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Discharge Characteristic of Chlorinated Methanes diluted in Atmospheric Air on Gliding Arc Plasma

Antonius Indarto† , Jae-Wook Choi, Hwaung Lee and Hyung Keun Song Korea Institute of Science & Technology, Clean Technology Research Center, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Abstract- Plasma processing of chloromethane compounds (methylene chloride (CH2Cl2), chloroform (CHCl3), and carbon tetrachloride (CCl4)) diluted in the atmospheric air using gliding arc have been studied. Various values of injected initial chloromethane concentrations, total gas flow rates, and power frequency were used as the variables to investigate their discharge characteristic. This paper evaluates the plasma process phenomena of chloromethane by gliding arc plasma.

Key words: Plasma, Gliding Arc, chloromethane, AC wave form, equilibrium voltage, voltage breakdown

1. Introduction

The applications of gliding arc for destructing toxic materials are widely used now. †

Corresponding author: E-mail:[email protected], Tel:+82-19-352-1981

Many dangerous emissions, such as: H2S [1], N2O[2], CHCl3 and CCl4 [3-4], have been investigated and studied. Usually, high percentage of destruction efficiency could be achieved using this method. The gliding arc creates arcs which were started at the shortest distance the electrodes and arcs move together with gas flow at the same direction. The number of arc that would be produced is dependent on many factors, such as: frequency of the power supply, flowing gas, and total gas flow rate. During this movement, plasma reaction is occurred simultaneously. Plasma arc usually has powerful energy to destruct strong molecule-bond or initiate the reaction of stable gas material due to many advanced characteristics such as higher flame temperature, higher electron density, and etc. However, not many papers discuss on the discharge behaviour of gliding arc plasma. Theoretical and numerical study of gliding arc to describe it has been published with showing many mathematical equations [5-9]. In this paper, the physical plasma characteristic of chloromethane compounds diluted in compressed air was tried to be explained. The experiment was carried out with two triangular stainless steel electrodes which were electrically charged by AC power supply. According to EPA report, chloromethane has been categorized as high thermal stability compound to be destructed [10]. The analysis was focusing on discharge phenomena, such as: equilibrium voltage, breakdown voltage, and voltage-current-power (V-I-W) profile as the influence of various concentration of chloromethane, total gas flow rate, and power frequency.

2. Experiment setup

The schematic diagram of experimental setup is shown in Fig.1. Chloromethane compounds and atmospheric air have been used as source gas. Details of each part of the system are described in the next section.

2.1. Plasma reactor and applied power system

Figure 1

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

2.2. Input gas

Chlorinated methanes, as the starting material, are: a. Methylene chloride: CH2Cl2, molecular weight 84.93, purity 99.0%, purchased from Junsei Chemical Co., Ltd., concentration 1, 2, 3, 4 % volume/volume b. Chloroform: CHCl3, molecular weight 119.38, purity 99.0%, purchased from Junsei Chemical Co., Ltd., concentration 1, 3, 5, 8% v/v. c. Carbon tetrachloride: CCl4, molecular weight 153.82, purity 99.5%, purchased from Kanto Chemical Co., Inc., concentration 1, 3, 5, 8% v/v. 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 Nl/min. Before entering the reactor, atmospheric air was passed through a scrubber first and directly mixed with chloromethane compound. The chloromethane compounds were introduced by syringe pump (KD Scientific, Model 100). Heater tape has been attached surrounding the line stream to maintain the input stream temperature higher than vaporized temperature of compounds.

2.3. Measurement system

The supplied power and AC voltage-current (V-I) wave form were measured by a digital oscilloscope (Agilent 54641A) having analog bandwidth of 350 MHz through a high voltage probe (Tektronix P6015A) and current monitor (Pearson 4997). The consumed power was also calculated by a watt meter (Metex M-3860M). The oscilloscope value is the real power used in the reactor only and defined as:

Discharge power = ∫ (V (t ) × I (t ) ) dt × frequency Watt

(1)

In this study, each data of experiment was taken after 30 minutes from the start of gliding plasma operation refers to the stability of bulk gas outlet temperature which has been measured by thermocouple.

3. Result and Discussion 3.1. Characteristic of Power Supplies

Figure 2

Figure 3

The special characteristic of gliding arc is the initial breakdown of the moving gas will begin the cycle of the gliding arc production. Initial breakdown voltage is higher than equilibrium voltage. Figure 2 shows the arc movement along the electrode plates. The number of produced arcs could be easily detected from the equilibrium waveform of the voltage and current. As shown in Figure 3, arcs were produced by over-current value. In this study, the applied AC power supply voltage and current were determined by gliding system. After achieving initial breakdown, power voltage and current were decreased into equilibrium state which those variables could not be adjusted or changed by power supply controller. Power supply frequency was only the adjustable independent variable. Frequency gave the important role in amount of arc production.

