2015 Volume 55 Issue 10 Pages 2105-2114
In the integrated steel works, huge quantities of coal and other fossil materials are consumed as reducing agents and energy resources. The steel industry must now deeply reexamine its utilization of carbon and energy from the viewpoints of global warming and energy security. Against this background, this paper focuses on the progressive design of an ambitious blast furnace for the future.
Several blast furnace processes including the current blast furnace and the oxygen blast furnace were examined by using a material and energy balance model of the integrated steel works. First, the current blast furnace was evaluated considering expanded use of hydrogen-rich injectants. Next, the oxygen blast furnace with top gas recycling was examined, and the characteristics of the carbon and energy balances were clarified from the viewpoints of CO2 mitigation and the energy balance in the steel works as a whole. Although the results confirmed that CO2 emissions can be reduced by intensifying top gas recycling, the energy supply to the downstream processes became seriously insufficient.
Then, the applicability of an oxygen blast furnace with a high injection rate of a hydrogen-rich gas such as natural gas instead of top gas injection was evaluated. This process, thanks to the intensified hydrogen reduction, enables the CO2 mitigation while maintaining the energy balance in the steel works. Based on the evaluation, the concept of the advanced oxygen blast furnace as a next-generation low carbon blast furnace with high energy flexibility was proposed.
The integrated steel works is a complex system in which huge quantities of coal and other fossil materials are consumed as reducing agents and energy resources in the upstream process, that is, the ironmaking process centering on the blast furnace, and the gases generated by the ironmaking process are supplied to downstream processes as energy. These systems have been highly optimized to produce steel products from the viewpoint of energy utilization. However, in order to address the issues of global warming and energy security, the steel industry must now deeply review its utilization of carbon and energy. Since the steel industry depends on coal as the main reductant in the production of steel products, efforts to decrease carbon consumption are being pursued from the mid- and long-term viewpoints on global warming.1,2) Currently, mitigation of CO2 emissions is an urgent issue in every industry, and a fundamental revision of the steel production process should also be actively carried out in the steel industry.
In the integrated steel works, coal is input to the ironmaking process as a reductant and energy source. Coal is charged to the blast furnace in the form of coke after carbonization in the coke oven. Because blast furnace performance basically determines the carbon requirement rate, improvement of the blast furnace process can be regarded as a high priority issue for suppressing carbon consumption, while also considering the energy balance of the integrated steel works. Currently, new unconventional energy resources such as shale gas are attracting special attention around the world. From the viewpoints of global warming and energy security, introduction of new energy resources, including hydrogen-rich gases such as shale gas, seems to be attractive even in the steel industry. Use of hydrogen-rich reductants leads to a decrease in CO2 emissions. In addition to the issue of global warming, energy flexibility, making it possible to respond to current conditions, can also be regarded as an important subject for strengthening the steel industry.
Looking at the history of the blast furnace, various efforts have been made to decrease the coke rate because coke consumption has a large influence on steel production costs. At first, combined blast consisting of oil injection and oxygen enrichment was applied in Europe, and those technologies spread on the global scale in the 1960s.3) Subsequently, pulverized coal injection was substituted for oil injection through all-coke operation. In parallel with the development of these combined blast operations, reducing gas injection processes were studied, particularly in the United States and Japan.4,5,6,7,8) In these processes, reducing gas produced by partial oxidation and reforming of oil, natural gas or COG is injected into the lower shaft of the blast furnace in order to drastically decrease the coke rate through intensified indirect reduction. Although these technologies were not commercialized for economic reasons, they nevertheless provided many significant results, later leading to the top gas recycling and oxygen blast furnace. In particular, the development of gas injection technology contributed to expanding reduction control in the blast furnace. The history of these technologies is shown in Fig. 1.
History of blast furnace for reducing coke rate and CO2 mitigation.
Since the 1980s, the oxygen blast furnace has been developed so as to enable injection of a large amount of pulverized coal.9,10,11,12,13,14,15) Later, various process researches on application of the oxygen blast furnace were also done.16,17) The top gas recycling process with shaft gas injection based on the oxygen blast furnace has attracted special attention for reducing CO2 emissions. The shaft gas injection in this process is similar to the reducing gas injection described above.18,19,20) For example, top gas recycling was involved in the ULCOS project in Europe.18,19) These processes indicated new aspects of innovation in the blast furnace and established the basic technologies for the next-generation blast furnace. Considering realistic solutions to global warming, more importance should be placed on consistency in energy diversification and flexibility in the integrated steel works in addition to mitigation of CO2 emissions.
The authors already reported the aspects of a desirable ironmaking process for the future in 2006.21) However, recent conditions surrounding the steel industry require an evolutionary review of the ironmaking process from the above viewpoints. This article focuses on the progressive design of an ironmaking process suitable for CO2 mitigation and energy flexibility, using a total material and heat balance model of the integrated steel works. Various simulations for analyzing the carbon and energy flows in the ironmaking process by using the above-mentioned model were carried out in order to design an ambitious new ironmaking process for the future. Finally, the concept of a next-generation blast furnace based on the oxygen blast furnace was proposed.
