Method for operating a direct reduction furnace and method for producing reduced iron

Abstract

Provided is a direct reduction furnace operation method that enables production of reduced iron having excellent energy efficiency when being melted in an electric furnace while reducing CO2 emission. The present invention involves: charging iron oxide and a carbonaceous material into a direct reduction furnace from the top of the furnace; and blowing a reducing gas having a H2 concentration of 80 vol% or more into the direct reduction furnace.

Description

[Technical Field]

[0001] This invention relates to a method for operating a direct reduction furnace and a method for producing reduced iron. [Background technology]

[0002] In recent years, steel mills have been strongly urged to conserve energy due to 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 steel mills. The most common reduction process used 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) at the tuyeres. This reaction generates CO and H2, which are reducing gases, and these reducing gases reduce the iron ore in the furnace. Due to recent improvements in blast furnace operation technology, the reducing agent ratio (the amount of coke and pulverized coal used per ton of molten iron produced) has been reduced to about 500 kg / ton, and the reducing agent ratio has already reached almost its lower limit. Therefore, further significant reductions in the reducing agent ratio cannot be expected.

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

[0004] The direct reduction ironmaking method is as follows: A direct reduction furnace is loaded with iron oxide raw material (hereinafter simply referred to as iron oxide), such as lumpy iron ore or pellets (powdered iron ore compressed into spheres). A reducing gas is then blown into the furnace to reduce the iron oxide and obtain reduced iron. The obtained reduced iron is then cooled in the region below the reducing gas injection point (cooling zone) of the furnace. Finally, the reduced iron is discharged from the bottom of the furnace. The reduced iron discharged from the furnace is then melted in an electric furnace.

[0005] Here, a shaft furnace is mainly used as the direct reduction furnace. Also, natural gas is generally used as the gas source for the reducing gas, such as Midrex® or Hyl®. In this case, the natural gas is reformed with the exhaust gas discharged from the top of the direct reduction furnace (hereinafter also called the top gas) to produce a reducing gas containing CO and H2. Then, the reducing gas is blown into the reduction furnace and iron oxide is reduced according to the following equation to obtain reduced iron. Fe2O3 + 3CO → 2Fe + 3CO2ΔH 298 = -247 kJ / kg - Fe (i) Fe2O3 + 3H2 → 2Fe + 3H2O ΔH 298 = 858 kJ / kg-Fe (ii)

[0006] Thus, in the general direct reduction ironmaking method, a reducing gas containing CO and H2 is used, and according to equation (i) above, a large amount of CO2 is generated and emitted.

[0007] In recent years, the need to reduce CO2 emissions on a global scale has increased, and further reductions in CO2 emissions are required in the steelmaking process. Therefore, a technology that uses a reducing gas mainly composed of hydrogen in the direct reduction steelmaking method is being considered. By using a reducing gas mainly composed of hydrogen, the main component of the furnace top gas becomes H2O, and a significant reduction in CO2 emissions can be expected.

[0008] As an example of such technology, Patent Document 1 describes: In a shaft furnace type direct reduction furnace operation method for producing reduced iron using a hydrogen-based reducing gas, A method for operating a direct reduction furnace using preheated raw materials, characterized by directly charging the furnace with iron oxide raw materials that have been preheated in advance. This has been disclosed.

[0009] Patent Document 2 contains: In a shaft furnace type direct reduction furnace operation method for producing reduced iron using a hydrogen-based reducing gas, A method for operating a direct reduction furnace that circulates top gas, characterized by injecting a portion of the gas discharged from the top of the furnace into the middle section of the furnace. This has been disclosed.

[0010] Patent Document 3 contains: A method for producing reduced iron by reducing iron oxide charged into a shaft furnace, A method for producing reduced iron, characterized by blowing a heated mixed gas containing 90% or more by volume of hydrogen gas and nitrogen gas into the shaft furnace. This has been disclosed. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2012-102371 [Patent Document 2] Japanese Patent Publication No. 2012-102372 [Patent Document 3] International Publication No. 2021 / 230307 [Overview of the project] [Problems that the invention aims to solve]

[0012] Incidentally, in the direct reduction ironmaking method, as mentioned above, the reduced iron obtained in the direct reduction furnace must be discharged from the furnace and then melted in an electric furnace. However, when the reduced iron obtained by the technologies described in Patent Documents 1 to 3 is melted in an electric furnace, there is a problem that the energy efficiency (hereinafter also referred to as the energy efficiency during electric furnace melting) is lower compared to when the reduced iron obtained by the general direct reduction ironmaking method is melted in an electric furnace.

