Abstract: The invention relates to a fluidized-bed reactor having a higher power density and to an exchangeable insert for said fluidized-bed reactor, which insert makes the higher power density possible. The supply and pre-heating of the fluidizing agent, especially air, to the fluidized bed is combined with the cooling of the reactor vessel wall owing to the preferably exchangeable insert in the fluidized-bed reactor. The “cold” fluidizing agent is guided in at least one flow channel in the metal jacket surrounding the fluidized bed and is preheated and said channel and is then injected into the fluidized bed in an appropriate location. The reactor vessel wall is cooled by pre-heating the fluidizing agent. The desired and required values for cooling the reactor vessel and the desired pre-heating of the fluidizing agent can be adjusted by suitably selecting the parameters ‘length’ and ‘volume’ of the flow channels in the insert and the flow rate of the fluidizing agent in the flow channels.