Scrubbing CO₂ Cheaply

A new carbon-capture method will be tested at a German coal plant.

A new process for scrubbing carbon dioxide (CO₂) from power-plant exhaust gases could make carbon capture a more affordable option for the energy industry. The process, which is to be tested in Germany this summer, promises to remove up to 90 percent of CO₂ from flue gases while using far less energy than other methods.

Catching carbon: This illustration shows the layout of a plant with carbon capture and storage. Credit: Siemens

Existing carbon-capture methods reduce a plant's efficiency by about 11 percent. The new process, developed by Siemens, could reduce this efficiency loss to just 9.2 percent. This may not seem like much of an improvement, "but in a power plant, that's a huge benefit," says Tobias Jockenhoevel, head of the project at Siemens, in Erlangen, Germany. Capturing CO₂ will always consume a certain amount of energy, says Jockenhoevel, so the aim is to find ways to keep these losses to a minimum.

In theory, 99.9 percent of the CO₂ emitted from a power plant could be removed using the process, but Jockenhoevel says that 90 percent is the economic optimum in terms of infrastructure costs and how much energy is required: "The last 10 percent costs too much."

In August, the Siemens process will be put to the test at a pilot facility built by Siemens and the energy company E.ON: the Staudinger coal-fired plant, near Frankfurt. The plant will be adapted so that part of its exhaust gases are fed into a chimney containing a 25-meter-high column that gives off a solvent mist that reacts with CO₂ under pressure. As the flue gases pass through the mist, the CO₂ is chemically absorbed, leaving residual gases to pass out of the chimney. The CO₂ can then be separated from the solvent, which can be reused.

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"It's basically like washing the gases," says Jockenhoevel. It is a standard approach to scrubbing CO₂; the novelty of the process comes down to the solvent used and the way it is recovered, says Jockenhoevel.

"There's a lot of research looking for new solvents," says Amparo Galindo, a physical chemist in the Carbon Capture and Storage group at Imperial College London. Currently, the most preferred solvent is monoethnolamine (MEA). "MEA reacts very strongly, but the difficulty with it lies in recapturing it so you can reuse it," which requires a lot of energy, she says.