Method for producing reduced iron

Abstract

To provide a novel and improved method for producing reduced iron capable of reducing a reduction gas unit requirement even when using a reduction gas containing hydrogen gas at a high concentration.SOLUTION: In a method for producing the reduced iron, the reduced iron is produced by reducing the iron oxide charged into a shaft furnace, and a mixed gas containing a reducing gas containing 90 vol% or more of hydrogen gas and a nitrogen gas at a predetermined temperature is blown into the shaft furnace.SELECTED DRAWING: Figure 1

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. 2020-85078, filed on May 14, 2020, the contents of which are incorporated herein by reference. [Background technology]

[0002] The method for producing reduced iron using a shaft furnace (shaft furnace operation) is a representative example of a direct reduction process for producing reduced iron from an iron oxide raw material, and is widespread mainly in regions where natural gas is available at low cost (oil-producing countries). Here, an outline of the existing shaft furnace operation will be described based on FIG. 14. In the example of FIG. 14, an iron oxide raw material (e.g., iron oxide pellets) 200 is charged from above the shaft furnace 100, and a reducing gas 300 is blown into the shaft furnace 100 from below. Here, the reducing gas is heated to a predetermined temperature (e.g., about 900 to 950°C) and then blown into the shaft furnace 100. Then, the reducing gas 300 blown into the shaft furnace 100 reduces the iron oxide raw material 200 in the shaft furnace. By such a direct reduction process, reduced iron 210 is produced. The reduced iron 210 is discharged from the bottom of the shaft furnace 100 and cooled. A furnace top gas 400 containing hydrogen gas, CO gas, water vapor, and CO2 gas is discharged from the top of the shaft furnace 100.

[0003] The reducing gas 300 used in the shaft furnace 100 is obtained by reforming a raw gas (e.g., natural gas, coke oven gas, etc.) 310 containing carbon, using steam, oxygen, etc., and its main components are hydrogen gas (H2) 300a and CO gas (CO) 300b. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] "DIRECT FROM MIDREX 3RD QUARTER 2017" (https: / / www.midrex.com / dfm-newsletter / 3q-2017-direct-from-midrex / ) Summary of the Invention [Problem to be solved by the invention]

[0005] In existing shaft furnace operation, the H2 / CO volume ratio of the reducing gas is generally in the range of 1.5 to 4.0. Therefore, even though it is an existing shaft furnace operation, it is considered to be a better steelmaking process than the blast furnace-converter process in terms of reducing CO2 emissions. However, in order to aim for zero-CO2 emission steelmaking, which will be required in the future, it is necessary to further increase the volume ratio of hydrogen gas contained in the reducing gas.

[0006] Various technologies related to shaft furnace operation have been proposed so far, but most of the technologies use natural gas containing carbon or coke oven gas as the raw material gas for the reducing gas. However, recently, some reduced iron manufacturers have proposed a process in which natural gas, the main raw material gas for the reducing gas, is replaced with hydrogen gas (i.e., operation using a reducing gas containing hydrogen gas at a high concentration close to 100% by volume), claiming to achieve zero CO2 emissions (see Non-Patent Document 1).

[0007] Although an operation using a reducing gas containing a high concentration of hydrogen gas is possible in a stoichiometric discussion based on heat and material balance, it is not necessarily problem-free in practice. Therefore, the present inventors have examined whether an operation using a reducing gas containing a high concentration of hydrogen gas can be achieved practically without any problems as an extension of the existing shaft furnace operation. As a result, it has become clear that there are technical problems to be solved. Although the details will be described later, it has become clear that a problem occurs in that if a reducing gas containing a high concentration of hydrogen gas is simply used in the existing shaft furnace operation, a large amount of hydrogen gas is discharged from the top of the furnace without being used for reduction, and the reducing gas consumption rate (the amount of hydrogen gas injected into the furnace required to produce 1 ton of reduced iron) excessively increases. Such a problem is not taken into consideration at all in Non-Patent Document 1.

[0008] The present invention has been made in view of the above problems, and an object of the present invention is to provide a new and improved method for producing reduced iron, which is capable of reducing the reducing gas consumption rate even when a reducing gas containing a high concentration of hydrogen gas is used. [Means for solving the problem]

[0009] The present inventors have examined whether operation using a reducing gas containing a high concentration of hydrogen gas can be achieved practically without problems as an extension of existing shaft furnace operation. The examination method was a simulation using a mathematical model of a shaft furnace. The model was constructed based on the chemical engineering method described in non-patent literature (e.g., Hara et al.: Tetsu to Hagane, Vol. 62 (1976), No. 3, p. 315; Yamaoka et al.: Tetsu to Hagane, Vol. 74 (1988), No. 12, p. 2254), and can theoretically analyze and estimate heat and mass transfer in a shaft furnace, such as chemical reactions and heat transfer phenomena, including the reduction reaction of iron oxide by reducing gas. Using this mathematical model, a shaft furnace operation using a reducing gas containing a high concentration of hydrogen gas was simulated to evaluate the macroscopic heat and mass transfer.

[0010] Table 1 shows the prerequisites (calculation conditions) established for the case study. In light of the objective of evaluating macroscopic heat and mass transfer, the calculation conditions were set based on representative operating conditions so as not to impair the generality of the results. In addition, the calculation target of the mathematical model was the space equivalent to the reduction reaction zone of the shaft furnace.

