Absorption–reduction technique assisted by ozone injection and sodium sulfide for NOx removal from exhaust gas

A two-stage process consisting of an ozonizing chamber and an absorber containing a reducing agent solution was proposed to remove nitrogen oxides (NOx) from exhaust gas. In the ozonizing chamber where the exhaust gas is treated by ozone, NO (the main component of NOx) was oxidized to NO2, and in the next step, NO2 was reduced to N2 by sodium sulfide (Na2S) used as the reducing agent. The NOx removal efficiency obtained by this two-stage process was higher than 95%. It was found that about three fourth of sodium sulfide was consumed without reacting with NOx, and accordingly the amount of sodium sulfide required should be four times that of NOx to be removed. The problem regarding the emission of H2S from Na2S was deeply related to the pH of the reducing agent solution, which could be solved by adjusting the pH using a basic reagent (NaOH). The concentrations of NO2− and NO3− in the reducing agent solution after processing were analyzed, which showed that about 25% of the NOx removed from the exhaust gas was simply dissolved in the reducing agent solution, i.e., about 75% of the NOx removed was reduced to N2.