By Pierce Hoenigman, Winter 2022.

Multiple studies have shown that humanity must limit global warming to under 1.5 to 2 degrees Celsius to prevent a climate catastrophe involving mass extinction and weather instability. [1] Unfortunately, global temperatures are projected to rise approximately 2.7 degrees Celsius by the end of the century due to human emissions, even if all current measures for carbon reduction stay in place. [1] There have been many proposed methods for resolving this issue. In particular, the idea of recapturing carbon dioxide from the atmosphere has recently begun to gain traction. Instead of attempting to mitigate carbon emissions, carbon capture techniques aim to scrub existing CO2 from the air, thereby reducing the greenhouse effect. Yet to capture carbon is not enough; it must be either stored or reused in some way to prevent its recirculation in the atmosphere. There are multiple ways to implement carbon capture; however, the success or failure of all of these ideas has so far been beholden to cost. No single country will pay to clean up the atmosphere if other countries are not paying. It is an unfortunate inevitability that even with so much on the line, governments refuse to take measures to limit global warming unless they are affordable. 

By far the largest carbon capture plants are those that are attached to refinery and power-producing facilities for oil, coal, and other traditional fuels. [4] In 2020, the Alberta Carbon Trunk Line opened, recapturing millions of metric tons of carbon each year and depositing it into fossil fuel caverns, producing an impact equivalent to eliminating 2.6 million cars. [5] The system works by leveraging a technique known as gasification. [5] Instead of simply burning fuel to power a steam turbine, then trying to capture CO2 out of the flue gas, the fuel is first gasified using oxygen so that it becomes a mixture of hydrogen gas and carbon dioxide. The hydrogen is then used as fuel to produce power by igniting it with oxygen, the only byproduct being water, while the CO2 is compressed. The recaptured carbon is then pumped into oil reservoirs for better oil recovery or to be stored indefinitely. [6] While these systems are impressive, it’s also worth noting that they wouldn’t exist without fossil fuel power, so the ACTL and other similar projects are ways of making fossil fuels less damaging rather than actually cleaning the atmosphere.

On the other hand, the technology that most people think of when they imagine carbon capture (and the one that has recently been making headlines) is carbon filtration via direct air capture. The Orca system is the world’s largest facility using this technology, despite being only the size of a few shipping containers. Designed and produced by Climeworks and first constructed in Iceland, this process of carbon capture begins with fans that collect air into a chamber. A special filter in this chamber then removes all the carbon dioxide and the rest of the air flows out the back. After the filter becomes clogged with carbon, the chamber closes to the outside and is heated to approximately the boiling point of water so that the CO2 comes off the filter, is piped away and mixed with water. After this step, the solution is pumped into the ground. The whole system is powered geothermally, though in locations with less geothermal potential, other power sources are also feasible. [7]

A less sophisticated method of carbon capture is the use of the most prolific carbon capturer to date: trees. The world’s forests provide an annual net carbon sink of approximately 7.6 billion metric tons of carbon dioxide annually, equivalent to over twenty percent of human emissions. [8][9] To become carbon neutral by planting trees would take nearly all the world’s land area. To put this into perspective, three small Orca plants could capture the equivalent of a square mile of forest. [10][11] Additionally, storage would need to be made permanent so that these trees do not release their carbon back into the atmosphere after they die. One method of doing this would be to switch to primarily timber construction. This would have the added benefit of reducing the need for concrete, which contributes to 8% of global emissions. [12] Considering the hundreds of thousands of tons of material that make up large buildings, this method of carbon sequestration would be incredibly helpful. Still, due to the large area required, the use of trees would likely be one of many techniques that form a complete solution.

