Emissions from diesel marine engines are significant contributors to the emissions inventories of commercial ports. Prior to 1998, these emissions were unregulated and current EPA regulations apply predominantly to new engines. Considering that the useful life of marine engines in work vessels, such as tugboats, may be 20 years or longer, retrofit emission reduction technologies are needed for these legacy engines. Oxides of nitrogen (NOx) have negative health and environmental impacts and are difficult to reduce substantially without aftertreatment. A scrubber system for NOx reduction was proposed; the presented research focuses on the verification of operating principles and the quantification of possible NOx reduction from this system. Major elements of the proposed scrubber system are exhaust heat exchangers, a catalyzed particulate filter (CPF), a diesel oxidation catalyst (DOC), and a packed bed wet scrubber. The system works on the principle of absorption of NOx species into water. The majority of engine-out NOx is in the form of nitric oxide (NO) which is relatively insoluble in water. A CPF and a DOC are utilized to convert up to 80% of the NO into nitrogen dioxide, NO2. NO2 and NO exist in equilibrium with N2O3 and N2O4, species of NOx that are highly soluble in water. The use of a CPF and DOC also reduces carbon monoxide, hydrocarbons, and particulate matter, reducing possible scrubber contamination. The scrubber liquor operates on a closed loop with zero discharge, its final composition is weak nitric acid; a byproduct of capturing the NOx. Research to support this design was conducted on a Mack E7 298 kW, 12 liter engine operating over 8 steady state points. Modal NOx absorption ranged from 4–66%. Cycle average NOx absorption ranged from 15–58%. It was concluded that NOx absorption varies with gas residence time, absorption surface area, temperature, and NOx concentration. Separately, a system was constructed and operated to convert the stored concentrated NOx into diatomic nitrogen, carbon dioxide, and water.

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