Abstract:
A fermenter for a biogas plant which operates with wet fermentation includes a fixed casing which surrounds a reaction space, an inlet for biomass and an outlet for the fermentation reside. A wall is arranged inside the casing, which separates the inlet from the outlet.

Description:
RELATED APPLICATION 
       [0001]    This is the U.S. national phase filing of PCT/EP2010/001321, filed Mar. 3, 2010, which claims priority to German Application Serial No. 10 2009 012 418.7, filed Mar. 10, 2009. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to a fermenter for a biogas plant which operates with wet fermentation (up to 15% dry matter), comprising a fixed casing which surrounds a reaction space, an inlet for biomass and an outlet for the fermentation residue. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the fermenter, the biomass is fermented under anaerobic conditions, wherein biogas is obtained. With regard to an optimum efficiency it is desirable when the biomass introduced into the fermenter only leaves the same when it is fermented completely. In practice, however, this can only be achieved with great difficulty. In the usually employed fermenters, which operate by the wet fermentation process, the problem exists that during removal of the fermentation residue, some unfermented biomass always also leaves the fermenter (“loss of fresh weight”). This loss of fresh weight could not be avoided in the previously used fermenters, which operate by a pot process. Fermenters should be improved such that the loss of fresh weight is reduced or even avoided completely. 
       SUMMARY OF THE INVENTION 
       [0004]    A fermenter is formed inside a casing a wall to separate the inlet from the outlet. This prevents freshly introduced biomass from directly reaching the outlet such that the biomass cannot leave the fermenter without being fermented completely. 
         [0005]    In one example, the casing wall extends spirally. In this way, a comparatively long path can be defined inside the fermenter, which the biomass must cover from the inlet to the outlet. 
         [0006]    In accordance with an exemplary embodiment of the invention it is provided that the wall defines a spiral path for the biomass inside the reaction space, wherein the inlet is arranged at one end of the path and the outlet is arranged at the other end. When the biomass is supplied continuously, this design allows adjusting a slow movement of the biomass along the path towards the outlet, with the retention time of the biomass in the fermenter being determined such that the biomass is fermented completely when it reaches the outlet. In this way, an optimum efficiency is obtained, as on the one hand no unfermented biomass can leave the fermenter and on the other hand biomass fermented already does not stay in the fermenter for an excessively long period, occupying volume therein which might otherwise be utilized for the fermentation process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention will be described below with reference to a preferred embodiment which is represented in the attached drawings, in which: 
           [0008]      FIG. 1  shows a cross-section through a fermenter according to the invention; and 
           [0009]      FIG. 2  shows a section along plane II-II of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIGS. 1 and 2  show a fermenter  10  which is part of a biogas plant. The fermenter  10  is formed by a fixed casing  12  which is arranged on a foundation  14 . The casing  12  is cylindrical, with the central axis of the cylinder being vertical to the foundation  14 . In the interior of the casing  12  a wall  16  is arranged, which extends spirally. An outer end  16 A of the wall  16  touches the casing  12 , whereas the inner end  16 B of the wall  16  ends in the vicinity of the central axis of the cylindrical casing  12 . By the spiral wall  16  a path is formed which extends helically from the outside, i.e. from the casing  12 , towards the center. This path extends in a vertical direction from the upper edge of the wall  16  down to the foundation  14 . On the upper surface of the casing  12  a lid  15  is arranged, so that in the interior of the casing  12  a sealed reaction space is formed. The lid  15  is located at a distance from the upper edge of the wall  16 , so that in the upper region of the fermenter a continuous gas collecting space is formed. 
         [0011]    Both the casing  12  and the spiral wall  16  can be made of metal and/or concrete. The lid  15  here also is a heavy construction. Alternatively, a plastic hood can also be used. 
         [0012]    In the vicinity of the outer end  16 A of the spiral wall  16  an inlet  18  for biomass is arranged, which extends through the casing  12 . Approximately in the middle of the reaction space of the fermenter enclosed by the casing  12  and hence in the interior of the spiral formed by the wall  16 , an outlet  20  is arranged for the fermentation residue, i.e. for the fermented biomass. Due to the spiral wall  16 , the biomass moves along a predetermined path through the fermenter, in order to get from the inlet  18  to the outlet  20 . Along the path a plurality of stirrers  22  are arranged, which ensure a uniform vertical intermixing. Furthermore, the fermenter is provided with a heater H, which ensures a constant temperature in the interior of the fermenter. 
         [0013]    In operation, biomass is supplied to the fermenter via the inlet  18 . Corresponding to the quantity supplied, completely fermented biomass is discharged through the outlet as fermentation residue. Therefore, a very slow flow of the biomass is obtained from the inlet  18  to the outlet  20  in direction of the arrow P of  FIG. 2 . The resulting different levels of the biomass along the path formed are indicated in  FIG. 1  with the arrows N. While proceeding from the inlet  18  to the outlet  20 , the biomass is regularly circulated by the stirrers  22  in a vertical direction, so that the fermentation process proceeds uniformly. The biogas obtained during the fermentation is removed via a gas vent  24 . 
         [0014]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.