FR3072667A1 - METHOD AND DEVICE FOR PROCESSING DIGESTAT OF METHANIZATION UNIT AND METHANIZATION UNIT COMPRISING SUCH A DEVICE - Google Patents

Abstract

The process for treating crude or liquid digestate resulting from a phase separation step leaving an anaerobic digestion unit comprises: a step of passage of air in or on the surface of said digestate contained in a tank to extract at least one ammonia to the atmosphere of the tank and - a step of passage of the atmosphere of the tank in a treatment system. In embodiments, the method comprises a step of reinjecting the digestate after passage of air, to the material preparation system. In embodiments, the method comprises, after the step of air passage in or on the surface of said digestate, a step of passing biogas into said tank to reduce the pH of the digestate.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and a device for treating raw digestate or liquid digestate resulting from a phase separation step coming from an anaerobic digestion unit and a methanization unit comprising such a device.

STATE OF THE ART

The methanation units currently known produce a large quantity of raw or liquid digestate resulting from a phase separation stage, the treatment of which is very expensive and polluting.

These anaerobic digestion units are most often faced with the following problems:

- progressive inhibition of anaerobic digestion caused by the recirculation of raw digestate or liquid digestate and

- conservation of nutrients, the main one being nitrogen.

To counteract these problems, the treatment of the raw or liquid digestate resulting from a phase separation stage is most often essential in order to limit its inhibitory nature, in particular due to the often too high concentrations of ammonium ions (NH4 ⁺ ). This recirculation must not impact the biological reactions which intervene in the methanisation process. The objective is also to extract the fertilizing compounds it contains in order to be able to make the most of them.

In addition, these anaerobic digestion units consume a lot of fresh water in order to dilute the inputs so that they reach a target dry matter content depending on the process used, which has a significant impact on the environment and the operating cost. of the methanization unit.

The digestate represents the digested material leaving the methanization unit. This organic product has a good fertilizing value. It undergoes, on most of the installations, a solid liquid separation by means of a screw press. The liquid fraction of the digestate represents the largest part of the volume and contains the majority of the mineral nitrogen (in the form of ammonium) serving as fertilizer. However, if it is spread, the use as it is of this liquid fraction causes a volatilization of part of this nitrogen, and specific techniques (landfilling) are required to minimize these losses, which generates an additional cost. In addition, in regions already experiencing an excess of nitrogen in the soil, the use of this product is made difficult.

The simplest solutions used to control the ammonia content consist of carrying out a treatment on the raw or liquid digestates. Despite their a priori simplicity, these solutions are not widely used in agricultural, territorial or industrial methanation.

Most of the techniques used apply to the liquid fraction of the digestate and come from the treatment of pig manure, a product comparable to the liquid digestate. The objective is to eliminate its nitrogen content, on the one hand to reduce the nuisance during spreading, and on the other hand to recover this nitrogen in the form of a fertilizer which can be used separately and in a controlled manner. Applied to anaerobic digestion these techniques hardly allow recirculation at the head of the process of the treated digestates.

In addition, it is often necessary to include a centrifugation step beforehand because the dry matter content at the start of treatment must be as low as possible. It is a separation / concentration type process, which also has its origins in the treatment of slurry or wastewater. This technique is quite expensive and generates high energy consumption (in electrical form).

The current methods which make it possible to greatly reduce the nitrogen content (more than 50%) are:

- membrane filtration by ultrafiltration / reverse osmosis,

- ammonia "stripping",

- evaporation / condensation,

- thermal drying,

- the precipitation of struvite (This approach consists in forcing the precipitation of an ammonium magnesium phosphate hexahydrate complex (MgNH4PO4.6H2O) called struvite, reference: S. Uludag-Demirer, GN Demirer, C. Frear, S. Chen, Anaerobic digestion of dairy manure with enhanced ammonia removal. Journal of environment Management 86 (2008) 193-200),

- composting and

- biological treatments.

These techniques are more or less easy to use, but most of them require a significant supply of thermal energy such as membrane filtration, stripping and biological treatment which show the greatest consumption of electricity. Only the struvite precipitation method involves a significant consumption of chemicals used in the process.

After treatment, four methods make it possible to conserve nitrogen well, namely membrane filtration, stripping with acid washing, evaporation and struvite precipitation.

The cost of the majority of these techniques makes them unprofitable on anaerobic digestion plants. For example, the operating cost (to remove ammonia and produce non-toxic dilution juices for anaerobic digestion) may turn out to be higher than the cost of purchasing potable water. The treatment techniques strongly impacting ΓΟΡΕΧ (operating expenses) then the CAPEX (investment expenses) of the facilities (due to the investments then significant energy and electrical costs).

In addition, the impact on the physicochemical nature of the solutions treated often makes them incompatible with a return to the methanization process. For example, stripping, as it is carried out today, requires the addition of base to increase the efficiency of ammonia elimination, due to the favorable impact of a basic pH. . However, these high pH values, obtained at the end of the treatment, prevent any return to the anaerobic digestion process.

None of the existing technologies makes it possible to treat the ammonia obtained in the anaerobic digestion process, where the technologies must be inexpensive and without impact on the nature of the raw or liquid digestates resulting from a phase separation step recirculated at the head of digester.

In addition, part of the ammonia is produced during the organic matter reduction stages. The biochemical composition of a protein can be written C53H6O28N12S3.

According to Buswell's equation, it is then possible to estimate the ammonia levels in the medium (Buswell, AM and HF Mueller (1952) Mechanism of methane fermentation. Industrial and Engineering Chemistry 44 (3): 550-552) .

CcHhOoNnSs + 1/4 (4c-h-2o + 3n + 2s) H ₂ O = 1/8 (4c + h-2o-3n-2s) CH ₄ +

1/8 (4c-h + 2o + 3n + 2s) C02 + nNFh + SH2S (A)

Depending on the physicochemical conditions of the environment, such as the hydrogen potential (acronym "pH") and the temperature in degrees Celsius, the ammonia is found in free form (NH3) or ionized (NH4 +). Each of these forms seems to have an inhibiting action on the methanisation process. This inhibition occurs at different stages of the reaction, and can affect one or more metabolic pathways. The mechanism of inhibition of the anaerobic digestion reaction chain remains poorly understood.

Many studies, such as those of Kayhanian in 1999 and Sprott and Patel in 1986, implicate NH3 in its free form, which by passive diffusion through the membranes, would cause a change in intracellular pH, inducing a significant energy need for cellular maintenance, accompanied by an intracellular depletion of ionized potassium. This inhibition would therefore be non-exclusive.

The study carried out by Shumei Gao in 2015 showed that the hydrolysis step of the methanization reaction was little impacted by a concentration of free NH3 with a concentration of less than 0.5 grams per liter (g / L). On the other hand, the methanogenesis stage seems to preferentially take the hydrogenotrophic pathway. This versatility has been demonstrated by the increased activity of cofactor 420, an electron transporter specific to hydrogenotrophic populations.

If the concentration of ionized ammonia increases above 3 g / L, ionized ammonia can become an inhibitor of the methanogenesis reaction. This is called ammonia (NH3) or alkalosis poisoning.

NH3 would inhibit acidogenic and acetogenic bacteria; hydrolysis products, such as amino acids and volatile fatty acids, accumulate which can lead to a drop in pH in the digester, or acidosis.

Inhibition can also occur in the methanogenesis pathway, which results in an accumulation of acetate and a decrease in pH.

In general, inhibition results in a more or less significant reduction in the amount of biogas produced.

