Technical Field of the Invention

The present invention relates to a digester for production of biogas in a dung pit, a method for production of biogas in a dung pit and a method for converting a dung pit to a digester for production of biogas.

Technical Background

The manure available within agriculture is an energy source that in general not is utilized. The manure is usually used as a fertilizer and thereby spread on the fields. The manure has a low ratio of ammonium nitrogen compared with total nitrogen, which implies a high risk of leakage of nutrients from the fields. Within agriculture, the manure is generally stored in a dung pit located at a farm. In general, the dung pit is not artificially covered and thus ammonia is dispersed into the ambient air leading to environmental load. In addition, leakage of the greenhouse gas methane to the atmosphere is normal, since a normal dung pit without an artificial cover will when the temperature is high enough, e.g. during the summer, generate a substantial amount of methane, which will be dispersed into the ambient air.

The manure may be used for production of biogas, since when biological material is anaerobically degraded under certain conditions, methane is produced. The energy source in biogas is essentially methane. In addition, anaerobic degradation increases the amount of ammonium nitrogen (NH -N), i.e. the amount of nitrogen in form of ammonium, in the manure. Thus, the quality of the manure as a fertilizer is increased and the demand for artificial fertilizers is decreased.

For production of biogas, biological material, such as manure, is usually degraded in a digestion tank to which biological material is transferred from e.g. a dung pit. The digestion tank is normally equipped with an agitator for mixing the manure. The produced biogas is accumulated at the roof of the digestion tank where it is collected. The energy content of the produced biogas is most efficiently used for production of electricity in a gas motor, but when combusting biogas in gas motors substantial amounts of heat are produced simultaneously. Production of biogas from biological material in a digestion tank requires installation of a complete digestion tank including necessary auxiliary equipment in order to start up biogas production at a farm. Thus, the initial investment is high and is not economically profitable for small and medium sized farms, since small and medium sized farms cannot use all the produced heat. Such a traditional biogas plant may only be profitable for large estates having a large number of buildings to heat. Also larger farms can have difficulties to achieve a satisfactory result due to that they cannot use all of the heat produced in parallel to the electric power production.

DK 95 00408 discloses a reaction tank that is positioned in an existing or new storage tank for manure. The storage tank is equipped with a gas sealed cover. This saves space compared to the digestion tank above, but it still requires installation of a complete digestion tank including necessary auxiliary equipment in order to start up biogas production at a farm. Thus, the initial investment is still high and is not economically profitable for small and medium sized farms. In addition, in order to install the reaction tank in an existing storage tank for manure, the storage tank has to be emptied.

EP 2 095 701 discloses a cover for a slurry tank, which is mounted on the upper edge of the tank. The cover may be gastight and is insulated facilitating utilization of biogas. However, in EP 2 095 701 the degradation of biological material is inefficient and the possibly produced biogas has a low content of methane.

Thus, there still exist a need for a biogas production plant comprising simple and inexpensive equipment that achieves efficient degradation of biological material and high content of methane in the produced biogas.

Summary of the Invention

An object of the present invention is to provide simple equipment for production of biogas. An object of the present invention is to provide inexpensive equipment for production of biogas. An object of the present invention is to provide equipment that enables economically profitable production of biogas at a farm. An object of the present invention is to provide equipment that enables efficient degradation of biological material. An object of the present invention is to provide equipment that enables a high content of methane in produced biogas. An object of the present invention is to provide equipment that enables reduction of emission of methane and/or ammonia from a dung pit into the ambient air.

These and further objects are achieved by a digester for production of biogas in a dung pit containing biological material, wherein the digester comprises a dung pit containing biological material, which dung pit comprises a bottom, wherein the digester further comprises a roof covering the dung pit, and at least one insulating element at least partly insulating the bottom of the dung pit.

The digester according to the present invention provides simple equipment for production of biogas. The digester according to the present invention provides inexpensive equipment for production of biogas. The digester according to the present invention provides equipment that enables economically profitable production of biogas at a farm. The digester according to the present invention provides equipment that enables efficient degradation of biological material. The digester according to the present invention provides equipment that enables a high content of methane in produced biogas. The digester according to the present invention enables production of biogas in an existing dung pit. The digester according to the present invention enables production of biogas in an existing dung pit without having to empty the dung pit before starting biogas production in the dung pit. The digester according to the present invention enables production of biogas without a digestion tank. The digester according to the present invention enables reduction of emission of methane and ammonia from a dung pit into the ambient air.

The above and further objects are also achieved by an insulating element as defined above. This also applies to a use of an insulating element as defined above for production of biogas in a dung pit.

An object of the present invention is to provide a method for production of biogas using inexpensive equipment. An object of the present invention is to provide a method that enables economically profitable production of biogas at a farm. An object of the present invention is to provide a method that enables efficient degradation of biological material. An object of the present invention is to provide a method that enables a high content of methane in produced biogas. An object of the present invention is to provide a method that enables reduction of emission of methane and/or ammonia from a dung pit into the ambient air.

These and further objects are achieved by a method for production of biogas in a dung pit containing biological material, wherein said dung pit comprises a bottom and is provided with a roof covering the dung pit and at least one insulating element, which is placed on the bottom of the dung pit and at least partly insulates the bottom of the dung pit, wherein the method comprises collecting produced biogas at the roof covering the dung pit.

The method for production of biogas according to the present invention provides a method for production of biogas using inexpensive equipment. The method for production of biogas according to the present invention provides a method that enables economically profitable production of biogas at a farm. The method for production of biogas according to the present invention provides a method that enables efficient degradation of biological material. The method for production of biogas according to the present invention provides a method that enables a high content of methane in produced biogas. The method for production of biogas according to the present invention provides a method that enables production of biogas in an existing dung pit. The method for production of biogas according to the present invention provides a method that enables production of biogas without a digestion tank. The method for production of biogas according to the present invention provides a method that enables reduction of emission of methane and ammonia from a dung pit into the ambient air.

