Operation method of direct reduction furnace and method for producing reduced iron

Abstract

Provided is an operation method of a direct reduction furnace capable of reducing a CO2 discharge amount, avoiding troubles such as decrease in productivity and operation stop, and performing stable operation. The circulation furnace top gas is blown into a direct reduction furnace, and the blowing position of the circulation furnace top gas in the vertical direction is set to a height position where the reduction progress rate A(y) is 30-60%.

Description

Direct reduction furnace operation method and reduced iron production method

[0001] The present invention relates to a method for operating a direct reduction furnace and a method for producing reduced iron.

[0002] In recent years, steelworks have been strongly required to save energy against the backdrop of global environmental issues and the depletion of fossil fuels. The raw material for iron is mainly iron oxide such as iron ore, and a reduction process to reduce this iron ore is essential in steelworks. The most common reduction process that is widespread worldwide is the blast furnace. In a blast furnace, coke or pulverized coal reacts with oxygen in hot air (air heated to about 1200°C) in the tuyere. This reaction produces CO and H, which become reducing gases. ２ These reducing gases reduce iron ore and other materials in the furnace. Thanks to recent improvements in blast furnace operation technology, the reducing agent rate (the amount of coke and pulverized coal used to produce 1 ton of molten iron) has been reduced to about 500 kg / t, which is already at its lower limit. Therefore, a further significant reduction in the reducing agent rate cannot be expected.

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

[0004] The direct reduction ironmaking process is as follows. Specifically, 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 (spherical iron ore powder). A reducing gas is then injected into the direct reduction furnace to reduce the iron oxide and obtain reduced iron. The obtained reduced iron is then cooled in a region (cooling zone) below the reducing gas injection position of 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 (registered trademark) or Hyl (registered trademark), is generally used as the source of the reducing gas. In this case, the natural gas is reformed with the exhaust gas (hereinafter also referred to as top gas) discharged from the top of the direct reduction furnace to produce CO and H. ２ The reducing gas is then blown into a reduction furnace to reduce the iron oxide according to the following formula, thereby obtaining reduced iron: ２ O ３ +3CO→2Fe+3CO ２ ΔH ２９８ =-247kJ / kg-Fe (i) Fe ２ O ３ +3H ２ → 2Fe + 3H ２ O ΔH ２９８ = 858 kJ / kg-Fe (ii) Here, kJ / kg-Fe is a unit that represents the amount of change in enthalpy (kJ) per kg of Fe (iron).

[0006] In this way, in the general direct reduction ironmaking process, CO and H ２ Since a reducing gas containing ２ is produced and discharged out of the system.

[0007] In recent years, CO ２ There is a growing need to reduce CO emissions, and the steelmaking process is also ２ Therefore, in the direct reduction ironmaking process, a technology using a reducing gas whose main component is hydrogen is being considered. By using a reducing gas whose main component is hydrogen, the main component of the furnace gas becomes H. ２ O and CO ２ A significant reduction in emissions can be expected.

[0008] As an example of such a technique, Patent Document 1 discloses, "A method for operating a direct reduction furnace using a shaft furnace system for producing reduced iron using a reducing gas mainly containing hydrogen, the method comprising the step of charging a raw material iron oxide that has been preheated in advance into the direct reduction furnace."

[0009] Patent Document 2 discloses "a method for operating a direct reduction furnace using a shaft furnace system to produce reduced iron using a reducing gas mainly composed of hydrogen, the method comprising the step of blowing a portion of the gas discharged from the top of the furnace into the middle of the furnace, and circulating the furnace top gas."

[0010] Patent Document 3 discloses "a method for producing reduced iron by reducing iron oxide charged into a shaft furnace, characterized in that a heated mixed gas containing a reducing gas containing 90% by volume or more of hydrogen gas and nitrogen gas is blown into the shaft furnace."

[0011] JP 2012-102371 A JP 2012-102372 A International Publication No. 2021 / 230307

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

[0013] The present invention has been developed to solve the above problems, and ２ An object of the present invention is to provide a method for operating a direct reduction furnace that reduces waste and enables stable operation by avoiding problems such as reduced productivity and shutdowns. Another object of the present invention is to provide a method for producing reduced iron by using the above-mentioned method for operating a reduction furnace. In this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively.

[0014] The inventors have conducted extensive research into the above-mentioned problems and have come to the following conclusions. (1) In direct reduction ironmaking, iron oxide, such as pellets, may be pulverized during the reduction process (hereinafter referred to as reduction pulverization). In particular, when iron oxide is reduced using a reducing gas primarily composed of hydrogen, reduction pulverization is more pronounced than when natural gas is used as the reducing gas, which is likely to lead to increased pressure loss in the direct reduction furnace. As a result, poor descent, such as raw material hanging, occurs, leading to problems such as reduced productivity and operational shutdowns. (2) Furthermore, when iron oxide is reduced using a reducing gas primarily composed of hydrogen, fine, streaky cracks are likely to occur in the iron oxide crystal lattice, and these cracks become the starting point for fractures. (3) Specifically, in a direct reduction furnace, iron oxide is charged through an iron oxide charging port at the top of the furnace and gradually lowered. Reducing gas is then injected through a reducing gas inlet to reduce the iron oxide and obtain reduced iron. In this way, the reduction of the iron oxide charged into the direct reduction furnace progresses as it descends. Therefore, during operation of the direct reduction furnace, the reduction rate of iron oxide varies depending on the height position y of the direct reduction furnace. Hereinafter, the reduction rate of iron oxide at height position y in the direct reduction furnace is referred to as the reduction progress rate A(y) of iron oxide. (4) Herein, the cracks described above occur significantly when iron oxide remains at a height position where the reduction progress rate A(y) is 30 to 60%, particularly at a height position where the reduction progress rate A(y) is 30 to 40%. In other words, to suppress reduction disintegration, it is effective to increase the reduction reaction rate at the height position (hereinafter also referred to as the reduction reaction rate) and narrow the range of the height position; in other words, to shorten the residence time of iron oxide at the height position. (5) To increase the reduction reaction rate at the height position, it is effective to increase the temperature at the height position. To this end, it is essential to utilize a portion of the top gas (hereinafter also referred to as circulating top gas), which is the gas discharged from the top of the direct reduction furnace, specifically, to inject the circulating top gas into the direct reduction furnace at the above-mentioned height position in the vertical direction. The present invention was completed based on the above findings and further investigations.

