(19)
(11)EP 2 989 233 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
22.07.2020 Bulletin 2020/30

(21)Application number: 13711915.2

(22)Date of filing:  26.03.2013
(51)International Patent Classification (IPC): 
C25B 15/00(2006.01)
C25B 1/00(2006.01)
C25B 9/18(2006.01)
C25B 15/08(2006.01)
C25B 9/00(2006.01)
C25B 15/02(2006.01)
(86)International application number:
PCT/EP2013/056411
(87)International publication number:
WO 2014/154253 (02.10.2014 Gazette  2014/40)

(54)

A PROCESS FOR PRODUCING CO FROM CO2 IN A SOLID OXIDE ELECTROLYSIS CELL

VERFAHREN ZUR HERSTELLUNG VON CO AUS CO2 IN EINER FESTOXID-ELEKTROLYSEZELLE

PROCÉDÉ DE PRODUCTION DE CO À PARTIR DE CO2 DANS UNE CELLULE D'ÉLECTROLYSE À OXYDE SOLIDE


(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(43)Date of publication of application:
02.03.2016 Bulletin 2016/09

(73)Proprietor: Haldor Topsøe A/S
2800 Kgs. Lyngby (DK)

(72)Inventors:
  • JAKOBSSON, Niklas Bengt
    26023 Kågeröd (SE)
  • FRIIS PEDERSEN, Claus
    2720 Vanløse (DK)
  • BØGILD HANSEN, John
    3050 Humlebaek (DK)

(74)Representative: Haldor Topsøe A/S 
Haldor Topsøes Allé 1
2800 Kgs. Lyngby
2800 Kgs. Lyngby (DK)


(56)References cited: : 
EP-A2- 0 129 444
US-A- 6 106 966
US-A1- 2008 023 338
WO-A2-2013/131778
US-A1- 2006 130 647
US-A1- 2011 253 551
  
  • JOHN B. HANSEN ET AL: "Production of Sustainable Fuels by Means of Solid Oxide Electrolysis", ECS TRANSACTIONS, vol. 35, no. 1, 1 May 2011 (2011-05-01), pages 2941-2948, XP055067864, ISSN: 1938-5862, DOI: 10.1149/1.3570293
  • EBBESEN S D ET AL: "Electrolysis of carbon dioxide in Solid Oxide Electrolysis Cells", JOURNAL OF POWER SOURCES, ELSEVIER SA, CH, vol. 193, no. 1, 19 March 2009 (2009-03-19), pages 349-358, XP026150424, ISSN: 0378-7753, DOI: 10.1016/J.JPOWSOUR.2009.02.093 [retrieved on 2009-03-19] cited in the application
  
Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


Description


[0001] This invention belongs to the field of electrolysis conducted in solid oxide electrolysis cell (SOEC) stacks. 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. Fig. 1 shows the principle build-up of a solid oxide electrolysis cell system. It 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.

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] US 2011/0253551 relates to a method and an apparatus for producing a CO containing product, where cathode and anode sides of an electrically driven oxygen separation device are contacted with CO2 and a reducing agent, respectively. The CO2 is reduced to CO through ionization of oxygen, and the reduction agent lowers the partial pressure of oxygen at the anode side to partially drive oxygen ion transport within the device through consumption of the oxygen and also to supply heat. Lowering of oxygen partial pressure reduces the voltage, and therefore electrical power is to be applied to the device, and the heat is supplied to heat the device to the operation temperature and to reduce the CO2 occurring at the cathode side.

[0008] US 2008/0023338 discloses high temperature (about 500 to 1200°C) SOEC electrolysis for syngas production, where syngas components H2 and CO are formed by decomposition of CO2 and H2O or steam. One or more of the components for the process, such as steam, energy or electricity, may be provided using a nuclear power source.

