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 leftover from the distillation and cracking of crude oil in oil refineries. Generally, it has a boiling temperature between 350 and 500 °C and a significantly increased viscosity compared to diesel. As it is created through the extraction of more valuable components of its petroleum precursor, HFO contains various undesirable compounds and elements, which includes 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.
As the fuel is cheap, it is predominantly utilized for marine vessel propulsion in marine diesel engines due to its relatively low cost compared to 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, which are common due to their universal and dominant usage in marine transportation.
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