Method and device for producing biogas

Abstract

The invention relates to a method and a biogas plant for producing biogas, preferably from rice straw, wherein a substrate is fermented in two reactors (1, 2) in a circulating manner, so that a methane production from cellulose-and/or lignocellulose-containing substrate can be improved.

Claims

1. A method for producing biogas comprising the following steps: at least partial fermenting of a cellulose-containing substrate for a first residence time at a temperature in the region of 20° C. to 55° C. in a first reactor with mesophilic bacteria suitable for producing methane from acetic acid; transferring a part of the at least partially fermented substrate out of the first reactor into a second, heatable reactor with hyperthermophilic bacteria, wherein the hyperthermophilic bacteria is suitable for breaking down the at least partly fermented substrate; incubating the at least partly fermented substrate at a temperature in the region of 55° C. to 80° C. for a second residence time, wherein acetic acid is formed at least partly; returning the substrate with acetic acid out of the second reactor into the first reactor; incubating the substrate with acetic acid in the first reactor; and isolating methane-containing biogas out of the first reactor; wherein the method is carried out in a biogas facility, the facility comprising: a first reactor containing mesophilic bacteria which are suitable for methane production from acetic acid, hydrogen and carbon dioxide; a second heatable reactor comprising hyperthermophilic bacteria which are suitable for the preferably anaerobic fermentation of cellulose-containing substrates into acetic acid; and a circulation device for producing a biomass circulation between the first reactor and the second reactor, wherein the circulation device comprises a conveying device and at least one connection conduit which connects the first reactor and the second reactor, wherein the connection conduit comprises an aeration device, and wherein the conveying device is configured for conveying substrate through the connection conduit from the first reactor into the second reactor as well from the second reactor into the first reactor.

2. The method according to claim 1, wherein the cellulose-containing substrate has an ammonium nitrogen concentration of at least 100 mg NH4-N/1.

3. The method according to claim 1, comprising: transferring a part of the at least party fermented substrate out of the first reactor into a second, heatable reactor; and transferring the part of the at least partly fermented substrate out of the first reactor into an aeration device for inactivating anaerobic bacteria.

4. The method according to claim 1, comprising repeating at least once the steps of: transferring a part of the at least partly fermented substrate out of the first reactor into a second, heatable reactor with hyperthermophilic bacteria, wherein the hyperthermophilic bacteria are suitable for breaking down the at least partly fermented substrate; and returning the substrate with acetic acid out of the second reactor into the first reactor.

5. The method according to claim 1, wherein fermentation residues which remain in the first reactor and which after a fermentation in the first reactor are essentially not further broken down by the mesophilic bacteria are led out of the first reactor and drained, wherein an accordingly separated process liquid is at least partially fed back into the first reactor via a discharge conduit.

6. The method according to claim 1, wherein the mesophilic bacteria are selected from the group of archaea and/or that the hyperthermophilic bacteria are selected from the group of Clostridiaceae and/or Thermotogaceae.

7. The method according to claim 1, wherein the cellulose-containing substrate comprises lignocellulose and/or that the lignocellulose-containing substrate comprises straw.

8. The method according to claim 1, wherein a pH-value of the substrate in the first reactor lies in the neutral to slightly alkaline range and/or that a pH-value of the substrate in the second reactor is at least 4.0 and/or at the most 6.5.

9. A biogas facility for producing biogas, comprising a first reactor containing mesophilic bacteria which are suitable for methane production from acetic acid, hydrogen and carbon dioxide; a second heatable reactor comprising hyperthermophilic bacteria which are suitable for the preferably anaerobic fermentation of cellulose-containing substrates into acetic acid; and a circulation device for producing a biomass circulation between the first reactor and the second reactor, wherein the circulation device comprises a conveying device and at least one connection conduit which connects the first reactor and the second reactor, wherein the connection conduit comprises an aeration device, and wherein the conveying device is configured for conveying substrate through the connection conduit from the first reactor into the second reactor as well from the second reactor into the first reactor.

10. The biogas facility according to claim 9, further comprising: a first one-way valve located upstream of the aeration device; and a second one-way valve located downstream of the aeration device.

11. The biogas facility according to claim 9, further comprising: a drainage device for separating a process liquid from a fermentation residue upon a drainage of the fermentation residue after a fermentation in the first reactor, wherein the first reactor is connected to the drainage device via a discharge conduit, for discharging the fermentation residues.

