Ž .Thin Solid Films 316 1998 111]116

Ar torch plasma characteristics and its application to wastetreatment

Tsuginori InabaU , Yoshimoto Watanabe, Masayoshi Nagano, Takayuki Ishida,Masao Endo

Chuo Uni¨ersity, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan

Abstract

The effect of the axial gas flow-rate on the characteristics of torch plasma, which is a DC arc plasma produced by atransferred type torch, has been investigated. The voltage gradient is approximately 2]4 Vrcm. It increases with the axial gasflow-rate and decreases with the current. However, it becomes constant near a current of 200 A and an axial gas flow-rate of 4Nlrmin. The axial temperature is not affected by the axial gas flow-rate and increases with the current. The radialtemperature distribution becomes flatter when the current increases and the axial gas flow-rate decreases. An application ofthe torch plasma was carried out to melt fly ash into a glassified slag with a hardness of 6 Mohs. The leakage tests of thesolution in pH 6 or 8 were examined. Q 1998 Elsevier Science S.A.

Keywords: Torch plasma; Flow-rate; Electrode; Photo-radius

1. Introduction

The problem of the disposal of hazardous wasteshas become very important in recent years. The appli-cation of arc plasma to waste treatment has attractedattention in Japan, also in the world, because of thevolume decrease and being a sharply harmless compo-

w xsition 1 .Torch plasma is one of the stabilized arc plasmas

w xwhich is easy to control 2 . The characteristics of thetorch plasma have been examined considering theinfluence of various parameters, such as the currentor the plasma gas flow-rate. Fly ash, for example, hasbeen melted and harmlessly treated by torch plasma.Considering the pH and increasing inspection itemsof the leaching analysis in the solution, fly ash hasbeen examined for its characteristics.

U Corresponding author. Tel.: q81 3 3817 1860; fax: q81 3 38171847; e-mail: inaba@elect.chuo-u.ac.jp

2. Experimental equipment of torch plasma

Major specifications of the experimental equipmentof torch plasma employed in this study are as follows:The maximum rate of the actual electric power sourceis DC 150 V, 400 A in current, with a no load voltageof 300 V. The torch plasma is produced between anegatively charged Th-W tip electrode and a posi-tively charged water-cooled stainless-steel thick discthrough a water-cooled nozzle. The size of the plasmachamber is 50 cm in diameter and 60 cm in length.The working gas of the torch plasma is argon, theflow-rate is varied from 0 to 20 Nlrmin, as shown in

w xFigs. 1 and 2 2 .

3. Influence of gas flow-rate on terminal voltage andvoltage gradient

3.1. Terminal ¨oltage

Ž .As the gas flow-rate F Nlrmin through the inner

0040-6090r98r$19.00 Q 1998 Elsevier Science S.A. All rights reservedŽ .P I I S 0 0 4 0 - 6 0 9 0 9 8 0 0 3 9 9 - X

( )T. Inaba et al. r Thin Solid Films 316 1998 111]116112

Fig. 1. Electrode arrangement.

Ž .nozzle increases, the terminal voltage V V of theatorch plasma increases nearly in proportion to the gasflow-rate. The longer the appearance plasma lengthL the smaller the voltage gradient E. Fig. 3 showsathe characteristics of the terminal voltage vs. gasflow-rate at 100 A of the arc current I . This tendencyais held in a wide range of the arc current, from 50 to150 A. The gradient of the curves is f sdV rdFF aŽ .VrNlrmin , which is 0.71 VrNlrmin at L s3 cm.aThe following equations are obtained:

Ž .V sV q f ?F 1a a0 F

Ž .V sV qEL, LsL qL 2a f i a

where V and f are a function of I and L ,a0 F a arespectively, V is the voltage drop of both electrodes,fL is the inner plasma length within the torch and Li

Ž .is the total plasma length. Eq. 1 confirms that con-vectional loss increases in proportion to the gas

w xflow-rate 3 .

Fig. 2. Structure of plasma torch.

