Reduced iron production method and reduced iron production device

Abstract

This reduced iron production method comprises: an iron oxide charging step for charging a shaft furnace with an iron oxide raw material; a partial combustion step for partially combusting a raw material gas with oxygen gas to raise the temperature of the raw material gas to 800°C or higher, and generating a reducing gas; and a blowing step for blowing the reducing gas into the shaft furnace. In the blowing step, when the temperature of the reducing gas after the partial combustion step is not lower than 800°C but lower than 900°C, the water vapor concentration of the reducing gas is set to 21 vol% or less. When the temperature of the reducing gas after the partial combustion step is not lower than 900°C but lower than 1000°C, the water vapor concentration of the reducing gas is set to 23 vol% or less. When the temperature of the reducing gas after the partial combustion step is not lower than 1000°C, the water vapor concentration of the reducing gas is set to 24 vol% or less.

Description

Reduced iron manufacturing method and reduced iron manufacturing device

[0001] The present invention relates to a method and an apparatus for producing reduced iron. This application claims priority to Japanese Patent Application No. 2023-213399, filed on December 18, 2023, the contents of which are incorporated herein by reference.

[0002] The method for producing reduced iron using a shaft furnace (shaft furnace operation) is a typical example of a direct reduction process for producing reduced iron from iron oxide raw materials, and is widespread mainly in regions where natural gas is available at low cost (oil-producing countries). Here, an overview of the method for producing reduced iron using a shaft furnace will be explained. First, iron oxide raw materials (e.g., iron oxide pellets) are charged from the top of the shaft furnace, and reducing gas is blown into the shaft furnace from the bottom. The reducing gas is heated to a predetermined temperature (e.g., about 900 to 950°C) and then blown into the shaft furnace. The reducing gas blown into the shaft furnace then reduces the iron oxide raw materials in the shaft furnace. Reduced iron is produced by this direct reduction process. The reduced iron is discharged from the bottom of the shaft furnace and cooled. Hydrogen gas, CO gas, water vapor, and CO are blown into the shaft furnace from the top of the shaft furnace. ２ After steam is removed from the furnace top gas, the hydrogen gas and CO gas in the furnace top gas are reused as part of the raw material gas. ２ Gas may also be removed.

[0003] The reducing gas used in the shaft furnace (injected into the shaft furnace) is a carbon-containing raw material gas (e.g., natural gas, coke oven gas, etc.) that is mixed with steam or CO ２ It is a gas obtained by reforming the raw material gas using carbon dioxide, oxygen gas, etc. However, when reforming the raw material gas in the shaft furnace, the raw material gas may be used as the reducing gas without being reformed. The main component of the reducing gas is hydrogen gas (H ２ ), CO gas (CO) and CH ４ It's gas.

[0004] By the way, in recent years, CO ２In order to further reduce the amount of gas emissions, techniques for increasing the concentration of hydrogen gas in the reducing gas have been studied. For example, Patent Document 1 discloses a technique using a reducing gas with a hydrogen gas concentration of 70 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.

[0005] International Publication No. 2022-169392

[0006] Incidentally, partial combustion of hydrogen gas generates water (steam), which generally retards the reduction reaction of the iron oxide raw material in the shaft furnace, and therefore reduces the reduction rate (productivity) of the reduced iron.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a new and improved method for producing reduced iron, which is capable of producing reduced iron with a high reduction rate even when hydrogen gas is partially combusted, and an apparatus for producing reduced iron suitable for the method.

