Process for performing methanation under intermittent operating conditions

Abstract

A method for operating a methanation zone comprising three or more methanation reactors. The method comprises: providing a feed gas stream to the methanation zone, when the operating capacity of the methanation zone falls below a turndown threshold at least due to a decreased amount of hydrogen in the feed stream, continuing to operate the methanation zone. During such operation, when a temperature of any reactor falls below 250 °C, heating at least a portion of a product stream from one or more reactors to produce a heated recycle stream; and providing the heated recycle stream to the methanation zone, preferably as part of the feed gas stream to maintain the temperature of all reactors in methanation zone at or above 250 °C at least until the hydrogen amount increases to above the turndown ratio.

Description

PROCESS FOR PERFORMING METHANATION UNDER INTERMITTENT OPERATING CONDITIONSField of the Invention

[0001] The present specification generally relates to the field of methanation to produce synthetic natural gas, and more specifically to systems and processes for producing the synthetic natural gas under intermittent operating conditions.Background of the Invention

[0002] This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of any prior art.

[0003] Renewable or “green” energy sources, such as electricity generated by photovoltaic panels or wind turbines, has garnered increasing interest in efforts to decarbonize global energy consumption. Also, of great interest is “green” hydrogen which includes hydrogen generated by water electrolysis powered by renewable energy or by low-carbon power. The green hydrogen can be used as a feedstock in other applications, such as methanation, to further facilitate the energy transition. Generally, methanation is a process that converts carbon monoxide (CO) and carbon dioxide (CO2) into methane (CFU) by using a catalyst and hydrogen (H2). This process is commonly used to produce synthetic natural gas (SNG). Methanation can play a substantive role in the energy transition as it offers the option to convert carbon dioxide emissions from industrial processes, along with a renewable resource, such as green hydrogen, into a useful fuel source.

[0004] The methanation reaction, also known as the Sabatier reaction, is a chemical reaction that occurs in the presence of a catalyst, usually a metal such as nickel or cobalt, and is exothermic. The general equation for the methanation reaction is;CO2+ 4H2CH4+ 2H2O

[0005] The reaction proceeds in two steps. In the first step, carbon dioxide is reduced to carbon monoxide (CO) by reacting with hydrogen gas:CO2+ 2H2^ CO + 2H2O

[0006] In the second step, carbon monoxide reacts with additional hydrogen gas to produce methane and water vapor: CO + 3H2^ CH4 + H2O

[0007] Typically, “g electricity generated by a renewable power source, such as wind or solar, is routed to a utility power grid system to provide to hydrogen production facilities. As such, the “green” hydrogen is produced indirectly from green electricity rather than directly from the renewable energy sources themselves. There are advantages to receiving the green electricity directly due to drawbacks associated with depending on the electric power grid, such as being subject to interruptions in the electric power grid system, including blackouts and other grid disturbances (e.g. voltage dip, power quality, voltage oscillation etc) and faults. Renewable energy sources include, but are not limited to wind, hydroelectric power, geothermal, solar, and tidal wave action. Some of these alternative energy sources provide intermittent, time-variable power, and / or power-variable power, which can pose potential challenges to receiving the green or renewable electricity directly. For example, the rate of hydrogen production generally depends on the availability of electricity. Intermittent availability and / or varied power of the renewable energy tends to lead to intermittent and / or varied amounts of electrical generation, which can translate to intermittent and / or varied hydrogen production amounts.

[0008] Various processes are known for producing SNG, such as WO2016139451 and WO2016139452, which provide various reactors arrangements in the methanation process to perform the reaction under different conditions.

[0009] These disclosures, however, do not address challenges associated with the potential intermittent availability hydrogen. As such, there is still a need to provide improvements to the methanation process. These and other objectives will become apparent from the disclosure provided herein. Summary of the Invention

[0010] The present disclosure provides methods and systems allow the methanation reactors to continue to operate even when the hydrogen amount is below the turndown or shutdownthreshold, which enables the methanation production to resume when the hydrogen amount rises above the turndown or shutdown threshold without engaging in the startup process.

