A Study on the Efficient Degradation of Sulfur Hexafluoride by Pulsed Dielectric Barrier Discharge Synergistic Active Gas

Abstract

SF₆ is a strong greenhouse effect gas, which is widely used in high-voltage electrical equipment such as circuit breakers and high-voltage switchgear because of its excellent insulation performance and arc extinguishing ability. In recent years, the use and emission of SF₆ have been rising, and with the proposal of the dual carbon strategic goal, its harmless degradation has become an urgent problem to be solved. In this paper, SF₆ was degraded by pulsed DBD plasma technology and O₂. Studies have shown that the addition of O₂ can effectively promote the degradation of SF₆. With the increase in the added O₂ content, the DRE and EY of SF₆ first increased and then decreased. Under the conditions of the input power of 50 W, SF₆ concentration of 2%, and gas flow rate of 50 mL/min, the reaction system obtained the highest DRE and EY of 58.40% and 5.24 g/kWh when the O₂ content was 1%, respectively. In the SF₆/Ar/O₂/H₂O system, the addition of H₂O could improve the product selectivity of SO₂F₂, and when the O₂ concentration was 1%, the highest selectivity of SO₂F₂ was 48.96%, and the concentration was 8006.76 ppm. The addition of O₂ inhibited the production of SO₂, and with the addition of the O₂ system, SO₂F₂ and SOF₄ were the main components of degradation products; however, there were also SOF₂, SO₂, SiF_4, SF₄, etc. In this paper, the decomposition path of O₂ under SF₆ was analyzed in detail according to infrared spectroscopy and decomposition products.

1. Introduction

Sulfur hexafluoride (SF₆) is an artificial inert gas synthesized by French chemists Moissan and Lebeau in 1900. Under normal conditions, it is a colorless, odorless non-toxic non-flammable gas, slightly soluble in water, ethanol, and diethyl ether. Its insulation capacity is more than 2.5 times that of air and nitrogen, and its arc extinguishing capacity is 100 times that of air [1,2,3]. Because of its excellent electrical insulation and arc extinguishing characteristics, it is widely used in high-voltage electrical equipment such as circuit breakers, relays, high-voltage switchgear, etc. It is also used in metal smelting, aerospace, medical, and other fields [4,5,6]. However, due to its strong greenhouse effect, SF₆ was listed as a greenhouse gas in the Kyoto Protocol in 1997 [7]. Its global warming potential (GWP) is 23500 times greater than that of CO₂, and it takes 3200 years to degrade in the atmosphere, making it impossible to degrade naturally [8].

