The global economy relies heavily on the shipping industry to move 90% of all traded goods across oceans. This fleet requires a large supply of fuel, primarily a category of petroleum products known as bunker oil. Bunker oil is a dense, heavy hydrocarbon mixture, derived as a byproduct of the oil refining process. Historically inexpensive, it has been the fuel of choice for international shipping, but its composition presents unique challenges that have led to significant global regulations.
Defining Bunker Oil
Bunker oil is the umbrella term for any fuel used to power a ship’s engine. The name is a historical carryover from steam-powered vessels, where coal was stored in compartments called “bunkers.” When the industry transitioned to liquid petroleum fuels, the term was repurposed for the ship’s fuel tanks and the fuel inside. Although generic, the term most commonly refers to the heavy, residual fuel types used by large commercial vessels like container ships and oil tankers. This fuel is thicker and more viscous than the gasoline or diesel used in cars. Unlike lighter petroleum products, bunker oil often requires heating, sometimes up to 100°C, just to be pumped and prepared for injection into marine engines.
How Bunker Oil is Produced and Its Composition
Bunker oil is a residual fuel, the heavy remainder left after crude oil has undergone the refining process. Crude oil is heated in a distillation column, where lighter products such as gasoline, jet fuel, and diesel distill off as vapors at lower temperatures. The thick liquid remaining at the bottom is the primary component of traditional Heavy Fuel Oil (HFO). This residual nature results in high viscosity and density, making the fuel dark and sticky at room temperature.
Its chemical composition includes long-chain hydrocarbon molecules and complex polyaromatic structures. The heaviest components of the original crude oil—including naturally occurring sulfur, nitrogen, and heavy metals like vanadium and nickel—become concentrated in this residual fraction. The presence of these elements, particularly sulfur, is a defining characteristic of traditional bunker oil.
Classification and Grades of Marine Fuels
The marine fuel industry uses the ISO 8217 specification to classify and specify the properties of bunker oil. Marine fuels are divided into residual fuels and distillate fuels. Residual fuels, designated with an ‘RM’ prefix (e.g., RMG 380), are heavy fuel oils that require heating and are defined by their maximum kinematic viscosity at 50°C.
Distillate fuels, designated with a ‘DM’ prefix (e.g., DMA, DMB), are lighter, cleaner-burning oils like Marine Gas Oil (MGO), similar to diesel fuel. These fuels do not require pre-heating and are used mostly in smaller vessels or auxiliary engines. The ISO standards incorporate sulfur content as a key parameter due to global environmental regulations. Newer grades like Very Low Sulfur Fuel Oil (VLSFO) and Ultra Low Sulfur Fuel Oil (ULSFO) have been introduced to meet stricter limits. VLSFO typically contains a maximum of 0.50% sulfur by mass. These compliant fuels are often complex blends of residual and low-sulfur distillate components.
Environmental Concerns and Global Regulations
The high sulfur content in traditional bunker oil poses significant environmental and public health risks. When combusted in ship engines, the sulfur is released as sulfur oxides (SOx). SOx emissions contribute to acid rain, harming ecosystems and are linked to respiratory illnesses in coastal populations. The combustion process also releases harmful particulate matter and heavy metals concentrated in the residual fuel.
In response, the International Maritime Organization (IMO) implemented a global mandate known as IMO 2020. This regulation, effective January 1, 2020, reduced the maximum allowable sulfur content in marine fuel used outside of designated Emission Control Areas (ECAs) from 3.5% to 0.50%. Ships operating within ECAs, such as those around North America and the Baltic Sea, must adhere to an even stricter limit of 0.10% sulfur.
Ship operators have three main options to comply with the IMO 2020 mandate:
- Switching from high-sulfur heavy fuel oil to compliant fuels like VLSFO or MGO.
- Installing an Exhaust Gas Cleaning System, commonly called a “scrubber,” which removes SOx from the exhaust, allowing the continued use of high-sulfur fuel.
- Converting the vessel to run on alternative, cleaner fuels, such as Liquefied Natural Gas (LNG).