[0008] The gist of the present invention is as follows: [1] A method for producing reduced iron according to one aspect of the present invention includes an iron oxide charging step of charging an iron oxide raw material into a shaft furnace, a partial combustion step of partially combusting the raw material gas with oxygen gas to raise the temperature of the raw material gas to 800°C or higher and generating a reducing gas, and an injection step of injecting the reducing gas into the shaft furnace, wherein in the injection step, when the temperature of the reducing gas after the partial combustion step is 800°C or higher and lower than 900°C, the water vapor concentration of the reducing gas is 21% by volume or lower, when the temperature of the reducing gas after the partial combustion step is 900°C or higher and lower than 1000°C, the water vapor concentration of the reducing gas is 23% by volume or lower, and when the temperature of the reducing gas after the partial combustion step is 1000°C or higher, the water vapor concentration of the reducing gas is 24% by volume or lower. [2] In the method for producing reduced iron according to [1], in the blowing step, the water vapor concentration of the reducing gas may be 10% by volume or less when the temperature of the reducing gas after the partial combustion step is 800°C or more and less than 900°C, the water vapor concentration of the reducing gas may be 17% by volume or less when the temperature of the reducing gas after the partial combustion step is 900°C or more and less than 1000°C, and the water vapor concentration of the reducing gas may be 19% by volume or less when the temperature of the reducing gas after the partial combustion step is 1000°C or more. [3] In the method for producing reduced iron according to [1], the raw material gas may contain hydrogen gas as a main component. [4] In the method for producing reduced iron according to [2], the raw material gas may contain hydrogen gas as a main component. [5] In the method for producing reduced iron according to any of [1] to [4], in the partial combustion step, the raw material gas and the oxygen gas may be introduced into a heating furnace, and the raw material gas may be partially combusted in the heating furnace. [6] In the method for producing reduced iron according to [3] or [4], in the partial combustion step, the temperature of the raw material gas at 100°C or less may be increased to 897°C or higher by partially combusting it with the oxygen gas. [7] In the method for producing reduced iron according to [3] or [4], in the partial combustion step, the temperature of the raw material gas at 100°C or less may be increased to 969°C or higher by partially combusting it with the oxygen gas. [8] The method for producing reduced iron according to any of [1] to [4] may include a preheating step of preheating the raw material gas, and in the partial combustion step, the preheated raw material gas may be partially combusted with the oxygen gas.[5] In the method for producing reduced iron described in [8], the preheating step may preheat the raw material gas using sensible heat contained in the exhaust gas of the shaft furnace.

[10] In the method for producing reduced iron described in [9], the preheating step may include a circulation step of introducing hydrogen gas obtained by dehydrating the exhaust gas after the preheating step into a heating furnace.

[11] A method for producing reduced iron according to another aspect of the present invention includes an iron oxide charging step of charging an iron oxide raw material into a shaft furnace, a partial combustion step of partially combusting the raw material gas with oxygen gas to raise the temperature of the raw material gas to 800°C or higher and generating reducing gas, and an injection step of injecting the reducing gas into the shaft furnace, wherein the relationship between the water vapor concentration, which is the concentration of the water vapor in terms of volume fraction of the hydrogen gas and water vapor in the reducing gas, and the reduction rate of reduced iron is determined for each temperature of the reducing gas, and the water vapor concentration is controlled based on the relationship in accordance with the target reduction rate and the temperature of the reducing gas, or the temperature of the reducing gas is controlled in accordance with the target reduction rate and the water vapor concentration in accordance with the relationship.

[12] A reduced iron manufacturing apparatus according to another aspect of the invention is an apparatus for manufacturing reduced iron using the reduced iron manufacturing method described in [6], and includes a heating furnace connected to a pipe and configured to heat and partially combust the introduced raw material gas and oxygen gas to obtain reducing gas, and a shaft furnace into which the reducing gas obtained in the heating furnace is blown, and the heating furnace is configured so that the raw material gas is supplied directly from a raw material gas supply source through the pipe.

[0009] According to the above aspects of the present invention, it is possible to provide a method for producing reduced iron that can produce reduced iron with a high reduction rate even when hydrogen gas is partially combusted, and an apparatus for producing reduced iron that is suitable for this method.

[0010] 1 is a graph showing the correlation between the hydrogen gas concentration (volume %) in the reducing gas and the reduction rate (%) of reduced iron (DRI) for each temperature of the reducing gas. FIG. 2 is a schematic diagram showing the configuration of an apparatus for producing reduced iron according to a first embodiment. FIG. 3 is a schematic diagram showing the configuration of an apparatus for producing reduced iron according to a second embodiment. FIG. 4 is a graph showing the correlation between the preheating temperature of a raw material gas and the partial combustion rate of the raw material gas. FIG. 5 is a schematic diagram showing the configuration of an apparatus for producing reduced iron according to a third embodiment.

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0012] <1. Findings of the Inventors> First, the findings of the inventors, which form the basis of this embodiment, will be described. The inventors investigated the correlation between the water vapor concentration (volume %) in reducing gas and the reduction rate (%) of reduced iron. Specific investigations were performed by simulation using a mathematical model of a shaft furnace. This model was constructed based on chemical engineering techniques described in non-patent literature (e.g., Hara et al., "Iron and Steel," Vol. 62 (1976), No. 3, p. 315; Yamaoka et al., "Iron and Steel," Vol. 74 (1988), No. 12, p. 2254). This model is capable of theoretically analyzing and predicting heat and mass transfer within a shaft furnace, including chemical reactions and heat transfer phenomena, such as the reduction reaction of iron oxide raw materials by reducing gas. Using this mathematical model, a simulation of shaft furnace operation using a reducing gas containing a high concentration of hydrogen gas was performed to evaluate macroscopic heat and mass transfer.

