FR3045663A1 - METHANIZATION UNIT EQUIPPED WITH A "U" -shaped LINEAR DIGESTER WITH INTERNAL THERMAL EXCHANGER - Google Patents

Abstract

The invention relates to a linear digester (2) of an anaerobic digestion unit (1), configured to degrade organic matter and generate biogas, which comprises an inlet (7), an outlet (12), means for routing organic material from the inlet to the outlet, a first compartment (9) and a second compartment (10). These compartments communicate in an end portion (11) and are separated by a partition wall (13) configured to provide thermal conduction. The inlet and the outlet are located respectively in the first compartment and in the second compartment, opposite said end portion. This design is used to heat the fresh organic material located upstream of the digester by means of the digestate located downstream of said digester. The invention also relates to the methanation unit (1) which optionally comprises a post-digester (20) in addition to the digester (2).

Description

METHANIZATION UNIT EQUIPPED WITH A "U" -shaped LINEAR DIGESTER WITH INTERNAL THERMAL EXCHANGER

Technical area

The present invention relates to the field of anaerobic digestion units that make it possible to valorize solid organic materials by generating biogas, which is injected onto the gas network after purification, and the digestate, which is used for fertilization and soil restructuring. . The invention relates in particular to the design of the digester and the post-digester which is optionally associated with said digester.

State of the art

The methanation units are known to those skilled in the art. Some allow the recovery of liquid organic materials, comprising less than 15% dry matter. The others allow the valorization of pasty or solid organic matter, comprising more than 15% of dry matter.

These methanation units include a digester in which are inserted organic materials that can be of agricultural, agro-industrial and / or municipal. The natural degradation of these organic materials can generate in the enclosure of the digester, biogas and digestate.

Biogas is essentially composed of methane (CH4), carbon dioxide (CO2) and hydrogen sulphide (H2S). The methane content can vary from 50 to 70%. The carbon dioxide content oscillates between 30 and 50%. The hydrogen sulphide content is generally less than 1%. Their respective proportions vary according to the nature of the waste and the operating conditions. After purification of the biogas by means of a treatment system present on the anaerobic digestion unit, the methane is injected onto the distributor's gas network, for example to heat dwellings or to produce electricity.

The digestate consists of juice and dry matter which, after separation by means of a filtration system, are operated separately. The juices are spread in the fields for soil fertilization, and the dry materials are spread in the fields for soil restructuring.

The anaerobic digestion process generally requires that the temperature of the fresh organic matter be raised to a temperature of the order of 30 ° C for the mesophilic systems, and at a temperature of about 60 ° C for the thermophilic systems, in order to promote the development of methanogenic bacteria.

On existing biogas plants, this temperature contribution is implemented by means of a system for heating organic matter, which uses about 15% of the primary energy of the crude biogas produced. This energy requirement therefore strongly impacts the overall efficiency of these methanation units. Hydrogen sulphide is a toxic gas for humans and highly corrosive to the equipment of the methanation unit. In order to reduce the hydrogen sulphide content present in the biogas, the methanization units are generally equipped with a bio-filtration system consisting in injecting a small quantity of oxygen or air into the biogas, this mixture being exploited by sulphate-reducing bacteria which lower the sulfur content and form sulphate (SO42).

In a first known embodiment, the bio-filtration system is placed inside the digester. In this case, the air or oxygen is injected directly into the gaseous atmosphere of the digester enclosure and sulphate-reducing bacteria develop on the surface of the digester, for example on the surface of the ceiling, which is often made of wood. , on the surface of the walls of the digester, or even on the surface of the membrane covering sealing the chamber of the digester. This implementation has several disadvantages, namely: The filtration efficiency of hydrogen sulfide is very limited; The filtration efficiency is unstable due to the random drop of sulfur compounds in the digester;

The development of sulphate-reducing bacteria is limited by a too small surface area;

The development of sulphate-reducing bacteria is limited because of the absence of nutrients, vitamins and trace elements; The effectiveness of methanogenic bacteria is competed by the supply of oxygen in the digester which generates an anaerobic medium too rich in oxygen; The effectiveness of methanogenic bacteria is competed by the formation of sulphate favoring a predominance of sulphate-reducing bacteria; The effectiveness of methanogenic bacteria is competed by the acidification of the medium;

The digester degrades prematurely because of the formation of sulfuric acid on the walls.

