BIOGAS

FIELD OF THE INVENTION

This invention relates to an anaerobic digester for digesting organic matter and producing biogas. This invention also relates to an apparatus for digesting organic matter and producing biogas and a method for digesting organic matter and producing biogas. Further the invention also extends to a feed/discharge arrangement for a digester, a drum for a biogas digester and a plant installation for the production of biogas

This invention relates particularly but not exclusively to an anaerobic digester for digesting organic matter that is animal waste obtained from livestock on farms such as cows and producing biogas from this waste for use as a distributed energy source. It will therefore be convenient to hereinafter describe the invention with reference to this example application. However at the same time it must be recognized that the invention is capable of broader application. For example the invention could also be used for digesting other biomass such as green waste or sewage.

DEFINITIONS

In the specification the term "comprising " shall be understood to have a broad meaning similar to the term "including" and will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the term "comprising" such as "comprise" and "comprises".

Further in this specification the term 'drum' shall be understood to have a broad meaning and shall include all containers that can be rotated about a longitudinal axis. It shall include barrels and cylindrical containers within its scope but shall not be limited to these containers.

In the specification the term "blade or blades" shall be understood to have a broad meaning and shall include all manner and shape of fixed mixing formations on the internal surface of a rotating drum. It shall be understood to include flights on the internal surface of the drum.

BACKGROUND TO THE INVENTION

Biogas typically refers to the gas produced by the biological breakdown of organic matter in the absence of oxygen. This is called the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material and energy crops. Biogas is typically composed of about 55 to 75 percent methane. Most of the rest of the biogas is carbon dioxide, with a small amount of hydrogen sulfide and other trace gases. It will be appreciated that the precise composition of any amount of biogas will depend on the organic matter that is treated and the precise conditions within that biogas reactor. Some advantages of biogas or biogas- based energy generation include the following:

• Biogas has a high Calorific value and is an excellent fuel for heating and generating electricity.

• Biogas is a renewable and sustainable energy source.

· Biogas provides for distributed energy generation, thereby providing much- needed energy for remote locations and villages;

• The production of biogas also produces a plant nutrient or fertilizer that is returned to the soil.

• The use of biogas as a fuel reduces greenhouse gas emissions. · The production of biogas removes organic waste from the environment that would otherwise over time release large amounts of nitrous oxide and methane into the atmosphere.

• A revenue stream can be created from methane production and greenhouse gas emissions offsets.

The digestion process of organic matter occurs in four basic stages (hydrolysis, acidogenesis, acetogenesis, and methanogenesis). It is the final stage, methanogenesis that breaks down the intermediate compounds to produce methane. This is accomplished using mesophilic or thermophilic methanogens as is known in the art.

Currently, although there are substantially different types of Biogas Digesters, their working principle is very much the same. One example type of prior art biogas digester is shown in Figure 1 in which raw organic matter enters the digester tank (fermentation chamber) through an inlet pipe either directly or after a mixing pit. Biogas is collected above the slurry and leaves the tank through a gas pipe in the top of the cover. In the case of a fixed dome type of digester as shown in Figure 1 the top is made of concrete or bricks and the rest of the digester is located below the ground level. The floating cover type digester has a steel cover floating on the slurry, which is above ground, while the rest of the digester is located below ground level. The digested slurry leaves the digester through an outlet pipe and is collected in an outlet pit or hydraulic chamber. One drawback of this type of digester relates to the material handling of the matter and the handling of the digested slurry or digestate which is difficult. The raw organic matter has poor flow characteristics which make it difficult to load raw organic matter into the reactor and also to unload processed slurry from the digester after treatment has been completed. A further shortcoming is that it is difficult to access an interior space of the digester when it is received in the ground.

One practice for dealing with these difficulties is to add water or other liquid to improve the flowability. Consequently, this dramatically increases the volume of solids and liquid within the digester without a concomitant increase in the raw organic matter that generates biogas. As a result the actual biomass is only about 20-30% of the total volume and the efficiency of the digester is relatively low. The construction, operational and maintenance costs of producing a unit volume of biogas is thus increased.

More advanced anaerobic digesters may include a heating system to maintain a preferred digester temperature of about 34 to 39 degrees Celsius at which the bacteria flourish and this produces biogas more efficiently than non-heated digesters. However the cost of heating a digester of increased size is proportionately greater.

Further, stirring of the contents of a digester is desirable to prevent settling of solids and crust formation, as well as to ensure an even temperature within the digester. Typically, mixing is carried out by mechanical stirrers or by biogas recirculation. However in a larger digester with a larger volume of slurry, the cost of generating the heat to maintain suitable temperatures for digestion is very high. This is particularly the case in cold climates and in the winter season.

Another shortcoming of adding a large volume of water to improve flowability is that the increased volume of slurry within the digester is more difficult to stir. In fact when the volume is large it is impractical to effect full scale stirring of the slurry.

Consequently many prior art digesters do not have an arrangement for agitating the slurry or an arrangement for maintaining an increased temperature in the digester that is favourable to chemical reactions occurring within the digester. This is a definite shortcoming in prior art digesters.

