FR3092769A1 - PROCESS FOR THE PRODUCTION OF BIOMETHANE WITH MUTUALIZED PURIFICATION FOR INJECTION INTO THE NATURAL GAS NETWORK - Google Patents

Abstract

The invention relates to a process for the pooled production of biomethane for injection into the natural gas network using one or more sites Pi producing biogas (i = 1 to n), comprising the following steps: a) Production of biogas (101 ) on each Pi producer site; b) Pretreatment on site Pi of the biogas produced to obtain a pre-treated biogas (102); c) Optional partial purification on site Pi to obtain a partially purified biogas (103) with a methane CH4 content of at most 60% by volume; d ) Conditioning of biogas in high pressure or supercritical gaseous form on site Pi; e) Collection and transport of conditioned biogas (105) to an advanced purification site near said natural gas network; f) Advanced mutualized purification of conditioned biogas (105 ) to obtain a biomethane (106), for injection into natural gas networks or usable as bioNGV fuel. Figure 1 to publish.

Description

PROCESS FOR THE PRODUCTION OF BIOMETHANE WITH SHARED ADVANCED PURIFICATION FOR INJECTION INTO THE NATURAL GAS NETWORK

The invention relates to the field of the production of biomethane by methanation of agricultural waste or other organic waste, in particular for injection into the natural gas network.

Anaerobic digestion (also called anaerobic digestion) is a process of degradation by microorganisms of organic matter under controlled conditions and in the absence of oxygen. This degradation leads to the production of a digestate (wet product rich in partially stabilized organic matter) and of a biogas, a water-saturated gaseous mixture composed mainly of methane and CO ₂ . Depending on the organic materials and the techniques used, biogas contains about 30 to 75% by volume of methane, 15 to 60% of CO ₂ , water (water-saturated biogas) and some trace elements (such as NH ₃ , N ₂ , O ₂ , H ₂ S, VOCs, siloxanes).

The inputs in a methanation process are biodegradable organic substances. These inputs are, for example, livestock effluents, plant matter (dedicated crops, intermediate crops for energy purposes or crop residues), industrial effluents from, for example, the agri-food industry, bio-waste resulting from the collection of household waste , collective catering or mass distribution, sludge from wastewater treatment plants, etc.

Methanisation makes it possible to meet several environmental challenges: waste management, production of renewable energy and reduction of greenhouse gas emissions.

Depending on the level of purification envisaged, many ways of recovering biogas from methanization are possible, such as:
the combustion of biogas to produce heat, the combustion of biogas to produce electricity with or without heat recovery (cogeneration), the production of biomethane for injection into gas networks (authorized since November 2011), the production of biomethane for vehicle fuel (BioNGV), production of biomethane to produce liquefied natural gas (BioLNG).

In the remainder of the description, “biomethane” means purified, high-purity biogas which has characteristics similar to natural gas and which can be injected into the gas network or used as a vehicle fuel called bioNGV (generally contains more than 95% by volume of methane, or even more than 98% by volume of methane).

In France, anaerobic digestion is increasingly moving towards the injection of biomethane into gas networks as a renewable substitute for natural gas thanks to numerous incentives. The Energy Transition Law for Green Growth (LTECV) reinforces the ambitions attributed to the injection of biomethane by setting a national target of 10% renewable gas in consumption by 2030.

Nevertheless, this recovery sector is hampered by the geographical distance between many mobilisable deposits and gas networks of suitable capacity, the agricultural sector in particular having the greatest potential for mobilizable deposits with around 80% of the total methanogenic potential.

French farms are indeed often too remote and/or too small to envisage a connection to the network. Farms that raise livestock often have limited herd sizes of the order of 50 to 100 animals on average which do not allow, using available technologies, to produce sufficient quantities of biogas for its recovery in biomethane for injection into the network. Farms with several hundred animals (of the “1,000 cow farm” type) are then necessary to develop economically profitable methanation projects to recover biomethane by reinjection.

