A method and device for cyclic plasma nitrogen fixation by double dielectric barrier discharge

Abstract

The application belongs to the technical field of low-temperature plasma nitrogen fixation, and particularly relates to a circulating plasma nitrogen fixation method and device of double dielectric barrier discharge, which comprises the following steps: introducing air into a spatial dielectric barrier discharge system to perform first discharge activation, generating plasma activation gas containing active nitrogen-oxygen species, introducing the gas into a liquid container to perform gas-liquid reaction in the form of bubbling with the liquid, and setting a surface dielectric barrier discharge area above the liquid to perform secondary discharge activation on residual gas. Through a gas circulation channel, the residual gas reenters the spatial dielectric barrier discharge system, realizing the circulation utilization of the same gas in the system through multiple discharges and multiple gas-liquid reactions. Through the above two-electrode discharge and gas reflux mechanism, the generation amount of active nitrogen-oxygen species and the conversion efficiency of the active nitrogen-oxygen species into liquid phase can be significantly improved, direct discharge of residual gas is reduced, the gas utilization rate is improved, and the application is suitable for the field of plasma nitrogen fixation.

Description

Technical Field

[0001] This application belongs to the field of low-temperature plasma nitrogen fixation technology, specifically a cyclic plasma nitrogen fixation method and apparatus using dual dielectric barrier discharge. Background Technology

[0002] Plasma nitrogen fixation technology utilizes low-temperature plasma to activate nitrogen and oxygen in the air, generating a gas rich in reactive nitrogen and oxygen species, which further generates nitrate ions in the water. ) and nitrite ions ( Compared to traditional high-temperature and high-pressure nitrogen fixation methods, such as the Haber-Bosch process, this technology can operate under ambient or near-ambient temperature and pressure conditions. The reaction conditions are mild, the equipment structure is relatively simple, and it has potential application value in agricultural irrigation water preparation, liquid fertilizer production, and environmental remediation.

[0003] In existing technologies, space dielectric barrier discharge devices are typically used to generate plasma-activated gas, which is then introduced into water via bubbling or gas delivery structures. Reactive nitrogen and oxygen species undergo dissolution, hydration, and oxidation reactions at the gas-liquid interface, generating nitrate and nitrite ions. However, due to the limited lifetime of reactive nitrogen and oxygen species, they are prone to decay or recombination reactions during migration from the gas phase to the liquid phase, resulting in a limited proportion of reactive species actually entering the water and low utilization efficiency.

[0004] To address this issue, a surface dielectric barrier discharge device was proposed to be installed on the water surface, causing the discharge to occur near the gas-liquid interface, thereby shortening the transport distance of active species and improving the gas-liquid reaction efficiency. However, in practical applications, the single discharge method still has the following shortcomings: Firstly, the gas only undergoes one discharge, and some gas may not be fully activated, directly entering the space above the water or being discharged from the system after the gas-liquid reaction; secondly, even if a discharge area is set near the gas-liquid interface, some residual gas is still not further activated or fully participates in subsequent reactions, resulting in low utilization rates of active species and energy. Therefore, how to enable the gas to participate in the reaction multiple times and be recycled while ensuring the gas-liquid reaction efficiency, and to increase the generation of active nitrogen and oxygen species and their conversion rate in the liquid phase, has become an urgent problem to be solved in existing plasma nitrogen fixation technology. Summary of the Invention

[0005] This application provides a cyclic plasma nitrogen fixation method and apparatus using dual dielectric barrier discharge, which solves the problems of low utilization rate of active nitrogen and oxygen species, insufficient gas-liquid reaction, and energy waste caused by direct emission of residual gas in the prior art.

[0006] The first aspect of this application provides a cyclic plasma nitrogen fixation device with dual dielectric barrier discharge, comprising: The surface dielectric barrier discharge system and plasma activated water container system are installed inside the exhaust shell, along with the space dielectric barrier discharge system and the gas circulation system that enables gas circulation between the exhaust shell and the space dielectric barrier discharge system. The surface dielectric barrier discharge system has a surface dielectric discharge plate installed on the top surface inside the exhaust shell. Under the action of the alternating electric field applied by the high-frequency power supply, surface discharge is formed on the surface of the surface dielectric discharge plate. The plasma activated water container system includes a cooling water tank placed inside the exhaust shell, a liquid container in the middle of the cooling water tank, an aeration structure at the bottom of the liquid container, and the liquid surface in the liquid container facing the surface dielectric discharge plate. The space dielectric barrier discharge system includes a cylindrical shell, a ground electrode layer disposed within the inner layer of the cylindrical shell, a quartz dielectric layer disposed within the inner layer of the ground electrode layer, and a high-voltage electrode disposed within the inner layer of the quartz dielectric layer. The cylindrical shell is provided with a gas passage. The gas circulation system includes a low-flow air pump for extracting gas from the gas extraction shell and delivering it to the space dielectric barrier discharge system. The plasma activation gas generated in the space dielectric barrier discharge system is introduced into the water in the liquid container through an aeration structure to form bubbles.

[0007] Furthermore, the surface dielectric discharge plate includes a metal foil sheet as a high-voltage electrode on the upper layer, a mesh electrode as a grounding electrode on the lower layer, and a dielectric layer between the metal foil sheet and the mesh electrode.

[0008] Furthermore, the surface of the dielectric layer of the surface dielectric discharge plate is provided with a non-planar undulating structure, so that the area not covered by the mesh electrode forms a tortuous gas flow channel.

[0009] Furthermore, the aeration structure includes an aeration head for forming bubbles from the gas entering the liquid container; a microporous plate is provided in the bubble rising path above the aeration head to form a secondary control structure for the bubbles.

