Abstract
Although sulfur hexafluoride (SF6) is widely used in gas-insulated electrical equipment due to the excellent electrical insulation and arc-interruption behaviors, its strong greenhouse effect with high glow warming potential (GWP) has been driving the research on environmentally friendly gases for SF6 alternatives. Recently, the heptafluoro-isobutyrontrile (CF3)2CFCN (3M NovecTM 4710) is reported to have the potential to replace SF6. The decomposition characteristic of gas insulating medium is one of the basic conditions to measure whether it has the feasibility of replacing SF6. However, the decomposition characteristic of (CF3)2CFCN and the formation mechanism of by-products are still unclear. In this paper, a series of AC corona discharge experiments were performed, and the gas by-products of (CF3)2CFCN mixed with CO2, N2 and air were qualitatively analyzed by gas chromatography mass spectrometry (GC/MS) method, respectively. Then, the decomposition mechanism of (CF3)2CFCN molecule and the formation mechanism of gas by-products were studied via density functional theory (DFT) method. The results show that the major gas by-products are CO, CO2, C2O3F6, CF4, C2F6, C3F6, C3F8, C2F4, (CF3)3CF, CF3CN, C2F5CN and CNCN, in addition to trace amounts of F3CC≡CCF3 and CF3CF=CFCF3. Among the three initial dissociation pathways of (CF3)2CFCN molecule, a C-CF3 bond fission pathway that produces CF3CFCN and CF3 radicals is identified as the thermodynamically favorable channel. The bond energy of C–CF3 bond calculated at M06-2X/def2-QZVP//M06-2X/6-311G(d,p) level is 94.7 kcal/mol.
Highlights
Due to the highly stable molecular structure, excellent insulation and arc extinguishing properties, SF6 gas is extensively used in gas insulated electrical equipment, such as gas insulated switchgear (GIS) and gas insulated line (GIL)
Pohlink et al.6 studied the thermal stability of (CF3)2CFCN/CO2 mixture experimentally, and the results showed that CO gas is the one of the main byproducts; in addition, low concentrations of C2F6, COF2, CF3CN and C2F5CN gases were detected at 800 ○C
We found that C2F4+CF2 complex (IM1) can directly produce C3F6 through a transition state of 50.2 kcal/mol, which is quite different from the above literatures
Summary
Due to the highly stable molecular structure, excellent insulation and arc extinguishing properties, SF6 gas is extensively used in gas insulated electrical equipment, such as gas insulated switchgear (GIS) and gas insulated line (GIL). The 100-year global warming potential (GWP100) value of SF6 gas is about 23,500 times that of CO2 gas.. SF6 has a long lifetime in the atmosphere of 3200 years and severe greenhouse effect.. The 1997 Kyoto Protocol clearly listed SF6 gas as one of the six restricted gases used worldwide. One of the best ways of reducing SF6 emissions is to limit its usage. In order to reduce or even eliminate the use of SF6 gas, finding environmentally friendly gases for the replacement has gradually become the research hotspot of power industry nowadays.
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