Abstract

Aerosol emissions from marine traffic are known to release substantial amount of air pollutants, such as SO2 and fine particulate matter (PM2.5). In the past two decades, the International Maritime Organisation (IMO) reduced the fuel sulphur content (FSC) to directly lower emissions of SO2 and indirectly of PM2.5. Since 2020, a global FSC cap of 0.5% was set, together with sulphur emission control areas (SECA) with maximum FSC of 0.1%. It was anticipated that ship owners change from heavy fuel oil (HFO) to “cleaner” diesel-like fuels or install open or closed-loop sulphur scrubbers to comply with sulphur regulation. However, compliant hybrid fuels from blending (hydrotreated) residues from the crude oil vacuum distillation with middle distillates, either from straight run gas oil or cycle oil from fluid catalytic cracking, became more established (Ershov et al, 2022).
To investigate the PM2.5 composition from different marine fuels, we analysed emissions from a four-stroke medium-speed marine engine with common rail injection and 80 kW nominal power. The engine was operated over four different loads (25, 50, 75 and 100%) on three SECA-compliant fuels hydrotreated vegetable oil (HVO), marine gas oil (MGO), and ultralow sulphur aromatic-rich HFO (ULS-HFOar); a compliant low-sulphur HFO (LS-HFO) and two non-compliant high sulphur HFOs (HS-HFO, HS-HFOsyn) which may be used combined with sulphur scrubbers (Jeong et al, 2023). Filter samples of PM2.5 were collected and analysed by multi-wavelength thermal optical carbon analysis (Chen et al, 2015) coupled to resonance-enhanced multiphoton ionisation time-of-flight mass spectrometry (Diab et al, 2015). Moreover, complementary fuel analysis was carried out to obtain bulk properties and chemical composition.
Optical properties of PM2.5 by means of the Angström Absorption Exponent (AAE) were close to unity for all fuels after weighting according to the IMO engine cycle. Emission factor (EF) of elemental and organic Carbon (EC, OC) revealed significant differences between SECA-compliant fuels and LS-/HS-HFOs, but not among the SECA-compliant fuels (Figure 1). However, the EF of total polycyclic aromatic hydrocarbon (PAH) was similar between the ULS-HFOar and non-SECA-compliant fuels and even more than one order of magnitude higher than for MGO and HVO; estimated toxicity equivalents of carcinogenic PAH (TEQPAH) for ULS-HFOar double the TEQPAH for MGO and HVO. The LS-HFO did not emit less PAH and TEQPAH than the HS-HFO.
The composition of PM2.5 emissions are closely related to the fuel composition because unburned fuel is a major contributor. Therefore, HFOs of different FSC may not be different in PAH emissions and TEQPAH. Moreover, hybrid SECA-compliant fuels from blending with cycle oil may not give the reduction in air pollutant emissions as anticipated from fuel switching to MGO.

This research was funded by the Federal Ministry for Economic Affairs and Climate Action (project SAARUS 03SX483D), dtec.bw-Digitalization (funded by European Union – NextGenerationEU) and Technology Research Center of the Bundeswehr (projects LUKAS and MORE).

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