Effects of ambient gas on cold atmospheric plasma discharge in the decomposition of trifluoromethane

D. B. Nguyen and W. G. Lee*
Department of Chemical Engineering, Kangwon National University, Chuncheon, Kangwon 24341, Republic of Korea. E-mail: wglee@kangwon.ac.kr

Received 18th January 2016 , Accepted 3rd March 2016

First published on 4th March 2016


Abstract

The effect of dilution gases (He, Ar and N2) on plasma discharge in the decomposition of trifluoromethane (CHF3) was investigated in dielectric barrier discharge at atmospheric pressure. A comparison among these dilution gases was performed in terms of active power, apparent power, power factor, impedance, and power efficiency for the decomposition of CHF3. He dilution showed the most homogeneous and stable discharge among the dilution gases. However, Ar dilution ignited the generation of streamers in the plasma discharge, resulting in a relatively superior power efficiency in CHF3 decomposition. In addition, due to the plasma reactions between O2 and N2, N2 as a dilution gas had a disadvantaging tendency to form nitric oxide compounds (harmful compounds). Complete decomposition of CHF3 could be performed under the following conditions; Ar dilution, active power ≥ 40 W, a CHF3 fraction of 0.2% and a total flow rate of 1000 ml min−1.


Introduction

In this decade, there has been a lot of interest in the abatement of greenhouse gases (GHGs).1 Among GHGs, trifluoromethane (HFC-23, CHF3) has the second highest global warming potential, 11[thin space (1/6-em)]700 (for a time horizon of 100 years) times higher than CO2, and has a lifetime of 264 years.2 CHF3 emissions come from commercial refrigeration, air conditioning, polymer industries, and semiconductor industries. In the year 2010, the worldwide emission of hydrofluorocarbons (including CHF3), perfluorocarbons, and sulfur hexafluoride was 1[thin space (1/6-em)]015[thin space (1/6-em)]443 thousand metric tons.3 It accounted for about 2.0% of total GHG emissions in that year.4 Therefore, an abatement of CHF3 in gas waste is worth implementing regarding GHG emissions. Several methods are commonly employed for the decomposition of CHF3, such as thermal processes, catalysts, plasma or plasma-catalysts, and conversion to environmentally benign compounds.2

The thermal process is an effective method for the decomposition of CHF3. This method has been widely used in industry for the decomposition of CHF3 under the clean development mechanism methodologies (CDM).5,6 In the CDM, a mixture of CHF3, fuel (LNG) and air is burned in an incineration furnace at a high operation temperature (1200 °C).6 Another concern present with this method is the presence of HF acid and fluorinated compounds in the exhaust gas stream. Consequently, further stages are required for treatment of the exhaust gas, stages which are for cooling, filtering, absorption of the acids, and decomposition of fluorinated compounds.6 These additional necessary processes show that the thermal process is costly and requires a resistant material reactor. Conventional catalysts have succeeded in reduced operating temperatures (∼500 °C) for the decomposition of CHF3.7–9 However, HF formation and then deactivation of the catalyst still occur, which are challenges of the use of a conventional catalyst for the abatement of CHF3. These drawbacks can be ignored with a noble metal as a catalyst, but to do so is costly.9,10 In plasma technology such as a dielectric barrier discharge (DBD), high-energy electrons can be generated in the discharge zone. These electrons initiate the dissociation of gas molecules at a relatively low temperature and atmospheric pressure, through electron impact reactions. Thus, the decomposition of CHF3 can be performed at low operating cost: relative low temperature, ambient atmospheric pressure, fast conversion, and easy realization.11–13 However, DBD for gas treatment results in low energy efficiency, which may be due to the considerable input power used to heat up a plasma system, usually electrodes and barriers.14 Adding dilution gas such as Ar, He or N2 is suggested to improve the efficiency of the plasma reaction in a DBD; this method allows the increase of reaction opportunity between activated dilution gas molecules and the reactant molecules. The different levels of dilution gas in the feed would change the physics of the discharge; this method also provides a different way to obtain a plasma reaction performance.15–20

Although the decomposition of CHF3 would increase with a high wall temperature in the DBD,21 we suggested that a reactor immersed in an oil bath be used for CHF3 decomposition. This configuration would improve the energy efficiency due to the prevention of micro-arcing on the barrier surface, preventing rapid heating during the plasma reaction.22,23 In this study, the decomposition of CHF3 was carried out in a coaxial DBD reactor immersed in an electrically insulating oil bath. The effects of adding Ar, He, or N2 on the decomposition of CHF3 were investigated, and were considered in terms of the physical features of the discharge, decomposition rate, and selectivity of CO and CO2 in the gas outlet. Spectra of plasma discharge were analyzed to suggest a pathway reaction for the CHF3 decomposition in a DBD reactor.

