Reduced iron manufacturing method

Abstract

Provided is a reduced iron production method with which energy saving and CO2 emission reduction can be simultaneously achieved. This method involves: constituting a system of circulating and reusing a top gas, the system including a blow-in step, a reduction step, a distribution step, a synthesis step, and a reformulation step; and controlling a distribution ratio of the top gas in the distribution step according to a ratio of the amount of H2 to the amount of CO consumed in the reduction step, i.e., consumed H2 / CO.

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 synthesis step of synthesizing regenerated methane gas from the furnace top gas discharged from the reduction furnace and hydrogen gas; a reforming step in which the regenerated methane gas is used as a raw material gas and a reducing gas is obtained from the raw material gas; 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. To achieve stable operation that simultaneously achieves energy savings and CO2 emission reductions, it is important to maintain a healthy material balance in the above circulation system, especially the C (carbon) material balance. To achieve this, it is important to control the gas balance in the circulation system so that the ratio of the amount of H2 to the amount of CO in the reducing gas supplied to the reducing furnace (hereinafter also referred to as H2 / CO in the reducing gas) is always maintained within a certain range without large fluctuations. Note that the H2 / CO in the reducing gas is a volume ratio under standard conditions (also referred to as a flow rate ratio under standard conditions). The same applies to the consumed H2 / CO ratio, etc., which will be described later.

[0013] The H2 / CO ratio of the reducing gas has an optimal range in terms of energy depending on the furnace temperature, pressure, furnace size, and other factors of the direct reduction furnace. In addition to natural gas, which is used in typical direct reduction furnaces such as Midrex® and Hyl®, other sources of reducing gas include coke oven gas, hydrocarbons, waste plastics, and combustion gases from biomass. When these gases are used, the H2 / CO ratio of the reducing gas varies depending on the composition of the gas. Currently operating direct reduction furnaces are operated within a range of H2 / CO suitable for each plant. However, the composition of the furnace top gas may fluctuate when the quality of the iron oxide used as the raw material for reduced iron or the furnace temperature fluctuates. In such cases, the H2 / CO ratio of the reducing gas also fluctuates, making it difficult to maintain a healthy material balance in the above-mentioned circulation system.

[0014] As a result of further investigations into the above points, the inventors have found that by performing the following control, it is possible to maintain a healthy material balance in the circulation system, that is, to stably perform an operation that simultaneously achieves energy savings and reductions in CO2 emissions during the production of reduced iron. The top gas discharged from the reduction furnace is divided into a first top gas and a second top gas. At this time, the distribution ratio of the furnace top gas in the distribution process is controlled according to the consumed H2 / CO, which is the ratio of the amount of H2 to the amount of CO consumed in the reduction process. The present invention was completed based on the above findings and further investigations.

[0015] That is, the gist of the present invention is as follows.

[0016] 1. A charging step of charging iron oxide into a reduction furnace, An injection step of injecting 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 the top gas discharged from the reduction furnace into a first top gas and a second top gas, A synthesis step of synthesizing a reformed methane gas from the first top gas and hydrogen gas, A reforming step of obtaining the reducing gas from the raw material gas using the reformed methane gas and the second top gas as the raw material gas, [[ID=I6]]which has A method for producing reduced iron, which controls the distribution ratio of the top gas in the distribution step according to the ratio of the amount of H2 to the amount of CO consumed in the reduction step, that is, the consumed H2 / CO.

[0017] 2. Further having a separation step of separating the first top gas into carbon dioxide gas and the remaining gas between the distribution step and the synthesis step, The method for producing reduced iron according to 1 above, wherein the carbon dioxide gas is used as the first top gas in the synthesis step.

