Method for producing reduced iron

Abstract

Provided is a reduced iron manufacturing method that makes it possible to achieve energy saving and reduction of CO2 emissions at the same time. The present invention forms a system for circulating furnace top gas for reuse, the system having a blowing step, a reduction step, a distribution step, a synthesis step, and a reformation step. Additional H2O is supplied in the synthesis step, and the amount of the additional H2O supplied in the synthesis step is controlled according to the amounts of CO and H2O contained in a first furnace top gas.

Description

[Technical field]

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

[0002] In recent years, energy conservation has been strongly demanded in steelworks against the backdrop of global environmental problems and fossil fuel depletion problems. The raw material of iron is mainly iron oxide, and a reduction process to reduce this iron oxide is essential in steelworks. The most common reduction process that is widespread worldwide is the blast furnace. In a blast furnace, coke or pulverized coal reacts with oxygen in hot air (air heated to about 1200°C) at the tuyere. This reaction produces reducing gases CO and H2, which are used to reduce iron ore and other materials in the furnace. With the recent improvement of blast furnace operation technology, the reducing agent ratio (the amount of coke and pulverized coal used to produce 1 ton of molten iron) has been reduced to about 500 kg / t, which is already almost at the lower limit. Therefore, no further significant reduction in the reducing agent ratio is expected.

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

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

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

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

[0007] [Patent Document 1] JP 2017-88912 A [Patent Document 2] Patent No. 6190522 Summary of the Invention [Problem to be solved by the invention]

[0008] In the method described in Patent Document 1, natural gas supplied from an external source is used to produce the reducing gas, and therefore, although the amount of CO2 emitted is lower than that emitted from a blast furnace, a certain amount of CO2 is unavoidable.

[0009] Moreover, the method described in Patent Document 2 produces reducing gas using coke oven gas or converter gas generated in a steelworks. Here, in an integrated steelworks, coke oven gas and converter gas are required as fuel gas for downstream processes such as heating furnaces and annealing furnaces. Therefore, if coke oven gas or converter gas is diverted to the reduced iron production process, it will cause a fuel gas shortage in the downstream processes. As a result, natural gas will be supplied from outside to compensate for the shortage of fuel gas in the downstream processes. In other words, even with the method described in Patent Document 2, energy saving and reduction of CO2 emissions cannot be achieved at the same time, and problems remain.

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

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

[0012] Furthermore, the inventors conducted further studies and obtained the following findings. To stably perform operations that simultaneously achieve energy savings and reduced CO2 emissions, it is important to maintain a healthy material balance in the circulation system, especially the C (carbon) material balance. In other words, it is important to control the gas balance in the circulation system so that the ratio of the H2 amount to the CO amount in the reducing gas supplied to the reducing furnace (hereinafter also referred to as H2 / CO of the reducing gas) is always kept within a certain range without large fluctuations. Note that the H2 / CO of the reducing gas referred to here is the volume ratio under standard conditions (also referred to as the flow rate ratio under standard conditions). The same applies to CO2 / CO, which is the ratio of the CO2 amount to the CO amount contained in the furnace top gas, which will be described later. To achieve this, it is necessary to synthesize CH4 stably and in high yield during the synthesis process.

[0013] Incidentally, the furnace gas usually contains, in addition to CO2, a certain amount of CO that was not used in the reduction of iron oxide in the reduction furnace. Therefore, in the synthesis process, CH4 is synthesized from CO in addition to CO2. Here, when CH4 is synthesized from CO2, CH4 is synthesized according to the methanation reaction formula (i) below. When CH4 is synthesized from CO, CH4 is synthesized according to the methanation reaction formula (ii) below. CO2+4H2→ CH4+2H2O ΔH=-165kJ / mol ···(i) CO+3H2→ CH4+H2O ΔH=-206kJ / mol ···(ii)

[0014] However, when a CO methanation reaction is carried out using a general methanation catalyst, catalyst poisoning occurs, and the catalyst becomes deactivated over the extended operation period, resulting in a decrease in methane yield.

[0015] Here, to separate the CO2 and CO contained in the top gas, a huge PSA (pressure swing adsorption) separation device or the like is required. However, depending on the equipment specifications, it may not be possible to introduce such a huge device. Even if such a device could be introduced, it is not necessarily desirable in terms of compactness of the entire facility and energy efficiency.

