Oxygen carrier material, method for producing same, and carbon dioxide reduction system

Abstract

An oxygen carrier material according to the present disclosure, which contains an oxygen carrier and a support and which modifies carbon dioxide into carbon monoxide, uses iron oxide as the oxygen carrier and iron-doped calcium titanate as the support. A method for producing the oxygen carrier material comprises a first step for generating the support, and a second step for generating the oxygen carrier material using the support, wherein: in the first step, an iron-doped calcium titanate is generated by heating a mixture of iron oxide, calcium carbonate, and titanium oxide; and in the second step, a mixture of the iron-doped calcium titanate and iron oxide is heated.

Description

Oxygen carrier material, method for producing the same, and carbon dioxide reduction system

[0001] This disclosure relates to oxygen carrier materials, methods for producing the same, and carbon dioxide reduction systems.

[0002] In recent years, research and development have been underway on technologies to recover carbon dioxide (CO2) from exhaust gases and the atmosphere, and to convert the recovered CO2 into valuable substances, with the aim of preventing global warming and realizing a carbon-cycle society. For example, there is a technology to produce a product gas containing carbon monoxide (CO) by contacting a raw material gas containing CO2 with a reducing agent containing a metal compound that reduces CO2, and the catalyst used in this process has been disclosed (see, for example, Patent Document 1).

[0003] International Publication No. 2019 / 230855

[0004] To improve the efficiency of gas production, it is important to use metal compounds with high reaction efficiency with carbon dioxide as catalyst materials. Conventional catalysts improve gas production efficiency by using cerium (Ce) as a raw material. However, because Ce is a rare earth element, it is costly, and the unstable supply of raw materials makes it difficult to produce a stable catalyst.

[0005] This disclosure is made to solve the above-mentioned problems and aims to provide an inexpensive oxygen carrier material with high gas production efficiency (conversion efficiency of CO2 to CO) without using rare earth elements, a method for producing the same, and a carbon dioxide reduction system.

[0006] The oxygen carrier material according to this disclosure is an oxygen carrier material comprising an oxygen carrier and a carrier that reduces carbon dioxide to carbon monoxide, wherein the oxygen carrier is iron oxide and the carrier is iron-doped calcium titanate CaTi 1―x Fe x O 3―δ (x is a real number between 0 and 1, and δ is a real number between 0 and 3) (δ represents a small amount of oxygen deficiency).

[0007] The method for producing an oxygen carrier material according to this disclosure is a method for producing an oxygen carrier material, characterized in that it includes a first step of producing the carrier and a second step of producing the oxygen carrier material using the carrier.

[0008] The carbon dioxide reduction system according to this disclosure is a carbon dioxide reduction system using the above-mentioned oxygen carrier material, and is characterized by comprising: a first reactor that reduces carbon dioxide to carbon monoxide by an oxidation reaction of the oxygen carrier material; and a second reactor that reduces the oxidized oxygen carrier material with a reducing gas.

[0009] According to the oxygen carrier material and method for manufacturing the same described herein, it is possible to provide an oxygen carrier material that is inexpensive and has high gas production efficiency (conversion efficiency of CO2 to CO) without using rare earth elements.

[0010] This is a flowchart showing the manufacturing process for the carrier used in the oxygen carrier material according to Embodiment 1. This is a flowchart showing the manufacturing process for the oxygen carrier material according to Embodiment 1. This is a diagram showing an example configuration of a carbon dioxide reduction system according to Embodiment 2. Figures 4A to 4C show the X-ray diffraction spectra of the carrier used in the oxygen carrier material according to Embodiment 1 in Example 1. This is a diagram showing the gas production efficiency of oxygen carrier particles produced using the oxygen carrier material production method according to Embodiment 1 in Example 3.

[0011] Embodiment 1. The oxygen carrier material according to Embodiment 1 is composed of an oxygen carrier and a carrier. The oxygen carrier material is used in a chemical loop method to produce a product gas containing carbon monoxide by contacting it with a raw material gas containing carbon dioxide to reduce the carbon dioxide. The oxidized oxygen carrier material can be reduced by contacting it with a reducing gas. In this case, the oxygen carrier material of this disclosure undergoes the conversion of carbon dioxide to carbon monoxide and regeneration of the oxygen carrier material by the reducing gas upon contact with the raw material gas and the reducing gas.

