Production method of reduced iron

Abstract

To provide a novel and improved method for producing reduced iron capable of reducing a reducing gas unit consumption and improving thermal efficiency even when using a reducing gas containing a hydrogen gas at a high concentration.SOLUTION: A method for producing reduced iron by reducing iron oxide charged into a shaft furnace includes: blowing a heated mixed gas including a reducing gas containing 90 vol.% or more of a hydrogen gas and a nitrogen gas into the shaft furnace from a tuyere provided in a lower part of a reducing zone of the shaft furnace; blowing at least a part of the reducing gas into a cooling zone of the reduced iron provided in a lower part of the shaft furnace at a normal temperature; and using the reducing gas raised in the cooling zone for reducing the iron oxide.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. 2020-93139, filed on May 28, 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 iron oxide raw material, and is widespread mainly in regions where natural gas is available at low cost (oil-producing countries). Here, an overview of existing shaft furnace operation will be described with reference to FIG. 18. In the example of FIG. 18, the upper side of a shaft furnace 100 is a reduction zone 100a, and the lower side is a cooling zone 100b. The reduction zone 100a is a zone where iron oxide is reduced to produce reduced iron, and the cooling zone 100b is a zone where the produced reduced iron is cooled. A tuyere 100c for injecting reducing gas into the shaft furnace 100 is provided below the reduction zone 100a.

[0003] In shaft furnace operation using such a shaft furnace 100, 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 a tuyere 100c provided at the bottom of the reduction zone 100a. Here, the reducing gas 300 is heated to a predetermined temperature (airflow temperature, e.g., about 900 to 1000°C) and then blown into the shaft furnace 100. The iron oxide raw material 200 is reduced by the reducing gas 300 rising from the tuyere 100c during the process of descending in the reduction zone 100a, and the reduction rate is approximately 100% when it reaches the tuyere level (the same height as the installation position of the tuyere 100c), and the temperature is raised to the level of the airflow temperature. By such a direct reduction process, reduced iron 210 is produced. The reduced iron 210 is cooled in the cooling zone 100b below the shaft furnace 100, and then discharged from the bottom of the shaft furnace 100. Non-Patent Document 1 discloses a technique for simultaneously cooling and carburizing the reduced iron 210 by injecting a hydrocarbon gas (e.g., natural gas) 500 into the cooling zone 100b. Depending on the form of the final product, the reduced iron may be subjected to hot agglomeration. Meanwhile, top gas 400 containing hydrogen gas, CO gas, water vapor, and CO2 gas is discharged from the top of the shaft furnace 100.

[0004] 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 is mainly composed of hydrogen gas (H2) and CO gas (CO). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2010-046055 A [Non-patent literature]

[0006] [Non-Patent Document 1] "DIRECT FROM MIDREX 3RD QUARTER 2017" (https: / / www.midrex.com / dfm-newsletter / 3q-2017-direct-from-midrex / ) [Non-Patent Document 2] S.Hosokai, Y.Kashiwaya, K.Matsui, N.Okinaka & T.Akiyama:Environ. Sci. Technol.(2011),45,p.821-826 Summary of the Invention [Problem to be solved by the invention]

[0007] 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.

[0008] 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).

[0009] 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.

[0010] Furthermore, in order to more effectively achieve zero-CO2 emission steel production, it is necessary not only to increase the volume fraction of hydrogen gas contained in the reducing gas (i.e., to advance the decarbonization of the reducing material), but also to improve the thermal efficiency of the process (i.e., to save energy).

[0011] 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 and improving thermal efficiency even when a reducing gas containing a high concentration of hydrogen gas is used. [Means for solving the problem]

[0012] 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.

[0013] Table 1 shows the assumptions (calculation conditions) made for the case study. In light of the objective of evaluating macroscopic heat and mass transfer, the calculation conditions were set based on typical operating conditions so as not to impair the generality of the results.

[0014] [Table 1]

[0015] Fig. 14 is a graph showing the minimum amount of heat (hereinafter also referred to as "unit heat 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 reducing gas. In this specification, " / t-Fe" indicates "value per ton of reduced iron." The reduction rate of reduced iron is defined by the following formula. (Reduction rate) = (1 - (amount of unreduced oxygen in reduced iron) / (amount of reduced oxygen in oxidized iron raw material)) x 100 (%)

[0016] In Fig. 14, "product DRI take-out sensible heat" is the sensible heat taken out of the furnace by the product, i.e., reduced iron, "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. 14, the heat consumption rate increases as the H2 / CO volume ratio of the reducing gas increases. Here, the ratio of 80 / 20 to 66 / 33 corresponds to the typical reducing gas composition in existing shaft furnace operation. Also, Fig. 15 shows the minimum amount of reducing gas (i.e., reducing gas consumption rate) (Nm 3 15 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. 15, the reduction gas consumption rate increases as the H2 / CO volume ratio of the reducing gas increases.

[0017] The reason for the results shown in Figures 14 and 15 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)

[0018] 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.

[0019] 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. 16. Here, FIG. 16 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.

[0020] 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. 317 is a graph showing the relationship between the blast temperature (℃) of the reducing gas 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. 17, in order to operate the furnace at the same reducing gas consumption rate as that of the existing shaft furnace, 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 that of the existing shaft furnace. 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, in which the reduced iron particles in the furnace stick to each other, may occur. Furthermore, since high-temperature hydrogen gas is handled, problems such as increased equipment costs may occur in order to ensure the safety of the operation and to deal with hydrogen embrittlement.

