Kinetics of cellulose pyrolysis modelled by three consecutive first-order reactions

Abstract

Isothermal thermogravimetry can be used to help elucidate complex reaction mechanisms, particularly if data are represented as ‘DMM plots’, i.e., as - d M′/d t vs. M′, where M′ is dimensionless mass and t is time. In a previous work it was shown that the simplest homogeneous reaction scheme consistent with the DMM curves of cellulose pyrolysis consists of three consecutive first-order reactions. In this work it is further shown that other mechanisms, such as nucleation and growth (Avrami-Erofeev equation) and diffusion, are also inconsistent with the experimental DMM curves. The experimental isothermal temperature range of the previous work is extended up to around 350°C to allow the calculation of the kinetic parameters of each of the three consecutive reactions. The stoichiometric parameters obtained by optimization for each of the reactions confirm former conclusions: the first reaction releases some volatile matter; the second reaction releases practically no volatiles, probably consisting of a rearrangement or activation of the solid; the third reaction releases most of the total volatile matter. Activation energies obtained vary between 139 and 209 kJ/mol.