Method of operating direct reduction furnace and method of producing reduced iron

Abstract

Provided is a method for operating a direct reduction furnace, the method attaining a reduction in CO2 emission and also making a stable operation possible by avoiding troubles such as an operation stop and a decrease in production efficiency. A high-temperature gas is blown into the direct reduction furnace, separately from a reducing gas. The blowing position of the high-temperature gas along the vertical direction is made in an upper position than the blowing position of the reducing gas along the vertical direction.

Description

DESCRIPTION TITLEMETHOD OF OPERATING DIRECT REDUCTION FURNACE AND METHOD OF PRODUCING REDUCED IRON TECHNICAL FIELD

[0001] The present disclosure relates to a method of operating a direct reduction furnace and a method of producing reduced iron. BACKGROUND

[0002] In recent years, there has been a strong demand for energy savings in steelworks against the backdrop of global environmental issues and fossil fuel depletion issues. The raw material for iron is mainly iron oxide such as iron ore, and a reduction process to reduce this iron ore is essential at steelworks. The most widespread and common reduction process worldwide uses a blast furnace. In a blast furnace, coke or pulverized coal reacts with oxygen in hot blast (air heated to about 1200 °C) from a tuyere. This reaction produces CO and H2 as reducing gases, which are used to reduce the iron ore and the like in the furnace. Recent improvements in blast furnace operation technology have decreased the reducing agent rate (the amount of coke and pulverized coal used per tonne of hot metal produced) to about 500 kg / t, and the reducing agent rate has already almost reached a lower limit. Therefore, no further significant decrease in the reducing agent rate is expected.

[0003] On the other hand, direct reduction ironmaking (also called direct reduction or direct ironmaking) has been developed as a different reduction process than a blast furnace.

[0004] Direct reduction ironmaking is as follows. A direct reduction furnace is charged with iron oxide raw material (hereinafter simply referred to as iron oxide) such as lump iron ore (lump ore) or pellets (pulverized iron ore solidified into a spherical shape). A reducing gas is then injected into the direct reduction furnace to reduce the iron oxide and obtain reduced iron. The resulting reduced iron is then cooled in a region (cooling zone) below the position where the reducing gas is injected into the direct reduction furnace. The reduced iron is then discharged from the bottom of the direct reduction furnace. The reduced iron discharged from the direct reduction furnace is then melted in an electric furnace.

[0005] Here, a shaft furnace is mainly used as the direct reduction furnace. Furthermore, natural gas, such as Midrex® (Midrex is a registered trademark in Japan, other countries, or both) or Hyl® (Hyl is a registered trademark in Japan, other countries, or both), is generally used as the source of reducing gas. In this case, natural gas is reformed with exhaust gas discharged from the furnace top of the direct reduction furnace (hereinafter also referred to as furnace top gas) to generate a reducing gas containing CO and H2. The reducing gas is then injected into the reduction furnace to reduce the iron oxide according to the following formula to obtain reduced iron.Fe2O3 + 3CO ® 2Fe + 3CO2 ΔH298 = −247 kJ / kg-Fe (i)Fe2O3 + 3H2 ® 2Fe + 3H2O ΔH298 = 858 kJ / kg-Fe (ii)Note that kJ / kg-Fe is a unit that represents the amount of change in enthalpy (kJ) per kg of Fe (iron).

[0006] As described above, in general direct reduction ironmaking, a reducing gas containing CO and H2 is used, and therefore a large amount of CO2 is produced according to the above formula (i) and discharged outside the system.

[0007] In recent years, there has been a growing need to reduce CO2 emissions on a global scale, and further reductions in CO2 emissions are being sought in the ironmaking process as well. Therefore, in direct reduction ironmaking, techniques using a reducing gas containing hydrogen as the main component are being considered. By using a reducing gas containing hydrogen as the main component, the main component of the furnace top gas becomes H2O, which is expected to significantly reduce CO2 emissions.

[0008] As examples of such technology, Patent Literature (PTL) 1 describes:"In a method of operating a direct reduction furnace by a shaft furnace method to produce reduced iron using a reducing gas mainly consisting of hydrogen,a method of operating a direct reduction furnace using a preheated raw material, characterized in that preheated raw material iron oxide is charged into the direct reduction furnace".

[0009] PTL 2 describes:"In a method of operating a direct reduction furnace by a shaft furnace method to produce reduced iron using a reducing gas mainly consisting of hydrogen,a method of operating a direct reduction furnace in which furnace top gas is circulated, characterized in that a portion of gas discharged from the furnace top is injected from a middle section of the furnace".

[0010] PTL 3 describes:"A method of producing reduced iron by reducing iron oxide charged in a shaft furnace,characterized in that a heated mixed gas including nitrogen gas and a reducing gas containing 90 vol% or more of hydrogen gas is injected into the shaft furnace". CITATION LISTPatent Literature

[0011] PTL 1: JP 2012-102371 APTL 2: JP 2012-102372 APTL 3: WO 2021 / 230307 A1 SUMMARY(Technical Problem)

[0012] However, in the techniques of PTL 1 to 3, an increase in pressure loss in the direct reduction furnace can cause improper descent, such as hanging of the raw material, which can lead to problems such as reduced productivity and shutdown of operations.

[0013] The present disclosure has been developed to solve the above problems, and an aim thereof is to provide a method of operating a direct reduction furnace that reduces CO2 emissions and enables stable operation by avoiding problems such as reduced productivity and shutdown of operations. Another aim of the present disclosure is to provide a method of producing reduced iron that produces reduced iron by the aforementioned method of operating a reduction furnace. Hereinafter, any numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower limit value and the upper limit value, respectively.(Solution to Problem)