On the other hand, ionized ammonia is at the origin of the formation of hydrogen carbonates, therefore of the buffering capacity of the anaerobic fermentation medium. At basic pH, the volatile fatty acids are preferably in their ionized form, thus attenuating their inhibitory effect. The diffusion of volatile fatty acids in the non-ionized form through cell membranes, induces a high concentration in the intracellular medium, thus creating an inhibition of microorganisms. Ammonia, below a concentration of 3 g / L, is therefore beneficial in this respect.

It is therefore essential to regulate the amount of ammonia present in the digester. It is classically accepted that the factors on which it is possible to act, to regulate the ammonia concentration, are:

- the nature of the substrates introduced:

- a material with a low carbon to nitrogen ratio, called "C / N >>, and / or a high level of organic nitrogen, called" N _or g >>, leads to a high level of ammonia in the reactor anaerobic digestion (or anaerobic digestion), due to normal degradation of this nitrogenous compound which is a conventional process of mineralization of the degraded material;

- a material already rich in ammonia also leads by additive action to a high rate of ammonia in the methanizer;

- diluting the methanizer with fresh water, which reduces the ammonia levels in the methanizer,

- the recirculation of liquids from anaerobic digestion, which dissolve a significant part of the ammonia, in fact, without specific treatment, this leads to an increase in the ammonia concentration by additive action.

- the average hydraulic residence time: the greater it is, the more complete mineralization of the organic nitrogen Norg is observed, thereby inducing an increase in the ammonia concentration in the methanizer.

- the temperature and pH pair: for example a temperature of 55 ° C and a pH of 8 favor the NH3 form, harmful to microorganisms, while a temperature of 37 ° C and a pH below 7.4 lead to NH4 form, less inhibitory.

The stability of an anaerobic digester therefore depends on controlling the ammonia concentration.

OBJECT OF THE INVENTION

The present invention aims to remedy all or part of these drawbacks.

To this end, the present invention relates, according to a first aspect, to a process for treating raw or liquid digestate resulting from a phase separation step, leaving a continuous methanization unit for thick or liquid routes, the unit anaerobic digestion comprising a material preparation system, which comprises:

a step of passing air into or on the surface of said digestate contained in a tank to extract at least ammonia towards the atmosphere of the tank and

- a stage of extraction of the ammonia contained in the atmosphere.

Thus, at the end of treatment, the treated digestate has a considerably reduced ammonium concentration, no longer posing a recirculation problem and therefore making it possible to limit the quantities of fresh water to be supplied in order to dilute the inputs.

On the other hand, the process makes it possible in certain cases to obtain an ammonium salt which is a fertilizer usable in agriculture. The creation of this byproduct stabilizes the nitrogen, which makes it less volatile and therefore reduces the risk of volatilization during spreading.

In addition, the regulations aim to introduce three new products into the mineral fertilizer standard NF U42-001-1, including ammonium sulfate and phosphate solutions. The proposed treatment process therefore makes it possible to create a new marketable product.

It should be noted that the ammonia treatment method which is the subject of the invention does not require the addition of reagents to the digestates to be treated. It eliminates the ammonia (without changing the pH when the steps are coupled), thus allowing significant recirculation on the anaerobic digestion processes.

In addition, the injection of biogas in the second stage is a real asset because, at the end of this stage, the gas has a high concentration of methane and, whatever the type of recovery of the biogas implemented, the best is to '' have the highest possible methane content.

In embodiments, the method includes a step of reinjecting the digestate after passage of air, to the system for preparing the material.

In embodiments, the method comprises, after the step of passing air into or on the surface of said digestate, a step of passing biogas through said digestate to reduce the pH of the digestate.

In embodiments, during the step of passing biogas into the digestate, a biogas is injected having a methane concentration between 45 and 70% and a carbon dioxide concentration between 30% and 55%.

In embodiments, the method comprises a step of reinjecting the digestate after passage of the biogas, to the system for preparing the material.

In embodiments, during the step of passing air into or at the surface of said digestate, the temperature of the digestate is between 25 ° C. and 85 ° C.

In embodiments, during the step of air passage over the surface of said digestate, the rate of renewal of the atmosphere in the tank is between 0.5 and 5 per hour.

In embodiments, during the step of passing air over the surface of said digestate, the ratio of the gas / liquid exchange surface to the volume of digestate to be treated is between 0.006 m ² per ml of digestate to treat and 0.017 m ² / mL of digestate to be treated.

In embodiments, during the step of passage through the digestate, the ratio of flow rate of injected air to volume of digestate to be treated is between 0.020 and 0.200 L of air / ml of digestate to be treated.

In embodiments, the method which is the subject of the invention further comprises the following steps:

- methanation reaction by fermentation of a substrate in at least one compartment, the reaction producing an atmosphere comprising at least methane and ammonia,

- extraction of the atmosphere from at least one compartment,

- washing of the atmosphere extracted by injection of a purification liquid,

- extraction of the washed gas, the ammonia concentration of which has decreased and

- injection of said gas into at least one compartment.

The present invention relates, according to a second aspect, to a device for treating raw or liquid digestate resulting from a phase separation step leaving a methanization unit, which comprises:

a means of air passage in or on the surface of said digestate contained in a tank to extract at least ammonia towards the atmosphere of the tank and

- a means of extracting ammonia from the atmosphere.

In embodiments, the device comprises means for reinjecting the digestate after passage of air, to the system for preparing the material.

In embodiments, the device comprises a means for passing biogas through said digestate to reduce the pH of the digestate, after the passage of air in or at the surface of said digestate.

The aims, advantages and particular characteristics of this device being similar to those of the process which is the subject of the present invention, they are not repeated here.

The present invention also aims to control the ammonia concentration in an atmosphere of a methanization compartment to improve the speed and the stability over time of a chemical reaction. The present invention also aims to orient the methanization reaction to obtain the desired form of ammonia.

To this end, in embodiments, the methanization device comprises:

a methanization reactor comprising at least one compartment into which a fermentation substrate is inserted, producing an atmosphere comprising at least methane and ammonia,

- a unit for extracting the atmosphere from at least one compartment, the unit comprising:

- a first duct for extracting the atmosphere from at least one compartment,

- a washing column into which the first conduit opens and which comprises at least one injector of a purification liquid,

- At least a second extraction conduit for the washed gas, the ammonia concentration of which has decreased and for injecting said gas into at least one compartment.

Thanks to these provisions, the balance of dissolved ammonia is regulated by simultaneous actions on:

- reactor temperature,

- the pH of the reactor and

- the partial pressure of ammonia gas in the atmosphere of the reactor.

In embodiments, the injection pipe has at least one gas injection orifice close to the bottom of a compartment.

These embodiments make it possible to inject the gas, the ammonia concentration of which is reduced at the bottom of the compartment, in order to stir the fermentation substrate and pass through this substrate according to a process called "bubbling".

In embodiments, the cleaning liquid comprises sulfuric or nitric or phosphoric acid.

The advantage of using acid is to transform the ammonia from the extracted atmosphere into ammonium salts of the ammonium sulfate type, which is a fertilizer, by reaction with sulfuric acid.

In embodiments, the methanization reactor comprises at least two compartments, each compartment comprising a gas injection duct for which the quantity of gas injected into each compartment is governed by a valve, each valve being controlled independently.

Thanks to these arrangements, the reactors comprising several compartments, each compartment performing a different stage of the methanization reaction. The temperatures and the pHs can be adapted in the different compartments as a function of these stages, the ammonia concentration of the atmosphere of at least one compartment being independently regulated.

