WO2023126177A1

WO2023126177A1 - Biological methanation reactor utilising a microbial flora in suspension and process for employing such reactor

Info

Publication number: WO2023126177A1
Application number: PCT/EP2022/085831
Authority: WO
Inventors: Sébastien LEMAIGRE; Jimmy ROUSSEL; Philippe Delfosse
Original assignee: Luxembourg Institute Of Science And Technology
Priority date: 2021-12-28
Filing date: 2022-12-14
Publication date: 2023-07-06
Also published as: EP4457331A1; LU501102B1

Abstract

The invention relates to a biological methanation reactor, comprising a tank (1) comprising a liquid or semi-liquid medium (2), an inlet (5) for anoxic gas for treatment, comprising especially carbon dioxide, a microbial flora (20), at least one non-microporous tubular membrane (6) immersed at least partially in said liquid or semi-liquid medium, said at least one membrane comprising a conduit into which a gas may be injected, said reactor comprising at least one gas injector (7) connected to said conduit of said membrane and an outlet (10) for recovering methane. The reactor is notable in that said flora is in suspension in said liquid or semi- liquid medium and comprises hydrogenotrophic methanogenic Archaea and/or comprises homoacetogenic bacteria and acetotrophic methanogenic Archaea, in that the gas injector (7) is a pure dihydrogen injector, and in that the reactor comprises a gas collection and storage space (3), said outlet (10) being connected to said space (3), and said gas injector (7) and said inlet (5) for anoxic gas for treatment being two independent gas inlets.

Description

BIOLOGICAL METHANATION REACTOR UTILIS ING A MICROBIAL

FLORA IN SUSPENS ION AND PROCESS FOR EMPLOYING SUCH

REACTOR

FIELD OF THE INVENTION

The invention relates to an ex si tu biological methanation reactor which enables the biological conversion of carbon monoxide ( CO) , carbon dioxide ( CO2 ) and dihydrogen (H2 ) to methane (CH4 ) , the latter being useful as a source of energy .

PRIOR ART

"Biological methanation" refers to a process for converting carbon dioxide and/or carbon monoxide to produce methane , using highly specialised microorganisms ( the Archaea ) , in a technical plant . Methanation should not be confused with (bio- ) methanisation, which relates to the natural process of the breakdown of organic matter in the absence of oxygen .

Biological methanation so makes it possible to derive value , industrially, from carbon dioxide , from biogas or from syngas for example , to produce methane . In order to accomplish this , it is necessary to provide the carbon dioxide with dihydrogen . This reaction may be carried out either thermochemically or biologically - the present invention relates only to production of methane by the biological route .

More particularly, the invention is concerned with the conversion of dihydrogen and carbon dioxide to methane by hydrogenotrophic methanogenic anaerobic Archaea microorganisms (microbial flora ) .

There are two technical challenges which biological methanation reactors must be able to meet . Firstly, the solubility of dihydrogen in water is very low . Secondly, the methanogenic Archaea which are responsible for the methanation reaction have a very low renewal rate , meaning that they are slow to reproduce . For this reason, there are reactors which house membranes which enable a large quantity of microorganisms to be secured in the reactor in the form of a biofilm, and which are traversed by a gas stream containing carbon monoxide and dihydrogen, allowing the components of the gas to be delivered directly to the microorganisms contained in the biofilm, which converts them into ethanol and other soluble products . Patent application US 8 058 058 describes one reactor of this type , comprising a tank containing an aqueous bath in which microporous or non-microporous tubular membranes are immersed, these membranes being covered at least partially with a biofilm . A gas mixture comprising carbon monoxide , carbon dioxide and dihydrogen is introduced into the tubular membranes , at one end . The rest of the gas mixture is recovered at the other end of the tubular membranes . The product converted by the microorgani sms making up the biofilm is obtained in the liquid product of the tank, and extracted in the upper part of the tank . The product converted is therefore also liquid .

It is noted that the purpose of a device of this kind, although it uses carbon dioxide or monoxide and also dihydrogen, is not to produce methane from these inputs . Rather than a methanation reactor, therefore , it is a reactor which utilises bacteria ( and not Archaea ) in order to generate biochemical acetogenesis reactions that lead to the production of compounds such as ethanol or acetic acid . It should nevertheless be noted that these compounds are produced in a form soluble in a liquid and not in gaseous form .

There are devices which utilise membranes as a microbial biofilm support for generating production of methane by biological methanation .

Some of these devices operate in si tu, meaning that the biological methanation reaction proceeds in a biomethanisation reactor whose purpose is to produce biogas ( a gas mixture containing methane , but also carbon dioxide ) via the anaerobic digestion of complex organic matter, usually in a non-hydrolysed form (manure , slurry, biowaste , primary sludges from sewage plants , etc . ) . Thus patent EP 2 771 472 describes an in si tu process of biological methanation for reducing the level of CO2 in the biogas produced by a biomethanisation reactor, by inj ecting dihydrogen into the liquid content of said reactor using hollow fibre membranes . There are two drawbacks to this type of reactor . Firstly, it requires the inj ection of an acidic product into the reactor in order to maintain the pH value at between 7 and 8 in the liquid medium, owing to the in si tu configuration of the methanation reactor, which requires the preservation of all of the microbial classes included in the anaerobic digestion consortium, in particular the hydrolytic, acidogenic and acetogenic bacteria . Secondly, the hollow fibre membranes recommended are subj ect to rapid colonisation of their microporosity by the microorganisms present in the liquid medium, so giving rise to a rapid decrease in the rate of gas permeation through this type of membrane , causing a rapid drop in the methanation yield of the reactor unless its membranes are regularly replaced . Such regular replacement , however, entails substantial reactor operating costs , since the bundles of hollow fibre membranes are very expensive , and also entails repeated interference with the anaerobic conditions beneficial to biological methanation, in light of the need for the reactor to be opened in each of these procedures .

There are also ex si tu processes that utilise membranes as a support for microbial securement for biological methanation . In these processes , the reactor hosting the methanation reaction is a reactor entirely dedicated to the realisation of methanation, and not to that of biomethanisation . The ex si tu methanation reactors exhibit a better yield than the in si tu methanation reactors , since they enable production of the conditions beneficial to the development of methanogenic Archaea, without any need to preserve the other microbial classes involved in the anaerobic digestion consortium . Hence the device described in patent EP 3 555 258 combines a biomethanisation reactor with an ex si tu biological methanation reactor in the form of an MBfR (Membrane Biofilm Reactor ) . In the tank of the MBfR reactor, dihydrogen and carbon dioxide are inj ected as a mixture into a liquid medium via a bundle of membranes which are permeable to these gases , and development of a biofilm which realises the methanation reaction is stimulated on the surface of the membranes . The MBfR reactor described here has a number of limitations . Indeed, it requires a permanent flow of liquid through it in order to remove the methane it produces in a solubilised form ( or in the form of micro-bubbles ) , since the reactor does not incorporate a dedicated gas outlet . Because methane has a very low solubility in water, the flow rate of liquid traversing the reactor must be high, thereby necessitating high investment costs and operating costs for the liquid pumps required, these costs increasing in line with the viscosity of the liquid pumped . Moreover, the stream of liquid sweeping the surface of the membranes prevents the utilisation of a liquid medium containing solid elements , which would risk impairing the membranes and the biofilm they contain . Lastly, the reactor described requires a permanent exchange of liquid with a biomethanisation reactor, hence restricting its use to speci fic sites specialised in the production of biogas or of biomethane .

Interestingly, in a study evaluating an MBfR reactor that utilises hollow fibre membranes for the purposes of biological methanation, the authors demonstrated that the biofilm formed on the membranes contributed only to 22-36% of the dihydrogen consumption of the reactor, the maj or part of the dihydrogen being consumed, accordingly, by the microbial flora in suspension in the liquid medium surrounding the membranes ( Luo et al . , 2013 ) .