[0013] The calculation conditions are as shown in Table 1. In this calculation, the reducing gas was assumed to consist of hydrogen gas. The residence time in Table 1 refers to the time that the oxidized iron raw material charged from the furnace top remains in the region where it comes into contact with the reducing gas (the region where the oxidized iron raw material is reduced by the reducing gas). This region is specifically the region between the reducing gas inlet and the top charging position. The gas flow rate is the total flow rate of the reducing gas and steam ascending through the shaft furnace. Here, the value is shown per ton of reduced iron. The gas flow rate may vary depending on factors such as the reduction rate. The top pressure refers to the air pressure at the top of the shaft furnace. The raw material particle size refers to the particle size (mm) of the oxidized iron raw material. Particle size can be measured by sieving. Porosity is the ratio of the pore volume within a particle to the apparent volume of the raw material pellet, and the measurement method is specified in JIS M8716:1990. The hydrogen reduction rate constant represents the rate of the reduction reaction by hydrogen and is defined as the movement speed (m / s) of the reaction interface within the particle per unit time. When the hydrogen gas concentration in the reducing gas is less than 100% by volume, the gas other than hydrogen gas is assumed to be water vapor. Therefore, assuming an actual machine, the hydrogen gas, which is the raw material gas, is partially combusted to raise the temperature of the reducing gas to the desired temperature. However, if partial combustion alone is insufficient to achieve the required heating temperature, this can be achieved by a separate heating furnace or a heat exchanger that uses the sensible heat contained in the exhaust gas. The temperature of the reducing gas was set to 800°C or higher. This is because if the temperature of the reducing gas is lower than 800°C, it will take longer to achieve the target reduction rate of 80% or higher or 92% or higher (described below), resulting in a decrease in production efficiency. The residence time, gas flow rate, furnace top pressure, raw material particle size, and porosity were set to the above ranges because these ranges are preferable for achieving a high reduction rate.

[0014]

[0015] The results of the simulation are shown in Figure 1. The horizontal axis of Figure 1 represents the hydrogen gas concentration (volume %) in the reducing gas (displayed on the bottom) and the water vapor concentration (volume %) in the reducing gas (displayed on the top). In this simulation, when the hydrogen gas concentration in the reducing gas is less than 100 volume %, the gas other than hydrogen gas was assumed to be water vapor. Therefore, the denominator in the formula for calculating the hydrogen gas concentration is the sum of the volumes of hydrogen gas and water vapor, and the water vapor concentration (volume %) in the reducing gas is the value obtained by subtracting the value on the horizontal axis from 100. The vertical axis represents the reduction rate of reduced iron. The reduction rate is expressed as a percentage (mass of oxygen removed from iron oxide by hydrogen in reduced iron / mass of oxygen in iron oxide originally present in the raw material).

[0016] On the other hand, the target lower limit of the reduction ratio of reduced iron used as blast furnace feedstock is 80%. This is because a reduction ratio below 80% reduces the effect of improving the reducing agent ratio and production volume of the blast furnace. As shown in FIG. 1 , the water vapor concentration required to achieve a reduction ratio of 80% or more varies depending on the temperature of the reducing gas. Specifically, when the temperature of the reducing gas (temperature at the time of injection) is 800°C or higher and lower than 900°C, the water vapor concentration needs to be 21% by volume or less to achieve a reduction ratio of 80% or more. When the temperature of the reducing gas is 900°C or higher and lower than 1000°C, the water vapor concentration needs to be 23% by volume or less to achieve a reduction ratio of 80% or more. When the temperature of the reducing gas is 1000°C or higher, the water vapor concentration needs to be 24% by volume or less to achieve a reduction ratio of 80% or more.

[0017] On the other hand, the lower limit of the target reduction ratio of reduced iron used as an electric furnace feedstock is 92%. This is because if the reduction ratio is less than 92%, it becomes difficult to melt the reduced iron in the electric furnace in the next process. As shown in Figure 1, the water vapor concentration required to achieve a reduction ratio of 92% or more varies depending on the temperature of the reducing gas. Specifically, when the temperature of the reducing gas (temperature at the time of blowing) is 800°C or higher and lower than 900°C, the water vapor concentration must be 10% by volume or less to achieve a reduction ratio of 92% or more. When the temperature of the reducing gas is 900°C or higher and lower than 1000°C, the water vapor concentration must be 17% by volume or less to achieve a reduction ratio of 92% or more. When the temperature of the reducing gas is 1000°C or higher, the water vapor concentration must be 19% by volume or less to achieve a reduction ratio of 92% or more. If the water vapor concentration exceeds 10% by volume when the temperature of the reducing gas is 800°C or higher and lower than 900°C, if the water vapor concentration exceeds 17% by volume when the temperature of the reducing gas is 900°C or higher and lower than 1000°C, or if the water vapor concentration exceeds 19% by volume when the temperature of the reducing gas is 1000°C or higher, the reduction rate decreases, and if the calculation conditions in Table 1 are assumed, the reduction rate will be less than 92%.

