Biogas plant and operation method

Abstract

The present invention relates to a method for use in a biogas plant having at least one digester (12) and a thermal treatment unit (50), the method comprising the steps of : a ) anaerobically digesting an organic biomass (18) in at least one digester (12) through microorganisms at a temperature below 50 ° C to produce methane-containing biogas (16) and a digestate (14); b) removing a major fraction (32) of the digestate (14) from the digestion process after a fast retention time t and subjecting at least part of said major digestate fraction (32) to a thermal treatment in a thermal treatment unit (50) at a temperature above 175 ° C for at least one minute, wherein the thermal treatment step involves at least one treatment selected from the group consisting of pyrolysis, torrefaction, gasification, hydrothermal gasification, hydrothermal carbonization and hydrothermal liquefaction; and c ) retrieving a minor fraction (34) of the digestate (14) from the digestion process after an inoculum retention time T that is longer than the fast retention time t, returning said minor digestate fraction (34) into the digestion process and mixing the minor digestate fraction with the organic biomass (18) within the digester (12).

Description

[0001] Biogas Plant and Operation Method

[0002] The present invention relates to a method for use in a biogas plant according to Claim 1 and a biogas plant according to Claim 13 or Claim 14 .

[0003] In general , the technical field of the present invention is the anaerobic digestion - sometimes also referred to as " fermentation" - of biomass , i . e . organic matter present in various types of wastes ( sludge , agro-industrial wastes , energy crops ) , in an anaerobic biogas plant . The technology of anaerobic digestion is a biochemical process accomplished by the combined action of a group of several types of microorganisms , which metabolize the organic materials into a gaseous mixture consisting of mainly methane and carbon dioxide (biogas ) under anaerobic conditions .

[0004] Anaerobic digestion generally consists of four steps : hydrolysis , acidogenesis , acetogenesis , and methanogenesis . High molecular weight substrates are first hydrolyzed by fermentative bacteria into low molecular weight and water soluble organic intermediate products such as glucose , fatty acids and amino acids . During acidogenesis , volatile fatty acids (VFAs ) are derived, along with the generation of byproducts including NH3, CO2 and H2S . The VFAs are then converted into acetate , more CO2 , H2 , etc . , via acetogenesis . It is the acetate that methanogens act on to produce biomethane during the final methanogenesis process . Among the four steps , hydrolysis is the slowest and is therefore the rate-determining step .

[0005] The biogas is generally upgraded and then either sent to a natural gas network or used directly for the production of

[0006] A24215WO / 29 . 11 . 2024 / electricity, heat and / or transportation fuel . The final digestate remaining after the digestion process is used as soil conditioner and fertilizer .

[0007] To provide the required anaerobic conditions , vertical or horizontal digestion tanks are generally used . One example of a hori zontal digester that is operated in plug-flow mode and is suitable for anaerobic digestion of biogenic wastes is described in European patent EP 0 621 336 . The digester is an elongated, horizontal tank having an inlet at one end and an outlet at the opposite end . Chopped biogenic wastes are supplied through the inlet and are inoculated by means of digested material and press water from the post-processing after digestion . By this , the substrate to be digested is enriched with methanobacteria . In the digester, the biogenic wastes are now decomposed under controlled mixing forming biogas and are subsequently, after exiting through the outlet, supplied to post-processing incl . dewatering and aerobic rotting .

[0008] The worldwide demand for anaerobic biogas plants with larger capacities led to the construction of increasingly large digesters . Nowadays , horizontal digester tanks of a total length of 50 meters and having a diameter of more than 10 meters can be reali zed . This increase in size came along with technical difficulties associated with the transport of the digestate through the digester , as well as with the control of the digestion process . Specific issues concern the provision of a certain homogeneity of the digestate and the prevention of the sedimentation of heavy solids , such as in particular sand and stones , at the bottom of the digester tanks . Sedimentation prevention ensures continuous discharge of the

[0009] A24215WO / 29 . 11 . 2024 digested substrate . Even when operating the digester in plugflow mode , the heavier goods tend to sink because of the low flow velocity and thus cannot be discharged . As a result , the increase in size of the digester often came at the expense of a decreased digestion quality and longer retention time resp . the passing time of the biogenic wastes through the digester from the inlet to the outlet .

[0010] Also , the increased size of the digesters comes with a higher energy expenditure for heating because temperature is one of the most important parameters influencing the performance of anaerobic digestion processes . Depending on the temperature , the following types of digestion are distinguished : psychrophilic digestion ( 10-20 ° C ) ; mesophilic digestion ( 20- 45 ° C ) ; and thermophilic digestion ( 50-70 ° C ) . The conventional operational temperature levels for anaerobic digesters are mesophilic and thermophilic .

[0011] Thermophilic anaerobic digestion processes have several benefits compared to mesophilic anaerobic digestion : thermophilic AD processes have been shown to produce more biogas in a shorter time and to achieve good hygienization of the waste feedstock by reducing several indicator organisms , including Escherichia coli and Salmonella, and by inactivating plant seeds in the resulting digestate . In particular if the digestate is to be used as a fertilizer, it generally needs to be hygienized, meaning that pathogenic microorganisms are removed from the digestate . Temperatures above 50 ° C for the digestion is one ef fective way to achieve hygienization . On the other hand, thermophilic conditions require significantly higher energy input and thus increase operational costs . Further, at higher temperature , not only methane production

[0012] A24215WO / 29 . 11 . 2024 can be increased but also the generation of free ammonia, which can have an inhibitory ef fect on the digestion performance .

[0013] Therefore , while mesophilic systems allow to reduce the energy consumption and operational costs , they come with the drawbacks of lower biogas production , longer retention times ( i . e . "dwell time" of the digestate ) and lack of hygienization ( compared to thermophilic systems ) .

[0014] When trying to increase the efficiency of the digestion process , it was found that the addition of some digested substrate to the mass of fresh digestate introduced into the digester leads to a reduction of the overall retention time . In accordance therewith, EP 1 930 404 discloses the use of some digested substrate from the digester itself for inoculation of the fresh biomass introduced into the digester (" self-inoculation" ) . Some of the digested biomass retrieved after a total retention time is therefore returned to the inlet area of the digester .

[0015] The biogas plant and conversion method disclosed in European Patent EP 2 948 537 , which is owned by the Applicant of the present application , makes use of the self-inoculation technology while aiming at increasing the overall throughput of biomass through the biogas plant . The method of EP ' 537 involves the return of some substrate from an inoculum fermenter chamber operated in the plug-flow mode back to the inoculum fermenter chamber by means of a return path . In addition , the remaining, not returned substrate is supplied to at least one further fast fermenter chamber - also operated in the plug-flow mode - by means of a fast inoculation path and is mixed there with additional fresh fermentation substrate . This means the at least one fast fermenter chamber does not

[0016] A24215WO / 29 . 11 . 2024 require a self-inoculation , which allows for achieving a significantly shorter dwell time in the fast fermenter chamber and, as a result , for increasing the overall throughput of raw fermentation substrate of the biogas plant compared to a biogas plant having the same number / volume of fermenter chambers operated in parallel . An alternative system requiring only a single digester while achieving the same benefits was developed by the Applicant and disclosed in European Patent EP 3 750 862 .

[0017] Not disregarding the progress made in the described prior art , there is still room for improvement with respect to maximizing biogas production , decreasing energy input and making efficient use of the resulting digestate .

[0018] In view of the above , the problem solved by the present invention is thus to provide a method and a system for efficient conversion of organic biomass into biogas by anaerobic digestion at a reduced input of energy and chemicals and which enables versatile use of the resulting digestate .

[0019] This obj ect is achieved by the method as defined in Claim 1 and the biogas plant defined in Claims 13 and 14 of this application . Preferred embodiments are subj ect of the dependent claims .

