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Journal: Plasma Devices and Operations

Article ID GPDO 149472

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Plasma Devices and Operations Vol. 00, No. 00, Month 2006, 1–12

Discharge characteristics of a gliding-arc plasma in chlorinated methanes diluted in atmospheric air ANTONIUS INDARTO*, JAE-WOOK CHOI, HWAUNG LEE and HYUNG KEUN SONG Korea Institute of Science and Technology, Clean Technology Research Centre, PO Box 131, Cheongryang, Seoul 130-650, South Korea (Received 18 October 2004) Plasma processing of the chloromethane compounds (methylene chloride (CH2 Cl2 ), chloroform (CHCl3 ) and carbon tetrachloride (CCl4 )) diluted in atmospheric air using a gliding arc has been studied. Various injected initial chloromethane concentrations, total gas flow rates and power frequencies were used as the variables to investigate the discharge characteristics. This paper evaluates the phenomenon of chloromethane processing by gliding-arc plasma. Keywords: Plasma; Gliding arc; Chloromethanes;Alternating-current waveform; Equilibrium voltage; Voltage breakdown

1.

Introduction

The plasma of a gliding arc is widely used now to destroy toxic materials. Many dangerous emissions, such as H2 S [1], N2 O [2], CHCl3 and CCl4 [3, 4], have been investigated and studied. Usually, a high destruction efficiency can be achieved by using this method. The device for the generation of a gliding arc consists of a pair of flat electrodes, which are connected to a power supply. In operation, the arc is ignited at the narrowest part of the gap between the electrodes immediately after breakdown. Breakdown takes place when the electric field in the gap is high enough to ignite the arc. The current of the arc increases very rapidly at moderate voltages sufficient to create a powerful arc that expands upwards along the surface of electrodes and elongates until it can no longer be maintained. At this point, the arc is extinguished and the process is repeated [5]. The number of arcs that will be produced depends on many factors, such as the frequency of the power supply applied, the species of flowing gas and the total gas flow rate. During this arc movement, the destruction of the molecules of hazardous materials simultaneously occurs. Plasma arcs usually have an energy high enough to break strong molecular bonds or to initiate a reaction of stable gas material owing to the high temperature of the flame, high electron density, etc. *Corresponding author. Email: [email protected]

Plasma Devices and Operations ISSN 1051-9998 print/ISSN 1029-4929 online © 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/10519990500494898

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However, there have been only a few papers that have discussed the behaviour of a gliding arc. The results of theoretical and numerical studies performed using many mathematical equations describing a gliding arc have been published in the literature [6–10]. In this paper, we try to explain the physical characteristics of a plasma of compressed air with chloromethane compounds diluted in it. The experiment was carried out with two triangular stainless steel electrodes, which were electrically charged from an alternatingcurrent (AC) power supply. According to a report by the US Environmental Protection Agency, the chloromethane to be destroyed was categorized as a compound of high thermal stability [11]. An analysis was carried out, which was focused on the discharge parameters, such as the equilibrium voltage, breakdown voltage and voltage–current–power (V –I–W ) profile as functions of the chloromethane concentration, total gas flow rate and power frequency.

2.

Experimental set-up

A schematic diagram of the experimental set-up is shown in figure 1. Chloromethane compounds and atmospheric air were used as input gases. The system and the components of the set-up are described in detail in the following sections.

Figure 1.

Schematic diagram of the experimental set-up: MFC, mass flow controller.