[0010] Furthermore, the surface of the microporous plate is integrally formed, and the lower surface is provided with dense, pointed, cone-shaped bubble-breaking protrusions; before the rising large-sized bubbles pass through the microporous plate, they first come into contact with the pointed, cone-shaped bubble-breaking protrusions and break, and the small-sized bubbles that do not break are then dispersed by the microporous plate to form uniform bubbles that diffuse back into the liquid container.

[0011] Furthermore, a polytetrafluoroethylene hydrophobic membrane is provided at the opening above the liquid container, and a square slot is provided along the upper edge of the liquid container for limiting the polytetrafluoroethylene hydrophobic membrane; the polytetrafluoroethylene hydrophobic membrane is embedded in the square slot around its perimeter and is fitted and fixed with the central opening of the cooling water tank cover, the central opening facing the surface dielectric discharge plate.

[0012] Furthermore, the high-voltage electrode adopts a spiral structure.

[0013] Furthermore, the vacuum casing is provided with a pressure relief hole, and a one-way air supply structure is provided at the pressure relief hole to introduce external gas when the internal pressure of the vacuum casing is lower than the external air pressure.

[0014] A second aspect of this application provides a cyclic plasma nitrogen fixation method using dual dielectric barrier discharge, comprising: The space dielectric barrier discharge device generates low-temperature plasma from the incoming air under the action of a high-voltage alternating electric field, producing plasma-activated gas containing active species. The plasma-activated gas is introduced into a liquid container set in a cooling water tank through an aeration structure, so that it forms bubbles in the liquid to be treated, and generates nitrate ions and nitrite ions through hydration and oxidation reactions. The remaining gas that is not completely dissolved continues to enter the surface dielectric barrier discharge device above the liquid surface to be treated for secondary activation, and the active species are transferred into the liquid through the gas-liquid interface. The residual gas that has not fully reacted is collected by the gas circulation system and reintroduced into the space dielectric barrier discharge device through the return channel for further discharge treatment, thus forming a gas circulation.

[0015] Furthermore, it also includes: by comparing and analyzing the discharge waveforms of the surface dielectric barrier discharge system and the space dielectric barrier discharge system, determining the excitation intensity distribution of the gas in different discharge regions, and adjusting the gas flow rate and discharge parameters according to the excitation intensity distribution.

[0016] Compared with the prior art, the advantages of this application are as follows: This application employs a dual-discharge structure combining spatial dielectric barrier discharge and surface dielectric barrier discharge. Through the synergistic effect of volume discharge and surface discharge, the gas is excited by an electric field in stages within different discharge regions. During gas-phase discharge and gas-liquid interface reaction, reactive nitrogen and oxygen species are continuously generated, thereby increasing the overall amount of reactive species generated in the gas. The dual-discharge structure can form complementary discharge modes in space, allowing gas that was not fully activated in the previous stage of discharge to be reactivated in subsequent discharge regions, thus improving the overall activation level of the gas and the generation efficiency of reactive nitrogen and oxygen species. Through the bipolar discharge and gas recirculation mechanism, the generation amount of reactive nitrogen and oxygen species and their conversion efficiency into the liquid phase can be significantly improved, reducing the direct emission of residual gas and improving gas utilization, making it suitable for the field of plasma nitrogen fixation.

[0017] This application constructs a gas circulation system that creates a closed-loop path for the gas between the space dielectric barrier discharge system, the gas-liquid reaction region, and the surface dielectric barrier discharge system. Through multiple gas circulations within the system, residual gas can repeatedly enter the discharge region and the liquid reaction region to continue participating in the reaction, reducing energy waste caused by direct emission of residual gas. This increases the generation of reactive nitrogen and oxygen species and the cumulative concentration of liquid products under unit energy consumption conditions, thereby enhancing the overall gas utilization rate and energy efficiency of the system.

[0018] This application incorporates a double-layer aeration structure in the liquid reaction unit. The porous aeration head enables the gas to form a large number of uniform bubbles in the liquid. Subsequently, a microporous plate positioned above the aeration head blocks, buffers, breaks up, and guides the bubbles, further refining them and allowing them to diffuse uniformly back into the main liquid region. This significantly increases the gas-liquid interface area, prolongs the gas residence time in the liquid phase, improves the gas-liquid mass transfer efficiency, and allows active nitrogen and oxygen species to dissolve more fully and participate in the reaction, promoting the generation of nitrate and nitrite ions and improving the generation efficiency of liquid-phase nitrogen fixation products.

