An improved method of operating a solid oxide electrolysis cell with carbon dioxide electrolysis

Abstract

The invention relates to a method for converting carbon dioxide into carbon monoxide in high-temperature, dry solid-oxide electrolysis, which improves the lifetime of SOECs and SOEC stacks by solving the coking problem while ensuring the highest possible CO production per electrolysis cell or stack of electrolysis cells.

Description

Technical Field

[0001] This invention proposes an improved method for operating a solid oxide electrolyzer (SOEC) using CO2 (carbon dioxide) electrolysis. This invention also proposes an improved method for operating a solid oxide electrolyzer (SOEC) stack using CO2 (carbon dioxide) electrolysis. Background Technology

[0002] Electrolyzers can be used to electrochemically convert H2O (water) to H2 (hydrogen), CO2 to CO (carbon monoxide), or a combination of H2O and CO2 to syngas (a combination of CO, CO2, H2, and H2O). Carbon monoxide is traditionally produced from fossil fuels, such as through steam reforming of natural gas. Therefore, CO production using conventional methods is associated with significant CO2 emissions. Alternatively, carbon monoxide can be produced by electrolyzing CO2. If 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 eliminated. Therefore, electrolysis is a potentially more sustainable method of CO production compared to traditional fossil fuel-based methods.

[0003] Methods for CO2 electrolysis can generally be divided into two types: low-temperature CO2 electrolysis and high-temperature CO2 electrolysis. Low-temperature CO2 electrolysis (often referred to as electrochemical CO2 reduction (eCO2R)) is a process in which carbon dioxide is reduced (CO2 + H2O + 2e-). - →CO+2OH - The process occurs in an aqueous solution at the cathode of a battery, where water molecules participate in the electrochemical reaction, converting H₂O to OH⁻ at the cathode. - This can be called "wet electrolysis." Under atmospheric pressure, the operating temperature of a cryogenic CO2 electrolyzer is limited to 100°C because the reaction fluid is primarily water. In cryogenic electrolysis, the diffusion coefficient of CO2 is relatively low, and the applied overpotential is high. The electrolyte is typically a polymer or even a liquid, as it does not need to withstand high temperatures. Examples of products obtainable through cryogenic electrolysis include hydrogen, carbon monoxide, methanol, ethylene, and formic acid. The main challenges of cryogenic wet CO2 electrolysis include electrode instability, poor selectivity for CO production, high power consumption, and low current density. Examples of cryogenic electrolysis can be found, for example, in WO18228723.

[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⁻) occurs... - →CO+O 2-The process takes place in the gas phase on the surface of a suitable catalyst. Typical operating temperatures for SOEC are between approximately 600°C and 1000°C. In high-temperature electrolysis, the diffusion coefficient of CO2 is relatively high, and the applied overpotential is low. The electrolyte is typically made of ceramic materials, such as stabilized zirconium oxide or doped cerium oxide, which become oxygen ion conductors at high temperatures.

[0005] High-temperature electrolysis can be carried out with or without steam. When steam is present, it is called "wet electrolysis." An example of a product obtainable through high-temperature wet electrolysis is syngas (a combination of CO, CO2, H2, and H2O). Examples of high-temperature wet electrolysis can be found, for example, in WO 18206235. When steam is absent, it is called "dry electrolysis." An example of a product obtainable through high-temperature dry electrolysis is carbon monoxide gas, especially high-purity carbon monoxide gas (essentially a combination of CO and CO2). Examples of high-temperature dry electrolysis can be found, for example, in WO 2018 / 228716, WO 2016 / 091636, WO 2015 / 014527, WO 2014 / 154253, and WO 2018 / 206235.

[0006] Specifically, WO 2018 / 206235 discloses, in its Example 1, the operating point for dry electrolysis in an SOEC stack consisting of 75 cells. It operates at an average temperature of 700°C, with pure CO2 supplied to the cathode at a flow rate of 100 Nl / min, while an electrolysis current of 50 A is applied. The gas exiting the cathode side of the stack consists of 26% CO and 74% CO2.

[0007] SOECs used for high-temperature electrolysis typically include a fuel electrode, a solid electrolyte, an oxygen electrode, and an optional contact layer to increase in-plane conductivity and provide improved contact with adjacent interconnects. In the context of this invention, the term "fuel side" refers to an SOEC that includes a fuel electrode and in which the CO2 reduction reaction (CO2 + 2e⁻) occurs. - →CO+O 2- The oxygen side. The term "oxygen side" refers to the side of the SOEC containing the oxygen electrode where the O2 evolution reaction (2O) occurs. 2- →O2+4e -On one side of the cell stack (called an SOEC stack), multiple cells are typically connected in series electrically and in parallel fluidly. The cells are usually arranged at spaced locations by inserting interconnect plates (also called interconnects). The interconnect plates provide electrical contact between adjacent cells and provide flow fields for fuel and oxygen, respectively, for the fuel electrode and oxygen electrode. In CO2 electrolysis systems, multiple SOEC stacks typically operate simultaneously to achieve the desired CO production rate. In systems for high-temperature CO2 electrolysis, the stacks are typically arranged in parallel fluid connection, and preferably, they are arranged around an inlet manifold for simultaneously supplying fuel gas to each SOEC stack. Summary of the Invention

[0008] Minimizing operating expenses (OPEX) and capital expenditures (CAPEX) of CO2 electrolysis systems is generally desirable. Intuitively, OPEX can be minimized by operating the SOEC stack at the maximum possible conversion rate. In dry CO2 electrolysis, this means converting the highest possible amount of CO2 feed into carbon monoxide (CO) products. The maximum possible CO2 conversion rate is thermodynamically determined and varies with the CO / CO2 ratio in the gas, absolute pressure, and temperature. For example, in dry CO2 electrolysis at 800°C and 1 bar, the maximum CO / CO2 ratio is approximately 11.8. At higher CO / CO2 ratios, higher pressures, and lower temperatures, the Boudouard reaction (2CO = CO2 + C) becomes thermodynamically favorable. The Boudouard reaction leads to build-up formation (also known as coking or carbonization) in the cell, which can in turn cause cell and stack failure. Therefore, operating an SOEC stack becomes more complex.