[0015] That is, the gist of the present invention is as follows: 1. A method for operating a direct reduction furnace, comprising: a charging step of charging iron oxide into the direct reduction furnace; a first injection step of injecting a reducing gas mainly composed of hydrogen into the direct reduction furnace; a second injection step of injecting circulating top gas into the direct reduction furnace; and a reduction step of reducing the iron oxide in the direct reduction furnace to obtain reduced iron, wherein the circulating top gas is a part of the top gas, and the top gas is a gas discharged from the top of the direct reduction furnace, and in the second injection step, the circulating top gas is injected at a vertical position at which a reduction progress rate A(y) is 30 to 60%, where the reduction progress rate A(y) is the reduction progress rate of iron oxide at height position y in the direct reduction furnace.

[0016] 2. The method for operating a direct reduction furnace according to 1 above, wherein the vertical injection position of the circulating furnace top gas is set to a height position at which the reduction progress rate A(y) is 30 to 40%.

[0017] 3. The method for operating a direct reduction furnace according to 1 or 2 above, wherein the reduction progress rate A(y) is determined based on planned operating conditions of the direct reduction furnace.

[0018] 4. The method for operating a direct reduction furnace according to any one of 1 to 3 above, wherein the temperature of the circulating furnace top gas is controlled before the second blowing step.

[0019] 5. The method for operating a direct reduction furnace according to any one of 1 to 4 above, wherein the amount of the circulating furnace top gas is 10 to 50 volume % of the total amount of the furnace top gas.

[0020] 6. H in the reducing gas ２ 6. The method for operating a direct reduction furnace according to any one of 1 to 5, wherein the concentration is 80% by volume or more.

[0021] 7. The method for operating a direct reduction furnace according to any one of 1 to 6 above, further comprising a heat supply step of supplying heat to the direct reduction furnace.

[0022] 8. The method for operating a direct reduction furnace according to 7 above, wherein heat from combustion of biomass is used as the heat source for the heat supply step.

[0023] 9. A method for producing reduced iron, comprising producing reduced iron by the method for operating a direct reduction furnace according to any one of 1 to 8 above.

[0024] According to the present invention, CO is generated by using a reducing gas containing hydrogen as the main component. ２ It is possible to stably operate a direct reduction furnace while reducing emissions and avoiding problems such as reduced productivity and shutdowns. Furthermore, the method for operating a reduction furnace of the present invention is extremely advantageous in terms of improving reaction efficiency.

[0025] Fig. 1 is a schematic diagram showing an example of the configuration of a direct reduction furnace used in a method for operating a direct reduction furnace according to an embodiment of the present invention. Fig. 2 is a schematic diagram showing an example of a conventional process for producing reduced iron. Fig. 3 is a schematic diagram showing an example of a method for producing reduced iron according to an embodiment of the present invention. Fig. 4 is a diagram showing an example of the relationship between the height position y of a direct reduction furnace and the reduction progress rate A(y).

[0026] [1] Method for Operating a Direct Reduction Furnace Hereinafter, a method for operating a direct reduction furnace according to one embodiment of the present invention will be described.

[0027] A method of operating a direct reduction furnace according to one embodiment of the present invention comprises: a charging step of charging iron oxide into the direct reduction furnace; a first injection step of injecting a reducing gas mainly composed of hydrogen into the direct reduction furnace; a second injection step of injecting a circulating top furnace gas into the direct reduction furnace; and a reduction step of reducing the iron oxide in the direct reduction furnace to obtain reduced iron, wherein the circulating top furnace gas is a part of the top furnace gas, and the top furnace gas is a gas discharged from the top of the direct reduction furnace, and in the second injection step, the injection position of the circulating top furnace gas in the vertical direction is set to a height position at which a reduction progress rate A(y) is 30 to 60%.

[0028] 1 is a schematic diagram showing an example of the configuration of a direct reduction furnace (shaft furnace) used in a method for operating a direct reduction furnace according to one embodiment of the present invention. In the figure, reference numeral 1 denotes a shaft furnace, 1a denotes iron oxide, 1b denotes reduced iron, 1c denotes a 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 outlet, 4 denotes a circulating furnace top gas inlet, 5 denotes a reducing gas inlet, 6 denotes a temperature control device, 7 denotes a cooling gas inlet, 8 denotes a cooling gas suction port, and 9 denotes a reduced iron discharge port.

