A method for producing a catalyst for the direct decomposition of hydrocarbons by regenerating its catalytic function.

Abstract

Converting carbon to methane increases the options for utilizing carbon. [Solution] A method for converting carbon to methane includes the steps of preparing an aggregate of multiple particles containing 4.3% to 54.3% by mass of iron and 45.7% to 95.7% by mass of carbon, wherein the multiple particles contain cementite, and contacting the aggregate with hydrogen.

Description

[Technical Field]

[0001] This disclosure relates to a method for converting carbon to methane and a method for producing a catalyst for the direct decomposition of hydrocarbons using this method. [Background technology]

[0002] Currently, the production of various energy sources heavily relies on fossil fuels such as oil, coal, and natural gas. However, from the perspective of protecting the global environment, the increase in carbon dioxide emissions released by the combustion of fossil fuels is a cause for concern. The Paris Agreement, agreed upon in 2015, calls for a reduction in carbon dioxide emissions to address climate change, and reducing carbon dioxide emissions from the combustion of fossil fuels remains a crucial issue for thermal power plants and other similar facilities. While processes for separating and capturing emitted carbon dioxide are being actively investigated, technologies for producing energy without emitting carbon dioxide using alternative fuels to fossil fuels are also being explored.

[0003] Therefore, hydrogen, a clean fuel that does not emit carbon dioxide when burned, is attracting attention as an alternative to fossil fuels. Hydrogen can be produced, for example, by steam reforming methane contained in natural gas. However, this manufacturing method produces carbon monoxide as a byproduct, and carbon monoxide is ultimately oxidized and emitted as carbon dioxide. On the other hand, methods such as water electrolysis and photocatalysis are being considered as ways to produce hydrogen from water without using fossil fuels, but these methods require a great deal of energy and are economically problematic.

[0004] In response to this, methods for producing hydrogen and carbon by directly decomposing hydrocarbons (hereinafter referred to as "direct hydrocarbon decomposition methods") have been developed. For example, the applicant of this disclosure has developed a direct hydrocarbon decomposition method using an unsupported catalyst, which is an aggregate of iron particles, as described in Patent Document 1. The features of the direct hydrocarbon decomposition method are that it has the potential to obtain hydrogen fuel without emitting carbon dioxide, and that the carbon in the hydrocarbon raw material gas can be recovered as solid by-product carbon after hydrogen purification. Therefore, from the perspective of decarbonization, it has the potential to be easier to handle and less expensive compared to recovering and storing gaseous carbon dioxide. Further effective utilization of this by-product carbon is desired. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 7089235 [Overview of the project] [Problems that the invention aims to solve]

[0006] When hydrocarbons are directly decomposed, the resulting carbon deposits adhere to the catalyst, yielding a graphite-coated catalyst with reduced activity. If carbon can be efficiently separated from this graphite-coated catalyst, it will lead to more effective utilization of unsupported catalysts and carbon. In this regard, the inventors of this disclosure have demonstrated that a graphite-coated catalyst, after being subjected to a direct hydrocarbon decomposition method using the catalyst described in Patent Document 1, retains methanation activity.

[0007] In general, the synthesis of methane from hydrogen and carbon dioxide is often called methanation. However, in this disclosure, the reaction in which carbon and hydrogen are reacted to convert carbon into methane, as shown in the reaction equation (1) below, will be defined as "methanation." C+2H2→CH4···(1) By converting the carbon in a graphite-coated catalyst to methane through methanation, the carbon can be effectively utilized as methane. Furthermore, if the carbon is removed from the graphite-coated catalyst, the unsupported catalyst obtained by carbon removal can be reused in direct hydrocarbon decomposition methods.

[0008] In view of the circumstances described above, at least one embodiment of this disclosure aims to increase the options for carbon utilization by converting carbon to methane. [Means for solving the problem]

[0009] To achieve the above objective, the method for converting carbon to methane according to the present disclosure comprises the steps of preparing an aggregate of a plurality of particles comprising 4.3% to 54.3% by mass of iron and 45.7% to 95.7% by mass of carbon, wherein the plurality of particles include cementite, and contacting the aggregate with hydrogen.

[0010] Furthermore, the method for converting carbon to methane according to the present disclosure includes the steps of preparing an aggregate of a plurality of particles containing 4.3% to 54.3% by mass of iron and 45.7% to 95.7% by mass of carbon, and contacting the aggregate with hydrogen, wherein the step of preparing the aggregate includes the steps of preparing an unsupported catalyst containing a plurality of particles made of iron or iron oxide, and contacting the unsupported catalyst with a gas containing hydrocarbons. [Effects of the Invention]

