An Intro to Carbon Sequestration & What it Means for Aviation
As efforts to mitigate and curb emissions resulting from aviation continue to ramp up, there is an increasing focus towards addressing the high carbon dioxide (CO2) levels that are expected to still be emitted as well as the prior emissions that are already in the atmosphere. Even if CO2 emissions were eliminated tomorrow, the ecosystem would still take centuries or millennia to remove the excess CO2 in the atmosphere by way of natural processes.
This is where Carbon Capture and Sequestration (CCS) can play a significant role. Also referred to as “carbon removals”, CCS is a process designed to capture and store carbon over longer or near-permanent time scales. Before being extracted and consumed, fossil-based crude oil sequestered carbon on time scales over millions of years. The goal of CCS is to find solutions to re-achieve CO2 storage on those types of time horizons. The challenge with CCS is identifying efficient methods to capture, effective ways to store, and reliable means for long-term monitoring. This paper will summarize some of the different sequestration approaches and discuss where these types of projects fit into addressing aviation’s emissions footprint.
To better understand the different types of approaches, it is helpful to separate the capture of carbon from the storage of carbon within CCS. Generally, there are three ways either the capture or sequestration process can be categorized: biological, geological, and technological. Some approaches will use the same type of method for both capturing and sequestering, but most approaches we think about for CCS combine different approaches for the capture versus sequestration process.
Carbon Capture
Let’s start by focusing on the different approaches for first capturing the carbon.
Biological
Biological methods harness the intrinsic ability of plants and soils to absorb and retain carbon dioxide through photosynthesis and the natural carbon cycle – a process in operation for hundreds of millions of years. As plants or biomass grow, they capture carbon from the ambient atmosphere as part of their growth process.
Pathways for sequestration of macroalgae carbon into the deep sea. As macroalgae grow, they removes carbon dioxide from the atmosphere. Most of the carbon sequestered by macroalgae is sent to the deep sea either in the form of dissolved carbon or in the form of plant detritus which easily floats out to sea thanks to gas-filled bladders. This figure was adapted from Krause-Jensen and Duarte, 2016.
Projects linked to this method include afforestation (planting new forests), reforestation (restoring existing forests), cultivating kelp or algae in ocean-based projects, and creating enhanced wetland ecosystems. A highlight of this method is its ability to be done relatively simply and inexpensively. Also worth noting is that this process can be done on smaller scales, requiring minimal to no infrastructure. Examples of success stories can be seen in Africa, particularly northern Ethiopia, where communities banded together to control livestock grazing and wood cutting, which allowed vegetation to naturally regenerate1. Farming kelp and other macroalgae can help capture CO2 in the ocean, which is particularly helpful where land usage may be constrained or where forestry projects aren't feasible. It is estimated that macroalgae alone capture 200 million tons (181.44 million metric tons) of CO2 from the atmosphere annually, which is roughly 0.5% of global emissions2.
However, these types of projects are primarily focused on the capture of CO2, as they only provide short-term sequestration of the CO2. Once the plants die and decay (or are cut down and burned), a majority of the captured CO2 can be re-released into the atmosphere.
Geological
Geological capture primarily revolves around enhancing or accelerating existing natural processes that sequester CO2. In the natural process of mineralization, CO2 reacts with certain minerals to form stable carbonates like limestone.
By exposing air and seawater to minerals like olivine or serpentine, CO2 can be pulled from out of the atmosphere and converted into solid compounds that are then stored in the ocean or underground, locking it away from the atmosphere. Mineralization integrates capture and storage very well and is capable of very long-term carbon storage within stable minerals.
Technological
Capturing CO2 directly from the air using mechanical processes, filters, or solvents is usually referred to as Direct Air Capture (DAC). A related technique would be using the same process through scrubbing CO2 at the point of emissions, known as “point capture”.
Usually, either a solid or liquid is used to capture the CO2 once it is drawn in from the ambient air. Solid sorbent filters bind with the CO2 as it passes through and, once saturated, are sealed off and heated, where the concentrated CO2 is then ready to be sequestered and stored. Liquid technology may use a chemical solution to react and bind with the CO2 that goes through a similar heating process once saturated.
DAC systems can be deployed anywhere, allowing for localized carbon removal; however, it is inherently expensive and inefficient – think about how difficult it is to sort out 1 specific molecule from every 2,500 in the air or 400 molecules for every million (400ppm). That said, it is a straightforward process for removing CO2 already in the air.
On the other hand, point-capture can be a more efficient process due to the higher concentration of CO2 it deals with. Many industrial processes release significant amounts of CO2. Point-capture involves capturing this CO2 at the point of emission. Since this capture point is tied directly to the emission of CO2, instead of grabbing just 1 molecule out of 2,500, you are trying to capture 2,400 of 2,500… or 990,000 of every million molecules. Making the process intrinsically more efficient and cost-effective. However, it does not address prior emissions already in the atmosphere as it is inherently tied to the new emission of CO2. Nevertheless, it may be one of the few powerful ways to address emissions from hard-to-decarbonize and carbon-intensive industries like steel and concrete production.
