WO2025248009A1 - Reactor and method for catalytic methanation induced by nanosecond plasma coupled with a radio-frequency plasma - Google Patents

Abstract

The invention relates to a reactor (1) for catalytic methanation using non-thermal plasma, comprising: - an enclosure (2) comprising a reaction chamber (3), supplied upstream with carbon dioxide and with dihydrogen, - a catalytic structure (6) housed in the reaction chamber and comprising at least one catalyst (7) promoting the methanation reaction, - an RF generator (12) electrically connected to at least one RF electrode (13) arranged around the reaction chamber, the RF generator being configured to apply an oscillating radio-frequency voltage to the RF electrode, so as to generate radio-frequency discharges in the reaction chamber, - an NS generator (15) electrically connected to an NS electrode (16) that is arranged around the reaction chamber, the NS generator being configured to transmit electrical pulses to the NS electrode, so as to generate nanosecond discharges in the reaction chamber.

Description

Description

Title: Reactor and process of catalytic methanation induced by nanosecond plasma coupled with a radiofrequency plasma.

technical field

The present invention relates to the field of obtaining methane by catalytic methanation in particular, activated by non-thermal plasma.

Its main aim is to improve the yield of this reaction.

Previous technique

To reduce the environmental impact of carbon dioxide (CO2) emissions into the air, various CO2 valorization processes have been researched to transform CO2 into synthetic fuel. Among these processes is the hydrogenation of CO2 to obtain methane (CH4). CO2 hydrogenation is a reaction of CO2 with dihydrogen (H2) to produce CH4. It is thus possible to recycle CO2 by converting it into methane, a combustible gas. This reaction is also called methanation and follows the following reaction equation:

_CO2 + _4H2 = _CH4 + _2H2O .

Non-thermal plasma catalytic methanation is a well-known methanation process that uses a plasma in conjunction with a catalyst to enhance methanation performance, i.e., to facilitate the reaction of CO2 with dihydrogen [1]. Non-thermal plasma catalytic methanation also has the advantage of operating at temperatures below 550°C, even below 250°C, and at atmospheric pressures, for example, pressures up to 10 bar, making this methanation process easily applicable on an industrial scale.

For example, patent applications EP 3 050 865 Al, US 10 968 410 Bl, US 2023/0234018 Al, EP 4237 140 Al, US 2022/0040664 Al, and WO 2023/041627 Al each describe a reactor and a process for dielectric barrier discharge plasma catalytic methanation, also known as DBD plasma. DBD plasma is a plasma generated by an electrical discharge created between two electrodes separated by a dielectric material. However, the efficiency of reactors and catalytic methanation processes using a DBD plasma remains low and therefore unsatisfactory, efficiency being the ratio between the quantity of methane produced and the energy required to implement the process.

There is therefore a need for a reactor and a non-thermal plasma catalytic methanation process capable of producing a large quantity of methane with a low energy input.

The purpose of the invention is to meet, at least in part, this need.

Description of the invention

To this end, the invention relates to a reactor for catalytic methanation by non-thermal plasma, comprising:

- an enclosure comprising a reaction chamber, intended to be fluidly connected upstream to a carbon dioxide supply source and a dihydrogen supply source,

- a catalytic structure housed in the reaction chamber and comprising at least one catalyst promoting the methanation reaction,

- a radio frequency generator, called an RF generator, electrically connected to at least one electrode, called an RF electrode, arranged around the reaction chamber, the RF generator being configured to apply a voltage oscillating in the radio frequencies to the RF electrode so as to generate radio frequency discharges in the reaction chamber,

- a nanosecond pulse generator, called NS generator, electrically connected to an electrode, called NS electrode, arranged around the reaction chamber, the NS generator being configured to transmit electrical pulses to the NS electrode so as to generate nanosecond discharges in the reaction chamber.

Preferably, the catalytic structure includes a retaining structure holding the catalyst(s) in the reaction chamber by forming a passage between the inlet and outlet of said reaction chamber.

Preferably, the reactor includes a matching box electrically connecting the RF generator to the RF electrode. Preferably, the reactor includes a bandstop filter electrically linking the NS generator to the NS electrode, the bandstop filter being adapted to filter electrical signals at the oscillation frequency of the voltage applied by the RF generator.