[0021] 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 of 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 could be reduced, and the blowing temperature of the reduction gas could also be reduced.

[0022] Furthermore, the present inventors have focused on the sensible heat of reduced iron from the viewpoint of improving thermal efficiency. That is, the iron oxide raw material that reaches the tuyere level is reduced by almost 100% to become reduced iron. The temperature of this reduced iron is very high, approximately the same as the blast temperature. The present inventors thought that if such sensible heat of reduced iron could be used to heat the reducing gas, the thermal efficiency could be further improved. Therefore, the present inventors decided to blow at least a part of the reducing gas supplied from the outside into the cooling zone and cool the reduced iron with this reducing gas. The reducing gas blown into the cooling zone cools the reduced iron while rising in the cooling zone. Accordingly, the reducing gas is heated by the sensible heat of the reduced iron. By appropriately adjusting the amount of reducing gas blown into the cooling zone (the adjustment method will be described later), the reducing gas can be heated to approximately the blast temperature level when it reaches the tuyere level. Then, the reducing gas heated by the sensible heat of the reduced iron is used for reducing the iron oxide. This allows at least a portion of the reducing gas used in the reduction of iron oxide to be heated by the sensible heat of the reduced iron, thereby reducing the heating load of the reducing gas and improving thermal efficiency.

[0023] While it is expected that the social demand for hydrogen gas will expand explosively, it is unclear whether hydrogen gas to meet this demand will be available stably (not only in terms of quantity but also in terms of price). Furthermore, hydrogen gas is very unstable, so transportation must be carried out with extreme care. For this reason, in order to commercialize the hydrogen reduction process, it is necessary to diversify hydrogen gas supply sources, and preferably to secure a hydrogen gas supply source with high portability. Therefore, the present inventors focused on ammonia gas as a hydrogen gas supply source and considered injecting ammonia gas into the cooling zone. Ammonia is mass-produced industrially as a raw material for chemical fertilizers, and can be easily liquefied, making it a hydrogen carrier with excellent portability.

[0024] Furthermore, the ammonia gas blown into the cooling zone cools the reduced iron while rising in the cooling zone. Accordingly, the ammonia gas is heated by the sensible heat of the reduced iron. After that, the ammonia gas decomposes into nitrogen gas and hydrogen gas using the reduced iron as a catalyst. That is, the decomposition reaction of ammonia gas is an endothermic reaction, but the heat required for the decomposition reaction is provided by the reduced iron. In addition, the reduced iron itself acts as a catalyst to promote the decomposition of ammonia gas. The nitrogen gas and hydrogen gas generated by the decomposition of ammonia gas rise while being further heated by the sensible heat of the reduced iron, and reach the tuyere. By appropriately adjusting the amount of ammonia gas blown into the cooling zone (the adjustment method will be described later), the nitrogen gas and hydrogen gas can be heated to approximately the blowing air temperature level when they reach the tuyere level. Then, the hydrogen gas and nitrogen gas generated by the decomposition of ammonia gas are used as part of the mixed gas described later. Therefore, by blowing ammonia gas into the cooling zone, the ammonia gas can be used as a hydrogen gas supply source, and the thermal efficiency can also be improved. The present invention was made based on these findings.

[0025] 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 from a tuyere provided at the bottom of a reduction zone of the shaft furnace, while at least a portion of the reducing gas is blown at room temperature into a cooling zone for reduced iron provided at the bottom of the shaft furnace, and the reducing gas that has risen through the cooling zone is used for reducing the iron oxide.

[0026] 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.

[0027] 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, 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, while ammonia gas is blown at room temperature into a cooling zone for reduced iron provided in the lower part of the shaft furnace, and the hydrogen gas and nitrogen gas generated by the decomposition of the ammonia gas as it rises in the cooling zone are used as part of the mixed gas.

[0028] Here, a process may be included in which at least unreacted hydrogen gas and nitrogen gas are separated and recovered from the top gas of the shaft furnace, and a portion of the separated and recovered hydrogen gas and nitrogen gas are reused as part of the mixed gas, and the remainder is used as fuel gas when heating the mixed gas. Effect of the Invention

[0029] According to the above-mentioned aspect 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 and the thermal efficiency can be improved. [Brief description of the drawings]

[0030] [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] 1 is a graph showing a correlation between the unit consumption of hydrogen gas injected into a cooling zone and the temperature and reduction rate of reduced iron. [Figure 6] 1 is a graph showing the state inside a shaft furnace. [Figure 7] 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 8] 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 9] 1 is a graph showing the correlation between the unit consumption of hydrogen gas injected into the cooling zone and the temperature and reduction rate of reduced iron for each amount of added nitrogen gas. [Figure 10] FIG. 4 is an explanatory diagram showing a process flow of a method for producing reduced iron according to a second embodiment. [Figure 11] 1 is a graph showing the correlation between the reaction temperature and the volume ratio and equilibrium constant of each gas when ammonia gas is decomposed into nitrogen gas and hydrogen gas. [Figure 12] 1 is a graph showing the correlation between the blowing temperature of the mixed gas and the unit consumption of ammonia gas blown into the cooling zone, the unit consumption of hydrogen gas generated by decomposition of ammonia gas, and the unit consumption of hydrogen gas blown from the outside. [Figure 13] 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 14] 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 15] 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 16] 1 is a graph showing the utilization rate (%) of the reducing gas for each H2 / CO volume ratio of the reducing gas. [Figure 17] 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 18] FIG. 1 is an explanatory diagram showing the process flow of an existing shaft furnace operation. [Figure 19]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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] 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.

