Methods and associated reactors and systems for plasma-based methane conversion

Abstract

A method is disclosed for plasma-based conversion of methane in a plasma reactor, the plasma reactor (1) comprising a reaction chamber and an energy source to create the plasma in the reaction chamber, the reaction chamber being fluidly connected to a separate post-plasma reaction chamber, the plasma reactor comprising a gas inlet, the post-plasma reaction chamber comprising a gas outlet, the method comprising: a) generating a plasma in the reaction chamber by activating the energy source; b) providing a first gas flow comprising a CH4 component through the gas inlet and into the reaction chamber, allowing plasma-based conversion of the CH4 component into gaseous H2 and solid carbon and allowing the continuous removal of produced H2 through the post-plasma reaction chamber and through the gas outlet; c) in consideration of a solid carbon deposit being formed on a predetermined portion of an inner wall of the post-plasma reaction chamber, replacing the first gas flow with a second gas flow, the second gas flow comprising a plasma-activated CO2 component, allowing the oxidation of the deposited carbon, and allowing the continuous removal of produced CO through the gas outlet until the deposited carbon is substantially removed; and associated reactors and reactor systems

Description

[0001] Methods and associated reactors and systems for plasma-based methane conversion.

[0002] Technical field

[0003] The present disclosure relates to methods and associated reactors and systems for plasma-based methane conversion, for instance, but not only, for the production of syngas.

[0004] Background art

[0005] Plasma-based methane (CH4) conversion has been intensely investigated as a potentially green hydrogen (H2) production route. This H2can be used as a fuel directly or combined with carbon monoxide (CO) to form syngas, a key precursor for many industrial chemical processes. Pure plasma-based CH4conversion is typically limited by the formation of solid carbon, which can disrupt the discharge and interrupt the process.

[0006] As a solution, CH4is often diluted with a secondary gas, which may be an inert gas such as argon (Ar), which does not influence the products other than diluting the effluent stream. This increases the operation time of the reactor, but still results in the production of solid carbon which requires removal after time.

[0007] Another secondary discharge gas option is carbon dioxide (CO2), which results in the direct production of syngas (CO / H2) in a process known as dry reforming of methane (DRM). DRM does aid in soot suppression at lower CH4fractions, albeit with the potential formation of other unwanted products such as water. In addition to this, higher CH4fractions still result in carbon deposition, eventually destabilizing and disrupting the plasma discharge (and hence the potential continuous operation of the process).

[0008] There exists a need in industry for improved methods, reactors and reactor systems that solve the above-cited problems.

[0009] Summary of the disclosure It is an objective of the present disclosure to provide a method according to claim 1 , and to provide associated reactors and reactor systems.

[0010] In a first aspect of the present disclosure, a method is disclosed for plasma-based conversion of methane in a plasma reactor, the plasma reactor comprising a reaction chamber and an energy source to create the plasma in the reaction chamber, the reaction chamber being fluidly connected to a, preferably separate, post-plasma reaction chamber, preferably by means of a fluid connection comprising an opening or open tube arranged in a common separation wall between the reaction chamber and post-plasma reaction chamber, the plasma reactor comprising a gas inlet, the post-plasma reaction chamber comprising a gas outlet. The method comprises: a) generating a, preferably continuous, plasma in the reaction chamber by activating the energy source; b) providing a first gas flow comprising a CH4component through the gas inlet and into the reaction chamber, allowing (e.g. plasma based) conversion of the CH4component (at least to a large extent, for instance between 40 and 100%, or between 80 and 100%) into gaseous H2and solid carbon and allowing the continuous removal of produced H2through the post-plasma reaction chamber and through the gas outlet; c) in consideration of, or as a function of, a solid carbon deposit being formed on a predetermined portion of an inner wall of the post-plasma reaction chamber, the predetermined portion (preferably comprising a web or grid structure) preferably arranged within substantial alignment with the fluid connection such that more than 50% of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is passing through it, replacing the first gas flow with a second gas flow, the second gas flow comprising a (e.g. plasma-activated) CO2component, wherein preferably the CO2component in the second gas flow is within the range of 60% to 100%, wherein preferably a distance between the discharge arc and the solid carbon deposit is smaller than 10mm, allowing the oxidation of the deposited carbon, and allowing the continuous removal of produced CO through the gas outlet until the deposited carbon is substantially removed (e.g. on at least the predetermined portion of the inner wall).

[0011] To solve the issue of carbon deposition disrupting the process continuity of CH4conversion in plasma reactors, embodiments of the present disclosure establish an innovative looping system, wherein a pure CO2or low CH4fraction DRM mixture is activated once a sufficient amount of solid carbon has been formed. This process simply switches the discharge gas within the same reactor and provides the advantage of reducing the downtime of the reactor required for cleaning.

[0012] The reaction chamber and post-plasma reaction chamber are preferably separate chambers which are preferably connected by means of a fluid connection, or instance defined by an opening or open tube arranged in a common separation wall between the reaction chamber and post-plasma reaction chamber. A cross-section of the opening or open tube has a diameter preferably larger than 5 mm, for instance preferably within the range of 5mm to 50cm, or within the range of 5mm to 10cm.

[0013] The predetermined portion of the inner wall of the post-plasma reaction chamber can be a portion of the inner side of the outer wall of the post-plasma reaction chamber but can be defined on a separate structure arranged or positioned within the internal volume of the post-plasma reaction chamber. The separate structure or predetermined portion thereof can for instance comprise a web or grid structure arranged within substantial alignment with the fluid connection, such that, preferably, most (for instance more than 50%, or more than 60%, or more than 70%, or more than 80%, or more than 90%) of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is passing through it. Preferably, such a separate structure allows gas flow to go through when no or only a limited amount of solid carbon is deposited thereon.

[0014] In preferred embodiments, the separate structure comprises a grid or web structure that is arranged within the post-plasma reaction chamber and provides a closed connection between the fluid connection’s outlet and the grid or web structure, such that most (more than 95% or more than 99%) or all of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is obliged to pass through it. In such embodiments, a differential pressure increase over the bed, grid or web structure may occur during solid carbon loading, a parameter which can be a measure of the amount of solid carbon being deposited.

[0015] According to preferred embodiments, the method comprises repeating steps (b) and (c) alternati ngly , to alternatingly replace the first gas flow with the second gas flow and vice versa. This process provides the advantage of further reducing the downtime of the reactor required for cleaning. It is a further advantage that the reaction chamber and the postreaction chamber do not strictly need to be separable and could be permanently fixed to one another. Also, if the reaction chamber and the post-reaction chamber are separable, they need to be separated less frequently.

[0016] According to preferred embodiments, the method comprises detecting or measuring an amount of the carbon being deposited in the post-plasma reaction chamber during step (b) and switching to step (c) if the solid carbon amount reaches, or reaches above a first predetermined threshold value.

[0017] According to preferred embodiments, the amount of carbon being deposited in the postplasma reaction chamber is an amount being deposited at a predetermined location of maximal deposit, such as for instance at a bed, web or grid structure.

[0018] According to preferred embodiments, measuring the amount of carbon deposited can comprise optical or electrical measurements, or can comprise determining a differential pressure measurement over a bed, web or grid that is loaded with solid carbon in step (b) and cleaned from solid carbon in step (c).

