Facility and method for producing biomethane by natural separation of gases from bacterial digestion of solid waste containing fermentable organic matter

Abstract

The invention relates to a facility for producing biomethane from solid waste containing fermentable organic matter, the facility comprising: - at least one enclosure (10) arranged to be open in order to receive a mass of waste (16) to be treated, or closed in a gas-and liquid-tight manner; and - a first capture means positioned inside the enclosure in an upper zone for accumulating light gases which is located above the mass of waste and is commonly referred to as the ceiling of the enclosure, the first capture means being configured to capture light gases originating from a bacterial digestion of the mass of waste inside the enclosure, the facility being characterised in that it also comprises a lower zone for accumulating heavy gases which is located beneath the mass of waste. The invention is useful for producing biogas from solid fermentable waste.

Description

Description Title of the invention: Installation and process for the production of biomethane by natural separation of gases from the bacterial digestion of solid waste containing fermentable organic matter technical field [1] The invention relates to the field of methanization, that is to say the production of biomethane by the fermentation of waste containing fermentable organic matter (also called "methanizable matter"). [2] More particularly in this field, the invention relates to the production of biomethane of suitable quality for efficient energy recovery in the form of injection of biomethane into a gas distribution network, in the form of bioNGV (natural gas for vehicles) or in the form of fuel for a fuel cell. Technological background [3] The injection of biomethane into a distribution network is subject to strict quality criteria. In Switzerland, for example, the gas must be 96% methane (CH4) and contain a maximum of only 2% hydrogen (H2) and 5 parts per million hydrogen sulfide (H2S). In France, the purification rate is set at 97% methane, but without a specific limit for hydrogen. For use as bio-CNG, the CH4 content can be between 80% and 97%. For the use of biomethane in a fuel cell, additional constraints apply in order to eliminate volatile organic compounds that are particularly harmful to the catalysts of external reformers or to the fuel cell anode. [4] In the field of methanization, all known digesters (also called methanizers) have the same configuration: - the materials to be treated, present in liquid or solid form, occupy the lower part of the sealed enclosure where anaerobic digestion (methanation) takes place - The upper part of the enclosure, left open, constitutes the accumulation zone for the produced gas, commonly called the "sky" of the digester. In all operational digesters in Europe, the biogas produced is extracted by suction from the gas present in the sky. This Suction is achieved through one or more valves connected to a network of tubes, forming the gas manifold. The gas is then drawn out and transported outside the enclosure by creating a vacuum in the manifold using a dedicated pump. [5] The pump(s) draw in the gas accumulated in the sky of the digester and pump it to equipment external to the enclosure which will process this gas to extract biomethane of appropriate quality for energy recovery by injection into a gas distribution network or by use in a fuel cell. [6] Biogas extracted from a conventional digester comprises on average: 45 to 55% methane CH4, 55% to 45% carbon dioxide CO2, some 0.1% dihydrogen H2, dioxygen O2, dinitrogen N2, and hydrogen sulfide H2S, and traces of carbon monoxide CO, sulfur compounds other than H2S, volatile organic compounds of silicon COVSi (such as octamethylcyclotetrasi oxane commonly called D4) and chlorinated compounds (such as C2H2C12). [7] The processing equipment at the output of the digester is designed to produce biomethane from the gaseous mixture extracted from the digester. [8] We start by removing carbon dioxide (CO2) from the biogas produced, which represents 45% to 55% of the raw biogas. [9] Currently, this removal is achieved by physical separation of CH4 molecules and CO2 molecules. Several technologies are available to carry out this operation: membrane purification, PSA (Pressure Swing Adsorption) process, amine washing process, where the biogas is passed through a chemical washing solution which fixes the carbon dioxide and thus separates it from the rest.

[0010] All of these solutions have the disadvantage of being expensive, both to purchase and to operate.

[0011] On average in France, biomethane purification and injection equipment represents between 20% and 25% of the capital expenditure (CAPEX) of a medium-sized methanizer, and up to 35% for a small unit.

[0012] Furthermore, the treatment carried out by these separation solutions generates high energy consumption (as with membranes or the PSA process) and / or uses expensive consumables (as with membranes and the amine process). On average in France, the operating costs (OPEX) of the biomethane purification / injection stages represent approximately 28% to 38% of the OPEX of a methanizer operating according to state-of-the-art technology.

