WO2012074395A1

WO2012074395A1 - Biogas system

Info

Publication number: WO2012074395A1
Application number: PCT/NL2011/050818
Authority: WO
Inventors: Mirik Castro; Samuel Castro; Chandler Elizabeth Hatton
Original assignee: Simgas B.V.
Priority date: 2010-12-01
Filing date: 2011-11-30
Publication date: 2012-06-07
Also published as: ZA201303993B; CN103237881A; AP2013006892A0; AP4013A; NL2005793C2; CN103237881B

Abstract

The invention relates to a biogas system comprising a cylindrical digestion vessel having a bottom wall, a first circumferential wall and a top opening to define a digestion chamber to be filled with a digestive mixture, and a cover having a top side, a second circumferential wall and a bottom opening to define a gas storage chamber, wherein the cover is inserted in the digestion vessel via the top opening with its bottom opening directed towards the bottom wall to capture gas that is released inside the digestion chamber, wherein the digestion vessel comprises first elongated guides and the cover comprises second elongated guides, wherein the first elongated guides are at least partly inserted into the second elongated guides.

Description

Biogas system

BACKGROUND

The invention relates to a biogas system. Biogas systems comprise a digestion chamber to contain a digestive mixture of manure and vegetables that is partly resolved in water in order to produce methane gas . The methane gas can be used for household or small industrial purposes.

A known small size biogas system is made of a cylindrical plastic barrel having a top opening over its full width and a bucket shaped cover that is inserted into the barrel via the top opening. The cover floats on the digestive mixture with its opening facing downwards to capture a major part of the methane gas. The cover is connected to a gas hose to feed the methane gas to a household burner. The methane gas is pressurized by stones that rest on top of the floating cover.

A disadvantage of the known small size biogas system is that the floating cover is made top-heavy by the stones, whereby the cover topples over to some extend inside the barrel. The cover thereby releases some captured gas or jams whereby the gas pressure immediately drops. Toppling over of the cover is especially disadvantageous when the level of the digestive mixture inside the barrel raises high enough to push a part of the cover through the top opening. The jamming can also occur when the tolerances of the outer diameter of the cover not properly adapted to the tolerances of the inner diameter of the vessel.

It is an object of the present invention to provide a reliably operating biogas system.

It is an object of the invention to provide a biogas system that maintains the methane gas pressure substantially equal.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides a biogas system comprising a cylindrical digestion vessel having a bottom wall, a first circumferential wall and a top opening to define a digestion chamber to be filled with a digestive mixture, and a cover having a top side, a second circumferential wall and a bottom opening to define a gas storage chamber, wherein the cover is inserted in the digestion vessel via the top opening with its bottom opening directed towards the bottom wall to capture gas that is released inside the digestion chamber, wherein, preferably, the first circumferential wall comprises first cylindrical wall sections that merge into first elongated guides, which first elongated guides extend in the longitudinal direction of the first circumferential wall and project inwardly with respect to the first cylindrical wall sections, and wherein, preferably, the second circumferential wall comprises second cylindrical wall sections that merge into second elongated guides, which second elongated guides extend in the longitudinal direction of the first circumferential wall and are recessed inwardly with respect to the second cylindrical wall sections to provide an elongated insertion space that is accessible from outside, wherein the first elongated guides are at least partly inserted into the second elongated guides.

The cooperating first and second guides enable the cover to solely slide upwards and downwards inside the digestion vessel during capturing gas whereby jamming is mitigated. This ensures reliable operation of the biogas system.

In an embodiment the digestion vessel has a constant wall thickness over the first cylindrical wall sections and the first guides. As the first guides project inwardly with respect to the first cylindrical wall sections, the first circumferential wall is provided with incorporated reinforcements at the area of constant wall thickness .

In an embodiment thereof the digestion vessel has a constant wall thickness over the first circumferential wall and the bottom wall. Therefore the digestion vessel has a constant wall thickness over its entirety, which offers manufacturing advantages.

In an embodiment the first guides have a semicircular or U-formed cross section, whereby each first guide has two wall sections that merge into first circumferential wall sections which extend directly aside that guide.

In an embodiment the first circumferential wall comprises a slurry section and a gas capture section above the slurry section, wherein the first guides extend along the gas capture section and comprise an end wall at its lower end that extends at a distance from the bottom wall. The first guides extend along the gas capture section to guide the cover during its gas capturing. The first guides end at a distance from the bottom wall to optimize the size of the digestion chamber at the slurry section where the gas is mainly produced.