[0019] In embodiments comprising an optical measurement or visual or optical inspection, the post-plasma reaction chamber may comprise a transparent window in a sidewall thereof. For instance, such a window can be placed in a sidewall of the post-plasma reaction chamber opposite the common sidewall and can for instance extend within the angular range of 30° to 60° with respect to the longitudinal axis of the plasma reactor, defined with respect to a central portion of the distal end (most central end) of the bed, grid or web structure. Alternatively, such a transparent window can for instance be provided in a sidewall of the post-plasma reaction chamber adjacent to the common wall between the reaction chamber and post-plasma reaction chamber and can for instance extend within the angular range of 30° to 60° with respect to the longitudinal axis of the plasma reactor, defined with respect to a central portion of the distal end (most central end) of the bed, grid or web structure.

[0020] According to preferred embodiments, the method comprises measuring a concentration of an O2 component in the gas outlet and switching to step (b) if the O2 concentration reaches a second predetermined threshold value. According to preferred embodiments, the at least one replacing of one of a first and a second gas flow with the other one of the first and the second gas flow is performed in a gradual manner, for instance by continuously or stepwise increasing and decreasing respective components. In such embodiments, during a transition phase, respective components of the first and second gas can be fed into the reactor at the same time, for a short period of time (for instance for less than 1 minute, or less than 45 or less than 30 seconds).

[0021] It is an advantage that, for instance in case of plasma discharge, no shut-down and reignition of the reactor is required and the reactor can perform continuously and without being interrupted. This reduces overall downtime of the reactor and increases efficiency. In certain embodiments, the first and second gas flows can be mixed (e.g. by a mixing means) during the transition phase.

[0022] According to preferred embodiments, the at least one replacing of one of a first and a second gas flow with the other one of the first and the second gas flow is performed in a discontinuous or discrete manner. For instance, the first gas flow can be switched off completely at the same time that the second gas flow is switched on. This allows for a clear separation of product gas flows. In such embodiments, the plasma may be extinguished when the first gas flow is switched off and reignited when the second gas flow is switched on.

[0023] According to preferred embodiments, the plasma reaction chamber has an internal volume within the range of 5 cm3to 5000 cm3.

[0024] According to preferred embodiments, the post-plasma reaction chamber has an internal volume within the range of 1 cm3to 1000 cm3.

[0025] According to preferred embodiments, the method allows a gas flow throughput within the range of 1 L / min to 100 L / min, and utilizes a plasma or energy source having a power output within the range of 0.1 kW to 50 kW.

[0026] According to preferred embodiments, a specific energy input of the reactor is within the range of 1 to 10 kJ / L, more preferably 4 to 10 kJ / L during step (b). Preferably, it is larger than 4 kJ / L during step (c). According to preferred embodiments, the CH4component in the first gas flow is within the range of 10% to 100%, more preferably within the range of 50% to 100%.

[0027] According to preferred embodiments, the first gas flow further comprises an inert gas (e.g. Ar / He) and / or a co-reactant gas (e.g. CO2 to enable “dry reforming of methane”).

[0028] In some embodiments only an inert gas is added to the CH4component in the first gas flow (and no co-reactant gas). The inert gas component is then complementary to the CH4component (i.e. constitutes 100% minus the CH4component). Preferably, the inert gas component is smaller than 95% or smaller than 75%, or smaller than 50% or more preferably smaller than 25%, even more preferably smaller than 5%.

[0029] In some embodiments only a co-reactant gas is added to the CH4component in the first gas flow (and no inert gas gas). The co-reactant gas component is then complementary to the CH4component. Preferably, the co-reactant gas component is smaller than 75%, more preferably smaller than 50%, even more preferably smaller than 25%.

[0030] In some embodiments both inert gas and co-reactant gas are added to the CH4component. Preferably, their combined presence is smaller than 95% or smaller than 75%, or smaller than 50% or more preferably smaller than 25%, even more preferably smaller than 5%. Preferably, the inert gas component concentration is larger than the co- reactant component concentration.

[0031] According to preferred embodiments, the CO2 component in the second gas flow is within the range of 1% to 100%, 1% to 99%, 60% to 100%, 60% to 99%, 80% to 100%, or 80% to 99%.

[0032] According to preferred embodiments, the second gas flow further comprises an inert gas (e.g. Ar / He) and / or a co-reactant such as CH4in a lower fraction (i.e. less than 50%).

[0033] In some embodiments only an inert gas is added to the CO2 component in the second gas flow (and no co-reactant gas). The inert gas component is then complementary to the CO2 component. Preferably, the inert gas component is smaller than 95% or smaller than 75%, or smaller than 50% or more preferably smaller than 25%, even more preferably smaller than 5%.

[0034] In some embodiments only a co-reactant gas is added to the CO2 component in the second gas flow (and no inert gas gas). The co-reactant gas component is then complementary to the CO2 component. Preferably, the co-reactant gas component is smaller than 50%, more preferably smaller than 25%, even more preferably smaller than 5%.

[0035] In some embodiments both inert gas and co-reactant gas are added to the CO2 component. Preferably, their combined presence is smaller than 95% or smaller than 75%, or smaller than 50% or more preferably smaller than 25%, even more preferably smaller than 5%. Preferably, the CO2 component concentration is larger than the co- reactant component concentration.

[0036] According to preferred embodiments, a temperature of the post-plasma reaction chamber is within the range of 700 °C to 6000 °C, more preferably between 1200°C and 6000°C during all or most of step (c). This allows a process known as the reverse Boudouard reaction to take place (CO2+C^2CO).

[0037] According to preferred embodiments, the gas temperature at the solid-gas interface between the gas and the deposited solid carbon in the post-plasma reaction chamber is larger than 700°C.

[0038] According to preferred embodiments, a temperature of the reaction chamber is within the range of 700°C to 10000 °C, more preferably within the range of 1000°C to 10000°C more preferably within the range of 2000°C to 8000°C, more preferably within the range of 6000°C to 8000°C.

[0039] According to preferred embodiments, the plasma is of a thermal or quasi-thermal type.

[0040] According to preferred embodiments, the switching frequency between providing the first and second gas flows is within the range of 1 / day and 1 / minute, more preferably between 2 / hour to 10 / hour. The typical time periods for the provisioning of the first gas flow and the second gas flow can depend on the flow rate for the respective gas flows.

[0041] For instance, if the flow rate of the first gas flow is about equal to the flow rate of the second gas flow, the duration of the second step is preferably shorter than the duration of the first step of the process.

[0042] For instance, if the flow rate of the first gas flow is smaller than the flow rate of the second gas flow, the duration of the second step is preferably shorter than the duration of the first step times the ratio of first gas flow rate over the second gas flow rate.

[0043] For instance, if the flow rate of the first gas flow is larger than the flow rate of the second gas flow, the duration of the second step is preferably shorter than the duration of the first step times the ratio of second gas flow rate over the first gas flow rate.

[0044] In a second aspect of the present disclosure, a plasma reactor is disclosed comprising a reaction chamber having a gas inlet, a preferably separate post-plasma reaction chamber fluidly connected to the reaction chamber preferably by means of a fluid connection comprising an opening or open tube arranged in a common separation wall between the reaction chamber and post-plasma reaction chamber, the post-plasma reaction chamber comprising a gas outlet and an energy source to create the plasma in the reaction chamber, the plasma reactor further comprising a means for measuring an amount of carbon deposited in a predetermined portion of an inner wall of the postplasma reaction chamber, the predetermined portion (preferably comprising a web or grid structure) preferably arranged within substantial alignment with the fluid connection such that more than 50% of the gas flow flowing from the reaction chamber into the postplasma reaction chamber is passing through it.