The above objects relating to providing equipment and further objects are also achieved by a method for converting a dung pit containing biological material to a digester for production of biogas, wherein the dung pit comprises a bottom, and wherein the method comprises immersion of at least one insulating element as defined above into the dung pit, placement of said at least one insulating element, and covering of the dung pit with a roof. The above disclosed advantages of the digester for production of dung pit containing biological material according to the present invention also apply to the method for converting a dung pit containing biological material to a digester for production of biogas according to the present invention. The same applies to use of an insulating element as defined above for production of biogas in a dung pit.

Brief Description of the Drawings

Fig 1 is a side view of an embodiment of a digester according to the present invention shown without roof.

Fig 2 is a top view of the embodiment of a digester shown in fig 1 .

Fig 3 is a side view of an embodiment of a digester according to the present invention.

Fig 4 is a top view of the embodiment of a digester shown in fig 3.

Fig 5 is a side view of an embodiment of an insulating element according to the present invention.

Fig 6 is a top view of the embodiment of an insulating element shown in fig 5.

Fig 7 is a side view of an embodiment of a safety roof according to the present invention.

Fig 8 is a cross sectional top view of the embodiment of a safety roof shown in fig 7, wherein the cross section is taken along the liquid level shown in fig 7.

Detailed Description of the Invention

As said above, the present invention relates to a digester (1 ) for production of biogas in a dung pit (2) containing biological material, wherein the digester (1 ) comprises a dung pit (2) containing biological material, which dung pit (2) comprises a bottom, wherein the digester (1 ) further comprises a roof (3) covering the dung pit (2), and at least one insulating element (4, 4'), which is placed on the bottom (5) of the dung pit (2) and at least partly insulates the bottom (5) of the dung pit (2). Embodiments of a digester according to the present invention are shown in figures 1 -4.

In the present invention, the at least one insulating element is placed on the bottom of the dung pit. By placing said at least one insulating element on the bottom of the dung pit, the dung pit may be insulated after building of the dung pit. This enables insulation of an existing dung pit and erection of a digester according to the present invention in an existing dung pit. The fact that the at least one insulating elements is placed on the bottom of the dung pit simplifies the construction of a digester for production of biogas and reduces the cost for a digester for production of biogas.

When operating the digester according to the present invention, initiating biological material may be supplied to at least one region above the at least one insulating element. Preferably, initiating biological material is supplied to the center region above the at least one insulating element. The initiating biological material may be supplied continuously or batch-wise, wherein the batches may be separated by a time interval. The initiating biological material may for example be supplied once every twenty-four hours. The initiating biological material may be preheated. The supply of possibly preheated initiating biological material will initiate and/or increase the degradation of biological material in the dung pit. Alternatively, if the ambient temperature is high enough degradation may be maintained without preheating initiating biological material.

In some embodiments of the present invention, the digester (1 ) comprises a dung pit (2). In some embodiments, the digester (1 ) comprises a dung pit (2) containing biological material. In some embodiments, the dung pit (2) comprises a bottom (5) and at least one wall (14). A dung pit is a container, which within agriculture is used for storage of biological material, such as manure from farmyard animals. A dung pit is usually an open container.

In some embodiments, the dung pit (2) is an existing dung pit. By an existing dung pit is meant a dung pit that already exist, i.e. a dung pit that already is present for example at a farm. The fact that the dung pit is existing implies that the dung pit exists when the digester according to the present invention is erected. This may also be expressed as that the dung pit is converted to a digester. By using an existing dung pit the production of the digester is simplified. In addition, the cost for the digester is decreased and the economically profitability of the production of biogas at a farm is increased. The present invention enables production of biogas in an existing dung pit without having to empty the dung pit before starting biogas

production in the dung pit. The at least one insulating element will at least partly insulate the bottom of the dung pit and thus strongly increase the efficiency of the degradation of biological material, since the degradation is more efficient at higher temperatures. Without insulating elements substantial amounts of heat would leak to the soil through the bottom of the dung pit and the degradation would slow down or terminate. Thus, the insulating elements increase the efficiency of the degradation, reduce the need for supply of heat and/or enable degradation at a specific level of heat supplied by means of the preheated initiating biological material. The effect of the present invention is in particular perceivable at lower temperatures, e.g. during the autumn, winter and/or spring.

The initiating biological material may be preheated in a preheating tank located outside the dung pit. The initiating biological material may be preheated to 10-80°C, such as 20-75°C, such as 30-60°C, such as 35-50°C, such as 38-50°C, such as 38-45°C, such as 40-45°C. When preheating the initiating biological material to at least about 70°C the initiating biological material is hygienized. The initiating biological material may be preheated to 35-75°C, such as 40-75°C. The preheating tank may be equipped with an agitator agitating the initiating biological material. The agitator homogenizes the initiating biological material and/or distributes the heat essentially evenly in the initiating biological material. The preheating tank may be substantially smaller than a digestion tank of a conventional biogas plant considering that in the present invention only an amount of initiating biological material necessary for initiating and/or maintaining the digestion in the dung pit is necessary, whereas the digestion tank of a conventional biogas plant have to accommodate all the biological material to be degraded.

The initiating biological material may be supplied from the preheating tank to the biological material contained in the dung pit manually, by gravity or by means of a pump. The initiating biological material may be supplied from the preheating tank to the biological material contained in the dung pit by gravity by location of the preheating tank at an altitude above the level at which the initiating biological material is supplied to the biological material contained in the dung pit. The initiating biological material may be supplied from the preheating tank to the biological material contained in the dung pit in a piping. The piping may end in an outlet located above the at least one insulating element. The initiating biological material may be supplied to the biological material contained in the dung pit by an outlet located above the at least one insulating element. The outlet may be located above the center of the at least one insulating element. The outlet may be located in the center of the dung pit.

In some embodiments of the present invention, the digester comprises supply means for supply of initiating biological material. In some

embodiments, the supply means comprises at least one outlet located above the at least one insulating element.

