Fuel oils are made primarily from hydrocarbons—molecules consisting of hydrogen and carbon. The exact composition depends on the grade of the fuel oil and the crude oil from which it was refined. Here’s an overview of some key components: Alkanes (Paraffins): Saturated hydrocarbons that make up a large part of fuel oils, contributing to its energy content. Aromatics: Unsaturated hydrocarbons with ring structures, which influence the fuel\'s combustion properties. Sulfur Compounds: Fuel oil can contain sulfur, a significant environmental concern because, when burned, it forms sulfur dioxide (SO₂), which contributes to acid rain and air pollution. The sulfur content varies significantly among grades of fuel oil. Asphaltenes: Heavier hydrocarbons that contribute to the high viscosity of some grades, especially the heavier grades like No. 6. Trace Elements: Other compounds can include nitrogen, oxygen, and metals (like vanadium and nickel), which can impact engine performance and emissions. Refining Process Fuel oil is a product of refining crude oil, and the process varies depending on the desired product. The crude oil is distilled to separate its components based on boiling points. The heavier fractions (that don\'t boil until higher temperatures) are used to produce the heavier grades of fuel oil (such as No. 5 and No. 6), while lighter fractions contribute to gasoline and diesel fuels. Further refining processes such as visbreaking or coking can be used to further reduce the viscosity of heavy oils. Fuel Oil Grades and Their Characteristics No. 1 (Kerosene) Viscosity: Low viscosity, similar to kerosene. Uses: Residential heating, jet engines (aviation fuel), and some industrial heating. Combustion: Burns very cleanly with a low carbon content. Properties: Typically clear and colorless. It is very low in sulfur and can be used in regions with strict environmental regulations. No. 2 (Heating Oil) Viscosity: Slightly thicker than kerosene but still relatively light. Uses: Common in residential heating and light industrial heating systems. It’s used in furnaces and boilers in homes and small buildings. Combustion: Burns efficiently but may produce more soot and carbon than lighter oils. Properties: Often dyed red in many countries to denote its tax-free status for heating purposes. No. 4 (Heavy Heating Oil) Viscosity: Higher viscosity than No. 2, making it more difficult to burn without preheating. Uses: Often used in larger commercial buildings or industrial boilers, as it provides more energy. Combustion: Requires preheating or additives to flow properly and burn efficiently. It can produce more particulate matter than lighter oils. No. 5 (Bunker A) Viscosity: Much thicker and heavier than the above grades, requiring significant heating before use. Uses: Used in industrial boilers and some large heating systems, as well as certain power generation systems. Combustion: Burns at high temperatures, making it suitable for large-scale energy production.