This artificial gas was once used to develop nuclear weapons, but now exists all around you

Audit expert: Gan Qiang

Lecturer at Beijing Institute of Technology, PhD in Applied Chemistry

It is a synthetic gas with a history of 100 years. It was once used in the Manhattan Project to develop nuclear weapons in the United States. It is also widely used in various power equipment because of its excellent insulation properties. You may not feel its existence, but this gas is indeed around you all the time. It is the gas hidden behind the lights of thousands of homes - sulfur hexafluoride.

But few people know that sulfur hexafluoride is a strong greenhouse gas that is extremely harmful to the earth's environment.

Properties of sulfur hexafluoride Sulfur hexafluoride is a gas material that was first obtained by two French chemists, Professors Moissan and Lebeau, by burning sulfur in fluorine gas in 1900. Its chemical formula is SF6, which is a colorless, odorless, non-toxic, non-combustible, and non-corrosive gas at room temperature and pressure.

SF6 is one of the substances with the best chemical stability known so far. It does not decompose at temperatures between 500 and 600 °C, and does not react with acids, alkalis, water, etc. It will only decompose into atomic gases of sulfur and fluorine under the action of a high-voltage electric arc of several thousand degrees. However, once the arc is released, they will recombine into SF6 in a very short time.

SF6 molecular model

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SF6 has excellent insulation properties because it has a strong negative charge that surpasses similar gases. It can easily adsorb free discrete electrons in the electric field to form negative ions with larger mass. This enables it to effectively reduce ionization collisions in the gas and actually plays an insulating role.

In a normal electric field, the insulation strength of SF6 can be more than twice that of air, and as the gas pressure increases, the insulation performance will also improve; at the same time, due to the stability of SF6 gas, it also has a cooling effect on the arc that far exceeds that of air.

In fact, in practical applications, scientists have found that the insulation capacity of SF6 is hundreds of times greater than that of air. Because of these excellent properties, SF6 has great uses in various industries, especially the electric power industry.

Application of sulfur hexafluoride At present, SF6 is widely used in electrical, aerospace, chemistry, physics, meteorology, laser, medicine and other fields.

High-purity SF6 plays an important role in cutting-edge scientific research and production sectors and is recognized as a much-needed specialty gas.

In the military field, it is used as an etchant and cleanser in torpedoes or submarine propellants; in the microelectronics industry, it is used as a cleaning gas and etching gas; in the manufacture of optical fibers, it is used as an isolation layer dopant; in the alloy smelting process, high-purity SF6 and its mixed gas can be used as a protective gas to prevent the alloy from being oxidized; in the medical field, SF6 can be used as a safe contrast agent for ultrasound angiography, especially for liver tumor angiography. Generally, SF6 is injected intravenously and then slowly excreted through breathing...

In the power industry, SF6 has become the third insulating cooling medium after air and oil in the electrical industry due to its good insulation, arc extinguishing and chemical stability, and is widely used in electrical equipment series. At present, among medium-voltage power distribution equipment, there are 30 million sets of medium-voltage switchgear that use sulfur hexafluoride as arc extinguishing medium. Such as gas-insulated fully enclosed combination appliances, circuit breakers, current transformers, power switches, enclosed capacitor banks, transformers, high-power cables, lightning rods, etc.

The use of SF6 enables power equipment to be miniaturized and lightweight, saves materials, reduces costs, ensures the safety of equipment operation, reduces noise, and improves the working environment.

However, when sulfur hexafluoride was first discovered, people did not realize the powerful greenhouse effect of this gas and the harm it caused to the environment.

Environmental hazards of sulfur hexafluoride As the use of SF6 gas continues to increase, the content of SF6 in the atmosphere continues to rise. At present, the total amount of SF6 gas emitted worldwide each year is equivalent to 125 million tons of carbon dioxide (CO2), and it is still growing at a rate of 10% per year. Therefore, SF6 gas was listed as one of the six restricted greenhouse gases by the parties to the United Nations Climate Change Convention in the Kyoto Protocol signed in 1997. The main reasons for its terrible greenhouse effect include the following two aspects:

Exhaust Emissions

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First, there is the astonishing global warming potential of SF6. The global warming potential (GWP) is an indicator that directly compares the impact of various gases on the global greenhouse effect. The global warming potential refers to the greenhouse effect produced by different greenhouse gases relative to the mass of carbon dioxide with the same effect within a limited 100-year time frame. It uses the greenhouse effect produced by 1 kg of CO2 as a unit to compare the impact of different greenhouse gases on the environment. Scientists have found that releasing 1 kg of SF6 is equivalent to releasing 23,900 kg of CO2, that is, the climate warming power of SF6 is 23,900 times that of CO2.

In 2019, the BBC reported that although CO2 is recognized as one of the main causes of global warming, SF6 is the most terrible greenhouse gas known to mankind. Just 1 kilogram of SF6 has the same effect on global warming as the CO2 emitted by 24 people flying back and forth between London and New York.

Secondly, SF6 is extremely difficult to decompose, and its existence period can be as long as 3,000 years. It continues to accumulate, which also has a considerable boost to the greenhouse effect. Since SF6 is a synthetic gas and has quite stable chemical properties, it is very difficult to react with other substances when it is discharged into the air. As it continues to accumulate in the atmosphere, the greenhouse effect it brings is also continuously increasing.

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At present, although SF6 is widely used in various industries and plays a huge role, it also has great harm to the atmospheric environment and is well-deserved to be the "most terrible greenhouse gas". Therefore, in recent years, the international community has begun to pay attention to the emission of SF6, and various countries have introduced some policies to regulate the use of SF6 and strengthen the recovery and reuse of SF6 gas. At the same time, some SF6 application industries have gradually made technical improvements or studied other alternative gases to reduce the use of SF6. However, the consumption of SF6 in the power and electrical industry is still a huge number, and based on current research results, there is still no single gas or gas mixture that can completely replace SF6 in the power industry.

Therefore, there is still a long way to go to address the greenhouse effect challenge of SF6, or sulfur hexafluoride.