What is arc plasma?

In nature, thunder is a famous seasonal feature of summer. Thunder is a type of arc plasma, possessing ultra-high temperatures of up to several thousand degrees. However, thunder takes place in an extremely short period of time in the range of several hundred-thousandths of a second. Although thunder exerts instantaneous impact on electrical systems, the amount of work capacity, or energy, that it possesses is extremely small. Arc plasma can be described as the stable maintenance and control of thunder over a long-term period.

When ice is heated, it changes into water and then into steam. When exposed to even higher temperatures, the steam separates into oxygen and hydrogen atoms. It then emits electrons and becomes plasma, which is the 4th state of matter. The term arc is the general name for the plasma phenomenon in the electric industry. Since it is difficult to differentiate between arc and plasma, the two terms are often used in combination to form the term arc plasma.

Arc plasma can be classified into different types such as atmospheric thermal plasma and low-pressure high-frequency plasma. Among these different types, thermal plasma is notable for possessing characteristics similar to thunder. These characteristics include extremely ultra-high temperatures, high energy performance and high brightness. When controlling the flow of electric current, it is also possible to realize high controllability in the range of milliseconds. By effectively utilizing these characteristics, thermal plasma can be applied to a variety of fields including processing such as welding, cutting and thermal spraying, metallurgy such as the reduction and refinement of metals, chemical engineering, large-scale lighting, and ultra-fine nanometer technology.

Confronting environmental issues --utilization in the processing of waste products--

Since the COP3 Protocol Climate Conference was held in Kyoto approximately 10 years ago, environmental measures that include measures to prevent global warming have become a social issue on a global scale. This change in social consciousness has resulted in dramatic advancements in environmental reform technology. As part of these technological advances, arc plasma is attracting attention for its use in the processing and volume reduction of waste products. The effect of volume reduction is particularly great in Japan, a country with a small land area. Arc plasma also exhibits great effectiveness in the detoxification and partial detoxification of medical waste and hazardous materials. ⁽¹⁾

Examples of research and development in the electric industry

Chi-no-Kairo (The Corridor of Wisdom), an educational program broadcast by Chuo University, (No. 72: Research and Application of Arc Plasma) introduces specific examples of specific research and development being performed in the electric industry. Such examples include volume-reduction processing and dry surface decontamination processing for radioactive waste, creation of ultra-fine particles and other new materials, and detoxification and recycling for asbestos, a material which is an outstanding insulator but has been discovered to be a carcinogen. The program also clarifies the fundamental characteristics of arc plasma, which is responsible for supporting the research and development discussed above.

As an evolution technology for the melting of materials, arc plasma displays unparalleled capability in the creation of new materials. Arc plasma is also appropriate for cleaning the surface of base materials, and can also be used to lightly decontaminate the surface layer of radioactive waste products generated when dismantling nuclear reactors. Also, arc plasma has also been used to create a rough, uneven finish on the surface of materials to which a sprayed film will be applied. This processing successfully increases the strength of application on materials to 3 or 4 times the conventional strength. In this way, the range of application for arc plasma continues to expand.

Social contribution to environmental purification

Thermal plasma (plasma at ultra-high temperatures of 5,000 degrees or higher) possesses the characteristics of ultra-high temperatures, high brightness and high energy performance. Thermal plasma has demonstrated a wide variety of applications and uses, including welding, cutting, thermal spraying, material synthesis, lighting, surface processing, and processing of waste products. Currently, a variety of research is being conducted regarding thermal plasma. In particular, research themes in recent years include new areas such as the advancement of modeling (calculation/simulation) technology, the development of compact waste processing equipment, the development of compact thermal spraying equipment, and ultra-high speed surface processing. ⁽²⁾

