Method for determining a biochemical methane potential of a substrate and a quantity of an at least one VOC in a biogas produced from said substrate

Abstract

The invention relates to a method (100) for determining a biochemical methane potential of a substrate together with a quantity of at least one volatile organic compound (VOC) in a biogas produced from said substrate, the biogas being generated during anaerobic digestion, said method (100) comprising: - a step of determining the biochemical methane potential (130) of the substrate, - a step of sampling (140) the generated biogas in a container dedicated to gas headspace analysis, said sampling occurring during the step of determining the biochemical methane potential (130); and - a step of determining the quantity of at least one volatile organic compound (VOC) (150) in the generated biogas, said step comprising injecting a sample from the container into an analytical system through a temperature-controlled injection setup.

Description

[0001] METHOD FOR DETERMINING A BIOCHEMICAL METHANE POTENTIAL OF A

[0002] SUBSTRATE AND A QUANTITY OF AN AT LEAST ONE VOC IN A BIOGAS PRODUCED FROM SAID SUBSTRATE

[0003] Field of the invention

[0004] [1] The present invention relates to the field of biogas. In particular, the invention relates to the field of analysis of volatile organic compound (VOCs) in a biogas.

[0005] [2] This invention provides a new method for determining a biochemical methane potential of a substrate and a quantity of at least one volatile organic compound in a biogas produced from said substrate.

[0006] Description of Related Art

[0007] [3] Current energy requirements necessitate the adoption of alternative sources that are both sustainable and capable of reducing reliance on non-renewable resources. Renewable energy sources are critical in addressing environmental impacts associated with conventional energy production. Biomethane, derived from biogas, represents a potential renewable energy source due to its capability for sustainable production and lower environmental footprint.

[0008] [4] Biogas is generally produced via anaerobic digestion of organic materials, including agricultural waste, manure, municipal waste, and plant material. It primarily consists of methane and carbon dioxide. The conversion of waste into energy through biogas not only reduces waste but also decreases methane emissions from organic decomposition. This process thus contributes to reducing the overall greenhouse gas emissions.

[0009] [5] The broader application of biogas technology is hindered by significant challenges, especially in the purification of biogas into biomethane. This stage is crucial to remove contaminants such as volatile organic compounds (VOCs), which vary significantly in concentration depending on the digestion process and input materials. VOCs are a group of heterogeneous molecules and are usually considered pollutants. VOCs can be generated in different ways and in particular during the degradation of organic matter during the generation of biogas.

[0010] [6] According to studies by Gomez et al. (2016) and Rasi et al. (2007), VOC concentrations can range between 35 to 1731 mg.Nm3and 5 to 268 mg.Nm3of biogas, respectively. The variability in VOC content is influenced by factors such as the nature of the methanization unit (e.g., on-farm, territorial, industrial methanization, or WWTP sludge) and the substrates utilized. Notably, the use of substrates that contain fats and oils can lead to the presence of specific VOCs such as toluene and benzene, compounds commonly found in industrial applications as solvents and synthesis intermediates. Manufacturers in the sector are therefore obliged to carry out biogas treatments to remove the VOCs. Hence, the biogas treatment units require specific sizing depending on the quantity of biogas produced and the quantity of pollutants present.

[0011] [7] There is therefore a need for a solution allowing on the one hand to quantify the biogas produced by a substrate and its energy potential and on the other hand to be able to identify and quantify the VOCs produced by said substrate. Obviously, such a solution should be part of the energy and ecological transition.

[0012] Summary of the invention

[0013] [8] The following sets forth a simplified summary of selected aspects, embodiments and examples of the present invention for the purpose of providing a basic understanding of the invention. However, this summary does not constitute an extensive overview of all the aspects, embodiments and examples of the invention. Its sole purpose is to present selected aspects, embodiments and examples of the invention in a concise form as an introduction to the more detailed description of the aspects, embodiments and examples of the invention that follow the summary.

[0014] [9] The invention aims to overcome the disadvantages of the prior art. In particular, the invention proposes a method for determining a biochemical methane potential of a substrate together with a quantity of at least one volatile organic compound (VOC) in a biogas produced from said substrate, the biogas being generated during anaerobic digestion, said method comprising:

[0015] - a step of determining the biochemical methane potential of the substrate,

[0016] - a step of sampling the generated biogas in a container dedicated to gas headspace analysis, said sampling occurring during the step of determining the biochemical methane potential; and

[0017] - a step of determining the quantity of at least one volatile organic compound (VOC) in the generated biogas, said step comprising injecting a sample from the container into an analytical system through a temperature-controlled injection setup.

[0018]

[0010] The applicant has developed a method capable of estimating in a combined manner the quantity of biogas and VOCs produced from the fermentation of a substrate or a mixture of substrates. This can be used to better sizing the treatment units or to optimize substrates uses to limit the quantity of VOCs generated. In particular, it relates to a quantification of the biochemical methane potential of a substrate, along with a simultaneous quantification of the production of volatile organic compounds (VOCs) by this substrate. In particular, this can also be formulated as a new method combining an analysis of the biochemical methane potential of a substrate and an analysis of the quantity of at least one volatile organic compound (VOCs) in the biogas generated from this substrate.

[0019]

[0011] Moreover, as it is illustrated in example, the applicant has been able to identify the analytical conditions improving the reliability of the method. In particular, the use of a temperature-controlled injection setup improves the method.

[0020]

[0012] The invention makes it possible to screen many substrates and to simultaneously estimate the quantity of VOCs and the production of biomethane for different types of substrates.

[0021]

[0013] The invention can thus be used for improving the management of an industrial plant for biomethane production. For example, the invention can be used to select the accepted substrates and to adapt the purification capacity of VOCs in this industrial digester to the selected substrates. Hence, a digester operator will be able to identify and refuses substrates that generated VOC emissions that are incompatible with the purification capacities of the installation.

[0022]

[0014] According to other optional features of the process according to the invention, it can optionally include one or more of the following characteristics alone or in combination:

[0023] - the step of determining the biochemical methane potential of the substrate includes pressure measurement or volumetric measurement. Such methods are rapid and combine effectively with the step of determining the quantity of at least one volatile organic compound (VOC) in the generated biogas according to the invention.

[0024] - the step of determining the biochemical methane potential of the substrate includes a quantification of the biogas produced over time and quantification of at least CH4and CO2 in the biogas. Such a step allows for better characterization of the potential of a substrate in relation to its VOC production.

[0025] - the at least one VOC comprises 2-butanone, p-cymene, toluene, m,p-xylene, n- propylbenzene, 2-butanol, methyl ethyl ketone, tetrahydrofuran, 1 ,4-dioxane or R- limonene or combination thereof. These VOCs are important to monitor when determining the potential of a substrate in relation to its VOC production.

[0026] - the substrate is selected from agricultural waste, industrial organic waste, municipal organic waste, and / or other organic waste; and combination thereof. Indeed, the present method can be used on a wide variety of substrates.

[0027] - the biochemical methane potential of the substrate is determined using an inoculum from a digester, such as an industrial digester, operating in mesophile or thermophile conditions. Indeed, this method can directly use an inoculum from a digester for a result adapted to a given installation.

[0028] - the step of sampling the generated biogas is done using a glass gastight syringe. As described later, this allows for better measurement of the VOCs generated.

[0029] - the container dedicated to gas headspace analysis is a silanized container equipped with a tight-fitting septum. As described later, this allows for better measurement of the VOCs generated.

[0030] - the step of determining the quantity of at least one volatile organic compound (VOC) in the generated biogas uses a gas chromatography coupled to mass spectrometry. The method can be implemented with robust analysis methods such as GC-MS.

[0031] - it is conducted on several substrates. Indeed, the method can be repeated for several substrates successively or even be carried out on a mixture of substrates.

[0032] - it further comprises a step of computing for the substrate, a methane production to VOC generation index.