The invention therefore proposes an ex si tu reactor for optimising the utilisation of microbial flora present in suspension in a liquid or semi-liquid medium for biological methanation . The reactor forming the subj ect of the invention utilises membranes (which are not microporous ) , not for promoting the development of a biofilm, but instead for providing dihydrogen local ly at all points in a volume of liquid or semi-liquid medium, so as to compensate for its low solubility . It therefore has features intended to prevent the loss of yield due to microbial plugging of the membranes . Moreover, the reactor forming the subj ect of the invention enables the recovery of methane in gaseous form, thereby avoiding the need to maintain a continuous flow of liquid through the reactor, and enables the utilisation of a liquid medium which is viscous and/or contains solid elements . Lastly, the reactor forming the subj ect of the invention does not require exchange of high volumes of liquid or semi-liquid medium with a biomethanisation reactor, hence allowing it to be installed on a site not specialised in the production of biogas or biomethane , such as an industrial site that generates gaseous outputs containing carbon dioxide and/or carbon monoxide .

SUMMARY OF THE INVENTION

The invention relates to a biological methanation reactor, comprising a tank comprising : a liquid or semi-liquid medium, an inlet for anoxic gas for treatment , comprising carbon dioxide and optionally carbon monoxide , a microbial flora comprising methanogenic Archaea, at least one non-microporous tubular membrane which is permeable to dihydrogen, immersed at least partially in said liquid or semi-liquid medium and at least partially in contact with said microbial flora, said at least one membrane comprising a membrane conduit into which a gas may be inj ected, said reactor comprising at least one gas inj ector connected to said conduit of said at least one membrane , and a dedicated gas outlet for the recovery of methane generated by said biological methanation and produced in the tank by said methanogenic Archaea microbial flora in contact with said gas having passed through said at least one membrane .

Moreover, said tank comprising a second inlet for feeding the liquid or semi-liquid medium with nutrients , and also a second outlet for discharge of ef fluents .

The methanation reactor in accordance with the invention is notable in that said microbial flora utilised is an anaerobic microbial flora in suspension in said liquid or semi-liquid medium and comprises hydrogenotrophic methanogenic Archaea and/or comprises homoacetogenic bacteria and acetotrophic methanogenic Archaea, in that the gas inj ector is a pure dihydrogen inj ector, and in that said reactor comprises a gas collection and storage space , said gas comprising methane from the methanation reaction generated by said microbial flora in contact with said pure dihydrogen inj ected into the liquid or semi-liquid medium through said at least one membrane , said outlet for recovering the methane being a gas outlet and being connected to said gas collection and storage space , and said gas inj ector and said inlet for anoxic gas for treatment being two independent gas inlets in said reactor .

The combined features of a microbial flora in suspension and of separate inj ection of pure dihydrogen and of an anoxic gas for treatment prevent the formation of a thick biofilm on the tubular membranes which would risk plugging of said membranes and reducing the ef ficacy of their permeation by gases . The reason is that the hydrogenotrophic methanogenic are Archaea lack a local carbon source for developing on the outer surface of the membranes , which would not be the case i f the anoxic gas for treatment ( containing carbon dioxide or monoxide ) was inj ected into the liquid or semi-liquid medium as a mixture with dihydrogen via the tubular membranes , as is often practised with MBfR reactors . The reactor in accordance with the invention therefore allows the rate of dihydrogen supply to the microbial flora in suspension in the liquid or semi-liquid medium to be maximised, while being less expensive to maintain than the current MBfR reactors .

The methanation reactor in accordance with the invention may also comprise the following features , taken separately or in combination :

Advantageously, the gas collection and storage space is within said tank, between the surface of said liquid or semi-liquid medium and a tank top cover . This gas collection and storage space , which is not present in the MBfR reactors , allows the biomethane produced by the microbial flora to be collected in gas form and extracted from the reactor in this same form via the dedicated outlet for treated gas . The presence of this gas collection and storage space therefore avoids the need for the reactor forming the subj ect of the invention to be traversed by a continuous flow of liquid in order to extract the biomethane produced in a soluble form ( or in the form of micro-bubbles ) , and allows a liquid medium to be utilised that is viscous and/or contains solid elements , something which is di f ficult with the MBfR reactors . Moreover, the presence of this gas collection and storage space avoids the need for the reactor forming the subj ect of the invention to exchange large volumes of liquid or semi-liquid medium with a biomethanisation reactor, so allowing it to be installed on a site not specialised in the production of biogas or biomethane , such as an industrial site generating gaseous outputs containing carbon dioxide and/or carbon monoxide . Lastly, the presence of this gas collection and storage space produces an increase in the performance of the biological methanation in the reactor, by increasing the residence time of the anoxic gas for treatment in said reactor, this having the ef fect of increasing the time for which carbon dioxide and/or carbon monoxide are/ is available for the microbial flora in suspension in the liquid or semi-liquid medium and responsible for the conversion of said gases/gas to biomethane .

The inlet for anoxic gas for treatment preferably opens out into said gas collection and storage space .

More preferably, the inlet for anoxic gas for treatment and the outlet for recovering the methane each comprise a non-return valve .

Furthermore , and according to one advantageous embodiment , the methanation reactor according to the invention comprises an agitator device for generating an agitation in said liquid or semi-liquid medium .

In the context of this embodiment , said agitator device preferably comprises at least one gas di f fuser immersed in the liquid or semi-liquid medium, preferably provided in the bottom of said tank . By siting the gas di f fuser in the bottom of the tank, the di f fused gas ascends the whole of the column of the liquid or semi-liquid medium and hence comes into contact with the entirety of the microbial flora : this promotes the contacting of the microbial flora with the gas and also the ef fective solubilisation of the carbon dioxide and/or carbon monoxide in the liquid or semi-liquid medium .

Said agitator device preferably comprises a recirculation pump which is connected on the one hand to said gas collection and storage space , for extraction of gas from said space , and which is connected on the other hand to said gas di f fuser for feeding said gas di f fuser with said gas extracted from said gas collection and storage space . In this way, the anoxic gas for treatment is inj ected by the di f fuser into the liquid or semi-liquid medium, and the carbon dioxide ( and optional ly carbon monoxide ) is contacted with the microbial flora, allowing it to perform the methanation reaction in the presence of dihydrogen . The inj ection of the anoxic gas for treatment by the gas di f fuser, and not by the membrane as is customary in known solutions , makes it possible to prevent the formation of biofilm on the membranes , hence preserving them from plugging . Indeed, by virtue of the solution in accordance with the invention, the hydrogenotrophic methanogenic Archaea lack a local carbon source for developing on the surface of the tubular membranes and a local dihydrogen source for developing on the carbon dioxide-rich gas di f fusers .

Said at least one di f fuser is advantageously placed under said at least one tubular membrane .

Furthermore , said agitator device extends preferably over substantially 0 . 05 m² per m³ of liquid or semi-liquid medium in said tank . This enables distribution of the gas for treatment throughout the volume of the liquid or semi-liquid medium . Moreover, the ascent of the gas bubbles prevents the local accumulation of dihydrogen at the surface of the tubular membranes - local accumulation reduces the permeation flow rate of dihydrogen through the wall of the tubes .

According to one working example of the invention, the reactor comprises at least one bundle of membranes comprising at least two tubular membranes , the axes of said at least two tubular membranes comprised in said at least one bundle of membranes being substantially parallel to one another, and each of said at least two tubular membranes of said at least one bundle of membranes comprising respectively a membrane conduit fed with pure dihydrogen by said gas inj ector .

The tank may accommodate one or more groups of bundles of tubular membranes , to optimise the methanation reaction . According to the arrangement of the groups of membrane bundles in the tank, and according to their orientation, the tank may accommodate a greater or lesser length of tubular membrane , so promoting the methanation yield . This yield will be increased in line with the extent to which the configuration of the network of tubular membranes enables dihydrogen to be supplied locally at any point in the volume of liquid or semiliquid medium ( large length of tubular membranes relative to the volume of liquid or semi-liquid medium, and uni form arrangement of said membranes ) . By virtue of this dense and uni form configuration of the network of tubular membranes , it is possible advantageously to compensate for the low solubility of dihydrogen in the liquid or semi-liquid medium and to maximise the derivation of value from the dihydrogen by the microbial flora present in suspension in said medium .

