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I am just wondering if supplying compressed hydrogen to a gas turbine (combustor) would be a better fuel as it has three times the octane rating of kerosene. It just seems better all around.

0scar
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Skyhawg
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    Related: Are there any programs to build a hydrogen powered airplane? – fooot Jan 21 '20 at 17:47
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    "It just seems better all around." I would think the pressurized tanks to hold enough hydrogen would really be a factor in not using it. – CrossRoads Jan 21 '20 at 17:51
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    A lot of aircraft engines have commercial variants, and sometimes they will be configured to run on whatever is convenient. GE makes a version of the CF-34 that runs on natural gas and is widely used as a pipeline pump. – John K Jan 21 '20 at 18:16
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    Is octane rating even a factor in turbines? I don't think so, because it's a measure of how readily a fuel detonates ("knock") when compressed in a piston engine. Since many power generating turbines run on natural gas, the only obstacle to using hydrogen would seem to be the cost. – jamesqf Jan 21 '20 at 18:18
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    @jamesqf nor is it a factor with hydrogen, as hydrogen doesn't contain any octane and therefore it's "octane rating" is 0 by definition (the octane rating being derived from the percentage of octane in the fuel historically, albeit the formulas have changed a bit over time to account for different additives). – jwenting Jan 22 '20 at 06:09
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    @jwenting That is not true, octane rating can be defined for hydrogen as well since the modern measurement methods are independent of actual octane content. According to Wikipedia, hydrogen has a RON of > 130. – Bianfable Jan 22 '20 at 10:52
  • @Bianfable so the definition of "octane rating" has changed over time from its original meaning of being the percentage of octane in the fuel, as I postulated. Under the original definition hydrogen (or methane too for example) would have an octane rating of 0 because it contains 0% octane. That's what the name octane rating comes from in the first place. – jwenting Jan 22 '20 at 11:06
  • @jamesqf Given that the source of hydrogen is natural gas, it would be kind of pointless to use hydrogen instead of natural gas, yeah. – Luaan Jan 22 '20 at 11:51
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    @jwenting I'm not aware that octane rating ever meant the proportion of octane in the fuel (I suppose there might have been variations in different countries?). Do you have any references to the contrary? The name comes simply from (a particular species of) octane being the benchmark of the conditions in which the fuel self-ignites. – Luaan Jan 22 '20 at 11:56
  • OK guys, I edited my answer. -NN – niels nielsen Jan 22 '20 at 18:23
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    @jwenting at no time did octane rating refer to the percentage of octane in the fuel itself. The very first standardized tests for it, used the number to refer to the percent of octane in a reference blend of iso-octane and n-heptane that began to knock at the same compression ratio as the fuel under test. Actual amount of octane in the tested fuel was not relevant. See here for the history of octane rating testing: https://tandfonline.com/doi/abs/10.1080/17581206.2016.1223940?src=recsys&journalCode=yhet20 – mbrig Jan 22 '20 at 18:34
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    @Luaan: Natural gas is not the only possible source of hydrogen, though it might be (or might not - I don't really know) the source of most commercial hydrogen. You can, as you may have learned in high school chemistry, produce it by electrolysis of water, or by many other reactions. – jamesqf Jan 22 '20 at 19:23
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    @jwenting: Octane rating does not mean that the fuel contains octane, it just measures the ability to resist knock compared to pure octane, which has its rating arbitrarily set to 100. So for instance ethanol, which contains not octane at all, has an octane rating of ~113, natural gas is about 130, &c. – jamesqf Jan 22 '20 at 19:30
  • Does "knock" actually happen in a Turbine as in a piston engine? – Skyhawg Jan 22 '20 at 19:58
  • I was assuming that a steady flow of fuel in a mixing chamber was just a steady burn, Does "knock" actually happen in a Turbine as in a piston engine. I thought it was a steady burn in a single chamber vs multiple cylinders and so has NO knock but with a 3 times higher "power" rating than fossil fuel.... So much to learn! – Skyhawg Jan 22 '20 at 20:08
  • Oh yes! The Space Shuttle main engines were preceded by a "topping cycle" turbine which ran on the main engine' LH and LOX fuels. – Russell McMahon Jan 22 '20 at 21:10
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    @Skyhawg I think you might be interested in this video on the history and future of hydrogen-powered aircraft. I believe it answers your question and provides additional context. – jayhendren Jan 23 '20 at 06:17
  • You might also be interested in the Pulse Detonation Engine which uses hydrogen and oxygen as fuel. – JPhi1618 Jan 23 '20 at 17:14