[0011] [Table 1]

[0012] FIG. 10 is a graph showing the minimum amount of heat (hereinafter also referred to as "heat unit consumption") (MJ / t-Fe) required to produce 1 ton of reduced iron (100% reduction rate) using reducing gas at 900°C for each H2 / CO volume ratio of the reducing gas. In this specification, " / t-Fe" indicates "value per 1 ton of reduced iron (100% reduction rate)". In FIG. 10, "product DRI take-out sensible heat" is the sensible heat taken out of the furnace by the reduced iron product, "top gas take-out sensible heat" is the sensible heat taken out of the furnace by the top gas, and "reduction reaction heat" is the heat required for the reduction reaction of iron oxide. As is clear from FIG. 10, the heat unit consumption increases as the H2 / CO volume ratio of the reducing gas increases. Here, the ratios of 80 / 20 to 66 / 33 correspond to typical reducing gas compositions in existing shaft furnace operations. FIG. 11 also shows the minimum amount of reducing gas (i.e., reducing gas consumption rate) (Nm 3 11 is a graph showing the reduction gas consumption rate (H2 / CO volume ratio) for each H2 / CO volume ratio of the reducing gas. As is clear from Fig. 11, the reduction gas consumption rate increases as the H2 / CO volume ratio of the reducing gas increases.

[0013] The reason for the results shown in Figures 10 and 11 is that the reduction reaction with CO gas is an exothermic reaction, while the reduction reaction with hydrogen gas is an endothermic reaction, as shown in the following equations (1) and (2). Fe2O3+3H2→2Fe+3H2O -854MJ / t-Fe (1) Fe2O3+3CO→2Fe+3CO2+246MJ / t-Fe ····(2)

[0014] In other words, the more the volume ratio of hydrogen gas in the reducing gas increases, the more heat input (reduction reaction heat) is required to cover the reduction reaction heat caused by hydrogen gas. If the blowing temperature of the reducing gas is not changed, the reducing gas consumption rate must be increased as shown in FIG.

[0015] What should be noted here is the deterioration of the reducing gas utilization rate that occurs with an increase in the reducing gas consumption rate. The reducing gas utilization rate calculated from the furnace gas composition is shown in FIG. 12. Here, FIG. 12 is a graph showing the reducing gas utilization rate (%) for each H2 / CO volume ratio of the reducing gas. The reducing gas utilization rate is obtained by dividing the total volume of steam and CO2 gas contained in the furnace gas by the total volume of hydrogen gas, steam, CO gas, and CO2 gas contained in the furnace gas. Since the reduction reaction amount (in other words, the amount of deoxidization) required to produce 1 ton of reduced iron (reduction rate 100%) is the same, it is natural that the reducing gas that does not participate in the reduction reaction increases if the reducing gas consumption rate is increased, and the reducing gas, i.e., hydrogen gas, is wasted for heat supply. In other words, the more the volume ratio of hydrogen gas in the reducing gas increases, the more hydrogen gas as a heat supply source needs to be supplied to the shaft furnace in order to cover the reduction reaction heat by hydrogen gas. Furthermore, as a result of blowing a large amount of hydrogen gas into the shaft furnace, most of the hydrogen gas does not react in the shaft furnace and is discharged as furnace top gas. Therefore, the utilization rate of the reducing gas decreases. Thus, if a reducing gas containing a high concentration of hydrogen gas is simply used in the existing shaft furnace operation, a large amount of hydrogen gas is consumed without being used for reduction, resulting in a technical problem of excessively increasing the hydrogen gas consumption rate.

[0016] On the other hand, it is theoretically possible to cover the reduction reaction heat by hydrogen gas by increasing the blowing temperature of the reducing gas. 313 is a graph showing the relationship between the reduction gas temperature (℃) and the reduction gas blast temperature (℃) for each H2 / CO volume ratio of the reducing gas. When using a reducing gas containing hydrogen gas at a high concentration of 90 volume % or more, as shown in FIG. 13, in order to operate at the same reducing gas consumption rate as that of the existing shaft furnace operation, it is necessary to significantly increase the blast temperature by at least 100°C (200°C or more when the H2 / CO volume ratio is 100 / 0) compared to the existing shaft furnace operation, roughly speaking. However, when the blast temperature of the reducing gas containing hydrogen gas at a high concentration is significantly increased, there is a concern that the so-called sticking phenomenon will occur, in which the reduced iron particles in the furnace stick to each other. Furthermore, since high-temperature hydrogen gas will be handled, problems such as increased equipment costs may occur in order to ensure the safety of the operation and to deal with hydrogen embrittlement.

[0017] In short, the fundamental problem when operating a shaft furnace using reducing gas containing a high concentration of hydrogen gas is how to cover the heat of the reduction reaction by hydrogen gas. The inventor came up with the idea of ​​blowing nitrogen gas, which does not affect the reduction reaction in the shaft furnace, into the shaft furnace together with the reducing gas as a method for solving this fundamental problem. The inventor then decided to have nitrogen gas cover at least a part of the heat required for the reduction reaction by hydrogen gas. As a result, the reduction gas consumption rate can be reduced, and the blowing temperature of the reduction gas can also be reduced. The present invention was made based on this knowledge.

[0018] That is, according to one aspect of the present invention, there is provided a method for producing reduced iron by reducing iron oxide charged into a shaft furnace, characterized in that a heated mixed gas containing a reducing gas containing 90 volume % or more of hydrogen gas and nitrogen gas is blown into the shaft furnace.

