Thermal properties

Abstract

The traditional question at the start of a class on thermal properties of wood is, “Does wood burn?” The students have all been warmed in front of a wood-burning fire before, so they are sure the answer is yes—but since the professor asked the question, there must be some hidden trick to the obvious answer. Going with their experience, their answer is “yes, wood burns.” But, the actual answer is no, wood does not burn. In short, wood undergoes thermal degradation as it heats up, giving rise to volatile, flammable gases which burn when they contact a source of ignition. So it is the flammable gases that burn, not the wood itself. This is, of course, an oversimplified explanation of the pyrolysis and burning processes which are the subject of this chapter. Lignocellulosic materials decompose on heating and when exposed to an ignition source by two different mechanisms. The first, dominant at temperatures below 300˚C, degrades polymers by the breaking of internal chemical bonds; dehydration (elimination of water); formation of free radicals, carbonyl, carboxyl, and hydroperoxide groups; formation of carbon monoxide and carbon dioxide; and finally, the formation of reactive carbonaceous char. Oxidation of the reactive char results in smoldering or glowing combustion, and further oxidation of the combustible volatile gasses gives rise to flaming combustion. The second mechanism, which takes over at temperatures above 300˚C, involves the cleavage of secondary bonds and formation of intermediate products such as anhydromonosaccharides, which are converted into low molecular weight products (oliosaccharides and polysaccharides), which lead to carbonized products.

Citation

Rowell, Roger M. 2005. Thermal properties. Handbook of wood chemistry and wood composites. Boca Raton, Fla. : CRC Press, 2005: pages 121-138.