To overcome this constraint, alternative solutions have been studied, such as:
- Road transport of raw materials to be methanized to a shared methanization site close to an injection point on the network. However, the logistics of collecting, transporting and storing solid raw materials to the methanization site is complicated with a limited range of action. Furthermore, the raw materials to be methanized, such as manure, lose their methanogenic power if they are stored for too long. In addition, a return of digestates from methanation to farms must be ensured for spreading activities.
-The routes carried out which aim at the production and purification of biogas on the site of the agricultural exploitation with a transport of the biomethane produced in liquid or gaseous form under pressure by road, for an injection on gas networks having greater carrying capacity. We then speak of carried injection.

If the carried injection concerns a single methanisation unit and a single injection point, we speak of individual carried injection. If several anaerobic digestion units carry their biomethane and share an injection point, the term shared carried injection is used.

The scenarios for the injection of biomethane are therefore multiple and depend on the mode of purification on the production site (no purification, total purification with complete decarbonation or simple pre-treatment without decarbonation, fixed or mobile) and the type of packaging for the transport (in gaseous form at high pressure, in liquid form at high or low pressure).

The patent application FR3011750 describes in particular a method for supplying biomethane to a natural gas network from n biogas producing sites comprising the stages of biogas production in each of the sites, the high pressure storage of the biogas produced in each of the sites , the collection of all the biogas stored using a mobile means of collection, then the purification and injection of the biomethane into the natural gas network. The device for purifying the biogas collected can be mobile or fixed, and preferably placed close to the station for injecting the biomethane into the network. In this case, no pre-treatment or purification of the biogas is carried out before collection.

Moreover, whatever the scenario envisaged, the profitability of a carried injection project for small methanization units is called into question because of the significant additional costs generated by the purification and carrying stages. The profitability of injecting biomethane into the gas network has only been proven with known technologies if 50 to 75 Nm ³ /h of biomethane is injected, whereas an average farm in France generally has the capacity to produce around 10 to 30 Nm ³ /h of biomethane (ie 20 to 60 Nm ³ /h of biogas).

To date, there is therefore no solution adapted to biomethane recovery such as, for example, reinjection into the networks for small agricultural production units, whereas these represent the vast majority of French agricultural resources.

The present invention aims to propose a solution for the injection of biomethane, shared, therefore applicable to any size of methanization unit, located close or not to an injection point, which ensures producers the valuation of their productions. of biogas considered too low and/or too far from the networks to be valued individually.

French patent application FR18/52.693 filed by the Applicant on March 28, 2018 describes a process for the shared production of biomethane suitable for injection into the natural gas network using one or more _{biogas producing sites P i} , and comprising the following steps: production of biogas by methanation of an organic load on each production site P _i ; partial purification, on each producer site i, of the biogas produced including partial decarbonation to obtain a partially purified biogas with a methane CH _{4 content} greater than 60% by volume on each producer site P _i ; conditioning of the partially purified biogas in gaseous or supercritical form on each production site Pi; collecting and transporting all of the partially purified biogas from each of the production sites P _i using a collection device to a complementary purification site close to said natural gas network; pooled additional purification of the partially purified biogas collected on said site of additional purification to obtain a pooled flow of biomethane.

Surprisingly, the Applicant has discovered that a limited purification, or even a simple on-site pre-treatment to obtain a biogas with a methane CH _{4 content of} less than or equal to 60% by volume on each production site P _{i made it} possible to facilitate conditioning of the biogas produced by each production site and to limit the investment costs of each production site.

The approach proposed according to the invention thus consists in coupling:
-A step of pre-treatment of the biogas produced on each production site to eliminate the pollutants present in trace amounts such as H ₂ S, VOCs, siloxanes, NH ₃ and possibly including dehydration, in order to obtain a pre-treated biogas;
-An optional purification step comprising a partial decarbonation of the biogas on each production site, for example using a membrane module sized to carry out a partial decarbonation of the biogas, to obtain a partially purified biogas containing a maximum of 60% in volume of methane CH ₄ . ;
- A step of advanced purification, comprising a shared decarbonation on the injection site, which completes, if necessary, the purification possibly carried out on the production site, for example using an amine washing process to obtain a biomethane meeting specifications.

