Production method for reducing gas and reduction method for iron ore

Abstract

Provided is a method for producing a reducing gas that includes carbon monoxide from a starting material gas that includes carbon dioxide and a sulfur compound. This production method for a reducing gas uses a starting material gas that is one or more types of byproduct gas selected from the group that consists of blast furnace gas, hot blast stove exhaust gas, coke oven exhaust gas, heating furnace exhaust gas, annealing furnace exhaust gas, and power generation facility exhaust gas. The production method includes a dry desulfurization step for removing a sulfur compound from the starting material gas to obtain a purified gas and an electrolysis step for using a solid oxide electrolyzer cell to electrolyze the purified gas and obtain a reducing gas.

Description

Method for producing reducing gas and method for reducing iron ore

[0001] The present invention relates to a method for producing a reducing gas and a method for reducing iron ore.

[0002] In recent years, there has been a demand for reducing greenhouse gas emissions as a measure against global warming. Much of our energy demand is met by burning fossil fuels, and carbon dioxide, a major greenhouse gas, is generated as a result. Discussions are underway to reduce the use of fossil fuels and use renewable energy sources such as wind, solar, and geothermal energy instead. However, the use of renewable energy presents cost challenges, and it is currently unrealistic to meet all energy demands with renewable energy. Therefore, technologies that separate, capture, and store carbon dioxide generated by fossil fuel combustion (Carbon Capture and Storage: CCS) and convert the captured carbon dioxide into valuable resources such as fuels and chemical products (Carbon Capture and Utilization: CCU) are being considered.

[0003] As an invention related to CCU, for example, Patent Document 1 describes an invention of a method for synthesizing methane, which includes a first reaction step of reacting carbon dioxide with hydrogen to obtain carbon monoxide, and a second reaction step of reacting the produced carbon monoxide with hydrogen to obtain methane. Patent Document 2 describes an invention related to a method for co-electrolyzing carbon dioxide and water using a solid oxide electrolysis cell to ultimately synthesize hydrocarbons, and the structure of the solid oxide electrolysis cell.

[0004] If the feed gas supplied to a solid oxide electrolysis cell contains sulfur components, the catalyst surface of the electrode may react with the sulfur, resulting in performance degradation. Therefore, technologies for removing sulfur components from the feed gas have been investigated. For example, Patent Literature 3 describes an invention of a high-temperature fuel cell stack in which natural gas and water are supplied to a solid oxide electrolysis cell to reform a synthesis gas containing hydrogen, methane, carbon monoxide, and water, and the resulting synthesis gas is then supplied to a solid oxide fuel cell to generate electricity. This high-temperature fuel cell stack uses hydrogen generated by the solid oxide fuel cell to hydrodesulfurize the sulfur components contained in natural gas. Hydrodesulfurization is a method for removing sulfur from linear hydrocarbon chains by reacting the sulfur contained in the linear hydrocarbon chain with hydrogen to produce hydrogen sulfide. The generated hydrogen sulfide can be easily removed by reacting it with a metal to form a metal sulfide.

[0005] JP 2012-140382 A JP 2023-50701 A International Publication No. 2011 / 137916

[0006] In the blast furnace process, a typical steelmaking process, carbon dioxide is produced during the reduction of iron oxide using coke, so the separation of carbon dioxide from blast furnace gas by physical adsorption has been investigated. The carbon dioxide separated from blast furnace gas contains residual sulfur components, but unlike natural gas, the form of these compounds is different: in addition to hydrogen sulfide, they exist in the form of sulfur compounds such as carbonyl sulfide and carbon disulfide.

[0007] According to the inventors' investigations, when attempting to perform hydrodesulfurization on carbon dioxide separated from blast furnace gas using a solid oxide electrolysis cell as described in Patent Document 3, it was found that it is difficult to decompose and remove sulfur compounds, particularly carbonyl sulfide and carbon disulfide. As described above, if the feed gas supplied to the solid oxide electrolysis cell contains sulfur compounds, the performance of the electrodes may deteriorate. This has led to the problem of limiting the use of carbon dioxide derived from blast furnace gas in CCU.

[0008] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a method for producing a reducing gas containing carbon monoxide from a raw material gas containing carbon dioxide and a sulfur compound.

[0009] The gist and configuration of the present invention are as follows.

[0010] [1] A method for producing a reducing gas containing carbon monoxide from a raw material gas containing carbon dioxide and sulfur compounds, wherein the raw material gas comprises one or more by-product gases selected from the group consisting of blast furnace gas, hot stove exhaust gas, coke oven exhaust gas, heating furnace exhaust gas, annealing furnace exhaust gas, and power generation facility exhaust gas, the method comprising: a dry desulfurization step of removing the sulfur compounds from the raw material gas to obtain a purified gas; and an electrolysis step of electrolyzing the purified gas using a solid oxide electrolysis cell to obtain the reducing gas.

[0011] [2] The method for producing a reducing gas according to the above [1], wherein the sulfur compound includes at least one of carbonyl sulfide and carbon disulfide.

[0012] [3] The method for producing a reducing gas according to the above [2], further comprising a hydrolysis step of hydrolyzing at least one of carbonyl sulfide and carbon disulfide, located upstream of the dry desulfurization step.

[0013] [4] The method for producing a reducing gas according to any one of the above [1] to [3], wherein the dry desulfurization step includes an operation of removing the sulfur compounds using a dry desulfurization agent.

