Catalytic process with lignin could enable sustainable aviation fuel | Hacker News Reader

Catalytic process with lignin could enable sustainable aviation fuel

nrel.gov

60 pointsph0rque81 comments

This seems like more organic chemistry research, not a giant breakthrough. Still, we need sustainable fuels in aviation for the foreseeable future. Given the physics of flight on this planet, batteries just won’t cut it.

Unfortunately even with a 100% renewable fuel, you still have additional greenhouse effect from the condensation trails. I doubt there’s a reasonable way to capture jet exhaust, and even if it did it would increase the weight of the aircraft.

So basically, the future is steampunk dirigibles, and don’t let anyone tell ya different.

I almost didn't believe you that the effect of contrails could be that large, but I searched and it sounds crazy: https://e360.yale.edu/features/how-airplane-contrails-are-he...

For what it's worth, contrails only form when conditions are right at certain altitudes, so if we had renewable fuels we could theoretically move aircraft outside the contrail altitude window. It would piss off controllers and burn slightly more fuel sometimes but it could be done.

I believe it's not actually contrails specifically but rather the effects of releasing exhaust (CO2) at a high altitude (which would happen whether the contrails form or not).

I’m not sure what you’re talking about, but the thread and article are about contrails, specifically.

CO2 is just the basic evil everyone talks about. Water is a lot more evil (much broader range of wave lengths of light absorbed than the comparatively tiny range of CO2), if it is in the air.

Water changes it behavior depending on the relative amount in the air. A sufficiently high amount will act on visible light (contrails, clouds, fog, ...), but even in its "transparent" form it messes with wave lengths outside of the visible range.

Contrails are created by local changes of temperature at the wing tips (turning the invisible water into visible, temporarily) and by adding water as part of the exhaust. Burning CxHy leaves some H2O, visible or not.

I would have guessed that the visible form is contributing more to cooling than the (probably more common) invisble form. Possibly a matter of when they turn invisible, which might be more likely in the morning than at night (those produced during one day staying visible during the night helping with heating but vanishing next day, not helping with cooling).

Modelling water properly is quite surely complicated, with bigger effects than CO2. Not only for contrails.

Contrails are ice from water in the exhaust, unrelated to the wingtips. You will sometimes see vortices at high angles of attack on takeoff or landing, but those don’t turn into contrails.

This became a minor fake controversy recently when Google changed their carbon footprint measures on flight to stop estimating these non carbon effects.

The reason they did so was they were just using a rule of thumb that applies to all flights and they want to be able to encourage airlines to adopt the known practices that can avoid these extra emissions (shifting time of flights is the big one I think).

There's no standard yet for estimating them though, but they are working on one:

https://airqualitynews.com/2022/09/01/is-google-airbrushing-...

I guess it’s unintuitive because the contrails only last a few hours, so how could they have any long term effect? But aviation is continuous, so they get replaced quickly, so there’s probably a constant contribution to warming.

I’m not sure how the contrail effect interacts with Earth’s increasing cloudiness.

I want to also say I didn't believe them but because I thought he was referring to the conspiracy theory not the actual water vapor trails

"Chem trails" is the terminology usually associated with the conspiracy theories.

> I doubt there’s a reasonable way to capture jet exhaust, and even if it did it would increase the weight of the aircraft.

Just taking it with you is a non-starter.

If your fuel is mostly carbon by weight, then the CO2 will weigh 2-3x as much. At that point batteries are starting to look appealing by comparison. There are battery techs with around 400Wh/kg and various almost-working ideas in the 500-600 region. Once you run 11000Wh/kg hydrocarbons through a turbine at 30% and carry the 2 Oxygen atoms for every C you might only be getting 1000Wh per kg of exhaust you carry.

Does commercial aviation carry an oxidizer though? I thought jets were air breathing.

OP's point is that if they wanted to capture the exhaust, then they would be carrying the oxidizer...

You are correct. I was speaking of the exhaust which is oxidised (that is, after all why you burn the fuel). If you wanted to do CCS on a jet the H2O can be vented, but the CO2 (which you have to carry to your destination) weighs 44/12 times the carbon in the fuel (which is a large portion of the fuel mass -- abput 3/4ths iirc).

I think the hydrogen-electric [1] development is more promising here because it has the potential to deliver long haul flights with no emissions or contrails.

While H2 isn't great for decarbonization at scale, in specific industries like aviation it might be applicable assuming it's made cleanly.

[1] https://www.zeroavia.com/

What's the justification to expect that 25-50% of human climate impact will be from aviation by 2050?

