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 of CO2 emissions. A system for recycling furnace top gas, which has a blowing step, a reduction step, a distribution step, a synthesis step and a reforming step, is configured, and the distribution ratio of furnace top gas in the distribution step is controlled on the basis of consumed H2 / CO, which is the ratio of the amount of H2 consumed in the reduction step to the amount of CO consumed in the reduction step.

Description

Technical Field

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

[0002] In recent years, in steel mills, 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 mills, a reduction process for reducing this iron oxide is essential. The most common reduction process widely used in the world 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 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, a further significant reduction in the reductant cannot 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 reduced iron is manufactured. 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 to 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 to the reduction of iron oxide, is discharged from the reduction furnace, for example, from the top. Moreover, 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 outside the system.

[0005] As such a reduced iron manufacturing process, for example, Patent Document 1 discloses 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, thereby manufacturing 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 Patent Application Laid-Open 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 emission is less than that of a blast furnace, a certain amount of CO2 emission cannot be avoided.

[0012] In addition, the method described in Patent Document 2 uses coke oven gas or converter gas generated in a steel mill to produce a reducing gas. Here, in an integrated steel mill, 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, leaving a problem.

[0013] The present invention has been completed 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 have repeatedly studied to simultaneously achieve energy saving and reduction of CO2 emissions, and have developed a system for recycling top gas.

[0015] That is, the inventors have developed a system for recycling and reusing top gas (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] Synthesis process: synthesizing a regenerated methane gas from the top gas discharged from the reduction furnace and hydrogen; and

[0019] Reforming process: using the regenerated methane gas as a raw material gas to obtain a reducing gas from the raw material gas.

[0020] In addition, the inventors and others further conducted repeated research and obtained the following insights.

[0021] · In order to stably perform operations that simultaneously achieve energy conservation and reduction of CO2 emissions, it is important to maintain a sound material balance, particularly the material balance of C (carbon), in the above-mentioned circulation system.

[0022] · For this purpose, it is important to control the gas balance of the above-mentioned circulation system so that the ratio of the amount of H2 to the amount of CO in the reducing gas supplied to the reduction furnace (hereinafter, also referred to as H2 / CO of the reducing gas) always remains within a certain range without significant fluctuations. It should be noted that the H2 / CO of the reducing gas is the volume ratio under standard conditions (it can also be said to be the flow ratio under standard conditions). The same applies to the consumed H2 / CO and the like described later.

[0023] However, the H2 / CO of the reducing gas has an optimal range in terms of energy depending on the in-furnace temperature, pressure, furnace size, etc. of the direct reduction furnace. In addition, as raw materials for the reducing gas, in addition to natural gas used in representative direct reduction furnaces such as Midrex (registered trademark) and Hyl (registered trademark), for example, coke oven gas, hydrocarbons, waste plastics, and combustion gases of biomass can be used. When using these gases, the H2 / CO of the reducing gas also varies to various values depending on their composition. In fact, even in currently operating direct reduction furnaces, operations are carried out within the range of H2 / CO of the reducing gas suitable for each plant. However, when the quality of iron oxide as a raw material for reduced iron changes or the in-furnace temperature changes, there are cases where the composition of the top gas changes. In such cases, the H2 / CO of the reducing gas also changes, and it is difficult to maintain a sound material balance in the above-mentioned circulation system.

[0024] Regarding the above aspects, the inventors and others further conducted repeated research and obtained the following insights: By performing the following control, a sound material balance can be maintained in the above-mentioned circulation system, that is, when manufacturing reduced iron, operations that simultaneously achieve energy conservation and reduction of CO2 emissions can be stably performed.

[0025] · Distribute the top gas discharged from the reduction furnace into a first top gas and a second top gas.

[0026] · At this time, control the distribution ratio of the top gas in the distribution process according to the ratio of the amount of H2 consumed to the amount of CO consumed in the reduction process, that is, the consumed H2 / CO.

[0027] The present invention was completed through further research based on the above insights.

[0028] That is, the gist of the present invention is constituted as follows.

[0029] 1. A method for manufacturing reduced iron, comprising:

[0030] A filling step of filling iron oxide into a reduction furnace,

[0031] A blowing step of blowing a reducing gas into the reduction furnace,

[0032] A reduction step of obtaining reduced iron by reducing the iron oxide with the reducing gas in the reduction furnace,

[0033] A distribution step of distributing the top gas discharged from the reduction furnace into a first top gas and a second top gas,

[0034] A synthesis step of synthesizing a recycled methane gas from the first top gas and hydrogen, and

[0035] 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,

[0036] Controlling the distribution ratio of the top gas in the distribution step according to the ratio of the amount of H2 consumed to the amount of CO consumed in the reduction step, i.e., the consumed H2 / CO.