In a second known embodiment, overcoming these aforementioned drawbacks, the bio-filtration system is implemented outside the digester after extraction of the biogas. In this case, the air is injected on the biogas extracted at the exit of the digester, and the bacteria develop on a plastic lining which offers a large specific surface on which sulphate-reducing sulphate-producing bacteria develop. This implementation has the disadvantage of being very expensive. Summary of the invention

The present invention overcomes the aforementioned drawbacks of existing methanation units, the main objective being to optimize the production efficiency of methane sent to the gas distribution network.

This optimization of the methane production efficiency firstly requires reducing the consumption of raw biogas used to heat the organic matter in the digester enclosure. For this purpose, the invention uses a linear digester of an anaerobic digestion unit, configured to degrade organic matter and generate biogas. The digester has an inlet, through which the fresh organic material enters the interior of the digester. The digester also includes an outlet, through which is removed from the enclosure of the digester, the digestate resulting from the degradation of organic matter. The digester comprises means for conveying the organic matter from the inlet to the outlet. These conveying means make it possible to push the organic material into the enclosure, which ensures a linear and continuous displacement progressively allowing the entry of fresh organic matter and the exit of digestate. Remarkably, the digester comprises a first compartment and a second compartment which communicate with each other in an end portion, the inlet and the outlet being located respectively in the first compartment and in the second compartment, opposite said end portion. Thus, the chamber of the digester has a linear configuration in the shape of a "U". In addition, these two compartments are separated by a separation wall configured to provide thermal conduction. The degradation of organic matter allows a gradual increase in temperature until a digestate is reached at a temperature of about 55 ° C, while the fresh organic material has a temperature of the order of 15 ° C. The "U" shaped configuration of the linear digester, with separation by a wall providing thermal conduction, allows the organic material present in the second compartment, whose degradation is more advanced or has reached the state of digestate before its outlet of the digester, to heat the organic material present in the first compartment, whose degradation is less advanced or is still in the state of fresh organic matter at its entry into the digester. Thus, it is no longer necessary to use a portion of the raw biogas produced to heat the organic material in order to promote its degradation and the production of biogas, which makes it possible to achieve the aforementioned main objective.

In order to optimize the heating of the fresh organic matter, as soon as it enters the enclosure of the digester according to the invention, said digester comprises a heat exchange circuit arranged in a serpentine close to the inlet in the first compartment and to near the exit in the second compartment. In addition, the heat exchange circuit contains a coolant, circulating means being configured to convey said coolant in the heat exchange circuit of the second compartment to the first compartment. The organic material is fresh and has a temperature of the order of 15 ° C, when entering the enclosure. Conversely, the digestate resulting from the degradation of the organic matter, has a temperature of the order of 55 ° C at the outlet of the enclosure. The circulation of the coolant against the flow direction of the organic matter in the digester enclosure, optimizes the warming of organic matter from its entry into the digester, and thus to promote the rapid degradation of this organic matter and therefore methanation, as soon as entering the digester enclosure. This supply of thermal energy is carried out without any consumption of raw biogas produced previously by said digester. This optimization of the heating makes it possible to increase the flow of the organic matter in the digester.

It is also possible to provide a heat exchange circuit which extends over the entire length of the thermal conduction wall in the first compartment and in the second compartment, the heat transfer fluid circulating counter-current to the direction of advance of the material. organic in the digester enclosure.

According to the digester object of the invention, it comprises means for mixing the organic material in the first compartment and in the second compartment. This makes it possible to homogenize the organic matter throughout its thickness as it moves around the enclosure, thus promoting the rapid degradation of all the organic matter and the formation of biogas.

