Non-equilibrium plasma gas decomposition apparatus

Abstract

This invention provides a gas decomposition apparatus using non-equilibrium plasma, which efficiently cools the decomposition gas and offers excellent decomposition efficiency. [Solution] The gas decomposition apparatus 1 using non-equilibrium plasma according to the present invention comprises a reaction tube 7 that generates non-equilibrium plasma inside, a gas supply tube 9 connected to one end of the reaction tube 7 to supply reaction gas, and a gas recovery / cooling tube 11 inserted into the reaction tube 7 from the other end to recover decomposition gas and cool the decomposition gas. The gas recovery / cooling tube 11 is characterized by having a gas flow tube 13 through which decomposition gas flows in the center, and a cooling tube 15 on the outside through which a cooling liquid flows to cool the decomposition gas flowing through the gas flow tube 13.

Description

Technical Field

[0005] , ,

[0001] The present invention relates to a gas decomposition apparatus using non-equilibrium plasma for decomposing a reaction gas containing carbon dioxide by non-equilibrium plasma.

Background Art

[0002] As a gas decomposition apparatus using microwave plasma which is an example of non-equilibrium plasma, for example, a "gas conversion system" disclosed in Patent Document 1 can be mentioned.

[0003] The gas conversion system of Patent Document 1 includes a microwave waveguide for propagating microwaves, a gas flow tube passing through the microwave waveguide, temperature control means for controlling the temperature of the microwave waveguide, and a generator which is arranged in the vicinity of 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 Patent Document 1

[0004] reference). [[ID=ID=19]] And in Patent Document 1, it is said that it can be used for the conversion of carbon dioxide (CO2) to carbon monoxide (CO) and oxygen (O2) (see 【0**011】 of Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] <0000**029>Since the gas temperature rises during plasma generation, there is a problem that a reverse reaction occurs in which CO and O decomposed by the plasma recombine, and the decomposition efficiency of CO2 decreases. In this regard, in Patent Document 1, it is described that as temperature control means, not only the microwave waveguide but also the gas flow tube (reaction tube) is cooled from the outside (see

[0012] of Patent Document 1).

[0006] However, cooling the gas flow tube from the outside results in poor cooling efficiency, and sufficient cooling cannot be obtained, making it difficult to adequately suppress the decrease in decomposition efficiency.

[0007] This invention was made to solve the aforementioned problems and aims to provide a gas decomposition apparatus using non-equilibrium plasma that efficiently cools the decomposition gas and has excellent decomposition efficiency. [Means for solving the problem]

[0008] (1) The gas decomposition apparatus using non-equilibrium plasma according to the present invention comprises a reaction tube that generates non-equilibrium plasma inside, a gas supply tube connected to one end of the reaction tube to supply reaction gas, and a gas recovery / cooling tube inserted into the reaction tube from the other end of the reaction tube to recover decomposition gas and cool the decomposition gas, The aforementioned gas recovery and cooling pipe is characterized by having a gas flow pipe in the center through which decomposition gas flows, and a cooling pipe outside of it through which a cooling liquid flows to cool the decomposition gas flowing through the gas flow pipe.

[0009] (2) Furthermore, in the case described in (1) above, the gas recovery / cooling pipe has a triple-pipe structure further comprising a coolant recovery pipe on the outside of the cooling pipe for recovering the coolant after heat exchange, The outlet of the gas passage pipe, the coolant inlet of the cooling pipe, and the coolant outlet of the coolant recovery pipe are located away from the inlet into which the gas recovery / cooling pipe is inserted into the reaction pipe, and the position of the inlet into which the decomposition gas flows into the gas passage pipe can be adjusted by adjusting the amount into which the gas recovery / cooling pipe is inserted into the reaction pipe. [Effects of the Invention]

[0010] According to the gas decomposition apparatus using non-equilibrium plasma of the present invention, the decomposition gas can be efficiently cooled by the gas recovery and cooling tube, thereby increasing the decomposition efficiency of the reaction gas. [Brief explanation of the drawing]

[0011] [Figure 1] This is an explanatory diagram of the gas decomposition apparatus according to this embodiment. [Figure 2] This is an explanatory diagram of the internal structure of the gas recovery and cooling pipe according to this embodiment. [Figure 3] Figure 2 is a plan view of the gas recovery and cooling pipe shown. [Figure 4] This is an explanatory diagram illustrating the reaction process in the reaction tube of the gas decomposition apparatus according to this embodiment. [Figure 5] This is an explanatory diagram illustrating the relationship between gas flow velocity and the installation location of gas recovery and cooling pipes. [Figure 6] This is an explanatory diagram illustrating the relationship between microwave intensity and the installation location of the gas recovery and cooling pipes. [Modes for carrying out the invention]

