Method for production of methane

Abstract

The present disclosure relates to a process for production of a methane rich gas having a production mode of operation and a standby mode of operation, in which standby mode involves directing a standby process gas comprising hydrogen and less than 0.1 vol% carbon oxides to contact said material catalytically active in methanation.

Description

DescriptionTitle of Invention : Method for production of methane Technical Field

[0001] The present invention relates to a process for the production of a methane rich gas from a synthesis gas rich in carbon oxides and hydrogen, in which the hydrogen is supplied from an intermittent source, such as electrolysis of water by electricity using a non-constant source, e.g. solar energy, wind energy and other renewable electricity sources.Background Art

[0002] It is known to produce synthetic natural gas (SNG) from a synthesis gas containing carbon oxides (CO and CO2) and hydrogen by the passage of such synthesis gas through a methanation stage including one or more methanation reactors each comprising a fixed bed of catalyst, where the synthesis gas is prepared by for instance gasification of a carbonaceous material. The present invention relates to such production where the supply of reactant is intermittent, e.g. due to hydrogen being produced by solar energy during daytime but not being produced during night time, or due to fluctuations in electricity prices due to the balance between supply and demand.

[0003] The operation of chemical reactions - especially at elevated temperatures - benefits from minimal variation of conditions and production rates, but when the availability of a single reactant is intermittent, alternative means must be considered.

[0004] The intuitive solution to the intermittent supply is the provision of stored reactant - in this case stored hydrogen. While this is technically possible, the storage of large amounts of hydrogen is complicated and costly, so this is undesired.

[0005] Another solution is to shut down the chemical plant, by lowering pressure and temperature to ambient condition or alternatively by not providing the reactant in demand - in this case by replacing hydrogen with nitrogen. However, for processes requiring elevated temperature, such a shut down is a source of instability and the transition to shut down and from standby back to production will require significant time.Summary of Invention

[0006] We have now identified an alternative process for this situation. By stopping the feeding of the reactant hydrogen, the process will stop, but the increased concentration of carbon oxides and methane causes a risk of coke deposition on the catalyst surface. Therefore, during the lack of availability of hydrogen, the solution is to maintain hydrogen in the process, but block or reduce the inflow of carbon oxides. The carbon oxides in the process are allowed to react with hydrogen, and a stand-by mode with less than 50 vol% methane is established, in which the carbon oxides have reacted to a concentration approaching zero, such as below 0.1 vol%.

[0007] Similar approaches may also be applied for other catalytic high temperature processes, where appropriate review over catalyst sensitivities may result in beneficial stable standby operation.Definitions

[0008] For the present application the following terms shall be understood according to the definitions below.

[0009] A material catalytically active in methanation shall be understood as a material catalyzing the methanation reactions shown below, including but not limited to nickel or ruthenium as a catalytically active constituent, provided on a support comprising alumina, such as one or more constituents from the group consisting of alumina, MgAI spinel, alumina-zirconia, and calcium aluminates.

[0010] Where concentrations are stated in % this shall be understood as volumetric (molar) % on a wet basis.

[0011] The term “carbon oxides” shall be construed as covering one or both of carbon monoxide and carbon dioxide.

[0012] The term “reactants” shall be construed as a mixture comprising carbon oxides and hydrogen, in any ratio.

[0013] The term “products” shall be construed as a mixture comprising methane and water, in any ratio.

[0014] The term ignition of the methanation process refer to the temperature and other conditions where quantitative conversion of reactants to products occurs.Detailed description

[0015] In methanation processes the formation of methane from carbon oxides and hydrogen proceeds quickly to equilibrium in the presence of a catalyst if the inlet temperature is sufficient for activating the reaction. The reaction will thus proceed in accordance with either one or both of the following reaction schemes:CO+3H2<=> CH4+H2O (1 )CO2+4H2 <=> CH4+2 H2O (2)

[0016] These reactions are coupled to equilibrium between carbon monoxide and carbon dioxide as follows:CO+H2O <=> CO2+H2 (3)

[0017] The reactions are commonly catalysed by metals of the groups 8, 9 and 10, most commonly nickel or ruthenium as a catalytically active constituent, provided on a support comprising alumina, such as one or more constituents from the group consisting of alumina, MgAI spinel, alumina-zirconia, and calcium aluminates. US 9,757,714 discloses examples of such catalysts.