Gliding-arc plasma in chlorinated methanes

2.1

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Plasma reactor and power supply

The reactor was made from a quartz-glass tube of inner diameter 45 mm and length 300 mm. The upper part and the bottom of the reactor equipped with Teflon seals consisted of two electrodes made of stainless steel. The electrodes were 150 mm in length. The separation of electrodes in the narrowest section was 1.5 mm. The gas mixture was injected between the electrodes through a capillary (a nozzle tube) of 0.8 mm inner diameter. A thermocouple, located 10 cm above the electrodes, was provided to measure the temperature of the outlet gas. A high-frequency AC power supply (Autoelectric A1831) with a maximum voltage of 10 kV and a maximum current of 100 mA was connected to the gliding-arc electrode to generate a plasma. The frequency of the power supply could be adjusted from 10 to 20 kHz. 2.2

Input gas

The following chlorinated methanes were used as the initial material: (i) methylene chloride: CH2 Cl2 ; molecular weight, 84.93; purity, 99.0%; purchased from the Junsei Chemical Co., Ltd; concentrations, 1, 2, 3 and 4 vol.%; (ii) chloroform: CHCl3 ; molecular weight, 119.38; purity, 99.0%; purchased from the Junsei Chemical Co., Ltd; concentrations, 1, 3, 5 and 8 vol.%; (iii) carbon tetrachloride: CCl4 ; molecular weight, 153.82; purity, 99.5%; purchased from the Kanto Chemical Co., Inc.; concentrations, 1, 3, 5 and 8 vol.%. Atmospheric air was used as the carrier gas and controlled by a calibrated mass flow controller (Tylan FC-280S). The flow rates were 3, 4 and 5 l min−1. Before entering the reactor, atmospheric air first passed through a scrubber and then was mixed with the chloromethane compound. Chloromethane compounds were injected using a syringe pump (KD Scientific model 100). The temperature of the input stream was maintained higher than the temperature of vaporization of the compounds by means of a heating tape surrounding the streamline. 2.3 System of measurements The power supplied and the AC voltage–current (V –I) waveform were recorded using a digital oscilloscope (Agilent 54641A) with a high-voltage probe (Tektronix P6015A) having an analogue bandwidth of 350 MHz and a current monitor (Pearson 4997). The power consumed was also recorded using a wattmeter (Metex M-3860M). The power measured with the oscilloscope was the real value absorbed in the reactor only and was defined as
discharge power = [V (t) × I (t)] dt × frequency W. (1) In this study the experimental data were taken 30 min after initiation of the plasma of the gliding arc referred to the onset outlet temperature of the bulk gas measured with a thermocouple.

3.

Results and discussion

3.1 Characteristics of the power supply The specific characteristic of the gliding arc is the initial breakdown of the moving gas that initiates this arc. The initial breakdown voltage was higher than the equilibrium voltage.

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Figure 2.

Movement of the gliding arc along the electrode plate recorded by a high-speed camera.

Figure 2 shows the arc movement along the electrode plates. The number of arcs produced could be easily found from the waveform of the voltage and current (figure 3). Following [12], the breakdown at arc ignition results from the over-current at the shortest distance between the pair of electrodes. In this study, the AC supply voltage applied and the current at breakdown and in the equilibrium state were determined by various parameters, such as the distance between the electrodes, the material of the electrodes and the gas flow rate, and could not be manually adjusted by varying the power supply parameters. On achieving initial breakdown, the supply voltage and current decreased in the equilibrium state to a stationary value, which could not be adjusted or changed by varying the power supply parameters. The frequency of the power supply was the only adjustable independent parameter. However, this frequency played an important role in the number of arcs produced.

Gliding-arc plasma in chlorinated methanes

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Figure 3. Typical waveform of the AC power supply. The phenomena of arc production can be clearly seen from the fluctuations in current waveform.

3.2

Influence of the chloromethane compounds

The power consumed or applied played the main role in maintaining the stability or instability of the gliding plasma. Although the concentration and flow rate were kept the same, different compounds of injected material gave different power consumptions. Figure 4 shows the result of the measurements of voltage carried out using an oscilloscope for different gases. Slight differences in the voltage and positions of the maxima occurred. With increasing concentration of the chloromethane in the inlet stream, the difference increased rapidly, which is clearly shown in figure 5. From figure 5, it can be concluded that the compound containing CCl4 consumed the highest discharge power. The power consumed by different compounds was as follows (in descending order): CCl4 > CH2 Cl2 > CHCl3 . A good analytical explanation can be given on the basis of the Paschen law, according to which the potential is a function of the product of pressure and gap length [13]: V = f (p, d).

