Method and system for producing gas and solid carbon by plasma pyrolysis of a hydrocarbon or a hydrocarbon mixture

Abstract

The invention relates to a method for producing hydrogen and carbon by plasmalysis of a hydrocarbon or a mixture of hydrocarbons, implementing a step of injecting the hydrocarbon or the mixture of hydrocarbons in gaseous form into an active zone (30) of a reaction chamber (15) of a plasmalysis reactor (10) that is controlled to produce a plasma. The method comprises a repetition of steps applied to the gas stream during its path between the inlet and the outlet of the active zone (30): generating a plasma pulse for a pulse duration until a target heating temperature is reached; maintaining an optimal activation regime of the heated plasma in the active zone (30) for a predetermined residence time; and cooling the resident plasma for a predetermined cooling time.

Description

Process and system for the production of gas and solid carbon by plasma pyrolysis of a hydrocarbon or a mixture of hydrocarbons FIELD OF INVENTION The invention belongs to the field of devices for the production of gas, and more particularly hydrogen, and solid carbon by hydrocarbon reforming. STATE OF THE ART Hydrocarbon reforming is commonly used to produce hydrogen on a large scale, often employed as a feedstock in the petrochemical industry and other industrial applications. For example, hydrogen is essential in the manufacture of products such as ammonia, methanol, and other chemical compounds, as well as in applications such as hydrogen as a clean fuel. In a plasma pyrolysis process, also called plasmalysis, there is also production of solid carbon. Although solid carbon extracted by hydrocarbon reforming is not in principle a sought-after product in this process because it is generally considered an undesirable by-product, carbon black remains a versatile material with high added value which finds various applications in different industries depending on the quality obtained (tires, polymers, inks and paints, batteries, catalysts, electronic applications, …). Carbon production is one of the goals of the invention. However, the solid carbon that forms during reforming also causes operational problems by clogging catalysts and reactors, thus reducing process efficiency. Therefore, reforming operations are generally designed to minimize the formation of solid carbon and prevent its accumulation. In the state of the art, the methods of cleaning carbon or decoking are mainly of two types: mechanical cleaning of coke, chemical cleaning of coke. Mechanical coke cleaning methods can involve removing the coking material with a scraper or cleaning the coke with shock waves. For example, cleaning systems using hydraulic means, such as high-pressure water jets or hydraulic sandblasting, are widely used and can fracture the coke layer. Oxidizing chemical cleaning by a mixture of superheated air and steam, possibly with the addition of hydrogen, is also known. But all these processes have the constraint of the reactor being shut down due to decoking of the installation. The overall reaction is CH4 => 2 H2 + C(s) if the input hydrocarbon is methane. The challenge is to control carbon growth to prevent the formation of carbon bridges in undesirable locations, particularly between the two electrodes, which could cause a reactor short circuit and halt plasmalysis. This control must be performed continuously while the plant is operating normally. The pyrolysis reaction of methane CH4→C(s)+2 H2 allows the production of hydrogen without CO2 emissions since the carbon is produced in solid form (without oxidation). The formation of solid carbon is the desired outcome of the process; however, this carbon can form at the wrong time or in the wrong place, for example by depositing on the electrodes, which ultimately short-circuits the system and interrupts the operation: this is the carbon bridge. Carbon deposition on the walls or electrodes is one of the major difficulties reported in the literature: "solid carbon or soot deposits on flow channel walls," for example in Fincke et al. [2]. This document suggests diluting the input methane by injecting hydrogen to reduce carbon formation, which comes at the expense of energy efficiency. Carbon bridges are frequently encountered in the academic literature. (Sun et al. 2016) [7] also report problems with carbon bridges blocking the operation of a thermal plasma generated by a DC arc. In another DC arc system, (Li et al. 2017) [4] demonstrates the phenomenon of growth on the electrode. (Gao et al. 2018) [3] mentions coking problems characteristic of intermediate temperature (warm) plasmas, typically gliding arc, spark, microwave or RF. They use pulsed plasmas to limit this problem, but still encounter carbon deposition problems on the electrodes for certain geometries (gap of 8 to 10 mm). (Maqueo, Coulombe, and Bergthorson 2019) testifies to carbon growth on the anode of their NRP plasma system in a point / plane configuration. (Delikonstantis et al. 2020) [1] mentions mechanical solutions and alternatively suggests using alternating air plasmas to enable decoking. This solution, which oxidizes carbon and thus eliminates the entire benefit of the pyrolysis process for decarbonizing hydrogen, is not acceptable here. In microwave plasmas, carbon deposition in the chamber is also a problem that prevents continued operation (Zherlitsyn, Shiyan, and Demchenko 2016) [8]. Carbon deposition on the walls makes them opaque to waves. Finally, the problems of carbon bridges and carbon deposition have also been encountered by industry. Document WO2010037237A1 [5] explicitly mentions carbon bridges (Fig. 14, Fig. 15). Documents US9812295B1 and US10332726B2 mention carbon deposition on the chamber walls, typically made of quartz, which makes them opaque, reduces the power deposited in the plasma, and limits operating times. This is therefore a known problem often overcome by interrupting the process or by diluting, to the detriment of process performance. In thermal torch processes, the problem is circumvented by using a plasma gas primarily composed of a component other than hydrocarbons, preferably hydrogen, which is then mixed with hydrocarbons. This prevents carbon growth on the electrodes. However, the hydrocarbons are consequently diluted in an additional gas to be heated, increasing energy consumption and reducing energy efficiency. When the diluent gas is hydrogen, this also shifts the chemical equilibrium towards rehydrogenation of the products, requiring a higher operating temperature compared to operation without hydrogen diluent, which further increases energy consumption. For example, in document WO 2015 / 116943

[0008] It is mentioned that the plasma torch is usable with a plasma gas which is composed of at least 60% H2 by volume. US20210245133 discloses a plasma-based hydrocarbon gas processing system for the production of acetylene and hydrogen. This system primarily comprises: a distribution subsystem consisting of a distribution duct and a gas injector that combines the hydrocarbon gas stream with a hydrogen gas stream and an optional auxiliary gas stream, and which directs and disperses, through an arrangement of nozzles, the different streams in directions and at velocities such that a vortex mixing of the three separate streams is produced in the plasma reaction chamber.a tube-shaped plasma reaction chamber where gases are excited by microwave energy, a microwave subsystem directing microwave energy into the plasma reaction chamber, a cooling subsystem to protect the reactor tube from thermal stress, a gas mixture separation subsystem using an adsorption process. The maintenance measure recommended in this document is a periodic shutdown of the reactor to remove the accumulation of carbon soot either by manual cleaning or by using a cleaning gas. US patent 10332726 discloses a microwave plasma-based device for chemically treating hydrocarbon-containing gas, comprising a waveguide receiving two gas streams: a feed stream and a treatment stream. Plasma is generated in the feed gas, and its average energy is controlled by adjusting the pulse frequency to convert the treatment gas into separate components. This microwave plasma-based device aims to enhance the efficiency of the coupling between microwave radiation and the gas flow within the reaction chamber in order to improve energy absorption by the circulating gas, leading to better decomposition of gas molecules into their constituent species. It is demonstrated that controlling the energy delivered to the plasma by adjusting the