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For other uses, see Fire (disambiguation).
A forest fire

Fire is a rapid oxidation process that releases energy in varying intensities in form of light and heat and often creates smoke. It is commonly used to describe either a fuel in a state of combustion (e.g., a campfire, or a lit fireplace or stove) or a violent, destructive and uncontrolled burning (e.g., in buildings or a wildfire). The discovery of how to make fire is considered one of humankind's most important advances, allowing higher hominids to ward off wild animals, cook food, and control their own source of light and warmth.

Chemistry

Broadly speaking there are two types of fire, flaming and smoldering fires, and they exhibit traits unique to themselves.

Flaming

Flaming fires involve the rapid oxidation of a fuel (combustion or release of energy) with associated flame, heat, and light. The flame itself occurs within a region of gas where intense exothermic reactions are taking place. An exothermic reaction is a chemical reaction that takes place within a substance whereby heat and energy are released as the substance changes to a simpler chemical form. As chemical reactions occur within the fuel being burned, light is usually emitted as photons are released by the oxidation of the fuel. Depending upon the specific chemical and physical change taking place within the fuel the flame may or may not emit light. For example, burning alcohol is usually invisible although the heat given off is tremendous. The visible flame has no mass. What we see as visible flame is actually energy (photons) being released in the form of light by the oxidation of the fuel. The color of the flame is dependent upon the energy level of the photons emitted. Lower energy levels produce colors toward the red end of the light spectrum while higher energy levels produce colors toward the blue end of the spectrum. The hottest flames are white in appearance. The color of a fire may also be affected by chemical elements in the flame, such as barium giving a green flame color.

Smoldering

The latter example, a smoldering fire, is a flameless form of combustion, deriving its heat from oxidations occurring on the surface of a solid fuel. Two common examples are glowing coals and cigarettes. Smolder propagates in a creeping fashion over solid fuels or inside porous fuels, and its temperature and heat released are low in comparison. The difference between flaming and smoldering combustion is that the latter occurs on the surface of the solid rather than in the gas phase.

Chemical Reaction

The fire triangle

Fires start when both a flammable and/or a combustible material with an adequate supply of oxygen or another oxidizer is subjected to enough heat. This is commonly called the fire triangle. No fire can exist without all three elements being in place.

The common fire-causing sources of heat include:

Once ignited, fires can sustain their own heat by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of oxygen and fuel.

Fire can be extinguished by removing any one of the elements of the fire triangle. The traditional extinguishant of water acts by cooling the combusting material to stop the reaction, whereas a Carbon Dioxide extinguisher acts by starving the fire of oxygen.

The unburnable solid remains of a combustible material left after a fire are called ash, soot or cinder.

Flame

Main article: Flame

A flame is an exothermic, self-sustaining, oxidizing chemical reaction producing energy and glowing hot matter, of which a very small portion is plasma. It consists of reacting gases and solids emitting visible and infrared light, the frequency spectrum of which depends on the chemical composition of the burning elements and intermediate reaction products.

In many cases, such as the burning of organic matter, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of 'fire'. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. For reasons currently unknown by scientists, the flame produced by exposure of zinc to air is a bright green, and produces plumes of zinc oxide. Usually oxygen is involved, but hydrogen burning in chlorine also produces a flame, producing hydrogen chloride (HCl). Other possible combinations producing flames, amongst many more, are fluorine and hydrogen, and hydrazine and nitrogen tetroxide.

The glow of a flame is complex. Black-body radiation is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and on chemical makeup for the emission spectra. The dominant color in a flame changes with temperature. The photo of the forest fire is an excellent example of this variation. Near the ground, where most burning is occurring, the fire is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.

