Analyses of the impact of torrefaction processes on hydrolysis lignin samples through chemical and morphological investigations

Hydrolysis lignin (HL) samples were torrefied in a fixed bed reactor under nitrogen, during 30 or 60 min, and under four isothermal temperatures (225, 250, 275, or 300 °C). The impact of these torrefaction conditions on the material was analyzed through chemical, thermal, and biochemical analyses and scanning electronic microscopy (SEM) investigations. The H/C and O/C ratios were observed to decrease from 1.33 to 1.09 or 1.05 and from 0.43 to 0.27 or 0.24, with the isothermal temperature or residence time, because the carbon contents of the torrefied samples were significantly increased, leading to materials close to chars. The lower heating values were increased to 23.5 or 25.3 MJ kg−1 with the torrefaction process when compared with that (21.6 MJ kg−1) of raw HL. The highest energy yield was reached for the lowest temperature and residence time. The potassium percentage was highly increased in the torrefied samples. SEM investigations revealed that the sheeted structure of the raw material led to fragments whose assembly became chaotic when increasing the isothermal temperature and residence time. ANOVA computations proved that the residence time had no significant impact on the characterizations of the torrefied samples.