Experimental

A schematic diagram of the experimental setup is shown in Fig. 1(a). The coaxial DBD reactor has been previously described elsewhere,24 and has a discharge gap and discharge volume of 1.0 mm and 10 ml, respectively. The reactor was immersed in an electrical insulating oil bath (transformed oil provided by Michang Oil, KSC2301). The volume of transformed oil used to cover all parts of the reactor was about 4000 ml. The feed gas was a mixture of CHF3 and O2, and the dilution gas was one of Ar and He or N2, with a purity of approximately 99.99%, which was introduced into the discharge zone by mass flow controllers (MKP, TSC-210) at room temperature (∼20 °C). Volume ratio among the feed gases was 0.2% of CHF3, 1.0% of O2, and 98.8% of dilution gas. The surface temperature of the ground electrode during the plasma reaction was measured by a thermal sensor, and was always lower than 75 °C in all experiments.
image file: c6ra01485b-f1.tif
Fig. 1 (a) Schematic diagram of the experimental setup, with 1 = mass flow controller, 2 = power electrode, 3 = quartz tube, 4 = ground electrode, 5 = oil bath, 6 = AP plasma power supply, 7 = Tektronix 2012B used P6015A passive high voltage probe, 8 = gas chromatograph, 9 = optical emission spectrometer, 10 = camera (back camera of Samsung Galaxy Note 3), (b) cross-sectional view of reactor, and (c) equivalent circuit model, with Rg = equivalent resistance of the reactor, Cg = capacitance of the discharge gap, Cq = capacitance of quartz, Co = capacitance of oil, Cd = absolute capacitance of the dielectric layers.

A high voltage was supplied by an AC pulse power supply (HVP, AP Plasma Power Supply). This power has a supplied power, voltage and frequency of up to 2 kW, 15 kVp and 30 kHz, respectively. The power was controlled by a manual adjustment of the amplitude of applied voltage. All electrical data were recorded by an oscilloscope (Tektronix TDS2012B, 2 channels). The applied voltage was measured by a passive high voltage probe (Tektronix, P6015A), while a Rogowski coil (Pearson, 410) was used to measure the total current. A root mean square of voltage (VRMS) and current (IRMS), and active power (integration of voltage and current) were the products of the oscilloscope (see ESI, Fig. S1). The electrical data were collected with five-time measurements for a reliable analysis. Typical electrical waveforms are shown in Fig. 2.


image file: c6ra01485b-f2.tif
Fig. 2 Effect of dilution gases on (a) current waveform and (b) voltage waveform (flow rate of CHF3, O2 and total flow rates were 2, 10 and 1000 ml min−1, respectively; frequency = 30 kHz; applied voltage = 6.5 kVp).

The composition of the gas products was analyzed by a gas chromatograph (GC, Younglin YL6100GC) with a Carboxen™ 1010 PLOT capillary column as the GC column. The GC system used a thermal conductivity detector (TCD) and flame ionization detector (FID). The GC analysis detected the gas product of the reaction, including CO, CO2, and CHF3. The GC analyses have been done twice in order to be assured of reproducible results. The emission of plasma was measured by an optical emission spectrometer (AvaSpec-2048 XL). According to the analysis of the experimental results, several parameters were defined as follows:

 
image file: c6ra01485b-t1.tif(1)
 
image file: c6ra01485b-t2.tif(2)
 
image file: c6ra01485b-t3.tif(3)
 
image file: c6ra01485b-t4.tif(4)
 
Apparent power, Papparent (VA) = VRMSIRMS (VA) (5)
 
image file: c6ra01485b-t5.tif(6)
 
image file: c6ra01485b-t6.tif(7)
where RMS is a root mean square of voltage or current, and φ is the phase angle between the voltage and current.

Results and discussion

Effect of dilution gases on electrical properties

The effects of typical dilution gases on the characteristic of electrical power were investigated under the variation of applied voltage from 5.0 to 6.5 kVp. The plasma reaction was performed with a frequency of 30 kHz and total flow rate of 1000 ml min−1 in the feed gas, which included 2 ml min−1 CHF3, 10 ml min−1 O2, and 988 ml min−1 dilution gas.