[0014] [5] The method for producing a reducing gas according to the above [4], wherein the dry desulfurization agent contains zinc oxide.

[0015] [6] The method for producing a reducing gas according to any one of the above [1] to [5], further comprising a dehydration step for removing water and water vapor contained in the raw material gas, upstream of the dry desulfurization step.

[0016] [7] The method for producing a reducing gas according to any one of the above [1] to [6], further comprising a dust removal step for removing dust contained in the raw material gas, upstream of the dry desulfurization step.

[0017] [8] The method for producing a reducing gas according to any one of [1] to [7] above, further comprising a hydration step of adding steam to the purified gas downstream of the dry desulfurization step and upstream of the electrolysis step, wherein the reducing gas contains hydrogen.

[0018] [9] The method for producing a reducing gas according to any one of the above [1] to [8], wherein the raw material gas comprises the blast furnace gas.

[0019]

[10] A method for reducing iron ore, comprising: a dry desulfurization step of removing sulfur compounds from a by-product gas containing carbon dioxide and sulfur compounds generated in a blast furnace to obtain a purified gas; an electrolysis step of electrolyzing the purified gas using a solid oxide electrolysis cell to obtain a reducing gas; and a reduction step of supplying the reducing gas to the blast furnace to reduce the iron ore.

[0020] According to the method for producing a reducing gas of the present invention, in the process of producing a reducing gas containing carbon monoxide using a solid oxide electrolytic cell from a feed gas containing carbon dioxide and sulfur compounds, deterioration of the solid oxide electrolytic cell due to sulfur compounds can be suppressed, thereby making it possible to easily produce a reducing gas from a feed gas containing carbon dioxide and sulfur compounds derived from, for example, blast furnace gas.

[0021] Fig. 1 is a flowchart showing a method for producing a reducing gas according to the present invention; Fig. 2 is a schematic diagram showing a process for electrolysis of carbon dioxide using a solid oxide electrolysis cell; Fig. 3 is a flowchart showing a method for producing a reducing gas according to the prior art; Fig. 4 is a flowchart showing options upstream of the dry desulfurization step in the method for producing a reducing gas according to the present invention; Fig. 5 is a flowchart showing options downstream of the dry desulfurization step in the method for producing a reducing gas according to the present invention; Fig. 6 is a flowchart showing a method for reducing iron ore according to the present invention;

[0022] Hereinafter, embodiments of the present invention will be described in detail. The component composition of a gas in this specification is expressed in volume percentage (vol%) or parts per million by volume (ppm). For precision, the component composition of a gas may be the component composition at standard state. Standard state refers to a state in which the gas temperature is 0°C and the gas pressure is 1 atmosphere.

[0023] 1. Method for Producing Reducing Gas In one embodiment, the present invention provides a method for producing a reducing gas containing carbon monoxide from a raw material gas containing carbon dioxide and sulfur compounds, wherein the raw material gas comprises one or more by-product gases selected from the group consisting of blast furnace gas, hot stove exhaust gas, coke oven exhaust gas, heating furnace exhaust gas, annealing furnace exhaust gas, and power plant exhaust gas, and the method comprises: a dry desulfurization step of removing sulfur compounds from the raw material gas to obtain a purified gas; and an electrolysis step of electrolyzing the purified gas using a solid oxide electrolysis cell to obtain a reducing gas. Figure 1 is a flowchart showing the method for producing a reducing gas according to the present invention. In the method for producing a reducing gas according to the present invention, the raw material gas is treated in the dry desulfurization step S1 to obtain a purified gas, and the obtained purified gas is then treated in the electrolysis step S2 to produce a reducing gas.

[0024] <Feedstock Gas> The feedstock gas used in the method for producing a reducing gas according to the present invention is a gas containing carbon dioxide and sulfur compounds. A typical feedstock gas is a by-product gas generated from a blast furnace, i.e., blast furnace gas. As described above, a large amount of carbon dioxide is generated in the ironmaking process, typified by the blast furnace method, and therefore separation of carbon dioxide from blast furnace gas by physical adsorption has been studied. Storing the separated carbon dioxide as it is (CCS) poses a cost problem. Converting the carbon dioxide generated from a blast furnace into a valuable resource using the method for producing a reducing gas according to the present invention and effectively utilizing it in the ironmaking process (CCU) is preferable in terms of supply and demand balance.

[0025] During the operation of a blast furnace, by-product gases containing carbon dioxide and sulfur compounds are generated. The reducing gas production method according to the present invention is suitable as a means for producing a reducing gas containing carbon monoxide using such by-product gases. The reducing gas produced by the reducing gas production method according to the present invention can be used, for example, as a gas for promoting or assisting the reduction reaction in a blast furnace.

[0026] The raw material gas used in the method for producing a reducing gas according to the present invention comprises one or more by-product gases selected from the group consisting of blast furnace gas, hot stove exhaust gas, coke oven exhaust gas, heating furnace exhaust gas, annealing furnace exhaust gas, and power generation facility exhaust gas. All of these by-product gases contain carbon dioxide and sulfur compounds. These by-product gases may be used alone or in combination of two or more.

[0027] Carbon dioxide contained in the feed gas is a typical greenhouse gas, and as mentioned above, reduction in emissions is desired. According to the method for producing a reducing gas of the present invention, the carbon dioxide contained in the feed gas is electrolyzed using a solid oxide electrolysis cell to convert it to carbon monoxide, thereby promoting the use of carbon dioxide in a CCU. If the proportion of carbon dioxide in the feed gas is 15 vol% or more, carbon dioxide becomes the main component of the feed gas, which is preferable in terms of the yield of the reducing gas. There is no particular upper limit to the proportion of carbon dioxide, and it may be, for example, 99.9 vol% or less.