[0011] According to one aspect, there is provided a method for operating a methanation zone (100) comprising three or more methanation reactors in fluid communication. The method comprises: providing a feed gas stream to the methanation zone. When the feed gas stream comprises hydrogen and carbon dioxide in an amount to provide at least one reactor (102) with hydrogen and carbon dioxide, the methanation zone is operated to convert the hydrogen and carbon dioxide in the respective reactors to a product stream from each reactor in the methanation zone, wherein each respective product stream comprises methane. At least a portion of a product stream is recycled to the methanation zone, preferably as part of the feed gas stream, to facilitate controlling a temperature of one or more reactors to below 650 C, preferably below 630 C, more preferably below 620 C. When the operating capacity of the methanation zone falls below a turndown threshold at least due to a decreased amount of hydrogen in the feed stream, the methanation zone is continued to be operated. During such operation, when the temperature of any reactor falls below 250 ^C, heating at least a portion of a product stream from one or more reactors to produce a heated recycle stream; and providing the heated recycle stream to the methanation zone, preferably as part of the feed gas stream, to maintain the temperature of all reactors in methanation zone at or above 250 ^C at least until the hydrogen amount increases to above the turndown ratio.

[0012] Optionally, in certain embodiments, availability of the hydrogen in the fresh feed fluctuates such that the hydrogen in the feed gas fluctuates between above and below the turndown ratio over a period of operation of the methanation zone.

[0013] Optionally, in certain embodiments, the steps of providing sequentially less hydrogen, providing a portion of a product stream, and optionally, providing a heated recycle stream are performed instead of turning down the methanation zone when the operating capacity falls below the turndown ratio at least due to a decreased amount of hydrogen in the feed stream.

[0014] Optionally, in certain embodiments, at least three methanation reactors are connected in series.

[0015] Optionally, in certain embodiments, at least three methanation reactors are connected in parallel.

[0016] Optionally, in certain embodiments, the recycle stream is heated using a heating component selected from a group consisting of electrical, steam, waste heat recovery streams, and any combination thereof.

[0017] Optionally, in certain embodiments, the feed gas stream consists essentially of the heated recycle stream.

[0018] Optionally, in certain embodiments, the feed gas stream comprises green hydrogen.

[0019] Optionally, in certain embodiments, a majority of hydrogen in the feed gas stream being green hydrogen.

[0020] Optionally, in certain embodiments, fresh hydrogen in the feed gas consists essentially of green hydrogen.

[0021] Optionally, in certain embodiments, the method further comprises: prior to providing the feed gas comprising hydrogen in an amount at or above the turndown ratio, if any reactor in the methanation zone is below 250 ^C, providing a heated inert stream to the methanation zone to heat the reactor(s) to at least 250 ^C.

[0022] Optionally, in certain embodiments, the turndown ratio is less than 20%, preferably less than 30%, and preferably less than 40%. Brief Description of the Drawings

[0023] FIG. 1 depicts a diagram of an exemplary embodiment of a methanation zone that comprises three methanation reactors connected in series.

[0024] FIG. 2 depicts a diagram of an exemplary embodiment of a methanation zone that comprises one methanation reactor in parallel with three methanation reactors connected in series.

[0025] FIG.3 depicts a diagram of an exemplary embodiment of operation of the methanation zone of FIG.1 during intermittent availability of hydrogen according to aspects disclosed in the present disclosure.

[0026] FIG. 4 depicts a diagram of an exemplary embodiment of a methanation system that comprises four methanation zones.

[0027] FIG.5 depicts a diagram of an exemplary embodiment of operation of the methanation system of FIG.4 during intermittent availability of hydrogen according to aspects disclosed in the present disclosure.

[0028] FIG. 6 depicts a diagram of another exemplary embodiment of operation of the methanation system of FIG. 4 during intermittent availability of hydrogen according to aspects disclosed in the present disclosure.

[0029] FIG. 7 depicts a diagram of yet another exemplary embodiment of operation of the methanation system of FIG. 4 during intermittent availability of hydrogen according to aspects disclosed in the present disclosure. Detailed Description of the Invention

[0030] The present invention will now be described in detail with reference to embodiments thereof as illustrated in the accompanying drawings. References to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the field, and which would be apparent to those skilled in the art, are within the spirit and scope of the invention.

[0031] Although the description herein provides numerous specific details that are set forth for a thorough understanding of illustrative embodiments, it will be apparent to one skilled in the art that embodiments may be practiced without some or all of these specific details. In other instances, well known process steps and / or structures have not been described in detail in order to not unnecessarily obscure the present invention. The features and advantages of embodiments may be better understood with reference to the drawings and discussions that follow.

