Hybrid Dipping–Shower Wet Scrubber for Multi-Pollutant Emission Reduction From Marine Diesel Exhaust: Experimental Assessment of a Low-Cost Prototype as an Auxiliary Exhaust Gas Cleaning Approach

Marine diesel engines are major contributors to atmospheric pollution, emitting SO 2 , CO 2 , NO x , and CO in quantities subject to increasingly stringent International Maritime Organization (IMO) regulations.Wet scrubbers are a cost-effective exhaust gas cleaning option, yet most existing systems employ either immersion (dipping) or spray (shower) gas-liquid contact in isolation, and experimental data on a structurally combined dipping-shower contact pattern under marine diesel conditions remain scarce.In this study, the term "hybrid" denotes this structural combination of bubble-immersion and spray contact, not the open-loop/closed-loop switching systems commonly called hybrid scrubbers in shipping.A prototype hybrid wet scrubber was fabricated from recycled industrial oilstorage drums and tested on a Dongfeng diesel engine at four engine-speed settings (25%, 50%, 75%, and 100% of rated speed).Concentrations of CO 2 , SO 2 , NO 2 , and CO were measured at the scrubber inlet and outlet with an Internet of Things (IoT)-based multi-gas sensor node under three configurations: dipping-only, shower-only, and hybrid (dipping + shower), with five consecutive sensor readings per condition, and removal efficiency (η) was reported as mean ± standard deviation.Averaged over the four engine speeds, the hybrid configuration gave the highest removal of all four gases, with mean gains of about 6 and 10 percentage points for SO 2 and about 3 and 10 percentage points for CO relative to dipping-only and shower-only operation, respectively; it was not superior at every individual condition (e.g., CO at 100% and CO 2 at 75% of rated speed).SO 2 achieved the highest removal (75-79% in hybrid mode, coefficient of variation below 7%), and CO peaked at 78% at 25% rated speed, declining at higher speeds because of reduced gas-liquid contact time.By contrast, CO 2 removal remained inherently limited (8-11%) owing to its low aqueous solubility, and NO 2 removal was below 10%, reflecting the poor water solubility of NO that dominates NO x .The overall efficiency trend was SO 2 > CO > CO 2 > NO 2 .These results indicate that the hybrid dipping-shower scrubber is best regarded as a low-cost, transitional emission-control device suited mainly to SO 2 and partial CO abatement; it cannot meet deep CO 2 capture or stringent NO x -control requirements without alkaline absorbents, oxidants, or multi-stage treatment.Future work should address chemically enhanced scrubbing liquids, repeated long-duration testing, and characterization of the wash water to manage the risk of transferring pollution from air to water.