EP4720249A1 - Method and system for the hygienization of a digestate in the production of biogas - Google Patents

Abstract

The present application relates to a method and system for hygienization of a digestate (14) in the production of a biogas from biomass 16. The method involves a) an anaerobic digestion step involving breakdown of organic biomass (16) through microorganisms in the absence of oxygen at a temperature below 50°C within an anaerobic digester (10) to produce methane-containing biogas (12) and a digestate (14); and b) a thermal treatment step involving treatment of at least part of the digestate (14, 22, 24) at a temperature above 175°C for at least one minute. The thermal treatment step involves one treatment selected from the group of pyrolysis, torrefaction, gasification, hydrothermal gasification, hydrothermal carbonization and hydrothermal liquefaction.

Description

Method and system for the hygienization of a digestate in the production of biogas

The present invention relates to the field of waste and effluent treatment, and in particular to a method and system for the production of biogas, including the sanitization ("hygienization") of pathogen-containing digestate formed in said production of biogas and originating from the feedstock.

Anaerobic digestion (AD) is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is generally carried out in four stages: hydrolysis; fermentation and acidification; acetogenesis, and methanogenesis. During these four stages, the organic matter in the waste and wastewaters is transformed to biogas, a mix of methane (CH4) and carbon dioxide (CO2) and a nutrient rich sludge (digestate) . The raw biogas can be used directly as fuel or upgraded to natural gas-quality biomethane. The raw or upgraded biogas can be utilized in combined heat and power gas engines; it has therefore a large potential as a renewable energy source. The nutrient-rich digestate can be composted and used as fertilising soil amendment in agriculture. Thus, anaerobic digestion can be used for industrial or domestic purposes to manage waste and to produce fuels .

Anaerobic digesters can be designed and engineered to operate using a number of different configurations and can be categorized into batch vs. continuous process mode, mesophilic vs. thermophilic temperature conditions, high vs. low portion of solids, and single stage vs. multistage processes. Important digester parameters include temperature, pH, f eedstock-to- microorganism ratio, organic loading rate, hydraulic and solids retention time, adequate mixing, and others.

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.

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 coii 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 effective 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 can be increased but also the generation of free ammonia, which can have an inhibitory effect on the digestion performance.

Therefore, mesophilic systems are preferable in view of reduced energy consumption and operational costs, but they come with the drawbacks of lower biogas production and lack of hygienization (compared to thermophilic systems) . When operating at mesophilic conditions, hygienization may be achieved by either specific hygienization treatments, e.g. pasteurization, a prolonged residence time within the digester, e.g. 15 days and longer, and/or high pH.

EP3150569A1 discloses a method for producing a fertilisation product from anaerobic digestate by pasteurization of the digestate in specific tanks, in which the digestate is heat treated at a temperature of from 70°C to 80°C. This temperature is chosen to ensure killing of harmful microorganisms while minimizing the degradation of organic materials in the digestate. Thus, specific equipment and treatment conditions are required exclusively to achieve hygienization.

No special equipment is needed if hygienization is achieved by prolonged residence of the digestate within the digester. However, lower residence times mean lower throughput and thus a decreased biogas production over time as well as the necessity to build larger and thus more expensive plants.

The pH within the fermenter can be increased e.g. by the addition of lime, caustic soda, ammonia, or urea. The recommendation for high pH treatment of sewage products requires pH 12 for at least 3 months. However, it was found that microbial activity tends to decrease at high pH levels. When ammonia is present, lower pH and shorter treatment times may suffice. At the same time, the use of ammonia creates an environmental issue and constitutes an occupational hazard.

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 biomass into biogas by anaerobic digestion at a reduced input of energy and chemicals and which achieves effective and economical hygienization of the resulting digestate .

This obj ect is achieved by the method as defined in Claim 1 and the system as defined in Claim 12 of this application . Preferred embodiments are subj ect of the dependent claims .