Further digesters are prone to accumulating sand, stones or other inert materials therein over time. Often it is difficult to remove these materials from the digester particularly when it is received within the ground. Over a period of time these materials will considerably decrease the volume of the digester and hence reduce its productivity because there is a correspondingly reduced space within which to receive raw organic matter for digestion. Further a quantity of non-degradable organics within a digester also increases over time and these form either a sludge that sinks to the bottom of the digester or a crust at the top of the slurry. Bother of these outcomes reduce the effective volume of the digester which can receive organic matter. A crust at the top of the slurry can also block the gas flow to the gas storage which is very problematic.

SUMMARY OF THE INVENTION

According to one aspect of this invention there is provided an anaerobic digester for digesting organic matter and producing biogas, the digester including:

a support;

a drum having one end and an opposed end that is mounted for rotation on the support, the drum having an internal surface and defining an interior space that is closed off from an external environment outside of the drum whereby to resist oxygen from entering the drum and to facilitate anaerobic digestion of organic matter within the drum, the drum including at least one fixed internal matter displacing formation mounted on the internal drum surface of the drum and which projects from the internal drum surface into the interior space, each internal matter displacing formation acting to mix organic matter within the drum when the drum rotates; and

a feed/discharge arrangement on the drum towards said one end thereof, through which raw organic matter to be treated can be introduced into the interior space and through which treated solid or liquid organic matter can be discharged from the interior space.

The anaerobic digester may include a biogas off-take arrangement for removing biogas generated by anaerobic digestion from the drum.

Each internal matter displacing formation may be in the form of a mixing blade that extends in a curved manner along the internal drum surface and which extends generally in a direction of length of the drum.

The anaerobic digester may have two said mixing blades, and the two mixing blades may be arranged so that they are substantially diametrically opposed to each other on the internal surface of the drum, and each mixing blade may extend in a generally helical configuration in the direction of length of the drum.

Each mixing blade may also extend generally from one end of the drum towards the other end of the drum. Thus the mixing blades are fixed to the drum and only interact with the organic matter to cause it to be displaced when the drum rotates. That is they do not move independently of the drum. Each mixing blade may have a width of 30mm to 120mm, e.g. about 60 to 90mm, and may extend substantially orthogonally away from the internal surface of the drum. Further each mixing blade may have one edge that is attached to the internal drum surface and another edge that is free and is spaced away from the internal drum surface.

The drum may have an axis of rotation and the drum may be orientated such that its axis of rotation extends at an angle of 0-30 degrees to a horizontal orientation.

The drum may be orientated such that its axis of rotation extends at an angle of 5- 20 degrees to a horizontal orientation. In particular the axis of rotation of the drum may extend at an angle of 10-17 degrees to a horizontal orientation, e.g. at about 13.5 degrees to a horizontal orientation. This orientation assists in mixing organic matter within the drum by causing it to flow towards one end under the influence of gravity and the mixing blades lift up the matter and drop it down to generally mix it with other organic matter.

The anaerobic digester may include a drum orientation adjustment arrangement for enabling the orientation of the drum relative to a horizontal orientation to be adjusted to suit the properties of the organic matter being treated within the drum. The drum orientation adjustment arrangement may include a hydraulic ram that acts bears against the support thereby to raise or lower the support towards one end of the drum, thereby to adjust the axis of rotation of the drum to achieve optimum mixing of the organic matter within the drum.

The anaerobic digester may include a drive arrangement on the support for rotating the drum on the support in both directions, e.g. both clockwise and anti-clockwise directions. In particular the drive may be capable of selective operation by an operator whereby to cause the drum to rotate in either of clockwise or anti-clockwise directions for a period of time, and then to stop rotation of the drum.

The drive arrangement may include a motor that drives a shaft which in turn drives a drive flange which is driveably coupled to the other end of the drum whereby to rotate the drum.

The support may include a base for mounting on a support surface, and two longitudinally spaced support formations projecting up from the base on which the drum is supported towards each end thereof.

One said support formation may include two laterally spaced rollers on which the drum rotates. The drum may have a circumferential track ring and the rollers may engage the track ring and roll across the track ring as the drum rotates. The other support formation may include the drive flange which is driveably coupled to said drum.

The feed/discharge arrangement may be mounted on one end of the drum and may project away from said one end of the drum substantially in the direction of the longitudinal axis of the drum.

The feed/discharge arrangement may define an opening through which nongaseous raw matter to be treated can be fed into the digester and through which nongaseous treated matter can be discharged from the digester, and a seal for sealing off the opening from an external environment when raw matter is fed into the drum and when treated matter is withdrawn from the drum.

The seal may be a water seal that is positioned between the interior space of the container and the external environment to resist the passage of air from the outside environment into the drum.

The feed/discharge arrangement may be mounted on one end of the drum and project away from said one end of the drum in a direction of the longitudinal axis thereof. Further the feed/discharge arrangement may have two ends.

The feed/discharge arrangement may include at least one feed/discharge matter displacing formation on the internal drum surface that feeds matter into the interior space when the drum is rotated in one direction and which discharges matter from the interior space when the drum is rotated in the opposite direction. This way operation of the feed/discharge arrangement is effected by rotation of the drum as distinct from rotation of a screw conveyor or an auger which rotates independently of the drum. The helical configuration of the displacing formation like a screw or auger acts to feed raw organic matter into the drum when the drum is rotated in one direction and to discharge organic matter from the drum when it is rotated in said other direction.

Each feed/discharge matter displacing formation may be a screw formation having a helical configuration extending from one end of the feed discharge arrangement to the other end such that rotation of the drum in said one direction causes the helical formation to physically displace organic matter towards the interior space and rotation of the drum in the opposite direction causes the screw formation to physically discharge organic out of the interior space.

