Carbon Capture and Sequestration

By Bill Clayton

Electron-microscope images showing graphite layers on nickel catalyst (above), and carbon nanotubes that form on surface of nickel catalyst during steam reforming of the graphite layers (below). (Photo courtesy of Johannes Schwank)

Earth's atmosphere is a "skin" of air about 370 miles thick. Keeping it healthy will determine whether the planet turns into a barren space rock or remains a viable human habitat.

Greenhouse gases pose a particular threat. According to Christian Lastoskie, associate professor in the departments of Civil and Environmental Engineering and Biomedical Engineering, a "good percentage of greenhouse gases such as carbon dioxide (CO2) find their way into the atmosphere through the natural carbon cycle. Without their heat-trapping ability, we'd have subzero temperatures and inhospitable conditions on much of the Earth's surface."

However, burning fossil fuels like coal, oil and natural gas has created alarming volumes of CO2 - in the last 200 years, atmospheric CO2 has increased 27 percent. "Automobile emissions get the most publicity, but thirty percent of carbon dioxide emitted into the atmosphere originates from fossil-fuel fired power plants."

Nature removes carbon dioxide from the atmosphere by photosynthesis, a process in which trees and other plants absorb and transform the gas. "It's a natural hedge against global warming," Lastoskie said, "but it can't keep up with accelerated rates of CO2 formation from human activities." As atmospheric greenhouse gas concentrations increase and the impacts of climate change become more apparent, carbon capture and storage (CCS) technologies are attracting attention as a way to slow the increase of, and eventually stabilize, atmospheric CO2. "CCS strategies focus on locking away carbon dioxide, not preventing its creation. But they'll be increasingly important as society makes a gradual transition to energy technologies that don't rely upon fossil fuels."

One CCS strategy, geologic storage, involves capturing CO2 and sequestering it underground in places such as depleted oil and gas reservoirs or coal seams that can't be mined. If emissions remain at present levels, there's enough global geologic storage space to accommodate CO2 for at least the next century. But there are uncertainties, the biggest of which is leakage of the CO2 into the biosphere.

There are other sequestration schemes, such as injecting captured CO2 into oceans. However, research in this area has been slow to develop because of concerns about possible ecosystem damage.

Lastoskie and his team have been investigating the capacity of nanoporous carbons to adsorb and sequester CO2. (In adsorption, a substance such as a gas or liquid condenses on the surface of a solid. This is often confused with absorption, a process in which a material soaks up a substance.)

"We've simulated, on the molecular scale, the uptake of CO2 on model organic surfaces with various pore geometries, chemistries and frameworks," Lastoskie said. "The efficient capture of CO2 requires adsorbents that recognize and grab CO2 from a mixture of other gases. To store the captured CO2, we need large-capacity 'spongelike' adsorbents that retain the injected gas for long timescales. We're currently identifying and developing these adsorbents, which include naturally occurring materials such as coal, and synthetic materials such as activated carbons or metal-organic frameworks."

The team used Monte Carlo algorithms - a widely used class of computer simulations - to evaluate adsorption of CO2 and other gas mixtures on model coal macromolecules. Using statistical methods from thermodynamics, the researchers determined how much CO2 can potentially be stored in the coal seams, and how strongly the CO2 will remain bound to the coal particles if other gases, such as methane or water vapor, are present.

Lastoskie said that coal matter "has a lot of pores in the two-nanometer range and these pores provide a lot of room for CO2 adsorption. Also, electrostatic interactions between the CO2 molecules and the coal help to preferentially bind CO2 to the coal particles and displace other adsorbed gases such as methane." (The energy industry already uses this technique to recover natural gas.)

One of the problems under investigation is the swelling that occurs when CO2 molecules crowd onto the elastic coal particles. Too much swelling pinches the coal seam shut, so that no more CO2 can be injected into the formation. "To make geological storage of CO2 work, we have to figure out how to work around problems like that," Lastoskie said.

The research is one more vital piece of Lastoskie's overall objective: developing CCS strategies that will help stabilize the atmosphere and, quite literally, life as we know it. - E