Discharge Characteristics of DC Arc Water Plasma for Environmental Applications

Transcription

1 Plasma Science and Technology, Vol.14, No.12, Dec Discharge Characteristics of DC Arc Water Plasma for Environmental Applications LI Tianming ( ), Sooseok CHOI, Takayuki WATANABE Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Yokohama , Japan Abstract A water plasma was generated by DC arc discharge with a hafnium embedded rodtype cathode and a nozzle-type anode. The discharge characteristics were examined by changing the operation parameter of the arc current. The dynamic behavior of the arc discharge led to significant fluctuations in the arc voltage and its frequency. Analyses of the high speed image and the arc voltage waveform showed that the arc discharge was in the restrike mode and its frequency varied within several tens of kilohertz according to the operating conditions. The larger thermal plasma volume was generated by the higher flow from the forming steam with a higher restrike frequency in the higher arc current conditions. In addition, the characteristics of the water plasma jet were investigated by means of optical emission spectroscopy to identify the abundant radicals required in an efficient waste treatment process. Keywords: thermal plasma, water plasma torch, arc movement, restrike frequency PACS: Hn DOI: / /14/12/11 1 Introduction Thermal plasma technology has been established as playing an important role in many industrial fields such as plasma cutting and welding, plasma spraying, waste treatment, and synthesis of nanoparticles [1 4]. The most widely used thermal plasma source is the DC plasma torch operated in the non-transferred arc mode at atmospheric pressure. A conventional nontransferred arc is generated with the injection of plasma-forming gas between a rod-type cathode and a nozzle-type anode, which has to be cooled sufficiently with water to avoid the melting problem on the anode surface caused by a strong heat flux from the arc discharge [5]. For this reason the typical thermal plasma generation system requires complex equipment such as a gas supply unit for the plasma forming gas and a coolant circulation unit for preventing electrode melting. Thus, an efficient and simple plasma-generation system is required for the practical applications of a thermal plasma. A new plasma torch used in the present work generates 100% water plasma without any additional steam generator [6,7]. The way of generating plasma supporting steam in the water plasma torch is that water in the reservoir is heated up and evaporated in the anode region by strong heat transferred from the arc discharge. At the same time, the electrodes are cooled by the evaporated water. Therefore, such an electrode configuration and plasma forming gas generation do not require additional water cooling and gas supply units. In addition, the distinctive steam-generation method provides the portable lightweight plasma generation system with a significantly high energy efficiency of more than 86% [6]. These unique features of the proposed plasma-generation method have not been readily achieved by other methods, and they allow simple and efficient water plasma generation. The water plasma is a kind of thermal plasma which provides high enthalpy to enhance reaction kinetics and chemical reactivity. Moreover, the water plasma is sustained by 100% steam as the plasma forming gas which is suitable for waste treatment in the industrial field due to its abundant oxygen and hydrogen radicals [6]. The purpose of this study was to investigate the discharge characteristics of DC water plasma in order to find the proper operating conditions for the waste treatment process. Special attention was devoted to the examination of the dynamic behavior of the arc discharge and restrike frequency of the water plasma system. Those show the unique characteristics different from the conventional non-transferred DC plasma torch. 2 Experiment The experimental setup of the water plasma generation system is shown in Fig. 1. In the cathode, a small hafnium rod with 1 mm in diameter was embedded into the main copper rod, and the nozzle-type copper anode was a 1.5 mm in inner diameter. The arc current