[0018] 3. The method for producing reduced iron according to 1 or 2 above, wherein the distribution ratio Y×100 (%) of the first top gas in the distribution step is controlled within the range of the reference distribution ratio Y0×100 (%)±5%. Here, When X≦1, Y0 = 1 / (2η) When 1<X<4η - 1, Y0=(X + 1) / (4η) When X≧4η - 1, Y0 = 1 is. Also, X: Consumed H2 / CO η×100: CO2 conversion rate (%) by methane synthesis in the synthesis step is. Furthermore, if the range of the reference distribution ratio Y0×100±5% exceeds 100%, the upper limit of the range shall be 100%.

[0019] 4. The method for producing reduced iron according to 3 above, wherein the distribution ratio Y×100(%) to the first furnace gas in the distribution step is controlled to the reference distribution ratio Y0×100(%).

[0020] 5. The method for producing reduced iron according to 3 or 4 above, wherein when the consumed H2 / CO ratio is less than 1, excess H2 is recovered from a circulation system having the injection step, the reduction step, the distribution step, the synthesis step, and the reforming step.

[0021] 6. The method for producing reduced iron according to 3 or 4, wherein when the consumed H2 / CO>4η-1, additional H2 is supplied to a circulation system having the injection step, the reduction step, the distribution step, the synthesis step, and the reforming step.

[0022] 7. The method for producing reduced iron according to any one of 1 to 6 above, wherein at least one of dedusting and dehydration of the furnace gas is carried out prior to the distribution step.

[0023] 8. The method for producing reduced iron according to any one of 1 to 7 above, wherein the regenerated methane gas is dehydrated prior to the reforming step. [Effects of the Invention]

[0024] According to the present invention, it is possible to stably carry out an operation that simultaneously achieves energy saving and reduction in CO2 emissions when producing reduced iron. [Brief explanation of the drawings]

[0025] [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. [Figure 3]FIG. 10 is a diagram showing the relationship between consumed H2 / CO:X and the distribution ratio Y of the first furnace top gas. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, a method for producing reduced iron according to one embodiment of the present invention will be described with reference to the drawings.

[0027] 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.

[0028] 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 of it 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, where it is burned, for example, with oxygen from the air. After being burned as heating fuel, the furnace gas is usually discharged outside the system, still containing CO2.

[0029] 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. 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 hydrogen gas is used. In the diagram, reference numeral 10 denotes a hydrogen supply unit, 11 denotes a methane synthesis unit, 12 denotes a CO2 separation unit, 13 denotes a steam supply unit, and 14 denotes a furnace top gas distribution 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.

[0030] ·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. It is important to control the distribution ratio to the first furnace gas in accordance with the ratio of consumed H / CO, which is the ratio of the amount of H to the amount of CO consumed in the reduction step. The reason for this will be described later. 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 or the like may be used.

[0031] ·Separation process Between the distribution step and the synthesis step described below, a separation step may be optionally performed, for example, using a CO2 separator to separate the first furnace gas into carbon dioxide gas and a residual gas. The CO2 separation method 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 methods, such as the amine absorption method, and adsorptive separation methods, such as the PSA method, have a long history of use in chemical plants and industrial CO2 production. The CO2 concentration in the carbon dioxide gas is preferably 90% by volume or more. The CO2 concentration in the carbon dioxide gas may also be 100% by volume. The residual gas may be introduced, for example, together with the second furnace gas, into the reforming step described below.

[0032] ·Synthesis process In the synthesis step, for example, in a methane synthesis apparatus, regenerated methane gas is synthesized from the first furnace gas distributed in the distribution step and hydrogen gas. When the separation step is performed, it is preferable to use carbon dioxide gas as the first furnace gas in this step. CH4 is synthesized from at least one of CO2 and CO contained in the first furnace gas and H2 according to the methanation reaction formulas (i) and (ii) below. 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 carbon dioxide gas obtained in the above separation step is used, the composition of the first furnace gas will be the same as that of the carbon dioxide gas. Furthermore, when the above separation step is not performed, the composition of the first furnace gas is basically the same as that of the furnace gas. In this case, the composition of the first furnace gas is CO: 5-50 vol%, CO2: 5-30 vol%, H2: 5-80 vol%, HO: 0-35 vol%, and the balance: 0-20 vol%. The composition of the second furnace gas, which will be described later, is also 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 carbon dioxide gas and the remaining gas, and then the 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] Furthermore, the amount of hydrogen gas supplied in the synthesis step (the amount of hydrogen gas supplied to the methane synthesis apparatus) is preferably stoichiometric with respect to the amount of CO contained in the first furnace top gas, based on the methanation reaction formula shown in the above formula (i).