[0016] Therefore, the inventors further investigated a method for preventing catalyst poisoning in the synthesis process and stably synthesizing CH4 at a high yield without separating the CO2 and CO contained in the top gas in the above-mentioned circulation system. As a result, the following findings were obtained. That is, H2O is supplied separately to the synthesis process, and the amount of additional H2O supplied to the synthesis process is controlled according to the amounts of CO and H2O contained in the first top gas. This makes it possible to prevent catalyst poisoning in the synthesis process and stably synthesize CH4 at a high yield without separating the CO2 and CO contained in the top gas. As a result, it becomes possible to perform operations in the production of reduced iron that simultaneously achieve further energy savings and reductions in CO2 emissions under highly stable operation.

[0017] The inventors consider the above mechanism as follows. That is, during the methanation reaction of CO2, a compound having an aldehyde (CHO) group is generated as an intermediate. Meanwhile, during the methanation reaction of CO, C is generated as an intermediate. This C precipitates on the catalyst and blocks the catalytic active sites, causing catalyst poisoning. Here, if a sufficient amount of HO is present in the synthesis process, C produced as an intermediate during the methanation reaction of CO reacts with HO to produce a compound having an aldehyde (CHO) group. This makes it possible to prevent catalyst poisoning in the synthesis process and stably synthesize CH4 at a high yield without separating the CO2 and CO contained in the top gas. As a result, it becomes possible to carry out operations in the production of reduced iron that simultaneously achieve further energy savings and reductions in CO2 emissions under highly stable operation. The present invention was completed based on the above findings and through further investigation.

[0018] That is, the gist and configuration of the present invention are as follows.

[0019] 1. A charging step of charging iron oxide into a reduction furnace; a blowing step of blowing a reducing gas into the reduction furnace; a reduction step of reducing the iron oxide with the reducing gas in the reduction furnace to obtain reduced iron; A distribution step of distributing a top gas discharged from the reduction furnace into a first top gas and a second top gas; a synthesis step of synthesizing regenerated methane gas from the first furnace top gas and hydrogen gas; An additional supply step of supplying additional HO to the synthesis step; a reforming step in which the regenerated methane gas and the second furnace gas are used as raw material gases to obtain the reducing gas from the raw material gas; having an amount of additional H2O supplied to the synthesis step being controlled according to an amount of CO and an amount of H2O contained in the first top gas.

[0020] 2. The method for producing reduced iron according to item 1, wherein an amount of additional HO supplied to the synthesis step is controlled so that the sum of an amount of HO contained in the first top gas and an amount of additional HO supplied to the synthesis step is 1.0 equivalent or more and 3.0 equivalents or less relative to an amount of CO contained in the first top gas.

[0021] 3. The method for producing reduced iron according to item 1, wherein an amount of additional HO supplied to the synthesis step is controlled so that the sum of an amount of HO contained in the first top gas and an amount of additional HO supplied to the synthesis step is 1.0 equivalent or more and 2.0 equivalents or less relative to an amount of CO contained in the first top gas.

[0022] 4. The method for producing reduced iron according to any one of 1 to 3 above, wherein CO2 / CO, which is a ratio of the amount of CO2 to the amount of CO contained in the top gas, is 0.25 or more and 4.0 or less.

[0023] 5. The method for producing reduced iron according to any one of 1 to 4 above, wherein the top gas or the first top gas is subjected to dedusting prior to the synthesis step.

[0024] 6. The method for producing reduced iron according to any one of 1 to 5 above, wherein the regenerated methane gas is dehydrated prior to the reforming step. Effect of the Invention

[0025] According to the present invention, in the production of reduced iron, it is possible to carry out an operation that simultaneously achieves further energy saving and reduction in CO2 emissions under highly stable operation. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 is a diagram showing a conventional process for producing reduced iron. [Diagram 2] FIG. 2 is a diagram showing an example of a process for producing reduced iron according to a method for producing reduced iron according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Hereinafter, a method for producing reduced iron according to an embodiment of the present invention will be described with reference to the drawings.

[0028] First, a conventional manufacturing process for reduced iron (hereinafter also referred to as the conventional manufacturing process) will be described. Fig. 1 is a diagram showing a schematic configuration of an example of the conventional manufacturing process. In the figure, reference numeral 1 denotes a reduction furnace, 1a denotes iron oxide, 1b denotes reduced iron, 3 denotes a dust remover, 4 denotes a dehydrator, 5 denotes a natural gas supply section, 6 denotes an air supply section, 7 denotes a reformer, and 9 denotes a reducing gas blowing device.