[0019] This application arranges a surface dielectric barrier discharge system above the liquid surface inside a liquid container, enabling the gas activated by the discharge to enter the liquid phase system within a short distance to participate in the reaction. This shortens the migration distance of active species in the gas phase, reduces attenuation losses during the transport process, thereby improving the efficiency of the transfer of active nitrogen and oxygen species to the liquid phase system and further promoting the formation of nitrate and nitrite ions. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall structure of the device provided in the embodiments of this application; Figure 2 This is a schematic diagram of the surface dielectric barrier discharge system provided in the embodiments of this application; Figure 3 This is a schematic diagram of the structure of the plasma-activated water container system provided in the embodiments of this application; Figure 4 This is a schematic diagram of the structure of the space dielectric barrier discharge system provided in the embodiments of this application; Figure 5 This is a schematic diagram of the circulating water system provided in the embodiments of this application; 1-Evacuation casing, 2-Surface dielectric discharge plate, 3-Grounding electrode wire, 4-Cooling water tank, 5-Base, 6-Aerator head, 7-Water tank outlet, 8-Water tank inlet, 9-Metal foil, 10-High voltage electrode wire, 11-Low flow air pump, 12-Air pump inlet pipe, 13-Air pump inlet, 14-Air pump outlet, 15-Air pump outlet pipe, 16-Gas passage inlet, 17-Cylindrical casing, 18-Cooling water inlet, 19-Outlet pipe, 20-Cooling water outlet, 21-Gas... 22-High voltage electrode, 23-Device base, 24-Experimental base, 25-Ground electrode layer, 26-Quartz dielectric layer, 27-Liquid container, 28-Aeration tower head, 29-Liquid container air inlet, 30-Polytetrafluoroethylene hydrophobic film, 31-Support pad, 32-Cooling water tank cover, 33-Grounding terminal, 34-High voltage terminal, 35-Mesh electrode, 36-Outer shell pressure relief hole, 37-Outer shell air outlet, 38-First circulating water machine, 39-Second circulating water machine, 40-Microporous plate. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0022] This application provides a cyclic plasma nitrogen fixation method and apparatus based on the synergistic effect of space dielectric barrier discharge and surface dielectric barrier discharge, combined with gas recycling. By constructing a closed-loop gas circulation path within the system, the air entering the system is no longer discharged after completing one discharge and one gas-liquid reaction in a single pass. Instead, it circulates repeatedly between the space dielectric barrier discharge system and the surface dielectric barrier discharge system, allowing the same gas to undergo the discharge activation process multiple times and participate in the gas-liquid reaction multiple times. This gradually increases the proportion of active nitrogen and oxygen species generated in the gas and their conversion efficiency in the liquid phase.

[0023] Compared with traditional single-stage discharge nitrogen fixation methods, this application does not determine the degree of nitrogen conversion based on a single discharge. Instead, it constructs a gas reflux path, allowing the gas to form multiple reciprocating flow paths within the system. This expands the single discharge process into a multi-discharge accumulation process, enabling the gas to alternately receive energy input under different discharge forms. This breaks through the limitation of the activation degree by a single discharge structure and improves the overall utilization efficiency of a unit gas within the system.

[0024] See Figures 1-5 As shown, a cyclic plasma nitrogen fixation device with dual dielectric barrier discharge provided in this application embodiment includes: The surface dielectric barrier discharge system and the plasma activated water container system are installed inside the exhaust shell 1, along with the space dielectric barrier discharge system and the gas circulation system that enables gas circulation between the exhaust shell 1 and the space dielectric barrier discharge system. The surface dielectric barrier discharge system has a surface dielectric discharge plate 2 installed on the top surface of the exhaust shell 1. Under the action of the alternating electric field applied by the high-frequency power supply, surface discharge is formed on the surface of the surface dielectric discharge plate 2. The surface dielectric discharge plate 2 has a three-layer structure, including a metal foil 9 as a high-voltage electrode on the upper layer, a mesh electrode 35 as a grounding electrode on the lower layer, and a dielectric layer between the metal foil 9 and the mesh electrode 35. The metal foil 9 serves as the high-voltage electrode of the surface dielectric barrier discharge system, and the mesh electrode 35 serves as the grounding electrode. Both are electrically connected to the power supply and ground, respectively. The mesh electrode 35 is folded into multiple parallel line structures to form a covering surface. The covering surface formed by the mesh electrode 35 does not completely cover the dielectric layer. The surface of the dielectric layer has a non-planar undulating structure, which creates a tortuous gas flow channel in the area not covered by the mesh electrode 35, thereby prolonging the residence time of gas in the surface dielectric barrier discharge device and increasing the number of contacts between the gas and the discharge area. In one embodiment, the dielectric layer is an epoxy resin dielectric layer, and the dielectric layer with the non-planar undulating structure is formed by 3D printing technology; the metal foil 9, the mesh electrode 35, and the dielectric layer are integrated using printed circuit board technology. The non-planar undulating structure of the discharge region is parallel to the liquid surface, which not only causes the discharge to occur near the gas-liquid interface but also prolongs the residence time of the gas in the discharge region, thereby improving the migration efficiency of active species. The metal foil 9 of the dielectric barrier discharge device is fixed to the vacuum casing 1 via a high-voltage connector and then connected to a frequency power supply. The mesh electrode 35 is connected to the power supply to ensure stable discharge. The dielectric layer material is selected from quartz, which is resistant to high voltage, heat, and has good chemical stability, to ensure discharge uniformity and gas activation effect.

[0025] The plasma-activated water container system includes a cooling water tank 4 housed within an exhaust shell 1. A liquid container 27 is located in the center of the cooling water tank 4, and an aeration structure is installed at the bottom of the liquid container 27. In one example, the aeration structure is a double-layer aeration structure. The central opening of the cooling water tank cover 32 faces the surface dielectric discharge plate 2. The cooling water tank 4 has a water inlet 8 and a water outlet 7, which are connected to a first circulating water machine 38 via water pipes passing through the exhaust shell 1, ensuring the water remains within a controlled temperature range. The water level in the cooling water tank 4 is lower than the liquid level in the liquid container 27. The double-layer aeration structure in the liquid container 27 is used to introduce plasma-activated gas generated by the space dielectric barrier discharge device into the water to form bubbles. The double-layer aeration structure includes an aeration head 6, which is a porous structure used to form bubbles from the gas entering the liquid container 27. A microporous plate 40 is installed in the bubble rising path above the aeration head 6, forming a secondary control structure for the bubbles. This results in double-layer aeration. The mixed gas, after being treated by the space dielectric barrier discharge device, is connected to an aeration tower head 28 through a gas pipeline. The aeration head 6 of the aeration tower head 28, with its porous structure, enters the liquid container 27, and is then introduced into the water body by the aeration head 6 within the liquid container 27, forming a large number of microbubbles. This allows the gas to enter the liquid phase system in the form of a dispersed phase. As the bubbles rise in the liquid, they continuously break and renew themselves as they pass through the microporous plate 40, forming a stable and continuously renewing gas-liquid interface on its surface. At this interface, active nitrogen and oxygen species undergo dissolution, hydration, and further oxidation reactions, gradually transforming into nitrate ions and nitrite ions, which then enter the liquid phase, thereby realizing the transformation of gaseous active species into liquid products.