[0045] According to preferred embodiments, the predetermined portion of the inner wall of the post-plasma reaction chamber is embodied as or on a separate structure arranged or positioned within the internal volume of the post-plasma reaction chamber.

[0046] According to preferred embodiments, the separate structure or predetermined portion thereof can for instance comprise a bed, web or grid structure arranged within substantial alignment with a fluid connection between the plasma reaction chamber and the post-plasma reaction chamber, for instance along the longitudinal axis of the plasma reactor, and for instance (defining a plane) perpendicular on the longitudinal axis.

[0047] Preferably, the separate structure is arranged and adapted such that most (for instance more than 50%, or more than 60%, or more than 70%, or more than 80%, or more than 90%, or more than 95%) of a gas flow flowing from the reaction chamber into the postplasma reaction chamber would have to pass through it.

[0048] Preferably, the separate structure is suitable for allowing gas flow to go through when no or only a limited amount of solid carbon is deposited thereon (for instance when less than 50% of the open volume of the bed, grid or web is occupied), and suitable for collecting solid carbon particles from the first gas flow.

[0049] In preferred embodiments, the separate structure comprises a web, grid or web structure that is arranged within the post-plasma reaction chamber and provides a closed connection between the fluid connection’s outlet and the grid or web structure, such that most (more than 95% or more than 99%) or all of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is obliged to pass through it.

[0050] According to preferred embodiments, the post-plasma reaction chamber comprises a differential pressure sensor arranged and adapted for measuring a pressure difference over the bed, web or grid structure, i.e. between opposed sides of the bed, web or grid structure.

[0051] According to preferred embodiments, the support structure comprises comprise lateral bypass openings in the sidewalls of the support structure arranged parallel to the reactor’s longitudinal axis to allow a small amount of gas flow to bypass the bed, grid or web structure, for instance to allow pressure release when the bed, grid or web structure becomes clogged or substantially clogged.

[0052] According to preferred embodiments, the reactor further comprising a means for measuring an O2 concentration of a gas flow passing through the gas outlet.

[0053] According to preferred embodiments, the reactor is of a thermal or quasi thermal plasma type. According to preferred embodiments, the reaction chamber and the post-plasma reaction chamber are fixedly connected.

[0054] According to preferred embodiments, the post-plasma reaction chamber comprises a transparent window.

[0055] In a third aspect of the present disclosure, a system for plasma-based conversion of methane is disclosed comprising a plasma reactor according to any of the embodiments of the second aspect, which further comprises a means for providing a gas flow to the inlet, a means for switching between providing a first gas flow comprising CH4and providing a second gas flow comprising CO2 at the inlet and a control unit for controlling the means for switching to switch, preferably continuously, preferably repeatedly, between providing the first flow of gas and providing the second flow of gas based on or at least on the measured amount of carbon deposited and / or the O2 concentration measured at the gas outlet.

[0056] According to preferred embodiments, the system comprises a first, inlet tube system comprising a first set of one or more valves to control the flow of respective gas flows towards the gas inlet.

[0057] According to preferred embodiments, the system comprises a second, outlet tube system comprising a second set of one or more valves fluidly connected to the gas outlet. For instance, the second, outlet tube system may comprise a switching valve directing the outlet gas flow (for instance the first and second gas flow respectively) towards first and second subsystems of the outlet tube system.

[0058] According to preferred embodiments the first and second set of valves are controlled by the control unit.

[0059] According to preferred embodiments, the control unit is adapted for controlling the movement of the first and second set of valves in a synchronized manner.

[0060] Features and advantages disclosed for one of the above aspects of the present disclosure are hereby also implicitly disclosed for the other aspects, mutatis mutandis, as the skilled person will recognize. For instance, features disclosed for the method are considered to be also disclosed for the reactor and system, the latter comprising means or being arranged and / or adapted to perform the corresponding method step.

[0061] When, in the present disclosure, gas flow components are expressed as percentages of a respective gas flow, these are supposed to be in volume percent (volume %).

[0062] Brief description of the drawings

[0063] The disclosure will be further elucidated by means of the following description and the appended figures.

[0064] Fig. 1 illustrates a system and a first state of a method according to embodiments of the present disclosure.

[0065] Fig. 2 illustrates the same system as in Fig. 1 and a second state of a method according to embodiments of the present disclosure.

[0066] Fig. 3 illustrates an embodiment wherein a window is provided in the post-plasma reaction chamber for optical measurement or inspection of the deposited carbon.

[0067] Figure 4 shows a schematic overview of the reactor setup for all CRG stages. Gas lines are indicated with dotted lines, and electric connections with black lines.

[0068] Figure 5 shows average outlet concentrations of CO, CO2, CH4, O2 and H2as a function of time for three repetitions. Plasma-based DRM was implemented for 2 min, followed by plasma extinguishing and inlet gas switching for 2 min, and subsequent CO2 plasma for 1 min. This cycle was repeated three times. Zones I indicate plasma-off and zones II represent plasma-on (either DRM or CO2).

[0069] Figure 6 shows conversion (left) and energy cost (right) for DRM (solid carbon formation) and CO2 (solid carbon oxidation) phases, averaged over three cycles and three repetitions.

[0070] Figure 7 shows average outlet concentrations of CO, CO2, CH4, O2 and H2as a function of time over three repetitions. Plasma-based CH4pyrolysis was implemented for 2 min, followed by a CO2 plasma for 1.5 min. The cycle was repeated thrice. Zones I indicate plasma-off and zones II represent plasma-on (either Ar / CH4or CO2).

[0071] Figure 8 shows CH4and CO2 conversion (left) and energy cost (right) for CH4pyrolysis (solid carbon formation) and CO2 (solid carbon oxidation) phases, averaged over three cycles and three repetitions. Detailed description of preferred embodiments

[0072] The present disclosure will be described with respect to particular embodiments and with reference to certain drawings, but the disclosure is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the disclosure.

[0073] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order.

[0074] The various embodiments, although referred to as "preferred" are to be construed as examples in which the disclosure may be implemented rather than as limiting the scope of the disclosure.

[0075] To solve the issue of carbon deposition disrupting the process continuity of CH4conversion in plasma reactors, the current disclosure establishes an innovative looping system, wherein a pure CO2 or low CH4fraction DRM mixture is activated once a sufficient amount of solid carbon 3 has been formed. This process switches the discharge gas within the same reactor, reducing the downtime of the reactor required for cleaning.

[0076] Fig. 1 illustrates a plasma reactor 1 according to embodiments of the present disclosure. The plasma reactor 1 comprises a reaction chamber 10 and an energy source 12 to create a plasma 2 in the reaction chamber 10.

[0077] The reaction chamber comprises a gas inlet 100 for feeding an incoming gas flow into the reaction chamber 10.

[0078] The reaction chamber is (fluidly) connected to a post-plasma reaction chamber 11 by means of a fluid connection 13, which comprises for instance an opening in a common separating wall or tube or similar structure. The post-plasma reaction chamber 11 comprises a gas outlet 110 to allow removal of the gas from the reactor. The gas is led from the inlet 100 to the outlet 110 of the plasma reactor 1 .

