Assessment of various kinetic models for the pyrolysis of a microgranular cellulose

The kinetics of pyrolysis of a micro-crystalline cellulose in nitrogen were studied from TGA and DTG data, obtained with two different modes of heating: a dynamic mode at constant heating rates between 1 and 11 degreesC/min and an isothermal mode at various temperatures, kept constant between 280 and 320 degreesC. In isothermal mode, it appeared very clearly that the mass depletion shows a sigmoid profile characteristic of an auto-accelerated reaction process. This behaviour is consistent with kinetics of nuclei-growth, well represented by the models of Avrami-Erofeev (A-E) and of Prout-Tompkins (P-T) type. All the other kinetic models commonly applied to the thermal decomposition of solids revealed unsatisfactory. The TGA and DTG data were, thus, found ideally simulated from a reaction scheme consisting in two parallel reactions, termed 1 and 2, each one described by the kinetic law: dx/dt = -A(-E/RT)X(n)(1 - 0.99x)(m). Reaction 1 is related to the bulk decomposition of cellulose and is characterised by the set of parameters: E-1 = 202kJ/mol; n(1) = 1;m(1) = 0.48. Reaction 2 is related to the slower residual decomposition, which takes place over approximately 350degreesC and affects only 16% by weight of the raw cellulose. With in, constrained to 1, the optimised parameters of this reaction were: E-2 = 255 kJ/mol; n(2) = 22. Finally, the proposed model allowed to correctly fit not less than to 10 sets of ATG-DTG data, isothermal and dynamic. (C) 2004 Elsevier B.V. All rights reserved.