[0009] On the one hand, there is a desire to maximize CO2 conversion and operate at a very high CO / CO2 ratio at the product stream / stacking outlet to minimize OPEX. However, this operating strategy can lead to cell failures and costly stack replacements due to coking. On the other hand, if the stack operates at a very low CO2 conversion rate, it will not be optimally utilized, and more stacks will be needed to achieve the desired CO production rate.

[0010] We found that one of the main challenges of high-temperature CO2 electrolysis is coking (carbon formation).

[0011] The purpose of this invention is to increase the lifetime of SOEC and SOEC stacks by solving the coking problem, while ensuring that each electrolyzer or electrolyzer stack produces the highest possible amount of CO.

[0012] Chemical processes involving the presence of CO and CO2 tend to produce carbonization (coke). It is well known that the probability of coking (carbon formation, C) in a CO / CO2 system is thermodynamically determined by the Boudouard reaction (2CO → CO2 + C). Thermodynamics determines the tendency for the reaction to occur, while kinetics determines the reaction rate. Therefore, it is known that in a CO-CO2 gas mixture, the probability of coking (from CO to C) increases with a higher CO / CO2 ratio, higher pressure, and lower temperature. For example, in a dry CO / CO2 mixture at 800°C and atmospheric pressure, a CO / CO2 ratio above 7.8 is thermodynamically favorable for coking. At 750°C, a CO / CO2 ratio above 3.6 is thermodynamically favorable for coking, and at 700°C, a CO / CO2 ratio above 1.7 is thermally favorable for coking.

[0013] Therefore, it is also known from the Boudouard reaction that for a given dry CO / CO2 system with a specific CO / CO2 ratio, a critical temperature can be determined below which the tendency for CO carbonization increases significantly. This temperature can be called the Boudouard temperature. According to the values ​​listed above, the Boudouard temperature for a CO-CO2 system with a CO / CO2 ratio of 7.8 is 800℃, the Boudouard temperature for a CO-CO2 system with a CO / CO2 ratio of 3.6 is 750℃, and the Boudouard temperature for a CO-CO2 system with a CO / CO2 ratio of 1.7 is 700℃.

[0014] Therefore, the inventors believed that operating SOEC at temperatures above Boudouard's could avoid the problem of coking inside the SOEC. However, the inventors have now discovered that even when operating SOEC at temperatures above Boudouard's, it is still prone to coking, which adversely affects its lifespan. The inventors have also found that there is no clear pattern as to when coking can be avoided and when it occurs when operating at temperatures above Boudouard's.

[0015] The inventors have now discovered that in SOEC, the probability of coking is determined not only by the combination of operating temperature and the CO / CO2 ratio leaving the cell or stack, but also that the conditions under which coking is more likely to occur in SOEC depend primarily on the combination of the following key parameters: fuel gas inlet temperature (T), space velocity (SV). 燃料 ) and CO concentration (X) CO ); inlet temperature (T) and space velocity (SV) of the flushing gas 冲洗 ); and the electrolytic current density (i) on the electrolyte in SOEC.

[0016] In a first aspect, the present invention relates to a method for converting carbon dioxide into carbon monoxide in a high-temperature, dry solid oxide electrolysis process, the method comprising the following steps:

[0017] 1) Provide a fuel gas stream containing 70-100 vol% CO2 and 0-30 vol% CO, wherein the mole fraction of CO (x) CO Within the range of 0 to 0.3;

[0018] 2) Provide flushing gas flow;

[0019] 3) Provide solid oxide electrolyzers (SOECs) with fuel and oxygen sides;

[0020] 4) Heat the fuel gas stream and flushing gas stream to a gas stream inlet temperature T in the range of 600°C to 1000°C, for example, 700°C to 850°C;

[0021] 5) In 2s -1 up to 30s -1 airspeed SV within the range 燃料 Supply fuel gas streams to the fuel side of SOEC;

[0022] 6) In 0.1s -1 up to 20s -1 airspeed SV within the range 冲洗 The flushing gas stream is supplied to the oxygen side of the SOEC;

[0023] 7) Apply a flux of -0.2 A / cm to the solid electrolyte. 2 Up to -1A / cm 2 The current density i is used for electrolysis current to electrolyze a portion of CO2 into CO on the fuel side of SOEC and to generate O2-rich flushing gas on the oxygen side of SOEC.

[0024] This process achieves a high CO2 conversion rate in the battery without affecting SOEC lifetime.

[0025] The inventors discovered that adjusting the relationship between the following five process parameters—inlet temperature of fuel gas, space velocity, CO concentration of fuel gas, space velocity of flushing gas, and electrolytic current density on SOEC—provides conditions that will avoid or greatly reduce coking in SOEC.

[0026] For industrial-scale production of CO-rich gases, SOECs are typically arranged within SOEC stacks. Multiple stacks can be further arranged into an SOEC system.

[0027] In dry solid oxide electrolysis, the fuel gas stream should be dry, meaning it contains only about 0-1 vol% H2O. This is to avoid the reaction of water with carbon monoxide to form hydrogen and carbon dioxide (water-gas shift reaction). Separating CO from a mixture of CO2, CO, H2O, and H2 is much more difficult than separating CO from a mixture of CO and CO2 (see, for example, WO18228716). Furthermore, it is preferred that the fuel gas stream contains only about 0-1 vol% H2. This is to avoid the reaction of hydrogen with carbon dioxide to form 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 CO and CO2. The fuel gas may include small amounts of other non-reactive components.

[0028] It is generally preferred to maintain a fairly uniform temperature within the battery, between batteries (i.e., within the battery stack), and between battery stacks (i.e., within a system for high-temperature CO2 electrolysis). However, in practice, small temperature gradients may exist between and within the batteries. For example, the temperature variation within the battery may be 25°C, 30°C, 40°C, or even 50°C. In particular, without departing from the present invention, the difference between the inlet and outlet temperatures can be as high as 50°C, or even as high as 75°C or 100°C. The inlet temperatures of the fuel gas flow and the flushing gas flow can also vary. If the flushing gas and the fuel gas have different inlet temperatures, or if the inlet temperatures vary between different inlets, then T should use the average inlet temperature (i.e., the arithmetic mean of the inlet temperatures). This applies to all aspects of the present invention.

