Phil Renforth on Carbon Sequestration
Transcript
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Oliver Strimpel
This is Geology Bites with Oliver Strimpel. For many years, efforts to limit climate change have focused on curtailing anthropogenic emissions of greenhouse gases. But it is increasingly clear that such curtailment will not, on its own, be able to prevent the damaging effects of global warming. Therefore, more attention is now directed to mitigating climate change by enhancing the removal or sequestration of greenhouse gases from the atmosphere. As a result, our climate change goals are now often specified in terms of when we plan to reach net 0 emissions, rather than just when we can reach emission reduction targets. Before we get into detail about the sequestration methods, let me summarize the scale of the problem we're addressing by comparing naturally occurring processes that inject and remove greenhouse gases with human emissions. When discussing these flows on a global scale, a convenient unit is a billion tonnes, or a gigaton, per year. So before the industrial revolution, the naturally occurring processes were roughly balanced with about 200 gigatons of carbon dioxide being removed from the atmosphere each year. And the corresponding amount being returned to it. Just over half of that flow came from the land, driven by photosynthesis removing the CO2 and decay of plant matter re-releasing it. The oceans generated the remainder of that flow to give us a sense of scale. The River Thames flows about two gigatonnes of water per year into the North Sea. Phil Renforth is an expert on carbon sequestration. He is especially interested in enhancing the weathering of rocks and has performed in-depth investigations of geochemical techniques of removing atmospheric carbon dioxide. He is an Associate Professor in the School of Engineering and Physical Sciences at Herriot Watt University. Phil Renforth, welcome to Geology Bites.
Phil Renforth
Thanks for having me, Oliver.
Oliver Strimpel
In my introduction, I talked about the naturally occurring biochemically generated flows of carbon dioxide. But on top of that, we have volcanic emissions. How significant are those?
Phil Renforth
So, volcanic emissions are quite low. That's on the order of 0.1 or 0.2 gigatons of carbon PA. Compared to photosynthesis, that's quite small, but photosynthesis is essentially balanced, taking up CO2, balanced by organic carbon degrading. What's interesting about the volcanic emissions is that, while it's quite small, it can really drive the climate. It can put more CO2 into the atmosphere, and the balance to that volcanic emission is essential. Over geological time periods on the orders of thousands to millions of years, the weathering of rocks on the land surface and in the ocean removes carbon dioxide from the atmosphere, so it helps to balance some of the emissions from volcanic degassing.
Oliver Strimpel
That's interesting. So the biological processes were balanced at a rate of about 200 gigatons of carbon dioxide per year, and the physical processes, either volcanic emissions and over geological time, consequent increased weathering of rocks were roughly balanced as well. But at flow rates about 1000 times smaller. So now that we've got some context, let's move on to the human carbon dioxide emissions that are causing climate change and driving the need for carbon sequestration. In 2021, humans emitted about 36 gigatons of carbon dioxide. So to have a meaningful impact, carbon sequestration methods have to be able to remove multiple gigatons of carbon dioxide per year. As I said in the introduction, you're especially interested in enhanced weathering. What exactly is enhanced weathering?
Phil Renforth
Enhanced weathering is a term that's used to describe taking rock and crushing it up into a fine powder. We know that these reactions take CO2 out of the atmosphere over long periods of time. We're trying to increase the rate at which that happens by creating new surface area. So if you take a material and you crush it up, you increase its surface area. If you do that enough, you can create enough surface area that they dissolve on meaningful time scales for us. The other side of that picture is where the minerals dissolve. The idea with enhanced weathering is you spread the minerals into places or environments that favor mineral weathering. Places that have been proposed include agricultural land, so spreading crushed rocks onto cropping soils. To spread the minerals and the rocks onto coastal environments and spreading minerals onto other environments like forested catchments, places where we're trying to grow forests. So trying to combine a bunch of different ways of removing CO2 from the atmosphere.
Oliver Strimpel
What are the various ways of enhanced weathering that you've explored?
