The dragon, through a gland, produces a flammable oil from the fat of the animals it has eaten, the oil is stored in a bag, the blood circulation around the bag increases, heating the oil to the point that it catches fire on contact with the air, the oil comes out from two mouth tubes, one on each side, the tubes are in the mouth to help you aim better.
Details: My dragon is the same as the Game of Thrones Drogon when Khaleesi first flew it, that's the maximum size the species can get (however, with bigger wings to be more proportionate as, from what I've researched, the Those dragons' wings aren't big enough compared to their bodies, so they couldn't be 100% physically correct, plus my dragons have more membrane on their wings following the logic of bat wings). The dragon is hot blooded. I still don't have a way to explain how the dragon will withstand a hot oil inside him without disastrous consequences. If you like, you can suggest another way for the dragon to light the oil.