Results from exploratory project on NOx emissions from Ocean Going Vessels (OGV) using remote sniffer measurements (

Rapport, 2024

The NOx emissions from ships are governed by the International Maritime Organization (IMO) through MARPOL's Annex VI. This annex is part of the International Convention for the Prevention of Pollution from Ships. It establishes emission limits in the form of Tiers, which vary depending on the construction date of the ships and their engines. Our study concentrates on ships classified under Tiers 0 to II, as these are currently the most common vessels navigating the waters.
In the study, mass- and brake-specific emissions of NOx at different ship engine loads were determined from six years of remote sniffer measurements at the Great Belt Bridge in Denmark. This bridge is situated over the main shipping channel connecting the Baltic Sea and the North Sea, with over 25,000 ships passing through annually. The analyzed data predominantly corresponds to 721 container ships, 425 RoRo vessels, 127 Reefers (refrigerated cargo), and 892 crude oil tankers, which are highly relevant for the ship types traversing Southern California waters to call Port of Los Angeles and the Port of Long Beach. Additionally, since vessels tend to reduce speed when approaching the Great Belt Bridge, the operational profiles of these ships are also very relevant for specific areas like Southern California, where multiple federal and local programs exist to encourage ships to voluntarily adopt reduced speeds as a strategy to either avoid whale strikes or decrease total emissions by reducing fuel consumption for the same distance travelled.
The finding reveal that Tier II container ships have the highest brake-specific NOx emissions among all ship types and tiers and that they display a different emission load dependency. The Tier II engines are tuned to minimize emissions at higher engine loads. However, emissions significantly increase at lower loads, being 30 % higher compared to those at 70 % engine load. This is unlike the older Tier 0 and Tier I engines. Noteworthy is that the requirement in the IMO technical code is based on a weighted average which is heavily weighted against higher loads, typically 80 %, and therefore Tier II container vessels are still in compliance with the IMO rules when being measured on a test bed. It should also be noted that the emissions at engine loads below 25 % are not included for any of the tiers in the IMO rules.   
An observation from the study is that the obtained average emission factor (Eavg) for Tier II container ships was 17.7 g/kWh. While this is considerably lower than the 19.2 g/kWh observed for Tier 0 ships, it is significantly higher when compared to the NOx emission limit of 14.4 g/kWh as outlined in the NOx technical code. Part of this discrepancy might be attributed to uncertainties in the calculation of Specific Fuel Oil Consumption (SFOC) for the container ships and differences in the calculation of the weighted average, necessitating further investigation. It is noteworthy that the average emission factor (Eavg) for other ship types such as Reefers, RoRo, and crude oil tankers is significantly lower than that of container ships and are generally consistent with the requirement of being below 14.4 g/kWh.

 In conclusion, Tier II container ships, despite being optimized for reduced emissions at higher engine loads, display substantially increased emissions at lower engine loads compared to Tier I and Tier 0 container ships. Notably, ships have low engine loads when they navigate through nearshore shipping lanes to enter or exit a port. This is of concern for Vessel Speed Reduction (VSR) programs in areas like Southern California, since the elevated NOx emissions at lower loads could neutralize or even surpass the emission reductions these programs aim to achieve. We suggest that it would be beneficial if Tier II emissions could be abated through modifications in engine design or optimization, or potentially through retrofitting. This could be achieved through amendments to the IMO regulations and updates to the associated engine certification procedures. For example, this might involve giving more weight to emissions at low engine loads when calculating the emission limit, and including emissions below 25% engine load in the assessment.