[0028] The sulfur compounds contained in the feed gas include at least one of hydrogen sulfide, carbonyl sulfide, and carbon disulfide. As described above, supplying a feed gas containing sulfur compounds to a solid oxide electrolysis cell presents the problem of making it difficult to electrolyze carbon dioxide. To solve this problem, the feed gas used in the method for producing a reducing gas according to the present invention is limited to a feed gas containing both carbon dioxide and sulfur compounds. Since the proportion of sulfur compounds in the feed gas exceeds 0.010 ppm, desulfurization is necessary, thereby achieving significant effects of the present invention. Furthermore, a proportion of sulfur compounds of 1.0 vol% or less does not significantly impede the use of carbon dioxide. Therefore, the proportion is preferably greater than 0.010 ppm and less than 1.0 vol%. Note that the sulfur compounds in the present invention are limited to those in a gaseous state during the process of carrying out the method for producing a reducing gas according to the present invention. Furthermore, in this specification, the term "proportion of sulfur compounds" refers to the total proportion of all gaseous sulfur compounds contained in the feed gas.

[0029] In addition to carbon dioxide and sulfur compounds, the feed gas may contain other gas components, such as carbon monoxide, hydrogen, and a small amount of water vapor, to the extent that they do not significantly interfere with the electrolysis of carbon dioxide in the solid oxide electrolysis cell.

[0030] 1, the method for producing a reducing gas according to the present invention includes an electrolysis step S2 in which a purified gas is electrolyzed using a solid oxide electrolysis cell to obtain a reducing gas. The purified gas is a gas obtained by removing sulfur compounds from a raw material gas, and will be described in detail later.

[0031] The technology underlying the method for producing a reducing gas according to the present invention is a method for obtaining carbon monoxide by electrolyzing carbon dioxide using a solid oxide electrolyzer cell. Figure 2 is a schematic diagram showing the process of electrolyzing carbon dioxide using a solid oxide electrolyzer cell. The solid oxide electrolyzer cell 1 has a structure in which a solid electrolyte 2 is sandwiched between a cathode 3 and an anode 4. Hereinafter, the solid oxide electrolyzer cell may be referred to as an "SOEC."

[0032] The solid electrolyte 2 may be made of, for example, yttria-stabilized zirconia, which has ion conductivity. The solid electrolyte 2 is maintained at a high temperature of 650°C or higher by a heating means (not shown). By creating a potential difference between the cathode 3 and the anode 4 using an external DC power supply 5, oxygen ions migrate from the cathode 3 side to the anode 4 side within the solid electrolyte 2. The cathode 3 may be made of, for example, a porous material in which fine nickel particles are mixed with yttria-stabilized zirconia. The nickel particles function as a catalyst during the electrolysis of carbon dioxide. The anode 4 may be made of, for example, a porous support on which a thin film of La—Sr-based oxide is formed. Specific examples of La—Sr-based oxides include La—Sr—Co-based oxides, La—Sr—Co—Fe-based oxides, and La—Sr—Fe—Mn-based oxides.

[0033] When carbon dioxide is supplied to the surface of the cathode 3 in the SOEC 1 having the above configuration, the carbon dioxide receives electrons from the cathode 3 and is electrolyzed into carbon monoxide and oxygen ions. The chemical formula (1) of the electrochemical reaction at the cathode 3 is shown below. [Chemical Formula 1] CO ２ + 2e － → CO + O ２－ ...(1)

[0034] Carbon monoxide generated by this reaction is extracted from the surface of the cathode 3 to the outside of the SOEC 1 and is used as a reducing gas in the present invention. Meanwhile, oxygen ions move inside the solid electrolyte 2 toward the anode 4, release electrons to the anode 4, and become oxygen. The chemical formula (2) of the electrochemical reaction at the anode 4 is shown below. [Chemical Formula 2] 2O ２－ → O ２ + 4e － ... (2)

[0035] The oxygen generated by this reaction is extracted from the surface of the anode 4 to the outside of the SOEC 1 and is used as a by-product in the present invention. In order to extract the oxygen generated at the anode 4 to the outside, air may be introduced into the anode 4 from the outside.

[0036] The electricity used in the DC power supply 5 of the SOEC 1 is preferably electricity obtained from renewable energy.

[0037] <Dry Desulfurization Process> If the raw material gas supplied to the surface of the cathode 3 of the SOEC 1 contains carbon dioxide and sulfur compounds, it becomes difficult to electrolyze carbon dioxide using the SOEC 1. Nickel particles are dispersed on the surface of the cathode 3. These nickel particles function as a catalyst that reduces the activation energy when carbon dioxide is electrolyzed. In addition, the surfaces of the nickel particles are electrically active and play a role in transferring electrons to carbon dioxide. However, if sulfur compounds such as hydrogen sulfide are mixed into the raw material gas, nickel sulfide, which is not electrically active, is formed on the surfaces of the nickel particles. As a result, electrons cannot be transferred to carbon dioxide, making it difficult to electrolyze carbon dioxide.

