Reduced iron manufacturing method

Abstract

Provided is a reduced iron production method capable of achieving both saving of energy and a reduction in the CO2 discharge amount. A system is constructed which has a blowing step, a reduction step, a distribution step, a synthesis step, and a reforming step, and circulates and recycles a furnace top gas. The gas amounts introduced in the distribution step, the synthesis step, and the reformation step are controlled according to the CO2 conversion rate due to a methanation reaction in the synthesis step.

Description

[Technical Field]

[0001] The present invention relates to a method for producing reduced iron. [Background technology]

[0002] In recent years, steelworks have been strongly demanding energy conservation in the face of global environmental issues and the depletion of fossil fuels. Iron is primarily made from iron oxide, and a reduction process to reduce this iron oxide is essential in steelworks. The most common reduction process worldwide is the blast furnace. In a blast furnace, coke or pulverized coal reacts with oxygen in hot air (air heated to approximately 1200°C) in the tuyere. This reaction produces reducing gases, CO and H2, which are used to reduce iron ore and other materials in the furnace. Recent improvements in blast furnace operation technology have reduced the reducing agent ratio (RAR) (the amount of coke and pulverized coal used to produce 1 ton of molten iron) to approximately 500 kg / t, which is already close to its lower limit. Therefore, further significant reductions in the RAR are not expected.

[0003] Meanwhile, in regions where natural gas is produced, a method of producing reduced iron using a vertical reduction furnace (hereinafter also referred to as a shaft furnace) is also commonly used. In this method, a reduction furnace is filled with agglomerated iron ore (hereinafter simply referred to as iron oxide), such as sintered ore or pellets, as the iron oxide raw material. Then, a reducing gas containing CO and H2 is injected into the reduction furnace to reduce the iron oxide and produce reduced iron. In this method, natural gas or the like is used as the raw material gas for the reducing gas. This raw material gas, together with the top gas, is heated and reformed in a reformer. This produces reducing gas. Here, the top gas is the gas used after the reduction of iron oxide in the reduction furnace and is generally discharged from the top of the reduction furnace. The generated reducing gas is injected into the reduction furnace and reacts with iron oxide supplied from the top of the reduction furnace. The iron oxide is then reduced to produce reduced iron. The reduced iron is then cooled in a region below the position where the reducing gas is injected into the reduction furnace and then discharged from the bottom of the reduction furnace.

[0004] As described above, the furnace top gas, which is the gas remaining after the reduction of iron oxide, is discharged from the reduction furnace, for example, from the furnace top. After dust collection and cooling, the furnace top gas is partially fed to the reformer as a raw material for the reformed gas. The remaining furnace top gas is used as fuel gas for the reformer. In this method, the furnace top gas used as fuel gas for the reformer is usually discharged outside the system.

[0005] As such a reduced iron production process, for example, Patent Document 1 describes a method in which exhaust gas from a reducing furnace and natural gas are reformed in a reformer to generate a reducing gas mainly composed of CO and H, and this reducing gas is blown into a reducing furnace to reduce iron oxide in the reducing furnace, thereby producing reduced iron.

[0006] Furthermore, Patent Document 2 describes a method for producing reduced iron by reforming coke oven gas and the top gas of a reducing furnace from which CO2 has been removed to produce reducing gas, and then injecting the reducing gas into a reducing furnace. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-88912 [Patent Document 2] Patent No. 6190522 Summary of the Invention [Problem to be solved by the invention]

[0008] The method described in Patent Document 1 uses natural gas supplied from an external source to produce the reducing gas, which results in the problem of unavoidable CO2 emissions to a certain extent, although at a lower level than in a blast furnace.

[0009] Furthermore, the method described in Patent Document 2 produces reducing gas using coke oven gas or converter gas generated within a steelworks. In an integrated steelworks, coke oven gas or converter gas is required as fuel gas for downstream processes such as heating furnaces and annealing furnaces. Therefore, diverting coke oven gas or converter gas to the reduced iron production process would result in a fuel gas shortage in the downstream processes. As a result, natural gas must be supplied from an external source to compensate for the fuel gas shortage in the downstream processes. In other words, the method described in Patent Document 2 cannot simultaneously achieve energy conservation and CO2 emission reduction, and thus, issues remain.