Speci fically, the method of the present invention i s directed to a conversion of organic biomass into biogas and a digestate under mesophilic conditions , whereby hygieni zation of digestate is achieved by a subsequent heat treatment step . In line with the present invention, the method involves a ) an anaerobic digestion step involving breakdown of organic biomass through by microorganisms in the absence of oxygen at a temperature below 50 ° C within an anaerobic digester to produce methane-containing biogas and a digestate ; and b ) a heat treatment step involving treatment of at least a fraction of the digestate at a temperature above 175 ° C for at least one minute , wherein the thermal treatment step involves one treatment selected from the group of pyrolysis , torref action, gasi fication, hydrothermal gasi f ication, hydrothermal carboni zation and hydrothermal liquefaction .

Thus , in the anaerobic digestion step, organic biomass is anaerobically digested with the aid of bacteria in an anaerobic digester . In the following it will simply be referred to "the digester" , without always speci fying that it is an anaerobic type .

In accordance with the present invention, the temperature within the digester is kept below 50 ° C, preferably within the temperature range of 20 ° C to about 48 ° C, and most preferably within the range of 40 ° C to 48 ° C, which means that the digestion within the digester takes place at or close to mesophilic conditions . This provides an advantage because the balance between ammonium and ammonia tilts toward ammonium, resulting in reduced inhibition of the anaerobic digestion (AD) process compared to the presence of free ammonia . As a result , it allows for an increased organic loading rate without compromising the stability of the process . The inventive method thus allows for more organic feedstock to be converted, and thus to achieve a high biogas production rate , without digestion inhibition due to increased ammonia concentration .

In line with the present invention the resulting digestate is then treated in a heat treatment step, which involves a heat treatment at a temperature of at least 175 ° C for at least one minute .

The heat treatment of the digestate has the e f fect that pathogens (viruses and bacteria ) are ef fectively ki lled or at least inactivated . It has been found that even a very short time of one minute is suf ficient for this purpose . Thus , any pathogens , bacteria, viruses or the like that are present in the organic waste can be killed thoroughly and reliably during the heat treatment , which eliminates the need for temperatures above 50 ° C and/or for keeping the digestate within the digester for a prolonged residence time in order to achieve hygieni zation . As a result , the residence time of the organic biomass in the anaerobic digester can be reduced signi ficantly - compared to normal anaerobic digestion under thermophilic conditions - and will only be determined by the speed of the bacterial conversion of the organic biomass to biogas .

As mentioned, the thermal treatment step involves one of pyrolysis , torrefaction, gasi fication, hydrothermal gasi fication, hydrothermal carboni zation and hydrothermal liquefaction .

The term pyrolysis generally refers to a process of thermal decomposition of - generally organic - materials at elevated temperatures , usually above 350 ° C, often in an inert atmosphere . It involves a change of chemical compos ition as it generally involves heating of the material above its decomposition temperature . In general , pyrolysis of organic substances produces volatile products and leaves char, a carbon-rich solid residue .

Torrefaction as used herein refers to a thermochemical treatment of biomass at usually 200 to 350 ° C . It is similar to pyrolysis but carried out at lower temperatures , generally under atmospheric pressure and in the absence of oxygen . During the torrefaction process , the water contained in the biomass as well as superfluous volatiles are released, and the biopolymers ( cellulose , hemicellulose and lignin) partly decompose , giving of f various types of volatiles . The final product is the remaining solid, dry, blackened material that is of char-like structure and is referred to as torrefied biomass or (bio ) coal .

( Traditional ) Gasi fication as used herein refers to a process that converts biomass- or foss il fuel-based carbonaceous materials into gases , in particular carbon monoxide, hydrogen, and carbon dioxide . This is achieved by reacting the feedstock material at temperatures of typically above 700°C, without or incomplete combustion, via controlling the amount of oxygen atoms present in the reaction by adding one or more of air, oxygen, steam and carbon dioxide. During gasification process, a synthesis gas (often referred to as "syngas") is produced together with different tar components. Syngas can be used as a fuel due to the flammability of its main components hydrogen and carbon monoxide. In addition, it may also be used as the hydrogen source in fuel cells. Several types of gasifiers are currently available for commercial use: counter-current fixed bed gasifiers, co-current fixed bed gasifiers, and fluidized bed gasifiers. For the latter, it is further distinguished between Bubbling Fluidized Bed (BFB) , Circulating Fluidized Bed (CFB) and Dual Fluidized Bed (DFB) gasifiers.