[0013] The present invention has been developed to solve the above-mentioned problems, and an object of the present invention is to provide an operating method for a direct reduction furnace that makes it possible to produce reduced iron excellent in energy efficiency during melting in an electric furnace while reducing CO₂ emissions. Another object of the present invention is to provide a method for producing reduced iron, which produces reduced iron by the above-mentioned operating method for a reduction furnace. In the present disclosure, any numerical range represented by using "~" means a range including the numerical values described before and after "~" as the lower limit and the upper limit, respectively. [Means for Solving the Problems]

[0014] The inventors first conducted repeated studies on the cause of the above-mentioned problems, and obtained the following findings. (1) In the technologies of Patent Documents 1 to 3, a reducing gas mainly containing hydrogen is used. That is, the reducing gas contains almost no CO. (2) On the other hand, in a general direct reduction ironmaking method, the reducing gas contains CO. Therefore, in the direct reduction furnace, carburization of reduced iron proceeds during the reduction of iron ore through the Boudouard reaction (2CO→C+CO₂). (3) That is, when using a reducing gas mainly containing hydrogen, particularly a reducing gas with an H₂ concentration of 80% by volume or more, as in the technologies of Patent Documents 1 to 3, carburization of reduced iron does not occur during the reduction of iron ore. Here, when reduced iron is carburized, the following effects can be obtained. · Lowering the melting point of reduced iron. · Reducing iron oxide remaining in reduced iron. · When reduced iron is melted in an electric furnace, carbon is burned by oxygen. This generates a large amount of energy and shortens the melting time of reduced iron. (4) As described in (3) above, the above-mentioned effects can be obtained by carburizing reduced iron. For reduced iron obtained by the technologies of Patent Documents 1 to 3, carburization does not proceed during the reduction of iron oxide, which results in decreased energy efficiency during melting in an electric furnace.

[0015] Therefore, based on the above findings, the inventors further conducted repeated studies to solve the above problems and obtained the following findings. (5) It is important to charge, from the furnace top of a direct reduction furnace, a carbonaceous material, preferably at least one of biomass and plastic, in addition to iron oxide. Thereby, even when a reducing gas with an H₂ concentration of 80% by volume or more is used, carburization into reduced iron proceeds, which makes it possible to produce reduced iron excellent in energy efficiency during melting in an electric furnace. Further, by using carbon-neutral biomass, particularly semi-carbonized biomass, as the carbonaceous material, CO₂ emissions can be substantially zero, and it becomes possible to produce reduced iron that is particularly excellent in energy efficiency during melting in an electric furnace under excellent reaction efficiency in the direct reduction furnace. The present invention has been completed through further studies based on the above findings.

[0016] That is, the gist configuration of the present invention is as follows.

[0017] 1. A method for operating a direct reduction furnace, comprising: a charging step of charging iron oxide and a carbonaceous material from the furnace top into the direct reduction furnace; a blowing step of blowing a reducing gas having an H₂ concentration of 80% by volume or more into the direct reduction furnace; a reduction step of reducing the iron oxide to obtain reduced iron in the direct reduction furnace; A method for operating a direct reduction furnace, comprising the above steps.

[0018] 2. The method for operating a direct reduction furnace according to the above 1, wherein at least one of biomass and plastic is used as the carbonaceous material.

[0019] 3. The method for operating a direct reduction furnace according to the above 1 or 2, wherein semi-carbonized biomass is used as the carbonaceous material.

[0020] 4. The method for operating a direct reduction furnace according to any one of the above 1 to 3, wherein a charging amount of the carbonaceous material into the direct reduction furnace is controlled in accordance with the content of C contained in the reduced iron.

[0021] 5. A method for operating a direct reduction furnace according to any one of 1 to 4 above, wherein the amount of carbon contained in the reduced iron is controlled to be 1.0% by mass or more and 5.0% by mass or less, by controlling the amount of carbon charged into the direct reduction furnace.

[0022] 6. A method for operating a direct reduction furnace according to any one of 1 to 5, further comprising a heat supply step for supplying heat to the direct reduction furnace.

[0023] 7. Using biomass as the heat source for the heat supply process Grill A method for operating the direct reduction furnace described in 6 above, which uses heat.

[0024] 8. Any of the above items 1 to 7 directly A method for producing reduced iron, which involves producing reduced iron by operating a reduction furnace. [Effects of the Invention]

[0025] According to the present invention, it becomes possible to produce reduced iron with excellent energy efficiency during electric furnace melting while substantially reducing CO2 emissions. [Brief explanation of the drawing]

[0026] [Figure 1] This is a schematic diagram showing an example of a direct reduction furnace. [Figure 2] This is a schematic diagram illustrating an example of a conventional reduced iron production process. [Figure 3] This is a schematic diagram illustrating an example of a reduced iron production process according to one embodiment of the present invention. [Modes for carrying out the invention]

[0027] The following describes an operating method for a direct reduction furnace according to one embodiment of the present invention.

[0028] A method for operating a direct reduction furnace according to one embodiment of the present invention is: The charging process involves charging iron oxide and carbon material into the aforementioned direct reduction furnace from the top of the furnace, The blowing process involves blowing a reducing gas with an H2 concentration of 80% by volume or more into the aforementioned direct reduction furnace. In the aforementioned direct reduction furnace, a reduction step is performed in which the iron oxide is reduced to obtain reduced iron, It holds.

[0029] Figure 1 is a schematic diagram showing an example of the configuration of a shaft furnace, which is a direct reduction furnace. In the figure, reference numeral 1 denotes the shaft furnace, 1a denotes iron oxide, 1b denotes reduced iron, 1c denotes the top of the furnace, 1d denotes the cooling zone, 2 denotes the reduction gas inlet, 3 denotes the top gas outlet, 4 denotes the cooling gas inlet, 5 denotes the cooling gas suction port, 6 denotes the iron oxide charging inlet, 7 denotes the reduced iron outlet, and 13 denotes the briquette machine.

