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Power plant turns its carbon emissions into stone

Section of rock core from the CO2 storage reservoir showing vesicular basalt with a well-defined fracture with calcium carbonate mineralization.
Source: Annette K. Mortensen
Section of rock core from the CO2 storage reservoir showing vesicular basalt with a well-defined fracture with calcium carbonate mineralization.

A power plant in Iceland is the first in the world to turn its carbon emissions to stone, providing a new method for battling pollution and climate change.

And the team working on the project accomplished the feat in just a few years, instead of the thousands of years many had believed would be needed.

Moreover, the rock they bury the carbon in is basalt, a type of rock common all over the world, so the method has the potential to be scaled up and put into wide use.

"The current consensus is that it would take up to thousands of years to mineralize CO2," said Sigurdur Gislason, one of the co-authors of the study, and a professor at the University of Iceland's Institute of Earth Sciences. That was based on experiences with one of the dominant methods of carbon capture — burying the gas in underground basins formed by sedimentary rock. "But we can show you can mineralize CO2 within two years," Gislason told CNBC in an interview. "That is pretty amazing, because mineralizing is the safest way of storing CO2."

Gislason is one of the members of the CarbFix project, an Iceland-based group of engineers and scientists from several universities in multiple countries and the Icelandic energy company Reykjavik Energy. Since 2007, the team has been honing its process for injecting water and CO2 into a basalt-lined underground well near the Hellisheidi geothermal power plant in Iceland.

Capturing carbon dioxide and sequestering it underground has been discussed as one of the ways of reducing carbon emissions in the atmosphere, which degrade air quality and contribute to climate change.

Proposals for CO2 storage sites have included placing the gas in empty underground aquifers, old oil wells and unminable coal seams, among other places.

The team behind the CarbFix project, however, has devised a novel method of mixing the CO2 with water and then injecting it into basalt rock underground, where it reacts with minerals and forms a new, solid rock that does not leak and poses no apparent threat to the environment.

The team published its results in the peer-reviewed journal Science on Thursday.

Here is how the process works.

First, the carbon dioxide is mixed with water as it is injected into a deep well containing basalt.

When the CO2 mixes with water, the combination becomes very acidic. Basalt is a reactive rock, and it dissolves in a matter of minutes when it comes into contact with the acidic CO2/water solution.

As the basalt dissolves, it begins leaching minerals, such as calcium, iron and magnesium. That process neutralizes the water, killing the acidity, and the metals combine with the CO2 to form a type of solid rock called calcite, or calcium carbonate.

In the beginning, the team was concerned about the potential environmental effects of the process and the resulting rock. Basalt contains aluminum and chromium, which can be environmental hazards.

But Gislason said that the water pulled back out of the ground once the process is complete is safe enough to drink by European health standards.

"In fact this could be one way of cleaning water," he said. "If you start with water that is not fit for human consumption, because of let's say, heavy metals, you could clean the water by taking it through the carbonization process and pulling it back out downstream."

The process is not yet a silver bullet for carbon pollution. It needs to be studied further, and it needs to be implemented on a much larger scale than it is in Iceland.

One disadvantage to the CarbFix process is that it requires a lot of water, about 15-25 tons for each ton of carbon. Just pumping that amount of water will require a lot of energy on its own.

However, Gislason noted that this can easily be done using ocean water, and basalt formations are found throughout the ocean floor, minimizing the impact on land use and freshwater sources.

There are also certain types of underground microbes that have been shown feeding off minerals produced in this process, and they emit methane, another harmful greenhouse gas, according to a release from Columbia University's Lamont-Doherty Earth Observatory accompanying the study. Researchers are currently evaluating the impact the microbes might have on the CarbFix calcite.

Gislason said the process can be scaled up to meet the needs of larger plants. The Hellisheidi plant produces only about 5 percent of the CO2 of the average coal-fired plant.

The process also costs a lot less than other carbon sequestration plans, said Reykjavik Energy's Edda Aradottir, one of the co-author's of the study, in the same release. The CarbFix solution can store carbon for about $30 a metric ton, compared with between $60 to $130 a metric ton for other methods.

The best opportunities for this would be along coastlines, where there is both abundant seawater, and basalt, Gislason said. The paper says that a site could be set up off the coast of the Pacific Northwest of the United States, for example.

Despite concerns that injecting water into the ground (as is done with wastewater from hydraulic fracturing or "fracking") can lead to earthquakes, Gislason says the CO2-charged water is not buoyant, and thus does not affect rock underground in the same way.

The team has been injecting about 10,000 tonnes of emissions per year since 2014, and plan to double the size of the operation later this year.