Source
r/AskEngineers•
blackdowney

Recently graduated MEEN that wants to build direct air carbon capture device.

Discussion

It’s pretty easy to see that humanity is doomed if we don’t curve our emissions and sequester carbon for good. I’ve been fortunate enough to graduate with no loans and have invested my money wisely so that working is optional.

I’d like to turn my attention towards beginning a new journey of trying to fix things (even if it’s a waste of time). I’ve been trying to come up with ways of capturing carbon from the air, that may be inefficient from a power perspective but cheap to build.

Would it be possible to use a jet engine’s compressor and 0 degrees centigrade ambient temps to obtain liquid CO2 from the air? I assume it isn’t done due to an incredible amount of inefficiencies, but if I could build a solar powered system, despite its inefficiencies, I think it would at least offset my own carbon footprint.

Thoughts?

Posted Dec 16, 2021, 11:14 AM UTC

115
saywherefore•

Why not put that same solar energy into the grid and so reduce the amount of fossil fuel burnt to generate electricity? That would almost certainly have a far higher carbon reduction effect.

3
blackdowneyOP•

Yes of course. This would likely have a larger net carbon reduction. I just want something that removes it from the air while everyone else works to revamp the grid.

2
saywherefore•

This will sound harsh, but if you want to do some good then there is no point pursuing a vanity project like this. If you can afford to work without pay then look into the various charities and not-for-profits that are working on scalable solutions to eco problems.

3
blackdowneyOP•

This may be the way in terms of getting to a shortcut on learning / avoiding pitfalls. It’s not harsh at all btw.

2
Robertusa123•

Forget carbon capture.... the real invitation is finding something marketable to do with all the carbon your going to capture

2
blackdowneyOP•

I was thinking of a custom HVAC system that can store power using liquid CO2 somehow. It seems that if temps will continue increasing, then an HVAC that can also store thermal energy may be a possible use case.

This whole thread is going to come across like I learned nothing in 6 years. I know I’m being impractical.

2
Robertusa123•

Dosent sound energy efficient. It takes alot of energy to condence co2 into a liquid state. Not to menchen the engory needed to keep it stores in a liquid form

2
gankedbyewoks•

Info on R-744 (CO2)

https://emersonclimateconversations.com/2015/07/16/co2-as-a-refrigerant-comparison-of-r744-with-other-refrigerants/

2
dmills_00•

The problem with atmospheric carbon capture is that the atmosphere is very much mostly NOT CO2...

Were I trying for carbon capture I think I would try to sneak up on it from behind.

Consider for example that biomass captures carbon just fine, but then returns it if you let the biomass rot or if you burn in the conventional way, it does however deal with the 'get reasonably concentrated carbon' bit of the problem. Fast growing plants that you then process to capture the carbon are probably the way to go.

So, lets see we want to burn the stuff in a way that results in an almost pure CO2 waste stream that we can then do SOMETHING (TBD) with. Ideally this process should be a net energy producer so we can make the whole thing pay...

Burning carbon biomass with air gets you about 20% CO2 in the exhaust (Because air is ~20% oxygen), better then trying to capture from ambient air, but still not wonderful, so lets look elsewhere.

How about the water gas reaction? Endothermic, but the resulting hydrogen can be burned in air and should more then provide the heat required.

C+2H2O => 2H2+CO2 (Plus some CO in reality), separate the product gasses and you have a pure CO2 waste stream and a mostly H2 stream, burn some of the H2 to provide the heat to run the reaction, and do whatever you were going to do with the CO2 stream, run the rest of the H2 as feedstock for the H-B process to make fertiliser for the biomass production?

The real problem is what do you do with the CO2 once you have it?

1
willowgardener•

I mean... Hemp and bamboo are very fast growing and have a number of uses... Seems like engineering machines to further automate their processing would be an effective climate change fighting tool

1
dmills_00•

What is it about fucking hemp?

Hint, the stuff does NOT make good rope, ask anyone who has sailed tall ships, there is a reason we all use modern synthetics that just look like the old stuff, but don't give you splinters and don't rot from the middle.

In any case at scale land based plants are going to be WAY too labour intensive, think algal blooms instead, you can move them around for processing with pumps not people.

1
willowgardener•

I wouldn't use hemp for rope (although one advantage hemp has over synthetics in that area is that it decomposes, and thus does not contribute to ocean plastics). I'd be more inclined to use it for clothing, building materials like hempcrete, and paper--areas where synthetics are not viable or have disadvantages. I'd be inclined to use it to replace cotton, for instance, because it has a higher yield per acre, requires fewer pesticides and less water, and can be grown in a wider variety of climates--but, as you mentioned, it has a high labor cost to process. This is why I mentioned engineering machines to further automate its processing. I'd be inclined to use it to replace wood-based products to reduce deforestation, thus capturing carbon further.

In regard to "what is it about fucking hemp", the answer for me is its relatively high tensile strength, decomposability, ability to be grown in a wide variety of environments, and fast growth (and thus fast carbon capture). Certainly there are materials that can capture more carbon, like algae, but have few uses (beyond burning it for fuel, which negates the carbon capture); and there are materials that are in some ways better, like polymers, but have destructive environmental impacts. It strikes a nice balance between utility and helpful environmental impact.

1
bgraham111Mechanical Engineering / Design Methodolgy•

An interesting thought experiment.

Capture CO2, even if it is energy inefficient.

So where is your energy coming from to capture the CO2? Burning coal? Burning natural gas? Solar? Wind? Hydro? Nuclear? Will your energy source release more than is prevented/captured?

What is a better use of the energy you have? Capturing CO2? Or preventing the release of more CO2?

Why a turbine to compress mostly Nitrogen (78% of air), to only get a little CO2 (0.04% of air)? Are there more effective ways to capture CO2?

Is the goal to reduce CO2 in the air? To capture CO2 that is already out there? To specifically use a turbine to capture CO2, even if it's not the best solution? Trying to understand the goal, the purpose.

Maybe figure out what you are really trying to do, then run some numbers. Start simple in a giant excel spreadsheet to do some rough calculations, to see if this warrants more study.

1
davidquick•

How are you going to pay for it? I suspect that you will find that after you set everything up you will be net negative once you figure in the cost of manufacture of all the parts and apparatus as well as the CO2 generated acquiring the wealth to purchase all the equipment much less operate it. I'd guess you'd be 10-20 years in the hole at ideal electrical efficiency.

1
YogurtIsTooSpicy•

Most oil and gas majors now have teams dedicated to carbon capture processes. If you're serious about working in that field, then ironically Shell, Exxon, Chevron, bp etc. are going to be your best bet.

What you're describing is technically possible and is called cryogenic distillation. It's currently used to produce pure component gasses like oxygen and nitrogen from air, but is prohibitively expensive for CO2. Other separation technologies like amine absorption, pressure swing adsorption, or membrane separation show more promise but are still not yet cost-effective.

Another consideration is the source. Atmospheric air is something like 0.04% CO2 while boiler flue gas is more like 15%. It seems to make more sense to me to do carbon capturing further upstream rather than waiting until your working volume is, you know, the entire atmosphere.

1