The invention relates to a process for the pooled production of biomethane suitable for injection carried into the natural gas network, implementing one or more _{biogas production sites P i} , i being between 1 and n, comprising the following steps:
a) Production of biogas (101) by methanation of an organic load on each production site P _i;
b) Pretreatment of the biogas produced by elimination of the pollutants present in trace amounts such as H ₂ S, VOCs, siloxanes, NH ₃ to obtain a pretreated biogas (102);
c) Optional partial purification, on each producer site i, of the pretreated biogas (102) including partial decarbonation to obtain a partially purified biogas (103) with a methane CH _{4 content} of 60% by volume at most on each producer site P _i ;
d) Conditioning of the pretreated biogas (102) obtained in step b) or of the partially purified biogas (103) obtained in step c) in high pressure or supercritical gaseous form on each producer site Pi to obtain a conditioned biogas (105 );
e) Collection and transport of all the pretreated or partially purified biogas conditioned (105) in step d) from each of the production sites P _i using a collection device (8) to a purification site thrust near said natural gas network;
f) Pooled advanced purification of the pretreated or partially purified conditioned biogas (105) collected in step e) on said advanced purification site to obtain a pooled stream of biomethane (106).

The method may include a step of dehydrating or dehumidifying the biogas produced at any time before or during step d) of conditioning.

The organic load can be chosen from agricultural waste, sewage treatment plant sludge, bio-waste, effluent from the food industry, alone or in a mixture.

The optional partial purification of step c) is advantageously carried out by membrane separation in one or more membrane modules (5) each comprising one or more separation stages, preferably a single stage, to produce a partially purified biogas (103) enriched in methane with a CH ₄ methane content of at most 60% by volume and a residual gas (104) depleted in methane.

Advantageously, the heat of the residual gas (104) resulting from the membrane separation is used to provide the heat necessary for the methanation and for the other heat requirements of the production site Pi allowing the production of biogas from step a).

The shared deep purification of step f) can be carried out in a deep purification device (10) implementing at least one of the techniques chosen from washing with water, separation by pressure-modulated adsorption (PSA), separation in a cryogenic system, membrane separation, washing with amines.

Preferably, the extensive purification is carried out by amine washing under pressure in an amine washing unit.

In one embodiment, the conditioning of the pretreated (102) or partially purified (103) biogas from step d) is carried out by compression of the pretreated (102) or partially purified (103) biogas at a pressure of between 200 and 300 bar and cooling to ambient temperature to place the conditioned biogas (105) under high pressure gaseous conditions.

In another embodiment, the pretreated (102) or partially purified (103) biogas is conditioned in supercritical form in step d) by compression at a pressure of between 40 and 90 bar and/or cooling to a temperature of between -82°C and 30°C in the pretreated (102) or partially purified (103) biogas in order to place the conditioned biogas (105) under supercritical conditions corresponding to the residual CO _{2 content of} said pretreated (102) or partially purified (103).

Advantageously, the conditioned biogas (105) in high-pressure gaseous form or in supercritical form is expanded and/or reheated on the advanced purification site before step f) of shared advanced purification.

The heat needed to heat the conditioned biogas (105) can be provided by cooling a cold water circuit of the shared advanced purification unit.

At least some of the n biogas producing sites can be agricultural sites.

Advantageously, the biomethane produced (106) has a methane content greater than 95% by volume, preferably greater than 98% by volume, very preferably greater than 99% by volume.

At least part of the biomethane produced (106) can be injected into the natural gas distribution networks or transport networks, and/or directly supplies local consumers, and/or supplies a bioNGV fuel station.

List of Figures

Other characteristics and advantages of the method according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the appended figures and described below.

Figure 1 shows the block diagram of the process according to the invention.

Figure 2 represents a phase diagram (P, T) for a binary mixture having a given composition, on which the various phase domains are specified: gas, liquid, gas+liquid and supercritical.

More specifically, the present invention relates to a process for producing biomethane for injection carried into a natural gas network (transport network or distribution network) from n biogas production facilities (for example methanizers), in which each of the facilities produces, pre-treats, possibly partially purifies and stores biogas. This pretreated and optionally partially purified biogas on each production site P _i has a methane CH _{4 content of} at most 60% by volume; it is then collected and transported by a mobile system to the advanced purification site common to the n production facilities, near the point of injection into the gas network. The biogas collected then undergoes a shared purification step pushed close to the injection site so as to produce biomethane that complies with the standards for injection into a natural gas network.

In other words, the present invention couples a step of pretreatment and optional partial purification; said purification allowing partial decarbonation of the biogas on the n production facilities and a pooled advanced purification step on the injection site.