[0030] In the shaft furnace 1, iron oxide 1a is charged from the top of the furnace 1c, particularly from the iron oxide charging inlet 6, and the iron oxide 1a is gradually lowered. High-temperature reducing gas is then blown in from the reducing gas inlet 2 located in the middle of the shaft furnace 1 to reduce the iron oxide 1a and obtain reduced iron 1b. The reduced iron 1b is then discharged from the reduced iron outlet 7 located at the bottom of the shaft furnace 1. Meanwhile, top gas is discharged from the top gas outlet 3. The temperature of the reducing gas blown in from the reducing gas inlet 2 is, for example, 700°C to 1200°C.

[0031] Furthermore, a cooling gas inlet 4 and a cooling gas suction port 5 are located in the cooling zone 1d at the bottom of the shaft furnace 1. Cooling gas is blown into the interior of the cooling zone 1d from the cooling gas inlet 4. The cooling gas suction port 5 suctions these cooling gases to prevent them from entering the furnace top 1c. For example, N2 is used as the cooling gas.

[0032] Furthermore, in the operation method of a direct reduction furnace according to one embodiment of the present invention, it is important to charge carbon material into the direct reduction furnace from the top of the furnace in addition to iron oxide.

[0033] [Charging process] In an operation method for a direct reduction furnace according to one embodiment of the present invention, carbon material is charged into the direct reduction furnace from the top of the furnace, particularly from the iron oxide charging inlet, in addition to iron oxide. This allows carburization of the reduced iron to proceed even when using a reducing gas with an H2 concentration of 80% by volume or higher, making it possible to produce reduced iron with excellent energy efficiency during electric furnace melting. Specifically, the carbon material charged into the direct reduction furnace undergoes thermal decomposition as it is heated by the gas in the furnace. As a result, carbonization progresses in the carbon material while generating volatile gases. The generated carbon comes into contact with Fe3O4, FeO, and Fe, which are produced during the reduction of iron oxide, and carburization proceeds. Furthermore, if the volatile gases generated from the carbon material contain CO, CH4, and H2, in addition to the Boudoir reaction (CO → C + CO2), the CH4 → C + 2H2 reaction and the CO + H2 → C + H2O reaction occur, further promoting carburization of the reduced iron. Furthermore, additional carbon material may be charged directly into the reduction furnace from locations other than the top of the furnace, if desired.

[0034] Here, carbon material is charged into the direct reduction furnace together with iron oxide from the furnace top 1c, particularly from the iron oxide charging inlet 6 (hereinafter also referred to as the first method). In addition to the first method, the following methods may also be implemented: charging the carbon material together with cooling gas from the cooling gas inlet 4 (hereinafter also referred to as the second method), providing tuyeres in the middle of the furnace and performing gas-phase transport by gas (hereinafter also referred to as the third method), and pre-heating the solid carbon material to generate volatile gas components containing C, and charging only the volatile gas into the direct reduction furnace 1 (hereinafter also referred to as the fourth method). However, in the second method, since the temperature of the cooling zone is lower than that of the upper part of the furnace, the volatilization of volatile gases contained in the carbon material may not proceed sufficiently, and the amount of carburization into the reduced iron may decrease. Also, in the third method and others, energy and costs may be required separately for transporting the carbon material. For these reasons, the first method alone may be implemented as the method for charging carbon material into the direct reduction furnace. On the other hand, if there are circumstances such as a limited amount of charcoal material that can be charged from the top of the furnace, multiple methods may be combined as appropriate. Charcoal material is a substance that contains some carbon (C) atoms. In addition to carbon (C) atoms, charcoal material may also contain other atoms such as hydrogen (H) atoms or oxygen (O) atoms. For example, a substance in which the mass of carbon atoms accounts for 30-100%, the mass of hydrogen atoms for 0-30%, and the mass of oxygen atoms for 0-60% of the total mass of charcoal material is preferred.

[0035] From the perspective of effectively carburizing reduced iron, suitable carbon materials include biomass and plastics. Among these, biomass is particularly advantageous because it is a carbon-neutral carbon material, making it possible to reduce CO2 emissions to virtually zero. Plastics can be new or used. Used plastics, conceptually, include waste plastics, plastics not intended for disposal, and factory scraps. When using waste plastics, the carbon material that would otherwise be landfilled or incinerated can be effectively utilized, thereby reducing the environmental burden. However, when using plastics, it is not possible to reduce CO2 emissions to zero, so it is preferable to use the CO2 contained in the furnace top gas for CCU or CCS, as described later.

[0036] Here, biomass refers to a general term for a certain amount of accumulated animal and plant resources and waste derived therefrom (excluding fossil resources). In the operation method of a direct reduction furnace according to one embodiment of the present invention, any biomass that produces carbonized material by thermal decomposition, such as agricultural, forestry, livestock, fisheries, and waste materials, can be used. In particular, biomass with a high effective calorific value is preferred, and woody biomass is preferred, for example.

[0037] Examples of woody biomass include forestry biomass.

[0038] Examples of forestry-related biomass include, • Papermaking by-products such as pulp black liquor and chip dust, • By-products of lumbering such as bark and sawdust, forest residues such as branches, leaves, treetops, and leftover timber, Special forest products such as thinned timber from cedar, cypress, and pine trees, and spent logs used for cultivating edible fungi. • Firewood forests such as oak, sawtooth oak, and pine; short-rotation forestry timber such as willow, poplar, eucalyptus, and pine. These are some examples.

