Fuel Study Guide for McGraw-Hill's Firefighter Exams (page 5)

Fuel (Combustible Matter—Solids, Liquids, And Gases)

Combustible matter may be in a solid, liquid, or gaseous state.

Solids

Solids are materials with defined volume, size, and shape at a given temperature. Examples are wood and wood products (paper, cardboard), carbon-containing materials (coal, charcoal), plastics (polyvinyl chloride, epoxies), textiles (cotton, wool, rayon), and combustible metals (magnesium, aluminum).

Wood and Wood Products

Wood and wood products are the most common solids encountered by firefighters. They are considered Class A-type materials and require water or water solutions to cool them below their ignition temperature and extinguish them. The average ignition temperature of wood is approximately 400°F. Major components are carbon, hydrogen, and oxygen.

Factors Affecting the Ignition and Combustibility of Wood and Wood Products

Many factors influence the ignition and combustibility of wood and wood products, the most important of which are cited below.

• Physical form (size, form, shape, mass)—The greater the mass in relation to surface area, the more heat energy will be required to ignite it and the slower the rate of burning will be once ignited.
• Thermal inertia—Resistance to heating, generally based on the specific gravity and density of the material, is known as its thermal inertia. Materials with a low thermal inertia (low specific gravity and density) will heat up and ignite more readily than materials with a high thermal inertia, high specific gravity, and density.
• Moisture content—Wet wood is more difficult to ignite than dry wood. Wood is very difficult to ignite when the moisture content rises above 15 percent.
• Species—Low-density softwoods (pine) will ignite at lower temperatures than high-density hardwoods (oak).
• Ignition temperature—The minimum temperature to which a material must be heated for it to ignite and be self-sustaining without an external input of heat is known as the ignition temperature.
• Piloted-ignition temperature—Ignition temperature caused with the assistance of an external (flame, spark) heat source is known as the piloted-ignition temperature. It is usually considerably lower than the ignition temperature.
• Arrangement—The term arrangement refers to the spacing of the fuel material. Tightly stacked lumber is much more difficult to ignite and will burn at a slower rate than lumber loosely arranged.
• Time—Wood and wood products must be exposed to heat for a certain period of time before combustible vapors are produced and ignite.
• Heat Source—A heat source provides heat. For wood products heat sources include steam pipes, matches, and a blowtorch.
• Rate of Heating—The rate or speed at which a substance becomes heated may be constant or sporadic.
• Oxygen—Oxygen-enriched atmospheres (greater than 21 percent in air) enhance burning, whereas oxygen-deficient atmospheres (less than 15 percent in air) will generally not support combustion.

Carbon And Carbon Containing Materials

Carbon and carbon-containing materials have ignition temperatures in the range of 600° to 1,400°F, depending on the amount of carbon in them. Coal and charcoal burn hotter than wood and wood products, and they generate large quantities of toxic and flammable carbon monoxide gas. They are classified as Class A-type materials.

Plastics

Plastics are other common combustible solids, although they may be produced as a liquid or foam. Most plastics are petroleum based (hydrocarbons). They can be soft or hard and be electrically conductive or nonconductive (insulators). Manufactured plastics usually contain additives (colorants, stabilizers, lubricants), which change the chemical nature and combustibility of the original plastic. Pyrolysis doesn't occur as readily in plastics as in wood and, therefore, plastics tend to have a higher ignition temperature than wood and wood products.

Plastics can be divided into two categories: thermoplastics and thermosets. Thermoplastics (polypropylene, polyvinyl chloride) are formed by heat and pressure and can be reshaped repeatedly by heat and pressure. In a fire, they will melt and flow like a liquid. Thermosets (alkyds, epoxies), on the other hand, may only be formed by heat and pressure once. When subjected to the heat from a fire, they will decompose and burn. They are classified as Class A-type materials.

Textiles

Textiles include clothing, bedding, upholstery, and carpeting. In general, all textile fibers are combustible. Textiles can be divided into two categories: natural fiber and synthetic fiber.