The structure of the model used in this research is shown in Fig. 2. The model elements consist of the sintering machine, coke oven and blast furnace and also include the power plant, hot stoves and oxygen plant. In addition to these facilities, various other equipment such as blowers for the blast furnace is also included in this model. The model of the blast furnace is based on a Rist diagram. The original Rist diagram does not include preheating gas injection and shaft gas injection, which are introduced as top gas recycling techniques. Therefore, in this study, the original Rist diagram was partly modified. Since strict modification of the Rist diagram considering the penetration behavior in shaft gas injection is essentially difficult, the effect of reducing gas injection through the tuyere gas is included in the heat and material balance in both the upper and lower shaft, and shaft gas and preheating gas injection are considered in the heat and material balance only in the upper shaft.8,22) This simplified modification of the Rist diagram apparently expresses the difference of the penetration effect of the reducing gas by the tuyere gas injection and shaft gas injection. The details of this phenomenon are described in Section 4.1. This formulation is relevant to the practical behavior of shaft gas injection accompanied by ununiform penetration in the cross section on the basis of the Rist diagram.
Material and energy flow model of integrated steel works for use in simulation. (Online version in color.)
The calculation domain is represented in Fig. 2. Considering the facilities and equipment shown in Fig. 2, the material and energy balances in the integrated steel works for each case were calculated. Excess energy (E) is defined as generated energy (blast furnace gas (BFG) +coke oven gas (COG)) minus energy consumed in the ironmaking process (fuel for the coke oven, sintering machine, hot stove and power plant).
The details of the material flows for the various blast furnace processes are shown in Fig. 3. Figure 3(a) is a conventional blast furnace, (b) is an oxygen blast furnace with top gas recycling with and without a CO2 capture process and (c) is the advanced oxygen blast furnace, meaning a revised oxygen blast furnace process which is suitable for diversified energy sources. Namely, a hydrogen-rich injectant such as natural gas is introduced together with pulverized coal through the tuyere. In the previously-proposed oxygen blast furnace,12) tuyere gas injection including CO2 gas was partly introduced to control the flame temperature; however, in this study, natural gas was applied due to its large decomposition heat and large hydrogen content. By utilizing these features, it is possible to control the flame temperature at the proper level, and it is also possible to decrease in carbon input.
Several blast furnace processes for use in simulations.
In this study, standard calculation conditions were set as follows. Shaft efficiency and thermal reserve zone temperature were set at 0.94 and 1000°C respectively for both conventional blast furnace and oxygen blast furnaces. Heat loss was set at 0.42 GJ/thm for conventional blast furnace and smaller value of 0.25 GJ/thm was applied for oxygen blast furnaces owing to its high productivity. Limiting conditions for the operations were evaluated mainly by two factors, that is, top gas temperature (Tg) and flame temperature (Tf). The criterions of those values were 100°C<Tg<250°C for both conventional and oxygen blast furnaces, 2000°C<Tf<2500°C for blast furnace and 2000°C<Tf<2800°C for oxygen blast furnaces respectively. In the case of oxygen blast furnaces, although the upper limit of Tf was not clarified, the operations based on the similar values were actually adopted in the experimental furnace condition.12) The main objective is an evaluation of the oxygen blast furnace processes by comparison with the conventional blast furnace.
2.2. Approaches to Achieving Low CarbonThe reduction mechanism in the blast furnace, as shown in Fig. 4, provides the basic concept of approaches to achieving low carbon. As is well known, the reduction mechanism in the blast furnace is divided into three steps, i.e., CO gas reduction, H2 gas reduction and direct reduction, respectively. The reducing agent depends on the distribution of these steps. On the basis of these reduction steps, the approaches to achieving low carbon can be represented as described below. First, one favorable approach is improvement of gas utilization. This can be attained by use of high reactivity coke to lower the thermal reserve zone temperature. Many studies have examined the use of high reactivity coke such as ferro-coke.23,24,25) Second, recycling of the unused CO gas included in the top gas through CO2 capture leads to a decrease in the reducing direct reduction ratio, which implies a decrease in the coke rate. This is well known as the concept of the top recycling process.18,19) Basically, the essential issue is the decrease in the coke rate, even if the reducing agent increases. Intensified hydrogen reduction is also available for reduction of the coke rate. However, it must be noted that the selection of reductants including hydrogen is sensitive to the heat balance in the blast furnace because hydrogen reduction is endothermic. Natural gas can be regarded as a promising reductant for the blast furnace. In addition to these concepts for achieving low carbon, attention must be paid to the total energy balance in the steel works.
Concept of approaches for achieving low carbon in blast furnace.
