EP3642321A1

EP3642321A1 - Method and device for producing biomethane in a compartmentalised reactor by viscous process

Info

Publication number: EP3642321A1
Application number: EP18752530.8A
Authority: EP
Inventors: Michel Bonhomme
Original assignee: Individual
Current assignee: B H M
Priority date: 2017-06-20
Filing date: 2018-06-20
Publication date: 2020-04-29
Also published as: FR3067719B1; FR3067719A1; WO2018234695A1

Abstract

The present invention concerns a method and a device for the sectorised stirring of viscous anaerobically digested substrates and enrichment of the recirculated biogas into biomethane by reducing the carbon dioxide reduced by endogenous and exogenous hydrogen. The device consists of a plurality of compartments with closed gas blankets. Controlling the pH of the first hydrolysis and acidogenesis compartment to 5.5 at hyperthermophilic temperature promotes the production of CO2 and H2. The biogas produced in the first compartments and enriched with H2 is recirculated in the last compartment kept at a partial pressure that controls the solubility of the CO2 and keeps the pH below 7.3. The CO2 is reduced by the hydrogen reintroduced into the last compartment through microporous ramps integrated into the floor. The substrate is stirred at intervals by compressed biogas expanded into the stirring sectors of each compartment at a pressure, dependent on the measured viscosity, through injectors situated under by-pass plates integrated into the floor. The pH is also controlled by controlling the transfer of the substrate on either side of the walls of each compartment by the suction of the substrate by the pressurised gas stream close to the wall. Introducing exogenous dihydrogen through the microporous ramps of the last compartment makes it possible to produce biomethane with at least 95% CH4.

Description

METHOD AND DEVICE FOR PRODUCING BIOMETHANE

COMPARTIMENT REACTOR IN VISCOUS PATHWAY

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and a device for methanization multi-compartmented, viscous, fermentable organic substrates for the production of biomethane by recirculation of biogas enriched in hydrogen produced in the first fermentation compartments maintained hyperthermophilic temperature and thermophilic in the last gas compartments maintained at different pressures to regulate the pH and solubility of gases. The punctual stirring device with compressed biogas expanded at a pressure depending on the viscosity of the fermenting substrate in injectors under the floor-integrated branch plates makes it possible to keep the viscous substrate homogeneous.

An exogenous supply of dihydrogen increases the methane content of the biomethane and allows its reinjection into the natural gas network.

STATE OF THE ART

Anaerobic digestion is a complex microbial process. It involves many microbial groups that each take part in the decomposition process.

The main stages of the process are hydrolysis, fermentation activity, acetogenesis and methanogenesis.

Hydrolysis is an extracellular process in which organic substances made of complex molecules (polysaccharides, cellulose, lipids, proteins.) Are broken down into simple soluble compounds (amino acids, simple sugars, fatty acids, glycerol.

The fermentation pathway or acidogenesis metabolizes the substrates into different fermentation products. PH- and temperature-related dissolved dihydrogen levels guide the dominant metabolic pathway, type of volatile fatty acids, lactate, ethanol, and so on.

The acetogenous route converts the metabolites derived from the acidogenic phase by acetogenic bacteria. At low pH the production of acetate and butyrate and dihydrogen is favored, as is the production of ethanol.

The hydrogen must be evacuated or consumed by homoacetogenic bacteria to produce acetate or by hydrogenophilic bacteria reducing carbon dioxide by dihydrogen or degassed in the gas sky.

The acetogenesis step is carried out by two families of bacteria:

Acidogenic producing hydrogen-producing bacteria capable of producing acetate and hydrogen from metabolites derived from acidogenesis. According to the type of acid, the temperature and the partial pressure of hydrogen, the multiplication times of these bacteria are more or less long.

- Homoacetogenic bacteria or non-syntrophic acetogenic bacteria.

The metabolism of these bacteria is mainly oriented towards the production of acetate. They can act as partners of hydrogenotrophs. A first group would produce acetate, butyrate from simple compounds. The second group uses hydrogen and carbon dioxide to produce acetate.

Methanogenesis involves converting acetate, hydrogen and carbon dioxide to methane. There exist two principal ways using methanogens strict or arched methanogens:

Methanogens acetoclasts: acetate + H2-►CO2 + CH4.

Hydrogenophilic methanogens: CO 2 + 4H 2 -> H 2 O + CH 4.

In view of these two routes, we see all the interest of directing the fermentation process to the hydrogenotrophic pathway that directly produces methane without carbon dioxide.

During the hydrolysis and acidogenesis step, an acidic pH is sought insofar as it promotes the production of dihydrogen and limits the increase in pH of the next step of acetogenesis. In the compartments reserved for methanogenesis, a pH that is too basic, starting from 7.5, slows down the kinetics of degradation and the growth rate of the bacteria. A basic pH also favors the production of ammonia, a gas which inhibits the activity of methanogens.

The flow of fermentation material in piston or semi-piston mode until its extraction from the fermentor makes it possible to respect the steps of the anaerobic digestion and to tend towards a more optimized operation of each stage of the methanation.

In a methanization system composed of several phases, including a phase reserved for hydrolysis and acidogenesis in hyperthermophilic temperature and several phases reserved for acetogenesis and methanogenesis, thermophilic temperature, the kinetics of degradation on all phases are greatly improved. However, the gas produced during the hydrolysis and acidogenesis phase essentially contains carbon dioxide with a low methane level. This penalizes the quality of the biogas produced on all phases with a methane level of the order of 50 to 55% only. This rate varies according to the nature of the substrate.

The increase in the hydraulic residence time tends to increase the pH and to increase the rate of CH4 of the biogas. This is accompanied by a decrease in the degradation kinetics of the substrate and the production of inhibiting gases such as ammonia.

We are talking about anaerobic digestion in liquid or in the dry route. The liquid pathway is characterized by the easily pumpable nature of the substrates. The mixing is either mechanical or by the force of the gas injected through a gas booster. The fermentation tanks are thus infinitely mixed. The dwell times of the material are then random. Residual liquid effluents at the end of fermentation are important. Dilution of waste to operate at values of less than 8% dry matter, for example, gives rise to phase separations between the fibers and the liquid. This lack of dry matter requires almost continuous mixing to avoid phase separation. It is also a limiting factor of fermentation due to lack of support for bacteria.

The term "dry route", commonly used, is in our view inappropriate because the very thick material with a high dry matter content has a wet part, present in the cells of the material to be fermented. The bound water is rapidly released by the enzymatic activity and the rise of the temperature of the medium. This liquid part, external to the solid structure of the substrate, is necessary for the exchange of metabolites that dissolve in the liquid to be accessible to bacteria.

Before entering the fermentor the material is in a solid form that does not flow, only a liquid juice can possibly get out of the mass and flow. We have to deal with an elastic and viscous non-Newtonian fluid close to the shear threshold that can be characterized as a threshold fluid.

When the substrate is too solid - for example more than 20% dry matter for a manure or more than 25% dry matter for the fermentable part of the household waste, it should be diluted slightly with a liquid effluent, water or a liquid from the downstream processing of the digestate. This liquid is collected by pressing, or by evaporation with production of liquid distillate or other methods of separating the solid phase from the liquid phase.

Agitation of a thick substrate is necessary for the flow of the substrate. It avoids phase separation.

But the methanization in dry or viscous way faces the difficulties of mixing a material close to the non-flow thresholds. This requires too much mixing power to mix a viscous material on the whole of the fermentation enclosure. This justifies dividing the fermenter into a stirring zone in order to be able to apply a lower stirring power to each sector of the fermenter.

In the prior art, the limits of the stirring of a thick material and the flows thereof, with a strong influence on the fermentation processes, are noted. Methods for mechanical agitation of the material with moving blades, worms, moving rolls or processes using compressed biogas to stir the material are known. Gas brewing has the advantages of being able to inject pressurized gas into different sectors of a fermentation enclosure subdivided into as many sectors as necessary in order to mix and homogenize large volumes of material. However homogenization by the injection of compressed gas successively relaxed in each sector has limitations.

Stirring with gas must be sufficiently powerful to stir a thick material in a volume and a determined sector. The main difficulty is to agitate a thick material from bottom to top over the height of the filler substrate and over a wide area of material. Indeed the peculiarity of a gas flow, necessarily from a low point, is to widen in its ascent and therefore leave the lowest part of the material to be stirred outside the stirring motion.

This generates unstirred zones in the lower part of the gas flows and gradually settlements of inert material. This phenomenon is more particularly present on the periphery of the fermenters because of the loss of load due to the friction of the material striking the walls. Thus, on the bottom of fermentors and on the walls, on a greater or lesser thickness, sediments of elements of higher density are encountered. The solid-liquid contact absolutely necessary for the bacterial activity is made difficult in the weakly brewed zones, which disturbs the exchanges and the bacterial balances.

Agitation has another impact that is often underestimated. It allows the desorption or degassing of certain metabolites such as carbon dioxide, ammonia and hydrogen sulphide.

By modifying the total pressure and the partial pressure of each gas component in the gas sky, the solubility of these elements is affected.

For example, by modifying the solubility of the carbon dioxide, the acid-base balance is modified.

The interest of sectorization for the biochemistry and fluid mechanics of fermenters is obvious.

