Dual Torch Plasma Arc Furnace for Medical Waste Treatment

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2007 Plasma Sci. Technol. 9 709

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Dual Torch Plasma Arc Furnace for Medical Waste Treatment

LIU Bin (��)1, M. KIKUCHI1, LI Heping (���)2, T. IWAO3, T. INABA1

1 Graduate School of Science and Engineering, Chuo University, Tokyo 112-8551, Japan2 Department of Engineering Physics, Tsinghua University, Beijing 100084, China3 Faculty of Engineering, Musashi Institute of Technology, Tokyo 158-8557, Japan

Abstract In this paper, characteristics of a dual torch plasma arc used for hazardous wastetreatment and operated at atmospheric pressure are studied, and also compared with those of themulti-torch plasma arc and the single torch plasma arc. The dual torch plasma arc is generatedbetween the cathode and anode with argon as the working gas. The temperature distributionsof the plasma arc are measured using a spectroscope and line pair method with the assumptionof local thermodynamic equilibrium (LTE) for the DC arc current I = 100 A and argon flow rateQ= 15 slpm. The measurements show that the temperatures of the dual torch arc plasma in theregions near the cathode, the anode and the center point are 10,000 K, 11,000 K and 9,000 K,respectively. And the high temperature region of the multi torch plasma arc is of double or muchwider size than that of a conventional dual torch plasma arc and single plasma torch. Based onthe preceding studies, a dual torch plasma arc furnace is developed in this study. The measuredgas temperature at the center region of the argon arc is about 11,000 K for the case of I = 200 Aand Q =30 slpm operated in atmosphere.

Keywords: dual torch, plasma arc furnace, medical waste

PACS: 52.77.-j

1 Introduction

Plasma arcs have a lot of outstanding features,such as high temperature [1∼5], intensive radiation [6,7]

and high energy, etc. Although different conven-tional plasma torches, e.g., the transferred and non-transferred arc plasma torches, have been developed,there are still some problems for ignition and genera-tion of high temperature and highly intense arcs for haz-ardous waste treatment [8∼12]. For the non-transferredtorches although the plasma jet in a non-electricallytransferred type can treat electrically non-conductingmaterial, it is difficult to generate a high temperaturearc. In the case of the plasma torch of a transferredtype [5], although a high temperature arc can be gen-erated, it is difficult to treat the non-conducting ma-terial. Therefore, it is important to solve this problemimmediately. Specifically, medical waste includes dif-ferent kinds of materials, such as knives, injectors, in-jector needles, diaper, blood etc. Normally the plasticis non-conducting material, while metals such as knivesand injector needles are electrically conducting. Be-cause the injector needle is very sharp, it is danger-ous for workers and citizens. It is not enough to treatthe injector needle by the normal incinerator, becausethe temperature is too low to treat the injector needlemade of stainless steel. On the other hand the off-gas should be clean. The multi torch plasma arc cangenerate high temperature and also can treat the non-conducting material, because it has a higher tempera-

ture arc column, even if it is a kind of non-transferredtype. The multi torch plasma arc is a more effectivedevice for treating the hazardous waste. In this paper,the temperature distribution of the multi torch plasmaarc at a current near 100 A is measured for the treat-ment of hazardous waste [1,2]. The result is comparedwith those of the conventional dual torch [1] and singletorch [12]. However, the dual torch plasma arc is one ofthe non-transferred type arcs, i.e. it is not producingJoule heating of materials themselves. Therefore, thetemperature of the arc column must be high enough totreat the metals. In addition, a dual torch furnace wasdeveloped for hazardous waste treatment.

2 Experimental arrangement

Fig. 1 shows the experimental setup of dual torchplasma arc. The crossing angle between cathode (‘�’)and anode (‘⊕’) is 98 degrees. The argon is filled in thechamber at a pressure of 0.1 MPa. The multi, dual andsingle torch plasma arc is generated between the cath-ode and anode. The multi torch plasma arc has morethan three gas flows from two electrode nozzles. In thispaper a 3rd nozzle at the middle top of the chamberis adopted. The conventional dual torch plasma archas only two gas flows from the nozzles. In the caseof multi and dual torch the working gas is argon andits flow rates at cathode and anode, Fc and Fa, are setsteadily to be 15 slpm from the nozzle electrode. In thecase of single torch plasma arc the working gas is also

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Fig.1 Schematic diagram of the experimental setup

Fig.2 Multi and dual torch plasma arc mode

argon and its flow rate at cathode is set steadily to be15 slpm. The middle nozzle gas in the case of multitorch plasma arc is also argon. The maximum DC arccurrent of the dual and multi torch plasma arc is 100 A.The total arc length L is 3 cm to 5 cm.

3 Arc mode

The arc mode is changed by changing the gas flowsurrounding the arc [13,14]. Fig. 2 shows the multi andconventional dual torch plasma arc modes. The arc ap-pearance changes from a V (dual torch) to a Y (multitorch) mode. This means that the high temperatureregion increases from V mode to Y mode. The elec-trode jet from the anode is larger than that from thecathode. Therefore, the arc mode from anode is alwaysstrong. The size of the anode spot was observed to belarger than that of the cathode spot. Fig. 3 shows anarc image of the single torch mode taken by a camera.

