Introduction to Domestic and International Sulfur Hexafluoride (SF 6 ) Greenhouse Gas Emission Reduction Technologies for Power Grid Enterprises

: Since its introduction in 1930, sulfur hexafluoride ( SF 6 ) has rapidly become the primary insulation material for high-voltage electrical equipment such as transmission lines, switches, transformers, circuit breakers, and reactors, owing to its exceptional electrical properties. However, the flip side of its excellent performance is its extremely high global warming potential (GWP), which is over 23,000 times that of carbon dioxide ( CO 2 ). Managing SF 6 has thus become a key aspect of emission reduction efforts. The primary strategies for reducing SF 6 emissions include minimizing gas leakage during the production, operation, and maintenance of electrical equipment, as well as purifying and recycling emitted SF 6 gas. Various purification and monitoring methods have been developed for routine operations and have already been implemented by power grid companies globally. In addition to ongoing daily monitoring, research is also underway concerning the transformation, degradation, and substitution of SF 6 gases. Degradation and transformation methods primarily focus on high-temperature degradation, photocatalysis, and arc degradation. In the context of substitution, the idea is to use conventional gases or various organic compounds to form SF 6 mixtures or alternative gases, with the aim of reducing effect of leakage while maintaining performance. Finally, recommendations are made for power grid enterprises to reduce SF 6 emissions, emphasizing the importance of source control and the exploration of alternatives. Effective equipment management to prevent gas leakage, comprehensive monitoring and assessment, as well as the proper handling of unusable SF 6 devices, are also crucial steps in emission reduction efforts.


1.Introduction to Sulfur Hexafluoride Gas
Sulfur hexafluoride ( SF 6 ) is an ultra-high-voltage insulation material that also possesses excellent arc extinguishing properties.Since its introduction in 1930, SF 6 quickly became a preferred insulation material for various high-voltage electrical equipment, including highvoltage transmission lines, circuit breakers, transformers, switches, and reactors.It is widely used as an insulating material in these high-voltage electrical devices.a  According to reports from the Intergovernmental Panel on Climate Change (IPCC), nearly 80% of globally produced SF 6 gas is applied in the power industry, aside from its usage in magnesium metal industry (4%), electronics industry (8%), related insulation applications (3%), as well as in particle accelerators, fiber optics production, biomedical applications, etc4.
With technological advancements, scientists have discovered other beneficial properties of SF 6 , such as its non-flammable nature, non-corrosiveness, and effective a* First author: Wangchao Dong, dwchhf@live.comb* Corresponding author: aiguningkesi@mail.ustc.edu.cninsulation against air, which have expanded its applications into areas like metal smelting, integrated circuit manufacturing, and refrigerant production.
However, while SF 6 boasts various excellent electrical properties, it is also recognized as one of the most potent greenhouse gases with severe global warming potential (GWP).Its GWP is over 23,000 times that of carbon dioxide (CO2), and it can persist in the atmosphere for approximately 3,200 years, making natural degradation within a human lifespan nearly impossible.Consequently, SF 6 is classified as one of the six greenhouse gases that require strict control of emissions by the Kyoto Protocol.countries around the world have undertaken full lifecycle monitoring of SF 6 electrical equipment.Environmental and power departments in various countries have also implemented numerous technologies to restrict the release of SF 6 gas into the atmosphere, and China is no exception.Currently, the annual usage of SF 6 gas in China's power system alone reaches 5,000 to 6,000 tons and is increasing at a rate of 20% annually.The existing stock of SF 6 gas in active equipment within the power industry is approximately 18,000 tons, with a potential recycling capacity of about 600 tons per year.When converted to CO2 equivalent, this amounts to over 14 million tons1.During the Copenhagen World Climate Conference, China pledged to reduce CO2 emissions per unit of GDP by 40% to 45% by 2020 compared to 2005 levels.With the rapid development of the national power grid, especially high-voltage transmission and distribution lines, and the increasing use of GIS and other switchgear, the consumption of SF 6 gas in the power system has significantly increased.Therefore, the rational and proper recovery, purification, and reuse of SF 6 gas has become an urgent issue to address.

