Carbon capture hopes set in stone

By Tim Radford

LONDON, 13 June, 2016 – Scientists in Iceland have concrete evidence that carbon capture and sequestration can be made to work. They have buried carbon dioxide in the rocks and watched it turn to stone.

There is no guarantee that what works on a small scale at a geothermal plant in south-west Iceland could be practical or economical for the world’s giant fossil fuel power plants.

But the study, backed by US partners and launched only in 2012, shows that one problem with carbon dioxide capture as a way of reducing the greenhouse effect can be sidestepped.

Environmentalists, engineers and climate scientists have all argued that carbon dioxide buried underground and under pressure was likely to find a way to escape to the surface. Some have declared the technology impractical.

Unexpected speeds

But a consortium from Europe, Australia and the US report in Science journal that – at least in certain geological conditions – the gas turns into rock, and does so at unexpected speeds.

The Carbfix pilot project began as an answer to a renewable energy problem. The Hellisheidi power plant is the world’s biggest geothermal energy generator, where water of volcanic origin runs turbines to power Iceland’s capital city, Reykjavik, and other settlements. But more than just water comes from the ground: there’s a lot of carbon dioxide and hydrogen sulphide as well.

So Carbfix experimenters began mixing the gases with water from subterranean sources and then re-injecting the solution into the basalt below.

When basalt is exposed to carbon dioxide and water, a series of chemical reactions begins, and the carbon precipitates into a chalky mineral. In human terms, geological processes can be achingly slow. But, in fact, the researchers watched 95% of the injected carbon solidify within two years.

“This means we can pump down large amounts of CO₂ and store it in a very safe way over a very short period of time,” says one of the research team, Martin Stute, professor of environmental science at the Lamont-Doherty Earth Observatory at Columbia University in the US.

“In the future, we could think of using this for power plants in places where there’s a lot of basalt – and there are many such places.”

About 10% of continental rocks are basalt, and almost 100% of the planet’s ocean floor is made of basalt, so there should be no problem finding suitable geological conditions. But that doesn’t mean the technology can be easily exported to other carbon dioxide sources.

More potent gas

Hellisheidi generates 40,000 tons of CO₂ a year, but any coal-burning power station could produce 20 times that. Hellisheidi also has vast quantities of water at its disposal, but other power generators might not be so well-served.

And there are suggestions that subterranean microbes might turn the carbonate back into methane – a far more potent greenhouse gas than carbon dioxide. So there are questions to resolve.

But the experiment is proof in principle that one objection to carbon sequestration may be needless.

The scientists pumped 250 tons of CO₂ and water and hydrogen sulphide down to as deep as 800 metres, and monitored the reaction. The experimenters had hoped that solidification would happen within eight to 12 years. To their surprise, it happened within months.

Juerg Matter, associate professor of geoengineering at the University of Southampton in the UK, who led the study, says: “We need to deal with rising carbon emissions. This is the ultimate permanent storage – turn them back into stone.” – Climate News Network