A METHOD FOR SUPPLYING OXYGEN-ENRICHED GAS TO AN OXYGEN-CONSUMING PROCESS

Abstract

In a method for supplying oxygen-enriched gas to an oxygen consuming process, in which the oxygen-enriched gas with a low nitrogen content is generated by supplying an anode-side feed gas comprising CO.sub.2 to the anode side of a solid oxide electrolysis cell, oxygen is generated on the anode side of the solid oxide electrolysis cell. This way, an anode-side product gas is formed, in which the oxygen-enriched gas comprises at least a part. The oxygen-enriched gas has a low nitrogen content, and the temperature of the oxygen-enriched gas exiting the solid oxide electrolysis cell is between 600 and 1000° C. The method has multiple advantages, first of all as regards energy saving.

Claims

1. A method for supplying oxygen-enriched gas to an oxygen-consuming process that is an oxy-calcining process, in which at least one operating solid oxide electrolysis cell is provided having a cathode side and an anode side, and a) a cathode-side feed gas stream comprising steam or CO.sub.2 or a mixture thereof is supplied to the cathode side of the at least one solid oxide electrolysis cell, b) at least part of the cathode-side feed gas stream is electrochemically reduced in the solid oxide electrolysis cell, thereby forming a cathode-side product gas stream that is enriched in hydrogen, carbon monoxide or a mixture thereof, c) at least part of the cathode-side product gas stream is supplied to a hydrogen- and/or carbon monoxide-consuming process, d) an anode-side feed gas stream comprising CO.sub.2 is supplied to the anode side of the solid oxide electrolysis cell, and e) oxygen is electrochemically generated on the anode side of the solid oxide electrolysis cell, thereby forming an anode-side product gas stream enriched in oxygen, wherein an oxygen-enriched gas comprising at least part of the anode-side product gas stream enriched in oxygen is fed to the oxygen-consuming process, the oxygen-enriched gas has a low nitrogen content, the content of nitrogen being below 10 vol %, and the oxygen-enriched gas exiting the solid oxide electrolysis cell has a temperature in the range of between 600° C., and 1000° C.

2. The method according to claim 1, wherein the temperature of the oxygen-enriched gas exiting the at least one solid oxide electrolysis cell is between 600° C., and 900° C.

3. The method according to claim 1, wherein the hydrogen or carbon monoxide or the mixture of hydrogen and carbon monoxide is electrochemically generated on the cathode-side of the at least one solid oxide electrolysis cell and the oxygen electrochemically generated on the anode-side of the at least one solid oxide electrolysis cell are generated at a molar ratio of (H2+CO):O2 of 2:1.

4-6. (canceled)

7. The method according to claim 1, wherein the anode-side product gas enriched in oxygen has an oxygen content of 0<O.sub.2≤100%.

8. The method according to claim 1, wherein at least part of the anode-side product gas is recycled and used as at least part of the anode-side feed gas.

9. The method according to claim 1, wherein the at least part of the cathode-side product gas is recycled and used as at least part of the cathode-side feed stream.

10. The method according to claim 1, wherein a flue gas stream is obtained from the oxy-calcining process comprising carbon dioxide and is recycled and used as at least part of the cathode-side feed stream and/or the anode-side feed gas that is fed to the solid oxide electrolysis cell.

11. The method according to claim 1, wherein the hydrogen- and/or carbon monoxide-consuming process includes methanol production processes, ammonia production processes, hydrotreating processes, methanation processes, hydrogenation processes, carbonylation processes, hydroformulation(oxo synthesis) processes, or oxidative carbonylation processes.

12. The method according to claim 1, wherein the cathode-side feed stream comprises CO.sub.2 and at least part of the CO.sub.2 in the cathode-side feed gas stream and/or in the anode-side feed gas stream originates from one or more of the following: metallurgy processes, cement production, carbon capture processes, direct air capture processes and carbon-based fuel combustion processes, including combustion of non-fossil fuels, or other processes where CO.sub.2 is generated in one or more streams.

