Process and Device for the Production of Methane

Abstract

The present invention relates to a process for producing methane as well as the device for producing methane which allows increasing the methane content of the outgoing gas and preferably simultaneously increasing the methane content of the outgoing gas and the productivity of the reactor.

Description

Title of the invention: Method and Device for the production of methane Technical field

[0001] The invention relates to methods and devices for the production of methane. Prior art

[0002] Biological methanation technology consists of one or more reactors, containing microorganisms in an aqueous liquid medium. These microorganisms are fed on the one hand by streams of carbon dioxide (CO2) in the gaseous state, and on the other hand by streams of hydrogen (H2) also in the gaseous state. It is the metabolism of the microorganisms that allows the transformation of CO2 and H2 molecules into methane (CH4) and water (H2O) molecules by methanation reaction.

[0003] Methanation makes it possible to transform industrial gases containing CO2 into methane, such as biogas produced by methanization, syngas produced by pyrogasification or combustion effluents. These gases can thus be injected into natural gas networks or used directly for applications such as mobility.

[0004] The methanation reaction is exothermic.

[0005] Many parameters influence biological methanation technology.

[0006] Typically, specific strains and variants are developed to improve methanation processes, such as for example those described in the international patent application WO2012094538 of the University of CHICAGO, and more particularly the isolated Methanothermobacter thermautotrophicus strain UC 120910, deposited on December 21, 2010 at the American Type Culture Collection (ATCC®) under the patent filing identification number ATCC® PTA-11561.

[0007] Also mentioned as a parameter influencing biological methanation technology is the composition of the medium comprising the microorganisms. Typically, application EP2959003 describes the influence of nitrogen concentration on the productivity rate of methane and biomass contained in the bioreactor.

[0008] It is also known that temperature, pH and pressure have a direct influence on methane production. For example, document EP2675904 describes specific hydrogen partial pressure / CO2 partial pressure ratios for optimizing methane production. Application WO2013 / 060331 also describes that maintaining the pH in the bioreactor between 7 and 8 would increase the amount of methane obtained.

[0009] The gas feed rate is also an important parameter. Typically, patent application EP2872637 describes injection rates of hydrogen and carbon dioxide into a reactor greater than 2.1 volumes of gas per volume of medium per minute (vvm). According to the present application, high flow rates allow an increase in productivity. However, the methane content in the resulting mixture is low.

[0010] The various states of the art mostly make it possible to increase productivity, that is to say the quantity of methane produced for a given reactor volume, but do not make it possible to obtain gases with a very high methane content, in particular a methane content of 90% or more. The increase in productivity is at the expense of the methane content. Conversely, certain processes making it possible to obtain a high methane content have low productivity.

[0011] However, the methane content obtained is a key factor. For a gas to be injected into the natural gas network, it must meet specifications defined by the natural gas network operators. These specifications vary from one country to another and include numerous criteria. These different criteria generally imply a methane content greater than 95%, or even more. Similarly, the ISO 15403 standard, which defines the specifications for natural gas that can be used as compressed natural gas for vehicles, implies a high methane content.

[0012] Obtaining a high methane content at the outlet of the biological methanation process therefore makes it possible to limit, or even eliminate, the post-treatment of the gas necessary to achieve the specifications allowing it to be recovered. Technical problem

[0013] While some prior art processes allow good productivity to be achieved, the methane content obtained is low. While other processes allow good methane enrichment to be obtained in the gas mixture obtained, the productivity is lower. Thus, the solutions described in the prior art do not allow both high productivity and high methane content to be obtained. Also, the process and the device according to the present invention propose increasing the methane content of the gas for at least a productivity identical to that of the prior art of the reactor, and advantageously simultaneously increasing the methane content of the outgoing gas and the productivity of the reactor. Summary of the invention

[0014] The methane production method and the device according to the present invention make it possible to increase the methane content of the outgoing gas and preferably to simultaneously increase the methane content of the outgoing gas and the productivity of the reactor. Statement of the invention

[0015] A process for producing methane is thus proposed comprising:

[0016] A step (a), in a first bioreactor comprising methanogenic microorganisms in a liquid medium, of producing a gas mixture comprising methane, consisting of bringing said microorganisms into contact with incoming gases;

[0017] A step (b), in a second bioreactor, of enriching the gas mixture obtained in step (a) with methane, consisting of transferring, at least in part, from the first bioreactor to the second bioreactor, on the one hand the gas mixture obtained in step (a), on the other hand the liquid medium contained in the first bioreactor, so as to increase the methane content in the gas mixture.

[0018] Very advantageously, step a) of the process according to the invention makes it possible to increase productivity, i.e. to increase the quantity of methane produced in the first bioreactor.

[0019] Advantageously, step b) of the process according to the invention allows the enrichment of the methane content in the gas mixture in order to obtain a methane content greater than 90%.

[0020] Thus, steps a) and b) of the process according to the invention make it possible to simultaneously increase the methane content of the outgoing gas and the productivity of the reactor.

