Method for producing reduced iron

Abstract

Provided is a method for producing reduced iron with which it is possible to achieve both energy conservation and reduction in the amount of CO2 emissions. A system for recycling furnace top gas, which comprises a blowing step, a reduction step, a distribution step, a synthesis step and a reforming step, is configured, and the amount of gas introduced into the distribution step, the synthesis step and the reforming step is controlled in accordance with the CO2 conversion rate of a methanation reaction in the synthesis step.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing reduced iron. Background Art

[0002] In recent years, in steel plants, against the backdrop of global environmental problems and fossil fuel depletion problems, there has been a strong demand for energy conservation. The main raw material of iron is iron oxide, and in steel plants, a reduction process for reducing this iron oxide is essential. The most common reduction process worldwide is the blast furnace. In the blast furnace, coke, pulverized coal, and oxygen in hot air (air heated to about 1200 °C) react at the tuyeres. Through this reaction, CO and H2, which are reduction gases, are generated, and these reduction gases are used to reduce iron ore and the like in the furnace. With the improvement of blast furnace operation technology in recent years, the reductant ratio (the amount of coke and pulverized coal used per ton of hot metal produced) has been reduced to about 500 kg / t, and the reductant ratio has almost reached the lower limit. Therefore, no further significant reduction in the reductant ratio can be expected.

[0003] On the other hand, in regions where natural gas is produced, a method for manufacturing reduced iron using a vertical reduction furnace (hereinafter also referred to as a shaft furnace) is also often used. In this method, lumpy iron ore such as sintered ore and pellet ore (hereinafter also simply referred to as iron oxide) is filled in the reduction furnace as an iron oxide raw material. Moreover, a reduction gas containing CO and H2 is blown into the reduction furnace to reduce the iron oxide and manufacture reduced iron. In this method, natural gas or the like is used as a raw material gas for the reduction gas. This raw material gas is heated and reformed together with the top gas in a reforming device. Thereby, a reduction gas is generated. Here, the top gas is the gas after being supplied for the reduction of iron oxide in the reduction furnace and is generally discharged from the top of the reduction furnace. The generated reduction gas is blown into the reduction furnace and reacts with the iron oxide supplied from the upper part of the reduction furnace. Then, the iron oxide is reduced to reduced iron. Next, the reduced iron is cooled in a region lower than the position where the reduction gas is blown in the reduction furnace and then discharged from the lower part of the reduction furnace.

[0004] In addition, as described above, the top gas, which is the gas after being supplied for the reduction of iron oxide, is discharged from the reduction furnace, for example, from the top. Then, after the top gas is dust-collected and cooled, a part of it is sent to the reforming device as a raw material for the reforming gas. In addition, the remaining top gas is used as a fuel gas for the reforming device. In this method, the top gas used as the fuel gas for the reforming device is usually discharged out of the system.

[0005] As such a reduced iron manufacturing process, for example, Patent Document 1 describes the following method: The exhaust gas of the reduction furnace and natural gas are reformed in a reforming device to generate a reduction gas mainly composed of CO and H2, and this reduction gas is blown into the reduction furnace to reduce the iron oxide in the reduction furnace and manufacture reduced iron.

[0006] In addition, Patent Document 2 describes the following method: a reducing gas is produced by reforming coke oven gas and the top gas of a reduction furnace from which CO2 has been removed, and the reducing gas is blown into the reduction furnace to produce reduced iron.

[0007] Prior art documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2017-88912

[0010] Patent Document 2: Japanese Patent No. 6190522 Summary of the Invention

[0011] In the method described in Patent Document 1, natural gas supplied from the outside is used to produce a reducing gas. Therefore, there is a problem that although the CO2 emissions are less than those of a blast furnace, a certain amount of CO2 emissions cannot be avoided.

[0012] In addition, the method described in Patent Document 2 uses coke oven gas or converter gas generated in a steelworks to produce a reducing gas. Here, in an integrated steelworks, coke oven gas or converter gas is required as fuel gas for the next process such as a heating furnace or an annealing furnace. Therefore, if coke oven gas or converter gas is diverted to the reduced iron manufacturing process, there will be a shortage of fuel gas in the next process. As a result, in order to compensate for the shortage of fuel gas in the next process, natural gas will be supplied from the outside. That is, even in the method described in Patent Document 2, it is impossible to simultaneously achieve energy saving and reduction of CO2 emissions, and there is a problem.

[0013] The present invention has been made in view of the above situation, and an object thereof is to provide a method for manufacturing reduced iron that can simultaneously achieve energy saving and reduction of CO2 emissions.