The inj ector advantageously feeds the conduit of each of said tubular membranes with pure dihydrogen by means of a group of connection tubes , each of the connection tubes being connected to a main inj ection conduit of said inj ector . The multiplication of the dihydrogen inj ection modules allows the intake of dihydrogen by one o f them to be closed of f , in the event of leakage , for example . A single large dihydrogen inj ection module could be employed in the reactor without departing from the scope of the invention, but the slightest leakage would entail the total halting of dihydrogen inj ection . The result would be a large loss of revenue for the operator . With a large number of modules , the process is able to remain profitable in the event of leakage in one or more modules , it being possible to replace the defective modules during an annual maintenance procedure .

The tank more preferably has a tank volume dedicated to the liquid or semi-liquid medium; said at least two membranes of said at least one group of membranes have a total speci fic length of at least 1000 m per cubic metre of said volume dedicated to the liquid medium and they occupy a speci fic volume of less than substantially 0 . 2 m³ per m³ of said volume dedicated to the liquid or semiliquid medium ( or less than 0 . 2 m³ per m³ of said volume dedicated to the liquid or semi-liquid medium) .

Furthermore , said at least one membrane comprises silicone . The reason is that this material has a high gas permeability ( especially for dihydrogen) . As an example , a length of 1 metre of silicone tube with an inner diameter of 8 mm and a wall thickness of 1 mm, subj ected to a difference in partial pressure of dihydrogen of 1 bar between the membrane conduit and the outside of the membrane, is able to diffuse a dihydrogen flow rate of ~ 1.4 L/day. Silicone, furthermore, has an excellent durability, a reduced cost by comparison with hollow fibre membranes, and a dense, non-microporous structure, which offers a low attachment surface area to the microorganisms. This latter feature enables effective stability of time of the rate of gas permeation through the membrane, with no risk of the efficacy of this membrane being reduced by the development of a thick biofilm presenting a physical barrier to the passage of the gas .

In the context of one advantageous embodiment, the membrane comprises at least one silicone tube or a plurality of silicone tubes aligned with one another and held by their ends of tubes in gas distributor elements, the gas distributor elements and the ends of tubes being embedded in a polyurethane or polyepoxide resin.

With further preference, said conduit of said tubular membrane is subject to an internal pressure of between substantially 0.5 and 1.5 bar (or between 0.5 and 1.5 bar) , an intramembrane pressure range that enables a suitable rate of dihydrogen supply to the microbial flora present in suspension in the liquid or semi-liquid medium. Moreover, subjecting said tubular membrane conduit to a regulated pressure within this range prevents contamination of the hydrogen circuit with gases such as ammonia (NH3) or hydrogen sulfide (H2S) which are potentially toxic for the microbial flora.

With further preference, the methanation reactor comprises a plurality of levels each comprising at least one tubular membrane, with the internal conduits of the tubular membranes of each level being subject to different pressures. The reason is that, in the case of a tall reactor, this configuration makes it possible to compensate for the negative effect of the hydrostatic pressure (due to the column of liquid or semi-liquid medium) on the rate of permeation of dihydrogen through the wall of the tubular membranes , by subj ecting the membranes immersed most deeply to an intra-membrane pressure which is greater than that to which the less deeply immersed membranes are subj ected .

The methanation reactor advantageously comprises a first flowmeter/ flow regulator device which is connected to said dihydrogen inj ector .

More advantageously, the methanation reactor comprises a second flowmeter/ flow regulator device , which is connected to said inlet for anoxic gas for treatment .

The reactor preferably further comprises , according to one embodiment , a module for controlling one or other or the two first and second flowmeter/ flow regulator devices .

Furthermore , the reactor preferably comprises an instrument for analysing the composition of the gas at said gas outlet and the composition of the gas at said inlet for anoxic gas for treatment opening out into said gas collection and storage space .

These latter features enable the implementation of a process in accordance with the invention, for a methanation reactor as defined above , which comprises the following steps :

- measuring a first composition of the anoxic gas for treatment at said inlet for anoxic gas to be treated by means of said analytical instrument , said first composition of the anoxic gas for treatment comprising a concentration of carbon dioxide and/or carbon monoxide ,

- measuring a second composition of gas at said gas outlet , said second composition of gas comprising a concentration of methane and of dihydrogen, and

- comparing said concentration of methane with a predetermined threshold value for methane and comparing said concentration of dihydrogen with a predetermined threshold value of dihydrogen, - comparing said concentration of carbon dioxide and/or of carbon monoxide with a predetermined threshold value of carbon dioxide and/or of carbon monoxide , and increasing a first setpoint value of said first flowmeter/ flow regulator device and/or reducing a second setpoint value of said second flowmeter/ flow regulator device and/or increasing a service time of said recirculation pump i f said concentration of carbon dioxide and/or of carbon monoxide is greater than said predetermined threshold value of carbon dioxide and/or of carbon monoxide , and vice versa, or decreasing said first setpoint value of said first flowmeter/ flow regulator device and/or increasing a second setpoint value of said second flowmeter/ flow regulator device and/or increasing a service time of said recirculation pump i f said concentration of dihydrogen at said outlet is greater than said predetermined threshold value of dihydrogen, and vice versa .

The invention likewise relates to a computer program comprising instructions which when executed by computer implement all of the steps of the process defined above .

FIGURES

Further advantages and features of the invention will emerge on examination of the detailed description of one embodiment , which is in no way limiting, and of the attached drawings , in which :

[ Fig . 1 ] is a schematic representation of a biological methanation reactor in accordance with the invention, seen in section,

[ Fig . 2 ] shows a reactor in accordance with a first embodiment according to the invention, seen in perspective ,

[ Fig . 3 ] shows the reactor of Figure 2 , seen in perspective , in which a cover has been removed and a tank is illustrated transparently, for better identi fication of its constituent elements , [Fig. 4] shows the reactor of Figure 2, seen in perspective, in which the tank, the cover and a pump have been removed,

[Fig. 5] shows the tank of the reactor of Figure 2, seen in perspective, and also an agitating device connected to the pump, a pipe for feeding the reactor with anoxic gas for treatment, and the presence of a gas collection and storage space situated above the level of the liquid or semi-liquid medium in the tank,

[Fig. 6] shows an example of a membrane that the tank of the reactor in accordance with the invention may comprise, seen in perspective and in longitudinal section,

[Fig. 7] is an enlargement of the zone VII shown in Figure 6,

[Fig. 8] shows a variant embodiment of a reactor in accordance with the invention, illustrating a tank seen in section, with a cover formed by a deformable membrane of low gas permeability,

[Fig. 9] illustrates a plan view of the tank shown in Figure 8, and

[Fig. 10] shows a part of the reactor illustrated in Figure 8, seen in perspective.

DESCRIPTION OF AN EMBODIMENT

Figure 1 provides a schematic illustration of one embodiment of a biological methanation reactor in accordance with the invention.

The reactor comprises a tank 1 containing a liquid or semi-liquid medium 2.

The liquid or semi-liquid medium 2 comprises a microbial flora 20 comprising hydrogenotrophic methanogenic Archaea and/or comprising homoacetogenic bacteria and acetotrophic methanogenic Archaea in suspension in said medium 2.

Because of the level of the liquid or semi-liquid medium 2 comprising said flora in suspension, there is a free volume of liquid above the liquid or semi-liquid medium in the tank; expressed alternatively, the liquid or semiliquid medium 2 comprising said microbial flora 20 in suspension does not fill the whole of the volume of the tank 1 .

This volume left free above the level of the liquid or semi-liquid medium constitutes a gas collection and storage space 3 . This volume is closed of f in the top part of the tank 1 by a top cover 4 , advantageously having a domed shape ( see Figure 2 , for example ) to increase the volume of the gas collection and storage space 3 .

