Sulfurous fuel and CaCO3-containing sorbent are combusted in a furnace of a circulating fluidized bed boiler. A dry circulating fluidized bed scrubber includes a reactor with water and Ca(OH)2 feeds for converting SO2 in the exhaust gas to CaSO3 and CaSO4 and a dust separator in gas flow connection with the reactor. A discharge removes CaO-containing bottom ash from the furnace. A classifier classifies a portion of the removed CaO-containing bottom ash into coarse and finer portions. A fine ash channel conveys some of the finer bottom ash portion from the classifier to a grinder. A ground ash channel conveys some of the ground bottom ash portion from the grinder to a hydrator to hydrate CaO in the ash to Ca(OH)2. A hydrated ash channel conveys some of the Ca(OH)2 from the hydrator to the dry circulating fluidized bed scrubber as a sorbent.