Reduced iron manufacturing method

Abstract

This method for producing reduced iron includes an iron oxide raw material preheating step of preheating an iron oxide raw material to a predetermined preheat temperature, an iron oxide charging step of charging the preheated iron oxide raw material into a shaft furnace, a partial combustion step of generating reducing gas by partially burning a raw material gas with oxygen gas, and an injection step of injecting the reducing gas into the shaft furnace in which the preheated iron oxide raw material has been charged.

Description

[Technical Field]

[0001] The present invention relates to a method for producing reduced iron. This application claims priority based on Japanese Patent Application No. 2024-005112, filed on January 17, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] The method of producing reduced iron using a shaft furnace (shaft furnace operation) is a representative 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, we will provide an overview of the method of producing reduced iron using a shaft furnace. First, oxidized iron raw materials (e.g., iron oxide pellets) are charged from above the shaft furnace, and reducing gas is injected from below the shaft furnace into which the oxidized iron raw materials have been charged. The reducing gas is heated to a predetermined temperature (e.g., approximately 900 to 950°C) and then injected into the shaft furnace. The reducing gas injected into the shaft furnace then reduces the oxidized iron raw materials in the shaft furnace. This direct reduction process produces reduced iron. The reduced iron is discharged from the bottom of the shaft furnace and cooled. Top gas (exhaust gas) containing hydrogen gas, CO gas, water vapor, and CO2 gas is discharged from the top of the shaft furnace. After water vapor is removed from the top gas, the hydrogen gas and CO2 gas in the top gas are reused as part of the raw material gas. In some cases, CO2 gas is removed from the top gas after water vapor is removed.

[0003] The reducing gas used in shaft furnaces is obtained by reforming raw gas containing carbon (such as natural gas or coke oven gas) with steam, CO2 gas, or oxygen gas. Alternatively, the raw gas can be used as the reducing gas without reforming it. The main components of reducing gas are hydrogen gas (H2), CO gas (CO), and CH4 gas.

[0004] Recently, in order to further reduce CO2 gas emissions, techniques for increasing the concentration of hydrogen gas in reducing gas have been studied. For example, Patent Document 1 discloses a technique using reducing gas with a hydrogen gas concentration of 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. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022-169392 Summary of the Invention [Problem to be solved by the invention]

[0006] Incidentally, when hydrogen gas is partially combusted, water (steam) is generated. The steam is discharged as top furnace gas. The inventors have found that an increase in the steam concentration in the top furnace gas reduces the reduction potential at the furnace top, making the iron oxide raw material more susceptible to reduction disintegration. When 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 unstable operation.

[0007] Therefore, the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a new and improved method for producing reduced iron that can suppress reduction disintegration of an oxidized iron raw material even when a raw material gas containing hydrogen gas is partially combusted. [Means for solving the problem]

[0008] The gist of the present invention is as follows. [1] Iron oxide raw material at 300°C or higher Preheat temperaturea partial combustion step of partially burning a raw material gas containing hydrogen gas with oxygen gas to generate a reducing gas; and a blowing step of blowing the reducing gas into the shaft furnace in which the preheated iron oxide raw material has been charged, wherein X is a water vapor concentration in volume % of the furnace gas of the shaft furnace; Prediction and Y, a thermal temperature, satisfy the following formula (5): Y>10X+300 (X≧30) (5) [2] Iron oxide raw material at 300°C or higher Preheat temperature a partial combustion step of partially burning a raw material gas containing hydrogen gas with oxygen gas to generate a reducing gas; and a blowing step of blowing the reducing gas into the shaft furnace in which the preheated iron oxide raw material has been charged, wherein X is a water vapor concentration in volume % of the furnace gas of the shaft furnace; Prediction A method for producing reduced iron, characterized in that the heat temperature Y satisfies the following formula (1): Y≧10X+400 (X≧20) (1) [3] The method for producing reduced iron according to [1] or [2], wherein the raw material gas contains 90% by volume or more of hydrogen gas. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a new and improved method for producing reduced iron, which can suppress reduction disintegration of an oxidized iron raw material even when a raw material gas containing hydrogen gas is partially combusted. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a test device simulating a reduced iron manufacturing device. [Figure 2] 1 is a graph showing a stable operational region (acceptable region) A1, an unstable operational region (unacceptable region) B1, and a boundary line A between the two regions. [Figure 3] 1 is a schematic diagram showing a reduced iron manufacturing apparatus according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[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 Inventor> First, the inventor's findings, which form the basis of this embodiment, will be described. The inventor assumed that preheating the oxidized iron raw material would be an effective means of suppressing reduction disintegration of the oxidized iron raw material, and conducted various experimental studies on the correlation between the water vapor concentration in the furnace top gas and the preheating temperature of the oxidized iron raw material. Based on the results, the inventor identified the preheating temperature required for the oxidized iron raw material depending on the water vapor concentration in the furnace top gas, and completed the reduced iron manufacturing method according to this embodiment. According to the reduced iron manufacturing method according to this embodiment, reduction disintegration can be suppressed by adjusting the preheating temperature of the oxidized iron raw material depending on the water vapor concentration in the furnace top gas. As a result, operation is stable.