[0014] The inventors of the present invention have repeatedly conducted research to simultaneously achieve energy saving and reduction of CO2 emissions, and have developed a system for recycling top gas.

[0015] That is, the inventors of the present invention have developed a top gas recycling and reuse system (hereinafter also simply referred to as a recycling system), which has the following processes:

[0016] Blowing process: blowing a reducing gas into a reduction furnace;

[0017] Reduction process: reducing iron oxide with the reducing gas in the reduction furnace to obtain reduced iron;

[0018] Distribution process: distributing the top gas discharged from the reduction furnace into a first top gas and a second top gas;

[0019] Synthesis process: synthesizing a regenerated methane gas from the first top gas and hydrogen; and

[0020] Reforming process: Using the recycled methane gas and the top gas from the second furnace as raw material gases to obtain a reducing gas from these raw material gases.

[0021] In addition, the inventors have obtained the following insights through further repeated research.

[0022] · In the above-mentioned circulation system, control the gas volumes introduced into the distribution process, the synthesis process, and the reforming process according to the CO2 conversion rate η m [-] (hereinafter also simply referred to as the CO2 conversion rate) of the methanation reaction in the synthesis process.

[0023] · Thereby, it is possible to produce reduced iron with high operating stability while achieving further energy conservation and a reduction in CO2 emissions without inputting excessive energy.

[0024] The present invention has been completed through further in-depth research based on the above insights.

[0025] That is, the main gist of the present invention is constituted as follows.

[0026] 1. A method for manufacturing reduced iron, comprising the following processes:

[0027] Filling process: Filling iron oxide into a reduction furnace;

[0028] Blowing process: Blowing a reducing gas into the above reduction furnace;

[0029] Reduction process: Reducing the above iron oxide with the above reducing gas in the above reduction furnace to obtain reduced iron;

[0030] Distribution process: Distributing the top gas discharged from the above reduction furnace into a first top gas and a second top gas;

[0031] Synthesis process: Synthesizing recycled methane gas using the above first top gas and hydrogen; and

[0032] Reforming process: Using the above recycled methane gas and the above second top gas as raw material gases to obtain the above reducing gas from these raw material gases,

[0033] Control the gas volumes introduced into the above distribution process, the above synthesis process, and the above reforming process according to the CO2 conversion rate η m [-] of the methanation reaction in the above synthesis process.

[0034] 2. The method for manufacturing reduced iron according to 1 above, wherein the gas volumes introduced into the above distribution process, the above synthesis process, and the above reforming process are controlled in a manner that satisfies the following formulas (1) and (2).

[0035]

[0036] Among them,

[0037] W CO2 : The amount of CO2 contained in the top gas introduced into the distribution process [Nm 3 / t],

[0038] W H2 : The amount of H2 contained in the top gas introduced into the distribution process [Nm 3 / t],

[0039] E H2 : The amount of H2 contained in the hydrogen gas introduced into the synthesis process [Nm 3 / t],

[0040] V CO2 : The amount of CO2 contained in the gas introduced into the reforming process [Nm 3 / t],

[0041] V H2 : The amount of H2 contained in the gas introduced into the reforming process [Nm 3 / t].

[0042] 3. The method for manufacturing reduced iron according to 1 or 2 above, wherein, in the above distribution process, the above top gas is separated into separated carbon dioxide gas and first remaining gas,

[0043] The separated carbon dioxide gas is distributed as the above first top gas,

[0044] The above first remaining gas is distributed as the above second top gas.

[0045] 4. The method for manufacturing reduced iron according to 1 or 2 above, wherein, in the above distribution process, the above top gas is separated into separated carbon dioxide gas and first remaining gas,

[0046] The separated carbon dioxide gas is distributed as the above first top gas,

[0047] The above first remaining gas is separated into separated hydrogen and second remaining gas,

[0048] The separated hydrogen is introduced into the above synthesis process,

[0049] The above second remaining gas is distributed as the above second top gas.

[0050] 5. The method for manufacturing reduced iron according to 1 or 2 above, wherein, in the above distribution process, the above top gas is separated into separated hydrogen and third remaining gas,

[0051] Introduce the separated hydrogen gas described above into the synthesis process described above.

[0052] Separate the above-mentioned third remaining gas into separated carbon dioxide gas and fourth remaining gas.

[0053] Distribute the above-mentioned separated carbon dioxide gas as the above-mentioned first top gas, and distribute the above-mentioned fourth remaining gas as the above-mentioned second top gas.

[0054] According to the present invention, when manufacturing reduced iron, it is possible to perform operations that simultaneously achieve further energy conservation and reduction of CO2 emissions with high operating stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is a diagram showing a conventional manufacturing process of reduced iron.

