Direct Air Capture – The future of CO2 removal?

During the height of the COVID-19 pandemic in 2020, the entire world ground to a halt in many ways. Travel, both nationally and internationally, was all but stopped; planes were grounded and the roads were quiet. Consequently, CO2 emissions fell drastically at the time with a peak reduction of around 17% in places(1). By the end of the year however emissions had picked up again, resulting in an overall global drop in CO2 emissions of 6.4% in 2020(2). Though this sounds like a step in the right direction, the atmospheric CO2 level still increased in 2020, albeit at a reduced rate. This reduction however is relatively small, meaning the impact of the pandemic cannot be distinguished from natural year-on-year variability in atmospheric CO2 levels(1).Evidently if we wish to reduce global warming, we need to take drastic action to reduce the level of CO2 in the atmosphere.

In order to limit global warming, we as a planet have set a target, detailed in the Paris Agreement that the global temperature should be no greater than 1.5°C compared to pre-industrial levels(3). We are currently on track to reach this temperature at around 2034(4), so to prevent further increases much more needs to be done to reduce the amount of carbon dioxide emissions. Simply reducing the carbon output of existing processes is not enough, we need ‘negative emissions’ (greenhouse gas removal), where overall carbon is taken in as opposed to being emitted(5). This is where direct air capture comes in.

Direct air capture (DAC) involves extracting air and filtering the carbon dioxide out using chemical reactions, which can then be stored long term consequently keeping it out of the atmosphere(6). DAC is not a new concept and has been used as far back as the 1950s in submarines and spacecraft to keep the carbon dioxide levels of sealed spaces at safe levels. Perhaps most famously carbon capture was crucial during the Apollo 13 mission, when an explosion meant that three astronauts were forced to modify a CO2 ‘scrubber’ designed for a different module in order to survive in a module built for just two(7). Today however the technology is much more refined, and some companies are beginning to implement large scale direct air capture operations with big ambitions.

One such company is ‘Carbon Engineering’, who use a solution of potassium hydroxide to react with the CO2 in the air. The carbon dioxide is then trapped in a liquid solution as a carbonate salt, from which stage it can be heated to release pure CO2 gas; meanwhile the filtered air is released back into the atmosphere, containing considerably less CO2 (process seen in figure 1 below)(6). Once the pure carbon dioxide is obtained, it can either be stored long-term underground or even sold since CO2 has various uses, such as in the process of making jet fuel(8). The ‘pellets’ used to capture the CO2 can also be re-used once the CO2 is removed, lowering the cost of the process. This process, outlined in their 2018 paper, can be seen in figure 1 below(9).

Figure 1: The process which Carbon Engineering use to filter out carbon dioxide from the atmosphere.

Direct air capture plants like these are powerful, for example just one of these proposed plants can capture the same amount of CO2 in a year as 40 million trees(6). Currently, they only have small scale, prototype plants. From 2022 however, they plan to begin constructing the first large scale commercial plants in various locations around the globe. On this large scale, DAC plants have the potential to remove exceedingly large quantities of CO2 from the atmosphere. In fact, Carbon Engineering believe that by 2050, DAC facilities will be playing a “significant role in the global effort to achieve next zero emissions”(10).

A second company, ‘Climeworks’, use a different method to capture CO2. Once ambient air is drawn in, the carbon dioxide is captured on the surface of a solid filter material containing amine chemical groups(11). Once the filter is concentrated with CO2 it is heated and the gas is liberated. To store the CO2 and prevent it from reaching the atmosphere again, it is mixed with water and pumped deep underground where it can mineralise. This process can be seen in the short video below(12).

The work and development of these companies is not going unnoticed. Direct air capture is becoming more widely recognised and accepted as having the potential to be a significant contributor to limiting global warming(13). Both companies are on route to deploy their first large scale DAC plants in the near future, and have partnered with other companies to develop plants in multiple locations around the globe, with the aim of becoming prevalent worldwide(10, 12). In fact, earlier this year Microsoft chose Climeworks as part of their plan to offset their own emissions (both current and historical)(14). As they gain recognition, these DAC plants will spread worldwide and kickstart the journey towards negative emissions.

It may now seem like direct air capture is an ‘easy fix’ for climate change and we should focus our efforts on implementing the technology as quickly as possible(11). This however would be a naïve approach; while DAC has many benefits, it also has its drawbacks. Since the concentration of CO2 in air is relatively low, a large amount of energy is required to filter the carbon dioxide from the air. Using computer models to explore the feasibility of DAC(13), challenges were highlighted relating to the large-scale, rapid deployment of DAC in terms of energy and raw material consumption(13). By 2100, to scale DAC up to a level where it can significantly reduce our net carbon emissions would require a quarter of the world’s entire energy demand, an inconceivable amount for just one industry. Furthermore, production of the CO2 sorbent (material which captures the gas) would require a much larger manufacturing effort to keep up with the demand(13, 15).

A further drawback is the cost of DAC. Previous estimates have had a large degree of uncertainty, mainly due to the different methods of DAC or whether the CO2 is stored or sold, however they all show that DAC is expensive. Most estimates state that it costs $100-1000 per tonne of CO2 removed from the atmosphere(13), though in Carbon Engineering’s paper, they estimate that their process will be cheaper at a cost of $94-232 per tonne(9). There are several other methods of removing carbon dioxide from the atmosphere (such as afforestation and soil carbon sequestration)(16), and despite the recent cheaper estimates, DAC is still among the most expensive methods of extracting CO2 from the atmosphere. However, it is hoped that as the technology progresses, the costs will be reduced as the method becomes more efficient, as seen with previous climate technologies such as solar and wind power.(15)

Despite the higher relative cost, DAC still seems one of the most feasible greenhouse gas removal options to implement on a large scale. The potential to remove CO2 from the atmosphere is great,  and the technology is already developed and available, all that remains is to scale up(6). Unlike other methods DAC can be implemented virtually anywhere, so DAC plants will not take up space which could be used for farming or other important activities. Furthermore, DAC requires very little land and water usage, something which some other methods of greenhouse gas removal require extensively(5). Therefore, the pros and cons must be weighed up when considering the jump to implementing DAC on a large scale.

In conclusion, it appears that DAC has great potential, but it should be used with caution and further refinements may be necessary. This is supported by computational models performed by Realmonte et. al (13). They concluded that we should not risk assuming that DAC can be deployed at such a large scale, as if we later find out it is not feasible at such a large scale after heavily relying on it, that would be catastrophic and lead to a large increase in global temperature(13). Therefore, the best approach would be to implement DAC alongside other methods, as opposed to instead of. This is also important as we should not use DAC as an excuse to live a ‘carbon-rich’ lifestyle, we should still be continually implementing policies to reduce our carbon emissions in other ways, as depending on DAC alone is extremely inadvisable.(15) Nonetheless, hopefully in the future DAC turns out to be feasible at the large scale as it has the potential to significantly aid our mission to reduce global warming.

 

Author: Oliver Pearce

 

References:

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  14. Climeworks. Climework’s solution is part of Microsoft’s plan to reach negative emissions: Climeworks,; 2021 [Available from: https://climeworks.com/news/this-negative-emission-plan-by-microsoft-marks-an-important.
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