Abstract:
A combined installation is disclosed for the production of biogas and compost, including a dry fermentation fermenter for producing biogas in a batch mode, a biogas outlet, a purging gas inlet, a biogas line connected to the biogas outlet, a waste gas line, a waste gas chimney connected to the biogas outlet via a first biogas/waste gas line, a waste gas flare connected to the biogas outlet via a second biogas/waste gas line, a fresh air line connected to the purging gas inlet, a control means for connecting the biogas outlet to the biogas line or the biogas/waste gas chimney via the first biogas/waste gas line or the waste gas flare via the second biogas/waste gas line and for connecting the purging gas inlet to the waste gas line or the fresh air line, and a measurement means connected to the control means for detecting methane and carbon dioxide concentrations.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a combined installation for the production of biogas and compost from biomass, including at least one fermenter according to the principle of dry fermentation, according to claim  1 , and to a method of switching a fermenter in a like installation between biogas production and composting, according to claim  13 . 
         [0002]    So-called “dry fermentation” allows pourable biomasses from agriculture, from biological waste and from communal cultivated areas to be converted to methane without having to convert the materials to a liquid substrate which can be pumped. Biomasses having a dry substance content of up to 50% can be fermented. This dry fermentation method is described, for example, in EP 0 934 998. 
         [0003]    In the case of “dry” fermentation, the material to be fermented is not stirred into a liquid phase as is the case, for example, with liquid fermentation of organic waste. Instead, the fermentation substrate which has been introduced into the fermenter is kept moist all the time by drawing off the percolate at the bottom of the fermenter and spraying it over the biomass again. This results in optimum living conditions for the bacteria. During recirculation of the percolate, the temperature can moreover be regulated, and it is possible to add additives for process optimisation. 
         [0004]    From WO 02/06439 a bioreactor or fermenter having the form of a prefabricated garage is known, which is operated according to the principle of dry fermentation in the so-called batch process. In this case, after seeding with already fermented material, the fermenter is filled with the fermentation substrate by means of tractor shovels. The fermentation container is constructed in the form of a garage and is closed by a gastight door. The biomass is fermented with air being excluded, with no further thorough mixing being performed during the process, and with no additional material being supplied. The percolate which seeps out of the material being fermented is drawn off via a drainage groove, is temporarily stored in a tank, and is again sprayed over the fermentation substrate, in order to moisturize it. The fermentation process takes place in the mesophilic temperature range between 34 and 37° C., with temperature equalisation being carried out with the aid of floor heating and wall heating. 
         [0005]    The resultant biogas can be used to obtain electricity and heat in a block-type thermal power station (BHKW; Blockheizkraftwerk). In order to ensure that sufficient biogas is always available for the block-type thermal power station, a plurality of fermentation containers are operated with offset timings in the dry fermentation installation. At the end of the dwell time, the fermenter area is emptied completely and then refilled. The fermented substrate is subsequently supplied to composting, resulting in the production of an organic fertiliser that is comparable to conventional composts. 
         [0006]    Such fermenters for the production of biogas according to the principle of dry fermentation are further known from DE 203 19 847 U1 and from EP 1 681 274 A2. From DE 34 38 057 it is known to produce compost from the used or fermented biomass from a biogas installation. 
         [0007]    Batch operation makes it necessary to shut down the individual fermenters from time to time, i.e., after the biomass present in the fermenter was subjected to complete anaerobic conversion; in other words, 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 has to be resumed. This involves the drawback that it is necessary, for safety reasons, to prevent an explosive biogas/air mixture from being created while the individual fermenters are being loaded and unloaded. 
         [0008]    To this end, it is known from EP 1301583 B to flood a fermenter during its operation with waste gas containing carbon dioxide from the block-type thermal power station that is being operated with biogas, in the event of an explosion risk, that is to say if air has entered the fermenter. Subsequently the fermented biomass may be removed without any risk from the fermenter and supplied to a composting installation. 
         [0009]    It is therefore the object of the present invention to further develop a biogas installation as known from EP 1301583 B in such as way that post-composting of the spent biomass is simplified. 
         [0010]    This object is achieved through the features of claims  1  and  13 . 
         [0011]    Due to the fact that the spent biomass is composted in the fermenter by switching over from anaerobic fermentation to aerobic composting, it is no longer necessary to convert the spent biomass in a separate composter. The combined installation according to claim  1  includes the necessary components in order to enable safe switching, shutting down and unloading, as well as a safe start-up of a fermenter. The fermenter of the invention is configured such that the entire fermentation process, which consists of anaerobic fermentation and aerobic composting, may unfold inside it before it becomes necessary to remove the spent biomass and again charge the fermenter with fresh biomass. 
