Assume you fill a cupola to the brim with charcoal. The cupola is a just a tall vertical tube with an intake for air at the base. The charcoal at the bottom is ignited and the hot gasses flow upwards between the chunks of cold charcoal, heating them, such that the smoke that leaves the top is relatively cold after having passes through some cold charcoal.
As combustion progresses the charcoal at the base of the stack is slowly consumed and the column sinks, the smoke keeps heating the charcoal at the higher levels but it doesnt burn due to a lack of oxygen (it gets consumed at the lower levels).
As charcoal is burned, it gets replaced by fresh hot charcoal from near the bottom of the stack, because this charcoal was already hot before burning it produces hotter combustion gasses, further heating the column in a positive feedback loop.
You keep the top of the column cold by chucking in additional fresh cold charcoal. The hot gasses cool down as they pass through this charcoal, so all the heat is kept in the stack, not much energy goes out with the smoke.
What is the maximum theoretical temperature of the cupola?