Intrinsic kinetics and devolatilization of wheat straw during torrefaction

Torrefaction is a mild thermal treatment (200-300 degrees 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 x 10(4) and 4.34 x 10(3) 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 degrees C. (C) 2012 Elsevier B.V. All rights reserved.