[0079] In alignment or substantial alignment with the outlet of the fluid connection 13, for instance along the longitudinal axis of the plasma reactor, the post-plasma reaction chamber 11 comprises a bed, web or grid structure 111 , such that most or all of the gas flowing from reaction chamber 10 to post-plasma chamber 11 flows through the bed, web or grid structure 111. The bed, grid or web structure can for instance extend along a plane. Such plane is preferably arranged perpendicularly on the longitudinal axis. Preferably, the reactor comprises a fixed bed attached directly to the end of the discharge zone, as shown in Figure 1. The discharge zone can be seen as the area of the reactor wherein the plasma is created due to electrical breakdown of the gas and then sustained by the power supply unit.

[0080] The bed, web or grid structure 111 is for instance supported by a support structure 1111 arranged between the inner sidewalls of the post-plasma reaction chamber and the bed, web or grid structure. This support structure 1111 can provide a closed or substantially closed connection between the fluid connection’s outlet and the bed, grid or web structure 111 , such that most (more than 95% or more than 99%, or 99,9%, or 100%) or all of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is obliged to pass through it. In such embodiments, a differential pressure increase over the bed, grid or web structure may occur during material loading of the structure, which would reduce the average opening or pore diameter in the bed, web or grid structure, or close the pores or openings completely. A differential pressure over the structure 111 is a preferred parameter as a measure of the amount of material (here solid carbon 3, see below) being deposited on the structure 111. The support structure 1111 may comprise lateral bypass openings 1112 in the sidewalls of the support structure 1111 arranged parallel to the reactor’s longitudinal axis to allow a small amount of gas flow to bypass the bed, grid or web structure 111 , for instance to allow pressure release when the bed, grid or web structure 111 becomes clogged or substantially clogged.

[0081] A plasma 2 is generated in the reaction chamber 10 by activating said energy source 12, whereby a sufficiently large potential difference between two electrodes in the reaction chamber 10 is applied. For instance, a first electrode 15 can be provided at a wall of the plasma reaction chamber 10 opposite to the common sidewall 14. A second electrode 16 can be provided at the common sidewall 14, in the plasma reaction chamber 10, for instance close to or adjacent the opening or fluid connection 13. Suitable electrical isolation means or structures are provided to the reaction chamber 10 to isolate the first and second electrodes 15 and 16 from the sidewalls of the reaction chamber 10. Alternatively, the plasma 2 can be generated by microwave (MW) or radio frequency (RF) electromagnetic waves or by an alternating magnetic field (i.e. electromagnetic induction).

[0082] A first gas flow G1 is provided in this step through the gas inlet 100 and into the reaction chamber 10. The first gas comprises pure CH4or CH4combined with a secondary gas. The secondary gas may be inert (e.g. Ar) or oxidizing (e.g. CO2), but should not, or not entirely, suppress the formation of soot (e.g. does not comprise any of H2, H2O, O2), as this would jeopardize the second step of the process. Hereby conversion of the CH4component into gaseous H2and solid carbon 3 occurs. A continuous removal of produced H2through the post-plasma reaction chamber 11 and through said gas outlet 110 is taking place.

[0083] After a while solid carbon 3 deposit is formed on a predetermined portion of the inner wall of the post-plasma reaction chamber 11 , here the bed, web or grid structure 111. In this fixed bed, the solid carbon 3 produced during the decomposition of CH4is accumulated. Preferably, most of the solid carbon 3 is captured, for instance more than 50%, or more than 75% or more than 99%. The more solid carbon 3 that is captured, the more efficient the process will be. The plasma utilized in this first step may be nonthermal (“cold”) (i.e. all temperature components of the system are not equal, e.g.

[0084] Te>Tv>Tg wherein Teis the electron temperature, Tvis the vibrational temperature and Tgis the gas temperature), quasi-thermal (“warm”) (i.e. some temperature components of the system are not equal, e.g. Te>Tg=Tvwherein Tvis the vibrational temperature, Teis the electron temperature and Tgis the gas temperature) or thermal (i.e. all temperature components of the system are equal, e.g. Te=Tg=Tvwherein Tvis the vibrational temperature, Teis the electron temperature and Tgis the gas temperature). Preferably, the discharge in the first stage is “warm” or thermal, as this type is more favorable for the second stage (see below) and would enable a single power supply for both stages. In consideration of a solid carbon 3 deposit being formed on this predetermined portion of an inner wall of said post-plasma reaction chamber 11 , the first gas flow G1 is replaced with a second gas flow G2 in a second step. This can be performed in a discontinuous or discrete manner (direct switch, from 100% first gas flow G1 / G2 to 100% second gas flow G2 / G1) or in a gradual manner, for instance by continuously or stepwise decreasing / increasing the respective flows.

[0085] The formation of carbon deposit can be measured by means of an increase in differential pressure over the bed, grid or web. Alternatively, electrical or optical measurements, or visual or optical inspection can be used. The measured values can be mapped to corresponding thicknesses of a deposited carbon layer in a predetermined area. For instance, a first threshold value or thickness T 1 can be used to trigger the second step.

[0086] The second gas flow G2 comprises a CO2 component, allowing the oxidation of the deposited carbon. Continuous removal of produced CO through the gas outlet 110 is then allowed to take place until the deposited carbon is removed or substantially removed (for instance until more than 80%, or more than 90%, or more than 95%). In this second step, carbon dioxide (CO2) plasma 2 thus reacts with the solid carbon 3 produced in the first step to remove oxygen O2from the effluent and boost CO production. In some embodiments, a full removal of the deposited carbon may not be needed as it may perform sub-optimal and may reduce overall process efficiency. For instance, only 80 to 90% of the deposited carbon can be removed in the second step.

[0087] In this second step, the same reactor and bed, web or grid 111 are used, but the discharge gas is changed, illustrated in Fig. 2. This provides the advantage that parts of the reactor do not need to be dismounted, replaced or cleaned.

[0088] A pure CO2 discharge is thereby created within the reactor domain by continuing the application of a large potential difference between the electrodes (or constant application of microwave or radio frequency electromagnetic waves, or constant electromagnetic induction). For instance, the process gas can be changed in a gradual, continuous manner by decreasing the CH4fraction while simultaneously increasing the CO2 content until no CH4remains. This process can be advantageous in case a production of syngas is intended. Alternatively, the plasma power supply 12 can be turned off, gases can be switched and a reignition can take place in pure CO2. The elected or preferred method can depend on factors such as power supply characteristics, ignition protocols, etc. The latter process can be advantageous when a clear separation of the outlet gas streams (G1*, G2*) is required.

[0089] The discharge leads to the oxidation of the deposited carbon via the following reactions:

[0090] O2+2C ^2CO (R1)

[0091] O+C ^CO (R2)

[0092] CO2+C^2CO (R3)

[0093] Reaction R1 utilizes the molecular oxygen (O2) formed during CO2plasmolysis and should occur as long as the distance between the end of the discharge (e.g. extreme axial end of an arch (last place where to find free electrons and plasma) and the carbon deposit is not too long (for instance smaller than 5 cm, more preferably smaller than 1 cm; otherwise recombination with CO will dominate and the O2will reform CO2).

