Sciencemadness Discussion Board

Future of carbon-free aviation ?

metalresearcher - 31-1-2021 at 08:24

Aviation is one of the biggest CO2 producers. Now many companies and institutions are researching on alternatives, now more than ever.

These alternatives are:
* Hydrogen. Energy to mass ratio is much better than Jet-A1 (current kerosene fuel), but energy to volume ratio is much worse. H2 tanks take lots of space, regardless whether compressed to 700 bar or liquefied (which requires additionally cryogenic tanks of -252 C).
* Batteries (i.e. electric planes). Tesla batteries (Li-ION) are 400Wh/kg, which is way too heavy. When it becomes e.g. 1000Wh /kg, then only short haul (< 1000km) flights are feasible, but forget about intercontinental flights up to 10000km.
* Ammonia. Nice article: https://newatlas.com/aircraft/reaction-engines-ammonia-carbo... but I think, it emits NOx as well and much more than current kerosene based aircraft. And liquid NH3 should be kept under -33 C, also when on the ground as well.
* CCS, Carbon Capture and Storage. This article advocates it https://www.sciencedaily.com/releases/2021/01/210113100810.h... as 'sustainable', but putting CO2 in the ground does indeed not heat up the atmosphere (assumed it is not leaking), but it is still creating waste and not a circular economy. Moreover, there is already wasteless CCS and that are devices called 'trees'. But we are logging all these green lungs of Earth in Brasil, Congo, Indonesia. So at least stop this first.
* CCU or Synfuel. This filters CO2 from the air as seen in these videos.
https://www.youtube.com/watch?v=_kVT77n67UY
https://www.youtube.com/watch?v=bvwnvYvMPyo
https://www.youtube.com/watch?v=s1-soaZn4B0
The CO2 will react with water with a CeO2 catalyst which created syngas (CO + H2). This will be processed with the Fischer-Tropsch process (which ia already used commercially for several decades to make oil from coal gas) and that results synthetic crude oil of which Jet-A1 (and other petroleum based products) can be made.
Currently, it is expensive and requires renewable energy (otherwise it is useless), but maybe in the 30s or 40s (before 2050) it might be feasible.
This requres no redesign of aircraft as it is almost the same fuel, with the difference it is not fossil.

What are your ideas ?

DBX Labs - 31-1-2021 at 08:47

I think that biofuels will be a big development in upcoming years. I’m not sure which route would be used to turn produced ethanol into lengthier hydrocarbons for use in jet engines, but like you said, plants are the best carbon capture we have (even if it only reduces CO2 to carbohydrates).

Fulmen - 31-1-2021 at 10:03

Quote: Originally posted by metalresearcher  
Aviation is one of the biggest CO2 producers.


No it's not. Heat and electricity is the biggest with 40%, road transport produces appr 15%. Aviation barely registers with some 2-3%.

Fyndium - 31-1-2021 at 11:26

Heating, transportation, concrete and metal industries prioritize in CO2 emissions. CO2 produced is high in aviation per unit, but overall it is a small percentage.

Combustible fuels can be made through synthetic or biosynthetic means and they can be directly utilized to existing, matured technology. Hydrogen was never an option, as it's density is mere 70kg/M3 and energy density per kg only a bit over 2x of LPG.

Possibly some simple compound that could be mass produced from syngas or some other single step scalable industrial process that is energy intensive, that is liquid at big temp range and possesses high energy density with very clean combustion would be an option in the future. Not sure what it would be, though.

metalresearcher - 31-1-2021 at 12:21

Quote: Originally posted by Fyndium  
Heating, transportation, concrete and metal industries prioritize in CO2 emissions. CO2 produced is high in aviation per unit, but overall it is a small percentage.

Combustible fuels can be made through synthetic or biosynthetic means and they can be directly utilized to existing, matured technology. Hydrogen was never an option, as it's density is mere 70kg/M3 and energy density per kg only a bit over 2x of LPG.

Possibly some simple compound that could be mass produced from syngas or some other single step scalable industrial process that is energy intensive, that is liquid at big temp range and possesses high energy density with very clean combustion would be an option in the future. Not sure what it would be, though.


