bunker fuel

Bunker fuel is the thick, dark residue left over after refineries have extracted lighter products like gasoline and diesel from crude oil. It sits at the bottom of the barrel, literally and economically: the heaviest fractions, with viscosity often exceeding 700 centistokes at 50 degrees Celsius. Ships use it because it is cheap, abundant, and energy-dense, not because it flows easily or burns cleanly. A large container vessel might burn 200 tonnes per day at sea, making fuel cost the second-largest operating expense after labour.

The term bunker originated in coal-fired steamships, where fuel was stored in compartments called bunkers. When petroleum replaced coal, the name stuck to the fuel itself rather than the storage space. Marine fuels are classified by ISO 8217 standard: marine gas oil (MGO), marine diesel oil (MDO), and heavy fuel oil (HFO) spanning ranges from 2 to 700 centistokes. Most large ships run HFO, a black-brown liquid that must be heated to 40 to 50 degrees Celsius in storage tanks and to 120 to 150 degrees Celsius in fuel lines before it will flow through injectors.

Bunker fuel contains sulfur at concentrations from 0.5 percent to 3.5 percent by mass, along with vanadium, nickel, and other heavy metals that corrode cylinder liners and form abrasive ash in combustion chambers. This is why marine engines are built with thicker materials and oversized cooling systems compared to land-based diesel engines. The fuel also carries water, sediment, and incompletely refined hydrocarbons that settle out during storage, requiring regular tank cleaning and fuel polishing (centrifugal separation) to avoid filter blockages and injector damage.

Why shipping chose bunker fuel

Bunker fuel survives in marine transport because the economics are brutal: a tonne costs roughly one-third the price of marine diesel oil. For a 20,000 tonne deadweight cargo ship, that difference amounts to tens of thousands of dollars per voyage. Refineries deliberately produce heavy bunker fractions to maximize yield from each barrel of crude; the alternative would be to crack or blend them into lighter products at higher cost. Ships burn bunker because refineries optimize around shipping's appetite for cheap, heavy feedstock.

The International Maritime Organization introduced the 2020 sulphur cap, limiting bunker fuel sulfur content to 0.5 percent globally (down from 3.5 percent in previous decades). This drove adoption of scrubbers, which remove sulfur oxides by spraying seawater into exhaust gases, and shift to compliant fuels. Some operators switched to marine gas oil or liquefied natural gas; others installed scrubber systems costing millions of dollars. The regulation shrank the heavy fuel market but did not eliminate it, because the price advantage of 0.5 percent sulfur HFO still undercuts lighter alternatives for cost-sensitive operators.