The feed/discharge arrangement may include an outer cylindrical formation and an inner cylindrical formation and an annular space defined between the outer and inner cylindrical formations. Each screw formation may extend across the annular space between the outer and inner cylindrical formations and be sealed to the outer and inner cylindrical formations.

The feed/discharge arrangement may comprise two screw formations and each screw formation is connected to a said mixing blade such that it effectively forms an extension of the associated mixing blade within the interior space of the drum.

The biogas off-take arrangement may include a gas collection head and an offtake conduit operatively coupled to the gas collection head for drawing biogas off an operatively upper region of the drum that is above the surface of solid or liquid matter within the drum. The biogas off-take arrangement may further include a non-return valve in line with the off-take conduit for resisting flow of biogas in a reverse direction back into the drum and a manually operable shut-off valve for shutting off the off-take conduit.

The barrel may have a broadly circular cross sectional shape at a plurality of points along its length. Conveniently the barrel may have one frusto-conical section towards one end thereof and another substantially frusto-conical section towards the other end thereof and a substantially cylindrical section intermediate the conical sections.

The digester defined above with its rotating drum and the mixing blades interacting with the feed/discharge arrangement is very efficacious at mixing the organic matter within the drum thereby to promote the anaerobic reactions. In particular the fixed mixing blades which lift and displace organic matter, greatly assist with the process.

The digester defined above is also efficacious at feeding raw matter into, and discharging treated matter from, the interior space of the drum. This confers a great working advantage compared to a known biogas digester which requires a lot of water to be added to the raw matter when it is fed into the digester to get it to move into the body of the digester. However the addition of water is disadvantageous because it reduces the weight of biomass that can be processed and utilises energy.

According to another aspect of this invention there is provided an apparatus for the anaerobic digestion of organic matter and the production of biogas, the apparatus including:

an anaerobic digester including:

a support;

a drum having one end and an opposed end that is mounted for rotation on the support, the drum having an internal surface and defining an interior space that is closed off from an external environment outside of the drum whereby to resist oxygen from entering the drum and to facilitate anaerobic digestion of organic matter within the drum, the drum including at least one fixed internal matter displacing formation mounted on the internal drum surface of the drum and which projects from the internal drum surface into the interior space, each internal matter displacing formation acting to mix organic matter within the drum when the drum rotates; and

a feed/discharge arrangement on the drum towards said one end thereof, through which raw organic matter to be treated can be introduced into the interior space and through which treated solid or liquid organic matter can be discharged from the interior space; and

an outer container defining a container interior space within which the anaerobic digester is received, and which enables the digester to be lifted and moved around.

The outer container may be in the form of an elongate rectangular container having a container interior space that can receive the digester therein with sufficient clearance for the digester to rotate within the outer container.

The outer container may have a heating arrangement for engineering the temperature within the digester so as to promote a favourable rate of anaerobic digestion within the digester. By engineering the temperature within the digester is meant providing an ability to increase the temperature within the drum to a desired minimum temperature or temperature range.

The outer container may include a fluid that is air surrounding the digester and the heating arrangement may heat up the air surrounding the digester and thereby the temperature of the organic matter within the interior space of the drum.

The heating arrangement may include a heat exchanger including heat transfer tubes within the outer container, and a heat transfer fluid can be passed through the heat transfer tubes to heat up the air within the outer container. The heat transfer fluid may include heat transfer gases from a generator or a boiler that is located proximate to the apparatus. For example the heat transfer gases may be exhaust gases from a generator or an engine that operates by combustion of a carbon fuel, such as a biogas engine or generator.

The heating arrangement for heating the interior space of the surround container may include a solar heat exchanger that is exposed to solar rays from the sun which is operatively connected to the heat exchanger within the outer container whereby to use solar energy to heat up the air within the interior space of the outer container. The outer container may have a top wall, a bottom wall and a side wall comprising a plurality of side wall portions, and the outer container may include a thermal insulating material extending across the top wall, bottom wall and the side wall to retard the loss of heat from the container interior space through the walls of the outer container.

The outer container may have attachment formations, e.g. at or towards each end thereof, to enable it to be lifted up by a lifting apparatus and mounted on a transport vehicle and be transported from one location to another location. Thus the apparatus may be mounted on a support surface such as the ground in an outdoor environment and may be transportable from one location to another.

In some forms the surround container may be an intermodal shipping container which enables it to be handled and be shipped from one location to another using standard container shipping and handling equipment, e.g. a twenty foot or a forty foot intermodal shipping container. The advantage of using a standard shipping container is that the transport infrastructure already exists to lift and lower these shipping containers and also transport them from one place to another.

The digester may include a biogas off-take arrangement for removing biogas generated by anaerobic digestion from the drum, and the apparatus may include a biogas storage vessel positioned outside of the outer container that is operatively coupled to the off-take conduit for storing biogas that is generated in the drum of the digester.

The biogas off-take arrangement may include any one or more of the features of the off -take arrangement defined above in the first aspect of the invention.

Conveniently the biogas storage vessel may be in the form of a tank that is mounted on a support surface such as the ground adjacent to the outer container. The biogas storage vessel, e.g. a tank, may be proximate to the digester and the outer container, e.g. outside of and adjacent to the outer container. For example the biogas storage vessel may be mounted on a support surface such as the ground adjacent to the outer container.

