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A CHEAP mineral membrane that filters carbon dioxide from power station flue gases could make it easier to dispose of the greenhouse gas in old oil wells.

But while the power industry is excited by the prospect, environmentalists say it will only serve to delay more fundamental action on greenhouse gas emissions.

The mineral membrane acts as a molecular sieve: it exploits the fact that CO₂ molecules are very slightly smaller than molecules of nitrogen, the main component of flue gas. With a diameter of 0.33 nanometres, CO₂ passes through the new membrane over 100 times as fast as N2, which has a diameter of 0.36 nanometres. The filter works at temperatures of up to 200 掳C, making it suitable for use with the hot exhaust gases of power stations. 鈥淚t has proved to be very cheap and efficient,鈥� says Ken-ichi Okamoto, a member of the team at Yamaguchi University in Japan that made the filter.

The researchers made the membrane from a zeolite, a mineral composed of aluminium oxide and silicon oxides. After coating a cylindrical ceramic scaffold with zeolite seed crystals, they kept it at 100 掳C in a solution of zeolite for 30 hours (Chemical Communications, 2003, p 2154).

None of the other ways of sequestering CO₂ from flue gases is proving even remotely economical. Until now the top contender has been amine scrubbing. This uses the solvent monoethanolamine to strip out CO₂ as exhaust gases bubble through. The CO₂ is then removed and the solvent recycled. Other materials such as activated charcoal have been tried but because these work by absorbing CO₂ rather than filtering it, they have to be regularly replaced or recycled which is expensive.

The only method in large-scale use involves liquefying all the waste gases and distilling the CO₂. But this consumes vast amounts of energy. Norwegian company Statoil uses this technology to remove about 1 million tonnes of CO₂ a year from natural gas. The CO₂ is then pumped into the disused Sleipner oilfield in the North Sea. But without tax breaks provided by the Norwegian government this would be too expensive to be practical.

By contrast, the CO₂ filtered through the new membrane can be pumped straight into the ground. Michael Anderson, a chemist at the University of Manchester Institute of Science and Technology in the UK says that the molecular sieve appears to have overcome many of the drawbacks of other methods. 鈥淭his filter could become economic very quickly,鈥� he says.

But environmentalists are unhappy about the possibility of sequestering more CO₂. Paul Johnston, a scientist at Greenpeace Research Laboratories in Exeter, UK, says: 鈥淧umping CO₂ into holes in the North Sea is just a quick fix with a huge energy penalty. This will inevitably be used as a prescription for business as usual.鈥