[0020] According to the present invention, a method for use in a biogas plant having at least one digester and a thermal treatment unit is disclosed, the method comprises the steps of : a ) anaerobically digesting an organic biomass in at least one digester through microorganisms at a temperature

[0021] A24215WO / 29 . 11 . 2024 below 50 ° C to produce methane-containing biogas and a digestate ; b) removing a maj or fraction of the digestate from the digestion process after a fast retention time t and subj ecting at least part of said digestate fraction to a thermal treatment at a temperature above 175 ° C for at least one minute, wherein the thermal treatment step involves at least one treatment selected from the group consisting of pyrolysis , torref action, gasification, hydrothermal gasification, hydrothermal carbonization and hydrothermal liquefaction ; c) retrieving a minor fraction of the digestate from the digestion process after an inoculum retention time T that is longer than the fast retention time t , returning said minor digestate fraction into the digestion process and mixing the minor digestate fraction with the organic biomass within the digester .

[0022] In a first aspect the present invention thus relates to a specific method for digesting organic biomass in an anaerobic biogas plant , which is designed to improve the efficiency and effectiveness of biogas production . The steps of said method will be discussed in more detail in the following paragraphs .

[0023] Step a ) of the inventive method starts with anaerobically digesting organic biomass . This means that microorganisms break down the organic biomass in the absence of oxygen, producing methane-containing biogas and a residue known as digestate . The organic biomass generally refers to digestible matter, such as plants , agricultural residues , organic sludge , animal waste , foodwaste etc . It may be supplied to the digester

[0024] A24215WO / 29 . 11 . 2024 as a mixed feedstock that includes organic biomass and other non-digestible components , such as e . g . stones or sand, or said non-digestible components may be removed from the mixed feedstock prior to supplying the organic biomass into the digester .

[0025] In accordance with the present invention , the digestion occurs at a temperature below 50 ° C, which means that it is mesophilic digestion - in contrast to thermophilic digestion occurring at temperatures above 50 ° C . As mentioned in the introductory portion of the description, mesophilic conditions provide the benefits of requiring less energy for heating compared to thermophilic digesters , leading to lower operational costs . In addition , while thermophilic conditions can increase the rate of biogas production , mesophilic conditions provide a steady and reliable biogas output , which can be advantageous for systems where a consistent energy supply is essential .

[0026] After a fast retention time ( t ) , a maj or fraction of the digestate is removed from the digestion process . (Notably, the terms "maj or fraction of digestate" and "maj or digestate fraction" are used interchangeably . ) At least part of this maj or digestate fraction undergoes thermal treatment at a temperature above 175 ° C for at least one minute . Such high temperatures may significantly alter the properties of the digestate .

[0027] The thermal treatment includes at least one of the following :

[0028] Pyrolysis : generally involving thermal decomposition of materials at high temperatures in an inert atmosphere .

[0029] Torref action : generally involving mild thermal treatment for drying and partial decomposition to improve the properties

[0030] A24215WO / 29 . 11 . 2024 of the treated material .

[0031] Gasification : generally involving conversion of organic materials into gases - so-called "syngas" including carbon monoxide , hydrogen , and carbon dioxide .

[0032] Hydrothermal Gasification : generally involving gasification using water at high temperatures and pressures .

[0033] Hydrothermal Carbonization : generally involving conversion of biomass into hydrochar using hot compressed water .

[0034] Hydrothermal Liquefaction : generally involving conversion of biomass into liquid biofuels using water at elevated temperatures and pressures .

[0035] In line with the present invention , a minor fraction of the digestate is held within the digestion process for a longer retention time ( T ) , which is longer than the fast retention time ( t ) . This minor fraction is then reintroduced into the digestion process and mixed with incoming organic biomass . Thus , a portion of the digestate is used as an inoculum to help maintain microbial populations and enhance ongoing digestion . Specifically, thanks to said inoculum, rapid onset of a balanced microbial population within the digester is accomplished, with the effect that the digestion of the organic biomass within the digester is accelerated, meaning that less retention time is required to produce a certain amount of biogas compared to digesters not using an inoculum . In consequence , the anaerobic biogas plant of the present invention allows for increasing the overall throughput of organic biomass and thus maximizing the amount of biogas produced within the digester without expanding the digester size .

[0036] A first end product of the method of the present invention is

[0037] A24215WO / 29 . 11 . 2024 biogas resulting from the degradation of the organic biomass within the digester . Said biogas can be retrieved from the digester in a known manner . The digestate is a further product that is retrieved from the digester . Reusing a minor fraction of the digestate as inoculation substrate allows to sustain microbial activity in the digester, improving overall efficiency and stability of the digestion process . By thermally treating a maj or fraction of the digestate , the hygienization of said maj or digestate fraction is achieved and the digestate can be further converted into additional useful products . Depending on the type of thermal treatment , these products include e . g . biochar, syngas , hydrochar and / or bio-oil . Notably, some or all of the maj or digestate fraction may directly be thermally treated or it may first be separated into a liquid digestate portion and a solid digestate portion . In the latter case , at least the solid digestate portion will be thermally treated in line with the inventive method .

[0038] The inventive method thus optimi zes anaerobic digestion for biogas production by combining anaerobic digestion with advanced thermal processing techniques to enhance biogas yield and obtain valuable products from organic waste .

[0039] In a preferred embodiment , the minor fraction of digestate retrieved from the digestion process is returned back into the digestion process without further processing .

[0040] As mentioned above , the minor fraction of the digestate remains in the digester for a longer retention time (T ) compared to the maj or fraction (t ) to ensure it is well-decomposed . This prolonged inoculum retention time allows for the build-up of a more robust microbial population .

[0041] A24215WO / 29 . 11 . 2024 After the inoculum retention time, the retrieved minor fraction of digestate is directly reintroduced into the digester - or (at least partly) into another digester -, where it mixes with the fresh incoming organic biomass. By doing this, it helps to inoculate the fresh organic biomass and to enhance the microbial population required for effective anaerobic digestion. This is because the reintroduced digestate carries active microorganisms that can immediately start breaking down the organic biomass, thereby maintaining the efficiency of the digestion process. It also ensures a steady supply of active microorganisms, which can result in more stable and consistent biogas production.

[0042] In a preferred embodiment, the anaerobic digestion step takes place at a temperature in the range of 20°C to 48°C, preferably 32°C to 48°C, most preferably 42°C to 48°C. The digestion process thus preferably takes place in the higher mesophilic range, which still significantly reduces energy input and thus operational costs compared to the normally used thermophilic conditions (above 50°C) . In addition, it was found that digesters that operate in the range of 20°C to 48°C generally have a higher ammonia tolerance compared to digesters that operate at higher temperatures. An increased ammonia tolerance allows higher organic loading of the digester to maximize biogas production.

[0043] In a particularly preferred embodiment, the temperature within the anaerobic digester in which the inventive digestion method takes place is kept in the range of 42°C to 48°C. It has surprisingly been found that the speed of the anaerobic digestion process is particularly high in the temperature range of 42°C to 48°C. In fact, it was found that the digestion

[0044] A24215WO / 29.11.2024 process for some feedstocks at these temperatures is the same or slightly faster than at 55 ° C , i . e . under thermophilic conditions . Without wanting to be bound by the theory, it is assumed that in the range of 42 ° C to 48 ° C, a higher ratio of nitrogen is present in the form of ammonium rather than ammonia in the digester content compared to thermophilic conditions where ammonia inhibition of the anaerobic digestion process is more pronounced .

[0045] In some embodiments , the temperature within the at least one digester may be seasonally adj usted . Specifically, during the colder months of winter, plant growth typically diminishes , resulting in a lower volume of green waste , which serves as an important organic co-substrate for anaerobic digesters . As such, it may be preferable to lower the temperature within the digester, which results in energy savings and higher tolerance towards ammonia inhibition despite some loss of hygienization during the digestion process .