pulse frequency, duty cycle, and pulse shape can be used to select specific reaction pathways for the creation of separate components from a treatment gas. US patent 9812295B1 discloses a chemical processing method employing pulsed microwave radiation through a waveguide, with the microwave radiation propagating in one direction along the waveguide. The pressure inside the waveguide is at least 0.1 atmospheres. A feed gas is provided at a first location along a length of the waveguide, with the majority of the feed gas flowing in the direction of microwave radiation propagation. A plasma is generated in the feed gas, and a process gas is added to the waveguide at a second location downstream of the first location. The majority of the process gas flows in the direction of microwave propagation at a velocity greater than 5 slm (standard liters per minute).An average plasma energy is controlled to convert the process gas into separate components, by controlling at least one pulse frequency of the pulsed microwave radiation and / or one duty cycle of the pulsed microwave radiation. To prevent carbon buildup, which is considered a waste product in this document, a windowless device between the waveguide and the reaction chamber is proposed, thus increasing the volume where carbon can be deposited—a larger volume compared to similar devices in the prior art. Document WO 2024 / 184026 A1 discloses a method and system for dissociating components of carbon dioxide and / or methane using a microwave plasma. A microwave plasma environment in the form of a non-thermal plasma, which is created as a pulsed microwave plasma by applying microwave pulses to the plasma, is arranged in a reactor chamber. Components of carbon dioxide and / or methane are introduced into the reactor chamber. The non-thermal pulsed microwave plasma is designed to dissociate the components of carbon dioxide and / or methane into dissociation products, preferably selected from the group consisting of carbon monoxide, hydrocarbons, hydrogen, solid carbon, syngas, fuels, and / or oxygenated compounds. Document WO 03 / 010088 A1 discloses a process for producing hydrogen and carbon by decomposing natural gas or methane in a non-thermal plasma field obtained by dielectric barrier discharge. The device for implementing the process comprises two elongated concentric electrodes, an inner and an outer one, and a dielectric barrier situated between them and arranged to form a suitable gap between the inner electrode and the barrier. A high-voltage pulse generator connected to the electrodes, when energized, produces the non-thermal plasma by dielectric barrier discharge in the gas passing through the gap, thereby decomposing the gas into its constituents, namely hydrogen and carbon. Document EP 1226343 B1 discloses a Plasma Fuel Converter comprising an electrically conductive structure forming a first electrode, a second electrode arranged to create an air gap relative to the first electrode in a reaction chamber, a fuel-air mixture residing in the air gap, and a power supply which controls the current and is connected to the first and second electrodes to provide voltage in a range of about 100 volts up to 40 kilovolts and limits the current in the range of about 10 milliamperes up to 1 ampere to generate a discharge to reform the fuel. US document 2015 / 223314 A1 discloses several plasma torch geometries, aimed at improving the efficiency and effectiveness of the torch, reactor and manufacturing process. One aim of the invention is to provide a technical solution that can prevent the formation of carbon bridges in a plasma pyrolysis gas production process, and that can be implemented continuously while the installation is operating normally (i.e., without stopping the reactor). The objective is thus to avoid the formation of carbon bridges in undesirable locations, particularly between the two electrodes, as such carbon bridges can cause a reactor short circuit and the pyrolysis process to stop. REFERENCES Delikonstantis, Evangelos, Marco Scapinello, Orelie Van Geenhoven, and Georgios D. Stefanidis. 2020. “Nanosecond Pulse d Discharge-Driven Non-Oxidative Methane Coupling in a Plate-to-Plate Electrode Configuration Plasma Reactor.” Chemical Engineering Journal 380 (August): 122477. https: / doi.org / 10.1016 / j.cej.2019.122477.Fincke, James R., R.P. Anderson, T. Hyde, R. Wright, R. Bewley, D. C. Haggard, and W. D. Swank. 2000. “SER Thermal Conversion of Methane to Acetylene Final Report.” Idaho National Engineering and Environmental Laboratory, no. January: 69.Gao, Yuan, Shuai Zhang, Hao Sun, Ruixue Wang, Xin Tu, and Tao Shao. 2018. “Highly Efficient Conversion of Methane Using Microsecond and Nanosecond Pulsed Spark Discharges.” Applied Energy 226 (September): 534–45. https: / doi.org / 10.1016 / j.apenergy.2018.06.006.Li, Tianyang, Christophe Rehmet, Yan Cheng, Yong Jin, and Yi Cheng. 2017. “Experimental Comparison of Methane Pyrolysis in Thermal Plasma.” Plasma Chemistry and Plasma Processing 37 (4): 1033–49. https: / doi.org / 10.1007 / s11090-017-9806-x.Liu, Zhuomin, Thomas Whidden, Tobie Boutot, and Yun Yang. 2008. Apparatus and Method for Effecting Plasma-Based Reactions. WO2010037237A1_I, issued 2008.Maqueo, P. D.G. G, S. Coulombe, and J. M. Bergthorson. 2019. “Energy Efficiency of a Nanosecond Repetitively Pulsed Discharge for Methane Reforming.” Journal of Physics D: Applied Physics 52 (27): 274002. https: / doi.org / 10.1088 / 1361-6463 / ab199b.Sun, D.L., F. Wang, R.Y. Hong, and C.R. Xie. 2016. “Preparation of Carbon Black via Arc Discharge Plasma Enhanced by Thermal Pyrolysis.” Diamond and Related Materials 61 (January): 21–31. https: / doi.org / 10.1016 / j.diamond.2015.11.004.Zherlitsyn, Alex G., Vladimir P. Shiyan, and Paul V. Demchenko. 2016. “Microwave Plasma Torch for Processing Hydrocarbon Gases.” Resource-Efficient Technologies 2 (1): 11–14. https: / doi.org / 10.1016 / j.reffit.2016.04.001. This objective is achieved with a process for the production of hydrogen and carbon by plasmalysis of a hydrocarbon or a mixture of hydrocarbons, implementing a step of injecting this hydrocarbon or mixture of hydrocarbons in gaseous form into the active zone of a reaction chamber within a controlled reactor to produce a plasma, characterized in that it comprises a repetition of the following steps: production of a plasma pulse (E1) for a pulse duration until a target heating temperature is reached, maintenance of an optimal activation regime (E2) of the heated plasma in the active zone of the reaction chamber, for a predetermined residence time, cooling (E3) of the resident plasma in the active zone of the reaction chamber for a predetermined cooling time. According to the invention, the injection step includes channeling the flow of the incoming gas stream towards the active zone, and ejecting the products of plasmalysis from the active zone, by implementing a nozzle arranged to provide a secondary flow of the gas stream, separate from that of the incoming hydrocarbon subjected to plasmalysis, in the vicinity of the electrodes. The residence time, or duration, is the time during which the volume of gas to be treated is exposed to the plasma. This time affects the rate of chemical reactions: a longer residence time can allow for more complete reactions, while a shorter residence time can prevent the decomposition or dissociation of certain products and inhibit some recombination. The active zone of the reactor's reaction chamber is the volume of gas treated by the plasma in the space between the reactor's electrodes. The optimal activation regime is defined as the point at which the plasma reaches stable conditions to maximize interactions with the treated gas. It should be noted that the production process according to the invention is designed to treat both pure and diluted hydrocarbons. The gas stream entering a reactor is composed mainly of hydrocarbons, i.e., the gas stream is composed of hydrocarbons at a concentration of more than 50%, but a dilution gas can be introduced at concentrations of 10%, 20%, 30%, or even 40%. This dilution gas can be H2, or non-combustible gases such as nitrogen, argon, or helium. The hydrocarbon gas stream can be injected into an active zone of a reaction chamber of a plasmalysis reactor comprising electrodes whose power supply is controlled to generate a pulsed plasma in the active zone of the reaction chamber, and the plasma cooling step, for a cooling time, can be triggered by interrupting the power supply to the electrodes for a predetermined interruption time. The hydrocarbon gas flow passing through the active