Hacker News Reader

Comment by foerbert | Hacker News Reader

They said carbon neutral, so I'm assuming the idea would be to use carbon dioxide and hydrogen to make kerosene to begin with. Then you can burn it and get the carbon dioxide back to start over. It's not carbon from the process entirely, but instead simply not adding additional carbon dioxide to the atmosphere.

View on HN

ok. I see. Due to conservation of energy it takes energy to synthesize fuel hydrocarbons. So you might start with carbon dioxide, run it through some energy consuming process exposed to hydrogen and get hydrocarbons and water as the byproduct from the excess oxygen. I don't know enough chemistry to know what the exact synthesis method would be but I do know it would require substantial energy input. Thus the kerosene serves as a kind of battery. It is not a primary energy source any more. If this is feasible why not make carbon-neutral gasoline too?

Too much energy loss. For airplanes it might still be worthwhile because the weight-savings are so subatantial.

Yes, exactly. There are/have been various small scale projects along those lines. Hydrogen and CO2 to methane, to longer-chain hydrocarbons that can be put into existing engines. As you say, the synthetic fuel is just an energy (and carbon) carrier, you have to put all of that energy into the system somewhere — ideally from solar or wind or nukes or something.

New Zealand synthesized fuel like this in the 80's during an oil crisis, as did Nazi Germany in the 1940's. It's well-understood chemistry, just too expensive right now. Probably will remain that way until it's too late unless we can put a ~$200/t tax on carbon emissions globally.

My understanding is Nazi Germany made synthetic fuels from carbon monoxide, eg from coking coal, which is a more reactive substance than carbon dioxide.

Right! The idea is that kerosene is a more convenient form of energy storage than many alternatives, despite the lower efficiency and higher capital costs for recharging it. Compared to, say, Li-ion, kerosene has numerous order-of-magnitude advantages; for example:

- safety: lithium-ion batteries explode if you look at them funny, and are full of poisons, while kerosene can put out a lit match and can be safely imbibed;

- materials availability: carbon is about ten times as abundant in earth's crust as lithium, and there are readily available ways to concentrate carbon from the air such as planting forests, while concentrating lithium from seawater is somewhat trickier;

- specific energy: Li-ion batteries can store 0.5 MJ/kg, and kerosene is 43 MJ/kg, which is a huge advantage for things like trucking and aviation;

- energy density: Li-ion batteries store 1 MJ/liter, kerosene is 35 MJ/liter, which is also important for things like aviation; and

- capital cost of storage facilities: you can store 200 liters of kerosene in a US$10 barrel, while 200 liters of lithium-ion batteries would be about 12,000 18650s, which costs on the order of US$50,000.

Gasoline does indeed share most of these advantages and can also be used in cheaper engines that can rev faster. Other fuels that might be reasonable to synthesize in the same way include hydrogen itself, methane, LP gas (propane/butane), and ammonia (!!).

The best way to compare batteries is energy density per unit mass, energy input and output per unit cost, and cost of production/charging. Long chain hydrocarbons have a much higher density than Li and are easy to pump and store and work in existing vehicles, but a higher cost of production and they are essentially one time use, non rechargeable since the discharge mechanism is combustion.

The ideal solution would be efficient solar bioreactors where bioengineered algea converted solar energy into fats as a bioproduct of a photosynthetic process sequester CO2 that could be esterfied into alkanes or that directly produced alkanes. Long chain alkanes, my understanding is, are not toxic to life. It is the short chain stuff and aromatic ring based stuff like benzene that is hazardous.

https://sfamjournals.onlinelibrary.wiley.com/doi/10.1111/175...

That's a valid way to look at it if you think of the battery as analogous to the kerosene, rather than the tank. (BTW, lithium-ion batteries don't contain metallic lithium, so the much higher energy density of Li is not in play here.) I was thinking of the species that are created and destroyed within batteries as it's charged and discharged, like lead and lead dioxide, as being more closely analogous to the kerosene.

Algae have very low capital costs but also fairly low photonic efficiency; it's reasonable to expect non-algae-based designs to be superior. (BTW, you can't esterify things into alkanes; alkanes aren't esters.) You're right about the toxicity profiles, although even things like hexane and octane are relatively low toxicity; hexane's oral LD50 is like tens of grams per kilogram, but they have higher toxicities if you breathe or inject them.

Ok. I was thinking of things like biodiesel. My understanding is that involves an esterfication process but I read about it 10 years ago and never took orgo. I accept esters are not alkanes my ignorance shows...thanks for correcting the record for other readers.

I was using Li as shorthand for Litium Ion batteries which I assume contain salts or oxides or hydrides of lithium. I know it is not elemental lithium. My point was that H-C bonds contain more energy than whatever is in Li batteries in the charged state per mol.

Reply on news.ycombinator.com