[0012] Plasma is broadly classified into two types: thermal equilibrium plasma (or thermal plasma) and non-equilibrium plasma (or non-thermal plasma). In a non-equilibrium plasma, ionized ions and electrons exist alongside unionized molecules. A characteristic of non-equilibrium plasma is that the temperatures of these elements differ; electrons reach tens of thousands of degrees Celsius, while other molecules and ions remain at lower temperatures, sometimes around room temperature. In this invention, the decomposition efficiency is enhanced by utilizing the characteristic of non-equilibrium plasmas, where molecules and ions are at low temperatures, and by cooling the decomposed reaction gas using a gas recovery and cooling tube before its temperature rises due to the release of decomposition enthalpy and excitation energy obtained in the plasma. To create such non-equilibrium plasmas, methods are known that involve discharging a gas at room temperature or low pressure using microwaves, radio waves, dielectric barriers, gliding arcs, etc. In this embodiment, an example using microwaves will be described below.

[0013] As shown in FIG. 1, the gas decomposition apparatus 1 according to the present embodiment includes a microwave generator 3, a microwave waveguide 5, a reaction tube 7 that generates plasma inside, a gas supply tube 9 connected to one end side of the reaction tube 7 to supply reaction gas, and a gas recovery / cooling tube 11 that is inserted into the reaction tube 7 from the other end side of the reaction tube 7 to recover the decomposed gas and cool the decomposed gas. Hereinafter, each component will be described in detail.

[0014] <Microwave Generator> The microwave generator 3 generates microwaves.

[0015] <Microwave Waveguide> The microwave waveguide 5 is a tube that guides the microwaves generated by the microwave generator 3 to the reaction tube 7. A reflector 12 that reflects microwaves is provided on the side of the microwave waveguide 5 opposite to the side where the microwave generator 3 is provided. By adjusting the position of the reflector 12, the position of the standing wave of the microwaves in the reaction tube 7 can be adjusted, and thereby the generation position of the plasma can be adjusted. [[ID=?]] [[ID=?]]

[0016] [[ID=?]] <Reaction Tube> The reaction tube 7 is made of a transparent hollow quartz tube through which the microwaves guided by the microwave waveguide 5 can pass.

[0017] <Gas Supply Tube> The gas supply tube 9 is connected to the upper end of the reaction tube 7 and supplies, for example, carbon dioxide gas as the reaction gas to the reaction tube 7. The gas supply tube 9 may be connected to the reaction tube 7 via a swirling flow generation unit (not shown) that supplies the reaction gas to the reaction tube 7 as a swirling flow.

[0018] <Gas Recovery / Cooling Tube> The gas recovery / cooling tube 11 is inserted into the reaction tube 7 from the insertion port 7a at the lower end of the reaction tube 7 to recover the decomposed gas and cool the decomposed gas obtained by decomposing the reaction gas by plasma. As shown in Figures 1, 2, and 3, the gas recovery and cooling pipe 11 has a triple-pipe structure comprising a gas flow pipe 13 through which decomposition gas flows in the center, a cooling pipe 15 through which coolant flows to cool the decomposition gas flowing through the gas flow pipe 13, and a coolant recovery pipe 17 outside the cooling pipe 15 for recovering the coolant after heat exchange.

[0019] The gas passage pipe 13 has openings at both its upper and lower ends. The upper opening is an inlet 13a through which reaction gases flow in, and the lower opening is an outlet 13b through which decomposition gases are discharged. A decomposition gas recovery pipe 19 for recovering the decomposition gases is connected to the outlet 13b of the gas passage pipe 13. The decomposition gas recovery tube 19 is connected to a pressure regulating valve 20 and a vacuum pump 21. The pressure regulating valve 20 adjusts the pressure in the reaction tube 7, and the decomposition gas is sucked in and recovered by the suction force of the vacuum pump 21.

[0020] The cooling pipe 15 has a gas passage pipe 13 inserted through its lower end and a cover 15f that seals the surrounding area, while its upper end is open, allowing the coolant to flow out. A coolant inlet 15a is provided on the lower side of the cooling pipe 15, and a coolant inlet pipe 23 is connected to the inlet 15a.

[0021] The coolant recovery pipe 17 is provided so as to cover at least the upper part of the cooling pipe 15 from the outside, and its lower and upper ends are sealed. An outlet 17a is provided on the lower side of the coolant recovery pipe 17 for discharging the coolant after heat exchange to the outside, and a coolant discharge pipe 25 is connected to the outlet 17a.