[0037] 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.

[0038] 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.

[0039] Furthermore, the methanation catalyst exhibits a stable high conversion rate during CO methanation. However, there have been reported cases where the formation of C intermediates during CO methanation causes C precipitation, resulting in catalyst poisoning. Therefore, it is preferable to include a separation step between the distribution step and the synthesis step, in which the first furnace gas is separated into carbon dioxide gas and a residual gas. In other words, it is preferable to use the carbon dioxide gas separated from the first furnace gas in the separation step as the first furnace gas in the synthesis step.

[0040] 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.

[0041] 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. The CH4 concentration in the regenerated methane gas from which HO has been removed may be 100% by volume.

[0042] 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 the regenerated methane gas and the second furnace gas are 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). Note that the reforming reaction shown in formula (iv) proceeds by supplying steam to the reformer. CH4+CO2→ 2CO+2H2ΔH=247kJ / mol ···(iii) CH4+H2O → CO+3H2ΔH=206kJ / mol ···(iv)

[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 energy source for the reforming reaction, 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] Top gas distribution ratio in the distribution process In the method for producing reduced iron according to one embodiment of the present invention, it is important to control the distribution ratio of the furnace gas in the distribution step in accordance with consumed H / CO, which is the ratio of the amount of H to the amount of CO consumed in the reduction step (hereinafter simply referred to as consumed H / CO). Here, the consumed H2 / CO is calculated, for example, by the following formula. Consumed H2 / CO = ([H2 content in reducing gas blown into reduction furnace (Nm 3 / t)]-[H2 content in furnace gas (Nm 3 / t)]) / ([Amount of CO contained in the reducing gas blown into the reduction furnace (Nm 3 / t)]-[CO content in top gas (Nm 3 / t)]) In addition, Nm 3 / t is the basic unit per ton of reduced iron (DRI) produced.

[0048] For example, the distribution ratio Y×100 (%) to the first top gas is controlled around the reference distribution ratio Y0×100 (%) calculated according to X. For example, Y×100 (%) is controlled within a range of preferably Y0×100 (%) ± 5%, more preferably Y0×100 (%) ± 3%, and even more preferably Y0×100 (%) ± 1%. Most preferably, the distribution ratio Y×100 (%) to the first top gas is controlled to Y0×100 (%). Here,[[]]END]] When X ≤ 1, Y0 = 1 / (2η) When 1 < X < 4η - 1, Y0 = (X + 1) / (4η) When X ≥ 4η - 1, Y0 = 1 That is.[[]]END]] [[ID=十二]]Also,[[]]END]] X: Consumption H2 / CO η×100: CO2 conversion rate (%) by methane synthesis in the synthesis process That is.[[]]END]] Furthermore, when the ranges of Y0×100 ± 5%, Y0×100 ± 3% and Y0×100 ± 1% each exceed 100%, the upper limit of the range shall be 100%.

[0049] [[ID=二十四]]Note that Y is defined by the following formula.[[]]END]] Y = [Amount of top gas distributed to the first top gas in the distribution process (Nm 3 / t)] ÷ [Amount of top gas introduced into the distribution process (Nm 3 / t)][[]]END]] The distribution ratio to the second top gas is also defined in the same manner as the above formula. Since the top gas is basically distributed to the first top gas and the second top gas, by appropriately controlling the distribution ratio to the first top gas, the distribution ratio to the second top gas is also appropriately controlled. In this case, the distribution ratio to the second top gas is 1 - Y. In addition, if it is 10% by volume or less of the amount of top gas, a part of the top gas may be supplied to other loads according to operating conditions, etc. Note that the amount of top gas supplied to other loads is not included in the amount of top gas introduced into the distribution process.[[]]END]] [[ID=3十四条]]

