Investigation of biomass torrefaction based on three major components: Hemicellulose, cellulose, and lignin

In this work, torrefaction of hemicellulose, cellulose, and lignin were studied at a series of torrefaction temperatures (210, 240, 270, and 300 °C) based on the properties of their three-phase products, namely solid, liquid (water and tar), and gaseous products. Among the three biomass components, significant difference of torrefaction characteristics was found due to their different molecular structures. For the solid product, hemicellulose presented lowest yield from 85.65% to 41.54% as the temperature increased because of the poor thermostability, thereby showing obvious variations in carbon and oxygen contents. For the gaseous product, CO2, followed by CO, were the dominant gaseous components in all torrefaction conditions including different feedstock and temperatures; also a small number of other components (H2 and CH4) were produced during torrefaction of lignin. The liquid product was composed of water and a small amount of tar. For the tar product, acids and ketones were the dominant components in the torrefaction of hemicellulose, while anhydrosugar in the torrefaction of cellulose, and phenols in the torrefaction of lignin. As the temperature increased from 210 °C to 300 °C, about 19.76–71.11%, 5.85–33.27%, and 16.28–44.89% of oxygen from hemicellulose, cellulose, and lignin, respectively, was transformed into the liquid and gaseous products. Dehydration reaction and volatilization of the oxygenated gaseous product were the two dominant deoxygenation pathways, and the water, CO2, and CO were the dominant carriers of oxygen migration. In contrast to oxygen, only a small amount of carbon was transferred into tar product, followed by gaseous product, consequently the solid samples still retained most of the energy.