How realistic is 'air fuel synthesis' as a petroleum substitute?

I saw this on one the financial news shows I watch and it sounds like pie-in-the-sky to be honest. I’m sure it’s possible from a technical point of view, but I can’t image it could be anywhere near being cost effective - ever. Unfortunately a quick search didn’t turn anything up in that regard.

The idea has 2 parts. First you capture CO2 from the atmosphere. Second you break water into hydrogen and oxygen and capture the hydrogen. With those components, you have the ingredients to make a renewable hydrocarbon fuel.

You can read more about the process here.

I’m not that interested in the chemistry unless there might be issues that affect cost. For example with biofuels, I think one of the problems is that they still have to be refined and that adds to the cost of the final product (although I’m not really clear on that). So if there are similar issues here, as there seem to be, that would would be interesting if the discussion isn’t unduly technical.

My main interest would be estimating the cost per liter or gallon for producing fuel this way as well as the extent to which it would be a drop in replacement for existing fuels. For example the link above mentions converting the reactants to syngas but that further reactions are necessary to create synthetic petroleum and gasoline - thus adding to the cost. Obviously syngas can’t be considered a drop-in replacement.

It is always going to take more energy to build burnable molecules than you can get from burning them as fuel.

All projects of this type are variations on perpetual motion, and all fail due to the laws of thermodynamics. You can’t get something for nothing.

Burning Hydrocarbons releases energy due to the strength of the bond between Carbon and Oxygen, and Hydrogen and Oxygen. To “capture” the Carbon from CO2 and the Hydrogen from H2O, you first have to apply just as much energy to break those bonds as the reverse reaction will yield.

BUT—and here’s where thermodynamics ruins the party—no process is 100% efficient, and it will take more energy to “capture” the Carbon and Hydrogen reactants than burning them will produce.

Yeah, it could be done, if you had a huge surplus of energy to spare. Maybe some day, after we finally get fusion power plants working or something, this is how we’ll power airplanes (hydrocarbons do, after all, have a lot of advantages as an energy-storage medium-- They have a very high energy density and are quite safe, relatively speaking). Right now, though, pure pie-in-the-sky.

Right, but people always tend to talk about these issues in absolutes. The problem is that the markets always sees them on a continuum of costs. What is the cost of refitting existing fleets with a new type of engine for example. Is it technically feasible. What type of energy or fuel will you use. And so on.

I can imagine that at some point, electricity production costs could drop to a point where the input cost required for the process might not be prohibitive. I’d just like to get a handle on that.

For example, I think solar panels are getting down into the $1/watt range and I think efficiency is around 20% - but that might be the high end - not sure. The $1 figure I think is for thin film printable panels. The point is that at some point you’re probably going to have a more decentralized grid system with people and companies feeding into the grid rather than just drawing power from it. So the idea of cheap electricity isn’t completely ridiculous - at least I don’t think it is.

This step right here will require more energy input than will be contained in the resulting fuel.

I’m pretty sure this is a given. I think the point is to use cheap electricity to create a liquid fuel that is equivalent to gasoline. I’m just curious as to how cheap electricity would have to be and at would point it would be cost effective. IOW, how expensive would oil have to be vs how cheap would electricity have to be - assuming that these are the only relevant factors.

If you want to produce liquid fuels for transportation, starting off with air is a pretty silly way to do it. Start off with coal, or biomass.

I mean, air synthesis is how plants do it–they take in water and CO2 and sunlight and produce sugar and O2. Photosynthesis for the win. Except we can skip a step by getting plants to do the air synthesis for us, harvesting the plants, and use the plant biomass to manufacture liquid fuel.

The most common liquid fuel we create today in this manner is ethanol, Brazil makes a metric shitload of it. Biodiesel is another. But we could end up with any arbitrary organic molecule we like given enough money and energy for synthesis.

I do understand that there are a lot of other options. That’s not what I’m asking. If you want to get into some of them you can look at the recent research into using ambient noise to create quantum coherence the same way plants to.

That’s actually irrelevant. Petrol, unlike electricity, is easily portable. Currently, that is.

Fuel from air is pretty much the holy grail and whoever invents an economic solution will make billions if not trillions.

If electricity is cheap enough to make this process economically viable, I seriously doubt we will need hydrocarbon fuels. It would probably take Fusion Power to get electricity cheap and abundant enough to power this scheme.

I suspect this has more to do with Carbon offsets and Taxes, than an actual Fuel Source.