Phil Renforth
The idea would be to extract and spread basic rocks, basalts, or gabbro, or ultrabasic rocks, so dunites, peridotites or kimberlite. You need about five times the amount of basic rock to the amount of CO2 that you want to capture. So, for instance, if you want to remove a gigaton of CO2 from the atmosphere you'll need something on the order of five gigatons of rock. It sounds a lot of rock, and it is. There's no getting away from. You know, there's a large amount of material, but if you look at what we currently do, rock extraction is one of the largest things we do as a species. Global rock extraction, but on the order of 50 gigatons of rock per year. It's essentially rock that's extracted for construction aggregate. It's the largest thing we do as a species, and that is going to increase as we meet the demand for a growing population. So you can say, well, maybe an extra 2 to 10 gigatons of rock per year over the next 20-30 years, it seems achievable. It's marginal on what we do already.
Oliver Strimpel
What is the geochemical reaction that happens when you spread fine ultrabasic or basic rock particles over land?
Phil Renforth
The first thing that happens is that CO2 dissolves in water. The next step is you either get something called congruent dissolution of the mineral, where essentially the whole mineral just dissolves in water and creates the elemental products of that mineral, for instance, calcium, magnesium, or silicon, aluminium. Or you get hydrolysis, where you get the mineral partly dissolving and some of the solid transforming into things like clays or of the secondary minerals. The results of those processes are essentially drainage waters that are rich in calcium and magnesium ions, which are positively charged, and that positive charge is balanced by negative charges from bicarbonate ions. So that's the CO2 in the water is being neutralized, so the carbonic acid in the water has been neutralized to form this bicarbonate. And then that gets washed into the ocean and then eventually, over time, some of that carbon will be turned into shell, where it will then be incorporated into sediment, eventually becoming limestone.
Oliver Strimpel
So for that to happen, first you have to put these basic rocks over a suitable land area. Then water has to dissolve them. Then the ions have to get into the sea, and then that has to form mineral skeletons of microorganisms. So are all those things guaranteed to happen in the locations that you would expect to spread this rock?
Phil Renforth
So for carbon sequestration to happen, you just need to form these bicarbonate ions. So another way of describing it is alkalinity providing that water reaches the ocean, that alkalinity will be stable for thousands to potentially hundreds of thousands of years. The formation of new minerals, new carbonate minerals isn't necessarily required, but it's another storage location for carbon. A lot more permanent carbonate minerals obviously last for millions of years.
Oliver Strimpel
Are there suitable locations where we can extract of the order of gigatons of basic rocks like basalt and gabbro and suitable adjacent land area where we can take that crushed rock and spread it around with bulldozers or agricultural equipment?
Phil Renforth
Basalt is pretty ubiquitous globally. The UK, for instance, has enormous basalts, so basic rock outcrops. We did a calculation trying to sort of work out how much carbon that could store. So on the order of hundreds of gigatons of carbon, which globally is not unreasonable, but nationally it's an enormous resource. Another way of putting it, if you look globally, you could capture all of the carbon in the earth system. So not just all of the carbon in the atmosphere, which we would never want to do anyway, but all of the carbon in the earth system a few times over without exhausting that resource. So there's enough rock out there. It's a case of where it is in relation to where it's needed to be to dissolve. Well, that then starts to become not just a factor of the location of those two things, the location of the resource and the location of the application site. But it's also the method of transporting the material, and that would depend on future transport emissions intensity, which mode of transport. We did a glorified back-of-the-envelope calculation in a paper a few years ago where we took ultrabasic rock from the ophiolite in Oman, which is a really big ultrabasic rock deposit, and you could move it to the corn belt and even further into the Canadian prairies before you ended up releasing more CO2 than you could draw down from the mineral dissolving. Now you never, you never moved that rock that far, but it kind of makes the point that you can move material quite far without blowing the carbon budget on the weathering. But it's more of an optimization problem than a feasibility problem.