[0032] <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. Furthermore, in the first embodiment, at least a portion of the reducing gas 31 is blown at room temperature into a cooling zone for reduced iron provided in the lower part of the shaft furnace 10.

[0033] More specifically, the upper side (generally the upper half) of the shaft furnace 10 is the reduction zone 10a, and the lower side (generally the lower half) is the cooling zone 10b. Of course, the division between the reduction zone 10a and the cooling zone 10b is not limited to this example, and for example, the reduction zone 10a may be set longer. A tuyere 10c for blowing the mixed gas 30 into the shaft furnace 10 is provided at the lower part of the reduction zone 10a (the boundary between the reduction zone 10a and the cooling zone 10b). Although not shown, a tuyere for blowing the reducing gas into the cooling zone is provided at the lower end of the shaft furnace 10 in addition to a discharge port for reduced iron.

[0034] 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, a step of blowing the heated mixed gas 30 into a shaft furnace 10, and a step of blowing at least a part of the reducing gas 31 at room temperature into a cooling zone 10b for reduced iron provided in the lower part of the shaft furnace 10. The other steps may be similar to those of the existing shaft furnace operation.

[0035] For example, the reducing gas 31 and the nitrogen gas 32 supplied from the outside are introduced into the heating furnace 50, and the reducing gas 31 and the 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 the mixed gas 30, and the mixed gas 30 is heated to a predetermined temperature.

[0036] 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 (i.e., the reducing gas 31 is composed only of hydrogen gas). In addition, the method of heating the mixed gas 30 is preferably by an electric heater, and in the case of combustion heating, it is preferable that the fuel gas be mainly hydrogen.

[0037] 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. In the following description, unless otherwise specified, the hydrogen gas concentration of the reducing gas 31 is assumed to be 100 volume %, that is, composed only of hydrogen gas.

[0038] 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).

[0039] 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.

[0040] 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.

[0041] In the heating furnace 50, the mixed gas 30 is heated to a predetermined temperature (the temperature of the mixed gas 30 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 when 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 a range in which the shaft furnace operation according to the first embodiment is possible, but may be, for example, about 750°C.

[0042] 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 above the shaft furnace 10. There is no particular restriction on the type of the iron oxide raw material 20, and it may be the same as that used in existing shaft furnace operations. The iron oxide raw material 20 may be, for example, iron oxide pellets. The iron oxide raw material 20 charged into the shaft furnace 10 falls through the reduction zone 10a. The arrow X in the figure indicates the direction in which the iron oxide raw material 20 or reduced iron 21 moves within the shaft furnace 10.

[0043] The mixed gas 30 injected into the shaft furnace 10 rises in the reduction zone 10a of the shaft furnace 10. The reducing gas 31 in the mixed gas 30 reduces the iron oxide raw material 20 falling in the reduction zone 10a 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 reduction rate of the reduced iron 21 is approximately 100% when the reduced iron 21 reaches the tuyere level (the same height as the tuyere 10c), and the temperature of the reduced iron 21 is raised to the level of the blowing air temperature. A top gas 40 is discharged from the top of the shaft furnace 10. The top gas 40 contains unreacted hydrogen gas as well as water vapor and nitrogen gas 32.

[0044] Thereafter, the reduced iron 21 falls in the cooling zone 10b. Meanwhile, at least a part of the reducing gas 31 supplied from outside is blown into (the lower part of) the cooling zone 10b at room temperature. That is, in the first embodiment, a part of the reducing gas 31 supplied from outside (the reducing gas 31 supplied through the supply line (a) in FIG. 1) is blown into the cooling zone 10b at room temperature, and the remainder (the reducing gas 31 supplied through the supply line (b) in FIG. 1) is heated together with the nitrogen gas 32 and blown into the reduction zone 10a of the shaft furnace 10 from the tuyere 10c. The reducing gas 31 blown into the cooling zone 10b cools the reduced iron 21 while ascending in the cooling zone 10b. Accordingly, the reducing gas 31 is heated by the sensible heat of the reduced iron 21. Then, the reducing gas 31 heated as it ascends in the cooling zone is used for the reduction of the oxidized iron raw material 20. Here, since the sensible heat of the reduced iron 21 is also the sensible heat imparted to the mixed gas 30 by the heating furnace 50, the reducing gas 31 blown into the cooling zone 10b recovers a part of the sensible heat imparted to the mixed gas 30 by the heating furnace 50. Therefore, the heating load of the reducing gas is reduced, and the thermal efficiency is improved.

[0045] Here, the range of "room temperature" is not particularly limited as long as it is a temperature range recognized as room temperature in the technical field to which the present invention belongs, but may be within a range of, for example, about 25±10°C. Furthermore, the unit consumption (injection amount) of the reducing gas 31 injected into the cooling zone 10b (Nm 3 / t-Fe) is preferably set so that the reduction rate of the reduced iron 21 discharged from the shaft furnace 10 is 100% (or a value close to that, for example, 95% or more) and the temperature of the reduced iron 21 is cooled to about room temperature (for example, room temperature to room temperature + 30°C). 3 In the case of / t-Fe, it is approximately 450 to 550 Nm 3 / t-Fe (see Figures 4 and 5).

