JPH04351899A - Microwave heat plasma reaction device - Google Patents

Abstract

PURPOSE:To restrict generation of an arc, and improve a heat resistance by forming inorganic compound having a good high-temperature heat resistance and a good high-temperature stability on the inner side of an outer tube around a plasma torch generating part. CONSTITUTION:A moulded body 12 is formed of baron nitride having a good high-temperature heat resistance and a good high-temperature stability to be assembled in an outer tube 2. The outer tube 2 is formed thin at a bottom part 22 of the moulded body 21, a cooling water sump 23 is provided through a metal wall, and cooling is performed by the good heat conductivity of the boron nitride moulded body 21. Gas dissosiated at a forward end 5 thus becomes a strong plasma flame 7 at a torch generating part 4. Powdery reaction material is charged from reaction material charging ports 16, 12 toward the plasma flame 7 over it to be fused for causing high-temperature chemical reaction to be jetted through a wall 16 of a plasma chamber into the inside of the plasma chamber at a high temperature. Stable and strong plasma can thus be generated, and good quality reaction products can be obtained. A discharge preventing effect, an insulation effect, and a heat resistance are improved. and an applicable microwave power can be improved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microwave thermal plasma reaction apparatus for synthesizing highly pure substances with high efficiency, with the aim of producing new materials and developing new fields of application.

0002

[Prior Art] In a plasma torch generator using a conventional coaxial tube circuit, the tip of the inner tube is made sharp and the inner diameter of the metal outer tube is narrowed to increase the microwave electric field in this area and create a spiral state. A certain gas was ionized to generate plasma in the atmosphere. To explain this based on the drawings, FIG. 3 is a longitudinal sectional view of a conventional microwave thermal plasma reactor. In the figure, 1 is an inner tube, and 2 is an outer tube, both of which are made of metal and form a coaxial tube circuit for transmitting microwave power. A thin tube 11 for introducing a reactant passes through the center of the inner tube 1, and the reactant is introduced from a reactant inlet 16 at the top of the thin tube 11. A partition pipe 3 for the cooling waterway is placed on the outside of this
is provided, and cooling water enters through the inlet 17 and exits through the outlet 18 to cool the inner tube 1. The tip 5 of the inner tube 1 is sharp, and the microwave power transmitted in the direction of arrow A within the coaxial tube circuit is concentrated to form a plasma torch generating section 4. The reactant is introduced into the upper part of the generated plasma flame 7. The tip of the outer tube 2 is constricted inward and has the narrowest part 6, forming a plasma torch generating part. A cooling water reservoir 13 is provided inside the outer tube 2 around this plasma torch generation part,
Cooling water flows from a cooling water inlet 14 to an outlet 15. In order to increase the stability and controllability of the plasma, two systems of gas ejection ports, one for the circumferential direction and one for the radial direction, are provided above the plasma torch generating section. In the figure, the jet ports 9 for the circumferential direction and the jet ports 10 for the radial direction are located at 91 on the circumference, respectively.
Several holes like 101 are made to converge the gas in the center of the coaxial tube in a spiral state. Since the microwave electric field is at its maximum in the plasma torch generating section 4, gas is ionized at the tip 5 of the inner tube 1 of the coaxial tube and plasma is generated. Once generated, the plasma is supplied with energy by the electric field formed around the torch generating portion, and the plasma flame 7 is maintained stably. The powdered reactant is in the capillary 11
Reactant inlet 16 on the top and reactant inlet 1 on the side
2 into the upper part of the plasma flame 7, it is melted by the plasma flame, a high-temperature chemical reaction occurs, and it is ejected into the plasma chamber in a high-temperature state. 19 is a plasma chamber wall.