6 Answers6

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Yes, indeed the plan to build a hydrogen-powered jet laid the groundwork for using hydrogen in the Centaur rockets and the upper stages of the Saturn 5. While hydrogen was used only experimentally in test rigs, the Soviet Union built a derivative of a regular airliner, the Tupolev 155, for testing hydrogen and natural gas in flight.

EDIT: NACA also did in-flight testing. Please see @jayhendren's answer for details.

So yes, the Tu-155 was indeed flown on several occasions with one hydrogen-powered turbofan. While the left and center engine remained NK-8s, the right engine was replaced by a NK-88 which was adapted for LNG and hydrogen. Other projects like one for a hydrogen-powered supersonic airliner sadly ended with the Soviet Union itself.

Tu-155 Cutaway view

Tu-155 Cutaway view (picture source)

Hydrogen is also the prime propellant in hypersonic ramjets, but those are not turbines.

Hydrogen has a wide mixing ratio with air where it will burn. Also, being gaseous, it mixes much more quickly with air, so the combustion chamber can be small. When a converted J-57 was experimentally run on hydrogen in 1957,

The test engineers were agreeably surprised by the ease of engine operation. They ran it at full power and throttled back so far that the air fan was revolving so slowly the individual blades could be counted. Under this latter condition, the throttle could be opened and the engine would quickly and smoothly accelerate to full power. They found that the temperature distribution was good and there were no major problems.

But Niels is correct - its low density makes hydrogen problematic. As Alexis W. Lemmon, Jr., reported in May 1945 in his report on possible jet fuels (from history.nasa.gov):

"Although the liquid hydrogen-liquid oxygen system has by far the highest specific impulse performance of any system considered in this report, the low average density of the fuel components almost completely eliminates this system from all but very minor applications."