According to another aspect of this invention there is provided a feed/discharge arrangement for a digester including a drum defining an interior space that is closed off from air outside the drum and that is mountable on a support such that it can be rotated on the support, the feed/discharge arrangement including an opening through which nongaseous raw matter to be treated can be fed into the digester and through which nongaseous treated matter can be discharged from the digester. The feed/discharge arrangement may include at least one feed/discharge matter displacing formation on the internal drum surface that feeds matter into the interior space when the drum is rotated in one direction and which discharges matter from the interior space when the drum is rotated in the opposite direction.

Each feed/discharge matter displacing formation may be in the form of a screw formation have a helical configuration as defined in the first aspect of the invention above. Rotation of the drum in said one direction causes the screw formation to physically displace organic matter towards the interior space and rotation of the drum in the opposite direction causes the helical formation to physically discharge organic away from the interior space.

The feed/discharge arrangement may further include a seal for sealing the interior space within the barrel off from the outside air while still permitting non-gaseous matter to be introduced into or withdrawn from the digester. The seal may have the same function as a water seal with a water lock interposed between the interior space of the container and the outside air.

The feed/discharge arrangement, and the screw formation, may further include any one or more of the features of the feed/discharge arrangement defined in the first aspect of the invention defined above.

According to yet another aspect of this invention there is defined a drum for an anaerobic digester as defined in the first aspect of the invention.

According to yet another aspect of the invention there is provided a biogas production plant for producing biogas including an apparatus as defined in the second aspect of the invention above.

The apparatus of the plant may include any one or more features of the apparatus defined in the second aspect of the invention above.

According to another aspect of this invention there is provided a method of carrying out anaerobic digestion of organic matter to generate biogas, the method including:

carrying out anaerobic digestion of organic matter within a drum which has an interior space which is sealed off from an external environment, and which has at least one fixed blade mounted on the drum which projects into the interior space of the drum; rotating the drum at least periodically whereby to cause the blade/s to mix the organic matter within the interior space;

removing biogas which is generated in the drum; and discharging treated solid or liquid organic wastes from the drum.

The method may include feeding raw organic waste into the interior space through a feed/discharge arrangement, which displaces organic matter into the interior space when the drum is rotated in one direction, and discharges organic matter from the interior space when the drum is rotated in an opposite direction.

The feed/discharge arrangement may include a helical matter displacing formation and rotation of the drum in one direction may displace organic matter away from the outside environment and towards the interior space, and rotation of the drum in the opposite direction may displace organic matter in a direction away from the interior space of the drum into the outside environment.

The feeding raw organic waste into the interior space through the feed/discharge arrangement may include passing the organic waste through a water seal whereby to resist oxygen from the external environment from entering the interior space.

The method may include treating the organic matter in a digester as defined in the first aspect of the invention above.

The method may include raising the temperature within the digester to a temperature above that of the external environment whereby to increase the rate at which anaerobic digestion occurs within the interior space of the drum.

There may be an outer container having a container space within which the digester is contained and raising the temperature within the digester may include heating the air in the container space.

Heating the air in the container space may comprise heating by means of a heat exchanger using heat obtained from solar energy or heat obtained from combustion of carbon in a generator or an engine.

DETAILED DESCRIPTION OF THE INVENTION

An anaerobic digester for digesting organic matter and an apparatus for digesting organic matter and producing biogas in accordance with this invention may manifest itself in a variety of forms. It will be convenient to hereinafter describe several embodiments of the invention in detail with reference to the accompanying drawings. The purpose of providing this detailed description is to instruct persons having an interest in the subject matter of the invention how to carry the invention into practical effect. However it is to be clearly understood that the specific nature of this detailed description does not supersede the generality of the preceding broad description. In the drawings: Figure 1 is a schematic front view of an anaerobic digester for treating organic waste and producing biogas in accordance with the prior art;

Figure 2 is a front view of part of an anaerobic digester for treating organic waste and producing biogas in accordance with one embodiment of the invention;

Figure 3 is a rear perspective view of the anaerobic digester of Figure 2;

Figure 4 is a rear perspective view showing greater detail of a feed/discharge arrangement for the anaerobic digester of Figure 2;

Figure 5 is a rear perspective view of the apparatus of Figure 2 excluding the feed/discharge arrangement showing how rotation of the anaerobic digester in one direction can be used to introduce material into the digester and rotation of the digester in another direction discharges material from the digester;

Figure 6 is an upper perspective view from one end of an apparatus for producing biogas by the anaerobic digestion of organic matter including a digester like that shown in Figure 2 above;

Figure 7 is an upper perspective view from the other end of the apparatus in

Figure 6;

Figure 8 is a side perspective view of an apparatus for producing biogas in accordance with another embodiment of the invention;

Figures 9A to 9C are a sequence of schematic drawings showing how the apparatus of Figures 6 and 7 can be lifted off the ground at one location, be loaded onto a transport vehicle for shipping containers and transported to another remote location where it can be used to process organic matter at the other location;

Figure 10 is a schematic flow sheet of a plant installation including an apparatus for producing biogas in accordance with the invention;

Figure 1 1 is a schematic drawing showing how three digesters can be arranged adjacent to each other in parallel with each other, and how the drums of the three digesters can be driven by a single drive arrangement; and

Figure 12 is a front view of an anaerobic digester in accordance with another embodiment of the invention.

