Can we suck CO₂ back out of the air?

From https://carbonengineering.com/our-technology/

I was recently told that by 2050, we should be able to take CO₂ out of the atmosphere for $50 per metric ton.* That would be remarkable news if true. Each year, the UK is responsible for about 800 million tons of CO₂-equivalent greenhouse gases, or about 12 tons per person. I say ‘CO₂-equivalent’ here because e.g. N₂O has the same effect on the climate as 300 tons of CO₂, meaning we can think of it as ‘300 tons of a CO₂ equivalent greenhouse gas’.

*1,000kg is a tonne in Europe and a metric ton in the US. A UK ton is 1,016kg and a US ton is 907kg. They’re all pretty close, so I’ll be writing ton for readability.

By sucking those 800 million tons out of the air for $50 a ton, we could cancel out a year’s carbon emissions for $40 billion (£31 billion), or $600 (£470) per person in the UK.

From https://en.wikipedia.org/wiki/Government_spending_in_the_United_Kingdom

That’s a lot of money, but the chart above shows it’s within reach; it’s about what the UK government spends on transport. In fact, renewable energy, electric cars and other low-carbon technology should lower emissions by 2050, so that the actual cost will be lower.

With that in mind, I dove into the research on Direct Air Capture (DAC) of CO₂ to see how realistic that estimate of $50/ton really was. Note that DAC is only one of several ‘negative emissions’ technologies, and refers to capture of CO₂ from the atmosphere with self-contained machines, as in the top screenshot. DAC mainly requires heat and electricity to run; this makes it stand out from other approaches, which need huge quantities of land or materials to run.

The Good: Technology

The first piece of good news is that the chemistry behind DAC isn’t experimental: it has been used on submarines since the 1950s. There are two main methods, and one uses high school chemistry, so I’ve included a schematic below for interest.

The ‘high temperature’ method. From here.

Don’t worry if you don’t follow this diagram — all that matters is that air goes in at the left and concentrated CO₂ comes out at the right. That last point is important; DAC doesn’t get rid of CO₂; it just captures it for us, and we need to decide what to do with it. More on that later.

The second good point is that DAC has seen much more interest from startups than other methods for ‘climate engineering’. This seems to be because other methods have side effects or social consequences which they are nervous of. For example, capturing CO2 using crops uses land which could be used for food, driving up food prices.

Climeworks’ commercial DAC machine

Only one of these startups, Climeworks, has commercial technology, but others such as Carbon Engineering have pilot plants which show their technologies are viable.

What can we say about costs? Cost estimates from the startups often “do not include all cost components” and so should be taken with a pinch of salt. This 2019 paper recalculates costs for the technologies with pilot plants and finds they are in the range of $230-$450 per ton of CO₂. That matches well with a 2011 review from the American Physical Society which estimates $430/ton ±50%. On the other hand, Climeworks is offering to trap CO2 for $1,100 per ton right now, so that may be a more reliable starting point.

We’d definitely expect costs to go down over time, just like the costs of any new technology, as people find better methods and as economies of scale kick in. How much they would come down by is another question. DAC companies are fond of quoting the falling cost of solar panels…

From https://blog.nationalgeographic.org/2017/06/22/solar-energys-rapid-growth-to-save-the-oceans/

But this is a cherry-picked case, and we shouldn’t expect that much of a drop in DAC costs. Some more datapoints…

  • The $50/ton (‎‎€40/ton) projection is from here, and is for the ‘low temperature’ method used by Climeworks, which is the cheapest assuming a free source of waste heat. Waste heat is heat that would otherwise be dumped into the environment by e.g. a steel plant.
  • An analysis based on the underlying physical limits (‘thermodynamics’) tells us that at current prices removal at $50/ton is physically possible — always a good check to run!
  • Climeworks website used to list a target of $100/ton, which is 11x cheaper than they can currently manage. Given that they actually have commercial technology, I’m inclined to take this more seriously than the $50 estimate.

To sum up: while $100/ton or so seems more realistic, $50/ton is not an outrageous estimate.

The Bad: We have CO₂ on our hands

Earlier I said this…

DAC doesn’t get rid of CO₂; it just captures it for us, and we need to decide what to do with it.

And remember, the $100/ton or $50/ton figure is just for capturing CO₂; if we spend more money to transport the captured CO₂, etc., we need to adjust the figure.

So… options:

  • We can use CO₂ to push oil out of the ground — enhanced oil recovery
  • We can use CO₂ to produce other chemicals — carbon capture and utilisation
  • We can store CO₂, probably underground — carbon capture and storage

Enhanced Oil Recovery

This paper estimates an additional cost of €-4 to €56 ($-5 to $66) per ton CO₂ for enhanced oil recovery. The negative cost is possible because oil producers will pay you up to $30/ton to deliver them CO₂.

The extra cost will be noticeable, but there’s a more important point: using CO₂ to recover more oil is counterproductive. More precisely: by making it easier to extract oil we would push down the cost of oil. That would in turn increase the amount of oil extracted. And whether the oil is used for plastics or burned, that would increase the amount of CO₂ in the atmosphere.