Peter Kämpf
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  • Thank you.... I was thinking compressed gas as similar to the auto makers solution.... Two tanks @ 60 liters each .... Is 55kg total weight – Skyhawg Jan 22 '20 at 01:35
  • Tu-155 had only one engine (#3) running on LH/LNG, so it was just (a) hydrogen-powered turbofan. – Zeus Jan 22 '20 at 05:51
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    Airbus and Boeing also were planning hydrogen powered experimental jetliners, but AFAIK neither ever even built a prototype as it would be utterly impractical as the hydrogen tanks would have taken up the entire passenger cabin and cargo compartment. – jwenting Jan 22 '20 at 06:14
  • @Zeus: I did not know that fact, but it turns out you are right. Corrected. – Peter Kämpf Jan 22 '20 at 10:40
  • @Skyhawg The low density already takes compression into account. LPG isn't compressed gas - it's liquefied (it's even in the name - liquified petroleum gas). The same is true of oxygen-hydrogen tanks in rockets. Liquids cannot be compressed much. The density of liquid hydrogen is mere 70g/L - a kilogram of liquid hydrogen requires a 14 litre tank. And on top of that, it's much easier to liquify natural gas and keep it that way. – Luaan Jan 22 '20 at 12:02
  • @Luann They're talking about Liquid Natural Gas (LNG), not Liquid Petroleum Gas (LPG). The former is cryogenically-liquified while the latter, being mostly propane and other short-chain hydrocarbons, liquifies without cryogenic treatment. Any time you're dealing with cryogenic liquids, the tanks, lines, fittings etc. get more complicated. – Meower68 Jan 22 '20 at 15:19
  • I was not thinking LNG nor LPG, just straight up H in compressed gaseous form. Hydrogen is Hydrogen, the number one on the periodic table, the most abundant molecule in the Universe. – Skyhawg Jan 23 '20 at 00:24
  • @Skyhawg When people use Hydrogen as fuel they're not using the gaseous form because that's nowhere near dense enough to be feasible. They're using liquid hydrogen which is far denser than compressed hydrogen gas and cannot be further compressed (in terms of volume, you can of course increase fluid pressure but all you'll get is a higher psi/kpa value without a significant decrease in volume) – slebetman Jan 23 '20 at 04:55
  • "While hydrogen was used only experimentally in test rigs, the Soviet Union built a derivative of a regular airliner..." this is not quite correct. NACA performed in-flight testing of a hydrogen-powered turbojet in 1956, well before the Soviet program. See my answer for more details. – jayhendren Jan 23 '20 at 05:00
  • @slebetman I believe you are correct, after reading and learning more, a PEM fuel cell uses gaseous molecules to convert to electricity. This seems more viable than burning. So Turbine use is not so good as electric prop would be. Sigh. – Skyhawg Jan 23 '20 at 05:07
  • @jayhendren: Thank you for the information! I did not read the history.nasa.gov pages carefully enough. – Peter Kämpf Jan 23 '20 at 09:43
  • If you put the hydogen tank into the fuselage (as suggested by the cutaway), wings tanks become dead space. – Manu H Jan 26 '20 at 09:27
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For fuels intended for use in aircraft, the key performance parameter is the energy density of the fuel: how much potential work is stored in how many liters of stuff. High energy density means the fuel tanks will be small and the energy released upon burning a liter of it will be large. The problem with using hydrogen as aircraft fuel is that its energy density is way lower than that of kerosene or diesel (because a liter of hydrogen at atmospheric pressure contains far less chemical potential energy than does a liter of kerosene), and squeezing it down to reduce its volume requires cryogenic refrigeration which hugely increases cost and weight, and the octane rating advantage of H2 is not balanced by these disadvantages.

Specifically regarding burning hydrogen in brayton cycle turbines, this is possible but economically impractical because the cost to make a liter of hydrogen is far greater than the cost to refine a liter of jet kerosene from crude oil.

niels nielsen
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    Energy/mass of H2 is great (not include mass of tanks). The problem with H2 gas is energy / volume. (litres not kilograms). Your answer defines energy density as energy / kilogram and then talks about volume >.<. https://en.wikipedia.org/wiki/Energy_density does define that term as energy / volume, so yes, energy density in that sense is the problem. Energy/mass is more properly called specific energy. – Peter Cordes Jan 22 '20 at 04:36
  • @PeterCordes yes, which is why Airbus and Boeing gave up on their plans for hydrogen powered airliners. – jwenting Jan 22 '20 at 06:17
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    @jwenting: I'm commenting only about terminology / wording, not the main point of the answer. (That it's impractical because of volume reasons, and/or mass of tanks / cryo equipment). Would be happy to upvote this answer once it's worded properly to actually say what it means :P – Peter Cordes Jan 22 '20 at 06:24
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    You're right - energy density is the key. However, hydrogen has GREAT energy density; as the question states. The problem is that the hydrogen container is heavy; which means that the effective energy density is less – UKMonkey Jan 22 '20 at 14:38
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    I want to point out that Wikipedia defines it the same way I do: "Energy density is the amount of energy stored in a given system or region of space per unit volume. Colloquially it may also be used for energy per unit mass, though the accurate term for this is specific energy." Based on this, hydrogen does NOT have GREAT energy density. – Philip Ngai Jan 22 '20 at 17:38
  • So then instead of "burning".... The Fuel cell option of converting to electricity is a better solution for energy/density and would also seem to solve emissions concerns as well? – Skyhawg Jan 22 '20 at 19:30
  • @Skyhawg no, because a fuel cell is also just added weight and doesn't solve the inherent lack of energy density. Not to mention I suspect the weight of a fuel cell which could output as much energy as a turbine would probably weigh more than the whole plane. – Turksarama Jan 23 '20 at 01:27
  • I am confused then.... A 100KW fuel cell @ 53KG and two tanks that hold 60 litres compressed @ 700bar each @ 55kg total = 108KG. 158HP max. Six hrs of power. Seems as though a single or two person Personal VTOL is doable. – Skyhawg Jan 23 '20 at 01:38
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Von Ohain's first prototype of his HeS 3 turbojet, the HeS 1, burned hydrogen in the first runs. Only after some modifications was he able to make it work with a liquid fuel.

