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ONE way to combat global warming is by sequestering the carbon dioxide belched out by power stations, locking it away in buried vaults. A big problem, though, is that only about a tenth of the gas produced by burning fossil fuels is CO₂. Most of the rest is nitrogen, which is not a greenhouse gas and would needlessly take up space in the vault. But separating the two gases can be a costly affair.

鈥淪eparating nitrogen from CO₂ when sequestering carbon can be costly鈥�

Now a team led by at the University of Wyoming in Laramie say they have designed a cheap filter that could capture 90 per cent or more of the CO₂ emitted by power stations. 鈥淭his is a way to capture CO₂ for about $20 a tonne 鈥� less than half the cost of current methods,鈥� says Radosz.

Most existing ways of separating CO₂ from nitrogen are not very effective at the high temperatures and low pressures typical of flue gas. The requires the gas to be compressed or chilled before passing it through a membrane or solvent that traps CO₂ 鈥� a complex process that costs about $47 per tonne of CO₂ captured.

The new filter contains specially prepared pieces of carbon about the size of a pinhead 鈥� much like ground-up charcoal. The highly porous surface of these grains traps and bonds with CO₂ from flue gas at temperatures of up to 50 掳C, while allowing the nitrogen to escape. Heating the carbon up to 100 掳C releases the CO₂ for storage (Industrial & Engineering Chemistry Research, ). The researchers plan to start trials of the filter at coal-fired power plants in 2009.

Radosz suggests that the captured gas could even be injected into oil reservoirs and coal seams. This would make the extraction of oil and methane more efficient, but has not been economically viable until now.

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