Intrinsic kinetics and devolatilization of wheat straw during torrefaction

Torrefaction is a mild thermal treatment (200–300 °C) in an inert atmosphere, which is known to increase the energy density of biomass by evaporating water and a proportion of volatiles. In this work, the degradation kinetics and devolatilization of wheat straw was studied in a thermogravimetric analyzer by coupling with a mass spectrometer. The kinetic parameters obtained by applying a two-step reaction in series model and taking initial dynamic heating period into account can accurately describe the experimental results with different heating programs. Activation energies and pre-exponential parameters obtained for the two steps are: 71.0 and 76.6 kJ mol−1, 3.48 × 104 and 4.34 × 103 s−1, respectively. The model and these parameters were also proven to be able to predict the residual mass of wheat straw in a batch scale torrefaction reactor. By analyzing the gas products in situ, the formation of water, carbon monoxide, formic acid, formaldehyde, methanol, acetic acid, carbon dioxide, methyl chloride, traces of hydrogen sulfide and carbonyl sulfide were found at torrefaction temperatures of 250 and 300 °C.

► The mass loss during heating was taken into account when deriving kinetic parameters. ► Model can predict the residual mass of torrefaction in a batch-scale reactor well. ► Gas evolution was monitored in situ at 250 and 300 °C for wheat straw torrefaction. ► At 300 °C, water accounts for almost half of the overall mass loss.