PURPOSE: To reduce the quantity of desulfurizing agent lower than a stoichiometric quantity, and thereby improving denitrification and desulfurization rates, by combining a fluidized combustion boiler with an active carbon absorption type desulfurzation equipment, such that a flue gas is supplyed to a desulfurization equipment after desulfurized inside the boiler furnace so as to be denitificated and desulfurized under the presence of ammonia.

CONSTITUTION: A coal grain and a desulfurizing agent are supplyed to a fluidized combustion boiler 1 wherein a part of SO2 gas developed during a fluidized combustion of coal grains is fixed in the desulfurizing agent. Quantity of this desulfurizing agent is 1.0 time or less of a stoichiometric quantity required for fixing the SO2 gas. A combustion process is effected in two steps. Primary air ratio is around 1.0 and total air ratio is around 1.2. Hence, it is possible to suppress NOx content of flue gas to be less than 150ppm and its SOx content to be less than 400ppm, particularly advantageous for effecting a denitrification simultaneously by a subsequence denitrification equipment. A desulfurizing agent and an unburnt coal and the like which are collected inside a cyclone 2 is returned to a boiler 1, and then the flue gas is fed to an air heater wherein the flue gas is cooled down to about 140°C and any foreign material is removed by a dust collector 4. Then, the flue gas is guided into an active carbon absorption type desulfurization equipment 5 together with an ammonia gas of 200W300ppm. After desulfurized and denitrificated, the flue gas is discharged into air from a stack 6.