[0026] The porous aerator head 6 is used to generate a large number of uniform bubbles, and the gas flow rate is adjustable to improve the initial contact efficiency between the plasma-activated gas and the liquid. The microporous plate 40 disposed above it is used to block, buffer, break up and guide the rising bubbles, so that the bubbles are further refined and dispersed before and after passing through the microporous plate.

[0027] The microporous plate 40 is positioned in the path of the rising bubbles, below the height of the liquid surface. The surface of the microporous plate 40 is integrally formed, and the lower surface is provided with dense, pointed, cone-shaped bubble-breaking protrusions. Before the rising large bubbles pass through the microporous plate 40, they first come into contact with the pointed, cone-shaped bubble-breaking protrusions and break. The small bubbles that do not break are then dispersed by the microporous plate 40 to form uniform bubbles that diffuse back into the liquid container 27.

[0028] The surface of the microporous plate 40 is provided with densely packed pointed protrusions arranged in the direction of bubble ascent. These pointed protrusions are conical with a small radius of curvature at their apex, designed to create localized stress concentration areas during bubble contact, thus prompting larger bubbles to break and redisperse as they pass through the microporous plate. The pointed structures not only improve bubble refinement and increase the gas-liquid contact surface area, but also extend the bubble residence path in the liquid by disturbing the local flow field, thereby improving the mass transfer efficiency of reactive nitrogen and oxygen species into the liquid phase.

[0029] According to the above embodiment, the dispersed bubbles after secondary regulation by the microporous plate 40 continue to diffuse within the liquid bulk region, increasing the residence time of bubbles that did not undergo sufficient gas-liquid mass transfer during the first bubbling process, thereby increasing the gas-liquid contact opportunity, improving gas-liquid mass transfer efficiency, and enhancing the generation efficiency of nitrate and nitrite ions. Simultaneously, residual gas that has not fully participated in the reaction after passing through the liquid phase continues to migrate upwards towards the liquid to be treated after passing through the microporous plate, continuously providing a source of reactivated gas for the surface dielectric barrier discharge device positioned above the liquid surface, thereby improving the subsequent secondary discharge activation efficiency and enhancing the overall gas recycling capacity of the system.

[0030] A polytetrafluoroethylene (PTFE) hydrophobic membrane 30 is disposed above the liquid container 27. The upper edge of the liquid container 27 is provided with a square groove for limiting the PTFE hydrophobic membrane 30. The edge of the PTFE hydrophobic membrane 30 is embedded in the square groove and fitted and fixed with the central square opening of the cooling water tank cover 32 to prevent displacement during bubbling disturbance and gas circulation, while preventing droplets from splashing and contacting the surface dielectric discharge plate 2, and allowing gas to pass through.

[0031] The cooling water tank 4 is equipped with an open cooling water tank cover 32. The cooling water tank cover 32 cooperates with the polytetrafluoroethylene hydrophobic membrane 30 and together with the extraction shell 1 forms a closed reaction space to reduce the escape of active species. It should be noted that the extraction shell 1 is designed with a removable sealing cover. After the sealing cover is opened, the liquid used for the reaction is replaced. The extraction shell 1 is equipped with a shell pressure relief hole 36. The shell pressure relief hole 36 is equipped with a one-way gas supply structure and a pressure relief structure. The one-way gas supply structure is used to introduce external gas when the internal pressure of the extraction shell 1 is lower than the external air pressure, so as to prevent the low-flow air pump 11 from continuously sucking and causing a negative pressure to form inside the extraction shell 1, and to maintain the stable gas flow in the surface dielectric barrier discharge device. The one-way gas supply structure can be, for example, a spring-loaded type, which uses the preload force of the spring to press the valve core onto the valve seat. It will only open when the inlet pressure is sufficient to overcome the spring force. The spring parameters are selected according to the required pressure difference. The pressure relief structure includes a pressure relief hole, which is used to release excess gas in a timely manner when the internal pressure of the system rises abnormally, preventing local pressure accumulation from affecting discharge stability and system safety. It is implemented using a conventional structure and will not be described in detail here. The unidirectional gas supply structure is equipped with a check valve, which only allows external air to enter unidirectionally when a negative pressure is formed inside the system. This is to compensate for gas loss caused by reaction consumption and minor leakage during the circulation process, prevent excessive negative pressure from forming inside the system, and ensure stable circulating gas flow and continuous discharge.

[0032] A space dielectric barrier discharge system includes a cylindrical housing 17, a ground electrode layer 25 disposed within the cylindrical housing 17, a quartz dielectric layer 26 disposed within the ground electrode layer 25, and a high-voltage electrode 22 disposed within the quartz dielectric layer 26. The cylindrical housing 17 has a gas passage and a cooling water passage, wherein the gas passage includes a gas inlet 16 and a gas outlet 21, and the cooling water passage includes a cooling water inlet 18 and a cooling water outlet 20. The cylindrical housing 17 is mounted on a device base 23. The quartz dielectric layer 26, disposed between the high-voltage electrode 22 and the ground electrode layer 25 in the space dielectric barrier discharge system, forms a dielectric barrier discharge structure. The high-voltage electrode 22 in the space dielectric barrier discharge system adopts a spiral structure to increase the discharge area and improve discharge uniformity.

