Inputs of solid-state NMR to evaluate and compare thermal reactivity of pine and beech woods under torrefaction conditions and modified atmosphere

Abstract

In this work, solid 13C NMR investigations are carried out to highlight some transformations induced by torrefaction into the wood structure. A first set of experiments is dedicated to a comparative study of the changes into respectively pine and beech wood samples, torrefied under the same conditions at different temperatures ranging from 200°C to 300°C in a lab-scale device. The main transformations which can be put into evidence through NMR investigations are depletion of hemicelluloses, demethoxylation, cleavage of β-O-4 structures, and formation of new chemical structures correlated with the hemicelluloses depletion. For both type of wood some differences between beech and pine wood are highlighted, mainly attributed to differences in the compositions of their lignin and hemicellulose fractions, which results in a higher reactivity of beech wood.The influence of the presence of steam (20%v) or of oxygen (3%v) into the gas atmosphere in the torrefaction device is studied in a second set of experiments. The gas compositions have been chosen to mimic the conditions of an industrial furnace, where strictly neutral conditions could not be set. 13C NMR characterizations of wood samples torrefied at the same temperatures and under different gas atmospheres are compared. It is concluded that below 280°C, the presence of either steam or oxygen has no effect on the characteristics of torrefied solid. Above 280°C, the presence of H2O promotes hemicellulose de-acetylation and to a lesser extent lignin demethoxylation. Above 290°C, the presence of oxygen promotes cellulose degradation.