That is indeed Kerosene in MJ/kg *and* MJ/liter which can be mass produced this way. Kerosene is 35MJ/liter and L H2 10MJ/liter.
A full A380 (do they still fly ?) with 180m3 fuel for a long haul flight would require 630m3 L H2 which should be cooled to -252C. 200C below the already -50C ambient temperature at cruise altitude. That extra 450m3 costs at least 20-30m of its fuselage length. sacrificing 200 of the 400 passenger space.
Admitted, 180m3 Jet A-1 weight 144 tons and 630m3 L H2 only 44 tons which saves 100 tons payload weight and hence less H2 fuel is required.
For smaller, still extensively used long haul planes like the A350 or B787 Dreamliner. the figures are smaller.
But anyway, it requires a stark fuselage (and wing?) design.

https://en.wikipedia.org/wiki/Energy_density#In_chemical_reactions_(oxidation)



[Edited on 2021-1-31 by metalresearcher]

mysteriusbhoice - 31-1-2021 at 12:33

produce the fuel using carbon capture and solar energy as the power source!!

Fyndium - 31-1-2021 at 13:05

There will probably be new energy storage technologies in the future that allow for a much higher energy densities than are known for now. Conceptually, as long as there is matter to manipulate, as in the atmosphere, only a source of energy would be needed to utilize (nitrogen, oxygen, etc) as a kinetic energy countermass.

In space, on the other hand, I don't know if they ever got the microwave engine to produce any thrust.

Grizli7 - 1-9-2021 at 10:37

Once upon a time, during the Cold War in the USSR, engineers were tasked with building a bomber that could fly for an unlimited time. If my memory serves me, one plane was built that flew. A nuclear reactor was used as a power plant. Thermonuclear energy is on its way, who knows, wait and see ...
As for new ways of storing energy. About 7 years ago, I saw a TV report that scientists discovered a way to store electricity by orders of magnitude superior to the existing ones. With the same overall dimensions, the battery capacity will grow hundreds of times and will be charged almost instantly. But the problem is technology.

karlos³ - 1-9-2021 at 10:58

Quote: Originally posted by Grizli7  
Once upon a time, during the Cold War in the USSR, engineers were tasked with building a bomber that could fly for an unlimited time. If my memory serves me, one plane was built that flew. A nuclear reactor was used as a power plant. Thermonuclear energy is on its way, who knows, wait and see ...

As far as I know, that was an american project, they built the reactor in and flew around with it, but it wasn't connected to the internal power of the plane.

Although, some of the new russian hypersonic missiles are said to contain something like that?
At least there was an accident and it lead to some radiation casualties, is what was claimed.
No idea.

I don't understand the whole "carbon-free" thing, as I fertilise my aquarium plants for example with CO2 and the same is true for plants.
Second best carbon capture device in existance.... they are called "trees".
Best is called "planktonic algae".
But thats almost political I guess, just wanted to say my bit and will not participate any more in this.

[Edited on 1-9-2021 by karlos³]

metalresearcher - 2-9-2021 at 02:24

Quote: Originally posted by karlos³  
Quote: Originally posted by Grizli7  
Once upon a time, during the Cold War in the USSR, engineers were tasked with building a bomber that could fly for an unlimited time. If my memory serves me, one plane was built that flew. A nuclear reactor was used as a power plant. Thermonuclear energy is on its way, who knows, wait and see ...

As far as I know, that was an american project, they built the reactor in and flew around with it, but it wasn't connected to the internal power of the plane.

Although, some of the new russian hypersonic missiles are said to contain something like that?
At least there was an accident and it lead to some radiation casualties, is what was claimed.
No idea.

I don't understand the whole "carbon-free" thing, as I fertilise my aquarium plants for example with CO2 and the same is true for plants.
Second best carbon capture device in existance.... they are called "trees".
Best is called "planktonic algae".
But thats almost political I guess, just wanted to say my bit and will not participate any more in this.

But these devices are being destroyed on large scale (rainforest logging in Brasil, Indonesia, etc). That should be stopped first.

clearly_not_atara - 2-9-2021 at 06:37

I feel like you're overselling the space problem and underselling the chemistry problem with hydrogen.

Modern planes are already full of empty space. Loads of it. The mass-to-weight ratio of a compressed H2 tank makes it less efficient than gas, but that's not the real point.

Jet turbines are made of high-temperature alloys inevitably based on nickel (Ni3Al, Ni3Ti) or titanium (TiAl). Problem: these metals undergo hydrogen embrittlement and thermal hydriding. Hydriding is already a problem for a car-style engine, and when you consider the even higher forces and temperatures on a jet turbine you quickly realize how damaging it would be. Burning hydrogen a lot harder than it sounds. I literally can't think of a single suitable alloy for building a hydrogen-burning jet.

