Fuel Oil (also known as Furnace Oil/Heating Oil/Heavy Oil/Gasoil) – is a fraction obtained from the distillation of crude oil. It includes distillates (the lighter fractions) and residues (the heavier fractions).

Fuel oil is a broad term that could refer to a number of different refined products ranging in density from kerosene to residual fuel oil. However, it typically refers to residual fuel oil or No. 6 fuel oil.

Heavy Fuel Oil is a residual fuel product of the distillation. This type of fuel oil is used to power ships and power stations. It is also sometimes known as Marine Fuel/Bunker Fuel/No.6 Fuel Oil.

To achieve various specifications and quality levels, these residual fuels are blended with lighter fuels such as Marine Gasoil (MGO) or Marine Diesel Oil (MDO). The resulting blends are also referred to as intermediate fuel oils (IFO) or marine diesel oil. They are classified and named according to their viscosity, IFO 180 and IFO 380, with viscosities of 180 mm²/s and 380 mm²/s, respectively.

A key differentiator of heavy fuel oils is their sulphur oxides (SOx) content. According to ISO 8217, their maximum sulphur content must not exceed 3.5%. The following main classes with regard to the sulphur content can be distinguished:

  • High sulphur fuel oil (HSFO): max. 3.5% SOx – Heavy fuel oils designated as high-sulphur fuel oils (HSFO), with a maximum sulphur content of 3.5% as permitted under ISO 8217.
  • Low sulphur fuel oil (LSFO): max 1.0% SOx – Usually these are marine fuel types IFO 180 or IFO 380, which have been desulfurized.
  • Ultra low sulphur fuel oil (ULSFO): max 0.1% SOx – Usually refers to marine gasoil. It is composed exclusively of distillates.

Mazut is a residual fuel oil often derived from petroleum sources and is either blended with lighter petroleum fractions or burned directly in specialized boilers and furnaces. It is also used as a petrochemical feedstock.