Published online 20 November 2008 | Nature | doi:10.1038/news.2008.1244

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Briny mix could stop carbon dioxide leaks

Engineers hope salt-water technique could make coal-fired power plants a cleaner energy option.

power stationMixing carbon dioxide with salt-water might help to make coal-fired power plants a little greener.Punchstock

Dissolving carbon dioxide in salt water could improve the prospects of storing the greenhouse gas in underground aquifers, say Texan engineers.

The rise in CO2 concentrations from fossil-fuel burning has prompted a variety of proposals for how to capture and store the gas. Systems that produce a stream of CO2 could be integrated into power stations, for instance, allowing the gas to be compressed and then injected directly underground into water-bearing porous rocks.

But no-one really knows whether CO2 will remain stored in this way over many decades, or whether it will slowly leak from the aquifers it is pumped into.

Steven Bryant, a chemical engineer at the University of Texas in Austin, is hoping to develop a method that ensures the gas stays put.

His team's solution is to pump salt water out of a suitable aquifer, dissolve the CO2 into the brine in a mixing tank above ground, then inject the CO2-laden solution back into the aquifer.

Bryant unveiled his team's feasibility study at the ninth annual International Conference on Greenhouse Gas Control Technologies in Washington DC, on 19 November.

The briny deep

Bryant thinks the method would reduce the chances of CO2 escaping from aquifers because the mixture is more dense than the surrounding water, and less likely to rise towards the surface. But it would also require more upfront investment.

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A 500-megawatt power plant using the dissolution method would need 50 pairs of injection–extraction wells — and enough land to site them — along with roughly 200 million litres of salt water per day, he estimates.

The technique would capture 90% of the plant's CO2, roughly 10,000 tonnes of the gas every day — the same amount expected from an injection-only technique.

The US Department of Energy estimates that setting up the carbon capture and storage facility for a coal-fired plant costs about $300,000 per megawatt of power generation capacity. Pre-mixing CO2 with salt-water would roughly double that capital cost, Bryant estimates, with day-to-day running costs about the same. But his scheme could also save untold expense incurred by the need to monitor and mitigate CO2 leaks, he adds.

Bryant admits that much more work is needed. The group is about to start experiments to see if carbonate minerals precipitate out of the CO2-salt solution. That should help to determine whether the technique could be deployed on a large scale without fouling equipment or causing unwanted environmental effects.

But the method "gives us another acceptable option to prevent sequestration leaks", says environmental engineer Peter McGrail of the Pacific Northwest National Laboratory in Richland, Washington. 

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