[0094] The reaction of atomic oxygen (O) with the solid carbon 3, R2, is possible as long as the discharge is close enough to the solid carbon 3, for instance at a distance smaller than 10 mm, or smaller than 5 mm (recombination of O into O2is on the spatial order of several mm at atmospheric pressure).

[0095] Reaction R3, known as the reverse Boudouard reaction, occurs at elevated temperatures (> 700°C, or > 1000°C), so preferably a quasi-thermal or thermal plasma will be used in the second stage.

[0096] At a certain moment in time, when enough solid carbon 3 will have been removed from the bed, grid or web structure 111 , and again a switch is made from gas flow G2 to gas flow G1.

[0097] For instance, the oxygen concentration in the outlet gas flow G2* can be measured or monitored, and a switch from second gas flow G2 to second gas flow G1 can be performed when a second threshold T2 of the oxygen concentration in the outlet gas flow is reached. Alternatively, the differential pressure over the bed, grid or web structure 111 can be monitored and when this differential pressure reaches below a predetermined third threshold T3, the switch can be performed. Still alternatively, optical or electrical measurements can be performed to trigger this switch. In the case of optical measurements, the post-plasma reaction chamber 11 may comprise a window 1113 (see Fig. 3) providing optical access to the post-plasma reaction chamber. In still alternative embodiments a combination can be made of any of the beforementioned gas flow switching trigger means.

[0098] In case a transparent window 1113 is provided for an optical measurement or visual or optical inspection, the window is preferably placed in a sidewall of the post-plasma reaction chamber such that the deposited carbon is optically or visually accessible. For instance, the window can be arranged opposite the common sidewall, and can extend within the angular range of 0 between 30° to 60° with respect to the longitudinal axis of the plasma reactor, the angle 0 defined with respect to a central portion of the distal end (most central end) of the bed, grid or web structure 111.

[0099] Depending on the desired products, the effluent from the two stages (gas flow G1* and gas flow G2*) can either be combined or kept separate. If pure CH4or CH4diluted with an inert gas is used, then the effluent will be H2(or H2with an inert gas and / or coreactant gas), possibly with some remainder of unconverted CH4. If a DRM mixture is used at this initial stage, a syngas mixture will be produced. The second stage will produce an oxygen free CO / CO2mixture, which can be separated using PSA (or a similar technology) to the constituent components. This can either be combined with the H2or syngas produced in the first stage to increase the syngas yield.

[0100] In order to provide the respective gas flows G1 and G2, and optionally a combination thereof during a transition phase, to the inlet 100 of the plasma reactor 1 , the system 1000 may comprise one or more valves 5 to control the respective gas flows.

[0101] In embodiments where a clear gas separation is required between the respective gas flows G1*, G2* at the gas outlet 110, further valves may be provided in an outlet tube system (not depicted) fluidly connected to the gas outlet 110. For instance, the outlet tube system may comprise a switching valve 5 directing the outlet gas flow towards first and second subsystems of the outlet tube system. This is preferably performed in a synchronized manner, i.e. at corresponding moments in time, with the switching of the respective inlet gas streams. The system 1000 can comprise a control unit 6 for controlling the respective valves accordingly. This can result in lower downstream gas separation costs. The disclosed plasma-based cyclic reforming-gasification (CRG) process, consisting of alternating carbon formation and oxidation phases in a single gliding arc plasmatron (GAP) reactor has been tested.

[0102] The first phase involved solid carbon formation through either dry reforming of methane (DRM) with a high methane (CH4) fraction or CH4pyrolysis, producing also valuable syngas or H2, respectively. The second phase involved the oxidation of the formed solid carbon in a fixed bed by the high-temperature effluent of a carbon dioxide (CO2) plasma. The CRG process was demonstrated in both a discontinuous and continuous manner, each having associated benefits and drawbacks. The former was implemented with the DRM reaction, producing a syngas ratio (H2 / CO) of 1.7, while the subsequent oxidation phases led to increasing CO concentration and decreasing O2 concentration with each pass, thus reducing separation costs. Continuous CRG with CH4pyrolysis resulted in consistent absolute CH4conversion around 60%, with solid carbon and H2being the main products. The subsequent oxidation phases resulted in similar trends to the post- DRM oxidation phases, reaching a maximum O2 concentration of only 0.5 vol% after three passes. The results demonstrate the capability to turn the typically undesirable formation of solid carbon by-product into a useful reactant for improving the CO2 conversion and CO concentration in the effluent of a CO2 plasma. The self-cleaning nature of the CRG process increases the operational efficiency and time-on-line of biogas or CH4reforming by plasma technology.

[0103] Materials and methods

[0104] One and the same plasma reactor for each stage of the process has been used, namely a gliding arc plasmatron (GAP) reactor, having a setup as presented in Figure 4.

[0105] A gas chromatograph equipped with a TCD detector (Agilent, 990 Micro GC) was used to analyse the effluent and the flux ratio was determined using either N2(added postreactor for DRM case) or Ar (CH4pyrolysis) as the internal standard. The latter is needed to properly account for gas “expansion” or “contraction” due to changes in the volumetric flow rate when the reactions proceed. The GC was used in series with multiple in-line non-dispersive infrared (NDIR) detectors to measure CO / CO2 / CI- (Emerson, XSTREAM XEGP Continuous Gas Analyser) and an optical sensor for O2 (Pyroscience, FDO2). The first phase utilises either a DRM mixture with a high CH4content (CO2 / CI- = 40 / 60) or an Ar / CH4(90 / 10) mixture, to produce either syngas or hydrogen (H2) in addition to solid carbon. The total flow rate for all phases was fixed at 10 L / min. The solid carbon formed was captured in an insulated fixed bed

[0032] , with the bed containing inert Sasol AI2O3 beads (diameter = 1.5 mm), connected to the plasma reactor outlet.

[0106] The implemented cyclic switching between the first and second stages occurred after a set period. In both cases, the plasma was operated for 2 min for the solid carbon formation phase, while the pure CO2 plasma was operated for 1 or 1 .5 min in the carbon oxidation phase.

[0107] The process can be either discontinuous or continuous, with the former entailing plasma extinguishment and reignition between the two stages. This discontinuous process has the advantage of a clearly defined separation of product streams, but the disadvantage of reduced process operation time.

[0108] In the case of continuous switching, the inlet gas mixture is changed without extinguishing the arc. This has the clear advantage of reduced process downtime, with the disadvantage of less defined product separation (depending on chamber evacuation rates and downstream switch capabilities).

[0109] The reactant conversion was calculated from the measured detected concentrations defined above, and besides conversion, also the energy cost (EC) of the conversion determines the performance. In terms of energy metrics, energy efficiency has been shown to be difficult to accurately quantify for systems with competing reactions such as DRM, so the more straightforward metric of EC was used. Each CRG experiment was performed in triplicate, with the error shown comprising of the standard deviation between the measured values.