[0021] Preferably, the incoming gases are hydrogen (H2) and carbon dioxide (CO2).

[0022] The CO2 used in the process according to the invention may be a pure gas or come from a mixture of gases comprising CO2. Generally speaking, the CO2 may come from any source. Typically, the pure gas may be a synthesis gas obtained by any manufacturer known to those skilled in the art. The methane production process according to the present invention may also be supplied with CO2 by adding biogas from the digestion of organic waste in the context of methanization.

[0023] Methanization can be, by way of illustration, dry or liquid methanization, a process for methanization of sludge, agricultural waste, green waste, agri-food and household waste.

[0024] The methane production process according to the present invention can be supplied with biogas directly at the outlet of the methanization digester, without recourse to any particular purification or after treatment of impurities (H2S, Volatile Organic Compounds (VOCs), siloxanes, etc.).

[0025] According to one embodiment, the CO2 comes from landfill biogas produced by the burial of organic waste.

[0026] CO2 can also come from gases produced by thermal treatment processes chemical processing of dry biomass or solid waste (pyrolysis or gasification processes producing syngas).

[0027] The CO2 may also be fatal CO2. If necessary, the fatal CO2 is purified in order to remove pollutants and / or obtain a high CO2 concentration.

[0028] In one embodiment, the CO2 supply may consist of a mixture of the different sources as previously described.

[0029] The hydrogen used in the process according to the invention may be a pure gas or come from a mixture of gases comprising hydrogen. Generally, the hydrogen may come from any source. Typically, the pure gas may be a synthesis gas obtained by any manufacturer known to those skilled in the art.

[0030] According to one embodiment, the hydrogen can be produced by electrolysis of water from electricity drawn from the electrical network or from a renewable electricity production source. Electrolysis can be alkaline electrolysis, PEM (membrane) electrolysis, or high temperature electrolysis (SOEC).

[0031] According to one embodiment, the hydrogen may be fatal hydrogen. If necessary, the fatal H2 is purified in order to eliminate pollutants and / or increase the H2 concentration.

[0032] According to one embodiment, the hydrogen can also come from gases produced by thermochemical treatment processes of dry biomass or solid waste (syngas).

[0033] According to one embodiment, the hydrogen is obtained from organisms by photosynthesis or by fermentation in a light-free environment (a process known as “dark fermentation”) in a separate reactor.

[0034] In one embodiment, the H2 supply may consist of a mixture of the different sources as previously described.

[0035] Thus, carbon dioxide and hydrogen are injected as gas into the first bioreactor and consumed by the microorganisms in the liquid phase or reaction medium in order to generate a gas mixture in the first bioreactor.

[0036] The term “methanogenic microorganisms” means all microorganisms capable of producing methane, preferably from hydrogen and carbon dioxide.

[0037] The organisms are mainly hydrogenotrophic methanogens, homoacetogens and acetoclastic methanogens.

[0038] Hydrogenotrophic methanogens directly metabolize methane from hydrogen and carbon dioxide. The methanation reaction can be represented as follows: [Math. 1] 4 H2 + C02 CH4 + 2 H20

[0039] Homoacetogens and acetoclastic methanogens use acetate as an intermediate substrate for the production of methane. The methanation reaction is carried out in several successive and inseparable stages, for example: [Math. 2] h2+co2 -> ch3cooh ch3cooh -> ch4 + co2

[0040] Thus, and according to one embodiment, the microorganisms will be chosen from hydrogenotrophic methanogenic microorganisms, homoacetogenic microorganisms, acetoclastic methanogenic microorganisms or a mixture of these microorganisms.

[0041] Hydrogenotrophic and acetoclast methanogenic microorganisms are Archaea (or archea), strictly anaerobic prokaryotic unicellular microorganisms that belong to the kingdom of euryarchaeotes. They can be chosen from four classes of archaea: Methanobacteria, Methanomicrobia, Me-thanococci and Methanopyri.

[0042] Homoacetogenic microorganisms are bacteria (anaerobic prokaryotic unicellular microorganisms) which mainly belong to the class Clostridia and produce acetate from CO2 and H2.

[0043] These bacteria belong in particular to the genera Clostridium, Acetobacterium, Sporomusa, Acetogenium, Acetoanaerobicum, Pelobacter Butyribacterium, Eu-bacterium.

[0044] In a preferred embodiment, the methanogenic microorganisms are selected from the classes Methanobacteria and Clostridia.

[0045] The terms "biomass" or "cell culture" are understood to mean all the microorganisms as previously defined, methanogenic or not, of one or more species, used in the process according to the invention to produce methane from carbon dioxide and hydrogen. These microorganisms are placed within the first reactor by inoculation of the liquid medium contained in the first reactor or by inoculation of a liquid medium which will subsequently be introduced into the first reactor according to the present invention.

[0046] Thus, and advantageously, the microorganisms are brought into contact with the CO2 and the H2 in a liquid medium in which the microorganisms are maintained.

