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mxaxai
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Synthetic aviation fuels - the future for environmentally friendly aviation?

Fri Jun 18, 2021 7:41 pm

At an aviation conference today, German chancellor Angela Merkel pushed for rapid change towards renewable energy in aviation.
Airlines must move ″rapidly″ to using renewable energies, German Chancellor Angela Merkel told a national aviation conference at Berlin airport on Friday.

Merkel cited synthetic fuel in particular as a key component of a "disruptive renenwal" process required in the industry.

In response, Lufthansa chief executive Carsten Spohr asked for state help because synthetic fuels, generated using electricity, which itself needs stem from renewable sources in order for there to be a major environmental gain, currently cost 10 times more than standard fuel. [She] said the shift to renewable energy sources must be made "not at some point, but as rapidly as possible, because production cycles in the aviation industry are very long."

https://www.dw.com/en/angela-merkel-pus ... a-57953799

Which is a good point. Neither hydrogen nor batteries or any other alternative are capable or mature enough to provide a 1-for-1 replacement for current kerosene-fueled planes. While other industry sectors are aiming for decarbonization in the coming decades, and even developing countries are rapidly increasing their renewable energy output, aircraft manufacturers have yet to present a viable plan. Even if there was a breakthrough discovery of some super-battery right now, it would take a decade or more until the first battery airliner is delivered.

Rolls Royce is betting on sustainable - synthetic - aviation fuels as well.
Rolls-Royce, the jet engine maker, has said that all products launched after 2030 will be capable of running with net zero carbon emissions as part of decarbonising plans that rely heavily on replacing fossil fuels with synthetic alternatives that are yet to be approved.
...
Rolls-Royce is instead pinning its hopes on synthetic fuels, which the industry calls “sustainable aviation fuels”, or SAF. Almost identical chemically, but produced from non-oil sources, the fuels could theoretically result in significantly less or even zero new carbon emissions across their lifecycle.

https://www.theguardian.com/business/20 ... zero-plans

Fundamentally, SAF appears to be viable. Test flights have operated successfully, an A350 as given in the article above, a H145 helicopter, and perhaps others. Studies even show that the 'cleaner' synthetic fuel burns with less soot, leading to fewer contrails - another impact of aviation on the climate.
 
frmrCapCadet
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Fri Jun 18, 2021 9:42 pm

But if we figure out how to use hybrids for those shorter than 1 (even 2) hour flights the fossil fuel used in aviation will drop dramatically. Likely it might be cheaper for longer flights to use a percent of synthetic fuel. We just need to make significant improvements every year.
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Starlionblue
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Sat Jun 19, 2021 12:44 am

frmrCapCadet wrote:
But if we figure out how to use hybrids for those shorter than 1 (even 2) hour flights the fossil fuel used in aviation will drop dramatically. Likely it might be cheaper for longer flights to use a percent of synthetic fuel. We just need to make significant improvements every year.


Agreed. Instead of targeting all aviation immediately, significant gains can be made by targeting those flights where hybrid or even entirely electric propulsion looks to be somewhat practical.


Apart from rockets, aircraft are probably the hardest vehicle type for which to implement other propulsion than the current carbon emissions-intensive one, due to the paramount importance of weight. It is difficult to beat the energy density of hydrocarbons. Granted, these can be synthetic, but that brings its own issues as mentioned above.

In a car, lorry, or even a ship, weight has much less of an impact. Hence hybrid or entirely electric propulsion is much easier to implement. Emissions from the production of electricity aimed at industry and households are also easier to make carbon-neutral since the change "only" needs to be implemented at the source.

Not discounting that aviation needs to do its part, so to speak, but even if aviation became carbon neutral tomorrow, the majority of carbon emissions would still be ongoing.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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Aaron747
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Sat Jun 19, 2021 1:29 am

Starlionblue wrote:
frmrCapCadet wrote:
But if we figure out how to use hybrids for those shorter than 1 (even 2) hour flights the fossil fuel used in aviation will drop dramatically. Likely it might be cheaper for longer flights to use a percent of synthetic fuel. We just need to make significant improvements every year.


Agreed. Instead of targeting all aviation immediately, significant gains can be made by targeting those flights where hybrid or even entirely electric propulsion looks to be somewhat practical.


Apart from rockets, aircraft are probably the hardest vehicle type for which to implement other propulsion than the current carbon emissions-intensive one, due to the paramount importance of weight. It is difficult to beat the energy density of hydrocarbons. Granted, these can be synthetic, but that brings its own issues as mentioned above.

In a car, lorry, or even a ship, weight has much less of an impact. Hence hybrid or entirely electric propulsion is much easier to implement. Emissions from the production of electricity aimed at industry and households are also easier to make carbon-neutral since the change "only" needs to be implemented at the source.

