Catalytic and char-promoting effects of potassium on lignocellulosic biomass torrefaction and pyrolysis

Abstract

Torrefaction is a potential pretreatment method to produce biochar used as a solid fuel or as an environmentally friendly construction material. However, the torrefaction performance varies when wood composition and mineral content change. Potassium (K) is an abundant mineral present in plants and is claimed to influence the degradation of lignocellulosic biomass. Therefore, a full understanding of the role of potassium is required to predict torrefaction behavior. Samples of beech, celluloses, hemicelluloses, and lignin were impregnated with different potassium carbonate K2CO3 concentrations (0.004, 0.008, and 0.012 M). Thermogravimetric analysis was utilized to analyze their thermal degradation characteristics under different temperatures. At 250 °C, the catalytic effect of potassium was weak. At 300 °C, potassium acted as a catalyst until a precise mass loss (65 wt%) and torrefaction duration (105 min) promoted char formation. The changed effect of this weight loss was possibly related to the relative increase in lignin content compared to cellulose and hemicelluloses While at a higher temperature (380 °C), the catalytic effect of potassium was rapid, and the increased char content was better observed. The potassium was found to act mainly on cellulose and hemicelluloses. After pyrolysis, the char weight increased with K up to 25.86 and 15.23 % for heating rates of 10 and 20 K min−1, respectively. FTIR and XRD showed that potassium did not significantly impact the structure or the crystallinity of wood at ambient conditions. The obtained results provide a novel insight into the role of potassium in the thermal treatment of biomass, with a better understanding of the thermal degradation mechanisms. This information could help to reduce the operating time and/or temperature, thus rendering torrefaction a green method for waste wood valorization.