[0037] 2. The method for manufacturing reduced iron according to 1 above, wherein, between the distribution step and the synthesis step, there is further a separation step of separating the first top gas into a carbon dioxide gas and the remaining gas,

[0038] Using the carbon dioxide gas as the first top gas in the synthesis step.

[0039] 3. The method for manufacturing reduced iron according to 1 or 2 above, wherein, controlling the distribution ratio Y×100(%) of the first top gas in the distribution step within the range of the reference distribution ratio Y0×100(%) ± 5%.

[0040] Here,

[0041] When X≤1, Y0 = 1 / (2η)

[0042] When 1<X<4η-1, Y0 = (X + 1) / (4η)

[0043] When X≥4η-1, Y0 = 1.

[0044] In addition,

[0045] X: consumed H2 / CO

[0046] η×100: CO2 conversion rate (%) of methane synthesis in the synthesis step.

[0047] Further, when the range of the reference distribution ratio Y0×100±5% exceeds 100%, the upper limit of this range is 100%.

[0048] 4. The method for manufacturing reduced iron according to item 3 above, wherein the distribution ratio Y×100 (%) of the first top gas in the above distribution process is controlled to the reference distribution ratio Y0×100 (%).

[0049] 5. The method for manufacturing reduced iron according to item 3 or 4 above, wherein when the consumption H2 / CO < 1, the remaining H2 is recovered from the circulation system having the above blowing process, the above reduction process, the above distribution process, the above synthesis process, and the above reforming process.

[0050] 6. The method for manufacturing reduced iron according to item 3 or 4 above, wherein when the consumption H2 / CO > 4η - 1, H2 is additionally supplied to the circulation system having the above blowing process, the above reduction process, the above distribution process, the above synthesis process, and the above reforming process.

[0051] 7. The method for manufacturing reduced iron according to any one of items 1 to 6 above, wherein at least one of dust removal and dehydration of the top gas is performed before the above distribution process.

[0052] 8. The method for manufacturing reduced iron according to any one of items 1 to 7 above, wherein dehydration of the regenerated methane gas is performed before the above reforming process.

[0053] According to the present invention, it is possible to stably perform operations that simultaneously achieve energy saving and reduction of CO2 emissions during the manufacture of reduced iron. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 is a diagram showing a conventional manufacturing process of reduced iron.

[0055] Figure 2 is an example of a manufacturing process of reduced iron showing a method for manufacturing reduced iron according to an embodiment of the present invention.

[0056] Figure 3 is a diagram showing the relationship between the consumption H2 / CO: X and the distribution ratio Y of the first top gas. DETAILED DESCRIPTION OF THE INVENTION

[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 1It is a schematic diagram showing an example of a conventional manufacturing process. In the figure, symbol 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 the reduction furnace and slowly descends. High-temperature reducing gas is blown into it from the middle part of the reduction furnace to reduce the iron oxide. Then, the 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. The top gas is dust-removed by the dust removal device, and a part of it is sent to the reforming device after moisture adjustment as a raw material gas. A gas containing hydrocarbons such as natural gas from the natural gas supply section is supplied to the reforming device together with the moisture-adjusted top gas. Then, 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, the reducing gas is blown into the reduction furnace. In addition, the remaining part of the top gas is used as heating fuel in the combustion chamber of the reforming device after dehydration, for example, burned by oxygen in the air. The top gas burned as heating fuel is usually discharged to the outside of the system in a state containing CO2.

[0060] On the other hand, in the method for manufacturing reduced iron according to an embodiment of the present invention, for example, as Figure 2 shown, 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. Moreover, methane gas synthesized from the first top gas and hydrogen (hereinafter, also referred to as recycled methane gas) is used instead of hydrocarbon gases such as natural gas externally supplied in the above-mentioned Figure 1 conventional process. In the figure, symbol 10 is a hydrogen supply section, 11 is a methane synthesis device, 12 is a CO2 separation device, 13 is a water vapor supply section, and 14 is a top gas distribution section. Here, the recycled methane gas generated by the methane synthesis device 11 is supplied to the reforming device 7 as a raw material gas for the reducing gas.