Oliver Strimpel
Just to come back to the geochemistry for a moment and the ultrabasic rocks. So you talked about releasing cations like magnesium and calcium. Is it really the olivine in these rocks that is the active mineral, and we also talked about weathering limestone, and there you have calcium carbonate? So are those the actual minerals that wind up producing the bicarbonate?
Phil Renforth
In ultrabasic rock like dunites will be rich in olivine, which is the main mineral that's doing the work for you. So we want to choose rocks with high concentrations of the more reactive minerals, and olivine is the mineral that weathers the quickest.
Oliver Strimpel
What about other methods of enhanced weathering? So far we've really talked about taking naturally occurring materials and exposing them or exposing more surface area. What other methods involve enhanced weathering?
Phil Renforth
To facilitate mineral weathering, you can do it either by increasing surface area, or you can put the mineral into an environment in which it dissolves quicker. So you can create an environment that facilitates mineral dissolution. I'll give you an example. It's called the accelerated weathering of limestone, and the idea is that you can take limestone which is full of calcium carbonate and you put that into a reactor with elevated CO2, which will help the mineral dissolve, and you flush the reactor with sea water. So you cycle seawater through the reactor and then back into the ocean. The product is an alkaline-rich sea water flowing back into the ocean. It's almost like doing what a river does. Another idea is around using electrochemistry. You create a voltage across an electrochemical cell, and then at one side of that cell you have an acidic environment and then the other side you have a basic environment. When you use the acidic environment to dissolve your rock and the basic environment you can use to remove CO2 directly from the atmosphere, or you can add that alkaline water to the ocean. There's a third category there, and it's around creating minerals that are more reactive. You could take a limestone, for instance, and you could put it into a kiln or a furnace and evolve the CO2. And if you capture that CO2 as it evolves, you do something called carbon capture and storage where you inject the CO2 underground. What you're left with is a reactive mineral that can remove CO2 from the atmosphere. And you could either spread that out onto a site, or you can add it to the ocean. So there's ideas around using minerals that we specifically create to react with CO2.
Oliver Strimpel
When we talked earlier, you mentioned the idea of using industrial waste products as a source material for some of this active enhanced weathering process.
Phil Renforth
We already create materials that are reactive to CO2. The most voluminous of those is cement. Cement will react with CO2 and you can see evidence of that if you go to historic buildings. You can sometimes see stalactites forming underneath the old concrete bridges, sometimes on the base of old buildings. So there are materials that we already create for other purposes that do have a reaction with CO2 and over their life cycle will react with CO2. There's also waste materials that we create that we could also use to react with CO2. Probably the most voluminous of those are mine wastes from particular ultrabasic rock deposits. Particularly things like nickel, diamonds and some copper deposits produce waste tailings that could react with CO2 and also slag from the steel industry react with CO2. So there's a whole part of the research that we do that explores ways of taking those materials and reacting them with CO2 and finding the best ways of doing that.
Oliver Strimpel
And what do you do with the product of that reaction with the industrial waste?
Phil Renforth
There are companies currently operating and some in the UK that turn that into construction material. There are some high-value products that you could potentially create, but most of those really don't scale beyond a few million tons per year globally. Construction products do have that scalable potential. We do gigatons worth of construction per year, so that does seem quite an interesting and scalable area.
Oliver Strimpel
So these would be breeze blocks made of atmospheric carbon dioxide with slag and other waste products combined with it.
Phil Renforth
That's right. Yes, there's a company in the UK called OCO technology. They produce construction blocks out of a waste material and CO2. There's also creating aggregate, just crushed rock or sand, that you can then introduce into the construction industry as a secondary material.
Oliver Strimpel
You've already alluded to alkalinity, and I know that one approach that you've explored in depth has been to deposit significant volumes of material into the oceans to increase the ocean's alkalinity. Can you explain what alkalinity is exactly, and then how doing that would draw down atmospheric carbon dioxide? And then, of course, I'm interested in how much you'd need to add to make a difference.