In embodiments, the device includes a sensor for the rate of ammonia in the fermenting material and a control unit configured to trigger the operation of the extraction unit to reduce the concentration of ammonium ion when the concentration captured is higher 4 g / L of fermentation material.

In embodiments, the control unit is configured to trigger the operation of the extraction unit to reduce the concentration of ammonium ion when the concentration captured is greater than 3 g / L of fermented material.

In embodiments, the device comprises a plurality of fermentation compartments, the atmospheres of which are made independent by being separated by at least one partition.

In embodiments, the device includes a piston system for moving the fermenting material from one compartment to another.

In embodiments, the device comprises a material to be fermented, the dry matter content of which is greater than 15%.

In embodiments, the device comprises a material to be fermented, the dry matter content of which is greater than 20%.

In embodiments, the device comprises at least one chimney descending into the fermenting material for injecting the washed gas into a compartment from which the gas to be washed has been extracted by the extraction unit.

In embodiments, the device comprises means for pressurizing the gas to be injected through a chimney at a pressure between 5 and 8 bars.

In embodiments, the extraction unit is configured to extract part of the atmosphere into a compartment where the majority of acid-forming bacteria are found.

In embodiments, the extraction unit is configured to extract part of the atmosphere into a compartment where the majority of the acetogenic bacteria are found.

In embodiments, the extraction unit is configured to extract part of the atmosphere in a compartment where methanogenesis is carried out.

In embodiments, at least one compartment in which the extraction unit extracts part of the atmosphere has a temperature between 50 ° C and 60 ° C.

In embodiments, at least one compartment in which the extraction unit extracts part of the atmosphere has a hydrogen potential of between 7.5 and 8.7.

In embodiments, at least one compartment in which the extraction unit extracts part of the atmosphere has a hydrogen potential of between 7.9 and 8.7.

In embodiments, at least one compartment in which the extraction unit extracts part of the atmosphere has a methane content of between 52% and 65%.

These compartments particularly benefit from the regulation of the ammonia level.

BRIEF DESCRIPTION OF THE FIGURES

Other advantages, aims and characteristics of the invention will emerge from the following description of at least one particular embodiment of the method and of the device for generating a personalized and adapted program of physical and sports activities, with regard to attached drawings, in which:

FIG. 1 schematically represents a methanization unit of known type,

FIG. 2 schematically represents a particular embodiment of the device which is the subject of the invention,

FIG. 3 schematically represents a packed column operating against the current,

FIG. 4 schematically represents a functional diagram relating to a first step of the process which is the subject of the invention,

FIG. 5 represents, diagrammatically, a functional diagram relating to a second step of the process which is the subject of the invention,

FIG. 6 schematically represents a test reactor,

FIG. 7 represents experimental points tested with an injection of air into the substrate,

FIG. 8 represents experimental points tested with an air injection on the substrate surface,

FIG. 9 represents, in the form of a flow diagram, steps of a particular embodiment of the method which is the subject of the invention,

FIG. 10 schematically represents a particular embodiment of the device which is the subject of the present invention,

FIG. 11 represents, diagrammatically and in the form of a flowchart, a succession of particular steps of a method which is the subject of the present invention,

FIG. 12 schematically represents a methanization unit in the dry process,

- Figure 13 shows, schematically, a gas washer and

- Figure 14 shows a multi-compartment device with sensors and control means.

DESCRIPTION OF EXAMPLES OF EMBODIMENT OF THE INVENTION

Note that the figures are not to scale.

Some agricultural substrates have high nitrogen contents. However, highly nitrogenous substrates slow down or even inhibit the methanization process.

Among the methanisable deposits, there are several sources of nitrogen:

- mineral nitrogen, preferably from urine (mainly present in manure and slurry) and

- organic nitrogen from proteins contained in organic matter.

A significant part of the ammonia is produced during the stages of reduction of organic matter. Depending on the physicochemical conditions of the environment (pH and Temperature), the ammonia is found in free form (NH3) or ionized, ammonium ion (NH4 ⁺ ). Each of these forms has an inhibiting action on the methanization process; however, the NH3 free form is known to be the most harmful. This inhibition occurs at different stages of the reaction, and can affect one or more metabolic pathways. If the concentration of ammonium ions increases above 3 g / L, it can become an inhibitor of the methanogenesis reaction. We then speak of poisoning with ammonia (NH3) or alkalosis.

The inhibition then results in a more or less significant reduction in the amount of biogas produced.

It is therefore essential to regulate the amount of ammonia (NH3 or NH4 ^{+ form} ) present in the digester. It is classically accepted that the factors on which it is possible to act, to regulate the ammonia concentration, are:

- the nature of the substrates introduced:

- dilution of the inputs or the methanizer, with fresh water, which makes it possible to reduce the level of ammonia in the methanizer. At the start of anaerobic digestion, the inputs are diluted in order to reach an optimal dry matter rate depending on the technology chosen (liquid or thick route). One of the objectives of the present invention is to use as little fresh water as possible, by recirculating as much as possible the juices produced after separation of the phases of the raw digestate at the outlet of methanization.

- the recirculation of process juices, which dissolve a significant part of the ammonia. Without specific treatment, this however leads to an increase in the concentration of ammonia by additive action.

- the average hydraulic residence time: the greater it is, the more a complete mineralization of the organic nitrogen Norg is observed, thereby inducing an increase in the concentration of ammonia in the methanizer and

- the temperature and pH pair: for example, a temperature of 55 ° C and a pH of 8 favor the NH3 form, harmful to microorganisms, while a temperature of 37 ° C and a pH below 7.4 lead to NH4 ⁺ form, less inhibitory.

The stability of an anaerobic digester therefore depends on controlling the concentration of ammonia.

The present invention aims to:

- be able to use substrates with an imbalance in total nitrogen. This would open up the current market to many substrates / wastes that are currently little used in these processes (because they cause ammonia inhibition). This would also reduce (or even eliminate) the use of dilution water used in current processes, greatly minimizing "aqueous" discharges from the methanizer outlet.

Three applications are identified:

- small-scale individual farming, where the approach of German manufacturers is the most convincing (we limit ourselves to the supply of equipment and the management of works, which makes it possible to offer low prices),

- the medium-sized individual farmer (often led by "agro managers") and the small collective and

- collective territorial projects and anaerobic digestion of household waste and sludge.

The elimination and control of the ammonia proposed here concerns the crude digestates and liquid digestates resulting from a phase separation stage, used to dilute the substrates used.

External treatment of ammonia on raw and liquid digestates from a phase separation stage.

FIG. 1 represents a methanization unit 20 of known type which may include one or more anaerobic digestion tanks.

An input preparation system 21 receives fresh water through an inlet 22. The inputs are introduced into the methanizer 23 which may include one or more anaerobic digestion tanks, which supplies biogas to a recovery unit 24. The digestate in outlet of the methanizer, which may include one or more tanks, is separated by a separator 26 into a solid part, which is stored in a storage 25, and a liquid part, or "juice". This liquid part is distributed between a storage 27 of liquid digestate and a processing unit 28, the outlet 29 of which is reinjected into the material preparation system 21.

The process which is the subject of the invention aims to reduce, even eliminate, the ammonia contained in the raw or liquid digestates resulting from a phase separation stage while being able to create a value-added product according to the following stages:

The first step, illustrated in Figure 2, consists of:

a) shift the NH4 ⁺ (aq) θ Nhhg balance in favor of ammonia (gaseous form) thanks to optimal operating conditions (temperature and pH);

b) inject air into or on the digestate surface (raw or liquid) and

c) treat the atmosphere resulting from the passage of air in contact with the digestate.

