|---|---|
| Energy saving ratio derived from material strength relaxation | ||||||||
| 0% | −5% | −10% | ||||||
| Sintering Machine | Sinering Energy | kg-C/t-Sr | 50.0 | 50.0 | 50.0 | 47.5 | 45.0 | |
| MJ/thm | 1841 | 1841 | 1841 | 1631 | 1434 | |||
| Coke Oven | Carbonization Energy | MJ/t-coal | 2469 | 2469 | 2469 | 2343 | 2218 | |
| MJ/thm | 1440 | 1440 | 1051 | 965 | 883 | |||
| Blast Furnace | Shaft Efficiency | – | 0.94 | 0.94 | 0.98 | 0.98 | 0.98 | |
| TRZ Temp. | °C | 1000 | 1000 | 950 | 950 | 950 | ||
| Heat Loss | MJ/thm | 418 | 251 | 251 | 251 | 251 | ||
| Ore | Sinter | % | 80 | 80 | 80 | 75 | 70 | |
| Lump Ore | % | 20 | 20 | 20 | 25 | 30 | ||
| Total Mass | kg/thm | 1593 | 1593 | 1593 | 1583 | 1572 | ||
| Reducing Agent | PCR | kg/thm | 150 | 300 | 229 | 234 | 238 | |
| NG | kg/thm | 0 | 0 | 47 | 44 | 42 | ||
| CR | kg/thm | 345 | 250 | 221 | 219 | 216 | ||
| RAR | kg/thm | 495 | 550 | 497 | 497 | 496 | ||
| Blast | Temp. | °C | 1150 | 25 | 25 | 25 | 25 | |
| Air | Nm3/thm | 1038 | 0 | 0 | 0 | 0 | ||
| Oxygen | Nm3/thm | 27 | 300 | 285 | 283 | 282 | ||
| Moisture | g/Nm3 | 15 | 2 | 2 | 2 | 2 | ||
| Tuyere Gas Injection | Volume | Nm3/thm | 0 | 49 | 0 | 0 | 0 | |
| Temp. | °C | 25 | 25 | 25 | 25 | 25 | ||
| Preheat Gas Injection | Volume | Nm3/thm | 0 | 294 | 250 | 249 | 246 | |
| Temp. | °C | – | 1000 | 1000 | 1000 | 1000 | ||
| Bosh Gas | Volume | Nm3/thm | 1430 | 836 | 818 | 810 | 804 | |
| Flame Temp. | °C | 2250 | 2698 | 2587 | 2596 | 2595 | ||
| BFG | Volume (wet) | WNm3/thm | 1619 | 1173 | 1044 | 1037 | 1030 | |
| Temp. | °C | 159 | 151 | 150 | 150 | 150 | ||
| Calorific Value | MJ/WNm3 | 3.00 | 6.40 | 5.68 | 5.67 | 5.66 | ||
| Gas Utilization Efficiency | % | 51.1 | 48.4 | 54.1 | 54.2 | 54.3 | ||
| Reduction | Indirect (CO) | % | 63 | 69 | 65 | 65 | 65 | |
| Direct | % | 28 | 19 | 14 | 14 | 15 | ||
| Indirect (H2) | % | 9 | 12 | 21 | 21 | 21 | ||
| Power Plant | Efficiency | – | 0.350 | 0.350 | 0.385 | 0.385 | 0.385 | |
| Oxygen Plant | Total Oxygen Consumption | Nm3/thm | 27 | 326 | 308 | 306 | 305 | |
| Energy Requirement | kWh/Nm3 | 0.43 | 0.43 | 0.43 | 0.43 | 0.43 | ||
| MJ/thm | 42 | 505 | 477 | 474 | 472 | |||
The energy flow in the ironmaking processes are illustrated in Figs. 11, 12 and 13. The total input energy, energy consumption and energy supply are shown in Fig. 14. The energy flow in the ironmaking process based on the conventional blast furnace is shown in Fig. 11. The input energies derived from coking coal and pulverized coal are 16.86 GJ/thm and 4.74 GJ/thm, respectively, and input energy derived from other raw materials such as ore and coal for sintering machine is 0.51 GJ. That is, the total input energy is 22.11 GJ/thm. At the same time, mixed gas of BFG and COG (M gas) is generated from the coke oven and blast furnace at the rate of 8.64 GJ/thm. Of this, 3.98 GJ/thm of the M gas is consumed in the ironmaking process, and the remainder of the M gas, 4.66 GJ/thm, is supplied to downstream processes.
Energy flow in ironmaking process based on conventional blast furnace.
Energy flow in ironmaking process based on previous oxygen blast furnace.