[0022] As described above, because the outlet of the gas passage pipe 13, the coolant inlet 15a of the cooling pipe 15, and the coolant outlet 17a of the coolant recovery pipe 17 are located away from the inlet 7a of the reaction pipe 7, the gas recovery / cooling pipe 11, the coolant inlet pipe 23, and the coolant outlet pipe 25 do not interfere with the reaction pipe 7, and the amount of gas recovery / cooling pipe 11 inserted into the reaction pipe 7 can be adjusted. In other words, in the example shown in Figure 2, the position of the outlet 17a of the coolant recovery pipe 17 only needs to be a distance from the inlet 13a on the tip surface of the gas recovery / cooling pipe 11 that is necessary to adjust the insertion amount.

[0023] Here, we will explain why the gas recovery / cooling tube 11 is provided and why the amount of gas recovery / cooling tube 11 inserted into the reaction tube 7 can be adjusted. Figure 4 illustrates the reaction state of carbon dioxide (CO2), the reaction gas, in reaction tube 7. When the reactive gas CO2 is turned into plasma by microwaves, carbon dioxide dissociates into oxygen atoms (O) and carbon monoxide (CO). As shown in Figure 4, the dissociated O and CO undergo two reactions: 2O + M → O2 + M (reaction (i)) and CO + O + M → CO2 + M (reaction (ii)). Here, M in the reaction equations is a third substance, which is a surrounding molecule (CO2, CO, O2, etc.) that removes the heat of reaction generated in reactions (i) and (ii). When the reactor wall is relatively close, the surface of the wall acts as the third substance. After carbon dioxide dissociates, reaction (ii) becomes more vigorous than reaction (i) in the high-temperature region. Therefore, if the gas temperature does not decrease, reaction (ii) continues to proceed, and the conversion rate of carbon dioxide decreases. However, as mentioned above, unlike thermal equilibrium plasmas, non-equilibrium plasmas have lower temperatures for the gas that functions as the third body and the tube walls. Therefore, by rapidly cooling them before their temperatures rise, reaction (ii) (reverse reaction) can be suppressed, and the conversion rate of CO2 can be increased. Therefore, a gas recovery and cooling pipe 11 is provided to rapidly cool the gas temperature before it rises after the carbon dioxide has been decomposed.

[0024] Here, the conversion rate is a measure of decomposition efficiency, representing the percentage of the gas (in this case, CO2) supplied to reaction tube 7 that has been 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. To increase the conversion rate, it is necessary to rapidly cool the reaction gas to suppress reaction (ii), and as described above, the gas recovery / cooling tube 11 has this function. However, if the reaction gas undergoes reaction (ii) after dissociation and is then introduced into the gas recovery / cooling tube 11, the effect of suppressing the decrease in conversion rate cannot be obtained. Therefore, it is necessary to position the inlet of the gas passage pipe 13 in the gas recovery / cooling pipe 11 so that the gas can be introduced immediately after gas decomposition by plasma.

[0025] Thus, in order to maintain a high conversion rate, it is desirable to allow the dissociation reaction of CO2 due to plasma formation to proceed sufficiently and to start cooling before reaction (ii) (reverse reaction) proceeds. For this reason, there is an optimal location for the installation of the gas recovery / cooling pipe 11 where cooling begins.

[0026] Figure 5 shows the case where the microwave intensity is the same but the gas flow velocity is different, with the gas flow velocity in Figure 5(b) being faster than in Figure 5(a). If the gas flow rate is high, the reaction time required for dissociation does not change, but the gas and plasma flow further downstream in the same amount of time, so the plasma region expands downstream. For this reason, as shown in Figure 5(b), it is preferable to install the gas recovery and cooling pipe 11 further downstream than in the case of Figure 5(a).

[0027] Figure 6 shows the case where the gas flow velocity is the same but the microwave intensity is different, with the microwave intensity being stronger in Figure 6(b) than in Figure 6(a). Even with the same gas flow velocity, if the microwave intensity is strong, plasma formation will continue further downstream. Therefore, as shown in Figure 6(b), the gas recovery and cooling pipe 11 needs to be installed further downstream than in the case of Figure 6(a).

[0028] As described above, even when using the same equipment, if there are changes in gas flow rate or microwave intensity, it is desirable to be able to adjust the installation position of the gas recovery / cooling pipe 11 to a suitable position in order to increase the conversion rate. The optimal placement of the gas recovery / cooling pipe 11 is determined by deciding the amount of the gas recovery / cooling pipe 11 to be inserted into the reaction pipe 7 based on the condition that the conversion rate of the supplied carbon dioxide is maximized, and adjusting its position in advance.