[0009] J.B. Hansen et al.: "Production of Sustainable Fuels by Means of Solid Oxide Electrolysis", ECS Transactions 35(1), 2941-2948 (2011) has its main focus on the production of sustainable fuels by pressurized SOEC operation. This option is attractive because it is considered more efficient to generate the required syngas pressure by evaporating water and operate the SOEC under pressure than compressing the syngas downstream the stacks.

[0010] In WO 2013/131778 belonging to the Applicant, an apparatus for production of high purity CO is disclosed, said apparatus making it possible to produce CO at a low cost without having to use cryogenic techniques. More specifically it becomes technically and economically feasible to produce high purity CO in small quantities, for example at the consumption site or at local distribution centers.

[0011] 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.

[0012] Besides the above-mentioned patents and patent applications, the concept of electrolysing CO2 in solid oxide electrolysis cells is described in "Modeling 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).

[0013] Specifically the present invention relates to 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, and wherein the content of CO in the output from the SOEC stack is 20-80 wt%, 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, 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, and
  • the carbon dioxide from a gas separation unit, which still contains some carbon monoxide, is recycled to the fuel side of the SOEC and tied to the CO2 feed stream upstream from a feed gas purification unit.



[0014] The principle underlying the present invention 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).

[0015] 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.

[0016] 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. This embodiment is shown in Fig.15. 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.

[0017] This basic principle of the invention is illustrated in Fig. 2. Various embodiments of the process according to the invention will appear from the following detailed description of the invention.

[0018] In the above layout according to the basic principle of the invention 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.

[0019] 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.

[0020] 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.

[0021] 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. The apparatus layout corresponding to this embodiment of the process is illustrated in Fig. 3.

[0022] 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 is utilized to heat the inlet gas. This allows 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. The apparatus layout corresponding to this embodiment of the process is illustrated in Fig. 4.

[0023] 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 illustrated in Fig.4. As a heat effective alternative, the configuration shown in Fig.5 can be used. Here, no flushing on the oxygen side is used, and feed gas (CO2) is provided by two individually controlled flows. One flow (a) shares a heat exchanger (A) with the output flow from the SOEC fuel side, and the other flow (b) shares a heat exchanger (B) with the output flow from the oxygen side of the SOEC. By adjusting the flows (a) and (b) while maintaining the desired total input (a + b) 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).

[0024] 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.

[0025] 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.

[0026] 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 as shown in Fig. 6. 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.

[0027] The gas connections for the heating and the cooling flows may be identical.

[0028] 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; see Fig. 7.

[0029] 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.

[0030] 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.

[0031] 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; see Fig. 8.

[0032] 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.

[0033] 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.

[0034] 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. The apparatus layout corresponding to this embodiment of the process is illustrated in Fig. 9.

[0035] 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. Fig. 10 shows an apparatus layout corresponding to that shown in Fig. 9, but with a feed gas purification unit added.

[0036] 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)



[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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. This embodiment of the invention is illustrated in Fig. 13.

[0041] 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 and the product purification unit.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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. This embodiment of the invention is illustrated in Fig. 14 a and b.

[0046] 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



[0047] 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.

[0048] 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.

[0049] 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.

[0050] O2 can be drawn conveniently from the O2-CO2 mix on the anode side of the SOEC.

[0051] 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. 11, 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.

[0052] 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.

[0053] 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.

[0054] To avoid such a scenario, one preferred system lay-out illustrated in Fig. 12 makes it 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.

[0055] 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.




Claims

1. 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, and wherein the content of CO in the output from the SOEC stack is 20-80 wt%, 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,

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, and

- the carbon dioxide from a gas separation unit, which still contains some carbon monoxide, is recycled to the fuel side of the SOEC and tied to the CO2 feed stream upstream from feed a gas purification unit.


 
2. The process according to claim 1, wherein the product stream from the SOEC stack is subjected to a separation process in a separation unit, said separation unit 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).
 