The effect of temperature on the degradation kinetics of the material is well known. This temperature can be mesophilic, about 38 ° C, thermophilic, about 55 ° C, or hyperthermophilic, between 65 and 72 ° C for example. The higher the temperature, the faster the hydrolysis of the material. A first step in hyperthermophilic temperature promotes the solubility of the substrate. It is accompanied by a production of volatile fatty acids, which causes a decrease in pH. But it is incompatible with the activity of acetogenic and methanogenic bacteria. The production of methane is essentially at mesophilic or thermophilic type temperatures. In hyperthermophile there is eradication of the development of methanogenic bacteria. Hence the importance of compartmentalization of fermenters that can operate at different temperature regimes.

In a continuous fermentor and so-called dry process there are limits in the hydrolysis phase, phase in which the fermenting substrate is poorly hydrolyzed, which gives it a higher viscosity at a given solids content. This leads to limiting the diffusivity of the elements in a water-poor environment. The open water dissolves the nutrients and ensures their diffusion to the bacterial sites.

But we also know that a low dry matter content causes rapid phase separation. It is therefore necessary to agitate the medium almost permanently, which generates the quasi-constant degassing of the substrate, in particular the degassing of carbon dioxide, which participates in the process of producing the biogas by the hydrogenotrophic route.

To overcome these aspects, either the level of dry matter in the first compartment is reduced to levels largely below 20% dry matter, variable rate depending on the nature of the substrates, or two hydraulic phases are integrated, reducing the rate of dry matter in the first compartment well below the non-flow threshold, but above the fast phase separation threshold. The substrate which has hydrolyzed during the hydrolysis and acidogenesis phase is then reconcentrated.

The re concentration generates a liquid from acidogenesis which can be advantageously used to dilute the incoming substrate and reduce the pH of the acidogenesis phase.

We know that an acid pH around 5.5 in hyperthermophilic fermentation, see thermophilic favors the production of dihydrogen which can advantageously favor the hydrogenotrophic methanization if the gas is recycled in the process of methanogenesis. We also know that the viscosity of a substrate decreases with increasing temperature and decreasing pH.

The consideration of the partial pressure acting on the solubility of the gases is essential to control the equilibrium of the fermentation.

This balance depends in particular:

- Biochemical exchanges between the aqueous phase and the solid phase thanks to the free water allowing dissolution and solubilization of the metabolites introduced. The viscosity should not be too high so as not to reduce enzymatic and thermal diffusivity.

- The maintenance of a liquid solid homogeneous medium favorable to biochemical exchanges. Maintaining homogeneity requires a sufficiently high viscosity of the fermentation medium to minimize the rate of phase separation between the liquid phase and the solid phase.

- pH regulation in a range favorable to the methanogenic activity between 6.8 and 7.3 for example by the dissolution of carbon dioxide in the fermentation substrate. We know that the pH of the medium decreases by the increase of soluble carbon dioxide. As an indication, in water, when the partial pressure of carbon dioxide is doubled above water, there is a decrease in pH of the order of 0.3 units.

- The conversion of carbon dioxide, depending on the pH, the temperature and the partial pressure in carbonic acid, hydrogen carbonate and carbonate. The pH regulation should maintain the system in a pH range favorable to bacterial activity and avoid the carbonate form that precipitates.

- The evacuation of the mineralized nitrogen and salts produced. Their gradual concentration leads to an increase in pH, which is accompanied by the formation of ammonia at basic pH, a toxic element of methanogens.

- Piloting the transfer of substrate from one compartment to another compatible with the maintenance of biochemical equilibrium and the steps of anaerobic digestion

Ideally, a good continuous methanization system in thick or viscous material would be to have an independent fermenter for each of the steps that are hydrolysis, acidogenesis, acetogenesis and methanogenesis. This last step is advantageously controlled in two steps.

The tanks would be interconnected by a transfer system. Each tank could have a volume corresponding to the optimum hydraulic residence time which is a function of the substrate and biochemical equilibrium specific to each step.

This system must also make it possible to control the interactions between physical devices related to fluid mechanics and the rheology and biochemistry of fermentation inducing the selectivity and development of the most adapted bacterial populations.

We know the biogas injection technology in the bottom of tanks developed in the 80s by the company Valorga created by the inventor of the present patent application, technology today in the public domain.

For a 3000 m3 fermenter, for example, there are thus 12 agitating sectors agitated successively. The system consists of compressing the gas by a low-power compressor for eight minutes, for example biogas in a 20 m3 box, for example, several bars, 7 bars for example, to relax it intermittently in a few seconds from 7 to 5 bars. for example, 40 m3 relaxed in 8 seconds for example. This generates a very powerful vertical flow of biogas from bottom to top and a high gas injection rate of 5 m3 per second, for example in a stirring sector of an area of the order of 22 m2 receiving the gas. under pressure by thirty injectors distributed on the surface of the sector on the bottom of the tank This technology requires a particularly important civil engineering structure to access the gas pipes leading to the bottom of the fermentation tank. Galleries located under the fermenter are necessary to access the injectors that open at the bottom of the tank. These pressurized gas injection ducts are necessarily narrow to prevent backflow of material into the gas piping, resulting in a narrow, ascending gas jet, limiting the impact on the stirring of a thick material, which can to favor the sedimentation on the floor of heavy elements such as pebbles or glasses and to cause, depending on the quality of the substrates and in time, volumes of sedimented matter which hinder the good functioning of this type of system.

We know the limits of the ARKOMETHA system developed by ARKOLIA ENERGIES, a system created by the inventor of the present patent application. The material, after any dilution before entering the fermenter, is introduced by an introduction device such as worm into the fermenter to be mixed with the fermenting material in the first sector of the first compartment, operating at a temperature level preferably hyperthermophilic (65 ° C to 72 ° C). The mixing of a very thick material mixing with a material already present in the first part of the fermenter and having undergone thermo-enzymatic hydrolysis is useful to the process. The viscosity of the material changes under the influence of thermo-enzymatic hydrolysis (conjunction of temperature and enzymatic action of bacteria).

But a viscosity too close to the non-flow thresholds does not favor:

- The enzymatic diffusivity on the one hand and the production of volatile fatty acids which need a sufficient dilution liquid.

- Obtaining a homogeneous substrate at the exit of hydrolysis and acidogenesis.

- A homogeneous flow of the substrate during transfer to the acetogenesis compartment.

The mixing system ARKOMETHA system, close to the Valorga system, now in the public domain, is to be considered. It rests on chimneys spread over the whole fermenter. Several chimneys are activated at the same time and constitute a sector of agitation. Each compartment of the fermenter is composed of several areas of agitation. Compressed biogas is injected into the chimneys. Each chimney leaves above the ceiling to open out of the floor. The injection of pressurized gas opens near the floor to go up thanks to the density differential, thus creating a stirring movement of the substrate in powerful fermentation.

The height between the floor and the surface of the fermenting substrate is important depending on the size of the fermenters. This is at least 6 meters and most often greater than 8 meters. The gas bubbles leading to the floor are smaller because of the pressure. They widen during the ascent thus generating a much larger gas cone near the surface than at the bottom of the fermenter at the mouth of each chimney. The mouth of these small chimneys, 3 to 4 cm in diameter, for example, lets the pressurized gas which collapses on the floor to go up quickly in the mass in fermentation, thus creating a convective movement around the flow of ascension gas. The cone of gas widening on the floor remains limited.

Anchoring chimneys requires special devices. Chimneys from the roof and through the material require a network of piping gas distribution on the roof of the fermenter to bring the gas under pressure in the many chimneys. A chimney for 12 m2 of floor, of the order of 25 chimneys for a digester of 3000 m3 divided into 7 sectors of agitation for example. The distribution network of these chimneys and their anchorage is complex and expensive. The chimneys opening above the fermentation substrate create thermal bridges which make it more difficult to keep the fermenter temperature between 72 and 65 ° C (hyperthermophilic temperature) and 55 ° C (thermophilic temperature).

The reduction of carbon dioxide by dihydrogen through the activity of hydrogenophilic bacteria is not new in itself and even less inventive.

The inventive character lies in the judicious interactions of the system between the stirring of a substrate in viscous fermentation which is the object of bacterial support making it possible to avoid phase separation, the need to have a substrate depleted of biodegradable organic material to favor the Hydrogenophilic activity, the partial pressures of carbon dioxide necessary for its solubilization in the substrate under-saturation of carbon dioxide and the diffusion of dihydrogen by microporous ramps on the floor of the fermenter

We insist that the process concerns a viscous fermentation substrate. The solids content must be sufficient to avoid rapid phase separation between the so-called solid fibers and the liquid that remains necessary for the diffusivity and dissolution of the biodegradable elements.

The substrate to ferment and ferment viscous is transferred from one compartment to another. The transfers between these compartments must be controlled in order to maintain the biochemical conditions and the viscosity of the substrates necessary, in particular to maintain a pH of preferably less than 7.3.

Compartmentalization makes it possible to control the pH of the entire system. The dihydrogen necessary for the enrichment of the biogas is preferentially produced at a pH around 5.5 by the hyperthermophilic temperature in the compartment of hydrolysis and acidogenesis.

The reduction of carbon dioxide by dihydrogen can be effective only if the solubilization substrate is homogeneous and if the solubility differential between CO 2 and ΓΗ 2 is reduced by the set of pressures and by the biochemical form of the dependent carbon dioxide. in particular pH. Carbon dioxide is dissolved in various forms depending on the pH, temperature and partial pressure of it in the gas sky.

