Direct-reduced iron production device and direct-reduced iron production method

Abstract

This direct-reduced iron production device comprises: a shaft furnace for producing direct-reduced iron using hydrogen gas; a dust removal device that removes dust from exhaust gas from the shaft furnace; a dehydration device that dehydrates the dust-removed exhaust gas to obtain a circulation gas; a hydrogen gas introduction device that introduces the hydrogen gas into the circulation gas to obtain an unheated gas containing the circulation gas and the hydrogen gas; a heating device that heats the unheated gas to obtain a reducing gas; and a hydrocarbon gas introduction device that introduces a hydrocarbon gas into at least one of the circulation gas, the unheated gas, and the reducing gas.

Description

Direct reduced iron manufacturing apparatus and direct reduced iron manufacturing method

[0001] The present invention relates to an apparatus for manufacturing direct reduced iron and a method for manufacturing direct reduced iron. This application claims priority to Japanese Patent Application No. 2024-059691, filed on April 2, 2024, the contents of which are incorporated herein by reference.

[0002] Currently, reduced iron is produced by shaft furnace operation using natural gas (NG) as a reducing gas, as in the HyL process and the Midrex process. In order to lower the melting point of the reduced iron and facilitate melting in an electric furnace in the subsequent process, it is preferable that the reduced iron contain about 4% carbon.

[0003] For example, in the case of direct reduced iron (DRI), which is produced without molding and cooled after reduction, the carbon concentration is generally increased by carburizing with a gas such as methane during the cooling process in a shaft furnace (Patent Document 1, Non-Patent Document 1). When reduced iron is used as pellets, the pellets need to be cooled, so cooling with methane gas is not a problem. On the other hand, when transporting reduced iron by sea, it is mandatory to process it into hot briquette iron (HBI) for disaster prevention reasons. When producing hot briquette iron (HBI) by hot agglomeration outside a shaft furnace, the reduced iron must be discharged outside the furnace at a temperature of approximately 700°C, making the above-mentioned cooling process impossible. Therefore, when producing HBI, it is difficult to increase the carbon concentration in the reduced iron after reduction.

[0004] The carbon concentration of HBI produced with methane gas is typically 0.5% to 1.5%, less than half the carbon concentration of DRI carburized during the cooling process. ２ From the perspective of reduction, H ２ is preferably used as the reducing gas, but H ２ When HBI is used as a reducing gas, the carbon concentration does not increase during the reduction process because the reducing gas does not contain carbon, and there is a problem that the HBI contains almost no carbon.

[0005] As a technique for efficiently adding carbon in the cooling zone, Patent Document 1 discloses a technique in which natural gas, which is the raw material gas, is not decomposed and methane is left in the cooling gas.

[0006] Japanese Patent Publication No. 62-263911

[0007] ENERGIRON-Technology-General Overview (2012)

[0008] However, the technology disclosed in Non-Patent Document 1 uses natural gas as the raw material gas, and therefore CO ２ The reduction effect is low.

[0009] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide an apparatus and method for producing direct reduced iron that are capable of carburizing reduced iron even when the reduced iron is discharged at a high temperature in a direct reduction furnace that uses hydrogen as a raw material gas.

[0010] In order to solve the above problems, the present invention proposes the following means. (1) A first aspect of the present invention provides an apparatus for producing direct reduced iron, comprising: a shaft furnace that produces direct reduced iron using hydrogen gas; a dust remover that removes dust from the exhaust gas of the shaft furnace; a dehydration device that dehydrates the dust-removed exhaust gas to obtain a circulating gas; a hydrogen gas introducing device that introduces hydrogen gas into the circulating gas to produce a pre-heated gas containing the circulating gas and the hydrogen gas; a heating device that heats the pre-heated gas containing the circulating gas and the hydrogen gas to produce a reducing gas; and a hydrocarbon gas introducing device that introduces a hydrocarbon gas into at least one of the circulating gas, the pre-heated gas, and the reducing gas. (2) A second aspect of the present invention provides the apparatus for producing direct reduced iron of the first aspect, further comprising: a gas analyzer that measures the concentration of a tracer gas in the circulating gas; and a control device that controls the amount of hydrocarbon gas introduced by the hydrocarbon gas introducing device in accordance with the concentration of the tracer gas. (3) A third aspect of the present invention provides the apparatus for producing direct reduced iron of the second aspect, further comprising: a gas analyzer that measures the concentration of a tracer gas in the circulating gas; and a control device that controls the amount of hydrocarbon gas introduced by the hydrocarbon gas introducing device in accordance with the concentration of the tracer gas. ２(4) Aspect 4 of the present invention relates to the direct reduced iron manufacturing apparatus of any one of Aspects 1 to 3, further comprising a carbon analyzer for measuring a carbon concentration in the direct reduced iron. (5) Aspect 5 of the present invention relates to a direct reduced iron manufacturing method using a shaft furnace and hydrogen gas, and includes: a dedusting step of dedusting an exhaust gas from the shaft furnace; a dehydration step of dehydrating the exhaust gas from which dust has been removed in the dedusting step to obtain a circulating gas; a hydrogen gas introducing step of introducing the hydrogen gas into the circulating gas to obtain a pre-heating gas containing the circulating gas and the hydrogen gas; a heating step of heating the circulating gas and the pre-heating gas containing the hydrogen gas to obtain a reducing gas; and a hydrocarbon gas introducing step of introducing a hydrocarbon gas into at least one of the circulating gas, the pre-heating gas, and the reducing gas.