[0033] - it further comprises a step of selection within a plurality of substrates the one or those having a generation of VOCs reduced to the generation of biomethane lower than a predetermined threshold. Such a step allows to quickly compare the potential of different substrates or mixtures of substrates.

[0034]

[0015] In another aspect, the invention relates to a method for producing biomethane, comprising: measuring a C, N content of a substrate;

[0035] - selecting the substrate for biomethane production based on the measured C, N content of the substrate, when C / N ratio of the substrate is from 15 to 30; and

[0036] - fermenting the selected substrate in an anaerobic digester alone or with other substrates.

[0037]

[0016] In another aspect, the invention relates to a method for producing biomethane, comprising:

[0038] - measuring a H, N content of a substrate;

[0039] - selecting the substrate for biomethane production based on the measured H, N content of the substrate, when H / N ratio of the substrate is higher than 3; and

[0040] - fermenting the selected substrate in an anaerobic digester alone or with other substrates.

[0041]

[0017] Such a method can be used to improve the ratio of biogas produced on VOCs generated. Moreover, it can be used in some embodiments to adapt the substrates to the purification capacity of the system comprising the anaerobic digester.

[0042]

[0018] According to other optional features of the method according to the invention, it can optionally include one or more of the following characteristics alone or in combination:

[0043] - the selecting step comprises selecting the substrate based on the purification capacity of the anaerobic digester in which the fermentation is conducted. Indeed, depending on the purification equipment available, certain installations will be more suited to the use of highly energetic substrates but which generate substantial amounts of VOCs.

[0044] - the measuring step comprises measuring a C, H, N, content of a substrate and the selecting step comprises selecting the substrate when the H / N ratio of the substrate is higher than 3 and the C / N ratio of the substrate is higher than 20. As presented in the examples, this allows us to have the best ratio of VOC production to biogas production.

[0045] Brief description of the drawings

[0046]

[0019] The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

[0047]

[0020] Figure 1 illustrates a method for determining the biochemical methane potential of a substrate together with a quantity of at least one volatile organic compound (VOC) in a biogas produced from said substrate according to the invention.

[0048]

[0021] Figure 2 illustrates a step of preparing an inoculum according to the invention.

[0049]

[0022] Figure 3 illustrates a step of preparing a substrate according to the invention.

[0050]

[0023] Figure 4 illustrates a step of determining the quantity of at least one volatile organic compound (VOC) in the generated biogas according to the invention.

[0051]

[0024] Figure 5 illustrates a method for producing biomethane according to the invention.

[0052]

[0025] Several aspects of the present invention are disclosed with reference to flow diagrams and / or block diagrams of process, devices and systems.

[0053]

[0026] On the figures, when present, the flow diagrams and / or block diagrams show the architecture, the functionality and possible implementation of devices or systems or processes, according to several embodiments of the invention.

[0054]

[0027] In some implementations, the functions associated with the box may appear in a different order than indicated in the drawings. For example, two boxes successively shown, may be performed substantially simultaneously, or boxes may sometimes be performed in the reverse order, depending on the functionality involved.

[0055] Detailed description

[0028] A description of example embodiments of the invention follows.

[0056]

[0029] In the following description “biogas” can refer to a gas mixture that is naturally produced from the decomposition of organic waste including but not limited to agricultural, industrial, and domestic waste. In an oxygen-free environment (anaerobic), these materials break down, emitting a gas predominantly composed of methane (CH4) and carbon dioxide (CO2), typically in the range of 50-70% and 30-50% respectively, along with trace amounts of other gases like hydrogen sulfide (H2S).

[0057]

[0030] As used herein, the term “biomethane” can refer to methane produced during the decomposition of organic matter process, for example during an anaerobic fermentation.

[0058]

[0031] As used herein, the terms “fermentation” or “digestion” are used interchangeably and can refer to the biochemical processes through that organic matter is broken down by microorganisms, often resulting in the production of various compounds such as gases, acids, or alcohols.

[0059]

[0032] As used herein, the term "together" in the context of this invention explicitly denotes the concurrent execution of both actions for BMP assessment and actions for VOCs quantification within the same experimental framework. While it is acknowledged that these analyses may not occur simultaneously in real-time, they are preferably conducted within the confines of a single experimental setup to ensure data comparability and relevance. This enhances the efficiency and efficacy of substrate characterization for anaerobic digestion applications according to the invention.

[0060]

[0033] As used herein, the terms “digester” and “biogas reactor” can refer, within the meaning of the invention to a system intended for production of biogas.

[0061]

[0034] As used herein, the term “substrate” can refer to the organic materials that are processed, preferably in an anaerobic digester to produce biogas and digestate, the substrate can be solid and / or liquid.

[0062]

[0035] As used herein, the term “inoculum” can refer to a substance containing microorganisms, for example Bacteria or Archaea, preferably methanogens. For example, in the context of the present invention, an inoculum can be added to an anaerobic digester to initiate or enhance the breakdown of organic substrate materials for efficient biogas production.

[0063]

[0036] As used herein, the expression “volatile organic compounds” or “VOCs” can refer, within the meaning of the invention to pollutants of biogas, preferably a group of heterogeneous molecules can be generated in different ways and in particular during the degradation of organic matter during the generation of biogas. The VOCs present in biogas are most of the time linked to specific substrates.

[0064]

[0037] As used herein, “BMP” can refer to the Biochemical Methane Potential, which quantifies the maximum amount of methane that can be produced from the anaerobic digestion of organic substrates under controlled laboratory conditions. The BMP is quantified in normal cubic meter of methane (abbreviated as Nm3) per unit of matter. The matter can be raw matter, total solids and / or volatile solids.

[0065]

[0038] As used herein, the expressions, “experimental BMP” or “BMPeXp” can refer to the biochemical methane potential determined by a biochemical methane potential test so-called BMP test which will be detailed later.

[0066]

[0039] As used herein, the expressions “theorical BMP” or “BMPth” can refer to the estimated biochemical methane potential, calculated based on the properties of a given substrate such as chemical composition.

[0067]

[0040] As used herein, the expression “Bl” refers to the biodegradability index, which is the capacity of a substrate to be degraded and corresponds to the BMPeXp / BMPth ratio.

[0068]

[0041] As used herein, the expression “gas headspace” can refer, within the meaning of the invention, to the volume occupied by a gas in a container, for example a digester or a bioreactor, said gas is preferably a biogas.

[0069]

[0042] As used herein, “container” can refer, within the meaning of the invention, to any receptacle or vessel designed to hold, store, and maintain the integrity of a sample, substance, or material. A container according to the invention will preferably ensure the containment and preservation of the sample's physical and chemical properties, preventing contamination or loss of volatiles. Containers can be made of various materials, including glass, plastic, or metal. Preferably, the containers used in the present invention are glass containers. The containers used in the present invention can be selected from: absorbent cartridges, vials such as headspace vials, bottles, or gas sampling bags. Preferably, the containers used in the present invention are selected from: absorbent cartridges, or vials such as headspace vials.

[0070]

[0043] As mentioned, the biogas can be a renewable energy source due to its capability for sustainable production and lower environmental footprint. However, some substrates can induce a high production of VOCs that need to be removed before use of the biomethane. The inventors developed a solution allowing on the one hand to quantify the biogas produced by a substrate and its energy potential and on the other hand to be able to identify and quantify the VOCs produced by said substrate.

[0071]

[0044] According to a first aspect, the invention relates to a method 100 for determining the biochemical methane potential of a substrate together with a quantity of at least one volatile organic compound (VOC) in a biogas produced from said substrate.

[0072]

[0045] The biogas is a gas from fermentation such as anaerobic fermentation or aerobic fermentation. The biogas is preferably generated from at least one substrate during anaerobic fermentation. The biogas can comprise methane, carbon dioxide, sulfur, hydrogen, nitrogen, and / or oxygen. Moreover, the biogas will generally comprise volatile organic compounds in trace concentrations.