[0014] We have conducted further studies on the aforementioned problem and have made the following discoveries.(1) In direct reduction ironmaking, iron oxide such as pellets may be disintegrated during the reduction process (hereinafter also referred to as reduction disintegration). In particular, when iron oxide is reduced using a reducing gas containing hydrogen as the main component, reduction disintegration becomes more pronounced than when natural gas is used as the reducing gas, which is likely to lead to an increase in pressure loss within the direct reduction furnace. Improper descent, such as hanging of the raw material, consequently occurs, leading to problems such as reduced productivity and shutdown of operations.(2) When iron oxide is reduced using a reducing gas containing hydrogen as the main component, fine streak-like cracks are likely to occur in the crystal lattice of the iron oxide, and these cracks serve as initiation points for fracture.(3) That is, in a direct reduction furnace, iron oxide is charged through an iron oxide charging port at the furnace top, and the iron oxide is gradually allowed to fall. A reducing gas is injected into the furnace through a reducing gas injection port to reduce the iron oxide and obtain reduced iron. In this way, the iron oxide charged into the direct reduction furnace is reduced as it descends. Therefore, during operation of the direct reduction furnace, the reduction degree of the iron oxide varies depending on the height position y in the direct reduction furnace. Hereinafter, the reduction degree of the iron oxide at a height position y in the direct reduction furnace will be referred to as the progress reduction degree A(y) of the iron oxide.(4) Here, the aforementioned cracks are prominently generated when iron oxide remains in the upper to middle section of the direct reduction furnace higher than the vertical injection position of the reducing gas, particularly at a height position at which the progress reduction degree A(y) is 30 % to 60 %, and even more so at a height position at which the progress reduction degree A(y) is 30 % to 40 %. Hence, to control reduction disintegration, it is effective to increase the rate of the reduction reaction of iron oxide at the aforementioned height position (hereinafter also referred to as the reduction reaction rate) and narrow the range of the aforementioned height position, in other words, to shorten the residence time of iron oxide at the aforementioned height position.(5) To increase the reduction reaction rate at the aforementioned height position, it is effective to increase the temperature at the aforementioned height position. To this end, it is essential to inject a high-temperature gas containing nitrogen as the main component into the direct reduction furnace separately from the reducing gas, and to set the vertical injection position of the high-temperature gas higher than the vertical injection position of the reducing gas.The present disclosure is based on these discoveries and further studies.

[0015] The primary features of the present disclosure are as follows.1. A method of operating a direct reduction furnace, the method comprising:a charging process of charging iron oxide into the direct reduction furnace;a first injection process of injecting a reducing gas containing hydrogen as a main component into the direct reduction furnace;a second injection process of injecting a high-temperature gas containing nitrogen as a main component into the direct reduction furnace; anda reducing process of reducing the iron oxide in the direct reduction furnace to obtain reduced iron, whereina vertical injection position of the high-temperature gas in the second injection process is higher than a vertical injection position of the reducing gas in the first injection process.

[0016] 2. The method of operating a direct reduction furnace according to 1, wherein the vertical injection position of the high-temperature gas is set to a height position at which a progress reduction degree A(y) is 30 % to 60 %, andthe progress reduction degree A(y) is a reduction degree of iron oxide at a height position y in the direct reduction furnace.

[0017] 3. The method of operating a direct reduction furnace according to 2, wherein the vertical injection position of the high-temperature gas is set to a height position at which the progress reduction degree A(y) is 30 % to 40 %.

[0018] 4. The method of operating a direct reduction furnace according to 2 or 3, wherein the progress reduction degree A(y) is determined based on operating conditions planned for the direct reduction furnace.

[0019] 5. The method of operating a direct reduction furnace according to any one of 1 to 4, wherein a H2 concentration of an injection gas consisting of the reducing gas and the high-temperature gas is 60 vol% or more.

[0020] 6. The method of operating a direct reduction furnace according to any one of 1 to 5, wherein an amount of the high-temperature gas is 10 vol% to 60 vol% of an amount of the reducing gas.

[0021] 7. The method of operating a direct reduction furnace according to any one of 1 to 6, wherein a H2 concentration of the reducing gas is 80 vol% or more.

[0022] 8. The method of operating a direct reduction furnace according to any one of 1 to 7, further comprising a heat supply process of supplying heat to the direct reduction furnace.

[0023] 9. The method of operating a direct reduction furnace according to 8, wherein heat from combustion of biomass is used as a heat source in the heat supply process.

[0024] 10. A method of producing reduced iron, the method comprising producing reduced iron by the method of operating a direct reduction furnace according to any one of 1 to 9.(Advantageous Effect)

[0025] According to the present disclosure, the use of a reducing gas containing hydrogen as the main component makes it possible to reduce CO2 emissions, while enabling stable operation of a direct reduction furnace by avoiding problems such as reduced productivity and shutdown of operations. Furthermore, the method of operating a reduction furnace according to the present disclosure is extremely advantageous in terms of improving reaction efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the accompanying drawings:FIG. 1 is a schematic diagram illustrating an example of the configuration of a direct reduction furnace used in a method of operating a direct reduction furnace according to an embodiment of the present disclosure;FIG. 2 is a schematic diagram illustrating an example of a conventional reduced iron production process;FIG. 3 is a schematic diagram illustrating an example of a method of producing reduced iron according to an embodiment of the present disclosure;FIG. 4 is a diagram illustrating an example of the relationship between the height position y of the direct reduction furnace and the progress reduction degree A(y), and the relationship between the height position y of the direct reduction furnace and the iron oxide temperature (temperature distribution of iron oxide in the shaft furnace); andFIG. 5 is a diagram illustrating an example of the result of computational simulation of the progress reduction degree A(y) and the temperature of iron oxide at each height position y in a direct reduction furnace in the case of producing reduced iron by the method of operating a direct reduction furnace according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0027] [1] Method of operating a direct reduction furnaceThe following describes a method of operating a direct reduction furnace according to an embodiment of the present disclosure.

[0028] The method of operating a direct reduction furnace according to an embodiment of the present disclosure includes:a charging process of charging iron oxide into the direct reduction furnace;a first injection process of injecting a reducing gas containing hydrogen as a main component into the direct reduction furnace;a second injection process of injecting a high-temperature gas containing nitrogen as a main component into the direct reduction furnace; anda reducing process of reducing the iron oxide in the direct reduction furnace to obtain reduced iron, whereina vertical injection position of the high-temperature gas in the second injection process is higher than a vertical injection position of the reducing gas in the first injection process.

[0029] FIG. 1 is a schematic diagram illustrating an example of the configuration of a direct reduction furnace used in a method of operating a direct reduction furnace (shaft furnace) according to an embodiment of the present disclosure. In the drawing, reference numeral 1 denotes a shaft furnace, 1a denotes iron oxide, 1b denotes reduced iron, 1c denotes the furnace top, 1d denotes a reduction zone, 1e denotes a cooling zone, 2 denotes an iron oxide charging port, 3 denotes a furnace top gas discharge port, 4 denotes a high-temperature gas injection port, 5 denotes a reducing gas injection port, 7 denotes a cooling gas injection port, 8 denotes a cooling gas suction port, and 9 denotes a reduced iron discharge port.