[0032] In addition, when like elements are used in one or more figures, identical reference characters will be used in each figure, and a detailed description of the element will be provided only at its first occurrence. Some features or components of the systems or processes described herein may be omitted in certain depicted configurations in the interest of clarity.

[0033] While specific embodiments have been described herein, it is understood that such descriptions are not intended to limit the described embodiments. Instead, any combination of the features and elements provided above, whether related to different embodiments or not, iscontemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the aspects, features, embodiments and advantages described herein are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s).

[0034] As noted above, there are potential challenges to operating a methanation facility that receives a major portion (greater than 50%, including essentially all) of hydrogen for the methanation from an intermittently available source, such as a green hydrogen production facility that is receiving renewable energy directly. If the equipment to produce synthetic natural gas receives hydrogen from such a green hydrogen production facility, then the methanation reaction can be subject to the intermittent availability, including varied amounts, of the green hydrogen. When the hydrogen amount decreases to a certain amount, the methanation reaction generally stops. When the hydrogen amount increases to an adequate level, the methanation reaction can begin again. However, in an industrial facility setting, it can be challenging for the methanation production equipment to readily respond to such fluctuations due to the desire to heat up the methanation reactor and catalyst bed to accelerate the start of the exothermic methanation reaction process. If the reactor vessel and catalyst bed are not sufficiently heated prior to the introduction of the carbon dioxide and hydrogen gas, the reaction may proceed slowly, if at all. This is because the reaction requires a certain amount of thermal energy to overcome the activation energy barrier and reach its equilibrium state. Referring to FIGS.1 and 2, optionally, if the temperature of any reactor(s) 102 in methanation zone 100 is below 250C, the start-up process may desirably include providing a heated inert stream 108 to methanation zone 100 to ensure all reactors 100 have a temperature of 250C or higher before feed stream 104 is provided to reactors 102. Suitable examples of inert stream 108 includes a nitrogen stream, such as comprising at least 80 vol% N2.

[0035] Typically, when the amount of hydrogen in the feed stream decreases to below a turndown ratio for any reason, the facility transitions from production mode to shutdown mode. As used herein, the term “turndown” as used herein is a selected ratio between the minimum operating capacity and the maximum operating capacity of the facility. During the shutdown mode, the reactors and catalyst beds begin to cool down due to heat losses, likely to below the desired starting reaction temperature. When the hydrogen amount returns to above the selectedturndown ratio, the facility can transition back to production mode from shutdown mode. Starting up production, however, can involve long heating time to bring the reactors and catalyst beds to the desired starting temperature. Such time investment may be acceptable if the shutdown and start up cycles are infrequent, typically a few times a year, such as for maintenance or unexpected events. That may not be the case if the shutdowns occur frequently and over a prolonged operational period, such as due to the fluctuations in availability of the hydrogen. Beyond the time lost to heating up the reactors that leads to loss of production, frequent shutdown and start up cycles can increase thermal stress on the equipment that can lead to shorter operational lifespan.

[0036] The present disclosure provides methods and systems that allow the methanation reactors to continue to operate even when the hydrogen amount is below the turndown or shutdown threshold, which enables the methanation production to resume when the hydrogen amount rises above the turndown or shutdown threshold without engaging in the startup process. That is, when the hydrogen amount in a feed stream to the methanation reactors falls below the turndown or shutdown threshold (e.g., the threshold that triggers shutdown of the system / reactors), the methanation reactors do not shutdown and restart. Instead, the methanation reactors continue operating in accordance with the method disclosed herein. This provides an option to continue to operate the methanation reactors and avoid the turndown / shutdown and startup cycle as the hydrogen amount fluctuates above and below the turndown or shutdown threshold.

[0037] Referring to FIG.1, there is provided methanation zone 100 which comprises three or more methanation reactors 102 in fluid communication with one another. As shown, the reactors 102 are connected in series. Additionally or alternatively, FIG. 2 shows methanation zone 200 with two reactors 102 connected with each other in parallel, and subsequently in series with the remaining reactors. It is understood that FIGS.1 and 2 are examples of potential arrangements for methanation reactors102 in methanation zone 100 and that the embodiments described herein apply to other arrangements as well. Additional examples include those disclosed in WO2016139451 and WO2016139452.