2.1.1Reducing Gas Leaks from 𝑆𝑆𝑆𝑆 6 Electrical Equipment During Production, Operation, and Maintenance
In general, the reasons for SF 6 gas leaks include: ①Leaks at weld seams; ②Leaks at shell defects; ③Leaks at flange joints; ④Leaks in gasket sealing systems; ⑤Leaks at relay connections; ⑥Leaks in GIS (Gas Insulated Switchgear) enclosures.These leaks can occur due to improper manufacturing processes or inadequate gas refilling operations.The time distribution of SF 6 gas leaks can be divided into three stages: ①During delivery and acceptance, leaks caused by material, processing, and assembly defects.
② During the first winter and summer cycle, leaks resulting from the impact of temperature changes on sealing materials.
③ Leaks occurring as equipment approaches or exceeds its expected service life due to aging and deterioration of sealing materials.
To address this issue, companies like State Grid Corporation of China have made improvements in processes such as efficient synthesis reactions, crude product purification, and fluoride production processes.They have also strengthened the systematic regulations for operational procedures and equipped themselves with SF 6 leak detectors, online monitoring systems, and recovery devices.For instance, State Grid Corporation of China's Hubei branch has improved the method for detecting SF 6 gas leaks in GIS equipment.They comprehensively monitor the SF 6 emissions from the production, operation, and maintenance processes of SF 6 electrical equipment, ensuring compliance with the national standard for industrial sulfur hexafluoride (GB/T 12022-2014).

2.1.2Purification and Recovery of Emitted 𝑆𝑆𝑆𝑆 6 Gas
During the use of electrical equipment containing SF 6 , there is a potential for SF 6 to undergo decomposition, resulting in the formation of toxic and harmful byproducts, including low fluorides.This can pose risks to both human health and the proper functioning of equipment.To address this concern, it is essential to purify and recover the emitted SF 6 gas to eliminate these impurities.In response, State Grid Corporation of China has established multiple SF 6 gas purification and recovery facilities (as shown in Figure 1).

Figure 1 The number of large-scale SF6 gas purification and recovery facilities in China
The working process of this facility mainly involves collecting the gas from the electrical equipment into the tank through the recovery system of the facility, transporting it to the purification system, removing impurities through the purification system, and then filling the purified gas back into the electrical equipment.This process helps to reduce the emissions of SF 6 and impurities from the electrical equipment to the external environment.
Apart from China, many other countries have also made efforts in the purification and recovery of SF 6 gas.For instance, Solvay Fluor in Germany introduced the "ReUse" program in Europe, proposing on-site treatment of SF 6 waste gas through gas purification vehicles.The Japan Electric Power Company and manufacturers' association have implemented SF 6 recovery plans4; The United States Environmental Protection Agency (USEPA) established SF 6 emission control plans for electrical equipment, magnesium production, die-casting, and other applications as early as 19973.

2.2.1Conversion and degradation of SF6 exhaust gases
SF 6 , as a chemically stable non-CO2 gas, has a long lifecycle with natural degradation mechanisms mainly relying on photolysis and settling, which are inefficient.Therefore, scientists have proposed various artificial degradation methods to convert SF 6 exhaust gases into non-toxic and harmless products, aiming to reduce the emission of SF 6 gases into the atmosphere.These artificial degradation methods include high-temperature degradation, photocatalytic degradation, arc degradation, etc 错误!未找到引用源。.In this paper, one of the most advanced degradation methods, the rectifying SiC-Fe2O3 heterojunction structure for SF 6 degradation in air, is briefly introduced.
Scientists achieved this by depositing FeOOH onto SiC through self-assembled chemical deposition and designed an in-situ formed SiC-Fe2O3 heterojunction structure during the heating process of SiC-FeOOH composite to catalytically degrade SF 6 .The results showed that at 700°C, the initial degradation rate of SF 6 reached 93.6%, with a lifetime of 470 minutes, which is 1.8 times and 1.2 times higher than at 600°C and 650°C, respectively.The degradation amount was 99.0 mL•g -1 .
Especially for high-concentration SF 6 (35 vol%), the degradation amount reached 615.0 mL•g -1 .Through electron coupling within the heterojunction structure, electrons transferred from SiC to SF 6 through Fe2O3, thereby reducing the S-F bond energy.This non-uniform electron distribution heterojunction structure exhibits significant potential in the field of SF 6 degradation in the future4 (as shown in Figure 2).

