[mcguire]

That gas holder article says they contained methane or coal gas. Methane's density is 0.657 kg/m³; hydrogen's is 0.08375 kg/m³.

[Robotbeat]

Coal gas is a mix that (by energy) is about half hydrogen, as I said. And hydrogen gas a specific energy of 142MJ/kg vs 55.5MJ/kg for methane.