Ciaran McKeon, of Frazer-Nash Consultancy, explores the environmental potential of direct air capture technologies that actively remove CO2 from the air
The Committee on Climate Change’s Sixth Carbon Budget is clear that to achieve the UK’s net zero carbon emissions goal by 2050 will require engineering solutions to remove greenhouse gas emissions. Moreover, as the November date for the United Nations’ Climate Change Conference (COP26) in Glasgow draws ever closer, member nations’ will be considering how to capture emissions in their own plans to meet the targets of the Paris Agreement.
Enter direct air carbon capture and storage (DACCS), one of the negative emission technologies (NETs) highlighted by the Sixth Carbon Budget and being trialled around the world. It‘s a simple solution: direct air capture (DAC) technologies use low carbon energy to take carbon dioxide (CO2) emissions straight from the air for either storage in geological sinks or reuse. But as the technology is in its infancy and unproven at scale, does DAC offer a silver bullet to the climate change conundrum?
What is direct air capture and how does it work?
DAC technologies remove the CO2 content from ambient air – effectively, they act as an artificial tree. However, while some of the CO2 captured by a tree can be released into the atmosphere when it dies, all of the CO2 captured by DAC can be permanently stored (or re-used). ‘Traditional’ Carbon Capture and Storage (CCS) will capture carbon dioxide from the flue gas of large industrial sources, and can support the generation of energy such as hydrogen; whereas DAC captures carbon dioxide from ambient air. At present, DAC companies are using a process that chemically separates the CO2 from air[1] – although both cryogenic processes, which freeze the CO2 out of the air, or membrane technology could be possible alternatives.
DAC is a four-stage process: in stage 1, the ambient air is directed towards the material used to adsorb or absorb the carbon dioxide – the sorbent. This can be enhanced by using fans to pull the air through. In Stage 2, the CO2 comes into contact with a capture agent, which can be either a liquid or a solid, that captures it from the air. Structured packing maximises the contact between air and the capture medium. At the third stage, the CO2 is released from the capture agent into a storage facility. From an environmental point of view, it is important that this release can be achieved in such a way that the capture agent can be re-used repeatedly. Finally, the high-purity CO2 is compressed, before transportation to either a sequestration site – it can be permanently stored within geological formations such as saline aquifers or depleted reservoirs – or for reuse. Carbon dioxide can be used in a range of industrial processes: in chemicals’ and refrigerants’ creation; in the food industry as a component of fizzy drinks; or as a feedstock for synthetic fuels (the current CO2 market is 230 Mt per year[2]).
DAC systems, are less limited by location, require less space and use less water than other negative emissions technologies
Why not just plant trees? Planting trees – afforestation – can be a complementary greenhouse gas removal option and offer a number of environmental and social benefits. However, in areas where land space is at a premium, like the UK, trees may end up competing for land space with food production, potentially resulting in increased food prices. ‘Artificial’ trees, the manufactured DAC systems, are less limited by location, require less space and use less water than other NETs.
What is the future role of DAC?
Focusing on energy efficiency, developing renewable and nuclear power, and investing in ‘traditional’ carbon capture, utilisation and storage (CCUS) remain the most viable options in reducing CO2 emissions and meeting our climate change targets. However, as CCUS technologies are further developed, DAC plants could easily feed into the same transportation and storage infrastructure, offering an additional means of increasing CO2 capture.
The aim of the UK’s net zero cluster approach is either for areas to exist that produce no CO2, or to offset the CO2 that is produced by using NETs such as DACCS. The current technologies being developed to capture carbon at source are aiming for efficiencies of approximately 95%: could DAC be used to capture the residual 5%, enabling net zero to be achieved within a ‘cluster’? This is already being considered by technology provider, Carbon Engineering, which wants to develop a commercial scale DAC plant, potentially linked to its industry- and government-funded Acorn CCS and hydrogen project in the North East of Scotland[3].
Another use for DAC could be in more rural areas, where the concentration of industrial CO2 sources is more sparse and thus ‘traditional’ CCS is not an option? The construction and operation of a DAC plant would increase employment, whilst captured CO2 could be reused to increase the yield of greenhouse crops, supporting the local farming community. As the UK looks to become a global leader in renewables, there is also the potential for DAC be used flexibly within the wider energy system. For example, when demand for energy is lower, surplus clean energy could be used for carbon capture through the DAC process described previously.
