EP2940773A1 - Ejector for solid oxide electrolysis cell stack system - Google Patents
Ejector for solid oxide electrolysis cell stack system Download PDFInfo
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- EP2940773A1 EP2940773A1 EP14166323.7A EP14166323A EP2940773A1 EP 2940773 A1 EP2940773 A1 EP 2940773A1 EP 14166323 A EP14166323 A EP 14166323A EP 2940773 A1 EP2940773 A1 EP 2940773A1
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- soec
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- 238000005868 electrolysis reaction Methods 0.000 title claims abstract description 30
- 239000007787 solid Substances 0.000 title claims abstract description 21
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- 238000000034 method Methods 0.000 claims abstract description 105
- 229910002092 carbon dioxide Inorganic materials 0.000 claims abstract description 86
- 239000001569 carbon dioxide Substances 0.000 claims abstract description 85
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 claims abstract description 83
- 229910002091 carbon monoxide Inorganic materials 0.000 claims abstract description 83
- 229910052760 oxygen Inorganic materials 0.000 claims description 51
- 239000001301 oxygen Substances 0.000 claims description 51
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims description 50
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- 238000004519 manufacturing process Methods 0.000 description 8
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- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 6
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- 238000010744 Boudouard reaction Methods 0.000 description 3
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- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 2
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 description 2
- 239000002250 absorbent Substances 0.000 description 2
- 230000002745 absorbent Effects 0.000 description 2
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- 239000004215 Carbon black (E152) Substances 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 239000004793 Polystyrene Substances 0.000 description 1
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- HNPSIPDUKPIQMN-UHFFFAOYSA-N dioxosilane;oxo(oxoalumanyloxy)alumane Chemical compound O=[Si]=O.O=[Al]O[Al]=O HNPSIPDUKPIQMN-UHFFFAOYSA-N 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
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- XLYOFNOQVPJJNP-ZSJDYOACSA-N heavy water Substances [2H]O[2H] XLYOFNOQVPJJNP-ZSJDYOACSA-N 0.000 description 1
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- JCXJVPUVTGWSNB-UHFFFAOYSA-N nitrogen dioxide Inorganic materials O=[N]=O JCXJVPUVTGWSNB-UHFFFAOYSA-N 0.000 description 1
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Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B1/00—Electrolytic production of inorganic compounds or non-metals
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
- C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
- C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
Definitions
- This invention belongs to the field of electrolysis conducted in solid oxide electrolysis cell (SOEC) stacks. More particular, the invention relates to a Solid Oxide Electrolysis Cell (SOEC) stack system for producing CO comprising an ejector.
- SOEC Solid Oxide Electrolysis Cell
- a solid oxide electrolysis cell is a solid oxide fuel cell (SOFC) run in reverse mode, which uses a solid oxide or ceramic electrolyte to produce e.g. oxygen and hydrogen gas by electrolysis of water.
- a solid oxide electrolysis cell system comprises an SOEC core wherein the SOEC stack is housed together with inlets and outlets for process gases.
- the feed gas often called the fuel gas, is led to the cathode part of the stack, from where the product gas from the electrolysis is taken out.
- the anode part of the stack is also called the oxygen side, because oxygen is produced on this side.
- the present invention relates to a process for producing carbon monoxide (CO) from carbon dioxide (CO 2 ) in a solid oxide electrolysis cell (SOEC) or SOEC stack, wherein CO 2 is led to the fuel side of the stack with an applied current and excess oxygen is transported to the oxygen side of the stack, optionally using air or nitrogen to flush the oxygen side, and wherein the product stream from the SOEC, containing CO mixed with CO 2 , is subjected to a separation process.
- SOEC solid oxide electrolysis cell
- US 8,138,380 B2 describes an environmentally beneficial method of producing methanol by reductively converting carbon dioxide, said method including a step in which recycled carbon dioxide is reduced to carbon monoxide in an electrochemical cell.
- syngas components hydrogen and carbon monoxide may be formed by decomposition of carbon dioxide and water or steam in a solid oxide electrolysis cell to form carbon monoxide and hydrogen, a portion of which may be reacted with carbon dioxide to form carbon monoxide utilizing the so-called reverse water gas shift (WGS) reaction.
- WGS reverse water gas shift
- US 2012/0228150 A1 describes a method of decomposing CO 2 into C/CO and O 2 in a continuous process using electrodes of oxygen deficient ferrites (ODF) integrated with a YSZ electrolyte.
- ODF oxygen deficient ferrites
- the ODF electrodes can be kept active by applying a small potential bias across the electrodes.
- CO 2 and water can also be electrolysed simultaneously to produce syngas (H 2 + CO) and O 2 continuously. Thereby, CO 2 can be transformed into a valuable fuel source allowing a CO 2 neutral use of hydrocarbon fuels.
- US 8,366,902 B2 describes methods and systems for producing syngas utilising heat from thermochemical conversion of a carbonaceous fuel to support decomposition of water and/or carbon dioxide using one or more solid oxide electrolysis cells. Simultaneous decomposition of carbon dioxide and water or steam by one or more solid oxide electrolysis cells can be employed to produce hydrogen and carbon monoxide.
- gas pressure means such as compressors may be necessary to provide flow of the process gasses through the stack system. Since such gas pressure means entails a need for a surplus energy input to propel the pressure means, the overall efficiency of the SOEC stack system and CO production drops with increasing necessary energy input. The parasitic loss in the system increases with increasing need for surplus energy.
- a Solid Oxide Electrolysis Cell (SOEC) stack system for producing CO is disclosed as seen on Fig. 2 .
- the system comprises an SOEC stack.
- the electrolysis cells in the stack each has a fuel side and an oxygen side, hence, when all the cells are put on top of each other to form a stack, the stack has a fuel side and an oxygen side.
- the stack has a fuel side inlet and outlet and an oxygen side inlet and outlet.
- the configuration of the inlets and outlets may be of any kind known in the art such as internal manifolded, external manifolded or a combination of these.
- This cleaning of the process gas is done in a process gas separator downstream the SOEC stack.
- the process gas separator has a process gas inlet and a first and a second process gas outlet.
- the process gas is fed to the process gas separator inlet via process gas piping when it exits the SOEC stack from the fuel side outlet.
- Cleaned process gas, CO with a purity approaching 100% exits the process gas separator via the first process gas outlet.
- Separated process gas which comprises CO2 and CO exits the process gas separator from the second process gas outlet.
- the process gas separator demands a certain pressure of the process gas entering.
- an ejector is provided downstream the SOEC stack and upstream of the process gas separator.
