WO1999067176A1

WO1999067176A1 - System for anaerobic treatment of fluid organic material

Info

Publication number: WO1999067176A1
Application number: PCT/DK1999/000357
Authority: WO
Inventors: Johannes Jacobus Mallan; Jakob Fenger; Mikael Ramus Nielsen; Bjørn Bjørnestjerne Reuter CHRISTIANSEN
Original assignee: Supergas A/S
Priority date: 1998-06-23
Filing date: 1999-06-23
Publication date: 1999-12-29
Also published as: AU4602899A; CN1306497A

Abstract

System for anaerobic treatment of fluid organic material, in particular waste generating methane, and comprising a fermentation tank (1), and a displacement tank (2) arranged at a higher level than the fermentation tank (1) and connected with a gas storage tank (3). A gas conduit (6) connects an upper portion of the fermentation tank (1) with an upper portion of the displacement tank (2). At least one connecting conduit (4) connects the fermentation tank (1) with the displacement tank (2) for transferring fluid material between said tanks. A normally closed pressure-equalising device (7) is provided in the gas conduit (6). The pressure-equalising device (7) is adapted to open without supply of external power at a given pressure difference between the fermentation tank (1) and the displacement tank (2) and subsequently to close with a given time delay.

Description

Title: System for anaerobic treatment of fluid organic material.

Technical Field

The invention relates to a system for anaerobic treatment of fluid organic material, in particular waste generating methane, and comprising a fermentation tank adapted to be filled with organic material to a certain level; a displacement tank arranged at a higher level than the fermentation tank and connected with a gas storage tank; and a gas conduit connecting an upper portion of the fermentation tank with an upper portion of the displacement tank, at least one connecting conduit connecting the fermentation tank with the displacement tank for transferring fluid material between said tanks, and the gas conduit being provided with a normally closed shut-off device which opens for equalising the pressure difference between the fermentation tank and the displacement tank.

Background Art

Industrial biogas systems are known provided with various motor-driven means for treating supernatant layer and sediment or bottom sludge formed in operation of fermentation tank. Such systems are unsuitable in areas with no electricity as in many rural areas in the developing countries.

Furthermore biogas systems of the above type are known in which fluid material is displaced from the fermentation tank to the displacement tank during operation thereof and the displaced liquid subsequently flows back to the fermentation tank due to a pressure equalisation between said two tanks. When the liquid flows back a an agitation of the liquid in the fermentation tank is produced. Systems of this type are inter alia described in EP-A1-0013538, EP-A1-0211451, DE-OS 32 28 782, WO 85/00584 and WO 95/05451. The present invention is based on the prior art dis- closed in the latter international publication. A common feature of the said systems is that the shut-off device arranged in the gas- or pressure-equalising conduit is electrically operated and/or electricity is required for operating the system. This entails that the systems cannot be used in areas with no electricity.

Brief Description of the Invention

The object of the invention is to provide a biogas system of the above type which does not require a supply of electricity.

The system according to the invention is characterised in that the shut-off device is a pressure-controlled pressure-equalising device having a delay element for control- ling the closure and adapted to open without being supplied with external power at a given pressure difference between the fermentation tank and the displacement tank and subsequently to close with a given time delay.

When gas is generated the fermentation tank, a portion of the fluid material is displaced to the displacement tank. When the pressure difference between the fermenta- tion tank and the displacement tank or the gas storage tank communicating directly herewith, has reached a predetermined maximum, the pressure-equalising device opens automatically. By opening the pressure-equalising device, the pressure between the fermentation tank and the displacement tank or the gas storage tank is equalised. As a result the fluid material displaced to the displacement tank flows back to the fermentation tank within a few seconds. The fluid material flowing back produces a heavy agitation and turbulence in the fluid material and a processing of any supernatant layers. On completion of the pressure equalisation and the return-flow of the fluid material, the pressure-equalising device closes automatically. The gas generated in the fermentation tank then produces a pressure difference between the fermentation tank and the displacement tank or the gas storage tank. The above described cycle is subsequently repeated. According to the invention the pressure-equalising device may comprise a lower container partially filled with a liquid and interconnected with a portion of the gas conduit extending from the fermentation tank; and an upper container arranged above the lower container and to which a portion of the gas conduit leading to the displace- ment tank is connected above a maximum liquid level in the upper container, said containers further being interconnected by means of blow-out conduit and a return conduit, the blow-out conduit being connected to the upper container above the maximum liquid level therein and connected through a first water trap with the lower container at a first level, and the return conduit being connected to the upper con- tainer at a level below the maximum liquid level therein and to the lower container at a second level being lower than the said first level for the connection of the blowout conduit with the lower container and having a lowermost point being lower than the first water trap of the blow-out conduit.