[0046] Next, among the effects of the first embodiment, the effect of the mixed gas 30 will be described. The inventors simulated shaft furnace operation according to the first embodiment using the above-mentioned mathematical model. For comparison, a shaft furnace operation without adding nitrogen gas 32 was also simulated. The results are shown in Figs. 2 and 3. The calculation conditions were the same as those in Table 1. The hydrogen gas concentration of the reducing gas 31 was set to 100% by volume. The reduction rate of the reduced iron 21 was set to 100% at the tuyere level. In this simulation, the entire amount of the reducing gas 31 was charged into the heating furnace 50.

[0047] Figure 2 shows the relationship between the blowing temperature (℃) and hydrogen gas consumption (Nm 3 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 15, 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).

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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 3This means that the amount of hydrogen gas in the sintered iron (sintered iron) 21 is roughly equivalent to that in the sintered iron (sintered iron) 22 in terms of heat and reaction operation. From another perspective, the amount of nitrogen gas 32 added can be adjusted under the condition of a constant blast temperature, thereby controlling the production amount of reduced iron 21. For example, the amount of nitrogen gas 32 added (Nm 3 Increasing the amount of reduced iron produced per unit time increases the amount of reduced iron produced.

[0052] 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).

[0053] 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.

[0054] Here, the saturation of the effect due to the excessive introduction of nitrogen gas 32 will be described with reference to FIG. 19. The definitions of the vertical and horizontal axes in FIG. 19 are the same as those in FIG. 3. However, the horizontal axis in FIG. 19 indicates a larger amount of added nitrogen gas than that in FIG. 3. That is, the horizontal axis in FIG. 19 is an extension of the horizontal axis in FIG. 3. The graphs depicted in FIG. 19 are the same as the graphs L4 to L6 in FIG. 3. The blowing 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.

[0055] 19 and

[0039] , 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.

[0056] Next, the effect from the viewpoint of thermal efficiency will be described. FIG. 4 is a graph similar to FIG. 3, but shows the effect of the amount of nitrogen gas 32 added (Nm 3 / t-Fe) and hydrogen gas consumption rate (Nm 3 The graphs show the relationship between the temperature of the blast and the amount of nitrogen gas 32 added (t-Fe) and the blast temperature (t-Fe) from the top to the bottom. 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. The simulation conditions for obtaining FIG. 4 were the same as those for FIG. 2 and FIG. 3. According to FIG. 4, for example, when the blast temperature is 900°C and the amount of nitrogen gas 32 added is 330 Nm 3 In the case of / t-Fe, the blowing amount of the reducing gas 31 (here, hydrogen gas) from the tuyeres 10c is 1620 Nm 3 By using / t-Fe, reduced iron 21 with a reduction rate of 100% can be produced.

[0057] The inventor has achieved 1620Nm 3 A simulation was performed in which a part of the reducing gas 31 of the t-Fe / t-Fe was blown into the cooling zone 10b at room temperature (30°C in this case) and the reducing gas 31 rising through the cooling zone 10b was used for the reduction of the iron oxide raw material 20. The simulation was performed using the mathematical model described above, and the calculation conditions were the same as those in Table 1. The amount of added nitrogen gas 32 was 330 Nm 3 The gas temperature was 1000° C., and the hydrogen gas concentration of the reducing gas 31 was 100% by volume. The results are shown in FIG. 5. The horizontal axis of FIG. 5 represents the blowing amount (Nm m) of the reducing gas 31 into the cooling zone 10b. 3 / t-Fe), and the vertical axis indicates the temperature (°C) or reduction rate (%) of the reduced iron 21 discharged from the lower part of the shaft furnace 10. Graph L20 indicates the amount (Nm 3 / t-Fe) and the reduction rate (%) of the reduced iron 21. Graph L21 shows the correlation between the amount of reducing gas 31 blown into the cooling zone 10b (Nm 3 The correlation between the amount of reduced iron (Ir / t-Fe) and the temperature (℃) of reduced iron 21 is shown.

[0058] 5, the reduced iron 21 is reliably cooled to room temperature by increasing the amount of reducing gas 31 injected into the cooling zone 10b. However, if the injection amount is excessively large, the temperature of the reducing gas 31 in the cooling zone 10b is not sufficiently increased, and the temperature of the reducing gas 31 in the reduction zone 10a decreases, which may result in the reduction rate of the reduced iron 21 being less than 100%.

[0059] Considering the actual operation of the shaft furnace, it is preferable that the reduction rate of the reduced iron 21 discharged from the shaft furnace 10 is 100% (or a value close to that, for example, 95% or more) and the temperature of the reduced iron 21 is cooled to about room temperature (for example, room temperature to room temperature + 30°C). Therefore, in the example of FIG. 5, the amount of the reducing gas 31 injected into the cooling zone 10b is about 450 to 550 Nm 3 In this case, the reduction rate of the reduced iron 21 can be set to 95% or more, and the temperature of the reduced iron 21 can be set to room temperature (30°C) + 30°C or less. A more preferable blowing amount is 500 Nm 3 In this case, the reduction rate of the reduced iron 21 can be set to 100%, and the temperature of the reduced iron 21 can be set to room temperature (30°C). In other words, the total blowing amount of the reducing gas 31 can be increased to 1620 Nm3 without affecting the reduction efficiency in the reduction zone 10a. 3 / t-Fe, which is about 30% of the total, is 500Nm 3 The heat treatment of the reducing gas 31 of / t-Fe can be omitted.