[0012] The oxygen carrier used is iron oxide, a metal oxide that reduces carbon dioxide. By selecting iron oxide as the oxygen carrier, the efficiency of converting carbon dioxide to carbon monoxide (gas production efficiency) can be improved. Iron (iron oxide) is abundant in the earth, making it inexpensive and readily available. Furthermore, it has low toxicity and is easy to handle throughout the entire product lifecycle, from manufacturing to use and disposal.

[0013] The oxygen carrier is preferably a substance that readily adsorbs carbon dioxide. The adsorption of carbon dioxide may be physical or chemical. Furthermore, the oxygen carrier is preferably supported on a carrier, and is preferably a different substance from the carrier.

[0014] The support is a compound that is reduced upon contact with a reducing gas and oxidized upon contact with a source gas containing carbon dioxide, and is an oxide ion conductor having a perovskite or spinel structure, and is an iron-doped calcium titanate (CaTi) obtained by doping rare earth-free calcium titanate with iron Fe as a metal ion. 1―x Fe x O 3―δ It is mainly composed of ). Known methods can be used for doping. Doping with iron can improve the oxide ion conductivity of the carrier. All the elements that make up iron-doped calcium titanate are abundant in the earth, so it is inexpensive and readily available. In addition, it has low toxicity and is easy to handle throughout the entire product lifecycle, from manufacturing to use and disposal. This composition makes it possible to provide an inexpensive oxygen carrier material with high gas production efficiency (conversion efficiency of CO2 to CO) without using rare earth elements.

[0015] The molar ratio of iron element contained in iron-doped calcium titanate as a carrier is preferably 1% or more and less than 50%, and particularly preferably 5% or more and 30% or less. By adjusting this molar ratio, the gas production efficiency is improved. When it is less than 1%, the amount of iron is insufficient, so the oxide ion conductivity is not improved and the gas production efficiency is not improved. Also, when it exceeds 50%, since the amount of iron is excessive, there is a concern that the stability of the carrier may decrease during repeated redox reactions.

[0016] The mass ratio of iron oxide as an oxygen carrier contained in the oxygen carrier material is preferably 20% or more and less than 100%. More preferably, it is 20% or more and 70% or less, and particularly preferably 20% or more and 30% or less. By adjusting this mass ratio, the gas production efficiency and the durability of the oxygen carrier material are improved. When it is less than 20%, the amount of the oxygen carrier is small, so the gas production efficiency decreases.

[0017] The oxygen carrier material is preferably granular, flaky, or pellet-shaped, and more preferably granular. Also, it is preferable to have a specific range of particle sizes by operations such as classification. The average particle size of the oxygen carrier material is preferably 100 μm or more and 1000 μm or less, and more preferably 100 μm or more and 400 μm or less. By adjusting this particle size, particle collapse due to sintering can be suppressed and the contact efficiency with the gas can be improved. When it is less than 100 μm, if the particle size is extremely small, sintering will progress and the desired particle size cannot be maintained, resulting in a decrease in gas production efficiency. Also, when it exceeds 1000 μm, there is a concern that the contact efficiency with the gas may decrease because the particle size is too large.

[0018] The specific surface area of the oxygen carrier material is 2 preferably 1 m 2 / g or more and 100 m 2 / g or less. By adjusting this specific surface area, the contact area between the gas and the oxygen carrier material can be increased, and the gas production efficiency of reducing carbon dioxide to carbon monoxide can be improved. When it is less than 1 m 2If the value exceeds [amount] / g, there is a concern that the mechanical strength of the particles may decrease.

[0019] The porosity of the oxygen carrier material is preferably between 0.1 and 0.6. Adjusting the porosity to this range improves the gas contact efficiency. If it is less than 0.1, the gas contact efficiency decreases because there are fewer voids inside the oxygen carrier particles. If it exceeds 0.6, there is a concern that the mechanical strength of the particles will decrease.