12. The biogas facility according to claim 9, further comprising: a mixing device which is arranged upstream of the first reactor and is connected to the first reactor by a feed conduit, for mixing substrate with a process fluid in a manner such that a dry matter content of the substrate which is feedable through the feed conduit to the first reactor can be adjusted.

13. The biogas facility according to claim 11, further comprising: a return device for returning the process fluid into the first reactor.

14. The biogas facility according to claim 9, wherein at least one of: the connection conduit comprises a two-way valve or the circulation device comprises two connection conduits between the first reactor and the second reactor.

15. The biogas facility according to claim 9, wherein at least one of: the circulation device comprises at least one pump for delivering substrate or the biogas facility comprises at least one of: at least one stirring device or at least one pump device for intermixing the substrate in at least one of the first reactor or the second reactor.

16. The biogas facility according to claim 9, wherein at least one of: the first reactor is heatable in a manner such that a temperature of at least 30° C. can be set or the second reactor is heatable in a manner such that a temperature of at least 65° C. can be set.

Description

(1) Embodiment examples of the present invention are hereinafter explained with reference to the accompanying drawings. There are shown in:

(2) FIG. 1a: a schematic representation of a biogas facility,

(3) FIG. 1b: a schematic representation of the biogas facility with an aeration device,

(4) FIG. 2a: a schematic representation of a biogas facility with a 2-way valve arrangement,

(5) FIG. 2b: a schematic representation of a biogas facility with a 2-way valve arrangement and an aeration device;

(6) FIG. 3: a schematic representation of a biogas facility with a drainage device,

(7) FIG. 4: a schematic representation of a biogas facility with a mixing device, a size-reduction device and with a drainage device, and

(8) FIG. 5: a schematic representation of a method for producing biogas.

(9) FIG. 1a shows a biogas facility with a first reactor 1 and with a second reactor 2. A content of the first reactor 1 contains mesophilic bacteria and a cellulose-containing and lignocellulose-containing substrate, wherein the cellulose-containing and lignocellulose-containing substrate comprises rice straw, rice straw silage and chicken manure. In the shown example, the first reactor 1 is heatable and the contents of the first reactor 1 have been heated to a temperature of 33° C. However, it is also conceivable for the first reactor 1 not to be heatable and for a temperature between 20° C. and 40° C. to set in due to an exothermic fermentation process by the contained mesophilic bacteria. The mesophilic bacteria—in the present example these are archaea, and specifically Methanobacteriales, Methanococcales, Methanomicrobiales, Methanocellales—are suitable for producing methane from acetic acid. The substrate which is contained in the first reactor 1 resides for example 30 days in the first reactor 1 and at least partly ferments, so that methane is generated. A part of the at least partially fermented substrate is led out of the first reactor 1 into the second reactor 2 by way of a circulation device 3. The circulation device herein comprises a first 4 and a second connection conduit 5, wherein the at least partly fermented substrate is transported via the first connection conduit 4 from the first reactor 1 into the second reactor 2 by way of a pump device, preferably an eccentric screw pump. The second reactor 2 is heatable and the contents of the second reactor 2 are heated to a temperature of 68° C. Hyperthermophilic bacteria, for example clostridia, preferably Clostridium aceticum, Clostridium thermocellum and/or Clostridium stecorarium are in the second reactor 2. The hyperthermophilic bacteria are suitable for further breaking down the at least partly fermented substrate, in particular lignocellulose. For this, the at least partly fermented substrate remains in the second reactor for a residence time of at the most 3 days. There, the lignocellulose is converted at least partly into acetate, butanol and further acids/alcohols by the hyperthermophilic bacteria. The substrate with acetic acid is led back from the second reactor 2 into the first reactor 1 via the second connection conduit 5 by way of an eccentric screw pump. The substrate with the acetic acid is incubated in the first reactor 1 by the mesophilic bacteria and methane-containing biogas with at least 50% methane, 30% carbon dioxide, 1000 ppm hydrogen sulphide and trace gases arises. The biogas is at least partly isolated and transferred out of the first reactor 1 via a discharge device 6, i.e. a gas conduit. The remaining, at least partly fermented substrate is again transferred into the second reactor 2 via the first connection conduit 4 and there is further broken down, in order after a further residence time of 5 days in the second reactor 2 to be led again into the first reactor 1 via the second connection conduit 5 and to be incubated there again. The substrate is therefore fermented and further broken down in the first 1 as well as in the second reactor 2. This substrate circulation is repeated so often until almost no or only little biogas can be obtained from the substrate.