The tank 1 of the methanation reactor comprises an inlet 5 for gas for treatment , which also opens out into the gas collection and storage space 3 ; the gas for treatment is anoxic ( that is , devoid of oxygen) and comprises carbon dioxide ( CO2 ) and optionally carbon monoxide ( CO) . The inlet 5 for anoxic gas for treatment is equipped with a non-return valve ( reference 50 ) .

The reactor in accordance with the invention further comprises a group of non-microporous tubular membranes 6 , which are immersed in the liquid or semi-liquid medium 2 comprising the microbial flora 20 ; accordingly, the non-microporous tubular membranes 6 are in contact with the liquid or semi-liquid medium 2 and also with the microbial flora 20 in suspension in the medium .

Each tubular membrane 6 comprises an internal conduit 60 , also called "membrane conduit" , which is blind (meaning that one of the ends of the tubular membrane is closed of f ) .

Each membrane conduit 60 is fed with dihydrogen (H2 ) via a feed conduit 70 connected to a pure dihydrogen inj ector 7 .

"Pure" dihydrogen refers to a gas comprising at least 95% dihydrogen, or even less : the volume concentration of dihydrogen at the inj ector is at least 80% , with the remaining 20% ( or substantially 20% ) comprising neither carbon dioxide nor carbon monoxide . Each membrane 60 is permeable to dihydrogen such that the dihydrogen inj ected into the membrane 60 conduits passes through the wall of each membrane 60 to penetrate the liquid or semi-liquid medium 2 comprising the microbial flora 20 in suspension .

The microbial flora 20 is therefore in contact with the dihydrogen . The microbial flora 20 generates the methanation reaction, which involves conversion of carbon dioxide and optionally of carbon monoxide (which are present in the anoxic gas for treatment ) into methane and water in the presence of dihydrogen, according - respectively - to the following reactions ( 1 ) and ( 2 ) : C0₂ + 4 H2 CH₄ + 2 H2O (1)

CO + 3 H_z ■> CH₄ + H₂0 (2).

So that the microbial flora 20 is also in contact with the anoxic gas for treatment , the methanation reactor in accordance with the invention comprises at least one gas di f fuser 8 , provided in the bottom of the tank 1 , which takes the gas from the gas collection and storage space 3 and reinj ects it in the form of fine bubbles into the liquid or semi-liquid medium 2 by means of a pump 9 outside the tank 1 . The reason is that the anoxic gas for treatment , in the context of this exemplary embodiment , is inj ected into the gas collection and storage space 3 . It is therefore from this space 3 that the gas for treatment is withdrawn by the pump 9 to feed the gas di f fuser 8 , which inj ects it into the liquid or semiliquid medium 2 comprising the microbial flora in suspension .

In so doing, the gas di f fuser 8 likewise constitutes a device which creates an agitation in the liquid or semiliquid medium, with the gas reinj ected by the di f fuser 8 ascending the whole of the column of liquid 2 and creating movement in the microbial flora 20 , thereby preventing said flora from stagnating in the liquid or semi-liquid medium and depositing on the membranes 6 , to prevent formation of a biofilm on the membranes 6 . The advantage of preventing formation of biofilm is that it prevents the plugging of the membranes 6 , partially inhibiting the di f fusion of dihydrogen in the liquid or semi-liquid medium . Accordingly, by preventing the formation of biofilm, the presence of dihydrogen in the liquid or semi-liquid medium is promoted, as is , consequently, the methanation reaction by the microbial flora in suspension in said medium .

The gas di f fuser 8 thus has a dual function : introducing the anoxic gas for treatment into the liquid or semiliquid medium comprising the microbial flora in suspension ( the genesis of the methanation reaction) , and more ef fectively inhibiting the formation of biofilm on the membranes 6 .

The formation of biofilm is also prevented by the fact that the gas inj ected by the membranes into the liquid or semi-liquid medium contains no carbon dioxide ( solely dihydrogen) - in the absence of carbon dioxide di f fused through the membrane , and, by creation of an agitation in the liquid or semi-liquid medium, the microbial flora develops neither on the membranes 6 nor on the gas di f fuser 8 .

The reactor in accordance with the invention, there fore , comprising a dihydrogen inlet independent of the inlet for anoxic gas for treatment , makes it possible to prevent the formation of biofilm, and enables optimum utilisation of the microbial flora in suspension for the realisation of the methanation .

The methanation reaction by the microbial flora 20 in suspension takes place as follows :

The methane generated by the methanation reaction is collected in the gas collection and storage space 3 , above the liquid or semi-liquid medium 2 . It is evacuated from the tank 1 of the reactor by an outlet 10 made in the cover 4 in the context of this example .

The outlet 10 is equipped, like the inlet 5 for anoxic gas for treatment , with a non-return valve 100 . In order to maintain the richness and the activity of the microbial flora 20 in suspension in the liquid or semiliquid medium 2 , a second inlet 22 is provided in the tank 1 , for feeding the liquid or semi-liquid medium 2 with nutrients . I f the source of these nutrients is an anaerobic sludge from a methanisation digester, the inlet 22 additionally feeds the liquid or semi-liquid medium with clean microbial flora and with additional soluble inorganic carbon (HC0>3~, C0>3²~ and H2CO3 ) which can be converted into methane by the methanation reaction .

The second inlet 22 , used for supplying nutrients , is made in the sidewall of the tank 1 , in the top part of said tank .

The tank also has a second outlet 21 , for evacuating the liquid/ semi-liquid ef fluents from the reactor . This outlet is made in the tank bottom so as to allow the ef fluents to be evacuated under the ef fect of gravity .

In contrast to the MBfR (membrane biofilm) reactors , the reactor forming the subj ect of the invention does not require a permanent exchange of liquid medium with another reactor or a gas extractor . This feature enables the adoption of a high hydraulic residence time of the liquid or semi-liquid medium in the reactor, ensuring the maintenance of a high density of methanogenic Archaea in suspension in the liquid or semi-liquid medium, in spite of the low reproduction rate inherent in this microbial class . The nutrients , furthermore , may be introduced into the reactor in dried or lyophilised form, hence maximising the hydraulic residence time of the reactor while minimising the flow of liquid or semi-liquid medium between the inlet 22 and the outlet 21 .

Figures 2 to 5 show one particular embodiment of a reactor in accordance with the invention, and will now be described .

The tank 1 shown in Figure 2 is substantially cylindrical in shape and has a capacity of substantially 800 L ( litres ) . The wall of the tank may be equipped with heating means , allowing the temperature of the liquid or semi-liquid medium 2 it comprises to be increased, so as to promote the reaction .

The temperature of the liquid or semi-liquid medium 2 is ideally between substantially 35 ° C and 42 ° C ( or between 35 ° C and 42 ° C ) , or else between substantially 50 ° C and 65 ° C ( or between 50 ° C and 65 ° C ) .

The tank 1 is isolated from the ground by being mounted on four feet 11 .

The tank 1 encloses a group of membranes 6 , all of which are tubular, and they extend in rectilinear directions with axes which are substantially all parallel with one another .

The cover 4 is substantially in the form of a dome and is secured on the edge of the upper end of the tank 1 , which is open, by means of a bolted double flange 40 comprising an EPDM seal .

Figure 5 illustrates the position of the bolted double flange 40 relative to the level of the liquid or semiliquid medium 2 in the tank, without the cover 4 , so as to more ef fectively identi fy the distance separating the level of the liquid or semi-liquid medium in the tank from the level of the opening (upper end) of the tank 1 . It is therefore seen that the bolted double flange 40 is at a distance d above the level of the liquid or semiliquid medium 2 .

It is also seen that the inlet 5 for anoxic gas for treatment is made through the sidewall of the tank, at a level situated between the level of the liquid or semiliquid medium 2 and the level of the upper end of the tank 1 .