[0011] The method for converting carbon to methane according to this disclosure allows for the conversion and utilization of carbon contained in an aggregate of multiple particles containing iron and carbon, thereby increasing the options for carbon utilization. Furthermore, when hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst containing multiple iron particles, the generated carbon coats the unsupported catalyst, reducing its activity. By converting the carbon contained in the unsupported catalyst with reduced activity into methane and reusing the resulting methane in the direct hydrocarbon decomposition method, the options for carbon utilization can be increased. Additionally, by reusing the unsupported catalyst obtained by removing the carbon in the direct hydrocarbon decomposition method, the amount of unsupported catalyst discarded can be reduced. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of the configuration of a hydrogen production apparatus for carrying out the method for converting carbon to methane according to the present disclosure. [Figure 2] This is a schematic diagram of the experimental apparatus for verifying the effectiveness of the carbon-to-methane conversion method disclosed herein. [Figure 3] This graph shows the experimental results for Examples 1-3. [Figure 4] These are the X-ray diffraction patterns of the methanation catalysts before the experiment for Examples 1 and 2 and Comparative Example 1. [Figure 5] This graph shows the relationship between temperature and methanation activity in Example 2. [Figure 6] This is a scanning electron microscope image of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 500°C. [Figure 7] This is a scanning electron microscope image of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 600°C. [Figure 8] This is a scanning electron microscope image of the surface of a particle aggregate after methanation at a pressure of 1 ata and a temperature of 680°C. [Figure 9]This is a photograph taken by a scanning electron microscope of the surface image of an aggregate of particles after methaneation at a pressure of 1 ata and a temperature of 700 °C. [Figure 10] This is a photograph taken by a scanning electron microscope of the surface image of an aggregate of particles after methaneation at a pressure of 1 ata and a temperature of 800 °C. [Figure 11] This is a photograph taken by a scanning electron microscope of the surface image of an aggregate of particles after methaneation at a pressure of 1 ata and a temperature of 900 °C. [Figure 12] This is a graph showing the relationship between the specific surface area of an aggregate of particles after methaneation and the temperature of methaneation. [Figure 13] This is a graph showing the experimental results of Examples 2 and 4. [Figure 14] This is a graph showing the change over time of the cumulative value of the hydrogen production amount generated by directly decomposing methane using an aggregate of particles after methaneation at a pressure of 1 ata and a temperature of 500 °C. [Figure 15] This is a graph showing the change over time of the cumulative value of the hydrogen production amount generated by directly decomposing methane using an aggregate of particles after methaneation at a pressure of 1 ata and temperatures of 600 °C and 680 °C, respectively. [Figure 16] This is a graph showing the change over time of the cumulative value of the hydrogen production amount generated by directly decomposing methane using an aggregate of particles after methaneation at a pressure of 1 ata and temperatures of 800 °C and 900 °C, respectively.

Mode for Carrying Out the Invention

[0013] Hereinafter, a method for converting carbon to methane according to an embodiment of the present disclosure will be described based on the drawings. The embodiments described below show one aspect of the present disclosure, do not limit this disclosure, and can be arbitrarily changed within the scope of the technical idea of this disclosure.

[0014] <Configuration of an apparatus for implementing a method for converting carbon to methane> As shown in Figure 1, the methanation apparatus 10, in which a method for converting carbon to methane is carried out, is equipped with a reactor 11 in which methanation occurs. The reactor 11 contains an aggregate 12 of multiple particles containing iron and carbon. The multiple particles constituting the aggregate 12 contain 4.3% to 54.3% by mass of iron and 45.7% to 95.7% by mass of carbon, and some of the iron and carbon constitute cementite. In other words, the multiple particles contain cementite. Regarding the conditions for the amount of cementite contained in the multiple particles, the quantitative values ​​of the peaks attributed to cementite and the peaks attributed to alpha iron (αFe) obtained by X-ray diffraction analysis of the aggregate are determined using the RIR (Reference Intensity Ratio) method, and if the quantitative values ​​of the peaks attributed to cementite and αFe are I1 and I2, respectively, then it is preferable that the ratio I1 / (I1+I2) is 0.17 or more and 1.0 or less.

[0015] To supply hydrogen for methanation into reactor 11, a hydrogen-containing gas line 13 through which at least hydrogen-containing gas, i.e., hydrogen-containing gas, flows is connected to reactor 11. The hydrogen-containing gas is not limited to a gas containing only hydrogen, but may also contain methane, other hydrocarbons, noble gases, or nitrogen in addition to hydrogen. Furthermore, a methane-containing gas line 14 is connected to reactor 11 so that methane-containing gas, which is a mixture of methane produced by methanation (described later) and unreacted hydrogen (including any other components present in the hydrogen-containing gas), can flow out of reactor 11. A compressor 15 may be provided in the methane-containing gas line 14.

[0016] While not limited to this, one example of the assembly 12 may be an unsupported catalyst (graphite-coated catalyst) coated with carbon produced by directly decomposing hydrocarbons into hydrogen and carbon. Inside the reactor 2 for directly decomposing hydrocarbons into hydrogen and carbon, there is an unsupported catalyst 3 containing multiple particles made of iron or iron oxide. The reactor 2 is equipped with a heating device 4 (for example, a jacket through which steam flows or an electric furnace, etc.) for raising the temperature of the inside of the reactor 2, particularly the unsupported catalyst 3. The reactor 2 is connected to a raw material gas line 5 through which a gas containing hydrocarbons (raw material gas) flows. As the raw material gas, for example, natural gas, compressed natural gas (CNG), city gas, liquefied petroleum gas, naphtha, etc. can be used. The reactor 2 is connected to a product gas distribution line 6 at a position opposite to where the raw material gas line 5 is connected to the reactor 2, relative to the unsupported catalyst 3.

[0017] In order to transfer the unsupported catalyst 3 (i.e., graphite-coated catalyst) after the hydrocarbons have been directly decomposed into hydrogen and carbon in reactor 2 to the methanation unit 10, reactor 2 and reactor 11 may be connected by a catalyst transfer line 7. In addition, in order to make the methane-containing gas usable as part of the raw material gas supplied to reactor 2, the methane-containing gas line 14 may be configured to be connected to the raw material gas line 5.

[0018] As mentioned above, methane-containing gas typically contains hydrogen that is not consumed during methanation, so a portion of the methane-containing gas may be used as part of the hydrogen-containing gas. That is, a recycling line connecting the methane-containing gas line 14 and the hydrogen-containing gas line 13 may be provided. Furthermore, as will be described later, the product gas circulating in the product gas distribution line 6 also contains hydrogen, so a portion of the product gas may be used as part of the hydrogen-containing gas. That is, a recycling line connecting the product gas distribution line 6 and the hydrogen-containing gas line 13 may be provided. Moreover, considering the concept of a direct decomposition method of hydrocarbons without carbon dioxide emissions, it is preferable to use hydrogen produced using renewable energy, for example, hydrogen produced by electrolysis of water using electricity generated from solar energy, as at least part of the hydrogen-containing gas.