In a compact embodiment thereof the biogas system comprises an inlet pipe that extends inside one of the first guides and passes through the end wall. The first guides provide some space when embodied hollow, which space is optimally used by passing the inlet pipe through the first guide . In an embodiment thereof the inlet pipe extends fully inside the outer circumference of the first cylindrical wall sections, whereby the inlet pipe is fully incorporated into the digestion vessel.

In an embodiment the biogas system comprises an inlet funnel that is connected to the inlet pipe, wherein the inlet funnel comprises an inlet opening that at least partly extends outside the outer circumference of the first cylindrical wall sections. The inlet opening is easily accessible from outside to fill the digestion vessel with the digestive mixture.

In a compact embodiment the biogas system comprises an outlet pipe that extends inside one of the first guides and passes through the end wall. The first guides provide some space when embodied hollow, which space is optimally used by passing the outlet pipe through the first guide.

In an embodiment thereof the outlet pipe extends fully inside the outer circumference of the first cylindrical wall sections, whereby the outlet pipe is fully incorporated into the digestion vessel.

In an embodiment the biogas system comprises a discharge extension that is connected to the outlet pipe, wherein the discharge extension comprises an discharge edge that at least partly extends outside the outer circumference of the first cylindrical wall sections to enable the discharge of superfluous mixture in a controlled manner.

In an embodiment the end wall merges into a spout that extends inside the digestion chamber towards the bottom wall, wherein the inlet pipe or the outlet pipe is inserted into said spout. In this manner the position of the inserted pipe is fixated.

In an embodiment thereof the spout comprises at the side that faces the bottom wall an connection section that is provided with inwardly directed crush ribs, wherein the crush ribs engage the inserted pipe. The crush ribs tighten the connection between the spout and the inserted pipe, whereby the fixation of the inserted pipe is secured.

In a combined embodiment the inlet pipe extends opposite to the outlet pipe in circumferential direction.

In a combined embodiment the inlet pipe extends deeper into the slurry section than the outlet pipe, whereby newly added mixture is inserted at low level inside the digestion vessel.

In a combined embodiment the inlet opening is positioned higher than the discharge edge, whereby superfluous inserted mixture is discharged in a controlled manner and even helps to flush the system.

In an embodiment the biogas system comprises a canvas reinforcement sleeve that extends around the first circumferential wall sections of the digestion vessel.

In an embodiment the cover has a constant wall thickness over the second cylindrical wall sections and the second guides. As the second guides are recessed inwardly with respect to the first cylindrical wall sections to provide the elongated insertion space, the second circumferential wall is provided with incorporated reinforcements at the area of constant wall thickness.

In an embodiment thereof the cover has a constant wall thickness over the second circumferential wall and the top side. Therefore the cover has a constant wall thickness over its entirety, which offers manufacturing advantages.

In an embodiment the second guides have a semicircular or U-shaped cross section, whereby each second guide has two wall sections that merge into first circumferential wall sections which extend directly aside that guide.

In an embodiment the top side comprises substantially straight top wall sections that extend substantially transverse to the centre line of the second circumferential wall, wherein the top wall sections merge into a raised reinforcement cross that merges into the second circumferential wall.

In an embodiment thereof the reinforcement cross and the top wall sections define accommodation spaces wherein a ballast body has been inserted or confined. The ballast bodies are well kept in position on top of the cover. The downward pressure exerted by the ballast bodies keeps the captured gas on pressure.

In an embodiment thereof the ballast body comprises a lower protrusion having a shape that is complementary to the shape of the accommodation space. The protrusion keeps the ballast block inserted, whereby the remainder of the ballast block can be of any suitable shape.

In an embodiment the ballast body is a hollow body with a closable filling hole so as to be to be filled with a liquid or sand. The hollow body can be filled at the installation site, thereby keeping its transport weight low.

In an embodiment that is suitably made by means of rotation moulding the hollow body has a constant wall thickness .

In an embodiment the ballast body comprise or are made of solid cement or concrete.

In an light weight and thereby easily transportable embodiment the digestion vessel and the cover are made of a thermoplastic resin, preferably polyethylene.

According to a second aspect, the invention provides a method for manufacturing a biogas system by means of rotation moulding of a thermoplastic resin, wherein the biogas system comprises a cylindrical digestion vessel having a bottom wall, a first circumferential wall and a top opening to define a digestion chamber, and a cover having a top side, a second circumferential wall and a bottom opening to define a gas storage chamber, wherein the cover is inserted in the digestion vessel via the top opening with its bottom opening directed towards the bottom wall to capture gas that is released inside the digestion chamber, wherein the casting mould comprises a closed inner space that forms the outer shape of the digestion vessel that on its top is continued into the cover, wherein the method comprises forming a hollow body comprising the digestion vessel and the cover inside the casting mould, releasing the hollow body from the casting mould, and making at least one cut to separate the cover from the digestion vessel to enable the cover to be inserted in the digestion vessel.