13. The method according to claim 1, wherein the nitrogen content in the oxygen-enriched stream is less than 1%.

14. The method according to claim 1, wherein the nitrogen content in the oxygen-enriched stream is less than 0.1%.

15. The method according to claim 1, wherein the solid oxide electrolysis cell is operating at thermoneutral voltage or within ±0.2 V/cell from the thermoneutral voltage.

16. A plant comprising: a solid oxide electrolysis cell having an anode side and a cathode side; an oxygen-consuming unit that is an oxy-calcining unit; and a hydrogen- and/or carbon monoxide-consuming unit, wherein the anode side of the cell is in fluid connection with the oxygen-consuming unit and the cathode side of the cell is in fluid connection with the hydrogen- and/or carbon monoxide-consuming unit, and wherein the plant is configured to operate the method according to claim 1.

17. The plant according to claim 16, wherein the oxygen-consuming unit and the hydrogen- and/or carbon monoxide-consuming unit is one and the same oxy-calcining unit.

18. The plant according to claim 16, wherein the plant comprises control means configured to control the flow from the anode side of the solid oxide electrolysis cell to the oxygen-consuming unit.

19. The plant according to claim 16, wherein the plant comprises control means configured to control the flow from the cathode side of the solid oxide electrolysis cell to the hydrogen- and/or carbon monoxide-consuming unit.

Description

EXAMPLE 1 (COMPARATIVE EXAMPLE)

[0057] In FIG. 1, a method (1) according to current state-of-the-art is presented, wherein oxygen-enriched gas (104) is fed to an oxy-calciner (10) and the oxygen-enriched gas (104) originates from a cryogenic air separation unit (11). More specifically, an air stream (101) is fed to a cryogenic air separation unit (11) and is thereby separated into an oxygen-enriched gas stream (102) and an oxygen-deficient gas stream (103). The gas stream (102) is pre-heated (e.g. to around 650° C.) using a preheater (12), and the preheated stream (104) is thereafter fed to the oxy-calciner (10). Simultaneously, a stream of fuel (105) is fed to the oxy-calciner (10), and the stream is optionally preheated (not shown). A stream of solid material comprising calcium carbonate (106) is pre-heated (e.g. to around 650° C.) using a second preheater (13) and the pre-heated stream of solid material (107) is fed to the oxy-calciner (10). Fuel (105) reacts with the oxygen in the oxygen-enriched gas stream (104), and the exothermic combustion reaction raises the temperature in the oxy-calciner (10) to around 900° C., which causes the calcium carbonate in the solid material (107) to decompose into calcium oxide and carbon dioxide in the oxy-calciner (10). A suitable oxy-calciner design for the purpose is a circulating fluidized bed (CFB) calciner. The output stream (108) from the oxy-calciner, comprising calcium oxide, carbon dioxide and steam, is fed to a first separator (14), such as a cyclone, where the stream is separated into a stream comprising calcium oxide (109) and a stream comprising carbon dioxide and steam (110). The stream (110) is further fed to a second separator (15), such as a water knockout vessel, where the stream (110) is separated into a stream comprising H.sub.2O (111) and a stream comprising carbon dioxide (112).

[0058] An oxy-calciner with a capacity to calcine 300 tonnes calcium carbonate per hour requires approximately 13 tonnes methane or natural gas per hour as fuel and approximately 60 tonnes oxygen-enriched gas per hour as oxidant, assuming the oxygen content in the oxygen-enriched stream (102,104) is approximately 95%. The main impurity in the oxygen-enriched stream (102,104) originating from an air separation unit (11) is nitrogen. The higher the required oxygen content in the oxygen-enriched stream (102), the lower is the efficiency of the air separation unit (11). Output streams from a 300 tonnes/h oxy-calciner are, for example, a stream comprising calcium oxide (109) with a flow rate of 165 tonnes/hr, a stream comprising H.sub.2O (111) with a flow rate of 30 tonnes/hr, and a stream comprising carbon dioxide (112) with a flow rate of approximately 170 tonnes/hr. The composition of the stream comprising carbon dioxide (112) is for example 97% CO.sub.2, 1% O.sub.2, 1.5% N.sub.2 and 0.01% H.sub.2O. Although the pre-heaters (12,13) and separator units (14,15) are shown in FIG. 1 as separate units, pre-heating and separation may be carried out in units that combine the functions of pre-heaters and the functions of separators, e.g. in solid-gas cyclones. For example, the pre-heating of oxygen-enriched stream (102) may be carried out in a cyclone in the presence of calcium oxide product from the oxy-calciner.

