The science of bonfires

[Barry]

Can anybody explain to me in simple terms how bonfires work? I was never very good at science when at school, but I would like to better understand he whole chemical process of fire. For example, what exactly are flames? I suppose they must be the transfer of the solid thing being burned into gas. But why do flames lengthen whenever there is a breeze? Too much wind too soon and a fire goes out; lots of wind when a fire is really going and the whole conflagration is considerably speeded up. Why?
Whenver I go to professional firework displays and there is a bonfire, there is always a tunnel beneath the wood. I suppose this is to enable air to circulate beneath. But why is this necessary?
I am trying to burn some rotten pallets I have found on my plot. These are quite damp, but previous experiences suggests that if the fire gets hot enough, they will burn. However, they are refusing to catch light, which suggests that my fire isn't really hot enough!

[oldherbaceous]

Gosh Barry, thats all to much for me, i'm just going to stick to knowing that it does work.

[Cider Boys]

Hello Barry

I will let more enlightened contributors explain the chemical process, but simply you need three requirements for a fire.

1. Combustible material
2. A source of ignition (heat)
3. Oxygen (from the air)

To put a fire out you only need to remove any one of the above.

Barney

[Allan]

Try this one for your answers

http://en.wikipedia.org/wiki/Fire

Allan

[peter]

A non-comprehensive reply.
Fire is an exothermic chemical reaction.
It gives out heat from the chemical reaction.
At a point that varies according to the materials and local conditions a fire becomes selfsustaining, the heat given out heats up the naterial not yet burning enough for it to burn.

For example, what exactly are flames?
They those parts of the stuff being burnt that become gaseous due to the heat and then combine with the oxygen in the air physically away from the original material.

But why do flames lengthen whenever there is a breeze?
The gas is blown away from its source and continues to combine with the oxygen.

Too much wind too soon and a fire goes out.
The heat that turns solid to gas is blown away and without the gas to combine with oxygen the fire goes out.

Lots of wind when a fire is really going and the whole conflagration is considerably speeded up.
When the hot mass is too large for the wind to cool and continues to produce lots of hot gases, a burst of wind drags them out and makes the flames much much larger.

Whenver I go to professional firework displays and there is a bonfire, there is always a tunnel beneath the wood.Usually this is there to let you light the bonfire at its centre. At any rate that is what my dad used to do with his boarding school's huge fire. It means it burns quicker and brighter as the heat comes from the centre rather than working in from the outside.

I am trying to burn some rotten pallets I have found on my plot. These are quite damp, but previous experiences suggests that if the fire gets hot enough, they will burn. However, they are refusing to catch light, which suggests that my fire isn't really hot enough!
Spot on. In addition to getting hot enough to burn first the wood has to dry out, water turns to steam at 100C and you need to dry it out enough for the burning process to become selfsustaining.

Other odd things.

Charcoal burners get the logpile burning well and then exclude the air as much as possible. With modern kit that means a big pressure cooker with blue flames coming out of the vent. Keeping the oxygen out makes all the gas "boil off" leaving just the carbon.

Coke at the blacksmiths furnace is just carbon, as it is made from coal in a process bit like charcoal making, you will see very little flame, except the part where the air from the bellows is forced through, but it is hot enought to make steel drippy.

Slow or fast.
Forest fires whip along.

Human bodies can burn slowly, media call it spontaneous human combustion, but given the right conditions the fat in the body supports the burning of the flesh and bones, apart from the extremities (hands and feet) where there is not enough fat to generate the heat to burn the flesh and bones.

[Jenny Green]

Flippin' heck Peter you are clever!
The bits I don't understand about spontaneous human combustion are a) why don't they put themselves out? and b) why doesn't the surrounding area catch fire too?

[Barry]

Thank you everybody for that!
So, if I am able to get a small fire going, but then add a large chunk of perfectly dry wood to the top of the flames, why won't it burn? It might char a bit, but most often it will sit there quite happily in the middle of the fire and not burst into flames. What is stopping it from being consumed?

[peter]

The fire retardant sprayed on it by the builder?

It needs to get up to temperature first, but it could have been treated if it is building timber.

[peter]

Flippin' heck Peter you are clever!
The bits I don't understand about spontaneous human combustion are a) why don't they put themselves out? and b) why doesn't the surrounding area catch fire too?

No not really, wish I could organise myself to get successional crops sorted for at least one occassion!

Cos they are dead or unconcious then dead.

Think of a candle, mostly wick that burns all the wax before it gets away from the wick.
Combined with:
A charcoal burners oven, where there is a low oxygen level, so it smoulders rather than burns.