Growing population? Lower cost of access to flight? Other industries decarbonizing and causing aviation impact to just be a larger fraction of overall impact?

IIRC it's a combination of air travel continuing to expand at the roughly 2.5% yearly rate it has been doing for the past decades, and other sectors of the economy successfully decarbonizing.

Isn't a contrail a condensation trail? In a hydrogen fuel cell the waste product is H2O so surely there is a contrail.

I believe contrail formation requires nucleation sites, which is normally provided by soot particles from the burning fuel.

Good point, thanks.

It doesn’t in a really moist layer. Ice condensation nuclei are relatively rare (thus the silver nitride seeding idea), so a lot of times ice clouds form when a parcel of air gets supersaturated. This is pretty common.

Even more, because all the energy is from "burning" H2 only, no Cs to burn. Whether or not visible contrails form is a separate question. Even transparent water is highly efective outside of the visible range.

If you burn the hydrogen in a turbine, you need to worry about this. If you use a fuel cell you can easily capture the water.

Presumably at the cost of the airplane getting heavier as it flies rather than lighter as today?

Depends on whether the plane currently takes off with any water that could instead be replaced mid-flight and/or if dumping the excess water at different temperatures/places could avoid the issues.

I would be shocked if the airplane has use for water in the same order of magnitude as the combusted fuel multiplier.

Jet-A has around 43 MJ per kg. An A320 burns around 2.5 tons of fuel per hour. Hydrogen is about 3x as dense on an energy basis, so call it 0.8 tons of hydrogen per hour. Hydrogen combustion to water has a mass ratio of around 9:1, meaning an A320 would be creating over 7 tons of water (~1500 gallons) per hour of cruise flight. The potable water tanks on an A320 are around 50 gallons or around 2 minutes of cruise flight (that’s potable water tanks only). It’s a different order of magnitude.

Having listened to both Boeing and Airbus recently, neither seem to see any alternatives to SAF for long haul though.

Hydrogen has it's challenges. I heard someone did the math on running Heathrow exclusively on it who came to the conclusion that you'd basically need 3-4 reactors to produce it.

Hydrogen production, though, is one of the workable applications of wind turbines.

I'm not a subject matter expert, but think much of the value stems from lignin being so abundant because it's a byproduct of paper making rather than some advancement in chemistry, itself.

There are indications that SAF burn cleaner and thus produce less than half the contrails: https://www.nasa.gov/press-release/nasa-dlr-study-finds-sust... and lowers the effeet on global warming.

This should be the top reply to me. Cool research. I wouldn’t have thought about it (though I did worse in my atmospheric chemistry class than anything else I studied in atmospheric science, so not surprising).

Batteries make sense for shorter flight today and will slowly increase their range, future plane designs will likely use hydrogen fuel cells, but yes, for existing planes and the next couple of decades the industry is looking at a range of Sustainable ways to make e-kerosene to burn in existing engines, several of which are at pilot (no pun intended) stages.

Wait what? "lignin underutilised", (used)"only in low-value applications"? If by low value applications they mean individual house heating then perhaps. Could those factories that make wood products and currently dispose of their waste wood(sawdust etc) by selling it to pellet makers make much more by selling it for aviation fuel? I seriously doubt it.

The most recently I bought wood pellets I paid about ~$30/GJ aviation fuel(jet a) now appears to be around ~$56/GJ. It is more, but not orders of magnitude more. Add to this the cost of chemicals, reactors, disposal of byproducts and I doubt this new use will provide more profit.

I believe there is some kind of black liquid that is a waste product of paper production. I've read a few ideas for reusing this. I think traditionally it's been burnt to power the paper processes.

https://en.m.wikipedia.org/wiki/Black_liquor

> In the United States, paper companies have consumed nearly all of the black liquor they produce since the 1990s.[6] As a result, the forest products industry has become one of the United States' leading generators of carbon-neutral renewable energy, producing approximately 28.5 terawatt hours of electricity annually.

So, assuming those land based processes can be substituted for renewables energy, that frees this up to be used in transport.

(And yes this also applies to wood burnt in electricity generation, CHP and home heating. Burning stuff is inefficient, costly and polluting, we should be doing less of it whenever we can. Get a heat pump and some insulation).

Interestingly lignin is also the thing that caused us to have this whole carbon problem in the first place. When it first evolved there was nothing that could break it down and that's part of the reason why we have all these deposits millions of years later.

https://www.pnas.org/doi/10.1073/pnas.1517943113 claims that lignin was being broken down even during coal-forming periods. I did not verify this article though.