[0060] FIG. 6 shows the state inside the furnace when the above simulation was performed. The vertical axis of FIG. 6 shows the depth (m) from the top end of the shaft furnace 10, and the horizontal axis shows the temperature (°C) of each component, the reduction rate (%) of the reduced iron 21, or E H2 (%), where E H2 is called the "hydrogen gas utilization rate" and means the H2O / (H2+H2O) volume ratio in the gas, but in this simulation, 100% by volume of hydrogen gas is injected into the shaft furnace 10, so it simply indicates the H2O concentration. Graph L30 shows the temperature of the reducing gas 31 at each depth position in the shaft furnace 10. Graph L31 shows the temperature of the reduced iron 21 (or the iron oxide raw material 20) at each depth position in the shaft furnace 10. Graph L32 shows the reduction rate of the reduced iron 21 at each depth position in the shaft furnace 10. Graph L33 shows the E H2 "Tuyere" indicates the installation position of the tuyere 10c. According to the graph L30, the reducing gas 31 injected into the cooling zone 10b is heated by the reduced iron 21 as it rises in the cooling zone 10b, and when it reaches the tuyere level, it is heated to approximately the same temperature as the reducing gas 31 (=the temperature of the mixed gas 30) injected from the tuyere 10c. Therefore, it can be said that the oxidized iron raw material 20 can be reduced without affecting the reduction efficiency in the reduction zone 10a. Also, according to the graph L31, it can be seen that the temperature of the reduced iron 21 (or the oxidized iron raw material 20) is approximately the same as the temperature of the reducing gas 31. It can also be seen that the reduction rate of the reduced iron 21 is approximately 100% at the tuyere level.

[0061] The preferable amount of reducing gas 31 to be blown into the cooling zone 10b may vary depending on the operating conditions (e.g., the blowing temperature, the amount of nitrogen gas 32 added, etc.). Therefore, a graph similar to that shown in Fig. 5 may be created for each operating condition to specify the preferable amount of reduction gas 31 to be blown.

[0062] Therefore, by blowing the reducing gas 31 at room temperature into the cooling zone 10b, the sensible heat of the reduced iron 21 can be recovered, and the thermal efficiency can be improved. Furthermore, by adjusting the amount of reducing gas 31 blown into the cooling zone 10b so that the reduction rate of the reduced iron 21 is 95% or more and the temperature of the reduced iron 21 is room temperature (30°C) + 30°C or less, the thermal efficiency can be improved without affecting the reduction efficiency in the reduction zone 10a (and the temperature of the discharged reduced iron 21).

[0063] <1-1. First modified example> Next, a first modified example of the first embodiment will be described with reference to Fig. 7. In an 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.

[0064] 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 cooled to room temperature. 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.

[0065] 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.

[0066] 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.

[0067] Here, when the hydrogen gas concentration of the reducing gas 31 is 100% by volume, according to the above formula (1), the stoichiometric minimum amount of the reducing gas 31 is 600 Nm 3 Therefore, when operating under the operating conditions shown in Figure 5, for example, 500 Nm 3 The reducing gas 31 of / t-Fe (here, hydrogen gas) is blown into the cooling zone 10b from the outside, and the remaining 100 Nm 3 It is preferable to supply the reducing gas 31 of / t-Fe from the outside to the heating furnace 50. In this case, the reduction rate of the reduced iron 21 can be made 100% with the stoichiometrically minimum amount of reducing gas 31, and the temperature of the reduced iron 21 can be made room temperature (30°C).

[0068] In this case, the shaft furnace 10 is fed with a total of 1620 Nm 3 Since it is necessary to blow in reducing gas 31 of / t-Fe (see Figure 4), 1020 Nm 3 The reducing gas 31 of / t-Fe is blown in. 100 Nm 3The reducing gas 31 of / t-Fe is supplied, so the remaining 920 Nm 3 / t-Fe will be covered.

[0069] As described above, according to the first modified example, the reducing gas 31 and the nitrogen gas 32 can be effectively utilized.

[0070] <1-2. Second modified example> Next, a second modified example of the first embodiment will be described with reference to Fig. 8. In the second modified example, like the first modified example, 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. However, in the second modified example, the entire amount of reducing gas 31 supplied from the outside is blown into the cooling zone 10b. In other words, the supply line (b) is omitted. The amount of reducing gas 31 blown into the cooling zone 10b is, for example, the stoichiometric lower limit value of 600 Nm3 mentioned above. 3 The reducing gas 31 injected into the cooling zone 10b cools the reduced iron 21 while rising in the cooling zone 10b (the reducing gas 31 itself is heated). Then, the heated reducing gas 31 reduces the oxidized iron raw material 20 in the reduction zone 10a.

[0071] As described above, in a system in which the furnace gas 40 is circulated as in the first modified example, ideally, the stoichiometrically minimum amount of reducing gas 31 should be supplied from the outside. For example, if the operating conditions under which FIG. 5 was obtained are applied directly to the first modified example (FIG. 7), the reduction gas 31 of 500 Nm 3 The reducing gas 31 of / t-Fe (here, hydrogen gas) is blown into the cooling zone 10b from the outside, and the remaining 100 Nm 3 The reducing gas 31 for / t-Fe may be supplied to the heating furnace 50 from outside.

[0072] If the entire amount of reducing gas 31 supplied from the outside could be blown into the cooling zone 10b at room temperature, the thermal efficiency would be further improved and the supply line (b) would be omitted, which would be extremely advantageous in terms of operation.