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

[0062] · Distribution process

[0063] In the distribution process, for example, at the top gas distribution section of the furnace, the top gas discharged from the reduction furnace is distributed into a first top gas and a second top gas. Moreover, at this time, it is important to control the distribution ratio of the first top gas according to the ratio of the amount of H2 consumed to the amount of CO consumed in the reduction process, that is, the consumed H2 / CO. It should be noted that the reason will be described later. In addition, the method for distributing and controlling the flow rate of the top gas is not particularly limited, and a conventional method can be used. For example, a mass flow controller can be used.

[0064] · Separation process

[0065] Optionally, a separation process as follows can be further provided between the above-mentioned distribution process and the synthesis process described later: The first top gas is separated into carbon dioxide gas and the remaining gas by using, for example, a CO2 separation device. The method for 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-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. It should be noted that the CO2 concentration in the carbon dioxide gas is preferably 90% by volume or more. The CO2 concentration in the carbon dioxide gas can also be 100% by volume. In addition, the remaining gas can be introduced into the reforming process described later together with the second top gas, for example.

[0066] · Synthesis process

[0067] In the synthesis process, for example, in a methane synthesis device, a regenerated methane gas is synthesized from the first top gas distributed in the above-mentioned distribution process and hydrogen. In addition, in the case where the above-mentioned separation process is performed, it is preferable to use the carbon dioxide gas as the first top gas in this process. It should be noted that CH4 is synthesized from at least one of CO2 and CO contained in the first top gas and H2 according to the methanation reaction formulas (i) and (ii) below.

[0068] CO2 + 4H2 → CH4 + 2H2O ΔH = -165 kJ / mol ··· (i)

[0069] CO + 3H2 → CH4 + H2O ΔH = -206 kJ / mol ··· (ii)

[0070] 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, CH4 is synthesized in the methane synthesis device by the reaction of at least one of the above formulas (i) and (ii). The synthesis conditions of CH4 and the like are not particularly limited, and a conventional method can be used.

[0071] When the composition of the first top gas is the same as that of the carbon dioxide gas obtained using the above separation process, it becomes the same composition as that of the carbon dioxide gas. Additionally, without going through the above separation process, the composition of the first top gas is basically the same as that of the top gas. In this case, the composition of the first top gas is as follows: CO: 5 - 50 vol%, CO2: 5 - 30 vol%, H2: 5 - 80 vol%, H2O: 0 - 35 vol%, balance: 0 - 20 vol%. The same applies to the composition of the second top gas described later.

[0072] 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, specifically blast furnace gas (BFG) and coke oven gas (COG). Additionally, other gases can be introduced into the above separation process together with the first top gas, separated into carbon dioxide gas and the remaining gas, and then the carbon dioxide gas can be supplied to the synthesis process.

[0073] Furthermore, the supply source of hydrogen used in the synthesis process is not particularly limited, and it can be supplied and produced by any method. As a method for producing hydrogen, for example, synthesis by electrolysis of water, synthesis by decomposition reaction of ammonia or hydrocarbons, 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 perspective of further reducing CO2 emissions, it is preferable to use at least one of electrolysis of water and decomposition of ammonia for synthesis. Additionally, when producing hydrogen by electrolysis of water, by using green hydrogen produced using electricity obtained from green energy sources such as sunlight, wind, and geothermal energy, CO2 emissions can be made zero. The H2 concentration of hydrogen is not particularly limited, preferably 90 vol% or more, more preferably 95 vol% or more. It should be noted that the H2 concentration of hydrogen can also be 100 vol%.

[0074] In addition, the supply amount of hydrogen in the synthesis process (the supply amount of hydrogen to the methane synthesis device) is preferably stoichiometric with respect to the amount of CO2 contained in the first top gas based on the methanation reaction formula shown in the above formula (i).

[0075] In the synthesis of CH4, generally used methanation catalysts can be utilized. Specifically, transition metal-based catalysts such as Fe, Ni, Co, and Ru can be used. Among them, the Ni-based catalyst has higher activity. Additionally, the Ni-based catalyst also has higher heat resistance and can be used at a temperature of around 500 °C. Therefore, the Ni-based catalyst is particularly preferred. Additionally, iron ore can also be used as a catalyst. In particular, high-crystalline water ore has an increased specific surface area after removing the crystalline water and can be suitably used as a catalyst.

[0076] As the reactor of the methane synthesis apparatus 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 and a reforming apparatus.