[0110] Results

[0111] A. Discontinuous plasma-based CRG

[0112] As defined above, the process can be operated in a discontinuous manner, wherein the plasma is extinguished after the carbon formation stage and subsequently reignited for the carbon oxidation stage after a defined period. The reaction implemented in the first stage was DRM with a high CH4fraction (60 vol%), while the subsequent stage utilised a pure CO2 plasma to oxidise the formed carbon deposit. The DRM stage was applied for 2 min, followed by 2 min period without plasma and then a 1 min period with a CO2 plasma. This cycle was repeated thrice, with the resulting reactant and product concentrations shown in Figure 5. The plasma-based DRM (i.e. , carbon formation) phases result in consistently high absolute CH4and CO2 conversions, reaching values around 81 % and 75%, respectively, yielding an overall (i.e., weighted average) conversion of 78% (see Figure 6 below). Indeed, the CH4and CO2 concentrations start at 60 and 40 vol%, respectively, but both quickly drop to below 20 vol% (see Figure 5). At the same time, the H2and CO concentrations rise, to about 43 and 24 vol%, respectively. The resulting syngas ratio during these periods was approximately 1 .7, which is in the range required for aldehyde (H2 / CO = 1), alcohol or higher hydrocarbon synthesis (H2 / CO = 2) via the Fischer- T ropsch (FT) process. The concentrations (and thus also conversion and syngas ratio) were basically constant for the three cycles.

[0113] After each of the three DRM stages, plasma extinguishment enables the CH4and CO2 levels to return to the initial values (60 and 40 vol%), while the H2and CO concentrations drop back to zero. This is followed by the removal of CH4from the inlet stream and an increase in CO2 flow rate (from 4 to 10 L / min) in the second half of the plasma-off phase.

[0114] Once this period elapses and a stable CO2 signal was detected, the plasma was reignited. This results in a drop in CO2 concentration to about 90 vol% (corresponding to ca. 6% conversion, accounting for gas expansion), and a rise in CO concentration, reaching maximum values around 10 vol%. Importantly, these peak CO values are accompanied by a distinct lack of O2, as the solid carbon formed in the first stage is oxidised, both by the O2 produced in the plasma by CO2 splitting (partial / full oxidation) and by unconverted CO2(reverse Boudouard reaction) in the plasma effluent.

[0115] As the carbon trapped in the bed was consumed, the detected O2 concentration rose slightly, as plasma-based CO2 dissociation became the primary CO production route. Interestingly, the O2 concentration rose much more slowly with each repetition, reaching a peak value around 2 vol% after the first cycle, which was reduced to a maximum of 0.5 vol% after the third repetition. This effect would presumably be further pronounced with an increased number of repetitions, producing a near 02-free effluent stream. A recycling concept for the 02-free effluent can further increase the CO2 conversion, and CO concentration produced, reducing the need for separation downstream to reach a utilisable purity.

[0116] The (overall) conversion and energy cost (EC) for both stages in the discontinuous CRG process are shown in Figure 6. For the applied specific energy input (SEI, i.e., ratio of power over flow rate) of 125 kJ / mol, an appreciably high average overall conversion was achieved in the DRM stage (ca. 78%), which resulted in a significantly low EC around 0.16 MJ / mol of reactants converted (13.9 kWh / t syngas). These metrics are among the best of plasma-based DRM works published in literature, with high conversion and low EC simultaneously realised (see comprehensive overview by Wanten et al. (8. Wanten, S. Maerivoet, C. Vantomme, J. Slaets, G. Trenchev, A. Bogaerts, Dry reforming of methane in an atmospheric pressure glow discharge: Confining the plasma to expand the performance, Journal of CO2Utilization 56 (2022), https: / / doi.org / 10.1016 / jjcou.2021.101869).

[0117] In contrast, at an approximately equal SEI (132 kJ / mol), the average conversion obtained with the CO2 plasma oxidising the solid carbon deposit was only around 6%. The corresponding EC was also much higher in this period than in the solid carbon formation stage, reaching an average value of 2.2 MJ / mol, or 24.4 kWh / t CO. While the metrics for the CO2 conversion are worse than the best values reported in literature related to post-plasma carbon beds for boosting CO2 conversion, the significant reduction of O2 concentration in the effluent, the excellent metrics of the DRM process, and the possibility of applying high CH4fractions without plasma destabilisation, convincingly demonstrate the potential of the disclosed CRG process. Further optimisation of the process parameters (e.g. cycling period, SEI) and bed design can improve both conversion and energy cost.

[0118] B. Continuous plasma-based CRG

[0119] If the inlet mixture is switched without plasma extinguishment, the process can be considered as continuous, which has the evident benefit of reduced process downtime. In our study, the reaction implemented in the first stage is CH4pyrolysis, while the second stage again utilises a pure CO2 plasma to oxidise the formed carbon deposit. The CH4pyrolysis stage is applied for 2 min, followed by 1.5 min CO2 plasma. This CRG protocol is repeated three times, with the resulting concentrations of reactants and products shown in Figure 7.

[0120] Upon plasma ignition (after ca. 1 min) in the Ar / CH4(90 / 10) stream, the CH4outlet concentration dropped more than two-fold to a relatively steady value around 4 vol%, corresponding to an absolute conversion around 60 %. This is in line with the formed H2concentration detected by the GC halfway through this period, which is ca. 8 vol%. The H2concentration was obtained by GC measurement of the effluent. While this produced only a single data point per solid carbon formation phase, it still provided a good indication of the H2production. The CH4and H2values detected during the second and third cycles of CH4pyrolysis (starting at ca. 4.5 and 8 min) are similar, demonstrating a relatively consistent process.

[0121] During the oxidation phases in pure CO2plasma, the CO2concentration reached again 90 vol%, corresponding to a conversion of 6%, with CO concentration around 9 vol% in the first cycle, but slightly increasing to 10 vol% in the third cycle. At the same time, the efficiency of O2removal improved with increasing number of cycles. In the first oxidation stage (after ca. 3 min), the detected O2concentration rose more slowly than the CO concentration, pointing towards full oxidation (i.e. C(s) + O2CO2). By the end of this first oxidation stage, the O2rose to ca. 2.5 vol% as the produced carbon was consumed. In the following oxidation stage (after ca. 6 min), a noticeable peak in CO concentration was seen, indicating a significant increase in the RBR rate and oxidation of the carbon deposit with CO2. By the end of this period, the O2concentration rose to just 0.5 vol%. Finally, in the third oxidation period (after ca. 9.5 min), a peak in CO concentration was observed once again, with the peak spanning a slightly longer timescale, and consequently the production of O2being delayed. The O2concentration only began to rise after about 45 sec of this third oxidation phase, reaching a peak value around 0.5 vol% by the end of the phase.

[0122] The CH4and CO2conversion and the EC for the CH4pyrolysis and CO2plasma stages of the continuous CRG process are shown in Figure 8.

[0123] While the absolute conversion of CH4was relatively high (ca. 61 %), the abundance of diluting gas (90% Ar) resulted in a relatively low overall conversion of 6%. Increasing the CH4content (and hence reducing the Ar content) is feasible, but this would increase the rate and yield of solid carbon formation and thus decrease the timescale of plasma destabilisation. To implement this efficiently, the first period would have to be much shorter, the optimisation of which has not been considered yet. Alternatively, an SEI lower than that applied in this work (59 kJ / mol) could also decrease the rate of solid carbon formation. The EC of 0.98 M J / mol CH4converted is obviously related to the relatively low conversion but is still quite reasonable. In the solid carbon oxidation stage, a CO2conversion was obtained similar to that realised in the discontinuous CRG process above, around 6%. However, the EC for this condition was lower (ca. 1 .7 M J / mol) and more in-line with literature values for post-plasma carbon bed works. As the conversion was approximately equal, the improved EC is attributed to the lower SEI implemented in this setting (ca. 101 kJ / mol). Interestingly, when the product streams from all cycles and both phases were collected, a syngas ratio around 1.2 was achieved with the continuous CRG process. Evidently, the plasma-based CRG process can also be successfully operated in a continuous manner with alternating carbon formation and oxidation periods. The length of these periods can be tailored with the process parameters to improve the total syngas ratio or the CH4conversion (and EC), and to ensure the O2 concentration remains below the value of 1 vol% during all oxidation stages. For example, the latter could be achieved if a higher power or higher CH4fraction is applied in the formation phase, resulting in more significant carbon deposition. This simple change would reduce the time of the formation phase and could increase the period of the oxidation stage.

