In the Game of Thrones TV series the fire of the dragons is "biological" and comes from two tubes in the dragons' mouth. In the second last episode of Season 6, the dragons Viserion and Rhaegal break the stone walls of the great pyramid using their fire breath. Later it is said the fire is even hot enough to melt stone.
This raises the question: if a dragon produces fire through chemical reactions (instead of magic) how potent would its fire be? Would it be as potent as the series?
Another example of how potent the fire is in Game of Thrones -- if turned upon a person, the flame burns the person turns to ash before they have time to scream.
The dragons in my world have oil-based fire breath. They are carnivorous and metabolize the oil from animal fat. It is produced from a gland in the mouth and ignited by a spark in the mouth. The gland also produces methane gas and magnesium, as I believe these materials lead to hotter flame.
For reference the dragon's body is about twice the size of a horse. It weighs about 300kg and has a wingspan of about 20m.