Carbon Dioxide Removal: Direct Air Capture (DAC)

Carbon dioxide removal (CDR) technologies, such as Direct Air Capture (DAC), need to be developed so that they are efficient and effective enough to be used as one of the many available tools needed to combat climate change. When used alone, it is unlikely that dioxide removal technologies can be a complete solution to the climate crisis due to the vast amounts of carbon dioxide (CO₂) contained within the atmosphere. CDR technologies, and more specifically DAC, have significant barriers such as energy requirements, the challenges of CO₂ storage, and the capital/operational costs. But in order to avoid the worst-case outcomes of climate change, every possible action must be undertaken, including the use of direct air capture carbon dioxide removal.

Carbon dioxide removal is the process of removing CO₂ from the atmosphere, which should not be confused with carbon capture and storage (CCS), which captures carbon at the point source of emissions. Traditional CDR practices typically involved changes to land and agricultural practices such as afforestation and reforestation, forest management, enhanced soil carbon storage, and more recently, Biomass Energy with carbon capture and sequestration (BECCS). Land-based CDR practices are a vital tool for combating climate change and all efforts should be made to adopt them; however, even as they are relatively cheap and readily available, they are not without their own challenges. Many land-based CDR methods require large amounts of land, and BECCS requires large volumes of organic matter as feedstock, which would need to be grown and harvested, potentially displacing food production and pristine nature. The enormous land requirement for terrestrial CDR makes it difficult to scale up to the levels needed to make a significant impact in lowering atmospheric CO₂ levels, meaning that other solutions need to be adopted in conjunction with land-based CDR practices, opening the door for direct air capture carbon dioxide removal.

It is almost universally accepted that high levels of CO₂ removal from the atmosphere will be required to avert the most catastrophic outcomes of climate change. In the most well-known global climate treaty of the 21st century, the Paris Agreement, ratified by all nations except Iran, Yemen, Libya, and recently withdrawn by the United States, it is agreed that the world will need to limit global warming to well below 2°C above pre-industrial levels. In order to achieve this goal, large-scale atmospheric carbon dioxide removal of around 1,300 Gigatons (1,300,000,000,000 tonnes) will be required by the end of the century, in addition to rapid decarbonization of the global economy. Under the current global emissions trajectories in which global decarbonization is moving vastly slower than necessary, even more atmospheric CO₂ would need to be removed to meet the Paris Agreement goals.

With such a large amount of CO₂ needing to be removed from the atmosphere, it is important to consider the feasibility of direct air capture carbon dioxide removal. The process of direct air capture involves selectively finding and capturing CO₂ from the ambient air where it is present at low concentrations of around 420 parts per million (ppm). Once captured, the CO₂ needs to be concentrated after which it can be stored, transported, or utilized. Because the CO₂ in the air is present at such low concentrations, special materials known as 'sorbents' are used to capture the CO₂ due to their ability to selectively bind to CO₂ molecules. These bonds need to be strong to attract the CO₂ molecules and hang on to them, meaning that a very significant amount of energy is required to break these bonds so the CO₂ can be detached and concentrated. Because the CO₂ in the atmosphere is a gas, it is possible to calculate the minimum energy required to capture this CO₂ assuming an unrealistic 100% efficient process. Estimates of the minimum energy required to capture 1,300 Gigatons of CO₂ lie around 540 EJ (exajoules), which is over 5 times the level of global electrical energy supply in 2019. If a more realistic efficiency value is used, which is nearer to 10% thermal efficiency, the energy required rises significantly.

Another framework for thinking about the effectiveness of CO₂ direct air capture is in future scenarios where it can be utilized to counteract hard-to-abate sectors, assuming that the world is able to navigate past the impending climate catastrophe. High CO₂-emitting sectors such as aviation, shipping, and the steel industry are all sectors that are difficult to decarbonize, and DAC could be used to counteract these emissions by removing the CO₂ from the air before it is released into the atmosphere or by providing the carbon feedstock for "carbon-neutral" synthetic fuels. It is crucial to recognize that the fundamental requirement for direct air capture is to remove CO₂ that is already in the atmosphere, and not to be used to counteract emissions from hard-to-abate sectors or be utilized for other purposes that result in CO₂ being released back into the atmosphere. If capital investments for direct air capture are initially focused on applications such as synthetic fuels, there is a risk that the technology will be developed in a way that is not conducive to the removal and long-term storage of CO₂ from the atmosphere due to a lack of capital investment in storage infrastructure. However, counteracting hard-to-abate sectors and providing the carbon feedstock for synthetic fuels is a future use case for DAC.

What is carbon capture and how is it different from carbon dioxide removal?