[0018] If the process involves sustainable production of methane, the hydrogen would typically originate from an electrolyzer receiving water and sustainable electricity and the carbon oxides would typically be purified biogas or a waste carbon dioxide stream; preferably of biogenic origin. The carbon oxide may also be captured carbon dioxide of fossil origin, but in this case the process will not be considered sustainable as it involves a net greenhouse gas emission, but nevertheless, the capture of carbon dioxide for conversion to methane may still be a contribution to reduction greenhouse gas emission and be counted as being a process with beneficial low carbon intensity.

[0019] For traditional methanation, the feed gas has been a synthesis gas comprising H2 in combination with carbon oxides, mainly CO, which may have been adjusted to the optimal ratio between CO and H2, which is close to 3:1 , or when the “module” is defined including H2, CO and CO2 it becomes ((H2-CO2) / (CO+CO2)=3). However, when hydrogen and carbon oxides originate from distinct sources, each may befed separately, and possibly purified and otherwise pretreated separately. If the carbon oxide feed is dominated by CO2 the definition of the module means the optimal ratio between CO2 and H2, is 4:1. In general the methanation process is carried out at elevated pressure such as 20 to 30, 40 or 60 barg.

[0020] In production of synthetic natural gas from intermittent supply of hydrogen the process will not be operational during periods with absence of hydrogen, irrespectively whether this is due to wind or solar electricity not being produced or a commercial decision due to the cost of hydrogen due to electricity price.

[0021] As storage of large volumes of hydrogen is complex and expensive, it may be more cost effective to have a regime of operation involving a stand-by mode, in which no production occurs.

[0022] The net reaction of methane formation whether by reaction (1 ) or (2), or both, is highly exothermic, and accordingly, the temperature of the reactants and products will increase during the passage through a bed of a material catalytically active in methanation in an adiabatic reactor. However, such increasing temperature will tend to displace the carbon oxide / methane equilibrium towards lower methane concentration. Consequently, complete, or close to complete, reaction will only be possible if the inlet temperature is sufficient for activating the reaction, and the temperature increase is limited by cooling the reacting gas in one way or another, for instance by recycling of cooled product gas.

[0023] The methanation process may also been conducted in a cooled or isothermal reactor, such as a boiling water reactor, to increase the methane yield by displacing the chemical equilibrium towards methane.

[0024] It is also known to carry out the reaction in multiple reactors, to control the temperature of the methanation reaction, by cooling between each reactor. The reaction in the first reactor(s) is termed “bulk methanation” as a majority of the methane is produced in these steps, and the reaction in the final reactor is termed “final methanation”. The reaction temperature during bulk methanation may commonly be above 500°C or 550°C, such as up to 650°C, 700°C or higher. Especially the first bulk methanation reactor reaching the highest temperatures requires a thermally stable catalyst, with a stabilized support, e.g. comprising rare earths. Furthermore reactivity and thus temperatures may be controlled by recycleof product and addition of steam to shift the equilibrium or stepwise addition of feed to limit the extent of reaction. Accordingly, the composition at the inlet of each of the individual reactors in a methanation process may vary according to process design.

[0025] Final methanation is characterized by the presence of only small amounts of reactants, primarily CO2 and H2, such that the equilibrium between reactants and products shifts away from reactants. To shift the equilibrium further towards products, produced water may be removed, and the inlet temperature may also be reduced, and thus more active and less robust catalysts are required in this step. Commonly the final methanation catalyst is more active, so the feed temperature may be around 230°C. The upper reaction temperature during final methanation may commonly be above 300°C, such as above 350°C and commonly below 500°C, such as below 450°C.

[0026] In addition to the maintenance of process temperature in the optimum range for a favorable equilibrium, methanation processes are also sensitive to formation of coke deposits on the catalyst as well as to condensation of water, which may cause a partial or even complete deactivation of the catalyst. The catalysts applied in methanation, have the potential to decompose CH4 to free carbon, which irreversibly may block the catalyst surface. If an elevated H2 concentration is provided, this decomposition is avoided. The conditions avoiding carbon formation are determined by thermodynamics, with CH4 and temperature being significant drivers of carbon formation, moderate CO and CO2 presence will slightly lower the temperature where carbon formation occurs and increasing pressure will increase the temperature of carbon formation. Thermodynamic calculation tools are available for evaluating the correlation between composition, temperature, and pressure, but may have to be adjusted to accurately match the specific catalyst.

