FR2929268A1 - DIGESTER WITH MEANS FOR CONTAINING BIOGAS - Google Patents

Abstract

The invention relates to a digester (30) for carrying out an organic waste methanation treatment (36) for the production of a biogas and a digestate, said digester comprising a tank part ( 31) for containing said organic waste (36), said bowl portion housing a removable stirrer (32), and a roof portion (33) closing said bowl portion (31), said roof portion (33) having a first portion (331) traversed by an opening (34) for passage of said agitator (32). According to the invention, said agitator comprises means for confining (332, 61) said biogas to maintain said biogas inside said digester ( 30) during extraction and after extraction of said removable stirrer (32).

Description

Digester with means of confinement of a biogas. FIELD OF THE INVENTION The field of the invention is that of the treatment of organic waste, in particular of those produced during the treatment of water.

More specifically, the invention relates to digesters which are used for the implementation of methanization treatments of organic waste. 2. Prior Art Wastewater contains a certain proportion of soluble and particulate organic pollution. The particulate part can be partly removed by simple decantation, the mixture of particles and water obtained is called primary sludge. Soluble organic pollution can be treated, at least in large part, by the implementation of biological treatment processes. The biological treatment of water consists in putting the water to be treated in contact with microorganisms that consume, for their growth, the organic pollution dissolved in these waters. The biological treatment of water is accompanied by the formation of sludge, called biological sludge or secondary sludge which constitutes organic waste. The mixture of primary sludge and secondary sludge is called mixed sludge. In order to treat these sludges in order to break them down and make them rot and harmless, various techniques have been proposed. Anaerobic digestion is one of them. The methanisation of organic waste is a natural process that consists of biologically decomposing organic waste by subjecting it to anaerobic fermentation. It leads to the combined production of gas convertible into energy (biogas) and digestate usable for example as compost. Methanization is commonly carried out inside the reactor called digesters.

As represented in FIGS. 1 and 2, these digesters may, for example, have a tank 10 of essentially cylindrical shape intended to contain organic waste 11 to be treated and housing a vertical stirrer 12 for stirring these wastes in order to improve the process of methanation. The tank of these digesters is closed by means of a roof which, according to the models, can be flat 13 or curved 20 and which has a plug 14 for the insertion and extraction of the stirrer 12. Means of Evacuation of a gas mixture formed during methanization extends above the maximum level of organic waste. An empty space 17 exists between this maximum level 16 and the roof 13, 20, inside which the gas mixture is concentrated before being discharged from the digester. 3. Disadvantages of the Prior Art In the course of the use of these digesters, it proves regularly necessary to carry out maintenance campaigns in order to maintain their level of performance. These campaigns may include the intervention of the agitator, which requires the extract of the tank. The extraction of the stirrer requires the opening of the stopper 14.

Thus, the opening of the plug 14 causes the concentrated gas mixture in the empty space 17 to escape outside the digester to be distributed in the ambient atmosphere. The gas mixture formed during the methanation process is essentially composed of a mixture of methane (up to 85% of the volume of biogas produced) and carbon dioxide. Methane and oxygen may constitute, when combined, an explosive mixture. Thus, bringing the gas mixture into contact with the atmosphere can produce such a mixture, which represents a potential danger. To prevent the formation of such an explosive gas mixture, the technicians working to maintain these digesters must enforce compliance with a rigorous intervention protocol that includes an essential step preceding the opening of the digester: the inerting of the digester. The inerting of the digester is to strongly lower the methane content of the gaseous sky formed in the upper part of the digester. For this purpose, the digester is emptied of the organic waste it contains so as to stop the formation of biogas (emptying step). Nitrogen is then blown into the digester to expel the methane it contains. The digester can then be opened to extract the agitator. This technique of the prior art is effective because it prevents the formation of an explosive gas mixture at the opening of the digester. It thus contributes to making safer the use of this type of digester for the implementation of an anaerobic digestion process. The implementation of this type of digester however has a number of disadvantages.

These disadvantages are mainly due to the maintenance protocol. Indeed, this maintenance protocol, to which the maintenance technicians can not derogate for obvious security reasons, is particularly long and leads to the observation of periods unproductive in terms of treatment. To this unavailability of the digestion plant is added the often important cost of the inerting operation (injection of a so-called neutral gas in large quantities). In other words, this maintenance protocol generates relatively high operating costs. In addition, it can happen that, negligently or by precipitation, maintenance technicians do not respect or poorly this maintenance protocol resulting in the occurrence of accidents. The use of this type of digester can thus remain dangerous. 4. OBJECTIVES OF THE INVENTION The object of the invention is notably to overcome these disadvantages of the prior art.

