Novel NO_x Emission Reduction Technology for Diesel Marine Engines

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 (NO_x) have negative health and environmental impacts and are difficult to reduce substantially without aftertreatment. A scrubber system for NO_x reduction was proposed; the presented research focuses on the verification of operating principles and the quantification of possible NO_x 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 NO_x species into water. The majority of engine-out NO_x 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 NO_x 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 NO_x. Research to support this design was conducted on a Mack E7 298 kW, 12 liter engine operating over 8 steady state points. Modal NO_x absorption ranged from 4–66%. Cycle average NO_x absorption ranged from 15–58%. It was concluded that NO_x absorption varies with gas residence time, absorption surface area, temperature, and NO_x concentration. Separately, a system was constructed and operated to convert the stored concentrated NO_x into diatomic nitrogen, carbon dioxide, and water.