[0020] Furthermore, the oxygen carrier material of this disclosure may contain a porosity agent and a binder. Examples of porosity agents include graphite and activated carbon. Examples of binders include ethylcellulose and terpineol.

[0021] Next, the method for manufacturing the oxygen carrier material according to Embodiment 1 will be described with reference to Figures 1 and 2. Figure 1 is a flowchart showing the manufacturing process for the carrier used in the oxygen carrier material according to Embodiment 1. Figure 2 is a flowchart showing the manufacturing process for the oxygen carrier material using the carrier manufactured in Figure 1.

[0022] The oxygen carrier material of this disclosure can be manufactured by a solid-phase method. The manufacturing method consists of a step for manufacturing a carrier (Figure 1) and a step for manufacturing the oxygen carrier material (Figure 2).

[0023] In the process for manufacturing the carrier shown in Figure 1, first, one material is selected from the group consisting of iron and iron oxide (Fe2O3, FeO, Fe3O4, etc.), one from the group consisting of calcium, calcium oxide (CaO), and calcium carbonate (CaCO3), and one from the group consisting of titanium and titanium oxide (TiO2), and then weighed (step S101). At this time, it is preferable to select iron oxide, calcium carbonate, and titanium oxide as raw materials. Furthermore, the molar ratio of iron contained in the iron-doped calcium titanate used as the carrier is adjusted to be between 5% and 30%. Adjusting to this molar ratio improves the gas production efficiency.

[0024] Next, the material is crushed and mixed (step S102), and then calcined in air at a temperature of 900°C to 1200°C (step S103). The method of crushing and mixing is not particularly limited, but it is preferable to use a ball mill. A ball mill has a simple structure and can perform processing inexpensively and stably. The calcination is preferably carried out at 1200°C for 10 hours. The calcination method is not particularly limited, but it is preferable to use a rotary furnace, and it is more preferable that the rotation speed is 0.1 rpm or more and less than 20 rpm. By calcining under these conditions, a uniform carrier can be obtained.

[0025] Finally, the resulting mixture is crushed (step S104) to obtain the desired carrier, iron-doped calcium titanate (step S105).

[0026] In the process for manufacturing the oxygen carrier material shown in Figure 2, first, the iron-doped calcium titanate used as the carrier and the iron oxide used as the oxygen carrier, obtained in the process for manufacturing the carrier shown in Figure 1, are weighed (step S201). At this time, the mass ratio of iron oxide contained in the oxygen carrier material is adjusted to be between 20% and 30%. Adjusting to this mass ratio improves the gas production efficiency and the durability of the oxygen carrier material.

[0027] Next, a porosity agent and a binder are added (step S202). Graphite, activated carbon, etc., can be used as the porosity agent. Ethyl cellulose, terpineol, etc., can be used as the binder. Mixing the porosity agent and the binder improves the uniformity of the oxygen carrier material.

[0028] Next, the material is crushed and mixed (step S203), dried (step S204), and then calcined in air at a temperature of 800°C to 1000°C (step S205). The method of crushing and mixing is not particularly limited, but it is preferable to use a ball mill. A ball mill has a simple structure and can process the material stably at low cost. For calcination, it is preferable to calcinate at 950°C for 3 hours. The calcination method is not particularly limited, but it is preferable to calcinate using a rotary furnace, and it is more preferable that the rotation speed is 0.1 rpm or more and less than 20 rpm. By calcining under these conditions, a uniform oxygen carrier material can be obtained.

[0029] Finally, the obtained mixture can be classified (step S206) to form particles with a specific particle size range, thereby obtaining the oxygen carrier material of this disclosure (step S207). The particle size of the oxygen carrier material is preferably 100 μm or more and 1000 μm or less. More preferably 100 μm or more and 400 μm or less. This particle size improves the contact efficiency with the gas and suppresses particle breakdown due to sintering.