https://en.wikipedia.org/wiki/Heinkel_HeS_1

Peter Kämpf
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xxavier
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This is not fully on topic ('Aviation'), but answers the question in some way:

Hydrogen is currently considered the 'fuel of the future' for existing and new gas turbines in power plants, for small and large gas turbines equally. The industry is working to enable all existing turbine lines to handle that, and the modifications are minor (well, minor compared to the complexity of a modern gas turbine). Many models are already successfully enabled and proven, and the market is expecting to get the first requests soon.
Overall, it is quite easy to use hydrogen - just maybe not on an airplane.

The main reason to go for hydrogen burning is that it enables hydrogen storage as a battery - when there is extra power available (preferably solar or wind), it gets converted to hydrogen, and when the power is needed, it gets burned in existing gas turbines. This would allow to save trillions of existing investments in power plants by converting them into zero-emission hydrogen-burning plants.

[Disclaimer: I have a professional relationship to such a company; however, this is not restricted information]

Aganju
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  • Solar/wind/hydro conversion to stored Hydrogen is beautiful :) – Skyhawg Jan 23 '20 at 05:15
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    For non-aviation purposes the expected use is in fuel cells, not turbines. Fuel cells produce electricity, not heat. This frees them from the Carnot efficiency limit. Even in hydrogen cars, the weight penalty of a fuel cell plus electric motor is so small that the efficiency gains are worth it. In aviation, fuels cells would still mean you can have a smaller, lighter hydrogen tank, but the weight remains problematic either way. – MSalters Jan 23 '20 at 11:14
  • Yes, but on a mega macro scale it's perfect, and ground based electricity turbines can have huge hydrogen tanks. Windmills generate electricity, excess is used to split water and hydrogen, hydrogen is stored for wind lull. The bigger the better. Liquid Natural Gas will be much easier for aircraft (particularly ones that operate between terminals) and now is extremely inexpensive. – Robert DiGiovanni Jan 23 '20 at 17:10
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NACA (the predecessor to NASA), has burned hydrogen in a turbojet engine in flight.

There is an extensive history of NACA's and NASA's experimentation with hydrogen as a fuel on the NASA history website.

In the 1950s NACA burned hydrogen in a turbojet engine on a modified B-57 aircraft as part of Project Bee:

Project Bee

A technical report on the in-flight performance of the hydrogen-powered turbine was published and is available for download on the NASA website.

The experiments with hydrogen as a fuel source for turbine engines were successful enough that the Lockheed CL-400 high-altitude reconnaissance aircraft was designed to use hydrogen fuel:

enter image description here

Ultimately, however, the CL-400 was cancelled, although some of the research for this project made its way into the now-famous SR-71.

jayhendren
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This 2018 paper might be what you're looking for: https://www.sciencedirect.com/science/article/abs/pii/S0360319917349005

Airbus suggests that an hydrogen fed turbofan would achieve a better efficiency than kerosene through higher compression

High auto-ignition temperature: Hydrogen’s high auto-ignition temperature enables higher compression ratios in a hydrogen engine compared to a hydrocarbon engine. A higher compression ratio results in greater thermal efficiency, or less energy loss during combustion.

However your question misses some points regarding hydrogen. High pressure is pretty much excluded so you'll have to consider liquid hydrogen at -252.9C, which makes storage a pain. If you don't insulate you'll have different mechanical properties due to low temperature. If you do insulate you add weight and points of failure.

Also, it has indeed a higher specific energy per kg but it's also much less dense. Combined with storage nightmare, gains from using hydrogen are not that straightforward.

Also, production losses (including energy to liquify) do not help hydrogen's case cost-wise.

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