EXAMPLE 2

[0059] In FIG. 2, a preferred embodiment of the method (2) according to the invention is presented, wherein oxygen-enriched gas (206) is fed to an oxy-calciner (10), the oxygen-enriched gas (206) is obtained by flushing the anode (oxy) side (17A) of at least one operating solid oxide electrolysis cell (17) with a feed gas (205) comprising CO.sub.2, and wherein at least part of the first cathode-side product gas (203) comprising carbon monoxide and/or hydrogen, is supplied to a hydrogen- and/or carbon monoxide-consuming process (18). More specifically, a first cathode-side feed stream (201), comprising water or steam or CO.sub.2 or a mixture thereof, is pre-heated using a cathode-side preheater (16) and the preheated first cathode-side feed stream (202) is thereafter fed to the cathode side (17C) of at least one solid oxide electrolysis cell (17).

[0060] Simultaneously, a first anode-side feed gas comprising carbon dioxide (204) is pre-heated using an anode-side preheater (19) and the pre-heated first anode-side feed gas (205) is fed to the anode (oxy) side (17A) of the solid oxide electrolysis cell. External voltage is applied to the solid oxide electrolysis cell (17), thereby providing a driving force for the electrochemical reduction of at least part of the carbon dioxide and/or steam in the first cathode-side feed stream (202) into carbon monoxide and/or hydrogen, thereby forming a first cathode-side product gas (203) that is enriched in hydrogen, carbon monoxide or a mixture thereof. Part of (not shown) or all of the first cathode-side product gas (203) is supplied to a hydrogen- and/or carbon monoxide-consuming process (18), such as a methanol production process, ammonia production process, a hydrotreating process, a methanation process, a hydrogenation process, a carbonylation process, a hydroformulation (oxo synthesis) process, or an oxidative carbonylation process. The externally applied voltage drives an electrochemical oxidation reaction on the anode side (17A) of the solid oxide electrolysis cell, whereby oxygen ions (O.sup.2−) are converted into molecular oxygen (O.sub.2). The electrochemically generated O.sub.2 is mixed with the pre-heated first anode-side feed gas comprising carbon dioxide (205), thereby forming a first anode-side product gas, the oxygen-enriched gas (206), with a low nitrogen content. The operating temperature of the solid oxide electrolysis cell (17) is generally between 600° C., and 1000° C., and preferably between 600° C., and 900° C. 700° C., and 850° C. Due to the high operating temperature of the solid oxide electrolysis cell, the oxygen-enriched stream (206) does not need to be heated further before being fed into the oxy-calciner (10) but may be passed through heat exchangers (not shown). It is further understood that other aforementioned streams may be passed through heat exchangers for better thermal integration.

