Circulating fluidized bed boiler and a method of assembling a circulating fluidized bed boiler

A circulating fluidized bed boiler comprising:

• a rectangular furnace, which is horizontally enclosed by sidewalls, for combusting fuel and combustion gas therein and generating a stream of flue gas and particles, the sidewalls comprising first and second short sidewalls and first and second long sidewalls, wherein a common width of the first and second long sidewalls is greater than a common width of the first and second short sidewalls;
• multiple particle separators arranged on the side of each of the first and second long sidewalls, inlets of the multiple particle separators being connected to an upper portion of the adjacent long sidewalls, for separating particles from the stream of flue gas and particles discharged from the furnace, wherein each of the particle separators comprises a vertical gas outlet tube for discharging cleaned flue gas from the particle separator;
• a back pass arranged on the side of the second short sidewall of the furnace, the back pass being horizontally enclosed by back pass walls comprising a first back pass wall facing the second short sidewall of the furnace;
• and
• a horizontally extending cross over duct system directly connected to the vertical gas outlet tubes of the particle separators for conducting the cleaned flue gas to the back pass, wherein the cross over duct system provides a straight gas flow that is angled relative to the sidewalls of the furnace from each of the vertical gas outlet tubes of the particle separators to openings in the first back pass wall.