[0013] In total, these high CAPEX and OPEX costs combine to make the biomethane purification / injection stage the main cost item in the cost price of one MWh of decarbonized gas energy, such as biomethane or bioCNG.

[0014] After CO2 removal, further purification is necessary if use in a fuel cell is considered. Organic sulfur compounds, which appear alongside hydrogen sulfide (H2S) when bacteria digest proteins containing sulfur atoms, as well as other harmful compounds, must be removed so that the resulting gas contains no more than 1ppm (parts per million) of sulfur compounds.

[0015] Sulfur compounds can be removed using activated carbon filters or zeolite. Zeolite offers the best efficiency in removing H2S, but its cost remains higher than activated carbon, the most widely used and cost-effective solution.

[0016] Studies highlight the influence of adsorbent bed height, pollutant concentration, gas flow rate, the presence of trace compounds in the mixture, and humidity on the purification performance of these removal techniques. Maintaining an H2S concentration below the tolerance threshold of 1 ppmvH2S for the reformer used in SOFC fuel cells is possible, but at a prohibitive cost.

[0017] For the removal of silica compounds (VOCSi), silica gel is the most suitable for 1. D4 Adsorption. The physicochemical actions of the adsorbents, composed of zeolite + H2S and silica gel + D4 couples, allow H2S to be adsorbed and then oxidized to elemental sulfur on the zeolite surface. During adsorption onto silica gel, D4 is likely to polymerize on the surface. These two phenomena prevent the regeneration of the adsorbents.

[0018] This results in a still very high cost of fine purification, which prevents any economic feasibility of using biomethane in heat pumps, according to the state of the art of available techniques. Statement of the invention

[0019] The invention aims to significantly reduce the production costs of biomethane of sufficient quality for energy recovery in a distribution network or as fuel for a fuel cell.

[0020] With this objective in mind, the invention proposes a biomethane production plant using solid waste containing fermentable organic matter, comprising: - at least one enclosure (10) arranged to be open to receive a mass of waste (16) to be treated, or closed in a manner that is airtight against gases and liquids, and - a first means of capture positioned inside the enclosure in an area upper accumulation of light gases located above the waste mass and commonly called the containment sky, the first means of capture being configured to capture light gases from bacterial digestion of the waste mass inside the containment.

[0021] The installation according to the invention is characterized in that it also includes a lower zone for the accumulation of heavy gases located below the mass of waste.

[0022] The installation thus exploits the physical properties of gases (molar mass difference) for a static separation of gases within the installation itself and before capturing the gases produced within the installation, as will be seen more clearly later in examples of the installation.

[0023] The invention is based on the observation that there is a single distinguishing factor that allows the energy fraction of biogas (methane) to be separated from all other undesirable gases or compounds; this distinguishing factor is the molar mass of each component of the biogas produced, as will be explained in more detail later. Based on this physical property of gases, the invention proposes to separate the gases within the methanization plant itself, before capturing the gases produced within the plant, as will be described in more detail later.

[0024] In the installation according to the invention, the complete separation of the gases constituting the biogas is carried out within the digester itself in a purely static manner: the lighter gas accumulates in the headspace of the chamber, and the heavier gas accumulates by sedimentation in the lower gas accumulation zone, as will be described in more detail. Positioned inside the chamber in the headspace above the waste mass, the first collection device captures the accumulated lighter gas, which contains methane and traces of dihydrogen (H2). The lighter gas is hereafter referred to as HR gas.

[0025] The light gas produced by the installation according to the invention is biomethane, which can be used as an energy source, with the same applications as biomethane produced by the purification processes mentioned in the "Technological Background" section, namely: injection into a distribution network, bio-CNG, or use in a fuel cell. No further gas treatment outside the facility is required; the cost and time required to produce methane from an installation according to the invention are thus significantly lower than the cost and time required to produce methane from existing biogas plants.

[0026] The installation is advantageously complemented by a second collection device positioned in the lower gas accumulation zone and configured to capture heavy gas, which consists mainly of carbon dioxide (CO2). Heavy gas is subsequently referred to as LP gas.