It should be possible to quantify this effect. I can’t find a justified estimate,* but this notes that CO₂ accounts for 25-50% of the cost of oil, so the effect will be significant. Without actual numbers I’m loathe to trust this method; emissions from oil mean that your $50 will pay for less than a ton of emission reduction, and it’s even possible that net emissions will increase.

*This paper estimates that if you maximise the amount of CO₂ you are pushing underground, which oil producers don’t currently do, the emissions from the oil correspond to about half the CO₂ used in enhanced oil recovery. But that doesn’t tell us how the ensuing change in oil prices would affect consumption and so emissions.

What’s less obvious is that carbon capture and utilisation has the exact same problem just discussed. CO₂ in fizzy drinks and food has to be very pure, but all the other industrial users of CO₂ are buying from the same industrial sources that oil producers are, such as power generation plants. In other words, there’s a market where people buy and sell CO₂. If you sell more CO₂ on that market, the price of CO₂ goes down. That makes every process using CO₂ cheaper, including oil recovery. That means more emissions. You can’t get around this by only selling CO₂ to, say, firefighters; they will just buy less CO₂ in the market, pushing down prices and making enhanced oil recovery cheaper.

Again, it should be possible to quantify this effect. You can look at all the buyers of CO₂, estimate their emissions per ton used and average to see the effect on emissions. But I can’t find such an analysis and without it, can’t be sure how much carbon capture and utilisation reduces emissions.

What about fizzy drinks and food? Climeworks are working with Coca-Cola, so we know the CO₂ from DAC is pure enough. Just one problem: as soon as you open up your Coke, the CO₂ goes back into the atmosphere. And the same is true for any use in food. Arguably there will still be net reductions, because the food industry won’t be getting CO₂ from other sources, but this article is not about that kind of offsetting. Besides, there’s a much bigger issue with utilising captured CO₂, which I’ll return to below.

The only real way to store CO₂ is to pump it underground, for example into empty oil fields (which has a certain irony). Sites with higher capacity tend to be offshore or otherwise harder to reach; this estimates a cost of $7-$24 per ton stored in sites with reasonably high capacity. Of course, we also need to get the CO₂ to the storage site, which costs $4-$7/ton using offshore pipelines or per ton or $13-$19/ton using ships.

These transportation and storage costs add a bit to our estimate of $100/ton, but not too much. So it looks like we can get rid of all that CO₂. There’s just one more thing to consider…

The Ugly: Scaling up

There is one very, very big caveat to everything I’ve written so far. Let’s look back at our starting point, and update it with our new estimate:

  • The UK produces about 800 million tons of emissions each year
  • Removing CO₂ with DAC costs $100 per ton
  • So it will take $80,000 million = $80 billion to cancel a year’s emissions

Hidden in this is the idea that if you can capture 1 ton of CO₂ for $100, you can capture 10 tons for $1,000, 100 tons for $10,000 and so on, all the way up to 100,000,000 tons for $10,000,000,000 and finally 800,000,000 tons for $80,000,000,000.

Intuitively, this kind of ‘linear’ scaling makes sense because once we have one working DAC plant, we can make lots of identical ones. 10 plants should suck out 10 times the CO₂ at 10 times the price; 100 plants should suck out 100 times the CO₂ at 100 times the price; and so on. (In fact economies of scale mean costs per ton should go down as we scale up, but we already took that into account in our $100/ton estimate.)

Reasoning about prices like this is natural because it’s what we do in everyday life. If I can buy a pen for $1, I can buy 10 pens for $10, or 100 pens for $100, or 1,000 pens for $1,000.

Not nearly a million pens. From here.

But… what happens if I try to buy a million pens? What happens if I try to buy a billion? At some point something will break down — pen prices will go up, or perhaps pens just won’t be available whatever I pay. As with pens, so with DAC. Let’s look at the different issues…

From here. Gt means gigaton, or billion tons.

The graph above, from this analysis, shows how much CO₂ we could potentially get rid of in different ways. The whole world currently produces around 50Gt of CO₂ equivalents each year. You can see immediately that the scope of CCU (utilisation) is negligible. Oil recovery (EOR) could use about 5Gt each year, or 10% of current emissions, albeit with the problems noted earlier.

More usefully, storage (CCS) could handle about 8Gt each year, or about 15% of current emissions. That’s more useful than it may seem, because emissions will be lower by 2050. That said, it’s clear that they won’t be 85% lower than now. The UK was originally aiming to cut its 2050 emissions to about 35% of current levels; it’s not on target to do that. It’s plausible that we’ll halve global emissions by 2050; then DAC and storage could handle about 30% of the remainder. Not ‘zero carbon’, but not useless either.