         [0012]    In accordance with a preferred aspect of the invention according to claim  10 , a first purging gas inlet opens into the fermenter in the area above the biomass. 
         [0013]    In accordance with a preferred aspect of the invention according to claim  11 , the fermenter comprises a floor plate having provided therein purging gas passages that are connected to a second purging gas inlet. 
         [0014]    In accordance with a preferred aspect of the invention according to claim  12 , the purging gas passages are configured for discharging seepage liquids seeping from the biomass during the production of biogas. 
         [0015]    Due to the provisions of claim  13 , biogas production and processing are maintained for as long as possible even while the fermentation process is terminated by purging with waste gas containing carbon dioxide, i.e., the biogas/waste gas mixture of the fermenter continues to be supplied to the biogas consumer until the quality of this mixture drops below a predetermined degree, before the fermenter is then switched over for composting of the fermented biomass contained in it. Only when the methane concentration in the biogas outlet drops below an upper limit, the biogas line leading to the biogas consumer is disconnected from the biogas outlet. After this, the biogas/waste gas mixture containing only a small quantity of methane is discharged via a waste gas chimney. This is carried out until the methane concentration has dropped to a lower limit at which virtually no methane is contained in the biogas/waste gas mixture any more. Afterwards the fermenter is purged not with waste gas containing carbon dioxide but with fresh air, and discharging the waste gas/biogas/fresh air mixture via the waste gas chimney is continued until the carbon dioxide concentration in the waste gas/biogas/fresh air mixture has dropped to a first limit. Only then the fermenter is switched over for composting. After termination of the composting process, the fermenter may be opened in order to unload the spent biomass and again charge the fermenter with fresh biomass. As a result of composting following fermentation, it is possible to open the fermenter for its unloading and reloading in the absence of any risk. 
         [0016]    In accordance with a preferred aspect of the invention according to claim  14 , the biogas/waste gas mixture is not emitted to the environment via the waste gas chimney when the upper limit of the methane concentration is reached, but is fed to a waste gas flare and burnt there. Optionally the waste gas flare may be supplied with additional fuel, so that combustion will take place in any case. Combustion of the biogas/waste gas mixture is performed until the methane concentration in the biogas/waste gas mixture becomes less than a medium limit that is situated between the upper and lower limits. 
         [0017]    In accordance with preferred aspects of the invention according to claims  15  and  16 , the composting process is controlled by adjusting the quantity and/or the temperature of the fresh air supplied via the fresh air line, to thus obtain an optimal process medium. 
         [0018]    In accordance with a preferred aspect of the invention according to claim  17 , the gas mixtures discharged from the fermenter are filtered. As a result of filtering, substances possibly detrimental to the consumers, which might result in clogging of valves, for instance, are removed to the largest possible extent. 
         [0019]    In accordance with a preferred aspect of the invention according to claim  18  and  19 , an explosive biogas/air mixture is safely prevented from being formed during start-up. 
         [0020]    This fermenter which has been started again is connected to the biogas line at a fourth methane concentration limit, which is equal to the upper limit—claim  20 . 
         [0021]    The waste gas for purging the fermenter is provided, for example, by an internal combustion engine—claim  21 . 
         [0022]    In accordance with a preferred embodiment of the invention according to claim  22 , the waste gas containing carbon dioxide is provided from a biogas processing means disposed downstream of the at least one fermenter. 
         [0023]    The remaining subclaims relate to advantageous aspects of the invention. 