The outlet 10 , which enables recovery of a methane- enriched gas , is made through the cover 4 : the non-return valve 12 is illustrated only in Figure 5 , at a distance from the edge of the tank that is situated between the edge of the tank and its centre , in a position diametrically opposite to the position of the inlet for anoxic gas for treatment .

The pure dihydrogen inj ector 7 is a cylinder which is placed near to the tank 1 , and which feeds (by virtue of the feed conduit 70 ) a network of tubes connected to the internal conduits of the membranes 6 . This dihydrogen source may be replaced by the pressurised dihydrogen reservoir of a water electrolyser .

The group of conduits feeding the membranes 6 is illustrated in greater detail in Figures 3 and 4 .

Each internal membrane conduit is connected by a feed tube 72 to a main inj ection conduit 71 ( distribution conduit ) which is toroidal in shape and which extends around the tank, this tube being itsel f fed by the inj ection conduit 70 .

It is seen in Figure 3 that each tube is connected to the main conduit 71 by opening and closing devices 73 , which can be controlled independently of one another . Therefore , i f a leakage was detected on a connecting tube , it is possible to halt the feed of dihydrogen to the membrane associated with the tube , but without having to halt the feeding of the other tubes connected to the other membranes .

The gas reintroduction pump 9 is also sited close to the tank .

It provides a feed to seven gas di f fusers 8 , which equip the bottom of the tank 1 .

It should be appreciated that the invention is not limited to the presence of seven gas di f fusers and that the reactor could comprise more or fewer of them .

The gas di f fusers preferably extend over a surface area of more than substantially 0 . 05 m² per m³ of liquid or semi-liquid medium contained in said tank, to create an agitation suf ficient for preventing the formation of biofilms and so that the entirety of the microbial flora in suspension can be in contact with the carbon dioxide and the carbon monoxide of the anoxic gas for treatment that is inj ected . In the context of the example represented, the gas di f fusers 8 cover 34 % of the surface area of the bottom of the tank 1 .

The gas di f fusers 8 are placed under the tubular membranes shown in more detail in Figures 3 and 4 .

A first conduit 90 withdraws the gas present in the gas collection and storage space 3 , and a second conduit 91 reinj ects the gas withdrawn into the gas di f fuser 8 . Figure 5 shows in more detail the conduits 90 and 91 and the gas di f fusers 8 placed at the bottom of the tank 1 . The methanation reactor in accordance with the invention and shown in Figures 2 to 5 comprises thirty-seven non- microporous tubular membranes 6 .

An example illustrating one membrane 6 in more detail is illustrated in Figures 6 and 7 .

Each membrane 6 is actually composed of a bundle of thirty-six silicone tubes 61 , each tube itsel f being a membrane , and the tubes 61 being positioned vertically and in parallel against one another, and held together by an upper distribution element 62 and a lower end element 63 . A central sti f fening rod 64 is also provided within each bundle of membranes 6 , in the middle of the silicone tubes 61 , to maintain the integrity of the upper distribution element 62 and the lower end element 63 , in particular by screw connection of the ends of the rod 64 in the distribution element 62 and end element 63 .

The distribution element 62 and end element 63 and also the ends of the silicone tubes which are connected therein are embedded in a polyurethane resin which seals the connection between the sil icone tubes and the distribution element 62 and end element 63 ( see reference 69 in Figure 6 ) .

The lower end of the silicone tubes is connected to the lower end element 63 by notched nipples 65 ( Figure 7 ) .

The upper distribution element 62 of each bundle of membranes comprises an inlet 66 for connection of a feed tube 71 , for introducing pure dihydrogen into the membrane conduit of each of the 36 silicone tubes included in the bundle of membranes . To accomplish this , the upper distribution element 62 comprises a hollow space 67 connected to the inlet 66 , having 36 outlets which allow the pure dihydrogen to di f fuse into the membrane conduit of each silicone tube 61 via a notched nipple 65 .

The upper distribution elements 62 and the lower end elements 63 may be produced by 3D printing .

The pressure obtained by inj ection of the gas into the membrane conduit of each silicone tube 61 is between 0 . 5 and 1 . 5 bar ( the di f fusion of pure dihydrogen through the wall of each tubular membrane 61 is dependent on the internal pressure in the membrane conduit - the higher this pressure , the higher the rate of permeation of dihydrogen through the wall of the silicone tube ) .

It will be appreciated from the description above that the membrane conduit into which the dihydrogen is inj ected may be realised by a single conduit ( a passage with walls belonging to it ) or by other means , such as by spaces defined between parallel silicone tubes , positioned against one another, without departing from the scope of the invention .

In the context of the example presented, the silicone tube included in the thirty-seven bundles of membranes 6 has a total specific length of 1000 m per m³ of volume of dedicated liquid or semi-liquid medium comprising the microbial flora in suspension . However, the speci fic volume occupied by the silicone tube is less than 0 . 2 m³ per m³ of dedicated volume for the liquid or semi-liquid medium comprising the microbial flora in suspension .

It is noted in Figures 3 and 4 that all of the tubular membranes 61 are oriented in the same way, paral lel to the axis of the tank .

Reference will now be made to other elements of the reactor in accordance with the invention, which is illustrated in Figures 1 to 7 , these elements allowing further optimisation of the functioning of said reactor . Provision is made for the pure dihydrogen feed conduit 70 to be equipped with a first device 31 which provides the functions both of a flowmeter and a flow regulator . The inlet for anoxic gas for treatment is likewise equipped with a second device 32 also providing the functions of a flowmeter and of a flow regulator .

The first and second flowmeter/ flow regulator devices 31 and 32 are combined in their functioning with a control module 30 , which the methanation reactor in accordance with the invention likewise comprises .

Provision is also made for the gas collection and storage space to be equipped with an instrument 33 for analysing the gas composition at the inlet for anoxic gas for treatment , and for the outlet ( enabling extraction of the methane-enriched gas ) to be also equipped with an instrument 34 for analysing the methane composition of the gas .

The instrument 33 serves to identi fy the proportion of carbon dioxide and carbon monoxide in the anoxic gas for treatment that is introduced into the reactor .

The instrument 34 serves to identi fy the proportion of dihydrogen and methane in the gas extracted from the reactor .

The control module serves to police the rate o f pure dihydrogen that is introduced and the rate of anoxic gas for treatment that is introduced, depending on the compositional data supplied by the instruments 33 and 34 . More particularly, the control module 30 implements a computer program which comprises instructions which, when they are executed, implement all of the steps o f the process which will now be described :

Using the instrument 33 , a first measurement of the composition of the anoxic gas for treatment at said gas inlet is obtained - the first measurement of the composition of the anoxic gas comprises a concentration of carbon dioxide and/or carbon monoxide .

In parallel and/or simultaneously, using the instrument 34 , a second measurement is obtained of the composition of the gas at said gas outlet 10 , the second measurement of the composition of gas comprising a concentration of methane and of dihydrogen .

Two comparisons are made by the control module : i ) The methane concentration of the second gas composition measurement is compared with a predetermined threshold value of methane , and the dihydrogen concentration of the second gas composition measurement is compared with a predetermined threshold value of dihydrogen . ii ) The carbon dioxide and/or carbon monoxide concentration of the first measurement of the composition of anoxic gas for treatment is compared with a predetermined threshold value of carbon dioxide and/or carbon monoxide .

I f via this comparison it is found that the dihydrogen concentration at the outlet from the reactor is greater than said predetermined threshold value of dihydrogen, said first setpoint value of said first flowmeter/ flow regulator device 31 is reduced and/or the second setpoint value of said second flowmeter/ flow regulator device 32 is increased, and/or the service duration of said recirculation pump 9 is increased, and vice versa .

I f via this comparison it is found that said carbon dioxide and/or carbon monoxide concentration is greater than said predetermined threshold value of carbon dioxide and/or carbon monoxide , the first setpoint value of said first flowmeter/ flow regulator device 31 is increased and/or the second setpoint value of said second flowmeter/ flow regulator device 32 is reduced, and/or the service duration of said recirculation pump 9 is increased .