[0029] In one example of the conventional manufacturing process, iron oxide is charged from the top of the reduction furnace and gradually lowered. High-temperature reducing gas is blown into the furnace from the center to reduce the iron oxide. The reduced iron is then discharged from the bottom of the reduction furnace. At this time, furnace top gas containing mainly CO, CO2, H2, and H2O is discharged from the top of the reduction furnace. This furnace top gas is dusted in a dust collector, and a portion of the top gas is adjusted in moisture and sent to the reformer as a raw material gas. A gas containing a hydrocarbon, for example, natural gas from a natural gas supply unit, is supplied to the reformer together with the moisture-adjusted top gas. The supplied gas is then heated in the reformer. Then, a reforming reaction occurs, and a high-temperature reducing gas containing mainly CO and H2 is generated. This reducing gas is then blown into the reduction furnace. The remaining part of the top gas is dehydrated and then used as a heating fuel in the combustion chamber of the reformer, and is burned, for example, with oxygen in the air. After being burned as a heating fuel, the top gas is usually discharged outside the system, still containing CO2.

[0030] On the other hand, in a method for producing reduced iron according to an embodiment of the present invention, as shown in Fig. 2, for example, a furnace top gas distribution section distributes a furnace top gas discharged from a reducing furnace into a first furnace top gas and a second furnace top gas. Then, instead of a hydrocarbon gas such as natural gas supplied from the outside in the conventional process of Fig. 1, methane gas (hereinafter also referred to as regenerated methane gas) synthesized from the first furnace top gas and hydrogen gas is used. In the figure, reference numeral 10 denotes a hydrogen supply section, 11 denotes a methane synthesis unit, 12 denotes an H2O supply section, and 13 denotes a furnace top gas distribution section. Here, the regenerated methane gas generated in the methane synthesis unit 11 is supplied to a reformer 7 and used as a raw material gas for the reducing gas. Each step of the method for producing reduced iron according to one embodiment of the present invention will be described below. Note that the filling step, the blowing step, and the reducing step may be performed in a conventional manner, for example, in the same manner as in the above-mentioned conventional production process, and therefore the description thereof will be omitted here.

[0031] ·Distribution process In the distribution step, for example, in the top gas distribution section, the top gas discharged from the reducing furnace is distributed into a first top gas and a second top gas. The distribution and flow rate control means of the top gas are not particularly limited, and may be in accordance with a conventional method. For example, a mass flow controller or the like may be used. The distribution ratio of the top gas is not particularly limited. For example, the distribution ratio of the top gas may be appropriately determined by a material balance calculation depending on the composition of the reducing gas and the top gas. In addition, a part of the top gas may be supplied to another load depending on the operating conditions, etc., as long as it is 10 volume % or less of the amount of the top gas.

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

[0033] The composition of the first furnace gas and the second furnace gas described later is basically the same as that of the furnace gas. The composition of the furnace gas can be exemplified as follows: CO: 5-50 volume %, CO2: 5-30 volume %, H2: 5-80 volume %, H2O: 0-35 volume %, and the balance: 0-20 volume %.

[0034] In addition to the first furnace gas, any other gas containing at least one of CO and CO2 (hereinafter also referred to as other gas) can also be used in the synthesis step. Examples of other gases include gases produced as by-products in the iron-making process, specifically, blast furnace gas (BFG) and coke oven gas (COG).

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

[0036] Furthermore, the amount of hydrogen gas supplied in the synthesis step (the amount of hydrogen gas supplied to the methane synthesis apparatus) is preferably stoichiometric with respect to the amount of CO and CO contained in the first furnace gas, based on the methanation reaction formulas (i) and (ii) shown above.

[0037] In the synthesis of CH4, a commonly used methanation catalyst can be used. Specifically, transition metal catalysts such as Fe, Ni, Co, and Ru can be used. Among them, Ni-based catalysts have high activity. In addition, Ni-based catalysts have high heat resistance and can be used up to temperatures of about 500°C. Therefore, Ni-based catalysts are particularly preferred. Iron ore may also be used as a catalyst. In particular, high water of crystallization ores have an increased specific surface area when the water of crystallization is dehydrated, and can be suitably used as a catalyst.

[0038] The reactor of the methane synthesis apparatus used in the synthesis process may be a fixed bed reactor, a fluidized bed reactor, an air-flow reactor, or the like. The physical properties of the catalyst may be appropriately selected depending on the type of reactor. A heat exchanger may be disposed in the gas flow path downstream of the reactor to recover the reaction heat (gas sensible heat) of the methanation reaction in each reactor. The recovered thermal energy may be returned, for example, to the upstream side of the reactor and used to keep the reactor heated. The recovered thermal energy may also be used for heating other processes such as the reforming process.

