High-Temperature Conversion of SO₂ to SO₃: Homogeneous Experiments and Catalytic Effect of Fly Ash from Air and Oxy-fuel Firing

Abstract

The reaction of SO 2 with fly ash in the presence of O 2 and H 2 O involves a series of reactions that lead to the formation of SO 3 and eventually H 2 SO 4 . Homogeneous experiments were conducted to evaluate the effects of the procedural variables, i.e., temperature, gas concentrations, and residence time, on the post-combustion conversion of SO 2 to SO 3 . The results were compared to existing global kinetics and found to be dependent upon SO 2, O 2, residence time, and temperature and independent of H 2 O content. For a residence time of 1 s, temperatures of about 900 °C are needed to have an observable conversion of SO 2 to SO 3 . Literature suggested that the conversion of SO 2 to SO 3 is dependent upon the iron oxide content of the fly ash. Experiments using three different fly ash samples from Australian sub-bituminous coals were used to investigate the catalytic effects of fly ash on SO 2 conversion to SO 3 at a temperature range of 400–1000 °C. It was observed that fly ash acts as a catalyst in the formation of SO 3, with the largest conversion occurring at 700 °C. A homogeneous reaction at 700 °C, without fly ash present, converted 0.10% of the available SO 2 to SO 3 . When fly ash was present, the conversion increased to 1.78%. The catalytic effect accounts for roughly 95% of the total conversion. Average SO 3 /SO 2 conversion values between fly ash derived from air and oxy-fuel firing and under different flue gas environments were found to be similar.