[0077] In addition, the above-mentioned methanation catalyst shows a stable high conversion rate during the methanation of CO2. However, cases have been reported in which C precipitation occurs due to the formation of C intermediates during the methanation of CO, resulting in catalyst poisoning. Therefore, it is preferable to have a separation process that separates the first top gas into carbon dioxide gas and the remaining gas between the above-mentioned distribution process and the synthesis process. In other words, in the synthesis process, as the first top gas, the carbon dioxide gas separated from the first top gas in the above-mentioned separation process is used.

[0078] It should be noted that if the H2O by-produced through methane synthesis is introduced into the reforming apparatus, there will be a situation where H2O is in excess in the reforming process described later. Therefore, it is preferable to appropriately dehydrate the recycled methane gas using a dehydration device before the reforming process described later in consideration of the material balance of the entire circulation system.

[0079] The CH4 concentration of the recycled methane gas is not particularly limited. In the recycled methane gas from which H2O has been removed, for example, the recycled methane gas on the outlet side of the methane synthesis apparatus dehydrated 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 recycled methane gas from which H2O has been removed can also be 100% by volume.

[0080] ·Reforming process

[0081] In the reforming process, the above-mentioned recycled methane gas and the above-mentioned second top gas are used as raw material gases to obtain a reducing gas from the raw material gases. For example, the recycled methane gas and the second top gas are introduced into a reforming apparatus, and the recycled methane gas and the second top gas are heated in the reforming apparatus. Then, in the reforming apparatus, 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 by formula (iv) is carried out by supplying steam to the reforming apparatus.

[0082] CH4 + CO2 → 2CO + 2H2 ΔH = 247 kJ / mol ··· (iii)

[0083] CH4 + H2O → CO + 3H2 ΔH = 206 kJ / mol ··· (iv)

[0084] It should be noted that 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.

[0085] From the fact that the enthalpies of formation of the above formulas (iii) and (iv) are positive, 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 energy source during the reforming reaction, it is theoretically possible to achieve zero CO2 emissions.

[0086] 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 using 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.

[0087] In addition, before the above-mentioned distribution process, it is preferable to perform at least one of dust removal and dehydration of the top gas. Any dust removal device can be used as the dust removal device for dust removal. In addition, any dehydration device can be used as the dehydration device for dehydration. It should be noted that the order of dust removal and dehydration is not particularly limited. In Figure 2 In the example shown, for the top gas, after dust removal using a dust removal device, dehydration is performed using a dehydration device, and then the top gas is distributed into the first top gas and the second top gas.

[0088] · Distribution ratio of the top gas in the distribution process

[0089] Moreover, in the method for manufacturing reduced iron according to an embodiment of the present invention, it is important to control the distribution ratio of the top gas in the distribution process according to the ratio of the amount of H2 consumed to the amount of CO consumed in the reduction process, that is, the consumed H2 / CO (hereinafter, also simply referred to as consumed H2 / CO).

[0090] Here, the consumed H2 / CO is calculated, for example, by the following formula.

[0091] Consumed H2 / CO = ([Amount of H2 contained in the reducing gas blown into the reduction furnace (Nm 3 / t)] - [Amount of H2 contained in the top gas (Nm 3 / t)]) / ([Amount of CO contained in the reducing gas blown into the reduction furnace (Nm 3 / t)] - [Amount of CO contained in the top gas (Nm 3 / t)])

[0092] It should be noted that Nm 3 / t is the raw material unit per 1 t of reduced iron (DRI) produced.

[0093] For example, the distribution ratio Y×100(%) of the first top gas is controlled around the reference distribution ratio Y0×100(%) calculated based on X. For example, Y×100(%) is controlled within a range preferably of Y0×100(%)±5%, more preferably of Y0×100(%)±3%, and further preferably of Y0×100(%)±1%. Most preferably, the distribution ratio Y×100(%) of the first top gas is controlled to be Y0×100(%).

[0094] Here,

[0095] When X≤1, Y0 = 1 / (2η)

[0096] When 1<X<4η-1, Y0 = (X + 1) / (4η)

[0097] When X≥4η-1, Y0 = 1.

[0098] In addition,

[0099] X: Consumption of H2 / CO

[0100] η×100: CO2 conversion rate (%) based on methane synthesis in the synthesis process.

[0101] In addition, when the ranges of Y0×100±5%, Y0×100±3%, and Y0×100±1% exceed 100% respectively, the upper limit of the range is 100%.