[0011] According to the above aspects of the present invention, it is possible to provide a direct reduced iron manufacturing apparatus and a direct reduced iron manufacturing method that are capable of carburizing reduced iron even when discharged at a high temperature in a direct reduction furnace that uses hydrogen gas.

[0012] FIG. 1 is a flow chart showing an example of a direct reduced iron manufacturing apparatus according to a first embodiment of the present invention; FIG. 2 is a flow chart showing an example of a direct reduced iron manufacturing apparatus according to a second embodiment of the present invention; FIG. 3 is a diagram for explaining the relationship between the amount of hydrocarbon gas introduced and the carbon concentration in reduced iron; FIG. 4 is a diagram for explaining the relationship between the amount of hydrocarbon gas introduced and the carbon concentration not contained in reduced iron; and FIG. 5 is a diagram for explaining the relationship between the amount of hydrocarbon gas introduced and the tracer gas (CO ２ 10(a) is a diagram illustrating the relationship between the amount of hydrocarbon gas introduced and time; FIG. 10(b) is a diagram illustrating the relationship between the carbon concentration in reduced iron and time; FIG. 10(c) is a diagram illustrating the relationship between the tracer gas concentration in the circulating gas and time; and FIG. 10(c) is a flow diagram illustrating an example of an apparatus for producing direct reduced iron according to a third embodiment of the present invention.

[0013] First Embodiment A direct-reduced iron manufacturing apparatus 100 and a method for manufacturing direct-reduced iron according to a first embodiment will be described below with reference to the accompanying drawings. FIG. 1 is a flow diagram illustrating an example of the direct-reduced iron manufacturing apparatus according to the first embodiment of the present invention. The direct-reduced iron manufacturing apparatus 100 includes a shaft furnace 20 that uses hydrogen gas to produce direct-reduced iron, a dust removal device 30 that removes dust from the exhaust gas of the shaft furnace 20, a dehydration device 40 that dehydrates the dust-removed exhaust gas to obtain a circulating gas, a hydrogen gas introduction device 65 that introduces hydrogen gas into the circulating gas to obtain a pre-heating gas containing the circulating gas and hydrogen gas, a heating device 50 that heats the pre-heating gas to produce a reducing gas, and a hydrocarbon gas introduction device 60 that introduces a hydrocarbon gas into at least one of the circulating gas, the pre-heating gas, and the reducing gas. The hydrogen gas may contain impurities such as nitrogen gas as long as they do not inhibit the reduction reaction. The hydrogen gas has a concentration of, for example, 90 vol% or more.

[0014] (Shaft furnace 20) The shaft furnace 20 includes a raw material charging section 24 for charging the raw material iron oxide, a reduced iron discharge section 25 for discharging reduced iron, a reducing gas inlet 28 located at the bottom of the shaft furnace 20 for blowing in reducing gas, and an exhaust gas outlet 29 located at the top of the shaft furnace 20 for discharging exhaust gas. The raw material iron oxide is charged into the shaft furnace 20 through the raw material charging section 24 at the top. The iron oxide is reduced by hydrogen gas as it descends inside the shaft furnace 20 to become reduced iron. The reduced reduced iron is discharged from the reduced iron discharge section 25 in a high-temperature state. The reaction between iron oxide and hydrogen gas is as shown in formula (1A) below. As shown in formula (1A) below, hydrogen gas reacts with iron oxide to form water (water vapor). Furthermore, iron oxide is reduced by carbon monoxide (CO) generated by the reaction of hydrocarbon gases, which will be described later, as shown in formula (1B) below. In this reaction, carbon dioxide (CO ２ ) occurs. ２ O ３ +3H ２ → 2Fe + 3H ２ O (1A) Fe ２ O ３ +3CO→2Fe+3CO ２ (1B)