[0073]

[0046] As mentioned, the invention relates to combined analyses of a biochemical methane potential of a substrate and a quantity of at least one volatile organic compound (VOC) in a biogas produced from said substrate. In the specific context of the invention and, in a preferred embodiment, "together" implies that the biogas for VOCs analysis is sampled during the biochemical methane potential determination. For example, the gas headspace container is filed during the biochemical methane potential determination.

[0074]

[0047] As shown in figure 1 , a method 100 according to the invention comprises: a step of determining the biochemical methane potential 130 of the substrate, a step of sampling 140 the generated biogas and a step of determining a quantity of at least one VOC 150 in the biogas.

[0075]

[0048] A method 100 according to the invention can also comprise: a step of preparation of the inoculum 110; a step of preparation of the substrate(s) 120; a step of computing 160 for a substrate, a methane production index on its VOC generation; and / or a step of selection 170 within a plurality of substrates the one or those having a generation of VOCs reduced to the generation of biomethane lower than a predetermined threshold.

[0076]

[0049] As illustrated in figure 1 , a method 100 according to the invention can comprise a step of preparation of the inoculum 110. This step can be used to determine the composition of said inoculum, determine its capacity to produce biogas on its own and finally put it in the best conditions for a BMP test.

[0077]

[0050] The invention relates to the concomitant determination of a BMP of a substrate and its VOCs production. In a preferred embodiment, this determination can be specific to a given microbial community. Hence, the invention can advantageously be used to determine the potential of substrate for a given microbial community, thus a given inoculum.

[0078]

[0051] As illustrated in figure 2, the step of preparation of an inoculum can comprise the following steps: a step of recovering 111 an inoculum for example from a biogas plant, a step of acclimatizing 112 the inoculum and / or a step of characterizing the inoculum 113.

[0079]

[0052] An inoculum according to the invention is a microbial consortium able to conduct a fermentation (or a digestion) process from a substrate. More preferably, the inoculum according to the invention can produce at least one gas.

[0080]

[0053] The inoculum can comprise at least one microorganism or a mixture of microorganisms. In particular, the at least one microorganism can be selected from bacteria, archaea, yeasts, virus, or fungi. Generally, the inoculum comprises a mesophilic or thermophilic microbial consortium.

[0081]

[0054] When recovering 111 an inoculum of interest, the inoculum can come from a biogas production site, and it can be recovered from a digester (or an anaerobic digester). The inoculum can be recovered from a mesophile or thermophile system, operating with different substrates.

[0082]

[0055] The step of preparation of the inoculum 110 can comprise a step of acclimatizing

[0083] 112 the inoculum. This step 112 allows the inoculum to be stored and to be stabilized in order to be tested.

[0084]

[0056] The acclimatization can be done aerobically or anaerobically based on the inoculum. Preferably, the acclimatization is anaerobic.

[0085]

[0057] The step of acclimatizing 112 the inoculum can be carried out at a temperature ranging from 35°C to 55°C. For example, if the inoculum is selected from a mesophile strain, the step of acclimatizing 112 can be carried out at a temperature ranging from 35°C to 42°C. If the inoculum is selected from a thermophile strain, the step of acclimatizing 112 can be carried out at a temperature ranging from 50°C to 55°C.

[0086]

[0058] The step of preparation of the inoculum 110 can comprise a step of characterizing

[0087] 113 the inoculum. The step of characterizing 113 the inoculum can include measurement of inoculum's composition, including pH, redox potential, and nutrient content. This characterization aids in predicting biogas yield, optimizing digestion efficiency, and ensuring substrate compatibility. It also provides a baseline for repeatable experiments, allowing for consistent results and comparability.

[0088]

[0059] Several parameters of the inoculum can be measured. For example, the density of the inoculum can be measured in grams per millilitre (g / mL). This parameter provides insight into the overall mass of the inoculum relative to its volume, which is important for maintaining consistency in anaerobic digestion experiments. The Total Solid (TS) content of the inoculum is measured in grams per 100 grams of wet mass (g / 100g wet mass). This represents the solid portion of the inoculum, providing a baseline for understanding its composition and potential contribution to the digestion process. The volatile solids content of the inoculum is measured in grams per 100 grams of total solids (g / 100g TS). This indicates the organic portion of the total solids, which is important for assessing the microbial activity and the potential yield of biogas in the digestion process. The redox potential of the inoculum is measured in millivolts (mV), providing insight into its oxidation-reduction state. This parameter reflects the electron-accepting or donating capacity of the inoculum, which influences the microbial community and digestion efficiency. The pH, buffer capacity, and ammonium concentration of the inoculum can also be measured.

[0089]

[0060] As illustrated in figure 1 , a method 100 according to the invention can comprise a step of preparing 120 the substrate(s). This step can be used to ensure accurate characterization of its mechanical and chemical properties. This can be useful to facilitate accurate estimation of its Biochemical Methane Potential both theoretical and experimental.

[0061] A substrate for fermentation can be selected from any material producing biogas when fermented by an inoculum, with a preference for materials that can undergo anaerobic fermentation. In particular, the substrate to be fermented according to the invention can produce a large quantity of biomethane and a minimal quantity of pollutants such as VOCs.

[0090]

[0062] Preferably, the substrate according to the invention can be selected from wastes or by-products such as agricultural wastes or by-products, industrial organic wastes or byproducts, municipal organic wastes, and / or other organic wastes or by-products. The substrate can be selected from recycling channels such as industrial byproducts and / or household wastes, in order to minimize the amount of material sent to landfills and reduce greenhouse gas emissions associated with waste decomposition.

[0091]

[0063] For example, the substrate can be selected from: agricultural waste: animal manure (e.g. manure from cows, pigs, and poultry); crop residues (e.g. straw, corn stalks, vegetable peels, husk, bark, ears, cob, and inflorescence); livestock feed and its by-products (e.g. fodder, hay,); silage (e.g. fermented crops like maize or grass); wastes from agricultural storage, such as rotten fruits and vegetables;

[0092] - industrial organic waste: waste from food and beverage production (e.g. waste from bakery, pastry, confectionery, dairy, brewery, distillery, and other food processing sectors); agrifood industry wastes (e.g. by-products from cereal processing, sugar refinery, fruit and vegetable processing, aromatic processing, dried fruit processing, cacao, tea, coffee, and oilseed); aquaculture sector wastes (e.g. wastes from fish processing, including wastewater and by-products), meat sector wastes, milkmaid sector wastes (e.g. dairy industry waste, including spoiled dairy products); meat sector wastes (e.g. slaughter waste, pieces of butcher carcasses unfit for consumption); lipid-rich waste (e.g. waste oils from restaurants or food processing industries, by-products from various food processing industries); municipal organic waste: fermentable fraction of household waste; green wastes (e.g. organic waste from parks, gardens, and landscaping activities); sludge from wastewater treatment plants;

[0093] - other organic wastes: paper, cardboard, textile, compost, microalgae, macroalgae...

[0094]

[0064] As illustrated in figure 3, the step of preparation of a substrate 120 can comprise the following steps: a step of characterizing 121 the substrate; and / or a step of estimating 122 the theoretical Biochemical Methane Potential (BMPth) of the substrate.

[0095]

[0065] The characterization 121 of the substrate preferably involves determining various parameters for assessing its suitability for anaerobic fermentation. For example, the characterization can include measuring or calculating one or several of the following parameters: Raw Matter (RM): The total mass of the substrate, including both organic and inorganic materials; Total Solids (TS): The weight of the substrate after removing all water content through a heat treatment at 105°C for 24 hours; Volatile Solids (VS): The organic portion of the Total Solids, determined by further heating at 550°C for 4 hours; COD: The Chemical Oxygen Demand, a measure of the oxidizable organic matter, which indicates the potential for biogas production (for liquid substrates); Elemental Composition: Determining the percentage composition of Carbon, Hydrogen, Nitrogen, Oxygen, and Sulfur (%CHNOS), which provides insight into the energy content and biodegradability of the substrate; and pH: The acidity or alkalinity of the substrate, which can affect the microbial activity in anaerobic digestion.