[0010] The present invention has been made in consideration of the above-described current situation, and has an object to provide a method for producing reduced iron that can simultaneously achieve energy saving and reduction in CO2 emissions. [Means for solving the problem]

[0011] The inventors have conducted extensive research to simultaneously achieve energy savings and reductions in CO2 emissions, and have developed a system that circulates and reuses furnace top gas. That is, the inventors a blowing step of blowing a reducing gas into a reduction furnace; a reduction step of reducing iron oxide with a reducing gas in a reduction furnace to obtain reduced iron; a distribution step of distributing a furnace top gas discharged from the reducing furnace into a first furnace top gas and a second furnace top gas; a synthesis step of synthesizing regenerated methane gas from the first furnace top gas and hydrogen gas; a reforming step in which the regenerated methane gas and the second furnace top gas are used as raw material gases to obtain a reducing gas from the raw material gases; We have developed a furnace gas circulation and reuse system (hereinafter simply referred to as the circulation system) that has the following features.

[0012] Furthermore, the inventors have conducted further studies and have obtained the following findings. In the above circulation system, the CO2 conversion rate η due to the methanation reaction in the synthesis process mThe amounts of gas introduced into the distribution step, synthesis step, and reforming step are controlled according to [-] (hereinafter also referred to simply as CO2 conversion rate). This makes it possible to produce reduced iron under highly stable operation without inputting excess energy, thereby achieving further energy savings and reducing CO2 emissions at the same time. The present invention was completed based on the above findings and further investigations.

[0013] That is, the gist and configuration of the present invention are as follows.

[0014] 1. A charging step of charging iron oxide into a reduction furnace; a blowing step of blowing a reducing gas into the reduction furnace; a reduction step of reducing the iron oxide with the reducing gas in the reduction furnace to obtain reduced iron; a distribution step of distributing a furnace top gas discharged from the reducing furnace into a first furnace top gas and a second furnace top gas; a synthesis step of synthesizing regenerated methane gas using the first furnace top gas and hydrogen gas; a reforming step in which the regenerated methane gas and the second furnace top gas are used as raw material gases to obtain the reducing gas from the raw material gases; and CO2 conversion rate η by the methanation reaction in the synthesis process m The method for producing reduced iron controls the amounts of gas introduced into the distribution step, the synthesis step, and the reforming step according to [-].

[0015] 2. The method for producing reduced iron according to 1 above, wherein the amounts of gas introduced into the distribution step, the synthesis step, and the reforming step are controlled so as to satisfy the following formulas (1) and (2):

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number

[0016] 3. In the distribution step, the furnace gas is separated into a separated carbon dioxide gas and a first remaining gas, Distributing the separated carbon dioxide gas as the first furnace top gas; 3. The method for producing reduced iron according to 1 or 2 above, wherein the first remaining gas is distributed as the second furnace top gas.

[0017] 4. In the distribution step, the furnace gas is separated into a separated carbon dioxide gas and a first remaining gas, Distributing the separated carbon dioxide gas as the first furnace top gas; separating the first remaining gas into a separated hydrogen gas and a second remaining gas; introducing the separated hydrogen gas into the synthesis step; 3. The method for producing reduced iron according to 1 or 2 above, wherein the second remaining gas is distributed as the second furnace top gas.

[0018] 5. In the distribution step, the furnace gas is separated into a separated hydrogen gas and a third remaining gas, introducing the separated hydrogen gas into the synthesis step; Separating the third remaining gas into a separated carbon dioxide gas and a fourth remaining gas; 3. The method for producing reduced iron according to 1 or 2 above, wherein the separated carbon dioxide gas is distributed as the first furnace top gas, and the fourth remaining gas is distributed as the second furnace top gas. [Effects of the Invention]

[0019] According to the present invention, it is possible to carry out an operation in which reduced iron is produced with high operational stability and at the same time achieves further energy saving and reduction in CO2 emissions. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing a conventional process for producing reduced iron. [Figure 2] 1 is a diagram showing an example of a process for producing reduced iron by a method for producing reduced iron according to an embodiment of the present invention. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, a method for producing reduced iron according to one embodiment of the present invention will be described with reference to the drawings.

