Petroleum product burned to generate motive power or heat
Fuel oil is any of various fractions obtained from the distillation of petroleum (crude oil). Such oils include distillates (the lighter fractions) and residues (the heavier fractions). Fuel oils include heavy fuel oil (bunker fuel), marine fuel oil (MFO), furnace oil (FO), gas oil (gasoil), heating oils (such as home heating oil), diesel fuel, and others. 
The term "fuel oil" generally includes any liquid fuel that is burned in a furnace or boiler to generate heat (heating oils), or used in an engine to generate power (as motor fuels). However, it does not usually include other liquid oils, such as those with a flash point of approximately , or oils burned in cotton- or wool-wick burners. In a stricter sense, "fuel oil" refers only to the heaviest commercial fuels that crude oil can yield, that is, those fuels heavier than gasoline (petrol) and naphtha.
Fuel oil consists of long-chain hydrocarbons, particularly alkanes, cycloalkanes, and aromatics. Small molecules, such as those in propane, naphtha, gasoline, and kerosene, have relatively low boiling points, and are removed at the start of the fractional distillation process. Heavier petroleum-derived oils like diesel fuel and lubricating oil are much less volatile and distill out more slowly.
Uses.
 Oil has many uses; it heats homes and businesses and fuels trucks, ships, and some cars. A small amount of electricity is produced by diesel, but it is more polluting and more expensive than natural gas. It is often used as a backup fuel for peaking power plants in case the supply of natural gas is interrupted or as the main fuel for small electrical generators. In Europe, the use of diesel is generally restricted to cars (about 40%), SUVs (about 90%), and trucks and buses (over 99%). The market for home heating using fuel oil has decreased due to the widespread penetration of natural gas as well as heat pumps. However, it is very common in some areas, such as the Northeastern United States.
Residual fuel oil is less useful because it is so viscous that it has to be heated with a special heating system before use and it may contain relatively high amounts of pollutants, particularly sulfur, which forms sulfur dioxide upon combustion. However, its undesirable properties make it very cheap. In fact, it is the cheapest liquid fuel available. Since it requires heating before use, residual fuel oil cannot be used in road vehicles, boats or small ships, as the heating equipment takes up valuable space and makes the vehicle heavier. Heating the oil is also a delicate procedure, which is impractical on small, fast moving vehicles. However, power plants and large ships are able to use residual fuel oil.
Use of residual fuel oil was more common in the past. It powered boilers, railroad steam locomotives, and steamships. Locomotives, however, have become powered by diesel or electric power; steamships are not as common as they were previously due to their higher operating costs (most LNG carriers use steam plants, as "boil-off" gas emitted from the cargo can be used as a fuel source); and most boilers now use heating oil or natural gas. Some industrial boilers still use it and so do some old buildings, including in New York City. In 2011 New York City estimated that the 1% of its buildings that burned fuel oils No. 4 and No. 6 were responsible for 86% of the soot pollution generated by all buildings in the city. New York made the phase out of these fuel grades part of its environmental plan, PlaNYC, because of concerns for the health effects caused by fine particulates, and all buildings using fuel oil No. 6 had been converted to less polluting fuel by the end of 2015.
Residual fuel's use in electrical generation has also decreased. In 1973, residual fuel oil produced 16.8% of the electricity in the US. By 1983, it had fallen to 6.2%, and as of 2005[ [update]], electricity production from all forms of petroleum, including diesel and residual fuel, is only 3% of total production. The decline is the result of price competition with natural gas and environmental restrictions on emissions. For power plants, the costs of heating the oil, extra pollution control and additional maintenance required after burning it often outweigh the low cost of the fuel. Burning fuel oil, particularly residual fuel oil, produces uniformly higher carbon dioxide emissions than natural gas.
Heavy fuel oils continue to be used in the boiler "lighting up" facility in many coal-fired power plants. This use is approximately analogous to using kindling to start a fire. Without performing this act it is difficult to begin the large-scale combustion process.
The chief drawback to residual fuel oil is its high initial viscosity, particularly in the case of No. 6 oil, which requires a correctly engineered system for storage, pumping, and burning. Though it is still usually lighter than water (with a specific gravity usually ranging from 0.95 to 1.03) it is much heavier and more viscous than No. 2 oil, kerosene, or gasoline. No. 6 oil must, in fact, be stored at around heated to before it can be easily pumped, and in cooler temperatures it can congeal into a tarry semisolid. The flash point of most blends of No. 6 oil is, incidentally, about . Attempting to pump high-viscosity oil at low temperatures was a frequent cause of damage to fuel lines, furnaces, and related equipment which were often designed for lighter fuels.
For comparison, BS 2869 Class G heavy fuel oil behaves in similar fashion, requiring storage at , pumping at around and finalizing for burning at around .
Most of the facilities which historically burned No. 6 or other residual oils were industrial plants and similar facilities constructed in the early or mid 20th century, or which had switched from coal to oil fuel during the same time period. In either case, residual oil was seen as a good prospect because it was cheap and readily available. Most of these facilities have subsequently been closed and demolished, or have replaced their fuel supplies with a simpler one such as gas or No. 2 oil. The high sulfur content of No. 6 oil—up to 3% by weight in some extreme cases—had a corrosive effect on many heating systems (which were usually designed without adequate corrosion protection in mind), shortening their lifespans and increasing the polluting effects. This was particularly the case in furnaces that were regularly shut down and allowed to go cold, because the internal condensation produced sulfuric acid.