[0046] In a preferred embodiment , prior to the thermal treatment in step b) , the maj or digestate fraction is subj ected to a solidliquid phase separation to obtain a solid digestate portion and a liquid digestate portion, whereupon at least part (preferably all ) of the solid digestate portion is subj ected to the thermal treatment .

[0047] The terms "liquid" and "solid" digestate portions are used to differentiate a portion with a higher moisture content , i . e . the liquid digestate portion, from a portion with a lower moisture content , i . e . the solid digestate portion . As such, in comparison , the liquid digestate portion has a higher moisture content than the solid digestate portion , but solid

[0048] A24215WO / 29 . 11 . 2024 parts can still be present in the liquid digestate portion and the solid digestate portion may also contain liquid components .

[0049] The inclusion of a solid-liquid phase separation of the maj or digestate fraction before thermal treatment is a strategic enhancement to the anaerobic digestion process . It optimizes the subsequent thermal treatment step by focusing on the solid digestate portion , resulting in better resource utili zation, higher efficiency, and improved product quality . After thermal treatment , the solid digestate portion may yield valuable byproducts like biochar , which can be used e . g . as a soil amendment , or syngas for further energy production .

[0050] The liquid digestate portion , on the other hand, can be managed separately, which might involve less energy-intensive treatments such as nutrient recovery, water recycling, or anaerobic / aerobic treatment for further digestate stabilization . The separated liquid digestate portion can be used for various purposes , like irrigation , i f it meets regulatory standards , or it can be further processed to recover nutrients or produce additional biogas .

[0051] The solid-liquid phase separation is preferably a mechanical dehydration treatment . Heating up a liquid generally requires more energy than heating of a solid material . A solid-liquid separation of some or all of the maj or digestate faction prior to the thermal treatment allows a differentiation in the application and / or the duration of the thermal treatment of the resulting liquid and solid digestate portion .

[0052] Preferably, the solid-liquid phase separation is carried out by pressing, sieving or centrifugation . Pressing, e . g . with the aid of a screw press , is preferred for most types of

[0053] A24215WO / 29 . 11 . 2024 digestate , as it highly effective in producing a solid digestate portion with a low moisture content in a short time . In addition , the moisture content in the solid digestate portion can be controlled by adj usting the pressing pressure applied . For instance , in case of a screw press the mode of operation can be adj usted to set to moisture content of the solid digestate portion to a desired level , which is typically around 60-70 % . In a specific embodiment , the operational pressure of the screw press can be adj usted with the aid of a pressure controller .

[0054] I f the digestate-to-be-separated contains abrasive components or only very little solid material , sieving or centrifugation can be used as separation technique . An adj ustment of the rotational speed of the centrifuge device also allows controlling the moisture content of the resulting solid digestate portion . For some types of digestates , in particular those that contain mostly organic fractions of municipal solid waste (OFMSW) , a solid-liquid separation can be achieved by sieving . One specific example is the use of vibrating sieves with two or more stages . By mounting dif ferent sieving elements (with variable pore size ) , the moisture content of the solid digestate portion can be adj usted .

[0055] A solid-liquid phase separation prior to the thermal treatment is particularly preferred if moisture content of the digestate is above 75% and if the thermal treatment is one of pyrolysis , torrefaction or gasification . Hydrothermal treatments , such as hydrothermal carbonization , hydrothermal liquefaction or hydrothermal gasification , on the other hand, have been found to work with digestates having a higher moisture content , such as 50% to 95% humidity, preferably about 75% humidity . For

[0056] A24215WO / 29 . 11 . 2024 that reason , a dehydration treatment , such as solid-liquid phase separation , is usually not required for hydrothermal thermal treatment .

[0057] The moisture content in the digestate can be assessed by experienced operators , if a rough estimation is considered sufficient . Alternatively, moisture measurement methods are known to the skilled person . One example is the thermogravimetric method, which by measures the weight loss of a sample loss on drying; here , the sample is heated and the resulting weight loss from moisture evaporation is recorded .

[0058] In a preferred embodiment , at least some of the solid digestate portion and at least some of the liquid digestate portion are both thermally treated - either in the same thermal treatment unit or in dif ferent thermal treatment units . The liquid digestate portion may be treated for only a few minutes or even j ust one to two minutes with the aim to achieve hygienization thereof . Notably, if at least a part of the liquid digestate portion is to be used for humidification of the digester feedstock, said part may not even in every case require a thermal treatment , at all .

[0059] The solid digestate portion may be heat-treated longer than the liquid digestate portion . This is preferably the case unless the thermal treatment is performed by fluidized bed gasification . For any of the other above mentioned thermal treatments , e . g . pyrolysis , torref action, gasification or solid bed gasification, the solid digestate portion is preferably treated for a time span that allows for adequate conversion of the treated solid digestate portion into high- quality fuel or char . This time span is dependent from the

[0060] A24215WO / 29 . 11 . 2024 chosen thermal treatment , in particular the thermal treatment temperature , but also from the moisture content of the treated digestate and the average particle si ze . Since the solid digestate portion has a reduced water content compared to the maj or digestate fraction (prior to solid-liquid-separation ) , the thermal treatment time for achieving essentially full conversion of the solid digestate portion can be reduced ( compared to thermal treatment of the maj or digestate fraction ) . Therefore , if a prior liquid-solid phase separation is performed on the maj or digestate fraction, 5 to 10 minutes of pyrolysis or torrefaction of the resulting solid digestate portion may suffice to achieve essentially full conversion of the latter .

[0061] In the aforementioned preferred embodiment involving a liquidsolid separation , it is further preferred that waste heat produced during the thermal treatment of the solid digestate portion is recovered . The recovered heat is preferably used within the biogas plant . For example , the recovered heat may be used for heating the anaerobic digester and / or for thermally treating at least part of the liquid digestate portion to achieve hygienization thereof .

[0062] Hygienization of the liquid digestate may be performed in the same thermal treatment unit as the one used for the thermal treatment of the solid digestate . Preferably, however, hygienization of the liquid digestate is performed in a separate unit , e . g . a sanitizer, that uses the recovered (waste ) heat from the thermal treatment of the solid digestate portion . While in this embodiment , two thermal treatment units are required, it provides the benefit of heat integration, such that hygieni zation of the liquid digestate portion ( some

[0063] A24215WO / 29 . 11 . 2024 or all thereof) can be achieved without additional energy demands . This approach helps to reduce the overall energy consumption and associated costs of the biogas plant, as it leverages waste heat that would otherwise be lost.

[0064] Also, by reusing heat within the system, the biogas plant can minimize its reliance on external energy sources. This contributes to a lower carbon footprint and enhances the plant's sustainability.

[0065] In a preferred embodiment, the thermal treatment involves pyrolysis at a temperature of at least 450°C, preferably 450°C to 850°C, for at least 15 minutes, in particular 15 to 180 minutes, preferably 20 to 140 minutes. Pyrolysis of the digestate at 450°C or more for at least 15 minutes was found to enable effective conversion of the heat-treated digestate into fully carbonized products, such as biochar, coke, carbon and charcoals that can be used as such in industrial applications or as soil improver or as carbon sequestration means, thereby contributing to the economic viability and sustainability of the anaerobic digestion process.

[0066] In a more specific embodiment, the thermal treatment involves directly heated pyrolysis. In this specific embodiment, in the pyrolysis unit the heat is applied directly to the material, rather than being conducted through a surface such as a unit wall. This allows for rapid heating and decomposition of the material into products like bio-oil, gas, and charcoal. The direct heating approach is often more efficient and can process a wide range of feedstocks, including organic biomass and waste materials .