Just after a pressure equalisation, ie when the pressure in the fermentation tank and the displacement tank is the same, the pressure in the upper and lower containers of the pressure-equalising device is the same. Consequently, the blow-out conduit and the return conduit are filled with liquid up to a level corresponding to the liquid level in the lower container. The liquid closes the connection between the gas in the two containers. At generation of gas in the fermentation tank, an overpressure is formed in the lower container and liquid therefrom is displaced to the blow-out conduit and the return conduit. At a continuously increasing pressure in the lower container, liquid is displaced to the upper container through the return conduit and the liquid level in the blow-out conduit increases further up to its connection with the upper container. When the liquid level in the lower container reaches the connection point with the blow-out conduit, ie it reaches the first level, a liquid communication no longer exists between the blow-out conduit and the lower container. At a continuously increasing gas pressure in the lower container, liquid is also displaced from the lower container to the upper container through the return conduit. At some point in time the balance is broken between the liquid level in the blow-out conduit and the pressure in the lower container. This occurs when the liquid in the blow-out conduit reaches the bottom of the first water trap. As a result substantially all liquid in the blow-out conduit is blown-out to the upper container. In this connection it should be noted that the blow-out conduit has such a flow area that the gas rate through said conduit at the equalising pressure is sufficiently high to entrain substantially all liquid in the blow-out conduit. As known by experts in the field, the gas rate necessary for carrying out said process must be in the order of at least 40 m/s. Blowing-out the liquid in the blow-out conduit creates an open connection between the lower container and the upper container, whereby gas from the lower container and thus from the fermentation tank flows to the upper container and thus to the displacement tank and the gas storage tank. Accordingly a pressure equalisation occurs between the lower container and the upper container and thus between the fermentation tank and the displacement tank or the gas storage tank. By this pressure equalisation fluid material flows from the displacement tank to the fermentation tank as described above. Subsequent to the pressure equalisation between the lower container and the upper container, liquid from the upper container flows back to the lower container via the return conduit. The time delay for the closure of the pressure-equalising device is thus primarily determined by the difference in height between the connection points of the blow-out conduit and of the return conduit with the lower container and the flow area of the return conduit.

Furthermore according to the invention the return conduit may be connected with the lower container through a second water trap arranged at a lower level than the water trap of the blow-out conduit.

Moreover according to the invention the return conduit may be provided with an adjustable valve means for adjusting the effective flow area thereof. It is thus possible to adjust the pressure-equalising time by regulating the return flow rate of the liquid and consequently the closure of the water trap of the blow-out conduit. Furthermore according to the invention the pressure-equalising device may be formed such that liquid has flowed from the lower container through the blow-out conduit to the upper container before the predetermined pressure difference is reached. The liquid column corresponding to the height of the blow-out conduit then determines the pressure difference at which the pressure-equalising device opens. At the same time it is ensured that substantially all liquid in the blow-out conduit is blown out and an opening is effected.

Furthermore according to the invention the system may further comprise a feed vessel for new organic material and arranged at a higher level than the fermentation tank and connected therewith through a feed conduit. In connection with the pressure equalisation between the fermentation tank and the displacement tank new organic material is automatically pulled into the fermentation tank and mixed with existing material and active micro-organisms in the fermentation tank.

Moreover according to the invention the fermentation tank may near its bottom be provided with a sediment or sludge outlet opening connected to a sediment-removal conduit having a shut-off device. When opening the shut-off device, sediment or bottom sludge in the fermentation tank flows out through the sediment outlet opening and is thereby removed via the sediment-removal conduit.

