A Cheap Trick Enables Energy-Efficient Carbon Capture

A simple new method has been shown to remove carbon-dioxide emissions from power plant exhaust while consuming half the energy needed by the best existing carbon-capture approach. Testing of the technology, which uses cheap limestone-derived material to trap carbon dioxide, has been underway at two different megawatt-scale pilot plants in Spain and Germany.

Bernd Epple, who has been testing the technology for four years at a plant at the Technical University of Darmstadt in Germany, says that it costs less than €20 to capture each ton of carbon dioxide using limestone, compared with about €50 for traditional solvents.

No commercial power plant today captures carbon dioxide for the purpose of stowing it away to combat climate change. The main reason is the prohibitive cost of today’s carbon-capture technologies. Researchers are now seeking more energy-efficient, low-cost methods for carbon capture.

The most researched method of trapping carbon dioxide from power plants involves injecting cooled-down exhaust gas into the bottom of tall “scrubbing towers” full of packing material while feeding liquid amine-based solvents through the top. Carbon dioxide dissolves into the solvent, which is then removed and heated in another chamber to about 130 °C to remove the carbon dioxide.

Lime, or calcium oxide, soaks up more carbon dioxide than other solvents per unit of weight, says Paul Fennell, a lecturer in clean energy at Imperial College, London, who is involved in the 1.7-megawatt CaOling project in northern Spain. And it is cheap because it is derived from abundant limestone.

The limestone-based process uses fluidized bed reactors, which are common in many industries and are therefore easier and cheaper to install at existing power plants than amine scrubbing towers. What’s more, once the lime has lost its reactivity, the spent material can be used to make cement.

The biggest advantage of the new method is that it takes place at higher temperatures, which is the reason it’s twice as energy-efficient as using amines. In the first of two reactors, temperatures reach 650 °C because the reaction between lime and carbon dioxide, which forms calcium carbonate, releases heat. In the second reactor, the calcium carbonate is heated to 900 °C to remove the carbon dioxide. Heat from both reactor vessels is used to generate steam that drives a turbine to produce electricity.

Using lime to absorb carbon dioxide is not a new idea. But drawbacks of the amine system have recently spurred interest in the technology, says Jasmin Kemper, a project officer at the International Energy Agency’s Greenhouse Gas program. While lime has many benefits over amine solvents, one limitation is that the material is not very stable and cannot be recycled as readily. According to Epple, the material can be reused 100 times before it has to be replaced. However, the low price of lime and the value of the spent material could help make up for this relatively short useful lifespan, she says.

There is one important thing that the new carbon-capture technique lacks: experience. “The preliminary results from the current one-to-two-megawatt-scale pilot plants are really good and promising,” Kemper says. “But to go towards commercial scale, you would need at least a 50-megawatt demonstration unit.”

This story was updated on December 12 to correct the figure from 15MW to 50MW in Jasmin Kemper’s last quote.