Method of producing reduced iron

Abstract

We propose a method that enables energy conservation and a reduction in CO2 emissions when manufacturing reduced iron from iron oxide. In the method, in a circulation system in which an iron oxide filling process for filling iron oxide into a reduction furnace, a reduction gas injection process for injecting a reduction gas into the reduction furnace, a reduction process for reducing the iron oxide by the reduction gas within the reduction furnace, a methane synthesis process for synthesizing a gas having methane as the main component from a portion of the furnace gas discharged from the furnace furnace and hydrogen gas, and a gas reforming process in which the remaining portion of the methane and the furnace gas is used as raw material gas and the raw material gas is heated to reform into the reduction gas, the operation is continued while preserving the carbon that is insufficient during operation in the circulation system at one or more of the reduction gas injection process, the reduction process, the methane synthesis process, and the gas reforming process, or at a connection point between any two or more of the reduction gas injection process, the reduction process, the methane synthesis process, and the gas reforming process.

Description

Technology Field

[0001] This invention relates to a method for producing reduced iron, in particular to a method for producing reduced iron using a vertical furnace. Background Technology

[0002] Recently, there is a strong demand for energy conservation in steel mills, against the backdrop of global environmental issues and the depletion of fossil fuels.

[0003] Therefore, the raw material for iron is mainly iron oxide, and a reduction process to reduce this iron oxide becomes essential. The most widespread and common reduction process in the world is the blast furnace. In this blast furnace, oxygen in the hot air (air heated to about 1200°C) reacts with coke or pulverized coal at the tuyere to produce CO and H2 gases (reducing gases), and these reducing gases are used to reduce iron ore and other materials inside the furnace. Due to recent improvements in blast furnace operation technology, the reducing agent ratio (the amount of coke and pulverized coal used per ton of hot metal produced) has been reduced to about 500 kg / t, but the reducing agent ratio has already reached its lower limit, so further significant reduction in the reducing agent ratio cannot be expected.

[0004] Meanwhile, in regions where natural gas is produced, a method of producing reduced iron is also frequently used in which agglomerated iron ore, such as sintered ore or pellets, is filled into a water-type reduction furnace as a raw material for iron oxide, and a reducing gas containing hydrogen and carbon monoxide is blown in to reduce the raw material for iron oxide. In this method, natural gas is used as the raw material gas for the reducing gas. The raw material gas is heated and reformed in a heating reforming device (7) together with the furnace top gas discharged from the furnace top of the reduction furnace to produce reducing gas. The generated reducing gas is blown into the reduction furnace and reacts with the raw material for iron oxide supplied from the top of the reduction furnace, thereby reducing the iron oxide and becoming reduced iron. The produced reduced iron is cooled in an area below the location where the reducing gas is blown into the reduction furnace, and then discharged from the bottom of the reduction furnace.

[0005] In addition, the gas after being provided for the reduction of iron oxide is discharged as furnace gas from the furnace of the reduction furnace, and after dust collection and cooling, a portion of the furnace gas is sent to the heating reforming device (7) as a raw material for the reforming gas. In addition, the remaining furnace gas (the remainder of the furnace gas) is used as fuel gas for the heating and heating reforming device (7). The remaining furnace gas used as fuel gas for the heating and heating reforming device (7) is generally discharged outside the system.

[0006] As a process for producing reduced iron as described above, for example, Patent Document 1 describes a method of producing reduced iron by reforming the exhaust gas of a reduction furnace and natural gas in a reformer to generate a reducing gas consisting mainly of CO and H2 gas, and blowing this reducing gas into a reduction furnace to reduce the iron oxide inside the reduction furnace. Prior art literature

[0007] Japanese Patent Publication No. 2017-88912 The problem to be solved

[0008] In the method for producing reduced iron described in Patent Document 1, since natural gas is used to produce reducing gas, there is a problem in that some amount of CO2 emission cannot be avoided, although it is lower than that of a blast furnace. Since the gas emitted in this way contains a certain amount of CO2, this method for producing reduced iron can reduce the amount of CO2 emission to a certain extent, but it cannot be made zero.

[0009] The present invention is made in consideration of the above phenomenon and aims to propose a method that enables energy conservation and a reduction in CO2 emissions when producing reduced iron from iron oxide. means of solving the problem

[0010] The inventors, aiming to reduce CO2 emissions to zero in the above-described method for manufacturing reduced iron, have discovered that energy saving and CO2 emission reduction are possible by synthesizing regenerated methane by supplying CO2-free hydrogen from the outside to the furnace gas, which is the exhaust gas from the reduction furnace, and by using the regenerated methane to produce reducing gas. That is, in this method for manufacturing reduced iron, all gaseous carbon sources, such as CO2 or CO contained in the exhaust gas of the reduction furnace, are circulated and reused through a process for synthesizing regenerated methane. Therefore, it is possible to reduce the amount of carbon emitted as CO2 to zero outside the circulation (reuse) system, which is composed of an iron oxide charging process into the reduction furnace, a reducing gas injection process into the reduction furnace, a reduction process within the reduction furnace, a methane synthesis process that generates regenerated methane gas from the furnace gas of the reduction furnace, and a gas reforming process that generates reducing gas from the regenerated methane gas and the furnace gas.