With the rapid development of the electric power industry in recent years, the demand and emission of SF₆ have been rising year by year, and the concentration of SF₆ increased dramatically in the atmosphere [9]. With the proposal of a double carbon strategic goal, the degradation and emission reduction of SF₆ has become an urgent problem to be solved. Many scholars at home and abroad have carried out extensive research on SF₆ degradation, and at present, there are mainly thermal catalytic degradation, photocatalytic degradation, plasma degradation, and so on. Thermal catalytic degradation is more effective and has a larger degradation amount, but its energy utilization rate is small [10]. Photocatalytic degradation reaction conditions are mild and easy to control, but the degradation amount and destruction and removal efficiencies (DREs) in this method are low [11]. Plasma degradation has the advantages of good degradation effect, high energy utilization, and ease of control [12]. Plasma degradation technology generates a large number of electrons and active particles in the plasma region to react with SF₆, causing it to gradually break bonds and decompose. Plasma degradation techniques are divided into gliding arc plasma [13], radiofrequency plasma [14], and dielectric barrier discharge (DBD) plasma [15,16,17], according to the plasma generation method. The gliding arc plasma can quickly produce a large amount of plasma, the phenomenon is more intense, and the processing rate is fast, but its energy utilization is low, and the reaction conditions are harsh [18]. Radiofrequency plasma is generated by the radiofrequency source of high-frequency alternating magnetic field excitation of the ionization of the gas undergoing reaction for its production, with low power consumption, and high degradation rate, but the equipment is more complex, and the method is not conducive to industrial applications [19]. DBD plasma has the advantages of mild reaction, easy control, high energy utilization, and adaptability [20,21]. In 2007, Shen et al. [22], used AC plasma to investigate the degradation law of SF₆ under different input voltages and found that the DRE gradually increased with the increase in power. The addition of water vapor can effectively improve the DRE; the initial partial pressure ratio of water vapor to SF₆ is close to 1:1, which can lead to a DRE of close to 90%, and the addition of water vapor of 28.2 kPa and 1.8 kPa of air can lead to SF₆ degradation of 92%. The addition of 28.2 kPa water and 1.8 kPa air can degrade SF₆ by 92%. In 2014, Zhuang et al. used an AC power supply to drive a star-shaped electrode DBD reactor to degrade 820 ppm SF₆ gas and found that the microdischarge generated at the tip of the star-shaped electrode effectively promoted the decomposition of SF₆ gas [23]. Zhang et al. [24,25] used DBD plasma to study the effects of different background gases and different filling dielectric on SF₆ degradation and found the degradation effect He > Ar > N₂ > air and the degradation effect Al₂O₃ > glass spheres > no filler with different filling dielectric materials, as well as the maximum DRE of 2% SF₆ at 110 W in the system filled with γ-Al₂O₃ particles. The maximum DRE of SF₆ reached 85.97%, and the energy yield (EY) reached 9.17 g/kWh. The pulsed power supply is a switching power supply system, and compared with the AC power supply, it will not continuously apply voltage to the reactor, effectively avoiding the thermal effect generated between two pulses. More discharge energy is used to ionize the gas so that the temperature of the reactor is maintained at a relatively low level, and at the same time, the narrow pulse rise time can inhibit the generation of the fine filament discharge and improve the uniformity of the discharge [26]. Shao et al. used nanosecond pulsed plasma and bimetallic nickel–iron catalyst coupling to synergistically catalyze CO₂ methanation and found that the Fe/Ni ratio at 231 °C had a CO conversion and energy efficiency of 67.5% and 57,823 μg/kJ, and a C-H selectivity of 99%, with complete suppression of the side reactions of CO generation [27]. Dong et al. established a multi-needle-plate high-pressure pulsed gas–liquid two-phase discharge plasma for the degradation of phenolic wastewater, and the results showed that under the conditions of a pulse voltage of 26 kV, a pulse frequency of 70 Hz, and an aeration volume of 4 L/min, the best degradation effect was achieved by the addition of 0.05 g of the Fe-TiO₂ catalyst coupled with the discharge plasma at a roasting temperature of 500 °C. In addition, the addition of active gas can effectively improve the DRE and EY and can change the product distribution to some extent [28]. Zhang et al. [29,30] explored the effects of the addition of H₂O and NH₃ on SF₆ degradation and found that the addition of H₂O can improve the selectivity of SO₂, and the addition of NH₃ can convert SF₆ gas into solid products such as sulfur monomers, NH₄HF₂, and NH₄F, which is convenient for harmless treatment of the products.

In order to further investigate the effect of adding active gas O₂ on SF₆ decomposition, in this study, an experimental platform of pulsed DBD was developed to analyze the effect of O₂ on the degradation of SF₆ by pulsed DBD. The decomposition path and promotion mechanism of SF₆ under the participation of O₂ were analyzed, and the findings provide an experimental basis for the efficient degradation of SF₆ waste gas.

2. Experimental Platforms

In this study, a pulsed DBD experimental platform was built, as shown in Figure 1, which mainly consisted of a gas distribution system, a plasma degradation system, a decomposition product detection system, and a decomposition product absorption processing system. Without special instructions, the SF₆ concentration was 2%, the gas flow rate was 50 mL/min, the input power was 45 W, the input voltage was 15 kV, and the pulse frequency was 15 kHz.

In this study, a GC500 (Jiangsu Electric Technology Co., Zhenjiang, Jiangsu, China) Ltd. four-channel intelligent dynamic gas distributor was used to control the input gas concentration and flow rate. The water gas generator was customized by (Suzhou Vorand Experimental Equipment Co., Ltd., Suzhou, China), and the range of the input gas flow rate was 0~500 mL/min with an instrumental accuracy of 1% F.S. The distribution concentration and output gas flow rate can be set in the gas distributor. A Fourier transform infrared spectrometer (FTIR) was used for qualitative analysis of SF₆ gas products. Gas chromatography–mass spectrometry (GC-MS) was used to quantify the main degradation products SO₂, SOF₂, SO₂F_2, and SOF₄. An MX2500+ three-channel spectrometer produced by (Ocean Optics Co., Ltd, Quadrangle Blvd Orlando, FL, USA) was used to detect the plasma emission spectra. In order to improve the reproducibility of the experiments, all the experiments in this study were repeated three times, and the results were averaged.