According to one embodiment, the reactor may include at least one ground electrode electrically connected to ground and forming, with the RF electrode, a capacitor so that the radiofrequency discharges generated are capacitive discharges.

Preferably, the reactor comprises several RF electrodes, each electrically connected to the RF generator, and/or several ground electrodes, each electrically connected to an electrical ground, the RF electrodes being arranged alternately with the ground electrodes so as to form a plurality of capacitors.

According to another embodiment, the RF electrode is in the form of a coil, one end of which is electrically connected to the RF generator and the other end is electrically connected to an electrical ground, so that the radio frequency discharges generated are inductive discharges.

Preferably, the reactor comprises several RF electrodes, each in the form of a coil, one end of which is electrically connected to the RF generator and the other end is electrically connected to an electrical ground, so that the generated radiofrequency discharges are inductive discharges.

Preferably, the reactor includes a cooling system configured to circulate a cooling fluid along the containment.

The invention also relates to an installation for non-thermal plasma catalytic methanation, comprising at least one reactor according to the present invention, the reaction chamber of each reactor being fluidly connected to a carbon dioxide supply source and to a dihydrogen supply source.

Preferably, the installation comprises a plurality of reactors according to the present invention.

Preferably, the reactor containment structures are distributed regularly angularly around the longitudinal axis of the installation with a pitch equal to 27t/n, n being the number of reactors.

Preferably, the RF generator and/or the NS generator is/are common to the reactors. Preferably, the carbon dioxide supply source and/or the hydrogen supply source is/are common to the reactors.

The invention also relates to a non-thermal plasma catalytic methanation process, preferably implemented by an installation according to the present invention, the process comprising the following steps: a) supplying a reaction chamber with carbon dioxide and dihydrogen, the reaction chamber housing a catalytic structure comprising a catalyst promoting the methanation reaction, b) generating nanosecond electrical discharges in the reaction chamber, creating a plasma, called NS plasma, with the gas circulating in the reaction chamber, c) generating radiofrequency electrical discharges in the reaction chamber, creating a plasma, called RF plasma, with the gas circulating in the reaction chamber, step c) being successive to step b).

Preferably, steps b) and c) are repeated in a plurality of cycles, each cycle comprising a step b) followed by a step c), the cycles preferably being periodic, preferably with a period between 0.1 and 100 ms.

The present invention essentially consists of a reactor and a process for catalytic methanation using non-thermal plasmas, the non-thermal plasmas being a nanosecond (NS) plasma created by nanosecond discharges and an RF plasma created by radiofrequency discharges. The successive use of these two plasmas considerably increases the amount of methane produced relative to the energy input for their creation. Indeed, the NS and RF plasmas facilitate the dissociation of carbon dioxide by exciting its molecules. The NS plasma can also facilitate the creation of the RF plasma by reducing the voltage required to ignite the RF plasma.

The present invention thus makes it possible to increase the ratio between the quantity of methane produced and the energy required to implement the process. In particular, the inventor measured a conversion rate of 40% when using the reactor or process according to the present invention. The conversion rate corresponds to the ratio of the quantity of methane produced to the quantity of carbon dioxide supplied. The inventor also measured that the reactor according to the present invention consumes 8 kWh per kilogram of carbon dioxide supplied. Brief description of the drawings

Other advantages and features will become clearer upon reading the detailed description, provided for illustrative purposes only and not as a limitation, with reference to the following figures:

[Fig 1] Figure 1 is a schematic view of a nanosecond plasma-induced catalytic methanation reactor and radiofrequency plasma according to the present invention;

[Fig 2A], [Fig 2B] Figures 2A and 2B are perspective views of a reactor enclosure according to the present invention, the enclosure delimiting a reaction chamber in which is housed a catalytic system comprising catalysts and a retaining structure holding the catalysts;

[Fig 3] Figure 3 is a schematic view of a nanosecond plasma-induced catalytic methanation reactor and a radiofrequency plasma according to an embodiment in which the reactor comprises a single RF electrode and a single ground electrode forming a capacitor;