[0029] If there is no change in gas flow velocity or microwave intensity, the installation position of the gas recovery / cooling pipe 11 does not need to be adjustable if it has been set to the optimal position in advance.

[0030] Furthermore, if the gas recovery / cooling tube 11 is too close to the microwaves, the following problems may occur. The structural material of the gas recovery / cooling tube 11 absorbs microwaves. Furthermore, the coolant flowing inside the gas recovery / cooling tube 11 also absorbs microwaves. In both of the above cases, the microwave electromagnetic field is disturbed, weakening the plasma, reducing the dissociation efficiency, and consequently lowering the conversion rate. Therefore, care must be taken to ensure that the gas recovery / cooling tube 11 is not positioned too close to the microwaves.

[0031] Next, the operation of this embodiment configured as described above will be explained. When decomposing carbon dioxide, carbon dioxide gas is supplied to the reaction tube 7 from the gas supply pipe 9 shown in Figure 1. Next, the vacuum pump 21 is started to evacuate the reaction tube 7 through the gas passage pipe 13, reducing the pressure, and the pressure inside the reaction tube 7 is adjusted to a predetermined pressure by the pressure regulating valve 20.

[0032] Next, the microwave generator 3 is activated 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 transparent 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 within the reaction tube 7. When carbon dioxide is turned into plasma, it dissociates into oxygen atoms and carbon monoxide. The dissociated decomposition gas (oxygen atoms and carbon monoxide) is introduced into the gas flow pipe 13 in the gas recovery / cooling pipe 11 and cooled by the cooling pipe 15.

[0033] As mentioned above, the gas recovery and cooling pipe 11 is installed in an optimal position before reaction (ii) occurs. Therefore, the decomposition gas is cooled before reaction (ii) occurs, suppressing the decrease in conversion rate due to reaction (ii), and is recovered in the gas recovery pipe. Therefore, it becomes possible to recover the decomposed gas with a high conversion rate.

[0034] In the above example, a method using microwaves was given as a way to generate non-equilibrium plasma, but the present invention is not limited to this, and also includes methods of 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 via coaxial cables or high-voltage cables instead of microwave waveguides 5, causing the carbon dioxide inside the reaction tube to be ionized and turned into plasma.

[0035] Furthermore, while the above example described the case in which carbon dioxide gas (CO2) is decomposed as a reaction gas to produce CO and O2, the present invention is not limited to this and also includes, for example, the case in which H2 and O2 are produced from water vapor (H2O).

[0036] Furthermore, although the above example shows a triple-tube structure for the gas recovery / cooling pipe 11, as mentioned above, if positional adjustment of the gas recovery / cooling pipe 11 is not required, a coolant discharge pipe may be connected to the upper end of the cooling pipe 15 to discharge the coolant to the outside from the side wall of the reaction pipe 7. [Explanation of symbols]

[0037] 1. Gas decomposition apparatus 3. Microwave generator 5. Microwave waveguide 7. Reaction tube 7a Insertion port 9 Gas supply pipe 11. Gas recovery and cooling pipes 12 Reflector 13 Gas flow pipe 13a Inlet 13b Outlet 15 Cooling pipe 15a Inlet 15f Lid body 17 Coolant recovery pipe 17a Outlet 19. Decomposition gas recovery pipe 20 Pressure regulating valve 21 Vacuum pump 23 Coolant inlet pipe 25 Coolant drain pipe

Claims

1. The system comprises a reaction tube that generates a non-equilibrium plasma internally, a gas supply tube connected to one end of the reaction tube to supply reaction gas, and a gas recovery and cooling tube inserted into the reaction tube from the other end to recover decomposition gas and cool the decomposition gas. The gas recovery and cooling tube is characterized by having a gas flow tube in the center through which the decomposed gas flows, and a cooling tube on the outside through which a cooling liquid flows to cool the decomposed gas flowing through the gas flow tube. This is a gas decomposition apparatus using non-equilibrium plasma.

2. The gas recovery and cooling pipe has a triple-tube structure, further comprising a coolant recovery pipe on the outside of the cooling pipe for recovering the coolant after heat exchange. The gas decomposition apparatus using non-equilibrium plasma according to claim 1, characterized in that the outlet of the gas passage pipe, the coolant inlet of the cooling pipe, and the coolant outlet of the coolant recovery pipe are located away from the inlet into which the gas recovery / cooling pipe is inserted into the reaction pipe, and the position of the inlet into which the decomposed gas flows into the gas passage pipe can be adjusted by adjusting the amount into which the gas recovery / cooling pipe is inserted into the reaction pipe.

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