FIG. 2 shows a methanizer which may include one or more anaerobic digestion tanks 31, a separator 32 leading to a storage 34 for solid digestate and an outlet 43 for liquid digestate. This liquid part is distributed between a storage 33 of liquid digestate and a treatment unit 30. This treatment unit 30 comprises a tank 36 which receives the liquid digestate and air from a pressurized air inlet 37, through, depending on the variants:

Choice n ° 1: of a gas distributor 38 at the bottom of the tank 36 and / or

Choice n ° 2: of an air inlet 44 in the atmosphere 39, at the top of the tank 36.

In the three variants (choice 1, choice 2, choice 1 and 2), the tank 36 has a gas outlet pipe 40, essentially air charged with ammonia, and a liquid phase outlet which can lead to the second step of the process illustrated in FIG. 4 and / or of the system for preparing the inputs 42 at the top of the methanizer. Line 40 leads the gas to a treatment system 41.

The atmosphere treatment technique used may include at least one of the following (non-exhaustive list):

Physico-chemical absorption techniques:

- Washes,

- Columns with packing,

- Spray reactors,

- Sprayer.

Biological absorption techniques:

- Treatment by bio filter.

Among these different treatment techniques, the most used technologies are packed column contactors. Note that a pre-treatment of the gases is often necessary before introducing them into these contactors. A packed column 50 (FIG. 3) consists of the following elements:

- a packing support grid 54 enabling the weight of the packing 53 and the liquid retained on this packing to be supported and ensuring distribution of the liquids at the bottom of the column,

- a gas inlet 55.

- a liquid distribution system 52, with redistribution if necessary. The liquid can be, in this case of application, a water solution containing sulfuric, phosphoric or nitric acid.

- a demister 58 for removing drops of liquids which could be entrained by the gas at outlet 59.

Operation is generally against the current: the gas is rising and the washing liquid flows by gravity onto the lining. The choice of packing, an essential element of this type of contactor, is dictated by the contact surface offered between the gas and the liquid used, the calculation of the pressure drops and its price. The linings can be of various shapes (rings, saddles ...), of different materials (ceramic, glass, metal ...) and be stored or arranged in bulk. FIG. 3 represents a packed column 50 operating against the current.

The effectiveness of a gas treatment by an absorption process depends on the nature of the compounds, the washing solution used, the type of contactor selected and the operating conditions.

The main advantages of absorption:

- Adaptable to many industrial sectors, notably the chemical, parachemical and pharmaceutical industries, rendering, urban water treatment plants, organic fertilizer manufacturing, etc.

- Good abatement efficiency (> 95%) over a wide range of flow rates, concentrations and compounds. The cleaning performance by washing very often exceeds 95%, reaching even more than 98% in many cases. In addition, it should be noted that the field of application of absorption processes is very wide.

- Adaptable to high concentrations of pollutants and to sudden load variations.

- Flexibility and regularity of treatment

- Immediate process start-up and restart.

It is noted that the device which is the subject of the invention may require a thermal contribution in order to heat the liquid digestates in order to be placed in the best possible operating conditions.

The injection of air into or on the surface of the liquid digestate causes an increase in pH induced by the desorption of the initially dissolved carbon dioxide. Unable to recirculate excessively basic digestates which could inhibit the first stages of anaerobic digestion, the pH of the digestates is lowered during the second stage, before the juice can be used as a diluent towards a system for preparing the material.

The second optional step, illustrated in Figure 4, consists of:

d) injecting biogas with methane concentrations between 45 and 70% and carbon dioxide between 30% and 55% and

e) the return of the digestates treated in the material preparation system, at the head of the methanizer, in order to dilute the inputs.

FIG. 4 shows a methanization unit 60 comprising a methanizer 61 which may include one or more anaerobic digestion tanks, a pipeline 62 carrying biogas containing concentrations of methane between 45 and 70% and carbon dioxide between 30% and 55%. Line 62 injects this biogas through a gas distributor 63 at the bottom of the tank, which opens into a tank 64 comprising the digestate from the first step (Figure 3). The tank 64 has an upper outlet 65 for biogas containing methane concentrations between 70 and 90% and carbon dioxide between 10% and 30%. The tank 64 also has an outlet 66 for the return of the treated digestate to the material preparation system. This second step dissolves the CO2 contained in the biogas, which causes a drop in pH. The outgoing biogas therefore has a low carbon dioxide concentration but a large proportion of methane.

To illustrate the advantages of implementing an ammonia treatment process and device, several simulations have been carried out using sizing tools. The type of project taken in this example corresponds to a 250 kWe / d installation treating a mixture composed of bovine manure (60%), grass silage (30%) and cereal waste (10%) (Percentages mass).

In the table below, scenario 1 corresponds to the basic operating mode, of a unit operating with the addition of water and concentration of NH4 ⁺ in the methanizer of 2.75 g.L ' ¹ . As shown in scenarios 2 and 3, the increase in dryness at the start of digestion induces a significant increase in the level of NH4 ⁺ in the medium, close to the inhibition thresholds. Remember that the dryness is the mass percentage of dry matter. Thus a sludge with a dryness of 10% has a humidity of 90%, by mass.

Scenario 7 corresponds to a favorable functioning found between the input dryness, the recirculation rate and the ammonia treatment rate. Indeed with the operating parameters of scenario 7, the annual water supply becomes zero, the rate of NH4 ⁺ at the end of digestion is acceptable (2.63 g.L ' ¹ ). The impact on tank volumes is improved by around 30% for the methanizer and by 50% for the press juice storage tank.

Scenario 1 2 3 4 5 6 7 Input dryness (% DM) 20 25 30 25 25 25 30 Recirculation rate of digestate (%) 0 0 0 65 65 65 27 NH3 evaporation (%) 0 0 0 0 15 25 35 NH4 output (g / L) 2.75 3.40 4.03 4.52 3.25 2.32 2.63 Water (m ³ / day) 13.0 6.2 1.7 0.0 0.0 0.0 0.0 Methanizer tank (m ³ ) 1669 1316 1081 2054 2054 2054 1200 Storage tank (m ³ ) 4357 3142 2332 1991 1991 1991 2017

Reducing anaerobic digestion and storage volumes can lead to a 15% reduction in the investment to be made to build a methanation unit.

Optimizing the treatment of ammonia on the crude or liquid digestates resulting from a phase separation step comprises optimizing the treatment of ammonia on the crude or liquid digestates resulting from a phase separation step, according to the characteristics of the digestates, and to optimize the quantities of air and biogas injected, with study of the operating conditions.

Depending on the dryness of the material present in the methanizer, the treatment can be applied directly to the raw digestate, as in device 70 illustrated in FIG. 5.

FIG. 5 shows the methanization unit 70 comprising the methanizer 71 which may include one or more anaerobic digestion tanks, the raw digestate of which is conveyed, by a pipe 72 to a treatment unit identical to that illustrated in FIG. 2 , even if the values of its operating parameters may be different.

The dry matter content of the material to be treated can reach up to 17%.

Tests have already been carried out on the step of injecting air into or on the surface of the digestates in order to remove the ammonia present in the digestates, which also results in a desorption of the dissolved CO2.