[0029] In a shaft furnace, iron oxide is charged from the top of the furnace, particularly through the iron oxide charging port, and gradually falls in the reduction zone. High-temperature reducing gas, primarily composed of hydrogen, is injected into the shaft furnace through a reducing gas inlet provided in the reduction zone, thereby reducing the iron oxide. The reduced iron is then discharged through a reduced iron outlet located at the bottom of the shaft furnace. Meanwhile, top gas is discharged through a top gas outlet. Furthermore, circulating top gas, a portion of the top gas, is re-injected into the shaft furnace through the circulating top gas inlet. Before injecting the circulating top gas, the temperature of the circulating top gas is optionally controlled using a temperature control device. Note that the hydrogen-based reducing gas is not injected into areas other than the reduction zone (e.g., the cooling zone, the reduced iron outlet, etc.).

[0030] In addition, a cooling gas inlet and a cooling gas suction port are arranged in the cooling zone located at the bottom of the shaft furnace. Cooling gas is blown into the cooling zone from the cooling gas inlet. The cooling gas suction port sucks the cooling gas so that it does not enter the furnace top. For example, N ２ can be used.

[0031] Hereinafter, each step of the method for operating a direct reduction furnace according to one embodiment of the present invention will be described in detail. Note that the charging step can be performed in a conventional manner, and therefore a description thereof will be omitted here.

[0032] [First Injection Step] In the first injection step, a reducing gas containing hydrogen as a main component is injected into the direct reduction furnace. ２ The concentration is preferably 80% by volume or more, more preferably 90% by volume or more, and even more preferably 95% by volume or more.２ The upper limit of the concentration is not particularly limited. ２ The concentration may be 100% by volume. ２ The type of the remaining gas other than the above is not particularly limited. The remaining gas may be, for example, N ２ , H ２ O, CO and CO ２ In addition, gases produced as by-products in the steelmaking process (hereinafter also referred to as 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 CO ２ If the furnace gas contains CO ２ In this case, the CO contained in the furnace gas ２ As will be described later, it is preferable to use the resulting wastewater for chemical synthesis (CCU) or storage (CCS).

[0033] The temperature of the reducing gas blown through the reducing gas blowing port is, for example, 700°C to 1200°C. If the temperature of the reducing gas is less than 800°C, the heat inside the furnace may be insufficient, and the reduction reaction may be delayed. 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.

[0034] [Second Injection Step] In the second injection step, circulating furnace top gas is injected into the direct reduction furnace. It is extremely important that the vertical injection position of the circulating furnace top gas is at a height where the reduction progress rate A(y) is 30 to 60%.

[0035] Vertical Injection Position of Circulating Top Gas: Height at Which the Reduction Progress Rate A(y) is 30 to 60% By injecting circulating top gas vertically at a height at which the reduction progress rate A(y) is 30 to 60%, reduction disintegration can be significantly suppressed. Specifically, when iron oxide is reduced using a reducing gas primarily composed of hydrogen, fine streak-like cracks form in the crystal lattice of the iron oxide, and these cracks become the starting point for fracture. These cracks occur most frequently when iron oxide resides at heights at which the reduction progress rate A(y) is 30 to 60%, particularly at heights at which the reduction progress rate A(y) is 30 to 40%. In other words, reducing disintegration can be effectively suppressed by increasing the reduction reaction rate at these heights and narrowing the range of these heights—in other words, by shortening the residence time of iron oxide at these heights. Increasing the temperature at these heights can effectively increase the reduction reaction rate at these heights. For this purpose, it is essential to inject the circulating furnace top gas into the direct reduction furnace at the height position mentioned above. This significantly suppresses reduction disintegration. Furthermore, it also effectively contributes to improving the reaction efficiency. Therefore, the vertical injection position of the circulating furnace top gas is set at a height position where the reduction progress rate A(y) is 30 to 60%, preferably a height position where the reduction progress rate A(y) is 30 to 40%.

[0036] The inventors believe that the reason why the cracks occur prominently at the height position is as follows: In the reduction of iron oxide with hydrogen, the reaction progresses as hydrogen atoms diffuse into the crystal lattice of the iron oxide. ２ O ３ From Fe ３ O ４ As the reduction to FeO progresses, oxygen (O) is removed from the iron oxide. Therefore, the volume of iron oxide (cm) per mole of Fe ３ / mol) decreases, and internal stress occurs due to volumetric shrinkage. This internal stress is alleviated by the generation 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. ２ O ３ From Fe ３ O４ As reduction progresses to FeO and Fe, the volume shrinkage due to reduction increases, and fine streaky cracks are more likely to occur. In particular, in a structure mainly composed of FeO, where the amount of Fe produced is small, fine streaky cracks are more likely to occur. On the other hand, when Fe begins to be produced, a solid-phase sintering phenomenon between Fe particles progresses, which has the effect of sealing the cracks. Here, the iron oxide before the start of reduction becomes Fe ２ O ３ When the reduction progress rate A(y) is 11%, the iron oxide is substantially Fe ３ O ４ Furthermore, when the reduction progress rate A(y) is 33%, the iron oxide is substantially composed of FeO. If the reduction progresses further than this, Fe begins to be produced. Therefore, the inventors believe that the above-mentioned cracks and reduction disintegration occur significantly when iron oxide remains at a height position where the reduction progress rate A(y) is 30 to 60%, particularly at a height position where the reduction progress rate A(y) is 30 to 40%.

