Bunker fuel, also known as residual fuel or heavy fuel oil (HFO), is the thick, viscous residue left over from the crude oil refining process after more valuable products like gasoline, diesel, and jet fuel have been extracted. Its name originates from the coal bunkers of steam-powered ships, which were repurposed to hold this liquid fuel during the transition to oil. Characterized by its high viscosity and density, bunker fuel must be pre-heated significantly before it can be pumped and injected into a ship’s engine. This cost-effective fuel has been the lifeblood of the global shipping industry for decades, powering the massive marine engines of container ships, tankers, and bulk carriers that facilitate over 80% of world trade.

Despite its economic advantages, bunker fuel is notoriously polluting. Its combustion releases significant amounts of sulfur oxides (SOx) and nitrogen oxides (NOx), which contribute to acid rain and respiratory problems. Furthermore, it is a major source of particulate matter, including black carbon, and emits high levels of greenhouse gases like carbon dioxide. The environmental impact of bunker fuel became a primary driver for international regulation, leading to the International Maritime Organization (IMO) imposing increasingly strict limits on the sulfur content in marine fuels.

The most significant of these regulations is the IMO 2020 rule, which came into effect on January 1, 2020. This landmark policy slashed the allowable sulfur content in fuel oil from 3.50% to just 0.50% globally, a dramatic reduction aimed at improving human health and environmental outcomes. To comply, ship operators have adopted three main strategies: switching to very low-sulfur fuel oil (VLSFO) or marine gas oil (MGO), installing exhaust gas cleaning systems known as "scrubbers" to clean high-sulfur fuel emissions, or transitioning to alternative fuels like liquefied natural gas (LNG).

Looking toward the future, the role of traditional bunker fuel is diminishing as the maritime industry faces immense pressure to decarbonize. The IMO has set ambitious targets to reduce the sector's greenhouse gas emissions by at least 50% by 2050 compared to 2008 levels. This goal is accelerating the development and adoption of zero-carbon bunkering options such as green methanol, ammonia, and hydrogen. While heavy fuel oil remains in use, particularly on vessels with scrubbers, the industry is steadily moving away from this conventional bunker fuel toward a more diverse and sustainable array of energy sources to power global shipping.