Role of Thiosulfate in NO2 Absorption in Aqueous Sulfite Solutions

NOx emissions continues to be a major challenge in order to reduce the environmental impact of thermal conversion of non-recyclable waste and fuels with high nitrogen contents. Oxidation of NO to NO2 followed by absorption of NO2 in aqueous solutions with sulfur compounds as additives has recently gained more attention in order to reduce NOx emissions from flue gases. One major challenge in NO2 absorption is the high consumption of sulfite, requiring sulfite oxidation inhibitors such as thiosulfate. The present study clarifies the chemistry and consumption of thiosulfate in wet scrubbing of NO2 with buffered solutions under well-controlled conditions at neutral pH at 25 °C. Absorption rates for the reaction of thiosulfate and NO2 in the scrubber were determined, and the ability of thiosulfate to inhibit the sulfite oxidation was investigated and quantified. With 1 mM thiosulfate (no sulfite) in the scrubber solution, the NO2 absorption rate increased by 30% as compared to pure water and by 160% with 100 mM thiosulfate. The absorption rate increased with time in the presence of oxygen. With 1 mM sulfite (no thiosulfate), the initial NO2 absorption rate increased by 200% as compared to pure water. However, the absorption rate decreases significantly after short periods of time due to the high consumption of sulfite through undesired oxidation by O2. In tests with sulfite and thiosulfate as an oxidation inhibitor, high removal efficiencies could be maintained over extended periods of time. With a 10 mM sulfite solution, 1 mM thiosulfate decreased the sulfite oxidation rate from 130 to 31 mg·L–1·min–1, and 2 mM thiosulfate decreased the rate to 16 mg·L–1·min–1. For the investigated conditions, the consumption of thiosulfate did not change significantly when higher sulfite concentrations were used. The results of the study demonstrate high NO2 absorption rates for low concentrations of sulfite and thiosulfate at neutral pH.