In the current blast furnace, pulverized coal injection is a usual practice, and natural gas injection is also used in some regions. Although pulverized coal injection is economical, other hydrogen-rich injectants are more favorable for mitigation of CO2 emissions. Here, injection of coke oven gas (COG), natural gas (CH4) and hydrogen in the conventional blast were evaluated, although these are conventional technologies.26,27,28) Hydrogen is not a primary energy source, but is artificially produced from fossil fuels such as oil and coal. Thus, it should be noted that hydrogen utilization is accompanied by CO2 generation in the hydrogen production process. In this simulation, hydrogen is treated as a symbolic carbon-free material, assuming that supplies of hydrogen become commonplace in the future.
Table 1 shows the calculation conditions and some calculated results. Pulverized coal injection is used commonly in all the cases. The effect of gaseous injectants on the coke rate is shown in Fig. 5. Oxygen enrichment is also shown in Fig. 5, assuming a constant flame temperature in the raceway. At the same injection volume, natural gas is the most effective for decreasing the coke rate, and hydrogen shows less effect than natural gas and COG. Although direct injection of hydrogen surely increases indirect reduction in the blast furnace, hydrogen cannot replace carbon in the raceway due to its lack of an exothermic effect. Although injection of COG was used in the distant past,26) high COG injection disturbs the consistency of energy utilization in the steel works.
| Case | Unit | Base | BF-CG1 | BF-CG2 | BF-NG1 | BF-NG2 | BF-H1 | BF-H2 | |
|---|---|---|---|---|---|---|---|---|---|
| Shaft efficiency | – | 0.94 | 0.94 | 0.94 | 0.94 | 0.94 | 0.94 | 0.94 | |
| TRZ temp. | °C | 1000 | 1000 | 1000 | 1000 | 1000 | 1000 | 1000 | |
| Burden | Sinter | kg/thm | 1274 | 1274 | 1274 | 1274 | 1274 | 1274 | 1274 |
| Lumpy ore | kg/thm | 319 | 319 | 319 | 319 | 319 | 319 | 319 | |
| Reducing agent | PCR | kg/thm | 150 | 150 | 150 | 150 | 150 | 150 | 150 |
| Injection gas | – | COG | COG | NG | NG | H2 | H2 | ||
| Gaseous injectant | kg/thm | 0 | 25 | 50 | 50 | 100 | 13 | 26 | |
| Nm3/thm | 0 | 57 | 113 | 70 | 140 | 150 | 300 | ||
| Coke rate | kg/thm | 345 | 322 | 300 | 290 | 235 | 312 | 279 | |
| Reducing agent | kg/thm | 495 | 497 | 500 | 490 | 485 | 475 | 455 | |
| Blast condition | Blast temperature | °C | 1150 | 1150 | 1150 | 1150 | 1150 | 1150 | 1150 |
| Oxygen enrichment | % | 2 | 6 | 12 | 13 | 38 | 8 | 22 | |
| Specific bosh gas volume | Nm3/thm | 1430 | 1368 | 1283 | 1283 | 1112 | 1329 | 1205 | |
| H2/(H2+CO) in bosh gas | % | 15 | 23 | 29 | 28 | 37 | 32 | 43 | |
| Flame temperature | °C | 2250 | 2225 | 2225 | 2225 | 2225 | 2225 | 2225 | |
| Top gas temperature | °C | 159 | 150 | 126 | 131 | 79 | 148 | 122 | |
| Slag volume | kg/thm | 300 | 297 | 295 | 293 | 286 | 296 | 292 | |
Pig iron temp: 1500 °C, Heat loss: 0.42 GJ/thm, Shaft efficiency: 0.94, TRZ: Thermal reserve zone, NG: Natural gas
Calculated results for conventional blast furnace with hydrogen-rich injectants.
While the replacement ratio of natural gas is the most favorable, oxygen enrichment exceeding 30% in a case of BF-NG2 is required to compensate for the decomposition heat of natural gas in the raceway, as shown in Fig. 5, leading to the top gas temperature drop to lower than 100°C. Moreover, as practical issues, oxygen enrichment exceeding 30% in the hot blast causes safety problems and equipment damage in the conventional hot air blowing system (oxidation of metallic pipes or lances, etc.). It is estimated that the injection rate of natural gas is limited at 50 kg/thm (case BF-NG1).
In the case of hydrogen injection, it seems that no limitation exists from the viewpoints of Tg and Tf presented in Table.1, however, an important point to note is that reduction of iron oxide by hydrogen is endothermic and a smooth energy exchange in the shaft should be considered. Here, the staged heat balance in the blast furnace with hydrogen injection is calculated as shown in Fig. 6. In the case of 300 Nm3/thm hydrogen injection (case BF-H2), the heat demand of the solids in the shaft is very close to the energy supply from gas due to the endothermic nature of the hydrogen reduction reaction, and in this case, heat exchange becomes stagnant in the shaft. Therefore, from the staged heat balance, it is considered that approximately 150 Nm3/thm injection of hydrogen (case BF-H1) is the upper limit.