[0027] The BP gas can be released into the atmosphere: the CO2 it contains is of biogenic origin, making it emission-neutral. Alternatively, the BP gas can be recovered by extracting the CO2 it contains. This CO2 can then be used as a food-grade gas, a cooling gas, a plant growth promoter, injected into greenhouses, or as a chemical component for industry. In this case, the biogas plant becomes a carbon sink.

[0028] The invention also relates to a method of implementing an installation as described above. Presentation of the drawings

[0029] The invention will be better understood, and other features and advantages of the invention will become apparent from the following description of examples of implementations of the invention. These examples are given by way of non-limiting example. The description should be read in conjunction with the accompanying drawings in which: - [Fig. 1] Figure 1 is a diagram of an installation according to the invention - [Fig. 2] Figure 2 is a table listing the various gases and volatile compounds that may be found in biogas from a methanizer.

[0030] Throughout the description, the terms "light gas" and "heavy gas" are defined with respect to the molar mass of nitrogen gas (N2), which has a value of 28g / mol; a light gas (respectively a heavy gas) has a molar mass less than (respectively greater than) this value.

[0031] Throughout the description the terms above / below, lower / upper, top / bottom are used according to their usual definitions to position objects, in absolute terms or in relation to each other, along a vertical axis Z represented in the drawing plane of figure 1; heights are defined along this same vertical axis from bottom to top. Detailed description of the invention

[0032] As previously stated, the invention relates to a biomethane production plant using solid waste containing fermentable organic matter, comprising: - at least one enclosure (10) arranged to be open to receive a mass of waste (16) to be treated, or closed in a manner that is airtight against gases and liquids, and - a first capture means positioned inside the containment in an upper zone of light gas accumulation located above the waste mass and commonly called the containment ceiling, the first capture means being configured to to capture light gases from bacterial digestion of the waste mass inside the enclosure.

[0033] At the bottom of the enclosure, the example shown also provides means for evacuating liquids from the decomposition of organic matter, for example an opening 12a closed by an opening / closing valve.

[0034] By decomposition of organic matter, we mean here the bacterial digestion or fermentation of fermentable organic matter.

[0035] Several types / forms of installation can be envisaged according to the invention.

[0036] Figure 1 shows a schematic example of a chamber 10 forming a small-scale bioreactor. The chamber shown has a generally cylindrical shape, made of metal, concrete, etc., with an opening for loading the waste to be treated and unloading the residues at the end of treatment. The opening can be on the top (Fig. 1), which can be closed, for example, by a sealed lid or by placing a sealed cover (Fig. 1); the opening can also be on the side, which can be closed, for example, by a sealed side door (variant not shown). The chamber in Fig. 1 has a generally cylindrical shape with a diameter of approximately 1 m to about 3 m and a height of approximately 2 m to about 6 m, for the treatment of a waste volume of approximately 1.5 m³ 3 up to approximately 43 m 3 and the production of a volume of biogas on the order of 0.1 m³ 3 per hour up to 4 m 3per hour (with a high degree of variability depending on the nature of the waste being treated) on average during a methane production phase (called the methanogenic phase) of the anaerobic treatment of waste.

[0037] Patent document Dl = FR3038532 describes another type of installation also falling within the scope of the invention. The installation in Dl comprises a plurality of similar compartments: each compartment is delimited by walls of concrete or equivalent materials and has dimensions and an opening large enough to load waste and unload residues using otherwise known motorized handling equipment. A cover is used to seal each compartment airtight. Each compartment, once sealed, forms a sealed enclosure in which a methanation reaction can be carried out. Such an installation is obviously much larger than that shown in Figure 1 and is suitable for processing very large volumes of solid waste.

[0038] The installation according to the invention is characterized in that it also comprises a lower heavy gas accumulation zone located beneath the waste mass

[0039] The table in Figure 2 lists the various gases and volatile compounds likely to be found in biogas from a digester, with their molar mass (in g / mol) and their average proportion by volume in the biogas produced. The table clearly shows that:

[0040] - The energetic gases (H2 and CH4) have a molar mass less than 14g / mol (top of the table), a fortiori less than the molar mass of gaseous nitrogen N2 (28g / mol).

