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Takayuki Watanabe

Professor
Department of Chemical Engineering
Kyushu University

Field of study : Plasma Chemistry, Energy Engineering
Key words : Plasma Processing, Modeling of Plasma Flow, Nanomaterials, Waste Treatment, Lunar Resource Utilization
Homepage : http://www.chem-eng.kyushu-u.ac.jp/lab5/index-e.html/

September 19, 2017 Up

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Waste Treatment by Atmospheric Pressure Plasmas can be downloaded from here.



1 The subject and aims of research

Thermal plasmas have simply been used as a high temperature source. This indicates that thermal plasmas may have more capabilities in material processing, especially production of high-quality and high-performance materials, if thermal plasmas are utilized effectively as chemically reactive gases. Therefore we developed the numerical analysis to investigate non-equilibrium characteristics in thermal plasmas. These results can be utilized for the nano-material synthesis as well as waste treatment using thermal plasmas.

2 Related recent research topics

Modeling of Reactive Plasma Flows

The induction thermal plasma approach has been applied for many fields, including treatment of harmful waste materials, recovery of useful material from waste, and production of high-quality and high-performance materials, such as synthesis of nanoparticles, deposition of thin films, and plasma spraying. Induction thermal plasmas offer unique advantages including high enthalpy to enhance reaction kinetics, high chemical reactivity, oxidation and reduction atmospheres in accordance with required chemical reactions, and rapid quenching. These advantages increase the advances and demands in plasma chemistry and plasma processing.

However, thermal plasmas have been simply used as high-temperature source, because argon is typically used as the plasma gas. In some applications such as reactive plasma spraying, material synthesis, and waste treatment, thermal plasmas with adding reactive gas are desirable to enhance the chemical reactivity of the plasma.

Thermal plasmas have been mainly treated as equilibrium conditions even though sophisticated modeling considering chemical reactions has been required for industrial applications. In previous works, estimation of thermodynamic and transport properties was oversimplified, therefore, more sophisticated models are required. The oversimplified estimation, such as use of equilibrium properties and use of the first-order approximation of Chapman-Enskog method, would cause errors in the numerical results.

In order to improve the accuracy of thermodynamic and transport properties, higher-order approximation of Chapman-Enskog method was applied for estimation of the transport properties of induction thermal plasmas. The thermal conductivity and the electrical conductivity by higher-order approximation differ from those of the first-order approximation especially over 10000 K.

In this study, a non-equilibrium modeling of induction thermal plasmas was developed without chemical equilibrium assumptions. This formulation including the finite-rates of dissociation and ionization is presented using higher-order approximation of Chapman-Enskog method for estimation of the transport properties.


RF plasma temperature distribution for nanoparticle synthesis.

Comparison of temperature distributions obtained from the chemical equilibrium model (CE) and from the chemical non-equilibrium model (CNE) for argon-oxygen induction plasmas.


Development of Innovative In-Flight Glass Melting Technology for Energy Conservation

In-flight melting process for vitrification using RF thermal plasmas or multi-phase arc was developed. The followings were performed to investigate the in-flight melting process of vitrification.

In-flight melting by RF thermal plasmas
Powders as the starting material were injected into RF thermal plasmas for the vitrification. Injected powders were melted, followed by the vitrification during in-flight treatment. Especially, the injection location of the powders into the RF plasmas and the powder size were examined to investigate the in-flight process. From the bubble analysis of the prepared glass revealed that the in-flight process provide the rapid vitrification with high efficiency. Moreover, the quenched powders were collected in the downstream region to estimate the vitrification. The vitrification of almost 100% was completed during a few ? seconds of the flight time for about 100?mm as the starting powders.

In-flight melting by multi-phase arc
Powders as the starting material were injected into multi-phase arc for the vitrification. Injected powders were melted, followed by the vitrification during in-flight treatment. Multi-phase arc provide larger high-temperature region than RF thermal plasmas, then more rapid with higher efficiency for vitrification were performed using the multi-phase arc.


High-quality glass (porosity < 0.01%) were produced from in-flight melted powders.


Development of new glass melting technology that can rapidly melt atomized andhomogenized granular glass materials.


Generation of Steam Plasmas under Atmospheric Pressure

Applications for the destruction of hazardous and waste materials such as halogenated hydrocarbons by reactive thermal plasmas are investigated. For halogenated hydrocarbon decomposition, key technologies are the stable generation of DC steam plasmas and the off-gas treatment after the decomposition of halogenated hydrocarbon.

Therefore, DC 100%-steam plasma characteristics were investigated for the application of halogenated hydrocarbon decomposition. The developed steam plasma system is a portable light-weight plasma generation system that does not require any gas supply. The system has high energy-efficiency since cooling water is not needed. In addition, a dry process was developed for halogenated hydrocarbon decomposition and simultaneously adsorbing fluorine and bromine with solid alkaline reactants.


Water plasma system for CFC destruction.


Photographs for water plasma, methanol-water plasma, and ethanol-water plasma for waste treatments.