Hydrothermal gasification is a thermochemical process that involves converting wet biomass and/or organic waste into useful gases, such as hydrogen, methane, and carbon monoxide, along with liquid and solid by-products. This process typically occurs under high temperature (typically between 200°C and 600°C) and pressure conditions (typically between 10 and 25 MPa) in the presence of water. Preferably, the hydrothermal gasification is conducted at supercritical conditions, at temperatures above 374°C and pressures above 221 bar. Hydrothermal gasification offers the advantage of the ability to process wet or high-moisture feedstocks without the need for extensive drying, reducing preprocessing costs and energy requirements .

Hydrothermal carbonization generally refers to a thermochemical conversion process which is used to convert biomass and/or organic waste in the presence of water into solid biofuel, hydrochar, liquid, and gaseous products. Hydrochar is regarded as the primary product, because it can be used in a variety of applications, such as a solid fuel or as an adsorbent for removing pollutants from water/wastewater streams. Hydrothermal carbonization is usually conducted at a temperature above 175°C, often in the range of 180°C to 220°C. It has the benefit of allowing treatment of rather wet digestate, e.g. digestates having a moisture content above 50%, such as around 75% or higher. The term "moisture content" equally refers to the humidity or water content of the digestate .

Hydrothermal liquefaction refers to a thermochemical conversion process that transforms wet biomass and/or organic waste into a liquid hydrocarbon-rich product known as bio-oil. This process occurs under moderate to high temperature (typically between 250°C and 400°C) and pressure conditions (typically between 10 and 25 MPa) in the presence of water. It provides the benefit of allowing processing of wet or high- moisture feedstocks without the need for extensive drying, reducing preprocessing costs and energy requirements. It enables the production of a liquid bio-oil product with properties similar to petroleum crude oil.

Each of these treatments enables hygienization of the digestate, thereby allowing for the use of the treated digestate as fertilizer or for other purposes. In addition, these treatments allow conversion of the digestate to produce valuable products, in particular solid products with high calorific value, such as e.g. char, fuel coal and syngas, for use as energy source. Separate devices used exclusively for hygienization are no longer required, which saves construction costs and energy.

During the above-mentioned thermal treatment processes, gases are generated and said gases typically contain a mixture of combustible gases such as hydrogen, carbon monoxide, and methane, along with tar vapors and other volatile organic compounds. When these gases are cooled, the tar vapors and volatile components partly or completely condense, forming a liquid condensate. This condensate may contain water, organic compounds, and tars derived from the thermal decomposition of the biomass. It may be further treated in an evaporator. For instance, the condensate may be treated in multistage vacuum systems utilize a series of vacuum stages to create and maintain low-pressure conditions, allowing for the removal of water and other volatile components from the condensate at lower temperatures compared to atmospheric evaporation. After such treatment, the purified condensate is collected and can be recycled, reused, or discharged as appropriate for the specific application.

While pyrolysis and (normal) gasification are preferably used for heat treatment of solid material with a low water content, hydrothermal gasification, hydrothermal carbonization and hydrothermal liquefaction are also well suited for a heat treatment of liquids, including liquid digestate and digestate (non-separated) directly out of the digester.

The term digestate, as used throughout this application refers to the material obtained after at least partial digestion of organic biomass, whereby the latter may include food waste, sewage sludge, livestock manure, agricultural by-products, etc. Preferably, the organic biomass includes components having high carbon contents, such as carbohydrates, preferably cellulose, hemicellulose, lignin, etc.

As regards the digestion process it is preferred that the anaerobic digestion step takes place at a temperature in the range of 20°C to 48°C, preferably 32°C to 45°C. Most preferably the temperature for the anaerobic digestion is maintained in the range of 42°C to 48°C. The digestion process thus 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) to achieve hygienization . In addition, 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.

As mentioned above, in a particularly preferred embodiment, the temperature within the anaerobic digester in which the digestion process 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 process at these temperatures is even 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 feedstock compared to thermophilic conditions where ammonia inhibition of the anaerobic digestion process is more pronounced. In some embodiments , the temperature within the anaerobic digester may be seasonally adj usted . Speci fically, during the colder months of winter, plant growth typically diminishes , resulting in a lower volume of green waste , which serves as organic feedstock 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 hygieni zation during the digestion process .

