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August 2011

Water Films Expedite the Underground Lockup of Climate Change's Chief Culprit

Carbon dioxide is more quickly turned to stone when water is present

Water and Spectra
A thin film of water more quickly solidifies gaseous carbon dioxide, according to a fundamental study of the common pollutant. Enlarge Image

Results: A thin film of water more quickly solidifies gaseous carbon dioxide, according to scientists at Pacific Northwest National Laboratory who are studying underground storage of the common pollutant. Keeping carbon dioxide deep underground and out of the atmosphere removes it as a player in climate change. But, how would water influence reactions with underground minerals? The PNNL team found that nanometer-thick films of water coat the minerals and accelerate the reactions to solidify carbon dioxide. When the films are thinner than a nanometer, the reactions are slower.

"The reaction rate depends on the thickness of these thin films," said Dr. John Loring, a geochemist at PNNL who worked on the project.

Why It Matters: Power plants that burn coal and other fossil fuels emit more than 40 percent of the nation's carbon dioxide. These emissions contribute to changes in weather patterns, affecting cities and crops. Reducing or storing emissions could ease these changes. By understanding the fundamental reactions that sequester carbon away from the atmosphere, scientists can inform industry and policymakers about the feasibility and safety of different options.

Methods: One option for storing carbon dioxide is to capture the gas and inject it deep underground in porous rock formations. The gas exists in a supercritical fluid state at the temperatures and high pressures that are found in these underground formations. A supercritical fluid has a mixture of the properties of a liquid and a gas. The team wanted to determine how fast supercritical carbon dioxide that contains dissolved water reacts with underground rocks to form carbon-containing minerals. This process is known as carbonation.

The team used an infrared spectrometer equipped with a high-pressure reaction vessel, located in Sigma 5 at Pacific Northwest National Laboratory. On one of the reaction vessel's windows, they deposited a thin film of the mineral forsterite.  Forsterite contains magnesium, silicon, and oxygen and exists at geologic repositories.

They exposed the forsterite to water-free supercritical carbon dioxide at conditions similar to those found in underground repositories targeted as storage sites: 50 degrees Celsius and 180 atmospheres of pressure. They collected the data from the instrument and used it as a baseline.

Next, they added water to the supercritical carbon dioxide and mixed it with the forsterite. They collected data for several hours, until the reactions stopped. In addition, they used ex situ x-ray diffraction to study the samples. They repeated the experiments at different levels of water, up to the saturation limit of the supercritical carbon dioxide. When there was no water present, carbonation didn't happen. When water was present the reaction occurred, but the amount of water mattered.

When water was present but did not saturate the supercritical carbon dioxide, an ångstrom-thick water film formed on the minerals. An angstrom is one-tenth of a nanometer. Less than 1% of the carbon was taken up into the mineral formation.

"Apparently, these super-thin films allowed some initial reactivity during the first 3 hours of our experiments," said Loring. "But the angstrom-thin films can't support a continuous transformation reaction over 24 hours."

When more water was added, a nanometer-thick water film was detected. The carbonation reaction happened continuously, picking out 2 to 10% of the carbon dioxide and incorporating it into a mineral.

What's Next? The team will conduct more experiments on the carbonation of other minerals that are typically located in underground repositories and found in caprocks, which are the formations over the top of the repositories. The experiments will be done using EMSL resources, including atomic force microscopy, nuclear magnetic resonance spectroscopy, infrared spectroscopy, and x-ray diffraction.

Acknowledgments: Funding was provided by DOE's Basic Energy Sciences Geosciences Program through a Single Investigator Small Group Research grant and the Carbon Sequestration Initiative, a Laboratory Directed Research and Development program at PNNL. Part of this work was performed at EMSL, a national scientific user facility.

The work was done by John Loring, Christopher Thompson, Zheming Wang, Alan Joy, Deborah Sklarew, H Todd Schaef, Eugene Ilton, Kevin Rosso, and Andrew Felmy of PNNL.


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