Reduction furnace operation method and reduced iron production method

Abstract

Provided is a reduced iron production method with which a further reduction in CO2 emission is rendered possible. A reducing gas is made to have an H2 concentration of 90 vol% or higher. Iron oxide in a reduction furnace is made to have such a particle diameter distribution along the furnace-radial direction that the average particle diameter of the iron oxide located at the center of the reduction furnace is larger than the average particle diameter of the iron oxide located by the wall part of the reduction furnace.

Description

Method for operating a reduction furnace and method for producing reduced iron

[0001] The present invention relates to a method for operating a reduction furnace and a method for producing reduced iron.

[0002] In recent years, steelworks have been strongly required to save energy against the backdrop of global environmental issues and the depletion of fossil fuels. The raw material for iron is mainly iron oxide, and a reduction process to reduce this iron oxide is essential in steelworks. The most common reduction process that is widespread worldwide is the blast furnace. In a blast furnace, coke or pulverized coal reacts with oxygen in hot air (air heated to about 1200°C) in the tuyere. This reaction produces CO and H, which become reducing gases. ２ These reducing gases are used to reduce iron ore and other materials in the furnace. Thanks to recent improvements in blast furnace operation technology, the reducing agent rate (the amount of coke and pulverized coal used per ton of molten iron produced) has been reduced to approximately 500 kg / t, which is already at its lower limit. Therefore, a further significant reduction in the reducing agent rate cannot be expected.

[0003] Meanwhile, in areas where natural gas is produced, a method of producing reduced iron using a vertical reduction furnace (hereinafter also referred to as a shaft furnace) is also commonly used. In this method, a reduction furnace is filled with iron ore agglomerates such as sintered ore or pellets as iron oxide. Then, CO and H generated from natural gas are added to the reduction furnace. ２ A reducing gas containing Fe is blown in to reduce iron oxide in accordance with the following formula, thereby producing reduced iron: ２ O ３ +3CO→2Fe+3CO ２ ... (i) Fe ２ O ３ +3H ２ → 2Fe + 3H ２ O... (ii)

[0004] An example of a process for producing reduced iron is shown in Figure 1. In the figure, reference numeral 1 denotes a reduction furnace, 1a denotes iron oxide, 1b denotes reduced iron, 3 denotes a dust removal device, 4 denotes a dehydration device, 5 denotes a natural gas supply section, 6 denotes an air supply section, and 7 denotes a reformer.

[0005] In this process of producing reduced iron, iron oxide is charged into the top of the reduction furnace and gradually lowered. High-temperature reducing gas is blown into the furnace from the center to reduce the iron oxide. Reduced iron is discharged from the bottom of the reduction furnace. Meanwhile, gases, mainly CO and CO2, are discharged from the top of the reduction furnace. ２ , H ２ , H ２ The furnace top gas containing O is discharged. After dust collection and cooling, a portion of the furnace top gas is sent to the reformer as a raw material for reformed gas. In the reformer, a reforming reaction occurs between the furnace top gas and natural gas supplied from the outside, and mainly CO and H are produced. ２ This reducing gas is then blown into the reducing furnace. The remaining part of the furnace top gas is dehydrated and then used as heating fuel in the combustion chamber of the reformer, where it is burned, for example, with oxygen in the air.

[0006] As a technology related to such a process for producing reduced iron, for example, Patent Document 1 discloses the following: "A method for producing reduced iron by reducing iron oxide, comprising: a reduced iron producing step of reducing the iron oxide by bringing the iron oxide into contact with a reducing gas while causing it to fall from a top of a reduction furnace to reduce the iron oxide to produce reduced iron, and discharging the reduced iron from the bottom of the reduction furnace; a reformed gas producing step of extracting furnace top gas from the reduction furnace, adjusting the moisture content thereof, and performing dust removal treatment thereon to produce a process gas, and supplying at least the process gas into a reformer to produce a reformed gas containing carbon monoxide and hydrogen in the reformer; a reducing gas supplying step of supplying the produced reformed gas to the reduction furnace as the reducing gas; a cooling step of introducing a cooling gas into a cooling region set in a lower part of the reduction furnace to cool the cooling region; and a reformed gas introducing step of extracting a portion of the reformed gas and introducing it into the cooling region to increase the carbon content of the reduced iron passing through the cooling region."