Fig. 3. Terminal voltage vs. Ar gas flow-rate.

3.2. Voltage gradient

In Fig. 4, the voltage gradients of the torch plasmaat Fs4, 12 and 20 Nlrmin are plotted as a functionof the arc current I . The voltage gradient is decidedaas dV rdL along the column. As the current in-a acreases or the gas flow-rate decreases, the voltagegradient decreases. However, this effect becomes lessas the current increases or the gas flow-rate de-creases. It is expected that the voltage gradient willremain mostly unchanged at)200 A of current andat-4 Nlrmin of gas flow-rate.

4. The estimation of radial temperature distributionwithin the plasma by using photo-radius

4.1. Relation of photo-radius ¨s. current and gas flow-rate

Fig. 5 shows the characteristics of the photo-radiusŽ .R cm vs. current at Fs4, 12 and 20 Nlrmin. Thep

photos of the plasma were taken in a specified condi-

Fig. 4. Voltage gradient vs. current.

( )T. Inaba et al. r Thin Solid Films 316 1998 111]116 113

Fig. 5. Photo-radius vs. current, X is the length from the front ofthe torch.

tion with TMAX-400 KODAK film, ND-400 filter,fs5.6 exposure scale and 1r125 s exposure time inorder to define distinctly the radius of the plasma. Ineach case, the photo-radius is gradually saturated asthe current increases. As the gas flow-rate increases,the photo-radius narrows by a constant value. When

2Ž .the average current density of the plasma J Arcmis constantly maintained, the photo-radius should beproportional to the square root of the current as

' Ž .'R s Irp J A I 3p

However, the actual average current density is notconstant, but a small amount increases with the arccurrent. So, the photo-radius increases a little lessthan the square of the arc current.

4.2. Estimation of a¨erage temperature

Assuming the torch plasma has a constant tempera-ture distribution over the cross-section, the average

2Ž .arc current density. J Arcm can be calculatedŽ .from Eq. 3 as follows:

2 Ž .JsI r p R 4� 4a p

Ž .Using the voltage gradient E Vrcm at each cur-rent and gas flow-rate in Fig. 4, the average electrical

Ž .conductivity s Srcm can be evaluated as

Ž .ssJrE 5

Ž . Ž .T 'T s 61

From the relationship of s and T reported byw xDevoto in Fig. 6 4 , we can obtain the average tem-

perature T , as a function of s .1

Fig. 6. Electrical conductivity and thermal conductivity vs. temper-ature of Ar gas.

4.3. Estimation of radial temperature distribution

The energy balance in the arc column consists ofElenbaas]Heller’s equation, radiant energy and con-vection loss as follows:

1 ­ ­ T 2 Ž .rk yQqs E yP s0 7cž /r ­ r ­ r

Ž .where r cm is the radial distance from the columnŽ .center, E Vrcm is the axial voltage gradient mea-

Ž 3.sured in this time, Q Wrcm is the radiant energyŽ . Ž .density, T K is the temperature, k Wrcm?K is the

Ž .thermal conductivity, s Srcm is the electrical con-Ž 3.ductivity and P Wrcm is the convection loss. Onc

this experimental condition, Q can be ignored be-cause of the relatively lower temperature. P is as-csumed to be negligible inside the surface of thephoto-radius, but significant outside it due to the highspeed of the gas flow. P is proportional to thectemperature difference between in and out of theboundary, to the peripheral length of the boundaryand to the gas flow-rate.

Employing the boundary condition at the center ofrs0 that

­ T Ž .TsT , s0 at rs0 80 ­ r

where T is the central temperature, the radial tem-0perature distribution from rs0 to R in the arcpcolumn can be estimated. By calculating the circum-ferential integral of the temperature distribution, the

Ž .averaged temperature T K in the arc column can2be shown as follows:

R1 p Ž . Ž .T ' 2p rT r dr 9H2 2p R 0p

( )T. Inaba et al. r Thin Solid Films 316 1998 111]116114

Ž .Fig. 7. Temperature distribution in column influence of current .