[0050] The CO2 conversion rate η×100 (%) due to methane synthesis in the synthesis process is defined by the following formula: [CO2 conversion rate η × 100 (%)] = (1 - [the amount of CO2 contained in the regenerated methane gas emitted after the synthesis of CH4 in the synthesis process (Nm 3 / t)] ÷ [Amount of CO2 introduced into the synthesis process (Nm 3 / t)]) × 100

[0051] Next, we will explain the above-mentioned standard distribution ratio Y0 × 100 (%). First, when iron oxide is reduced using a reducing gas that is a mixture of H2 and CO, the amounts of H2 and CO consumed are determined by the equilibrium constant and reaction rate constant of the following equations (v) and (vi). For example, if the grade of the raw iron ore decreases, the activation energy of H2 reduction increases, which is expected to slow the progress of equation (vi). In this way, the amounts of H2 and CO consumed in reduction vary depending on the operating conditions. 3CO+Fe2O3→3CO2+2Fe ···(v) 3H2+Fe2O3→3H2O+2Fe ···(vi)

[0052] In this regard, in a method for producing reduced iron according to one embodiment of the present invention, H and CO equivalent to the amounts consumed by reduction are synthesized by reforming CH to compensate for the amount, thereby making it possible to maintain a constant composition of the reducing gas injected into the reduction furnace. That is, in the above formula (v), the amount of CO consumed by reduction is equal to the amount of CO produced by reduction. A portion of this CO is methanated as the first top gas in the synthesis step and then supplied to the reforming step. The remaining CO is supplied as the second top gas to the reforming step without passing through the synthesis step. That is, in terms of the mass balance of the entire C, in the above circulation system, the amount of CO consumed by the reduction of iron oxide in the reduction step is substantially equal to the amount of CO produced in the reforming step.

[0053] Here, as shown in the above formulas (iii) and (iv), when CH4 is reformed with CO2, H2 and CO are produced in a ratio of 1:1. On the other hand, when CH4 is reformed with HO, H2 and CO are produced in a ratio of 3:1. The distribution ratio to the first top gas is synonymous with the ratio of CO2 contained in the top gas that is supplied to the synthesis process. Similarly, the distribution ratio to the second top gas is synonymous with the ratio of CO2 contained in the top gas that is supplied to the reforming process. In other words, the distribution ratio to the second top gas that is supplied to the reforming process can be said to indicate the ratio of the reforming reaction by CO2 in formula (iii) above. Therefore, the distribution ratio of the top gas is an important factor in maintaining a healthy material balance in the above circulation system, especially the material balance of the entire C.

[0054] Here, if the total carbon equivalent of CO2 contained in the furnace gas introduced into the distribution process is set to 1, the carbon equivalent of CH4 in the regenerated methane gas synthesized in the synthesis process is expressed as Yη, which is the product of the distribution ratio Y of the first furnace gas and the CO2 conversion rate η due to methane synthesis in the synthesis process.

[0055] Furthermore, the CO2 introduced into the reforming process is the sum of the CO2 contained in the second furnace gas and the CO2 contained in the first furnace gas that remains without being converted into methane in the synthesis process. Therefore, if the total carbon equivalent of the CO2 contained in the furnace gas introduced into the distribution process is set to 1, the carbon equivalent of the CO2 introduced into the reforming process can be expressed by the following formula. 1-Y+Y(1-η)=1-Yη

[0056] For example, when X=1, that is, when the ratio of the amount of H2 to the amount of CO consumed in the reduction step is 1:1, allowing the reaction of the above formula (iii) to proceed 100% produces reducing gas with a H2:CO=1:1 ratio, thereby compensating for the amounts of H2 and CO consumed in the reduction step. In this case, it is necessary to supply CO2 in an amount equivalent to the CH4 contained in the regenerated methane gas to the reforming step. Yη:1-Yη=1:1 When this is transformed, Y=1 / (2η) This becomes: That is, when X=1, Y0=1 / (2η). Even when X<1, that is, when the amount of CO consumed in the reduction step is greater than the amount of H2, it is optimal to allow the reaction of equation (iii) above to proceed 100% in order to maintain the material balance. Therefore, in this case too, Y0=1 / (2η).