[0047] By "liquid medium" or "reaction medium" is meant the liquid medium in which the microorganisms are maintained, to generate a mixture of gases, and in which the CO2 and H2 will be injected and dissolved, whether or not this medium allows the production of biomass. In the context of the invention, the reaction medium comprises at least water, nutrients, CO2 and dissolved H2.

[0048] Preferably, the liquid medium or reaction medium is composed of sources of nutrients (nitrogen, calcium, potassium, sulfur, phosphorus, magnesium) and trace elements (iron, zinc, copper, cobalt, nickel, molybdenum, iodine and boron) necessary for microbial growth and activity.

[0049] The liquid medium according to the invention may also comprise acetate.

[0050] Preferably, the liquid medium in the first bioreactor is a continuous liquid phase.

[0051] For the purposes of the present invention, the term "continuous liquid phase" means a liquid volume exhibiting physical continuity, as opposed to a discontinuous liquid volume consisting of a set of liquid phases without contact with each other, such as liquid drops percolating in a gas phase.

[0052] For the purposes of the present invention, the term "gas mixture" means the gas mixture generated within the first reactor. Preferably, the gas mixture comprises at least hydrogen (H2), carbon dioxide (CO2) and methane (CH4).

[0053] Advantageously, step (a) according to the method of the invention makes it possible to generate a gas mixture comprising at least H2, CO2, and CH4 and to obtain optimal conditions for the growth of methanogenic microorganisms. Advantageously, step (a) ensures high productivity, thanks to a high incoming gas flow rate. In a particularly advantageous manner, the first bioreactor makes it possible to convert more than 80% of the hydrogen contained in the incoming gases.

[0054] Advantageously also, step (b) according to the method of the invention makes it possible to reduce the hydrogen and carbon dioxide content in the gas mixture obtained in step (a) and to increase the methane content in the gas mixture obtained in step a). This enrichment step is made possible by the introduction into the second bioreactor (step (b)) of the liquid medium taken from the first bioreactor (step (a)) as well as the transfer of the gas mixture.

[0055] These two stages of the process according to the invention thus make it possible to simultaneously optimize and increase the productivity of the process and the methane content of the outgoing gas, i.e. to obtain an adequate productivity / methane content pair.

[0056] For the purposes of the present invention, the term "productivity" means the flow rate of methane produced per useful volume of reactor. Productivity is thus expressed in NmVh of methane produced per useful m3 of reactor, a unit commonly expressed vvh by those skilled in the art. Productivity may also be expressed in NL / h of methane produced per liter of reactor.

[0057] The measurement of productivity is carried out by any technique known to those skilled in the art. Typically, the measurement of productivity can be carried out by measuring the total outgoing gas flow rate using a Pitot probe and by measuring the methane content using an infrared spectrometry gas analyzer. The flow rate of methane calculated from these data is reduced to the useful volume of the reactor mobilized by the reaction.

[0058] According to one embodiment, the productivity is at least 0.1 vvh, preferably at least 1 vvh, preferably at least 5 vvh, preferably at least 10 vvh, preferably at least 20 vvh and preferably at least 30 vvh.

[0059] The methane content within the meaning of the present invention is understood as the molar fraction of methane in the gas leaving the reactor.

[0060] According to one embodiment, the high methane content means an outgoing gas containing at least 80% methane, preferably 85%, even more preferably 90%, preferably 91%, preferably 92%, preferably 93%, preferably 94% and particularly preferably at least 95%, preferably at least 96%, preferably at least 97%, preferably at least 98%, and preferably at least 99%.

[0061] Thus, and advantageously, the process according to the invention makes it possible to obtain biomethane, directly injectable into the natural gas network, without an additional purification step.

[0062] The measurement of the methane content of an outgoing gas is carried out by any technique known to those skilled in the art. Typically, the measurement of the methane content is carried out using a gas analyzer by infrared spectrometry or gas chromatography. The methane content is expressed on dry gas.

[0063] Advantageously, the method according to the invention makes it possible to obtain a productivity of at least 0.15 vvh at atmospheric pressure and a methane content in the outgoing gas of at least 90%.

[0064] The features set out in the following paragraphs may optionally be implemented. They may be implemented independently of one another or in combination with one another:

[0065] According to the method of the invention, the transfer of the liquid medium is carried out by taking said liquid medium from the first bioreactor and then by injecting said liquid medium into the upper part of the second bioreactor so that said liquid medium circulates by gravity in the second bioreactor and is recovered in the first bioreactor.

[0066] Preferably, the liquid medium circulates, in the second bioreactor, through a packing. The packing may be a bulk packing or a structured packing. By way of illustration, the packing may be made with PALL® rings.

[0067] Advantageously, the packing makes it possible to increase the contact surface between the liquid medium and the gas mixture.

[0068] Advantageously, the injection of the liquid medium into the second bioreactor and the circulation of the liquid medium by gravity in this second bioreactor, preferably through a packing, will allow the liquid medium to circulate by a percolation phenomenon which will increase the contact surface between the liquid and the gas mixture obtained in step (a). The hydrogen and carbon dioxide content of the gas mixture obtained in step (a) will therefore decrease and the gas mixture will be enriched in methane.