[0018] Although the upper limit of the target reduction rate is not particularly limited, it is preferably 95% whether used in a blast furnace or an electric furnace. This is because a reduction rate exceeding 95% reduces the production efficiency of reduced iron and increases production costs. As can be seen from FIG. 1 , the water vapor concentration at which the reduction rate is 95% or less varies depending on the temperature of the reducing gas. Specifically, when the temperature of the reducing gas is 800°C or higher and lower than 900°C, the water vapor concentration may be 6% by volume or higher to achieve a reduction rate of 95% or less. When the temperature of the reducing gas is 900°C or higher and lower than 1000°C, the water vapor concentration may be 11% by volume or higher to achieve a reduction rate of 95% or less. When the temperature of the reducing gas is 1000°C or higher, the water vapor concentration may be 17% by volume or higher to achieve a reduction rate of 95% or less.

[0019] The above is summarized in Table 2.

[0020]

[0021] As described above, the present inventors have found that it is possible to specify a range of water vapor concentration that is practically acceptable (i.e., that allows a target reduction rate to be achieved) for each reducing gas temperature. By designing operation within such a range of reducing gas temperature and water vapor concentration, the loads of preheating and heating the raw material gas can be reduced. When the relationship between the reduction rate and the water vapor concentration in terms of the volume fraction of hydrogen gas and water vapor in the reducing gas is determined for each reducing gas temperature, as shown in FIG. 1 , during operation, the water vapor concentration can be controlled according to the target reduction rate and the reducing gas temperature, and the reducing gas temperature can also be controlled according to the target reduction rate and the water vapor concentration. The method for producing reduced iron according to this embodiment has been devised based on the above findings.

[0022] 2. First Embodiment (2-1. Reduced Iron Manufacturing Apparatus) Next, a first embodiment will be described. As shown in FIG. 2 , a reduced iron manufacturing apparatus 1A according to the first embodiment includes a shaft furnace 1 and a heating furnace 2. The shaft furnace 1 is the same as a conventional one. That is, an oxidized iron raw material is charged into the shaft furnace 1 from above. The type of oxidized iron raw material is not particularly limited, as long as it is the same as in existing shaft furnace operation. Examples of the oxidized iron raw material include iron oxide pellets. Meanwhile, reducing gas obtained by partially burning the raw material gas is injected into the shaft furnace 1 from the lower side of the shaft furnace 1 (for example, from a position corresponding to the cooling zone when the shaft furnace has a reduction zone, a transition zone, and a cooling zone, in that order from the furnace top). The reducing gas injected into the shaft furnace 1 reduces the oxidized iron raw material in the shaft furnace 1 while rising within the shaft furnace 1. As a result, reduced iron (DRI) is produced. The produced reduced iron is discharged from the lower side of the shaft furnace 1 and cooled. Meanwhile, furnace top gas (exhaust gas) is discharged from the top of the shaft furnace 1. The exhaust gas contains unreacted hydrogen gas as well as water vapor, dust, and the like.

[0023] A raw material gas and oxygen gas are introduced into the heating furnace 2. The raw material gas is a gas containing hydrogen gas (natural gas, furnace gas, hydrogen gas, or the like). Preferably, the raw material gas contains hydrogen gas as a main component (for example, 90% by volume or more of hydrogen gas and 10% by volume or less of gases other than hydrogen gas, 95% by volume or more of hydrogen gas and 5% by volume or less of gases other than hydrogen gas, or 99% by volume or more of hydrogen gas and 1% by volume or less of gases other than hydrogen gas), and more preferably, is composed of hydrogen gas. Examples of gases other than hydrogen gas contained in the raw material gas include CO gas, CO ２ Gas, CH ４ Examples of hydrogen gas that can be used as the raw material gas include electrolytic hydrogen gas, coke oven gas (COG), and hydrogen gas obtained by separating hydrogen from gas obtained by an aqueous gas shift reaction using a PSA method or a membrane separation method.

[0024] In the heating furnace 2, the raw material gas is partially combusted using oxygen gas. Specifically, the heating furnace 2 mainly partially combusts hydrogen gas in the raw material gas. In this way, the heating furnace 2 heats the raw material gas and generates reducing gas. Here, in this embodiment, partial combustion refers to the partial combustion of H in the raw material gas when the raw material gas reacts with oxygen. ２ This means that 40 mass % or more of gas and CO gas remain. The heating furnace 2 heats the raw material gas to at least 800°C or higher. As described above, in order to achieve a reduction rate of 80% or higher or 92% or higher for the reduced iron, it is necessary to set the water vapor concentration within a predetermined range depending on the temperature range of the reducing gas, or to set the temperature of the reducing gas within a predetermined range depending on the range of the water vapor concentration.