4 Arc voltage

The arc voltage V as a function of arc current at dif-ferent arc lengths is shown in Fig. 4. The arc voltageof single torch plasma arc is almost constant of around30 V against the arc current for L=4 cm. However, thevoltage of the multi and dual torch plasma arc is about1.3∼ 1.5 times higher than that of the single torch arcand decreases with arc current. These characteristics

Fig.3 Plasma arc mode of single torch

Fig.4 Arc voltage as function of arc length and current

are similar to a wall-stabilized arc. Because the anodehas a nozzle, the radius of plasma arc is constricteddue to the pipe type gas flow. Therefore, the voltage ofmulti and dual torch plasma arc is larger than that ofsingle torch plasma arc. So both higher voltage and in-put power can be obtained in dual and multi arc cases.The voltage and input power of multi torch plasma arcis 1.1, 1.4 times than that of the dual torch plasma arcand the single torch plasma arc, respectively, in the caseof L= 3 cm and I = 100 A. The arc voltage increaseswith the arc length.

5 Temperature of plasma arc

The temperature of dual torch plasma arc at 100 Aunder a condition of LTE was measured by using a spec-troscope. The intensity of spectrum Ar II line emittedfrom the torch plasma is shown in Fig. 5. The intensityof Ar I is about 3 times as high as that of Ar II. The tem-perature distribution was calculated by using the linepair method. Fig. 6 shows that the temperature distri-butions of the (a) single, (b) dual, and (c) multi-torchplasma arc at an arc current of 100 A. The temperatureof the single torch near the cathode is about 12,000 K.The temperature of dual torch is ca. 10,000 K near thecathode and ca. 11,000 K near the anode. The centerpoint of the plasma arc, which is the attachment pointbetween the plasma arc from the cathode and anode, is

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Fig.5 Intensity of spectrum Ar II

Fig.6 Temperature distribution of torch plasma arc in ar-

gon at 0.1 MPa

ca. 9,000 K. In the multi torch plasma case the tem-perature is about 10,000 K or less at the center point.The temperature distribution area of multi plasma arcis double or much wider than those of the dual andsingle torch plasma arc.

Fig.7 Heating efficiency as a function of current

6 Heating efficiency of torch

plasma

Fig. 7 shows the heating efficiency as a function ofarc current for different torches. da is a distance be-tween the arc center and the neutral bar. The heatingefficiency ηh is calculated by

ηh = Qwater/P, (1)

where Qwater is the heating power to the centerpole/anode [in W] and P = V I is input arc power[in W].

The heating efficiency of dual torch is 12% to 17%,while the heating efficiency of single torch is 16% to23%. On the other hand, for the dual torch the arc volt-age fall at the electrode is 6 V which is higher than thatfor the single torch. This means that the efficiency ofthe dual torch could be much higher. Furthermore, thedual torch plasma arc can treat different kinds of ma-terials, such as non-conducting materials, much morethan the single torch arc can treat. Therefore, the dualtorch plasma and multi torch plasma can be expectedto achieve higher efficiency in the future.

7 Development of dual torch

plasma furnace

Fig. 8 shows a dual torch plasma arc furnacefor�medical waste treatment developed at the ChuoUniversity, Musashi Institute of Technology and ShinwaIndustry Co. Inc. in Tokyo, Japan, supported byTokyo Metropolitan Government. Fig. 9 shows adual torch plasma arc mode. The chamber size isφ1500 mm× 2200 mm (length). The furnace has twoelectrodes, cathode and anode. The crossing angle ofthe electrodes is 60 degrees. The power source capacityis DC 300 kW. This furnace, power source, gas treat-ment device, and utility of gas and water can be put ona commercial 5.5 ton truck to allow treatment of wasteson site. Therefore, this system contributes to avoid theexpansion of infection. This furnace is also designed tohave a third nozzle called a multi-torch plasma arc to

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Fig.8 Dual torch plasma arc furnace

Fig.9 Dual torch plasma arc mode

treat waste efficiently and obtain a wider high-temperature region. Temperature of dual torch plasmaarc at the center point was measured to be 11,000 K inthe case of 200 A in arc current and 30 slpm in plasmagas flow rate of argon gas in air atmosphere at 0.1 MPa.

8 Summary

The main conclusions obtained in this study are asfollows:

a. The arc appearance in the dual and multi plasmatorches changes from a V mode (dual torch) to a Ymode (multi torch). The high temperature regionincreases significantly in the case of the multi torchplasma arc.

b. The temperature of the dual torch is ca. 10,000 Knear the cathode, ca. 11,000 K near the anode, andca. 9,000 K at the central point between the electrodesat an arc current of 100 A.

c. The temperature of the multi torch plasma isabout 10,000 K or less at the central point between theelectrodes at an arc current of 100 A.

d. The temperature distribution of the multi plasmaarc is double or much wider than those of the dual andsingle torch plasma arc.

e. The heating efficiency of the dual torch arc to thecentral pole is about 12% to 17% in the experiment.

f. A dual torch plasma arc furnace was developedfor medical waste treatment and its temperature at thecentral point was measured to be 11,000 K in the caseof an arc current of 200 A and an argon plasma gas flowrate of 30 slpm for an air atmosphere at 0.1 MPa.

Acknowledgement

The authors wish to thank Prof. J. HEBERLEIN ofUniversity of Minnesota and Prof. I. MIYACHI of theAichi Institute of Technology for their fruitful sugges-tions. This research was supported by the Institute ofScience & Engineering of Chuo University, Musashi In-stitute of Technology, Shinwa Industry Co. Inc., TokyoMetropolitan Government, and the Japan Society forPromotion of Science.

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(Manuscript received 5 June 2007)E-mail address of LIU Bin: lbyf@hotmail.com

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