[coderenegade]

All the best to them, but I'll believe it when I see it.

I'm also skeptical that petrol / diesel / long chain hydrocarbons are even the right fuel to make. If the average length of the carbon chain for something like diesel is around eight, you can make roughly eight times the number of methanol molecules for the same carbon input. Hydrogen requirements are also lower, so the cost limitation there is reduced as well. It just seems like an inherently cheaper $/kWh pathway for storing energy, especially when you consider that there are already amateurs doing methanol conversions for cars for a few thousand dollars.

Obviously this doesn't work for aviation, but the calculus there is a bit different. LH2 has a number of advantages for aircraft, and depending on how much cheaper it is than synthetic kerosene, it may prove to be the better option.

On the subject of direct air capture -- have any studies been done on its efficacy relative to fast growing plants? Some seaweeds can grow at a rate of a meter a day, which obviously requires pulling carbon from the water (i.e. indirectly from the atmosphere). Similarly, it seems like there are pre-existing (and potentially cost effective) pathways for shorting the carbon cycle by, for example, using sewerage as a source, since all of that carbon was at one time pulled from the atmosphere by a vegetable.

[ncmncm]

The sweet spot for synthetic fuel, in most cases, is anhydrous ammonia. It stores in liquid form at room temperature under mild compression. Ammonia can be burned in place of natural gas in generators, and in place of bunker oil in ships given retrofitted tankage and plumbing. It is probably practical for retrofitted freight trucks, rail locomotives, and farm machinery. Its volumetric energy density is lower than kerosene's, but usually tolerably so. It is unlikely to find use in cars.

Anyplace where LH2 aircraft operate, kerosene-powered airframes will be simply unable to compete. It is not clear that existing airframes can be retrofitted, and build-out of LH2 craft may take a long time. By 2040, if civilization has not collapsed yet, probably the majority will be LH2, and old kerosene airframes will be on marginal routes.

Synthetic hydrocarbon fuel will have strong demand for at least a decade or two, maybe longer depending on many factors including various costs, taxes, and regulations.

[coderenegade]

Maybe. Ammonia is certainly the best carbon-free fuel, the only real issue with it is its toxicity. Methanol is a lot easier to handle since it's a liquid at SSL conditions, and generally less dangerous, but it has it's own downsides (it's hygroscopic, for example). But I agree with you in the sense that, to me at least, the two most sane options for an e-fuel are methanol and ammonia. I think the path of trying to make synthetic petrol and diesel is probably the wrong choice.

[jabl]

> Ammonia is certainly the best carbon-free fuel, the only real issue with it is its toxicity

If we don't care about toxicity, might as well go with hydrazine, which is liquid at room temperature and has higher energy density than ammonia. ;)

(No idea about the cost of hydrazine synthesis, though)

> most sane options for an e-fuel are methanol and ammonia

The article seems to say their process produces ethanol. If it really works as well as they claim it does, it could be a gamechanger.

[ncmncm]

We do, in fact, care about toxicity, so do not e.g. expect to see ammonia powering cars.

NASA and ESA are phasing out use of hydrazine because of its unfortunate handling characteristics. But solid hydrazone might still find uses.

Among the chief attractions of ammonia is that it is very simply synthesized with free feedstock. You bond hydrogen stripped from water to nitrogen from air. Although toxic, it is a lot lighter than air, so if it leaks it goes up, and does not hang about poisoning people in a broadening area.

The main problem with hydrocarbons as synthetic fuel is that you need the carbon, which in air is at below 0.05% concentration. It is certainly possible, but seems unlikely to approach ammonia in cost.

[jabl]

> NASA and ESA are phasing out use of hydrazine because of its unfortunate handling characteristics.

I read about various 'green propellant' efforts over the years, but it seems in practice hydrazine is still used for orbital manouvering systems. Of course for those the volumes are small compared to the launch rockets themselves. For launch vehicles it seems only old Russian and Chinese designs still use hydrazine.

> Although toxic, it is a lot lighter than air, so if it leaks it goes up, and does not hang about poisoning people in a broadening area.

Eventually yes it will dissipate upwards, but typically ammonia accidents result in a vapour cloud traveling close to the ground. E.g. https://www.youtube.com/watch?v=qIi4_Poo2HY

> The main problem with hydrocarbons as synthetic fuel is that you need the carbon, which in air is at below 0.05% concentration. It is certainly possible, but seems unlikely to approach ammonia in cost.

There's certainly a largish cost to concentrating CO2 from the atmosphere. Carbon Engineering, one of the companies in the DAC space, claims 8.8 GJ/ton. Just some back of the napkin comparison to the enthalpy of formation for CO2 and H2O (+ adding an assumed 70% efficiency for CO2 dissociation and water electrolysis), and assuming we're building hydrocarbons with a 2:1 H:C ratio, that would mean a roughly 40% energy penalty compared to starting with a concentrated CO2 feedstock.

One the plus side you get a fuel with cheaper and safer handling, better energy density, and compatibility with existing equipment. Hard to say which approach will win. Might well be as you said, that for large industrial users that can take appropriate precautions like maritime shipping or peaker power plants ammonia will be a better solution, but for other smaller scale usage synthetic hydrocarbons will win.

[ncmncm]

They still loft hydrazine on birds already designed, but plan to use a non-toxic, and otherwise actually better alternative in new designs.

Hydrazine is troublesome because it is readily absorbed through the skin, in liquid form, or the lungs, in vapor form, whence it destroys the liver and other internal organs. As vapor it is slightly heavier than air, so its vapor spreads out from point of release.

When you see dramatic videos of spilled ammonia, that is generally liquid, either boiling anhydrous or dissolved in water and spreading out on the ground. Purely gaseous leaks go up. But liquid spills can be pretty bad.

All that said, synthetic hydrocarbon fuels will clearly be better for small and consumer-grade use in places where batteries do not suffice.

[coderenegade]

"I'm pretty sure we can go through that iceberg." --Captain of the Titanic, probably.

"Might as well go with hydrazine." --Unknown, heard before explosion.

I missed the bit on ethanol when I skimmed through the first time, but you're right. That's actually fairly exciting if it's something that can be scaled.

The part that baffles me is that if you have a workable fuel like ethanol, from an efficient process that doesn't compete with arable land, why try to make petrol from it? Ethanol is perfectly fine, and inherently cheaper than any hydrocarbon you would make from it.

[jabl]

> The part that baffles me is that if you have a workable fuel like ethanol, from an efficient process that doesn't compete with arable land, why try to make petrol from it? Ethanol is perfectly fine, and inherently cheaper than any hydrocarbon you would make from it.

Indeed. Even replacing the current usage of ethanol in the gasoline pool would be a huge market, and AFAIU most Otto engines can be relatively cheaply modified to work on up to E85 fuel.

Perhaps they're trying to fly under the radar of the corn ethanol lobby?