Bunker fuel and maritime fuel oil standards

Bunker fuel is technically any type of fuel oil used aboard ships. It gets its name from the containers on ships and in ports that it is stored in; in the days of steam they were coal bunkers but now they are bunker fuel tanks.  When ships began to transition from solid fuels like coal to liquid petroleum fuels the storage areas on board kept the name bunker and the name stuck like a tar ball.

Maritime fuel oil standards evolved and different grades were classified by viscosity. Viscosity remains the defining characteristic of today’s standards but there are also factors of density and contamination.

Recently, international standards have come into place and are slowly replacing the old national and regional designations.

Early Maritime Fuel Standards

In order to understand why maritime fuels are classified as they are you should know a little bit about the engines that burn the fuel.

Large ships use residual engine heat to warm the fuel in the bunker and then just before burning. If an engine is shut down and cold then a heating plant, usually powered by low viscosity oil, is used to generate the heat to warm the oil to fire the engine. Once the oil is up to operating temperature the auxiliary fuel heating system is turned off.

Most of the fuels used in shipping are cheap, low grade types that need to be kept hot to flow and atomize properly in the combustion chamber. At room temperature low grade fuels will be more like a solid than a liquid. They may resemble something like toothpaste or peanut butter rather than an internal combustion fuel.

Early oil powered ships used a configuration called a pan burner to fire the boiler that drove the steam works of the vessel. This was often adapted engineering used to retrofit coal fired ships so they could burn the cheaper liquid fuels. Another cost savings was the labor reduction in material handling from onshore storage to heating the boiler. This was a major cost reduction since many ships were not equipped with automatic coal feeders and used shovels and manpower to stoke the boilers.

The oil feed system of early ships was often a steam powered ram pump which used great force to deliver gooey fuel to the pan burner. This pan burner system is an open flame. When ships began using internal combustion engines the need to atomize fuel through a high pressure injector necessitated the need for more precise viscosity measurements.

International Fuel Standards

At first every country or region had its own standard for maritime fuel. This caused plenty of confusion and at least a few ruined engines.

One example is the American Society for Testing and Materials which has very familiar classifications since they were used for all types of fuel oil including those used to heat homes. The classification system is number from one to six with Type 1 being the least viscous and most expensive. Home heating units usually burn Type 2 but some old commercial buildings on the East Coast of the United States still burn Types 5 or 6 resulting in high particulate pollution.

Since shipping is the oldest globalized industry it was only natural to move towards an international standard for fuel grades.

In the 1980’s the International Organization of Standardization fuel classifications took root throughout the maritime industry and the new standards became fully adopted. Vestiges of the old system still remain here and there, for example one of the earliest terms for liquid fuel, bunker, is still used in commodity markets.

The ISO Standards for Maritime Fuel

The ISO standard for distillate and residual maritime fuels is 8217 which was most recently amended in 2010.

There are many variables in determining the grade of fuel under ISO 8217 so the specific information is best expressed in a table. Tables for Distillate Fuel (PDF) and Residual Fuel (PDF) standards make the comparison much easier.

Standards are set for viscosity, pour point, flash point, sulfur content, contaminant content, and density.

Contamination with water and mineral soil is acceptable for some grades of heavy fuel oil. Those same grades might also contain some aluminum or silica used in the refining process.

All of the unwanted materials must be removed from the fuel stream as it reaches the engine. This is accomplished using centrifugal separators that keep heavier components like water or particulate away from the center of the chamber where the purified fuel is drawn off to be burned.

This is also the reason why density is important. There are newer centrifugal separators that have a small advantage since they can remove contaminants from slightly more dense fuels. The older type of separator cannot remove contaminants and could lead to engine damage or failure. These are the basic divisions of maritime fuel form lightest to heaviest. There are subdivisions of these groups but these are the most recognizable designations.

  • MGO – Marine gas oil – Roughly equivalent to No. 2 fuel oil, made from distillate only
  • MDO – Marine diesel oil – A blend of heavy gasoil that may contain very small amounts of black refinery feed stocks, but has a low viscosity up to 12 cSt so it need not be heated for use in internal combustion engines
  • IFO – Intermediate fuel oil – A blend of gasoil and heavy fuel oil, with less gasoil than marine diesel oil
  • MFO – Marine fuel oil – same as HFO (just another “naming”)
  • HFO – Heavy fuel oil – Pure or nearly pure residual oil, roughly equivalent to No. 6 fuel oil

The ISO standards break this down even further. These are the measurements used by engineers and are the most modern and professional way to describe ship fuel. You may see many references to the terms above but they are falling out of use within the industry.

These are divided into distillate and residual types. They are listed from least to most viscous.

Distillate types are; DMX, DMA, DMB, and DMC.

Residual types are; RMA 30, RMB 30, RMD 80, RME 180, RMF 180, RMG 380, RMH 380, RMK 380, RMH 700, RMK 700.

The Future of Maritime Fuels

The future of ship fuel is already here in places like the European Union and California. The requirement to use low sulfur fuels to reduce particulate pollution is difficult for an industry which must rebuild a supply chain based on the low sulfur model and retrofit existing ships to burn the cleaner fuel.

It is estimated that as many as 80,000 deaths are caused annually by particulate emission from ships.


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