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 (NO x ) from exhaust gas. In the ozonizing chamber where the exhaust gas is treated by ozone, NO (the main component of NO x ) was oxidized to NO 2 , and in the next step, NO 2 was reduced to N 2 by sodium sulfide (Na 2 S) used as the reducing agent. The NO x 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 NO x , and accordingly the amount of sodium sulfide required should be four times that of NO x to be removed. The problem regarding the emission of H 2 S from Na 2 S 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 NO 2 − and NO 3 − in the reducing agent solution after processing were analyzed, which showed that about 25% of the NO x removed from the exhaust gas was simply dissolved in the reducing agent solution, i.e., about 75% of the NO x removed was reduced to N 2 .