[0019] The methanation plant 10 may be installed in a plant separate from the plant where the reactor 2 is installed. In this case, the catalyst transfer line 7 may be connected to a container for transporting the graphite-coated catalyst to the reactor 11, rather than to the reactor 11, and the methane-containing gas line 14 may be connected to a tank for storing methane-containing gas or to a consumption facility for consuming methane-containing gas, rather than to the raw material gas line 5.

[0020] <How to convert carbon into methane> Next, the operation of the methanation apparatus 10 (the method of converting carbon to methane) will be described. After the aggregate 12 is placed in the reactor 11, hydrogen-containing gas flows into the reactor 11 via the hydrogen-containing gas line 13. The hydrogen-containing gas that flows into the reactor 11 passes through the aggregate 12 while in contact with it. At this time, the hydrogen in the hydrogen-containing gas reacts with the carbon contained in the aggregate 12 through the catalytic action of the aggregate 12 and is converted to methane. The catalytic action of the aggregate 12 for methanation will be demonstrated in the embodiments described later. In this way, since the carbon contained in an aggregate of multiple particles containing iron and carbon can be converted to methane and utilized, the options for utilizing carbon can be increased.

[0021] The methane-containing gas, including the methane produced by methanation, flows out of the reactor 11 and through the methane-containing gas line 14, being transported to storage and consumption facilities. In this way, the carbon contained in an aggregate of multiple particles including iron and carbon can be converted into methane and utilized, thus increasing the options for carbon utilization.

[0022] As shown in the configuration in Figure 1, if the methane-containing gas line 14 is connected to the raw material gas line 5, the methane-containing gas can be used as part of the raw material gas supplied to reactor 2. Also, as shown in the configuration in Figure 1, if reactor 2 and reactor 11 are connected by a catalyst transfer line 7, the graphite-coated catalyst can be transferred from reactor 2 to reactor 11 as an assembly 12. After transferring the graphite-coated catalyst to reactor 11, a new unsupported catalyst 3 can be packed into reactor 2, or the catalyst after methanation in reactor 11 (i.e., regenerated catalyst) can be packed into reactor 2 to carry out the direct decomposition of hydrocarbons in reactor 2. The graphite-coated catalyst is an unsupported catalyst coated with carbon generated by the direct decomposition of hydrocarbons, so its activity in the direct decomposition of hydrocarbons is reduced. If the carbon contained in the graphite-coated catalyst is converted to methane and the resulting methane is reused in the direct decomposition of hydrocarbons, the options for using carbon can be increased, and if the unsupported catalyst obtained by removing the carbon is reused in the direct decomposition of hydrocarbons, the amount of unsupported catalyst that is discarded can be reduced.

[0023] In the examples described later, it is verified that the catalyst after methanation can be used as a catalyst for the direct decomposition of hydrocarbons. Thus, the operation of regenerating the graphite-coated catalyst to obtain a catalyst for the direct decomposition of hydrocarbons can also be said to be an operation of producing a catalyst for the direct decomposition of hydrocarbons (unsupported catalyst 3), although it is an operation of regenerating the graphite-coated catalyst. The specific surface area of ​​the iron particles contained in the multiple particles constituting the aggregate 12, which is the catalyst produced by this operation (method of producing the catalyst), is 0.52 m². 2It is preferable that the specific surface area is 1 / g or more. The detailed method for calculating this specific surface area will be explained in detail in the examples described later.

[0024] Furthermore, if the pressure is 0.098 MPa (1 ata) or higher, it is preferable to contact the aggregate 12 with hydrogen in the temperature range of 450°C to 680°C, and even more preferable to contact the aggregate 12 with hydrogen in the temperature range of 500°C to 680°C. The preferred temperature range for methanation will be verified in the examples described later.

[0025] <Method for direct decomposition of hydrocarbons> Next, the operation of the hydrogen generation apparatus 1 will be described. The raw material gas flows into the reactor 2 via the raw material gas line 5. The raw material gas that flows into the reactor 2 passes through the unsupported catalyst 3 while in contact with it. At this time, the hydrocarbons in the raw material gas are directly decomposed into hydrogen and carbon. Taking methane as an example of the hydrocarbon in this direct decomposition reaction, the reaction represented by the following reaction equation (2) occurs in the reactor 2. The mechanism by which the unsupported catalyst 3 causes the direct decomposition reaction of hydrocarbons is described in detail by the applicant of this disclosure in Patent Document 1. CH4 → 2H2 + C ... (2)

[0026] Carbon generated by the direct decomposition method of hydrocarbons adheres to the unsupported catalyst 3, and the generated hydrogen, along with unreacted hydrocarbons, flows out of reactor 2 as product gas and circulates through product gas distribution line 6. Carbon recovery can be performed by stopping the supply of raw material gas to reactor 2, recovering the graphite-coated catalyst from reactor 2, and, if necessary, removing the carbon attached to the graphite-coated catalyst using a carbon removal device. Hydrogen recovery is performed by recovering the product gas circulating through product gas distribution line 6. If the hydrogen concentration in the product gas is low, a hydrogen generation device may be provided to purify the product gas and increase the hydrogen concentration.