[0007] JP 2017-88912 A

[0008] However, in the conventional reduced iron production process as disclosed in Patent Document 1, a certain amount of CO ２In recent years, from the perspective of preventing global warming, CO ２ Therefore, CO emissions are being further reduced in the production process of reduced iron. ２ The current situation is one in which further reductions in emissions are required.

[0009] The present invention has been developed in view of the above-mentioned circumstances, ２ It is an object of the present disclosure to provide a method for producing reduced iron that can achieve a further reduction in the amount of waste. Note that in this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower limit and upper limit, respectively, except when written as "greater than" or "less than."

[0010] The inventors have conducted extensive research to solve the above problems and have found the following: (1) As described above, in the conventional reduced iron production process such as that described in Patent Document 1, CO and H ２ The reducing gas usually contains a CO concentration of about 40% and H ２ The concentration of the reducing gas is about 60%. As shown in FIG. 2, heat exchange (heat transfer) and a reduction reaction occur between the reducing gas and iron oxide. This produces reduced iron (Fe) according to the above formulas (i) and (ii). At the same time, CO ２ (2) The inventors focused on the above reduction reaction and reduced the CO concentration of the reducing gas while increasing the H ２ In other words, the ratio of the reduction reaction according to the above formula (i) in the total reduction reaction is decreased, while the ratio of the reduction reaction according to the above formula (ii) is increased. ２ (3) However, the reduction gas H ２ It was found that excessively increasing the concentration, especially above 90% by volume, causes the reduction reaction to stagnate in the center of the reduction furnace, significantly reducing the reaction efficiency. It was also found that the gas flow rate near the furnace wall increases, increasing heat loss to the outside of the reduction furnace.

[0011] Therefore, the inventors have conducted extensive research into the cause of the above (3) and have come to the following findings: (4) The inertial force F of the fluidｉ is generally expressed as follows: ｉ ∝ ρv ２ In this equation, ρ is the density of the fluid [kg / m ３ ], v is the fluid velocity [m / s]. From this formula, when the fluid velocity is the same, the magnitude relationship of the inertial force is determined by the magnitude of the fluid density. ２ A gas with a concentration of 60% (hereinafter referred to as CO-H ２ (also called mixed gas) and H ２ A gas with a concentration of 100% (hereinafter referred to as H ２ When compared with gas, H ２ Since the molecular weight of is smaller than that of CO, ２ The density of gas is lower. For example, at 900°C and 101.3 kPaG, CO-H ２ Mixture of gases and H ２ The density of the gas is 0.58 kg / m ３ and 0.04 kg / m ３ Therefore, the inertial force is also H ２ The gas is CO-H ２ (5) As shown in FIG. 2, the reducing gas injected into the reducing furnace is attenuated and its velocity decreases as it approaches the center of the furnace due to friction with the iron oxide particles packed in the furnace. The degree of this velocity decrease of the reducing gas depends on the inertial force of the gas. As mentioned in (4) above, H ２ The inertial force of the gas is CO-H ２ The inertial force of the mixed gas is smaller than that of the reducing gas. ２ When gas is used, the reducing gas is not sufficiently supplied to the vicinity of the center of the reducing furnace. As a result, the reduction reaction stagnates in the center of the reducing furnace, significantly reducing the reaction efficiency. Furthermore, as the reducing gas is not sufficiently supplied to the vicinity of the center of the reducing furnace, the gas flow rate near the furnace wall increases, and heat loss to the outside of the reducing furnace increases.

[0012] Based on the above findings, the inventors conducted further research and discovered the following: (6) In the particle size distribution of iron oxide (iron oxide filled in the reducing furnace) in the radial direction of the reducing furnace, it is effective to make the average particle size of iron oxide located in the center of the reducing furnace larger than the average particle size of iron oxide located in the furnace wall of the reducing furnace. This increases the gaps between the iron oxides located in the center of the reducing furnace, reduces friction between the reducing gas and the iron oxide in that area, and ultimately improves the ease of gas flow (permeability) in the center of the reducing furnace. Therefore, when H is used as the reducing gas, ２ Even when gas is used, the reducing gas is easily supplied to the center of the reducing furnace. As a result, stagnation of the reduction reaction in the center of the reducing furnace is suppressed, and desired reaction efficiency can be obtained. In addition, heat loss to the outside of the reducing furnace can be suppressed.

