# 2.3: DAC

The National Academies report has a detailed chapter on DAC if you want to read more.Chemical direct air capture machines are not some crazy new technology. They have been used on submarines and space stations for quite some time, and you can even buy one (for your underground bunker or whatever).

Interestingly it seems that Prometheus Fuels may have a more energy efficient way to electrolytically generate fuel, specifically for the part dealing with the separation of the generated fuel from the surrounding water — this kind of thing could be fantastic for jet fuel specifically, which is otherwise difficult to decarbonize.)The idea has a long history. A group including Freeman Dyson, from 1977, cites a 1965 proposal by Beller and Steinberg. This was an era when people were bullish on abundant nuclear energy, and they proposed to use this energy to extract CO2 from the air to produce chemical fuel such as methanol locally for the army, by reacting adsorbed CO2 with hydrogen generated electrolytically from water. They estimated a need for 5700 processing plants, each burning 1 GigaWatt, to compensate for emissions. That’s about 6 TeraWatts or about ⅓ of civilization’s current global energy consumption.

This paper, by Klaus Lackner, covers the basic physics of carbon capture really well, e.g., how long an absorption column for CO2 needs to be, and so on. He also explains why some chemical capture approaches may be more energy efficient (not to mention much more land efficient) than biological capture approaches:Same for any other approach that generates an energy-dense fuel“Biomass generation is, however, a very inefficient approach because it is coupled with the reduction of the carbon which requires as much energy as was released in the combustion”.

Lackner also explains where the energy requirement actually shows up (we know it must show up somewhere from the thermodynamics argument above) in these processes:

“Most of the energy demand for an absorption process is in the recovery of the absorbent”.

In other words, getting the CO2 back off of the thing it stuck to. What is then the main reason why we can’t easily reach the ideal thermodynamic limit above? Lacker explains:

“In practice, most effective absorbents will bind much more strongly then is required strictly by thermodynamic considerations.”

In the paper, Lacker proposes industrial facilities to generate CaCO3 bricks by reacting CO2 with Ca(OH)2, which would then be recycled to continue the process. Recycling the Calcium to re-run this process then consumes much of the energy. The binding energy to the absorber is about 8 times too high in this case, compared what would be needed to approach the thermodynamic limit.In practice, basically one needs to heat the carbon-capturing materials to cause them to give back their captured CO2, and this heating is where the energy is needed — and, because the binding to the material is stronger than it absolutely needs to be thermodynamically, you pay extra energy for the heating. So, we have understood the fundamentals of carbon capture!

Land use

Trees are great and all, but planting enough trees would take up a large land area all across the world. Direct air capture using industrial facilities would have a much smaller land area footprint, and would arguably have a much smaller ecological footprint overall. Here is an image of it from the recent EFI report:

To convince ourselves of the landuse differences we can do a dumb back of the envelope calculation for this. Suppose you need to capture 10 GtCO2 per year.

We can first figure out roughly what % of the mass of a given unit of air CO₂ is. The atmosphere is mostly N2 which weighs 28 g/mol, CO2 weights 44 g/mol so at 410ppm we can roughly state that (410*44 / (1e6 * 28)) = 0.064% of the atmosphere's mass comes from CO₂.

We can then use V=M/density to get that
$V = \dfrac{10Gt}{density \space of\space air * 0.00064} = 1e16m³$  of air processed per year.

The fact that the BECCS area is a lot larger than that should be cautionaryIf your single DAC plant processes 100 m^3 per second of air and takes up the size of a football field, then our total area of DAC plants needs to be: 5 million acres, or 6x the total area of Rhode Island.

## Commercialization

You can exapand the dropdowns below to get more details on some of the companies currently working on DAC technology.

Lackner and Allen B. Wright have started a company called Global Research Technologies to work on this. As the Scientific American article explained, “Lackner and his partner, Allen B. Wright of Global Research Technologies (GRT) in Tucson, Ariz., have developed a proprietary plastic that grabs CO2 from the atmosphere the way flypaper grabs flies. When the CO2-enriched plastic is rinsed with water vapor, a stream of pure CO2 forms that can be sequestered underground”.

(Incidentally, another potential process is sodium hydroxide (NaOH) + water to absorb CO2, then recycle the alkaline NaOH by addition of lime (CaO) to make calcium carbonate, which is then heated to recycle the lime.)