[0057] On the other hand, as the value of X increases, it is necessary to increase the ratio of the HO reforming reaction of the above formula (iv). That is, it is necessary to increase the ratio of CO2 introduced into the synthesis step relative to the CO2 contained in the top gas by increasing the distribution ratio Y to the first top gas in the distribution step. Here, if Y=1, Among the reforming reactions in the reforming step, the ratio of the CO reforming reaction of the above formula (iii) is 1-η (×100(%)) It is expressed as: The ratio of the HO reforming reaction in the above formula (iv) is: η-(1-η)=2η-1 (×100(%)) It is expressed as: Therefore, the H2 / CO ratio of the reducing gas obtained in the reforming step is expressed by the following formula: {2×(1−η)+3(2η-1)} / {2×(1−η)+2η-1}=4η-1 Therefore, when X=4η-1, Y=1, that is, distributing the entire amount of top gas as the first top gas in the distribution process and promoting the reforming reaction of the above formula (iv) as much as possible according to η, is optimal for maintaining the material balance. Therefore, in this case, Y0=1.

[0058] Even when X>4η-1, that is, when the consumed H2 / CO ratio in the reduction step exceeds 4η-1, it is optimal to proceed with the reaction of the above formula (iv) as much as possible in order to maintain the material balance. Therefore, in this case, Y0=1.

[0059] Also, when 1 < X < 4η - 1, the reference distribution ratio Y0 to the first top gas is, as shown in FIG. 3, a straight line Y0 = aX + b connecting two points with Y0 = 1 / (2η) when X = 1 and Y0 = 1 when X = 4η - 1 as thresholds. Here, a and b are as follows. a = 1 / (4η) b = 1 / (4η) That is, when 1 < X < 4η - 1, Y0 = (X + 1) / (4η).

[0060] Then, the distribution ratio Y×100(%) to the first top gas is controlled before and after the reference distribution ratio Y0×100(%) calculated according to X. For example, Y×100(%) is preferably controlled within the range of Y0×100(%) ± 5%, more preferably within the range of Y0×100(%) ± 3%, and even more preferably within the range of Y0×100(%) ± 1%. Most preferably, the distribution ratio Y×100(%) to the first top gas is controlled to Y0×100(%). Thereby, H2 / CO of the reducing gas can be maintained within a certain range without large fluctuations, and the gas balance of the above circulation system can be controlled.

[0061] In addition, when X < 1, even if the reaction of the above formula (iii) proceeds 100%, excess H2 (hereinafter also referred to as excess H2) will occur in the circulation system. Therefore, in this case, it is preferable to recover the excess H2 from the circulation system. The step of recovering the excess H2 can be, for example, immediately before the reforming step, or after the reforming step and before the blowing step. The recovery amount of the excess H2 is preferably determined such that the amount of H2 contained in the reducing gas blown into the reduction furnace is kept constant and the reaction of the above formula (iii) proceeds 100%. Therefore, from the above viewpoints, the step of recovering the excess H2 is preferably after the reforming step and before the blowing step.

[0062] Furthermore, when X>4η-1, even if the reaction of the above formula (iv) proceeds to 100%, there will be a shortage of H2 in the circulation system. Therefore, in this case, it is preferable to additionally supply H2 to the circulation system. The amount of additional H2 to be supplied is preferably determined so that the amount of H2 contained in the reducing gas blown into the reducing furnace is kept constant. Therefore, it is preferable that the step of additionally supplying H2 be performed, for example, immediately before the reforming step, or after the reforming step and before the blowing step.

