Fire

A

1

Fuel and Oxygen

Most combustible fuels begin as solids, such as wood, wax, and plastic. Many fuels that people burn for energy, including gasoline and methane (natural gas), begin as either a liquid or a gas. Any fuel must be in a gaseous state (so that it can react with oxygen) before a fire can occur. Heat from the fire’s ignition source, and later from the fire itself, decomposes solid and liquid fuels, releasing flammable gases called volatiles. Some solids, such as the wax in a candle, melt into a liquid first. The liquid then evaporates, giving off volatiles that may then burn. Other solids, such as wood and cotton, decompose and evaporate directly. In a wood fire, gases given off by the decomposing wood enter the flame, combine with oxygen from the surrounding air, and ignite. The heat from the flame decomposes more wood, thus adding more flammable gases to the flame and creating a self-supporting process.

Most common fuels consist of compounds containing the elements carbon and hydrogen. Fuels often also contain oxygen, nitrogen, chlorine, and sulfur. Cellulose is the principle combustible compound in wood, paper, and cotton. It contains carbon, hydrogen, and oxygen. Plastics that burn, such as polyvinylchloride (PVC), polystyrene, polymethyl methacrylate (PMMA), nylon, and polyurethane, are composed mostly of carbon and hydrogen. Liquid fuels include oil and gasoline, while gaseous fuels include methane, propane, and hydrogen. All of these fuels (except pure hydrogen) contain both carbon and hydrogen.

A

2

Ignition Source

A fire can start when a fuel becomes so hot that it releases sufficient flammable gases for combustion to occur. At this temperature, called the fuel’s piloted ignition temperature, a spark or flame will start the combustion reaction. One source of piloted ignition is an open flame, such as that from a match or lighter. Sparks, such as those generated by electricity, may also ignite a fire. Engineers and scientists usually use the term piloted ignition to refer to solid fuels. Liquid fuels have, instead, a flash point temperature. At a liquid’s flash point, an ignition source will cause a flame to flash across the surface of the liquid.

The unpiloted ignition temperature of a fuel, also called its spontaneous ignition temperature, is the temperature the fuel must reach to ignite on its own. It is higher than the piloted ignition temperature, because a flame or spark is not present to provide the extra heat needed to kick-start the chemical reaction. Heat within the fuel provides this energy. Some fuels do not have a spontaneous ignition temperature because they break down into other substances before they can ignite on their own. Flammable gases have just one ignition temperature. They will ignite at this temperature if they are present in the right concentrations.

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Ignition depends not only on a fuel’s ignition temperature but also on the way the fuel absorbs heat. This absorption determines how heat will affect the fuel’s temperature. A fuel’s capacity to absorb heat depends on the type of fuel involved and its arrangement. Thick logs, for example, can absorb a large amount of heat before they reach their ignition temperature. Small twigs, however, need just a small amount of heat to reach the same ignition temperature. Fuels also need to absorb heat at or above a certain rate for ignition to occur. (The absorption rate can be expressed as units of heat absorbed per unit of time.) At the minimum absorption rate, the fuel will eventually reach its ignition temperature. A piece of wood will never ignite if the ignition source produces heat at a rate slower than the minimum rate required for ignition.

A

3

Chain Reaction

The final requirement for a fire is a chemical chain reaction. The heat of the ignition source starts the reaction, and heat from the fire’s flame continues the reaction. The flame needs to heat the fuel and make it release enough flammable gases to continuously support the chemical reaction. A common example of combustion is the burning of wood. When an ignition source heats wood to a sufficient temperature, about 260°C (500°F), the cellulose in the wood decomposes, producing volatile gases and char. The average composition of the gases can be represented by the compound CH₂O, where C stands for carbon, H stands for hydrogen, and O stands for oxygen. Under ideal conditions, CH₂O reacts with oxygen in the air and produces carbon dioxide (CO₂) and water vapor (H₂O). In the real world conditions are not ideal, so fires often produce other products as well, such as carbon monoxide (CO) and soot. The following equations show the two main stages involved in burning wood. The italicized letters represent numbers that depend on the conditions of the fire, such as how quickly the fire burns and the specific composition of the wood.

B

Burning Rate

Different kinds of fires burn at different rates—one fire may slowly smolder, while another may quickly use up its fuel. The rate at which a fire burns depends on the composition of the fuel, the surface area of the fuel, and the amount of oxygen that is available.

Most plastics burn at twice the rate of cellulose fuels, such as wood and leaves, because of the different chemical reactions involved. The burning rate of the same fuel, however, can also vary depending on how much of the fuel’s surface is exposed to the air. As the exposed surface of a fuel increases in comparison to its volume, the burning rate of the fuel increases as well. When the fuel’s gases have more surface area from which to escape, they can come into contact with more air. The increased exposure to air increases the amount of oxygen available for combustion. For example, people often use small twigs and pieces of wood called kindling to start a campfire. Kindling has a large amount of surface area compared to its volume. Its relatively large surface area to volume ratio also means that kindling heats and ignites more easily than thicker pieces of wood. Once ignited, kindling burns very quickly.

C

Products of Combustion

The products that a fire releases, and the rate at which it releases them, depend on the fuel and on the fire’s burning rate. Some fuels will produce more heat than others as they burn, and some will produce different kinds of gases. A fire that burns slowly may produce different products than one that burns quickly. The burning rate also affects the rate at which a fire releases products.

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