[0027] The carbon produced by the direct decomposition method of hydrocarbons may be recovered not only by the recovery method described above, but also, as described above, by transferring at least a portion of the graphite-coated catalyst in reactor 2 to reactor 11 via catalyst transfer line 7, and converting it to methane by methanation in the methanation apparatus 10. Alternatively, the graphite-coated catalyst may be temporarily stored in a storage container connected to catalyst transfer line 7, the container may be transferred to the methanation apparatus 10, and the graphite-coated catalyst in the container may be filled into reactor 11 of the methanation apparatus 10 to perform methanation. [Examples]

[0028] <Experimental apparatus for performing the method of converting carbon to methane according to this disclosure> Figure 2 shows the configuration of an experimental apparatus for verifying the effectiveness of the carbon-to-methane conversion method of this disclosure. The experimental apparatus 20 includes a quartz glass reactor 23 with an inner diameter of 22 mm and a length of 700 mm, which houses a methanation catalyst 22. The reactor 23 is heated in an electric furnace 24. The reactor 23 is connected to a hydrogen supply line 25 for supplying hydrogen and a reaction gas flow line 26 through which the reaction gas containing methane produced by methanation flows after it exits the reactor 23. The reaction gas flow line 26 is connected to a gas chromatograph 27 for measuring the composition of the reaction gas.

[0029] <Preparation of the methanation catalyst in Example 1> 3.5 g (bulk density 3, volume 1.2 cc) of aggregates of reduced iron particles (K-100T, particle size 100 μm to 180 μm) obtained from JFE Steel Corporation was weighed and packed into a quartz glass reactor with an inner diameter of 22 mm and a length of 700 mm. This aggregate of reduced iron particles was maintained at 800°C, and a gas containing 99 vol% or more methane was introduced for 10,000 h. -1The aggregate of reduced iron particles was exposed to a space velocity (GHSV) (methane gas flow rate of 12 L / h) for 14 hours. This exposure caused a portion of the methane to decompose directly into hydrogen and carbon, yielding an aggregate of reduced iron particles with carbon attached. This aggregate was then purged with nitrogen and cooled to room temperature. The composition of this aggregate was calculated from the mass of the aggregate before the experiment (containing only reduced iron) and the mass of the aggregate after the experiment (containing both reduced iron and carbon). It was confirmed that the aggregate contained 54.3 mass% iron and 45.7 mass% carbon. The aggregate thus obtained was designated as the methanation catalyst 22 of Example 1.

[0030] <Preparation of methanation catalysts in Examples 2 and 3> The methanation catalyst 22 of Example 2 was prepared under the same conditions as the preparation method of the methanation catalyst 22 of Example 1, except that the contact time between the methane-containing gas and the aggregate of reduced iron particles was 90 hours. The methanation catalyst 22 of Example 3 was prepared under the same conditions, except that the contact time between the methane-containing gas and the aggregate of reduced iron particles was 329 hours. Upon examination of the composition of the methanation catalyst 22 of Example 2, it was confirmed that it contained 6.8 mass% iron and 93.2 mass% carbon. Upon examination of the composition of the methanation catalyst 22 of Example 3, it was confirmed that it contained 4.3 mass% iron and 95.7 mass% carbon.

[0031] <Preparation of the methanation catalyst for Comparative Example 1> A mixture of graphite (MG-1) obtained from Nippon Graphite Industries Co., Ltd. and the reduced iron used in Examples 1-3 was used as the methanation catalyst 22 for Comparative Example 1.

[0032] <Experimental conditions for Examples 1-3 and Comparative Example 1> The methanation catalysts 22 obtained in Examples 1-3 and Comparative Example 1 were placed in the reactor 23 of the experimental apparatus 20. The temperature and pressure in the reactor 23 were adjusted to 600°C and 1ata, respectively, and methanation was carried out by supplying hydrogen to the reactor 23 at a rate of 83 Ncc / min. The compositions of the catalysts in Examples 1-3 and Comparative Example 1 are summarized in Table 1 below.

[0033] [Table 1]

[0034] <Experimental results of Examples 1-3 and Comparative Example 1> The experimental results for Examples 1 to 3 are shown in Figure 3. Figure 3 shows the change over time in the carbon-to-methane conversion rate in methanation using each of the methanation catalysts 22 in Examples 1 to 3. The carbon-to-methane conversion rate is defined by the following equation (3). Note that the "amount of methane in the reaction gas" in the following equation (3) can be calculated from the measurement results of gas chromatography 27. Conversion rate = (Amount of methane in reaction gas) × 2 / Hydrogen supply × 100 ... (3)

[0035] As can be seen from Figure 3, in Examples 1 and 2, the conversion rate increased immediately after the start of hydrogen supply, indicating that methanation occurred quickly. Approximately 4 hours after the start of hydrogen supply, the conversion rate reached a maximum of 22.1% in Example 1 and a maximum of 30.4% in Example 2. After that, the conversion rate began to decrease, becoming almost 0% approximately 15 hours after the start of hydrogen supply in Example 1 and approximately 18 hours after the start of hydrogen supply in Example 2. In Example 3, the conversion rate remained at 0% and no methanation occurred until approximately 12 hours after the start of hydrogen supply, but since the conversion rate increased thereafter, it can be seen that methanation started approximately 12 hours after the start of hydrogen supply. In Example 3, the conversion rate reached a maximum of 18.5% approximately 22 hours after the start of hydrogen supply, and then began to decrease, becoming almost 0% approximately 45 hours after the start of hydrogen supply.

[0036] Observing the appearance of the methanation catalyst 22 in Example 1 before and after the experiment, it was observed that after the experiment, almost all of the carbon attached to the reduced iron particles in the methanation catalyst 22 was removed by methanation, leaving only iron (reduced iron). This suggests that almost all of the carbon attached to the reduced iron particles was converted into methane.

[0037] Observing the appearance of the methanation catalyst 22 in Example 2 before and after the experiment, it was observed that although some carbon remained on the reduced iron particles in the methanation catalyst 22 after the experiment, most of the carbon had been removed from the reduced iron particles. Therefore, it is considered that most of the carbon attached to the reduced iron particles was converted into methane.

[0038] Although not shown in Figure 3, when the methanation catalyst 22 of Comparative Example 1 was used, the conversion rate remained at 0%. In other words, almost no methanation occurred even when the methanation catalyst 22 of Comparative Example 1 was used. Despite the iron concentration being almost the same for the methanation catalyst 22 of Comparative Example 1 and the methanation catalyst 22 of Example 2, almost no methanation occurred in the former, suggesting that the bonding state and contact state between iron and carbon in the catalyst may be important factors in the activity of methanation.

