St George, Call Your Office
The magazine called Discover: Science for the Curious is playing the what-if? game this month with an article entitled "The Science Behind Mythical Dragons".
Well, you have to take a look, doncha.. Especially at the bit about the fire-breathing kind. To wit:
The most dramatic aspect of dragons is arguably the most scientifically plausible. The bombardier beetle (Stenaptinus insignis), for example, produces the toxin benzoquinone and byproduct heat in a kind of internal combustion chamber, and then squirts it at any perceived threat. If the bombardier could evolve so complex a defense, it’s not much of a stretch to imagine a dragon with venom glands filled with a gas that could ignite when released with just a bit of friction.
Diethyl ether, an organic solvent, is Gee’s top candidate.
“It’s really easy to make by ‘drying’ ethanol with sulfuric acid,” notes Gee, cautioning that making it is best left to dragons and scientists — don’t try this at home. “It’s so hard to handle without it catching fire spontaneously. A tiny spill and rivers of fire would stream across my lab bench at tremendous speed.”
Gee envisions how a dragon might biologically synthesize diethyl ether.
“Yeasts and other organisms produce ethanol as a waste product, and there are bacteria that excrete sulfuric acid (they’re responsible for corroding concrete). I could imagine a microbial community in which diethyl ether is made as a waste product and exploited by dragons to breathe fire.”
Because it produces copious amounts of vapor, a little diethyl ether yields an impressive amount of flame. As it does not mix with water, ether’s fires are not quick to extinguish and could easily cause the kind of destruction for which Smaug is legendary.
So now you know.
There's more here. (They might be able to fly, too.)