According to the invented method two essential parts for the biogas system, which are the biogas vessel and the cover, are manufactured by separating them from the same hollow body. In this manner any small production deviation such as uneven shrinkage or unround shapes that occur will be present to the same extend at de lower side of the cover and the upper side of the digestion vessel, whereby after making the cut optimal conditions are provided to insert the cover in the digestion vessel.

In an embodiment thereof the casting mould comprises a closed inner space that forms the outer shape of the digestion vessel that on its top is continued into the cover via an additional bridge, wherein the additional bridge bridges the difference in diameter of the digestion vessel and the cover at the additional bridge, wherein the method comprises forming a hollow body comprising the digestion vessel, the additional bridge and the cover inside the casting mould, releasing the hollow body from the casting mould, making a first cut to form the bottom opening and to separate the cover from the additional bridge, and making a second cut to form the top opening and to separate the digestion vessel from the additional bridge. The removal of the additional bridge by two cuts ensures that the cover gently slides into the digestion vessel.

In an embodiment the casting mould comprises basic mould parts and insert moulds that are inserted between the basic mould parts, wherein the basic mould parts form the base of the digestion vessel, the continuation of the digestion vessel into the cover, and the top side of the cover, wherein the insert moulds provide additional height to the digestion vessel and the cover. The insert moulds are inserted to obtain alternative heights for the produced digestion systems. The basic mould parts are used for all heights. In this manner biogas systems with alternative heights can be produced while using the same basic mould parts .

The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, in particular the aspects and features described in the attached dependent claims, can be made subject of divisional patent applications .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be elucidated on the basis of an exemplary embodiment shown in the attached drawings, in which :

figure 1 shows a biogas system according to a first embodiment of the invention;

figures 2A and 2B show separately the lower part and the upper parts of the biogas system according to figure 1;

figures 3A and 3B show longitudinal sections of the biogas system according to figure 1, in two outermost opposite operative positions of the movable parts;

figure 4 shows a longitudinal section of an alternative biogas system according to a second embodiment of the invention;

figures 5A and 5B show the upper parts of the alternative biogas system according to figure 6;

figure 5C shows alternative upper parts of the alternative biogas system according to figure 6;

figures 5D and 5E show further alternative upper parts of the alternative biogas system according to figure 6.

figure 6 show a manufacturing phase of two parts of the biogas system according to figure 1; and

figure 7 show a manufacturing phase of two other parts of the biogas system according to figure 1;

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a biogas system 1 according to a first embodiment of the invention. Figures 2A and 2B show some of its parts in more detail. The interior of the biogas system 1 is shown in figures 3A and 3B.

As shown in figures 1 and 2A the biogas system 1 comprises a thin walled digestion vessel 10 comprising a circumferential wall 11 having a centre line A and a bottom wall 12 both having everywhere a constant wall thickness of 4 millimetres. The cylindrical wall 11 defines a digestion chamber 19 and can be generally divided into a lower slurry section 13 and an upper gas capture section 14. The digestion vessel 10 is made of a thermoplastic resin, such as polyethylene, by means of rotation moulding as will be described later in more detail.

At the slurry section 13 the circumferential wall

11 comprises first cylindrical wall sections 15 that merge into in totally four groups of two inwardly directed reinforcement ribs 16 that extend in the longitudinal direction of the vessel 10. At the upper side the first cylindrical wall sections 15 and the reinforcement ribs 16 merge into an outwardly directed, circumferential reinforcement rib 17.

At the gas capture section 14 the circumferential wall 11 comprises second cylindrical wall sections 20 that merge into in totally four inwardly directed first guides 21 that extend in the longitudinal direction of the vessel 10. The first guides 21 have an U-shaped cross section and are evenly distributed over the circumference of the vessel 10. At the lower side the first guides 21 each merge into a straight end wall section 22 that extends parallel to the bottom wall 12. At the top side the second wall sections 20 merge into an outwardly directed, circumferential reinforcement rib 24 that defines the circular vessel opening 23.

