Improved method for operating a solid oxide electrolytic cell in carbon dioxide electrolysis

Abstract

The present invention relates to a method for converting carbon dioxide to carbon monoxide in high temperature dry solid oxide electrolysis, which provides increased lifetime for SOECs and SOEC stacks by addressing the problem of coking, while ensuring as much CO production as possible from each cell or stack.

Description

[Technical Field]

[0001] This invention proposes an improved method for operating a solid oxide electrolytic cell (SOEC) in CO2 (carbon dioxide) electrolysis. The invention also proposes an improved method for operating a solid oxide electrolytic cell (SOEC) stack in CO2 (carbon dioxide) electrolysis. [Background technology]

[0002] Electrolytic cells can be used to electrochemically convert H2O (water) to H2 (hydrogen), CO2 to CO (carbon monoxide), or a combination of H2O and CO2 to synthesis gas (a combination of CO, CO2, H2, and H2O). Carbon monoxide is traditionally produced from fossil fuels, for example, through steam reforming of natural gas. Therefore, the production of CO using conventional methods is associated with significant CO2 emissions. Alternatively, carbon monoxide can be produced by the electrolysis of CO2. When electricity from low-carbon energy sources (wind, solar, nuclear, etc.) is used in the electrolysis process, CO2 emissions associated with CO production can be minimized, or even negative. Therefore, electrolysis is a potentially more sustainable method for producing CO compared to traditional fossil fuel-based methods.

[0003] Methods for CO2 electrolysis can generally be classified into two groups: low-temperature CO2 electrolysis and high-temperature CO2 electrolysis. Low-temperature CO2 electrolysis, often called electrochemical CO2 reduction (eCO2R), involves the carbon dioxide reduction reaction (CO2 + H2O + 2e⁻¹). - →CO+2OH - ) takes place in an aqueous solution on the cathode of the cell, and water molecules participate in the electrochemical reaction, with H2O being converted to OH on the cathode. -This is a process that converts CO2 into CO2. This is sometimes called "wet electrolysis." At atmospheric pressure, the operating temperature of a low-temperature CO2 electrolytic cell is limited to 100°C due to the reaction fluid, which is mainly water. In low-temperature electrolysis, the diffusion coefficient of CO2 is relatively low and the applied overpotential is large. The electrolyte is generally a polymer or even a liquid because it does not need to withstand high temperatures. Examples of products that can be obtained by low-temperature electrolysis are hydrogen gas, carbon monoxide, methanol, ethylene, and formic acid. The main challenges in low-temperature wet CO2 electrolysis include electrode instability, low selectivity for CO production, high power consumption, and low current density. An example of low-temperature electrolysis is, for example, WO2018 / 228723 This can be found in (Patent Document 1).

[0004] High-temperature CO2 electrolysis refers to CO2 electrolysis in a solid oxide electrolytic cell (SOEC). In an SOEC, the carbon dioxide reduction reaction (CO2 + 2e - →CO+O 2- The process is carried out in the gas phase on a suitable catalyst surface. Typical operating temperatures for SOEC are approximately 600°C to 1000°C. In high-temperature electrolysis, the diffusion coefficient of CO2 is relatively high, and the applied overpotential is low. The electrolyte is generally made of a ceramic material that becomes an oxygen ion conductor at high temperatures, such as stabilized zirconia or doped ceria.

[0005] High-temperature electrolysis can be carried out with or without the presence of vapor. When vapor is present, it may be called "wet electrolysis." An example of a product that can be obtained by high-temperature wet electrolysis is synthesis gas (a combination of CO, CO2, H2, and H2O). An example of high-temperature wet electrolysis is, for example, WO2018 / 206235It can be found in (Patent Document 2). When there is no steam, it can be referred to as "dry electrolysis". Examples of products that can be obtained by high-temperature dry electrolysis are carbon monoxide gas, especially high-purity carbon monoxide gas (essentially a combination of CO and CO2). Examples of high-temperature dry electrolysis are, for example, WO2018 / 228716 (Patent Document 3), WO2016 / 091636 (Patent Document 4), WO2015 / 014527 (Patent Document 5), WO2014 / 154253 (Patent Document 6), WO2018 / 206235 (Patent Document 2 ) that can be found in.

[0006] In particular, WO2018 / 206235 discloses in its Example 1 the operating points for dry electrolysis in a SOEC stack consisting of 75 cells. It is operated at an average temperature of 700 °C using pure CO2 supplied to the cathode at a flow rate of 100 Nl / min while passing an electrolysis current of 50 A. The gas emerging from the cathode side of the stack consists of 26% CO and 74% CO2.

[0007] SOECs for high-temperature electrolysis generally include a fuel electrode, a solid electrolyte, an oxygen electrode, and optionally a contact layer, increasing the in-plane conductivity and providing improved contact to adjacent interconnects. In the context of the present invention, the term "fuel side" includes the fuel electrode and represents the side of the SOEC where the CO2 reduction reaction (CO2 + 2e - → CO + O 2- ) is taking place. The term "oxygen side" includes the oxygen electrode and refers to the side of the SOEC where the O2 generation reaction (2O 2- → O2 + 4e - ) is taking place. The SOEC can be arranged in a stack of cells (referred to as a SOEC stack). In a SOEC stack, multiple cells are generally electrically connected in series and fluidly connected in parallel. The cells are generally arranged at spaced positions by interposing interconnect plates (also called interconnects). The interconnect plates provide electrical contact between adjacent cells and Each plays a role in providing flow fields for fuel and oxygen to the fuel and oxygen electrodes, respectively. In a CO2 electrolysis system, multiple SOEC stacks are typically operated simultaneously to reach the desired CO production rate. In systems for high-temperature CO2 electrolysis, the stacks are generally arranged to be fluidly connected in parallel, and preferably they are positioned around an inlet manifold to simultaneously supply fuel gas to each of the SOEC stacks. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] WO2018 / 228723 [Patent Document 2] WO2018 / 206235 [Patent Document 3] WO2018 / 228716 [Patent Document 4] WO2016 / 091636 [Patent Document 5] WO2015 / 014527 [Patent Document 6] WO2014 / 154253 [Overview of the project] [Problems that the invention aims to solve]

