Mechanism of synergistic removal of NO and SO₂ by sodium bicarbonate

Sodium bicarbonate (NaHCO₃) is considered to be an effective alkaline adsorbent for SO₂ removal and surprisingly, the concentration of NO is significantly reduced along with the generation of NO₂ during its desulfurization. Unfortunately, the mechanism of NO interaction with NaHCO₃, SO₂ and O₂ is ambiguous. In this work, the effects of absorption gas and absorber composition on SO₂/NO absorption performance were explored, the absorption products were characterized using XPS and SEM, and the Gibbs free energy of the inferred reaction path was calculated based on density functional theory (DFT). The results showed that SO₂ and O₂ synergistically promoted the absorption and removal of NO by NaHCO₃, which could completely remove SO₂ and absorb 90% of NO at 160 °C. Sodium metabisulfite (Na₂S₂O₅) and sodium dithionate (Na₂S₂O₆) were identified as the active substances responsible for efficient NO absorption, and the oxidation of Na₂S₂O₅ to Na₂S₂O₆ is the controlling step of the NO removal reaction. Specifically, Na₂S₂O₅ is an intermediate produced by the reaction of NaHCO₃ with SO₂, and subsequently reacts with O₂ to produce Na₂S₂O₆, which releases reactive oxygen species to oxidize NO to NO₂. In addition, when the S/N ratio is greater than 1 and the O₂ content is greater than 5%, both SO₂ and NO can maintain removal efficiency higher than 90%, indicating that the absorption reaction of SO₂ and NO by NaHCO₃ is highly adaptable to the flue gas composition.