Figure 1 illustrates a layout of an apparatus for producing biogas in accordance with the prior art. This apparatus has been discussed in some detail in the background to the invention above and will not be described further in this detailed description. In Figures 2 to 5, reference numeral 12 refers generally to an anaerobic digester for treating organic waste and producing biogas in accordance with one embodiment of the invention.

The digester 12 is rotatably mounted on a support 14 about an axis of rotation 41 . The digester 12 in turn comprises a digester container or reactor which is in the form of a drum 16 and a feed/discharge arrangement which is indicated generally by the numeral 40 mounted on the drum 16. The drum 16 has a drum internal surface 19 and defines a drum interior space 20 that is closed off from the external environment including air outside of the drum 16 to facilitate anaerobic digestion of raw matter to be treated within the drum 16. The feed/discharge arrangement 40 is designed to be able to feed raw organic matter into the drum 16 without letting air from the atmosphere into the drum interior space 20. Naturally it is also able to discharge treated organic matter from the interior space 20 of the drum 16, once again also without letting in air from the external environment.

The drum 16 includes one frusto-conical section 22 towards one end 23 thereof and another substantially frusto-conical section 24 towards the other end 25 thereof and a substantially cylindrical section 26 intermediate the frusto-conical sections 22 and 24. The drum 16 has two internal matter displacing formations in the form of fixed mixing blades 32 and 34 on its internal surface 19. Each mixing blade 32, 34 has one longitudinal edge that is attached to the internal surface 19 and projects away therefrom to a free longitudinal edge that is spaced away from the internal surface 19. Each blade 32 and 34 extends in a curved path and has a helical configuration with the helix extending generally in the direction of length of the drum 16. Each mixing blade has a width of about 60 to 90mm, and extends substantially orthogonally away from the internal surface of the drum. The blades 32 and 34 lift, and thereby mix, organic matter within the interior space 20, on rotation of the drum 16.

In the embodiment shown in the drawings the two helical mixing blades 32 and 34 are arranged so that they are substantially diametrically opposed to each other on the internal surface 19. That is they are on opposite sides of the internal surface 19 at each point along the length of the drum 16.

The axis of rotation of the drum indicated by reference numeral 41 is orientated at an angle of about 10-17 degrees to a horizontal orientation, which is indicated in the drawings by the reference numeral 43. In the illustrated embodiment the axis 41 is orientated at an angle of about 13.5 degrees to the horizontal orientation. This orientation assists in mixing organic matter within the drum 16 by causing it to flow towards the end 25 under the influence of gravity. The mixing blades 32, 34 lift up the matter and then drop it down to effect mixing with other matter. As shown in Figure 2 of the drawings, the blades 32, 34 have vent holes 36 that enable gaseous communication between adjacent spaces in the head space above the organic matter when digestion is occurring. This avoids isolate pockets of gas forming in the headspace above the slurry in the drum

The feed/discharge arrangement 40 is mounted on the end 23 of the drum 16 and defines an opening 45 through which non-gaseous raw matter to be treated can be fed into the drum 16 and through which treated organic matter can be removed from the drum 16 after it has been treated. The feed/discharge arrangement 40 projects away from the end 23 of the drum 16 along the longitudinal axis 41 of the drum 16. The feed/discharge arrangement 40 is fixedly mounted on the drum 16 and rotates together with the drum 16 during operation of the digester 12.

As shown in some detail in Figure 4, the feed/discharge arrangement 40 includes two ends and an outer cylindrical formation 35 and an inner cylindrical formation 37 extending between the two ends. An annular space is defined between the outer and inner cylindrical formations 35 and 37. The feed/discharge arrangement 40 includes two feed/discharge matter displacing formations 42 and 44 on the inner surface of the outer cylindrical formation 35 that feeds matter into the interior space 20 when the drum 16 is rotated in one direction and which discharges matter from the interior space 20 when the drum 16 is rotated in the opposite direction. More specifically each feed/discharge matter displacing formation 42 and 44 comprises a feed/discharge screw formation that extends in a helical configuration from one end of the feed/discharge arrangement 40 to the other. Each screw formation 42, 44 extends across the annular space between the outer and inner cylindrical formations 35, 37 and is sealed to the outer and inner cylindrical formations 35, 37 along its length. In particular the screw formations 42, 44 are welded to each of the outer and inner cylindrical formations 35, 37 along their length. This way operation of the feed/discharge arrangement 40 is effected by drum rotation as distinct from rotation of a screw conveyor or an auger independently of a stationary drum 16. The helical configuration of the displacing screw formation 42, 44 functions much like a screw to feed raw organic matter into the drum 16 when the drum 16 is rotated in one direction, and to discharge organic matter from the drum 16 when it is rotated in said other direction. Each feed/discharge screw formation 42, 44 is connected to a said mixing blade 32, 34 such that it effectively forms an extension of the associated mixing blade 32, 34 within the interior space 20 of the drum 12. Once the drum contains sufficient organic matter, the feed/discharge arrangement 40 includes a water seal for sealing off the opening from an external environment when raw matter is fed into the drum 16 and when treated matter is withdrawn from the drum 16.