zone of the plasmalysis reactor reaction chamber can undergo a progressive increase in temperature on its path between the inlet and outlet of the active zone. The hydrocarbon gas stream injected into the active zone of the reaction chamber of the plasmalysis reactor can be a methane gas stream, said active zone being the site of a series of reactions (R1-R4) of respective dissociations of the injected methane, ethane, ethylene, and acetylene, and the cooling step is triggered before the initiation of the (R4) decomposition step of acetylene, for a predetermined cooling time. The repetition N of steps (E1, E2, E3) applied to the gas flow during its path between the inlet and outlet of the active zone can stop when the initiation of step (R4) coincides with the ejection of the gas flow from the active zone. Preferably, the repetition N of steps (E1, E2, E3) is periodic. In a particular embodiment of the invention, the production process may further include: a step to preheat the gas injected into the plasmalysis reactor (10) until a plasma temperature is reached within a target heating temperature range, so that the first three dissociation reactions (R1, R2, R3) are initiated and triggered; a step of interrupting the plasma by controlling the electrodes (13, 14) for a predetermined interruption time such that the first three dissociation reactions are sufficiently advanced and the acetylene dissociation step (R4) is not fully initiated; a step of cooling the plasma until a plasma temperature is reached within a target cooling temperature range, followed by a return to the preheating step. Cooling occurs during the interruption period during which the plasma no longer receives energy to maintain its temperature. Cooling is achieved during this same period by heat diffusion, advection of fresh gas, or by radiative exchange of the plasma with the reactor walls. The cooling time is equivalent to the plasma interruption time. The target heating temperature range is preferably within the range [1500 – 3000 K], it can advantageously be within the range [1800 K – 2000 K]. The target cooling temperature range is preferably within the range [1000 K – 1500 K]. When the catalytic effect of the plasma, i.e., the cumulative effect of the energetic excitation of methane molecules, the formation of reactive species, the localized temperature effect, and the thermodynamic non-equilibrium, causes the first methane dissociation reaction, the process may further include the following steps: a step to control the electrode supply so that the energy input from the plasma leads to a plasma temperature such that the second and third dissociation reactions are faster than the acetylene dissociation reaction; a step of interrupting the plasma by controlling the electrode supply for a predetermined interruption time, before initiating the acetylene dissociation step (R4); a step of cooling the plasma until it reaches a plasma temperature within a target cooling temperature range, followed by a return to the preheating step.followed by a return to the order stage. The predetermined interruption time can be between 10µs and 100µs. The incoming gas stream may also include a methane dilution gas. The dilution gas can be chosen from dihydrogen H2, Argon or helium He. According to another aspect of the invention, a system for the production of hydrogen and carbon by plasmalysis of a hydrocarbon or a mixture of hydrocarbons is proposed, comprising a reactor having a reaction chamber including an active zone and provided for receiving an incoming gaseous flow of this hydrocarbon or mixture of hydrocarbons, a pair of electrodes powered by a pulse generator controlled by control means, piping means provided for channeling the flow of the incoming gaseous flow towards the active zone and for ejecting the products of the plasmalysis from the active zone, implementing the production process according to the invention. The control means are programmed to generate a pulsed plasma in the active zone of the reaction chamber. According to the invention, the channeling means are arranged to avoid stationarity of the gaseous products from plasmalysis on the surface of the electrodes. The electrodes can be arranged to produce a homogeneous electric field over the active area. The channeling means may advantageously include a nozzle arranged to provide a secondary flow of a gaseous stream distinct from that of incoming hydrocarbon subjected to plasmalysis, in the vicinity of the electrodes. The nozzle can be made of an electrically conductive material and has a floating electrical potential relative to either of the electrodes so as to act as a relay for plasma discharges between said electrodes. In a first configuration of an electrode-nozzle system implemented in the invention, the nozzle is provided to surround a first electrode so as to channel the incoming gas flow between said first electrode and the internal wall of said nozzle, said nozzle comprising at its end opposite said first electrode an opening provided to allow said channeled gas flow to pass through. In this first configuration, the nozzle can also have an hourglass shape comprising a first flared nozzle part surrounding a first electrode and a second flared nozzle part surrounding the second electrode within the reaction chamber, said first and second nozzle parts being connected by a conduit arranged to constitute a first active zone for the gas flow. In a second configuration of the electrode-nozzle system, the nozzle comprises a frustoconical cylinder interposed between a first electrode of substantially conical shape and a second hollow electrode of substantially frustoconical shape, said frustoconical cylinder forming the nozzle comprising: a first opening opposite said first electrode and separated from the first electrode by a first electrically insulating element having a central opening of a diameter greater than that of the interface end of said first electrode, so as to provide an annular gap to allow passage of a channeled gas flow along the external surface of said first electrode, a second opening separated from an open end of said second electrode by a second electrically insulating element having a central opening of a diameter substantially equal to that of the interface end of said second electrode,in order to channel the gas flow exiting the nozzle into said second electrode. In this second configuration, the nozzle can advantageously have a convergent shape from the first insulating element to the second insulating element. The control means of the production system according to the invention can be programmed to produce a pulsed plasma at a frequency between 1 kHz and 1 MHz. It can also be predicted that the control means are programmed to produce a pulsed plasma with a pulse duration of less than 100 ns for a gas pressure between 100 mbar and 1 bar, less than 1 µs for a gas pressure between 10 mbar and 100 mbar, and less than 10 µs for a gas pressure less than 10 mbar. Plasma control means can also be provided to: produce a plasma pulse (1) for a pulse duration until a target heating temperature is reached, maintain this heated plasma fully active in the active zone (30) of the reactive chamber (15) of the reactor (10), for a predetermined residence time, cool (3) the resident plasma for a predetermined cooling time. When the production system according to the invention is implemented for the production of solid carbon and dihydrogen from methane, the active zone of the reaction chamber of the reactor being the site of a series of reactions (R1-R4) of respective dissociation of injected methane, ethane, ethylene, and acetylene, the control means are programmed to interrupt the plasma before the (R4) acetylene dissociation step, for a predetermined interruption time. The production system according to the invention may then include means for preheating the gas injected into the plasmalysis reactor until it reaches a plasma temperature within a target heating temperature range, so that the first three dissociation reactions are initiated and triggered. The control means can be programmed to control the supply of the electrodes so that the energy input from the plasma leads to a plasma temperature such that the second and third dissociation reactions are faster than the dissociation reaction of acetylene. The control means can be provided to provide in operation a modulation of the plasma pulse frequency and / or a modulation of the voltage applied to the electrodes. The predetermined interruption time can be between 10µs and 100µs. The electrodes are preferably arranged to