Carbon capture is the process of capturing CO₂ from large emitters, often called Carbon Capture and Storage (CCS). CO₂ is a highly abundant greenhouse gas in the Earth's atmosphere with around 420 parts per million (ppm) by volume in 2024. CO₂ has one of the highest radiative forcing of the greenhouse gases and has an atmospheric residence time that can last for centuries, making it imperative to reduce or stabilize atmospheric CO₂ levels. Carbon capture as a concept and technology has existed for almost a century, being used to extract CO₂ from raw natural gas as well as other industrial processes. However, in the early 1970s, it was discovered that CO₂ is highly effective for use in oil well stimulation known as Enhanced Oil Recovery (EOR), which to this day accounts for the majority of CO₂ capture and storage. Only more recently has the idea of using carbon capture as a method of reducing CO₂ emissions to combat anthropogenic climate change been considered. As humans have been emitting CO₂ into the atmosphere at unprecedented rates through the burning of fossil fuels for the past 100 years, the idea of capturing CO₂ from the atmosphere is an appealing one, yet it is a monumental task considering the amount of CO₂ that needs to be captured. Many governments and companies have invested heavily in CCS since the turn of the century to capture CO₂ from large emitters such as natural gas refineries, and although CCS has not been a complete failure, many consider the low payoff from investments a story of unmet expectations. More recently, the focus has shifted to carbon dioxide removal technologies such as direct air capture as a means of capturing CO₂ directly from the atmosphere, as even if the world transitioned to net zero emissions today, the greenhouse effect caused by the amount of accumulated CO₂ already in the atmosphere could still lead to increased global temperatures throughout the next century.. In order for DAC to become a viable method of removing CO₂ from the atmosphere, three key obstacles need to be considered: the thermodynamic minimum energy requirement, the carbon storage requirement, and the cost of deployment and operation.

What is the goal of carbon dioxide removal?

When considering the feasibility of dioxide removal technologies and more specifically DAC, it is important to clarify what is trying to be achieved and what is achievable. The amount of carbon dioxide that would need to be removed from the atmosphere to return to preindustrial levels is almost inconceivable. However, dioxide removal technologies could be used as a tool to achieve a net zero economy under specific emissions pathways. The Intergovernmental Panel on Climate Change (IPCC), which is the leading body in charge of analyzing and assessing the field of climate change research, utilizes Representative Concentration Pathways (RCPs) as a framework for estimating future warming and atmospheric CO₂ concentration pathways based on different scenarios and assumptions. There are 7 main RCPs ranging from RCP1.9 to RCP8.5, with the naming based on the radiative forcing in Watts per meter squared (W/m²) that would be applied to the climate system in the year 2100. As a comparison, the current radiative forcing caused by anthropogenic greenhouse gases is estimated to be around +4 W/m². RCP2.6 is considered a high intervention pathway with emissions beginning to decline in 2020 and reaching zero by 2100, while RCP8.5 is considered a high emissions scenario where we carry on as usual and do not transition to a "green" economy. RCP4.5, still considered an ambitious target by 2024 standards, involves global emissions reductions in the latter half of the century resulting in stabilized CO₂ concentrations by 2100.

Direct Air Capture (DAC) technology

Direct Air Capture (DAC) is a technology that involves capturing CO₂ from the air using a combination of physical and chemical processes. A DAC system typically consists of a fan that is used to draw air into the system, a water removal system depending on the sorbent material used, a sorbent (capture) material that selectively removes the CO₂ from the air, a desorb (release) phase where the CO₂ is released from the sorbent material, and a regeneration phase where the sorbent material is treated so that it can be used again. Depending on the desired final outcome of the CO₂, whether it is sequestration or utilization, further steps may be needed to prepare the CO₂ for transportation or storage.

For current state-of-the-art DAC technology, the critical step and challenge of the system is the choice of sorbent material. Sorbents are defined as materials that can hold or capture gases and liquids either through physical or chemical processes. As CO₂ is an acidic gas, sorbents that rely on a chemical binding process are often alkaline in nature. Preparation and maintenance of sorbents are where the largest energy input is required, making them the main limiting factor in the efficiency of a DAC system. This is because the bonds between the sorbent need to be very strong to "trap" the CO₂ that is passing by, meaning that a lot of energy is required to break these bonds. Sorbents come in two main forms:

Liquid sorbents capture CO₂ through the process of absorption, where the CO₂ is dissolved into the liquid sorbent material. Liquid sorbents mostly rely on chemical bonding and are therefore usually an alkaline solution (also known as amines). After the CO₂ has been absorbed, the liquid sorbent material needs to be treated with heat and other solvents to release the CO₂.

Solid sorbents capture CO₂ through the process of adsorption, where the CO₂ is held on the surface of a solid material. Solid sorbents are usually a porous material that has a high surface area to volume ratio, which allows for a large number of CO₂ molecules to pass through and be held on the surface of the sorbent. Solid sorbents can also be coated in materials such as amines which chemically bond with CO₂.