[0069] According to one embodiment, in the method according to the invention, the liquid medium is a continuous liquid phase into which the incoming gases are injected and the second bioreactor contains a continuous gas phase.

[0070] Advantageously, the injection of the gases entering the continuous liquid phase makes it possible to disperse the gases in the continuous liquid phase.

[0071] For the purposes of the present invention, the term "continuous gas phase" means a gas phase exhibiting physical continuity, as opposed to a discontinuous gas phase consisting of a set of gaseous media without contact with each other, such as gas bubbles dispersed in a liquid medium.

[0072] According to one embodiment of the method according to the invention, the gas obtained in step a) is transferred from the first bioreactor to the second bioreactor by creating a pressure differential between the two bioreactors.

[0073] The gas obtained in step a) can thus be transferred into the second bioreactor in order to be enriched there according to step b) of the process.

[0074] Any system known to those skilled in the art allowing the creation of a pressure differential between the first bioreactor and the second bioreactor may be used. Typically, a compressor may be used.

[0075] According to one embodiment of the method according to the invention, H2 and CO2 can further be injected into the second bioreactor.

[0076] The additional injection of H2 and CO2 into the second bioreactor makes it possible to modify the stoichiometry of the gases in order to work with different stoichiometric conditions in the two reactors. Advantageously, the gas mixture injected into the first reactor has an excess of H2, to promote its transfer, and CO2 is added to the second reactor to ensure a high methane content.

[0077] A person skilled in the art is able to adjust the temperature, pressure and flow rate parameters to operate the method according to the invention.

[0078] For purely illustrative purposes, the temperature may be between 50 and 70°C, typically between 60 and 70°C.

[0079] For illustration purposes, the pressure will be between 1 and 20 bars, preferably between 2 and 18 bars, preferably between 3 and 16 bars, preferably between 4 and 14 bars, preferably between 5 and 12 bars.

[0080] The invention also relates to a methane production device comprising: - a first bioreactor comprising methanogenic microorganisms in a liquid medium; said liquid medium being a continuous liquid phase; - a second bioreactor comprising a continuous gas phase and a system allowing increased gas exchange; - a device for injecting the incoming gases into said continuous liquid phase contained in the first bioreactor; - at least one means for supplying liquid medium contained in the first bioreactor cooperating with the second bioreactor, said means being capable of supplying liquid medium to the second bioreactor, said supply means comprising pumping means ensuring the circulation of the liquid medium contained in the first bioreactor towards the second bioreactor, said liquid medium circulating by gravity on said system allowing the increase of gas exchanges, said liquid medium being recovered in said first bioreactor; and; - at least one means of transferring the gas mixture contained in the first bioreactor to the second bioreactor.

[0081] Typically, the circulation of the liquid medium contained in the first bioreactor to the second bioreactor can be done via pumping means such as a peristaltic pump or any other means known to those skilled in the art making it possible to ensure the circulation of the liquid from the first bioreactor to the second bioreactor.

[0082] Preferably, the liquid medium supply means is configured to supply liquid to the upper part of said second bioreactor. Even more preferably, the liquid is sprayed into the upper part of the second bioreactor. Typically, the spraying can be carried out by means of a spray.

[0083] According to one embodiment, the means for transferring the gas mixture contained in the first bioreactor to the second bioreactor is a device making it possible to create a pressure differential between the two bioreactors.

[0084] Typically, said means for transferring the gas mixture is a compressor.

[0085] By way of illustration, the compressor will be located upstream of the first bioreactor and will create a pressure differential between the two bioreactors to allow the transfer of the gas mixture obtained in the first bioreactor to the second bioreactor. Thus, the gas is transferred either directly in the case where the two reactors are two stages within the same reactor, or by means of a pipeline if the two reactors are separate.

[0086] According to one embodiment, the device according to the invention further contains a means for supplying incoming gases, preferably H2 and CO2, into the second bioreactor.

[0087] Advantageously, this additional injection makes it possible to modify the stœ- chiometrics of gases.

[0088] Typically, this supply means can be implemented with a compressor and a flow meter allowing the flow of additional gas injected to be adjusted.

[0089] According to one embodiment, the first bioreactor is chosen from a bubble column, a mechanically stirred column, an infinitely mixed reactor or an airlift reactor.

[0090] According to one embodiment, the device for injecting the incoming gases into the continuous liquid phase is chosen from fine bubble diffusers such as a porous column bottom diffuser, a perforated tube, a porous membrane made of polymers or ceramic material, a valve bubbler, or from bubble-free membrane contactors such as hollow fiber membranes, or from hydroejectors or static mixers.

[0091] According to one embodiment, the device for injecting the incoming gases into the continuous liquid phase is a fine bubble diffuser. According to this embodiment, the first bioreactor may further comprise a structured packing. The structured packing makes it possible to disperse the bubbles.

