[Marauder77]

Again. Its been a while since I've had chemistry, seeing as I study civil engineering, but there are chemical reactions that can look like fire that doesn't produce the same heat, or for that matter uses oxygen.

And as far as I understood the dialog the "atmospheric converter" was made by Rattigan and his brain-trust based on Sontaran technology, not made by the Sontarans themselves, and Rattigan was after all extremly gifted. It just need a few adjustments by the Doctor to be calibrated on the specific gases from the Atmos devices and power from the sonic screwdriver.