Reference will now be made to the operation of the methanation reactor which has j ust been described :

The tank is filled beforehand with a volume of 800 L of anaerobic liquid or semi-liquid medium comprising a hydrogenotrophic methanogenic Archaea microbial flora and/or homoacetogenic bacteria and acetotrophic methanogenic Archaea .

The volume of the gas collection and storage space remaining is approximately 200 L : it therefore corresponds substantially to a quarter of the volume dedicated to the liquid or semi- liquid medium, or to a fi fth of the total volume of the tank .

The tank is heated or maintained at a temperature of substantially 37 ° C or of substantially 55 ° C .

The anoxic gas for treatment is introduced into the gas collection and storage space 3 of the tank 1 via the gas inlet 5 , this gas comprising carbon dioxide and optionally carbon monoxide .

The recirculation pump 9 withdraws the gas present in the gas collection and storage space 3 in order to feed the gas di f fusers 8 at the tank bottom, and the di f fusers 8 inj ect this gas into the liquid or semi-liquid medium 2 comprising the microbial flora in suspension - accordingly, the microbial flora is contacted with the carbon dioxide and, optionally, carbon monoxide .

In parallel , pure dihydrogen is introduced into each of the bundles of membranes 6 .

The tubular membranes 61 included in the bundles of membranes 6 di f fuse the dihydrogen into the liquid or semi-liquid medium 2 comprising the microbial flora 20 ; this flora is thus also contacted with the dihydrogen required for the methanation reaction .

The microbial flora 20 then generates methane ( CH4 ) from the carbon dioxide ( and optionally from the carbon monoxide ) and from the dihydrogen with which it is supplied independently .

The methane , which has low solubility in water, ascends naturally in gaseous form at the surface of the liquid or semi-liquid medium, and enriches the gaseous medium in the gas collection and storage space 3 .

The methane-enriched gas is evacuated via the outlet 10 . The instruments for compositional analysis 33 and 34 , provide information to the control module 30 regarding, respectively, the composition of the carbon dioxide ( and optionally carbon monoxide ) gas introduced into the gas collection and storage space , and the dihydrogen and methane composition of the gas which is evacuated via the outlet 10 .

Permanently or at regular intervals , the control module 30 compares these carbon dioxide ( and optionally carbon monoxide ) , methane and dihydrogen compositions with threshold values which condition the operation of the devices 31 and 32 ( acting as flowmeter and as flow regulator ) in order to regulate the flow of anoxic gas for treatment that is introduced into the gas collection and storage space , and the flow of the dihydrogen that is introduced into the tubular membrane conduits .

Depending on whether the concentration of the anoxic gas for treatment that is introduced into the gas collection and storage space increases or reduces , and according to whether the methane and dihydrogen concentration of the methane-enriched gas increases or reduces , the control module modi fies the setpoint values of the devices 31 and 32 in order to increase or decrease the rate of anoxic gas for treatment that is introduced, or that of dihydrogen introduced .

The control module also acts on the operating mode for the pump 9 , to increase or reduce the di f fusion of gas (withdrawn from the gas storage space ) into the liquid or semi-liquid medium 2 .

From the description above , it is appreciated that the invention promotes the methanation reaction, in particular by preventing the formation of biofilm on the tubular membranes 61 and by policing the flow rate of the two gases ( anoxic gas for treatment and dihydrogen gas ) which are introduced separately into the liquid or semiliquid medium comprising the microbial flora in suspension .

It should be appreciated, however, that the invention is not speci fically limited to the embodiment presented in the figures , and that it extends to the use of any equivalent means .

For example , the inlet 5 for anoxic gas for treatment could be realised elsewhere : indeed, the inlet for anoxic gas for treatment can be provided at any point in the gas recirculation loop .

For further example , provision could be made to engage additional pumps - a first with a discharge conduit connected to the inlet 5 , and a second with an intake conduit connected to the outlet 10 , without departing from the scope of the invention .

Furthermore , Figures 8 and 9 illustrate a variant embodiment whereby the tank may contain 125 m³ of liquid or semi-liquid medium 2 :

One favoured application for a reactor of this scale is upgrading the biogas ( i . e . obtaining biomethane which can be inj ected into the natural gas network, starting from biogas ) produced by a small biogas production plant ( i . e . a plant producing from 100 to 150 Nm³ of biogas per day) . The chamber of the reactor described possesses the features of those equipping the most widespread design of a biogas production digester or post-digester ( cylindrical reactor with vertical axis ) . Consequently, a post-digester equipping a biomethanisation plant can be easily converted into a methanation reactor in accordance with the invention, thereby saving on the cost of installing a new reactor on the site .

A reactor of this kind is shown in Figures 8 and 9 : the cylindrical tank 1 has an internal diameter of 6100 mm and an internal height of 6200 mm, and has a flat bottom . It is constructed in reinforced concrete and closed of f hermetically at its top by a stretchable and detachable membrane 12 made from an elastomer selected for its low gas permeability (butyl , for example ) .

One favoured embodiment involves burying the tank in the ground over part of its height . This buried configuration provides both thermal insulation by the ground, and ready access to the interior of the tank from its top when the membrane has been removed, with the formate of the reactor enabling internal access with a construction machine . This makes it easier to install the internal elements of the reactor initially and to maintain them during maintenance operations .

The reactor is heated by means of a piping coil through which a heat trans fer fluid runs , said coil being embedded in the concrete of the sidewall of the tank (not illustrated) . The upper part of the tank, above the ground, is provided with a heat insulation 13 applied to the outer face of its reinforced concrete wall and protected by a rigid casing .

The 125 m³ capacity of the reactor corresponds to a height of substantially 4250 mm of liquid or semi-liquid medium 2 beneficial to the development of an anaerobic microbial flora comprising a population of hydrogenotrophic methanogenic Archaea and/or a population comprising both homoacetogenic bacteria and acetroptrophic methanogenic Archaea .

This medium may be the digestate from a digester intended for biogas production .

The nutrient content , microbial flora content and soluble inorganic carbon (HCG>3~, CG>3²~ and H2CO3 ) content of the reactor may be regenerated via the further introduction of liquid or semi-liquid medium via a dedicated inlet made in the sidewall , j ust above the level 14 of the liquid or semi-liquid medium 2 in the reactor .

The conduit 15 leading to this inlet comprises a pump 16 with a nominal delivery rate of substantially 10 m³/day . The liquid or semi-liquid ef fluents may be discharged via a dedicated outlet 17 which is made in the sidewall and is situated as close as possible to the bottom of the reactor . This outlet 17 is connected to a vertical conduit 18 , allowing the ef fluents to be brought back up above the level of the ground . The conduits 15 and 18 which are dedicated to the circulation of liquid or semi-liquid medium are stainless steel pipes .

The volume of the concrete tank corresponding to the top 1950 millimetres of its internal height does not contain liquid or semi-liquid medium and constitutes a gas collection and storage space 3 with a volume of substantially 58 m³, equivalent to about 46% of the volume dedicated to the liquid or semi-liquid medium .

In this space , the sidewall of the tank comprises an inlet via which the gas containing CO/CO2 to be converted to methane may be introduced into the reactor . The conduit 5 for gas for treatment that leads to this inlet comprises a gas pump with a nominal delivery rate of 150 Nm³/day ( at atmospheric pressure ) and an electric cut-of f value , which are placed in series .

The gas collection and storage space 3 comprises two dedicated gas outlets . Via the first outlet , the methane- enriched gas can be evacuated from the reactor, and this outlet is connected to a discharge conduit equipped with a second gas pump with a nominal delivery rate of 150 Nm³/day ( at atmospheric pressure ) , pump 92 , which enables active discharge of the treated gas from the reactor, and also an electrical cut-of f valve 93 .

