Plasma-based gas conversion device

Abstract

To provide a gas conversion device using plasma that exhibits excellent gas conversion efficiency. [Solution] The plasma gas conversion apparatus 1 according to the present invention comprises a reaction tube 7 that generates plasma inside, a gas supply tube 9 connected to the reaction tube 7 to supply reaction gas, a high conversion gas recovery tube 11 located downstream of the plasma generation region 16 in the reaction tube 7 to recover high conversion gas with a relatively high conversion rate other than the gas near the inner wall of the reaction tube 7, and a low conversion gas recovery tube 13 located downstream of the high conversion gas recovery tube 11 to recover low conversion gas with a relatively low conversion rate that was not recovered by the high conversion gas recovery tube 11, wherein the recovery port 11a of the high conversion gas recovery tube 11 is located between it and the inner wall of the reaction tube 7 with a gap extending around its entire circumference.

Description

Technical Field

[0001] The present invention relates to a gas conversion device for converting a reaction gas containing carbon dioxide by plasma.

Background Art

[0002] As a gas conversion device using microwave plasma, for example, there is a "gas conversion system" disclosed in Patent Document 1. The gas conversion system of Patent Document 1 includes a microwave waveguide for propagating microwaves, a gas flow tube (reaction tube) passing through the microwave waveguide, temperature control means for controlling the temperature of the microwave waveguide, and a generator disposed near the gas flow tube and configured to generate plasma in the gas flow tube so that the plasma converts the gas flowing in the gas flow tube during operation (see

[0004] of Patent Document 1).

[0003] And in Patent Document 1, it is stated that it can be used for the conversion of carbon dioxide (CO2) to carbon monoxide (CO) and oxygen (O2) (see

[0011] of Patent Document 1).

[0004] As something to improve gas conversion efficiency, Patent Document 2 proposes "an apparatus for performing a gas reaction, including a plasma reactor using a passing flow of gas, having a plasma chamber, particularly a cylindrical plasma chamber, wherein a flow forming element for forming a gas flow is disposed in front of and / or inside and / or behind the plasma reactor to form at least one, particularly a central, region where the flow decreases in the gas flow."

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] When plasma is generated using electromagnetic waves such as microwaves or RF waves, standing waves are usually used to deliver more energy to the target gas and cause it to dissociate. In this case, strong and weak electromagnetic fields alternate along the wave's propagation direction, so the reaction rate within the reactor changes in the cross-sectional direction, regardless of where the reactor is placed along the wave's path. Furthermore, when plasma is generated by discharge from electrodes, the electromagnetic field strength changes depending on the position from the electrodes, resulting in a distribution in the magnitude of the reaction rate. In any case, plasma density results in a spatial distribution caused by the generator and reactor shape.

[0007] Furthermore, the plasma density distribution is not only formed by the generating device, but can also be changed by controlling the airflow, as disclosed in Patent Document 2. Due to the mechanism described above, the plasma generates a concentration distribution in the cross-sectional direction, and if this distribution is stable, the gas conversion rate will always have areas with high and low concentrations.

[0008] As described above, since plasma exhibits a concentration distribution in the cross-sectional direction, a concentration distribution is also observed in the cross-sectional direction of the gas flow tube, and therefore the gas conversion rate also exhibits a distribution in the cross-sectional direction of the gas flow tube. Here, the conversion rate is a measure of decomposition efficiency, representing the percentage of the supplied gas (in this case, CO2) that is transformed. For example, if 1 kg of CO2 is supplied and the conversion rate is 10%, then 0.1 kg of CO2 will be transformed into CO and O2, and the remaining 0.9 kg will remain as CO2. Figure 7 shows a typical plasma-based gas conversion apparatus 31, where 3 is a microwave generator, 5 is a microwave waveguide, 7 is a reaction tube, 15 is a reflector, 33 is a converted gas recovery tube, 17 is a pressure regulating valve, and 19 is a vacuum pump. Figure 8 schematically shows the gas conversion rate in section A of reaction tube 7 in Figure 7.

[0009] As shown in Figure 8, in this example, the gas conversion rate is highest in the central part of the reaction tube 7, and decreases as you move radially outward.

[0010] Conventionally, all reaction gases were recovered from the reaction tube, including gases with low conversion rates near the inner wall of the reaction tube. Therefore, even if the conversion rate was increased in a part of the radial direction of the reaction tube 7, for example, in the central part, there was a limit to how much the overall conversion rate of the recovered gas could be increased.