2 Plasma Science and Technology, Vol.14, No.12, Dec was set between 5 A and 7 A, and the arc voltage was recorded within a range of between 100 V and 300 V fluctuating strongly. Since the arc discharge was unstable in the early stage of the plasma generation according to the water feeding rate, it was fixed after 30 min from the moment of arc ignition to obtain a stable arc discharge and reliable experimental data. plasma torch. The axial movement of the arc was confirmed from the pictures taken by the high speed camera. The arc was bent in the axial direction and an anodic arc root appeared on the nozzle surface when the arc came out from the water plasma torch interior. According to these high speed images, the water plasma has a unique arc voltage fluctuation mechanism that is different from the conventional non-transferred plasma torch. The spot of arc attachment on the anode surface is moved by the drag force lengthening the arc column, and it causes an increase in the arc voltage in the typical non-transferred plasma torch [5]. On the other hand, the anodic arc spot of the water plasma is fixed on the nozzle exit surface after the arc comes out from the inner chamber of the water plasma torch. Instead of movement of the anodic arc spot, the arc column is more bent, increasing its length and the arc voltage until it reaches a maximum length. Fig.1 Experimental setup of the water plasma system (color online) The arc voltage was recorded by a digital oscilloscope (KEYENCE, GR-7000) at a sufficiently high sample rate of s 1 considering the arc fluctuation frequency of several tens of kilohertz. A high speed video camera (NAC, MEMRECAM GX-8) was used to observe the arc movement at the nozzle exit with a high frame rate of s 1. A thermocouple was inserted into the water reservoir to monitor the water temperature during the experiment because the vapor pressure which affects the stability of steam supply would change along with the water temperature. Optical emission spectrometry (OES) of the DC arc water plasma jet was performed to measure the chemical radicals which are useful in the waste treatment process. In the OES measurement, a spectrometer and an optical system were used instead of the high-speed camera. The measurement point on the central axis of the plasma jet above the nozzle exit had been focused using a lens and an optical fiber which was 200 µm in diameter. The spectrometer (ihr 550, HORIBA) was set for a 0.02 mm resolution with a grating of 150 mm 1 and 1800 mm In the voltage waveform presented in Fig. 3, a periodic sawtooth shape is clearly found. The arc voltage fluctuation indicates the dynamic behavior of the arc discharge in the DC arc plasma torch. The arc movement is influenced by two forces; the gas drag force pulling the arc outside the torch is caused by the incoming gas flow and the electromagnetic Lorentz force pushing the arc inside the torch is caused by the arc current density and self-induced magnetic field [8,9]. However, the arc column has been mainly operated by the drag force in the water plasma torch due to a significantly small arc current in contrast to the typical DC Results and discussion Arc movement The side views of the plasma arc ejected from the nozzle exit are shown in Fig. 2. The time interval between photographs was 3.3 µs. In order to obtain a clear image of the arc in the plasma torch exterior, a relatively high arc current of 13 A and accordingly a high water feeding rate of 500 ml/h were used to generate a strong arc discharge, cooling down the water 1098 Fig.2 Images of the periodic variation of the plasma arc from a side view at the nozzle exit at time intervals of 3.3 µs with torch operation of 13 A and 500 ml/h for arc current and water feeding rate, respectively Fig.3 Voltage waveform at an arc current of 13 A and water feeding rate of 500 ml/h

3 LI Tianming et al.: Discharge Characteristics of DC Arc Water Plasma for Environmental Applications plasma torch operated with several hundred amperes. Therefore, the arc voltage is increased until the maximum point without remarkably disturbing the Lorentz force in a piece of the sawtooth. The periodic frequency of the arc voltage from valley to peak is several tens of kilohertz according to the data recorded by the oscilloscope, and it is in good agreement with the photographs of the plasma arc column in Fig. 2. Therefore, it has been clarified that the drag force of the gas flow enhances the arc length and increases the arc voltage as shown in Fig. 4. plasma becomes larger at a higher arc current. The picture of the water plasma jet consists of pixels and each pixel has 8 bits. Luminescent areas for different arc current conditions are presented in Fig. 6 after calculations in binary for the water plasma jet pictures with a threshold at 220 in brightness. It is considered that high brightness indicates high temperature. An enhanced high temperature area by increasing the arc current is important in the waste treatment process. A high decomposition rate for organic waste treatment had been obtained under high-arc-current conditions due to a large high-temperature area of the water plasma jet [2]. Fig.5 Photographs of the water plasma jet according to the arc current at a water feeding rate of 248 ml/h (color online) Fig.4 Arc movement mechanism (color online) From the measured voltage waveform, it is found that the ratio between the maximum value from valley to peak and the mean voltage is more than 100%. Therefore, the arc discharge of the water plasma can be defined as the restrike mode [10]. In the restrike mode, the anodic arc spot is first placed at a point closest to the cathode inside the water plasma torch. At this time, the arc voltage reaches its minimum value, because the arc length between the cathode and anode is the shortest. Then, the arc column is lengthened by the drag force of the steam flow in the axial direction towards the nozzle exit, causing an increase in the arc voltage. Finally, the arc voltage reaches the highest point with the largest curvature and the longest column length. An electric field is provided between the edge of the arc column and the anode wall, and a breakdown process may occur when the given electric field is higher than a critical field at the longest arc column, which distributes the highest electrical potential between the cathode tip and the anode wall [5]. Therefore, the reattachment of a new arc column is created between the cathode tip and the anode surface with a short length of the arc column. In this way, the arc repeats the movement with a periodic voltage fluctuation. 3.2 Effect of arc current The photographs of the plasma jet at different arc currents are shown in Fig. 5. The arc jet of the water Fig.6 Luminance area of the water plasma jet according to the arc current at a water feeding rate of 248 ml/h The voltage waveforms at different arc currents are also shown in Fig. 7. This figure indicates that the period between the valley and peak of the arc voltage is shortened by increasing the arc current, and such an effect of the arc current on the fluctuation frequency of the arc voltage is directly shown in Fig. 8. The arc frequency increases with the arc current, because the higher Joule heating acting on the water to be evaporated leads more easily to an enhancement of the steam drag force, thus the higher arc current increases the arc length in a relatively short time. In a conventional plasma torch, the arc frequency is decreased by increasing the arc current owing to the increase in the electromagnetic force which prevents the growth of the arc column. In our water plasma torch, however, the effect of the arc current on the restrike frequency is directly 1099

