Abstract: A method and apparatus for cooling hot gas in a reactor in which the lower section of the reactor is provided with an inlet for hot gas and a chamber encompassing a bubbling fluidized bed. A central section of the reactor is provided with a riser, and the upper section with a gas outlet, and the reactor has heat transfer surfaces for recovering heat from solid particles. The riser is defined by vertical walls which are disposed above the bubbling fluidized bed so that they divide the fluidized bed into concentric outer and inner parts. From the inner part of the fluidized bed solid particles are supplied to the hot inlet gas for cooling thereof. The gas containing solid particles is conveyed through the riser into the upper section of the reactor, where solid particles are separated from the gas in a particle separator and returned to the fluidized bed into the outer part thereof.