METHOD AND SYSTEM FOR TRANSFORMING A GAS MIXTURE USING PULSED PLASMA

Abstract

Method for transforming a gas mixture into a gas mixture of higher added value, comprising a step of injecting a gas mixture into a pulsed plasma reactor, a dissociation step using pulsed discharges to generate a shock wave between two electrodes to produce gases, and a step of releasing the produced gases to an area where they can be cooled down and/or separated and/or collected. The dissociation step is also designed to provide passive re-ignition of the plasma in the event that the latter is blown out by the continuous stream of gas in the reactor.

Claims

1. A method for producing gases from a dissociation of a gas mixture, comprising: a step of injecting a gas mixture into a pulsed plasma reactor comprising a structure defining a chamber containing a first electrode and one or more other electrodes of opposite polarity facing the first electrode; a dissociation step of the gas mixture, using isochoric discharges between the first electrode, of a given polarity, and the one or more other electrodes; a step of releasing the produced reactive gases from the dissociation step to an area where they can be cooled down and/or separated and/or collected; wherein the first electrode and the one or more other electrodes define an inter-electrode gap characterized by a variable inter-electrode distance and formed of an ignition area, and wherein the dissociation step comprises, in the event that the plasma produced in the reactor is blown out by a continuous stream of the gas mixture entering the reactor, a step for providing passive re-ignition of the plasma, the passive re-ignition step being performed within the ignition area in an area protected from the continuous stream of gas, the protected area resulting from the arrangement of an insulating block in the structure and having an inter-electrode distance allowing ignition of the plasma sheltered from the continuous stream of gas.

2. The method according to claim 1, wherein the step of passive re-ignition of the plasma further comprises, at the outlet of the ignition area, an entry of the plasma into a propagation area having an increasing distance and then decreasing distance between the second electrode and the structure connected to the first electrode, in the direction of propagation of the plasma, and then into a stable operating area arranged to create an electric field and having an inter-electrode distance less than the distance in the propagation area.

3. The method according to claim 1, wherein the dissociation step further comprises a plasma discharge between the first and second electrodes to produce an asymmetric shock wave.

4. The method according to claim 3, further comprising an increase in the reduced electric field intensity at one of the two electrodes to produce a reduced electric field asymmetry.

5. The method according to claim 4, further comprising heating one of the electrodes to produce a reduced electric field asymmetry.

6. The method according to claim 1, wherein the dissociation step further comprises a step for generating a high-voltage signal greater than 10 kV for controlling repetitive discharges by combining a very-high-voltage signal greater than 130 Td over short times less than 20 ns to ionize the gas and a high-voltage signal between 50 and 100 Td over long times less than 1 s to excite the molecules into excited vibrational levels.

7. A system for transforming a gas, using the production method according to claim 1, comprising: a pulsed plasma reactor comprising a structure defining a chamber containing a first electrode and one or more other electrodes of opposite polarity facing the first electrode; means for injecting a gas mixture into the pulsed plasma reactor so as to provide a substantially continuous inflow of gas into the pulsed plasma reactor; a dissociation stage comprising the pulsed plasma reactor receiving the inflow of gas at the inlet, the first long electrode of a given polarity, and the one or more other electrodes of opposite polarity, facing the first electrode, the first electrode and the one or more other electrodes defining an inter-electrode gap, characterized by a variable inter-electrode distance, and arranged so as to subject the flow of gas to isochoric discharges so as to produce reactive gases; an interface for releasing the reactive gases to an area where they can be cooled and/or separated and/or collected; and an insulating block creating an ignition area protected from the flow of gas the inter-electrode distance of which allows a passive re-ignition of the plasma in the event that the latter is blown out by a continuous stream of the gas mixture entering the plasma reactor.

8. The system according to claim 7, wherein the pulsed plasma reactor further comprises: an area of increasing distance and then decreasing distance between the second electrode and a structure connected to the first electrode in the direction of propagation of the plasma, known as the propagation area; and an area of inter-electrode distance less than the distance the propagation area, known as the stable operation area, arranged to create an electric field.

9. The system according to claim 7, wherein the stable operation area is substantially parallel to the direction of the gas flow.

10. The system according to claim 7, wherein the stable operation area is substantially transverse to the direction of the gas flow.

11. The system according to claim 7, further comprising means for controlling the direction of flow of the reactive gases in the plasma discharge, the direction control means comprising means for increasing the reduced electric field at one of the two electrodes.

12. The system according to claim 11, wherein the means for increasing the reduced electric field use a point-effect electrode.

