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Coaxing Carbon Dioxide into Tiny Underground Voids

New study shows how water in the right place can enhance carbon sequestration

November 2015

Coal-fired power plants work hard to heat and cool homes, but burning coal releases carbon dioxide that will need to be managed in the future. Sequestering the gas can mean capturing it, stripping it away from other pollutants, compressing it, and then injecting it into underground reservoirs. The conventional picture is that, in the last step, the carbon dioxide flows underground where it encounters water and forms carbonic acid, leading to mineral formations that trap the carbon. At Pacific Northwest National Laboratory, four scientists just turned that wisdom upside down.

Using extremely detailed computer simulations, they found the desired minerals, or carbonates, can form even without water-slurping carbonic acid. Instead, just a small amount of water is needed, but it needs to be in the right spot. Coaxed out of the carbon dioxide liquid, even at the low concentrations of 1 water molecule for every 10,000 carbon dioxides, the water forms an extremely thin layer around the target mineral, in this case, anorthite. Once the layer forms, water starts eating away at the mineral surface, leaving tiny atomic voids. Enter carbon dioxide, filling in those empty spots and mineralizing in minutes.

Why It Matters

With support from the DOE Office of Fossil Energy and the Office of Science, the PNNL research team ran extremely detailed simulations to understand the mineralization process. The studies included trajectory simulations that tracked the motion of thousands of electrons and nuclei of hundreds of atoms in millions of different structures to estimate the reaction free energy.

The simulations showed how water molecules floating around in the supercritical carbon dioxide self-organize in a film and dissolve part of the anorthite surface along the way. The water pulls out positively charged calcium cations to create holes, or defects. Around these defects sit negatively charged oxygen atoms. These atoms quickly connect up with the incoming carbon dioxide and form the desired mineral.

The water layer forms slowly at low concentrations, but once in place, it prepares the stage for carbon dioxide to transform quickly.

Next, from these simulations, the team wanted to extract the necessary data to determine the energy needed to form the water layer. They found that the reaction occurs without an energetic push, but the position of the water molecules as they come in to form first "puddles" of water that connect into a film as they grow bigger.

What’s Next

Carbon sequestration is just one benefit that could come from this study. The work provides insights into buried interfaces, hard to access by experiment alone. This study shows how to simulate these interfaces and calculate the energy involved—potentially leading to more efficient batteries, biofuel reactors, and solar cells.

Read the full story here.

PNNL Research Team: Mal-Soon Lee, B. Peter McGrail, Roger Rousseau, and Vassiliki-Alexandra Glezakou.

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