[0029] It should be understood that in SOEC, the fuel side and oxygen side must be in ion contact via a solid electrolyte in order to operate.

[0030] In a second aspect of the invention, a method is provided for selecting operating conditions for high-temperature, dry CO2 electrolysis in a solid oxide electrolyzer (SOEC), the SOEC having a fuel side and an oxygen side with ion contact via a solid electrolyte, the method comprising the following steps:

[0031] i. in 2s -1 up to 30s -1 airspeed SV within the range 燃料 A fuel gas stream is supplied to the fuel side of the SOEC, wherein the fuel gas stream comprises 70-100 vol% CO2 and 0-30 vol% CO, wherein the mole fraction of CO (x CO Within the range of 0 to 0.3;

[0032] ii. in 0.1s -1 up to 20s -1 airspeed SV within the range 冲洗Supply flushing gas stream to the oxygen side of SOEC

[0033] ii. By heating the fuel and flushing gas streams to a gas stream inlet temperature T ranging from 600°C to 1000°C, heat is supplied to the SOEC, and then...

[0034] iii. at -0.2A / cm 2 Up to -1A / cm 2 The current density i within the range is applied to the electrolyte of SOEC by an electrolytic current.

[0035] T and SV are selected through the following iterative process. 燃料 SV 冲洗 The values ​​of , and i:

[0036] a) T, SV 燃料 SV 冲洗 The operation conditions for , and i are set to initial values;

[0037] b) Determine the local temperature and local gas composition at multiple different locations within the battery;

[0038] c) Estimate local temperatures (local Boudouard temperatures) based on local gas composition, where carbon formation via the Boudouard reaction is thermodynamically favorable at each location below this local temperature.

[0039] d) Subtract the local Boudouard temperature from the measured local temperature to obtain the Boudouard margin, and

[0040] e) Change the gas flow rate, inlet temperature, and / or electrolysis current density until the Boudouard margin at each location is greater than zero.

[0041] This method of selecting operating conditions for high-temperature, dry CO2 electrolysis in solid oxide electrolyzers can be used as a trial-and-error procedure to obtain a set of operating parameters for a given SEOC or SOEC stack. Their combination will provide a method for converting carbon dioxide to carbon monoxide in high-temperature, dry solid oxide electrolysis outside the Boudouard temperature range, which means that significant coking will not occur.

[0042] Brief description of the attached figures

[0043] Figure 1 This is a schematic diagram of SOEC.

[0044] Figure 2 This is a block diagram showing an implementation scheme for a system that includes SOEC.

[0045] Figure 3This is a schematic diagram of a repeating cell in an SOEC stack.

[0046] Figure 4 a, 4b, 4c, 4d, and 4e describe the coking potential (CP) and various key process parameters (T, SV). 冲洗 SV 燃料 The relationship between %, CO, and i) demonstrates the effect of each parameter under other fixed conditions.

[0047] Figure 5 It shows based on V 燃料 V 氧气 And the assumptions about the effective area of ​​the battery, for various possible SV 燃料 SV 冲洗 Combined with i, an exemplary junction coke potential is calculated using equation (I). In case #1, V 燃料 V 氧气 The effective area (A) value of the battery is taken from Example 1 of this application.

[0048] Detailed description of this disclosure

[0049] This paper discloses a method for converting carbon dioxide into carbon monoxide in a high-temperature, dry solid oxide electrolysis process as defined above.

[0050] In this context, the term "inlet temperature" refers to the temperature near the location where the gas enters the battery or battery stack, such as the temperature in the gas manifold below the battery or battery stack. Experimentally, the inlet temperature can be easily determined using a thermocouple. If the flushing gas and fuel gas have different inlet temperatures, or if the inlet temperature varies between different inlets, the average inlet temperature (i.e., the arithmetic mean of the inlet temperatures) should preferably be used.

[0051] The term "purge gas" refers to the gas supplied to the oxygen side of an SOEC electrolyzer or electrolyzer stack. The purge gas is used to remove oxygen formed during the anodic reaction from the electrolyzer or electrolyzer stack. The purge gas flow can further be used to stabilize the temperature gradient within the electrolyzer or electrolyzer stack.

[0052] The term "fuel gas" is intended to refer to the gas supplied to the fuel side of an SOEC electrolyzer or electrolyzer stack. In this context, when the SOEC operates in dry CO2 electrolysis mode (i.e., converting CO2 to CO and O2), the fuel gas includes CO2 and optionally CO. Additionally, it may include trace amounts of other components, such as H2, N2, and H2O, totaling less than 5%.

[0053] The term "product gas" is intended to refer to the gas leaving the SOEC electrolyzer or electrolyzer stack on the fuel side. In this context, when SOEC operates in dry CO2 electrolysis, the product gas includes CO and CO2. As long as current flows through the SOEC electrolyzer or electrolyzer stack, the CO content in the product gas will be higher than the CO content in the fuel gas. In addition, the product gas may also include trace amounts of other components, such as H2, N2, and H2O, in total amounts not exceeding 5%.

[0054] "Space velocity" is a widely used parameter to describe the gas flow rate through a reactor (such as an SOEC or SOEC stack). As used herein, space velocity SV is defined as the reactor volume of feed passing through the reactor per unit time. Therefore, "flushing gas space velocity" refers to the volumetric flow rate of the flushing gas divided by the total volume of the oxygen-side compartments of the SOEC electrolyzer or stack, and "fuel gas space velocity" refers to the volumetric flow rate of the fuel gas divided by the total volume of the fuel-side compartments of the SOEC electrolyzer or stack. The volumetric flow rate of the gas can be easily determined using a gas flow meter or rotor flow meter, and the values ​​given herein are measurements taken under normal conditions (0°C, 1 atm). The total volume of the fuel-side (cathode) and oxygen-side (anode) compartments of the SOEC electrolyzer or stack can be determined from the technical drawings. Alternatively, the total volume of the fuel compartments in the SOEC stack (V0) is used to describe the gas flow rate. 燃料 It can be estimated based on the following equation:

[0055] V 燃料 =W·L·H 燃料.av ·n

[0056] Where W is the width of the effective SOEC area, L is the length of the effective SOEC area, and H... 燃料,av This is the average height of the gas passages on the fuel side, and n is the number of gas passages in the reactor. If a single SOEC is being tested, then n equals 1. Similarly, the total oxygen-side volume (V0) of the SOEC... 氧气 It can be determined according to the following equation:

[0057] V 氧气 =W·L·H 氧气.av ·n

[0058] Where H 氧气,av This is the average height of the oxygen-side gas channel. W, L, H 燃料,av and H 氧气,av Geometric equations can be used for measurement or calculation.

