Fuel Study Guide for McGraw-Hill's Firefighter Exams (page 3)

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.

Gases

Gases are the third stage of matter. The volume of a given amount of gas is dependent on its temperature and the surrounding pressure. An important concept for firefighters to understand regarding gases and vapors being emitted from a liquid is vapor density. Vapor density is the relative density of the gas or vapor as compared to air. The vapor density of air is 1. A gas or vapor with a vapor density more than 1 (butane, 2.1) will be heavier than air and travel along the ground surface in search of an ignition source. A gas or vapor with a vapor density less than 1 (methane, 0.55) will rise and disperse readily into the air. Gases are classified as Class B-type materials.

Chemical Properties of Gases

Gases can be classified according to their chemical properties as flammable (burn in air), inert (will not burn in air or in any concentration of oxygen and will not support combustion), oxidizer (will not burn in air or in any concentration of oxygen but will support combustion), toxic (poisonous or irritating when inhaled), and reactive (can rearrange chemically when exposed to heat or shock and explode or can react with other materials and ignite).

• Flammable—A gas that will burn in normal concentrations of oxygen in air is a flammable gas. When discussing flammable gases (or flammable vapors boiling off a liquid) mixing with air, the concept of flammable range must be understood. The flammable range is defined as the ratio of gas or vapor in air that is between the upper and lower flammable limits. The upper flammable limit is the maximum ratio of flammable gases/vapors above which ignition will not occur; it is too rich a mixture. The lower flammable limit is the minimum ratio of flammable gases/vapors in air below which ignition will not occur; it is too lean a mixture. Examples of flammable gases include acetylene, hydrogen, and propane.
• Inert—An inert gas is a nonflammable gas that will not support combustion. Examples include helium, nitrogen, and argon.
• Oxidizer—A nonflammable gas that will support combustion is known as an oxidizer. Examples include oxygen and chlorine.
• Toxic—Gases that cause harm to living tissue via chemical activity are called toxic gases. They can endanger the lives and health of those who inhale or come into skin contact with it. Examples include hydrogen cyanide, carbon monoxide, and ammonia.
• Reactive—Gases that react internally and with other materials are reactive gases. They can be heat sensitive and shock sensitive and also react with organic and inorganic substances to cause combustion. Examples include fluorine and vinyl chloride.