The initiating biological material may be any degradable biological material. The initiating biological material may comprise easily degradable material. The initiating biological material may comprise biological material derived from plants and/or animals. The initiating biological material may comprise green material or manure, compost or plant manure, or any mixture thereof. The manure may comprise animal manure such as manure from cattle. The initiating biological material may comprise liquid manure, semi- liquid manure, slurry manure or any mixture thereof.

In general, the at least one insulating element does not insulate the complete bottom of the dung pit. In some embodiments, the at least one insulating element insulates 5-80 %, such as 5-50 %, such as 10-30 % of the bottom of the dung pit.

The region above the at least one insulating element composes the primary region in which degradation of biological material will occur. The region outside the primary region composes the secondary region. In the secondary region a temperature gradient will occur and the highest temperature will be next to the primary region and the lowest temperature will be at the periphery of the dung pit. The biological material in the secondary region will insulate the primary region. A dung pit is in general rather shallow, typically 3-4 m, and thus the amount of biological material below where the initiating biological material is supplied into the dung pit is not sufficiently large to serve as satisfactory insulation in the same way as the biological material in the secondary region does. In addition, mixing, mainly vertical mixing, will occur in the primary region due to temperature differences in the primary region and without insulating elements also the biological material below where the initiating biological material is supplied into the dung pit, i.e. the biological material that could have served as insulation, would have been mixed with the biological material in the primary region, which would have implied a lower temperature and decreased or terminated degradation. Thus, by at least partially insulating the bottom of the dung pit, the insulating elements substantially increase the efficiency of the degradation in the dung pit.

Equilibrium will be established in the primary region defined above. In particular, equilibrium will be established in the primary region when initiating biological material is supplied to the biological material contained in the dung pit continuously or batch-wise separated by a fixed and steady time interval. In addition, supply of initiating biological material having a temperature within a fixed and steady temperature interval contributes to the establishment of a state of equilibrium in the primary region. The equilibrium temperature in the primary region will depend on the temperature of the initiating biological material supplied to the biological material contained in the dung pit. When supplying initiating biological material having a temperature within the interval 38-50°C, the temperature within the primary region will typically be 35-37°C.

In some embodiments of the present invention, the at least one insulating element (4, 4') comprises placement means (6) for stable placement of the at least one insulating element (4, 4') on the bottom (5) of the dung pit (2) containing biological material. By means of the placement means (6), the at least one insulating element (4, 4') stands stable on the bottom (5) of the dung pit (5) despite that the at least one insulating element (4, 4') is immersed into and placed on the bottom of the dung pit (2) already containing biological material. Embodiments comprising placement means are shown in figures 1 , 3 and 5.

In some embodiments, the placement means (6) comprises projections (7) on the underside of the at least one insulating element (4, 4'). The projections (7) may be accomplished by at least one vault (15) on the underside of the at least one insulating element (4, 4'). In some embodiments, the placement means (6) comprises at least one vault (15) on the underside of the at least one insulating element (4, 4'). The at least one vault (15) may preferably be two or more elongated curves on the underside of the at least one insulating element (4, 4'). Such embodiments are shown in figures 1 , 3 and 5. In some embodiments, the underside of the at least one insulating element (4, 4') is corrugated. The projection of the at least one insulating element implies that the projection of the at least one insulating element is in contact with the bottom of the dung pit. Alternatively, only a thin layer of biological material is present between the projection and the bottom of the dung pit. Thus, the at least one insulating element stands stable on the bottom of the dung pit. The parts of the underside of the at least one insulating element not having a projection, such as the upper parts of the at least one vault, may accommodate some of the biological material present in the dung pit when the at least one insulating element is immersed into and placed on the bottom of the dung pit. Without placement means, such as when the underside of the at least one insulating element is substantially flat, a thicker layer of biological material is present between the whole underside of the at least one insulating element and the bottom of the dung pit, which may result in an unstable construction.

In some embodiments of the present invention, the digester (1 ) comprises at least two insulating elements (4, 4'), see figures 1 -3. By using at least two insulating elements, the size of each element may be reduced.

Thereby, the handling of the at least one insulating element is facilitated. Both the handling during installation of the elements and during transport of the elements from the production and/or storage site to the installation site is facilitated. In particular, the transportation may be facilitated, since in many cases the use of only one insulating element that is sufficiently effective for biogas production would imply that the insulating element is too large to be transported on an ordinary truck.

In some embodiments, the at least one insulating element is 2-8 insulating elements, such as 4-6 insulating elements. These figures have been identified as suitable when considering such factors as handling, transportation, installation and production of the insulating elements.

Handling, transportation and installation may be difficult if too large insulating elements are to be used. On the other hand handling, transportation and installation may be extensive if too many insulating elements are to be used. Production is rendered difficult and thus expensive if too large elements are to be used at the same time as production costs are increased, e.g. due to the use of considerable numbers of incorporated components such as guiding means, if too many insulating elements are used.

Each insulating element of the present invention may be in the form of a circular sector, see figures 2 and 6. Then, the insulating elements of a digester according to the present invention typically together form a complete circle. A person skilled in the art readily realizes that insulating elements also may be in other geometrical forms, such as triangular, rectangular etc. The geometrical form achieved by combining insulating elements may also be in other forms, such as triangular, rectangular, pentagonal, hexagonal etc. For example, six triangular insulating elements may together form a hexagonal area of insulating elements.

In some embodiments, the at least one insulating element covers the central parts of the bottom of the dung pit. Typically, a single insulating element or a combination of two or more insulating elements covers the central parts of the bottom of a dung pit. In case of insulating elements in the form of circular sectors, the complete circle of insulating elements may cover the central parts of the bottom of a dung pit and may be positioned in the center of the dung pit. In most cases a dung pit is essentially circular.

Insulating elements in the form of circular sectors together forming a complete circle that is positioned in the center of a circular dung pit implies that the secondary region mentioned above is in the form of a cylinder with equally thick insulating "walls" of biological material.