[0011] Furthermore, the inventors examined the actual application of the above-described method for producing reduced iron to a steel mill and realized that it is necessary to secure carbon in the above-described circulation (reuse) system (hereinafter simply referred to as the system). That is, a certain amount of carbon is discharged outside the system in a form other than gas, such as carbon consumed in the carburization of the reduced iron within the reduction furnace, or carbon contained in the dust that is removed from the exhaust gas (furnace gas) by a dust collector accompanying the exhaust gas from the reduction furnace. In addition, during abnormal situations, such as when operational troubles occur, there is a possibility of discharging carbon outside the system in a form that does not occur during normal operation, such as atmospheric release of the exhaust gas from the reduction furnace. When considering such carbon discharge outside the system, the amount of carbon circulating within the system tends to decrease. In that case, the proportion of the iron oxide reduction reaction by CO within the iron oxide reduction reaction in the reduction furnace decreases, and the proportion of the reaction by H2 increases. While the reduction of iron oxide by CO is an exothermic reaction, the reduction of iron oxide by H2 is an endothermic reaction; therefore, if the latter reaction increases, there is a lack of heat in the reduction furnace, making it difficult to obtain reduced iron with a sufficiently high reduction rate.

[0012] Although there is a method to preheat the raw iron oxide for heat compensation, if operation continues without supplying a carbon source and all the reducing gas turns into hydrogen, the preheating temperature of the raw iron oxide becomes very high, making it difficult to handle the raw iron oxide. Furthermore, since preheating of the raw material was unnecessary in the original reduction furnace operation, additional equipment for preheating the raw material is required, resulting in an economically disadvantageous operation. On the other hand, regarding the method of raising the reducing gas temperature for heat compensation, if the reducing gas temperature rises, the raw material is prone to fusing together inside the reduction furnace, making normal lowering and discharge of the raw material difficult, and also making it prone to causing trouble in the reducing gas injection equipment. In addition, raising the reducing gas temperature compared to the original reduction furnace operation requires unnecessary energy, making it an economically disadvantageous operation. For the reasons above, the means of raising the reducing gas temperature is also unsuitable as a means of compensating for the endothermic component caused by H2 reduction, just like the preheating of the raw iron oxide. Therefore, it has been newly discovered that in the production of reduced iron in a recycling (reuse) system, in order to continue producing reduced iron with a sufficiently high reduction rate, it is necessary to supply a certain amount of carbon source to compensate for the carbon lost from within the carbon recycling and reuse system.

[0013] As described above, the inventors, having conducted diligent research to solve the aforementioned problems of the conventional technology and to further realize the production of reduced iron at a high reduction rate in a recycling (reuse) system, have developed a novel method for producing reduced iron described below.

[0014] 1. An iron oxide filling process for filling iron oxide into a reduction furnace, and

[0015] A reducing gas injection process for injecting reducing gas into the above-mentioned reduction furnace, and

[0016] A reduction process for reducing the iron oxide by the reduction gas in the above reduction furnace, and

[0017] A methane synthesis process for synthesizing a gas with methane as the main component from a portion of the furnace gas discharged from the furnace furnace of the above-mentioned reduction furnace and hydrogen gas, and

[0018] A gas reforming process that uses the above methane and the remainder of the above furnace gas as a raw material gas, and heats the said raw material gas to reform it into the said reducing gas.

[0019] As a method for producing reduced iron in this interconnected circulating system,

[0020] A method for producing reduced iron, wherein the carbon deficient during operation in the above-mentioned circulation system is supplemented at a connection point between any one or two or more of the above-mentioned reducing gas injection process, the above-mentioned reduction process, the above-mentioned methane synthesis process, and the above-mentioned gas reforming process, or between any two or more of the above-mentioned reducing gas injection process, the above-mentioned reduction process, the above-mentioned methane synthesis process, and the above-mentioned gas reforming process.

[0021] 2. A method for producing reduced iron described in 1, wherein carbon is preserved in each connection portion of either or both of the above-mentioned methane synthesis process and the above-mentioned gas reforming process, or three processes including the reduction process added to both of the above-mentioned processes.

[0022] 3. A method for producing reduced iron as described in 1 or 2, wherein the carbon is combustion gas of waste plastic.

[0023] 4. A method for producing reduced iron as described in 1 or 2, wherein the carbon is a combustion gas of biomass. Effects of the invention

[0024] The present invention enables the realization of operation while supplying an insufficient carbon source in a method for producing reduced iron having a closed system that circulates carbon, wherein CO2-free hydrogen is supplied from the outside to the furnace gas of a reduction furnace to synthesize regenerated methane, and the regenerated methane is used to produce a reduction gas. Therefore, since fluctuations in the amount of carbon circulating within the system are avoided, stable operation free of CO2 emissions can be carried out. Furthermore, when carbon-neutral raw materials such as waste plastics or biomass are used as carbon sources, in addition to the above effects, there is also the effect of further reducing the environmental burden of the process. Brief explanation of the drawing

[0025] Figure 1 is a diagram showing the process of producing reduced iron by a general shaft furnace. Figure 2 is a diagram showing the process of manufacturing reduced iron according to the present invention. Figure 3 is a diagram showing another reduced iron manufacturing process of the present invention. Specific details for implementing the invention

[0026] (Form for carrying out the invention)

[0027] Hereinafter, the method of the present invention will be described in detail with reference to the drawings. First, the conventional reduced iron manufacturing process, which is contrasted with the method of the present invention, will be described in order.

