Patent Application
20230294065
The present disclosure belongs to the field of gas production devices, and, in particular, reforming devices for the production of products of higher added value.
Plasma discharges present an electrophysical alternative to the transformation of gas mixtures into gas mixtures of higher added value by thermal approaches (pyrolysis), thermocatalytic approaches (reforming reactions), or electrochemical approaches (electrolysis).
Document U.S. Pat. No. 6,395,197 B1 discloses a method and system for producing hydrogen and elemental carbon from natural gas and other hydrocarbons. Diatomic hydrogen and unsaturated hydrocarbons are produced as reactor gas in a fast quench plasma reactor. During fast quenching, the unsaturated hydrocarbons are further decomposed by heating the reactor gases. Other gases can be added at different stages of the process to form a desired final product and prevent back-reactions. The product is a hydrogen fuel and elemental carbon that can be used in powder form as a feedstock for a number of industrial processes.
Document U.S. Pat. No 5,409,784 discloses a plasmatron-fuel cell system for generating electricity, wherein the plasmatron receives a hydrocarbon fuel and reforms the hydrocarbon fuel to produce a hydrogen-rich gas.
The use of pulses makes it possible to produce plasmas with an equivalent density of reactive species while reducing the heating compared to non-pulsed plasmas. The energy efficiency of the method is improved.
In the case where these methods use plasma discharges in high-velocity gas streams, the gas residence time may become comparable to or less than the characteristic ionization times. In this case, the chemical reaction might not occur, and the plasma might not ignite.
Active systems are already known to ignite the plasma, which make it possible to increase the electric field above the breakdown value. These active systems can use an increase of the voltage applied to the electrodes, a decrease of the gas pressure, an increase of the gas temperature, or a decrease of the inter-electrode distance by a mobile mechanical system.
These active systems presented above have industrial limitations. They are not suitable for pulse-generated plasmas, which means it is not possible to benefit from their advantageous energy efficiency. Indeed, the decrease of the pressure and the increase of the temperature require an interruption of the process. The increase of the voltage requires an oversizing of the voltage generator (additional cost). The presence of moving parts leads to additional maintenance and sealing costs. Moreover, feedback systems require sensors, thus measurement systems (electrical measurement, optical measurement) and a processing circuit for feedback.
Document WO 2013/078880 A1 discloses a multi-stage plasma reactor system including (i) hollow cathodes for cracking carbonaceous material, each stage comprising hollow cathodes and hollow anodes cooled by recycling cooling agent or refrigerant fluid, (ii) one or more working gas inlets, (iii) one or more inlets for carbonaceous material and carrier gas as feedstock, and (iv) reaction tubes connected to the anode or to the cathode.
Document CN 109663555 A discloses a system and a method for synergistically converting greenhouse gas and biochar by pulsed jet plasma. A discharge arc formed between an inner and an outer electrode is driven by an ascending CO2 spiral airflow and sequentially passes through a tapered nozzle and an air distribution plate to form a plurality of uniformly distributed plasma microjets. The microjets drive the biochar particles to form a gas-solid fluidization reaction area.
The purpose of the present disclosure is to propose a method and system for pulsed plasma gas transformation that allows better operational continuity and lower maintenance costs than current methods and systems.
This objective is achieved with a method for producing gases from a gas mixture comprising:
According to the present disclosure, 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 two other areas, and the dissociation step comprises, in the event that the plasma produced in the reactor is blown out by a continuous stream of gas in the reactor, a step for providing passive re-ignition of the plasma, the passive re-ignition step being performed in an ignition area providing an area protected from the continuous stream of gas and having an inter-electrode distance allowing ignition of the plasma sheltered from the continuous stream of gas.
The re-ignition technique used in the system/method according to the present disclosure is passive and therefore reliable.
It should be noted that this configuration of the dissociation reactor could also be used in plasma-assisted combustion chambers for which the control of the reactive area in high-flow media can pose a real problem.
The passive re-ignition of the plasma can further advantageously comprise, at the outlet of the ignition area (1), an entry of the plasma into a propagation area having an increasing inter-electrode distance (2) and then decreasing inter-electrode distance (3) in the direction of propagation of the plasma, and then into a stable operating area (4) arranged to create an electric field and having an inter-electrode distance less than the distance in the propagation area.
The passage from the area (1) to the area (2) then (3) is obtained advantageously by using the flow induced by discharges producing a shock wave, referred to as isochoric discharges. This shock wave is created passively by the isochoric discharges.
Another problem solved in the gas transformation method according to the present disclosure is the need to control the gas flow within the plasma reactor.
The inflow of gas (overall flow) is transformed by passing through a reactive area (a reaction transforms the incoming materials into products), which generates its own flow (induced flow). If the products of the reaction are convected upstream of the overall flow, they can be transformed again in the reactive area, and the energy efficiency drops.
These isochoric discharges produced during the dissociation step between the first electrode of given polarity and the other electrode of opposite polarity generate an asymmetric shock wave that contributes to control of the direction of the flow of the reactive gases in the plasma discharge.
