Oxford Sparks

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Emily Elias: Carbon capture is a way to help mitigate emissions produced by heavy industries and power stations from being released up into the atmosphere. But what happens when you pump that stuff down underground?

On this episode of the Oxford Sparks Big Questions podcast, we’re looking at carbon capture and we’re asking, how do you convert CO2 to rock?

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Emily: Hello, I’m Emily Elias, and this is the show where we seek out the brightest minds at the University of Oxford, and we ask them the big questions. And for this one, we’ve reached a researcher who’s ready to rock.

Shurui Miao: I am Dr Shurui Miao. I am a physical chemist, and I study liquids and solutions under very, very small spaces.

Emily: Okay. And so for this big question we have come to you because we’re, we’re kind of talking a bit about carbon capture when it comes to putting liquids into small spaces. Okay, walk me through this. How does carbon capture work?

Shurui: Yes. So the idea is really to achieve ultimately achieving negative emission. And what by that we mean that we are constantly emitting CO2 into air by combusting fossil fuels, generating power, clearing land, deforestation, etc. All of these are sometimes justified. So we have to have a mean of removing the carbon footprint, and this is what carbon capture is all about. It can be roughly divided into two parts the carbon capture side and the storage side.

So the capture side is operated by these gigantic fans that see through air and extract the little CO2 molecules from air, and then enrich them into some form. We can then worry about storage. So storage, there are two main ways of doing this. The current most sort of at scale way of doing this is to pressurise the enrich the CO2 to form a thing called supercritical CO2, which is thought to behave almost like a liquid. And then we pump underground into those cavities in the sort of earth crust. And then we hope that it stayed there.

So the caveat here is that it’s very easy for it to leak and escape back into the air. So by doing that, then we sort of wasted all the energy of pressurising it, pumping it – we haven’t really achieved a stable, secure storage.

So the technology that I work on is really not only that we pump underground, but we’re trying to create the environment for the CO2 to rapidly react with mineral deposits so that they turn into solids, which then sort of, you know, most mundane object could be a limestone, almost a mineral, which then is stable and will stay there for thousands of years.

Emily: Okay. So the big idea is that you take the CO2 through the capture process. You’re more concerned about the storage process and figuring out, hey, can we turn this CO2, this liquidy CO2, into limestone.

Shurui: Yes. So really is converted from a gas, which is sort of very easy to escape. And can we store it into a more stable form, into the solid?

Emily: But like, how does that work? I’m assuming you don’t just have like witchcraft on your side, there’s real chemistry going into this. How do you turn something that’s like a liquid state into limestone?

Shurui: So the good news is that thermodynamics or, you know, the word nature is on our side. So if you put calcium, water and CO2 sort of in the same space, when they meet each other, it is a naturally favourable process for them to form calcium carbonate, which is sort of the main ingredients of the calcium mineral, or the carbonate minerals. And so this is a naturally favourable process.

All we’re trying to do is to find out the right environments and conditions – so temperature pressure surface area and calcium magnesium abundances, sort of trying to find the right combination of those conditions to encourage this process to happen at scale.

Emily: So have you done this in the lab?

Shurui: Yes. So I’ve sort of done a little proof of concept, which is we have some, uh, basaltic, which is a volcanic rock rich in calcium and magnesium. We have some sample provided by our colleagues from Earth Sciences. So all I did was ground it up, add a drop of water, expose it to air, came back the next day and put it under a microscope. We see beautiful crystals of calcite. So that’s just the overnight experiment.

Emily: Geez, that’s that seems pretty crazy to me. I mean, obviously there’s a difference between doing something in a lab and then doing something at scale with these, like big industries. How do you make something translate from the lab to the real world?

Shurui: So this is actually already happening, which is very encouraging. So there are pilot plants, uh, at least within the European Union. The focus is in Iceland. There are companies sort of trying to operate this gigantic carbon capture and storage plants at scale, And so to name a few. The, uh, what’s one of the startups in the, in the field is called Carbfix. And they collaborate with Climeworks, which is sort of more focused on the capture side. Carbfix focus on the storage side. And together they build this infrastructure in Iceland, which right now, I think the biggest project they have is Silverstone. And so they’re aiming talking about, uh, 40,000 tonnes per year by 2030.

Emily: Can you put that into context for me? What does 40,000 tonnes a year mean? Is that like, boom, we solved climate change? Or is that a small drop in the ocean?

Shurui: No, it’s unfortunately not yet the silver bullets. So human emission right now we’re averaging about forty gigatonnes or around that per year. So forty thousand tonnes is only about one millionth of what we emit every year. But the facts that we can do this at scale, uh, renewably and sustainably is really, really encouraging.

Emily: Okay, so you kind of mentioned there’s some variables to this at play of like making this conversion from liquidised carbon dioxide into limestone. What are some of the variables that you want to sort of like try and manipulate to make this process bigger and faster and more scalable?