[0028] [Previous Process]

[0029] That is, FIG. 1 is a diagram showing the configuration of a process for manufacturing reduced iron using a conventional shaft-type reduction furnace. In FIG. 1, reference numeral 1 is a reduction furnace, 1a is iron oxide, 1b is reduced iron, 2 is furnace gas discharged from the reduction furnace (1), 3 is a dust removal device for furnace gas (2), 4 is a dehydration device, 5 is natural gas supplied from the outside, 6 is air, 7 is a heat reforming device that heats and reforms the remainder of the furnace gas (2) mixed with natural gas using heat obtained by mixing and burning a portion of the furnace gas (2) with air (6) to produce a reducing gas (8) containing carbon monoxide gas and hydrogen gas, and 9 is a reducing gas injection device that supplies the reducing gas to the reduction furnace (1).

[0030] In the reduction furnace (1), which is the center of the reduced iron manufacturing process, bulk iron oxide raw material (1a), such as sintered ore or pellets, is charged from the top and slowly lowered. Then, by blowing high-temperature reducing gas (8) from the middle of the furnace, the iron oxide raw material (1a) is reduced, and reduced iron (1b) is discharged from the bottom of the furnace. At this time, furnace gas (2), mainly composed of CO, CO2, H2, and H2O, is discharged from the top of the furnace. This furnace gas (2) is cleaned by a dust removal device (3), and some of it is moisture-adjusted as raw material gas and sent to a heating and reforming device (7). In the heating and reforming device (7), a gas containing hydrocarbons, such as natural gas (5), is supplied along with the aforementioned moisture-adjusted furnace gas (2) and heated. In the heating reforming device (7), a reforming reaction occurs, and a high-temperature reducing gas (8), mainly composed of CO and H2 gas, is produced and blown into the reduction furnace (1). In addition, the remaining portion of the furnace gas (2) is used as a heating fuel together with air (6) in the combustion chamber of the heating reforming device (7) after dehydration. The path for supplying and discharging the heating fuel used for heating reforming is indicated by a dotted line in FIG. 1.

[0031] [First form]

[0032] Regarding the conventional process shown in FIG. 1 above, in the present invention, as shown in FIG. 2, instead of hydrocarbon gas supplied from the outside, such as natural gas (5), regenerated methane produced in a methane synthesis reactor within the process is used. That is, in the configuration of the reduced iron manufacturing process shown in FIG. 2, a methane synthesis device (11) is newly installed to synthesize a gas with methane as the main component from a portion of furnace gas (2) and hydrogen (10). The regenerated methane produced in this methane synthesis device (11) is supplied to a heat reforming device (7) together with the remainder of the furnace gas to be used as a raw material gas for the reduced gas (8).

[0033] In addition, in the present invention, the "gas having methane as the main component" synthesized in the methane synthesis process may include, in addition to methane, for example, water vapor (H2O), hydrogen (H2), nitrogen (N2), CO, CO2, hydrocarbons, etc.

[0034] As raw materials for methane synthesis in the methane synthesis apparatus (11), hydrogen gas (10) supplied from the outside and a gas containing at least one selected from CO and CO2 are used. Any gas available within the steel mill can be used as the gas containing at least one selected from CO and CO2, but for example, as shown in FIG. 2, exhaust gas obtained by mixing and burning a portion of the furnace gas (2) with a supporting gas in the combustion chamber of the heating reforming apparatus (7) is used. Furthermore, when burning the furnace gas (2) in the combustion chamber of the heating reforming apparatus (7), it is preferable to use oxygen gas (12) rather than air as the supporting gas so that nitrogen is not mixed in. Here, the oxygen gas (12) used as the supporting gas does not necessarily have to be pure oxygen with an oxygen concentration of 100%, and may contain some gases other than oxygen, such as nitrogen, carbon dioxide, argon, etc. However, if the oxygen concentration is excessively low, the gas volume increases, and since a problem arises where the heating reforming device (7) or the reactor for methane synthesis must be enlarged, the oxygen concentration is preferably 80% or higher. The path for supplying the heating fuel used for heating reforming and using the exhaust gas as a raw material for methane synthesis is shown by a dotted line in FIG. 2.

[0035] In addition, the above hydrogen gas (10) and oxygen gas (12) can, in principle, have zero CO2 emissions if CO2-free power is used in their production. Here, as CO2-free power, it is preferable to use, for example, power generated by solar power generation or nuclear power generation.

[0036] In the operation shown in FIG. 2 above, carbon in the circulation system gradually decreases due to carburization of the reduced iron, leakage of gas from the furnace body or piping constituting the process, and emission of gas within the system during abnormal operation. Therefore, it is necessary to compensate for the decrease in carbon. Accordingly, in the present invention, stable production of reduced iron in the circulation system is realized by operating while replenishing the decrease in carbon at the connection point between any one or two or more processes among the reducing gas injection process, the reduction process, the methane synthesis process, and the gas reforming process, or between any two or more processes among the reducing gas injection process, the reduction process, the methane synthesis process, and the gas reforming process. That is, in the closed circulation system of the present invention, since the decrease in carbon is, for example, only about 30 kg per ton of reduced iron, carbon compensation is possible without significantly altering the existing equipment. Accordingly, carbon conservation can be performed in one or more of the above-mentioned reduction gas injection process, the above-mentioned reduction process, the above-mentioned methane synthesis process, and the above-mentioned gas reforming process, or in a connecting part connecting two or more of the above-mentioned processes.