[0092] According to one embodiment, the second bioreactor is selected from a percolation reactor, a bulk packed column, a structured packed column, a spray column, a falling film column or a tray column.

[0093] According to one embodiment, the system allowing the increase of gas exchanges of the second bioreactor is a packing system. Typically the packing system means any system allowing the contact surface between the liquid phase and the gas phase to be increased in order to increase the exchanges in the second bioreactor.

[0094] For illustrative purposes, the packing system may be a loose packing consisting of Pall® rings, preferably 5 / 8 inch Pall® rings having a diameter of 15 mm and a height of 15 mm.

[0095] Preferably, the first bioreactor is a bubble column and the second bioreactor is a percolation reactor. Brief description of the drawings

[0096] Other characteristics, details and advantages of the invention will appear on reading the detailed description below, and on analyzing the appended drawings, in which: Fig.l

[0097] [fig.l] shows a device for the production of methane according to the state of the art; Fig. 2

[0098] [fig-2] shows a device for the production of methane according to a method of rea embodiment of the invention in which the two bioreactors are combined (“two-stage system”); Fig. 3

[0099] [fig.3] shows a device for the production of methane according to another method of embodiment of the invention in which the two bioreactors are connected by a means for transferring the gas mixture.; Fig. 4

[0100] [fig.4] shows a curve representing the productivity in NL CH4 / Lutile / h in function of the methane content (%CH4) of a state-of-the-art process. (“1 stage: bubble column”) and of the process according to the present invention (“2 stages: bubble column + percolation”) as shown in Figure 2; Fig. 5

[0101] [fig.5] shows a curve representing the composition of the outgoing gas in CH4, CO2, H2 as a function of time of the process according to the present invention (“two-stage system”) as shown in Figure 5. The composition is expressed on dry gas. Description of the embodiments

[0102] The drawings and the description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand the present invention, but also contribute to its definition, where appropriate.

[0103] Reference is now made to Figure 1. The device 10 as shown in Figure 1 is a state-of-the-art methanation reactor for producing methane and comprising a single 22-litre bubble column reactor 11 (with a useful volume of 18 litres), gas-tight and thermally insulated. The temperature within the bubble column reactor is maintained at approximately 55°C by the presence of a water circulation jacket 12.

[0104] A mixture of incoming gases 13, H2 and CO2, is injected into the bubble column 11 by a fritted type fine bubble diffuser 14 in the lower part of the bubble column 11.

[0105] The upper part of the reactor is made of a polyvinyl chloride (PVC) plate pierced with 7 orifices (not shown) allowing the passage of probes. The device 10 also comprises a gas outlet equipped with a condenser 15, a gas outlet 26 towards a meter, a gas loop 16 connected to analyzers 17 (analysis of the outgoing gases allowing the quantification of the CO2, H2 and CH4 contents respectively), a gas recirculation loop 24 from the upper part of the reactor to the lower part, an orifice 21 for mixing the recirculated gases with the incoming gases, an inlet 18 for the supply of nutrient solution, and a purge of the liquid medium 27. A three-way valve 22 allows sampling gas in order to check the composition of the gas by gas chromatography and to regenerate the gas and the anoxic environment in the gaseous atmosphere.

[0106] The concentration of dissolved carbon dioxide, the redox potential and the pH are measured by probes 23 immersed in the liquid medium (directly in the reactor). The probe for measuring the pH also allows the temperature to be measured.

[0107] The bubble column comprises a liquid medium consisting of hydrogenotrophic methanogenic, acetoclastic methanogenic and homoacetogenic microorganisms, nutrients and trace elements.

[0108] According to this embodiment of the state of the art, the gas supply was carried out via two synthetic gas cylinders each comprising H2 and CO2. Mass flow meters allow the inlet flow rates of the incoming gases to be finely adjusted. A recirculation of the gases 24 from the top to the bottom of the column is carried out at a constant speed, by means of a valve pump 25. This recirculation 24 has the purpose of increasing the gas retention rate and the gas retention time in order to increase the dissolution of the H2 and the consumption by the microorganisms in order to reduce the residual H2 concentration in the outgoing gas mixture.

[0109] The flow rate of the outgoing gas mixture is measured by a Ritter gas meter. The gases leaving the reactor pass through a condenser 15 maintained at 4°C. The condensation water is partially reintroduced into the reactor in order to maintain the volume of the liquid medium.

[0110] The reactor is continuously supplied with incoming gas (H2 and CO2). On the other hand, the supply of nutrients and the purging of the liquid medium are carried out discontinuously. The dosage of sulfur is carried out by a piston syringe system. The collection of liquid samples for the analysis of the compounds is carried out in the lower part of the reactor. Typically, the nutrients are injected from a concentrated nutrient solution, in particular NH4C1 at 20g / L, KH2PO4 at 10g / L, MgCl2 at 2g / L, CaCl2 at 1g / L, Na2S at 26.7g / L and NaHCO3 12.4g / L.