While DAC, to date, has focused on the capture of CO2 from air, further developments in this technology could offer the opportunity to expand its remit. Methane (CH4), for example, is a heavy contributor to the global warming effect, and is considerably more potent than CO2: removing one molecule of CH4 could potentially deliver more impact than removing one CO2 molecule. Research into this area is limited at present, in part due to methane being much less concentrated in air than CO2, and in part due to the lack of revenue opportunity. But, as climate change climbs higher up world government’s agendas, this is a potential area that may warrant further exploration.
Is DAC ready for use?
DAC technologies are currently estimated to be at Technology Readiness Levels (TRLs) of between 4 and 7. A number of developers believe that active megaton capacity DAC plants, capturing 1 MtCO2/year and with a thirty-year lifespan, could be running at a viable cost of $100/tCO2 within the next 10-15 years. Indeed, Carbon Engineering, Climeworks and Global Thermostat have concepts already at demonstration and pilot scales, capturing up to 1000 tCO2/year.
If we are to meet our global carbon budgets, it seems likely that NETs such as DAC will be necessary
As is often the case, the main driver in whether DAC will be a noteworthy contributor in the greenhouse gas removal arena will be cost. Investors will need to judge whether this technology is commercially viable, based on their assessments of technology cost reduction and market conditions. This can often be a challenge for novel technologies, and to help clarify and de-risk their investment, they will need to ensure that their due diligence includes independent verification, through technical assessments of both the engineering and commercial aspects of equipment and infrastructure.
Governments will also have a role to play in the success of DAC technology. They must not only set binding emissions targets and provide innovation funding to support a nascent industry but also create robust policy that considers emissions removals in the price of carbon and supports the development of a market for NETs. This could create the right conditions for DAC to demonstrate its potential.
While DAC may not offer a silver bullet solution to achieving all our global climate change targets, neither is it a red herring. If we are to meet our global carbon budgets, it seems likely that NETs such as DAC will be necessary. It is one element in the jigsaw that will deliver the energy transition, and will need to fit in alongside other parts of the puzzle to produce a full picture of decarbonisation.
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[1] David Sandalow, Julio Friedmann, Colin McCormick, Sean McCoy. Direct Air Capture of Carbon Dioxide ICEF Roadmap 2018. Tokyo : Innovation for Cool Earth Forum (ICEF), 2018.
[2] IEA. Putting CO2 to Use. IEA. [Online] September 2019. [Cited: 25 October 2020.] Paris. https://www.iea.org/reports/putting-co2-to-use.
[3] ‘UK’s first commercial-scale direct air capture plant could be built in Aberdeenshire’. Aberdeen Business News. [Online] Aberdeen Business News, 17 September 2020. [Cited: 22 October 2020.] https://aberdeenbusinessnews.co.uk/uks-first-commercial-scale-direct-air-capture-plant-could-be-built-in-aberdeenshire/.
An interesting viewpoint on storage of CO2. Hwoever, in the real world, man is emitting about 34 Gt/y (BP 2020 report) . To remove even the 1 % of this (the most that the UK could be blamed for), would be 340 Mty : a scale up of 340 times.
It is also worth noting that trees can be planted anywhere in the world with equal effect as the atmosphere is well mixed regarding CO2, so there is no need to conflict with dense populations and useful financial terms could be agreed with some developing countries where more trees would be really beneficial and money gained. Equally true of the robotic trees advocated.
Mechanical means may well become cost-effective if someone somewhere decides to pay for it, but why go for this instead of natural biology?
Zillions of years ago, it was trees that took all the CO2 from the atmosphere and replaced it with Oxygen. Even though we have spent the last 200 years digging up those old trees and burning them we can still have the option of reforesting large areas of the planet that’s risk-free and beneficial to all effects of climate change such as water management and soil erosion.
Full marks for the Engineering – but I think Nature has a better answer.
