Monday, February 14, 2011

The science of fire

Many of you are aware of the fire triangle and the fire pyramid (yes they are different) but how many of you REALLY understand the science behind these catchy terms?

A quick review: The fire triangle is heat, fuel, and oxygen or sometimes referred to as air.  Picture a triangle and if you remove any one of the sides the triangle loses support and collapses.  Remove any piece of the fire triangle and the fire goes out.  This is a fortunate effect as you will understand soon, if you don't already.

And the fire pyramid which is tinder, kindling, and fuel not to be confused with the pyrimid fire that is unrelated to this article.  A pyrimid is unlike a triangle in that it is built on a stable platform and can support itself.

Fire is a chemical process known as oxidation:
In this process oxygen combines with hydrogen and carbon, together the atoms rearrange and form water and carbon dioxide.  This energy causes heat, the same process takes place when metal rusts but the apparent lack of heat is due to a much lengthier time involved. 

But who cares about that nerd stuff here is some more nerd stuff to ponder!

Wood gas is a syngas, also known as producer gas, which is produced by thermal gasification of biomass or other carbon-containing materials such as coal in a gasifier or wood gas generator. It is the result of two high-temperature reactions (above 700 °C (1,292 °F)): an exothermic reaction where carbon burns to CO2 but is then reduced partially back to CO (endothermic); and an endothermic reaction where carbon reacts with steam, producing carbon monoxide (CO), molecular hydrogen (H2), and carbon dioxide (CO2).

In several gasifiers, the actual gasification process is preceded by pyrolysis, where the biomass or coal turns into char, releasing methane (CH4) and tar rich in polycyclic aromatic hydrocarbons (PAH). Other gasifiers are fed with previously pyrolysed char. Wood gas is flammable because of the carbon monoxide, hydrogen, and methane content. According to Wikipedia...I love Wikipedia.

Now for the important parts...When heat is applied to carbon fuel IE: wood, it immediately begins to break it down to the pyrolysis process (char) which brakes down further as described above, releasing wood gas that is ignited by the extreme heat.  When we interrupt this process such as when making char cloth we create a fuel that requires very little energy from heat to initiate the rest of the process because it does not have to first dry out the fuel and then break it down, it is already there. 

NOTE:  Remember that the process of breaking down carbon releases basically water and carbon dioxide, this will be revisited when we talk about food and the survival diet.

So when we use a small heat source on a small fuel IE: tinder it will be sufficient to complete the process were it is insufficient on larger materials.  This why we use tinder, then kindling, and lastly larger fuel wood in a gentle slow progress that supports itself and allows the fire to grow.  This is exactly why you cannot light a log with a little match.  This is important to us because in order to BUILD a fire for survival we have to understand where we can take shortcuts and where physics will simply not allow short cuts no matter how cold or in need you are.  Fire does not respond to tantrums and believe me I have seen some tantrums. 

In a friction fire, material selection aside we have to be able to create sufficient tinder (the char) heat from friction and air flow based on the size of the char particles and the arrangement that they fall in (the lay).  When you practice friction fire and find success pay close attention to the char particles shape size and lay.  That is what you are trying to reproduce time and time again.  It is the most under studied portion of the friction fire process and in my opinion the most important.

So heat oxidized the carbon eventually releasing woodgas that is ignited by the heat creating more heat.  the greater heat is capable of braking down larger fuel etc. etc. 

Have you ever noticed that when a fire is burning it appears to dance on top of the log not actually touching it.  That is the wood gas escaping the biomas (fuel) and carborizing (mixing with oxygen) to allow combustion.  sometime you may even see a gas pocket burn out of the side of the fuel like a little torch, that is because it is escaping the fuel under pressure, just like turning up the bellows fan on a forge.  If you can crank up the airflow you increase the heat, you can even increase the heat to forging temperature if you are clever.  Of course this will also increase the fuel consumption and reduce the waste IE: less smoke. 

There is an equal amount of science involved in controlling the smoke on the fire to create more or less as your needs change.  I may visit that topic in the future.  And of course there are more factors to consider when using chemical fuels to generate a fire but these are the basics and are true in all carbon fuel cases.  Honestly it is amazing to me that a cigarette tossed carelessly out of a car window can create such large wildfires if you think about all the factors that you must consider in order to light  a campfire.


The next time that you are sitting around watching the woodsman's TV.  Pay close attention to all the little chemical processes that are happening in all the stages.  It is more educational than the Discovery Channel.