[0102] It should be noted that Y is defined by the following formula.

[0103] Y = [Amount of top gas distributed as the first top gas in the distribution process (Nm 3 / t)]÷[Amount of top gas introduced into the distribution process (Nm 3 / t)]

[0104] The distribution ratio of the second top gas is also defined in the same manner as the above formula. It should be noted that since the top gas is basically distributed into the first top gas and the second top gas, by appropriately controlling the distribution ratio of the first top gas, the distribution ratio of the second top gas can also be appropriately controlled accordingly. In addition, in this case, the distribution ratio of the second top gas is 1 - Y. Furthermore, if it is 10% by volume or less of the amount of top gas, a part of the top gas can be supplied to other loads according to operating conditions, etc. It should be noted that the amount of top gas supplied to other loads is not included in the amount of top gas introduced into the distribution process.

[0105] In addition, the CO2 conversion rate η×100(%) of methane synthesis in the synthesis process is defined by the following formula.

[0106] [CO2 conversion rate η × 100(%)] = (1 - [amount of CO2 contained in the regenerated methane gas discharged after synthesizing CH4 in the synthesis process (Nm 3 / t)] ÷ [amount of CO2 introduced into the synthesis process (Nm 3 / t)]) × 100

[0107] Next, an explanation will be given based on the above-mentioned reference distribution ratio Y0 × 100(%). First, when reducing iron oxide with a reducing gas that is a mixed gas of H2 and CO, the consumption amounts of H2 and CO are determined by the equilibrium constants and reaction rate constants of the following formulas (v) and (vi). For example, assuming that the quality of the raw iron ore deteriorates, the activation energy of H2 reduction increases, thereby slowing down the progress of formula (vi). Thus, the amounts of H2 and CO consumed in the reduction vary depending on the operating conditions.

[0108] 3CO + Fe2O3 → 3CO2 + 2Fe ··· (v)

[0109] 3H2 + Fe2O3 → 3H2O + 2Fe ··· (vi)

[0110] At this point, in the method for manufacturing reduced iron according to an embodiment of the present invention, by synthesizing and compensating for H2 and CO equivalent to the amount consumed in the reduction by reforming CH4, the composition of the reducing gas blown into the reduction furnace can be kept constant. That is, in the above formula (v), the amount of CO consumed in the reduction is equal to the amount of CO2 generated by the reduction. Moreover, a part of this CO2 is methanated in the synthesis process as the first top gas and supplied to the reforming process. In addition, the remaining CO2 is supplied to the reforming process as the second top gas without passing through the synthesis process. That is, if we focus on the overall material balance of C, in the above cycle system, the amount of CO consumed in the reduction of iron oxide in the reduction process is substantially equal to the amount of CO generated in the reforming process.

[0111] Here, as shown in the above formulas (iii) and (iv), in the case of reforming CH4 with CO2, H2 and CO are generated in a ratio of 1:1. On the other hand, in the case of reforming CH4 with H2O, H2 and CO are generated in a ratio of 3:1. The distribution ratio of the first top gas can be said to have the same meaning as the ratio of CO2 in the top gas that is supplied to the synthesis process. In addition, the distribution ratio of the second top gas has the same meaning as the ratio of CO2 in the top gas that is supplied to the reforming process. In other words, it can be said that the distribution ratio of the second top gas supplied to the reforming process represents the ratio of the reforming reaction based on CO2 in the above formula (iii). Therefore, the distribution ratio of the top gas becomes an important factor in maintaining the material balance of the above-mentioned sound cycle system, especially the material balance of the entire sound C.

[0112] Here, if the total carbon equivalent amount of CO2 contained in the top gas introduced into the distribution process is set to 1, the carbon equivalent amount of CH4 in the regenerated methane gas synthesized in the synthesis process is represented by Yη, which is obtained by multiplying the distribution ratio Y of the first top gas by the CO2 conversion rate η based on methane synthesis in the synthesis process.

[0113] In addition, the CO2 introduced into the reforming process is the sum of the CO2 contained in the second top gas and the CO2 remaining in the CO2 contained in the first top gas without synthesizing methane in the synthesis process. Therefore, if the total carbon equivalent amount of CO2 contained in the top gas introduced into the distribution process is set to 1, the carbon equivalent amount of CO2 introduced into the reforming process is represented by the following formula.

