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Is Direct Air Capture the future?

Almost all scenario thinking that relates to the goal of net-zero emissions during the second half of this century has to consider the role of negative emission technologies. These are mechanisms and approaches which result in the removal of carbon dioxide from the atmosphere and its sequestration in the biosphere (e.g. trees) or lithosphere (i.e. geological storage). This is necessary because we are very unlikely to see out the century with a complete end to fossil fuel use, industrial processes and land change practices all of which lead to the release of carbon dioxide into the atmosphere. Further, the application of carbon capture and storage on facilities such as cement plants and steel mills won’t deal with remaining emissions from mobile sources such as aviation and shipping so removal elsewhere must be done to balance these remaining sources. In addition, many scenarios utilise negative emission technologies as a way to correct the overshoot of goals from earlier in the century, effectively mopping up carbon dioxide released earlier.

There are a number of ways in which carbon dioxide can be removed from the atmosphere, with the simplest being an expansion of the biosphere through reforestation. But as was illustrated in the Shell Sky scenario, even very large scale reforestation isn’t sufficient to balance ongoing fossil fuel use. Global reforestation of some 700-800 million hectares of land (an area the size of Brazil) shifted the outcome in 2100 from 1.75°C (midpoint of a range reflecting uncertainty) to 1.5°C, which required a sink of some 10 Gt carbon dioxide per annum (current fossil fuel use results in some 32 Gt of carbon dioxide emissions – Source: IEA).

In addition to reforestation, the Sky scenario utilises CCS for industrial facilities and incorporates bioenergy production with carbon capture and storage (BECCS) to act as a negative emission technology (see illustration below), giving a total geological based sink of about 10 Gt per annum. BECCS hardly exists in practice today but a 1 million tonne per annum facility is operating in the USA. The technology is well understood and effectively a commercial proposition given the right CO2 pricing system.

Apart from reforestation and BECCS, another technology exists to remove carbon dioxide from the atmosphere, known as direct air capture (DAC). This technology captures the carbon dioxide from the very low concentration in the atmosphere and then makes it available for use or geological storage (DACCS). A small demonstration plant is running in Iceland as part of a much larger geothermal power complex and I was fortunate to be able to visit it a few weeks ago.

Sitting within the ON Power Geothermal facility sits a single Climeworks air capture unit. It takes in air with a carbon dioxide concentration of some 410 ppm (ambient atmospheric conditions) and vents air with a concentration at about 100 ppm. An amine system acts as the sorbent and 4-6 times per day the unit recharges itself by using geothermal energy to heat the amine sorbent and release the carbon dioxide under controlled conditions.

That carbon dioxide then joins a larger carbon dioxide stream (from the geothermal plant) and is injected into the subsurface where it reacts with various minerals to form carbonates, effectively fixing itself into the geology.

This single unit captures and stores approximately 50 tons per annum of carbon dioxide, which is about enough to balance the emissions of eight Icelanders, but only three American citizens. This is very much a pilot unit for demonstration and proof of concept purposes, with plans by Climeworks for scaling the technology.

In the Sky Scenario we chose to use BECCS rather than DACCS as our negative emission technology because BECCS is visible and scalable today. This is because all the related processes and practices like biomass collection and use, geological storage and carbon dioxide transport are all scalable or have been scaled. As such, a scaled systems approach for BECCS could be envisaged in the decades ahead. In the case of DACCS, the scope is potentially huge, but the development pathway for this technology probably has some way to go. This is illustrated by the debate underway in academic circles about the cost of DACCS. DAC is challenged simply by the vast quantity of air that must be processed to extract every ton of carbon dioxide. Today, BECCS can be visualised as a cost effective technology whereas that is not yet the case for DACCS.

Ultimately DAC may also have another use, that being the manufacture of synthetic fuels and materials. Even if society eventually stops extracting fossil fuels, it’s very unlikely that we will stop using hydrocarbons, they are just too useful. But manufacturing them from scratch needs a source of carbon and a source of hydrogen, both of which could come eventually come from renewable energy powered processes. For carbon, it would be DAC and for hydrogen it would be electrolysis of water. Combined and with enough energy, you can make pretty much anything. But scaling this technology is a daunting prospect, which I wrote about a few years ago with reference to the manufacture of synthetics Jet A1 for aviation. All of these technologies will also require years or perhaps decades of development to see significant cost improvements emerge.

It was fascinating to see this technology in action, albeit at a very small scale. Whatever finally emerges, it probably won’t look anything like the plant in Iceland, but we shouldn’t underestimate the ability to innovate in the face of real need and commercial opportunity. It will likely take a long time, but later in the century it may well be the case that planes are flying on air in more than one sense.

Further reading: For a very comprehensive look at greenhouse gas removal technologies, a recent report from the Royal Society is worth a look.

Note: Scenarios are not intended to be predictions of likely future events or outcomes and investors should not rely on them when making an investment decision with regard to Royal Dutch Shell plc securities. Please read the full cautionary note in


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