Hydrogen to be solubilized must be diffused into diffusion ramps that prevent the coalescence of the hydrogen and its evacuation into the gas sky.

It appears obvious that the system can not work if the convergence of the parameters of the substrate viscosity, the fermentation temperatures, the carbon dioxide partial pressure dependent pH to regulate the form of the dioxide is convergent. of dissolved carbon, especially in the form of carbonic acid and hydrogen carbonate and the solubilization and diffusivity of dihydrogen to reduce solubilized carbon dioxide.

Biogas enrichment technologies are known by the injection of dihydrogen in a two-stage technology to obtain biogas plus methane. The first reactor produces biogas, then the biogas produced is recycled to a second reactor in which dihydrogen is injected to reduce the carbon dioxide of the biogas to methane by promoting the activity of the hydrogenophilic bacteria. The hydraulic residence time is in the second reactor of the order of fifteen days with an organic load of 1 gram of volatile solid per liter per day. The organic load is low and the hydraulic residence time is high. Such methods are described in particular in the following scientific publications:

Biogas Upgrading via Hydrogenotropic Methanogenesis in Two Stage Continuous Stirred Tank Reactors at Mesophilic and Thermophilic Conditions. 30 Maria Bassini, Panagiotis G, Kougias, Laura Treu, and Irini Angelidaki. Danemarc.

• "Simultaneous Hydrogen Use and In Situ Biogas Upgrading in an Anaerobic Reactor". Posted on November 8, 201 1. Gian Luo, Sara Johanson, Kanokwan Boe, The Xie, Qi Zhou, Irini Angelidaki

The international application WO 2014/027165 describes a continuous methanization device in the dry process, comprising at least one compartment, in which at least one stack makes it possible to inject gas under pressure near the bottom of the compartment, in order to stir the material located in this compartment. This gas under pressure can in particular be biogas resulting from the fermentation of the material. The disadvantages of injecting gas into the compartment via a chimney have been presented previously.

French patent application FR 2 551 457 describes a fermenter making it possible to carry out a methanogenesis, comprising biogas outlets as well as reinjection ducts for said biogas in the fermentation tank. This reinjection is performed by a plurality of ducts which open into the bottom of the tank. These conduits consist of tubes provided longitudinally with a plurality of injection orifices. In the prior art search there are numerous patents relating to methanation, or electro-ethane reactors having an anode, a cathode and a plurality of methanogenic microorganisms, see in particular the international application WO2016071192.

There are also several patent documents in which, by the electrolysis of the water of the fermentation substrate in the reactor, is provided dihydrogen reducing the carbon dioxide by contacting with a species of microorganisms hydrogenotrophic methanogen to produce methane. International applications W0201 1000084, WO2015120983 and WO2014154250 relate to this process.

There are several patent documents which show a simple injection of dihydrogen into the methanation reactor to improve the methane level in the biogas production.

Other documents describe processes in which the biogas and dihydrogen are introduced into a dedicated bioreactor for this purpose, as for example in the international application WO2014187985.

The method and device improve the economics of gas methanation, particularly in the biochemical production of methane from carbon dioxide and dihydrogen. He understands :

- The preparation of a reactor filled with biofilm supports on the surface of which are immobilized micro-organisms producing methane.

- The runoff of a liquid to be treated on the biofilm supports. In this process, it is not viscous, but in liquid way.

- The introduction into the reactor of a gas mixture by choosing the mixing ratio so that the dissolution of the gaseous substrates in the liquid gives a stoichiometric ratio of gaseous substrates dissolved in the liquid treatment.

Carbon dioxide is highly soluble, unlike the poorly soluble dihydrogen. In the implementation of this process, the risk is great to create hydrogen bubbles that are evacuated into the gas sky. The dissolution of gases and the chemical form they take depends on the pH. PH control is necessary for acid-base balance and the avoidance of inhibitory elements. The difference in solubility means that the gas mixture will necessarily generate a significant dihydrogen deficiency.

The reduction of carbon dioxide through a support or bacterial bed causes an increase in pH. The regulation of the pH passes by the regulation of the solubilization of the carbon dioxide.

The method relates to biofilm supports made in the form of rotating disks and arranged in the direction of the longitudinal axis of the cylindrical zone of the reactor.

This system is only applicable on a liquid substrate slightly laden with degradable material. In the state of the description the operation of the system does not appear obvious. It does not integrate the solubility differential between carbon dioxide and dihydrogen. The solubility of the carbon dioxide introduced depends on its undersaturation in the substrate or fermentation support and the partial pressure of carbon dioxide in the gas sky to maintain equilibrium as the soluble carbon dioxide is reduced.

The substrate must also provide the nitrogen and trace elements necessary for the development and renewal of bacteria.

In the international application WO 2014/128300, the disclosed method is based on at least one bioreactor with a reaction vessel adapted to the growth, fermentation and culture of methanogenic microorganisms, a feed device for dihydrogen and dioxide of carbon and a device for measuring nitrogen in the liquid phase inside the reaction vessel. Nitrogen measurement is fundamental. But this process remains within the limits of the other processes mentioned.

Indeed none of the processes mentions the partial pressure ratio of carbon dioxide on the total pressure in the gas sky and the importance of pH regulation according to the fermentation step. In the first stage of fermentation, the physicochemical and thermodynamic conditions must be favorable for the production of dihydrogen, while in the other stages the support substrate for the reduction of carbon dioxide must have a pH close to neutrality. In the last compartment, most of the biodegradable organic matter has been reduced to methane, which tends to increase the pH by the disappearance of the biodegradable organic matter and the production of ammonium. The pH can reach values higher than eight, which leads to the production of NH3, a gas inhibiting the development of methanogenic bacteria.

In contrast, in the present invention, the pH is regulated by the solubility of CO 2 and its chemical conversion to carbon species by the action of CO 2 partial pressure. The regulation of the pH by the regulation of the CO 2 partial pressure is possible at each stage only thanks to compartmentalisation. The present invention responds to the complexity of the process by assembling means taking into consideration the interactions between the control of biochemistry and the fluid mechanics posing the necessary control of the viscosity of the mixtures and the phase separation of the constituent elements.

OBJECT OF THE INVENTION

The present invention aims to remedy all or part of the disadvantages of devices and methanization processes previously described in the prior art. The present invention relates to a continuous methanization process in a viscous manner in a fermenter divided into compartments or fermentation tanks with closed gas skies and variable pressure, characterized in that it comprises the steps of:

Introduction and transfer of a material to be fermented, said substrate, with a high viscosity comprising at least 12% of dry matter, in a first compartment maintained at a hyperthermophilic temperature, preferably between 65 ° and 72 ° C., then in several compartments maintained at a temperature preferably thermophilic,

Mixing the fermentation substrate in the stirring sectors of each compartment by compressed biogas injection expanded under branch plates fixed to the floor of each compartment,

Regulating the transfer of the fermentation substrate from one compartment to another by the suction created by the flow of gas under pressure on either side of the partition wall of the compartments, and

Regulating the pH of the first compartment for hydrolysis and acidogenesis to about 5.5, preferably hyperthermophilic temperature, to produce dihydrogen,

Adjusting the partial pressure of carbon dioxide in the closed gas sky of the other compartments to contain the pH, preferably below 7.3,

Recirculating biogas containing dihydrogen produced in the first compartments through microporous ramps located on the floor preferably in the last compartment, and

Optionally, introduction of exogenous dihydrogen into the last compartment gas sky by the microporous ramps located on the floor of the fermenter of the last compartment to produce at least 95% biomethane CH4.

According to one aspect of the invention, in the process used, the mixture of the viscous substrate is carried out by punctually and successively relaxing in each stirring sector a volume of gas under a determined pressure, depending on the measured viscosity of the substrate. , in several gas injectors under floor-integrated branch plates.

According to another aspect of the invention, the method comprises measuring the viscosity of the thick material in fermentation to adjust the flow rate of the stirring gas in the sectors of each compartment.

According to another aspect of the invention, the adjustment of the flow rates, the duration and the stirring frequency by the compressed gas expanded in each stirring sector is carried out as a function of the measured viscosity.

According to another implementation of the process of the invention, the measurement of the viscosity is carried out by measuring the flow rate of compressed gas in a caisson at a given volume and pressure pressure in a stirring sector over a given period of time, values of viscosity associated with the pressures, times and flow rates having been previously predetermined and calibrated by an industrial viscometer.

According to another implementation of the method of the invention, the expansion of the compressed biogas is carried out in a gas injector composed on the one hand of several tips at the end of pressurized gas supply pipes sufficiently narrow that the viscous substrate can not flow back into the gas pipe and secondly a loose gas bypass plate.

According to another implementation of the method of the invention, the recirculation of the gas is carried out by overpressure of the biogas containing hydrogen produced in the first compartments in at least the last compartment intended for hydromethanization. According to another implementation of the method of the invention, the diffusion of the biogas containing dihydrogen is carried out by microporous gas diffusion ramps, integrated in the floor of at least said last compartment to prevent the coalescence of evacuated gas bubbles. in the gas sky.

According to another implementation of the method of the invention, the regulation of the pH of the fermentation substrate in at least the last compartment is effected by adjusting the partial pressure of carbon dioxide in the gas sky.

According to another implementation of the method of the invention, the regulation of the partial pressure of carbon dioxide is carried out by adjusting the total pressure as a function of the carbon dioxide content and the gas volume of the gas by the means of valves with variable opening on each gas sky.

