What's going on with mining carbon from the atmosphere?

[stefan r]

As you might have figured out, what interests me is building O'Neil structures. Don't get me wrong, I like the idea of the Earth cooling down a lot, but it isn't the only thing I'm thinking of. Carbon, being the lightest and strongest of materials to build with, is the logical material to build these structures from. That being (in my mind anyway) a given, it is a matter of getting the Carbon up where it can be used.

Build O'Neil cylinders using asteroids. The raw materials are already up there. Trying to get anything out of Earth's gravity well is going to be a huge energy drain.

https://periodictable.com/Properties/A/MeteoriteAbundance.html
Calcium is 1.1% of the mass of asteroids. Calcium is not very useful as a construction material. Magnesium is 12% of the mass of asteroids. Magnesium is extremely useful as construction material. However, we might be able to sacrifice some of it. Calcium and Magnesium react with carbon dioxide in the air. This ultimately creates limestone and dolomite. This material can be made into artificial islands. Sort of like what China is doing in the Spratly Islands. Or perhaps you could just let it burn up on reentry.

For power supply use orbital ring systems. The kinetic energy contained in objects in low Earth orbit LEO is roughly equivalent to the energy in hydrocarbons like butane. This is not including the oxygen mass. We have well developed magnetic braking technology. Some of it is deployed in electric cars. You need an elliptical orbital ring system that contacts Earth's surface in order to de-orbit objects while delivering power. The electric supply is much better than crude oil because magnetic brakes recover better than 90% of the energy. Power plants are limited by the Carnot cycle.

Lime (calcium oxide) is a major component of Lunar highlands (~15%). This should be sent to Earth's orbital ring system and then used as for concrete and mortar in construction projects. Concrete is labeled as a CO2 source but that is only in the production of portland cement. Concrete absorbs carbon dioxide during its lifetime.

If there is no ring system the mass can still be used by space stations with momentum exchange tethers. Catapulting the calcium into a lower orbit boosts the station to a higher orbit. The gives us momentum that can be used to catapult missions out of Earth's orbit. Most of the fuel used in launching deep space missions is wasted lifting fuel to LEO. It does not matter what sort of material you send down from Luna. Only the total mass matters. You could build mass into a useable products and use the products and then de-orbit when you are done with it.

Magnesium is weaker than steel by cross section but is about equal by weight. Magnesium is stiffer. That would be a defect in some cases like automobiles but for space applications like huge telescopes or frames for solar farms it is a feature.

I'm not certain what effects calcium (or magnesium) particles in the upper atmosphere would have on climate. They should rain out fairly quickly once they sink to cloud level. While the particles are up in the stratosphere they help cool things down. May also cause havoc in the ozone and most agriculture. Should do more research before trying this.

 

[DrStupid]

There is no serious thought about turning CO2 back into C.

Where does the black stuff comes from that burns in your barbecue?

 

[Khatti]

Harvesting carbon from CO2 in low Earth orbit is awfully inefficient. The highest-altitude data I could find is 100 km, with a concentration of ~240 ppm (extrapolated). You need at least twice that height for a reasonable orbit, which should drop the concentration significantly more, but let's use 250 ppm. At 10-10 kg/m3 your giant 1000 m2 scoop collects 750 mg of gas per second or 60 kg per day. Out of that ~20 gram is CO2, which means 6 g of carbon per day or 200 grams per year.

Would it be possible to drop a large tube or hose into the atmosphere from low orbit down to--say-- seventy kilometers and cycle atmosphere up into orbit where the oxygen and carbon could be separated? In theory cheap power would be more feasible in orbit than on Earth. I can see stability issues with a free hanging tube, but it may not matter at that height. I'm sure there are other technical issues, but then if we knew how to do it we wouldn't be having this conversation in the first place.

 

[DrStupid]

Would it be possible to drop a large tube or hose into the atmosphere from low orbit down to--say-- seventy kilometers and cycle atmosphere up into orbit where the oxygen and carbon could be separated?