[0013] (1-1. Test equipment) FIG. 1 is a schematic diagram showing the configuration of a test apparatus 100 simulating a reduced iron manufacturing apparatus. The present inventors conducted tests using this test apparatus 100. This test apparatus 100 includes a small shaft furnace 110, a raw material 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 analyzer 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 functions similarly to a conventional shaft furnace. Specifically, iron oxide raw material is charged from above the small shaft furnace 110. Subsequently, reducing gas is injected into the small shaft furnace 110 from the lower side of the small shaft furnace 110 into which the iron oxide raw material has been charged. The reducing gas injected into the small shaft furnace 110 rises within the small shaft furnace 110. The reducing gas reduces the iron oxide raw material in the small shaft furnace 110 to produce reduced iron (DRI). The reduced iron is discharged from the bottom of the small shaft furnace 110 and cooled. Furnace gas (exhaust gas) is also discharged from the top of the small shaft furnace 110. The top gas contains unreacted hydrogen gas, 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 equipped with multiple heaters and thermocouples along its height, allowing temperature adjustment and temperature measurement at each vertical position.

[0015] The raw material gas supply device 120 is a device capable of supplying various raw material gases (hydrogen gas, CO gas, CO gas, CH gas, N gas, etc.) to the heating furnace 130 via a water vapor supply device 125. In this test, hydrogen gas (100% by volume of hydrogen gas) was used as the raw material gas.

[0016] In addition to the raw material gas, steam is introduced into the heating furnace 130 from the steam supply device 125 to simulate oxygen combustion. In this test, the temperature of the reducing gas containing the raw material gas and steam 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 this pipe. 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 pipe and the small shaft furnace 110, and the temperature of the reducing gas is measured using this thermocouple.

[0017] The raw material hopper 140 stores the iron oxide raw material and charges it into the small shaft furnace 110 from above. In this test, fired pellets with an average particle size of 12.5 mm and a reduction degradation index (RDI) of 5% were used as the iron oxide raw material. The particle size distribution was measured according to the method specified in JIS M8706:2015, and the average particle size was measured according to the method specified in Annex J of the same JIS. The reduction degradation index was measured according to the method specified in ISO11257:2022. This reduction degradation index is the upper limit that meets the current quality standards.

[0018] A heating furnace (not shown) is disposed upstream of the raw material hopper 140 , and 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 below the small shaft furnace 110 to the reduced iron hopper 160. The reduced iron hopper 160 cools the reduced iron while storing it. The reduced iron hopper 160 discharges the cooled reduced iron downward.

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

[0021] (1-2. Test Method) The inventors performed shaft furnace operation using the test apparatus 100 by varying the water vapor concentration in the furnace top gas and the preheating temperature of the oxidized iron raw material, and evaluated the operating conditions. The water vapor concentration in the furnace top gas was adjusted by adjusting the amount of water vapor generated from the water vapor supply device 125. Cases where operation was stable (i.e., no clogging due to reduction disintegration occurred) for more than five hours after the start of operation were evaluated as A. Cases where the small shaft furnace 110 became clogged within five hours after the start of operation and reduced iron could no longer be discharged were evaluated as B. Case B is considered to indicate reduction disintegration in the small shaft furnace 110. The test results are summarized in Table 1. When the water vapor concentration in the furnace top gas was less than 20% by volume, reduction disintegration did not occur, regardless of the preheating temperature of the oxidized iron raw material.

