Thermal decomposition properties of fluoronitriles-N₂ gas mixture as alternative gas for SF₆

Highlights

• Thermal decomposition of C₄F₇N-N₂ generated C₃F₆ firstly at 350℃.

• CF₄ and C₃F₈ were generated at temperatures higher than 450℃.

• C₃F₆, C₃F₈, CF₄, CF₃CN, (CN)₂ and COF₂ are the main decomposition products.

• Thermal stability of C₄F₇N-N₂ gas mixture at high gas pressure conditions is great.

Abstract

We explored the thermal decomposition properties of Fluoronitriles-N₂ (C₄F₇N-N₂) gas mixture using as SF₆ substitute gas in GIE. The yield of main decomposition products of 10 %C₄F₇N-90 %N₂ gas mixture under different overheating temperature and gas pressure conditions were detected and analyzed. We found that thermal decomposition of C₄F₇N-N₂ mainly produces C₃F₆, C₃F₈, CF₄, CF₃CN, (CN)₂ and COF₂. C₃F₆ is the first detected by-product at 350℃, while CF₄ and C₃F₈ were generated at temperatures higher than 500℃. The yield and formation rate of C₃F₆, (CN)₂ increases with temperatures lower than 450℃ then decreases at higher temperatures, while the generation of C₃F₈, CF₄, CF₃CN and COF₂ has positive correlation with temperature. C₄F₇N-N₂ gas mixture has great thermal stability at high pressure. The increase of gas pressure could effectively reduce the decomposition of C₄F₇N, which ensures the insulation strength of gas mixture.

Introduction

Sulphur hexafluoride (SF₆) is widely used in all kinds of electrical equipment since 1960s because of its excellent dielectric strength (DS) and arc-extinguishing capabilities [1]. However, SF₆ has been recognized as one of the most greenhouse gases (GHG) with the extremely high global warming potential (GWP) of 23500 (100-year horizon) and long atmospheric lifetime of 3200 years [2]. It is reported that power industry accounts for 80 % sales of SF₆ worldwide [3]. Naturally, seeking alternative gases for power industry has become a hot spot.

Recently, considerable attention has been paid to C₄F₇N (fluoronitriles), the last generation insulation gas with DS twice that of SF₆ and GWP value of 2090 [4,5]. C₄F₇N needs to be mixed with other carrier gases such as CO₂, N₂ or dry air for engineering application due to its high boiling point (-4.7℃) [6]. In addition, C₄F₇N is not classified as CMR (Carcinogens Mutagenic Reprotoxic) substance with the median lethal concentration (LC50, 4 h, rats) in the range of 12,000–15,000 μL/L (ppm, part per million). It is reported that the LC50 (4 h, rats) for 4 %C₄F₇N-96 %CO₂ and 6 %C₄F₇N-94 %CO₂ gas mixture is about 160,000 and 95,500 μL/L. Thus, no specific safety label is needed [7].

At present, several researches have been conducted on the insulation properties and arc-extinguishing capabilities of C₄F₇N gas mixture [6,[8], [9], [10]]. Owens et al. tested the DS of C₄F₇N gas mixture under relatively uniform field and found that mixtures with 20 % C₄F₇N in N₂ or air display DS comparable to SF₆. Similar results are found with CO₂ mixtures except that at higher pressures the SF₆ curve is more closely replicated with a 15 % mixture [6]. Zhong et al. explored the effect of buffer gases on plasma properties and arc decaying characteristics of C₄F₇N-N₂ and C₄F₇N-CO₂ gas mixture. It is reported that the descending order of the thermal recovery ability for the gases is: SF₆>C₄F₇N>75 %C₄F₇N-25 %N₂>75 %C₄F₇N-25 %CO₂. While, the descending order of the pre-dielectric recovery ability for the gases is: SF₆>C₄F₇N>75 %C₄F₇N-25 %CO₂>75 %C₄F₇N-25 %N₂. Hence, the addition of N₂ shows better thermal recovery ability while inferior pre-dielectric recovery ability than that of CO₂ [10]. The partial discharge (PD), surface flashover discharge and lightning impulse characteristics of C₄F₇N gas mixture was also investigated, which confirms that C₄F₇N gas mixture has great application potential [[11], [12], [13]]. As for engineering application, General Electric (GE) has developed the 420 kV gas insulated busbars (GIB), the 145 kV gas insulated switchgear (GIS) and 245 kV current transformer (CT) using C₄F₇N−CO₂ gas mixture as the insulating medium in 2016 [4].