3. The process according to claim 2, 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.
 
4. The process according to claim 2, 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.
 
5. The process according to claim 2, 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.
 
6. The process according to claim 1, 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.
 
7. The process according to Claim 1, 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.
 
8. The process according to claim 7, wherein a compressor is placed between the SOEC stack and the purification unit.
 
9. The process according to claim 7, wherein a purge stream is imposed on the recycle stream 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.
 
10. 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.
 
11. The process according to claim 10, wherein the quench is carried out with an inert gas, such as N2, or preferably with CO2.
 
12. The process according to claim 10, 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.
 
13. The process according to any of the preceding claims, 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.
 
14. The process according to claim 13, 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.
 
15. The process according to claim 13, wherein the H2S is added to the feed gas immediately downstream from the SOEC stack to protect the downstream equipment from carbon formation and metal dusting.
 
16. The process according to any of the preceding claims, 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.
 
17. The process according to any of the preceding claims, wherein small amounts of H2 are added to obtain a system which is not dependent on a recycle stream.
 
18. The process according to any of the preceding claims, 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.
 
19. The process according to claim 18, 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.
 


Ansprüche

1. Verfahren zur Herstellung von Kohlenmonoxid (CO) aus Kohlendioxid (CO2) in einem Festoxid-Elektrolyse-Zellenstapel (SOEC), wobei CO2 bei angelegtem Strom zur Brennstoffseite der SOEC geführt wird, und wobei der Gehalt an CO im Auslass des SOEC Stapels 20-80 Gew.-% beträgt, wobei das Verfahren ferner umfasst:

Erwärmen des Einlassgases auf der Brennstoffseite mittels einer Heizeinheit, um der SOEC Wärme zuzuführen, wobei die Betriebstemperatur der Heizeinheit mindestens die Betriebstemperatur des Zellenstapels minus 50°C, bevorzugt mindestens die Betriebstemperatur des Zellenstapels, ist und

Erwärmen des Einlassgases auf der Sauerstoffseite mittels einer Heizeinheit, um der SOEC Wärme zuzuführen, wobei die Betriebstemperatur der Heizeinheit mindestens die Betriebstemperatur des Zellenstapels minus 50°C, bevorzugt mindestens die Betriebstemperatur des Zellenstapels ist,

wobei

- eine geeignete Betriebstemperatur für die SOEC mit Zustromabflusswärmetauschern aufrechterhalten wird, die sowohl auf der Sauerstoffseite als auch auf der Brennstoffseite der SOEC eingebaut sind, und

- das Kohlendioxid aus einer Gastrenneinheit, das noch etwas Kohlenmonoxid enthält, zur Brennstoffseite der SOEC zurückgeführt und stromaufwärts von der Speisegasreinigungseinheit mit dem CO2-Zufuhrstrom verbunden wird.


 
2. Verfahren gemäß Anspruch 1, wobei der Produktstrom aus dem SOEC-Stapel einem Trennverfahren in einer Trenneinheit unterzogen wird, wobei die Trenneinheit ausgewählt ist aus Druckwechseladsorption (PSA), Temperaturwechseladsorption (TSA), Membrantrennung, Tieftemperaturtrennung und Flüssigkeitswäschertechnologie, z. B. Waschen mit N-Methyldiethanolamin (MDEA).
 
3. Verfahren gemäß Anspruch 2, wobei die Druckwechseladsorptionseinheit (PSA) einen Adsorptionsschritt umfasst, der aus zwei oder mehr Adsorptionssäulen besteht, die jeweils Adsorbentien mit selektiven Adsorptionseigenschaften gegenüber Kohlendioxid enthalten.
 
4. Verfahren gemäß Anspruch 2, wobei die Druckwechseladsorptionseinheit (PSA) einen Adsorptionsschritt umfasst, der aus zwei oder mehr Adsorptionssäulen besteht, die jeweils Adsorbentien mit selektiven Adsorptionseigenschaften gegenüber Kohlenmonoxid enthalten.
 