[0039] Furthermore, some waste-derived biomass, for example, • General waste such as pruned branches from street trees in municipalities and garden trees in private homes, • Pruning branches from street trees owned by the national or prefectural government, trees in corporate gardens, etc., industrial waste such as construction and building waste, etc. It can also be suitably used as woody biomass.

[0040] Furthermore, some agricultural biomass, for example, • Rice husks, wheat straw, rice straw, sugarcane residue, palm oil, etc., which are generated from waste and by-products, Rice bran, rapeseed, soybeans, etc., which are derived from energy crops. It can also be suitably used as woody biomass.

[0041] Biomass is composed of C, O, and H atoms. However, biomass itself has a high water content and low density, resulting in low strength. Therefore, if biomass is directly charged into a reduction furnace as is, it may cause pulverization and shearing within the furnace, potentially stifling the reduction reaction. For this reason, it is preferable to use semi-carbonized biomass as the carbon material.

[0042] Here, semi-carbonized biomass refers to biomass that is not completely carbonized but is partially carbonized, preferably with a density of 700-850 kg / m³. 3 This is biomass. Semi-carbonized biomass can also be described as biomass that has been subjected to heat treatment, such as woody biomass, to reduce its moisture content and increase its density and strength by promoting carbonization.

[0043] Partially carbonized biomass has excellent storability and transportability. Furthermore, because it is not completely carbonized, volatile gas components remain. Therefore, when partially carbonized biomass is charged into a furnace and heated, it generates volatile gases such as CO, H2, CO2, CH4, and O2. Of these volatile gases, CO, CH4, and H2 promote carburization of reduced iron. In particular, CO and H2 have reducing properties, thus promoting the reduction of iron oxide and preventing reaction stagnation due to endothermic heat during hydrogen reduction. Moreover, because the heat treatment temperature for producing partially carbonized biomass is mild, the total energy consumption is also reduced. For these reasons, it is preferable to use partially carbonized biomass as carbon material. Hereafter, biomass that has not been carbonized will also be referred to as raw biomass. Biomass that has been completely carbonized will also be referred to as carbonized biomass. Furthermore, since the temperature at the top of the shaft furnace is approximately 300°C, if raw biomass is charged from the top of the furnace, some degree of carbonization will occur within the furnace. However, as mentioned above, there is a concern that this may cause pulverization or shelf-hanging at the top of the furnace. Therefore, it is preferable to use semi-carbonized biomass.

[0044] The method for producing semi-carbonized biomass is not particularly limited. For example, it is preferable to produce it by performing low-temperature heat treatment at 200-300°C on raw biomass and then compacting it. This makes it possible to further increase the energy density. According to the above production method, for example, the density is 200 kg / m³. 3 When using woody biomass (raw biomass) as a material, the density is 750 kg / m³. 3 It becomes possible to produce a certain amount of carbonized biomass.

[0045] Furthermore, it is preferable to control the amount of carbon material directly charged into the reduction furnace (hereinafter also referred to as the amount of carbon material charged) according to the amount of carbon contained in the reduced iron. For example, it is preferable to control the amount of carbon material charged so that the amount of carbon contained in the reduced iron is 1.0% by mass or more and 5.0% by mass or less.

[0046] In other words, if the amount of charcoal charged is insufficient, the carburization of the reduced iron may not proceed sufficiently. On the other hand, if the amount of charcoal charged is excessive, it does not contribute to the carburization reaction, and the amount of carbon discharged from the bottom of the furnace along with the reduced iron increases. Furthermore, reduced iron is often compressed into HBI (Hot Briquetted Iron) to improve transportability and handling. For maritime transport of HBI, according to international regulations (IMSBC: International Maritime Solid Bulk Cargoes), the apparent density must be 5.0 g / cm³. 3 The above is required. In this regard, if there is an excess of carbon in the reduced iron, the apparent density of HBI decreases, and the yield decreases. In addition, an increase in the amount of carbon dissolved in Fe particles and an increase in the amount of carbon present at the interface of Fe particles leads to a decrease in the strength of HBI.

[0047] Therefore, from the viewpoint of achieving both excellent energy efficiency during electric furnace melting and excellent HBI performance (apparent density and strength), it is preferable to control the amount of carbon charged in the reduced iron, in particular, so that the amount of carbon in the reduced iron is 1.0% by mass or more and 5.0% by mass or less. Furthermore, the amount of carbon charged in the reduced iron is controlled so that the amount of carbon in the reduced iron is more preferably 2.0% by mass or more, even more preferably 3.0% by mass or more, and even more preferably 4.0% by mass or more. The amount of carbon in the reduced iron can be measured, for example, using reduced iron directly discharged from the reduction furnace, according to the procedure described in Evaluation 1 of Example 1 below. In addition, the carbon material to be used may be selected according to the amount of carbon in the reduced iron.