[0037] Here, the reduction progress rate A(y) can be calculated by the following formula: [Reduction progress rate A(y) (unit: %)] = ([oxygen content of iron oxide before reduction (unit: mass %)] - [oxygen content of iron oxide at height position y of the direct reduction furnace (unit: mass %)]) / [oxygen content of iron oxide before reduction (unit: mass %)] × 100 The oxygen content (mass %) of iron oxide before reduction is determined by the amount of FeO and Fe ２ O ３ is the total amount of oxygen (mass%) contained in Fe ２ O ３ The amount of oxygen contained in the iron oxide before reduction is, for example, the amount of Fe obtained by subtracting the amount of Fe (mass%) in FeO from the total mass (mass%) of Fe atoms. ２ O ３ It is assumed that it exists as Fe ２ O ３ The total amount of oxygen (mass%) contained in the iron oxide at the height position y in the direct reduction furnace (hereinafter also referred to as iron oxide at the height position y) can be calculated by the sum of FeO and Fe ３ O ４For example, the amount of Fe (mass%) in FeO and M.Fe (metallic Fe) minus the amount of Fe (mass%) in FeO and M.Fe (metallic Fe) from the total mass (mass%) of Fe atoms in iron oxide at height position y is ３ O ４ It is assumed that it exists as Fe ３ O ４ The total amount of oxygen (mass%) contained in the

[0038] The reduction progress rate A(y) is preferably determined based on the planned operating conditions of the direct reduction furnace (hereinafter also referred to as planned operating conditions). Examples of methods for determining the reduction progress rate A(y) include chemical analysis of iron oxide, analysis of the gas concentration in the furnace, and a method using computational simulation. Of these, the method using computational simulation is preferred. Specific examples of each method are as follows:

[0039] Chemical analysis of iron oxide: Preliminary operation of the direct reduction furnace is carried out according to the planned operating conditions, and during the operation, the discharge of reduced iron and the injection of reducing gas are suddenly stopped. ２ Gas is blown into the direct reduction furnace to quench the iron oxide inside the furnace. The iron oxide is then discharged little by little from the bottom of the furnace. Iron oxide that is thought to have accumulated at various vertical heights in the furnace based on the volume of the discharged iron oxide is then sampled and the reduction progress rate A(y) is determined by chemical analysis. In an alternative embodiment to the above, the direct reduction furnace is pre-operated according to the planned operating conditions. Iron oxide samples during reduction are then collected through sampling ports installed at various vertical heights in the furnace, and the reduction progress rate A(y) of the iron oxide is determined by chemical analysis.

[0040] Analysis of gas concentration in furnace gas: The direct reduction furnace is pre-operated according to the planned operating conditions. Then, the furnace gas is sampled from gas sampling pipes installed 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 amount of hydrogen and other components reduced from the reducing gas before it was injected into the direct reduction furnace, and the reduction progress rate A(y) of iron oxide is calculated.

[0041] Computational Simulation Computational simulation (for example, a two-dimensional DEM-CFD method that combines a one-dimensional mathematical model or a DEM (Discrete Element Method) that calculates the behavior of solid particles with a CFD (Computational Fluid Dynamics) method that performs gas fluid calculations) is performed in accordance with the planned operating conditions to calculate the reduction progress rate A(y) of iron oxide.

[0042] 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 component composition). Furthermore, when determining the reduction progress rate A(y), the circulation of the furnace top gas (injection of circulating furnace top gas) is not taken into consideration. The above-mentioned chemical analysis of iron oxide and analysis of the gas concentration in the furnace gas may be performed during actual operation of the direct reduction furnace (rather than during pre-operation), and the reduction progress rate A(y) may be determined based on these results.

[0043] In addition to the above-mentioned methods, the reduction progress rate A(y) may be determined based on, for example, past operational data of a direct reduction furnace similar to the planned operating conditions, literature, or the like.

[0044] In addition, when the reduction rate of iron oxide at height position y of the direct reduction furnace varies (is not constant) in the furnace radial direction, the average value of the reduction rates 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 rate of iron oxide at height position y of the direct reduction furnace. Here, the dimensionless radius of the direct reduction furnace is the 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.

[0045] The vertical injection position of the circulating furnace top gas can be adjusted, for example, by providing a plurality of circulating furnace top gas inlets in the height direction (vertical direction) of the direct reduction furnace and determining the circulating furnace top gas inlet to be actually used depending on the reduction progress rate A(y) of iron oxide. The vertical injection position of the circulating furnace top gas can also be adjusted by providing the circulating furnace top gas inlet with a lifting function (a function for changing the position in the vertical direction) or a function for adjusting the injection angle.

[0046] The representative position (vertical middle position) of the vertical injection positions of the circulating furnace top gas may be a height position at which the reduction progress rate A(y) is 30 to 60%, preferably a height position at which the reduction progress rate A(y) is 30 to 40%. In particular, it is preferable that the entire vertical injection position of the circulating furnace top gas is included in the range of height positions.

[0047] Optionally, the temperature of the circulating top gas may be controlled before being blown into the direct reduction furnace. As described above, increasing the temperature at the height position is effective in suppressing reduction disintegration. To achieve this, it is effective to raise the temperature of the circulating top gas higher than the temperature at the height position, preferably at least 50°C higher than the temperature at the height position at which the reduction progress rate A(y) is 60%, and more preferably at least 100°C higher than the temperature at the height position at which the reduction progress rate A(y) is 60%. The upper limit of the temperature of the circulating top gas is not particularly limited. For example, the temperature of the circulating top gas is preferably 1200°C or lower, more preferably 1000°C or lower. The temperature control method for the circulating top gas is not particularly limited, and a general gas heating device may be used. In one example, the temperature of the top gas is 300 to 400°C. The temperature at the height position at which the reduction progress rate A(y) is 60% may be measured, for example, using a thermometer installed in the direct reduction furnace, or may be determined by the above-mentioned calculation simulation.