More specifically, an object of the invention is to provide a digester for the implementation of an organic waste methanization treatment that is simple maintenance. Another object of the invention is to implement such a digester which is safe, especially with regard to its maintenance. The invention also aims to provide such a digester which is reliable. The invention also aims to provide such a digester which is simple in design and whose production is cheap. 5. Objective of the invention These objectives, as well as others which will appear subsequently, are achieved by means of a digester intended for the implementation of a methanisation treatment of organic waste in order to producing a biogas and a digestate, said digester comprising a tank portion for containing said organic waste, said bowl portion housing a removable agitator, and a roof portion enclosing said tank portion, said portion forming roof having a first portion through which a passage opening of said agitator. According to the invention, said digester comprises means for confining said biogas to maintain said biogas inside said digester during the extraction and after extraction of said removable stirrer. Thus, the invention is based on a completely innovative approach which consists in providing an interior confinement space formed inside the digester so that the gases produced during methanation are concentrated and remain there. confined when the digester is open in particular to extract the agitator. This characteristic thus makes it possible to avoid compliance with the maintenance protocol that the techniques of the prior art impose on the technicians who are responsible for it.

In other words, the implementation of the invention leads to allow the extraction of the stirrer without the need to drain the digester of the waste it contains, or to perform its inerting, it that is to say to substitute biogas for a so-called neutral gas (for example nitrogen).

The invention thus contributes to considerably facilitate the realization of maintenance campaigns of the digester. As a result, it reduces the time required for these campaigns and therefore the costs associated with them. It also leads to the supply of digesters which are safe insofar as their maintenance is not subject to the implementation of certain rules whose non-compliance could lead to the occurrence of accidents (explosions) likely to have serious consequences. as to the safety of people, at least potentially. According to an advantageous aspect of the invention, said confinement means define an interior space for confining said biogas between the surface defined by the upper level reached by said organic waste inside said digester and said roof portion. The waste contained in the digester tank is thus used as a means for sealing the containment volume which makes it possible to prevent the gas trapped therein from escaping through the opening formed in the roof of the digester for the extraction of the agitator. According to one embodiment of the invention, said confinement means comprise a substantially tubular element which extends from said opening towards the bottom of said digester and which has a lower end in contact with said organic waste. This implementation makes it possible to simply but effectively produce an interior hermetic confinement space for trapping the gases produced during the treatment of the waste. According to another embodiment of the invention, said confinement means comprise an elevated portion formed in said roof portion, and defining an interior confinement space with the surface defined by the upper level reached by said organic waste, this level extending in the extension of said first portion. This implementation constitutes a second simple solution for creating an interior hermetic confinement space for the biogas. A digester according to the first embodiment of the invention preferably comprises a weir whose upper part extends at least in the extension of said lower part of said substantially tubular element and whose lower part extends outside said digester. The implementation of such a weir makes it possible to maintain the level of sludge contained in the digester at the level of the first portion. This leads to prevent on the one hand that the inner space of containment fills with sludge, and on the other hand that the level of sludge in the digester extends below the first portion. This would result in the formation of a gas pocket 15 above the sludge that would be likely to escape outside the digester upon opening. A digester according to the second embodiment of the invention advantageously comprises a spillway whose upper part extends at least in the extension of said first portion and the lower portion 20 extends outside said digester. The implementation of such a weir makes it possible to maintain the level of waste in the digester tank so that the lower part of the tubular element is immersed in the waste, or at least in contact with it, and the interior containment space is watertight. It is understood that if the lower part of the tubular element was not in contact with the upper surface defined by the waste, the gas contained in this inner confinement space could escape through the tubular element during extraction of the digester. According to an advantageous characteristic of the invention, said raised portion extends over a fraction of the cross section of said bowl portion.

This fraction can be chosen so as to define the volume of the internal confinement space according to the needs, in particular as a function of the quantity of sludge that can be treated inside the digester. This volume can be obtained taking into account the space available.

Thus, the same volume can be defined by varying on the one hand the surface of the cross-section fraction and on the other hand the height of the raised portion. According to an advantageous aspect, a digester according to the invention comprises means for protecting said digester.

In this case, said protection means preferably comprise a valve capable of connecting said interior confinement space outside said digester. The implementation of such protection means prevents that the pressure prevailing inside the inner confinement space exceeds a value beyond which the digester could be deformed to produce an explosion. According to a preferred feature, a digester according to the invention comprises means for extracting said biogas contained in said interior confinement space.