[0067] A24215WO / 29.11.2024 In an alternative preferred embodiment, the thermal treatment step b) involves torrefaction at a temperature of at least 200°C, more preferably 250°C to 350°C, for a treatment time of preferably at least 40 minutes. Compared to pyrolysis, torrefaction provides a milder but more efficient thermal treatment option. This process enhances the properties of the digestate, making it more energy-dense, hydrophobic, and grindable, while potentially reducing operational costs and environmental impact compared to higher-temperature processes like pyrolysis. Torrefaction thereby optimizes the digestate, in particular the solid digestate fraction, for various applications, improving the overall performance and sustainability of the anaerobic digestion system.

[0068] In a further alternative preferred embodiment, the thermal treatment step b) involves gasification that is conducted at a temperature of preferably at least 600°C, more preferably at least 700°C, in particular in the range of 700-1200°C. Gasification is a process that converts organic materials into syngas (a mixture of carbon monoxide, hydrogen, and carbon dioxide) through partial oxidation at high temperatures. The syngas produced is a valuable energy product that can be used for energy generation, either by combustion in gas engines or turbines or as a feedstock for producing chemicals and liquid fuels. Since most of the treated digestate is converted into gaseous products, gasification can minimize solid (waste) material production.

[0069] The treatment time for effective syngas production by gasification depends on the type of gasification. Traditional solid-bed or fixed-bed gasification often requires a longer treatment time than a few minutes, but highly depends on the

[0070] A24215WO / 29.11.2024 type and moisture content of the digestate . Pre-drying of the to-be-heat-treated digestate can be employed to reduce the minimum treatment time . The newer process of f luidi zed-bed gasification can convert digestate into syngas in a time as short as one minute or even less . In addition, it was found that gasification , in particular fluidized bed gasification, provides the benefit of being particularly effective in reducing the plastic content in product obtained after the gasification .

[0071] Alternatively, hydrothermal gasification, preferably at supercritical conditions can be employed . This is a process that converts organic materials into gas by reacting them with water at high temperatures and pressures , preferably beyond the critical point of water . The critical point of water is at a temperature of 374 ° C and a pressure of 22 . 1 megapascals (MPa ) , i . e . 221 bar . Beyond this point , water enters a supercritical state where it is neither a distinct liquid nor a gas but exhibits properties of both . In this state , supercritical water acts as a powerful solvent and can greatly enhance the reactivity of organic materials . The hydrothermal gasification technique is promising for converting wet organic biomass or waste into energy-rich gases without the emissions of other more conventional gasification techniques that work at lower temperatures and pressures .

[0072] Alternatively, the thermal treatment step may involve hydrothermal carbonization (HTC) for at least 30 minutes , preferably at least one hour, more preferably at least 2 hours , in particular 2-5 hours , preferably at a temperature within the range of 175 ° C to 350 ° C . HTC is a process that uses hot , pressurized water to convert the digestate into a carbon-rich

[0073] A24215WO / 29 . 11 . 2024 solid product known as hydrochar . Hydrochar has properties similar to activated carbon and can be used as a fuel , soil amendment , or for carbon sequestration . It can also be employed in wastewater treatment , adsorption processes , or as a material for further chemical applications .

[0074] Operating at moderate temperatures compared to pyrolysis or gasification , HTC can be more energy-efficient and easier to manage . While one minute thermal treatment has been found to be effective for hygienization purposes , treating the digestate by hydrothermal carbonization for at least 2 hours allows effective conversion of the digestate into valuable products , such as high quality fuel .

[0075] Hydrothermal carbonization has the benefit that it allows treatment of digestate with various moisture contents without pre-drying, which saves energy and costs for drying before processing . Thus , it is possible to treat digestate directly out of the digester or either or both of liquid and solid digestate portions obtained after a solid-liquid separation . This makes it a good match for digestate from anaerobic digestion , which can have a high moisture content .

[0076] In a preferred embodiment , the minor fraction of digestate removed from the digestion process is less than 50% , preferably less than 40% , more preferably about 30% or less , of the amount of organic biomass fed into the digestion process . This means that the minor fraction of the digestate removed from the digestion process and then reintroduced ( recycled) into the digester preferably constitutes a relatively small percentage of the total organic biomass fed into the digestion process . This avoids accumulation of recalcitrant materials , potential

[0077] A24215WO / 29 . 11 . 2024 buildup of inhibitory compounds , or reduced ef ficiency in processing new organic biomass . The most preferred proportion of 30% or less was found to provide an optimal balance between maintaining sufficient microbial inoculum within the digester and ensuring that most of the digestate undergoes further processing (e . g . , thermal treatments , nutrient recovery) .

[0078] In a preferred embodiment , the fast retention time t is less than 50 % , preferably less than 40 % , more preferably about a third of the inoculum retention time T . This means that the fast retention time t ( the period the maj or fraction of the digestate stays in the digester before being removed) is preferably significantly shorter than the inoculum retention time T ( the period the minor fraction of the digestate stays in the digester before being recycled) . A shorter retention time for the maj or digestate fraction means faster processing and removal , allowing the system to handle a higher throughput of organic biomas s . This enhances overall process efficiency . A shorter retention time of the maj or fraction also reduces the risk of inhibition by intermediate fermentation products , improving overall stability . On the other hand, the longer retention time for the minor fraction ensures thorough digestion and stabili zation, which provides a stable inoculum and helps in maintaining a healthy microbial community by reintroducing only well-digested material .

[0079] Preferably, the fast retention time t is less than 10 days , preferably between 5 and 8 days . A retention time of less than 10 days for the maj or fraction of the digestate ensures fast turnover, efficient digester use , and continuous organic biomass processing, contributing to overall system efficiency and stability . By adhering to these retention times , the plant

[0080] A24215WO / 29 . 11 . 2024 can effectively balance the need for rapid initial decomposition with the demands for subsequent thermal treatment , ensuring an ef ficient , flexible , and high- performance anaerobic digestion operation .

[0081] It is also preferred that the inoculum retention time T is at least 7 days , more preferably at least 14 days , most preferably about 21 days . This preferred inoculum retention time ensures the development of a mature and active microbial community within the anaerobic digester - which promotes improved process stability and optimized biogas yield and quality .

[0082] The method may additionally include a step of condensing steam and vaporized compounds from the digestate with the aid of a condensing system that may involve passing the vapor through a series of cooled pipes or surfaces . As the vapor cools , the water and some of the organic compounds condense back into the liquid phase . This liquid, known as the condensate , is then collected . It may require further treatment to separate water from other condensed organic compounds , depending on its intended use or disposal requirements . In one embodiment the condensing system includes a multistage vacuum system that is designed to ef ficiently condense and separate various components from the vaporized compounds under reduced pressure , with the goal of producing a clean condensate , e . g . for use in industrial processes or for disposal with minimal environmental impact .

[0083] In some embodiments , the thermal treatment is carried out at ambient pressure . This reduces the construction and operational costs .

[0084] A24215WO / 29 . 11 . 2024 Preferably, the pH within the digester is kept below 12 . While it has been suggested in the prior art to increase the pH within the digester over 12 to achieve hygienization, the method of the present invention does not require such measures .