[0033] In the space dielectric barrier discharge system, air undergoes dielectric barrier discharge under the action of a high-voltage alternating electric field, generating active nitrogen and oxygen species and forming a low-temperature plasma gas. The high-voltage electrode 22 is spirally arranged inside the quartz dielectric layer 26 to form a uniform and dense discharge gap. The high-voltage electrode 22 is spirally wound inside the quartz dielectric layer 26, and then a slurry is prepared by mixing high-purity quartz powder with a binder. This slurry is injected into a mold using a vacuum injection process to ensure that the gaps of the high-voltage electrode 22 are completely filled without air bubbles, and then cured at a low temperature. High-temperature sintering densifies the quartz.

[0034] The gas circulation system includes a low-flow air pump 11, used to extract gas from the extraction shell 1 and deliver it to the space dielectric barrier discharge system. The plasma-activated gas generated within the space dielectric barrier discharge system is introduced into the water in the liquid container 27 via an aeration structure to form bubbles. The low-flow air pump 11 is connected to the air pump inlet 13 via an air pump inlet pipe 12, and its air pump outlet 14 is connected to the gas passage inlet 16 of the space dielectric barrier discharge system via an air pump outlet pipe 15, for re-delivering the gas recovered from the extraction shell 1 to the space dielectric barrier discharge system. In this device, air is introduced into the exhaust housing 1 through the air pump inlet pipe 12, driven by a low-flow air pump 11. When the air pressure inside the exhaust housing 1 is lower than the external air pressure by a certain value, external gas is introduced through the housing pressure relief hole 36. This gas is then transported to the gas passage inlet 16 of the space dielectric barrier discharge device via the air pump outlet pipe 15. Finally, after passing through the flow-guiding and pressure-stabilizing structure located at the gas passage inlet 16, it enters the discharge space inside the cylindrical housing 17. Inside the cylindrical housing 17, the high-voltage electrode 22 and the ground electrode layer 25 are isolated by a quartz dielectric layer 26. Under the action of the alternating high-voltage electric field applied by the high-frequency power supply, a dielectric barrier discharge region is formed. This causes nitrogen and oxygen molecules in the air to undergo dissociation, excitation, and recombination reactions under the bombardment of high-energy electrons and the action of a strong electric field, generating reactive nitrogen and oxygen species such as nitric oxide, nitrogen dioxide, and ozone.

[0035] A flow guiding and pressure stabilizing structure is installed in the return channel of the gas circulation system. The flow guiding and pressure stabilizing structure includes a check valve component, which is used to directionally transport, pressure buffer, and stabilize the flow field of the residual gas recovered by the surface dielectric barrier discharge device, so as to improve the flow uniformity and reactivation efficiency of the residual gas when it re-enters the space dielectric barrier discharge system.

[0036] The gas circulation system recovers the residual gas generated in the liquid container 27 and the surface dielectric barrier discharge area, and reintroduces it into the space dielectric barrier discharge system to achieve multi-round gas circulation activation.

[0037] Since dielectric barrier discharge is a non-thermal plasma discharge form, its discharge region is mainly distributed within the limited space between the electrode and the dielectric layer. The effective residence time of the gas in this region is determined by both the gas flow rate and the volume of the discharge region. Therefore, it is difficult for all the gas entering the cylindrical shell 17 to be fully ionized and excited during a single pass; some gas may only undergo weak excitation or not participate fully in the reaction. This residual gas, along with the generated active gas, is discharged through the gas passage outlet 21 of the spatial dielectric barrier discharge system and enters the subsequent reaction unit.

[0038] The mixed gas, after being treated by the space dielectric barrier discharge system, enters the liquid container 27 through the aeration tower head 28. Then, it is introduced into the water body through the aeration head 6 within the liquid container 27, forming a large number of microbubbles. This allows the gas to enter the liquid phase system in the form of a dispersed phase. As the bubbles rise in the water body, they continuously burst and renew themselves as they pass through the microporous plate 40, forming a stable and continuously renewing gas-liquid interface on its surface. At this interface, active nitrogen and oxygen species undergo dissolution, hydration, and further oxidation reactions, gradually transforming into nitrate and nitrite ions, which then enter the liquid phase, thus realizing the transformation of gaseous active species into liquid products.

[0039] Due to limitations in the residence time of air bubbles in water and the gas-liquid mass transfer rate, the gas entering the liquid container 27 cannot completely dissolve or fully participate in the reaction during a single gas-liquid contact process. Some gas still accumulates in the space above the liquid container 27 without undergoing an effective reaction. To prevent liquid splashing or water mist from entering the upper discharge area, a polytetrafluoroethylene (PTFE) hydrophobic membrane 30 is installed above the liquid container 27. This PTFE hydrophobic membrane 30 allows gas to pass through while blocking liquid droplets from rising, thereby ensuring the stable operation of the upper discharge structure without affecting gas transport.

