Effect of ZnO/γ-Al2O3 Catalyst

Packing on the Degradation of SF6 in a Dual-Stage DBD Plasma Reactor

Authors: Chi Zhang, Ying Zhang, Jianbo Shi, Huiyu Tan, Yalong Li, Lei Yu

Abstract

Sulfur hexafluoride (SF6) gas is widely used in the electric power industry as an excellent insulating gas. However, SF6 is a potent greenhouse gas, and its degradation is an ideal method for emission reduction. In the field of SF6 degradation, due to the harsh decomposition conditions, it is often difficult to achieve both a high degradation rate and favorable product distribution. To address this issue, a double-stage dielectric barrier discharge (DBD) series degradation experimental platform for SF6 gas was designed in this study. A ZnO catalyst was packed in the first-stage reactor to improve the SF6 degradation rate, while a γ-Al2O3 catalyst was packed in the second-stage reactor to optimize the product distribution. Additionally, the selectivity of SF6 degradation products was regulated by externally adding H2O. Experimental results showed that under an input power of 100 W, the SF6 degradation rate reached 100% in the ZnO/γ-Al2O3 double-stage packed system, achieving complete degradation of SF6. As the input power increased, the energy yield showed a gradually decreasing trend. The addition of an appropriate concentration of H2O effectively improved both the degradation rate and energy yield of SF6, with the maximum energy yields of the two-stage reactors reaching 23.9 and 17.9 g/kWh, respectively. With regard to decomposition products, the types of SF6 decomposition products in both reactors remained unchanged, including SO2, SO2F2, SOF2, and SOF4. The main product in the first-stage reactor was SO2F2, whereas in the second-stage reactor, SO2 was predominant. The addition of H2O also effectively increased the yield of SO2 while suppressing the formation of SO2F2, which is more difficult to treat. This study provides experimental support for SF6 exhaust gas degradation and offers a research direction for industrial applications.

Introduction

SF6 is a colorless, odorless, nontoxic, nonflammable inert gas, −

which exhibits excellent electrical insulation and arc-quenching properties, making it widely used in gas-insulated equipment. However, SF6 was listed as one of the six greenhouse gases in the Kyoto Protocol in 1997. , Its global warming potential (GWP) is 23,500 times that of carbon dioxide (CO2), and its atmospheric lifetime reaches up to 3200 years. , The direct emission of SF6 waste gas poses a significant threat to the ecological environment. Therefore, the degradation and conversion of SF6 are effective measures to reduce its emission. At present, mainstream methods for SF6 degradation include thermal (catalytic) decomposition, photocatalytic degradation, and nonthermal plasma degradation. −

Among them, nonthermal plasma technology represented by dielectric barrier discharge (DBD) has received widespread attention due to its ability to utilize high-energy electrons and active species in the plasma to react with SF6, thereby breaking its molecular bonds and achieving decomposition in a short time.

Studies have shown that during the DBD degradation of SF6, the addition of different packing materials and active gases can effectively improve the SF6 degradation rate and energy yield, as well as optimize the distribution of the byproducts. In 2014, Zhuang et al. applied DBD to degrade SF6 and found that the addition of air and CO2 significantly increased the degradation rate, achieving up to 94% for 3600 ppm of SF6. However, a large amount of SO2F2 was produced, which was unfavorable for subsequent harmless treatment of the byproducts. Kim et al. introduced α-Al2O3 pellets as a catalyst in a DBD reactor for SF6 degradation. However, due to the low activity of α-Al2O3, a reaction temperature above 400 °C was required to provide sufficient surface active sites to improve the SF6 degradation rate. Zhang et al. studied the effects of packing catalysts such as ZnO and γ-Al2O3, as well as active gases like H2O and O2, on the DBD degradation of SF6. Their study revealed that ZnO could effectively increase the discharge voltage and current amplitude, promote filamentary discharge, and significantly enhance the degradation rate of SF6 in the packed bed reactor. Meanwhile, γ-Al2O3, with its large specific surface area, rich pore structure, and abundant active surface sites, was effective in regulating product distribution. , Regarding active gases, the addition of H2O or O2, either individually or in combination, was found to improve the SF6 degradation rate. These gases also influenced the distribution of decomposition the presence of H2O tended to promote the formation of SO2, while O2 favored the generation of SO2F2. , Therefore, both packing catalysts and active gases play a positive role in enhancing the DBD degradation of SF6. However, it remains challenging to achieve both high degradation efficiency and precise control over product distribution under these conditions. To address this issue, some researchers have proposed using multistage DBD reactors in series for SF6 degradation. For example, when DBD reactors were connected in series and packed with CaO, both the SF6 degradation rate and the selectivity toward CaF2 and CaSO4 were improved.