Natural fiber textiles can be divided into fibers derived from plants (cotton, linen, hemp) and those derived from animals (wool, mohair, camel hair). Plant fiber is composed mostly of cellulose, which consists of carbon, hydrogen, and oxygen. During a fire situation, plant fiber will decompose and burn but will not melt. The ignition temperature of cotton is approximately 750°F. Animal fiber, however, is chemically different from plant textile material. Protein is the major component. Animal fiber, with an ignition temperature of approximately 1,100°F, will not ignite as readily as plant fiber.

Synthetic fiber (rayon, nylon, polyester) is material woven from artificial fiber (plastic, hydrocarbon, metal, glass). Burning characteristics of synthetic fiber include decomposition, burning, and melting. Synthetic fiber can, however, be made flame retardant and various kinds of synthetic fiber (Nomex, Kevlar) are used in the production of "fireproof" clothing for firefighters. Synthetic fibers are classified as Class A-type materials.

Factors Affecting Ignition and Combustibility of Textile Products

Various factors affect the ignition and combustibility of textile products. Some of these are listed below.

• chemical composition
• weight of the fabric
• type of weave
• finishing treatments

Combustible Metals

The elements that will combine with oxygen, reach their ignition temperature, and burn are known as combustible metals. Metals do not, however, undergo pyrolysis to produce combustible vapors when heated. They burn on their surface with no flaming combustion. Metals that do burn produce an abundance of heat energy. When water is applied, and the water molecule separates, steam and hydrogen explosions can occur. For this reason, water, unless in large amounts, is not recommended as an extinguishing agent on combustible metals. Specific extinguishing agents (graphite, salts) have been developed to cover the surface of the burning metal and exclude oxygen. Combustible metals are classified as Class D-type materials.

Liquids

Liquids make up the stage of matter between solids and gases. A liquid has definite volume but takes the shape of the container it is being stored in. Liquids that produce vapors that burn can be divided into two categories: combustible liquids (kerosene, diesel, heavy fuel oils) and flammable liquids (gasoline, methyl alcohol, acetone). Liquids can present other hazards to firefighters besides fire (corrosiveness and toxicity). In general, liquids that burn are classified as Class B materials; however, vegetable oils used in cooking and the preparation of foods are classified as Class K materials.

Some key characteristics to understand concerning liquids that burn are the flash point, boiling point, specific gravity, solubility, and viscosity.

• Flash point—The flash point is the minimum temperature of a liquid at which it emits vapors to form an ignitable mixture with air. For firefighters, the flash point is the most important property of liquids that burn. The degree of hazard will be determined by the flash point of the liquid because it is the vapors of the liquid that burn, not the liquid itself. Liquids are classified as combustible (flashpoint of 100° F or more) and flammable (flashpoint of less than 100°F).
• Boiling point—The boiling point is the temperature of the liquid at which it will liberate the most vapors. It is the temperature at which the vapor pressure of the liquid equals atmospheric pressure. The normal boiling point of a liquid is the temperature at which it boils at sea level, usually recorded as 14.7 pounds per square inch absolute (psia). It is impossible to raise the temperature of a liquid above its boiling point, except if it is under pressure.
• Specific gravity—The specific gravity of a liquid is the ratio of the weight of the liquid to the weight of an equal volume of water. The specific gravity of water is 1. A liquid (gasoline, 0.8) with a specific gravity less than water will float on water, whereas a liquid with a specific gravity more than 1 (sulfuric acid, 1.8) will sink.
• Solubility—The solubility of a liquid is the percentage by weight of the liquid that will dissolve in water. The solubility of a liquid ranges from negligible (less than one tenth of 1 percent) to complete (100 percent).
• Viscosity—Viscosity is a measure of a liquid's flow (through an opening or into a container) in relation to time. Thick liquids (molasses, asphalt, wax) are on the borderline between liquids and solids and are considered viscous.