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

Daniel Schmid,Mikko Hupa,Oskar Karlström,Ilari Vuorinen,Mikko Paavola,Virva Kuvaja

doi:10.1021/acs.energyfuels.1c03914

Daniel Schmid, Mikko Hupa + Show 4 more

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https://doi.org/10.1021/acs.energyfuels.1c03914

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Abstract

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 °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 % O2 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 % O2. 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.

Highlights

NOx emissions resulting from thermal conversion of solid and liquid fuels, and from other industrial processes, continue to be a major challenge for environmental reasons and due to increasingly strict emission limits
The present study investigates the role of sulfite oxidation in absorption of NO2 in aqueous sulfite solutions at neutral pH
NO2 absorption experiments were performed with and without oxygen in the atmosphere to investigate the role of sulfite oxidation in the overall sulfite consumption and how sulfite oxidation can be prevented by the addition of thiosulfate to the scrubber solution

Summary

Introduction

NOx emissions resulting from thermal conversion of solid and liquid fuels, and from other industrial processes, continue to be a major challenge for environmental reasons and due to increasingly strict emission limits. One alternative NOx reduction strategy is based on absorption in aqueous solutions.[14,15] The solubility of NO in water is low, but various additives have been investigated to improve the solubility and removal of NO.[16−20] Alternatively, to the direct absorption of NO, NO can be selectively oxidized to NO2, which has a higher solubility in water.[16,21,22] NO can be selectively oxidized NaClO3, O3, etc.[23−27] to NO2 by, e.g., ClO2, NaClO, even with the higher

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