[0038] Referring to FIGS. 1 – 2, during operation, after startup, feed gas 104 is provided to methanation zone 100, which may be achieved by providing it as shown in FIGS. 1 and 2 or according to the reactor configuration being employed. Typically, feed gas 104 comprises hydrogen and carbon dioxide in an M-ratio of at least 3, where the M-ratio being ^^^^^^^−^^^^^^ ^^^^^^^^^^^^=^2 ^^^^2^^^^^^^^^^+^^^^^^^^^^2. Potential sources for the hydrogen and carbon dioxide include fresh feed106 and recycle stream 112. It is understood that the term “fresh” may be used to describe fresh or new hydrogen being provided to the zone or system and not the un-reacted hydrogen that may be in the recycle stream. When the amount of hydrogen in the feed gas 104 is at or above a selected turndown ratio, production mode can take place in methanation zone 100 in one or more reactors 102 where hydrogen reacts with carbon dioxide and carbon monoxide to form methane to produce a product stream 110. Optionally, the selected turndown ratio is less than 0.1 (10%), preferably less than 0.2 (20%), more preferably less than 0.3 (30%), and most preferably less than 0.4 (40%). In other words, the selected turndown ratio below which a facility would be shutdown can be when the facility is being operated at less than 10%, less than 20%, less than 30%, or less than 40% the maximum operating capacity of that facility, A portion of the hydrogen in the reactors 102 typically remains unreacted because there is an equilibrium limitation on the extent of conversion, and the remaining hydrogen can be recycled for further conversion.

[0039] As shown in FIGS.1 – 2, at least a portion of the product stream 110 can be provided back to the methanation zone 100 via recycled stream 112, which can be provided as part of the feed gas 104. In addition to allowing remaining hydrogen to be further converted, recycling a portion of the product stream 110 can provide an optional mechanism to control the temperature of the product stream, particularly at least by allowing for heat recovery and gas recirculation, which can facilitate maintaining the exothermic reactions in reactors 102 under a desirable temperature to avoid an excessive temperature inside the reactors. Optionally, the desirable temperature includes below 650 C, preferably below 630 C, more preferably below 620 C. Excessive temperature may damage the reactor itself and / or the catalyst. Heat recovery may be provided by means known to one of ordinary skill, such as heat exchangers cooling the hot gas stream at the outlet of one or more, including each reactor, and producing high pressure steam. Recycling or recirculation of at least a portion of the product stream can be done by means known to one of ordinary skill, including suitable gas compressors. Recirculation or recycling of at least a portion of the product stream also provides another option to control the reaction rate and the temperature inside the reactors 102, which is by dilution of the fresh feed 106 in the feed gas 104 that enters the first reactor 102 in the series with the portion 112 being recycled. Amounts and rates of recycling or recirculation of at least a portion of the product stream can be determined by one of ordinary skill to achieve at least these objectives.

[0040] Recycling loop 120 can allow for heat recovery of the portion 112 of the product stream110 where portion 112 is cooled before it is combined with feed gas 104. Such cooling may be achieved using known means, such as passing through one or more heat exchangers, including ashell and tube heat exchanger fed with a cooling medium. Recirculation loop 120 may beconfigured using known methods such as using a recycle compressor or by using a steam ejector. One of ordinary skill can select the volume ratio between portion 112 and feed gas 104 based on the particular compositions, pressure, and temperature of both streams: portion 112 and feed gas 104.

[0041] While FIGS.1 – 2 show that recycle stream 112 being taken from the last reactor 102 in the series, it is understood that recycle stream 112 can come from any reactor 102 and it can come from one or more reactors. The selection of the reactor(s) for recycle stream 112 is a design choice for one of ordinary skill. Also, opting to provide the recycle stream 112 to methanation zone 100 as part of feed gas 104 can provide another control point for modifying the composition and temperature of feed gas 104.

[0042] Typically, the methanation catalyst used in reactors 102 is desirably a nickel- or ruthenium- methanation catalyst. The same or different methanation catalyst may be present in methanation reactors 102. The methanation catalyst may be in the form of pellets or extrudates, but may also be a foam, monolith or coating on an inert support. The flow through the catalyst in the methanation reactors 102 may be axial-flow, radial flow or axial-radial flow.

