23E06
Thermal Plasma Characterization on Long DC Arc Discharge for Waste Treatment
廃棄物処理用のロングDCアーク放電における熱プラズマ特性
Sooseok Choi1, TianMing Li1, Takayuki Watanabe1, Takashi Nakayama2, and Koji Otsuki2
崔 秀錫, 李 天明，渡辺隆行，中山貴志，大槻浩司
1

Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology
4259 G1-22, Nagatsuta, Yokohama, Kanagawa 226-8502, Japan
東京工業大学大学院総合理工学研究科 〒226-8502 横浜市緑区長津田町4259番地G1-22
2
Clean Technology Co., Ltd.
3209-1, Hineno, Izumisano-shi, Osaka 298-0021, Japan
クリーン・テクノロジー株式会社 〒289-0021 大阪府泉佐野市日根野3209-1
Characteristics of thermal plasma generated by the long DC arc discharge system for waste treatment werenumerically analyzed. Effects of arc current, gas flow rate, confinement tube diameter, and electrode gap
distance on the thermal plasma were investigated. The numerical results reflect the experimental data wellaccording to operating conditions. Uniformly high temperature distribution is predicted with large diameterof the plasma confinement tube, because less intrusion of cold gas into the center region leads broad current
density profile in radial direction. Input power level is controllable precisely by adjusting electrode gap
distance because of the linearity of arc voltage on the arc length.

1. Introduction
Thermal plasma has received a great attention
for the waste treatment application, because it
provides high energy density and control over theprocess chemistry with high decomposition
efficiency [1]. In order to improve the thermal
plasma waste treatment, novel long DC arc wasstudied in the present work.
Since long electrode gap distance of 400 mm
and large plasma confinement tube diameter of 43mm are employed in the experiment, largerplasma volume and longer waste residence time
are achievable when compared with those in aconventional thermal plasma system. For asuccessive application, thermodynamic andelectric properties of the long DC arc plasma
were characterized by numerical analysis invarious operation and design conditions.
2. Features of the Long DC Arc System
Photograph of the long DC arc system is shownin Fig. 1 (a). This system is constructed by Clean
Technology Co., Ltd. to decompose global warming
PFCs exhausted from the semiconductor industry.
Arc channel is placed between tungsten imbedded
rod-type cathode and copper-tungsten alloy used
dome-type anode. Thermal plasma is confined by
the quarts tube which has main diameter of 43 mm
except anode region of 90 mm to avoid thermaldamage. Auxiliary metal wire is used to breakdown
in long electrode gap distance.
Computational domain for the thermal plasma
region is illustrated in Fig. 1 (b). Nitrogen plasma

forming gas is introduced with a swirl motion fromthe top of the confinement tube. Since the change ofsystem geometry in the real experiments requires
time and cost, effects of electrode gap distance andtube diameter on the thermal plasma characteristics
were numerically analyzed in the range presented inTable 1. Influence of operating parameters of arc
current and N2 flow rate were also investigated.

Fig.1. (a) photograph of the long DC arc system, and (b)computational domain on the plasma region.
Table 1. Design and operating parameters.
Electrode Gap Distance
L (mm)
Tube Diameter
D (mm)Arc Current
I (A)Gas Flow Rate
G (lpm)

300, 350, 400, 450, 500
30, 35, 40, 43, 45, 50
6, 7, 8, 9, 1030, 40, 50, 60

3. Numerical Model
Axis symmetric 2-dimensional, steady state,laminar flow, and thermal plasma in LTE (localthermodynamic equilibrium) were assumed.
Conservations of mass, momentum, energy, andcurrent were used as governing equations where
electromagnetic Lorentz force, joule heating, andradiation loss were also considered as the MHD(magnetohydrodynamics) fluid analysis method [2].
4. Results and Discussion
Numerically calculated arc voltages arecompared with experimental data in Fig. 2 bychanging the arc current. The voltage difference
from 7% to 18% is thought to come from the LTE
assumption and steep increase of electric
conductivity of N2 gas around 7,000 K which iscorresponding to the center area temperature [3].
However, present numerical simulates suitably thevoltage decrease by increasing arc current due tothe enhancement of plasma temperature and electric
conductivity, especially in the high arc current
where energies of electron and heavy particles arecloser than in the low arc current condition.
Numerical results on the plasma gas flow effects
are also in good agreement with experimentallymeasured data. The higher arc voltage was obtainedby increasing flow rate, because increased cold
plasma forming gas reduces temperature andelectric conductivity.
Since the operating conditions are usuallypredetermined and have the limitation in theindividual treatment process, design variables areimportant to control thermal plasma characteristics.
Temperature distributions are presented in Fig. 3
for different confinement tube diameters. Relatively
uniform high temperature distribution in radial
direction is achieved with a large tube diameter
owing to uniform Joule heating by current density
profiles in large diameter as shown in Fig. 4.

Fig.2. Arc voltage comparison between experimental andnumerical results according to arc current.

Fig.3. Comparison of temperature distributions between
D=30 and 50 at operating conditions of 10 A and 30 lpm.

Fig.4. Radial profiles of arc current density on the middleof the arc channel (z=200) for different tube diameters.

The arc voltage is linearly increased by 1.61
V/mm with lengthening the electrode gap distance.
Therefore, the electrode gap distance can be utilized
to control the input power level precisely at fixed
operating conditions and power requirement.
5. Conclusion
Thermal plasma generated by the long DC arc
discharge has been characterized by numericalanalysis. The numerical results are in good
agreement with the experimental data. Temperaturefield and input power level required in the thermal
plasma waste treatment can be predicted andcontrolled by the design variables.
References[1] J. Heberlein and A.B. Murphy: J. Phys. D: Appl.
Phys. 41 (2008) 053001.
[2] S. Choi, J.M. Park, W.T. Ju, and S.H. Hong: J.
Therm. Sci. Tech. 6 (2011) 210.
[3] A.B. Murphy and C.J. Arundell: Plasma Chem.
Plasma Process. 14 (1994) 451.