[0022] First, a conventional reduced iron production process (hereinafter also referred to as the conventional production process) will be described. Figure 1 is a diagram showing a schematic configuration of an example of the conventional production process. In the figure, reference numeral 1 denotes a reduction furnace, 1a denotes iron oxide, 1b denotes reduced iron, 3 denotes a dust remover, 4 denotes a dehydration device, 5 denotes a natural gas supply unit, 6 denotes an air supply unit, 7 denotes a reformer, and 9 denotes a reducing gas injection unit.

[0023] In one example of a conventional manufacturing process, iron oxide is charged into a reduction furnace from the top and gradually lowered. High-temperature reducing gas is injected into the furnace from the center to reduce the iron oxide. The reduced iron is then discharged from the bottom of the reduction furnace. At this time, furnace top gas containing mainly CO, CO2, H2, and H2O is discharged from the top of the reduction furnace. This furnace top gas is then cleaned in a dust collector, and a portion is adjusted for moisture content and sent to a reformer as a feed gas. A hydrocarbon-containing gas, such as natural gas from a natural gas supply, is supplied to the reformer together with the moisture-adjusted furnace top gas. The supplied gas is then heated in the reformer. A reforming reaction occurs, producing high-temperature reducing gas containing mainly CO2 and H2. This reducing gas is then blown into the reduction furnace. The remaining furnace top gas is dehydrated and used as heating fuel in the combustion chamber of the reformer. After being burned as heating fuel, the furnace gas is usually discharged outside the system while still containing CO. When reduced iron is produced using this example of the conventional production process, approximately 1 ton of CO is discharged outside the circulation system for each ton of reduced iron produced.

[0024] On the other hand, in a method for producing reduced iron according to one embodiment of the present invention, as shown in Fig. 2, the furnace top gas discharged from the reducing furnace is distributed into a first furnace top gas and a second furnace top gas in a furnace top gas distribution unit. Then, instead of a hydrocarbon gas such as natural gas supplied from an external source in the conventional process shown in Fig. 1, methane gas (hereinafter also referred to as regenerated methane gas) synthesized from the first furnace top gas and H gas is used. In the diagram, reference numeral 10 denotes a hydrogen supply unit, 11 denotes a methane synthesis unit, 13 denotes a heat source, 14 denotes a furnace top gas distribution unit, and 15 denotes an oxygen supply unit. Here, the regenerated methane gas generated in the methane synthesis unit 11 is supplied to a reformer 7 and used as a raw material gas for the reducing gas. Each step of the method for producing reduced iron according to one embodiment of the present invention will be described below. Note that the filling step, blowing step, and reducing step may be performed in accordance with a conventional method, for example, in the same manner as in the conventional production process described above, and therefore, description thereof will be omitted here.

[0025] ·Distribution process In the distribution step, for example, in a furnace gas distribution section, the furnace gas discharged from the reducing furnace is distributed into a first furnace gas and a second furnace gas. The distribution and flow rate control means for the furnace gas are not particularly limited and may be conventional. For example, a mass flow controller may be used.

[0026] In the distribution step, the furnace gas may be distributed with its composition intact, or specific gas species such as CO2 and H2 may be separated and distributed, for example, as follows: (a) The furnace gas is separated into a separated carbon dioxide gas and a first remaining gas (CO2 separation), and the separated carbon dioxide gas is distributed as the first furnace gas and the first remaining gas is distributed as the second furnace gas. (b) The furnace top gas is separated into a separated carbon dioxide gas and a first remaining gas (CO2 separation), and the separated carbon dioxide gas is distributed as a first furnace top gas. The first remaining gas is separated into a separated hydrogen gas and a second remaining gas (H separation). The separated hydrogen gas is then introduced (supplied) to the synthesis step, and the second remaining gas is distributed as a second furnace top gas. (c) The furnace top gas is separated into separated hydrogen gas and a third remainder gas (H separation). Next, the separated hydrogen gas is introduced (supplied) to the synthesis step, and the third remainder gas is separated into separated carbon dioxide gas and a fourth remainder gas (CO separation). Then, the separated carbon dioxide gas is distributed as the first furnace top gas, and the fourth remainder gas is distributed as the second furnace top gas.

