Abstract: An apparatus for supplying fluidized bed material to a circulating fluidized bed boiler having a primary zone with a fuel feed point and a furnace zone above the primary zone, a mixing chamber for receiving solid fuel and a portion of the total combustion air needed in the boiler. The combustion air and circulating fluidized bed solids are supplied tangentially into the chamber which is advantageously in the form of a horizontally extending cylinder. The combustion air and solids are intimately mixed with each other before they are discharged into the primary zone of the boiler. The primary zone of the boiler is shaped with an upwardly increasing cross sectional area so that the already mixed combustion air and solids diffuse evenly through the fluidized bed boiler.