[0061] Simultaneously, a stream of fuel (105) is being fed to the oxy-calciner (10) and the stream is optionally preheated (not shown). A stream of solid material comprising calcium carbonate (106) is pre-heated (e.g. to around 650° C.) using a preheater (13), such as a solid-gas cyclone, and the preheated stream of solid material (107) is fed to the oxy-calciner (10). Fuel (105) reacts with the oxygen in the oxygen-enriched stream (206) and the exothermic combustion reaction raises the temperature in the oxy-calciner (10) to around 900° C., which causes the calcium carbonate in the solid material (107) to decompose into calcium oxide and carbon dioxide in the oxy-calciner (10). The output stream (108) from the oxy-calciner, comprising calcium oxide, carbon dioxide and steam is fed to a first separator (14), such as a cyclone, where the stream is separated into a stream comprising calcium oxide (109) and a stream comprising carbon dioxide and steam (110). The stream (110) is further fed to another separator (15), such as a water knockout vessel, where the stream is separated into a stream comprising H.sub.2O (111) and a stream comprising carbon dioxide (112).

[0062] An oxy-calciner with a capacity to calcine 300 tonnes of calcium carbonate per hour requires approximately 13 tonnes of methane or natural gas per hour as fuel and approximately 61 tonnes of oxygen-enriched gas per hour as oxidant, assuming the oxygen content in the oxygen-enriched stream (206) is 95 vol %, balance CO.sub.2. The required flow rate of the first anode-side feed stream (204) depends on the desired oxygen-content in the oxygen-enriched stream (206). In order to feed 61 tonnes of an oxygen-enriched gas (206) comprising 95 vol % O.sub.2 in CO.sub.2 to the oxy-calciner, approximately 4 tonnes of CO.sub.2 have to be supplied to the anode side (17A) of the electrolysis unit (17). The amount of oxygen produced by the solid oxide electrolysis cell (17) is determined by Faraday's law: for producing 57 tonnes per hour of oxygen, the required electric current through the electrolysis unit is approximately 191 000 000 A. Typical electrolysis currents for solid oxide electrolysis cells range from 0.5 A/cm.sup.2 to 1 A/cm.sup.2. The required electrode area for the electrolysis unit (17) under abovementioned conditions ranges therefore between 19100 m.sup.2 and 38200 m.sup.2. Such an electrolysis unit would produce approximately 7 tonnes of hydrogen per hour on the cathode side of the cell, when pure water or steam is used as the first cathode-side feed stream (201) to the cell or approximately 100 tonnes of CO per hour on the cathode side of the cell, when pure CO.sub.2 is used as the first cathode-side feed stream (201) to the cell. Highest system efficiencies are achieved when the electrolysis unit (17) is operated close to the thermoneutral voltage.

[0063] The nitrogen content of the oxygen-enriched gas stream (206) is determined by the nitrogen content in the first anode-side feed gas stream (204). For example, if the first anode-side feed gas stream (204) has a nitrogen content of 1 vol % and the desired oxygen content in the oxygen-enriched stream (206) is 95 vol %, then the resulting nitrogen content in the oxygen-enriched gas stream (206) is approximately 0.05 vol %.

EXAMPLE 3

[0064] In FIG. 3, a preferred embodiment of the method (3) according to the invention is presented, wherein oxygen-enriched gas (312) is fed to an oxy-calciner (10), the oxygen-enriched gas (312) is obtained by flushing the anode (oxy) side (17A) of at least one solid oxide electrolysis cellsolid oxide electrolysis cell (17) with a anode-side feed gas (308,309) comprising CO.sub.2, and wherein at least part of the first cathode-side product gas (304,305), enriched in carbon monoxide and/or hydrogen, is supplied to a hydrogen- and/or carbon monoxide-consuming process (18), and where part of either or both of the product gas streams (304,310) from the solid oxide electrolysis cellsolid oxide electrolysis cell (17) are recycled back to the cell. More specifically, a first cathode-side feed stream (301), comprising water or steam or CO.sub.2 or a mixture thereof, is pre-heated using a cathode-side preheater (16). The preheated first cathode-side feed stream (302) is optionally mixed with a first cathode-side recycle stream (306), thereby obtaining a second cathode-side feed stream (303), which is fed to the cathode side (17C) of at least one solid oxide electrolysis cell (17). External voltage is applied to the solid oxide electrolysis cell (17), thereby providing a driving force for the electrochemical reduction of at least part of the carbon dioxide and/or steam in the second cathode-side feed stream (303) into carbon monoxide and/or hydrogen, thereby forming a first cathode-side product gas (304) that is enriched in hydrogen, carbon monoxide or a mixture thereof. Optionally, part of the first cathode-side product gas (304), is recycled back to the electrochemical cell (17) as a first cathode-side recycle stream (306). The remainder of the first cathode-side product gas (305) is supplied to a hydrogen- and/or carbon monoxide-consuming process (18). Although not specifically shown in FIG. 3, the splitting of the first cathode-side product gas (304) into streams (305) and (306) may be carried out in a separation unit, such as a pressure-swing adsorber or a temperature-swing adsorber or a separation membrane. Additional blowers or compressors may be included to increase the pressure of the stream (304).

