Is water a possible fuel for jet engines?

Question

Jet engines can run on almost any fuel, and the operating temperatures of modern jet engines' hottest sections are anywhere between 3000 and 3150 degrees F (1648 and 1732 degrees Celsius). Does that mean that a hydrogen on-demand system could work on modern jets?

Water is pumped and heated first by the exhaust section, then directed towards the hotter sections of the engine (when hot enough to not cause cooling and lower engine efficiency) where it's broken down into hydrogen and oxygen at a heat above 1472 degrees F (800 degrees Celsius), then those gases are pumped into the engine for combustion.

The advantages are that firstly, water is abundant and therefore cheap. Even sea water could be used because at those temperatures it's easy to design a system that would get rid of the impurities that would otherwise corrode critical engine parts.

Secondly, it would save on manufacturing costs given that non-heat critical parts in the exhaust section would not need to be made of sophisticated and expensive materials and alloys given the cooling effect of water.

Thirdly, the costs of the fuel weight would be reduced given that the energy density of hydrogen is twice that of fossil fuels, so less would need to be carried. And most importantly, the environment problem would be solved in aviation given that there would be little or no carbon dioxide emissions.

Answer 1

You're proposing to use the heat of the engine to break down water into hydrogen and oxygen, and then burn the hydrogen in the oxygen, turning it back into water. And, remember, when I said "the heat of the engine", I mean the heat you obtain by... burning the last lot of hydrogen and oxygen you made.

Even if this process was 100% efficient, you'd just be converting back and forth between water and hydrogen/oxygen. No energy would be created, so there'd be no thrust. And it's not 100% efficient: you'd lose energy, rather than gain it.

Answer 2

No. Water is not a fuel, it is hydrogen that has burned already. It is a very stable oxidation product, so stable that it is used to extinguish fires. Commercial production of hydrogen is not done by heating water. From the wiki:

There are four main sources for the commercial production of hydrogen: natural gas, oil, coal, and electrolysis; which account for 48%, 30% 18% and 4% of the world’s hydrogen production respectively.[5] Fossil fuels are the dominant source of industrial hydrogen.

There is quite some interest in R&D projects to produce hydrogen from water, because like the OP states water is very abundant. Many production methods are researched: electrolysis, radiation, chemical reaction. Heat is one of the least promising techniques, since temperatures of up to 3000 °C are required. From this wiki:

Thermal water splitting has been investigated for hydrogen production since the 1960s.[16] The high temperatures needed to obtain substantial amounts of hydrogen impose severe requirements on the materials used in any thermal water splitting device. For industrial or commercial application, the material constraints have limited the success of applications for hydrogen production from direct thermal water splitting and with few exceptions most recent developments are in the area of the catalysis and thermochemical cycles.

And that is in a production facility in a research lab. Now we're going to do this in an aircraft, where safety, weight and reliability are of prime importance. Water has been used for propulsion before (by injecting it into the hot exhaust gas of a turbojet) but that went out of the door: it does very little for high-bypass engines, not enough to justify the extra weight.

But most inventions start with an outlandish idea which is dismissed by the grey eminences at first. The OP has two seedlings that may have merit:

• Use waste heat that is produced anyway to generate useful power or thrust. This is done in power stations already: in remote communities in Australia one can find gas turbines driving alternators, next to a steam turbine that uses the waste heat from the gas turbine. But this is not viable for aircraft, too heavy and no redundancy since the steam turbine cannot work without the gas turbine.
• Use water that is on board anyway for propulsion. Waste water from the toilets! Inject that into the hot exhaust gases for an environmentally friendly solution.

Answer 3

This answer shows that:

The top gas temperature in a modern [civilian] jet turbine is more like 1500°C, and the turbine blades tolerate temperatures of around 1200°C.

On the other hand, thermal decomposition of water at 2200°C only splits 3% of the water. 50% for 3000°C, which remains the toughest challenge because of the materials required.

Liquid hydrogen however, is more promising.

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Temperatures [and apparatuses] aside, to fix the mentioned perpetual motion problem (it's either splitting or thrust, not both), you'll need a jet fuel gas turbine for the splitting, the un-split water to be recycled, and then separate hydrogen-burning jet engines for propulsion.

This is exactly like having an airplane carry crude oil and an oil refinery to produce its own fuel by burning fuel. The only difference is that breaking down hydrocarbons is much easier than breaking down water.

Answer 4

^{@DavidRicherby has demonstrated how breaking down water, and then burning hydrogen back to recreate water is a process with losses that cannot generate a positive budget of energy. But let's go further, as if this was not the case, and see which quantity of water is involved.}

(All figures are approximated to make them simple to compute mentally.)

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Let's evaluate the quantity of water you would need to replace fuel, based on the energy that can be extracted from hydrogen and from fuel. We need to compare the specific energy of both.

For a familiar reference: 1kWh = 3.6 MJ, this is what would consume a steam iron in an hour.

To get 1 kg of hydrogen from water, you need to break down, in the most optimistic case, about 10 kg of water (atomic mass of $H_2$ = 2 , of $O$ = 16, of $H_2O$ = 18).

• If hydrogen and oxygen could be separated, by some mean, 1 kg of water would produce 100 g of hydrogen. Hydrogen has a specific energy of 120 MJ/kg, that is 12 MJ/100 g.

• But the specific energy of fuel is much greater: 42 MJ/kg.

Assuming you have the water breakdown reaction for free, which is indeed not the case, you still need 42/12 = 3.5 kg of water to replace 1 kg of fuel.

Not really a good deal... the aircraft needs tanks about 3 times larger and needs a whole redesign to increase the maximum takeoff mass (which in turn will require a lot more hydrogen).

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^{That's without counting fuel which, in real life, is needed to breakdown water and extract hydrogen. With this fuel you reach insane masses.}

^{However... There are many processes for water splitting, some are really not very efficient... but if it's free...}

^{The energy required to extract hydrogen could be solar energy, which you get for free. There is a process known as thermochemical cracking which use cells with catalyst materials exposed to solar rays.
This process is extremely slow and not very efficient (7%), but it works!
While this is not usable for aircraft propulsion, it's possible to use it for large scale solar energy storage, converting solar energy into hydrogen chemical energy which can be used later.
See Almeria Spanish solar power plant.}

Answer 5

There are two ways of get this working, but both seem only very remotely possible.

1. Venus planet, or any other with the surrounding temperature above the water boiling temperature. In such environment, liquid water would have energy: while boiling, it can produce compressed steam able to rotate a turbine. A compressor, similar to the compressor of the jet engine, could help getting more hot air into the system to boil the water. Same way liquid nitrogen may work on Earth.

2. There is plenty of nuclear energy in the water, it contains hydrogen that powers the Sun by fusing into helium. A machine the could use such an energy is currently being built while I think initially it will power more something like a large ship.

You cannot decompose the water into oxygen and hydrogen and then burn these components because exactly the amount of energy required to decompose is released during the later burning process. Due inevitable losses, the machine eventually will stop after exhausting the initial energy pool.