Proposal of Treatment for Hazardous Wastes Using the Highly Concentrated Radiation from Torch Plasma

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1 , pp Proposal of Treatment for Hazardous Wastes Using the Highly Concentrated Radiation from Torch Plasma Toru IWAO, Hirokazu MIYAZAKI, Takayuki ISHIDA, Yafang LIU and Tsuginori INABA The Graduate School of Science and Engineering, Chuo University, Tokyo, Japan. (Received on July 5, 1999; accepted in final form on October 20, 1999) The plasma torch that can usually reduce the waste and dissolve the iron and so on has very useful characteristics. It is a kind of the stabilized arc plasma that can be easily controlled. It has been examined considering the influences or various parameters, such as the current, the gas flow-rate and the plasma length. In this paper, we measured the radiation power emitted from the Ar plasma torch. The radiation power increased in proportion to the th power of the current at 1, 2 cm of appearance plasma length. And it increased in proportion to the th power of the gas-flow rate between 4 20 Nl/min. Then, we measured the temperature of the radiation spot collected by reflectors using the thermocouple. It increased in proportion to the th power of the current at 1 cm and 2 cm in the plasma length. Moreover, the total temperature is calculated as K in a torus model. We proposed the treatment for the hazardous wastes using the highly intense radiation of Ar torch plasma collected by reflectors and concentrated by the lens. And, we proposed the plasma treatment of the high temperature and radiation power for the hazardous wastes by using the plasma torch. The plasma torch ordinary has the high temperature and plenty of radiation power. Therefore, when both of the characteristics are combined usefully, we can get the many more benefits; the in-put power is used little, hazardous wastes are treated by the high temperature and hazardous gas from the wastes is treated by the radiation power. KEY WORDS: arc plasma; plasma torch; radiation power; high temperature; plasma treatment. 1. Introduction The problems of the disposal hazardous wastes have especially become very important in recent years. The application of the arc plasma to waste treatment has attracted attention in Japan, 1) and the world, 2) because of the volume decrease and being a sharply harmless composition. The plasma torch that can usually reduce the waste and dissolve the iron and so on has very useful characteristics. We have examined the plasma treatment for fly ash and asbestos using the high temperature characteristics of the torch plasma. The slag can be harmless at ph 6 8 at the leakage tests of the solution. 1) The radiation power has a lot of useful characteristics such as a good conduction in the vacuum as well as in the air, a clear circumstance with no chemical combustion needing any oxygen gas, a focusing on the radiation flux into a small spot and so on. We can use it as a clean source for heating materials, especially treating hazardous wastes. In this case, it s expected that radiation becomes greater. However there have been few reports 3) on the radiation power from the torch plasma in the past. Actually, while the radiation power is a clean source and has a lot of rays, it has been ignored as a loss, so far. So, we have examined considering the influences or various parameters, such as the current, the gas flow-rate and the plasma length to get the benefits from the radiation power. 2. Experimental Equipment to Generate Torch Plasma Major specifications of the experimental equipment, the plasma torch, to generate torch plasma employed in this study are as follows: The maximum rating of the actual electric power source is DC 150 V, 400 A in current, with a no load voltage of 300 V. The torch plasma is produced between a negatively charged Th W tip electrode and a positively charged the water-cooled stainless-steel thick disc through a water-cooled nozzle. The size of the plasma chamber is 0.5 m in diameter and 0.6 m in length. The working gas of the torch plasma is Ar, the gas flow-rate is varied from 0 to 20 Nl/min, as shown in Fig. 1. The appearance plasma length is varied from 0 to 0.1 m. 4) 3. Principles of Plasma Torch Operations Figure 2 shows the experimental arrangement used in this study. First, the torch is settled just over the 0.06 m-diameter stainless steel anode electrode inside a crucible filled with Ar gas in the plasma chamber. The reason for using Ar gas is to eliminate the influence of chemical change caused by ISIJ