[0027] Here, the method of CO2 separation is not particularly limited, and various methods can be used, such as chemical absorption, physical absorption, adsorptive separation, membrane separation, cryogenic separation, oxyfuel combustion, and chemical looping combustion. Among these, chemical absorption, typified by amine absorption, and adsorptive separation, typified by PSA, have a long history of use in chemical plants and industrial CO2 production. These methods also have a track record of recovering gas with a CO2 concentration of 99% or more by volume. Therefore, these methods are preferred. The CO2 concentration in the separated carbon dioxide gas is preferably 90% or more by volume. The CO2 concentration in the separated carbon dioxide gas may be 100% by volume.

[0028] The method for separating H2 is not particularly limited, and various methods such as adsorption separation and membrane separation can be used. The concentration of H2 in the separated hydrogen gas is preferably 90% by volume or more. The concentration of H2 in the separated hydrogen gas may be 100% by volume.

[0029] The compositions of the first, second, and fourth remaining gases distributed as the second furnace top gas vary depending on the amounts of CO2 and H2 separated, but are, for example, CO: 5 to 70 volume %, CO2: 0 to 25 volume %, H2: 0 to 75 volume %, H2O: 0 to 50 volume %, and balance: 0 to 30 volume %.

[0030] In addition, when the furnace gas is distributed into the first furnace gas and the second furnace gas with the same composition in the distribution step, a separation step of separating specific gas species such as CO2 and H2 from the first furnace gas and the second furnace gas may be optionally included between the distribution step and the synthesis step described below. For example, as in (a) and (b) above, the first furnace gas may be separated into a separated carbon dioxide gas and a first residual gas (CO separation), and the separated carbon dioxide gas may be used as the first furnace gas in the synthesis step. The first residual gas may be directly combined with the second furnace gas. Alternatively, the first residual gas may be separated into a separated hydrogen gas and a second residual gas (H separation), and then the separated hydrogen gas may be introduced into the synthesis step, and the second residual gas may be combined with the second furnace gas. Alternatively, as in (c) above, the first furnace top gas is separated into a separated hydrogen gas and a third remainder gas. The separated hydrogen gas is then introduced into the synthesis step, and the third remainder gas is separated into a separated carbon dioxide gas and a fourth remainder gas (CO separation). The separated carbon dioxide gas may then be used as the first furnace top gas in the synthesis step, and the fourth remainder gas may be merged with the second furnace top gas.

[0031] Furthermore, as shown in Figure 2, from the viewpoint of material balance, a portion of the furnace gas may be used as fuel for heating a reformer or the like before being introduced into the distribution process. For example, in the combustion chamber of the heating device, a portion of the furnace gas is combusted using oxygen supplied from an oxygen supply device, for example, pure oxygen produced by a cryogenic separation process driven by CO2-free electricity. The combusted furnace gas is then dehydrated as necessary and returned to the original line to be introduced into the distribution process.

[0032] ·Synthesis process In the synthesis step, for example, in a methane synthesis apparatus, regenerated methane gas is synthesized from the first furnace top gas distributed in the distribution step and hydrogen gas. CH4 is synthesized from at least one of CO2 and CO contained in the first furnace top gas and H2 according to the following methanation reaction formulas (i) and (ii). CO2+4H2→ CH4+2H2O ΔH=-165kJ / mol ···(i) CO+3H2→ CH4+H2O ΔH=-206kJ / mol ···(ii) For example, the first furnace gas and hydrogen gas supplied from outside the circulation system are introduced into a methane synthesis apparatus. Then, in the methane synthesis apparatus, CH4 is synthesized by at least one of the reactions represented by the above formulas (i) and (ii). The synthesis conditions for CH4 are not particularly limited, and may be performed according to conventional methods.

[0033] When the separated carbon dioxide gas obtained in the distribution step or separation step is used, the composition of the first furnace gas will be the same as that of the separated carbon dioxide gas. Furthermore, when the furnace gas is distributed in the distribution step with its composition intact and does not undergo the separation step, the composition of the first furnace gas is essentially the same as that of the furnace gas introduced into the distribution step. In this case, the composition of the first furnace gas is, for example, 5-50% by volume of CO, 5-30% by volume of CO, 5-80% by volume of H, 0-35% by volume of H0, and the balance: 0-20% by volume. The composition of the second furnace gas, which will be described later, is similar.

