Abstract:
Disclosed is a bioreactor, especially a plug flow bioreactor, including an agitator running along a longitudinal axis for thoroughly mixing a dry solid biomass that is to be outgassed in a fermentation chamber. In said bioreactor, an agitator shaft is mounted exclusively in two opposite end walls of the fermentation chamber while at least one free end of the agitator is connected to a drive unit outside the fermentation chamber.

Description:
PRIORITY CLAIM 
     This is a U.S. national stage of application No. PCT/DE2007/002190, filed on Dec. 6, 2007. Priority is claimed on German Application No. 10 2006 058 030.3, filed Dec. 7, 2006 and German Application No. 10 2007 014 417.4, filed Mar. 22, 2007, the contents of both of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a bioreactor, especially a plug flow bioreactor, which has a long-shaft agitator for thoroughly mixing a biomass with a high solids content that is to be degassed in a fermentation chamber. 
     2. Description of the Prior Art 
     The use of bioreactors in biogas plants is well known and has proven effective. In this connection, bioreactors are used for the fermentation of biowaste, green waste, food waste or the like, which has a high solids content, for example, more than 40%, without the addition of liquid manure. The resulting fermentation is dry mesophilic or thermophilic fermentation at moderate temperatures, for example, 37-41° C. or 50-60° C. 
     In addition, a plug flow bioreactor can be continuously charged, which is equivalent to a large input of biomass to be fermented, which in turn results in a high gas yield. The agitator ensures optimal degassing and a uniform temperature distribution, and the results plug flow ensures a well-defined residence time of the substrate in the bioreactor. 
     Plug flow bioreactors of the type under discussion are comparatively complicated in their design and normally require a large number of additional buildings, for example, operations buildings, gas accumulators, etc. 
     The capital expenses of biogas plants operated in this way are correspondingly high. 
     SUMMARY OF THE INVENTION 
     Against this technical background, an objective of the invention is to develop a bioreactor that can be built simply and thus inexpensively in a short construction time and which has a high gas output and yet is very easy to maintain. 
     In a embodiment, a bioreactor, especially a plug flow bioreactor, having a long-shaft agitator for thoroughly mixing a biomass with a high solids content that is to be degassed in a fermentation chamber is disclosed. A shaft of the agitator is supported in two opposite end walls of the fermentation chamber and at least one free end of the agitator is connected to a drive outside the fermentation chamber. 
     Due to a preferably exclusive mounting of the long-shaft agitator in the end walls, undisturbed plug flow develops, which ensures uniform material throughput and uniform temperature distribution in the substrate to be degassed. Dead water at an additional center bearing of the agitator shaft is avoided, since a center bearing of this type rotates completely in the fermentation substrate. 
     The arrangement of the drive outside the fermentation chamber allows simple inspection of the chamber without the need for personnel to make a dangerous entrance into the chamber on a regular basis. In accordance with one embodiment of the invention, entrance of personnel into the fermentation chamber of a bioreactor thus becomes a rare occurrence. 
     In one embodiment of the bioreactor the torque of the drive decoupled from a rising end wall of the fermentation chamber is introduced into a foundation. This end wall can then also be correspondingly weakly dimensioned. 
     Alternatively, the torque of the drive, is introduced into a rising end wall of the fermentation chamber by a torque support. 
     Both alternatives allow safe absorption of the high drive power of about 30 kW at a torque of about 100,000 Nm without any weakening of the wall in the immediate vicinity of the opening for the bearing or the shaft due to a large number of fastening bolts, heavy-duty dowels or the like. 
     Furthermore, both bearings of the agitator are preferably accessible and replaceable from outside the fermentation chamber, for inspection as well as in the event of damage, naturally, after the substrate level in the fermentation chamber has been lowered sufficiently. Entrance into the fermentation chamber is therefore not necessary. 
     A central shaft of the agitator is preferably several axial sections of reduced diameter at each end wall. Accordingly, the bearings in the end walls can also have a small diameter, so that the end walls are weakened only slightly by the corresponding openings for the bearings. 
     In a bioreactor accordingly to one embodiment of the invention, the walls are segmented precast concrete units rest on continuous footings. The construction of the bioreactor with segmented precast concrete units does not require an expensive foundation but rather only the construction of cost-effective continuous footings to reduce capital expenses. The short construction time and a large degree of independence from weather contribute further to reduced capital expenses. Beyond that, especially a very high, constant concrete quality, which withstands even aggressive substrates and gases at all times, is guaranteed. 
