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When wood is heated, the first thing to ignite are the "burnable gasses" released by the heating process (producing flames). Some time later, the combustion of these gasses heat the wood to the point that the carbon eventually burns (seen as embers).

What is this gas which is given off by the wood when it is heated?

To clarify:

  • There are two broad stages of combustion, which can be summarized as before and after the carbon in the wood is heated enough to combust directly. I am asking about the combustible compounds consumed during the first stage (before carbon ignites).
  • We are all familiar with the end result of a fire: smoke, steam, etc. These contain many chemical elements, such as CO2 and nitrous oxides. I am not talking about these byproducts.
  • When making char cloth, a flammable gas is given off. At night, this often appears as a blue (or green) jet of flame extending from the charring container. This (I suspect) is the same or remarkably similar to the gas released by burning wood. What is it?
  • Gasifiers can be used to power vehicles or other machinery by heating wood in an environment with limited oxygen. What type of flammable gas is produced by a gasifier?
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    This might be a stretch, but maybe the people over at chemistry.SE might be able to provide better answers than us over here? – anderas Apr 24 '17 at 6:20
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    This is answered by the Wikipedia article on pyrolysis – Chris H Apr 24 '17 at 7:33
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Wood is not a simple chemical. It releases a number of different compounds when heated.

Try distilling wood some time. Put a few slivers into a test tube and heat it. Pipe the result into another test tube that is in cool water. You'll probably be surprised at all the stuff that collects.

There are many varieties of wood, and each of those can be in different states, different moisture levels, etc. It is therefore impossible to give a accurate account of what you will get with any one chunk of wood.

However, common emissions of heated wood include water vapor, methanol, and various other volatile organics. Obviously the methanol will burn, and so will many of the volatile organics. The water vapor tends to make the result harder to burn because it won't burn itself, but will displace oxygen necessary to burn other components.

Note that methanol was called "wood alcohol" long ago, since that was the primary way to make it.

  • Many of those volatile organics are to some extent bad for you. Many also taste good (they're what makes food taste smoked). In my experiments with smoking garlic and chillies I've certainly observed a lot of tar as they condense. – Chris H Apr 24 '17 at 16:52
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The gasifier probably produces a different mix than cooler processes in your other answer. Heat wood enough without oxygen and you get mainly hydrogen and carbon monoxide, leaving behind charcoal. The H2 and CO mix is a fuel.

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    I find it hard to believe you get free hydrogen from heating wood, at least not without really high temperatures (more than just a Bunsen burner heating a test tube). Do you have a reference for that? – Olin Lathrop Apr 24 '17 at 20:37
  • @Olin I linked it in my comment under the question. en.m.wikipedia.org/wiki/Pyrolysis. It's not free: the process is endothermic (requires heat), which could be your tinder burning and then the gases themselves. Temperatures start below glowing hot. – Chris H Apr 25 '17 at 6:20
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Very complex. I am not an expert, but these links might give a hint.

Smoke - Chemical composition: https://en.wikipedia.org/wiki/Smoke#Chemical_composition

Lignin, essentially the crude oil of the plant world: https://en.wikipedia.org/wiki/Lignin#Composition

See the image on the Wikipedia lignin page to get an idea of how complex it is.

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