Method for producing reduced iron

Abstract

This method for producing reduced iron includes: an iron oxide starting material preheating step for preheating an iron oxide starting material to a predetermined preheating temperature; an iron oxide charging step for charging the preheated iron oxide starting material into a shaft furnace; a partial combustion step for generating a reducing gas by partially combusting a starting material gas with an oxygen gas; and a blowing step for blowing the reducing gas into the shaft furnace into which the preheated iron oxide starting material has been charged.

Description

DESCRIPTIONTITLE OF INVENTION: METHOD OF PRODUCING REDUCED IRON TECHNICAL FIELD

[0001] The present invention relates to a method of producing reduced iron.Priority is claimed on Japanese Patent Application No. 2024-005112, filed January 17, 2024, the content of which is incorporated herein by reference.BACKGROUND ART

[0002] A method of producing reduced iron using a shaft furnace (shaft furnace operation) is a representative of a direct reduction process of producing reduced iron from an iron oxide raw material, and is widespread mainly in regions (oil-producing countries) where natural gas is available at low cost. Here, an outline of the method of producing reduced iron using a shaft furnace will be described.First, an iron oxide raw material (for example, iron oxide pellets) is charged from an upper part of the shaft furnace, and a reducing gas is blown from a lower part of the shaft furnace into which the iron oxide raw material has been charged. Here, the reducing gas is heated to a predetermined temperature (for example, about 900°C to 950°C) and then blown into the shaft furnace. The reducing gas blown into the shaft furnace reduces the iron oxide raw material in the shaft furnace. Reduced iron is produced by such a direct reduction process. The reduced iron is discharged from the lower part of the shaft furnace and cooled. From a furnace top of the shaft furnace, a top gas (exhaust gas) including hydrogen gas, CO gas, water vapor, and CO2 gas is discharged. After water vapor is removed from the top gas, hydrogen gas or CO gas in the top gas is reused as a part of a feed gas. In addition, in some cases, the CO2 gas is removed after the water vapor is removed from the top gas.

[0003] The reducing gas used in the shaft furnace is obtained by reforming a feed gas (for example, natural gas, coke oven gas, or the like) containing carbon using water vapor, CO2 gas, oxygen gas, or the like. Alternatively, the feed gas is not reformed, the feed gas is used as it is as the reducing gas. Main components of the reducing gas are hydrogen gas (H2), CO gas (CO), and CH4 gas.

[0004] By the way, in recent years, in order to further reduce the amount of CO2 gas emitted, a technique for increasing a concentration of hydrogen gas in a reducing gas has been studied. For example, Patent Document 1 discloses a technique using a reducing gas in which a concentration of hydrogen gas is 70% by volume or more. In the technique disclosed in Patent Document 1, the reducing gas is heated by heat exchange with exhaust gas, heating in a heating furnace, and partial combustion of the reducing gas using oxygen gas.Citation ListPatent Document

[0005] Patent Document 1: PCT International Publication No. WO2022 / 169392SUMMARY OF INVENTIONTechnical Problem

[0006] Incidentally, water (water vapor) is generated in a case where hydrogen gas is partially combusted. Water vapor is discharged as top gas. The present inventor has found that a reduction potential of the furnace top is lowered by an increase in water vapor concentration of the top gas, and thus reduction disintegration of the iron oxide raw material is likely to occur. When the reduction disintegration of the iron oxide raw material occurs, the flow of the iron oxide raw material in the shaft furnace is hindered, resulting in an unstable operation.

[0007] Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a novel and improved method of producing reduced iron, in which it is possible to suppress reduction disintegration of an iron oxide raw material even in a case where a feed gas containing hydrogen gas is partially combusted.Solution to problem

