Thursday, February 22, 2007

Capturing Carbon with Enzymes

A new process turns the greenhouse gas into useful materials.

By Tyler Hamilton

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A CO2 Solution bioreactor, like the one shown in the diagram above, contains an enzyme-bonded packing material that interacts with a water solution pumped in from the top and smokestack emissions that enter and bubble up through the bottom. The carbon dioxide in the emissions stream is captured by the enzymes on the surface of the packing material and converted into bicarbonate ions. Cleaned-up air then escapes from the top while the bicarbonate solution exits the bottom. The bicarbonate, in a separate process, can be extracted from the solution and made into compounds, such as limestone, for use by industry.
Credit: CO2 Solution Inc.

A new way to capture carbon dioxide from smokestacks produces a raw material that can be sequestered underground or turned into substances such as baking soda, chalk, or limestone. CO₂ Solution, of Quebec City, Canada, has already tested its process on a small municipal incinerator and an Alcoa aluminum smelter. Its scientists are now working with power-plant equipment giant Babcock and Wilcox on ways to adapt the technology to a coal-fired generating station.

The company has genetically engineered E. coli bacteria to produce an enzyme that converts carbon dioxide into bicarbonate. The enzyme sits at the core of a bioreactor technology that could be scaled up to capture carbon-dioxide emissions from power plants that run on fossil fuels--a timely development as political support grows for cap-and-trade schemes that assign a market value to carbon.

"So far we have a small prototype," says Sylvie Fradette, vice president of research and development at CO₂ Solution. "Next we have to look at what's necessary for a very large prototype or pilot plant."

The enzyme, called carbonic anhydrase, ordinarily processes carbon dioxide produced in organisms. In E. coli it plays an essential role in metabolism. As a doctoral student in the 1990s, Fradette led a research team at the University of Laval that isolated the enzyme, immobilized it, and figured out how to reproduce it. After getting her Ph.D. in chemical engineering, she joined CO₂ Solution in 1998. "We thought it would be interesting to use this enzyme to replicate what it already does so well in nature," Fradette says. "We found it was very efficient in doing the CO₂ transformation."

There are many other ways of capturing carbon dioxide from industrial and power-plant flue stacks, but separation of the carbon-dioxide stream from other gases and pollutants makes most of these approaches energy intensive and therefore quite costly. Fradette says CO₂ Solution's approach--which does not require separation of carbon dioxide from other gases--can be applied to any gaseous effluent that contains carbon dioxide, so it would be ideal for both conventional coal plants and newer "clean coal" facilities based on gasification.

The bioreactor is a long cylinder containing a packing material that acts as a solid support for the enzyme. The surface of this material has been chemically modified so that the enzymes attach securely. At the top of the cylinder, a water solution is pumped in and flows around the packing material, while gases from a smokestack enter the bottom of the cylinder and bubble up through the solution. The carbon dioxide is absorbed into the solution and then interacts with the enzymes, which convert the greenhouse gas into bicarbonate ions. To end the process, cleaned up air escapes from the top while the bicarbonate solution is extracted for further processing--either back into pure carbon dioxide for long-term geological storage or into a carbonate compound, such as limestone, that can be used by industry.

Capturing Carbon with Enzymes

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