[0027] Now, according to the present invention, to allow for efficient production of methane from an intermittent hydrogen source, if a standby mode is employed, this must involve a sufficient temperature for fast ignition of the process and conditions avoiding coke deposits.

[0028] We have identified that blocking the inflow of hydrogen when it is not available will put the process at risk of extensive coke deposits, while circulation of a processgas comprising hydrogen and absence or a very low level of carbon oxides is safely avoiding coke deposits, even at a minimum viable temperature, and presence of up to 95 vol%, 70 vol%, 60 vol% or 50 vol% methane, if the process is allowed to cool before high methane levels are allowed. This minimum viable temperature may be sufficient for ignition of the methanation process or it may also be a lower temperature sufficient for keeping the catalyst stable, although inactive, and allowing the transition from standby mode to production mode to be done in sufficiently short time. In practice this may be obtained by a transition mode, involving stopping the addition of carbon oxides to the process and circulating recycled product such that all carbon oxides are allowed to react with hydrogen and an amount of methane rich gas is withdrawn from the process until the level of methane is less than the required limit and balanced by an amount of hydrogen under the conditions for a standby mode. Since methanation will not occur during standby mode, a thermal loss to the surroundings will cause the temperature to decrease unless heating is provided.

[0029] In order to be able to ignite the process at the time of hydrogen production, such heating to a minimum viable temperature is provided. The heating may be provided by electrical heating, heat integration with nearby processes, such as heating by a steam circuit, by a molten salt circuit e.g. heated by exported energy during operation mode or by any other heaters of appropriate nature. Beneficially the heat source used for starting up the process, is configured for allowing heating at regular intervals during standby mode of the process. The temperature maintained as the minimum viable elevated threshold temperature may for the typical bulk methanation catalysts be around 300°C and the safe elevated threshold temperature may be lower, such as 250°C or 280°C. For the typical final methanation catalysts the temperature of ignition may be around 230°C and the safe minimum viable temperature may be lower, such as 200°C or 180°C. The minimum viable temperature during standby will commonly not be above 320°C or 330°C. These sufficient temperatures for methanation will as described by Pedersen K, Skov A, Rostrup-Nielsen JR (1980) ACS Div Fuel Chem 25:89-100 (Preprints) depend on the catalyst, and may be 200°C for non-nickel catalysts and 300°C for nickel based catalysts. However, in addition to the catalyst composition the process gas composition will also define the range of stable operation, and fora given process gas composition and catalyst, this ignition temperature or operational methanation temperature is dependent on the specific catalytically active material used in the process and will be simple to determine experimentally for the skilled person. Commonly the presence of water, will increase the sufficient temperature for methanation, possibly by about 50°C.

[0030] For ultimate safety of operation, the composition in the reactor must be safe with respect to carbon formation at all conditions. This may be obtained by a standby composition avoiding carbon formation at the highest temperature of the reactors, which is the outlet temperature of the first methanation reactor. This temperature is commonly 675°C and the upper methane concentration may be around 70 vol% at this temperature. However, if a transition mode is allowed, where temperature decreases, the allowable amount of methane may be significantly higher than 70 vol%, such that the only requirements to the standby mode will be minimal amounts of carbon oxides and a moderate presence of H2, such as 2 vol%, 5 vol%, 30 vol%, 40 vol% or 50 vol%. For an efficient and safe change from standby mode to production mode the temperature must be sufficient for allowing heating of the reactor with sufficiently short notice, which will be limited as it is recommended that heating rate is kept below 50°C / hour, and finally the temperature and composition in the reactor at the time of introduction of carbon oxides must be such that metal carbonyls are not formed and methane is not decomposed to carbon.

[0031] During the standby mode other process parameters such as space velocity and pressure may also be modified to passivate the reaction and to minimize cost of operation, such as energy consumption for heating and compressors, but it may also be preferred to keep these conditions similar to process conditions to minimize thermal and mechanical stress. Still when operating the process with regular transition from production mode to standby mode, it is recommended to consider thermal stress in the design of equipment and selection of materials.