[0038] Referring again to FIG. 1 , the method for producing a reducing gas according to the present invention includes a dry desulfurization step S1 in which sulfur compounds are removed from a raw material gas to obtain a purified gas. In the dry desulfurization step, sulfur compounds are removed from a raw material gas containing carbon dioxide and sulfur compounds. In this specification, the gas from which the sulfur compounds have been removed is referred to as a purified gas. The purified gas from which the sulfur compounds have been removed is supplied to the cathode 3 of the SOEC 1 to perform the electrolysis step S2, thereby maintaining the electrical activity of the nickel particles. This allows for stable and continuous electrolysis of carbon dioxide while preventing deterioration of the cathode 3 of the SOEC 1.

[0039] The proportion of sulfur compounds in the purified gas is preferably 0.10 ppm or less, since a proportion of sulfur compounds of 0.10 ppm or less can suppress cathode deterioration. A more preferred proportion of sulfur compounds in the purified gas is 0.010 ppm or less. The lower the proportion of sulfur compounds in the purified gas, the better, so no particular lower limit is set, although the lower limit may be zero. It is more preferred that the proportion of sulfur compounds be below the detection limit of the analytical means.

[0040] <Sulfur Compounds> In a preferred embodiment, in the method for producing a reducing gas according to the present invention, the sulfur compounds include at least one of carbonyl sulfide and carbon disulfide. These sulfur compounds are particularly difficult to desulfurize using conventional hydrodesulfurization. FIG. 3 is a flowchart showing a method for producing a reducing gas according to the prior art. This prior art process employs the hydrodesulfurization described in Patent Document 3. When a source gas containing sulfur compounds is hydrodesulfurized, methanation of carbon dioxide tends to proceed. This will be explained using carbonyl sulfide as an example. In the method for producing a reducing gas according to the prior art shown in FIG. 3, when the hydrodesulfurization step S1' is performed using a source gas containing carbon dioxide and carbonyl sulfide, carbon dioxide reacts with hydrogen to produce methane and water. The chemical formula (3) of this reaction is shown below. [Chemical Formula 3] CO ２ + 4H ２ → CH ４ + 2H ２ O... (3)

[0041] On the other hand, carbonyl sulfide reacts with hydrogen to produce carbon monoxide and hydrogen sulfide. The chemical reaction is shown below in chemical formula (4): [Chemical formula 4] COS + H ２ → CO + H ２ S... (4)

[0042] The chemical reaction represented by chemical formula (3) is the methanation of carbon dioxide, also known as the Sabatier reaction. This chemical reaction is exothermic, with a free energy change ΔG of approximately -130 kJ / mol. Therefore, the chemical reaction represented by chemical formula (3) tends to proceed spontaneously in the presence of a catalyst such as nickel particles. On the other hand, the free energy change ΔG of the hydrodesulfurization of carbonyl sulfide represented by chemical formula (4) is nearly zero. Therefore, this chemical reaction is unlikely to proceed spontaneously unless special operations such as increasing the partial pressure of hydrogen are performed.

[0043] As a result, in the hydrodesulfurization step S1', most of the hydrogen is consumed in the methanation of carbon dioxide, while hydrodesulfurization of carbonyl sulfide hardly progresses. Furthermore, since the carbon dioxide contained in the raw material gas is consumed by methanation, the yield of the target reduced gas containing carbon monoxide decreases. Furthermore, when a purified gas containing methane is treated in the electrolysis step S2 in the presence of water vapor, methane reacts with water to produce carbon monoxide and hydrogen, and during this process, a large amount of heat is lost, causing a decrease in the temperature of the SOEC. Similarly, when the sulfur compound is carbon disulfide instead of carbonyl sulfide, methanation of carbon dioxide proceeds preferentially.

[0044] The present invention aims to solve the above-mentioned problems associated with conventional hydrodesulfurization processes. In a preferred embodiment, the sulfur compounds contained in the feed gas are limited to those containing at least one of carbonyl sulfide and carbon disulfide, which are particularly effective in achieving the effects of the present invention.

[0045] <Dry Desulfurization Agent> In a preferred embodiment, in the method for producing a reducing gas according to the present invention, the dry desulfurization step includes removing the sulfur compounds using a dry desulfurization agent. The dry desulfurization agent is a granular substance used in dry desulfurization. In dry desulfurization, the source gas is passed through a container filled with the dry desulfurization agent, and the sulfur compounds are physically or chemically adsorbed onto the surface of the dry desulfurization agent, thereby removing the sulfur compounds from the source gas. Among the sulfur compounds that may be contained in the source gas, for example, hydrogen sulfide can be removed by reacting it with a metal to form a metal sulfide, as described above. Furthermore, hydrogen sulfide is easily soluble in water, making it relatively easy to remove. On the other hand, carbonyl sulfide and carbon disulfide are difficult to remove by these methods, and therefore, it is effective to remove them using a dry desulfurization agent. Examples of dry desulfurization agents that can be used in this preferred embodiment include known dry desulfurization agents, such as iron-based desulfurization agents such as iron oxyhydroxide, copper-based desulfurization agents, and zinc oxide.

