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What is Fire, What is Smoke?

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What is a fire?

A wood fire is a type of chemical reaction, in which wood combines with oxygen, so that if it is burning well, the only products will be carbon dioxide (CO2), water (H2O) and a small amount of ash, which cannot burn any further. Carbon dioxide is a gas and the water appears as steam.

Wood

Wood is "organic", which means that it contains a lot of carbon. It also contains hydrogen, oxygen and small amounts of many other elements. When wood is heated to a high enough temperature, it begins to decompose, which is another way of saying that it falls apart. When it decomposes wood releases gases, vapours and very small particles which then mix with the oxygen in the air and creates a lot of heat. 

The particles become so hot that they glow for a short while as they escape up in to the air. There are billions of these tiny particles glowing at once, and it is this glowing mixture of particles and gases that we call flames. As soon as all the particles are burned in a flame, the glowing stops, and we just see a heat haze above the fire.

So when we light a fire, it is best to use small twigs and paper first to heat up the wood enough so that it can release all of the gases, vapours and particles. This is why a fire will produce a lot of smoke when first lit as there is not enough heat yet to burn the gases, vapours and particles properly. Also to keep a fire burning hot enough it needs a good supply of air. When air is restricted the fire will smoulder producing a lot of unburnt gases, vapours and particles or 'wood-smoke'.

What is smoke?

Smoke is a collection of very small carbon particles and tiny drops of oils and tars. Individual particles are too small to see with the naked eye, but many particles together become easily seen as wood-smoke. It arises because the fuel is not burnt properly. Smoke is therefore wasted fuel.

Charcoal

The final phase of wood combustion, or decomposition, is when wood turns into charcoal. Almost half of the heat released by wood comes from the glowing coals. When the wood has been burning properly for a while most of the gases vapours and particles (smoke) have been released leaving only charcoal which is almost pure carbon. The charcoal glows because it still has contact with the oxygen in the air (burning) and is extremely hot. 

No flames are generated from glowing coals because charcoal only produces carbon dioxide which, unlike gases, vapours and particles, cannot be burnt any further, therefore, there are no flames. Burning charcoal is much cleaner than burning wood due to the little amount of smoke produced. So, the quicker a fire is reduced to glowing coals the hotter it will be and the less smoke it will produce.

During this phase, very little smoke is emitted because charcoal is a very clean burning fuel. It is essential that before turning a fire down that only glowing charcoal remains. If it is turned down before then, the air is cut off that is needed to burn all the gases properly, and will generate smoke again.

For further information

Air Specialist
134 Macquarie Street
Hobart TAS 7000
Phone: 03 6165 4599 Fax: 03 6233 3800
Email: EnvironmentEnquiries@environment.tas.gov.au