According to another embodiment of the invention, the method comprises the regulation of the acid-base balance by the point opening of the valve on the transfer line between the compartments during the agitation of the proximity sector generating the suction from one compartment to another and thus the flow and then the reflux of the substrate on the other side of the wall.

According to another implementation of the method of the invention, the regulation of a pH close to 5.5 in the 1 ° compartment is achieved by maintaining at a hyperthermophilic temperature, preferably between 65 and 70 ° C., by l. adaptation of the hydraulic residence time of the fermentation substrate between one and three days.

According to another implementation of the process of the invention, the recirculation of the biogas produced in the upstream compartments and containing dihydrogen is carried out in the last downstream compartment by means of microporous ramps located on the floor of said compartment to enrich the methane level. of biogas by the activity of hydrogenophilic bacteria.

According to another implementation of the method of the invention, the introduction of exogenous dihydrogen into the installation in the recirculated gas in the substrate of the last compartment is carried out by the microporous diffusion ramps. According to another implementation of the process of the invention, the calculation of the volume of dihydrogen to be introduced by the microporous ramps is carried out according to the equation of four moles of hydrogen per mole of carbon dioxide by measuring the volume of dioxide. solubilized carbon obtained by calculating the volume of carbon dioxide introduced and maintaining the necessary partial pressure of carbon dioxide.

According to another implementation of the method of the invention, the control to adjust the volume of hydrogen is achieved by integrating the continuous analyzes of the composition of the gas in the gas sky, the measurement of the total pressure in the sky of gas, the calculation of the volume of the gas sky, the volume of compressed gas in the gas sky integrating the total pressure and the calculation of the partial pressure of carbon dioxide calculated by the carbon dioxide content in the confined gas.

According to another implementation of the method of the invention, corrective actions to produce a biomethane concentrating in the gas sky with at least 95% methane content are achieved by adjusting the total pressure in the gas sky of the last compartment as a function of the partial pressure of carbon dioxide and the flow of exogenous dihydrogen introduced by the microporous ramps.

The present invention also relates to an anaerobic digestion device designed to implement the continuous methanization process in a viscous manner as described above, characterized in that it comprises a fermentor divided into compartments or fermentation tanks with gas skies. closed and with variable pressure.

In particular, this anaerobic digestion device is characterized in that it comprises microporous ramps integrated into the floor, said microporous ramps being intended to prevent the coalescence of the recirculated gases and the introduced exogenous dihydrogen.

DETAILED DESCRIPTION OF THE INVENTION

For this purpose the present invention aims, according to a first aspect, a methanization process continuously viscous in a fermentor subdivided into separate compartments gas sky, maintained at temperatures, preferably hyperthermophilic in the 1 ° compartment and thermophilic in the other compartments, to increase the kinetics of degradation and to produce a biogas enriched in methane, characterized in that it consists of means:

- Introduction of the material to be fermented, at a high viscosity, corresponding to at least 12% of dry matter in a first compartment of hydrolysis and acidogenesis, then a means for transferring the hydrolysed material in successive compartments reserved for acetogenesis and methanogenesis. - Successive stirring in each fermentation sector of the viscous substrate by pressurized biogas expanded in injectors integrated into the floor under gas branch plates.

- Adjustment of substrate transfer in fermentation between the compartments from upstream to downstream or in the opposite direction by the controlled suction of the substrate from one compartment to another by the agitation of the proximity sector.

- Adjustment of the injected biogas flow rate during each stirring in the sectors of each compartment according to the measured viscosity of the substrate in the compartment.

- Measurement of the viscosity in the different compartments by measuring the resistance to the passage of gas by measuring the rate of passage of the gas injected into a sector at a given initial and final pressure.

- Recirculation of the gas produced in the first compartments in the other compartments by diffusers integrated into the floor.

- Adjustment of the partial pressure of carbon dioxide in the closed gas skies of acetogenesis and methanogenesis compartments.

Thanks to these provisions, all of the viscous material in fermentation is brewed sector by sector, without there being sedimentation. Thanks to these provisions, the fermentation substrate is kept homogeneous.

Thanks to these provisions the stirring creates a lateral movement sweeping the floor and an upward and convective movement of the fermenting substrate in each sector. Thanks to these viscosity measurement arrangements, the dry matter content of the incoming substrate and the fermenting substrate can be adjusted.

Thanks to these arrangements, the transfer of the fermentation material and the acid-base balance in the compartments are managed by stirring near a transfer line.

Thanks to these provisions, the substrate is solubilized in the first compartment, an acidic fermentation and the lowering of the pH, the value of which will be between 4.5 and 6.

Thanks to these provisions, the production of dihydrogen in the gas is particularly favored in the first compartment for hydrolysis and acidogenesis and the first compartment for acetogenesis.

Thanks to these provisions, several phases of the fermentation are formed under thermodynamic conditions favoring the kinetics of degradation of the fermentation substrates.

Thanks to these dispositions the volume of fermenters is reduced. Thanks to these provisions, the methane content of the biogas produced by the recirculation of the biogas produced in diffusers and the adjustment of the partial pressure of carbon dioxide is increased.

Thanks to provisions, the acid-base balance of the fermentation substrate is controlled. Thanks to these provisions, the gas produced in the compartment for hydrolysis and acidogenesis and acetogenesis contains carbon dioxide and dihydrogen.

Thanks to these provisions for adjusting the partial pressure of carbon dioxide, the pH of the fermentation substrate is regulated, preferably between 6.8 and 7.3 by the solubilization of carbon dioxide in the last compartment intended for methanogenesis. hydrométhanisation.

Thanks to these provisions reintroduction of biogas product containing dihydrogen in the last compartment for hydromethanization increases the methane content of the biogas produced.

Thanks to all these provisions, the dry matter content of the substrates introduced into the fermentor, the kinetics and the rate of degradation of the material, the production and the quality of the biogas and the yield per ton of organic matter introduced are increased.

The present invention aims, according to a second aspect, a continuous methanization process in a viscous manner in a multi-compartment fermenter, characterized in that it consists of:

- Decrease of the viscosity by dilution of the fermentation substrate in the first compartment of hydrolysis and acidogenesis.

- Concentration of the fermentation substrate at the outlet of the hydrolysis compartment and acidogenesis before its introduction into the 2nd compartment to produce acidic juices.

- Recirculation of acidic juices to dilute the incoming substrate.

- Adaptation of acid juice production.

- Adaptation of the hydraulic residence time in the first compartment of hydrolysis and acidogenesis between 1, 5 and 3 days.

- Diffusion of the biogas product enriched in hydrogen in the first compartments in the last compartments by diffusers distributed on the floor.

- Adjustment of the partial pressure of carbon dioxide in closed gas skies, including methanogenesis compartments.

Thanks to these provisions, the stirring in the first compartment of a material not yet hydrolysed is made easier.

Thanks to these provisions the enzymatic diffusivity and the solubilization are favored. Thanks to these provisions the fermentation substrate is more easily and rapidly heated, especially at a hyperthermophilic temperature between 65 and 70 ° C to be more easily cooled to 55 ° C after the 1 ° compartment before being reconcentré and introduced into the other compartments at a thermophilic temperature.

By virtue of these arrangements, the substrate of the first compartment is diluted to a solids content of, for example, between 10% and 12% by acidic juices resulting from the phase separation at the outlet of the first compartment of hydrolysis and acidogenesis.

Thanks to these arrangements, the hydrolysed and fermented substrate is reconcentrated at the outlet of the first compartment at a solids content of between 18 and 25%, for example depending on the substrate, in order to reduce the separation rate between the solid phase and the liquid phase. Thanks to these provisions by reconcentrating the substrate before being introduced into the 2nd compartment, the volume of compartments reserved for acetogenesis and methanogenesis is reduced.

Thanks to these provisions, acidic juices are produced which are reintroduced into the 1st compartment.

Thanks to these arrangements the volume of acidic juice is regulated to obtain preferably a pH of the order of 5.5 in the first compartment by adjusting the dilution ratio and the dry matter content.

With these pH control arrangements around a value of 5.5 in the first compartment, the production of dihydrogen in the gases produced is maximized.

Thanks to the regulation provisions for the partial pressure of carbon dioxide in the last compartments, the pH around the neutrality (from 6.8 to 7.3) is controlled by the solubilization of the carbon dioxide in its acid form and hydrogen carbonate ..

Thanks to these provisions by promoting the decrease in pH to a value between 4.5 and 6, preferably around 5.5, at a hyperthermophilic temperature in the first compartment is produced dihydrogen in the gas from the fermentation.

Thanks to these provisions the gas produced in the compartment for hydrolysis and acidogenesis and acetogenesis preferentially contains carbon dioxide and dihydrogen.

Thanks to these provisions by reintroducing carbon dioxide and dihydrogen, the hydrogenophilic activity is thus promoted.

Thanks to these provisions, the solubilized carbon dioxide in the last compartments is reduced by the hydrogen produced and reintroduced to produce methane.

The present invention aims, according to a third aspect, a methanization process continuously viscous in a multi-compartmented fermenter in which is introduced into the last compartment for hydromethanization on the one hand biogas produced at a partial pressure of dioxide carbon-promoting a pH of less than 7.3 for example and exogenous dihydrogen, characterized in that it consists of means: - Introduction into the last compartment of both enriched biogas and produced in the other compartments (compartments 2, 3 and 4 of FIG. 1) and the exogenous dihydrogen in a ratio compatible with the hydrogenotrophic reaction. - Adjustment and maintenance of the pressure in the gas sky according to the calculated partial carbon dioxide pressure.