[0030] In the shaft furnace, iron oxide is charged from the furnace top, particularly from an iron oxide charging port, and the iron oxide is gradually lowered in the reduction zone. The iron oxide is reduced by injecting high-temperature reducing gas, having hydrogen as the main component, in through the reducing gas injection port provided in the reduction zone of the shaft furnace. The reduced iron is then discharged from the reduced iron discharge port located at the bottom of the shaft furnace. On the other hand, the furnace top gas is discharged from the furnace top gas discharge port. A high-temperature gas containing nitrogen as the main component is injected into the shaft furnace through a high-temperature injection port, separately from the reducing gas. The reducing gas containing hydrogen as the main component is not injected into areas other than the reduction zone (such as the cooling zone and the reduced iron discharge port).

[0031] Furthermore, a cooling gas injection port and a cooling gas suction port are arranged in the cooling zone located at the lower portion of the shaft furnace. The cooling gas is injected into the cooling zone through the cooling gas injection port. Furthermore, the cooling gas suction port sucks in the cooling gas so that it does not enter the furnace top. The cooling gas may, for example, be N2.

[0032] The following is a detailed description of each process of the method of operating a direct reduction furnace according to an embodiment of the present disclosure. The charging process can be carried out by a conventional method and therefore will not be described here.

[0033] [First injection process]In the first injection process, a reducing gas containing hydrogen as the main component is injected into the direct reduction furnace. Here, "containing hydrogen as the main component" means that the H2 concentration is preferably 80 vol% or more. The H2 concentration is more preferably 90 vol% or more. The H2 concentration is even more preferably 95 vol% or more. No particular upper limit is placed on the H2 concentration in the reducing gas, and the H2 concentration may be 100 vol%. The type of the remaining gas other than H2 is not particularly limited. As the remaining gas, for example, a gas produced as a by-product in the ironmaking process (hereinafter also referred to as by-product gas) can also be used, in addition to N2, H2O, CO, and CO2. Examples of by-product gases include blast furnace gas (BFG) and coke oven gas (COG). However, if the remaining gas contains CO or CO2, the furnace top gas will contain CO2. In this case, the CO2 included in the furnace top gas is preferably used for basic chemical synthesis (CCU) or storage (CCS), as described below.

[0034] The temperature of the reducing gas injected through the reducing gas injection port is, for example, 700 °C to 1200 °C. Furthermore, if the temperature of the reducing gas is less than 800 °C, the heat in the furnace may be insufficient, which may delay the reduction reaction. On the other hand, if the temperature of the reducing gas exceeds 1000 °C, fusion of the reduced iron particles (called clustering) may progress, making it difficult to discharge the reduced iron. Therefore, the temperature of the reducing gas is preferably 800 °C to 1000 °C.

[0035] [Second injection process]In the second injection process, it is extremely important to inject a high-temperature gas containing nitrogen as the main component into the direct reduction furnace and to set the vertical injection position of the high-temperature gas higher than the vertical injection position of the reducing gas. The high-temperature gas is supplied from outside the direct reduction furnace system. In other words, the high-temperature gas does not include the gas obtained by circulating the furnace top gas discharged from the direct reduction furnace.

[0036] By injecting high-temperature gas containing nitrogen as the main component into the direct reduction furnace, it is possible to increase the reduction reaction rate at a predetermined height and significantly reduce reduction disintegration. Furthermore, nitrogen does not participate in the reduction reaction in the direct reduction furnace and is also advantageous from the perspectives of cost, storability, and safety. Therefore, in the second injection process, high-temperature gas containing nitrogen as the main component is injected into the direct reduction furnace.

[0037] Here, "containing nitrogen as the main component" means that the N2 concentration is 70 vol% or more. The N2 concentration is preferably 90 vol% or more. The N2 concentration is more preferably 95 vol% or more. No particular upper limit is placed on the N2 concentration in the high-temperature gas, and the N2 concentration may be 100 vol%. The remaining gas is not particularly limited, and in addition to H2, H2O, CO, and CO2, by-product gases can also be used. Examples of by-product gases include blast furnace gas (BFG) and coke oven gas (COG). However, if the remaining gas contains CO or CO2, the furnace top gas will contain CO2. In this case, the CO2 included in the furnace top gas is preferably used for basic chemical synthesis (CCU) or storage (CCS), as described below.

[0038] Vertical injection position of high-temperature gas: higher than the vertical injection position of the reducing gasBy setting the vertical injection position of the high-temperature gas higher than the vertical injection position of the reducing gas, reduction disintegration can be significantly suppressed. The vertical injection position of the high-temperature gas is preferably at a height position at which the progress reduction degree A(y) is 30 % to 60%. That is, when iron oxide is reduced using a reducing gas containing hydrogen as the main component, fine streak-like cracks are likely to occur in the crystal lattice of the iron oxide, and these cracks serve as initiation points for fracture. The aforementioned cracks are prominently generated when iron oxide remains in the upper to middle section of the direct reduction furnace higher than the vertical injection position of the reducing gas, particularly at a height position at which the progress reduction degree A(y) is 30 % to 60 %, and even more so at a height position at which the progress reduction degree A(y) is 30 % to 40 %. Hence, to suppress reduction disintegration, it is effective to increase the reduction reaction rate at the aforementioned height position and narrow the range of the aforementioned height position, in other words, to shorten the residence time of iron oxide at the aforementioned height position. To increase the reduction reaction rate at the aforementioned height position, it is effective to increase the temperature at the aforementioned height position. To this end, it is essential to inject a high-temperature gas containing nitrogen as the main component into the direct reduction furnace separately from the reducing gas, and to set the vertical injection position of the high-temperature gas higher than the vertical injection position of the reducing gas. This can significantly suppress reduction disintegration. Furthermore, this also effectively contributes to improving the reaction efficiency. Therefore, the vertical injection position of the high-temperature gas is set higher than the vertical injection position of the reducing gas. The vertical injection position of the high-temperature gas is preferably set to a height position at which the progress reduction degree A(y) is 30 % to 60 %. The vertical injection position is more preferably set to a height position at which the progress reduction degree A(y) is 30 % to 40 %. When a plurality of injection positions exists for the reducing gas, the vertical injection position for the high-temperature gas is set higher than the uppermost vertical injection position for the reducing gas.

