A circulating fluidized bed boiler (10), comprising a rectangular furnace (12) which is horizontally enclosed by a front wall (16), a back wall (16') and two sidewalls (14, 14'), multiple particle separators (18, 18') connected to the upper portion of each of the front wall (16) and the back wall (16'), wherein each particle separator comprises a gas outlet (34, 34'), and a flue gas duct system (26) connected to the gas outlets for conducting cleaned flue gas to a back pass (28),wherein the particle separators are arranged in pairs of particle separators, wherein each pair of particle separators includes a front separator (18) arranged adjacent to the front wall (16) and a back separator (18') arranged adjacent to the back wall (16'), and in that the flue gas duct system comprises cross over ducts (32, 32', 32''), each cross over duct connecting the gas outlet (34) of a front separator (18) of a pair of particle separators, across and over the furnace, to the gas outlet (34')of the back separator (18') of the same pair of particle separators, and to the back pass (28), which back pass (28) is arranged on the back wall side of the furnace (12).