The world is facing a major climate crisis and it is widely accepted that the biggest contributor to this crisis is the build-up of GHG (greenhouse gases) in the atmosphere. In the face of this, the IPCC (Intergovernmental Panel on Climate Change) advocates several methods by which we can cut emissions of greenhouse gases; for example, renewable energy, restoring nature and technologies that can capture and store carbon dioxide - commonly known as CCS (Carbon Capture & Storage). Despite this, atmospheric CO2 currently measures ~418ppm (parts per million) versus ~280ppm in the pre-industrial era. The IPCC acknowledges that current methods used to reduce emissions towards zero are not likely to be sufficient in keeping the cumulative GHG emissions to a level which would cap the global temperature rise to the desired 1.5 ˚C outlined in the Paris Agreement , or for that matter even 2.0 ˚C. With 350ppm seen as the threshold needed to avoid a climate tipping point with adequate certainty we have no other choice but to find ways to reduce overall atmospheric CO2 levels, even if we were to stop emitting CO2 .
DAC has a role as a CDR (Carbon Dioxide Removal) technology
CCS, CCU and CDR are all related concepts, but they are different in important ways. CCS is a set of industrial methods for the chemical capture of CO2 into a pure stream and its subsequent geological storage for the future. Carbon Capture and Utilisation (CCU) is a set of industrial methods for the chemical capture of CO2 and its conversion into products such as fuels, plastics, and aggregates.
CDR involves both the capture of CO2 from the atmosphere combined with durable storage (see Figure 3). This can be achieved by producing a useful product, for example, biologically in trees or in some geological storage underground that guarantees the captured CO2 is not reintroduced into the atmosphere for many years or even centuries (see Figure 2).
In short, CDR technologies reduce the level of GHG present in the atmosphere while CCS and CCU reduce how much GHG we add to the atmosphere.
The scale of this task cannot be overestimated. All scenarios for limiting warming to well below 2˚C involve removing hundreds of billions of metric tonnes of CO2 from the atmosphere over the course of this century.
The recognised CDR techniques by the IPCC are bioenergy with carbon capture and storage (BECCS), afforestation and reforestation, soil carbon sequestration, ocean fertilization, enhanced weathering, ocean alkalinisation and DAC.
What is DAC and what is its role?
Direct Air Capture is a technology that captures carbon dioxide directly from the ambient air and stores it underground or uses it in industrial processes. It involves a chemical (as opposed to a biological) reaction.
Several different DAC technologies are currently being developed and tested, and the broad categories are:
Solid sorbent-based: uses solid sorbents to capture CO2 from the air. The sorbent material is usually a metal-organic framework (MOF) or an amine-functionalised silica gel. The captured CO2 can then be released and collected for storage or utilization.
Liquid sorbent-based: uses liquid sorbents, such as amines or alkali metal hydroxides to capture CO2 from the air. The liquid sorbent is typically sprayed onto a contactor surface, where it reacts with CO2 to form a solution. The CO2-rich solution can then be processed to release the CO2 for storage or utilisation, allowing the sorbent to be used again.
Membrane-based: uses selective membranes to capture CO2 from the air. The membranes are typically made from polymers or ceramics and are designed to selectively permeate CO2 while blocking other gases. The captured CO2 can then be concentrated and processed for storage or utilisation.
Direct mineralisation: uses naturally occurring minerals, such as olivine or serpentine, to capture CO2 from the air. The minerals react with CO2 to form stable carbonates, which can be stored or utilised.
Cryogenic: uses cryogenic temperatures to capture CO2 from the air. The air is cooled and condensed to remove water vapor, then further cooled to capture CO2 as a solid. The CO2 can then be sublimated and processed for storage or utilisation.
Currently, DAC is an order of magnitude more expensive than alternative CCS technologies such as capturing CO2 from concentrated exhaust streams (i.e., power plants) or other CDR technologies such as BECCS, afforestation and reforestation or enhanced weathering.
However, DAC is a group of technologies which have advantages which cannot be easily replicated by other CDR approaches. These advantages include the ability to:
Close the carbon cycle for fuels – provide high-quality feedstock for carbonaceous energy carriers such as methanol, synthetic diesel, or gasoline. (This is certainly not a CDR application).
Create a carbon feedstock for industrial processes – for curing cement, feedstock for plastics, carbon fibres and composites, soil enhancers, etc. (This is not a CDR technology per se, but the carbon stored in the products will be kept out of the atmosphere for decades).
Offer insurance against gas leakage – DAC cannot stop acute damages associated with gas loss, but it provides a way to recapture leaked CO2. By attaching a financial cost to this, it can make the leakage of CO2, at least theoretically, insurable.
