CLEANTECH: Capturing Flue Gas

The press to control greenhouse gas emissions has set off a horse race to develop the best technology for capturing carbon dioxide from coal power plants so it can be sequestered underground from the atmosphere. Coal power plays a big role in the power industry, producing half the electricity used in the U.S. Even in California, coal plants provide 17 percent of the electricity used and state restrictions on global warming are forcing utilities to phase out purchase of coal power unless the plants can be successfully retrofitted to capture and sequester their CO2 emissions. As restrictions on greenhouse gas emissions take root, unless carbon capture and sequestration technology materializes coal power plants ultimately would have to be replaced by renewable energy or low emissions plants. Many believe that developing carbon capture and sequestration technology would be less expensive. In Menlo Park, SRI International is working under a recent Department of Energy grant to develop a solid sorbent system to capture CO2 from coal plant flue gas. Sorbent capture systems consist of a bed of material which bonds with the CO2 in flue gas. The sorbent is then decarbonated through a reaction process, giving off the CO2 so it can be captured and sequestered. DOE late last month provided SRI $1.8 million toward a $2.2 million project aimed at developing a high-capacity carbon sorbent capture system. The aim is to bench test a system to evaluate its technical and economic merits. The project is slated to run 36 months and if successful be scaled up to pilot test in an actual coal plant, said Gopala Krishnan, SRI associate director of materials research. A pilot scale project could be operable within four years, he said. SRI envisions using either a moving bed or a fluidized bed of carbon sorbent. In the moving bed, according to Krishnan, particles of carbon sorbent about a centimeter in diameter would flow down a cylinder through the flue gas and then drop to a decarbonation zone, which would release adsorbed CO2 for capture. In a fluidized bed system, the carbon sorbent particles would be smaller, with diameters of about a half millimeter. SRI plans to use a sorbent that consists of almost pure carbon, he said. It would be able to adsorb 90 percent of the CO2 in coal plant flue gas. Each 10 pounds of the sorbent material would be capable of adsorbing one pound of CO2. However, Krishnan said that the technology faces a key hurdle: minimizing the parasitic use of energy produced by a coal plant. To decarbonate the sorbent, he said, it must be heated. Along with other equipment needed to operate the carbon sorbent capture system, SRI’s goal is to keep the parasitic energy loss to no more than 15 percent of a coal plant’s total output, he said. Yet, he said, SRI is confident that it can be done. He compared the emergence of carbon sorbent capture technology, along with membranes and other techniques to capture and sequester CO2, to the state of sulfur scrubbing technology circa 1970. People thought it was too expensive he recalled, but within 10 years scrubbers became commercially available. “It’s going to be driven by policy,” said Barbara Heydorn, director of SRI’s Center of Excellence in Energy. She stressed that the potential for capturing and sequestering carbon from conventional coal power plants “has really become significant.”

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CLEANTECH: Capturing Flue Gas

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