Bunker fuel is the heavy, dense fuel that powers most of the world’s cargo ships, tankers, and container vessels. It sits at the very bottom of the crude oil refining process, making it the cheapest and thickest petroleum product available at commercial scale. Nearly every product you buy that traveled by sea got there burning some form of bunker fuel.
How Bunker Fuel Is Made
When crude oil is refined, it gets heated inside a tall distillation column. Lighter products rise to the top: gasoline forms near the top at temperatures between 25°C and 60°C, while diesel condenses in the middle at roughly 250°C to 350°C. Below those sit the bunker fuels, with boiling points ranging from 350°C to 500°C. At the very bottom of the column, you find bitumen, the tar-like material used to pave roads.
Bunker fuel is essentially what’s left over after refineries have extracted all the more valuable, lighter products. This residual oil is thick, dark, and so viscous at room temperature that it needs to be heated before it can even be pumped into a ship’s engine. It contains heavy molecular compounds like asphaltenes and resins that resist breaking down, which is part of what makes it both useful as a long-burning fuel and problematic when it escapes into the environment.
Types of Bunker Fuel
The term “bunker fuel” covers a range of marine fuels, but the most common grades fall into three categories:
- Marine Gas Oil (MGO) is the lightest option, closer to diesel. It’s cleaner-burning and used in smaller vessels or in waters with strict emission rules.
- Intermediate Fuel Oil (IFO) blends heavy residual oil with lighter distillates to achieve a specific viscosity. IFO 180 and IFO 380 are common grades, with the number referring to the fuel’s thickness measured in centistokes.
- Heavy Fuel Oil (HFO), also called Bunker C, is the thickest and cheapest grade. For decades, this was the default fuel for large ocean-going vessels.
Why Ships Use It
The shipping industry moves roughly 80% of global trade by volume, and fuel costs are one of the biggest expenses for any vessel operator. Bunker fuel’s appeal is simple: it’s the bottom of the barrel, literally, which makes it far cheaper per ton than diesel or gasoline. A large container ship can burn 150 to 300 tons of fuel per day on a transoceanic crossing, so even small price differences per ton translate to enormous savings. Ship engines are also specifically designed to handle heavy, viscous fuels that would destroy a car engine.
Sulfur Limits and IMO 2020
Traditional heavy bunker fuel is loaded with sulfur, and burning it produces sulfur oxides that contribute to acid rain, respiratory illness, and particulate pollution. For years, ships were allowed to burn fuel with sulfur content up to 3.5% by mass. That changed on January 1, 2020, when the International Maritime Organization’s “IMO 2020” rule cut the global sulfur limit to 0.50%, a reduction of more than 85%. The IMO estimates this led to a 70% drop in total sulfur oxide emissions from shipping.
In designated Emission Control Areas, the rules are even tighter. Waters like the Baltic Sea, North Sea, and most recently the Mediterranean Sea enforce a sulfur limit of just 0.10%. Ships operating in these zones either burn much cleaner (and more expensive) low-sulfur fuel or install exhaust gas cleaning systems, commonly called scrubbers, that strip sulfur from the exhaust before it leaves the smokestack.
To comply, shipowners have taken three main approaches: switching to compliant low-sulfur fuel blends, installing scrubbers that let them keep burning cheaper high-sulfur fuel while cleaning the exhaust, or converting engines to run on liquefied natural gas.
Environmental Impact of Spills
When lighter fuels like marine diesel spill into the ocean, nature handles much of the cleanup. Diesel loses about 40% of its volume to evaporation within 48 hours, even in cold weather, and in rough seas most of a spill will disperse and evaporate within five days. Natural degradation in water and sediments takes days to months.
Heavy bunker fuel is a completely different problem. Bunker C is dense enough to sink rather than float, and it remains essentially unchanged even after long periods of time in water or along a shoreline. Chemical dispersants, which work on lighter oil spills, are not effective on Bunker C. Cleanup relies heavily on manual recovery, with crews physically scraping and collecting the fuel from rocks, sand, and wildlife. Studies of contaminated shorelines have found residual bunker fuel compounds, particularly heavy hydrocarbons and asphaltenes, persisting in sediments for 30 years or more, slowly biodegrading but never fully disappearing.
Alternatives Gaining Ground
The shipping industry is actively exploring fuels that could eventually replace traditional bunker oil. The five leading candidates are liquefied natural gas (LNG), methanol, ammonia, biofuel, and hydrogen. LNG is the most commercially mature of these options and already powers a growing number of new vessels. It produces virtually no sulfur emissions and significantly less particulate matter than heavy fuel oil, though it still releases carbon dioxide.
Methanol and ammonia are attracting attention because they can be produced from renewable sources, potentially making them carbon-neutral. Ammonia produces no carbon dioxide when burned, but it’s toxic and corrosive, which creates handling and safety challenges aboard ships. Hydrogen is the cleanest option on paper, producing only water when used in fuel cells, but storing enough of it to power a large vessel across an ocean remains a major engineering and cost hurdle. For now, conventional bunker fuel and its lower-sulfur variants still dominate the global fleet, but the transition is underway, driven by tightening regulations and shipping companies’ own decarbonization targets.