     In a further embodiment of the bioreactor of the invention, the undersides of the walls have a foot with an L-shaped bend and that the foot rests on a continuous footing. Without any anchoring, a segmented precast concrete unit of this is set onto the continuous footing and aligned. If all of the precast concrete units are correctly positioned and possibly braced, a bioreactor bottom plate, which may consist of multiple layers, is constructed by casting into the space between the walls. 
     The use of precast concrete units provides a simple for the longitudinal walls to taper towards the top. This takes the decreasing pressure by the substrate into account, and material can thus be saved. 
     Moreover, the use of segmented precast concrete units, for a wall includes at least longitudinal bracing, in the form of tension rods. A mechanically stable structure is guaranteed in this way. 
     The mechanical stability of the bioreactor is further enhanced if opposite segments of the precast concrete units of the longitudinal walls are joined on the upper side by a tension rod placed under a tensile load and a beam placed under a compressive load. Due to this distribution to a tension rod that can be placed under a tensile load and a beam that can be placed under a compressive load, the tension rod and the beam can be designed optimally and with the appropriate material. In particular, the tension rod is made of a metal, especially a high-grade steel, and that the beam is a wooden beam that is inserted in pockets in the walls. This further reduces capital expenses. 
     Mechanical stability of the bioreactor allows the fermentation chamber to be covered by a roof that serves as a gas accumulator designed with a semicircular cross section. This avoids an external gas accumulator. Furthermore, there is no need for a heavy and expensive concrete roof that seals the fermentation chamber at the top. In accordance with one embodiment of the invention, it is possible, for the roof to be constructed in the manner of an inflatable hall, in particular, for it to be formed by a sheet, and for it to be removed during operation. Inspection work is thus greatly simplified, and, when necessary, the long-shaft agitator can be removed and restored without any trouble when the roof has been removed. In particular, these measures also make it possible largely to avoid dangerous work inside a closed fermentation chamber. 
     Finally, the fermentation chamber is covered by a net. A net of this type serves effectively for the colonization of sulfur bacteria, which are useful in the fermentation processes in a bioreactor of the aforementioned type. In addition, the net, which is supported on the tension rods and/or stays, supports a membranous sheet on the underside of the gas accumulator and prevents the sheet from sagging into the fermentation chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below with reference to the figures, which show schematic representations of a specific embodiment of the invention. 
         FIG. 1  is a bioreactor; 
         FIG. 2  is a partial section of the bioreactor of  FIG. 1  in a top view; 
         FIG. 3  is a section along line III in  FIG. 2 ; 
         FIG. 4  is a section along line IV in  FIG. 2 ; 
         FIG. 5  is a section along line V in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is an off-center longitudinal section through a bioreactor  1  of one embodiment of the invention with an operations building  2  is connected to an end face of the bioreactor  1 . The bioreactor is designed as a plug flow bioreactor with a long-shaft agitator  3 , which has a central shaft  4  with a plurality of blades  5  that rotate spirally in the axial direction. The shaft  4  of the agitator  3  is supported on bearings exclusively in the two opposite end walls  6 ,  7 . A center bearing is preferably deliberately dispensed with. 
     The shaft  4  passes through the wall  7  on the end opposite the operations building  2  and is connected to an electric motor drive  9  outside the fermentation chamber  8 . The torque produced by the drive  9  is introduced into a foundation  11  via a triangular support structure  10  decoupled from the end wall  7  of the fermentation chamber  8 , (see also  FIG. 3 ). 
     The shaft  4 , which is supported in the bearings  12 ,  13  in the walls  6 ,  7 , is preferably divided into five axial sections. The two end sections  14 ,  15  of the shaft  4  shown in  FIG. 2  have a reduced diameter relative to the three middle sections. The bearings  12 ,  13  of the shaft  4 , as well as the drive  9 , are accessible and replaceable from outside the fermentation chamber  8 . 