[0019] According to another aspect of the present invention, there is provided a method for producing reduced iron by reducing iron oxide charged in a shaft furnace, the method comprising the steps of heating a mixed gas containing a reducing gas containing 90 volume % or more of hydrogen gas and nitrogen gas, and blowing the heated mixed gas into the shaft furnace.

[0020] Here, the method may include a step of separating and recovering at least unreacted hydrogen gas and nitrogen gas from the top gas of the shaft furnace, and a step of reusing the separated and recovered hydrogen gas and nitrogen gas as part of the mixed gas.

[0021] In addition, a part of the separated and recovered hydrogen gas and nitrogen gas may be used as a fuel gas when heating the mixed gas.

[0022] According to another aspect of the present invention, there is provided a method for producing reduced iron by reducing iron oxide charged in a shaft furnace, the method comprising the steps of: separately heating a reducing gas containing 90 volume % or more of hydrogen gas and nitrogen gas; mixing the heated reducing gas and nitrogen gas to prepare a mixed gas; and blowing the mixed gas into the shaft furnace.

[0023] Here, the heating temperature of the nitrogen gas may be set higher than the heating temperature of the reducing gas.

[0024] The method may also include a step of separately separating and recovering at least unreacted hydrogen gas and nitrogen gas from the top gas of the shaft furnace, and a step of reusing the separated and recovered hydrogen gas and nitrogen gas as reducing gas and nitrogen gas to be heated.

[0025] The mixed gas may contain nitrogen gas at a ratio of 90% by volume or less of hydrogen gas.

[0026] The temperature of the mixed gas when it is blown into the shaft furnace may be 900° C. or lower. Effect of the Invention

[0027] According to the above-mentioned aspects of the present invention, even when a reducing gas containing a high concentration of hydrogen gas is used, the reducing gas consumption rate can be reduced. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is an explanatory diagram showing a process flow of a method for producing reduced iron according to a first embodiment. [Diagram 2] 1 is a graph showing the relationship between the blowing temperature of the mixed gas and the hydrogen gas consumption rate for each added amount of nitrogen gas. [Diagram 3] 1 is a graph showing the relationship between the amount of nitrogen gas added and the hydrogen gas consumption rate for each blowing temperature of the mixed gas. [Figure 4] 1 is a graph showing the relationship between the amount of nitrogen gas added and the hydrogen gas consumption rate for each blowing temperature of the mixed gas. [Diagram 5] FIG. 4 is an explanatory diagram showing a process flow of a method for producing reduced iron according to a first modified example of the first embodiment. [Figure 6] FIG. 4 is an explanatory diagram showing a process flow of a method for producing reduced iron according to a second modified example of the first embodiment. [Figure 7] FIG. 4 is an explanatory diagram showing a process flow of a method for producing reduced iron according to a second embodiment. [Figure 8] 1 is a graph showing the relationship between the heating temperature (° C.) of hydrogen gas and the heating temperature (° C.) of nitrogen gas. [Figure 9] FIG. 11 is an explanatory diagram showing a process flow of a method for producing reduced iron according to a modified example of the second embodiment. [Figure 10] 1 is a graph showing the results of calculations of the heat consumption rate (MJ / t-Fe) when producing 1 ton of reduced iron using reducing gas at 900° C. for each H2 / CO volume ratio of the reducing gas. [Figure 11] 1 is a graph showing the results of calculation of the reducing gas consumption rate (Nm3 / t-Fe) when producing 1 ton of reduced iron using reducing gas at 900°C for each H2 / CO volume ratio of the reducing gas. [Figure 12]1 is a graph showing the utilization rate (%) of the reducing gas for each H2 / CO volume ratio of the reducing gas. [Figure 13] 1 is a graph showing the relationship between the reducing gas consumption rate (Nm3 / t-Fe) and the blowing temperature (°C) of the reducing gas for each H2 / CO volume ratio of the reducing gas. [Figure 14] FIG. 1 is an explanatory diagram showing the process flow of an existing shaft furnace operation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. A numerical range indicated by "to" means a range including the numerical values ​​before and after "to" as the lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range.

[0030] <1. First embodiment> First, a process flow of a method for producing reduced iron (shaft furnace operation) according to a first embodiment will be described with reference to Fig. 1. In the first embodiment, generally, a heated mixed gas 30 containing a reducing gas 31 containing 90 volume % or more of hydrogen gas and nitrogen gas 32 is blown into a shaft furnace 10.

[0031] More specifically, the method for producing reduced iron according to the first embodiment includes a step of heating a mixed gas 30 containing a reducing gas 31 and a nitrogen gas 32, and a step of blowing the heated mixed gas 30 into a shaft furnace 10. The other steps may be the same as those in the operation of an existing shaft furnace.

[0032] 1, for example, reducing gas 31 and nitrogen gas 32 are introduced into a heating furnace 50, and the reducing gas 31 and nitrogen gas 32 are heated together in the heating furnace 50. As a result, the reducing gas 31 and the nitrogen gas 32 are mixed in the heating furnace 50 to become a mixed gas 30, and the mixed gas 30 is heated to a predetermined temperature.

[0033] As described above, the reducing gas 31 contains hydrogen gas at 90% by volume or more (mass % relative to the total volume of the reducing gas 31). In other words, the hydrogen gas concentration of the reducing gas 31 is 90% by volume or more. From the viewpoint of zero CO2 emissions, the hydrogen gas concentration of the reducing gas 31 is preferably as high as possible within the range of 90% by volume or more, and is preferably 100% by volume. In addition, the method of heating the reducing gas is preferably by an electric heater, and in the case of combustion heating, it is preferable that the fuel gas be mainly hydrogen.