- Continuous calculation of the partial pressure of carbon dioxide.

- Adjustment of the partial pressure of carbon dioxide in the closed gas skies of the compartments of acetogenesis and methanogenesis (compartments 3 and 4 of the figure

1) to control the pH of the fermentation substrate to a value close to seven (7).

- Introduction of dihydrogen by microporous ramps at the floor of the last compartment for hydromethanization (compartment 5 of Figure 1).

- Gradual introduction of dihydrogen at a rate adjusted to the point of detection of trace of hydrogen in the gas sky.

- Adjustment of the partial pressure of carbon dioxide and the total pressure in the gas phase of the last compartment and the flow rate of hydrogen according to the rate of biomethane measured continuously.

Thanks to these provisions the pH is regulated by keeping it at a value between 6.8 and 7.3 by increasing the partial pressure of carbon dioxide.

Thanks to these provisions it promotes the solubilization of carbon dioxide in each compartment.

With these provisions, the pH is maintained in a range favorable to the activity of hydrogenotrophic bacteria, namely a pH of between 6.8 and 7.3.

Thanks to these provisions, a permanent undersaturation of carbon dioxide introduced continuously by its continuous reduction by the continuously diffused dihydrogen is created.

With these arrangements, methane is produced as the solubilized carbon dioxide is reduced by the hydrogen diffused by microporous ramps.

With these arrangements the flow rate of dihydrogen is adjusted to the rate of reduction of carbon dioxide to produce methane.

Thanks to these provisions, a biomethane with at least 95% methane is continuously produced.

CH4.

BRIEF DESCRIPTION OF THE FIGURES

Other advantages, aims and features of the present invention will be further described in the following description, made for explanatory and non-limiting purposes with reference to the accompanying drawings, in which:

Figure 1 shows schematically in section view from above, a first particular embodiment of the fermentation device object of the present invention, showing the path of the substrate and the mixing device and recirculation accompanying it.

FIG. 2 schematically represents, in longitudinal section, the first particular embodiment of the fermentation device which is the subject of the present invention, representing the path of the substrate and the stirring device of the fermentation substrate in the compartments.

Figure 3 schematically shows the general embodiment of the fermentation device object of the present invention, representing the path of the biogas produced and recirculation of biogas in the biogas diffusers for enriching the biogas methane.

Figure 4 shows schematically, in side section, a particular embodiment of the fermentation device object of the present invention, showing two biogas injection nozzles and the bypass plate thereof.

- Figure 5 shows schematically, a particular embodiment of the fermentation device object of the present invention, showing two biogas injection nozzles under two branch plates thereof.

FIG. 6 schematically represents, in lateral section, a pressurized gas injector, a gas bypass plate and the sketch of a convective motion of the substrate by the rise of the gas bubbles which enlarge during their ascent. FIG. 7 shows the pH regulating mechanism by adjusting the transfer of substrate by a transfer line between two compartments and the stirring system transferring the upstream upstream substrate. DESCRIPTION OF THE DEVICE FROM FIGURES

Figures 1 and 2

The device is composed of a closed cylindrical fermenter FIG. 1 (seen from above) or of parallelepipedal shape FIG. 2 (seen from the side), divided into compartments 2, 3, 4, 5 separated from top to bottom by walls 6, 7 , 8 for the parallelepipedal shape and 6, 7, 8 and 9 for the cylindrical shape, a floor 10 and a roof 1 1.

It receives the fermentation substrate via a pipe 12, the substrate is transferred to each compartment by simple gravity or by any means of transfer of a thick and viscous concrete pump type. The fermenting substrate successively passes from the first compartment 2, to a second compartment 3, then to a third 4 and a fourth 5 by a transfer line 13 Figure 1 and 2. The number of compartments is not limiting. Each compartment can be replaced by separate tanks connected to each other along the same path by a transfer pipe substrate from one tank to another.

Each fermentation compartment is filled with substrate to ferment to a high level 14 thus leaving a gaseous sky 15, 16, 17, 18 fill with gas to be evacuated Figure 2. Each day a quantity of material is introduced through a suitable pipe 12 in the compartment according to the expected residence time of the material over the entire volume of the tank. For example, the residence time can be 15 days. So we introduce every day a fifteenth of the volume of material present in the fermenter. Each day a corresponding amount of material is extracted through a pipe 21 provided with a valve (not shown).

The fermenting material moves daily depending on the material introduced to pass into the next compartment by simple gravity by a transfer pipe 13 having a sufficiently large diameter allowing a thick and viscous material to pass or by suction of the substrate through the agitation of proximity, (see description figure 7).

According to Figure 1, each transfer line is provided with a valve 22 to prevent the transfer of the substrate from one compartment to the other during each stirring. The power of the stirring system causes suction of the substrate of the adjacent compartments. With between 5 and 12 agitations per sector per sector depending on the compartments, a large flow would pass from one compartment to another which would not respect the forward march of the substrate and tend to that the compartmentalized reactor is closer to an infinitely mixed flow.

The transfer pipe can be replaced by an opening at the bottom of the partitions 6, 7, 8, Figure 2 can be closed by a self-closing door.

The displacement of a thick material, introduced into the fermenter, close to the non-flow thresholds is difficult. It is made possible by the hydrolytic and enzymatic activity and the solubilization of the biodegradable organic matter into volatile fatty acids.

Figures 2, 4, 5 and 6 schematize the stirring device of the substrate in particular inventive fermentation. It overcomes the problems encountered in continuous stirring systems of a highly viscous fermentation substrate.

In order for the fermentation substrate to move, the material must be stirred regularly to avoid phase separation between the liquid and the solid. In the case of phase separation, the liquid preferably transits, which rapidly causes blockages of the solid material in the fermenter which can no longer be transferred or extracted.

The brewing device is essential to ensure the flow of viscous material in fermentors of several hundred cubic meters, up to several thousand cubic meters.

The thick material is brewed regularly by the gas inlet under pressure.

The stirring device shown in FIGS. 1 and 2 is made of the pressurized box 31 connected to a pressurized gas distribution network 32 surrounding the fermenter and located in the lower part of the fermenter. The gas used is the biogas produced by the transiting fermenter 37 Figure 2. The biogas rotates in a loop, injection, recovery in the gasometer, compression, injection.

The device illustrated in FIGS. 1 and 2 implements a low power compressor 30 which sends the compressed gas into a pressurized gas box 31. The gas stored in the box of 10 m 3 for example is compressed over a period of several hours. minutes (10 minutes for example) at a pressure of 7 bars for example, or 70 m3 tablets. It is connected to the pressurized gas distribution network 32. This network surrounds the fermenter in the base portion and serves to supply pressurized gas to the biogas feed pipes 33 under the branch plates 34. The biogas at the branch plates is provided with a nozzle which allows the pressurized gas to pass under the branch plate of FIGS. 45 and 6 described hereinafter. Each pressurized gas supply pipe is provided with a controlled valve 40 FIG. 1 and 2 which, when it opens, allows the gas to escape. The volume of compressed gas in several minutes is released at each stirring in a few seconds, for example in 8 seconds.

The pressurized gas expands between 7 and 5 bar, for example at a high flow rate of 9000 m3 per hour under the branch plate 34 figure 2 and 7 figure 4 5 and 6 generating both a lateral and upward movement of the biogas and a convective motion of the substrate. During each stirring a valve 2 FIG 2 closing the agitated compartment located on each gas sky opens to let the gas escape through the pipe 36 which opens into a buffer gasometer 37 Figure 2 whose function is to cushion the massive arrival of biogas during each agitation. The valve 35 remains closed between each agitation.

The gas emerges at a high flow rate into one or more pressurized gas supply pipes 33 FIG. 1 provided at their end with an opening called an injection nozzle located under the branch plates 34 FIG. 1. The gas collides with a branch plate 34 in FIG. 2 shown in FIGS. 46 which generates both lateral and upward sweeping movement of the biogas creating a convective movement.

The initial volume of the chamber 30 and FIG. 1 is adapted to the desired gas flow rate. The pressure variations in the box to modulate the rate of agitation according to the viscosity and the volume of material to be stirred.

The periodic stirring system of the fermenting substrate to avoid phase separations is not to be confused with the gas recirculation system produced in Figure 3 described below.

The number of stirring sectors is not limited. They are determined by the simultaneous opening of several valves 40 1 and 2 on several pressurized gas supply pipes 33. This depends on the geometry, the size of the fermenter of the number of stirring sectors determining the number of ramps and pressurized gas injector opening at the same time. The biogas evacuation pipes have a sufficient diameter in order to limit the overpressure of biogas in the sky of each compartment of the fermenter. The biogas that has stirred the material and that has come out of the fermenter returns to the buffer gasometer 37 of sufficient size so that there is no overpressure in the gasometer.

The distribution network brings the compressed biogas to a starting pressure which determines the flow rate and this as a function of the viscosity of the material in fermentation. The starting pressure is strong in the 1 ^st hydrolysis compartment where the viscosity is very high and then decreases in each compartment, from sector to sector. The viscosity decreases as the organic matter is degraded and the dry matter content decreases. The viscosity of one compartment to another being decreasing, the pressure of expansion of the gas is decreasing from one compartment to the other in the direction of introduction 12 of the material until the extraction thereof Figures 1 and 2.