[0008] The gist of the present invention is as follows.[1] A method of producing reduced iron, including: preheating an iron oxide raw material to a predetermined preheating temperature; charging the preheated iron oxide raw material into a shaft furnace; generating a reducing gas by partially combusting a feed gas containing hydrogen gas with oxygen gas; and blowing the reducing gas into the shaft furnace in a state in which the preheated iron oxide raw material is charged.[2] The method of producing reduced iron according to [1], in which X, which represents a water vapor concentration of a top gas of the shaft furnace in % by volume, and Y, which is the predetermined preheating temperature, satisfy Expression (5),Y > 10X + 300 (X ≥ 30) (5).[3] The method of producing reduced iron according to [1], in which X, which represents a water vapor concentration of a top gas of the shaft furnace in % by volume, and Y, which is the predetermined preheating temperature, satisfy Expression (1),Y ≥ 10X + 400 (X ≥ 20) (1).[4] The method of producing reduced iron according to any one of [1] to [3], in which the feed gas contains hydrogen gas as a main component.Advantageous Effects of Invention

[0009] According to the present invention, it is possible to provide a novel and improved method of producing reduced iron, in which it is possible to suppress reduction disintegration of an iron oxide raw material even in a case where a feed gas containing hydrogen gas is partially combusted.BRIEF DESCRIPTION OF DRAWINGS

[0010] [FIG. 1] A schematic view showing a configuration of a test apparatus that simulates an apparatus for producing reduced iron.[FIG. 2] A graph showing an operation stable region (acceptable region) A1, an operation unstable region (unacceptable region) B1, and a boundary line A between the two regions.[FIG. 3] A schematic view showing an apparatus for producing reduced iron according to the present embodiment.DESCRIPTION OF EMBODIMENTS

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0012] <1. Findings by Present Inventor>First, the knowledge of the present inventor, which is the basis of the present embodiment, will be described. The present inventor assumed that preheating of an iron oxide raw material is effective for suppressing reduction disintegration of the iron oxide raw material, and conducted various experimental studies on a correlation between a water vapor concentration of a top gas and a preheating temperature of the iron oxide raw material. Based on the result, the required preheating temperature of the iron oxide raw material according to the water vapor concentration of the top gas was specified, and a method of producing reduced iron according to the present embodiment was completed. According to the method of producing reduced iron according to the present embodiment, it is possible to suppress reduction disintegration by adjusting the preheating temperature of the iron oxide raw material depending on the water vapor concentration of the top gas. As a result, a stable operation is achieved.

[0013] (1-1. Test Apparatus)FIG. 1 is a schematic view showing a configuration of a test apparatus 100 that simulates an apparatus for producing reduced iron. The present inventor conducted a test using the test apparatus 100. The test apparatus 100 includes a small shaft furnace 110, a feed gas supply device 120, a heating furnace 130, a raw material hopper 140, a discharge feeder 150, a reduced iron hopper 160, and a gas analysis device 170. Details of the test apparatus 100 are described in a non-patent document (Mizutani et al.: CAMP-ISIJ, 33 (2020), 483 “Development of an adiabatic countercurrent moving-bed shaft furnace simulator”).

[0014] The small shaft furnace 110 has the same function as a shaft furnace in the related art. That is, an iron oxide raw material is charged from above the small shaft furnace 110. Subsequently, a reducing gas is blown into the small shaft furnace 110 from a lower side surface of the small shaft furnace 110 in which the iron oxide raw material is charged. The reducing gas blown into the small shaft furnace 110 rises in the small shaft furnace 110. The reducing gas reduces the iron oxide raw material in the small shaft furnace 110 to generate direct reduced iron (DRI). The reduced iron is discharged from a lower side of the small shaft furnace 110 and cooled. In addition, a top gas (exhaust gas) is discharged from a furnace top of the small shaft furnace 110. The top gas includes unreacted hydrogen gas as well as water vapor, dust, and the like. The small shaft furnace 110 has an inner diameter of 0.1 m and a height of 4.0 m. The small shaft furnace 110 is provided with a plurality of heaters and thermocouples along a height direction thereof, allowing temperature adjustment and temperature measurement at each portion in the height direction.

[0015] The feed gas supply device 120 is a device capable of supplying various feed gases (hydrogen gas, CO gas, CO2 gas, CH4 gas, N2 gas, and the like) to the heating furnace 130 via the water vapor supply device 125. In the present test, hydrogen gas (100% by volume of hydrogen gas) was used as a feed gas.

