The Fool on the Hill: Lies, damned lies, and aviation fuel

Since my last post about aviation fuel last week, the commentariat, inspired by Rachel Reeves' gibberish about a third runway at Heathrow, has been engaging in a paean of magical thinking.

Wouldn't it be nice, say The Rest is Politics today, from Syria, to which they've flown, because of course they have, if we could make aviation fuel from waste? After all, the aviation industry say we can, so it must be true, mustn't it? Specifically, for example, GE Aerospace say

While [Sustainable Aviation Fuel] can have the same chemical compositon as petroleum-based jet fuel, it has lower lifecycle carbon emissions. This is because it can be made from renewable sources such as plant-based material, fats, oils and greases, alcohols, waste streams, captured carbon, and other alternative feedstocks.

(my emphasis)

And they're not lying. It can. How much of it?

Not very much.

Note the things other than waste streams that they say sustainable aviation fuel can be made of: 'plant-based material, fats, oils and greases...' We have a collective noun for these things in English, which is simpler to say than that long list. It is this:

Food.

That's what I argued in The potato famine and aviation fuel: we can make sufficient aviation fuel for the amount of air travel we do today. All it takes is enough food for three and three quarter billion people. After all, aviation fuel is refined hydrocarbon. Human food is refined hydrocarbon with a few necessary additives.

Waste words

But, what if we could make aviation fuel from waste?

What is waste, in this context? Waste is, hydrocarbon rich things which are not food.

After two billion years of continuous evolution, animals — we — have become extremely efficient at extracting hydrocarbon energy from organic material. The things we can't use as food are those whose energy cost of mastication and digestion is greater than the energy which can be released by burning the hydrocarbon it contains; or those things from which the energy cost of removing the components which are actually poisonous to us is uneconomically high (that's why we typically don't eat food made from coal or petrol, by the way).

We can, of course, grow crops which produce carbon rich material which is poisonous to us, and we could make that into aviation fuel. But we couldn't grow it on non-productive land, because that land is non-productive. And if it's productive land, then it could be used to grow food; and if it could be, it mostly is being.

Finally, there's another form of waste. There's shit. Shit is food which has passed through the gut of an animal and had all the hydrocarbon energy which can efficiently be extracted, extracted. You can then take that shit and use anaerobic digestion to extract a little bit more hydrocarbon, but it's a little bit more.

So, fundamentally, what sustainable aviation fuel can be made from is either food, waste food, or shit, with a very small side order of grossly poisonous or very unpalatable things. And frankly, there isn't very much of it; we're not that wasteful. As we grow more careful with the storage and transport of food, there's progressively less of it.

So, yes, if we want to fly, say, half a dozen long distance flights a day, worldwide, we can certainly do that on aviation fuel made from waste. We can make some aviation fuel from waste. We can make some aviation fuel literally from shit. It isn't impossible.

Hydrogen

Hydrocarbons are high energy fuels, because they contain both carbon and hydrogen atoms, both of which are highly reactive with oxygen. We call reacting a material with oxygen, 'burning' that material.

Burning a kilogram of carbon releases 33 megajoules of (heat) energy, and produces CO₂, carbon dioxide, which is a greenhouse gas. Burning a kilogram of hydrogen releases 141 megajoules of heat energy, and produces H₂O, water, which is not.

So obviously, it makes sense to run planes on hydrogen, rather than hydrocarbon?

Well, yes, if you have an economic, sustainable source of hydrogen.

You can split water into hydrogen and oxygen by electrolysis. The energy cost of doing this is higher than energy you get back from burning the same amount of hydrogen, but you can do it and all steps in the process are in principle clean. You could fly all the planes we fly now. You'd just need a staggering amount of energy (I'll run the numbers later; it's probable more than all the electricity generated on earth, but I'll have to do the calculations).