In this work, a dual-stage DBD platform was constructed for SF6 degradation. ZnO catalyst was packed in the first-stage reactor to enhance the SF6 degradation rate, while the γ-Al2O3 catalyst was packed in the second-stage reactor to improve the product distribution. On the basis of catalyst-packed dual tubes, H2O was added to further enhance the SF6 degradation rate and optimize the byproduct profile, providing a new approach for the industrial degradation of SF6 waste gas.

Experimental Platform and Methods

The dual-stage DBD experimental platform for SF6 degradation is shown in Figure . Since pure SF6 gas is difficult to ionize under discharge conditions, a gas distributor system is first employed to dilute SF6 and obtain a gas mixture with the desired concentration. The DBD reactor is powered by a high-voltage power supply, generating plasma to initiate the degradation reactions. The gas products generated after the degradation process are analyzed by a gas detection system. Finally, the exhaust gas is treated by absorption in a NaOH solution.

The experiments were conducted using a CTP-2000K AC power supply produced by Nanjing Suman Technology Co., Ltd., which, when paired with a voltage regulator, provides an AC output voltage adjustable from 0 to 30 kV, a frequency range of 1 to 100 kHz, and an output power range of 0 to 500 W. In this study, the operating frequency was approximately 8.5 to 9.0 kHz, and the input power of the power supply was kept below 120 W. The gas distribution system mainly consists of a dynamic gas distributor and a precision water vapor generator. The gas distributor used was the GC500 model from Jiangsu Tanggao Electric Co., Ltd., with a precision of up to 0.2% F.S. and a maximum dilution ratio of 1. The precision water vapor generator, custom-made by Suzhou Furande Experimental Equipment Co., Ltd., features a gas flow range of 0–500 mL/min and an accuracy of 1% F.S. The gases used in the experiment were supplied by Newrad Gas Co., Ltd..

The reactor used in the experiment adopts a double-layer quartz dielectric coaxial cylindrical structure, with the inner electrode being a copper rod and the outer electrode being a stainless steel mesh wrapped around the outer surface of the outer dielectric tube. The discharge region has a length of 20 cm, and the discharge volume is approximately 52 cm^3^. The detection system mainly consists of a gas chromatograph (GC), a Fourier transform infrared spectrometer (FTIR), and a gas chromatograph–mass spectrometer (GC-MS).

The experimental conditions are as the background gas is Ar, the initial SF6 concentration is 3%, and the gas flow rate is 150 mL/min. The reactor operated at atmospheric pressure. The degradation effect of SF6 is mainly evaluated using the following three degradation rate, energy yield, and the concentration of degradation products.

The calculation formula for the degradation rate (DR) is1DR(%)=Cin−CoutCin×100%where C in and C out represent the volume fractions of the target gas before and after degradation, expressed in % or ppm.

The calculation formula for the energy yield (EY) is2EY(g/kWh)=DR×Cin×Vg×60×146p×22.4where P is the plasma discharge power, V g is the gas flow rate during injection (in mL/min), 60 is the conversion factor from minutes to hours, 146 is the molar mass of SF6, and 22.4 is the standard molar volume. The unit of EY is g/kWh.