produce a homogeneous electric field over the active area. For the channeling of gas flows in a production system according to the invention, two types of piping are available, each with different functions: One has the function of bringing the gas into the active zone, the other has the function of extracting the treated gas from the active zone. The channels used can be: A channel channeled by a solid profile, A channel channeled by a gaseous flow. In a particular configuration of the production system according to the invention, the channel channeled by a solid profile is the incoming piping channel and includes a single-flow nozzle bringing the gas into the active zone. In another particular configuration of the production system according to the invention, the channel channeled by a solid profile is the incoming channel channel and includes a double-flow type nozzle bringing the gas into the active zone and arranged to provide a secondary flow of gas near the electrodes. With the plasma pulse technique implemented in the present invention, it is now possible to overcome the thermal runaway mechanism observed in prior art processes: a plasma with a high power density becomes resistive due to the high electron density and therefore heats up due to the Joule effect. This heating reduces the gas density and facilitates ionization, which further increases the electron density. The temperature rises sharply over very short periods. At atmospheric pressure, the temperature increase can occur in a few nanoseconds. Cooling on these timescales is not possible, and it is therefore necessary to interrupt the energy supply, i.e., pulse to control the temperature increase. This means ceasing the energy supply during the interval between pulses so that the plasma no longer receives energy to maintain its high temperature. The principle of the invention is thus to shift the growth phase of solid carbon, both temporally and spatially, downstream of the active plasma zone. This is achieved by pulsing for sufficiently short durations so that, in a single pulse, the threshold temperatures for carbon nucleation and growth are not exceeded. The production process according to the invention also makes it possible to avoid the effects of heat accumulation from pulse to pulse: with an electrode geometry designed to produce a homogeneous electric field over a large volume, by distributing the plasma filaments in space and thus avoiding the effects of local temperature concentration; with a mechanical flow control element preventing any stationary zone, i.e. a zone of product stagnation and a progressive increase in temperature. The process for producing solid carbon and dihydrogen according to the invention provides the following advantages: absence of short circuits and premature interruption of plasmalysis, guarantee of continuous operation over long periods, increased energy yield because there is no dilution with other gases, usable generation of carbon product in solid form, simplicity of use, no active or mechanical component to prevent the growth of carbon bridges. Carbon production in the gas is acceptable, but production on the electrode surface is not. The production process according to the invention makes it possible to produce carbon in volume while preventing this carbon production from occurring on the electrode surface. Carbon production is meaningful in the invention: it is one of the objectives of the device. It is therefore necessary to play on several parameters: find the temperature range which allows the production of carbon to be initiated while promoting the production of H2, distribute the hot zones to avoid a thermal accumulation which would create carbon and prevent a runaway mechanism, eliminate the stationary zones of the flow, expose the gas sufficiently so that the first three reactions are triggered but reduce the residence time relative to the carbon growth time. DESCRIPTION OF THE FIGURES This is a synoptic diagram of a first example of the realization of a system for the production of solid carbon and dihydrogen according to the invention; This is a synoptic diagram of a second example of the realization of a system for the production of solid carbon and dihydrogen according to the invention; Laest is a diagram representing the variation of reaction constants as a function of temperature for the main reactions implemented in the production process according to the invention; Lareprésente en synthetique des processus reactionnels implemented in the production process according to the invention; Lare represents a first configuration of an electrode-nozzle system implemented in a combustion reactor according to the invention, in a cross-sectional view (5A) and in a partial perspective view (5B); Laillustre on the views of the, the path of the gas flow and examples of arcs produced between the electrodes and the nozzle; Lare represents a second configuration of an electrode-nozzle system implemented in a combustion reactor according to the invention, in a partial perspective view (7A) and in a cross-sectional view (7B); Laillustre on the views of the, the path of the gas flow and examples of arcs produced between the electrodes and the nozzle; They represent several possibilities (9A-9D) for generating pulses, depending on the local parameters of the reactor; Laest is a diagram representing a relationship between production time and temperature, to cause a shift in the carbon growth phase, temporally and spatially, downstream of the plasma zone; This illustrates an example of control pulses where the pulse repetition frequency is between 10 and 100 kHz; and Laillustrates another configuration of an electrode-nozzle system implemented in a combustion reactor according to the invention, in a cross-sectional view, in which the nozzle is of the double-flow type. DETAILED DESCRIPTION A system 1 for the production of dihydrogen gas and solid carbon by plasma pyrolysis according to the invention comprises, with reference to the, a hydrocarbon source, for example methane 2, a plasma reactor 10 receiving via an inlet 3 this gas and producing at the outlet solid carbon stored in a chamber 5 connected via a collection conduit 4 downstream of the reactor 10 and dihydrogen gas in a storage unit 7 connected via an outlet conduit 6 downstream of the reactor 10. The hydrocarbon processed by the production system according to the invention can be supplied pure or in the form of hydrocarbon mixtures or even in a diluted form with a minor dilution gas i.e. in a proportion of less than 50%. The reactor 10 includes a first stage 11 for heating the incoming hydrocarbon gas, leading to an active zone 30 of a reaction chamber 15 of the reactor 10 comprising a pair of electrodes 13, 14, one of the electrodes 13 being surrounded by a pipe nozzle 12. In the following, we may refer to it interchangeably as a reaction chamber or a reactor. The electrodes 13,14 of the plasma reactor 10 have a geometry arranged to prevent any stationary zone, by distributing the discharges spatially so as to have a homogeneous electric field. In a second embodiment illustrated by the, for which identical components to those of the present identical references, the production system 1' comprises two reactors 10 coupled in parallel supplied with methane gas from a common feed unit 2 and whose respective outlets of H2 and solid carbon lead to a hot column 23 leading to a heat exchanger 24 intended to recover some of the heat to preheat the methane coming from a methane storage unit 25 and leads to the feed unit 2. The mixture of dihydrogen and solid carbon passing through the exchanger 24 is led to a separation unit 26 having on one side a gas outlet 4 connected to the inlet of a dihydrogen storage unit 7 and on the other side an outlet connected to a solid carbon recovery unit 5. The two reactors 10 each comprise a first heating stage 11 and a reaction chamber 15 comprising a pair of electrodes 13,14. The gas and solid carbon production system 1' according to the invention comprises a control device for the two reactors 10, including a unit 20 designed to control, on the one hand, a heating control device 21 and, on the other hand, a high-voltage power supply device 22 for each pair of electrodes. These two respective heating control and high-voltage power supply devices 21, 22 are respectively supplied with electrical energy 23. This second configuration is particularly suitable for the production of industrial equipment, with an installation of a plurality of containers arranged one on top of the other with a set of cells working in parallel and