[0056] Figure 2 It is a diagram showing an example of a manufacturing process of reduced iron related to a method for manufacturing reduced iron according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0057] Hereinafter, a method for manufacturing reduced iron according to an embodiment of the present invention will be described with reference to the drawings.

[0058] First, a conventional manufacturing process of reduced iron (hereinafter also referred to as a conventional manufacturing process) will be described. Figure 1 It is a diagram showing a schematic configuration of an example of a conventional manufacturing process. In the figure, reference numeral 1 is a reduction furnace, 1a is iron oxide, 1b is reduced iron, 3 is a dust removal device, 4 is a dehydration device, 5 is a natural gas supply section, 6 is an air supply section, 7 is a reforming device, and 9 is a reducing gas blowing device.

[0059] In an example of a conventional manufacturing process, iron oxide is charged from the upper part of a reduction furnace and made to slowly descend. The iron oxide is reduced by blowing high-temperature reducing gas into the middle of the reduction furnace. Then, reduced iron is discharged from the lower part of the reduction furnace. At this time, top gas mainly containing CO, CO2, H2, and H2O is discharged from the upper part of the reduction furnace. This top gas is dust-removed using a dust removal device, and a part of it is sent to a reforming device after moisture adjustment as a raw material gas. A gas containing hydrocarbons, such as natural gas from a natural gas supply section, is supplied to the reforming device together with the moisture-adjusted top gas. Next, in the reforming device, the supplied gas is heated. Then, a reforming reaction occurs to generate a high-temperature reducing gas mainly containing CO and H2. Then, this reducing gas is blown into the reduction furnace. In addition, the remaining part of the top gas, after dehydration, is used as heating fuel in the combustion chamber of the reforming device. The top gas after being burned as heating fuel is usually discharged out of the system in a state containing CO2. It should be noted that if reduced iron is manufactured by this example of the conventional manufacturing process, when manufacturing 1 t of reduced iron, about 1 t or more of CO2 is discharged out of the circulation system.

[0060] On the other hand, in a method for manufacturing reduced iron according to an embodiment of the present invention, for example, as Figure 2 shown, in a top gas distribution section, the top gas discharged from the reduction furnace is distributed into a first top gas and a second top gas. Moreover, methane gas synthesized from the first top gas and H2 gas (hereinafter also referred to as regenerated methane gas) is used instead of hydrocarbon gases such as natural gas supplied from the outside in the above-mentioned Figure 1 conventional process. In the figure, reference numeral 10 is a hydrogen supply section, 11 is a methane synthesis device, 13 is a heat source, 14 is a top gas distribution section, and 15 is an oxygen supply section. Here, the regenerated methane gas generated in the methane synthesis device 11 is supplied to the reforming device 7 as a raw material gas for the reducing gas.

[0061] The following describes each process of the method for manufacturing reduced iron according to an embodiment of the present invention. It should be noted that the filling process, the blowing process, and the reduction process are carried out according to a conventional method. For example, they can be carried out in the same manner as the above-mentioned conventional manufacturing process, so the description is omitted here.

[0062] · Distribution process

[0063] In the distribution process, for example, in the top gas distribution section, the top gas discharged from the reduction furnace is distributed into a first top gas and a second top gas. In addition, the means for distributing and controlling the flow rate of the top gas is not particularly limited and can be according to a conventional method. For example, a mass flow controller or the like can be used.

[0064] In the distribution process, the top gas can be distributed according to its original composition. Additionally, for example, specific gas species such as CO2 and H2 can be separated and distributed as follows.

[0065] (a) Separate the top gas into separated carbon dioxide gas and first remaining gas (CO2 separation), distribute the separated carbon dioxide gas as the first top gas, and distribute the first remaining gas as the second top gas.

[0066] (b) Separate the top gas into separated carbon dioxide gas and first remaining gas (CO2 separation), and distribute the separated carbon dioxide gas as the first top gas.

[0067] Additionally, separate the above-mentioned first remaining gas into separated hydrogen and second remaining gas (H2 separation). Then, introduce (supply) the separated hydrogen into the synthesis process, and distribute the second remaining gas as the second top gas.

[0068] (c) Separate the top gas into separated hydrogen and third remaining gas (H2 separation). Then, introduce (supply) the separated hydrogen into the synthesis process, and separate the third remaining gas into separated carbon dioxide gas and fourth remaining gas (CO2 separation). Next, distribute the separated carbon dioxide gas as the first top gas, and distribute the fourth remaining gas as the second top gas.