Not discounting that aviation needs to do its part, so to speak, but even if aviation became carbon neutral tomorrow, the majority of carbon emissions would still be ongoing.


Especially from seagoing cargo and agriculture. These two contribute far more than aviation ever can or will.
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Starlionblue
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Sat Jun 19, 2021 2:10 am

Aaron747 wrote:
Starlionblue wrote:
frmrCapCadet wrote:
But if we figure out how to use hybrids for those shorter than 1 (even 2) hour flights the fossil fuel used in aviation will drop dramatically. Likely it might be cheaper for longer flights to use a percent of synthetic fuel. We just need to make significant improvements every year.


Agreed. Instead of targeting all aviation immediately, significant gains can be made by targeting those flights where hybrid or even entirely electric propulsion looks to be somewhat practical.


Apart from rockets, aircraft are probably the hardest vehicle type for which to implement other propulsion than the current carbon emissions-intensive one, due to the paramount importance of weight. It is difficult to beat the energy density of hydrocarbons. Granted, these can be synthetic, but that brings its own issues as mentioned above.

In a car, lorry, or even a ship, weight has much less of an impact. Hence hybrid or entirely electric propulsion is much easier to implement. Emissions from the production of electricity aimed at industry and households are also easier to make carbon-neutral since the change "only" needs to be implemented at the source.

Not discounting that aviation needs to do its part, so to speak, but even if aviation became carbon neutral tomorrow, the majority of carbon emissions would still be ongoing.


Especially from seagoing cargo and agriculture. These two contribute far more than aviation ever can or will.


Precisely. Reducing carbon emissions in other sectors than aviation is both easier and has more impact.

Again, I'm not saying carbon emissions in aviation should not be targeted. They should. The problem is facts versus perception. In many places, there seems to be a tendency for aviation to be seen as the big bad in the carbon emissions context while in reality, it is a rather minority contributor. See the concept of flygskam https://en.wikipedia.org/wiki/Flight_shame. I find it ironic that so many people limit their own flying for environmental reasons, but at the same time have no problems ordering stuff online that is then flown in. Or taking an electric train with power generated by burning coal.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
mxaxai
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Sat Jun 19, 2021 12:25 pm

Aaron747 wrote:
Especially from seagoing cargo and agriculture. These two contribute far more than aviation ever can or will.

Agriculture is definitely a big contributor, though many people find it easier to avoid air travel than to eat less (or no) meat.

In absolute numbers, ships are a big contributor too but per ton of cargo they're very efficient. If you have the choice between air cargo and sea cargo, the latter is better for the climate. Of course it might be even better to not transport stuff at all. The range of ships makes batteries difficult to use but they're easier to convert to hydrogen than aircraft.

A single long haul flight emits as much CO2 as half a year of commute. With electric cars becoming increasingly common, the relative impact of such flights is bound to increase. SAF would be especially helpful for these flights, since hybrid or electric aircraft are unfeasible for any but the shortest routes. Long haul flying is also difficult to replace with other modes of transport.

Targeting short haul flights first is reasonable but I believe that a solution must be found for long haul as well. Perhaps a small percentage of renewable fuel / SAF that gets increased every year until we reach 100% by 2040 (or thereabout).
 
LH707330
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Sun Jun 20, 2021 2:42 am

I wrote a summary of this last year: https://www.merlins-corner.net/whysaf.html

One thing we should all nudge the airlines to do is add that checkbox to boost the demand, then the scaling of SAF will clear the hump faster.
 
frmrCapCadet
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Sun Jun 20, 2021 2:36 pm

Totally out of my fields, but could the excess energy that wind and solar often produce be used to 'juice up' any liquid flammable to get its energy density up to the kerosene range? Alternately, as gasoline demand declines, and I think we will be seeing this within 5 years, could that gas be reformulated into kerosene? I have only seen one discussion and understand it will be difficult.
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chimborazo
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Sun Jun 20, 2021 7:09 pm

frmrCapCadet wrote:
Totally out of my fields, but could the excess energy that wind and solar often produce be used to 'juice up' any liquid flammable to get its energy density up to the kerosene range? Alternately, as gasoline demand declines, and I think we will be seeing this within 5 years, could that gas be reformulated into kerosene? I have only seen one discussion and understand it will be difficult.


Pre-hearing fuel before loading with energy from a wind/solar system? Could have an effect but the fuel will cool in the wings so would not have too much of an effect except perhaps short-haul. Unless the fuel tanks were insulated (more weight… more fuel required to move the plane a given distance).
Currently oil/fuel heat exchangers are used on the way to the engine for most (all?) commercial jets. So if that oil heat is still there it will have to be transferred out of the engine another way- more heat in the atmosphere?

