The Science of Fire

Fire is used and has been used in one form of another by nearly every human being on the planet on a daily basis but what is it?

Humankind’s relationship with fire has spanned millennia the origins of our relationship with it lost in the mists of time. We have harnessed the energy of fire for comfort and warmth, to cook and preserve food, forge and strengthen our sense of community and self identity, and to light and protect our homes.  From the earliest of times it has been and still is used as essential tool in land management to clear land, fertilise it, and as a way to promote new growth. Fire has and does shape the very social, political, and economic environment we live in.  Fire across time and cultures has been used as a means to signal and celebrate good and bad news, to smelt, mould and shape metal, glass, wood and more recently plastics and polycarbonates. In a more modern context fire powers our homes, offices, shopping centres and hospitals through the use of steam driven generators in electric power stations. It lies at the heart of the power pack of the internal combustion engine. Fire, at one of the same time has been our greatest asset and friend, and our worst nightmare and foe, our supreme defender and most destructive aggressor, the greatest gift to humanity and one of nature’s most devastating destructive forces whilst through out history elevated to a spiritual and ceremonial icon. In summary our contemporary and historical lives are intertwined with and inseparable from our relationship with fire.   

The ancient Greeks understood fire to be one of the four elements earth, water, and air alongside   fire that make up the universe. Unlike earth, water, and air fire does not consist of material matter but is in fact a perceptible bi product of material matter in the process of changing form. In its simplest terms fire consists of a complex chemical reaction involving the rapid oxidation of material matter, in other words fuel, whether that material matter is biomass, fossil fuels such as gas, oil and

 

Fig 1:  Triangle of Fire

 

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coal or any other combustible material. The combustion process results in a greater release of the energy stored within the fuel as either heat or light than the combustion process needs to ignite or sustain itself (exothermic). One of the key words to this definition is the word rapid and therefore excludes slower forms of oxidisation such as rusting and even the bodily function of digestion.

Traditionally, fire has been understood in terms of a triangular relationship between the fuel, heat and oxygen (see fig 1).  Further research into fire, usually funded through the need to combat the destructive forces of an uncontrolled fire, revealed the inadequacy of such simplistic representation of the true complexity of what a fire is.  It is now commonly recognised and accepted that a fourth element is present and an essential component of a fire, the chemical chain reaction.  Without the four elements of fuel, heat, oxygen and the resulting chemical chain reaction being present there is no fire and if any one of the four components is removed the fire is extinguished. Thus the more simplistic  triangular representation of a fire was superseded by the more complex tetrahedron or pyramid see fig 2 a two dimensional depiction of a three dimensional tetrahedron or pyramid of fire.

 

Fig 2: The Tetrahedron or Pyramid of fire.

 

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Oxygen is needed to sustain the fire, heat to raise the temperature of the fuel sufficiently allowing     it to ignite. The fuel or combustible material must present to change form and subsequently an exothermic chemical chain reaction between the fuel and oxygen takes place whilst sufficient levels of heat are maintained to allow the process to takeplace.  As already alluded to earlier within this article exothermic means more energy is being released from the chemical reaction between the fuel,   the oxygen, and the necessary heat to allow combustion to take place than the system needs to sustain itself. This surplus energy is emitted in the form of radiation as heat and light and is the tangible by product of the chemical reaction of the combustible matter oxidising or changing form.  

 

By Phil Cleaver

 

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