In some embodiments of the present invention, the at least one insulating element comprises concrete, light concrete and/or fiber cement. In some embodiments, the at least one insulating element comprises concrete. Concrete and in particular light concrete is in itself insulating. However, in some embodiments the at least one insulating element comprises a specific insulating material. In some embodiments, the at least one insulating element comprises a main material and a specific insulating material. In some embodiments, the main material is concrete, light concrete and/or fiber cement. The specific insulating material may be an inert and/or rigid insulating material. By an inert insulating material is meant an insulating material that not is admixed with the main material of the insulating element, such as concrete. By a rigid insulating material is meant an insulating material that preserves its form when moderate forces are applied to the rigid insulating material. By an inert and/or rigid insulating material is meant an insulating material that may be enclosed in the main material of the insulating element, such as concrete, by moulding. The specific insulating material may be polystyrene and/or rigid polyurethane foam. In some embodiments, the at least one insulating element comprises concrete, light concrete and/or fiber cement and polystyrene and/or rigid polyurethane foam. In some

embodiments, the at least one insulating element comprises concrete and polystyrene. The at least one insulating element may comprise a core of an inert and/or rigid insulating material embedded in a main material of the insulating element. The at least one insulating element may comprise a core of polystyrene embedded in concrete. The at least one insulating element may comprise an upper layer (16), an intermediate layer (17) and a lower layer (18), see figure 5 for one such embodiment. In some embodiments, the upper layer (16) is made of concrete, light concrete and/or fiber cement, the intermediate layer (17) is made of an inert and/or rigid insulating material and the lower layer (18) is made of concrete, light concrete and/or fiber cement. In some embodiments, the upper layer (16) is made of concrete, the

intermediate layer (17) is made of polystyrene and the lower layer (18) is made of concrete.

In some embodiments of the present invention, the at least two insulating elements (4, 4') comprise guiding means (8, 8') for guiding one insulating element (4) in relation to one other insulating element (4') when placing the insulating elements on the bottom of the dung pit (2) containing biological material, see figures 1 and 2. The guiding means (8, 8') facilitate the installation of insulating elements (4, 4') in a dung pit (2) containing biological material, since when a first insulating element (4') is immersed into and placed on the bottom of the dung pit, it will in general be located below the surface of the biological material and thus it is not visible. The guiding means (8, 8') enables that a second insulating element (4) is positioned in relation to the first insulating element (4'). It is of course advantageous that the insulating elements are positioned next to each other such that they together form a large insulating area on the bottom of the dung pit. The guiding means are advantageous in creating the large insulating area by placing at least two insulating elements next to each other.

In some embodiments, the guiding means (8, 8') comprises at least one first guiding means (9, 9') located at at least one insulating element (4) and at least one second guiding means (10, 10') located at at least one other insulating element (4'), wherein said at least one first guiding means (9, 9') mates with said at least one second guiding means (10, 10'). One such embodiment is shown in figures 1 -3, 5 and 6. The two mating guiding means facilitate the positioning of one insulating element next to one other insulating element in a dung pit containing biological material.

In some embodiments, the at least one first guiding means (9, 9') is at least one guiding sleeve and the at least one second guiding means (10, 10') is at least one guiding rod, wherein said at least one guiding sleeve of one insulating element (4) runs down along said at least one guiding rod of one other insulating element (4') when said one insulating element (4) is placed on the bottom of the dung pit already comprising said one other insulating element (4'). The guiding rods are preferably sufficiently long to at least partly be seen above the surface of the biological material in the dung pit when a first insulating element has been immersed into a dung pit and been placed on the bottom of the dung pit. Such embodiments are shown in figures 1 -3, 5 and 6. When a second insulating element is to be immersed into the dung pit and placed next to the first insulating element on the bottom of the dung pit, the guiding sleeves of the second insulating element are positioned above the rods of the first insulating element such that the guiding sleeves of the second insulating element will run down along the guiding rods of the first insulating element when the second insulating element is lowered into the dung pit. Thus, the second insulating element will be positioned next to the first insulating element.

In some embodiments comprising insulating elements in the form of circular sectors, the first guiding means is located at one of the radii of the circular sector of the insulating elements and the second guiding means is located at the other radius of the circular sector of the insulating elements, see figures 2 and 6. The first guiding means of the insulating element to be immersed into and placed on the bottom of the dung pit first may be absent or removed before immersion in order to not be an obstacle for the insulating element to be immersed into and placed on the bottom of the dung pit last. Alternatively, the insulating element to be immersed into and placed on the bottom of the dung pit first may comprise second guiding elements on both the radii of its circular sector and consequently the insulating element to be immersed into and placed on the bottom of the dung pit last may comprise first guiding means on both the radii of its circular sector. The same may be applied to insulating elements of other shapes, where each of two sides of each insulating element faces one side of one other insulating element, which will be possible with insulating elements in the form of e.g. triangles and rectangles.

In some embodiments, the at least one insulating element is a first insulating element, at least one intermediate insulating element and a last insulating element. The at least one intermediate insulating element may be one or more insulating elements, typically the at least one intermediate insulating element is 2-6 insulating elements.