Furthermore according to the invention the conduits between the fermentation tank and the displacement tank may be visible and accessible, whereby the construction of the system is simplified, visual control is possible and the maintenance and operation are simplified.

Moreover according to the invention the at least one connecting conduit may be connected tangentially with the fermentation tank so as to provide an effective agita- tion of the fluid material in the fermentation tank when the liquid displaced to the displacement tank flows back to the fermentation tank. Finally according to the invention the fermentation tank, the displacement tank and the gas storage tank may be made of reinforced plastic sheeting, such as reinforced PVC sheeting. This embodiment of the invention is in particularly advantageous for smaller-sized systems having a fermentation tank volume ranging from 2 to 15m³ , the systems thereby being very cost-effective and easy to manufacture. This feature is particularly important in systems to be used in remote areas of developing countries, where transportation is effected by means of primitive means of transport or by means of bearers.

Brief Description of the Drawings

The invention is explained in detail below with reference to the drawings, in which

Fig. 1 is a diagrammatic side view of an embodiment of a small biogas system according to the invention,

Fig. 2 is a diagrammatic side view of an embodiment of a large biogas system according to the invention,

Fig. 3 is a diagrammatic side view of an embodiment of a pressure-equalising device used in the systems according to Figs. 1 and 2, the pressure-equalising device shown in the state prior to the beginning of a equalisation cycle after the conclusion of a previous pressure equalisation,

Fig. 4 illustrates the pressure-equalising device in Fig. 3 immediately prior to pressure equalisation, and

Fig, 5 illustrates the pressure-equalising device immediately after pressure equalisation.

Best Mode for Carrying Out the Invention The system shown in Fig. 1 for anaerobic treatment of fluid organic material comprises a fermentation tank 1, a displacement tank 2 arranged at a level above the fermentation tank 1 , and a gas storage tank 3 arranged above the fermentation tank 1. The fermentation tank 1 is interconnected with the displacement tank 2 through a connecting conduit 4 connecting the bottom portion of the fermentation tank 1 with the bottom portion of the displacement tank 2. The connecting conduit 4 is connected tangentially with the fermentation tank 1. A second conduit 5 is branched off from the connecting conduit 4 and connected tangentially with the fermentation tank 1. The second conduit 5 is connected with the fermentation tank 1 at the maximum level of the organic material M in the fermentation tank 1.

The fermentation tank 1 and the displacement tank 2 are further interconnected by means of gas conduit 6 opening into the fermentation tank 1 above the maximum liquid level b therein and in the displacement tank 2 above the maximum liquid level a' therein. A pressure-equalising device 7 is arranged in the gas conduit 6, said device 7 being formed as described below with reference to Figs. 3 to 5. The pressure-equalising device 7 divides the gas conduit 6 into a first gas conduit portion 6' between the fermentation tank 1 and the pressure-equalising device 7 and a second gas conduit portion 6" between the pressure-equalising device 7 and the displacement tank 2. An additional gas conduit 8 connects the second gas conduit portion 6" with the gas storage tank 3. As a result the gas storage tank 3 communicates continuously with the displacement tank 2 such that the pressure in these tanks is always the same.

An overflow pipe 9 provided with a water trap 10 extends into the interior of the displacement tank 2 and ends in an overflow funnel for determining the maximum liquid level a' .

New material is fed to the fermentation tank 1 from a feed trough 12 arranged at a higher level than the fermentation tank 1. The feed trough 12 is connected with the fermentation tank 1 via a feed conduit 13 opening tangentially into the fermentation tank 1 below the lowest liquid level a in the fermentation tank 1. The system is further provided with a sediment-removal conduit 14 having an inlet opening 15 immediately above the bottom of the fermentation tank 1. The sediment- removal conduit 14 is provided with a manually operated shut-off device 16 and extends to a sediment-collecting place (not shown). The overflow pipe 9 from the displacement tank 2 also extends to the sediment-collecting place.