[0015] Reducing gas heated by the heating device 50 is blown in through the reducing gas blowing port 28. Iron oxide is reduced by the reducing gas. The reduced reduced iron reacts with hydrocarbon gas introduced by the hydrocarbon gas introducing device 60 (described later), increasing the carbon concentration in the directly reduced iron. Here, the reaction will be explained using methane gas as an example of the hydrocarbon gas. The reaction between reduced iron and methane gas is as shown in formula (2A) below. As shown in formula (2A) below, hydrogen gas is generated by the reaction between methane gas and reduced iron. The generated hydrogen gas is discharged from the exhaust gas discharge port 29. Furthermore, methane gas reacts with water and carbon dioxide in the shaft furnace 20 to generate carbon monoxide and hydrogen as shown in formulas (2B) and (2C) below. The generated carbon monoxide and hydrogen are discharged from the exhaust gas discharge port 29. 3Fe+CH ４ →Fe ３ C+2H ２ (2A) H ２ O + CH ４ →CO+3H ２ (2B) CO ２ +CH ４ → 2CO + 2H ２ (2C)

[0016] (Dust remover 30) The dust remover 30 removes dust from the exhaust gas discharged from the exhaust gas outlet 29. There is no particular limitation on the dust removal method, and examples include a cyclone and a scrubber. ３ It is preferable to remove dust so that the dust content falls within the range of 1. The exhaust gas from which dust has been removed is sent to the dehydration device 40.

[0017] (Dehydration Device 40) The dehydration device 40 dehydrates the exhaust gas removed by the dust removal device 30 to obtain a circulating gas. The exhaust gas discharged from the exhaust gas outlet 29 contains unreacted hydrogen gas (exhaust hydrogen gas), hydrocarbon gas, water vapor, carbon monoxide, and carbon dioxide generated by the reduction reaction of iron oxide, etc. Because water inhibits the reduction reaction, it is preferable that the moisture concentration in the exhaust gas be as low as possible. For example, if the moisture concentration in the exhaust gas is 25 vol%, it is preferable to remove the moisture to a moisture concentration of 2 vol% or less by dehydration. The dehydration device 40 dehydrates the exhaust gas after dust removal, for example, by cooling it. The dehydrated circulating gas is pressurized by a compressor (not shown) to a pressure higher than the pressure inside the shaft furnace 20 and then introduced into the heating device 50.

[0018] (Hydrogen gas introduction device 65) The hydrogen gas introduction device 65 introduces hydrogen gas, which is a raw material gas, into the circulating gas before it is introduced into the heating device 50, thereby obtaining a pre-heated gas containing the circulating gas and hydrogen gas. The introduction of hydrogen gas into the circulating gas can be performed by a known method. The hydrogen gas supplied as the raw material gas is, for example, hydrogen gas produced by electrolysis of water.

[0019] (Heating device 50) The heating device 50 heats the circulating gas dehydrated in the dehydration device 40 and the pre-heating gas containing hydrogen gas to 700°C or higher, and injects the pre-heating gas into the reducing gas inlet 28 as reducing gas. In this embodiment, the pre-heating gas is composed of the circulating gas and hydrogen gas, which is the raw material gas. The temperature of the reducing gas injected is approximately 700 to 1000°C. The injection rate of the reducing gas is approximately 1000 to 2200 Nm ３ / t-DRI.

[0020] (Hydrocarbon Gas Introducing Device 60) The hydrocarbon gas introducing device 60 introduces a hydrocarbon gas into the reducing gas injected into the shaft furnace 20 from the reducing gas injection port 28. The reducing gas into which the hydrocarbon gas has been introduced is introduced into the shaft furnace 20 from the reducing gas injection port 28, and the introduced hydrocarbon gas is used in the above-mentioned reaction. The amount of hydrocarbon gas introduced can be changed appropriately depending on the raw material (iron oxide) used. The hydrocarbon gas is pressurized to a pressure equal to or higher than the pressure of the reducing gas and introduced into the reducing gas. The hydrocarbon gas may be introduced from a tank (not shown) that stores the hydrocarbon gas, or may be introduced via piping from a factory or the like external to the direct reduced iron manufacturing apparatus 100.

[0021] (Method for Producing Direct Reduced Iron) The method for producing direct reduced iron according to this embodiment uses a shaft furnace and hydrogen gas to produce direct reduced iron. The method includes a dust removal process for removing dust from the exhaust gas of the shaft furnace 20, a dehydration process for dehydrating the exhaust gas removed in the dust removal process to obtain a circulation gas, a hydrogen gas introduction process for introducing hydrogen gas into the circulation gas to obtain a pre-heated gas containing the circulation gas and hydrogen gas, a heating process for heating the pre-heated gas to obtain a reducing gas, and a hydrocarbon methane gas introduction process for introducing a hydrocarbon gas into at least one of the circulation gas, the pre-heated gas, and the reducing gas. The method for producing direct reduced iron using the direct reduced iron manufacturing apparatus 100 according to the first embodiment will be described below. Raw material (iron oxide) is introduced into the raw material charging section 24 at the top of the shaft furnace 20. The raw material is reduced in the shaft furnace 20 by reducing gas introduced through a reducing gas inlet 28 to produce reduced iron, which is then discharged from a reduced iron discharge section 25 of the shaft furnace 20. It is preferable that the raw materials used in the direct reduced iron manufacturing apparatus 100 according to the first embodiment have small variations in their components.