[0013] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0014] 1. A method for operating a reducing furnace, comprising: a charging step of charging iron oxide into the reducing furnace; an injecting step of injecting a reducing gas into the reducing furnace; and a reducing step of reducing the iron oxide with the reducing gas in the reducing furnace to obtain reduced iron, wherein the H ２ a concentration of iron oxide in the reducing furnace is 90% by volume or more, and in a particle size distribution of iron oxide in a radial direction of the reducing furnace, an average particle size of iron oxide located at a center portion of the reducing furnace is larger than an average particle size of iron oxide located at a furnace wall portion of the reducing furnace.

[0015] 2. The method for operating a reducing furnace according to 1, wherein a ratio of an average particle size of iron oxide located in a center portion of the reducing furnace to an average particle size of iron oxide located in a furnace wall portion of the reducing furnace is 2.00 or more.

[0016] 3. The method for operating a reducing furnace according to 1 or 2, wherein the particle size distribution of iron oxide in the radial direction of the reducing furnace satisfies the following formula (1): [Center particle size] > [Intermediate particle size] > [Furnace wall particle size] (1). Here, the [Center particle size], [Intermediate particle size], and [Furnace wall particle size] are the average particle sizes of iron oxide located in the center, intermediate, and furnace wall of the reducing furnace, respectively. The center, intermediate, and furnace wall of the reducing furnace are the regions of the dimensionless radius of the reducing furnace that are 0 to 0.57, greater than 0.57 to 0.81, and greater than 0.81 to 1.00, respectively. The dimensionless radius of the reducing furnace is the dimensionless radius in the radial direction of the furnace, with the center position of the reducing furnace being 0 and the furnace wall of the reducing furnace being 1.00.

[0017] 4. The method for operating a reducing furnace according to any one of 1 to 3, wherein in the charging step, the charging position of the iron oxide is a furnace wall portion of the reducing furnace.

[0018] 5. A method for producing reduced iron, comprising producing reduced iron by the method for operating a reduction furnace according to any one of 1 to 4 above.

[0019] According to the present invention, CO ２ This will enable the production of reduced iron while achieving further reductions in CO emissions, resulting in a higher level of CO ２ It will be possible to contribute to preventing global warming by reducing emissions.

[0020] 1 is a diagram showing an example of a process for producing reduced iron. 2 is a diagram schematically showing the heat exchange (heat transfer) between a reducing gas and iron oxide inside a reducing furnace and the reduction reaction. 3 is a diagram (contour diagram) showing the calculation results of the gas flow in the iron oxide packed bed inside the reducing furnace. 4 is a diagram showing the calculation domain of FIG. 3.

[0021] A method for operating a reducing furnace according to one embodiment of the present invention will now be described.

[0022] A method for operating a reducing furnace according to one embodiment of the present invention includes: a charging step of charging iron oxide into the reducing furnace; an injecting step of injecting a reducing gas into the reducing furnace; and a reducing step of reducing the iron oxide with the reducing gas in the reducing furnace to obtain reduced iron.

[0023] Here, the H of the reducing gas blown into the reduction furnace２ The concentration is set to 90% by volume or more. This significantly reduces the ratio of the reduction reaction according to the above formula (i) in the total reduction reaction, and ２ The amount of H generated as a reducing gas is also significantly reduced. ２ The concentration is 90% by volume or more. ２ The concentration is preferably 95% by volume or more, more preferably 98% by volume or more. ２ The upper limit of the concentration is not particularly limited, and may be 100% by volume. ２ The type of the remaining gas other than the above is not particularly limited, but for example, N ２ Examples include:

[0024] In the method of operating a reducing furnace according to one embodiment of the present invention, it is important that, in the particle size distribution of iron oxide (iron oxide filled in the reducing furnace) in the radial direction of the reducing furnace (hereinafter simply referred to as the particle size distribution in the furnace radial direction), the average particle size of iron oxide located at the center of the reducing furnace (hereinafter also referred to as the center particle size) is larger than the average particle size of iron oxide located in the wall portion of the reducing furnace (hereinafter also referred to as the furnace wall particle size).