[0034] In addition to the first furnace gas, any other gas containing at least one of CO and CO (hereinafter also referred to as other gas) can also be used in the synthesis step. Examples of other gases include gases produced as by-products in the steelmaking process, specifically blast furnace gas (BFG) and coke oven gas (COG). The other gas may be introduced into the separation step together with the first furnace gas, and separated into separated carbon dioxide gas and remaining gas, and then the separated carbon dioxide gas may be supplied to the synthesis step.

[0035] The source of hydrogen gas used in the synthesis process is not particularly limited, and it may be supplied and produced by any method. Examples of methods for producing hydrogen gas include synthesis by water electrolysis and synthesis by the decomposition reaction of ammonia, hydrocarbons, or organic hydrides. However, when hydrocarbons or organic hydrides are used as raw materials, CO2 is emitted during the hydrogen synthesis process. Therefore, from the perspective of further reducing CO2 emissions, synthesis by at least one of water electrolysis and ammonia decomposition is preferable. Furthermore, when producing hydrogen gas by water electrolysis, CO2 emissions can be reduced to zero by using green hydrogen produced using electricity obtained from green energy sources such as solar, wind, and geothermal energy. The H2 concentration of the hydrogen gas is not particularly limited, but is preferably 90% by volume or more, more preferably 95% by volume or more. The H2 concentration of the hydrogen gas may be 100% by volume.

[0036] Commonly used methanation catalysts can be used to synthesize CH4. Specifically, transition metal catalysts such as Fe, Ni, Co, and Ru can be used. Among them, Ni-based catalysts have high activity. Ni-based catalysts also have high heat resistance and can be used at temperatures up to about 500°C. Therefore, Ni-based catalysts are particularly preferred. Iron ore can also be used as a catalyst. In particular, high-water-of-crystallization ores have an increased specific surface area when the water of crystallization is dehydrated, making them suitable for use as catalysts.

[0037] The reactor used in the synthesis process for the methane synthesis apparatus can be a fixed-bed reactor, a fluidized-bed reactor, an entrained-bed reactor, or the like. The physical properties of the catalyst can be appropriately selected depending on the type of reactor. In addition, a heat exchanger can be disposed in the gas flow path downstream of the reactor to recover the reaction heat (gas sensible heat) of the methanation reaction in each reactor. The recovered thermal energy can be used, for example, to heat a reduction furnace or a reformer.

[0038] Furthermore, the methanation catalyst exhibits a stable and high conversion rate during CO methanation. However, during CO methanation, the formation of C intermediates causes C precipitation, resulting in catalyst poisoning. One measure to prevent catalyst poisoning during CO methanation is, for example, the addition of HO (water vapor). However, considering long-term catalyst use, a decrease in catalyst life is unavoidable. Furthermore, as shown in the above formulas (i) and (ii), the CO methanation reaction has a larger negative reaction enthalpy and generates a larger heat release than the CO methanation reaction. This results in a rapid increase in catalyst temperature during CO methanation. Furthermore, because the methanation reaction itself is an exothermic reaction, the conversion rate decreases over reaction time. Furthermore, an increase in catalyst temperature may accelerate catalyst deactivation. Therefore, in the synthesis process, it is preferable to use the separated carbon dioxide gas obtained in the distribution process or separation process as the first furnace gas.

[0039] If HO produced as a by-product of methane synthesis is introduced into the reformer, the amount of HO may become excessive in the reforming step described below. Therefore, it is preferable to dehydrate the regenerated methane gas using a dehydrator as appropriate, prior to the reforming step described below, while taking into consideration the material balance of the entire circulation system.

[0040] Furthermore, the CH4 concentration of the regenerated methane gas is not particularly limited. In the regenerated methane gas from which HO has been removed, for example, in the regenerated methane gas obtained by dehydrating the gas on the outlet side of a methane synthesis unit using a dehydrator, the CH4 concentration is preferably 80% by volume or more, more preferably 90% by volume or more. However, the CH4 concentration in the regenerated methane gas from which HO has been removed may be 100% by volume.

