Abstract:
The invention relates to a bioreactor for methanizing biomass, a biogas plant having a plurality of such bioreactors, and a method for operating such a bioreactor. Because the elongated reactor vessel includes both a loading gate and an unloading gate that are arranged at opposite ends of the elongated reactor vessel, it is possible to remove consumed biomass, which is harmless in terms of epidemiologic hygiene and plant hygiene due to thermophilic process control during the fermentation, from the reactor vessel through the unloading gate and to transfer this consumed biomass directly to the composting process. The bioreactor thus has a “clean” unloading gate and an “unclean” loading gate.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is filed under 35 U.S.C. §111(a) and is based on and hereby claims priority under 35 U.S.C. §120 and §365(c) from International Application No. PCT/EP2013/054339, filed on Mar. 5, 2013, and published as WO 2013/131876 A1 on Sep. 12, 2013, which in turn claims priority from German Application No. 202012100788.6, filed in Germany on Mar. 6, 2012, and from German Application No. 202012100816.5, filed in Germany on Mar. 7, 2012. This application is a continuation-in-part of International Application No. PCT/EP2013/054339, which is a continuation-in-part of German Application Nos. 202012100788.6 and 202012100816.5. International Application No. PCT/EP2013/054339 is pending as of the filing date of this application, and the United States is an elected state in International Application No. PCT/EP2013/054339. This application claims the benefit under 35 U.S.C. §119 from German Application Nos. 202012100788.6 and 202012100816.5. The disclosure of each of the foregoing documents is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a bioreactor for methanizing biomass, a biogas plant with a plurality of such bioreactors, and a method for operating such a bioreactor. 
       BACKGROUND 
       [0003]    A bioreactor of the type presented here is known from European Patent EP1301583 B1 to the present applicant. The complete contents of the disclosure of EP 1301583 B1 with regard to the constructive design is incorporated herein by reference. This known bioreactor is constructed in the manner of a prefabricated garage and has a front and a back end. The complete front end is open and is closed by a flap-shaped gate in an airtight manner. Fresh biomass can be introduced through the gate into the bioreactor and used biomass discharged from the bioreactor through it. The used or spent biomass is composted in a customary manner. According to the governing German biowaste regulations (BioabfV), compost may be discharged on open spaces and green areas only if the biowaste is harmless with regard to diseases and phytohygiene. For example, weed seeds and phytopathogenic germs, such as fire blight pathogens, must be killed. In order to kill diseases, weed seeds and phytopathogenic germs, the temperature in the composted matter must be maintained at a certain temperature range for a given time. This increases the requirements on the care that must be taken during the composting. If biogas is already produced in a thermophilic range (temperature &gt;55° C.), then less care must be taken during the subsequent composting of the spent biomass. However, there is the danger in the bioreactors known from EP1301583 B1 that the spent biomass will be re-contaminated as the bioreactor is discharged, and the spent biomass will have to be decontaminated before it is composted. 
         [0004]    Starting from the bioreactor according to EP1301583 B1, the present invention has the objective of designing a bioreactor in which the danger of contamination of the spent biomass with regard to diseases and phytohygiene is considerably reduced. Furthermore, the invention has the objective of providing a method for operating such a bioreactor. 
       SUMMARY 
       [0005]    A bioreactor that generates methane through dry fermentation of biomass includes an elongated reactor container, a biogas removal outlet, a loading gate and an unloading gate. The reactor container has a bottom plate, a cover plate, two side walls, a front end and a back end. The reactor container is made of steel-reinforced concrete and has a rectangular cross section. The biomass rests on the bottom plate between a first retention device and a second retention device. The bottom plate is flush with the ground allowing biomass to be pushed by a loader bucket across the ground, through the front end and onto the bottom plate. The methane exits the reactor container through the biogas removal outlet. The loading gate seals the front end in an airtight manner when the loading gate is closed, and the unloading gate seals the back end in an airtight manner when the unloading gate is closed. The loading and unloading gates cover substantially all of the cross-sectional area of the reactor container at the front and back ends, respectively. The gates are constructed as flaps that articulate from the top of the reactor container. 
         [0006]    The first retention device is disposed inside the reactor container adjacent to the loading gate, and the second retention device is disposed inside the reactor container adjacent to the unloading gate. The first retention device prevents the biomass from pressing against the loading gate, and the second retention device prevents the biomass from pressing against the unloading gate. A first percolate drainage system is disposed between the first retention device and the closed loading gate, and a second percolate drainage system is disposed between the second retention device and the closed unloading gate. Percolate that seeps out of the biomass during fermentation is collected using the percolate drainage systems. 
         [0007]    A method of operating a bioreactor that methanizes biomass involves loading a reactor container, sealing the ends of the container, generating methane and unloading the spent biomass. The bioreactor has a reactor container that includes a bottom plate, a cover plate, two side walls, a front end and a back end. The reactor container is made of steel-reinforced concrete and has a rectangular cross section. The reactor container is loaded with unfermented biomass only through a loading gate at the front end of the container. The biomass rests on the bottom plate. The loading of the reactor container is performed by pushing unfermented biomass in a loader bucket through the front end and onto the bottom plate that is flush with the ground. The biomass is prevented from pressing against the unloading gate by pushing the biomass against a retention device disposed inside the reactor container adjacent to the unloading gate. 