13. The system according to claim 11, wherein the means for increasing the reduced electric field use a heating mechanism included in one of the electrodes.

14. The system according to claim 7, further comprising means for generating a high-voltage signal greater than 10 kV for controlling repetitive discharges by combining a very-high-voltage signal greater than 130 Td over short times less than 20 ns to ionize the gas and a high-voltage signal between 50 and 100 Td over long times less than 1 s to excite the molecules into excited vibrational levels.

15. The system according to claim 7, wherein the system is configured to produce gaseous dihydrogen from hydrocarbon and CO2 mixtures or hydrocarbons, to inject the hydrocarbon and CO2 mixtures or of hydrocarbons at the inlet of the pulsed plasma reactor, and to collect gaseous dihydrogen at the outlet of the pulsed plasma reactor.

16. The system according to claim 15, wherein the isochoric discharges comprise nanosecond repetitively pulsed discharges.

17. The system according to claim 15, wherein the interface for releasing the reactive gases comprises: a stage for rapid cooling of the reactive gases; and a stage for separating the gaseous dihydrogen and carbon monoxide produced after the cooling of the reactive gases.

18. A method of using a system according to claim 7 to produce oxygen from carbon dioxide, comprising injecting carbon dioxide at the inlet of the pulsed plasma reactor and collecting oxygen at the outlet of the pulsed plasma reactor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The present disclosure will be better understood in the light of the description illustrated by the following figures:

[0070] FIG. 1 is an overview of a dihydrogen production system according to the present disclosure;

[0071] FIG. 2 is a cross-sectional view of an exemplary embodiment of a dihydrogen production system according to the present disclosure;

[0072] FIG. 3 is a larger view of FIG. 2, illustrating the key components of the system;

[0073] FIG. 4 is a partial cross-sectional view of an exemplary embodiment of a dissociation stage in a dihydrogen production system according to the present disclosure;

[0074] FIG. 5A is a partial cross-sectional view of a first configuration of the dissociation stage, in which the stable area is transverse to the gas flow;

[0075] FIG. 5B is a partial cross-sectional view of a second configuration of the dissociation stage, in which the stable area is transverse to the gas flow;

[0076] FIG. 6 illustrates the various locations of the ignition, propagation and stability areas within a dissociation stage;

[0077] FIG. 7 is an enlarged cross-sectional view of a dissociation stage, representing characteristic inter-electrode distances;

[0078] FIG. 8 illustrates three examples of characteristic profiles providing inter-electrode distance variations within a dissociation stage;

[0079] FIG. 9 illustrates schematically the phenomenon of re-injection of hot gases into the plasma within a reactor;

[0080] FIG. 10 is a partial cross-sectional view of a dissociation stage configured to avoid this re-injection phenomenon;

[0081] FIGS. 11A-11C illustrates three exemplary embodiments of axial electrodes adapted to avoid this re-injection phenomenon;

[0082] FIG. 12 is an overview of a device for generating a mixed signal for feeding the electrodes of a dihydrogen production system according to the present disclosure; and

[0083] FIG. 13 is an electrical diagram of a practical exemplary embodiment of the generating device of FIG. 12.

DETAILED DESCRIPTION

[0084] A system S for producing dihydrogen gas according to the present disclosure comprises, with reference to FIGS. 1 and 2, a dissociation stage DI receiving at the inlet a gaseous flow such as a mixture of methane CH.sub.4 and carbon dioxide CO.sub.2, an ultra-rapid cooling stage FQ (“Fast Quenching”), followed by a separation stage SE of the dihydrogen gas H2 and the carbon monoxide gas CO.

[0085] By way of practical example, the gas flow processed by this production system may be about 0.2 m.sup.3/hr or ˜3.5 liters/min.

[0086] For the stoichiometry of the gaseous inputs CO2:CH4, a ratio 50:50 to 30:70 corresponding to a biogas type mixture can be provided; and 0:100 for pure methane.

[0087] With reference to FIG. 3, a practical exemplary embodiment of a dihydrogen gas production system according to the present disclosure will now be described.

[0088] The dissociation stage 10 comprises a structure 12, cylindrical in shape and made of a stainless steel/aluminum alloy, having an inlet 21 for a gas inflow (CH.sub.4, CO.sub.2) and defining a first chamber 20 containing a first electrode 13 acting as an anode facing a second electrode 15 acting as a cathode arranged in the middle of an outlet opening 26 of the first chamber 20. This cathode can be made of tungsten. The dissociation stage 10 is also provided with a connector 11 that contains a supply cable for the electrode 13. The structure 12 contains an insulating block 14 arranged to avoid any occurrence of an electric arc due to the high-voltage supply of the electrode 13.