So, in this article the estimation was carried outunder the following assumptions:

1. The convection loss inside of R can be negligi-pble.

2. No electrically conductive region exists outside ofthe R .p

3. The torch plasma is uniform along the axis.

The propriety of these assumptions will be discussedin the future.

4.4. Estimation of radial temperature distribution

The influence of the current and the gas flow-rateon the temperature distribution is shown in Fig. 7 andFig. 8, respectively. From Fig. 7, it is estimated thatthe central temperature T is 11 700 K at I s100 A.0 aMoreover, T increases with current and decreases0

Ž .with radius r. Then T r becomes flatter and theradius of the arc becomes larger with the current. Itcan be seen in Fig. 8 that the central temperatures

ŽFig. 8. Temperature distribution in column influence of gas flow-.rate .

Ž .are the same regardless of the gas flow-rate and T rdecreases with radius r, more rapidly with the gasflow-rate, departing from a model of radially constanttemperature, because the radial temperature gradientis owing to the thermal conduction power or thevoltage gradient. The surface temperature of theplasma at the photo-radius is many thousands ofdegrees. Because the gas flow runs rapidly around theplasma, the peripheral thermal conductive part sur-rounding the central electric conductive area is re-vealed by the gas flow. Therefore the boundary of thetorch plasma is easily seen.

5. Comparison with characteristics of principle wall-stabilized arcs

5.1. Dependence on current and radius of centraltemperature

In case of wall-stabilized arcs of air and SF gases,6Ž .when normalized with I rR Arcm , the centrala p

Ž .temperature T K in the arc column can be de-0scribed by the relation

1Ž .I 1840 K SF3a 6 Ž .T sT , T s , 100 01 01 ½ž /R Ž .p 2300 K air

w xas shown in Fig. 9 5,6 . Though this torch plasma isstabilized by circumferential layers of gas flow, whichis different from a cylindrical wall, T f2190 K is01

Ž .estimated for the present torch plasma if Eq. 10should continue in the direction the dotted line shows.

5.2. Uni¨ersal characteristics of ¨oltage gradient ¨s.current normalized by the radius

Employing the photo-radius R as an arc radius, apuniversal curve of voltage gradient vs. arc current

Fig. 9. Axial temperature vs. currentrradius.

( )T. Inaba et al. r Thin Solid Films 316 1998 111]116 115

Fig. 10. Voltage gradient=radius vs. currentrradius. A is calcu-lated by Shindo, etc., B is measured by H.W. Emmons, C ismeasured by J. Kopainsky. They are in wall-stabilized arcs.

normalized by the radius is obtained in the form ofER ]I rR . Plotting the data of the present torchp a pplasma using solid lines in Fig. 10, it becomes evidentthat the curves are lower than those calculated andmeasured in the wall-stabilized arcs. The curve at 20Nlrmin gas flow-rate is nearly flat. As the gas flow-ratedecreases, the tendency of the curves becomes thesame as those in the wall-stabilized arcs.

6. Results of waste treatment in fly ash

w xFig. 11 7 shows the appearance of a glassified slagof molten fly ash, which is obtained by torch plasmatreatment.

6.1. Hardness of the surface

The slag is melted and the glass set to the inside.The surface has a skin with a hardness of 6 Mohs.

6.2. Solubility tests considering pH

The materials were stirred for 6 h in the waterwhich was adjusted to the pH to the pure distilled

Ž .Fig. 11. Slag of molten fly ash right .

water conforming to bulletin No.13 of the Environ-ment Agency, then the solubility tests were carriedout by using reagents. The test results with regard tonine inspection items on heavy metals, such as ironand copper, are shown in Table 1. Comparing all thesolution of the original fly ash and the solution of themolten slag, the values of the solution of the slag aremuch smaller than those of the solution of fly ash.Therefore it can be concluded that the melting methodusing torch plasma is superior to the ordinary treat-ment. The values partially differ a little according tothe pH of nitric acid, although it seems there is nonoticeable difference between pH 6 and 8. While, alittle leakage was seen in comparison with pure water

w xin pH 7 8 .