Ammonia, on the other hand, might be underrated. It contains most of the chemical energy of the hydrogen that goes into it. It can be liquefied at achievable pressures. Direct-ammonia fuel cells might make up for the lower energy density of NH3 vis-a-vis C10H22 and also prevent NOx combustion.

metalresearcher - 2-9-2021 at 22:14

@clearly_not_atara: But NH3 or kerosene also contain hydrogen which might result in embrittlement of these alloys ? The combustion of hydrocarbons is a rather complex set of reactions where also elemental hydrogen is released as intermediate product ?

Fyndium - 3-9-2021 at 00:14

It appears that chemically bonded hydrogen does not have similar properties than elemental hydrogen, which readily emigrates into most alloys. As far as I know, the whole chemistry is based on this very matter, the reason why you simply cannot mix benzene and acetone and be good.

People are only sold with the energy per weight of hydrogen. For example, my father just doesn't listen when I try to very politely explain him about how it requires either extreme pressures or extreme cryogenics to be stored, and even then it will be around 90kg/m3 with third or less of HC energy densities.

Metals possess energy densities comparable to hydrogen in volume. A liter of aluminum can release up to 140MJ of energy, hence if it could be turned into a battery, it could surpass energy densities of most HC.

Ammonia sounds like a feasible competitor as it has a well-established, large scale mass production already developed and in use, and mostly combusts well.

Nuclear energy in form of fission or yet better fusion would be the next generation, as it has energy density several orders of magnitude higher than any other form. The US had a plan about nuclear aircraft, and they built a concept plane, but apparently never flew it. The concept appears to be to generate heat, which simply used air as the reaction mass, hence allowing for practically an infinite specific impulse. A nuclear reactor could be ran without any full enclosure, if the environmental radiation is not an issue. This would be the case in space, and only a single shield would be needed in front of the reactor to protect the controls and cargo. This would make the reactor very lightweight compared to conventional reactors, but apparently running such a reactor in atmosphere would cause it to irradiate any active elemental traces on it's path, leaving a fallout of varying degree, perhaps not an issue in small scale, but definitely if ran for years with multiple units.

mayko - 3-9-2021 at 06:25

The nuclear aircraft was Project Pluto, which was primarily an effort to build a long-lasting cruise missile. The plan seems to have been for it to drop a dozen or so hydrogen bombs and then spend a few weeks flying low over populated areas (massive shock waves, unshielded reactor, and fallout trail were features rather than bugs), before finally crashing into a hillside somewhere.

Quote:

The success of this project would depend upon a series of technological advances in metallurgy and materials science. Pneumatic motors necessary to control the reactor in flight had to operate while red-hot and in the presence of intense radiation. The need to maintain supersonic speed at low altitude and in all kinds of weather meant that the reactor, code-named "Tory", had to survive high temperatures and conditions that would melt the metals used in most jet and rocket engines.


If you think that's bad, wait till you see the the lack of leg room!!

The spacecraft was Project Orion, which planned to kick atomic bombs out the back and get propelled forward by the blast. Carl Sagan called it "the best use I can think of for nuclear weapons".

clearly_not_atara - 3-9-2021 at 14:38

Elemental hydrogen in NH3 or CHx fuel fires might be present at a fraction of a percent of the partial pressure that it would have as a primary fuel. That would mean that the time needed for hydriding would be hundreds if not thousands of times as long before considering equilibrium effects. At that point, other wear mechanisms become more important. After all, nothing lasts forever.

rockyit98 - 3-9-2021 at 18:40

most likely with delta shaped aircraft with compressed H2 storage with hydrogen fuel cells used to run electric motors, maybe with perovskite solar cell that 40% to 50% efficient. maybe Hybrid Air Vehicle with H2 as the lighter-than-air gas that also used as fuel for slow speed travel.

metalresearcher - 3-9-2021 at 23:24

Quote: Originally posted by rockyit98  
most likely with delta shaped aircraft with compressed H2 storage with hydrogen fuel cells used to run electric motors, maybe with perovskite solar cell that 40% to 50% efficient. maybe Hybrid Air Vehicle with H2 as the lighter-than-air gas that also used as fuel for slow speed travel.

Would be OK for short range flights, but not for nonstop 10000+ km intercontinental flights.
Even with current kerosene fuel, the optimum nonstop distance is 5700km for a Boering 777. Longer flights consume more fuel to transport the extra fuel to finalize the flight.
With hydrogen this will be worse.