FR2629470A1 - Process for recovering the fuel gases produced by liquid-phase anaerobic fermentation of organic materials and fermenters for implementing this process - Google Patents

Abstract

The subject of the invention is a process for recovering the fuel gases produced by liquid-phase anaerobic fermentation of organic materials and fermenters for implementing this process. A fermenter 1 according to the invention comprises a cylindrical tank 2 made from prestressed reinforced concrete, which is open at its upper part. This tank is covered with a bell-shaped flexible pocket 9 which is open at its lower part. The lower edge 10a of the skirt 10 of the pocket is pasted in a leakproof manner onto the inner face of the tank, below the minimum level 6a of the liquid. The roof 11 of the pocket is fixed onto suspenders 16 attached to a supporting structure 17. One application is the production of methane-rich gas by anaerobic fermentation of ground domestic refuse, agricultural or industrial waste, buckets of plants for sewage treatment.

Description

Method for collecting gases
fuels produced by fermentation
anaerobic liquid phase of organic matter
and fermenters for the implementation of this process.

DESCRIPTION
The subject of the present invention is a process for collecting the combustible gases produced by anaerobic fermentation, in the liquid phase, of organic materials, in particular of ground household waste and of fermenters for the implementation of this process.

 The technical sector of the invention is that of the construction of high capacity anaerobic fermenters.

 Methods are known for producing a methane-rich combustible gas in a fermenter or digester, in which organic materials are fermented in the liquid phase, protected from air.

 In particular, large capacity fermenters, of the order of 3000 m3, were built to treat urban waste. These are first sorted to extract the glass and iron, then crushed, moistened, kneaded, reheated to 350 and injected continuously into a digester where anaerobic fermentation takes place.

 The fermenters known to date include a vertical tank into which the organic matter is injected from below.

 Advantageously, the tank has a vertical partition which extends over only part of a diameter of the tank.

 The organic matter is injected on one side of the partition and it progresses slowly to an outlet located on the other side of the partition, staying for about fifteen days in the tank. The temperature of the liquid is maintained at about 350.

 The tank is located above a technical vacuum and the floor of the tank has gas injectors which allow part of the gas which is recycled to be injected into the liquid. The recycled gas is injected under a pressure of the order of eight bars. It is used to stir the liquid phase. The gas injection is discontinuous and is located by a rotating sector.

 The fermentation of organic matter produces a so-called biogas combustible gas with a methane content of around 60%.

 One of the problems posed by this type of fermenter is that of the gas tightness of the tank.

 Another problem is that of resistance to corrosion by the combustible gas which contains 2 to 3% of hydrogen sulfide (SH2).

 Another problem is that of the fatigue of the materials under the effect of pressure oscillations due to the bursting of the gas bubbles injected at the lower part of the liquid phase.

When they burst on the surface, these bubbles cause pressure oscillations at high frequency, which vibrate the bottom of the walls and the cover of the tank and which cause microcracks of these through which the corrosive gas passes;
Boiler digesters have been built from welded metal sheets, but experience has shown that vibrations due to pressure oscillations tire welds which lose their tightness. In addition, the metal containers are corroded by the combustible gas.

 Metal containers therefore require a lot of maintenance and are reserved for small capacity fermenters.

 Reinforced concrete fermenters have also been constructed comprising a cylindrical skirt of prestressed concrete having a concrete floor and cover.

 Prestressed concrete has good liquid tightness.

However, the pressure oscillations tire the concrete and cause micro-cracks to appear in it through which the combustible gas can circulate by corroding the reinforcements of the concrete.

 To prevent fuel gas from leaking through the microcracks, prestressed concrete tanks were constructed with a sealing coating bonded to the internal walls of the tank. This solution does not prevent fatigue of the concrete due to high frequency pressure oscillations caused by the bursting of gas bubbles on the surface of the liquid.

 An objective of the present invention is to provide large capacity fermenters, in particular prestressed concrete fermenters, for producing a combustible gas by anaerobic fermentation, in the liquid phase, of organic materials, which comprise, above the liquid phase, sealed means for collecting the combustible gas, which resist corrosion well by the gas and which, moreover, quickly dampen the pressure oscillations due to the bursting of the gas bubbles and avoid the fatigue of the containers containing the liquid phase.