[0016] In addition to the feed gas, water vapor is introduced into the heating furnace 130 from the water vapor supply device 125 to simulate oxygen combustion. In this test, a temperature of the reducing gas including the feed gas and the water vapor was set to 900°C. The heating furnace 130 and the small shaft furnace 110 are connected by a pipe, and the reducing gas is blown into the small shaft furnace 110 through the pipe. Here, the temperature of the reducing gas is measured by, for example, a thermocouple. The thermocouple is provided at a connecting portion between the above-described pipe and the small shaft furnace 110, and the temperature of the reducing gas is measured using the thermocouple.

[0017] The raw material hopper 140 stores the iron oxide raw material and charges the iron oxide raw material into the small shaft furnace 110 from above the small shaft furnace 110. In this test, sintered pellets having an average particle size of 12.5 mm and a reduction disintegration index (RDI) of 5% were used as the iron oxide raw material. A particle size distribution was measured according to the method described in JIS M 8706:2015, and the average particle size was measured according to the method described in Annex J of the same JIS. The reduction disintegration index was measured by the method described in ISO 11257:2022. The reduction disintegration index is an upper limit that satisfies the current quality standards.

[0018] A heating furnace (not shown) is disposed upstream of the raw material hopper 140, and the iron oxide raw material preheated by the heating furnace is supplied to the raw material hopper 140.

[0019] The discharge feeder 150 supplies the reduced iron discharged from the lower side of the small shaft furnace 110 to the reduced iron hopper 160. The reduced iron hopper 160 cools the reduced iron while storing the reduced iron. The reduced iron hopper 160 discharges the cooled reduced iron downward.

[0020] The top gas discharged from the furnace top of the small shaft furnace 110 is introduced into the gas analysis device 170. The gas analysis device 170 analyzes a composition of the top gas. In the present test, a water vapor concentration of the top gas is particularly measured. The water vapor concentration is measured by a moisture meter installed in an exhaust gas pipe. A neutron moisture meter can be used as the moisture meter.

[0021] (1-2. Test method)The present inventor performed a shaft furnace operation using the test apparatus 100 while variously changing the water vapor concentration of the top gas and the preheating temperature of the iron oxide raw material, and evaluated an operation state. Here, the water vapor concentration of the top gas was adjusted by adjusting the amount of water vapor generated from the water vapor supply device 125. Then, a case where the operation could be continued stably (in other words, clogging due to reduction disintegration did not occur) for 5 hours or longer from the start of the operation was determined as A. On the other hand, a case where the small shaft furnace 110 became clogged within 5 hours after the start of operation and reduced iron could no longer be discharged was determined as B. In the case of B, it is considered that reduction disintegration occurs in the small shaft furnace 110. The test results are summarized in Table 1. In a case where the water vapor concentration of the top gas was less than 20% by volume, reduction disintegration did not occur regardless of the preheating temperature of the iron oxide raw material. 

[0022] [Table 1]Raw material preheating temperature (°C)Water vapor concentration of top gas (% by volume)102030405060400AAB   600AAB   700 AAB  800  AAB 900   AAB

[0023] When Table 1 is plotted as a graph, FIG. 2 is obtained. FIG. 2 shows an operation stable region A1, an operation quasi-stable region A2, and an operation unstable region B1, and a boundary line A between the operation stable region A1 and the operation quasi-stable region A2, and a boundary line B between the operation quasi-stable region A2 and the operation unstable region B1. As shown in Table 1 and FIG. 2, there is a correlation between the preheating temperature of the iron oxide raw material, the water vapor concentration of the top gas, and the presence or absence of reduction disintegration. According to FIG. 2, in a case where the X-axis (horizontal axis) represents % by volume of the water vapor concentration of the top gas and the Y-axis (vertical axis) represents the preheating temperature (°C) of the iron oxide raw material, each of the above-described regions is represented by the following expressions. The following expressions are derived from the relationship between the preheating temperature (Y) of the iron oxide raw material and the water vapor concentration (X) of the top gas at the boundary line A and the boundary line B in FIG. 2, which is a graph of Table 1, which shows the test results.The operation stable region A1 is represented by Expressions (1) and (2).Y ≥ 10X + 400 (X ≥ 20) (1)Y = any temperature (X < 20) (2)The operation quasi-stable region A2 is represented by Expressions (3) and (4).10X + 300 < Y < 10X + 400 (X ≥ 30) (3)Y < 10X + 400 (20 < X < 30) (4)

