Abstract:
A biogas plant produces both methane and liquid fertilizer by fermenting biomass. The plant includes a fermenter, a percolate tank and a sanitation tank located inside the percolate tank. Dry fermentation takes place in the fermenter and generates methane and a percolate. The percolate is circulated between the fermenter and the percolate tank. Percolate that is returned from the percolate tank to the fermenter is sprinkled over the biomass. A portion of the percolate is transferred from the percolate tank into the sanitation tank. The percolate in the sanitation tank is sanitized by heating to a Celsius temperature between 45° and 65° for a period of at least five days. The percolate in the sanitation tank is heated using both a heating device in the sanitation tank as well as heat generated from a thermophilic fermentation reaction occurring in the percolate tank. The sanitized percolate is used as liquid fertilizer.

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/EP2014/064338, filed on Jul. 4, 2014, and published as WO 2015/001091 A1 on Jan. 8, 2015, which in turn claims priority from German Application No. 1020132113258.1, filed in Germany on Jul. 5, 2013. This application is a continuation-in-part of International Application No. PCT/EP2014/064338, which is a continuation of German Application No. 1020132113258.1. International Application No. PCT/EP2014/064338 is pending as of the filing date of this application, and the United States is an elected state in International Application No. PCT/EP2014/064338. This application claims the benefit under 35 U.S.C. §119 from German Application No. 1020132113258.1. The disclosure of each of the foregoing documents is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method of producing a liquid fertilizer and a biogas plant for the realization of the method. 
       BACKGROUND 
       [0003]    Biogas plants serve to produce biogas by way of fermentation of biomass in so-called fermenters. Depending on the type of the biomass to be fermented, fermentation can be dry fermentation or wet fermentation. The biogas contains mainly methane. As is known from European patent EP1301583 B1, during dry fermentation, the biomass that is used is dry and contains interfering substances, such as sand, rocks, and woody or fibrous components. The biomass includes biodegradable waste (organic waste of animal or plant origin that can be decomposed by micro-organisms and/or enzymes), dung (a mixture of manure and litter) or grass. The biomass ingredients can cause problems in wet-fermentation biogas plants. In particular, the biomass used for dry fermentation can be stacked, but not pumped. Although referred to as “dry” fermentation, the “dry” is only relative to wet fermentation. The biomass (also called the substrate) in dry fermentation can have a water content of up to 70%. In most cases, the biomass is moistened with a liquid before and/or during fermentation in order to start and/or maintain the necessary microbial processes, i.e., the anaerobic decomposition that takes place in the fermenters. Water enriched with suitable bacterial cultures can also be used as the liquid. Often part of the seepage (also called the percolate) is used as the liquid. The percolate is removed from a bottom section of the fermenter and is again sprinkled over the biomass. 
         [0004]    However, by far the largest part of the percolate produced during fermentation is collected in tanks in order to be applied to agricultural fields for fertilization. Section 2.2.3 of the German Biodegradable Waste Ordinance (BioAbfV) of Apr. 4, 2013 defines mandatory “sanitation” procedures for handling percolate that is applied to agricultural fields. In particular, the percolate must be heated to a temperature of 70° C. for an hour in order to achieve the defined sanitary quality and environmental compatibility based on pathogen content, weed seeds and other undesirable components such as salmonella, clubroot and tomato seeds. 
         [0005]    Biogas plants with solid fermenters and percolate circulation are known from GB 2407088 A as well as from EP 2275526 A2. The fermentation process in the percolate tank can be thermophilic so that the percolate is sanitized in the tank. However, because the percolate tank is part of the percolate circuit, the sanitized percolate is returned to the fermenter and is contaminated again. A permanent sanitation of the percolate can only be achieved when no more percolate is added from the fermenter or returned to the fermenter. 
