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 °C/min and an isothermal mode at various temperatures, kept constant between 280 and 320 °C. 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/RTxn(1−0.99x)m. Reaction 1 is related to the bulk decomposition of cellulose and is characterised by the set of parameters: E1=202 kJ/mol; n1=1; m1=0.48. Reaction 2 is related to the slower residual decomposition, which takes place over approximately 350 °C and affects only 16% by weight of the raw cellulose. With m2 constrained to 1, the optimised parameters of this reaction were: E2=255 kJ/mol; n2=22. Finally, the proposed model allowed to correctly fit not less than to 10 sets of ATG–DTG data, isothermal and dynamic.