[0024] However, in a case where a shaft furnace operation is performed using the iron oxide raw material used in the present test, the iron oxide raw material may be preheated to a preheating temperature corresponding to the operation stable region A1 and the operation quasi-stable region A2 shown in Expressions (5) and (6) and then charged into the shaft furnace.Y > 10X + 300 (X ≥ 30) (5)Y = any temperature (X < 30) (6)More preferably, the iron oxide raw material is preheated to a preheating temperature corresponding to the operation stable region A1 shown in Expressions (1) and (2) and then charged into the shaft furnace. When the iron oxide raw material is preheated to the preheating temperature corresponding to the operation stable region A1 and then charged into the shaft furnace, it is possible to more reliably enjoy the effect of suppressing the reduction disintegration.

[0025] In actual operation, it is considered that the operation stable region may vary depending on the reduction disintegration index of the iron oxide raw material and the like. Therefore, the above-described test may be conducted for each iron oxide raw material actually used, and the preheating temperature (= predetermined preheating temperature) at which the operation stable region and the operation quasi-stable region are obtained may be specified. The above test results were obtained when the reduction disintegration index was 5%, but the same results can be obtained when the reduction disintegration index is less than 5%. In addition, the reduction disintegration index of the iron oxide raw material used in the above-described test is 5%, which is the upper limit that satisfies the current quality standards. Therefore, in a case where the reduction disintegration index of the iron oxide raw material to be used satisfies the current quality standards, it is considered that reduction disintegration can be suppressed by preheating the iron oxide raw material to satisfy at least Expressions (1) and (2) (substantially Expression (1)).An upper limit and a lower limit of the preheating temperature are not particularly limited, but from the viewpoint of obtaining the effect of suppressing reduction disintegration, the lower limit of the preheating temperature is preferably 300°C and more preferably 400°C.

[0026] <2. Apparatus for Producing Reduced Iron>Next, an apparatus 1A for producing reduced iron according to the present embodiment will be described. FIG. 3 is a schematic view showing a configuration of the apparatus 1A for producing reduced iron according to the present embodiment. As shown in FIG. 3, the apparatus 1A for producing reduced iron according to the present embodiment includes a shaft furnace 1, a heating furnace 2, a top gas treatment device 3, an iron oxide raw material preheating furnace 4, and a flow meter (mass flow meter or the like) 5.

[0027] The shaft furnace 1 is the same as in the related art. That is, an iron oxide raw material is charged from above the shaft furnace 1. The type of the iron oxide raw material is not particularly limited, and may be the same as in an existing shaft furnace operation. Examples of the iron oxide raw material include iron oxide pellets (sintered pellets and the like). Subsequently, the reducing gas is blown into the shaft furnace 1 from a lower side surface of the shaft furnace 1 in which the iron oxide raw material is charged. The reducing gas blown into the shaft furnace 1 rises in the shaft furnace 1. The reducing gas reduces the iron oxide raw material in the shaft furnace 1 to generate direct reduced iron (DRI). The reduced iron is discharged from a lower part of the shaft furnace 1 and cooled. In addition, a top gas (exhaust gas) is discharged from a furnace top of the shaft furnace 1. The top gas includes unreacted hydrogen gas as well as water vapor, dust, and the like.

[0028] A feed gas and oxygen gas are introduced into the heating furnace 2. The feed gas preferably contains hydrogen gas as a main component, and preferably consists of hydrogen gas. The expression that the feed gas contains hydrogen gas as a main component represents that the feed gas contains 90% by volume or more of hydrogen gas. The feed gas may contain a small amount (10% by volume or less) of gases other than hydrogen gas as long as the effect of the present embodiment is not impaired. Examples of the gases other than hydrogen gas include CO gas, CO2 gas, and CH4 gas. As hydrogen gas as the feed gas, a hydrogen gas obtained by separating hydrogen from electrolytic hydrogen gas, coke oven gas (COG), or a gas obtained by a water-gas shift reaction using a PSA method or a membrane separation method may be used.