Liquid and solid sorbents have their own unique advantages and disadvantages. Liquid sorbents are a proven technology that has been used in industrial gas scrubbing for many decades and they tend to have a high CO₂ capture efficiency. However, liquid sorbents require a high temperature (large energy input) in the desorption and regeneration phases. Solid sorbents have the advantage of being significantly less energy-intensive and are generally more robust and less complex systems. However, solid sorbents tend to have a slower capture rate and lower capture volume as they are difficult to run continuously.

Recently, there have been a handful of advancements in sorbent technologies that may assist in addressing the high energy requirements of the desorb and regeneration phases. These materials do not circumvent the high energy requirement that is explained in detail in the next section. However, they do require a significantly lower temperature to desorb the CO₂ compared to existing best practice technologies, which would mean that they could utilize low-quality heat such as the waste heat from the exhaust of an industrial process.

The minimum energy requirement: thermodynamic limits and Gibbs free energy

Many physical processes are dictated by unassailable laws of thermodynamics which cannot be violated, and carbon dioxide removal is no different. DAC technology can be simplified to one fundamental task: moving CO₂ that is in a low concentration in the air to a high concentration where it can be stored, transported, or utilized. Gases have a natural tendency to disperse (spread out) and mix with their surrounding medium, this process being related to the concept of entropy. It is for this reason that the concentration of oxygen in the atmosphere remains relatively constant when you are at similar altitudes throughout the Earth. As such, in order to counteract this natural tendency to disperse, some form of energy has to be applied in order to keep the CO₂ concentrated and separated from the surrounding air. This is known as a change of state of the gas. The absolute minimum amount of energy that needs to be applied to capture (change the state of) the CO₂ is dictated by the Gibbs free energy.

Gibbs free energy is a complex concept that can be hard to visualize or form analogs using real-world examples as it is based on a number of assumptions and it describes idealized conditions that often don't occur naturally, such as 100% efficiency, and reversible energy conversion. However, it is a useful concept to understand the thermodynamic limits as it provides a benchmark for the absolute minimum amount of energy required to capture CO₂. Analysis of the DAC process using Gibbs free energy shows that the energy required to capture non-trivial amounts of CO₂ lies within the realm of possibility, yet the energy required is still enormous. When utilizing energy efficiency conversions that are more reflective of current DAC technologies, i.e., much lower than 100% efficient, the energy requirement jumps to a level that is seemingly insurmountable. These energy requirements do not take into account the additional energy costs that would be required at a DAC facility, such as the energy required to compress the CO₂, transport it, and store it.

Carbon storage

Another issue that arises with carbon dioxide removal is where and how the CO₂ would be sequestered. Achieving pre-industrial concentrations of atmospheric CO₂ requires enormous efforts, and even if the goal were merely to remove the CO₂ produced annually, the amount of CO₂ needing storage would still be immense. CO₂ can either be sequestered (permanently stored), or it can be utilized as it is with contemporary enhanced oil recovery (EOR) techniques or through conversion into liquid fuels through the Fischer–Tropsch chemical process. The trouble with utilization methods is that they do not address the key goal of DAC, which is to remove CO₂ from the atmosphere, so they will not be covered in this section.

A practical advantage that DAC has over conventional CCS is that it doesn't need to be deployed at the source of CO₂ emissions, such as the site of a natural gas refinery. This technically could reduce the need for extensive infrastructure networks to move captured CO₂ to storage sites, meaning that DAC sites could be located at or near potential storage sites.

Some of the methods and locations where CO₂ can be stored include:

Saline Aquifer Storage: Saline aquifer storage is often considered the most feasible storage method as it has the greatest potential to store large volumes of CO₂. Saline aquifers are large underground reservoirs of briny water which have the ability to dissolve CO₂ so that it can be sequestered.

Depleted gas and oil reservoir storage: Depleted gas and oil reservoirs are often considered as a storage method for CO₂ as the technology has been developed and tested for enhanced oil recovery (EOR) by the oil industry and much of the infrastructure required already exists on site.

Basalt formations storage (Mineralization): Basalt rock formations are viewed as a promising CO₂ storage method as they are very abundant and the minerals in basalt have the ability to react with CO₂ to form solid carbonate minerals which can store CO₂ indefinitely. As with depleted oil and gas reservoirs, large amounts of water and energy are required to dissolve the CO₂ so that it can react with the basalt minerals.

Hydrate Storage: Hydrate storage is a method that involves capturing CO₂ as a gas and dissolving it in water under high pressure and low temperature to form a solid crystalline hydrate. CO₂ hydrate forms almost spontaneously under these conditions, but these conditions also need to be maintained in order to keep the CO₂ stored.

