The Lexington Collection
Pretty Polly
  Fire, Smoke & Coals.

The Combustion of Wood
Turns out that the combustion of wood is an interesting subject that has undergone quite a bit of study in recent years. Seems that wood is the world's most widely used industrial raw material and the third most commonly used fuel. The hardwoods used in the preparation of barbecue are composed primarily of cellulose (39%), hemicellulose (35%), and lignin (19.5%), extractives (3.1%) and ash (0.3%). The cellulose and lignin are primarily in the cell walls. The extractives, primarily in the cells and walls, are such things as tannins, starches, resins, oils, dyes, alkaloids, and sugars. These vary widely among wood species and give each its distinctive characteristics. Up to 80% of the weight of green wood is retained water. Dry wood reaches an equilibrium at 20%.

Wood does not burn directly. Rather, when heat is applied it first undergoes a process of thermal degradation called pyrolysis in which the wood breaks down into a mixture of volatiles and solid carbonaceous char. The cellulose and hemicellulose form mainly volatiles while the lignin mainly forms the char. Exactly what products are formed by each depends upon the temperature, heating rate, particle size, and any catalysts that might be present.

The solid char remains in place. What goes up with the volatiles are a gas fraction (carbon monoxide and dioxide, some hydrocarbons, and elemental hydrogen), a condensed fraction (water, aldehydes, acids, ketones, and alcohols), and -- here we go! -- a tar fraction (sugar resides from the breakdown of cellulose, furan derivatives, phenolic compounds, and -- pay attention here -- airborne particles of tar and charred material which form the smoke.

Wood undergoing pyrolysis with flame combustion of volatiles and glowing combustion of coals.

Smouldering & Flame Combustion
If oxygen is present and the temperature is sufficiently high, burning of the volatiles occurs. When temperatures are too low or when there is insufficient oxygen for complete combustion of the volatiles, smouldering occurs. This is characterized by smoking, the emission of unoxidized pyrolysis products. (This is the awful tasting stuff, creosote, that will give barbecue a bitter taste.) If the temperature is high enough and sufficient oxygen is present, then flaming combustion occurs with less smoking and more complete oxidation of the pyrolysis products. Further pyrolysis of volatiles during flaming combustion may cause char particles (soot) to form.

Coals & Glowing Combustion
The remaining lignin char burns in the presence of oxygen in glowing combustion. These are my beloved coals that yield the thin blue smoke that makes great barbecue! And, that's why it is so important to preburn the wood to coals.

Commercial charcoal, on the other hand, is produced by heating wood at about 1000 deg F in the absence of air to produce a porous solid containing 85-98% carbon. Accordingly, this stuff is fundamentally different from live coals and should not be expected to produce similar results.


The Lexington Collection