Since it's Steampunk, probably coal? 
Have you seen Aeromodeller2? Their website hasn't updated since 2016:
Aeromodeller2
Quote:
The Aeromodeller2 is a design for a zero-emission, autonomous, nomad hydrogen-based airship that will never land.
The ship regenerates its fuel, creating hydrogen from wind power and the rain on its skin.
When it depletes its energy reserves, the ship drops anchor and rests, in order to replenish its energy in a renewable way.
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