[0011] This invention was made to solve the aforementioned problems, and aims to provide a gas conversion device using plasma that exhibits excellent gas conversion efficiency. [Means for solving the problem]

[0012] (1) The plasma gas conversion apparatus according to the present invention comprises: a reaction tube that generates plasma inside; a gas supply tube connected to the reaction tube for supplying reaction gas; a high conversion gas recovery tube disposed downstream of the plasma generation region in the reaction tube for recovering high conversion gas with a relatively high conversion rate other than the gas near the inner wall of the reaction tube; and a low conversion gas recovery tube provided downstream of the high conversion gas recovery tube for recovering low conversion gas with a relatively low conversion rate that was not recovered by the high conversion gas recovery tube. The recovery port of the high-conversion gas recovery pipe is characterized by being positioned with a gap extending around its entire circumference between it and the inner wall of the reaction pipe.

[0013] (2) The device described in (1) above is further characterized by being equipped with a low conversion gas circulation line that supplies the low conversion gas recovered in the low conversion gas recovery pipe to the upstream side of the plasma generation region in the reaction tube.

[0014] (3) Further, it is characterized in that a plurality of the gas conversion devices described in (1) above are provided, and the gas recovered by the low-conversion gas recovery pipe of the gas conversion device arranged on the upstream side is supplied to the upstream side of the plasma generation region in the reaction pipe on the downstream side.

[0015] (4) Further, the plasma-based gas conversion device according to the present invention includes a reaction pipe for generating plasma therein, a gas supply pipe connected to the reaction pipe for supplying a reaction gas, a high-conversion gas recovery pipe arranged on the downstream side of the plasma generation region in the reaction pipe for recovering a reaction gas having a relatively high conversion rate other than the gas near the inner wall of the reaction pipe, and a low-conversion gas recovery pipe provided at the downstream end of the reaction pipe for recovering a relatively low-conversion gas that has not been recovered by the high-conversion gas recovery pipe. The recovery port of the high-conversion gas recovery pipe is arranged via a gap extending over the entire circumference between the high-conversion gas recovery pipe and the inner wall of the reaction pipe. It is characterized in that a plurality of sets of the plasma generation region and the high-conversion gas recovery pipe are provided from the upstream side to the downstream side of the gas flow in the reaction pipe.

Effects of the Invention

[0016] According to the present invention, in a gas conversion device using plasma, a gas conversion device excellent in gas conversion efficiency can be obtained.

Brief Description of the Drawings

[0017] [Figure 1] It is an explanatory diagram of the plasma-based gas conversion device according to Embodiment 1. <000008�>It is an enlarged view showing a part of FIG. [Figure 3] It is an explanatory diagram of another aspect of Embodiment 1. [Figure 4] It is an explanatory diagram of the plasma-based gas conversion device according to Embodiment 2. [Figure 5] It is an explanatory diagram of the plasma-based gas conversion device according to Embodiment 3. [Figure 6]It is an explanatory diagram of a gas conversion device using plasma according to Embodiment 4. [Figure 7] It is an explanatory diagram of a general gas conversion device using plasma. [Figure 8] It schematically shows the gas conversion rate at part A of the gas flow pipe in FIG. 7.

Embodiments for Carrying Out the Invention

[0018] [Embodiment 1] The present invention relates to a gas conversion device using plasma. To generate plasma, methods of discharging gas from atmospheric pressure to low pressure using microwaves, high frequencies, dielectric barriers, gliding arcs, etc. are known. The pressure range is preferably about 13 kPa for microwaves and about 5 kPa for high frequencies (RF). Although atmospheric pressure is usually used for gliding arcs and dielectric barriers, plasma can be generated in a pressure range from below 1 kPa to atmospheric pressure by any method. In this embodiment, an example using microwaves will be described below.

[0019] As shown in FIG. 1, the gas conversion device 1 using plasma according to this embodiment includes a microwave generator 3, a microwave waveguide 5, a reaction tube 7 for generating plasma inside, a gas supply tube 9 connected to the reaction tube 7 for supplying reaction gas, a high-conversion gas recovery tube 11 for recovering reaction gas with a high conversion rate, and a low-conversion gas recovery tube 13 for recovering reaction gas with a relatively low conversion rate that was not recovered by the high-conversion gas recovery tube 11. In FIG. 1, the same parts and corresponding parts as in FIG. 7 are labeled with the same reference numerals. Hereinafter, each component will be described in detail.

[0020] [Microwave Generator] The microwave generator 3 generates microwaves.

