Fire Fuels: Solids, Liquids & Gases

Fuel is the material or substance being oxidized or burned in the combustion process. In scientific terms, the fuel in a combustion reaction is known as the “reducing agent”. Most common fuels contain carbon along with combinations of hydrogen and oxygen. These fuels can be further broken down into hydrocarbon-based fuels (such as gasoline, fuel oil, and plastics) and cellulose-based materials (such as wood and paper).

Fuel may be found in any of three states of matter: solid, liquid or gas.

The initiation of combustion requires the conversion of fuel into the gaseous state by heating. Fuel gases are evolved from solid fuels by pyrolysis, the chemical decomposition of a substance through the action of heat. Fuel gases are evolved from liquids by vaporization. This process is the same as boiling water or evaporation of a pan of water in sunlight. In both cases, heat caused the liquid to vaporize. No heat input is required with gaseous fuels and this places considerable restraints on the control and extinguishment of gas fuel fires.

Solid fuels have definite shape and size. One primary consideration with solid fuels is the surface area of the material in relation to its mass. The larger the surface area for a given mass, the more rapid the heating of the material and increase in the speed of pyrolysis.

The physical position of a solid fuel is also of great concern to firefighting personnel. If the solid fuel is in a vertical position, fire spread will be more rapid than if it is in a horizontal position. This is due to increased heat transfer through convection and direct flame contact in addition to conduction and radiation.

Pyrolysis of wood by temperature

Liquid fuels have physical properties that increase the difficulty of extinguishment and hazard to personnel. Liquids will assume the shape of their container. When a spill occurs, the liquid will assume the shape of the ground (flat) and will flow and accumulate in low areas.

The density of liquids in relation to water is known as specific gravity. Water is given a value of one. Liquids with a specific gravity less than one are lighter than water, while those with a specific gravity greater than one are heavier than water. It is interesting to note that most flammable liquids have a specific gravity of less than one, therefore they would float on top of water.

The solubility of a liquid fuel in water is an important factor. Hydrocarbon liquids as a rule will not mix with water. Alcohol and polar solvents mix with water and if large volumes of water are used, they may be diluted to the point where they will not burn. Consideration must be given to which extinguishing agents are effective on hydrocarbons (insoluble) and which affect polar solvents and alcohol (soluble).

The volatility, or ease with which the liquid gives off vapor, influences fire control objectives. The density of gas or vapor in relation to air is of concern to volatile liquids and with gas fuels.

Gases tend to assume the shape of their container but have no specific volume. If the vapor density of a gas is such that it is less dense than air (air is given a value of one), it will rise and tend to dissipate. If a gas or vapor is heavier than air, it will tend to hug the ground and travel as directed by terrain and wind.

An easy way to remember those gases that are lighter than air is the acronym “HA HA MICEN”:

This is a significant property for evaluating exposures and where hazmat gas and vapor will travel.

The mixture of the fuel vapor and air must be within the flammable range. The upper and lower limits of concentration of vapor in air will allow flame propagation when contacted by a source of ignition.

The flammable range varies with the fuel and with the ambient temperature. Usually the flammable range is given for temperatures of 70°F (21°C).

Examples of flammable ranges (percent of vapor in air)

When the proper fuel vapor/air mixture has been achieved, it must be raised to its ignition temperature. See the fire behavior glossary for flash point, fire point and ignition temperature.