A novel combined system using Na2S2O8/urea to simultaneously remove SO2 and NO in marine diesel engine exhaust

•Na2S2O8/urea for simultaneously removing NO and SO2 from ships was investigated.•Urea could inhibit the generation of nitrate and improve the NO absorption.•NO and SO2 removal efficiencies achieved 100 %, and nitrate reached 8.56 mg/L.•Acidic condition (pH ≤ 5.5) was conducive to NO removal and nitrate inhibition. The novel combined system using Na2S2O8/urea was used to simultaneously absorb nitric oxide and sulfur dioxide emissions from marine diesel engines as well as inhibit the formation of nitrate in cleaning wastewater to meet the increasingly stringent requirements of regulations. The influences of reaction temperature, Na2S2O8 concentration, urea concentration, SO2 concentration, NO concentration and pH value on SO2 removal efficiency, NO removal efficiency and nitrate concentration were investigated. The experimental results showed that different reaction temperatures had different influences on SO2 removal efficiency, NO removal efficiency and nitrate concentration. An increase in Na2S2O8 could improve the absorption of NO. The addition of urea could effectively improve the removal efficiency of NO and reduce the nitrate concentration. The removal efficiencies of 1000 ppm NO and 1000 ppm SO2 achieved 100 % with 0.2 mol/L Na2S2O8 and 2 mol/L urea at 70℃, and the nitrate content was 8.56 mg/L which was far lower than the regulatory requirement of 60 mg/L. The acidic condition (pH ≤ 5.5) not only facilitated the absorption of NO but also reduced the generation of nitrate. According to the experimental results, the novel combined system was promising to be applied to the control technology of marine diesel engine exhaust.