[0025] As described above, the H of the reducing gas injected into the reduction furnace ２ If the concentration is increased to 90% by volume or more, the reduction reaction stagnates near the center of the reducing furnace, significantly reducing the reaction efficiency. Here, by making the particle diameter at the center larger than the particle diameter at the furnace wall, the gas flowability (permeability) near the center of the reducing furnace is improved. Therefore, the H ２Even when the concentration is 90% by volume or higher, the reducing gas is more likely to be delivered to the vicinity of the center of the reducing furnace. As a result, stagnation of the reduction reaction near the center of the reducing furnace is suppressed, and the desired reaction efficiency is achieved. Therefore, the particle size at the center is made larger than the particle size at the furnace wall. In other words, the ratio of the average particle size of iron oxide located at the center of the reducing furnace to the average particle size of iron oxide located at the furnace wall, [center particle size] / [furnace wall particle size], is made to exceed 1.00. [center particle size] / [furnace wall particle size] is preferably 2.00 or more, more preferably 2.40 or more. There is no particular upper limit to the ratio [center particle size] / [furnace wall particle size]. For example, [center particle size] / [furnace wall particle size] is preferably 3.40 or less.

[0026] Here, the center and wall of the reducing furnace are defined as regions of the dimensionless radius of the reducing furnace that are 0 to 0.57 and greater than 0.81 to 1.00, respectively. The dimensionless radius of the reducing furnace is a dimensionless radius in the furnace radial direction, with the center position of the reducing furnace being 0 and the wall of the reducing furnace being 1.00.

[0027] Furthermore, when the region of the dimensionless radius of the reducing furnace that is greater than 0.57 and less than 0.81 is defined as the intermediate portion, it is preferable that the particle size distribution in the radial direction of the furnace satisfy the following formula (1): [Particle size at the center] > [Particle size at the intermediate portion] > [Particle size at the furnace wall portion] (1) Here, [Particle size at the center], [Particle size at the intermediate portion], and [Particle size at the furnace wall portion] are the average particle sizes of iron oxide located at the center, intermediate portion, and furnace wall portion of the reducing furnace, respectively.

[0028] The central particle size, intermediate particle size, and furnace wall particle size may be measured, for example, by a model experiment. One example is as follows: A reduction furnace model of the same size as the actual furnace is manufactured. Approximately 10 kg of iron oxide is charged into the reduction furnace model under the same conditions as the actual furnace, using the same brand of iron oxide as that charged into the actual furnace. The iron oxide located in the regions of the reduction furnace model that correspond to the central, intermediate, and furnace wall areas is then sampled. The sampled iron oxide is then sieved through sieves with 1 mm pitch ranging from 1 mm to 25 mm, in descending order of mesh size, to determine the mass of the iron oxide remaining on each sieve and the product of the sieve mesh size. The sum of the calculated products is then divided by the total mass of the iron oxide sieved to determine the central particle size, intermediate particle size, and furnace wall particle size. This can be expressed by the following formula: D = (d ２５ ×25+d ２４ ×24+...+d １ × 1) / M where, D: center particle diameter, middle particle diameter or furnace wall particle diameter (mm) d ｎ : mass (kg) of iron oxide remaining on a sieve with nmm sieve openings; M: total mass (kg) of iron oxide sieved; n: sieve opening size, an integer from 1 to 25.

[0029] As described above, the iron oxide used may be, for example, agglomerated iron ore such as sintered ore or pellets (hereinafter also referred to as agglomerated ore, etc.). Agglomerated ore may contain components other than iron oxide, and the particle size of iron oxide referred to here means the particle size of particles such as agglomerated ore that also contain components other than iron oxide.

[0030] The above-described particle size distribution in the furnace radial direction can be obtained by adjusting the charging position of the iron oxide. For example, the charging position of the iron oxide can be set to the furnace wall of the reducing furnace. By adjusting the charging position of the iron oxide in this way, a mountain-shaped iron oxide packed layer is formed on the furnace wall of the reducing furnace. When iron oxide is further charged to the furnace wall of the reducing furnace in this state, the iron oxide with larger particle sizes rolls toward the center, while the iron oxide with smaller particle sizes does not move to the center but remains on the furnace wall or in the middle. Therefore, it is possible to achieve the above-described particle size distribution in the furnace radial direction. The above-described particle size distribution in the furnace radial direction can also be obtained by adjusting the shape of the feeder used to charge the iron oxide.

[0031] The conditions for each step other than those mentioned above are not particularly limited and may be in accordance with conventional methods. For example, the flow rate of the blown reducing gas is 200,000 to 400,000 Nm ３ The temperature of the blown reducing gas is preferably 850 to 1050°C.

[0032] The method for operating a reduction furnace according to one embodiment of the present invention is particularly directed to a method using a shaft furnace as a direct reduction ironmaking process. Shaft furnaces have the advantages of high production efficiency, availability, and operational stability. Examples of shaft furnace types include Midrex (registered trademark) and HyL (registered trademark).