Staged heat balance in conventional blast furnace with hydrogen injection.
Figure 7 shows the distribution of the reduction steps. In the conventional blast furnace, the ratios of direct reduction, CO gas reduction and H2 reduction are approximately 30%, 60% and 10%, respectively. It can be understood visually that hydrogen-rich injectants replace direct reduction as shown in Fig. 7, however the operational limit is given at the injection rate of 50 kg/thm for natural gas and 150 Nm3/thm for hydrogen as mentioned above, the possible decrease in the direct reduction ratio is estimated to be limited to the region up to 20% for both cases.
Distribution of reduction steps in conventional blast furnace with hydrogen-rich injectants.
Top gas recycling is attracting attention as an approach for mitigation of CO2 emissions in the steel works. It constitutes the essential part of the ULCOS project.18,19) In that process, the nitrogen-free blast furnace is a basic concept for effectively utilizing top gas recycling. The injection of reducing gas into the lower shaft activates indirect reduction and suppresses direct reduction, and as a result, the coke rate can be reduced. The injection gas temperature and the appropriate injection point are the key points. It is thought that the favorable point for injection is just below the indirect reduction zone.5) Since injection of high temperature gas at over 1000°C could rather activate coke gasification, the optimum temperature should be carefully selected so as to make effective use of the injection gas. Moreover, the important point to note in connection with shaft gas injection is the diffusion behavior of the gas injected into the shaft. According to previous studies, the gas penetration area is limited to the peripheral zone of the blast furnace.8,22) The mathematical model and cold model experiments indicated that the penetration area is dependent on the injected gas volume ratio to the total gas volume, and the diffusion effect is estimated to be slight.8,22) This dynamic behavior of shaft gas injection has a large influence on the effectiveness of top gas recycling, especially in large blast furnaces. On the contrary, in preheating gas injection, since the objects of gas injection are supply of heat and a proper gas volume to control the heat flow ratio in the upper shaft,12) diffusion of the injected gas to the center is not a serious problem. For the reasons mentioned above, the effect of shaft gas injection was simplified in the manner described in Section 2.1 in this model.
The calculation conditions are shown in Table 2. Case O-00 is a reference condition characterized by massive coal injection, which is a previous concept of the oxygen blast furnace that pursued the economic advantage of high gas generation in spite of high reducing agent consumption. Cases O-01 to O-04 were set as top gas recycling processes with CO2 capture. Previous to top gas injection into the shaft and tuyere, the CO2 included in the top gas was captured with 100% efficiency. Then some part of the top gas, which was heated to 1000°C, was injected into the shaft. The remainder was introduced into the tuyere as tuyere gas injection, and the flame temperature in the raceway was appropriately controlled. Basically, in the case of a higher injection rate of tuyere gas, the PC rate was decreased in order to maintain the flame temperature. As shown in Table 2, the recycling ratio of the top gas was increased in order to the right.
| Case | Unit | Oxgen BF | |||||
|---|---|---|---|---|---|---|---|
| O-00 | O-01 | O-02 | O-03 | O-04 | |||
| Shaft efficiency | – | 0.94 | 0.94 | 0.94 | 0.94 | 0.94 | |
| TRZ temp. | °C | 1000 | 1000 | 1000 | 1000 | 1000 | |
| Burden | Sinter | kg/thm | 1274 | 1274 | 1274 | 1274 | 1274 |
| Lumpy ore | kg/thm | 319 | 319 | 319 | 319 | 319 | |
| Reducing agent | PCR | kg/thm | 300 | 300 | 200 | 100 | 100 |
| Coke rate | kg/thm | 253 | 224 | 284 | 352 | 352 | |
| Reducing agent | kg/thm | 553 | 524 | 484 | 452 | 452 | |
| Blast condition | Oxygen | Nm3/thm | 303 | 287 | 276 | 267 | 267 |
| Moisture | g/Nm3 | 2 | 2 | 2 | 2 | 2 | |
| Shaft gas Injection | Injection rate | Nm3/thm | 300 | 350 | 350 | 350 | 550 |
| Injection temp. | °C | 1000 | 1000 | 1000 | 1000 | 1000 | |
| CO2 sequestration | Nm3/thm | 0 | 184 | 184 | 186 | 258 | |
| Tuyere gas Injection | Injection rate | Nm3/thm | 60 | 60 | 250 | 400 | 400 |
| CO2 sequestration | Nm3/thm | 0 | 32 | 131 | 213 | 187 | |
| Top gas recycling ratio | % | 30 | 52 | 70 | 82 | 88 | |
| Specific bosh gas volume | Nm3/thm | 857 | 798 | 912 | 989 | 989 | |
| H2/(H2+CO) in bosh gas | % | 17 | 19 | 15 | 9 | 9 | |
| Flame temperature | °C | 2638 | 2738 | 2578 | 2560 | 2563 | |
| Top gas temperature | °C | 164 | 126 | 144 | 133 | 238 | |
| Slag volume | kg/thm | 304 | 300 | 298 | 296 | 296 | |
Pig iron temp: 1500 °C, Heat loss: 0.25 GJ/thm, TRZ: Thermal reserve zone
