Environmental Trade-Offs in Nitrogen Oxide Removal from Ship Engine Exhausts

Abstract

The emissions of nitrogen oxides (NO _x ) from ship engines constitute an increasing part of the total global emissions of NO _x while the share from land-based sources is decreasing. In the coming years, new regulations will set limits to emissions in specific areas and various technical countermeasures will be developed and implemented. However, when taking technical measures against emissions on a local scale, there is always a risk of inducing an increase in the total environmental impact related to the technology used, owing to increased energy use and emissions in other places.

In the present study, an investigation of the difference in environmental impact from passenger transport vessels with and without catalytic NO _x emission purification was performed in a life cycle assessment. The production and transport of chemicals used in the catalyst are included in the study.

The study shows that the use of selective catalytic reduction (SCR) gives a considerable decrease in the environmental impact compared with using a diesel engine without a catalyst. The transport of urea solution over a 500 km distance makes a very small contribution to the total environmental impact. The global warming potential is the only impact category for which values are increased when including the urea production and transport. When looking at the contribution from urea to the total impact from the SCR process, the increase is less than 10 per cent for most impact categories.