The pyrolytic degradation of wood-derived lignin from pulping process

Abstract

Lignin is a key component in the biomass with a complex polymeric structure of the phenyl-C3 alkyl
units. The kraft lignin from the wood pulping process is tested in TG-FTIR and Py-GC-MS. The samples
are pyrolyzed in TGA coupled with FTIR from 30 to 900 C at the heating rate of 20 and 40 K/min. The
evolution of phenolic compounds in the initial pyrolysis stage of lignin is determined by FTIR, while
the second stage is mainly attributed to the production of the low molecular weight species. A benchscale
fast pyrolysis unit is employed to investigate the effect of temperature on the product yield and
composition. It is found that the guaiacol-type and syringol-type compounds as the primary products
of lignin pyrolysis are predominant in bio-oil, acting as the significant precursors for the formation of
the derivatives such as the phenol-, cresol- and catechol-types. A series of free-radical chain-reactions,
concerning the cracking of different side-chain structures and the methoxy groups on aromatic ring,
are proposed to demonstrate the formation pathways for the typical compounds in bio-oil by closely
relating lignin structure to the pyrolytic mechanisms. The methoxy group (–OCH3) is suggested to work
as an important source for the formation of the small volatile species (CO, CO2 and CH4) through the relevant
free radical coupling reactions.

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