managed in a fully automatic way. An industrial installation may, for example, provide for the installation of the cells in a production system according to the invention, in which the cells inject their gas into the hot column 23 where the R4 reaction of acetylene decomposition can continue, followed by the heat exchanger 24 to cool the temperature of the outlet gases and preheat the gas mainly composed of hydrocarbons to be reformed 25, and where the hydrogen and carbon products are separated by a gas-solid separation unit 26. Electrode geometry Electrode geometry plays a crucial role in preventing carbon bridge formation. A cone-shaped or pointed geometry for both electrodes promotes carbon bridging and should therefore be avoided. An asymmetrical geometry, particularly a concave one electrode, allows for a more even distribution of discharges. Placing a solid profile between the two electrodes, channeling the incoming gas flow and passing it through a bottleneck, improves the desired technical effect by avoiding any point of contact between the two electrodes. Several geometries, which will be described later, allow the inclusion of secondary flow to prevent the formation of stationary zones. It is observed that carbon bridges (junctions) form outside the main flow. It is also observed that secondary injection allows the growth point to be moved outside the nozzle edges, which are bathed by the secondary flow. The main flow is channeled by a secondary flow, avoiding any recirculation zones. The main flow is channeled by a mechanical component, avoiding any recirculation zones. It should be noted that the incoming gas to be treated may experience turbulence and have stationary zones. This has no impact because, since the gas has not been treated, it is not at risk of producing carbon. However, upon exiting the active zone, the treated gas is ready to produce carbon and no stationary zones exist. A flow tube or directed gas flow can be obtained either by means of a double flow nozzle or by means of an external flow. The geometries of the electrodes can be different. They have in common that they have rotational symmetry, but one can be a solid cone, while the other can be an open (or hollow) cone. The plasma pyrolysis gas production process according to the invention implements a temporal and spatial shift in the growth phase of solid carbon downstream of the plasma zone. Within the chain reactions triggered by pyrolysis, there are, with reference to the [reference to the invention], four main recombinations: R1. + (ethane) R2. + (ethylene) R3. (acetylene) R4. The goal is to slow down or delay carbon growth in reaction R4, i.e., during the decomposition of acetylene in the active zone. The amount of energy supplied to the plasma must be modulated over time to channel and select the reaction pathways, favoring reactions R1, R2, and R3 and delaying, or even preventing, the initiation of R4. The production process according to the invention is inherently an iterative process, with reference to the invention. Each gas molecule is subjected to a pulse from 1 to N times as it passes through the active zone of the reactor. This number N corresponds to the number of pulses of pulse duration t and can be between 1 and 1000, depending on the pulse frequency f, the length L of the distance between the two electrodes, and the velocity V of the gas flow through the active zone. During the time it takes for the gas molecule to travel from the inlet to the outlet of the active zone, multiple chemical dissociations and recombinations occur, and the temperature of the product of these various chemical reactions (hydrocarbon radicals, ionized particles, and other chemical combinations) increases non-linearly as they progress toward the outlet of the active zone. It is the combination of the five main parameters, namely the distance L between the two electrodes, the velocity V of the flux, the number N of pulses, the pulse duration t (or the duty cycle) and the frequency of the pulses, that will allow us to obtain the desired technical effect. In the synthesis illustrated by the, this means that the gas molecule entering the active zone will undergo an iterative process of N pulses during its journey between the entry and exit of the active zone. When the CH4 hydrocarbon molecule enters the active zone 30 at a temperature T0, it undergoes a first impulse for an impulse duration t. This impulse contributes to dissociating and recombining the CH4 molecule into first by-products (hydrocarbon radicals) and increases the temperature of the molecules resulting from this first dissociation / recombination (reactions R1 to R3) to a temperature which will rise throughout the duration of the impulse and then fall back (inter-impulse cooling) and stabilize at a temperature T1 greater than T0. The active volume cools between pulses through radiation and heat diffusion with its surroundings. This cooling cannot be adjusted during operation. A controllable cooling method is heat exchange with the secondary flow around the active zone. Varying the flow rate of the secondary flow can be used to modify the cooling of the active zone. When the products resulting from the first dissociation / recombination undergo the second pulse, a new dissociation / recombination (reactions R1 to R3) will take place and the products from these new reactions will see their temperature increase throughout the pulse and then fall back down (inter-pulse cooling) and stabilize at a temperature T2 higher than T1. Throughout this iterative process, gas molecules move from the entrance to the exit of the active zone. When the products that have undergone N pulses have reached a temperature T N and reach the threshold of the active zone exit, there is no more inter-pulse cooling because it is at this precise moment that the products from the N ième Plasma discharges are expelled from the active zone. This temperature T N is the temperature that initiates the R4 reaction of acetylene dissociation. The temperature of the products exiting the active zone continues to rise, and the R4 reaction proceeds outside the plasma zone. This is a self-sustaining exothermic reaction. Acetylene (C₂H₂) decomposes thermally to produce carbon black (C) and hydrogen (H₂). The production process according to the invention is therefore a process of N iterations applied to the hydrocarbon gas CH4 during its passage between the inlet and outlet of the active zone of the reaction chamber, of a three-step sequence: a step to produce a plasma pulse, a step to maintain the heated plasma in an optimal activation regime for the duration of the pulse during which certain dissociations / recombinations of the gas occur, a step to cool the plasma for a duration of plasma interruption, followed by ejection of the gas from the active zone at N ième iteration at the moment when the R4 reaction of acetylene decomposition is initiated. The cooling time is equivalent to the plasma interruption time. Gas nozzle design to prevent carbon bridge formation The gas nozzles are designed to interact with the electrodes in such a way as to prevent gas recirculation and thus the presence of carbon bridge precursors in a location conducive to carbon bridge formation. Furthermore, a geometric constraint has been added to the nozzle to prevent stray discharges and contamination of the solid carbon by the nozzle metal. It is observed that carbon bridges grow from a stationary zone, sheltered from the flow. A second gas injection, coaxial with the flare, is designed to eliminate these stationary zones. In the case of heterogeneous staged injection, the gas in question is different from the gas to be reformed. In this case, homogeneous staged injection is chosen, meaning that a gas identical to the gas to be reformed is used, i.e., pure CH4. In a first configuration of an electrode-nozzle system 5 illustrated by Figures 5 and 6, two first and second electrodes 51, 52, of hollow frustoconical shape, are arranged opposite each other. The first electrode 51 is surrounded by a nozzle 50 which has an end opening 53 machined to have a substantially concave shape, this end opening 53 being interposed between the first and second electrodes 51, 52. The specific concave shape of the nozzle interposed between the two electrodes has the remarkable technical effect of allowing a recirculation of the incoming flow, meaning that no channel is privileged. In operation, with reference to Figure 6A, a gas flow 6 is injected between the outer wall of the first electrode 51 and the inner wall of the nozzle 50, passes through the end opening 53 and