The DRE of SF₆ is calculated as follows:

$$DRE(\%)={\displaystyle \frac{C_{in}-C_{out}}{C_{in}}}\times 100\%$$

(1)

where $C_{in}$ is the concentration of SF₆ before degradation, and $C_{out}$ is the concentration of SF₆ remaining after the degradation reaction.

The EY of SF₆ is defined as the mass of SF₆ degraded per unit of input energy and is calculated using Equation (2).

$$EY(\mathrm{g}/\mathrm{kWh})={\displaystyle \frac{M_{S{F}_6}}{P\times t}}$$

(2)

where $M_{S{F}_6}$ is the mass of degraded SF₆ gas at a certain time, in g; P is the input power, in W; t is the degradation time; and EY is the energy efficiency, in g/kWh.

The product selectivity of the main degradation products of SO₂, SOF₂, SO₂F₂, and SOF₄ is calculated using Equation (3) as follows:

$$S_K={\displaystyle \frac{C_K}{C_{in}-C_{out}}}\times 100\%$$

(3)

where $C_K$ is the concentration of the decomposition product K gas after the SF₆ degradation reaction, and S_K indicates the concentration of the decomposition product K gas as a percentage of the total SF₆ degradation.

3. Results

3.1. DER and EY

The addition of H₂O and O₂ can change the decomposition path of SF₆; therefore, we explored the influence law of different O₂ concentrations on SF₆ degradation. Figure 2 shows the changing law of the DRE and EY of SF₆ by the O₂ concentration without the addition of H₂O and with the addition of 0.5% H₂O to the system. With the increase in the O₂ concentration, both the DRE and EY of SF₆ showed a trend of first increasing and then decreasing. In the system without additional H₂O, when the O₂ concentration was 1%, the DRE of SF₆ in the system was 58.40%, and the EY was 5.24 g/kWh. With the addition of 6% O₂, the DRE and EY of SF₆ were 48.45% and 4.35 g/kWh, respectively, which were higher than that of the case with no additional gas (42.65%, 3.82 g/kWh). The DRE and EY of SF₆ were significantly higher in the system with 0.5% H₂O added than in the system without the added H₂O, and the highest DRE and EY were observed at 2% O₂ concentration, namely at 84.2% and 7.3 g/kWh, respectively. In the system with 6% O₂, the DRE and EY of SF₆ were 68.94% and 5.99 g/kWh, which were lower than those in the system without the added O₂ (70.14% and 6.09 g/kWh). In the synergistic degradation of SF₆ by H₂O and O₂, excessive O₂ inhibited its degradation, probably because more energy was used to activate the decomposition of O₂, and thus less energy was used to decompose SF₆, resulting in a lower degradation rate of SF₆.

3.2. Active Particle Analysis

After investigating the influence of the O₂ concentration on the degradation of SF₆ by pulsed DBD, in order to further study the microscopic mechanism of O₂ promoting the decomposition of SF₆, we detected and analyzed the active particles in the decomposition process of SF₆ discharge with the addition of different concentrations of O₂.

Figure 3 shows the emission spectra of the SF₆/Ar/O₂ system with oxygen concentration at 1%. The main characteristic spectral lines in the 300~850 nm band include Ar and F spectral lines. The Ar spectral lines mainly include Ar 696.54 nm, Ar 750 nm, Ar 763.84 nm, Ar 772.78, and Ar 811.78 nm. The F spectral line produced by the free F atoms, which are generated by the bond breaking of (4) and (5), is F 738.51 nm. Weak N₂ out peaks were detected at 337.10 nm and 357.61 nm. Since there was no N₂ involved in the system, this may be due to the characteristic spectral line of the N₂ band generated by the N₂ excitation in the air between the spectrometer and the reactor.