The method according to the invention further provides for conditioning of the pretreated or partially purified biogas, either in gaseous form, advantageously at high pressure, or preferably under supercritical conditions with a view to its transport from the production sites to the site of injection.

The process according to the invention consists in producing biomethane, which can be intended for injection carried to supply a natural gas network, fromnotproduction sites P _I of biogas, withIvarying from 1 tonot, comprising at least the following steps:
-For each of the production sites P _I of biogas, one stage at _I biogas production;
-For each of the production sites P _I of biogas, one stage b _I biogas pre-treatment by eliminating trace elements (such as NH₃, NOT₂, O₂, H₂S, VOCs, siloxanes and possibly water (dehydration or dehumidification),
-For each of the production sites P _I of biogas, one stage vs _I optional partial purification of the biogas produced including partial decarbonation in order to obtain a methane content CH₄up to 60% by volume;
-For each of the production sites P _I of biogas, one stage d _I conditioning of pretreated or partially purified biogas;
- A step and collection and transport of all the biogas produced and stored at each of the production sites P _I using a mobile collection device (e.g. a truck);
-A step f advanced purification of the biogas collected so as to produce high purity biomethane, that is to say advantageously meeting the standards for injection into natural gas networks, for example a methane content greater than or equal to 92% by volume, preferably greater than or equal to 95% by volume, very preferably greater than 98% by volume, even more preferably greater than or equal to 99% by volume;
-A possible step g injection of biomethane into the natural gas network (possibly including biomethane odorization and metering).

The number n of shared production sites is specific to each biomethane injection project. It varies according to the size and the number of sites potentially producing biogas located within a given radius of the territory. It also depends on the reception capacity of the network at the level of the injection site.

Advantageously, the invention relates to at least two production sites, preferably a number of 5 to 10 biogas production facilities.

The production of each production site P _i is not necessarily identical. Furthermore, the composition of the gases produced may vary depending on the production site P _i .

A good understanding of the invention is based on the block diagram shown in Figure 1. For the sake of simplicity, only the essential elements useful for understanding and implementing the installation are indicated. All the steps necessary for the purification of biogas into biomethane and for the injection of biomethane into the gas network are not described, but are known to those skilled in the art.

According to Figure 1, each production site P _I implements an installationI _I methanization comprising a methanizer1, producing biogas101. Using a blower2, raw biogas101is sent to a preprocessing module3to remove trace pollutants such as H₂S, VOC, siloxanes or even NH₃. The technology used can be activated carbons, regenerative washing with soda or not, regenerable sieves or not, etc.

The pretreated biogas 102 removed from its trace pollutants is then compressed in a compressor 4 , at a pressure generally between 8 and 16 bar, before possibly being sent to a partial decarbonation stage using a membrane module 5 . The latter produces a partially purified biogas 103 (retentate) with a methane content of less than or equal to 60% by volume, and a residual gas 104 (permeate) with a methane content of between 10 and 30% by volume, preferably order of 20% by volume. The membrane module 5 thus preferably consists of a single stage of membranes, which constitutes an important asset in terms of investment and operation. The performance of the membrane separation (methane yield, methane content in the partially purified biogas 103 ) are adjusted according to the heat requirements of the methanizer 1 (the objective being to ensure self-consumption of the methanizer 1 ). Indeed, a thermal integration can be set up and aims to burn the residual gas 104 in a boiler in order to supply a heating system of the methanizer 1 , but also to meet the heat needs on the production site (hygienization digestates, heating of agricultural buildings for example). Depending on the methane content of the residual gas 104 , the installation of a low calorific value boiler (LCV) may be required. An alternative would consist in enriching the residual gas 104 with a small fraction of the raw biogas 101 or the pretreated biogas 102 in order to increase the methane content beyond 25%, preferably beyond 30%, to allow the use of a gas boiler of simple technology. The CO ₂ resulting from the purification of biogas can be recovered to enrich the atmosphere of greenhouses with CO ₂ .