0003

[Problem to be solved by the invention] In the above conventional device,
In order to prevent melting and damage to the metal surface 8 of the plasma torch generator 4 due to the high temperature plasma flame 7, a cooling water reservoir 13 was provided inside the metal outer tube 2 and cooling water was allowed to flow therethrough. However, with this structure, the microwave power that can be applied is limited to several kilowatts, and if more microwave power is applied, the metal surface 8 of the outer tube 2 near the plasma flame 7 will be melted and damaged even if the amount of cooling water is increased. Cheap. If the goal is to create new materials such as ceramics or develop new application fields, the applied microwave power will need to be 10 kilowatts or more, so it is necessary to suppress arcing in the torch generating part and improve heat resistance and insulation. There is a need. However, in the above-mentioned conventional apparatus, when the microwave power exceeds 5 kilowatts, the outer periphery of the torch generating part tends to melt and break, making it difficult to operate for a long time. In view of the above-mentioned conventional problems, the present invention aims to provide a microwave thermal plasma reactor that can withstand continuous use of strong microwave power.

0004

[Means for Solving the Problems] The present invention provides a coaxial tube circuit for transmitting microwave power consisting of an inner tube and an outer tube, and a plasma torch generating section formed by narrowing the tip of the inner tube. , a means for ejecting gas in the circumferential direction and radial direction of the inner tube, and a reactant inlet provided at the top of the coaxial tube circuit, in a microwave thermal plasma reactor having a reactant inlet provided inside the outer tube and This is a microwave thermal plasma reactor characterized by forming an inorganic compound insulator around the plasma torch generating part. The present invention suppresses arc generation in the plasma torch generation area and improves heat resistance by forming an inorganic compound with excellent high heat resistance and high temperature stability on the inside of the outer tube around the plasma torch generation area. , which achieves the intended purpose. The heat resistance can be particularly improved by using boron nitride as the inorganic compound and forming a molded body thereof or forming a film of boron nitride on the surface of the outer tube to cover it.

[0005]

[Function] High temperature heat-resistant inorganic compounds such as boron nitride are electrical insulators, so if they are formed on the outer tube around the plasma torch generation part, the voltage distribution will be more insulated than if it were made entirely of metal. Since it is divided into a body part and a space part, the microwave electric field strength in the space of the torch generating part decreases when the dimensions are the same. However, since the dielectric constant of the boron nitride molded body is about 3 to 4, the reduction is only about 10% within the range of use. Therefore, if necessary, the same result can be obtained by slightly reducing the inner diameter of the outer periphery (the part where the boron nitride molded body is formed). Also, since its thermal conductivity is comparable to that of iron, the cooling effect will be good if the bottom is water-cooled. All of the above features indicate that it is optimal as the outer periphery of the plasma torch generating section. Incidentally, even if a boron nitride suspension is injected onto the outer circumferential surface of the metal plasma torch generation part to form a thin film to cover it, discharge can be prevented and the applied microwave power can be increased.

[0006]

[Embodiment] Fig. 1 is a vertical cross-sectional view showing an embodiment of the present invention, and Fig. 2
FIG. 3 is a vertical cross-sectional view showing another embodiment. In the figure, the same reference numerals as in FIG. 1 indicate the same parts. FIG. 1 shows an example in which a molded body of boron nitride is used as the inorganic compound insulator. The characteristics of boron nitride (boron) are as follows. 1. The thermal conductivity is 25 to 80 W/mK at room temperature, which is close to that of iron. 2. It has high temperature stability up to 2200 degrees Celsius in an inert atmosphere. 3. It has chemical stability that makes it difficult to react with melts of various materials. 4. It has a low coefficient of thermal expansion and excellent thermal shock resistance. 5. The electrical dielectric strength is also excellent at 30 to 40 kV/mm. 6. Machining of the molded body is easy. 7. The suspension forms a uniform film (planar layer of powder). Therefore, if this boron nitride molded body is processed to form the outer periphery of the plasma torch generation part 4, even if the applied microwave power is 10 kilowatts or more, arc discharge will be less likely to occur than a metal conductor, and the heat resistance will be improved. I can do things. Furthermore, forming a thin film by spraying a boron nitride suspension on the outer peripheral surface of a torch generating part made of a metal conductor is also effective in improving insulating heat resistance and preventing discharge.