[0043] Reactors 102 may include bulk methanation reactors that are operated under adiabatic conditions. The outlet stream of the bulk methanation reactors may be recovered as the product stream 110 or it may be provided to one or more trim methanation reactors 102 that are also in methanation zone 100 as known to one of ordinary skill. Trim methanation reactors may be used to produce high-specification substitute natural gases. Methanation production can continue while the methanation zone 100 has hydrogen and carbon dioxide in the suitable stoichiometric amounts, such as in an M-ratio of at least 3.

[0044] When hydrogen availability fluctuates and becomes limited, feed gas 104 would contain less hydrogen, which leads to a lower operating capacity. If hydrogen available continues to be limited, the operation output can fall below a selected turndown ratio (below-turndown- operating-capacity), which can lead to transitioning into shutdown mode. Referring to FIG. 3, instead of shutting down the affected methanation zone 100 according to general industry protocol when the operation output falls below the selected turndown ratio, the present disclosure allowsthe affected zone to continue to operate in below-turndown mode. If the hydrogen in fresh feed stream 106 increases back within a relatively short time to provide a feed stream 104 of above the turndown ratio, the reactors 102 in zone 100 can promptly return to production methane. If the hydrogen availability remains below the turndown ratio for a prolonged period of time such that the temperature of one or more reactors 102 in zone 100 falls below 250 ^C, the temperature of the reactors 102 can be maintained at 250C or above by heating either (i) recycle stream 112 using heating element 124 to generate recycle stream 116 (as shown) and / or (ii) another product stream (not shown) from one or more reactors 102 can be heated, and providing stream 116 to the methanation zone 100, preferably as part of feed stream 104. This allows for shutdown of the methanation zone 100 to be avoided when the operating capacity is below the turndown ratio due to limited hydrogen while maintaining the temperature of reactors 102 at or above 250C for the duration of the period of limited hydrogen availability (i.e., below turndown ratio). When hydrogen returns to at or above the turndown ratio, normal production can resume reasonably promptly without needing to engage the start-up process. Suitable examples of heating component 124 include heating equipment configured to supply heat or thermal energy, preferably at a temperature of 280C or above, more preferably 300C or above, to increase the temperature of a fluid stream, preferably via a heat exchanger and a working fluid such as steam. Heat sources for heating component 124 can include combustion, steam, and / or waste heat recovery streams.

[0045] Reactors 102 suitably includes sensors throughout the respective reactor to provide temperature readings inside that reactor. The region inside reactor 102 that typically has a lower temperature than the rest of the reactor is the region around the inlet, particularly the region around the inlet of the initial reactor(s) receiving feed gas stream 104, in contrast to subsequent reactor(s) that receive a product stream of another reactor 102. That temperature reading is typically referred to as the “inlet temperature.” Reference to a reactor temperature includes any temperature reading inside the reactor, preferably the inlet temperature.

[0046] Feed gas stream 104 can comprise fresh hydrogen, if any is available. If fresh feed 106 is not available at certain times during this below-turndown mode, the feed gas stream 104 can consist essentially of heated recycle stream 116 during such times. The flow rate and temperature of heated recycle stream 116 for a particular facility under a certain set of conditions can be selected by one of ordinary skill to suitably meet the objectives as described herein, including maintaining the temperature of all reactors 102 in zone 100 at 250C or above for the duration ofthe period of limited hydrogen availability (i.e., below turndown ratio). Optionally and preferably, the pressure of heated recycle stream 116 is provided at a pressure that corresponds to the pressure of the reactor 102 to which stream 116 is being provided. Pressurization of stream 116 prior to providing it to a reactor 102 can be achieved by means known to one of ordinary skill, such as via one or more compressors. When hydrogen level resumes to above the turndown ratio, the conversion reaction also resumes, which produces exothermic energy as provided elsewhere. The heating and provision of heated recycle stream 116 can stop until it is needed again when the hydrogen level drops below the turndown ratio and temperature of at least one reactor 102 falls below 250C.

[0047] FIG.4 depicts an illustrative facility 400 with multiple methanation zones 100, such as the four methanation zones 100A – 100D as shown. Although not explicitly depicted, it is understood that facilities with more than one methanation zones 100 can apply the methods described in the foregoing paragraphs, particularly with respect to FIG.3, as desired or suitable to manage below-turndown-operating-capacity, for individual methanation zones 100. Additionally or alternatively, the present disclosure also provides another aspect to collectively manage multiple methanation zones 100 that are located at the same facility and preferably are in fluid communication with one another.