In a non-limiting embodiment of the digester according to the invention, the partition wall is made of a structural steel. Other materials providing good thermal conduction can also be envisaged, without departing from the scope of the invention. The invention also relates to an anaerobic digestion unit comprising a linear digester having the aforementioned characteristics, and making it possible to achieve the aforementioned main objective. The anaerobic digestion unit according to the invention comprises a filtration system configured to extract the hydrogen sulphide present in the raw biogas resulting from the degradation of the organic matter. In a preferred embodiment, the filtration system comprises means for injecting concentrated oxygen or air into the raw biogas, a perforated floor, and a porous lining coating said floor. The porous lining is designed in a material configured to bio-filter biogas by allowing the growth of sulphate-reducing bacteria.

In one embodiment of the anaerobic digestion unit according to the invention, the digester comprises the means for injecting concentrated oxygen, or even air, into the upper part of the compartments. In addition, the digester comprises the openwork floor covered with porous lining, which are arranged above the compartments of the enclosure, and a membrane arranged above the floor, sealingly relative to said compartments. The presence of the perforated floor and the porous lining allow the biogas present in the enclosure, to pass through said elements to be housed under the membrane. The concentrated oxygen injected in a small quantity into the biogas at the top of the compartments passes through the perforated floor through the perforated floor and penetrates the lining. Sulfato-reducing bacteria that develop in this packing, for example those of the family Thiobacillus, exploit oxygen (O2) and hydrogen sulphide (H2S) present in the biogas to form sulfate ions (S042). The evacuation of oxygen out of the enclosure, through the perforated floor, keeps all the efficiency of the methanogenic bacteria that develop in the compartments of the digester, which optimizes the methane content (CH4 ) biogas. Similarly, the development of sulfuric acid and sulfate is carried out on the porous lining, outside the enclosure, said lining having a high specific surface, since it extends over the entire floor. This greatly reduces the presence of sulfuric acid on the walls of the digester enclosure and the competition between methanogenic bacteria and sulphate-reducing bacteria. Thus, it avoids premature degradation of the digester and acidification of organic matter, and promotes the methanization of organic matter.

In a preferred embodiment of the anaerobic digestion unit according to the invention, the latter comprises a post-digester defining an enclosure, in addition to the digester. The post-digester allows temporary storage of the digestate. Transfer means are arranged between the digester and the post-digester to evacuate the digestate at the outlet of the digester to an inlet of the post-digester. In addition, communication means are configured to transfer the biogas from the enclosure of the digester to the enclosure of the post-digester, this transfer can be carried out directly or indirectly. This design advantageously makes it possible to implement bio-filtration in the post-digester rather than in the digester. This design avoids the degradation of the anaerobic conditions in the digester since the supply of oxygen takes place outside the enclosure of the digester. This also avoids sulfuric acid flows in the digester, thus reducing competition between methanogenic bacteria and sulphate-reducing bacteria, and acidification of the medium.

According to this preferred embodiment of the anaerobic digestion unit, the concentrated oxygen injection or air injection means are arranged in the area of the communication means between the digester and the post-digester. The injection of concentrated oxygen or air during the transfer of the biogas from the digester chamber to the post-digester enclosure makes it possible to homogenize the oxygen mixture with the biogas. However, an alternative embodiment could be provided with the concentrated oxygen injection or air injection means arranged directly in the post-digester enclosure.

According to this preferred embodiment of the anaerobic digestion unit, the post-digester comprises the perforated floor covered with the porous lining, which are arranged above its enclosure, and a membrane arranged above the floor in a sealed manner with respect to this pregnant. These characteristics correspond to those mentioned above, when they are implemented directly above the enclosure of the digester.