[0048] Furthermore, the amount of circulating top gas (amount injected into the direct reduction furnace) is preferably 10 to 50% by volume of the total amount of top gas (total amount of top gas discharged from the top gas outlet). If the amount of circulating top gas is less than 10% by volume of the total amount of top 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 circulating top gas exceeds 50% by volume of the total amount of top gas, the pressure loss of the gas increases around the inlet of the circulating top gas, and problems such as poor descent due to hanging of iron oxide may occur. In addition, the circulating top gas contains H ２ H produced by reduction of ２ Since it contains O, H ２The concentration may decrease, which may be unfavorable for the reaction equilibrium and may result in stagnation of the reduction reaction. Furthermore, the temperature of the reduced iron increases when it passes through the cooling zone of the direct reduction furnace, which may cause fusion of the reduced iron particles (also known as clustering), making it difficult for the reduced iron to be discharged from the lower part of the direct reduction furnace. Therefore, the amount of circulating top gas is preferably 10 to 50% by volume of the total amount of top gas. The amount of circulating top gas is more preferably 20% by volume or more of the total amount of top gas. The amount of circulating top gas is more preferably 40% by volume or less of the total amount of top gas. The amount of circulating top gas may be determined by performing the above-mentioned calculation simulation according to the planned operating conditions.

[0049] [Reduction Step] In the reduction step, iron oxide is reduced using a reducing gas containing hydrogen as a main component, for example, according to the above formula (i) to obtain reduced iron. The reducing gas is subjected to a reduction reaction in the direct reduction furnace and then discharged as top gas from the top of the direct reduction furnace. The temperature of the top gas is, for example, 300 to 400°C.

[0050] [Heat Supply Step] The method for operating a direct reduction furnace according to one embodiment of the present invention preferably further includes a heat supply step of supplying heat to the direct reduction furnace. As described above, in a general direct reduction ironmaking process, CO and H ２ A reducing gas containing H ２ On the other hand, in the method for operating a direct reduction furnace according to one embodiment of the present invention, a reducing gas containing hydrogen as a main component is used. Here, as shown in the above formulas (i) and (ii), the reduction reaction with CO is an exothermic reaction, while the reduction reaction with H ２ The reduction reaction by H is an endothermic reaction. ２ When a gas with a concentration close to 100% by volume is used, 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 viewpoint of improving the reaction efficiency by endothermic compensation.

[0051] The heat source is not particularly limited. For example, the heat from biomass combustion is suitable as the heat source. By utilizing the heat from biomass combustion, CO emitted from the direct reduction furnace can be reduced. ２ Not only that, but also the CO emitted throughout the manufacturing process２ It is also possible to reduce the amount of CO₂ generated to substantially zero. The biomass used is preferably semi-carbonized biomass or tar obtained during the pyrolysis of biomass. Carbonized biomass and tar have a high calorific value per unit volume and are considered promising alternative fuels to coal. The heat from the combustion of biomass can be supplied from a reducing gas inlet or a circulating furnace top gas inlet. However, when using a reducing gas containing hydrogen as the main component, the temperature tends to decrease from the upper to the middle of the direct reduction furnace. Therefore, supplying heat from the upper to the middle of the direct reduction furnace improves the reaction efficiency. Therefore, it is preferable to supply heat from a circulating furnace top gas inlet, for example.

[0052] Here, biomass is a general term for a certain amount of accumulated animal and plant resources and waste materials originating from these resources (excluding fossil resources). In a method for operating a direct reduction furnace according to one embodiment of the present invention, any biomass that produces charcoal through pyrolysis, such as agricultural, forestry, livestock, fisheries, and waste, can be used. In particular, biomass with a high effective calorific value is preferred, such as woody biomass.

[0053] An example of woody biomass is forestry biomass.

[0054] Examples of forestry biomass include: Papermaking by-products such as pulp black liquor and chip dust; Lumbering by-products such as bark and sawdust, and forest residues such as branches, leaves, treetops and offcuts; Special forest products such as thinned timber from cedar, cypress and pine species, and discarded logs for edible fungi; Firewood and charcoal forests such as castanopsis, oak and pine, and short-rotation forestry timber such as willow, poplar, eucalyptus and pine.

[0055] Furthermore, some waste-based biomass, for example, general waste such as pruned branches from roadside trees in municipalities and garden trees in private homes, and industrial waste such as pruned branches from roadside trees in national and prefectural governments and garden trees in companies, and construction and building waste, can also be suitably used as woody biomass.

[0056] Some agricultural biomass, such as waste and by-product sources such as rice husks, wheat straw, rice straw, sugarcane residue, palm oil, etc., and energy crop sources such as rice bran, rapeseed, and soybeans, can also be suitably used as woody biomass.

[0057] Semi-carbonized biomass is biomass that is not completely carbonized but is partially carbonized, and preferably has a density of 700 to 850 kg / m ３ Semi-carbonized biomass is biomass that has been subjected to a heat treatment to reduce the moisture content of biomass, such as woody biomass, and to promote carbonization to increase the density and strength of the biomass.