3.1. Influence of chloromethane compounds

Figure 4

Figure 5

Consumed or applied power has the main important role to hold the stability or instability of gliding plasma. Although the concentration and flow rate were kept same but difference compounds of injected material gives different power consumption. Figure 4 shows the comparison of oscilloscope result of average voltage. Slightly different of voltage area and maximum peak voltage have been occurred. With increasing concentration of chloromethane in inlet stream, the difference was getting more and more. It is clearly shown by figure 5. From figure 5, it can be conclude that CCl 4 consumed highest discharge power. Amount number of consumed discharged power follows: CCl4 > CH2Cl2 > CHCl3 A good analytical explanation could be found following the Paschen’s Law that the potential is a function of the product pressure and gap length [11]. V = f ( p, d )

(2)

In this experiment, the gap distance of electrode was kept constant and the pressure could be assumed constant also. Although it was mainly function of p and d, in the real experiment, some coefficient must be added to match the result between experiment and mathematical calculation [12]. Extended of equation Eq. 1 gives:

V=

B pd  A pd  ln    ln(1 / γ ) 

(3)

γ is secondary emission of electron of Townsend and followed: 1 =∈αd γ

(4)

Differentiating Eq. 2 and set the derivation equal to zero will give: ( pd ) m =

e 1 2.718 1 ln = ln A γ A γ

(5)

The minimum or maximum voltage was obtained by substituting Eq. 5 into Eq. 3: Vm = 2.718

B 1 ln A γ

(6)

Eq. 6 is usually called as voltage of breakdown (Vbd). In case gliding arc, Vm > V. Less information about constant of A and B under gliding arc plasma. The parameters A and B must be experimentally determined [13]. Routing Eq. 3 and 6, it could be said there is a relation of V to the V bd. Experiment result got the different value of Vbd when the chloromethane compounds were injected in the different ratio of concentration. In this study, to prove the relationship of V and Vbd, the algorithm followed this way: By re-arrangement of Eq. (6) into: A=

2.718 1 B ln Vm γ

and substituting into Eq. (3) will give:

(7)

V=

B pd  2.718 B  ln    Vm 

(8)

If we compare two different amount of concentration of chloromethane compounds: B1 p1 d 1  2.718 B1  ln   V m1  V1  = B2 p 2 d 2 V2  2.718 B 2  ln    Vm 2 

(9)

The experiment was occurred at the same condition of pressure and gap distance and it can be written: p1 = p2 and d1 = d2. Parameter B is function of effective ionization (V*) and pressure. This potential will be used to travel the electron through the gap to make ionization. Because we used the same gap distance, pressure, and very low concentration different of chloromethane compounds, it could be assumed that B1 ≈ B2. The remaining Eq. (9) will be: 1 ln   V1  = Vm 2  1  Vm1 ln   V2 

(10)

Figure 6

The comparison between calculation and experimental result is shown in Fig. 6. When

the experiment was done with concentration as the variable but at same compound of chloromethanes, the result was closed each other. The satisfaction result was also achieved when the experiment has been done in the different total gas flow rate and fixed amount of concentration and chloromethane species. However, this kind of result could not be found when we applied same flow rate in different compound of chloromethane. It means that the parameters A and C have specific number for each chloromethane gases and give the important role to initiate the production of arc in gliding system. Radu, et.al. and others have studied and mentioned about the effect of electron on breakdown initiation. Lack of free electrons, those are necessary to initiate a breakdown under ac condition, will lead to over-voltage across the short gap that will produce larger magnitude and more rapid rise times [14-17]. Taylor et al. has comparing the stability of these compounds and gave the order of stability under oxidative condition [18]: CCl4 = CH2Cl2 > CHCl3 And in the absence of oxygen: CCl4 > CH2Cl2 > CHCl3 Stability is depending on structure and chemical-bond of compounds. This factor could tell the reason why CCl4 gave the highest value of V and Vbd. This result was also match with other experiment result that gave CCl4 higher energy consumption than CHCl3 [4].

3.2. Influence of total gas flow rate

Figure 7

Un-adjustable equilibrium voltage and current by power supply controller after initial breakdown made the experiment little bit difficult to set in the exact same supplied voltage and current. In this case, the total gas flow rate was also a factor that must be counted as a variable. Fig. 7 shows the effect of total gas flow rate on power profile. By comparing, it could be easily that at 3 Nl/min, total discharge power that supplied to the system has the higher value compared to 4 and 5 Nl/min.

