Direct air capture: an industrial solution to the climate crisis?

Direct air capture is still too expensive for rollout at the scale required to meet net zero – but it need not be. Appetite for technology development is growing, safe in the knowledge that current demand for negative emissions technologies outstrips available capacity – and that future demand will be even greater.

According to the Intergovernmental Panel on Climate Change, for every 1,000 gigatonnes of carbon dioxide emitted due to human activity, the earth’s surface temperature warms by an estimated 0.45°C. This near-linear relationship is helpful for understanding our remaining “carbon budget” and the need for carbon capture technologies [1].

As of 2021, the earth had already warmed by a little over 1°C. This implies a remaining carbon budget of 400 gigatonnes, or – given stable global greenhouse gas emissions equivalent to about 50 gigatonnes carbon dioxide per year [2] – roughly until the end of the decade before we exceed the 1.5°C of warming beyond which the adverse impacts on human activity are predicted to be immense.

Even in the best case, emissions will come nowhere near reaching zero within this timeframe. This is where carbon dioxide removal enters the picture. Long considered controversial, for creating a temptation to leave the problem for another day, it is now widely seen as an essential tool for mitigating climate change by offsetting ongoing emissions. This is particularly true given that some carbon-intensive industries (such as steel, cement and chemicals production) are difficult to decarbonise [3].

The other purpose of carbon capture technologies is to claw back historic emissions to gradually undo some of the effects of climate change. According to some scenarios, by mid-century gigatonnes of carbon dioxide will need to be removed from the atmosphere each year [4].

Direct air capture (DAC) refers to any technology that extracts carbon dioxide out of regular air (rather than capturing the carbon dioxide at the point of generation such as in flue gasses). 

Once captured from the air, the carbon dioxide may be used directly (e.g. in drinks carbonation), chemically converted (e.g. into aviation fuel, diamonds or polymers) or sequestered underground. When paired with technology that permanently keeps this carbon out of the carbon cycle, DAC becomes an example of a negative emissions technology.

The longest-standing approach to DAC relies on liquid solvents, typically aqueous amine or hydroxide solutions, that preferentially absorb carbon dioxide when contacted with air. The solvent is regenerated in a closed system by heating, causing the captured carbon dioxide to be released for further processing. 

More recently, solid adsorbents have been exploited for DAC. In this approach, porous adsorbents act as filters to preferentially adsorb the carbon dioxide as it passes through. The porous adsorbent is then regenerated once saturated with carbon dioxide (typically by heating or applying a vacuum) to release and capture the carbon dioxide.

As an example of a commercial DAC company using a solid adsorbent technology, Climeworks has recently commissioned their Orca plant in Iceland, the largest DAC plant in operation for the time being. At this plant, the captured carbon dioxide is currently sequestered using an underground process provided by the company CarbFix, in which the carbon dioxide is converted to carbonate minerals at a rate of around 4,000 tonnes per year [5].

Direct air capture has made strides ahead in recent years, but the processing of vast volumes of air still translates into high costs. Climeworks has proven that there is commercial interest in carbon capture, and currently charges individuals €1,000 per tonne to offset their carbon emissions. For the technology to have wide-reaching impact, however, the process needs to be financially favourable to the largest industrial emitters.

One way to achieve this is to use the carbon dioxide product in a way that keeps it out of the carbon cycle – for example by using synthetic biology to convert carbon dioxide into chemical intermediates (that do not go on to be combusted) – and to generate revenue from these products.

Another impetus comes from carbon emission taxes or cap-and-trade schemes. Policy is inexorably increasing the scope and price of these taxes to nudge specific industries towards reducing their emissions, as shown by the Carbon Pricing Dashboard [6]. For industries that are unable to reduce their emissions, paying for DAC or other negative emissions technologies will be financially incentivised once this becomes cheaper per tonne than the associated carbon tax. 

A rough price point for where negative emissions could be competitive in the mid-term is thought to be around $100 per tonne. We at TTP believe that finding novel ways to improve DAC technologies to be cheaper to manufacture and consume less heat and electricity will be key to creating scalable solutions that can meet or exceed these price points.

As organisations contemplate investing in the development and scale-up of DAC methods, it is important to establish which methods deliver real, long-lasting benefits. While there is no standard method for assessing carbon dioxide removal technologies, Carbonplan (a non-profit organisation that offers approaches to evaluating climate solutions) uses a useful collection of metrics to assess projects [7]:

Given the need to scale up carbon removal solutions from kilotonnes today to gigatonnes in the not-too-distant future, other factors also need consideration. These include water use, land use, and the overall investment required.

Even though reducing the release of carbon dioxide at source may be simpler, cheaper and less harmful, there is a need to capture historical carbon dioxide emissions and those from processes that cannot be decarbonised. As such, negative emissions technologies such as DAC are born out of the necessity to limit climate change. 

Moreover, given the substantial quantities of carbon dioxide that need to be removed, coupled with ever-increasing carbon taxes, there will likely be significant financial incentives to both develop and use negative emissions technologies.

While there are challenges in developing commercially viable plants, DAC is also a huge opportunity for those that can develop technical solutions to make the process more efficient, lower the overall material costs, and reduce the amount of heat required for regeneration. Through methods such as advanced material selection and development, modelling of carbon dioxide adsorption for different sorbent geometries, and process optimisation, the cost and scalability of the process can be dramatically improved for the next generation of DAC.