[0022] The reducing gas introduced through the reducing gas inlet 28 of the shaft furnace 20 reduces the iron oxide in the shaft furnace 20 and is then discharged as exhaust gas from the exhaust gas outlet 29 of the shaft furnace 20. The discharged exhaust gas contains unreacted hydrogen gas and hydrocarbon gas, as well as water vapor, carbon monoxide, and carbon dioxide generated during the reduction reaction of the iron oxide. The exhaust gas discharged from the exhaust gas outlet 29 is removed from the exhaust gas by the dust removal device 30 (dust removal process), dehydrated in the dehydration device 40 to produce a circulating gas (dehydration process), and hydrogen gas is introduced into the circulating gas by the hydrogen gas introduction device 65 to produce a pre-heated gas containing the circulating gas and hydrogen gas (hydrogen gas introduction process). Next, the pre-heated gas containing the circulating gas and hydrogen gas is heated in the heating device 50 to produce a reducing gas (heating process), which is then introduced into the furnace from the bottom of the shaft furnace 20. A portion of the exhaust gas may be discharged outside the system.

[0023] Before being introduced into the shaft furnace 20 after heating, a hydrocarbon gas is introduced into the reducing gas by a hydrocarbon gas introduction device 60 (hydrocarbon gas introduction step). Examples of hydrocarbon gases include methane gas. The amount of hydrocarbon gas introduced is controlled to an amount sufficient for the carbonization reaction of formula (2A) depending on the raw material whose components have been analyzed in advance. Therefore, the hydrocarbon gas appropriately introduced from the hydrocarbon gas introduction device 60 can reduce excess hydrocarbon gas not used in the carbonization reaction, thereby reducing the amount of carbon dioxide generated by the reaction of the hydrocarbon gas.

[0024] The direct-reduced iron manufacturing apparatus 100 and the method for manufacturing direct-reduced iron according to the first embodiment have been described above. Conventional direct reduction equipment using natural gas as the raw material gas incorporates equipment for removing or decomposing carbon dioxide in the circulating gas, making it easy to inject hydrocarbon gas. However, when the raw material gas targeted by the present invention is hydrogen gas, equipment for removing or decomposing carbon dioxide is not necessary. Without equipment for removing or decomposing carbon dioxide, excessive injection of hydrocarbons can lead to the accumulation of carbon dioxide in the circulating gas. The direct-reduced iron manufacturing apparatus 100 enables carburization of reduced iron even at high-temperature discharge in a direct reduction furnace using hydrogen gas. Furthermore, by introducing an appropriate amount of hydrocarbon gas based on a pre-analyzed raw material, the amount of carbon dioxide generated by the reaction of the hydrocarbon gas can be reduced. The circulating gas may be used outside the direct-reduced iron manufacturing apparatus 100.

[0025] In the first embodiment, the hydrocarbon gas is introduced into the reducing gas after being heated by the heating device 50. However, the hydrocarbon gas may be introduced into at least one of the circulating gas, the pre-heating gas, and the reducing gas. The hydrocarbon gas may be introduced into the circulating gas or the pre-heating gas before being heated by the heating device 50. The hydrocarbon gas may be introduced into the circulating gas and the pre-heating gas, or into the pre-heating gas and the reducing gas, or into the circulating gas and the reducing gas, or into the circulating gas, the pre-heating gas, and the reducing gas. When introducing the hydrocarbon gas, a hydrocarbon gas that has been pressurized to a pressure equal to or higher than that of the reducing gas or the circulating gas to be introduced is used.

[0026] The reducing gas in the first embodiment is a heated mixture of circulating gas and hydrogen gas, but in the first embodiment, a hydrocarbon gas is introduced into the reducing gas before it is introduced into the shaft furnace 20. That is, the reducing gas introduced into the shaft furnace 20 contains circulating gas, hydrogen gas, and hydrocarbon gas.

[0027] Second Embodiment Next, a direct-reduced iron manufacturing apparatus 100A according to a second embodiment will be described. FIG. 2 is a flow diagram illustrating an example of a direct-reduced iron manufacturing apparatus according to the second embodiment of the present invention. The direct-reduced iron manufacturing apparatus 100A includes a shaft furnace 20 that uses hydrogen gas to produce direct-reduced iron, a dust removal device 30 that removes dust from the exhaust gas of the shaft furnace 20, a dehydration device 40 that dehydrates the dust-removed exhaust gas, a hydrogen gas introduction device 65 that introduces hydrogen gas into a circulating gas to obtain a pre-heated gas containing the circulating gas and hydrogen gas, a heating device 50 that heats the pre-heated gas to produce a reducing gas, a hydrocarbon gas introduction device 60 that introduces a hydrocarbon gas into at least one of the circulating gas, the pre-heated gas, and the reducing gas, a gas analyzer 80 that measures the concentration of a tracer gas in the exhaust gas or the circulating gas, and a control device 70 that controls the amount of hydrocarbon gas introduced by the hydrocarbon gas introduction device 60 depending on the concentration of the tracer gas in the exhaust gas or the circulating gas. Hereinafter, the same components will be designated by the same reference numerals, and descriptions thereof may be omitted.

