A recent study by the International Council on Clean Transportation (ICCT) has estimated the emissions of nitrogen oxides (NOx) from ships operating in Danish waters in 2019. The study collected data from 615 measurements taken from 545 unique ships using exhaust gas sampling devices attached to helicopters. The data revealed that the highest mean NOx emission rates were found at main engine loads below 25%, with emissions of 12 g/kWh. Emission rates decreased as engine loads increased, with mean emission rates of 8.1 g/kWh at loads greater than 75%. This challenges the assumption that marine engines primarily operate at higher loads.
Reducing NOx emissions from ships could have significant health benefits. In 2020, ship-source air pollution was estimated to have caused up to 266,000 premature deaths globally from lung cancer and cardiovascular disease. This includes the impacts of NOx, sulfur oxides (SOx), and particulate matter. The study highlights the need to address and control NOx emission rates during low load operations, which accounted for higher emission rates than expected by regulations.
The study suggests that the International Maritime Organization (IMO) should consider implementing not-to-exceed (NTE) standards for new and existing ships. These standards would focus on operations at low loads and include a test point below 25% load. This would provide a more comprehensive understanding of ship emissions, particularly during low load operations where emission rates were found to be higher than expected. Overall, the study emphasizes the importance of reducing NOx emissions from ships to mitigate the health burdens associated with the shipping industry.