[0048] As an example, it is preferable to control the amount of carbon material charged into the direct reduction furnace so that the amount of carbon in the reduced iron is between 1.0% by mass and 5.0% by mass, using the ratio of the amount of carbon (mass of carbon atoms) contained in the carbon material to the amount of iron oxide (mass of carbon atoms) contained in the carbon material charged into the direct reduction furnace (hereinafter also referred to as the C / Fe charging ratio). In this case, by controlling the amount of carbon material charged into the direct reduction furnace so that the C / Fe charging ratio is preferably in the range of 1.0 to 20.0%, more preferably in the range of 3.0 to 15.0%, the amount of carbon in the reduced iron can be made between 1.0% by mass and 5.0% by mass. For example, if the iron oxide charged into the direct reduction furnace consists solely of Fe2O3, and the mass of carbon atoms accounts for 50% of the total mass of the carbon material charged into the direct reduction furnace, then if the ratio of the amount of carbon material charged to the direct reduction furnace to the amount of iron oxide charged (= amount of carbon material charged [kg / h] / amount of iron oxide charged [kg / h] × 100) is in the range of 1.4 to 28.0%, then the C / Fe charging ratio will be in the range of 1.0 to 20.0%. Note that the mass of carbon atoms in the total mass of the carbon material is, for example, 30 to 100%.

[0049] In addition, the iron oxide used in the operation method of 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 (powdered iron ore compressed into spheres). The grade of the iron ore used as iron oxide, that is, the iron content, is not particularly limited, but from the viewpoint of reduction in a shaft furnace, it is generally preferable to have 65% by mass or more. However, in recent years, a price increase is expected for high-grade ore from South America and other sources. For this reason, for example, low-grade ore from Australia, which is an inexpensive and abundant resource (Fe content: 63% by mass or less), may also be used as needed.

[0050] [Blowing process] A reducing gas with an H2 concentration of 80% by volume or more is injected directly into the reduction furnace. Here, the H2 concentration is preferably 90% by volume or more, more preferably 95% by volume or more. The upper limit of the H2 concentration is not particularly limited and may be 100% by volume. The type of residual gas other than hydrogen is not particularly limited. As residual gas, for example, N2, H2O, CO and CO2, as well as gases produced as by-products in the steelmaking process (hereinafter also referred to as by-product gases), can be used. Examples of by-product gases include blast furnace gas (BFG) and coke oven gas (COG). However, if the residual gas contains CO or CO2, then the top gas will contain CO2. In this case, it is preferable to use the CO2 contained in the top gas for CCU or CCS, as described later.

[0051] [Reduction Process] In the reduction process, reduced iron is obtained by reducing iron with a reducing gas with an H2 concentration of 80% by volume or higher, according to equation (i) above. Iron oxide can also be reduced by volatile gases generated from carbon material directly charged into the reduction furnace.

[0052] [Heat supply process] In the operation method of a direct reduction furnace according to one embodiment of the present invention, it is preferable to further include a heat supply step for supplying heat to the direct reduction furnace. As described above, in a general direct reduction ironmaking method, a reducing gas containing CO and H2 (by volume ratio, H2 / CO = 1 to 3) is used. On the other hand, in the operation method of a direct reduction furnace according to one embodiment of the present invention, a reducing gas with an H2 concentration of 80% by volume or more is used. Here, as shown in equations (i) and (ii) above, the reduction reaction with CO is an exothermic reaction, while the reduction reaction with H2 is an endothermic reaction. Therefore, if a gas with an H2 concentration close to 100% by volume 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 to compensate for the endothermic reaction.

[0053] The heat source is not particularly limited. For example, the heat of combustion of biomass is suitable as a heat source. By utilizing the heat of combustion of biomass, it is possible to make virtually zero CO2 emissions not only from the reduction furnace but also from the entire manufacturing process. As for the biomass, the semi-carbonized biomass mentioned above and tar obtained in the process of pyrolysis of biomass are preferred. Carbonized biomass and tar have a high calorific value per unit volume and are considered promising as alternative fuels to coal.

[0054] Furthermore, the amount of heat supplied to the direct reduction furnace can be determined by, for example, calculating the difference between the enthalpy due to the reduction reaction calculated from the composition of the reducing gas used (for example, a gas with an H2 concentration of 100% by volume) using equations (i) and (ii) above, and the enthalpy due to the reduction reaction calculated from the composition of the reducing gas used in a general direct reduction ironmaking method (for example, a gas with a volume ratio of H2 / CO = 1 to 3), and then converting that difference into an amount of heat equivalent to the amount of reduced iron produced.

[0055] [Reduced Iron Production Process] Next, we will explain the entire process for producing reduced iron. Figure 2 shows an example of the configuration of a conventional reduced iron production process. In the figure, reference numeral 1 denotes a shaft furnace, 1a denotes iron oxide, 1b denotes reduced iron, 8 denotes a dust removal device, 9 denotes a dewatering device, 10 denotes a natural gas supply unit, 11 denotes an air supply unit, 12 denotes a heating and reforming device, and 14 denotes a reducing gas injection device.