         [0006]    A method of producing liquid fertilizer from percolate is sought that does not result in recontamination of the sanitized percolate as occurs in GB 2407088 A and EP 2275526 A2. A method is sought in which percolate can be drained from the biogas plant during operation of the fermenter. Furthermore, it is an object of the invention to provide a biogas plant for implementing the method. 
       SUMMARY 
       [0007]    The invention relates to a method of producing a liquid fertilizing agent and a biogas plant for performing the method. Percolate must be sanitized in order for the percolate to be applied to fields as liquid fertilizer. Conventional biogas plants have both a solids digester and a percolate circuit. However, the novel biogas plant sanitizes the percolate separately from the percolate circuit using heat generated by thermophilic bacterial processes acting on the percolate in the percolate circuit. The percolate tank is part of the percolate circuit, so sanitized percolate would be recontaminated if fed back into the digester. By providing a percolate tank and a separate sanitizing tank that are different components, the sanitizing can be performed independently of and in parallel with the percolate circuit in which percolate circulates between the digester and the percolate tank. Whenever excess percolate accumulates in the percolate circuit, the excess percolate is transferred to the sanitizing tank and sanitized there while separate from the percolate in the percolate circuit. Thus, surplus percolate that accumulates can be sanitized even while biogas is being produced in the digester. In addition, separating the percolate tank from the sanitizing tank prevents percolate that has already been sanitized from becoming recontaminated. Therefore, the sanitizing need be performed only once. 
         [0008]    A biogas plant produces both methane and liquid fertilizer by fermenting biomass. The liquid fertilizer has a sanitary quality that is acceptable for spraying on fields used to grow produce for human consumption. The plant includes a fermenter, a percolate tank and a sanitation tank. The sanitation tank is located inside the percolate tank in an elevated position coaxially to the percolate tank such that an upper end of the sanitation tank is level with a top end of the percolate tank. Dry fermentation takes place in the fermenter and generates methane and a percolate. The percolate is circulated through a percolate pipe between the fermenter and the percolate tank. Percolate that is returned from the percolate tank to the fermenter is sprinkled over the biomass. A portion of the percolate is transferred from the percolate tank into the sanitation tank. The percolate in the sanitation tank is heated to a Celsius temperature between 45° and 65° for a period of at least five days and is thereby sanitized. The percolate in the sanitation tank is heated using both a heating device in the sanitation tank as well as heat generated from a thermophilic fermentation reaction occurring in the percolate tank. The biogas plant includes a temperature adjustment device coupled to the heating device. The temperature adjustment device is used to maintain the percolate in the sanitation tank between 45° and 65° Celsius. The sanitized percolate is used as the liquid fertilizer 
         [0009]    A method of producing methane and liquid fertilizer in a biogas plant sanitizes the fertilizer using heat generated from thermophilic fermentation of percolate produced by fermenting biomass. The methane is generated by fermenting the biomass in a fermenter in which the percolate is a byproduct. The percolate is transferred from the fermenter to a percolate tank. The percolate is returned from the percolate tank to the fermenter and sprinkled over the biomass to promote a dry fermentation reaction. A first portion of the percolate is transferred from the percolate tank into a sanitation tank that is arranged concentrically inside the percolate tank. The percolate contained in the sanitation tank is sanitized by heating to a Celsius temperature between 45° and 65° for a period of at least five days. 
         [0010]    The first portion of the percolate passes through a valve from the percolate tank into the sanitation tank. The valve is closed during the period of at least five days while the percolate contained in the sanitation tank is heated to the Celsius temperature between 45° and 65°. This prevents the percolate in the sanitation tank from being recontaminated by unsanitized percolate from the percolate tank. The heating of the percolate contained in the sanitation tank is performed using heat generated from a thermophilic fermentation reaction of the percolate in the percolate tank as well as by using a heating device located inside the sanitation tank. The percolate is then drained from the sanitation tank and used as the liquid fertilizer. A second portion of the percolate is transferred from the percolate tank into the sanitation tank after the draining of the first portion of percolate from the sanitation tank. 