[0039] From a comparison of the experimental results of Examples 1 to 3 and Comparative Example 1, it was confirmed that when hydrogen is brought into contact with hydrogen hydrogen an aggregate of multiple particles containing 4.3% to 54.3% by mass of iron and 45.7% to 95.7% by mass of carbon, methanation occurs and carbon can be converted to methane, in order to rapidly generate methanation activity, it is preferable to bring hydrogen into contact with an aggregate of multiple particles containing 6.8% to 54.3% by mass of iron and 45.7% to 93.2% by mass of carbon.

[0040] Next, we present the results of our investigation into the physical properties of the methanation catalysts 22 in Example 2 and Comparative Example 1, which showed completely different results regarding methanation activity despite having almost identical iron concentrations. X-ray diffraction analysis was performed on both catalysts before the experiment. The X-ray diffraction analysis was performed in accordance with JIS K 0131 using a benchtop X-ray diffractometer (MiniFlex600) manufactured by Rigaku Corporation. Figure 4 shows the X-ray diffraction patterns of the methanation catalysts 22 of Examples 1 and 2 and Comparative Example 1 before the experiment. From the X-ray diffraction patterns, it was observed that the methanation catalysts 22 of Examples 1 and 2 before the experiment contained αFe and cementite. In contrast, from the X-ray diffraction pattern, it was observed that the methanation catalyst 22 of Comparative Example 1 before the experiment was mainly composed of αFe and graphite.

[0041] In the X-ray diffraction pattern shown in Figure 4, the quantitative values ​​I1 and I2 of the peaks attributed to cementite and αFe were determined using the RIR (Reference Intensity Ratio) method. In this method, the cementite and αFe peaks in Example 1 were set to the peaks at 2θ=47.6° and 2θ=77.2°, respectively, while the cementite peak in Example 2 was set to the peak at 2θ=53.8°. No αFe peak was detected in Example 2. When the cementite content in the particles of the methanation catalyst 22 was evaluated using the ratio I1 / (I1+I2), the ratio was 0 in Comparative Example 1, 0.17 in Example 1, and 1.0 in Example 2. Methanation activity was observed in the methanation catalysts 22 of Examples 1 and 2, while methanation activity was not observed in the methanation catalyst 22 of Comparative Example 1. This is thought to be because the former contains cementite, while the latter does not. Therefore, in order to exhibit methanation activity, the methanation catalyst 22 must contain cementite, and it was found that the cementite content ratio I1 / (I1+I2) is preferably 0.17 or higher and 1.0 or lower.

[0042] From the perspective of methanation activity, the reason why the methanation catalysts 22 of Examples 1-3 are more efficient than the methanation catalyst of Comparative Example 1 is that the carbon generated by the direct decomposition of hydrocarbons adheres to the iron surface of the methanation catalysts 22 of Examples 1-3 and grows thereafter, the main active metal (iron) is in a fine and highly dispersed state and is in solid-state contact with many carbons, and furthermore, some of the iron is carbidized (forming cementite), and hydrogen readily reacts with the carbon in the iron and the carbon attached to the iron, so it is thought that reaction equation (1) is promoted. Moreover, from a microscopic perspective, it is thought that methanation is promoted by the sequential occurrence of reaction equations (5) and (6) below, and the repeated oxidation-reduction behavior between carbide and iron. Fe3C + 2H2 → CH4 + 3Fe ... (5) 3Fe + C → Fe3C ···(6)

[0043] <Verification of the relationship between temperature, pressure, and methanation activity> Next, using the methanation catalyst 22 from Example 2, methanation was performed under conditions where the pressure inside the reactor 23 was 1 ata and the temperature inside the reactor 23 was 400°C, 450°C, 500°C, 550°C, 600°C, 680°C, 700°C, 800°C, and 900°C, and the changes in methanation activity were investigated. Furthermore, methanation was performed under conditions where the pressure inside the reactor 23 was 0.59 MPa (6 ata) and the temperature inside the reactor 23 was 400°C, 450°C, 500°C, 550°C, 600°C, 680°C, 700°C, and 900°C, and the changes in methanation activity were also investigated.

[0044] During methanation, the composition of the reaction gas was analyzed by gas chromatography 27, and the conversion rate to methane at each set temperature was calculated from the composition analysis results. The maximum conversion rate was determined from the change in the conversion rate over time under each condition. Figure 5 shows the relationship between the temperature of methanation and the maximum conversion rate at the respective pressures of 1ata and 6ata. As can be seen from Figure 5, in both the 1ata and 6ata pressures, methanation activity was exhibited at temperatures above 450°C, and the methanation activity increased with increasing temperature in the temperature range up to 550°C at the 1ata pressure and up to 600°C at the 6ata pressure. In the temperature range from 550°C to 900°C at the 1ata pressure and from 600°C to 900°C at the 6ata pressure, the methanation activity decreased with increasing temperature, and at 900°C, the conversion rate was approximately 3% at the 1ata pressure and approximately 17% at the 6ata pressure. This confirms that methanation activity is exhibited within the temperature range of 450°C to 900°C. In Figure 5, the equilibrium conversion rate to methane in 1ata is shown by a dashed line, and the equilibrium conversion rate to methane in 6ata is shown by a dashed line. It was found that at the pressure of 1ata, the conversion rate to methane is approximately the equilibrium conversion rate in the temperature range of 700°C to 900°C, and at the pressure of 6ata, the conversion rate to methane is approximately the equilibrium conversion rate in the temperature range of 800°C to 900°C.

