All’s fair in the fight against CO₂ emissions. One strategy is Direct Air Capture (DAC), where ambient air is sucked in and channelled over a liquid or solid chemical sorbent that selectively removes CO₂ from the air. Calcium carbonate is produced in several chemical reaction steps, which can be used to obtain carbon dioxide in a concentrated fashion and stored underground. The chemical sorbent is regenerated during the process and reused. Finally, the CO₂-free air is returned to the atmosphere.

That all sounds like a seductively simple solution to a complex problem. Particularly in the USA, there have been vociferous calls for such a technological solution to reduce the CO₂ content in the atmosphere.

Ineffective, inefficient

However, in the first in-depth scientific study on Direct Air Capture, an international committee of thirteen renowned scientists, including ETH-Zurich professor Marco Mazzotti as one of two European authors, now reveal that the method is currently not an option to reduce the CO₂ content. The scientists conclude that DAC is too ineffective and inefficient to mitigate climate change – and this won’t change much for decades to come, either. DAC will only play a minor role in reducing the CO₂ content, the researchers state.

With this study, which the scientists spent two years working on, they make it clear that there is little sense in extracting CO₂ from the ambient air with a major technical process. Instead, the greenhouse gas can be separated at large-scale sources, such as coal-fired power plants, much more cost-effectively and energy-efficiently since the CO₂ there is 300 times more concentrated.

Costs ten times higher

It currently costs ten times more to capture one ton of CO₂ from the air than directly at a large-scale source. ‘With the study, we’ve shown that DAC with chemicals only makes sense if all emissions from power plants or industrial installations are prevented first,’ says Mazzotti.

Moreover, DAC also requires energy to drive fans that suck in the air and channel it through the sorbent, for instance. Unless this energy comes from carbon-free sources, the net yield of CO₂ captured is rather poor. ‘When you capture a certain amount of CO₂, you need a certain amount of energy,’ explains the ETH-Zurich professor. This leads to an increase in the amount of carbon dioxide emitted, which is not just bad news from a climate perspective; this ratio also increases the net costs for capture from the ambient air. ‘The energy needed to operate such a plant therefore has to come from renewable energy sources,’ stresses the ETH-Zurich professor; ‘otherwise it makes no sense.’ This especially applies to the USA, where a lot of energy is still produced from coal.

At present, DAC plants would be extremely resource-intensive. After all, no one has built a large-scale plant yet; only a few laboratory plants and prototypes are in operation. In order to be effective, however, the plants would have to be constructed on an industrial scale. They would have to be of gigantic proportions, says Mazzotti, not to mention need incredible amounts of sorbent.

Technically feasible

The study does not doubt the feasibility of DAC. ‘Extracting CO₂ from the air, absorbing it and storing it in the ground is doable,’ states Mazzotti. However, the problem is that carbon dioxide in the air is so diluted that we would need a sorbent that can provide strong bonds with CO₂. What’s more, this binding agent also needs to be easy to recycle so as to facilitate thousands of cycles. ‘We mustn’t lose these chemicals,’ says the ETH-Zurich professor, ‘which is easier said than done’. There is nothing in the literature to suggest there are better materials or chemicals for this purpose.

DAC has primarily been put forward as a potential solution to climate change in the USA. A handful of people support the method; renowned journals are publishing articles on it. There has also been an ETH-Zurich spin-off based on DAC. With a view to assessing the further development and use of resources, the American Physical Society (APS) was thus asked to compile a study. ‘This we did based on scientific principles and as balanced and fairly as possible,’ says Marco Mazzotti.

Technology for mid-twenty-first century

DAC will only become interesting in a few decades’ time. ‘One day we should have power stations where CO₂ isn’t even produced in the first place. Or we won’t have coal-fired power plants anymore,’ says the professor. Only once all the possibilities of preventing and reducing CO₂ emissions have been exhausted will the method become interesting.

Whether DAC plans will now be shelved, however, Mazzotti cannot say. He won’t be conducting any research on it personally, but he is sure that other research teams will continue to work on the topic. Only, the study also reveals that no amount of tinkering will help DAC make a breakthrough. What it needs is a technological leap in the materials used as sorbents. Something radically new needs to be found here.

Further reading:

Direct Air Capture of CO₂ with Chemicals. A Technology Assessment for the APS Panel on Public Affairs, April 28, 2011.