The digester 12 includes a biogas collection head 89 and an off-take conduit 90 (not shown) towards the top of the drum 16 for drawing off biogas as it is produced by the digestion reactions. As illustrated in Fig 2 of the drawings, the biogas collection head 89 is received within the drum 16 and is positioned in an operatively upper region of the interior space. The collection head 89 is covered with a mesh which resists it from getting blocked. It also has a non-return valve 91 that resists the passage of gas in a reverse direction back into the head space of the drum 16. The conduit 90 extends from the gas collection head 89 vertically down to the axis 41 of the drum 16 and then turns through ninety degrees and travels in a longitudinal direction through the feed/discharge arrangement 40 and out through the opening 45. The conduit 90 is typically braced at one or more points along its length by means of bracing elements 93 to hold it firmly in position within the drum 16. The conduit 90 has a gas connector 92 outside of the drum 16 that enables it to be operatively coupled to a flexible gas conduit which leads to a gas storage tank. The gas connector 92 includes a manually operable shut-off valve with a handle, e.g. a gate valve, that enables it to be opened and closed off by an operator. The digester is operated such that the drum 16 is always stopped in a certain position after undergoing a period of rotation for mixing or feeding/discharging. The certain position has the gas collection head 89 in an operatively upper region of the interior space 20. Similarly in this certain position the vent holes 93 in the blades 32, 34 are positioned in the operatively upper region of the interior space, e.g. in the head space above the organic matter in the drum 16.

The apparatus 10 also includes a drive arrangement 60 on the support 14 for rotating the drum 16 on the support 14 in both directions, i.e. clockwise and anti-clockwise directions. An operator can select whether to rotate the drum in a clockwise or an anticlockwise direction and the operator can stop rotation of the drum 16 when it is no longer required. The drive arrangement includes a motor that drives a shaft which in turn drives a drive flange which is directly driveably coupled to the end 25 of the drum 16 whereby to drive rotation of the drum 16. The support 16 includes a base in the form of two spaced rails 52, 54 mounted on a support surface that extends longitudinally the length of the digester 12. The rails 52, 54 are spaced apart from each other to provide lateral stability and extend parallel to each other in a lengthwise direction. The support 16 also includes two longitudinally spaced support formations 53, 55 projecting up from the rails 52, 54 on which the drum 16 is supported towards each end thereof. The support formation 53 comprises two laterally spaced rollers supported above the rails 52, 54 that support the drum 16. The drum 16 has a circumferential track ring and the rollers engage the track ring and roll over the track ring as the drum 16 rotates. The other support formation 55 includes the drive flange described above which is driveably coupled to said drum 16.

Figures 5 and 6 show an apparatus for carrying out the anaerobic digestion of organic matter and the generation of biogas. In these drawings the apparatus is indicated generally by the reference numeral 10.

The apparatus 10 comprises an anaerobic digester 12 as described above and as illustrated in Figures 2 to 6 above and an outer container 70. The outer container 70 contains and encloses the digester 12. The outer container 70 also provides engineered heating for causing the anaerobic digester 12 to operate at temperatures that yield a favourable rate of anaerobic digestion of the organic matter therein. The outer container 70 is in the form of an elongate rectangular intermodal shipping container having a container interior space 72 that can receive the digester 12 therein with clearance such that the barrel 16 is able to rotate within the outer container 70. The container 70 has a pair of doors 73 at one end thereof which can be opened as shown in Fig 7.

The outer container 70 has a top wall 74, a bottom wall 76 and a side wall 78 extending between the top and the bottom walls 74, 76. The side wall 78 comprises a plurality of side wall portions that together with the doors 73 collectively enclose the side of the outer container 70. The support 14 of the digester 12 is mounted on the bottom wall 76 of the container 70. The outer container 70 is thermally insulated whereby to retard the loss of heat from the interior space 72 of the outer container 70. In particular each of the walls and wall portions has thermal insulating panels extending across the wall or wall portion for resisting heat loss through the walls 72, 74 and 76 of the outer container 70.

The outer container 70 also includes a heating arrangement shown generally by numeral 80 for raising the temperature within the container interior space 72 to a temperature in the range of about 20 to 45 degrees Celsius that is most favourable for anaerobic digestion depending on the methanogens that are being used. There are mesophilic and thermophilic methanogens and different methanogens even within these groups digest most effectively at different temperatures. The heating arrangement enables the digester temperature to be heated up to the most suitable temperature for that particular application.

In Figures 6 and 7 the heating arrangement 80 includes a container heat exchanger 82 for delivering heat into the interior space 72 of the outer container 70 and an external heat exchanger 86 for extracting heat from the external environment for delivering to the container heat exchanger 82. The container heat exchanger 82 includes heat transfer tubes 84 arranged within the container 70 for receiving a heat transfer fluid therein and transferring heat from the tubes 84 into the container interior space 72 whereby to heat air in the interior space 72. In particular in the illustrated embodiment the heat transfer tubes 84 are arranged on the inside of the walls of the outer container 70.

The external heat exchanger 86 includes a solar heat exchanger for collecting solar energy from the sun and transferring this heat into the container interior space 72 of the container 70. This solar heat exchanger 86 contains heat transfer tubes 87 that carry a heat transfer fluid. The heat transfer tubes 87 absorb solar energy from the sun which heats up the heat transfer fluid and this fluid is then transferred to the tubes 84 of the container heat exchanger 82 where it gives up its heat. The heating arrangement 80 also includes conduits (not shown) for directing the heat transfer fluid from the solar heat exchanger 86 to the container heat exchanger 82 and then back to the solar heat exchanger 86. The heating arrangement 80 thus forms a closed loop which extracts heat from outside of the outer container 70 such as energy from the sun and delivers it into the interior space 72 of the outer container 70.

