Effects of torrefaction on lignin-rich biomass (hazelnut shell): Structural variations

Although the role of torrefaction on individual macromolecules of biomass including hemicellulose, cellulose, and lignin has been studied in detail, the behavior of lignin-rich biomass during torrefaction has not been identified properly. Therefore, this study focuses on the determination of the effects of torrefaction on a high-lignin content hazelnut shell (HS) at 300 °C that is regarded as the upper temperature limit for torrefaction. In this way, the fuel properties, chemical composition, and the functional groups of biochar produced from torrefaction were compared with those of the untreated HS. Upon torrefaction, the carbon content increased 34.5% and the oxygen content decreased 33% which means an improvement in fuel properties at the expense of a 19.7% loss in hydrogen content. Accordingly, the torrefaction process changed the chemical composition of biomass from C₆H_9.95O_4.26 to C₆H_5.94O_2.12, which shows the enrichment of carbon per both hydrogen and oxygen atoms and this corresponds to a lignitic coal structure. As a result of oxygen loss mainly from the hemicellulose structure in the form of C - O - C, Fourier Transform Infrared spectroscopy bands of C=C, C - H, and C=O intensified in the biochar. Besides, the bands that predict the aromaticity due to lignin content did not lose their intensities. It is concluded that torrefaction is a promising method to improve the fuel characteristics of lignin-rich hazelnut shells.