In a preferred embodiment , prior to step b ) , the digestate is subj ected to a solid-liquid phase separation to obtain a solid digestate fraction and a liquid digestate fraction; wherein at least part , preferably essentially all , of the solid digestate fraction is subj ected to thermal treatment in step b ) .

The terms " liquid" and " solid" digestate fraction are used to di f ferentiate a fraction with a higher moisture content , i . e . the liquid digestate fraction, from a fraction with a lower moisture content , i . e . the solid digestate fraction . As such, in comparison, the liquid digestate fraction has a higher moisture content than the solid digestate fraction, but solid parts can still be present in the liquid digestate fraction and the solid digestate fraction may also contain liquid components .

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 the digestate prior to the heat treatment allows a di f ferentiation in the application and/or the duration of the heat treatment of the liquid and solid digestate fraction . 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 digestate, as it highly effective in producing a solid digestate with a low moisture content in a short time. In addition, the moisture content in the solid digestate fraction can be controlled by adjusting the pressing pressure applied. For instance, in case of a screw press the mode of operation can be adjusted to set to moisture content of the solid digestate fraction to a desired level, which is typically around 65-70%. In a specific embodiment, the operational pressure of the screw press can be adjusted with the aid of a pressure controller.

If the digestate contains abrasive components or only very little solid material, sieving or centrifugation can be used as separation technique. An adjustment of the rotational speed of the centrifuge device also allows controlling the moisture content of the resulting solid digestate fraction. For some types of digestates, in particular those that contain mostly organic fractions of municipal solid waste (OFMSW) , a solidliquid separation can be achieved by sieving. One specific example is the use of vibrating sieves with two or more stages. By mounting different sieving elements (with variable pore size) , the moisture content of the solid digestate fraction can be adjusted.

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 .

A solid-liquid phase separation prior to the heat treatment is particularly preferred i f moisture content of the digestate is above 75% and i f the heat treatment is one of pyrolysis , torrefaction or gasi fication . Hydrothermal treatments , such as hydrothermal carboni zation, hydrothermal liquefaction or hydrothermal gasi fication, 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 that reason, a dehydration treatment is usually not required for hydrothermal heat treatment .

In a preferred embodiment , all or some of the solid digestate fraction and all or some of the liquid digestate fraction are both thermally treated in step b ) . The liquid digestate fraction may be treated for only a few minutes or even j ust one to two minutes with the aim to achieve hygieni zation thereof . Notably, i f at least a part of the liquid digestate fraction is to be used for humidi fication of the digester feedstock, said part may not even in every case require a heat treatment , at all .

The solid digestate fraction may be heat-treated longer than the liquid digestate fraction . This is preferably the case unless the heat treatment is performed by fluidi zed bed gasi fication . For any of the other above mentioned heat treatments , i . e . pyrolysis , torrefaction, gasi fication or solid bed gasi fication, the solid digestate fraction is preferably treated for a time span that allows for adequate conversion of the solid digestate into high-quality fuel or char. This time span is dependent from the chosen heat treatment, in particular the heat treatment temperature, but also from the moisture content of the digestate fraction and the average particle size. Due to the prior dewatering of the digestate, the heat treatment time for essentially full conversion of the solid digestate fraction can be reduced. Therefore, if a prior liquid-solid phase separation is performed on the digestate, 5 to 10 minutes of pyrolysis or torrefaction may suffice to achieve essentially full conversion of the digestate.

In an alternative, particularly preferred embodiment, at least part of the solid digestate fraction is heat treated as described above, while the heat liberated during the heat treatment of the solid digestate fraction is recovered and used for thermally treating at least part of the liquid digestate fraction. Thus, in this preferred embodiment, the solid digestate fraction is heat treated for achieving hygienization thereof and for converting the digestate into valuable products, such as char, fuel, coal or syngas. During this heat treatment, energy is liberated in the form of (waste) heat. This heat is preferably recovered and used for hygienization of at least part of the liquid digestate fraction. Hygienization of the liquid digestate may be performed in the same unit as the one used for the heat 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 heat treatment of the solid digestate for heating the liquid digestate to achieve hygienization of the latter. While in this embodiment, two heat treatment units are required, it provides the benefit of heat integration, such that hygieni zation of the liquid digestate fraction ( some or all thereof ) can be achieved without additional energy demands .