then flows along the outer wall of the second electrode 52. The use of a lateral gas flow prevents the creation of any stationary zone from the central flow. The nozzle possesses a floating electrical potential and acts as a relay by redistributing the flux. This nozzle, which redistributes the received gas flux, can be defined as a neutrode in the sense of plasma physics. Its concave portion, interposed between the two electrodes, constitutes an arc foot, the attachment zone for electric arcs. The nozzle 50 thus acts as a neutrode and electric arcs 61 are established successively between the first electrode 51 and the neutrode nozzle 50, while other electric arcs 62.1,62.2,62.3 are established successively between the neutrode nozzle 50 and the second electrode 52. This results in a desired technical effect of non-stationarity of the gas within the active zone. Other nozzle configurations with primary and secondary flow control can be considered. The inter-electrode space includes an active zone that receives the gas to be treated, which is channeled by a solid profile. Downstream of this active zone is a carbon production zone surrounded by a gas flow channeling the treated gas exiting the active zone. The material used for the construction of a nozzle must be electrically conductive, for example, but not limited to, copper or graphite. In a particular embodiment, the electrodes may have a conical shape. Alternatively, a solid, hourglass-shaped nozzle profile can be considered, composed of two inverted cones connected by a thin cylinder, positioned inside the reaction chamber midway between the two electrodes. The constricted central section, or bottleneck, encloses the active zone in its middle. This profile allows for better flow control by creating a Venturi effect that influences the forces within the flow, preventing the formation of stationary zones in the active area. In a second configuration of an electrode-nozzle system 7 illustrated by figures 7 and 8, the two electrodes 71,72 are asymmetrical: a first electrode 71 has a conical shape designed so that a gas flows along its external surface, while a second electrode 72, coaxial with the first electrode 71, has a hollow frustoconical shape.The electrode-nozzle system 7 further comprises, between the first and second electrodes 71, 72,: a first electrically insulating element 73, of annular shape surrounding the exit end of the first electrode 71 while leaving an annular gap 75 intended to allow the passage of a gas flow, a nozzle 70 of truncated conical shape extending from the first insulating element 73, this nozzle acting as a neutrode and intended to receive via its large diameter opening the gas flow exiting the annular gap 75, a second electrically insulating element 74, of annular shape whose internal opening has a first surface on which is fixed the small diameter opening of the nozzle 70 and a second surface on which is fixed the small diameter opening of the second electrode 72. In this electrode-nozzle system 7 in operation, with reference to the, a gas flow 6 is injected along the external surface of the first electrode 71. This gas flow passes through the gap 75 of the first insulating element 73, crosses the interior of the neutrode nozzle 70, passes through the opening of the second insulating element 74 and then enters the second hollow electrode 72 via its small diameter opening. Electric arcs 61 are created successively between the first electrode 71 and the neutrode nozzle 70 and other arcs are created successively between the neutrode nozzle 70 and the second electrode 72. A first active zone 81 covers the region between the first and second electrodes 71,72 while three other active zones 82, sites of the reactions indicated in the figure, are generally located inside the second electrode 72, in the vicinity of its small diameter opening fixed to the second insulating annular element 74. In another configuration of the electrode-nozzle system 8 illustrated by Figure 1, in which elements common to Figures 5 and 6 are designated by identical reference numerals, the nozzle is of the dual-flow type with a first nozzle section acting as a neutrode 50 surrounding the electrode 51 and itself surrounded by a second nozzle section 54 designed to channel a separate secondary flow 80 from the gas flow, a primary flow 81 of which is channeled between the electrode 51 and the first nozzle section 50 so as to pass through an active volume 90 treated by the plasma. In this configuration, the separate secondary flow eliminates stationary zones and cools the active volume 90 treated by the plasma. Carbon growth control method We will now describe an example of the implementation of a process for controlling carbon growth. Use of a nanosecond pulsed plasma (pulse duration ≤ 10 ns) When using pulsed plasma, short plasma times (or residence times, i.e., the time the gas volume to be treated is exposed to the plasma) are used to increase the pulse power in order to maintain a constant average power over time, and short off-plasma times are used to limit the recombination of hydrogen atoms. Furthermore, a faster power rise time induces a faster propagation velocity of the plasma plume, allowing for the formation of a higher electric field and increased radical formation efficiency. Consequently, pulsed systems crack methane into hydrogen and other hydrocarbon radicals more efficiently than continuous-wave systems. The type of plasma generated is classified as non-thermal and atmospheric pressure. Pulsed plasma offers numerous advantages: The residence time of the gas in the plasma is shorter than the characteristic time for the production of solid carbon (reaction R4). Thanks to electron impact dissociation, the dissociation rate of methane molecules (reaction 1) is faster than that of the carbon production reaction (reaction 4), even at low temperatures. The pulsed nature of the plasma acts as a thermal low-pass filter, causing an uneven distribution of energy transfer: a large amount of energy is received by the electrons in a very short time, and their temperature therefore increases exponentially. The residual energy is absorbed by the gas, but over a longer period, and the average gas temperature increases only linearly. In other words, the strong electric field used allows for selectivity that limits heating: there is a strong transfer of energy from electrons towards the electronic excitation and vibrational modes of the molecules rather than towards the translational modes responsible for heating the gas These characteristics allow for a time lag between the production of solid carbon and the dissociation of gas molecules. Distribution of landfills The discharges must be spatially distributed over a wide area to avoid sharp local temperature increases and the accumulation of precursors responsible for the production of solid carbon. A changing redistribution with each pulse prevents localized carbon deposition in a single location and immediate fouling that could cause a short circuit. Several 9A-9D plasma pulse control waveforms can be considered, as illustrated in the. Pulsed plasma offers a key advantage: With each pulse, the initiation process creates a shock wave that induces its own flow dynamics and redistributes the reaction products throughout the space. The reaction products are also rapidly cooled by mixing with the surrounding gas. This allows for control of the average energy in the plasma. The average plasma energy must be controlled to improve energy absorption by the gas and to select the reaction pathways that best convert it into separate components. Indeed, radical production is directly linked to the pulse power density and its associated plasma energy, which control the gas temperature and thermal dissociation. To control the average energy in the plasma, it is necessary to control the duration and frequency of the radiation pulse, its duty cycle (i.e., the ratio between the period during which the pulsed radiation is activated and the period during which it is deactivated), its shape, and the average energy level output over time. It is essential to ensure that the following formula is respected: xL << τchemical carbon time Where f = frequency, t = pulse duration, V = flow velocity, L = distance between the 2 electrodes τ is the characteristic time of the carbon formation reaction (R4). Depending on the local parameters of the reactor, there are, with reference to the, several possibilities for generating pulses. The pulsed wave radiation used to control plasma generation has a power level that also depends on the voltage