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Energy

Capturing Carbon with Enzymes

A new process turns the greenhouse gas into useful materials.

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.

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.

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.

Don Langley, vice president and chief technology officer for Babcock and Wilcox, says the approach holds more promise for large-scale carbon capture than do some alternatives, such as carbon-dioxide-absorbing algae farms that require huge tracks of land. “We’re in serious discussions at the moment on how to move forward with a more formal relationship, which would potentially include licensing the technology and joint development,” Langley says, adding that the bioreactor must be scaled up substantially and operate more efficiently to be economical. “We’re kind of placing bets, if you will,” he says. “Obviously, we see some things in the CO2 Solution approach that I really wouldn’t want to disclose that make us pretty interested.”

Aluminum giant Alcoa tested CO2 Solution’s lab prototype in 2004 by attaching it to an air outlet from a smelter’s emissions-scrubbing system. Michel Lepage, Alcoa’s director of laboratories and environment, says the technology worked well. “The system removed 80 percent of the CO2, which is quite large,” says Lepage, emphasizing that it was a small-scale test. “But it told us it has a high potential.”

CO2 Solution also sees its technology serving some industrial niches. Last week the company was granted a European patent for the process of capturing carbon-dioxide emissions from cement factories and converting it into bicarbonate and eventually limestone–a key ingredient in manufacturing the cement itself. The company already holds a U.S. patent on the process. The bicarbonate is also useful for producing carbonate compounds for neutralizing industrial waste and some effluents.

For its part, Babcock and Wilcox is strictly interested in the carbon-dioxide removal, compression, and geologic sequestration; any end-stream products that may result would simply be a bonus. “Quite frankly, it just boils down to what’s the dollars-per-avoided-ton cost of CO2,” says Langley. “That’s our focal point.”

Langley emphasizes that CO2 Solution’s technology is still very much at an embryonic stage of development and that there are major economic and technical hurdles to overcome. “There’s not any technology today that’s really down to where we need to be,” he says.

The company’s techniques for reproducing the enzyme have already led to a significant drop in enzyme production costs. Perhaps the biggest challenge will be to produce enough of the enzymes to process the enormous quantity of carbon dioxide that would be emitted from a coal- or gas-fired power plant. CO2 Solution is determined to do it, particularly given the heightened profile of climate change and the likelihood that carbon regulation is around the corner.

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