Regarding the injection of air into the digestates (bubbling):

Tests were carried out in a glass reactor 80 with a double jacket, as illustrated in FIG. 6, representing the geometry of an industrial tank which can be heated. A quantity of liquid digestate 83 is introduced into the tank 82 of the reactor 80. The air flow entering via an inlet 85 is adjusted as a function of the volume of liquid digestate to be treated in order to obtain the Q / V ratio (flow of air injected on the volume of digestate to be treated) desired. The temperature in the double jacket is followed throughout the tests by a temperature probe 89 as well as the pH of the liquid digestates, by a pH probe 88. Different samples are taken during the 32 hours of test by a sampling point of liquid digestate 86. A water inlet 81 and a water outlet 87 allow a circulation of heating water from the tank 82. Finally, a gas outlet 84 allows the evacuation of the gases to be treated.

The conditions tested are:

- treatment temperature ranging from 25 to 55 ° C and

- Q / V ratio (flow of air injected on volume of digestate to be treated) of 0.006 to 0.135 Lair / mL of liquid digestate.

Experimental points 91 implemented are represented in the graph 90 of FIG. 7. It is observed, in FIG. 7, that the intervals of temperature values and of Q / V ratio are:

-25à60 ° C

- 0.020 to 0.200 L of air / mL of digestate to be treated.

Concerning the air injection on the surface of the digestates (sweeping):

Tests have been carried out on two cylindrical reactors of different capacities and internal diameters comprising a double jacket in which it is possible to circulate a fluid keeping the tank at constant temperature.

A quantity of digestate is introduced into the reactor vessel in order to obtain an S / V ratio:

- on the one hand between section S of the reactor

- and on the other hand the volume V of digestate

The small reactor allows to test a small section and different S / V depending on the filling of the tank. The large reactor makes it possible to test a large section and different S / Vs depending on the filling of the tank.

The incoming air flow is regulated according to the volume of gaseous sky, which gives a rate of renewal of this gaseous sky per hour.

The temperature in the double jacket is followed throughout the tests by a temperature probe as well as the pH of the digestates, by a pH probe. Different samples are taken during the 32 hours of testing by a digestate sampling point. A water inlet and a water outlet allow circulation of heating water from the tank. Finally, a gas outlet allows the evacuation of the gases to be treated.

The conditions tested are:

- Temperature: 55 ° C

- Renewal of the tank atmosphere: 5 per hour

- Reactor section: min = 80cm ² max = 320cm ²

- S / V ratio: min = 0.006 m ² / mL of digestate to be treated and max = 0.017 m ² / mL of digestate to be treated

Experimental points 96 used are represented in graph 95 of FIG. 8.

The ranges of renewal rate and Q / V ratio temperature values for this treatment are:

- Temperature: 25 ° C - 85 ° C

- Renewal of the tank atmosphere: 0.5-5 times per hour

- Q / V ratio: 0.006 - 0.017 m ² / mL digestate to be treated

We observe, in FIG. 9, a process for reducing, or even eliminating, the ammonia contained in the crude or liquid digestates resulting from a stage of phase separation of an anaerobic digestion unit which can comprise one or more anaerobic digestion tanks. by creating a value-added product, depending on the type of air treatment chosen. This process includes:

A / A first phase 100 comprising:

a) a step 101 of establishing operating conditions in temperature and in pH, favorable to the shift of the NH4 ⁺ (aq) θ NH3 (g) balance in favor of ammonia (gaseous form);

b) a step 102 of injecting air into or at the surface of the digestates by bubbling or scanning the gaseous sky and

c) a step 103 of upgrading the ammonia by an atmospheric treatment system.

B / A second step 104, comprising:

d) a step 105 of injecting biogas into the digestates, with a methane concentration between 45 and 70% and a carbon dioxide concentration between 30% and 55% and

e) a step 16 of returning the treated digestates to the material preparation system.

Embodiments are described below which add, to the method and the device described above, regulation of the level of ammonia in at least one compartment of a methanization reactor. These embodiments combine the advantages of treating ammonia in raw or liquid digestates and a more efficient and faster operation of the methanization reactor.

FIG. 10, which is not to scale, shows a schematic view of an embodiment of the device 110, which comprises:

a methanization reactor comprising a compartment 111 in which there is a substrate 112 in fermentation, producing an atmosphere 113 comprising at least methane and ammonia,

- a unit for extracting the atmosphere from compartment 111, the unit comprising:

- a first extraction duct 114 from the atmosphere of the compartment

111,

a washing column 115 into which the first conduit 114 opens and which comprises at least one injector 116 of a purification liquid,

- At least one second extraction duct 117 for the washed gas, the ammonia concentration of which has decreased and for injecting said gas into the compartment 111.

Substrate 112 contains bacteria for the fermentation process. The temperature, the hydrogen potential and the residence time of the substrate in the compartment are controlled and are maintained within a tolerance range defined by predetermined limit values. Such a control orients the methanization reaction to form ammonia in NH3 form. Control is carried out in each compartment of the reactor, in particular in the hydrolysis compartment.

The compartment 111 air extraction unit is configured to regulate the ammonia concentration in the atmosphere of each compartment. The regulation can be carried out by a servo-control of a valve on the extraction duct 114 and 117 as a function of the ammonia concentration. This concentration can either be estimated by calculation performed on the basis of temperature and pH conditions (according to the acid-base equilibria described below), or measured directly in the liquid or the gas. The control can be carried out by a control unit. The control can also depend on the methane concentration ratio (CHU) on the ammonia concentration (NH3) which must be kept close to a predetermined limit value. The methane concentration ratio (CHU) on the ammonia concentration (NH3) can be replaced by the carbon concentration ratio (C) on the nitrogen concentration (N).

Preferably, when the methanization reactor comprises at least two compartments 111, each compartment 111 comprises a gas injection pipe 117 for which the quantity of gas injected into each compartment 111 is governed by a valve, each valve being controlled independently. The control of each valve can correspond to the embodiment described above.

In preferred embodiments, the pressure in the extraction ducts 114 and 117 is equal to the pressure of the atmosphere 113 of the compartment 111. Alternatively, the pressure in at least one extraction duct, 114 and / or 117 is lower than the pressure of the atmosphere 113 of the compartment 111. Where, the pressure in at least one extraction duct 114 and / or 117 is higher than the pressure of the atmosphere 113 of the compartment 111.

The pressure in the washing column can also be modified such that the gases in the washing column are compressed relative to the atmosphere 113 of compartment 111. These embodiments make it possible to at least partially remove carbon dioxide (CO2 ) in the atmosphere of compartment 111. In particular, the elimination of carbon dioxide is improved when the purification liquid is water. Such removal of carbon dioxide is a method of regulating the hydrogen potential in compartment 111. Therefore in these embodiments, the ammonia concentration and the carbon dioxide concentration are regulated simultaneously.

In embodiments, the temperature of the scrubbing liquid is optimized to wash the gas more efficiently.

In embodiments, the injection conduit 117 has an injection orifice 118 for the gas close to the bottom 119 of the compartment 111 and therefore into the fermentation substrate 112. The injection of the gas close to the bottom 119 of the compartment 111 allows stir the substrate with a gas so the ammonia concentration is reduced.

In embodiments, the injection pipe 117 has an orifice 118 in the atmosphere 113 of the compartment 111. The embodiments of gas injection can be combined and the washing unit can comprise at least two pipes injection port 117, one having an injection port in the atmosphere of compartment 111 and the other having an injection port close to the bottom 119 of compartment 111.

In embodiments, the scrubbing liquid includes sulfuric, nitric or phosphoric acid to transform the washed ammonia into fertilizer.

Concerning ammonia, the ionic equilibrium in solution in the reactor is governed by the equation:

NH ₃ d + H2O NH4OH * 5 NH ₄ ⁺ + OH- (B)

For example, by placing it at 55 ° C and at a pH greater than 7.5, the balance is oriented towards the dissolved NH3 form (NH3d) in the reactor. The dissolved form is inhibitory above a certain concentration threshold in the liquid.

