Abstract: A fluidized bed combustion apparatus in which in the free-board above the bed at least one system of plates or baffles inclined with respect to the vertical is arranged so that at least one upwardly directed channel having internally at least one group of two opposed arrays of superimposed inclined plates is formed, which plates at least alternately extend obliquely downwardly from the outside to the inside and are closely connected with the outer boundary of the channel. Preferably all plates of each array extend obliquely downwardly at an angle with the vertical within 20.degree.-50.degree. from the outside to the inside from a vertical wall defining said channel whereas the plates of the one array are off-set over a distance "s" in the vertical direction with respect to the plates of the other array.