NOx removal from simulated marine exhaust gas via wet absorption using TBAB/PBS composite solution and subsequent reduction by adding Na2SO3

With the aim to develop wet denitration process for marine application, it is still challenging to simultaneously achieve high-efficiency NO_x removal from flue gas and low nitrate concentration in scrubbed solution. Herein a novel two-step method was proposed to solve this issue: phosphate buffer solution (PBS) with organic additive was used to absorb NO_x efficiently from flue gas, and in next step, Na₂SO₃ as a reductant was added in the scrubbed solution to reduce NO_x off-line. Five typical organic additives were chosen to compose with PBS. The tetrabutylammonium bromide and phosphate buffer solution (TBAB/PBS) composite solution exhibited superior NO_x absorption performance. Furthermore, the effects of TBAB concentration, NO_x oxidation degree, solution pH, temperature and O₂ concentration on NO_x absorption performance of TBAB/PBS composite solution were investigated systematically, and a high NO_x absorption efficiency of >90% could be achieved. It was mainly attributed to a physical complex between PBS and NO₂, $N{O}_3^{-}$, $N{O}_2^{-}$, together with a mass transfer enhancement by TBAB. A long-run test showed that nitrate concentration was much lower than the limit value of IMO. Cyclic absorption and regeneration tests confirmed that Na₂SO₃ could effectively regenerate the scrubbed solution in the absence of O₂.