A circulating fluidized bed boiler includes a furnace for combusting solid carbonaceous fuel in a fast fluidized bed. A solids separator is adjacent to a sidewall of the furnace and separates solids entrained with exhaust gas discharged via an outlet channel. A gas seal conveys at least a portion of the separated solids to a first fluidized bed heat exchange chamber that is arranged downstream of the gas seal and has internal heat exchange surfaces. A first lift channel has a lower end connected to a bottom portion of the first fluidized bed heat exchange chamber and an upper end connected to an upper end of a first return channel for discharging solids from the first fluidized bed heat exchange chamber and taking the cooled solids to a lower portion of the furnace. A second fluidized bed heat exchange chamber is arranged adjacent to a lower sidewall of the furnace.