David Keith et al also have a startup on this, called Carbon Engineering. It appears they are using a refinement of the Lackner process. From a recent paper:

“As with any industrial technology, there is a sharp distinction between the ease of developing “paper” designs and the difficulty of developing an operating plant. To paraphrase Rickover: an academic plant is simple, cheap, and uses off-the-shelf components; whereas, a practical plant is complicated, expensive, and “is requiring an immense amount of development on apparently trivial items.”24 [Carbon Engineering] has now spent roughly 100 person-years on such apparently trivial items to develop a process proposed almost two decades ago by Klaus Lackner and collaborators…”

Their proposed 94 to 232 $/t-CO2 is very expensive, however: to do 30 gigatons per year is 3 trillion dollars per year, about 4% of world GDP. Still, this is much lower than previous estimates. Where is Carbon Engineering going to get the energy inputs to this process? One option is to burn natural gas, but do so cleanly, using the same type of carbon capture chemistry they use for ambient air capture to simultaneously sequester CO2 from the burning of the natural gas and thus stay strongly net-negative. This TED talk suggests that that is what they are doing. (One recent paper proposed to derive the energy from bioenergy with carbon capture and storage (BECCS) which would make the overall process even more strongly CO2 net-negative.) Another company in the space, called Global Thermostat, thinks they can bring it to$50 per tonne of CO2. They have some estimates online of what their system can do: “20-500 tonnes of CO2/yr/m^2 or more, depending on the embodiment used”. This is to be compared with emissions of 35.9 GtCO2 of carbon dioxide per year. So to offset current emissions, we need, assuming an embodiment with 200 tonnes of CO2/yr/m^2, to have a total area of a square 14 kilometers on a side, or 44 km on a side if it only captures 20 tonnes CO2 per yr per m^2. Let’s call it 100 km on a side conservatively, about the area of the state of Connecticut. Here is a paper from this group, and some discussion of the economics.

Climeworks is another company in the space. Their site has a really nice graphic on comparison of carbon capture techs, citing this and this. It appears from Jennifer Wilcox‘s TED talk that they want to get the energy input to their process from geothermal, or from industrial waste heat.

In any case, commercial efforts in this space currently seem like a way to increase the “tech readiness level” proactively, anticipating a larger-scale effort later on, for instance with policy changes that put a price on CO2 emissions and improve the economics. Richard Branson has a prize offering for a commercially viable large-scale effort in this area, which has yet to be awarded. (Back of the envelope says it is just possible. An industry roughly the size of the automotive industry would do it.)

## Some emerging DAC technologies

Metal Organic Frameworks (MOFs)

MOFs are one of the most impressive areas of modern atomically precise nanotechnology. The reason we care about them here is that they may be an alternative to some of the more traditional substances used to actually capture the CO₂ in DAC.

I can see where nano-porous solid structures like MOFs could be helpful in terms of space efficiency: we don’t want a CO2 molecule in the air to have to diffuse very far before it hits the absorbing surface. The main question I had about this at first was the feasibility of later extracting the carbon from this dense 3D environment, but reassuringly this JACS paper also observed low capacity loss (~0.2%) after 50 cycles of temperature-driven CO2 desorption. This work is being pushed forward by the company Mosaic. On their website, they point at the fundamental issue we identified above that limits energy costs for carbon capture and prevents reaching near to the thermodynamic limit — regenerating the capture material, i.e., getting the CO2 back off once it is stuck: “cooperative-binding technology allows the CO2-loaded materials to be regenerated using only moderate temperature or pressure changes, substantially increasing energy efficiency and decreasing costs”. There are other impressive companies in the MOF space as well, one of which I got to visit a year or so ago.

Electro swing

Also in the realm of new approaches, instead of regenerating the adsorbent with heat, there are variants that are electrically switchable in their CO2 binding capacity. This seems to open very fertile ground for nanotechnology exploration. Electrical rather than thermal cycling could potentially be done at ambient temperature and pressure and with improved energy efficiency. I’m sure there are plenty of practical challenges but this looks super interesting.

