Selective SO2 Absorption in CO2-Containing Simulated Marine Exhaust Gas: Optimization of Sodium-Based Buffer-Type Mixed Absorbents

SO2 removal in marine exhaust gas cleaning systems is evaluated together with the CO2 concentration. Absorbent design must therefore consider selective SO2 removal in the presence of CO2. This study compared the SO2 absorption performance, pH stability, and SO2/CO2 selectivity of NaOH, NaHCO3, and Na2CO3 under SO2-only conditions and in the presence of 8% CO2 using simulated marine exhaust gas. At an equal compound molarity of 0.25 M under SO2-only conditions, Na2CO3 showed the highest conversion retention and pH stability. Its average SO2 conversion was 99.6%, the duration at pH ≥ 7 was 98.6 min, and t90 exceeded 120 min. Na2CO3 also exhibited the highest overall gas-phase mass transfer coefficient, KG, of 8.932 × 10–3 mol·m–2 · s–1 · kPa–1 in wetted-wall column (WWC) tests. In the presence of 8% CO2, all three absorbents maintained approximately 99% SO2 conversion. NaHCO3 showed the highest uptake-based apparent SO2/CO2 selectivity of 0.213. These results guided the design of NaHCO3–Na2CO3–NaOH buffer-type mixed absorbents. The 1%-A and 5%-B formulations were selected from the 1 and 5 wt % groups, respectively, as candidates for subsequent wet scrubber evaluation. Both maintained pH ≥ 7 for 180 min in the bubble reactor. Their apparent CO2 uptakes were 36.6 and 22.9 mmol·L-solution–1, respectively. In the wet scrubber, 1%-A and 5%-B achieved SO2 conversions of 96.5% and 94.6%, respectively, while the outlet CO2 concentration remained close to the inlet level. The study identifies bicarbonate-rich, low-NaOH buffer-type formulations that maintain SO2 removal while showing a small decrease in outlet CO2 concentration.