Simultaneous removal of SO2 and NOx from flue gas using (NH4)2S2O3/steel slag slurry combined with ozone oxidation

Abstract

The use of thiosulfate salt (S2O32−), a low-cost reductant, as an absorbent for NO2 removal has been rarely reported. In this study, we propose a method that combines ozone oxidation and (NH4)2S2O3/steel slag slurry spraying to simultaneously remove sulfur dioxide (SO2) and nitrogen oxides (NOx) from flue gas. We investigated the effects of operation conditions on the simultaneous removal efficiencies of SO2 and NOx. Results showed that SO2 removal efficiency was affected by pH, whereas NOx removal efficiency was mostly influenced by (NH4)2S2O3 concentration, reaction temperature, and pH level. Under optimal operation conditions, removal efficiencies of almost 100% for SO2 and above 78.0% for NOx were achieved. NH4+ in the steel slag slurry could tolerate high NO2− concentrations (up to 3.00 mol/L), and the co-existence of NH4+ and S2O32− had a significant synergistic effect on NOx removal. Together with S2O32−, Mg2+ that leached from the steel slag also had a synergistic effect on NO2 removal. We established the reaction mechanisms for NOx removal using (NH4)2S2O3/steel slag slurry. In this process, S2O32− acted as a reducing agent and an oxidation inhibitor of SO32−, thereby enhancing NO2 removal. Moreover, NH4+ inhibited the denitration product (NO2−) from decomposing into NO and NO2, thereby accelerating NOx removal.