SF6 abatement in a packed bed plasma reactor: Role of zirconia size and optimization using RSM

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Abstract

This work describes plasma destruction of SF6 in a zirconia (ZrO2) packed bed plasma reactor (PBR). Electric signals, equivalent parameters, emission spectra and degradation results have been utilized to evaluate the influence of ZrO2 size on PBR discharge characteristics and SF6 degradation. The results present that the size of ZrO2 has a significant impact on PBR discharge characteristics due to its influence on physical parameters. Moreover, bigger ZrO2 beads packing shows a better performance in SF6 degradation because of longer gas residence time. In addition, the key operating parameters including flowrate, SF6 concentration, oxygen concentration, and water vapor concentration were optimized by response surface methodology (RSM) with Box-Behnken design (BBD). The proposed optimization model shows satisfactory correlation between the predicted and actual results. For energy yield (EY), the mutual effect of flowrate, SF6 concentration and water vapor concentration was significant. For SO2F2 selectivity, the mutual effect of SF6 concentration and water vapor concentration was significant. The optimum SF6 abatement was predicted from RSM as 14.64 g/kWh and 14.42% for EY and SO2F2 selectivity at flowrate of 200 mL/min, SF6 concentration of 3%, oxygen concentration of 0.83% and water vapor concentration of 1.89%.

Introduction

Sulfur hexafluoride (SF6), a colorless, odorless and non-flammable inert gas, has been widely used in the power industry because of its excellent insulation and arc extinguishing properties [1]. However, it is also one of the most potent greenhouse gases with a global warming potential value (GWP) of 23,500 [2], [3], [4]. In 1997, SF6 was defined as a restricted-usage gas because it poses serious hazards to the ecological system [5]. Recently, some environmental-friendly insulating gases such as C4F7N and C6F12O have been proposed to replace SF6, but they still suffer from the problems of biological toxicity, low liquefaction temperature, and poor stability [6], [7], [8]. Therefore, the necessity of SF6 abatement still shows significance in the field of environmental protection.

Among the present methods for SF6 degradation, nonthermal plasma (NTP) has been considered as one of the most effective and energy-saving methods, such as dielectric barrier discharge (DBD) [9], [10], microwave discharge [11], and radio frequency discharge [12]. Benefited from its non-equilibrium nature, stable discharge features, powerful operability and simple experimental setup, DBD plasma has attracted increasing interest in the field of exhaust gas treatment [13], [14]. The presence of dielectric barrier can restraint the formation of spark discharge, induce strong electric field and generate plenty of reactive species, which is able to destroy the chemical bonds and drive a range of chemical reactions, resulting in the destruction of pollutant molecules [15], [16], [17], [18], [19], [20].

It seems that effective SF6 abatement using DBD plasma needs a maximization of energy efficiency. As a result, the optimization of the reactor geometry, power parameters and carrier gases becomes an approach to enhance the performance of the plasma degradation process [21], [22]. Furthermore, additive gases including H2O, O2 and NH3 have been employed to enhance the plasma performance because blocking the recombination of low-fluoride sulfide and fluorine atoms has been considered to be extremely crucial in SF6 degradation process [23], [24]. Moreover, the SF6 products distribution has also been changed by the introduction of additive gases, which provides a feasible approach to adjust its products. These studies are very instructive, but less involved in multifactor interaction. However, in practice application, many methods would be combined together to achieve the final goals. Therefore, study for multifactor interaction on SF6 degradation is indispensable for both scientific and engineering point of view.

Recently, the packing materials in the packed bed plasma reactor (PBR) are expected to enhance the removal ability significantly [15], [16], [25]. In our previous work, glass beads and γ-Al2O3 pellets were employed as packing materials to improve SF6 degradation. Remarkable SF6 removal efficiency and energy yield (EY) were achieved because of the higher reduced electric field and mean electron energy they brought [26], [27]. Besides, many research has shown that the size of packing materials also plays an important role in PBR discharge characteristics and pollutant molecules destruction [28], [29]. However, there is still no fundamental law accounting for its influence on the removal of pollutant. Therefore, targeted research should be conducted for SF6 degradation because PBRs’ performance is extremely hard to predict.

In our previous work, many degradation products such as SO2F2, SOF2, SOF4, SO2 and SF4 were detected [23]. However, the packing materials were easily corroded by some of them, which resulting in high frequency replacement [26]. Different from the glass beads and γ-Al2O3 pellets, the excellent corrosion resistance of zirconia (ZrO2) may make it a more promising packing material. Response surface methodology (RSM) is a powerful tool for solving multivariable problems by establishing a multivariate quadratic regression equation to fit the relationship between variables and responses, and analyzing the regression equation to find the optimal experimental parameters. Recently, RSM has been applied to the optimization of plasma process and remarkable effect has been achieved [30], [31], [32]. However, to the best of our knowledge, similar research for SF6 abatement has not been reported.

Inspired by the above mentioned studies, in this paper, the effect of the ZrO2 beads size on PBR discharge characteristics and SF6 abatement has been investigated. Moreover, RSM model based on Box-Behnken design (BBD) has been employed to further investigate the single and mutual effect and make optimization of four key process parameters (flowrate, SF6 concentration, oxygen concentration and H2O concentration) in terms of energy yield (EY) and SO2F2 selectivity, which is expected to be a powerful tool for the optimization and prediction of PBR degradation of SF6.

Section snippets

Experimental setup and measurement apparatus

The experimental setup of SF6 degradation in the PBR system was described in Fig. 1, which comprised of a SF6 feeding system, a power supply system and a series of measurement apparatus.

The SF6 feeding system consisted of a gas distribution instrument (GC400, maximum dilution ratio 300: 1, gas distribution accuracy ±1% FS), a precision humidity generator (FD-HG, accuracy ±1% RH) and gas cylinders (99.999% SF6, 99.999% O2 and 99.999% Ar). Ar was used as the background gas because SF6 destruction

Effect of ZrO2 size on PBR discharge characteristics

Fig. 3 shows the electric signals of the PBR discharge with 5.3 mm, 3.0 mm and 1.8 mm ZrO2 beads packed. The current pluses can be observed in three current waveforms, which is a typical packed-bed effect with both filamentary discharge and surface discharge. First, the number of pluses decreased dramatically with the decrease of ZrO2 size. The similar finding has also been reported in the simulation of [29] where the pluses amplitude presents an opposite trend with beads size. [15] stated that

Conclusion

In this paper, SF6 was degraded based on ZrO2-packed PBR. The effect of ZrO2 size on the discharge characteristics and SF6 abatement was studied, and four key operating parameters (flow rate, SF6 concentration, oxygen concentration, and water vapor concentration) of PBR degradation of SF6 were optimized using response surface methodology. The results show that ZrO2 size can significantly affect the discharge characteristics of PBR, which is attributed to the change in physical parameters of the

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This study is funded by National Natural Science Foundation of China (NSFC, funding number is 51777144) and State Grid Science and Technology Project (SGHB0000KXJS1800554).

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