MIT engineers make converting CO2 into useful products more practical

I have not thought about this too carefully so I might be overlooking something. With that out of the way, a quick search indicates that we burn about 90 % of gas, oil, and coal for one purpose or another. Let's round this and pretend we burn it all. To undo this we will essentially need the same amount of energy again that we got out of it when we burned it, we would need to use all the energy coming from fossil fuels to undo burning them. Conservation of energy essentially.

Which makes it obvious that the entire idea is pretty pointless, burn fossil fuels to generate energy to then use it to unburn fossil fuels. To do it with renewable energy, we still need the same capacity as the fossil fuel capacity and when we have that - ignoring issues like fluctuations in renewable sources - it makes more sense to just use the renewable sources directly instead of using them to undo burning fossil fuels.

If you want to use the process to pull carbon out of the atmosphere, then you first have to replace all fossil fuels with renewable ones, then you can use additional renewable capacity to remove carbon. Add additional 10 % capacity to the world energy capacity to undo one year of carbon emissions every decade, at least to a first approximation.

To come back to the initial question, you essentially need an industry the same order of magnitude as the fossil fuel industry to have a meaningful impact. Not going to happen anytime soon.

The point that you're missing is that changes this equation a bit is that burning fossil fuels wastes most of the energy as heat another waste of energy is the amount of FFs we use to ship FFs to other places. So together that means we don't need the same amount of electric power to do the same amount of work. That being said, keeping fossil fuels in the ground will always be better than removing CO2 for the reasons you said. We also seem to be growing energy demands instead of shrinking or stabilizing them which also makes the transition harder.

Lots of solar on-site that doesn't need to transfer it's power elsewhere could be used; maybe the real winner would be 100% solar-powered solar panel factory :)

>>we would need to use all the energy coming from fossil fuels to undo burning them

This would true if we need to re-create the original molecule with it's stored energy (plus losses of course).

However, it seems this is a misapprehension of the task. Instead of trying to recover the entire hydrocarbon molecule, we're "just" trying to extract or recombine the CO2 reactant.

Without doing the chemistry or the math, it seems likely that a variety of methods of either preferentially attracting CO2, or combining it into simpler lower-energy-dense molecules to be collected, would require less energy as was in the original hydrocarbon, often substantially less.

Seems it should be an inequality, not an equality. Or am I missing something?

> To undo this we will essentially need the same amount of energy again that we got out of it when we burned it

Amine based carbon capture at the smokestack captures about 90% of CO2 with a 20% energy penalty. There's a new natural gas turbine design that captures 100% at no energy penalty (Allam cycle).

Both those technologies do not undo the burning process, they just capture the carbon dioxide. Like putting a gigantic balloon on top of the smoke stack to trap the flue gases. The real problem here is what do you do with all your captured carbon dioxide? You are producing it at the same rate as you are consuming fossil fuels, for each tanker, pipeline, or train delivering fossil fuels, you will need an equally sized tanker, pipeline, or train transporting the carbon dioxide to some storage facility. For each well or mine extracting fossil fuels you need a equally sized hole in the ground to dump the carbon dioxide into.

If you can use the renewable energy directly, that's far better, but often you can't.

Synthetic hydrocarbons let you use renewable energy shifted in time, space, modality or avoid capital costs.

- applications that can't use batteries, like long distance plane flights

- applications where it's cheaper to spend 6X as much for fuel than it is to buy a new vehicle

- for storage more than a few days

etx

Sure, if you capture carbon dioxide from the atmosphere to use it as a storage or transportation medium for renewable energy, that is perfectly fine. Trade some efficiency for overcoming some of the limitations of renewable sources like their variability or the difficulty to store electricity.

But that is not really carbon removal from the atmosphere, you take some out and later put it back. The article however frames the endeavor as removing carbon from the atmosphere, either the one we are currently burning or even the one we burnt in the past. Carbon removal by definition means we can not burn it later somewhere else, we have to permanently store it somewhere. There is no point in turning the carbon into some high quality product if we then just bury it somewhere, you want something cheap to make and easy to store.

As a non-fossile source for chemicals it makes sense but that is just a small fraction of our problem as we just burn most of the stuff.

While you are right that capturing the carbon dioxide can be done with relatively little energy, that is not what the article is about. If you capture it, you end up with tons and tons of waste, essentially as much as the fuel you burned, what are you going to do with it? The article is about [...] converting CO2 into useful products [...] so that you do not end up with waste but useful products and the requires as much energy as you got from burning the stuff, at least to a first approximation, you would of course not try to recreate the exact same stuff you just burned.

If you capture the carbon dioxide, then for every supertanker full of oil you burn you need to permanently get rid of a supertanker full of liquid carbon dioxide. This is of course a project of insane scope given that we burn billions of tons every year. So in order to not have to deal with the waste, what if we just turn it into something useful that people will pay for? Because that costs a lot of energy, the energy we just extract. And now you want to put it back in? To get back what you just burned or at least something similar that you could almost certainly produce more efficiently directly from the oil?

OK, yes, building larger-molecule more-useful-stuff will take more energy, and I'll go with the first approximation that it's a similar quantity of energy re-input (some useful things less, some more). And yes, all that product will take substantial volume. Thx for clarifying.

That said, it still seems an extremely useful measure, even if we keep using only single-digit percentages for long-use plastics instead of hydrocarbon fuels.

Let's assume that for the next century or so a bunch of applications will continue to require the convenience and energy-density of liquid hydrocarbons. In order to avoid extracting more and further increasing CO2 levels, we'll have to input significant energy to reconstitute them from CO2. Obviously, inputting that energy from more fossil fuels defeats the purpose, but using renewables will work; and now they are even cheaper energy inputs.

The result would be a cycle of newly fabricated hydrocarbon fuels, which can be custom-optimised for each application. No new CO2 would enter the atmosphere and the existing levels would be reduced by the amount of hydrocarbon fuels (and plastics, etc.) fabricated and in existence throughout the entire chain of existence, fabrication, storage, distribution, transport, in-vehicle, right up to the moment it is burned. With cheaper renewable energy inputs and optimized custom fabrication, it would likely get cheaper than the existing drill/pump/transport/refine process. And, it's permanently sustainable, and as liquid hydrocarbon fuel use declines, custom production can be converted to storable materials.

Totally agree, it makes sense to use renewable sources to produce hydrocarbons from the air, whether to burn them or for chemical products. But to significantly remove carbon from the atmosphere as suggested in the article it makes no sense.