[0045] As described above, it was confirmed that methanation activity occurs in a pressure range of 1 ata to 6 ata and a temperature range of 450°C to 900°C. Next, we consider the preferred temperature range for methanation. If we assume that the conditions in which the maximum conversion rate is approximately 10% or more are preferred for methanation activity, then, based on Figure 5, a temperature range of approximately 470°C to approximately 770°C is preferred at a pressure of 1 ata. Furthermore, a temperature range of 500°C to 900°C is preferred at a pressure of 6 ata. Note that in Figure 5, the curve obtained by regression of the plot for the 6 ata conditions is located above the curve obtained by regression of the plot for the 1 ata conditions in the temperature range of approximately 510°C and above, so it is thought that as the pressure increases, the curve representing the relationship between the conversion rate and the temperature of methanation shifts upward. Thus, at pressures of 1 ata or higher, at least the preferred temperature range for 6 ata, i.e., 500°C to 680°C, can be said to be the preferred temperature range for methanation.

[0046] As described in Patent Document 1, which is a prior application of the applicant of this disclosure, when hydrocarbons such as methane come into contact with iron particles, the hydrocarbons are directly decomposed into hydrogen and carbon, and in the process, the iron constituting the iron particles is decomposed into fine particles on the order of submicrons. When hydrogen is brought into contact with particles to which carbon is attached to iron in this state, as described above, methanation occurs, and the carbon attached to the iron is removed from the iron as methane, and iron particles having a fine mesh-like structure are obtained. Next, we will consider a more preferable temperature range for performing methanation, based on the view of the structure of such iron particles obtained after methanation.

[0047] Figures 6-11 show surface images of particle aggregates after methanation (removal of attached carbon by direct decomposition of methane) using the methanation catalyst 22 of Example 2 at a pressure of 1 ata and temperatures of 500°C, 600°C, 680°C, 700°C, 800°C, and 900°C, respectively, taken with a scanning electron microscope (SEM) at 3000x magnification. These images confirm that methanation yielded particle aggregates with a fine mesh-like structure. From these images, the specific surface area of ​​iron particles contained in the methanated particles was calculated using the method described below.

[0048] First, from a photograph of the surface image of the aggregate of particles after methanation, the diameter of each individual iron mesh structure is determined at multiple points, and their average value d [μm] is calculated. Next, assuming that the cross-section of a single mesh structure is circular, the area CSA [μm] of the circular cross-section with the average value d as its diameter is calculated. 2 ] is calculated using the following formula (7). CSA = π × (d 2 / 4) ···(7) Next, assuming the mesh structure is a single cylindrical structure, the mass of the particles W [g] and the density of iron ρ [g / cm³] are given. 3 From the cross-sectional area CSA, the length L [m] of a single cylindrical structure is calculated using the following formula (8). L = W / (CSA·ρ) × 10 -6 ...(8) Next, the surface area SA[m] of the cylindrical structure, excluding the circular surfaces at both ends. 2 ] is calculated using the following formula (9). SA = π × d × L × 10 -6 ...(9) Finally, the specific surface area SSA [m2 / g] is calculated from the surface area SA and the particle mass W using the following formula (10). SSA = SA / W ... (10)

[0049] Figure 12 shows a graph plotting the specific surface area (SSA) calculated in this way against the methanation temperature. In Figure 12, although the slope of the line obtained by linear regression of the three plots at 500°C, 600°C, and 680°C is almost the same as the slope of the line obtained by linear regression of the three plots at 700°C, 800°C, and 900°C, the two lines appear to be discontinuous between 680°C and 700°C, and the two lines do not form a single line. When both lines are extrapolated to a temperature of 400°C, the intercept on the vertical axis is larger for the former line than for the latter line. In the relationship between methanation temperature and specific surface area, it can be said that the specific surface area estimated from the methanation temperature is smaller for the latter line than for the former line, based on the three data points from 500°C to 680°C that are regressiond by the former line and the three data points from 700°C to 900°C that are regressiond by the latter line. The reason for this is thought to be that as the methanation temperature increases, carbon is removed and some of the exposed iron particles sinter. In Figures 6-8, no sintering of the iron particles is observed, and all have a fine mesh-like structure. However, in Figures 9-11, the iron particles are relatively larger compared to Figures 6-8, and the surface of the mesh-like structure is smoother, which supports the idea that sintering did occur.

[0050] Therefore, in terms of the specific surface area of ​​the aggregate of particles after methanation, the upper limit of the preferred temperature range for methanation is less than 700°C, preferably 680°C or lower. The lower limit of the methanation temperature is 450°C, which is the temperature at which methanation activity is expressed, and 500°C is a preferred lower limit, which is the condition under which the maximum conversion rate to methane is approximately 10% or more. Thus, the temperature range for methanation is 450°C to 680°C, preferably 500°C to 680°C.

[0051] <Preparation of the methanation catalyst in Example 4> Methanation catalyst 22 for Example 4 was prepared in the same manner as in Example 1, except that 1.47 g of Australian hematite (JSS806-1, particle size 100 μm to 180 μm) was sampled, the temperature of the hematite was maintained at 800 °C, and the hematite was contacted with a gas containing 99 vol% or more of methane at a flow rate of 83 cc / min for 25 hours. The iron concentration of methanation catalyst 22 for Example 4 was 9.8 mass% (mass of iron: 0.37 g, total mass: 3.78 g).

[0052] <Experimental conditions for Example 4> The methanation catalyst 22 of Example 4, obtained in this manner, was placed in the reactor 23 of the experimental apparatus 20. Methanation was carried out by supplying hydrogen to the reactor 23 at a rate of 83 Ncc / min under conditions where the pressure inside the reactor 23 was 1 ata and the temperature inside the reactor 23 was 400°C, 500°C, 550°C, 600°C, 680°C, 700°C, and 900°C. During methanation, the composition of the reaction gas was analyzed by gas chromatography 27, and the conversion rate to methane at each set temperature was calculated from the composition analysis results. The maximum value of the conversion rate was determined from the change in the conversion rate over time under each condition. The relationship between the methanation temperature and the maximum value of the conversion rate is shown in Figure 13. Figure 13 also shows the relationship between the methanation temperature and the maximum value of the conversion rate obtained by performing methanation using the catalyst 22 of Example 2.