Figure 2
The degradation proportion of SF 6 in SiC, FeOOH, and SiC-FeOOH composite materials.

2.2.2Substituting gases with lower Global Warming Potential (GWP) for SF6
Due to the Kyoto Protocol explicitly listing SF 6 as one of the six greenhouse gases to be restricted in emissions, seeking alternative gases with excellent insulation and ideal arc-extinguishing properties to replace SF 6 has naturally become one of the research directions in the global scientific community.Currently, there are two main categories of SF 6 substitute gases: ① SF 6 mixed gases Including dry air and its constituent gases: oxygen, nitrogen, carbon dioxide, and methane.The advantages of these gases lie in their environmental friendliness, safety, and low liquefaction temperature (boiling point).However, their insulation performance is only about 0.3 times that of SF 6 .As a result, they cannot be used alone but must be mixed with SF 6 or other substitute gases.The addition of other gases reduces the boiling point of SF 6 , addressing the issue of gas liquefaction in cold regions.When SF 6 is mixed with SF 6 in a certain proportion, their insulation strength exhibits a strong synergistic effect.Among them, the mixture of SF 6 and nitrogen in a molar ratio of 7:13 shows the strongest synergistic effect5.Therefore, this mixture is often used in the fields of Gas-Insulated Lines (GIL) and Gas Circuit Breakers (GCB) in power equipment (These will be shown in Table 1).Furthermore, CF 4 (carbon tetrafluoride) is a compound that exhibits good arc extinguishing performance and a relatively low liquefaction temperature.By adding an appropriate amount of CF 4 to SF 6 , the liquefaction temperature of the gas can be reduced, while slightly sacrificing insulation and arc extinguishing performance.This adjustment allows the gas mixture to meet the requirements of usage in cold regions.Various mature SF 6 -CF 4 gas mixtures for circuit breakers have been developed, and they can stably operate under severe cold conditions of -40°C.
Saturated halogenated hydrocarbons primarily rely on the strong electronegativity of molecules to adsorb electrons during the discharge process, thereby enhancing the electrical insulation properties.In chemical processes, hydrogen atoms in compounds can be replaced by halogen atoms to increase the electronegativity of gas molecules.Fluorine and chlorine atoms are commonly used as halogen substitutes to ensure low boiling points of gases.Additionally, bromine and iodine atoms offer advantages at the microscopic level, with multiple electron cloud energy levels and larger collision interfaces, enhancing the ability to block discharges 错误!未找到引用源。.The insulating properties of individual saturated halogenated hydrocarbons are shown in Table 2.  3), perfluoroisobutylene (C 4 F 7 N), octafluorocyclobutane (c-C 4 F 8 ), trifluoroiodomethane (CF 3 I), etc., are being explored as potential substitutes for SF 6 .These gases have characteristics including higher insulation strength compared to SF 6 and lower Global Warming Potential (GWP).However, they also have higher boiling points, which necessitates mixing them with gases like N 2 and CO 2 as mentioned earlier.
Additionally, fluorinated compounds with carbon can generate carbon deposits after discharge, potentially leading to reduced insulation performance.Among these gases, worth mentioning is trifluoroiodomethane (CF 3 I).It has an extremely short atmospheric lifetime (0.005 years) and great development potential.However, it is highly carcinogenic and produces elemental iodine during discharge, which reduces equipment lifespan and poses health and safety risks.
The ideal substitute for SF 6 gas needs to possess high insulation and arc extinguishing strength, low GWP, and a low boiling point.Further research and exploration are required for gases with such characteristics.