[0028] (Gas analyzer 80) The gas analyzer 80 measures the concentration of the tracer gas in the circulating gas. The gas analyzer 80 can be appropriately selected depending on the type and concentration of the tracer gas in the exhaust gas. It is preferable to measure the concentration of the tracer gas continuously. For example, a non-dispersive infrared analyzer, a gas chromatograph, etc. can be used as the gas analyzer 80. Here, the tracer gas is a gas that remains after the hydrocarbon gas has been decomposed and reacted in the furnace, and is a gas containing H ２ The tracer gas may be at least one of the group consisting of methane, carbon monoxide, and carbon dioxide. Carbon dioxide (CO ) is preferably used as the tracer gas because its concentration in the gas may inhibit reduction. ２ The measured tracer gas concentration is sent to the control device 70.

[0029] (Control device 70) The control device 70 controls the amount of hydrocarbon gas introduced by the hydrocarbon gas introducing device in accordance with the concentration of the tracer gas in the exhaust gas measured by the gas analyzer 80. For example, when the limit point G in Fig. 5 is exceeded, the control device 70 controls, for example, the valve 62 to reduce the amount of hydrocarbon gas introduced. The control device 70 also controls the hydrogen gas introducing device to introduce hydrogen gas equivalent to the amount consumed in the reduction.

[0030] The control device 70 may be configured using, for example, a processor such as a CPU (Central Processing Unit) and a memory. The control device 70 operates the valve 62 and controls the amount of hydrocarbon gas introduced by the processor executing a program. Note that all or part of the functions of the control device 70 may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above program may be recorded on a computer-readable recording medium. The computer-readable recording medium is, for example, a portable medium such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, a semiconductor storage device (for example, an SSD (Solid State Drive)), or a storage device such as a hard disk or semiconductor storage device built into a computer system. The above program may be transmitted via a telecommunications line.

[0031] (Method for producing direct reduced iron) A method for producing direct reduced iron using the direct reduced iron manufacturing apparatus 100A according to the second embodiment will be described below. Raw materials (iron oxide) are charged into the shaft furnace 20 through the raw material charging section 24 at the top of the shaft furnace 20. The raw materials are reduced in the shaft furnace 20 by a reducing gas introduced through a reducing gas inlet 28 to produce reduced iron, which is then discharged from the reduced iron discharge section 25 of the shaft furnace 20.

[0032] The reducing gas introduced through the reducing gas inlet 28 of the shaft furnace 20 reduces iron oxide in the shaft furnace 20 and is then discharged as exhaust gas from the exhaust gas outlet 29 of the shaft furnace 20. The discharged exhaust gas contains hydrogen gas, the aforementioned water vapor gas, carbon dioxide, carbon monoxide, and the like. The exhaust gas discharged from the exhaust gas outlet 29 is dusted in the dust removal device 30 (dust removal process), dehydrated to form a circulating gas (dehydration process), and hydrogen gas is introduced into the circulating gas to obtain a pre-heated gas containing the circulating gas and hydrogen gas (hydrogen gas introduction process). The pre-heated gas is heated in the heating device 50 to form a reducing gas (heating process), which is introduced into the furnace from the bottom of the shaft furnace 20. A portion of the exhaust gas may be discharged outside the system. The gas analyzer 80 measures the concentration of the tracer gas in the circulating gas. The amount of hydrogen gas introduced into the circulating gas is preferably equivalent to the amount consumed for reduction.

[0033] Before being introduced into the shaft furnace 20, a hydrocarbon gas corresponding to the concentration of the tracer gas is introduced into the heated reducing gas by the hydrocarbon gas introduction device 60 and the control device 70 (hydrocarbon gas introduction step). Here, each reaction will be explained using an example in which methane gas is used as the hydrocarbon gas. Methane gas introduced into the reducing gas reacts with reduced iron according to the above formula (2A). If more methane gas is introduced than is consumed in this reaction, the carbon dioxide concentration in the circulating gas increases, which hinders the reduction reaction of iron oxide. In a direct reduction facility that uses natural gas as the raw material gas, the CO ２ However, when the raw material gas targeted by the present invention is hydrogen, CO ２ Therefore, in the direct reduced iron manufacturing apparatus 100A, it is important to appropriately control the amount of hydrocarbon gas introduced so as to prevent carbon dioxide from accumulating in the circulating gas. A method for adjusting the amount of hydrocarbon gas introduced based on the concentration of the tracer gas will be described below.