(2)

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Figure 4. Voltage profile.

In this experiment the interelectrode gap was kept constant, and the pressure could also be assumed constant.Although the potential was a function of p and d, in the real experiment, some coefficients must be introduced to ensure a match between the results of the experiment and mathematical calculations [14]. By rearranging equation (2) and inserting some coefficients, we have Bpd V = , (3) ln[Apd/ ln(1/γ )] where γ is the Townsend secondary emission coefficient of electrons, which is written as follows: 1 (4) = eαd . γ

Figure 5. Effect of the injected chloromethane compounds (species, concentration and total gas flow rate) on the discharge power.

Gliding-arc plasma in chlorinated methanes

By differentiating equation (2) and setting the derivative equal to zero, we have     1 2.718 1 e (pd)m = ln = ln . A γ A γ

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(5)

The minimum or maximum voltage was obtained by substituting equation (5) into equation (3):   B 1 Vm = 2.718 ln . (6) A γ The voltage given in equation (6) is usually called the breakdown voltage Vbd . In the case of a gliding arc, Vm > V . Less information about the constants A and B is available in the case of a gliding-arc plasma. The parameters A and B must be determined experimentally [15]. From equations (3) and (6), it can be seen that there is a relation between V and Vbd . When chloromethane compounds were injected with different concentrations, the values of Vbd obtained from the experiment were different. In this study, to check the relationship between V and Vbd , we used the following algorithm. By rearranging equation (6) in the form   2.718 1 A= B ln (7) Vm γ and substituting it into equation (3) we have V =

Bpd . ln(2.718B/Vm )

(8)

For two different concentrations of chloromethane compounds, we obtain V1 B1 p1 d1 / ln(2.718B1 /Vm1 ) = V2 B2 p2 d2 / ln(2.718B2 /Vm2 )

(9)

The experiment was carried out under the same pressures and gap distances: p1 = p2 and d1 = d2 . The parameter B is a function of the effective ionization potential V ∗ and pressure. This potential ensures that the transport electrons move through the gap and thus ionization is produced. As we used the same gap distances, the pressures and concentrations of chloromethane compounds differed only slightly, and it could be assumed that B1 ≈ B2 . In this case, equation (9) can be written in the form ln(1/V1 ) Vm2 = ln(1/V2 ) Vm1

(10)

A comparison between the calculation and experimental results is shown in figure 6. When the experiment was carried out with the same chloromethane compounds but with different concentrations, the results were close. A satisfactory result was also achieved when the experiment was carried out with different total gas flow rates and fixed concentration and chloromethane species. However, the result was not satisfactory when we applied the same rates with different chloromethane compounds. This means that the parameters A and C have specific values for each chloromethane gas and play an important role in the initiation of arc cycle production. The chemical stability of molecules, which determines the emission of such species as electrons and ions, has an appreciable effect on the breakdown process. Bartnikas and co-workers [16–19] have studied and described in their work the effect of electrons on the initiation of breakdown. The lack of free electrons that are necessary to initiate the breakdown

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Figure 6.

Comparison between the calculation and experimental value of Vbd .

will lead to an over-voltage across the electrode gap, which will result in a higher voltage and current and shorter rise times. Taylor and Dellinger [20] have compared these compounds and graded these compounds with respect to stability under oxidation conditions [20] according to CCl4 = CH2 Cl2 > CHCl3 and in the absence of oxygen according to CCl4 > CH2 Cl2 > CHCl3 . The stability depends on the structure and chemical bonds of the compounds. A higher chemical bond stability than those of other chlorinated methanes could explain why CCl4 gave the highest value of V and Vbd . This result also matched other experimental results testifying that CCl4 has a higher energy consumption than CHCl3 [4].

Figure 7. Power profile as a function of the total gas flow rate. The data were obtained using 1 vol.% CCl4 as the injected compound at a power frequency of 20 kHz.