[0039] We think that the reason why the cracks occur significantly at the aforementioned height position is as follows. In the reduction of iron oxide with hydrogen, the reaction proceeds as hydrogen atoms diffuse into the crystal lattice of the iron oxide. During this process, as the reduction of Fe2O3 to Fe3O4 and FeO progresses, oxygen (O) is removed from the iron oxide. Therefore, the volume (cm3 / mol) of iron oxide per mole of Fe decreases, and internal stress occurs due to volume shrinkage. This internal stress is relieved by the formation of cracks in the crystal lattice. Therefore, when the internal stress due to volumetric shrinkage increases, fine cracks occur mainly along the direction in which hydrogen atoms diffuse in the crystal lattice. As the reduction proceeds from Fe2O3 to Fe3O4, FeO, and Fe, the volume shrinkage due to reduction increases, and fine streak-like cracks are more likely to occur. In particular, in a structure mainly composed of FeO in which the amount of Fe formed is small, fine streak-like cracks are likely to occur. On the other hand, once Fe begins to be formed, solid-phase sintering of Fe particles progresses, which has the effect of sealing the cracks. Here, in a case in which the iron oxide before the start of reduction is composed of Fe2O3, the iron oxide will be substantially composed of Fe3O4 when the progress reduction degree A(y) is 11 %. When the progress reduction degree A(y) is 33 %, the iron oxide will be substantially composed of FeO. If the reduction proceeds further, Fe begins to form. Therefore, we think that the aforementioned cracks and reduction disintegration are prominently triggered when iron oxide remains in the upper to middle section of the direct reduction furnace higher than the vertical injection position of the reducing gas, particularly at a height position at which the progress reduction degree A(y) is 30 % to 60 %, and even more so at a height position at which the progress reduction degree A(y) is 30 % to 40 %.

[0040] Here, the progress reduction degree A(y) can be calculated by the following formula.[progress reduction degree A (y) (unit: %)] = ([amount of oxygen in iron oxide before reduction (unit: mass%)] − [amount of oxygen in iron oxide at height position y in direct reduction furnace (unit: mass%)]) / [amount of oxygen in iron oxide before reduction (unit: mass%)] × 100The amount of oxygen (mass%) in iron oxide before reduction is the total amount of oxygen (mass%) included in the FeO and Fe2O3 contained in the iron oxide before reduction. The amount of oxygen included in Fe2O3 can be calculated by, for example, assuming that the Fe obtained by subtracting the amount of Fe (mass%) in FeO from the T. Fe (the total mass (mass%) of Fe atoms) of the iron oxide before reduction is present as Fe2O3, and then calculating the total amount of oxygen (mass%) included in the Fe2O3. Furthermore, the amount of oxygen (mass%) in the iron oxide at height position y in the direct reduction furnace (hereinafter also referred to as iron oxide at height position y) is the total amount of oxygen (mass%) included in FeO and Fe3O4 contained in the iron oxide at height position y. For example, assuming that the Fe obtained by subtracting the amount of Fe (mass%) in FeO and M. Fe (Fe present as metal) from the T. Fe (the total mass (mass%) of Fe atoms) of the iron oxide at the height position y is present as Fe3O4, the total amount of oxygen (mass%) included in the Fe3O4 can be calculated.

[0041] The progress reduction degree A(y) is preferably determined based on the operating conditions planned for the direct reduction furnace (hereinafter also referred to as planned operating conditions). The progress reduction degree A(y) can be determined by, for example, chemical analysis of the iron oxide, analysis of the gas concentration in the furnace gas, or computational simulation. Among these, a method using computational simulation is preferred. Examples of specific aspects of each method are as follows.

[0042] - Chemical analysis of iron oxideThe direct reduction furnace is pre-operated according to the planned operating conditions, and during this operation, the discharge of reduced iron and the injection of reducing gas are suddenly stopped. N2 gas for cooling is then injected into the direct reduction furnace to rapidly cool the iron oxide in the direct reduction furnace. Next, the iron oxide is gradually discharged from the bottom of the direct reduction furnace. Iron oxide considered to have been retained at each height position in the vertical direction of the direct reduction furnace based on the discharged volume is sampled and subjected to chemical analysis to determine the progress reduction degree A(y). As an alternative to the above method, the direct reduction furnace may be pre-operated according to the planned operating conditions. Partially reduced iron oxide samples are then collected from sampling ports provided at various height positions in the vertical direction of the direct reduction furnace, and the progress reduction degree A(y) of the iron oxide is determined by chemical analysis.

[0043] - Analysis of gas concentration in furnace gasThe direct reduction furnace is pre-operated according to the planned operating conditions. The gas inside the furnace is then sampled from gas sampling pipes provided at various height positions in the vertical direction of the direct reduction furnace, and the gas concentration is analyzed by gas chromatography to determine the reduction in the amount of hydrogen and other components from the reducing gas before injection into the direct reduction furnace, thereby determining the progress reduction degree A(y) of iron oxide.

[0044] - Computational simulationA computational simulation (for example, a one-dimensional mathematical model, or a two-dimensional DEM-CFD method combining a DEM (Discrete Element Method) for calculating the behavior of solid particles with a CFD (Computational Fluid Dynamics) method for performing gas fluid calculations) is performed in accordance with the planned operating conditions to calculate the progress reduction degree A(y) of iron oxide.

[0045] The planned operating conditions include the shape of the direct reduction furnace, the production rate of reduced iron (charged amount of iron oxide), the composition, temperature, pressure, and injection amount (gas flow rate) of the reducing gas (in a steady-state operating state), and the type of iron oxide (particle size and chemical composition). In addition, when determining the progress reduction degree A(y), the injection of high-temperature gas is not taken into consideration. The chemical analysis of iron oxide and the analysis of the gas concentration in the furnace gas may be carried out during actual operation of the direct reduction furnace (rather than during pre-operation) to determine the progress reduction degree A(y).

[0046] In addition to the above-described methods, the progress reduction degree A(y) may be determined based on, for example, past operational data of a direct reduction furnace under similar operating conditions to the planned operating conditions, or on literature and the like.