Phil Renforth
Alkalinity is the ability of a solution to neutralize charge. Think of it in terms of carbonate alkalinity, which is the balance between positive charges and negative charges in solution, and you've got positive charges like calcium and your magnesium, sodium, potassium. And when we're talking about carbonate alkalinity, we're talking about that being balanced with mainly bicarbonate and carbonate ions. So if you add additional calcium and magnesium or sodium or potassium to water through weathering, mineral dissolution, that will then be balanced by greater amount of bicarbonate ions, where that carbon in that carbonate ion has come from CO2. The ocean has a large amount of alkalinity within it. It's the largest carbon pool at the Earth's surface. Something on the order of 40,000 gigatons of carbon. And because of that large pool, we wouldn't need to change it by that much to have a really large lever on carbon storage. For comparison, the amount of carbon that's in the atmosphere in total is about 860 gigatons of carbon, so small changes in the alkalinity in the oceans could have a large amount of storage within the Earth system. The first idea that was put forward was taking limestone, heating it up in a kiln to produce lime, hydrating that lime, and adding that hydrated lime to the ocean, and that would dissolve in the same way that a mineral would dissolve on the land surface. It would release its calcium and hydroxide, so that's the other part of the mineral. And that hydroxide would increase the seawater pH, will remove CO2 and essentially neutralize that CO2 to form bicarbonate ions.
Oliver Strimpel
Wow, so this is, I guess, called liming of the oceans. How would we get the lime into the ocean?
Phil Renforth
That's still an open question. Some folks explore the idea of adding it from the back of a ship. So you'd take the lime from the lime kiln, you'd add it to a ocean-going vessel, and then you'd add it from the back of the vessel where the wake of the ship would help mix the water. The supply chain would be that you take limestone, you put it into a lime kiln at 1000 degrees, you would evolve CO2 And for this process to work, you'd almost certainly need to capture that CO2 and stop it from going into the atmosphere by injecting it underground. The carbon balance wouldn't work at all, really, if you didn't do that. Other ideas use solar-powered calciners and electric calciners powered by renewable energy, for instance. So we need to capture the CO2 that was devolved from that process, which is obviously the CO2 from the limestone, but also the CO2 from the fuel that's used in driving the reaction. You then take the lime, hydrate it to produce hydrated lime, and then add that to your ocean-going vessel. I really don't think there's consensus on the method of how that's added. It's still an active area of research.
Oliver Strimpel
Let's talk a bit about in situ carbonation, as it's called, which you've already referred to a little bit when you've talked about pumping any waste CO2 that's produced or byproduct CO2 that's produced as a result of some of the processes you discussed. Pumping those underground. So how would that kind of sequestration work, and what scale could that operate at?
Phil Renforth
What I've previously spoken about is the idea of injecting CO2 into unreactive rock formations, saline aquifers or oil and gas. And the idea is that the CO2 will be physically trapped. There's ideas out there that proposed the injection of CO2 into basic or ultrabasic rocks or reactive rock formations where the CO2 wouldn't be trapped physically, but would react with the rock underground to form carbonate mineral and new mineral. That's currently being demonstrated in Iceland. And there are other projects, one that's being proposed in Oman at the moment. The project in Iceland seems to be pretty convincing in its findings. The CO2 was injected, and it reacted with the rock, and it formed a new carbonate mineral. There is pretty strong evidence that that process works. The thing that needs to be developed is the engineer and the cost and can we optimize it. With in situ mineralization, we're not limited by the amount of rock that's out there. There's an enormous amount of suitable rock deposits. There will be places that are better than others, the right sort of permeability in the rocks that help the flow of CO2. But it will come down to an engineering optimization. Finding the best way to inject CO2 with water into the rock formation. How much of that rock formation can be carbonated before you need to drill a new bore hole.
Oliver Strimpel
Would those reactive rocks be the ultrabasic and the basic rocks like the basalts and the peridotites and so on that you talked about earlier with your passive methods?