3.Conclusion and Suggestions for 𝐒𝐒𝐒𝐒 𝟔𝟔 Emission Reduction in Power Grid Enterprises
This paper compiles research findings from both domestic and international reports and provides the following recommendations for SF 6 gas emission reduction in power grid enterprises in China: Firstly, tackling the source is of paramount importance.This involves two main aspects.On one hand, it is essential to minimize gas leakage from SF 6 electrical equipment during usage, production, operation, and maintenance.On the other hand, there is a need to enhance the degradation and purification recovery of emitted SF 6 gas.Preventing gas leakage can be addressed from three angles: ① For newly acquired SF 6 electrical equipment, a comprehensive documentation system should be established upon receipt.This system should rigorously monitor gas quality, conduct stringent inspections to verify compliance, and closely review essential information such as producer and production date.Simultaneously, strict management measures should be implemented for gas storage areas7.
② For the already operational SF 6 electrical equipment, comprehensive live monitoring is necessary.This encompasses gas composition analysis within the equipment, detection and assessment of gas leakage levels.Maintenance should be carried out for equipment that does not meet application requirements, and regular reevaluations are essential.
③ For unusable SF 6 electrical equipment, prompt retrieval and proper disposal should be conducted.In terms of degradation and purification of gases, the current SF 6 decomposition methods mainly involve arc decomposition, corona spark and point discharge-induced decomposition, and high-temperature decomposition8.It is also possible to employ high temperature, photolysis, discharge, and catalytic methods to purify SF 6 waste gas into completely non-toxic and harmless gases, ultimately achieving the harmless degradation of SF 6 waste gas.
Next, the search for gases with lower GWP to replace SF 6 is crucial.Currently, there are three main categories of substitute gases that have been extensively studied: ①Conventional Gases: This includes compressed air, N 2 (nitrogen), CO 2 (carbon dioxide), etc. Research has indicated that compressed air and N 2 can be used as substitutes for SF 6 in medium-voltage switchgear insulation, with their application expanding to higher voltage levels.
②Perfluorocarbon (PFC) Gases: Various PFC gases have been considered as substitutes for SF 6 in highvoltage equipment.However, their drawback lies in their high liquefaction temperature, which leads to elevated costs.To address this, a solution is to create mixed buffering gases by combining these PFC gases with N 2 or CO 2 .
③ Trifluoromethyl Iodide ( CF 3 I ) Gas: CF 3 I gas is another category9, known for its high dielectric strength.
It has a global warming potential equivalent to CO 2 and decomposes rapidly in the atmosphere.
These alternative gases offer potential solutions for SF 6 replacement, each with its own advantages and challenges that need to be further explored.
Finally, due to varying costs associated with different mitigation strategies, electric grid enterprises should make reasonable choices based on their own budgets, regardless of the selected emission reduction approach.Governments should also implement robust policies and regulations to provide support.Given the urgent need to address global climate change, and considering SF 6 's exceptionally high global warming potential and extended lifespan, the academic community, industries, and governments worldwide must unite their efforts.This collective action should stem from a perspective of long-term human interests and the common destiny of humanity, as we collectively confront the challenges posed by SF 6 's greenhouse gas effect10.

)=1: 4 A
GIL with a voltage level of 550kV and a transmission capacity of 300MW has been developed.Hyosung (Republic of Korea) n(SF 6 ):n(N 2 )=1:3 In the experiment of 170kV/50kA compressed air GCB, its breaking capacity is equivalent to pure SF 6 .The University of Tokyo n(SF 6 ):n(N 2 )=1:3 In the experiment of blowing air GCB, its breaking capacity is close to 80% of pure SF 6 .Manitoba Hydro(Ca nada) n(SF 6 ):n(N 2 )=1:1 A high-voltage GCB with a voltage level of 115kV and a breaking current of 40kA has been developed.

Table 1
The composition and performance of SF6 mixed gases n(SF 6 ):n(N 2

Table 2
Insulation Performance of Saturated Halogenated C 6 F 12 O ,PFK-6112, the physical and chemical properties of these two perfluorinated ketones are demonstrated in Table

Table 3
Comparison of Physicochemical Properties of