[0063] If the distribution ratio Y to the first furnace gas does not match the reference distribution ratio Y0, the consumed H2 / CO and the H2 / CO of the reducing gas obtained in the reforming process will not strictly match. However, as described above, this is acceptable as long as the distribution ratio Y×100(%) to the first furnace gas is within the range of the reference distribution ratio Y0×100±5%. In this case, for example, H2 may be supplied or recovered as appropriate so that the amount of H2 contained in the reducing gas is kept constant.

[0064] Furthermore, the present invention focuses on the mass balance of the CO2 circulation. In other words, since CO and H2 are assumed to remain constant before and after the synthesis step and the reforming step, when the CO methanation reaction according to the above formula (ii) occurs in the synthesis step, the H2 / CO ratio of the reducing gas changes. Therefore, as described above, it is preferable to have a separation step in which the first furnace gas is separated into carbon dioxide gas and a residual gas, and to use the carbon dioxide gas obtained in the separation step as the first furnace gas in the synthesis step. Furthermore, it is preferable that the CO2 concentration in the carbon dioxide gas is high, preferably 90% by volume or more. Furthermore, the CO2 concentration in the carbon dioxide gas is optimally 100% by volume. When CO methanation occurs in the synthesis step, for example, H2 may be supplied or recovered as appropriate so that the amount of H2 contained in the reducing gas remains constant.

[0065] The distribution ratio to the first top gas in the distribution process may be constant or may be changed at any timing. For example, the components of the reducing gas and the top gas may be measured, and the distribution ratio to the first top gas in the distribution process may be changed according to fluctuations in X: consumed H2 / CO. Furthermore, the distribution ratio to the first top gas at the time of equipment startup may be determined, for example, from past operating history, and thereafter, the distribution ratio to the first top gas may be appropriately controlled according to fluctuations in X: consumed H2 / CO.

[0066] In the method for producing reduced iron according to one embodiment of the present invention, the flow rate of the reducing gas supplied to the reduction furnace is 1500 Nm 3 / t or more 3500Nm 3 / t or less is preferable. In other words, if the flow rate of the reducing gas supplied to the reducing furnace is too low, it will lead to a decrease in production volume and a deterioration in product properties. On the other hand, if the flow rate of the reducing gas supplied to the reducing furnace is too high, the gas flow resistance in the reducing furnace will increase, and the raw material (pellets) in the furnace will not descend. Therefore, the flow rate of the reducing gas supplied to the reducing furnace should be 1500 Nm 3 / t or more 3500Nm 3 / t or less is preferable.

[0067] 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 iron ore formed by solidifying powdered iron ore). The quality of the iron ore used as the iron oxide raw material, i.e., the iron content, is not particularly limited, but from the viewpoint of reduction in a shaft furnace, it is generally preferable that the iron content be 65% by mass or more. However, in recent years, high-quality iron ore, such as that produced in South America, is expected to become depleted, leading to a rise in its price. Therefore, it is preferable to use low-quality iron ore with an iron content of 63% by mass or less, as needed. Such low-quality iron ore is inexpensive and abundant, and examples of sources include Australia.

[0068] Additionally, in the method for producing reduced iron according to one embodiment of the present invention, a method using a shaft furnace as a direct reduction ironmaking method has been described. 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]

[0069] Examples will be described below. In the circulation system shown in Figure 2, reduced iron was produced under the conditions listed in Table 1. Under all conditions, the flow rate of the reducing gas supplied to the reduction furnace was 2200 Nm 3 The operation period was 28 days. In the distribution process, the entire amount of introduced top gas was distributed to the first and second top gases, and it was not supplied to other loads. In the separation process, the first top gas was separated into carbon dioxide gas (CO2 concentration: 99% by volume or more) and the remaining gas, and the carbon dioxide gas was used as the first top gas in the synthesis process. The remaining gas was introduced into the reforming process together with the second top gas. When excess H2 was recovered or additional H2 was supplied, it was done after the reforming process and before the injection process. Note that conditions other than those described above and in Table 1 were standard practice. In Table 1, the flow rates of the reducing gas and top gas are rounded to the nearest whole number. Therefore, the (total) flow rate of the reducing gas may not match the sum of the H2 and CO2 amounts in the reducing gas. The same applies to the top gas.