         [0011]    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 
         [0012]    The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
           [0013]      FIG. 1  is a schematic illustration of principles used in a first embodiment of the invention. 
           [0014]      FIG. 2  is a biogas plant according to a second embodiment of the invention. 
           [0015]      FIG. 3  is a biogas plant according to a third embodiment of the invention. 
           [0016]      FIG. 4  is a biogas plant according to a fourth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
         [0018]      FIG. 1  shows a biogas plant  10  in which a percolate tank  11  and the sanitation tank  12  are different components, which allows the sanitation function of the biogas plant to be carried out independently and in parallel with the circulation of percolate  13  between the fermenter  14  and the percolate tank  11 . Separating the percolate tank  11  and the sanitation tank  12  allows excess percolate  13 , whenever produced, to be transferred to the sanitation tank  12  and sanitized there separately from the percolate in the percolate circuit between the fermenter  14  and the percolate tank  11 . As a result, excess percolate  13  produced during biogas generation in the fermenter  14  can be sanitized. The separation of percolate tank  11  and sanitation tank  12  also prevents a recontamination of the percolate that was already sanitized. Thus, sanitation need be carried out only once. 
         [0019]    The decontaminated percolate  15  from the sanitation tank  12  can be stored temporarily in a storage tank  16 . The transfer of contaminated percolate  13  from the percolate tank  11  to the sanitation tank  12  is demand-based, i.e., either excess percolate  13  must be drained from the percolate tank  11 , or there is a demand to use the percolate as liquid fertilizer. The contaminated percolate  13  is drained into the sanitation tank  12  based on the fill level in the percolate tank  11  and/or in the sanitation tank  12 . The transfer of contaminated percolate  13  from the percolate tank  11  into the sanitation tank  12  is realized depending on the fill level of the percolate in the sanitation tank  12  and/or the fill level of the contaminated percolate  13  in the percolate tank  11  and/or the fill level of the sanitized percolate  15  in the storage tank  16 . The transfer of contaminated percolate  13  from the percolate tank  11  into the sanitation tank  12  is always realized only after complete drainage of the sanitized percolate  15  from the sanitation tank  12 . 
         [0020]      FIG. 2  illustrates a biogas plant  17  with multiple fermenters  18 , each of which operates in batch mode to generate methane. The biogas plant  17  includes a percolate tank  19 , a sanitation tank  20 , and a temperature adjustment device  21 , which includes a heating device  22 . The percolate tank  19  is connected to each fermenter  18  to form a percolate circuit. The sanitation tank  20  for the storage of percolate  15  is connected to the percolate tank  19 . The percolate tank  19  and the sanitation tank  20  are separate and different components, although in the embodiment of  FIG. 2 , the sanitation tank  20  is located inside the percolate tank  19 . 
         [0021]    The temperature adjustment device  21  maintains the Celsius temperature T inside the sanitation tank  20  in the range of 65°&gt;T&gt;45°, preferably 60°&gt;T&gt;50°. This temperature range allows for the thermophilic temperature treatment of the percolate contained in the sanitation tank  20 . As there are different types of “thermophilic” fermentation reactions, those reactions that take place in the Celsius temperature range of 65°&gt;T&gt;45°, preferably 60°&gt;T&gt;50°, take place inside the sanitation tank  20 . Although the disclosed sanitation temperature range is lower than that specified by the German Biodegradable Waste Ordinance (BioAbfV), the same sanitation result can be achieved if the duration of the thermophilic fermentation sanitation step is extended. The energy and heating costs of the sanitation step are thereby reduced. An acceptable sanitary quality for producing liquid fertilizer for use on fields growing produce for human consumption can be achieved by heating the percolate in the sanitation tank  20  to above 45° Celsius for at least five days. This sanitary quality has killed undesirable components such as salmonella bacteria, clubroot, tomato seeds and weed seeds. 