[0033] In addition, a method for producing reduced iron according to one embodiment of the present invention produces reduced iron by the above-described method for operating a reduction furnace. Note that conditions other than those described above are not particularly limited and may be those according to conventional methods.

[0034] Using a two-dimensional mathematical model, calculations were made of the gas flow in the iron oxide packed bed inside the reduction furnace when the reduction furnace was operated under the conditions in Table 1. The analysis conditions were as follows. A representative example of the calculation results is shown in Figure 3. The conditions of the reference example in Table 1 are assumed to be the operating conditions of a general shaft furnace. Figure 4 shows the calculation domain of Figure 3. (Analysis conditions) - Calculation domain size: 2m x 4m - 25 x 66 divisions - Size of gas injection hole for reducing gas: Φ150mm - Injection speed of reducing gas: 100m / s - Void fraction inside furnace: 0.3 - The equation of continuity and the Navier-Stokes equation were discretized using the SMAC method and numerically analyzed.

[0035]

[0036] 3, in the inventive example, the velocity of the reducing gas near the center of the reducing furnace was increased and the velocity of the reducing gas near the furnace wall was decreased compared to the comparative example. Furthermore, in the inventive example, the velocity of the reducing gas near the center of the reducing furnace was increased and the velocity of the reducing gas near the furnace wall was decreased compared to the reference example.

[0037] In addition, when the reduction furnace was operated under the conditions in Table 1 in the actual reduction furnace, the CO ２ It was possible to produce reduced iron with reaction efficiency comparable to that of the reference example while significantly reducing the amount of H2 emitted from the reducing gas. ２ When reduced iron was produced under various conditions in which the iron oxide particle size distribution in the radial direction of the reducing furnace was varied with a concentration of 90% by volume or more, the same results as above were obtained in all cases in which the particle size at the center was larger than that at the furnace wall. Furthermore, in the examples of the invention shown in Table 1, the above formula (1) was satisfied, and therefore reduced iron was produced with particularly excellent reaction efficiency.

[0038] REFERENCE SIGNS LIST 1 reduction furnace 1a iron oxide 1b reduced iron 3 dust removal device 4 dehydration device 5 natural gas supply section 6 air supply section 7 reformer

Claims

1. A method for operating a reduction furnace, comprising: a charging step of charging iron oxide into the reduction furnace; a blowing step of blowing a reducing gas into the reduction furnace; and a reduction step of reducing the iron oxide with the reducing gas in the reduction furnace to obtain reduced iron, wherein the H ２ concentration of the reducing gas is 90% by volume or more, and in the particle size distribution of iron oxide in the furnace radius direction in the reduction furnace, the average particle size of the iron oxide located at the center of the reduction furnace is larger than the average particle size of the iron oxide located at the furnace wall portion of the reduction furnace.

2. The operation method of the reduction furnace according to claim 1, wherein the ratio of the average particle size of iron oxide located at the furnace wall portion of the reduction furnace to the average particle size of iron oxide located at the central portion of the reduction furnace is 2.00 or more.

3. The operation method of the reduction furnace according to claim 1 or 2, which satisfies the following formula (1) in the particle size distribution of iron oxide in the radial direction of the furnace radius in the reduction furnace. [Central portion particle size] > [Intermediate portion particle size] > [Furnace wall portion particle size] ・・・(1) Here, [Central portion particle size], [Intermediate portion particle size], and [Furnace wall portion particle size] are the average particle sizes of iron oxide located at the central portion, intermediate portion, and furnace wall portion of the reduction furnace, respectively. In addition, the central portion, intermediate portion, and furnace wall portion of the reduction furnace are the regions of dimensionless radius of the reduction furnace of 0 to 0.57, the region of more than 0.57 to 0.81, and the region of more than 0.81 to 1.00, respectively. The dimensionless radius of the reduction furnace is the dimensionless radius in the furnace radius direction with the center position of the reduction furnace being 0 and the furnace wall of the reduction furnace being 1.

00.

4. The operation method of the reduction furnace according to any one of claims 1 to 3, wherein in the filling step, the filling position of the iron oxide is the furnace wall portion of the reduction furnace.

5. A method for producing reduced iron, which produces reduced iron by the operation method of the reduction furnace according to any one of claims 1 to 4.

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