Carbon capture: transforming greenhouse gas into rock

iceland_geothermal_plant

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A few months ago, after drilling a well 400 meters deep, scientists in Iceland were repeatedly frustrated that the well kept breaking down. Retrieving the pump from the depths of the earth, they found its base covered in a scaly green and white material that clogged the end of the machine. Instead of feeling dismay over the equipment failure, the scientists celebrated. The crusty residue was calcite, a hopeful sign that the researchers had developed a new method to chemically trap carbon dioxide (CO2) underground and mitigate climate change.

A promise always a decade away

The idea of carbon capture and storage (CCS) has had many advocates and critics. The goal is simple: Find a way to grab all the CO2 emitted during fossil fuel combustion, and permanently sequester it somewhere on the planet, either in geologic formations or the deep ocean, where it cannot escape into the atmosphere. Supporters claim that the technology can create “clean coal,” allowing society to continue to burn away fossil fuels using existing infrastructure. Proponents have carried out a host of pilot projects, the most common using sandstone or depleted oil wells as storage sites, because engineers and scientists already know how these sites operate from the oil and gas business. Unfortunately, no feasible path to cheap and reliable CCS has come from this work.

The problem with current CCS technology is twofold. First, current state-of-the-art technology using old well sites is estimated to cost 50 to 100 dollars per ton of CO2 stored. Imagine that an initial goal of the technology would be to store just 20 percent of the 35.9 Gigatons of global CO2 emissions from 2014. This puts a minimum cost of such CCS at $359 trillion!

Price is not the only concern. Even if CO2 could be affordably stored in sandstone, the molecule’s probability of escape is too high. Rock layers typically cap these storage sites and are sensitive to seismic activity. Fissures in the rocks could allow fluid to leak through and provide the perfect vehicle for CO2 to escape back into the atmosphere. Researchers have not been able to find a storage site that prevents the possibility of this type of leak, making the long-term application of the technology infeasible.

The chemistry of carbon storage

Remember those happy Icelandic scientists? They just may have found a solution. Twenty-five miles east of Reykjavik, a team of scientists from Iceland, the United States, and Europe collected hot, bubbling CO2 from a nearby geothermal plant and injected it 500 meters underground to test a new carbon storage system. The destination: a basalt rock formation.

Why basalt?  Unlike sandstone formations, basalt, an igneous rock, contains many metals, such as calcium, magnesium, or iron, that facilitate a chemical reaction with CO2 known as carbonation. Carbonation transforms the greenhouse gas into calcite, a white and green rock. In contrast to sandstone, which physically constrains CO2underground in a densified gas phase, basalt chemically—and permanently—stores CO2. Fissures and water leaks could no longer allow the greenhouse gas to escape into the atmosphere.

The idea of basalt storage had been considered before, but most computer models suggested that calcite formation would take a decade or even hundreds of years. The scientists in Iceland wanted to try it anyway, at least to gain more information about the chemical process. After doping CO2 from the geothermal plant with heavy carbon to monitor its movement, the scientists plunged 220 tons of CO2 deep into the basalt formation along with a lot of extra water. The addition of water is crucial, because gaseous CO2 by itself could easily escape the injection site before reacting with the metals in the basalt. CO2 is not buoyant and dissolves in water, allowing the chemical reactions to complete before the CO2 degasses.

After the injection, all the scientists could do was watch and wait. Eighteen months after injection, the well broke down, and the elated scientists found the green and white, scaly calcite stuck to the bottom. The chemical reactions necessary for storage took only a little over a year, an order of magnitude less than that predicted by computer models. Over 95 percent of the injected carbon had been transformed into rock.

The speed of the carbonation “means this method could be a viable way to store CO2 underground—permanently and without risk of leakage,” says Juerg Matter, lead author of the study and geologist at the University of Southampton in the United Kingdom.

The difficult path to commercialization

The results from this experiment suggest basalt storage could solve the gas leakage problem intrinsic to most other CCS technologies. Once chemically bonded to form calcite, the CO2 is permanently stored as part of the rock formation. The findings are not specific to Iceland, either. Similar injections into basalt formations have succeeded in Wallula, Washington, and giant basalt formations in the Pacific Northwest make it a perfect location for upscaling the technology.

But challenges remain. More tests are required to confirm that carbonation of injected CO2 is consistently fast and that the Icelandic results were not an anomaly due to particular chemical conditions at the initial test sites. Speed could also be a drawback. If carbonation occurs too quickly, CO2 will not have time to spread throughout the entire basalt formation, limiting storage to locations near the injection site.

Even if these technological issues are addressed, price is still a concern. It is not clear whether basalt storage methods will reduce the price tag currently restraining the market growth of other sequestration technologies. As energy analyst Vaclav Smil has discussed, carbon capture technology requires a tremendous amount of infrastructure. Assuming we want to store just 20 percent of 2010 emissions, Smil estimates this would require transporting 8 billion cubic meters of compressed CO2 gas to storage sites. In comparison, the oil industry transported 4.7 billion cubic meters of crude oil in 2010. This means that carbon sequestration would require “an entirely new worldwide absorption-gathering-compression-transportation-storage industry” that is 70 percent larger than the global crude oil industry developed over multiple decades. Such a challenge would require global cooperation and infrastructure development at an unprecedented pace to prevent CO2from rising above 450 parts per million.

Any new CCS method must be viewed with a hopeful but critical eye. Basalt storage appears to be a technological leap that could lead to its adoption in certain cases, but including carbon sequestration as a major part of any climate mitigation plan will be limited more by politics and global infrastructure than by the viability of any one specific technology.

References

Matter J.M. et al. “Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions.” Science, 352(6291), 1312-1314, 2016.

Kintisch, E. “New solution to carbon pollution?Science, 352(6291), 1262-1263, 2016.

Smil, V. “Global energy: the latest infatuations.” American Scientist, 99, 212-219, 2011.

Photo Credits

Figure of Nesjavellir Geothermal Power Station courtesy of Gretar Ivarsson

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