Gas from the gas storage tank 3 flows to the place of use via a feed conduit 17.

In smaller-sized systems of a fermentation tank volume ranging from 2 to 15 m³ the fermentation tank, the displacement tank and the gas storage tank are advantageously made of reinforced plastic sheeting such as PVC sheeting.

The system shown in Fig. 1 operates in the following manner from a stage in which the pressure in the fermentation tank 1 and the displacement tank 2 has been equalised by the pressure-equalising device 7. It is further implied that the system has been in operation for some time such that the material in the fermentation tank comprises a supernatant layer, a middle substrate layer and a lower sediment layer.

An increase in pressure takes place, when gas is generated in the fermentation tank 1. The liquid level in the fermentation tank 1 then decreases gradually from the maximum level b to the minimum level a, whereby the substrate is gradually displaced to the displacement tank 2 and the substrate level therein is gradually increased from the lowermost level b' , substantially corresponding to the level b in the fermentation tank, to the maximum level a' . Just prior to the maximum level a' in the displacement tank 2 is reached, the pressure-equalising device 7 opens, said device being adapted to open at a pressure difference between the fermentation tank and the displacement tank slightly lower than the level difference between the liquid levels a' and a. When the pressure-equalising device 7 opens, gas flows from the fermentation tank 1 to the gas storage tank 3 and the displacement tank 2 via the gas conduit 6 and the additional gas conduit 8, whereby the same pressure exists in the said tanks. The substrate in the displacement tank 2 flows to the fermentation tank 1 through the connecting conduit 4 and the second conduit 5 and thereby produces an agitation of the existing material in the fermentation tank and a percolation of the supernatant layer. During pressure equalisation, new material is supplied from the feed trough 12 via the feed conduit 13 to the fermentation tank 1. This new material is mixed with the existing material in the fermen- tation tank. The pressure-equalising device 7 is provided with a delay element for controlling the closure thereof such that the closure is effected with a predetermined time delay in order to provide sufficient time for the substrate displaced to the displacement tank 2 to flow back to the fermentation tank and induce the intended agitation. When the the pressure-equalising device 7 has been closed, a new cycle commences as described above.

The system shown in Fig. 2 comprises a fermentation tank 21 and a displacement tank 22 arranged thereabove and separated from the fermentation tank 21 by means of a partition wall 19. The fermentation tank 21 and the displacement tank 22 form a unit made from an iron sheet. The gas storage tank 23 is made of plastic sheeting, such as PVC sheeting, and connected with the displacement tank 22 in an rim portion 20 thereof. A connecting conduit 24 extends from a bottom portion of the displacement tank 22 to a bottom portion of the fermentation tank 21. A second conduit 25 is branched off from the connecting conduit 24 and extends into the fermentation tank 21 in an area corre- sponding to the maximum liquid level b therein.

A feed conduit 33 extends from a feed trough 32 to the lower end of the connecting conduit 24 and is connected therewith. Consequently new organic material from the feed trough 32 is mixed with the substrate flowing from the displacement tank 22 to the fermentation tank 21 at pressure equalisation.

A sediment-removal conduit 34 provided with an inlet opening 35 immediately above the bottom of the fermentation tank 21 extends to a sediment collecting place (not shown). The sediment-removal conduit 34 is provided with a shut-off valve 36. The fermentation tank 21 and the displacement tank 22 are interconnected above their respective maximum liquid levels b and a' by means of a gas conduit 26 provided with a pressure-equalising device 27, which may be formed as described below with reference to Figs. 3 to 5. Finally the system is provided with a supply conduit 37 leading a place of use from an area above the maximum liquid level a' in the displacement tank 22 and further an overflow pipe 29 having a water trap 30 and an overflow funnel 31 which is arranged in the displacement tank 22.

The system in Fig. 2 operates in the same manner as the system in Fig. 1, for which reason reference is made to the description of the function described in connection with Fig. 1.