[0114] 1 - Y + Y(1 - η) = 1 - Yη

[0115] Moreover, for example, when X = 1, that is, the ratio of the amount of H2 and CO consumed in the reduction process is 1:1, by allowing the reaction of the above formula (iii) to proceed 100%, a reducing gas with H2:CO = 1:1 can be generated to compensate for the amount of H2 and CO equivalent to that consumed in the reduction process. In this case, it is necessary to supply CO2 equivalent to the CH4 contained in the regenerated methane gas in the reforming process.

[0116] Yη:1 - Yη = 1:1

[0117] If it is transformed, it becomes Y = 1 / (2η).

[0118] That is, when X = 1, it becomes Y0 = 1 / (2η).

[0119] It should be noted that when X < 1, that is, when the amount of CO consumed in the reduction process is more than the amount of H2, it is also preferable to carry out the reaction of the above formula (iii) by 100% in terms of maintaining the material balance. Therefore, in this case, it also becomes Y0 = 1 / (2η).

[0120] On the other hand, as the value of X increases, it is necessary to increase the ratio of the H2O reforming reaction of the above formula (iv). That is, it is necessary to increase the proportion of CO2 introduced into the synthesis process for the CO2 contained in the top gas by increasing the distribution ratio Y of the first top gas in the distribution process.

[0121] Here, when Y = 1,

[0122] The ratio of the CO2 reforming reaction of the above formula (iii) in the reforming reaction of the reforming process is represented by 1 - η (×100(%)).

[0123] In addition, the ratio of the H2O reforming reaction of the above formula (iv) is represented by η - (1 - η) = 2η - 1 (×100(%)).

[0124] Therefore, the H2 / CO ratio of the reducing gas obtained in the reforming process is represented by the following formula.

[0125] {2×(1 - η) + 3(2η - 1)} / {2×(1 - η) + 2η - 1} = 4η - 1

[0126] Therefore, when X = 4η - 1, Y = 1, that is, in the distribution process, the total amount of the top gas is distributed as the first top gas, and it is preferable to carry out the reforming reaction of the above formula (iv) as much as possible according to η in terms of maintaining the material balance. Therefore, in this case, it becomes Y0 = 1.

[0127] It should be noted that when X > 4η - 1, that is, when the consumed H2 / CO in the reduction process exceeds 4η - 1, it is also preferable to carry out the reaction of the above formula (iv) as much as possible in terms of maintaining the material balance. Therefore, in this case, it becomes Y0 = 1.

[0128] In addition, when 1 < X < 4η - 1, as Figure 3 shown, the reference distribution ratio Y0 of the first top gas uses Y0 = 1 / (2η) as the threshold value when X = 1 and Y0 = 1 as the threshold value when X = 4η - 1, and the straight line connecting the two points is Y0 = aX + b. Here, a and b are as follows.

[0129] a = 1 / (4η)

[0130] b = 1 / (4η)

[0131] That is, when 1 < X < 4η - 1, it becomes Y0 = (X + 1) / (4η).

[0132] Moreover, the distribution ratio Y×100(%) of the first top gas is controlled around the reference distribution ratio Y0×100(%) calculated from X. For example, Y×100(%) is controlled preferably in the range of Y0×100(%)±5%, more preferably in the range of Y0×100(%)±3%, and further preferably in the range of Y0×100(%)±1%. Most preferably, the distribution ratio Y×100(%) of the first top gas is controlled to Y0×100(%). Thereby, the H2 / CO of the reducing gas can be maintained within a constant range without significant fluctuations, maintaining the gas balance of the above-mentioned circulation system.

[0133] It should be noted that when X < 1, even if the reaction of the above formula (iii) proceeds 100%, excess H2 (hereinafter also referred to as excess H2) will be generated in the circulation system. Therefore, in such a case, it is preferable to recover the excess H2 from the circulation system. The process of recovering the excess H2 can be, for example, before the reforming process or after the reforming process and before the blowing process. It should be noted that 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 remains constant and the reaction of the above formula (iii) proceeds 100%. Therefore, from the above viewpoints, the process of recovering the excess H2 is preferably after the reforming process and before the blowing process.

[0134] In addition, when X > 4η - 1, even if the reaction of the above formula (iv) proceeds 100%, there will be a shortage of H2 in the circulation system. Therefore, in such a case, it is preferable to additionally supply H2 to the circulation system. The additional supply amount of H2 is preferably determined such that the amount of H2 contained in the reducing gas blown into the reduction furnace remains constant. Therefore, the process of additionally supplying H2 is preferably, for example, before the reforming process or after the reforming process and before the blowing process.