[0034] When the hydrogen gas concentration of the reducing gas 31 is 90 volume % or more and less than 100 volume %, the reducing gas 31 may contain a reducing gas other than hydrogen gas. Such reducing gases include, for example, CO gas and hydrocarbon gas. The hydrocarbon gas generates CO gas in the shaft furnace.

[0035] The nitrogen gas 32 is an inert gas that is not directly involved in the reduction reaction in the shaft furnace, and simply functions as a carrier that carries sensible heat into the shaft furnace 10. Therefore, according to the first embodiment, it is not necessary to apply a heating load only to the hydrogen gas, and therefore it is possible to operate the shaft furnace at an appropriate blowing temperature (predetermined temperature).

[0036] The amount of nitrogen gas 32 added to the reducing gas 31 will be described in detail later, but the effects of this embodiment (reduction in hydrogen gas consumption and reduction in hydrogen gas blowing temperature) can be obtained even by adding a small amount of nitrogen gas 32 to the reducing gas 31. On the other hand, if an excessive amount of nitrogen gas 32 is added, the hydrogen concentration in the mixed gas 30 decreases, and this slows down the reduction reaction rate of the iron oxide, exceeding the compensation effect of the reduction reaction heat due to the heat supply from the nitrogen gas 32. In this case, the effects of this embodiment become saturated. From this viewpoint, the amount of nitrogen gas 32 added is preferably 90 volume % or less of the reducing gas 31.

[0037] The mixed gas 30 is preferably composed of only the reducing gas 31 and the nitrogen gas 32 described above, but may contain gases other than the reducing gas 31 and the nitrogen gas 32 as long as they do not affect the effects of this embodiment.

[0038] In the heating furnace 50, the mixed gas 30 is heated to a predetermined temperature (the temperature of the mixed gas when it is blown into the shaft furnace, i.e., the blowing temperature). The predetermined temperature may be adjusted as appropriate depending on the operating conditions of the shaft furnace, but as described below, the predetermined temperature can be lower than the case where the nitrogen gas 32 is not added. This is because the nitrogen gas 32 functions as a carrier of sensible heat. The predetermined temperature is preferably 900°C or lower. The lower limit of the predetermined temperature is not particularly limited as long as it is within the range in which the shaft furnace operation according to the first embodiment is possible, but may be, for example, about 750°C.

[0039] The mixed gas 30 is heated to a predetermined temperature and then blown into the shaft furnace 10. Meanwhile, the iron oxide raw material 20 is charged from the top of the shaft furnace 10. The type of the iron oxide raw material 20 is not particularly limited, and may be the same as that of the existing shaft furnace operation. The iron oxide raw material 20 may be, for example, iron oxide pellets. The mixed gas 30 blown into the shaft furnace 10 rises in the shaft furnace 10. The reducing gas 31 in the mixed gas 30 reduces the iron oxide raw material 20 in the shaft furnace 10 to produce reduced iron 21. The reduction reaction by hydrogen gas is an endothermic reaction, but the heat of the reduction reaction is provided by the sensible heat of the reducing gas 31 as well as the sensible heat of the nitrogen gas 32. The reduced iron 21 is discharged from the bottom of the shaft furnace 10 and cooled. Meanwhile, the furnace top gas 40 is discharged from the top of the shaft furnace 10. The furnace top gas 40 contains unreacted hydrogen gas as well as water vapor and nitrogen gas 32.

[0040] Next, the effects of the first embodiment will be described. The inventors simulated shaft furnace operation according to the first embodiment using the above-mentioned mathematical model. For comparison, they also simulated shaft furnace operation without adding nitrogen gas 32. Note that this simulation corresponds to an example and a comparative example of the first embodiment. The results are shown in Figs. 2 and 3. Note that the calculation conditions were the same as those in Table 1. Also, the hydrogen gas concentration of the reducing gas 31 was set to 100% by volume.

[0041] Figure 2 shows the relationship between the blowing temperature (℃) and hydrogen gas consumption (Nm3 The graph L1 shows the above relationship when no nitrogen gas was added, and the graph L2 shows the relationship when nitrogen gas was added to the reducing gas 31 at 250 Nm 3 The above relationship when / t-Fe is added, graph L3 shows the nitrogen gas reduced gas 31 at 500 Nm 3 2 shows the above relationship when t-Fe is added. Therefore, graphs L2 and L3 correspond to the shaft furnace operation according to the first embodiment. Here, since the hydrogen gas concentration of the reducing gas 31 is 100 volume %, the hydrogen gas consumption unit can be read as the reducing gas consumption unit. According to graphs L2 and L3 in FIG. 2, the reducing gas consumption unit when the blast temperature is 900°C is approximately 1500 to 1700 Nm 3 On the other hand, looking at Figure 11, in the existing shaft furnace operation (H2 / CO volume ratio of reducing gas = 80 / 20 to 66 / 33), the reducing gas consumption rate when the blast temperature is 900°C is approximately 1200 to 1400 Nm 3 Therefore, by adding nitrogen gas 32 to the reducing gas 31, even if the hydrogen gas concentration of the reducing gas 31 is high (here, 100% by volume), the blast temperature level (e.g., 900°C) is the same as that of the existing shaft furnace operation, and the reducing gas consumption rate level (e.g., 1500 to 1700 Nm 3 This makes it possible to produce reduced iron at low temperatures (approximately 1000 / t-Fe).

