Substitutes for Potent Greenhouse Gases

5 Consumption of SF6 and substitution possibilities

SF6 (sulphurhexafluoride) is gaseous at normal temperatures and atmospheric pressures. SF6 is used in some noise-reduced double glazed windows, where SF6 in an argon mixture fills the space between the panes of glass. The purpose is to absorb acoustic waves and thus secure against noise from the outside.

According to the Danish Environment Protection Agency an amount of 7.2 tonnes of SF6 was used in 1997 for this purpose. This examination has mainly been prepared according to information from suppliers and importers of SF6. The consumption of SF6 for this purpose is declining, and in 1995 and 1996 the amount used was 13.5 and 9.4 tonnes, respectively.

Some of the production is sold in Denmark. There are app. 30 producers of this type of noise-insulated double glazed windows in Denmark.

According to a survey made by the Danish Environment Protection Agency a direct emission of SF6 occurs during charging of the windows. This loss varies between 10 and 20% depending on the equipment and the procedures used. Previously, the amount of emission was much bigger.

Initially, SF6 is accumulated in the windows, however, when the windows puncture, the substance will leak out into the atmosphere.

As no collection or recovery arrangements exist, which would be difficult whatsoever, the entire amount of SF6 will probably end up in the atmosphere. As this type of window has been produced for some years (15-20 years), it is expected that some emission from old windows with SF6 will occur in connection with puncture or scrapping of the windows. If we assume that the lifetime for these windows is 20 years, we are about to reach the point where the actual emission corresponds with the raw material consumption.

DTI Energy has consulted Peter Vestergaard from DTI Building Technology and manufacturer representatives. The information received hereby was that:

DTI Energy has contacted Delta Acoustics and Vibration with the purpose of formulating a project within the area.

A project proposal has been prepared, in which the initial part of the project (subproject 1) contains a more precise description of the production of noise-insulated windows, e.g. type of window, number and application concerning the extent of traffic noise, expected life time etc. Laboratory measurements on 8-10 windows with SF6 will be carried out together with parallel measurements on similar windows not containing SF6. Thus, the aim of subproject 1 is to demonstrate the importance of using SF6 as a noise-insulating medium, where after targets for the following part of the project (subproject 2) will be decided upon. Finally, new concepts of windows not containing SF6 will be prepared in co-operation with manufacturers.

5.2 Protective gases in light-metal foundries

According to the Danish EPA the consumption in 1997 of SF6 used as protective gas for light metal casting, was 0.6 tonnes. The consumption in 1995 and 1996 was 1.5 and 0.4 tonnes.

The production takes place at the company Metallic A/S. Here SF6 is used in a mixture of other gases (CO2 and atmospheric air) to protect liquid magnesium from igniting, when casting the metal for machinery parts. Liquid magnesium is highly flammable and will ignite when exposed to air.

The same method is used in other countries. A search on the Internet reveals that a number of different magnesium casting machines exists, all protected with SF6 systems. SF6 will be released from this source to the environment.

Metallic A/S also casts goods in aluminium, zinc and brass, but the use of SF6 exclusively takes place when casting magnesium.

According to Lars Feldager Hansen, Metallic A/S, magnesium is a very light and strong metal. Consequently, the use of magnesium parts is increasing in the car industry.

Metallic A/S is currently rebuilding their factory in order to terminate the use of SF6. Application of SO2 in closed machinery parts will be introduced instead. Partial implementation of this technology has already been carried out and full implementation is expected during the next year. The new technology has been introduced in co-operation with Norsk Hydro.

Aluminium production
According to Preben Norgaard Hansen, DISA A/S, SF6 is used for degassing liquid aluminium before casting. Previously, chlorine-containing gases were used for this purpose, however due to the working environment this caused problems.

SF6 is introduced into the liquid metal in small bubbles where gas, including hydrogen, diffuses into the bubbles, which rise to the surface to be released in the atmosphere.

On a global level app. 20 Disamatic automatic casting machines for aluminium production exist. This market is growing steadily, as the use of aluminium for car parts is increasing.

DISA has previously tested this technology at their test foundry in Denmark, but is presently not using SF6 for this purpose. Per Norgaard Hansen is not aware of the use of SF6 for aluminium casting in Denmark.

5.3 Insulating gas in electric power switches

SF6 has a remarkable dielectric value. Because of this, the substance is used as insulating gas in certain high voltage installations. In principle, there are two different fields of application:

According to figures registered by the Danish EPA, the consumption of new SF6 for these purposes was app. 1.4 tonnes in 1995, 1 tonne in 1996 and in 1997 the consumption was app. 4.2 tonnes. Probably, the installed amount is much higher, but the emission is limited because the gas is kept in closed equipment and because the gas is collected and recycled when maintaining or disassembling the equipment. Thus emission only occurs by accident or unexpected leakages.