[0059] In this context, the term "mole fraction" (x) refers to the amount of gaseous substance in a gas. COx is the number of moles of a gaseous substance in a gas mixture divided by the total number of moles in the gas mixture. For example, when the mole fraction of CO in a fuel gas is 5 mol%, then x CO =0.05.

[0060] The term "electrolysis current density" is defined as the total electrolysis current flowing through the electrolytic cell or electrolytic cell stack divided by the effective area of ​​the SOEC. The "effective area of ​​the SOEC" refers to the geometric region of the electrochemically active SOEC, i.e., the geometric region of the SOEC participating in the electrochemical reaction. For an SOEC with a rectangular effective area, the effective area can be estimated by multiplying W by L.

[0061] In the context of this invention, the term "inert" refers to a gaseous substance that does not participate in the chemical or electrochemical reactions in SOEC at the relevant temperature. Inert substances typically include nitrogen, argon, helium, etc. In some cases, such as when an inert gas is used as a purging gas, the inert gas may also include CO2, air, vapor, etc.

[0062] In this context, "local" means smaller than 12x12x12cm 3 Preferably smaller than 1x1x1cm 3 And more preferably, less than 0.1 x 0.1 x 0.1 cm 3 The volume.

[0063] Typical operating temperatures for SOECs range from approximately 600°C to 1000°C: high temperatures are required to achieve sufficient oxide ionic conductivity in the ceramic membrane used as the electrolyte. Commonly used electrolyte materials include stabilized zirconium oxide, such as yttrium-stabilized zirconium oxide (YSZ), doped cerium oxide, and doped lanthanum gallate. Commonly used oxygen electrode materials include perovskite materials, such as Sr-doped LaMnO3 (LSM), Sr-doped LaFeO3 (LSF), Sr-doped LaCoO3 (LSC), Sr-doped La(Co,Fe)O3 (LSCF), and Sr-doped SmCoO3. Perovskite materials are often further mixed with doped cerium oxide to form a composite oxygen electrode (SOEC anode). Dopants other than Sr, such as Ca and Ba, and materials other than perovskite, such as the Ruddlesden-Popper phase, are known. Commonly used fuel electrode materials include composites of metallic Ni and stabilized zirconium oxide, such as Ni-YSZ, or metallic Ni and doped cerium oxide.

[0064] The inventors have now discovered an operating window for SOEC stacks in CO2 electrolysis that allows the SOEC stack to operate safely at the highest possible CO / CO2 ratio. Specifically, the inventors have defined the coking potential (CP) of the stack, and they have further discovered that the coking potential (CP) can serve as a reference for T, SV. 冲洗SV 燃料 x CO The function of i is estimated and is defined by the following equation (I):

[0065]

[0066] In one implementation, the SOEC or SOEC stack operates under conditions of CP ≤ -50. This provides a high CO2 to CO conversion rate and a long SOEC lifetime of over one year.

[0067] Simultaneously, economic profitability is maximized when the SOEC stack operates with a CP ≤ -15. This provides a high CO2-to-CO conversion rate and a long SOEC lifetime exceeding one year, while also offering high profitability. In one embodiment of the invention, the CP is in the range of -75 to -15. There is no practical lower limit for CP. However, the lower limit of CP can be -100, -80, or -75. Equation (I) is determined empirically at atmospheric pressure. Therefore, empirical equation (I) is applicable to pressures close to atmospheric pressure, at least for absolute pressures from 0.5 bar to 2 bar, for example, from 0.7 bar to 1.8 bar.

[0068] In one embodiment, the fuel gas stream comprises 80-100 vol%, for example, 88-98 vol% CO2. In another embodiment, the fuel gas stream comprises 0-20 vol%, for example, 1-12 vol% CO. In yet another embodiment, the fuel gas stream comprises 80-100 vol%, for example, 88-98 vol% CO2 and 0-20 vol%, for example, 1-12 vol% CO. In one embodiment, the mole fraction of CO in the fuel gas stream (x...) CO The concentration 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 comprises 15-95 vol% CO, for example, 15-90 vol%, 20-80 vol%, 20-70 vol%, 20-60 vol%, 20-50 vol% CO or 30-50 vol% CO. In one embodiment, any residual gas present in the fuel gas stream is an inert gas (e.g., N2 or a rare gas).

[0069] In one 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, wherein the mole fraction of CO (x) COThe concentration is in the range of 0 to 0.2, for example, 0.01 to 0.15 or 0.05 to 0.1. The advantage of such a fuel gas stream is that a product gas stream can be obtained, which includes CO in the range of 20 vol% to 50 vol% and low contents of H2O and H2.

[0070] 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, wherein the mole fraction of CO (x) CO The concentration is in the range of 0 to 0.2. The advantage of such a fuel gas stream is that a product gas stream can be obtained, which includes CO in the range of 20 vol% to 50 vol% and low contents of H2O and H2.

[0071] In another implementation, formula (I) is used to select T and SV. 燃料 SV 冲洗 X CO The initial settings of i satisfy CP = _15, and then proceed according to the guide for selecting operating parameters as described in claim 15 to initiate the method for selecting operating conditions for a specific SOEC stack design.

[0072] Detailed description of the attached figures

[0073] Figure 1 This is a schematic diagram of an SOEC. The SOEC (1) includes a fuel side (11), an oxygen ion conducting electrolyte (12), and an oxygen 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 a potential supplied by a power supply unit (20). The resulting product gas stream (102) has a higher CO content than the fuel gas (i.e., CO-rich), and this CO-rich product gas stream is collected from the fuel side of the SOEC.

