Fuel oil, also known as heavy oil, marine fuel or furnace oil, consisting mainly of residues from crude-oil distillation, referring to the heaviest commercial fuel. It is produced by fractional distillation of crude oil between 370 oC (700 0F) and 600 oC (1112 0F) boiling range. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash point of approximately 40 °C and oils burned in cotton or wool-wick burners. In this sense, diesel is a type of fuel oil. Fuel oil is made of long hydrocarbon chains, particularly alkanes, cycloalkanes and aromatics. The term fuel oil is also used in a stricter sense to refer only to the heaviest commercial fuel that can be obtained from crude oil, heavier than gasoline and naphtha. It is used primarily for steam boilers in power plants, aboard ships, and in industrial plants. Commercial fuel oils usually are blended with other petroleum fractions to produce the desired viscosity and flash point. Flash point is usually higher than that of kerosene. The term fuel oil ordinarily does not include such fuels as kerosene.
Fuel oil is classified into six classes, numbered 1 through 6, according to its boiling point, composition and purpose. The boiling point, ranging from 175-600 °C, and carbon chain length, 9 to 70 atoms, of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil has to be heated in order to get into liquid state. Price usually decreases as the fuel number increases.
Because the sulfur contained in the crude oil is concentrated in the residue material, fuel oil sulfur levels are naturally high. The sulfur level is not critical to the combustion process as long as the flue gases do not impinge on cool surfaces (which could lead to corrosion by the condensation of acidic sulfur trioxide). However, in order to reduce air pollution, most industrialized countries now restrict the sulfur content of fuel oils. Such regulation has led to the construction of residual desulfurization units or cokers in refineries that produce these fuels.
Residual fuels may contain large quantities of heavy metals such as nickel and vanadium; these produce ash upon burning and can foul burner systems. Such contaminants are not easily removed and usually lead to lower market prices for fuel oils with high metal contents.