Anyone wanna take a bet that kwh required to create and process a pound of lignin into the desired format exceeds its useful energy output at all low, medium, and high scale?

Add enough penalties for using fossil fuels, and the market will switch to renewable or synthetic fuels.

There's an issue here. 80% of the global sulfur supply comes from removal of sulfur from fossil fuels. Among countless other vital industrial uses without which life as we know it ceases, sulfur is used for making lithium batteries and fertilizer.

What's the issue? Even if oil is the only feasible supply of sulphur, it can still be used for that purpose. Just don't burn it without capturing the exhaust.

Obviously this has consequences in form of higher prices. But the alternative leads to global warming.

How is this a problem? If the energy is gathered via renewables, then why not produce it at 5x the energy it yields when burned airborne?

This would be a problem if it was the kWh of energy that was causing climate change, rather than the carbon.

In anything you can electrify, just the electrification cuts energy use by a factor 3 but some things are actually hard to electrify in the short term so cheap green renewables being used to make carbon neutral fuels makes a lot of sense as we have access to lots of cheap energy, and even more as we electrify more and more stuff but have only a limited carbon budget.

Sustainable? Yes, in theory, but no in practise. That's my gut feeling because I doubt that wood (or lignin producers) will grow as fast as our travel (and heating) will use them up. Just skimmed some studies (in German) describing these consequences.

And here in Europe there's currently the "trend" to switch from gas heating to pellets (or just wood stoves in private buildings) duento the "energy crisis" which results from the war in Ukraine (and missing or failed long term strategies in politics earlier) . But calculations show that this would kill our forests fast, because of over consumption, again. And, last but not least, studies show that wood stoves produce "second hand smoke 2.0" which isn't healthy either.

So just changing fuels from dead dinosaurs to dead trees won't help us in the long range, I expect.

If anyone is interested in the full text, there's a PDF copy linked from some of the co-authors' research group here https://www.romangroup.mit.edu/publications

I don't get it. The fuel is still a hydrocarbon. Note the "carbon" in hydrocarbon. When it burns it's still emitting CO2. Where the fuel comes from is not relevant to atmospheric CO2.

Edit: Let's go through this carefully.

1. pump jet fuel out of the ground. Burn it. CO2 goes into atmosphere. Bury plants with an equivalent amount of C.

2. Convert plants with an equivalent about of C into jet fuel. Burn it. CO2 goes into atmosphere.

So, which scenario reduces the amount of CO2 into the atmosphere? Neither. They're the same.

This whole scheme of "sustainable biofuels" is a complete Emperor Has No Clothes sham, to put it mildly.

The plants used for the fuel would have pulled the CO2 from the atmosphere within the last year or so.

How is that relevant? A C atom from a million years ago is the same as a C atom from today.

(Yes, I know Carbon 14 decays, but that's not relevant to this discussion.)

A C atom from today is not magically different.

The difference is that carbon from millions of years ago was buried deep underground. Digging up and burning fuel increases atmospheric carbon. Cycling atmospheric carbon into fuel and back to the atmosphere is a one for one replacement.

That's why I wrote burying the equivalent plant C mass, which is a lot cheaper than trying to convert it into fuel.

I‘m not sure transporting plants to a dump site and burying them there is cheaper than converting to fuel and selling it. Excavation would be quite CO2 intensive, too.

Instead of transporting them to the fuel conversion site? There are landfills and pit mines everywhere.

Just putting stuff in landfills results in methane production, which would gradually leak out and cause even more global warming than CO2. You‘d need to invest in making things air tight, ideally over hundreds of years.

Methane has a half life in the atmosphere of 12 years. Gradually leaking methane is way down the list of problems.

Methane is one of the top problems and low hanging fruit to tackle.

https://www.epa.gov/gmi/importance-methane

> Methane is the second most abundant anthropogenic GHG after carbon dioxide (CO2), accounting for about 20 percent of global emissions. Methane is more than 25 times as potent as carbon dioxide at trapping heat in the atmosphere. Over the last two centuries, methane concentrations in the atmosphere have more than doubled, largely due to human-related activities. Because methane is both a powerful greenhouse gas and short-lived compared to carbon dioxide, achieving significant reductions would have a rapid and significant effect on atmospheric warming potential.

https://www.globalmethanepledge.org/

> In advance of the 2021 UN Climate Change Conference in Glasgow (COP 26), the United States and the European Union jointly launched the Global Methane Pledge, which asked countries to band together and commit to a collective goal of reducing global methane emissions at least 30% from 2020 levels by 2030

It's almost relevant, because C14 decays into N14 (which is what we measure for carbon dating). N14 on the ground is a stable fertilizer. But if it is airborne and makes it to the planet's uppermost layers, cosmic rays eventually convert it back to C14 again. It's an interesting cycle.