(10) On transferring the at least partly fermented substrate further from the first reactor 1 into the second reactor 2, most mesophilic bacteria die in the second reactor 2 due to the high operating temperature. The hyperthermophilic bacteria die in the first reactor, at least for the most part, when these go through the second connection conduit 5 into the first reactor 1, due to the lower temperature. The first reactor 1 as well as the second reactor 2 each comprise a stirring device 7, 8, wherein the first stirring device 7 continuously intermixes the contents of the first reactor 1 and the second stirring device 8 the contents of the second reactor 2 in order to prevent deposits and adhesions of the contents and to keep the substrate in a homogeneous as possible state. In the first reactor, the substrate in weight parts is twenty times larger than the substrate in the second reactor 2.

(11) FIG. 1b shows a biogas facility which corresponds essentially to FIG. 1a, but comprises an aeration device 28 in the connection conduit 4. A one-way valve 29 and a pump 30 are arranged upstream of the aeration device 28. A further one-way valve 30′ is arranged downstream of the aeration device. The pump 30 pumps the substrate out of the first reactor 1 into the aeration device given an opened valve 29 and a closed valve 31. The valve 29 is then closed. The substrate is aerated in the aeration device. Herein, the predominantly anaerobic bacteria from the first reactor 1 which are located in the substrate which is to be aerated are inactivated or killed by the aeration. The valve 31 is subsequently opened and the aerated substrate is led into the second reactor 2. One therefore succeeds largely preventing a methane-forming fermentation from taking place in the second reactor 2. No aeration is necessary for the substrate in the return conduit from the second reactor 2 into the first reactor 1 since the bacteria of the reactor 2, predominantly clostridia, are inactivated or die due to the temperature difference in the second reactor 2 and the first reactor 1, since these bacteria require higher temperatures, corresponding to the temperatures in the reactor 2, for survival and/or for actively metabolising.

(12) The second reactor of FIG. 1b further comprises a feed conduit 32. Easily fermentable substances can be admixed to the reactor contents by way of this feed conduit 32.

(13) FIG. 2a shows a further embodiment example of the biogas facility of FIG. 1, wherein a pump device 9 for intermixing the substrate is used in the first reactor. The contents of the second reactor 2 are thoroughly stirred with a stirring device 8. A combination of stirring device 8 and/or pumping device 9 in the first 1 and/or the second reactor 2 is therefore likewise conceivable for a biogas facility. In particular, this can be advantageous in order to be able select a mixing device which is as inexpensive and energy-saving as possible in accordance with the size of the reactors, in order to reduce the manufacturing and operating costs of the biogas facility.

(14) FIG. 2b shows a biogas facility which corresponds essentially to the biogas facility of FIG. 2a, but comprises a connection conduit 10 which is additionally provided with an aeration device 28, two pumps 30 and two two-way valves 11. The substrate can be pumped from the first into the second reactor 2 and vice versa by way of a respective opening and closing of the valves 11. The substrate is preferably aerated on pumping from the first reactor 1 into the second reactor 2. For this, the two-way valve 11 (analogously to the one-way valve in FIG. 1b) downstream of the aeration device is first closed and the substrate is pumped from the first reactor 1 into the aeration device 28. The two-way valve 11 upstream of the aeration device 28 is then closed and the substrate is aerated in the aeration device 28. The two-way valve 11′ downstream of the aeration device is opened after the aeration process, so that the substrate is led into the second reactor 2. The two-way valves 11 and 11′ can be opened in a manner such that the substrate can only flow in the direction of the first reactor 1, for returning the substrate from the second reactor 2 into the first reactor, in order to permit a circulation. A further pump 30′ is provided for this, said further pump pumping the substrate out of the second reactor 2 into the first reactor 1. Furthermore, a further valve which is closed given a return can be provided on the aeration device 28, so that the substrate is not possibly unnecessarily aerated again on being led back into the first reactor. Such a valve is not represented in FIG. 2b but can be optionally added.

