Molecular Mechanisms in the Thermochemical Conversion of Lignins into Bio-Oil/Chemicals and Biofuels

Abstract

11.1 Introduction Lignin accounts for 20–30 % by weight of lignocellulosic biomass , and is a promis-ing renewable resource for the production of aromatic chemicals and bio-fuels [ 1 ]. It is composed of phenylpropane units containing three different aromatic ring sub-stitution patterns: p -hydroxyphenyl (H), guaiacyl (4-hydroxy-3-methoxyphenyl , G) and syringyl (3,5-dimethoxy-4-hydroxyphenyl , S) [ 1 ]. Softwoods contain a greater proportion of G units and smaller amounts of the H type, whereas hardwood s con-sist of G and S units while herbaceous species contain G, S and H units [ 1 ]. These monomers are linked together through ether (C–O) and condensed (C–C) bonds. Accordingly, lignin pyrolysis proceeds heterogeneously, depending on the plant species . This is in contrast to the pyrolysis of cellulose, a homogeneous polymer of D-glucose units connected via β-1 → 4 linkages . In this chapter, molecular mechanisms involved in lignin pyrolysis are discussed with the focus being primarily on G-lignin after the brief analysis of the devolatil-ization temperature of lignin and the product compositions. Although many papers have reported theoretical investigations of specifi c pyrolysis reactions of lignins, this chapter concentrates on the results of experimental investigations conducted by the author's research group.