The effect of dilution gas on the electrical waveforms is shown in Fig. 2. The current profile of CHF3/O2/He plasma was more homogenous than that of Ar or N2 due to the uniform discharge effect from the addition of He. The result is comparable with the previous reports,18 which showed that the plasma discharge of He/CO2 is more homogenous than the plasma of CO2 only or a CO2/Ar mixture. The difference in IRMS values between CHF3/O2/He (0.415 A) and CHF3/O2/Ar (0.418 A) discharges was small. However, those values were higher than that of the CHF3/O2/N2 discharge (0.354 A). The same trend was also observed in the voltage waveforms, as shown in Fig. 2(b). The results show that the influence of dilution gases on the homogeneity of discharge can be listed in the order of N2 < Ar < He. The condition with He dilution produced the most homogeneous and stable electrical properties.

The differences in dilution gases induced the differences of the root mean square of voltage and current, as well as the electrical waveforms in the plasma discharges. The dilution gas can also affect apparent power (the magnitude of complex power) and active power (average power or power consumption), as shown in Fig. 3. The active and apparent powers linearly increased with the increase of applied voltage from 5.0 to 6.5 kVp for Ar or He dilution, whereas N2 dilution showed a rapid and non-linear increasing trend of these powers. Hence, the active power of N2 dilution was higher than that of He or Ar dilution at an applied voltage of 6.5 kVp. In the case of He and Ar dilutions, the values of apparent power were almost the same level as each other, but higher than that of N2 dilution under the same conditions. Although the apparent power of He dilution was similar to Ar dilution, the active power of Ar dilution was higher than that of He dilution. This phenomenon is due to the higher power factor of Ar dilution, compared to that of He dilution.


image file: c6ra01485b-f3.tif
Fig. 3 Effect of dilution gases on (a) active power and (b) apparent power (flow rates of CHF3, O2 and dilution gas in feed were 2, 10 and 988 ml min−1, respectively; frequency = 30 kHz; Y error bar is the standard deviation of a sample).

The impedance of N2 dilution decreased rapidly with the increase of applied voltage from 5.0 to 6.5 kVp, whereas the impedance of He or Ar dilution decreased gradually with the range of applied voltage, as shown in Fig. 4. The level of impedance was He ≈ Ar < N2. This result is comparable to the effect of dilution gas on the active power, as shown in Fig. 3(a). The correlation between active power and impedance could be explained by the computation of active power as a function of impedance following eqn (8).25 It is suggested that a decrease in impedance leads to an increase in the active power.

 
image file: c6ra01485b-t7.tif(8)


image file: c6ra01485b-f4.tif
Fig. 4 Effect of dilution gases on (a) impedance and (b) power factor (flow rates of CHF3, O2 and dilution gas in feed were 2, 10 and 988 ml min−1, respectively; frequency = 30 kHz; Y error bar is the standard deviation of a sample).

Regarding the action mechanism of dilution gas on impedance and power, we considered a computation of the impedance as a function of resistance and capacitance. A cross-sectional view of the DBD reactor showing three dielectric layers, a gas gap, a dielectric barrier, and a small oil gap, is shown in Fig. 1(b). The complex impedance Z of the equivalent circuit, which is provided in Fig. 1(c), can then be calculated by the following eqn (9).25,26

 
image file: c6ra01485b-t8.tif(9)

Rg is the equivalent resistance of the reactor and Cx is the capacitance of the dielectric layers.

image file: c6ra01485b-t9.tif

image file: c6ra01485b-t10.tif

image file: c6ra01485b-t11.tif

image file: c6ra01485b-t12.tif
ε0 is the electric constant (8.854 × 10−12 F m−1), εx is the relative permittivity (dielectric constant) of the material, l is the length of the discharge.

Replace |Z| and Re(Z) = Rg/(1 + ω2Cg2Rg2) in eqn (8), the active power as a function of Rg, Cq and Cg is given in eqn (10).

 
image file: c6ra01485b-t13.tif(10)

The relative permittivity of Ar, He, and N2 was 1.0005172, 1.0000650 and 1.0005480, respectively.27 Then the capacitances of the gas gap (Cg) with pure Ar, He, or N2 at 20 °C were 88.9401, 88.8999 and 88.9428 pF, respectively. The fractions of CHF3 and O2 were fixed at 0.2% and 1%, respectively, in the experiment. Therefore, we supposed that the Cg of feeds did not significantly change with Ar, He or N2 as a dilution gas. Consequently, the active power and impedance were dependent on the equivalent resistance of the reactor (Rg). The experimental data indicated that Rg with N2 as a dilution gas significantly decreased with the increase of applied voltage, whereas Rg was only slightly changed when He or Ar was used as a dilution gas.

