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

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.

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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, 11 700 (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 1 015 443 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.

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 R_g = equivalent resistance of the reactor, C_g = capacitance of the discharge gap, C_q = capacitance of quartz, C_o = capacitance of oil, C_d = 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 kV_p 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 (V_RMS) and current (I_RMS), 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.

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 kV_p).

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:

Apparent power, P_apparent (VA) = V_RMSI_RMS (VA) (5)

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 kV_p. 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 I_RMS 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 kV_p 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 kV_p. 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.

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 kV_p, 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.

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

R_g is the equivalent resistance of the reactor and C_x is the capacitance of the dielectric layers.

ε₀ is the electric constant (8.854 × 10⁻¹² F m⁻¹), ε_x is the relative permittivity (dielectric constant) of the material, l is the length of the discharge.

Replace |Z| and Re(Z) = R_g/(1 + ω²C_g²R_g²) in eqn (8), the active power as a function of R_g, C_q and C_g is given in eqn (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 (C_g) 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 C_g 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 (R_g). The experimental data indicated that R_g with N₂ as a dilution gas significantly decreased with the increase of applied voltage, whereas R_g 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 kV_p, 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 (φ_He) was larger than that of Ar (φ_Ar) 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.

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.

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 (C³Π_u → B³Π_g), 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, H_β, CO₂, and CO bands between 200 to 600 nm, as shown in Fig. 7(a). The bands of H_α, 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.

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.

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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