Add in the recent Rice University system for making graphene downstream.
https://www.youtube.com/watch?v=hzm5AMPFMqs
Hidebound thinking tends to dismiss such ideas but in reality, try thinking it through taking the employment & manufacturing elements as lead aspirations. Loss of employment under current job types can be replaced with a variety of new ones at many skill levels to bring these novel solutions to life. Investment needs people to be alive as consumers to close the loop. No people, no return on capital, so why invest? To make a difference of course. Crudely the Government of the day will have to issue Bonds to get the investment going
How much energy does it take to drive the fans? What are the magic adsorbers / absorbers used and how is the CO2 released from them (more energy?)
The article contains absolutely no information of the efficiency of what has been achieved today and what is projected in the future. Written for a readership of engineers?
I don’t think so.
Until I see published details of the heat and mass balance, along with costings and economic justification I am dubious about the benefits of carbon capture even for the ‘low hanging fruit’ that is end-of-stack flue gas, at perhaps 10% (100,000 ppm) CO2 content. The idea of extracting CO2 from a source which is 250 times more dilute (i.e., atmospheric air @ 400 ppm) is risible. The fans alone to move 2,500 tonnes of air per one tonne of CO2 extracted will consume a colossal amount of energy (and associated carbon footprint) ~ and that is before considering the balance of plant or the small matter of what to do with it once collected. Ciaran McKeon’s suggestion that it could be used for carbonated drinks is half-baked: not only is the volume, in global terms, nugatory but he does not appear to release the CO2 is still released to atmosphere – either from the glass or sooner (or rather) later from the drinker
Frankly I’m disappointed to see this report from a respected consultancy such as Fraser-Nash
I’m always puzzled why afforestation means the same as forestation (planting trees) https://www.yourdictionary.com/forestation, when asymmetric means lacking symmetry and asymptomatic means without symptoms
Investors should realise that to do nothing will result in there being (eventually)no humans left to breathe/fart less CO2 as the climate won’t support life but hey ho,their bank balances will be healthy.
So would you put YOUR pension savings into this project?
I fully agree with Trevor’s comments and was similarly discombobulated by the use of “afforestation” in an apparently opposite meaning from what one would expect. Isn’t the English language magnificent!
Direct air capture of CO2 is proven technology, and when it is used to produce fossil free hydrocarbons with the Fischer-Tropsch process, the products will pay for the cost of green electricity to make the hydrogen and run the plant. See https://ineratec.de/en/home/ CCUS used to allow the continued used of fossil methane is only up to 95% efficient and the cost of deep geological storage not proven at scale, however it is a valuable stepping stone to allow the production of blue hydrogen until 2050 when it must be phased out. In the UK over 2.5 million tons of hydrogen is produced in refineries and petrochemicals every year which cannot be stopped over-night, so this tech is valuable to reach net zero fossil carbon emissions by 2050.
In simple chemistry terms 16 kg methane produce 6 kg hydrogen and 44 kg CO2. The energy conversion is low and the hydrogen cost at present is about 3 times that of the methane going in. With, as yet unproven large scale CCS, the cost is increased further and efficiency reduced.
Unless a massive valuation is put on the CO2 (a tax that will have no benefit for the people taxed, but only to “save the planet”), hydrogen is going to impoverish the western world and drive more industry East.
DAC could be useful would to make biofuels , such as used at the Drax power station, carbon neutral; as otherwise CCCS at gas fuelled power stations would be a more obvious (and easier?) choice for carbon neutrality.
Running costs are part of affordability but there needs to be adequate storage (which would compete for hydrogen storage) and so details about the capacity, in the UK, for storage – and the number of years before these are full are really necessary (no point of capturing CO2 if there is no storage)
Julian.
Bad example, Drax uses wood pellets that are sourced by deforestation – Greenwash.
Trevor is correct, the claimed benefits should be quantified, extracting anything from a low concentration source is inherently less efficient than from a high concentration one.
For how many years would a system have to operate to capture all the CO2 emitted to manufacture & install it?
The late David McKay’s “Sustainable energy without the hot air” (available to read for free on the net) shows the rankings of options & much more besides. DAC appears to be a distraction. Why build them while coal is still burnt?