Bunker fuel is a type of liquid fuel which is fractionally distilled from crude oil. Also known as fuel oil, this material can be broken down into different categories based on its chemical composition, intended purpose, and boiling temperature. In comparison with other petroleum products, bunker fuel is extremely crude and highly polluting.
After crude oil is extracted from the ground and brought to a refinery, it goes through a process called fractional distillation. During fractional distillation, the oil is heated, causing different types of oil within the crude to separate as they have different boiling points. Classically, fractional distillation is accomplished in a distillation column, which siphoned off various fractions as they precipitated out. During fractional distillation, oil refineries can also use catalysts to “crack” the hydrocarbon chains in the crude oil to create specific oil fractions.
Small molecules like those in propane gas, naptha, gasoline for cars, and jet fuel have relatively low boiling points, and they are removed at the start of the fractional distillation process. Heavier petroleum products like diesel and lubricating oil precipitate out more slowly, and bunker oil is literally the bottom of the barrel; the only thing more dense than bunker fuel is the residue which is mixed with tar for paving roads and sealing roofs.
The hydrocarbon chains in bunker fuel are very long, and this fuel is highly viscous as a result. It is also heavily contaminated with various substances which cannot be removed, so when it is burned, it pollutes heavily. The thick fuel is difficult for most engines to burn since it must be heated before it will combust, so it tends to be used in large engines like those on board ships. Ships have enough space to heat this type of fuel before feeding it into their engines, and their extremely sophisticated engines are capable of burning a wide range of fuels, including low quality bunker fuel.
Many oil spills have involved bunker fuel, leading some environmental organizations to call for a ban on the substance. It is extremely difficult to clean up and it coats birds and shorelines very effectively, because it's so dense. Because it also carries a range of contaminants, it can represent a serious environmental hazard when it spills. However, bunker fuel is also extremely cheap, and many shipping companies would lobby against any proposed ban out of concern for a sudden jump in shipping costs.