Carbon emissions turn to stone underground

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Solidified carbonate, precipitated from carbon dioxide, in basalt rock.
Kevin Krajick / Lamont-Doherty Earth Observatory

Carbon dioxide pumped into volcanic basalt rock turns into stone in a matter of months, a new study shows.

An international team led by the UK’s University of Southampton’s Juerg Matter mixed 250 tonnes of carbon dioxide with water and injected it into rocks 400-800 metres beneath the Hellisheidi power plant in Iceland.

After only two years, they found 95% of the carbon had solidified into minerals – much faster than estimations that the process could take anywhere between eight and hundreds – even thousands – of years.

“This means that we can pump down large amounts of carbon dioxide and store it in a very safe way over a very short period of time,” said study co-author Martin Stute, a hydrologist 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.” Volcanic basalt comprises pretty much the entire sea floor and around 10% of continental land.

One way of tackling carbon emissions from dirty coal-fired power plants is carbon capture and storage, but amid high costs of capturing the carbon and safety concerns surrounding sequestration, it has gained little large-scale traction.

And it’s not just coal-fired power plants that produce carbon emissions.

Geothermal facilities, such as the Hellisheidi power plant, pump volcanically heated water to spin turbines. But with that water comes loads of volcanic gases, including carbon dioxide and rotten egg gas, or hydrogen sulfide.

Much research into carbon storage has focused on refilling spent gas wells or injecting the gas with water at high pressure into saline aquifers, more than a kilometre underground. The Otway project in Victoria, Australia, is one facility tracking how carbon dioxide moves and reacts in those layers.

In Iceland in 2007, the company running the Hellisheidi plant, Reykjavik Energy, joined forces with universities to find a local solution to their own gassy problem.

When basalt is exposed to atmospheric carbon dioxide and water, the carbon precipitates into a pale, chalky mineral. But no one knew how fast this might happen with high concentrations of carbon dioxide.

So in 2012-13, they injected a mix of volcanic gases and water into the rocks below at two sites. After two years, Matter and colleagues dug up rock cores and to their surprise, found 95% of the injected carbon had mineralised.

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Rock cores pulled up from the injection site in October 2014.
Juerg Matter

But before we all go pumping carbon-dioxide-laden water into basalt, remember: the process may be a solution best suited for Iceland.

Geothermal plants don’t produce nearly as much carbon dioxide as coal-fired plants, and not all carbon-emitting plants have handy access to basalt. (Iceland, being highly volcanic, is mostly basalt.)

But the main hurdle beyond the basalt, says Sigurdur Gislason, a University of Iceland geologist and study co-author, is the water needed – around 25 tonnes per tonne of carbon dioxide.

While the Hellisheidi plant could use pre-existing infrastructure to pipe carbon-loaded water into the ground, costing only around $30 per tonne, this could blow out to around $130 per tonne of injected carbon dioxide for a coal-fired plant.

Still, it’s a win for the residents around the plant, who don’t have to put up with as much stinky hydrogen sulfide gas in the air.

And the researchers and company are already scaling up the technology, “where up to 5,000 tonnes of carbon dioxide per year are captured and stored in a basaltic reservoir”, Matter says.

The work was published in Science.

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