I've got an area set aside for burning yard waste, about 10'x15', surrounded by corrugated sheet metal on edge.
Had a pile of debris in there one time, must have been 9' high. It was wet, so I decided to use a small quantity of gas to get 'er going, about a cup or so. Poured it on the pile at about 5' high on one of the long sides, stepped back & tossed in a lit wad of paper.
Blew out the corner of the surround.
DW came out the back door wondering WTF!! (House is a good 75 yards from the pile).
Fortunately I didn't loose any hair.
And the damn fire didn't start. Had to use the propane weed burner to get it lit. I still use the weed burner - every time.
Yeah, not a good idea to use gas in an enclosed area. Those fumes will build up quick. And release their energy even quicker.