[0030] As described above, the oxygen carrier material according to Embodiment 1 is an oxygen carrier material comprising an oxygen carrier and a carrier that reduces carbon dioxide to carbon monoxide, wherein iron oxide is used for the oxygen carrier and iron-doped calcium titanate is used for the carrier, so that the mass ratio of iron oxide contained in the oxygen carrier material is 20% or more and 30% or less, and the molar ratio of iron contained in the carrier is 5% or more and 30% or less.

[0031] Furthermore, according to the method for producing an oxygen carrier material according to Embodiment 1, the method for producing the oxygen carrier material includes a first step of producing the carrier and a second step of producing the oxygen carrier material using the carrier, wherein in the first step, a mixture of iron oxide, calcium carbonate, and titanium oxide is heated to produce iron-doped calcium titanate, and in the second step, a mixture of the iron-doped calcium titanate and iron oxide is heated.

[0032] This makes it possible to provide an inexpensive oxygen carrier material with high gas production efficiency (conversion efficiency of CO2 to CO) without using rare earth elements.

[0033] Embodiment 2. The carbon dioxide reduction system according to Embodiment 2 will be described. The carbon dioxide reduction system of this disclosure is a carbon dioxide reduction system using the oxygen carrier material according to Embodiment 1, and applies the chemical loop method.

[0034] Figure 3 shows the configuration of the carbon dioxide reduction system 100 according to Embodiment 2. As shown in Figure 3, the carbon dioxide reduction system 100 is a system for implementing a chemical loop method to produce a gas containing carbon monoxide by circulating an oxygen carrier material using a transfer system 3 between a first reactor 1 that reduces carbon dioxide and a second reactor 2 that reduces the oxygen carrier material. The oxygen carrier material consists of a carrier and an oxygen carrier, with the carrier being iron-doped calcium titanate and the oxygen carrier being iron oxide.

[0035] In the chemical loop method, a product gas containing carbon monoxide is produced by repeatedly carrying out a redox reaction in which an oxygen carrier material is oxidized by carbon dioxide and reduced by hydrogen. The oxygen carrier material is used in the reverse water-gas shift reaction. The reverse water-gas shift reaction is a reaction that produces carbon monoxide and water from carbon dioxide and hydrogen. When the chemical loop method is applied, the reverse water-gas shift reaction is carried out by dividing it into the reduction reaction of the oxygen carrier material by hydrogen (first process) and the oxidation reaction of the oxygen carrier material by carbon dioxide (second process), which are shown in equations (1) and (2) below, respectively.

[0036] 4H2 (gas) + Fe3O4 (solid) → 4H2O (gas) + 3Fe (solid) ... (1) 4CO2 (gas) + 3Fe (solid) → 4CO (gas) + Fe3O4 (solid) ... (2)

[0037] The carbon dioxide reduction system 100 is a system for producing a product gas 4 containing carbon monoxide by bringing a raw material gas containing carbon dioxide into contact with the above-described oxygen carrier material for reducing carbon dioxide. In the reduction reaction of the oxygen carrier material, hydrogen is oxidized to produce water 5. In the oxidation reaction of the oxygen carrier material, carbon dioxide is reduced to produce carbon monoxide.

[0038] The carbon dioxide reduction system 100 includes a first reactor 1 for reducing carbon dioxide to carbon monoxide and a second reactor 2 for reducing the oxidized oxygen carrier material with a reducing gas, which are installed as independent facilities. The oxygen carrier material is enclosed in each reactor. A raw material gas supply unit 6 for supplying the raw material gas is connected to the first reactor 1, and a reducing gas supply unit 7 for supplying the reducing gas is connected to the second reactor 2. A heating device 8 for heating the reducing gas may be provided downstream of the reducing gas supply unit 7.

[0039] A transfer system device 3 for transferring the oxygen carrier material is provided between the first reactor 1 and the second reactor 2. The reducing gas is preferably hydrogen, and therefore, it is preferable that the reducing gas supply unit 7 includes a hydrogen generator. For example, a solid oxide electrolysis cell or the like is used for the hydrogen generator.

[0040] An impurity removal device 9 for removing impurities (such as oxygen) contained in the raw material gas may be provided downstream of the raw material gas supply unit 6, that is, upstream of the first reactor 1. A heating device 10 for heating the raw material gas may be provided downstream of the impurity removal device 9.

