Investigating SO3 Formation from the Combustion of Heavy Fuel Oil in a Four-Stroke Medium-Speed Test Engine

The validation of detailed models, in terms of SO 3 formation in large marine engines operating on sulfur-containing heavy fuel oils (HFOs), relies on experimental work. The requisite is addressed in the present work, where SO 3 is measured in the exhaust gas of an 80 kW medium-speed single-cylinder HFO-fuelled test engine. SO 3 formation is triggered by running the engine at altered operational conditions and speeds within 1050–1500 rpm. The test engine does not represent a large low-speed marine engine; however, the nature of high-temperature SO 3 formation may well be explored with the current engine and serve as reference for further modeling studies. SO 3 is measured using a continuous SO 3 monitor from PENTOL GmbH. The monitor offers online SO 3 readings and short sampling times, in contrast to other extractive methods. The measurement is based on SO 3 capture in isopropanol prior to chemical conversion and indirect detection via light absorption in a photometer. Present results show that SO 3 formation is favored by elevated pressure histories, premixed combustion, and reduced speeds. The fraction of fuel sulfur converted to SO 3 is measured to be on the order of 0.5%–2.4%, corresponding to 4–14 ppmv. SO 3 and NO x are not comparable, according to thermodynamic considerations, yet both species involve the radical pool and are studied in parallel. Resulting emissions of SO 3 and NO x in the exhaust gas follow a comparable trend throughout the experiments.