[0096]

[0066] These values can be used to estimate 122 the theoretical Biochemical Methane Potential (BMPth) of the substrate. In particular, the estimation of the theoretical Biochemical Methane Potential (BMPth) of the substrate can comprise calculating the potential methane yield using the Buswell formula, which combines the elemental composition of the substrate to predict the volume of methane that could be generated. This equation reflects the stoichiometric conversion of organic material into methane (CH4) and carbon dioxide (CO2) based on its carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) content. The theoretical methane potential is usually calculated by assuming complete conversion of the biodegradable organic matter into these gases.

[0097]

[0067] Further refinement of the BMPth estimation can involve corrections based on the volatile solids (VS) content.

[0098]

[0068] In addition, the pH and COD values are integrated into the model to adjust for environmental conditions.

[0099]

[0069] Moreover, the preparation of the substrate can include several other steps such as: sanitizing the substrate to eliminate unwanted microorganisms or pollutants, this can be done by heating at temperatures ranging from 70°C to 90°C, or through chemical treatments, depending on the nature of the substrate; or freezing the substrate to preserve the substrate and its characteristics by slowing down microbial and enzymatic activity, preventing degradation before analysis or experimentation.

[0100]

[0070] As illustrated in figure 1 , a method 100 according to the invention can comprise a step of determining the biochemical methane potential 130 of the substrate. This step 130 can be used to measure the maximum methane yield from the substrate(s) through anaerobic digestion. This step 130 provides comparable data on the substrate's energy potential, enabling comparison between substrates.

[0101]

[0071] This step 130 as well as the whole method 100 according to the invention can be implemented with one specific substrate to characterize the VOCs production of one specific substrate. However, the present invention also encompasses its implementation with several substrates to characterize the VOCs production of a mix of substrates. Hence, when the invention describes a step of determining the biochemical methane potential 130 of the substrate, it can encompass determining the biochemical methane potential 130 of a mix of substrates.

[0102]

[0072] As mentioned previously, the substrates can be derived from various organic materials, such as agricultural waste, food waste, or sewage sludge. The inoculum, sourced from previously fermented material, can be selected based on its compatibility with the substrate, ensuring efficient fermentation.

[0103] The step of determining the biochemical methane potential 130 of the substrate is preferably carried out following the instruction detailed in “Calculation of Biochemical Methane Potential (BMP)” from Standard BMP Methods Document 200, Version 1 .6 by Hafner, S.D., Astals, S., Holliger, C., Koch, K., Weinrich, S. It can also be determined according to the standard DBFZ, 2024. Standard BMP Methods, https: / / www.dbfz.de / en / projects / bmp (accessed 21 July 2024).

[0104]

[0073] In particular, the method 100 can comprise a step of mixing the inoculum and substrate for fermentation. The mixture ratio of substrate to inoculum can range from 1 :1 to 10:1 , depending on the substrate type. For example, agricultural waste may require a higher inoculum proportion to accelerate decomposition, while food waste may ferment more efficiently with a balanced ratio. Preferably, the substrate / inoculum ratio is calculated from a calculated ratio of the organic matter content in the substrate to the organic matter content in the inoculum. Typically, the substrate / inoculum ratio is ranging from 0.25 to 0.5. This ratio ensures a balanced microbial activity during fermentation.

[0105]

[0074] In a preferred embodiment, sealed containers are incubated at a controlled temperature for a set duration, often ranging from a few days to several weeks. This allows sufficient time for the microbial community to break down the organic matter. More preferably, the step of determining BMP 130 involves an anaerobic fermentation process, preferably performed in a sealed container to maintain low oxygen levels. The temperature can range from 15°C to 60°C, depending on the inoculum's optimal range. The process is carried out under a pressure of 1 bar to 1 .1 bar for stable anaerobic digestion. In certain embodiments, a thermophilic inoculum may be used, operating at higher temperatures around 55°C to 60°C. Alternatively, a mesophilic inoculum can be employed, functioning at temperatures ranging from 15°C to 45°C.

[0106]

[0075] In a preferred embodiment, during the incubation period, the gas produced is collected and analysed periodically to measure the volume and composition, particularly the methane content. The total amount of methane produced is indicative of the substrate's biochemical methane potential. Hence, more preferably, the step of determining BMP 130 preferably comprise a quantification of the biogas production over time. This quantification can encompass a volume quantification and a composition analysis. The volume quantification can comprise measuring the cumulative volume of biogas produced as a function of time, expressed in terms of cubic meters per kilogram of total solids (m3 / kg TS), cubic meters per kilogram of volatile solids (m3 / kg VS), or per unit of substrate weight. The composition analysis can encompass identifying the biogas composition using chromatographic analysis, preferably gas chromatography, or a portable infrared detector.

[0107]

[0076] As illustrated in figure 1 , a method 100 according to the invention can comprise a step of sampling 140 the generated biogas. This step 140 can be used to obtain biogas for VOCs analysis. This is particularly relevant to understand the relationship between VOCs content and biogas production, with a focus on methane production in regard to VOCs production. This relation is particularly influenced by the substrate(s) which is / are used during fermentation.

[0108]

[0077] In a preferred embodiment, the biogas sampling is conducted together with sampling for the BMP (Biomethane Potential) test. This can be done for example at the same timing. Sampling is preferably done at discrete intervals, although continuous sampling is also possible. Typically, the biogas samples are collected 2 to 3 times per week, with each sampling event conducted in duplicate.

[0109]

[0078] The step of sampling 140 the generated biogas, as part of the method 100, involves collecting gas samples for VOCs analysis.

[0110]

[0079] The method 100 can comprise the use of sampling tool(s). The sampling tool(s) can be selected among gas-tight syringe; or inert gas-tight bags.

[0111]

[0080] A gas-tight syringe, often made of glass or specialized plastic, can be used for direct sampling from the headspace of the biogas reactor. The syringe's tight seal prevents gas leakage, ensuring sample integrity. This system prevents air contamination and preserves the biogas composition. Alternatively, inert gas-tight bags, made from materials such as Teflon® or aluminized Mylar®, are suitable for collecting larger gas samples directly from the reactor. They maintain sample integrity over longer periods and can be easily transported for analysis. Vacuum-driven systems can also be employed. These systems use a vacuum pump to draw biogas into a sealed chamber or container, minimizing gas loss and ensuring accurate sampling.

[0112]

[0081] As mentioned in the example, the step of sampling 140 the generated biogas is advantageously conducted using a glass gas-tight syringe. Using a glass gas-tight syringe provides the most accurate and repeatable results. During the step of sampling 140 a sufficient sample volume to allow for comprehensive analysis is collected, typically between 5 mL to 50 mL, depending on the analytical method used. The step of sampling 140 can be done at ambient T°C and under pressure conditions, preferably more than 1 bar.

[0113]

[0082] Once the biogas is sampled, it is transferred to a glass container, preferably a silanized glass container, more preferably a crimped silanized glass container. For example, a vial equipped with a septum, preferably a 3mm thick butyl and PTFE type septum.

[0114]

[0083] The step of transferring is preferably carried out for transferring a gas space into a container at a pressure between 1 and 2 bar.

[0115]

[0084] In particular, the sampling is conducted in a container dedicated to gas headspace analysis. Hence, the sampling is conducted in a container dedicated to gas headspace analysis, such as a vial, preferably a glass vial. As it will be mentioned in example, the use of silanized glass vial is preferred.

[0116]

[0085] After collection, the containers containing the sample should be handled with care to prevent gas loss or contamination. The containers can be stored at a stable temperature, ideally between 15°C and 25°C, to prevent gas expansion or contraction that could lead to leakage. The collected samples can be stored for typically up to 48 hours, before analysis. However, if possible, the samples are analysed immediately after collection to preserve their integrity.

[0117]

[0086] A method 100 according to the invention can be used to compare the methane production in regard to VOCs production for several substrates. Hence, preferably the sampling 140 step comprises at least one collection of the biogas produced by each of the tested substrates during a BMP test.