[0047] When the reduction degree of iron oxide at the height position y in the direct reduction furnace varies (is not constant) in the furnace radial direction of the direct reduction furnace, the average value of the reduction degree of iron oxide at the dimensionless radius of the direct reduction furnace of 0, 0.50, and 1.00 may be taken as the reduction degree of iron oxide at the height position y in the direct reduction furnace. Here, the dimensionless radius of the direct reduction furnace is a dimensionless radius in the furnace radial direction, with the center position of the direct reduction furnace being 0 and the furnace wall of the direct reduction furnace being 1.00.

[0048] Furthermore, to set the vertical injection position of the high-temperature gas higher than the vertical injection position of the reducing gas, the high-temperature gas injection port may, for example, be provided in the direct reduction furnace so that the position of the high-temperature gas injection port is higher than the position of the reducing gas injection port in the height direction (vertical direction) of the direct reduction furnace. Additionally, the vertical injection position of the high-temperature can be adjusted by, for example, providing a plurality of high-temperature gas injection ports in the height direction (vertical direction) of the direct reduction furnace and determining the high-temperature gas injection port actually to be used in accordance with the progress reduction degree A(y) of iron oxide. The vertical injection position of the high-temperature gas can also be adjusted by providing the high-temperature gas injection port with a lifting function (vertical position changing function) or an adjustment function of injection angle.

[0049] A representative position (vertical middle position) of the vertical injection position of the high-temperature gas is preferably a height position at which the progress reduction degree A(y) is 30 % to 60 %. The representative position is more preferably a height position at which the progress reduction degree A(y) is 30 % to 40 %. In particular, the entire vertical injection position of the high-temperature gas is preferably included in the range of this height position.

[0050] Here, high-temperature gas refers to gas at 700 °C to 1000 °C. As described above, increasing the temperature at the aforementioned height position is effective in controlling reduction disintegration. For this purpose, the temperature of the high-temperature gas is preferably set to be equal to or higher than 100 °C above the temperature at the height position at which the progress reduction degree A(y) is 60 %. The temperature of the high-temperature gas is more preferably set to be equal to or higher than 200 °C above the temperature at the height position at which the progress reduction degree A(y) is 60 %. The temperature at the height position at which the progress reduction degree A(y) is 60 % may, for example, be measured by a thermometer installed in the direct reduction furnace or be determined by the aforementioned computational simulation.

[0051] The H2 concentration in the (total) injection gas consisting of the reducing gas and the high-temperature gas is preferably 60 vol% or more. The H2 concentration is more preferably 80 vol% or more. This is because if the H2 concentration in the injection gas is less than 60 vol%, the reducing gas potential of H2 will decrease, which may result in a decrease in the reduction reaction rate.

[0052] Furthermore, the amount of high-temperature gas (injection amount into the direct reduction furnace) is preferably 10 vol% to 60 vol% of the amount of reducing gas (injection amount into the direct reduction furnace). If the amount of the high-temperature gas is less than 10 vol% of the amount of reducing gas, it may be difficult to obtain the effect of increasing the reduction reaction rate as described above. On the other hand, if the amount of the high-temperature gas exceeds 60 vol% of the amount of the reducing gas, the pressure loss of the gas increases around the injection area of the high-temperature gas, which may cause a problem of improper descent, such as hanging of iron oxide. Therefore, the amount of high-temperature gas is preferably 10 vol% to 60 vol% of the amount of reducing gas. The amount of the high-temperature gas is more preferably 20 vol% or more of the amount of the reducing gas. The amount of the high-temperature gas is more preferably 50 vol% or less of the amount of the reducing gas.

[0053] [Reducing process]In the reducing process, iron oxide is reduced according to the above formula (i), for example, using a reducing gas containing hydrogen as the main component, to obtain reduced iron. The reducing gas is subjected to a reduction reaction in the direct reduction furnace and is then discharged as furnace top gas from the furnace top of the direct reduction furnace. The temperature of the furnace top gas is, for example, 300 °C to 400 °C.

[0054] [Heat supply process]The method of operating a direct reduction furnace according to an embodiment of the present disclosure preferably further includes a heat supply process of supplying heat to the direct reduction furnace. As described above, in the method of operating a direct reduction furnace according to an embodiment of the present disclosure, a reducing gas containing hydrogen as the main component is used, and a high-temperature gas is injected into the direct reduction furnace separately from the reducing gas. As illustrated in the above formulas (i) and (ii), the reduction reaction with CO is an exothermic reaction, whereas the reduction reaction with H2 is an endothermic reaction. Therefore, when a gas having a H2 concentration close to 100 vol% is used as the reducing gas, the temperature inside the direct reduction furnace may decrease. In this case, it is preferable to supply heat to the direct reduction furnace from the perspective of improving the reaction efficiency by compensating for endothermic heat.

[0055] The heat source is not particularly limited. For example, the heat generated by burning biomass is suitable as a heat source. By utilizing the heat generated by burning biomass, it is possible to reduce not only the CO2 emitted from the direct reduction furnace but also the CO2 emitted from the entire production process to substantially zero. Semi-carbonized biomass and tar that is obtained in the process of pyrolysis of biomass are preferred as the biomass. Carbonized biomass and tar have a high amount of heat generated per unit volume and are considered promising alternative fuels to coal. The combustion heat of the biomass can be supplied from the reducing gas injection port or the high-temperature gas injection port. However, when a reducing gas containing hydrogen as the main component is used, the temperature tends to decrease from the upper section to the middle section of the direct reduction furnace. Therefore, the reaction efficiency is improved by supplying heat from the upper section to the middle section of the direct reduction furnace. Heat is thus preferably supplied from, for example, the high-temperature gas injection port.

[0056] Here, biomass is a generic term for a certain amount of accumulated plant and animal resources and waste derived from these resources (excluding fossil resources). In the method of operating a direct reduction furnace according to an embodiment of the present disclosure, any biomass that produces charcoal when pyrolyzed, such as agricultural, forestry, livestock, fisheries, and waste biomass, can be used. In particular, biomass with a high effective amount of heat generated, such as woody biomass, is preferred.

[0057] An example of woody biomass is forestry-derived biomass.

[0058] Examples of forestry-derived biomass include:- papermaking by-products such as pulp black liquor and chip dust,- sawmill by-products such as bark and sawdust, and forest residual material such as branches, leaves, tops, and short pieces of lumber,- thinned timber such as cedar, cypress, and pine, and material from specialty forest products such as waste logs from edible fungi cultivation, and- firewood and charcoal trees such as castanopsis, oak, and pine, and short-rotation forestry trees such as willow, poplar, eucalyptus, and pine.