[0041] Modification process In the reforming process, the regenerated methane gas and the second furnace gas are used as raw material gases to obtain a reducing gas from the raw material gases. For example, the regenerated methane gas and the second furnace gas are introduced into a reformer, and then heated in the reformer. Then, in the reformer, a reducing gas containing CO and H2 is generated by the reforming reactions shown in the following formulas (iii) and (iv). The reforming reaction shown in formula (iv) proceeds when water vapor is supplied to the reformer. CH4+CO2→ 2CO+2H2ΔH=247kJ / mol ···(iii) CH4+H2O → CO+3H2ΔH=206kJ / mol ···(iv)

[0042] The raw material gas may be heated, for example, in a reformer. The heating temperature and heating means of the raw material gas are not particularly limited and may be conventional. For example, the heating temperature of the raw material gas may be 300 to 700°C. Furthermore, the temperature of the reducing gas (blowing into the reducing furnace) may be 750 to 1100°C.

[0043] The gas composition of the reducing gas is, for example, CO: 1 to 60% by volume, H2: 40 to 99% by volume, and the balance: 0 to 30% by volume.

[0044] As can be seen from the fact that the enthalpy of formation of the above formulas (iii) and (iv) is positive, both of the reactions of the above formulas (iii) and (iv) are endothermic reactions. Therefore, by using green energy such as solar, wind, or geothermal energy as the heat source for the reformer, it is possible, in principle, to reduce CO2 emissions to zero.

[0045] Next, the reducing gas is introduced into the reducing furnace through an injection process. For example, the reducing gas is introduced into the reducing furnace using a reducing gas injection device. Then, in the reducing furnace, the iron oxide is reduced by the reducing gas to obtain reduced iron. Meanwhile, the reducing gas after being used to reduce the iron oxide is discharged from the reducing furnace as furnace top gas.

[0046] Furthermore, it is preferable to perform at least one of dedusting and dehydration of the top gas prior to the above-mentioned distribution step. Any dust removal device can be used as the dust removal device. Any dehydration device can be used as the dehydration device. The order of dedusting and dehydration is not particularly limited. In the example shown in Figure 2, the top gas is dedusted by the dust removal device and then dehydrated by the dehydration device, and then the top gas is distributed into a first top gas and a second top gas.

[0047] Controlling the amount of gas introduced into the distribution process, synthesis process and reforming process In a method for producing reduced iron according to one embodiment of the present invention, it is important to control the amounts of gases introduced (supplied) to the distribution step, synthesis step, and reforming step in accordance with the CO conversion rate. This makes it possible to stabilize the composition of the reducing gas blown into the reducing furnace, particularly the ratio of H to CO contained in the reducing gas (hereinafter also referred to as H / CO of the reducing gas). As a result, it becomes possible to produce reduced iron under high operational stability without inputting extra energy, that is, while simultaneously achieving further energy savings and reductions in CO emissions.

[0048] Here, the CO conversion rate indicates the rate at which CO contained in the first furnace gas introduced into the synthesis step is converted to CH by the methanation reaction. For example, the CO conversion rate can be calculated by the following formula. CO2 conversion rate η m [-] = 1-(the amount of CO2 contained in the regenerated methane gas emitted after the synthesis of CH4 in the synthesis process [Nm 3 / t]) ÷ (the amount of CO2 contained in the first furnace gas introduced into the synthesis process [Nm 3 / t]) The CO2 conversion rate can be adjusted mainly by the reactor used in the synthesis step, for example, the type and amount of catalyst used in the reactor, the volume of the reactor, the temperature of the reactor, etc. In addition, when there is a gas introduced into the synthesis step other than the first furnace gas, such as separated hydrogen gas, the amount of CO2 contained in that gas (the amount of CO2 contained in the first furnace gas introduced into the synthesis step [Nm 3 / t]). Also, Nm 3 / t and kg / t are the basic units per ton of reduced iron (DRI) produced.

[0049] It is preferable to control the amounts of gas introduced into the distribution step, synthesis step, and reforming step so as to satisfy the following formulas (1) and (2).

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number

[0050] The left side of the above equation (1) is (W CO2 -V CO2 ) / W CO2 This can be rewritten as: , which represents the proportion of CO2 converted to CH4 out of the CO2 introduced into the distribution process. If the above formula (1) is not satisfied, the amount of CO2 converted to CH4 in the synthesis process will be insufficient, resulting in excess CO2 in the circulation system. As a result, in order to operate stably under a healthy material balance, it may be necessary to release CO2 outside the circulation system.