[0118]

[0087] The sampling can also be conducted with an absorbent cartridge, thus replacing vial sampling. Absorbent cartridges allow for the direct capture of VOCs from the biogas, which can then be analysed using thermal desorption. Cartridges are portable, easy to handle, and do not require the careful manipulation needed for glass containers, reducing the risk of sample loss or contamination.

[0119]

[0088] As illustrated in figure 1 , a method 100 according to the invention can comprise a step of determining the quantity of at least one volatile organic compound 150 in the generated biogas. This step 150 is essential to the invention given that it makes it possible to correlate the presence of VOCs in the biogas produced with biochemical methane potential of the substrate for a given substrate. In particular this step 150 , allows for the quantitative and qualitative analysis of VOCs in biogas.

[0120]

[0089] In a biomethane production context, the Volatile organic compounds (VOCs) are generally a group of chemical compounds that have high vapor pressures under normal conditions, which means they can easily become vapours or gases. The diversity of VOCs in biomethane primarily arises from the varied feedstocks used in the anaerobic digestion process. These substrates contain a complex mixture of organic materials, leading to the production of different VOCs during digestion, common categories of VOCs found in biomethane includes alkanes, alkenes, aromatics, alcohols, ketones, aldehydes, esters, ethers, halogenated compounds, amines, and / or terpenes. In particular, VOCs analysed in the context of the present invention can comprise compounds selected from: alkanes: methane, propane, and butane; alkenes: ethylene, propylene, butadiene, and isoprene;

[0121] - aromatics: benzene, toluene, styrene, p-cymene, m,p-xylene, and n-propylbenzene;

[0122] - alcohols: methanol, ethanol, isopropanol, and 2-butanol; ketones: acetone, methyl ethyl ketone, 2-butanone, and cyclohexanone; aldehydes: formaldehyde; acetaldehyde, and benzaldehyde; esters: ethyl acetate, methyl methacrylate, and isopentyl acetate; ethers: dimethyl ether, diethyl ether, tetrahydrofuran, and 1 ,4-dioxane; halogenated compounds: chloroform, trichloroethylene, and methyl chloride; amines: methylamine, ethylamine, and aniline; and

[0123] - terpenes: limonene, r-limonene, alpha-pinene and beta-pinene.

[0124]

[0090] As illustrated in the figure 4, the step of determining the quantity of at least one volatile organic compound 150 in the generated biogas can comprise steps for calibration 151 , sample preparation 152, injection 153 of a sample, compound separation 154, and VOC identification 155.

[0125]

[0091] The calibration 151 of the analytical instruments can be done using gas standards. These standards are prepared by diluting a certified standard gas (containing VOCs in a methane matrix) with methane.

[0126]

[0092] The step of determining the quantity of at least one volatile organic compound 150 in the generated biogas can comprise a step of sample preparation 152. The sample preparation 152 can comprise with a stabilization time of between 10 and 60 minutes at a temperature over 40°C, such as 50°C.

[0127]

[0093] The step of determining the quantity of at least one volatile organic compound 150 in the generated biogas can comprise an injection 153 of a sample from the container (such as a headspace vial or an absorbent cartridge) into an analytical system.

[0128]

[0094] This injection in preferably conducted through a gas injection setup such as a headspace or a desorption.

[0129]

[0095] The injection in advantageously conducted using a temperature-controlled injection setup. The temperature-controlled injection setup can for example be selected from a headspace oven or a desorption unit. The sample are heated at a temperature of at least 30°C before injection, preferably at least 40°C, more preferably at least 50°C; which improves the sensitivity and reproducibility of the results. In particular, for absorbent cartridge desorption, the desorption unit heat the cartridge to release the analytes. Similarly, for headspace vial injection, while the analytes are already volatilized into the headspace, the heating improve the results. Also, the injection step involves the use of carrier gas to transport the analytes from the sample introduction point into the GC column.

[0130]

[0096] The step of determining the quantity of at least one volatile organic compound 150 in the generated biogas can comprise a separation 154 of VOCs. The separation 154 of VOCs from the biogas sample can be preferably achieved through gas chromatography. This technique, based on the differences in the chemical properties of the compounds, allows for an effective separation before further analysis. The gas chromatography (GC) can be a conventional GC, Multidimensional GC or Fast GC.

[0131]

[0097] Each of these configurations can be coupled with different detectors depending on the specific requirements of the analysis, such as mass spectrometry (MS) for identification or flame ionization detectors (FID).

[0132]

[0098] The step of determining the quantity of at least one volatile organic compound 150 in the generated biogas can comprise a step of VOC identification / quantification 155. The VOC identification 155 and their quantification can be done using mass spectrometry coupled with gas chromatography (GC-MS). This method provides high precision and specificity, enabling the detection of various VOCs within the complex biogas matrix. Alternative detection systems such as Flame Ionization Detectors (FID) can also be employed, depending on the specific requirements of the analysis.

[0133]

[0099] Also, the VOC identification 155 and their quantification can be done without a preliminary step of separation 153 of VOCs. For example, the VOC can be identified and quantified by using direct injection mass spectrometers (DIMS). In particular, a method according to the invention can use proton-transfer-reaction mass spectrometry (PTR-MS) or selected ion-flow-tube mass spectrometry (SIFT-MS). Alternatively, the VOC can be identified and quantified by Ion mobility spectrometry (IMS).

[0134]

[0100] In a more preferred embodiment, the step of determining the quantity of at least one volatile organic compound 150 in the generated biogas comprise the quantification of VOCs using a mass spectrometry coupled to gas chromatography.

[0135]

[0101] When absorbent cartridges are used, the step of determining the quantity of at least one volatile organic compound 150 in the generated biogas can comprise a step of thermal desorption. This method is particularly advantageous due to its ability to concentrate VOCs from gaseous samples, allowing for more sensitive detection. The thermal desorption can involve heating a sample to release VOCs, which are then swept into a separation system (e.g. GC) or analytical system by an inert carrier gas.

[0136]

[0102] As illustrated in figure 1 , a method 100 according to the invention can comprise a step of computing 160 a VOC generation index on biogas production for a substrate. This step 160 is particularly advantageous as it can be used to discriminate the substrates, from a reproducible experiment, on their capacity to produce high quantity of biogas with low concentration of VOCs. In particular, it can be used to analyse the production of biogas in regard of particularly unwanted VOCs for a plurality of substrates.

[0137]

[0103] Hence, the step of computing 160 a VOC generation index on biogas production can generate a value expressed in quantity of VOC (the quantified ones) per volume of biogas (such as in mg per Nm3of biogas).

[0138]

[0104] The VOC generation index (VGI) can be computed using the following formula: VGI=Mass of VOCs generated (mg) I Volume of Biogas Produced (Nm3)

[0139]

[0105] The VOC generation index can be based on methane production and thus can be computed using the following formula: VGI =Mass of VOCs generated (mg) I

[0140] Volume of Methane Produced (Nm3):

[0141]

[0106] This index provides a normalized value that reflects the efficiency of biogas production, preferably methane production, relative to the generated VOCs.

[0142]

[0107] This step can be used to compute VGI values for multiple substrates under identical conditions. These values can then be compared to identify the most efficient substrates, i.e., those that produce the highest biogas or methane yield with the least quantity / mass of VOCs.

[0143]

[0108] As illustrated in figure 1 , a method 100 according to the invention can comprise a step of selecting 170, within a plurality of substrates, the one or those having a VOC generation index on biogas production lower than a predetermined threshold. This step 170 is particularly advantageous as it can be used to ensure the selection of substrates that are most efficient in biogas production, thereby facilitating the optimization of substrate selection for improved biogas yield and sustainability.

[0144]

[0109] The predetermined threshold for the VOC generation index can be based on historical data for a given digester, specific experimental results or purification capacities of a digester. This threshold can be considered as representing the minimum acceptable efficiency for a substrate to be considered efficient for biogas production.