[0039] In addition, the above methanation catalyst stably exhibits a high conversion rate during methanation of CO2. However, as described above, during methanation of CO, C is generated as an intermediate. This causes C precipitation and catalyst poisoning. In order to prevent such catalyst poisoning, it is important to supply additional H2O to the synthesis process and control the amount of this additional H2O according to the amount of CO and H2O contained in the first furnace gas. Details are as described in the [Additional Supply Process] below.

[0040] Furthermore, the temperature of the reactor of the methane synthesis apparatus where the methanation reaction occurs in the synthesis step is preferably controlled in the range of 200 to 550 ° C. If the temperature of the reactor of the methane synthesis apparatus is less than 200 ° C, the reaction rate is slowed down and the activity of the methanation catalyst is insufficient. On the other hand, if the temperature of the reactor of the methane synthesis apparatus exceeds 550 ° C, the catalyst temperature may be excessively increased, and the deactivation of the catalyst may be accelerated. In addition, as shown in the above formulas (i) and (ii), the methanation reaction is an exothermic reaction. Therefore, if the catalyst temperature is excessively increased, the reaction equilibrium is biased to the left (CO2 and CO side) of the above formulas (i) and (ii), and the methane yield decreases. Therefore, the temperature of the reactor of the methane synthesis apparatus is preferably controlled in the range of 200 to 550 ° C.

[0041] In addition, the pressure of the reactor of the methane synthesis apparatus where the methanation reaction occurs in the synthesis process is preferably controlled in the range of 3 to 10 atm. As shown in the above formulas (i) and (ii), the methanation reaction is a molecular reduction reaction. Therefore, according to Le Chatelier's principle, the higher the pressure of the reactor of the methane synthesis apparatus, the more likely the reaction equilibrium is to be biased to the right (CH4) of the above formulas (i) and (ii), and the methane yield is improved. However, if the pressure of the reactor of the methane synthesis apparatus becomes excessively high, there may be a problem in terms of safety. Therefore. The pressure of the reactor of the methane synthesis apparatus is preferably controlled in the range of 3 to 10 atm.

[0042] In addition, if HO generated as a by-product of methane synthesis is introduced into the reformer, there may be an excess of HO in the reforming step described below. Therefore, it is preferable to dehydrate the regenerated methane gas using a dehydrator as appropriate, prior to the reforming step described below, while taking into consideration the material balance of the entire circulation system.

[0043] The CH4 concentration of the regenerated methane gas is not particularly limited. In the regenerated methane gas excluding H2O, for example, the regenerated methane gas obtained by dehydrating the gas on the outlet side of the methane synthesis unit using a dehydrator, the CH4 concentration is preferably 80% by volume or more, more preferably 90% by volume or more. The CH4 concentration in the regenerated methane gas may be 100% by volume.

[0044] ·Additional supply process The additional supply step is a step of supplying additional H2O to the synthesis step. Here, the additional H2O is H2O that is supplied to the synthesis step separately from the first furnace gas.

[0045] In this additional supply step, it is important to control the amount of additional H2O depending on the amount of CO and H2O contained in the first furnace top gas.

[0046] As described above, the top gas usually contains a certain amount of CO that was not used in the reduction of iron oxide in the reduction furnace in addition to CO2. Therefore, in the synthesis process, CH4 is synthesized from CO in addition to CO2. When CH4 is synthesized from CO2, CH4 is synthesized according to the methanation reaction formula (i) above. When CH4 is synthesized from CO, CH4 is synthesized according to the methanation reaction formula (ii) above. However, when a methanation reaction of CO occurs using a general methanation catalyst, catalyst poisoning occurs, and the catalyst becomes deactivated as the operation period extends, resulting in a decrease in methane yield.

[0047] During the methanation reaction of CO2, a compound with an aldehyde (CHO) group is generated as an intermediate. Meanwhile, during the methanation reaction of CO, C is generated as an intermediate. This C precipitates on the catalyst and blocks the catalytic active sites, causing catalyst poisoning.

[0048] Here, if a sufficient amount of HO is present in the synthesis process, C produced as an intermediate during the methanation reaction of CO reacts with HO to produce a compound having an aldehyde (CHO) group. This makes it possible to prevent catalyst poisoning in the synthesis process and stably synthesize CH4 at a high yield without separating the CO2 and CO contained in the top gas. As a result, it becomes possible to carry out operations in the production of reduced iron that simultaneously achieve further energy savings and reductions in CO2 emissions under highly stable operation.