5. Verfahren gemäß Anspruch 2, wobei die Druckwechseladsorptionseinheit (PSA) mindestens zwei Adsorptionsschritte umfasst, von denen der erste Schritt zwei oder mehr Adsorptionssäulen umfasst, die jeweils Adsorbentien mit selektiven Adsorptionseigenschaften gegenüber Kohlendioxid enthalten, während der zweite Schritt zwei oder mehr Adsorptionssäulen umfasst, die jeweils Adsorbentien mit selektiven Adsorptionseigenschaften gegenüber Kohlenmonoxid enthalten.
 
6. Verfahren gemäß Anspruch 1, wobei keine Spülung an der Sauerstoffseite verwendet wird und Speisegas in Form von CO2 durch zwei einzeln geregelte Ströme bereitgestellt wird, von denen einer einen Wärmetauscher mit dem Ausgangsstrom der Brennstoffseite des Stapels teilt und der andere einen Wärmetauscher mit dem Ausgangsstrom der Sauerstoffseite des Stapels teilt.
 
7. Verfahren gemäß Anspruch 1, wobei die Abkühlgeschwindigkeit des Systems geregelt wird und wobei eine schnelle Abkühlung auf unter 300°C in weniger als 24 Stunden durch Zugabe eines Kühlmediums zum System im Falle eines Stromausfalls sichergestellt wird.
 
8. Verfahren gemäß Anspruch 7, wobei ein Kompressor zwischen dem SOEC-Stapel und der Reinigungseinheit angeordnet ist.
 
9. Verfahren gemäß Anspruch 7, wobei dem Rückführstrom ein Spülstrom überlagert wird, um eine Ansammlung von unerwünschten inerten Komponenten zu vermeiden, wobei der Spülstrom zu einem katalytischen Oxidationsmittel geleitet wird, um CO zu CO2 zu oxidieren, oder vor dem Erreichen der Umgebung zu einem thermischen Oxidator.
 
10. Verfahren gemäß einem der vorhergehenden Ansprüche, wobei das von der Kathodenseite der SOEC kommende Gas auf eine Temperatur von etwa 400-600°C abgeschreckt wird, um Metallstaubbildung (Metal Dusting) zu vermeiden.
 
11. Verfahren gemäß Anspruch 10, wobei das Abschrecken mit einem Inertgas durchgeführt wird, wie z.B. N2, oder bevorzugt CO2.
 
12. Verfahren gemäß Anspruch 10, wobei der Zustromabflusswärmetauscher Wärme aus einem Temperaturbereich zwischen 400-600°C, bevorzugt zwischen 450-550°C, statt der SOEC-Betriebstemperatur nutzt, um Metallstaubbildung zu reduzieren.
 
13. Verfahren gemäß einem der vorhergehenden Ansprüche, wobei dem Speisestrom H2S auf einen Gehalt von zwischen 50 ppb und 2 ppm, bevorzugt zwischen 100 ppb und 1 ppm, zugesetzt wird, um die Kohlenstoffbildung im System zu unterdrücken.
 
14. Verfahren gemäß Anspruch 13, wobei das H2S dem Speisegas unmittelbar stromabwärts der Speisegasreinigungseinheit zugesetzt wird, um den SOEC-Stapel und die nachgeschaltete Ausrüstung vor Kohlenstoffbildung und Metallstaubbildung zu schützen.
 
15. Verfahren gemäß Anspruch 13, wobei das H2S dem Speisegas unmittelbar stromabwärts des SOEC-Stapels zugesetzt wird, um die nachgeschaltete Ausrüstung vor Kohlenstoffbildung und Metallstaubbildung zu schützen.
 
16. Verfahren gemäß einem der vorhergehenden Ansprüche, wobei eine Speisegasreinigungseinheit unter Verwendung von Adsorbentien auf der Basis von Aktivkohle, Aluminiumoxid, ZnO, Ni oder Cu eingebaut wird, um eine Vergiftung der SOEC zu vermeiden.
 
