Understanding the chemical and structural transformations of lignin macromolecule during torrefaction

Torrefaction is an efficient method to recover energy from biomass. Herein, the characteristics (mass yield, energy yield, physical, and chemical characteristics) of torrefied bamboo at diverse temperatures (200-300 degrees C) were firstly evaluated by elemental analysis, XRD, and CP-MAS C-13 NMR methodologies. Under an optimal condition the terrified bamboo has a relative high energy yield of 85.7% and a HHV of 20.13 MJ/kg. The chemical and structural transformations of lignin induced by thermal treatment were thoroughly investigated by FT-IR and solution-state NMR techniques (quantitative C-13 NMR, 2D-HSQC, and P-31-NMR methodologies). The results highlighted the chemical reactions of the native bamboo lignins towards severe torrefaction treatments occurred, such as depolymerization, demethoxylation, bond cleavage, and condensation reactions. NMR results indicated that aryl-ether bonds (beta-O-4) and p-coumaric ester in lignin were cleaved during the torrefaction process at mild conditions. The severe treatments of bamboo (275 degrees C and 300 degrees C) induced a dramatic enrichment in lignin content together with the almost complete disappearance of beta-O-4, beta-beta, and beta-5 linkages. Further analysis of the molecular weight of milled wood lignin (MWL) indicated that the average molecular weights of torrefied MWL were lower than those of control MWL. It is believed that understanding of the reactivity and chemical transformations of lignin during torrefaction will contribute to the integrated torrefaction mechanism. (C) 2014 Elsevier Ltd. All rights reserved.