Figure 8 shows the coke rate, reducing agent and carbon input with an intensified recycling ratio of top gas. The recycling ratio of top gas and the energy supply to downstream are also shown in Fig. 8. In Case O-00, the reducing agent is higher than in the base case of the conventional blast furnace because cold oxygen is supplied to the tuyere in place of hot blast with high sensible heat. The reducing agent decreases with the recycling ratio, and the coke rate is naturally influenced by pulverized coal injection. The carbon input decreases with intensified top gas recycling, corresponding to the change in the reducing agent. However, it should be noted that the energy supply to the downstream processes decreases as a result of intensified top gas recycling. In this calculation, the energy demand required for CO2 sequestration is not included because a large-scale CO2 sequestration technique suitable for the ironmaking process is still under development. However, according to a report on CO2 sequestration, the energy consumption of this process is estimated to be 2.5–4.0 GJ/t-CO2.29) Considering the energy demand for CO2 sequestration, the energy supply to the downstream processes will decrease even more, and the CO2 mitigation effect will be worsened. Accordingly, although top gas recycling is beneficial for achieving low carbon in the ironmaking process, the consistency of the self-completing energy system in the integrated steel works is a practical concern.
Calculated results for oxygen blast furnace with top gas recycling.
Various oxygen blast furnace processes have already been proposed, and JFE Steel verified its effectiveness with an experimental blast furnace characterized by massive coal injection and preheating gas injection into the upper shaft.9,10,11,12,13,14) Although the basic concept is still valid, considering the current needs for low carbon and energy flexibility, further development suitable for injection of gaseous materials such as natural gas is desired. As described in Section 3, although a high rate of hydrogen-rich gas injection requires high oxygen enrichment, the oxygen blast furnace with preheating gas injection enables flexible injection of natural gas. The decomposition heat of the injected natural gas can be just balanced by the oxygen supply. The problem of the higher heat flow ratio accompanying high oxygen enrichment can be compensated by preheating gas injection into the upper shaft.
In the case of the top gas recycling, it was necessary to use a large amount of shaft gas and co-injection of tuyere gas with pulverized coal and oxygen. Moreover, since it is estimated that a CO2 sequestration system will require more sophisticated engineering techniques, it might be relatively complicated to construct the total process. Therefore, a new process design is proposed here. As shown in Fig. 3(c), the advanced oxygen blast furnace is equipped with only preheating gas injection and co-injection of pulverized coal and natural gas, and does not employ a CO2 sequestration process and top gas recycling to the tuyere.
In order to clarify the features of the advanced oxygen blast furnace related to energy source flexibility, several calculations were made, as shown in Table 3. Cases O-10 to O-13 were calculated using the same values for shaft efficiency, thermal reserve zone temperature and raw material conditions as in Case O-00, and the natural gas and pulverized coal injection rates were changed to examine the effect of the carbon contents from the injectants on the carbon input to the steel works.
| Case | Unit | Advanced Oxgen BF | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| O-10 | O-11 | O-12 | O-13 | O-20 | O-21 | O-22 | O-23 | |||
| Shaft efficiency | – | 0.94 | 0.94 | 0.94 | 0.94 | 0.98 | 0.98 | 0.98 | 0.98 | |
| TRZ temp. | °C | 1000 | 1000 | 1000 | 1000 | 950 | 950 | 950 | 950 | |
| Burden | Sinter | kg/thm | 1274 | 1274 | 1274 | 1274 | 1101 | 1101 | 1101 | 1101 |
| Lumpy ore | kg/thm | 319 | 319 | 319 | 319 | 472 | 472 | 472 | 472 | |
| Reducing agent | PCR | kg/thm | 350 | 250 | 150 | 50 | 350 | 250 | 150 | 50 |
| Natural gas rate | kg/thm | 0 | 50 | 100 | 150 | 0 | 50 | 100 | 150 | |
| Coke rate | kg/thm | 189 | 226 | 262 | 299 | 159 | 196 | 232 | 269 | |
| Reducing agent | kg/thm | 539 | 526 | 512 | 499 | 509 | 496 | 482 | 469 | |
| Blast condition | Oxygen | Nm3/thm | 292 | 301 | 311 | 320 | 276 | 285 | 294 | 304 |
| Moisture | g/Nm3 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | |
| Preheat gas Injection | Injection rate | Nm3/thm | 250 | 200 | 200 | 200 | 250 | 200 | 200 | 200 |
| Injection temp. | °C | 1000 | 1000 | 1000 | 1000 | 1000 | 1000 | 1000 | 1000 | |
| CO2 sequestration | Nm3/thm | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Tuyere gas Injection | Injection rate | Nm3/thm | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| CO2 sequestration | Nm3/thm | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Specific bosh gas volume | Nm3/thm | 774 | 870 | 966 | 1063 | 741 | 837 | 933 | 1029 | |
| H2/(H2+CO) in bosh gas | % | 21 | 29 | 34 | 39 | 22 | 30 | 36 | 41 | |
| Flame temperature | °C | 2683 | 2539 | 2386 | 2222 | 2575 | 2426 | 2267 | 2097 | |
| Top gas temperature | °C | 122 | 131 | 184 | 225 | 137 | 145 | 196 | 231 | |
| Slag volume | kg/thm | 301 | 295 | 290 | 284 | 276 | 270 | 264 | 259 | |
