Heavy fuel oil is a residual fuel incurred during the distillation of crude oil. It is used to generate motion and/or heat that have a particularly high viscosity and density. Heavy fuel oil is mainly used as a marine fuel.
The quality of the residual fuel depends on the quality of the crude oil. To achieve various specifications and quality levels, these residual fuels are blended with lighter fuels such as marine gasoil or marine diesel oil. 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.

In the MARPOL Marine Convention of 1973, heavy fuel oil is defined either by a density of greater than 900 kg/m³ at 15°C or a kinematic viscosity of more than 180 mm²/s at 50°C. Heavy fuel oils have large percentages of heavy molecules such as long-chain hydrocarbons and aromatics with long-branched side chains.

The ISO 8217 international standard divides marine fuels into distillate marine fuels and residual marine fuels (RMA). The latter are collectively called heavy fuel oils. An exception is the lowest viscous quality level, RMA 10, which is no longer referred to as an HFO, as its proportion of heavy fuel oil is so small. ISO 8217 stipulates that residual fuels, and therefore all heavy fuel oils, may not contain old oil or lubricating oils. A key differentiator of heavy fuel oils is their sulfur oxides (SOx) content. According to ISO 8217, their maximum sulfur content must not exceed 3.5%.

The following main classes with regard to the sulfur content can be distinguished:

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