[0145]

[0110] In the step of selecting 170 one or several substrates preferably comprise the comparison of their VOCs generation indexes against the predetermined threshold to identify substrates with an VGI lower than the threshold. These substrates are considered more efficient in terms of biogas production relative to the amount of VOCs generated.

[0146]

[0111] Reducing the use of inefficient substrates can decrease the generation of VOCs leading to lower purification costs and reduced environmental impact. Hence, this selection process contributes to a more streamlined and effective biogas production system.

[0147]

[0112] in another aspect, the invention relates to an improved method 200 for producing biomethane. Indeed, based on the results generated as part of the implementation of the method of determining a biochemical methane potential of a substrate together with a quantity of at least one volatile organic compound (VOC) in a biogas, the inventors propose an improved method for producing biomethane.

[0148]

[0113] As illustrated in the figure 5, the method for producing biomethane 200 comprises: measuring 210 the C, H, N content of a substrate; selecting the substrate 220 for biomethane production; and fermenting the selected substrate 230 in an anaerobic digester alone or with other substrates.

[0149]

[0114] As illustrated in figure 5, a method 200 according to the invention can comprise a step of measuring 210 measuring the carbon (C), hydrogen (H), and / or nitrogen (N) content of a substrate.

[0150]

[0115] This step 210 can bring information on the chemical composition of the substrate, which influences its suitability and efficiency for biogas production in a context of VOCs generation.

[0151]

[0116] As disclose in the present invention, the knowledge of the C, H, and N, content enables better control over the anaerobic digestion process, leading to improved methane production and reduced VOCs production. In particular, the knowledge of the N and H content is necessary to implement this method of the invention. Preferably, the knowledge of the C, N and H content can be used to implement preferred embodiments of this method of the invention.

[0152]

[0117] Measuring the elemental composition is generally a fast and low-cost method that provides reproducible data on chemical composition of a substrate.

[0153]

[0118] This step 210 can comprise preparation of the samples of the substrate including: drying or grinding. The elemental analysis can be done using elemental analyzers, commonly known as CHN analyzers or advanced elemental analyzers.

[0154]

[0119] As illustrated in figure 5, a method 200 according to the invention can comprise a step of selecting 220 the substrate for biomethane production based on the measured C, H, N content measured.

[0120] As illustrated in example, the selecting step 220 can comprise selecting a substrate when the H / N ratio of the substrate is higher than 3, preferably higher than 5, more preferably higher than 8 and even more preferably higher than 10.

[0155]

[0121] Also, as illustrated in example, the selecting step 220 can comprise selecting a substrate when the C / H ratio of the substrate is lower than 6, preferably lower than 5.5, and more preferably lower than 5.

[0156]

[0122] Also, as illustrated in example, the selecting step can comprise selecting a substrate when the C / N ratio of the substrate is higher than 20, preferably higher than 25, more preferably higher than 30, and even more preferably equal or higher than 35. It can for example be from 15 to 30.

[0157]

[0123] More preferably, the selecting step 220 can comprise selecting a substrate when the H / N ratio of the substrate is higher than 3 and the C / N ratio of the substrate is higher than 20, preferably the H / N ratio of the substrate is higher than 5 and the C / N ratio of the substrate is higher than 25, more preferably the H / N ratio of the substrate is higher than 8 and the C / N ratio of the substrate is higher than 30, and even more preferably the H / N ratio of the substrate is higher than 10 and the C / N ratio of the substrate is equal or higher than 35.

[0158]

[0124] Advantageously, the selecting step 220 can comprise selecting a substrate based on the purification capacity of the anaerobic digester in which the fermentation is conducted. Preferably, the selecting step 220 can comprise selecting a substrate based on the results of a method 100 according to the invention for determining a biochemical methane potential of a substrate together with a quantity of at least one volatile organic compound (VOC) in a biogas.

[0159]

[0125] Advantageously, the selecting step 220 can comprise selecting the substrate having a experimental BMP higher than 650 Nrr CH tMSV.

[0160]

[0126] As illustrated in figure 5, a method 200 according to the invention can comprise a step of fermenting 230 the selected substrate in an anaerobic digester alone or with other substrates. This fermentation, using selected substrate(s) should produce biogas with reduced VOCs production and in some embodiments with VOCs production adapted to the purification capacities of the anaerobic digesters.

[0161]

[0127] A method 200 of the invention can include pre-treatment steps for the selected substrate(s) such as thermal, chemical, or mechanical processes to enhance the biodegradability of these substrates.

[0162]

[0128] The selected substrate(s) are typically loaded into the anaerobic digester which is sealed and designed to maintain anaerobic conditions such as temperature (mesophilic: e.g. 30-40°C or thermophilic: e.g. 55-60°C), pH (neutral to slightly alkaline), and moisture content to support the microbial activity.

[0129] The fermentation step 230 can include monitoring of parameters such as temperature, pH, gas production rate, and VOCs concentration.

[0163]

[0130] As mentioned, the fermentation step 230 can be implemented on a mixture of several substrates to improve the overall biogas production compared to digesting a single substrate.

[0164] EXAMPLES

[0165]

[0131] The invention is further described in detail by reference to the following experimental examples. These examples are provided for purposes of illustration only and are not intended to be limiting unless otherwise specified. Thus, the invention should in no way be construed as being limited to the following illustrative examples, but rather, should be construed to encompass any and all variations which become evident as a result of the teaching provided herein. Without further description, it is believed that one of ordinary skills in the art can, using the preceding description and the following illustrative examples, practice the claimed processes. The following examples, therefore, specifically point out the preferred embodiments of the present invention and are not to be construed as limiting in any way the remainder of the disclosure.

[0166] Materials and methods

[0167]

[0132] Substrate analysis and preparation

[0168]

[0133] For each of the 7 substrates used in the study, the analysis of total solids (TS) and volatile solids (VS) were conducted.

[0169]

[0134] The total solids (TS)measurement was carried out at a temperature of 105°C [UN 55 universal oven, Memmert™] and that of the volatile solids (VS) at 550°C [L9 / L11 SKM muffle furnace, Nabertherm™] by gravimetry according to the standard method NF EN 15934.

[0170]

[0135] After drying at 38°C [Oven 100-800, Memmert™] and grinding to <10mm [Ultra centrifugal mill ZM 200, Retsch™], the elemental composition (% CHNSO) was measured.

[0171]

[0136] Inoculum analysis and preparation

[0172]

[0137] The inoculum was acclimatized in a 5L glass pilot reactor for 5 days at 38°C with continuous stirring at 70 rpm. The inoculum had the following features: density of the inoculum: 1.024 g / mL; TS content: 7.18 g / 100g raw matter (RM); VS content: 57.93 g / 100g total solids; pH: 7.8; Redox Potential: less than -300 mV; Buffer Capacity (TAG): 15.11 g CaCO3 / L; Ammonium Concentration: 2.46 g NH4 L.

[0173]

[0138] Theoretical BMP Calculation:

[0139] The theoretical BMP provides an estimate of the maximum amount of methane that can be produced from a given substrate. It can be calculated based on the elemental composition of the substrate, particularly its carbon, hydrogen, and oxygen content.

[0174]

[0140] The BMP (Biochemical methane potential) corresponds to the maximum specific methane production of a substrate, in units of normal cubic meter of CH4per unit of raw material (RM), TS or VS. The biodegradability index (Bl) is an indicator of how easily a substrate degrades and corresponds to the ratio between its experimental BMP (BMPexp) and its theoretical BMP (BMPth). The BMPthis calculated from the properties of the substrate and the BMPexp is determined using a biochemical methane potential test. The BMPth of solid substrates was calculated from the elemental composition (using the equation of Buswell et al.) [1].

[0175]

[0141] Experimental Setup:

[0176]

[0142] Duplicate reactors without substrate were set up as blanks and duplicate reactors with microcrystalline cellulose (CMC) were set up as controls in order to validate the good biological activity of the inoculum used.

