Torrefaction, pyrolysis and two-stage thermodegradation of hemicellulose, cellulose and lignin

Abstract

This study aims to investigate the thermal degradation mechanisms of hemicellulose, cellulose, and lignin using pyrolysis–gas chromatography/mass spectrometry. To characterize the decomposition behaviors, the torrefaction, pyrolysis, and their combination of the three constituents are performed using single-shot and double-shot reactions, with emphasis on the influence of torrefaction upon pyrolysis. The analysis suggests that O-acetyl and pentose units contained in the hemicellulose are thermally degraded into acetic acid and furfural in torrefaction, so acetic acid and furfural contents decrease significantly in the pyrolysis of torrefied hemicellulose. The impact of torrefaction upon cellulose is insignificant, resulting from the high thermal stability of the crystalline structure in the cellulose. When the pyrolysis temperature is higher than 300 °C, the cellulose starts to decompose and form organic volatile products. The products from lignin mild torrefaction at 250 °C are not obvious, except for vanillin. The severe torrefaction at 300 °C has a significant influence on lignin pyrolysis, and the pyrolysis of torrefied lignin results in an increase in aromatic compounds. Overall, the analysis suggests that torrefaction can be applied for biomass pyrolysis to stabilize pyrolysis bio-oil, make its composition more uniform, and separate certain chemicals. Based on the thermal degradation of the three main biomass components from torrefaction, pyrolysis, and their combination analyzed by Py-GC/MS, the obtained findings have provided useful insights into the applications of torrefaction, pyrolysis, and their combination for biofuel production.