[0058] The method for producing semi-carbonized biomass is not particularly limited. For example, semi-carbonized biomass is preferably produced by subjecting uncarbonized biomass (hereinafter also referred to as raw biomass) to low-temperature heat treatment at 200 to 300°C and then compacting it. This makes it possible to further increase the energy density. According to the above production method, for example, semi-carbonized biomass with a density of 200 kg / m ３ When woody biomass (raw biomass) is used as the material, the density is 750 kg / m ３ It is possible to produce semi-carbonized biomass of this size.

[0059] The amount of heat supplied to the direct reduction furnace is determined, for example, as follows: That is, the enthalpy of the reduction reaction calculated from the composition of the reducing gas used (for example, a gas with a hydrogen concentration of 100% by volume) using the above formulas (i) and (ii) is calculated based on the enthalpy of the reduction reaction calculated from the composition of the reducing gas used (for example, a gas with a hydrogen concentration of 100% by volume) and the enthalpy of the reduction gas used in a general direct reduction ironmaking process (for example, a gas with a hydrogen concentration of 100% by volume). ２ The enthalpy difference due to the reduction reaction is calculated from the composition of the oxidized gas (CO = 1 to 3), and the enthalpy of the circulating furnace top gas is subtracted from this enthalpy difference. The resulting value is converted into a heat quantity equivalent to the amount of reduced iron produced, and this value is used as the heat quantity to be supplied to the direct reduction furnace.

[0060] [Raw Material] The iron oxide used in the method for operating a direct reduction furnace according to one embodiment of the present invention is, for example, iron ore. Specific examples include lump iron ore (lump ore) and iron oxide pellets (spherical iron ore powder). The grade of the iron ore used as the iron oxide, i.e., the iron content, is not particularly limited, but from the perspective of reduction in a shaft furnace, it is generally preferable that the iron content be 65% by mass or more. However, in recent years, the price of high-grade ore from South America, for example, is expected to rise. For this reason, low-grade ore (Fe content: 63% by mass or less), which is an inexpensive and abundant resource, such as from Australia, may also be used as needed.

[0061] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0062] [2] Method for Producing Reduced Iron Next, a method for producing reduced iron will be described. The method for producing reduced iron according to one embodiment of the present invention produces reduced iron by the above-described method for operating a direct reduction furnace.

[0063] Fig. 2 is a schematic diagram showing an example of a conventional reduced iron production process. Fig. 3 is a schematic diagram showing an example of a reduced iron production method (production process) according to an embodiment of the present invention. In the figure, reference numeral 1 denotes a shaft furnace, 1a denotes iron oxide, 1b denotes reduced iron, 6 denotes a temperature control device, 10 denotes a dust removal device, 11 denotes a dehydration device, 12 denotes a natural gas supply unit, 13 denotes an air supply unit, 14 denotes a heating reformer, 15 denotes a reducing gas injection device, and 16 denotes a CO ２ 17 is a hydrogen supply unit, 18 is a gas heater, and 19 is a methanol synthesis unit.

[0064] In the conventional reduced iron manufacturing process shown in Figure 2, iron oxide is charged into the shaft furnace from the top and gradually lowered. High-temperature reducing gas is blown into the shaft furnace from the middle to reduce the iron oxide. The reduced iron is then discharged from the bottom of the shaft furnace. At this time, mainly CO, CO ２ , H ２ , H ２A furnace top gas containing O is discharged. This furnace top gas is subjected to dust removal in a dust removal device, and a portion thereof is fed as raw material gas to a dehydration device after moisture adjustment. A hydrocarbon-containing gas, for example, natural gas from a natural gas supply unit, is supplied to the thermal reformer together with the moisture-adjusted furnace top gas. Next, the supplied gas is heated in the thermal reformer. Then, a reforming reaction occurs, and mainly CO and H are produced. ２ This produces a high-temperature reducing gas containing the above-mentioned elements. This reducing gas is then blown directly into the reducing furnace through a reducing gas blower. The remaining furnace top gas is dehydrated and then used as heating fuel in the combustion chamber of the thermal reformer.

[0065] On the other hand, in the method for operating a direct reduction furnace according to one embodiment of the present invention, as described above, a reducing gas containing hydrogen as a main component is used. Therefore, in the method for operating a direct reduction furnace according to one embodiment of the present invention, a thermal reforming device is not required. Instead, as shown in the example of FIG. 3, hydrogen gas (H ２ The hydrogen gas (gas with a concentration of preferably 80% by volume or more, more preferably 90% by volume or more, and even more preferably 95% by volume or more) is heated by a gas heater. The heating temperature of the hydrogen gas is not particularly limited, but is preferably, for example, 900 to 1200°C. Next, the heated hydrogen gas is blown into the shaft furnace as a reducing gas by a reducing gas blowing device. After being subjected to a reduction reaction inside the shaft furnace, the reducing gas is mainly converted into H ２ and H ２ The circulating top gas is discharged from the shaft furnace as a furnace top gas containing O. Then, a part of the furnace top gas, ie, a circulating top gas, is re-injected into the shaft furnace through the circulating top gas inlet. As in the example of FIG. 3, the temperature of the circulating top gas may be optionally controlled by a temperature control device before being injected. In the example of FIG. 3, the remaining furnace gas other than the circulating top gas is removed from the dust collector and then dehydrated in a dehydrator. Then, the newly introduced H ２ and is blown into the shaft furnace as a reducing gas.