Referring now to Figs. 3 to 5 a preferred embodiment of a pressure-equalising device 7,27 is described in detail. The pressure-equalising device comprises a lower container 40 and an upper container 41 arranged thereabove. In its top the upper container 41 is connected to the second gas conduit portion 6" ,26" of the gas conduit 6, said member being connected to the displacement tank 2,22 and the gas storage tank 3,23 respectively. In its top the lower container 40 is connected to the first gas conduit portion 6 ' ,26' of the gas conduit 6,26, said portion opening into the fermentation tank 1 ,21. The two containers 40,41 are further interconnected by means of blow-out conduit 42 and a return conduit 43. The pressure-equalising device is partially filled with liquid which can be added through a filling conduit 44 opening into the lower container 41 and being provided with a filling funnel 45. The funnel 45 is covered by a web 46 to prevent impurities from entering the pressure-equalising device.

The blow-out conduit 42 is connected with the upper container 41 above the maximum liquid level d therein (confer Fig. 4) and connected with the lower container 40 via a first water trap 47 in form of a U-bend at a first level e. The return conduit 43 is connected with the upper container 41 in a lower portion thereof and connected with the lower container 40 via a second water trap 48 in form of a U-bend in a second level f below the first level e. The return conduit 43 is further provided with an adjusting valve means 50 for adjusting the effective flow area of the return conduit 43. When the return conduit 43 is a soft hose, the valve means 50 may be formed of simple hose clamp. Finally the lower container 40 is provided with a liquid-level indicator 49 such that the liquid level in the lower contained may be determined.

The pressure-equalising device operates as follows from the stage shown in Fig. 3: In this state the pressure is the same in the two containers 40 and 41 just as the pressure is the same in the fermentation tank 1,21 and the displacement tank 2;22 or the gas storage tank 2;23, respectively. The liquid level g is the same in the blow-out conduit, the lower container 40 and the return conduit 43 due to their interconnection. When gas is generated in the fermentation tank 1 ;21 , the pressure therein and thus also in the lower container 40 increases. The liquid level in container 40 then decreases gradually and liquid is displaced to the blow-out conduit 42 and the return conduit 43. At a further pressure increase in the lower container 40, at some point in time liquid flows up into the upper container 41 through the return conduit 43. Further at some point the liquid level in the lower container 40 reaches the level e corresponding to the connection point of the blow-out conduit 42 with the lower container 40. Subsequently hereto at a further pressure increase additional liquid does not flow from the lower container 40 to the blow-out conduit 42. At a continued increase of the pressure in the lower container 40, the liquid level therein reaches the level f corresponding to the connection point of the return conduit with the lower container 40. The liquid level in the branches 51 ,52 of the water traps 47,48 then decreases gradually until the state shown in Fig. 4 is reached. In this state, in which the liquid level in the branch 51 of the first water trap 47 is just above the bottom the water trap, the blow-out conduit is completely filled with liquid, liquid possibly already having flowed to the upper container 41 through the blow-out conduit 42.

The effective liquid column H in the blow-out conduit has now reached its maximum. At a further increase of the pressure in the lower container 40?, the balance in pressure is broken and liquid in the blow-out conduit is blown out and into the upper container 41 , the blow-out conduit 42 having such a flow area that the gas rate during the blow-out ensures that substantially all liquid is blown out, ie. a gas rate of at least 30-40 m/s. It should further be noted that the lowermost point of the return conduit 43, ie the bottom of the second water trap 48, is at such a level that the liquid column L, to which it is subjected, exceeds the maximum liquid column H in the blow-out conduit. In other words liquid must be present in the branch 52 of the water trap 48. Otherwise the return conduit 43 and not the blow-out conduit is blown out.