Because CO₂ is already transported in pipelines for use in oil recovery, we can get an idea of the scale of what’s needed by comparison to the oil industry. I’m going to quote directly here:

in 2050 the CCS industry will need to be larger by a factor of 2–4 in volume terms than the current global oil industry. In other words, we have 35 years to deploy an industry that is substantially larger than one which has been developed over approximately the last century, resulting in the sequestration of 8–10 GtCO₂ per annum by 2050 […] This is an exceptionally challenging task, similar in scale to wartime mobilization, but it is a task we should not be daunted by.

From The role of CO capture and utilization in mitigating climate change

I don’t generally feel qualified to say what is or isn’t politically viable. But I’m going to go out on a limb and say that anything that’s ‘similar in scale to wartime mobilization’ is somewhat ambitious.

When I showed you the chemistry behind DAC, I left out one rather important thing…

From here.

Energy.

Correction: I got the numbers wrong in the first v. of this subsection. Some authors use Ceq to mean carbon-equivalent, and others to mean CO₂-equivalent; I didn’t spot the difference.

Energy needs to go in as a mix of electricity and heat. The mix needed varies between the two main DAC methods, but they both require substantial energy inputs. This paper suggests 45 GJ (gigajoules) per ton of carbon removed, transported and stored, which is about 12 GJ per ton of CO₂. We estimated we could store 8 Gt (gigatons) of CO₂ a year by 2050; that’s 8,000,000,000 tons. Multiplying that by 45 GJ/ton we find that in 2050 DAC and storage would use 96,000,000,000 GJ of energy.

That’s 96 exajoules.

If you haven’t heard of an exajoule before, that’s because ‘exa-’ is a prefix used for very, very large numbers. (kilo, mega, giga, tera, peta, exa.) Energy measurements are always large, but this is still a huge figure. Current world energy production is around 550 exajoules; DAC would need about a fifth of that.

I should say that the 96 exajoules figure might not take into account the free ‘waste heat’ used by the low-temperature method mentioned earlier in the article. This review estimates that the method needs 1750 kWh (kilowatt-hours) of heat to capture a ton of CO₂. That’s about 6 GJ per ton, or half the energy required. So in this case, we halve the energy requirements; DAC would need about 10% of the world’s current energy supply to capture 15% of current emissions.

I was a little worried that I had put too much weight on that 45 GJ/ton estimate, so I looked for alternative analyses. This paper suggests ramping up DAC fairly slowly until 2100. They estimate it will then need ‘a quarter of global energy demand to provide power and heat for DACCS technologies by the end of the century’ (my emphasis).

Where’s all that energy going to come from? The paper just mentioned suggests natural gas. But that will of course mean more warming. Here’s what they suggest…

This graph has a target of 1.5° C by 2100, but the graph for 2° C looks very similar.

The darker line is emissions without DAC; the lighter line with DAC. They’re proposing a huge increase in emissions until about 2070, to build and run gas-powered DAC plants, followed by a sharp decrease from 2070 onwards. Which means a lot more warming until 2070, then rapid temperature drops. Given the uncertainties inherent in any new technology, this seems to me to be an extremely risky strategy.

What about using renewable energy instead? The problem is that renewables are already expanding as fast as possible; we won’t have spare renewable energy for DAC for decades. Remember that the low cost estimates depended on ramping up DAC starting now, and you can see this just doesn’t fit together.

The DAC machines contain chemicals, such as sodium hydroxide, which are used to absorb CO₂. Scaling DAC to huge levels therefore needs huge amounts of chemicals. This short paper looks at the analyses I cited above and finds they ignore this requirement. That pushes up the energy/electricity requirements even more (likely by more than current annual world energy/electricity production), but let’s set that aside and just look at the quantities that would be needed:

(Corrected:) This tells you how what chemicals we’d need to produce to run DAC at a scale of 30 gigatons of CO₂ a year. NaOH (sodium hydroxide) is used by the high temperature method; NH₃ (ammonia) and EO (ethylene oxide) are used to produce the monoethanolamine used in the low temperature method. In all cases we’d need about 100 years worth of the current world production.

Those estimates are based on capturing 30 gigatons of CO₂ a year, which is about 60% of current emissions. Even if you scale down to the 8 gigatons/year of CO₂ we can store underground (~15% of current emissions), you’d need about 25 years worth of the current world production of those chemicals.

These aren’t chemicals used in small quantities. Sodium hydroxide is used to make soap and paper; ammonia is used to make fertiliser. Both have huge markets already — about $30 billion for sodium hydroxide and $50 billion for ammonia. There’s no way we can magically scale up these markets to obtain the extra quantities required for DAC.

Oh, did I mention that producing that much sodium hydroxide would also produce 50 to 100 times the chlorine gas the world can currently use? So we would have to store gigatons of that somewhere. As you probably know, chlorine gas is a chemical weapon.

I can’t see any way this could possibly go wrong.

tl;dr

Nice idea, but there’s no way we can scale it up to meaningful levels by 2050.

Thank you to the denizens of /r/slatestarcodex, especially /u/Terrible_Amphibian_4, for thoughts and corrections.

Key science, with sources. Minus bad statistics. Minus shaky methodology. Minus politicisation, left or right.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store