The agitation sectors are determined by the number of injectors or nozzles for injecting pressurized biogas under the bypass plates activated at the same time by the opening of a valve 40 FIGS. 1 and 2. The number depends on the surface area to be shaken. Most often two, three or four valves open at the same time to allow the passage of pressurized gas to the injection nozzles of pressurized gas. The number of injection nozzles in action depends on the volume and geometry of the fermenter. The flow rate of the expanded pressurized gas depends on the state of the fermentation material measured punctually by a viscometer.

The stirring rate of the stirring sectors of a viscous material depends on the viscosity, the rate of phase separation and the swelling of the gasification material depending on the nature of the fermenting substrate. For example, agitation zones must be stirred every two hours for the first compartment reserved for hydrolysis and acidogenesis, while the other compartments may be agitated every five or six hours.

Pressurization in the gas skies reduces the rate of phase separation and swelling and thus reduces the need for frequent agitation.

The pressurized gas diffusion pipes are fixed on the floor of the fermenter by flanges 3, 4 and 5 fixed on the floor of the fermenter by screws cast in the concrete of the raft. These pipes can also be introduced into ducts embedded in the floor at the design of the equipment.

The sheath receives one or more pipes 6 to supply pressurized gas which stops at the gas diffusion point 5 FIG.

FIG. 2 shows that each diffusion pipe is connected to the pressurized gas supply feeder 32, itself connected to the compressed biogas box 31. Each pipe is provided with a valve 40 which, when opened, leaves pass the pressurized gas that relaxes in the fermenter at a rate of several thousand cubic meters per hour. The head of the nozzle 5 is provided with an orifice FIG. 4 and 5 with a surface of the order 5 to 7 cm 2, for example arranged in such a way that the viscous material can not flow back into the pipe, or that the refluxed material is systematically evacuated during injections with high gas flow. The material being very viscous can only penetrate a short distance. When gas is injected, the force exerted by the compressed and relaxed gas returns the material to the fermenter.

The gas opening through the tips is upward. It leaves through the end 5 of the pipe 6 4 and 5 of injection of the gas under pressure. The gas collides with a branch plate 7. The branch plate 7 Figures 4, 5 and 6 generates a lateral movement of gas Figure 6 which sweeps the bottom to a diameter of 4 to 5 m for example. The width of the sweep depends on the start pressure of the expanded gas, the flow rate, the material height and the viscosity of the material. The differential density of the gas relative to the material causes an upward movement while creating gas bubbles becoming increasingly important as one approaches the surface. This upward force displaces the material that is driven by the movement of the gas while creating a strong suction of the material on the sides of the ascending gas column. Bubbles causing the matter to explode on the surface. This results in a strong convective movement of the material around the ascending gas column generated by the bypass plate 7 FIGS. 4, 5 and 6.

Alternatively each pressurized gas supply pipe may be replaced by two pipes of a section corresponding to the section of a single pipe to reduce the risk of reflux of the material into the pipe.

Recirculation of the gas.

The present invention is based on a device for recirculating the gas produced in the first compartments 2 and 3 in the last two or more compartments 4 and 5 to be enriched in methane. During hydrolysis and acidogenesis, especially at a hyperthermophilic temperature, the methane content of the gas produced is low, which results in a methane content in the biogas of 50 to 60% depending on the hydraulic residence time. In the device represented in FIG. 1, the gas produced in the first compartment 2 and the second compartment 3 for hydrolysis and acidogenesis and acetogenesis FIG. 3 is overpressed by a booster 30 FIG. 3 to be reintroduced by diffusion ramps. 31 opening on the floor in the third compartment 4 by orifices 51. The gas produced in the third compartment 4 is enriched in methane. It is recirculated and supercharged preferably in the fourth compartment 5. Although the orifices 51 are not shown in Figure 1, it is understood that according to the invention, these orifices 51 may also be present in the compartment 5. The gas produced in the last compartment is enriched in methane thanks to the hydrogenotrophic activity which generates the reduction of the carbon dioxide of the biogas by the hydrogen produced in the hydrolysis and acidogenesis compartment maintained at a temperature of preferably hyperthermophilic 65 at 72 ° C and at a pH of the order of 5.5 and in the second compartment in which the pH is low, of the order of 6.8 to 7.2 for example.

The gas sky of the compartments 3, 4 and 5 is maintained under pressure according to a set point, by a variable opening valve 32, FIG. 3 whose opening depends on the pressure setpoint so that the partial pressure of carbon dioxide in the gas sky generates the process of solubilizing carbon dioxide and lowering the pH.

In a conventional biogas process the pH increases as the biodegradable organic matter is reduced to biogas. This is accompanied by an increase in nitrogen in its ammonium form. The fermentation substrate is basified to reach a pH level in the last part of the process, around 8 and above. This slows the dynamics of reduction of volatile fatty acids not yet reduced. This slowing of the kinetics of degradation is most often compensated by high hydraulic residence times.

In the process according to the invention, the pH of the first compartment is maintained at a pH of the order of 5.5 by any means such as hyperthermophilic temperature and / or recirculation of more or less acidic juice. In the other compartments the pH is maintained in a range of preferably between 6.8 and 7.3 by regulating the partial pressure of carbon dioxide. Carbon dioxide solubilized by the partial pressure of carbon dioxide is reduced to methane by recirculation of the hydrogen produced in the gas of the first compartments, especially in the hydrolysis and acidogenesis step and contained in the recirculated gas.

At the same time, the kinetics of degradation are improved by maintaining the pH, preferably between 6.8 and 7.3 throughout the process. As a result, the reduction rate of the substrate is increased, thus producing more methane in the biogas in a reduced hydraulic residence time.

By regulating the pH and reducing the hydraulic residence time in the last compartment in which the biogas enriched at each stage is recirculated, the development of the hydrogenotrophic activity is promoted. Hydrogenophilic bacteria have growth and renewal rates of a few hours and less than twelve hours. A hydraulic residence time of less than about three days does not allow the renewal of the acetoclast bacteria.

In one embodiment the substrate ¹ compartment 2 is a hyperthermophilic temperature between 65 and 72 ° for example. This temperature level favors the destructuring of the cellulosic material and the acidogenic activity which lowers the pH to pH values between 5 and 6 depending on the nature of the substrate and the level of NH 4 ⁺ and salts in the medium. This drop in pH promotes the production of dihydrogen and a better balance acid-base in the next compartment. Moreover a temperature at 70 ° C can eradicate pathogens.

The first compartment 2 figures 2 and 3 is reserved for hydrolysis and more particularly for thermo-enzymatic hydrolysis 65 ° -72 ° C .; it is separated from the rest of the fermenter by the wall 6. The wall made of insulating material rises to the ceiling which limits the heat transfer with the second methanogenesis compartment 3 maintained at a thermophilic temperature at 55 ° C.

The substrate passes from the first compartment 2 to the second compartment 3 through the opening of a valve 22 Figure 1 and a pipe with a large section 13 double jacket in which the substrate is cooled. The coolant reduced to 62 ° C is used to heat the dilution liquid of the incoming substrate. The cooled substrate is introduced by a pump into the second compartment 3 at 55 ° C, which temporarily raises the temperature of the substrate between 57 ° C and 58 ° C. This is brought very quickly not simple thermal diffusion with the substrate in fermentation in the second compartment and between the uninsulated walls between the second 6, third 7 and fourth compartment 8 at 55 ° C Figure 1, 2 and 3. The walls outside are sufficiently insulated to limit thermal losses.

Heat exchangers are placed on the wall of the third and fourth compartments to compensate for heat losses through the walls and the water vapor discharged into the escaping gas for the stirring of the substrate and the production of biomethane. It should be noted that there is only one gas outlet 38 in FIG. 2 in the system according to the invention, which limits the losses by evacuation of the gas, in particular gas at 70 ° C. produced in the first compartment. The latter being recirculated by diffusers located on the floor thus warms the other compartments.

In a conventional anaerobic digestion system with increasing pH, the NH 4 + / NH 3 - equilibrium becomes more favorable to the more volatile NH 3 - form, which can generate a dynamics of inhibition of the bacterial flora resulting in a slowing down of the activity. On the other hand, the increase in pH causes losses of nitrogen in its gaseous NH 3 - form, especially at a thermophilic temperature.

Transfers from one compartment to another 2, 3, 4, and 5 are preferably carried out by external pipes 13 of a diameter large enough to allow the passage of a thick and viscous material FIGS. 1 and 2. diameter is of the order of 0.35 m. These pipes are provided with a guillotine valve Figure 3 opening during the loading of a new substrate, which allows the transfer by gravity of the compartment to the other. As already stated, in order to mix a mass of several tens or even hundreds of cubic meters per stirring sector, a stirring system, organized in agitation zones, is used which is very powerful by injecting gas under pressure by means of injectors located on the floor generating gas flows of several cubic meters per second. This system generates a very powerful movement of the fermentation substrate in the sector requested. The level of the substrate rises in the stressed area of one to two meters, which creates an increase in volume above the base level without agitation, of more than 15 m ³ for a stirring area of one hundred meters. cubes of substrate for example. This movement creates a powerful suction of substrate in the transfer line 7 from one compartment to the other, close to the agitated sector FIG. 7. The transfer line has a large diameter to allow a thick and viscous substrate to pass through. order of 35 to 40 cm in diameter, for example. At each agitation, if the transfer line is not closed, a large amount of substrate is transferred from one compartment to another.