[0177]

[0143] Duplicate reactors with the substrates of interest were set up for each test which was concluded when gas production was less than 1% over three consecutive days. The test was validated if: the BMP of CMC (BMPCMC) ranged from 340-395 Nm3 / tMSV and the relative standard deviation of CMC was less than 6%.

[0178]

[0144] Incubation

[0179]

[0145] 580 mL plasma bottles with a working volume of 300 mL were used as reactors, sealed with septa and aluminium crimp seals. The substrate-to-inoculum ratio (S / X) was 0.5 gMSV / gMSV. The reactors were inerted with nitrogen (1 bar) for 3 minutes to remove oxygen and were incubated at a mesophilic temperature of 38°C with continuous stirring at 70 rpm [lnnova®44 / 44R Shaker, New Brunswick™].

[0180]

[0146] Biogas Analysis for BMP and main constituents

[0181]

[0147] The volume of biogas produced was measured manometrically [Manometer LEO2, Keller 0-4 bars]. The composition of the biogas was analysed by gas chromatography [MicroGC 990, Agilent] connected to an associated PC, measuring the percentages of CH4, CO2, H2, O2, N2, and H2S.

[0182]

[0148] Collecting biogas for VOCs analysis

[0149] The VOCs are sampled at the time of the manometric readings of the BMPs analyses on each of the bottles. Briefly, it comprises collection of 10 mL of biogas in a glass syringe then manual injection into a pre-crimped silanized vial (10mL) then use of the vials for analysis by GC-MS.

[0183]

[0150] It was decided to use pre-crimped headspace vials [Chromoptic VVC20C1946, glass, V=10 mL, D=20mm, PTFE / butyl septa] equipped with a magnetic capsule. The vials have not been inerted with nitrogen to reduce the number of taps and therefore the possibility of leaks.

[0184]

[0151] VOCs separation and analysis

[0185]

[0152] The biogas sample contained in a crimped and sealed silanized vial was placed in a headspace oven where it underwent a stabilization phase at 50°C. The vial was then pierced by the injection needle and the gas, under slight overpressure, was sent to the gas chromatograph via a heated transfer line.

[0186]

[0153] The different compounds were separated on the capillary column then detected by the mass spectrometer. The analyses focused mainly on 10 VOCs: 2-Butanone, 2-Butanol, THF, Benzene, 1 ,4-Dioxane, Toluene, Xylene, n-propylbenzene, Limonene, Cymene.

[0187]

[0154] Sample injection is carried out using an Agilent 8697, with the following parameters: initial vial temperature: 50°C; vial equilibration: 2 minutes; injection loop temperature: 160°C; transfer line temperature: 170°C; and injection duration: 0.5 minutes.

[0188]

[0155] Gas chromatography analysis is carried out with an Agilent 8890, using helium as carrier gas, at a constant flow rate of 1 mL / min. The injector parameters are: split 1 / 10, with a split flow rate of 10 mL / min; injector temperature: 250°C; the column used is a DB 624, 30 m long, with an internal diameter of 250 pm and a film thickness of 1 .4 pm. The oven parameters are: initial temperature: 40°C; ramp 1 : 5°C / min up to 60°C, maintained for 5 minutes; ramp 2: 5°C / min up to 160°C

[0189]

[0156] Mass spectrometry analysis is performed with an Agilent 5977C. The settings are as follows: transfer line temperature: 250°C; MS source temperature: 230°C; quadrupole temperature: 150°C; injection time: 3 minutes. The acquisition is done in SIM mode, with the characteristic compounds and ions detected in the following time windows: 3 to 6.5 min: 2- Butanone (43 ; 72 m / z), 2-Butanol (45 ; 59 m / z), THF (42 ; 72 m / z); 6.5 to 10.5 min: Benzene (51 ; 78 m / z), 1 ,4-Dioxane (58 ; 88 m / z); 10.5 to 20 min: Toluene (65 ; 91 m / z), Xylene (91 ; 106 m / z) ; 20 to 29 min: n-Propylbenzene (91 ; 120 m / z), Limonene (93 ; 121 ; 136 m / z), Cymene (119 ; 134 m / z).

[0157] The 10 selected VOCs were quantified using a calibration line produced for each compound to be analysed. The calibration lines were obtained by analysing 4 dilutions made from a gas standard mixture certified for VOCs. This standard gas contained the 10 selected VOCs. The dilutions were carried out using methane: 2-Butanone: 1 to 55 ppm; 2-Butanol: 0.1 to 5 ppm; 1 ,4-Dioxane: 1 to 65 ppm; Tetrahydrofuran (THF): 0.5 to 25 ppm; Toluene: 0.1 to 5 ppm; Xylene: 0.2 to 10 ppm; PropylBenzene: 0.1 to 5 ppm; Limonene: 1 to 60 ppm; Cymene: 0.5 to 25 ppm.

[0190] RESULTS & DISCUSSION

[0191]

[0158] Optimizing VOCs analysis

[0192]

[0159] During their studies, the inventors found that the peak areas of VOCs (Volatile Organic Compounds) are more intense in silanized pre-sealed vials than in pre-sealed glass vials. The affinity of the desired VOCs is lower for the silanized vials than for the glass vials. Thus, the use of silanized vials is preferable.

[0193]

[0160] To determine the influence of the syringe material (glass or plastic) on the adsorption of VOCs during the transfer of biogas from BMP tests to silanized pre-sealed vials, the transfer of biogas to pre-sealed vials was compared using a glass syringe and a plastic syringe. The headspace sampled by both syringes came from the same source. After transferring 10 mL of the headspace into silanized pre-sealed vials, they were analyzed by HS-GC-MS.

[0194]

[0161] Upon comparing the chromatographic profiles of the biogas samples, it was found that the peak areas of VOCs are more intense in the gas collected using the glass syringe than in that collected using the plastic syringe. The loss of VOCs during transfer with the glass syringe is therefore less significant than with the plastic syringe. Hence, the use of a glass syringe is preferable.

[0195]

[0162] Comparing the substrates

[0196]

[0163] The results of the analyses carried out on the 7 substrates are gathered in table 1 below.

[0197] Table 1

[0198]

[0164] Comparing the BMP

[0199]

[0165] The values of experimental BMPs are reported in the table 2 below.

[0200] Table 2

[0201]

[0166] The values of the experimental BMPs measured vary between 261 Nm3CH4 / tVS for substrate n°2 and 796 Nm3CH4 / tVS for substrate n°12.

[0202]

[0167] The biodegradability index (Bl) varies between 52% (“substrate n°2”) and 100% (“substrates n°1 , 5, 7, 12). Fat type substrates have a high BMP (> 500 Nm3CH4 / t VS) and a Bl greater than 90%. From the BMP values obtained and taking into account biodegradability, it appears that the most interesting substrate in terms of methane production and biodegradability seem to be: “substrate n°1”, “substrate n°5”, “substrate n°7” and “substrate n°12.”

[0203]

[0168] In terms of raw materials, the most interesting substrate in terms of biochemical methane potential is substrate n°12 and greasy effluents.

[0204]

[0169] Analysing VOCs production by substrates

[0205]

[0170] Based on the BMP data of different substrates and the concentrations of VOCs obtained by GC-MS, the cumulative production of VOCs during the BMP test, expressed in relation to the biogas produced by the substrate, was calculated.

[0206]

[0171] To do this, the quantity of the 10 VOCs produced (mg) by the different substrates was calculated by subtracting the production of the blank. Then the cumulative production of VOCs was normalized to the cumulative volume of biogas (Nm3) produced by the substrate during the BMP test.

[0207]

[0172] The results are presented in Table 3 below.