[0040] Residual gas escaping from above the liquid container 27 enters a surface dielectric barrier discharge system installed inside the extraction housing 1. The surface dielectric barrier discharge system includes surface dielectric discharge plates 2 mounted on four support pads 31 on the extraction housing 1. The metal foil 9 of the surface dielectric discharge plates 2 is connected to a high-voltage terminal 34, and the mesh electrode 35 is connected to a grounding terminal 33. Under the action of an alternating electric field applied by a high-frequency power supply, a surface discharge is formed on the surface of the surface dielectric discharge plates 2, causing the residual gas passing through this area to be bombarded again by high-energy electrons and excited by the electric field, thereby achieving a reactivation process.

[0041] Surface discharge is characterized by a wide discharge area and a large plasma coverage area, which allows gas to be subjected to an electric field over a large spatial scale when passing through the area. This provides supplementary excitation to gas molecules that were not fully excited during the previous stage of space dielectric barrier discharge, giving them the opportunity to be transformed into active nitrogen and oxygen species again.

[0042] Even after surface dielectric barrier discharge treatment, the gas may still contain some incompletely converted components. Therefore, an air outlet 37 is provided on the exhaust housing 1, which is connected to the air pump inlet pipe 12. A one-way air supply structure is provided at the pressure relief hole 36 of the housing. With the help of a low-flow air pump 11, a stable backflow driving force is formed, so that the residual gas flows back to the gas passage inlet 16 of the space dielectric barrier discharge system through the air pump inlet 13 and the air pump inlet pipe 12. This forms a closed loop path for the gas to flow from the space dielectric barrier discharge area, the liquid container 27, the surface dielectric barrier discharge area and back to the dielectric barrier discharge area.

[0043] The aforementioned circulation path ensures that the gas entering the system is no longer converted in a one-time process, but rather undergoes alternating effects of space dielectric barrier discharge and surface dielectric barrier discharge multiple times within the system. It also continuously participates in the gas-liquid reaction process when passing through the liquid container 27 multiple times, thereby gradually increasing the proportion of gas converted into active nitrogen and oxygen species and entering the liquid phase, thus achieving multiple uses and step-by-step activation of the gas.

[0044] In this cycle, the space dielectric barrier discharge system mainly undertakes the function of initial excitation and activation of the air entering the system, so that nitrogen and oxygen in the air are converted into intermediate species with higher reactivity; the liquid container 27 mainly undertakes the function of converting gaseous active species into liquid phase, so that they are gradually converted into nitrate ions and nitrite ions; the surface dielectric barrier discharge system mainly undertakes the function of supplementing excitation of residual gas, so that the gas that did not undergo effective reaction in the previous stage can have another opportunity for conversion; the exhaust shell 1 and the low flow air pump 11 jointly undertake the function of gas circulation and transportation, so that the gas forms a stable circulation flow state inside the device.

[0045] Through this synergistic effect, the plasma nitrogen fixation process is transformed from the traditional single-discharge conversion method to a multi-discharge cumulative conversion. The number of gas activation cycles in the device increases, which improves the total amount of active nitrogen and oxygen species generated and their conversion efficiency in water without increasing the gas intake. At the same time, it reduces the energy loss caused by the direct emission of underutilized gas and improves the overall energy utilization rate.

[0046] Furthermore, since the gas alternately passes through the space dielectric barrier discharge region and the surface dielectric barrier discharge region during the circulation process, it can periodically switch between the volume discharge region and the surface discharge region. This disperses the discharge load of different discharge regions during continuous operation, avoiding the problem of local discharge concentration and instability caused by long-term operation of a single discharge structure, which is beneficial to improving the continuity of the discharge process and the stability of system operation.

[0047] In the aforementioned cyclic nitrogen fixation process involving multi-stage gas discharge and multiple gas-liquid reactions, each functional module forms a stable operating system through structural coupling and parameter coordination, enabling gas activation, mass transfer reaction, and reflux transport to form a continuous closed loop. The spatial dielectric barrier discharge system and the surface dielectric barrier discharge system are spatially separated, but form a sequential relationship along the gas flow path, allowing the gas to alternately receive electric field effects under different discharge forms, thus avoiding the insufficient activation problem caused by relying solely on a single discharge structure. By combining volume discharge and surface discharge forms, the gas alternately undergoes excitation processes in high-field-strength local regions and large-area uniform discharge regions, thereby achieving a phased distribution of energy input on a time scale and a dispersed configuration of discharge regions on a spatial scale. Simultaneously, combined with the tortuous gas flow channel inside the exhaust shell 1 and the unidirectional gas replenishment structure at the pressure relief hole 36 of the shell, the flow state of the circulating gas within the surface dielectric barrier discharge region can be further stabilized, reducing the impact of local stagnation or instantaneous negative pressure on the circulation efficiency.

[0048] During device operation, discharge parameters and gas circulation parameters jointly determine the effective activation degree of the gas within the system. By adjusting the operating power of the low-flow air pump 11, the gas circulation rate within the device can be controlled, thereby altering the residence time of the gas in the space dielectric barrier discharge region and the surface dielectric barrier discharge region, allowing the gas to achieve different degrees of ionization and excitation. By adjusting the output voltage and frequency of the high-frequency power supply, the electric field intensity distribution in the surface dielectric barrier discharge region and the space dielectric barrier discharge region can be changed respectively, allowing the gas to receive energy inputs of different intensities at different stages, thereby improving the generation efficiency of reactive nitrogen and oxygen species while ensuring discharge stability.

[0049] To ensure the system maintains a stable discharge state under long-term operation, both the space dielectric barrier discharge system and the liquid container 27 are equipped with independent temperature control loops. The first circulating water pump 38 and the second circulating water pump 39 are connected to the cooling water tank 4 and the internal water channels of the cylindrical shell 17 of the space dielectric barrier discharge system, respectively, ensuring that the water and discharge structure remain within a controlled temperature range. This prevents the discharge mode from transitioning from stable discharge to arc discharge due to localized heat accumulation. This temperature control structure not only extends the service life of the electrodes and dielectric materials but also helps maintain non-thermal plasma characteristics, ensuring that active species are primarily generated by electron excitation rather than relying on overall gas heating, thereby improving energy utilization efficiency.

