Thermokinetic investigation of cellulose pyrolysis -: impact of initial and final mass on kinetic results

Cellulose pyrolysis has been investigated by combined TGA/DTA applying constant heating rates between 0.14 and 105 K min(-1). The experiments entailed variation of initial mass and initial bulk density of the cellulose samples. The final mass depended on the heating rate as well as on the initial mass and bulk density and was reproducible when these parameters were carefully controlled. Mass transport resistance is a dominating factor increasing the char yield even in samples of 1 mg significantly at all heating rates. At higher initial sample masses, the char yield is determined by meshed heat and mass transport phenomena. Exothermic secondary reactions form additional char and change the overall heat of reaction. The heat released by secondary reactions promotes the volatilization of primary tars which in turn reduces the char yield. Fitting global reaction models to the experimental TGA results assuming a single first-order reaction yielded apparent kinetic parameters which were strongly correlated, but varied in a broad range depending in a complicated manner on heating rate and initial sample mass. This, together with the inherent impossibility to predict changes in product distribution, results in the restricted applicability of a single first-order reaction as predictive model for reactor analysis and design. Multi-step models with a modified Broido-Shafizadeh mechanism are more successful in describing experimental results for small sample masses. Nevertheless, they can not be extended to conditions under which secondary char is formed due to the lack of kinetic data in the literature. (C) 2002 Elsevier Science B.V. All rights reserved.