The bottom line is that the most flammable thing on board or even off board is the gas. If it goes in the intake there will be flames out the exhaust which can find its way back to the source.
Although unlikely, the name of this thread is "It might burn..." and the most likely thing to burn is propane.

The vid just shows how easy it is to ignite the gas and how it can make someone dance and scream like a little girl.