Authored by Isabella Corpora, Research Fellow, Institute for Carbon Removal Law and Policy
Prepared for the Institute for Carbon Removal Law and Policy
A variety of igneous rocks and minerals are currently under evaluation as potential prospects to facilitate permanent carbon sequestration. Olivine, serpentine, and peridotite are some of the many that bind with carbon dioxide to form carbonates, hence providing a more permanent removal of captured carbon. This post is about a rock that deserves more attention: basalt.
Basalt is an igneous rock that is widely abundant globally and can bind to carbon dioxide more quickly than other options. Different forms of basalt stand as serious contenders for large-scale subsurface or in situ mineral carbonation, so why are they not a hotter topic in the carbon removal world?
To set this up, there are distinct rocks used for removing versus storing carbon. Those evaluated to capture carbon can do so through the process of enhanced weathering. Enhanced weathering is usually completed on surface levels, such as with the mineral olivine that binds with ambient carbon dioxide along seashores. For storage purposes, rocks can be evaluated based on their presence in different layers below Earth’s crust. For both processes, the process of mineralization can occur where silicate materials and gases, like carbon dioxide, bind to form products like carbonates. Basalt is usually viewed for its storage potential. Carbon dioxide would need to first be captured through other technological processes and then injected into a basalt aquifer for carbonation. Though there is potential for basalt to be used for enhanced weathering purposes, the emphasis through this post will be on in situ storage.
Changing atmospheric carbon dioxide into rocks is a complex chemistry trick. The ability of a rock to function as a carbon-storer is a function of surface porosity levels, gaseous pressure requirements, and temperature levels, among other factors. Some minerals, including wollastonite, tend to be less available in nature and have stricter requirements needed for successful carbonation to occur. Basalt, however, provides us a unique opportunity due to its scalability and characteristics as an igneous rock. Basalt forms from lava flows, notably along ridges in the ocean. These ridges span globally, making basalt the Earth’s most abundant bedrock and providing an occasion for further exploration and testing. It has a relatively higher porosity level (10-15%, compared to peridotite around 1%), allowing for larger amounts of carbon dioxide to bind in pores on its surface, and with a higher density in carbonate form can more easily fall to the ocean floor for storage.
More than 8% of Earth’s surface includes basalt, and Sanna et al. noted the ocean basaltic storage potential can be as high as 8238 gigatons available (at 2700m deep and with 200m of sediments forming a cap layer for trapping). This ocean storage is attractive because sedimentary layers in the deep sea would provide an additional natural permeability layer to maintain the carbon underground and decrease potential escapage, essentially acting as a “lid” to trap the carbon beneath it. Trapping is important because it helps prevent gas escapage while mineralization occurs. With other forms of rock, increased temperatures (think: energy requirements), higher carbon dioxide purification levels, and elevated pressures could all have stricter requirements for permanence below the Earth’s surface. This is an important consideration as proper storage can reduce the risk for escapage back into the atmosphere or ocean.
It has recently been identified that mineralization rates in basalt are also much faster than previously anticipated. The CarbFix project near the Hellisheidi power plant in Iceland conducted a test study in 2012 and injected more than 175 tons of pure carbon dioxide into a basalt aquifer. The original expectation was for the mineralization process to take several years, yet the study found that carbonate material was formed in just two years, impressing scientists for the fast turnaround time and storage potential. The sequestration prices are also generally cheaper than have been seen with other mineralizaton options. In areas like Wallula, WA and near Hellisheidi, sequestration is being achieved at about $10-30 per ton of carbon sequestered. Though the price of deep storage in ocean basalt is higher, early estimates suggest that it can be achieved at $200 per ton. By contrast, serpentine, with higher pressure, temperature, and purification requirements, can range from $200-600 per ton sequestered.
As the Earth is now approaching 420ppm carbon dioxide in the atmosphere, the need for long-term carbon sequestration is becoming more pressing. Other forms of rock continue to stand as competitive contenders for carbon removal due to their quick binding rates with carbon dioxide and potential to be brought to areas where carbon removal is conducted, rather than transporting captured carbon to at times distant injection sites. Processes like enhanced weathering can also both capture and store carbon, uniting both goals and requiring less resources. Yet, many of these above-ground methods can have direct environmental impacts in ecosystems due to their surface exposure, such as with olivine. In situ basalt storage has few external impacts due to deep injections, quick mineralization rates, and natural trapping mechanisms.
A second CarbFix project has started and should provide additional findings on carbonation rates in basalt in the coming years. Perhaps with further usage of the CarbFix technology, we have a widely scalable solution on our hands. Other externalities of basalt should also be considered, but while greater investigation is being conducted, basalt seems to be a promising contender. It could be expected that we will see more basalt in our futures.
References
Goldberg, David S, and Angela L Slagle. “A Global Assessment of Deep-Sea Basalt Sites for Carbon Sequestration.” Energy Procedia, vol. 1, no. 1, Feb. 2009, pp. 3675–3682., doi:10.1016/j.egypro.2009.02.165.
Goldberg, David S, et al. “Carbon Dioxide Sequestration in Deep-Sea Basalt.” Proceedings of the National Academy of Sciences, vol. 105, no. 29, 22 July 2008, pp. 9920–9925., doi:10.1073/pnas.0804397105.
Matter, Juerg M, et al. “Rapid Carbon Mineralization for Permanent Disposal of Anthropogenic Carbon Dioxide Emissions.” Science, vol. 352, no. 6291, 10 June 2016, pp. 1312–1314., doi:10.1126/science.aad8132.
National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: https://doi.org/10.17226/25259.
Sanna, Aimaro, et al. “A Review of Mineral Carbonation Technologies to Sequester CO2.” Chem. Soc. Rev., 2014, pp. 8049–8080., doi:10.1039/c4cs00035h.