Fuel oil (also known as heavy oil, marine fuel or furnace oil) is a fraction obtained from petroleum distillation. Any liquid fuel that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power can be defined as Fuel Oil up to a flash point of approximately 42 °C (108 °F). The term fuel oil is also used in a stricter sense to refer only to the heaviest commercial fuel that can be obtained from crude oil, i.e., heavier than gasoline and naphtha. Small molecules like those in propane, naphtha, gasoline for cars, and jet fuel have relatively low boiling points, and they are removed at the start of the fractional distillation process. Heavier petroleum products like diesel fuel and lubricating oil are much less volatile and distil out more slowly, while bunker oil is literally the bottom of the barrel; in oil distilling, the only things denser than bunker fuel are carbon black feedstock and bituminous residue (asphalt), which is used for paving roads and sealing roofs.
The boiling point and carbon chain length of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil must be heated for it to flow. Price usually decreases as the fuel number increases.
Mazut is a residual fuel oil often derived from Russian 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. In the Russian practice, though, "mazut" is an umbrella term roughly synonymous with the fuel oil in general, that covers most of the types mentioned above, except US grades 1 and 2/3, for which separate terms exist (kerosene and diesel fuel/solar oil respectively — Russian practice doesn't differentiate between diesel fuel and heating oil). This is further separated in two grades, "naval mazut" being analogous to US grades 4 and 5, and "furnace mazut", a heaviest residual fraction of the crude, almost exactly corresponding to US Number 6 fuel oil and further graded by viscosity and sulphur content.
In the maritime field another type of classification is used for fuel oils:
CCAI and CII are two indexes which describe the ignition quality of residual fuel oil, and CCAI is especially often calculated for marine fuels. Despite this, marine fuels are still quoted on the international bunker markets with their maximum viscosity (which is set by the ISO 8217 standard - see below) due to the fact that marine engines are designed to use different viscosities of fuel. The unit of viscosity used is the centistoke (cSt) and the fuels most frequently quoted are listed below in order of cost, the least expensive first.
| Residual Fuel | Kin. Viscosity [mm²/s] at 50°C | Density [g/cm³] at 15°C |
|---|---|---|
| RMA 30 | < 30 | < 0.960 |
| RMB 30 | < 30 | < 0.975 |
| RMD 80 | < 80 | < 0.980 |
| RME 180 | < 180 | < 0.991 |
| RMF 180 | < 180 | < 0.991 |
| RMG 380 | < 380 | < 0.991 |
| RMH 380 | < 380 | < 0.991 |
| RMH 700 | < 700 | < 0.991 |
| RMK 380 | < 380 | < 1.010 |
| RMK 700 | < 700 | < 1.010 |
A key differentiator of heavy fuel oils is their sulphur 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:
| Marine fuel | Max. sulphur content |
|---|---|
| High sulphur fuel oil (HSFO) | 3.5% |
| Low sulphur fuel oil (LSFO) | 1.0% |
| Ultra low sulphur fuel oil (ULSFO) | 0.1% |
Low sulphur fuel oil (LSFO)
Heavy fuel oils are referred to as low sulphur fuel oil (LSFO) if their sulphur content is below 1%. Usually these are marine fuel types IFO 180 or IFO 380, which have been desulphurised. Until the end of 2014, ships could still travel through Emission Control Areas (ECAs) with this type of marine fuel.
Ultra-low sulphur fuel oil (ULSFO)
Since January 1, 2015, in accordance with Annex VI of the MARPOL Conventions, ship emissions must contain no more than 0.1% sulphur in such protected areas (ECAs). Due to these tightened restrictions, LSFOs no longer play an appreciable role in these areas and have been virtually replaced with the ultra-low sulphur fuel oil (ULSFO) marine fuel, which complies with those limits. Theoretically, heavily desulphurised IFO fuels could also be used here, but in practice the desulphurization of such heavy fuel oils is too expensive to make economic sense. For this reason, today the term ultra-low sulphur fuel oil usually refers not to desulphurised heavy fuel oils, but to marine gasoil, which is already low in sulphur. It is composed exclusively of distillates and has a sulphur content of under 0.1%. This marine fuel is also known as ultra-low marine gasoil. ULSFO is used in medium- to high-speed diesel engines. When converting from LSFO to ULSFO, it must be ensured that the engine technology is compatible with ULSFO.
High-sulphur fuel oil (HSFO)
The alternative to using marine fuels with such low sulphur content in ECAs is the use of scrubbers. This technology involves injecting water into the exhaust stream to reduce sulphur and other emissions. However, refitting a ship with this technology costs several million euros and means that the ship is docked for a period of time, which leads to a loss in revenue and income for the shipowners. On the other hand, a scrubber allows higher-sulphur marine fuels to be used. In this context, such heavy fuel oils are designated as high-sulphur fuel oils (HSFO), which have a maximum sulphur content of 3.5% as permitted under ISO 8217.