- the ejector provides a pressure increase to the process gas exiting the SOEC stack by means of injection of high pressure CO2. This is feasible since the CO2 feed is normally stored in liquid form under elevated pressure, whereas the SOEC unit operates close to ambient conditions and therefore the feed gas pressure is usually reduced over a reduction valve up-stream from the SOEC unit. In-stead of the use of a reduction valve, at least a part of the high pressure feed gas may thus be lead through the ejector for pressure reduction and thereby increasing the process gas pressure upstream the process gas separator.
- the ejector hence provides pressure for the process gas separation, but also reduces the CO content in the feed gas for the process gas purification due to the injection of CO2. This may result in a lower yield of CO of the purification, but since the ejector is replacing a compressor, it provides a cheaper and less complicated unit with a lower footprint.
- a further second ejector is arranged between the process gas separator and the SOEC stack as seen on Fig. 3 , providing recycling of at least a part of the process gas from the second process gas outlet, via this second ejector to the fuel side inlet of the SOEC stack.
- Piping connects the second process gas outlet with the ejector and further connects the ejector to the fuel side of the SOEC stack.
- the recycle of at least a part of the process gas from the second process gas outlet to the fuel side of the SOEC stack reduces the consumption of CO2 and also lowers the waste of CO2 from the system.
- the recycling requires a slight increase in pressure of the recycle gas to overcome the pressure drop in the system. This pressure increase is provided by the second ejector.
- carbon monoxide is produced from carbon dioxide (CO2) in a solid oxide electrolysis cell (SOEC) stack, wherein CO 2 is led to the fuel side of the SOEC with an applied current, said process further comprising:
- the principle underlying the present embodiment consists in leading CO 2 to the fuel side of an SOEC with an applied current to convert CO 2 to CO and transport the oxygen surplus to the oxygen side of the SOEC. Air, nitrogen or carbon dioxide may be used to flush the oxygen side. Flushing the oxygen side of the SOEC has two advantages, more specifically (1) reducing the oxygen concentration and related corrosive effects and (2) providing means for feeding energy into the SOEC, operating it endothermic.
- the product stream from the SOEC contains mixed CO and CO 2 , which is led to a separation process such as pressure swing adsorption (PSA), temperature swing adsorption (TSA), membrane separation, cryogenic separation or liquid scrubber technology, such as wash with N-methyl-diethanolamine (MDEA).
- PSA pressure swing adsorption
- TSA temperature swing adsorption
- MDEA cryogenic separation
- liquid scrubber technology such as wash with N-methyl-diethanolamine (MDEA).
- PSA is especially suitable for the production of high purity CO according to the present invention.
- Carbon dioxide is the most abundant impurity.
- trace amounts of N 2 and H 2 may be present in the feed gas to the PSA unit.
- an adsorption comprising at least two adsorption columns, each containing adsorbents exhibiting selective adsorption properties towards carbon dioxide, can be used to remove CO 2 from the gas mixture.
- a second adsorption step can be employed to further remove carbon dioxide in addition to other pollutants such as nitrogen.
- This adsorption step comprises at least two adsorption columns, each containing adsorbents exhibiting selective adsorption properties towards carbon monoxide.
- Such an adsorption step may be used alone or as a second step in combination with the above mentioned adsorption step selective towards CO 2 .
- Adsorbents being selective regarding carbon monoxide adsorption include activated carbon, natural zeolites, synthetic zeolites, polystyrene or mixtures thereof.
- addition of copper or aluminium halides to any of the materials mentioned above to introduce monovalent copper ions and/or trivalent aluminium onto the materials is beneficial with respect to carbon monoxide selectivity and capacity.
- the addition of Cu or Al can be combined with impregnation of carbon onto the carrier to preserve the oxidation stage of Cu and Al.
- copper ions can be introduced into the zeolite material by ion exchange to increase the carbon monoxide selectivity and capacity.
- N 2 is difficult to separate effectively from CO in the downstream purification process which, as mentioned, uses PSA, TSA, membrane separation, cryogenic separation or liquid scrubber technology, such as wash with N-methyl-diethanolamine (MDEA).
- MDEA N-methyl-diethanolamine
- the electrolysis process in the SOEC requires an operating temperature between 650 and 850°C.
- the overall operation can consume heat (i.e. be endothermic), it can be thermoneutral or it can generate heat (i.e. be exothermic). Any operation carried out at such high temperatures also leads to a significant heat loss. This means that typically it will require external heating to reach and maintain the desired operating temperature.
- external heaters are used to heat the inlet gas on the oxygen side and the fuel side in order to supply heat to the SOEC stack, thereby mitigating this issue.
- Such external heaters are also useful during start-up as they can provide heat to help the SOEC reach its operating temperature. Suitable feed gas temperatures would be around 700 to 850°C.
- the external heaters can be electrical, but gas or liquid fuelled external heaters may also be used.
- the hot exhaust gas on the oxygen side and the fuel side may be utilized to heat the inlet gas. This is another way to maintain a suitable operating temperature for the SOEC and at the same time reduce the load on the heaters.
- mass (O 2 ) is transferred from the fuel side to the oxygen side, which leads to a limitation on the maximum temperature that can be reached in the feed effluent heat exchanger on the fuel side alone.
- a third feed effluent heat exchanger is implemented, said third heat exchanger transferring heat from the hot outlet side of the feed effluent heat exchanger on the oxygen side to the cold inlet of the feed effluent heat exchanger on the fuel side.
- feed effluent heat exchangers increases the efficiency with respect to power consumption of the plant, and it also greatly reduces the load on the high temperature heaters.
- the feed effluent heat exchangers will slow down and restrict the maximum rate of cooling by insertion of cold gases at the feed and purge inputs.
- fast cooling is desirable when electrical anode protection (EAP) is used during a power failure, where the electrical protection is provided by a battery back-up.
- EAP electrical anode protection
- the stack should be cooled to a temperature below the cathode/nickel oxidation temperature (e.g. 400°C) before the battery back-up power is used.
- a tie-in point is designed in between the high temperature heater and the SOEC, where a cooling medium such as air, N 2 or CO 2 can be added to the system and thus the cooling down rate can be increased and independently controlled.
- This tie-in point can be introduced on the anode side as well as on the cathode side of the SOEC.
- the gas connections for the heating and the cooling flows may be identical.
- the feed effluent heat exchanger employed on the cathode side of the SOEC may be subject to corrosion due to carbon formation in the carbon monoxide-rich atmosphere present on this side.
