A Burning Scientific Question

[bearbait]

One log won't burn.
Two logs will burn.
Three logs make a fire.

An adage I've known for a many a long time. The Indian Fire Lay, or Star Fire Lay works on this principle: push the logs together to increase the fire; pull them apart slightly to slow it.

How come that barbeque charcoal briquettes don't seem to readily follow this rule when they're apparently burning, or glowing, just the same as logs? For example when I bake in the Dutch Oven I have a few briquettes scattered on the lid of the oven all some distance from each other but they seem to happily burn away for considerable time without the need for contact.

Any ideas? Is it because they're already in the charcoal state whereas wood has to get rid of moisture, etc. and get to that state first for combustion?

 

[Janne]

Excellent question1
(I do not have a clue!)
Could it be that the briquettes are porous enough so the surface that 'burns' is very large? So surface area?

How about charcoal chunks?

Once ignited, I tilt the one log at a 45 degree angle - fire at the bottom area, and usually it burns OK. I use thin logs though, split sticks maybe 1-3 inches across before splitting. I like small fires. I am lazy.

 

[sunndog]

Sommat about free radicals mebbe

 

[Klenchblaize]

Quantum physics: As in the "Entanglement Theory".

Don't ask for more as my brain is way too small!

K

 

[bopdude]

Inward burning charcoal as outward burning wood ? although both decrease in size the charcoals heat is centered to the core whereas wood expels a flame outward, just a guess mind you ?

 

[Janne]

Sommat about free radicals mebbe

No Politics please!

 

[C_Claycomb]

I think I know, or at least I think I can have a pretty good guess.

What we call burning is an exothermic chemical reaction that takes fuel and oxygen and releases energy (light and heat are on the same spectrum, just different wave lengths), carbon dioxide and possibly other gases. The reaction requires an initial kick of energy, ignition, and continues when the energy released as heat is great enough to maintain the reaction. More oxygen tends to result in more heat and fewer large molecules in the waste products gases. More heat can also result in a more efficient use of available oxygen.

Charcoal is mainly carbon and minerals. As mentioned, it has high surface area which makes it more readily reactive. It does not need to be turned into a gas in order to react and release heat and CO2. It does not need a lot of air to continue its reaction since the carbon is so readily available and it has so much area.

Wood on the other hand is made of all sorts of long chain molecules that must be broken down and turned into a gas before they can react and release heat. I am not sure of exact temperatures, but there is plenty of info out there for folk who are interested, such as this lot about ignition temperatures:
http://www.tcforensic.com.au/docs/article10.html

Wood starts to release gasses at a lower temperature than those gasses will ignite. In order for the gas/air mixture to ignite, there must be some additional heat, a source of ignition. Then in order for the reaction to continue the temperature of the gas as it is released from the wood must be maintained. The hottest part of a flame is higher up, where the reaction with oxygen is taking place most actively, but if the wood down at the bottom of the gas column is not kept hot, it will either cease producing gases, or they will become cooler and eventually there won't be enough heat in the gas to maintain the reaction.

Pushing wood together, with the right amount of spacing, allows the wood to share radiant heat, to heat both gases and air flow and prevent too much heat being lost to the environment. This keeps the wood generating combustible gases. Moisture interferes with combustion because heat that could be used to liberate combustible gases is getting used to to make steam. Wood does tend to have more moisture than charcoal, but one would see the same thing happen even with really dry wood, just to a lesser extent.

Hope that makes some sort of sense.

 

[C_Claycomb]

My burning question is why I seem to get more soot on the bottom of my pan when using something like the Bushbuddy or Solostove, which has twin walls and is meant to mix air with smoke and give more complete combustion, than when the same pan is hung over a camp fire. The stove uses less wood to boil a given quantity of water, and tends to burn all the wood to ash very efficiently, but the soot on the pan is invariable heavy.

Could be that on a fire the soot gets baked on more, so it is not so noticeable coming off on everything, like it does when using a stove, but I am not sure this is the case.