In FIG. 1, 21 is a boron nitride molded body whose inner diameter is narrowed at the narrowest part 6, and is incorporated into the outer tube 2. At the bottom 22 of the molded body 21, the outer tube 2
A cooling water reservoir 23 is provided across the thin metal wall of the outer tube 2 to provide sufficient cooling by utilizing the good thermal conductivity of the boron nitride molded body 21. As a result, the gas ionized at the tip 5 becomes a powerful plasma flame 7 at the torch generating section 4. The powdered reactant is introduced into the upper part of the plasma flame 7 through the reactant inlet 16 and the side inlet 12, where it is melted, causes a high-temperature chemical reaction, and is ejected into the plasma chamber at a high temperature through the wall 16 of the plasma chamber. Ru.

FIG. 2 shows an embodiment in which a suspension of boron nitride is sprayed onto the metal surface 8 of the metal conductor torch generation part in FIG. 3 to form a thin film coating 25 for protection, which is effective in preventing discharge. be. This method also made it possible to increase microwave power.

In all of the above embodiments, the inner diameter of the outer tube of the coaxial portion was 40 mm, the outer diameter of the inner tube was 26 mm, and the minimum diameter of the outer periphery of the torch generating portion was 20 mm. Applying 10kW of microwave power with a frequency of 2.45GHz to this,
When a mixed gas of hydrogen and argon was flowed through it, a strong and stable plasma was generated and high-quality reactants were produced.

0010

Effects of the Invention With conventional microwave thermal plasma generators, it has not been possible to obtain a high-output reactor using powerful microwave power of 10 kW or more. However, if the microwave thermal plasma reactor of the present invention is used, this problem can be solved because a powerful plasma flame can be stably generated and maintained. The features of the microwave thermal plasma reactor of the present invention are as follows. (1) In a microwave thermal plasma reactor using the boron nitride molded body or coating of the present invention in the torch generating part,
By improving the discharge prevention effect and insulation heat resistance, the microwave power that can be applied has been significantly increased compared to conventional methods. (2) In the microwave thermal plasma reactor of the present invention, a reactant containing no impurities can be obtained. (3) The electric power required to maintain atmospheric pressure pure hydrogen plasma is significantly lower than that of DC arc jets, high frequency plasmas, etc. That is, in 4 MHz high frequency plasma, 50
Microwaves require 1 kW or more, whereas microwaves require 1 kW or more.
．． A low power of about 8 kW is sufficient. (4) Experiments have shown that the microwave thermal plasma reactor is suitable for material synthesis because both the temperature gradient and the concentration gradient in the plasma are considered to be quite small. (5) Since there is almost no melting of the electrodes, there are fewer problems with the electrodes. (6) Generates a strong and stable plasma flame for a long time and efficiently produces high-purity products.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of an embodiment of the present invention in which a boron nitride molded body is used.

FIG. 2 is a longitudinal cross-sectional view of another embodiment in which a thin film coating of boron nitride is used.

FIG. 3 is a longitudinal cross-sectional view of a conventional microwave thermal plasma reactor.

[Explanation of symbols]

1 Inner pipe 2 Outer tube 3 Division pipe 4 Plasma torch generation part 5 Tip 6 Narrowest part 7 Plasma flame 8 Metal surface 9, 91 Circumferential jet nozzle 10, 101 Radial jet nozzle 11 Thin tube 12, 16 Reactant input port 13, 23 Cooling water reservoir 14, 17 Cooling water inlet 15, 18 Cooling water outlet 19 Plasma chamber wall 21 Molded body 22 Bottom 25 Thin film coating

Claims (3)

[Claims] 1. A coaxial tube circuit for transmitting microwave power consisting of an inner tube and an outer tube, a plasma torch generating section formed by narrowing the tip of the inner tube, and a circumferential direction of the inner tube. In a microwave thermal plasma reactor having a means for ejecting gas in the radial direction, and a reactant inlet provided at the top of a coaxial tube circuit, an inorganic material is provided inside the outer tube and around the plasma torch generating part. A microwave thermal plasma reaction device characterized by forming a compound insulator. 2. The microwave thermal plasma reactor according to claim 1, wherein the inorganic compound insulator is boron nitride. 3. The plasma reactor according to claim 1, wherein the inorganic compound insulator is a film of boron nitride.