2 Fig. 1. Profile of the plasma torch. Fig. 3. Settlement of power meter. Fig. 2. Experimental arrangement. Fig. 4. Settlement of lens. interaction of the processed materials with the surrounding atmosphere. In order to generate the arc plasma used in the torch, a pilot arc is started by a high-frequency discharge initiated between the cathode electrode and the nozzle. Then, using the electric conductivity of the pilot arc, the main arc is started in the electrode-crucible gap. Once the main arc is started, the pilot arc is usually turned off and the torch is upward moved to keep the specified gap from the anode. This main arc is ejected to the opposite anode from the nozzle by the pressure flow of the arc gas and operating gas (Ar gas); however, in this case, the high temperature plasma current is enhanced by efficient use of the thermal pinch produced by the nozzle and the operating gas. In order to prevent the cathode electrode from being worn out by oxidation, Ar gas was used as the plasma gas. 4. Experimental Arrangement Figure 3 shows the settlement of the power meter. The radiation power is measured by the power meter. Figure 4 shows the settlement of the lens. The radiation power emitted from the arc is concentrated by the lens. 5. Radiation Power 5.1. Experimental Methode for Changing Appearance Plasma This experiment is held to be Ar in plasma gas, 1atm in surrounding pressure in thechamber, F 12 Nl/min in gas flow-rate and 5 mm in nozzle diameter of the torch. The experiment was carried out as follows, L a is 0.01 m and 0.02 m, X is 0.58 m and 0.68 m and the current is A at intervals of 25 A. Originally, the radiation emitted from the arc plasma covers the wavelength range from ultraviolet rays, visual rays to infrared rays. The used power meter can measure the wide wavelength range mentioned above Experimental Principle The whole radiation power F is calculated by the following equation in a sphere model: 4pX 2 f (radiation power density) F...(1) where f is radiation power density measured in a unit area of the power meter and X is measurement distance. When the appearance plasma length, L a, is too shorter than X, the light source is regarded as a point one. Because L a is 0.01 and 0.02 m, and L a /X is less than 1/20, we can consider the light source as a point in this report. Collecting both the measured values at X of two distances could decrease the measurement error or dispersion. The longer a measuring distance, X, is, the smaller a measured radiation power density, f, is. But when f is inserted into Eq. (1), F is same value in different X s Radiation Power with Appearance Plasma Length Figure 5(a) shows the experimental value of radiation power emitted from the appearance plasma. The more current increases, the more radiation power increases in proportion to the th power of the current. The more L a increases, the more radiation power increases. The power of nth in the relation of E In can be derived as 2.0 in several tens ampare, 1.7 in several hundreds ampare, 1.2 in several thousands ampare at the stabilized arc radiation power in theory. 5,6) Measurement value of nth to be is almost same to the theoretical one to be 2.0 in the same current range at A Experimental Methode for Changing Gas Flow Rate The Ar flow-rate in plasma gas, at 1 atm in surrounding pressure in the chamber, is held to be F 4, 12, 20 Nl/min in gas-flow rate at 5 mm in nozzle diameter of the torch ISIJ 276