Avoid expensive pipelines that take a long time to build – DAC systems can be built close to remote sequestration sites, close to electrical power sources or close to the site of CO2 usage.
Minimise the land footprint compared to biogenic CDR technologies – DAC needs a relatively small land footprint, although the size of secondary facilities providing electricity, thermal energy and water, pipelines and injection wells (if the CO2 is to be sequestered) should not be underestimated. Indeed, powering 13 GtCO2/year DAC installations requires 100 million acres of solar energy according to the National Academies of Sciences .
Essential key attributes for a successful DAC implementation
A DAC solution requires the following five key attributes to succeed:
Low-cost air contactor that minimises pressure drop – the low concentration of CO2 in air (~418 ppm today) requires passage of large air volumes through the contactor.
Rapid sorption / desorption kinetics – fast sorption and desorption cycling times needs less sorbent for the same output; the sorbent must be able to withstand thousands of sorption-desorption cycles.
Low sorbent regeneration energy – the CO2 binding energy must be high enough to achieve a good uptake capacity, but not so high that the release of the CO2 has unacceptably high energy (heat) requirements; effective process design must minimise the mass of the DAC equipment that needs to be heated and cooled together with the sorbent without adding to the performance of the system.
Low capital costs – the expected lifetime of the sorbents plays a key role in minimising the lifecycle costs of the sorbents as well as the use of low-cost and readily available sorbents.
Challenges and promises of DAC
The primary challenge facing DAC technology is cost, with current estimates from studies and manufacturers predicting that DAC facilities would cost between $100 and $300 per metric ton of CO2 captured. Estimated costs have been going down rapidly and economies of scale, its associated learning curve and technological advances are likely to continue driving down the cost. Several DAC companies are claiming that massive roll-out of their technology would cost less than $100 per metric ton, but evidence of this is still scarce and unverified for the moment.
A widely cited application for DAC is its use as a feedstock for synthetic fuels, also known as PtL (Power to Liquid). The Clean Skies for Tomorrow study by McKinsey argues that there will be a need for up to 280 million metric tons of SAF (Sustainable Aviation Fuel) by 2040 (see Figure 3). Since every metric ton of PtL produced requires ~3.15 metric ton of carbon dioxide this would require that 882 million metric tons of carbon dioxide be captured, just to serve the aviation industry.
It is important to note that the use of CO2 as a fuel does nothing to reduce the amount of CO2 in the atmosphere – this can only be achieved through long-term storage. There are currently very few large-scale underground carbon storage facilities in operation, and the political interest in and public acceptance of such projects is still a concern, especially in highly populated areas such as Western Europe.
It is claimed by the proponents of DAC technology that DAC installations can be easily set up anywhere in the world, but this is only true in theory. A specific DAC installation can produce more GHG emissions than can be removed from the air across its lifecycle if the wrong implementation scenario is chosen. Their efficiency and potential for net GHG emission reduction is influenced by several external parameters. Indeed, one of the few available LCA (Life Cycle Analysis) studies concludes that the CDR potential of projects based on Climeworks technology, one of the leading DAC companies, is highly dependent on its location as well as the electricity and heat sources that it uses for its operation. The efficiency of DAC projects can swing from only 9% (Greece, grid, HTHP) to 97% (Norway, grid, waste heat) (see Figure 5).
A material advantage of DAC is that it allows for granular MRV (monitoring, reporting and verification) of net carbon dioxide removal, enhancing its transparency and attractiveness to investors who want to avoid a ‘greenwashing’ label.
Risks associated with DAC
The first risk that is important to highlight is not a technical risk per se, but more a political one. Non-biogenic CDR technologies such as DAC could potentially lead to false expectations and undeservedly be presented as an ‘easy’ solution to reduce the level of GHG in the atmosphere. With this being said:
Thermodynamics dictate that DAC is (and will stay) very energy intensive (see Figure 6). To capture a metric tonne of CO2, Carbon Engineering’s (a Canadian DAC developer) has an energetic requirement equivalent to either 8.81 GJ of natural gas or 5.25 GJ of natural gas coupled with 366 kWh of electricity.
Putting unwarranted expectations on DAC technology to take CO2 out of the atmosphere could lead to neglecting crucial emissions reduction.
The enormous investments required for DAC from an R&D, resources and capital perspective could hamper the efforts to reach a carbon neutral society through alternative ‘natural’ CDR solutions as it draws away crucial resources from solutions such as (re)forestation, soil improvement and BECCS (Bioenergy with Carbon Capture and storage). The capital cost of constructing the Climeworks’ Hinwil DAC system, which captures 900 tCO2/yr., was $3-4 million.
CDR technology requires that the captured CO2 be permanently sequestered, leading to general, storage related concerns:
Transporting and injecting CO2 into geological reservoirs for storage raises concerns about pipelines, CO2 leakage, seismic activity, and water pollution.
