NO2 Absorption in Aqueous Solutions with Phenolic Additives: Hydroquinone and 4‑Methylaminophenol Sulfate

NO ₂ absorption has recently gained attention as a promising alternative to conventional NO _x reduction methods. One key challenge in NO ₂ absorption is that high amounts of additives are required to maintain satisfactory absorption efficiency. The present study investigates the absorption of NO ₂ from flue gases in aqueous solutions with low concentrations of phenolic additives. Absorption tests with hydroquinone and 4-methylaminophenol sulfate were performed in a laboratory-scale absorption system with test gases containing 0 or 5% O ₂ (N ₂ as a balance gas) and 50, 100, or 150 ppm of NO ₂, representing typical flue gas conditions in a range of industrial processes. The results show that NO ₂ was absorbed efficiently with small concentrations of 4-methylaminophenol sulfate, both with 0 or 5% O ₂ in the test gas. Hydroquinone also enhanced NO ₂ absorption but to a lesser degree than 4-methylaminophenol sulfate. Interestingly, the presence of O ₂ did not influence the consumption of the phenolic additives, in contrast to commonly used additives such as sulfite that require oxidation inhibitors such as thiosulfate. With a 2 mM solution of 4-methylaminophenol sulfate, more than 90% of the incoming NO ₂ was absorbed. Thus, NO ₂ absorption with 4-methylaminopheno sulfate appears to be able to remove NO _x emissions from flue gases with high efficiency.