[0034] "Relationship between the Amount of Hydrocarbon Gas Introduced, the Carbon Concentration in Reduced Iron, and the Amount of Carbon Dioxide Gas" First, the relationship between the amount of hydrocarbon gas introduced, the carbon concentration in reduced iron, and the amount of carbon dioxide gas will be described. FIG. 3 is a diagram for explaining the relationship between the amount of hydrocarbon gas introduced and the carbon concentration in reduced iron. The horizontal axis of FIG. 3 indicates the amount of hydrocarbon gas introduced, and the vertical axis indicates the carbon concentration in reduced iron. FIG. 4 is a diagram for explaining the relationship between the amount of hydrocarbon gas introduced and the carbon concentration not contained in reduced iron. The horizontal axis of FIG. 4 indicates the amount of hydrocarbon gas introduced, and the vertical axis indicates the carbon concentration not contained in reduced iron. FIG. 5 is a diagram for explaining the relationship between the amount of hydrocarbon gas introduced and the amount of tracer gas (CO ２ 5 is a diagram illustrating the relationship between the amount of hydrocarbon gas introduced and the carbon concentration in the reduced iron. The horizontal axis of FIG. 5 represents the amount of hydrocarbon gas introduced, and the vertical axis represents the concentration of the tracer gas in the circulating gas. Up to the limit point G, the carbon concentration in the reduced iron increases with the amount of hydrocarbon gas introduced (FIG. 3). When the circulating gas is not discharged outside the system, there is a correlation between the amount of hydrocarbon gas introduced and the carbon concentration in the reduced iron. Beyond the limit point G, the increase in the carbon concentration in the reduced iron becomes gradual. When the increase in the carbon concentration in the reduced iron becomes gradual, increasing the amount of hydrocarbon gas introduced is not incorporated into the reduced iron, and the concentration of carbon not contained in the reduced iron (e.g., methane gas and decomposition products of methane gas) increases (FIG. 4). The amount of carbon not incorporated into the reduced iron can be determined by analyzing the tracer gas. Meanwhile, carbon dioxide is generated from the excess hydrocarbon gas, causing a rapid increase in the tracer gas concentration (e.g., carbon dioxide concentration) in the shaft furnace 20 (FIG. 5). A certain amount of carbon dioxide is consumed in the reaction (2C) above, but any carbon dioxide in excess of this amount of consumption accumulates in the circulating gas.

[0035] Next, this process will be explained using a diagram with time as the horizontal axis. FIG. 6( a) illustrates the relationship between the amount of hydrocarbon gas introduced and time, FIG. 6( b) illustrates the relationship between the carbon concentration in reduced iron and time, and FIG. 6( c) illustrates the relationship between the tracer gas concentration in the circulating gas and time. Here, the amount of hydrocarbon gas introduced is increased in proportion to the elapsed time ( FIG. 6( a)). As the amount of introduced hydrocarbon gas increases, the carbon concentration in reduced iron increases proportionally. As the carbon concentration in reduced iron approaches its limit, the increase in the carbon concentration in reduced iron becomes more gradual over time ( FIG. 6( b)). When the increase in the carbon concentration in reduced iron becomes more gradual, carbon dioxide is generated from the excess hydrocarbon gas that is not incorporated into the reduced iron. Therefore, when the increase in the carbon concentration in reduced iron becomes more gradual, the tracer gas concentration (e.g., carbon dioxide concentration) in the shaft furnace 20 increases rapidly over time ( FIG. 6( c)).

[0036] The hydrocarbon gas introduction rate can be adjusted, for example, by the following method. The correspondence between the hydrocarbon gas introduction rate, the reduced iron carbon concentration, and the tracer gas concentration is determined in advance, and the hydrocarbon gas introduction rate B, for example, is determined based on the target reduced iron carbon concentration A ( FIG. 3 ). Next, the tracer gas concentration in the circulating gas is continuously measured using a gas analyzer 80. If the measured value is within the range of variation of the estimated value C in FIG. 5 , which is estimated based on the hydrocarbon gas introduction rate B, the system is determined to be operating normally, and operation is continued. If the measured value exceeds the range of variation of the estimated value C, the system is determined to be operating abnormally, and the hydrocarbon gas introduction rate is increased or decreased. It is possible to reduce the tracer concentration by discharging the circulating gas outside the system, but it is preferable not to discharge the circulating gas in order to avoid energy loss.

[0037] The amount of hydrocarbon gas introduced can also be adjusted by the following method. The carbon accumulation state in the shaft furnace 20 is estimated from the amount of hydrocarbon gas introduced, the carbon concentration of the reduced iron, and the tracer concentration of the circulating gas. If it is determined that carbon is accumulating in the shaft furnace 20 (for example, if the tracer gas concentration of the circulating gas increases rapidly), the amount of hydrocarbon gas introduced is reduced. Although it is possible to reduce the tracer concentration by discharging the circulating gas outside the system, it is preferable not to discharge it in terms of energy loss. As described above, by controlling the amount of hydrocarbon gas introduced, the carbon concentration in the circulating gas can be kept within a certain range.

