How to Store Carbon Dioxide? Lock It Up in Rocks.

Stories from New Scientist.
July 27 2014 3:08 PM

Rock Solid Solution

A new project stashes carbon dioxide in the form of minerals.

Lava columns and sea stacks
Lava columns and sea stacks in Iceland.

Photo by Ben Husmann/Flickr Creative Commons

This article originally appeared in New Scientist.

How can we get rid of excess CO2? Geologist Juerg Matter of the University of Southampton, U.K. is a principal investigator of the Iceland-based project CarbFix, whose recent results show it has safely stored nearly 170 tons of carbon dioxide underground by reaction with minerals—stashing it in rock so it can’t leak out again. The next step is to go big.

How did you start working on a project to lock away carbon dioxide in rocks?
When I was working in Oman, I saw these really blue, alkaline rock pools with white deposits at the bottom. The rocks were mantle peridotite, which reacts with CO2 to form white carbonates.

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Later I moved to Columbia University’s Lamont-Doherty Earth Observatory in New York, where I met Taro Takahashi and Dave Goldberg. They told me they wanted to look at a way of locking up CO2 using mineral carbonation. I said, “I've seen that in Oman.”

How does your carbon storage method differ from conventional techniques?
The standard method of carbon storage is injecting pure CO2 deep into the Earth’s crust. But the risk with that approach is that the gas could leak back out. So in our pilot project in Iceland, called CarbFix, we take CO2 and wastewater from the same geothermal power plant and inject them together. The CO2 dissolves and, like in a bottle of sparkling water, it stays dissolved as long as it's sealed. It then reacts with calcium and magnesium silicates in rocks to form carbonates.

What are the advantages of this method?
It's more permanent, safer, and needs less monitoring than conventional carbon storage. If you inject pure CO2 into the subsurface, like they do in the North Sea, then you have a bubble sitting there that can potentially leak out. When CO2 is dissolved in water it’s less likely to escape into the atmosphere, and reacts with rock to form carbonate minerals much more quickly, meaning it needs less monitoring. That reduces costs.

You've been using this approach in your pilot project in Iceland. What have you learned?
In less than a year, about 85 percent of the CO2 we injected had converted to carbonates, which is faster than in our lab studies. This is a really big result, but we’re still working out the details of why it happened so quickly.

What are the benefits of such rapid conversion of CO2?
It could be a disadvantage, because you don’t want to clog up the pores in the rock before the space is full: That could mean drilling more injection wells at extra cost. The advantage is that, if it doesn't fully block up the pore space, a fast reaction reduces the need for long-term monitoring. You know the CO2 is safely stored.

Where can you find the right types of rock?
There are two types, basalt and mantle peridotite, and you can find both on every continent. Peridotite is found in Oman and a lot of other places, but basalt is more common: Two-thirds of the rocks on Earth are basaltic, and the whole ocean floor is basalt. In Iceland there is basalt on the surface, and there are huge basalt regions in India, Siberia, the Northwestern United States and Brazil.

Which type of rock is better?
Reactivity with CO2 is faster in mantle peridotite, but if you want to inject large volumes of dissolved CO2 then you need pore space in the rocks. Basalt has a much higher porosity than peridotite.

Are there enough of these rocks in every country for this to work as a global strategy?
Not on land, but in Europe almost all CO2 storage is offshore anyhow. Planned pilot projects on land—Barendrecht in the Netherlands was a big one—were canceled because of negative public perception. People protested because they didn't want it in their backyards.

So Europe is looking for offshore storage reservoirs, and if you think about offshore storage you immediately think about basalt. The entire ocean floor is made of it.

Could changing the composition of rocks shift land masses or lead to earthquakes?
Anything you inject into the subsurface—CO2, liquid chemical waste—could produce tremors. When CO2 reacts with silicate it makes a less dense carbonate that fills more space. If porosity is limited in the subsurface you could raise the ground. We have a lot of CO2 to put under the Earth’s surface, and if you inject vast amounts of it, the subsurface could rise. In Algeria, the surface lifted at a conventional carbon storage project called In Salah, but by millimeters, not meters. It could happen with this approach too.

You need energy to pump CO2 underground. How do the emissions produced by that compare with what you're storing?

For CarbFix the electricity comes from geothermal power, so the emissions penalty for injecting dissolved CO2 is on the order of 0.2 percent. With electricity from a coal-fired power plant, the penalty would be much higher, over 10 percent of the volume stored.

How do the costs of mineral storage compare with those of conventional carbon storage?
For the injection and storage process at the CarbFix site in Iceland it’s about $17 to $30 per ton. (For context, a car produces roughly a ton of CO2 every 5,000 kilometers.) At $17, it's about twice as much as direct CO2 injection, but these costs don't include monitoring or capture.

The bulk of the cost for both methods is in the capture process, which is $55 to $112 per ton of CO2. And overall it comes out the same because if the CO2 mineralizes you don't need long-term monitoring. We get criticized for injecting CO2 dissolved in water, because it means we are likely to need more injection wells due to the extra volume from the water. But if we inject even deeper we can use less water and reduce costs again.

When will this technology be ready for use on a large scale?
Our work in Iceland shows that it's already feasible in basalt. The next step would be to upscale from the several hundred tons we've injected to a million tons per year. Basalt could be ready on that industrial scale in five to 10 years. Mantle peridotite will take a little bit longer, 10 to 15 years, because we still have to prove that key processes happen.

If this technique were to take off, could we use up the supply of ocean floor basalt?

Dave Goldberg found that just one ocean ridge site—admittedly covering tens of thousands of square kilometers—could potentially store the total amount of CO2 we’ve already emitted into the atmosphere. The storage potential of all the ocean ridges is around 10 times larger than the CO2 emissions we’d get from burning all of Earth’s fossil fuel. There is no way we will use up this resource. Although how much can be practically used may depend more on politics and economics than science.

Could this be our main way to store CO2?
That is more about what’s lacking for all carbon capture and storage: Policymakers have to address climate mitigation and provide an economic model, which means us having to pay for CO2 emissions. From a science and engineering point of view, we're way beyond the first-generation system: Now we are ready to move on and improve. 

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