The biogas can thus be transported outside the digester to a retention zone in which it will be stored before being at least partially reused as a source of energy. In this case, said means for extracting said biogas preferably comprise an antifoam pot so as to avoid the formation of foam in the circulation circuit of the biogas. According to an advantageous characteristic of the invention, the agitator comprises at least one lower blade and an upper blade, said upper blade extending close to said opening. The lower blade optimizes the mixing of fresh sludge newly introduced into the lower region of the digester with the sludge already present. The upper blade, positioned in such a position, prevents the formation of a relatively compact agglomerate of organic waste, also called cake, at the opening, cake whose presence could affect the extraction of the agitator . Said tank portion advantageously has a bottom provided with a purge, the implementation of which can easily drain the digester when necessary. 6. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and nonlimiting example, and the accompanying drawings, among others. which: - Figure 1 shows a diagram of a digester, which according to the prior art, implements a flat roof; - Figure 2 illustrates a diagram of another digester, which according to the prior art, implements a domed roof; - Figure 3 shows a sectional view of the assembly of a tank and a roof of a digester according to a first embodiment of the invention; FIG. 4 illustrates another sectional view of an assembly of a digester according to the first embodiment of the invention; - Figure 5a is a top view of the tank and roof shown in Figure 3; - Figure 5b shows a top view of a variant of the tank and roof 25 shown in Figure 3; FIG. 6 illustrates a second embodiment of the invention in which the confinement means comprise a substantially tubular element. 7. Description of an embodiment of the invention 7.1. Recall of the Principle of the Invention The general principle of the invention is based on the use, within a digester, of means for confining a biogas produced during the methanization treatment of organic waste in order to maintain said biogas inside said digester during the extraction and after extraction of the removable stirrer which it is provided. The implementation of such confinement means allows maintenance technicians to overcome compliance with the maintenance protocol imposed by the techniques of the prior art. The invention thus contributes to providing digesters whose maintenance is safe, less burdensome and less expensive, compared to the techniques of the prior art. 7.2. Examples of digesters according to the invention 7.2.1 Raised portion roof A. Architecture 15 In relation to FIGS. 3 to 5b, a first embodiment of a digester according to the invention is presented. As shown in these figures, such a digester 30 includes a tank portion 31. This tank portion 31 is substantially cylindrical in shape. It has a main axis A which is intended to extend substantially vertically when the digester 30 is in operation. This tank portion 31 is closed in a lower part by a bottom 40, and in an upper part by a roof portion 33. It is intended to contain organic waste, for example sludge 25 from the water treatment used for the purposes of anaerobic digestion. For this purpose, the digester 30 comprises sludge introduction means inside the tank portion 31. These introduction means are here represented in the form of a pipe 43 which extends along the side wall of the tank portion 31 and bottom 40.

The pipe 43 has a bottom portion 431 which opens at the bottom of the tank portion 31 essentially near its center. The lower part 431 thus positioned makes it possible to optimize the mixing of the newly introduced fresh sludge with the digested sludge, at least partially, present in the digester 30. The bottom 40 essentially has the shape of a cone whose apex points below The digester 30. This bottom has a purge 41 which can make it possible to empty the digester 30 organic waste 36 it contains, or digestate produced during the implementation of their methanization. This purge 41 may for example comprise a drain plug which has a threaded portion intended to be screwed into a complementary threaded portion formed in the bottom 40. The roof portion 33 has a first portion 331. This first portion is essentially flat and is traversed by an opening 34 allowing the passage of an agitator 32 for insertion or extraction of the digester 30. The middle of this opening 34 substantially coincides with the main axis A of the tank portion 31. The 20 roof part 33 further has a second portion 332. This second portion 332 extends in elevation relative to the first portion 331 so that it defines an interior confinement space 35. As it appears in FIG. 5a, the elevated portion 332, which for the sake of clarity is here delimited by thick lines, extends over a fraction of the cross-section of the tank portion 31. In this case, it represents a cylinder section. More precisely, this fraction represents, in this embodiment, a little less than half of the cross-sectional area of the tank portion 31. However, it can be provided in other embodiments that this fraction is smaller. or larger depending on the desired volume for the inner containment space 35. It can also be expected that this raised portion has another shape. Thus, in the variant illustrated in Figure 5b, the raised portion 332 has a substantially rectangular section. The volume defined by the inner confinement space 35 preferably represents between 1 and 10% of the volume of the tank 31 of the digester. The digester 30 comprises means for maintaining the required level N of the organic waste 36 at the height of the first portion 331 of the roof portion 33. These holding means comprise a weir 37. In a variant, it may be provided that the N level extends slightly above the first portion 331. The tank portion 31 is traversed by a hole allowing the passage of this weir 37. This weir 37 comprises a pipe element whose upper portion 371 extends essentially in the extension of the first portion 331 of the roof portion 33. This channel element further comprises a lower portion 372 which extends outside the digester 30. The implementation of this weir 37 allows to maintain the level N of waste 36 contained in the digester at the first portion 331 of the roof portion 33. Thus, the interior space confinement, which is delimited by the surface sup waste, does not communicate with the opening 34 formed in the roof portion 33. In addition, the implementation of this weir 37 prevents the interior confinement space 35 delimited by the second portion 332 is filled 36. When the quantity of organic waste 36 is too large, the amount likely to fill, at least in part, the internal confinement space 35 flows into the channeling element of the spillway 37. This portion is then stored in a retention zone (not shown). The digester 30 also comprises means for protecting the digester.