[0021] [Microwave Waveguide] The microwave waveguide 5 is a tube that guides microwaves generated by the microwave generator 3 to the reaction tube 7. A reflector 15 that reflects microwaves is provided on the side of the microwave waveguide 5 opposite to the side where the microwave generator 3 is installed. By adjusting the position of the reflector 15, the position of the microwave standing waves inside the reaction tube 7 can be adjusted, thereby adjusting the position where the plasma is generated. In Figure 1, the rugby ball-shaped area shown in gray inside the reaction tube 7 represents the plasma generation region 16.

[0022] <Reaction tube> The reaction tube 7 consists of a hollow quartz tube through which microwaves guided by the microwave waveguide 5 can pass.

[0023] <Gas supply pipe> The gas supply pipe 9 is connected to the upper end of the reaction pipe 7 and supplies a reaction gas, such as carbon dioxide gas, to the reaction pipe 7. The gas supply pipe 9 may be connected to the reaction pipe 7 via a swirling flow generating unit (not shown) that supplies the reaction gas to the reaction pipe 7 as a swirling flow.

[0024] <High-conversion gas recovery pipe> The high conversion gas recovery pipe 11 is located in the central downstream part of the plasma generation region 16 in the reaction pipe 7, and recovers high conversion gas with a relatively high conversion rate, excluding the gas near the inner wall of the reaction pipe 7. Recovering high conversion gas means recovering gas located closer to the center of the reaction tube 7, without recovering gas near the inner wall of the reaction tube 7. Therefore, as shown in Figure 2, the high conversion gas recovery tube 11 has its end bent upward, and the recovery port 11a opens upward. The recovery port 11a is positioned with a gap extending around its entire circumference between it and the inner wall of the reaction tube 7. This prevents the recovery of low-conversion gas, which has a low conversion rate and is located near the inner wall of the reaction tube 7, from being recovered from the gas converted in the plasma generation region 16, thereby improving the overall conversion rate of the recovered gas.

[0025] The size of the gap between the recovery port 11a of the high-conversion gas recovery pipe 11 and the inner wall of the reaction pipe 7 should be determined by the desired conversion rate of the recovered gas. In other words, if the goal is to recover gas with a high conversion rate, the gap should be widened, or in other words, the diameter of the recovery port 11a should be reduced to recover gas closer to the center. Conversely, if a very high conversion rate is not desired, the gap should be narrowed to recover more gas.

[0026] However, since the objective of the present invention is to achieve excellent gas conversion efficiency, it is desirable that the cross-sectional area of ​​the high-conversion gas recovery pipe 11 be 80% or less of the cross-sectional area of ​​the reaction pipe, and more preferably 50% or less. Furthermore, it is preferable that the position of the recovery port 11a in the gas flow direction be near the downstream end of the plasma generation region 16.

[0027] A pressure regulating valve 17 and a vacuum pump 19 are connected to the high-conversion gas recovery pipe 11. The pressure regulating valve 17 adjusts the pressure in the high-conversion gas recovery pipe 11, and the vacuum pump 19 draws in and recovers the gas. Therefore, the amount of gas recovered can be adjusted by adjusting the pressure in the high-conversion gas recovery pipe 11 using the pressure regulating valve 17, independently of the diameter of the recovery port 11a. The amount of gas suction in the high-conversion gas recovery pipe 11 should be determined by the desired conversion rate of the recovered gas. That is, if the goal is to recover gas with a high conversion rate, the suction amount should be reduced to recover gas closer to the center. Conversely, if a very high conversion rate is not desired, the suction amount should be increased to recover more gas.

[0028] As described above, the proportion of the total gas recovered from the high-conversion gas recovery pipe 11 can be adjusted by the ratio of the cross-sectional area of ​​the high-conversion gas recovery pipe 11 to the cross-sectional area of ​​the reaction pipe 7, and the amount of suction from each pipe, thereby adjusting the conversion rate of the recovered high-conversion gas. However, in reality, the cross-sectional area of ​​the high-conversion gas recovery pipe 11 also affects factors other than the suction volume. For example, if the cross-sectional area of ​​the high-conversion gas recovery pipe 11 is small, it becomes easier to quickly cool the high-conversion gas generated in the plasma generation region, thereby suppressing the decrease in conversion rate due to the reverse reaction and further utilizing the advantages of recovering only the high-conversion gas. On the other hand, if the cross-sectional area of ​​the high-conversion gas recovery pipe 11 is too small, the pressure loss will be large, requiring greater pressure to be applied to ensure the suction volume, which leads to increased costs. Although the reaction tube 7 and the high-conversion gas recovery tube 11 are usually circular, in reality, both the reaction tube 7 and the high-conversion gas recovery tube 11 can be rectangular or elliptical, and they do not have to be the same shape.

