Scientists turn CO2 to stone in just two years: a solution for climate change?

Researchers at the world's largest geothermal power plant have found a way to store carbon dioxide underground and turn it to rock.

Courtesy of Árni Sæberg
Reykjavik Energy's Hellisheidi geothermal power plant, seen here, currently emits 40,000 tons of carbon dioxide and 12,000 tons of hydrogen sulfide per year. The CarbFix I pilot CO2 injection site is connected to the powerplant via a pipeline that injects some of the CO2 and H2S gases into a basaltic storage reservoir about a quarter mile below surface.

Nature can turn carbon dioxide into rock, but it takes thousands upon thousands of years. Scientists in Iceland may have just figured out how to do it in less than two.

Carbon capture and storage (CCS) holds enormous potential to slow climate change, taking carbon dioxide gas out of the atmosphere and storing it underground – in theory, anyway.

But efforts to turn theory into reality have faced huge technical challenges, not least concerns that the liquid carbon dioxide being pumped down into the Earth might leak back out.

In a radical new approach, described Thursday in the journal Science, scientists mix carbon dioxide with water and then inject the slurry into basaltic rock, where it solidifies into veins.

In essence, the researchers managed to turn carbon dioxide into stone – and quickly.

“The biggest surprise of the whole project was that the reaction between carbon dioxide and basaltic rock was really, really fast,” lead author Juerg Matter, an associate professor in geoengineering at the University of Southampton, England, tells The Christian Science Monitor in a telephone interview. “Over two years, 95 percent to 98 percent was mineralized.”

To put this in perspective, the Intergovernmental Panel on Climate Change (IPCC) in a 2005 report predicted that it would take between 100 to 1,000 years for this mineral carbonation to take place. Even at that rate, it acknowledged that CCS, which it described as “an immature technology,” holds significant appeal due to the “permanence of storage of CO2 in a stable solid form.”

“This is a great step forward in proving the practicality of the process,” says Robert Zierenberg, a geology professor at the University of California, Davis, who was not involved in the research, in a phone interview with the Monitor. “People knew it would work but didn’t know the timescale – whether it would be rapid enough to be useful.

“You need a large-scale project to prove that, and that’s exactly what they’ve done.”

It began at the biennial State of the Planet Conference held at Columbia University in New York, when Dr. Matter was still a professor at that institution. The event brings together world leaders, CEOs, scientists, and other influential thinkers to consider the planet's most urgent challenges.

One of the attendees was the president of Iceland. Matter had already started working with the idea of storing carbon dioxide in basaltic rock, which the island nation is built of. The two got to talking, and the project was born.

The formal collaboration began in 2007, when various universities partnered with the Hellisheidi power plant, the world’s largest geothermal energy installation. 

Partnering with a geothermal energy plant means the deep boreholes, which are expensive to drill, are already in place. In addition, the power plant provides a source of carbon dioxide, pumping out 40,000 tons a year.

While this may seem something of a paradox, as geothermal is a "clean" energy, these emissions are only five percent of what an equivalent coal-fired plant would produce. And even this is at the high end for geothermal plants, many of which produce no emissions.

So, the scientists had the infrastructure, the carbon dioxide, and an enormous quantity of basalt.

In a 2012-2013 pilot project, christened Carbfix, they disposed of 250 tons of carbon dioxide, mixed with water and the other pollutant emitted by Hellisheidi, hydrogen sulfide. They sunk the cocktail 400 to 800 meters (a quarter mile to half a mile) below ground, where it began reacting with the minerals in the basalt and solidifying. Two years later, it had almost entirely turned to stone.

They immediately scaled up the project. In 2015, the energy company sequestered 5,000 tons of carbon dioxide, and they are on track to double that this year. Their eventual goal is to capture and sequester all their emissions in the same way.

“This is one of the most exciting aspects of the work for me: to have a partnership with industry,” says coauthor Martin Stute, an environmental science professor at Columbia University, in a phone interview with the Monitor. “They’ve actually upscaled this and are using it.”

Could this, then, be an answer to global warming?

We asked Jason Veysey, a senior scientist with the Stockholm Environment Institute who was not involved with this research. He notes that, of the IPCC's hundreds of climate models, almost all that illustrate pathways to a brighter future talk of CCS. 

“This paper opens up a new reservoir for CCS which is potentially enormous,” says Mr. Veysey. “It represents the tip of the iceberg.”

A note of caution

Sequestering carbon dioxide in basalt may not be a silver bullet, as questions remain about how widely the technique can be applied.

Chief among the concerns is the amount of water required: about 25 tons for every ton of carbon dioxide turned to stone. As many fossil fuel-heavy areas also suffer water scarcity, notes Veysey, this could be problematic. 

One solution could be the use of seawater, though salt water's efficacy remains untested, says Columbia's Dr. Stute.

The other major limitation is the availability of basalt. Only about 10 percent of continental rock is composed of this porous stone, though it accounts for almost all the seafloor. If seawater does prove to be effective, then coastal power plants could prove ideal hosts for the technique.

The final hurdle is cost.

“With our method, storage cost is about $17 per ton of carbon dioxide,” says Matter, “which is a little bit higher than traditional carbon dioxide injection – about $5 to $10.”

But, as the professor points out, because this pioneering method turns the carbon dioxide to stone, storing it in a permanent fashion, it eliminates costs associated with monitoring for leakage.

Additionally, he says, storage costs are dwarfed by the pricetag for the “capture” phase, which can cost up to $150 per ton of carbon dioxide.

This is also a nascent technology, so costs could fall as the methods are refined.

Nevertheless, as Matter says, the most popular disposal site for emissions remains free to power companies: the atmosphere.

As long as companies don't bear the direct costs for pollution, he worries, profit-seeking industries may not be willing to invest in CCS.

Hellisheidi's commitment to the new technology reflects Icelanders' willingness "to invest in the future," says UC Davis's Dr. Zierenberg, "and not just take a short-term approach to making profits."

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