Anyone?

 

[Janne]

Soot = carbon= incomplete combustion due to lack of Oxygen.
Try to use less fuel at a time. Or make the distance flame to vessel smaller.

A flame is in fact not hottest at the tip. Lit a match in an environment that is completely still, no air movement and look how the flame looks like.
You see two quite distinct parts. The hottest area is at the tip of the inner part.

Experiment with a vessel over the flame, change the distance.

I did this years ago as I find the oily soot a pure p.i.t.a. when you use containers that stack into each other like a Trangia.

 

[bearbait]

Many thanks C_Claycomb for your answer. I've read it though two or three times now and am generally getting it.

FWIW I've found that a mix of regular wood fire embers and of charcoal briquettes makes quite a flexible heat base to cook with. And sit by...

 

[leon-1]

Wood is well wood and wood contains a lot of water, you require a lot of energy to burn off the moisture in wood, this is normally released as smoke which contains a massive amount of steam bonded with a level of carbon and nitrogen. So when you heat up a wood molecule that contains carbon, nitrogen, lignin, cellulose, hydrogen and oxygen all of the atoms within that molecule start to vibrate at differing frequencies. The radicals or bonds that hold the molecule together cannot stand upto all of the different vibration frequencies running through them and as such some of the bonds are broken and some of the atoms break away as smoke. When the smoke gets hot enough it will seperate into its component atoms leaving at least 2 volatile gases which will ignite. As we cannot see pure gases as they burn what we see as flames are infact the remaining elements of the smoke also being ignited so the flames are actually taking their colour from the nitrogen and carbon that have been bonded into the steam given off. The ash that remains after fire is the result of complete combustion.

Charcoal is on the other hand wood that has been taken to the point of pyrolisis. It has now undergone an irreversible chemical change due to heat, because oxygen levels have been retarded it is impossible for the wood to go through complete combustion after all moisture / volatile gases have been released so when heat is taken away you are left with the result of incomplete combustion which is charcoal.

Charcoal is like charred cloth (with the omission of lignin) in as much as it's packed full of free radicals and they will combine with most anything if the conditions are right, so when you subject it to large amounts of heat in the open air it will strip oxygen from it's surrounding to achieve complete combustion and because there is little to no moisture to burn off it doesn't require more heat from it's surroundings to perpetuate the cycle.

 

[Mr Wolf]

Basically comparing refined to unrefined fuel.
Sorry to simplify.
Besides you cant have charcoal without a fire

 

[leon-1]

My burning question is why I seem to get more soot on the bottom of my pan when using something like the Bushbuddy or Solostove, which has twin walls and is meant to mix air with smoke and give more complete combustion, than when the same pan is hung over a camp fire. The stove uses less wood to boil a given quantity of water, and tends to burn all the wood to ash very efficiently, but the soot on the pan is invariable heavy.

Could be that on a fire the soot gets baked on more, so it is not so noticeable coming off on everything, like it does when using a stove, but I am not sure this is the case.

Anyone?

Not sure on this one Chris, but you are focusing all of the waste from fire that has been enhanced by mixing wood gas with oxygen from the air, the stove itself should be providing a greater thermal column than an open fire and as such probably creating a greater draw and probably the faster it burns fuel. Because it is far hotter there will be a lot of carbon thrown up and probably not getting a chance to get to complete combustion as they are coming into contact with something that in essence is cool and a bit like boiling water in a plastic bottle the conduction of heat through your pot probably is stopping them from reaching complete thermal degredation.

 

[leon-1]

Basically comparing refined to unrefined fuel.
Sorry to simplify.

It depends on how you look at it. Remember that the gasses given off by wood when it's heated are a fuel in their own right and that for charcoal to burn it has to be able to oxidise with the oxygen in the air around it, the more you provide (a bellows for instance) the hotter it becomes.

Beyond that charcoal is a better fuel in many ways as it provides more heat for less weight, but it is still a by product of an inefficient fire and incomplete combustion.