[0066] In addition, reducing gases such as CO and CH ４ The furnace gas contains CO ２When the circulating furnace top gas contains CO, the CO contained in the remaining furnace top gas other than the circulating furnace top gas is ２ It is preferable to separate and recover the CO from the remaining top gas other than the circulating top gas, and use it for basic chemical synthesis (CCU) or storage (CCS). ２ CO by the separation device ２ Separation is carried out, and then the separated CO ２ This shows an example in which CO emitted from the entire reduced iron production process is supplied to a methanol synthesis unit. ２ Emissions can be reduced to virtually zero.

[0067] The heat source of the gas heater is not particularly limited. For example, the heat from biomass combustion is suitable as the heat source. By utilizing the heat from biomass combustion, the CO emitted from the direct reduction furnace can be reduced. ２ Not only that, but also the CO emitted throughout the manufacturing process ２ It is possible to reduce the carbon dioxide emission to substantially zero. As the biomass, the above-mentioned semi-carbonized biomass and tar obtained in the process of pyrolysis of biomass are preferable.

[0068] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0069] In the method for operating a direct reduction furnace and the method for producing reduced iron according to one embodiment of the present invention, a method using a shaft furnace has been described in particular. However, the type of direct reduction furnace is not limited to this, and methods using a fluidized bed, rotary kiln, rotary hearth furnace (RHF), etc. may also be used. Note that a shaft furnace is preferred as the direct reduction furnace because of its high production efficiency, availability, and operational stability. Furthermore, the majority of direct reduction furnaces operating worldwide are shaft furnace-type Midrex (registered trademark) and Hyl (registered trademark).

[0070] Hereinafter, a method for operating a direct reduction furnace and a method for producing reduced iron according to one embodiment of the present invention will be specifically described with reference to examples. However, the present invention is not limited to the examples described below.

[0071] A test for producing reduced iron was conducted by operating a direct reduction furnace using the reduced iron production process shown in Figure 3. The test conditions are as shown in Table 1 and below. Conditions not specified were those according to conventional methods or the general description in the specification. The vertical injection position of the circulating furnace top gas in Table 1 is a representative position (middle position in the vertical direction). In some cases, the temperature of the circulating furnace top gas was controlled so that the temperature was in the range of 600 to 700°C. Height of direct reduction furnace (shaft furnace): 10 m Inner diameter of direct reduction furnace: 5.5 m Reduced iron production rate: 200 t / h Operation period: 28 days Composition of reducing gas: H ２ Concentration 90% by volume (volume ratio, H ２ :N ２ = 9:1) Reducing gas temperature: 1000°C Reducing gas pressure: 200 kPaG Reducing gas blowing amount: 2200 Nm ３ / t-DRI (amount of reducing gas injected per ton of reduced iron produced) Iron oxide: Brazilian iron oxide pellets with a particle size of 10.0 to 15.0 mm Main component composition of iron oxide: in mass %, T. Fe: 66%, FeO: 0.63%, SiO ２ :2.0%, CaO:2.1%, Al ２ O ３ : 0.5%, MgO: 0.16%, C: 0.1%

[0072] Furthermore, prior to the operation of the direct reduction furnace, a computational simulation was performed using the DEM-CFD method with the above test conditions as the planned operating conditions, and the reduction progress rate A(y) in the shaft furnace was determined. The results are shown in Figure 4. As shown in Figure 4, the height position at which the reduction progress rate A(y) was 30 to 60% was a position where the height y of the direct reduction furnace was approximately 2.5 to 6.0 m. Furthermore, the height position at which the reduction progress rate A(y) was 30 to 40% was a position where the height y of the direct reduction furnace was approximately 4.0 to 6.0 m. The height y of the direct reduction furnace was set to 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).

[0073] Furthermore, in some examples, in order to compensate for endothermic heat during hydrogen reduction, biomass was combusted using a boiler, and the combustion heat was supplied directly to the reduction furnace.

[0074] Then, the pressure drop (hereinafter also referred to as pressure drop) and reaction efficiency in the direct reduction furnace were evaluated in the following manner.

[0075] [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, for example, according to the Ergun equation. In Evaluation 1, the degree of reduction disintegration was evaluated based on the pressure drop in the direct reduction furnace.

[0076] That is, the pressure (kPaG) of the reducing gas at the reducing gas inlet and the pressure (kPaG) of the furnace top gas at the furnace top gas outlet were measured, and the difference between these pressures was calculated as the pressure loss in the direct reduction furnace.

[0077] The evaluation results are also shown in Table 1. Here, the meanings of A to D in the column for Evaluation 1 in Table 1 are as follows: A: Pressure loss in the direct reduction furnace is less than 50 kPa B: Pressure loss in the direct reduction furnace is 50 kPa or more and less than 70 kPa C: Pressure loss in the direct reduction furnace is 70 kPa or more and less than 100 kPa D: Pressure loss in the direct reduction furnace is 100 kPa or more

[0078] Generally, when the pressure drop in the direct reduction furnace is 100 kPa or more, poor descent such as hanging occurs in the direct reduction furnace, and the productivity of reduced iron decreases. Therefore, cases A, B, and C were evaluated as pass (reduction disintegration was sufficiently suppressed, and stable operation was possible).

