How to stash carbon dioxide: Turn it into stone.
Dissolved in water and pumped underground, CO2 becomes limestone.
The good news is that mineral carbonation promises to lock away CO2 – a powerful greenhouse gas – in a much more stable form than simply pumping it into an underground geologic formation. The bad news: Sequestering CO2 in this way is resource-intensive.
Scientists the world over are exploring ways to capture carbon dioxide and pump it deep underground as a compressed gas. But what’s to keep the buoyant gas from working its way up through fractured rock and reentering the atmosphere?
Ensuring that such a leak never occurs could require monitoring the injection site for a very long period of time – from hundreds to tens of thousands of years.
“How long can you afford, or how long can you plan on a government being in place to be able to watch over something?” says Travis McLing, a technical lead researcher and geochemist at the Idaho National Laboratory in Idaho Falls. “Name a government that’s been stable or intact for even 1,000 years. Long-term stewardship is a real issue.”
One potential way to prevent CO2 from leaking out after injection is to quickly turn the gas into minerals that also occur naturally, such as calcium carbonate, which is the main component in limestone. Many questions remain about the mineralization process, from how quickly it would occur to the cost of required resources and infrastructure, but a handful of scientists are bent on unearthing the answers.
Next month in Reykjavík, Iceland, a pilot project will, for the first time, inject carbon dioxide-saturated water into a formation of basalt rock more than 1,300 feet below ground. The project includes Icelandic, French, and American scientists in partnership with Reykjavík Energy, a geothermal energy company. They hope to learn how quickly the carbon dioxide and water mixture reacts with the basalt rock to form calcium carbonate. Under laboratory conditions, mineralization began within four to six weeks and had occurred extensively within months.
In most sequestration projects, compressed carbon dioxide is injected into a geologic formation already filled with water. In order for mineralization to start, the gas must first dissolve into the water. Scientists don’t know how long that natural process will take, but it is considered the limiting factor, according to Dr. McLing, and the CO2 could leak out during that time. Scientists at Reykjavík want to bypass the slow process of CO2 absorption into water and the risks it poses by mixing the gas with water before injecting it underground.
“How fast you can convert CO2 into a mineral – that is critical,” says Juerg Matter, a geochemist at Columbia University’s Lahmont-Doherty Earth Observatory in New York. “We have to expedite it because of the global-warming issue.”
Dr. Matter is involved in another similar study of peridotite, an igneous rock, where it occurs in a large formation in the Middle Eastern country of Oman. Like basalt, peridotite chemically reacts with carbon dioxide-saturated water to quickly form minerals, some of which contain carbon. Matter and his colleague, geologist Peter Kelemen, estimate that with accelerated mineralization, the 16,000 cubic-mile formation could absorb some 4 billion tons of CO2 a year, about 12 percent of the world’s annual CO2 emissions – if it could be collected and transported there.
The scientists are in early discussions with a large oil company in Oman about field tests. They also have a pending patent on part of the injection process they are developing. Professor Kelemen says any proceeds would be the property of Columbia University.
Peridotite occurs in sizable and accessible quantities on every continent except Antarctica, according to Kelemen. Basalt is one of the Earth’s most common rock formations, making up 10 percent of the continental crusts.
Basalt is also found on the seafloor surrounding most continents, as well as inland. There is storage space in geologic formations worldwide for at least 2 trillion tons of carbon dioxide, according to the United Nations’ Intergovernmental Panel on Climate Change. The human population produces roughly 30 billion tons of CO2 annually.
As Kelemen says, “There is a huge amount of rock, compared with CO2, in the atmosphere.”
But even if suitable geologic formations abound, the question remains: Does CO2 mineralization make economic and practical sense?
Capturing carbon dioxide and injecting the dry gas into the ground takes large amounts of energy and money – as much as $110 per ton of CO2 to capture and transport, according to a recent paper by Carnegie Mellon University environmental engineer Edward Rubin. But to start the mineralization quickly by dissolving CO2 into water before injection would also require a lot of water.
“When you talk about sequestration at scale for commercial operations,” says Peter McGrail, an environmental engineer with the Pacific Northwest National Laboratory, “you end up having to deal with extraordinarily large volumes of water.” To dissolve a ton of CO2 requires about 27 tons of water, as well as significant pressure and heat.
Another potential problem occurs after injection, as carbonate minerals precipitate from the water solution into the porous rock. The minerals might plug up areas into which the water solution is migrating. If this happens, new wells and pipelines would be required to continue the sequestration project.
“This is really the core problem with this idea,” says Dr. McGrail. “It may be possible [to surmount it], but whether it’s cost-effective to do so and whether the risk associated with having to drill many more wells early in a project [is] hard to say.”
The process of injecting CO2 underground – whether combined with water or just as a gas – uses relatively little energy. It is carbon capture, compression, and transport that demand significant amounts of power.
According to researcher McLing, conservative estimates find that with most of our existing coal-fired power plants, at least 25 percent of the electricity produced would go toward capture and compression technology. So for every three electricity plants built with carbon capture capabilities, a fourth would be needed just to power the process.
“I’m pretty strongly pro-CO2 capture and sequestration,” says McLing. “But ... is it the right thing in terms of resource conservation? To get rid of the carbon dioxide, we have to burn an additional 20 to 40 percent more coal or natural gas or petroleum. What was formerly a 200-year supply of a resource may now only be a 100-year supply of a resource.
“Those are things we have to balance,” McLing concludes. “Are conservation and sequestration compatible?”