Depending on the compartment and the sector, there may be between 4 and 12 agitations per day per sector from upstream to downstream. On a day is between 8 and 24 agitations on both sides of the partition walls of the compartments, which generates significant flows from one compartment to another.

The substrate transferred from one compartment to another is mixed with the substrate of the receiving compartment. The mixture will take several minutes before flowing back into the original compartment. Over time this phenomenon tends to mix compartments, which goes against the process of walking forward and control of the biochemical balance. This justifies that between each compartment there is a closed guillotine-type valve 22 FIG. 1 and 8 FIG. 7. These valves open only when the substrate is loaded into the first compartment and during the transfer by gravity. one compartment to another once a day. They also open, if necessary, according to the acid-base balance during agitation to transfer the substrate by suction from one compartment to the other in a direction from upstream to downstream or in the opposite direction of downstream upstream by suction during agitation.

The regulation of the transfer participates in the acid-base balance. For example, if a compartment has a pH that is too acidic, the substrate is sucked from the compartments further downstream to the upstream compartments. It is represented by FIG. 7. This represents the system of flow and reflux of the substrate between two compartments 1 and 2 FIG. 7 filled with fermentation substrate 3 and 4.

Figure 7 shows the flow of the substrate from downstream to upstream. On each compartment is shown the brewing system made of a gas branch plate 5 under which opens a pipe 6 to bring compressed compressed gas. The two compartments are connected by a transfer pipe 7 provided with a transfer valve 8. When the transfer valve is closed there is no flow between the two compartments. When there is a stirring of a sector close to the transfer wall 9 and 10 and the transfer valve is open there is a flow of material from the unstirred compartment to the stirred compartment. In an industrial system the walls 9 and 10 are only one wall. The transfer line 7 is adapted to the geometry of the fermenters and their compartmentalization. FIG. 7 represents a flow from the downstream compartment 2 towards the upstream compartment 1. This flow generates a temporary increase in the volume of substrate in the upstream compartment 2 which is accompanied by an increase in the high level of substrate 1 1 compared to the outside level. However, the level of substrate 13 in the downstream compartment 2 decreases temporarily. Then the substrate of the upstream compartment 1 reflux in the downstream compartment to find a new equilibrium 12 in the two compartments. The movement is reversed when activating the agitation of the proximity sector of the downstream compartment. The substrate coming from the upstream compartment 1 is sucked into the downstream compartment 2.

We know that the substrate in fermentation during the acidogenesis is rather acidic to become more and more basic at the exit of the fermenter as the process of methanisation evolves. This explains why when the transfer valve is closed, the upstream substrate is more acidic than the more basic downstream substrate. But with each stirring when the transfer valve is open the flow and reflux of the substrate change the pH of the substrate on either side of the wall between the compartments. When aspirating more basic substrate, the substrate mixes with a more acidic substrate, which slightly lowers its pH before it refluxes into the more basic substrate. Conversely, agitation near a transfer line in a more basic medium causes aspiration of a more acidic substrate which mixes with the more basic substrate before refluxing upstream in the acid substrate.

By renewing the substrate transfers several times a day, the biochemical equilibria are considerably modified.

We can see the whole point of the system which makes it possible to regulate the evolution of the pH, if necessary, by playing on the opening or the closing of the transfer valves outside the transfer once a day during the loading of new substrate.

This contributes to the means of regulating the acid-base equilibrium of a fermenter to maintain it in an optimum operating range. For example, if the substrate is too acidic, the system is operated in the direction of increasing the pH. If the substrate is too basic, the system is operated in the direction of decreasing pH. This system of regulation is useful but turns out, in certain situations, insufficient since the process of anaerobic digestion inexorably tends in its forward march to generate a basic pH. PH management through the management of the partial pressure of carbon dioxide remains one of the essential bases of innovation.

Introduction of exogenous dihydrogen.

The enrichment process of the methane level can be advantageously improved by the introduction of exogenous dihydrogen 52 by the microporous recirculation ramps The dihydrogen is produced by any method, for example by electrolysis of water.

The dihydrogen is introduced by microporous ramps to avoid the coalescence of hydrogen bubbles. This reduces the carbon dioxide recirculated and solubilized into methane, which keeps the carbon dioxide under-saturation. The less soluble methane rises in the gas sky. The supply of dihydrogen is adjusted to the rate of introduction of carbon dioxide biogas introduced and maintained at a partial pressure favorable for solubilization and the production of a methane at least 95% methane. The supply of dihydrogen is regulated by a gas analyzer 32 detecting any traces of dihydrogen in the gas sky 15 Figure 3 to adjust the flow so that there is more traces of hydrogen.

The present invention also relates to a device for continuous methanization in a viscous manner in a fermenter divided into closed gas and variable pressure gas compartments characterized in that it comprises means:

Introduction and transfer of a thick material to ferment said "substrate", high viscosity comprising at least 12% dry matter in a first compartment for hydrolysis and acidogenesis maintained at a hyperthermophilic temperature, preferably between 65 ° and 72 ° C, then in several compartments for acetogenesis and methanogenesis maintained at a thermophilic temperature, preferably about 55 ° C,

- regulating the pH of the first compartment to about 5.5 to produce dihydrogen.

The invention is characterized in particular by the use of bypass plates to improve the stirring of the substrate in fermentation.

Figures 2, 4, 5 and 6 schematize the stirring device of the substrate in particular inventive fermentation. It overcomes the problems encountered in continuous stirring systems of a highly viscous fermentation substrate, containing at least 12% dry matter. In order for the fermentation substrate to move, the material must be stirred regularly to avoid phase separation between the liquid and the solid. In the case of phase separation, the liquid preferably transits, which rapidly causes blockages of the solid material in the fermenter which can no longer be transferred or extracted.

In the device according to the invention, the thick material is brewed regularly by the inlet of gas under pressure. The gas opening through the tips is upward.

As shown in Figures 4, 5 and 6, the gas exits through a nozzle 5 of the pipe 6 for injection of gas under pressure. The gas collides with a branch plate 7. The branch plate 7 causes a lateral movement of the gas which sweeps the bottom to a diameter of 4 to 5 m for example. The width of the sweep depends on the start pressure of the expanded gas, the flow rate, the material height and the viscosity of the material. The differential density of the gas relative to the material causes an upward movement while creating gas bubbles becoming increasingly important as one approaches the surface. This upward force displaces the material that is driven by the movement of the gas while creating a strong suction of the material on the sides of the ascending gas column. Bubbles causing the matter to explode on the surface. This causes a powerful convective movement of the material around the ascending gas column generated by the branch plate 7.

According to a particular aspect, the device according to the invention further comprises means for recirculating the gas containing the hydrogen produced in the first compartments through microporous ramps located on the floor of a last compartment for hydromethanization.

According to another particular aspect, the device according to the invention further comprises means for adjusting the CO 2 partial pressure in the closed gas space of the compartment intended for hydromethanisation, in order to maintain the pH of said compartment between 6 , 8 and 7.5.

According to another particular aspect, the device according to the invention further comprises means for introducing exogenous dihydrogen into the gas sky of the last compartment intended for hydromethanization, said means comprising microporous ramps located on the floor of the last compartment to produce at least 95% biomethane CH4.

According to another particular aspect, the device according to the invention is characterized in that the means for mixing the fermenting substrate comprise a gas injector consisting on the one hand of several tips at the end of pressurized gas supply pipes sufficiently narrow so that the viscous substrate can not flow back into the gas pipe and secondly of a relaxed gas branch plate .

Preferably, said means for mixing the fermenting substrate comprise nozzles located under the branch plates.

According to one particular embodiment, the means for mixing the fermenting substrate comprise branch plates located between 5 and 20 centimeters above the floor of each compartment.

According to another particular aspect, the device according to the invention comprises a means for measuring the viscosity of the thick material in fermentation, making it possible to adjust the flow rate of the stirring gas in the sectors of each compartment.

Preferably, the viscosity measuring means is a means for measuring the flow rate of compressed gas in a box with a given volume and pressure in a stirring sector over a given period of time, the viscosity values associated with the pressures. , durations and flow rates having been previously predetermined and calibrated by an industrial viscometer.

According to another particular aspect, the device according to the invention comprises a means of regulating the acid-base balance in the compartments, consisting of valves closing the transfer lines between the compartments which can be opened during stirring, this generating suction from one compartment to another and thus the flow and then the reflux of the substrate on the other side of the wall.

The present invention also relates to a continuous methanization process in the viscous process, comprising the following steps:

a) Introduction into a fermentor divided into compartments closed gas skies and variable pressure of a thick material to ferment said "substrate", high viscosity comprising at least 12% dry matter;

b) hydrolysis and acidogenesis of the substrate at a hyperthermophilic temperature (preferably between 65 ° and 72 ° C) and at a pH of about 5.5;

c) Acetogenesis and Methanogenesis of the Substrate at a Thermophilic Temperature

(About 55 ° C.) and at a pH of between 6.8 and 7.5;

(d) recovery of the biogas thus produced;

characterized in that the mixing of the fermenting substrate in the stirring sectors of each compartment is effected by compressed compressed biogas injection under bypass plates fixed to the floor of each compartment.