 Another object of the present invention is to provide means for collecting and containing the combustible gas above the liquid phase, which means have a variable capacity and serve as a buffer between the fermenter and the gas distribution network.

 Another objective of the invention is to provide means for collecting the combustible gas above the liquid phase, which make it possible to lighten the reinforced concrete container and, in particular, to remove a large part of the skirt located at the -above the maximum level of the liquid phase and to replace the concrete cover of the tank with a light metal support structure, carrying a light roof and an insulating under-roof, resulting in a significant gain in concrete volume and weight of the structure which affects the foundations all the more if one is building in a seismic zone.

 The present invention relates to a process for collecting the combustible gases produced by a fermenter comprising, in a known manner, a container, for example a tank of prestressed reinforced concrete, in which anaerobic fermentation takes place, in the liquid phase, of organic matter. , for example crushed urban waste.

 The objectives of the invention are achieved by a method according to which the container is capped with a deformable pocket, in the shape of a bell, open at its lower part, the lower edge of which is sealed in sealed manner on the internal faces of the walls. vertical of said container at a level below the minimum level of the liquid phase and the roof of said pocket is suspended from straps which are hooked to a support structure which surmounts said container and which supports a mechanical protection cover and a layer of 'thermal insulation.

 A fermenter according to the invention comprises, at its upper part, a deformable pocket in the shape of a bell open at its lower part, the lower edge of which is sealingly bonded to the internal wall of said container at a level situated below. the minimum level of said liquid phase, so that said combustible gas is released inside said pocket.

 Advantageously, the roof of the deformable pocket is suspended from straps which are hung on a carrying structure which overcomes said container.

 According to a preferred embodiment, the deformable pocket comprises, above its lower edge, accordion folds which constitute a deformable bellows and which are connected together by vertical strips of elastic material.

 The invention results in new anaerobic fermenters, in the liquid phase, of organic materials intended to produce a combustible gas rich in methane.

 An advantage of the fermenters according to the invention, provided at their upper part with a deformable pocket which collects the combustible gas, resides in the fact that the elastic deformations of the pocket create oscillating variations in the volume which very quickly absorb the oscillations' of pressure due to bursting of recycle gas bubbles on the surface of the liquid. This results in a significant reduction in the fatigue of the materials which constitute the structure, in particular in the prestressed concrete tank and a reduction in micro-cracks in the concrete.

 In addition, the pocket which is made up of a thin deformable membrane is very light, resulting in a significant reduction in the weight of 1 item.

 In the frequent case of a fermenter comprising a vertical cylindrical tank, of prestressed concrete, of large capacity, which contains the liquid phase, it becomes possible to reduce the height of this tank by adopting a total height such that the upper edge of the tank is located about a meter above the maximum level of liquid and cover the concrete tank with a deformable cover which replaces the top of the tank and which also removes the concrete cover of the tank. It is then sufficient to suspend the pocket from a light metal frame and to place on it a roof in self-supporting trays provided with an insulating under-roof.

 The following description refers to the appended drawings which represent, without any limiting character, exemplary embodiments of the invention.

 Figure 1 is an axial section of a fermenter according to the invention.

 Figure 2 is a horizontal section along II-II of Figure 1.

 Figure 3 is a vertical section on a large scale of the junction between the bottom of the skirt and a concrete wall.

 Figure 4 is an axial section of a flexible gas discharge pipe.

 Figures 5 and 6 are axial vertical sections of two other embodiments of fermenters according to the invention.

 FIG. 7 is a horizontal section of a fermenter in the form of a buried channel.

 FIG. 8 is a vertical section of a fermenter according to FIG. 7.

 Figure 1 is an axial vertical section of a first embodiment of a fermenter according to the invention.

 This fermenter or digester comprises a vertical tank 1 comprising a cylinder of prestressed reinforced concrete 2 and a concrete floor 3, placed above a technical void, in which there are pipes 4, which supply injectors 4a making it possible to inject a portion of the recycled combustible gas through the floor 3.

 The tank has a vertical partition 5 which is situated in a diametral plane and which extends over approximately two thirds of the diameter from the vertical wall 2, so that it divides the tank 1 into two compartments, which communicate between them by a vertical opening delimited by the free end of the partition 5 and by the vertical wall 2.