[0009] Generally, it is desirable to reduce the operating expenditure (OPEX) and capital expenditure (CAPEX) of a CO2 electrolysis system. Intuitively, OPEX can be minimized by operating the SOEC stack at the maximum possible conversion rate. In dry CO2 electrolysis, this means converting as much of the CO2 supply material as possible into carbon monoxide (CO) products. The maximum possible CO2 conversion rate is determined thermodynamically and is a function of the CO / CO2 ratio in the gas, absolute pressure, and temperature. For example, in dry CO2 electrolysis at 800°C at 1 bar, the maximum CO / CO2 ratio is approximately 11.8. At higher CO / CO2 ratios, the Boudouard reaction (2CO=CO2+C) becomes thermodynamically favorable, as the pressure and temperature increase. The Boudouard reaction causes carbon formation (also called coking or carbonization) within the cell, which can then lead to cell and stack failures. Consequently, the task of operating an SOEC stack becomes more complex.

[0010] On the one hand, it is desirable to maximize the CO2 conversion rate and operate at a very high CO / CO2 ratio at the product stream / stack outlet to minimize OPEX, but such an operating strategy can lead to cell failures due to coking and expensive stack replacements. On the other hand, if a stack is operated at a very low CO2 conversion rate, it is not optimally utilized, and more stacks are required to reach the desired CO production rate. [Means for solving the problem]

[0011] The inventors have found that one of the main challenges in high-temperature CO2 electrolysis is coking (carbon formation).

[0012] The objective of this invention is to extend the lifespan of SOECs and SOEC stacks by addressing the coking problem, while ensuring the highest possible CO generation from each cell or stack.

[0013] Chemical processes involving CO and CO2 tend to result in carbonization (coking). The probability of coking (formation of carbon, i.e., C) in a CO / CO2 system is known to be determined by the thermodynamics of the Boudouard reaction (2CO → CO2 + C). Thermodynamics determines the tendency of the reaction to occur, while kinetics determines the rate of the reaction. Therefore, in CO-CO2 gas mixtures, the probability of coking (formation of C from CO) is known to increase at higher CO / CO2 ratios, higher pressures, and lower temperatures. For example, in dry CO / CO2 at 800°C and atmospheric pressure, coking is thermodynamically favorable whenever the CO / CO2 ratio exceeds 7.8. At 750°C, coking is thermodynamically favorable when the CO / CO2 ratio exceeds 3.6, while at 700°C, coking is thermodynamically favorable when the CO / CO2 ratio exceeds 1.7.

[0014] Therefore, it is known that the Boudouard reaction can be used to determine the critical temperature below which, for a given dry CO / CO2 system with a specific CO / CO2 ratio, the tendency for CO to carbonize increases significantly. This temperature is sometimes called the Boudouard temperature. Based on the values ​​mentioned above, the Boudouard temperature for a CO-CO2 system with a CO / CO2 ratio of 7.8 is 800°C, the Boudouard temperature for a CO-CO2 system with a CO / CO2 ratio of 3.6 is 750°C, and the Boudouard temperature for a CO-CO2 system with a CO / CO2 ratio of 1.7 is 700°C.

[0015] Therefore, the inventors hypothesized that if the SOEC was operated above the Boudouard temperature, the coking problem could be avoided within the SOEC. However, the inventors found that even when the SOEC was operated above the Boudouard temperature, there was still a tendency for coking, and therefore an adverse effect on its lifespan. The inventors also found that there is no clear pattern regarding when coking can be avoided and when coking occurs when operated above the Boudouard temperature.

[0016] The inventors have now found that in SOEC, the likelihood of coking is not determined solely by the combination of operating temperature and the CO / CO2 ratio exiting the cell or stack, but rather that the conditions in SOEC where coking is far more likely to occur depend primarily on the following combination of critical parameters: fuel gas inlet temperature (T), space velocity (SV). fuel ) and CO concentration (X CO ); flash gas inlet temperature (T) and space velocity (SV) flush ); and combinations of electrolytic current density (i) passing through the electrolyte in SOEC.

[0017] In a first embodiment, the present invention relates to a method for converting carbon dioxide to carbon monoxide in high-temperature dry solid oxide electrolysis, comprising the following steps: 1) A step of supplying a fuel gas stream containing 70-100 volume% CO2 and 0-30 volume% CO, wherein the mole fraction of CO (x CO A step where ) is in the range of 0 to 0.3; 2) Providing a flash gas stream; 3) Providing a solid oxide electrolytic cell (SOEC) having a fuel side and an oxy side; 4) Heating the fuel gas stream and flash gas stream to a gas stream inlet temperature T in the range of 600 to 1000°C, for example, 700 to 850°C; 5) 2-30s -1 Spatial velocity SV in the range fuel Next, the fuel gas stream is supplied to the fuel side of the SOEC; 6) 0.1~20s -1 Spatial velocity SV in the range flush Next, the flash gas stream is supplied to the oxy side of the SOEC; 7) -0.2 A / cm 2 ~-1A / cm 2 The step involves passing an electrolytic current having a current density i in the range through a solid electrolyte to electrolytically convert a portion of CO2 to CO on the fuel side of the SOEC and to generate an O2-enriched flash gas on the oxy side of the SOEC.

[0018] This method results in a high CO2 conversion rate within the cell without compromising the lifespan of the SOEC.

[0019] The inventors have found that adjusting the relationships between combinations of five process parameters provides conditions for avoiding or significantly reducing coking in SOEC: inlet temperature, fuel gas space velocity, fuel gas CO concentration, flash gas space velocity, and electrolytic current density through SOEC.

[0020] For the industrial-scale production of CO-enriched gases, SOECs are typically placed in SOEC stacks. Multiple stacks may be further arranged within the SOEC system.