According to the anaerobic digestion unit object of the invention, for the aforementioned variants, this comprises means for spraying a nutritional solution on the surface of the porous lining, so as to promote the development of sulphate-reducing bacteria in the porous packing. The anaerobic digestion unit according to the invention also comprises means for extracting the biogas after carrying out the bio-filtration. When the bio-filtration is carried out directly at the level of the digester, or when the bio-filtration is carried out at the level of the post-digester and that the biogas is transferred from the enclosure of the digester directly towards the enclosure of the post-digester the extraction means are arranged to extract the desulfurized biogas present under the membrane. However, it is possible to envisage a variant in which the communication means transfer the raw biogas present in the digester enclosure, below the membrane, said crude biogas then being desulfurized by passing through the porous lining and the perforated floor to descend in the post-digester enclosure. According to this variant, the extraction means are then arranged to extract the desulfurized biogas present in the enclosure of the post-digester, and not under the membrane. This extraction of the desulphurized biogas also makes it possible to suck the raw biogas, which is then forced through the floor and the porous lining, for bio-filtration.

In a preferred embodiment, the porous lining consists of vulcanized coconut fibers, preferably in the form of plates adjoining each other over the entire surface of said lining. It is also possible to envisage vulcanized hemp or linen fibers or even other ligneous-type organic materials. However, other organic or synthetic porous carriers could be envisaged, the essential objective being to increase the development surface of sulphate-reducing bacteria. Vulcanized coconut fibers, or even vulcanized hemp fibers, will be favored because they contribute to the thermal insulation of the digester and / or the post-digester, depending on the design. The anaerobic digestion unit according to the invention also comprises means for purifying the extracted biogas, making it possible to separate the methane from the carbon dioxide. The anaerobic digestion unit according to the invention also comprises means for grinding the organic materials before they are injected into the digester. This makes it possible to reduce the length of the fibers present in the organic material, so as to promote the movement and the mixing thereof in the enclosure. The anaerobic digestion unit according to the invention also comprises a hopper equipped with a mixer, in which the organic materials are inserted before being injected into the digester. This allows to homogenize the fresh organic matter. The anaerobic digestion unit according to the invention also comprises means for separating liquids and dry matter contained in the digestate which is discharged at the outlet of the digester or the post-digester, when said treatment unit is equipped with it. Thus, liquids or juices can be spread in the fields to fertilize the soil and, separately at another time, the dry matter can be spread in the fields for soil restructuring.

Brief description of the figures

The characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment based on figures, among which:

FIG. 1 schematizes, in a three-dimensional view, an anaerobic digestion unit according to a preferred embodiment of the invention;

Figure 2 shows schematically, in top view, the digester according to one embodiment of the invention;

Figure 3 shows schematically, in a cross-sectional view, the digester;

FIG. 4 illustrates a temperature rise curve of the organic material with the digester according to the invention;

FIG. 5 schematizes, in a view in longitudinal section, the digester or the post-digester;

Figure 6 shows a part of the floor implemented above the enclosure of the digester or above the enclosure of the post-digester, according to the configuration of the anaerobic digestion unit.

detailed description

As illustrated in FIG. 1, the anaerobic digestion unit 1 comprises a digester 2 which allows the degradation of organic materials (not shown) for the creation of biogas. The organic materials are preferably derived from agriculture, for example manure. Before being conveyed into the digester 2, these organic materials are ground by means of a mill 3, in order to obtain a grain size preferably less than 5 cm. Once ground, these organic materials are inserted into a hopper 4 which is equipped with a mixer 5 for homogenizing the organic materials. The hopper 4 comprises a piston pump or a worm arranged in the bottom of this hopper 4, to advance the organic material without incorporating air, given its layout in the bottom. This hopper 4 also comprises a weighing platform (not shown) to weigh the mass of the fresh organic material before sending it to the digester 2.

The fresh organic material leaves the hopper 4 and circulates in a conduit 6 until reaching an inlet 7 (illustrated by the arrow in FIG. 2) implemented by a first orifice (not illustrated) arranged in the lower part of the wall 8 of the digester 2. This orifice opens into a first compartment 9 inside the chamber 2a of the digester 2. The digester 2 also comprises a second compartment 10 which communicates with the first compartment 9, at the level of the end portion 11 located on the opposite side to the end wall 8. This second compartment 10 comprises an outlet 12 (illustrated by the arrow in FIG. 2) implemented by a second orifice (not illustrated) arranged in the lower part of the end wall 8 of the digester 2. The two compartments 9, 10 are physically separated by a thermal conduction wall 13, preferably made of structural steel, which extends over almost the entire length the chamber 2a of the digester 2, except in said end portion 11. Thus, the digester 2 provides a linear displacement in the form of "U" of the organic material inside its chamber 2a. The organic material is moved in the chamber 2a by means of a hydraulic pump (not shown) arranged on the duct 6, at the inlet 7 in the digester 2.