[0032] The standby mode may be implemented for the plant as a whole, with heating of standby process gas to a common temperature in a single position. An alternative implementation may involve heating of the standby process gas for each reactor, to a temperature appropriate for that reactor. This could involve controlling the standby process gas temperature to a temperature ranging from40°C or 20°C during production mode below the inlet temperature of that specific reactor to 40°C or 20°C during production mode above the inlet temperature of that specific reactor. The standby mode may also involve more than heating the standby process to a single standby temperature without a standby temperature for each reactor, such as a standby temperature for the first bulk methanation reactor, with the following bulk methanation reactors having the temperature resulting from the upstream reactors, and similar with active heating of the first final methanation reactor, with the second final methanation reactor having the inlet temperature resulting from the outlet of the first final methanation reactor. Naturally, intermediate configurations may involve heating the standby process gas in more than two positions. Finally, the circulation of standby process gas may be recirculation around the full reactor train, or around selected reactors, such as a recirculation from a position upstream a second heater to a position upstream a first heater, and a recirculation from a position downstream the last reactor to a position upstream a second heater. The configuration may also involve a recirculation loop in which there are multiple heaters. In all cases the temperature during standby mode may range from 40°C or 20°C below the inlet temperature of that specific reactor during production mode to 40°C or 20°C above the inlet temperature of that specific reactor during production mode. The ranges of standby mode temperature may be selected individually for each heater.

[0033] The standby process described above is mainly intended for moderate periods of time, such as a single night or possibly a few days, but if the process must be idle for longer periods, such as one or more weeks, it is expected that an idle state in which the process gas is replaced with an inert gas such as nitrogen. Commonly such a state would involve the nitrogen being provided at moderate pressure, such as 5 barg.Advantageous Effects of Invention

[0034] A first embodiment of the present disclosure relates to a process for production of a methane rich gas having a production mode of operation and a standby mode of operation, in which production mode involves directing a flow of production mode process gas comprising hydrogen and carbon oxides to contact a material catalytically active in methanation at an operational methanation temperature and in which standby mode involves directing a standby process gas comprisinghydrogen and less than 0.1 vol% carbon oxides having a temperature being less than 40°C or less than 20°C cooler than the operational methanation temperature to contact said material catalytically active in methanation.

[0035] This has the associated benefit of providing a process with rapid and robust change from standby to production mode. Beneficially the standby process gas may also comprise methane, as this will reduce the amount of hydrogen held in the system, but this is not a requirement. The change from standby to production mode will require a catalytically active material at sufficient temperature for methanation. The standby period may depend on e.g. weather influencing renewable electricity availability, and may commonly last at least 4 hours.

[0036] A second embodiment involves a process according to the first embodiment in which standby mode further involves provision of thermal energy by an external source of thermal energy, such as an electrical heater, heat exchange with steam or melted salt to the standby process gas.

[0037] This has the associated benefit of providing the energy for operation in an efficient way, possibly by integration with another process or by storing thermal energy from production mode to standby mode.

[0038] A third embodiment involves a process according to the second embodiment above in which the temperature of the standby process gas at the inlet of at least one reactor is at least 200°C, 280°C or 300°C and less than 330°C or 320°C.

[0039] This has the associated benefit of such temperatures being cost effective and sufficient for heating the process to the ignition temperature of the bulk methanation process in a short time. The minimum viable temperature for the final methanation step may beneficially be lower, such as down to 230°C.

[0040] A fourth embodiment involves a process according to the second or third embodiment in which the external source of thermal energy is configurable for heating process gas during a phase of process start up.

[0041] This has the associated benefit of using the equipment required during startup throughout the lifetime of the process plant, and may involve several such heaters.

[0042] A fifth embodiment involves a process according to an embodiment above in which production mode involves a net export of thermal energy.

[0043] This has the associated benefit of energy efficient processing, for instance if the hydrogen is provided by solid oxide electrolysis, requiring heat up of water.

[0044] A sixth embodiment involves a process according to an embodiment above, in which hydrogen is provided from a source of fluctuating hydrogen production, optionally electrolytically produced hydrogen receiving water and a renewable electricity source, such as wind power, solar power, water power or tidal power.

[0045] This has the associated benefit of such a hydrogen source being sustainable, and the provided methane also being a sustainable source of fuel, in a process configured for standby operation when electricity is unavailable.

[0046] A seventh embodiment involves a process according to an embodiment above, in which input from one or more of a predictive model, historical data and current data of external conditions related to energy production, such as meteorological data, irradiance data, energy consumption and value and cost of electricity and methane, is used for determining timing of a change between production mode operation, transition mode operation and standby mode operation.

[0047] This has the associated benefit of being able to plan and adjust the process conditions optimally. The input may beneficially be qualified by one or more machine learning models and / or human user interaction, and modelling of direct or indirect production costs including feed composition and catalyst activity may also be involved in the planning of process operation.