[0050] By comparing and analyzing the two-stage discharge waveforms, the distribution of excitation intensity of gas in different discharge regions can be determined, and the gas flow rate and discharge parameters can be adjusted accordingly to make the two-stage discharge complementary in the overall energy distribution, avoiding the situation where the load in one discharge region is too large or the energy utilization in another discharge region is insufficient.

[0051] In this device structure, the liquid container 27 not only serves as a gas-liquid reaction unit but also as a buffer space in the gas circulation path, allowing for repeated material exchange between the gas and liquid phases during multiple cycles. Since the gas passes through the liquid container 27 again in each cycle, the generated active nitrogen and oxygen species continuously transfer from the gas phase to the liquid phase, thereby reducing the effective concentration of active species in the gas phase and promoting the continued generation of new active species during subsequent discharges. This process forms a cumulative pattern on a macroscopic scale, causing the overall system conversion efficiency to gradually increase with the number of cycles.

[0052] By constructing a multi-stage reaction system with clearly defined structural coupling and functional division, the role of gas within the system changes from a one-time consumption to a cyclical utilization model. This allows gases that did not fully participate in the reaction in the previous stage to continue to be converted in subsequent discharges and reactions, thereby reducing the gas consumption and energy input required per unit product. Compared to traditional nitrogen fixation devices that rely solely on single-stage discharges or single gas-liquid reactions, this structure significantly increases the number of times gas participates in the system by introducing a gas circulation path. The generation process of reactive nitrogen and oxygen species changes from instantaneous generation to cyclic cumulative generation, thus achieving higher yields of liquid-phase nitrate and nitrite ions under the same operating conditions.

[0053] Meanwhile, the repeated flow of gas within the device, and its alternating movement between different discharge and reaction regions, helps to homogenize the discharge load, reducing the risk of long-term high-energy-density operation in localized areas, thereby improving the overall system's safety and operational stability. By forming a tortuous gas flow channel within the exhaust housing 1 and installing a unidirectional gas replenishment structure at the housing pressure relief hole 36, the device can maintain a stable internal pressure distribution and gas flow state during cyclic operation, avoiding negative or overpressure states caused by gas consumption, local blockage, or instantaneous suction fluctuations, thus ensuring the continuity and reliability of the gas circulation path.

[0054] Therefore, this application introduces a space dielectric barrier discharge system, a surface dielectric barrier discharge system, and a gas circulation system into the same system, enabling the gas to form a multi-stage excitation, multiple reactions, and multiple recirculation working mode inside the device. This transforms the plasma nitrogen fixation process from a linear process to a cyclic process, so that the generation of active nitrogen and oxygen species is no longer limited by the conversion capacity of a single discharge process, but is gradually accumulated through multiple repetitions. This achieves a simultaneous improvement in gas utilization and energy utilization without significantly increasing the structural complexity of the device.

[0055] This application sets up an adjustable circulating water system to continuously cool the space dielectric barrier discharge system, the surface dielectric barrier discharge system and related structures, so that each area maintains a stable operating temperature during operation, avoids local overheating from affecting the discharge mode and equipment structure, and improves the reliability and stability of the device in long-term operation.

[0056] Furthermore, the overall device adopts a modular design, with each functional unit including a space dielectric barrier discharge system, a surface dielectric barrier discharge system, a gas circulation system, and a circulating water system. These units operate independently yet collaboratively, resulting in a clear system structure, convenient installation and maintenance, and stable operation under ambient temperature and pressure conditions. This device is not only suitable for plasma nitrogen fixation processes but can also be used in various applications such as plasma-activated water preparation, demonstrating excellent applicability and promotional value.

[0057] In another aspect, this application provides a cyclic plasma nitrogen fixation method based on the above-mentioned device using dual dielectric barrier discharge, comprising: The space dielectric barrier discharge system generates low-temperature plasma from the incoming air under the action of a high-voltage alternating electric field, producing plasma-activated gas containing active species. The plasma-activated gas is introduced into the liquid container 27 located in the cooling water tank 4 through an aeration structure, so that it forms bubbles in the liquid to be treated, and generates nitrate ions and nitrite ions through hydration and oxidation reactions. The remaining gas that is not completely dissolved continues to enter the space dielectric barrier discharge system above the liquid surface to be treated for secondary activation, and the active species are transferred into the liquid through the gas-liquid interface. The residual gas that has not fully reacted is collected by the gas circulation system and reintroduced into the space dielectric barrier discharge device through the return channel for further discharge treatment, thus forming a gas circulation.

[0058] It also includes: by comparing and analyzing the discharge waveforms of surface dielectric barrier discharge system and space dielectric barrier discharge system, determining the excitation intensity distribution of gas in different discharge regions, and adjusting the gas flow rate and discharge parameters according to the excitation intensity distribution.

