Thermal behaviour and kinetic study for woody biomass torrefaction and torrefied biomass pyrolysis by TGA

• Thermal behaviour and kinetics of woody biomass torrefaction were studied.
• Torrefied biomass showed a different decomposition behaviour from raw biomass.
• First-order one-step global model was developed for the torrefied biomass pyrolysis.

The thermal decomposition behaviour and kinetics of Douglas fir sawdust torrefaction and torrefied sawdust pyrolysis were investigated using a thermogravimetric analyser (TGA). The mass loss of samples in torrefaction was highly related to the torrefaction temperature. The two-step reaction model fitted well for Douglas fir sawdust torrefaction. The activation energies of the first and second reaction stages were 112 kJ mol−1 and 150 kJ mol−1, respectively. Torrefied biomass exhibited different thermo decomposition behaviours compared to untreated biomass. The start point of torrefied biomass decomposition was shifted and the degree of shift increased with the severity of torrefaction. The final biochar yield of torrefied biomass was also increased with the increase of torrefaction temperature. Derivative thermogravimetric (DTG) curves showed that the shoulder of hemicelluloses decomposition in torrefied biomass pyrolysis was eliminated. The decomposition rate of torrefied biomass has a decreasing trend due to the mass depletion in torrefaction. A first-order one-step global model with the average activation energies in the range of 195–204 kJ mol−1 can describe the raw and torrefied biomass pyrolysis. The kinetic analysis also showed that the torrefied biomass pyrolysis from high torrefaction temperature might be multiple-step reactions.