[0029] In the heating furnace 2, the feed gas is partially combusted using oxygen gas. Specifically, in the heating furnace 2 the hydrogen gas in the feed gas is mainly partially combusted. The partial combustion means that when the feed gas and oxygen are reacted with each other, H2 gas and CO gas in the feed gas are not completely combusted but a certain amount or more of H2 gas and CO gas in the feed gas remains. The certain amount is, for example, a proportion of 50% by volume of H2 gas and CO gas being combusted. As a result, the heating furnace 2 heats the feed gas and generates the reducing gas. A heating temperature is not particularly limited, and may be the same as that in the shaft furnace operation in the related art. For example, the heating temperature may be 800°C to 1,150°C.

[0030] The heating furnace 2 and the shaft furnace 1 are connected by a pipe, and the reducing gas is blown into the shaft furnace 1 through the pipe. Here, a temperature of the reducing gas is measured by, for example, a thermocouple.The thermocouple is provided at a connecting portion between the above-described pipe and the shaft furnace 1, and the temperature of the reducing gas is measured using the thermocouple.

[0031] In a case where the temperature of the reducing gas cannot be heated to a desired temperature only by the partial combustion of the feed gas, a separate heating unit (a heater or the like) may be provided in the heating furnace 2, and the reducing gas may be heated by the heating unit.

[0032] The top gas treatment device 3 collects a top gas discharged from a furnace top of the shaft furnace 1, and performs dehydration and dust removal. In a case where a feed gas containing hydrogen gas as a main component is used, the treated top gas is generally hydrogen gas, and thus the treated top gas may be reused as the feed gas or may be reused as a fuel gas in another process.

[0033] The iron oxide raw material preheating furnace 4 is provided upstream of a raw material hopper (not shown) and preheats the iron oxide raw material to a predetermined preheating temperature in advance (before the iron oxide raw material is charged into the shaft furnace 1). Here, the predetermined preheating temperature is a preheating temperature specified by the above-described test method. That is, the predetermined preheating temperature is a preheating temperature at which the shaft furnace operation can be stably performed for 5 hours or longer (clogging due to reduction disintegration does not occur).

[0034] For example, the iron oxide raw material is preheated such that X, which represents the water vapor concentration of the top gas of the shaft furnace 1 in % by volume, and Y, which is a predetermined preheating temperature, satisfy Expressions (1) and (2) (substantially Expression (1)).Y ≥ 10X + 400 (X ≥ 20) (1)Y = any temperature (X < 20) (2)

[0035] The preheating temperature Y of the iron oxide raw material satisfies Expression (1) and is preferably close to the boundary line A (Y = 10X + 400), and thus the water vapor concentration X of the top gas can be reduced, that is, the degree of partial oxidation can be reduced. A method of measuring the water vapor concentration X will be described later.

[0036] A specific preheating method by the iron oxide raw material preheating furnace 4 is not particularly limited. As the preheating method, for example, electricity, gas, microwaves, and top gas sensible heat or waste heat can be used. A preheating type is not particularly limited, and may be a batch type, a rotary kiln type, a shaft furnace type, or the like. The iron oxide raw material may be preheated in the raw material hopper. Alternatively, in a case where the iron oxide raw material has sensible heat generated at the time of the production thereof (for example, sintered pellets immediately after the production is in a high temperature state), such sensible heat may be used. The iron oxide raw material preheated in the iron oxide raw material preheating furnace 4 is charged into the shaft furnace 1.

[0037] The iron oxide raw material after preheating is once stored in the raw material hopper and then charged into the shaft furnace 1. The preheating temperature of the iron oxide raw material can be measured by a thermocouple or a radiation thermometer provided in the raw material hopper. The iron oxide raw material preheating furnace 4 is controlled such that the preheating temperature measured by these measuring units is a predetermined preheating temperature.