[0029] <Low-conversion gas recovery pipe> The low conversion gas recovery pipe 13 recovers low conversion gas, which has a relatively low conversion rate, that was not recovered by the high conversion gas recovery pipe 11.

[0030] As shown in Figure 1, the low-conversion gas recovery pipe 13 is connected to the downstream end of the reaction pipe 7 and recovers all the gas flowing through the reaction pipe 7 that is not recovered by the high-conversion gas recovery pipe 11. Furthermore, the low-conversion gas recovery pipe 13 is connected to a pressure regulating valve 17 and a vacuum pump 19, similar to the high-conversion gas recovery pipe 11. The pressure regulating valve 17 adjusts the pressure in the reaction pipe 7, and the gas is drawn in and recovered by the suction force of the vacuum pump 19. In the example shown in Figure 1, the gas recovered in the low-conversion gas recovery pipe 13 is to be discarded, but it may be used for other purposes.

[0031] Next, the operation of this embodiment configured as described above will be explained. When carbon dioxide is being converted, carbon dioxide gas is supplied to the reaction tube 7 from the gas supply pipe 9 shown in Figure 1. Next, the vacuum pump 19 is started to evacuate the reaction tube 7 through the high-conversion gas recovery pipe 11 and the low-conversion gas recovery pipe 13, reducing the pressure, and the pressure inside the reaction tube 7 is adjusted to a predetermined pressure by the pressure regulating valve 17.

[0032] Next, the microwave generator 3 is started and outputs microwaves of a predetermined output. The microwaves propagate through the microwave waveguide 5 and irradiate the reaction tube 7. The irradiated microwaves enter the reaction tube 7, which is made of quartz, and irradiate the carbon dioxide supplied to the reaction tube 7. The carbon dioxide irradiated by the microwaves is ionized and turned into plasma inside the reaction tube 7. When carbon dioxide is turned into plasma, it is converted into oxygen and carbon monoxide. At this time, as shown in Figure 8, the gas conversion rate is highest in the central part of the reaction tube 7, and decreases as you move radially outward. In this state, the high-conversion gas recovery pipe 11 recovers high-conversion gas, for example, with a conversion rate of 50%, and downstream of it, the low-conversion gas recovery pipe 13 recovers the gas that was not recovered by the high-conversion gas recovery pipe 11. As a result, the high conversion gas recovery pipe 11 can recover conversion gas at a conversion rate that was not possible with conventional methods.

[0033] In the example shown in Figure 2, the recovery port 11a of the high-conversion gas recovery pipe 11 is shown to be open upwards, but the arrangement of the recovery port 11a of the high-conversion gas recovery pipe 11 in the present invention is not limited to this. For example, as shown in Figure 3, the high-conversion gas recovery pipe 11 may be inserted into the reaction pipe 7 from the side, and the recovery port 11a may be open to the side. In short, the recovery port 11a of the high conversion gas recovery pipe 11 does not need to recover the low conversion gas flowing along the inner wall of the reaction pipe 7, and to achieve this, the recovery port 11a is positioned with a gap extending around its entire circumference between it and the inner wall of the reaction pipe 7.

[0034] [Embodiment 2] In Embodiment 1, the gas recovered by the low-conversion gas recovery pipe 13 was either disposed of or used for other purposes, and the destination of the low-conversion gas recovery pipe 13 was not particularly limited. In contrast, the plasma-based gas conversion device 21 of this embodiment, as shown in Figure 4, is equipped with a low-conversion gas circulation line 23 that supplies the low-conversion gas recovered in the low-conversion gas recovery pipe 13 to the upstream side of the plasma generation region 16 in the reaction pipe 7. In Figure 4, the same reference numerals are used for parts that are the same as or corresponding to parts in Figure 1. This is also true for Figures 5 and 6, which describe embodiments 3 and 4 later.

[0035] The gas recovered in the low-conversion gas recovery tube 13 has a low conversion rate and therefore contains a large amount of carbon dioxide. This low-conversion gas is then returned to the upstream side of the plasma generation region 16 in the reaction tube 7 for further conversion. This further increases the conversion rate to the supply gas.