It is therefore advisable to pass the ammonia from the liquid to the gas, according to the balance:

NH3d NH _3g (C)

According to Henry's law, the maximum concentration of a gas in solution, in equilibrium with an atmosphere containing this gas, is proportional to the partial pressure of this gas at this point:

^ ΛΉ3εϊ ⁼ PlOiSg ^X (0) with

C ^s NH3d: maximum concentration (at saturation) of ammonia in the liquid pNH3g: partial pressure of ammonia in the gas

Hnh3: Henry's constant for ammonia.

According to equation D, it is therefore necessary to reduce the partial pressure of ammonia in the atmosphere 113 of compartment 111 of the methanization reactor. This operation is carried out by recirculating the biogas produced in compartment 111 of the methanisation reactor, with elimination of the ammonia NH3g in the recirculation loop.

Part of the biogas produced and making up the atmosphere 113 in compartment 111 is treated in the washing unit which is configured to remove NH3g. Then the biogas is reinjected with a reduced concentration and NH3 in compartment 111. This operation has the effect of reducing the amount of NH3 in dissolved form by lowering the partial pressure in the atmosphere 113 of compartment 111, NH3 takes a form gas. The atmosphere 113 then enriched in NH3g is again treated in the washing unit, and the cycle is repeated until the desired concentrations of ammonia are obtained in the substrate 112 and in the atmosphere 113.

FIG. 11 schematically represents a methanization process 120, comprising the following steps:

reaction 121 of methanization by fermentation of a substrate 112 in a compartment 111, the reaction producing an atmosphere 113 comprising at least methane and ammonia,

- extraction 122 from atmosphere 113 of compartment 111,

- washing 123 of the atmosphere extracted by injection of a purification liquid,

extraction 124 of the washed gas, the ammonia concentration of which has decreased and

- injection 125 of said gas into compartment 111.

The process for removing ammonia NH3g from the atmosphere 113 of compartment 111 is detailed below.

The washing column 115 performs the washing step 123, for example with a purification liquid comprising sulfuric acid. The NH3g is transformed into ammonium sulphate, which is a fertilizer, then the ammonium sulphate is recovered at the bottom of column 115. The gas purified from ammonia is recovered at the head of the washing column before being reinjected into compartment 111.

We observe, in FIG. 13, a washer comprising, from upstream to downstream of the displacement of the gas to be washed:

- an entrance 141; and

- a washing column drum 142 comprising o a lining 143, o sprinklers 144 and o an outlet 145.

Against gas, a liquid is pumped by a main pump 146 at the base of the washing column 142 and injected into the sprinklers 144. A secondary pump 148 pumps acid into a tank 147 and adds it to the circulating liquid .

An outlet 149 at the bottom of the washing column 142 makes it possible to recover the liquid in a compartment 150. This liquid in the compartment 150 can in particular be used as fertilizer.

The washing column 142 here takes the form of a packed column, which generally comprises the following elements:

- A packing support grid 152 for supporting the weight of the packing and the liquid retained thereon and ensuring distribution of the liquids at the bottom of the column.

- a demister (not shown) for removing drops of liquids which could be entrained by the gas.

The operation here is against the current: the gas is rising and the washing liquid flows by gravity onto the lining. The choice of packing, an essential element of this type of washer, is dictated by the contact surface offered between the gas and the liquid used, the calculation of the pressure drops and its price. The linings can be of various shapes (rings, saddles ...), of different materials (ceramic, glass, metal ...) and be stored or arranged in bulk. In the column illustrated in FIG. 13, the atmosphere sampled in the methanization unit enters through a lower inlet and goes up the barrel of washing column 142 to the outlet of washed gas 145, after having successively passed through the support grid and lining 143. In the opposite direction, the washing solution enters the washing column barrel 142 through an upper inlet, passes through the lining 143 and the support grid before being evacuated via a valve. (not shown), to a recirculation or compartment outlet.

Depending on the conditions of use of the washing column 115, all or part of the CO2 present in the atmosphere 113 extracted can be eliminated. Operation with only water, or under pressure, results in a depletion of the atmosphere in CO2.

Preferably, according to the equations of chemical and thermodynamic equilibria described above, the process for removing ammonia is carried out under the following conditions:

the temperature in reactor 112 is between 50 ° C and 60 ° C and

- the hydrogen potential in reactor 112 is between 7.5 and 8.5.

Usually, the combination of these operating conditions is avoided in anaerobic digestion processes, in particular operating points with a temperature above 55 ° C and hydrogen potential greater than 8, since these operating points are likely to be strongly inhibitors of anaerobic digestion process.

The process 120, for in situ removal of ammonia makes it possible to work with such temperatures and such hydrogen potentials, making it possible to maximize the kinetics of the methanization process.

The residence times of the substrate in a compartment with the method 120 are:

for a reactor operating in piston or semi-piston mode, that is to say for a continuous methanisation process, the residence time of the substrate in the hydrolysis compartment is between 0.5 and 5 days and the residence time of the substrate in the digestion compartment is between 5 and 25 days; and

- For a reactor operating in infinitely mixed, the residence time of the substrate in the digestion compartment is between 30 and 60 days.

In a reactor operating in piston or semi-piston mode, the acid-base balance of the methanisation process 120 is controlled in real time by a servo control (as described above), by acting on a specific zone within the 'flow. Partitioning of the substrate 112 and / or of the atmosphere 113, along the flow of the piston, makes it possible to act in particular independently either on the hydrolysis compartment, or on another compartment downstream of the hydrolysis. Control of the acid-base balance can also be optimized by eliminating part of the carbon dioxide from the atmosphere.

The present invention can be implemented in a thick methanization unit, for example as described in international application PCT / FR2013 / 051938 published under the reference WO 2014/027165, incorporated here by reference, or in a unit for more classic two-phase anaerobic digestion in a liquid process known as "infinitely mixed".

In FIG. 12, a thick methanization unit 130 is observed, of which only two compartments 131 and 132, separated by a partition 140, are shown and only a chimney, respectively 134 and 135, is located in each compartment.

Although this embodiment has chimneys descending from the ceiling of the compartment, it can also be implemented with nozzles or gas injection pipes starting from a side wall or from the floor of a compartment.

A gas inlet 133 distributes gas, via valves 136 and 137 independently controlled, in the chimneys 134 and 135. The injection of gas under pressure causes the gas bubbles 138 to rise, which entrains the material into a convection movement, for example by following the path indicated by the arrows 139. A gas outlet 151 allows the sampling of parts of the atmospheres of the different compartments.

In the thick methanization unit, the size of the installations required most of the time requires the lowest possible methanizer volume, implying on the one hand a high dryness (greater than 15% and preferably at 17%) and, on the other hand, a thermophilic operating temperature (about 55 ° C). These operating conditions (high concentrations and high temperature) make the bacterial flora more fragile vis-à-vis ammonia, a compound inherent in the anaerobic digestion process. This greatly limits the use of nitrogenous substrates / waste, and consequently the development of the sector.

The two-phase liquid way in infinitely mixed processes are low dryness processes (less than 10%). These processes naturally require a significant recirculation of the juices at the outlet of the methanizer, to minimize the quantities of dilution water and the aqueous discharges at the outlet of the methanization process. These processes are the most used in agricultural environment. However, in order to be able to use highly nitrogenous substrates (such as fattening manures for example), it is necessary to introduce carbon-based co-substrates, and to minimize the recirculation of the juices from the process, thereby increasing the amounts of water for dilution and aqueous waste.