[0111] The composition (proportion of H2, CO2, CH4) of the outgoing gas mixture is measured continuously by sampling in the upper part of the column.

[0112] Reference is now made to Figure 2 reproducing a device 30 for the production of methane according to an embodiment of the invention. The elements shown in Figure 2 and bearing the same references as those in Figure 1 represent the same objects, which are not described again below.

[0113] The methanation device or reactor 30 is composed of a bubble column 31 and a percolation reactor 32, gas-tight and thermally insulated.

[0114] The 22-litre bubble column 31 (with a useful volume of 18 litres) is connected to the percolation reactor 32 by a PVC part 33. The bubble column and the reactor thus connected are tightened by a collar. The percolation reactor 32 is packed with 5 / 8 inch Pall® rings having a diameter of 15 mm and a height of 15 mm (Techim France).

[0115] The bubble column 31 comprises a liquid medium comprising hydrogenotrophic methanogenic, acetoclastic and homoa-ketogenic methanogenic microorganisms, nutrients and trace elements. The liquid medium from the bubble column 31 is pumped into the lower part of the bubble column 31 and is conveyed via a peristaltic pump 40 into the upper part of the percolation reactor 32. The liquid medium is injected by spraying using a spray 38 present in the upper part of the percolation reactor. The liquid medium circulating by gravity on Pall® rings will percolate through the rings in order to increase the contact surface between the liquid and the gas, until it falls back into the bubble column 31. The gas mixture generated in the bubble column 31 diffuses into the percolation reactor, through a stainless steel grid 34 retaining the Pall® rings of the reactor 32.

[0116] Methane productivity is high, in particular due to a relatively high hydrogen partial pressure, and a high flow rate thus allows high microbial growth. The gas mixture thus generated will diffuse, according to a pressure differential applied between the inlet of the bubble column 31 and the outlet of the percolation reactor 32, into the percolation reactor 32 in which the hydrogen and carbon dioxide will be transformed into methane to achieve a high methane content in the outgoing gas mixture. According to this embodiment, more than 80% of the hydrogen contained in the incoming gases is converted.

[0117] Part 33 allows the connection between the bubble column 31 and the percolation reactor 32 and includes tappings, 331, 332, allowing analysis by gas chromatography and / or liquid sampling. Part 33 in which tappings 331 and 332 are shown schematically is an enlargement, in Figure 2, of this part to illustrate the tappings.

[0118] The concentration of dissolved carbon dioxide, the pH and the redox potential are measured by probes 36, 37 immersed in the liquid, either directly in the reactor (not shown), or in a cell 35 connected to the reactor as shown in Figure 2. The pH probe also allows the temperature to be measured.

[0119] In the same way as before, the gas supply is carried out via two synthetic gas cylinders, each comprising H2 and CO2. Mass flow meters allow the inlet flow rates of the incoming gases to be finely adjusted. The flow rate of the outgoing gas mixture is measured by a Ritter gas meter. The gases leaving the reactor pass through a condenser maintained at 4°C. The condensation water is partially reintroduced into the reactor in order to maintain the volume of the liquid medium.

[0120] The reactor is continuously supplied with incoming gases (H2 and CO2). On the other hand, the supply of nutrients and the purging of the liquid medium from the reactor are carried out discontinuously. The sulfur dosage is carried out by a piston syringe system. The collection of liquid samples for the analysis of the compounds is carried out in the lower part of the reactor.

[0121] The composition (proportion of H2, CO2, CH4) of the outgoing gas mixture is measured continuously by sampling in the upper part of the column, using the same analyzers mounted in series as previously described.

[0122] Reference is now made to Figure 3. The elements shown in Figure 3 and bearing the same references as those in Figure 2 represent the same objects, which are not described again below.

[0123] In this embodiment, the bubble column 31 and the percolation reactor 32 are connected to each other by pipes and peristaltic pumps. This embodiment makes it possible in particular to use reactors of different diameters and to reduce the height of the reactor. The liquid medium contained in the bubble reactor 31 is pumped into the lower part of the bubble column 31 via a peristaltic pump 40 to be injected by spraying at the top of the percolation reactor 32 using a spray 38. The liquid medium, in the same way as in the embodiment described in Figure 2 will circulate by percolation on the Pall® rings and will be collected and reinjected through the tube 41 into the bubble column 31. The outgoing gas mixture generated in the bubble column 31 is transferred into the percolation reactor 32 by applying a pressure differential between the two bioreactors, if necessary using a compressor.

[0124] In the same way as before, the gas supply is carried out via two synthetic gas cylinders each comprising H2 and CO2. Mass flow meters allow the inlet flow rates of the incoming gases to be finely adjusted. The flow rate of the outgoing gas mixture is measured by a Ritter gas meter. The gases leaving the reactor pass through a condenser maintained at 4°C. The condensation water is partly reintroduced into the bubble column 31 in order to maintain the volume of the liquid medium.