[0021] In dry solid oxide electrolysis, the fuel gas stream should be dry, meaning it should contain only about 0-1 volume% H2O. This is to avoid the reaction between water and carbon monoxide, which forms hydrogen and carbon dioxide (water-gas shift reaction). Separating CO from a mixture of CO2, CO, H2O, and H2 is far more difficult than separating CO from a mixture of CO2 (for example, WO2018 / 228716 (See [reference]). Furthermore, the fuel gas stream preferably contains only about 0-1 volume% H2. This is to avoid the reaction between hydrogen and carbon dioxide forming water and carbon monoxide (reverse water-gas shift). Separating CO from a mixture of CO2, CO, H2O, and H2 is more difficult than separating CO from a mixture of CO2. The fuel gas may also contain small amounts of other unreactive components.

[0022] In general, it is preferable that the temperature be kept extremely uniform within cells, between cells (i.e., within stacks), and between stacks (i.e., within the system for high-temperature CO2 electrolysis). However, in practice, small temperature gradients may exist both between cells and within cells. For example, temperature fluctuations within cells may be 25, 30, 40, or even 50°C. In particular, the difference between the inlet and outlet temperatures may be up to 50°C, or even up to 75°C or 100°C, without departing from the present invention. The inlet temperatures of the fuel gas stream and flash gas stream may also fluctuate. If the flash gas and fuel gas have different inlet temperatures, or if the inlet temperature fluctuates across different inlets, the average inlet temperature (i.e., the arithmetic mean of the inlet temperatures) should be used for T. This is the same for all aspects of the present invention.

[0023] In SOEC, it should be understood that the fuel side and the oxy side must be in ionic contact via a solid electrolyte in order to be operational.

[0024] A second aspect of the present invention provides a method for selecting operating conditions for high-temperature dry CO2 electrolysis in a solid oxide electrolytic cell (SOEC) having a fuel side and an oxy side that are in ionic contact via a solid electrolyte, comprising the following steps: i. Direct the fuel gas stream to the fuel side of the SOEC for 2-30 seconds. -1 Spatial velocity SV in the range fuel A step of supplying, wherein the fuel gas stream contains 70-100 volume% CO2 and 0-30 volume% CO, and the mole fraction of CO (x CO ) is a step in the range of 0 to 0.3; ii. Direct the flash gas stream to the oxi side of the SOEC for 0.1 to 20 seconds. -1 Spatial velocity SV in the range flush The step of supplying, iii. A step of supplying heat to the SOEC by heating the fuel and flash gas stream to a gas stream inlet temperature T in the range of 600 to 1000°C, and continuing iv. The electrolytic current i is -0.2 A / cm through the electrolyte of SOEC. 2 ~-1A / cm 2 A step of applying current at a current density within the range of, Includes T, SV fuel , SV flush The value of i is determined by the following iterative process: a) T, SV fuel , SV flush Set the operating conditions of i to their initial values; b) Determining local temperature and local gas composition at multiple locations that are diversely distributed within the cell; c) Based on the local gas composition described above, estimate for each of the above locations the local temperature (local Boudouard temperature) below which carbon formation by the Boudouard reaction becomes thermodynamically favorable. d) Subtracting the local Boudouard temperature from the measured local temperature to obtain the Boudouard difference, and e) Varying the gas flow rate, inlet temperature(s) and / or electrolytic current density until the Boudouard difference for each of the aforementioned positions is greater than zero. The method selected by is provided.

[0025] This method for selecting operating conditions for high-temperature dry CO2 electrolysis in solid oxide electrolytic cells can be used as a trial-and-error procedure to obtain a set of operating parameters for a given SEOC or SOEC stack. This procedure, combined, provides a method for converting carbon dioxide to carbon monoxide in high-temperature dry solid oxide electrolysis, which is outside the Boudouard temperature range and means that no significant coking occurs.

[0026] figure: [Brief explanation of the drawing]

[0027] [Figure 1] Figure 1 is a schematic diagram of SOEC. [Figure 2]Figure 2 is a block diagram showing one embodiment of a system equipped with SOEC. [Figure 3] Figure 3 is a schematic diagram of the SOEC stack iteration unit. [Figure 4] Figures 4a, 4b, 4c, 4d, and 4e show the relationship between the coking potential (CP) and various critical treatment parameters (T, SVflus, SVfuel, %CO, and i), demonstrating the effect of each parameter under otherwise fixed conditions. [Figure 5] Figure 5 shows exemplary coking potentials calculated by equation (I) for several possible combinations of SVfuel, SVflush, and i, based on assumptions regarding Vfuel, Voxygen, and the active area of ​​the cell. In case #1, the Vfuel, Voxygen, and cell active area (A) were obtained from Example 1 of this application.

[0028] Detailed description of the invention This specification discloses a method for converting carbon dioxide to carbon monoxide in a high-temperature dry solid oxide electrolysis as defined above.

[0029] In this context, the term "inlet temperature" is intended to represent the temperature near the location(s) where the gas enters the cell or stack, for example, in a gas manifold below a cell or stack. Experimentally, the inlet temperature is readily determined using thermocouples. If the flash gas and fuel gas have different inlet temperatures, or if the inlet temperatures differ across different inlets, the average inlet temperature (i.e., the arithmetic mean of the inlet temperatures) should preferably be used.

[0030] The term "flash gas" is intended to refer to the gas supplied to the oxy side of an SOEC cell or stack. Flash gas is used to transport oxygen formed in the anode reaction out of the cell or stack. The flash gas flow can also be used to smooth out temperature gradients within the cell or stack.

[0031] The term "fuel gas" is intended to refer to the gas supplied to the fuel side of an SOEC cell or stack. In this context, when an SOEC is operated using a dry CO2 electrolysis method (i.e., converting CO2 to CO and O2), the fuel gas contains CO2 and optionally CO. It may also contain trace amounts of other components, such as H2, N2, and H2O, up to less than 5% of the total.

[0032] The term "product gas" is intended to refer to the gas emanating from the fuel side of an SOEC cell or stack. In this context, when an SOEC is operated by dry CO2 electrolysis, the product gas contains CO and CO2. As long as current flows through the SOEC cell or stack, the CO content in the product gas will be higher than the CO content in the fuel gas. The product gas may also contain trace amounts of other components, such as H2, N2, and H2O, up to less than 5% of the total.