Pig iron temp: 1500°C, Heat loss: 0.25 GJ/thm, TRZ: Thermal reserve zone
In Cases O-20 to O-23, the shaft efficiency, thermal reserve zone temperature and raw material conditions were set to improved values. It was reported that the permeability in the lower part of blast furnace, in particular, in the cohesive zone, was much improved owing to the accelerated reduction rate by hydrogen derived from natural gas.30,31) This means the usable ratio of low reducibility iron ore such as lumpy ore can be increased. In addition, since hydrogen reduction replaces direct reduction, degradation of coke is suppressed owing to a decrease in the solution loss reaction. As favorable results of these changes, use of low strength and high reactivity coke is possible, and the thermal reserve zone temperature decreases. Moreover, the accumulation of powder near the raceway is restrained by the rapid consumption of coke powder, which is related to the high hydrogen content.30) Therefore, shaft efficiency will also be improved through improvement of the gas flow behavior in the blast furnace.
5.2. Features of Advanced Oxygen Blast FurnaceFigure 9 shows the calculated results of the reducing agent, carbon input, BFG calorific value and energy supply to downstream process. In both Cases O-10 to O-13 and O-20 to O-23, the reducing agent gradually decreases as the natural gas injection rate increases. In particular, the carbon input decreases remarkably owing to the increased hydrogen input. As mentioned above, the previous oxygen blast furnace aimed at massive coal injection and generation of a large amount of gas, and accordingly, input carbon was high.12,13) In addition, the calorific value and hydrogen content in the BFG increased by two times or more in the advanced oxygen blast furnace in comparison with the conventional blast furnace.
Calculated results for advanced oxygen blast furnace.
Figure 10 shows the relationship between input energy and input carbon to the steel works. Every case except Case O-00 and Case O-10 shows a smaller carbon input than in the conventional blast furnace, and the input energy of Cases O-10, O-20 and O-21 is also lower than that of the conventional blast furnace. Although natural gas injection can surely reduce carbon input, input energy gradually increases with the natural gas injection rate because it is necessary to compensate for the decomposition heat of the natural gas. However, since the hydrogen included in the natural gas acts advantageously on the material balance of the total reductant, the carbon input decreases. These behaviors of the reduction mechanism in the blast furnace can be graphically understood in the Rist diagram shown in Fig. 11, which shows the conventional blast furnace (Base), typical oxygen blast furnace (Case O-00) and advanced oxygen blast furnace (Cases O-22 and O-23). Case O-23 corresponds to the maximum injection of natural gas, namely, 150 kg/thm. In the Rist diagram, point E in Fig. 11 in the conventional blast furnace moves downwards as a result of cold oxygen supply and natural gas injection. This implies an increase in specific oxygen demand due to the lack of sensible heat in the hot blast. However, with this movement, direct reduction corresponding to point B moves downwards due to the active reduction gas supply from the raceway. Here, it is noted that, on a molar basis, the reducing agent estimated from the gradient of the operation line increases in every case. However, the molar basis reductant must be translated to the net carbon rate considering the hydrogen input. As a result of this translation procedure, the highest reducing agent rate on a molar basis, i.e., Case O-23, provides the lowest reducing agent rate on a weight basis. This is the reason why both the input energy and supply energy to downstream increase in Case O-23. Injection of preheating gas into the upper shaft over the W point has no influence on the reduction behavior of iron oxide. Moreover, the points U and V respectively move to U’ and V’ due to the influence of the increase in the hydrogen supply and the decomposition heat of the natural gas.
Relationship between input energy and carbon input in advanced oxygen blast furnace.
Rist diagram for conventional blast furnace and advanced oxygen blast furnace.
The hydrogen ratio (H2/(H2+CO)) in the bosh gas is shown in Tables 1, 2, 3. In the case of top gas recycling, the hydrogen ratio is similar to that of the conventional blast furnace because both cases are based on CO gas reduction as shown in Table 2. However, in the advanced oxygen blast furnace as shown in Table 3, the hydrogen ratio increases up to 40%. In the heat balance, hydrogen reduction of iron oxide has a negative effect due to the endothermic nature of the reaction, but an accelerated reduction rate in the blast furnace can be expected as far as preheating gas is supplied.12,15) The MIDREX process, which is a representative direct reduction process based on natural gas, is also considered to be favorable for CO2 mitigation.32,33) These expansions of the operating area in the gas composition by hydrogen lead to CO2 mitigation.