[0049] Therefore, in this additional supply step, the amount of additional H2O is controlled according to the amount of CO and H2O contained in the first furnace gas. In particular, the amount of additional H2O is controlled so that the sum of the amount of H2O contained in the first furnace gas and the amount of additional H2O supplied to the synthesis step is preferably 1.0 equivalent or more relative to the amount of CO contained in the first furnace gas. However, the methanation reaction of CO2 and CO in the above formulas (i) and (ii) is an equilibrium reaction, and if H2O increases excessively, the equilibrium shifts to the left (CO2 and CO side), and the methane yield decreases instead. Therefore, the amount of additional H2O is controlled so that the sum of the amount of H2O contained in the first furnace gas and the amount of additional H2O supplied to the synthesis step is preferably 3.0 equivalent or less, more preferably 2.0 equivalent or less, relative to the amount of CO contained in the first furnace gas.

[0050] The term "equivalent" used here means molar equivalent. The fact that the sum of the amount of HO contained in the first furnace gas and the amount of additional HO supplied to the synthesis step is 1.0 equivalent to 3.0 equivalent to the amount of CO contained in the first furnace gas can also be expressed by the following formula. 1.0≦([H2O content in first furnace gas (Nm 3 / t)] + [Additional amount of H2O supplied to the synthesis process (Nm 3 / t)]) / [CO content in first furnace gas (Nm 3 / t)]≦3.0 Hereinafter, ([H2O content in the first furnace gas (Nm 3 / t)] + [Additional amount of H2O supplied to the synthesis process (Nm 3 / t)]) / [CO content in first furnace gas (Nm 3 / t)] is also called HO / CO. That is, H2O / CO is preferably 1.0 or more. Also, H2O / CO is preferably 3.0 or less, more preferably 2.0 or less.

[0051] In addition, the form of H2O supplied to the synthesis step in the additional supply step is preferably water vapor.

[0052] The HO supply source used in the additional supply step is not particularly limited and may be supplied by any method. For example, HO supplied from outside the circulation system may be used, or HO dehydrated by a dehydrator described later may be reused.

[0053] In addition, as described later, for example, when wet dust removal is performed on the furnace gas using a scrubber or the like, H2O equivalent to the amount of saturated water vapor is present in the furnace gas after dust removal, and further in the first furnace gas. The amount of H2O contained in the first furnace gas also contributes to preventing catalyst poisoning. Therefore, the amount of additional H2O is controlled taking into consideration the amount of H2O contained in the first furnace gas as well as the amount of CO contained in the first furnace gas. The amount of saturated water vapor is a value determined by the temperature and flow rate of the furnace gas. In addition, the method for measuring the amount of H2O present in the first furnace gas is not particularly limited, and may be measured using, for example, a dew point meter or a moisture meter.

[0054] Modification process In the reforming step, the regenerated methane gas and the second furnace gas are used as raw material gases to obtain a reducing gas from the raw material gases. For example, the regenerated methane gas and the second furnace gas are introduced into a reformer, and the regenerated methane gas and the second furnace gas are heated in the reformer. Then, in the reformer, a reducing gas containing CO and H2 is generated by the reforming reactions of the following formulas (iii) and (iv). The reforming reaction shown in formula (iv) proceeds by supplying steam to the reformer. CH4+CO2→ 2CO+2H2ΔH=247kJ / mol ···(iii) CH4+H2O → CO+3H2ΔH=206kJ / mol ···(iv)

[0055] The gas composition of the reducing gas is, for example, CO: 1 to 60 volume %, H2: 40 to 99 volume %, and the balance: 0 to 30 volume %.

[0056] As can be seen from the fact that the formation enthalpy of the above formulas (iii) and (iv) is positive, both of the above formulas (iii) and (iv) are endothermic reactions. Therefore, by using green energy such as solar, wind, and geothermal energy as the energy source for the reforming reaction, it is possible in principle to reduce CO2 emissions to zero.

[0057] Next, the reducing gas is introduced into the reducing furnace through an injection process. For example, the reducing gas is introduced into the reducing furnace using a reducing gas injection device. Then, in the reducing furnace, the iron oxide is reduced by the reducing gas to obtain reduced iron. Meanwhile, the reducing gas after being used to reduce the iron oxide is discharged from the reducing furnace as a top gas.