[0070] [Table 1]

[0071] In all of the examples of the invention in which the distribution ratio of the furnace gas in the distribution process was controlled according to the consumed H2 / CO, stable operation was possible over the entire 28-day operation period under a sound material balance in the circulation system shown in Figure 2, i.e., the system in which the furnace gas is circulated and reused, which is extremely advantageous for realizing energy savings. In addition, CO2 emissions from the circulation system were also reduced to zero. For comparison, reduced iron was produced in No. 12 under the same conditions as No. 6, except that the top gas distribution ratio was not controlled and all of the top gas was introduced into the synthesis process. However, during the operation period, stable operation under a sound material balance could not be achieved, and the operation was forced to be suspended. [Explanation of symbols]

[0072] 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 12 CO2 separator 13 Steam supply section 14 Furnace top gas distribution section

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 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 the reducing gas from the raw material gases; and The ratio of H to the amount of CO consumed in the reduction step ２ The ratio of the amount consumed H ２ a distribution ratio Y×100(%) to the first furnace gas in the distribution step is controlled within a range of a reference distribution ratio Y 0 ×100(%)±5% according to the amount of CO in the first furnace gas. where: When X≦1, Y 0 =1 / (2η) When 1<X<4η-1, Y 0 =(X+1) / (4η) If X≧4η−1, Y 0 =1 is. Also, X: consumption H 2 / CO η×100: CO 2 conversion rate (%) due to methane synthesis in the synthesis step is. Furthermore, if the range of the reference distribution ratio Y 0 ×100±5% exceeds 100%, the upper limit of the range is set to 100%.

2. The method further comprises a separation step between the distribution step and the synthesis step, of separating the first furnace gas into carbon dioxide gas and a remaining gas, The method for producing reduced iron according to claim 1 , wherein the carbon dioxide gas is used as the first furnace gas in the synthesis step.

3. The distribution ratio Y×100(%) to the first furnace gas in the distribution step is defined as the reference distribution ratio Y ０ 3. The method for producing reduced iron according to claim 1, wherein the temperature is controlled to be 100%.

4. The consumption H ２ When / CO<1, excess H is removed from the circulation system having the injection step, the reduction step, the distribution step, the synthesis step, and the reforming step. ２ The method for producing reduced iron according to claim 1 or 2, wherein

5. The consumption H ２ When / CO<1, excess H is removed from the circulation system having the blowing step, the reduction step, the distribution step, the synthesis step, and the reforming step. ２ The method for producing reduced iron according to claim 3, wherein

6. The consumption H ２ When / CO>4η-1, H is introduced into the circulation system having the injection step, the reduction step, the distribution step, the synthesis step, and the reforming step. ２ The method for producing reduced iron according to claim 1 or 2, further comprising the step of:

7. The consumption H ２ When / CO>4η-1, H is introduced into the circulation system having the injection step, the reduction step, the distribution step, the synthesis step, and the reforming step. ２ The method for producing reduced iron according to claim 3 , further comprising the step of:

8. The method for producing reduced iron according to claim 1 or 2, wherein at least one of dedusting and dehydration of the furnace gas is carried out prior to the distribution step.

9. The method for producing reduced iron according to claim 3 , wherein at least one of dedusting and dehydration of the furnace gas is carried out prior to the distribution step.

10. 3. The method for producing reduced iron according to claim 1, wherein the regenerated methane gas is dehydrated prior to the reforming step.

11. The method for producing reduced iron according to claim 3 , wherein the regenerated methane gas is dehydrated prior to the reforming step.

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