According to the literature [31], the main dissociation process of O₂ molecules in the DBD reaction is shown in (6)~(9), and the O radicals produced by dissociation can provide the system with O elements and promote the SF₆ degradation reaction. However, O₂ is also an electronegative gas, which will adsorb electrons in the reaction and reduce the probability of collision reaction between free electrons and SF₆ gas molecules.

$${\mathrm{e}}^{*}{+\mathrm{SF}}_6\to {\mathrm{SF}}_{\mathrm{x}}+(6-\mathrm{x})\mathrm{F}+\mathrm{e}(1\le \mathrm{x}\le 5)$$

(4)

$${\mathrm{Ar}}^{*}+{\mathrm{SF}}_6\to {\mathrm{SF}}_x+(6-\mathrm{x})\mathrm{F}+\mathrm{Ar}(1\le \mathrm{x}\le 5)$$

(5)

$${\mathrm{e}}^{*}+{\mathrm{O}}_2\to \mathrm{O}+\mathrm{O}+\mathrm{e}$$

(6)

$${\mathrm{e}}^{*}+{\mathrm{O}}_2\to \mathrm{O}+{\mathrm{O}}^{-}$$

(7)

$${\mathrm{e}}^{*}+{\mathrm{O}}_2\to {\mathrm{O}}^{+}+\mathrm{O}+2\mathrm{e}$$

(8)

$${\mathrm{O}}^{-}\to \mathrm{O}+\mathrm{e}$$

(9)

$$\mathrm{e}+\mathrm{Ar}\to {\mathrm{Ar}}^{*}+\mathrm{e}$$

(10)

In addition, the spectral intensity of the Ar 763.84 nm spectral line under different O₂ concentration conditions was recorded in this study. Figure 4 shows the relative intensity of the Ar 763.84 nm spectral line under different O₂ concentration conditions. With the gradual increase in O₂ concentration, the intensity of the Ar 763.84 nm spectral line in the system gradually decreased, which was mainly caused by the fact that the increase in the O₂ concentration led to a decrease in the Ar gas in the system; thus, the reaction process of (7) was inhibited, and the content of the excited-state Ar* in the reaction system decreased, which led to the weakening of the intensity of the Ar spectral line. Although O radicals could promote the SF₆ degradation process and inhibit the recombination of primary decomposition products of SF₆, when the O₂ concentration was excessive, the Penning ionization process in the DBD reaction was weakened, and the plasma density decreased, which reduced the opportunity for the interaction of active substances such as high-energy electrons, excited-state particles, and other active substances with SF₆ gas molecules, which then resulted in a decrease in the DRE of SF₆.

3.3. Product Analysis

In order to qualitatively analyze the decomposition products of SF₆ under the added O₂ system, FTIR was used to detect and analyze the system with and without added O₂. Figure 5, Figure 6 and Figure 7 show the FTIR spectra of the degradation products in the systems without the addition of O₂ and with the addition of 1% O₂, and 2% O₂/0.5% H₂O, respectively. The main degradation products included SOF₄, SO₂F₂, SOF₂, SF₄, OF₂, and SiF₄. The addition of 2% O₂/0.5% H₂O system showed a significant decrease in the absorbance of SF₆ and a significant increase in SO₂F₂ and SiF₄ compared with the no-addition system and 1% O₂ system. The results are consistent with the GCMS quantitative detection results in Figure 8 and Figure 9. The addition of O₂ and H₂O significantly initiated the degradation of SF₆, which was decomposed into SO₂F₂. The reason for the detection of SiF₄ in the SF₆ decomposition products is that the reactor material used in the experiment is SiO₂ quartz glass. The F ions produced by the ionization of SF₆ gas in the plasma region etched with SiO₂ and generated SiF₄. However, when different concentrations of O₂ were added, no characteristic peaks of SO₂ appeared in the FTIR spectra, indicating that SO₂ is relatively difficult to generate when O₂ is added. There were more obvious SO₂ characteristic peaks in the system with the addition of 0.5% H₂O, indicating that the addition of H₂O can improve the product selectivity of SO₂. The formation pathways of each product were hypothesized based on the main degradation products, as shown in (11) to (17). It can be seen that the energy of the reaction path for the formation of S-O-F intermediate products such as SOF₄ and SOF₂ during the reaction was low, and these reactions were more likely to occur. The S-O-F intermediate products could further generate SO₂F₂ and the final products SO₂ and HF through the reaction.