I would now like to discuss surface processing performed by decompression arcs with a pressure that is approximately 1/100 of the atmospheric pressure. There are cases in which hazardous substances exist only in the surface layer of some waste products. A representative example of materials that possess hazardous substances in the surface layer is metal waste products which contain an oxidized layer in a steel surface. Also, high-capability films which are resistant to corrosion, friction and heat are difficult to process and inhibit recycling efforts. Therefore, when considering technology for effective removal of the surface layer only, one noteworthy option is the implementation of a surface processing or surface cleaning method that uses arcs in a decompressed environment. Currently, the main methods of surface cleaning include chemical processing that uses acidic/alkaline solutions and machine processing that uses blasting (spraying with ultra-fine particles). However, both of these methods generate large amounts of secondary hazardous materials such as waste solutions, sludge (sticky waste solution containing solids), dust and noise. In contrast, the arc cleaning technology offers the advantages of processing that is performed in a dry and enclosed environment, of being environmentally friendly, and of enabling the collection of secondary materials. The program Chi-no-Kairo introduces metal surface processing, surface cleaning, and pre-thermal spraying processing characteristics that utilize the cathode spots of direct current arc plasma.

Current status and new advancements for waste product processing using thermal plasma

Next, I would like to discuss the current status of waste product processing that uses thermal plasma, as well as new advancement for such processing. When working with thermal plasma, it is possible to obtain characteristics such as ultra-high temperature, high energy density, and active chemical reaction, and to perform rapid plasma control. It is also possible to freely select from among inert atmospheres using argon or helium, oxidized atmospheres using oxygen, and reduced atmospheres using nitrogen or hydrogen. This makes thermal plasma ideal for waste product processing in which separation is performed for various types of materials. When discussing the current status and new advancements for waste product processing using thermal plasma, Chi-no-Kairo first begins by summarizing the current status. The program then proceeds to summarize the current status, technology and issues for various types of processing including incinerated ash, radioactive waste products, chlorofluorocarbons, PCBs, asbestos and medical waste. In recent year, efforts have been made to increase the practicality, scale, simplicity and efficiency of waste product processing using thermal plasma, and further advances are expected for the technology.

Orientation of future environmental research -Reduction of CO₂ gas through the use of electric vehicles-

On a different topic, I would like to discuss the reduction of CO₂ gas through the implementation of electric vehicles. On average, when replacing gasoline-powered vehicles with electric vehicles, it is possible to reduce the amount of CO₂ gas emissions by almost 50% for equal distances traveled. However, this reduction assumes a number of conditions. In the worst case scenario, driving is performed entirely through coal-fired power and there is absolutely no reduction in CO₂. The emission of CO₂ gas is reduced to approximately 80% of coal-fired power when using oil-fired power, and to approximately 70% when using natural gas-fired power. Of course, CO₂ emissions are reduced to almost zero when using nuclear power or natural energy generation such as water power, wind power or solar power. In such cases, outstanding reductions in CO₂ gas are achieved. ⁽³⁾

During this period of rapid development in the application of arc plasma, I am happy that our university's educational program Chi-no-Kairo introduced the possibilities of arc plasma. I believe that this introduction of arc plasma has great meaning in terms of contribution to both the industrial world and society. In the future, I expect arc plasma to make further social contributions and to revolutionize technology in variety of ways. For example, arc plasma can contribute to improved capability in the insulating materials of electrical transmission/distribution systems that support power transport, to the development of equipment for the removal of nuclear power plants, and to other high value-added technology that spans a wide range of applications.

Chi-no-Kairo Research and Application of Arc Plasma, the educational program supervised by the author, can be accessed here:

Reference Literature

• (1) Inaba, New Utilization Technology for Arc Plasma, Chuo University Kusa-no-Midori (The Greenness of Grass), No. 110, pp. 24-27 (1997)
• (2) Inaba/Iwao, New Developments in Thermal plasma that Support Material Processing, Plasma/Nuclear Fusion Research, Vol. 82, No. 8, pp. 470-471 (2006)
• (3) Ryu/Inaba, Dynamic Reduction Results in the Amount of CO2 Gas Emissions through the Implementation of Electric Cars, Journal of the Institute of Electrical Engineers of Japan, Vol. 129-D, No. 2, pp. 232-233 (2009)