[0056] In the example of the reduced iron production process shown in Figure 2, iron oxide is charged into the shaft furnace 1 from the top and gradually lowered. High-temperature reducing gas is then blown in from the middle of the shaft furnace 1 to reduce the iron oxide 1a. Reduced iron 1b is then discharged from the bottom of the shaft furnace 1. At this time, top gas mainly containing CO, CO2, H2, and H2O is discharged from the top of the shaft furnace 1. This top gas is dust-removed by a dust removal device 8, and a portion of it is dehydrated in a dehydration device 9 and supplied to the heating and reforming device 12 as raw material gas. Along with the moisture-adjusted top gas, a gas containing hydrocarbons, such as natural gas from a natural gas supply unit, is supplied to the heating and reforming device 12. Next, the supplied gas is heated in the heating and reforming device 12. A reforming reaction occurs, and high-temperature reducing gas mainly containing CO and H2 is generated. This reducing gas is then blown into the reduction furnace. Furthermore, the remaining portion of the furnace top gas is dewatered and then used as heating fuel in the combustion chamber of the heating and reforming unit 12.

[0057] In the operation method of a direct reduction furnace according to one embodiment of the present invention, as described above, a reducing gas with an H2 concentration of 80% by volume or more is used. Therefore, in the operation method of a direct reduction furnace according to one embodiment of the present invention, a heating reforming device is not required. Instead, as shown in Figure 3, hydrogen gas (gas with an H2 concentration of 80% by volume or more, preferably 90% by volume or more, more preferably 95% by volume or more) supplied from the hydrogen supply unit 15 is heated by a gas heater 16. The heating temperature of the hydrogen gas is not particularly limited, but for example, 900 to 1200°C is preferred. Then, the heated hydrogen gas is blown into the shaft furnace 1 as a reducing gas by a reducing gas injection device 14. After being subjected to a reduction reaction in the furnace, the reducing gas is discharged from the shaft furnace 1 as a top gas mainly containing H2 and H2O. The H2 that was not used in the reduction reaction is recycled and reused. In the example shown in Figure 3, the top gas is first dusted by the dust removal device 8, then dehydrated by the dewatering device 9, mixed with newly introduced H2, and then blown into the shaft furnace 1 as a reducing gas.

[0058] Furthermore, if the top gas contains CO2, it is preferable to separate and recover the CO2 contained in the top gas and use it for basic chemical synthesis and utilization (CCU) or storage (CCS). Figure 3 shows an example in which CO2 is separated from the top gas using a CO2 separation device 17, and then the CO2 is supplied to a methanol synthesis device 18 as CCU. In particular, when non-carbon neutral carbon materials such as waste plastics are used as carbon materials, this makes it possible to reduce CO2 emissions to virtually zero.

[0059] The heat source for the gas heater 16 is not particularly limited. For example, the heat of combustion of biomass is suitable as the heat source. By utilizing the heat of combustion of biomass, it is possible to reduce not only the CO2 emitted directly from the reduction furnace but also the CO2 emitted from the entire manufacturing process to virtually zero. As the biomass, the semi-carbonized biomass mentioned above and tar obtained in the process of thermal decomposition of biomass are preferred.

[0060] Other than the conditions mentioned above, there are no particular limitations; you may follow the usual law.

[0061] In the description of the operation method of a direct reduction furnace according to one embodiment of the present invention, a method using a shaft furnace was particularly described. However, the type of direct reduction furnace is not limited to this, and methods using fluidized bed, rotary kiln, rotary hearth furnace (RHF), etc. are also possible. In addition, a shaft furnace is preferred as a direct reduction furnace because it offers high production efficiency, uptime, and operational stability. Furthermore, the majority of direct reduction furnaces operating worldwide are shaft furnace types, such as Midrex® and Hyl®.

[0062] Furthermore, a method for producing reduced iron according to one embodiment of the present invention involves producing reduced iron by the operation method of the direct reduction furnace described above. Other conditions are not particularly limited and may be handled according to conventional methods.

[0063] As described above, the present invention makes it possible to produce reduced iron with excellent energy efficiency during electric furnace melting while substantially reducing CO2 emissions. Furthermore, according to a preferred embodiment of the present invention, it is possible to produce reduced iron that achieves both excellent energy efficiency during electric furnace melting and excellent HBI (Hot Briquetted Iron) performance while substantially reducing CO2 emissions. [Examples]

[0064] The following examples will specifically describe the operation method of a direct reduction furnace and the method for producing reduced iron according to one embodiment of the present invention. However, the present invention is not limited to the examples described below.

[0065] (Example 1) A reduced iron production test was conducted using a bench-scale shaft furnace of the reduced iron production process shown in Figure 3. The test conditions were as follows. Conditions not explicitly stated were those in accordance with the standard method or the general description section of the specification. Production rate of reduced iron: 15 kg / h Operating period: 7 days Reducing gas composition: H2 concentration 90% by volume (volume ratio: H2:N2 = 9:1) Injection rate of reducing gas: 2200 Nm³ 3 / t-DRI (amount of gas per ton of reduced iron) Iron oxide: Brazilian iron oxide pellets with a particle size of 10.0-15.0 mm. Main component 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%

[0066] In some cases, the carbon material shown in Table 1 was charged along with iron oxide through the iron oxide charging inlet at the top of the shaft furnace.

[0067] Then, the energy efficiency during the melting of the manufactured reduced iron in an electric furnace and the reaction efficiency in a direct reduction furnace were evaluated according to the following procedure.