The National Aeronautics and Space Administration (NASA) of the United States has recently found that gravity plays a role. Modifying the gravity causes different flame types.[1] The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity, such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient (although it will go out if not moved steadily, as the CO2 from combustion does not disperse in microgravity, and tends to smother the flame). There are several possible explanations for this difference, of which the most likely is that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs.[2] Experiments by NASA reveal that diffusion flames in microgravity allow more soot to be completely oxidized after they are produced than diffusion flames on Earth, because of a series of mechanisms that behave differently in microgravity when compared to normal gravity conditions.[3] These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.

In combustion engines, various steps are taken to eliminate a flame. The method depends mainly on whether the fuel is oil, wood, or a high-energy fuel such as jet fuel.

Typical temperatures of fires and flames

  • Oxyhydrogen flame: 2000 °C or above) (3645 °F) [4]
  • Bunsen burner flame: (min. to max. setting) 1300 to 1600 °C (2372 to 2912 °F) [5]
  • Blowtorch flame: 1800 °C (3272 °F) [6]
  • Candle flame: 760 °C (1400 °F)
  • Smoldering cigarette:
    • Temperature without drawing: side of the lit portion; 400 °C (750 °F); middle of the lit portion: 585 °C (1110 °F)
    • Temperature during drawing: middle of the lit portion: 700 °C (1290 °F)
    • Always hotter in the middle.

Controlling fire

The ability to control fire is one of humankind's great achievements. Fire making to generate heat and light made it possible for people to migrate to colder climates and enabled people to cook food — a key step in the fight against disease. Archaeology indicates that ancestors or relatives of modern humans might have controlled fire as early as 6,000 years ago. The Cradle of Humankind site has evidence for controlled fire from 6,000 to 5,000 years ago.[7] By the Neolithic Revolution, during the introduction of grain based agriculture, people all over the world used fire as a tool in landscape management. These fires were typically controlled burns or "cool fires", as opposed to uncontrolled "hot fires" that damage the soil. Hot fires destroy plants and animals, and endanger communities. This is especially a problem in the forests of today where traditional burning is prevented in order to encourage the growth of timber crops. Cool fires are generally conducted in the spring and fall. They clear undergrowth, burning up biomass that could trigger a hot fire should it get too dense. They provide a greater variety of environments, which encourages game and plant diversity. For humans, they make dense, impassable forests traversable.

A blacksmith's fire, used primarily for forging iron.

The first technical application of the fire may have been the extracting and treating of metals. There are numerous modern applications of fire. In its broadest sense, fire is used by nearly every human being on earth in a controlled setting every day. Users of internal combustion vehicles employ fire every time they drive. Thermal power stations provide electricity for a large percentage of humanity.

The use of fire in warfare has a long history. Hunter-gatherer groups around the world have been noted as using grass and forest fires to injure their enemies and destroy their ability to find food, so it can be assumed that fire has been used in warfare for as long as humans have had the knowledge to control it. Homer detailed the use of fire by Greek commandos who hid in a wooden horse to burn Troy during the Trojan war. Later the Byzantine fleet used Greek fire to attack ships and men. American and British warplanes destroyed the German city of Dresden on February 14, 1945 by creating a firestorm, in which a ring of fire surrounding the city was drawn inward by an updraft caused by a central cluster of fires. In the Vietnam War, the Americans dropped napalm from the air. More recently many villages were burned during the Rwandan Genocide. Aerial bombing of cities, including firebombing using incendiary bombs, was also used frequently during World War II. Molotov cocktails are cheap to construct and are commonly used as well.

Fire and Fuel

A coal-fired power station in the People's Republic of China.

Setting fuel aflame releases usable energy. Wood was a prehistoric fuel, and is still viable today. The use of fossil fuels, such as petroleum, natural gas and coal, in power plants supplies the vast majority of the world's electricity today; the International Energy Agency states that nearly 80% of the world's power comes from these sources.[8] The fire in a power station is used to heat water, creating steam that drives turbines. The turbines then spin an electric generator to produce power.