[0037] In the reduced iron manufacturing process shown in FIG. 2, for example, carbon can be replenished at one or more of the maintenance points (1) to (3) shown in FIG. 2. Maintenance point (1) is a connection point between a dehydration device (4) and a heat reforming device (7) on a path extending from a methane synthesis device (11), maintenance point (2) is a connection point between a reduction furnace (1) and a dust removal device (3) on a path extending from a reduction furnace (1), and maintenance point (3) is a connection point between a dehydration device (4) and a heat reforming device (7) on a path extending from a reduction furnace (1). Maintenance point (1) can be described as a connection point between a methane synthesis process and a gas reforming process, maintenance point (2) as a connection point between a reduction process and a methane synthesis process and a connection point between a reduction process and a gas reforming process, and maintenance point (3) as a connection point between a reduction process and a methane synthesis process.

[0038] Here, regarding carbon conservation, since a carbon shortage within the system, as mentioned in the tactical example, increases the likelihood of operational instability, it is preferable to conserve carbon continuously rather than replenish it intermittently. In other words, it is desirable to initiate carbon conservation as soon as a carbon shortage within the system is identified and to continue conserving it continuously until the shortage is resolved.

[0039] At that time, the method for determining the amount of carbon deficiency is not particularly limited, but methods such as calculating it from the flow rate and composition of the reducing gas and furnace gas blown into the reduction furnace, or calculating it from the carbon content of the reduced iron product and the amount of reduced iron produced, are considered. Considering the speed of measurement, the former method of calculating it from the flow rate and composition of the reducing gas and furnace gas is preferable. The method of calculating it from the flow rate and composition of the reducing gas and furnace gas is a method of calculating the amount of carbon atoms contained in each gas from the measurement results of the flow rate and composition of the reducing gas and furnace gas, and taking the difference as the amount of carbon emitted from the system, i.e., the amount of carbon deficiency. Furthermore, the method of calculating it from the carbon content and production of the reduced iron product is a method of measuring the carbon content and production of the reduced iron product and taking the amount of carbon emitted as carbon in the reduced iron (carburization amount) calculated therefrom as the amount of carbon deficiency.

[0040] Meanwhile, fluctuations in the amount of carbon in the system due to carbon conservation can be identified, for example, by determining the amount of carbon contained in the reducing gas from the flow rate and composition of the reducing gas and monitoring its trend over time. If the amount of carbon in the system shows an increasing trend, the amount of carbon being conserved is in excess, so the amount of carbon being conserved is reduced. If the amount of carbon in the system shows a decreasing trend, the amount of carbon being conserved is insufficient, so the amount of carbon being conserved is increased. As described above, by monitoring fluctuations in the amount of carbon in the system and adjusting the amount of carbon being conserved according to such fluctuations, it is possible to continue operations while conserving carbon in the system without excess or shortage.

[0041] When varying the amount of carbon maintained according to fluctuations in the amount of carbon within the system, it is desirable to set the standard amount of carbon within the system to a condition where the reduction rate of the product reduced iron (product reduction rate) is greater than or equal to a reference value. At that time, by setting the reference value of the reduction rate to 90% or higher, the energy required in the subsequent process of melting DRI (direct reduced iron) can be minimized. More preferably, by setting the reference value of the reduction rate to 93% or higher, the energy required in the subsequent process can be further reduced.

[0042] In addition, as a method for replenishing carbon, it is acceptable to introduce a gaseous carbon source, such as natural gas, directly into the system, or to combust a gaseous, liquid, or solid carbon source and introduce the combustion gas. When introducing combustion gas, it is desirable to utilize the combustion heat of the carbon source as a heat source required in the system, for example, a heat source for a heating reforming device (7), in order to improve energy efficiency. However, when introducing the combustion gas of the carbon source into the system, it is desirable to remove the combustion gas before introducing it into the system, as there is a possibility that the combustion gas may contain unburned solids and liquids, such as soot and tar. When removing the combustion gas, a new dust collector may be installed for the carbon source combustion gas, but it is more suitable to effectively utilize existing equipment, such as by using a dust collector for furnace gas in combination.

[0043] In particular, using waste plastics as a solid carbon source allows for the effective utilization of carbon sources that would otherwise have been disposed of through landfill or incineration, thereby reducing the environmental burden of the process. The same applies when using biomass, which is a carbon-neutral carbon source, as a solid carbon source. For the reasons above, using waste plastics or biomass as carbon sources is more suitable for embodiments of the present invention than using fossil fuels such as natural gas, coal, or petroleum.

[0044] In conventional reduced iron manufacturing processes, it has been proposed to supply a carbon source for purposes different from carbon conservation in the circulation (reuse) system of the present invention, such as to increase the carbon content of the reduced iron. For example, Japanese Patent Publication No. Sho 49-129616 describes a method of carbonizing reduced iron while cooling it by injecting LPG from below the reduction gas injection position of the reduction furnace. In the case of supplying a carbon source from the bottom of the reduction furnace as proposed in this publication, the supply location of the carbon source is outside the carbon circulation / reuse system. That is, the carbon supplied from the bottom of the reduction furnace carburizes within the reduced iron before being supplied into the circulation / reuse system, and since it cannot compensate for the shortage within the circulation / reuse system, this technology could not be applied to the present invention.