[0033] "Space velocity" is a widely used parameter to describe the rate of gas flow through a reactor such as an SOEC or SOEC stack. When used herein, space velocity SV is defined as the number of reactor volumes of feed material passing through the reactor per unit time. Thus, "space velocity of flash gas" represents the volumetric flow rate of flash gas divided by the total volume of the oxy-side compartment of the SOEC cell or stack, and "space velocity of fuel gas" represents the volumetric flow rate of fuel gas divided by the total volume of the fuel-side compartment of the SOEC cell or stack. The volumetric flow rate of gas is readily determined using a gas flow meter or rotometer, and the values ​​shown herein represent values ​​measured under standard conditions (0°C, 1 atm). The total volumes of the fuel-side (cathode) and oxy-side (anode) compartments of the SOEC cell or stack can be determined from the drawings (technical drawings). Alternatively, the total volume of the fuel compartment in the SOEC stack (V) can be determined from the drawings. fuel ) can be estimated according to the following formula:

[0034]

number

[0035]

number

[0036] The "mole fraction" of gas species in gases (x CO In this context, the term ) represents the number of moles of a gas species in a gas mixture divided by the total number of moles in that gas mixture. For example, if the mole fraction of CO in the fuel gas is 5 mol%, then x CO = 0.05.

[0037] The term "electrolytic current density" is defined as the total electrolytic current flowing through a cell or stack, divided by the SOEC active area. "SOEC active area" represents the geometric area of ​​an SOEC that is electrochemically active, i.e., involved in electrochemical reactions. For an SOEC with a rectangular active area, the active area can be estimated by multiplying W by L.

[0038] In the context of this invention, the term "inert" refers to a gas species that does not participate in chemical or electrochemical reactions in the SOEC at the relevant temperature. Inert species typically include nitrogen, argon, helium, and the like. In some situations, for example, when the inert gas is used as a flash gas, the inert gas may also include CO2, air, vapor, and the like.

[0039] In this context, "local" means 12 × 12 × 12 cm 3 Smaller than, preferably 1 x 1 x 1 cm 3 Smaller than, more preferably 0.1 × 0.1 × 0.1 cm 3 This means representing a volume smaller than [a certain value].

[0040] The typical operating temperature for SOEC is approximately 600°C to 1000°C: high temperatures are necessary to achieve sufficient oxide ion conductivity in the ceramic film used as the electrolyte. Commonly used electrolyte materials include stabilized zirconia, e.g., yttria-stabilized zirconia (YSZ), doped ceria, and doped plantan gallate. Commonly used oxy electrode materials include perovskite materials, e.g., Sr-doped LaMnO3 (LSM), Sr-doped LaFeO3 (LSF), Sr-doped LaCoO3 (LSC), Sr-doped La(Co,Fe)O3 (LSCF), Sr-doped SmCoO3, and many others. Perovskite materials are also commonly mixed with doped ceria to form composite oxy electrodes (SOEC anodes). Dopants other than Sr, such as Ca and Ba, are known to form non-perovskite materials, such as the Ruddlesden-Popper phase. Commonly used fuel electrode materials include composites of metallic nickel and stabilized zirconia, such as Ni-YSZ, or composites of metallic nickel and doped ceria.

[0041] The inventors have now found an operating window for SOEC stacks in CO2 electrolysis that enables safe operation of the SOEC stack at the highest possible CO / CO2 ratio. In particular, the inventors defined a coking potential (CP) for the stack, and further defined a coking potential (CP) of T, SV flush , SV fuel , x CO And it can be estimated as a function of i, and we found that it is defined by:

[0042]

number

[0043] Simultaneously, economic profitability is maximized when the SOEC stack is operated such that CP ≤ -15. This results in a high conversion rate of CO2 to CO, a long lifespan of the SOEC exceeding one year, and high profitability. In one embodiment of the present invention, CP is in the range of -75 to -15. With respect to CP, there is no actual lower range. However, lower ranges of CP may be -100, -80, or -75. Equation (I) was experimentally determined at atmospheric pressure. Thus, empirical equation (I) is applicable at pressures near atmospheric pressure, at least 0.5 to 2 bar, for example, 0.7 to 1.8 bar absolute pressures.

[0044] In one embodiment, the fuel gas stream contains 80-100%, for example, 88-98 volume% CO2. In one embodiment, the fuel gas stream contains 0-20 volume%, for example, 1-12 volume% CO. In one embodiment, the fuel gas stream contains 80-100%, for example, 88-98 volume% CO2 and 0-20 volume%, for example, 1-12 volume% CO. In one embodiment, the mole fraction of CO in the fuel gas stream (x CO ) is in the range of 0 to 0.2, for example, 0.01 to 0.15 or 0.05 to 0.1. In one embodiment, the product gas stream contains 15 to 95 volume% CO, for example, 15 to 90, 20 to 80, 20 to 70, 20 to 60, 20 to 50 volume% CO, or 30 to 50 volume% CO. In one embodiment, some remainder of the gas present in the fuel gas stream is an inert gas (e.g., N2 or a noble gas).

[0045] In a particular embodiment, the fuel gas stream consists of 80-100 vol% CO2, 0-20 vol% CO, 0-1 vol% H2O, and 0-1 vol% H2, with the remainder being inert, and the mole fraction of CO (x COThe value is in the range of 0 to 0.2, for example, 0.01 to 0.15 or 0.05 to 0.1. Such a fuel gas stream has the advantage of being able to produce a product gas stream containing CO in the range of 20 to 50 volume percent, as well as low content of H2O and H2.

[0046] In another specific embodiment, the fuel gas stream consists of 88-98 vol% CO2, 1-12 vol% CO, 0-1 vol% H2O, and 0-1 vol% H2, with the remainder being inert, and the mole fraction of CO (x CO The value is in the range of 0 to 0.2. Such a fuel gas stream has the advantage of being able to produce a product gas stream containing CO in the range of 20 to 50 volume percent, as well as low content of H2O and H2.

[0047] In another embodiment, a method for selecting operating conditions for a particular SOEC stack design is to use equation (I) to satisfy CP = 15, T, SV fuel , SV flush , X CO The process begins by selecting an initial set of and i, and then proceeds according to guidance for selecting operating parameters as claimed in claim 15.