[Fig 4] Figure 4 is a schematic view of a nanosecond plasma-induced catalytic methanation reactor and radiofrequency plasma according to an embodiment in which the reactor comprises a single RF electrode and two ground electrodes arranged on either side of the RF electrode;

[Fig 5], [Fig 6] Figures 5 and 6 are schematic views of a nanosecond plasma-induced catalytic methanation reactor and radiofrequency plasma according to an embodiment in which the reactor comprises several RF electrodes and several ground electrodes, the RF and ground electrodes being alternated with each other;

[Fig 7], [Fig 8] Figures 7 and 8 are schematic views of a nanosecond plasma-induced catalytic methanation reactor and radiofrequency plasma according to an embodiment in which the RF electrode(s) are in the form of coils;

[Fig 9] Figure 9 is a schematic perspective view of an installation comprising several reactors according to the present invention;

[Fig 10] Figure 10 is a perspective view of an example of the realization of an installation comprising several reactors according to the present invention; [Fig 11] Figure 11 is a perspective view of the central block of the installation illustrated in Figure 10, the central block comprising the reaction chambers of the reactors;

[Fig 12] Figure 12 is a front view of a mounting plate for the installation illustrated in Figure 10;

[Fig 13] Figure 13 is a perspective view of the assembly of the RF electrode and ground electrodes around a reaction chamber of the installation illustrated in Figure 10;

[Fig 14] Figure 14 is a perspective view of an envelope intended to house within it the central block of the installation illustrated in Figure 10;

[Fig 15] Figure 15 is a perspective view of a distribution flange of the power supply unit of the installation illustrated in Figure 10;

[Fig 16] Figure 16 is a perspective view of a collection flange of the recovery block of the installation illustrated in Figure 10.

Detailed description

For clarity, the different elements of the figures are represented to a free scale, the actual dimensions of the different parts not necessarily being respected.

Throughout this application, the terms "inlet", "outlet", "downstream" and "upstream" are to be understood in reference to the direction of gas flow from their entry into the reactor to their exit from it.

Figure 1 illustrates a reactor 1 according to the present invention. The reactor 1 comprises a housing 2 delimiting a reaction chamber 3. The housing 2 is in the form of a hollow tube with a diameter between 10 and 40 mm, preferably between 26 and 28 mm, and/or a length between 10 and 100 cm, for example, 70 cm. The housing may be made of an insulating material, for example, glass, preferably borosilicate glass, quartz, alumina, silicon nitride ceramic, or boron nitride ceramic.

The enclosure 2 is connected upstream with a carbon dioxide, CO2, supply line 4 and a hydrogen, H2, supply line 5. In particular, the airtight connection of the lines 4 and 5 with the enclosure 2 can be achieved by flanges, for example KF 16 type flanges. A catalytic system 6 is housed within the reaction chamber 3. The catalytic system 6 includes catalysts 7 to promote the methanation reaction of CO2 with H2 to methane, CH4. The catalysts 7 can be made of transition metals such as nickel, cobalt, or iron. The catalysts 7 can be in the form of beads.

Figures 2A and 2B illustrate a containment 2 of a reactor 1 according to two different embodiments of catalytic system 6. In each of Figures 2A and 2B, the catalytic system 6 includes a retaining structure 8 to retain the catalysts 7 in the reaction chamber 3 by delimiting a passage 9 between the inlet and outlet of the gases in said reaction chamber 3.

In each of Figures 2A and 2B, the support structure 8 comprises bars 10, parallel to the longitudinal axis of the enclosure 2, the bars 10 being spaced apart to delimit a cylinder, the space between two adjacent bars 10 being less than the size of a catalyst 7.

The support structure 8 also includes support rods 11 for holding the bars 10 in the enclosure 2, preferably in a centered manner. The support structure 8 may be made of glass, preferably quartz glass.

According to the embodiment of Figure 2A, the catalysts 7 are retained inside the cylinder delimited by the bars 10, and the passage 9 is defined between the wall of the enclosure 2 and said cylinder.

According to the embodiment of Figure 2A, the catalysts 7 are retained outside the cylinder formed by the bars 10, between the wall of the enclosure 2 and said cylinder, and the passage 9 is delimited by the inside of said cylinder.