[0046] In a more preferred embodiment, the dry desulfurization agent is a desulfurization agent containing zinc oxide. Among the dry desulfurization agents exemplified above, iron-based dry desulfurization agents have difficulty in completely removing carbonyl sulfide and carbon disulfide. Copper-based dry desulfurization agents can remove carbonyl sulfide and carbon disulfide, but they also react with carbon monoxide and water to produce carbon dioxide and hydrogen, making them unsuitable as dry desulfurization agents in the present invention. Zinc oxide does not have these drawbacks and is therefore more preferred as a dry desulfurization agent in the present invention. Chemical formulas (5), (6), and (7) of the reaction for desulfurizing hydrogen sulfide, carbonyl sulfide, and carbon disulfide using zinc oxide are shown below. [Chemical Formula 5] ZnO + H ２ S → ZnS + H ２ O...(5) [Chemical formula 6] ZnO + COS → ZnS + CO ２ ...(6) [Chemical formula 7] 2ZnO + CS ２ → 2ZnS + CO ２ ... (7)

[0047] The zinc oxide-containing desulfurizing agent is preferably in the form of a cylinder obtained by extruding a slurry containing a powder of the active ingredient, followed by drying and solidification. The outer diameter of the cylindrical desulfurizing agent is preferably 3.0 mm or more, since a diameter of 3.0 mm or more does not interfere with the flow of the raw material gas, and a diameter of 5.0 mm or less ensures the surface area required for absorption. In dry desulfurization using a zinc oxide-containing desulfurizing agent, it is preferable to maintain the temperature of the desulfurizing agent at 350°C or more and 450°C or less in terms of the absorption efficiency of sulfur compounds. The desulfurizing agent is preferably heated by heat exchange with the purified gas after desulfurization or the reducing gas after electrolysis. Since the adsorption efficiency of a zinc oxide-containing desulfurizing agent decreases after use for a certain period of time, it is preferable to replace it with a new one.

[0048] Even when the dry desulfurization agent is heated as described above, the temperature of the dry desulfurization agent may decrease due to contact with a raw material gas at a lower temperature, resulting in a decrease in the absorption efficiency of sulfur compounds. To prevent such a problem, a pre-desulfurization heating step for heating the raw material gas may be provided before the dry desulfurization step. The temperature of the raw material gas heated in the pre-desulfurization heating step is preferably controlled to be equal to or slightly higher than the temperature of the heated desulfurization agent.

[0049] <Hydrolysis Step> Next, several steps that can be implemented as options upstream of the above-mentioned dry desulfurization step will be described. FIG. 4 is a flowchart including options upstream of the dry desulfurization step S1. This upstream option may be implemented in combination with a downstream option described below, or the upstream option may be implemented alone. In a preferred embodiment, the method for producing a reducing gas according to the present invention has a hydrolysis step O2 upstream of the dry desulfurization step S1, in which at least one of carbonyl sulfide and carbon disulfide is hydrolyzed. Carbonyl sulfide and carbon disulfide react with water or steam to produce carbon dicarbonide and hydrogen sulfide. Chemical formulas (8) and (9) of this reaction are shown below. [Chemical Formula 8] COS + H ２ O → CO ２ + H ２ S...(8) [Chem.9] CS ２ + 2H ２ O → CO ２ + 2H ２ S... (9)

[0050] The chemical reactions represented by chemical formulas (8) and (9) are likely to proceed in the presence of an alumina catalyst heated to, for example, 200°C, and do not involve methanation. Furthermore, the reactions proceed even in the presence of carbon dioxide or carbon monoxide. Therefore, the hydrolysis step is suitable for removing carbonyl sulfide and carbon disulfide from the raw material gas. By providing an additional hydrolysis step upstream of the dry desulfurization step, carbonyl sulfide and carbon disulfide can be more completely removed. This can prevent a phenomenon known as sulfur slip, in which sulfur compounds flow into downstream processes such as the electrolysis step.

[0051] 4, when the dehydration step O3, which will be described later, is provided upstream of the dry desulfurization step S1, the hydrolysis step O2 is preferably provided upstream of the dry desulfurization step S1 and upstream of the dehydration step O3. Furthermore, when the dedusting step O1, which will be described later, is provided in addition to the dehydration step O3, the hydrolysis step O2 is preferably provided downstream of the dedusting step O1 and upstream of the dehydration step O3.

[0052] <Dehydration Step> In a preferred embodiment, the method for producing a reducing gas according to the present invention includes a dehydration step O3 upstream of the dry desulfurization step S1, which removes water and water vapor contained in the raw material gas. If water vapor is contained in the raw material gas to be subjected to dry desulfurization, the desulfurization will not proceed smoothly. This will be explained using zinc oxide as an example. Chemical formula (5) representing the desulfurization of hydrogen sulfide by zinc oxide is reproduced below. [Chemical Formula 10] ZnO + H ２ S → ZnS + H ２ O... (5)

[0053] When the partial pressure of water vapor contained in the feed gas is high, the chemical equilibrium of the chemical reaction expressed by chemical formula (5) shifts toward the left side, preventing the chemical absorption of hydrogen sulfide by zinc oxide. This tendency is also observed for sulfur compounds other than hydrogen sulfide. Therefore, in a preferred embodiment, the water or water vapor contained in the feed gas is removed in advance to lower the partial pressure of water vapor, thereby shifting the chemical equilibrium of chemical formula (5) toward the right side. This improves the efficiency of desulfurization and enables the electrolysis process in the SOEC to be carried out more stably.

[0054] Specific means for carrying out the dehydration step include, for example, a method in which the raw material gas is passed through the surface of a pipe cooled with water or other refrigerant to condense and remove the moisture, a method in which an adsorbent such as hydrophilic zeolite is used, etc. When there is a surplus heat source that can be used for heating to remove the water adsorbed by the adsorbent and regenerate the adsorbent, the method in which an adsorbent is used is preferred because of its high efficiency.