[0022] [Table 1]

[0023] Figure 2 is obtained by graphing Table 1. Figure 2 shows the stable operational region A1, metastable operational region A2, and unstable operational region B1, as well as the boundary line A between the stable operational region A1 and metastable operational region A2, and the boundary line B between the metastable operational region A2 and unstable operational region B1. As shown in Table 1 and Figure 2, there is a correlation between the preheating temperature of the oxidized iron raw material, the water vapor concentration in the furnace gas, and the occurrence of reduction disintegration. According to Figure 2, when the X-axis (horizontal axis) represents the water vapor concentration in the furnace gas (volume %) and the Y-axis (vertical axis) represents the preheating temperature of the oxidized iron raw material (°C), each of the above regions can be expressed by the following equation. The following equation was derived from the relationship between the preheating temperature of the oxidized iron raw material (Y) and the water vapor concentration in the furnace gas (X) at boundary line A and boundary line B in Figure 2, which is a graph of the test results in Table 1. The stable operational region A1 is expressed by the following equations (1) and (2). Y≧10X+400 (X≧20) (1) Y = Any temperature (X<20) (2) The operational metastable region A2 is expressed by the following equations (3) and (4). 10X+300 <Y<10X+400 (X≧30) (3) Y<10X+400 (20 <X<30) (4)

[0024] Therefore, when operating a shaft furnace using the iron oxide raw material used in this test, the iron oxide raw material should be preheated to the preheating temperatures corresponding to the stable operational region A1 and the metastable operational region A2 shown in the following equations (5) and (6) before being charged into the shaft furnace. Y>10X+300 (X≧30) (5) Y = Any temperature (X<30) (6) More preferably, the oxidized iron raw material is preheated to a preheating temperature corresponding to the stable operation region A1 shown in the above formulas (1) and (2) and then charged into the shaft furnace. If the oxidized iron raw material is preheated to a preheating temperature corresponding to the stable operation region A1 and then charged into the shaft furnace, the effect of suppressing reduction disintegration can be more reliably enjoyed.

[0025] In actual operation, the stable operational region is likely to vary depending on the reduction degradation index of the iron oxide raw material. Therefore, the above-described test should be performed for each iron oxide raw material actually used to identify the preheating temperatures (=predetermined preheating temperatures) that result in the stable operational region and the semi-unstable operational region. The results of the above test were obtained with a reduction degradation index of 5%, but similar results can be obtained if the reduction degradation index is less than 5%. Furthermore, the reduction degradation index of the iron oxide raw material used in the above-described test was 5%, which is the upper limit that meets the current quality standards. Therefore, if the reduction degradation index of the iron oxide raw material used meets the current quality standards, it is likely that reduction degradation can be suppressed by preheating the iron oxide raw material so that at least equations (1) and (2) (effectively equation (1)) are satisfied. The upper and lower limits 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, more preferably 400°C.

[0026] <2. Reduced iron manufacturing equipment> Next, a description will be given of a reduced iron manufacturing apparatus 1A according to this embodiment. Fig. 3 is a schematic diagram showing the configuration of the reduced iron manufacturing apparatus 1A according to this embodiment. As shown in Fig. 3, the reduced iron manufacturing apparatus 1A according to this embodiment includes a shaft furnace 1, a heating furnace 2, a furnace top gas treatment device 3, an oxidized iron raw material preheating furnace 4, and a flow meter (such as a mass flow meter) 5.

[0027] The shaft furnace 1 is the same as a conventional one. That is, the iron oxide raw material is charged from above the shaft furnace 1. There is no particular restriction on the type of iron oxide raw material, as long as it is the same as in existing shaft furnace operation. An example of the iron oxide raw material is iron oxide pellets (such as fired pellets). Next, reducing gas is injected into the shaft furnace 1 from the lower side of the shaft furnace 1 into which the iron oxide raw material has been charged. The reducing gas injected into the shaft furnace 1 rises within the shaft furnace 1. The reducing gas reduces the iron oxide raw material in the shaft furnace 1 to produce reduced iron (DRI). The reduced iron is discharged from the bottom of the shaft furnace 1 and cooled. In addition, top gas (exhaust gas) is discharged from the top of the shaft furnace 1. The top gas contains unreacted hydrogen gas as well as water vapor, dust, and the like.