Reactor 1 also includes a radio frequency generator 12, called RF generator, connected to at least one electrode 13, called RF electrode, arranged around reaction chamber 3.

The RF generator 12 is configured to apply a voltage to the RF electrode 13, an electrical voltage oscillating at an oscillation frequency between 400 kHz and 50 MHz, so as to generate radio frequency discharges creating a plasma, called RF plasma, with the gas circulating in the reaction chamber 3. Preferably, the oscillation frequency of the applied electrical voltage is approximately 13.56 MHz. Advantageously, such an oscillation frequency exhibits good discharge penetration capability in materials and low dielectric heating losses. Furthermore, an oscillation frequency of approximately 13.56 MHz allows for efficient coupling between the electromagnetic field and the RF plasma because it is high enough to permit good energy transfer to electrons in the RF plasma, thus facilitating gas ionization and sustaining the radiofrequency discharges. Moreover, such an oscillation frequency is low enough to allow for relatively simple power electronics control while being high enough to avoid excessive interference with common electrical systems.

Preferably, the amplitude of the oscillations of the electrical voltage applied by the RF generator 12 is less than 6000 V.

Reactor 1 may include a matching box 14 to electrically connect the RF generator 12 to the RF electrode 13. The matching box 14 allows the impedance at the output of the RF generator 12 to be adapted, thus increasing the power transmitted to the RF electrode 13.

Reactor 1 also includes a nanosecond pulse generator 15, referred to as the NS generator, connected to an electrode 16, also referred to as the NS electrode, arranged around the reaction chamber 3. The NS generator 15 is configured to transmit electrical pulses to the NS electrode 16 so as to generate nanosecond discharges creating a plasma, referred to as the NS plasma, with the gas circulating in the reaction chamber 3. The electrical pulses can have a duration of between 20 and 500 ns, preferably between 50 and 150 ns. The voltage of the electrical pulses can be between 2 and 20 kV. The repetition frequency of the pulses can be between 100 Hz and 10 kHz, for example, approximately 1 kHz.

The NS 16 electrode can be in the form of an annular ribbon, wound around the chamber 2 or wound inside the chamber 2 against its inner wall. Preferably, the width of the ribbon forming the NS 16 electrode is between 0.2 and 3 cm. The NS 16 electrode can also be in the form of a point made of conductive material passing through the wall of the chamber 2 with one end housed in the reaction chamber 3.

The NS 16 electrode can be made of copper. A band-stop filter 17 can be arranged on the electrical path connecting the NS generator 15 to the NS electrode 16, the band-stop filter 17 being adapted to filter electrical signals at the oscillation frequency of the voltage applied by the RF generator 12. The band-stop filter 17 thus prevents an electrical current, created by the radio frequency discharges, from reaching the NS generator 15.

A control unit 42 can be electrically connected with the RF generator 12 and the NS generator 16 in order to control them.

As illustrated in figures 3 to 8, the possible configurations for the RF electrode(s) 13 and the NS electrode 16 are varied.

In particular, the RF electrode(s) 13 can be adapted to generate radio frequency capacitive discharges, as illustrated in Figures 3 to 6. In this case, the reactor 1 also includes at least one ground electrode 18, connected to ground 19, to form at least one capacitor with the RF electrode(s) 13.

The RF electrode(s) 13 and/or the ground electrode(s) 18 may each have the form of an annular ribbon, wrapped around the enclosure 2. Preferably, the width of the ribbon forming each RF electrode 13 is between 1 mm and 3 cm, for example equal to 1 cm.

The RF electrode(s) 13 and/or the ground electrode(s) 18 may be made of copper.

Figure 3 illustrates an embodiment of a reactor 1 in which there is a single RF electrode 13 connected to the RF generator 12 and a single ground electrode 18. The RF electrode 13 and the ground electrode 18 then operate as a pair.

According to another embodiment, illustrated in Figure 4, the reactor 1 can comprise a single RF electrode 13 connected to the RF generator 12 and two ground electrodes 18, sandwiching the RF electrode 13. The RF electrode 13 and the ground electrodes 18 then operate as a triplet, which has the advantage of exposing the gas circulating in the reaction chamber 3 for a longer time to the RF plasma.