(15) The contents of the first reactor have a pH-value of 7. The contents of the second reactor 2 have a pH-value of 5.5. These pH-values correspond to the preferred pH-values of the bacteria which are brought into the first and the second reactor 1, 2.

(16) In FIG. 2, the first reactor 1 has a volume which is twenty times larger than the volume of the second reactor 2. The two reactors 1, 2 in FIGS. 2a and 2b are connected to a circulation device 3, wherein the circulation device comprises a transport or delivery device in the form of an eccentric screw pump and a connection conduit 10 with a two-way valve arrangement 11. A through-flow direction of the substrate can be set via the two-way valve arrangement 11, so that a substrate flow from the first reactor 1 into the second reactor 2, as well as a substrate flow from the second reactor 2 into the first reactor 1 can run through the connection conduit 10. As has already been described and shown in FIGS. 1a and 1b, biogas can be isolated and led out via the discharge device 6. A circulation of substrate can of course also be generated by way of transferring substrate out of the one reactor, for example reactor 1, into an intermediate container and from the intermediate container into the other, for example the second reactor 2. This can be advantageous for example if the first reactor and the second reactor with regard to location are to be installed remotely from one another and long, connection conduits with complex paths would be necessary.

(17) Furthermore, the residence time can be effectively related to the reactor volumes. The first reactor has a volume which is twenty times larger than the volume in the second reactor. Only a part-quantity of the substrate in the first reactor can therefore fit into the second reactor, in this example a twentieth. Since the entire contents of the first reactor is to run through the second reactor and is to incubate in the second reactor for 2.5 days at a time, a first residence time in the first reactor of 50 days results. The substrate for example as fermentation residue is then led out of the first reactor after 50 days.

(18) FIG. 3 shows a biogas facility with a drainage device 12 which is connected to the first reactor 1 via a discharge conduit 13. Fermentation residues which arise after fermentation in the first reactor 1 and/or the second reactor 2 are then led via the discharge conduit 13 into the drainage device 12. This can be effected for example at a point in time when a biogas production drops and a volume flow in the discharge device 6 drops below a measured limit value, for example on dropping 5-10% below the value of the previous volume flow of biogas [Nm.sup.3/h]. Of course, the fermentation residues can also be discharged out of the first reactor into the drainage device after a first residence time, here for example after 50 days. There, a process fluid is separated from the fermentation residues. This process fluid is led at least partly into the first reactor via a return device 14 and is mixed there with the substrate by way of the stirring device 7. One can therefore counteract a clotting of the substrate and a ratio of a liquid content of the substrate to a dry matter content of the substrate can be adjusted. A solid fermentation residue, i.e. the drained fermentation residue can be further discharged out of the drainage device 12 via a discharge element 15, for example a tube. The at least partly fermented substrate is thoroughly mixed (intermixed) in the second reactor 2 by way of a pumping device 16, preferably in a continuous manner.

(19) A biogas facility which corresponds to the construction of the biogas facility in FIG. 2a is represented in FIG. 4. However, it comprises further elements such as for example a drainage device 12, a mixing device 16, a pre-treatment device 17, and a silicic acid discharge device 20. Before substrate is fed to the first reactor 1, it is reduced in size in the pre-treatment device 17 to a fibre length of preferably 40 mm, preferably by way of a mechanical cutter or cutting mill. The substrate fibres, in particular rice straw and wheat straw which are reduced in size to 40 mm are led through a tube 18 into the mixing device. A drainage device 12 is provided as also in FIG. 3. This leads the separated process fluid via the return device 14 into the mixing device 16. The size-reduced substrate and process fluid are mixed in the mixing device 16 before it is led further into the first reactor 1 via a feed conduit 19. In the shown example, apart from the size-reduced substrate and the process fluid, no further substances are fed to the mixing device 16. Of course, further substance constituents, for example manure, can be led to the mixing device 16 via further conduits.