[0036] Furthermore, in order to supply the low-conversion gas recovered in the low-conversion gas recovery pipe 13 to the upstream side of the plasma generation region 16 of the reaction tube 7, the downstream end of the low-conversion gas circulation line 23 can be connected to the gas supply pipe 9 and supplied to the reaction tube 7 in a mixed state with newly supplied carbon dioxide gas. Alternatively, the downstream end of the low-conversion gas circulation line 23 may be directly connected to the upper end of the reaction vessel without going through the gas supply pipe 9, thereby supplying the gas to the reaction pipe 7 independently of the newly supplied carbon dioxide gas.

[0037] [Embodiment 3] In Embodiment 2, the low-conversion gas recovered in the low-conversion gas recovery pipe 13 was circulated and supplied to the upstream side of the plasma generation region 16 in the same reaction pipe 7. In contrast, the plasma-based gas conversion device 25 of this embodiment supplies the low-conversion gas recovered in the low-conversion gas recovery pipe 13 to the upstream side of the plasma generation region 16 in another reaction pipe 7.

[0038] The gas conversion device 25 according to this embodiment will be described with reference to Figure 5. As shown in Figure 5, the gas conversion device 25 of this embodiment has multiple gas conversion devices 1 (two in this example) as shown in Embodiment 1, and the gas recovered in the low-conversion gas recovery pipe of the gas conversion device 1 located on the upstream side is supplied to the upstream side of the plasma generation region 16 in the reaction tube 7 on the downstream side.

[0039] According to this embodiment, similar to Embodiment 2, the conversion rate to the supply gas is further increased. The downstream reaction tube 7 may be supplied with not only the recovered gas but also newly supplied carbon dioxide gas. Furthermore, in order to supply the low-conversion gas recovered in the low-conversion gas recovery pipe 13 to the upstream side of the plasma generation region 16 of the reaction tube 7, the low-conversion gas recovered in the low-conversion gas recovery pipe 13 can be connected to the gas supply pipe 9 and supplied to the reaction tube 7 in a mixed state with newly supplied carbon dioxide gas. Alternatively, the low-conversion gas recovered in the low-conversion gas recovery pipe 13 may be supplied directly to the upper end of the reaction pipe 7 without going through the gas supply pipe 9, thereby supplying it to the reaction pipe 7 independently of the newly supplied carbon dioxide gas.

[0040] In the example shown in Figure 5, two gas conversion devices 1 are used, but the present invention is not limited to this, and three or more may be used. Furthermore, in the example shown in Figure 5, microwaves are supplied in parallel to multiple gas conversion devices 1 by branching the microwave waveguide 5. However, the present invention is not limited to this, and instead of branching the microwave waveguide 5, microwaves may be supplied in series to multiple gas conversion devices 1 by arranging reaction tubes 7 in series at the antinodes of multiple microwave standing waves formed along a single microwave waveguide 5. Alternatively, multiple microwave generators 3 may be used. Furthermore, although the gas recovered in the low-conversion gas recovery pipe 13 of the gas conversion device 1 located at the downstream end is to be discarded in the example shown in Figure 5, it may also be supplied to the reaction pipe 7 of any of the gas conversion devices 1 located upstream.

[0041] [Embodiment 4] In this third embodiment, the low-conversion gas recovered in the low-conversion gas recovery pipe 13 was supplied to the upstream side of the plasma generation region 16 in another reaction pipe 7. In contrast, in the plasma-based gas conversion device 27 of this embodiment, as shown in Figure 6, multiple sets (two sets in this example) of plasma generation regions 16 and high-conversion gas recovery pipes 11 are provided in a single reaction tube 7, extending from the upstream to the downstream side of the gas flow in the reaction tube 7.

[0042] According to this embodiment, similar to embodiments 2 and 3, the conversion rate to the supply gas is further increased. Furthermore, by changing the diameter and suction volume of the recovery port 11a of the high-conversion gas recovery pipe 11 on the upstream and downstream sides, the conversion rate of the gas recovered in a single reaction pipe 7 can also be changed.

[0043] In the example shown in Figure 6, two sets of plasma generation regions 16 and high-conversion gas recovery pipes 11 are provided, but the present invention is not limited to this, and three or more sets may be provided. Furthermore, in the example shown in Figure 6, microwaves are supplied in parallel to multiple plasma generation regions 16 by branching the microwave waveguide 5. However, the present invention is not limited to this. Instead of branching the microwave waveguide 5, a single microwave waveguide 5 may be bent multiple times in a zigzag pattern, allowing a straight reaction tube 7 to pass through the microwave waveguide 5 multiple times. By aligning the penetration points with the antinodes of microwave standing waves, microwaves can be supplied in series to multiple plasma generation regions 16. Alternatively, instead of bending the microwave waveguide 5, a single reaction tube 7 may be bent multiple times in a zigzag pattern, allowing the reaction tube 7 to pass through a straight microwave waveguide 5 multiple times. By aligning the penetration points with the antinodes of microwave standing waves, microwaves can be supplied in series to multiple plasma generation regions 16. Alternatively, multiple microwave generators 3 may be used.