[0085] In a further aspect , the present invention relates to a biogas plant comprising an anaerobic digester and a thermal treatment unit , the anaerobic digester being configured for the digestion of organic biomas s to produce a methane-containing biogas and a digestate , said digester having a main inlet in an entry section at a first end for receiving organic biomass , a rear section at a second opposite end, a middle section arranged between the entry section and the rear section when seen along a common longitudinal axis , a gas outlet for discharging the biogas ; and an agitation device comprising a shaft extending in the direction from the first end to the second end and having at least one agitating arm for mixing the digestate within the digester, wherein the middle section of the digester includes a main outlet for removal of a maj or fraction of the digestate after a fast retention time t , the rear section includes an additional outlet for retrieval of a minor fraction of the digestate after an inoculum retention time T , wherein the inoculum retention time T is longer than the fast retention time t ,

[0086] A24215WO / 29 . 11 . 2024 the digester includes a separate return line leading from the additional outlet back to the entry section of the digester for allowing a direct return of the minor digestate fraction retrieved from the rear section back into the entry section of the digester; and the main outlet of the digester being connected with the thermal treatment unit for delivery of at least part of the maj or digestate fraction, wherein the digester includes a temperature controller to keep the temperature within the digester below 50 ° C , and the thermal treatment unit includes at least one unit selected from the group consisting of a pyrolysis unit , a torrefaction unit, a gasification unit , a hydrothermal gasification unit , a hydrothermal carbonization unit and a hydrothermal liquefaction unit .

[0087] The above-described first alternative of the inventive anaerobic biogas plant design includes a single type of an anaerobic digester operated under mesophilic conditions .

[0088] By operating under mesophilic conditions , the anaerobic digester offers a reliable , cost-effective , and energyefficient solution for organic waste management and biogas production . In addition, it was found that mesophilic digesters generally exhibit a higher tolerance to ammonia compared to thermophilic digesters . This means that the microbes operating at mesophilic conditions can better withstand higher concentrations of ammonia . This increased tolerance to ammonia helps in maintaining a stable microbiological environment in mesophilic digesters , leading to fewer disruptions in the process .

[0089] A24215WO / 29 . 11 . 2024 The main inlet at the first end is the entry point for fresh organic biomass into the digester . The rear section is located at the far end of the digester, primarily serving for the retrieval of the minor fraction of digestate . The middle section between entry and rear sections serves as the main processing area where the maj or fraction of the digestate is removed after the fast retention time t .

[0090] The agitation device ensures thorough mixing of the organic biomass within the digester , promoting even distribution of microorganisms and consistent digestion . The agitating arm ( s ) help in breaking up any clumps and maintaining homogeneous conditions within the digester .

[0091] The main outlet in the middle section allows for the removal of the maj or fraction of digestate after the designated fast retention time t , ensuring efficient flow and preventing overloading in the digester .

[0092] The additional outlet in the rear section is used for retrieving the minor digestate fraction after the prolonged inoculum retention time T , supporting microbial reinoculation .

[0093] The return line from the additional outlet to the entry section facilitates the reintroduction of the minor digestate fraction back into the entry section of the digester, maintaining the microbial population and enhancing the digestion of fresh organic biomass .

[0094] In line with the present invention , at least part of the maj or fraction of the digestate is directed from the digester ' s main outlet to the thermal treatment unit for further processing . The plant can thus efficiently convert some or all the maj or

[0095] A24215WO / 29 . 11 . 2024 fraction of digestate into valuable products , enhancing resource utilization and economic viability .

[0096] On the other hand, recycling the minor digestate fraction helps maintaining a stable and active microbial population , supporting consistent biogas production and digestion efficiency . This allows for increasing the overall throughput of organic biomass and thus maximi zing the amount of biogas produced within the digester without expanding the digester size .

[0097] Also , owing to the specific design of the digester with an additional outlet and a separate return line , the anaerobic biogas plant of the present invention allows for the inoculation of organic biomass as well as the preparation of an inoculum digestate within one and the same digester and nevertheless securing the incessant operation of the anaerobic digestion with systematic removal and recycling of digestate fractions . An additional inoculation digester is therefore not required . The maj or advantages of the inventive anaerobic biogas plant therefore lie in the shortened retention time and in its simplicity in construction and operation of the biogas plant .

[0098] Notably, the trans fer of some or all of the maj or digestate fraction from the digester to the thermal treatment unit may be a direct trans fer , i . e . without additional treatment , or the transfer may occur via an interposed treatment unit , in particular via an interposed solid-liquid separation device . In the latter case , the solid-liquid separation device is connected to both, the main outlet of the digester and the thermal treatment unit . Throughout this application , the term

[0099] A24215WO / 29 . 11 . 2024 "connected" hereby means that material can be transferred from one unit to the other unit . For instance , a connection may be established by trans fer pipes or other material trans fer equipment .

[0100] I f a solid-liquid separation device is present , it preferably includes a screw press and is configured for separating digestate discharged from the main outlet of the middle section of the digester into a liquid digestate portion and solid digestate portion . Trans fer pipes between the solid-liquid separation device and the thermal treatment unit are in this case provided for allowing transfer of at least the solid digestate to the thermal treatment unit for thermal treatment . By thermally treating primarily the solid digestate portion, i . e . material that has a reduced water-content compared to the maj or digestate fraction, the energy consumption of the thermal treatment unit can be reduced and the treatment time of the digestate to produce valuable products , such as char , hydrochar, syngas etc . can be shortened significantly .

[0101] In an alternative aspect , the present invention relates to a biogas plant comprising a digestion system for the anaerobic digestion of organic biomass and a thermal treatment unit . The digestion system produces a digestate and a methane-containing biogas as primary products . To this end, the digestion system comprises two separate digesters : an inoculum digester and a fast digester . The use of two different types of digesters allows for dif ferentiated treatment stages , optimizing retention times and microbial dynamics to enhance overall biogas production and process ef ficiency .

[0102] The inoculum digester and the fast digester are both supplied

[0103] A24215WO / 29 . 11 . 2024 with organic biomass for anaerobic digestion . To this end, each of the inoculum digester and the fast digester includes a respective inlet in an entry section at a first end of the respective digester, and at least one outlet in a rear section at an opposite end of the respective digester . Both digesters further include a respective agitation device to maintain homogeneity within the organic biomass , ensuring even microbial activity and preventing sedimentation or stratification .

[0104] Both digesters preferably operate in plug-flow mode , meaning the organic biomass moves through each digester in a sequential manner, ensuring uniform exposure to microbial activity and consistent processing . Operating in the plug-flow mode also avoids common issues like channeling and allows for more predictable and controlled digestion progress across both digesters .

[0105] In line with the present invention at least the fast digester is configured for anaerobically digesting the organic biomass at a temperature below 50 ° C . To this end, at least the fast digester includes a temperature controller to keep the temperature within the fast digester below 50 ° C . Thus , the fast digester ( and preferably also the inoculum digester) is configured to operate at mesophilic temperatures which is energy-efficient and easier to manage compared to higher- temperature processes due to a lower risk of process inhibition by free ammonia ( see discussion above in connection with the inventive method) .

[0106] After an inoculum retention time T, a first and a second digestate fraction are discharged from the rear section of the

[0107] A24215WO / 29 . 11 . 2024 inoculum digester . In other words , after the inoculum retention time T , a certain amount of digestate is discharged from the inoculum digester . Said discharged amount is divided into two fractions : a first and a second digestate fraction .

[0108] The inoculum digester includes a return line to allow return of the first digestate fraction directly back into the entry section of the inoculum digester . The returned first digestate fraction acts as self-inoculating substrate and thus ensures a high concentration of active microorganisms within the inoculum digester . The inoculum digester is thus designed to develop and sustain a robust microbial population that can effectively inoculate the organic biomass . This inoculum digester operates with an extended inoculum retention time T .

[0109] The inoculum digester and the fast digester are connected via a transfer line to allow transfer of the second digestate fraction to the entry section of the fast digester . The second digestate fraction thereby brings a rich microbial inoculum into the fast digester to enhance the digestion process of the organic biomass therein .

[0110] The fast digester and the thermal treatment unit are connected to allow trans fer of a fast digestate discharged from the rear section of the fast digester after a fast retention time t to the thermal treatment unit . Looking at the total digestate produced by the inoculum digester and the fast digester within the digestion system, the returned first digestate fraction constitutes a minor fraction of the total digestate , whereas the fast digestate discharged from the rear section of the fast digester after the fast retention time t constitutes a maj or fraction of the total digestate produced in the digestion

[0111] A24215WO / 29 . 11 . 2024 system . Since the fast retention time t is shorter than the inoculum retention time T , the fast digester focuses on the rapid processing of the biomass , thereby ensuring efficiently processed material ready for further treatment .