[0069] Here, the method of CO2 separation is not particularly limited, and various methods such as chemical absorption method, physical absorption method, adsorption separation method, membrane separation method, cryogenic separation method, oxy-fuel combustion method, and chemical looping combustion method can be used. Among them, in particular, the chemical absorption method represented by the amine absorption method and the adsorption separation method represented by the PSA method have a long history of use in chemical plants, industrial CO2 production, etc. Additionally, these methods also have a record of recovering gases with a CO2 concentration of 99 vol% or more. Therefore, these methods are preferred. It should be noted that the CO2 concentration in the separated carbon dioxide gas is preferably 90 vol% or more. The CO2 concentration in the separated carbon dioxide gas can be 100 vol%.

[0070] The method of H2 separation is also not particularly limited, and various methods such as adsorption separation method and membrane separation method can be used. The concentration of H2 in the separated hydrogen is preferably 90 vol% or more. The concentration of H2 in the separated hydrogen can be 100 vol%.

[0071] Additionally, the compositions of the first remaining gas, the second remaining gas, and the fourth remaining gas distributed as the second top gas vary depending on the amounts of CO2 and H2 separated, and are, for example, CO: 5 - 70 vol%, CO2: 0 - 25 vol%, H2: 0 - 75 vol%, H2O: 0 - 50 vol%, balance: 0 - 30 vol%.

[0072] It should be noted that when the top gas is distributed into the first top gas and the second top gas according to the original composition in the distribution process, a separation process for separating specific gas species such as CO2 and H2 from the first top gas and the second top gas can be arbitrarily provided between the above-mentioned distribution process and the subsequent synthesis process.

[0073] For example, as described in the above (a) and (b), the first top gas can be separated into separated carbon dioxide gas and the first remaining gas (CO2 separation), and the separated carbon dioxide gas is used as the first top gas in the synthesis process. The above-mentioned first remaining gas can be directly merged with the second top gas. In addition, the above-mentioned first remaining gas can be separated into separated hydrogen and the second remaining gas (H2 separation), and then, the separated hydrogen is introduced into the synthesis process, and the second remaining gas is merged with the second top gas.

[0074] In addition, as described in the above (c), the first top gas can be separated into separated hydrogen and the third remaining gas. Then, the separated hydrogen is introduced into the synthesis process, and the third remaining gas is separated into separated carbon dioxide gas and the fourth remaining gas (CO2 separation). Then, the separated carbon dioxide gas is used as the first top gas in the synthesis process, and the fourth remaining gas is merged with the second top gas.

[0075] In addition, as Figure 2 shown, from the perspective of material balance, before the top gas is introduced into the distribution process, a part of it can be used as heating fuel for a reforming device or the like. For example, in the combustion chamber of the heating device, using the oxygen supplied from the oxygen supply device, as an example, using pure oxygen generated by a cryogenic separation process driven by CO2-free electricity, a part of the top gas is burned. Next, the burned top gas is dehydrated as needed and then returned to the raw material production line and introduced into the distribution process.

[0076] ·Synthesis process

[0077] In the synthesis process, for example, in a methane synthesis device, regenerated methane gas is synthesized from the first top gas and hydrogen distributed in the above-mentioned distribution process. It should be noted that CH4 is synthesized by the methanation reaction formula of at least one of CO2 and CO contained in the first top gas and H2 according to the following formulas (i) and (ii).

[0078] CO2 + 4H2 → CH4 + 2H2O ΔH = -165 kJ / mol ··· (i)

[0079] CO + 3H2 → CH4 + H2O ΔH = -206 kJ / mol ··· (ii)

[0080] For example, the first top gas and hydrogen supplied from the outside of the above-mentioned circulation system are introduced into the methane synthesis device. Then, in the methane synthesis device, CH4 is synthesized by at least one of the reactions in the above formulas (i) and (ii). The synthesis conditions of CH4 and the like are not particularly limited, and conventional methods can be used.

[0081] When using the separated carbon dioxide gas obtained in the above-mentioned distribution process or separation process, the composition of the first top gas is the same as that of the separated carbon dioxide gas. In addition, when the top gas is distributed according to the original composition in the distribution process without passing through the separation process, the composition of the first top gas is basically the same as that of the top gas introduced into the distribution process. The composition of the first top gas in this case is, for example, CO: 5 to 50% by volume, CO2: 5 to 30% by volume, H2: 5 to 80% by volume, H2O: 0 to 35% by volume, balance: 0 to 20% by volume. The composition of the second top gas described later is the same.