According to a preferred embodiment, the process according to the invention further comprises a step of hydromethanisation by re-circulation of the biogas produced during the steps in a compartment for hydromethanization, via microporous ramps located on the floor of said compartment.

According to another specific implementation, the process according to the invention is characterized in that the partial pressure of carbon dioxide in the closed gas space of the compartment intended for hydromethanization is adjusted so as to maintain a pH in this compartment. between 6.8 and 7.5.

According to this implementation, the partial pressure of carbon dioxide can be regulated by the adjustment of the total pressure as a function of the carbon dioxide content and the volume of gas sky, by means of variable opening valves on each gas sky. According to another specific implementation, the process according to the invention is characterized in that the acid-base balance of the compartments is regulated by the opening of the valves closing the transfer lines between the compartments during agitation, this generating suction from one compartment to another and thus the flow and then the reflux of the substrate on the other side of the wall.

HYDROMETHANIZATION COMPARTMENT

The term "hydromethanization" refers to the chemical transformation represented by the following equation: CO2 + 4H 2 -> H 2 O + CH 4.

In the hydromethanization compartment, hydrogen H 2 is introduced in order to promote the conversion of C 2 carbon dioxide into CH 4 methane, which makes it possible to increase the proportion of CH 4 in the biogas produced in the fermentor according to the invention. The present invention relates to a continuous methanization device in a viscous manner in a fermenter divided into closed gas and variable pressure gas compartments characterized in that it comprises means:

Mixing the fermentation substrate in the agitation sectors of each compartment by compressed biogas injection,

Regulating the transfer of the fermentation substrate from one compartment to another by the suction created by the flow of gas under pressure on either side of the partition wall of the compartments, Regulating the pH of the first compartment to about 5.5 to produce dihydrogen, and

Recirculation of the gas containing dihydrogen produced in the first compartments through microporous ramps located on the floor of a final compartment for hydromethanization.

Preferably, this device further comprises means for adjusting the CO 2 partial pressure in the closed gas space of the compartment intended for hydromethanisation, in order to maintain the pH of said compartment between 6.8 and 7.5 . Even more preferably, this device further comprises means for introducing exogenous dihydrogen into the gas sky of the last compartment intended for hydromethanization, said means comprising microporous ramps located on the floor of the last compartment to produce biomethane at least 95% CH4. PH REGULATION

Means for maintaining a certain pH value in the different compartments are essential means for the implementation of the invention.

Indeed, in a conventional methanation process, the pH increases as the biodegradable organic matter is reduced to biogas. This is accompanied by an increase in nitrogen in its ammonium form. The fermentation substrate is basified to reach in the last part of the process a pH level most often greater than 8. It slows down the dynamics of reduction of volatile fatty acids not yet reduced. This slowing of the kinetics of degradation is most often compensated by high hydraulic residence times.

The present invention provides a device and a method in which the pH value is regulated:

a) in the compartment or compartments intended for the hydrolysis and acidogenesis step, so that it is between 4.5 and 6, preferably between 5 and 6, and

b) in compartments for acetogenesis and methanogenesis, so that the pH is between 6.8 and 7.5, and preferably between 6.8 and 7.3.

The means for obtaining this maintenance of the appropriate pH are described in detail in the present application.

In the device and method according to the invention, the pH is maintained in a range preferably between 6.8 and 7.3 in the compartments intended for acetogenesis and methanogenesis, by the solubilization of the carbon dioxide generated by a pressure partial carbon dioxide adaptation and constant, especially in the last two compartments. Part of the solubilized carbon dioxide due to the sufficient partial pressure of carbon dioxide is reduced in methane concomitantly with the generated hydrogen, especially in the hydrolysis and acidogenesis step and contained in the recirculated gas.

At the same time, the kinetics of degradation are improved by maintaining the pH, preferably between 6.8 and 7.3 throughout the process. As a result, the reduction rate of the substrate is increased, thus producing more methane in the biogas in a reduced hydraulic residence time. The volume of fermenters is smaller.

By regulating the pH and reducing the hydraulic residence time in the last compartment in which the biogas which is enriched at each stage is recirculated, the development of the hydrogenotrophic activity is promoted. Hydrogenophilic bacteria have growth and renewal rates of a few hours and less than twelve hours. A hydraulic residence time of less than three days does not allow the renewal of acetoclast bacteria whereas the hydrogenophilic bacteria have growth and renewal rates of a few hours.

Claims

1. Process for methanization continuously in a viscous manner in a fermentor divided into closed fermentation compartments or tanks for closed gas and at variable pressure, characterized in that it comprises the steps of:

Introduction and transfer of a material to be fermented, said substrate, with a high viscosity comprising at least 12% dry matter, in a first compartment maintained at a hyperthermophilic temperature, preferably between 65 ° and 72 ° C., then in several compartments maintained at a temperature preferably thermophilic, - mixing the fermenting substrate in the stirring sectors of each compartment by compressed biogas injection expanded under branch plates fixed to the floor of each compartment,

2. Method according to claim 1 wherein the mixture of the viscous substrate is carried out by punctually and successively relaxing in each stirring sector a volume of gas under a determined pressure, depending on the measured viscosity of the substrate, in several gas injectors. under branch plates in the floor.

3. Method according to claim 2 which comprises measuring the viscosity of the thick material in fermentation for adjusting the flow rate of the stirring gas in the sectors of each compartment.

4. Method according to claim 2 or 3 wherein the adjustment of the flow rates, the duration and the stirring frequency by the compressed gas expanded in each stirring sector is performed according to the measured viscosity.

5. The method of claim 4 wherein the measurement of the viscosity is performed by measuring the flow rate of compressed gas in a box volume and a given pressure expanded in a stirring sector over a given time, the viscosity values. associated pressures, times and flow rates that were previously predetermined and calibrated by an industrial viscometer.

6. Method according to one of claims 1 to 5 wherein the expansion of the compressed biogas is carried out in a gas injector composed on the one hand of several tips at the end of pressurized gas supply pipes sufficiently narrow to that the viscous substrate can not flow back into the gas pipe and secondly a relaxed gas bypass plate.

7. Method according to one of claims 1 to 6 wherein the recirculation of the gas is effected by overpressure of the biogas containing hydrogen produced in the first compartments in at least the last compartment for hydromethanization.

8. The method of claim 7 wherein the diffusion of the biogas containing dihydrogen is carried out by microporous gas diffusion ramps, integrated in the floor of at least said last compartment to prevent the coalescence of gas bubbles discharged into the sky. gas.

9. Method according to one of claims 1 to 8 wherein the regulation of the pH of the fermenting substrate in at least the last compartment, is effected by adjusting the partial pressure of carbon dioxide in the gas sky.

10. The method of claim 9 wherein the regulation of the partial pressure of carbon dioxide is effected by adjusting the total pressure as a function of the carbon dioxide level and the gas volume of the sky by means of valves. variable opening on each gas sky.

1 1. Process according to one of claims 1 to 10 which comprises the regulation of the acid-base balance by the punctiform opening of the valve on the transfer line between the compartments during agitation of the proximity sector generating suction from one compartment to another and thus the flow and then the reflux of the substrate on the other side of the wall.

12. Method according to one of claims 1 to 1 1 wherein the regulation of a pH close to 5.5 in the 1 ° compartment is achieved by maintaining a hyperthermophilic temperature, preferably between 65 and 70 ° C, by adapting the hydraulic residence time of the fermentation substrate between one and three days.

13. Method according to one of claims 1 to 12 wherein the recirculation of the biogas produced in the upstream compartments and containing dihydrogen is performed in the last downstream compartment by means of microporous ramps located on the floor of said compartment to enrich the rate of biogas methane by the activity of hydrogenophilic bacteria.

14. Method according to one of claims 1 to 13 wherein the introduction of exogenous dihydrogen to the installation in the recirculated gas in the substrate of the last compartment is performed by the microporous diffusion ramps.

15. The method of claim 14 wherein the calculation of the volume of dihydrogen to be introduced by the microporous ramps is carried out according to the equation of four moles of hydrogen per mole of carbon dioxide by measuring the volume of solubilized carbon dioxide obtained. by calculating the volume of carbon dioxide introduced and maintaining the required partial pressure of carbon dioxide.

16. Method according to one of claims 1 to 15 wherein the control to adjust the volume of hydrogen is achieved by integrating the continuous analysis of the gas composition in the gas sky, the measurement of the total pressure in the sky of gas, the calculation of the volume of the gas sky, the volume of compressed gas in the gas sky integrating the total pressure and the calculation of the partial pressure of carbon dioxide calculated by the carbon dioxide level in the confined gas.

17. Method according to one of claims 1 and 16 wherein corrective actions to produce a biomethane concentrating in the gas sky with at least 95% methane level are achieved by adjusting the total pressure in the gas sky of the last compartment depending on the partial pressure of carbon dioxide and the flow of exogenous dihydrogen introduced by the microporous ramps.

18. An anaerobic digestion device designed to implement the continuous methanization process in a viscous manner according to any one of claims 1 to 17, characterized in that it comprises a fermentor divided into compartments or fermentation tanks with gas skies. closed and with variable pressure.

19. A methanation device according to claim 18, characterized in that it comprises microporous ramps integrated in the floor, said microporous ramps being intended to prevent the coalescence of the recirculated gas and exogenous dihydrogen introduced.