In a preferred embodiment according to the present disclosure, the shock waves are obtained by repetitive pulsed nanosecond discharges, produced between the first electrode of given polarity and the other electrode or electrodes of opposite polarity or neutral.
The direction control may advantageously comprise an increase in a reduced electric field at one of the two electrodes.
A heating included in one of the electrodes could also be provided to produce a reduced electric field asymmetry.
The shock wave caused by the pulsed discharge and the associated hydrodynamic expansion have been the subject of several scientific works [1] [2] [3]. The novelty of the method according to the present disclosure lies in the stability of the flow control obtained.
It is noted that the ignition of a plasma is driven by the reduced electric field E/N, where E is the electric field and N the number of molecules per unit of volume. E/N is expressed in Townsends (1 Td=10−17 V·cm2).
The hydrodynamics generated by a shock wave can take two forms:
In the present disclosure, the regime must be non-diffusive. Dumitrache's theory [5] provides a criterion for achieving a non-diffusive regime, which depends on the dimensionless number π:
where E is the energy deposited in thermal form in the plasma, d is the inter-electrode distance, R is the radius of the discharge, and P is the gas pressure.
In the non-diffusive regime, the discharge creates a shock wave that can be modeled by a cylindrical shock wave centered on the inter-electrode axis, and two spherical shock waves substantially centered in front of each of the electrodes. Under axisymmetric initial conditions, the spherical shock waves diffuse with the same velocity and the hot gases are ejected along a torus. Under non-symmetric initial conditions, one of the two shock waves is faster and the hot gases are ejected on the side of the faster shock wave.
The propagation speed of a shock wave is proportional to the pressure gradient. In an isochoric discharge (energy deposition<<hydrodynamic times), the pressure gradient is proportional to the temperature gradient at the end of the discharge. In isochoric discharges, the temperature increase is due to the predissociation of excited electronic states (ultrafast heating).
The excitation of electronic states increases with the reduced electric field E/N. Therefore, if one of the two electrodes is initially hotter, the reduced electric field will be higher. Consequently, the excitation and therefore the predissociation will be higher. Consequently, the temperature in the discharge will be higher, and thus the pressure, and so the shock wave will be faster at this electrode. Consequently, the hot gases will be ejected from the side of the hot electrode. The electrode will remain hot, hence the stability.
In a particular exemplary embodiment of the present disclosure, the heating of one of the electrodes is produced directly by the impact of the ions on the electrode and by the reduction of thermal diffusion. The heating of one of the two electrodes can be increased by choosing for this electrode a material with low thermal diffusivity.
To understand the mechanisms controlling the flow induced by a single nanosecond discharge generated between a pair of electrodes and leading to the formation of the two hydrodynamic regimes observed, it may be useful to refer to document [6].
To understand the impact of the recirculation of gas flows on the temporal development of the species and the temperature of the gases in the vicinity of the discharge area generating a shock wave, it may be useful to refer to document [7].
For a numerical study of the fluid dynamics induced by the plasmas produced by two laser pulses for the ignition of combustible mixtures, it may be useful to refer to document [8].
In the method, the geometry and thermophysical properties of the electrodes are controlled to generate the induced flow and to convectively direct the outgoing gases away from the reactive area and downstream of the overall flow.
A novel approach is also proposed for the generation of voltage signals applied to the electrodes of the plasma reactor using the gas transformation method according to the present disclosure.
Indeed, it is known that plasmas are characterized by the reduced electric field (E/N) applied in the discharge (expressed in Townsends: Td). Different types of plasma (microwave, nanosecond, DBD, etc.) correspond to different ranges of reduced electric fields. Each range of reduced electric field corresponds to a different excitation mode of the molecule.
The dissociation of molecules (CO2, hydrocarbons) by plasma requires both a generation of a sufficient density of electrons and an excitation of these electrons at the vibrational energies of the molecules.
The production of electrons is obtained by ionization at strong electric fields (>130 Td). The vibration of molecules is obtained for intermediate electric fields (50-100 Td).
The aim is to combine different signals in an efficient way to obtain a strong ionization followed by a vibration of the molecules by combining an electric pulse of reduced field >130 Td followed by an electric pulse of intermediate field (50-100 Td).
The dissociation step may further comprise a step for generating a high-voltage signal for controlling repetitive discharges by combining a very-high-voltage signal over short times to ionize the gas and a high-voltage signal over medium times to excite the molecules into excited vibrational levels.
According to another aspect of the present disclosure, there is proposed a system for transforming a gas mixture, using the production method according to the present disclosure, comprising:
The pulsed plasma reactor according to the present disclosure may advantageously comprise:
an area of increasing inter-electrode distance and then decreasing inter-electrode distance in the direction of propagation of the plasma, known as the propagation area, and
The isochoric discharges produced between the first electrode of given polarity and the other electrode or electrodes of opposite polarity generate a shock wave that contributes to controlling the direction of the reactive gases.
The first electrode may advantageously have a point effect arranged so as to generate, in the stable operation area, a reduced electric field greater than that generated in the ignition area or in the propagation area.