[0053] As can be seen from FIG. 13, in the temperature range of 600° C. or lower, although the activity of the methanation using the methanation catalyst 22 of Example 4 is slightly lower than the activity of the methanation using the methanation catalyst 22 of Example 2, it was confirmed that the activity of methanation is exhibited in the temperature range from a temperature exceeding 400° C. to 900° C. even in the methanation using the methanation catalyst 22 of Example 4. Therefore, when preparing an aggregate of a plurality of particles containing iron and carbon, not only a configuration in which a gas containing a hydrocarbon is brought into contact with a non-supported catalyst containing a plurality of iron particles (for example, reduced iron), but also a configuration in which a gas containing a hydrocarbon is brought into contact with a non-supported catalyst containing a plurality of particles of iron oxide (for example, iron ore) can be adopted. It has become clear that even when a configuration in which a gas containing a hydrocarbon is brought into contact with a non-supported catalyst containing a plurality of particles made of iron oxide is adopted, there is no difference from the case where a configuration in which a gas containing a hydrocarbon is brought into contact with a non-supported catalyst containing a plurality of iron particles in that the particles obtained after methanation are iron particles. Therefore, the methanation temperature range in the former case is 450° C. to 680° C., preferably 500° C. to 680° C.

[0054] <Use of the Catalyst after Methanation for the Direct Decomposition Method of Hydrocarbons> After methanation was carried out using the methanation catalyst 22 of Example 2 under the conditions of a pressure of 1 ata and each temperature of 500° C., 600° C., 680° C., 800° C., and 900° C., it was verified whether the obtained iron particles had activity as a catalyst for the direct decomposition of hydrocarbons. The experiment of the direct decomposition of hydrocarbons was carried out using the apparatus used to prepare the methanation catalysts 22 of Examples 1 to 3. The reactor of the apparatus was filled with 0.27 g of each iron particle, the pressure in the reactor was maintained at 1 ata and the temperature at 800° C., and a gas containing 99 vol% or more of methane was supplied to the reactor at a weight hourly space velocity (WHSV) of 0.307 min -1 ·g -1 −1·g−1 (the gas flow rate was 83 Ncc / min), and the gas and each iron particle were brought into contact for 20 hours. The amount of hydrogen generated in each experiment under each condition was measured, and FIGS. 14 to 16 show the change over time of the cumulative value of the amount of hydrogen generated in each experiment under each condition.

[0055] As can be seen from Figures 14-16, hydrogen was generated in experiments using each iron particle, confirming that each iron particle has the activity to directly decompose hydrocarbons, that is, that each iron particle can be used as a catalyst for the direct decomposition of hydrocarbons. From this, it can be said that a catalyst for the direct decomposition of hydrocarbons can be produced by removing carbon from the above-mentioned aggregate of multiple particles containing iron and carbon using methanation (the method for converting carbon to methane as disclosed herein).

[0056] As can be seen from Figures 14 and 15, in experiments using iron particles obtained after methanation with the methanation catalyst 22 of Example 2 at a pressure of 1 ata and temperatures of 500°C, 600°C, and 680°C respectively (hereinafter referred to as "particles obtained under low-temperature conditions"), hydrogen was generated well immediately after the start of the experiment, meaning that the direct decomposition activity of hydrocarbons was well exhibited. On the other hand, as can be seen from Figure 16, in experiments using iron particles obtained after methanation with the methanation catalyst 22 of Example 2 at a pressure of 1 ata and temperatures of 800°C and 900°C respectively (hereinafter referred to as "particles obtained under high-temperature conditions"), hydrogen generation was slower compared to experiments using particles obtained under low-temperature conditions, meaning that the direct decomposition activity of hydrocarbons was slower to manifest. Based on these results, it is preferable to use a method for producing catalysts for the direct decomposition of hydrocarbons in which carbon is removed from an aggregate of iron and carbon particles using methanation in a temperature range of 450°C to 680°C, preferably 500°C to 680°C.

[0057] As shown in Figure 12, the specific surface area of ​​iron particles obtained by methanation at temperatures below 680°C increases as the methanation temperature decreases. Therefore, the specific surface area of ​​iron particles contained in a catalyst for the direct decomposition of hydrocarbons, produced by removing carbon from an aggregate of iron and carbon particles using methanation, is equal to the specific surface area of ​​iron particles obtained by methanation at 680°C, which is 0.52 m². 2 It is preferable that the amount is 1 / g or more.

[0058] The contents described in each of the above embodiments can be understood, for example, as follows:

[0059] [1] A method for converting carbon to methane according to one embodiment is: A step of preparing an aggregate (12) of multiple particles containing 4.3 mass% to 54.3 mass% iron and 45.7 mass% to 95.7 mass% carbon, wherein the multiple particles contain cementite. The steps include bringing hydrogen into contact with the aforementioned assembly (12) and Includes.

[0060] The method for converting carbon to methane according to this disclosure allows for the conversion and utilization of carbon contained in an aggregate of multiple particles containing iron and carbon, thereby increasing the options for carbon utilization. Furthermore, when hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst containing multiple iron particles, the generated carbon coats the unsupported catalyst, reducing its activity. By converting the carbon contained in the unsupported catalyst with reduced activity into methane and reusing the resulting methane in the direct hydrocarbon decomposition method, the options for carbon utilization can be increased. Additionally, by reusing the unsupported catalyst obtained by removing the carbon in the direct hydrocarbon decomposition method, the amount of unsupported catalyst discarded can be reduced.

