A circulating fluidized bed boiler comprising - a furnace for combusting solid carbonaceous fuel in a fast fluidized bed, the furnace having walls made of water/steam tube panels and used for evaporating the water fed therein, - a solids separator arranged adjacent a sidewall of the furnace for separating solids entrained with exhaust gas discharged via an outlet channel from an upper portion of the furnace, - a gas seal for conveying at least a portion of the separated solids to a first fluidized bed heat exchange chamber arranged downstream of the gas seal and having internal heat exchange surfaces, - a first lift channel, having 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 arranged adjacent a lower sidewall of the furnace and having internal heat exchange surfaces, an inlet channel arranged between the second fluidized bed heat exchange chamber and said furnace for introducing hot solids from the furnace to the second fluidized bed heat exchange chamber, a second lift channel having a lower end connected to a bottom portion of the second fluidized bed heat exchange chamber and an upper end connected to discharge the solids to the lower portion of the furnace, - the first fluidized bed heat exchange chamber being positioned above the second fluidized bed heat exchange chamber, characterized in that the first fluidized bed heat exchange chamber has two first lift channels and two first return channels arranged at the lateral sides thereof such that the second fluidized bed heat exchange chamber is situated between the lower ends of the two first return channels. - a furnace for combusting solid carbonaceous fuel in a fast fluidized bed, the furnace having walls made of water/steam tube panels and used for evaporating the water fed therein, - a solids separator arranged adjacent a sidewall of the furnace for separating solids entrained with exhaust gas discharged via an outlet channel from an upper portion of the furnace, - a gas seal for conveying at least a portion of the separated solids to a first fluidized bed heat exchange chamber arranged downstream of the gas seal and having internal heat exchange surfaces, - a first lift channel, having 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 arranged adjacent a lower sidewall of the furnace and having internal heat exchange surfaces, an inlet channel arranged between the second fluidized bed heat exchange chamber and said furnace for introducing hot solids from the furnace to the second fluidized bed heat exchange chamber, a second lift channel having a lower end connected to a bottom portion of the second fluidized bed heat exchange chamber and an upper end connected to discharge the solids to the lower portion of the furnace, - the first fluidized bed heat exchange chamber being positioned above the second fluidized bed heat exchange chamber, characterized in that the first fluidized bed heat exchange chamber has two first lift channels and two first return channels arranged at the lateral sides thereof such that the second fluidized bed heat exchange chamber is situated between the lower ends of the two first return channels.