The bottom end of the tube would get quite hot if it dives through the atmosphere with a speed of 7.8 km/s.

How does that "cycling" works?

In order to keep the speed of the orbiter, you would need to compensate drag and coriolis force (in the co-rotating frame). How are you going to do that? Maybe waste oxygen can be used as reaction mass for a rocket motor. But I don't think that would be trivial.

Maybe the drag and heat problem can be solved if the tube is not hanging straight down but rotating around the orbiter and diving into the atmosphere with almost zero relative velocity. But that would bring some other problems (e.g. the required strength of the material).

 

[Rive]

Would it be possible to drop a large tube or hose into the atmosphere from low orbit down to--say-- seventy kilometers and cycle atmosphere up into orbit where the oxygen and carbon could be separated?

Any place dense enough would mean significant atmospheric drag what you must compensate with fuel.
But if you have fuel (mass) at hand already then you don't need atmosphere anymore.

 

[mfb]

Any place dense enough would mean significant atmospheric drag what you must compensate with fuel.
But if you have fuel (mass) at hand already then you don't need atmosphere anymore.

Different use. The atmosphere scoop would use modified ion engines and expels most of the mass it collects to stay in orbit. The rest is accumulated and can be used in a conventional rocket engine to leave Earth quickly.

A rotating tether for ... gas collection? That's a curious approach, but I fear torque will ruin that concept. It would spin up until it breaks (unless you have a few breakthroughs with carbon nanotubes).

 

[mfb]

Let's keep this discussion about the science as good as possible.

 

[Jarvis323]

Is it really such a stretch to mine carbon from the atmosphere? Couldn't you just manufacture a practically arbitrary number of tiny carbon capturing objects (as much as needed), using solar energy in outer space, and then let them drop to the earth, capturing some carbon along the way? Is there a catch, because this type of approach seems pretty easy and feasible to me? Heck, you could probably even manufacture little bots capable of dipping down, grabbing some CO2, and then coming back into space. There should be plenty solar energy in space to not have to worry about energy efficiency. I guess just make sure to do it on the dark side of the Earth to avoid blocking the Sun.

 

[Rive]

Couldn't you just manufacture a... in outer space, and then let them drop to the earth...

If we could, it would be better to simply replace Earth based manufacturing instead of all that drone-and-whatnot for keeping the CO2-generating Earth based manufacturing running too.

Is it really such a stretch to mine carbon from the atmosphere?

Yes, it is: at least, it takes more resource and footprint than use the carbon directly from mines.

 

[Jarvis323]

If we could, it would be better to simply replace Earth based manufacturing instead of all that drone-and-whatnot for keeping the CO2-generating Earth based manufacturing running too.Yes, it is: at least, it takes more resource and footprint than use the carbon directly from mines.

But extracting it from the atmosphere using energy and resources acquired in space has some major benefits.

1) A practically limitless supply of resources and energy to use.

2) The energy used doesn't add anything at all to the carbon footprint, and the resources used would not be taken from our supply on Earth.

3) Reduction of CO2 in the atmosphere to reduce global warming (to any level we choose).

 

[Khatti]

Let's keep this discussion about the science as good as possible.

That is more than fair enough.

I freely admit that there is a Holy Grail aspect to this whole discussion. If when all is said and done, it comes to naught, fine. There is no guarantee that you will find a technical solution or scientific breakthrough if you're looking for it. But it's even more unlikely that you will find a technical solution or scientific breakthrough if you're not looking for it.

 

[member 656954]

Has Carbon Engineering been raised already? They're the Direct Air Capture leaders AFAIK. But there are also lots of research approaches, like this one, that are been mooted as solving the problem of cost-effective extraction from the air, or this one that uses micropore methods to trap the CO2.

Also, technically, these examples generate 'carbon' as an output, often as a fuel but for the last example, as polyurethane.

I do wonder if any CO2 atmospheric extraction method is going to be economic, perhaps unless a high carbon tax is applied that provides a profit mechanism for the method.