[0061] [2] A method for converting carbon to methane according to another embodiment is the method for converting carbon to methane according to [1], The quantitative values ​​of the peaks attributed to cementite and alpha iron obtained by X-ray diffraction analysis of the aggregate (12) are determined using the RIR method, and if the quantitative values ​​of the peaks attributed to cementite and alpha iron are denoted as I1 and I2, respectively, then the ratio I1 / (I1+I2) is 0.17 or greater and 1.0 or less.

[0062] This configuration allows for the conversion of carbon contained in an aggregate of multiple particles, including iron and carbon, into methane for use, thereby increasing the options for utilizing carbon.

[0063] [3] A further embodiment of a method for converting carbon to methane is the method for converting carbon to methane according to [1] or [2], The step of preparing the aforementioned assembly (12) is: The steps include preparing an unsupported catalyst (3) which is an aggregate of multiple iron particles, The steps include bringing a gas containing hydrocarbons into contact with the unsupported catalyst (3) and Includes.

[0064] When hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst consisting of aggregates of iron particles, the generated carbon adheres to the unsupported catalyst, forming a graphite-coated catalyst, which reduces the activity of the direct decomposition of hydrocarbons. If the carbon contained in the unsupported catalyst with reduced activity is converted into methane and the resulting methane is reused in the direct decomposition method of hydrocarbons, the options for utilizing carbon can be increased. Furthermore, if the unsupported catalyst obtained by removing the carbon is reused in the direct decomposition method of hydrocarbons, the amount of unsupported catalyst that is discarded can be reduced.

[0065] [4] A method for converting carbon to methane according to another embodiment is: 4. A step of preparing an aggregate (12) of multiple particles containing 4.3 mass% to 54.3 mass% of iron and 45.7 mass% to 95.7 mass% of carbon, The steps include bringing hydrogen into contact with the aforementioned assembly (12) and Includes, The step of preparing the aforementioned assembly (12) is: The steps include preparing an unsupported catalyst (3) containing multiple particles made of iron or iron oxide, The steps include bringing a gas containing hydrocarbons into contact with the unsupported catalyst (3) and Includes.

[0066] When hydrocarbons are directly decomposed into hydrogen and carbon using an unsupported catalyst containing multiple iron or iron oxide particles, the generated carbon adheres to the unsupported catalyst, forming a graphite-coated catalyst, which reduces the activity of the direct decomposition of hydrocarbons. If the carbon contained in the unsupported catalyst with reduced activity is converted into methane and the resulting methane is reused in the direct decomposition method of hydrocarbons, the options for utilizing carbon can be increased. Furthermore, if the unsupported catalyst obtained by removing the carbon is reused in the direct decomposition method of hydrocarbons, the amount of unsupported catalyst that is discarded can be reduced.

[0067] [5] A method for producing a catalyst for the direct decomposition of hydrocarbons according to one embodiment is: The method includes removing carbon from the aggregate (12) using a method of replacing any of the carbons in [1] to [4] with methane.

[0068] By using a catalyst obtained by the method for producing a catalyst for the direct decomposition of hydrocarbons according to this disclosure, the catalyst can exhibit good activity immediately after the start of direct decomposition.

[0069] [6] A method for producing a catalyst for the direct decomposition of hydrocarbons according to another embodiment is the method for producing the catalyst for the direct decomposition of hydrocarbons according to [5], The specific surface area of ​​the iron particles contained in the aforementioned plurality of particles is 0.52 m². 2 It is 1 / g or more.

[0070] With this configuration, activity can be expressed effectively immediately after the start of direct degradation. [Explanation of Symbols]

[0071] 3. Non-supported catalysts 12 aggregates

Claims

1. 4. An aggregate of multiple particles containing 4.3% to 54.3% by mass of iron and 45.7% to 95.7% by mass of carbon, wherein the aggregate of multiple particles contains cementite, The steps of bringing the aggregate into contact with hydrogen and A method for converting carbon into methane, including the following.

2. The quantitative values ​​of the peaks attributed to cementite and alpha iron obtained by X-ray diffraction analysis of the aforementioned aggregate were determined using the RIR method, and the quantitative values ​​of the peaks attributed to cementite and alpha iron were determined as follows: １ and I ２ Therefore, ratio I １ / (I １ +I ２ The method for converting carbon to methane according to claim 1, wherein the ratio is 0.17 or more and 1.0 or less.

3. The step of preparing the aforementioned assembly is: The steps include preparing an unsupported catalyst consisting of an aggregate of iron particles, The steps include bringing a gas containing hydrocarbons into contact with the unsupported catalyst and A method for converting carbon to methane according to claim 1 or 2, comprising:

4. 4. A step of preparing an aggregate of multiple particles containing 4.3% to 54.3% by mass of iron and 45.7% to 95.7% by mass of carbon, The steps of bringing the aggregate into contact with hydrogen and Includes, The step of preparing the aforementioned assembly is: A step of preparing an unsupported catalyst containing multiple particles made of iron or iron oxide, The steps include bringing a gas containing hydrocarbons into contact with the unsupported catalyst and A method for converting carbon into methane, including the following.

5. The method for converting carbon to methane according to any one of claims 1, 2, or 4, wherein the step of contacting the aggregate with hydrogen is performed at a pressure of 0.098 MPa (1 ata) or more and at a temperature range of 450°C to 680°C.

6. A method for producing a catalyst for the direct decomposition of hydrocarbons, comprising the step of removing carbon from the aggregate using a method for converting carbon to methane as described in any one of claims 1, 2, or 4.

7. The specific surface area of ​​the iron particles contained in the aforementioned plurality of particles is 0.52 m². ２ A method for producing a catalyst for the direct decomposition of hydrocarbons according to claim 6, wherein the amount is 1 / g or more.

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