[0073] However, as shown in Fig. 5, if the entire amount of the reducing gas 31 supplied from outside is simply injected into the cooling zone 10b at room temperature, there is a concern that the reduction rate of the reduced iron 21 may decrease. In order to inject the entire amount of the reducing gas 31 supplied from outside into the cooling zone 10b at room temperature while maintaining the reduction rate of the reduced iron 21 at 95% or more, a suitable range of the injection amount as shown in Fig. 5 (suitable range of the injection amount of the reducing gas 31 into the cooling zone 10b. In the example of Fig. 5, the suitable range is 450 to 550 Nm 3 / t-Fe) is the lower stoichiometric limit of 600Nm 3 It would be better if the preferred range could be shifted to the right to include / t-Fe.

[0074] The reason why the above-mentioned preferable range is determined is that if an excessive amount of reducing gas 31 is injected into the cooling zone 10b, the temperature of the reduction zone 10a drops, and the reduction rate of the reduced iron 21 drops. Therefore, if such a temperature drop can be compensated for by a separate means, the amount of reducing gas 31 injected into the cooling zone 10b can be increased, and the preferable range of the injection amount can include the stoichiometric value. The present inventor has focused on increasing the amount of nitrogen gas 32 added (in other words, the amount circulating in the circulation system including the shaft furnace 10) as such a means.

[0075] For example, when the amount of nitrogen gas 32 added is increased under the operating conditions of FIG. 5, the graph shown in FIG. 9 is obtained. Graph L40 shows the results when the amount of nitrogen gas added is 500 Nm 3 The amount of reducing gas 31 blown into the cooling zone 10b when / t-Fe is obtained (Nm 3 / t-Fe) and the reduction rate (%) of reduced iron 21. Graph L41 shows the correlation between the amount of nitrogen gas added and the reduction rate (%) of reduced iron 21. 3 The amount of reducing gas 31 blown into the cooling zone 10b when the reducing gas 31 becomes / t-Fe (Nm 3 / t-Fe) and the reduction rate (%) of reduced iron 21. Graph L42 shows the correlation between the nitrogen gas injection amount and the reduction rate (%) of reduced iron 21. 3 The amount of reducing gas 31 blown into the cooling zone 10b when / t-Fe is obtained (Nm 3 The correlation between the amount of reduced iron (Ir / t-Fe) and the temperature (℃) of reduced iron 21 is shown.

[0076] According to FIG. 9, the amount of nitrogen gas 32 blown in was changed from 330 to 500 to 800 Nm 3 / t-Fe, the reduction rate of the reduced iron 21 relative to the amount of the reducing gas 31 blown into the cooling zone 10b can be increased. 3 / t-Fe, 600Nm 3 When the reducing gas 31 of t-Fe is blown into the cooling zone 10b, the reduction rate of the reduced iron 21 can be made 95%. Furthermore, the blowing amount of the nitrogen gas 32 is set to 800 Nm 3 / t-Fe, 600Nm 3 When the reducing gas 31 of 500 Nm3 / t-Fe is blown into the cooling zone 10b, the reduction rate of the reduced iron 21 can be made 100%. In this way, by increasing the amount of nitrogen gas 32 added, the stoichiometric value can be included in the preferable range of the blowing amount. 3 By setting the amount of nitrogen gas 32 to 800 Nm3 / t-Fe or more, the reduction rate of the reduced iron 21 can be made 95% or more even if the entire amount of the reducing gas 31 in the stoichiometric value is blown into the cooling zone 10b. 3 If the nitrogen gas 32 is cooled to 95% or more by using the nitrogen gas 32 / t-Fe, the reduction rate of the reduced iron 21 can be made 100%. Note that the reducing gas 31 may be blown into the cooling zone 10b in an amount equal to or greater than the stoichiometric amount. In this case, it is preferable to further increase the amount of nitrogen gas 32 added and maintain the reduction rate of the reduced iron 21 at 95% or more, preferably 100%.

[0077] Of course, the preferred amount of nitrogen gas 32 to be added may vary depending on the operating conditions. Therefore, a graph such as that shown in Fig. 9 may be created for each operating condition to identify the preferred amount of nitrogen gas 32 to be added. In other words, the amount of nitrogen gas 32 to be added may be adjusted so that the reduction rate of the reduced iron 21 can be maintained at 95% or more even if a stoichiometric amount (or more) of reducing gas 31 is injected into the cooling zone 10b.

[0078] In this way, according to the second modified example, by adjusting the amount of added nitrogen gas 32, the entire amount of reducing gas 31 from the outside can be blown into the cooling zone 10b, and further, the reduction rate of reduced iron 21 can be made 95% or more, preferably 100%.

[0079] <2. Second embodiment> Next, a second embodiment of the present invention will be described. Fig. 10 is an explanatory diagram showing a process flow of a method for producing reduced iron according to the second embodiment. As shown in Fig. 10, the second embodiment is roughly characterized in that ammonia gas 33 is blown into the cooling zone 10b instead of the reducing gas 31. The ammonia gas 33 is blown into the cooling zone 10b at room temperature. That is, in the second embodiment, the ammonia gas 33 is used as a hydrogen gas supply source.