[0079] [Evaluation of Reaction Efficiency (hereinafter also referred to as Evaluation 2)] Evaluation 2 was performed based on the reduction rate of the reduced iron obtained as a product. Here, the reduction rate is defined by the following formula: [Reduction rate (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

[0080] The oxygen content (mass%) of iron oxide before reduction is the amount of FeO and Fe ２ O ３Here, the Fe content (mass%) in FeO is calculated by subtracting the Fe content (mass%) in FeO from the T.Fe (mass%) of iron oxide before reduction. ２ O ３ It is assumed that it exists as Fe ２ O ３ The total amount of oxygen (mass%) contained in the reduced iron (obtained after reduction) was calculated. The amount of oxygen (mass%) contained in the reduced iron was calculated based on the total amount of FeO and Fe ３ O ４ Here, the Fe content (mass%) obtained by subtracting the Fe content (mass%) in FeO and M.Fe from the T.Fe (mass%) of reduced iron is ３ O ４ It is assumed that it exists as Fe ３ O ４ The total amount of oxygen (mass%) contained in the sample was calculated.

[0081] The evaluation results are also shown in Table 1. The meanings of A to D in the column for evaluation 2 in Table 1 are as follows: A: Reduction rate is 98% or more B: Reduction rate is 95% or more and less than 98% C: Reduction rate is 90% or more and less than 95% D: Reduction rate is less than 90%

[0082] The reduced iron discharged from the direct reduction furnace is then melted in an electric furnace. Therefore, it is generally required that the reduction rate of the reduced iron obtained as a product is 90% or more. Therefore, cases A, B, and C were evaluated as having excellent reaction efficiency.

[0083]

[0084] As shown in Table 1, in the inventive example, reduction disintegration was sufficiently suppressed, the pressure loss in the direct reduction furnace was small, and the reaction efficiency was also excellent. ２The reduction degradation was also reduced to virtually zero. Furthermore, by adjusting the vertical injection position of the circulating top gas at a height where the reduction progress rate A(y) was 30 to 40%, reduction degradation was more effectively suppressed, and pressure loss in the direct reduction furnace was further reduced. Furthermore, by adjusting the amount of circulating top gas to 10 to 50 volume % of the total amount of top gas, reduction degradation was more effectively suppressed, and pressure loss in the direct reduction furnace was further reduced. At the same time, this effectively contributed to improving the reaction efficiency. In addition, supplying heat to the direct reduction furnace also effectively contributed to improving the reaction efficiency. In particular, in Nos. 10 and 11, in which the vertical injection position of the circulating top gas was adjusted to a height where the reduction progress rate A(y) was 30 to 40% and the amount of circulating top gas was adjusted to 10 to 50 volume % of the total amount of top gas, pressure loss in the direct reduction furnace was very small, and reduction degradation was more effectively suppressed. Furthermore, the reaction efficiency was also extremely excellent.

[0085] On the other hand, in all of the comparative examples, reduction disintegration was not sufficiently suppressed, and the pressure loss in the direct reduction furnace increased. In addition, the reaction efficiency was sometimes insufficient.

[0086] In addition, the reducing gas H ２ Similar results were obtained when reduced iron was produced under conditions in which the concentration was 80% by volume or more and the vertical injection position of the circulating top gas and the amount of circulating top gas were variously changed.

[0087] DESCRIPTION OF SYMBOLS 1 Shaft furnace 1a Iron oxide 1b Reduced iron 1c Furnace top 1d Reduction zone 1e Cooling zone 2 Iron oxide charging port 3 Furnace top gas outlet 4 Circulating furnace top gas inlet 5 Reducing gas inlet 6 Temperature control device 7 Cooling gas inlet 8 Cooling gas suction port 9 Reduced iron outlet 10 Dust removal device 11 Dehydration device 12 Natural gas supply section 13 Air supply section 14 Heating reforming device 15 Reducing gas inlet 16 CO ２ Separator 17 Hydrogen supply unit 18 Gas heater 19 Methanol synthesis unit

Claims

1. A method for operating a direct reduction furnace, the method for operating the direct reduction furnace comprising: a charging step of charging iron oxide into the direct reduction furnace; a first blowing step of blowing a reducing gas mainly composed of hydrogen into the direct reduction furnace; a second blowing step of blowing circulating top gas into the direct reduction furnace; and a reduction step of reducing the iron oxide in the direct reduction furnace to obtain reduced iron, wherein the circulating top gas is a part of the top gas, the top gas is a gas discharged from the top of the direct reduction furnace, and in the second blowing step, the vertical blowing position of the circulating top gas is set to a height position where the reduction progress rate A(y) is 30 to 60%. Here, the reduction progress rate A(y) is the reduction rate of iron oxide at the height position y of the direct reduction furnace.

2. The method for operating a direct reduction furnace according to claim 1, wherein the vertical blowing position of the circulating top gas is set to a height position where the reduction progress rate A(y) is 30 to 40%.

3. The method for operating a direct reduction furnace according to claim 1 or 2, wherein the reduction progress rate A(y) is determined based on the operating conditions planned in the direct reduction furnace.

4. The method for operating a direct reduction furnace according to any one of claims 1 to 3, wherein the temperature of the circulating top gas is controlled before the second blowing step.

5. The method for operating a direct reduction furnace according to any one of claims 1 to 4, wherein the amount of the circulating top gas is 10 to 50% by volume of the total amount of the top gas.

6. The H of the reducing gas ２ The operating method of the direct reduction furnace according to any one of claims 1 to 5, wherein the concentration is 80% by volume or more.

7. The method for operating a direct reduction furnace according to any one of claims 1 to 6, further comprising a heat supply step of supplying heat to the direct reduction furnace.

8. The method for operating a direct reduction furnace according to claim 7, wherein the combustion heat of biomass is used as the heat source in the heat supply step.

9. A method for producing reduced iron, wherein reduced iron is produced by the method for operating a direct reduction furnace according to any one of claims 1 to 8.

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