[0025] 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 this pipe.

[0026] Here, the temperature of the reducing gas is measured by, for example, a thermocouple. A thermocouple is provided at the connection between the above-mentioned piping and the shaft furnace 1, and the temperature of the reducing gas is measured using this thermocouple. The water vapor concentration is calculated by the following formula: volume of water vapor / volume of reducing gas x 100 (vol %). Since water vapor is generated by the chemical reaction shown in the following chemical formula, the volume of water vapor can be calculated as 2 x volume of oxygen gas. 2H ２ +O ２ →2H ２ O

[0027] The volume of oxygen gas is measured by providing a flow meter (such as a mass flow meter) in the oxygen gas pipe connected to the heating furnace 2. The volume of the reducing gas is calculated by measuring the volume of the raw material gas introduced into the heating furnace 2 (gas volume at the inlet of the heating furnace) with the flow meter and using the volume of the oxygen gas using the following formula: Volume of reducing gas = gas volume at the inlet of the heating furnace + oxygen gas volume

[0028] The oxygen gas may be heated by a separate heating means (such as a heater) provided in the heating furnace 2, or may be heated by heat exchange with the exhaust gas.

[0029] (2-2. Method for Producing Reduced Iron) Next, a method for producing reduced iron using the reduced iron production apparatus 1A (a method for producing reduced iron according to a first embodiment) will be described. The method for producing reduced iron according to the first embodiment includes an iron oxide charging step of charging an iron oxide raw material into the shaft furnace 1, a partial combustion step of partially combusting the raw material gas with oxygen gas in the heating furnace 2 to raise the temperature of the raw material gas to 800°C or higher and generate reducing gas, and an injection step of injecting the reducing gas into the shaft furnace 1.

[0030] When the water vapor concentration is controlled in accordance with the temperature of the reducing gas, the water vapor concentration of the reducing gas is set to 21% by volume or less when the temperature of the reducing gas is 800°C or higher but lower than 900°C, the water vapor concentration of the reducing gas is set to 23% by volume or less when the temperature of the reducing gas is 900°C or higher but lower than 1000°C, and the water vapor concentration of the reducing gas is set to 24% by volume or less when the temperature of the reducing gas is 1000°C or higher. On the other hand, when the temperature of the reducing gas is controlled in accordance with the water vapor concentration, the reducing gas temperature is set to 800°C or higher (preferably lower than 900°C) when the water vapor concentration of the reducing gas is 21% by volume or less, the reducing gas temperature is set to 900°C or higher (preferably lower than 1000°C) when the water vapor concentration of the reducing gas is 23% by volume or less, and the reducing gas temperature is set to 1000°C or higher when the water vapor concentration of the reducing gas is 24% by volume or less. This makes it possible to achieve a reduction rate of 80% or higher of reduced iron.

[0031] On the other hand, in order to achieve a reduction rate of 92% or more of reduced iron, when the water vapor concentration is controlled in accordance with the temperature of the reducing gas, it is preferable to set the water vapor concentration of the reducing gas to 10% by volume or less when the temperature of the reducing gas is 800° C. or more and less than 900° C., to 17% by volume or less when the temperature of the reducing gas is 900° C. or more and less than 1000° C., and to set the water vapor concentration of the reducing gas to 19% by volume or less when the temperature of the reducing gas is 1000° C. On the other hand, when the temperature of the reducing gas is controlled in accordance with the water vapor concentration, it is preferable to set the reducing gas temperature to 800° C. or more (preferably less than 900° C.) when the water vapor concentration of the reducing gas is 10% by volume or less, to 900° C. or more (preferably less than 1000° C.) when the water vapor concentration of the reducing gas is 17% by volume or less, and to set the reducing gas temperature to 1000° C. or more when the water vapor concentration of the reducing gas is 19% by volume or less.

[0032] It is preferable that the water vapor concentration be 6% or more when the temperature of the reducing gas is 800°C or higher and lower than 900°C, that the water vapor concentration be 11% or more when the temperature of the reducing gas is 900°C or higher and lower than 1000°C, and that the water vapor concentration be 17% or more when the temperature of the reducing gas is 1000°C or higher. This makes it possible to achieve a reduction rate of reduced iron of 95% or lower. The reduction rate of reduced iron can also be achieved by controlling the temperature of the reducing gas in accordance with the water vapor concentration.

[0033] The upper limit of the temperature of the reducing gas is not particularly limited and may be adjusted appropriately depending on the performance of the shaft furnace 1, for example, but may be 1150°C.

