From 1d4chan

Promethium is an umbrella term used for liquid and gaseous fuels in the Imperium of Man generally synthetic space gasoline used to power the vehicles and as flamethrower fuel. That said, 'promethium' can also refer to things like liquid hydrogen, coal, and many other fossil fuels or equivalents. The most common type, at least on the tabletop, is the flamethrower fuel, which is functionally identical to what we could call napalm- a jellied flammable chemical that can burn in most atmospheres and sticks to things.

Confusingly, there is a real element called Promethium- it's number 61 on the periodic table, is nonexistent in nature in its pure form (it's a lanthanide, so when it bonds with other things it forms salts), all of its isotopes are radioactive (which is very common) and it has very few practical uses.

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