3 Fig. 6. Increasing temperature of radiation spot. Fig. 5. Radiation power from appearance plasma vs. current. Fig. 7. Sum estimated temperature of radiation spots. The experiment was carried out as follows: L a is 0.02 m, X are 0.63 m and 0.68 m and the current is A at intervals of 25 A Radiation Power with Gas Flow-rate Figure 5(b) shows the experimental value of the radiation power emitted from the appearance plasma as functions of the gas-flow rate. The more current and gas-flow rate increases, the more radiation power increases in proportion to the th power of the current. The more gas-flow rate increases, the more radiation power increases. 6. Radiation Spot Concentrated by the Lens When the hazardous wastes are treated by the radiation power, high temperature is most important. The radiation energy can be collected by the reflectors and concentrated by the lens into a small spot. In this paper, when it was collected by the lens, the temperature of it was measured by thermocouple Temperature of Radiation Spot The radiation power emitted from the arc is concentrated by the lens. Therefore, the temperature of the radiation spot is higher than that of the atmosphere. Figure 6 shows the dtemp that is the increment of temperature. The dtemp increases in proportion to the 2.0th 2.1th power of the current. The dtemp for 0.02 m is higher than that for 0.01 m, because the radiation power at 0.02 m is higher than that at 0.01 m. When the current is 150 A at 0.02 m, the dtemp is about 10 K. If the whole radiation power is completely collected at one spot by the reflectors, the sum estimated dtemp, SdT, can be calculated as following equation in a case of 0.02 m and 150 A. Torus model 7) 2 2 π A Area ratio: a times...(2) 2 a π 2 SdT adt 2 a 10 K K,...(3) where A is the measuring distance between the lens and the arc to be 1.73 m, the diameter of the lens, a, to be is 0.15 m. The SdT of the radiation spot concentrated by the lens are estimated to be K at the torus model. Figure 7 shows the Sdt. 7. Proposal of Plasma Treatment Using Highly Intense Radiation Technologies for Hazardous Wastes The problem of the disposal hazardous wastes have especially become very important in recent years. An application of the torch plasma was carried out to melt fly ash or ISIJ

4 Fig. 8. Equipment for plasma-radiation treatment of hazardous waste. asbestos into a glassified slag with a hardness of 6 Mohs. The leakage tests of the solution in ph 6 or 8 were examined. 1) In addition, it seems to be able to solve the problems of some hazardous gases such as CO 2, FCl n, SF 6, liquid such as pure PCB, an impure oil contaminated with PCB, small powder such as asbestos, solids such as dioxin, As, Ka used in MHD. The radiation power emitted from an Ar torch plasma is easily controlled as functions of the gas-flow rate, appearance plasma length and current. When the reduction of waste and dissolution of iron are carried out by using the plasma torch, the radiation power has been ignored as a loss. Nevertheless, the radiation power is a clean source for heating materials. In this case, it s expected that the radiation power becomes greater. 8) 7.1. Plasma Treatment with Radiation Power It is a clean source with little off-gas to heat the hazardous wastes. 9) The equipment of the treatment for hazardous wastes using the radiation power emitted from the Ar torch plasma is proposed as shown in Fig ) The radiation power collected by reflectors is concentrated on hazardous wastes in the chamber(ii) by the lens. The SdT increase dtemp is estimated to become K at 150 A, 0.02 m at torus model. The torch plasma and hazardous wastes can be separated. There is plasma gas such as Ar. That is to say, there is no oxygen gas and then high temperature radiation in the chamber(i). Therefore, the hazardous materials will be melted without any combustion. It causes no off-gas in the chamber(i), which should be dealt with an expensive special facilities. The reason for using Ar gas is to eliminate the influence of chemical reactions caused by interaction of the processed materials with the surrounding atmosphere. The radiation emitted from the arc plasma covers the wavelength range from ultraviolet rays, visual rays to infrared rays. It looks like the sun light. 