Options to reduce SF6 from other SF6 applications

Winfried Schwarz, Öko-Recherche, Frankfurt am Main
 

Different to the well known SF6 applications as presented before the "other" sectors were underestimated in the past. This is in contrast to their importance as SF6 emissions sources. For example, in the year 1996 the american company Nike in the US filled by its own quite 288 tons SF6 in soles of sport shoes. This quantitiy equals roughly the annual atmospheric SF6 emissions of all 15 EU countries together. Nike is now phasing out SF6.

It is a common feature of all of the so called "other" SF6 applications, that the replacement is much easier to realize than in the core sector of SF6 use like the electrical equipments, etc.

Which are in Europe important "other" SF6 consumption sectors?

Within the EU-15 Germany might be the country with the highest volume of "other" SF6 applications. Let's have a statistical look at the German domestic SF6 consumption based on informations from the SF6 producers and traders. In 1998 they supplied to customers in Germany about 570 tons.

Diagram 1, left side, shows: The biggest part was used by the manufacturers of electrical equipments: 400 tons. In addition 3 tons by magnesium casters and 11 tons by the semiconductor industry.

In addition to these "hard" SF6 applications the "soft" sectors of sound proof windows and car tires demanded 150 tons: 120 tons for filling soundproof windows and 30 tons for filling tires. Further 6 tons SF6 were used in other small sectors: 3 tons for particle accelarators, 1 tonne for military aircraft radar, nearly 1 tonne for electron microscopes, portable radio materiology and for medical radiotherapy. Apart from this half a tonne SF6 as tracer gas and half a tonne for cleaning secundary aluminium casting. As can be seen, the most of these small applications are using SF6 as inert gas for electrical insulation. Therefore we subsume them under "electrical equipment" treated in the speeches before.

Thus the real important "other SF6 applications" to be treated in the following are soundproof windows and car tires, covering 26% of the annual SF6 consumption.

Quite different the SF6 emissions. They totalled in 1998 about 226 tons from all applications. The point of interest: More than 80 percent stem from car tires (125 tons) and soundproof windows (60 tons), the so called "soft" SF6 sectors.

Diagram 2 shows that also in the long run, at least until 2010, the SF6 emissions from these two sources will be much more than from other applications.

Emissions abatement policies, so the first conclusion, does well paying high attention to these both "other" SF6 applications.

1. SF6 in car tires

Car drivers in Germany and some other european countries can have their tires filled with SF6. The filling for a set of 4 tires is somewhat over 1 kg. The SF6 molecules, being larger than those of air, diffuse more slowly through the rubber. The inventor of SF6 as tire filling gas, the German company Continental, thus promised "stable tire pressure for 1 year and longer".

The system offered under the name of "Air Safe" can prevent deviations from the correct filling pressure over almost the entire service life of a tire, as long as it is not made leaky by mechanical damage or defects in the tire.

In practice, SF6 filling can lead to neglect of regular pressure inspection. The ecological concerns: With a delay of three years after filling, when the used tire is disposed of, the SF6 banked in it is released completely to the atmosphere.

The diagram 3 shows a clear trend downwards. Consumption dropped 95-98 from 125 to 30 t. However, due to the three-year delay, emissions still continued to rise over that period from 100 t to 125 t. The development for the three years after 1998 is predetermined: emissions drop to 30 t by 2001. This is the consumption of the year 1998.

This trend reflects to a large extent the reaction to the ecological critique against SF6 in tires since 1995. Continental stopped its own sales of SF6. But because the company left their trading companies free to continue to meet customer demand the sales kept going on - however on a much lower level of 30 tons per year. In our business-as-usual scenario this consumption level of 1998 is assumed to remain constant in the future, thus emissions drop by 2001 to this value, where they remain until 2010 (cf. Diagram 3).

The reduction scenario assumes that SF6 will be completely phased out as a tire gas. It is assumed that consumption reaches zero in 2007, so that by 2010 at the latest there are no emissions any more.

This assumption is not unrealistic. The most tire trade chains have dispensed with SF6 and are offering nitrogen as an alternative for the car drivers who find air too common. In the meantime, some 1000 tire traders have nitrogen flasks for tire filling.

The alternative to SF6 tire filling is air. It has not been possible to find measurement results giving an indication of the extent to which or indeed whether at all air, with its 20% oxygen content, diffuses differently through rubber tires than 100% nitrogen.

