The pyrolytic degradation of wood-derived lignin from pulping process

Lignin is a key component in the biomass with a complex polymeric structure of the phenyl-C3 alkylunits. The kraft lignin from the wood pulping process is tested in TG-FTIR and Py-GC-MS. The samplesare pyrolyzed in TGA coupled with FTIR from 30 to 900 C at the heating rate of 20 and 40 K/min. Theevolution of phenolic compounds in the initial pyrolysis stage of lignin is determined by FTIR, whilethe second stage is mainly attributed to the production of the low molecular weight species. A benchscalefast pyrolysis unit is employed to investigate the effect of temperature on the product yield andcomposition. It is found that the guaiacol-type and syringol-type compounds as the primary productsof lignin pyrolysis are predominant in bio-oil, acting as the significant precursors for the formation ofthe 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 closelyrelating lignin structure to the pyrolytic mechanisms. The methoxy group (–OCH3) is suggested to workas an important source for the formation of the small volatile species (CO, CO2 and CH4) through the relevantfree radical coupling reactions