An apparatus is presented for burning carbonaceous deposits off of fluidized particles. The apparatus is preferably employed as the catalyst regeneration zone of a fluidized catalytic cracking process. Heat pipes are used to cool the particles and transfer heat to an external steam generation zone. Preferably, the heat pipes extend vertically upward from an external catalyst transfer conduit to the steam generation chamber. The water level in the steam generation chamber or the density of the fluidized particles in contact with the heat pipes can be used to control the rate of heat removal from the combustion zone.