[0048] Detailed description of the drawing Figure 1 is a schematic diagram of the SOEC. The SOEC (1) comprises a fuel side (11), an oxygen ion conductive electrolyte (12), and an oxy side (13). Fuel gas (101) is supplied to the fuel side (11), where a portion of the CO2 present in the gas stream is electrochemically converted to CO. The driving force for the electrochemical reaction is provided by the potential supplied by the power supply unit (20). The resulting product gas stream (102) has a higher CO content than the fuel gas (i.e., is enriched with CO), and this CO-enriched product gas stream is collected from the fuel side of the SOEC.

[0049] A stream of oxygen ions passes through the electrolyte (103) to the oxi side (13) of the SOEC. On the oxi side of the SOEC, an anodic reaction occurs, oxidizing ionic oxygen. A flash gas (104) is used to carry the oxygen formed by the anodic reaction out of the SOEC. As a result of the electrochemical reaction on the oxi side, the flash gas stream (104) is enriched with oxygen. The oxygen-enriched flash gas stream (105) is collected from the oxi side of the SOEC.

[0050] Figure 2 is a block diagram showing one embodiment of a system (2) equipped with an SOEC. The gas stream (201) is optionally mixed with a CO2-enriched gas stream (203), and the resulting fuel gas stream (101) is supplied to the fuel side (11) of the SOEC (10). A portion of the CO2 present in the fuel gas stream is electrochemically converted to CO. The resulting CO-enriched product gas stream (102) is collected from the fuel side of the SOEC. The product gas stream (102) can optionally be supplied to a gas purification unit (30), where it is divided into a first CO-enriched product gas stream (202) and a second CO2-enriched product gas stream (203). The CO2-enriched product gas stream (203) can optionally be recycled back to the fuel side of the SOEC as described above.

[0051] The flash gas stream (104) is supplied to the oxy side (13) of the SOEC (10). As a result of electrochemical reactions on the oxy side, the flash gas stream (104) is enriched with oxygen. The oxygen-enriched flash gas stream (105) is collected from the oxy side of the SOEC.

[0052] Figure 3 is a schematic diagram of an SOEC stack repeating unit. More specifically, it shows how interconnects (40) are arranged between adjacent SOECs (10). In this figure, the oxy side of the SOEC faces upward and the fuel side of the SOEC faces downward. The width W and length L of the SOEC are schematically shown. Two different arbitrary interconnect shapes are shown. In Figure 3a, the average height H of the fuel-side gas channel is shown. fuel,av and the average height H of the oxy gas channel oxygen,av This is lower than in Figure 3b, which is due to the difference in shape between the interconnect designs in the two figures.

[0053] Figures 4a, 4b, 4c, 4d, and 4e show the coking potential (CP) and various critical processing parameters (T, SV). flush , SV fuel This shows the relationship between %, %CO, and i) and demonstrates the effect of each parameter under otherwise fixed conditions. Figure 4 is explained in more detail in Examples 4-8.

[0054] This method will be explained in more detail in the following non-limiting example. [Examples]

[0055] example Example 1 Although coking is not expected based on thermodynamic considerations, the SOEC stack was operated in dry CO2 electrolysis under the condition that the aforementioned coking potential is positive.

[0056] More specifically, an electric heater was used to heat the fuel gas (food-grade CO2), thereby raising the inlet temperature (T) to 725°C. The fuel gas composition was ≥99.9% CO2, and the CO content was at the ppm level (i.e., x CO (=0). The SOEC stack contains 75 cells connected in series, each measuring 108 cm². 2It had an active surface area. Total volume of the fuel compartment (V fuel ) is 243cm 3 And on the other hand, the total volume of the oxy side compartment (V oxygen ) is 405cm 3 That was it. Fuel gas, 1.26s -1 The space velocity (SV) of the fuel gas fuel ) corresponds to 1.1Nm 3 The stack was supplied with a volumetric flow rate of / h. Air was used as the flash gas on the oxy side of the stack, 1.03s -1 The space velocity of the flash gas (SV flush ) corresponds to 1.5Nm 3 The stack was supplied with a volumetric flow rate of / h. The stack current was -0.37 A / cm². 2 The electrolytic current density (i) was fixed at -40A. Under these conditions, 60% of the CO2 supplied to the stack was converted to CO, which corresponds to a CO / CO2 ratio of 60 / 40 = 1.5 near the stack outlet.

[0057] In a dry CO / CO2 mixture at 725°C, coking is thermodynamically favorable whenever the CO / CO2 ratio exceeds 2.47, which corresponds to a CO2 conversion rate of 71.2%. In other words, based on thermodynamic considerations, carbon should not have formed inside the stack. However, the stack underwent considerable coking and failed after only 12.5 hours of operation.

[0058] The coking potential (CP) under the above conditions was 72.3, meaning that coking is actually very favorable in the stack, which explains why the stack failed after only a few hours of testing. Carbon formation in the stack was also confirmed by post-test analysis using Raman spectroscopy.

[0059] Example 2 Based on thermodynamic considerations, coking was not expected, and the SOEC stack was operated in dry CO2 electrolysis under the conditions where the coking potential was negative, i.e., according to the method of the present invention.

[0060] More specifically, an electric heater was used to heat the fuel gas (food-grade CO2), thereby raising the inlet temperature (T) to 750°C. The fuel gas was a mixture of food-grade CO2 and CO. The CO content in the fuel gas was 3.6% (i.e., x CO (=0.036). The SOEC stack contains 75 cells connected in series, each measuring 108 cm². 2 It had an active surface area. Total volume of the fuel compartment (V fuel ) is 243cm 3 And on the other hand, the total volume of the oxy side compartment (V oxygen ) is 405cm 3 That was it. Fuel gas, 9.14s -1 The space velocity (SV) of the fuel gas fuel ) corresponds to 8Nm 3 The stack was supplied at a volumetric flow rate of / h. A mixture of air and CO2 was used as the flash gas on the oxy side of the stack, and the process took 9.81s. -1 The space velocity of the flash gas (SV flush ) corresponds to 14.3Nm 3 The stack was supplied with a volumetric flow rate of / h. The stack current was -0.46 A / cm². 2 The electrolytic current density (i) was fixed at -50A. Under these conditions, 19.6% of the CO2 supplied to the stack was converted to CO, which corresponds to a CO / CO2 ratio of (19.6 + 3.6) / (100 - 19.6 - 3.6) = 0.30 near the stack outlet.