[0041] It is desirable that the first reactor 1 and the second reactor 2 are equipped with a heating system for heating to a predetermined temperature. The heating temperature is set to be not higher than the firing temperature of the oxygen carrier material. The heating temperature is preferably 600°C or higher and lower than 1000°C, more preferably 900°C. By setting the temperature to this value, the gas production efficiency is improved. With this system configuration, a gas containing carbon monoxide can be produced from a raw material gas containing carbon dioxide.

[0042] Also, the first reactor 1 and the second reactor 2 for enclosing the oxygen carrier material may be one. In that case, it is desirable to have a gas switching unit that alternately switches and supplies the raw material gas and the reducing gas to the reactor. Further, there may be two or more first reactors 1 and second reactors 2. For example, while the reduction of the oxygen carrier material oxidized by the reducing gas is being carried out in the second reactor 2, the reduction of the raw material gas can be carried out in a plurality of first reactors 1. Note that the larger the number of reactors, the more the flow rate fluctuations associated with the switching of the reactors can be suppressed.

[0043] As described above, according to the carbon dioxide reduction system according to Embodiment 2, in the carbon dioxide reduction system using the above oxygen carrier material, a first reactor that reduces carbon dioxide to carbon monoxide by the oxidation reaction of the oxygen carrier material, and a second reactor that reduces the oxidized oxygen carrier material with a reducing gas are provided. By repeating the reaction in the first reactor and the reaction in the second reactor, CO2 can be converted to CO, and an efficient carbon monoxide production reaction can be advanced.

[0044] Examples will be given below to more specifically describe the oxygen carrier material of the present disclosure. Note that the oxygen carrier material of the present disclosure is not limited to these examples.

[0045] [Example 1] Using the method for producing a carrier used in the oxygen carrier material according to Embodiment 1, iron-doped calcium titanate was produced. The molar ratios of the iron element contained in the carrier were set to 20%, 50%, and 80%. For each of the obtained carriers, an XRD spectrum was measured using an X-ray diffractometer (MiniFlex600, manufactured by Rigaku Holdings Co., Ltd.).

[0046] Figures 4 to 6 show the XRD spectra of the support material at molar ratios of iron element of 20%, 50%, and 80%, respectively. From Figure 4, it can be seen that a single-phase peak of calcium titanate is detected when the molar ratio of iron element is 20%. On the other hand, from Figures 5 and 6, when the materials were manufactured with 50% and 80% iron element, in addition to the calcium titanate peak, a peak of the impurity phase (Ca3Fe2TiO8) was detected.

[0047] These results suggest that when the support material is manufactured with a molar ratio of iron element of 50% or more, the impurity phase peak appears, potentially reducing gas production efficiency compared to the 20% ratio where only a single-phase peak was detected.

[0048] [Example 2] Oxygen carrier particles were produced using the method for producing oxygen carrier material according to Embodiment 1, with the mass ratio of iron oxide set to 0%, 10%, 20%, 30%, and 50%. CO2 gas was supplied to the obtained oxygen carrier particles, and the particle performance (CO processing amount) was evaluated.

[0049] First, 5 g of oxygen carrier particles were packed into a quartz tube with an inner diameter of 30 mm and a length of 800 mm. Nitrogen gas was supplied into the quartz tube at a flow rate of 3000 mL / min, and the quartz tube was heated to 900°C at a heating rate of 20°C / min.

[0050] Next, hydrogen gas was supplied at a flow rate of 90 mL / min and nitrogen gas at a flow rate of 2910 mL / min for 2 hours to reduce the oxygen carrier particles. After that, nitrogen gas was supplied into the quartz tube at a flow rate of 3000 mL / min for 5 minutes, and then CO2 gas was supplied at a flow rate of 90 mL / min and nitrogen gas at a flow rate of 2910 mL / min to reduce the CO2. At this time, the generated gas discharged from the quartz tube contained CO gas.

[0051] (Table 1)

[0052] Table 1 summarizes the amount of CO (mol) processed by each particle at 900°C. From Table 1, it can be seen that the amount of CO processed increases with increasing mass ratio of iron oxide. Considering practical application, a CO processing amount of 0.040 mol or more per cycle is required, so the mass ratio of iron oxide must be at least 20%.

