The specification describes a method and arrangements for operating fluidized bed combustors in particular boilers. In the described method and arrangements thermal energy carried away from the combustor by the flue-gas emitted therefrom is recuperated by passing the flue-gas through a scrubber in which it is scrubbed with water. The scrubbing water subsequently being used to pre-heat fluidizing gas fed to the fluidized bed in the combustor and/or working fluid fed to the boiler. Additional description is given of using a heat exchanger through which both flue-gases from the combustor and the working fluid fed to the combustor are fed such that the heat carried away from the combustor in the flue-gases is given directly to the working fluid fed to the combustor. In the described method and arrangements the fluidizing gas may be charged with water vapor and/or steam easing control of the bed.