- This type of corrosion is generally renowned as metal dusting, and it may be mitigated by choosing an appropriate material or coating with respect to the heat exchanger and the heat exchanger conditions.
- An alternative solution to the metal dusting issue is to simply quench the gas coming from the cathode side of the SOEC to a temperature around 400-600°C, where metal dusting is kinetically inhibited.
- the quench should be performed with an inert gas such as N 2 , H 2 O, but most preferably with CO 2 .
- the feed effluent heat exchanger is still in service, but now utilizing the heat from a temperature range within 400-600°C, most preferably within 400-550°C, instead of from the SOEC operating temperature. This obviously reduces the overall efficiency of the plant with respect to heat and CO 2 consumption, but it does mitigate the metal dusting issue and it is an alternative to using more exotic materials on the cathode side.
- the SOEC unit together with the preheaters on the cathode side and the anode side as well as the feed effluent heat exchangers placed directly downstream from the SOEC unit comprise an entity called the SOEC core.
- This core is encapsulated and thermally insulated towards the surroundings to mitigate heat loss from and thermal gradients within these units which are operating at high temperatures.
- the core shell can be connected to the PSA purge line in order to assure that any leakage of CO is oxidized to CO 2 in the oxidation unit.
- the outlet stream from the oxygen side (anode side) of the SOEC is led to the oxidation unit to ensure that any leakage of CO into the oxygen side of the system is also oxidized into CO 2 .
- separate oxidation units may be established for the SOEC core purge and for the oxygen side outlet of the SOEC unit.
- these two streams may also share one common oxidizing unit.
- this catalytic oxidizing unit would include a catalytic oxidation reactor utilizing a catalyst.
- Said catalyst comprises a noble metal catalyst, such as Pt or Pd optionally combined with V 2 O 5 and WO 3 on a TiO 2 or alumina carrier, and the catalyst operates at temperatures above 100°C, preferably between 150 and 250°C.
- the CO 2 source is available at elevated pressure, whereas the SOEC is operating close to atmospheric pressure.
- a compressor between the SOEC and the separation process such as pressure swing adsorption (PSA)
- PSA pressure swing adsorption
- Adsorbents or absorbents are used upstream from the SOEC to remove undesired contaminants in the gas.
- Sulfur species and siloxanes in particular, but also other contaminants, such as halogens and higher hydrocarbons (e.g. benzene), are known to poison solid oxide cells.
- Such compounds can be absorbed, e.g. with active carbon or absorbents based on alumina, ZnO, Ni or Cu, such as Topsoe HTZ-51, Topsoe SC-101 and Topsoe ST-101.
- Carbon formation can also be suppressed by addition of H 2 S. Both carbon formation and metal dusting are normally considered to take place through the following reactions: 2CO ⁇ C + CO 2 (Boudouard reaction) and H 2 + CO ⁇ H 2 O + C (CO reduction)
- H 2 S does not affect the thermodynamic potential for metal dusting, but it pacifies the metal surfaces so that the sites, where the carbon-forming reactions would take place, are blocked.
- H 2 S to the feed stream to a level of H 2 S between 50 ppb and 2 ppm, most preferably between 100 ppb and 1 ppm, would effectively suppress carbon formation in the SOEC stack, i.e. in the Ni-containing cathode, and also protect downstream equipment from metal dusting attacks.
- the relatively low level mentioned above is enough to suppress the formation of carbon, and at the same time it does not cause any detrimental effects on the SOEC stack performance.
- H 2 S can be added to the feed gas just downstream from the feed gas purification unit to protect the SOEC and the downstream equipment from carbon formation and metal dusting.
- H 2 S can be added just downstream from the SOEC to only protect the downstream equipment from metal dusting.
- the same adsorbents as used for the feed gas purification can be used, i.e. active carbon or adsorbents based on alumina, ZnO, Ni or Cu, such as Topsoe HTZ-51, Topsoe SC-101 and Topsoe ST-101.
- the purification unit is preferably placed between the product gas compressor or ejector and the product purification unit.
- H 2 can be removed by selective oxidation of hydrogen: 2H 2 + O 2 ⁇ 2H 2 O
- the water formed is easily separated using cooling and condensation. This will make it possible to use H 2 in any SOEC operation where the target product is CO.
- H 2 is oxidized over oxidation catalysts at a lower temperature than CO.
- the applicable temperature level depends on the catalyst.
- a Pd or Pt catalyst can be expected to oxidize H 2 at temperature levels from ambient temperature to 70°C, whereas temperatures above 150°C are needed to oxidize CO.
- the CO/CO 2 product stream is effectively cleaned from H 2 .
- O 2 can be drawn conveniently from the O 2 -CO 2 mix on the anode side of the SOEC.
- the compartment around the stack may be purged with CO 2 .
- a heater is installed to bring the inlet CO 2 gas, utilized as a compartment purge, up to the operating temperature of the SOEC stack or above.
- This heater could for example be applied as a radiant heater, where the heater is incorporated in the CO 2 purge gas manifold, simultaneously heating the physical perimeter of the stack and the inlet CO 2 purge gas.
- the radiant heater can replace the oxygen side inlet heater, or alternatively it can be used as an additional heater which is used to reduce the time for cold start-up.
- the current invention focuses on applications, where carbon monoxide is the desired product, but the principles applied and the process configurations are also valid for the cases, where a mixture of CO 2 and steam comprises the feed stock and a mixture of hydrogen and CO is the desired product.
- a mixture of CO 2 and steam comprises the feed stock and a mixture of hydrogen and CO is the desired product.
- steam will follow CO 2 and H 2 will follow the CO product gas.
- the final PSA step would separate H 2 from CO and is thus only applicable in cases where splitting H 2 from CO is desired for the downstream process.
- steam is preferably removed from the product stream upstream from the product gas separation unit.
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- Inorganic Chemistry (AREA)
- Carbon And Carbon Compounds (AREA)
Abstract
The invention concerns a process for producing carbon monoxide (CO) from carbon dioxide (CO2) in a solid oxide electrolysis cell (SOEC) or SOEC stack system where an ejector provides a surplus pressure to feed the process gas to a process gas separator an possibly a further ejector provides surplus pressure to recycle at least a part of the waste gas stream from the process gas separator to the inlet side of the SOEC stack.
Description
- This invention belongs to the field of electrolysis conducted in solid oxide electrolysis cell (SOEC) stacks. More particular, the invention relates to a Solid Oxide Electrolysis Cell (SOEC) stack system for producing CO comprising an ejector.