         [0024]    Further details, features and advantages of the invention will become evident from the following description of exemplary embodiments with reference to the drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIGS. 1 to 7  show schematic representations of various operating states during termination of the fermentation process in a fermenter of a combined installation and during (re-)starting of the fermenter in accordance with a first embodiment of the invention; 
           [0026]      FIG. 8  shows a schematic illustration of a second embodiment of the invention including a fermenter; 
           [0027]      FIGS. 9 to 15  show schematic representations of various operating states of a combined installation including three fermenters during termination of the fermentation process in a fermenter of a combined installation and during (re-)starting of a fermenter; 
           [0028]      FIG. 16  is a representation corresponding to  FIG. 1  of a third embodiment of the invention including a waste gas and fresh air supply, respectively, from the floor plate of the fermenter; 
           [0029]      FIG. 17A  is a top view of the floor plate having purging gas passages of the fermenter in accordance with the embodiment of  FIG. 16 ; 
           [0030]      FIG. 17B  is a sectional view along line B-B of  FIG. 17A  with transverse passage and the purging gas passages; and 
           [0031]      FIG. 17C  is a sectional view along line C-C of  FIG. 17A  with the purging gas passages. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]      FIGS. 1 to 7  show a first embodiment of a combined installation according to the present invention including a single fermenter  2 . The fermenter  2  has a cuboid shape and is constructed approximately in the form of a prefabricated garage. The fermenter  2  can be filled with biomass  6  and emptied again by means of a tractor shovel through a loading and unloading opening  4  which extends over one of the end faces of the cuboid fermenter  2 . Reference is made to WO 02/06439 with regard to details of the construction of the fermenter  2 . 
         [0033]    The fermenter  2  further includes a biogas outlet  8  adapted to be connected via valves  10  to a biogas line  12 , a first biogas/waste gas line  14  and a second biogas/waste gas line  16 . The biogas line  12  leads to a block-type thermal power station  18  constituting a biogas-utilising means. The first biogas/waste gas line  14  leads to a biogas/waste gas chimney  20 . The second biogas/waste gas line  16  leads to a waste gas flare  22 . Furthermore, the fermenter  2  includes a purging gas inlet  24  adapted to be connected via valves  10  to a waste gas line  26  or to a fresh air line  28 . A waste gas fan  27  is arranged in the waste gas line  26  and may be used to pump waste gas into the fermenter  2 . A fresh air fan  29  for sucking in fresh air from the environment is arranged in the fresh air line  28 . Waste gas containing carbon dioxide is passed into the fermenter  2  as purging gas via the waste gas line  26 , and fresh air is passed into the fermenter  2  via the fresh air line  28 . 
         [0034]    The valves  10  are connected to a control means  30  and are opened or closed by means of the control means  30 . The control means  30  is also connected to a first measurement sensor  32  which is arranged in the biogas outlet  8  and detects the methane concentration in the respective gas mixture. The control means  30  is furthermore connected to a second measurement sensor  34  which is likewise arranged in the biogas outlet  8  and detects the carbon dioxide concentration in the respective gas mixture. The control means  30  is also connected to a third measurement sensor  36  which is arranged in the biogas outlet  8  and detects the gas volume flow in the biogas outlet. Optionally the extraction of gas from the fermenter  2  can be assisted by a fan  38  which is arranged in the biogas outlet. 
         [0035]      FIGS. 1 to 7  show various phases of terminating the fermentation process inside the fermenter  2  and starting the fermenter  2 , with active lines and positions of components being illustrated by solid lines, while lines and positions of components which are inactive or shut off, respectively, are illustrated by dashed lines. In accordance with the invention, aerobic conversion of the fermented biomass follows immediately after the fermentation process in the same fermenter by suitably switching the latter over to composting before the fermenter is then opened, unloaded, reloaded and started up again. 
         [0036]      FIG. 1  shows the first phase of terminating the fermentation process inside the fermenter  2 , in which waste gas containing carbon dioxide is pumped into the interior of the fermenter  2  via the waste gas line  26  and the purging gas inlet  24 . The biogas outlet  8  is still connected to the biogas line  12 , so that the biogas/waste gas mixture continues to be passed on to the block-type thermal power station  18 . 
         [0037]    Only when the methane concentration detected by the first measurement sensor  32  in the biogas outlet  8  has dropped below an upper limit, the valve  10  in the biogas line  12  is closed by the control means  30  and the valve  10  in the second biogas/waste gas line  16  is opened in a second phase, as is illustrated in  FIG. 2 . In this second phase of terminating the fermentation process in the fermenter  2 , the biogas/waste gas mixture is burnt in the waste gas flare  22 . Optionally this combustion process can be assisted by adding additional fuel. 
         [0038]    When the methane concentration detected by the first measurement sensor  32  in the biogas outlet  8  has dropped below a medium limit, the valve  10  in the second biogas/waste gas line  16  is closed by the control means  30  and the valve  10  in the first biogas/waste gas line  14  is opened in a third phase, as is illustrated in  FIG. 3 . In this third phase of terminating the fermentation process inside the fermenter  2 , the biogas/waste gas mixture is emitted to the environment via the waste gas chimney  20 . 