[0044] In the above example, a method using microwaves was given as a way to generate plasma, but the present invention is not limited to this, and also includes methods for generating non-equilibrium plasma by discharging using high frequency, dielectric barriers, gliding arcs, etc. In high-frequency dielectric barrier discharge, a high-frequency power supply is used instead of the microwave generator 3. In gliding arc discharge, low-frequency or DC power supplies can also be used in addition to high-frequency power supplies. Then, from these plasma generation power supplies, voltage is applied to electrodes installed inside or outside the reaction tube 7 via coaxial cables or high-voltage cables instead of microwave waveguides 5, causing the carbon dioxide inside the reaction tube 7 to be ionized and turned into plasma.

[0045] Furthermore, while the above example described the case of decomposing carbon dioxide gas (CO2) as a reaction gas to produce CO and O2, the present invention is not limited to this, and includes, for example, the case of producing H2 and O2 from water vapor (H2O), the case of producing H2 from NH3, and the case of producing synthesis gas (a mixed gas of CO and H2) from CO2 and CH4. In addition, it includes the case of synthesizing methane (CH4) or ethylene (C2H4) from CO2 and H2, and the case of synthesizing synthesis gas, formic acid (CH2O2), formaldehyde (CH2O), methanol (CH4O), ethanol (C2H6O), etc. from CO2 and H2O or CO2 and H2 or a mixed gas thereof. [Explanation of Symbols]

[0046] 1. Gas conversion device 3. Microwave generator 5. Microwave waveguide 7. Reaction tube 9 Gas supply pipe 11. High-conversion gas recovery pipe 11a Collection port 13 Low-conversion gas recovery pipe 15 Reflector 16 Plasma generation region 17 Pressure regulating valve 19 Vacuum pump 21 Gas conversion device (Embodiment 2) 23 Low-conversion gas circulation line 25 Gas conversion device (Embodiment 3) 27 Gas conversion device (Embodiment 4) 31. Gas conversion device (conventional example) 33. Conversion gas recovery pipe

Claims

1. The device comprises a reaction tube that generates plasma internally, a gas supply tube connected to the reaction tube for supplying reaction gas, a high conversion gas recovery tube positioned downstream of the plasma generation region in the reaction tube for recovering high conversion gas with a relatively high conversion rate other than the gas near the inner wall of the reaction tube, and a low conversion gas recovery tube provided downstream of the high conversion gas recovery tube for recovering low conversion gas with a relatively low conversion rate that was not recovered by the high conversion gas recovery tube. A plasma-based gas conversion apparatus characterized in that the recovery port of the high-conversion gas recovery tube is positioned with a gap extending around its entire circumference between it and the inner wall of the reaction tube.

2. The plasma-based gas conversion apparatus according to claim 1, further comprising a low-conversion gas circulation line that supplies the low-conversion gas recovered in the low-conversion gas recovery pipe to the upstream side of the plasma generation region in the reaction tube.

3. A plasma-based gas conversion apparatus having a plurality of gas conversion devices as described in claim 1, wherein the gas recovered in the low-conversion gas recovery pipe of the gas conversion device located on the upstream side is supplied to the upstream side of the plasma generation region in the reaction tube on the downstream side.

4. The device comprises a reaction tube that generates plasma internally, a gas supply tube connected to the reaction tube for supplying reaction gas, a high conversion gas recovery tube positioned downstream of the plasma generation region in the reaction tube for recovering reaction gas with a relatively high conversion rate other than the gas near the inner wall of the reaction tube, and a low conversion gas recovery tube provided at the downstream end of the reaction tube for recovering low conversion gas with a relatively low conversion rate that was not recovered by the high conversion gas recovery tube. The recovery port of the high-conversion gas recovery pipe is positioned with a gap extending around its entire circumference between it and the inner wall of the reaction pipe. A plasma-based gas conversion apparatus characterized in that multiple sets of the plasma generation region and the high-conversion gas recovery pipe are provided from the upstream side to the downstream side of the gas flow in the reaction pipe.

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