[0042] Therefore, even when a reducing gas containing a high concentration of hydrogen gas is used, the reducing gas consumption rate can be reduced, and the blowing temperature of the mixed gas 30 (i.e., the blowing temperature of the reducing gas 31) can be reduced. Furthermore, it can be seen that the more nitrogen gas 32 is added, the more the reducing gas consumption rate is reduced, and the blowing temperature of the reducing gas 31 is also reduced. By reducing the blowing temperature of the reducing gas 31, for example, sticking and the like are suppressed.

[0043] FIG. 3 shows the relationship in FIG. 2 with the amount of nitrogen gas 32 added (Nm 3 / t-Fe) and hydrogen gas consumption rate (Nm 3 / t-Fe). In other words, FIG. 3 shows the relationship between the amount of nitrogen gas 32 added (Nm 3 / t-Fe) and hydrogen gas consumption rate (Nm 3 / t-Fe) for each blowing temperature (°C) of mixed gas 30. Graph L4 shows the above relationship when the blowing temperature is 800°C, graph L5 shows the above relationship when the blowing temperature is 900°C, and graph L6 shows the above relationship when the blowing temperature is 1000°C. It can be seen from graphs L4 to L6 that the hydrogen gas consumption rate is reduced by simply adding a small amount of nitrogen gas 32 at any blowing temperature.

[0044] Furthermore, looking at graphs L5 and L6, 330Nm 3 By adding nitrogen gas of 1 / t to the reducing gas 31, the blowing temperature of the mixed gas 30 can be lowered from 1000°C to 900°C while maintaining the hydrogen gas consumption rate. Therefore, by adding nitrogen gas 32 to the reducing gas 31, the blowing temperature can be lowered, and sticking can be suppressed.

[0045] Furthermore, when looking at graph L5 (air temperature 900°C), the nitrogen gas 32 is mixed with the reducing gas 31 at 330 Nm 3 / t-Fe addition reduces hydrogen gas consumption by 300Nm 3 / t-Fe can be reduced by 330Nm 3 / t-Fe nitrogen gas 32 and 300Nm 3 This means that the amount of hydrogen gas in the blast furnace is roughly equivalent to that in the blast furnace for / t-Fe in terms of heat and reaction operation. From another perspective, the amount of reduced iron produced can also be controlled by adjusting the amount of nitrogen gas 32 added under the condition of a constant blast temperature. For example, the amount of nitrogen gas 32 added (Nm 3 Increasing the ratio of t-Fe to t-Fe increases the amount of reduced iron produced per unit time.

[0046] The reason why this seemingly inexplicable equivalence relationship exists, or why seemingly mysterious operations are possible, is that when a shaft furnace is operated using a reducing gas containing a high concentration of hydrogen gas, a reduction reaction occurs in the shaft furnace 10 that is rate-limited by heat (i.e., the amount of heat brought into the shaft furnace 10 keeps the temperature inside the furnace at an appropriate level, allowing the reduction reaction to proceed smoothly).

[0047] As described above, according to the first embodiment, the mixed gas 30 of the reducing gas 31 and the nitrogen gas 32 is blown into the shaft furnace 10, so that the nitrogen gas 32 can be used as a sensible heat carrier. As a result, even when the shaft furnace is operated using the reducing gas 31 containing a high concentration of hydrogen gas, for example, as shown in Figures 2 and 3, the reducing gas consumption rate can be reduced, and the blowing temperature of the mixed gas 30 can be reduced.

[0048] Here, the saturation of the effect due to the excessive introduction of nitrogen gas 32 will be described with reference to FIG. 4. The definitions of the vertical and horizontal axes in FIG. 4 are the same as those in FIG. 3. However, the horizontal axis in FIG. 4 indicates a larger amount of added nitrogen gas than that in FIG. 3. That is, the horizontal axis in FIG. 4 is an extension of the horizontal axis in FIG. 3. The graphs depicted in FIG. 4 are the same as the graphs L4 to L6 in FIG. 3. The blast temperatures in these graphs are 800°C, 840°C, 860°C, 880°C, 900°C, 920°C, 940°C, 960°C, 980°C, 1000°C, 1020°C, 1050°C, and 1100°C from the top.

[0049] 4 and

[0036] , the effect of this embodiment is saturated when an excessive amount of nitrogen gas 32 is added to the reducing gas 31. The condition for reaching saturation varies depending on the blowing temperature condition, but it can be said that the effect of this embodiment can be obtained as long as the condition that the volumetric flow rate (addition amount) of nitrogen gas 32 is 90 volume % or less of the volumetric flow rate of reducing gas 31 (i.e., the mixed gas 30 contains nitrogen gas 32 at a ratio of 90 volume % or less of the reducing gas 31) is satisfied.

[0050] <1-1. First modified example> Next, a first modified example of the first embodiment will be described with reference to Fig. 5. In actual shaft furnace operation, it is also important to effectively utilize the reducing gas 31 and the nitrogen gas 32. Therefore, in the first modified example, the unreacted hydrogen gas 31a and the nitrogen gas 32 are separated and recovered from the top gas 40 and reused as part of the mixed gas 30.

[0051] Specifically, the furnace gas 40 is introduced into the separation and recovery device 60, where the furnace gas 40 is cooled. Preferably, the furnace gas 40 is further de-dusted. As a result, water vapor is removed from the furnace gas 40 as water 65, and unreacted hydrogen gas 31a and nitrogen gas 32 are separated and recovered as the circulation gas 70. When the reducing gas 31 contains a reducing gas other than hydrogen gas (CO, etc.), the circulation gas 70 may contain oxides of the reducing gas (CO2, etc.) in addition to the unreacted reducing gas, but there is no problem in operation even if the circulation gas 70 contains these gases. As the separation and recovery device 60, for example, a device that separates and recovers unreacted reducing gas from the furnace gas of a blast furnace can be used. Then, the circulation gas 70 is reused as a part of the mixed gas 30. That is, the circulation gas 70 is introduced again into the heating furnace 50 and heated.

