[gus_massa]

> Plants do it, after all. It's not impossible.

The problem is that to reverse CO2 -> C + O2 you need the same amount of energy that you get burning coal C + O2 -> CO2.

But burning coal, like half of the energy is lost as unuseful heat.

The reaction of plants is different, but plants only has a 2% of efficiency. The chemical reaction of plants is more complicated, so let's be optimistic and assume this reaction has a 10% of efficiency.

If they use a coal plant to power the CO2 -> C + O2 conversion, they will release like 20 times the amount of CO2 absorbed.

If they use a renewable source, it's better to close the absortion plant and also 20 coal plants.

Until we close all the coal plants and we get very cheep carbon-free energy, it's bad for the environment to try a CO2 -> C + O2 conversion.

[jjk166]

There are reasons you would want to use fossil fuels and use renewables to reverse the process. Fossil fuels have high energy density, can be transported and stored with minimal losses, and consumed anywhere anytime. There are lots of things that are difficult to directly electrify like say planes. You can't simply stick a bunch of batteries on a plane to replace a jet engine and its fuel tanks. But building an excess of solar panels on the ground to mop up the CO2 equivalent of the jet engine's exhaust is simply a matter of cost. E-fuels are basically just really dense chemical batteries.

[gus_massa]

I agree. I'd prefer using something like biodiesel in planes, but I'm not sure if there is a technical limitation.

[BizarroLand]

Seems like the chemical process has been a solved equation for a while. Rather than directly splitting the molecule, they use high pressure and heat to incorporate a hydrogen atom into it, converting it into Methane which can then be broken down with high heat to carbon and hydrogen.

From the page https://chemistry.stackexchange.com/questions/915/how-can-ca...

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React carbon-dioxide with hydrogen in the Sabatier (https://en.wikipedia.org/wiki/Sabatier_reaction) process to get methane. 400 °C, high pressure and Nickel catalyst needed. The process is slightly exothermic so it can keep going on its own:

CO2+4H2→CH4+2H2O

This process have been proposed to generate fuel on Mars, and used on the ISS to process exhaled carbon-dioxide.

Split the resulting water (use electrolysis or some other thermochemical cycle) take the oxygen, bring the hydrogen back to Step 1:

2H2O→2H2+O2

Electricity for this may come off solar panels.

Methane decomposes at high temperatures. The process goes to completion around 1200°C. Collect the condensed carbon, bring the hydrogen back to step 1.

CH4→C+2H2

This process is proposed as an emissions free alternative to produce hydrogen from natural gas. Heat may come from concentrated solar light.

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The missing ingredients are excess CO2 from the atmosphere and a machine system to do the full process with, and of course the math about economies of scale regarding how fast a machine that uses this method combined with the powder in the article can actually strip carbon from CO2 in the atmosphere.

It likely would neither be cheap nor fast, but since it uses nickle as a catalyst at least it is not fancy rarefied expensive hard to find materials, just more an issue of getting all of the parts together and making the whole machine run as efficiently and effectively as possible.

Also, if the methane is generated from CO2, then burning it is nearly carbon neutral, so in places where fossil fuels are the ideal energy source it would allow us to use Green energy to make transportable low-carbon fossil fuels for them to use.