[0045] [2nd form]

[0046] A second embodiment of the present invention is shown in FIG. 3. This second embodiment differs in that, among the furnace gas (2) generated from the reduction furnace (1), the amount used for heating the heat reforming device (7) in the first embodiment (Fig. 2) is introduced into a methane synthesis device (11) that performs methane synthesis and is used as a raw material for regenerated methane. By changing the flow of furnace gas (2) within this system, the amount of regenerated methane required for the heat reforming device (7) can be synthesized. At this time, since fuel for heating the heat reforming device (7) is not supplied, it is preferable to use an external CO2-free heat source, for example, a heat source (13) using CO2-free electricity. The heat supply path from the heat source (13) is indicated by a dotted line in FIG. 3. In this method, if CO2-free electricity is used for heating the heat reforming device (7) or for hydrogen production, CO2 emissions can be made zero in principle.

[0047] In this second form as well, since it is necessary to preserve the reduction of carbon from within the system, stable production of reduced iron within the circulation system can be realized by preserving the carbon source in the same way as in the first form. Furthermore, in this second form, by using the preserved carbon source as a heating fuel for the heating reforming device (7), the amount of CO2-free heat source used can be reduced. Also, as in the first form, by using carbon sources such as waste plastic or biomass for preservation, the environmental burden of the process can be reduced.

[0048] In the reduced iron manufacturing process shown in FIG. 3, for example, carbon can be replenished at one or more of the maintenance locations (4) to (6) shown in FIG. 3. Maintenance location (4) is a connection part between the methane synthesis device (11) and the dehydration device (4) on a path extending from the methane synthesis device (11), maintenance location (5) is a connection part between the reduction furnace (1) and the dust removal device (3) on a path extending from the reduction furnace (1) (the same location as (2) in FIG. 2), and maintenance location (6) is a connection part between the dust removal device (3) and the dehydration device (4) on a path extending from the reduction furnace (1). The maintenance section (4) can be described as a connection section between the methane synthesis process and the gas reforming process, the maintenance section (5) as a connection section between the reduction process and the methane synthesis process and a connection section between the reduction process and the gas reforming process, and the maintenance section (6) as a connection section between the reduction process and the methane synthesis process.

[0049] In the above, carbon retention sites (1) to (6) in the form shown in FIGS. 2 and FIGS. 3 have been exemplified, but carbon retention sites are not limited to these and any process and connection part between processes, excluding the iron oxide filling process, can be targeted. However, it is preferable to exclude from carbon retention sites the part from the outlet side of the heating reforming device (7) to the inlet side of the reduction furnace (1), that is, the reduction gas injection process performed by the reduction gas injection device (9), the reduction process performed in the reduction furnace (1), the connection part between the heating reforming process and the reduction gas injection process, and the connection part between the reduction gas injection process and the reduction process. This is because, while maintaining the composition of the reduction gas constant to suppress fluctuations in the iron oxide reduction reaction behavior within the reduction furnace is indispensable for stable operation of the reduction furnace, carbon retention in the part from the outlet side of the heating reforming device (7) to the inlet side of the reduction furnace (1) can cause fluctuations in the composition of the reduction gas injected into the reduction furnace.

[0050] In addition, when using the combustion gas of a liquid or solid carbon source as a carbon source, it is more desirable to make the part in front of the dust collector on the outlet side of the reduction furnace (e.g., (2) in FIG. 2, (5) in FIG. 3)) the carbon storage location, as this eliminates the need to install a new dust removal facility. Alternatively, it is more effective to make the heating part of the heating reforming device, or the part upstream thereof (e.g., (3) in FIG. 2) the carbon storage location, and utilize the sensible heat of the combustion gas of the carbon source as the heat required for the reforming reaction.

[0051] In addition, in the configuration of FIGS. 2 and FIGS. 3 of the present invention, if a small amount of unnecessary gas other than CO, CO2, H2, H2O, and hydrocarbons, such as nitrogen gas for purging, is mixed in, the unnecessary gas gradually accumulates within the process, causing a problem in which the concentration of reducing gas decreases. Therefore, it is preferable to periodically monitor the nitrogen concentration within the system, and when the nitrogen concentration rises to a certain level—for example, when the nitrogen concentration becomes 20% or higher—it is preferable to perform an operation to temporarily discharge the furnace gas or combustion exhaust gas introduced into the methane synthesis reactor to the outside of the system. At this time, since regenerated methane cannot be produced, it is preferable to perform an operation to temporarily inject natural gas, etc., instead of regenerated methane.

[0052] In addition, regarding the H2 supplied from an external source as a raw material for methane synthesis, a method that minimizes CO2 generation is desirable. For example, the electrolysis of water is suitable. In this case, the H2 gas does not necessarily have to be H2 gas with a concentration of 100%, but a higher H2 concentration is preferable to maintain a high methane concentration in the generated regenerated methane gas. Preferably, the H2 concentration is 80 vol% or higher.

[0053] Examples

[0054] The following describes embodiments of the present invention. Additionally, the operating specifications are described here as the consumption rate per ton of reduced iron (DRI). For example, when 1,300 kg of sintered ore is used per ton of reduced iron (DRI), the amount of sintered ore used is expressed as 1,300 kg / t. When considering a reduced iron plant with a capacity of 3,000 ton / day, multiplying the following by 3,000 yields the specifications per day. Here, a value of 90% or more of the product reduction rate was set as a standard for maintaining normal operation and from the perspective of the melting and smelting efficiency of DRI in subsequent processes. The product reduction rate is an indicator expressed as a percentage of the ratio of the atomic weight of oxygen derived from iron oxide (mass%) contained in the product to the atomic weight of oxygen derived from Fe2O3 (mass%) assuming that all of the total iron content (T.Fe) in the product is Fe2O3. Representative specifications among the operating conditions and operating results of Invention Examples 1 to 6 and Comparative Examples are shown in Table 1.

