Abstract: To make it possible to burn brown coals with cinders that vary in composition in the fluidized bed (6) of one and the same steam generator, one of the flue surfaces (12 or 13) accommodated in flues (2 and 3) downstream of the bed is exploited by way of communicating lines (43, 44, and 45), which can be blocked off, as either economizers or superheaters as desired. The economizer mode is employed when a prescribed volume of flue gas is recirculated through a recirculation line (29) to maintain a fuel-type dictated low bed temperature of preferably 730.degree. to 760.degree. C. The superheater mode is employed when less flue gas is recirculated to maintain a fuel-type dictated higher bed temperature of preferably 820.degree. to 805.degree. C.