The tank 1 contains an organic material in the liquid phase, for example urban waste or ground household waste mixed with water. The level of the liquid in the tank varies between a minimum level 6a and a maximum level 6b
The liquid phase arrives at the bottom of the tank through an inlet pipe 7, located on one side of the partition 5 and leaves it through an outlet pipe 8, located on the other side of the partition 5, so that the liquid progresses slowly from the inlet to the outlet and that the organic materials remain about fifteen days in the tank, in which the liquid is maintained at a temperature of the order of 300C. The residence time varies with the temperature. The liquid contained in the tank undergoes an anaerobic fermentation which gives rise to combustible gases containing about 60% of methane which are collected at the top of the tank and sent to a distribution network, after have been purified and compressed.

 Part of the gas produced is recycled and inject at the base of the liquid by injectors located in the floor 3. This injection is carried out under a pressure of the order of eight bars, in successive bursts and in an angular sector which moves by turning around the axis of the tank.

 This gas injection is used to stir and homogenize the liquid phase which tends to stratify in successive layers of different density.

 As a result of the viscosity of the liquid phase, the recycled gas forms bubbles 4b which burst on the surface of the liquid and which cause pressure oscillations in the volume located above the surface of the liquid. These pressure oscillations cause the tank to vibrate and cause the reinforced concrete to fatigue, which risks cracking, hence the risk of fuel gas leaking through the cracks with the risk of explosion.

 In addition, the combustible gas contains 2 to 3% of hydrogen sulphide and it is corrosive, which risks rapidly leading to an enlargement of the cracks and the rate of leakage of combustible gas.

 A fermenter according to the invention comprises, in its upper part, a deformable pocket 9, having the general shape of a bell open at its lower part. This pocket 9 comprises a skirt of generally cylindrical shape 10 having a diameter substantially equal to the inside diameter of the concrete skirt 2 and a roof 11 which closes the upper end of the skirt. The skirt further comprises, over its entire height, two vertical walls 12 which surround the central partition 5 and which meet beyond the free end.

 The upper edge 2a of the concrete skirt 2 is located approximately one meter above the maximum level of liquid 6b.

 The deformable skirt 10 plunges into the liquid phase. The lower edge 10a of the skirt 10 is one to two meters below the minimum level of liquid 6a. It is glued against the internal face of the concrete tank 2 over a height of approximately 50 cm.

 The bonding associated with the pressure of the liquid which presses the deformable skirt against the concrete tank 2 ensures the liquid tightness of the junction between the deformable pocket and the reinforced concrete tank.

 The deformable skirt 10 is independent of the reinforced concrete tank over its entire height, with the exception of the strip 10a which is glued against the reinforced concrete tank. Thus, the flexible pocket can deform freely.

 To facilitate deformation, the deformable skirt 10 advantageously comprises, immediately above the strip 10a which is glued to the internal wall of the tank, accordion folds 13, which form a deformable bellows. These folds are connected together by elastic vertical bands or straps 13a which stretch when the bellows lengthens and which resiliently recall the bellows to its initial position. Thus, each time a bubble of recycling gas 4b bursts on the surface of the liquid, it creates inside the deformable pocket 9 pressure waves.

 The increase in pressure results in the elongation of the bellows which is resiliently returned to its original position, hence an oscillation of the bellows 13 which very quickly absorbs pressure oscillations.

 Figure 3 is a partial vertical section, on a larger scale, which represents the lower end of the deformable skirt 10, which is glued against the inner wall of the concrete tank 2 over a height 10a.

 The lower edge of the strip 10a is pinched between the tank 2 and a plastic plinth 14 which is fixed to the concrete tank by screws or dowels 15.

 This figure shows the pleated zone 13 forming a deformable bellows and one of the elastic bands 13a which is glued against the internal wall of the deformable membrane 10. The bands or straps 13a are for example bands of elastic fabric, the l elastic stretch can reach 30% of the length. The strips 13a have a width of a few centimeters and the spacing between strips is for example of the order of 50 cm.

 FIG. 1 represents a preferred embodiment in which the deformable pocket 9 extends above the upper edge 2a of the reinforced concrete skirt.

 In this embodiment, the roof 11 of the deformable pocket is suspended from straps 16, uniformly distributed over the entire surface of the roof.

 The straps 16 are attached to a metal frame 17 which serves as a supporting framework.

 The frame 17 is supported by metal posts 18 which bear on the upper edge 2a of the concrete tank 2.