[0058] The ratio of the amount of CO2 to the amount of CO contained in the furnace gas, CO2 / CO, is preferably 0.25 or more and 4.0 or less. That is, the ratio of the amount of CO2 to the total amount of CO2 and CO contained in the top gas, CO2 / (CO2+CO), represents the degree of reaction progress in the reduction reaction of iron oxide by CO in the following formula (v). In addition, CO2 / (CO2+CO) depends on the equilibrium constant and reaction rate constant of the formula (v). 3CO+Fe2O3→3CO2+2Fe ΔH=-247kJ / kg-Fe ···(v) 3H2+Fe2O3→3H2O+2Fe ΔH=858kJ / kg-Fe ···(vi)

[0059] For example, if the grade of the raw material iron ore decreases, the activation energy of reduction by H2 increases. This decreases the reaction rate of the above formula (vi) while increasing the reaction rate of the above formula (v). As a result, the value of CO2 / (CO2+CO) increases. In addition, the reduction by CO in the above formula (v) is an exothermic reaction, whereas the reduction by H2 in the above formula (vi) is an endothermic reaction. Therefore, when the temperature inside the reduction furnace increases, the reaction rate of the above formula (vi) increases, while the reaction rate of the above formula (v) decreases. As a result, the value of CO2 / (CO2+CO) decreases.

[0060] From the viewpoint of stable operation, CO2 / (CO2+CO) is preferably 0.2 or more, more preferably 0.4 or more. CO2 / (CO2+CO) is preferably 0.8 or less, more preferably 0.6 or less. That is, 0.2≦CO2 / (CO2+CO) Transforming it gives us 0.25≦CO2 / CO It becomes. CO2 / (CO2+CO)≦0.8 Transforming it gives us CO2 / CO≦4.0 It becomes. Also, 0.4≦CO2 / (CO2+CO) Transforming it gives us 0.67≦CO2 / CO It becomes. Also, CO2 / (CO2+CO)≦0.6 Transforming it gives us CO2 / CO≦1.5 It becomes. That is, the CO2 / CO ratio is preferably 0.25 or more, more preferably 0.67 or more, and is preferably 4.0 or less, more preferably 1.5 or less.

[0061] The method for measuring CO2 / CO, which is the ratio of the amount of CO2 to the amount of CO contained in the top gas, is not particularly limited, but can be measured, for example, as follows. That is, the flow rate of the top gas (converted to standard conditions) is measured using a flow meter at an arbitrary point in the top gas flow section. In addition, the top gas is sampled from a sampling port, and the volume concentrations of CO2 and CO in the gas are measured by gas chromatography. Then, the volume concentration of CO2 in the gas is divided by the volume concentration of CO to obtain CO2 / CO. The flow rate of the top gas (converted to standard conditions) [Nm 3 / h] is the flow rate (standard condition conversion) [Nm 3 / t]. The flow rates of other gases can also be measured in a similar manner.

[0062] Furthermore, if the first furnace gas introduced into the synthesis process contains dust, it may cause problems such as clogging. Therefore, it is preferable to remove dust from the furnace gas or the first furnace gas prior to the synthesis process. In particular, it is preferable to remove dust from the furnace gas prior to the distribution process. Any dust removal device can be used for the dust removal. For example, in a blast furnace, dry dust removal is performed on the furnace gas using a cyclone or the like, and then wet dust removal is performed using a scrubber or the like. In wet dust removal, the dust is washed away with water, and the furnace gas is also cooled at the same time. As a result, most of the water vapor contained in the furnace gas is condensed and collected together with the wastewater. However, in this case, the furnace gas contains H2O equivalent to the amount of saturated water vapor depending on the temperature of the furnace gas after cooling. As described above, the amount of H2O contained in the first furnace gas also contributes to preventing catalyst poisoning. Therefore, it is preferable to perform, for example, wet dedusting so that H2O equivalent to the amount of saturated water vapor is present in the furnace gas.

[0063] In the method for producing reduced iron according to an embodiment of the present invention, the flow rate of the reducing gas supplied to the reduction furnace is 1500 Nm 3 / t or more 3500Nm 3 In other words, 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 deterioration of product properties. On the other hand, if the flow rate of the reducing gas supplied to the reduction furnace is too high, the gas flow resistance in the reduction furnace will increase, and the raw material (pellets) in the reduction furnace will not descend. Therefore, the flow rate of the reducing gas supplied to the reduction furnace should be 1500 Nm 3 / t or more 3500Nm 3 / t or less is preferable.