11) Figure 9 shows the example of intensity due to the Ar torch plasma. Therefore, if this wavelength is used effectively, it can be applied for a lot of fields Plasma Treatment with High Temperature and Highly Intense Radiation by Using Plasma Torch The Ar torch plasma has a characteristic of high temperature such as K at the central part of the arc column 12) even the boundary of the arc is around K at Fig. 9. Intensity of radiation power from torch plasma. Fig. 10. Plasma torch with reflectors. 150 A, 0.02 m. In addition, the radiation power emitted from Ar torch plasma is highly intense such as K of the SdT at 150 A, 0.02 m at the torus model. If the characteristics of the high temperature and the highly intense radiation are used at same time, the efficiency of the plasma torch could be better. In addition, the radiation power has a lot of rays of intensity from the ultraviolet rays to the infrared rays. So, if the hazardous gases are generated, the radiation power could attack them. The electron beam is used at the thermal power station to attack them. 13) Figure 10 shows the equipment of the plasma torch with reflectors. Therefore, if the plasma torch is used, we can get the benefits as follows: (1) CO 2 in the off-gas is reduced because the power is used little and O 2 is not used in the fuel. (2) Hazardous wastes are treated by the high temperature. (3) Hazardous gas could be treated by the radiation power. 8. Future In the future, the radiation spot should be collected by the reflector and then be concentrated by the lens. Moreover, the temperature should be measured at these arrangement. Then, the hazardous wastes should be treated by the radiation spot ISIJ 278

5 9. Conclusions 1) The whole radiation power could be calculated to be 4pX 2 f in a sphere model, where X is measuring distance and f is radiation power density measured by a power meter. 2) The radiation power increases in proportion to the power of th of the current, which is near to 2.0 in theory for wall-stabilized arcs. 3) The radiation power increases with the appearance plasma length and gas flow-rate. 4) The radiation power can be easily to be controlled as functions of the appearance plasma length, current, gas flow-rate and so on. 5) The increment of temperature, dtemp, of the radiation spot concentrated by the lens is 10 K at the torus model at 150 A, 0.02 m and increase in proportion to the th power of the current. 6) The sum estimated temperature, Sdt, of the radiation spot concentrated by the lens and reflectors is K at the torus model at 150 A, 0.02 m and increase in proportion to the th power of the current. 7) The equipment for the plasma treatment of hazardous wastes due to radiation power emitted from torch plasma is proposed. The torch plasma is easily controlled by the current, lens, appearance plasma length and gasflow rate. When they are changed, the radiation power and dtemp could be also changed. Acknowledgments The authors wish to thank Prof. I. Miyachi, Prof. M. Yoda of the Aichi Institute of Technology, Prof. T. Sakuta of the Kanazawa University, Dr. Y. Goda, Mr. K. Ikeda, Ms. S. Furukawa of CRIEPI, Mr. T. Hayashi, Mr. T. Hirano and Mr. H. Yoshida of Chuo University for their fruitful suggestion. This research was supported by the High-Tec Project on the Ultra High Temperature Plasma by the Institute of Science and Engineering of Chuo University. REFERENCES 1) T. Inaba, Y. Watanabe, M. Nagano, T. Ishida and M. Endo: Thin Solid Films, 316 (1998), ) T. Inaba, Proc. Int. Symp. on Environmental Technologies, Atlanta, 1 (1995), 13. 3) S. Fur ukawa, T. Amano, K. Adachi and M. Shibuya: Trans. IEEJ, A119 (1999), ) T. Inaba, Y. Watanabe, T. Ishida and M. Endo: Trans. IEEJ, A118 (1998), ) T. Iwao, T. Inaba and M. Endo: Trans. IEEJ, A118 (1998), ) T. Inaba, S. Kusunoki, T. Iwao and M. Endo: Trans. IEEJ, A118 (1998), 10. 7) T. Iwao, T. Inaba and M. Endo: Proc. IEEJ-A. Ann. Meet., Osaka, (1997), ED ) T. Inaba, S. Kusunoki and M. Endo: Proc. IEEE IAS, 31st Ann. Meet., IEEE, San Diego, (1996), ) K. Adachi, K. Ikeda, T. Amakawa and T. Inaba: CRIEPI, W91302 (1992), ) Y. Ozaki: High Voltage and Power Engineering, Denki Shoin, Tokyo, (1997), ) D. M. Camm and B. Lojek: 2nd Int. Rapid Thermal Conf., (1994). 12) T. Iwao, T. Ishida, T. Hayashi, T. Hirano, M. Endo and T. Inaba: Trans. JIM, 63 (1998), ) Horii: Hoden Handbook on Electrical Discharge, IEEJ, Tokyo, (1998), ISIJ