[0051] In addition, the left side of the above equation (2) is (W H2 +E H2 -V H2 ) / (W H2 +E H2 ) and can be said to represent the proportion of H2 converted to CH4 out of the total H2 introduced between the distribution process and the reforming process. If the CO2 conversion rate is less than the value on the left side of the above equation (2), the reaction efficiency in the synthesis process will be insufficient, resulting in an excess of H2 in the circulation system. As a result, stable operation under a sound material balance may not be possible.

[0052] The right side of the above equation (2) is (W H2 +E H2 -V H2 ) / E H2 This can be rewritten as: and can be said to represent the ratio of the amount of H2 converted to CH4 to the amount of H2 introduced into the synthesis process from outside the circulation system. Here, if the CO2 conversion rate η exceeds the value on the right side of the above formula (2), excessive H2 will be consumed in the synthesis process, which is undesirable from the viewpoint of energy conservation.

[0053] From the above, it is preferable to control the amounts of gas introduced into the distribution step, synthesis step, and reforming step so as to simultaneously satisfy the above formulas (1) and (2).

[0054] The left side of the above equation (1) is 1-V CO2 / W CO2 A value of +0.015 is more preferable. The left side of the above equation (2) is 1-V H2 / (W H2 +E H2 ) +0.015 is more preferable. The right side of the above equation (2) is 1 + (W H2 -V H2 ) / E H2 A value of -0.015 is more preferable.

[0055] The amounts of gas introduced into the distribution step, synthesis step, and reforming step may be constant or may be changed at any timing as long as they simultaneously satisfy the above formulas (1) and (2). The amounts of gas introduced into the distribution step, synthesis step, and reforming step at the time of equipment startup may be determined, for example, from past operating history, and the amounts of gas introduced into the distribution step, synthesis step, and reforming step may be changed and controlled as appropriate depending on the subsequent operating conditions.

[0056] Furthermore, the iron oxide raw material used in the method for producing reduced iron according to one embodiment of the present invention is, for example, iron ore. Specific examples include lump iron ore (lump ore) and pellets (spherical solidification of iron ore powder). The quality of the iron ore used as the iron oxide raw material, i.e., the iron content, is not particularly limited, but is generally preferably 65 mass% or more from the viewpoint of reduction in a shaft furnace.

[0057] Additionally, in the method for producing reduced iron according to one embodiment of the present invention, a method using a shaft furnace has been described as a direct reduction ironmaking method. However, the type of 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 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). [Example]

[0058] Examples will be described below. Reduced iron was produced in the circulation system shown in Figure 2 under the conditions listed in Table 1. Under all conditions, the operation period was 28 days. Table 1 lists the operating specifications in terms of the basic unit per ton of reduced iron produced. For example, if 1,300 kg of iron oxide pellets are used to produce 1 ton of reduced iron, the amount of iron oxide pellets used is expressed as 1,300 kg / t. If 3,000 t / day of reduced iron is produced, multiply this amount by 3,000 to obtain the daily specifications.

[0059] In both cases, the iron oxide pellets were charged into the reduction furnace at a rate of 1,394 kg / t in the charging process. In the injection process, reducing gas heated to 980°C was injected into the center of the reduction furnace to reduce the iron oxide pellets and obtain reduced iron. The furnace gas discharged from the reduction furnace was then dedusted and appropriately dehydrated to achieve a balanced mass balance. From the perspective of mass balance, a portion of the furnace gas was used as heating fuel. The furnace gas branched off as heating fuel was combusted in the combustion chamber of the reformer using oxygen generated by a cryogenic separation process driven by CO2-free electricity. The entire exhaust gas from the combustion chamber of the reformer was then recovered, dehydrated, and recombined with the furnace gas. The combined furnace gas was introduced into the distribution process and distributed into the first and second furnace gases. In the distribution step, the top gas introduced into the top gas distribution section was appropriately separated to obtain carbon dioxide gas by any of the above-mentioned modes (a) to (c) so as to balance the material balance, and the separated carbon dioxide gas was distributed as the first top gas. The separated hydrogen gas was also introduced into the synthesis step as the first top gas together with the separated carbon dioxide gas. Furthermore, the remaining gas (the first remaining gas, the second remaining gas, or the fourth remaining gas) was distributed as the second top gas. Next, the first top gas (partially including the separated hydrogen gas) and hydrogen gas from outside the circulation system were introduced into a methane synthesis unit, and regenerated methane gas was synthesized in the reactor of the methane synthesis unit. After dehydration of the synthesized regenerated methane gas, the regenerated methane gas and the second top gas were introduced into a reformer as raw gases, and a reducing gas was obtained from the raw gas. The total amount of gas introduced into the reforming step in Table 1 is the total amount of the regenerated methane gas and the second top gas. Conditions other than those described above and in Table 1 were in accordance with conventional methods.

