Monday, March 09, 2009

Scrubbing CO₂ Cheaply

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

By Duncan Graham-Rowe

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

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.

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.

"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.

The Siemens system uses a solvent made from amino acid salts instead. CO₂ can be removed and the solvent recovered by applying energy to break apart the chemical bonds formed between the two. This means simply boiling the solvent off, but the chemistry involved allows this to happen at lower temperatures. Amino acid salt formulations are also more stable than MEA and less likely to react with oxygen and sulphur dioxide in the exhaust gases; virtually none of the solvent should escape into the atmosphere along with residual gases. While the supply of other solvents needs to be regularly topped up because of these losses, this isn't the case with the amino acid salts, says Jockenhoevel. Instead of heating the solvent in one location to remove the CO₂, it is divided into two streams that are heated separately in a way that requires less energy.

Jim Watson, director of the Sussex Energy Group at the University of Sussex, in Brighton, U.K., cautions that the cost of carbon capture has to be balanced against the relatively low cost of buying carbon credits. He adds that developing the technology is expensive, and storing sequestered carbon reliably is an as yet unsolved problem.

However, Watson believes that the project is a positive step. "Anything that gets the efficiency losses down is important," he says. "The loss in efficiency is a very significant barrier to anyone taking up this technology."

Jockenhoevel suggests that the efficiency loss must be below 10 percent--any higher, and the cost of capture becomes more expensive for utility companies than paying for carbon-offsetting certificates, he says.

The technology will work with any kind of power plant that runs on fossil fuel and can be retrofitted to existing facilities, says Jockenhoevel. However, even if this summer's tests go according to plan, it will be years before the technology is deployed, partly because of the difficulty of storing CO₂, and partly because of the price of carbon on the carbon-exchange markets.

"It is low and very volatile," says Watson. Unless the cost of offsetting carbon increases, he says, carbon capture will remain a very pricey alternative.