[0082] It should be noted that in the synthesis process, in addition to the first top gas, any other gas containing at least one of CO and CO2 (hereinafter also referred to as other gas) can also be used. As other gases, for example, gases by-produced in the steelmaking process can be cited, and specifically, blast furnace gas (BFG) and coke oven gas (COG) can be cited. In addition, other gases and the first top gas can be introduced into the above-mentioned separation process together, and after separating into separated carbon dioxide gas and the remaining gas, the separated carbon dioxide gas is supplied to the synthesis process.

[0083] In addition, the supply source of hydrogen used in the synthesis process is not particularly limited, and it can be supplied and manufactured by any method. As a method for manufacturing hydrogen, for example, synthesis by electrolysis of water, decomposition reactions of ammonia or hydrocarbons, and organic hydrides can be cited. However, if hydrocarbons or organic hydrides are used as raw materials, CO2 is discharged during the hydrogen synthesis process. Therefore, from the viewpoint of further reducing the CO2 discharge amount, it is preferable to synthesize by at least one of electrolysis of water and decomposition of ammonia. In addition, when manufacturing hydrogen by electrolysis of water, the CO2 discharge can be made zero by using green hydrogen manufactured by using electric power obtained from green energy such as sunlight, wind power, and geothermal energy. The H2 concentration of hydrogen is not particularly limited, and it is preferably 90% by volume or more, more preferably 95% by volume or more. It should be noted that the H2 concentration of hydrogen can be 100% by volume.

[0084] In the synthesis of CH4, commonly used methanation catalysts can be utilized. Specifically, transition metal-based catalysts such as Fe, Ni, Co, Ru, etc. can be used. Among them, Ni-based catalysts have high activity. In addition, Ni-based catalysts also have high heat resistance and can be used at a temperature of around 500 °C. Therefore, Ni-based catalysts are particularly preferred. In addition, iron ore can also be used as a catalyst. Especially for high-crystalline water ore, if the crystal water is removed, the specific surface area increases, making it suitable as a catalyst.

[0085] As the reactor of the methane synthesis device used in the synthesis process, a fixed-bed reactor, a fluidized-bed reactor, a entrained-flow reactor, etc. can be used. The physical properties of the catalyst can be appropriately selected according to the form of these reactors. In addition, a heat exchanger can be arranged in the gas flow path on the downstream side 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, for heating a reduction furnace or a reforming device.

[0086] In addition, the above-mentioned methanation catalyst stably shows a high conversion rate during the methanation of CO2. However, during the methanation of CO, C precipitation occurs due to the formation of C intermediates, resulting in catalyst poisoning. As a countermeasure to prevent catalyst poisoning during the methanation of CO, for example, adding H2O (steam) can be cited. However, considering the long-term use of the catalyst, a decrease in catalyst life cannot be avoided. In addition, as shown in the above formulas (i) and (ii), compared with the methanation reaction of CO2, the methanation reaction of CO has a larger negative reaction enthalpy and a large heat release. Therefore, during the methanation of CO, the catalyst temperature rises sharply. In addition, since the methanation reaction itself is an exothermic reaction, the conversion rate decreases as the reaction time prolongs. Furthermore, the deactivation of the catalyst may also be accelerated as the catalyst temperature rises. Therefore, in the synthesis process, it is preferred to use the separated carbon dioxide gas obtained in the above-mentioned distribution process or separation process as the first top gas.

[0087] It should be noted that if the H2O by-produced during the methane synthesis is introduced into the reforming device, H2O may be excessive in the subsequent reforming process. Therefore, it is preferred to appropriately dehydrate the regenerated methane gas using a dehydration device before the subsequent reforming process in consideration of the overall material balance of the circulation system.

[0088] In addition, the CH4 concentration of the regenerated methane gas is not particularly limited. In the regenerated methane gas after removing H2O, for example, in the regenerated methane gas obtained by dehydrating the gas on the output side of the methane synthesis device using a dehydration device, the CH4 concentration is preferably 80% by volume or more, more preferably 90% by volume or more. It should be noted that the CH4 concentration in the regenerated methane gas after removing H2O can be 100% by volume.

[0089] ·Reforming process

[0090] In the reforming process, the above-mentioned regenerated methane gas and the above-mentioned second top gas are used as raw material gases, and a reducing gas is obtained from these raw material gases. For example, the regenerated methane gas and the second top gas are introduced into a reforming device, and the regenerated methane gas and the second top gas are heated in the reforming device. Then, in the reforming device, a reducing gas containing CO and H2 is generated through the reforming reactions of the following formulas (iii) and (iv). It should be noted that the reforming reaction shown in formula (iv) is carried out by supplying steam to the reforming device.