[0124] The present disclosure thus highlights for the first time the potential of plasma-based cyclic reforming-gasification (CRG) processes. Conditions that typically result in flow- stabilised plasma disruption due to solid carbon formation, such as DRM with high CH4fractions or CH4pyrolysis, can now be successfully achieved without plasma destabilisation due to the cyclic removal of solid carbon deposit by oxidation with CO2 plasma effluent. Even more, the solid carbon deposit is used to enhance the CO2conversion and produce more CO, thus enhancing the overall performance.

[0125] In a discontinuous manner, wherein inlet gas streams are switched between plasma extinguishment and reignition, it was demonstrated that a syngas ratio of 1.7 can be repeatedly realised from a DRM mixture containing 60:40 CH4:CO2. In a continuous manner, where the inlet mixture is switched seamlessly and without plasma extinguishment, effective CH4pyrolysis (60% absolute conversion) can produce high- purity H2without excessive downtime for reactor maintenance (due to excessive carbon deposition). In both cases, the second phase of carbon oxidation removed most of the O2 produced by CO2 plasmolysis (<1 vol% after second pass), thus reducing postplasma separation costs. The tailoring of the phase periods can be further optimised and used to yield a flexible output, dependent on the downstream application.

[0126] The production of syngas of an appreciably high fraction in the DRM phases can serve as a route for biogas valorisation, while plasma-based CH4pyrolysis produces H2as energy carrier coupled to renewable energy supplies. Overall, CRG allows both reactions to be conducted on extended timescales without plasma destabilisation, with the by-product of solid carbon being intermittently utilised in-situ for increasing the performance of CO2 plasmolysis.

[0127] The following could for instance be claimed:

[0128] 1. A method for plasma-based conversion of methane in a plasma reactor (1), said plasma reactor (1) comprising a reaction chamber (10) and an energy source (12) to create the plasma (2) in said reaction chamber (10), said reaction chamber (10) being fluidly connected to a separate post-plasma reaction chamber (11) by means of a fluid connection comprising an opening or open tube arranged in a common separation wall between the reaction chamber (10) and post-plasma reaction chamber (11), said plasma reactor (1) comprising a gas inlet (100), said post-plasma reaction chamber (11) comprising a gas outlet (110), the method comprising: a) generating a, preferably continuous, plasma (2) in said reaction chamber (10) by activating said energy source (12); b) providing a first gas flow (G1) comprising a CH4component through said gas inlet (100) and into said reaction chamber (10), allowing plasma-based conversion of said CH4component into gaseous H2and solid carbon and allowing the continuous removal of produced H2through said post-plasma reaction chamber (11) and through said gas outlet (110); c) in consideration of a solid carbon deposit being formed on a predetermined portion of an inner wall of said post-plasma reaction chamber (11), the predetermined portion, preferably comprising a web or grid structure, arranged within substantial alignment with the fluid connection such that more than 50% of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is passing through it, replacing said first gas flow (G1) with a second gas flow (G2), said second gas flow comprising a plasma-activated CO2component, wherein the CO2component in the second gas flow (G2) is within the range of 60% to 100%, wherein a distance between the discharge arc and the solid carbon deposit is smaller than 10mm, allowing the oxidation of the deposited carbon, and allowing the continuous removal of produced CO through said gas outlet (110) until said deposited carbon is substantially removed.

[0129] 2. A method according to item 1 , comprising repeating steps (b) and (c) alternatingly, to alternatingly replace the first gas flow (G1) with the second gas flow (G2) and vice versa. 3. A method according to item 2, further comprising detecting or measuring an amount of said carbon being deposited in said post-plasma reaction chamber (11) during step (b) and switching to step (c) if said carbon amount reaches a first predetermined threshold value (T1).

[0130] 4. A method according to item 3, wherein said amount of carbon (3) being deposited in said post-plasma reaction chamber (11) is an amount being deposited at a predetermined location of maximal deposit, such as for instance at a bed, web or grid structure (111).

[0131] 5. A method according to any of the previous items, further comprising measuring a concentration of an O2 component in said gas outlet (110) and switching to step (b) if said O2 concentration reaches a second predetermined threshold value (T2).

[0132] 6. A method according to any of the previous items, wherein said at least one replacing of one of a first and a second gas flow (G1 , G2) with the other one of the first and the second gas flow (G2, G1) is performed in a gradual, for instance continuous or stepwise, manner.

[0133] 7. A method according to any of the previous items, wherein said at least one replacing of one of a first and a second gas flow (G1 , G2) with the other one of the first and the second gas flow (G2, G1) is performed in a discontinuous manner.

[0134] 8. A method according to any of the previous items, allowing a gas flow throughput within the range of 1 L / min to 100 L / min, and having a plasma or energy source having a power output within the range of 0.1 kW to 50 kW.

[0135] 9. A method according to any of the previous items, wherein a specific energy input is within the range of 1 to 10 kJ / L, more preferably 4 to 10 kJ / L during step (b) and is larger than 4 kJ / L during step (c).

[0136] 10. A method according to any of the previous items, wherein said CH4component in said first gas flow (G1) is within the range of 10% to 100%, more preferably within the range of 50% to 100%.

[0137] 11. A method according to any of the previous items, wherein said first gas flow (G1) further comprises an inert gas and / or a co-reactant gas.

[0138] 12. A method according to any of the previous items, wherein said CO2 component in said second gas flow (G2) is within the range of 1 % to 100%

[0139] 13. A method according to item 11 , wherein said second gas flow (G2) further comprises an inert gas and / or co-reactant such as CH4in a lower fraction. 14. A method according to any of the previous items, wherein a temperature of said post-plasma reaction chamber (11) is within the range of 700 °C to 6000 °C, more preferably between 1200°C and 6000°C during all or most of step (c).

[0140] 15. A method according to any of the previous items, wherein a temperature of said reaction chamber (10) is within the range of 700°C to 10000 °C, more preferably within the range of 1000°C to 10000°C, more preferably within the range of 2000°C to 8000°C, more preferably within the range of 6000°C to 8000°C.

[0141] 16. A method according to any of the previous items, wherein said plasma (2) is of the thermal or quasi-thermal type.

[0142] 17. A plasma reactor (1) comprising a reaction chamber (10) having a gas inlet (100), a separate post-plasma reaction chamber (11) fluidly connected to said reaction chamber (10) by means of a fluid connection comprising an opening or open tube arranged in a common separation wall between the reaction chamber (10) and postplasma reaction chamber (11), said post-plasma reaction chamber (11) comprising a gas outlet (110) and an energy source (12) to create the plasma (2) in said reaction chamber (10), the plasma reactor (1) further comprising a means (112) for measuring an amount of carbon deposited in a predetermined portion of an inner wall of said postplasma reaction chamber, said predetermined portion, preferably comprising a web or grid structure, arranged within substantial alignment with the fluid connection such that more than 50% of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is passing through it.