Once you've split water, and extracted pure hydrogen, you then have to store it. All atoms are small, of course, but hydrogen atoms are particularly small. Being particularly small, they can pass through the gaps between the atoms of solids like, for example, steel. So it's quite hard stuff to store. It also makes steel brittle. Consequently, although hydrogen is extremely light, safe containment of hydrogen is not. And you want to keep hydrogen safely contained.

So even if you could economically generate enough hydrogen to fly jetliners, you might still not be able to fly jetliners with it.

Batteries

Similarly, you can fly planes using electric motors powered by batteries. You can do that now. The problem is twofold. First, we don't have any battery technology with the energy density of hydrocarbons; that's partially mitigated by the fact that electric motors are much more energy efficient than internal combustion engines. But secondly, you have to carry the whole weight of the battery for the full duration of the flight, whereas with with fossil fuels the weight decreases quite rapidly in the initial climb to cruising altitude (because a lot of the total fuel is burned then), so for the rest of the flight there's a lot less to carry.

So I'm not claiming, now, that it would be impossible to fly, in future, much as we do today, in electric aircraft. I don't think it will be practical, for the same reason as electrolysed hydrogen will not be practical: the round trip efficiency of putting electricity into a battery and taking it out again is less than 100% (although lithium ion batteries are better than 95%, so close). And powered flight requires an extraordinary amount of energy.

Manipulated truth

No. Aviation fuel from waste; hydrogen as aviation fuel; battery powered airliners; these are all conscious, deliberate, intentional, damnable lies. But they are a special kind of lies. They are in fact, two distinct special kinds of lies. In the first place, they are lies which are true at the margins.

1. We can, in fact, fly some planes on fuel made from waste;
2. Hydrogen would be a good aviation fuel, if we could isolate it cleanly at reasonable cost, and solve the problems of storing and transporting it;
3. We can certainly fly small planes on short routes using battery power.

But behind these marginal truths is the much bigger and far more obvious truth that we're all trying not to notice: hurling a four hundred and forty two ton plane ten kilometres into the air to transport just four hundred and sixty seven passengers (almost one ton per passenger) is an extraordinarily, a recklessly, profligate thing to do. A thing so profligate that it would not, were we not burning through literally millions of years worth of fossilised sunlight, be possible to do at all.

So they are also a second, special, sort of lie; a sort described by George Monbiot as 'perceptionware' — fake solutions to real problems in order to convey the perception that something is being done, that there is a technofix coming down the track, that the problem is, in fact, manageable — when it is not.

Examples of 'perceptionware' that Monbiot cites are carbon capture and storage, oil from algae, and, of course, mumbo-jumbo jets.

But perhaps the clearest example of perceptionware is the repeated unveiling, across the past 25 years, of mumbo-jumbo jets. Throughout this period, fossil fuel and airline companies have announced prototype green aircraft or prototype green fuels, none of which has made any significant dent in emissions or, in most cases, materialised at all. Their sole effect so far has been to help companies avoid legislative action.

The clearest indication that sustainable aviation fuel is greenwashing — or 'perceptionware' — is the rate at which targets for its deployment are being missed. In their 2024 report 'Greenwashing the skies', the Institute for Policy Studies reports:

The aviation industry has a twenty-year history of missing its SAF production benchmarks. The International Air Transport Association (IATA) announced an aggressive climate goal in 2007, asserting that SAFs would account for 10 percent of all jet fuel consumed by the aviation sector within a decade. The target was never met. IATA proceeded for the next ten years to make their goals less ambitious while still failing to meet them. It predicted SAFs would hold a 3 percent market share by 2020, but SAFs currently account for just 0.2 percent of the total jet fuel supply.

So: sustainable aviation fuel was going to be 10% of all aviation fuel by 2017, then 3% by 2020, then 0.2% in 2024. And we're going to reach 'net zero' — which must mean that sustainable fuel is 100% of all fuel (not just aviation fuel) by that time — by 2050?

The other leg has bells on it.

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