Anne McCaffrey wrote a series of novels (Dragonriders of Pern) based around fire breathing alien dragons. Their method of producing fire was to digest phosphine

Anne McCaffrey wrote a series of novels (Dragonriders of Pern) based around fire breathing alien dragons. Their method of producing fire was to digest phosphine rich rocks in a dedicated stomach and then emit phosphine gas which would spontaneously combust when expelled and mixed with air. In short, fire breath without the need for fire in the belly.