Other very new technological approaches look at using electrical processes to separate CO2 from seawater in novel applications that could be tacked on to desalination facilities3 or combined with green hydrogen production. These types of technologies will likely take longer to fully mature, but represent very intriguing carbon sequestration opportunities that have additional social or energy-related benefits.
Carbon Sequestration
Now, let’s talk about the different ways that carbon can be sequestered long-term.
Biological
Despite short-term storage of CO2 in biomass during the life of that biomass, that short-term storage can be extended through the growth of new biomass in its place that through a constant cycle then becomes a longer-term storage option. For example, think about a degraded land that could host a forest. If a new forest is planted, some carbon will be sequestered in the plants and tress that inhabit that forest. Although those plants will die one day, new plants will take their place, creating a cycle of carbon storage that is higher than the baseline of when that land was just degraded land. This is a harder method to guarantee will be permanent because of the potential of future deforestation, land-use changes, or climate risks but still shows how biomass can be a longer-term storage solution for carbon.
A similar method would be through enhanced soil sequestration, using certain agricultural practices, like no-till farming, cover cropping, and agroforestry to enhance the amount of CO2 stored in the soil. Scientists estimate that these agricultural soils could sequester one billion tons of carbon per year4.
The other biological approach involves using the ocean. Algae, kelp, and phytoplankton can absorb CO2 from the atmosphere as they grow. These organisms can be encouraged to flourish through various techniques, and when they die, they sink to the ocean floor, effectively sequestering the carbon in a longer-term ocean carbon cycle. Growing and sinking kelp forests can store up to 20 times more carbon per acre than land forests. However, there are concerns about potential ecological impacts and unintended consequences of large-scale implementation without a better understanding of the ocean’s long-term carbon cycle.
Geological
UIC Class VI wells inject CO2 for long-term storage to reduce emissions to the atmosphere. Source: EPA
In addition to the long-term storage of CO2 through mineralization processes, geological storage is frequently used in combination with technological capture methods like DAC. Geological sequestration involves taking captured CO2 emissions, liquefying or solidifying the gas, and injecting it into depleted oil formations, permeable rock, or underground salt caverns for long-term storage. Based on an International Energy Agency assessment, the total global geological sequestration storage capacity has been estimated to be between 8 and 55 gigatonnes5. Most of that storage capacity is onshore in salt caverns, deep saline formations—large porous formations filled with brine or salty water—and depleted oil and gas fields. The advantage is that these storage solutions are expected to work for thousands of years to permanently sequester carbon; however, this process can be costly and require long-term monitoring and financial commitment to ensure the CO2 does not escape or leak from the storage area.
Technological
Technological sequestration uses captured CO2 and repurposes it as an input material for industrial or consumer products6. While it may not sequester the carbon as long as certain biological or geological methods, it can easily sequester the carbon for tens of years or longer decadal time scales. Mixing point-captured CO2 from the smokestacks of concrete plant kilns with minerals that get placed in cement can store the captured CO2 within the buildings it helps create. The CO2 is stored within the concrete of the structure itself, sequestering it until the concrete is destroyed (which may even be beyond the life of the building).
Furthermore, CO2 captured can be utilized in industries such as enhanced oil recovery or converted into useful materials like graphene, which has many commercial uses, including aircraft electronics and avionics. These types of technological storage are newer, more innovative methods that are economical and easy to monitor for leakage.
Crossing Methods for Carbon Capture and Storage
When most people think about CCS, they envision a Direct Air Capture plant that stores carbon within geological formations, a process that combines a technological capture process with a geological storage. The truth is that the field of different CCS approaches out there is quite varied. Each with their own benefits, challenges, and developmental progress.
One of the more interesting CCS processes for aviation is called Bioenergy with Carbon Capture and Storage (BECCS). BECCS combines bioenergy production (such as burning organic materials like wood or waste material from crops for energy or the production of biofuels) with carbon capture and storage. The CO2 is captured during the growth of the biomass and then sequestered and stored during the biofuel production process, cost-effectively removing carbon from the atmosphere. The process of capturing the carbon is performed by the growth of the original biomass (a biological process) and then taking the CO2 it captured and combining it with long-term storage (either a geological or technological process).
| Carbon Capture | Carbon Storage | |
|---|---|---|
| Biological | Biochar, Biomass Growth | Biomass Growth, Ocean |
| Geological | Mineralization | Salt Caverns, Oil Fields |
| Technological | DAC, Point-capture | Concrete, Consumer Products |
The process is more efficient than DAC today (biomass is more efficient at selecting out CO2 than mechanical processes, for now) and produces a valuable product, so it is expected to be cheaper than DAC. BECCS can be used in combination with sustainable aviation fuel (SAF) production to help sequester additional CO2 during the production process, enhancing the carbon reduction capabilities of SAF.