[0059] The gas is first ionized and excited in the space dielectric barrier discharge system, forming a plasma-activated gas containing various active species. This activated gas enters the liquid container 27 in the form of bubbles through bubbling, contacting the liquid to be treated for initial gas-liquid mass transfer. Subsequently, the residual gas enters the surface dielectric barrier discharge system, where it is re-excited by surface discharge and simultaneously acts on the gas-liquid interface region near the liquid surface, achieving re-activation and further gas-liquid mass transfer of the residual gas. Finally, the gas that did not fully participate in the reaction in the surface dielectric barrier discharge system is transported back to the space dielectric barrier discharge system by the circulating gas system, forming multiple rounds of cyclic activation, thereby continuously promoting the generation and reaction of active species. Simultaneously, the device is equipped with a circulating water and temperature control system to regulate the temperature of the discharge area, ensuring continuous operation of the device under stable conditions and improving the overall treatment effect.

[0060] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A circulating plasma nitrogen fixation device with dual dielectric barrier discharge, characterized in that, include: The surface dielectric barrier discharge system and the plasma activated water container system are installed inside the exhaust shell (1), the space dielectric barrier discharge system is installed, and the gas circulation system realizes gas circulation between the exhaust shell (1) and the space dielectric barrier discharge system, wherein: The surface dielectric barrier discharge system has a surface dielectric discharge plate (2) set on the top surface inside the exhaust shell (1). Under the action of the alternating electric field applied by the high frequency power supply, a surface discharge is formed on the surface of the surface dielectric discharge plate (2). The surface of the dielectric layer of the surface dielectric discharge plate (2) has a non-planar undulating structure, so that the area not covered by the mesh electrode (35) forms a tortuous gas flow channel. The plasma activated water container system includes a cooling water tank (4) placed inside the vacuum shell (1), a liquid container (27) is provided in the middle of the cooling water tank (4), an aeration structure is provided at the bottom of the liquid container (27), and the liquid surface inside the liquid container (27) faces the surface dielectric discharge plate (2). The space dielectric barrier discharge system includes a cylindrical shell (17), a ground electrode layer (25) disposed in the inner layer of the cylindrical shell (17), a quartz dielectric layer (26) disposed in the inner layer of the ground electrode layer (25), and a high voltage electrode (22) disposed inside the quartz dielectric layer (26). The cylindrical shell (17) is provided with a gas passage. The gas circulation system includes a low-flow air pump (11) for extracting gas from the gas extraction shell (1) and transporting it to the space dielectric barrier discharge system. The plasma-activated gas generated in the space dielectric barrier discharge system is introduced into the water in the liquid container (27) through the aeration structure to form bubbles. The aeration structure includes an aeration head (6) for forming bubbles from the gas entering the liquid container (27). A microporous plate (40) is provided in the bubble rising path above the aeration head (6) to form a secondary control structure for the bubbles. The surface of the microporous plate (40) is integrally formed, and the lower surface is provided with dense pointed cone-shaped bubble-breaking protrusions. Before the rising large-sized bubbles pass through the microporous plate (40), they first come into contact with the pointed cone-shaped bubble-breaking protrusions and break. The small-sized bubbles that do not break are then dispersed by the microporous plate (40) to form uniform bubbles and diffuse back into the liquid container (27).

2. The cyclic plasma nitrogen fixation device with dual dielectric barrier discharge according to claim 1, characterized in that, The surface dielectric discharge plate (2) includes a metal foil (9) on the upper layer as a high voltage electrode, a mesh electrode (35) on the lower layer as a ground electrode, and a dielectric layer between the metal foil (9) and the mesh electrode (35).

3. The cyclic plasma nitrogen fixation device with dual dielectric barrier discharge according to claim 1, characterized in that, A polytetrafluoroethylene hydrophobic membrane (30) is provided at the opening above the liquid container (27). A square slot is provided on the upper edge of the liquid container (27) for limiting the polytetrafluoroethylene hydrophobic membrane (30). The polytetrafluoroethylene hydrophobic membrane (30) is embedded in the square slot and is fitted and fixed with the central opening of the cooling water tank cover (32) of the cooling water tank (4). The central opening faces the surface dielectric discharge plate (2).

4. The cyclic plasma nitrogen fixation device with dual dielectric barrier discharge according to claim 1, characterized in that, The high-voltage electrode (22) adopts a spiral structure.

5. The cyclic plasma nitrogen fixation device with dual dielectric barrier discharge according to claim 1, characterized in that, The vacuum casing (1) is provided with a pressure relief hole (36), and a one-way air supply structure is provided at the pressure relief hole (36) for introducing external gas when the internal pressure of the vacuum casing (1) is lower than the external air pressure.

6. A cyclic plasma nitrogen fixation method using dual dielectric barrier discharge, employing the cyclic plasma nitrogen fixation apparatus using dual dielectric barrier discharge as described in any one of claims 1-5, characterized in that, include: The space dielectric barrier discharge system generates low-temperature plasma from the incoming air under the action of a high-voltage alternating electric field, producing plasma-activated gas containing active species. The plasma-activated gas is introduced into the liquid container (27) set in the cooling water tank (4) through the aeration structure, so that it forms bubbles in the liquid to be treated, and generates nitrate ions and nitrite ions through hydration and oxidation reactions; The remaining gas that is not completely dissolved continues to enter the surface dielectric barrier discharge system above the liquid surface to be treated for secondary activation, and the active species are transferred into the liquid through the gas-liquid interface. The residual gas that has not fully reacted is collected by the gas circulation system and reintroduced into the space dielectric barrier discharge device through the return channel for further discharge treatment, thus forming a gas circulation.

7. The cyclic plasma nitrogen fixation method based on dual dielectric barrier discharge according to claim 6, characterized in that, Also includes: By comparing and analyzing the discharge waveforms of surface dielectric barrier discharge system and space dielectric barrier discharge system, the excitation intensity distribution of gas in different discharge regions is determined, and the gas flow rate and discharge parameters are adjusted according to the excitation intensity distribution.

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