Marine Fuels (Bunker Fuels)

Marine fuels (cf. DIN ISO 8217), also called bunker fuels, are generally divided into two different classes:

• Heavy fuel oil (HFO) and 
• Distillates

The latter are colloquially known as marine gasoil (MGO). The first group, the heavy fuel oils, also includes other products such as LSFO, ULSFO and HSFO. The blends of heavy fuel oil and distillates frequently used in practice are described as marine diesel oil (MDO) or intermediate fuel oils (IFO). In the narrower sense of the term, “marine diesel oil” refers specifically to blends with a very low proportion of heavy fuel oil. This type of marine diesel oil is therefore also categorized as a distillate in some of the literature. Large ships can run on heavy fuel oil as well as marine diesel oil. Smaller vessels such as barges are not designed to run on heavy fuel oil.

The various marine fuels in accordance with ISO 8217 are produced from crude oil by means of fractional distillation in refineries. In this process, the crude oil is gradually heated to just below the temperature at which it would decompose. If during the heating process, the boiling point of a particular component of the crude oil – a “fraction” – is exceeded, it passes into the gas phase. Fractions with smaller molecules evaporate at lower temperatures than those with long-chain (and therefore larger) molecules. The individual fractions are then cooled to below their boiling point again, and thus liquefied. The fractions separated from the crude oil in this way are called distillates.

The remaining residue in the fractional distillation, which does not pass into the gas phase, is referred to as residual fuel or heavy fuel oil. The sulfur content of this heavy fuel oil can be reduced by further processes.

Since 1987, the requirements to be met by petroleum-based fuels used in diesel engines and boilers in the shipping industry have been specified by the International Maritime Organization (IMO) in the ISO 8217 standard “Petroleum Products – Fuel (class F) – Specifications of marine fuels.” It is an accepted global standard for the specification of marine fuels, and distinguishes them into residual fuels and distillate fuels according to their main components. In addition, ISO 8216 classifies categories for marine fuels.

Depending on whether the fuel was produced through distillation or accrued as a residue in the oil refinery, it is classified as a distillate (or “distillate fuel” according to the standard) or a residual fuel. In accordance with ISO 8217, residue fuels are divided into six fuel types depending on their viscosity (kinematic viscosity) – RMA, RMB, RMD, RME, RMG and RMK – in combination with their max. kinematic viscosity limit value at 50°C. The viscosity is given in square millimeters per second (mm²/s). Large values such as 700 describe very viscous residue fuels. The lower the kinematic viscosity value, the thinner the fuel. As a rule of thumb, the thinner the viscosity, the higher the quality of the marine fuel. Residual fuels are used in large, medium to slow-speed marine engines. Provided that the ship is not in a zone with special emissions restrictions (Emission Control Area, or ECA), this will usually be an intermediate fuel oil (IFO) 380 marine fuel type with the ISO 8217 designation RMG 380 or RMK 380.

Smaller ships usually use lighter marine fuels such as distillate fuels and less viscous residual fuel oils, as only larger engines can efficiently combust residual fuels with high proportions of heavy fuel oil. In accordance with ISO 8217, distillate fuels are divided into four classes: DMX, DMA, DMB and DMZ. DMX is a distillate that is used only in smaller engines (lifeboats/emergency units) and is intended for use outside the engine room. DMA and DMB differ mainly in that DMB may contain traces of residual fuel. The fourth distillate class, DMZ, must not contain residual fuel constituents, has a higher aromatics content and a slightly increased viscosity at 40°C compared with the other distillate fuels. This is to ensure that the fuel injection can continue to cool and lubricate when switching from a low-quality marine fuel to DMZ (such as when moving into an ECA).

In practice, mixtures of distillate fuels and residual fuels are mostly used, i.e. intermediate fuel oils (IFO). IFO 380 and IFO 180 (RMG) are the fuels most commonly used in shipping.

The ISO 8217 standard also defines a marine fuel’s quality parameters, e.g. ignitibility (Calculated Carbon Aromaticity Index/CCAI), maximal acid and metal content, and sulfur content limits. In principle, compliance with the emission limits can also be guaranteed with additional equipment (filter systems, scrubbers). Here the ship-owner is responsible for complying with emission limits by purchasing the appropriate marine fuel for the respective ship’s technology.

Liquefied natural gas (LNG) is now being used by ships, too, as an alternative marine fuel – also and precisely because it lowers emissions. LNG is regarded as the marine fuel of the future and as an important way of meeting stricter environmental regulations. This fuel’s carbon footprint and emissions of sulfur and nitrogen compounds are significantly better than those of marine fuels based on crude oil. As it can be assumed that the thresholds for nitrogen oxide and sulfur dioxide emissions will continue to be lowered to protect the environment, experts expect that LNG will increasingly be used as a marine fuel. To date there are no internationally binding rules and standards for the worldwide use and storage of LNG, however these are currently being developed by various committees and organizations, including the Society for Gas as Marine Fuel (SGMF), IMO and ISO. For this reason, LNG as a marine fuel has not yet been covered or defined in DIN ISO 8217 even though it is used to fuel ships.

ISO 8217 2010 FUEL STANDARD FOR MARINE RESIDUAL FUELS