To further understand the distribution of the main degradation products, four gases, namely SO₂, SOF₂, SO₂F₂, and SOF₄, were quantitatively detected by GCMS, and the selectivity distributions of the four products are shown in Figure 8 and Figure 9. In the SF₆/Ar/O₂ system, the product selectivity of SO₂F₂ and SOF₄ dominated, and only a small amount of SO₂ and SOF₂ was detected. With the increase in O₂ content, the selectivity of SO₂F₂ showed a rising trend as a whole, and the selectivity of SO₂F₂ after the addition of O₂ was higher than that of the system without the addition of O₂. The selectivity of SOF₄ showed a trend of increasing and then decreasing, as shown in (15), and the selectivity of SO₂F₂ and SOF₄ with the addition of O₂ was higher than that of the system without O₂. The product selectivity of SO₂F₂ and SOF₄ was higher than that of the system without O₂, which indicated that O₂ improved the generation of S-O-F products at 1% O₂ concentration; the selectivity of SO₂F₂ was 34%, and its concentration was 3978.83 ppm at this time. The concentration of SOF₄ was 408.29 ppm at 1% O₂ concentration. The decomposition products such as SO₂, SOF₄, and SO₂F₂ are acidic gases, which can be absorbed by the saturated NaOH solution at the end of the reaction system, as shown in Figure 1, thus effectively minimizing their harmful effects on the environment and human safety. The gases absorbed by lye no longer contained atmospheric pollutants, thus achieving the purpose of harmless emission.

In the SF₆/Ar/O₂/H₂O system, the main products were SO₂F₂, and trace SO₂ with SOF₂ and SOF₄. Compared with the SF₆/Ar/O₂ system, the addition of O₂ improved the product selectivity of SO₂F₂. With the increase in the O₂ content, the product selectivity of SO₂F₂ showed a trend of increasing and then decreasing, and in the case of 1% O₂, the selectivity of SO₂F₂ was 48.96%; the highest selectivity was 48.96% at a concentration of 8006.76 ppm. SOF₂ increased with the increase in the O₂ content, as shown in Equation (14).

In the SF₆/Ar/O₂ system, the free O radicals, produced according to Formula (6), reacted with the low-fluorine sulfide produced by gradual dissociation to form the main degradation products such as SO₂F₂, in which SOF₄, SOF₂, SO₂F₂, and SO₂ were mainly produced according to Equations (11)–(17) in

Table 1. Formation reactions of the products and their reaction heat with O₂.

NO.	Reaction	Reaction Heat (kcal/mol)
(11)	${\mathrm{SF}}_5+\mathrm{O}\to {\mathrm{SOF}}_4+\mathrm{F}$	−177.94
(12)	${\mathrm{SF}}_4+\mathrm{O}\to {\mathrm{SOF}}_4$	−529.82
(13)	${\mathrm{SF}}_3+\mathrm{O}\to {\mathrm{SOF}}_2+\mathrm{F}$	−324.56
(14)	${\mathrm{SF}}_2+\mathrm{O}\to {\mathrm{SOF}}_2$	−547.64
(15)	${\mathrm{SOF}}_4+\mathrm{O}\to {\mathrm{SO}}_2{\mathrm{F}}_2+2\mathrm{F}$	−365.64
(16)	${\mathrm{SOF}}_2+\mathrm{O}\to {\mathrm{SO}}_2{\mathrm{F}}_2$	−246.46
(17)	${\mathrm{SOF}}_2+\mathrm{O}\to {\mathrm{SO}}_{2}2\mathrm{F}$	−285.89

[32]. SOF₄ is the intermediate product of the degradation reaction, which will be further oxidized by O₂ to SO₂F₂, which is the reason why the content of SOF₄ is lower than that of SO₂F₂. The formation process of SOF₂ is shown in Equations (13) and (14); this process requires SF₆ gas molecules to combine with O after breaking 3~4 S-F bonds. In addition, SOF₂ gas also combines with O to form further products such as SO₂F₂, resulting in a relatively low content, while SO₂ has a relatively low content due to its formation pathway requiring more steps and energy in the reaction system.