[0028] A raw material gas and oxygen gas are introduced into the heating furnace 2. The raw material gas preferably contains hydrogen gas as a main component, and is preferably composed of hydrogen gas. "The raw material gas contains hydrogen gas as a main component" means that the raw material gas contains 90% by volume or more of hydrogen gas. The raw material gas may contain trace amounts (10% by volume or less) of gases other than hydrogen gas, as long as the effects of this embodiment are not impaired. Examples of gases other than hydrogen gas include CO gas, CO2 gas, and CH4 gas. The hydrogen gas used as the raw material gas may be electrolytic hydrogen gas, coke oven gas (COG), or hydrogen gas obtained by separating hydrogen from gas obtained by an aqueous gas shift reaction using a PSA method or a membrane separation method.

[0029] The heating furnace 2 partially combusts the raw material gas using oxygen gas. Specifically, the heating furnace 2 mainly partially combusts the hydrogen gas in the raw material gas. Partial combustion means that when the raw material gas and oxygen are reacted, the H2 gas and CO gas in the raw material gas are not completely burned, but rather a certain amount or more of H2 gas and CO gas remain in the raw material gas. The certain amount means, for example, that the proportion of H2 gas and CO gas burned is 50% by volume. In this way, the heating furnace 2 heats the raw material gas and generates a reducing gas. The heating temperature is not particularly limited as long as it is the same as that of conventional shaft furnace operation. For example, the heating temperature may be 800 to 1150°C.

[0030] The heating furnace 2 and the shaft furnace 1 are connected by a pipe, and the reducing gas is blown through this pipe into the shaft furnace 1. 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.

[0031] If the reducing gas cannot be heated to a desired temperature by only partial combustion of the raw material gas, a separate heating means (heater, etc.) may be provided in the heating furnace 2, and the reducing gas may be heated by this heating means.

[0032] The top gas treatment device 3 recovers, dehydrates, and removes dust from the top gas discharged from the top of the shaft furnace 1. When a raw material gas containing hydrogen gas as a main component is used, the top gas after treatment becomes mostly hydrogen gas, and therefore may be reused as the raw material gas or as fuel gas in other processes.

[0033] The oxidized iron raw material preheating furnace 4 is provided upstream of a raw material hopper (not shown), and preheats the oxidized iron raw material to a predetermined preheating temperature in advance (before being charged into the shaft furnace 1). Here, the predetermined preheating temperature is a preheating temperature determined by the above-mentioned test method. In other words, the predetermined preheating temperature is a preheating temperature at which the shaft furnace can be operated stably for 5 hours or more (without clogging due to reduction disintegration).

[0034] For example, the iron oxide raw material is preheated so that X, which is the water vapor concentration in the furnace top gas of the shaft furnace 1 expressed in volume %, and Y, which is a predetermined preheating temperature, satisfy the following formulas (1) and (2) (substantially formula (1)). Y≧10X+400 (X≧20) (1) Y = Any temperature (X<20) (2)

[0035] The preheating temperature Y of the oxidized iron raw material satisfies the above formula (1) and is preferably closer to the boundary line A (Y = 10X + 400), because this allows the water vapor concentration X in the furnace top gas to be reduced, i.e., the degree of partial oxidation to be reduced. The method for measuring the water vapor concentration X will be described later.

[0036] The specific preheating means used in the iron oxide raw material preheating furnace 4 is not particularly limited. For example, electricity, gas, microwaves, and furnace top gas sensible heat or waste heat can be used as preheating means. The preheating method is also not particularly limited, and may be a batch type, rotary kiln type, shaft furnace type, or the like. The iron oxide raw material may be preheated in a raw material hopper. Alternatively, if the iron oxide raw material has sensible heat generated during its production (for example, fired pellets immediately after production are 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 preheated iron oxide raw material is temporarily stored in a raw material hopper and then charged into the shaft furnace 1. The preheat temperature of the iron oxide raw material can be measured by a thermocouple or a radiation thermometer or the like provided in the raw material hopper. The iron oxide raw material preheating furnace 4 is controlled so that the preheat temperature measured by these measuring means becomes the predetermined preheat temperature.