         [0022]    The sanitation tank  20  is arranged inside the percolate tank  19 . This arrangement is possible in most cases because the sanitation tank  20  is normally smaller than the percolate tank  19 , and it is advantageous for structural reasons because only a single foundation is required. Furthermore, no or only short connecting pipes are necessary between the two tanks. Finally, the energy required to maintain the temperature of the percolate  13  in the sanitation tank  20  is reduced in comparison to locating the sanitation tank  20  outside the percolate tank  19 . Energy can be saved even if the sanitation tank  20  is located only partially inside the percolate tank  19 . However, the sanitation tank  20  is preferably completely integrated into the percolate tank  19  for thermal reasons so that the sanitation tank  20  is completely surrounded by contaminated percolate  13 . 
         [0023]    In one embodiment, the percolate tank  19  and the sanitation tank  20  are coaxially oriented and have a common vertical axis. Thus, the tanks  19 - 20  are arranged concentrically to each other. Although, the sanitation tank  20  can be located anywhere inside the percolate tank  19 , a concentric arrangement is preferred for structural reasons. The tanks  19 - 20  can be less expensively manufactured if they are shaped as straight cylinders with circular bases. Alternatively, the tanks can be elongated and rectangular with square bases. The shape of the percolate tank  19  need not be identical to the shape of the sanitation tank  20 . 
         [0024]    The sanitation tank  20  is directly connected to a fermenter  18  for the intake of percolate  13  through a percolate pipe  23 . Thus, the sanitation tank  20  is connected to the fermenter  18  directly via the percolate pipe  23  and indirectly via the percolate tank  19 . As a result, an interim storage of percolate in the percolate tank is not necessary. The connection between the sanitation tank  20  and the fermenter  18  is opened and closed on a demand basis or situation basis. 
         [0025]    A manually adjustable valve or an electromagnetic valve (or several valves) is used for demand-based activation. The automatic filling of the sanitation tank  20  directly from the fermenter  18  is preferably performed depending on the fill level of the percolate in the sanitation tank  20  or in the percolate tank  19 . For example, from a technical standpoint it is easier with a fully filled percolate tank  19  not to begin by draining percolate from the percolate tank  19  into the sanitation tank  20  and then to refill the percolate tank  19  with percolate  13  from the fermenter  18 . 
         [0026]    The temperature adjustment device  21  includes the heating device  22 . Depending on the arrangement of the sanitation tank  20  and the percolate tank  19  (next to each other or one inside the other), the heating device  22  for temperature control of the percolate  15  is designed in different ways. The heating device  22  is designed so that it extends into at least a part of the bottom and/or at least a part of the wall of the sanitation tank  20  in order to achieve and maintain the desired temperature in a way that is as consistent as possible. Alternatively, heating elements can be arranged inside the sanitation tank  20 , preferably directly in contact with the percolate  15 . With a concentric inter-arrangement of the sanitation tank  20  inside the percolate tank  19 , heating elements inside the sanitation tank  20  in combination with the heating through the percolate  13  in the percolate tank can ensure a consistent heating of the sanitized percolate  15  in the sanitation tank. In such a case, the heating power can be reduced. The temperature adjustment device  21  also includes a cooling device that can, for example in the summer, prevent the overheating of the percolate  15  in the sanitation tank  20 . 
         [0027]    In addition, the temperature adjustment device  21  includes a heat exchanger for heat transfer between the percolate tank  19  and the sanitation tank  20  and/or between the fermenter  18  and the sanitation tank  20 . Depending on the marginal conditions (climatic conditions, size and design of the tanks, etc.), the heat transmitted from the percolate tank  19  via the heat exchanger to the sanitation tank  20  can be sufficient to achieve and maintain the thermophilic temperature, or heat from the above-mentioned heating device must be added. In case of the above-mentioned arrangement of the sanitation tank  20  inside the percolate tank  19 , in particular when the sanitation tank  20  is encompassed by percolate  13 , at least part of the wall of the sanitation tank  20  functions as an indirect heat transmitter. In case of metal tanks, naturally the entire wall acts as a heat exchanger. 