[0048] Referring to FIG.4, all the methanation zones 100 are in standard operation mode when hydrogen availability is above the turndown ratio. That is, each zone receives its respective feed stream 104 (e.g., 104a, 104b, etc.) that comprises the respective fresh feed 106 (e.g., 106a, 106b, etc.) to produce its respective product stream 110 (e.g., 110a, 110b, etc.). Referring to FIG. 5, when the hydrogen amount fluctuates and decreases such that the operation capacity of at least one methanation zone, such as zone 100D, decreases below the turndown ratio, that zone continues to operate in below-turndown mode (“non-producing methanation zone”) instead of transitioning into shutdown mode. For the time period during which the hydrogen amount is inadequate to support standard operation of all zones, available fresh hydrogen 106 is sequentially provided to less methanation zones so that the fresh hydrogen can be aggregated for distribution to maximize the number of zone(s) that can be operated in standard operation (above turndown “producing methanation zones”). This means fresh feed 106 can be withheld from certain zone(s) 100, such as 100D, and more zone(s) 100 can stop receiving fresh feed 106 if the level of hydrogen continues to decrease. If the hydrogen in fresh feed stream 106 increases back to above the turndown ratiowithin a relatively short time (e.g., before a reactor 102 cools to below 250 ^C), the reactors 102 in the zone(s) 100 with adequate hydrogen can promptly return to production of methane. If the hydrogen availability remains below the turndown ratio for at least one zone 100 (e.g., 100D) for a prolonged period of time such that the temperature of one or more reactors 102 in zone 100D falls below 250 ^C, the temperature of those reactors 102 can be maintained at 250 ^C or above by providing at least a portion of the product stream 110 of one or more of the remaining zones (e.g., 110a, and / or 110b, etc.) as heated stream 154 (154a, 154b, and / or 154c). Preferably, heated stream 154 is provided as the feed stream 104.

[0049] Continuing to run the affected methanation zone(s) in below-turndown mode (capacity) and keeping the reactors at 250C or above while hydrogen availability remains limited (i.e., below turndown ratio) avoids shutdown of the methanation zone 100D and maintains a desirable condition for a prompt return to production mode when the adequate hydrogen availability resumes without needing to engage the start-up process.

[0050] If the hydrogen availability continues to decrease such that additional methanation zones 100 (e.g., 100B, and / or 100C) are at or below the turndown ratio, the principles described above can be applied to aggregate the available fresh feed 106 to maintain as many methanation zone(s) 100 in system at or above turndown ratio as can be supported by the available hydrogen.

[0051] For instance, referring to FIG. 6, in addition to zone 100D being operated in below- turndown mode, if the hydrogen continues to decrease such that zone 100C is also below the turndown ratio, both zones 100C and 100D continue to operate in below-turndown mode rather transition to shutdown mode. For the time period during which the hydrogen amount is inadequate to support standard operation of both zones 100C and 100D, both zones do not receive any fresh feed 106. That way, all the available fresh feed 106 is aggregated and provided to the remaining zone(s) (e.g., 100A and 100B) in system 400. This allows for of maximizing the number of zones 100 that are operating at above the turndown ratio, as well as increasing the overall production rate by aggregating fresh feed to producing zone(s) where the hydrogen can be efficiently converted to methane rather than being provided to a sub-optimal operating zone that is at below-turndown capacity.

[0052] If the hydrogen in fresh feed stream 106 increases back to above the turndown ratio for one or both zones in below-turndown mode (100A and / or 100B) within a relatively short time, the reactors 102 in the zone(s) 100 with adequate hydrogen can promptly return to production ofmethane. If the hydrogen availability remains limited for a prolonged period of time such that the temperature of one or more reactors 102 in zones 100C and 100D falls below 250 ^C, the temperature of those reactors 102 can be maintained at 250C or above by providing at least a portion of the product stream 110 of one or more of the remaining zones (e.g., 110a, and / or 110b, etc.) as heated stream 154 (154a, and / or 154b). Preferably, heated stream 154 is provided as the respective feed stream 104.

