By Michael Klein, The University of Texas at Austin, Electrochemical Energy Laboratory and MRS Student Chapter
Big Picture – Accelerating the carbon cycle
In a recent Perspectives article in Science, Sigurdur Gislason of the University of Iceland and Eric Oelkers of the Universite Paul-Sabatier and UCL discuss efforts to capture atmospheric carbon dioxide in basaltic rock. This is a subfocus within the larger field of subsurface carbon storage. As the article deftly points out: The Earth’s carbon cycle continuously redistributes carbon between the atmosphere and geological deposits. Human intervention (primarily the burning of fossil fuels) in the last two centuries has accelerated the transfer of carbon from underground sources to the atmosphere.
The lack of a corresponding increase in the rate of carbon incorporation from the atmosphere to the crust has yielded the catastrophic increase in atmospheric CO2 levels which is driving global warming. A natural avenue to pursue is thus a means to artificially accelerate the other half of the cycle—to drive the incorporation of carbon into subsurface rock.
Overview – Injecting CO2 in to basaltic rock
The earliest efforts in subsurface carbon storage have primarily focused on sedimentary rocks (such as sandstone), which have several disadvantages, as discussed in the article. The sedimentary rocks are largely unreactive to CO2—with appreciable rates of carbonation occurring on timescales of tens of thousands of years. Additionally, gaseous CO2 is buoyant, so its natural tendency is to diffuse toward the surface. As a result, this method depends on the existence of an impermeable caprock to contain the carbon—a dependence that may be unreliable over geologic timescales.
A promising alternative currently being explored is to inject CO2 into basaltic rock. The mineral composition of most basalts promotes relatively rapid carbonation—ensuring containment over geological timescales. Additionally, by injecting CO2 as a solution in water, as is currently being demonstrated by the CarbFix project in Iceland, the buoyancy concern can be alleviated. An alternative project, the Big Sky Carbon Sequestration Partnership (BCSP) in Washington State, is injecting CO2 at large depths in basaltic formations, but not first dissolving it in water. This beneficially increases the concentration of CO2 that can be injected, but it is dependent on a caprock to maintain the sequestration.
The persistent problem with CO2 injection into basaltic rock, as with all carbon sequestration schemes, remains cost. The technologically appealing CarbFix project has sequestration costs of $17/ton CO2, and the BCSP project is about half of that. Combined with expensive CO2 separation operations, the cost of carbon capture and storage greatly exceeds the price of carbon in even the most tightly regulated markets. As a result, despite the technological promise of carbon sequestration in basaltic rocks, we are unlikely to see any widespread implementation absent broad policy changes.