[0052] As described above, the nitrogen gas 32 functions as a carrier that carries sensible heat into the shaft furnace 10, and is therefore not consumed in the shaft furnace 10. Therefore, the nitrogen gas 32 circulates in a circulation system that connects the heating furnace 50, the shaft furnace 10, and the separation and recovery device 60. Therefore, once an amount of nitrogen gas 32 required to produce a desired amount of reduced iron is introduced into this circulation system, ideally there is no need to introduce nitrogen gas 32 from outside thereafter. Of course, additional nitrogen gas 32 may be supplied from outside.

[0053] On the other hand, since the reducing gas 31 is consumed in the shaft furnace 10, the reducing gas 31 is insufficient with only the circulated hydrogen gas 31a. Therefore, the reducing gas 31 may be supplied from outside to make up for the shortage. This makes it possible to ideally produce reduced iron 21 with the stoichiometrically minimum amount of reducing gas 31. Of course, the reducing gas 31 may be supplied from outside in an amount exceeding the stoichiometric amount.

[0054] As described above, according to the first modified example, the reducing gas 31 and the nitrogen gas 32 can be effectively utilized.

[0055] <1-2. Second modified example> Next, a second modified example of the first embodiment will be described with reference to Fig. 6. In the second modified example, like the first modified example, the unreacted hydrogen gas 31a and nitrogen gas 32 are separated and recovered from the top gas 40 and reused as part of the mixed gas 30. In the second modified example, in addition to performing the same process as the first modified example, the amount of nitrogen gas 32 circulating in the circulation system can be adjusted.

[0056] More specifically, in the second modified example, the circulating gas 70 separated and recovered by the separation and recovery device 60 is introduced into the branch pipe 80. In the branch pipe 80, a part of the circulating gas 70 is supplied to the heating furnace 50 as a fuel gas 85 for the heating furnace 50. The heating furnace 50 generates heat by burning the fuel gas 85, and heats the gas in the heating furnace 50 with this heat. The exhaust gas 85a (including water vapor and nitrogen gas 32) after combustion is dissipated to the outside. The remaining circulating gas 70 is reused as a part of the mixed gas 30, as in the first modified example. That is, the circulating gas 70 is introduced again into the heating furnace 50. The other processes are the same as those in the first modified example. In addition, if the nitrogen gas 32 is not introduced from the outside, the nitrogen gas 32 circulating in the circulation system (the circulation system in which the circulating gas 70 circulates) will gradually decrease. Conversely, by introducing the nitrogen gas 32 from the outside, the amount of the nitrogen gas 32 circulating in the circulation system can be maintained or increased.

[0057] As described above, according to the second modified example, the reducing gas 31 and the nitrogen gas 32 can be effectively utilized, and the amount of the nitrogen gas 32 circulating in the circulation system can be adjusted.

[0058] <2. Second embodiment> Next, a process flow of a reduced iron production method (shaft furnace operation) according to a second embodiment will be described with reference to Fig. 7. In the second embodiment, similarly to the first embodiment, a mixed gas 30 containing a reducing gas 31 containing 90 volume % or more of hydrogen gas and nitrogen gas 32 and having a predetermined temperature is blown into a shaft furnace 100. However, the reduced iron production method according to the second embodiment differs from the first embodiment in that the reducing gas 31 and the nitrogen gas 32 are heated separately and then mixed.

[0059] More specifically, the method for producing reduced iron according to the second embodiment includes a step of separately heating the reducing gas 31 and the nitrogen gas 32, a step of preparing a mixed gas 30 at a predetermined temperature by mixing the heated reducing gas 31 and the nitrogen gas 32, and a step of blowing the mixed gas 30 at the predetermined temperature into the shaft furnace 10. Other steps may be similar to those in the operation of an existing shaft furnace.

[0060] For example, as shown in FIG. 7, the reducing gas 31 is introduced into a reducing gas heating furnace 51 and heated, and the nitrogen gas 32 is introduced into a nitrogen gas heating furnace 52 and heated. As a result, the reducing gas 31 and the nitrogen gas 32 are heated separately. The characteristics of the reducing gas 31 and the nitrogen gas 32 are the same as those of the first embodiment. For example, the reducing gas 31 contains hydrogen gas at 90% by volume or more (mass % relative to the total volume of the reducing gas 31). That is, the hydrogen gas concentration of the reducing gas 31 is 90% by volume or more. The hydrogen gas concentration of the reducing gas 31 is preferably as high as possible within a range of 90% by volume or more, and is preferably 100% by volume. When the hydrogen gas concentration of the reducing gas 31 is 90% by volume or more and less than 100% by volume, the reducing gas 31 may contain a reducing gas other than hydrogen gas. Such reducing gases include, for example, CO gas and hydrocarbon gases. The mixed gas 30 is preferably composed of only the reducing gas 31 and the nitrogen gas 32 described above, but may contain gases other than the reducing gas 31 and the nitrogen gas 32 as long as they do not affect the effects of this embodiment.

