Abstract: Air flow is controlled in a PFBC plant comprising a combustor and a pressure vessel, a two-shaft gas turbine consisting of a low-pressure compressor LC and a low-pressure turbine LT interconnected by a shaft and a high-pressure compressor HC and a high-pressure turbine interconnected by a shaft. Air is sucked into LC, is passed on to HC and via an intercept valve into the pressure vessel and the combustor, whereafter the return flow is passed via the intercept valve to HT and further to LT, from where the return flow is released into the free environment. To control the air flow the set value (G.sub.set) of the air flow control is calculated on the basis of the of the bed level and the actual value (G.sub.