         [0039]    When the methane concentration detected by the first measurement sensor  32  in the biogas outlet  8  has dropped below a lower limit, the valve  10  in the waste gas line  26  is closed and the valve  10  in the fresh air line  28  is opened in a fourth phase, as is illustrated in  FIG. 4 . In this fourth phase of terminating the fermentation process inside the fermenter  2 , fresh air is pumped into the fermenter  2  via the fresh air line  28  and the purging gas inlet  24 . The waste gas/air mixture continues to be emitted to the environment via the biogas outlet  8  and the first biogas/waste gas line  14  in the waste gas chimney  20 . 
         [0040]    When the carbon dioxide concentration detected by the second measurement sensor  34  in the biogas outlet  8  has dropped below a first limit, the fermenter is switched over to an aerobic process management, so that the fermented biomass present in the still-closed fermenter is composted. At the end of composting, the valve  10  in the fresh air line  28  is closed by the control means  30  and the loading and unloading opening  4  is opened, as is illustrated in  FIG. 5 . 
         [0041]    Once the fermenter  2  has been again charged with fresh biomass, the loading and unloading opening  4  is closed, the connection between biogas outlet  8  and waste gas chimney  20  via the first biogas/waste gas line  14  is maintained, and the control means  30  opens the valve  10  in the waste gas line  26 , so that waste gas containing carbon dioxide is pumped into the fermenter  2 —see  FIG. 6 . This is continued until the carbon dioxide concentration in the biogas outlet  8  as detected by the second measurement sensor  34  reaches or exceeds a second limit. 
         [0042]    When this second limit for the carbon dioxide concentration has been reached, the control means  30  closes the valve  10  in the waste gas line  26  and in the first biogas/waste gas line  14  and opens the valve  10  in the biogas line  12 , as is illustrated in  FIG. 7 . The biogas production phase has thus been reached again, and the biogas produced in the fermenter  2  is supplied to the block-type thermal power station  18  via the biogas line  12 . 
         [0043]    In the embodiment described above, all of the measurement sensors  32 ,  34 ,  36  are arranged in the biogas outlet  8 . According to a second embodiment of the present invention, the second and third measurement sensors  24 ,  36  may alternatively also be arranged in the first and second biogas/waste gas line  14 , 16 , respectively.  FIG. 8  shows an alternative aspect of the invention which differs from the embodiment shown in  FIGS. 1 to 7  in that the first and second biogas/waste gas lines  14 , 16  are combined to form a common biogas/waste gas line  40  before they open into the biogas outlet  8 . The second measurement sensor for detection of the carbon dioxide concentration is arranged in the common biogas/waste gas line  40 , and the third measurement sensor  36  is arranged in the first biogas/waste gas line  14 . For the rest, this second embodiment of the invention corresponds to the first embodiment. The operation is also identical. 
         [0044]      FIGS. 9 to 15  show a third embodiment of a combined installation according to the present invention, in which three fermenters  2 - 1 ,  2 - 2  and  2 - 3  (in the following collectively designated as “ 2 - i ”) are provided in parallel operation. Mutually corresponding components are provided with the same reference symbols. In the combined installation shown in  FIGS. 9 to 15 , each of the three fermenters  2 - i  is provided with a purging gas inlet  24 - 1 ,  24 - 2  and  24 - 3 , respectively, each of which may be shut off by a valve  10 . The three purging gas inlets  24 - i  are combined to form a common purging gas inlet  42 . A waste gas line  26  and a fresh air line  28 , each of which may be shut off by a valve  10 , open into the common purging gas inlet  42 . 