[0038] The direct reduced iron manufacturing apparatus 100A and the method for manufacturing direct reduced iron according to the second embodiment have been described above. The direct reduced iron manufacturing apparatus 100A allows carburization of reduced iron even at high temperatures in a direct reduction furnace using hydrogen gas. The amount of hydrocarbon gas introduced is controlled according to the tracer gas concentration, so the amount of carbon dioxide generated can be reduced even when raw materials with large variations in composition are used.

[0039] In the direct reduced iron manufacturing apparatus 100A, the tracer gas concentration is measured using the circulating gas after dehydration in the dehydration device 40, but the timing of measuring the tracer concentration is not particularly limited as long as it is measured before heating in the heating device 50. For example, the exhaust gas after dust removal in the dust removal device 30 may be measured.

[0040] In the second embodiment, the hydrocarbon gas is introduced into the reducing gas, but the hydrocarbon gas may also be introduced into the circulating gas or the pre-heated gas. When introducing the hydrocarbon gas, the hydrocarbon gas is pressurized to a pressure equal to or higher than that of the reducing gas or circulating gas to be introduced. When the hydrocarbon gas is introduced into the circulating gas, the gas analyzer 80 measures the concentration of the tracer gas in the circulating gas before the hydrocarbon gas is introduced into the circulating gas. The gas analyzer 80 may also measure the amount of hydrogen gas.

[0041] Third Embodiment Next, a direct-reduced iron manufacturing apparatus 100B according to a third embodiment will be described. Fig. 7 is a flow diagram showing an example of the direct-reduced iron manufacturing apparatus according to the third embodiment of the present invention. The direct-reduced iron manufacturing apparatus 100B includes a shaft furnace 20 that uses hydrogen gas to produce direct-reduced iron, a dust removal device 30 that removes dust from the exhaust gas of the shaft furnace 20, a dehydration device 40 that dehydrates the dust-removed exhaust gas to obtain a circulating gas, a hydrogen gas introducing device 65 that introduces hydrogen gas into the circulating gas to obtain a pre-heating gas containing the circulating gas and hydrogen gas, a heating device 50 that heats the pre-heating gas to obtain a reducing gas, a hydrocarbon gas introducing device 60 that introduces a hydrocarbon gas into at least one of the circulating gas, the pre-heating gas, and the reducing gas, a gas analyzer 80 that measures the concentration of a tracer gas in the exhaust gas or the circulating gas, a control device 70 that controls the amount of hydrocarbon gas introduced by the hydrocarbon gas introducing device 60 depending on the concentration of the tracer gas in the exhaust gas or the circulating gas, and a carbon analyzer 90 that measures the carbon concentration in the direct-reduced iron. Hereinafter, the same components will be given the same reference numerals and descriptions thereof may be omitted.

[0042] (Carbon analyzer 90) The carbon analyzer 90 measures the carbon concentration in the direct reduced iron discharged from the reduced iron discharge unit 25. The carbon concentration in the direct reduced iron can be measured, for example, in accordance with JIS G 1211-3, the combustion-infrared absorption method. The measured carbon concentration is sent to the control device 70.

[0043] (Control device 70) Control device 70 controls the amount of hydrocarbon gas introduced by hydrocarbon gas introducing device 60 in accordance with the concentration of the tracer gas in the circulating gas or exhaust gas and the carbon concentration in the direct reduced iron measured by gas analyzer 80. Control device 70 controls the amount of hydrocarbon gas introduced, for example, by controlling valve 62. Control device 70 also controls the hydrogen gas introducing device to introduce hydrogen gas equivalent to the amount consumed in reduction.

[0044] (Method for producing direct reduced iron) A method for producing direct reduced iron using the direct reduced iron manufacturing apparatus 100B according to the third embodiment will be described below. Raw materials (iron oxide) are charged into the shaft furnace 20 through the raw material charging section 24 at the top of the shaft furnace 20. The raw materials are reduced in the shaft furnace 20 by a reducing gas introduced through a reducing gas inlet 28 to produce reduced iron, which is then discharged from the reduced iron discharge section 25 of the shaft furnace 20.

[0045] The reducing gas introduced through the reducing gas inlet 28 of the shaft furnace 20 reduces iron oxide in the shaft furnace 20 and is then discharged as exhaust gas from the exhaust gas outlet 29 of the shaft furnace 20. The discharged exhaust gas contains hydrogen gas, the above-mentioned water vapor gas, carbon dioxide, carbon monoxide, etc. The exhaust gas discharged from the exhaust gas outlet 29 is dusted in a dust removal device 30 (dust removal process), dehydrated to form a circulating gas (dehydration process), and hydrogen gas is introduced into the circulating gas to obtain a pre-heating gas containing the circulating gas and hydrogen gas (hydrogen gas introduction process). The pre-heating gas is heated in a heating device 50 to form a reducing gas (heating process). The reducing gas is introduced into the furnace from the bottom of the shaft furnace. A portion of the exhaust gas may be discharged outside the system. The amount of hydrogen gas introduced into the circulating gas is preferably equivalent to the amount consumed for reduction.