Figure 8

To study deeply about this effect, we have tried to capture the real voltage-current profile at equilibrium condition. Fig. 8 shows the behavior of voltage-current wave. Calculation of both real and average value of voltage wave gave the result that total supplied voltage would be lower at lower total gas flow rate. But, the different was not significantly high. This phenomenon has clearly explained by previous explanation of Paschen’s law [12]. Usually increasing flow rate would increase the pressure to the system. Increasing pressure could increase the breakdown-voltage (Vb) in term of producing initial arc and equilibrium voltage in term of stabilizing arc cycle production. Current wave form effect could be suspected as the main reason of the increasing or decreasing value of total discharge power. Comparing fig 8 (b), (d), and (e), it shows that at 3 Nl/min the number of sudden-fluctuated pulse was higher that two others. It means at 3 Nl/min the system produced higher number of arc compared to 4 and 5 Nl/min. As mention before, as effect of increasing flow rate refers to increasing

pressure, the possibility to produce arc was getting decrease. That is why the number of sudden-fluctuated pulse was lower and lower with the increment of total gas flow rate. However, sudden-fluctuated pulses also gave significant contribution to the calculation of average total supply current to the system. Compared to average current when plasma was off, the value of total average current when plasma on was 5 ~ 10 times higher.

3.3. Effect of frequency

Figure 9

Figure 10

Power supply frequency was adjustable factor in this experiment. Fig. 9 shows the effect of frequency on power profile. Integration calculation by Eq. 1 gave the total discharge power increased linier with increasing number of frequency. When the condition was kept constant, number of arc was also increased. Radu et. al. mentioned that changing the frequency will change the basic Townsend breakdown mechanism [14]. Increasing frequency would increase the number of sudden-fluctuated pulsed current and voltage peak per cycle. Examined the power waveform, increasing number of peak per cycle would give more supply of energy, Fig. 10. Measurement using wattmeter was also giving the same trend as oscilloscope measurement but little bit higher. Oscilloscope just measured the netto energy that was supplied to the plasma system. In the other hand, wattmeter measured the total power that needed by all instrument, including total power to operate the power supply.

4. Conclusion

The power discharge characteristic of gliding arc plasma using chloromethane compounds has been studied. Various concentration, total gas flow rate, and frequency have been used to investigate the behavior of voltage-current-power (V-I-W). Different kind of chloromethane compounds gave significant different value of discharge power, equilibrium voltage, and breakdown voltage also which CCl4 gave the highest value of them. In case of different concentration and total gas flow rate, the phenomena were following Paschen’s law which gave relation between equilibrium voltage and breakdown voltage. Increasing amount of total gas flow rate would degrease the discharge power. It would reduce the number of production of arc that would reduce the sudden-fluctuated pulse in the current wave. Discharge power would also increase with the increasing value of frequency.

Acknowledgement

This work was supported by National Research Laboratory program of Korea Minister of Science and Technology.

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Fig. 1. Schematic diagram of experimental set up

Figure 2. Gliding Arc movement along the electrode plate. Captured by high-speed camera

6000

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Figure 3. Typical waveform of AC power supply. The arc production phenomena could be seen clearly from fluctuation of current wave form.

4000 3000

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2000 1000 0 -1000 CH2Cl2

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CCl4 CHCl3

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230 220 210 CCl4, 3 Nl/min CCl4, 4 Nl/min CCl4, 5 Nl/min CHCl3, 3 Nl/min CHCl3, 4 Nl/min CHCl3, 5 Nl/min CH2Cl2, 3 Nl/min CH2Cl2, 4 Nl/min

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

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3 Nl/min, calculation 4 Nl/min, calculation 5 Nl/min, calculation CCl4, experiment

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Concentration (% v/v) Figure 6. Comparison of calculated and experimental value of Vbd.

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Figure 7. Power profile as effect of total gas flow rate. Data was taken using 1% of CCl 4 as injected compound and power frequency 20 kHz.

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Voltage (V)

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(f)

Figure 8. Voltage-Current behavior at 1% of injected CCl4, power frequency 20 kHz. (a) V-3 Nl/min (b) I-3 Nl/min (c) V-4 Nl/min (d) I-4 Nl/min (e) V-5 Nl/min (f) I-5 Nl/min

1000 15 kHz 16.25 kHz 17.5 kHz 18.75 kHz 20 kHz

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Fig 9. Effect of applied power supply frequency on power profile. Data was taken using 10% of CHCl3 at total gas flow rate 2.5 Nl/min

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Discharge power (Watt)

320 300 280 260 240 220 Wattmeter Oscilloscope

200 180 14

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Fig 10. Effect of applied power supply frequency on total discharge power. Data was taken using 8% of CHCl3 at total gas flow rate 2.5 Nl/min