[0074] The current carrying oxygen ions passes through the electrolyte (103) and reaches the oxygen side of the SOEC (13). On the oxygen side of the SOEC, an anodic reaction occurs to oxidize ionic oxygen. A flushing gas (104) is used to transport the oxygen formed in the anodic reaction out of the SOEC. As a result of the electrochemical reaction on the oxygen side, the flushing gas stream (104) becomes oxygen-rich. The oxygen-rich flushing gas stream (105) is collected from the oxygen side of the SOEC.

[0075] Figure 2This is a block diagram showing an embodiment of the system (2) including an SOEC. A gas stream (201) is optionally mixed with a CO2-rich 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. A CO-rich product gas stream (102) is collected from the fuel side of the SOEC. The product gas stream (102) may optionally be supplied to a gas purification unit (30), in which the product gas stream (102) is separated into a CO-rich first product gas stream (202) and a CO2-rich second product gas stream (203). The CO2-rich product gas stream (203) may optionally be recycled back to the fuel side of the SOEC, as described above.

[0076] A flushing gas stream (104) is supplied to the oxygen side (13) of the SOEC (10). As a result of the electrochemical reaction on the oxygen side, the flushing gas stream (104) becomes oxygen-rich. The oxygen-rich flushing gas stream (105) is collected from the oxygen side of the SOEC.

[0077] Figure 3 This is a schematic diagram of a repeating unit in an SOEC reactor. More specifically, it shows how interconnects (40) are positioned between adjacent SOECs (10). In the figure, the oxygen side of the SOEC faces upward, while the fuel side faces downward. The width W and length L of the SOEC are schematically shown. Two different arbitrary interconnect geometries are shown. Figure 3 In section a, the fuel-side gas passage H 燃料,av Average height and oxygen-side gas passage H 氧气,av The average height is lower than Figure 3 b, this is due to the geometric differences between the interconnect designs in the two figures.

[0078] Figure 4 a, 4b, 4c, 4d, and 4e describe the coking potential (CP) and various key process parameters (T, SV). 冲洗 SV 燃料 The relationship between each of , %CO, and i) demonstrates the effect of each parameter under other fixed conditions. Figure 4 A more detailed explanation is given in Examples 4-8.

[0079] The method is described in more detail in the following non-limiting embodiments. Example

[0080] Example 1

[0081] The SOEC stack is operated by dry CO2 electrolysis, under the condition that coking is not expected to occur based on thermodynamic considerations, but the coking potential is positive.

[0082] More specifically, an electric heater is used to heat the fuel gas (food-grade CO2), thereby raising the inlet temperature (T) to 725°C. The fuel gas composition is ≥99.9% CO2, and the CO content is at the ppm level (i.e., x). CO =0). The SOEC stack consists of 75 cells connected in series, each with an effective area of ​​108 cm². 2 Fuel side compartment (V 燃料 The total volume is 243 cm³. 3 The oxygen-side compartment (V 氧气 The total volume is 405 cm³. 3 The fuel gas was delivered at a rate of 1.1 Nm. 3 A volumetric flow rate of / h is supplied to the stack, which corresponds to 1.26 s⁻¹. -1 fuel gas space velocity (SV) 燃料 Air is used as the flushing gas on the oxygen side of the stack, and at 1.5 Nm³. 3 A volumetric flow rate of / h is supplied to the stack, which corresponds to 1.03s. -1 flushing gas space velocity (SV) 冲洗 The stack current is fixed at -40A, which corresponds to -0.37A / cm. 2 The electrolysis current density (i). Under these conditions, 60% of the CO2 supplied to the pile is converted into CO, which corresponds to a CO / CO2 ratio of 60 / 40 = 1.5 near the pile outlet.

[0083] In a dry CO / CO2 mixture at 725°C, coking is thermodynamically favorable when the CO / CO2 ratio is above 2.47, corresponding to a CO2 conversion rate of 71.2%. In other words, based on thermodynamic considerations, carbon should not form inside the pile. However, severe coking occurred in the pile, leading to failure after only 12.5 hours of operation.

[0084] Under the above conditions, the coking potential (CP) was 72.3, which is indeed very favorable for coking in the pile, thus explaining why the pile failed after only a few hours of testing. Post-test analysis using Raman spectroscopy also confirmed carbon formation in the pile.

[0085] Example 2

[0086] The SOEC stack is operated by dry CO2 electrolysis, provided that no coking is expected based on thermodynamic considerations and the coking potential is negative, i.e., according to the method of the present invention.

[0087] More specifically, an electric heater is used to heat the fuel gas (food-grade CO2), thereby raising the inlet temperature (T) to 750°C. The fuel gas is a mixture of food-grade CO2 and CO. The CO content in the fuel gas is 3.6% (i.e., x CO =0.036). The SOEC stack consists of 75 cells connected in series, each with an effective area of ​​108 cm². 2 Fuel side compartment (V 燃料 The total volume is 243 cm³. 3 The oxygen-side compartment (V 氧气 The total volume is 405 cm³. 3 The fuel gas was delivered at 8 Nm 3 A volumetric flow rate of / h is supplied to the stack, which corresponds to 9.14 s. -1 fuel gas space velocity (SV) 燃料 A mixture of air and CO2 was used as the flushing gas on the oxygen side of the reactor, at a concentration of 14.3 Nm. 3 A volumetric flow rate of / h is supplied to the stack, which corresponds to 9.81s. -1 flushing gas space velocity (SV) 冲洗 The stack current is fixed at -50A, which corresponds to -0.46A / cm. 2 The electrolysis current density (i). Under these conditions, 19.6% of the CO2 supplied to the pile is converted to CO, corresponding to a CO / CO2 ratio of (19.6+3.6) / (100-19.6-3.6) = 0.30 near the pile outlet.

[0088] In a dry CO / CO2 mixture at 750°C, coking is thermodynamically favorable when the CO / CO2 ratio is above 3.6, corresponding to a CO2 conversion rate of 78.4%. In other words, from a thermodynamic perspective, carbon should not form inside the pile during operation. Under the above conditions, the coking potential (CP) is -59.9, meaning no coking occurs.

[0089] Example 3

[0090] The SOEC stack is operated by dry CO2 electrolysis, provided that no coking is expected based on thermodynamic considerations and the coking potential is negative, i.e., according to the method of the present invention.

