The fuel probably can't be a hydrocarbon (methane, propane, octane), as those are mostly inorganic. It'd have to be an organic compound like a fat, since fat is mostly oil, which is flammable. Dragons might have huge oil deposits in their bodies so that they combust and exhale out as fire. The fat would have to be transferred from the deposits to the lungs to catch and burn as the dragon exhales. There would also have to be a spark ignition as soon as the dragon exhales to catch the transferred fat on fire and then exit the dragon's windpipe.
But I don't know, I'm no expert.