[0091] CH4 + CO2 → 2CO + 2H2 ΔH = 247 kJ / mol ··· (iii)

[0092] CH4 + H2O → CO + 3H2 ΔH = 206 kJ / mol ··· (iv)

[0093] It should be noted that the raw material gas can be heated in, for example, a reforming device. The heating temperature and heating means of the raw material gas are not particularly limited, and conventional methods can be used. For example, as the heating temperature of the raw material gas, 300 to 700 °C can be exemplified. In addition, as the temperature of the reducing gas (blown into the reduction furnace), 750 to 1100 °C can be exemplified.

[0094] In addition, 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.

[0095] Based on the positive enthalpy of formation of the above formulas (iii) and (iv), it can be seen that the reactions of the above formulas (iii) and (iv) are both endothermic reactions. Therefore, by using green energy such as sunlight, wind power, and geothermal energy as the heat source of the reforming device, it is theoretically possible to make the CO2 emission zero.

[0096] Next, the reducing gas is introduced into the reduction furnace via a blowing process. For example, a reducing gas blowing device is used to introduce it into the reduction furnace. Then, in the reduction furnace, the iron oxide is reduced by the reducing gas to obtain reduced iron. On the other hand, the reducing gas after being used for the reduction of iron oxide is discharged from the reduction furnace in the form of top gas.

[0097] In addition, it is preferable to carry out at least one of dust removal and dehydration of the top gas before the above-mentioned distribution process. Any dust removal device can be used for the dust removal device used in the dust removal. In addition, any dehydration device can be used for the dehydration device used in the dehydration. It should be noted that the order of dust removal and dehydration is not particularly limited. Figure 2In the example shown, after the top gas is dust-removed using a dust-removing device, dehydration is performed using a dehydration device. Thereafter, the top gas is distributed into a first top gas and a second top gas.

[0098] · Control of the gas amounts introduced into the distribution process, synthesis process, and reforming process

[0099] Moreover, in the method for manufacturing reduced iron according to an embodiment of the present invention, it is important to control the gas amounts introduced (supplied to) the distribution process, synthesis process, and reforming process according to the CO2 conversion rate. Thereby, the composition of the reducing gas blown into the reduction furnace can be stabilized, particularly the ratio of the amount of H2 to the amount of CO in the reducing gas, i.e., H2 / CO (hereinafter also referred to as the H2 / CO of the reducing gas). As a result, reduced iron can be manufactured with high operation stability while achieving further energy saving and reduction in CO2 emissions without inputting excessive energy.

[0100] Here, the CO2 conversion rate represents the proportion of CO2 contained in the first top gas introduced into the synthesis process that is converted into CH4 by the methanation reaction. For example, the CO2 conversion rate can be calculated by the following formula.

[0101] CO2 conversion rate η m [-] = 1 - (amount of CO2 [Nm [[ID=!5]] 3 / t] contained in the regenerated methane gas discharged after synthesizing CH4 in the synthesis process) ÷ (amount of CO2 [Nm 3 / t] contained in the first top gas introduced into the synthesis process)

[0102] It should be noted that the CO2 conversion rate can be mainly adjusted by the reactor used in the synthesis process, for example, the type, amount, volume of the catalyst used in the reactor, the temperature of the reactor, etc. In addition, when there is a gas introduced into the synthesis process other than the first top gas such as separated hydrogen, the amount of CO2 contained in this gas is also included in (amount of CO2 [Nm 3 / t] contained in the first top gas introduced into the synthesis process).

[0103] In addition, Nm 3 / t and kg / t are basic units per ton of reduced iron (DRI) produced

[0104] Then, for example, it is preferable to control the gas amounts introduced into the distribution process, synthesis process, and reforming process in a manner that satisfies the following formulas (1) and (2).

[0105]

[0106] Among them,

[0107] WCO2 : The amount of CO2 contained in the top gas introduced into the distribution process [Nm 3 / t]

[0108] W H2 : The amount of H2 contained in the top gas introduced into the distribution process [Nm 3 / t]

[0109] E H2 : The amount of H2 contained in the hydrogen introduced into the synthesis process [Nm 3 / t]

[0110] V CO2 : The amount of CO2 contained in the gas introduced into the reforming process [Nm 3 / t]

[0111] V H2 : The amount of H2 contained in the gas introduced into the reforming process [Nm 3 / t].

[0112] It should be noted that the gas introduced into the reforming process is basically the recycled methane gas and the second top gas. In addition, the hydrogen introduced into the synthesis process is the hydrogen introduced from outside the circulation system, excluding the separated hydrogen and the H2 in the first top gas mentioned above.

