Role of Sulfite Oxidation in NO2 Absorption in a pH-Neutral Scrubber Solution

Nitrogen dioxide (NO2) absorption in aqueous sulfite solutions has recently gained more attention to reduce NOx emissions. The NO2 removal efficiency is strongly influenced by the competing reactions of NO2 absorption and sulfite oxidation. The present study investigates the role of sulfite oxidation in NO2 absorption at neutral pH. The effects of sulfite concentration, thiosulfate, pH, NO2 inlet concentration, oxygen concentration on NO2 absorption efficiency, and sulfite oxidation rates were investigated under well-controlled conditions at 25 degrees C in a laboratory-scale wet scrubbing system. NO2 absorption rates showed a strong dependence on the sulfite concentration and increased linearly with increasing NO2 inlet concentrations between 25 and 150 ppm. At the highest investigated sulfite concentration, 20 mM, the NO2 removal efficiency was around 80%. Oxidation rates of sulfite during NO2 absorption in the presence of 5 vol % O-2 increased with increasing sulfite concentrations in the scrubbing solution and ranged from 20 mg.L-1.min(-1) with 1 mM sulfite to 200 mg.L-1.min(-1) with 20 mM sulfite. The oxidation rate decreased significantly when thiosulfate was added, while the oxidation rates were independent of the oxygen concentration between 2-10 vol % O-2. The NO2 absorption rate decreased with decreasing pH between pH 6 and 8 proportionally to the sulfite concentration as predicted by the sulfite-bisulfite equilibrium. The implication from these results is that significantly less sulfite feeding is needed in industrial NO2 absorption if the pH can be maintained at neutral pH, instead of lower pH such as in simultaneous SO2 and NO2 scrubbing.

Automated summary

The role of sulfite oxidation in NO2 absorption at neutral pH was investigated, and it was found that sulfite concentration significantly influenced NO2 absorption rate, while oxidation rate increased with increasing sulfite concentration. Maintaining neutral pH can reduce the need for sulfite feeding in industrial NO2 absorption.