[0112] In accordance with the present invention, the thermal treatment unit includes at least one unit selected from the group consisting of a pyrolysis unit , a torrefaction unit , a gasification unit , a hydrothermal gasification unit , a hydrothermal carbonization unit and a hydrothermal liquefaction unit . With respect to the specifics and benefits associated with each of the listed types of thermal treatment units it is referred to the discussion thereof above in connection with the inventive method .

[0113] In analogy to the embodiment involving a single digester type as described above , digestate discharged from the rear section of the fast digester after the fast retention time may be trans ferred directly to the thermal treatment unit or indirectly via an interposed treatment unit , in particular an interposed solid-liquid separation device . In the latter case the solid-liquid separation device is connected to the fast digester and the thermal treatment unit .

[0114] I f a solid-liquid separation device is present , said separation device preferably includes a screw press and is configured for separating the digestate - this means either the maj or digestate portion discharged from the middle section of the ( single-type ) digester or the fast digestate discharged from the rear section of the fast digester - into a liquid digestate portion and solid digestate portion . Connection means , e . g . trans fer pipes , are in this case provided between the

[0115] A24215WO / 29 . 11 . 2024 respective digester and the solid-liquid separation device as well as between the solid-liquid separation device and the thermal treatment unit for allowing transfer of digestate to the solid-liquid separation device , and of the solid digestate portion to the thermal treatment unit , respectively . With respect to the benefits of performing a solid-liquid separation of the digestate prior to the thermal treatment it is referred to the paragraphs above in connection with the discussion of the single-digester-type embodiment .

[0116] Preferably, both the inoculum digester and the fast digester are dry or semi-dry digesters operated in plug flow mode . There are two main types of anaerobic digestion processes for treatment of biodegradable wastes , namely "wet anaerobic digestion systems" , which use organic material with consistency of 10-20% dry matter or less , and "dry ( or semidry) anaerobic digestion systems" for organic matter with consistency of 20 to 40% dry matter or more . Digestate with a dry matter content of at least 20% is generally preferred as the solid portion of the digestate can be converted into valuable solid or gaseous products , such as fuel or char . I f the digestate has a high viscosity ( i . e . reduced water content ) , plug flow digesters have a higher specific throughput capacity compared to stirred digesters .

[0117] The system may further include a biogas treatment unit for upgrading the biogas produced in the above-described anaerobic digester or digester system . The biogas produced normally contains methane , carbon dioxide , hydrogen sulfide , water, and other contaminants . After upgrading the biomethane can be used as a vehicle fuel or inj ected into the natural gas grid network . There are several methods for biogas upgrading,

[0118] A24215WO / 29 . 11 . 2024 including water scrubbing, chemical scrubbing, pressure swing adsorption ( PSA) , and membrane separation . Depending on the type of biogas treatment unit , it may require energy in form of heat and / or in the form of electrical power . For instance , in the pressure swing adsorption technique , biogas is compressed and fed into a column containing an adsorbent material that selectively retains CO2 . This process requires energy for compressing the biogas and for the periodic regeneration of the adsorbent material , which is achieved by reducing the pressure in the column . Instead of a PSA it is preferred that the biogas treatment unit is an amine upgrader . An amine upgrader typically involves amine gas treatment , which removes carbon dioxide , hydrogen sulfide , water, and other contaminants from biogas through chemical absorption of these gases by aqueous solutions of amines : The biogas is passed through a solution where the impurities react with the amine , forming a non-volatile compound that can be separated . The cleaned gas , now with reduced levels of CO2 and H2S , can be used for further applications , and the amine solution can be regenerated for reuse by heating to release the absorbed gases .

[0119] Preferably, the biogas plant employs heat integration between the thermal treatment unit and the biogas treatment unit . For instance , waste heat from the thermal treatment unit can be directly trans ferred to the biogas treatment unit or it may be converted into electrical power through e . g . the Rankine cycle or a micro gas turbine . In case that an amine upgrader ( amine scrubber) is used as biogas treatment unit , heat energy produced from the thermal treatment unit can be used for the regeneration of the amine solution by directly trans ferring it to the amine upgrader .

[0120] A24215WO / 29 . 11 . 2024 The present invention is further described in connection with the enclosed drawings , which show purely schematically :

[0121] Fig . 1 a schematic view of a first embodiment of an inventive biogas plant including an anaerobic digester for the digestion of organic biomass and a thermal treatment unit ; and

[0122] Fig . 2 a schematic view of a second embodiment of an inventive biogas plant including a digestion system for the anaerobic digestion of organic biomass and a thermal treatment unit , the digestion system comprising an inoculum digester and a fast digester .

[0123] In the attached figures , rectangular boxes are used to designate units or devices , whereas circles are used to designate materials that are produced or treated within the biogas plant .

[0124] The anaerobic biogas plant 10 shown in Fig . 1 is operated in accordance with a first embodiment of the present invention and includes at least one anaerobic digester 12 of a single type for the production of a digestate 14 and a biogas 16 from organic biomass 18 . The organic biomass 18 is supplied to the digester 12 via a supply line 20 and enters the digester 12 through an inlet 21 in an entry section 24 at a first end 25 of the digester 12 . The digester 12 further comprises a main outlet 22 in a middle section 26 , an additional outlet 26 in a rear section 28 at an opposite end 30 , a gas outlet 29 for discharging biogas the 16 , and a return line 31 connecting the

[0125] A24215WO / 29 . 11 . 2024 additional outlet 27 with the inlet 21 . After entering the digester 12 through the inlet 21 in the entry section 24 , the organic biomass 18 is transported in the plug-flow mode through the middle section 26 and further towards the rear section 28 of the digester 12 . Since the main outlet 22 is located in the middle section 26 , a maj or fraction 32 of the digestate 14 is not transported all the way to the rear section 28 of the digester 12 but is removed from the middle section 26 via the main outlet 22 after a fast retention time t of 7 days . Only a minor fraction 34 of the digestate 14 is transported into the rear section 28 and retrieved via the additional outlet 27 after a prolonged inoculum retention time T of 14 days . After being discharged through the additional outlet 27 , the minor digestate fraction 34 directly returned back into the entry section 24 of the digester 12 via the return line 30 to act as self-inoculation substrate . Once returned into the digester 12 , the minor digestate fraction 34 is mixed with freshly supplied organic biomass 18 with the aid of the agitation device (not shown ) to help the digestion process .

[0126] Although not shown in the schematic figure , the digester comprises a temperature controller to keep the temperature within the digester 10 below 50 ° C , specifically in the range of 42 ° C to 48 ° C .

[0127] In addition to the digester 12 , the biogas plant 10 further includes a solid-liquid-separation device 40 and a thermal treatment unit 50 . In the shown embodiment , the solid-liquid separation device 40 is a screw press and is interposed between the digester 12 and the thermal treatment unit 50 , meaning that the solid-liquid separation device 40 is connected to both the digester 12 and the thermal treatment unit 50 in a

[0128] A24215WO / 29 . 11 . 2024 way that enables trans fer of material from one unit to the other .