In use raw organic matter to be treated is introduced into the interior space 20 of the digester 12 through the feed/discharge arrangement 40. The raw organic matter is often in a slurry form and includes some solid material. The drum 16 of the digester 12 is filled, up to a level of about 70 to 85 % of its volumetric capacity, with raw organic matter. The digester 12 contains micro-organisms that promote various reactions that lead to the breakdown on the raw organic matter and ultimately to the generation of a biogas and a solid treated waste that is suitable for use as a fertiliser. As these reactions would be known to persons skilled in the art and do not form part of this invention they will not be described further in this specification. The raw material is fed into the interior space 20 by means of the feed/discharge arrangement 40. To do this the drum 16 is rotated in a direction that causes the matter displacing screw formations 42, 44 of the feed/discharge arrangement 40 to actively displace the organic matter through the opening and into the interior space 20. The water seal that is formed prevents any significant amount of air from entering the interior space 20 of the drum 16.

Thereafter the batch of organic matter is subjected to anaerobic digestion within the drum 16. During the course of this digestion the drum 16 is rotated periodically to mix the organic matter being treated which helps to promote the digestion reactions. When this occurs the mixing blades 32 and 34 lift up organic matter. When the organic matter is lifted to a certain height it drops back down onto other matter within the drum thereby effecting a mixing action. This enables the matter to be mixed effectively without having to have a high percentage of water within the drum 16.

Further air within the container interior space 72 is heated using the heating arrangement 80. This in turn raises the temperature within the interior space 20 of the digester 12 and heats up the organic matter within the digester 12. In this environment applicant has found that the digester can accomplish digestion of a batch of organic matter within a significantly reduced time frame when compared with prior art digesters. As biogas is generated by the anaerobic digestion reactions, it collects in a head space within the drum 16 above the surface of the solid or liquid matter within the drum 16. The biogas is drawn off from the head space through the biogas collection head 89 and the biogas off-take conduit 90. The biogas off-take conduit has a non-return valve 92 to resist biogas from flowing in a reverse direction back into the drum 16.

Once digestion is complete the treated solid matter within the drum 16 can be discharged from the drum interior space 20 by the feed/discharge arrangement 40. This is accomplished by rotating the drum 16 in a reverse direction to the direction which feeds matter into the drum 16. Rotation of the drum 16 causes the screw formations to move the treated matter through the annular passage and out through the opening 45. The treated solid material is useful as a fertiliser for adding to soils, particularly those used to grow crops.

Figure 8 illustrates an apparatus that is similar to that shown in Figures 6 to 7 and accordingly unless otherwise illustrated the same reference numerals will be used to refer to the same components. The following description will focus on the differences between this embodiment and the earlier embodiment. Figure 8 shows an off-take conduit 90 having a non-return valve 92 and a manually operable shut-off valve coupled in line therewith leading to a biogas storage tank 94. The storage tank 94 has a domed top 96 and is of adjustable volume that increases as the tank 94 fills with biogas.

Another difference from the embodiment in Figures 6 and 7 is that the heating arrangement 80 which heats up the digester is different to that in the earlier embodiment.

In this embodiment the hot gases are provided by exhaust gases from a generator such as a diesel generator or a biogas generator. These hot gases contain waste heat that is used to heat up the air surrounding the digester 12 within which the digestion reactions are occurring. The hot gases are pumped through a heat exchanger having a set of heat transfer tubes within the outer container and give up heat to the air within the container 70 thereby to heat up the digester.

Figure 9 comprises a sequence of schematic drawings that show how the apparatus 10 can be lifted up off a ground surface on which it is positioned. The apparatus 10 can be transported on a shipping container road vehicle 100 to another location where it can be used to digest organic matter at that other location. Once the organic matter that is available at the other location has been treated it can be transported to a yet further location where the process can be repeated. This way the productivity of the digester and its overall utilisation is not tied to the availability of raw organic matter for digestion at that particular site or location.

Applicant envisages that the apparatus 10 could be transported from one farm to another to treat accumulated raw organic matter such as animal waste at each location and then be moved on to the next location. This way one apparatus 10 could be used to treat raw matter in a number of different locations. Thus the applicant envisages transporting the apparatus to a location where raw matter is available for treatment rather than shipping organic waste from distributed farms to a central processing plant for treatment.

Figure 10 is a flowsheet illustrating one example plant installation for producing biogas including an apparatus 10 in accordance with one embodiment of the invention.

The installation which is indicated generally by numeral 101 includes a biogas digester 12 which provides biogas to a generator 103 to supply power to a drive arrangement 104 which produces mechanical drive for the apparatus 10. Optionally on start up a power supply 1 10 from the grid can be used to initially drive the drive arrangement 104 until the biogas is generated from the apparatus 10 to then supply biogas to drive the generator 103 which then supplies the power for the drive arrangement 104. The installation 101 also includes solar panels 106 which are used to heat the water to supply hot water to the heat exchanger on the walls of the outer container 70 for housing the digester 12.