Companies like Carbfix have developed technologies to reduce the risks of CO2 storage but improved regulations and continued R&D will be necessary to ensure safe CO2 storage, as is the case for any CCS and CDR technology.
Burying CO2 has by itself no financial benefits, so DAC companies and their customers are incentivised to sell the CO2 providing no CDR benefit at all or to locate sites solely based on prevailing government incentives - rather than on the best implementation scenario for each technology.
The DAC lifecycle will itself produce CO2 and could become a problem, instead of a solution, if the permanent removal of CO2 is not properly rewarded. Currently there is no carbon price anywhere in the world large enough to make DAC with carbon storage economically viable as a standalone and commercial business case.
DAC pilot projects
In September 2022 there were only 18 direct air capture plants operating worldwide, capturing a tiny 0.01 Mt CO2/year . Oxy is in the process of building the biggest DAC plant in the world which is expected to capture up to 500,000 metric tons of carbon dioxide per year with the capability to scale up to 1 million metric tons at a projected cost of $1 billion, and CarbonCapture plans to be able to remove 5 million metric tons of carbon dioxide a year at its Wyoming facility by 2030 .
Compare this with growing energy-related CO2 emissions (increasing by 0.9% or 321 Mt in 2022), reaching a new high of over 36.8 Gt, and it is easy to see that DAC deployment needs to be scaled up dramatically to make a material difference in the amount of CO2 in the atmosphere.
DAC is not mentioned much in the scientific literature, where it makes up ~2% of the available CDR methods, but it does receive a lot of attention in the innovation and investment space, especially in the United States where proposed or demonstration hubs under construction account for the vast majority of (traceable) public funding ($3.5 billion). The Inflation Reduction Act (IRA) provides a directly paid credit for DAC combined with geological storage of $180 per metric tonne of CO2.
The exact percentage of GHG emissions that need to be compensated by CDR technologies will depend on a range of factors, including the level of ambition in worldwide emissions reduction targets, the rate of technological progress in negative emissions, and the ability and cost to implement these technologies at scale.
DAC technology companies
Many DAC companies and universities are focusing on producing better sorbents, including improved stability, longevity, and sorbent cycle duration. Others are developing enhanced contactors by maximising the sorbent surface exposure with minimal addition to heating requirements and minimising pressure drops while moving large volumes of air through the contactor. Research topics in this area are diverse, focusing on MOFs (Metal Organic Frameworks), aerogels, (honeycomb) monoliths, silica fibres and many others.
More and more attention is also being focused on engineering. For DAC to be able to remove gigatonnes of CO2 per year from the atmosphere, as would be required by 2050, it is essential that costs fall by standardising global supply chains, fundamental improvements in design and manufacturing of the technology are achieved (learning-by-doing) and the production of the sorbents is increased manifold. Large engineering and chemical companies are arguably best placed to take on these challenges .
DAC offers a promising solution for addressing some of the challenges associated with climate change. It has several applications in CCU (e.g., PtL), CDR applications and several pilot projects underway to scale up the technology.
Some of the main conclusions of our research are:
DAC installations do not require as much land as biogenic technologies, but we do need to make sure that we include all the secondary, supporting infrastructure.
One of the major benefits of DAC is that it can be used to capture carbon dioxide from any source, including emissions from industrial processes, transportation, and even natural sources such as volcanoes, making it a highly versatile solution for reducing greenhouse gas emissions.
Direct Air Capture is a natural companion for renewable energy sources such as wind and solar power to create a carbon-neutral energy system, but only in the right location with the right mix of electricity and heat sources.
DAC should not be used as a substitute for emissions reduction, since we do not yet fully understand which technologies are realistically suitable for massive upscaling (if any), what is a realistic time frame and what side-effects are to be expected.
DAC is currently only capable of capturing a small fraction of the carbon dioxide emissions that are produced each year. To make a significant impact on CO2 in the atmosphere, the technology must be scaled up significantly. DAC deployment at large scale requires massive capital investment and significant R&D spend, but a handrail for the enabling environment for cost reduction (knowledge, systems, frameworks, infrastructure, major public and private financing, and regulation) can be followed from the now successful solar PV and battery industries.
The enormous need for heat and low-cost power for GtCO2 scale DAC will have big implications on the future composition of the electricity sector, for which it is not prepared.
While several challenges remain, the potential benefits of DAC technology combined with other carbon removal methods must be recognised. DAC could play a vital role in mitigating global warming by providing negative emissions. We are very unlikely to reduce our CO2 to virtually zero, and even if we do, we will have no other option than to suck huge amounts of CO2 back out of the atmosphere through DAC. We must have DAC ready in our toolbox as we will most likely need it.
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