[0125] The reactor is continuously supplied with incoming gas (H2 and CO2). On the other hand, the supply of nutrients and the purging of the liquid medium from the reactor are carried out discontinuously. The sulfur dosage is carried out by a piston syringe system. The collection of liquid samples for the analysis of the compounds is carried out in the lower part of the bubble column 31.

[0126] The composition (proportion of H2, CO2, CH4) of the outgoing gas mixture is measured continuously by sampling in the upper part of the column, using the same analyzers mounted in series as previously described. Examples

[0127] Other advantages, aims and particular characteristics of the present invention will emerge from the examples which follow, given for explanatory and in no way limiting purposes.

[0128] In the examples which follow, the different parameters were measured by the techniques detailed below:

[0129] Measurement of productivity with respect to the methane produced Productivity with respect to methane produced is calculated using the following measure: lMath.31 % C HiOUt XQ With: PCH4 xl03= Productivity of p _ _______J___________g, or ? (dry) ^4 V useful methane in NL of CH4 / L of useful volume / h %CH4out = percentage of methane in the outgoing gas expressed on dry gas Qgout = outgoing gas flow rate in NmL / h expressed on dry gas Vutiie = useful volume of the reactor in which the reaction takes place

[0130] Measurement of the concentration of microbial biomass (MVS) The measurement of biomass concentration is estimated once a week by measuring suspended volatile matter (SVM) according to the Afnor NF T90-105-2 standard. The principle consists of taking a sample of a known volume (75 mL in our case). After centrifugation for 15 minutes, at 13,200 rpm and 4°C, the pellet is placed in an aluminum cup that has been previously dried and weighed. The cup is then placed in an oven at 105°C for 24 hours. Once the water has evaporated, only the suspended solids (SS) remain in the cup. The cup is then weighed after cooling in a desiccator. The difference in mass between the empty cup and the cup after its passage through the oven therefore corresponds to the SS contained in the sample. Considering the initial volume of liquid, the measurement is expressed in gL *. The cup is then placed in an oven at 550°C for 2 hours. After cooling, the cup, which now contains only the mineral matter, is weighed again. The mass of MVS is obtained by the difference between the mass of MES and the mass of mineral matter.

[0131] Measurement of the volume of outgoing gas The outgoing gas volume is measured by volumetry using a Ritter® brand drum gas meter (TG 05 Model 5). The volume is expressed as dry gas.

[0132] Measurement of H CO 2 and CH a contents The H2, CO2, CH4 composition of the outgoing gas is measured using different serially mounted analyzers: H2 is measured by thermal conductivity using a Rosemount® Binos 100 2M analyzer. CO2 and CH4 are measured by a non-dispersive infrared (NDIR) gas analyzer using the Rosemount® X-stream analyzer.

[0133] Measurement of pH and temperature pH and temperature are measured using a Mettler Toledo® probe and transmitter.

[0134] Measurement of the productivity / methane content pair The productivity with respect to the methane produced is calculated by the formula detailed above and the CH4 content is measured using the appropriate analyzer. Once this information was obtained for different operating points, graphs representing the CH4 content as a function of productivity were plotted and as shown in Figures 4 and 5.

[0135] Operating conditions Temperature: 52 to 57°C Pressure: atmospheric pressure The incoming gas flow rate ranged from 6.3 to 43.6 NL / h.

[0136] Example 1: Evaluation of biogas enrichment and productivity according to a state-of-the-art process

[0137] In the present example, the bioreactor used is the bioreactor as described in Figure 1.

[0138] The bioreactor was inoculated with microorganisms (biomass) from organic waste methanizers. After growth of the biomass, the concentration of microorganisms was then regulated around 3 g / L (MVS), by regular and appropriate liquid and biomass purges.

[0139] The pilot was continuously supplied with synthesis gases, H2 and CO2. The ratio between the H2 and CO2 flow rates was kept constant and the total flow rate gradually increased with the process performance.

[0140] The reactor operated for 300 days and the performance of the bioreactor was evaluated with respect to: - The methane content in the gaseous sky; - Methane productivity in NL of CH4 per L of reactor per hour (PCH4 NL / Lreac / h).

[0141] The results are presented in the table below for different incoming gas flow rates: [Table 1] QyOW (SOC) (NL / h) %CH4 PCH4>'4 6.4 1.2 97.7 63.9 7.4 1.4 92.3 63.4 94 2.0 82.2 814 9.4 2.2 76.8 94.8

[0142] Qgin (NL / h) corresponding to the flow rate of hydrogen and carbon dioxide entering in NL / h and Qgout (NL / h) corresponding to the flow rate of gases leaving in NL / h, expressed on dry gas.

[0143] Example 2: Evaluation of biogas enrichment and productivity according to the method and device of the invention

[0144] In the present example, the bioreactor used is the bioreactor as described in Figure 2.

[0145] The bioreactor was inoculated with microorganisms (biomass) from organic waste methanizers. After growth of the biomass, the concentration of microorganisms was then regulated around 3 g / L (MVS), by regular and appropriate liquid and biomass purges.

