Exhaust Gas Treatment in Marine Compression Ignition Engines: Design and Efficiency Assessment of a Wet Scrubber System

Abstract

Abstract. Under the Convention for the Prevention of Pollution from Ships, particulate matter (PM) and nitrogen oxides (NOx) emissions from marine engines are controlled, whereas hydrocarbon (HC) emission is not explicitly regulated. PM is mainly managed indirectly through limits on fuel sulfur content, known as the 'global sulfur cap'. In addition, regional frameworks such as the European Union impose further limits on marine engines, while NOx is addressed through Tier I–III standards. In response to increasingly stringent emission requirements, this study evaluates the feasibility and design optimization of a wet scrubber for diesel engine exhaust treatment. High-speed shadowgraph imaging was used to study multiphase interactions in the scrubber. A Photron Fastcam (up to 200,000 fps), high-frequency LED, and a ground-glass diffuser provided uniform backlighting. Micro lenses achieved 3.8 µm/pixel resolution, enabling detailed analysis of spray cone angle, droplet size distribution, and liquid-sheet breakup length. The optimized design minimized unfavorable spray–spray and spray–wall interactions, improving gas–liquid contact efficiency. Three scrubbing media—water, sodium chlorite (NaClO2), and hydrogen peroxide (H2O2)—were investigated. Experimental results achieved soot removal efficiencies of up to 70%, primarily through impaction and coagulation mechanisms. Optimal performance occurred at 75% engine load with a liquid-to-gas ratio of 5 l/Nm3. However, gaseous pollutant removal remained limited, with maximum reductions of 15% for NOx using NaClO2 and 4% for HC. These findings support effective soot reduction but indicate that hybrid or catalytic solutions are needed for comprehensive emission compliance.