[0055] <Dust Removal Step> In a preferred embodiment, the method for producing a reducing gas according to the present invention includes a dust removal step O1, which removes dust contained in the raw material gas, upstream of the dry desulfurization step S1. Blast furnace gas generated in the steelmaking process contains a large amount of dust. The dust contained in the blast furnace gas is very fine and light, so it does not fall due to gravity but remains suspended in the blast furnace gas. If dust-containing blast furnace gas is used as the raw material gas of the present invention, the dust may adhere to the dry desulfurization agent or the surface of the electrodes, reducing the efficiency of the treatment. Therefore, it is preferable to remove dust contained in the raw material gas in advance upstream of the dry desulfurization step S1.

[0056] Any means may be used to remove dust. For example, it is preferable to separate and remove dust from the raw material gas using a device such as a wet dust collector (wet scrubber). The temperature of the water used to wash the raw material gas in the wet dust collector is preferably 30°C or higher, which provides excellent dust collection effectiveness, and 60°C or lower, which prevents the amount of water or water vapor contained in the raw material gas from being too large and reduces the processing burden when performing the dehydration step O3 downstream. Therefore, it is preferable that the water temperature is 30°C or higher and 60°C or lower. A more preferable range is 30°C or higher and 40°C or lower. A known device such as a Venturi scrubber can be used as the wet dust collector.

[0057] When the wet dedusting step O1 is performed, it is unavoidable that water vapor will be mixed into the raw material gas even if the water temperature is lowered. For this reason, it is preferable to perform the above-mentioned dehydration step O3 downstream of the dedusting step O1 and upstream of the dry desulfurization step S1, as shown in Figure 4.

[0058] <Hydration Step> Next, a step that can be implemented as an option downstream of the dry desulfurization step will be described. FIG. 5 is a flowchart including an option downstream of the dry desulfurization step S1. This downstream option may be implemented in combination with the upstream option described above, or the downstream option alone may be implemented. In a preferred embodiment, the method for producing a reducing gas according to the present invention includes a hydration step O4, downstream of the dry desulfurization step S1 and upstream of the electrolysis step S2, in which water vapor is added to the purified gas, and the reducing gas contains hydrogen. By adding water vapor to the purified gas, electrolysis of water occurs simultaneously with electrolysis of carbon dioxide in the electrolysis step S2. Specifically, when water vapor is supplied to the surface of the cathode 3 in the SOEC 1, water molecules receive electrons from the cathode 3 and are electrolyzed into hydrogen and oxygen ions. The chemical formula (10) of the electrochemical reaction at the cathode 3 is shown below. [Chemical Formula 11] H ２ O + 2e － → H ２ + O ２－ ...(10)

[0059] Hydrogen is produced at the cathode 3 by the reaction shown in chemical formula (10). At the cathode 3, carbon dioxide and water are simultaneously electrolyzed (co-electrolyzed), resulting in the production of carbon monoxide and hydrogen. Because hydrogen is a reducing gas like carbon monoxide, a synthesis gas containing carbon monoxide and hydrogen is also a reducing gas. Meanwhile, as shown in chemical formula (2), oxygen ions move inside the solid electrolyte 2 toward the anode 4, release electrons to the anode 4, and become oxygen. The oxygen derived from the water vapor produced at the anode 4 mixes with the oxygen derived from carbon dioxide and is extracted to the outside of the SOEC 1 and used as a by-product.

[0060] The water vapor added to the purified gas in the hydration step is preferably generated by heating water with as high a purity as possible. For example, the water vapor used in the hydration step is preferably produced by indirectly heating ion-exchanged water.

[0061] <Blower> In a preferred embodiment, the method for producing a reducing gas according to the present invention has a blower for blowing the raw material gas. The blower is effective as a means for generating pressure for sending the raw material gas to the dry desulfurization step. The blower can be configured, for example, by a blower equipped with a fan. The allowable temperature at which the drive mechanism for rotating the fan can be used is not very high. For this reason, the blower is preferably provided upstream of the dry desulfurization step, where the purified gas may be heated to a high temperature.

[0062] 2. Iron Ore Reduction Method In another embodiment, the present invention is an invention of a method for reducing iron ore, comprising: a dry desulfurization step of removing sulfur compounds from a by-product gas containing carbon dioxide and sulfur compounds generated from a blast furnace to obtain a purified gas; an electrolysis step of electrolyzing the purified gas using a solid oxide electrolysis cell to obtain a reducing gas; and a reduction step of supplying the reducing gas to a blast furnace to reduce iron ore.

[0063] As described above, the reducing gas produced using the reducing gas production method according to the present invention contains carbon monoxide obtained by reducing carbon dioxide. Furthermore, the water vapor originally contained in the by-product gas and the water vapor added to the purified gas in the hydration step in a preferred embodiment are reduced to hydrogen in the electrolysis step S2. Both carbon monoxide and hydrogen have reducing properties and can therefore be used as gases for promoting or assisting the reduction reaction in a blast furnace.

[0064] Fig. 6 is a flowchart showing the iron ore reduction method according to the present invention. The dry desulfurization step S1 and the electrolysis step S2 have already been described, and therefore a detailed description thereof will be omitted here. However, as shown in Fig. 6, in the iron ore reduction method according to the present invention, it should be noted that the target from which sulfur compounds are removed in the dry desulfurization step S1 is not a general raw material gas, but a by-product gas generated from a blast furnace in the reduction step S3. The by-product gas is converted into a purified gas after sulfur compounds are removed in the dry desulfurization step S1. It goes without saying that the above-mentioned options may be implemented upstream and downstream of the dry desulfurization step S1.

