Abstract: A method and apparatus for cooling hot gas in a reactor which is provided with a mixing chamber for hot gas and a chamber encompassing a bubbling fluidized bed in the lower section of the reactor, a riser in the middle section, a gas outlet in the upper section of the reactor, and heat transfer surfaces for recovering heat from solid particles. The hot gas is introduced through the mixing chamber into the lower section of the reactor. In the mixing chamber solid particles are supplied to the hot gas for cooling the gas. The gas containing solid particles is conveyed into a separator. From the separator, the gas is conveyed through the riser into particle separators and, when purified, out of the reactor. The solid particles separated in the particle separator are returned to the lower section of the reactor, preferably both directly into the mixing chamber and into the fluidized bed. From the fluidized bed, cooled solid particles are conveyed to the mixing chamber for cooling of hot gas.