Lock it in Rock

Charles Fergus
September 01, 2003
Mercedes Maroto Valer

In this image captured by scanning electron microscope, ground-up olivine reacted with CO2 yields a stable end product: magnesite. A Penn State professor of energy and geo-environmental engineering is working to speed up the reaction.

The dust, gray, has been ground to particles 200 micrometers, or eight thousandths of an inch, thick—"about what coal is pulverized to before being burned in a power plant," says Mercedes Maroto-Valer, a chemical engineer and assistant professor of energy and geo-environmental engineering at Penn State.

The material in the small vial she displays is not, however, coal: it is serpentine, a magnesium silicate common in many regions of the globe. Through a chemical reaction, serpentine can bind the carbon dioxide produced by burning coal and other fossil fuels. The reaction occurs in nature, but "It's a slow process," Maroto-Valer says. "Geologicaltime slow: on the order of hundreds of millions of years." Which is part of the reason why the end-product, magnesite, is so stable. "Our research is aimed at speeding up the reaction," she continues, "so that it can take place in an hour or less."

There's a pressing need to develop such a technology. CO2 constitutes some 82 percent of all greenhouse gases. In the atmosphere, these pollutants act like the glass in a greenhouse, trapping light rays and converting them to heat. CO2 comes from vehicles' internal combustion engines; from factories such as cement plants ("They burn fuel to create the high temperatures needed to produce cement"); and from power plants, which emit a third of human-generated CO2 world-wide. Explains Maroto-Valer, "Since power plants are large stationary sources, it makes sense to target them when trying to reduce emissions."

First, the CO2 must be separated from other flue gases such as nitrogen, oxides of nitrogen and sulfur, and water vapor. (Another area of Maroto-Valer's research is the development of advanced technologies for capturing CO2.) The gas is placed in a pressurized tank—Maroto-Valer uses a one-liter vessel in her lab—containing pulverized serpentine. The tank is agitated. The reaction begins.

Several strategies can speed up the process. The particles can be ground finer, creating a larger surface area on which the reaction can take place, but that approach requires more energy, upping expenses while releasing more CO2. The mineral can be heat-treated, but that too calls for adding energy.

"The option we're exploring is to increase the surface area without breaking down the serpentine into smaller pieces," says Maroto-Valer. "We do this by using an acid to make holes in the particles: like Swiss cheese. The holes give the CO2 greater access to the mineral. We call this ëactive carbonation,' because we end up with an activated particle, one that's more responsive to CO2.

"Right now we're trying to find out how much larger a particle can be and still develop enough perforations to let the reaction proceed quickly." The federal Department of Energy, which funds the research, checks the resulting magnesite to see how much CO2 has been sequestered.

Maroto-Valer says the process has "no environmental legacy. It's a closed system: no gases or acids are released." Nor does the acid become incorporated into the mineral. The resulting magnesite is the most stable form of carbon known. No longer the gray of serpentine, it is white and looks like baking powder. Some sand (silicon dioxide) is also produced, plus water and heat: up to 2,300 BTUs per pound of sequestered carbon, heat that can generate power or further accelerate the reaction. The sand and magnesite can be recycled as aggregates for roadbuilding and fill for reclaiming mined land. Some researchers have baked the powder into bricks.

In Europe, industries and utilities receive carbon credits and pay carbon taxes. Although CO2 is not currently regulated in the United States, we're headed in that direction. Maroto-Valer predicts that CO2 reduction will come about "from various technologies. A utility in a given area will use what's cost-effective." Since Lock It in Rock serpentine is abundant along both U.S. coasts and in the Upper Midwest, utilities in those regions could tie up their CO2 in the mineral, which costs only $3 to $5 per ton to mine. The gas could be piped to reactors at mines, or the pulverized mineral could be hauled to the power plants. To bind a ton of CO2 requires one and a half to two tons of serpentine; another magnesium silicate, olivine, can be substituted in the process.

Notes Maroto-Valer, "The deposits of olivine and serpentine discovered to date are more than enough to sequester the CO2 that would be produced by the combustion of all our known fossil-fuel reserves."

M. Mercedes Maroto-Valer, Ph.D., is assistant
professor of energy and geo-environmental engineering in the College of Earth and Mineral Sciences and program coordinator for sustainable energy in The Energy Institute; 122 Hosler Bldg., University Park, PA 16802; 814-863-8265; mmm23@psu.edu. The U.S. Department of Energy is funding her CO2 sequestration research, and Penn State has filed a provisional patent on the work. Collaborators include Matthew E. Kuchta, M.Sc., a graduate student in geo-environmental engineering who received his master's degree based on the research; and Yinzhi Zhang, Ph.D., postdoctoral researcher.

Last Updated September 01, 2003