These protection means comprise a valve 38 which is capable of connecting the internal confinement space 35 with the outside of the digester when the pressure therein is too great. The digester 30 further comprises means for extracting the gas contained in the inner containment space 35. These extraction means comprise pipes which are connected to means for retaining the extracted gas (not shown). These means further comprise an antifoam pot 39, the implementation of which prevents the formation of foam in the biogas circuit. The biogas is extracted from the digester under the effect of its own pressure.

The bowl portion 31 houses a removable stirrer 32. This stirrer 32 extends substantially along the main axis A of the bowl portion 31. It has a lower blade 321 which extends into a lower region of the portion forming a tank 31, and an upper blade 322 which extends near the opening 34 formed in the first portion 331 of the roof portion 33.

The opening 34 can be closed by means of a plug member 42. This plug member 42 is traversed by a bore 421 for rotating the stirrer 32. B. Operation In operation, the portion forming The tank 31 of the digester 30 is filled to the level N with organic waste. This level N is maintained by the implementation of the weir 37. The agitator 32 is rotated. The process of methanization of the organic waste 36 contained in the digester 30 is then initiated. During this methanization, a succession of reactions occurs within the digester 30 so as to ferment the organic waste 36. The methanization of the organic waste 36 leads to the formation of a mixture of gas and a digestate. The produced gas mixture rises to the surface and is concentrated in the inner confinement space 35. The digestate concentrates on the bottom of the tank portion 31.

The gas mixture which is concentrated in the inner confinement space 35 is continuously discharged by the extraction means, and is thus conveyed into a retention zone. When the anaerobic digestion process is complete, the digestate produced can be removed from the digester by the purge 41 and collected in a retention zone. If, during the anaerobic digestion process, the pressure within the interior containment space exceeds a certain threshold beyond which it could be considered that a risk is incurred, a part of the mixture of gases that it encloses evacuates outside the digester 30 by means of the valve 38.

The upper blade 322 of the agitator 32 leads to avoid the formation of a cake, that is to say a relatively compact agglomerate of organic waste at the opening 34. The formation of such a cake could make it more difficult to extract the stirrer 32. C. Maintenance During the uses of this digester 30, it may be necessary to carry out maintenance campaigns in order to maintain its level of performance. These campaigns may include the intervention of the agitator, which requires the extract of the tank. In order to extract the agitator 32 from the digester 30, the operation of the agitator 32 must first be stopped. The plug 42 closing the digester 30 must then be deposited so as to release the opening 34. Because the level N of the waste 36 is maintained at the height of the first portion 331 of the roof portion 33, the gas contained in the interior confinement space 35 does not leak through the opening 34 as a result of its release.

The gas that is likely to be formed in the digester while the opening 34 is disengaged, furthermore continues to concentrate in the inner confinement space 35. Only a negligible, and therefore safe, part of the gas mixture produced by the organic waste 36 located directly below the opening 34 escapes from the digester 30 from this opening.

After the opening 34 is released, the stirrer 32 can be easily extracted from the digester 30. 7.2.2 Tubular element A. Architecture A second embodiment of a digester according to the invention is presented with reference to FIG. on which the numerical references similar to those of the preceding figures designate identical elements. Only the essential differences between this second embodiment and the first embodiment which has just been described will be highlighted here.

