A small amount of carbon dioxide does actually emerge from the process of Iceland’s geothermal energy production, however, originating in volcanic rocks and traveling along with other gases in the steam that drives turbines at geothermal plants. So in 2007, Reykjavik Energy — whose Hellisheidi plant is among the largest geothermal energy production facilities in the world, with 303 megawatts of capacity — started a project called CarbFix.

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The goal was to take some of the relatively small emissions of carbon dioxide from their geothermal plants (along with emissions of hydrogen sulfide, a dangerous gas) and stow them away back in the rocky ground 400 to 800 meters in depth. The Hellisheidi plant emits about 40,000 tons of carbon dioxide annually. (The United States, for comparison, emits more than 5 billion.)

It’s not the first time that anyone has tried to store excess carbon underground — but in this case, there was an added twist. The carbon wasn’t to be preserved in a gaseous or fluid (supercritical) form, such that it might someday escape back into the air. Rather, it would be dissolved in large volumes of water and pumped into porous, basaltic rock — a dark-colored volcanic rock that forms from the cooling of lava — and undergo a chemical reaction, aided by all the water, that would turn the carbon dioxide into a carbonate by binding it with calcium, magnesium or iron that naturally occurs in high amounts in basalt.

And now, in a new paper in Science, representatives of Reykjavik Energy and a team of scientists from a large number of universities, including the University of Southampton in the U.K. and the Lamont-Doherty Earth Observatory at Columbia University, show not only that the process of injecting carbon dioxide into basalt rock at the Iceland site works, but that moreover, the carbon dioxide is mineralized, or turned into rock, very rapidly. In two years, they report, over 95 percent of injected carbon dioxide had become mineral.

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“When we actually did it, it turned out to be two years,” said Martin Stute, a geochemist at Lamont-Doherty and a study author (the research was led by Stute’s colleague Juerg Matter, who also has an appointment at the University of Southampton). “We didn’t really expect that to happen ourselves, and we were very surprised.” The timeline is much different from what is generally assumed when scientists talk about the process of geological weathering, in which, over very vast timescales, carbon dioxide eventually becomes absorbed into rocks — what is generally considered to be the slowest part of the Earth’s carbon cycle.

“We demonstrate that by using this method, you can permanently remove the CO2, store it in the rock, and the rock isn’t going anywhere, it stays there for geological timescales,” adds Edda Aradottir, who works with Reykjavik Energy and is also an author of the study. “So I would hope that other types of industries would be interested in this method.”

The finding could be quite important because of the ubiquity of basalt in the world. The researchers say that 10 percent of the rocks that make up continents are basalt, and so is “most of the ocean floor.” So is 90 percent of Iceland, according to CarbFix.

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The injection reported in the new study was small, to be sure, and only amounted to a pilot project, Stute said. About 250 tons of gas was injected, mostly carbon dioxide but also a substantial volume of hydrogen sulfide. Since then, however, Reykjavik Energy has scaled up and injected 5,000 tons per year, and it is now looking to double that, Aradottir said.

Ultimately, it could be possible to store all the plant’s CO2, or 40,000 tons per year. “The capacity is, for sure, in the ground to take all the CO2, and much more, actually,” she said.

Other experts involved in the carbon capture and sequestration hailed the results reported by the group Thursday.

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“Carbon storage is a critical piece of the global initiative to reducing carbon dioxide emissions and realization of the Mission Innovation goals set out in Paris,” said Sallie Greenberg, a scientist with the Illinois State Geological Survey who has collaborated with Archer Daniels Midland on a project to store large volumes of carbon dioxide in Illinois sandstone layers. “This work on demonstrates carbon mineralization has potential to increase our ability to permanently store CO2.”

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“They made very nice progress and this paper represents a breakthrough,” added Klaus Lackner, who heads the Center for Negative Carbon Emissions at Arizona State University and who says he knows the researchers involved and helped get the project started but is not an author of the paper. “It is an important result that the CO2 forms carbonates in a short time. Mineral sequestration underground is only useful if it happens on a human time scale. If it takes 5,000 years, as it might in silicate formations, there really is not much of an advantage to forming minerals. It does not add much to the safety of the CO2 storage because on the relevant time scale the CO2 is still mobile and can come back.”

Lackner, along with other scientists involved in trying to study how to remove carbon dioxide from the air and store it, has researched a related idea dubbed “enhanced weathering.” In this scheme, the natural weathering reaction, in which carbon in water interacts with rocks to form carbonates, is sped up by crushing up lots of rock and spreading them out across the land.

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Stute said that in truth, both enhanced weathering and the basalt injection approach probably have their role in helping to fix the carbon problem. “A lot of these studies have been looking at just collecting rock materials and exposing them to CO2,” he said. “The chemical reactions are very similar, but it’s bringing the rocks to the CO2, versus bringing the CO2 to the rocks,” as happened in the current experiment.

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The researchers are enthusiastic about their possible solution, although they caution that they are still in the process of scaling up to be able to handle anything approaching the enormous amounts of carbon dioxide that are being emitted around the globe — and that transporting carbon dioxide to locations featuring basalt, and injecting it in large volumes along with even bigger amounts of water, would be a complex affair. Still, the potential is there, they say.

“You could basically sequester all the CO2 humans emit in basalts, if you want to, but you have to capture the CO2 and get it to these places,” Stute said. “That’s what the problem really is.”

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