In some embodiments, each of the first, intermediate and last insulating elements are in the form of circular sectors. The first insulating element may comprise second guiding elements on both the radii of its circular sector. The last insulating element may comprise first guiding elements on both the radii of its circular sector. The at least one intermediate insulating element may comprise first guiding means on one of the radii of its circular sector and second guiding means on the other radius of its circular sector. If the at least one insulating element is a first insulating element, two intermediate insulating elements and a last insulating element according to the above, i.e. in total four insulating elements, the immersion of these insulating elements into a dung pit and placement of these insulating elements on the bottom of the dung pit may be described as follows: The first insulating element is immersed into the dung put and positioned on the bottom of the dung pit such that the collection of four insulating elements make up a suitable insulating area at the bottom of the dung pit, preferably the central parts of the bottom of the dung pit. One of the intermediate insulating elements is positioned such that the first guiding means of said one intermediate insulating element mates with the second guiding means on one of the radii of the circular sector of the first insulating element when said one intermediate insulating element is lowered into the dung pit. The other intermediate insulating element is positioned such that the first guiding means of said one other intermediate insulating element mates with the second guiding means of said one intermediate insulating element when said one other intermediate insulating element is lowered into the dung pit. The last intermediate insulating element is positioned such that the first guiding means on one of the radii of the circular sector of the last insulating element mates with the second guiding means of said one other intermediate insulating element and the first guiding means on one other of the radii of the circular sector of the last insulating element mates with the second guiding means on one other of the radii of the circular sector of the first insulating element when the last insulating element is lowered into the dung pit.

In some embodiments, the at least one insulating elements comprises at least two first guiding means and at least two second guiding means. In some embodiments, each of the intermediate insulating elements comprises at least two first guiding means and at least two second guiding means. One such embodiment can be seen in figures 5 and 6.

In some embodiments, on each radius of the circular sector of the at least one insulating element having said at least one guiding rod, one guiding rod is locate at the arc of the circular sector, i.e. at one of the outer corners of the circular sector. One such embodiment can be seen in figures 5 and 6.

In some embodiments, the at least one insulating element (4, 4') is movable. It is apparent from above that the at least one insulating element is movable. For example, the description of immersion of one or several insulating elements into a dung pit and placement of said element(s) illustrates the movability of the insulating element(s). Also the advantages of movable insulating element(s) are apparent from the above description. For example, it is possible to immerse the insulating element(s) into a dung pit and place the insulating element(s) on the bottom of the dung pit, in particular it is important to be able to immerse the insulating elements into an existing dung pit and to be able to place the insulating elements on the bottom of the existing dung pit. The movability of the elements also implies that the at least one insulating element is removable from the dung pit.

The roof covering the dung pit will secure that biogas produced in the dung pit not will escape but may be collected. Besides, the roof contributes to the anaerobic environment in the digester.

In some embodiments of the present invention, the roof covering the dung pit comprises a membrane roof. In some embodiments, the roof covering the dung pit is a membrane roof.

In some embodiments of the present invention, the roof (3) covering the dung pit comprises a membrane roof (12) located above the at least one insulating element and a rigid roof (13) covering the parts of the dung pit not covered by the membrane roof. One such embodiment is shown in figures 3 and 4. The membrane roof located above the at least one insulating element may be located above the at least one insulating element and cover essentially the same area as the at least one insulating element. The membrane roof located above the at least one insulating element may be located above the primary region mentioned above. The rigid roof covering the parts of the dung pit not covered by the membrane roof may be a rigid roof made of concrete, steel, such as stainless steel, and/or plastics, such as polyethene and/or polypropene.

In some embodiments, the membrane roof is a double membrane roof. Such roofs are available from e.g. MT-Energie GmbH as air-supported foils or Tragluftfolienabdeckung.

In some embodiments, the roof covering the dung pit is affixed to at least some of the guiding rods of the insulating elements. In some embodiments, the membrane roof is affixed to at least some of the guiding rods. In some embodiments, the rigid roof is affixed to at least some of the guiding rods. In some embodiments, both the membrane roof and the rigid roof are affixed to at least some of the guiding rods.

In particular, the roof covering the dung pit may be affixed to the guiding rod located closest to the arc of the circular sector of each insulating element having at least one guiding rod. One such embodiment is shown in figure 3.

In some embodiments, the roof (3) covering the dung pit comprises a safety roof (19) comprising a safety roof body (20), a U-shaped flume (21 ) and liquid (22) contained in the U-shaped flume, which safety roof body (20) is arranged in the U-shaped flume (21 ) containing liquid (22). One such embodiment is shown in figures 7 and 8. The safety roof is under normal conditions, e.g. normal pressure in the covered dung pit, gas tight due to that the liquid serves as a seal. Besides, the safety roof enables evacuation of gas when the pressure in the covered dung pit reaches a certain level, since the safety roof body will lift when the pressure in the covered dung pit reaches a certain level, which is dependent on the weight of the safety roof body. In addition, the safety roof enables measurement of the pressure in the covered dung pit, since the safety roof body will rise gradually as the pressure in the covered dung pit increases. To facilitate the measurement of the pressure in the covered dung pit, the safety roof may have scale marks, e.g. in mm, indicating the level of the safety roof body. The scale marks may be located on the safety roof body, preferably on the parts of the safety roof body that may be covered by liquid, and thus the liquid level (23) on the safety roof body may be read and thus the level of the safety roof body is known. The level of the safety roof body may be used to determine the pressure in the covered dung pit, e.g. by calculating the pressure by means of the weight of the safety roof body or taking reference measurements of the pressure by a pressure gauge. Alternatively, the relative pressure variations may be registered by registration of the relative variations of the level of the safety roof body readable on the scale marks. In some embodiments of the present invention, the roof covering the dung pit comprises a safety roof located above the at least one insulating element and a rigid roof covering the parts of the dung pit not covered by the membrane roof. The rigid roof may be as described above. The safety roof located above the at least one insulating element may be located above the at least one insulating element and cover essentially the same area as the at least one insulating element. The safety roof located above the at least one insulating element may be located above the primary region mentioned above. In some embodiments, the U-shaped flume is circular. In some embodiments, the U-shaped flume is affixed to the at least some of the guiding rods. In some embodiments, the U-shaped flume is affixed to the guiding rod located closest to the arc of the circular sector of each insulating element having at least one guiding rod.

In some embodiments of the present invention, the digester comprises collection means for collection of produced biogas. The collection means may be located in the upper part of the covered dung pit. The collection means may be located at the roof covering the dung pit. The collection means may be located at the membrane roof. In case of a double membrane roof, the collection means may be collected at the inner membrane of the two membranes of the double membrane roof.