The power factor of N2 dilution increased sharply with the increase of applied voltage from 5.0 to 6.5 kVp, as shown in Fig. 4(b). This was due to the increase in active power of N2 dilution gas according to the applied voltage, as shown in Fig. 3(a). Also, the phase angle of He (φHe) was larger than that of Ar (φAr) at the same applied voltage (see ESI, Fig. S2). The power factor of Ar dilution showed a relative higher value than that of He dilution. Conclusively, the power consumption and the stability of discharge could be affected by the degree of the power factor, which in turn depends on the dilution gas. The adoption of He dilution gas is adequate for a stable discharge and low power consumption.

Effect of dilution gas on decomposition of CHF3

The decomposition of CHF3 and the selectivity of CO and CO2 as a function of active power are shown in Fig. 5. It is clear that the decomposition of CHF3 with Ar as a dilution gas was superior to other dilution gases at the same active power. Thus, the effectiveness of dilution gases on CHF3 decomposition can be listed as Ar > He > N2. A completed decomposition of CHF3 was accomplished with Ar dilution with an active power ≥ 40 W. CO2 selectivity was graded as He ≈ Ar > N2, whereas selectivity of CO was N2 > Ar ≈ He. Therefore, the carbon mass balance (sum of selectivity of CO and CO2) with Ar or He as a dilution gas was higher than that of N2 dilution at the same active power, as shown in Fig. 5(c), suggesting that the balance increased with the increase of active power. The carbon mass balance reached 93% under an active power of above 80 W with Ar or He as a dilution gas. In the case of N2, the dilution gas showed a lower carbon mass balance as well as a low decomposition of CHF3. However, the decomposition of CHF3 in the N2 dilution gas is an attractive and valuable process.23,28 This is due to the fact that the feed gas is composed similar to an emission of CHF3 in air. The result shows that Ar as a dilution gas for the decomposition of CHF3 in a DBD would be energy-effective.
image file: c6ra01485b-f5.tif
Fig. 5 Effect of dilution gas on (a) conversion of CHF3, (b) selectivity of CO or CO2, and (c) carbon mass balance (carbon mass balance = selectivity of [CO + CO2]; flow rate of CHF3 = 2 ml min−1; flow rate of O2 = 10 ml min−1; total flow rate = 1000 ml min−1; frequency = 30 kHz).

The discharge profiles under active power fixed at 60 W are shown in Fig. 6 or the ESI (Video 1). The intensity of streamers in the discharges is ordered as N2 > Ar > He. The dissociation energies of CHF3 molecules are relatively high for instance, H–CF3 (4.61 eV) and F–CHF2 (5.53 eV).29 Thus the plasma process for the decomposition of CHF3 should require a high level of electron energy. The streamers process at relatively high current density, enhancing plasma reactivity due to the generation of high-energy electrons (hot electrons).19 This phenomenon was a possible reason for the fact that Ar dilution was more effective than He dilutions in CHF3 decomposition. In contrast, even though N2 dilution generates a large amount of streamers in the discharge, the reaction effect for CHF3 decomposition was low. This is due to the competition of the chemical reactions between oxygen and nitrogen forming nitrogen oxide compounds.


image file: c6ra01485b-f6.tif
Fig. 6 An illustration of the typical plasma discharges with various dilution gases (a) CHF3/O2/He, (b) CHF3/O2/Ar, and (c) CHF3/O2/N2 (flow rate of CHF3 = 2 ml min−1; flow rate of O2 = 10 ml min−1; total flow rate = 1000 ml min−1; frequency = 30 kHz; active power was fixed at 60 W).

Analysis of optical emission spectra

A comparison of the emission spectrum from 200 to 900 nm with various dilution gases is shown in Fig. 7. Under the absence of N2 in the feed gases with Ar or He dilution, the spectra revealed N2 lines in the second positive system (C3Πu → B3Πg), as shown in Fig. 7(a). It seems that the results came from N2 contaminant of our plasma system. This phenomenon was also observed in previous reports.30,31 Several species of CHF3/O2/He spectrum can be attributed for CF, CF2, OH, Hβ, CO2, and CO bands between 200 to 600 nm, as shown in Fig. 7(a). The bands of Hα, F and O atoms were found in the wavelength from 600 to 900 nm in Fig. 7(b). He dilution seems to be more accessible to analyze plasma chemistry for the decomposition of CHF3 in the atmospheric plasma.
image file: c6ra01485b-f7.tif
Fig. 7 Emission spectrum of the CHF3/O2/X atmospheric plasma (a) 200–600 nm and (b) 600–900 nm (X stands for dilution gases: Ar, He or N2; flow rate of CHF3 = 2 ml min−1; flow rate of O2 = 10 ml min−1; total flow rate = 1000 ml min−1; frequency = 30 kHz; active power was fixed at 60 W).