(20) The biogas facility which is shown in FIG. 4 further comprises a silicic acid discharge device 20. This is preferably assembled onto the second reactor 2, so that silicic acid which arises partly due to clostridia on decomposition of the lignocellulose and can act in a manner inhibiting the methane formation, can be brought out before transferring the substrate further into the first reactor 1. One can therefore succeed in a methane formation not being inhibited in the first reactor by the mesophilic bacteria and the efficiency of the biogas facility can therefore be improved. The silicic acid can be separated for example by a silicate filter and silicic acid filter or also be separated out of the liquid phase by way of prior precipitation reactions, and subsequently discharged from the second reactor 2.

(21) FIG. 5 schematically describes an exemplary sequence of the method for producing biogas with the described biogas facility. In a first step 21, first a cellulose-containing substrate, containing rice straw and bovine manure is at least partly fermented in a first reactor at a temperature of 30 to 48° C. for a first residence time of 30 days. Mesophilic bacteria which can metabolise the substrate are contained in the first reactor.

(22) In a second step 22, the at least partially fermented substrate is led out of the first reactor into a second reactor. The second reactor is heated and its contents have a temperature of 68° C. Hyperthermophilic bacteria, which are suitable for breaking down the at least partly fermented substrate, in particular the lignocellulose, are in the second reactor.

(23) In a third step 23, the hyperthermophils incubate the at least partly fermented substrate in the second reactor during a residence time of preferably 2 days. The hyperthermophilic bacteria, in particular bacteria of the genus Clostridia, herein break down the partially decomposed substrate, in particular cellulose and lignocellulose and herein preferably produce acetic acid, apart from other organic acids.

(24) The at least partially decomposed substrate is then led out of the second reactor into the first reactor in a fourth step 24.

(25) In a fifth step 25, the led-back substrate is incubated again by the mesophilic bacteria in the first reactor. Herein, methane-containing biogas arises at least in part, and this is isolated and led out of the first reactor in a sixth step 26.

(26) Remaining, at least partially decomposed substrate is led further back into the second reactor and the described steps 22, 23, 24, 25 and 26 repeated, so that a loop 27 is created. Steps 22 to 26 are repeated until almost no more methane-containing biogas can be extracted, for example after a threefold repetition. Fermentation residues can thereafter be discharged, the biogas facility fed afresh with substrate and the method carried out once again.

(27) Of course the steps—in particular an incubation in the first and second reactor, i.e. the steps 21, 23 and 25—in part can also take their course at the same time. Furthermore, it is also conceivable for a circulation of substrate to take place, the substrate to be incubated and the methane to be discharged, in a continuous manner. All steps 21 to 26 can therefore also take their course in time intervals (so-called fed batch method).

(28) A typical substrate composition in the first reactor and in the second reactor in the course of the method is hereinafter explained in more detail in an embodiment example. Of course, the details specified hereinafter can also differ in other embodiment examples.

(29) A further embodiment example is explained in more detail hereinafter. A stationary operation is assumed on considering the method. At the beginning of the method, the first reactor comprises a content for example of 3000 m.sup.3 of substrate. The necessary microorganisms for the biogas process are located therein. Temperatures of 40-48° C. are ensured via a heat source. The substrate is mixed in time intervals by way of stirrers and/or mixing devices and the formed biogas is driven out of the liquid phase. A new distribution of biomass/bacteria suspension is further produced.

(30) 100 t of substrate are fed to the first reactor daily. The solid matter feed can herein be 14 t per feed. This in turn is composed of 60-65% dry mass % (DM %) of rice straw in the mixture (with 15% water content), 20-25 DM % of bovine manure, 10-20 DM % of recirculate with mesophilic bacteria. Approx. 100 t of substrate/sludge with a dry mass share of 10-12% DM % has been fed to the second reactor at the beginning of the observation time period after a first transfer. The solid matter share can be composed for example of 60-65 DM % of rice straw, 25 DM % weight percent of bovine manure. The remaining share consists of residual substances of short-circuit flows as well as of inorganic substances, ash as well as other foreign matter. Disregarding the 10 t of solid matter, the feed stream comprises up to 90 t of water.

(31) Up to 30% of the dry mass is broken up into soluble constituents for a residence time of for example of 3 days in the second reactor. This is typically realised by the performance capability of the hyperthermophilic microorganism which can be cultivated at 55-80° C. The temperature optimum depends on the addition of the substrate and its composition. Herein, up to 10 g/l of acetic acid can accumulate during the breakdown process, and this is led back into the first reactor 1 and converted into biogas.