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