[0113] The left side of the above formula (1) can be rewritten as (W CO2 -V CO2 ) / W CO2 , which can be said to represent the proportion of CO2 converted into CH4 in the CO2 introduced into the distribution process. Here, when the above formula (1) is not satisfied, the conversion amount of CO2 to CH4 in the synthesis process is insufficient, and CO2 remains in the circulation system. As a result, in order to perform stable operation under a sound material balance, it is sometimes necessary to discharge CO2 outside the circulation system.

[0114] In addition, the left side of the above formula (2) can be rewritten as (W H2 +E H2 -V H2 ) / (W H2 +E H2 ), which can be said to represent the proportion of H2 converted into CH4 in the total H2 introduced between the distribution process and the reforming process. Here, when the CO2 conversion rate is less than the left side value of the above formula (2), the reaction efficiency in the synthesis process is insufficient, and H2 remains in the circulation system. As a result, it is sometimes impossible to perform stable operation under a sound material balance.

[0115] The right side of the above formula (2) can be rewritten as (W H2 +E H2 -V H2 ) / EH2 , which can be said to represent the ratio of the amount of H2 converted to CH4 in the synthesis process to the amount of H2 introduced from outside the circulation system. Here, when the CO2 conversion rate η is greater than the right-hand side value of the above formula (2), H2 is excessively consumed in the synthesis process, which is not preferred from the perspective of energy conservation.

[0116] In summary, it is preferable to control the gas amounts in the introduction distribution process, synthesis process, and reforming process in a manner that satisfies the above formulas (1) and (2).

[0117] It should be noted that the left side of the above formula (1) is more preferably 1 - V CO2 / W CO2 + 0.015.

[0118] The left side of the above formula (2) is more preferably 1 - V H2 / (W H2 + E H2 ) + 0.015.

[0119] The right side of the above formula (2) is more preferably 1 + (W H2 - V H2 ) / E H2 - 0.015.

[0120] It should be noted that the gas amounts in the introduction distribution process, synthesis process, and reforming process only need to satisfy the above formulas (1) and (2) simultaneously, and can be constant or changed at any time. In addition, the gas amounts in the introduction distribution process, synthesis process, and reforming process at the start of equipment can be determined according to, for example, past operation experience, etc., and the gas amounts in the introduction distribution process, synthesis process, and reforming process can be appropriately changed and controlled according to the subsequent operating conditions.

[0121] In addition, the iron oxide raw material used in the method for manufacturing reduced iron based on an embodiment of the present invention is, for example, iron ore. As a specific example, lump iron ore (lump ore), pellet (a substance obtained by solidifying powdered iron ore into a spherical shape), etc. can be cited. The quality of the iron ore used as the iron oxide raw material, that is, the iron content, is not particularly limited, and from the perspective of reduction in a shaft furnace, it is generally preferably 65% by mass or more.

[0122] In addition, in the method for manufacturing reduced iron based on an embodiment of the present invention, the method using a shaft furnace has been particularly described as a direct reduced iron steelmaking method. However, the type of reduction furnace is not limited thereto, and it can also be a method using a fluidized bed, rotary kiln, rotary hearth furnace (RHF), etc. It should be noted that from the perspective of high production efficiency, operation rate, and operation stability, a shaft furnace is preferably used as the reduction furnace. In addition, most of the direct reduced iron furnaces operating globally are of the shaft furnace type, such as Midrex (registered trademark) and Hyl (registered trademark).

[0123] Example

[0124] The following describes the examples.

[0125] In Figure 2 In the shown circulation system, reduced iron is manufactured according to the conditions described in Table 1. Regardless of the conditions, the operation period is 28 days. In Table 1, the operation specifications are recorded in the basic unit per 1 t of reduced iron manufactured. For example, when 1300 kg of iron oxide pellets are used to manufacture 1 t of reduced iron, the usage amount of iron oxide pellets is expressed as 1300 kg / t. When manufacturing 3000 t / day of reduced iron, this amount is multiplied by 3000 times as the daily specification.

