● We need to remove 15 billion tonnes of carbon from the atmosphere by 2050. This will require natural and technological solutions.
● DAC is an exciting and highly scalable technology, separating and capturing the CO₂ from normal air.
● Geologically storing the captured carbon is an opportunity to permanently remove it from the atmosphere.
It is estimated that we will need to remove 15 gigatonnes (i.e. 15 billion tonnes) of carbon from the atmosphere by 2050 if we are to limit global warming to within 1.5 degrees Celsius from pre-industrial levels. Trees and other nature-based forms of sequestration have a big part to play in achieving this target. However, with growing pressures on land use and an increasing risk of nature-based sinks becoming sources, alternative technological solutions are required to meet these targets.
Direct Air Capture (DAC) technology draws CO₂ directly from the atmosphere. Geologically sequestering the captured carbon underground could play a significant role in global carbon removal requirements.
Gotta catch it all
In very basic terms, there are three main stages to the DAC process: finding the CO₂ in the air, separating it from everything else and then working out what to do with it.
Despite the levels of CO₂ in the air steadily rising, capturing it from atmospheric air is challenging. The concentration of CO₂ in the air is around 420 parts per million (ppm), or just 0.042%. This is equivalent to a single cubic meter of a whole Olympic swimming pool.
This is why carbon capture technologies have largely been focused on industrial processes to date. The CO₂ concentration in the combustion product of oil- and gas-fired power plants or steel and cement production can range from 3–20%², making it between 70x — 475x greater than in atmospheric air.
Plucked out of thin air
To capture CO₂ directly from the atmosphere, DAC uses either solid sorbents or liquid solvents. When these come into contact with air, they selectively react with and bind the CO₂, while allowing other components of air to pass through.
Carbon Engineering (CE) uses a closed loop, liquid-based approach to remove CO₂ from the air — using the same chemicals over and over again to minimize waste and consumables. In the first step, atmospheric CO₂ reacts with potassium hydroxide solution in modular air contactors. This non-toxic solution chemically binds with the CO₂ molecules to form potassium carbonate, removing them from the air and trapping them in the solution. The second step causes the CO₂ to precipitate out of solution in the form of calcium carbonate pellets. These pellets are then heated to release the CO₂ for capture, with the leftover calcium being reused in the process (Figure 2)³.
A typical DAC facility requires 3,000 tonnes of air to be processed in order to yield one tonne of CO₂ captured (assuming a 75% CO₂ capture rate). Other DAC providers, such as Climeworks and Global Thermostat, use a solid sorbent method which utilises amine absorbents in small, modular reactors. CO₂ is released from the solid sorbent by cycling the temperature in a vacuum chamber. Solid sorbents are consumables that must be periodically replaced and properly disposed.
There are two options for the captured CO₂: re-use it or find somewhere to store it.
Emit, capture, process, repeat
CO₂ can be used as an input in producing fuels, chemicals, foods, beverages and building materials and usage to enhance yields of biological processes. Currently, the biggest use globally of pure CO₂ is for enhanced oil recovery (EOR). This is where the carbon is used to recover more oil from a well than can be done using traditional oil extraction methods. The majority of the CO₂ used then remains sequestered underground, reducing the lifecycle emissions of oil recovered in this way.
Carbon can also be combined with renewably-sourced hydrogen to create synthetic fuels. Compared to the more prevalent biofuels, synthetic fuels created from DAC burn more cleanly, leading to less need for engine maintenance and require 100 times less land than biofuels⁵. While electrification is gaining traction in the automotive industry, synthetic fuels could be used in sectors such as aviation and heavy machinery which are harder to electrify and require the high energy density of liquid fuels.
Please return after use
In order for the captured CO₂ to be permanently removed from the atmosphere, it must be stored somewhere secure. Having dug deep into the ground to pull up one form of carbon (oil), it seems fitting that the existing wells are an ideal location to store the captured carbon. Saline formations also provide appropriate sites for geologic storage⁶.
To geologically sequester the CO₂, it is pumped into porous and highly permeable geological layers at least 1km deep. The location of these layers is key, as they should be capped by an impermeable layer (i.e. caprock) to prevent the upward migration or ‘leakage’ of any CO₂. Natural mineralization causes the CO₂ to gradually turn into stone in anywhere from a decade to a thousands of years depending on the geological characteristics of the well.
DAC combined with this form of permanent sequestration is an exciting opportunity to achieve negative emissions. However, it requires a significant amount of infrastructure (even beyond the DAC process) to sequester the carbon in this way. Transporting the captured carbon can be costly (both monetarily and environmentally), meaning new DAC plants are best situated in the proximity of existing underground storage reservoirs, including oil wells.
In part 2 of our diDACtic series, we will take a look at DAC removal units as an offset and how they rate in the BeZero framework.