To provide a method and an apparatus for flue gas desulfurization, which produces ammonium sulfate inexpensively by ozone oxidation of sulfur dioxide gas and ammonia gas which are generated when fossil fuel is burnt.

Ammonium sulfate is produced by reacting two moles of ammonia NH₃ with one mole of sulphuric acid H₂SO₄ as is apparent from the chemical formula (NH₄)₂SO₄. Ammonia gas approximately twice a gas containing sulfur dioxide is brought into gas-gas contact with the gas containing sulfur dioxide, and oxidized with ozone to precipitate a clear and colorless crystal. The crystal is ammonium sulfate or abbreviated as Ryuuan. A more effective gas-gas contact is achieved using a dimple spherical cyclone which is improved on the applicant's patent SPHERICAL CYCLONE [No.2420865 US 4908049 UK 0307821B1 (European Union Patent) FR 0307821B1 GR P, 3876910.7-08 AU606207 MX 172006 CA 1332048 BRP 18801778-8].