[0041] - All gases or compounds to be reduced or eliminated have a molar mass greater than 34g / mol. (bottom of the table), a fortiori greater than the molar mass of gaseous nitrogen.

[0042] Between these two groups, in the middle of the table, are some gases present occasionally (particularly when air enters the methanation chamber) that are completely neutral in the energy recovery processes considered. These neutral gases—carbon monoxide (CO), nitrogen gas (N2), and oxygen gas (O2)—present in very small quantities, are considered negligible in the following description.

[0043] The invention exploits the difference in molar mass (and therefore density) between the two main gases produced by the methanation reaction. Indeed, methane (CH4) has a molar mass of 14 g / mol and carbon dioxide has a molar mass of 44 g / mol. One mole of CO2 is thus 2.75 times heavier than one mole of CH4, and each occupies the same volume (22.41 liters per mole of gas, under normal conditions, according to Avogadro's law). Since both gases are produced simultaneously in the waste mass during the methanation reaction, the lighter CH4 tends to rise and accumulate at the top of the containment, while the heavier CO2 tends to sink and accumulate at the bottom.Experience shows that the methanation reaction of solid waste in a closed (anaerobic) enclosure, lasting from one to eight weeks, lasts long enough to allow natural settling of the gas produced and natural separation of the gases contained in the gas produced simply because of their difference in molar mass.

[0044] It should be noted that, since a low-pressure gas accumulation zone is essential for the implementation of the invention, the methanation of solid waste (solid waste with a particle size > 13 mm and containing more than 20% dry matter) is the only method that allows the invention to be implemented. The methanation of liquid or pasty waste (containing less than 20% dry matter) does not allow the implementation of the present invention.

[0045] The present invention can be implemented, in particular, in an installation conforming to that described in patent application FR2209257 of the same applicant, relating to the treatment of solid waste operated in discontinuous mode (batch treatment) in a multi-compartment installation, and according to a process allowing the injection of volumes determined liquids according to a sequence of injections having a predefined duration and injection frequency to optimize biogas production and ensure control of biochemical balances within each compartment where a batch of waste is treated.

[0046] The first gas collection means is positioned in the upper gas collection zone and preferably in a top wall of the enclosure; in the example shown, the first gas collection means includes a high exhaust opening 22. Such a collection means may be sufficient for a small enclosure. The first gas collection means may also include a first perforated tube (not shown), which is, for example, fixed to the top wall of the enclosure and has one end connected to the high exhaust opening. Alternatively, the first gas collection means may include a first tube network (not shown) comprising perforated tubes joined together, one end of the tube network being connected to the high exhaust opening.

[0047] The installation includes a second capture device positioned in the lower accumulation zone and configured to capture heavy gases.

[0048] According to the implementation method shown, the second collection device is positioned at the bottom of the enclosure. It includes, at a minimum, a low-level discharge opening 26 positioned as low as possible in a side wall of the enclosure, but above any liquid accumulation zone (Fig. 1), if one exists. The second collection device may also include a second perforated tube (not shown), positioned, for example, under the waste pile, with one end connected to the low-level discharge opening.

[0049] The lower gas accumulation zone can be delimited in the enclosure by a perforated wall 15 positioned at a predefined height above the bottom of the enclosure, the waste mass resting on the perforated wall. In addition to delimiting the lower gas accumulation zone, the perforated wall can facilitate the unloading of residues after the methanation of the waste mass.

[0050] Alternatively, the lower gas accumulation zone can consist of a structural layer 13 comprising non-fermentable structural elements 13a, the interstices between the non-fermentable structural elements 13a constituting pockets for the accumulation of heavy gases, the waste mass resting on the structural layer 13. Since the structural elements 13a are non-fermentable, the lower gas accumulation zone retains its gas storage capacity throughout the decomposition of the waste mass, which can last several days or several weeks. Also, the waste mass waste can be deposited directly on the structural layer, without the need for a perforated wall between the two.

[0051] The structuring elements, for example, have an elongated shape with a length equal to 5 to 10 times a width or diameter of the elongated shape, with interstices between the fermentable elements and the structuring elements defining a continuous gaseous space in which gases can circulate.

