Torrefaction of wood: Part 2. Analysis of products

Torrefaction is a mild pyrolysis process carried out at temperatures ranging from 225 to 300 °C, in which hemicellulose, the most reactive fraction of wood, is decomposed. Dehydration and decarboxylation reactions cause a mass loss of the wood, whereas the lower heating value of the wood is largely conserved. Deciduous wood types (beech and willow) and straw were found to produce more volatiles than coniferous wood (larch), especially more methanol and acetic acid. These originate from acetoxy- and methoxy-groups present as side chains in xylose units present in the xylan-containing hemicellulose fraction. The torrefied wood product has a brown/black color, reduced volatile content and increased energy density: 20.7 MJ/kg (after 15 min reaction time at 270 °C) versus 17.7 MJ/kg for untreated willow. It has favorable properties for application as a fuel for gasification and/or (co-)combustion.