       FIG. 1  also shows that the end walls  6 ,  7 , which in one embodiment, have essentially constant material thickness over their height, are each preferably formed by at least three horizontally subdivided precast concrete units. The lowermost precast concrete units  16 ,  17  preferably have L-shaped feet by which they are supported on the foundation or continuous footings  11 ,  18 . 
     Similarly to the end walls  6 ,  7 , the longitudinal walls  19 ,  20  shown in  FIG. 2  are formed by precast concrete units, which likewise are preferably horizontally and vertically segmented. The lowermost rows of the precast concrete units of the longitudinal walls are likewise supported on continuous footings  21 ,  22  as shown in  FIG. 3 . When at least the lowermost rows of the precast concrete units have been placed so that they are positioned on the continuous footings  11 ,  18 ,  21 ,  22 , a multilayer floor  23  that joins the walls  6 ,  7 ;  19 ,  20  is cast. 
     The end walls  6 ,  7  preferably have an essentially constant cross section over their height. Like the end walls  6 ,  7 , the longitudinal walls  19 ,  20  are also made to seal the fermentation chamber  8  and have an insulating layer that faces outward and is covered by a plate. 
     In addition to an opening for holding the bearing  13 , the end wall  7  preferably has one opening  24  for a manhole. The wall  6  is provided with a number of other openings, through which a variety of different conduction systems can be passed into the operations room  2 . For example, as shown in  FIG. 4 , openings  25 ,  26  in end wall  7  are preferably lined with fiber-cement sheaths and carry out the feed and return of a plurality of heaters  27  to  30  installed on the longitudinal walls  19 ,  20  that are vented via pipelines  51  that pass through the wall  6  to the outside. 
     Heaters  29  are installed at a level below a minimum level  31  of the substrate introduced into the fermentation chamber  8 . This minimum level  31  is indicated by a dot-dash line. The upper level  32 , which is also indicated by a dot-dash line, lies above the upwardly directed blades  5  of the agitator  3 . 
     In one embodiment, the biomass is introduced through a supply line  33  in the longitudinal wall  19 . In the immediate vicinity of this supply line  33 , liquid substrate is introduced through another supply line  34 . Accordingly, at the other end of the bioreactor  1 , a lower substrate outlet  35  and an upper substrate outlet  36  are provided in the wall  6 .shown in  FIG. 4 . 
     Openings  37 ,  38  in the wall  6  allow measurements of the level of filling and/or the temperature. Openings  39 ,  40  enclosed by transparent glass make it possible to look into the fermentation chamber  8  from a work platform  41  on the roof of the operations building  2 , which can be reached by stairs  42 . 
     Another opening  43  serves the purpose of negative and positive pressure safety, and still another opening  44  formed above the upper level  32  serves for the passage of a pipe system  45  for the removal of gas. 
     The longitudinal walls  19 ,  20  have a cross section that preferably tapers towards the top as shown in  FIGS. 3 and 4 . To obtain good mechanical bracing of the precast concrete units, the two longitudinal walls  19 ,  20  have a plurality of longitudinal stays  46 ,  47 , configured as tension rods, for longitudinally bracing the lateral walls  19 ,  20 . 
     In addition, opposite segments of precast concrete units of the two longitudinal walls  19 ,  20  are joined on the upper side by a tension rod  48  that is preferably made of metal, especially high-grade steel, that can be placed especially under a tensile load and by a wooden beam  49  that can be placed under a compressive load and is merely inserted in pockets of the segments of the longitudinal walls  19 ,  20 . This ensures that the walls  6 ,  7 ,  19 ,  20  have sufficient stability to allow a roof  50  to be placed on them. The roof  50  shown in  FIG. 5  is designed with a semicircular cross section and is provided as a gas accumulator, from which the gas formed in the fermentation chamber  8  can be removed through the pipe system  45 , which opens above the upper level  32  of the substrate. 
     The roof  50  is preferably formed in the manner of an inflatable hall, whose gas-tight sheet can also be removed during operation, so that the fermentation chamber  8  is practically freely accessible from above, for example, for inspection work. 
     A net  60  that serves as a growth surface for sulfur bacteria and thus allows effective internal desulfurization is preferably also provided between the roof  50  and the fermentation chamber  8 . In addition, the net, supported on the tension rods  48  and/or the beam  49 , prevents a membranous sheet on the underside of the gas accumulator from sagging into the fermentation chamber  8 . 
     Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.