[0061] Next, the heated reducing gas 31 and the nitrogen gas 32 are mixed in a mixing section 55 (for example, a junction of a pipe for the reducing gas 31 and a pipe for the nitrogen gas 32). That is, the heated nitrogen gas 32 is added to the heated reducing gas 31. This produces a mixed gas 30 at a predetermined temperature. Next, the mixed gas 30 is blown into the shaft furnace 10. The subsequent steps are the same as those in the first embodiment. Therefore, the heating temperatures of the reducing gas 31 and the nitrogen gas 32 are adjusted so that the temperature of the mixed gas 30 after mixing becomes a predetermined temperature. Here, the predetermined temperature is preferably 900°C or less as described above. The lower limit of the predetermined temperature is not particularly limited as long as it is within a range in which the shaft furnace operation according to the second embodiment is possible, but may be, for example, about 750°C.

[0062] The heating temperature of the reducing gas 31 and the heating temperature of the nitrogen gas 32 are not particularly limited as long as the temperature of the mixed gas 30 is within a range of a predetermined temperature, but it is preferable to set the heating temperature of the nitrogen gas 32 higher than that of the reducing gas 31. This allows the heating temperature of the reducing gas 31 to be reduced, so that the heating load on the reducing gas 31 can be further reduced. Here, as described in the first embodiment, the more the amount of nitrogen gas 32 added, the more the blowing temperature of the mixed gas 30 (i.e., the predetermined temperature) can be reduced. Therefore, the more the amount of nitrogen gas 32 added, the more the heating load on the reducing gas 31 can be reduced. Furthermore, as will be described in detail later, the more the amount of nitrogen gas 32 added, the more the reduction in the heating temperature of the reducing gas 31 relative to the increase in the heating temperature of the nitrogen gas 32 increases. That is, the more the amount of nitrogen gas 32 added, the more the heating temperature of the reducing gas 31 can be reduced by only slightly increasing the heating temperature of the nitrogen gas 32. Therefore, the more the amount of nitrogen gas 32 added, the greater the effect of separation heating. Of course, as explained in the first embodiment, the amount of nitrogen gas 32 added is preferably 90 volume % or less with respect to the total volume of the reducing gas 31.

[0063] Next, the effects of the second embodiment will be described in detail. The inventors have simulated shaft furnace operation according to the second embodiment using the above-mentioned mathematical model. For comparison, they have also simulated shaft furnace operation similar to that of the first embodiment, and shaft furnace operation without adding nitrogen gas 32. Note that this simulation corresponds to an example and a comparative example of the second embodiment. The results are shown in FIG. 8. Note that the calculation conditions were the same as those in Table 1. The hydrogen gas concentration of the reducing gas 31 was 100% by volume.

[0064] 8 is a graph showing the relationship between the heating temperature of hydrogen gas (i.e., the heating temperature of the reducing gas 31) (°C) and the heating temperature of nitrogen gas (°C). In FIG. 8, the hydrogen gas consumption rate (reducing gas consumption rate) is 1620 Nm 3 / t-Fe. Graph L10 shows the above relationship when reducing gas 31 and nitrogen gas 32 are mixed and heated (i.e., a heat treatment similar to that in the first embodiment is performed). Since the hydrogen gas basic unit is constant, the amount of nitrogen gas 32 added is different at each point on graph L10 (see Figures 2 and 3). At point P1 on graph L10, the amount of nitrogen gas added is zero. Graphs L11 and L12 both show the relationship between the heating temperature of hydrogen gas (i.e., the heating temperature of reducing gas 31) (°C) and the heating temperature of nitrogen gas (°C) when reducing gas 31 and nitrogen gas 32 are heated separately. However, in graph L11, the amount of nitrogen gas 32 added is 330 Nm 3 / t-Fe, and in graph L12, the amount of nitrogen gas 32 added is 500 Nm 3 / t-Fe.

[0065] Hydrogen gas consumption is 1620Nm 3 / t-Fe, and the amount of nitrogen gas 32 added is 330 Nm 3 3, the blowing temperature of the mixed gas 30 is about 900° C. When the reducing gas 31 and the nitrogen gas 32 are mixed and heated, the heating temperature of the reducing gas 31 and the nitrogen gas 32 is 900° C., which is the same as the blowing temperature of the mixed gas 30 (point P2).

[0066] Hydrogen gas consumption rate is 1620Nm 3 / t-Fe, and the amount of nitrogen gas 32 added is 500 Nm 3 3, the blowing temperature of the mixed gas 30 is 900°C or less (the calculated value by the mathematical model is 872°C). Therefore, when the reducing gas 31 and the nitrogen gas 32 are mixed and heated, the heating temperature of the reducing gas 31 and the nitrogen gas 32 is 872°C, which is the same as the blowing temperature of the mixed gas 30 (point P4).

[0067] Therefore, when 100% by volume hydrogen gas with a blast temperature of 1000°C is blown into the shaft furnace 10 (point P1), the amount of nitrogen gas 32 added is 330 Nm 3 By using / t-Fe, the blowing temperature of the mixed gas 30 can be reduced to 900°C (point P2) without increasing the hydrogen gas consumption rate. Furthermore, as shown in graph L11, by heating the reducing gas 31 and the nitrogen gas 32 separately and increasing the heating temperature of the nitrogen gas 32 to about 1350°C, the heating temperature of the reducing gas 31 can be reduced to about 800°C (point P3).

