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 of the air flow control is calculated on the basis of the of the bed level and the actual value is calculated from the available process parameters , and both are supplied to the regulator of the air flow control system , the output of which regulator controls the member for executing the control which is in the form of a guide vane ring with adjustable guide vanes and is provided at the inlet to lt .