[0052] Non-fermentable materials can be organic matter, preferably woody or lignocellulosic organic matter, which retains its mechanical strength long enough for bacterial digestion and complete fermentation of the waste mass. Examples include pieces of branches, sugarcane, bamboo, etc. Using organic matter facilitates unloading the plant at the end of fermentation; the organic matter in the structural layer can be removed along with the waste mass residues after methanation, for example, to a composting area.

[0053] Alternatively, the structural layer 13 may comprise a plurality of compartments (13b) containing non-fermentable materials, the compartments being placed side-by-side to form a continuous structural layer at the bottom of the enclosure. The presence of the compartments limits the admittedly small risk of compaction of the structural layer during the methanation stage, and thus ensures that the heavy gas accumulation pockets remain in the lower accumulation zone. The compartments are preferably perforated (such as gabions) to facilitate the flow of gases to and within the structural layer. The compartments can be filled outside the enclosure upstream and then installed at the bottom of the enclosure during the loading of the installation according to the invention. The construction of the structural layer within the enclosure is thus facilitated and completed more quickly.

[0054] According to another aspect of the invention, the enclosure may include a drainage layer 14 at the bottom of the enclosure, located below the lower gas accumulation zone; the drainage layer 14 forms an accumulation zone for liquids produced by the decomposition of the waste mass; it avoids the need for continuous removal of the liquid from the enclosure. In one example, the drainage layer comprises pebbles, which have the advantage of not deteriorating upon prolonged contact with the liquid, while also exhibiting good mechanical resistance to compaction.

[0055] For example, in the installation in Fig. 1, the following are positioned within the enclosure, in order from the bottom: a drainage layer 14, a grid 15, a structural layer 13, and a waste mass 16 comprising two substrate layers 16a Fermentable layers are separated by a non-fermentable intermediate structural layer 16b. This intermediate structural layer 16b improves the stability of the mass by limiting the effects of settling during its decomposition; however, it is not essential. In the example shown in Figure 1, the drainage layer is delimited by the grid 15, which supports the structural layer 13, composed of adjacent compartments 13b. Alternatively, the drainage layer could be made of pebbles, for example, and the structural layer 13 could be placed directly on the pebbles, in which case the grid 15 would not be necessary. The first means of collection consists of the upper drainage opening 22 located in the upper wall of the enclosure. The second means of capture consists of the low evacuation opening 26 located in the wall of the enclosure, at the boundary between the drainage layer and the structural layer.The small size of the installation enclosure according to figure 1 makes the use of tube networks for gas capture unnecessary.

[0056] Experience shows that if the methanogenic phase of anaerobic waste treatment continues for a minimum of 1 to 3 days, then it is possible to capture, using the first capture method, a light gas comprising at least 80 to 97% CH4, the remainder being primarily hydrogen (H2) and, to a lesser extent, CO2. Similarly, the second capture method yields a heavy gas comprising at least 80 to 97% CO2, the remainder being primarily CH4 and trace gases, as listed in the table in Figure 2.

[0057] According to another aspect of the invention, the height S of the waste mass within the containment is limited such that a ratio (C / S) between the height of the containment ceiling (C) and the height of the waste mass (S) is between 0.08 < C / S < 0.4, preferably between 0.12 < C / S < 0.25, and ideally is chosen to be C / S = 0.18 for a cylindrical or parallelepiped-shaped containment. For a containment with a predefined cross-section, the height S of the waste mass allows for estimating the quantity of waste and, based on the methanogenic potential, the quantity of gas per unit time likely to be produced during the methanogenic phase. The height of the ceiling allows for estimating the volume of the upper zone for the accumulation of light gases, CH4 and H2. The C / S ratio allows for optimizing the dimensions of the installation and its operation.Increasing the ceiling height increases the storage volume for the produced gas, thus limiting the risk of overpressure within the containment if the decomposition reaction accelerates. However, increasing the ceiling height also reduces the amount of waste that can be processed simultaneously. A C / S ratio of 0.4 therefore marks a limit of the installation's usefulness. Choosing a C / S ratio is thus a compromise between maximizing the containment's capacity and increasing the capacity of the containment. for maximum production of light gases, and a light gas storage buffer zone limiting the risks of overpressure, optimizing the natural separation of light and heavy gases, and optimizing the extraction of light gases.

