A Direct Air Capture plant starts up in Iceland

A recent story in the Washington Post (1) celebrated the start up of a climate busting facility in Iceland that removes carbon dioxide from air allowing it to be either stored underground or recycled as a feedstock allowing the production of climate neutral products.

The Climeworks Direct Air Capture facility in Iceland (from climeworks.com/orca )

The new facility, which could prove to be the first of many installed around the globe, is an example of Direct Air Capture (DAC). A simple overview of the process is provided in the diagram below. Ambient air, which currently contains about 410 ppm CO2, is drawn through a filter that removes CO2 while allowing the other constituents in the air to pass through. Once the carbon dioxide filter is “full” it is heated (to 100 C in the diagram) causing the trapped CO2 to be released in essentially pure form.

CDR Technologies

DAC is one of several Carbon Dioxide Removal (CDR) technologies that have been flagged by the UNIPCC (2) as potentially important tools in meeting global climate goals. The key part of this technology grouping is that they enable CO2 to be removed from the atmosphere. This approach becomes increasingly important as the deployment of other critical decarbonisation pathways such as renewable generation, electric vehicles and industrial hydrogen use is delayed or proceeds too slowly to prevent dangerously high levels of carbon dioxide accumulating in the atmosphere. Put simply, the longer we continue to emit significant quantities of CO2 the greater the need for CDR technologies to stabilise the atmosphere at a level consistent with climate targets

Other CDR technologies include :

Afforestation – growing trees to sequester carbon on grasslands and other areas that have not historically had forests.
Reforestation – growing trees (native or otherwise) on land that has previously supported forests
Bio Energy with Carbon Capture and Storage (BECCS) – a complex system that involves growing energy crops for power generation with the resultant carbon dioxide captured and geologically stored
Soil Carbon optimisation – maximising the amount of carbon that can be stored as organic matter in the soil

All of the approaches above are based on the fact that trees, crops and vegetation convert atmospheric CO2 into cellulose or plant matter more generally. In the case of BECCS a bio crop is harvested, dried and used to produce electricity with the post combustion CO2 stream being stored or recycled in the same way as the output of a DAC facility. In soil carbon optimisation, agricultural and conservation practices are modified so that carbon from decaying plant matter is preferentially retained in soils rather than generating CO2 which adds to atmospheric levels.

CDR – scale and land use

One of the key points to note when considering CDR technologies is that to meaningfully move the climate needle they will need to remove CO2 at a gigatonne scale. With annual anthropogenic CO2 emissions of about 35 gigatonnes, around 1,000 gigatonnes of CO2 has been added to the atmosphere in the three decades since the UNIPCC was first established to investigate climate change. This figure obviously continues to climb. A CDR sector with a cumulative removal capability of less than, say 2 gigatonnes/year will clearly be incapable of making a major contribution.

At this scale, afforestation, reforestation and especially BECCS, will require a massive land area. The 2018 UNIPCC report (2) which outlines potential decarbonisation pathways includes a scenario in which an aggressive BECCS roll out requires 7.2 million square kilometers to produce bio energy crops. If you think this sounds a lot then you are right! This represents almost 25% of the globe’s arable land.

This may be an extreme scenario but the message is clear, relying on growing lots of trees, whether to simply store carbon or as a feedstock to power generation facilities requires lots of land with an obvious tension and competition with food production.

Among CDR technologies, DAC is the only technology that does not compete for arable land and depending on how it is configured should have relatively modest land requirements. For this reason alone, proponents of DAC see it as an important tool in reaching net zero carbon.

The WaPo article reports the newly commissioned facility will remove about 4,000 tonnes of CO2/year. This is obviously many orders of magnitude below what might be needed but all technologies need to start somewhere so the Icelandic plant should be considered a small pilot facility rather than a major decarbonisation asset.

DAC – cost and location

The Climeworks demonstration plant is not cheap to run. The cost of capturing CO2 is reportedly around US$700/tonne CO2. If the technology is more widely used one would reasonably expect the costs to decline steeply as has happened with wind and solar generation. Even if the cost of DAC based CO2 removal drops to US$100/tonne, there will still need to be either a subsidy for removing carbon ( i.e. some sort of carbon tax) or the value of CO2 as a raw material needs to cover the cost of its capture. Perhaps there will need to be a combination of both.

Given atmospheric CO2 levels are essentially constant around the globe, from an air input perspective there are no constraints or advantages to where future DAC facilities should be situated. It would, however, be beneficial for large-scale DAC deployments to have access to both cheap, green electricity to power the fans used to draw air into the carbon removal vessels and industrial waste heat for the “desorption” step releasing the captured CO2 from the filter system. In summary, rather than competing for arable land with food production as will be the case for other CDR technologies, DACwill ideally be sited next to a major renewable energy hub that generates and supplies green hydrogen to adjacent industrial facilities such as steel and cement production.

The optimum location for DAC will also depend on how the captured CO2 is to be treated. There are two different strategies – geologic storage and conversion into usable products. Each of these has its own set of requirements.

Captured CO2 as a raw material

The thought of taking harmful CO2 out of the atmosphere and using it to produce a valuable and climate friendly product is very appealing but one that has to overcome a few hurdles. Firstly CO2 is a very stable molecule that lasts for thousands of years in the environment, so is not the obvious choice as a building block for more complex chemical products. This is not an insurmountable problem but means producing common organic commodities such as plastics, paints, dyes and glues will not be cheap and even in a fossil fuel constrained world there may be cheaper production pathways. Another consideration is that if DAC is built out at a gigatonne scale the quantity of CO2 produced would be so great that it is hard to conceive of a range of products that could realistically consume this vast quantity of CO2. Having too much of a raw material doesn’t sound like too great a problem but if the challenge is to avoid geologic storage then it could become problematic.