Conventional dung pits without cover will when the ambient

temperature is high enough, e.g. during the summer, generate a significant amount of methane, which then is released into the ambient air. The release of methane into the ambient air is a potential environmental load due to that methane is a greenhouse gas. By covering the dung pit with a roof and collecting the produced biogas according to the invention, the otherwise spontaneously produced biogas is not released into the ambient air. Instead, according to the present invention, optimized conditions for production of biogas is created all year around and due to the covering, the produced methane containing biogas is captured and may be used as an energy source.

In some embodiments of the present invention, the produced biogas has a high content of methane. In some embodiments, the produced biogas has a methane content of at least 50 %, such as at least 60 %, such as at least 65 %, such as at least 70 %.

The biological material contained in the dung pit may be any

degradable biological material. The biological material may comprise biological material derived from plants and/or animals. The biological material may comprise silage, green material, glycerol, fat, oil, carbohydrate

containing material such as starch, sugar, cellulose and hemicellulose, protein containing material, manure, compost or plant manure, or any mixture thereof. The manure may be animal manure, such as manure from cattle. The biological material may typically be farmyard manure. The biological material may comprise liquid manure, semi-liquid manure, slurry manure or any mixture thereof. The biological material may also comprise solid manure in combination with less viscous biological material and/or a liquid such as water. The biological material may contain hay. Hay is frequently contained in farmyard manure. Wood-chips and/or planing shavings may also be admixed with the other biological material contained in the dung pit.

The biological material contained in the dung pit may comprise biological material that is difficult or impossible to degrade, such as wood- chips and/or planing shavings. Despite the possible presence of biological material that is difficult to degrade in the biological material contained in the dung pit, the degradation of the biological material contained in the dung pit is efficient and the methane content in the produced biogas is high.

The biological material in the dung pit will occasionally be replaced because of addition of new biological material, such as addition of manure from the cattle at the farm, or removal of biological material e.g. for spreading as fertilizer at the fields.

The present invention may increase the ability to degrade biological material that is difficult to degrade, at least compared with conventional digestion tanks of biogas plants, since the mean retention time in a digester according to the present invention will be longer than in a conventional digestion tank. In addition, in a conventional digestion tank, the biological material is cooled directly after leaving the digestion tank in order to prevent further degradation of biological material and thereby reduce the dispersion of methane to the ambient air.

In some embodiments of the present invention, the digester (1 ) comprises at least one recirculation means (1 1 , 1 1 ') partly recycling produced biogas to the biological material contained in the dung pit (2). By recirculation of produced biogas a partial mixing of biological material in the region of where the recycled biogas is supplied to the biological material is achieved. Thus, the degradation is increased and the amount of produced methane is increased.

In some embodiments, the at least one recirculation means comprises a plurality of openings through which recycled produced biogas is supplied to the biological material contained in the dung pit. In some embodiments, the at least one recirculation means comprises a mesh through which recycled produced biogas is supplied to the biological material contained in the dung pit. The use of a mesh through which recycled biogas is supplied to the biological material implies that the area to which recycled biogas is supplied may be substantial. The mesh is reliable, since the use of a mesh secures the supply of recycled biogas since if one or some of the openings in the mesh are clogged by biological material, recycled biogas may still be supplied through several other openings in the mesh. In addition, the mesh is self- cleaning in respect of the biological material, since if some of the openings of the net are clogged, the pressure will increase and when the pressure is sufficiently high, the clogging material will be blown away from the openings.

In some embodiments, the at least one recirculation means supplies recycled produced biogas to the biological material located above the at least one insulating element. By supply of recycled biogas to the biological material located above the at least one insulating element, the biological material in the primary region defined above is mixed and thus the degradation of biological material in this region is increased and also the amount of produced methane is increased.

In some embodiments, the at least one recirculation means is located above the at least one insulating element or is integrated in the upper surface of the at least one insulating element. By placing the recirculation means above the at least one insulating element the specific position of the recirculation means may be varied in order to achieve the desired mixing for each operating condition. In addition, the number of recirculation means may be varied in order to achieve the desired mixing for each operating condition. The number of recirculation means and their specific position are readily determined by a person skilled in the art. By integrating the recirculation means in the upper surface of the at least one insulating element, the installation of the recirculation means are facilitated since the recirculation means are automatically immersed into and positioned in the dung pit. This is in particular advantageous considering that the recirculation means are located below the surface of the biological material contained in the dung pit. Preferably, the at least one recirculation means is integrated in the upper surface of said at least one insulating element.

In some embodiments, at least one recycling means is located above or is integrated in the upper surface of each of the insulating elements. The recycling means of each of the insulating elements, i.e. the recycling means located above or integrated in each insulating element, may be operated sequentially. Recycled biogas may be supplied by the recycling means of one insulating element at a time. Recycled biogas may be supplied by the recycling means of one other insulating element directly when the supply by the recycling means of said one insulating element has terminated or recycled biogas may be supplied by the recycling means of one other insulating element at a time separated from when the supply by the recycling means of said one insulating element has terminated. The time separated from when the supply by the recycling means of said one insulating element has terminated may be after when the supply by the recycling means of said one insulating element has terminated. Alternatively, the recycling of biogas to the recycling means of different insulating elements may overlap, e.g. the supply by recycling means of one other insulating element may start before the supply by the recycling means of one insulating element has terminated. The supply of biogas by recycling means of different insulating elements may be controlled, e.g. by a PLC. In some embodiments, several or all recycling means are operated simultaneously.

In some embodiments, only one recycling means is located above or is integrated in the upper surface of each of the insulating elements.

The digester is self-cleaning in respect of material that is difficult or impossible to degrade, such as planing shavings, since such material will be transported over the edge of the insulating elements due to the movements in the biological material located above the insulating elements and thereby such material will accumulate in the secondary region mentioned above.

