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.
Re: 28 - Not sure how these barns were constructed, but in an operation I worked at, scrapers would push manure into an enclosed underfloor pit at the end of one of the barns. Depending on circumstances it could be aerated prior to being loaded into a manure tank, so that the solids mixed with liquids so that the resulting slurry was more easily loaded into a tank and spread onto fields.
If such a setup, perhaps the level of methane was so high that a nearby spark or flame caused the explosion.
We were always very careful in/around enclosed spaces regarding methane. It will overcome you very fast if a high enough level.