- A solid oxide electrolysis cell is a solid oxide fuel cell (SOFC) run in reverse mode, which uses a solid oxide or ceramic electrolyte to produce e.g. oxygen and hydrogen gas by electrolysis of water. A solid oxide electrolysis cell system comprises an SOEC core wherein the SOEC stack is housed together with inlets and outlets for process gases. The feed gas, often called the fuel gas, is led to the cathode part of the stack, from where the product gas from the electrolysis is taken out. The anode part of the stack is also called the oxygen side, because oxygen is produced on this side.
- The present invention relates to a process for producing carbon monoxide (CO) from carbon dioxide (CO2) in a solid oxide electrolysis cell (SOEC) or SOEC stack, wherein CO2 is led to the fuel side of the stack with an applied current and excess oxygen is transported to the oxygen side of the stack, optionally using air or nitrogen to flush the oxygen side, and wherein the product stream from the SOEC, containing CO mixed with CO2, is subjected to a separation process.
- It is known that CO may be produced from CO2 by electrolysis. Thus,
US 2007/0045125 A1 describes a method for preparing synthesis gas (syngas comprising carbon monoxide and hydrogen) from carbon dioxide and water using a sodium-conducting electrochemical cell. Syngas is also produced by co-electrolysis of carbon dioxide and steam in a solid oxide electrolysis cell. -
US 8,138,380 B2 describes an environmentally beneficial method of producing methanol by reductively converting carbon dioxide, said method including a step in which recycled carbon dioxide is reduced to carbon monoxide in an electrochemical cell. - From
US 2008/0023338 A1 a method for producing at least one syngas component by high temperature electrolysis is known. The syngas components hydrogen and carbon monoxide may be formed by decomposition of carbon dioxide and water or steam in a solid oxide electrolysis cell to form carbon monoxide and hydrogen, a portion of which may be reacted with carbon dioxide to form carbon monoxide utilizing the so-called reverse water gas shift (WGS) reaction. -
US 2012/0228150 A1 describes a method of decomposing CO2 into C/CO and O2 in a continuous process using electrodes of oxygen deficient ferrites (ODF) integrated with a YSZ electrolyte. The ODF electrodes can be kept active by applying a small potential bias across the electrodes. CO2 and water can also be electrolysed simultaneously to produce syngas (H2 + CO) and O2 continuously. Thereby, CO2 can be transformed into a valuable fuel source allowing a CO2 neutral use of hydrocarbon fuels. - Finally,
US 8,366,902 B2 describes methods and systems for producing syngas utilising heat from thermochemical conversion of a carbonaceous fuel to support decomposition of water and/or carbon dioxide using one or more solid oxide electrolysis cells. Simultaneous decomposition of carbon dioxide and water or steam by one or more solid oxide electrolysis cells can be employed to produce hydrogen and carbon monoxide. - Besides the above-mentioned patents and patent applications, the concept of electrolysing CO2 in solid oxide electrolysis cells is described in "Modelling of a Solid Oxide Electrolysis Cell for Carbon Dioxide Electrolysis", a publication by Meng Ni of the Hong Kong Polytechnic University, and also by Sune Dalgaard Ebbesen and Mogens Mogensen in an article entitled "Electrolysis of Carbon Dioxide in Solid Oxide Electrolysis Cells", Journal of Power Sources 193, 349-358 (2009).
- When producing carbon monoxide (CO) from carbon dioxide (CO2) by means of an SOEC stack system, gas pressure means such as compressors may be necessary to provide flow of the process gasses through the stack system. Since such gas pressure means entails a need for a surplus energy input to propel the pressure means, the overall efficiency of the SOEC stack system and CO production drops with increasing necessary energy input. The parasitic loss in the system increases with increasing need for surplus energy.
- Therefore a need exist to lover the demand for surplus energy in an SOEC stack system for producing CO. In particular, when producing CO with a high purity in an SOEC stack system it is necessary to clean the produced CO in a process gas separator downstream the SOEC stack. The pressure to propel the process gas through the gas separator may be provided by a gas compressor which demands a surplus energy input as described above, as can be seen in
Fig. 1 . This problem may be solved in the present invention according to the claims. - In an embodiment of the invention a Solid Oxide Electrolysis Cell (SOEC) stack system for producing CO is disclosed as seen on
Fig. 2 . The system comprises an SOEC stack. The electrolysis cells in the stack each has a fuel side and an oxygen side, hence, when all the cells are put on top of each other to form a stack, the stack has a fuel side and an oxygen side. To enable gas to pass the cells, the stack has a fuel side inlet and outlet and an oxygen side inlet and outlet. The configuration of the inlets and outlets may be of any kind known in the art such as internal manifolded, external manifolded or a combination of these. When producing CO from CO2 in the SOEC stack, a 100% clean output of CO is not feasible to achieve. There is therefore a need to subsequent cleaning of the process gas after it exits the SOEC stack to approach the CO content of the process gas to 100%. - This cleaning of the process gas is done in a process gas separator downstream the SOEC stack. The process gas separator has a process gas inlet and a first and a second process gas outlet. The process gas is fed to the process gas separator inlet via process gas piping when it exits the SOEC stack from the fuel side outlet. Cleaned process gas, CO with a purity approaching 100% exits the process gas separator via the first process gas outlet. Separated process gas which comprises CO2 and CO exits the process gas separator from the second process gas outlet.
- The process gas separator demands a certain pressure of the process gas entering. To ensure the necessary process gas pressure is available, an ejector is provided downstream the SOEC stack and upstream of the process gas separator. The ejector provides a pressure increase to the process gas exiting the SOEC stack by means of injection of high pressure CO2. This is feasible since the CO2 feed is normally stored in liquid form under elevated pressure, whereas the SOEC unit operates close to ambient conditions and therefore the feed gas pressure is usually reduced over a reduction valve up-stream from the SOEC unit. In-stead of the use of a reduction valve, at least a part of the high pressure feed gas may thus be lead through the ejector for pressure reduction and thereby increasing the process gas pressure upstream the process gas separator. The ejector hence provides pressure for the process gas separation, but also reduces the CO content in the feed gas for the process gas purification due to the injection of CO2. This may result in a lower yield of CO of the purification, but since the ejector is replacing a compressor, it provides a cheaper and less complicated unit with a lower footprint.
