THERMAL DEGRADATION OF LIGNIN - A REVIEW

INTRODUCTION Lignin, a valuable resource for chemicals and energy, is a main component of wood, together with cellulose and hemicellulose. It is the second large source of organic raw material, constituting about 4-35 wt% of most biomass, 16-25 wt% of hardwoods and 23-35 wt% of softwoods. As the most abundant natural aromatic polymer, lignin has a highly branched three-dimensional phenolic structure including three main phenylpropane units, namely p-coumaril, coniferyl and sinapyl (Fig. 1). Softwood lignin contains relatively fewer sinapyl units and consists mainly of guaiacyl structures, while hardwood lignin contains guaiacylsyringyl structures. As a by-product of the paper industry, lignin is most often used by paper mills as a fuel for the recovery of its energy content. However, due to the very large generated quantities, lignin is increasingly considered as a potential source of chemicals, and studies on its thermal degradation receive much interest. “Pyrolytic lignin”, the organic phase obtained from the pyrolysis of wood or of other biomass resources, consists of a brown tar containing high molecular weight compounds derived from lignin, while the water-soluble fraction, accounting for 60-70 wt% of the whole oil, contains lower molecular weight substances.