[0080] To explain in more detail, ammonia gas is industrially mass-produced as a raw material for chemical fertilizers, and can be easily liquefied, making it a highly portable hydrogen carrier. Patent Document 1 and Non-Patent Document 2 show that ammonia can be used to reduce iron oxide. However, these documents only verify the basic phenomenon in a laboratory and do not show any specific process images. Here, the present inventor focused on the fact that ammonia can directly reduce iron oxide, but on the fact that reduced iron acts as a catalyst to decompose ammonia (Equation (a)).

[0081] [ka]

[0082] A countercurrent moving layer is formed in the shaft furnace 10 (i.e., the reducing gas 31 and the nitrogen gas 32 rise in the shaft furnace 10, while the oxidized iron raw material 20 and the reduced iron 21 fall in the shaft furnace 10). For this reason, if the ammonia gas 33 is blown in from the cooling zone 10b, the ammonia gas 33 can be decomposed by the reduced iron 21 acting as a catalyst while rising in the shaft furnace 10. To explain in more detail, the ammonia gas 33 blown into the cooling zone 10b cools the reduced iron 21 while rising in the cooling zone 10b. Accordingly, the ammonia gas 33 is heated by the sensible heat of the reduced iron 21. Thereafter, the ammonia gas 33 is decomposed into nitrogen gas and hydrogen gas by the reduced iron 21 acting as a catalyst. In other words, the decomposition reaction of the ammonia gas 33 is an endothermic reaction, but the heat required for the decomposition reaction is provided by the reduced iron 21. In addition, the reduced iron 21 itself acts as a catalyst to promote the decomposition of the ammonia gas. The nitrogen gas 32 and hydrogen gas generated by the decomposition of the ammonia gas 33, that is, the mixed gas 30, rises while being heated by the sensible heat of the reduced iron 21, and reaches the tuyere level. When the mixed gas 30 reaches the tuyere level, it is heated, for example, to the blowing air temperature level. This mixed gas 30 rises in the reduction zone 10a, like the mixed gas 30 blown in from the tuyere 10c, and reduces the iron oxide raw material 20 in the reduction zone 10a. That is, the hydrogen gas and nitrogen gas 32 generated by the decomposition of the ammonia gas 33 while rising in the cooling zone 10b are used as part of the mixed gas 30. Therefore, by blowing the ammonia gas 33 into the cooling zone 10b, the ammonia gas 33 can be used as a hydrogen gas supply source, and the thermal efficiency can also be improved. Furthermore, since a part of the reducing gas 31 (mainly hydrogen gas) supplied from the outside can be replaced with the ammonia gas 33, the amount of the reducing gas 31 supplied from the outside can be reduced.

[0083] Next, the effects of the second embodiment will be considered in detail. The equilibrium constant Kp of equation (a) is defined by the following equation (b). Here, the hydrogen gas concentration of the reducing gas 31 is set to 100% by volume.

[0084]

number

[0085] Here, p NH3 , p H2 , p N2 are the partial pressures of ammonia gas, hydrogen gas, and nitrogen gas, respectively, T is the reaction temperature, and R is the gas constant. Also, △G 0 (T) is the standard free energy of the decomposition reaction of ammonia gas33, and can be calculated based on the physical properties of ammonia gas, hydrogen gas, and nitrogen gas.

[0086] Figure 11 shows the results of plotting the ammonia gas decomposition rate and hydrogen production rate at equilibrium by reaction temperature. The horizontal axis of Figure 11 shows reaction temperature, and the vertical axis shows the volume ratio of each gas (volume of each gas / total volume of all gases) (%) or the equilibrium constant Kp. Graph L50 shows the volume ratio of ammonia gas at each reaction temperature, graph L51 shows the volume ratio of hydrogen gas at each reaction temperature, and graph L52 shows the volume ratio of nitrogen gas at each reaction temperature. Graph L53 shows the equilibrium constant Kp at each reaction temperature.

[0087] According to Fig. 11, most of the ammonia gas can be decomposed into hydrogen gas and nitrogen gas at a reaction temperature of 600°C or higher. In the shaft furnace 10, for example, as shown in Fig. 6, a reduced iron catalyst that satisfies this temperature condition exists in the cooling zone 10b. Therefore, when ammonia gas 33 is blown into the cooling zone 10b, the ammonia gas 33 can be decomposed into hydrogen gas and nitrogen gas 32 in the cooling zone 10b.

[0088] Considering that this decomposition reaction is an endothermic reaction (ΔH=46110 J / mol / K), a simulation was performed on the amount of ammonia gas that can be injected into the cooling zone 10b based on the heat and material balance of the cooling zone 10b under the following preconditions: (1) ammonia gas 33 is completely decomposed in the cooling zone 10b, (2) heat exchange between the reduced iron 21 and ammonia gas 33 in the cooling zone 10b proceeds ideally, and (3) the state inside the furnace of the reduction zone 10a is not affected (i.e., the temperature of the mixed gas 30 generated by the decomposition reaction at the outlet side (tuyere level) of the cooling zone 10b is approximately equal to the blast temperature, and the reduction rate of the reduced iron 21 is 100% at the tuyere level). The simulation was performed using the mathematical model described above, and the calculation conditions were the same as those in Table 1. The amount of ammonia gas 32 added was 330 Nm 3 The hydrogen gas concentration of the reducing gas 31 supplied from the outside was set to 100% by volume. The results are shown in FIG.

