Geothermal Power Plant Turns Carbon Emissions to Stone

Hellisheidi seen from the drill site. (Image courtesy of Kevin Krajick.)

It sounds like something out of ancient myth: a dire threat to the world is imprisoned deep beneath the earth, locked away for all time. In reality, the threat is carbon dioxide and the prison is volcanic basalt.

The heroes are a team of scientists and engineers who demonstrated that carbon dioxide emissions can be pumped into the basalt and chemically converted into a solid within months—radically faster than anyone previously predicted.

Reducing Geothermal Carbon Emissions

The project was carried out at Iceland’s Hellisheidi power plant, one of the world’s largest geothermal facilities with a capacity of 303 MW of electricity and 133 MW of hot water.

Reykjavik Energy, which runs the facility, began mixing carbon dioxide and hydrogen sulfide with water and reinjecting the resulting solution into the volcanic basalt below the plant.

University of Iceland geologist Sandra Snaebjornsdottir displays a core of porous basalt heavily laced with carbonate minerals. (Image courtesy of Kevin Krajick.)

Carbon naturally precipitates into a whitish, chalky mineral when mixed with basalt and water, but until now no one knew the speed of the chemical reactions that lead to this result. Although previous studies predicted that this process would take hundreds or even thousands of years, the research team found that 95 percent of the carbon injected below the plant solidified in under two years.

In a pilot project, the plant pumped 250 tons of carbon dioxide mixed with water and hydrogen sulfide 400-800 m below the surface. Fast-changing compositions of carbon isotopes in water samples taken from adjacent wells signaled that much of the carbon had mineralized within months.

Left to right, engineer Magnus Thor Arnarson, hydrologist Martin Stute and project leader Edda Sif Arradotir inspect the downgoing piping system. (Image courtesy of Kevin Krajick)

Based on these initial signs of success, the plant started injecting carbon dioxide at the rate of 5,000 tons per year. Ongoing monitoring indicates that mineralization has kept pace according to Edda Aradottir, who heads the project for Reykjavik Energy.

Cheap Carbon Dioxide Capture: Only in Iceland?

Hellisheidi produces roughly 40,000 tons of carbon dioxide per year, which is roughly five percent of the yearly emissions from a comparable coal plant. It also has the advantage of being situated on basalt containing calcium, iron and magnesium, which are needed to precipitate the carbon.

Hellisheidi geothermal power plant. (Image courtesy of Kevin Krajick.)

Another major impediment to the widespread implementation of this approach is the amount of water that needs to be mixed with the carbon dioxide—approximately 25 tons per ton of carbon dioxide—although seawater is an option.

One final issue involves subterranean microbes that feed on carbonate minerals and produce methane. Although these organisms were believed to exist only on the ocean floor, they were recently discovered in a California spring. If those microbes get a hold of the solidified carbon dioxide, they could undo the entire solidification process.

Nevertheless, these results could help to address the fear that captured and stored carbon dioxide emissions could seep back into the air or even explode if buried underground.

The results of this research are published under the title “Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions" in the journal Science.