Via the second outlet it is possible to draw in the gas contained in the gas collection and storage space via an intake conduit 51 which is connected to the inlet of a third gas pump 52 having a nominal delivery rate of ~ 35 Nm³/min ( at atmospheric pressure ) . The outlet of this pump discharges the gas via a conduit 53 connected to an inlet made in the sidewall of the concrete tank, below the level of liquid or semi-liquid medium, at 75 mm above the bottom of the tank .

This inlet supplies one hundred and twenty bubble di f fusers 80 of microperforated EPDM membrane disc type with gas via a distribution network 81 lying within a plane parallel to the bottom of the tank, in the direct proximity of the tank . The total effective surface area of these gas diffusers is 6.832 m², representing 0.055 m² per m³ of liquid or semi-liquid medium.

The group composed of the gas collection and storage space, the outlet, the gas intake conduit 51, the third pump 52, the discharge conduit 53, the inlet, the distribution network 81, the gas diffusers 80 and the liquid or semi-liquid medium 2 forms a recirculation loop which allows for an improvement in the contacting of the CO/CO2 fraction of the gas for treatment with the microbial flora in suspension in the liquid or semiliquid medium 2.

The conduit 5 for introducing the gas for treatment into the reactor, the piping network 51, 53, 81 that allows it to be recirculated in the liquid or semi-liquid medium 2, and the conduit for discharge of the treated gas are PVC pipes with an outer diameter of 100 mm and a thickness of 2.5 mm.

Added to the fixed gas collection and storage volume 3 is a variable volume 40 delimited by the top of the tank and the stretchable membrane 12 insulating said tank from the environment outside the reactor.

The volume of this space 40 can vary between 0 m³ (slack membrane 12) and around 59 m³ (taut membrane 12 forming a half-sphere - see figure 8) , this maximum volume corresponding to around 47% of the volume dedicated to the liquid or semi-liquid medium.

This variable-volume space 40 makes it possible to maximise the methane content of the treated gas by varying the residence time of the gas in the reactor in proportion to the level of carbon monoxide/carbon dioxide in the gas to be treated or to the rate of injection of this gas into the reactor.

This volume may be increased by injecting the gas for treatment into the reactor, by activation of the gas inj ection pump, or decreased by discharging the treated gas from the reactor, by activation of the gas extraction pump 92 .

This device is also able to treat volumes of gas in the reactor by successive cycles , using the stretchable membrane 12 in the manner of a lung : inflating the stretchable membrane 12 with the gas containing carbon monoxide/carbon dioxide ; waiting for the time needed to obtain a level of methane which is compatible with inj ection of the treated gas into the natural gas network (when using the reactor to upgrade biogas ) ; deflating the stretchable membrane 12 and releasing the biomethane to the natural gas network; recommencing the cycle .

Another embodiment for this variable-volume gas collection space is to insert , into the gas recirculation loop, one or more reservoirs containing an

( incompressible ) volume of liquid with a free liquid space above it . The gas storage capacity of said reservoir or reservoirs may be increased by reducing the part of the volume that is occupied by the liquid therein with the aid of a first pump, which withdraws the liquid therefrom and discharges it into a liquid storage tank which is independent of the variable-volume gas storage reservoir or reservoirs . Conversely, the gas storage capacity of said reservoir or reservoirs may be reduced by inj ecting liquid therein with the aid of a second pump, which withdraws liquid from the independent liquid storage tank and discharges it into the variable-volume gas storage reservoir or reservoirs .

Pure dihydrogen, which may be produced by a water electrolyser (not illustrated) , is inj ected into the liquid or semi-liquid medium 2 by means of tubular membranes 6 , which operate on the same principle and have the same mode of manufacture as that represented schematically in Figure 6 and described above . The upper and lower end elements 62 and 63 with which each membrane 6 is equipped have a parallelepipedal shape in the context of this example, and accommodate one hundred and fifty-seven silicone tubes 61 with internal and external diameters of 8 mm and 10 mm respectively. The tubes 71 connecting the dihydrogen injection modules are arranged such that the axes of the tubular membranes 6 are oriented in a horizontal direction (see Figure 10) , thereby enabling more effective vertical diffusion of the gas bubbles containing carbon monoxide/carbon dioxide into the entirety of the volume of liquid or semi-liquid medium 2.

The reason for this is that the end elements 62 and 63 of the membranes 6 form less of a hindrance to the ascent of the gas bubbles in the liquid or semi-liquid medium 2, since they are included in planes which are no longer perpendicular but are parallel to the vertical path of the gas bubbles.

The dihydrogen injection modules are distributed in twelve radial vertical dihydrogen injection units 100, each comprising a stainless steel chassis 101 with trapezoidal section, which supports one hundred and eight dihydrogen injection modules.

Each vertical unit 100 is suspended via chains from two metal cross-members 102, which have one end resting on the upper edge of the sidewall of the tank, and the other end resting on the top of a central reinforced concrete post .

Within one vertical dihydrogen injection unit 100, the one hundred and eight injection modules are distributed in two sub-units A and B, which are superposed vertically (i.e. one upper sub-unit A and one lower sub-unit B) each comprising six stages each comprising nine injection modules (or membranes 6) with different formats, corresponding to nine lengths of silicone tubes.

These nine injection module formats correspond to 240, 340, 500, 630, 760, 890, 1020, 1150 and 1280 millimetres of silicone tube lengths , respectively ( see references

Ml to M9 in Figure 10 ) .

The distribution network feeding the membranes 6 included in the 12 upper sub-units A with dihydrogen is independent from the network which supplies the membranes 6 in the 12 lower sub-units B .

Via this feature it is possible to compensate the negative ef fect of the hydrostatic pressure on the rate of permeation of dihydrogen through the wall of the silicone tubes , by subj ecting the membranes included in the 12 lower sub-units B, immersed to a depth of between 2 and 4 metres , to an intra-membrane pressure greater than that to which the membranes 6 included in the 12 upper sub-units A, immersed to a depth of between 0 and 2 metres , are subj ected .

The reactor must be fed by a pressurised source of dihydrogen, which may be the dihydrogen reservoir of a water electrolyser . The pressurised reservoir of dihydrogen feeds a general conduit on which the following control members are mounted in series : a general cut-of f valve for the dihydrogen feed, a first safety valve , a first pressure sensor, a general pressure regulator ( first-expansion valve ) , a second safety valve , a second pressure sensor, a f lowmeter/mass flow regulator device , and a third pressure sensor .

The general conduit feeds dihydrogen to as many secondary conduits as there are sub-units A or B superposed in each vertical dihydrogen inj ection unit ( two secondary conduits in the present case in which there are two subunits ) .

Each secondary conduit includes , in series , the following control members : a secondary cut-of f valve , a secondary pressure regulator ( second-expansion valve ) which allows the intra-membrane pressure of the membranes 6 to be adapted to the depth of immersion of the sub-unit A or B that includes these membranes , and a pressure sensor . Each secondary conduit feeds dihydrogen to as many tertiary conduits as there are sub-units A or B included in each level of depth of immersion ( twelve tertiary conduits in the present case ) . Each tertiary conduit includes , in series , the following control members : a tertiary cut-of f valve , a safety valve , a f lowmeter/mass flow regulator device , and a pressure sensor .

Each tertiary conduit feeds dihydrogen to the inj ection modules included in a sub-unit A or B via the following : a rigid conduit passing through the wall of the reactor and equipped with a connection to each of its ends ; a flexible internal pipe made of stainless steel , and an internal distribution network made of rigid stainless steel conduits .

The reactors forming the subj ect of the invention can be used j ointly . For example , a plurality of reactors may be used with their gas collection spaces mounted in parallel on the anoxic gas conduit containing the carbon dioxide for treatment , and thereby enable a multiplication in the value of the gas flow rate which can be treated by the number of reactors used . In order to maximise the methane content of the treated gas (where the treated gas is inj ected into the natural gas network) , it is also possible to employ a plural ity of reactors forming the subj ect of the invention, mounted in series .

