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The influences of torrefaction on the structure and pyrolysis behavior of lignin were investigated in this study. After torrefaction, the oxygen content of lignin was reduced, which led to the increase of its high heating value. The main reactions occurring during torrefaction were the cleavage of aryl ether linkages, demethoxylation, and dissociation of the aliphatic side chain, as indicated by Fourier transform infrared spectroscopy analysis. The distributed activation energy model with double combined Gaussian functions was adopted to analyze the pyrolysis kinetics of lignin. It was found that torrefaction increased the activation energy for degradation and condensation reactions. The devolatilization contribution of condensation reactions increased as well after torrefaction. Torrefaction also had a great effect on the pyrolytic product distribution of lignin. The yields of non-methoxylated phenols and phenols without a propyl side chain increased after torrefaction.

Aiming at promoting the industrial application of biomass pyrolysis technology, many approaches have been explored to reveal pyrolysis reaction mechanism. Among which the thermal analysis kinetics based on mechanism scheme receives significant attention. The kinetic models developed in past decades are classified into five types: one-step global model based on model-fitting method, global model based on model-free method, multi-step successive model, semi-global model and distributed activation energy model. In this review, the computation processes are introduced, and the advantages and disadvantages of these kinetic models are discussed and compared in detail. To obtain a universal kinetic model and simplify the intricate behaviors of biomass pyrolysis, the pyrolytic kinetics of the three major components, namely cellulose, hemicellulose and lignin, have been studied individually. Their pyrolysis behaviors are dramatically different in view of kinetics. It is recommended that the fundamental theory progress which has been made in the solid reaction kinetics should be applied in the biomass pyrolysis kinetics, and a lot of related traditional knowledge in this area needs to be updated urgently.

Cellulose and hemicellulose were isolated directly from softwood Pinus armandiiFranch by the alkali method for pyrolysis mechanism study. Structural characterization based on FTIR, 1H NMR, and 13C NMR showed that the hemicellulose was mainly composed of galactoglucomannan and arabinoxylan with abundant branches, while the cellulose was mainly composed of β-1,4-glucan with a few residual hemicellulose fragments. Pyrolysis of extracted cellulose showed high char yield due to the lower crystallinity. Hemicellulose pyrolysis showed a shoulder peak at low temperature and high char yield because of its abundant branches. A double-Gaussian distributed activation energy model was introduced for pyrolysis kinetics analysis. The devolatilization process for both cellulose and hemicellulose was dominated by a parallel decomposition reaction pathway. The distributions of their pyrolysis products were analyzed by Py-GC/MS. It is suggested that the utilization of isolated sample as model compound could give a better understanding of the biomass pyrolysis mechanism.