As best shown in figures 3A and 3B the biogas system 1 comprises an inlet pipe 30 that is made of a thermoplastic resin, such as polyvinylchloride . The inlet pipe extends within one of the first guides 21 and passes the end wall section 22 via a water tight seal that is not shown. The upper part 31 of the inlet pipe 30 extends fully inside the first guide 21 and the lower part 32 extends inside the slurry section up to a distance with respect to the bottom wall 12 that is substantially equal to the diameter of the inlet pipe 31. The inlet pipe 30 is connected to an inlet funnel 33 that is made of a thermoplastic resin, such as polyethylene. The inlet funnel 33 has an elongated funnel opening. The inlet funnel 33 extends with its funnel opening partly outside the outer circumference of the circumferential wall 11.

As shown in figures 3A and 3B the biogas system 1 comprises an outlet pipe 35 that is made of a thermoplastic resin, such as polyvinylchloride . The outlet pipe 35 extends within the first guide 21 that is at the opposite side of the inlet pipe 31. The outlet pipe 35 passes though the end wall section 22 via a water tight seal that is not shown. The outlet pipe 35 extends fully inside the first guide 21 and extends into the slurry section 13 over only a very short distance which is just enough for proper sealing. The outlet pipe 35 is connected to a funnel shaped discharge extension 36 made of a thermoplastic resin, such as polyethylene. The discharge extension has an elongated top side. The discharge extension 36 partly extends outside the outer circumference of an circumferential wall 11, wherein the extending portion is provided with a lowered discharge edge 37 to force the overflow at discharge to take place at the outermost extending edge of the discharge extension 36.

The inlet funnel 33 extends at a height E with respect to the bottom wall 12 that is some centimetres higher than the height B of the lowest part of the discharge edge 37. In this manner the digestion vessel 10 can be filled with a digestive mixture containing manure and vegetables that is partly solved in water via the inlet funnel 33 in a proper way, wherein excessive mixture is discharged via the discharge extension 36 when the maximum level for the mixture at the height B has been reached. Inside the digestion vessel 10 the slurry fraction lowers into the slurry section 13.

At the slurry section 13 the digestion vessel 10 comprises two tube connections 38, formed as a spout, in the lower half of two opposite cylindrical wall sections 15. By means of the tube connections 38 the digestion vessel 10 can be directly connected to a toilet that is installed at an appropriate height above the discharge extension to fill the vessel 10 with manure.

As shown in figure 1 the circumferential wall 11 has been reinforced at the upper gas capture section 14 by means of a tightly fitted canvas reinforcement sleeve 28 that is confined between the circumferential reinforcement ribs 17, 24. The canvas sleeve 28 comprises an opening 27 for the inlet funnel 35 and an opening 29 for the discharge extension 36.

As shown in figures 1 and 2B the biogas system 1 comprises a thin walled cover 40 having everywhere a constant wall thickness of 4 millimetres. The cover 40 has been inserted into the digestion vessel 10 via the vessel opening 23. The cover 40 defines a gas storage chamber 47 that is only freely accessible from below. The cover 40 is made of a thermoplastic resin, such as polyethylene, by means of rotation moulding as will be described later in more detail.

The cover 40 comprises cylindrical circumferential wall sections 41 that merge into four inwardly directed second guides 42 which extend in the longitudinal direction of the cover 40. The second guides 42 have an U-shaped cross section and are evenly distributed over the circumference of the cover 40. The outside dimensions of the second guides 42 are adapted to accommodate of the first guides 21 whereby the cover 40 can only freely slide upwards and downwards inside the vessel 10, that is without rotation around its longitudinal axis A and without tiling sideways or toppling over with respect to the digestion vessel 10. The cover 40 comprises multiple inwardly directed reinforcement ribs 43 extending through the circumferential wall sections 41 and the second guides 42 in circumferential direction of the cover 40.

At its top side the cover 40 is provided with a reinforcement cross 44 that widens towards its four extremities and that its four extremities merge into the second guides 42. Slightly below, the cover 40 comprises flat top walls 45 that together with the reinforcement cross 44 define four accommodation spaces 46 for confinement of four complementary formed concrete ballast blocks 50. The reinforcement cross 44 and the concrete ballast blocks 50 together form a smooth convex top face of the cover 40. The cover 40 comprises a hose connection 48 that connects the gas storage chamber 47 with a flexible gas hose 49. At its opposite side the hose 49 is connected to a household gas burner that is not shown. Alternatively, in the four accommodation spaces 46 four complementary hollow bodies have been accommodated. The hollow bodies can be filled with water or sand to provide the necessary weight.