[0048] An eighth embodiment involves a process according to the sixth or seventh embodiment in which the water directed to electrolysis is process steam, heated in a one or more of a boiler and a superheater by heat exchange receiving thermal energy released during methanation and said water optionally is provided as condensate from the methanation process.

[0049] This has the associated benefit of providing steam in an energy-efficient way for a high temperature electrolysis process such as solid oxide electrolysis.

[0050] A ninth embodiment involves a process according to an embodiment above, in which one or more pretreatments of said stream rich in hydrogen is carried out, such as removal of di-oxygen and removal of sulfur containing compounds.

[0051] This has the associated benefit of providing a pure hydrogen feed, to allow for efficient and robust methane production and to ensure stability of a reduced nickel based methanation catalyst.

[0052] A tenth embodiment involves a process according to an embodiment above, in which one or more pretreatments of said stream rich in carbon oxides is carried out, such as removal of sulfur containing compounds.

[0053] This has the associated benefit of providing a pure carbon oxide feed, to allow for efficient and robust methane production, e.g. when carbon oxides originate from a biogas or other contaminated feedstream.

[0054] A twelfth embodiment involves a process according to an embodiment above, in which, during standby mode an idle section of the production plant is allowed to cool to below operational temperature, and during transition mode or operational mode, an amount of process gas contained in said idle section is optionally released without contacting the material catalytically active in methanation.

[0055] This has the associated benefit of allowing robust process elements such as those required for CO2 purification to cool down, while more sensitive process elements, such as methanation reactors are protected actively by a standby mode.

[0056] A thirteenth embodiment involves a process according to any embodiment above, further comprising an idle mode, in which said material catalytically active in methanation is maintained in contact with an idle mode standby process gas composition, comprising hydrogen or nitrogen and less than 0.1 vol% carbon oxides having a temperature being more than 40°C cooler than the operational methanation temperature.

[0057] This has the associated benefit of providing a mode of operation during prolonged absence of attractive energy supply, while keeping the process plant in stable condition. This process mode may require consideration of low temperature hydrogen corrosion if the idle mode standby process gas composition involves high concentrations of hydrogen. The process may be chosen to operate in such an idle process mode more regularly than required by maintenance of the plant, such as at least once per 6 months, 3 months or monthly.Brief Description of Drawings

[0058] Fig. 1 shows a process layout suitable for carrying out the present disclosure.

[0059] Fig. 2 shows feedstock and product during production mode of a process according to the present disclosure.

[0060] Fig. 3 shows selected process characteristics in a process according to the present disclosure.Fig.1

[0061] [Fig.1 ] is a simplified process layout for production of a methane rich gas, which is configured for operation in accordance with the present disclosure, by appropriate configuration of valves V1 , V2, V3 and V4. In the figure V1 , V2 and V3 are shown in white, indicating open valves, and V4 in black indicating a closed valve, corresponding to operation in production mode. Here a H2 feedstock 2 is directed to a deoxygenation reactor DO, where any O2 present in the H2 feedstock is reacted to form H2O, such that the content of O2 is minimized. The purified H2 feedstock 8 is combined with a CO2 feedstock 6 and a recycle gas 14 compressed in compressor CMP to bulk methanation feed 8. The raw CO2 feedstock may originate from carbon capture of fossil combustion or from biogas, and commonly be purified (such as by sulfur removal) depending on the nature of origin, but this purification is not shown here. The bulk methanation feed 10 is at approximately 28 barg directed to a first bulk methanation reactor B1 at a temperature sufficient for ignition, such as around 330°C, optionally by pre-heating in heater H2. The intermediate bulk methanation stream 12 is cooled in heat exchanger H3 to around 340°C, to support further reaction in the second bulk methanation reactor B2. Similarly, the bulk methanation product is cooled to around 310°C in heat exchange H4 and split in recycle gas 14 and final methanation feed stream 16. The final methanation feed stream reacts in the first final methanation reactor F1 and is cooled to 100°C at 25 barg in heat exchanger H6. Heat exchanger H6 (and possibly other thermal equipment) in the figure symbolizes multiple thermal equipment required for this cooling; including multiple heat exchangers, a boiler and an air cooler. The cooled process gas stream 16 is directed to condensate separator CS, in which the majority of the water of stream 18 is condensed and withdrawn as condensate 20. The dried process gas 22 is heated to 230°C in heat exchangerH7 and directed to the second final methanation reactor F2, in which the absence of water provides a strong equilibrium push towards production of methane, which heats the process stream sufficiently to heat up the dried process gas 22 in heat exchanger H7. An optional addition of oxygen in combination with a catalytic oxygenation reactor may be employed to reduce the amount of hydrogen in the methane.