 The concrete tank has a thermal insulating coating 19 and the mechanical protection cladding thereof. The insulating coating is for example a stretched felt.

 The frame 17 supports a roof 20 in self-supporting metal trays. The roof 20 is provided with an insulating sub-roof, for example a glass wool sub-roof.

 The function of the roof 20 is to mechanically protect the deformable pocket from the weather and to support the insulating sub-roof which prevents heat losses by the deformable pocket.

 Alternatively, one can use a deformable pocket carrying an insulating coating or a double wall pocket.

 Figure 1 shows, in addition, flexible pipes 21, in the form of bellows, which connect the top of the bell 9 to gas distribution pipes.

 Figure 4 is an axial section of a flexible pipe 21. It is composed of a flexible sleeve 22 folded in accordion which is advantageously made of the same material as the pocket 9 and vertical elastic bands or straps 22a which are glued outside the sleeve.

 The lower end of the sleeve 22 and the lower ends of the strips 22a are pinched, at the same time as the roof 11 between two concentric rigid sleeves 23 and 24 extended respectively by two flanges 23a and 24a. The upper ends of the flexible sleeve 22 and the elastic bands 22a are pinched between a rigid sleeve 25 and a collar 25a. The flexible pipes 21 follow the oscillations of the roof 11 of the deformable pocket 9.

 Figure 5 is an axial section of an alternative embodiment of a fermenter according to the invention. The homologous parts are represented by the same references in FIGS. 1 and 5.

 The difference between this embodiment # and that of FIG. 1 lies in the fact that the cylindrical tank 2 extends higher and has a concrete roof 26 carrying a thermal insulating coating 26a. The deformable pocket 9 is always composed of a cylindrical skirt 10, a roof 11 and two walls 12 which surround the vertical partition 5. It is contained entirely inside the concrete tank. The roof li is suspended from the concrete roof 26 by lines 16.

 Figure 6 is an axial section of another embodiment of a fermenter according to the invention. The homologous parts are represented by the same references in FIGS. 1 and 6.

 In this embodiment, the upper edge 2a of the prestressed concrete cylindrical tank 2 is situated slightly above the maximum level 2a of the liquid. The deformable pocket 9 always comprises a skirt 10, the edge 10a of which is glued against the internal wall of the tank 2 and is surmounted by a pleated zone in the form of a bellows 13. It also comprises a flexible roof 11, which is fixed to hangers 16 attached to a metal frame 17, carried by metal posts 18 placed on the edge 2a of the tank.

 The roof 11 and the parts of the skirt 10 situated above the upper edge 2a are covered with a thermal insulating coating 27. This coating comprises along its lower edge, a flange 28, which is directed towards the outside and which rejects rainwater beyond the thermal insulating coating 19 placed against the external wall of the concrete tank 2.

 Figures 7 and 8 show respectively a partial horizontal section and a vertical section of another embodiment of a fermenter according to the invention.

 In this embodiment, the container in which the anaerobic fermentation takes place has the shape of a large gutter or channel of buried and open concrete, composed of two vertical side walls 29a, 29b, parallel to each other and of a bottom in V-shape composed of two walls 30a, 30b which converge downwards.

Channels 31 plunge into the liquid contained in the channel and make it possible to recycle in the bottom of the channel a plateau of the gas produced by the fermentation, which gas rises through the liquid in the form of bubbles 31a.

 Figure 7 shows a portion of a straight channel. It is specified that this channel can also have a hairpin shape made up of several rectilinear sections connected to one another by curved sections.

 The upper opening of the channel is capped, over its entire length, by a deformable pocket 32 which comprises, on the one hand, vertical walls 33 which plunge into the liquid and, on the other hand, a roof 34 which is hooked to lines 35 which are fixed to a frame 36 which is part of a metal or wooden frame 37 which carries a light roof 38.

 The frame 36, 37 is placed on posts 39, which rest on the upper edges of the concrete channel. A thermal insulating coating 40 is placed on the structure 37 and against the vertical walls situated above the ground.

 Flexible pipes 41 connect the top of the pocket 32 to external gas distribution pipes.

 As in the previous embodiments, the lower edges 33a of the walls 33 are glued against the internal faces of the side walls 29a, 29b as well as against the internal faces of the vertical walls 42 closing off the two ends of the channel and the walls 33 comprise, at -above their lower edge, a pleated bellows area 43, the folds of which are connected together by elastic straps 43a.