[0055]

[0056] Invention Examples 1 to 3 according to the present invention are cases of manufacturing reduced iron according to the reduced iron manufacturing process shown in FIG. 2. That is, 1342 kg / t of sintered ore is charged as raw material from the top of the reduction furnace (1). From the middle of the furnace, 2200 Nm³ of high-temperature reducing gas heated to 800°C is introduced. 3 It is assumed that / t (H2: 62%, CO: 38%) is injected. At this time, 2181 Nm from the top of the reduction furnace 3 A furnace gas of / t (H2: 48%, CO: 29%, CO2: 9%, H2O: 14%) is discharged. Here, the reason the volume of the discharged furnace gas is smaller than the volume of the reduction gas being blown in is that carbon is carburized into the reduced iron inside the reduction furnace.

[0057] After the furnace gas is cleaned, a portion is used as raw gas (indicated as “(raw gas)” in Table 1), and the remainder is used as heating fuel gas for the heating reforming device (7) (indicated as “(amount used for regenerative methane reaction after combustion in the heating furnace)” in Table 1). After dehydration, the heating fuel gas is combusted in the combustion chamber of the heating reforming device (7) using pure oxygen generated by a cryogenic separation process driven by CO2-free power. The exhaust gas from the combustion chamber of the heating reforming device (7) is entirely recovered and dehydrated, and the combustion exhaust gas (CO2: 100%) after dehydration is sent to the methane synthesis device (11) for methane synthesis. Here, regenerative methane gas is synthesized by adding hydrogen generated by electrolysis using CO2-free power (indicated as “hydrogen for regenerative methane” in Table 1). The synthesized regenerated methane gas, after dehydration, is introduced into a heating reforming device (7) together with the raw gas and used as a raw material for reducing gas. In this process, as described above, if there is a shortage of carbon in the system, it is necessary to supply a carbon source.

[0058] [Invention Example 1]

[0059] In Invention Example 1, natural gas was selected as the carbon source, and natural gas was supplied directly into the system from the location of the maintenance point (1) shown in FIG. 2. As a result of calculating from the carbon content in the product DRI and the carbon content in the furnace gas dust, the required supply amount of natural gas was 13.3 kg / t. Under these conditions, operating conditions were examined to inject high-temperature reducing gas having the above temperature and composition into the reduction furnace, and 1576 Nm³ in the furnace gas 3 / t as source gas, the remainder 605Nm 3 / t was used as the heating fuel gas for the heating reforming device (7). The raw gas was partially dehydrated to balance the hydrogen and oxygen atomic balance of the entire system. After dehydration, the heating fuel gas was combusted with pure oxygen and used in the heating reforming device (7). The exhaust gas after combustion was fully recovered and, after dehydration, injected into the methane synthesis device (11). 918 Nm inside the methane synthesis device (11) 3 A gas with methane as the main component was synthesized by reacting it with hydrogen at a rate of / t, and after dehydrating the synthesis gas, it was introduced into a heating reforming device (7) along with the raw gas and natural gas as a carbon source, and used as a raw material for reducing gas. It was confirmed that the reduced iron produced by the above method could continue normal operation by the method of Invention Example 1, as it achieved a product reduction rate of 92.9%.

[0060] [Invention Example 2]

[0061] In Invention Example 2, biomass made of wood was selected as the carbon source, and the combustion gas of the biomass was supplied directly into the system from the location of the preservation site (2) shown in FIG. 2. At the location of the preservation site (2) shown in FIG. 2, a dust collector for furnace gas can be utilized as a dust collector for the combustion gas of the biomass. As a result of calculations based on the carbon content in the product DRI, the carbon content in the furnace gas dust, and the carbon content in the biomass, the required supply amount of biomass was 11.7 kg / t. Under these conditions, operating conditions were examined to inject high-temperature reducing gas having the above temperature and composition into the reduction furnace, and 1522 Nm³ of the gas obtained by adding the combustion gas of the biomass to the furnace gas (shown as "flow rate before branching" in Table 1) 3 / t as source gas, the remainder 680Nm 3 / t was used as the heating fuel gas for the heating reforming device (7). The raw gas was partially dehydrated to balance the hydrogen and oxygen atomic balance of the entire system. After dehydration, the heating fuel gas was combusted with pure oxygen and used in the heating reforming device (7). The exhaust gas after combustion was fully recovered and, after dehydration, injected into the methane synthesis device (11). The exhaust gas was 1046 Nm³ within the methane synthesis device (11). 3 A gas with methane as the main component was synthesized by reacting it with hydrogen at a rate of / t, and the synthesized gas was dehydrated and then fed into a heating reforming device (7) together with the raw gas and used as a raw material for reducing gas. It was confirmed that the reduced iron produced by the above method could continue normal operation by the method of Invention Example 2, as it achieved a product reduction rate of 93.1%.

