Heavy fuel oil (HFO) is a fuel oil of a tar-like consistency. Also known as bunker fuel or residual fuel oil, HFO is the residual mixture left over from the distillation and cracking of crude oil in oil refineries. Generally, it has a boiling temperature between 350 and 500°C and is significantly more viscous than diesel fuel. As it is created through the extraction of more valuable components of its petroleum precursor, HFO contains various undesirable compounds and elements, which include aromatics, sulfur, nitrogen, vanadium, and others. These non-hydrocarbon contaminants significantly increase toxic gas and particulate emissions upon combustion, such as sulfur dioxide, carbon monoxide, and nitrogen oxides. It is predominantly used for marine vessel propulsion in marine diesel engines because it is cheaper than cleaner fuel sources such as diesel fuel or distillates. The emission-heavy nature of the fuel also contributes to this method of usage; marine vessels, such as oil tankers and cruise ships, are generally distant from population centers, sailing in open seas and oceans for the majority of the time, minimizing the exposure of humans to harmful aerosols and gaseous emissions. Ships utilizing heavy fuel oil may switch to cleaner alternatives such as diesel when approaching land. The use and carrying of HFO in seafaring vessels presents several environmental concerns, such as accidental oil spills due to adverse weather or routine handling. After the International Maritime Organization (IMO) implemented a global sulfur emissions cap in 2020, a growing number of ships have been equipped with scrubbers, which allow ships to continue using high-sulfur heavy fuel oil while meeting air quality regulations, shifting the environmental burden from air to water. HFOs are banned as fuel for ships travelling in the Antarctic as part of the IMO's International Code for Ships Operating in Polar Waters (the Polar Code). For similar reasons, an HFO ban in Arctic waters came into force in 2024.
Heavy fuel oil characteristics HFO consists of the remnants, or residual, of petroleum once the hydrocarbons of higher quality are extracted via processes such as thermal and catalytic cracking. Thus, HFO is also commonly referred to as residual fuel oil. The chemical composition of HFO is highly variable due to the fact that HFO is often mixed or blended with cleaner fuels; blending streams can include carbon numbers from C20 to greater than C50. HFOs are blended to achieve certain viscosity and flow characteristics for a given use. As a result of the wide compositional spectrum, HFO is defined by processing, physical and final-use characteristics. Being the final remnant of the cracking process, HFO also contains mixtures of the following compounds to various degrees: paraffins, cycloparaffins, aromatics, olefins, and asphaltenes as well as molecules containing sulfur, oxygen, nitrogen or organometals. HFO is characterized by a maximum density of 1010 kg/m3 at 15 °C, and a maximum viscosity of 700 mm2/s (cSt) at 50 °C according to ISO 8217.
Combustion and atmospheric reactions Given HFO's elevated sulfur contamination (maximum of 5% by mass), the combustion reaction results in the formation of sulfur dioxide (SO2).
Heavy fuel oil use and shipping Since the middle of the 20th century, HFO has been used primarily by the shipping industry due to its low cost compared with all other fuel oils, being up to 30% less expensive, as well as the historically lax regulatory requirements for emissions of nitrogen oxides (NOx) and sulfur dioxide (SO2) by the IMO. For these two reasons, HFO is the single most widely used engine fuel oil on-board ships. Data available until 2007 for global consumption of HFO at the international marine sector reports total fuel oil usages of 200 million tonnes, with HFO consumption accounting for 174 million tonnes. Data available until 2011 for fuel oil sales to the international marine shipping sector reports 207.5 million tonnes total fuel oil sales with HFO accounting for 177.9 million tonnes. Marine vessels with combustion engines can use a variety of different fuels for the purpose of propulsion, which are divided into two broad categories: residual oils or distillates. In contrast to HFOs, distillates are the petroleum products created through refining crude oil and include diesel, kerosene, naphtha and gas. Residual oils are often combined to various degrees with distillates to achieve desired properties for operational and/or environmental performance. Table 1 lists commonly used categories of marine fuel oil and mixtures; all mixtures including the low sulfur marine fuel oil are still considered HFO.
Arctic environmental concerns
The use and carriage of HFO in the Arctic is a commonplace marine industry practice. In 2015, over 200 ships entered Arctic waters carrying a total of 1.1 million tonnes of fuel with 57% of fuel consumed during Arctic voyages being HFO. In the same year, trends in carriage of HFO were reported to be 830,000 tonnes, representing a significant growth from the reported 400,000 tonnes in 2012. A report in 2017 by Norwegian Type Approval body DNV GL (Det Norske Veritas) calculated the total fuel use of HFO by mass in the Arctic to be over 75% with larger vessels being the main consumers. In the light of increased area traffic, and given that the Arctic is considered to be a sensitive ecological area with a higher response intensity to climate change, the environmental risks posed by HFO present concern for environmentalists and governments in the area. The two main environmental concerns for HFO in the Arctic are the risk of spill or accidental discharge and the emission of black carbon as a result of HFO consumption.
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