In summary, the O radicals provided by O₂ in the plasma reaction can react rapidly with the low-fluorinated sulfides produced by the gradual dissociation of SF₆ to form stable compounds, which facilitates the SF₆ degradation process. In order to more clearly show the path of O₂ involved in SF₆ decomposition, Figure 10 summarizes the decomposition of SF₆ and the generation of products.

3.4. Comparison of Degradation Methods and Harmless Treatment of SF₆ Degradation Products

3.4.1. Comparison of Degradation Methods

Table 2. Comparison of degradation performance of main methods.

Degradation Method	Degradation Rate	Energy Efficiency	Product Regulation
Thermal Degradation	High	Low	Converts to salts when reacting with corrosive materials
Thermal Catalysis	Moderate	Moderate	Generates toxic gases such as SO2 and SO3
Photocatalysis	Low	-	Produces harmless substances
DBD Plasma	High	High	Produces toxic substances such as SO2F2
Electrochemical Degradation	Extremely Low	-	Produces harmless substances

summarizes the performance of various degradation methods in terms of the degradation rate, energy efficiency, and product regulation. Overall, except for the DBD plasma technique used for the degradation of SF₆, catalytic degradation techniques suffer from low energy efficiency and small processing capacity, which presents many critical issues that need to be resolved before achieving large-scale industrial applications. The currently implemented high-temperature pyrolysis methods require significant heating and constant replacement of adsorption materials, resulting in high operational costs and increased danger. In contrast, DBD plasma technology boasts high degradation rates and high energy efficiency, with relatively strong safety and scalability. It can flexibly form portable and centralized degradation devices, making this technology a promising solution for the degradation of SF₆ in the power industry [33].

3.4.2. Harmless Treatment of SF₆ Degradation Products

The degradation products of SF₆, SOF₂, SO₂, SO₂F₂, and SOF₄ are all harmful substances. From a safety point of view, it is necessary to minimize the direct emission of harmful products. SOF₂ and SO₂ can be quickly absorbed by lye, and SOF₄ and SO₂F₂ can also be hydrolyzed and absorbed in a saturated NaOH solution. Therefore, 40% NaOH solution was used to absorb SF₆ degradation tail gas. The qualitative analysis of the treated exhaust gas by FTIR detection is shown in Figure 11. It can be seen that most of the degradation products of SF₆ are absorbed, and there is a small amount of SOF₄ gas and SO₂F₂ gas in the remaining gas except for some SF₆ gas, which is not involved in the reaction.

In order to detect the content of each substance in the SF₆ degradation tail gas treated with NaOH solution, the concentrations of SO₂, SOF₂, SO₂F₂, and SOF₄ were quantitatively detected by GCMS. The results are shown in Figure 12. SO₂ and SOF₂ were completely treated with the NaOH solution, and the concentrations of SO₂F₂ and SOF₄ were 100.9 and 23.6 ppm, respectively. According to Emission Standards of Pollutants for Inorganic Chemical Industry GB31573-2015 [34], the fluoride emission limit is 6 mg/m³, and the value converted into ppm is 134.4. Therefore, the exhaust gas absorbed by 40% NaOH solution meets the emission standards, thus achieving the purpose of harmless emission.

4. Conclusions

• The addition of O₂ could effectively promote the degradation of SF₆. With the increase in the addition of the O₂ content, the DRE and EY of SF₆ first increased and then decreased. The highest DRE and EY were obtained in the reaction system with 2% SF₆ at the input power of 45 W and the gas flow rate of 50 mL/min and in the reaction system with the O₂ content of 1%, which were 58.40% and 5.24 g/kWh, respectively.
• In the SF₆/Ar/O₂/H₂O system, the addition of H₂O improved the product selectivity of SO₂F₂, and the highest SO₂F₂ selectivity of 48.96% was obtained at an O₂ concentration of 1% and a concentration of 8006.76 ppm.
• The increase in the O₂ content led to a decrease in the Ar content, which in turn led to a decrease in the excited-state Ar* content, resulting in a decrease in the degradation rate in this system.
• The addition of O₂ inhibited the generation of SO₂, and in the addition of O₂, SO₂F₂ and SOF₄ were the main components in the degradation products. In addition, there were SOF₂, SO₂, SiF₄, SF₄, and OF₂.