17. Verfahren gemäß einem der vorhergehenden Ansprüche, wobei kleine Mengen von H2 zugesetzt werden, um ein System zu erhalten, das nicht von einem Rückführungsstrom abhängig ist.
 
18. Verfahren gemäß einem der vorhergehenden Ansprüche, wobei die Kammer um den SOEC-Stapel mit CO2 gespült wird und wobei eine Heizung installiert wird, um das als Kammerspülung verwendete CO2-Einlassgas auf die Betriebstemperatur des SOEC-Stapels oder höher zu bringen.
 
19. Verfahren gemäß Anspruch 18, wobei die Heizung als Strahlungsheizung angewendet wird, die in den CO2-Spülgasverteiler eingebaut ist und gleichzeitig den physischen Bereich des Stapels und das Einlass-CO2-Spülgas erwärmt.
 


Revendications

1. Procédé de production de monoxyde de carbone (CO) à partir de dioxyde de carbone (CO2) dans un empilement de cellules d'électrolyse à oxyde solide (SOEC), dans lequel le CO2 est conduit vers le côté combustible de la SOEC avec un courant appliqué, et dans lequel la teneur en CO dans la sortie de l'empilement de SOEC est comprise entre 20 et 80 % en poids, ledit procédé comprenant en outre:

le chauffage du gaz d'admission côté combustible au moyen d'une unité de chauffage, de manière à fournir de la chaleur à la SOEC, dans lequel la température de fonctionnement de ladite unité de chauffage est au moins la température de fonctionnement de l'empilement de cellules moins 50°C, de préférence au moins la température de fonctionnement de l'empilement de cellules, et

le chauffage du gaz d'admission côté oxygène au moyen d'une unité de chauffage, de manière à fournir de la chaleur à la SOEC, dans lequel la température de fonctionnement de ladite unité de chauffage est au moins la température de fonctionnement de l'empilement de cellules moins 50°C, de préférence au moins la température de fonctionnement de l'empilement de cellules, dans lequel

- une température de fonctionnement appropriée pour la SOEC est maintenue avec des échangeurs de chaleur charge-effluent incorporés à la fois du côté oxygène et du côté combustible de la SOEC, et

- le dioxyde de carbone d'une unité de séparation de gaz, qui contient encore du monoxyde de carbone, est recyclé vers le côté combustible de la SOEC et lié au flux d'alimentation en CO2 en amont d'une unité de purification de gaz d'alimentation.


 
2. Procédé selon la revendication 1, dans lequel le flux de produits de l'empilement de SOEC est soumis à un processus de séparation dans une unité de séparation, ladite unité de séparation étant choisie parmi une adsorption à variation de pression (PSA), une adsorption à variation de température (TSA), une séparation par membrane, une séparation cryogénique et une technologie d'épuration liquide, comme le lavage à la N-méthyl-diéthanolamine (MDEA).
 
3. Procédé selon la revendication 2, dans lequel l'unité d'adsorption à variation de pression (PSA) comprend une étape d'adsorption constituée de deux colonnes d'adsorption ou plus, chacune contenant des adsorbants ayant des propriétés d'adsorption sélective vis-à-vis du dioxyde de carbone.
 
4. Procédé selon la revendication 2, dans lequel l'unité d'adsorption à variation de pression (PSA) comprend une étape d'adsorption constituée de deux colonnes d'adsorption ou plus, chacune contenant des adsorbants ayant des propriétés d'adsorption sélective vis-à-vis du monoxyde de carbone.
 
5. Procédé selon la revendication 2, dans lequel l'unité d'adsorption à variation de pression (PSA) comprend au moins deux étapes d'adsorption, dont la première étape comprend deux colonnes d'adsorption ou plus, chacune contenant des adsorbants ayant des propriétés d'adsorption sélective vis-à-vis du dioxyde de carbone, tandis que la deuxième étape comprend deux colonnes d'adsorption ou plus, chacune contenant des adsorbants ayant des propriétés d'adsorption sélective vis-à-vis du monoxyde de carbone.
 