Each of these methods can be compared based on their respective costs per ton of carbon sequestered. Every year, the Alberta Carbon Trunk Line recaptures 14.6 million metric tons of carbon, while the Orca facility storesF four thousand metric tons. [3][5] Their respective construction costs are $1.2 billion and $15 million, not accounting for yearly operating costs. [5][13] Therefore, the price per metric ton of carbon dioxide captured per year is approximately $82 and $3750, respectively. Similar estimates for carbon sequestration by use of trees range from $35 to $100. [14] However, as previously suggested, each of these methods are likely to find their use in different situations; gasification is only useful for decreasing the emissions of fossil fuel usage, and planting forests takes up vast swaths of land. Additionally, the technology used by Climeworks is still young and has not yet had time to reach economies of scale to become lower in price (with the aim being $100 to $200 by the mid 2030s). [13] Thus, the answer to the question of which carbon capture and storage method will take over is complicated. As with most climate problems, the solution will undoubtedly require a combination of different approaches to reach a sustainable future.

[1] Climate Action Tracker. “Temperatures.” Last modified November 9, 2021. https://climateactiontracker.org/global/temperatures/.

[2] Panko, Ben. “World’s Largest Carbon Capture Plant Opens in Iceland.” Smithsonian Magazine, September 9, 2021. https://www.smithsonianmag.com/smart-news/worlds-largest-carbon-capture-plant-opens-iceland-180978620/.

[3] Cuéllar-Franca, Rosa M., Adisa Azapagic. 2015. “Carbon capture, storage and utilisation technologies: A critical analysis and comparison of their life cycle environmental impacts.” Journal of CO2 Utilization 9 (March): 82-102. https://www.sciencedirect.com/science/article/pii/S2212982014000626.

[4] Statista. “Capacity of operational large-scale carbon capture and storage facilities worldwide as of 2020.” Published February 1, 2021. https://www.statista.com/statistics/1108355/largest-carbon-capture-and-storage-projects-worldwide-capacity/.

[5] MIT. “Alberta Carbon Trunk Line Fact Sheet: Carbon Dioxide Capture and Storage Project.” https://sequestration.mit.edu/tools/projects/alberta_trunk_line.html.

[6] Wall, Terry F. 2007. “Combustion processes for carbon capture.” Proceedings of the Combustion Institute 31. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.5881&rep=rep1&type=pdf.

[7] Climeworks. “Direct air capture: a technology to remove CO2.” https://climeworks.com/co2-removal.

[8] Harris, Nancy L., David A. Gibbs, Alessandro Baccini, Richard A. Birdsey, Sytze de Bruin, Mary Farina, Lola Fatoyinbo, Matthew C. Hansen, Martin Herold, Richard A. Houghton, Peter V. Potapov, Daniela Requena Suarez, Rosa M. Roman-Cuesta, Sassan S. Saatchi, Christy M. Slay, Svetlana A Turubanova, Alexandra Tyukavina. 2021. “Global maps of twenty-first century forest carbon fluxes.” Nature Climate Change 11 (January): 234-240. https://www.nature.com/articles/s41558-020-00976-6.

[9] Ritchie, Hannah, Max Roser. Our World in Data. “CO2 emissions.” https://ourworldindata.org/co2-emissions.

[10] 10 Million Trees. “All About Trees.” http://www.tenmilliontrees.org/trees/.

[11] Loucks, Bryon. “Forest Management for Small Landowners.” https://www.wafarmforestry.com/sites/default/files/pdfs/Education/SFLO101/5-Planting.pdf.

[12] 2021. “Concrete needs to lose its colossal carbon footprint.” Nature 597 (September): 593-594. https://www.nature.com/articles/d41586-021-02612-5.

[13] UNESCO. “Carbon capture and storage plant becomes operational in Iceland.” Published September 20, 2021. https://en.unesco.org/news/carbon-capture-and-storage-plant-becomes-operational-iceland.

[14] Austin, K. G., J. S. Baker, B. L. Sohngen, C. M. Wade, A. Daigneault, S. B. Ohrel, S. Ragnauth, A. Bean. “The economic costs of planting, preserving, and managing forests to mitigate climate change.” Nature Communications 11 (2020). https://www.nature.com/articles/s41467-020-19578-z.