Co2 conversion method

Abstract

To provide a CO2 conversion method capable of efficiently obtaining ethylene from CO generated by an RWGS reaction.SOLUTION: A CO2 conversion method comprises generating CO and H2O by an RWGS reaction from CO2 and H2, generating C2H4 and H2 by CO electrolysis from the CO and H2O, and using the H2 generated by the CO electrolysis as the H2 of the RWGS reaction.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a CO2 conversion method. [Background technology]

[0002] Efforts aimed at mitigating or reducing the impact of climate change have been ongoing for some time, and research and development into the effective use of CO2 is being carried out to achieve this.

[0003] CO2 is produced by many industrial and biological processes. It is usually emitted into the atmosphere. However, because CO2 is recognized as a greenhouse gas, it is necessary to reduce CO2 emissions from these processes. In limited cases, CO2 can be used to enhance oil and gas recovery from oil wells, but most of the captured CO2 is released back into the atmosphere. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2023-527651 Summary of the Invention [Problem to be solved by the invention]

[0005] Meanwhile, in reducing CO2 emissions, the challenge is to effectively utilize CO2 as a raw material for other products without releasing it into the atmosphere.

[0006] One method for effectively utilizing CO2 is, for example, the reverse water gas shift (RWGS) reaction (CO2 + H2 → CO + H2O) used to convert CO2 to CO, as described in Patent Document 1, followed by the production of lower olefins such as ethylene (C2H4). With the CO (carbon monoxide) produced by the RWGS reaction and hydrogen from the electrolysis of water, useful chemical raw materials can be obtained through catalytic hydrogenation of carbon monoxide to hydrocarbons. However, this method has the drawback of requiring a large number of steps because it uses a catalytic reaction, resulting in the generation of by-products such as methanol, which must be converted into lower olefins. Another drawback is the high equipment costs and energy input.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a CO conversion method that can efficiently produce ethylene from CO produced in the RWGS reaction, thereby contributing to mitigating or reducing the impact of climate change. [Means for solving the problem]

[0008] The gist of the present invention is as follows. [1] CO and H2O are produced from CO2 and H2 by the RWGS reaction, generating C2H4 and H2 from the CO and H2O by CO electrolysis; A CO2 conversion method, characterized in that the H2 produced by the CO electrolysis is used as the H2 in the RWGS reaction. [2] The CO2 conversion method according to [1], characterized in that the C2H4 current efficiency in the CO electrolysis is set to 66.6% or less. [3] The CO conversion method according to [1] or [2], characterized in that the H2 produced by the CO electrolysis is used as the H2 and heat source for the RWGS reaction. [4] The CO2 conversion method according to [3], characterized in that the C2H4 current efficiency in the CO electrolysis is set to a value equal to or greater than a value at which excess H2 is not generated, depending on the H2 thermal efficiency. [Effects of the Invention]

[0009] According to the above-described aspects of the present invention, it is possible to provide a CO2 conversion method that can efficiently obtain ethylene from CO generated from the RWGS reaction. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a graph showing the relationship between the C2H4 current efficiency in a CO electrolysis cell and the amount of CO consumed / amount of H2 produced in the CO electrolysis cell. [Figure 2] 1 is a graph showing the relationship between H2 thermal efficiency and C2H4 current efficiency. [Figure 3] 1 is a graph showing the relationship between the C2H4 current efficiency in a CO electrolysis cell and the difference in heat quantity between H2 supply and demand, and surplus H2, when the H2 thermal efficiency is set to 75%. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the examples shown in the following description are merely examples, and the present invention is not necessarily limited to these examples. Appropriate modifications can be made within the scope of the present invention.

[0012] In the CO2 conversion method according to this embodiment, CO and H2O are produced from CO2 and H2 by the RWGS reaction, and C2H4 and H2 are produced from the CO and H2O using a CO electrolysis cell.

[0013] In the RWGS reaction, the following reaction occurs: The reactor and heating conditions for the RWGS reaction are not particularly limited. CO2+H2→CO+H2O(ΔH=41.2kJ / mol)

[0014] The following reaction occurs in CO electrolysis. For example, a CO electrolysis cell can be used for CO electrolysis. The specific configuration of the CO electrolysis cell is not particularly limited as long as it is configured to produce C2H4 and H2 through the following reaction. Furthermore, the HO used in the following reaction may be generated by the RWGS reaction or may be supplied from another source. 2CO+6H2O+8e - →C2H4+8OH - 2H2O+2e - →H2+2OH -

[0015] In the CO2 conversion method according to this embodiment, CO produced by the RWGS reaction can be converted into C2H4 by CO electrolysis, so the number of steps can be reduced and the equipment costs and energy input can be reduced.

[0016] In addition, in the CO conversion method according to this embodiment, H generated by CO electrolysis is used as H for the RWGS reaction, which makes it possible to omit the introduction of a water electrolysis device or the like for producing H for the RWGS reaction, and to omit the water electrolysis process.

[0017] Figure 1 is a graph showing the relationship between the C2H4 current efficiency of CO electrolysis in a CO electrolysis cell and the amount of CO consumed / H2 produced in CO electrolysis in a CO electrolysis cell. As shown in Figure 1, if the C2H4 current efficiency of CO electrolysis is 66.6% (and the H2 current efficiency is 33.4%), the molar amount of CO consumed in the RWGS reaction can be equal to the molar amount of H2 produced by CO electrolysis. In other words, by setting the C2H4 current efficiency of CO electrolysis to 66.6% or less (and the H2 current efficiency to 33.4% or more), the H2 required for the RWGS reaction can be supplied solely by the H2 produced by CO electrolysis, allowing continuous CO2 conversion without the need for a separate H2 supply source. Therefore, it is preferable to set the C2H4 current efficiency of the CO electrolysis cell to 66.6% or less.

