Heavy Fuel Oil

Heavy Fuel Oil, also called residual fuel or black oil, is the viscous bottom fraction remaining after lighter cuts have been removed during crude oil refining. With typical viscosities of 700 cSt or more at 50 °C and sulphur contents historically reaching 3.5% m/m, it must be heated to around 120–140 °C before injection into the main engine. The low cost per energy unit made HFO the economic choice for deep-sea trades for several decades, but its high emission profile — sulphur oxides, nitrogen oxides, and black carbon — drove progressive regulatory tightening under MARPOL Annex VI. IMO 2020, implemented under MARPOL Annex VI Regulation 14, effectively banned the combustion of HFO above 0.50% m/m sulphur without an approved scrubber. A separate HFO carriage ban applies in certain polar waters under MARPOL Annex I, reflecting the heightened environmental risk of an HFO spill in remote ice-covered regions. Chief engineers must verify the compatibility of any HFO blend with residual fuel already on board, since incompatible fuels can precipitate sludge that blocks purifiers and fuel lines.