In this embodiment, the roof portion 33 closing the digester 30 is substantially planar and does not have a raised portion as was the case in the previous embodiment. The containment means here has a substantially tubular element 61. The main axis of this substantially tubular element 61 coincides with the axis A of the tank portion 31 of the digester. This tubular element 61 extends from the roof portion 33 towards the bottom 40 of the digester 30 over a portion of the length of the tank 31. The upper part of the tubular element is hermetically bonded to the roof portion 33. These elements may for example be soldered to each other for this purpose. This digester comprises a weir 62 whose upper part 621 extends at least in the extension of the lower part 611 of the tubular element 61, and whose lower part extends outside the digester. The implementation of the weir 62 makes it possible to maintain the level reached by the organic waste 36 inside the tank 31 at a level N 'chosen so that the lower end 611 of the tubular element 61 is immersed in the waste or at least in contact with the upper surface of the waste. The upper surface of the waste 36, the outer contour of the tubular element 61, the inner part of the roof portion 33 and the peripheral contour of the vessel 31 delimit an interior confinement space 63 of substantially annular shape. In the same way as in the previous embodiment, the digester according to this second embodiment comprises digester protection means, means for extracting the gas contained in the internal confinement space 63 and a removable stirrer, which are not shown in this figure 6. B. Operation and maintenance The operation of the digester according to this second embodiment is similar to the operation of the digester according to the first embodiment. As the treatment of the organic waste contained in the digester progresses, the product gas is released into the interior confinement space 63. During the maintenance operation, an operator carries out the extraction. of the agitator, the gas contained in the inner confinement volume does not escape outside the inner confinement space 63 through the opening 34 through which the agitator is extracted. Indeed, because the level N 'of waste is maintained in the tank 31 so that the lower end 611 of the tubular element 61 bathes, and the junction between the upper part of the tubular element 61 and the roof portion 33 is sealed, the gas contained in the inner confinement space 63 can not escape from the digester in a manner other than by the implementation of the extraction means provided for this purpose. 7.3. Advantages The implementation of a digester according to the invention has a certain number of advantages. In particular, a digester according to the invention is particularly simple maintenance compared to the techniques of the prior art. Indeed, the extraction of the agitator requires neither the emptying of the digester, nor its inerting.

The maintenance of a digester according to the invention is simpler, requires less time and is therefore less expensive than that of the techniques of the prior art. In addition, a digester according to the invention is particularly safe. Its maintenance does not actually require the respect of a protocol whose non observance of the rules, for example by negligence or by precipitation, could lead to the occurrence of accident, as it is the case during the implementation of the techniques according to the prior art.

Claims (10)

REVENDICATIONS1. A digester (30) for carrying out an organic waste methanization process (36) for the production of a biogas and a digestate, said digester comprising a tank portion (31) for containing said organic waste (36), said bowl portion housing a removable stirrer (32), and a roof portion (33) closing said bowl portion (31), said roof portion (33) having a first portion (331) traversed by an opening (34) for passage of said agitator (32), characterized in that said agitator comprises means for confining (332, 10 61) said biogas to maintain said biogas inside said digester (30) during the extracting and extracting said removable stirrer (32). 2. Digester according to claim 1, characterized in that said confinement means (332, 61) define an interior confinement space (35, 63) of said biogas between the surface defined by the upper level (N, N ') reached by Said organic waste (36) within said digester (30) and said roof portion (33). 3. Digester according to any one of claims 1 and 2, characterized in that said containment means comprise a substantially tubular element (61) extending from said opening (34) towards the bottom (40) 20 of said digester (30) and having a lower end (611) in contact with said organic waste (36). 4. Digester according to claim 1, wherein said confinement means comprise a raised portion formed in said roof portion and defining an interior confinement space. with the surface defined by the upper level (N) reached by said organic waste (36), said level (N) extending at least in the extension of said first portion (331). 5. Digester according to claim 3, characterized in that it comprises a weir (62) whose upper part (621) extends at least in the extension of said lower part (611) of said essentially tubular element (61) and the lower part extends outside said digester (30). 6. Digester according to claim 4, characterized in that it comprises a weir (37) whose upper part (371) extends at least in the extension of said first portion (331) and the lower portion extends in outside said digester (30). 7. Digester according to any one of claims 4 and 6, characterized in that said raised portion extends over a fraction of the cross section of said bowl portion (31). 8. Digester according to any one of claims 1 to 7, characterized in that it comprises means for protecting said digester (30). 9. Digester according to claim 8, characterized in that said protection means comprise a valve (38) capable of connecting said inner confinement space (35) outside said digester (30). 10. Digester according to any one of claims 1 to 9, characterized in that it comprises means for extracting said biogas contained in said inner confinement space (35), said extraction means of said biogas comprising a pot antifoam (39).