After the blow-out the lower container 40 is communicates directly with the upper container 41 through the blow-out conduit 42 (confer Fig. 5). As a result gas from the lower container 40 and thus from the fermentation tank 1;21 connected thereto flows to the upper container 41 and from there on to the displacement tank 2;22 and the gas storage tank 2;23 connected thereto. This pressure equalisation entails that the substrate in the displacement tank 2; 22 flows back to the fermentation tank 1;21 and produces a heavy agitation of the fluid material therein as described above. The said pressure equalisation further causes the liquid in the upper container 41 of the pressure-equalising device to flow back to the lower container 40 through the return conduit 43. The rate of said reflow may be adjusted by means of the flow-adjusting device 50. Since the return conduit 43 is connected with the lower container 40 at a lower level f than the level of the blow-out conduit 42 (level e), some time passes before liquid begins to flow into the blow-out conduit 42 and closes the direct communication between the lower container 40 and the upper container 41. Finally the pressure-equalising device adopts the position shown in Fig. 3, in which liquid has flowed back from the upper container 41 to the lower container 40. A new pressure equalisation cycle then commences.

Claims

1. System for anaerobic treatment of fluid organic material , in particular waste generating methane, and comprising a fermentation tank (1;21) adapted to be filled with organic material (M) up to a certain level; a displacement tank (2;22) arranged on a higher level than the fermentation tank (1;21) and connected with a gas storage tank (3;23); and a gas conduit (6;26) connecting an upper portion of the fermentation tank (1;21) with an upper portion of the displacement tank (2;22), at least one connecting conduit (4 ; 24) connecting the fermentation tank ( 1 ; 21 ) with the displacement tank (2 ; 22) for transferring liquid material between said tanks, and the gas conduit (6; 26) being provided with a normally closed shut-off device which opens for equalising the pressure difference between the fermentation tank and the displacement tank, c h a r a c t e r i s e d in that the shut-off device is a pressure-controlled pressure-equalising device (7;27) having a delay element for controlling the closure and adapted to open without being supplied with external power at a given pressure differ- ence between the fermentation tank (1;21) and the displacement tank (2; 22) and subsequently to close with a given time delay.

2 System according to claim 1, c h a r a c t e r i s e d in that the pressure-equalising device comprises a lower container (40) partially filled with a liquid and interconnected with a portion of the gas conduit (6'; 26') extending from the fermentation tank (1;21); and an upper container (41) arranged above the lower container (40) and to which a portion of the gas conduit opening into the displacement tank (2;22) is connected above a maximum liquid level (d) in the upper container (41), said containers (40,41) further being interconnected by means of blow-out conduit (42) and a return conduit (43), the blow-out conduit (42) being connected to the upper container (41) above the maximum liquid level (d) therein, and connected through a first water trap (47) with the lower container (40) on a first level (e) and the return conduit (43) being connected to the upper container (41) on a level below the maximum liquid level (d) therein and to the lower container (40) on a second level (f) being lower than the said first level (e) for the connection of the blow-out conduit (42) with the lower container (40), and having a lowermost point being lower than the first water trap (47) of the blow-out conduit (42).

3. System according to claim, characterised in that the return conduit (43) is connected with the lower container (40) through a second trap seal (48) arranged on a lower level than the water trap (47) of the blow-out conduit (42).

4. System according to claim 2 or 3, characterised in that the return conduit (43) is provided with an adjustable valve means (50) for adjusting the effective flow area of the return conduit (43).

5. System according to one or more of the preceding claims, characterised in that the pressure equalising device is formed such that liquid has flowed from the lower container (40) through the blow-out conduit (42) to the upper container (41 ) before the predetermined pressure difference is reached.

6. System according to one or more of the preceding claims, characterised in that it comprises a feed vessel (12;32) for new organic material and arranged on a higher level than the fermentation tank (1 ;21) and connected therewith through a supply conduit (13;33).

7. System according to one or more of the preceding claims, characterised in that the fermentation tank (1 ;21) is near its bottom provided with a sediment or outlet opening (15;35) connected to a sediment-removal conduit (14;34) having a shut-off device (16;36).

8. System according to one or more of the preceding claims, characterised in that the conduits between the fermentation tank (1;21) and the displacement tank (2;22) are visible and accessible.

9. System according to one or more of the preceding claims, characterised in that the at least one connecting conduit (4; 24) is connected tangentially to the fermen- tation tank (1;21).

10. System according to one or more of the preceding claims, characterised in that the fermentation tank (1), the displacement tank (2) and the gas storage tank (3) are all made of reinforced plastic sheeting, such as reinforced PVC sheeting.