The circulating fluidized bed reactor device comprising a reactor chamber delimited horizontally by walls, a centrifugal separator and a back pass for heat recovery The reactor device allows for the introduction of a fluidizing gas into the reactor chamber and for maintaining a fluidized bed of particles in said chambers. The gas to be dedusted is transferred from the reactor chamber into the separator, and the separated particles are discharged from the separator and the dedusted gas is transferred from the separator into the back pass. The reactor chamber and the separator both have a common wall with the back pass.