According to Henrik Weldingh, DEFU (Research Institute of Danish Electric Utilities), an electric arc will be formed when switching off the power, and temperatures may reach extreme values (10,000 - 100,000 K). A substance is needed for breaking the electric arc by rapid and efficient cooling, so that power cut off is completed by the time the current reaches the zero point of the AC sine wave.
Several possibilities prevail, such as:

No Danish producers of this equipment exist. However, multinational companies like ABB, Siemens, Group Schneider etc. produce this type of equipment.

In Denmark about 600 transformer stations in the 10-20 kV range exist, which are either equipped with SF6 or vacuum switches. Prices are similar and competition is hard amongst the producers. Thus non-SF6 circuit switches for the 10-20 kV transformer stations are available. However, space related problems may arise when changing to this type, and rebuilding of the entire station may be necessary.

In addition, about 60,000 of 10 kW/400 V sub-stations exist. In this case the equipment may be based on SF6 both as switching and insulating agent, but other non-SF6 solutions are available. Because of the large number of sub-stations, parameters like reliability, maintenance and small physical size play a decisive role.
In the high voltage range from 60 kV and up there are no alternatives.

According to Henrik Weldingh, DEFU, new technology is not in sight. New semiconductors may be marketed in the future, but a technological break-through is required, as efficiency is too low with the known technology.

The other application within the electrical area is as insulating gas in compact transmission cables. As an example high voltage cables of 400 kV, from the generator and out of the power plant, are placed in pipes (for example in 20 m), filled with SF6. This prevents flashover to the pipe and thus short-circuiting power cables. Alternatively, the distance between the cables could be increased, allowing atmospheric air to become the insulating agent.

As no Danish manufacturers of this type of equipment exist, the initiation of development projects seems pointless within this area.

If application of a technology, not containing any strong green house gases is wanted, installation of non-SF6 switches in the 10 kV system is possible.

5.4 Tracer gas and other laboratory purposes

According to the Danish EPA, the consumption of SF6 by various research institutes is app. 0.6 tonnes per year.

DMU (the National Environmental Research Institute) uses a small amount of SF6 as tracer gas for tests of dispersal in the atmosphere. The purpose of these experiments is to test mathematical models for dispersal in the atmosphere. This kind of tests makes among others the foundation for standards of chimney heights, etc. Only small amounts are used, varying according to actual projects. According to Erik Lyck, DMU, an amount of 6 kg was used in 1995, in 1996 no amount was used, and in 1997 less than 100 g was used. In 1998 no application has taken place so far.

The application of SF6 as tracer gas is due to a number of special qualities of the substance, which makes it hard to replace. Among others, it is precisely and specifically detectable in very low concentrations and the concentration in atmosphere is very low. Foreign tests have been made with a PFC substance, however this causes environmental problems as well.

Erik Lyck estimates that there are no useable alternatives, however the amount used for tests has to be limited and controlled. The tracing equipment of DMU is that sensitive that the background level for SF6 is measurable. Erik Lyck has written an article about this subject.

In Denmark, app. 5 laboratories are performing ventilation tests. Small amounts of SF6 are used as tracer gas for indoor tests. The measurements are used for estimation of pollution dispersal, leakage from heat exchangers and estimation of short circuit between the airstreams, etc.

Christian Drivsholm, DTI Energy in Taastrup informs that 2 kg per year are used for these tests. Laughing gas (N2O) may be be used as well, however this is slightly problematic because of toxicity.

5.5 Car tyres

According to the survey by the Danish EPA no consumption of SF6 for car tyres is registered. According to various sources of information large amounts of SF6 are used in Germany (in the order of 100 tonnes per year) for the blowing of car tyres. Consequently, DTI Energy has tried to elucidate this use.

According to conversation with Rudolf Nielsen, DTI Energy, Torben Skovgaard, The Danish Tyre Safety Council and Jan Steen Hansen, Continental, the situation is as follows:

A German company named Messer Griesheim (near Hamburg) tried to sell a system, called Conti Air Safe to Continental, Denmark. The system was tested in 1990, but has not been introduced.

The sales argument was that the SF6 molecules, which are rather big, would be mixed with air in the car tyre, diffuse into the tire material and prevent/reduce diffusion of air out of the car tyre.

According to above mentioned persons no SF6 is used for this purpose in Denmark.

5.6 Other possible applications of SF6

At present DTI Energy has no knowledge of other applications of SF6 in Danish industry than the above mentioned.

However, DTI Energy knows that SF6 are used in the soles of Nike sports shoes. According to a letter from Sarah Severn, Director for Nike Environmental Action Team to Greenpeace Denmark (dated September 12, 1997), the consumption of SF6 from April 1, 1996 to March 31, 1997 was 288 tonnes. The substance is used in Nikes Air models, and the entire production of these soles is located in the USA.

At the same time Nike announced that a phase-out over three years of the use of SF6 is initiated and not later than year 2001 SF6 will be replaced with nitrogen.

According to the recent letter of August 17. 1998, from Sarah Severn Nike, the consumption of SF6 in 1997 was app. 276 tonnes and the consumption in 1998 is estimated to be app. 164 tonnes. This corresponds with a 40 % reduction.
Nike also states, that the consumers can not clearly differentiate between shoes produced with SF6 and with nitrogen.