In some embodiments of the present invention, the at least one insulating element comprises heating means. In some embodiments, the heating means is at least one pipe carrying a hot medium. The hot medium may be hot water. The at least one pipe may be incorporated in the insulating element. The at least one pipe may be embedded in the insulating element.

In some embodiments, the at least one insulating element is made of concrete. The at least one insulating element made of concrete enables easy incorporation of recycling means and/or heating means into the insulating element. The incorporation of recycling means and/or heating means into the insulating element may be enabled by embedding recycling means and/or heating means when moulding the at least one insulating element, in particular when moulding the at least one insulating element made of concrete. Also, the placement means, such as the projections, on the underside of the at least one insulating element are easily achieved when moulding the at least one insulating element e.g. of concrete.

As mentioned above, the present invention relates to a method for production of biogas in a dung pit containing biological material, wherein said dung pit comprises a bottom and is provided with a roof covering the dung pit and at least one insulating element, which is placed on the bottom of the dung pit and at least partly insulates the bottom of the dung pit, wherein the method comprises collecting produced biogas at the roof covering the dung pit.

In some embodiments of the method for production of biogas in a dung pit containing biological material, the dung pit is an existing dung pit. The advantages of an existing dung pit described above apply also to the method for production of biogas in a dung pit containing biological material according to the present invention.

In some embodiments of the method for production of biogas in a dung pit containing biological material, the method further comprises preheating initiating biological material, and supplying said preheated initiating biological material to at least one region above said at least one insulating element. By preheating initiating biological material and supplying the preheated initiating material to the biological material contained in the dung pit, enhanced degradation and thus also enhanced production of methane is achieved. The initiating biological material may be as defined above. The preheating of the biological material as well as the supply of preheated initiating biological material may be as described above. Thus, the corresponding advantages apply.

In some embodiments of the method for production of biogas in a dung pit containing biological material, the method further comprises recycling a portion of the produced biogas to the biological material contained in the dung pit. By recirculation of produced biogas a partial mixing of biological material in the region of where the recycled biogas is supplied to the biological material is achieved. Thus, the degradation is increased and the amount of produced methane is increased. The recirculation of produced biogas may be achieved using recirculation means as defined above and thus the

corresponding advantages apply.

In some embodiments of the method for production of biogas in a dung pit containing biological material, the at least one insulating element is as defined above.

As mentioned above, the present invention also relates to a use of an insulating element as defined above for production of biogas in a dung pit.

In some embodiments of the use of an insulating element as defined above for production of biogas in a dung pit, the dung pit is an existing dung pit. The advantages of an existing dung pit described above apply also to the use of an insulating element for production of biogas in an existing dung pit according to the present invention. As mentioned above, the present invention also relates to a method for converting a dung pit containing biological material to a digester for

production of biogas, wherein the dung pit comprises a bottom, and wherein the method comprises immersion of at least one insulating element as defined above into the dung pit, placement of said at least one insulating element on the bottom of the dung pit and covering of the dung pit with a roof.

In some embodiments of the method for converting a dung pit containing biological material to a digester for production of biogas, the dung pit is an existing dung pit. The advantages of an existing dung pit described above apply also to the method for converting a dung pit containing biological material to an existing dung pit according to the present invention.

In some embodiments of the method for converting a dung pit containing biological material to a digester for production of biogas, the roof may be the roof covering the dung pit as defined above and thus the corresponding advantages apply.

In some embodiments of the method for converting a dung pit containing biological material to a digester for production of biogas, the method further comprises providing the dung pit with recirculation means as defined above and thus the corresponding advantages apply.

One specific embodiment of the present invention relates to a digester

(1 ) for production of biogas in an existing dung pit (2) containing biological material, wherein the digester (1 ) comprises an existing dung pit (2) containing biological material, which dung pit (2) comprises a bottom, wherein the digester (1 ) further comprises a roof (3) covering the dung pit (2), and at least two insulating elements (4, 4'), which are placed on the bottom (5) of the dung pit (2) and at least partly insulate the bottom (5) of the dung pit (2),

wherein said at least two insulating elements (4, 4') are movable, wherein each of the at least two insulating elements (4, 4') comprise placement means (6) for stable placement of the at least two insulating elements (4, 4') on the bottom (5) of the dung pit (2) containing biological material, wherein the placement means (6) comprises projections (7) on the underside of the at least two insulating elements (4, 4'), wherein said at least two insulating elements (4, 4') comprise guiding means (8, 8') for guiding one insulating element (4) in relation to one other insulating element (4') when placing the insulating elements on the bottom of the dung pit (2) containing biological material, wherein said guiding means (8, 8') comprises at least one first guiding means (9, 9') located at at least one insulating element (4) and at least one second guiding means (10, 10') located at at least one other insulating element (4'), wherein said at least one first guiding means (9, 9') mates with said at least one second guiding means (10, 10'), wherein said first at least one guiding means (9, 9') is at least one guiding sleeve and said at least one second guiding means (10, 10') is at least one guiding rod, wherein said at least one guiding sleeve of one insulating element (4) runs down along said at least one guiding rod of one other insulating element (4') when said one insulating element (4) is placed on the bottom of the dung pit (2) already comprising said one other insulating element (4'),

wherein the digester (1 ) comprises at least one recirculation means (1 1 , 1 1 ') partly recycling produced biogas to the biological material contained in the dung pit (2), wherein the at least one recirculation means (1 1 , 1 1 ') comprises a mesh through which recycled produced biogas is supplied to the biological material contained in the dung pit (2), wherein the at least one recirculation means (1 1 , 1 1 ') supplies recycled produced biogas to the biological material located above said at least one insulating element (4, 4'), wherein the at least one recirculation means (1 1 , 1 1 ') is located above said at least one insulating element (4, 4') or is integrated in the upper surface of said at least one insulating element (4, 4'), and

wherein said roof (3) covering the dung pit (2) comprises a membrane roof (12) located above said at least one insulating element (4, 4') and a rigid roof (13) covering the parts of the dung pit (2) not covered by the membrane roof (12).

