A Better Way to Capture Carbon

New materials provide a potentially cheaper way to reduce carbon dioxide emissions from power plants.

Carbon-capturing crystals: This is an optical micrograph of a new material that can pull carbon dioxide from a stream of gases, making it possible to sequester the greenhouse gas. Credit: Omar Yaghi

Researchers have developed porous materials that can soak up 80 times their volume of carbon dioxide, offering the tantalizing possibility that the greenhouse gas could be cheaply scrubbed from power-plant smokestacks. After the carbon dioxide has been absorbed by the new materials, it could be released through pressure changes, compressed, and, finally, pumped underground for long-term storage.

Such carbon dioxide capture and sequestration could be essential to reducing greenhouse-gas emissions, especially in countries such as the United States that depend heavily on coal for electricity. The first stage, capturing the carbon, is particularly important, since it can account for 75 percent of the total costs, according to the Department of Energy.

The new materials, described this week in Science, were created by researchers at UCLA led by Omar Yaghi, a chemist known for producing materials with intricate microscopic structures. They absorb large amounts of carbon dioxide but do not absorb other gases.

Techniques already exist for capturing carbon dioxide from smokestacks, but they use large amounts of energy--15 to 20 percent of the total electricity output of a power plant, according to one estimate, Yaghi says. That is because existing materials, known as amines, need to be heated to release the carbon dioxide they've absorbed. Indeed, capturing and compressing carbon dioxide through these existing methods can add 80 to 90 percent to the cost of producing electricity from coal, says Thomas Feeley, a project manager at the National Energy Technology Laboratory.

Feeley says that Yaghi's materials "compare favorably" with other experimental materials that absorb carbon dioxide that are being developed to help bring down these costs. Yaghi says that his materials could lower costs considerably since they use less energy, although exactly how much will require testing the materials at power plants.

Beyond being potentially useful in smokestacks, the materials could be employed in coal gasification plants. In these plants, coal is first processed to produce a mixture of carbon dioxide and hydrogen gas. The hydrogen is then used to generate electricity. The carbon dioxide could be captured using a solvent that increases energy consumption. But as in the smokestack-based process, the new UCLA materials could require less energy.

The materials belong to a class called zeolitic imidazolate frameworks (ZIFs). They're made of metal atoms bridged by one of a number of ring-shaped organic molecules called imidazolates. Prior to Yaghi's research, 24 types of ZIFs had been developed over the course of 12 years. Yaghi made 25 new versions in just three months. These materials can be extremely versatile, since the metal atoms can act as powerful catalysts, and the organic molecules can serve as anchors for a number of functional molecules.

ZIF proliferation: New automated techniques allow researchers to quickly synthesize dozens of new materials called zeolitic imidazolate frameworks (ZIFs). Credit: Omar Yaghi

The new materials absorb carbon dioxide in part because they're extremely porous, which gives them a high surface area that can come into contact with carbon dioxide molecules. The most porous of the materials that Yaghi reports in Science contain nearly 2,000 square meters of surface area packed into one gram of material. One liter of one of Yaghi's materials can store all of the molecules of carbon dioxide that, at zero °C and at ambient pressure, would take up a volume of 82.6 liters.

While the exact mechanisms are not fully understood, Yaghi thinks that the slightly negative charge of organic molecules in his material attracts carbon dioxide molecules, which have a slightly positive charge. As a result, carbon dioxide is held in place, while other gases move through the material. This method of trapping carbon dioxide is better than some other methods because it does not involve strong covalent bonds, so it doesn't take much energy to release the gas.

The next step for the materials is commercialization. This means scaling up production and incorporating the materials into a system at a power plant, such as by packing the materials into canisters that can be filled with pressurized exhaust gases--something that the UCLA group says could be possible in two to three years. Yaghi estimates that the materials could easily be made in large quantities, since they are similar to other materials he has developed that can now be made by the ton by BASF, the giant chemical company. "Now it's in the hands of industry," Yaghi says. And he has developed automated techniques that could lead to more materials that could have even better properties.

http://www.technologyreview.com/Energy/20295/