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.
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