What matters is whether you put new carbon (which was not part of the carbon cycle) into the atmosphere.

I understand what you're saying, but it makes no sense. You can take the plant and bury it instead of converting it to jet fuel, and you have a FAR cheaper method of achieving exactly the same atmospheric CO2 result.

CO2 is CO2, regardless of whether it came from a new plant or a fossilized plant.

> CO2 is CO2, regardless of whether it came from a new plant or a fossilized plant.

There is a carbon cycle above ground, what we did is inject more and more carbon back into it that had been removed from it by slow natural processes over long periods of time long ago.

No. The problem we have is that we added all that carbon that was long ago buried deep back into the above ground carbon cycle. If we had only ever used carbon that already is in the above ground cycle we would not have a problem, we would not have increased atmospheric CO2 by much even if we temporarily - and it would always be just that - turned some more carbon lying around above ground into CO2. By using buried carbon we increased the amounts of carbon in the above ground cycle by a gigantic amount. In addition we burnet most of it directly too, but just that it is above ground, available to various processes including natural ones, would have increased CO2 even if we had not burnt it right away.

(disregarding that buried carbon does not only come from buried plants for this sentence) The problem is not that forests burn, it's that ancient long-buried forests burn.

Um, but you're burying the same amount of carbon that you're burning in the ordinary jet fuel, which achieves the same CO2 result. Without having to generate synthetic fuels at all.

But those two things are not the same. They are aiming to sustain flight while not putting more carbon into the atmosphere. Burying plants will maybe put carbon out of the Cycle but it won’t help you fly.

Yes, they are exactly the same. The exact same amount of CO2 is emitted into the atmosphere. The exact same amount of CO2 is in the ground.

People are evidently reading your original post far too quickly and skipping right over the “Bury plants with an equivalent amount of C.” step.

No, we saw that non sequitur.

How is it a non-sequitur?

Take summer 2022 CO2 into plants and bury them in an anaerobic environment. Burn Jet-A in carbon-equal amounts.

I don’t see an obvious way that that’s not carbon-neutral.

Ironically, I'm a big fan of this concept of "offsetting" carbon emissions.

It's a really useful, market based mechanism for tackling climate change, like carbon taxes.

Like every other thing that would make it easy to phase out fossil fuels, it's been relentlessly attacked though, and so people generally have been trained to hate it.

Of course, burying plants isn't the cheapest or best way to offset carbon emissions. And if airlines used cheaper, more effective offsets, they'd be attacked as "not really helping". And they do, and they have been.

So basically, you can't win, fossil fuel propaganda just has too much money behind it for us to do the obvious and easy stuff, and instead we'll have to slowly crawl towards a solution, pointlessly wasting money and killing people all along the way.

I think you’re underestimating how hard it would be to ensure that the buried carbon in the trees stays buried. Not to mention the colossal job of finding and managing these burial sites.

There’s an argument that we should be using way more (farmed) timber for building than we do since putting wood into a structure tends to ensure that it won’t all rot and release all that carbon.

I think the implied benefit is that the hydrocarbons could be synthetic. Why they're mentioning that is a bit suspect though. Basically they should be saying "you know those hydrocarbons that are 100 times more expensive than regular fuel? We can now use those for flying jets".

It would be a lot less appealing article then.

The way I see it there's only two ways we could have sustainable flying. Either using 100% hydrogen, or using some future battery breakthrough.

Synthetic fuels are cool, but in my opinion we should be storing captured CO2 in the ground, not releasing it in the air again.

Synthetic fuels, if I recall correctly, cost about 5x more than refined oil, which is why we pump oil instead of making synthetics.

As you say, there isn't going to be any magic solution for jet aircraft. It's an energy/weight problem, and jet fuel is really really hard to beat for that. For batteries it would not be a little breakthrough needed, it would need to be an incredible breakthrough, and nobody has any idea how that might work.

The fundamental issue is batteries, in essence, carry their own "oxidizer" with them, which is very heavy. With jet fuel, you just suck the oxidizer from the air as needed. Any battery that used air as its "oxidizer" would be, literally, burning it, and wouldn't be a battery.