         [0028]    The temperature of the interior of the fermenter  18  can be controlled using heating devices in the bottom or the wall for better control of the fermentation process. A particularly uniform and efficient temperature control of the interior of the fermenter  18  can be achieved using a heating device in the style of a floor heating in the bottom and/or in at least one wall. 
         [0029]    The biogas plant  24  of  FIG. 3  includes several fermenters  18  and/or several percolate tanks  19 ,  25  and/or several sanitation tanks  20 ,  26 . Each of the several percolate tanks  19 ,  25  is connected with at least one of the several fermenters  18  in a percolate circuit. Each of the several sanitation tanks is connected with at least one of the several percolate tanks  20 ,  26  for the intake of percolate  13 . Each of the sanitation tanks  20 ,  26  includes a temperature adjustment device  21  for maintaining the temperature of the percolate within a temperature range of 65°&gt;T&gt;45°, preferably 60°&gt;T&gt;50°, so as to support a thermophilic fermentation sanitation treatment of the percolate. The use of several fermenters  18  and/or several percolate tanks  19  and/or several sanitation tanks  20  not only increases the productivity of the fermentation and sanitation process, but also makes the biogas plant more flexible. This is advantageous in particular when the biomass  27  to be fermented is heterogeneous in its composition in order to produce biogas in an efficient and effective manner. In particular, it is advantageous to use different bacterial cultures for different biomass substrates. Preferably, each fermenter  18  is connected to each percolate tank  19 ,  25  and each sanitation tank  20 ,  26  to form a network of fermenters and tanks that cooperate with each other. Furthermore, the operation of the biogas plant  24  can be continued mostly unimpeded in case of maintenance work or in case of a malfunction incident. Alternatively there are several sanitation tanks  20  in one single percolate tank  19 , or several sanitation and percolate tanks can be connected with each other in a cascade. Although the biogas plant  24  can include any number of fermenters  18  and sanitation and percolate tanks, a complex biogas plant preferably includes sub-units each with one fermenter  18 , one sanitation tank  20 , and one percolate tank  19 . The sanitation tank  20  and percolate tank  19  that produce biogas from the biomass  27  based on the principle of dry fermentation can be part of a biogas plant for the production of biogas from biomass based on the principle of wet fermentation. 
         [0030]    The biogas plant  24  includes a control system  28  for controlling all its components. This is expedient in particular for maintaining the temperature of the percolate within a range that promoted a thermophilic treatment of the percolate. The components controlled by the control system  28  include temperature sensors  29 , the temperature adjustment devices  21 , valves  30 , fill level sensors  46 , leak detectors, devices for the registration of malfunctions in electrical installations, etc. 
         [0031]      FIG. 1  shows a schematic illustration of principles used by a first embodiment of the biogas plant  10 . Percolate  13  is circulated between the solid fermenter  14  containing biomass  27  and the percolate tank  11  using a first percolate pipe  31  and a second percolate pipe  32 . Excess percolate is discharged into the sanitation tank  12  via a third percolate pipe  33 . When the percolate  13  in the sanitation tank  12  has reached a sufficient fill level, the connection between the percolate tank  11  and the sanitation tank  12  is disconnected, and the percolate is sanitized in the sanitation tank  12  by thermophilic processes. Upon completion of sanitation, liquid fertilizer in the form of the sanitized percolate  15  is drained from the sanitation tank  12  via a fourth percolate pipe  34 . Via a fifth percolate pipe  35 , contaminated percolate  13  can also be discharged directly from the fermenter  14  into the sanitation tank  12 . During sanitation, the percolate pipe  35  is disconnected. The disconnection of the sanitation tank  12  from the percolate circuit with percolate tank  11  and fermenter  14  is performed by valves  30 . This safely prevents contaminated percolate  13  from entering the sanitation tank  12  during sanitation. Because percolate tank  11  and sanitation tank  12  are different components, biogas production in the fermenter  14  and sanitation in the sanitation tank  12  can be carried out in parallel. 