According to other embodiments, illustrated in Figures 5 and 6, the reactor 1 can comprise several RF electrodes 13 connected to the RF generator 12 and several ground electrodes 18, the ground electrodes 18 being alternated with the RF electrodes 13. The capacitors formed by the RF electrodes 13 and the ground electrodes 18 can be arranged upstream and/or downstream of the NS electrode 16. Indeed, the residence time of the gas in the reaction chamber 3 is significantly longer than the generation time of the RF plasma. For example, in the embodiment illustrated in Figure 6, the reactor 1 includes capacitors, formed by an RF electrode 13 connected to the RF generator and two ground electrodes 18, sandwiching the RF electrode 13, upstream of the NS electrode 16.

Preferably, at least one of the ground electrodes 18 is closer to the NS electrode 16 than the RF electrodes 13 are closer to the NS electrode 16. The distance between the NS electrode 16 and each of the ground electrodes 18 adjacent to said NS electrode 16 can be between 0.5 and 5 cm, for example, 1.5 cm. Advantageously, such a distance allows the ions generated by the NS plasma to come sufficiently close to the capacitors formed by the RF electrodes 13 and the ground electrodes 18, so that said ions act as charges facilitating the creation of the RF plasma by reducing the voltage required for its ignition.

For each of the RF electrodes 13, the distance between the RF electrode 13 and each of the ground electrodes 18 adjacent to said RF electrode 13 can be between 2 mm and 2 cm, for example equal to 5 mm.

The RE 13 electrode(s) can also be adapted to generate radio frequency inductive discharges, as illustrated in Figures 7 and 8. In this case, the reactor 1 includes at least one coil forming the RE 13 electrode, the coil 13 being connected at one end to the RF generator 12 and at the other end to ground 19.

In the embodiment illustrated in Figure 7, reactor 1 comprises a single RF electrode 13 in the form of a coil 13 downstream of the NS electrode 16. In the embodiment illustrated in Figure 8, reactor 1 comprises two RF electrodes 13 in the form of a coil 13, one of the RF electrodes 13 being upstream of the NS electrode 16 and the other of the RF electrodes 13 being downstream of the NS electrode 16.

The coil 13 consists of at least one turn wound around the enclosure 2. Preferably, the coil 13 comprises 4 or 5 turns wound around the enclosure 2. The turns may be made of copper.

The coil 13 can have a length between 1 and 10 cm, preferably between 1 and 5 cm, and even more preferably between 1.5 and 2 cm. The length of the coil 13 is defined as being the distance, measured parallel to the longitudinal axis of the enclosure 2, between its end connected to the RF generator 12 and its end connected to ground 18.

Preferably, the end of the coil 13 adjacent to the NS electrode 16 is the end connected to ground 18.

Preferably, the distance between coil 13 and electrode NS 16 is between 1 and 2 cm, for example equal to 1.5 cm.

The methanation process according to the present invention, implemented in particular by reactor 1 according to the present invention, comprises the following steps: a) supplying the reaction chamber 3 with carbon dioxide and dihydrogen, b) generating nanosecond electrical discharges in the reaction chamber 3, creating a plasma, called NS plasma, with the gas circulating in the reaction chamber 3, c) generating radiofrequency electrical discharges in the reaction chamber 3, creating a plasma, called RF plasma, with the gas circulating in the reaction chamber 3.

The flow rate of carbon dioxide and/or dihydrogen during step a) may be less than 50 L/min, for example between 5 L/min and 0.05 L/min.

The pressure in the reaction chamber 3, during the process, can be between 1 and 200 mbar, preferably between 60 and 100 mbar.

The temperature in reaction chamber 3 during the process can be below 550°C, or even below 400°C, for example, between 350 and 400°C. Preferably, the temperature is as low as possible because this reduces the power required to sustain the RF plasma. The minimum temperature depends on the catalyst 7.

Step b) can be carried out by transmitting electrical pulses to an electrode, referred to as electrode NS 16. The electrical pulses can have a duration of between 20 and 500 ns, preferably between 50 and 150 ns. The voltage of the electrical pulses can be between 2 and 20 kV. The pulse repetition frequency can be between 100 Hz and 10 kHz, for example, approximately 1 kHz.