When the fresh organic material enters the first compartment 9 of the digester 2, it is at a temperature of the order of 15 ° C. The natural degradation of the organic material allows its rise in temperature to reach about 55 ° C to 60 ° C in the digestate state where said organic material is degraded, near the outlet 12 in the chamber 2a. The methanation process operates through the development of methanogenic bacteria in the chamber 2a, this development being favorable when the temperature is of the order of 55 ° C. The presence of the thermal conduction wall 13 allows a heat exchange between the two compartments 9, 10, the 55 ° C digestate providing an energy supply to the fresh organic material at a temperature of 15 ° C. in the chamber 2a. . This ensures a faster temperature rise of the organic matter and, thus, the faster development of methanogenic bacteria. Thus, the digester 2a allows the establishment of the methanation process, without the need for additional energy intake that consumes some of the raw biogas produced previously, as provided by the prior methanization units.

In order to further accelerate the rise in temperature of the fresh organic matter following its entry into the first compartment 9 of the digester 2, the latter comprises a heat exchange circuit 14 which comprises a first portion 15 which extends in a serpentine along a length L close to the end wall 8, in the first compartment 9, and a second portion 16 which extends in a serpentine fashion near said end wall 8 on the length L, in the second compartment 9 The first and second portions 15, 16 are interconnected either by passing through, above or below the thermal conduction wall 13, or by circling said thermal conduction wall 13. heat exchange 14 contains a coolant (not shown) and is connected to a pump 17 which circulates the heat transfer fluid against the current, that is to say the second compartment 10 to the first compartment 9, so to transfer the thermal energy of the digestate, to the fresh organic matter.

As illustrated in FIGS. 1 to 3, the digester 2 also comprises a first set of blades 18 arranged along the length of the first compartment 9 and a second set of blades 19 arranged along the length of the second compartment 10. These two sets of blades 18, 19 are rotated respectively along two longitudinal axes XI, X2 in said compartments 9, 10, by means of rotation drive means implemented on each of the sets of blades, outside the wall of end 8. These rotating drive means consist, for example, of a notched wheel (not shown) which is integral with the shaft (not shown) of the set of blades 18, 19 arranged along the axis XI, X2, and a cylinder (not shown) configured so that its piston engages successively in the notches of said toothed wheel so as to rotate and cause the blade set 18 or 19. These sets of blades 18, 19 allow to homogenize the organic matter in the compa compartments 9, 10 over their entire height h and their widths 11, 12, as illustrated in FIG.

In a preferential and nonlimiting embodiment, by choosing a height h of the order of 5 meters and a width 11, 12 of the order of 5 meters, and a heat exchange circuit 15 of 400 meters long which s' coil extends over a length L of the enclosure 2a, of the order of 6 meters, the organic material reaches a temperature T1 of the order of 55 ° C at the end of this length L of 6 meters in the first compartment 9 as shown in Figure 4. The total length (L + L1 in Figure 2) of the chamber 2a is preferably of the order of 24 m. The methanation unit 1 preferably comprises a post-digester 20, illustrated in FIGS. 1 and 5. This post-digester 20 has an enclosure 20a making it possible to contain the digestate extracted from the digester 2. For this, a second conduit (not shown) is arranged between the outlet 12 of the digester 2 and an inlet (not shown) in the enclosure 20a of the post-digester 20, implemented by means of an orifice (not shown) arranged in the lower part of the end wall 21 of said post-digester 20, which is preferably in the extension of the end wall 8 of the digester 2. In addition, the digester 2 and the post-digester are separated by a common side wall 22, as shown in Figure 1.