Figure 1 1 is a diagrammatic installation showing a biogas plant installation 120 in which three digesters 122 as described above are used together in a modular fashion to provide a capacity to process greater amounts of organic waste and generate a greater amount of biogas. This installation 120 shows how the capacity to treat raw organic matter can be increased by simply adding additional digesters 122 which operate in parallel with each other and feed their biogas into a common biogas storage tank 124. The additional digesters 122 can be installed at the time the plant 120 is initially installed and commissioned, or they can be installed later. In particular they can be installed later on as capital becomes available to be invested in the installation or as the volume of raw matter to be treated increases.

In the arrangement shown in Figure 1 1 the three digesters 122 are driven by a single drive arrangement indicated generally by numeral 126. From the description above it will be appreciated that the drums 16 of the digesters 122 are not driven continuously during the digestion process. Instead the drums 16 are only required to be rotated driven at spaced time intervals to periodically mix the organic matter. The applicant has recognised that this enables a single drive arrangement 126 to be used to drive three digesters 12. As shown in the drawing the drive arrangement 126 has a coupling 128 that enables drive to be selectively applied to the three digesters 12 in turn. It will be appreciated that this considerably increases the utilisation of the drive arrangement 126 and the overall feasibility of the capital cost of the apparatus and the cost of the production of biogas.

Further the installation also shows a biogas outlet and take-off outlet on each digester feeding into a common manifold 129 which in turn extends to a biogas storage tank 124. Biogas generated in the digesters can be stored in the storage tank 124 until it is drawn off the tank 124 through the pipe 127 and used on site or transported away for use elsewhere.

Figure 12 illustrates a digester in accordance with another embodiment of the invention.

As this digester has many similarities with the digester in Figure 2 the same reference numerals will be used to indicate the same component unless otherwise indicated. The following description will focus on the difference between this embodiment and the Figure 2 embodiment.

The anaerobic digester in Figure 12 has a drum orientation adjustment arrangement indicated generally by reference numeral 130 for enabling the orientation of the drum 16 relative to a horizontal orientation to be adjusted to suit the properties of the organic matter being treated within the drum 16. The drum orientation adjustment arrangement 130 comprises a hydraulic lifting device 132 that is positioned towards the end 23 of the drum 16 for lifting the end 23 of the drum 16 up relative to the other end 25. The hydraulic device comprises a hydraulic lifting ram and a pivotal mounting at each end of the hydraulic ram for pivotally attaching it to each of the support 14 and the support surface on which the digester is mounted. It also includes a pivotal mounting 133 towards the end 25 of the drum 16 mounting the support 14 on the support surface on which the digester 12 is mounted. The hydraulic lifting device can be controlled by an operator using a hand operated controller. This feature enables an operator to adjust the axis of rotation of the drum 16 so that it is inclined at a suitable angle to a horizontal orientation indicated on the drawing by the reference numeral to achieve optimum mixing of the organic matter within the drum 16. Generally the more viscous and stiffer the organic matter within the drum 16 the greater is the desired angle of the drum 16 relative to a horizontal orientation.

An advantage of the anaerobic digester 12 as described above with reference to the drawings is that the organic matter is mixed very effectively by the rotating drum and the mixing blades and this promotes a favourable rate of digestion. This reduces the amount of water that needs to be added to the organic waste which is advantageous. This in turn improves utilisation of the digester 12 because the amount of organic matter which can be contained within each batch can be maximised.

A further advantage of the digester described above is that the feed/discharge arrangement 40 is very effective at feeding raw organic matter into the drum 16 and at discharging treated solid or liquid organic matter or digestate from the drum after digestion has occurred. The effective feed/discharge arrangement requires less water to displace the matter into the digester and this enables utilisation of the available space within the digester to be increased. A further advantage of the digester and apparatus described above is that the mixing arrangement with blades within the drum and the feed/discharge arrangement are integrated with each other and work together even though they are activated at different stages of the process. Essentially the same mechanism is used for both mixing and feeding/discharging and furthermore it is extremely effective at carrying out both functions.

A further advantage of the apparatus for producing biogas described above with reference to the drawings is that the temperature within the digester can be engineered to achieve a favourable rate of digestion reactions. It does this by heating the air within the outer container to a suitable temperature, say 36-40 degrees Celsius for mesophilic digestion, that is favourable to digestion reactions and this heat is then transferred through the wall of the drum and into the internal space of the drum.

A yet further advantage of the apparatus described above is that it is transportable like an intermodal shipping container. Thus the apparatus can be used at one location to treat a supply or organic waste and when that supply of waste has been exhausted it can be put on a truck and transported to another location where it can be used to treat another supply of organic waste. This feature provides an opportunity to obtain a greater utilisation or efficiency from a biogas producing apparatus than with a digester received within the ground.

A yet further advantage of the digester 12 is that it is mounted above the ground and is not received within the ground. As such it is accessible and maintenance can easily be carried out on the plant. For example access can be obtained to the interior space of the drum to remove residual material which is reducing the capacity of the drum. A further advantage of the apparatus for producing biogas described above with reference to the drawings is that the biogas that is generated in the digester can be used to drive a generator which is used to drive rotation of the drum. Thus the energy required to drive the digestion process can be provided by the actual biogas that is produced by the digestion process. A yet further advantage of the apparatus described above is that it is modular and additional modules can be added on, either at the time of installation or later on, to increase the volume of organic waste that can be processed and the amount of organic waste that can be produced.

It will of course be realized that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto, as would be apparent to persons skilled in the art, are deemed to fall within the broad scope and ambit of the invention as is herein set forth.