[0064] Furthermore, the iron oxide raw material used in the method for producing reduced iron according to one embodiment of the present invention is, for example, iron ore. Specific examples include lump iron ore (lump ore) and pellets (iron ore powder solidified into a spherical shape). The grade of the iron ore used as the iron oxide raw material, i.e., the iron content, is not particularly limited, but is generally preferably 65% ​​by mass or more from the viewpoint of reduction in a shaft furnace. However, in recent years, a price rise due to the depletion of high-grade iron ore from South America and the like is predicted. Therefore, it is preferable to use low-grade iron ore with an iron content of 63% by mass or less as needed. Such low-grade iron ore is inexpensive and abundant, and examples of places of origin include Australia.

[0065] In addition, in the method for producing reduced iron according to one embodiment of the present invention, a method using a shaft furnace as a direct reduction ironmaking method has been described. However, the type of reduction furnace is not limited to this, and a method using a fluidized bed, a rotary kiln, a rotary hearth furnace (RHF), or the like may also be used. Note that a shaft furnace is preferable as the reduction furnace because of its high production efficiency, operating rate, and operational stability. Also, most of the direct reduction furnaces operating around the world are shaft furnace type Midrex (registered trademark) and Hyl (registered trademark). EXAMPLES

[0066] Examples will be described below. Example 1 Reduced iron was produced under the following conditions in the circulation system shown in Figure 2. Iron ore pellets from Brazil were used as the raw material. Flow rate of reduction gas supplied to the reduction furnace: 2200 Nm 3 / t The reduction gas supplied to the reduction furnace is H2 / CO: 2.0 Operation period: 28 days Reduced iron production rate: 120t / h Gas composition in furnace top gas during normal operation (volume ratio): H2:CO:CO2:H2O=48:30:9:13

[0067] In the example of the invention, the amounts of CO, CO, and H2O contained in the top gas were measured so that the H2O / CO ratio was 1.0, and the amount of additional H2O supplied to the synthesis step was controlled according to the amounts of CO and H2O contained in the first top gas.

[0068] On the other hand, for comparison, in Comparative Example 1, reduced iron was produced without supplying additional H2O to the synthesis step. In addition, in Comparative Example 2, reduced iron was produced by supplying a constant amount of additional H2O to the synthesis step without controlling the amount of additional H2O according to the amounts of CO and H2O contained in the first top gas. In both Comparative Examples 1 and 2, the other conditions were the same as those in the above-mentioned invention example.

[0069] As a result, in the example of the invention, stable operation with high methane yield and prevention of catalyst poisoning was achieved under a healthy material balance in the circulation system shown in Figure 2, which is extremely advantageous for realizing energy savings over the entire operation period of 28 days, that is, the system in which the furnace gas is circulated and reused. In addition, CO2 emissions from the circulation system were reduced to zero.

[0070] On the other hand, in Comparative Example 1, catalyst poisoning due to the methanation reaction of CO occurred during the operation period, making it impossible to perform stable operation with a high methane yield, and the operation was forced to be suspended. When the locations where catalyst poisoning occurred were confirmed using the procedure described below, catalyst poisoning occurred in three or more locations. In Comparative Example 2, stable operation under a sound material balance could not be performed during the operation period, and the operation was forced to be suspended. The methane yield (from the start of operation to the suspension) evaluated as described below was less than 93%.

[0071] Example 2 Reduced iron was produced under the same conditions as those of the invention example of Example 1, except for the conditions shown in Table 1, particularly, the H2O / CO ratio being changed in various ways.

[0072] At this time, the methane yield was measured under each condition. Specifically, the methane yield was measured every hour over the entire operation period (28 days), and the average value of the measured methane yields was calculated. The methane yield was calculated using the following formula. [Methane yield (%)] = (1 - [Amount of CO2 and CO contained in the regenerated methane gas discharged after the synthesis of CH4 in the synthesis process (Nm 3 / t)]÷[Amount of CO2 and CO introduced into the synthesis process (Nm 3 / t)]) × 100 The methane yield was evaluated according to the following criteria. The evaluation results are shown in Table 1. 1 (Pass, Excellent): Methane yield is 97% or more 2 (pass, excellent): Methane yield is 95% or more but less than 97% 3 (Pass): Methane yield is 93% or more but less than 95% 4 (Fail): Methane yield is less than 93%