Phil Renforth
In the UK, there's not really suitable rocks for injection because the basalt that we have in the UK is relatively shallow. But there are large deposits of basalt. There's the Columbia flood basalts in Northwest America. Deccan traps in India and that's an enormous basalt deposit.
Oliver Strimpel
OK, as I said earlier, we're currently emitting about 36 gigatons of CO2 into the atmosphere. How much of this can we realistically remove if we deploy some of the sequestration methods you've described?
Phil Renforth
We can't balance the current emissions with removals, geophysically impossible. We can use CO2 removals to compensate for the residual CO2 that might be emitted from our civilization. The quantity of that depends on how quickly we can decarbonize, but the current estimates are between 10 and 20 gigatons of CO2 per year that we'll need to remove from the atmosphere. In terms of other ideas, growing trees, for instance, or land-management practices, at best, each one of those might be able to do one, possibly 2, gigatons CO2 per year. There's reasonable grounds that we can extract enough rock to be at the gigaton scale within decades. There will be limitations in terms of land area, in the amount of rain water that these rocks can dissolve into, and that will apply to the more passive approaches. The enhanced weathering approaches, current estimates still say that that could be on the order of gigatons per year.
Oliver Strimpel
So enhanced weathering, spreading basic rocks on agricultural land and so on could achieve of the order of a gigaton or two, which would be meaningful. What about the liming of the oceans or the in situ direct burial of carbon dioxide, either into passive or into reactive rocks?
Phil Renforth
Processes that require a lot of hardware, ocean liming for instance, would require a kiln or something that can heat the rock up to 1000 degrees. And there are other approaches that I mentioned about using reactors. If you're using hardware, the limitation there might be on the ability to create that hardware. They still don't think that is necessarily constraining because the cement industry has a similar challenge in that it takes rock and it heats it up to 1000 degrees. In the 1960s, it was producing about half a gigaton of cement per year. By the 1990s, early 2000s, that was up to several gigatons per year. So there's evidence that we can scale that type of technology up over decades to a gigaton scale. It's not saying that that isn't a challenge. It is an enormous challenge. But there's at least a precedent that we can do that type of technology on that type of scale over multi decades. What might limit ocean liming might be the rate at which we can add alkalinity to your lime to the ocean while not having a negative impact on ocean ecosystems.
Oliver Strimpel
You said that we don't have very long to do this, maybe a few decades at most. So could you rank the different sequestration methods that you've talked about in terms of how quickly we could scale them up if we decided to proceed?
Phil Renforth
I think alkaline materials, or anthropogenic materials, reacting them with CO2 to form carbonate will be the lowest-hanging fruit and some of the lowest cost as well. So taking things like slag from the steel industry or taking mine wastes and reacting them with CO2 to produce carbonate minerals. And you could do that within the site of the production facility without having secondary environmental impacts. That could be scaled up quite quickly. The addition of minerals to the land surface could be scaled up relatively quickly, but maybe not to gigaton scale, but certainly to megaton scale. In the production of rock aggregate, there's a fine material that's produced that's too small to sell to aggregate suppliers, called quarry fines, and that sometimes builds up on quarry sites. That could be a large source of material that could be used for enhanced weathering that would quickly get exhausted if this was scaled to gigaton scale. Additional extraction sites would almost certainly need to be opened to facilitate the transition to a gigaton scale.
Oliver Strimpel
So at the end of the day, given that we seem to have quite a host of different mitigation strategies at our disposal, are you optimistic about our climate prospects?
Phil Renforth
Everything that we do to try and solve climate change matters. We often think about climate impacts in terms of thresholds going over the safe amount of CO2 in the atmosphere, or trying to limit our CO2 emissions. But even if we don't meet those targets, we still save lives or improve people's lives from removing CO2 from the atmosphere. Every step is meaningful within trying to solve climate change. We can have a positive impact from even the small things that we do.
Oliver Strimpel
Phil Renforth, thank you very much.
Phil Renforth
Thanks a lot, Oliver.
Oliver Strimpel
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