Seawater/Seawater Cascade-Scrubbing Desulfurization Performance for Exhaust Gas of a 162-kW Marine Diesel Engine

To confirm the higher-availability and lower-resistance superiority of a proposed seawater/seawater cascade-scrubbing solution meeting the emission control area (ECA) requirements under harsh high-sulfur and low-alkalinity conditions, desulfurization experiments for the exhaust gas of a 162-kW marine diesel engine were compared between the cascade-scrubbing model and currently used once-through, open-loop solution. With the once-through seawater to scrub ${\mathrm{SO}}_2$ levels of $\mathrm{1,000}–\mathrm{2,860}\mathrm{mg}/{\mathrm{Nm}}^3$ (equal to about 1.8%–5% fuel-sulfur content), the desulfurization efficiency of the once-through system increased with the liquid-gas ratio and seawater alkalinity and decreased with ${\mathrm{SO}}_2$ concentration. At the harsh ${\mathrm{SO}}_2$ concentration of $\mathrm{2,860}\mathrm{mg}/{\mathrm{Nm}}^3$ where the once-through scrubbing thoroughly failed to meet the ECA’s requirements, an additional liquid-gas ratio increase above $8\mathrm{L}/{\mathrm{Nm}}^3$ was confirmed to be infeasible because of the potential for liquid flooding. In contrast, with a total liquid-gas ratio $\ge 9\mathrm{L}/{\mathrm{Nm}}^3$ (typically liquid-gas ratios of 7 and $2\mathrm{L}/{\mathrm{Nm}}^3$ in the main and auxiliary scrubbing sections, respectively), the cascade-scrubbing model easily met the ECA’s desulfurization requirements under harsh high-sulfur and low-alkalinity conditions, along with its lower packing pressure-drop levels allowing for a further liquid-gas ratio increase. Aside from the above superiority compared with the once-through open-loop solution, the achievement of high-efficiency seawater desulfurization in this work suggests that the proposed seawater/seawater cascade-scrubbing solution should be more economical in comparison to the current closed-loop solution and hybrid system, which both necessitate costly NaOH usage.