Despite the digestion process taking place at mesophilic conditions , the residence time of the organic biomass within the anaerobic digester is preferably less than 15 days . In particular, the residence of the organic biomass within the digester time may be less than 12 days . Thanks to the subsequent heat treatment step, prolonged residence times to achieve hygieni zation are not required . A shorter residence time equals higher throughput and thus higher ef ficiency of the digester, which lowers the capital expenditures .

In a preferred embodiment the thermal treatment step b ) involves treatment of the digestate by pyrolysis at a temperature of at least 450 ° C, in particular 450 ° C to 850 ° C, and for a treatment time of at least 15 minutes . In particular, the treatment time is preferably 15 to 180 minutes , more preferably 20 to 140 minutes . Pyrolysis of the digestate at 450 ° C or more for at least 15 minutes was found to enable ef fective conversion of the digestate into fully carboni zed products , such as coke , carbon, charcoals and chars that can be used as such in industrial applications or as soil improver or as carbon sequestration means .

In one embodiment , the thermal treatment referred to as step b ) involves directly heated pyrolysis . In this speci fic embodiment , in the pyrolysis reactor the heat is applied directly to the material , rather than being conducted through a surface such as a reactor 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 biomass and waste materials.

In another preferred embodiment, the thermal treatment step b) involves treatment of the digestate by 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. Digestate that has been treated above 200°C for at least 40 minutes can be used as fuel, with a higher effectiveness compared to dried solid digestate.

The above-defined preferred treatment times for pyrolysis and torrefaction are chosen in view of allowing conversion of the digestate. For other purposes, shorter treatment times can be applied. For instance, it was found that a treatment time of about 15 minutes or longer by pyrolysis or torrefaction (or gasification allows a significant reduction of the plastics content in the treated digestate.

In an 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. Apart from enabling effective hygienization, gasification can convert organic matter into a syngas that can serve as a fuel. 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 type and moisture content of the digestate. Pre-drying of the digestate can be employed to reduce the minimum treatment time. The newer process of f luidized-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 the heat-treated digestate.

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 biomass or waste into energy-rich gases without the emissions of other more conventional gasification techniques that work at lower temperatures and pressures.

In another embodiment of the inventive method, the thermal treatment step includes treatment of the digestate by hydrothermal carbonization for a treatment time of at least 30 minutes, preferably at least one hour, more preferably at least 2 hours, in particular 2-5 hours. While one minute heat 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. The temperature for hydrothermal carbonization is preferably within the range of 175°C to 350°C, more preferably 175°C to 280°C. 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 fractions.

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 efficiently 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.

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

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 hygieni zation, the method of the present invention does not require such measures .

In a further aspect , the present invention provides a system for the hygieni zation of a digestate in the production of a biogas . Said system includes

- an anaerobic digester configured to produce methane- containing biogas and a digestate through anaerobic digestion of organic biomass through microorganisms in the absence of oxygen, the anaerobic digester comprising an inlet for receiving organic biomass , a first outlet for discharging a biogas , a second outlet for discharging a digestate , and a temperature controller to keep the temperature within the digester below 50 ° C ;

- means for supplying at least part of the digestate from the digester to a heat treatment reactor, the heat treatment reactor being configured to heat at least part of the digestate to a temperature of at least 175 ° C and being selected from the group consisting of pyrolysis reactor, a gasi fication reactor, a torrefaction reactor, a hydrothermal gasi fication reactor, a hydrothermal liquefaction reactor, and a hydrothermal carboni zation reactor .

Prior to feeding the organic biomass into the anaerobic digester, the particle si ze of said organic biomass can be reduced by e . g . shredding, grinding or sieving . Reducing the particle si ze of the organic biomass will lead to a decreased viscosity of the digestate . This reduces the energy consumption and the wear on any agitator or mixing device installed within the digester for mixing or transporting the digestate within the digester .

Preferably, the digester is a dry or semi-dry digester operated in plug flow mode . There are two main types o f 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 semi-dry) 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 fraction of the digestate can be converted into valuable product , such as fuel or char . I f the digestate has a high viscosity ( i . e . reduced water content ) , plug flow digesters have a higher speci fic throughput capacity compared to stirred digesters .