[0089] The horizontal axis of FIG. 12 indicates the blowing temperature of the mixed gas 30 blown into the shaft furnace 10 from the tuyere 10c, and the vertical axis indicates the unit consumption (Nm m 3 / t-Fe), and the unit consumption of hydrogen gas produced by the decomposition of ammonia gas33 (Nm 3 / t-Fe), or the unit consumption (Nm 3 / t-Fe). Graph L60 shows the unit consumption (Nm 3 / t-Fe) and the air temperature. Graph L61 shows the unit consumption (Nm 3 / t-Fe) and the air temperature. Graph L62 shows the correlation between the unit consumption (Nm 3 / t-Fe) and the blowing air temperature.

[0090] As shown in FIG. 12, the amount of ammonia gas that can be decomposed in the cooling zone 10b is proportional to the sensible heat brought in by the reduced iron 21, so the higher the blowing temperature, the greater the amount of ammonia gas that can be decomposed (i.e., the greater the amount of ammonia gas that can be blown into the cooling zone 10b), and the greater the amount of hydrogen gas produced by the decomposition of ammonia gas 33. As a result, the amount of hydrogen gas supplied from the outside decreases. Hydrogen gas is produced by the decomposition of ammonia gas 33 at a rate of 200 Nm 3 If there is no furnace heat loss, the minimum amount of hydrogen gas required for reduction is 600 Nm 3 / t-Fe) can be covered by the decomposition of ammonia gas33.

[0091] The amount of ammonia gas that can be injected into the cooling zone 10b may differ depending on the operating conditions, but it is preferable to determine the amount of ammonia gas so that the reduction rate of the reduced iron 21 is 95% or more, preferably 100%.

[0092] According to the second embodiment, by blowing ammonia gas 33 into the cooling zone 10b, the ammonia gas 33 can be used as a hydrogen gas supply source, and the thermal efficiency can be improved. Furthermore, since a part of the reducing gas 31 (mainly hydrogen gas) supplied from the outside can be replaced with ammonia gas 33, the amount of the reducing gas 31 supplied from the outside can be reduced.

[0093] <2-1. Modifications> Next, a modified example of the second embodiment will be described with reference to Fig. 13. This modified example is roughly a combination of the first modified example of the first embodiment and the second embodiment. That is, in this modified example, the unreacted hydrogen gas 31a and nitrogen gas 32 are separated and recovered from the top gas 40 in the second embodiment, and are reused as part of the mixed gas 30.

[0094] However, in this modified example, since nitrogen gas 32 derived from ammonia gas flows into the system, it is necessary to constantly discharge an amount of nitrogen gas equivalent to this inflow amount to keep the amount of nitrogen gas circulating in the system constant.

[0095] Therefore, in this modification, 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 (containing hydrogen gas) is supplied to the heating furnace 50 as the fuel gas 85 for the heating furnace 50. The amount of the nitrogen gas 32 contained in the fuel gas 85 is set to be approximately equal to the amount of the nitrogen gas 32 that flows into the system due to the decomposition of the ammonia gas 33 (i.e., 1 / 2 the volume of the ammonia gas blown into the cooling zone 10b). Here, the amount of the nitrogen gas 32 in the fuel gas 85 can be grasped, for example, by measuring the flow rate of the fuel gas (volumetric method is common) and analyzing the nitrogen gas. 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 after combustion (containing water vapor and nitrogen gas 32) is released to the outside. The remaining circulating gas 70 is reused as part of the mixed gas 30. That is, the circulating gas 70 is introduced again into the heating furnace 50. Other processes are the same as those in the first modified example of the first embodiment and the second embodiment. As a result, the amount of nitrogen gas circulating in the system is kept constant, and the same effects as those in the first modified example of the first embodiment and the second embodiment described above can be obtained.

[0096] In addition, from the viewpoint of avoiding inhibition of reduction due to excessive introduction of nitrogen gas 32 as pointed out in

[0054] to

[0055] , it goes without saying that it is desirable to maintain the ratio of the amount of nitrogen gas circulating in the system to the amount of reducing gas 31 within the appropriate range shown in FIG. 19.

[0097] In the above-described example, the reducing gas 31 and the nitrogen gas 32 are heated together, but the present invention is not limited to such an example. For example, the reducing gas 31 and the nitrogen gas 32 may be heated separately, and then mixed and blown into the shaft furnace 10 from the tuyere 10c. In this case, it is preferable to set the heating temperature of the nitrogen gas 32 higher than the heating temperature of the reducing gas 31. This makes it possible to reduce the heating load of the reducing gas 31.

[0098] 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]

[0099] 10 Shaft furnace 10a Reduction Zone 10b Cooling Zone 20 Iron oxide raw materials 30 Mixed Gas 31 Reducing Gas 32 Nitrogen gas 33 Ammonia gas 40 Furnace gas 50 Furnace 60 Separation and recovery device 70 Circulating Gas 80 Branch Pipe 85 Fuel Gas

Claims

[Claim 1] A method for producing reduced iron by reducing iron oxide charged in a shaft furnace, comprising the steps of: A total amount of reducing gas supplied from outside is blown at room temperature into a cooling zone for reduced iron provided at a lower portion of the shaft furnace, A heated mixed gas containing the unreacted reducing gas and nitrogen gas separated and recovered from the furnace top gas is blown into the shaft furnace from a tuyere provided in a lower part of a reduction zone of the shaft furnace; The reducing gas contains 90% by volume or more of hydrogen gas, using the reducing gas that has risen through the cooling zone to reduce the iron oxide; 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 using the separated and recovered hydrogen gas and nitrogen gas as the mixed gas.

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