Comment by credit_guy | Hacker News Reader

Comment by credit_guy | Hacker News Reader

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> This is an important sanity check

You have a bug somewhere, I'm not sure where.

Roughly speaking the heat of combustion is proportional to the number of atoms of oxygen in the molecules coming out of the reaction.

So CH4 + 2 O2 = CO2 + 2 H20

If you hypothetically split CH4 first, you get

CH4 + 2 O2 = (C + 2 H2) + 2 O2 = (C + O2) + (2 H2 + O2) = CO2 + 2H20

The first reaction is endothermic, but it absorbs much less heat than the heat produced by the second part of the reaction.

So, you can roughly say that the energy coming from burning CH4 comes half from burning the Carbon and half from burning the Hydrogen.

Now, if you can make the reverse reaction CO2 -> C + O2 with 100% efficiency, then sure, you get to economically burn CH4 with zero emissions. But if that reaction has only 50% efficiency, then all your (energetic) profit has been wiped out.

The article doesn't say what efficiency this envisioned reaction has, but I'd be mightily surprised if it were 50%.

Much better to not burn the Carbon to begin with. That is what methane pyrolysis [1] tries to do.

[1] https://en.wikipedia.org/wiki/Pyrolysis#Methane_pyrolysis_fo...

These carbon capture and conversion technologies are far more promising as a means of feeding atmospheric carbon dioxide into industrial 'aerochemicals' (to replace petrochemicals) and materials, than as a means of mitigating fossil fuel combustion CO2 emissions.

Obtaining a pure stream of CO2 concentrated from 400 ppm atmospheric sources is the optional approach for industrial chemistry processes (and requires significant upfront energy investment), but from here one can go almost anywhere, to methane or jet fuel or graphite electrodes or carbon fiber building materials or synthetic dyes.

However, it's unlikely these technologies will have much effect on reducing atmospheric CO2 levels. They simply eliminate the need for natural gas / petroleum / coal as raw materials for synthesis of necessary products.

The article clearly states a claimed 92% efficiency.

Good point. But there are two problems with that claim.

One is about an order a magnitude, the other about a sign.

The first one: if you burn 1kg of pure Carbon, you get 3.67 kg of CO2 and 32.8 MegaJoules [1], which is the same as 32800 kJ or, 32800/3600 kWh. That's 9.11 kWh for each 3.67 kg of CO2, or 2.4848 kWh/kg. That's quite close to 2500 kWh/ton.

They are claiming it takes them 230 kWh/ton to reverse this reaction. You can see this is an order of magnitude wrong. Let's say they meant to say 2300 kWh/ton. If you divide that by 2500 kWh/ton you get exactly 92% that they claim.

But here's the second mistake: you need to put more, not less energy to split CO2, otherwise you'd get energy for free. 2300 kWh/ton is simply impossible. You need to use more than that, and actually more than 2500 kWh/ton if you don't want to violate the first principle of thermodynamics.

[1] https://en.wikipedia.org/wiki/Heat_of_combustion#Heat_of_com...

Maybe the researchers meant 230 Kwh above the reaction energy of pure carbon and oxygen. That makes the math check out, since 2,485/(2485+230)=91.5%, which could be rounded up to 92%. It would also square up with the estimated costs of $100/ton, since industrial electricity prices can go as low as 0.03 to 0.04 USD.

I wish I could read the source to verify, but unfortunately it's behind an academic publisher paywall.

> They are claiming it takes them 230 kWh/ton to reverse this reaction.

I don’t think that’s a claim in the article. If the carbon product was pure in the sense it could be burned again then it wouldn’t work out. But that’s not claimed. It’s not reduced to carbon!

To quote the article:

"The reactions break the carbon dioxide into oxygen gas, as well as carbonaceous sheets which ‘float’ to the surface of the container due to differences in density and can therefore be easily extracted."

and

"According to the research team, the process showed 92% efficiency in converting a tonne of CO2, using just 230kWh of energy."

Exactly. Carbonaceous.

Which is Carbon, because that's what left when you take away the O2 from CO2.

No it’s still oxidized carbon (graphene oxide) just less oxidized than CO2.

Otherwise, if it was unoxidized you could just burn that carbon again and regain 10x the energy you spent!

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