Figure 3A shows the highest possible position of the cover 40 with respect to the digester vessel 10. As indicated, the height D of the circumferential wall sections 41 of the cover 40 is substantially equal to the entire height C of the digestion vessel 10 minus the height B of the discharge edge 37 of the discharge extension 36 which determines the maximum level of the liquid inside the digestion vessel 10. In the extreme case that the gas storage chamber 47 is fully filled with methane gas and the cover 40 optimally floats on the liquid, and some excessive mixture has been discharged due to the height of the discharge edge 37, even then the second guides 42 are fully engaged by the first guides 21. The weight of the concrete ballast blocks 50 maintains the captured methane gas at an appropriate constant pressure in order to be fed to the gas burner. The maximum volume of the mixture at height B is twice the volume of the gas storage chamber 47.

Figure 3B shows the lowest position of the cover 40 inside the digestion vessel 10. In this position the cover 40 stably stands on the upper ends of the reinforcement ribs 16 of the slurry section 13. The height D of the circumferential wall sections 41 allows the gas storage chamber 47 to be fully filled with the mixture at its maximum level at height B without containing methane gas .

Figure 4 shows by means of a longitudinal section the interior of an alternative biogas system 101 according to a second embodiment of the invention. Figures 5A-E show various upper parts which can be used in the alternative biogas system 101 according to figure 4. Parts of the alternative biogas system 101 which are identical or substantially the same as the corresponding parts of the biogas system 1 as shown in figures 1-3 are indicated with the same reference numerals. The alternative biogas system 101 is provided with the canvas reinforcement sleeve 28, but for illustrating purposes the sleeve 28 has not been shown. Only the differentiating parts are provided with an alternative reference number and are discussed hereafter.

As shown in figure 4, the alternative biogas system 101 is provided with an inlet pipe 30 and an outlet pipe 35 which are placed within the first guides 21. The lower parts 32 of the inlet pipe 30 and the outlet pipe 35 pass through the end wall sections 22 via an inwardly directed inlet spout 134 and an inwardly directed outlet spout 135 in which the lower part 32 of the inlet pipe 30 and the lower part 39 of the outlet pipe 35, respectively, are received. In each of the first guides 21 multiple reinforcement ribs 51 may be formed that extend over the full length in the longitudinal direction of the first guides 21.

The spouts 134, 135 comprise an accommodation section 137 that downwardly merges into a smaller connection section 138. The upper side of the accommodation section 137 merges outwardly into the end wall sections 22 with a substantially constant radius along the circumference. Internally, the connection section 138 is provided with axially extending and radially inwardly directed crush ribs 139 that are distributed along the inner circumference. At the inwardly directed edges the crush ribs 139 define a diameter that is slightly smaller than the outer diameter of the lower parts 32, 39 of the pipes 30, 35. Once inserted, the lower part 32 of the inlet pipe 30 and the lower part 39 of the outlet pipe 35 displace the crush ribs and become jammed or pinched between them, thereby substantially fixing the position of the lower parts 32 relative to the spouts 134, 135. An additional resilient seal 136 is partly inserted in the accommodation section 137 and seals the connection between the pipe 30, 35 and the spout 134, 135. Accumulated slurry may further enhance the sealing.

As shown in figure 5A, the alternative biogas system 101 comprises a cover 140 which is inserted in the vessel 10 and which is guided by the first guides 21 so that it can only freely slide upwards and downwards in the vessel 10. The cover 140 comprises multiple inwardly directed reinforcement ribs 43 extending through the circumferential wall sections 41 in circumferential direction of the cover 140. In this alternative biogas system 101 the second guides 42 are smooth. The cover 140 is provided with a reinforcement cross 144 that widens towards its four extremities and at its extremities merges into the second guides 42. Slightly below, the cover 140 comprises substantially flat top walls 145 that together with the reinforcement cross 144 define four accommodation spaces 146. The cover 140 is provided with overflow channels 151 around its outer circumference for discharging precipitation which accumulates in the accommodation spaces 146 of the cover 140. To ensure that the precipitation flows towards the overflow channels 151, the flat top walls 145 are under a slight gradient that descents towards the outer circumference of the cover 140.

As shown in figures 4, 5A-5E, the alternative biogas system 101 can be provided with different types of hollow ballast bodies 150, 250, 350 which are placed on top of the cover 140. The hollow ballast bodies 150, 25, 350 are made of a thermoplastic resin, such as polyethylene. As shown in figures 5A and 5B, the first type of hollow ballast bodies 150 each have a semicircular shape that covers approximately halve of the cover 140, wherein the hollow ballast bodies 150 together substantially cover the entire cover 140. Each hollow ballast body 150 extends over two of the accommodation spaces 146 and is provided with two protrusions 154 which are complementary formed to and are to be confined within the accommodation spaces 146. In this manner, the hollow ballast bodies 150 can be placed on the cover 140 in a stable manner. In this example, a substantial part of the hollow ballast bodies 150 extends above and outside of the accommodation spaces 146, thereby substantially increasing the volume of the hollow ballast bodies 150 with respect to the volume of the accommodation space 146.