[0065] Simultaneously, a first anode-side feed stream comprising carbon dioxide (307) is pre-heated using an anode-side preheater (19). The pre-heated first anode-side feed stream (308) is optionally mixed with a first anode-side recycle stream (311), thereby obtaining a second anode-side feed stream (309), which is fed to the anode side (17C) of at least one solid oxide electrolysis cell (17). The externally applied voltage drives an electrochemical oxidation reaction on the anode side (17A) of the solid oxide electrolysis cell, whereby oxygen ions (O.sup.2−) are converted into molecular oxygen (O.sub.2). The electrochemically generated O.sub.2 is mixed with the second anode-side feed gas comprising carbon dioxide (309), thereby forming a first anode-side product gas (310) with a low nitrogen content. Optionally, part of the first anode-side product gas (310), enriched in oxygen, is recycled back to the electrochemical cell (17) as a first anode-side recycle stream (311). The remainder of the first anode-side product gas, the oxygen-enriched gas (312), is Oxy-calciner+SOEC, with CO.sub.2 recycle from flue gas to SOEC fed into the oxy-calciner (10). Although not specifically shown in FIG. 3, the splitting of the first anode-side product gas (310) into streams (311) and (312) may be carried out in a separation unit, such as a pressure-swing adsorber or a temperature-swing adsorber or a separation membrane. Additional blowers or compressors may be included to increase the pressure of the stream (310). Due to the high operating temperature of the solid oxide electrolysis cell, the oxygen-enriched stream (312) does not need to be heated further but may be passed through heat exchangers (not shown). It is further understood that other aforementioned streams may be passed through heat exchangers or additional pre-heaters for better thermal integration.

EXAMPLE 4

[0066] In FIG. 4, a preferred embodiment of the method (4) according to the invention is presented, wherein oxygen-enriched gas (312) is fed to an oxyfuel combustion chamber (20), the oxygen-enriched gas (312) is obtained by flushing the anode (oxy) side (17A) of at least one solid oxide electrolysis cell (17) with a anode-side feed gas (308,309) comprising CO.sub.2, and wherein at least part of the first cathode-side product gas (304,305), enriched in carbon monoxide and/or hydrogen, is supplied to a hydrogen- and/or carbon monoxide-consuming process (18), and where part of either or both of the output gas streams from the solid oxide electrolysis cell (17) are recycled back to the cell as described in Example 3. Advantageously, the oxygen content in the first anode-side product gas (310,312) is between 20 vol % and 40 vol %, for example 35 vol % to match the heat capacity of air and to obtain flame temperatures similar to flame temperatures in air-blown kilns. It is understood that the aforementioned streams may be passed through heat exchangers or additional pre-heaters for better thermal integration. Suitable oxyfuel combustion chamber designs for the purpose are pulverized fuel kilns or circulating fluidized bed (CFB) kilns. Simultaneously, a stream of solid fuel (401), comprising e.g. coal, wood or biomass, is optionally pre-heated using a preheater (21), thereby obtaining a pre-heated solid fuel stream (402). The pre-heated solid fuel stream (402) is fed to the oxyfuel combustion chamber (20). In the combustion chamber (20), solid fuel (402) reacts with the oxygen in the oxygen-enriched gas stream (312) and the exothermic combustion reaction raises the flame temperature near the burner (1 to 4 meters from the burner) to above 1100° C., and up to 1900° C. The output stream (403) from the oxyfuel combustion chamber (20), comprising solid combustion residues, carbon dioxide and steam is fed to a first separator (22), such as a cyclone, where the stream is separated into a stream comprising solid combustion residues (404) and a stream comprising carbon dioxide and steam (405). The stream (405) is further fed to a second separator (23), such as a water knockout vessel, where the stream is separated into a stream comprising H.sub.2O (406) and a stream comprising carbon dioxide (407). Further separation steps may be required and are known to those skilled in the art.