[0062] [Invention Example 3]

[0063] In Invention Example 3, waste plastics mainly composed of urethane were selected as the carbon source, and combustion gas of the waste plastics was supplied into the system after dust removal from the location of the maintenance point (3) shown in FIG. 2. As a result of calculating from the carbon content in the product DRI, the carbon content in the furnace top gas dust, and the carbon content in the waste plastics, the required supply amount of waste plastics was 15.7 kg / t. Under these conditions, operating conditions were examined to inject high-temperature reducing gas having the above temperature and composition into the reduction furnace, and 1576 Nm³ in the furnace top gas 3 / t as source gas, the remainder 605Nm 3 / t was used as the heating fuel gas for the heating reforming device (7). The raw material gas was partially dehydrated to balance the hydrogen and oxygen atomic balance of the entire system. After dehydration, the heating fuel gas was mixed with the combustion gas of waste plastic and combusted with pure oxygen for use in the heating reforming device (7). The exhaust gas after combustion was fully recovered and, after dehydration, injected into the methane synthesis device (11). The exhaust gas was 993 Nm³ within the methane synthesis device (11).3 A gas with methane as the main component was synthesized by reacting it with hydrogen at a rate of / t, and the synthesized gas was dehydrated and then fed into a heating reforming device (7) together with the raw gas and used as a raw material for reducing gas. It was confirmed that the reduced iron produced by the above method could continue normal operation by the method of Invention Example 3, as it achieved a product reduction rate of 93.1%.

[0064] As described above, according to the reduced iron manufacturing methods of Invention Examples 1 to 3, it is possible to achieve the target product reduction rate while operating with zero CO2 emissions.

[0065] Next, Invention Examples 4 to 6 according to the present invention are cases of manufacturing reduced iron according to the reduced iron manufacturing process shown in FIG. 3. That is, the conditions of the sintered ore charged into the reduction furnace (1), the reducing gas blown into the reduction furnace (1), the furnace top gas discharged from the furnace top, and the raw material gas used as a reforming raw material are the same as in Invention Example 1. The furnace top gas remaining after extracting the raw material gas is dehydrated and then sent to a methane synthesis device (11) as a raw material for regenerative methane synthesis (indicated as “(amount used for regenerative methane reaction)” in Table 1). Here, regenerative methane gas is synthesized by adding hydrogen generated by electrolysis using CO2-free power. The synthesized regenerative methane gas is introduced into a heating reforming device (7) and used as a raw material for the reducing gas. Here, in Invention Examples 4 to 6, since the heating fuel for the heating reforming device (7) is not supplied, CO2-free power is supplied from the outside instead to perform electric heating. In this process as well, similar to Invention Examples 1 to 3, if there is a shortage of carbon in the system, it is necessary to supply a carbon source.

[0066] [Invention Example 4]

[0067] In Invention Example 4, natural gas was selected as the carbon source, and natural gas was supplied directly into the system from the location of the maintenance point (4) shown in FIG. 3. As with Invention Example 1, the required supply amount of natural gas was 13.3 kg / t. Under these conditions, operating conditions were examined to inject high-temperature reducing gas having the above temperature and composition into the reduction furnace, and 1576 Nm³ in the furnace gas 3 / t was used as the source gas. The source gas was partially dehydrated to balance the hydrogen and oxygen atomic balance of the entire system. The remaining 605 Nm 3 / t was used as the raw gas for the regenerative methane reactor, and after dehydration, it was blown into the methane synthesis device (11). 455 Nm inside the methane synthesis device (11) 3 A gas with methane as the main component was synthesized by reacting it with hydrogen at a rate of / t, and natural gas was mixed with the synthesis gas as a carbon source. After dehydration, the mixture was fed into a heating reforming device (7) along with the raw gas and used as a raw material for reducing gas. The reduced iron produced by the above method achieved a product reduction rate of 93.3%, which confirmed that normal operation could be continued using the method of Invention Example 4.

[0068] [Invention Example 5]

[0069] In Invention Example 5, biomass made of wood was selected as the carbon source, and the combustion gas of the biomass was supplied directly into the system from the location of the preservation site (5) shown in FIG. 3. At the location of the preservation site (5) shown in FIG. 3, a dust collector for furnace gas can be utilized as a dust collector for the combustion gas of the biomass. As with Invention Example 2, the required supply amount of biomass was 11.7 kg / t. Under these conditions, operating conditions were examined to inject high-temperature reducing gas having the above temperature and composition into the reduction furnace, and 1522 Nm³ of the gas obtained by adding the combustion gas of the biomass to the furnace gas (Table 1 "Flow rate before branching") 3 / t was used as the source gas. The source gas was partially dehydrated to balance the hydrogen and oxygen atomic balance of the entire system. The remaining 680 Nm 3 / t was used as the raw gas for the regenerative methane reactor, and after dehydration, it was blown into the methane synthesis device (11). 529 Nm in the methane synthesis device (11) 3 A gas with methane as the main component was synthesized by reacting it with hydrogen at a rate of / t, and the synthesized gas was dehydrated and then fed into a heating reforming device (7) together with the raw gas to be used as a raw material for reducing gas. It was confirmed that the reduced iron produced by the above method could continue normal operation by the method of Invention Example 5, as it achieved a product reduction rate of 92.7%.