The pretreated or partially purified biogas 103 is then packaged on the production site P _i via a packaging system 6 . The objective is to reduce the volumes of biogas produced as much as possible to minimize storage and transport costs. Several packaging methods can be considered:
- Compression in the gaseous phase at high pressure, between 200 and 300 bar, and cooling to room temperature.
-Compression in supercritical phase (or called dense phase). The supercritical state is a state of matter which has physical properties (density, viscosity, diffusivity, etc.) intermediate between the gas phase and the liquid phase. The supercritical conditions are a function of the CO ₂ content in the partially purified biogas 103. The associated pressure and temperature conditions extend respectively from 40 to 90 bar and from -82 to 30°C. By way of example, the supercritical conditions are of the order of 90 bar and -20°C for a biogas pretreated with 50% volume of methane.

Packaging in liquid form is not advantageous here because of the excessively high CO _{2 content} in the pretreated or partially purified biogas 102 103 , which would run the risk of solidifying during the liquefaction of the biogas.

Conditioning the biogas in the supercritical phase constitutes an advantageous and preferred solution insofar as it makes it possible to densify the biogas to be transported without however compressing it to very high pressures. By supercritical we mean the state of matter when it is subjected to high pressure or temperature, in particular we speak of supercritical fluid when a fluid is heated beyond its critical temperature and when it is compressed above it. of its critical pressure. As indicated previously, the physical properties of a supercritical fluid (density, viscosity, diffusivity) are intermediate between those of liquids and those of gases. For a mixture (CH ₄ +CO2 type), the supercritical range (see Figure 2) is determined by:
- A pressure above the maximum pressure of the gas/liquid phase envelope
- A temperature between the critical temperature of the mixture and the maximum temperature of the gas/liquid phase envelope.

In the case of biogas, the conditioning pressure of pretreated or partially purified biogas in the supercritical phase is also close to the maximum operating pressure of natural gas transport networks (about 80 bar), unlike conditioning in gaseous form at high pressure. (200 to 300 bar). Compression and investment costs for storage and transport are thus saved when the pretreated or partially purified biogas is packaged in supercritical form.

The conditioning step in the conditioning system 6 advantageously includes prior drying of the pretreated 102 or partially purified 103 biogas (regenerable sieve, washing with glycol, etc.) in order to guard against any risk of formation of CO ₂ hydrates and /or CH ₄ . Preferably, the drying of the biogas is carried out on a regenerable sieve which is easier to operate for the farmer. In the case where the partial purification is carried out, the step of partial purification of the biogas can allow dehydration of the biogas which can reach a few ppmv H ₂ O in the partially purified biogas. So in this case, no drying unit is to be provided, or at least additional drying if necessary.

Biogas105preferably packaged in the supercritical phase, is stored in a tank7storage installed on the production site P _I . The total storage capacity of partially purified biogas on the production site P _I is adapted to the passing frequency of the collection device8(for example a truck) which rotates between all the production sites P _I and the shared injection site. Storage generally allows two to three days of production. However, its capacity can be adjusted according to the collection rotation and the regulatory constraints of biogas storage on the production site. P _I . The reservoir7for storing the biogas produced can be:
- mobile: these are mobile tanks equipped with a handling frame. The collection device8recovers the tank filled with pretreated or partially purified conditioned biogas105and replaces it with another empty tank. Several mobile tanks can thus be installed on the production site P _I within the limits of the regulations in force, to extend the period between two successive collections.
- fixed: biogas105content in the tank7of fixed storage is unloaded into another mobile tank installed on the mobile collection device8.

It is also possible to design a conditioning of the biogas102pretreated or103partially purified using a mobile system moving from production site to production site to collect the biogas produced. The packaging device can thus be installed on a truck and its packaging capacity is then a function of the total duration of mobilization acceptable on each production site. P _I . In this specific case, the producer can favor storage of the pretreated biogas102Where partially purified103at a higher pressure in order to limit the storage volume before collection.

Production sites P _I are advantageously individually equipped with a device for analyzing the methane content or measuring the PCS (superior calorific value) of the biogas produced to account for the recoverable production of each site P _I participating in the shared scope injection project.