[0059] Furthermore, some waste-based biomass, such as- general waste such as pruned branches from municipal roadside trees and garden trees of private homes, and- pruned branches from national or prefectural roadside trees and corporate garden trees, and industrial waste such as construction and building wastecan also be suitably used as woody biomass.

[0060] In addition, some agricultural biomass, such as- rice husks, wheat straw, rice straw, sugarcane waste, palm oil, and the like, which are generated from waste or by-products, and- rice bran, rapeseed, soybeans, and the like, which are generated from energy cropscan also be suitably used as woody biomass.

[0061] Semi-carbonized biomass is biomass that is not completely carbonized but is partially carbonized and preferably has a density of 700 kg / m3 to 850 kg / m3. Semi-carbonized biomass can also be said to be biomass with increased density and strength as a result of subjecting biomass, such as woody biomass, to heat treatment to reduce the moisture content and promote carbonization.

[0062] The method of producing semi-carbonized biomass is not particularly limited. For example, the semi-carbonized biomass is preferably produced by subjecting non-carbonized biomass (hereinafter also referred to as raw biomass) to low-temperature heat treatment at 200 °C to 300 °C and compacting the result. This makes it possible to further increase the energy density. According to the above production method, for example, when woody biomass (raw biomass) with a density of 200 kg / m3 is used as the raw material, semi-carbonized biomass with a density of approximately 750 kg / m3 can be produced.

[0063] The amount of heat to be supplied to the direct reduction furnace is determined as follows, for example. Using the above formulas (i) and (ii), the difference between the enthalpy due to the reduction reaction calculated from the composition of the reducing gas used (for example, a gas having a hydrogen concentration of 100 vol%) and the enthalpy due to the reduction reaction calculated from the composition of the reducing gas used in general direct reduction ironmaking (for example, a gas having a volume ratio of H2 / CO of 1 to 3) is obtained. The enthalpy of the high-temperature gas is then subtracted from this enthalpy difference, and the resulting value is converted into an amount of heat corresponding to the amount of reduced iron production and taken as the amount of heat to be supplied to the direct reduction furnace.

[0064] [Raw material]The iron oxide used in the method of operating a direct reduction furnace according to an embodiment of the present disclosure is, for example, iron ore. Examples include lumped iron ore (lump ore), iron oxide pellets (powdered iron ore hardened into a spherical shape), and the like. The grade of iron ore used as the iron oxide, that is, the iron content, is not particularly limited. From the perspective of reduction in a shaft furnace, the iron content is typically preferably 65 mass% or more. However, in recent years, an increase in the price of high-grade ores such as those from South America has been expected. Accordingly, low-grade ores (having an Fe content of 63 mass% or less), such as inexpensive and abundant Australian ores, may also be used as needed.

[0065] Conditions other than those described above are not particularly limited, and a conventional method may be used.

[0066] [2] Method of producing reduced ironNext, the method of producing reduced iron will be described. The method of producing reduced iron according to an embodiment of the present disclosure produces reduced iron by the above-described method of operating a direct reduction furnace.

[0067] FIG. 2 is a schematic diagram illustrating an example of a conventional reduced iron production process. FIG. 3 is a schematic diagram illustrating an example of a method of producing reduced iron (production process) according to an embodiment of the present disclosure. In the drawings, reference numeral 1 denotes a shaft furnace, 1a denotes iron oxide, 1b denotes reduced iron, 10 denotes a deduster, 11 denotes a dehydrator, 12 denotes a natural gas supply, 13 denotes an air supply, 14 denotes a heating reformer, 15 denotes a reducing gas injection device, 16 denotes a CO2 separator, 17 denotes a hydrogen supply, 18 denotes a gas heater, and 19 denotes a methanol synthesis device.

[0068] In the conventional process for producing reduced iron illustrated in FIG. 2, iron oxide is charged from the top of a shaft furnace and gradually lowered. There, iron oxide is reduced by injecting high-temperature reducing gas from a central portion of the shaft furnace. The reduced iron is then discharged from the bottom of the shaft furnace. In this process, furnace top gas containing mainly CO, CO2, H2, and H2O is discharged from the top of the shaft furnace. The furnace top gas is subjected to dust removal by the deduster, and a portion is subjected to moisture adjustment in the deduster and fed to the heating reformer as raw material gas. Gas containing hydrocarbons, for example, natural gas from a natural gas supply, is supplied to the heating reformer along with the furnace top gas that has been subjected to moisture adjustment. The supplied gas is then heated in the heating reformer. A reforming reaction then occurs, producing high-temperature reducing gas containing mainly CO and H2. This reducing gas is then injected into the direct reduction furnace through a reducing gas injection device. Furthermore, the remaining portion of the furnace top gas is dehydrated and subsequently used as fuel for heating in the combustion chamber of the heating reformer.

[0069] On the other hand, in the method of operating a direct reduction furnace according to an embodiment of the present disclosure, a reducing gas containing hydrogen as the main component is used, as described above. Therefore, in the method of operating a direct reduction furnace according to an embodiment of the present disclosure, a heating reformer is not required. Instead, as illustrated in the example of FIG. 3, hydrogen gas supplied from a hydrogen supplier is heated by a gas heater. The H2 concentration is preferably 80 vol% or more. The H2 concentration is more preferably 90 vol% or more. The H2 concentration is even more preferably 95 vol% or more. The heating temperature of the hydrogen gas is not particularly limited but is preferably 900 °C to 1200 °C, for example. Next, heated hydrogen gas is injected as a reducing gas into the shaft furnace by a reducing gas injection device. The reducing gas is subjected to a reduction reaction inside the shaft furnace and then discharged from the shaft furnace as furnace top gas containing mainly H2 and H2O. Separately from the reducing gas, a high-temperature gas containing nitrogen as the main component is injected into the direct reduction furnace. At this time, as described above, the vertical injection position of the high-temperature gas is set higher than the vertical injection position of the reducing gas. In the example of FIG. 3, the furnace top gas is subjected to dust removal by the deduster and dehydration by the dehydrator and is then mixed with newly introduced H2 and injected into the shaft furnace as reducing gas.