Why should Aviation care about CCS?
CCS will have several intersections with aviation. First, it offers an opportunity as a feedstock for SAF, with certain companies7 looking to take captured CO2 and synthesize it back into jet fuel. This process creates a truly circular emission of CO2, preventing any net new CO2 from being emitted into the atmosphere.
Second, aviation must deal with the residual emissions it will have as an industry. Even with leading SAF technologies, aviation is expected to have unaddressed CO2 emissions since SAF does not provide 100% CO2 reductions. Given the ramp-up of SAF and our 2050 goals, aviation must prepare for addressing significant residual emissions to achieve its 2050 net zero goal. CCS can provide opportunities for capturing and sequestering CO2 from the atmosphere to more effectively deal with these remaining emissions.
Third, every year that we fail to achieve significant reductions in CO2 also represents a future CO2 debt that must be repaid. CO2 remains in the atmosphere for hundreds of years and therefore the prior emissions of the industry (and what will be the prior emissions by 2050) should also be addressed. Thankfully, CCS can provide opportunities for addressing historical emissions as well. Companies like Microsoft have promised to remove all emissions that they have ever emitted from the formation of the company, through the use of CCS methods.
Future regulation for aviation is likely to require the use of CCS for addressing aviation emissions, but for today, CCS can be part of an offset purchasing strategy or combined with SAF to achieve a truer net zero carbon fuel (SAF still has other non-CO2 emissions that still need to be addressed).
The Challenges of CCS
Carbon sequestration holds great potential for mitigating climate change, but it also faces several challenges that must be addressed for these technologies to be effective and commercially sustainable.
Most carbon sequestration methods are currently very expensive to implement and operate. This limits their widespread adoption, especially without clear economic incentives or regulatory frameworks that value carbon removal. Whether there will be ways to lower the costs and find ways to make these technologies scalable and economically viable will determine the level of success of CCS.
Additionally, accurate monitoring and verification is crucial. There must be trust that the leakage or escape of CO2 will be avoided on long-term timeframes and that there will be a systematic standard for independent audits and verification of the methods, monitoring, and accounting practices used.
Some methods, such as ocean-based approaches, could have unintended ecological consequences and will require more complex monitoring to ensure the captured carbon does not make it back into the atmosphere or ecosystem. Altering ocean chemistry might harm marine ecosystems, causing negative feedback loops. Similarly, if not done carefully, afforestation or increasing bioenergy production could compete with land and resources that could be, or are currently, used for food production. Balancing these competing needs and taking a more holistic system view will be a paramount requirement for any of method.
Therefore, additional technological progress is needed to robustly scale up current approaches as well as develop new capture methods with better efficiency, robust monitoring, and effective long-term storage. On top of this, it is critical to build verification and monitoring methods that will be able to ensure the efficacy of different projects.
The Future of CCS
The work to sequester CO2 can be done in parallel with other efforts to reduce or offset emissions. CCS is almost always more expensive than reducing CO2 in the first place, which is a testament to where the focus should be for an organization. CCS is not meant to be a solution used instead of CO2 reductions but something used against the most expensive and difficult to decarbonize portions of a carbon footprint.
Thankfully, independent organizations and policymakers are taking notice and are providing incentives to pursue opportunities within CCS further. The International Emissions Trading Associate has released its High-Level Criteria for Crediting Carbon Geostorage Activities, which establishes six safeguards to underpin the issuance of carbon credits for geological sequestration projects8. In the US, the Inflation Reduction Act created a tax credit called the 45Q, which provides a direct financial incentive for using captured CO2 in oil recovery, industrial processes, or preferably in permanent storage9. Finally, government-supported investments are underway, with two DAC facilities along the US Gulf Coast receiving $3.5 billion in August of 2023 to accelerate their projects for capturing carbon10.
Financial incentives like these are continuing to increase and will hopefully spur the involvement required from key stakeholders to progress CCS into widespread use. It is important for end users to better understand what the options for CCS are and how that might fit into their sustainability programs. Doing so will demonstrate a more robust view of the long-term options for reducing the impacts already made by aviation.
Endnotes
https://www.wri.org/insights/how-ethiopia-went-famine-crisis-green-revolution
https://sitn.hms.harvard.edu/flash/2019/how-kelp-naturally-combats-global-climate-change/
https://news.mit.edu/2023/carbon-dioxide-out-seawater-ocean-decorbonization-0216
https://www.ieta.org/initiatives/high-level-criteria-for-carbon-geostorage-activities/