[0143] 18. A plasma reactor according to item 17, further comprising a means for measuring an O2 concentration (113) of a gas flow passing through said gas outlet (110).

[0144] 19. A plasma reactor (1 ) according to item 17 or 18, of the thermal or quasi thermal plasma type.

[0145] 20. A plasma reactor (1) according to any of items 17 to 19, wherein said reaction chamber (10) and said post-plasma reaction chamber (10) are fixedly connected.

[0146] 21 . A system (100) for plasma-based conversion of methane comprising a plasma reactor (1) according to any of items 17 to 20, further comprising a means for providing a gas flow to said inlet (4), a means (5) for switching between providing a first gas flow (G1) comprising CH4and providing a second gas flow (G2) comprising CO2 at said inlet (100) and a control unit (6) for controlling said means for switching (5) to switch, preferably continuously, preferably repeatedly, between providing said first flow of gas (G1) and providing said second flow of gas (G2) based on or at least on said measured amount of carbon deposited (3) and / or said O2 concentration measured at said gas outlet (110).

[0147] T1 Table of reference numbers.

Claims

Claims1. A method for plasma-based conversion of methane in a plasma reactor (1), said plasma reactor (1) comprising a reaction chamber (10) and an energy source (12) to create the plasma (2) in said reaction chamber (10), said reaction chamber (10) being fluidly connected to a separate post-plasma reaction chamber (11) by means of a fluid connection comprising an opening or open tube arranged in a common separation wall between the reaction chamber (10) and post-plasma reaction chamber (11), said plasma reactor (1) comprising a gas inlet (100), said post-plasma reaction chamber (11) comprising a gas outlet (110), the method comprising: a) generating a, preferably continuous, plasma (2) in said reaction chamber (10) by activating said energy source (12); b) providing a first gas flow (G1) comprising a CH4component through said gas inlet (100) and into said reaction chamber (10), allowing plasma-based conversion of said CH4component into gaseous H2and solid carbon and allowing the continuous removal of produced H2through said post-plasma reaction chamber (11) and through said gas outlet (110); c) in consideration of a solid carbon deposit being formed on a predetermined portion of an inner wall of said post-plasma reaction chamber (11), said predetermined portion, preferably comprising a web or grid structure, arranged within substantial alignment with the fluid connection such that more than 50% of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is passing through it, replacing said first gas flow (G1) with a second gas flow (G2), said second gas flow comprising a plasma-activated CO2component, wherein said CO2component in said second gas flow (G2) is within the range of 60% to 100%, wherein a distance between the discharge arc and the solid carbon deposit is smaller than 10mm, allowing the oxidation of the deposited carbon, and allowing the continuous removal of produced CO through said gas outlet (110) until said deposited carbon is substantially removed.

2. A method according to claim 1 , comprising repeating steps (b) and (c) alternatingly, to alternatingly replace the first gas flow (G1) with the second gas flow (G2) and vice versa.

3. A method according to claim 2, further comprising detecting or measuring an amount of said carbon being deposited in said post-plasma reaction chamber (11) during step (b) and switching to step (c) if said carbon amount reaches a first predetermined threshold value (T1).

4. A method according to claim 3, wherein said amount of carbon (3) being deposited in said post-plasma reaction chamber (11) is an amount being deposited at a predetermined location of maximal deposit, such as for instance at a bed, web or grid structure (111).

5. A method according to any of the previous claims, further comprising measuring a concentration of an O2 component in said gas outlet (110) and switching to step (b) if said O2 concentration reaches a second predetermined threshold value (T2).

6. A method according to any of the previous claims, wherein said at least one replacing of one of a first and a second gas flow (G1 , G2) with the other one of the first and the second gas flow (G2, G1) is performed in a gradual, for instance continuous or stepwise, manner.

7. A method according to any of the previous claims, wherein said at least one replacing of one of a first and a second gas flow (G1 , G2) with the other one of the first and the second gas flow (G2, G1) is performed in a discontinuous manner.

8. A method according to any of the previous claims, allowing a gas flow throughput within the range of 1 L / min to 100 L / min, and having a plasma or energy source having a power output within the range of 0.1 kW to 50 kW.

9. A method according to any of the previous claims, wherein a specific energy input is within the range of 1 to 10 kJ / L, more preferably 4 to 10 kJ / L during step (b) and is larger than 4 kJ / L during step (c).

10. A method according to any of the previous claims, wherein said CH4component in said first gas flow (G1) is within the range of 10% to 100%, more preferably within the range of 50% to 100%.

11. A method according to any of the previous claims, wherein said first gas flow (G1) further comprises an inert gas and / or a co-reactant gas.

12. A method according to any of the previous claims, wherein said CO2 component in said second gas flow (G2) is within the range of 1% to 100%.

13. A method according to claim 12, wherein said second gas flow (G2) further comprises an inert gas and / or co-reactant such as CH4in a lower fraction.

14. A method according to any of the previous claims, wherein a temperature of said post-plasma reaction chamber (11) is within the range of 700 °C to 6000 °C, more preferably between 1200°C and 6000°C during all or most of step (c).

15. A method according to any of the previous claims, wherein a temperature of said reaction chamber (10) is within the range of 700°C to 10000 °C, more preferably within the range of 1000°C to 10000°C, more preferably within the range of 2000°C to 8000°C, more preferably within the range of 6000°C to 8000°C.

16. A method according to any of the previous claims, wherein said plasma (2) is of the thermal or quasi-thermal type.

17. A plasma reactor (1) comprising a reaction chamber (10) having a gas inlet (100), a separate post-plasma reaction chamber (11) fluidly connected to said reaction chamber (10) by means of a fluid connection comprising an opening or open tube arranged in a common separation wall between the reaction chamber (10) and postplasma reaction chamber (11), said post-plasma reaction chamber (11) comprising a gas outlet (110) and an energy source (12) to create the plasma (2) in said reaction chamber (10), the plasma reactor (1) further comprising a means (112) for measuring an amount of carbon deposited in a predetermined portion of an inner wall of said postplasma reaction chamber, said predetermined portion, preferably comprising a web or grid structure, arranged within substantial alignment with the fluid connection such that more than 50% of the gas flow flowing from the reaction chamber into the post-plasma reaction chamber is passing through it.

18. A plasma reactor according to claim 17, further comprising a means for measuring an O2 concentration (113) of a gas flow passing through said gas outlet (110).

19. A plasma reactor (1 ) according to claim 17 or 18, of the thermal or quasi thermal plasma type.

20. A plasma reactor (1) according to any of claims 17 to 19, wherein said reaction chamber (10) and said post-plasma reaction chamber (10) are fixedly connected.21 . A system (100) for plasma-based conversion of methane comprising a plasma reactor (1) according to any of claims 17 to 20, further comprising a means for providing a gas flow to said inlet (100), a means (5) for switching between providing a first gas flow (G1) comprising CH4and providing a second gas flow (G2) comprising CO2 at said inlet (100) and a control unit (6) for controlling said means for switching (5) to switch, preferably continuously, preferably repeatedly, between providing said first flow of gas (G1) and providing said second flow of gas (G2) based on or at least on said measured amount of carbon deposited (3) and / or said O2 concentration measured at said gas outlet (110).

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