Influence of Nitrogen Dioxide Absorption on the Sulfite Oxidation Rate in the Presence of Oxygen: Online Raman Measurements

The oxidation of sulfite(aq) is investigated by passing O2, N2, and NO2 over a sulfite solution in a bubbling flask. The influence on sulfite oxidation of NO2 absorption and the presence of thiosulfate (an oxidation inhibitor) is investigated. The experiment is focused on conditions relevant to a combined SO2 and NO2 industrial flue gas cleaning system. Liquid composition is measured in situ using Raman spectroscopy equipped with immersion probes. Regression models are developed to quantify SO32–, HSO3–, S2O32–, SO42–, NO2–, NO3–, and CO32– also in mixtures of the mentioned chemicals. The results show that Raman spectroscopy is a possible method for liquid analysis of a NOx/SOx removal system. Speciation is successful within the limits of the experiment for most molecules. SO42–, CO32–, and S2O32– are quantified with high certainty; SO32– and HSO3– are quantified with some uncertainty and should be above 10 mM for quantitative measurements. NO3– concentration is below the limit of detection in the continuous experiments. The measured reaction rates of sulfite (SO32–) and bisulfite (HSO3–) oxidation with O2 are in agreement with the reviewed literature. Absorption of NO2(g) and the consequent formation of nitrite enhance sulfite oxidation. The addition of thiosulfate to the liquid reduces the rate of SO32– oxidation by ∼90% while maintaining NO2 absorption. The influence of NO2(g) and thiosulfate supports a previously proposed mechanism for sulfite oxidation via a radical chain mechanism.