Step c) can be carried out by applying a voltage to electrodes, referred to as RF electrodes 13, the voltage oscillating at an oscillation frequency between 400 kHz and 50 MHz, preferably between 400 kHz and 15 MHz. Preferably, the oscillation frequency of the voltage The applied voltage is approximately equal to 13.56 MHz. Preferably, the amplitude of the oscillations of the applied voltage is less than 6000 V.

Step a) can take place before and/or during steps b) and c).

Step c) follows step b). The time separating step c) from step b) is advantageously as short as possible. In particular, this time may be less than 150 ps, or even 50 ps.

Step b) can last between 50 and 150 ns.

Step c) can last between 20 ps and 100 ms, for example it can last 200 ps.

Steps b) and c) can be repeated in a plurality of cycles, each cycle comprising a step b) followed by a step c). Preferably, the cycles are carried out periodically, preferably with a period between 0.1 and 10 ms, for example equal to 1 ms.

The duration of step c) preferably represents between 10 and 80% of the duration of a cycle, preferably 25%. The inventors noted that such a duration ratio optimizes the ratio between the amount of methane produced and the energy required to maintain the RF plasma, while avoiding excessive heating of the gases within reaction chamber 3.

Figure 9 illustrates an installation 20 according to the present invention, the installation 20 comprising a plurality of reactors 1. Each reactor 1 is similar to those described previously. In particular, each reactor 1 comprises a chamber 2, a catalytic structure 6, at least one RF electrode 13, and at least one NS electrode 16, as described previously. The RF generator 12 and the NS generator 15 are common to each of the reactors 1. This advantageously limits the cost of the installation 20 and maximizes the efficiency, defined as the ratio of the energy supplied to the RF generator 12 and the NS generator 15 to the quantity of methane produced.

The containment structures 2 of each of the reactors 1 can be arranged parallel to each other. Specifically, the containment structures 2 can be arranged with rotational symmetry around the longitudinal axis of the installation 20. That is to say, the containment structures 2 can be distributed angularly around the longitudinal axis of the installation 20 with a spacing of 27t/n, where n is the number of reactors 1. Such a symmetrical arrangement has the advantage to standardize the operating conditions of reactors 1, so that they are identical for each of said reactors 1 knowing that reactors 1 influence each other by radiation.

Installation 20 also includes a ground rod 21 connected to the ground 19 of installation 20. The ground rod 21 is arranged parallel to the containment structures 2 of reactors 1. Preferably, the ground rod 21 is equidistant from each of the containment structures 2 of reactors 1. Preferably, the ground electrodes 18 of reactors 1 are electrically connected to the ground rod 21.

Installation 20 also includes retaining plates 22 to hold the containment structures 2 of each of the reactors 1. The retaining plates 22 include, in particular, holes into which the containment structures 2 of the reactors 1 are inserted. The retaining plates 22 can be connected to the ground 19 of installation 20.

Preferably, the carbon dioxide supply source and/or the dihydrogen supply source are common to each of the reactors 1. In particular, the installation 20 may include a distribution flange configured to connect the same carbon dioxide supply line 4 to each of the reaction chambers 3 of the reactors 1 and to connect the same dihydrogen supply line 5 to each of the reaction chambers 3 of the reactors 1.

Installation 20 may also include a collection flange configured to connect together the outlets of each of the reaction chambers 3 of the reactors 1 to a recovery conduit arranged downstream of said reaction chambers 3.

The reactors 1 also include a cooling system 23 for the reaction chambers 3. The cooling system 23 is configured, in particular, to circulate a cooling fluid along the containment walls 2 of the reactors 1, preferably the fluid flowing in the same direction as the gases circulating within the reaction chambers 3. Preferably, the cooling fluid is air. The cooling system 23 may be common to each of the reactors 1.

Figure 10 illustrates an example of an embodiment of an installation 20 according to the present invention. The installation 20 comprises a central block 24 including the containment structures 2 of the reactors 1 according to the invention. Figure 11 illustrates the central block 24 comprising the reactors 1. The containment structures 2 of the reactors 1 are arranged parallel to the longitudinal axis of the installation 20 and distributed angularly according to a rotational symmetry about said longitudinal axis. The central block 24 includes two support plates 22 connected to the installation's ground.