- In an embodiment of the invention, a further second ejector is arranged between the process gas separator and the SOEC stack as seen on
Fig. 3 , providing recycling of at least a part of the process gas from the second process gas outlet, via this second ejector to the fuel side inlet of the SOEC stack. Piping connects the second process gas outlet with the ejector and further connects the ejector to the fuel side of the SOEC stack. The recycle of at least a part of the process gas from the second process gas outlet to the fuel side of the SOEC stack reduces the consumption of CO2 and also lowers the waste of CO2 from the system. The recycling requires a slight increase in pressure of the recycle gas to overcome the pressure drop in the system. This pressure increase is provided by the second ejector. - In a further embodiment of the invention as seen in
Fig. 4 , carbon monoxide (CO) is produced from carbon dioxide (CO2) in a solid oxide electrolysis cell (SOEC) stack, wherein CO2 is led to the fuel side of the SOEC with an applied current, said process further comprising: - heating the inlet gas on the fuel side by means of a heating unit, so as to supply heat to the SOEC, wherein the operation temperature of said heating unit is at least the operation temperature of the cell stack minus 50°C, preferably at least the operation temperature of the cell stack, and
- heating the inlet gas on the oxygen side by means of a heating unit, so as to supply heat to the SOEC, wherein the operation temperature of said heating unit is at least the operation temperature of the cell stack minus 50°C, preferably at least the operation temperature of the cell stack. The content of CO in the output from the SOEC stack is preferably 20-80 wt%, and
- subjecting the product stream from the SOEC stack to a separation process in a process gas separator, said process gas separator being selected from pressure swing adsorption (PSA), temperature swing adsorption (TSA), membrane separation, cryogenic separation and liquid scrubber technology, such as wash with N-methyl-diethanolamine (MDEA), wherein process gas piping and an ejector is provided downstream the SOEC stack and upstream the process gas separator and said ejector is adapted to provide a pressure increase to the process gas provided to a process gas inlet of the process gas separator.
- The principle underlying the present embodiment consists in leading CO2 to the fuel side of an SOEC with an applied current to convert CO2 to CO and transport the oxygen surplus to the oxygen side of the SOEC. Air, nitrogen or carbon dioxide may be used to flush the oxygen side. Flushing the oxygen side of the SOEC has two advantages, more specifically (1) reducing the oxygen concentration and related corrosive effects and (2) providing means for feeding energy into the SOEC, operating it endothermic. The product stream from the SOEC contains mixed CO and CO2, which is led to a separation process such as pressure swing adsorption (PSA), temperature swing adsorption (TSA), membrane separation, cryogenic separation or liquid scrubber technology, such as wash with N-methyl-diethanolamine (MDEA).
- PSA is especially suitable for the production of high purity CO according to the present invention. Carbon dioxide is the most abundant impurity. However, due to impurities in the CO2 feed or due to leakage in the SOEC unit, trace amounts of N2 and H2 may be present in the feed gas to the PSA unit.
- In order to remove carbon dioxide an adsorption comprising at least two adsorption columns, each containing adsorbents exhibiting selective adsorption properties towards carbon dioxide, can be used to remove CO2 from the gas mixture. Furthermore, a second adsorption step can be employed to further remove carbon dioxide in addition to other pollutants such as nitrogen. This adsorption step comprises at least two adsorption columns, each containing adsorbents exhibiting selective adsorption properties towards carbon monoxide. Such an adsorption step may be used alone or as a second step in combination with the above mentioned adsorption step selective towards CO2. Adsorbents being selective regarding carbon monoxide adsorption include activated carbon, natural zeolites, synthetic zeolites, polystyrene or mixtures thereof. In particular, addition of copper or aluminium halides to any of the materials mentioned above to introduce monovalent copper ions and/or trivalent aluminium onto the materials is beneficial with respect to carbon monoxide selectivity and capacity. Optionally, the addition of Cu or Al can be combined with impregnation of carbon onto the carrier to preserve the oxidation stage of Cu and Al. In addition, in the case of a zeolite material, copper ions can be introduced into the zeolite material by ion exchange to increase the carbon monoxide selectivity and capacity.
- There is a significant risk that gas may leak from the oxygen side to the fuel side of the SOEC. In the case that air is used on the oxygen side, the oxygen is quickly consumed on the fuel side as carbon monoxide reacts with oxygen to form carbon dioxide. This may occur spontaneously at the elevated operating temperatures used in the cell (typically above 700°C) or on the Ni which is present as part of the fuel side.
- A more severe issue is that also nitrogen may leak over to the fuel side, and N2 is difficult to separate effectively from CO in the downstream purification process which, as mentioned, uses PSA, TSA, membrane separation, cryogenic separation or liquid scrubber technology, such as wash with N-methyl-diethanolamine (MDEA). This means that high purity CO is difficult to obtain. However, if CO2 is used on the oxygen side instead of air, this issue is mitigated and the gases present in the system are restricted to only CO, CO2 and O2.
- The electrolysis process in the SOEC requires an operating temperature between 650 and 850°C. Depending on the specific operating conditions, stack configuration and the integrity of the stack, the overall operation can consume heat (i.e. be endothermic), it can be thermoneutral or it can generate heat (i.e. be exothermic). Any operation carried out at such high temperatures also leads to a significant heat loss. This means that typically it will require external heating to reach and maintain the desired operating temperature.
- When the operation is carried out at a sufficiently large current in the SOEC stack, the necessary heat will eventually be generated, but at the same time the degradation of the stack will increase. Therefore, in another embodiment of the process external heaters are used to heat the inlet gas on the oxygen side and the fuel side in order to supply heat to the SOEC stack, thereby mitigating this issue. Such external heaters are also useful during start-up as they can provide heat to help the SOEC reach its operating temperature. Suitable feed gas temperatures would be around 700 to 850°C. The external heaters can be electrical, but gas or liquid fuelled external heaters may also be used.
- In addition to using inlet gas heaters to obtain the necessary operating temperature, the hot exhaust gas on the oxygen side and the fuel side may be utilized to heat the inlet gas. This is another way to maintain a suitable operating temperature for the SOEC and at the same time reduce the load on the heaters. Thus, by incorporating a feed effluent heat exchanger on both the oxygen side and the fuel side, the issues related to high temperature operation and heat loss are further mitigated. In accordance with the nature of the SOEC operation, mass (O2) is transferred from the fuel side to the oxygen side, which leads to a limitation on the maximum temperature that can be reached in the feed effluent heat exchanger on the fuel side alone. As a consequence of this, there will be an increase of mass through the SOEC on the oxygen side, which leads to the creation of an excess of heat in the SOEC oxygen outlet stream. This in turn leads to a surplus of heat in the outlet stream from the feed effluent heat exchanger on the oxygen side also. Thus, in order to utilize this excess heat on the oxygen side, a third feed effluent heat exchanger is implemented, said third heat exchanger transferring heat from the hot outlet side of the feed effluent heat exchanger on the oxygen side to the cold inlet of the feed effluent heat exchanger on the fuel side. By using electrical tracing in combination with high-temperature insulation on the connecting pipes between the heaters and the heat exchangers as well as between the heat exchangers, the heaters and the stack, the desired temperature level in the SOEC stack can be further conserved.