[0034] During operation, the water vapor concentration can be controlled by the mixing ratio of hydrogen gas and oxygen gas in the raw material gas introduced into the heating furnace 2. The reducing gas temperature can be controlled by the mixing ratio of hydrogen gas and oxygen gas in the raw material gas introduced into the heating furnace 2, and by preheating the raw material gas as described below.

[0035] As described above, the iron oxide raw material is reduced in the shaft furnace 1 to produce reduced iron. The reduced iron is discharged from the bottom of the shaft furnace 1 and cooled. Meanwhile, furnace top gas (exhaust gas) is discharged from the top of the shaft furnace 1. The exhaust gas contains unreacted hydrogen gas as well as water vapor, dust, and the like.

[0036] The reduced iron manufacturing apparatus 1A used in this embodiment has a heating furnace configured so that a raw material gas is supplied directly from a raw material gas supply source (outside the system and not shown) through a pipe, i.e., it does not have a preheating furnace like the reduced iron manufacturing apparatuses 1B and 1C used in the embodiments described later. Therefore, in the reduced iron manufacturing method according to this embodiment, preheating is not performed, and the raw material gas at 100°C or less is heated to a temperature at which a sufficient reduction rate is obtained by partially combusting it with oxygen gas.

[0037] As a result of the inventor's investigations, it was found that when a feed gas mainly composed of natural gas is used, a sufficient reduction rate cannot be obtained by partial combustion alone, but when the feed gas is mainly composed of hydrogen or consists of hydrogen gas, a sufficient reduction rate can be obtained by partial combustion alone. This is because the reduction reaction using natural gas is a reduction reaction caused by both carbon monoxide and hydrogen generated in the furnace, and since the reduction reaction using carbon monoxide is an exothermic reaction, the reduction reaction does not proceed at higher temperatures if the concentrations of carbon dioxide and water vapor generated by partial combustion are high. On the other hand, the reduction reaction using hydrogen is an endothermic reaction, so the reduction reaction proceeds at higher temperatures even if the water vapor concentration is high.

[0038] As a result of specific investigations, Table 3 shows the relationship between the reducing gas temperature, water vapor concentration, and reduction rate of the resulting reduced iron when hydrogen gas is used as the raw material gas and the temperature of the reducing gas is raised only by partial heating with oxygen. In this case, the water vapor concentration is equal to the concentration of consumed hydrogen. The calculation method for Table 3 is as described above.

[0039]

[0040] As can be seen from Table 3, to produce reduced iron with a reduction rate of 80% or higher for use as a blast furnace feedstock, it is sufficient to heat the feed gas to 897°C or higher by partial combustion (to provide a reducing gas with a temperature of 897°C or higher). In this case, the hydrogen concentration of the reducing gas is 11% by volume or higher. On the other hand, to produce reduced iron with a reduction rate of 92% or higher for use as an electric furnace feedstock, it is sufficient to heat the feed gas to 969°C or higher by partial combustion (to provide a reducing gas with a temperature of 969°C or higher). In this case, the water vapor concentration of the reducing gas is 12% by volume or higher.

[0041] As described above, according to the method for producing reduced iron according to the first embodiment using the reduced iron production apparatus 1A, if a gas mainly containing hydrogen is used as the raw material gas, reduced iron with a high reduction rate can be produced only by the partial combustion step of partially combusting the raw material gas with oxygen gas, even without the preheating step of preheating the raw material gas.

[0042] 3. Second Embodiment (3-1. Reduced Iron Manufacturing Apparatus) Next, a second embodiment will be described. As shown in Fig. 3, a reduced iron manufacturing apparatus 1B according to the second embodiment is configured by adding a preheating furnace 3 to the reduced iron manufacturing apparatus 1A according to the first embodiment.

[0043] A raw material gas is introduced into the preheating furnace 3. The composition of the raw material gas is the same as in the first embodiment. The raw material gas is preheated in the preheating furnace 3. The preheated raw material gas is introduced into the heating furnace 2. The preheating furnace 3 is equipped with any heating means (e.g., a heater) for preheating the raw material gas. The preheating temperature is not particularly limited, but considering that the raw material gas is heated (partially burned) in the preheating furnace 3, it is preferable that the preheating temperature be lower than the temperature of the reducing gas when it is blown into the shaft furnace 1.

[0044] Oxygen gas and preheated raw material gas are introduced into the heating furnace 2. As in the first embodiment, the heating furnace 2 partially combusts the raw material gas using oxygen gas. Specifically, the heating furnace 2 mainly partially combusts hydrogen gas in the raw material gas. As a result, the raw material gas is heated in the heating furnace 2 and a reducing gas is generated. In the heating furnace 2, the raw material gas is heated to at least 800°C or higher. As in the first embodiment, in the heating furnace 2, the water vapor concentration is set to a value within a predetermined range depending on the temperature range of the reducing gas, or the temperature of the reducing gas is set to a value within a predetermined range depending on the water vapor concentration.

