On-Board measurement of emissions on a dual fuel LNG powered cruise ship: A sea trial study

Abstract

The use of liquefied natural gas (LNG) in dual-fuel engines is expanding in the maritime industry, enabling compliance with sulfur and nitrogen oxide emissions regulations, while also reducing atmospheric emissions of particulate pollutants. LNG further allows for a reduction in direct CO₂ emissions compared to liquid fuels such as marine gas oil (MGO), which also helps meet greenhouse gas emission (GHG) reduction requirements in maritime transport. However, unburned methane emitted by dual-fueled engines remains an area for improvement. In this study, we monitored the atmospheric emissions of unburned methane, along with other gaseous and particulate pollutants, from a 4-stroke dual-fuel engine during sea trials of a new cruise ship, using both LNG and MGO fuels that these engines can operate on. During this ship's commissioning phase, we were able to perform measurements across a wide range of engine loads and confirm higher emission factors for unburned methane at low loads: the emission factor remains relatively stable at around 2.3 g.kWh⁻¹ between 60% and 95% engine load, but increases at lower loads, reaching 8.5 g.kWh⁻¹ at 25% load. For fine particulate emissions, characterized by the number of particles larger than 23 nm (PN₂₃), we established that switching from MGO to LNG results in a reduction factor of approximately 136 in PN₂₃ particles emitted by the engine at high load. The measured emission levels in MGO mode were relatively independent of engine load but tended to increase in LNG mode as engine load decreased. Based on our measurements of methane and CO₂, we propose a comparison of GHG emission levels as a function of engine load, which shows that switching from LNG to MGO at 75% load results in an 18% relative increase in CO₂ equivalent emissions. Finally, we had access to engine load monitoring (ELM) during commercial operation over a 3-month period. The load distribution obtained shows that the diesel-electric architecture, which adjusts the number of engines in operation and their load according to the vessel's energy demands, leads to a limitation in the use of the engine at low load. As a result, less than 7% of the engine operating time is spent at a load below 30%, a threshold beyond which the studied engine emits less than the default FuelEU value of 3.1%. When considering a usage-weighted average, based on the emission factors measured on board and the load distribution derived from the Engine Load Monitoring (ELM), we obtain an emission factor of 3.2 g.kWh⁻¹, or 1.7% of the fuel use. This value of 1.7% is lower than that specified by the FuelEU regulation, and it appears to be representative of the actual emissions of this vessel and its usage under operational conditions.