- Due to the transfer of oxygen ions from the fuel side to the oxygen side of the SOEC system the thermal mass of the fuel/oxygen input and output flows will be different when electrolysis is performed. As this difference will vary with the oxygen flow, which is proportional to the (possibly changing) current, it is in general not possible to recuperate all the heat from the SOEC output gases for all operating conditions. As a heat effective alternative, no flushing on the oxygen side is used, and feed gas (CO2) is provided by two individually controlled flows. One flow shares a heat exchanger with the output flow from the SOEC fuel side, and the other flow shares a heat exchanger with the output flow from the oxygen side of the SOEC. By adjusting the flows while maintaining the desired total input it is possible to assure equal thermal masses of the inputs to the two heat exchangers. This makes it possible to obtain an ideal recuperation of the heat from the SOEC for all CO production conditions desired (e.g. variations of CO production rate and CO/CO2 ratio in the fuel output gas).
- The introduction of feed effluent heat exchangers increases the efficiency with respect to power consumption of the plant, and it also greatly reduces the load on the high temperature heaters. However, with respect to the cooling-down rate in case of a plant trip or shut-down, the feed effluent heat exchangers will slow down and restrict the maximum rate of cooling by insertion of cold gases at the feed and purge inputs. In order to mitigate SOEC degradation during trip or shut-down it is beneficial to be able to control the cooling-down rate closely. In particular fast cooling is desirable when electrical anode protection (EAP) is used during a power failure, where the electrical protection is provided by a battery back-up. In this case the stack should be cooled to a temperature below the cathode/nickel oxidation temperature (e.g. 400°C) before the battery back-up power is used.
- In order to control the SOEC cooling rate precisely and with a higher degree of freedom a tie-in point is designed in between the high temperature heater and the SOEC, where a cooling medium such as air, N2 or CO2 can be added to the system and thus the cooling down rate can be increased and independently controlled. This tie-in point can be introduced on the anode side as well as on the cathode side of the SOEC.
- In the same way as a fast cooling can be desirable, there may also be many applications where it would be desirable to be able to heat the system fast to the stack operating temperature. This can for example be achieved by sending a relatively large flow of hot gases through the stack. To increase the in-flux of heat beyond the power level of the SOEC core heaters it can be advantageous to use external heaters connected to independent (large) gas flows. To avoid damage to the stack the flow and temperature of the external heaters can be controlled, for example to keep the temperature gradient across the stack below a given specified level.
- The gas connections for the heating and the cooling flows may be identical.
- The feed effluent heat exchanger employed on the cathode side of the SOEC may be subject to corrosion due to carbon formation in the carbon monoxide-rich atmosphere present on this side. This type of corrosion is generally renowned as metal dusting, and it may be mitigated by choosing an appropriate material or coating with respect to the heat exchanger and the heat exchanger conditions. An alternative solution to the metal dusting issue is to simply quench the gas coming from the cathode side of the SOEC to a temperature around 400-600°C, where metal dusting is kinetically inhibited. The quench should be performed with an inert gas such as N2, H2O, but most preferably with CO2. The feed effluent heat exchanger is still in service, but now utilizing the heat from a temperature range within 400-600°C, most preferably within 400-550°C, instead of from the SOEC operating temperature. This obviously reduces the overall efficiency of the plant with respect to heat and CO2 consumption, but it does mitigate the metal dusting issue and it is an alternative to using more exotic materials on the cathode side.
- In the gas purification step where CO is separated from CO2 (using e.g. a pressure swing adsorption unit), it is an inherent fact that some of the CO will follow the CO2 in the gas separation. By recycling this mix of CO and CO2, an increased utilization of the feedstock and thus an increased yield with respect to CO can be obtained. In order to avoid a build-up of unwanted inert components, a purge stream must be imposed on the recycle stream. This purge stream should be passed to a catalytic oxidizer to oxidize CO to CO2 or to a thermal oxidizer before reaching the surrounding environment.
- In this invention, the SOEC unit together with the preheaters on the cathode side and the anode side as well as the feed effluent heat exchangers placed directly downstream from the SOEC unit comprise an entity called the SOEC core. This core is encapsulated and thermally insulated towards the surroundings to mitigate heat loss from and thermal gradients within these units which are operating at high temperatures.
- In case of leakage of CO from the units within the SOEC core or from the tubes connecting the units within the SOEC core, the core shell can be connected to the PSA purge line in order to assure that any leakage of CO is oxidized to CO2 in the oxidation unit. To further mitigate leakage of CO into the surroundings; also the outlet stream from the oxygen side (anode side) of the SOEC is led to the oxidation unit to ensure that any leakage of CO into the oxygen side of the system is also oxidized into CO2.
- As an alternative, separate oxidation units may be established for the SOEC core purge and for the oxygen side outlet of the SOEC unit. Alternatively these two streams may also share one common oxidizing unit.
- In the case of a catalytic oxidizing unit, this catalytic oxidizing unit would include a catalytic oxidation reactor utilizing a catalyst. Said catalyst comprises a noble metal catalyst, such as Pt or Pd optionally combined with V2O5 and WO3 on a TiO2 or alumina carrier, and the catalyst operates at temperatures above 100°C, preferably between 150 and 250°C.
- In general, the CO2 source is available at elevated pressure, whereas the SOEC is operating close to atmospheric pressure. With respect to recycling, by arranging a compressor between the SOEC and the separation process, such as pressure swing adsorption (PSA), the need for a recycle compressor is omitted.
- In addition to the purification of the product outlet stream from the SOEC, also the CO2 feed gas on the fuel side may need to be purified. Adsorbents or absorbents are used upstream from the SOEC to remove undesired contaminants in the gas. Sulfur species and siloxanes in particular, but also other contaminants, such as halogens and higher hydrocarbons (e.g. benzene), are known to poison solid oxide cells. Such compounds can be absorbed, e.g. with active carbon or absorbents based on alumina, ZnO, Ni or Cu, such as Topsoe HTZ-51, Topsoe SC-101 and Topsoe ST-101.
