The combustion gas is cooled down to 50-70℃ in cooler and sent to absorbtion column where SO2 in the gas is absorbed and removed by dilute sulfuric acid from tower top. The dilute sulfuric acid absorbing SO2 will generate H2SO4 in oxidation tower and return to the absorbtion column. In order to ensure the proper concentration of dilute sulfuric acid, a certain amount of sulfuric acid will be sent to the crystallizer to generate gypsum in the crystallization. Gypsum is separated from the mother liquor through the centrifuge. The mother liquor return to the absorbtion column from mother liquor tank and the gypsum is dehydrated until it is easy to treat.