[0126] Here, regardless of the conditions, in the charging process, iron oxide pellets as raw materials are charged into the reduction furnace under the condition of 1394 kg / t. In the blowing process, reduction gas heated to 980 °C is blown from the middle of the reduction furnace to reduce the iron oxide pellets to obtain reduced iron. Then, after dust removal of the top gas discharged from the reduction furnace, dehydration is appropriately carried out to balance the material input and output. In addition, from the perspective of material balance, a part of the top gas is appropriately used as heating fuel. The top gas branched as heating fuel is burned in the combustion chamber of the reforming device using oxygen generated by a cryogenic separation process driven by CO2-free electricity. Then, all the exhaust gas from the combustion chamber of the reforming device is recovered, dehydrated, and then re-merged with the top gas. The merged top gas is introduced into the distribution process and distributed into the first top gas and the second top gas. In the distribution process, the top gas introduced into the top gas distribution section is appropriately separated into carbon dioxide gas in any one of the above (a) to (c) in a way that balances the material input and output, and this separated carbon dioxide gas is distributed as the first top gas. The separated hydrogen is introduced into the synthesis process together with the separated carbon dioxide gas as the first top gas. In addition, the remaining gas (the first remaining gas, the second remaining gas, or the fourth remaining gas) is distributed as the second top gas. Then, the first top gas (also including a part of the separated hydrogen) and hydrogen from outside the circulation system are introduced into the methane synthesis device, and regenerated methane gas is synthesized in the reactor of the methane synthesis device. After dehydration of the synthesized regenerated methane gas, the regenerated methane gas and the second top gas are introduced into the reforming device as raw material gas, and reduction gas is obtained from this raw material gas. It should be noted that the total amount of the introduced gas in the reforming process in Table 1 is the total amount of the regenerated methane gas and the second top gas. In addition, conditions other than those described above and in Table 1 can be carried out according to conventional methods.

[0127]

[0128] In the inventive examples, during the entire 28-day operation period, it is extremely advantageous in terms of energy conservation Figure 2 In the circulating system shown, that is, the system in which top gas is recycled, operation is stably carried out under a sound material balance. In addition, the CO2 emission amount from this circulating system can be made zero.

[0129] On the other hand, in the comparative examples where the gas volume is not controlled according to the CO2 conversion rate in the synthesis process, during the middle of the operation period, the composition of the reducing gas cannot be maintained within a certain range, the reaction in the reduction furnace is unstable, and the operation has to be interrupted.

[0130] Symbol Explanation

[0131] 1 Reduction furnace

[0132] 1a Iron oxide

[0133] 1b Reduced iron

[0134] 3 Dust removal device

[0135] 4 Dewatering device

[0136] 5 Natural gas supply section

[0137] 6 Air supply section

[0138] 7 Reforming device

[0139] 9 Reducing gas blowing device

[0140] 10 Hydrogen supply section

[0141] 11 Methane synthesis device

[0142] 13 Heat source

[0143] 14 Top gas distribution section

[0144] 15 Oxygen supply section

Claims

1. A method for manufacturing reduced iron, comprising the following steps: A filling step of filling 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 the top gas discharged from the reduction furnace into a first top gas and a second top gas; A synthesis step of synthesizing a recycled methane gas using the first top gas and hydrogen; And A reforming step of using the recycled methane gas and the second top gas as raw material gases to obtain the reducing gas from the raw material gases, According to the CO2 conversion rate η of the methanation reaction in the synthesis process m [-]Control the gas volume introduced into the distribution process, the synthesis process, and the reforming process.

2. The method for manufacturing reduced iron according to claim 1, wherein, Controlling the amounts of gases introduced into the distribution step, the synthesis step, and the reforming step in a manner that satisfies the following formulas (1) and (2), wherein, W CO2 : The amount of CO2 contained in the top gas introduced into the distribution process, in Nm 3 / t, W H2 : The amount of H2 contained in the top gas introduced into the distribution process, in Nm 3 / t, E H2 : The amount of H2 contained in the hydrogen introduced into the synthesis process, with the unit of Nm 3 / t, V CO2 : The amount of CO2 contained in the gas introduced into the reforming process, in Nm 3 / t, V H2 : The amount of H2 contained in the gas introduced into the reforming process, in Nm 3 / t.

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

4. The method for manufacturing reduced iron according to claim 1 or 2, wherein, In the distribution step, the top gas is separated into a separated carbon dioxide gas and a first remaining gas, The separated carbon dioxide gas is distributed as the first top gas, The first remaining gas is separated into a separated hydrogen gas and a second remaining gas, The separated hydrogen gas is introduced into the synthesis step, The second remaining gas is distributed as the second top gas.

5. The method for manufacturing reduced iron according to claim 1 or 2, wherein In the distribution step, the top gas is separated into a separated hydrogen gas and a third remaining gas, The separated hydrogen gas is introduced into the synthesis step, The third remaining gas is separated into a separated carbon dioxide gas and a fourth remaining gas, The separated carbon dioxide gas is distributed as the first top gas, and the fourth remaining gas is distributed as the second top gas.

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