         [0045]    The three fermenters  2 - i  are each provided with a respective biogas outlet  8 - 1   8 - 2  and  8 - 3  that are each adapted to be shut off by a respective valve  10 . The first biogas/waste gas line  14  to the waste gas chimney  20  and the second biogas/waste gas line  16  to the waste gas flare  22  are combined to form a common biogas/waste gas line  40  having a fan  38  arranged in it. Downstream from the fan  38 , the common biogas/waste gas line  40  splits into first, second and third biogas/waste gas line elements  40 - 1 ,  40 - 2  and  40 - 3 . The first biogas/waste gas line element  40 - 1  opens into the first biogas outlet  8 - 1  between the valve  10  and the first fermenter  2 - 1 . The second biogas/waste gas line element  40 - 2  opens into the second biogas outlet  8 - 2  between the valve  10  and the second fermenter  2 - 2 . The third biogas/waste gas line element  40 - 3  opens into the third biogas outlet  8 - 3  between the valve  10  and the third fermenter  2 - 3 . The three biogas/waste gas line elements  40 - 1 ,  40 - 2  and  40 - 3  may each be shut off by a respective valve  10 . The three biogas outlets  8 - 1 ,  8 - 2  and  8 - 3  open into a common biogas line  12  which leads to a block-type thermal power station  18 . An exhaust line  44  from the block-type thermal power station  18  opens into a second waste gas chimney  46 . The waste gas line  26  is connected via a 3-way valve  48  to the exhaust line  44 , i.e., the waste gas containing carbon dioxide which occurs in the block-type thermal power station  18  is used to purge a fermenter  2 - i  whose fermenting process is to be terminated and which is to be switched over to the composting process. The 3-way valve allows to regulate the volume flow of the waste gas which is sent via the waste gas line  26  for purging a fermenter  2 - i,  as well as the amount of waste gas which is emitted to the environment via the second waste gas chimney  46 . 
         [0046]    A first measurement sensor  32  for detection of the methane concentration is arranged in the common biogas line  12 . A second measurement sensor  34  for detection of the carbon dioxide concentration, a third measurement sensor  36  for detection of the volume flow, and a fourth measurement sensor  50  for detection of the methane concentration are arranged in the common biogas/waste gas line  40 , downstream from the fan  38  in the flow direction. The four measurement sensors  32 ,  34 ,  36 , and  50  are connected to a control means  30 . The various valves  10  are likewise connected to the control means. These control lines are not shown in  FIGS. 9 to 15  for reasons of clarity. 
         [0047]      FIGS. 9 to 15  illustrate termination of the fermentation process inside the fermenter  2 - 2  and restarting of the second fermenter  2 - 2  after the composting process which immediately follows the fermentation process and is initiated by switching over the fermenter  2 - 2 , with  FIGS. 9 to 15  representing the same phases and operating states as  FIGS. 1 to 7 . The biogas production of the first and third fermenters  2 - 1  and  2 - 3 , respectively, takes place continuously during termination of the fermentation process and of the composting process in the second fermenter  2 - 2  and during restarting of the second fermenter  2 - 2 . 
         [0048]      FIG. 9  shows the first phase of termination of the fermentation process inside the fermenter  2 - 2 , in which phase waste gas containing carbon dioxide from the block-type thermal power station  18  is pumped into the interior of the fermenter  2 - 2  via the 3-way valve  48  and the waste gas line  26 , the waste gas fan  27  and the second purging gas inlet  24 - 2 . As before, the second biogas outlet  8 - 2  is connected to the common biogas line  12 , so that the biogas/waste gas mixture continues to be supplied to the gas processing installation  44 . 
         [0049]    Only when the methane concentration detected by the first measurement sensor  32  in the common biogas line  12  has dropped below an upper limit, the control means  30  closes the valve  10  in the second biogas outlet  8 - 2  and opens the valve  10  in the second biogas/waste gas line element  40 - 2  and in the second biogas/waste gas line  16  in a second phase, as is illustrated in  FIG. 10 . In this second phase of terminating the fermentation process inside the fermenter  2 - 2 , the biogas/waste gas mixture is burnt in the waste gas flare  22 . Optionally this combustion process can be assisted by adding additional fuel. 
         [0050]    When the methane concentration detected by the fourth measurement sensor  50  in the common biogas/waste gas line  40  has dropped below a medium limit, the control means  30  closes the valve  10  in the second biogas/waste gas line  16  and opens the valve  10  in the first biogas/waste gas line  14  in a third phase, as is illustrated in  FIG. 11 . In this third phase of terminating the fermentation process inside the fermenter  2 - 2 , the biogas/waste gas mixture is emitted to the environment via the waste gas chimney  20 . 
         [0051]    When the methane concentration detected by the fourth measurement sensor  50  in the common biogas/waste gas line  40  has dropped below a lower limit, the control means  30  closes the valve  10  in the waste gas line  26 , appropriately switches the 3-way valve  48 , and opens the valve  10  in the fresh air line  28  in a fourth phase, as is illustrated in  FIG. 12 . In this fourth phase of terminating the fermentation process inside the fermenter  2 - 2 , fresh air is pumped into the fermenter  2 - 2  by the fresh air fan  29  via the fresh air line  28  and the purging gas inlet  24 . The waste gas/air mixture continues to be emitted to the environment via the second biogas outlet  8 - 2 , the second biogas/waste gas line element  40 - 2 , the common biogas/waste gas line  40 , and the first biogas/waste gas line  14  in the waste gas chimney  20 . Optionally this can be assisted by the fan  38 . 