The system preferably includes a dewatering device , preferably a screw press , for separating the digestate into a solid digestate fraction and a liquid digestate fraction . In this embodiment , the system further includes means for supplying digestate from the digester to the dewatering device , and means for supplying at least part of the solid digestate fraction to the heat treatment reactor .

Heat energy from the thermal treatment unit can be used for drying any of the solid and liquid digestate fractions or for heating the digester .

In the inventive system the heat treatment reactor is preferably a reactor selected from the group consisting of pyrolysis reactor, a gasi fication reactor, torrefaction reactor, hydrothermal gasification reactor, hydrothermal liquefaction reactor and hydrothermal carbonization reactor.

Most preferably, the reactor is a directly heated pyrolysis reactor configured to heat at least part of the digestate to a temperature of at least 450°C, in particular within the range of 450°C to 850°C.

In a particularly preferred embodiment, the system further includes a sanitizer; heat transfer means for transferring heat energy liberated from the heat treatment reactor to the sanitizer; and means for supplying at least part of the liquid digestate fraction to the sanitizer. The sanitizer is intended for heat treatment of the liquid digestate to achieve hygienization of the latter.

In the above preferred embodiment direct process heat integration can be used to directly transfer (waste) heat from the heat treatment reactor to the sanitizer using a heat exchanger or other common means for heat recovery, such as economizers and waste heat boilers.

The system may further include a biogas treatment unit for upgrading the biogas produced in the anaerobic digester. 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 injected into the natural gas grid network. There are several methods for biogas upgrading, 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 pres sure 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 treating, which removes carbon dioxide , hydrogen sul fide , water, and other contaminants from biogas . It involves the chemical absorption of these gases by aqueous solutions of amines . The gas 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 .

Preferably, the system employs heat integration between the heat treatment reactor and the biogas treatment unit . For instance , waste heat from the heat treatment reactor 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 liberated from the heat treatment reactor can be used for the regeneration of the amine solution by directly trans ferring it to the amine upgrader . An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, in which :

Figure 1 is a flow diagram showing the steps of a preferred embodiment of the inventive system . In particular, Fig . 1 shows the components of a system for the hygieni zation of a digestate in the production of a biogas in accordance with the present invention . Said system includes an anaerobic digester 10 configured to produce methane-containing biogas 12 and a digestate 14 through anaerobic digestion of organic biomass 16 through microorganisms in the absence of oxygen . The organic biomass 16 can be a variety of usually carbon-rich materials , such as urban wood waste , paper waste , cow manure , food waste , agricultural waste , etc . Although not shown in the schematic figure , the anaerobic digester 10 comprises an inlet for receiving the organic biomass , an outlet for discharging digestate , and a temperature controller to keep the temperature within the digester 10 below 50 ° C, speci fically in the range of 40 ° C to 48 ° C . The residence time of the organic biomass 16 within the anaerobic digester 10 is preferably 10 to 12 days . The digester is a plug- flow digester and may include agitators to aid mixing of the digestate within .

The system further includes trans fer or transportation means for supplying at least part of the digestate 14 that is discharged from the digester 10 to a dewatering device 20 . The dewatering device 20 is in this case a screw press that separates the digestate 14 from the digester 10 into a solid digestate fraction 22 and a liquid digestate fraction 24 . The solid digestate fraction 22 is supplied to a heat treatment reactor 30, e.g. through transfer pipes connecting the dewatering device 20 and the heat treatment reactor 30. The heat treatment reactor 30 is configured to heat the solid digestate fraction 22 to a temperature of at least 175°C. In particular, the heat treatment reactor can be a pyrolysis reactor, a gasification reactor, torrefaction reactor, hydrothermal gasification reactor, hydrothermal liquefaction reactor or hydrothermal carbonization reactor. In the shown embodiment, it is a pyrolysis reactor 30 (also called gasifier) . Within the pyrolysis reactor 30, the solid digestate fraction 22 is heated to at least 450°C for a few minutes, during which the solid digestate fraction is converted into a synthesis gas (syngas 32) and biochar 34. The syngas 32 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 reactor 30 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.

Heat liberated during the heat treatment of the solid digestate fraction 22 and also heat from syngas 32 is recovered with the aid of heat exchangers. The recovered heat 35 is transferred to a sanitizer 36.

