Eradicating CFC's with a plasma cask

Chlorofluorocarbons in spray cans and refrigerators may be a thing of the past but industry and sites for hazardous waste are still tackling the problem of their disposal.

Activated carbon filters only shift the problem, because the accumulated solvents - even if in concentrated form - still have to be dealt with when the filters are regenerated.

Incineration followed by pre-emission catalytic treatment is only said to make economic sense if pollutants burn as well.

The Fraunhofer Institute for Chemical Technology may have solved these problems with the development of the plasma 'cask'.

The reactor prototype consists of an elliptical metal chamber approximately 70 cm in diameter.

Microwaves are directed in one focal line, which ignite a plasma stream in the other.

'We are often surrounded by plasma in our daily lives,' said Dr Mathias Kaiser, head of the project. 'Fluorescent-light strips are a good example. Electrical discharges in gas produce not only light, but also an erratic mixture of atoms, electrons and gaseous ions. The density of plasma in the chamber, however, is nearly 10,000 times greater.'

Plasma only exists as long as energy is being supplied.

The Plasma in the reactor is produced using microwaves at the same frequency as household ovens, obtaining temperatures between 40 and 100 degrees centigrade, which are typical for fluorescent-light strips.

This is said to create two distinct advantages.

'On the one hand, the device is made of series components which are inexpensive and facilitate compliance with radiation emission standards,' explained Dr Kaiser. 'On the other, very little heat energy is lost since 'cold plasma' causes a negligible increase in the temperature of the gas stream.'

Highly charged particles in the plasma decompose pollutants. Ideally, the only substances remaining from the gaseous solvent are water vapour and carbon dioxide.

However, any hydrochloric and hydrofluoric acids that may be formed can be neutralised with a conventional gas washer, which is said to offer economic and environmental advantages.