[0070] Furthermore, when the furnace top gas includes CO2 because the reducing gas or the high-temperature gas includes CO or CH4, the CO2 included in the furnace top gas is preferably separated, recovered, and used for basic chemical synthesis (CCU) or storage (CCS). In the example of FIG. 3, an example of performing CCU is illustrated. That is, in the example of FIG. 3, the furnace top gas is subjected to CO2 separation by a CO2 separator, and the separated CO2 is then supplied to a methanol synthesis device. The CO2 emissions from the entire process of producing reduced iron can thus be reduced to substantially zero.

[0071] The heat source of the gas heater is not particularly limited. For example, the heat generated by burning biomass is suitable as a heat source. By utilizing the heat generated by burning biomass, it is possible to reduce not only the CO2 emitted from the direct reduction furnace but also the CO2 emitted from the entire production process to substantially zero. The aforementioned semi-carbonized biomass and the tar obtained in the process of pyrolysis of biomass are preferred as the biomass.

[0072] Conditions other than those described above are not particularly limited, and a conventional method may be used.

[0073] In the method of operating a direct reduction furnace and the method of producing reduced iron according to an embodiment of the present disclosure, a method using a shaft furnace has been described in particular. However, the type of direct reduction furnace is not limited to this and may be a fluidized bed, a rotary kiln, a rotary hearth furnace (RHF), or the like. Shaft furnaces are preferred as direct reduction furnaces because of their high production efficiency, ratio of utilization, and operational stability. Furthermore, the majority of direct reduction furnaces in operation worldwide are Midrex® and Hyl®, which are shaft furnace systems. EXAMPLES

[0074] Hereinafter, a method of operating a direct reduction furnace and a method of producing reduced iron according to an embodiment of the present disclosure will be specifically described with reference to examples. However, the present disclosure is not limited to the examples described below.

[0075] A test of producing reduced iron by operating a direct reduction furnace was carried out using the reduced iron production process illustrated in FIG. 3. The test conditions are as indicated in Table 1 and below. Conditions not specified were in accordance with conventional methods or the general description section of the specification. In addition, the vertical injection position of the high-temperature gas in Table 1 is a representative position (vertical middle position).Height of direct reduction furnace (shaft furnace): 10 mInner diameter of direct reduction furnace: 5.5 mProduction rate of reduced iron: 200 t / hOperation period: 28 daysComposition of reducing gas: H2 concentration 90 vol% (by volume ratio, H2:N2 = 9:1)Temperature of reducing gas: 1000 °CPressure of reducing gas: 200 kPaGInjection amount of reducing gas: 2200 Nm3 / t-DRI (injection amount of reducing gas per tonne of reduced iron produced)Vertical injection position (height) of reducing gas: 1.0 mTemperature of high-temperature gas: 900 °CIron oxide: Brazilian iron oxide pellets with a particle size of 10.0 mm to 15.0 mmMain chemical composition of iron oxide: in mass%, T. Fe: 66 %, FeO: 0.63 %, SiO2: 2.0 %, CaO: 2.1 %, Al2O3: 0.5 %, MgO: 0.16 %, C: 0.1 %

[0076] Prior to the operation of the direct reduction furnace, a computational simulation was carried out using a one-dimensional mathematical model with the above test conditions as the planned operating conditions, and the progress reduction degree A(y) in the shaft furnace was determined. In addition, a computational simulation was performed using the same one-dimensional mathematical model to determine the temperature distribution of iron oxide in the height direction inside the shaft furnace during operation. FIG. 4 illustrates the results. As indicated in FIG. 4, the height position at which the progress reduction degree A(y) was 30 % to 60 % was a position at which the height y of the direct reduction furnace was about 2.5 m to 6.0 m. The height position at which the progress reduction degree A(y) was 30 % to 40 % was a position at which the height y of the direct reduction furnace was about 4.0 m to 6.0 m. The height y of the direct reduction furnace is taken as 0 m at the lowest point of the reduction zone of the direct reduction furnace (the boundary between the reduction zone and the cooling zone).

[0077] Furthermore, in some examples, to compensate for endothermic heat during hydrogen reduction, biomass was combusted using a boiler, and the combustion heat was supplied to the direct reduction furnace.

[0078] The pressure loss (hereinafter also referred to as the pressure drop) and reaction efficiency in the direct reduction furnace were evaluated in the following manner.

[0079] [Evaluation of pressure drop in direct reduction furnace (hereinafter also referred to as Evaluation 1)]As reduction disintegration of iron oxide progresses, the particle size of the iron oxide decreases. Therefore, the pressure drop in the direct reduction furnace generally tends to increase according to the Ergun equation, for example. In Evaluation 1, the degree of reduction disintegration was evaluated based on the pressure drop in the direct reduction furnace.

[0080] That is, the pressure (kPaG) of the reducing gas at the reducing gas injection port and the pressure (kPaG) of the furnace top gas at the furnace top gas discharge port were measured, and the difference between these pressures was calculated as the pressure drop in the direct reduction furnace.

[0081] The evaluation results are listed in Table 1. Here, the meanings of A to D in the column of Evaluation 1 in Table 1 are as follows.A: Pressure drop in the direct reduction furnace is less than 50 kPaB: Pressure drop in the direct reduction furnace is 50 kPa or more and less than 70 kPaC: Pressure drop in the direct reduction furnace is 70 kPa or more and less than 100 kPaD: Pressure drop in the direct reduction furnace is 100 kPa or more

[0082] Generally, when the pressure drop in the direct reduction furnace is 100 kPa or more, improper descent, such as hanging, occurs in the direct reduction furnace, and the productivity of reduced iron decreases. Therefore, the cases of A, B and C were evaluated as pass (reduction disintegration was sufficiently controlled and stable operation was possible).

[0083] [Evaluation of reaction efficiency (hereinafter also referred to as Evaluation 2)]Evaluation 2 was performed based on the reduction degree of the reduced iron obtained as a product. Here, the reduction degree is defined by the following formula.[reduction degree (unit: %)] = {([amount of oxygen in iron oxide before reduction (unit: mass%)] − [amount of oxygen in reduced iron (unit: mass%)]) / [amount of oxygen in iron oxide before reduction (unit: mass%)]} × 100

[0084] The amount of oxygen (mass%) in iron oxide before reduction is the total amount of oxygen (mass%) included in the FeO and Fe2O3 contained in the iron oxide before reduction. Here, it was assumed that the Fe obtained by subtracting the amount of Fe (mass%) in FeO from the T. Fe (mass%) of the iron oxide before reduction is present as Fe2O3, and the total amount of oxygen (mass%) included in Fe2O3 was calculated.The amount of oxygen (mass%) in the reduced iron (obtained after reduction) is the total amount of oxygen (mass%) included in the FeO and Fe3O4 contained in the reduced iron. Here, it was assumed that the Fe obtained by subtracting the amount of Fe (mass%) in FeO and M. Fe from the T. Fe (mass%) of the reduced iron is present as Fe3O4, and the total amount of oxygen (mass%) included in Fe3O4 was calculated.