level. Depending on the available voltage, the duty cycle varies: From 5kV to 10kV ON / OFF < 20%; From 10kV to 15kV ON / OFF < 50%; From 15kV to 50kV ON / OFF < 90% To induce a shift in the carbon growth phase, both temporally and spatially, downstream of the plasma zone, one can proceed as follows, with reference to: [Procedure 1: local heating] The gas is heated to 2000 K < T < 3500 K, reactions (R1) to (R3) are initiated and triggered, The plasma is interrupted before (R4) for a time between 10 µs and 100 µs, The gas is cooled to 1000 K < T < 1500 K to return to zone I, Then we start again. Or [Procedure 2] The gas is not preheated. Reaction (R1) is carried out using the catalytic effect of the plasma. The energy input from the plasma is controlled so that the temperature, following the application of the plasma, places the gas in a zone where reactions (R2) and (R3) are faster than reaction (R4); i.e., a temperature T > ~1200 K. The plasma is interrupted before (R4) for a time between 10 μs and 100 μs. Then the process is repeated. The pulse width is between 1ns and 100ns. The pulse frequency must be between 10 MHz and 1 GHz, preferably around 100 MHz (10 ns width). The pulse repetition frequency is between 10 and 100 kHz (every 10 to 100 μs), with reference to the. Pulses for short durations prevent exceeding the threshold temperatures for carbon nucleation and growth. Since the repetition frequency is high, it is also necessary to control heat buildup between pulses, which requires controlling inter-pulse cooling. Several examples are given and the recommended radiation source is configured to activate and deactivate pulsed radiation at a frequency between 500 Hz and 1000 kHz and with a duty cycle of less than 90%. In most prior art plasmalysis devices, high pressure is used because it allows for greater efficiency. In contrast, low-pressure reactions allow for greater selectivity. Here, in the production process according to the invention, the reaction takes place at atmospheric pressure. The ideal pressure can be considered to be between 100 mbar and 2 bar absolute. Extension of operating times The combined hydrogen and carbon production process according to the invention makes it possible to considerably extend operating times, as illustrated by the three embodiment examples described below. Thus, in a first example of implementation corresponding to a frequency of 25 kHz, a voltage of 10 kV, a flow rate of 3 nl / min with a composition of 100% CH4, and a conventional nozzle, the process is interrupted by the growth of carbon on the electrodes or the nozzle after an average time of 30 s. In a second embodiment, a secondary flow of CH4 is added outside the nozzle to eject carbon from the active zone. The average time before process interruption due to carbon growth on the electrodes or nozzle remains 30 s. However, the carbon growth zones on the electrodes are affected by the presence of the external flow and move towards the inside of the nozzle, demonstrating that the control method is effective but not sufficient. In a third embodiment, the nozzle is shaped with a concave outlet face to eliminate stationary zones in the flow and is surrounded by an external flow. In this third embodiment, the average time before a process interruption due to carbon growth on the electrodes or the nozzle is 15 minutes. In a fourth embodiment corresponding to a frequency of 20 kHz, a voltage of 12 kV, a flow rate of 1 nl / min, and a composition of 100% CH4, an improved nozzle with a concave outlet face, as shown in the figure, has an average time before process interruption due to carbon growth on the electrodes or the nozzle of 180 min. Thus, flow control methods and the use of pulses have made it possible to go from an uninterrupted operating time of less than 30 s on a conventional nozzle to an uninterrupted operating time of 180 min on a prototype of the improved nozzle shown in the figure. This allows for the implementation of an industrial operation with alternating production cycles on the order of one hour of interrupted operation, combined with means for detecting the presence of carbon, and mechanical or chemical regeneration methods lasting from a few seconds to a few tens of seconds.The use of several cells in parallel as represented in the figure also makes it possible to guarantee continuous production at the installation level. Flow control without secondary flow Flow control with secondary flow Flow control with a concave nozzle Flow control with an improved nozzle (Figure 5) Frequency 25 kHz 25 kHz 25 kHz 20 kHz Voltage 10 kV 10 kV 10 kV 12 kV Flow rate 3 nl / min 3 nl / min 3 nl / min 1 nl / min Gas composition 100% CH4 100% CH4 100% CH4 100% CH4 Operating time without interruption 30 s 30 s 15 min 180 min. Pulse energy adjustment: For a given mass flow rate of gas to be treated, the flow velocity can be adjusted by changing the diameter of the nozzle used. A pulse frequency (PFR) is then selected. This frequency determines the cooling time between two pulses. A nomogram or table, such as the one shown below, is then used to determine the temperature obtained after the pulse, as a function of the flow velocity and the frequency used. The heating temperature can be measured directly at the plasma level using methods such as spectroscopy. Another industrial method for controlling the heating temperature is to control the energy deposited in each pulse. This requires knowledge of the thermodynamic properties of the gas, which is obtained by knowing the composition of the input gas, as well as the heating from previous pulses. From this, the energy to be applied to obtain a sequence of target temperatures between several pulses, ranging from a minimum to a maximum temperature, can then be deduced. The table below shows the minimum and maximum energy (MJ / kg / pulse) as a function of the flow velocity and pulse frequency when a stable plasma production zone is present. The energy per pulse, given as minimum and maximum values, is adjusted, for example, by modifying the generator setpoint voltage. Since the energy is expressed in MJ / kg / pulse, it must be converted to energy (J) by a production system operator based on the flow rate used. Tables for determining the heating temperature and energy per pulse can be constructed for specific residence lengths of the active zone. In the present embodiment, this residence length is 10 mm. A change in active length for another embodiment can be corrected by a proportional change in the flow velocity when reading the table. Flow velocity (m / s) Pulse and cooling frequency (kHz) Stable zone Minimum pulse heating (+°C) Maximum pulse heating (+°C) Minimum pulse energy (MJ / kg / pulse) Maximum pulse energy (MJ / kg / pulse) 3020X No stable zone 4020 Stable zone found 190930495.5555020X No stable zone 6020X No stable zone 6010X No stable zone 6020X No stable zone 6030 Stable zone found 190930845.5556040 Stable zone found 18433035776045X No stable zone 6050 Stable zone found 13872946456060 Stable zone found135827424.757010X No stable zone 7020X No stable zone 7030 Stable zone found 19983784577040 Stable zone found 1843316066.57045 Stable zone found 18953101777050 Stable zone found 184330517.57.57060 Stable zone found 13582929458010X No stable zone 8020X No stable zone 8030X No stable zone 8040 Stable zone found 190932445.568045 Stable zone found 1865319566.58050 Stable zone found 19363154778060X No stable zone 9010X No stable zone 9020X No stable zone 9030X No stable zone 9040 Stable zone found 1943329455.59045 Stable zone found190932555.569050stable zone found1883322466.59060stable zone found1843317677.510010Xno stable zone10020Xno stable zone10030Xno stable zone10040Xno stable zone10045stable zone found55.510050stable zone found192432855.5610060stable zone found190932646.5711010X no stable zone1887323511020X no stable zone11030X no stable zone11040X no stable zone11045 stable zone found5711050 stable zone found207139345611060 stable zone found1909342566.512010X no stable zone1909326912020X no stable zone12030X no stable zone12040X no stable zone12045X no stable zone12050 stable zone found5712060 stable zone found204239045.56.5. Of course, the present invention is not limited to the embodiments just described and many other variants can be considered without departing from the scope of the invention. These embodiments can be used with hydrocarbon mixtures, including, for example, methane, ethane, and ethylene, which will be decomposed into acetylene. The embodiments can also be used with a hydrocarbon mixture containing acetylene in a minor quantity, <10% by volume.