[0070] [Invention Example 6]

[0071] In Invention Example 6, waste plastic mainly composed of urethane was selected as the carbon source, and combustion gas of the waste plastic was supplied into the system after dust removal from the location of the maintenance point (6) shown in FIG. 3. As with Invention Example 3, the required supply amount of waste plastic was 15.7 kg / t. Under these conditions, operating conditions were examined to blow high-temperature reducing gas having the above temperature and composition into the reduction furnace, and 1576 Nm³ in the furnace gas 3 / t was used as the source gas. The source gas was partially dehydrated to balance the hydrogen and oxygen atomic balance of the entire system. The remaining 605 Nm 3 A mixed gas of / t and combustion gas of waste plastic was used as the raw material gas for a regenerated methane reactor, and after dehydration, was blown into a methane synthesis device (11). 529 Nm inside the methane synthesis device (11) 3A gas with methane as the main component was synthesized by reacting it with hydrogen at a rate of / t, and combustion gas of waste plastic was mixed with the synthesis gas as a carbon source. After dehydration, the gas was introduced into a heating reforming device (7) together with the raw material gas and used as a raw material for reducing gas. It was confirmed that the reduced iron produced by the above method achieved a product reduction rate of 93.2%, which allowed for continued normal operation using the method of Invention Example 6.

[0072] As described above, according to the methods for producing reduced iron in Invention Examples 4 to 6, it is possible to achieve the target product reduction rate while operating with zero CO2 emissions. In addition, although the energy source for the heating furnace in Invention Examples 4 to 6 was electricity, the energy source does not have to be electricity as long as it is CO2-free. For example, it is acceptable to heat using the combustion heat of hydrogen generated by electrolysis using CO2-free electricity.

[0073] [Comparative Example]

[0074] A comparative example of the present invention is described. In the reduced iron manufacturing process shown in FIG. 2, the conditions of the sintered ore charged into the reduction furnace, the reducing gas blown into the reduction furnace, the furnace gas discharged from the furnace, and the raw material gas used as a reforming raw material were started under the same conditions as in Invention Example 1. The difference from Invention Example 1 is that the operation was continued without supplying the carbon source originally required. As a result, at the beginning of the operation, the composition of the reducing gas was H2:CO = 62:38, but carbon atoms were gradually discharged from the system, and eventually, the operation had to be continued under the condition of H2:CO = 100:0. In the operation where all of this reducing gas became hydrogen, CO2, which is the raw material for methane, is not generated, and thus, since methane reforming is unnecessary, the methane synthesis device (11) and the heat reforming device (7) became unnecessary.

[0075] Specifically, as shown in Table 1, the furnace top gas of the reduction furnace is 2200 Nm 31821 Nm in / t(H2: 80%, H2O: 20%) 3 / t is source gas, 379Nm 3 By using / t as the combustion gas of the heating furnace and supplying hydrogen generated by the electrolysis of water by CO2-free power to the raw material gas, the total of 742 Nm³ is the hydrogen consumption due to the reduction of raw material sintered ore in the reduction furnace and the hydrogen consumption as a heat source for the heating reforming device (7). 3 Operation was carried out while compensating for hydrogen consumption of / t. As a result, the iron oxide reduction reaction in the reduction furnace became an endothermic reaction, and the amount of heat supplied to the reduction furnace was insufficient compared to the amount of heat required to sufficiently reduce the iron oxide, so the product reduction rate of the product DRI dropped to 85.4%. Since this value was below the standard product reduction rate of 90% for continuing normal operation, operation in the method of the comparative example had to be stopped.

[0076] As described above, it was confirmed that in the reduced iron manufacturing method of the comparative example, although the CO2 emission is 0, the target product reduction rate cannot be achieved, making it difficult to continue normal operation. Explanation of the symbols

[0077] 1 : Reduction furnace 1a : Iron oxide 1b : Reduced iron 2 : Rojeong Gas 3: Vibration damping device 4: Dehydration device 5 : Natural gas 6 : Air 7: Heating reformer 8: Reducing gas 9: Reducing gas injection device 10: Hydrogen 11: Methane Synthesis Unit 12: Oxygen gas 13 : Heat source

Claims

Claim 1 A method for producing reduced iron in a circulating system in which an iron oxide filling process for filling a furnace with iron oxide, a reducing gas injection process for injecting a reducing gas into the furnace, a reduction process for reducing the iron oxide by the reducing gas within the furnace, a methane synthesis process for synthesizing a gas mainly composed of methane from a portion of the furnace top gas discharged from the furnace top of the furnace and hydrogen gas, and a gas reforming process for using the methane and the remainder of the furnace top gas as raw materials and heating said raw materials to reform them into the reducing gas are interconnected, wherein the operation is continued while supplementing the carbon deficient during the operation in the circulating system at a connection point between any one or two or more of the reducing gas injection process, the reduction process, the methane synthesis process, and the gas reforming process, or between any two or more of the reducing gas injection process, the reduction process, the methane synthesis process, and the gas reforming process, and nitrogen within the circulating system A method for producing reduced iron, wherein when the concentration rises above a certain level, a portion of the furnace gas supplied to the methane synthesis process is discharged outside the circulation system, and natural gas is supplied instead of the synthesized methane to perform the gas reforming process. Claim 2 A method for producing reduced iron according to claim 1, wherein carbon is preserved in each connection portion of the three processes, which include either or both of the methane synthesis process and the gas reforming process, or the reduction process added to both of the processes. Claim 3 A method for producing reduced iron, wherein, in paragraph 1 or 2, the carbon is combustion gas of waste plastic. Claim 4 A method for producing reduced iron according to claim 1 or 2, wherein the carbon is a combustion gas of biomass.

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