4 Plasma Science and Technology, Vol.14, No.12, Dec contrary to the conventional plasma torch due to the enhanced steam pressure in high-arc-current conditions. The fluctuation of the electrical arc discharge causes non-uniform heating and transport of the injected plasma-forming gas. Since the temperature and velocity fields of the plasma jet are changed more slowly than the dynamic movement of the arc discharge, the high restrike frequency stabilizes the thermal plasma characteristics [8,9]. Therefore, the water plasma torch operating at a high current can provide a uniform hightemperature flame to obtain high decomposition efficiency with a reliable waste treatment process. because water had been dissociated into H and OH radicals in the plasma flame region. In addition, the excitation temperature of the water plasma was expected to be 4000 K in the nozzle exit [5]. The abundant radicals of H and OH at high temperature led to active chemical reactions. These features of the water plasma are very important for practical applications including organic waste treatment and efficient conversion of nondegradable hazardous waste into benign materials. Fig.9 Emission spectra of water plasma at an arc current of 6 A and a water feeding rate of 350 ml/h 4 Conclusions Fig.7 Measured voltage waveform according to the arc current at a water feeding rate of 248 ml/h (color online) Fig.8 Restrike frequency of the arc discharge according to the arc current at a water feeding rate of 248 ml/h 3.3 Spectroscopic characteristics The spectral lines of the water plasma jet were clearly observed at H γ (434.0 nm), H β (486.1 nm), H α (656.3 nm) and OH (306.4 nm) as shown in Fig. 9, The arc voltage of the DC arc water plasma is varied periodically by the axial movement of the arc column in the restrike mode. This fluctuation can be controlled by the operating parameter of the arc current which influences the steam drag force in the water plasma system. The restrike frequency is increased at the high arc current due to enhanced water evaporation, while it is decreased in the conventional plasma torch. The larger arc jet volume is obtained at the higher arc current, because the higher enthalpy is transferred into the steam increasing the gas pressure. An efficient and reliable waste-treatment process is expected by understanding the relationship between the thermal plasma characteristics and the operation parameter of the DC arc water plasma torch. References 1 Fauchais P. 2004, J. Phys. D: Appl. Phys., 37: 86 2 Narengerile, Yuan M H, Watanabe T. 2011, Chem. Eng. J., 168: Narengerile, Saito H, Watanabe T. 2009, Thin Solid Films, 518:

5 LI Tianming et al.: Discharge Characteristics of DC Arc Water Plasma for Environmental Applications 4 Yuan M H, Narengerile, Watanabe T, et al. 2010, Env. Sci. Technol., 44: Moreau E, Chazelas C, Mariaux G, et al. 2006, J. Therm. Spray Technol., 15: Watanabe T. 2005, ASEAN J. Chem. Eng., 5: 30 7 Watanabe T, Shimbara S. 2003, High Temp. Mater. Processes, 7: Leblanc L, Moreau C. 2001, J. Therm. Spray Technol., 11: Trelles J P, Pfender E, Heberlein J V R. 2007, J. Phys. D: Appl. Phys., 40: Duan Z, Heberlein J. 2002, J. Therm. Spray Techol., 11: 45 (Manuscript received 3 November 2011) (Manuscript accepted 6 January 2012) address of corresponding author Takayuki WATANABE: watanabe@chemenv.titech.ac.jp 1101