Is there any information regarding the combustion efficiency of LH2 over kerosene?

Question

Taking as reference the airbus A220 it needs around 22000 lt of kerosene to be filled. Is there a rough analogy of how much LH2 would the same aircraft need to operate in similar ranges. Of course there will be huge differences in the overall structure of the engine as well as the tanks to support and handle LH2. Doing the math with the LH2 properties the 22000 lt of kerosene correspond to 91900 lt of Hydrogen at 21K and a pressure of around 1.2 bar, but that doesn't mean that this is the amount of fuel the modified aircraft would need in terms of the LH2 combustion performance.

If I am missing something obvious here, please do correct me. Thank you in advance.

Answer 1

In order to combust, a fuel needs to be mixed with air. While kerosene needs first to be sprayed into the air stream to form as small droplets as possible which then will evaporate in the heat of the combustion process, liquid hydrogen only needs a bit of heat to become gaseous and will mix quickly. It therefore will burn more readily and more completely compared to kerosene. Hydrogen-powered engines need shorter combustors, and the heat of the compressed air entering the combustor will be plenty enough to heat it.

To quote from the linked answer:

The downsides, of course, are storage and the small molecular size of hydrogen. It is very hard to contain, and it needs big volumes to store a given amount of chemical energy. Pressurized storage at 200 bars holds only 18 kg/m³ or 45 times less than kerosene. With 142 MJ/kg, hydrogen holds three times as much chemical energy than kerosene, but then the volumetric efficiency of kerosene is still a factor of 13.3 better.

Cryogenic storage swaps pressure for low temperature: Below 33 K and above 13 bar, hydrogen becomes a liquid and storage density increases to 30 kg/m³. Still, cryogenic hydrogen storage needs 4 times the volume of the same amount of energy stored as kerosene, plus the isolation and the energy to cool it down and compress it.

Here it should be added that hydrogen is now regularly stored at 700 bar pressure. However, real gas effects make higher pressure less and less effective: Doubling the pressure from 350 bar to 700 bar will only pack 67% more hydrogen into the same volume.

Storing cryogenic fuel needs isolation if it isn't used up quickly. A rocket will burn its hydrogen within minutes, so if a bit boils off it can be tolerated. An airliner needs to fly for several hours, so keeping the hydrogen from boiling is essential if you want to land at your destination with the engines still running (and enough reserve fuel in the tanks).

So what you are missing is:

1. Tank weight,
2. the compression rsp. cooling energy, and
3. embrittlement prevention. Hydrogen will make the usual metals used in airplane construction brittle, so the long-term strength of tanks and pipes to store and transport hydrogen will be compromised if not designed well for that purpose.

Answer 2

A kilogram of kerosene contains a certain amount of stored chemical potential energy, as does a kilogram of hydrogen. These numbers are listed as heating values of fuels in combustion engineering handbooks, where perfect combustion conditions are assumed. These numbers allow you to determine how much of one fuel is needed to equal the performance of another fuel, which is the missing information you need here.