The hollow ballast bodies 150 are provided with first recesses 153, 157 which are aligned with respect to the second guides 42. The first recesses 153, 157 allow for passage of the first guides 21 when the cover 140 slides upwards or downwards through the vessel 10. Furthermore, the hollow ballast bodies 150 are each provided with a central second recess 158 which together keep the centrally located hose connection 152 of the cover 140 free and accessible once the hollow ballast bodies 150 have been placed.

The hollow ballast bodies 150 comprises a filling section 155 with a threaded nozzle 156 to which standard caps for stand water bottles or PET bottles can be fitted. The nozzle 156 can be used to fill the hollow ballast bodies 150 with water or sand to provide the necessary weight. Because of the additional volume of the hollow ballast bodies 150 above and outside the accommodation spaces 146, additional sand or water can be added to the hollow ballast bodies 150 compared to the ballast blocks 50 which are used in the biogas system 1 according to figures 1-5. The filling section 155 extends slightly above the rest of the body so that substantially the entire volume of the hollow ballast body 150 can be filled before it starts to overflow through the nozzle 156. Once filled, the nozzle 156 can be conveniently sealed by any suitable cap.

Figure 5C shows a second type of hollow ballast body 250 which is a variation on the hollow ballast bodies 150 as shown in figures 5A and 5B. This hollow ballast body 250 differs from the first type of hollow ballast bodies 150 as shown in figures 5A and 5B in that it is provided with radial reinforcement ribs 259 and concentric reinforcement ribs 260 in its upper surface. The reinforcement ribs 259, 260 strengthen the hollow ballast body 250 so that the structural integrity is improved.

Figures 4, 5D and 5E show a third type hollow ballast body 350 which is a variation on the hollow ballast bodies 150 as shown in figures 5A and 5B. This ballast body 350 differs from the hollow ballast bodies 150 as shown in figures 5A and 5B in that each ballast body 350 only covers a quarter of the cover 140. Each ballast body 350 is provided with one protrusion 154 with which it can be placed in one of the accommodation spaces 146. Four of these ballast bodies 350 together substantially cover the entire cover 140. In a similar manner as the hollow ballast bodies 150 of figures 5A and 5B, each hollow ballast body 350 is provided with first recesses 353, 357 and a second central recess 358 which are aligned with the second guides 42 and the hose connection 152, respectively, and an radially extending filling section 355 with the threaded nozzle 156. The use of these hollow ballast bodies 350 can reduce the weight per ballast body, thereby making it easier for the ballast bodies to be handled. The ballast body 350 can in its upper surface. also be provided with the radial reinforcement ribs 259 and concentric reinforcement ribs 260 like the ballast body 250 as shown in figure 5C.

Figure 6 shows a manufacturing phase of the biogas systems 1, 101 according to the invention. In particular, the combined manufacturing of the digestion vessel 10 and the cover 40 by means of a casting mould is shown. Figure 6 shows one part 60 of an opened rotation casting mould having in its closed state a fully closed inner space that forms the complete outer shape of subsequently the digestion vessel 10, an additional bridge 61 and the cover 40. The additional bridge 61 extends along the circumference and the inwardly located first guides 21 and second guides 42.

During manufacturing the casting mould is filled with pallets of thermoplastic resin and heated to melt the pellets, and rotated around three rotation axis in a manner known per se with rotation casting. The additional bridge 61 forms a temporary connection between the digestion vessel 10 and the cover 40. The additional bridge 61 bridges the difference between the outer diameter of the bottom of the cover 40 and the wider top side of the digestion vessel 10. After cooling down the rotated casting mould is opened and the shown hollow body comprising the digestion vessel 10, the additional bridge 61 and the cover 40 having a constant wall thickness is released. After release a first circumferential cut 62 (solid line) and a second circumferential cut 63 (dashed line) are made, which cuts 62, 63 then form the open lower side of the cover 40 and the vessel opening 23, respectively. In this manner any small production deviation such as uneven shrinkage or unround shapes that occur at the additional bridge 61 will be present to the same extent at de lower side of the cover 40 and the upper side of the digestion vessel 10, whereby the cover 40 will always fit into and slide with respect to the vessel 10. Alternatively only one of the cuts 61, 62 is made. Alternatively an additional bridge 61 is not applied and only one cut is enough to enable the cover 40 to be inserted into the digestion vessel 10. Alternatively the cuts 61, 62 are made by a single tool having a cutting thickness that is enough to remove the additional bridge 61 in its entirety. By means of rotation casting the digestion vessel 10 and the cover 40 everywhere have the same constant wall thickness of 4 millimeters. The appropriate constant wall thickness is in the range of 3-6 millimeters.

