Crude oil is a complex mixture of hydrocarbon molecules extracted from the Earth. Answering whether it burns is not a simple yes or no, as its flammability is highly variable. The ease and manner of ignition depend entirely on the oil’s specific composition, which varies widely depending on its source.
The Basics of Hydrocarbon Combustion
Crude oil is primarily composed of various chains of hydrogen and carbon atoms, known as hydrocarbons. These molecules store chemical energy released through combustion, a rapid chemical reaction. This process involves hydrocarbons reacting with oxygen, resulting in the release of heat, light, carbon dioxide, and water. For any fire to start, three elements must be present: fuel, an oxidizer (oxygen), and a sufficient heat source. The liquid oil itself does not burn; instead, the invisible vapors released from the oil’s surface ignite.
Flammability Differences in Crude Oil Grades
Crude oils are classified based on their density and sulfur content, which relates to their flammability. “Light” crude oils are less dense and more volatile because they contain a higher proportion of short-chain hydrocarbon molecules. These short chains evaporate easily, quickly producing enough flammable vapor to ignite, often at or below room temperature.
“Heavy” crude oils are thick and contain longer, more complex hydrocarbon chains. These chains are less volatile and require significant heating before they release enough vapor to catch fire. This difference is measured by the flash point, the minimum temperature at which a liquid gives off enough vapor to form an ignitable mixture. Light crude oils often have a flash point below 73°F (23°C), classifying them as highly flammable liquids.
The density is often described using the American Petroleum Institute (API) gravity scale. A higher API gravity indicates a lighter, more volatile oil that ignites more readily.
Fire Behavior and Associated Hazards
Once ignited, crude oil fires present unique hazards. The fire’s behavior is influenced by the oil’s wide range of boiling points, which can lead to a dangerous phenomenon known as “boil-over.” This occurs in large storage tanks containing water at the bottom. As the surface oil burns, a dense, superheated layer called a “heat wave” slowly progresses downward. When this hot zone contacts the water layer, the water rapidly turns to steam, expanding violently and forcing burning oil out of the tank in a sudden fireball.
The byproducts of a crude oil fire also pose significant health risks. These fires produce large amounts of thick, black smoke, which is uncombusted carbon particles (soot). Crude oil is also classified by its sulfur content; “sour” crude contains higher levels of sulfur compounds. When sour crude burns, it releases toxic gases, including sulfur dioxide and carbon monoxide.