Emissions from marine engines preparing for the upcoming sulphur cap limits

The International Maritime Organization (IMO) has implemented regulations to reduce emissions from ships. These regulations consider emissions of NOx and SOx. Global limit for fuel sulphur content of 0.5 wt-% will apply in 2020, while in the SOx Emission Control Areas (SECA) SOx is already limited to 0.1 wt-%. Tight Tier III NOx limits are applied for new builds in the NOx ECAs. So far, there is not explicit particulate matter (PM) emission limit, globally. However, there is a growing concern towards particle emissions and limits exist for PM in some special areas like inland waterways (in Europe), where also particle number (PN) emissions will be limited in 2020. In addition, IMO’s work related to black carbon (BC) at the moment, anticipates BC limit coming in near future. In addition, shipping is facing major challenges with decarbonising targets (2050).
In present study, we investigated how the different options in fulfilling the latest SECA limitations and the upcoming global sulphur limit influence the overall emissions. These technologies include lower sulphur level liquid fuels, conversion to natural gas and utilization of scrubbers. Various gaseous emissions, including e.g. SOx, NOx, CO, CO2 and hydrocarbons, and particle emissions, including PM, PN and BC, were studied. Experiments with low sulphur fuels i.e. with natural gas, marine gas oil (MGO) and marine diesel oil (MDO), were conducted with marine engine in laboratory. Emission measurements were also made on board two ships during regular cruising conditions. A modern cruise ship equipped with a hybrid sulfur scrubber was first tested. The second ship was a RoPax vessel equipped with an open loop seawater-operating scrubber. Results showed that changing high sulphur residual fuel to lower sulphur fuels resulted to lower SOx emissions, correspondingly. This fuel change had, though, no remarkable effect on NOx emissions. And the effect on CO2 was minor as well. The PM, on the other hand was found to decrease, which was expected due to the lower sulfate formation. However, the present results show that there are also other parts of the PM (like organic and elemental carbon) influenced by the fuel change. Using natural gas as a fuel, resulted to reduction of SOx emission by almost 100% and NOx emissions approximately by 70% compared to liquid marine gas oil. In addition, significantly lower PM, PN and elemental carbon was achieved when utilizing NG as a fuel. However, a simultaneously observed methane slip should be taken into account when evaluating the climatic impacts of NG fuelled engines. The on board measurements over scrubbers indicated scrubbers to be very effective in SOx reduction. In addition PM was found to decrease in scrubbers as well. However, further research is needed to solve their effect on particle number and size, especially when discussing the health of ship emissions with the growing trend in utilization of scrubbers.