[0045] Furthermore, in the second embodiment, the raw material gas introduced into the heating furnace 2 is preheated. Therefore, the partial combustion rate of the raw material gas (the water vapor concentration of the reducing gas) varies depending on the degree to which the raw material gas is preheated. FIG. 4 shows the correlation between the preheating temperature (°C) of the raw material gas and the hydrogen gas concentration and water vapor concentration (volume %) in the reducing gas when the raw material gas becomes hydrogen gas. In FIG. 4, the thermal efficiency is set to 100%, and the temperature of the reducing gas is set to 1000°C. Region A shows the partial combustion rate of the raw material gas when the raw material gas is not preheated, and region C shows the partial combustion rate of the raw material gas (0 volume %) when the raw material gas is preheated to 1000°C. As is clear from FIG. 4, the higher the preheating temperature of the raw material gas, the lower the partial combustion rate of the raw material gas. In other words, the heating load in the heating furnace 2 is reduced.

[0046] (3-2. Method for Producing Reduced Iron) Next, a method for producing reduced iron using the reduced iron manufacturing apparatus 1B (a method for producing reduced iron according to a second embodiment) will be described. The method for producing reduced iron according to the second embodiment is obtained by adding a preheating step of preheating a raw material gas to the method for producing reduced iron according to the first embodiment. In the partial combustion step, the preheated raw material gas is partially combusted with oxygen gas. In the preheating step, the raw material gas is preheated so that the partial combustion rate in the partial combustion step is greater than 0% by volume.

[0047] According to the second embodiment, in addition to obtaining the same effects as those of the first embodiment, the raw material gas is preheated in the preheating furnace 3, and therefore the heating load on the heating furnace 2 is reduced. Therefore, it is possible to reduce the water vapor concentration in the reducing gas in order to adjust the temperature of the reducing gas to a desired temperature.

[0048] 4. Third Embodiment (4-1. Reduced Iron Manufacturing Apparatus) Next, a third embodiment will be described. As shown in Fig. 5 , a reduced iron manufacturing apparatus 1C according to the third embodiment is configured by adding a dust removal device 4 and a dewatering device 5 to the reduced iron manufacturing apparatus 1B according to the second embodiment.

[0049] The exhaust gas discharged from the shaft furnace 1 is introduced into the dust removal device 4. The dust removal device 4 removes dust and the like in the exhaust gas by any dust removal means (dust removal filter) or the like. Furthermore, CO ２ The exhaust gas is then introduced into the preheating furnace 3. The preheating furnace 3 is a heat exchanger that preheats the raw material gas introduced into the preheating furnace 3 with the sensible heat contained in the exhaust gas. The raw material gas is then introduced into the heating furnace 2, and the exhaust gas is introduced into the dehydration device 5. The dehydration device 5 cools the exhaust gas to remove the water vapor in the exhaust gas as liquid water (i.e., dehydration). As a result, the exhaust gas is mainly composed of hydrogen gas (and in some cases CO gas and N ２ This gas is introduced into the heating furnace 2.

[0050] Preheated raw material gas, hydrogen gas recovered from the exhaust gas, and oxygen gas (at room temperature) are introduced into the heating furnace 2 (CO gas and N gas in the exhaust gas are removed). ２(Other gases may be introduced at the same time.) In the heating furnace 2, as in the first embodiment, the raw material gas and the like (hydrogen gas recovered from the raw material gas and exhaust gas) are partially combusted using oxygen gas. As a result, reducing gas is generated in the heating furnace 2. In the heating furnace 2, the raw material gas and the like are heated to at least 800°C or higher. In the heating furnace 2, as in the first embodiment, the water vapor concentration is set to a value within a predetermined range depending on the temperature range of the reducing gas, or the temperature of the reducing gas is set to a value within a predetermined range depending on the water vapor concentration.

[0051] (4-2. Method for Producing Reduced Iron) Next, a method for producing reduced iron using the reduced iron production apparatus 1C (a method for producing reduced iron according to a third embodiment) will be described. The method for producing reduced iron according to the third embodiment is different from the method according to the second embodiment in that the preheating step is changed and an exhaust gas circulation step is further added.

[0052] In the exhaust gas circulation process, the exhaust gas discharged from the shaft furnace 1 is de-dusted by the de-dusting device 4, and the de-dusted exhaust gas is introduced into the preheating furnace 3 to preheat the raw material gas by sensible heat (i.e., the preheating process is performed).The exhaust gas is then introduced into the dehydration device 5 to dehydrate the exhaust gas.The hydrogen gas obtained in this process is introduced into the heating furnace 2.