It’s such a complex situation. There are so many ways of increasing efficiency in one area that requires a deficit in another area. How do they balance out…? The engineers will tell us.
Add in lifting the fuel temperature closer to its flame point while in the plane… more complexity!
 
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Starlionblue
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Mon Jun 21, 2021 1:04 am

I don't think you can change kerosene into more energetic kerosene, but then again I'm not a petrochemical expert. Cars can add nitrous, but then you'd upset the rather delicate balance of jet engine combustion.

Heating or cooling the fuel will not change the energy density by weight. It will simply change the volume. That's why we need to take into account specific gravity when converting from litres to kg.

Related: This relationship between energy density by weight compared to volume is why SpaceX supercools the RP-1 (kerosene rocket fuel) when fueling the Falcon 9 rocket. They can get more weight into the same volume of tank, and thus more energy.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
LH707330
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Re: Synthetic aviation fuels - the future for environmentally friendly aviation?

Mon Jun 21, 2021 8:17 pm

Starlionblue wrote:
I don't think you can change kerosene into more energetic kerosene, but then again I'm not a petrochemical expert. Cars can add nitrous, but then you'd upset the rather delicate balance of jet engine combustion.

Heating or cooling the fuel will not change the energy density by weight. It will simply change the volume. That's why we need to take into account specific gravity when converting from litres to kg.

Related: This relationship between energy density by weight compared to volume is why SpaceX supercools the RP-1 (kerosene rocket fuel) when fueling the Falcon 9 rocket. They can get more weight into the same volume of tank, and thus more energy.

Shell made energy-denser fuel for the Qantas 747-400 delivery record, so it's been done before: http://www.aussieairliners.org/b-747/vh ... story.html

Petro-jet is basically a cut of several hundred different molecules with a spec for the aggregate properties. Synthetic or bio-derived fuels are usually a much smaller set of molecules, which is why they burn cleaner, because each can be designed closer to the engine's optimum burn temperature. I suspect Shell cherry-picked the longer ones in their blend, which made it more expensive, and not worth doing outside of a single stunt.

Energy density by weight and volume are a bit of a tradeoff: carbon-chain molecules have better energy/volume the longer they get (diesel, kerosene), but better energy/mass on a shorter chain (gasoline, methane). As a gross oversimplification, assume that a carbon atom has four bonds: typically it'll form a link with two adjacent carbon atoms in a chain, with a pair of hydrogen atoms on the side, and a hydrogen on each end of the chain, for a general formula of n*C+(2n+2)H. Because carbon has an atomic weight of 12, and hydrogen is 1, the molar mass of each link in the chain is 14, with 15 for each end piece. As an approximation, the energy gained from burning the molecule comes from breaking each of the links. Let's look at a few examples:

Methane: CH4: a single carbon with four hydrogens around it: mass=16, links=4, links/mass =4/16=.25
Octane: C8H18: an 8-chain with two hydrogen end-caps: mass=8*12+18=114, links=25, links/mass=25/114=.22

As the chains get longer, the 2n+2 effect approaches 2n in practice, so an infinitely long chain would have 3 links and weigh 15, for a links/mass of .2.

This page illustrates the trade well, though they got the 2n+2 bit somewhat wrong: https://neutrium.net/properties/specifi ... -of-fuels/

Rockets are probably the most weight-sensitive application out there, which is why some have gone to the trouble of cryogenically-cooled methane: you get the energy/mass and a decent energy/volume. Aviation has less weight sensitivity and lower risk tolerance, which is one reason jets burn kerosene.

The reason alcohols perform so poorly is that they contain oxygen, which is heavier (=16) and only has two bonds, so you take one of your hydrogen end caps and replace it with an oxygen and hydrogen (a hydroxyl) for a one-bond gain for a 16 mass gain. The other issue with the hydroxyls is that they mix with water (H2O), which is generally not good in fuels due to its lack of combustibility.

Regarding heating/cooling the fuel, if anything, you would want to cool the fuel before uplift to cram more in, but whatever few percent you gain from that is not worth the spillage when it warms up or the hassle and cost of cooling it, not to mention frost formation on the wings, even in summer. Longer-chain molecules (e.g. bunker fuel) are probably also out because they get gooey sooner as they cool, which means there's a pretty narrow range of molecule sizes that make sense in jet engines.

Synthetics might get you 5-10% densification, which may allow some co-optimization with the wing and engine structures, but I suspect the overall fuel burn improvement will be less than 5%. The big benefit will come from the lower soot emissions, and hence reductions in contrails and turbine cleaning requirements.

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