A video showing fire ignite and
extinguish
Fire is the rapid
oxidation of a
combustible material releasing
heat,
light, and various
reaction products such as
carbon dioxide and
water. If hot enough, the gases may become ionized to
produce
plasma. Depending on the
substances alight, and any impurities outside, the
color of the
flame and the fire's
intensity might vary. Fire in its most
common form can result in
conflagration, which has the potential to
cause physical damage through
burning.
Chemistry
Fires start when a flammable and/or a combustible material with an
adequate supply of
oxygen or another oxidizer
(e.g. nitrates, nitrites, inorganic peroxides, permanganates) is
subjected to enough
heat and is able to sustain
a
chain reaction. This is commonly
called the
fire tetrahedron. Fire
cannot exist without all of these elements being in place and in
the right proportions (though as previously stated, another strong
oxidizer can replace oxygen). For example, a flammable liquid will
start burning only if the fuel and oxygen are in the right
proportions.
Once ignited, a chain reaction must take place whereby 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 an oxidizer and fuel.
Fire can be
extinguished by removing
any one of the elements of the fire tetrahedron. Fire extinguishing
by the application of water acts by removing heat from the fuel
faster than combustion generates it. Application of
carbon dioxide is intended primarily to
starve the fire of oxygen. A forest fire may be fought by starting
smaller fires in advance of the main blaze, to deprive it of fuel.
Other
gaseous fire
suppression agents, such as
halon or
HFC-227, interfere with the chemical reaction
itself.
Flame
A flame is a combanation of reacting liquids and solids emitting
visible and
infrared dark, the
frequency spectrum of which depends on
the chemical composition of the burning material 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.
Usually oxygen is involved, but
hydrogen
burning in
chlorine also produces a flame,
producing
hydrogen chloride (HCl).
Other possible combinations producing flames, amongst many, 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 also plays a role in flame formation.
Modifying the gravity causes different flame types. 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
micro gravity 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 may go out if
not moved steadily, as the CO
2 from combustion does not
disperse as readily in micro gravity, and tends to smother the
flame). There are several possible explanations for this
difference, of which the most likely is that the temperature is
sufficiently evenly distributed that soot is not formed and
complete combustion occurs. Experiments by NASA reveal that
diffusion flames in micro gravity
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 micro gravity when compared to normal
gravity conditions. 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
Temperatures of flames by appearance
The temperature of flames with carbon particles emitting light can
be assessed by their color:
- Red
- Just visible:
- Dull:
- Cherry, dull:
- Cherry, full:
- Cherry, clear:
- Orange
- White
- Whitish:
- Bright:
- Dazzling:
Fossil record
Human control
The ability to
control
fire was a major change in the habits of early humans.
Making fire to generate heat and light made it
possible for people to
cook food, increasing
the variety and availability of nutrients. The heat produced would
also help people stay warm in cold weather, enabling them to live
in cooler climates. Fire also kept nocturnal predators at bay.
Evidence of cooked food is found from , although fire was probably
not used in a controlled fashion until 400,000 years ago. Evidence
becomes widespread around 50 to 100 thousand years ago, suggesting
regular use from this time; interestingly, resistance to
air pollution started to evolve in human
populations at a similar point in time. The use of fire became
progressively more sophisticated, with it being used to create
charcoal and to control wildlife from 'tens of thousands' of years
ago.
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"
which 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 autumn. 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. Fire was the basis of all
early thermal weapons.
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. In the
First World War, the first modern
flamethrowers were used by infantry,
and were successfully mounted on armoured vehicles in the Second
World War.
In the latter war, incendiary bombs were used by Axis and Allies alike, notably on Tokyo,
Rotterdam, London, Hamburg and, notoriously, at Dresden
, in the
latter two cases firestorms were
deliberately caused in which a ring of fire surrounding each city
was drawn inward by an updraft caused by a central cluster of
fires. The United States Army Air Force also extensively
used incendiaries against Japanese targets in the latter months of
the war, devastating entire cities constructed primarily of wood
and paper houses. In the
Second World
War, the use of
napalm and
molotov cocktails was popularized, though
the former did not gain public attention until the
Vietnam War. More recently many villages were
burned during the
Rwandan
Genocide.
Fire 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. 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
electricity.
The unburnable solid remains of a combustible material left after a
fire is called
clinker if its
melting point is below the flame temperature,
so that it fuses and then solidifies as it cools, and
ash
if its melting point is above the flame temperature.
Fire protection and prevention
Fire fighting services are provided in
most developed areas to extinguish or contain uncontrolled fires.
Trained
firefighters use
fire apparatus, water supply resources such
as
water mains and
fire hydrants or they might use A and B class
foam depending on what is feeding the fire.
Fire prevention is intended to reduce sources of ignition, and is
partially focused on programs to educate people from starting
fires. 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.
Model building
codes require
passive fire protection and
active fire protection systems to
minimize
damage resulting from a fire.
The most common form of active fire protection is
fire sprinklers. To maximize passive fire
protection of buildings,
building
materials and
furnishings in most
developed countries are tested for
fire-resistance,
combustibility and
flammability.
Upholstery,
carpeting
and
plastics used in
vehicles and
vessels
are also tested.
Human cost
[[Image:Fires world map - DALY - WHO2004.svg|thumb|
Disability-adjusted life year
for fires per 100,000 inhabitants in 2004.
]]
See also
References
- 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.
- "A Book of Steam for Engineers", The Stirling Company,
1905
- Fire & Life Safety Education, Manitoba Office of the Fire
Commissioner
Further reading
- Haung, Kai. 2009. Population and Building Factors That Impact
Residential Fire Rates in Large U.S. Cities. Applied Research
Project. Texas State University.
http://ecommons.txstate.edu/arp/287/
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