All of the biogas collected on each production site P _i is then stored and deconditioned on a single, shared storage site 9 located close to the place of injection into the network or of recovery of the biomethane by an alternative route. An advanced purification system 10 makes it possible to produce, from the collected biogas, biomethane 106 that complies with the standards for injection into the networks or for alternative recovery. The technology implemented in the advanced purification system 10 is preferably washing with amines under pressure, but can be any other technology suitable for the separation of CH ₄ and CO ₂ : washing with water, adsorption at modulated pressure (PSA), cryogenic system, membranes…

In the case of washing with amines, absorption processes are commonly used implementing an aqueous solution of amines to remove the acid compounds, in particular carbon dioxide (CO ₂ ), present in a gas. The gas is thus treated by bringing it into contact with the absorbent solution in an absorption column ("absorber"), then the absorbent solution is thermally regenerated in a regeneration column ("regenerator"). A gas depleted in acid compounds is then produced in the absorber, and a gas rich in acid compounds leaves the regenerator. Document US Pat. No. 6,852,144 describes, for example, a method for removing acid compounds from hydrocarbons. The method uses a water/N-methyldiethanolamine (MDEA) or water/triethanolamine absorbent solution containing a high proportion of at least one compound belonging to the following group: piperazine, methylpiperazine and morpholine.

The performance of acid gas deacidification processes by washing with amines is directly dependent on the nature of the nitrogen compound present in the absorbent solution. They may in particular be amines which may be primary, secondary or tertiary. They can have one or more equivalent or different amine functions per molecule, and can be used alone or as a mixture. Other compounds such as physical solvents can also be added.

The implementation of an aqueous solution comprising at least one nitrogen compound to deacidify a gaseous effluent is carried out schematically in the amine washing unit by carrying out an absorption step in an absorption column followed by a regeneration step in a regeneration column.

The CO ₂ absorption step can be carried out at a pressure in the absorption column of between 1 bar and 200 bar, preferably between 20 bar and 100 bar for the treatment of pretreated or partially purified biogas, and at a temperature in the absorption column between 20°C and 100°C, preferably between 30°C and 90°C, or even between 30 and 60°C.

The regeneration step consists in particular in heating and, optionally in expanding, the absorbent solution enriched in acid compounds (CO _{2 in} particular) in order to release the acid compounds in gaseous form.

The supercritical conditioning of the pretreated 102 or partially purified 103 biogas and its extensive purification by washing with amines under pressure make it possible to operate at a pressure close to that of the transport network (25 to 80 bar) limiting the loss of energy linked to the biogas decompression. Depending on the operating pressure of the network into which the biomethane produced is injected, recovery of the energy linked to the expansion can be envisaged.

The advanced purification of the collected biogas 105 into high-purity biomethane 106 is not ensured individually by each production site P _i but in a pooled manner by the common advanced purification device 10 installed on the injection site. The optional partial purification of the biogas on each production site P _i makes it possible, if necessary, to reduce the size and the cost of the advanced purification installation 10 . The process according to the invention coupling optional partial purification on the production sites P _i , pooling of the advanced purification on the injection site and conditioning of the biogas collected in the supercritical phase makes it possible to considerably reduce the cost of production of biomethane for the all biogas producers.

The invention also provides for thermal integration between the storage and deconditioning system 9 and the advanced purification device 10 of the biogas collected. Indeed, the first will be able to meet the cooling needs (cold utilities) of the second via an auxiliary cooling water circuit.

Biomethane106product at the outlet of the deep purification device10finally feeds the injection system11including, among other things, odorization, analysis and metering operations in order to be controlled, counted and injected into the gas network12to supply consumers with natural gas. The supply of the network is then made from biomethane106produced by all production sites P _I , making it more reliable both in terms of quality and volume injected. The gas network can be either a distribution network or a transmission network. The network pressure is advantageously between 2 and 6 bar for distribution, 15 and 25 bar for medium pressure distribution and 25 to 80 bar for transport.

The method according to the invention therefore presents the possibility of injecting into all types of gas networks, including the transmission network, thus making it possible to cover larger consumption areas. This geographical location flexibility is extremely interesting and beneficial for the system operator for whom the backflow of gas to higher pressure stages is no longer a problem.

Alternatively, at least part of the biomethane produced 106 can supply a BioNGV station (vehicle fuel) or directly supply local consumers with gas if there are needs in the territory (industrialists, communities, etc.).

Advantages of the invention

The invention makes it possible to reduce the cost of biomethane production by allocating the purification and transport costs between the different sites pooling their biogas production.