[0058] The highest level at which harmful gases or compounds that make up heavy gas (LP) can be generated is the upper surface of the waste mass. Once generated, the components of heavy gas (LP) cannot, under normal conditions, rise to the light gas (HR) collection zone because they are significantly heavier than the gases occupying the top of the digester.

[0059] However, if the light gas collection zone is located too close to the top surface of the waste mass, the suction pressure of the HR light gas flow can lead to the accidental aspiration of elements of the LP heavy gas. The height C of the ceiling must therefore be sufficient to prevent this undesirable effect. Since the volume of light gas aspirated corresponds to the volume of HR light gas produced, and since the volume of HR light gas depends on the volume of the treated mass, and the volume of the mass is a function of its height S, limiting the height S of the mass so that the C / S ratio is greater than 0.08 and preferably greater than 0.12 significantly reduces the accidental aspiration of heavy gas. In the case of a cylindrical or parallelepiped-shaped containment structure, tests have shown that a C / S ratio of 0.18 eliminates the risk of accidental aspiration.

[0060] In another respect, the height C of the ceiling is chosen so that the ceiling has a gas storage capacity of between 2 and 12 hours of light gas production, and preferably between 4 and 5 hours of light gas production for a cylindrical or parallelepiped-shaped enclosure. The storage capacity of the ceiling of the enclosure allows for the capture of light gas not continuously but intermittently, if necessary.

[0061] Finally, the installation may also include a means for analyzing the composition of the gas in the vicinity of the first capture means and a means for deactivating the first capture means when the gas in the vicinity of the first capture means contains a methane content below a predefined minimum level. Thus, when the methane content in the light gas present in the atmosphere is insufficient, gas capture can be temporarily interrupted until the methane content increases. The methane content is, for example, the proportion of methane in the light gas.

[0062] A method for implementing an installation as described above includes in particular an anaerobic bacterial digestion step during which an expected volume of light gas and an expected volume of heavy gas are captured via the first capture means and the second capture means respectively.

[0063] The process according to the invention can be optimized by control via digital control / command tools, making it possible to avoid the main situations presenting risks of disruption of the gravitational separation of gases and / or risks of malfunction of the respective means of capturing heavy and light gases.

[0064] Thus, preferably, in the context of an anaerobic bacterial digestion step comprising a hydrolysis phase, an acidogenesis phase, an acetogenesis phase and a methanogenesis phase, the expected volume of light gas and the expected volume of heavy gas are captured during the methanogenesis phase, each by their respective means of capture, because the gases generated during the phases prior to the methanogenesis phase still contain little valuable gas (CH4 and H2) and in practice present a high proportion of harmful volatile elements which will have almost totally disappeared when the methanogenic phase begins.

[0065] Preferably, during the bacterial digestion stage, at least one liquid injection step into the waste mass is performed, and light gas capture is interrupted during this liquid injection step. Injecting liquid into the waste mass improves its decomposition. The percolation of a large volume of liquid through the waste mass creates significant aerological turbulence, which can severely disrupt the natural separation of the gases produced throughout the percolation process. Interrupting light gas capture during the liquid injection steps prevents the capture of gas that is insufficiently rich in CH4.

[0066] Preferably, during the bacterial digestion stage, a methane concentration in the light gas is determined in the vicinity of the first capture device and, - if the methane level is below a minimum level, the capture of light gas is stopped.

[0067] The methane content in light gas is determined, for example, by the light gas composition analysis method described above. The minimum content can be set according to the intended use of the captured light gas, for example, 80% for use as bio-CNG, or 97% for use as fuel injected into a fuel distribution network.

[0068] Finally, the process according to the invention may also include an initialization step consisting of - determine parameters of the waste mass and - determine, from the parameters of the mass and a predictive model of anaerobic fermentation, an expected volume of light gas and an expected volume of heavy gas as the bacterial digestion stage progresses.

[0069] The use of a predictive fermentation model makes it possible to further optimize the anaerobic digestion stage in order to extract the largest possible quantity of energy gas from the waste mass in the shortest possible time.