Deep ocean storage: Deep ocean storage is a method that involves capturing CO₂ as a gas and dissolving it in the water of the ocean. The CO₂ can then be stored in the deep ocean where the pressure is high enough and temperatures are low enough to keep it dissolved in the water or in a hydrate form. Deep ocean storage carries the added risk of contaminating the ocean with the CO₂, which could have long-term effects on the ocean's ecosystem.

Other feasibility issues arise when considering the storage of CO₂ on a mass scale. Some storage methods rely on the CO₂ being stored in a liquid state or supercritical state (a state of matter between a gas and a liquid which is desirable for practical storage applications), which will require a significant energy input to compress and cool the gas. In the case of depleted oil and gas reservoirs, which function the same as enhanced oil recovery, large amounts of briny water are required to dissolve the CO₂ in order for it to be stored.

CO₂ leakage and safety risks

On top of concerns for energy and water usage, the storage of CO₂ also carries the risk of leakage, which could have long-term effects on the environment as well as local residents. Almost all proposed CO₂ storage methods carry some level of risk, relying on the use of natural formations as their main storage sites. The size and distributed nature of these formations can make them difficult to monitor and manage. Geological activity or large-scale rain events can also lead to shifting and erosion of the formations and transport infrastructure, which could lead to leakage. However, many of the storage methods rely on an eventual solidification of the CO₂ which would minimize leakage risks.

The potential safety risk of a massive CO₂ leak is also a concern when the leak is situated near a population center, which may lead to pushback from local residents. In 1986, a natural leak of between 100,000 and 300,000 tonnes of CO₂ from a volcanic crater lake named Lake Nyos in Cameroon, West Africa, killed around 1,800 people when the CO₂-rich water rose to the surface and released the CO₂. Because CO₂ is heavier than air, it initially will sink and linger in low-lying areas for many hours. This disaster is only one of two recorded cases of what is known as limnic eruptions. More recently, in 2020, heavy rains led to the rupture of a CO₂ pipeline from a CCS and EOR plant near the city of Satartia, Mississippi. The leak caused around 45 hospitalizations and no fatalities, but it was the inability of first responders to react which caused the most worry. This was due to the fact that combustion engines are unable to operate in the presence of high concentrations of CO₂, and poisoning from inhaling CO₂ can lead to impaired cognitive function, making it difficult to react to the situation and evacuate.

Cost of deployment

In addition to the minimum energy requirement for separating CO₂ from air, there are additional costs and energy requirements that would need to be considered when deploying DAC technology. As DAC technology is relatively new, the initial capital costs of construction are high for liquid sorbent systems and even higher for solid sorbent systems. A large portion of the operating costs of DAC plants depends on the source of energy used in the capturing process, with renewable energy currently being the most expensive source for DAC plants, while the use of fossil fuels is the cheapest. However, using contemporary technology and efficiency rates, it has been argued that DAC plants would not be practical if fossil fuel energy were used, as the CO₂ emissions from burning the fossil fuels would account for around half of the emissions captured by the DAC plant. Throughout the literature on DAC technology, there is an agreed-upon critical target cost of $100/tCO₂ for it to be considered economically viable. Yet even at this price, the cost of deploying DAC technology is still prohibitive.

Life Cycle Analysis (LCA)

As with all industrial processes, there are material inputs and waste outputs that need to be considered.

For a solid sorbent system, much of the construction of the plant involves relatively standard industrial processes requiring concrete and steel for building construction, while copper and polymers are used for internal components. It is with the production of the specialized solid sorbents where more novel materials and processes are required that can lead to a higher environmental impact.

Similarly, with liquid sorbent systems, the construction of the DAC facility is relatively standard, but the manufacture and waste management of the liquid sorbent (most commonly potassium hydroxide (KOH) or sodium hydroxide (NaOH)) and the auxiliary chemicals used for the desorption and regeneration phases are where environmental challenges arise. However, due to the extremely high energy inputs required by liquid sorbent carbon dioxide removal systems, it is unlikely that they will be utilized extensively outside of specific applications such as carbon capture and storage at refineries and energy production centers where waste heat and cheap energy are available. For this reason, we will be focusing on solid sorbent systems in the following sections.

Carbon dioxide removal from hard-to-abate sectors

Once global atmospheric carbon dioxide concentrations have been stabilized at a level that is sustainable for continued human existence, direct air capture could be used as a means to counteract the CO₂ emissions from hard-to-abate sectors. Hard-to-abate sectors is a general catch-all term used to refer to industries and processes that are the most difficult to decarbonize. Hard-to-abate sectors is somewhat of a misnomer as it implies that these sectors are not able to be decarbonized, when in fact they are able to be decarbonized with the right technology and policy. On a longer time scale, sectors that are considered "hard-to-abate" such as heavy-duty freight can be relatively simply decarbonized using contemporary electrification technologies. Direct air capture could also be utilized to counteract the CO₂ emissions from these very hard-to-abate sectors.