The burning of wood is often the first association to the word "fire". It is common in a developing country for wood to be the primary energy source as well. For instance, in Africa, 65% of the energy used comes from the burning of biomass, such as elephant dung. This has become much more abundant because the elephant population has tripled in the past few years.[9] What is less obvious is that wood burning power stations are less environmentally destructive than the fired oil power station in two major respects: first, wood is a renewable resource, especially if trees are grown in a modern, sustainable way; second, the carbon dioxide emissions are negligible because no more carbon dioxide can be produced by burning than would be produced by the natural rotting of wood. Thus, over a 100-year timescale, the effect is carbon-neutral.[10]. E.ON UK is soon to build a 44 megawatt wood fired power station in the United Kingdom for these reasons.[11]

Fire protection and prevention

A structure fire
Flammable gas warning
Main article: Fire protection

Codes and life safety codes offer fire fighting services to extinguish or contain uncontrolled fires. Trained firefighters use fire trucks, water supply resources such as water mains and fire hydrants, and an array of other equipment to combat the spread of fires.

To ensure fire safety of buildings, all building products, materials and furnishings in the United States must be tested for fire resistance, and researched so it causes no harm. combustibility and flammability before they can be used in construction. The same applies to upholstery, carpeting and plastics used in vehicles and vessels. Buildings, especially schools and tall buildings, often conduct fire drills to inform and prepare citizens on how to react to a building fire.

Purposely starting destructive fires constitutes arson and is a criminal offense in most jurisdictions.

Some jursidictions operate systems of classifying fires using code letters. Whilst these may agree on some classifications, they also vary. Below is a table showing the standard operated in Europe and Australasia against the system used in the United States.

Type of Fire European/Australasian Classification United States Classification
Fires that involve flammable solids such as wood, cloth, rubber, paper, and some types of plastics. Class A Class A
Fires that involve flammable liquids or liquifiable solids such as petrol/gasoline, oil, paint, some waxes & plastics, but not cooking fats or oils Class B Class B
Fires that involve flammable gases, such as natural gas, hydrogen, propane, butane Class C
Fires that involve combustible metals, such as sodium, magnesium, and potassium Class D Class D
Fires that involve any of the materials found in Class A and B fires, but with the introduction of an electrical appliances, wiring, or other electrically energized objects in the vicinity of the fire, with a resultant electrical shock risk if a conductive agent is used to control the fire Class E Class C
Fires involving cooking fats and oils. The high temperature of the oils when on fire far exceeds that of other flammable liquids making normal extinguishing agents ineffective. Class F Class K

Practical uses

  • Fire has been used for light, heat (for cooking, survival and comfort) and protection for thousands of years.
  • Fire-stick farming
  • Fire is used for cremation.
  • Fire is used for welding.
  • Fire is used for celebration (birthday candles)
  • Back-burning for fighting fires
  • Controlled burn-offs for preventing wildfires
  • Controlled burn-offs to clear land for agriculture

See also

References

Citations

  1. ^ Spiral flames in microgravity, National Aeronautics and Space Administration, 2000.
  2. ^ CFM-1 experiment results, National Aeronautics and Space Administration, April 2005.
  3. ^ LSP-1 experiment results, National Aeronautics and Space Administration, April 2005.
  4. ^ "Flame Temperature Measurement"
  5. ^ "Flame Temperatures"
  6. ^ "Pyropen Cordless Soldering Irons"
  7. ^ "UNESCO - Fossil Hominid Sites of Sterkfontein, Swartkrans, Kromdraai, and Environs"
  8. ^ "Share of Total Primary Energy Supply", 2002; International Energy Agency
  9. ^ "Energy in Africa - Chapter 3", United States Department of Energy information administration
  10. ^ The Straight Dope: What exactly is fire?. Adams, C. (2002). Retrieved December 19, 2004.
  11. ^ "How Can Burning Wood Help Reduce Global Warming", The Guardian

General references

  • , (2005). Climate Change Begins at Home. Palgrave Macmillan. ISBN 1-4039-4578-0
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