5.3. Carbon Input and Excess EnergyFigure 12 indicates the relationships between input energy and energy supply to downstream. In the typical integrated steel works, the downstream processes, including rolling mills for producing final steel products, require an energy supply of approximately 4 GJ/thm. The conventional blast furnace can supply just sufficient energy to these downstream processes. From Fig. 12, it can be understood that excess energy was secured except in Cases O-10, O-20 and O21. In the other cases, excess energy increases with the natural gas injection rate. Natural gas injection changes the heat balance in the lower part of the blast furnace due to the increased decomposition heat of hydrogen, as shown in the Rist diagram in Fig. 11. Associated with this phenomenon, the increase in input energy produces sufficient excess energy.
Relationship between input energy and energy supply to downstream processes with advanced oxygen blast furnace.
Figure 13 shows the relationship between the energy supply to downstream and input carbon in comparison with the oxygen blast furnace with the top gas recycling. As is obvious from Fig. 13, in the case of top gas recycling, the carbon input has a close relationship with excess energy because this process depends on the carbon-based reduction of iron oxide. In top gas recycling, a reduction of carbon input is attained at the cost of energy supply to downstream. On the contrary, in the advanced oxygen blast furnace represented in the hatched area in Fig. 13, the carbon input tends to decrease even though the energy supply to downstream increases. This implies that carbon-based reduction is gradually replaced by hydrogen reduction.
Relationship between energy supply to downstream processes and carbon input with various blast furnace processes.
The distribution of reduction steps in the blast furnace is shown in Fig. 14. The hatched area including the “Base” case corresponds to the conventional blast furnace condition described in Section 3. Although direct reduction connecting with a low coke rate can be controlled by injectants, the region for the conventional blast furnace has limits to some extent, as shown in Fig. 14, and the change of the reduction step is stagnant due to the hot blast condition including nitrogen. With intensified top gas recycling, the point corresponding to the reduction distribution moves upwards, namely, in parallel with the increase in CO gas reduction, and direct reduction decreases until 15% at the 89% top gas recycling ratio in Case O-04 in Table 2. CO gas reduction just replaces direct reduction. On the other hand, in the advanced oxygen blast furnace with natural gas injection, this point moves to the right. Unlike top gas recycling, hydrogen reduction replaces direct reduction. The direct reduction ratio in Case O-23 closely approaches 0%, while the CO gas reduction ratio remains constant. In this critical point, it is expected to avoid degradation of the coke particles by the solution loss reaction in the lower part of the furnace.
Distribution of reduction steps in various blast furnace processes.
Figure 15 shows the relationships between the energy supply to downstream and the CO2 emission ratio. In top gas recycling, mitigation of CO2 emissions is attained at the cost of energy supply to downstream. It is estimated that the limit of top gas recycling is almost equal to a 20% reduction in the CO2 emission ratio. In this situation, the energy supply to downstream approaches 0 GJ/thm, which is similar to the ultimate target of the New Blast Furnace of the ULCOS project.18,19) The crucial point in top gas recycling is that energy compensation for the downstream processes by some additional external energy source is required. Therefore, application of top gas recycling is restricted to certain steel works which have small-scale downstream processes. Since most steel works in Japan have large downstream processes, top gas recycling is not a suitable approach. Fig. 15 also shows the CO2 emissions ratio corresponding to the case with CCS (CO2 Capture and Storage), in which a remarkable decrease in CO2 emissions can be expected. However, CCS requires additional energy and costs, as mentioned above. The technological applicability and economic efficiency of CCS must be solved from the global viewpoint.
Relationship between energy supply to downstream processes and CO2 emissions with various blast furnace processes.
In the advanced oxygen blast furnace in the right part of Fig. 15, both CO2 mitigation and a sufficient energy supply can be attained. As was obvious in Fig. 13, the conventional intensification of the energy supply means an increase in carbon input. However, in the advanced oxygen blast furnace, CO2 mitigation is satisfied by energy conversion to a hydrogen-rich injectant. Especially in Case O-23, the CO2 mitigation ratio reaches a reduction of about 10% in comparison with the conventional blast furnace. The optimum point for CO2 mitigation and energy use will depend selectively on the situation of the actual steel works. For example, depending on the case, excess gas, including hydrogen, can be utilized as available energy for producing electricity at an outside power plant or as a chemical resource.