[0091] More specifically, an electric heater is used to heat the fuel gas (food-grade CO2), thereby raising the inlet temperature (T) to 745°C. The fuel gas is a mixture of food-grade CO2 and CO. The CO content in the fuel gas is 3.6% (i.e., x CO =0.036). The SOEC stack consists of 75 cells connected in series, each with an effective area of ​​108 cm². 2 Fuel side compartment (V燃料 The total volume is 243 cm³. 3 The oxygen-side compartment (V 氧气 The total volume is 405 cm³. 3 The fuel gas was delivered at 8 Nm 3 A volumetric flow rate of / h is supplied to the stack, which corresponds to 9.14 s. -1 fuel gas space velocity (SV) 燃料 A mixture of air and CO2 was used as the flushing gas on the oxygen side of the reactor, at a concentration of 14.3 Nm. 3 A volumetric flow rate of / h is supplied to the stack, which corresponds to 9.81s. -1 flushing gas space velocity (SV) 冲洗 The stack current is fixed at -70A, which corresponds to -0.65A / cm. 2 The electrolysis current density (i). Under these conditions, 27.4% of the CO2 supplied to the pile is converted to CO, corresponding to a CO / CO2 ratio of (27.4+3.6) / (100-27.4-3.6) = 0.45 near the pile outlet.

[0092] In a dry CO / CO2 mixture at 745°C, coking is thermodynamically favorable when the CO / CO2 ratio is above 3.37, corresponding to a CO2 conversion rate of 77.1%. In other words, from a thermodynamic perspective, carbon should not form inside the pile during operation. Under the above conditions, the coking potential (CP) is -30.5, meaning no coking occurs. Two piles operated under these conditions for more than a year without coking.

[0093] From an economic perspective, operating under the conditions listed in Example 3 is more profitable than operating under the conditions listed in Example 2 because more CO gas can be produced per pile per hour.

[0094] Example 4

[0095] Coking potential (CP) under a set of practical operating conditions was estimated. The purge gas space velocity was fixed at SV. 冲洗 =9s -1 The space velocity of the fuel gas is fixed at SV. 燃料 =7s -1 The mole fraction of CO in the fuel gas is x. CO =0.05, and the electrolysis current density is fixed at i = -0.5 A / cm. 2 The effect of changes in inlet temperature (T) was studied and plotted on... Figure 4In section a, under a selected set 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 reactor at high temperatures, provided that this does not impair the reactor lifetime through other mechanisms (such as interconnect corrosion).

[0096] Example 5

[0097] The CP was estimated under a set of actual operating conditions: the inlet temperature was fixed at T = 700℃, and the fuel gas space velocity was fixed at SV. 燃料 =7s -1 The mole fraction of CO in the fuel gas is x. CO =0.05, and the electrolysis current density is fixed at i = -0.5 A / cm. 2 The study investigated the variation of the flushing gas space velocity (SV). 冲洗 The impact of ) and in Figure 4 Plotting in b. Under the selected set of operating conditions, within 1 second... -1 At that time, CP = 6.9, at 9s -1 At that time, CP = 0.8, and at 29s -1 At that time, CP = -14.2. Generally, the higher the flush gas space velocity, the lower the CP. Therefore, to avoid carbon formation, it is desirable to operate the reactor at high flush gas space velocities, provided that this is not prohibitively expensive.

[0098] Example 6

[0099] The CP was estimated under a set of actual operating conditions: the inlet temperature was fixed at T = 700℃, and the purge gas space velocity was fixed at SV. 冲洗 =9s -1 The mole fraction of CO in the fuel gas is x. CO =0.05, and the electrolysis current density is fixed at i = -0.5 A / cm. 2 The study investigated the alteration of fuel gas space velocity (SV). 燃料 The impact of ) and in Figure 4 Plotting in C. Under a selected set of operating conditions, within 1 second... -1 At that time, CP = 134.9, at 7s -1 At that time, CP = 0.8, and at 13s -1 At that time, CP = -43.8. Generally, the higher the fuel gas space velocity, the lower the CP. Therefore, to avoid carbon formation, it is desirable to operate the reactor at high fuel gas space velocities, provided that this is not prohibitively expensive.

[0100] Example 7

[0101] The CP was estimated under a set of actual operating conditions: the inlet temperature was fixed at T = 700℃, and the purge gas space velocity was fixed at SV. 冲洗 =9s -1 The space velocity of the flushing gas is fixed at SV. 冲洗 =9s -1 Furthermore, the electrolysis current density is fixed at i = -0.5 A / cm. 2 The study investigated the variation of CO mole fraction (x) in fuel gas. CO The impact of ) and in Figure 4 Plot in d. Under the selected set of operational conditions, in x CO When x = 0.01, CP = -24.6. CO When x = 0.05, CP = 0.8, and at x CO When the CO molar fraction is 0.10, the CP is 2.6. Generally, the higher the molar fraction of CO in the fuel gas, the higher the CP. Therefore, to avoid carbon formation, it is desirable to operate the reactor at a low CO molar fraction, provided that the Ni in the fuel-side inlet of the reactor remains in a metallic state.

[0102] Example 8

[0103] The CP was estimated under a set of actual operating conditions: the inlet temperature was fixed at T = 700℃, and the purge gas space velocity was fixed at SV. 冲洗 =9s -1 The space velocity of the flushing gas is fixed at SV. 冲洗 =9s -1 And the mole fraction of CO in the fuel gas is x CO =0.05. The effect of changing the electrolysis current density (i) was studied, and... Figure 4 Plotting in e. Under the selected set of operating conditions, at -0.1 A / cm. 2 At that time, CP = -69.8, at -0.5 A / cm 2 At that time, CP = 0.8, and at -1.5 A / cm 2 At that time, CP = 81.8. Generally, the more negative the electrolysis current density, the higher the CP. Therefore, to avoid carbon formation, it is desirable to operate the stack at low electrolysis current densities (i.e., near-zero current). However, stack productivity is directly proportional to the absolute value of the electrolysis current, so high current densities (i.e., more negative currents) are needed to ensure commercially relevant benefits.

