Heavy fuel oil (HFO) is the thick, tar-like residue left over after crude oil has been refined into lighter products like gasoline, diesel, and kerosene. It is the cheapest and most energy-dense commercial fuel available, which is why the global shipping industry has relied on it for decades. As of 2022, HFO still made up 56% of the fuel burned by ships worldwide.
How Heavy Fuel Oil Is Made
Crude oil refining works by heating oil and collecting the vapors that rise at different temperatures, a process called distillation. Lighter products like gasoline and jet fuel evaporate first. What remains at the bottom after even vacuum distillation is a dense, sticky residue with a very high carbon-to-hydrogen ratio. This residue is the base of heavy fuel oil, but it’s far too thick to pump or burn on its own.
To make it usable, refiners blend this residue with a lighter “cutter stock,” a more volatile fuel that brings the viscosity down enough for engines to handle. The final product contains thousands of individual chemical compounds, broadly grouped into four families: saturates (ring-shaped molecules with chain-like branches), aromatics (compounds built around multiple interlocking carbon rings), resins, and asphaltenes. The high aromatic content is what gives HFO its dark color, strong odor, and stubborn resistance to breaking down in the environment.
Grades and Specifications
Not all heavy fuel oil is the same. The international standard ISO 8217 classifies marine fuels by viscosity, which is essentially a measure of how thick and resistant to flow the oil is. The two most commonly traded grades are IFO 380 and IFO 500. IFO 380 has a maximum viscosity of 380 centistokes at 50°C, and IFO 500 maxes out at 500 centistokes at the same temperature. For comparison, water at room temperature has a viscosity of about 1 centistoke. These fuels need to be heated to 120°C or more before they flow well enough to inject into an engine.
The specific gravity of HFO ranges from 0.95 to over 1.03, meaning some batches are lighter than seawater and some are heavier. That single detail has enormous consequences in a spill, because it determines whether the oil floats, sinks, or suspends somewhere in between.
Why Shipping Still Uses It
The answer is straightforward: cost and energy density. HFO is essentially the leftovers of the refining process, so it sells at a steep discount compared to cleaner distillate fuels. For a large container ship burning 150 to 300 tons of fuel per day, even a modest price difference per ton translates to millions of dollars a year. That economic reality has kept HFO dominant even as environmental regulations have tightened. According to the World Economic Forum, HFO’s share of the global shipping fuel mix actually rose from 49% in 2021 to 56% in 2022, partly because alternatives like liquefied natural gas still represent only about 6% of the market.
Environmental and Health Concerns
HFO’s biggest environmental problem is sulfur. Unregulated heavy fuel oil can contain sulfur levels dozens of times higher than road diesel. When burned, that sulfur becomes sulfur oxides, gases that cause acid rain, respiratory illness, and the formation of fine particulate matter. Ships running on HFO near coastlines have measurable effects on air quality in port cities.
The other major pollutant is black carbon, essentially soot. These tiny dark particles absorb sunlight and warm the atmosphere. In the Arctic, black carbon settles on ice and snow, darkening the surface and accelerating melting. Roughly two-thirds of the black carbon emitted by ships in the Arctic in 2015 came from burning HFO, and those emissions are projected to keep rising as Arctic shipping routes open up with retreating ice.
Spill Behavior
When HFO spills into water, it behaves very differently from lighter oils. Only 5 to 10% evaporates in the first hours, compared to 30 to 50% for a typical crude oil. The rest persists. It spreads into thick, dark slicks or breaks into tarballs ranging from several feet across to less than an inch, which can drift hundreds of miles on currents and are extremely hard to detect visually or with remote sensing. In areas with sediment, like rivers or beaches, floating HFO can pick up particles, become heavier, and sink, creating underwater tar mats that are even harder to clean up.
There is almost no natural dispersion of HFO, even in rough seas with winds of 20 knots. Shoreline cleanup can work if crews reach the oil quickly, but once it weathers and becomes stickier, removal gets dramatically harder. One partial silver lining: because HFO’s chemical components dissolve poorly in water, its direct toxicity to marine organisms is lower than that of diesel or lighter petroleum products. The greater danger is physical, coating birds, mammals, and shorelines in a persistent, suffocating layer.
The IMO 2020 Sulfur Cap
The most significant regulation targeting HFO is the International Maritime Organization’s 2020 rule, which dropped the allowable sulfur content in marine fuel from 3.5% to 0.50% by mass for ships operating outside designated emission control areas. Inside those areas, near certain coastlines, the limit is even stricter at 0.10%. This rule didn’t ban HFO outright, but it made the highest-sulfur blends non-compliant unless a ship takes additional steps to clean its exhaust.
Ship operators have three main options for compliance. First, they can switch to very low sulfur fuel oil (VLSFO), a more refined blend that meets the 0.50% cap. Second, they can burn alternative fuels like LNG. Third, they can keep burning traditional high-sulfur HFO if they install exhaust gas cleaning systems, commonly called scrubbers.
How Scrubbers Work
Scrubbers are devices fitted to a ship’s exhaust stack that remove sulfur oxides before they reach the atmosphere. There are three main types.
- Open-loop scrubbers pump seawater through the exhaust stream. The natural alkalinity of seawater neutralizes the sulfur oxides. The wash water is then discharged back into the sea, which has raised concerns about transferring pollution from the air to the ocean.
- Closed-loop scrubbers recirculate fresh water treated with a sodium hydroxide solution. Most of the scrubbing water is reused, with only a small amount discharged. This produces less ocean pollution but costs more to operate.
- Dry scrubbers pass exhaust through a bed of calcium hydroxide granules. The sulfur reacts with the granules to form gypsum, a solid byproduct that can be offloaded at port. No wash water is discharged at all, but the system requires storing and handling bulk material onboard.
Scrubbers have allowed a large portion of the global fleet to continue burning cheaper high-sulfur HFO while meeting the sulfur cap on paper. Whether this truly solves the environmental problem depends on which type of scrubber is used and how strictly wash water discharge is regulated, a debate that is still playing out in ports and regulatory bodies worldwide.
Arctic Restrictions
Beyond the global sulfur cap, the IMO has moved toward restricting HFO use in the Arctic specifically, driven by the dual threat of black carbon warming and the near-impossibility of cleaning up an HFO spill in ice-covered waters. An outright ban on the use and carriage of HFO in Arctic waters has been adopted, though implementation timelines include exemptions that delay full enforcement for some vessels. Environmental groups have argued that these exemptions undermine the ban’s purpose, since the ships most likely to spill are often the older vessels granted extra time to comply.
Replacing HFO in the Arctic with distillate fuels, LNG, or other alternatives would simultaneously reduce spill risk and cut black carbon emissions, addressing two problems at once. For now, the transition remains uneven, with cost and infrastructure gaps slowing the shift in one of the planet’s most climate-sensitive regions.