Results and Analysis

Electrical Parameters

In order to analyze the discharge behavior of different packing material systems, this article compares the Lissajous figures of the two-stage catalyst-packed reactor and the empty tube discharge system, as shown in Figure . Compared to the empty tube system, the addition of the two types of packing materials caused the discharge pattern shift from the parallelogram shape of the empty tube system to a more elliptical shape. The slopes of the DA and BC edges of the first- and second-stage reaction tubes increased, indicating an increase in the total system capacitance in the charging section. Additionally, the slopes of the AB and CD edges also increased, corresponding to an increase in the equivalent capacitance of the dielectric in the system. The calculated equivalent capacitances of the insulating dielectric in the first- and second-stage reaction tubes are shown in Table . Specifically, the equivalent capacitance of the first- and second-stage reaction tubes is 73.86 pF and 63.58 pF, respectively, which are significantly greater than the 52.61 pF of the empty tube. The equivalent capacitance of the first-stage reaction tube is greater than that of the second-stage reaction tube. Because compared to γ-Al2O3, ZnO as a semiconductor has a higher dielectric constant. This facilitates the formation of discharge channels and the accumulation of charges, thereby increasing the equivalent capacitance of the discharge region. Furthermore, the addition of the packing materials also increased the area enclosed by the Lissajous figure, indicating a significant increase in discharge power, which allows more energy to be used for activating and decomposing SF6. Therefore, it can be concluded that both types of packing in the first- and second-stage reaction tubes contribute to promoting the discharge process.

Effect of ZnO Catalyst Packing on Degradation

Rate and Energy Yield

To investigate the effect of the dual-stage DBD system on SF6 degradation, it is necessary to analyze the degradation rate and energy yield of the first-stage reaction tube and determine the optimal input power. During the degradation process in the dual-stage DBD system, the input power of the first-stage reaction tube is fixed to ensure that only a single experimental variable exists in the second-stage reaction tube. The influence of the ZnO-packed first-stage reaction tube on the degradation rate and energy yield of SF6 is shown in Figure .

It can be seen that as the input power increases, the degradation rate of SF6 shows a gradually increasing trend. When the input power is 70 W, the degradation rate of the first-stage reaction tube is 38.74%. As the input power increases to 110 W, the degradation rate reaches its maximum value of 85.03%. ZnO as the packed catalyst in the first-stage reaction tube can provide abundant active sites during the degradation of SF6, enabling the adsorption of SF6 molecules and their dissociation intermediates (such as SF~ x ~ and F^–^). This facilitates sufficient contact between the reactants and active species, thereby promoting the degradation of SF6. In the field of SF6 degradation, energy yield is an important parameter for evaluating the energy efficiency of the process and thus is of great significance. As shown in Figure , the energy yield of the first-stage reaction tube decreases as the input power increases. Specifically, when the input power increases from 70 to 110 W, the energy yield decreases from 21.45 to 15.93 g/kWh. Given that the degradation rate of SF6 increases significantly as the input power rises from 70 to 100 W, while energy yield decreases with increasing input power, an input power of 100 W is selected as the fixed input power for the first-stage reaction tube in the dual-stage DBD process for SF6 degradation.

Degradation Rate and Energy Yield in the Second-Stage

Reaction Tube

When the input power of the first-stage reaction tube was maintained at 100 W, the curves of the SF6 degradation rate and energy yield in the second-stage reaction tube as a function of input power are shown in Figure . It can be observed that the degradation rate in the second-stage reaction tube was significantly higher than that in the first-stage reaction tube. This is because 18.78% of SF6 remained undecomposed after the first stage, resulting in an SF6 concentration of only 5634 ppm entering the second-stage reaction tube, where energy available for SF6 degradation was relatively sufficient, thus leading to a higher degradation rate. In addition to further degrading the small amount of SF6 not decomposed in the first-stage reaction tube, the second-stage reaction tube also played a role in secondary degradation and regulation of the byproducts generated in the first stage. This observation is consistent with the experimental results reported in ref . When the input power was 70 W, the degradation rate in the second-stage reaction tube reached 90.1%. As the input power increased, the degradation rate further improved, reaching 100% at 90 W input power. This indicates that the overall degradation in the dual-stage DBD system achieved 100%, realizing complete degradation of SF6. γ-Al2O3 as the packed catalyst in the second-stage reaction tube possesses a rich porous structure and a large specific surface area, which helps enhance the contact between reactants and the catalyst surface and prolongs the residence time of reactants in the discharge region. This facilitates further conversion of SF6 that was not degraded in the first-stage reaction tube, thereby improving the overall degradation rate.