One specific embodiment of the present invention relates to a method for production of biogas in an existing dung pit containing biological material, wherein said dung pit comprises a bottom and is provided with a roof covering the dung pit and at least one insulating element, which is placed on the bottom of the dung pit and at least partly insulates the bottom of the dung pit, wherein the method comprises preheating initiating biological material, supplying said preheated initiating biological material to at least one region above said at least one insulating element, collecting produced biogas at the roof covering the dung pit and recycling a portion of the produced biogas to the biological material contained in the dung pit.

One specific embodiment of the present invention relates to a method for production of biogas in an existing dung pit containing biological material, wherein said dung pit is provided with a roof covering the dung pit and at least one insulating element at least partly insulating the bottom of the dung pit, wherein the method comprises preheating initiating biological material, supplying said preheated initiating biological material to at least one region above said at least one insulating element, collecting produced biogas at the roof covering the dung pit and recycling a portion of the produced biogas to the biological material contained in the dung pit, wherein the at least one insulating element is as defined in the specific embodiment relating to a digester disclosed above.

One specific embodiment of the present invention relates to an insulating element as defined in the specific embodiment relating to a digester disclosed above.

One specific embodiment of the present invention relates to a use of an insulating element as defined in the specific embodiment relating to a digester disclosed above for production of biogas in an existing dung pit.

One specific embodiment of the present invention relates to method for converting an existing dung pit containing biological material to a digester for production of biogas, wherein the dung pit comprises a bottom, and wherein the method comprises immersion of at least one insulating element as defined in the specific embodiment relating to a digester disclosed above into the dung pit, placement of said at least one insulating element on the bottom of the dung pit, and covering of the dung pit with a roof.

Examples

In the following examples 1 and 2 and reference examples 1 and 2 a circular reactor having a diameter of 2500 mm and a height of 700 mm is used. The reactor is equipped with a dome-shaped roof having a diameter of 800 mm and located above the center of the reactor. The reactor is also equipped with a flat roof between the dome-shaped roof and the jacket of the reactor, i.e. covering the remaining parts of the reactor. The reactor is also equipped with a supply tube with a diameter of 51 mm for supplying substrate to the reactor. The outlet of the supply tube is located in the center of the reactor at a level of 200 mm above the bottom of the reactor. Substrate is fed into the reactor until the hydraulic surface of the substrate is about 600 mm above the bottom of the reactor.

The substrate used in examples 1 and 2 and reference examples 1 -3 is manure slurry from cows at a diary farm. Substrate is also stored in a cooled and stirred intermediate bulk container. The temperature in the intermediate bulk container is about 5°. Possibly produced biogas is collected and the composition of the produced biogas is analyzed.

The examples 1 and 2 and the reference examples 1 -3 are carried out during the autumn, winter and/or spring in order to perform the evaluation during the colder part of the year when degradation of biological material is more intricate.

Example 1

The reactor is equipped with four insulating elements, each insulating element being in the form of a circular sector. The four insulating elements together form a circle having a diameter of 800 mm, which is located in the center of the reactor and at the bottom of the reactor. The insulating elements are made of an upper layer of Minerit ^® (fiber cement) and a lower layer of polystyrene. The height of each insulating element is 1 10 mm. The height of the Minerit ^® layer is 10 mm and the height of the polystyrene layer is 100 mm.

Manure slurry is transported from the intermediate bulk container to a supply vessel, where the slurry is preheated and thereafter supplied to the reactor by means of the supply tube once a day during a period of 25 days. Example 2

The reactor is equipped with insulating elements as in example 1 . In addition, located on top of each insulating element is a steel box having 60 holes in its upper surface, wherein each hole has a diameter of 3 mm. A flexible tube is connected to each steel box in order to enable supply of produced biogas.

As in example 1 , manure slurry is transported from the intermediate bulk container to a supply vessel, where the slurry is preheated and thereafter supplied to the reactor by means of the supply tube once a day during a period of 25 days. A portion of the produced biogas is recycled to the substrate in the reactor by means of the steel box having holes.

Reference example 1

In this reference example, there are no insulating elements in the reactor and no biogas is recycled.

Manure slurry is supplied directly from the intermediate bulk container to the reactor by means of the supply tube once a day during a period of 25 days.

Reference example 2

In this reference example, there are no insulating elements in the reactor and no biogas is recycled.

As in example 2, manure slurry is transported from the intermediate bulk container to a supply vessel, where the slurry is preheated and thereafter supplied to the reactor by means of the supply tube once a day during a period of 25 days.

Reference example 3

A conventional cylindrical reactor being insulated is used. The conventional reactor is equipped with an agitator. Initially, substrate is filled into the reactor to about 65-70 % of the volume of the reactor. The manure slurry is agitated by means of the agitator and heat is supplied in order to keep a temperature of about 37°C in the reactor. Once a day, manure slurry corresponding to 1/25 of the volume of the reactor is supplied from the intermediate bulk container. Once the reactor is full, the supply of manure slurry will result in that a corresponding amount is simultaneously emptied by means of an over-flow outlet.

The methane production in example 1 is higher than the methane production in reference example 2. Thus, it can be concluded that the presence of the insulating elements increases the methane production compared with the methane production in a reactor lacking insulating elements. The methane production in example 1 is also higher than the methane production in reference example 1 .

The methane production in example 2 is higher than the methane production in example 1 . Thus, it can be concluded that the methane production further increases when recycling portions of the produced biogas to the substrate in the reactor.

The methane production in example 2 is somewhat lower than the methane production in reference example 3. When comparing example 2 and reference example 3, it can be concluded that the methane production in a reactor having insulating elements, which reactor is mixed by recycled biogas, approaches the methane production in a conventional digester operated at optimal conditions.

The methane production in reference example 2 is higher than the methane production in reference example 1 , which demonstrates that preheating the initiating material enhances the methane production.