Gliding-arc plasma in chlorinated methanes

3.3

9

Influence of the total gas flow rate

After the initial breakdown of the discharge gap, we failed to control the equilibrium voltage and current by varying the power supply parameters, and the setting of a specified voltage and a specified current was difficult. In this case, the total gas flow rate was also a factor to be considered as a variable. Figure 7 shows the effect of the total gas flow rate on the power profile. It can be easily seen that, at 3 l min−1 , the total discharge power that was supplied to the system was higher than its values at 4 and 5 l min−1 .

Figure 8. Voltage–current behaviour using 1 vol.% CCl4 as the injected compound and a power frequency of 20 kHz: (a) V at a gas flow rate of 3 l min−1 ; (b) I at a gas flow rate of 3 l min−1 ; (c) V at a gas flow rate of 4 l min−1 ; (d) I at a gas flow rate of 4 l min−1 ; (e) V at a gas flow rate of 5 l min−1 ; (f) I at a gas flow rate of 5 l min−1 .

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To study this effect thoroughly, we tried to capture the real voltage–current profile under the conditions of equilibrium. Figure 8 shows the behaviour of the voltage–current wave obtained by experiment. The calculations of both the real and the average values of the voltage wave indicated that the total voltage supplied would be lower at lower total gas flow rates, but this difference was not noticeable. This phenomenon can be also explained by the Paschen law [14]. Usually, an increasing flow rate increases the pressure in the system. The increasing pressure can increase the breakdown voltage Vbd , which is higher than the voltage at low pressures. Moreover, the voltage under the equilibrium condition will also be higher than the voltage at low pressures. The effect of the current waveform can be supposed to be the main reason for increasing or reducing the value of the total discharge power. A comparison between figures 8(b), (d) and (e) shows that at 3 l min−1 the number of suddenly fluctuating pulses was higher than in the other two cases. This means that at 3 l min−1 the system produced more arcs than at 4 and 5 l min−1 . As mentioned above, the increasing flow rate resulted in a higher pressure; therefore, the possibility of producing arcs became weaker. That is why the number of suddenly fluctuating pulses became increasingly smaller with increasing total gas flow rate. However, suddenly fluctuating pulses also made a significant contribution to the calculated average total current fed to the system. Compared with the average current when no arc occurred, in the case with a plasma the value of the total average current was five to ten times higher. 3.4 Effect of the frequency The frequency of the power supply was an adjustable factor in this experiment. Figure 9 shows the effect of the frequency on the power profile. The integration of equation (1) shows that the total discharge power increased linearly with increasing frequency. When the conditions were kept constant, the number of arcs also increased. Radu et al. [16] mentioned that a change in the frequency would change the basic mechanism of the Townsend breakdown. An increasing frequency will lead to an increase in the sudden fluctuations in the current and voltage peaks per cycle. The integration calculation of the power waveform using equation (1) shows that a larger number of peaks per cycle will result in a higher supplied

Figure 9. Effect of the frequency of the applied power supply on the power profile. The data were obtained using 10 vol.% CHCl3 at a total gas flow rate of 2.5 l min−1 .

Gliding-arc plasma in chlorinated methanes

11

Figure 10. Effect of the frequency of the applied power supply on the total discharge power. The data were obtained using 8 vol.% CHCl3 at a total gas flow rate of 2.5 l min−1 .

energy (figure 10). The measurements carried out with a wattmeter showed the same tendency as the oscilloscope measurements but the power was slightly higher. An oscilloscope was used to measure only the energy that was supplied to the plasma. On the other hand, a wattmeter measured the total power needed for all instruments, including the total power to operate the power supply.

4.