[0089] The outlet opening 26 allows dissociated gases to enter the cooling area FQ formed of a second chamber 27 defined by a structure 23 with a cylindrical outer shape and a conical inner shape providing a continuous increase in the inner diameter of flow from the opening 26 to the outlet of the cooling area FQ.

[0090] With reference to FIGS. 2 and 3, the third stage SE of the dihydrogen gas production system 1 comprises a cylindrical structure 24 mechanically coupled to the outlet of the cooling stage FQ and a radial discharge duct 22. The separation chamber 19 inside the structure 24 is axially crossed by an electrical supply rod 25 having at its end the electrode 15 extending into the dissociation chamber 20.

[0091] Practical exemplary embodiments of the dissociation stage of a dihydrogen gas production system according to the present disclosure will now be described with reference to FIGS. 4 to 8.

[0092] This dissociation stage 40 comprises an anode 13 having a tapered and pointed shape at its end and a cathode 15, facing the anode 13, having a substantially rounded end and electrically connected to the inner wall of the dissociation chamber.

[0093] With reference to FIGS. 4 and 6, three characteristic areas can be identified within the dissociation stage: a so-called ignition area 1, AMO corresponding to a minimum inter-electrode distance, a propagation start area 2 where the plasma is just after ignition and in which the inter-electrode distance is increasing in the direction of plasma propagation, then a propagation area 3, PRO, in which the inter-electrode distance is decreasing, followed by a stability area 4, STA located between the tip of the anode 13 and the end of the cathode 15.

[0094] The insulating block 14, located upstream of the ignition area 1, has two functions: it prevents the occurrence of an electric arc and it creates this area 1 protected from the continuous stream of gas 5 in which the ignition will take place.

[0095] As illustrated in FIG. 7, the inter-electrode distance is variable, increasing and then decreasing, from a minimum value d1 in the ignition area 1 to a value d4 in the stability area 4 between the tip of the electrode 13 and the end of the electrode 15.

[0096] Two configurations of a dissociation stage of a gas transformation system according to the present disclosure, in which the gas stream is transverse to the electrode arrangement, will now be described with reference to FIGS. 5A and 5B.

[0097] In a first particular configuration of the dissociation stage 50A of a reactor arranged horizontally, illustrated by FIG. 5A in which the dashed lines delimit the flow area, the gas stream 55A flows perpendicular to the horizontal plane of the electrode arrangement 53, 57. The ignition area 1 is located outside the flow of the stream 55A and is therefore protected from this stream. During discharge in area 1, each spark can cause the induced flow to swing either to the left or to the right. Since the pulse frequency is high (about 1000 pulses per second), it is sufficient to wait for the spark that allows the flow to the right (in the direction of the electrode arrangement 53, 57), for there to be a correct ignition. A small flow bypass can also be provided to drive the plasma toward the electrode arrangement 53, 57. This induced flow will allow the plasma to be placed in the propagation start area 2 in the stream 55A, then the plasma will slowly move over the propagation area 3 to the stability area 4.

[0098] In a second particular configuration of the dissociation stage 50B of a horizontally arranged reactor illustrated by FIG. 5B, the gas stream 55B flows perpendicular to the vertical plane of the electrode arrangement 53, 57.

[0099] Several profiles of the propagation area can be considered as illustrated in FIG. 8. The efficiency of the profile depends on the ratio d1/d4 and the number 7C (related to the non-diffusive regime), which are chosen as a function of frequency and temperature.

[0100] With reference to FIGS. 9 to 11, embodiments of a dihydrogen gas production system according to the present disclosure will now be described, the system making it possible to solve the problem of re-injection of the produced gases into the plasma, as shown schematically in FIG. 9.

[0101] To control the gas flow in the reactor, the gas generation system according to the present disclosure thus comprises: [0102] two electrodes 13, 15 facing each other, as shown in FIG. 10, defining an inter-electrode area in which an electric field is created between the two electrodes to produce a plasma discharge generating a shock wave, hereinafter referred to as an isochoric discharge; [0103] a reactive area in which a high reduced field is promoted at one of the two electrodes, by using a point-effect electrode, with an increase in temperature, by a heating mechanism included in the electrode 13 and by reducing cooling mechanisms around the electrode.