In this context it seems to be a little strange, that the above mentioned company Nike is realizing its SF6 phase out in shoe soles by switching to nitrogen, too.

2. SF6 in Soundproof Windows

Since 1975 SF6 has been filled in the pane interspace of double glazing windows in order to improve their sound-insulating effect.

The glazing design on its own, i.e. increased interspace, different glass thickness, the use of cast-resin compound glass, etc. achieves sound reduction factors of 35 to 50 dB. SF6 can enhance sound reduction by an additional 2-4 dB. Roughly 5 dB is perceived by people as a halving of noise. In Germany usually seven per cent of insulating glazing is SF6-filled soundproof glazing.

SF6, being a very heavy gas and therefore reducing the speed of sound waves, is suited for sound insulation - but not for thermal insulation. In the latter function, SF6 is poorer than air. Argon is the gas used for thermal insulation in glazing. Argon has no sound reduction effect.

Argon is the reason for the significant drop of SF6 in new windows from 277 to 120 tons from 1995 to 1998. In the beginning soundproof windows were filled exclusively with SF6. Now thermal insulation requirements are rising. This is why many soundproof windows are receiving blends of argon and SF6. Over the 1995-1998 period, the average proportion of SF6 to argon in soundproof windows dropped from 75% in 1995 to 30% in 1998. Maybe this gas composition of 30% SF6/70% argon will have established itself as a longer-term compromise between acoustic and thermal insulation.

Now the emissions.

SF6 emissions from soundproof windows must be distinguished according to three different categories, whose quantitative impact changes over time.

• Filling losses occur exclusively in the year of fabrication and are directly proportional to annual SF6 consumption. Of the annual SF6 consumption for soundproof windows, 33% arise as filling emissions in the year of fabrication.
• Stock leakage emissions are the gas losses from the filled glazing throughout its entire lifetime, which averages 25 years. Stock leakage emissions are assumed to be 1% per year. This contains both continuous gas losses through the edge seal and gas losses through glass breakage before and during use.
• The emissions of the third category, disposal losses, only occur at the end of the use phase of the glazing - 25 years after the filling. There are no plans for a scheme to recover SF6 from end-of-life glazing, and this is not considered technically feasible. It thus must be expected that at the end of the lifetime of the soundproof glazing or of the window frame the filling gas escapes to the atmosphere.

As the first SF6 soundproof windows were installed in 1975, the first annual cohorts of disposal and gas losses are to be expected from the year 2000 onwards. These will then increase from year to year, peaking in 2020 at 142 t.

The diagram 4 shows the 1995-2010 SF6 emissions from soundproof windows according to the business-as-usual-scenario. It is assumed that until the year 2010 new annual SF6 consumption will remain at the same level as in 1998: 120 tons. The filling losses remain constantly at 40 tons. The stock leakage losses go on exceeding slightly 20 tons. The disposal emissions start their steep rise in the year 2000 with initially 3 tons.

Is SF6 phase out in sound proof windows possible? Yes, this is not only a possibility, but a growing reality. As above mentioned the SF6 filling only tops the sound reduction effect of the glazing design. Thus SF6 can be compensated for by increasing glass thickness or by increasing the pane interspace by approx. 2-4 mm. Moreover: SF6 has a distinct drop in effectiveness at low frequencies. With the modified European standard, which weights low frequencies (< 300 Hz) more strongly in the sound reduction factor, SF6 - according to expert's statements - no longer has any sound reduction benefit over air or argon.

In our reduction scenario it is assumed that the use of SF6 for soundproof windows is gradually phased out by 2005.

The diagram 5 shows the altered emissions. The main difference to Scenario I is the drop in overfilling (black area) to zero by the year 2005. This reduction potential (difference to Scenario I) in total emissions (uppermost white area) already amounts to 42 t in the year 2010.

To sum up: Even if from 2005 onwards SF6 is no longer used for new soundproof windows, the SF6 emissions (stock leakage losses and disposal losses) will last still 25 years more. The first year without any emissions from this application could be 2030. By 2010 the emissions would total about 80 tons. That is a slight reduction compared with the 126 tons according to the business as usual scenario.

But in any case, soundproof windows will be the largest SF6 emitter in Germany for the next 20 years.