Conclusion

The power discharge characteristics of a gliding-arc plasma have been studied using chloromethane compounds. Different concentrations, total gas flow rates and frequencies have been used to investigate the behaviour of the voltage–current–power (V –I–W ) characteristic. Different chloromethane compounds gave significantly different values of discharge power, equilibrium voltage and breakdown voltage; CCl4 gave the highest values. In the case of different concentrations and total gas flow rates the behaviour of chlorinated methane compounds followed the Paschen law, which gave the relation between the equilibrium voltage and the breakdown voltage. A higher total gas flow rate resulted in a decrease in the discharge power, a decrease in the number of arcs produced and, consequently, sudden fluctuations in the current wave. The discharge power also increased at higher frequencies. Acknowledgement This work was supported by the National Research Laboratory of the Ministry of Science and Technology of Korea. References [1] V. Dalaine, J.M. Cormier, P. Lefaucheux. A gliding discharge applied to H2 S destruction. J. Appl. Phys., 83, 2435 (1998). [2] K. Krawczyk, M. Młotek. Combined plasma–catalytic processing of nitrous oxide. Appl. Catal. B, 30, 233 (2001).

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[3] K. Krawczyk, B. Ulejczyk. Decomposition of chloromethanes in gliding discharges. Plasma Chem. Plasma Process, 23, 256 (2003). [4] K. Krawczyk, B. Ulejczyk. Influence of water vapour on CCl4 and CHCl3 conversion in gliding discharge. Plasma Chem. Plasma Process, 24, 155 (2004). [5] J.R. Roth, Industrial Plasma Engineering, Vol. 1, Principles (Institute of Physics Publishing, Bristol, 1995), pp. 385–386. [6] A. Fridman, S. Nester, L.A. Kennedy, et al. Gliding arc gas discharge. Prog. Energy Combust. Sci., 25, 211 (1999). [7] O.M. Yardimci, A.V. Saveliev, A.A. Fridman, et al. Thermal and nonthermal regimes of gliding arc discharge in air flow. J. Appl. Phys., 87, 1632 (2000). [8] I.V. Kuznetsova, N.Y. Kalashnikov, A.F. Gutsol, et al. Effect of ‘overshooting’ in the transitional regimes of the low-current gliding arc discharge. J. Appl. Phys., 92, 4231 (2002). [9] F. Richard, J.M. Cormier, S. Pellerin, et al. Physical study of a gliding arc discharge. J. Appl. Phys., 79, 2245 (1996). [10] S. Pellerien, F. Richard, J. Chapelle, et al. Heat string model of bi-dimensional dc Glidarc. J. Phys. D: Appl. Phys., 33, 2407 (2000). [11] P.H. Taylor, B. Dellinger, C.C. Lee. Development of a thermal stability-based ranking of hazardous organic compound incinerability. Environ. Sci. Technol., 24, 316 (1990). [12] C.S. Kalra, Y.I. Cho, A. Gutsol, et al. Gliding arc in tornado using a reverse vortex flow. Rev. Scient. Instrum., 76, 025 110 (2005). [13] V.F. Paschen. Ueber die zum Funkenubergang in Luft, Wasserstoff und Kohlensäure bei verschiedenen Druken erforderliche Potentialdifferenz. Wied. Ann., 37, 69 (1889). [14] J.D. Cobine, Gaseous Conductor Theory and Engineering Application (Dover Publications, New York, 1958), pp. 160–177. [15] J.R. Roth, Industrial Plasma Engineering, Vol. 1, Principles (Institute of Physics Publishing, Bristol, 1995), pp. 237–256. [16] I. Radu, R. Bartnikas, M.R. Wertheimer. Frequency and voltage dependence of glow and pseudoglow discharges in helium under atmospheric pressure. IEEE Trans. Plasma Sci., 31, 1363 (2003). [17] R. Bartnikas. Partial discharges. Their mechanism, detection and measurement. IEEE Trans. Dielect. Elect. Insulation, 9, 763 (2002). [18] J.P. Novak, R. Barnitas. Breakdown model of a short plane-parallel gap. J. Appl. Phys., 62, 3605 (1987). [19] R. Barnitas, J.P. Novak. Effect of overvoltage on the risetime and amplitude of PD pulses. IEEE Trans. Dielect. Elect. Insulation, 2, 557 (1995). [20] P.H. Taylor, B. Dellinger. Thermal degradation characteristics of chloromethane mixtures. Environ. Sci. Technol., 22, 438 (1988).