We present here an industrial solution for ammonia management on these two types of process.

The concentration is measured in the liquid part of the fermenting material: the rate of 3 g / L of ammonia is a value below which there are few inhibition problems. Beyond this, the risks are increased, but it is nevertheless possible to operate at higher values. Preferably, the slaving of the ammonia level aims to keep it below 4 g / L and, even more preferably, below 3 g / L.

The device described here makes it possible to regulate the amount of ammonia (NH3 or NH4 ^{+ form} ) present in the digester. The stability of an anaerobic digester depends on controlling the concentration of ammonia.

The device therefore makes it possible to use substrates having an imbalance in total nitrogen, under the conditions of a thick process (high dryness and high temperature). This allows, on the one hand, the development of this type of process and, on the other hand, to open the current market to many substrates / wastes that are currently little used in these processes (because they lead to 'ammonia). This also reduces (or even eliminates) the use of dilution water used in current processes, greatly minimizing "aqueous" discharges from the methanizer outlet. The two aforementioned advantages are likely to favor the development of thick methanization processes.

The device also makes it possible to control the rate of fertilizers (in particular nitrogen) when the product (digestate or press juice) is used in agricultural spreading.

The device improves the quality of the product from the methanizer in order to widen the destinations of outgoing products as commercial fertilizers.

The thick methanization process, for example as described in international application PCT / FR2013 / 051938 is characterized by:

- A compartmentalized fermenter allowing a separation of the different phases of anaerobic digestion (in order, hydrolysis and acidogenesis, acetogenesis and methanogenesis). The fermenter, subdivided into agitation sectors, thus corresponds to as many “independent reactors” as there are agitation sectors, each receiving a fermented substrate coming from the preceding sector in an appropriate inoculum. This cascade of reactors makes it possible both to optimize the reaction speed of the bacteria and to precisely control the entire fermenter.

- A stirring system made of chimneys starting from the roof of the fermenter up to the floor of the latter and letting biogas pass under pressure, at a high and variable flow rate, depending on the viscosity of the medium. This introduction of gas is carried out sequentially for a few seconds by generating a sweep of the bottom followed by a convective movement necessary for the homogenization of the substrate in fermentation. The chimneys are distributed over the entire fermenter and thus determine agitation sectors. The mixing of the material by the injection of biogas under pressure via the chimneys allows operation with a high rate of dry matter (between 20% and 30%) depending on the substrate and does not leave dead volumes. The dry matter content is an unreliable indicator of viscosity. The more inert the substrate contains (wood, glass, stones, plastics), the higher the permissible dry matter content. Each agitation sector is equipped with two to three chimneys which receive expanded biogas at a pressure between 5 and 8 bars generating a significant flow over a few seconds of the order of 8000 m ³ .h ' ¹ .

- By a kinetics of rapid degradation improved by thermoenzymatic hydrolysis and the piston path of the fermentation substrate making it possible to reduce the hydraulic residence time and therefore the volume of the fermenters.

- By controlling the acid-base balance made possible both by the thermodynamic conditions of the compartments and the sectoral development of the substrate.

The management of the concentration of ammonia in the fermenter can be envisaged by action on identified sectors (last sectors of the piston reactor, in particular). The acid-base balance can then be better controlled along the piston flow, thereby optimizing space for biogas production.

The ionic equilibrium in solution in the reactor is governed by the equation:

NH ₃ d + H ₂ O NH4OH * 5 NH ₄ ⁺ + OH

It is therefore possible to control the balance of dissolved ammonia (NH3d) by simultaneous actions on:

The methanizer temperature

The pH of the methanizer

The partial pressure of ammonia gas in the sky of the methanizer

However, on the thick process, the pH changes from 6.5 (first compartment) to 8.5 (last piston sector). The compartmentalization of the piston flow inside the methanizer also leads to a sectorization of the gaseous sky: thus, the gaseous skies of the last sectors are "rich" in ammonia concentration (temperature of 55 ° C and pH between 8 and 8, 5).

The temperature in these latter compartments is preferably between 50 ° C and 60 ° C. The hydrogen potential in these other compartments is preferably between 7.9 to 8.7.

Indeed, under these conditions, the balance is oriented towards the NH3d form. This is a huge difference compared to other methanisation processes, where the gaseous sky is uniform in composition because it is not compartmentalized.

Thus, on these latter sectors of the piston reactor, it is then possible to pass the ammonia from the liquid fraction to the gas fraction, according to the equilibrium:

NHad NH ₃ g

According to Henry’s law mentioned above, it is therefore necessary to reduce the partial pressure of ammonia in the gaseous sky of the methanizer. On the thick process, this operation can easily be carried out by recirculating the biogas used during the agitation sequences.

The device makes it possible to remove the ammonia NH ₃ g in the recirculation loop, as described with reference to FIGS. 10 and 13.

Part of the biogas produced is treated in a specific NH ₃ g elimination unit, then is reinjected without NH ₃ into the identified sector (s). This operation has the effect of stripping the NH ₃ d (NH ₃ d -> NH ₃ g) by lowering the partial pressure in the sky of the methanizer. The biogas enriched in NH ₃ g is again treated in the specific NH ₃ g elimination unit, and the cycle is repeated until the desired concentrations of ammonia in the liquid and in the gas are obtained.

The operating principle of the process for removing Nhhg ammonia from biogas is as follows. The gas / liquid absorption unit, shown in Figure 13, consists of an acid washing tower with the presence of packing. The biogas charged with Nhhg is sent against the flow of an acid flow in order to promote an acid-base reaction and its conversion into ammonium salt. For example, the use of sulfuric acid (H2SO4) as an eluent is of agronomic interest for operators of units since the product of its reaction leads to a solution of ammonium sulphate (fertilizer). The recovery of this charged solution is carried out at the bottom of the column. The biogas thus purified from the ammoniacal fraction is recovered at the head of the washing column before being reused in the methanization process.

Many academic works have already described this process of elimination of ammonia, works such as:

- Walker, M., lyer, K., Heaven, S., and Banks, C. J. (2011). “Ammonia removal in anaerobic digestion by biogas stripping: An evaluation of process alternatives using a first order rate model based on experimental findings”. Chemical Engineering Journal. 178 (15), 138-145; and

- F. Abouelenien, W, Fujiwara, Y. Namba, M. Kosseva N Nishio, Improved methan fermentation of chicken manure via ammonia removal by biogas recycle, Bioresour. Technol. 101 (2010) 6368-6373.

However, current methods are not suitable for carrying out this in situ treatment of ammonia. Indeed, due on the one hand to an absence of compartmentalization of the gaseous skies, and on the other hand to a homogeneous composition of the gas (induced by an operation in perfectly agitated reactor), the volumes of biogas to be treated are many too important.

The thick channel process illustrated in FIG. 12, in this area, has many advantages:

at. The piston operation induces a sectorization in which the NH3d concentrations are different (the pH only exceeds 7.8 on the second part of the reactor)

b. The gaseous skies are sectorized, the piloting being carried out by the management of methane and CO2 titles in the various sectors. The inventor has already observed that the NH3 concentrations in the gas, on the industrial pilot and in the last sectors of the reactor, reach values of between 0.5 and 1% (per 100 ppm in conventional processes). The thick process therefore naturally concentrates ammonia on fairly low volumes of biogas.