As shown in figure 6 the part 60 of the opened rotation casting mould is divided into basic mould parts 64, 66, 68, 70 having fluently connected insert moulds 65, 67, 69 in between. The basic mould parts 64, 66, 68, 70 and insert moulds 65, 67, 69 together form the biogas system 1 as disclosed in figures 1-3B. This biogas system 1 typically has a volume of the mixture at height B of 1500 liters. When the insert moulds 65, 67, 69 are removed and only the basic mould parts 64, 66, 68, 70 are connected together, an alternatively dimensioned biogas system is obtained having a reduced height C but still having a maximum volume of the mixture at height B that is twice the volume of the gas storage chamber of the cover. This alternative dimensioned biogas system has a volume of the mixture at the height B of 1000 liters. By providing only an alternative set of insert moulds 65, 67, 69 a volume of the mixture at the height B can be 700 liters while maintaining above mentioned relation in the volumes.

Figure 7 shows another manufacturing phase of the biogas systems 1, 101. In particular, the combined manufacturing of the inlet funnel 33 and the discharge extension 36 by means of a casting mould is shown. Figure 5 shows one half 80 of an rotation casting mould that is opened at its plane of symmetry. The rotation casting mould has in its closed state a fully closed inner space that forms the complete outer shape of the funnel 33 and the discharge extension 36 that are joined together at their longest upper edges. During manufacturing the casting mould is filled with thermoplastic pallets and heated to melt the pellets, and rotated around three rotation axis in a manner known per se with rotation casting. After cooling down the rotated casting mould is opened and the hollow body comprising the inlet funnel 33 and the discharge extension 36 having a constant wall thickness is released. After release a circumferential cut 81 is made form the inlet funnel 33 and the discharge extension 36. An additional cut will provide the lowest part of the discharge edge 37.

The manufacturing process of manufacturing the aforementioned biogas systems with a rotation casting mould allows for the biogas systems to manufactured in different scales .

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention .

Claims

C L A I M S

1. Biogas system comprising a cylindrical digestion vessel having a bottom wall, a first circumferential wall and a top opening to define a digestion chamber to be filled with a digestive mixture, and a cover having a top side, a second circumferential wall and a bottom opening to define a gas storage chamber, wherein the cover is inserted in the digestion vessel via the top opening with its bottom opening directed towards the bottom wall to capture gas that is released inside the digestion chamber, wherein the first circumferential wall comprises first cylindrical wall sections that merge into first elongated guides, which first elongated guides extend in the longitudinal direction of the first circumferential wall and project inwardly with respect to the first cylindrical wall sections, and wherein the second circumferential wall comprises second cylindrical wall sections that merge into second elongated guides, which second elongated guides extend in the longitudinal direction of the first circumferential wall and are recessed inwardly with respect to the second cylindrical wall sections to provide an elongated insertion space that is accessible from outside, wherein the first elongated guides are at least partly inserted into the second elongated guides.

2. Biogas system according to claim 1, wherein the digestion vessel has a constant wall thickness over the first cylindrical wall sections and the first guides.

3. Biogas system according to claim 2, wherein the digestion vessel has a constant wall thickness over the first circumferential wall and the bottom wall.

4. Biogas system according to any one of the preceding claims, wherein the first guides have a semicircular or U-formed cross section.

5. Biogas system according to any one of the preceding claims, wherein the first circumferential wall comprises a slurry section and a gas capture section above the slurry section, wherein the first guides extend along the gas capture section and comprise an end wall at its lower end that extends at a distance from the bottom wall.

6. Biogas system according to claim 5, comprising an inlet pipe that extends inside one of the first guides and passes through the end wall.

7. Biogas system according to claim 6, wherein the inlet pipe extends fully inside the outer circumference of the first cylindrical wall sections.

8. Biogas system according to claim 6 or 7, comprising an inlet funnel that is connected to the inlet pipe, wherein the inlet funnel comprises an inlet opening that at least partly extends outside the outer circumference of the first cylindrical wall sections.