[0104] The shock wave is created passively by the isochoric discharges.

[0105] Possible geometric profiles for the ignition, propagation and stabilization areas within a pulsed plasma reactor of a gas mixture transformation system according to the present disclosure will now be described.

[0106] First, it is important to note that an ideal one-dimensional (1D) propagation pattern is a straight profile forming an angle α with the direction of propagation, with the ideal angle α depending on the pulse frequency and the temperature reached. However, ignition at the beginning must play on the point effect, while stabilization at the end of the process requires reducing the inter-electrode gap.

[0107] An ideal theoretical profile [ignition+propagation+stabilization] would therefore be a combination of a point and two broken lines. As such, a theoretical profile is in practice difficult to machine; a profile using the same tangents as this ideal profile was used.

[0108] In this context, three cathode geometries designed to provide flow control are shown in FIGS. 11A-11C, with the objective of satisfying the following conditions: not blocking the flow direction, providing a replaceable cathode part, and being easily machinable.

[0109] In a first geometry (FIG. 11A), the cathode 15.1 has the form of a point at the end of the rod 25. In a second geometry (FIG. 11B), the cathode 15.2 has the form of a perforated disc arranged in the smaller diameter part of the rapid cooling area. In a second geometry (FIG. 11C), the cathode 15.3 has a complex geometry extending from the ignition area to the stability area. These cathodes 15.2 or 15.3 can be made of tungsten material using additive prototyping machines.

[0110] In a preferred mode of operation, the pulsed plasma generating a shock wave is generated by nanosecond repetitively pulsed (NRP) pulses, with a voltage of 10 kV and a repetition rate in the range of 5 to 500 kHz, preferably between 10 and 100 kHz.

[0111] An exemplary embodiment of a system for generating voltage signals that are applied to the plasma reactor electrodes of a gas generation system according to the present disclosure will now be described with reference to FIGS. 12 and 13. The voltage signals result from a combination of variably shaped high-voltage signals for generating plasma discharges, so as to excite different energy modes of a molecule to achieve a desirable chemical effect.

[0112] In the signal generation system 30, a very-high-voltage signal (>130 Td) over short times (0-20 ns), referred to as short pulse, is thus combined to ionize the gas with a high-voltage signal (50-100 Td) over long times (0-1 s), referred to as long pulse, to excite the molecules into vibrational levels. The long pulse is generated by a long pulse generator module 31, and the short pulse is generated by an NRP module 32. The two signals are combined with a mixing module 33.

[0113] The generation system 30 comprises: [0114] a DC module 31 generating a high-voltage pulse of duration 0-1 s, hereinafter referred to as long pulse, provided with an impedance adaptation, [0115] an NRP module 32 generating a high-voltage pulse of duration 0-20 ns, hereinafter referred to as short pulse, provided with an impedance adaptation, [0116] a module 33 for mixing short and long pulses, [0117] voltage probes 34 providing information about the signals actually applied to the electrodes of the reactor 10.

[0118] The long pulse generator module 31 is equipped with a protection realized by a first-order low-pass filter, while the short-pulse generator module 32 is equipped with a protection realized by a second-order high-pass filter.

[0119] The short-pulse generator module 32 provides a reduced electric field >100 Td and duration 0-20 ns, while the long pulse generator module 31 provides a reduced electric field of 50-100 Td and duration 0-1 s.

[0120] The signal generation system 30 is defined so that the reduced electric field of the long pulse is below the ionization threshold. The plasma is in the subcritical regime.

[0121] Kinetic calculations give the following: [0122] optimal E/N field: 50 Td or 4 kV/cm at a temperature of 900 K and 3 kV/cm at a temperature of 1200 K; [0123] target ranges: voltage [1-4 kV] and [0.5-30 A].

[0124] In a first example, the long-pulse generator module 31 is a DC generator of voltage 3 kV and of maximum current 1 A, and the short-pulse generator module 32 is a high-voltage NRP generator of voltage 10 kV. The NRP circuit is protected from the DC, and the DC circuit is protected from the NRP.

[0125] In another example, the short-pulse generator module 32 is a 10 ns nanosecond pulse generator, and the long-pulse generator module 31 is a 1 μs pulse generator.

[0126] The present disclosure is not limited to the exemplary embodiments just described and many other embodiments can be considered without departing from the scope of the present disclosure. In particular, the re-ignition technique set forth in the present disclosure could also be used in a plasma-assisted combustion system or for scramjets (supersonic combustion ramjet).

REFERENCES

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