Simultaneous absorption of SO2 and NOx with pyrolusite slurry combined with gas-phase oxidation of NO using ozone: Effect of molar ratio of O2/(SO2Â +Â 0.5NOx) in flue gas

NO in flue gas was oxidized into NO2 by ozone in gas-phase first, and then SO2 and NO2 were oxidized by redox reaction between MnO2 and SO2/NO2 and catalyzed oxidation between O2 and SO2/NO2 in absorption process. Molar ratio of O2/(SO2 + 0.5NOx) in flue gas had a decisive effect on catalyzed oxidation in absorption process, and eventually influenced reaction byproducts, Mn extraction rate and SO2/NOx absorption capability. Results showed that increasing O2/(SO2 + 0.5NOx) enhanced catalyzed oxidation and led to the lower reaction pH. When O2/(SO2 + 0.5NOx) ⩽ 18, SO2 and NOx were mainly oxidized by MnO2 with main products of MnSO4 and Mn(NO3)2, while when O2/(SO2 + 0.5NOx) ⩾ 18, SO2 and NOx were mainly oxidized by O2 with main products of H2SO4 and HNO3. Total absorption capability of SO2 and NOx increased with increasing O2/(SO2 + 0.5NOx). Mn extraction rate increased with the increase of O2/(SO2 + 0.5NOx) first and then decreased, the maximum Mn extraction rate of 91% was got when O2/(SO2 + 0.5NOx) at around 13. Both the SO2/NOx absorption capability and Mn extraction rate could be enhanced by adding rhodochrosite as pH regulator in the case of O2/(SO2 + 0.5NOx) ⩾ 18.