9. Biogas system according to any one of claims 5-8, comprising an outlet pipe that extends inside one of the first guides and passes through the end wall.

10. Biogas system according to claim 9, wherein the outlet pipe extends fully inside the outer circumference of the first cylindrical wall sections.

11. Biogas system according to claim 9 or 10, comprising a discharge extension that is connected to the outlet pipe, wherein the discharge extension comprises an discharge edge that at least partly extends outside the outer circumference of the first cylindrical wall sections.

12. Biogas system according to claim 6 or 9, wherein the end wall merges into a spout that extends inside the digestion chamber towards the bottom wall, wherein the inlet pipe and/or the outlet pipe is inserted into said spout .

13. Biogas system according to claim 12, wherein the spout at the side that faces the bottom wall comprises an connection section that is provided with inwardly directed crush ribs, wherein the crush ribs engage the inserted pipe.

14. Biogas system according to claims 6 and 9, wherein the inlet pipe extends opposite to the outlet pipe in circumferential direction.

15. Biogas system according to claims 6 and 9, wherein the inlet pipe extends deeper into the slurry section than the outlet pipe.

16. Biogas system according to claims 8 and 11, wherein the inlet opening is positioned higher than the discharge edge.

17. Biogas system according to any one of the preceding claims, comprising a canvas reinforcement sleeve that extends around the first circumferential wall sections of the digestion vessel.

18. Biogas system according to any one of the preceding claims, wherein the cover has a constant wall thickness over the second cylindrical wall sections and the second guides.

19. Biogas system according to claim 16, wherein the cover has a constant wall thickness over the second circumferential wall and the top side.

20. Biogas system according to any one of the preceding claims, wherein the second guides have a semicircular or U-shaped cross section.

21. Biogas system according to any one of the preceding claims, wherein the top side comprises substantially straight top wall sections that extend substantially transverse to the centre line of the second circumferential wall, wherein the top wall sections merge into a raised reinforcement cross that merges into the second circumferential wall.

22. Biogas system according to claim 21, wherein the reinforcement cross and the top wall sections define accommodation spaces wherein a ballast body has been inserted or confined.

23. Biogas system according to claim 22, wherein the ballast body comprises a lower protrusion having a shape that is complementary to the shape of the accommodation space .

24. Biogas system according to claim 22 or 23, wherein the ballast body is a hollow body with a closable filling hole so as to be to be filled with a liquid or sand.

25. Biogas system according to claim 24, wherein the hollow body has a constant wall thickness.

26. Biogas system according to any one of the claims 22-26, wherein the ballast body comprise or are made of solid cement or concrete.

27. Biogas system according to any one of the preceding claims, wherein the digestion vessel and the cover are made of a thermoplastic resin, preferably polyethylene.

28. Method for manufacturing a biogas system by means of rotation moulding of a thermoplastic resin, wherein the biogas system comprises a cylindrical digestion vessel having a bottom wall, a first circumferential wall and a top opening to define a digestion chamber, and a cover having a top side, a second circumferential wall and a bottom opening to define a gas storage chamber, wherein the cover is inserted in the digestion vessel via the top opening with its bottom opening directed towards the bottom wall to capture gas that is released inside the digestion chamber, wherein the casting mould comprises a closed inner space that forms the outer shape of the digestion vessel that on its top is continued into the cover, wherein the method comprises forming a hollow body comprising the digestion vessel and the cover inside the casting mould, releasing the hollow body from the casting mould, and making at least one cut to separate the cover from the digestion vessel to enable the cover to be inserted in the digestion vessel.

29. Method according to claim 28, the casting mould comprises a closed inner space that forms the outer shape of the digestion vessel that on its top is continued into the cover via an additional bridge, wherein the additional bridge bridges the difference in diameter of the digestion vessel and the cover at the additional bridge, wherein the method comprises forming a hollow body comprising the digestion vessel, the additional bridge and the cover inside the casting mould, releasing the hollow body from the casting mould, making a first cut to form the bottom opening and to separate the cover from the additional bridge, and making a second cut to form the top opening and to separate the digestion vessel from the additional bridge.

30. Method according to claim 28 or 29, wherein the casting mould comprises basic mould parts and insert moulds that are inserted between the basic mould parts, wherein the basic mould parts form the base of the digestion vessel, the continuation of the digestion vessel into the cover and the top side of the cover, and wherein the insert moulds provide additional height to the digestion vessel and the cover. o-o-o-o-o-o-o-o-

FG/HZ