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I came here to say essentially this. To your point about how low (in percentage terms) the concentration of CO2 is, direct carbon capture from the atmosphere still seems a fool's errand to me. The only way I see carbon capture being at all viable for this purpose in the short term is to scrub the waste gasses from industrial processes where carbon-containing compounds are extremely concentrated.

The green beaches thing seemed quite promising https://news.ycombinator.com/item?id=20403570

right, but that's for sequestering the carbon, not turning it into a valuable product like a fuel, unless I'm mistaken

Yes, but the main benefit of turning it into a valuable product is to make the process of sequestration cost effective. If sequestration could instead be made cost effective by just being really cheap (relative to other techniques), as will hopefully be the case with the olivine beaches, that could work too.

Cement (of certain types) relies on direct carbon capture from the atmosphere to complete curing (although conventional cement only carbonizes through a finite depth during its expected lifetime), so it's definitely possible. It just takes a lot of energy.

About 2 MWh (thermal, or ~7GJ) per tonne of CO2 captured (not counting storing that CO2) to regenerate the chemical CO2 capturing agent. Here's a review: https://www.sciencedirect.com/science/article/pii/S095965261...

(As an aside, a gallon of gasoline emits about 10kg of CO2, so to capture that CO2 from a gallon of gasoline would require about 20kWh worth of electricity.... Typically, a gallon of gasoline gives you about as much range as 10 kWh in an electric car, so you can go 2 times as far in an electric car just using the energy it'd take to capture the CO2 from a gasoline car. And that doesn't count the electricity required to refine/crack the gasoline from crude.)

And also, on board the International Space Station, CO2 is captured from the atmosphere regeneratively. Capturing CO2 in this way takes less energy at the higher concentrations on board ISS, but is still possible.

As an aside: Much of the ISS's CO2 is converted into a hydrocarbon (methane) using hydrogen (from water and solar electricity) and, amusingly, vented to space as a waste product. The point is to recover the oxygen (again, in the form of water) from the CO2.

In case someone else is wondering about the math on the aside about how a gallon of gas (3.8kg) can emit 10kg of CO2: https://www.fueleconomy.gov/feg/contentIncludes/co2_inc.htm

> And that doesn't count the electricity required to refine/crack the gasoline from crude.

If we can figure out how to extract atmospheric CO2 and convert it into hydrocarbons efficiently, crude oil won’t be bart of the equation any longer — our atmospheric carbon supply will be part of a closed loop. As a lay person, that’s an awfully appealing prospect.

I wonder how many kWh it takes to pump, transport and process a gallon of gasoline when sources from crude.

It’s doable, it just requires energy. You are really working against thermodynamics with CO2 sequestration.

If it's an unmoving device fixed in place, I could see that capture from air wouldn't be particularly efficient.

But what if it's a device that already exists that moves through air with great speed, filtering through huge amounts of air? Like, say, a jet engine?

Okay, but producing fuel requires a lot of energy, so how are you going to power that? With a different engine onboard your jet?

The point of this device is to take energy available on the ground and store it for use in the air, and use a storage method with a tremendous amount of energy per unit weight (hydrocarbons).

Planting forests and turning the trees into biochar seems like a cost effective way of carbon capture. You even get some energy out of it.

The biochar can then be used to plant more forests, for more biochar. It’s a virtuous cycle.

There are lots of point sources of CO2 that don't require as much energy to extract, but your point stands: the conversion takes a lot of energy (thermodynamics is a harsh mistress).

The tech works quite well though, CRI in Iceland makes methanol (a viable liquid fuel) from CO2 and electrolyzed H2 semi-commercially, taking advantage of cheap and abundant geomthermal electricity.

> To make a kg of hydrocarbon you need to process an awful lot of air to get enough CO2

Would replacing air with wood as the input alleviate the problem? (Trees, in effect, turn atmospheric carbon into hydrocarbons using solar energy.)

Empress trees capture roughly 100 tons of carbon per acre per year. But wood's energy density is roughly half that of oil (oil and coal are decomposed algae and wood respectively) so it's effectively 50 tons of potential jet fuel per acre of forest per year. A 10 hours 747 flight apparently burns about 250,000 pounds of fuel - roughly 125 tons. So we'd need about two and a half acres of forest for a ten hour 747 flight.

There has been some speculation that people could cultivate vats of algae illuminated by blue LEDs emitting light in the optimal photosynthesis spectrum. But this is probably only feasible with effectively limitless supplies of electricity.

The fuel in a single fully-fueled 747 contains about 150 tons of carbon. A forty-year-old tree contains about 1 ton of carbon.

If the jet's fuel tanks get filled about 200 times per year, then you need to cut down about 30,000 mature trees per year to supply the carbon to keep it supplied with fuel. This probably adds up to a few thousand acres of forest to continuously meet the needs of a single jumbo jet.

Well you and the rest of the naysayers are in luck. They just burned the amazon rainforest. That probably frees up plenty carbon into the air for the process.

Not trying to be a naysayer; I just wanted to make a rough estimate of just how many trees would be involved in this method. We already have the ability to plant and cut millions of acres of trees for lumber, so this is an entirely possible future. Whether it's more or less efficient than other methods of carbon capture is the real question.

Wood/biomass gasifiers were used during WW2 when gas was rationed in some areas. So they are practical, but with the current price of oil/gas I would not expect it is commercially viable. Wood is not the only source though, and there are current fuel sources that are wasted. Like it is common for farmers to burn the leftover straw after a harvest

> The obvious answer is the atmosphere. That's also the premise of carbon sequestration. The problem is that CO2 isn't actually particularly abundant. It's currently hovering around 400 parts per million - that's about 0.04% of air. To make a kg of hydrocarbon you need to process an awful lot of air to get enough CO2. That all takes energy. A lot of energy. Suddenly your round trip efficiency is practically zero. If these guys have cracked that problem, it would be fantastic. I hope they have, but I suspect they haven't.

Isn’t biofuel precisely this?

>To make a kg of hydrocarbon you need to process an awful lot of air to get enough CO2

Thinking laterally, instead of processing a lot of air, can't we just engineer a gaseous molecule that is good at capturing CO2 and can precipitate conditionally.

Imagine something like a buckyball that would let in a CO2 molecule. While it's sufficiently hot it stays under its gaseous form. Then once cold it falls to the ground.

You then release a lot of this engineered molecule in the atmosphere where it will come into contact and absorb the CO2 then fall to the ground where you can collect it efficiently by scrubbing it of surfaces like soot for processing and recycling.

If it is bio-safe and sufficiently stable you can even let it accumulate naturally in the water where you will retrieve it even more efficiently once it's concentrated enough. That's what we do in Rouen now and everyone is fine with it /s.

>The obvious answer is...

Did you know that there are massive streams of CO2 available for the taking?

https://www.reuters.com/article/us-australia-lng-carbon-fb/f...

Ammonia and hydrazine could work. I mean, as fuels, not as a PR stunt, obviously.

Maybe you put your device somewhere the winds blow for you. Think of the 200mph westerlies that blow through the Drake Passage.

Aside from the distance from civilization, you've got water, power, and air movement all right there.

Wind energy through a given cross section scales with the cube of air speed; kinetic energy per particle is proportional to the square of velocity, and mass flow is proportional to velocity. This makes high wind speeds difficult to design for, and most wind turbines will apply a brake and stop in high windspeed conditions in order to avoid overheating and other issues from the high energies involved.

I was thinking more of the need to move a lot of air for DAC, wind energy was somewhat secondary.