A method of reducing sulfur dioxide emissions of a circulating fluidized bed boiler plant, comprising the steps of: - feeding a first stream of sulfur-containing carbonaceous fuel at a first feeding rate to a furnace of the boiler; - feeding a second stream of calcium carbonate containing absorbent having a predetermined d50 particle size at a second feeding rate to the furnace; - feeding oxygen containing gas to the furnace for fluidizing a bed of particles forming in the furnace; - combusting the fuel with the oxygen, whereby the sulfur in the fuel is oxidized to sulfur dioxide; - calcining the calcium carbonate to calcium oxide in the furnace and utilizing a portion of the calcium oxide to sulfate a first portion of the sulfur dioxide to calcium sulfate in the furnace; - discharging flue gases, containing a second portion of the sulfur dioxide, and particles, including calcium oxide particles, entrained with the flue gases along a flue gas channel from the furnace; - separating a first portion of the entrained particles, including a first portion of the entrained calcium oxide particles, from the flue gases in a particle separator having a cut-off size, and returning at least a portion of the separated particles via a return duct to the furnace; - conveying a second portion of the entrained particles, including a second portion of the entrained calcium oxide particles, with the flue gases from the furnace to a semi-dry sulfur-reduction stage (38, 52) arranged downstream of the furnace, and - reducing the sulfur dioxide content of the flue gases in the semi-dry sulfur-reduction stage (38, 52), characterized in that the predetermined d50 particle size of the calcium carbonate containing absorbent is from 10 to 20 μm and smaller than 50 % of the cut-off size of the particle separator. - feeding a first stream of sulfur-containing carbonaceous fuel at a first feeding rate to a furnace of the boiler; - feeding a second stream of calcium carbonate containing absorbent having a predetermined d50 particle size at a second feeding rate to the furnace; - feeding oxygen containing gas to the furnace for fluidizing a bed of particles forming in the furnace; - combusting the fuel with the oxygen, whereby the sulfur in the fuel is oxidized to sulfur dioxide; - calcining the calcium carbonate to calcium oxide in the furnace and utilizing a portion of the calcium oxide to sulfate a first portion of the sulfur dioxide to calcium sulfate in the furnace; - discharging flue gases, containing a second portion of the sulfur dioxide, and particles, including calcium oxide particles, entrained with the flue gases along a flue gas channel from the furnace; - separating a first portion of the entrained particles, including a first portion of the entrained calcium oxide particles, from the flue gases in a particle separator having a cut-off size, and returning at least a portion of the separated particles via a return duct to the furnace; - conveying a second portion of the entrained particles, including a second portion of the entrained calcium oxide particles, with the flue gases from the furnace to a semi-dry sulfur-reduction stage (38, 52) arranged downstream of the furnace, and - reducing the sulfur dioxide content of the flue gases in the semi-dry sulfur-reduction stage (38, 52),