[0068] [Evaluation of energy efficiency during electric furnace melting (hereinafter also referred to as Evaluation 1)] Evaluation 1 was performed based on the amount of carbon (carburized amount) contained in the reduced iron produced. Specifically, the amount of carbon in the reduced iron discharged from the bottom of the shaft furnace was measured according to the infrared absorption method specified in JIS G 1211 (2018) "Iron and steel - Method for determining carbon content". This measurement of the amount of carbon in the reduced iron was performed daily, and the average value was taken as the amount of carbon in the reduced iron. The evaluation results are shown in Table 1. The meanings of A to E in the Evaluation 1 column of Table 1 are as follows. Furthermore, in the case of A to C, the energy efficiency during electric furnace melting was evaluated as excellent. A: The amount of carbon in reduced iron is 4.0% by mass or more. B: The amount of carbon in reduced iron is 3.0% by mass or more and less than 4.0% by mass. C: The amount of C contained in reduced iron is 1.0% by mass or more and less than 3.0% by mass. D: The amount of carbon in reduced iron is 0.5% by mass or more and less than 1.0% by mass. E: The amount of carbon in reduced iron is less than 0.5% by mass.

[0069] [Evaluation of reaction efficiency in a direct reduction furnace (hereinafter also referred to as Evaluation 2)] Evaluation 2 was performed using the return rate. Here, the return 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 Here, the amount of oxygen (O) in iron oxide before reduction (mass%) is the total amount of oxygen (O) contained in FeO and Fe2O3 in the iron oxide before reduction (mass%). Here, assuming that the amount of Fe obtained by subtracting the amount of Fe (mass%) in FeO from the total Fe (T.Fe) (mass%) of iron oxide before reduction exists as Fe2O3, the total amount of oxygen (O) contained in Fe2O3 was calculated. Also, the amount of oxygen (O) in reduced iron (obtained after reduction) is the total amount of oxygen (O) contained in FeO and Fe3O4 in the reduced iron (mass%). Here, assuming that the amount of Fe obtained by subtracting the amount of Fe (mass%) in FeO and M.Fe from the total Fe (T.Fe) of reduced iron exists as Fe3O4, the total amount of oxygen (O) contained in Fe3O4 was calculated.

[0070] The evaluation results are shown in Table 1. The meanings of A to D in the Evaluation 2 column of Table 1 are as follows: Generally, a reduction rate of 95% or higher is required for subsequent melting in an electric furnace. Therefore, cases A and B were evaluated as having excellent reaction efficiency in a direct reduction furnace. A: Return rate of 98% or higher B: Return rate is between 95% and 98% C: Return rate is between 90% and 95% D: Return rate less than 90%

[0071] [Table 1]

[0072] As shown in Table 1, in all of the inventive examples, the reduced iron produced exhibited excellent energy efficiency during electric furnace melting. Furthermore, in all of the inventive examples, the CO2 emissions from the entire process were substantially zero, and the reaction efficiency in the direct reduction furnace was also excellent. In particular, in No. 3, which used semi-carbonized biomass as the carbon material, the reduced iron produced exhibited particularly excellent energy efficiency during electric furnace melting, as well as particularly excellent reaction efficiency in the direct reduction furnace.

[0073] (Example 2) Except for the conditions shown in Table 2, reduced iron was produced under the same conditions as in Example 1. The amount of carbon in the produced reduced iron was measured in the same manner as in Evaluation 1 of Example 1. The reaction efficiency in the direct reduction furnace was also evaluated in the same manner as in Evaluation 2 of Example 1. The results are shown in Table 2.

[0074] Furthermore, a detailed evaluation of the energy efficiency during electric furnace melting of the manufactured reduced iron and an evaluation of the HBI performance were conducted according to the following procedure. The results are shown in Table 2.

[0075] [Detailed evaluation of energy efficiency during electric furnace melting (hereinafter also referred to as Evaluation 1')] To conduct a more detailed evaluation of the energy efficiency during electric furnace melting, in Evaluation 1', the energy efficiency during electric furnace melting was calculated using the reduced iron produced and following the method specified in JIS G 0703 (1995) "Heat Accounting Method for Arc Furnaces," using the following formula (1). The meanings of A to E in the Evaluation 1' column in Table 2 are as follows. Furthermore, cases A to C were evaluated as having excellent energy efficiency during electric furnace melting. Energy efficiency during electric furnace melting = (Heat contained in molten steel + Heat contained in slag + Heat of decomposition reaction - Heat contained in molten iron) / (Total heat input - Heat contained in molten iron) × 100 (%) ... (1) A: Energy efficiency of 80% or more during electric furnace melting. B: Energy efficiency during electric furnace melting is between 70% and 80%. C: Energy efficiency during electric furnace melting is between 50% and 70%. D: Energy efficiency during electric furnace melting is 40% or more and less than 50% E: Energy efficiency during electric furnace melting is less than 40%

[0076] [Performance evaluation of HBI] Using the produced reduced iron, HBI was briquetted according to the following conditions. Roll diameter of briquetting machine: 500 mmφ Linear pressure of rolls: 150 kN Rotation speed of rolls: 9 rpm Input amount of reduced iron: 60 kg Heating temperature of reduced iron: 700°C

[0077] Then, for the molded HBI, the apparent density and crushing strength were measured according to the following procedure, and the performance of HBI was evaluated. When both Evaluation 3 and Evaluation 4 described below are rated A to C (excluding the case where both Evaluation 3 and 4 are rated A to B), the HBI was evaluated as having excellent performance. In addition, when both Evaluation 3 and 4 described below are rated A to B, the HBI was evaluated as having particularly excellent performance. The results are also shown in Table 2.