In the embodiment including partial purification on the production site, the membrane modules are particularly suitable for coarse purification of the biogas including partial decarbonation, preferably with the use of a single stage of membranes; it is also possible to design small capacity modules (<20 Nm ³ /h of biomethane, i.e. <40 Nm ³ /h of biogas), the objective of this partial decarbonation being within the framework of the invention to produce a pretreated and partially purified biogas having a CH ₄ content of less than or equal to 60% by volume.

Partial purification (optional) of biogas which ensures part of the decarbonation on each production site P _I can allow, if necessary, to reduce the size and the cost of the installation of advanced purification shared.

The amine washing units allowing advanced purification have a biomethane production cost that varies little depending on the level of purification desired and are of great interest when the capacity approaches 200 Nm ³ /h of treated biogas.

The present invention further provides for the conditioning of the biogas (pretreated or partially purified) in the dense phase, preferably under supercritical conditions with a view to its transport from the production sites to the injection site. Said packaging is carried out either using a fixed system installed on the production site, or using a mobile system installed on a truck which performs a collection circuit between the various shared production sites. Said truck can advantageously use a bioNGV-type biomethane-based fuel.

Thus, the method according to the invention advantageously makes it possible to:
- Simplify or even eliminate the partial purification step on the production site;
- Have a less severe biogas conditioning temperature in the dense phase (in particular supercritical) (ie less cold) despite a larger volume of biogas to be conditioned (less CO ₂ partially eliminated on the production site).
- Eliminate methane losses from the partial purification stage if this is not implemented on the production site.

The advantages mentioned above make it possible, among other things, to limit the investment and operating costs of the unit, and thus increase the profitability of a project and the income of the biogas producer.

As part of the process according to the invention, the biomethane-producing sites, in particular agricultural holdings, are fully integrated into a circular economy, offering them not only a source of additional income, but also the possibility of practicing sustainable agriculture with the direct recovery of digestates from methanization at the right time for their crops, which also represents a significant saving on the purchase of fertilizers to fertilize the soil.

With regard to injection into the natural gas network, the network is supplied from the biomethane 106 produced by all the production sites P _i , thus making it more reliable both in terms of quality and volume injected.

An agricultural installation (production site) produces 50 Nm ³ /h of raw biogas to be treated containing 55% mol CH ₄ and 45% mol CO ₂ (O ₂ , N ₂ and other elements present in trace amounts)).

Three cases are considered:

Case 1A (according to the invention): only pretreatment of the biogas produced is carried out on site. The advanced purification to obtain biomethane that meets the specifications for injection into the network is carried out remotely.

Case 1B (according to the invention): pretreatment and partial purification including decarbonation to obtain a CH ₄ content of the biogas at the conditioning stage of 60% maximum are carried out on the production site. The advanced purification to obtain biomethane that meets the specifications for injection into the network is carried out remotely.

Case 2 (comparative): pre-treatment and partial purification including decarbonation to obtain a CH ₄ content of the biogas at the conditioning stage of 70% are carried out on the production site.

Case 3 (comparative): pre-treatment and partial purification to obtain a CH ₄ content of the biogas at the conditioning stage of 86% are carried out on the production site.

Raw biogas flow
(Nm3/h) CH4 content of the raw biogas produced by the methanizer (%mol) CH4 content of biogas at the conditioning stage (%mol) Daily volume to be packaged (Nm3) Daily volume to be stored in dense phase (m3) Dense phase temperature (°C) Case 1A (preprocessing) 50 55 55 1200 3.3 -30°C Case 1B (pre-treatment + on-site purification) 50 55 60 1100 3.1 -35°C Case 2
(pre-treatment + on-site purification) 50 55 70 943 2.6 -50°C Case 3
(pre-treatment + on-site purification)
bioNGV quality 50 55 86 767 2.1 -70°C

Table 1

The process according to the invention makes it possible to facilitate conditioning by having a conditioning temperature for the biogas (pretreated or partially purified under the conditions of the invention) in the dense (supercritical) phase that is less severe (ie less cold) despite a volume of biogas more important to condition since a smaller quantity of CO ₂ is eliminated on the production site (Cases 1A and 1B). In the embodiment without partial purification and with supercritical conditioning (Case 1A), the method according to the invention advantageously makes it possible to:

- Eliminate the partial purification step on the production site;
- Eliminate methane losses from the partial purification stage since this is not implemented on the production site.

