Influence of torrefaction on the devolatilization and oxidation kinetics of wood

Devolatilization and oxidation kinetics of torrefied wood have been studied by evaluating thermogravimetric curves measured in nitrogen and air at various heating rates. Samples consist of Norway spruce wood chips torrefied at several process temperatures and residence times. Data about untreated wood have also been obtained for comparison. Measured curves are well predicted by means of a five-reaction mechanism, consisting of three devolatilization reactions for the pseudo-components hemicellulose, cellulose and lignin and, in air, of two additional reactions for char devolatilization and combustion. The torrefaction pre-treatment only requires model modifications in the amounts of volatiles generated from the decomposition of pseudo-components, indicating that only their relative percentages and not their reactivities are modified. On the other hand, a slightly different thermal stability is found for the char generated from torrefied wood, which results in higher activation energy and lower reaction order for the oxidation step. Hence torrefaction conditions can affect the subsequent conversion characteristics of the char product.