- Carbon formation can also be suppressed by addition of H2S. Both carbon formation and metal dusting are normally considered to take place through the following reactions:
2CO → C + CO2 (Boudouard reaction)
and
H2 + CO → H2O + C (CO reduction)
- An addition of H2S does not affect the thermodynamic potential for metal dusting, but it pacifies the metal surfaces so that the sites, where the carbon-forming reactions would take place, are blocked.
- In the case of using SOECs for CO production, a high degree of conversion of CO2 to CO may result in a gas composition, with which there is a potential for carbon formation from the Boudouard reaction, and in the case of co-production of H2 and CO there may be a potential for carbon formation from the Boudouard reaction and from CO reduction. In particular, uneven flow distribution and current density etc. may cause local variation of the CO content above the potential limit for carbon formation.
- Adding H2S to the feed stream to a level of H2S between 50 ppb and 2 ppm, most preferably between 100 ppb and 1 ppm, would effectively suppress carbon formation in the SOEC stack, i.e. in the Ni-containing cathode, and also protect downstream equipment from metal dusting attacks. The relatively low level mentioned above is enough to suppress the formation of carbon, and at the same time it does not cause any detrimental effects on the SOEC stack performance.
- H2S can be added to the feed gas just downstream from the feed gas purification unit to protect the SOEC and the downstream equipment from carbon formation and metal dusting. As an alternative, H2S can be added just downstream from the SOEC to only protect the downstream equipment from metal dusting.
- To remove the sulfur from the product gas, the same adsorbents as used for the feed gas purification can be used, i.e. active carbon or adsorbents based on alumina, ZnO, Ni or Cu, such as Topsoe HTZ-51, Topsoe SC-101 and Topsoe ST-101. The purification unit is preferably placed between the product gas compressor or ejector and the product purification unit.
- The basic principle for feed gas purification is chemisorption of the sulfur compounds onto the active sites of the materials mentioned above. However, in the case of Ni and Cu these must stay in reduced state in order to maintain their performance with regards to feed gas purification. It should be noted, however, that pure CO2 is in essence an oxidizing environment, and there is thus a risk of oxidation with regards to Cu and Ni. The risk of oxidation is dependent on operating temperature, but for example Cu distributed over a high surface area carrier may oxidize also at temperatures close to ambient temperature.
- It is also essential to assure reducing conditions on the feed side, where the Ni-containing anode has to be kept in a reduced state at all times for temperatures above 400°C.
- In summary it is desirable to ensure reducing conditions with respect to feed gas purification and also with respect to the integrity of the SOEC. This can be accomplished by recycling CO from the SOEC.
- However, to obtain a system which is not dependent on a recycle stream, an addition of small amounts of H2 is a more practical solution from an operational point of view, as on-site storage of CO often provides challenges with respect to safety precautions due to the hazardous nature of this gas
- In order to avoid complicating the product purification process (PSA, TSA, membrane separation, cryogenic separation or liquid scrubber technology), H2 can be removed by selective oxidation of hydrogen:
2H2 + O2 → 2H2O
- The water formed is easily separated using cooling and condensation. This will make it possible to use H2 in any SOEC operation where the target product is CO.
- H2 is oxidized over oxidation catalysts at a lower temperature than CO. The applicable temperature level depends on the catalyst. A Pd or Pt catalyst can be expected to oxidize H2 at temperature levels from ambient temperature to 70°C, whereas temperatures above 150°C are needed to oxidize CO. By adding a stoichiometric level of the O2 required to oxidize the H2 present in the gas and passing the gas through a reactor containing an oxidation catalyst operating at a temperature, where H2 is selectively oxidized, the CO/CO2 product stream is effectively cleaned from H2.
- In practice it may be convenient to avoid close control of the H2 level in the gas and accurate dosing of O2, and thus a slight surplus (say 10 %) of oxygen may be applied and the remaining O2 removed in a second oxidizing reactor operating at a temperature above the oxidation temperature for CO. This assures full removal of O2 and provides an extra safety for complete removal of H2.
- O2 can be drawn conveniently from the O2-CO2 mix on the anode side of the SOEC.
- Finally, in order to avoid penetration of ambient air into the SOEC stack, the compartment around the stack may be purged with CO2. With the purpose of further utilizing this purge stream, a heater is installed to bring the inlet CO2 gas, utilized as a compartment purge, up to the operating temperature of the SOEC stack or above. This heater could for example be applied as a radiant heater, where the heater is incorporated in the CO2 purge gas manifold, simultaneously heating the physical perimeter of the stack and the inlet CO2 purge gas. In this configuration, which is shown in
Fig. 4 , the radiant heater can replace the oxygen side inlet heater, or alternatively it can be used as an additional heater which is used to reduce the time for cold start-up. - With respect to feed stock, the current invention focuses on applications, where carbon monoxide is the desired product, but the principles applied and the process configurations are also valid for the cases, where a mixture of CO2 and steam comprises the feed stock and a mixture of hydrogen and CO is the desired product. In all given embodiments and examples and for the case of utilizing a mix of CO2 and steam as feedstock, steam will follow CO2 and H2 will follow the CO product gas. However in the two-step PSA purification approach described above, the final PSA step would separate H2 from CO and is thus only applicable in cases where splitting H2 from CO is desired for the downstream process. With respect to product gas purification, steam is preferably removed from the product stream upstream from the product gas separation unit.
- In large systems, several stacks or stack sections will typically be used. Here it is a potential issue that if a stack leakage (e.g. a broken cell) appears in one stack, this may damage the neighboring stack. The mechanism here is that a crack in one cell leads to spontaneous combustion between the produced product gases and the produced oxygen. This will create a hot spot around the crack, which may create a thermal stress that enlarges the crack. This in turn leads to a large and very hot spot, which may cause a thermal stress also in neighboring cells, which again may lead to cracks in the cells. Eventually this can lead to the destruction of the entire stack and possibly also to the destruction of neighboring stacks.
- To avoid such a scenario it is possible to remove the electrolysis current selectively from failing stacks or failing stack sections. This can be done either by individual control (power supplies) for each stack (section) or by using electrical switches which can short-circuit failing stacks or stack sections.
- Once a stack or a stack section is switched off, the concentration of the desired product gas in the product gas flow will be reduced, and it is therefore desirable:
- ■ to use a gas separation unit (e.g. a PSA) with sufficient dynamic range to handle these changes in product gas compositions, and
- ■ to operate the system under conditions, where the current through the other stacks can be increased when a stack (section) is switched off. In this case the product gas composition can become more or less independent of the failure of one or even several stack (section) failures.
