Yeah I don't buy carbon removal as a practical solution. Humanity spent over 100 years pulling concentrated carbon out of the ground as fast as we could and got a bunch of useful energy out of it. Now all that carbon we pulled out of the ground and dumped into the atmosphere is a stable trace gas. The thermodynamic hurdle to reverse what we've done is huge. The thermodynamics is not there, the economics is not there.
Keep in mind that the energy expended in C02 sequestration is concentration. It's easily soluble in water when injected and then is reactive to produce mineral precipitates. And that's only the geologic sequestration. There are multiple approaches, both biological, engineered, and geologic. Either way, the hard part is going from dispersed to concentrated in most cases (or for the biologic cases, actually keeping the result in the biosphere and not decaying into the atmosphere).
You don't need to reverse the combustion reaction.
You don't have to spend anywhere near as much energy as you gained from combustion to sequester it. You're not turning it back into bare carbon or into a hydrocarbon.
You're breaking hydrogen - carbon bonds to gain energy during combustion. You're then sequestering C02 -- you don't need to chemically alter the C02. You do need to concentrate it, but in terms of thermodynamics, you get a large amount of energy from combustion and it takes less than that to concentrate the combustion products.
In principle (but often not reality to the practicalities of concentrating a dispersed gas) combustion and then sequestration can be energy positive overall. The thermodynamics isn't quite what folks often assume.
On the flip side, we're just starting the 100 years now trying to extract carbon dioxide out of the air as fast as we can to get a livable planet out of it.
Granted we (as a species) have huge challenges ahead of us, but with collective effort it could be possible.
Our CO2 output is increasing. Under the world's current political/economic regime, we can't keep our CO2 output stable, let alone decrease it. If we lack the seriousness to do that, we definitely lack the ability to do the much harder task of removing what we've already put there.