[0061] In a dry CO / CO2 mixture at 750°C, coking is always thermodynamically favorable when the CO / CO2 ratio exceeds 3.6, which corresponds to a CO2 conversion rate of 78.4%. In other words, based on thermodynamic considerations, carbon should not form inside the stack under operating conditions. The coking potential (CP) under the above conditions is -59.9, meaning that coking does not occur.

[0062] Example 3 Based on thermodynamic considerations, coking was not expected, and the SOEC stack was operated in dry CO2 electrolysis under the condition that the coking potential was negative, i.e., in accordance with the method of the present invention.

[0063] More specifically, an electric heater was used to heat the fuel gas (food-grade CO2), thereby raising the inlet temperature (T) to 745 °C. The fuel gas was a mixture of food-grade CO2 and CO. The CO content in the fuel gas was 3.6% (i.e., x CO = 0.036). The SOEC stack included 75 cells connected in series, each having an active area of 108 cm 2 . The total volume (V fuel ) of the fuel-side compartment was 243 cm 3 , while the total volume (V oxygen ) of the oxy-side compartment was 405 cm 3 . The fuel gas was supplied to the stack at a volume flow rate of 8 Nm -1 / h corresponding to a space velocity (SV fuel ) of the fuel gas of 9.14 s 3 . A mixture of air and CO2 was used as the flush gas on the oxy side of the stack and supplied to the stack at a volume flow rate of 14.3 Nm -1 / h corresponding to a space velocity (SV[[ID=?]] flush ) of the flush gas of 9.81 s 3 . The stack current was fixed at -70 A corresponding to an electrolysis current density (i) of -0.65 A / cm 2 . Under these conditions, 27.4% of the CO2 supplied to the stack was converted to CO, which corresponds to a CO / CO2 ratio of (27.4 + 3.6) / (100 - 27.4 - 3.6) = 0.45 near the stack outlet.

[0064] It should be noted that there seems to be a missing number in the tag for "SV" in the original text (the "?" in the translation). Please check and correct if necessary.In a dry CO / CO2 mixture at 745 °C, coking is always thermodynamically favorable when the CO / CO2 ratio exceeds 3.37, which corresponds to a CO2 conversion rate of 77.1%. In other words, based on thermodynamic considerations, carbon should not be formed inside the stack during operation. The coking potential (CP) under the above conditions is -30.5, that is, coking does not occur. Two stacks were operated under the above conditions for more than one year without coking. From an economic perspective, operation under the conditions listed in Example 3 is more advantageous than operation under the conditions listed in Example 2 because it can produce more CO gas per stack per hour.

[0065] Example 4 Estimate the coking potential (CP) for a series of actual operating conditions. The space velocity of the flash gas is SV flush = 9 s -1 is fixed, and the space velocity of the fuel gas is SV fuel = 7 s -1 is fixed. The mole fraction of CO in the fuel gas is x CO = 0.05, and the electrolysis current density is i = -0.5 A / cm 2 is fixed. The influence of the change in the inlet temperature (T) was investigated. It is plotted in Figure 4a. Under the selected series of operating conditions, CP = 7.9 at 675 °C, CP = 0 at 703 °C, and CP = -17.9 at 750 °C. Generally, the higher the inlet temperature, the lower the CP. Therefore, to avoid carbon formation, it is desirable to operate the stack at a high temperature, provided that this does not reduce the stack life due to other mechanisms (e.g., corrosion of the interconnect).

[0066] Example 5 Estimate CP for a series of actual operating conditions: Fix the inlet temperature at T = 700 °C, and fix the space velocity of the fuel gas at SV fuel = 7 s -1 The mole fraction of CO in the fuel gas is x CO = 0.05, and the electrolysis current density is i = -0.5 A / cm 2It was fixed at (SV). The spatial velocity of the flash gas (SV) flush The effect of the change in ) was investigated. This is plotted in Figure 4b. Under the selected series of operating conditions, 1s -1 CP=6.9, 9s -1 CP=0.8, and 29s -1 Therefore, CP = -14.2. Generally, the higher the space velocity of the flash gas, the lower the CP. Thus, to avoid carbon formation, it is desirable to operate the stack at a high flash gas space velocity, provided that this is not prohibitively expensive.

[0067] Example 6 CP is estimated for a set of practical operating conditions: with the inlet temperature fixed at T=700°C and the space velocity of the flash gas set to SV flush =9s -1 The mole fraction of CO in the fuel gas is fixed at x CO Assuming i = 0.05, the electrolytic current density is i = -0.5 A / cm². 2 It was fixed at (SV). fuel The effect of the change in ) was investigated. This is plotted in Figure 4c. Under the selected series of operating conditions, 1s -1 CP=134.9, 7s -1 CP=0.8, and 13s -1 The CP is -43.8. Generally, the higher the space velocity of the fuel gas, the lower the CP. Therefore, to avoid carbon formation, it is desirable to operate the stack at a high fuel gas space velocity, provided that this is not prohibitively expensive.

[0068] Example 7 We estimate CP for a series of practical operating conditions: fixing the inlet temperature at T=700°C and setting the space velocity of the flash gas to SV flush =9s -1 The space velocity of the fuel gas is fixed at SV. flush =9s -1 The current density is fixed at i = -0.5 A / cm². 2 Fixed at (x) the mole fraction of CO in the fuel gas. COThe effect of the change in ) was investigated. This is plotted in Figure 4d. Under the selected set of operating conditions, x CO =0.01, CP=-24.6, x CO =0.05, CP=0.8, and x CO At a value of 0.10, the CP is 2.6. Generally, the higher the mole fraction of CO in the fuel gas, the higher the CP. Therefore, to avoid carbon formation, it is desirable to operate the stack with a low CO mole fraction, provided that the Ni at the fuel inlet of the stack remains in a metallic state.

