Chlorine is already a chemical compound in the air of a few planets in canon, and Chaos canon. Alongside that is a few species breathe the chemical naturally. This in itself is the basis of an idea I have had for a time, but couldn't think of how to write properly.
Lets say a planet has Chlorine Triflouride, a type of Chlorine, as its main air agent. A species grows use to this, breathes it, lives in it. However, over a hundred thousand generations of trying to live off world, they have begun to fiddle with their genetics, invent tech, etcetera, that way to adapt to living with other chemical atmospherics.
Now they have managed to possess a sac based in their throat, through which oxygen or other atmospherics pass through and are merged with a heavily condensed liquid Chlorine Triflouride, that is turned to gas during that process, making it a sort of wean away from it system. To aid this, they have collars that hold canisters of the liquid that can be pushed through a needle administer system directly to the sac.
Of course being of a planet that had such elements, they would be a tougher reptilian species. From this i imagined their scaly, gravely tongues would be able to scrape a certain point in their teeth to create a spark. Now on the exhale, they can breathe fire. That in mind, over a millenia of this type of ordeal happening on accident, their mouths are further acclimated to the heat, so it doesn't harm them.
Basically imagine Abe from Hell Boy, only the opposite, the water in his collars are actually Chlorine Triflouride, and they can breathe fire because of the chemical they inhale.
Is this feasible?
Obviously I would be giving powerful weaknesses to counter such a heavy strength. Just this base idea, could you see it logically occurring and being approved.
Ok, so Codex judge whom reads this, bear with me a moment.