A method of combusting sulfurous fuel in a circulating fluidized bed boiler, comprising the steps of: a) feeding sulfurous fuel to a furnace of the boiler, combusting the fuel and generating SO 2 -containg exhaust gas; b) feeding CaCO 3 -containing sorbent to the furnace for converting the Ca-CO 3 to CaO and utilizing a portion of the CaO in the furnace to convert a portion of the SO 2 to CaSO 4; c) conveying the exhaust gas along an exhaust gas channel from the furnace to a dry circulating fluidized bed scrubber comprising a reactor having means for feeding water and Ca 2 for converting SO 2 in the exhaust gas to CaSO 3 and CaSO 4 and a dust separator in gas flow connection with the reactor for separating particulate matter from the exhaust gas, and d) removing CaO-containing bottom ash from the furnace, characterized by the steps of e) classifying at least a portion of the removed CaO-containing bottom ash into a coarse bottom ash portion and a finer bottom ash portion in a first stage classifier wherein the coarse bottom ash portion consists of particles having a particle size larger than a first predetermined limit; f) removing the coarse bottom ash portion from the boiler as a first combustion product, g) conveying the finer bottom ash portion from the first stage classifier to a second stage classifier to classify the finer bottom ash portion to a finest bottom ash portion and a middle size bottom ash portion, h) removing the finest bottom ash portion as a second combustion product, i) conveying the middle size bottom ash portion to a grinder so as to grind the bottom ash portion to have a median particle size of a second predetermined limit; j) conveying at least a portion of the ground bottom ash portion from the grinder to a hydrator so as to hydrate CaO in the ground bottom ash portion with a controlled amount of water or steam to Ca 2, and k) conveying at least a portion of the Ca 2 from the hydrator to the dry circulating fluidized bed scrubber as a sorbent. a) feeding sulfurous fuel to a furnace of the boiler, combusting the fuel and generating SO 2 -containg exhaust gas; b) feeding CaCO 3 -containing sorbent to the furnace for converting the Ca-CO 3 to CaO and utilizing a portion of the CaO in the furnace to convert a portion of the SO 2 to CaSO 4; c) conveying the exhaust gas along an exhaust gas channel from the furnace to a dry circulating fluidized bed scrubber comprising a reactor having means for feeding water and Ca 2 for converting SO 2 in the exhaust gas to CaSO 3 and CaSO 4 and a dust separator in gas flow connection with the reactor for separating particulate matter from the exhaust gas, and d) removing CaO-containing bottom ash from the furnace, characterized by the steps of e) classifying at least a portion of the removed CaO-containing bottom ash into a coarse bottom ash portion and a finer bottom ash portion in a first stage classifier wherein the coarse bottom ash portion consists of particles having a particle size larger than a first predetermined limit; f) removing the coarse bottom ash portion from the boiler as a first combustion product, g) conveying the finer bottom ash portion from the first stage classifier to a second stage classifier to classify the finer bottom ash portion to a finest bottom ash portion and a middle size bottom ash portion, h) removing the finest bottom ash portion as a second combustion product, i) conveying the middle size bottom ash portion to a grinder so as to grind the bottom ash portion to have a median particle size of a second predetermined limit; j) conveying at least a portion of the ground bottom ash portion from the grinder to a hydrator so as to hydrate CaO in the ground bottom ash portion with a controlled amount of water or steam to Ca 2, and k) conveying at least a portion of the Ca 2 from the hydrator to the dry circulating fluidized bed scrubber as a sorbent.