Abstract: A sodium carbonate supply device is provided that supplies sodium carbonate to flue gas at the outlet of a boiler 1, and an SO3 separation device 10 is provided that decreases the SO3 concentration in the flue gas while also separating and removing SO3 fraction from the flue gas at the inlet of a wet desulfurization device 7. The SO3 separation device is provided with a shell and tube type of heat exchanger that cools flue gas to below the dew point of H2SO4 gas by allowing flue gas to pass through the shell side and allowing boiler supply water to pass through the tube side. The SO3 component present in the flue gas is lowered in temperature and reacts with moisture in the flue gas to form H2SO4 gas, after which the H2SO4 gas condenses and adheres to the surface of the tubes. By then washing the liquid H2SO4 adhered to the tube surfaces with water, the SO3 component can be removed from the flue gas.

Abstract: In a coal-fired boiler, a heat recovery unit is located upstream of a dry electrostatic precipitator so as to reduce the temperature of exhaust gas flowing to the dry electrostatic precipitator and thus prevent reverse ionization in the electrostatic precipitator. The heat recovery unit located upstream of the dry electrostatic precipitator will in no way be corroded even if the concentration of dust is reduced in the dry electrostatic precipitator. A desulfurization unit does not require a cooling/dust removing section and eliminates the need for a wet electrostatic precipitator. The dry electrostatic precipitator includes a gas passage divided into a plurality of parallel passages and dampers operable to close the passages. This arrangement prevents dust from being dispersed as a result of hammering without charge.