Heavy Fuel Oil (HFO) is a dense, viscous, and low-grade residual fuel left over after crude oil refining processes have extracted lighter, more valuable hydrocarbons like gasoline and diesel. Often referred to as bunker fuel or residual fuel oil, it’s characterized by its high viscosity and density (defined by MARPOL as having a density over 900 kg/m³ at 15°C or a kinematic viscosity above 180 mm²/s at 50°C