Figure 12 illustrates a retaining plate 22. This plate 22 includes through holes 25 for the insertion of the containments 2 of the reactors 1. The retaining plate 22 also includes clips 26 for locking the containments 2 in the through holes 25. The clips 26 include orifices 27 allowing the passage of the cooling fluid, for example air, as close as possible to the outer wall of the containments 2.

Figure 13 illustrates an assembly of an RF electrode 13 and two ground electrodes 18 around the reaction chamber 3 of a reactor 1 included in the installation 20. The RF electrode 13 is in the form of a ribbon wrapped around the enclosure 2. A bar 28 clamps the ribbon forming the RF electrode 13 around the enclosure 2. The ribbon forming the RF electrode 13 is fixed at its ends to an electrically conductive rod, called the RF rod 29, which allows the RF electrode 13 to be electrically connected to the RF generator 12.

The RF rod 29 can be common to each of the reactors 1. This makes it easy to connect the same RF generator 12 to each of the RF electrodes 13 of the reactors 1.

Similarly, the ground electrodes 18 are in the form of a ribbon wrapped around the enclosure 2. Bars 30 tighten the ribbons forming the ground electrodes 18 around the enclosure 2. The ribbons forming the ground electrodes 18 are fixed at their ends to a ground rod 21, electrically conductive to electrically connect the ground electrodes 18 to the retaining plates 22.

The assembly of the NS 16 electrode around the reaction chamber 3 of a reactor 1 can be done similarly to that of the RF 13 electrode described above.

The installation 20 also includes a waterproof enclosure 31, in which the central block 24 is housed. The enclosure 31 contains the cooling fluid circulating inside the installation 20. The enclosure 31 also forms a Faraday cage around the central block 24.

Figure 14 illustrates the envelope 31 comprising two walls 32 that can be separated from each other, in order to open the envelope 31 and access the central block 24. Each wall 32 comprises a metal structure 33 and observation windows 34 fixed to the metal structure 33. The observation windows 34 may be made of poly(methyl methacrylate). The metal structure 33 may include a metal mesh, for example made of copper, covered by the observation windows 34.

Installation 20 also includes a fluidic supply unit 35 upstream of the central unit 24. The supply unit 35 includes inlets 36 for injecting a cooling fluid around the containments 2 of the reactors 1. The fluidic supply unit 35 may include one or more fans for drawing in and expelling the cooling fluid.

The fluidic feed unit 35 also includes the carbon dioxide feed line 4. In this example, the line 4 also serves as the hydrogen feed line 5. The feed unit 35 includes the distribution flange 37 connecting the line 4 to each of the reaction chambers 3 of the reactors 1. The distribution flange 37 is shown in Figure 15.

Installation 20 also includes a fluid recovery unit 38 downstream of the central unit 24. The fluid recovery unit 38 includes outlets 39 for discharging the cooling fluid. The fluid recovery unit 38 may include one or more fans for drawing in and discharging the cooling fluid.

The fluidic recovery block 38 also includes a conduit 40 for recovering gaseous effluents from the reactors 1. The fluidic recovery block 38 includes the collection flange 41 connecting the outlets of each of the reaction chambers 3 of the reactors 1 to the recovery conduit 40.

Other variations and improvements may be envisaged without departing from the scope of the invention as defined by the following claims. For example, in the embodiments illustrated herein, the installation 20 comprises six reactors 1, but this number is not limiting; for example, the installation 20 may comprise one, two, three, four, five, eight, twelve, fourteen, eighteen, twenty, twenty-two, or twenty-four reactors 1. Preferably, the installation 20 comprises fewer than 50 reactors 1, and preferably fewer than 20 reactors 1. Preferably, the number n of reactors 1 included in the installation 20 is a multiple of 3.

List of cited references [1] A. Salden et al.: “M eta-analysis of C 02 conversion, energy efficiency, and other performance data of plasma-catalysis reactors with the open access PIONEER database”, Journal of Energy Chemistry, vol. 86, pages 318-342, 2023.