[0067] A 0.21 MW pilot-scale coal oxycombustion unit requires a solid fuel (coal) stream (401) of 31 kg per hour and an oxygen-enriched stream (312) of 233 kg per hour, assuming the oxygen-content in the oxygen-enriched stream (312) is 35 vol %, balance CO.sub.2 and an oxygen excess of 5%. The output stream (403) from the oxyfuel combustion chamber (20) contains less nitrogen oxides (NO.sub.x) compared to air-blown kilns. NO.sub.x concentration is a function of the flame temperature, and thereby increases when oxygen-enriched streams (312) with higher oxygen contents are used. The nitrogen content in the solid fuel stream (401,402) also affects NO.sub.x concentration in stream (403).

EXAMPLE 5

[0068] In FIG. 5, another preferred embodiment of the method (5) according to the invention is presented, wherein oxygen-enriched gas (206) is fed to an oxy-calciner (10), the oxygen-enriched gas (206) is obtained by flushing the anode (oxy) side (17A) of at least one solid oxide electrolysis cell (17) with a first anode-side feed gas (205) comprising CO.sub.2, wherein at least part of the carbon monoxide and/or hydrogen containing gas (203) is supplied to a hydrogen- and/or carbon monoxide-consuming process (18), and wherein a flue gas stream comprising carbon dioxide (503) is recycled, i.e. at least a part of either or both of the feed gas streams (201,204) to the electrolysis unit comprise at least part of the flue gas stream comprising carbon dioxide (503). Specifically, the flue gas stream comprising carbon dioxide (501) is split into two equal or non-equal parts, wherein the first part of the stream comprising carbon dioxide (502) is led out of the process, while the second part of the stream comprising carbon dioxide (503) is used to supply the electrolysis stack with feed gas. More specifically, stream (503) is split into two equal or non-equal parts (504,505), wherein stream (504) is optionally mixed with a supplementary cathode-side feed stream (506) comprising water or steam or CO.sub.2 or a mixture thereof, thereby obtaining a first cathode-side feed stream (201). Stream (505) is optionally mixed with a supplementary anode-side feed stream (507) comprising CO.sub.2, thereby obtaining the first anode-side feed gas (204). In an embodiment of the method according to the invention, the supplementary gas stream (506) comprises steam and is low in CO.sub.2 content. It is understood that the stream comprising carbon dioxide (503) may be supplied to either or both sides of the solid oxide electrolysis cell (17).

EXAMPLE 6

[0069] In FIG. 6, another preferred embodiment of the method (δ) according to the invention is presented, wherein oxygen-enriched gas (206) is fed to an oxy-calciner (10), the oxygen-enriched gas (206) is obtained by flushing the anode (oxy) side (17A) of at least one solid oxide electrolysis cell (17) with a feed gas (205) comprising CO.sub.2, and wherein at least part of the first cathode-side product gas (203), comprising carbon monoxide and/or hydrogen, is supplied to a hydrogen- and/or carbon monoxide-consuming process (18), and wherein simultaneously a part of the carbon monoxide and/or hydrogen containing first cathode-side product gas (203) is supplied as fuel stream (602) to the oxy-calciner (10). More specifically, the first cathode-side product gas (203) is split into two equal or non-equal parts (601,602). Stream (601) is supplied to a hydrogen- and/or carbon monoxide-consuming process (18), whereas stream (602) is used as fuel in the oxy-calciner (10). The stream (602), enriched in hydrogen and/or carbon monoxide, may be mixed with additional fuel (603), such as methane, natural gas, hydrogen, carbon monoxide etc, thereby obtaining a combined fuel stream (604) that is fed into the oxy-calciner (10). It is understood that part of streams (203,206) may be recycled back to the corresponding sides of the solid oxide electrolysis cell, as described in Example 3 and 4 and not explicitly shown in FIG. 6.