The liquid digestate fraction 24 is collected and some of it is used for humidification of the digester feedstock, meaning that some of the liquid digestate fraction is returned to the digester 10 for increasing the moisture content of the organic biomass 16 within the digester 10. This adjustment 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 mixing/agitation means. The remaining part of the liquid digestate fraction 24 is transferred to the sanitizer 36 and is heat-treated therein for a duration of at least 1 hour at at least 70°C. The two digestate fractions 22, 24 are thus both heat-treated, but independently from one another, in different units and for different durations. The treatment of the liquid digestate fraction 24 can thus occur simultaneously to the treatment of the solid digestate fraction 22. Thanks to the heat treatment in the sanitizer 36, the liquid digestate 24 is hygienized, meaning that any pathogens other harmful organisms are effectively killed or at least deactivated. The treated liquid digestate 24 is released from the sanitizer 36 and can be used, for example, as fertilizer.

If 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 fraction and/or the syngas.

Claims

Claims

1. Method for hygienization of a digestate in the production of a biogas from biomass, the method involving a) an anaerobic digestion step involving breakdown of organic biomass (16) through microorganisms in the absence of oxygen at a temperature below 50°C within an anaerobic digester (10) to produce methane-containing biogas (12) and a digestate (14) ; and b) a thermal treatment step involving treatment of at least part of the digestate (14) at a temperature above 175°C for at least one minute, wherein the thermal treatment step involves one treatment selected from the group consisting of pyrolysis, torref action, gasification, hydrothermal gasification, hydrothermal carbonization and hydrothermal liquefaction.

2. Method according to any of the preceding claims, wherein 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.

3. Method according to any of the preceding claims, wherein prior to step b) , the digestate (14) is subjected to a solid-liquid phase separation to obtain a solid digestate fraction (22) and a liquid digestate fraction (24) , whereupon at least part of the solid digestate fraction (22) is subjected to the thermal treatment in step b) .

4. Method according to Claim 3, wherein heat (35) liberated during the heat treatment of the solid digestate fraction (22) is recovered and used for thermally treating at least part of the liquid digestate fraction (24) .

5. Method according to any of the preceding claims, wherein the residence time of the organic biomass (16) within the anaerobic digester (10) is less than 15 days.

6. Method according to any of the preceding claims, wherein the thermal treatment 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 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 .

8. Method according to any of Claims 1 to 6, wherein 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.

9. Method according to any of Claims 1 to 6, wherein the thermal treatment step 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 thermal treatment step is carried out at ambient pressure .

11. Method according to any of the preceding claims, wherein the pH within the digester (10) is kept below 12.

12. System for the hygienization of a digestate (14) in the production of a biogas (12) , the system including an anaerobic digester (10) configured to produce methane- containing biogas and a digestate through anaerobic digestion of organic biomass through microorganisms in the absence of oxygen, the anaerobic digester (10) comprising an inlet for receiving organic biomass (16) , a first outlet for discharging biogas (12) a second outlet for discharging digestate (14) , and a temperature controller to keep the temperature within the digester (10) below 50°C; means for supplying at least part of the digestate (14) from the digester (10) to a heat treatment reactor (30) , the heat treatment reactor (30) being configured to heat at least part of the digestate (14) to a temperature of at least 175°C, and being selected from the group consisting of a pyrolysis reactor, a gasification reactor, a hydrothermal gasification reactor, a torrefaction reactor, a hydrothermal carbonization reactor, and a hydrothermal liquefaction reactor.

13. System according to Claim 12, wherein the digester (10) is a dry or semi-dry digester operated in plug-flow mode.

14. System according to Claim 12 or 13, wherein the system includes a dewatering device (20) , preferably a screw press, for separating the digestate (14) into a solid digestate fraction (22) and a liquid digestate fraction (24) , the system further including means for supplying digestate (14) from the digester to the dewatering device (20) , and means for supplying at least part of the solid digestate fraction (22) to the heat treatment reactor (30) .

15. System according to Claim 14, further comprising a sanitizer (36) ; heat transfer means for transferring heat energy (35) liberated from the heat treatment reactor (30) to the sanitizer (36) ; and means for supplying at least part of the liquid digestate fraction (24) to the sanitizer (36) .