Role of Thiosulfate in NO2 Absorption in Aqueous Sulfite Solutions

Abstract

<p>NO_x 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 NO_x 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.<br></p>