Low-Carbon Cement

Results: Researchers at George Washington University have invented a solar-powered process that makes lime from limestone without emitting any carbon dioxide and demonstrated it in a proof-of-concept device. The researchers estimate that the process could cost less than the conventional one.

Why it matters: The new process could eliminate a major global source of carbon dioxide emissions. Lime is used to make cement, purify iron, treat soil, and produce glass, paper, sugar, and other things. Cement production alone emits 5 to 6 percent of total man-made greenhouse gases, and most of that comes from producing lime.

Methods: Rather than simply heating up limestone until it releases carbon dioxide, as in the conventional method, the process uses a combination of heat and electrolysis, which produces lime, oxygen, and either carbon or carbon monoxide, depending on the temperatures used. The researchers built a device that includes three Fresnel lenses for concentrating sunlight. Two of those heat a mixture of lithium carbonate and limestone (calcium carbonate). The third focuses light on a high-efficiency solar cell, which provides the electricity needed to electrolyze the carbonate mixture. High temperatures reduce the amount of electricity needed and cause the lime to precipitate out of the mixture, making it easy to separate.

Next Steps: The device works only when it’s sunny, and intermittent operation isn’t ideal for an industrial process. The researchers propose using molten salt to store heat, a system used in some solar thermal power plants. That would allow the process to run day and night.