Energy flow in ironmaking process based on energy saving oxygen blast furnace.
Comparison of input energy, energy consumption and energy supply to downstream processes of ironmaking processes.
The energy flow in the ironmaking process based on the previous oxygen blast furnace is shown in Fig. 12. This case is characterized by massive injection of pulverized coal into the blast furnace. As a result, the input energy derived from coking coal decreases remarkably. Apparently, for energy saving, it is preferable to replace coking coal with pulverized coal as far as possible because less carbonization heat is required. However, the results with the previous oxygen blast furnace tended to deviate from energy saving in blast furnace. The energy required in the blast furnace is 19.97 GJ/thm, which is significantly larger than that in the conventional blast furnace, 17.28 GJ/thm. This was mainly due to the increased oxygen requirement and massive pulverized coal injection, as shown in the movement of Point E in the Rist diagram of Fig. 3. Due to the injection of cold oxygen and massive pulverized coal injection, the reducing agent and oxygen consumption increase in spite of the lower coke rate. In particular, energy consumption for oxygen generation had a large effect on the increase in input energy. Moreover, the tuyere gas injection of BFG to control the flame temperature had a negative effect on energy consumption in the blast furnace. From the economic viewpoint, the previous oxygen blast furnace is favorable for suppressing the coke rate and increasing BFG generation, however has a negative effect on energy saving. Accordingly, as shown in Fig. 14, the total input energy is 22.87 GJ/thm, which is 3.4% larger than that in the conventional blast furnace.
To achieve energy saving in the ironmaking process, it is necessary to adjust the operation of the oxygen blast furnace and to promote energy saving taking advantage of the features of the oxygen blast furnace. The energy flow in the ironmaking process based on the energy saving oxygen blast furnace is shown in Fig. 13, where the energy saving ratio derived from relaxation of the material strength limitation is 10%. Improvements in blast furnace operation, such as the increase in shaft efficiency and lowering of the TRZ temperature by high reactivity coke, are also included. The input energies derived from the coking coal, pulverized coal, natural gas, and other raw materials are 10.91 GJ/thm, 7.52 GJ/thm, 2.29 GJ/thm and 0.46 GJ/thm, respectively. As a result, the total input energy is 21.18 GJ/thm, which is 7.4% less than the previous oxygen blast furnace and 4.2% less than the conventional blast furnace. In the energy saving oxygen blast furnace, natural gas is injected to control the flame temperature in the raceway instead of the tuyere gas injection of BFG used in the previous oxygen blast furnace. Natural gas injection is advantageous for a high replacement ratio of coke owing to its high calorific value and has a cooling effect owing to decomposition heat in the raceway. The energy inflow to the blast furnace is 18.55 GJ/thm, which is less than in the previous oxygen blast furnace. As an additional merit, further energy savings can be achieved in other processes such as the coke oven, sintering machine and electric power plant. M gas consumption in the coke oven, fine coke consumption in the sintering machine and M gas consumption in the power plant were reduced by 10% in this study. The effect of these total energy savings including the other processes is reflected in the reduction of coking coal consumption and pulverized coal consumption through the reduction of M gas consumption.
The input energy, energy consumption and energy supply to downstream processes are summarized for all the cases in Fig. 14. In the energy saving oxygen blast furnace, the figure shows the three cases with different energy saving ratio derived from burden strength relaxation, i.e., 0%, 5% and 10%. The bar at the left of each case in Fig. 14 means the input energy, which is the sum of the energy of coking coal, pulverized coal, natural gas and other raw materials. The bar at the right means the energy supply to downstream processes and energy consumption in the ironmaking process. The sum of the input energy and the sum of the energy supply to downstream processes and energy consumption in ironmaking should be equal. Thus, the energy consumption in the ironmaking process can be illustrated as shown by the gray bar in Fig. 14. In the previous oxygen blast furnace, energy consumption is 1.9% larger than that of the conventional blast furnace. In contrast to this, all three energy saving oxygen blast furnaces show less energy consumption than the conventional blast furnace. Even the energy saving oxygen blast furnace without burden strength relaxation (0% case) can achieve a 0.91% reduction in energy consumption. When the energy saving ratio derived from burden strength relaxation is increased to 5% and 10%, the reduction in energy consumption also increases to 3.2% and 5.3%, respectively. It should be noted that the energy supply to downstream processes by the energy saving oxygen blast furnaces is equal to that of the conventional blast furnace. That is, the energy saving effect can be achieved without decreasing the energy supply to the downstream processes.
