Methane. Lots of methane.
~~~
Yes, it is said that cows produce methane, but wouldn’t the methane concentration high enough for combustion also be considered noxious or toxic?
Methane is notably ‘lighter than air’ (specific gravity of .554). In an open-air environment it disperses rapidly, unlike propane.
I find it hard to believe that they would create an explosion risk by enabling enough methane to collect in a building. The lower explosive limit of Methane is ‘high’ compared to most gases, a little over 4% volume in air, whereas propane is closer to 2%. Methane is odorless natively, and is not TOXIC, but in high-enough concentrations it IS ‘suffocating’ if there’s not enough O2 in the mix.
And to have such a big explosion, that implies there was a LOT of methane within its explosive limits in the air.
Dunno.