Abstract: A boiler comprising a fluidized bed combustion chamber which communicates with an exchange enclosure containing water tubes heated by the combustion tubes and within which is arranged an exchange and dust removal chamber equipped with water tubes and containing a vertical internal partition dividing it into two half chambers communicating with one another. One of these half chambers communicates with the upstream part of the enclosure through an inlet orifice situated at the top part and constrains the fumes to follow a descending path along the partition, while the other communicates with the downstream part of the enclosure through an outlet orifice situated at the top part and constrains the fumes to follow an ascending path.