In addition, prolonged operation of GIS substations near or beyond their power limits induces an additional concern about the temperature stability of gas insulating medium with respect to heated-up current-carrying conductors [[14], [15], [16]]. High temperature spots may accelerate insulation deterioration and lead to the decomposition of gas insulating medium, which could pose a threat to the operation safety and service life of equipment, as well as the establishment of operation and maintenance regulations considering the toxicity of the decomposition by-products. Thus, it is necessary to explore the thermal stability and decomposition characteristics of C₄F₇N gas mixture. Kieffel et al. tested the thermal stability of C₄F₇N-CO₂ gas mixture and found that thermal decomposition of C₄F₇N began at 650℃. The target gas mixture with 400−600 μL/L C₄F₇N was pumped into a tube furnace at a rate of 1 L(liter)/min at high temperatures and the decomposition products were detected by Fourier transform infrared (FTIR) spectroscopy. Several by-products such as CO, COF₂, CF₃CN, C₂F₅CN, C₂F₆ was found [5]. Chen et al. explored the thermal decomposition properties of C₄F₇N-CO₂ gas mixture and the influence of pressure and temperature on the decomposition products are theoretically investigated based on the kinetic analysis model. It was pointed out that the C₄F₇N/CO₂ mixture undergoes significant decomposition above about 700 °C and the main products are CO, C₂F₆, C₃F₆, C₃F₈, C₂F₃N, C₃F₅N and C₂N₂. The tube furnace is used the heat source and the gas chromatography-mass spectrometer (GC–MS) was applied for by-product detection [17]. Li et al. tested the decomposition by-products of C₄F₇N-N₂ mixture after alternating current (AC) breakdown tests by GC–MS. It was pointed out that the decomposition of C₄F₇N-N₂ mixture produces CF₄, C₂F₆, C₃F₈, CF₃CN, C₂F₄, C₃F₆ and C₂F₅CN. And the yield of CF₄, C₂F₆ and CF₃CN is higher than the other products [18]. The thermal compatibility between C₄F₇N-N₂ gas mixture and copper, aluminum was also explored and it was found that reaction between C₄F₇N-N₂ gas mixture with heated copper at 220℃ produces C₃F₆ [19,20]. Kessler et al. explored the interaction between C₄F₇N gas mixture and several kinds of materials used in equipment such as metals and alloys, insulators, thermoplasts and duromers, lubricants, elastomers and desiccants [21]. Simka et al. and Zhao et al. and studied the decomposition characteristics of C₄F₇N gas mixture under partial discharge (PD) conditions. They found that the decomposition of C₄F₇N-air gas mixture mainly generates CO, CO₂, CF₄, C₂F₆, C₂F₄, C₃F₈, C₃F₆, C₄F₁₀, CF₃CN, (CN)₂, C₂F₅CN. The content of fluorocarbon and nitrile gases increased with the mixing ratio. And the main decomposition products for C₄F₇N-CO₂ gas mixture includes CO, CF₄, COF₂, C₂F₆, CHF₃, C₃F₈, CF₃CN, C₃F₆, C₂F₅CN, C₂N₂, C₄F₁₀, C₄F₆, C₂F₄O [22,23]. In addition, the decomposition mechanism of C₄F₇N was also explored based on density functional theory (DFT) by Zhang et al, Yu et al, and Rankovic et al. [[24], [25], [26]]. The decomposition pathways, reaction enthalpy, rate constants were calculated and analyzed. It was pointed out that the most abundant fragment anion of C₄F₇N is CF₃ [26].

Nowadays, studies on the decomposition properties of C₄F₇N gas mixture are mainly focused on the discharge conditions and there still exists some gaps in the study of thermal stability and decomposition properties of C₄F₇N gas mixture under local overheating fault. The work presented in this paper aims to investigate the thermal stability and decomposition properties of C₄F₇N gas mixture based on the local overheating fault simulation system. N₂ is selected as the buffer gas considering C₄F₇N-N₂ gas mixture could meet the dielectric and liquefaction temperature requirement for engineering application. Moreover, the cost of using N₂ as the buffer gas is quite lower than that of CO₂. And the decomposition of CO₂ in the discharge (arc discharge or AC breakdown) could generate amount of CO, which is a main toxic and harmful by-product. Thus, the use of N₂ as the buffer gas may be much safer, especially for medium-voltage (MV) equipment. The influence mechanism of temperature, gas pressure on the thermal decomposition properties were explored. Relevant results not only reveal the thermal stability and decomposition characteristics of C₄F₇N-N₂ gas mixture, but also provide reference for the further engineering application of this new gas insulating medium.