This is true. Another one: burying waste paper (and cutting down new trees) instead of recycling reduces both CO² and probably energy use too.

I think the argument against this is that these are not mutually exclusive alternatives. Reducing fossil fuels does not prevent us from burying biomass.

This is not true.

https://www.sciencefocus.com/science/is-recycling-paper-bad-...

> Recycling causes 35 per cent less water pollution and 74 per cent less air pollution than making new paper.

Ironically, one vaguely true point was that we used to burn wood waste in pulp mills, which is better than recycling with coal powered electrcity.

The fix was to stop burning coal for electricity though, not to stop recycling.

I think the reason everyone down thread is confused is that it was incredibly unclear that you were adding the whole sequester the net equivalent component.

It's an interesting thought though I'm curious how all the pieces would balance in practice.

Like, What's the cost of digging up the fuel? The cost of sequestering the equivalent. And there's pieces like reducing production reduces how much net sequestration will ever need to be done.

I don't think it's so hyper self evident that no hypothetical future sustainable biofuel has a place. No one's selling you a sham solution here, it's research after all.

> This whole scheme of "sustainable biofuels" is a complete Emperor Has No Clothes sham, to put it mildly.

I think that's unreasonably uncharitable.

> I'm curious how all the pieces would balance in practice

That's a reasonable thought. Though I find it hard to believe that processing plants into synthetic fuels would be cheaper than just burying it, like into an old coal mine.

> that's unreasonably uncharitable

It's reasonably charitable because every single article I've ever read about biofuels never mentions my observation, nor have I ever heard anyone else mention it. It's all like "if we burn fuels made from plants, we're not really contributing to CO2" nonsense. People have mentioned growing trees and then stuffing them into coal mines to sequester the C, but never in the context of biofuels.

Maybe I can draw attention to this by being uncharitable, which will make it worth while.

I think you just need better news sources:

https://theconversation.com/amp/ipcc-report-biofuels-alone-a...

This idea is called BECCS and is covered in detail by the IPCC reports:

https://en.m.wikipedia.org/wiki/Bioenergy_with_carbon_captur...

It and concepts like it are why we talk about 'net zero carbon' and not just 'zero carbon'.

It's generally assumed that some amount of this is required to stay below 1.5C so if this was the best solution for air travel, it would be used.

It's not as good as SAF and electrification though, so it's not used here, or for any other current process that has better options available.

When you bury plants, they decay and release their carbon as CO2 back into the atmosphere within a couple of years. Reliably sequestering CO2 by burying plants is difficult and inefficient. In addition, converting crude oil to jet fuel is also very energy-intensive; the energy required typically comes from fossil fuel sources, so you'd be releasing much more fossil carbon into the atmosphere than just what is burned in the jet turbines.

> so you'd be releasing much more fossil carbon into the atmosphere than just what is burned in the jet turbines

The energy efficiency of refining jet fuel is 95.3%.

Google "energy efficiency of refining jet fuel"

Who knows what the energy efficiency of refining plants into jet fuel is, but I bet it's a lot less than 95.3%. The article doesn't cite what it is.

The burning during the flight is the same (ignoring some mild advantages for the purer SAF) but from the well to wake there's other factors.

One big one on each side is methane escapes during oil drilling (which can be managed but often isnt) and the input for the biofuel being something that was grown on cleared forest.

Generally burning the methane and releasing the same carbon is better, and you get some use out of it. Today we do this for "renewable gas" but most land based uses for the output are in the process of being electeified. But we can still use then as GHG-negative feedstocks, or (roughly) GHG neutral fuels in aviation or shipping.

https://theicct.org/sites/default/files/publications/Alt-avi...

So this paper estimates Brazilian soy could be better or worse than fossil fuel, depending on land used. US maize would come out almost the same as fossil, while Brazilian Cane sugar would be better and EU Miscanthis grass better again.

And even better yet is using waste from agriculture or forestry or municipal Waste instead of something grown specifically for turning into fuel, some of which then make the process carbon negative.

This is great.

Right now all gasoline sold in the US needs to contain 10% bioethanol. That's just a handout scheme to corn farmers, who plant this corn on 30 million acres of good agricultural land. One out of three acres planted with corn is used for bioethanol.

If we convert this area to produce, let's say, bamboo, which could be converted to bio jet fuel, then that would make much more sense.

Surface traffic can and will be electrified in the years to come. Air traffic will never get electrified. We need a carbon neutral source for jet fuel.

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