As illustrated in FIGS. 1, 5 and 6, the post-digester 20 comprises rafters 23 which support a floor 24. This floor 20 is perforated over its entire surface and is coated with a porous lining, preferably vulcanized coconut fibers or vulcanized hemp fibers. This lining 25 can be implemented by means of vulcanized coconut fiber plates, adjacent to each other. Chevrons and a floor, similar to the rafters 23 and the floor 24, also extend above the chamber 2a of the digester 2, although this is not shown in FIG. 1 in order to allow the visualization of the elements to be seen. inside this enclosure 2a. However, above the chamber 2a of the digester 2, this floor is lined with a lining (not shown) more or less impervious to the gaseous fluid, for example raw wood planks adjoining each other, so as to to prevent the raw biogas from leaving the enclosure 2a of the digester 2, from above. The digester 2 and the post-digester 20 are covered tightly by a common membrane 26 of flexible, elastic and sealed material, for example a geo-synthetic material of the ethylene-propylene-diene monomer (EPDM) type, as illustrated in FIG. FIG. 5 only showing the post-digester portion 20. Openings (not shown) are implemented in the upper part of the common side wall 22, which enables the enclosure 2a of the digester 2 to communicate with the enclosure 20a of the post-digester 20. Thus, the biogas generated in the digester 2 is evacuated in the post-digester 20, passing through said openings. Ejection nozzles (not shown) are provided at these openings, preferably within the openings or, adjacent to the openings, at the post-digester side 20. These nozzles are configured to draw water from the water. concentrated oxygen, or even air, in the raw biogas, during its transfer from the chamber 2a of the digester 2 to the chamber 20a of the post-digester 20. The amount of concentrated oxygen injected is low, preferably in the range of 0.1% to 0.2%.

The biogas present in the enclosure 20a of the post-digester 20 and containing a small percentage of concentrated oxygen or air, is then transferred above the ceiling 27, under the membrane 26. During this transfer, the biogas passes through through the perforated floor 24, then through the porous lining, in which sulfate-reducing bacteria which consume oxygen (O 2) and hydrogen sulphide (H 2 S) present in the biogas, to form sulphate ions ( S042) on the surface of the lining 25. Thus, the biogas under the membrane 26 is partially purified and contains almost only methane (CH4) and carbon dioxide (CO2). In order to promote the development of sulphate-reducing bacteria in the lining 25, nozzles 28 are arranged under the membrane 26, as illustrated in FIG. 6, and make it possible to project the entire surface of said lining 25 with a nutritional solution 29, by example based on water and minerals, or even filtered juice from the digestate.

This partially purified biogas is then extracted by suction by means of a pump (not shown) and a conduit 30, illustrated in FIG. 6, to be sent via said conduit 30 to a purification station 31, illustrated in FIG. The extraction of the purified biogas also makes it possible to suck the raw biogas to force its passage through the floor 24 and the packing 25 ensuring the bio-filtration. The purification station 31 is configured to mix the biogas with water and to circulate the mixture in columns 32, which allows a separation of methane (CH4) and carbon dioxide (CO2). The methane is then sent to the gas network of the distributor 33 after checking its quality. The water used containing the carbon dioxide (CO2), is regenerated by evacuation of carbon dioxide, by means of a reset to atmospheric pressure, which allows to reuse this water loop. The anaerobic digestion unit 1 comprises other characteristics, in particular temperature sensors, level sensors, and an automaton for managing the various actuators present, in particular for the purpose of managing the flow of organic matter in the digester 2 and the pump 17 regulating the flow rate of the coolant circulating in the heat exchange circuit 14. The anaerobic digestion unit 1 also comprises a discharge duct (not shown) of the digestate present in the post-digester 20 and a pump (no illustrated) connected to this exhaust duct, for extracting the digestate through an outlet (not shown) arranged on the post-digester 20. The anaerobic digestion unit 1 also comprises a filtration system (not shown) connected to said duct. evacuation and allowing the separation of juices and dry matter present in the digestate, so as to separately value said products.