4.4. Relationship between Energy Supply to Downstream Processes and Energy Consumption in Ironmaking ProcessThe energy supply to downstream processes and energy consumption in the ironmaking process are mutually related through changes in operating conditions. In particular, the injectant into the tuyere has large effects on both the input energy to the ironmaking process and BFG generation. The relationship between energy consumption and energy supply to downstream processes was analyzed using the operating conditions of the energy saving oxygen blast furnaces in Table 2. The simulation results are shown in Fig. 15 for the three cases with the different energy saving ratios derived from material strength (0%, 5% and 10%), referring to the conventional blast furnace and previous oxygen blast furnace. The ratio of pulverized coal and natural gas was changed to control energy consumption and energy supply. When the pulverized coal rate was decreased, the natural gas rate and coke rate increased correlatively due to the heat balance in the lower part of the blast furnace. The specific oxygen consumption also changed correspondingly, and this change is related to BFG generation. The energy consumption in the ironmaking process decreases as the energy supply to downstream processes decreases. Furthermore, a much larger reduction in energy consumption is possible if the energy saving effect derived from material strength relaxation is promoted. From Fig. 15, it is obvious that the effect of the burden strength relaxation by downsizing has a huge impact on energy saving in the ironmaking process. Decreasing the energy supply to downstream processes as far as possible is favorable for minimizing the energy consumption in the ironmaking process. However, a sufficient energy supply should be secured in order to maintain the performance of the integrated steel works. Specifically, the energy supply to downstream processes should be maintained at the same level as with the conventional blast furnace, that is, 4.66 GJ/thm. As a result, it is estimated that an energy saving effect of 5.3% can be achieved as a result of a 10% relaxation of burden strength, as mentioned previously.
Relationship between energy supply to downstream processes and energy consumption of energy saving oxygen blast furnace depending on changes in operating conditions.
In addition to energy saving, careful attention should be paid to input carbon as an important evaluation point, since it is directly related to the CO2 emissions from the steel works. Input carbon was calculated using the same simulation model. The results are shown in Fig. 16. The operating conditions are the same as the conditions in Fig. 15. Intriguingly, input carbon decreases in parallel with the increase in the energy supply to downstream processes with the energy saving oxygen blast furnace, since CO gas derived from pulverized coal is replaced with H2 from the hydrogen-rich natural gas, and carbon consumption is suppressed. Input carbon can be reduced further by promoting the energy saving derived from the burden strength relaxation made possible by the downsized furnace profile. The input carbon in the energy saving oxygen blast furnace can be decreased by 8.1% when the 10% energy saving ratio derived from the material strength relaxation is applied. It is especially noteworthy that the behavior of input carbon change is opposite to that of energy consumption in the ironmaking process with the oxygen blast furnace proposed by this study. Although the optimum operation condition will depend on the situations of energy saving and CO2 mitigation, owing to the features of the oxygen blast furnace, injection of a high calorie hydrogen-rich gas such as natural gas contributes both to reducing input carbon and to insuring the energy supply to downstream processes.
Relationship between energy supply to downstream processes and input carbon of energy saving oxygen blast furnace depending on changes in operating conditions.
A study on the oxygen blast furnace, focusing on its potential for energy saving, was carried out based on a material and energy balance model of the integrated steel works, with particular attention to the advantages of high productivity and nitrogen-free operation in the oxygen blast furnace process. Although the previously proposed oxygen blast furnace was characterized by massive injection of pulverized coal and gas generation, the focus of the improved oxygen blast furnace with the downsized profile was reduction of energy consumption in the total ironmaking process, including the coke oven, sintering machine and electric power plant.
First, a DEM simulation was carried out to evaluate the effect of burden load reduction in the downsized oxygen blast furnace, as reduction of the burden load makes it possible to reduce the strength requirements for agglomerated burden materials, namely, sintered ore and coke. As a result, it was found that the compressive stress acting on the material particles in the downsized oxygen blast furnace was 20–30% less than that in the conventional blast furnace. Thanks to the relaxation of the burden strength requirements in the oxygen blast furnace, substantial energy savings can be expected in the sintering machine and the coke oven.
Next, the energy consumption of the total ironmaking process was quantitatively investigated by using the material and energy balance model of the integrated steel works. By appropriate control of the injection rates of pulverized coal and natural gas, the energy consumption of the ironmaking process based on the energy saving oxygen blast furnace could be reduced by 5.3% on the assumption that a 10% energy saving was possible in the sintering machine and coke oven, while also maintaining a sufficient energy supply to downstream processes. In addition to the downsized profile, flame temperature control by natural gas and the decrease in the solution-loss reaction in the nitrogen-free blast furnace have also contributed to the total energy saving.