[0208] Table 3

[0209]

[0173] Of the 7 substrates studied, substrate n°1 is by far the substrate with the highest cumulative production of VOCs with respectively 231 mg / Nm3biogas. Substrate n°1 has a high production of Limonene and Cymene (160 and 45 mg / Nm3biogas respectively) compared to the other substrate (on average 9 and 0.1 mg / Nm3biogas respectively). The substrate n°12 produces a small quantity of VOCs that are absent for other substrates such as toluene and xylene but remain relatively low < 1 mg / Nm3biogas.

[0210]

[0174] To date, there is no publication in the literature that has analysed the production of VOCs during a BMP test.

[0211]

[0175] Studying substrates properties in relation with VOCs production

[0212]

[0176] Furthermore, the inventors found characteristics of substrates that could be related to high amounts of VOCs produced or to unfavourable CH4 / VOCs ratios. The trend observed is that substrates with a high C / H ratio combined with a low C / N ratio showed higher VOCs content. This also appears when we look at an H / N ratio, where high values should likely be sought.

[0213]

[0177] The results are presented in Table 4 below.

[0214] Table 4

[0178] When we look at the ratio of the quantity of VOCs to the quantity of biogas emitted, we can observe that when the substrate exhibit H / N Ratio values that are greater than 3, the VOCs on Biogas index is lower than 25 mg / Nm3.

[0215]

[0179] Moreover, high H / N Ratio values and high C / N Ratio values such as those of the n°7 substrate (plant residue agri-food industry) are associated with a very low COV on Biogas index (mg / Nm3biogaz). Also, the COV on Biogas index (mg / Nm3biogaz) is low for substrate having a C / H Ratio lower than 6 or a C / N Ratio higher than 20.

[0216]

[0180] These results can serve as a guide for the selection of substrate or substrate combinations to ferment.

[0217]

[0181] Predicting VOCs production in digester plant

[0218]

[0182] Based on the VOC results obtained and the daily tonnage data of substrates in an industrial digester plant, the inventors have calculated the theoretical production of limonene over 6 months based on the BMP-VOC analyses and compared this calculated value with a measured value.

[0219]

[0183] To do this, the tonnage of each substrate on site over 6 months was calculated and then expressed in tVS. The methane production associated with each substrate (Nm3) was calculated via experimental BMP depending on the substrates studied during the trial, or from the values in the substrate database. This value was converted into biogas based on the %CH4.

[0220]

[0184] Finally, the production of limonene for each substrate was calculated using the limonene production (mg / Nm3) obtained during the trial, or for substrates not analysed during the trial, using an average limonene production value (9 mg limonene / Nm3biogas) and the previously determined biogas value.

[0221]

[0185] Thus, from the ratio of the total limonene (mg) produced by all substrates to the cumulative biogas production (Nm3) of the site over the same period, the average limonene value (mg / Nm3biogas) produced on the site was estimated.

[0222]

[0186] The values used to calculate this average are summarized in Table 5 below.

[0223] Table 5: Comparison of estimated / measured limonene production (mg / Nm3) in a biogas plant

[0224]

[0187] These data allow estimating a theoretical VOC production of 65 mg limonene / Nm3biogas, compared to the measured value on the industrial site, which on average obtains a value of 73 mg limonene / Nm3biogas over a similar time period.

[0225]

[0188] Thus, this method allows predicting production trends and identifying substrate typologies generating VOCs.

[0226]

[0189] The invention can be the subject of numerous variants and applications other than those described above. In particular, unless otherwise indicated, the different structural and functional characteristics of each of the implementations described above should not be considered as combined and I or closely and I or inextricably linked to each other, but on the contrary as simple juxtapositions. In addition, the structural and I or functional characteristics of the various embodiments described above may be the subject in whole or in part of any different juxtaposition or any different combination.

[0227] References

[0228] [1] A. Nielfa, R. Cano, et M. Fdz-Polanco, « Theoretical methane production generated by the co-digestion of organic fraction municipal solid waste and biological sludge », Biotechnol. Rep., vol. 5, p. 14-21 , mars 2015.

[0229] [2] Salazar Gomez, J. I., H. Lohmann, and J. Krassowski. 2016. ‘Determination of Volatile Organic Compounds from Biowaste and Co-Fermentation Biogas Plants by Single-Sorbent Adsorption’. Chemosphere 153 (June): 48-57.

[0230] [3] Rasi, S., A. Veijanen, and J. Rintala. 2007. ‘Trace Compounds of Biogas from Different Biogas Production Plants’. Energy 32 (8): 1375-80.

Claims

Claims1 . A method (100) for determining a biochemical methane potential of a substrate together with a quantity of at least one volatile organic compound (VOC) in a biogas produced from said substrate, the biogas being generated during anaerobic digestion, said method (100) comprising:- a step of determining the biochemical methane potential (130) of the substrate,- a step of sampling (140) the generated biogas in a container dedicated to gas headspace analysis, said sampling occurring during the step of determining the biochemical methane potential (130); and- a step of determining the quantity of at least one volatile organic compound (VOC) (150) in the generated biogas, said step comprising injecting a sample from the container into an analytical system through a temperature-controlled injection setup.

2. The method (100) according to claim 1 , wherein the step of determining the biochemical methane potential (130) of the substrate includes pressure measurement or volumetric measurement.

3. The method (100) according to claims 1 or 2, wherein the step of determining the biochemical methane potential (130) of the substrate includes a quantification of the biogas produced over time and quantification of at least CH4and CO2 in the biogas.

4. The method (100) according to anyone of claims 1 to 3, wherein the at least one VOC comprises 2-butanone, p-cymene, toluene, m,p-xylene, n-propylbenzene, 2-butanol, methyl ethyl ketone, tetrahydrofuran, 1 ,4-dioxane or R-limonene or combination thereof.

5. The method (100) according to anyone of claims 1 to 3, wherein the substrate is selected from agricultural waste, industrial organic waste, municipal organic waste, and / or other organic waste; and combination thereof.

6. The method (100) according to anyone of claims 1 to 5, wherein the biochemical methane potential (130) of the substrate is determined using an inoculum from a digester, such as an industrial digester, operating in mesophile or thermophile conditions.

7. The method (100) according to anyone of claims 1 to 6, wherein the step of sampling (140) the generated biogas is done using a glass gastight syringe.

8. The method (100) according to anyone of claims 1 to 7, wherein the container dedicated to gas headspace analysis is a silanized container equipped with a tight-fitting septum.

9. The method (100) according to anyone of claims 1 to 8, wherein the step of determining the quantity of at least one volatile organic compound (VOC) (150) in the generated biogas uses a gas chromatography coupled to mass spectrometry.

10. The method (100) according to anyone of claims 1 to 9, wherein it is conducted on several substrates.11 . The method (100) according to anyone of claims 1 to 10, wherein it further comprises a step of computing (160) for the substrate, a methane production to VOC generation index.

12. The method (100) according to anyone of claims 1 to 11 , wherein it further comprises a step of selection (170) within a plurality of substrates the one or those having a generation of VOCs reduced to the generation of biomethane lower than a predetermined threshold.

13. A method (200) for producing biomethane, comprising:- measuring (210) a C, N, content of a substrate;- selecting the substrate (220) for biomethane production based on the measured C, N, content of the substrate, when C / N ratio of the substrate is from 15 to 30; and- fermenting the selected substrate (230) in an anaerobic digester alone or with other substrates.

14. A method (200) for producing biomethane, comprising: measuring (210) a H, N, content of a substrate;- selecting the substrate (220) for biomethane production based on the measured H, N, content of the substrate, when H / N ratio of the substrate is higher than 3; and- fermenting the selected substrate (230) in an anaerobic digester alone or with other substrates.

15. The method (200) according to claim 13 or 14, wherein the selecting step (220) comprises selecting the substrate based on the purification capacity of the anaerobic digester in which the fermentation is conducted.

16. The method (200) according to anyone of claims 14 or 15, wherein the measuring step (210) comprises measuring a C, H, N, content of a substrate and the selecting step (220) comprises selecting the substrate when the H / N ratio of the substrate is higher than 3 and the C / N ratio of the substrate is higher than 20.

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