Could airplanes run on compressed natural gas?

[farmerben]

The main motivation for doing so would be reduced carbon emissions.  Natural gas has slightly more energy per kg than jet fuel.  Tankage is the issue.  Has this already been done?  What are the pros and cons?

[Lisias]

Yes, in theory they could.

Spoiler

As a matter of fact, it's possible to make airplanes using steam engines - believe it or not.

And with some interesting advantages, if you ask me.

And, YES, they are doing it.

https://www.planeandpilotmag.com/article/a-natural-gas-to-fly/

The problem is, as usual, weight and logistics. It's essentially the same reason we are not using Natural Gas automobiles everywhere neither.

A friend of mine bought a dual fuel (gasoline and natural gas) car once. He told me that the savings are not that great because the engine burned more natural gas per KM than gasoline, and he had to keep the gas tank filled up and so waste fuel with dead weight because there were very few Fuel Stations in the City with Natural Gas, and virtually no one in the country - so kiss bye bye Natural Gas while traveling to the beaches.

It's the same issue in the Aviation Industry - every airport has Jet Fuel and AvGas available, but until at least most of them would be able to deliver Natural Gas too, the airplanes will need to be dual fuel, and so they will waste weight due the gas tanks and the unavoidable Jet Fuel/AvGas ones too with some extra fuel for safety.

Edited August 5 by Lisias
Oh, yeah... Tyops...

[kerbiloid]

Since 1980s

Tu-155/Tu-156 (and based on it spaceplane design Tu-2000)

LNG + LH2

https://ru-m-wikipedia-org.translate.goog/wiki/Ту-155?_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp

https://ru-m-wikipedia-org.translate.goog/wiki/Ту-2000?_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp

https://topwar-ru.translate.goog/75222-unikalnye-gazovye-modifikacii-tu-154.html?_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp
_{(ignore the comments)}

 

Mi-8TG with TV2-117TG engines on buthane.
https://www-airwar-ru.translate.goog/enc/uh/mi8tg.html?_x_tr_sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp

[AckSed]

It has already been tried in smaller aircraft (a Beechcraft) with some modifications i.e. stick some methane tanks in behind the passenger seat: https://ntrs.nasa.gov/api/citations/20020091954/downloads/20020091954.pdf

And discussed and studied by NASA and Martin Marietta in the late 60s-early 70s RE: a Mach 3 commercial aircraft: https://ntrs.nasa.gov/citations/19680024112

Larger aircraft will need to be adapted or even designed around the tankage. The most recent study I've found is "Liquefied Natural Gas for Civil Aviation" (2020) Rompokos, Kissoon, Roumeliotis et al: https://www.mdpi.com/1996-1073/13/22/5925

On their model 737-800 they chose the "top tank" configuration: outside the aircraft, on top, in a fairing like a mini-Guppy:

Spoiler

Quote

The sized aircraft is heavier than the baseline due to the addition of the fuel storage system, and in combination with increased wetted area, requires more energy. However, due to the higher LHV of the LNG compared to Jet A-1, less mass of fuel is required for the same mission. Overall, the combined effect leads to a slightly heavier maximum take-off weight (MTOW) and maximum landing weight (MLW).

They also find that the model's CO2 emissions are 14-17% less than their Jet-A-fuelled baseline, and fuel costs (depending on which gas field it's sourced from) averaged 15-18% less.

[mikegarrison]

Kerosene is in a really good sweet spot between energy density by mass and energy density by volume. Gaseous fuels have some nice advantages in terms of combustion (no need to atomize the fuel, and can even pre-mix before the combustor), but they all have their own issues too, the main one being the energy density per volume.

[darthgently]

8 minutes ago, mikegarrison said:

Kerosene is in a really good sweet spot between energy density by mass and energy density by volume. Gaseous fuels have some nice advantages in terms of combustion (no need to atomize the fuel, and can even pre-mix before the combustor), but they all have their own issues too, the main one being the energy density per volume.

And add in the increased weight of the required pressure tank  over an ambient temp liquid tank.  Also not trivial is the inconvenience of the shape limits of high pressure tanks compared to the very loose shape constraints of a typical petro fuel tank

[kerbiloid]

7 hours ago, darthgently said:

And add in the increased weight of the required pressure tank  over an ambient temp liquid tank. 

That's why the gasocopter mentioned above was using buthane.

Methane causes higher internal pressure, while propane+buthane mix requires much weaker tanks (molar mass = 44 and 58 g/mol instead of 16 for the methane, so the pressure is three times lower), and at the same time is widely available as a by-product of every oil or gas refinery.

The advantage above the kerosene is that the aviation kerosene sometimes requires special oil from limited amount of oil rigs.
While propane+buthane can be made out of everything, from mineral coal to plastic toy dinosaurs.

Edited August 6 by kerbiloid

[magnemoe]

3 hours ago, kerbiloid said:

That's why the gasocopter mentioned above was using buthane.

Methane causes higher internal pressure, while propane+buthane mix requires much weaker tanks (molar mass = 44 and 58 g/mol instead of 16 for the methane, so the pressure is three times lower), and at the same time is widely available as a by-product of every oil or gas refinery.

The advantage above the kerosene is that the aviation kerosene sometimes requires special oil from limited amount of oil rigs.
While propane+buthane can be made out of everything, from mineral coal to plastic toy dinosaurs.

Yes butane is very easy to keep liquid, its stored in think metal containers, propane is also pretty easy but require more pressure.  Methane is more like oxygen, you have to pressurize it or go cryogenic. 
None want that. 

[Exploro]

There are drones that do use propane as fuel, The Lockheed Martin Striker for example. This particular example does not produce power via combustion though. Rather the propane is some how used to generate electricity via a fuel cell to drive the aircraft's electric motors.

Edited August 12 by Exploro