[0070] List of reference signs

[0071] 10: pregnant 11: sky 12: background 12a: Liquid drain opening 13: Structuring layer 13a: non-fermentable materials 13b: lockers 14: drainage layer 15: grid 16: Massive waste 16a: substrate layers 16b: intermediate structuring layer 22: High drainage opening 26: Low-level drain opening

Claims

Claims

1. Installation for producing biomethane from solid waste containing fermentable organic matter, installation comprising: - at least one enclosure (10) arranged to be open to receive a mass of waste (16) to be treated, or closed in a gas- and liquid-tight manner, and - a first capture means positioned inside the enclosure in an upper light gas accumulation zone located above the waste mass and commonly called the ceiling of the enclosure, the first capture means being configured to capture light gases originating from bacterial digestion of the waste mass inside the enclosure, the installation being characterized in that it also comprises a lower heavy gas accumulation zone located under the waste mass.

2. Installation according to claim 1 also comprising a second capture means positioned in the lower gas accumulation zone and configured to capture heavy gases.

3. Installation according to claim 1 or 2 in which the first capture means is positioned in a top wall of the enclosure and / or the second capture means is positioned at the bottom of the enclosure.

4. Installation according to one of claims 1 to 3, in which the lower accumulation zone is delimited in the enclosure by a perforated wall positioned at a predefined height above the bottom of the enclosure, the mass of waste resting on the perforated wall.

5. Installation according to one of claims 1 to 3 in which the lower accumulation zone is constituted by a structuring layer (13) comprising non-fermentable structuring elements (13a), the interstices between the non-fermentable structuring elements (13a) constituting pockets for accumulation of heavy gases, the mass of waste resting on the structuring layer.

6. Installation according to claim 5 in which the structuring layer (13) comprises a plurality of compartments (13b) containing the non-fermentable elements (13a), the compartments being juxtaposed to form a continuous structuring layer at the bottom of the enclosure.

7. Installation according to one of the preceding claims in which the enclosure comprises a draining layer (14) located below the lower gas accumulation zone, the draining layer forming an accumulation zone for liquids produced by the decomposition of the waste mass.

8. Installation according to one of the preceding claims in which a ratio (C / S) between a height of the sky of the enclosure (C) and a height of the waste mass (S) is between 0.08 < C / S < 0.4, is preferably between 0.12 < C / S < 0.25 and is ideally chosen equal to C / S = 0.

18.

9. Installation according to one of the preceding claims, in which a height of the sky is chosen so that the sky has a gas storage capacity of between 2 and 12 hours of production of light gases, and preferably of between 4 and 5 hours of production of light gases for a cylindrical or parallelepiped shaped enclosure.

10. Installation according to one of the preceding claims, also comprising a means for analyzing a composition of the gas in the vicinity of the first capture means and a means for deactivating the first capture means when the gas in the vicinity of the first capture means comprises a proportion by volume of methane less than a predefined minimum value.

11. Method for implementing an installation according to one of the preceding claims comprising a closed enclosure comprising a mass of waste to be treated and means for capturing gas originating at least in part from the decomposition of the mass of waste inside the enclosure, the capturing means comprising, inside the enclosure, at least two separate gas capturing means, a first capturing means being positioned in an upper gas accumulation zone to capture a light gas and a second capturing means being positioned in a lower gas accumulation zone located under the mass of waste to capture a heavy gas, the method comprising an anaerobic bacterial digestion step during which an expected volume of light gas and an expected volume of heavy gas are captured via the first capturing means and the second capturing means respectively.

12. The method of claim 11 wherein, during the anaerobic bacterial digestion step comprising a hydrolysis phase, an acidogenesis phase, an acetogenesis phase and a methanogenesis phase, the expected volume of light gas and the expected volume of heavy gas are captured during the methanogenesis phase.

13. Method according to one of claims 11 to 12, in which: - during the bacterial digestion stage, at least one stage of liquid injection into the waste mass is carried out and - the light gas capture is interrupted during the liquid injection step.

14. Method according to one of claims 11 to 13 in which: - during the bacterial digestion stage, a methane level in the light gas is determined in the vicinity of the first capture means and, - if the methane level is below a minimum level, light gas capture is interrupted.

15. Method according to one of claims 11 to 14 also comprising an initialization step consisting of - determine the parameters of the waste mass and - determine, from the parameters of the massif and a predictive model of anaerobic fermentation, an expected volume of light gas and an expected volume of heavy gas as the bacterial digestion stage progresses.

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