Up in the air 

Achieving net zero by 2050 has become the predominant priority of the 21st century. 

However, it presents a formidable challenge. There is simply no single silver bullet for the energy transition. While transforming the grid with clean energy and energy storage solutions is essential, several other critical components are necessary to close the gap and effectively combat global warming. 

Among these new technologies, carbon capture is poised to play a key role. In the words of the Intergovernmental Panel on Climate Change (IPCC), carbon dioxide removal is “part of all modelled scenarios that limit global warming to 2°C or lower by 2100.” 

Again, this segment of clean technology innovation comprises various sub-segments that are each crucial. Typically, carbon capture focuses on capturing carbon dioxide emissions from industrial sources. However, other branches, such as direct air capture (DAC), target atmospheric carbon dioxide, further diversifying the strategies available to combat climate change.  

Indeed, each strand will play its part in combatting climate change, as Dr Steve Smith, executive director of CO2RE – a UK Research and Innovation-funded gateway to research expertise on greenhouse gas removal (GGR) – and Oxford Net Zero at the University of Oxford, explains. 

“Global temperatures will keep rising for as long as we emit CO2 from fossil fuels,” he states. 

“Staying within climate limits means getting those emissions down to zero within a generation. We can definitely get most of the way there, but there might be a residual level of emissions, say 10%, that prove very hard to eliminate. Here, active CO2 removal will be very useful to balance out those emissions to reach net zero.” 

In the case of DAC, the premise is straightforward: extract CO2 from the atmosphere and either bury it underground or convert it into saleable products. After years of laboratory tests and experiments, these efforts have recently begun to gain commercial momentum. Indeed, according to the International Energy Agency (IEA), 27 DAC plants have been commissioned worldwide, with plans for 130 large-scale facilities currently in various stages of development. Furthermore, according to the Yale School of the Environment, there are now about 20 plants in operation across Europe, Canada and the US. 

The latter of these countries is playing a particularly prominent role in spearheading DAC expansions at present, with the US Department of Energy (DOE) having set aside US$3.5bn to support the development of four commercial-scale DAC hubs. As of April 2024, two of these facilities have been announced in the form of the South Texas DAC Hub in Texas and Project Cypress in Louisiana, with the former expected to begin absorbing 500,000 metric tons of atmospheric CO2 each year come 2025 – 125 times more CO₂ than the next-largest direct air capture plant. 

To fully grasp DAC’s potential, it’s essential to understand how the technology operates. Here, Benjamin Shindel, a specialist in materials for carbon capture at US-based Northwestern University, offers a succinct overview. 

“DAC operates by pulling air onto a solid adsorbent or a liquid absorbent where the CO2 is physically or chemically trapped,” he explains. “Once the sorbent is saturated, it is regenerated, typically with heat, pressure, or chemical means, releasing CO2 so that it can be concentrated and then sequestered permanently, generally through geological storage or conversion into useful products.” 

From a filtration and separation perspective, there are several ways in which this process is executed. As Shindel highlights, one method draws air through a filter using a fan which then traps the carbon dioxide particles. Once this is full, the collector closes, with temperatures then rising to 100°C, causing the filter to release the CO2 so it can be collected. 

Meanwhile, the second strategy developed by British Columbia-based Carbon Engineering absorbs CO2 into a liquid, before transferring it into limestones pellets using much higher temperatures (approximately 600°C). 

Of course, each method has its benefits and drawbacks. The first, for example, typically requires high amounts of energy, while the capture units are the size of shipping containers, making it hard to host several in one facility. Likewise, the second method – despite being well suited to large operations – requires a lot of water. 

However, it’s not just capturing the carbon that provides points of contention. “Additionally, what to do with the CO2 once it has been captured is another thorny problem,” Shindel continues. 

Currently, there are several different approaches. In terms of geological storage, Swiss DAC specialist Climeworks combines its DAC technology with CO2 storage (known as DAC+S) to safely bury captured carbon into suitable repositories deep underground – a permanent removal solution. Alternatively, companies like Airbus are pioneering efforts to convert captured CO2 into fertilisers, fuels, and carbon-negative materials. 

While both unique, innovative, these processes again come with their own problems. “Transporting CO2 long distances in pipelines to reach geological storage facilities is challenging, whereas converting it into fuel ultimately results in a net zero process, rather than a negative emission process, and these fuels are not yet cost-competitive,” Shindel explains. 

The cost conundrum 

Undoubtedly, there are several DAC-centric challenges which need to be overcome for the technology to become a truly critical and broadly adopted component of global net zero efforts. “Perhaps the most critical challenge is the need to reduce DAC costs substantially – efforts that will require many years of research and engineering design in scaling up DAC facilities,” Shindel explains. 