[0065] Next, the purified gas is converted into a reducing gas mainly composed of carbon monoxide in an electrolysis step S2, and the resulting reducing gas is supplied to a blast furnace in a reduction step S3 to be used to reduce iron ore. In the reduction step S3, the carbon monoxide contained in the reducing gas is converted into carbon dioxide, and the hydrogen is converted into water vapor. The carbon dioxide and water vapor generated from the blast furnace are recovered as by-product gases and sent back to the dry desulfurization step S1. In this way, according to the present invention, by converting the by-product gas generated from the blast furnace into a reducing gas and reusing it, it is possible to reduce carbon dioxide emissions into the environment as close to zero as possible.

[0066] The reducing gas used in the reduction step S3 may be all or a part of the reducing gas generated in the electrolysis step S2. In addition, when the reducing gas generated in the electrolysis step S2 is insufficient to provide the reducing agent required for reducing the iron ore in the reduction step S3, another reducing agent such as carbon monoxide generated from coke may be used in combination as a reducing agent.

[0067] As described above, in the electrolysis step S2, oxygen is generated as a by-product by the electrochemical reaction represented by chemical formula (2) at the anode 4 of the SOEC 1. The oxygen obtained here can be used for various purposes in the steelworks, for example, in a steelmaking process using an LD converter.

[0068] <Chemical Thermodynamic Calculations> Hereinafter, embodiments for carrying out the present invention will be described with specific numerical values. The embodiments shown here were not determined by actual experiments, but were all determined by performing chemical thermodynamic calculations based on the equilibrium constants of chemical reaction equations. Note that the embodiments of the present invention are not limited to the calculation examples shown below, and the embodiments of the present invention can be modified as desired without departing from the spirit and scope of the present invention.

[0069] (Calculation Example 1) First, a calculation example will be shown for a dry desulfurization process in which hydrogen sulfide is removed from a raw gas containing carbon dioxide and hydrogen sulfide using a desulfurization agent containing zinc oxide to obtain a purified gas. Chemical formula (5) representing this dry desulfurization process is reproduced below: [Chemical Formula 12] ZnO(s) + H ２ S(g) → ZnS(s) + H ２ O(g)...(5)

[0070] Here, the symbol g indicates a gas and the symbol s indicates a solid. By setting the temperature at which the dry desulfurization process is carried out at 400°C and using the known values ​​of the standard enthalpy of formation and standard entropy for these gases and solids, the Gibbs energy change ΔG of chemical formula (5) at 400°C can be calculated. From the calculated Gibbs energy change ΔG of chemical formula (5) and the gas constant, the equilibrium constant K of the chemical reaction represented by chemical formula (5) at 400°C can be calculated. This equilibrium constant K is equal to K expressed by the following formula (1). Here, [H ２ S] is the partial pressure of hydrogen sulfide, [H ２ O] is the partial pressure of water vapor. Assume that there is a sufficient amount of zinc oxide relative to the volume fraction of hydrogen sulfide. If the gas mixture is assumed to be an ideal gas, the ratio of the partial pressures in the gas mixture is equal to the ratio of the volume fractions.

[0071] Under the above assumptions, we calculated the volume fraction of hydrogen sulfide remaining in the purified gas obtained by desulfurizing the feed gas of Examples 1 to 3 shown in Table 1. Since the amount of water vapor generated by the reaction of chemical formula (5) is small compared to the amount of water vapor originally contained in the feed gas, the latter can be ignored and the volume fraction of hydrogen sulfide that is in equilibrium with the volume fraction of water vapor contained in the purified gas can be calculated by substituting the calculated value of K and the volume fraction of water vapor into formula (1). The results of this calculation are shown in Table 1.

[0072]

[0073] The results of chemical thermodynamic calculations shown in Table 1 indicate that the dry desulfurization process according to the present invention can reduce the volume fraction of hydrogen sulfide contained in the raw gas to approximately 1 / 100 or less. The volume fraction of hydrogen sulfide remaining in the purified gas decreases as the amount of water vapor contained in the raw gas decreases. The volume fraction of hydrogen sulfide in the purified gas of Example 2, which has a water vapor volume fraction of 5%, is 0.052 ppm, which is lower than the preferred upper limit of 0.10 ppm for sulfur compounds that can suppress cathode degradation. The volume fraction of hydrogen sulfide in the purified gas of Example 3, which has a water vapor volume fraction of 1%, is 0.010 ppm, which is equal to the more preferred upper limit for sulfur compounds that can suppress cathode degradation.

[0074] (Calculation Example 2) Next, a calculation example will be shown for a hydrolysis step in which carbonyl sulfide is removed from a source gas containing carbon dioxide and carbonyl sulfide using water vapor. Chemical formula (8) representing this hydrolysis step is reproduced as follows: [Chemical Formula 13] COS(g) + H ２ O(g) → CO ２ (g) + H ２ S(g)...(8)

[0075] Using the same procedure as in Calculation Example 1, calculate the Gibbs energy change ΔG and the equilibrium constant K of chemical formula (8) at 400°C. This equilibrium constant K is equal to K expressed by the following formula (2): where [COS] is the partial pressure of carbonyl sulfide, [CO ２] is the partial pressure of carbon dioxide. We also assume that there is a sufficient amount of water vapor relative to the volume fraction of carbonyl sulfide.