EXAMPLE 7

[0070] In FIG. 7, a preferred embodiment of the method (7) according to the invention is presented, wherein oxygen-enriched gas (703) is fed to an oxyfuel combustion chamber (20), wherein the oxygen-enriched gas (703) is obtained by mixing (in equal or non-equal parts) the following gas streams: a first anode-side product gas, enriched in oxygen (312), obtained by flushing the anode (oxy) side (17A) of at least one solid oxide electrolysis cell (17) with a anode-side feed gas (308,309) comprising CO.sub.2, and a flue gas stream (701), obtained by splitting the flue gas stream comprising CO.sub.2 (407) in two equal or non-equal parts (701,702). The stream (702) is led out of the system, while stream (701) is recycled, as explained above. Advantageously, the oxygen content in the first anode-side product gas (310,312) is between 90 vol % and 100 vol %, for example 95 vol %, to minimize total gas flow rate though the anode compartment of the solid oxide electrolysis cell. Advantageously, the oxygen content in the oxygen-enriched gas (703) is between 20 vol % and 40 vol %, for example 35 vol %, to match the heat capacity of air and to obtain flame temperatures similar to flame temperatures in air-blown kilns.

EXAMPLE 8

[0071] Two identical solid oxide electrolysis cell stacks, each comprising 75 cells with a total active area of approximately 8250 cm.sup.2, were operated for 120 hours in electrolysis mode (FIG. 8). A first cathode-side feed stream (301) comprising 99.9% CO.sub.2 was mixed with a first cathode-side recycle stream (306), comprising CO and CO.sub.2, thereby obtaining a second cathode-side feed stream (303). The second cathode-side feed stream (303) was simultaneously introduced onto the cathode-side compartments of both stacks at a temperature of 800° C. (FIG. 8a). An electrolysis current was passed through both stacks, resulting in a fraction of the CO.sub.2 in the cathode-side feed stream to be electrochemically converted into CO, thereby enriching the first cathode-side product stream (304) exiting the stacks with CO. The temperature of the stream (304) exiting the stack was 751-753° C. The gas was compressed and fed to a pressure-swing adsorber unit. The CO-rich exit stream (305) from the pressure-swing adsorber unit was collected as product to be used in e.g. phosgene plants, while the CO-lean exit stream (306) from the pressure-swing adsorber unit was mixed with the first cathode-side feed stream (301), as described above. A first anode-side feed stream (204), comprising ≥99.9% CO.sub.2, was pre-heated to 785° C., and the resulting preheated first anode-side feed stream (205) was fed to the anode-side of both stacks. As a result of the electrolysis current passing through the stacks, gaseous O.sub.2 was electrochemically generated on the anode sides of the cells in the stacks, whereby a first anode-side product gas (206), enriched in oxygen, was obtained. The first anode-side product gas (206) comprised CO.sub.2 and O.sub.2 and was notably low in nitrogen content. The precise nitrogen content was not measured, but was estimated to be below 50 ppm. The temperature of the first anode-side product gas (206) exiting the stacks was 791-793° C. during the experiment. The stack voltages required to maintain an electrolysis current of approximately 45 A were between 97 V and 103 V (FIG. 8b). Surprisingly, no significant performance degradation was observed while operating the solid oxide electrolysis cells using an anode-side stream comprising 99.9% CO.sub.2.