[0046] After heating, a hydrocarbon gas is introduced into the reducing gas by the hydrocarbon gas introducing device 60 and the control device 70 in an amount corresponding to the concentration of the tracer gas and the carbon concentration in the direct-reduced iron (hydrocarbon gas introducing step). If the hydrocarbon gas introduced into the reducing gas is methane gas, the introduced methane gas reacts with the reduced iron based on the above formula (2A). If more methane gas is introduced than is consumed in this reaction, the carbon dioxide concentration in the circulating gas increases, hindering the reduction reaction of iron oxide. Therefore, the direct-reduced iron manufacturing apparatus 100B controls the carbon dioxide concentration in the circulating gas to be within a predetermined range. Furthermore, because the target carbon concentration may not be achieved depending on the raw materials, the direct-reduced iron manufacturing apparatus 100B controls the amount of hydrocarbon gas introduced so that the carbon concentration of the direct-reduced iron is within a predetermined range.

[0047] The amount of hydrocarbon gas introduced can be adjusted, for example, by the following method. A correspondence relationship between the amount of hydrocarbon gas introduced and the carbon concentration of reduced iron and the tracer gas concentration is determined in advance, and the amount of hydrocarbon gas introduced B in FIG. 3 is determined based on the target value A of the carbon concentration of reduced iron. Next, the carbon concentration of the directly reduced iron is measured using a carbon analyzer 90. If the measured value is within the fluctuation range of the target carbon concentration, it is determined that the operation is normal, and operation is continued. If the measured value is higher than the target carbon concentration, the amount of hydrocarbon gas introduced is reduced; if the measured value is lower than the target carbon concentration, the amount of hydrocarbon gas introduced is increased. By controlling the amount of hydrocarbon gas introduced in this way, the carbon concentration of the directly reduced iron can be controlled within an appropriate range.

[0048] The direct reduced iron manufacturing apparatus 100B and the method for manufacturing direct reduced iron according to the third embodiment have been described above. The direct reduced iron manufacturing apparatus 100B allows carburization of reduced iron even at high temperatures in a direct reduction furnace using hydrogen gas. The amount of hydrocarbon gas introduced is controlled according to the carbon concentration in the direct reduced iron, so the carbon concentration in the direct reduced iron can be controlled within an appropriate range.

[0049] The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. In addition, the components in the above-described embodiments can be replaced with well-known components as appropriate, and the above-described modifications can be combined as appropriate, without departing from the spirit of the present invention.

[0050] The direct reduced iron manufacturing apparatus has high industrial applicability because it is possible to carburize reduced iron even at high temperature discharge in a direct reduction furnace using hydrogen gas.

[0051] 20 shaft furnace, 30 dust removal device, 40 dehydration device, 50 heating device, 60 hydrocarbon gas introduction device, 70 control device, 80 gas analyzer, 90 carbon analyzer, 100 direct reduced iron manufacturing device

Claims

1. An apparatus for producing direct reduced iron, comprising: a shaft furnace that produces direct reduced iron using hydrogen gas; a dust removal device that removes dust from exhaust gas from the shaft furnace; a dehydration device that dehydrates the dust-removed exhaust gas to obtain a circulation gas; a hydrogen gas introducing device that introduces hydrogen gas into the circulation gas to obtain a pre-heated gas containing the circulation gas and the hydrogen gas; a heating device that heats the pre-heated gas to form a reducing gas; and a hydrocarbon gas introducing device that introduces a hydrocarbon gas into at least one of the circulation gas, the pre-heated gas, and the reducing gas.

2. The direct reduced iron manufacturing apparatus according to claim 1, further comprising: a gas analyzer that measures the concentration of a tracer gas in the exhaust gas or the circulating gas; and a control device that controls the amount of hydrocarbon gas introduced by the hydrocarbon gas introducing device in accordance with the concentration of the tracer gas.

3. The tracer gas is CO ２ 3. The apparatus for producing direct reduced iron according to claim 2, wherein 4. The direct reduced iron manufacturing apparatus according to claim 1 or 2, further comprising a carbon analyzer for measuring the carbon concentration in the direct reduced iron.

5. A method for producing direct reduced iron using a shaft furnace and hydrogen gas, comprising: a dust removal step of removing dust from an exhaust gas of the shaft furnace; a dehydration step of dehydrating the exhaust gas from which dust has been removed in the dust removal step to obtain a circulation gas; a hydrogen gas introduction step of introducing the hydrogen gas into the circulation gas to obtain a pre-heated gas containing the circulation gas and the hydrogen gas; a heating step of heating the pre-heated gas to obtain a reducing gas; and a hydrocarbon gas introduction step of introducing a hydrocarbon gas into at least one of the circulation gas, the pre-heated gas, and the reducing gas.

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