Shurui: Yeah. So, um, in the lab, we’re really, really trying to understand which is the effect of confinement. So the process of is a sort of like many other chemical reactions. It’s a function of temperature, pressure, acidity, which is sort of the pH of the environment. And the other variable we identified to be important is confinement.

So in the lab we’re looking at how does this nucleation process or interactions of atoms change when we put them into very, very narrow spaces, which are often quite common in geological contexts. Um, so if you’re looking at the cores that they drilled to sample where they sort of perform this mineralisation, you see that this mineralisation often happens in very narrow channels, and we’re just trying to understand that topology or sort of, a sort of almost, a shape and continuity and surface area ratio. All of those which are not your conventional chemical or thermodynamic variables. How do those play a part in this process.

Emily: and the type of rock of where they’re doing this, this is like a volcanic rock, like you’d imagine sort of that sort of like black rock with lots of porous holes in it. Right. So how does that go into limestone? Does it then like the little porous holes seal up or what is it?

Shurui: Yeah. So that’s precisely the question we’re trying to understand. So you correct to point out that the volcanic rocks are porous black. And so instead of turning the entire, uh, let’s say, macroscopic entire column into a limestone, what really happens is that these calcium carbonate minerals forms in all the cavities within the volcanic rocks. And so when you look at pictures on, for example, Carbfix website, you see that they have these columns of centimetres diameters, which is mostly black with this silver sort of strings along, which is where the mineralisation happens. So by looking at the interactions of atoms and molecules under that confinement, the big question was really trying to understand is as soon as mineralisation happened, does it just block the cores or does it generate more fractures so that it can be a continuous sustainable process? Or does it just sort of destabilise the entire rock formation? So these are sort of the questions we’re trying to understand, which would be hugely important for scale up the technology.

Emily: So how do you go about tackling those questions? What is the stuff that you’re doing to figure that out?

Shurui: So we have a custom-built instrument in the labs, and that will allow us to measure interactions between atomically smooth surfaces across a very well controlled distances. So for example, we can easily study an interaction between those surfaces across a calcium rich liquid, which is where the mineralisation will happen over, let’s say, micrometre, which is about a sort of diameter of our hair. And then we can equally study the same interaction across a calcium and carbonate rich environment across a nanometre confinement, which is then we’re talking about really one thousandth of a hair diameter, sort of by understanding how the interaction changes as a function of the link scale, as well as a function of solution composition, will hope to answer this question.

Emily: Earlier in the conversation, you said that when it comes to storage, there’s a lot of hope. You pump stuff down there and you hope it stays. How would this method of turning it into limestone – are you able to sort of remove that hope and know that it’s staying down there?

Shurui: Yes. So this is a main challenge of the entire field. And what we currently do is that we apply isotopic analysis on the liquid we inject. So before mineralisation and then we analyse the water that we can extract from underground, miles and miles away from where we injected. And by comparing the difference in composition, we can back out estimate how much chemical change has happened for water between injection and extraction and sampling. And so that, from that, scientists have worked out that roughly 70 to 95 percent of injected CO2 have turned into minerals within two years.

Emily: So this does this mean that we’ve actually sort of like nailed carbon capture. We’ve figured it out. Or is this still sort of, we’re kicking the problem down the road a bit for future generations to deal with.

Shurui: So in my opinion, we’re definitely not kicking the problem down the future generation because we’re forming stable minerals. So once they form stable minerals, then the calcium and the carbon is going to stay there for centuries. So in that aspect, it’s very positive.

However, I also don’t think that this technology is going to be the solution to climate change. I think we are only part of a much, much bigger picture because, for example, the entire carbon capture and storage process is really, really expensive on the capture end. So we really, what we’re trying to do is help to make the entire process economically viable. We’re really trying to make the storage side as cheap as possible and as big as the scale as possible.

Emily: So are you optimistic for the future then?

Shurui: Yes, yes. I think we are making good progress. And ultimately, I think carbon capture and storage will be, again, play a huge role in achieving so-called negative emission, which is where we start removing CO2 from air, undoing all the damage we’ve done to the to the atmosphere and climate.

I really see what I work on as a way of decoupling technological advancement with emissions. So if you’re looking to human history, every big technological advancement comes with new ways of finding a better energy source, a more energy rich source, but all of which came with environmental impact. So the way I see my work is to decouple the two processes. So then that means future technological advancement, whether it’s a large data centre for AI or self-driving cars, or battery manufacturing and recycling, etc. all of those can potentially be decoupled to environmental damage, which to me is an incredible thing to achieve.

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Emily: This podcast was brought to you by Oxford Sparks from the University of Oxford, with music by John Lyons. And a special thanks to Dr Shurui Miao.

Tell us what you think about this podcast. We are on the internet at Oxford Sparks or go to our website, oxfordsparks.ox.ac.uk.

I’m Emily Elias. Bye for now.