A comparison of the energy yield curves in Figures and reveals that overall, the energy yield of the first-stage reaction tube was higher than that of the second-stage reaction tube. This is because a larger amount of SF6 was degraded in the first stage, where most of the SF6 was already decomposed. In contrast, the SF6 content in the second-stage reaction tube was relatively low, resulting in a smaller amount of decomposed SF6. Additionally, more energy in the second-stage reaction tube was consumed for the further decomposition of intermediate products formed in the first stage, which contributed to the lower energy yield. In both reaction tubes, the energy yield decreased as the input power increased. As the input power increased from 70 to 110 W, the energy yield in the second-stage reaction tube dropped from 15.55 to 9.12 g/kWh. According to relevant studies, SF6 exhibits excellent thermal stability and does not undergo thermocatalytic decomposition below 400 °C. However, in the present experimental system, the DBD degradation process generally ranges from 90 to 130 °C, which is far below the onset temperature for SF6 thermal decomposition. As the input power increased, part of the energy was dissipated as heat, which did not significantly promote SF6 decomposition. This ultimately led to a continuous decrease in energy yield.

Influence of ZnO/γ-Al2O3 Catalyst Packing on Decomposition Products

Figure presents the FTIR spectrum of the dual-stage packing system under an input power of 90 W. Under the impact of high-energy particles, SF6 molecules undergo progressive bond cleavage, as shown in reactions –. The resulting reactive species further combine with other particles to form sulfur-containing or fluorine-containing byproducts.3e+Ar→Ar*+e 4e*+SF6→SFx+(6‐x)F+e(1≤x≤5) 5Ar*+SF6→SFx+(6−x)F+Ar(1≤x≤5) 64HF+SiO2→SiF4+2H2OAfter searching and comparing the characteristic peaks, it was found that the main degradation products of SF6 after the two-stage reaction are the same in terms of species, namely, SO2F2, SO2, SOF2, and SOF4. This is consistent with the decomposition products identified in the literature for SF6 degradation in a single-stage process, indicating that the use of the dual-stage DBD for SF6 degradation does not change the types of degradation products. As for the oxygen (O) element in the degradation products, it mainly originates from SiO2 in a quartz glass reactor. Additionally, the presence of SiF4 was detected through the characteristic peaks, which is due to the corrosion of SiO2 by HF. The reaction process is shown in eq .

To further investigate the influence of the ZnO/Al2O3 dual-stage packed system on SF6 degradation products, quantitative analysis of SO2F2, SO2, SOF2, and SOF4 was performed by using GC-MS. The product concentration distribution is shown in Figure . As shown in Figure a, with the change in input power, the degradation product in the first-stage reaction tube is mainly SO2F2, and its concentration fluctuates between 15,989.76 and 11,953.26 ppm. The concentration of SO2 increases with the increase in input power, rising from 783.88 to 5355.63 ppm. However, its overall content is still lower than that of SO2F2 because in the first-stage reaction tube, reactions and dominate, while the content of SO2 generated by reactions and is relatively less. The content of SOF2 fluctuates between 2068.22 and 3743.55 ppm as the input power changes. SOF4 has the lowest concentration among the four degradation products, and its concentration gradually decreases with a change in input power. At 70 W, its concentration is 281.6 ppm, and when the input power reaches 110 W, SOF4 is almost undetectable. Analyzing reactions to shows that SOF2 and SOF4 mainly serve as intermediate products in the reactions. Both cannot remain stable for long periods during the reaction process; therefore, their concentrations are lower than those of SO2F2 and SO2.