[0104] Example 9

[0105] The SOEC stack, consisting of 75 cells, operates at an average temperature of 700°C. Pure CO2 is supplied to the cathode at a flow rate of 100 Nl / min, while an electrolysis current of 50 A is applied, converting CO2 into CO. This corresponds to 26% CO and 74% CO2 in the gas leaving the cathode side of the stack. This corresponds to the disclosure in Example 1 of WO 2018 / 206235.

[0106] The above embodiments describe the operating point of an SOEC reactor in dry CO2 electrolysis. The operating temperature (T) is 700°C, and the mole fraction of CO in the fuel gas stream (x) is... CO The value is 0. However, D1 does not disclose geometric information about the heap in order to make SV... 燃料 SV 冲洗 The estimation of SV becomes possible. For example, although the value of the volumetric flow rate of the fuel gas (100 Nl / min) is provided, in order to estimate SV... 燃料 The value requires knowing the total volume (V) of the fuel-side compartment. 燃料 Similarly, although a stack current value (50A) is provided, to estimate the value of i, the effective area of ​​each cell (and whether the cells are connected in series or parallel) needs to be known. Finally, in Example 1 of WO 2018 / 206235, there is no mention whatsoever of the flushing gas flow rate or space velocity, or even whether flushing gas is used.

[0107] exist Figure 5 Therefore, based on V 燃料 V 氧气 Based on the assumption of the effective area of ​​the battery, many possible SVs were calculated. 燃料 SV 冲洗 The junction potential of i combined with i.

[0108] In Case #1, V 燃料 V 氧气 The effective battery area (A) values ​​are taken from Embodiment 1 of this application. To avoid doubt, it should be emphasized that these do not represent preferred embodiments, and the SV and battery area mentioned in Embodiment 1 of WO 2018 / 206235 do not represent preferred embodiments either.

[0109] Cases #2 to #4 represent V 燃料 The changes led to SV 燃料 Changes. By adding SV 燃料 This can prevent coking. At low SV 燃料 Under these conditions, coking occurs.

[0110] Cases #5 and #6 represent changes in the effective battery area compared to #1. Increasing the effective battery area can prevent coking. Decreasing the effective battery area will cause coking.

[0111] Cases #7 and #8 represent changes in flushing gas flow rate. Increasing the flushing gas flow rate has a small positive effect, slightly reducing the likelihood of coking.

[0112] Cases #9 and #10 represent higher SV. 燃料 The change in the flushing gas flow rate. Increasing the flushing gas flow rate has a small positive effect, slightly reducing the likelihood of coking. However, coking still occurred.

[0113] In summary, these data clearly demonstrate that insufficient information was disclosed in Example 1 of WO 2018 / 206235 to incidentally predict the subject matter of this patent application. No suggestions or recommendations are given in this regard.

[0114] Implementation plan:

[0115] Implementation Scheme 1. A method for converting carbon dioxide into carbon monoxide in high-temperature, dry solid oxide electrolysis, the method comprising the following steps:

[0116] 1) Provide a fuel gas stream containing 70-100 vol% CO2 and 0-30 vol% CO, wherein the mole fraction of CO (x) CO Within the range of 0 to 0.3;

[0117] 2) Provide flushing gas flow;

[0118] 3) Provide a solid oxide electrolyzer stack (SOEC stack) comprising multiple solid oxide electrolyzers (SOECs), each electrolyzer having a fuel side and an oxygen side for ion contact via a solid electrolyte, and multiple SOECs being electrically connected in series and fluidly connected in parallel and separated by interconnects;

[0119] 4) Heat the fuel gas stream to 600-1000℃, for example, a fuel gas inlet temperature T in the range of 700℃ to 850℃;

[0120] 5) In 1-30s -1 airspeed SV within the range 燃料 The fuel gas stream is supplied to the fuel side of the SOEC;

[0121] 6) Within 0.1-20s -1 airspeed SV within the range 冲洗 The flushing gas stream is supplied to the oxygen side of the SOEC;

[0122] 7) Apply a current density i of -0.2 A / cm² to the solid electrolyte. 2 Up to -1A / cm 2The electrolysis current within the range is used to electrolyze a portion of CO2 into CO on the fuel side of the electrode side, and to generate an O2-rich flushing gas on the oxygen electrode side of the SOEC.

[0123] Claims 2-14 can be equivalently combined with embodiment 1.

[0124] Implementation Scheme 2. A method for converting carbon dioxide into carbon monoxide in high-temperature, dry solid oxide electrolysis, the method comprising the following steps:

[0125] 1) Provide a fuel gas stream containing 70-100 vol% CO2 and 0-30 vol% CO, wherein the mole fraction of CO (x) CO Within the range of 0 to 0.3;

[0126] 2) Provide flushing gas flow;

[0127] 3) Provide multiple solid oxide electrolyzer stacks (SOEC stacks) comprising multiple solid oxide electrolyzers (SOECs), each electrolyzer having a fuel side and an oxygen side for ion contact via a solid electrolyte, and the multiple SOECs arranged in the SOEC stack, the SOECs being electrically connected in series and fluidly connected in parallel and separated by interconnects; and 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;

[0128] 4) Heat the fuel gas stream to 600-1000℃, for example, a fuel gas inlet temperature T in the range of 700℃ to 850℃;

[0129] 5) In 1-30s -1 airspeed SV within the range 燃料 The fuel gas stream is supplied to the fuel side of the SOEC;

[0130] 6) Within 0.1-20s -1 airspeed SV within the range 冲洗 The flushing gas stream is supplied to the oxygen side of the SOEC;

[0131] 7) Apply a current density i of -0.2 A / cm² to the solid electrolyte. 2 Up to -1A / cm 2 An electrolysis current within a certain range is used to electrolyze a portion of CO2 into CO on the fuel side of the electrode side, and to generate an O2-rich flushing gas on the oxygen electrode side of the SOEC.

[0132] Claims 2-14 can be equivalently combined with embodiment 2.