6.2. Improvement of ProductivityThe factors which influence the productivity of the blast furnace are closely related to the phenomena which occur in the lower part of the furnace, such as slag flooding. The limit of productivity can be estimated from the flooding diagram by Fukutake,34) which was originally proposed by Sherwood,35) where the gas volume, slag properties and slag volume are involved. The calculated specific bosh gas volume for each case is shown in Tables 1, 2, 3. Basically, the specific gas volume of the nitrogen-free blast furnace is much lower than that of the conventional blast furnace. On the basis of these data, the operating conditions for the conventional blast furnace and Case O-23 are presented in the flooding diagram in Fig. 16. From Fig. 16, the limit of productivity for each case can be estimated. The maximum productivity in the conventional blast furnace is approximately 3.1, whereas that in Case O-23 increases to 5.2 owing to the smaller specific gas volume. This is an advantageous point of the nitrogen-free blast furnace. Even in the previously-proposed oxygen blast furnace, high productivity was confirmed with an experimental blast furnace.15) By fully utilizing the superiority of this feature, it is possible to reduce the blast furnace inner volume at the same production rate. The modification of blast furnace inner volume to a smaller size with a low height potentially enables the use of low grade, low strength burden materials, which also offers economic benefits.
Flooding diagrams for conventional blast furnace and advanced oxygen blast furnace.
A comparison of the conventional blast furnace with the advanced oxygen blast furnace is shown in Fig. 17. The total concept of the advanced oxygen blast furnace can be seen in the right part of Fig. 17. The calorific value of the top gas is approximately 2.0 times larger than that with the conventional blast furnace. In particular, the hydrogen content in the top gas is much higher, as shown in Fig. 9, making these gases very valuable for power plants and chemical processes. Moreover, if the blast furnace inner volume can be reduced owing to the high productivity of the oxygen blast furnace, the height of the furnace can be smaller than that of the conventional blast furnace. Thanks to the high hydrogen content in the bosh gas, it is estimated that the permeability of the cohesive zone is much improved;31) this point is helpful for productivity improvement. The direct reduction ratio is suppressed to the limit, and the solution loss reaction becomes extremely small. Because the burden stress from the upper part of the furnace is reduced, it is possible to relax burden property requirements related to physical strength. As outlined above, the concept of the advanced oxygen blast furnace has the various advantageous features of CO2 mitigation, energy use flexibility and relaxation of burden properties.
Comparison of conventional blast furnace and advanced oxygen blast furnace. (Online version in color.)
In this study all calculations were carried out based on the material and heat balance model. In order to evaluate the operational limitations of the actual oxygen blast furnaces, comprehensive judgments on the distributions of heat flow ratio, gas and solid temperatures and reduction degree and so on in the furnaces obtained by some kinetic approach are also important. In addition, it is necessary to design an appropriate burner for co-injection of pulverized coal/natural gas/oxygen, and is demanded to understand the injection limit by evaluating the combustion properties such as the relationship among injection rate, combustion efficiency and oxygen enrichment ratio when using such burners. These are main subjects remained for the future work.
The integrated steel works consumes huge quantities of coal and other fossil materials as reducing agents and energy resources in the ironmaking process. However, the steel industry must now conduct an exhaustive review of its utilization of carbon and energy from the viewpoints of global warming and energy security issues. Against this background, first, the limits of the current blast furnace were examined, after which the concept of a next-generation blast furnace based on the oxygen blast furnace was discussed by using a material and energy balance model of the integrated steel works. The following results were obtained:
(1) Even in the conventional blast furnace, hydrogen-rich injectants such as COG and natural gas can be applied in order to decrease carbon input. However, control of the direct reduction ratio relating to the coke rate is restricted to some extent, and high oxygen enrichment is required. These conditions restrict the expansion of the operating area for CO2 mitigation.
(2) In the oxygen blast furnace with top gas recycling, a decrease in carbon input can be attained in the region of the ironmaking process because top gas recycling implies the replacement of coke by CO gas derived from the top gas. In parallel with top gas recycling, the direct reduction ratio decreases by as much as 10%. The ultimate CO2 mitigation ratio is estimated to be approximately 20%. If CCS is introduced in the future, remarkable CO2 mitigation can be expected. However, intensified top gas recycling causes a shortage in the energy supply to the downstream processes, and compensating energy sources must be introduced in order to maintain the energy consistency of the integrated steel works.
(3) The oxygen blast furnace with preheating gas injection in the upper shaft makes it possible to inject a large amount of natural gas in addition to pulverized coal. Thanks to intensified hydrogen reduction, the direct reduction ratio decreases remarkably, leading directly to CO2 mitigation. With natural gas injection, although the energy input must be increased to control the heat balance in the lower part of the blast furnace, the process generates excess energy for the downstream processes. Balancing of input energy and excess energy is possible, depending on the situation of the steel works. For example, excess energy and gases can be supplied to an external power plant or the chemical industry. The proposed advanced oxygen blast furnace satisfies both CO2 mitigation and energy use flexibility. Moreover, high productivity can be expected, and assuming the furnace is downsized, taking advantage of this enhanced productivity, use of low grade burden materials also becomes possible. This is considered to be the concept of a next-generation blast furnace.
