PURPOSE:To reduce the electric resistance of dusts and enhance the dust collecting efficiency by converting SO2 obtained from a desulfurising apparatus to SO3 and introducing said SO3 into an exhaust gas containing dusts with high electric resistance at an inlet side of an electric dust collector. CONSTITUTION:An exhaust combustion gas 1 from a boiler A is introduced into a denitrification apparatus B to treat nitrogen oxides therein. Subsequently, said gas is introduced into an air preheater C to heat to about 250 deg.C or less and introduced into an electric dust collector D, but, prior to introduction, a SO3 gas is introduced carefully so as to be uniformly diffused. The dusts with electric resistance lowered by the introduction of SO3 is collected efficiently by the electric dust collector D and the exhaust gas issued from the electric dust collector is utilized in a gas/gas heater E to raise the temp. of an exhaust gas generated from an absorbing tower F and the temp. lowered exhaust gas is entered into an absorbing tower F. In said absorbing tower F, a SOx (mainly SO3) component in the exhaust gas is removed by an exhaust smoke desulfurizing method.
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