Turning CO2 into chalk and sand

New method could make carbon sequestration cheaper

Removing carbon dioxide from smokestacks and storing it permanently is one of the possible solutions to global warming, but remains expensive to do. A new technique could make carbon sequestration economical on a large scale, while producing useful materials on the side.

Dirk Van Essendelft, a chemical engineer at PennsylvaniaStateUniversity in University Park, described the method on August 19 in Philadelphia during a meeting of the American Chemical Society. He proposed a new way to mix CO2 with a type of mineral called serpentine, producing sand and another common mineral similar to chalk.

Capturing the CO2 from smokestacks requires energy. Van Essendelft said that, according to his calculations, a power plant that captures its emissions for storage into serpentine would suffer only a 10 percent loss of energy. If the technique can be applied on a large scale, storing carbon in minerals would become competitive with other proposals for carbon sequestration, such as pumping CO2 deep underground. “It puts mineral carbonation back in the game, as far as energy consumption,” he said.

Mercedes Maroto-Valer of the University of Nottingham in England says that the technique could be economical for large-scale carbon sequestration.

In his small-scale reactor, Van Essendelft grinds a serpentine rock mixed with water and acid. The combination of grinding and chemical action (mainly the acid) breaks down serpentine into magnesium and silica, which is essentially sand. He then adds ammonia and pumps CO2 in. The ammonia neutralizes the acid, allowing the CO2 to dissolve and react with magnesium, forming magnesium carbonate.

Magnesium carbonate is similar to chalk and has several applications. For example, Van Essendelft says, it could be used instead of limestone to produce cement.

Previous attempts at storing carbon in magnesium carbonate were energetically less favorable, since they required compressing CO2 before dissolving it, Van Essendelft says. His technique works at ordinary pressure.

He and others have now formed a start-up company, hoping to develop the technique into one that can be applied economically on a large scale. To do so, Van Essendelft says, the first step will be to build a reactor that can process CO2 continuously, rather than in batches as the current prototype does.

Serpentine is plentiful near the two U.S. coasts, but expensive to transport in large quantities, points out Chris Plum, the start-up’s president. Using it to store carbon would be economical in some geographic locations, while other carbon-sequestration techniques should be used in others, he says. “None of these processes is single-handedly going to solve the problem.”

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