[0060] [Table 1]

[0061] In all of the inventive examples, stable operation was achieved over the entire 28-day operation period under a sound material balance in the circulation system shown in Figure 2, which is extremely advantageous for achieving energy savings, i.e., the system in which the furnace top gas is circulated and reused. Furthermore, CO2 emissions from the circulation system were reduced to zero. On the other hand, in all of the comparative examples in which the gas amount was not controlled according to the CO conversion rate in the synthesis step, the composition of the reducing gas could not be maintained within a certain range during the operation period, and the reaction in the reduction furnace became unstable, which forced the operation to be suspended. [Explanation of symbols]

[0062] 1. Reduction furnace 1a iron oxide 1b Reduced iron 3 Dust removal equipment 4 Dehydration equipment 5. Natural Gas Supply Department 6 Air supply section 7. Reformer 9. Reducing gas injection device 10 Hydrogen supply unit 11 Methane synthesis unit 13 Heat source 14 Furnace top gas distribution section 15 Oxygen supply unit

Claims

1. a charging step of charging the iron oxide into a reduction furnace; a blowing step of blowing a reducing gas into the reduction furnace; a reduction step of reducing the iron oxide with the reducing gas in the reduction furnace to obtain reduced iron; a distribution step of distributing a furnace top gas discharged from the reducing furnace into a first furnace top gas and a second furnace top gas; a synthesis step of synthesizing regenerated methane gas using the first furnace top gas and hydrogen gas; a reforming step in which the regenerated methane gas and the second furnace top gas are used as raw material gases to obtain the reducing gas from the raw material gases; and CO from the methanation reaction in the synthesis process ２ Conversion rate η ｍ the amounts of gas introduced into the distributing step, the synthesizing step, and the reforming step are controlled according to [-] so as to satisfy the following formulas (1) and (2): [Equation 1] [Equation 2] where: W CO2 : Amount of CO 2 contained in the furnace gas introduced into the distribution process [Nm 3 / t] W H2 : Amount of H 2 contained in the furnace gas introduced into the distribution step [Nm 3 / t] E H2 : Amount of H 2 contained in hydrogen gas introduced into the synthesis step [Nm 3 / t] V CO2 : Amount of CO2 contained in the gas introduced into the reforming process [Nm 3 / t] V H2 : Amount of H 2 contained in the gas introduced into the reforming step [Nm 3 / t] is.

2. In the distribution step, the furnace gas is separated into a separated carbon dioxide gas and a first remaining gas, Distributing the separated carbon dioxide gas as the first furnace top gas; The method for producing reduced iron according to claim 1 , wherein the first remaining gas is distributed as the second top gas.

3. In the distribution step, the furnace gas is separated into a separated carbon dioxide gas and a first remaining gas, Distributing the separated carbon dioxide gas as the first furnace top gas; separating the first remainder gas into a separated hydrogen gas and a second remainder gas; introducing the separated hydrogen gas into the synthesis step; The method for producing reduced iron according to claim 1 , wherein the second remainder gas is distributed as the second top gas.

4. In the distribution step, the furnace gas is separated into a separated hydrogen gas and a third remaining gas, introducing the separated hydrogen gas into the synthesis step; Separating the third remaining gas into a separated carbon dioxide gas and a fourth remaining gas; 2. The method for producing reduced iron according to claim 1, wherein the separated carbon dioxide gas is distributed as the first furnace top gas, and the fourth remaining gas is distributed as the second furnace top gas.

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