The flame heats any surrounding fuel so it releases gases as well. When the flame ignites the gases, the fire spreads. On Earth, gravity determines how the flame burns. All the hot gases in the flame are much hotter and less dense than the surrounding air, so they move upward toward lower pressure.
This is why fire typically spreads upward, and it's also why flames are always "pointed" at the top. If you were to light a fire in a microgravity environment, say onboard the space shuttle , it would form a sphere!
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Mobile Newsletter chat subscribe. Prev NEXT. Environmental Science. Earth Science. When the volatile gases are hot enough about degrees F degrees C for wood , the compound molecules break apart, and the atoms recombine with the oxygen to form water, carbon dioxide and other products.
In other words, they burn. The carbon in the char combines with oxygen as well, and this is a much slower reaction. Wood, for example, contains molecules made from bound atoms of carbon, hydrogen and oxygen and smaller amounts of other elements.
When wood gets hot enough — such as when lightning hits or a log is tossed on an already burning fire — those bonds break. The process, called pyrolysis, releases atoms and energy. Unbound atoms form a hot gas, mingling with oxygen atoms in the air.
This glowing gas — and not the fuel itself — produces the spooky blue light that appears at the base of a flame. When carbon bonds with oxygen, it produces carbon dioxide — a colorless gas. When hydrogen bonds with oxygen, it produces water vapor — even as the wood burns. Fires burn only when all that atomic shuffling releases enough energy to keep the oxidation going in a sustained chain reaction. More atoms released from the fuel combine with nearby oxygen. That releases more energy, which releases more atoms.
This heats the oxygen — and so on. The orange and yellow colors in a flame appear when extra, free-floating carbon atoms get hot and begin to glow. These carbon atoms also make up the thick black soot that forms on grilled burgers or the bottom of a pot heated over a fire. By Stephen Ornes March 14, at am. Ignition and combustion of wood 2. Fire performance of wood 2. Under the influence of heat, wood produces easily substances that react eagerly with oxygen, leading to the high propensity of wood to ignite and burn.
Ignition and combustion of wood is mainly based on the pyrolysis i. When temperature increases, cellulose starts to pyrolyse. The decomposition products either remain inside the material or are released as gases. Gaseous substances react with each other and oxygen, releasing a large amount of heat that further induces pyrolysis and combustion reactions.
Pyrolysis and burning processes are illustrated in Figure 1. Depending on environmental conditions such as temperature, oxygen concentration, moisture, fire retardants, pH etc. In this case, pyrolysis produces a lot of tar including levoglucosan that decomposes easily into burning gases under the influence of heat see Figure 2b.
Thermal decomposition can take place also through char forming pathway. The pyrolysis of wood is dependent on external factors, such as the way of heating, warming-up rate of the material, etc.
Therefore, wood products do not have an explicit ignition temperature, but ignition takes place on a certain temperature range where the probability of ignition becomes large enough. The reaction-to-fire properties, such as ignitability, heat release and flame spread, are most relevant for fire retardant wood products.
Charring as a fire resistance characteristic property may also be influenced especially by surface protective layers. In order that wood can ignite, its temperature must rise so high that pyrolysis takes place strongly enough and the chemical reactions of combustion start. Therefore, the ignition of a wood product is dependent on the way of heating, that is, the thermal properties of the material, and the way of heat attack on the material.
The factors affecting the ignition of wood are well known in general: wet wood is difficult to ignite, thin pieces of wood ignite more easily than thick logs, and light wood species ignite quicker than heavy species. External factors having an influence on ignition are the intensity of heat exposure and its form of effect e. The moisture content of wood has an effect on ignition mainly as a heat sink.
Heating-up of the water and especially its vaporization consume heat energy. In addition, moisture increases the thermal inertia of the material.
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