[0069] Example 8 We estimate CP for a series of practical operating conditions: fixing the inlet temperature at T=700°C and setting the space velocity of the flash gas to SV flush =9s -1 The space velocity of the flash gas is fixed at SV. flush =9s -1 The mole fraction of CO in the fuel gas is fixed at x CO = 0.05 was used. The effect of the change in electrolytic current density (i) was investigated. This is plotted in Figure 4e. Under the selected series of operating conditions, -0.1 A / cm 2 CP = -69.8, -0.5 A / cm 2 CP = 0.8 and -1.5 A / cm² 2 CP = 81.8. Generally, the more negative the electrolytic current density, the higher the CP. Therefore, to avoid carbon formation, it is desirable to operate the stack at a low electrolytic current density (i.e., a current close to zero). However, the rate of stack formation is directly proportional to the absolute value of the electrolytic current, and therefore, a high current density (i.e., a more negative current) is required to ensure commercial viability.

[0070] Example 9 An SOEC stack consisting of 75 cells was operated at an average temperature of 700°C with an electrolytic current of 50A, supplied with pure CO2 at a flow rate of 100Nl / min to the cathode, and a CO2 to CO conversion rate corresponding to 26% CO and 74% CO2 in the gas emanating from the cathode side of the stack was obtained. This corresponds to the disclosure in Example 1 of WO2018 / 206235.

[0071] The above example describes the operating point of the SOEC stack in dry CO2 electrolysis. The operating temperature (T) is 700°C, and the mole fraction of CO in the fuel gas stream (x CO ) is 0. However, D1 is SV fuel , SV flush And in order to enable estimation of i, geometric information about the stack is not disclosed. For example, a value for the volumetric flow rate of the fuel gas (100 Nl / min) is given, but SV fuel To estimate the value of the fuel compartment, the total volume (V fuel It is necessary to know ). Similarly, a value for stack current (50A) is given, but in order to estimate the value of i, it is necessary to know the active area per cell (and whether the cells are connected in series or in parallel). Finally, neither the flash gas flow rate nor the space velocity is mentioned, and it is not mentioned at all in Example 1 of WO2018 / 206235 whether flash gas is used.

[0072] In Figure 5, as described above, V fuel , V oxygen And based on the assumptions regarding the active area of ​​the cell, the coking potential is several possible SV fuel , SV flush And the calculation is performed with respect to the combination of i.

[0073] In case #1, V fuel , V oxygenThe values ​​of the cell active area (A) and cell active area were obtained from Example 1 of this application. To avoid any doubt, it should be emphasized that these do not represent preferred embodiments and are not the SV, cell area, as referred to in Example 1 of WO2018 / 206235.

[0074] Cases #2 to #4 are SV fuel V brings about change fuel This represents variations of SV. fuel By increasing the SV, caulking can be avoided. fuel Then, caulking occurs.

[0075] Cases #5 and #6 show variations in the cell active area compared to #1. Increasing the cell active area can prevent caulking. Decreasing the cell active area causes caulking.

[0076] Cases #7 and #8 illustrate variations in flash gas flow rate. Increasing the flash gas flow rate has a slight positive effect, making caulking slightly less likely.

[0077] Cases #9~#10 are higher SV fuel This represents variations in flash gas flow rates. Increasing the flash gas flow rate has a slight positive effect, making caking slightly less likely. However, caking still occurs.

[0078] In summary, these data clearly demonstrate that Example 1 of WO2018 / 206235 discloses insufficient information to incidentally predict the invention of this patent application. The explanation contains no hints or suggestions whatsoever.

[0079] Embodiment: Embodiment 1. A method for converting carbon dioxide to carbon monoxide in high-temperature dry solid oxide electrolysis, comprising the following steps: 1) A step of supplying a fuel gas stream containing 70-100 volume% CO2 and 0-30 volume% CO, wherein the mole fraction of CO (x CO A step where ) is in the range of 0 to 0.3; 2) Providing a flash gas stream; 3) Providing a solid oxide electrolytic cell stack (SOEC stack) comprising a plurality of solid oxide electrolytic cells (SOECs), wherein each cell has a fuel side and an oxy side that are in ionic contact via a solid electrolyte, and the plurality of SOECs are electrically connected in series, fluidly connected in parallel, and separated by interconnectors; 4) Heating the fuel gas stream to a fuel gas inlet temperature T in the range of 600-1000°C, for example, 700-850°C; 5) 1-30s -1 Spatial velocity SV in the range fuel Next, the fuel gas stream is supplied to the fuel side of the SOEC; 6) 0.1~20s -1 Spatial velocity SV in the range flush Next, the flash gas stream is supplied to the oxy side of the SOEC; 7) -0.2 A / cm 2 ~-1A / cm 2 The step involves passing an electrolytic current having a current density i in the range through a solid electrolyte to electrolytically convert a portion of CO2 to CO on the fuel side of the electrode, and to generate an O2-enriched flash gas on the oxy electrode side of the SOEC.

[0080] Claims 2 to 14 can be similarly combined with Embodiment 1.

[0081] Embodiment 2. A method for converting carbon dioxide to carbon monoxide in high-temperature dry solid oxide electrolysis, comprising the following steps: 1) A step of supplying a fuel gas stream containing 70-100 volume% CO2 and 0-30 volume% CO, wherein the mole fraction of CO (x CO A step where ) is in the range of 0 to 0.3; 2) Providing a flash gas stream; 3) Providing a plurality of solid oxide electrolytic cell stacks (SOEC stacks) comprising a plurality of solid oxide electrolytic cells (SOECs), wherein each cell has a fuel side and an oxy side that are in ionic contact via a solid electrolyte, and the plurality of SOECs are arranged in an SOEC stack, the SOECs are electrically connected in series, fluidly connected in parallel, and separated by interconnects; each stack is mounted on a manifold for simultaneously supplying fuel gas streams to each stack and simultaneously receiving product gas streams from each stack; 4) Heating the fuel gas stream to a fuel gas inlet temperature T in the range of 600-1000°C, for example, 700-850°C; 5) 1-30s -1 Spatial velocity SV in the range fuel Next, the fuel gas stream is supplied to the fuel side of the SOEC; 6) 0.1~20s -1 Spatial velocity SV in the range flush Next, the flash gas stream is supplied to the oxy side of the SOEC; 7) -0.2 A / cm 2 ~-1A / cm 2 The step involves passing an electrolytic current having a current density i in the range through a solid electrolyte to electrolytically convert a portion of CO2 to CO on the fuel side of the electrode, and to generate an O2-enriched flash gas on the oxy electrode side of the SOEC.