The foregoing detailed description of a particular embodiment of the invention is in no way limiting. On the contrary, it aims to remove any imprecision as to its scope. Thus, many variants may be considered in the context of the invention. By way of example, the anaerobic digestion unit 1 may not have a post-digester 20. In this case, the oxygen ejection nozzles (not shown) would be distributed around the chamber 2, in the upper part of the latter, so as to mix the oxygen with the raw biogas in the upper part of the chamber 2a. In addition, the porous packing would be implemented on the perforated floor 24, directly above the enclosure 2a of the digester 2.

In another example, the methanation unit 1 comprises characteristics comparable to those mentioned above, with the difference that the openings (not shown) are not implemented on the side wall 22, and therefore do not allow the enclosure 2a of the digester 2 to communicate with the enclosure 20a of the post-digester 20. These openings are instead configured so that the chamber 2a of the digester 2 communicates with the space 34 disposed below the membrane 26. In addition, the conduit 30 for extracting the partially purified biogas is arranged, not in this space 34 below the membrane 26, but in the enclosure 20a of the post-digester 20. The circulation of the biogas in the post-digester 20 , generated by the suction during the extraction, is therefore reversed while maintaining the bio-filtration through the packing 25.

Claims (14)

A linear digester (2) of an anaerobic digestion unit (1), configured to degrade organic matter and generate biogas, the digester comprising an inlet (7), an outlet (12) and a conveyor means the organic material from the inlet to the outlet, characterized in that the digester comprises a first compartment (9) and a second compartment (10) which communicate in an end portion (11), the inlet and the outlet being respectively located in the first compartment and in the second compartment, opposite said end portion, said compartments being separated by a partition wall (13) configured to provide thermal conduction. 2. Digester (2) according to claim 1, which comprises a heat exchange circuit (14) arranged in a serpentine near the inlet (7) in the first compartment (9) and near the outlet (12) in the second compartment (10), the heat exchange circuit carrying a heat transfer fluid, circulation means (17) being configured to convey said fluid in the heat exchange circuit of the second compartment to the first compartment. 3. Digester (2) according to one of claims 1 or 2, which comprises means for mixing (18,19) of the organic material in the first compartment (9) and in the second compartment (10). 4. Digester (2) according to one of claims 1 to 3, wherein the partition wall (13) is made of a structural steel. 5. A methanation unit (1) comprising a linear digester (2) according to one of claims 1 to 4. 6. An anaerobic digestion unit (1) according to claim 5, which comprises a post-digester (20) defining an enclosure, and communication means configured to transfer the biogas from the digester (2) to the post-digester. 7. A methanation unit (1) according to one of claims 5 or 6, which comprises a filtration system configured to extract the hydrogen sulfide present in the raw biogas resulting from the degradation of the organic material. An anaerobic digestion unit (1) according to claim 7, wherein the filtration system comprises means for injecting concentrated oxygen or air into the raw biogas, a perforated floor (24), and a lining (25). ) porous coating said floor, the porous packing being designed in a material configured to ensure bio-filtration of biogas by allowing the development of sulphate-reducing bacteria. 9. A methanation unit (1) according to claim 8, which comprises means for spraying (28) a nutritional solution (29) promoting the development of sulphate-reducing bacteria in the lining (25) porous. 10. A methanation unit (1) according to one of claims 5 to 9, which comprises extraction means (30) of the biogas. 11. An anaerobic digestion unit (1) according to claim 10, which comprises means for purifying (31) the extracted biogas. 12. methanation unit (1) according to one of claims 5 to 11, which comprises grinding means (3) of the organic material before injection into the digester (2). 13. A methanation unit (1) according to one of claims 5 to 12, which comprises a hopper (4) provided with a mixer (5), in which are inserted the organic material before injection into the digester (2) . 14. A methanation unit (1) according to one of claims 5 to 13, which comprises means for separating the liquids and dry matter contained in the digestate which is discharged at the outlet of the digester (2) or the post-digester ( 20).