- Two calculations provide a comparison between the use of an ejector or a compressor to provide the necessary surplus pressure to feed the process gas from the SOEC stack to the process gas separator. It can be concluded that the ejector does provide a gas that can be handled in a PSA and that the specific CO2 consumption is increased by 66 % whereas the power consumption decreases by roughly 0.25 kwh/Nm3 CO produced. From a cost point of view this does increase the production cost slightly but the investment is reduced by up to 30 %.
Claims (23)
- A Solid Oxide Electrolysis Cell (SOEC) stack system for producing CO, comprising• an SOEC stack comprising a fuel side and an oxygen side, fuel side inlet and outlet and oxygen side inlet and outlet,• a process gas separator comprising a process gas inlet and a first and a second process gas outlet,• process gas piping,and further comprising an ejector adapted to provide a pressure increase to the process gas provided to said process gas inlet of the process gas separator.
- An SOEC stack system according to claim 1, comprising a further second ejector and piping which provides recycling of at least a part of the process gas from the second process gas outlet via said second ejector to the fuel side inlet of the SOEC stack, the second ejector is adapted to provide a pressure increase to the process gas which is recycled from the second process gas outlet to the SOEC stack.
- A process for producing carbon monoxide (CO) from carbon dioxide (CO2) in a solid oxide electrolysis cell (SOEC) stack, wherein CO2 is led to the fuel side of the SOEC with an applied current, said process further comprising:heating the inlet gas on the fuel side by means of a heating unit, so as to supply heat to the SOEC, wherein the operation temperature of said heating unit is at least the operation temperature of the cell stack minus 50°C, preferably at least the operation temperature of the cell stack, andheating the inlet gas on the oxygen side by means of a heating unit, so as to supply heat to the SOEC, wherein the operation temperature of said heating unit is at least the operation temperature of the cell stack minus 50°C, preferably at least the operation temperature of the cell stack, the content of CO in the output from the SOEC stack is 20-80 wt%,subjecting the product stream from the SOEC stack to a separation process in a process gas separator, said process gas separator being selected from pressure swing adsorption (PSA), temperature swing adsorption (TSA), membrane separation, cryogenic separation and liquid scrubber technology, such as wash with N-methyl-diethanolamine (MDEA), wherein process gas piping and an ejector is provided downstream the SOEC stack and upstream the process gas separator and said ejector is adapted to provide a pressure increase to the process gas provided to a process gas inlet of the process gas separator.
- The process according to claim 3, comprising a further second ejector and piping which provides recycling of at least a part of the process gas from a second process gas outlet of the process gas separator via said second ejector to a fuel side inlet of the SOEC stack, the second ejector is adapted to provide a pressure increase to the process gas which is recycled from the second process gas outlet to the SOEC stack.
- The process according to claim 4, wherein the pressure swing adsorption (PSA) unit comprises an adsorption step consisting of two or more adsorption columns, each containing adsorbents with selective adsorption properties towards carbon dioxide.
- The process according to claim 4, wherein the pressure swing adsorption (PSA) unit comprises an adsorption step consisting of two or more adsorption columns, each containing adsorbents with selective adsorption properties towards carbon monoxide.
- The process according to claim 3, wherein the pressure swing adsorption (PSA) unit comprises at least two adsorption steps, of which the first step comprises two or more adsorption columns, each containing adsorbents with selective adsorption properties towards carbon dioxide, while the second step comprises two or more adsorption columns, each containing adsorbents with selective adsorption properties towards carbon monoxide.
- The process according to claim 3, wherein electrical heaters are used to heat the inlet gas on the oxygen side and on the fuel side to supply heat to the SOEC stack to help it reach its operating temperature during start-up.
- The process according to claim 3, wherein no flushing on the oxygen side is used and feed gas in the form of CO2 is provided by two individually controlled flows, of which one shares a heat exchanger with the output flow from the fuel side of the stack and the other shares a heat exchanger with the output flow from the oxygen side of the stack.
- The process according to claim 3, wherein the cooling-down rate of the system is controlled, and wherein a fast cooling to below 300°C in less than 24 hours is secured through addition of a cooling medium to the system in case of power failure.
- The process according to claim 3, wherein a suitable operating temperature for the SOEC is maintained with feed effluent heat exchangers incorporated on both the oxygen side and the fuel side of the SOEC.
- The process according to claim 11, wherein excess heat on the oxygen side of the SOEC is utilized with a further heat exchanger transferring heat from the hot outlet side of the feed effluent heat exchanger on the oxygen side to the cold inlet of the feed effluent heat exchanger on the fuel side.
- The process according to claim 11 and 12, wherein a purge stream is imposed on the stream recycled from the second process gas outlet of the process gas separator to the fuel side inlet of the SOEC stack to avoid a build-up of unwanted inert components, said purge stream being passed to a catalytic oxidizer to oxidize CO to CO2 or to a thermal oxidizer before reaching the surrounding environment.
- The process according to any of the preceding claims, wherein the gas coming from the cathode side of the SOEC is quenched to a temperature of about 400-600°C to avoid metal dusting.
- The process according to claim 14, wherein the quench is carried out with an inert gas, such as N2, or preferably with CO2.
- The process according to claim 15, wherein the feed effluent heat exchanger utilizes the heat from a temperature range within 400-600°C, preferably within 450-550°C, instead of from the SOEC operating temperature in order to mitigate metal dusting.
- The process according to any of the claims 3 - 16, wherein H2S is added to the feed stream to a level between 50 ppb and 2 ppm, preferably between 100 ppb and 1 ppm, to suppress carbon formation in the system.
- The process according to claim 17, wherein the H2S is added to the feed gas immediately downstream from the feed gas purification unit to protect the SOEC stack and the downstream equipment from carbon formation and metal dusting.
- The process according to claim 17, wherein the H2S is added to the feed gas immediately downstream from the SOEC stack to protect the SOEC stack and the downstream equipment from carbon formation and metal dusting.
- The process according to any of the claims 3-19, wherein a feed gas purification unit utilizing adsorbents based on active carbon, alumina, ZnO, Ni or Cu is added to avoid poisoning of the SOEC.
- The process according to any of the claims 3 - 20, wherein small amounts of H2 are added to obtain a system which is not dependent on a recycle stream.
- The process according to any of the claims 3-21, wherein the compartment around the SOEC stack is purged with CO2, and wherein a heater is installed to bring the inlet CO2 gas, utilized as a compartment purge, up to the operating temperature of the SOEC stack or above.
- The process according to claim 22, wherein the heater is applied as a radiant heater, which is incorporated in the CO2 purge gas manifold, simultaneously heating the physical perimeter of the stack and the inlet CO2 purge gas.
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