[0053] When the hydrogen amount decreases such that one or more methanation zones is at or below the turndown ratio (such as 100D), the fresh feed stream 106 can be provided to sequentially less methanation zones 100 in system 400 to aggregate and divide the available fresh feed stream 106 across the remaining zones (such as 100A – 100C), thereby maximizing the number of zones in system 400 can be operated at above turndown with the available fresh feed 106. For the zone(s) that are operating in below-turndown mode, a portion of the product stream 110 from one or more producing zone(s) 100 is provided to maintain the temperature of its respective reactors 102 at 250C or above until production can resume.

[0054] When hydrogen level returns to above the turndown ratio, the conversion reaction also resumes, which produces exothermic energy as provided elsewhere. The provision of heated stream 154 to the now-producing zone(s) 100 can stop until heated stream 154 is needed again when the hydrogen level drops below the turndown ratio and temperature of at least one reactor 102 in a particular zone 100 falls below 250C.

[0055] In situations where the hydrogen level decreases to the extent that none of the methanation zones in a system can be sustained above turndown, a heated recycle stream can be provided to maintain the temperature of the reactors in these zones at 250C or above. For instance, FIG.7 depicts an embodiment where methanation zones 100B – 100D are not receiving any fresh feed 106 and methanation zone 100A receives fresh feed 106, if any, at a level below the turndown ratio. In such a scenario, at least a first portion of the product stream 110a can be heated to provide back to methanation zone 100A (preferably as part of the feed stream 104a) and a second portion of the product stream 110a is provided to the remaining zone(s) (e.g., 100B, 100C, etc.) in system 700. The temperature of heated recycle stream 116 is such that (i) the reactors 102 in methanation zone 100A is maintained at 250C or above and (ii) at least a portion, including most or all, of the product stream 110a can be used to maintain the temperature of the reactors 102 in all the zones 100.

[0056] While specific embodiments have been described herein, it is understood that such descriptions are not intended to limit the described embodiments. Instead, any combination of the features and elements provided above, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the aspects, features, embodiments and advantages described herein are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s).

Claims

CLAIMS1. A method for operating a methanation zone (100) comprising three or moremethanation reactors (102) in fluid communication, the method comprises:(a) providing a feed gas stream (104) to the methanation zone (100),(b) when the feed gas stream comprises hydrogen and carbon dioxide in an amount toprovide at least one reactor (102) with hydrogen an carbon dioxide : (b1) operating the methanation zone to convert the hydrogen and carbon dioxide in the respective reactors to a product stream (110) from each reactor (102) in the methanation zone, wherein the product stream comprises methane; (b2) recycling at least a portion of the product stream from one or more reactors (102) to the methanation zone, preferably as part of the feed gas stream, to facilitate controlling a temperature of the product stream below 650 C, preferably below 630 C, more preferably below 620 C;(c) when the operating capacity of the methanation zone falls below a turndown thresholdat least due to a decreased amount of hydrogen in the feed stream, (c1) continuing to operate the methanation zone; (c2) during at least step (c), when the temperature of any reactor falls below 250C, heating at least a portion of a product stream from one or more reactors (102) to produce a heated recycle stream (116); and providing the heated recycle stream (116) to the methanation zone, preferably as part of the feed gas stream, to maintain the temperature of all reactors (102) in methanation zone (100) at or above 250C at least until the hydrogen amount increases to above the turndown ratio.

2. The method of any prior claims, wherein availability of fresh hydrogen fluctuates suchthat the hydrogen in the feed gas fluctuates between above and below the turndown ratio over a period of operation of the methanation zone.

3. The method of any prior claims, wherein the methanation zone does not shutdown andrestart when the operating capacity falls below the turndown threshold.

4. The method of any prior claims, wherein at least three methanation reactors areconnected in series.

5. The method of any prior claims, wherein at least one methanation reactor is in parallelwith another methanation reactor.

6. The method of any prior claims, wherein the heating step is achieved using a heatingcomponent selected from the group consisting of electrical, steam, waste heat recovery streams, and any combination thereof.

7. The method of any prior claims, wherein the feed gas stream to the at least onemethanation zone consists essentially of the heated recycle stream.

8. The method of any prior claims, further comprising:prior to providing the feed gas to operate the methanation zone above the turndown ratio, if any reactor in a methanation zone is below 250 ^C, providing a heated inert stream (108) to the methanation zone to heat the reactor(s) to at least 250 ^C.

9. The method of any prior claims, wherein the turndown ratio is less than 20%, preferablyless than 30%, and preferably less than 40%.

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