[0129] Specifically, the maj or digestate fraction 32 that is discharged through the main outlet 22 is transferred to the solid-liquid separation device 40 via a transfer pipe 41 . The solid-liquid separation device 40 separates the digestate fraction 32 into a liquid digestate portion 42 and a solid digestate portion 44 . The solid digestate portion 44 is trans ferred to the thermal treatment unit 50 for thermal treatment . In the embodiment of Fig . 1 , the thermal treatment unit is a pyrolysis unit , within which the solid digestate portion 44 is heated to at least 450 ° C for a few minutes , during which the solid digestate portion 44 is converted into a synthesis gas ( syngas ) 46 and biochar 48 . The syngas 46 can be burned directly in gas engines , cooled to extract pyrolysis oil , used to produce methanol and hydrogen, or converted via the Fischer-Tropsch process into synthetic fuel . The pyrolysis unit is a directly heated pyrolysis reactor, specifically one that applies heat directly to the material , rather than being conducted through a surface such as a reactor wall .

[0130] Heat produced during the thermal treatment of the solid digestate portion and also heat from syngas is recovered with the aid of heat exchangers (not shown) . The recovered heat 52 is trans ferred to a sanitizer 54 .

[0131] At least part of the liquid digestate portion 42 is transferred to the sanitizer 54 and is heat-treated therein for a duration of at least 1 hour at a temperature of at least 70 ° C . The two digestate portions 42 , 44 are thus both heat-treated, but independently from one another, in different units and for

[0132] A24215WO / 29 . 11 . 2024 different durations . The treatment of the liquid digestate portion 42 can thus occur simultaneously to the treatment of the solid digestate portion 44 . Thanks to the thermal treatment in the sanitizer 54 , the liquid digestate portion 42 is hygienized, meaning that any pathogens other harmful organisms are effectively killed or at least deactivated . The hygienized liquid digestate portion can be used, for example , as fertilizer .

[0133] Although not shown in the figures , it is also possible that some of the liquid digestate portion is used for humidification of the digester feedstock, meaning that some of the liquid digestate portion is returned to the digester for increasing the moisture content of the organic biomass within the digester . This adj ustment of the moisture content and thus the viscosity of the digestate within the digester avoids excessive energy consumption for mixing or agitating the digestate and also reduces the wear on the agitation means .

[0134] I f desired, the system may further include an evaporator (not shown ) for condensing the liquid digestate . Such an evaporator is preferably also provided with heat energy recovered from the thermal treatment of the solid digestate portion and / or the syngas .

[0135] In the alternative embodiment shown in Fig . 2 , the biogas plant 100 comprises a digestion system 112 for the anaerobic digestion of organic biomass and a thermal treatment unit 114 . In analogy to the digester 12 in Fig . 1 , the digestion system 112 in Fig . 2 produces a total digestate 116 and a methane- containing biogas 118 as primary products . Contrary to the

[0136] A24215WO / 29 . 11 . 2024 digester 12 in Fig . 1 , however , the digestion system 112 comprises at least two digesters , namely an inoculum digester 120 and a fast digester 122 , that differ from one another in their construction and function . The two digesters 120 , 122 are connected, meaning that some type of transfer means is provided that allows trans fer of material from one digester to the other . Both digesters 120 , 122 are operated under mesophilic conditions , in particular in a temperature range of 42 ° C to 48 ° C . A temperature controller (not shown) is provided to keep the temperature within the digesters in the mesophilic range . In the shown embodiment , the size and thus the effective volume of the inoculum digester 120 is smaller than the one of the fast digester 122 . The biogas plant may optionally have several inoculum digesters and / or several fast digesters , which are operated as follows .

[0137] The inoculum digester 120 and the fast digester 122 are both supplied with organic biomass 124 for anaerobic digestion through microorganisms . More specifically, organic biomass 124 is supplied through a supply line 126 into an entry section 127 of the inoculum digester 120 via an inlet 128 at a first end 130 of the inoculum digester 120 . Analogously, organic biomass 124 is supplied through a supply line 131 into an entry section 133 at a first end 134 of the fast digester 122 via an inlet 132 .

[0138] Within both the inoculum digester 120 and the fast digester 122 , the organic biomass 124 is mixed with the aid of an agitation device (not shown ) and is moved in the plug-flow mode towards a rear section 138 , 140 at an opposite end 139 , 141 of the respective digester 120 , 122 .

[0139] A24215WO / 29 . 11 . 2024 Within the inoculum digester 120 , the organic biomass 124 is digested during an inoculum retention time T of 14 -21 days to produce a digestate - which will in the following be referred to as "inoculum digestate" 146 .

[0140] In the fast digester 122 , the organic biomass 124 is digested within a fast retention time t that is longer than the inoculum retention time T to produce a digestate - which will in the following be referred to as " fast digestate" 148 .

[0141] The inoculum digestate 146 and the fast digestate 148 together represent the digestate 116 of the digestion system 112 . Looking at the respective volumes , the inoculum digestate 146 ( or any part thereof ) constitutes a minor fraction of the digestate 116 and the fast digestate constitutes a maj or fraction of the digestate 116 of the digestion system 112 .

[0142] After the inoculum retention time T , a first 150 and a second fraction 151 of the inoculum digestate 146 are discharged through separate outlets from the rear section 138 of the inoculum digester 120 . The inoculum digester 120 includes a return line 154 to allow return of the first digestate fraction 150 from the rear section 138 directly back into the entry section 127 of the inoculum digester 120 . The returned first digestate fraction 150 acts as self-inoculating substrate and thus ensures a high concentration of active microorganisms within the inoculum digester 120 .

[0143] The inoculum digester 120 and the fast digester 122 are connected via a transfer line 155 to allow trans fer of the second digestate fraction 151 of the inoculum digestate 146 into the entry section 133 of the fast digester 122 . Thus , the second fraction 151 of the inoculum digestate 146 is introduced

[0144] A24215WO / 29 . 11 . 2024 into the fast digester 122 together with an amount of organic biomass 124 , such that an inoculation of the organic biomass 124 takes place in the fast digester 122 to enhance the digestion process therein . The second digestate fraction 151 is mixed with the introduced organic biomass 124 with the aid of the agitation device (not shown ) .

[0145] Notably, in the shown embodiment , the organic biomass and the second fraction 151 are introduced into the fast digester 122 through separate inlets . However, a common inlet is of course also possible .

[0146] As mentioned, the biogas plant 100 also includes a thermal treatment unit 114 . The fast digester 122 and the thermal treatment unit 114 are connected to allow transfer of the fast digestate 148 discharged from the rear section 140 of the fast digester 122 after a fast retention time t to the thermal treatment unit 114 . Thanks to the introduction of the second digestate fraction 151 of the inoculum digestate 146 the fast retention time t may be as short as 5-8 days and still allow for essentially full digestion of the organic biomass 124 within the fast digester 122 .

[0147] In the shown embodiment , the fast digestate 148 is directly trans ferred to the thermal treatment unit 114 . However, analogous to the embodiment shown in Fig . 1 , the fast digestate discharged from the fast digester may also first be separated into a liquid digestate portion 152 and a solid digestate portion 153 with the aid of a liquid-solid-separation device 155 prior to transferring at least the solid digestate portion 153 to the thermal treatment device 114 and the liquid digestate portion to a drying unit 156 used for drying - i . e .

[0148] A24215WO / 29 . 11 . 2024 reducing the volume - of the liquid digestate portion 152 and for sanitizing the same .

[0149] In the shown embodiment of Fig . 2 , the thermal treatment unit 114 is a gasifier that converts the solid digestate portion 153 into a syngas 158 . Heat 160 from the thermal treatment is recovered with the aid of a heat exchanger (not shown) and is trans ferred to the drying unit 156 .

[0150] Despite involving at least two digesters of dif ferent types , the embodiment shown in Fig . 2 is still in line with the basic concept of the present invention, since only a minor fraction of the digestate 116 of the digestive system 112 is retrieved and recycled for use as inoculum, whereas a maj or fraction of the digestate 116 of the digestive system 112 is removed and further processed in the thermal treatment unit 114 .