[0135] It should be noted that when the distribution ratio Y of the first top gas is inconsistent with the reference distribution ratio Y0, the consumed H2 / CO is not strictly consistent with the H2 / CO of the reducing gas obtained in the reforming process. However, as described above, as long as the distribution ratio Y×100(%) of the first top gas is in the range of the reference distribution ratio Y0×100±5%, it is acceptable. In this case, for example, the supply or recovery of H2 can be appropriately carried out to keep the amount of H2 contained in the reducing gas constant.

[0136] In addition, here, the material balance of the CO2 cycle is considered. That is, since CO and H2 remain unchanged before and after the synthesis process and the reforming process, if the methanation reaction of CO according to the above formula (ii) occurs in the synthesis process, the H2 / CO of the reducing gas changes. Therefore, as described above, it is preferable to have a separation process that separates the first top gas into carbon dioxide gas and the remaining gas, and use the carbon dioxide gas obtained in the separation process as the first top gas in the synthesis process. In addition, the higher the CO2 concentration in the carbon dioxide gas, the better, and it is preferably 90 vol% or more. In addition, the best case is that the CO2 concentration in the carbon dioxide gas is 100 vol%. It should be noted that in the case where the methanation of CO occurs in the synthesis process, for example, the supply or recovery of H2 can be appropriately carried out to keep the amount of H2 contained in the reducing gas constant.

[0137] It should be noted that the distribution ratio of the first top gas in the distribution process can be constant or changed at any time. For example, as long as the components of the reducing gas and the top gas are measured, the distribution ratio of the first top gas in the distribution process can be changed according to the change in X: the consumption of H2 / CO. In addition, as long as the distribution ratio of the first top gas at the start of equipment operation is determined according to the past operation history, etc., and then the distribution ratio of the first top gas is appropriately controlled according to the change in X: the consumption of H2 / CO.

[0138] In addition, in the method for manufacturing reduced iron according to an embodiment of the present invention, the flow rate of the reducing gas supplied to the reduction furnace is preferably 1500 Nm 3 / t to 3500 Nm 3 / t. That is, if the flow rate of the reducing gas supplied to the reduction furnace is too low, it will lead to a decrease in production volume and product properties. On the other hand, if the flow rate of the reducing gas supplied to the reduction furnace is too high, the ventilation resistance of the gas in the reduction furnace will increase, and the raw materials (pellets) in the reduction furnace will not drop. Therefore, the flow rate of the reducing gas supplied to the reduction furnace is preferably 1500 Nm 3 / t to 3500 Nm 3 / t.

[0139] In addition, the iron oxide raw material used in the method for manufacturing reduced iron according to an embodiment of the present invention is, for example, iron ore. As specific examples, massive iron ore (lump ore), pellet (a substance obtained by consolidating powdered iron ore into a spherical shape), etc. may be mentioned. 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 viewpoint of reduction in a shaft furnace, it is generally preferably 65% by mass or more. However, in recent years, due to the depletion of high-quality iron ore such as that produced in South America, the price is predicted to rise. Therefore, it is also preferable to use, as needed, low-quality iron ore having an iron content of 63% by mass or less. Such low-quality iron ore is inexpensive and abundant, and as the place of origin, for example, Australia, etc. may be mentioned.

[0140] In addition, in the method for manufacturing reduced iron according to an embodiment of the present invention, a method using a shaft furnace as a direct reduction steelmaking method has been particularly described. However, the type of reduction furnace is not limited thereto, and it may also be a method using a fluidized bed, a rotary kiln, a rotary hearth furnace (RHF), etc. It should be noted that a shaft furnace is preferably used as the reduction furnace because of high production efficiency, operation rate, and operation stability. In addition, most of the direct reduction furnaces operating in the world are of the shaft furnace type, Midrex (registered trademark) and Hyl (registered trademark).

[0141] Examples

[0142] Hereinafter, examples will be described.