         [0008]    The front end of the reactor container is sealed in an airtight manner by closing the loading gate. Methane is generated by fermenting the biomass using dry fermentation within a temperature range of 50° C. to 60° C. The spent biomass is unloaded from the reactor container only through an unloading gate at the back end of the reactor container. A retention device that retains the biomass is removed after the unloading gate is opened but before the spent biomass is unloaded from the reactor container. The unloading gate seals the back end in an airtight manner when the unloading gate is closed. The unloading gate covers substantially all of the cross-sectional area of the reactor container at the back end when the unloading gate is closed. The unloading gate is constructed as a flap that articulates from the top of the reactor container. 
         [0009]    The spent biomass is composted directly after unloading the spent biomass from the reactor container without first decontaminating the spent biomass after the spent biomass has been unloaded from the reactor container. The loading, sealing and unloading is repeated so as to generate methane in multiple batch operations. The generated methane is supplied to a cogeneration unit. 
         [0010]    Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
           [0012]      FIG. 1  is a schematic, vertical cross-sectional view of a bioreactor with two gates in accordance with the invention. 
           [0013]      FIG. 2  shows a perspective view of the bioreactor of  FIG. 1  from the front with an open loading gate. 
           [0014]      FIG. 3  shows a horizontal cross-sectional view through the bioreactor showing the pipelines of the wall heating and floor heating. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
         [0016]      FIG. 1  is a schematic, cross-sectional view of a bioreactor  10  according to the present invention. The bioreactor  10  has a loading gate  11  and an unloading gate  12  arranged at opposite ends of an elongated reactor container  13 . Elongated reactor container  13  has a rectangular cross section and is more than twice as long as the container ends are wide. Fresh and thus contaminated biomass is loaded into the reactor container  13  through the loading gate  11 . The bioreactor  10  generates methane from biomass using dry fermentation. Dry fermentation allows biomass from organic wastes, agricultural waster and communal garden and park areas to be methanized without transforming the materials into a pumpable, liquid substrate. It is possible to ferment biomasses having a dry substance fraction of up to 50%. The spent and thus thermally processed biomass is discharged from the reactor container  13  through the unloading gate  12  at the opposite end of the container  13 . The thermal or thermophilic processing during the digestion renders the spent biomass harmless with regard to diseases and phytohygiene. The subsequent composting is less troublesome as regards diseases and phytohygiene. Therefore, the bioreactor has a “clean” unloading gate  12  and an “unclean” loading gate  11 . The “clean” unloading gate  12  is exclusively used to discharge or unload spent and “clean” biomass from the reactor container  13 . The “unclean” loading gate  11  at the opposite end of the reactor container  13  is exclusively used to load unfermented, fresh and therefore hygienically contaminated biomass into the reactor container  13 . The length of the reactor container puts a significant distance between the “clean” unloading gate  12  and the “unclean” loading gate  11 . Therefore, it is less likely that the spent biomass will be re-contaminated as it is unloaded from the reactor container  13 . Undesired germs, seeds and bacteria from the area  14  around the loading gate  11  thus cannot pass into the “clean” area of the unloading gate  12 . It is possible by means of the bioreactor with two gates to spatially separate a “clean”, i.e., harmless with regard to diseases and phytohygiene, discharge zone from the loading zone  14 , which is not harmless with regard to diseases and phytohygiene. 
         [0017]    In thermophilic processing during the generation of biogas, including methane, the generation of biogas takes place with bacteria that optimally “work” in a temperature range of approximately 50° C. to 60° C. In mesophilic processing, however, the generation of biogas is performed by bacteria strains that optimally “work” in the range of approximately 30° C. and 35° C. Therefore, in the thermophilic processing during the generation of biogas, the spent biomass is already rendered harmless with regard to diseases and phytohygiene, and the directly following composting can be carried out with less supervision and care. 
         [0018]    The loading and unloading of the reactor container  13  is facilitated by the passable floor of the reactor container that is flush with the ground allowing the use of front-end loader buckets on both wheeled and tracked loading devices, such as tractors and caterpillars. To facilitate access of such wheeled or tracked loading devices, the two gates on opposite ends of the reactor container  13  open nearly the entire cross section of the reactor container. Thus, loading gate  11  covers substantially all of the cross-sectional area of reactor container  13  at the front end, and unloading gate  12  covers substantially all of the cross-sectional area of reactor container  13  at the back end. Because the floor of reactor container  13  is flush with the ground, biomass can be pushed by a loader bucket across the ground, through the front end and onto the bottom plate. Similarly, spent biomass can easily be removed from the reactor container  13  by pushing the spent biomass across the floor and out the back end. By constructing the reactor containers in a manner similar to prefabricated garage modules, the production costs for the reactor container are reduced. 
