Study pinpoints uses of most potent greenhouse gas

The study, by researchers at the Max Planck Institute, is an attempt to reconcile what is known about historic and current sales of SF₆ into different sectors with the rising concentrations of the gas in the atmosphere. Limited understanding of the mix of emissive and closed uses of the gas has so far inhibited the formulation of control strategies.

Production of SF₆ began in the USA in 1953, reaching 1,000 tons per year in the early 1970s. It is now running at an annual rate of around 8,500 tons.

The exceptional stability of SF₆ has made it attractive in specialised industrial applications. But the same property, alongside the efficiency of SF₆ in absorbing infrared radiation, make it a highly potent greenhouse gas. SF₆ has a very long atmospheric lifetime of 3,200 years, and a global warming potential 23,900 times greater than that of carbon dioxide over 100 years.

Monitoring of the build-up of SF₆ in the atmosphere since the late 1970s has enabled scientists to estimate annual global emissions. These now amount to 6,300 tons per year. The quantity may seem small - but it is equivalent in global warming terms to a quarter of the UK's annual CO₂ emissions of around 600 million tonnes. More than 95,000 tons of SF₆ are now present in the atmosphere.

Last year, the six main western producers provided the first aggregated figures on sales of SF₆ by end use. The firms are AlliedSignal, AGA, Asahi Glass, Air Products, Solvay and Kanto Denka Kogyo. Other manufacturers in China and Russia are believed to have produced 880 tons of SF₆ in 1996 - just over 10% of the global total. Consumption is split roughly evenly between North America, Europe and the rest of the world.

The figures support the common view that 80% of total sales of SF₆ are to manufacturers and users of electrical equipment. The gas is used as an insulating agent in products such as electricity cables, switchgear and circuit breakers. More than half of the reported sales to the sector are for refills.

However, the German researchers' detailed scrutiny of the manufacturers' sales data and sectoral consumption figures from other sources revealed that about 1,700 tons of SF₆ sales - 20% of the total - could not be allocated to particular end uses. The discrepancy was particularly large for North America, at 1,200 tons out of a current consumption of 3,000 tons.

The researchers believe that the explanation lies mainly in the magnesium industry, in which SF₆ is used as a degassing agent. The manufacturers' sales figures imply that less than two kilograms of the gas are used per ton of magnesium. But data provided by Norsk Hydro, a major magnesium producer, show that its consumption was running at 4kg/ton in the 1980s before technical and operational improvements cut it to 0.7kg/ton by 1996. Since the company is a pioneer in the field, the researchers suggest that SF₆ consumption elsewhere in the industry is at least 4kg/ton. This, along with other unaccounted for uses in the US military and space sector, explains the discrepancy in the figures.

The upshot is that the electrical sector is now thought to account for only 60% instead of 80% of total SF₆ consumption. The revised figure is more consistent with emission estimates derived from trends in SF₆ levels in the atmosphere, since the amount of the gas currently being "banked" in electrical equipment is now put at just over 2,000 tonnes per year rather than 3,000 tonnes.

The researchers go on to suggest that 90% of current emissions of SF₆ could be eliminated with available technology and good operating practices. By sector, their projections are:

Shoes and tyres: The "questionable" use of SF₆ in these sectors "could be given up". About 130 tons of the gas were used in 1995 for filling tyres - a practice largely confined to Germany. Twice this quantity is used by sportswear manufacturer Nike as a cushioning agent in some of its training shoes - though the company is working on alternatives (ENDS Report 265, pp 4-5 ).