Actually, one of the best arbitrary molecules to use as a liquid hydrocarbon fuel molecule is dimethyl ether. This is the most simple of all ethers, is almost biologically non-reactive, is non-persistent in the environment (unlike methane which is worse as a greenhouse gas than CO[SUB]2[/SUB]), can be stored at ambient temperature under mild pressure (unlike liquified natural gas which has to be maintained at cryogenic temperatures), has a low carbon load for the energy content, can be synthesized from any source of methane, methanol, or syngas, and is suitable for a wide range of applications where propane and diesel are currently used with minimal changes in infrastructure or combustion systems. It even has potential as a renewable fuel for liquid propellant rocket engines despite the low mass energy efficiency with future advances in pulsed detonation or continuous wave detonation engines.

Methanol is another possibility which can be synthesized directly from natural gas and used in automotive applications as an almost direct substitute for gasoline with just small modifications to the combustion map and swapping out of a few seals. In fact, many high performance cars use a fuel blend with a high methanol percentage in fuel (up to 85% methanol) or even pure methanol. It actually has several safety and thermodynamic performance advantages despite the somewhat lower specific energy content compared to gasoline, and in a trial program by the state of California showed fuel economy comparable to gasoline in engine systems original designed to operate on gasoline (with the appropriate combustion mapping).

As for sequestering CO[SUB]2[/SUB] directly from the air, the biggest problem is how sparse CO[SUB]2[/SUB] actually is, especially at ground level. However, carbon dioxide in the oceans and large bodies of water will come to equilibrium with the surrounding atmosphere, and sequestering the much denser concentrations of CO[SUB]2[/SUB] from these sources is potentially viable provided you have a non-polluting power source to do this.


Liquid or highly compressed gaseous fuels will still be required for mobile and transportation applications for the foreseeable future, whether we are using internal combustion engines or electrochemical fuel cells.


Absolutely, Sorry for the lack of clarity. I was trying to continue on others ideas that were expressed upthread. There are other methods of fuel generation that are more efficient WRT energy input vs energy output. I would think that these technologies would be used first (coal gas, biofuels etc) before we would resort to using this method.

Yes for the time being we need portable fuels, and for a long time coming.

I do suspect that this is more related to Carbon offsets and Taxes.


Never mind being profitable in energy terms – getting more out than you put in. That’s obviously impossible.

But could an economy work where big centrally located plants used sunlight, or fusion, to break water into hydrogen, and then the hydrogen is pumped to gas stations and used to fuel cars? This seems to be a model in some futuristic projections; Scientific American mentions it every now and then.

Yes, you’re losing real energy, but putting it into a useful form. We burn coal in big central plants and turn it into electricity in the same way: we lose energy – we could all have coal stoves in our homes! Much more efficient than turning the coal into electricity and the electricity into heat!

Hydrogen is not a good medium for mobile energy storage or a portable combustion fuel for internal combustion engines or fuel cells.


Well, for some transportation needs, at least. You could build a train system where the trains don’t need to carry any fuel at all, for instance, instead drawing their power from catenary lines. Still, sometimes there’s no substitute for an airplane, and practically speaking, no other power source than liquid fuel is currently good enough for them.

1 US gallon of gasoline contains about 120 MJ of energy.

1 KWh of electricity is 3.6 MJ.

So at 100% efficiency it would take 33.3 KWh to make 1 gallon of gasoline. At $0.10/KWh that’s $3.33/gallon.

In reality I’d think efficiency would be rather low, but I have no idea just how low. 20%? 5%?

So, it’d be significantly less efficient than an electric car, given that we have to generate electricity to separate out the gases. It still has the “advantage” that we can generate the gases overnight and store them, and pump them into vehicles as needed to fuel them up, rather than plugging in the cars to charge them.

Each step is difficult and expensive.
As has been pointed out, the only reason to do this is because you have a HUGE surplus of energy.
The only “efficient” way to do this is photosynthesis, which generally makes sugar rather than oil - so the additional next step is to ferment into alcohol. Photosynthesis - i.e. raising plants - requires a very controlled environment. Temperature and humidity, sunlight and watering etc. have to be good. this talks about creating oil (diesel) from coal and wood. Since it’s not done yet, obviously not cost effective versus $100/barrel oil.

The problem is the level of infrastructure investment versus return. We’d need the breakthrough solar power, acres of plants, water, processing plants, etc.