[0133] Reference Symbol List

[0134] 1: SOEC

[0135] 2: SOEC

[0136] 10: SOEC

[0137] 11: Fuel side

[0138] 12: Oxygen ion conduction electrolyte

[0139] 13: Oxygen side

[0140] 20: Power Supply Unit

[0141] 30: Gas purification unit

[0142] 40: Interconnectors

[0143] 101: Fuel gas / fuel gas flow

[0144] 102: Product gas flow

[0145] 103: Electrolytes

[0146] 104: Flushing gas / flushing gas flow

[0147] 105: Oxygen-rich flushing gas stream

[0148] 201: Gas Flow

[0149] 202: CO-rich product gas stream

[0150] 203: CO2-rich gas stream

Claims

1. A method for converting carbon dioxide into carbon monoxide in high-temperature, dry solid oxide electrolysis, the method comprising the following steps: 1) Provide a fuel gas stream containing 70-100 vol% CO2 and 0-30 vol% CO, wherein the mole fraction of CO is ( x CO Within the range of 0 to 0.3; 2) Provide a flushing gas flow; 3) Provide solid oxide electrolyzers (SOECs) with fuel and oxygen sides; 4) Heat the fuel gas stream and the flushing gas stream to a gas inlet temperature of 600°C to 1000°C. T ; 5) In 2 s -1 up to 30 s -1 airspeed within the range SV 燃料 The fuel gas stream is supplied to the fuel side of the SOEC; 6) In 0.1 s -1 up to 20 s -1 airspeed within the range SV 冲洗 The flushing gas stream is supplied to the oxygen side of the SOEC; 7) Apply a current density to the solid electrolyte. i -0.2 A / cm 2 Up to -1 A / cm 2 The electrolysis current is used to electrolyze a portion of the CO2 into CO on the fuel side of the SOEC, and to generate an O2-rich flushing gas on the oxygen side of the SOEC. Among them, the choice x CO , T , SV 燃料 , SV 冲洗 ,and i This causes the coking potential to be lower than the coking potential. CP ≤ -15, where CP is given by equation (I): 。 2. The method according to claim 1, wherein the coking potential CP of formula (I) during SOEC operation is in the range of -100 to -15.

3. The method according to claim 1 or 2, wherein the product gas stream comprises CO in the range of 15-95 vol%.

4. The method according to claim 1 or 2, wherein the fuel gas stream comprises 80-100 vol% CO2, 0-20 vol% CO, 0-1 vol% H2O, and 0-1 vol% H2, the remainder being inert, wherein the mole fraction of CO is ( x CO () in the range of 0 to 0.

2.

5. The method according to claim 1 or 2, wherein the fuel gas stream comprises 88-98 vol% CO2, 1-12 vol% CO, 0-1 vol% H2O, and 0-1 vol% H2, the remainder being inert, wherein the mole fraction of CO is ( x CO () in the range of 0 to 0.

2.

6. The method according to claim 1 or 2, wherein the flushing gas comprises air, dry air, O2, CO2, N2, steam, or a mixture thereof.

7. The method according to claim 1 or 2, wherein the solid oxide electrolyzer includes a fuel gas inlet leading 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 claim 1 or 2, wherein the solid oxide electrolyzer includes a flushing gas inlet leading to the oxygen side of the SOEC and a flushing gas outlet from the oxygen side of the SOEC.

9. The method according to claim 1 or 2, wherein an oxygen-rich flushing gas stream is collected from the oxygen side of the SOEC.

10. The method according to claim 1 or 2, wherein a CO-rich product gas stream is collected from the fuel side of the SOEC.

11. The method of claim 10, further comprising the step of separating the product gas stream into a first gas stream rich in CO and a second gas stream rich in CO2.

12. The method of claim 11, wherein the CO2-rich second stream is recycled to the fuel side of the SOEC.

13. The method of claim 1 or 2, wherein the fuel side of the SOEC comprises metallic nickel electrically connected to a power source.

14. The method according to claim 1 or 2, wherein the fuel gas stream and the flushing gas stream are heated to a gas stream inlet temperature of 700°C to 850°C. T .

15. The method according to claim 1 or 2, wherein the coking potential CP of formula (I) during the SOEC operation is in the range of -80 to -15.

16. The method according to claim 1 or 2, wherein the coking potential CP of formula (I) during SOEC operation is in the range of -60 to -15.

17. The method according to claim 1 or 2, wherein the product gas stream comprises 15-90 vol% CO.

18. The method according to claim 1 or 2, wherein the product gas stream comprises 20-80 vol% CO.

19. The method according to claim 1 or 2, wherein the product gas stream comprises 20-70 vol% CO.

20. The method according to claim 1 or 2, wherein the product gas stream comprises 20-60 vol% CO.

21. The method according to claim 1 or 2, wherein the product gas stream comprises 20-50 vol% CO.

22. The method according to claim 1 or 2, wherein the product gas stream comprises 30-50 vol% CO.

23. A method for selecting operating conditions for high-temperature, dry CO2 electrolysis in a solid oxide electrolyzer (SOEC), the SOEC having a fuel side and an oxygen side with ion contact via a solid electrolyte, the method comprising the following steps: i. in 2 s -1 up to 30 s -1 airspeed within the range SV 燃料 A fuel gas stream is supplied to the fuel side of the SOEC, wherein the fuel gas stream comprises 70-100 vol% CO2 and 0-30 vol% CO, wherein the mole fraction of CO is ( x CO Within the range of 0 to 0.3; ii. in 0.1 s -1 up to 20 s -1 airspeed within the range SV 冲洗 A flushing gas stream is supplied to the oxygen side of the SOEC; iii. By heating the fuel gas stream and flushing gas stream to a gas stream inlet temperature in the range of 600°C to 1000°C. T Heat is supplied to the SOEC, and then iv. Apply a current density to the electrolyte of the SOEC. i -0.2 A / cm 2 Up to -1 A / cm 2 Electrolysis current, T and SV are selected through the following iterative process. 燃料 SV 冲洗 The values ​​of , and i: a) The T, SV 燃料 SV 冲洗 、 and i The operation conditions are set to initial values; b) Determine the local temperature and local gas composition at multiple different locations within the electrolytic cell; c) Estimate the local temperature based on the local gas composition. The estimated local temperature is the local Boudouard temperature, below which carbon formation via the Boudouard reaction is thermodynamically favorable for each location. d) Subtract the local Boudouard temperature from the local temperature in step b) to obtain the Boudouard margin, and e) Change the gas flow rate, inlet temperature, and / or electrolysis current density until the Boudouard margin at each location is greater than zero.

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