Fuel oil is a refined petroleum product that serves as a fuel for engines, furnaces, and boilers. Derived from crude oil, fuel oil is primarily used in power generation, heating, and industrial applications. It is generally categorized by viscosity, boiling point, and flash point and is often referred to by its grade, such as No. 1 (kerosene), No. 2 (heating oil), No. 5, and No. 6 (heavy fuel oils). The heavier grades are commonly used in industrial and marine settings, while lighter grades are used in heating and smaller engines.

Applications:

Fuel oil, particularly heavy fuel oil, is used in thermal power plants to generate electricity by burning it in boilers to produce steam, which drives turbines.
Ships and large marine vessels use heavy fuel oil (often No. 6 oil, or bunker C) due to its high energy density and economic benefits for long-haul voyages.
Fuel oil is burned in industrial furnaces and boilers for heating processes, often in manufacturing and processing plants such as steel mills, cement plants, and glass factories.
Light fuel oils, especially No. 2 heating oil, are commonly used in residential and commercial heating systems to provide warmth during cold months.
Some older locomotives and certain rail transport systems use fuel oil, although this has largely been replaced by diesel fuel specifically refined for transportation.
Used in heavy machinery, such as excavators and mining trucks, in areas where diesel may not be available or feasible.