         [0032]      FIG. 2  shows a second embodiment of a biogas plant  17  with several fermenters  18 . In accordance with the operating processes illustrated in  FIG. 1 , the biogas plant  17  has a percolate tank  19  into which contaminated percolate  13  is emptied. The biogas plant  17  includes four fermenters  18  that are connected to the percolate tank  19  by percolate pipes  36  that all empty into a first main percolate pipe  37 . Percolate return pipes  38  run from the first main percolate pipe  37  back to the respective fermenters  18  in order to sprinkle the biomass  27  (the substrate) contained in the fermenters  18  with percolate  13  enriched with bacteria and to moisten the biomass during the dry fermentation process. Percolate  13  from the percolate tank  19  can also be sprinkled over the biomass  27 . A second main percolate pipe  39  supplies percolate  13  from the percolate tank  19  to the respective fermenters  18  via secondary percolate pipes  40 . 
         [0033]    In addition, the biogas plant  17  includes a sanitation tank  20  located inside the percolate tank  19  in an elevated position coaxially to the percolate tank  19  in such a way that its upper end is on the same level as the top end of the percolate tank  19 , as it is shown in  FIG. 2 . The biogas plant  17  also includes a storage tank  16  connected to the sanitation tank  20  via a percolate discharge pipe  41 . For this reason, the side panels and the bottom of the sanitation tank  20  are completely in contact with percolate when the percolate tank  19  is completely filled with percolate. 
         [0034]    The percolate  13  is caused to flow through the pipes  36 - 41  using pumps (not shown) and stop valves  30  that are each controlled by a control unit  28 . The direction of the flow of percolate  13  is shown in  FIG. 2  with arrows on the pipes  36 - 41 . The percolate pipe  23  represents a pipe that supplies contaminated percolate  13  from the fermenters  18  to the sanitation tank  20  for thermophilic sanitation (decontamination). 
         [0035]    The second embodiment of biogas plant  17  shown in  FIG. 2  includes four individual percolate circuits that operate using four separate percolate return pipes  38 . Biogas plant  17  also has a main percolate circuit that operates using the second main percolate pipe  39 . The percolate circuits can be switched separately from each other and in any combination. The percolate  13  can be supplied only to individual fermenters  18  or to all fermenters via the second main percolate pipe  39  by opening or closing the appropriate valves  30  in the secondary percolate pipes  40 . 
         [0036]      FIG. 3  shows a biogas plant  24  according to a third embodiment that differs from the second embodiment in that it includes a second percolate tank  25  and an additional storage tank  43 . Second percolate tank  25  includes a sanitation tank  26 . Second percolate tank  25  is similarly connected to each of the fermenters  18  by percolate pipe  37 , and the additional sanitation tank  26  is connected to the additional storage tank  43  by an additional percolate discharge pipe  42 . 
         [0037]    This means that the biogas plant  24  of the third embodiment is derived from the biogas plant  17  of the second embodiment by adding an additional one of the tank units  44  that is outlined by the dashed line in  FIG. 2 . The additional tank unit  44  with percolate tank  25  and storage tank  43  is connected in a functionally equivalent way to the first tank unit with percolate tank  19  and storage tank  16 . Consequently, the biogas plant  24  of the third embodiment includes an additional percolate circuit that returns percolate  13  from the additional percolate tank  25  to the fermenters  18 . 
         [0038]      FIG. 4  shows a biogas plant  45  according to a fourth embodiment that differs from the biogas plant  24  of the third embodiment in that the two sanitation tanks  20  and  26  are additionally cross connected with the two storage tanks  16  and  43 . 
         [0039]    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.