[0073] After the completion of the 28-day operation period, the methanation catalyst was evaluated for its resistance to catalyst poisoning (hereinafter also referred to as catalyst poisoning resistance) in the following manner. That is, after the operation period of 28 days was completed, the entire surface of the methanation catalyst was observed visually and by an optical microscope to confirm the presence or absence of discoloration. If a discoloration was confirmed, the presence or absence of fibrous material generation in the discoloration was confirmed by SEM, and the presence or absence of an increase in the intensity of C was confirmed by SEM-EDX analysis to determine whether catalyst poisoning occurred in the discoloration. The generation of fibrous material and the increase in the intensity of C on the surface of the methanation catalyst are phenomena caused by catalyst poisoning, and if at least one of these was confirmed in the discoloration, it was determined that catalyst poisoning had occurred. Then, the number of places where catalyst poisoning occurred on the surface of the methanation catalyst was counted, and the catalyst poisoning resistance was evaluated according to the following criteria. The evaluation results are also shown in Table 1. 1 (Pass, Excellent): No catalyst poisoning occurred 2 (Pass): Catalyst poisoning occurred in one or two places. 3 (Fail): Catalyst poisoning occurred in three or more places.

[0074] [Table 1]

[0075] As shown in Table 1, in all of No. 1 to 7, in which the amount of additional H2O supplied to the synthesis process was controlled according to the amount of CO and H2O contained in the first furnace gas, stable operation with high methane yield and prevention of catalyst poisoning was possible under a sound material balance in the circulation system shown in Figure 2, which is extremely advantageous for realizing energy savings, that is, the system in which the furnace gas is circulated and reused, over the entire operation period of 28 days. In addition, CO2 emissions from the circulation system were also reduced to zero. Among them, in Nos. 2 to 5, in which the amount of additional H2O supplied to the synthesis step was controlled so that the H2O / CO ratio was 1.0 or more and 2.0 or less, that is, the sum of the amount of H2O contained in the first furnace gas and the amount of additional H2O supplied to the synthesis step was 1.0 equivalent or more and 2.0 equivalent or less relative to the amount of CO contained in the first furnace gas, a particularly high methane yield and particularly excellent catalyst poisoning resistance were simultaneously obtained. [Explanation of symbols]

[0076] 1. Reduction furnace 1a Iron oxide 1b Reduced iron 3 Dust removal equipment 4 Dehydration equipment 5. Natural Gas Supply Department 6 Air supply section 7. Reformer 9. Reducing gas injection device 10 Hydrogen supply unit 11 Methane synthesis unit 12 H2O Supply Section 13 Top gas distribution section

Claims

1. a charging step of charging the iron oxide into a reduction furnace; a blowing step of blowing a reducing gas into the reduction furnace; a reduction step of reducing the iron oxide with the reducing gas in the reduction furnace to obtain reduced iron; A distribution step of distributing a top gas discharged from the reducing furnace into a first top gas and a second top gas; a synthesis step of synthesizing regenerated methane gas from the first furnace top gas and hydrogen gas; Addition of H to the synthesis process ２ an additional supply step of supplying O; a reforming step in which the regenerated methane gas and the second furnace gas are used as raw material gases to obtain the reducing gas from the raw material gas; having The additional H 2 O to be supplied to the synthesis step is adjusted so that the sum of the amount of H 2 O contained in the first furnace gas and the amount of additional H 2 O to be supplied to the synthesis step is 1.0 equivalent or more and 3.0 equivalent or less with respect to the amount of CO contained in the first furnace gas. ２ A method for producing reduced iron in which the amount of O is controlled.

2. H contained in the first furnace gas ２ The amount of O and additional H supplied to the synthesis step ２ The additional H is supplied to the synthesis step so that the total amount of H and O is 1.0 equivalents or more and 2.0 equivalents or less relative to the amount of CO contained in the first furnace gas. ２ The method for producing reduced iron according to claim 1 , wherein an amount of O is controlled.

3. The ratio of the amount of CO contained in the furnace gas to the amount of CO ２ The ratio of CO ２ The method for producing reduced iron according to claim 1 or 2, wherein / CO is 0.25 or more and 4.0 or less.

4. The method for producing reduced iron according to claim 1 or 2, further comprising the step of: prior to the synthesis step, removing dust from the top gas or the first top gas.

5. The method for producing reduced iron according to claim 3 , further comprising the step of: prior to the synthesis step, removing dust from the top gas or the first top gas.

6. 3. The method for producing reduced iron according to claim 1, further comprising the step of dehydrating the regenerated methane gas prior to the reforming step.

7. The method for producing reduced iron according to claim 3 , further comprising the step of dehydrating the regenerated methane gas prior to the reforming step.

8. The method for producing reduced iron according to claim 4 , further comprising the step of dehydrating the regenerated methane gas prior to the reforming step.

9. The method for producing reduced iron according to claim 5 , further comprising the step of dehydrating the regenerated methane gas prior to the reforming step.

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