Direct air capture (DAC) uses big machines to remove CO₂ straight from the air. When air passes through these machines, the chemicals inside react with CO₂, removing it and allowing the rest of the air to pass through unchanged . The chemicals which remove CO₂ are called a “capture agent”, and have to be separated from the CO₂ afterwards so they can be reused while the carbon dioxide is stored.
We’ve tried to make this work in lots of different ways; some we have given up on, while others are currently still being developed. In this chapter, let’s focus on two of the most promising types of capture agents :
Liquid solvents
Solid adsorbents
How do liquid solvents work?
A solvent is a substance (usually a liquid) that other materials can dissolve into.
In carbon removal, the air passes over a liquid solvent such as potassium hydroxide (KOH). This reacts with and removes CO₂ from the air, producing potassium carbonate (K₂CO₃) .
Carbon removal in liquid solvent step 1
Next, CO₂ must be separated from the potassium compound so that the liquid solvent can be used again to capture more CO₂.
We can do this by using calcium hydroxide (Ca(OH)₂), which separates the carbon from potassium carbonate by reacting with it to form calcium carbonate (CaCO₃, also known as limestone). The limestone can then be broken down to produce a stream of pure CO₂ that can either be stored or used directly .
Carbon removal in liquid solvent step 2
In both of these chemical cycles, the liquid solvents (KOH and Ca(OH)₂) are regenerated, meaning the process can capture CO₂ from the air again and again . Here is the whole process at once.
The entire process of liquid solvent DAC
How do solid adsorbents work?
So liquid solvents are one way to capture CO₂. The other one we mentioned was using ‘solid adsorbents’.
Absorption and adsorption are actually different things! Adsorption is when a thin layer of molecules (in this case CO₂) become stuck to the surface of a solid or liquid (in this case, the solid capture agent). The solid adsorbent, therefore, needs to be a material that CO₂ sticks to easily
Air is pulled into the collector using a fan until the filter is full of CO₂. Then the collector closes and the temperature increases to 80-100℃, which releases the CO₂ in a concentrated stream that can be stored or used directly .
The solid adsorbent process
How do these methods compare?
The removal of CO₂ from potassium and calcium carbonates (K₂CO₃ and CaCO₃) requires high temperatures (and thus a lot of energy) . Natural gas, a fossil fuel that itself releases CO₂, is often used to supply this energy, meaning that 50-90% of the CO₂ removed by this form of DAC is re-emitted !
Solid adsorbents need less energy because CO₂ binds less strongly to solids than liquids . This means less energy is needed to separate it into a pure stream . Because it operates at temperatures of around 100℃, free ‘waste heat’ from other facilities can be used to supply these relatively low energy requirements .
Will DAC have environmental impacts?
The liquids can be used again and again, but the solid filter gets worn out over time. This means it has to be replaced, and the old one becomes waste .
Apart from this, however, the environmental impacts of DAC are low compared to other ways of removing CO₂ from the atmosphere .
Weighing up the options
What else can we do?
Liquid solvents and solid adsorbents are the best-developed methods of DAC at the moment, but there are other methods in development too. These include cryogenic separation and organic membranes. Each of these is at a different stage of development and has different costs and energy requirements.
How much does DAC cost?
It’s tricky to estimate how much a technology will cost before it’s widespread . Estimates for DAC range from $30-1000/tCO₂. The cost depends on the type of DAC, how the captured CO₂ is used and how expensive the equipment is.
Costs for liquid solvents are expected to average between $94 and $232 per tonne of CO₂ captured. Solid adsorption is estimated slightly lower at $86 to $221 per tonne of CO₂ . This difference is mostly because liquid solvent DAC needs more energy.
How much CO₂ can DAC remove?
As far as we know, the main limit to how much CO₂ we can remove with DAC this century is the cost of the machines ! Carbon storage, unexpected environmental side-effects, and some land requirements may also limit DAC, though it needs far less land than other methods of carbon removal.
DAC’s high potential for carbon removal
Conclusions
DAC is an exciting, if expensive, new technology that might be able to remove large amounts of CO₂ from the atmosphere. But what do we do with the CO₂ once we capture it? In the next chapter, we will look at how captured CO₂ can be transported and stored.
. When air passes through these machines, the chemicals inside react with CO₂, removing it and allowing the rest of the air to pass through unchanged 