[0038] The flow meter 5 measures a flow rate of the top gas after the treatment by the top gas treatment device 3. The water vapor concentration of the top gas (before treatment) is calculated by Expression (7).Water vapor concentration of top gas = 1 - top gas flow rate after top gas treatment / flow rate of reducing gas (7)The flow rate of the top gas after the top gas treatment is measured by the flow meter 5. The flow rate of the reducing gas may be measured, for example, by using a flow meter provided in a pipe connecting the heating furnace 2 and the shaft furnace 1.

[0039] <3. Method of Producing Reduced Iron>Next, a method of producing reduced iron using the apparatus 1A for producing reduced iron will be described. The method of producing reduced iron according to the present embodiment includes: an iron oxide raw material preheating step of preheating an iron oxide raw material to a predetermined preheating temperature; an iron oxide charging step of charging the preheated iron oxide raw material into the shaft furnace 1, a partial combustion step of partially combusting a feed gas with oxygen gas to generate a reducing gas; and a blowing step of blowing the reducing gas into the shaft furnace 1 in a state in which the preheated iron oxide raw material is charged.

[0040] Here, the predetermined preheating temperature is a preheating temperature specified by the above-described test method as described above. That is, the predetermined preheating temperature is a preheating temperature at which the shaft furnace operation can be stably performed for 5 hours or longer (clogging due to reduction disintegration does not occur).

[0041] Preferably, the predetermined preheating temperature can also be calculated based on the water vapor concentration of the top gas. The water vapor concentration of the top gas is calculated based on Expression (7) and the flow rate of the top gas after the top gas treatment / the flow rate of the reducing gas measured by each flow meter.Although a case where the predetermined preheating temperature is calculated based on the water vapor concentration of the top gas has been described so far, the water vapor concentration of the top gas can also be calculated based on the preheating temperature of the iron oxide raw material. That is, reduction disintegration can also be suppressed by adjusting the water vapor concentration of the top gas to a predetermined range according to the preheating temperature of the iron oxide raw material. The water vapor concentration of the top gas is adjusted, for example, by the amount of oxygen gas for partial combustion (which may be expressed as a specific consumption) (Nm3-O2 / t-DRI) or the amount of hydrogen gas blown into the shaft furnace (which may be expressed as a specific consumption) (Nm3-H2 / t-DRI).

[0042] According to the present embodiment, even in a case where the reduction potential at the furnace top is reduced by water vapor contained in the reducing gas, reduction disintegration of the iron oxide raw material can be suppressed.

[0043] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to these examples. It is evident that a person skilled in the art of the present invention is able to conceive a variety of modification examples or correction examples within the scope of the technical concept described in the claims, and it is needless to say that such examples are also understood to be in the technical scope of the present invention.REFERENCE SIGNS LIST

[0044] 1 Shaft furnace2 Heating furnace3 Top gas treatment device4 Iron oxide raw material preheating furnace5 Flow meter (mass flow meter or the like)100 Test apparatus110 Small shaft furnace120 Feed gas supply device125 Water vapor supply device130 Heating furnace140 Raw material hopper150 Discharge feeder160 Reduced iron hopper170 Gas analysis device 

Claims

1. A method of producing reduced iron, the method comprising:preheating an iron oxide raw material to a predetermined preheating temperature;charging the preheated iron oxide raw material into a shaft furnace;generating a reducing gas by partially combusting a feed gas containing hydrogen gas with oxygen gas; andblowing the reducing gas into the shaft furnace in a state in which the preheated iron oxide raw material is charged.

2. The method of producing reduced iron according to Claim 1,wherein X, which represents a water vapor concentration of a top gas of the shaft furnace in % by volume, and Y, which is the predetermined preheating temperature, satisfy Expression (5),Y > 10X + 300 (X ≥ 30) (5).

3. The method of producing reduced iron according to Claim 1,wherein X, which represents a water vapor concentration of a top gas of the shaft furnace in % by volume, and Y, which is the predetermined preheating temperature, satisfy Expression (1),Y ≥ 10X + 400 (X ≥ 20) (1).

4. The method of producing reduced iron according to any one of Claims 1 to 3,wherein the feed gas contains hydrogen gas as a main component.

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