A fluidized bed boiler plant and a method of combusting sulfurous fuel in the fluidized bed boiler plant, a furnace of which plant is provided with a fluidized bed of particles. Sulfurous fuel, CaCO3-containing sulphur-binding agent and combusting air are introduced to the bed of particles, whereby fuel burns and generates flue gases and the sulphur-binding agent calcinates to CaO and binds SO2 generated in the combustion. Energy is recovered to a heat exchange medium circulating in heat exchange tubes of a condensing heat exchanger arranged in a flue gas channel, and a water solution of acid condensing on outer surfaces is neutralized by mixing it in a mixing vessel to a CaO-containing ash from a plant, preferably, fly ash collected by a dust separator.