[0151] A24215WO / 29 . 11 . 2024

Claims

Claims1. Method for use in a biogas plant (10, 100) having at least one digester (12, 120, 122) and a thermal treatment unit (50, 114) , the method comprises the steps of: a) anaerobically digesting an organic biomass (18, 124) in at least one digester (12, 120, 122) through microorganisms at a temperature below 50 °C to produce methane-containing biogas (16, 118) and a digestate (14, 116) ; b) removing a major fraction (32, 148) of the digestate(14, 116) from the digestion process after a fast retention time (t) and subjecting at least part of said major digestate fraction (32, 148) to a thermal treatment in a thermal treatment unit (50, 114) at a temperature above 175°C for at least one minute, wherein the thermal treatment step involves at least one treatment selected from the group consisting of pyrolysis, torref action, gasification, hydrothermal gasification, hydrothermal carbonization and hydrothermal liquefaction; c) retrieving a minor fraction (34, 150) of the digestate (14, 116) from the digestion process after an inoculum retention time (T) that is longer than the fast retention time (t) , returning said minor digestate fraction (34, 150) into the digestion process and mixing the minor digestate fraction (34, 150) with the organic biomass (18, 124) within the digester (12, 120, 122) .A24215WO / 29.11.20242. Method according to Claim 1, wherein the minor fraction(34, 118) of digestate retrieved from the digestion process is returned back into the digestion process without further processing.

3. Method according to Claim 1 or 2, wherein the anaerobic digestion in step a) takes place at a temperature in the range of 20°C to 48°C, preferably 32°C to 48°C, most preferably 42°C to 48°C.

4. Method according to any of the preceding claims, wherein prior to the thermal treatment, the major digestate fraction (32, 148) is subjected to a solid-liquid phase separation to obtain a solid digestate portion (44, 153) and a liquid digestate portion (42, 152) , whereupon at least part of the solid digestate portion (44, 153) is subjected to the thermal treatment in step b) .

5. Method according to Claim 4, wherein waste heat (52, 160) produced during the thermal treatment of the solid digestate portion (44, 153) is recovered and used for thermally treating at least part of the liquid digestate portion (42, 152) .

6. Method according to any of the preceding claims, wherein the thermal treatment in step b) involves pyrolysis at a temperature of at least 450°C, preferably 450°C to 850°C, for at least 15 minutes, in particular 15 to 180 minutes, preferably 20 to 140 minutes.

7. Method according to any of Claims 1 to 6, wherein the thermal treatment in step b) involves torrefaction at a temperature of at least 200°C, more preferably 250°C toA24215WO / 29.11.2024350°C, for a treatment time of preferably at least 40 minutes .

8. Method according to any of Claims 1 to 6, wherein the thermal treatment in step b) involves gasification that is conducted at a temperature of preferably at least 600°C, more preferably at least 700°C, in particular in the range of 700-1200°C.

9. Method according to any of Claims 1 to 6, wherein the thermal treatment step in step b) involves hydrothermal carbonization for at least 30 minutes, preferably at least one hour, more preferably at least 2 hours, in particular 2-5 hours, preferably at a temperature within the range of 175°C to 350°C.

10. Method according to any of the preceding claims, wherein the fast retention time t is less than 50%, preferably less than 40%, more preferably about a third of the inoculum retention time T.

11. Method according to any of the preceding claims, wherein the fast retention time t is less than 10 days, preferably between 5 and 8 days .

12. Method according to any of the preceding claims, wherein the inoculum retention time T is at least 7 days, more preferably at least 14 days, most preferably about 21 days .

13. Anaerobic biogas plant (10) comprising a single anaerobic digester (12) and a thermal treatment unit (50) , said digester (12) being configured for the digestion of organic biomass (18) to produce a methane-containingA24215WO / 29.11.2024biogas (16) and a digestate (14) , wherein the digester(12) comprises a main inlet (21) in an entry section (24) at a first end (25) of the digester for receiving an organic biomass (18) , a rear section (28) at a second opposite end (30) , a middle section (26) arranged between the entry section (24) and the rear section (28) when seen along a common longitudinal axis, a gas outlet (29) for discharging the biogas (16) ; and an agitation device comprising a shaft extending in the direction from the first end (25) to the second end (30) and having at least one agitating arm for mixing a digestion biomass within the digester, wherein the middle section (26) of the digester includes a main outlet (22) for removal of a major fraction (32) of the digestate (14) after a fast retention time t, the rear section (28) includes an additional outlet (27) for retrieval of a minor fraction (34) of the digestate (14) after an inoculum retention time T, wherein the inoculum retention time T is longer than the fast retention time t, the digester (12) includes a separate return line (31) leading from the additional outlet (27) back to the entry section (24) of the digester (12) for allowing a direct return of the minor digestate fraction (34) retrieved from the rear section (28) back into the entry section (24) of the digester (12) ;A24215WO / 29.11.2024the main outlet (22) of the digester (12) is connected with the thermal treatment unit (50) for delivery of the major digestate fraction (32) ; the digester (12) includes a temperature controller to keep the temperature within the digester below 50°C; and the thermal treatment (50) unit includes at least one unit selected from the group consisting of a pyrolysis unit, a torrefaction unit, a gasification unit, a hydrothermal gasification unit, a hydrothermal carbonization unit and a hydrothermal liquefaction unit.

14. Anaerobic biogas plant (100) comprising a digestion system (112) for the anaerobic digestion of organic biomass (124) through microorganisms to produce a methane-containing biogas (118) and a digestate (116) ; and further comprising a thermal treatment unit (114) , wherein the digestion system (112) includes an inoculum digester (120) and a fast digester (122) , which are both supplied with organic biomass (124) for anaerobic digestion; each of the inoculum digester (120) and the fast digester (122) includes an inlet (126, 132) in an entry section (127, 133) at a first end (130, 134) of the respective digester, and at least one outlet in a rear section (138, 140) at an opposite end (139, 141) of the respective digester (120, 122) ; the inoculum digester (120) and the fast digester (122) both include a respective agitation device for mixing a digestion biomass within the respective digester;A24215WO / 29.11.2024at least the fast digester (122) includes a temperature controller to keep the temperature within the digester below 50°C; and the inoculum digester (120) includes a return line (154) to allow return of a first digestate fraction (150) retrieved from the rear section (138) of the inoculum digester (120) after an inoculum retention time T, directly back into the entry section (127) of the inoculum digester ( 120 ) ; the inoculum digester (120) and the fast digester (122) are connected via a transfer line (155) to allow transfer of a second digestate fraction (151) retrieved from the rear section (138) of the inoculum digester (120) after the inoculum retention time T, into the entry section (133) of the fast digester (122) ; the fast digester (122) and the thermal treatment unit (114) are connected to allow transfer of a fast digestate (148) discharged from the rear section (140) of the fast digester (122) after a fast retention time t to the thermal treatment unit (114) ; and the thermal treatment unit (114) includes at least one unit selected from the group consisting of a pyrolysis unit, a torrefaction unit, a gasification unit, a hydrothermal gasification unit, a hydrothermal carbonization unit and a hydrothermal liquefaction unit.

15. Anaerobic biogas plant according to Claim 13 or 14, further comprising a solid-liquid separation device (40, 155) connected to and interposed between the digester (12) of Claim 13, or the fast digester (122) of Claim 14,A24215WO / 29.11.2024and the thermal treatment unit (50, 114) , wherein the solid-liquid separation device (40, 155) is configured for separating the digestate (14) discharged from the rear section (28) of the digester (12) or the fast digestate (148) discharged from the rear section(140) of the fast digester (122) into a liquid digestate portion (42, 152) and a solid digestate portion (44, 153) , whereby the connection between the solid-liquid separation device (40, 155) and the thermal treatment unit (50, 114) enables transfer of at least the solid digestate portion (44, 153) to the thermal treatment unit (50, 114) for thermal treatment.A24215WO / 29.11.2024

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