[0082] Claims 2 to 14 can be similarly combined with Embodiment 2. While this application relates to the invention described in the claims, it may also encompass the following other embodiments. 1. A method for converting carbon dioxide to carbon monoxide in high-temperature dry solid oxide electrolysis, comprising the following steps: 1) 70-100% by volume of CO 2 and a step of supplying a fuel gas stream containing 0 to 30 volume% of CO, wherein the mole fraction of CO (x CO A step where ) is in the range of 0 to 0.3; 2) Providing a flash gas stream; 3) Providing a solid oxide electrolytic cell (SOEC) having a fuel side and an oxy side; 4) Heating the fuel gas stream and flash gas stream to a gas stream inlet temperature T in the range of 600 to 1000°C, for example, 700°C to 850°C; 5)2~30s -1 Spatial velocity SV in the range fuel Next, the fuel gas stream is supplied to the fuel side of the SOEC; 6)0.1~20s -1 Spatial velocity SV in the range flush Next, the flash gas stream is supplied to the oxy side of the SOEC; 7) -0.2 A / cm 2 ~-1A / cm 2 An electrolytic current having a current density i in the range is passed through the solid electrolyte, and CO is generated on the fuel side of the SOEC. 2 A portion of it is converted to CO by electrolysis, and then O is used on the oxy side of SOEC. 2 Steps to generate enriched flash gas, A method that includes this. 2.x CO , T, SV fuel , SV flush The method according to 1 above, wherein i is selected such that the coking potential CP is ≤ -15, and CP is given by the following equation (I).

number

[0083] Reference Code List 1: SOEC 2: SOEC 10: SOEC 11: Fuel side 12: Oxygen ion conductive electrolytes 13: Oxy side 20: Power supply unit 30: Gas purification unit 40: Interconnect 101: Fuel gas / fuel gas stream 102: Product gas stream 103: Electrolyte 104: Flash gas / flash gas stream 105: Oxygen-enriched flash gas stream 201: Gasstream 202: CO-enriched product gas stream 203: CO2-enriched gas stream

Claims

1. A method for converting carbon dioxide to carbon monoxide in high-temperature dry solid oxide electrolysis, comprising the following steps: 1) 70-100% by volume of CO ２ and a step of supplying a fuel gas stream containing 0 to 30 volume percent of CO, wherein the mole fraction of CO (x ＣＯ A step in which ) is in the range of 0 to 0.3; 2) Providing a flash gas stream; 3) Providing a solid oxide electrolytic cell (SOEC) having a fuel side and an oxy side; 4) Heating the fuel gas stream and flash gas stream to a gas stream inlet temperature T in the range of 600 to 1000°C; 5) 2-30s －１ Space velocity SV in the range ｆｕｅｌ Next, the fuel gas stream is supplied to the fuel side of the SOEC; 6) 0.1 to 20 seconds －１ Space velocity SV in the range ｆｌｕｓｈ Next, the step of supplying the flash gas stream to the oxy side of the SOEC; 7) -0.2 A / cm ２ ~ -1 A / cm ２ An electrolytic current having a current density i within the range of is passed through a solid electrolyte, and a part of CO on the fuel side of the SOEC is electrochemically converted to CO, and O is ２ An oxygen-rich flash gas is generated on the oxygen side of the SOEC ２ Step of generating In a method including, x ＣＯ , T, SV ｆｕｅｌ , SV ｆｌｕｓｈ The method wherein i is selected such that the coking potential CP is ≤ -15, and CP is given by the following equation (I). [Math 1]

2. The method according to claim 1, wherein the coking potential CP of formula (I) is in the range of -100 to -15 during the operation of the SOEC.

3. The method according to claim 1 or 2, wherein the product gas stream obtained by the conversion in step 7) contains CO in the range of 15 to 95 volume percent.

4. The fuel gas stream contains 80-100% by volume of CO ２ , 0-20% by volume of CO, 0-1% by volume of H ２ O and 0-1 volume% H ２ It consists of the above, with the remainder being inert, and the mole fraction of CO (x ＣＯ The method according to any one of claims 1 to 3, wherein ) is in the range of 0 to 0.

2.

5. The fuel gas stream contains 88-98 volume percent CO ２ , 1-12 volume% CO, 0-1 volume% H ２ O and 0-1 volume% H ２ It consists of the above, with the remainder being inert, and the mole fraction of CO (x ＣＯ The method according to any one of claims 1 to 4, wherein ) is in the range of 0 to 0.

2.

6. Flash gas is air, dry air, O ２ CO ２ , N ２ The method according to any one of claims 1 to 5, comprising vapor or a mixture thereof.

7. The method according to any one of claims 1 to 6, wherein the solid oxide electrolytic cell comprises a fuel gas inlet to the fuel side of the SOEC and a fuel product gas outlet from the fuel side of the SOEC.

8. The method according to any one of claims 1 to 7, wherein the solid oxide electrolytic cell comprises a flash gas inlet to the oxy side of the SOEC and a flash gas outlet from the oxy side of the SOEC.

9. The method according to any one of claims 1 to 8, wherein the oxygen-enriched flash gas stream is collected from the oxy side of the SOEC.

10. The method according to any one of claims 1 to 9, wherein the product gas stream obtained by the conversion in step 7) is collected from the fuel side of the SOEC as a CO enrichment product gas stream.

11. The CO enrichment product gas stream is divided into a first CO enrichment gas stream and a second CO ２ The method according to claim 10, further comprising the step of splitting into an enriched gas stream.

12. Second CO ２ The method according to claim 11, wherein the enrichment stream is recycled to the fuel side of the SOEC and mixed into the fuel gas stream.

13. The method according to any one of claims 1 to 12, wherein the fuel side of the SOEC includes metallic nickel electrically connected to a power source.

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