The method according to the invention also makes it possible to limit the investment and operating costs of the unit, in particular those of the partial purification module and of the cooling module and thus increase the profitability of a project and the income of the biogas producer.

Claims (14)

Process for the shared production of biomethane suitable for injection into the natural gas network, implementing one or more _{biogas production sites P i} , i being between 1 and n, comprising the following steps:
a) Production of biogas (101) by methanation of an organic load on each production site P _i;
b) Pretreatment of the biogas produced by elimination of the pollutants present in trace amounts such as H ₂ S, VOCs, siloxanes, NH ₃ to obtain a pretreated biogas (102);
c) Optional partial purification, on each producer site i, of the pretreated biogas (102) including partial decarbonation to obtain a partially purified biogas (103) with a methane CH _{4 content} of 60% by volume at most on each producer site P _i ;
d) Conditioning of the pretreated biogas (102) obtained in step b) or of the partially purified biogas (103) obtained in step c) in high pressure or supercritical gaseous form on each producer site Pi to obtain a conditioned biogas (105 );
e) Collection and transport of all the pretreated or partially purified biogas conditioned (105) in step d) from each of the production sites P _i using a collection device (8) to a purification site thrust near said natural gas network;
f) Pooled advanced purification of the pretreated or partially purified conditioned biogas (105) collected in step e) on said advanced purification site to obtain a pooled stream of biomethane (106). Process according to claim 1 which comprises a step of dehydrating or dehumidifying the biogas produced at any time before or during the conditioning step d). Process according to one of the preceding claims, in which the organic load is chosen from agricultural waste, sludge from purification stations, bio-waste, effluents from the agro-food industry, alone or in a mixture. Process according to one of the preceding claims, in which the optional partial purification of step c) is carried out by membrane separation in one or more membrane modules (5) each comprising one or more separation stages, preferably a single stage, to produce a partially purified biogas (103) enriched in methane with a methane CH _{4 content of} at most 60% by volume and a residual gas (104) depleted in methane. Process according to claim 4, in which the heat of the residual gas (104) resulting from the membrane separation is used to supply the heat necessary for the methanization and for the other heat needs of the production site Pi allowing the production of biogas from step at). Process according to one of the preceding claims, in which the mutualized deep purification of step f) is carried out in a deep purification device (10) implementing at least one of the techniques chosen from washing with water, separation by adsorption pressure swing (PSA), separation in a cryogenic system, membrane separation, washing with amines. Process according to claim 6 wherein the extensive purification is carried out by amine washing under pressure in an amine washing unit. Process according to one of the preceding claims, in which the conditioning of the pretreated (102) or partially purified (103) biogas of stage d) is carried out by compression of the pretreated (102) or partially purified (103) biogas at a pressure comprised between 200 and 300 bar and cooling to ambient temperature in order to place the conditioned biogas (105) under high pressure gaseous conditions. Process according to one of Claims 1 to 7, in which the pretreated (102) or partially purified (103) biogas is conditioned in supercritical form in step d) by compression at a pressure of between 40 and 90 bar and/or cooling at a temperature between -82°C and 30°C in the pretreated (102) or partially purified (103) biogas in order to place the conditioned biogas (105) under supercritical conditions corresponding to the residual CO _{2 content of} said pretreated biogas (102) or partially purified (103). Process according to one of the preceding claims, in which the conditioned biogas (105) in high-pressure gaseous form or in supercritical form is expanded and/or reheated on the advanced purification site before stage f) of shared advanced purification. Process according to claim 10, in which the heat necessary for heating the conditioned biogas (105) is provided by cooling a cold water circuit of the mutualised advanced purification unit. Method according to one of the preceding claims, in which at least some of the n biogas-producing sites are agricultural sites. Process according to one of the preceding claims, in which the biomethane produced (106) has a methane content greater than 95% by volume, preferably greater than 98% by volume, very preferably greater than 99% by volume. Process according to one of the preceding claims, in which at least part of the biomethane produced (106) is injected into the distribution networks or the natural gas transport networks, and/or directly supplies local consumers, and/or supplies a bioNGV fuel station.