S4-5
Multi-Phase Plasma with Gas-Liquid-Solid for Environmental Application
気-液-固マルチフェーズプラズマによる環境応用プロセス
Takayuki Watanabe
渡辺隆行
Department of Chemical Engineering, Kyushu University744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
九州大学大学院工学研究院 〒4819-0395 福岡市西区元岡744
Thermal plasmas have attracted extensive attention due to their unique advantages, and it is expected to beutilized for a number of industrial applications. The advantages of thermal plasmas with multi-phaseconditions are beneficial for innovative processing based on the high enthalpy to enhance reaction kinetics,
high chemical reactivity, and oxidation or reduction atmospheres in accordance with required chemical
reactions.

1. Introduction
Thermal plasmas have attracted extensive
attention due to their unique advantages, and it isexpected to be utilized for a number of innovative
industrial applications such as decomposition ofharmful materials, recovery of useful materials fromwastes, and synthesis of high-quality and highperformance nanoparticles. The advantages ofthermal plasmas with multi-phase conditions
including high enthalpy to enhance reaction kinetics,
high chemical reactivity, and oxidation or reduction
atmospheres in accordance with required chemical
reactions are beneficial for innovative processing.
Recently, successful application based on gassolid reaction was developed for in-flight glass
melting. The granulated raw material with small
diameter is injected into thermal plasmas and thepowders contact fully with the plasma. The highheat-transfer and temperatures of the plasma melt theraw materials quickly. In addition, the decomposedgases of carbonates are removed during the in-flighttreatment to reduce the refining time considerably.
The example of gas-liquid reaction is the waterplasma generation for waste treatment. A DC arc
water plasma torch which is operated at atmosphericpressure by using evaporated cooling water asplasma forming gas directly. Since a large amount of
H, O, and OH radicals are generated in water plasma,decomposition reactions and syngas production areaccelerated in the treatment of water-soluble organiccompounds.
In spite of these experimental efforts for industrial
applications, thermal plasma generation with multiphase conditions and its characteristics remain to beexplored. Among various thermal plasma reactors,
arc plasma as an energy source with high energyefficiency has been applied in many applications.

2. Multi-Phase AC Arc for In-Flight Melting
Most glass is produced by the typical Siemenstype melter fired in air with heavy oil or natural gas
as the fuel. This type of melter has been used formore than 140 years because of its good large-scale
performance and continuous melting system. A longtime is required for the dissolution of SiO2 in themelting process and to allow the bubbles in the glass
to escape in the fining process. The key is to decreasethe melting and fining times to reduce the energy
consumed in glass production. The high temperatureof thermal plasmas makes it easy to melt raw glass
powders quickly. Reducing the formation of bubbles
in molten glass is an effective method of shortening
the fining time.

Fig. 1. Gas-solid reaction for in-flight glass melting

An innovative in-flight glass melting technology
shown in Fig. 1 was developed from the above pointof view [1]. Granulated raw material with a small
diameter is dispersed in the melter, where theparticles are in full contact with the plasma andburner flame. In addition, CO2 formed by thecarbonates decomposition is removed while the raw
material is still in-flight to reduce the fining time
considerably. The total vitrification time is evaluatedto be only 2 - 3 h for the same rate of productivity asthat obtained using the fuel-fired melter.
The multi-phase AC arc shown is suitable heatsource for the in-flight reaction between gas andsolid because it possesses many advantages such as
high energy efficiency, large plasma volume, andlong residence time of the treated materials [2]. The
arc discharge takes 20 ms for a periodic cycle while
the frequency of power source used for 12-phase AC
arc is 50 Hz. Most power sources for generating arc
plasma are accomplished by DC power supply,
however it takes more cost in the apparatus forconverting AC to DC. The single-phase or threephase AC power supplies have a characteristic ofintermittent discharge which limits the application ofarc plasma systems generated by AC power supply.
Therefore, a multi-phase AC power supply wasdeveloped to obtain a more effective arc plasmareactor.
The granulated raw materials of two types of glass,soda-lime glass and alkali-free glass, were heated by
the multi-phase AC arc to investigate the feasibilityof in-flight melting technology for glass production[3]. The vitrification degree, decomposition degree,
morphology, average diameter, and composition ofpowders were characterized. Also, the in-flightmelting behavior of particles was studied and theresults for different heating sources were compared
to provide guidelines for the glass industry.
Innovative in-flight melting technology wassuccessfully developed for multi-phase plasma
processing based on gas-solid reaction. The highvitrification degree achieved within milliseconds
reveals that the new in-flight melting technology canreduce energy consumption and shorten theproduction cycle.

process for gas-liquid reaction as shown in Fig. 2 [5].
The decomposition mechanisms as well as theintermediate behaviors were considered from thecomparative study on acetone and glycerine
decomposition. CH radical generated from both ofacetone and glycerine decomposition was oxidized
to form CO. Incomplete oxidation of CH3 leads toC2H2 formation as well as soot formation. The
negligible amount of soot generation from glycerine
decomposition even at high concentration indicatedthat oxidation of CH and CH3 was enhanced by OHradical.

3. Water Plasma for Waste Treatment
A water plasma torch was developed for waste
treatment processes based on gas-liquid reaction [4].
The hafnium embedded into a copper rod used ascathode material can overcome the erosion problems
and achieve a long operating time in oxidation
atmosphere. The torch can generate stable 100%water plasmas using DC discharge without
additional steam generator or gas supply system.
Using the water plasma produced by the plasma
torch, we succeed in decomposing liquid waste ofphenol, acetone, and alcohol solutions.
The roles of CH, CH3, and OH radicals in waterplasmas were investigated in the decomposition

References[1] T. Watanabe, et al.: Pure Appl. Chem. 82 (2010) 1337.
[2] T. Watanabe, et al.: Plasma Chem. Plasma Process. 34(2014) 443.
[3] M. Tanaka, et al.: Thin Solid Films 523 (2012) 67.
[4] Narengerile, et al: Plasma Chem. Plasma Process., 30(2010) 813.
[5] T. Watanabe et al.: Plasma Chem. Plasma Process., 32(2012) 123.

Fig.2. Optical emission spectra for organic solution
decomposition by multi-phase water plasma.