The stable operation area may be either substantially parallel to the direction of the gas flow, or substantially transverse to the direction of the gas flow.
In this transverse configuration, and if a horizontally arranged reactor is considered, the gas flow can be either perpendicular to a substantially horizontal plane through the electrodes or perpendicular to a substantially vertical plane through the electrodes.
In a preferred configuration of the present disclosure, the transformation system may further comprise 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.
The means for increasing the reduced electric field can use a point-effect electrode and/or a heating mechanism included in one of the electrodes.
The transformation system according to the present disclosure may further comprise 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.
According to yet a further aspect of the present disclosure, use of the system according to the present disclosure to produce gaseous dihydrogen from hydrocarbon and CO2 mixtures or hydrocarbons is proposed, comprising an injection of the hydrocarbon and CO2 mixtures or of hydrocarbons at the inlet of the pulsed plasma reactor, and a collection of gaseous dihydrogen at the outlet of the pulsed plasma reactor.
The isochoric discharges may advantageously comprise nanosecond repetitively pulsed (NRP) discharges.
The interface for releasing the reactive gases may comprise:
According to yet a further aspect of the present disclosure, use of the system according to the present disclosure to produce oxygen from carbon dioxide is proposed, comprising an injection of carbon dioxide at the inlet of the pulsed plasma reactor and a collection of oxygen at the outlet of the pulsed plasma reactor.
The present disclosure will be better understood in the light of the description illustrated by the following figures:
A system S for producing dihydrogen gas according to the present disclosure comprises, with reference to
By way of practical example, the gas flow processed by this production system may be about 0.2 m3/hr or ˜3.5 liters/min.
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.
With reference to
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 (CH4, CO2) 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.
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.
With reference to
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
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.
With reference to
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.
As illustrated in
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
In a first particular configuration of the dissociation stage 50A of a reactor arranged horizontally, illustrated by
In a second particular configuration of the dissociation stage 50B of a horizontally arranged reactor illustrated by
Several profiles of the propagation area can be considered as illustrated in
With reference to
To control the gas flow in the reactor, the gas generation system according to the present disclosure thus comprises:
The shock wave is created passively by the isochoric discharges.
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.
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.
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.
In this context, three cathode geometries designed to provide flow control are shown in
In a first geometry (
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.
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
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.
The generation system 30 comprises:
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.
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.
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.
Kinetic calculations give the following:
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.
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.
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).
[1] “Experimental study of the hydrodynamic expansion following a nanosecond repetitively pulsed discharge in air” (2011) Da A. Xu, Deanna A. Lacoste, Diane L. Rusterholtz, Paul-Quentin Elias, Gabi D. Stancu, and Christophe O. Laux.
[2] “Simulation of the hydrodynamic expansion following a nanosecond pulsed spark discharge in air at atmospheric pressure” (2013) Fabien Tholin and Anne Bourdon.
[3] Hydrodynamic Regimes Induced by Nanosecond Pulsed Discharges in Air: Mechanism of Vorticity Generation, (2019) Ciprian Dumitrache 1, Arnaud Gallant, Nicolas Minesi, Sergey Stepanyan, Gabi D Stancu and Christophe O Laux.
[4] Dumitrache, C.; Yalin, A. P. Numerical Modeling of the Hydrodynamics Induced by Dual-Pulse Plasma; In 2018 AIAA Aerospace Sciences Meeting; American Institute of Aeronautics and Astronautics: Reston, Virginia, 2018, 10.2514/6.2018-0689.
[5] Dumitrache, C.; Galant, A.; Minesi, N.; Stepanyan, S.; Stancu, G.-D.; Laux, C. O. Hydrodynamic regimes in NRP discharges (in preparation). Journal of Physics D: Applied Physics 2019.
[6] Two Regime Cooling in Flow Induced by a Spark Discharge. Bhavini Singh, Lalit K. Rajendran, Pavlos P. Vlachos, and Sally P. M. Bane. Phys. Rev. Fluids 5, 014501-Published 14 Jan. 2020.
[7] A 3-D DNS and experimental study of the effect of the recirculating flow pattern inside a reactive kernel produced by nanosecond plasma discharges in a methane-air mixture Maria Castela, Sergey Stepanyan (2017).
[8] Numerical Modeling of the Hydrodynamics Induced by Dual-Pulse Laser Plasma, Ciprian Dumitrache, Azer Yalin (2018).
[9] Mao et al 2018, “Numerical modeling of ignition enhancement of CH4/O2/He mixtures using a hybrid repetitive nanosecond and DC discharge,” doi/10.1016/j.proci.2018.05.106.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2005313 | May 2020 | FR | national |
This application is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/FR2021/050900, filed May 20, 2021, designating the United States of America and published as International Patent Publication WO 2021/234302 A1 on Nov. 25, 2021, which claims the benefit under Article 8 of the Patent Cooperation Treaty to French Patent Application Serial No. FR2005313, filed May 20, 2020.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2021/050900 | 5/20/2021 | WO |