[0053] [Example 3] Oxygen carrier particles were produced using the method for producing oxygen carrier material according to Embodiment 1, with the mass ratio of iron oxide set to 10%, 30%, and 50%. The gas production efficiency (yield: CO2 to CO conversion efficiency) of the obtained oxygen carrier particles was evaluated.

[0054] First, 5 g of oxygen carrier particles were packed into a quartz tube with an inner diameter of 30 mm and a length of 800 mm. Nitrogen gas was supplied into the quartz tube at a flow rate of 3000 mL / min, and the quartz tube was heated to 700°C at a heating rate of 20°C / min.

[0055] Next, hydrogen gas was supplied at a flow rate of 90 mL / min and nitrogen gas at a flow rate of 2910 mL / min for 2 hours to reduce the oxygen carrier particles. After that, nitrogen gas was supplied into the quartz tube at a flow rate of 3000 mL / min for 5 minutes, and then CO2 gas was supplied at a flow rate of 90 mL / min and nitrogen gas at a flow rate of 2910 mL / min to reduce the CO2. At this time, the generated gas discharged from the quartz tube contained CO gas.

[0056] Figure 5 shows the gas production efficiency of each particle at 700°C. This Example 3 has a lower reaction temperature compared to Example 2 (900°C). At such low reaction temperatures, although instantaneously, oxygen carrier particles with a low iron oxide mass ratio of 10% showed the highest gas production efficiency, followed by 30% and 50%. In particular, when the iron oxide mass ratio was 50%, the gas production efficiency was less than half that of 10%. From these results, it was found that at low reaction temperatures (~700°C), the gas production efficiency decreases significantly when the iron oxide mass ratio is 50% or higher.

[0057] While this application describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but are applicable individually or in various combinations to the embodiments. Accordingly, countless variations not illustrated are envisioned within the scope of the art disclosed herein. For example, these include modifying, adding or omitting at least one component, or even extracting at least one component and combining it with a component from another embodiment.

[0058] 1. First reactor, 2. Second reactor, 100. Carbon dioxide reduction system.

Claims

1. An oxygen carrier material comprising an oxygen carrier and a support that reduces carbon dioxide to carbon monoxide, wherein the oxygen carrier is iron oxide and the support is iron-doped calcium titanate.

2. The oxygen carrier material according to claim 1, characterized in that the mass ratio of iron oxide is 20% or more and 30% or less.

3. The oxygen carrier material according to claim 1 or 2, characterized in that the molar ratio of iron contained in the carrier is 1% or more and less than 50%.

4. The oxygen carrier material according to any one of claims 1 to 3, characterized in that the oxygen carrier material is granular and has an average particle size of 100 μm or more and 1000 μm or less.

5. The specific surface area of ​​the oxygen carrier material is 1 m² 2 / g or more, 100 m 2 The oxygen carrier material according to any one of claims 1 to 4, characterized in that it is less than or equal to / g.

6. The oxygen carrier material according to any one of claims 1 to 5, characterized in that the porosity of the oxygen carrier material is 0.1 or more and 0.6 or less.

7. A method for producing an oxygen carrier material according to any one of claims 1 to 6, comprising: a first step of producing the carrier; and a second step of producing the oxygen carrier material using the carrier.

8. The method for producing an oxygen carrier material according to claim 7, characterized in that, in the first step, a mixture of iron oxide, calcium carbonate, and titanium oxide is heated to produce iron-doped calcium titanate, and in the second step, a mixture of the iron-doped calcium titanate and the iron oxide is heated.

9. A method for producing an oxygen carrier material according to claim 7 or 8, characterized by being produced by a solid-phase method.

10. A carbon dioxide reduction system using an oxygen carrier material according to any one of claims 1 to 6, comprising: a first reactor that reduces carbon dioxide to carbon monoxide by an oxidation reaction of the oxygen carrier material; and a second reactor that reduces the oxidized oxygen carrier material with a reducing gas.

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