[0062] In standby mode, valves V1 , V2 and V3 are all blocked, and valve V4 is opened. During transition from production mode, valve V2 is closed such that CO2 is not added to the process, while an amount of hydrogen is added to the process in line 2, through valve V1 and the product gas 24 is split between an amount withdrawn and an amount recycled, by partial opening of valves V3 and V4 until an acceptable concentration of CH4 is present in the recycle gas 26. Since the exothermal methanation reaction will be passivated, the process temperature will gradually fall, and finally when the temperature approaches the threshold temperature, the process heaters will be configured for heating the process gas sufficiently for keeping the process gas above threshold temperature which in this illustration may be 280°C.

[0063] [Fig.2] shows the concentration of the four major constituents of the process gas at the feed (stream 8 of Fig 1 ), at the outlet of reactor 1 (stream 10), at the outlet of bulk methanation (stream 14) and the final product (stream 22). The removal of the water in the final methanation contributes both by the absence of one significant component and by shifting of the equilibrium.

[0064] [Fig.3] shows key process parameters during production mode, transition mode and stand-by mode in a simplified estimation. The initial 10 hours show production mode, where a feed of 14% CO2 and 56% H2 is converted to 84% CH4. The temperature out of R1 is 675°C which provides for export of 240 Gcal / h. The change to stand-by involves intermediate concentrations before the stable standby values shown are obtained (51 % H2 and 49% CH4, and substantial absence of other compounds), and during that period the reactor temperatures gradually decrease to 300°C. This temperature is sufficient for very rapid ignition when H2 is available, and CO2 is allowed into the process at 24 hr, and stable production is realized almost immediately.

Claims

Claims

1. A process for production of a methane rich gas having a production mode of operation and a standby mode of operation, in which production mode involves directing a flow of production mode process gas comprising hydrogen and carbon oxides to contact a material catalytically active in methanation at an operational methanation temperature and in which standby mode involves directing a flow of standby process gas comprising hydrogen and less than 0.1 vol% carbon oxides having a temperature being less than 40°C or less than 20°C cooler than the operational methanation temperature to contact said material catalytically active in methanation, and heating said standby process gas by an external source of thermal energy, such as an electrical heater, heat exchange with steam or melted salt for a period of at least 4 hours.

2. A process according to claim 2, in which the operational methanation temperature standby mode involves maintaining a minimum viable temperature of the standby process gas at the inlet of at least one reactor is at least 200°C, 280°C or 300°C and less than 330°C or 320°C.

3. A process according to claim 2 or 3, in which the external source of thermal energy is configurable for heating process gas during a phase of process start up.

4. A process according to any claim above, in which production mode involves a net export of thermal energy.

5. A process according to any claim above, in which hydrogen is provided from a source of fluctuating hydrogen production, optionally electrolytically produced hydrogen receiving water and a renewable electricity source, such as wind power, solar power, water power or tidal power.

6. A process according to claim 6, in which input from one or more of a predictive model, historical data and current data of external conditions related to energy production, such as meteorological data, irradiance data, energy consumption and value and cost of electricity and methane, is usedfor determining timing of a change between production mode operation, transition mode operation and standby mode operation.

7. A process according to claim 6 or 7, in which the water directed to electrolysis is process steam, heated in one or more of a boiler and a superheater by heat exchange receiving thermal energy released during methanation and said water optionally is provided as condensate from the methanation process.

8. A process according to any claim above, in which one or more pretreatments of said stream rich in hydrogen is carried out, such as removal of di-oxygen and removal of sulfur containing compounds.

9. A process according to any claim above, in which one or more pretreatments of said stream rich in carbon oxides is carried out, such as removal of sulfur containing compounds.

10. A process according to claim 9 above, in which during standby mode an idle section of the production plant is allowed to cool to below operational temperature, and during transition mode or operational mode, an amount of process gas contained in said idle section is optionally released without contacting the material catalytically active in methanation.

11. A process according to any claim above, further comprising an idle mode, in which said material catalytically active in methanation is maintained in contact with a passive mode standby process gas composition, comprising hydrogen or nitrogen and less than 0.1 vol% carbon oxides having a temperature being more than 40°C cooler than the operational methanation temperature.

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