6. Procédé selon la revendication 1, dans lequel aucun rinçage côté oxygène n'est utilisé et le gaz d'alimentation sous forme de CO2 est fourni par deux flux régulés individuellement, dont l'un partage un échangeur de chaleur avec le flux de sortie du côté combustible de l'empilement et l'autre partage un échangeur de chaleur avec le flux de sortie du côté oxygène de l'empilement.
 
7. Procédé selon la revendication 1, dans lequel la vitesse de refroidissement du système est régulée, et dans lequel un refroidissement rapide à moins de 300°C en moins de 24 heures est assuré par l'ajout d'un milieu de refroidissement au système en cas de panne de courant.
 
8. Procédé selon la revendication 7, dans lequel un compresseur est placé entre l'empilement de SOEC et l'unité de purification.
 
9. Procédé selon la revendication 7, dans lequel un flux de purge est imposé au flux de recyclage pour éviter une accumulation de composants inertes indésirables, ledit flux de purge étant passé sur un oxydant catalytique pour oxyder le CO en CO2 ou sur un oxydant thermique avant d'atteindre le milieu environnant.
 
10. Procédé selon l'une quelconque des revendications précédentes, dans lequel le gaz provenant du côté cathode de la SOEC est trempé à une température d'environ 400 à 600°C pour éviter la formation de poussière de métal.
 
11. Procédé selon la revendication 10, dans lequel la trempe est effectuée avec un gaz inerte, tel que le N2, ou de préférence avec le CO2.
 
12. Procédé selon la revendication 10, dans lequel l'échangeur de chaleur charge-effluent utilise la chaleur d'une plage de températures comprise entre 400 et 600°C, de préférence entre 450 et 550°C, au lieu de la température de fonctionnement de la SOEC afin d'atténuer la formation de poussière de métal.
 
13. Procédé selon l'une quelconque des revendications précédentes, dans lequel de l'H2S est ajouté au flux d'alimentation à un niveau compris entre 50 ppb et 2 ppm, de préférence compris entre 100 ppb et 1 ppm, pour supprimer la formation de carbone dans le système.
 
14. Procédé selon la revendication 13, dans lequel l'H2S est ajouté au gaz d'alimentation immédiatement en aval de l'unité de purification de gaz d'alimentation pour protéger l'empilement de SOEC et l'équipement en aval de la formation de carbone et de la formation de poussières de métal.
 
15. Procédé selon la revendication 13, dans lequel l'H2S est ajouté au gaz d'alimentation immédiatement en aval de l'empilement de SOEC pour protéger l'équipement en aval de la formation de carbone et de la formation de poussières de métal.
 
16. Procédé selon l'une quelconque des revendications précédentes, dans lequel une unité de purification de gaz d'alimentation utilisant des adsorbants à base de charbon actif, d'alumine, de ZnO, de Ni ou de Cu est ajoutée pour éviter l'empoisonnement de la SOEC.
 
17. Procédé selon l'une quelconque des revendications précédentes, dans lequel de petites quantités d'H2 sont ajoutées pour obtenir un système qui ne dépend pas d'un flux de recyclage.
 
18. Procédé selon l'une quelconque des revendications précédentes, dans lequel le compartiment autour de l'empilement de SOEC est purgé avec du CO2, et dans lequel un réchauffeur est installé pour amener le CO2 gazeux d'admission, utilisé comme purge de compartiment, jusqu'à la température de fonctionnement de l'empilement de SOEC ou au-delà.
 
19. Procédé selon la revendication 18, dans lequel le réchauffeur est appliqué en tant que réchauffeur rayonnant, qui est incorporé dans le collecteur de gaz de purge CO2, chauffant simultanément le périmètre physique de l'empilement et le gaz de purge CO2 d'admission.
 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



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Patent documents cited in the description




Non-patent literature cited in the description