A fluidized-bed furnace of the stationary type, comprising: a fluidized-bed reactor (A), constructed to form a fluidized bed, comprising: a first zone for heat recovery (B1), comprising a fluidized-bed section with immersion heating surfaces (E) and a fluidizing gas distributor (C1) with fluidization elements , and a second zone for primary combustion (B2), comprising a fluidized-bed section without immersion heating surfaces and a fluidizing gas distributor (C2) with fluidization elements ; a bed material discharge funnel (D) adjoining the area below the fluidizing gas distributor (C1, C2); and a free space above the fluidized-bed reactor (A) in which burn-out air nozzles are arranged within burn-out air levels; wherein the first zone for heat recovery (B1) and the second zone for primary combustion (B2) form a common fluidized bed, and wherein the fluidized-bed reactor is constructed to generate a fluidization of the fluidized-bed area of the second zone (B2) that has a higher speed than the one of the first zone (B1) so that a fluidized-bed internal circulation of material (F) is forming between the first and second zone along the immersion heating surfaces, and the cross-sectional area of the first zone (B1) comprises from 0.4 to 0.75 of the entire cross section of the fluidized-bed reactor; characterized in that the fluidized-bed furnace is constructed in a way that the total amount of oxygen Λ tot introduced to the fluidized-bed furnace is composed of the oxygen Λ B introduced to both zones of the fluidized bed B1 and B2 and of the oxygen fed to the free space, and wherein the oxygen portion Λ B that is fed directly to the fluidized bed is composed of the oxygen Λ B1 introduced to the fluidized-bed area of the first zone (B1) and of the oxygen Λ B2 introduced to the fluidized-bed area of the second zone (B2), each based on the stoichiometric oxygen demand O min for combusting the entire introduced combustible material, wherein Λ B1 is greater than Λ B2 . a first zone for heat recovery (B1), comprising a fluidized-bed section with immersion heating surfaces (E) and a fluidizing gas distributor (C1) with fluidization elements , and a second zone for primary combustion (B2), comprising a fluidized-bed section without immersion heating surfaces and a fluidizing gas distributor (C2) with fluidization elements ; a bed material discharge funnel (D) adjoining the area below the fluidizing gas distributor (C1, C2); and a free space above the fluidized-bed reactor (A) in which burn-out air nozzles are arranged within burn-out air levels; wherein the first zone for heat recovery (B1) and the second zone for primary combustion (B2) form a common fluidized bed, and wherein the fluidized-bed reactor is constructed to generate a fluidization of the fluidized-bed area of the second zone (B2) that has a higher speed than the one of the first zone (B1) so that a fluidized-bed internal circulation of material (F) is forming between the first and second zone along the immersion heating surfaces, and the cross-sectional area of the first zone (B1) comprises from 0.4 to 0.75 of the entire cross section of the fluidized-bed reactor; characterized in that the fluidized-bed furnace is constructed in a way that the total amount of oxygen Λ tot introduced to the fluidized-bed furnace is composed of the oxygen Λ B introduced to both zones of the fluidized bed B1 and B2 and of the oxygen fed to the free space, and wherein the oxygen portion Λ B that is fed directly to the fluidized bed is composed of the oxygen Λ B1 introduced to the fluidized-bed area of the first zone (B1) and of the oxygen Λ B2 introduced to the fluidized-bed area of the second zone (B2), each based on the stoichiometric oxygen demand O min for combusting the entire introduced combustible material, wherein Λ B1 is greater than Λ B2 . <EOS>