Abstract: A circulating fluidized bed boiler includes a furnace for combusting carbonaceous fuel, at least one outlet channel connected to the upper portion of the furnace for removing flue gas and solid particles generated in the combustion of the fuel, each outlet channel provided with a particle separator attached with a flue gas channel for transferring cleaned flue gas and a return duct for transferring separated solid particles to the lower portion of the furnace. The return duct is provided with a gas seal, a heat exchange chamber, a lift channel and an overflow conduit, in which solid particles exiting the gas seal are guided to the upper portion of the heat exchange chamber and from the lower portion of the heat exchange chamber through the lift channel to the furnace, or directly from the upper portion of the heat exchange chamber through the overflow conduit to the furnace.