[0143] In Figure 2 In the shown circulation system, reduced iron is manufactured under the conditions described in Table 1. Under any conditions, the flow rate of the reducing gas supplied to the reduction furnace is 2200 Nm 3 / t, and the operation time is 28 days. In the distribution process, the total amount of the introduced top gas is distributed into the first top gas and the second top gas, and no supply to other loads, etc. is performed. In addition, in the separation process, the first top gas is separated into carbon dioxide gas (CO2 concentration: 99% by volume or more) and the remaining gas, and the carbon dioxide gas is used as the first top gas in the synthesis process. In addition, the remaining gas and the second top gas are introduced into the reforming process together. In the case of recovering the remaining H2 and additionally supplying H2, both are performed after the reforming process and before the blowing process. It should be noted that the conditions other than those described above and those described in Table 1 are in accordance with the conventional method. In addition, the flow rates of the reducing gas and the top gas in Table 1 are expressed by rounding the first digit after the decimal point. Therefore, sometimes the (total) flow rate of the reducing gas does not match the sum of the H2 amount and the CO amount of the reducing gas. The same applies to the top gas.

[0144]

[0145] Invention examples of controlling the distribution ratio of top gas in the distribution process according to the consumption of H2 / CO are extremely advantageous in terms of energy conservation throughout the entire 28-day operation period. Figure 2 In the circulating system shown, which is a system for recycling top gas, stable operation is carried out under a sound material balance. Additionally, the CO2 emissions from this circulating system can be made zero.

[0146] Furthermore, for comparison, as No. 12, except for not controlling the distribution ratio of top gas and introducing the total amount of top gas into the synthesis process, the production of reduced iron was carried out under the same conditions as No. 6. As a result, stable operation under a sound material balance could not be carried out midway during the operation period, and the operation had to be interrupted.

[0147] Symbol Explanation

[0148] 1 Reduction Furnace

[0149] 1a Iron Oxide

[0150] 1b Reduced Iron

[0151] 3 Dust Removal Device

[0152] 4 Dewatering Device

[0153] 5 Natural Gas Supply Department

[0154] 6 Air Supply Department

[0155] 7 Reforming Device

[0156] 9 Reducing Gas Blowing Device

[0157] 10 Hydrogen Supply Department

[0158] 11 Methane Synthesis Device

[0159] 12 CO2 Separation Device

[0160] 13 Steam Supply Department

[0161] 14 Top Gas Distribution Department

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 from 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; Controlling the distribution ratio of the top gas in the distribution step according to the ratio of the amount of H2 consumed to the amount of CO consumed in the reduction step, i.e., the consumed H2 / CO.

2. The method for manufacturing reduced iron according to claim 1, wherein, Further, a separation step of separating the first top gas into a carbon dioxide gas and the remaining gas is provided between the distribution step and the synthesis step, And using the carbon dioxide gas as the first top gas in the synthesis step.

3. The method for manufacturing reduced iron according to claim 1 or 2, controlling the distribution ratio Y×100% of the first top gas in the distribution step 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, In addition, X: consumed H2 / CO η×100: CO2 conversion rate (%) of methane synthesis in the synthesis step, Furthermore, when the range of the reference distribution ratio Y0×100±5% exceeds 100%, the upper limit of this range is 100%.

4. The method for manufacturing reduced iron according to claim 3, wherein, Controlling the distribution ratio Y×100% of the first top gas in the distribution step to be the reference distribution ratio Y0×100%.

5. The method for manufacturing reduced iron according to claim 3, wherein, When the consumed H2 / CO<1, the remaining H2 is recovered by a circulation system having the blowing step, the reduction step, the distribution step, the synthesis step, and the reforming step.

6. The method for manufacturing reduced iron according to claim 4, wherein, When the consumed H2 / CO<1, the remaining H2 is recovered by a circulation system having the blowing step, the reduction step, the distribution step, the synthesis step, and the reforming step.

7. The method for manufacturing reduced iron according to claim 3, wherein, When the consumed H2 / CO>4η-1, H2 is additionally supplied to a circulation system having the blowing step, the reduction step, the distribution step, the synthesis step, and the reforming step.

8. The method for manufacturing reduced iron according to claim 4, wherein, When the consumed H2 / CO>4η-1, H2 is additionally supplied to a circulation system having the blowing step, the reduction step, the distribution step, the synthesis step, and the reforming step.

9. The method for manufacturing reduced iron according to claim 1 or 2, wherein, Before the distribution step, at least one of dust removal and dehydration of the top gas is performed.

10. The method for manufacturing reduced iron according to claim 3, wherein, Before the distribution step, at least one of dust removal and dehydration of the top gas is performed.

11. The method for manufacturing reduced iron according to claim 1 or 2, wherein, Before the reforming step, dehydration of the recycled methane gas is performed.

12. The method for manufacturing reduced iron according to claim 3, wherein, Before the reforming step, dehydration of the recycled methane gas is performed.

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