[0078] • Evaluation of apparent density of HBI (hereinafter also referred to as Evaluation 3) The apparent density of HBI was measured by the method described in "ISO 15968: Direct reduced iron - Determination of apparent density and water absorption of hot briquetted iron (HBI)". The measurement of the apparent density of HBI was carried out once a day. Specifically, 60 kg of reduced iron produced during the operation period was sampled every day, and HBI was produced from the sampled reduced iron. Five pieces of the produced HBI were sampled each time, and the apparent density of each HBI was measured. Then, the average value thereof was obtained, and the apparent density of HBI was evaluated. The meanings of A to D in the Evaluation 3 column in Table 2 are as follows. A: The apparent density of HBI is 5.5 g / cm 3 or higher B: The apparent density of HBI is 5.0 g / cm 3 or higher and less than 5.5 g / cm3 less than The apparent density of C:HBI is 4.5 g / cm³. 3 More than 5.0g / cm 3 less than D: The apparent density of HBI is 4.5 g / cm³. 3 less than

[0079] • Evaluation of HBI intensity (hereinafter also referred to as Evaluation 4) The crushing strength of HBI was measured using a general-purpose autograph at an applied rate of 1 mm / min. The crushing strength of HBI was also measured once a day. Specifically, 60 kg of reduced iron produced during the operating period was collected daily, and HBI was produced from the collected reduced iron. Five samples of the produced HBI were taken each time, and the crushing strength of each HBI was measured. The average value of these values ​​was then calculated to evaluate the crushing strength of HBI. The meanings of A to D in column 4 of Evaluation 4 in Table 2 are as follows. A: The crushing strength of HBI is 8000 kgf or more. B: The crushing strength of HBI is between 6000 kgf and 8000 kgf. C:HBI's crushing strength is between 4000 kgf and less than 6000 kgf. D: The crushing strength of HBI is less than 4000 kgf.

[0080] [Table 2]

[0081] As shown in Table 2, in all of the inventive examples, the reduced iron produced exhibited excellent energy efficiency during electric furnace melting. Furthermore, in the inventive example where the amount of carbon material charged was controlled so that the carburization amount of the reduced iron was in the range of 1.0 mass% to 5.0 mass%, the HBI performance was particularly excellent. [Explanation of Symbols]

[0082] 1. Shaft Furnace 1a Iron oxide 1b Reduced iron 1c Furnace top 1d Cooling Zone 2. Reducing gas inlet 3. Top gas outlet 4 Cooling gas inlet 5. Cooling gas intake port 6 Iron oxide coated entrance 7. Reduced iron outlet 8 Dust removal equipment 9 Dehydration equipment 10 Natural Gas Supply Department 11 Air supply unit 12 Heating and reforming apparatus 13 Briquette Machine 14. Reducing gas injection device 15. Hydrogen Supply Department 16 Gas heater 17 CO2 separation equipment 18. Methanol synthesis apparatus

Claims

1. A method for operating a direct reduction furnace, The charging process involves charging iron oxide and carbon material into the aforementioned direct reduction furnace from the top of the furnace, H to the direct reduction furnace ２ Concentration: A blowing process in which reducing gas of 80% by volume or more is injected, In the aforementioned direct reduction furnace, a reduction step is performed in which the iron oxide is reduced to obtain reduced iron, It has, A method for operating a direct reduction furnace, comprising controlling the amount of carbon material charged into the direct reduction furnace according to the amount of carbon contained in the reduced iron.

2. The method for operating a direct reduction furnace according to claim 1, wherein at least one of biomass and plastic is used as the carbon material.

3. The method for operating a direct reduction furnace according to claim 1, wherein semi-carbonized biomass is used as the carbon material.

4. The method for operating a direct reduction furnace according to claim 2, wherein semi-carbonized biomass is used as the carbon material.

5. A method for operating a direct reduction furnace according to any one of claims 1 to 4, wherein the amount of carbon material charged into the direct reduction furnace is controlled so that the amount of carbon contained in the reduced iron is 1.0% by mass or more and 5.0% by mass or less.

6. A method for operating a direct reduction furnace according to any one of claims 1 to 4, further comprising a heat supply step for supplying heat to the direct reduction furnace.

7. The method for operating a direct reduction furnace according to claim 5, further comprising a heat supply step for supplying heat to the direct reduction furnace.

8. The method for operating a direct reduction furnace according to claim 6, wherein the heat of combustion of biomass is used as the heat source for the heat supply process.

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

10. A method for producing reduced iron, comprising producing reduced iron by the operation method of a direct reduction furnace described in any one of claims 1 to 4.

11. A method for producing reduced iron, comprising producing reduced iron by the operation method of a direct reduction furnace described in claim 5.

12. A method for producing reduced iron, comprising producing reduced iron by the operation method of a direct reduction furnace described in claim 6.

13. A method for producing reduced iron, comprising producing reduced iron by the operation method of a direct reduction furnace described in claim 7.

14. A method for producing reduced iron, comprising producing reduced iron by the operation method of a direct reduction furnace described in claim 8.

15. A method for producing reduced iron, comprising producing reduced iron by the operation method of a direct reduction furnace described in claim 9.

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