Global kinetics of wood char devolatilization and combustion

Thermogravimetric curves in air of wood chars obtained from different species and conventional or fast pyrolysis show a low-temperature shoulder (devolatilization) followed by a high-temperature peak (combustion). A n-order global reaction provides a very poor description of the differential curves and, in agreement with previous literature, requires relatively low activation energies (114.5 and 167 kJ/mol for conventional and fast pyrolysis chars, respectively). The combination with an additional first-order reaction for the devolatilization stage produces accurate predictions of both integral and differential curves. It also gives rise to much higher activation energies of the combustion reaction (183 or 229 kJ/mol, depending on the severity of the pyrolysis conditions). The activation energy of the devolatilization reaction also increases with the severity of the pyrolysis conditions (from 114.5 to 218.5 kJ/mol), whereas the differences in the char reactivity deriving from the wood species can be taken into account by preexponential factors and order of the combustion reaction. The use of two first-order reactions for the devolatilization stage does not improve significantly the accuracy of the predictions.