A fluidized bed reactor, comprising a furnace, a heat exchange chamber, a substantially vertical discharge channel connected to the lower part of the heat exchange chamber for removing particulate material from the heat exchange chamber to the furnace, and a substantially vertical auxiliary channel for transferring particulate material from the heat exchange chamber to the furnace and from the furnace to the heat exchange chamber, the lower part of said auxiliary channel being provided with nozzles for fluidizing gas and with a flow conduit for connecting said auxiliary channel to the furnace and a flow conduit in the upper part for connecting said auxiliary channel to said heat exchange chamber. The furnace, heat exchange chamber, discharge channel and auxiliary channel preferably form an integrated structure having the discharge channel and the auxiliary channel adjacently arranged between the furnace and the heat exchange chamber.