Fire

C

1

Light and Heat

Once a material ignites, a flame forms. The flame consists of volatile gases moving upward, and it is the region in which the combustion reaction occurs. The gases in the flame move upward because they are hotter—and therefore lighter—than the surrounding air. The colors in the flame come from unburned carbon particles that glow and travel upward as the flame heats them.

The flame continues to burn as the volatile gases streaming from the fuel combine with oxygen from the surrounding air. Different parts of the flame have different temperatures. Most common fuels are compounds called hydrocarbons, and they produce about the same flame temperature, roughly 1200°C (2200°F). The maximum theoretical flame temperature for most hydrocarbons is about 1300°C (2400°F).

Different fuels produce varying amounts of heat. The rate at which a fire generates heat is equal to the burning rate of the fuel (measured in grams per second, or g/s) multiplied by the amount of heat produced by the combustion reaction. This second factor is called the effective heat of combustion, and scientists measure it in units of kilojoules per gram (kJ/g). When a gram of wood burns, for example, it produces 8 kJ of heat energy. Wood’s effective heat of combustion is therefore 8 kJ/g. Polyurethane’s effective heat of combustion is about 18 kJ/g. Polyurethane’s burning rate is also about twice that of wood under similar conditions. Multiplying the burning rates for these two substances by their effective heats of combustion, one finds that polyurethane fires produce heat at about 4.5 times the rate of wood fires under similar conditions.

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2

Gases

Fires can produce a number of different gases, including some that are harmless and some that are toxic. Carbon dioxide (CO₂) and water vapor (H₂O) are two relatively harmless gases produced by fires. Toxic gases from fires include carbon monoxide (CO), hydrogen cyanide (HCN), sulfur dioxide (SO₂), and hydrogen chloride (HCl).

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The specific gases and the amount of gas a fire produces depend on the type of fuel involved and the environment surrounding the fire. Different fuels will react differently in the combustion reaction, producing gases and amounts of gas specific to that type of fuel. For example, in well-ventilated conditions, polyurethane foam produces ten times more carbon monoxide for each gram burned than does wood. Fires that burn in an oxygen-rich environment will also produce less carbon monoxide than fires that burn where little oxygen is present. A well-ventilated fire has plenty of oxygen, so nearly all of the fuel’s volatile gases can take part in the combustion reaction, combining with oxygen in the air to produce carbon dioxide and water vapor. These fires produce less carbon monoxide because there is less carbon and oxygen left over from the initial combustion reaction to form carbon monoxide.

Fires that occur in an environment lacking sufficient oxygen will burn incompletely and smolder. These fires produce increasing amounts of carbon monoxide. For example, in an enclosed room, a fire will use up oxygen from the air as it progresses, decreasing the amount of oxygen in the room over time. Without sufficient oxygen, the volatile gases from the fire cannot fully take part in the combustion reaction. Some of the gases instead react to form carbon monoxide, which requires less oxygen than does combustion. Eventually, the amount of oxygen decreases below the level necessary for continued combustion, causing the fire to self-extinguish. Depending on the type of fuel, most fires self-extinguish at an oxygen concentration between 12 and 14 percent (by volume). By contrast, normal atmospheric air has an oxygen concentration of 21 percent.

C

3

Soot

As fires produce light, heat, and gases, they also produce soot, consisting of mostly carbon particles. Smoke may be defined either as just the soot particles given off by a fire, or as both the soot and the gaseous products of combustion.

The amount of soot produced by a fire depends on the type of fuel, the fuel’s burning rate, and environmental conditions. Most plastic fuels produce more soot than wood and other cellulose fuels. Plastics also usually burn more quickly than wood. Under similar conditions, for example, a slab of polyurethane will burn almost twice as fast as a slab of wood. The composition of plastic and plastic’s more rapid burning rate cause it to produce about 2.7 times as much soot as does wood. Fires also tend to produce more soot when they smolder and less soot when they burn freely in a well-ventilated area, with plenty of oxygen available.

VI

Destructive Force of Fire

Destructive fires can occur wherever fuel and oxygen are available, including in office buildings, homes, vehicles, and forests. Such fires may result in death or injury in addition to property damage. The great majority of structure fires in the United States and Canada occur in people’s homes.

Extinguishing a fire involves removing one of the requirements for combustion. Firefighters may physically remove the fuel from the fire by taking a burning item outside a structure. They can remove heat by cooling the fire with water or remove oxygen by smothering the fire with chemicals or a fire blanket. Interrupting the chemical chain reaction is more difficult but is typically done by applying special chemicals, such as halogenated compounds, to the fire. Halogenated compounds have been developed that are less damaging to the atmosphere’s ozone layer than earlier extinguishing chemicals in the group.

A

House Fires

Many people worry about being trapped in a hotel fire or a fire at their school or workplace. Yet the great majority of U.S. and Canadian fire fatalities are caused by fires in the home. Most house fires result from cooking accidents in the kitchen. Cigarettes have often been the cause of house fires that turn deadly. Smoking-related fires tend to smolder before they are discovered. Once a fire breaks out, it can envelop a room within minutes. Temperatures in the room may exceed 600°C (1100°F). While this heat alone would be deadly, the toxic gas in the smoke causes the majority of deaths and injuries. Almost half of all fatalities from fires are due to carbon monoxide poisoning, and more than a third are due to cardiopulmonary complications.

People protect themselves from the dangers of house fires in several ways. Fire extinguishers in homes enable people to put out fires before they become dangerous, while smoke detectors alert residents that a fire has broken out in the fire’s early stages. Communities support either a local fire department or a volunteer force, so people can call a phone number to summon firefighters to their home. The furniture, clothing, and building industries help prevent fires in the home by making products out of nonflammable materials or by treating materials with chemicals to make them less flammable.

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