[0068] In addition, the amount of nitrogen gas 32 added was 500 Nm 3 By using / t-Fe, the blowing temperature of the mixed gas 30 can be reduced to 872°C, which is below 900°C, without increasing the hydrogen gas consumption rate (point P4). Furthermore, as shown in graph L12, by heating the reducing gas 31 and the nitrogen gas 32 separately and increasing the heating temperature of the nitrogen gas 32 to about 1080°C, the heating temperature of the reducing gas 31 can be reduced to about 800°C (point P5).

[0069] Furthermore, comparing the graph L11 with the graph L12, the slope of the graph L12 is smaller than the slope of the graph L11. This means that the greater the amount of added nitrogen gas 32, the greater the reduction in the heating temperature of the reducing gas 31 relative to the increase in the heating temperature of the nitrogen gas 32.

[0070] As described above, according to the second embodiment, the reducing gas 31 and the nitrogen gas 32 are heated separately, so that the heating load of the reducing gas 31 can be further reduced. The effect of such separate heating is increased by increasing the heating temperature of the nitrogen gas 32 higher than the heating temperature of the reducing gas 31. Furthermore, the effect of separate heating is increased as the amount of added nitrogen gas 32 increases.

[0071] <2-1. Modifications> Next, a modified example of the second embodiment will be described with reference to Fig. 9. In this modified example, the first modified example of the first embodiment is applied to the second embodiment. That is, in this modified example, the unreacted hydrogen gas 31a and the nitrogen gas 32 are individually separated and collected from the top gas 40, and are reused as the reducing gas 31 and the nitrogen gas 32 to be heated.

[0072] Specifically, the furnace gas 40 is introduced into a separation and recovery device 60, and the furnace gas 40 is cooled in the separation and recovery device 60. Preferably, the furnace gas 40 is further de-dusted. As a result, water vapor is removed from the furnace gas 40 as water 65, and unreacted hydrogen gas 31a and nitrogen gas 32 are separated and recovered. Since the separated and recovered gas is a mixed gas of unreacted hydrogen gas 31a and nitrogen gas 32, a hydrogen gas separation process or a nitrogen gas separation process is performed on this mixed gas. As a result, the unreacted hydrogen gas 31a and nitrogen gas 32 are individually separated and recovered. Here, examples of the hydrogen gas separation process and the nitrogen gas separation process include the pressure swing adsorption method (PSA method) (https: / / www.jstage.jst.go.jp / article / jvsj1958 / 43 / 12 / 43_12_1088 / _pdf). The separated and recovered unreacted hydrogen gas 31a is introduced into a reducing gas heating furnace 51, and the separated and recovered nitrogen gas 32 is introduced into a nitrogen gas heating furnace 52. In this way, the unreacted hydrogen gas 31a and the nitrogen gas 32 are reused.

[0073] As in the first modified example of the first embodiment, the nitrogen gas 32 is not consumed in the circulation system (here, the circulation system connecting the nitrogen gas heating furnace 52, the shaft furnace 10, and the separation and recovery device 60). Therefore, once an amount of nitrogen gas 32 required for producing a desired amount of reduced iron is introduced into this circulation system, ideally, there is no need to introduce the nitrogen gas 32 from outside thereafter. On the other hand, since the reducing gas 31 is consumed in the blast furnace, the reducing gas 31 is insufficient with only the circulated hydrogen gas 31a. Therefore, the reducing gas 31 may be supplied from outside to cover the shortage.

[0074] As described above, according to this modification, the reducing gas 31 and the nitrogen gas 32 can be effectively utilized.

[0075] Although the preferred embodiment of the present invention has been described in detail above with reference to the accompanying drawings, the present invention is not limited to such an example. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention. [Explanation of symbols]

[0076] 10 Shaft furnace 20 Iron oxide raw materials 30 Mixed Gas 31 Reducing Gas 32 Nitrogen gas 40 Furnace gas 50 Furnace 60 Separation and recovery device 70 Circulating Gas 80 Branch Pipe 85 Fuel Gas

Claims

1. A method for producing reduced iron by reducing iron oxide charged in a shaft furnace, comprising the steps of: A heated mixed gas containing a reducing gas containing 90 volume % or more of hydrogen gas and nitrogen gas is blown into the shaft furnace; 2. A method for producing reduced iron, comprising the steps of: increasing the amount of nitrogen gas added, without changing the blowing temperature and the amount of hydrogen gas blown per unit time, thereby increasing the production amount of reduced iron per unit time.

2. A method for producing reduced iron by reducing iron oxide charged in a shaft furnace, comprising the steps of: heating a mixed gas containing a reducing gas containing 90 volume % or more of hydrogen gas and nitrogen gas; and blowing the heated mixed gas into a shaft furnace; a nitrogen gas supply amount of the hydrogen gas being increased without changing a blowing temperature of the hydrogen gas and an amount of the hydrogen gas being blown in per unit time, thereby increasing an amount of the reduced iron produced per unit time.

3. A step of separating and recovering at least the unreacted hydrogen gas and the unreacted nitrogen gas from the top gas of the shaft furnace; and reusing the separated and recovered hydrogen gas and nitrogen gas as part of the mixed gas.

4. 4. The method for producing reduced iron according to claim 3, wherein a part of the hydrogen gas and the nitrogen gas separated and recovered is used as a fuel gas for heating the mixed gas.

5. 5. The method for producing reduced iron according to claim 1, wherein the mixed gas contains the nitrogen gas in an amount of 90 volume % or less of the reducing gas.

6. The method for producing reduced iron according to any one of claims 1 to 5, wherein the mixed gas has a temperature of 900°C or lower when blown into the shaft furnace.

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