Waste Treatment by Reactive Thermal

Attractive thermal plasma processes have been proposed especially for waste treatment, because engineering advantages such as smaller reactor, lower capital cost, portability allowing on-site destruction, rapid start-up and shutdown offer efficient destruction of hazardous and waste materials.

Waste material can be efficiently degraded by thermal plasmas under reducing or oxidizing conditions. However, thermal plasmas have been mainly used as a high temperature source. This indicates that thermal plasmas may have more capability for waste treatment, if thermal plasmas are utilized effectively as chemically reactive gas.

In this research, application for destruction of hazardous and waste materials by thermal plasmas is developed. Radioactive waste treatment by thermal plasmas is an active research field, therefore plasma treatments of low-level radioactive wastes (LLW) as well as ion-exchange resin are investigated.

Moreover, reactive thermal plasma for halogenated hydrocarbon decomposition is developed. For halogenated hydrocarbon decomposition, the stable generation of DC steam plasmas is the important factor for industrial application. Steam plasmas are suitable for halogenated hydrocarbon decomposition because hydrogen and oxygen combine with the liberated halogen and carbon atoms to prevent recombination reactions that result in the reformation of halogenated hydrocarbons.

Another purpose is to develop a dry process for the off-gas treatment of halogenated hydrocarbon decomposition using solid alkaline reactants in Section 5. The off-gas treatment after the decomposition is the major concern, because the off-gas contains toxic and corrosive gases such as fluorine, chlorine, bromine, hydrogen fluoride, hydrogen chloride and hydrogen bromide.



Hydrogen generation from waste using thermal plasmas.



Thermal plasma het for waste destruction.





Cobalt doped resins after the plasma treatments.

Preparation of Functional Nanoparticles by Thermal Plasmas

Functional nanoparticles of silicide and boride were prepared by induction thermal plasmas. Silicide and rare-earth boride are attractive materials because of their high melting temperature, high electrical conductivity and low work function. Therefore these nanoparticles would be applied for electromagnetic shielding, and solar control windows with interaction with IR and UV light.

For the preparation of silicide, Si powder premixed with metal powder (Mo, Ti, Co, Fe, Cr, or Mn) was injected into the plasma. For the preparation of rare-earth boride, premixed powders of rare-earth oxide, B and C were introduced into the thermal plasma. In the thermal plasma, the injected powders were evaporated and reacted with boron. After the evaporation and reaction, the vapor was rapidly cooled after the plasma flame. The nanoparticles were prepared on condition that the vapor was quickly quenched by the water-cooled copper coil.

Another purpose is to investigate the condensation mechanism of mixture vapor of feed powders in thermal plasmas. The characteristics of the prepared nanoparticles are affected by the vapor pressure ratio of the constituent materials. Investigation of physical and chemical processes in thermal plasma processing is indispensable for Nanoparticle synthesis.



Induction thermal plasma system for nanoparticle synthesis.


LaB6 nanoparticles prepared from the La2O3-B-C powders injected into argon induction plasmas.

Development of Material Processing by Non-Equilibrium Atmospheric Plasmas

Atmospheric pressure plasma has been studied for the application of material surface cleaning, preparation of surface coating, and modification of polymer film. In this study, the atmospheric non-thermal plasmas ere applied to surface cleaning of waste plastic.

The paint attached to waste plastic surface causes low quality of recycled plastic. The paint removable is difficult by the conventional dry cleaning process, therefore new cleaning technology for waste-plastic recycle has been required.

The mechanism of the cleaning with atmospheric plasmas was investigated from the measurements of optical emission spectroscopy, the atomic concentrations of plastics surface were measured with electron spectroscopy for chemical analysis.

Furthermore, a kinetic model is developed to investigate ozone and oxygen radical production in dry air treated by a non-thermal plasma jet at atmospheric pressure.



Indium recovery from liquid crystal panel using atmospheric glow discharge.

Glow discharge of air under atmospheric pressure.


Seed treatment by atmospheric glow discharge.

Lunar Resources Utilization

Ground-engineering work on experimental missions for lunar resource utilization has been conducted. The goal of the research program is to conceptually design an experiment system for unmanned water production on the Moon, and to define essential technological breakthroughs.

As part of the research program, an experimental study on hydrogen reduction of lunar soil has been performed to design a chemical reactor of the water production. A fixed-bed reduction reactor and lunar soil simulants were prepared for our water-production experiments.

Over 20 processes of oxygen production on the moon have been proposed. Among these processes, oxygen production employing hydrogen reduction is the most feasible process. In this process, ilmenite contained in lunar soil is reduced with hydrogen producing water. Oxygen is subsequently produced by electrolysis. Hydrogen produced in reaction can be recycled.

Understanding the hydrogen reduction mechanism of ilmenite is important for the mission of utilizing lunar soil. The purpose of this work is to discuss the possibility and the mechanism of water production.



Experiment of water production from lunar soil simulant.



Oxygen production form lunar regolith.

Water production system by hydrogen reduction of lunar soil simulant.

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