Critical evaluation of global mechanisms of wood devolatilization

Thermogravimetric data on the devolatilization rate of beech wood are re-examined with the aim of incorporating the effects of high heating rates (up to 108 K min⁻¹) in the global kinetics. The mechanism consisting of three independent parallel reactions, first-order in the amount of volatiles released from pseudo-components with chief contributions from hemicellulose, cellulose and lignin, is considered first. It is found that the set of activation energies estimated by Gronli et al. M.G. Gronli, G. Varhegyi, C. Di Blasi, Ind. Eng. Chem. Res. 41 (2002) 4201-4208] (100, 236 and 46 kJ mol⁻¹, respectively) for one slow heating rate results in very high deviations between predicted and measured rate curves. The agreement is significantly improved by a new set of data consisting of activation energies of 147, 193 and 181 kJ mol⁻¹, respectively. In this case, the overlap is reduced between the reaction rates of the three pseudo-components whose chemical composition is also modified. In particular, instead of a slow decomposition rate over a broad range of temperatures, the activity of the third reaction is mainly explicated along the high-temperature (tail) region of the weight loss curves. The performances of more simplified mechanisms are also evaluated. One-step mechanisms, using literature values for the kinetic constants, produce large errors on either the conversion time (activation energy of 103 kJ mol⁻¹) or the maximum devolatilization rate (activation energy of 149 kJ mol⁻¹). On the other hand, these parameters are well predicted by two parallel reactions, with activation energies of 147 and 149 kJ mol⁻¹.