Environmental cleanups at such facilities are frequently complicated by the use of asbestos insulation on the fuel feed lines. No. 6 oil is very persistent, and does not degrade rapidly. Its viscosity and stickiness also make remediation of underground contamination very difficult, since these properties reduce the effectiveness of methods such as air stripping.
When released into water, such as a river or ocean, residual oil tends to break up into patches or tarballs – mixtures of oil and particulate matter such as silt and floating organic matter – rather than form a single slick. An average of about 5-10% of the material will evaporate within hours of the release, primarily the lighter hydrocarbon fractions. The remainder will then often sink to the bottom of the water column.
Health impacts.
Because of the low quality of bunker fuel, when burnt it is especially harmful to the health of humans, causing serious illnesses and deaths. Prior to the IMO's 2020 sulfur cap, shipping industry air pollution was estimated to cause around 400,000 premature deaths each year, from lung cancer and cardiovascular disease, as well as 14 million childhood asthma cases each year.
Even after the introduction of cleaner fuel rules in 2020, shipping air pollution is still estimated to account for around 250,000 deaths each year, and around 6.4 million childhood asthma cases each year.
The hardest hit countries by air pollution from ships are China, Japan, the UK, Indonesia, and Germany. In 2015, shipping air pollution killed an estimated 20,520 people in China, 4,019 people in Japan, and 3,192 people in the UK.
According to an ICCT study, countries located on major shipping lanes are particularly exposed, and can see shipping account for a high percentage of overall deaths from transport sector air pollution. In Taiwan, shipping accounts for 70% of all transport-attributable air pollution deaths in 2015, followed by Morocco at 51%, Malaysia and Japan both at 41%, Vietnam at 39%, and the UK at 38%.
As well as commercial shipping, cruise ships also emit large amounts of air pollution, damaging people's health. Up to 2019, it was reported that the ships of the single largest cruise company, Carnival Corporation &amp; plc, emitted ten times more sulfur dioxide than all of Europe's cars combined.
General classification.
United States.
Although the following trends generally hold true, different organizations may have different numerical specifications for the six fuel grades. The boiling point and carbon chain length of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil must be heated for it to flow. Price usually decreases as the fuel number increases.
Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners and high-performance/clean diesel engines. It is the kerosene refinery cut that boils off immediately after the heavy naphtha cut used for gasoline. This fuel is commonly known as diesel no. 1, kerosene, and jet fuel. Former names include: coal oil, stove oil, and range oil.
Number 2 fuel oil is a distillate home heating oil. Trucks and some cars use similar diesel no. 2 with a cetane number limit describing the ignition quality of the fuel. Both are typically obtained from the light gas oil cut. The name "gasoil" refers to the original use of this fraction in the late 19th and early 20th centuries—the gas oil cut was used as an enriching agent for carbureted water gas manufacture.
Number 3 fuel oil was a distillate oil for burners requiring low-viscosity fuel. ASTM merged this grade into the number 2 specification, and the term has been rarely used since the mid-20th century.
Number 4 fuel oil is a commercial heating oil for burner installations not equipped with preheaters. It may be obtained from the heavy gas oil cut. This fuel is sometimes known by the Navy specification of Bunker A.
Number 5 fuel oil is a residual-type industrial heating oil requiring preheating to for proper atomization at the burners. It may be obtained from the heavy gas oil cut, or it may be a blend of residual oil with enough number 2 oil to adjust viscosity until it can be pumped without preheating. This fuel is sometimes known by the Navy specification of Bunker B.
Number 6 fuel oil is a high-viscosity residual oil requiring preheating to . Residual means the material remaining after the more valuable cuts of crude oil have boiled off. The residue may contain various undesirable impurities, including 2% water and 0.5% mineral oil. This fuel may be known as residual fuel oil (RFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400.
United Kingdom.
The British Standard BS 2869, "Fuel Oils for Agricultural, Domestic and Industrial Engines", specifies the following fuel oil classes:
Class C1 and C2 fuels are kerosene-type fuels. C1 is for use in flueless appliances (e.g. lamps). C2 is for vaporizing or atomizing burners in appliances connected to flues.
Class A2 fuel is suitable for mobile, off-road applications that are required to use a sulfur-free fuel. Class D fuel is similar to Class A2 and is suitable for use in stationary applications, such as domestic, commercial, and industrial heating. The BS 2869 standard permits Class A2 and Class D fuel to contain up to 7% (V/V) biodiesel (fatty acid methyl ester, FAME), provided the FAME content meets the requirements of the BS EN 14214 standard.
Classes E to H are residual oils for atomizing burners serving boilers or, with the exception of Class H, certain types of larger combustion engines. Classes F to H invariably require heating prior to use; Class E fuel may require preheating, depending on ambient conditions.
Russia.
Mazut is a residual fuel oil often derived from Russian petroleum sources and is either blended with lighter petroleum fractions or burned directly in specialized boilers and furnaces. It is also used as a petrochemical feedstock. In the Russian practice, though, "mazut" is an