ASU Research E-Magazine: Set in Stone

Most carbon dioxide emissions come from the burning of fossil fuels. The likelihood that people will drastically cut their energy use or rapidly switch to alternative fuels anytime soon is slim. So what is a planet to do?

The problem is clear. There is too much carbon dioxide in the atmosphere, and every year people produce more. The gas traps the sun’s heat, helping to keep our planet warm. But too much of a good thing may wreak havoc on the global climate.

World leaders agree that we need to reduce CO₂ emissions or face extreme consequences. But most CO₂ emissions come from burning fossil fuels, and the likelihood that people will drastically cut their energy use or rapidly switch to alternative fuels is slim.

“You hear an awful lot about developing alternative energy sources. But the energy infrastructure of the world is heavily reliant on fossil fuels,” says Michael McKelvy, an ASU research scientist with the Center for Solid State Science (CSSS). “Currently, about 85 percent of worldwide energy comes from fossil fuels. It will be difficult to substantially replace those with alternative sources anytime soon.”

Imagine if we could take all that waste CO₂ and simply throw it away. What if we could catch it, imprison it in stone, and put it back in the ground from whence it came?

The ASU Carbon Sequestration Research Team is working to make that fantasy a reality. Team members combine extensive experience in chemistry, physics, geology, mineralogy, and electron microscopy. They also know multiphase fluid flow modeling. CSSS research scientist Andrew Chizmeshya leads the computer modeling effort.

Turning CO₂ into stone is one of several potential “sequestration” technologies. The idea is to store carbon harmlessly so it cannot enter the atmosphere and contribute to the greenhouse effect.

Carbon is sequestered naturally in many forms. For instance, it is stored in trees and in deposits of fossil fuels such as coal and oil. When people burn fossil fuels, or deplete old forests, that carbon is released into the atmosphere as CO₂.

Scientists are looking at ways to sequester carbon ourselves. Some are exploring methods to pump CO₂ underground into current or abandoned oil and gas wells as well as deep saline aquifers. However, this method only works if those repositories remain sealed. Other researchers are examining ocean sequestration—storing CO₂ at the bottom of the ocean or distributing it throughout the waters. Unfortunately, no one knows how this method will impact the ocean environment.

“Our efforts are novel. We want to dispose of the CO₂ as carbonate minerals,” says McKelvy. Carbon dioxide would be collected from a power plant and separated from other waste materials. The CO₂ then would be put through a chemical reaction with serpentine or olivine, two common minerals. The reactions would form the compounds magnesite and silica.

Magnesite is widely present in nature and is similar to the limestone found in the Grand Canyon. Silica is the sand used in glassmaking. Some of this end material could be put to practical use, but not all of it, McKelvy says. Where would all that rock go?

Right back into the ground.

“Practically speaking, you’d reclaim the [olivine or serpentine] mine as well as coal mines by putting minerals back into the ground,” says Chizmeshya. “The beauty of the process is that you end up with environmentally benign materials known to be stable on a geologic timescale. The important research question is can the process be made economically viable?”

So far the answer to that question has always been “no.”

Scientists at other institutions have figured out how to activate, or pretreat, serpentine and olivine so they will react quickly with CO₂. The pretreated mineral is placed in a solution of water, sodium chloride, and sodium bicarbonate. Then it is reacted with CO₂ at a high temperature. This method allows for 80 percent carbonation in less than an hour.

“These are the type of reaction conditions that could be economically viable. But the current pretreatment process is too expensive for the process to be cost competitive,” says McKelvy.

Currently, the minerals are pretreated through heating or intense grinding. McKelvy says these methods are too energy-intensive for practical use.

The ASU Carbon Sequestration Research Team is working to develop new activation processes and modified carbonation techniques to reduce cost. To do this, they are studying the carbonation process on the atomic level. They integrate in-situ experiments with advanced computational modeling.

For example, the team is studying a promising alternative to pretreating olivine. The researchers discovered that as olivine reacts with CO₂, it forms a hard coating, kind of like the candy shell surrounding an M&M chocolate. This coating drastically slows the reaction.

Pretreating the olivine prevents the formation of this coating, but it is too expensive. Instead, the researchers will explore ways to help break up the coating as the reaction occurs.

“Our research is really on the border of fundamental and applied science. Our focus is on fundamental research that has practical benefits for society,” says McKelvy.

The scientists use a variety of techniques to study carbonation, making heavy use of the facilities at the Goldwater Center for Science and Engineering and the Center for High-Resolution Electron Microscopy in CSSS. The group has also developed a unique patent-pending microreactor, which is providing the first real-time observations of the carbonation process.

The device is basically a tiny (1/10 ml) reaction cell with windows. It can produce temperatures up to 400 degrees Celsius, and pressures up to 300 atmospheres. ASU is collaborating with the Advanced Photon Source (APS) at Argonne National Laboratories to study the precise mechanisms that govern carbonation using this reactor. To date, the researchers have conducted X-ray synchrotron studies of carbonation using the GeoSoilEnviroCARS beamline at APS. They are currently using X-ray diffraction to explore the mineral carbonation process as it occurs.

“The cell can be adapted to work with several techniques,” explains McKelvy. “It allows us to explore the solid-fluid reaction while observing the CO₂-rich fluid in intimate contact with the aqueous phase.”

If the scientists can economize the carbonation process, they will have a promising method for large-scale CO₂ disposal. Olivine and serpentine are readily available in huge quantities.

“Global olivine reserves can carbonate a substantial amount of the CO₂ that could be generated from known coal reserves. Serpentine could cover all of it,” says Chizmeshya.

Mining, milling, and transporting the minerals would carry a cost, of course. Fortunately, serpentine and olivine mines already exist. The minerals cost about $5 per ton to mine and mill. The materials are found all over the world, which can help keep transportation costs low.

“Ideally, the mine and the power plant would be in close proximity,” says Chizmeshya. CO₂ could be shipped to the reaction plant via pipeline, the same way gas is transported.

Mineral sequestration may offer side benefits, as well. For example, it could help eliminate other dangerous substances. Asbestos is a form of serpentine that could be used to react with CO₂. Carbonation destroys asbestos, rendering it harmless to humans.

The process would integrate with the Department of Energy’s Vision 21 program. That program’s goal is to develop high-efficiency fossil fuel power plants that emit virtually no pollutants by 2015. It could also help the United States reduce its dependency on foreign oil.

“The United States has enough coal reserves to support our energy needs well into the next century,” says McKelvy. “That coupled with advances in power plant and transportation efficiency, hydrogen generation, and fuel cell technology could allow us to be energy independent with our own reserves. Carbon sequestration can enable us to use that in an environmentally sound way.”—Diane Boudreau

The ASU team is a member of the CO₂ Mineral Sequestration Working Group, managed by the Fossil Fuels Division of the U.S. Department of Energy. ASU members include McKelvy, Chizmeshya, Ray Carpenter, Renu Sharma, Hamdallah Béarat, and Jason Diefenbacher from CSSS; George Wolf and Brandon Doss from the Department of Chemistry and Biochemistry; Michael Kocher from Physics and Astronomy; and Dierdre Gormley, Ryan Nunez, and Youngchul Kim from the Science and Engineering of Materials Graduate Program. For more information, contact Michael McKelvy, Ph.D., 480.965.4535. Send e-mail to Michael.McKelvy@asu.edu