Pilot-scale investigation of mass transfer and performance of NO ₂ absorption in alkaline scrubbing systems

This study addresses the critical challenge of NO2 mass transfer efficiency in wet denitration systems through a pilot-scale investigation of oxidation-alkali-coupled absorption. Utilizing dual Φ600 mm packed towers equipped with polypropylene 350Y structured packing, the performance of NaOH (10 wt%) and NH3 (10–12 wt%) solutions was systematically evaluated under industrial conditions (50–55 °C, 0.5–2.5 m/s gas velocity). Gas velocity and liquid spray density exhibited unimodal relationships with NO2 removal efficiency (η) and gas-phase mass transfer coefficient (KGa), peaking at 1.5 m/s and 15–20 m3/(m2·h), respectively. An increase in inlet NO2 concentration (240–600 mg/m3) reduced η by 18–27% and KGa by 15–22%. Conversely, increasing the packing height (3–6 m) enhanced η by 35–42% with negligible KGa improvement. NaOH outperformed NH3 by 23–38% in η due to stronger alkalinity, achieving a maximum η of 89.2% under the favorable operating conditions identified in this study. The derived scale-up correlations reconcile laboratory insights with industrial implementation to facilitate ultra-low emission compliance (Nitrogen oxide ≤ 50 mg/m3), thereby providing actionable strategies for flue gas process design and operating-parameter selection. This work provides pilot-scale mass-transfer data for NO2 absorption in structured packed columns and clarifies the respective roles of hydrodynamic conditions and absorbent chemistry.