[0146] The pilot was continuously supplied with synthesis gases, H2 and CO2. The ratio between the H2 and CO2 flow rates was kept constant (and the total flow rate gradually increased with the process performance).

[0147] The reactor operated for 50 days and the performance of the bioreactor was evaluated with respect to: - The methane content in the gaseous sky; - Methane productivity in mL of CH4 / L of reactor per hour (PCH4 mL / Lreac / h)

[0148] The results are presented in the table below for different incoming gas flow rates: [Table 2] âgfc(NÜh) Qgcut (secj (NL / h) %C!-U PŒsxlQ3 14.5 2.9 94.5 76.7 14.5 3.0 93.9 77.9 29.2 6.1 90.8 153.6 43.6 9.7 83.3 225.0

[0149] Qgin (NL / h) corresponding to the flow rate of hydrogen and carbon dioxide entering in NL / h and Qgout (NL / h) corresponding to the flow rate of gases leaving in NL / h, expressed on dry gas.

[0150] The results obtained show that with only a bubble column (1-stage reactor), a significant compromise must be made between the methane content of the outlet gas and the productivity. The addition of a second percolation stage makes it possible to simultaneously increase the productivity of the process and the methane content of the gas produced. As an illustration, the bioreactor described in Figure 1 makes it possible to obtain a productivity of 91.4 x 10 4 NLCH4 / Lutiie / h for a CH4 content of 82.2% at the reactor outlet, while the bioreactor described in Figure 2 makes it possible to obtain a productivity of 153.6 x 10 4 NLCH4 / Lutiie / h for a CH4 content of 90.8% at the reactor outlet, i.e. higher productivity and CH4 content of 68.1% and 10.5% respectively.

Claims

Claims

1. A method for producing methane comprising: - A step (a), in a first bioreactor comprising methanogenic microorganisms in a liquid medium, of producing a gas mixture comprising methane, consisting of bringing said microorganisms into contact with incoming gases; - A step (b), in a second bioreactor, of enriching the gas mixture obtained in step (a) with methane, consisting of transferring, at least in part, from the first bioreactor to the second bioreactor, on the one hand the gas mixture obtained in step (a), on the other hand the liquid medium contained in the first bioreactor, so as to increase the methane content in the gas mixture.

2. Method according to claim 1, characterized in that the transfer of said liquid medium is carried out by taking said liquid medium from the first bioreactor and then by injecting said liquid medium into the upper part of the second bioreactor so that said liquid medium circulates by gravity in the second bioreactor and is recovered in the first bioreactor.

3. Method according to claim 1 characterized in that, in the first bioreactor, the liquid medium is a continuous liquid phase into which the incoming gases are injected, and in that the second bioreactor contains a continuous gas phase.

4. Method according to any one of claims 1 to 3, characterized in that the microorganisms are chosen from hydrogenotrophic methanogenic microorganisms, homoa-ketogenic microorganisms, acetoclastic methanogenic microorganisms or a mixture of these microorganisms.

5. Method according to any one of claims 1 to 4 characterized in that the incoming gases are CO2 and H2.

6. Method according to any one of claims 1 to 5, characterized in that H2 and CO2 can additionally be injected into the second bioreactor.

7. Methane production device characterized in that it comprises: - a first bioreactor comprising methanogenic microorganisms in a liquid medium, said liquid medium being a continuous liquid phase; - a second bioreactor comprising a continuous gas phase and a system allowing the increase of gas exchanges; - a device allowing the injection of the incoming gases into said continuous liquid phase contained in the first bioreactor; - at least one means for supplying liquid medium contained in the first bioreactor cooperating with the second bioreactor, said means being capable of supplying liquid medium to the second bioreactor, said supply means comprising pumping means ensuring the circulation of the liquid medium contained in the first bioreactor towards the second bioreactor, said liquid medium circulating by gravity on said system allowing the increase of gas exchanges, said liquid medium being recovered in said first bioreactor;and - at least one means for transferring the gas mixture contained in the first bioreactor to the second bioreactor.;

8. Device according to claim 7 characterized in that the second bioreactor contains a continuous gas phase.

9. Device according to any one of claims 7 to 8 characterized in that the device further contains a means for supplying incoming gases into the second bioreactor.

10. Device according to any one of claims 7 to 9 characterized in that the first bioreactor is chosen from a bubble column, a mechanically stirred column, an infinitely mixed reactor or an airlift reactor and in that the second bioreactor is chosen from a percolation reactor, a bulk packed column, a structured packed column, a spray column, a falling film column or a tray column.

11. Device according to any one of claims 7 to 10, characterized in that the device for injecting the incoming gases into said continuous liquid phase is chosen from fine bubble diffusers, such as a porous column bottom diffuser, a perforated tube, a porous membrane made of polymers or ceramic material, a valve bubbler, or bubble-free membrane contactors such as hollow fiber membranes, or a hydroejector or a static mixer.

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