         [0019]    Loading and unloading the reactor container  13  is facilitated by constructing the gates  11 - 12  as flaps so as to make both ends of the reactor container fully removable through hydraulic actuation of the two gates. Retention devices  15 - 16  are used inside the reactor container  13  to prevent the biomass from contacting the closed gates  11 - 12 . The retention devices  15 - 16  prevent excessive pressure from the expanding biomass from pressing against the gates. Accordingly, the gates can be constructed from lighter material and are less costly. The quasi-continuous generation of biogas in multiple batch operations is possible by using a plurality of biogas reactor containers in a biogas plant. 
         [0020]    The biogas reactor or bioreactor  10  of  FIG. 1  is made of steel-reinforced concrete or of ashlar-formed masonry blocks. The bioreactor  10  can be constructed in the manner of a prefabricated garage. The longitudinally extending reactor container  13  includes four planar concrete elements, namely, a bottom plate  17 , two side walls  18  and  19 , a cover plate  20  and an open front end  21 , an open back end  22  and a biogas removal connection  23 . The open front end  21  can be closed airtight by the loading gate  11 , and the open back end  22  can be closed airtight by the unloading gate  12 . Both gates  11 - 12  can be opened and closed hydraulically. Each of the gates  11 - 12  is constructed as a flap that articulates from the top of the reactor container  13 . For additional details regarding the design of a hydraulically actuated gate of a bioreactor, see U.S. Pat. No. 8,053,228 dated Nov. 8, 2011, entitled “Bioreactor Comprising a Retaining System,” which is incorporated herein by reference. 
         [0021]      FIG. 2  shows a perspective view of the front end  21  of the reactor container  13  with open loading gate  11 . A first retention device  15  in the form of a vertical wall element is provided in the reactor container  13  directly behind the loading gate  11 . In order to load the reactor container  13  with fresh biomass, the first retention device  15  is first removed. The biomass is then pushed up against a second retention device  16  provided adjacent to the unloading gate  12  at the back end of the reactor container  13 . Then the first retention device  15  is mounted into the reactor container  13  prior to closing the loading gate  11 . The first retention device  15  prevents the biomass from pressing directly against the loading gate  11 . The first retention device  15  therefore releases pressure from the airtight loading gate  11 . A first percolate drainage system  24  is provided in the bottom plate  17  between the closed loading gate  11  and the first retention device  15  to collect the percolate that seeps out of the biomass and through the first retention device  15 . Thus, the accumulation of percolate between the first retention device  15  and the closed loading gate  11  is prevented. The back end  22  of the bioreactor  10  is constructed similar to the front end  21  and includes a second percolate drainage system  25 .  FIG. 2  shows an open unloading gate  12  being shut. The second retention device  16  with a construction similar to that of the first retention device  15  is provided in front of the unloading gate  12  and also prevents the biomass from directly pressing against the unloading gate  12 . 
         [0022]      FIG. 3  is a cross-sectional view of reactor container  13 . Tubular hoses or pipes  26  are arranged inside the bottom plate  17  and the side walls  18 - 19 . A heating liquid such as hot water is pumped through the pipes  26 . The heating pipes  26  enable a thermophilic fermentation process to decontaminate the spent biomass. The heating pipes  26  run along the lengths of the two side walls  18 - 19  and bottom plate  17 . The heating liquid is circulated through the heating pipes  26  in order to maintain a temperature within the reactor container  13  above 55° C. 
         [0023]    For additional details regarding the construction of the reactor container  13 , see U.S. Pat. No. 8,105,823 dated Jan. 21, 2012, entitled “Biogas Installation for Production of Biogas from Biomass and Methods for Operation of the Biogas Installation,” which is incorporated herein by reference. The reactor container of the present invention, however, differs from the reactor container disclosed in U.S. Pat. No. 8,105,823 in that the present invention adds the second gate  12  for unloading, the second retention device  16  and the second percolate drainage system  25 . Spent biomass can be pushed out of the reactor container  13  through the unloading gate  12  without being contaminated in the loading zone  14 . 
         [0024]    U.S. Pat. No. 8,105,823 also provides additional details of a biogas plant that could be constructed from a plurality of reactor fermenters as described herein. Dry fermentation is carried out in each of the reactor containers  13  in a batch process. A constant supply of biogas can be supplied to a cogeneration unit by successively operating several fermenters in the dry fermentation biogas plant. Due to the batch-type operation, individual fermenters must be shut down from time to time. For example, the biogas production must be stopped, the fermented biomass must be removed from the respective fermenter, fresh biomass must be charged into the fermenter, and the biogas production is then resumed. Fresh biomass cannot be added in parallel to fermenters already running in the batch operation, as the quality of the biogas generated in each freshly charged fermenter is poor and would lower the quality of gas provided to the cogeneration unit. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
         
           
               10  bioreactor 
               11  first gate—loading gate 
               12  second gate—unloading gate 
               13  reactor container 
               14  contaminated loading zone 
               15  first retention device 
               16  second retention device 
               17  bottom plate 
               18 - 19  side walls 
               20  cover plate 
               21  open front end 
               22  open back end 
               23  biogas removal outlet 
               24  first percolate drainage 
               25  second percolate drainage 
               26  tubular heating hoses 
           
         
       
     
         [0041]    Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.