Two-step thermodegradation kinetics of cellulose, hemicelluloses, and lignin under isothermal torrefaction analyzed by particle swarm optimization

Highlights: • Torrefaction kinetic models of cellulose, hemicelluloses, and lignin are studied. • The two-step model and particle swarm optimization are combined. • Cellulose shows the greatest weight loss at the torrefaction temperature of 300 °C. • Lignin produces the most intermediate under torrefaction at temperatures lower than 300 °C. • CO and CO₂ produced are due to the cleavage of methoxyl, ether, carboxyl, and carbonyl groups. The recognition of the isothermal thermodegradation of cellulose, hemicelluloses, and lignin plays a vital role for torrefaction to upgrade lignocellulosic biomass and produce biochar. This study adopts a two-step model with particle swarm optimization (PSO) algorithm to calculate and predict the isothermal torrefaction kinetics of cellulose, hemicelluloses, and lignin under the torrefaction temperatures of 200, 250, and 300 °C. A thermogravimetric analyzer is coupled with Fourier Transform Infrared (TG-FTIR) spectrometer to analyze the instantaneous weight losses and released gaseous products. The predictions suggest that cellulose shows the greatest weight loss and generates the most volatile products (81.70%) followed by a final residue (18.29%) at the isothermal torrefaction temperature of 300 °C. Hemicelluloses have severe weight loss at 250 °C, owing to their relatively weak structure compared to cellulose. The final residue yield is in the range of 60.04–74.05%, and the second prevalent product is the intermediate ranging from 3.34 to 8.20%. Lignin shows higher thermal resistance to torrefaction and produces the most intermediate under the isothermal torrefaction at temperatures lower than 300 °C, accounting for 86.41–97.50%. The activation energies of cellulose, hemicelluloses, and lignin are in the range of 166–260, 48–55, and 59–70 kJ mol⁻¹, respectively. The FTIR spectra indicate that CO and CO₂ are the dominant gases in the torrefaction of the three model compounds due to the cleavages of methoxyl, ether, carboxyl, and carbonyl groups.