Claims

1. Biological methanation reactor, comprising a tank (1) comprising: a liquid or semi-liquid medium (2) , an inlet (5) for anoxic gas for treatment, comprising carbon dioxide and optionally carbon monoxide, a microbial flora (20) comprising methanogenic Archaea, at least one non-microporous tubular membrane (6) which is permeable to dihydrogen, immersed at least partially in said liquid or semi-liquid medium (2) and at least partially in contact with said microbial flora (20) , said at least one membrane (6) comprising a membrane conduit into which a gas may be injected, said reactor comprising at least one gas injector (7) connected to said conduit of said at least one membrane (6) , and an outlet (10) for the recovery of methane generated by said biological methanation and produced in the tank (1) by said methanogenic Archaea microbial flora (20) in contact with said gas having passed through said at least one membrane (6) , said tank (1) comprising a second inlet (21) for feeding the liquid or semi-liquid medium with nutrient, and comprising a second outlet for discharge of effluents (22) , characterised in that said microbial flora (20) utilised for the methanation reaction is an anaerobic microbial flora in suspension in said liquid or semi-liquid medium (2) and comprises hydrogenotrophic methanogenic Archaea and/or comprises homoacetogenic bacteria and acetotrophic methanogenic Archaea, in that the gas injector (7) is a pure dihydrogen inj ector , - 36 - and in that said reactor comprises a gas collection and storage space (3) , said gas comprising methane from the methanation reaction generated by said microbial flora (20) in contact with said pure dihydrogen injected into the liquid or semi-liquid medium (2) through said at least one membrane (6) , said outlet (10) for recovering the methane being a gas outlet and being connected to said gas collection and storage space (3) , and said gas injector (7) and said inlet (5) for anoxic gas for treatment being two independent gas inlets in said reactor . Biological methanation reactor according to Claim 1, characterised in that the gas collection and storage space (3) is within said tank, between the surface of said liquid or semi-liquid medium (2) and a tank top cover (4) . Biological methanation reactor according to Claim 1 or 2, characterised in that said inlet (5) for anoxic gas for treatment opens into said gas collection and storage space (3) . Biological methanation reactor according to Claim 1, 2 or 3, characterised in that said inlet (5) for anoxic gas for treatment and said outlet (10) for recovering the methane in gaseous form each comprise a non-return valve (50, 100) . Biological methanation reactor according to Claim 1, 2, 3 or 4, characterised in that said inlet (5) for anoxic gas for treatment and said outlet (10) for recovering the methane each comprise a pump . Biological methanation reactor according to any one of the preceding claims, characterised in that it comprises an agitator device (8, 9) for generating an agitation in said liquid or semi-liquid medium (2) . Methanation reactor according to Claim 6, characterised in that said agitator device (8, 9) comprises at least one gas diffuser (8) immersed in the liquid or semi-liquid medium. Methanation reactor according to Claim 7, characterised in that said agitator device (8, 9) comprises a recirculation pump (9) which is connected on the one hand to said gas collection and storage space (3) , for extraction of gas from said space, and which is connected on the other hand to said gas diffuser (8) for feeding said gas diffuser (8) with said gas extracted from said gas collection and storage space (3) . Methanation reactor according to Claim 7 or 8, characterised in that said at least one gas diffuser (8) is placed under said at least one tubular membrane ( 6 ) . Methanation reactor according to Claim 6 or either one of Claims 7 and 8 which is dependent on Claim 6, characterised in that said agitator device (8, 9) extends over substantially 0.05 m² per m³ of liquid or semi-liquid medium contained in said tank. Methanation reactor according to any one of the preceding claims, characterised in that it comprises at least one bundle of membranes (6) comprising at least two tubular membranes, the axes of said at least two tubular membranes comprised in said at least one bundle of membranes (6) being substantially parallel to one another, and each of said at least two tubular membranes of said at least one bundle of membranes (6) comprising respectively a membrane conduit (60) fed with pure dihydrogen by said gas injector (7) . Methanation reactor according to Claim 11, characterised in that said gas injector (7) feeds pure dihydrogen to the conduit of each of said tubular membranes (6) by means of a group of connection tubes (71) , each of the connection tubes

(71) being connected to a main injection conduit

(72) . Methanation reactor according to Claim 11 or 12, characterised in that said tank (1) has a tank volume dedicated to the liquid or semi-liquid medium (2) , in that said at least two membranes (6) of said at least one group of membranes have a total specific length of at least 1000 m per cubic metre of said volume dedicated to the liquid or semiliquid medium and occupy a specific volume of less than substantially 0.2 m³ per m³ of said volume dedicated to the liquid or semi-liquid medium. Methanation reactor according to any one of the preceding claims, characterised in that said membrane (6) comprises at least one silicone tube. Methanation reactor according to any one of the preceding claims, characterised in that said membrane comprises a plurality of silicone tubes

(61) aligned with one another and held by their tube ends in gas distributor elements (62, 63) , said gas distributor elements (62, 63) and the tube ends being embedded in a polyurethane or polyepoxide resin . Methanation reactor according to any one of the preceding claims, characterised in that said conduit - 39 - of said tubular membrane (6) is subject to an internal pressure of between substantially 0.5 and 1.5 bar . Methanation reactor according to any one of the preceding claims, characterised in that it comprises a plurality of levels each comprising at least one tubular membrane, and in that the internal conduits of the tubular membranes (6) are subject to different pressures. Methanation reactor according to any one of the preceding claims, characterised in that it comprises a first flowmeter/ flow regulator device (31) which is connected to said dihydrogen injector (7) . Methanation reactor according to any one of the preceding claims, characterised in that it comprises a second flowmeter/ flow regulator device (32) connected to said inlet (5) for anoxic gas for treatment . Methanation reactor according to Claim 18 or 19, characterised in that it comprises a module (30) for controlling one or other or the two first and second flowmeter/ flow regulator devices (31, 32) . Methanation reactor according to Claim 3 or according to any one of the preceding claims which is dependent on Claim 3, characterised in that it comprises a first instrument (34) for analysing the composition of the gas at said gas outlet (10) and a second instrument (33) for analysing the composition of the gas at said inlet (5) for anoxic gas for treatment opening out into said gas collection and storage space (3) . - 40 - Process for employing a methanation reactor according to Claims 8 and 21 , combined with the features of Claims 18 to 20 , characterised in that it comprises the following steps :

- obtaining a first measurement of the composition of the anoxic gas for treatment to be treated at said inlet ( 5 ) for anoxic gas by means of said second analytical instrument ( 33 ) , said first measurement of the composition of the anoxic gas for treatment comprising a concentration of carbon dioxide and/or carbon monoxide ,

- obtaining a second measurement of the composition of gas at said gas outlet ( 10 ) , by means of said first instrument ( 34 ) , said second measurement of the composition of gas comprising a concentration of methane and of dihydrogen, and

- comparing said concentration of methane with a predetermined threshold value for methane and comparing said concentration of dihydrogen with a predetermined threshold value of dihydrogen,

- comparing said concentration of carbon dioxide and/or of carbon monoxide with a predetermined threshold value of carbon dioxide and/or of carbon monoxide , and increasing a first setpoint value of said first flowmeter/ flow regulator device ( 31 ) and/or reducing a second setpoint value of said second flowmeter/ flow regulator device ( 32 ) and/or increasing a service time of said recirculation pump ( 9 ) i f said concentration of carbon dioxide and/or of carbon monoxide is greater than said predetermined threshold value of carbon dioxide and/or of carbon monoxide , and vice versa, or decreasing said first setpoint value of said first flowmeter/ flow regulator device ( 31 ) and/or increasing a second setpoint value of said second flowmeter/ flow regulator device ( 32 ) and/or increasing a service time of said recirculation pump - 41 -

(9) if said concentration of dihydrogen at said outlet (10) is greater than said predetermined threshold value of dihydrogen, and vice versa. 23. Computer program comprising instructions which when executed by computer implement all of the steps of the process according to Claim 22.