Oh. It wasn't "detonated," therefor it could not have exploded. Even though it was a chemical reaction, and we know those never result in explosions. And it didn't do damage to the surrounding barrels, another sure sign that a "chemical reaction" that "heated" the contents of the barrel, and resulted in "dislodging" the lid and "spewing" the contents was not an explosion. My goodness. Connecting the dots, I see.  

ex·plo·sion  (ĭk-splō′zhən)
n.
1. a. A release of mechanical, chemical, or nuclear energy in a sudden and often violent manner with the generation of high temperature and usually with the release of gases.

Gosh, that describes what the barrel did!