[0038] The flow meter 5 measures the flow rate of the top gas after treatment by the top gas treatment device 3. The water vapor concentration of the top gas (before treatment) is calculated by the following equation (7). Water vapor concentration in top gas = 1 - top gas flow rate after top gas treatment / reducing gas flow rate (7) The flow rate of the top gas after the top gas treatment is measured by a 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 for manufacturing reduced iron using the reduced iron manufacturing apparatus 1 A will be described. The method for manufacturing reduced iron according to this embodiment includes an iron oxide raw material preheating step of preheating an iron oxide raw material to a predetermined preheat temperature, an iron oxide charging step of charging the preheated iron oxide raw material into the shaft furnace 1, a partial combustion step of generating reducing gas by partially burning a raw material gas with oxygen gas, and an injection step of injecting the reducing gas into the shaft furnace 1 in which the preheated iron oxide raw material has been charged.

[0040] Here, the predetermined preheating temperature is the preheating temperature determined by the above-mentioned test method, as described above. That is, the predetermined preheating temperature is the preheating temperature at which stable operation (without clogging due to reduction disintegration) can be performed for 5 hours or more in shaft furnace operation.

[0041] Preferably, the predetermined preheating temperature can also be calculated based on the water vapor concentration of the top gas, which is calculated based on Equation (7) and the top gas flow rate / reducing gas flow rate after the top gas treatment measured by each flow meter. So far, we have explained the case where the predetermined preheating temperature is calculated based on the water vapor concentration of the top furnace gas. However, the water vapor concentration of the top furnace gas can also be calculated based on the preheating temperature of the oxidized iron raw materials. In other words, reducing disintegration can be suppressed by adjusting the water vapor concentration of the top furnace gas within a predetermined range depending on the preheating temperature of the oxidized iron raw materials. The water vapor concentration of the top furnace gas is adjusted, for example, by the amount of oxygen gas (or the basic unit) (Nm3-O2 / t-DRI) used during partial combustion or the amount of hydrogen gas (or the basic unit) (Nm3-H2 / t-DRI) injected into the shaft furnace.

[0042] According to this embodiment, even if the reduction potential at the furnace top is lowered by water vapor contained in the reducing gas, reduction disintegration of the oxidized iron raw material can be suppressed.

[0043] 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. [Explanation of symbols]

[0044] 1 shaft furnace 2 Furnace 3. Furnace gas treatment equipment 4. Iron oxide raw material preheating furnace 5. Flow meter (mass flow meter, etc.) 100 test devices 110 Small shaft furnace 120 Raw material gas supply device 125 Steam supply device 130 Heating Furnace 140 Raw material hopper 150 discharge feeder 160 Reduced iron hopper 170 Gas analyzer

Claims

1. an iron oxide raw material preheating step of preheating the iron oxide raw material to a preheating temperature of 300°C or higher; an iron oxide charging step of charging the preheated iron oxide raw material into a shaft furnace; a partial combustion step of generating a reducing gas by partially combusting a raw material gas containing hydrogen gas with oxygen gas; a blowing step of blowing the reducing gas into the shaft furnace in which the preheated iron oxide raw material is charged, wherein X, which is a water vapor concentration in volume %, of the furnace top gas of the shaft furnace, and Y, which is the preheating temperature, satisfy the following formula (5): Y>10X+300 (X≧30) (5)

2. an iron oxide raw material preheating step of preheating the iron oxide raw material to a preheating temperature of 300°C or higher; an iron oxide charging step of charging the preheated iron oxide raw material into a shaft furnace; a partial combustion step of generating a reducing gas by partially combusting a raw material gas containing hydrogen gas with oxygen gas; a blowing step of blowing the reducing gas into the shaft furnace in which the preheated iron oxide raw material is charged, wherein X, which is a water vapor concentration in volume %, of the furnace top gas of the shaft furnace, and Y, which is the preheating temperature, satisfy the following formula (1): Y≧10X+400 (X≧20) (1)

3. 3. The method for producing reduced iron according to claim 1, wherein the raw material gas contains 90% by volume or more of hydrogen gas.

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