Claims

Demands 1. Reactor (1) for non-thermal plasma catalytic methanation, comprising: - an enclosure (2) comprising a reaction chamber (3), intended to be fluidly connected upstream to a carbon dioxide supply source (4) and a dihydrogen supply source (5), - a catalytic structure (6) housed in the reaction chamber and comprising at least one catalyst (7) promoting the methanation reaction, - a radio frequency generator (12), called an RF generator, electrically connected to at least one electrode (13), called an RF electrode, arranged around the reaction chamber, the RF generator being configured to apply a voltage oscillating in the radio frequencies to the RF electrode, so as to generate radio frequency discharges in the reaction chamber, - a nanosecond pulse generator (15), called NS generator, electrically connected to an electrode (16), called NS electrode, arranged around the reaction chamber, the NS generator being configured to transmit electrical pulses to the NS electrode so as to generate nanosecond discharges in the reaction chamber. 2. Reactor according to claim 1, the catalytic structure comprising a retaining structure (8) retaining the catalyst(s) in the reaction chamber by forming a passage (9) between the inlet and outlet of said reaction chamber. 3. Reactor according to any one of the preceding claims, comprising a matching box (14) electrically connecting the RF generator to the RF electrode. 4. Reactor according to any one of the preceding claims, comprising a band-stop filter (15) electrically connecting the NS generator to the NS electrode, the band-stop filter being adapted to filter electrical signals at the oscillation frequency of the voltage applied by the RF generator. 5. Reactor according to any one of the preceding claims, comprising at least one ground electrode (18) electrically connected to a ground (19) and forming, with the RF electrode, a capacitor so that the radiofrequency discharges generated are capacitive discharges. 6. Reactor according to the preceding claim, comprising several RF electrodes, each being electrically connected to the RF generator, and/or several ground electrodes, each being electrically connected to an electrical ground, the RF electrodes being arranged alternately with the ground electrodes so as to form a plurality of capacitors. 7. Reactor according to any one of claims 1 to 4, the RF electrode being in the form of a coil, one end of which is electrically connected to the RF generator and the other end is electrically connected to an electrical ground, so that the radiofrequency discharges generated are inductive discharges. 8. Reactor according to the preceding claim, comprising several RF electrodes, each in the form of a coil, one end of which is electrically connected to the RF generator and the other end is electrically connected to an electrical ground, so that the radiofrequency discharges generated are inductive discharges. 9. Reactor according to any one of the preceding claims, comprising a cooling system (23) configured to circulate a cooling fluid along the containment. 10. Installation (20) for non-thermal plasma catalytic methanation, comprising at least one reactor (1) according to any one of the preceding claims, the reaction chamber (3) of each reactor being fluidly connected to a carbon dioxide supply source (4) and to a dihydrogen supply source (5). 11. Installation according to the preceding claim, comprising a plurality of reactors according to any one of claims 1 to 9. 12. Installation according to the preceding claim, the reactor enclosures (2) being regularly distributed angularly around the longitudinal axis of the installation with a pitch equal to 27t/n, n being the number of reactors. 13. Installation according to claim 11 or 12, the RF generator (12) and/or the NS generator (15) being common to the reactors. 14. Installation according to any one of claims 11 to 13, the carbon dioxide supply source (4) and/or the hydrogen supply source (5) being common to the reactors. 15. A non-thermal plasma catalytic methanation process, preferably carried out by an installation according to any one of claims 10 to 14, the process comprising the following steps: a) supplying a reaction chamber (3) with carbon dioxide and dihydrogen, the reaction chamber housing a catalytic structure (6) comprising a catalyst (7) promoting the methanation reaction, b) generating nanosecond electrical discharges in the reaction chamber, creating a plasma, called NS plasma, with the gas circulating in the reaction chamber, c) generating radiofrequency electrical discharges in the reaction chamber, creating a plasma, called RF plasma, with the gas circulating in the reaction chamber, step c) being successive to step b). 16. A method according to the preceding claim, wherein steps b) and c) are repeated in a plurality of cycles, each cycle comprising a step b) followed by a step c), the cycles preferably being periodic, preferably with a period between 0.1 and 100 ms.