7SOF2+O→SO2F2 8SOF4+O→SO2F2+2F 9SOF2+O→SO2+2F 10S+2O→SO2 11SF5+O→SOF4+F 12SF4+O→SOF4 13SF2+O→SOF2 14SF+O+F→SOF2 15SO2F2+2e→SO2+2FBased on the product concentration distribution of the second-stage reaction tube in Figure b, it can be observed that with the increase in input power, the content of SO2F2 gradually decreases. This is because reaction predominates in the second-stage reaction tube. At 70 W input power, its concentration is 15,105.38 ppm, but when the input power reaches 110 W, the concentration of SO2F2 sharply drops to 1869.94 ppm. However, the concentration of SO2 increases with the increase in input power. At 70 W, its concentration is 9302.68 ppm, and when the input power reaches 80 W, the concentration of SO2 increases to 15758.56 ppm. At this point, the concentration of SO2 in the products exceeds that of SO2F2, making SO2 the dominant degradation product. When the input power reaches 110 W, the concentration of SO2 reaches 25302.68 ppm, with most of the sulfur (S) in SF6 being converted to SO2. The variation trend of SOF2 is similar to that in the first-stage reaction tube, with its concentration fluctuating between 527.21 and 2135.18 ppm as the input power changes. The content of SOF4 is still relatively low compared to other products, and with the increase in input power, the concentration of SOF4 fluctuates between 297.12 and 158.29 ppm.

Effect of Additional H2O on the

Degradation Rate and Energy Yield of the ZnO/γ-Al2O3 Catalyst Packing System

Previous studies have shown that both the introduction of additional gases and catalyst packing can effectively enhance the degradation rate of SF6 and facilitate product regulation. To further improve the degradation performance of SF6, the effect of additional H2O was investigated on the basis of the catalyst packing. Under the conditions of an input power of 90 W, a gas flow rate of 150 mL/min, and Ar as the background gas, the influence of varying H2O concentrations on the degradation of SF6 by the ZnO/γ-Al2O3 dual-stage DBD system was studied.

Figure shows the effect of different H2O concentrations on the SF6 degradation rate and energy yield of the dual-stage DBD. It can be observed that, similar to the case without H2O addition, the degradation rate of the second-stage reaction tube is overall higher than that of the first-stage reaction tube. This is mainly due to the lower concentration of SF6 entering the second-stage reaction tube. As the concentration of additional H2O increases, the degradation rates in both reaction tubes first increase and then decrease. When the H2O concentration increases from 0.5 to 1.5%, the degradation rate in the second-stage reaction tube rises from 76.97 to 100%, indicating complete degradation of SF6 in the dual-stage system. However, with a further increase in the H2O concentration, the degradation rate begins to decline. Specifically, when the concentration increases from 1.5 to 2.5%, the degradation rate in the first-stage reaction tube decreases from 75.24 to 66.95%, while that in the second-stage reaction tube decreases from 100 to 71.29%. This phenomenon is mainly attributed to the fact that the addition of H2O absorbs energy in the reaction system and undergoes ionization, producing active species, such as OH and H radicals. These species can combine with SF~ x ~ and F^–^ generated from SF6 dissociation, thereby inhibiting recombination reactions and promoting SF6 degradation. However, H2O is also an electronegative gas. As its concentration increases, more high-energy electrons are captured by H2O, thus limiting the degradation reactions of SF6 and leading to a decrease in its degradation rate.

The analysis of the energy yield distribution under different concentrations of additional H2O reveals that the energy yield of the first-stage reaction tube is generally higher than that of the second-stage reaction tube. Both tubes exhibit a trend of initially increasing and then decreasing energy yields with increasing H2O concentration. As the H2O concentration increases from 0.5% to 1.0%, the energy yield of the first-stage reaction tube increases from 21.9 to 23.3 g/kWh, while that of the second-stage reaction tube increases to a peak of 17.9 g/kWh. When the H2O concentration reaches 1.5%, the energy yield of the second-stage reaction tube begins to decline to 16.7 g/kWh, while that of the first-stage reaction tube continues to increase slightly to 23.9 g/kWh. As the H2O concentration further increases from 2.0% to 2.5%, the energy yield of the first-stage reaction tube drops from 21.1 to 18.6 g/kWh, and that of the second-stage reaction tube decreases from 14.3 to 12.9 g/kWh. At lower concentrations of H2O, the energy yields of both tubes are enhanced due to the generation of reactive OH and H species during discharge, which effectively promote SF6 degradation. However, excessive H2O can reduce the energy yield due to the quenching effect and competition for electrons, leading to a decline in the degradation efficiency.

