Sulphur Hexafluoride (Global Warming)

SULPHUR HEXAFLUORIDE (SF6) is the most powerful of all the greenhouse gases recognized by the Kyoto Protocol and evaluated by the Intergovernmental Panel on Climate Change. Although its concentration in the atmosphere is low, the combination of a high global warming potential and a very long lifetime make emissions of SF, a considerable concern. It is primarily used as an electrical insulator in the high-voltage distribution network, and major industrial users are beginning to restrict the use and emission of SF .

The Kyoto Protocol requires developed nations to cut their emission of six greenhouse gases. The Intergovernmental Panel on Climate Change periodically assesses these gases and estimates a global warming potential (GWP) for each. The gasses, with their respective GWPs, are as follows: CO2 (1), CH4 (21), N2O (310), PFC (9200), HFC (11,700), and SF6 (23,900). In this list, HFC and PFC are groups of chemicals, and the value quoted is for the member of the group with the highest GWP. Despite the low atmospheric concentration of SFf (5.6 parts per trillion), its extremely high global warming potential and long lifetime (probably in excess of 1,000 years) mean that present emissions will have an effect on climate for a long time to come.

As SF6 gas is denser (heavier) and more electrically insulating than either dry air or dry nitrogen, it is an ideal electrical insulating material. The gas is used extensively in electrical applications, and its principal use is in the electrical generation and high-voltage distribution industry. There are two specific advantages of SFf in these applications. First, its highly insulating character means that less space is needed between high-voltage components, so that equipment can be made significantly smaller than is possible when air or nitrogen are use as insulators. Second, gas-insulated switch gear using SF6 rather than air demands a controlled environment, and the equipment is consequently more robust with regard to environmental pollutants and weathering than would be the case with simpler air-insulated equipment. In addition to these advantages, the gas is unreactive, nontoxic and nonflammable. In the United States, the electric power distribution industry works on a voluntary basis with the SFf Emissions Reduction Partnership for Electric Power Systems to identify and implement technologies for reducing SFf emissions.

Another large-scale use of SF6 is in magnesium metal manufacturing and casting. Magnesium metal is extremely reactive in air, particularly when hot or molten. The high density and low chemical reactivity of SF6 make it a suitable choice as a protective gas layer preventing contact of the molten, highly reactive metal with oxygen and water in the air. A voluntary SF6 Emission Reduction Partnership for the Magnesium Industry exists in the United States in association with the U.S. Environmental Protection Agency, which, together with the International Magnesium Association, is committed to eliminating SF6 emissions from the industry by 2011.

SF6 is also used in certain medical applications, including eye surgery and ultrasound scanning. Once again, it is the gas’s high density and low toxicity that are used. In eye surgery, the gas is commonly used to form a plug to seal the retina during surgery. Its high density means that the gas stays in place and does not enter the blood at an appreciable rate. The density of the gas also makes it an excellent contrast agent in medical ultrasound scanning.

Similar to the perfluorocarbons, SF6 is also used in the semiconductor industry, and there is concern about the growth of this industry leading to uncontrolled increases in the amount of SF6 released to the atmosphere.

Paradoxically, because of its high chemical stability, low toxicity, and low natural abundance, SF6 has been extensively used by atmospheric scientists as a tracer gas to understand the movements and mixing of air. The gas has, for instance, been injected into the exhaust plumes from power stations in an attempt to understand the origins of acid rain. In the United Kingdom, SF6 tracer experiments have demonstrated that power stations are capable of delivering acid rain pollution to Scandinavia. For similar reasons, SF is used to trace the movements of air within ventilation and air conditioning system tests. Recently, the gas was released on the London Underground in an attempt to understand the way toxic gases would spread throughout the system in the event of a terrorist attack.

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