Well it was about a year ago I created this thread and I caught myself thinking about it again. I found the article that brought this question up and below are some exerpts.
"Seems that wood is the world's most widely used industrial raw material and the third most coimmonly used fuel.v The hardwoods used in preparation of barbeque are composed primarily of cellulose (39%), hemicellulose (35%), and lignin (19.5%), extractivies (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 tanins, 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 themal degradation called pyrolysis in which the wood breaks down into a mixture of volitiles and solid carbonaceous char. The cellulose and hemicellulose mainly form volitiles while the lignin forms the char. Exactly what products are formed by each depends on the temperature, heating rate, particle size, and any catalysts that might be present.
The solid char remains in place. What goes up with the volitles 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 residues from the breakdown of cellulose furan and derivatives phenolic compounds, and -pay attention here - airborne particles of tar and charred material which form the smoke. If oxygen is present and the temp is sufficiently high, burning of the volitiles occurs. When temp are too low or when or when there is insufficient oxygen for complete combustion of the volitiles, smoldering occurs. This is characterized by smoking, the emission of unoxidized pyrolysis products. If the temperature is high enough, then flaming combustion occurs with less smoking and more complete oxidation of the pyrolysis products. Further pyrolysis of volitiles during combustion may cause char particles (soot) to form.
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 makews great barbeque! And that is why it is so important to pre-burn the wood to coals.