[0085] The evaluation results are listed in Table 1. The meanings of A to D in the column of Evaluation 2 in Table 1 are as follows.A: Reduction degree of 98 % or moreB: Reduction degree of 95 % or more but less than 98 %C: Reduction degree of 90 % or more but less than 95 %D: Reduction degree of less than 90 %

[0086] The reduced iron discharged from a direct reduction furnace is subsequently melted in an electric furnace. The reduction degree of the reduced iron obtained as a product is therefore generally desired to be 90 % or more. The cases A, B and C were thus evaluated as having excellent reaction efficiency.

[0087] [Table 1]

[0088] As illustrated in Table 1, in the Examples of the present disclosure, reduction disintegration was sufficiently controlled, the pressure drop in the direct reduction furnace was small, and the reaction efficiency was also excellent. The amount of CO2 emitted outside the reduced iron production process was also reduced to substantially zero. By setting the vertical injection position of the high-temperature gas to a height at which the progress reduction degree A(y) was 30 % to 40 %, reduction disintegration was more effectively controlled, and the pressure drop in the direct reduction furnace was further reduced. Furthermore, by setting the amount of the high-temperature gas to 10 vol% to 60 vol% of the amount of reducing gas, the reduction disintegration was more effectively controlled, and the pressure drop in the direct reduction furnace was further reduced. At the same time, this also effectively contributed to improving the reaction efficiency. In addition, supplying heat to the direct reduction furnace also contributed effectively to improving the reaction efficiency. In particular, in Nos. 9 and 10, in which the vertical injection position of the high-temperature gas was set to a height position at which the progress reduction degree A(y) was 30 % to 40 %, and the amount of high-temperature gas was 10 vol% to 60 vol% of the amount of reducing gas, the pressure drop in the direct reduction furnace was very small, and reduction disintegration was more effectively controlled. The reaction efficiency was also extremely high.

[0089] For reference, the progress reduction degree A(y) in the shaft furnace when high-temperature gas was injected under the conditions of No. 9 and the temperature distribution of iron oxide in the height direction inside the shaft furnace during operation were determined by computational simulation using the same one-dimensional mathematical model as above. FIG. 5 illustrates the results. From these results, it can be seen that, compared to the case in which high-temperature gas is not injected (FIG. 4), the range of height positions at which the progress reduction degree A(y) is 30 % to 60 % (the range in which the occurrence of the aforementioned cracks becomes noticeable) in particular is narrowed (from "y = approximately 2.5 m to 6.0 m" to "y = approximately 4.0 m to 7.0 m"). It is also clear that the reduction degree of the reduced iron obtained as a product (progress reduction degree A(0) at y = 0 m) is also significantly improved (from "about 80 %" to "about 95 %").

[0090] On the other hand, in all of the Comparative Examples, reduction disintegration was not sufficiently controlled, and the pressure drop in the direct reduction furnace increased. Furthermore, the reaction efficiency was sometimes insufficient.

[0091] Similar results were also obtained when reduced iron was produced under a set of conditions in which the H2 concentration of the reducing gas was set to 80 vol% or more, and the vertical injection position of the high-temperature gas, the N2 concentration and amount of the high-temperature gas, and the like were varied. REFERENCE SIGNS LIST

[0092] 1 Shaft furnace1a Iron oxide1b Reduced iron1c Furnace top1d Reduction zone1e Cooling zone2 Iron oxide charging port3 Furnace top gas discharge port4 High-temperature gas injection port5 Reducing gas injection port7 Cooling gas injection port8 Cooling gas suction port9 Reduced iron discharge port10 Deduster11 Dehydrator12 Natural gas supply13 Air supply14 Heating reformer15 Reducing gas injection device16 CO2 separator17 Hydrogen supply18 Gas heater19 Methanol synthesis device 

Claims

1. A method of operating a direct reduction furnace, the method comprising: a charging process of charging iron oxide into the direct reduction furnace; a first injection process of injecting a reducing gas containing hydrogen as a main component into the direct reduction furnace; a second injection process of injecting a high-temperature gas containing nitrogen as a main component into the direct reduction furnace; and a reducing process of reducing the iron oxide in the direct reduction furnace to obtain reduced iron, wherein a vertical injection position of the high-temperature gas in the second injection process is higher than a vertical injection position of the reducing gas in the first injection process.

2. The method of operating a direct reduction furnace according to claim 1, wherein the vertical injection position of the high-temperature gas is set to a height position at which a progress reduction degree A(y) is 30 % to 60 %, and the progress reduction degree A(y) is a reduction degree of iron oxide at a height position y in the direct reduction furnace.

3. The method of operating a direct reduction furnace according to claim 2, wherein the vertical injection position of the high-temperature gas is set to a height position at which the progress reduction degree A(y) is 30 % to 40 %.

4. The method of operating a direct reduction furnace according to claim 2 or 3, wherein the progress reduction degree A(y) is determined based on operating conditions planned for the direct reduction furnace.

5. The method of operating a direct reduction furnace according to any one of claims 1 to 4, wherein a H2 concentration of an injection gas consisting of the reducing gas and the high-temperature gas is 60 vol% or more.

6. The method of operating a direct reduction furnace according to any one of claims 1 to 5, wherein an amount of the high-temperature gas is 10 vol% to 60 vol% of an amount of the reducing gas.

7. The method of operating a direct reduction furnace according to any one of claims 1 to 6, wherein a H2 concentration of the reducing gas is 80 vol% or more.

8. The method of operating a direct reduction furnace according to any one of claims 1 to 7, further comprising a heat supply process of supplying heat to the direct reduction furnace.

9. The method of operating a direct reduction furnace according to claim 8, wherein heat from combustion of biomass is used as a heat source in the heat supply process.

10. A method of producing reduced iron, the method comprising producing reduced iron by the method of operating a direct reduction furnace according to any one of claims 1 to 9. 

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