         [0052]    When the carbon dioxide concentration detected by the second measurement sensor  34  in the common biogas line  40  has dropped below a first limit, the fermenter  2 - 2  is switched over to initiate composting, and the valve  10  in the fresh air line  28  is closed by the control means  30 . Following termination of composting it is possible to open the fermenter  2 - 2 , remove the spent biomass, and charge fresh biomass. 
         [0053]    Once the fermenter  2 - 2  has been recharged with fresh biomass, the loading and unloading opening is closed, the connection between the second biogas outlet  8 - 2  and the waste gas chimney  20  via the second biogas/waste gas line element  40 - 2 , the common biogas/waste gas line, and the first biogas/waste gas line  14  is maintained, and the control means  30  opens the valve  10  in the waste gas line  26  and switches the 3-way valve  48  in the exhaust line  44  of the block-type thermal power station  18 , so that waste gas containing carbon dioxide is pumped into the fermenter  2 - 2 —see  FIG. 14 . This process continues until the carbon dioxide concentration detected by the second measurement sensor  34  in the common biogas/waste gas line  40  has reached or exceeded a second limit. 
         [0054]    When this second limit for the carbon dioxide concentration has been reached, the control means  30  closes the valve  10  in the waste gas line  26 , switches the 3-way valve  38 , closes the valve  10  in the second biogas/waste gas line element  40 - 2 , and opens the valve  10  in the second biogas outlet  8 - 2 , as is illustrated in  FIG. 15 . Thus the second fermenter  2 - 2  has also once again reached the phase of biogas production, and the biogas produced in the fermenter  2 - 2  is supplied to the block-type thermal power station  18  via the biogas line  12  of the gas processing installation  44 . The biogas outlet  8 - 2  is not connected to the common biogas line  12  until the methane concentration detected by the fourth measurement sensor  50  has reached a fourth limit. This fourth limit coincides with the upper limit. 
         [0055]    The valve  10  in the waste gas line  26  may be omitted since its function can also be carried out by the 3-way valve  48 . 
         [0056]    In the following, exemplary numerical values for the various limits are given: 
         [0000]                                                    Methane concentration:   upper limit   30% to 50%               medium limit   10% to 20%               lower limit   0% to 3%               fourth limit   30% to 50%           Carbon dioxide concentration:   first limit   0.5% to 2%               second limit   5% to 15%                        
The waste gas volume flow in the waste gas line  26  is between 150 and 1000 m 3 /h, depending on the size of the fermenters and the amount of waste gas available. The fresh air volume flow in the fresh air line  28  is between 1000 and 5000 m 3 /h.
   FIG. 16  shows a representation corresponding to  FIG. 1  of a combined installation according to a fourth embodiment, which differs from the first embodiment according to  FIGS. 1 to 7  in that the purging gas having the form of waste gas or fresh air is supplied in the various operating states not only via the first purging gas inlet  24  in the area above the biomass  6 , but additionally or alternatively via a second purging gas inlet  25  in the area of the floor plate of the fermenter  2 . This has the effect that biogas present inside the biomass  6  is also “purged out” securely. Moreover this has the effect that the methane slip during loading and unloading of the fermenter is reduced further.
   FIG. 17A  shows a top view of the floor plate of the fermenter  2  in the embodiment according to  FIG. 16 . Purging gas passages  52  covered by a liquid- and gas-permeable grid  54  are provided in the floor plate of the fermenter  2  in a longitudinal direction. The various purging gas passages  52  that extend in parallel are interconnected in one ore several locations by a transverse passage  56  extending transversely to the longitudinal direction of the purging gas passages  52 . The second purging gas inlet  25  opens into this transverse passage  56 .  FIG. 17B  shows a sectional view along line B-B of  FIG. 17A  with transverse passage  56  and purging gas passages  52 .  FIG. 17C  shows a sectional view along line C-C of  FIG. 17A  with purging gas passages  52 .
 
In the embodiments of the invention according to  FIGS. 8 and 9  to  15 , the supply of purging gas may also be effected via purging gas passages  52  in the floor of the fermenter  2 . In the various embodiments the loading and unloading opening is provided on the left-hand side of the fermenter  2 . The loading and unloading opening may also be provided on the opposite side.