6.3. Electrical conducti¨ity

Fig. 12 and Table 2 show electrical conductivity ofthe solution, which indicates the degree of pollutionin the solution. The electrical conductivity of thesolution of the slag becomes 1r270 at pH 6 and1r460 at pH 8, in comparison with those of thesolution of the fly ash. In this connection, it is 1r640in pure water at pH 7. In all states, the amount ofleakage in the solution of both pH 6 and 8 is higher

Table 1Results of solubility tests of molten and pure fly ash

Limitation ofSolution of molten slag Solution of fly ashdetection.ppm pH 6 pH 8 pH 6 pH 8

Zn 0 0 0 0 0Fe 0 0 0 0 0.2Cu 0 0 0.5 0.5 0.5Nitrous acid 0.05 0.05 0.4 0.2 0.02COD 20 20 50 40 0Residual Cl 0 0 0.2 0.2 0.01

Ž .Cr VI 0 0 0.2 0.2 0.05Phosphoric acid ion 0 0 0.3 0.3 0.2Nitric acid 1 1 11 13 1

( )T. Inaba et al. r Thin Solid Films 316 1998 111]116116

Table 2Electric conductivity of molten and pure fly ash

Solution Pure distilled Solution of ca. 208 Cwater molten slag Solution ofŽ . Ž .mSrcm mSrcm fly ash

Ž .mSrcm

pH 6 } 189.2 51 400pH 7 1.21 24.6 15 700pH 8 } 107.8 49 200

than in pure water. So, slag has an extremely goodelusion-proof capability in regard to some heavy met-als.

7. Conclusions

The change of the terminal voltage and the voltagegradient of Ar torch plasma were examined as afunction of the gas flow-rate. The photo-radius wasobtained from photographic views and the radial tem-perature distribution in the torch plasma was esti-mated by using the measured voltage gradient. Fi-nally, the results of the torch plasma were comparedwith the characteristics of wall-stabilized arcs. Themain results are as follows:

1. The terminal voltage of the torch plasma is 30]60V at 50]150 A of current and 4]8 cm of plasmalength, increasing linearly with the increase of thegas flow-rate. This hints that convectional lossincreases in proportion to the gas flow-rate.

2. The voltage gradient of the torch plasma is 2]4Vrcm, increasing with the gas flow-rate and de-creasing with the current. However, it will remainmostly unchanged at)200 A of the current andat-4 Nlrmin of the gas flow-rate.

Fig. 12. Electric conductivity of molten and pure fly ash. PW ispure distilled water, SS is the solution of the molten slag and SF isthe solution of the fly ash.

3. The radial temperature distribution in the torchplasma becomes flatter and more similar to aradially same temperature model, as the gasflow-rate decreases, or as the current increases.The temperature of the surrounding surface ofthe plasma is thousands of degrees and it forms adistinct boundary.

4. The central temperature in the torch plasma de-pends little on the gas flow-rate. It increases

Ž .1r3nearly in proportion to I rR for the currenta p

I and the photo-radius R . This is the same asa pthe wall-stabilized arcs, such as air and SF gases.6

5. The following results were obtained by meltingthe disposal with torch plasma: Fly ash, by heatingit with torch plasma, can be molten into a slagwith a skin hardness of 6 Mohs. The slag has anextremely good characteristics against some heavymetals.

6. Though the solution of the original fly ash andthe solution of the molten slag were adjusted topH 6 and 8, a difference between the twoconcerning solubility and electrical conductivitywas not evident. However, a little leakage oc-curred in comparison with pure water in pH 7.

Acknowledgements

The authors wish to thank Prof. N. Igari for hisfruitful suggestions and the students belonging to thePower Energy Lab. of Chuo University for their ex-perimental efforts.

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