[0076] Based on the above assumptions, the volume fraction of carbonyl sulfide remaining in the gas obtained after hydrolysis by hydrolyzing the feed gas of Example 4 shown in Table 2 was calculated. Since the amounts of water vapor consumed and carbon dioxide produced by the reaction of chemical formula (8) are small compared to the amounts of carbon dioxide and water vapor originally contained in the feed gas, these are ignored. Furthermore, assuming that almost all of the 10 vol% carbonyl sulfide contained in the feed gas is hydrolyzed, the volume fraction of hydrogen sulfide contained in the gas after hydrolysis is estimated to be 10 vol%. The volume fraction of carbonyl sulfide contained in the gas after hydrolysis can be calculated by substituting the calculated value of K and the volume fractions of gases other than carbonyl sulfide into Equation (2). The results of this calculation are shown for Example 4 in Table 2.

[0077]

[0078] The results of chemical thermodynamic calculations shown in Table 2 indicate that the hydrolysis step according to the present invention can reduce the volume fraction of carbonyl sulfide contained in the raw material gas to approximately 1 / 500. The trace amount of hydrogen sulfide generated by the reaction of chemical formula (8) in the hydrolysis step can be removed in the dry desulfurization step, as shown in Calculation Example 1.

[0079] (Comparative Example) Next, a comparative example will be shown, which relates to a process for removing carbonyl sulfide from a feed gas containing carbon dioxide and carbonyl sulfide using conventional hydrodesulfurization without using a hydrolysis process, as in Example 4. Chemical formulas (3) and (4) representing this hydrodesulfurization process are reproduced below: [Chemical Formula 14] CO ２ (g) + 4H ２ (g) → CH ４ (g) + 2H ２ O(g) ...(3) [Chemical formula 15] COS(g) + H ２ (g) → CO(g) + H ２ S(g)...(4)

[0080] Using the same procedures as in Calculation Examples 1 and 2, the Gibbs energy change ΔG and the equilibrium constant K of chemical formulas (3) and (4) at 400°C can be calculated, and the volume fraction of the gas after hydrodesulfurization can be calculated. The results of this calculation are shown in Comparative Example 1 in Table 2. According to the results of the chemical thermodynamic calculations shown in Table 2, in Comparative Example 1, most of the hydrogen is consumed in the methanation of carbon dioxide represented by chemical formula (3), while hydrodesulfurization of carbonyl sulfide represented by chemical formula (4) hardly progresses, and therefore it can be seen that the volume fraction of residual carbonyl sulfide remains high compared to Invention Example 4.

[0081] REFERENCE SIGNS LIST 1 Solid oxide electrolysis cell (SOEC) 2 Solid electrolyte 3 Cathode 4 Anode 5 DC power supply S1 Dry desulfurization process S1' Hydrodesulfurization process (conventional technology) S2 Electrolysis process S3 Reduction process O1 Dust removal process O2 Hydrolysis process O3 Dehydration process O4 Hydration process

Claims

1. A method for producing a reducing gas containing carbon monoxide from a raw material gas containing carbon dioxide and sulfur compounds, wherein the raw material gas consists of one or more by-product gases selected from the group consisting of blast furnace gas, hot stove exhaust gas, coke oven exhaust gas, heating furnace exhaust gas, annealing furnace exhaust gas, and power generation facility exhaust gas, the method comprising: a dry desulfurization step of removing the sulfur compounds from the raw material gas to obtain a purified gas; and an electrolysis step of electrolyzing the purified gas using a solid oxide electrolysis cell to obtain the reducing gas.

2. The method for producing a reducing gas according to claim 1, wherein the sulfur compound includes at least one of carbonyl sulfide and carbon disulfide.

3. The method for producing a reducing gas according to claim 2, further comprising a hydrolysis step of hydrolyzing at least one of carbonyl sulfide and carbon disulfide, located upstream of the dry desulfurization step.

4. The method for producing a reducing gas according to any one of claims 1 to 3, wherein the dry desulfurization step includes an operation of removing the sulfur compounds using a dry desulfurization agent.

5. The method for producing a reducing gas according to claim 4, wherein the dry desulfurization agent contains zinc oxide.

6. The method for producing a reducing gas according to any one of claims 1 to 5, further comprising a dehydration step for removing water and water vapor contained in the raw material gas, located upstream of the dry desulfurization step.

7. The method for producing a reducing gas according to any one of claims 1 to 6, further comprising a dust removal step for removing dust contained in the raw material gas, located upstream of the dry desulfurization step.

8. A method for producing a reducing gas according to any one of claims 1 to 7, further comprising a hydration step of adding steam to the purified gas downstream of the dry desulfurization step and upstream of the electrolysis step, wherein the reducing gas contains hydrogen.

9. The method for producing a reducing gas according to any one of claims 1 to 8, wherein the raw material gas comprises the blast furnace gas.

10. A method for reducing iron ore, comprising: a dry desulfurization step of removing sulfur compounds from a by-product gas containing carbon dioxide and sulfur compounds generated in a blast furnace to obtain a purified gas; an electrolysis step of electrolyzing the purified gas using a solid oxide electrolysis cell to obtain a reducing gas; and a reduction step of supplying the reducing gas to the blast furnace to reduce the iron ore.

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