Predicting Direct Air Capture Performance, Come Rain or Shine

Process engineer Adam Ward is modelling DAC at Imperial College London. He explains his research to Aniqah Majid, the challenges of scaling carbon capture technology and why the UK’s famously unpredictable weather has a major bearing on performance

Aniqah Majid (AM): Tell us about your research into direct air capture

Adam Ward (AW): Our research team is looking at DAC in the UK, and specifically in the main industrial clusters. We got into it by making a proposal to the Industrial Decarbonisation Research and Innovation Centre (IDRIC), which is funded by UKRI (UK Research and Innovation) and is looking at all aspects of industrial decarbonisation. IDRIC are interested in DAC because it represents a really good opportunity to build a DAC plant somewhere where there’s a lot of infrastructure already there that can support the deployment.

The brief of our project was to look at optimising a DAC process under climatic conditions that you might see at these clusters.

The performance of a DAC process is dependent on what the weather is like in terms of how hot it is and how humid it is. It is very location-specific and IDRIC wanted to know, “What does the performance look like at these locations that we might want to deploy at?”, and they wanted us to optimise the system to look at aspects around resource usage.

They wanted us to investigate optimising the DAC in terms of how much electricity, steam, and water it needs, so that we can inform decision-making for policymakers and system planners and give them the information they need on how much DAC you could integrate at which industrial cluster. And, based on the weather performance, what would the performance of those DAC units look like in those locations?

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AM: Why is it important to get location and temperature right?

AW: DAC is basically a separation process. You take in and separate CO2 from everything else in the atmosphere. So, whatever the atmosphere is doing on a given day when you are operating the process, the process has to deal with that, and it is going to vary over time, and it is often going to be quite unpredictable over long timescales.

The weather has an impact on the performance of DAC in two ways: in the temperature and the humidity of the air that’s coming in. So how hot and how much it has been raining has a significant impact on performance.

But we have found two key outcomes in our work. The first is that the temperature is not necessarily as much of a factor as we thought it was previously. We identified that although temperature does have a significant effect on how the process performs, you can design the process in a way where you can tweak the way it’s operating, sort of on the fly to account for the temperature. That’s something that we can accommodate for very easily.

But we did see that the humidity of the air has a really big effect as it is difficult to counterbalance and change the way you operate the process at a given time. So, we were studying what the effects of different relative humidities would be, relative humidities being a zero to 100% scale of how moist the air is.

And we found values in a range of about 50 to 75% gave the best performance for DAC. And outside of that range, you actually see the performance of the system degrade quite significantly. Fortunately for the UK, [50 to 75%] is actually quite a common relative humidity range.

The weather at most of the UK’s industrial sites where we might be interested in deploying these kinds of things is strongly favourable for DAC performance.

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AM: How should the industry communicate to policymakers and taxpayers what DAC is and why it is needed, despite its high cost?

AW: From an engineering perspective, the concentration of CO2 in the air is low, it is actually around 400 parts per million or just a bit higher than that. Now that is extremely small, about 0.0004% CO2 in the air.

If you can imagine trying to sort through all the molecules in the air and pick out just the ones that are CO2 it is extremely difficult to do that. And that’s fundamentally why it’s so expensive when you’re trying to design a process that can be that selective in the way that it does the filtering. It’s really challenging. It’s really energy intensive to be able to sort the molecules out at that level.

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AM: What do you picture in terms of how DAC would look like on a large scale?

AW: We are seeing a trend towards commercialisation, but we have not figured out all the logistics of the scale that is needed. So, for example, the newest plant by Climeworks [in Iceland] will capture around 40,000 t of CO2 a year, and that will be the biggest plant that has ever been built. To put the scale into context, it is estimated the UK needs around 1m to 5m t/y of CO2 captured via DAC to meet its net zero target.

It is very much turning into a systems engineering problem more than a chemical engineering one. DAC is quite modular, and I think we can learn from what has been happening in the renewable energy industry where the deployment of standardised technologies like solar panels and wind turbines is mass-manufactured and then transported and built all over the world. Using that model, modularity in DAC would take something that we know how to do on a relatively modest scale and repeat and multiply that out to something that works for large scale.

The biggest engineering challenge right now is energy usage. DAC processes are very energy intensive. There is a concern that DAC emits more CO2 than it captures because of its energy usage

AM: What are the current challenges for engineers?

AW: The biggest engineering challenge right now is energy usage. DAC processes are very energy intensive. There is a concern that DAC emits more CO2 than it captures because of its energy usage.

It is a simple criticism, but it is something that scientists are paying a lot of attention to. Driving improvements in energy efficiency is a key touchpoint for companies to continue to develop and deploy DAC at a large scale.

A lot of DAC technologies require the input of not just electricity as energy, but also as heat. And that is harder to get in a low-carbon way.

There is this trend in industry towards electrification, like the electrification of steelmaking. It is a similar concept. You want to get heat from somewhere? Can you do it by just using electricity rather than burning gas? Also, technology developers will be looking at ways to use heat in their process where the heat comes from a medium that is at a lower temperature. We call this lower-grade heat. It is easier to electrify a process that runs on lower-grade heat.

Those are really the key ways that I will be looking to drive energy efficiency improvements, which, from an engineering standpoint, are the main blocks towards getting DAC to large scale.

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Adam Ward is a research assistant in the Department of Chemical Engineering and the Sargent Centre for Process Systems Engineering at Imperial College London. He is also a process engineer at Isometric, a carbon credit certification platform.