 The marks 6a and 6b represent the minimum and maximum levels of the liquid in the fermenter.

 The pockets 9 or 32 are composed of an impermeable fabric which has a woven frame, for example a frame of glass fiber or polyester fabric which is coated on both sides with a heat-sealable resin at low temperature, for example polyvinyl chloride or polyurethane.

 Polyvinyl chloride has the advantage of being able to be welded at a relatively low temperature, of the order of 500C, which does not break down the molecules, so that gas-tight welds are easily obtained while bonded junctions may not remain watertight due to the attack of the adhesive by the gas loaded with hydrogen sulfide.

Claims (10)

 1. Method for collecting the combustible gases produced in a fermenter of organic materials, in particular of crushed household waste, or of agricultural or industrial waste, which fermenter comprises a container (2) in which an anaerobic fermentation takes place in liquid phase, characterized by the following operations:  - The said container (2, 29a, 29b) is capped with a deformable waterproof pocket (9, 32), in the form of a bell open at its lower part, the lower edge of which is glued (10a, 33a), in a sealed manner; to the internal faces of the vertical walls of said container at a level below the minimum level (6a) of the liquid phase;  - And the roof (11, 34) of said pocket is suspended from straps (16, 35) which are hung on a carrying structure (17, 36) which overcomes said container and which supports a cover (20, 38) mechanical protection and a layer of thermal insulation.  2. Anaerobic fermenter in the liquid phase of organic matter, in particular ground household waste, with a view to producing a combustible gas rich in methane of the type comprising a container (2, 39a, 29b) containing said liquid phase maintained at a temperature of l 'order of 35 C, characterized in that said container comprises at its upper part a deformable pocket (9, 32), in the form of a bell open at its lower part, the lower edge (10a, 33a) of which is bonded, sealingly, at the internal wall of said container at a level situated below the minimum level (6a) of said liquid phase so that said combustible gas is released inside said pocket (9, 32).  3. Fermenter according to claim 2, characterized in that the roof (11, 34) of said deformable pocket is ## suspended from straps (16, 35) which are attached to a supporting structure (17, 36) which surmounted said container.  4. Fermenter according to claim 3, characterized in that said supporting structure is a metal frame which supports a roof (20) provided with a thermally insulating under-roof.  5. Fermenter according to any one of claims 2 to 4, characterized in that said deformable pocket comprises, above its lower edge (10a) a zone (13), pleated in accordion, which constitutes a deformable bellows, of which the folds are connected together by vertical strips (13a) of an elastic material.  6. Fermenter according to any one of claims 1 to 5, characterized in that said deformable pocket (9) is constituted by a membrane comprising a fabric reinforcement, for example of glass fiber or polyester fabric, coated on its two sides of a heat-sealable resin at a temperature below 80 C.  7. Fermenter according to claim 6, characterized in that said membrane is composed of a fabric frame coated on both sides with a layer of polyvinyl chloride.  8. Fermenter according to claim 2, characterized in that it comprises a cylindrical tank (2) of prestressed concrete, the upper edge (2a) of which is situated above the maximum level (6b) of the liquid and it comprises, in furthermore, a metal frame (17) carried by metal posts (18) which surmounts said tank and said deformable pocket (9) 'extends above said upper edge (2a) and its roof (11) is fixed to lines (16) which are attached to said metal frame.  9. Fermenter according to claim 2, characterized in that it comprises a cylindrical tank (2) of prestressed concrete closed by a roof (26) of concrete and said deformable pocket (9) is located inside de'ladite tank and its roof (11) is fixed to hangers (16) which are hooked to the roof (26) of the tank.  10. Fermenter according to claim 2, characterized in that it comprises a container in the form of a buried gutter delimited by parallel vertical walls (29a, 29b) and by a bottom constituted by two walls (30e, 30b) which converge towards the bottom, which channel is surmounted by a frame (36, 37, 39) carrying a roof (38) and an insulating layer (40) and further comprises, over its entire length, a deformable pocket (32) which covers said channel, which pocket has vertical walls (33) whose lower edge is glued against the vertical walls of said channel and a roof (34) which is fixed to hangers (35) attached to said frame (36).