A weak line of CF2 can be observed at 319.66 nm in two other spectra, with Ar and N2 as dilution gases. Bands of CF at 255 nm and OH at 307.8 nm are weak in the CHF3/O2/Ar spectrum. However, these bands could not be identified in the CHF3/O2/N2 spectrum. This result might be due to the fact that two bands of NO at 258.75 nm and N2 at 315.92 nm overlapped the CF and OH bands, respectively, in the CHF3/O2/N2 spectrum. The strong bands of NO in the A2Σ+ → X2Π system were observed in the CHF3/O2/N2 spectrum. Moreover, the bands of oxygen atoms were unclearly presented in the spectra of CHF3/O2/N2. These results suggest that nitric oxide compounds were generated during the plasma reaction with N2 dilution (R1)–(R15).20,32

 
O + O2 → O3 (R1)
 
N + O2 → NO + O (R2)
 
N + O*2 → NO + O (R3)
 
N + O3 → NO + O2 (R4)
 
N + O → NO (R5)
 
N*2 + 2O3 → NO + N + O2 (R6)
 
NO + O3 → NO2 + O2 (R7)
 
NO + O → NO2 (R8)
 
NO + N → N2O (R9)
 
NO2 + N → N2O + O (R10)
 
N*2 + O2 → N2O + O (R11)
 
NO2 + O3 → NO3 + O2 (R12)
 
2NO2 → N2O4 (R13)
 
N2O4 + NO3 → N2O5 + NO2 (R14)
 
NO3 + NO2 → N2O5 (R15)

A band of CH or C2 could not be identified in all these spectra, while C2 bands at 516.52 nm (0,0) were observed under these conditions: the absence of O2 in the feed and a flow rate of CHF3 and dilution gas at 8 and 988 ml min−1, respectively. The CH bands at 431.42 nm (4300 Å system) and 387.13 nm (3900 Å system) could be identified in the CHF3/Ar spectrum. However, these lines were not shown clearly in the CHF3/He and CHF3/N2 spectrum under these conditions. Further identification of spectra can be found in the ESI (Section S3).

Several species such as CF, CF2, CH, CO, CO2, C2, F, H, and Ar+/He+/N2+ (NO with N2 as a dilution gas) were detected in the spectra. Using a combination of spectra, analysis products by GC, and the kinetics of the chemical reactions as listed in the ESI (Section S4), we can suggest the possible reaction pathways for CHF3 decomposition in atmospheric plasma in combination with previous reports, as shown in Fig. 8.13,33 The pathways show that the main product is CO2, whose formation is made possible by the presence of O2. These dilution gases could be played as a generation of excited species (M). Furthermore, the kinetics data was used as a reference for suggesting types of pathways.


image file: c6ra01485b-f8.tif
Fig. 8 A flow diagram of the decomposition of CHF3 in the CHF3/O2/X atmospheric plasma (X stands for dilution gases: Ar, He or N2).

Conclusions

This study focused on the effect of dilution gases (He, Ar and N2) on the cold atmospheric plasma discharge for the decomposition of CHF3. The plasma reaction was performed in a coaxial dielectric barrier discharge at atmospheric pressure with a mixture of CHF3, O2, and one of these dilution gases as the feed gas. When He was used as a dilution gas, more homogenous and stable discharge could be obtained. However, the effectiveness of dilution gases for CHF3 decomposition could be listed as N2 < He < Ar. The generation of streamers, which can usually enhance the plasma reactivity, also happened in the plasma discharge in Ar dilution, producing an enhancement of CHF3 decomposition. However, in the case of N2 dilution, the reactions between N2 and O2 produce nitrogen oxide compounds, which decrease the plasma reaction for the decomposition of CHF3. A complete decomposition of CHF3 could be performed under the following conditions: Ar dilution, active power ≥ 40 W, a CHF3 fraction of 0.2%, and a total flow rate of 1000 ml min−1.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (NRF-2013R1A1A2005014).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: The ESI contains (1) schematic of electrical power measurement, (2) diagram of the phase angles between voltage and current for Ar and He as dilution gases, (3) identification of spectra of plasma discharges, (4) possible chemical reactions for the CHF3 decompositions in the atmospheric plasma, and (5) video of plasma discharge. See DOI: 10.1039/c6ra01485b

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