Verdox is a company founded by the authors of this paper which uses this electro swing approach. You can see more about how it works in this MIT news write up or in their video below:

Most governments, it seems, aren’t propping up carbon markets successfully at this point, and most current policy interventions aren’t really carbon taxes (e.g., there is an equivalent of a $1000/ton CO2 subsidy for electric cars in Norway, where if you go to Oslo nearly every other car is a Tesla). As mentioned in the previous post, France had a fuel tax of about$60/ton CO2 equivalent, but when Macron tried to double that over the next 4 years, the Yellow Vest protests happened. Canada did pass a carbon tax starting at below that level and rising to near it over a few years (“start low at $20 per ton in 2019, rising at$10 per ton per year until reaching $50 per ton in 2022”). And see the 45Q tax credit program in the USA. So there is recent progress and I think reason for hope that the economics will work out here. This post explores some of the broader potential applications of captured carbon.New markets for carbon also seem interesting in this context, at least as a way of jump-starting the economies of scale in the field, e.g., the company Blue Planet is using captured carbon for building and highway materials. According to their marketing, “If every new building for the next 30 years was made with the resulting product, humans could erase the globe-warming pollution they’ve sent into the atmosphere, said Fiekowsky…”. ## Electrofuels One thing some companies are doing is taking CO₂ removed from the air with DAC and using it to make various kinds of fuels. The basic idea is that you use a combination of DAC to get CO₂ from the air, electrolysis to get hydrogen from the air, and then combine these to generate hydrocarbons. It’s probably worth mentioning that the DAC part, while we’ve mentioned it being pretty energy intensive, is actually the least energy intensive one of these steps. Prometheus Fuels, for example, wishes to take atmospheric CO2 all the way back into a fuel for aircraft. This is going to cost an energy at least equal to the heat of combustion of the fuel, not to mention that associated with the concentration of the CO2 from the atmosphere. I was a bit surprised, but it is not actually in principle crazy to make gasoline this way even with the higher say 1000 kJ/mole energy requirement: if a gallon of gas requires/releases 20 lbs of CO2, and energy cost 5 cents per kWh, then we have a cost per gallon of 20 lbs / (44 grams per mole) * (1000 kJ / mole) * 5 cents per kWh =$2.86 / gallon, which is not an unreasonable gas price, and this closely matches what was said about this project.

Another recent entry into this space is Terraform industries. They position themselves as a lower capex higher opex approach than prometheus fuels. Basically, they want to use the same kind of approach (get CO₂ from DAC, get hydrogen from electrolysis, combine them to make hydrocarbons) but they aim for a lower efficiency system that has a higher percentage of costs in the energy input. This is largely a bet on the declining costs of renewables, specifically solar, if it continues to fall in price at its current rate then if most of your costs come from energy and not infrastructure you get to piggyback on that cost reduction.

While these technologies don’t really exist at any real scale today they do represent potentially the largest market for captured CO₂. This can be useful as if we can find profitable ways to use CO₂ from DAC then this can create a much better business model for these technologies to grow without needing government support. This is in the carbon removal section but obviously these technologies don’t have quite the same effect as a lot of the other things mentioned as they lead to the re-release of the CO₂ not the permanent sequestration. Still I think there are two benefits to this approach even if it doesn’t lead to the permanent sequestration of ​​CO₂, firstly there are lots of areas that will be very tricky to move off hydrocarbons so a carbon neutral way of providing these is a win there. Secondly, if these technologies can be profitable then they can rapidly scale up the use of DAC which hopefully can bring the cost of DAC down a lot which would have positive spillovers for using DAC purely for removal and sequestration.

The whole field is very nascent and really in the long run would only make sense to use for things where we don’t have a good electrically powered alternative. Why wouldn’t you use this for something you can power with electricity? Saul Griffith in his book points out that if you have an electric car you would store this in a battery (≈90% efficiency) and then power the car through a drivetrain (≈80% efficiency) giving you around 72% overall efficiency. If you used this to make gasoline then the process to make it would be around 50% efficient and the efficiency of then burning this gasoline would be around 20% efficient leaving you with a 10% efficiency rating on the same electricity. While it could in theory become cheaper than normal gasoline it will never be cheaper than straight up electricity.

Still I think this is an interesting area that will grow rapidly in size.

Market conclusions

Perhaps if CO2 is efficiently converted to certain specific forms like fuels, and in an economic environment with appropriate taxes on carbon emissions, or specifically on dirty fuels, it could be cost competitive — this seems to be Carbon Engineering’s plan. For large-scale deployment at the scale of the overall CO2 problem, the economics is at the forefront, and the economic incentives imposed collectively by society through carbon taxes and related mechanisms would be key, but see the previous paragraph regarding recent progress on this.

Overall, although some have been skeptical of the cost scaling of industrial direct air capture, there is an impressive amount of startup activity in the space. The Lawrence paper points out that truly widespread use of direct air capture may not naturally be employed anytime soon, since a more efficient approach would be to do carbon capture and storage directly at the output of fossil fuel burning power plants. But this is still a good way to get the technology moving… and commitments like the one made by Stripe could help bootstrap the field.

____________________________________________