Claims

A process for producing hydrogen and carbon by plasmalysis of a hydrocarbon or a mixture of hydrocarbons, implementing a step of injecting this hydrocarbon or mixture of hydrocarbons in gaseous form into the active zone of a reaction chamber within a reactor controlled to produce a plasma between two electrodes (13,14), comprising a repetition of the following steps: production of a plasma pulse (E1) for a pulse duration until a target heating temperature is reached, maintenance of an optimal activation regime (E2) of the heated plasma in the active zone of the reaction chamber, for a predetermined residence time, cooling (E3) of the resident plasma in the active zone of the reaction chamber for a predetermined cooling time.characterized in that the injection step includes channeling the flow of the incoming gas stream towards the active zone (30), and ejecting the products of the plasmalysis from the active zone (30), by implementing a nozzle arranged to provide a secondary flow of the gas stream, distinct from that of the incoming hydrocarbon subjected to plasmalysis, in the vicinity of the electrodes. A production method according to the preceding claim, wherein the hydrocarbon gas stream is injected into an active zone (30) of a reaction chamber (15) of a plasmalysis reactor (10) comprising electrodes (13,14) whose supply (22) is controlled to generate a pulsed plasma in the active zone (30) of the reaction chamber (15), wherein the plasma cooling step for a cooling time is triggered by the interruption of the supply (22) of the electrodes (13,14) for a predetermined interruption time. Production method according to any one of the preceding claims, wherein the hydrocarbon gas flow passing through the active zone (30) of the reaction chamber (15) of the plasmalysis reactor (10) undergoes a progressive increase in temperature on its path between the inlet and outlet of the active zone. A production method according to any one of the preceding claims, wherein the hydrocarbon gas stream injected into the active zone (30) of the reaction chamber (15) of the plasmalysis reactor (10) is a methane gas stream, said active zone being the site of a series of reactions (R1-R4) of respective dissociations of the injected methane, ethane, ethylene, and acetylene, characterized in that the cooling step is triggered before the initiation of the reaction (R4) of decomposition of acetylene, for a predetermined cooling time. Production method according to the preceding claim, characterized in that the periodic repetition N of steps (E1, E2, E3) applied to the gas flow during its path between the inlet and outlet of the active zone (30) stops when the initiation of step (R4) coincides with the ejection of the gas flow outside the active zone. A production method according to the preceding claim, characterized in that it further comprises: a step for preheating the gas injected into the plasmalysis reactor (10) until a plasma temperature is reached within a target heating temperature range, so that the first three dissociation reactions (R1, R2, R3) are initiated and triggered; a step of interrupting the plasma by controlling the electrodes (13, 14) for a predetermined interruption time such that the first three dissociation reactions are sufficiently advanced and the acetylene dissociation step (R4) is not completely initiated; a step of cooling the plasma until a plasma temperature is reached within a target cooling temperature range, followed by a return to the preheating step. Production method according to the preceding claim, characterized in that the target heating temperature range is within the range [1500 K – 3000 K]. Production method according to the preceding claim, characterized in that the target heating temperature range is within the range [1800 K – 2000 K]. Production method according to one of the three preceding claims, characterized in that the target cooling temperature range is within the range [1000 K – 1500 K]. A production method according to any one of claims 4 to 9, wherein the catalytic effect of the plasma causes the first dissociation reaction of methane, characterized in that it further comprises the following steps: a step to control the supply of the electrodes (13,14) so ​​that the energy input of the plasma leads to a plasma temperature such that the second and third dissociation reactions are faster than the dissociation reaction of acetylene, a step of interrupting the plasma by controlling the supply of the electrodes (13,14) for a predetermined interruption time, before initiation of the acetylene dissociation step (R4), a step of cooling the plasma until reaching a plasma temperature within a target cooling temperature range, followed by a return to the preheating step, followed by a return to the control step. Production method according to any one of claims 4 to 10, characterized in that the predetermined interruption time is between 10µs and 100µs. Production process according to any one of claims 4 to 11, characterized in that the inlet gas stream further comprises a methane dilution gas. Production process according to the preceding claim, characterized in that the dilution gas is chosen from dihydrogen H2, Argon or helium He. System (1,1') for the production of hydrogen and carbon by plasmalysis of a hydrocarbon or a mixture of hydrocarbons, comprising a reactor (10) having a reaction chamber (15) having an active zone (30) provided for receiving an incoming gaseous flow of this hydrocarbon or mixture of hydrocarbons, a pair of electrodes (13,14; 51,52;71,72) powered by a pulse generator (22) controlled by control means (20) programmed to generate a pulsed plasma in the active zone (30) of the reaction chamber (15), channeling means (12, 50, 70) provided to channel the flow of the incoming gas stream towards the active zone (30) and to eject the products of the plasmalysis from the active zone (30), implementing the production process according to any one of the preceding claims, characterized in that the channeling means (12, 50, 70) are arranged to avoid stationarity of the gaseous products from the plasmalysis on the surface of the electrodes (13,14; 51,52; 71,72).; Production system (1,1') according to the preceding claim, characterized in that the channeling means (50,70) comprise a nozzle arranged to provide a secondary flow of a gaseous stream distinct from that of incoming hydrocarbon subjected to plasmalysis, in the vicinity of the electrodes (51,52;71,72). Production system according to the preceding claim, characterized in that the nozzle (50,70) is made of an electrically conductive material and has a floating electrical potential with respect to one or the other of the electrodes (51,52;71,72) so as to constitute a relay for plasma discharges between said electrodes. Production system according to the preceding claim, characterized in that the nozzle (50) is provided to surround a first electrode (51) so as to channel the incoming gas flow between said first electrode (51) and the inner wall of said nozzle (50), said nozzle (50) comprising at its end opposite said first electrode an opening (53) provided to allow said channeled gas flow to pass through. Production system according to claim 15, characterized in that the nozzle has an hourglass shape comprising a first flared nozzle part surrounding a first electrode and a second flared nozzle part surrounding the second electrode, said first and second nozzle parts being connected by a conduit arranged to constitute a first active zone for the gas flow. Production system according to claim 15, characterized in that the nozzle (70) comprises a frustoconical cylinder interposed between a first electrode (71) of substantially conical shape and a second hollow electrode (72) of substantially frustoconical shape, said frustoconical cylinder forming nozzle (70) comprising: a first opening opposite said first electrode (71) and separated from the first electrode (71) by a first electrically insulating element (73) having a central opening of diameter greater than that of the interface end of said first electrode (71), so as to provide an annular gap to allow a channeled gas flow to pass along the external surface of said first electrode (71),a second opening separated from an open end of said second electrode (72) by a second electrically insulating element (74) having a central opening with a diameter substantially equal to that of the interface end of said second electrode (72), so as to channel the gas flow exiting the nozzle into said second electrode (72). Production system according to the preceding claim, characterized in that the nozzle (70) has a convergent shape from the first insulating element (73) to the second insulating element (74). Production system according to claim 15, characterized in that the nozzle has a concave outlet face. Production system (1) according to any one of claims 14 to 21, characterized in that the control means (20) are programmed to produce a pulsed plasma at a frequency between 1 kHz and 1 MHz. Production system (1) according to the preceding claim, characterized in that the control means (20) are programmed to produce a pulsed plasma with a pulse duration of less than 100 ns for a gas pressure between 100 mbar and 1 bar, less than 1 µs for a gas pressure between 10 mbar and 100 mbar, and less than 10 µs for a gas pressure less than 10 mbar. Production system (1) according to one of the two preceding claims, characterized in that the electrodes (13,14) are arranged to produce a homogeneous electric field over the active area (30). Production system (1) according to any one of claims 14 to 24, characterized in that the plasma control means are provided to: produce a plasma pulse for a pulse duration until a target heating temperature is reached, maintain this heated plasma fully active in the active zone (30) of the reaction chamber (15) of the reactor (10), for a predetermined residence time, cool the resident plasma for a predetermined cooling time. Production system (1) according to the preceding claim, characterized in that it further comprises means (11) for preheating the gas injected into the plasmalysis reactor until it reaches a plasma temperature within a target heating temperature range, so that the first three dissociation reactions are initiated and triggered, Production system (1) according to any one of claims 25 or 26, characterized in that the control means (20) are programmed to control the supply of the electrodes (13,14) so ​​that the energy input from the plasma leads to a plasma temperature such that the second and third dissociation reactions are faster than the dissociation reaction of acetylene. Production system (1) according to any one of claims 14 to 27, characterized in that the control means (20) are provided to provide in operation a modulation of the plasma pulse frequency and / or a modulation of the voltage applied to the electrodes (13,14).

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