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 (S 2 O 3 2− ), a low-cost reductant, as an absorbent for NO 2 removal has been rarely reported. In this study, we propose a method that combines ozone oxidation and (NH 4 ) 2 S 2 O 3 /steel slag slurry spraying to simultaneously remove sulfur dioxide (SO 2 ) and nitrogen oxides (NOx) from flue gas. We investigated the effects of operation conditions on the simultaneous removal efficiencies of SO 2 and NOx. Results showed that SO 2 removal efficiency was affected by pH, whereas NOx removal efficiency was mostly influenced by (NH 4 ) 2 S 2 O 3 concentration, reaction temperature, and pH level. Under optimal operation conditions, removal efficiencies of almost 100% for SO 2 and above 78.0% for NOx were achieved. NH 4 + in the steel slag slurry could tolerate high NO 2 − concentrations (up to 3.00 mol/L), and the co-existence of NH 4 + and S 2 O 3 2− had a significant synergistic effect on NOx removal. Together with S 2 O 3 2− , Mg 2+ that leached from the steel slag also had a synergistic effect on NO 2 removal. We established the reaction mechanisms for NOx removal using (NH 4 ) 2 S 2 O 3 /steel slag slurry. In this process, S 2 O 3 2− acted as a reducing agent and an oxidation inhibitor of SO 3 2− , thereby enhancing NO 2 removal. Moreover, NH 4 + inhibited the denitration product (NO 2 − ) from decomposing into NO and NO 2 , thereby accelerating NOx removal.