[0018] The H2 produced by CO electrolysis is preferably used not only as H2 for the RWGS reaction but also as a heat source for the RWGS reaction. Because the RWGS reaction is an endothermic reaction, heat is required for the reaction. Therefore, by burning the H2 produced by CO electrolysis and using it as a heat source for the RWGS reaction, it becomes unnecessary to supply heat from another source. This further reduces the input energy for CO2 conversion.

[0019] Here, the C2H4 current efficiency in CO electrolysis is preferably set to a value equal to or greater than the value at which excess H2 is not generated, depending on the thermal efficiency (H2 thermal efficiency) when H2 is used as a heat source. When H2 is used as a heat source, the H2 consumption range in the CO2 conversion method according to this embodiment can be expressed by the following formula.

[0020] RWGS endothermic enthalpy (kJ / mol) × CO production during RWGS reaction (mol / h) ≧ H2 combustion enthalpy (kJ / mol) × H2 production by electrolysis (mol / h) × H2 thermal efficiency (%)

[0021] Here, the RWGS endothermic enthalpy is ΔH = 41.2 kJ / mol, and the H2 combustion enthalpy is ΔH = 241.82 kJ / mol.

[0022] The amount of CO produced during the RWGS reaction is equal to the amount of CO consumed by electrolysis, and is equal to the amount of C2H4 produced by electrolysis × 2. The amount of C2H4 produced by electrolysis can be expressed by the following formula: Cell current (A) / Faraday constant ((A·sec.) / mol-e) × C2H4 current efficiency (%)

[0023] The amount of electrolytic H2 produced can be expressed by the following formula: Cell current (A) / Faraday constant ((A·sec.) / mol-e) × H2 current efficiency (%)

[0024] From the H2 consumption range obtained by the above method, the relationship between H2 thermal efficiency and C2H4 current efficiency is shown in Figure 2. The approximate curve shown in Figure 2 is y = -0.0021x 2This is the curve +0.3654x+46.557. When a certain H2 thermal efficiency is specified, if the C2H4 current efficiency in CO electrolysis is lower than the curve in Figure 2, excess H2 will be generated by CO electrolysis. The excess H2 must be consumed in a separate device, which increases the number of processes. Therefore, it is preferable to set the C2H4 current efficiency in CO electrolysis to a value equal to or higher than the H2 thermal efficiency at which excess H2 is not generated.

[0025] For example, when the H2 thermal efficiency is 75%, as shown in Figure 2, the generation of excess H2 due to CO electrolysis can be suppressed by setting the C2H4 current efficiency to 62.0% or higher.

[0026] When H2 is burned as a heat source, 241.82 kJ / mol of thermal energy is generated through the following reaction: 1 / 2O2 + H2 → H2O (ΔH = -241.82 kJ)

[0027] If the thermal efficiency of H2 when used as a heat source is assumed to be 75%, then 241.82 x 0.75 = 181.37 kJ / mol of heat can be obtained, and the RWGS reaction can provide 181.37 / 41.2 = 4.4 kJ / mol of heat per 1 mol of CO.

[0028] Figure 3 shows the relationship between the C2H4 current efficiency in CO electrolysis, the difference in heat supply and demand for H2, and surplus H2 when the H2 thermal efficiency is set to 75%. A positive value for the difference in heat supply and demand for H2 indicates that the amount of H2 supplied from CO electrolysis to the RWGS reaction is greater than the amount of H2 required for the RWGS reaction (both the H2 used for the reaction and the H2 used as a heat source). A positive value for surplus H2 indicates that the amount of H2 generated by CO electrolysis is greater than the amount of H2 required for the RWGS reaction (excluding the H2 used as a heat source).

[0029] As shown in Figure 3, if the H thermal efficiency when using H as a heat source is set to 75%, then by setting the C2H4 current efficiency in CO electrolysis to 62.0 to 66.6% (and the H2 current efficiency to 33.4 to 38.0%), CO2 conversion can be performed more efficiently without the need for H2 supply from another H2 source and without the generation of excess H2 by CO electrolysis. Therefore, when the H2 thermal efficiency is set to 75%, the C2H4 current efficiency in CO electrolysis is preferably set to 62.0 to 66.6%.

Claims

[Request 1] CO ２ and H ２ From this, CO and H are produced by the RWGS reaction. ２ O and The CO and H ２ From O, CO electrolysis produces C ２ H ４ and H ２ Generate The H generated by the CO electrolysis ２ The H ２ CO ２ How to convert. Request 2 C in the CO electrolysis ２ H ４ 2. The CO according to claim 1, wherein the current efficiency is 66.6% or less. ２ How to convert. Request 3 The H generated by the CO electrolysis ２ The H ２ and a heat source. ２ How to convert. Request 4 C in the CO electrolysis ２ H ４ The current efficiency is ２ Surplus H depending on thermal efficiency ２ 4. The CO concentration as claimed in claim 3, wherein the CO concentration is set to a value at which the CO concentration is not generated. ２ How to convert.

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