[0053] According to the third embodiment, in addition to obtaining the same effects as those of the first and second embodiments, the sensible heat of the exhaust gas is used to preheat the raw material gas, and furthermore, the hydrogen gas in the exhaust gas is reused, so that the thermal efficiency of the entire system can be improved and the amount of hydrogen gas consumed can be reduced.

[0054] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0055] According to the present invention, it is possible to provide a method for producing reduced iron that can produce reduced iron with a high reduction rate even when hydrogen gas is partially burned, and therefore the present invention has high industrial applicability.

[0056] 1 Shaft furnace 2 Heating furnace 3 Preheating furnace 4 Dust removal device 5 Dehydration device 1A, 1B, 1C Reduced iron manufacturing device

Claims

1. A method for producing reduced iron, comprising: an iron oxide charging step of charging an iron oxide raw material into a shaft furnace; a partial combustion step of partially combusting the raw material gas with oxygen gas to raise the temperature of the raw material gas to 800°C or higher and generating a reducing gas; and an injection step of injecting the reducing gas into the shaft furnace, wherein in the injection step, when the temperature of the reducing gas after the partial combustion step is 800°C or higher and lower than 900°C, the water vapor concentration of the reducing gas is set to 21 volume% or less; when the temperature of the reducing gas after the partial combustion step is 900°C or higher and lower than 1000°C, the water vapor concentration of the reducing gas is set to 23 volume% or less; and when the temperature of the reducing gas after the partial combustion step is 1000°C or higher, the water vapor concentration of the reducing gas is set to 24 volume% or less.

2. The method for producing reduced iron as described in claim 1, characterized in that in the blowing step, the water vapor concentration of the reducing gas is set to 10 volume % or less when the temperature of the reducing gas after the partial combustion step is 800°C or more and less than 900°C, the water vapor concentration of the reducing gas is set to 17 volume % or less when the temperature of the reducing gas after the partial combustion step is 900°C or more and less than 1000°C, and the water vapor concentration of the reducing gas is set to 19 volume % or less when the temperature of the reducing gas after the partial combustion step is 1000°C or more.

3. The method for producing reduced iron according to claim 1, wherein the raw material gas contains hydrogen gas as a main component.

4. The method for producing reduced iron according to claim 2, wherein the raw material gas contains hydrogen gas as a main component.

5. The method for producing reduced iron according to any one of claims 1 to 4, characterized in that in the partial combustion step, the raw material gas and the oxygen gas are introduced into a heating furnace, and the raw material gas is partially combusted in the heating furnace.

6. The method for producing reduced iron according to claim 3 or 4, characterized in that in the partial combustion step, the raw material gas having a temperature of 100°C or less is heated to 897°C or higher by partially combusting it with the oxygen gas.

7. The method for producing reduced iron according to claim 3 or 4, characterized in that in the partial combustion step, the raw material gas having a temperature of 100°C or less is heated to 969°C or higher by partially combusting it with the oxygen gas.

8. The method for producing reduced iron according to any one of claims 1 to 4, further comprising: a preheating step of preheating the raw material gas; and in the partial combustion step, the preheated raw material gas is partially combusted with the oxygen gas.

9. The method for producing reduced iron according to claim 8, characterized in that in the preheating step, the raw material gas is preheated using sensible heat contained in the exhaust gas from the shaft furnace.

10. The method for producing reduced iron according to claim 9, further comprising a circulation process for introducing hydrogen gas obtained by dehydrating the exhaust gas after the preheating process into a heating furnace.

11. A method for producing reduced iron, comprising: an iron oxide charging step of charging an iron oxide raw material into a shaft furnace; a partial combustion step of partially combusting the raw material gas with oxygen gas to heat the raw material gas to 800°C or higher and generate a reducing gas; and an injection step of injecting the reducing gas into the shaft furnace, wherein a relationship between a water vapor concentration, which is the concentration of the water vapor in terms of volumetric ratio of the hydrogen gas and water vapor in the reducing gas, and a reduction rate of reduced iron is determined for each temperature of the reducing gas, and the water vapor concentration is controlled in accordance with a target reduction rate and the temperature of the reducing gas based on the relationship, or the temperature of the reducing gas is controlled in accordance with a target reduction rate and the water vapor concentration based on the relationship.

12. An apparatus for producing reduced iron using the method for producing reduced iron as defined in claim 6, comprising: a heating furnace connected to piping for heating and partially combusting the introduced raw material gas and oxygen gas to obtain reducing gas; and a shaft furnace into which the reducing gas obtained in the heating furnace is blown in, wherein the heating furnace is configured so that the raw material gas is supplied directly from a raw material gas supply source through the piping.

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