Fire is an oxidation process that releases energy in varying intensities in the form of light (with wavelengths also outside the
visual spectrum) 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" cocktails contain a small amount of flammable
high-proof alcohol which is
ignited prior to consumption.
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 whereby heat and
energy are released as a substance changes to a more stable chemical form (in the case of fire, usually generating carbon dioxide
and water). 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 in the visible spectrum. For example, burning alcohol or burning hydrogen are usually invisible although the
heat given off is tremendous.
The visible "clear" flame has no mass. What we see as a 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. The flame
color depends also on the unoxidized carbon particles. In some cases there is a partial fuel oxidation due to oxygen lack in the
central part of the flame, where combustion reactions take place. In such cases the unoxidized hot carbon particles emit
radiation in the light spectrum, resulting in a yellow/red flame, such that of common house fireplace.
Smoldering
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 the temperature and heat released are low in comparison to a flaming.
Chemical Reaction
Fires start when 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.
Burns
Fire causes extreme pain in forms of 1st 2nd and 3rd degree burns. 1st burns only some of the epidermis, 2nd burns all of the
epidermis and some of the dermis, and 3rd burns clean through the dermis and epidermis and kills all nerve receptors. This is why
3rd degree burns often do not hurt.
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
The incomplete burn of a camp fire produces the common red-orange glow
-
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: 1300 to 1600 °C (2372 to 2912 °F) [5]
- Blowtorch flame: 1,300 °C (2372 °F) [6]
- Candle flame: 1000 °C (1832 °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.
Temperatures of flames by appearance
The temperature of flames with carbon particles emitting light can be assessed by their color: [7]
- Red
- Just visible: 977 °F (525 °C)
- Dull: 1290 °F (700 °C)
- Cherry, dull: 1470 °F (800 °C)
- Cherry, full: 1650 °F (900 °C)
- Cherry, clear: 1830 °F (1000 °C)
- Orange
- Deep: 2010 °F (1100 °C)
- Clear: 2190 °F (1200 °C)
- White
- Whitish: 2370 °F (1300 °C)
- Bright: 2550 °F (1400 °C)
- Dazzling: 2730 °F (1500 °C)
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 790,000 years ago. The Cradle of Humankind site has
evidence for controlled fire from 1 to 1.8 million years ago.[8] 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.
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
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.[9] 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.[10] 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
was removed by photosynthesis during production of the wood. Thus, over a 100-year timescale, the effect is
carbon-neutral.[11]. E.ON
UK is soon to build a 44 megawatt wood fired power station in the United Kingdom
for these reasons.[12]
Fire protection and prevention
-
Fire fighting services are provided in most developed areas 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.
Model building Codes require passive fire
protection and active fire protection systems to minimize damage resulting
from a fire. To maximize fire safety of buildings, building products, materials and furnishings in the United States are tested for fire resistance,
combustibility and flammability. 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 jurisdictions 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
- ^ Spiral flames in microgravity, National Aeronautics and Space
Administration, 2000.
- ^ CFM-1 experiment
results, National Aeronautics and Space Administration, April 2005.
- ^ LSP-1 experiment results, National Aeronautics and Space Administration, April
2005.
- ^ "Flame Temperature Measurement"
- ^ "Flame Temperatures"
- ^ "Pyropen Cordless Soldering Irons"
- ^ "A Book of Steam for Engineers", The Stirling Company, 1905
- ^ "UNESCO - Fossil Hominid Sites of Sterkfontein, Swartkrans, Kromdraai, and Environs"
- ^ "Share of Total Primary Energy Supply", 2002; International Energy Agency
- ^ "Energy in Africa - Chapter 3", United States Department of Energy information administration
- ^ The Straight Dope: What exactly is fire?. Adams, C. (2002). Retrieved December 19, 2004.
- ^ "How Can Burning Wood Help Reduce Global Warming", The
Guardian
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