Effect of Additional H2O on the

Decomposition Products in the ZnO/γ-Al2O3 Catalyst Packing System

Figure shows the variation in concentrations of decomposition products in the dual-stage catalyst packing system under different concentrations of additional H2O. During the degradation process, H2O molecules undergo ionization through collision or electron attachment, forming negative ions that subsequently decompose into H and OH radicals, as shown in reactions –. With the gradual increase in the concentration of added H2O, the SO2 concentration in the first-stage reaction tube increased from 5391.71 to 14,924.83 ppm, while the concentration of SO2F2 decreased from 13,372.8 to 5784.42 ppm. When the H2O concentration reached 1.5%, the SO2 concentration reached 13752.17 ppm, exceeding that of the SO2F2. This is primarily because SO2F2 reacts with the H radicals generated from the ionization of H2O, forming SO2, as illustrated in reaction . In addition, in the presence of added H2O, the concentrations of SOF2 and SOF4 generated in the first-stage reaction tube remained low. This is because SOF2 and SOF4 can also react with H2O to form SO2 and SO2F2, as described in reactions and .16e*+H2O→H+OH 17e*+H2O→H2O− 18e*+H2O−→H+OH+e 19SO2F2+2H→SO2+2HF 20SOF2+H2O→SO2+2HF 21SOF4+H2O→SO2F2+2HF

The trend of product concentration change in the second-stage reaction tube is similar to that in the first-stage reaction tube. As the added H2O concentration increases, the product concentration of SO2 gradually increases from 12,382.98 ppm to 23,222.25 ppm, while the product concentration of SO2F2 gradually decreases from 14,145.55 ppm to 1187.99 ppm. Additionally, the concentrations of SOF2 and SOF4 are not significantly affected by changes in the added H2O concentration. The product concentration of SOF2 remains between 242.82 ppm and 2718.73 ppm, while the concentration of SOF4 is the lowest. When the added H2O concentration is 2%, the maximum product concentration of SOF4 is 122.06 ppm.

Without the addition of H2O, when the input power was 90 W, the degradation rate of SF6 in the dual-stage system reached 100%, indicating complete degradation, with SO2 as the main degradation product, which is relatively easy to handle. When H2O was introduced at a concentration of 1.5% while maintaining the input power at 90 W, the degradation rates in both the first-stage and second-stage reaction tubes reached their maximum values. The overall degradation rate of the dual-stage system also remained at 100%, and the concentration of SO2 was further increased. Therefore, this study concludes that the optimal experimental condition for SF6 degradation is an input power of 90 W combined with an H2O concentration of 1.5%.

Results and Discussion

In the DBD SF6 degradation experiment, facing the challenge of balancing the high degradation rate and product control, a dual-stage DBD series system was used. The first-stage reaction tube focused on improving the SF6 degradation rate, while the second-stage reaction tube focused on product control. This approach enables high degradation rates while maintaining good product selectivity. In this article, the dual-stage DBD degradation experimental platform was used to investigate the effects of ZnO/γ-Al2O3 catalyst packing and added H2O on the degradation rate, energy yield, and degradation products of SF6 in the DBD dual-stage system. The following conclusions were (1)As the input power increases, the SF6 degradation rate of the ZnO/γ-Al2O3 dual-stage packing system gradually increases. At an input power of 100 W, the overall SF6 degradation rate of the dual-stage system can reach 100%. However, the energy yield of both the first-stage and second-stage reaction tubes gradually decreases as the input power increases.(2)The main degradation products of SF6 in the ZnO/γ-Al2O3 dual-stage packing system are SO2F2, SO2, SOF2, and SOF4. The γ-Al2O3 packing in the second-stage reaction tube can significantly regulate the distribution of SF6 degradation products, promoting the decomposition of SO2F2 in the second-stage reaction system, resulting in SO2 as the dominant degradation product.(3)In the ZnO/γ-Al2O3 dual-stage packing system, as the concentration of added H2O increases, both the degradation rate and energy yield first increase and then decrease. At the same time, the addition of H2O can further improve the product concentration distribution at lower input power.