While there may well be opportunities to convert captured CO2 into stable, valuable products, the WaPo article’s comment that it can be used to “feed plants” (presumably increasing hothouse CO2 levels) or to add “fizz” to carbonated drinks needs to be taken with a grain of salt.

One counter intuitive application that has received some publicity is combining the captured CO2 with hydrogen to produce synthetic fossil fuels. Obviously using these fuels would generate CO2, so the idea of putting the CO2 back into the environment after all the cost and effort to remove it seems a bit strange. The rationale for this suggestion is that it may not be possible to completely eliminate all fossil fuels, even in a best case decarbonisation world. If, for example, battery powered planes are not considered safe, the aviation industry may be forced to continue using hydrocarbon fuels. Rather than keep a small fraction of the existing oil and gas industry open to supply this requirement, captured CO2 could be used to produce a carbon “neutral” fuel.

A schematic of the proposed process is given below – this highlights how DAC in this application would be integrated with renewable generation, green hydrogen production and a synthetic fuel facility. This starts to look like a major industrial complex with significant employment potential.

Schematic of DAC facility combined with synthetic hydrocarbon production using captured Carbon Dioxide (3)

At this stage, production of synthetic diesel or aviation fuel is just a suggestion, one that face opposition from both advocates who reject the notion that fossil fuels will be needed in a decarbonised future and those promoting the use of starch, vegetable oils or cellulose as a superior feedstock if synthetic fuels are required.

DAC with Carbon Storage

The alternative DAC configuration is to inject the captured CO2 into stable geologic formations. In this arrangement it becomes a Carbon Capture and Storage (CCS) technology. There are some advantages and disadvantages to this approach. The advantages include the fact that this is a simpler configuration (if one considers storing gigatonnes of CO2 underground simple) as it doesn’t require integration with a downstream CO2 reprocessing facility making diesel, carbon fibre or something else. There could also be a potential advantage in being able to partner with remote electricity generators, providing a guaranteed power offtake and minimising transmission requirements.

The biggest technical constraint for DAC with underground CO2 storage is that it should be logically built adjacent to suitable geologic formations to minimise costs to build transport pipelines. Ideally the CO2 would be stored under the land owned and controlled by the DAC operator. While this will rule out plenty of potentially attractive locations, data from the US Department of Energy suggested there should be ample areas with sufficient storage capacity as indicated in the figure below

Areas the US Department of Energy considers suitable for carbon storage (4)

Political opposition to geologic sequestration will be a major hurdle for DAC with storage. Some of this opposition will be local pushback to large quantities of CO2 being stored below houses, schools and businesses and there will also be opposition to industrial scale DAC installations being built and operated close to wilderness and national parks. One imagines that these legitimate concerns can be reasonably navigated and the experience using CO2 for enhanced oil recovery suggests the risk of catastrophic CO2 release is actually extremely small.

There remains, however, a section of the climate community that is strongly opposed to CCS on more strategic grounds, believing that the fossil fuel industry is promoting coal and gas power plants fitted with CCS as a ploy to allow these plants to continue to operate. Political opposition based on CCS being a trojan horse for the fossil fuel industry remains an important hurdle for DAC proponents to overcome. Perhaps this will encourage initial DAC focus to be on using the captured CO2 as a feedstock, which seems to be the track Climeworks is taking. If, however, GHG emissions continue to track above the levels needed to remain below 2 C, anti CCS commentators may need to recognise that DAC with geologic storage is needed to achieve climate targets.

What to look for next

CDR technologies including DAC will get some publicity in the upcoming COP26 meeting in Glasgow but it is likely to remain a background issue until emissions from power generation and light vehicles are substantially eliminated. This work is the obvious pathway for the first part of the journey to zero carbon. DAC and CDR more generally will be needed when wind, solar and electric vehicles have halved carbon emissions and global focus turns to the second phase of the decarbonisation process. Substantial deployment will need the political and economic support that is currently enjoyed by renewables and other more mature decarbonisation options.

DAC will be seen as a neat technical solution by some, excess CO2 being simply drawn out of the atmosphere and safely stored or reused, supporting the continuation of a low/zero carbon version of the lifestyle currently enjoyed by many in the developed world. Activists convinced that the current high growth, high consumption global economic model is flawed will not want to see DAC used in this way. They will want to see deep cuts to both GHG emissions themselves as well as the consumption of energy, food and natural resources first, with DAC likely to be viewed as a final step or even a last resort. This debate is a decade or so away but the battlelines are starting to be drawn.

Finally, it is not obvious that synthetic diesel or other carbon based products will be manufactured from captured atmospheric CO2 but it is an interesting subplot that highlights the interconnectedness of deep decarbonisation. The level of CDR and DAC needed will depend, as discussed above, on progress with other decarbonisation pathways. BECCS will be the preferred CDR approach if zero carbon baseload electricity is needed, though nuclear energy could be the preferred baseload option for nations not willing to allocate arable land to biocrops. If new base load is not a priority, presumably because a combination of renewables plus storage proves adequate, DAC would be a more palatable option to both balance out residual GHG emissions or to decrease CO2 levels if they have exceeded levels required for climate stabilisation.

As is the case with many aspects of the journey to zero carbon there will be elements of both technical innovation and political trade offs still to play out for CDR and DAC. The start up of the Iceland facility is a positive first step for DAC providing policy makers with real life data for a technology that will need to be considered in the future.