Indeed, the figures are staggering. According to Yale, constructing and operating an air capture plant costs approximately 50 times more per ton of CO2 removed compared to planting trees. Government incentives, such as the US$3.5bn funding from the US DOE, aim to mitigate this gap. However, unless these costs decrease, the economic viability of DAC will continue to face a substantial uphill battle. 

“With renewable energy technologies becoming remarkably economical over the last few years, DAC is currently a less cost-effective method of lowering emissions than building out renewable infrastructure,” Shindel comments. “This will change (perhaps rapidly) over the coming decades as the transition away from fossil fuels ramps up. However, another challenge will be making sure DAC technologies are used appropriately, to lower emissions rather than offset increased fossil fuel use,” he adds. 

Given that the IEA’s pathway to net zero by 2050 requires more than one billion metric tons of CO2 to be pulled from the air annually – a target that will require tens of megaton-scale plants to be built every year – it is vital that DAC becomes a more commercially attractive technology quickly.

Scaling up efforts to explore DAC innovation 

Fortunately, there are several innovative efforts underway, with various stakeholders striving to bridge the current gap. One of the most promising opportunities for DAC lies in the production of synthetic fuels, a sector highlighted by the IEA as advancing early commercial efforts to create aviation fuels using air-captured CO2 and hydrogen. With the agency’s net zero pathway estimating that around 35% of air-captured CO2 is projected to be utilized for synthetic fuels by 2050, it is vital that efforts in this arena from innovators such as Canada’s Carbon Engineering and UK’s AtmosFUEL continue to progress. 

In addition to this, several innovator schemes have been deployed to find new potential markets and applications for DAC, as well as addressing key challenges such as cost. Here, the US DOE’s Carbon Negative Shot program stands as a prime example. Launched in 2021, it aims to reduce the cost of direct air capture to below US$100 per metric ton – a feat that would be a significant drop from current estimates that range between approximately US$600 and US$1000 per metric ton. 

These efforts are not limited to the public sector; several companies are also stepping up to the challenge. Exxon Mobil, for example, announced in April 2024 that it is actively developing technology to directly remove carbon dioxide from the atmosphere, with the ambitious goal of halving current high costs associated with DAC. 

Meanwhile, universities are also playing their role. Dr Smith, for example, recently led a team of 20 global carbon dioxide removal experts in producing a 120-report, exploring the rise of DAC innovation. Likewise, Shindel and Northwestern University have been researching a new innovative “moisture-swing” technique, which captures CO2 at low humidities and releases it at high humidities. 

“More cost effective and less energy intensive filtration technologies can improve the efficiency of DAC,” Shindel comments. “Sorbents that can be regenerated without high temperatures is one path of research, while sorbents that have even higher capacities for CO2 and better kinetics is another frontier.” 

“Meanwhile, moisture-swing DAC relies on materials that capture CO2 under dry conditions and release CO2 under humid conditions. This is a different methodology for cyclical use rather than employing temperature, pressure, or electricity to regenerate sorbents. It can be used much like any other DAC technology, but with humidity as a lever for regeneration,” he adds. 

A diverse portfolio of solutions 

Undoubtedly, this diversity in techniques and technologies is extremely crucial. With the urgent imperative of achieving net zero emissions, continuous innovation is essential to scale these solutions to required levels. 

“We are likely to need CO2 removal at the scale of billions of tons per year by mid-century,” explains Dr Smith. “Currently there are several different removal technologies that show potential, all at early stages. Developing that portfolio increases the amount of CO2 removal we can achieve. It also avoids over-reliance on any one solution which may turn out to be less effective, more costly, or have damaging environmental impacts.” 

While renewables are currently driving commercial viability, it is imperative that DAC becomes a more integral part of the energy mix. Indeed, its unique approach is critically important for addressing emission challenges and combating climate change effectively. 

“Even if the world manages to renewably produce its energy without greenhouse gas emissions, there’s still the issue of historical emissions,” Shindel adds. “CO2 stays in our atmosphere for thousands of years, contributing to global warming, so removing the CO2 we’ve already emitted will be essential. 

“There are also sectors of the global economy where limiting emissions is far more challenging, ranging from aviation and cattle farming to the production of concrete, where CO2 is emitted as part of the chemical reaction that forms the material itself. 

“The world is large and different solutions work best in different locations and under different economic conditions and incentives. Therefore, having a broad range of engineering tools to reduce and negate emissions is essential,” Shindel continues. 

There is simply no silver bullet for DAC. As with any problem, collaboration and different innovations and research avenues will be key. Within this, DAC will be part of the answer. 

About the author 

This article was written by Jonathan Dyble, a freelance writer from WD Editorial.

This article first appeared in the July 2024 issue of Filtration+Separation magazine. To read the full issue, click here