In the hydrolysis compartment (1 ^st sector of the piston reactor), the methane content is preferably between 30% and 45%. This titer gradually increases in the following sectors of the piston reactor. In the last sector, the methane titer is in the following range: between 52% and 65%. The CO2 titles are included in additional intervals: between 70% and 55% for the hydrolysis compartment, and between 35% to 48% in the last sector of the piston reactor.

vs. The partial ammonia pressure in the gas head can easily be controlled. Indeed, gas is natively reinjected into the methanizer, to perform the mixing and sectoring functions. It is therefore not necessary to add a source of gas injection, as may be the case with the other mechanically agitated processes.

The thick process is therefore the preferred process in which this elimination can be carried out. The robustness of the operation of this thick track process can thus be greatly improved, giving it an advantage compared to other thick track technologies. Today recognized as a mature industrial process, the thick process becomes a process presenting a real technological breakthrough in anaerobic digestion. The device described here makes it possible to rediscover the robustness of a mesophilic process, while remaining on a thermophilic temperature level.

In the case of an infinitely mixed liquid anaerobic digestion unit, the operation is similar to what is described in the present description: the biogas is injected into the liquid of the digester (for example the company's gas mixing process Degremont (registered trademark), on sludge treatment digesters from urban water treatment) and can be treated in a gas washing column, as described above, before being reinjected into the digestion liquid.

FIG. 14 shows a schematic view of a particular embodiment of the methanisation device 160, which includes:

a methanization reactor comprising, for example, three compartments 174, 175 and 176 in which there is a fermentation substrate, producing an atmosphere comprising at least methane and ammonia,

- a unit for extracting the atmosphere from each compartment, the unit comprising:

- the first extraction ducts 141 and 168, 169, 170, respectively, from the atmosphere of the compartments 174, 175 and 176,

- a washing column (see FIG. 13) into which the first conduit 141 opens and which comprises at least one injector of a purification liquid,

- second extraction ducts 145 of the washed gas, the ammonia concentration of which has decreased and of injection 171, 172 and 173 of the said gas into the compartments 174, 175 and 176, respectively.

The substrate, or fermenting material, circulates successively from compartment 174 to compartment 175 then to compartment 176, for example under the action of pistons (not shown). For example, in order, within compartment 174 there is hydrolysis and acidogenesis, in compartment 175, acetogenesis and, in compartment 176, methanogenesis.

The compartment 174, upstream, is associated with a pump or a valve 165 which receives gas from the outlet 145 of the washing column 142 and which injects gas into the second injection conduit 171 which opens into the fermentation substrate. . The compartment 174 is associated with the first extraction duct 168 provided with a pump or a valve 162 which delivers gas from the atmosphere of the compartment 174 at the inlet 141 of the washing column 142.

The intermediate compartment 175 is associated with a pump or a valve 166 which receives gas from the outlet 145 of the washing column 142 and which injects gas into the second injection pipe 172 which opens into the fermentation substrate. The compartment 175 is associated with the first extraction duct 169 provided with a pump or a valve 163 which delivers gas from the atmosphere of the compartment 175 at the inlet 141 of the washing column 142.

The downstream compartment 176 is associated with a pump or a valve 167 which receives gas from the outlet 145 of the washing column 142 and which injects gas into the second injection conduit 173 which opens into the fermentation substrate. The compartment 176 is associated with the first extraction duct 170 provided with a pump or a valve 164 which delivers gas from the atmosphere of the compartment 176 at the inlet 141 of the washing column 142.

Each of the compartments 174, 175 and 176 comprises sensors, physical or software, of concentration of ammonia in the substrate 178, of temperature 179, of pH 180 and of pressure of the atmosphere 177. A software sensor, or “observer of 'state >> is a program which estimates a physical quantity without directly measuring it. It uses information from other physical sensors for this. A state observer is an extension of a model represented as a state representation. When the state of a system is not measurable, an observer is designed to reconstruct the state from a model of the dynamic system and measurements of other quantities.

The atmospheres of compartments 174, 175 and 176 are independent, for example because they are in different tanks or separated by partitions, as illustrated in Figure 12.

The second injection pipes are shown here as crossing the floor of the compartments. Alternatively, they pass through the side walls or the ceiling of the compartment, as illustrated in Figures 10 and 12.

A control unit 161, taking for example the form of a server receives the signals from the sensors 177 to 180 and controls the operation of the device, in particular the operation of the pumps or valves 146, 148 and 162 to 167. The pumps and valves associated with the different compartments are independently controlled by the control unit 161. Preferably, the washed gas injected into a compartment comes from the same compartment.

Preferably, the second injection pipes 171, 172 and 173 have at least one gas injection orifice close to the bottom of the compartments 174, 175 and 176, respectively.

The control unit 161 is configured to trigger the operation of the extraction unit to reduce the ammonium ion concentration when the captured concentration is greater than 4 g / L of fermented material and, preferably, when the captured concentration is greater 3 g / L of fermentation material.

The material to be fermented is infinitely diluted or has a dry matter content greater than 15%, preferably greater than 17% and, even more preferably greater than 20%.

Preferably, at least one compartment (in particular the methanogenesis compartment), in which the extraction unit extracts part of the atmosphere and injects washed gas, has:

- A temperature between 50 ° C and 60 ° C, a hydrogen potential between 7.5 and 8.7 and, more preferably, between 7.9 and 8.7 and / or an atmosphere with a methane titer included between 52% and 65%.

Claims (12)

1. Process for treating raw or liquid digestate resulting from a phase separation step, leaving a continuous methanization unit for thick or liquid routes, the methanization unit comprising a system for preparing the material, characterized in what it includes: a step of passing air into or on the surface of said digestate contained in a tank to extract at least ammonia towards the atmosphere of the tank and - a stage of extraction of the ammonia contained in the atmosphere. 2. Method according to claim 1, which comprises a step of reinjecting the digestate after passage of air, to the material preparation system. 3. Method according to one of claims 1 or 2, which comprises, after the step of passing air into or on the surface of said digestate, a step of passing biogas through said digestate to reduce the pH of the digestate. 4. The method of claim 3, wherein, during the step of passing biogas through the digestate, injecting a biogas having a methane concentration between 45 and 70% and a carbon dioxide concentration between 30 % and 55%. 5. Method according to one of claims 3 or 4, which comprises a step of reinjecting the digestate after passage of the biogas, to the system for preparing the material. 6. Method according to one of claims 1 to 5, wherein during the step of passing air into or on the surface of said digestate, the temperature of the digestate is between 25 ° C and 85 ° C. 7. Method according to one of claims 1 to 6, wherein during the step of passing air over the surface of said digestate, the rate of renewal of the atmosphere of the tank is between 0.5 and 5 per hour. 8. Method according to one of claims 1 to 7, in which, during the step of passing air through the digestate, the ratio of flow rate of injected air to volume of digestate to be treated is between 0.020 to 0.200 L of air / mL of digestate to be treated. 9. Method according to one of claims 1 to 7, wherein during the step of passing air over the surface of said digestate, the ratio of the section of the treatment tank to the volume of digestate to be treated is between 0.006 m ² per ml of digestate to be treated and 0.017 m ² / ml of digestate to be treated. 10. Device for treating raw or liquid digestate resulting from a phase separation step leaving a methanization unit, characterized in that it comprises: a means of air passage in or on the surface of said digestate contained in a tank to extract at least ammonia towards the atmosphere of the tank and - a means of extracting ammonia from the atmosphere. 11. Device according to claim 10, which comprises a means for reinjecting the digestate after passage of air, to the system for preparing the material. 12. Device according to one of claims 10 or 11, which comprises a means for passing biogas through said digestate to reduce the pH of the digestate, after the passage of air in or at the surface of said digestate.